Materials for Supercapacitor Applications [1 ed.] 0128198583, 9780128198582

Table of contents :
Cover
MATERIALS FOR
SUPERCAPACITOR
APPLICATIONS
Copyright
Preface
Chapter 1 - Supercapacitor: an introduction
Outline
1.1 - Supercapacitor—an emerging electrical energy storage device
1.2 - Historical perspective
1.3 - Supercapacitors and batteries as electrical energy storage devices—a comparison
1.3.1 - Faradaic and non-Faradaic processes
1.3.2 - Types of capacitors and batteries
1.3.3 - Electrochemical capacitors and batteries: comparative properties
1.4 - Outlook and scope of the monograph
References
Chapter 2
- Fundamentals and energy storage mechanisms—overview
Outline
2.1 - Introduction
2.2 - Fundamentals
2.3 - Supercapacitors: types
2.3.1 - Electrochemical double layer capacitors (EDLCs)
2.3.2 - Pseudocapacitors
2.3.3 - Hybrid capacitors
2.3.3.1 - Composite supercapacitors
2.3.3.2 - Asymmetric supercapacitors
2.3.3.3 - Battery-type supercapacitors
2.4 - Electric double layer
2.4.1 - Helmholtz model
2.4.2 - Gouy-Chapman or diffuse model
2.4.3 - Stern model
2.4.4 - Grahame model
2.4.5 - Bockris-Devanathan-Müller (BDM) model
2.4.6 - Trasatti-Buzzanca-Conway model
2.4.7 - Marcus model
2.4.8 - Electric double layer in supercapacitors
2.5 - Pseudocapacitance
2.6 - Summary and outlook
References
Chapter 3
- Electrode materials for supercapacitors
Outline
3.1 - Introduction
3.2 - Electrode materials
3.2.1 - Carbon materials in supercapacitors
3.2.1.1 - Activated carbon
3.2.1.2 - Mesoporous carbon
3.2.1.3 - Carbide-derived carbons (CDC)
3.2.2 - Carbon nanomaterials in supercapacitors
3.2.2.1 - EDLCs
3.2.2.1.1 - CNTs in EDLCs
3.2.2.1.2 - Graphene in EDLCs
3.2.2.1.3 - Hybrid carbon nanomaterials in EDLCs
3.2.2.2 - Pseudocapacitors (PCs)
3.2.2.2.1 - CNTs in PCs
3.2.2.2.2 - Graphene in PCs
3.2.2.2.3 - Hybrid carbon nanomaterials in PCs
3.2.2.3 - Carbon-based hybrid supercapacitors
3.2.2.3.1 - Carbon-based bendable supercapacitors (film-/fiber-shaped)
3.2.2.3.2 - Carbon-based stretchable and twistable supercapacitors (film-/fiber-shaped)
3.2.2.3.3 - Carbon-based ultrafast supercapacitors for ac-line filtering
3.3 - Perspectives on carbon for SC electrodes
3.4 - Transition metal oxides/hydroxides
3.4.1 - RuO2
3.4.2 - RuO2-based composites
3.4.2.1 - Mixed-oxide composites
3.4.2.2 - RuO2/carbon composites
3.4.2.3 - RuO2/polymer composites
3.4.3 - Manganese dioxide (MnO2) for PCs
3.4.3.1 - Recent R&D Advancements in MnO2
3.4.4 - Cobalt oxides and hydroxide for supercapacitors
3.4.4.1 - Co3O4
3.4.4.2 - Co(OH)2
3.4.5 - Nickel oxide/hydroxide (NiO/Ni(OH)2)
3.4.5.1 - NiO
3.4.5.2 - Ni(OH)2
3.4.6 - Nickel cobaltite (NiCo2O4)
3.4.7 - Tin oxide
3.4.8 - Vanadium oxides-based materials
3.4.8.1 - Vanadium pentoxide
3.4.8.2 - Other elemental metal doped vanadium pentoxide composites
3.4.8.3 - Other vanadium pentoxide composites
3.4.8.4 - Vanadium pentoxide/compound-carbon material composites
3.4.8.4.1 - Vanadium pentoxide/activated carbon or carbon fiber material composites
3.4.8.4.2 - Vanadium pentoxide/carbon nanotubes composites
3.4.8.4.3 - Vanadium pentoxide/graphene composites
3.4.8.5 - Vanadium pentoxide/conducting polymer composites
3.4.8.6 - Vanadium dioxide
3.4.8.7 - Vanadium trioxide
3.4.8.8 - Mixed valence vanadium oxide and its composite
3.4.8.9 - Nitrides
3.4.8.9.1 - Vanadium nitride
3.4.8.9.2 - Vanadium nitride/compound-carbon material composites
3.4.8.9.3 - Vanadium nitride/titanium nitride composites
3.4.8.10 - Vanadium sulfide
3.4.8.10.1 - Vanadium disulfide
3.4.8.10.2 - Vanadium tetrasulfide
3.4.8.10.3 - Silver vanadium sulfide
3.4.8.10.4 - Mixed metal vanadates
3.4.8.11 - Vanadyl phosphate
3.4.9 - Iron oxide-based materials
3.4.9.1 - Influence of preparation routes
3.4.9.1.1 - Hydrothermal method
3.4.9.1.2 - Solvothermal method
3.4.9.1.3 - Electrodeposition method
3.4.9.1.4 - Spin coating technique
3.4.9.1.5 - Electrospinning technique
3.4.9.1.6 - Sol-gel method
3.4.9.1.7 - Precipitation method
3.4.9.1.8 - Successive ionic layer adsorption and reaction (SILAR) method
3.4.9.2 - α-Fe2O3-based composites
3.4.9.2.1 - α-Fe2O3-carbon composites
3.4.9.2.2 - α-Fe2O3-conducting polymer composite
3.4.9.2.3 - α-Fe2O3-metal oxide/hydroxide composite
3.4.9.2.4 - Ternary nanocomposite
3.4.9.3 - Cell performance of α-Fe2O3
3.4.9.3.1 - Symmetric supercapacitor of α-Fe2O3
3.4.9.3.2 - Asymmetric supercapacitor (ASC) of α-Fe2O3
3.5 - Perspectives on transition metal oxides for SC electrodes
3.6 - Conclusions and outlook
References
Chapter 4 - Electrolyte materials for supercapacitors
Outline
4.1 - Introduction
4.2 - Influence of electrolytes on the performance factors of ESs
4.2.1 - Capacitance
4.2.2 - Energy density and power density
4.2.3 - Equivalent series resistance
4.2.4 - Cycle life
4.2.5 - Self-discharge rate
4.2.6 - Thermal stability
4.3 - Electrolyte materials and compositions for electrochemical supercapacitors
4.3.1 - Aqueous electrolytes
4.3.1.1 - Strong acid electrolytes
4.3.1.1.1 - Acid electrolytes for electrical double-layer capacitors
4.3.1.1.2 - Acid electrolytes for pseudocapacitors
4.3.1.1.3 - Acidic electrolytes for hybrid capacitors
4.3.1.2 - Strong alkaline electrolytes
4.3.1.2.1 - Alkaline electrolytes for electrical double-layer capacitors
4.3.1.2.2 - Alkaline electrolytes for pseudocapacitors
4.3.1.2.3 - Alkaline electrolytes for hybrid capacitors
4.3.1.3 - Neutral electrolytes
4.3.1.3.1 - Neutral electrolytes for electrical double-layer capacitors
4.3.1.3.2 - Neutral electrolytes for pseudocapacitors
4.3.1.3.3 - Neutral electrolytes for hybrid capacitors
4.3.2 - Organic electrolytes
4.3.2.1 - General composition, properties, and ES performance of organic electrolytes
4.3.2.1.1 - Organic electrolytes for electrical double-layer capacitors
4.3.2.1.2 - Organic electrolytes for pseudocapacitors
4.3.2.1.3 - Organic electrolytes for hybrid capacitors
4.3.2.2 - Organic solvents
4.3.2.2.1 - Single organic solvents for electrolytes
4.3.2.2.2 - Solvent mixtures for electrolytes
4.3.2.3 - Conducting salts for electrolytes
4.3.2.3.1 - Effect of conducting salt on ES performance
4.3.2.3.2 - Exploration of new conducting salts
4.3.3 - Ionic liquid-based ES electrolytes
4.3.3.1 - General composition, properties and ES performance of ionic liquid electrolytes
4.3.3.2 - Solvent-free ionic liquids
4.3.3.2.1 - Solvent-free ionic liquids for EDLCs
4.3.3.2.1.1 - Aprotic ionic liquids
4.3.3.2.1.2 - Protic ionic liquids
4.3.3.2.1.3 - Mixture of ionic liquids
4.3.3.2.2 - Solvent-free ionic liquids for pseudocapacitors
4.3.3.2.3 - Solvent-free ionic liquids for hybrid electrochemical capacitors
4.3.3.3 - Mixtures of ionic liquids and organic solvents
4.3.4 - Solid- or quasi-solid-state electrolytes for ESs
4.3.4.1 - Gel polymer electrolytes
4.3.4.1.1 - Hydrogel polymer electrolytes
4.3.4.1.1.1 - Hydrogel polymer electrolytes for carbon-based electrodes
4.3.4.1.1.2 - Hydrogel polymer electrolytes for pseudocapacitors and hybrid capacitors
4.3.4.1.2 - Organogel electrolytes
4.3.4.1.3 - IL-based solid-state electrolytes
4.3.4.1.4 - Environmentally friendly gel polymer electrolytes
4.3.4.1.5 - Structural electrolytes
4.3.4.2 - Inorganic solid-state electrolytes
4.3.5 - Redox-active electrolytes
4.3.5.1 - Redox-active aqueous electrolytes
4.3.5.1.1 - Redox-active aqueous electrolytes for carbon-based ESs
4.3.5.1.2 - Redox-active aqueous electrolytes for pseudocapacitive electrodes
4.3.5.2 - Redox-active nonaqueous electrolytes
4.3.5.3 - Redox-active solid electrolytes
4.4 - Electrolyte compatibility with inactive components of ESs
4.4.1 - Compatibility with current collectors
4.4.2 - Binders
4.4.3 - Separators
4.5 - Electrolyte performance validation using supercapacitor test cells
4.6 - Challenges in the development of ES electrolytes
4.7 - Summary and future research directions
References
Chapter 5 - Characterization methods for supercapacitors
Outline
5.1 - Introduction
5.2 - Evaluation of supercapacitors performance
5.2.1 - Overview of test procedures
5.2.2 - Electrochemical apparatus
5.2.3 - Electrochemical cell
5.2.4 - Electrochemical interface: supercapacitors
5.3 - Transient techniques
5.3.1 - Cyclic voltammetry
5.3.2 - Galvanostatic cycling
5.3.2.1 - Constant current charge or discharge
5.3.2.2 - Constant potential charge or discharge
5.3.2.3 - Constant power charge or discharge
5.3.2.4 - Leakage current and self-discharge behavior
5.3.3 - Stationary technique
5.3.3.1 - Electrochemical impedance spectroscopy
5.3.3.2 - Supercapacitor impedance
5.4 - Key scaling parameters
5.4.1 - Capacitance
5.4.2 - Evaluation of CT
5.4.3 - Evaluation of CS
5.4.4 - Major influencing factors
5.5 - Equivalent series resistance
5.5.1 - Evaluation of RESR
5.5.2 - Key influencing factors
5.6 - Operating voltage, Vo
5.6.1 - Evaluation of Vo
5.6.2 - Major factors influencing Vo
5.7 - Time constants
5.8 - Power and energy densities
5.8.1 - Power density
5.8.2 - Energy density
5.9 - Leakage and maximum peak currents
5.10 - Cycle life and capacitance retention rate
5.11 - Inconsistencies in evaluation of SCs
5.11.1 - Causes for the inconsistencies
5.11.2 - Device performance versus material property
5.11.3 - Rate dependence
5.12 - Other test procedures
5.13 - Summary
References
Chapter 6 - Supercapacitors: prospects and future direction
Outline
6.1 - Prospects and possible future research directions
References
Index
Back Cover

Citation preview

MATERIALS FOR SUPERCAPACITOR APPLICATIONS M. Aulice Scibioh Department of Chemistry, Madras Christian College, Chennai, Tamil Nadu, India

B. Viswanathan National Center for Catalysis Research (NCCR), Department of Chemistry, Indian Institute of Technology Madras (IITM), Chennai, Tamil Nadu, India



Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www. elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-819858-2 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisitions Editor: Kostas Marinakis Editorial Project Manager: Kelsey Connors Production Project Manager: Prem Kumar Kaliamoorthi Cover Designer: Mark Rogers Typeset by Thomson Digital

Preface The available energy sources are being examined by the population of the world from various points of view, such as sustainable availability, low cost, universal equi-distribution, and above all non-polluting. Electricity is considered as one of the appropriate options from many points of view. Among these possibilities, the prominent ones that are receiving the attention of researchers today are batteries, supercapacitors, and fuel cells. Fuel cells and batteries are well known electrochemical energy sources over a century, though they are still threatening to become world-wide-spread distribution. Supercapacitors are yet another option for electrochemical energy sources, the material selection, and design with high capacitance value appear to be the main purpose in this endeavor. Many materials have been examined for supercapacitor applications and the current level of research in this search of material with high value of capacitance has thrown many possible solutions but the goal has not yet been reached. The compilation presented in this monograph is an attempt to synthesize the lines of thought in this direction. The authors are fully aware of their shortcomings in this endeavor. Still this is an attempt to compile the available data in a single source so that it will help the researchers to at least rationalize the materials to be tested further. In presenting this material, the authors kept in mind that this document should help future search for materials for superconducting applications. The material scientists should be in a position to rationalize their choice of material precursors and the synthesis strategy that can be adopted for making them suitable for supercapacitor applications. We wish to thank our fellow coworkers in the National Centre for Catalysis Research, (NCCR) Indian Institute of Technology, Madras, and the colleagues from Madras Christian College, Tambaram for their constructive suggestions and comments, which have helped us to revise our original draft. We are particularly grateful to Mr. N. Hariprasad for his help in compiling the text into a presentable form. The authors will be grateful for any suggestions and corrections so that the material presented will be useful for a wider audience. The authors are grateful to the Department of Science and Technology, Government of India for their continuous support to the National Centre for Catalysis Research (NCCR) at IIT, Madras. The authors must admit that their motivation in compiling this material in the form of a book is to assimilate the scientific literature and spread the information on a topic like supercapacitors. We only hope our wish and desire might have been fulfilled in this endeavor. Chennai  M. Aulice Scibioh May 2019    B. Viswanathan

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C H A P T E R

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Supercapacitor: an introduction O U T L I N E 1.3.1 Faradaic and non-Faradaic processes 1.3.2 Types of capacitors and batteries 1.3.3 Electrochemical capacitors and batteries: comparative properties

1.1 Supercapacitor—an emerging electrical energy storage device 1 1.2 Historical perspective 1.3 Supercapacitors and batteries as electrical energy storage devices—a comparison

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6 7 8

1.4 Outlook and scope of the monograph 9

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1.1  Supercapacitor—an emerging electrical energy storage device Changing climatic patterns, depletion of fossil fuels, and uneven distribution of energy sources necessitate us to opt for alternate, renewable sources to meet the ever-growing energy demands of human race around the globe. To cope up with such transiting energy paradigm, it is inevitable to develop and realize affordable, efficient, and reliable energy storage technologies in order to harvest energy from the unlimited but intermittent renewable energy sources, such as wind, tide, and sunlight. In general, electrical energy storage (EES) systems are classified based on the form of energy used, as shown in Scheme 1.1, into mechanical, thermal, chemical, electrochemical, and EES systems. Electrochemical supercapacitors gain prominence as electrochemical energy storage devices due to their matchless, superior characteristics, such as high power densities at reasonably high energy densities and significantly long cycle life. A plot of power versus energy density—known as Ragone plot for the important energy storage systems is shown in Fig. 1.1 [1]. As can be seen, while a supercapacitor is employed as an energy storage device, the value of specific power indicates the fastness with which it can deliver energy, and the value of specific energy implies on how far it can deliver the energy by a single charging. The time constrains of the devices mentioned in the figure are derived by dividing the energy density by the power. Owing to the attractive characteristics of the supercapacitors, they are presently employed Materials for Supercapacitor Applications. http://dx.doi.org/10.1016/B978-0-12-819858-2.00001-9 Copyright © 2020 Elsevier Inc. All rights reserved.

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1.  Supercapacitor: an introduction

SCHEME 1.1  Classification of electrical energy storage systems according to energy form. Electrical energy storage systems Mechanical

Thermal

• Pumped hydrostorage • Thermochemical • Compressed air energy • Sensible thermal storage (CAES) • Latent thermal • Liquid air energy storage (LAES) • Flywheels

Chemical

Electrochemical

Electrical

• Hydrogen storage • Synthetic natural gas (SNG)

• Lithium-ion • Supercapacitors battery • Lead acid battery • NaS BATTERY • Redox flow battery

Source: World Energy Council.

FIGURE 1.1  Specific power against specific energy, also called a Ragone plot for various energy storage devices. Source: Reproduced with permission from Ref. [1].

in a variety of applications ranging from power electronics, large-scale vehicular applications, such as buses and subway trains, store energy at intermittent generators in windmills to smart grid applications catering to stability, frequency smoothing and regulation, and for peak shifting. The future market for batteries and the supercapacitors appears to grow exponentially. 



1.2  Historical perspective

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Capacitor is originally termed as a condenser, in which a passive electrical terminal is employed to store the energy electrostatically in an electric field separated by an insulator (dielectric). A supercapacitor differs from a regular capacitor owing to its very high capacitance, and is rated in Farads, which is several orders of magnitude higher than the electrolytic capacitor. Differential voltage application on the positive and negative plates, charge the capacitor, which is just like the building up of electrical charge while walking on a carpet and the release of energy through fingers while touching an object. Supercapacitors are considered to be ideal energy storage devices due to their ability to undergo frequent charge–discharge cycles at high current in a short duration. The charging time of a supercapacitor is nearly 10 s and the self-discharge of a supercapacitor is substantially higher that of an electrostatic capacitor and little higher than a battery—the main contributor to this phenomenon is organic electrolyte. In general, the energy stored in a supercapacitor falls to nearly 50% in duration of nearly 30 days; while a nickel-based battery self-discharges 10%–15% in a month but Li-ion battery discharges only to an extent of 5% per month.

1.2  Historical perspective In ancient times, the idea to store electrical charge in surface has been emerged from the phenomena associated with rubbing of amber. In the mid-18th century, the effect of such phenomena was understood when physics of “static electricity” was being investigated and during that period, various electrical machines, such as the Electrophorus and the Wimshurst machine were being developed. Excellent examples of these, as well as Leyden jars, can be witnessed in the Museum of Science in Florence. However, an understanding of electricity at the molecular electronic level did not evolve until 140 years later, starting indirectly with the work of Michael Faraday and later with that of J. J. Thomson and of Millikan on the electron. The development of the Leyden jar and the discovery of the principle of charge separation and charge storage on the two surfaces of the Leyden jar, separated by a layer of glass were considered as milestones in the physics of electricity and later for electrical technology, electronics, and electrochemical engineering. The discovery of the Leyden jar, referred to in early works as well as in technological applications until the mid-20th century as the “condenser” is attributed either to Dean Kleist at Leyden or contemporarily to Musschenbroek at Kamin, Pomerania. In later terminology, the device in various embodiments is referred to as a “capacitor” and its capability for charge storage per volt, is referred to as its “capacitance,” C. It has to be noted that the term “capacity,” used in battery terminology indicates the extent of Faradaic charge storage (in units of coulombs or Watt-hours), which is different from the term “capacitance” applied in the field of capacitors (given in units of Farads). Uncertainty prevailed for years regarding the nature of electricity after the independent discoveries of “animal electricity” by Galvani in 1776 and “voltaic electricity” by Volta in 1800, in spite of the work by Faraday, including his finding of the chemical equivalence of electrical charge. It was not until the discoveries by J. J. Thomson on the charge-to-mass ratio of the ubiquitously produced negative charge carriers arising in the ionization of low-pressure gases—first investigated by Crookes [2], and the work by Millikan [3], and by Townsend [4] on the absolute value of the charges borne by such particles that a modern view of the



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1.  Supercapacitor: an introduction

nature of electricity could be proposed in terms of accumulation or deficiency of such charges, and the dynamics of their motion. In 1881, Johnstone Stoney [5] coined the name “electron” (from the Greek word eAliKrOV for amber) for such negatively charged particles, which are the natural units of electricity, the electron charge being 4.80 × 10−10 electrostatic units (esu) of charge or 1.60 × 10−20 absolute electromagnetic units (emu) of charge. It is interesting to note from history that Faraday himself has failed to reach the conclusion on his laws relating the extent of a charge’s passage (current X time = coulombs) to quantitatively determinable extents of chemical change associated with the electrolysis of aqueous acid and metal–salt solutions, implied an atomic unit of electricity. However, the importance of his laws was not diminished, as it unambiguously demonstrated in a quantitative way the equivalence of electrical charge to the extent of generated chemical change that was dependent on the chemical identity of the element concerned and its equivalent weight or oxidation state in solution. Much later, von Helmholtz [6] in his Faraday Lecture of 1881 arrived at the key conclusion that Faraday’s laws implied a fundamental unit of electrical charge. This concept has paved way for the development of quantitative and more fundamental science of electrochemistry and for a quantitative science of electrical nature of matter. This work was further elucidated through spectroscopy and theoretical advances of Bohr, Sommerfeld, Schroedinger, and Heisenberg on electron energy states in atoms and molecules, and the significance of ionization and its relation to solvolysis in solution and the state of solid salts [7]. Although the experimental phenomenology of electrical charging of surfaces, including those of the Leyden jar condenser was well understood in the mid and later part of the 18th century, the physical significance of charging or discharging processes at the plates of capacitors could not be fully understood until the atomic nature of electricity, the electron, was characterized and its properties were directly determined [3, 4, 8]. Similarly, the charging or discharging of capacitors by a flow of electrical charges in wires could not have been understood until the electron theory of metals had been developed and the mechanism of current flow in metal conductors was understood clearly. Even the physical and chemical significance of the charging of amber could not be understood clearly until the satisfactory ideas about the ionization of molecules and macromolecules (through frictional or triboelectrical effects) had been formulated, partially through the results on spectroscopic ionization limits of electron energy states in molecules or atoms. Thus, the understanding on the mechanisms of electrical charge storage in capacitors remained incomplete at the atomic physical level until ∼ 140 years after the development of the Leyden jar capacitor and related contemporary electrical machines. Nevertheless, it was Faraday who had some of the first (in principle) correct ideas about polarization in dielectrics and the significance of dielectric strain and lines of (electrical) force in the dielectric materials of charged condensers. At this point it has to be mentioned that the charging of metallic plates of a capacitor involves the production of an excess (negative plate) or deficiency (positive plate) of the density of the delocalized electron plasma of the metal over a short distance (ca. 0.1–0.2 nm, the Thomas–Fermi screening distance) from the formal outer surface of the metal plate. Howev­ er, each plate has its own electric potential (an equipotential) throughout its material, except very closer region to its surface. Hence, local charge density variation within the so-called “Thomas–Fermi screening distance” has to arise according to the Poisson relation that expresses the field gradient in terms of the local space charge density and the Gauss relation that expresses the field as a function of surface charge density. At an insulator such as amber, the excess charge density that arises upon charging has a different origin that is associated 



1.2  Historical perspective

5

with localized molecular ionization (localized oxidation) of the insulator material at its surface, or in some cases is due to negative ionization by localized uptake of electrons at electron acceptor sites on the surface (localized reduction). These latter phenomena are the subject of triboelectricity. The principle that electrical energy can be stored in a charged capacitor was known since 1745; at a voltage difference, V, established between the plates accommodating charges +q and −q, the stored energy, G, is ½ CV2 or ½ qV, G being a Gibbs free energy, which increases as the square of V. The utilization of this principle to store electrical energy for practical purposes, as in a cell or battery of cells seems to have been first proposed and claimed as an original development in the patent granted to Becker in 1957 [9]. The patent described the EES by means of the charge held in the interfacial double layer at a porous carbon material perfused with an aqueous electrolyte. The principle involved was charging of the capacitance, Cdl of the double layer, which arises at all solid/electrolyte interfaces, such as metal, semiconductor, and colloid surfaces and also at the phase boundary between two immiscible electrolyte solutions [10]. In 1957, a team of General Electric Engineers experimenting with devices using porous carbon electrode observed the effect of electric double layer capacitor. They have inferred that energy was in store at the carbon pores, as it showed an exceptionally high capacitance. Later in 1966, a group of researchers at Standard Oil of Ohio accidentally rediscovered the effect while working on fuel cell designs. Their cell design contained two layers of activated charcoal separated by a thin porous insulator, and the mechanical design remained the same for most electric double layer capacitors to date. In 1978, NEC finally introduced the term supercapacitor and its application was used to provide backup power for maintaining computer memory [11]. Due to its application, supercapacitor captured the attention of researchers in diverse fields, which led to the development of other composite materials for constructing electrodes, such as metal oxides and conducting polymer, etc. Among the challenges faced in this century is unquestionably energy storage. Therefore, it is relevant and important that new, environmentally friendly and low-cost energy storage systems be developed in order to respond to the needs of emerging ecological concerns and modern society [12]. In summary, supercapacitors (also called electric double-layer capacitors or ultracapacitors) are energy storage devices with very high capacity and a low internal resistance that are able to store and deliver energy at relatively higher rates as compared to batteries due to the mechanism of energy storage which involves a simple charge separation at the interface between the electrode and the electrolyte [13, 14]. A supercapacitor consists of two electrodes, an electrolyte, and a separator, which isolates the two electrodes electrically. Electrode material is the most important component of a supercapacitor [15, 16]. Some of the benefits of supercapacitors when compared with other energy storage devices are long life, high power, flexible packaging, wide thermal range (−40 to 70°C), low maintenance, and low weight [17]. Supercapacitors can best be utilized in areas that require applications with short load cycle and high reliability, for example, energy recapture sources, such as forklifts, load cranes and electric vehicles, power quality improvement [18]. Among the promising applications of supercapacitors is in fuel cell vehicles and low emission hybrid vehicles [19, 20]. Supercapacitors with its unique qualities when used with batteries or fuel cells they can serve as temporary energy storage devices providing high power capability to store energy from braking [21, 22]. Due to its high power capability a bank of supercapacitors, can bridge the short time duration between a power failure and the start up of backup power generators. Even though 

6

1.  Supercapacitor: an introduction

energy density of supercapacitor is greater than that of conventional capacitors; it is considerably lower than batteries or fuel cells. Electrochemical performances of an electrode material strongly rely on factors, such as surface area, electrical conductivity, wetting of electrode, and permeability of electrolyte solutions [23]. Passive components are required in all electronic applications to store electrical energy in volume and weight as small as possible. The power needed by an application as well as the speed of storage process determines the type of energy storage device to be used. Essentially, when it comes to applications the ones that need faster discharge rate opt for capacitor while the slower ones opt for batteries. From Fig. 1.1, it can be seen that a batteries are capable of attaining up to 150 Wh kg−1 of energy density, around 10 times what an electrochemical capacitor is capable of. In terms of power density, batteries do not have the capability of reaching the values of electrochemical capacitors. Batteries hardly reach 200 W kg−1 which is about 20 times less than the expected electrochemical capacitor performance. Batteries experience weaknesses like rapid decrease in performances due to fast charge discharge cycles or cold environmental temperature, they are expensive to maintain and have a limited lifetime [24]. One of the frequently asked question is how can the electrochemical supercapacitor technology be compared to the battery technology? At the moment, the state of art supercapacitors available till date cannot be employed as a substitute for the battery technology; however, it could work as a supplement in terms of momentary and temporary power outage by providing the instantaneous current requirement thereby reducing the battery current. In large-scale battery units, electrochemical supercapacitors can be installed in parallel to compensate for momentary and temporary interruptions, and this would greatly repress the undue stress put on batteries from short term interruptions [25].

1.3  Supercapacitors and batteries as electrical energy storage devices—a comparison In this section, we discuss on some of the similarities and differences between electrochemical capacitors and batteries in terms of the electrochemical processes that are involved in their discharge and recharge cycling, and in their potential uses as EES devices. In particular, the fundamentally different mechanisms of charge storage that are normally involved will be emphasized, along with the consequent, usually different, relations between the extents of charge accommodation at the electrodes and the electric potential differences (cell voltage) between pairs of electrodes having conjugate, ±, polarities. One of the main and kinetically significant differences between capacitors and batteries is that the electrodes of the latter usually undergo substantial phase changes during discharge and recharge processes (minimally though for the intercalation systems), which lead to kinetic and thermodynamic irreversibility. On the other hand, capacitors of the double-layer type require only electrostatic charge accommodation with virtually no phase change, though some small but significant reversible electrostriction of the electrolyte can arise upon charging.

1.3.1  Faradaic and non-Faradaic processes A fundamental difference between the operational mechanism of electrochemical capacitors and battery cells is that for the double-layer type of capacitor, the charge storage process 



1.3  Supercapacitors and batteries as electrical energy storage devices—a comparison

7

is non-Faradaic, it means that ideally no electron transfer takes place across the electrode interface and the storage of electric charge and energy is electrostatic. In battery-type processes, the essential process is Faradaic, that is the transfer of electron does take place across the double layers, with a consequent change of oxidation state, and hence the chemistry of the electroactive materials. Intermediate situations arise where Faradaic charge transfer occurs, but owing to special thermodynamic conditions that apply, the potential, V, of the electrode is certain continuous function of the quantity of charge, q, passed leading to the occurrence of a derivative, dq/dV. This is equivalent to and measurable as a capacitance and is designated as a pseudocapacitance. A slightly different situation arises when chemisorptions of ions or molecules takes place with partial charge transfer [26–28]. For example, in a process such as (1.1) M + A − ↔ M / A (1−δ )− + δ e (in M) Such a reaction at the surface of an electrode M usually gives rise to a potential dependent pseudocapacitance and the quantity δe is related to the so-called “electrosorption valence.” Hence, the important differences in the charge storage processes are as follows: In non-Faradaic process, the charge accumulation is achieved electrostatically by positive and negative charges residing on two interfaces separated by a vacuum or a molecular dielectric (the double layer or, e.g., a film of mica, a space of air or an oxide film, as in electrolytic capacitors). In Faradaic process, the charge storage is achieved by an electron transfer that produces chemical or oxidation state changes in the electroactive materials according to Faraday’s laws (hence the term) related to electrode potential. Pseudocapacitance can arise in some cases. The energy storage is indirect and is analogous to that in a battery. In a battery cell, every electron charge is Faradaically neutralized by charge transfer, resulting in a change of oxidation state of some redox electroactive reagent. For example, in the cathode of Ni-Cd battery [29], the following reaction takes place. (1.2) Ni 3+ ⋅ O 2 − ⋅ OH − + e − + H + → Ni 2 + ⋅ 2OH − In a capacitor, actual electron charges, either in excess or deficiency, are accumulated on the electrode plates with lateral repulsion and no involvement of redox chemical changes. However, in certain cases of double-layer charging, some partial electron transfer does occur, leading to pseudocapacitance, for example, when chemisorption of electron-donative anions, such as Cl−, Br−, I−, or CNS−, takes place as represented in Eq. (1.1). The electrons involved in double-layer charging are the delocalized conduction-band electrons of the metal or carbon electrode, while the electrons involved in Faradaic battery-type processes are transferred to or from valence-electron states (orbitals) of the redox cathode or anode reagent, although they may arrive in or depart from the conduction-band states of the electronically conducting support material. In certain cases, the Faradaically reactive battery material itself is metallically conducting (e.g., PbO2, some sulfides, RuO2), or else is a wellconducting semiconductor and a proton conductor, for example, Ni·O·OH.

1.3.2  Types of capacitors and batteries Various types of capacitors and their mode of energy storage—electrostatic or Faradaic (when pseudocapacitance arises), are summarized in Table 1.1. Here, we intend to provide only the overview of different kinds of capacitors and it is a matter for detailed discussion in 

8

1.  Supercapacitor: an introduction

TABLE 1.1  Types of capacitors and mode of energy storage [11]. Type

Basis of charge or energy storage

Examples

Vacuum

Electrostatic

—

Dielectric

Electrostatic

Mica, Mylar, paper

Oxide electrolytic (thin film)

Electrostatic

Ta2O5, Al2O3

Double-layer

Electrostatic (charge separation at double-layer at electrode interface)

Carbon powders, fibers

Colloidal electrolyte

Electrostatic (special double-layer system)

—

Redox oxide film

Faradaic charge transfer (pseudocapacitance)

RuOx, IrOx, Co3O4

Redox polymer film

Faradaic charge transfer (pseudocapacitance)

Polyaniline, polythiophene

Soluble redox system

Faradaic charge transfer (pseudocapacitance)

V2+/V3+/VO2+, Fe(CN)64−- Fe(CN)63

Chapter 2. Normally, capacitors function as elements of electronic circuits or communications equipment, or as ballast for starting electric motors or electric discharge tubes, such as fluorescent lights. Also, devices of very large capacitance are now available for storing electric energy in various applications. In Table 1.2, a few distinguished types of batteries are summarized. These are generally classified as primary (nonrechargeable) or secondary (multiply rechargeable) batteries. The discharge or recharge mechanism is mainly Faradaic, although all electrode interfaces exhibit a double-layer capacitance that is reversibly chargeable. For batteries the latter mechanism accommodates about 2%–5% of the total charge accepted. In a different class from the battery systems listed in Table 1.2 are fuel cells in which the anode and cathode (oxygen or air) reactants are supplied on a continuous basis from external reservoirs, and the electrode surfaces provide an interface for either electrocatalytic oxidation or reduction of the reagents supplied. The primary metal-air cells are operated as semi-fuel cells, but the “fuel” is an easily oxidizable base metal and a gas-diffusion catalyzed air or O2 cathode is employed. Such cells using Al are not rechargeable except by mechanical replacement of the metal anodes. However, if Zn is used, electrochemical recharging is possible, but requires a bifunctional catalyzed counter electrode capable of evolving H2 on recharge or reducing O2 (air) on discharge.

1.3.3  Electrochemical capacitors and batteries: comparative properties The behavior and properties of electrochemical capacitors and batteries are summarized in this section in tabular forms. The advantages and disadvantages of employing electrochemical capacitor for EES are listed in Table 1.3. Comparative electrical characteristics of battery and electrochemical capacitor behavior are given in Table 1.4, and the essential difference of thermodynamic behavior of ideal battery and electrochemical capacitor materials are given in Table 1.5. The overall comparison of electrochemical capacitor and battery characteristics under charge-recharge conditions is presented in Table 1.6.





1.4  Outlook and scope of the monograph

9

TABLE 1.2  Types of batteries and mode of energy storage [11]. Type

Basis of charge or energy storage

Primary Leclanche, Zinc-MnO2

Faradaic

Alkaline, Zinc-MnO2

Faradaic

Mg-AgCl

Faradaic

Mg-PbCl2

Faradaic

Li-SOCl2 and other cathodes

Faradaic

Li-CFx

Faradaic

Al-air (catalyzed)

Faradaic

Alkaline, Zinc-MnO2

Faradaic

Mg-AgCl

Faradaic

Mg-PbCl2

Faradaic

Li-SOCl2 and other cathodes

Faradaic

Li-CFx

Faradaic

Al-air (catalyzed)

Faradaic

Zn-air (catalyzed)

Faradaic

Secondary Lead-acid, Pb-PbO2

Faradaic

Nickel-cadmium, Ni·O·OH-C

Faradaic

Nickel-hydrogen, Ni·O·OH-metal hydride

Faradaic

Nickel-zinc, Ni·O·OH-Zn

Faradaic

Mercuric oxide-zinc, HgO-Zn

Faradaic

Silver oxide(s)-zinc, AgO-Zn

Faradaic

Zinc-air (catalyzed)

Faradaic

Li-TiS2

Faradaic (exhibiting intercalative psuedocapacitance)

Li-MoS2

Faradaic (exhibiting intercalative psuedocapacitance)

Li-MnO2

Faradaic (exhibiting intercalative psuedocapacitance)

Li-CoO2

Faradaic (exhibiting intercalative psuedocapacitance)

Li-C-CoO2 and other cathodes

Faradaic (exhibiting intercalative psuedocapacitance)

Li-iron sulfides

Faradaic (exhibiting intercalative psuedocapacitance)

Na-S

Faradaic (exhibiting intercalative psuedocapacitance)

1.4  Outlook and scope of the monograph Supercapacitors emerge as promising candidates for power devices for future genera­ tions. Apart from niche applications, these devices are expected to find several future applications in hybrid electric vehicles and other larger power devices and systems. In order for



10

1.  Supercapacitor: an introduction

TABLE 1.3  Advantages and disadvantages of employing electrochemical capacitor for electrical energy storage [11]. Advantages

Disadvantages

• Long cycle life, > 100,000 cycles; A few up to 106 • Good power density (under certain conditions, limited by IR or equivalent series resistance (esr) complexity of equivalent circuit) • Simple principle and mode of construction (can employ battery construction technology) • Cheap materials (for aqueous embodiments) • Combines state-of-charge indication, • Q = j(V) • Can combined with rechargeable battery for hybrid applications (electric vehicles)

• Limited energy density • Poor volume energy density • Low working voltages (compared with electrolytics; satisfactory compared with batteries) • Aq. voltage range 0–1.4 V; non-aq. to 4.5 V. In practice, 3.5 V • Non-aq. embodiments require pure, H2O-free materials; more expensive. • Requires stacking for high potential operation (electric vehicles) • Hence, good matching of cell units is necessary

TABLE 1.4  Electrical characteristics of battery versus electrochemical capacitor [11]. Battery

Electrochemical capacitor

Ideally has single-valued free energies of components

Has continuous variation of free energy with degree of conversion of materials or extent of charge held

EMF is ideally constant with degree of charge and discharge, except for nonthermodynamic incidental effects, or phase changes during discharge

Potential is thermodynamically related to state of charge through log [X/1 − X] factor, in a continuous manner for a pseudocapacitor, or directly to Q for a double-layer capacitor

Behavior is not capacitative, except in very general sense

Behavior is capacitative

Irreversibility is usual behavior (materials irreversibility and kinetic irreversibility)

High degree of reversibility is common (10−4–10−6 cycles with RuO2 or C double-layer capacitors)

Response to linear modulation of potential gives irreversible i vs. V profile with nonconstant currents

Response to linear modulation of potential gives more or less constant charging current profile but with some dependence on materials

Discharge at constant current arises at a more or less constant potential except for intercalation Li batteries

Discharge at constant current gives mainly linear decline of potential with time, which is characteristic of a capacitor

TABLE 1.5  Basic differences in thermodynamic behavior of ideal battery and electrochemical capacitor materials [11]. Battery

Supercapacitor

During discharge or recharge has unique, singlevalued free energies ∆G of the electroactive phases involved ∆G = constant

Has continuously changing free energy of electroactive material with extent of charge and discharge. G = ½ CV2 or ∆G = ∆G0 + RT ln [X/(1 − X)] for pseudocapacitance

Has corresponding single-valued potential on discharge

Has corresponding continuous variation of potential during charge and discharge

Usually not reversible, in the sense that recharge curve is not mirror image of discharge curve, for example, in cyclic voltammetry

Recharge and discharge curves are mirror images of one another, that is, in cyclic voltammetry. Highly reversible





1.4  Outlook and scope of the monograph

11

TABLE 1.6  Overall comparisons of electrochemical capacitor and battery characteristics [11]. S. No

Battery

Capacitor

1.

Ideally has constant (thermodynamic) discharge or recharge potential, except for Li intercalation systems

Has intrinsically sloping charge and discharge curve

2.

Does not have good intrinsic state-of-charge indication except for Li intercalation systems

Due to earlier reason, it has good intrinsic stage-of-charge indication

3.

Has moderate or good energy density, depending on equivalent weights and electrode potentials of active materials

Has relatively poor energy density

4.

Has relatively poorer power density, depending on kinetics

Has good power density

5.

Has less cycle life by a factor of 1/100 ∼ 1/1000 due to irreversibility of redox and phase-change processes in three dimensions

Has excellent cyclability or cycle life due to simple addition or withdrawal of charges (in double-layer type)

6.

Has internal IR due to electrolyte and active materials

Has internal IR due to high-area matrix + electrolyte

7.

Has significant T -dependent activation polarization (Faradaic resistance)

Has little or no activation polarization but C may be temperature-dependent

8.

Has poorer lifetime due to degradation or reconstruction of active materials

Has long lifetime except for corrosion of current collectors, etc.

9.

Electrolyte conductivity can decrease or increase on charging, depending on chemistry of cell reactions, for example, with Pb-acid

Electrolyte conductivity can diminish on charging due to ion adsorption

Can be constructed in bipolar configuration

Can be constructed in bipolar configuration

10.

supercapacitors to attain such status, it is important that their energy and power densities need to be maximized. A direct and important approach to address the present challenges is to develop advanced electrode materials and methods to fabricate these materials for supercapacitors. The recent years have witnessed enormous interest in the research of numerous materials and methods for the synthesis, for applications in supercapacitor electrode technology. In the constantly changing technological scenario, it is relevant to review the various aspects of supercapacitor devices. In order to push the frontiers in this area, it is necessary to carefully and critically evaluate the materials that are employed for this purpose, and it is essential to identify and develop advanced materials and fabrication methods for these devices, with an eye on recent developments in these areas, and their implications on the future of supercapacitor technology. This monograph also addresses the principal technological challenges facing the development efforts in the future. The aim of this treatise is to give a comprehensive view, present critical analyses, and to examine the principle involved in the choice of materials for electrode and electrolyte development and their influence on the overall performance of the supercapacitor, covering both the fundamental science (physics, chemistry), and engineering aspects. In order to provide the necessary background in this field to the reader, some fundamental aspects and principles of electrochemistry is presented in Chapter 2. In Chapter 3, the basic principles involved in the selection of materials for electrode fabrication, status report on the various types of materials that are employed are discussed and analyzed, along with various synthetic strategies adapted 

12

1.  Supercapacitor: an introduction

for their preparation. In Chapter 4, to facilitate the research and development of electrolytes for electrochemical supercapacitors (ESs), a comprehensive overview of the development and recent trends concerning materials for electrolytes in ESs are provided. Various types of electrolytes reported in the literature are summarized and classified into aqueous, organic, ILs, solid-state or quasi-solid-state, and redox-active electrolytes. The influences of the electrolyte properties on the ES performance are discussed in depth. Principles and methods to design and optimize electrolytes for ESs are proposed, emphasizing the interplay between the electrolytes and electrode materials. Considerable attention is bestowed on various characterization techniques, which have been employed to assess the performance of supercapacitors in Chapter 5, discussing on basic conceptual and theoretical background concerning the improvement of capacitance value of the supercapacitors to advance their applications by maximizing their energy and power densities. The final chapter of this monograph—Chapter 6, presents the challenges still remaining in this area and the possible propositions to surmount the difficulties, in order to realize this technology in practical applications.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

[15] [16] [17] [18] [19]

P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. W. Crookes (1879), quoted by S. Glasstone in Textbook of Physical Chemistry, Van Nostrand, New York, 1940. R.A. Millikan, On the Elementary Electrical Charge and the Avogadro Constant., Phys. Rev. 2 (1913) 143. J. S. Townsend, Electricity in Gases (1879), quoted by S. Glasstone in Textbook of Physical Chemistry, Van Nostrand, New York, 1940. G. Johnstone Stoney, On the cause of double lines and equidistant satellites in the spectra of gases, Phil. Mag. 11 (1881) 381; Sci. Trans. Roy. Soc. Dublin 4 (1891) 583. H. von Helmholtz, On the modern development of Faraday’s conceptions of electricity, J. Chern. Soc., Lond. 39 (1881) 277. B.E. Conway, Ionic Hydration in Chemistry and Biophysics Studies in Physical and Theoretical Chemistry, vol. 12, Amsterdam, Elsevier, (1981). J. Thomson, On the Charge of Electricity carried by a Gaseous Ion, Phil. Mag. 5 (1903) 346; see also J. J. Thomson, The Electron in Chemistry, Franklin Institute Lectures, Chapman and Hall, London, 1923. H. E. Becker, The rst supercapacitor device, based on double-layer charge storage (using carbon electrodes), U.S. patent 2,800,616 (to General Electric Co.), 1957. Z. Samec, Charge transfer between two immiscible electrolyte solutions: Part III. Stationary curve of current vs. potential of electron transfer across interface, J. Electroanal. Chern. 103 (I) (1979). B.E. Conway, Electrochemical Supercapacitors Scientific Fundamentals and Technological Applications, Plenum Press, New York, (1999). A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nanostructured materials for advanced energy conversion and storage devices, Nat. Mater. 4 (2005) 366. M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Rouf, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498. H.P. Wu, D.W. He, Y.S. Wang, M. Fu, Z.L. Liu, J.G. Wang, H.T. Wang, Graphene as the electrode material in supercapacitors, 2010 8th International Vacuum Electron Sources Conference and Nanocarbon, IEEE (2010) 465. M.H. Ervina, B.S. Miller, B. Hanrahan, B. Mailly, T. Palacios, A comparison of single wall carbon nanotube electrochemical capacitor electrode fabrication methods, Electrochim. Acta 65 (2012) 37. M.A. Pope, S. Korkut, C. Punckt, I.A. Aksay, Supercapacitor electrode produced through evaporative consolidation of graphene oxide-ionic liquid gels, J. Electrochem. Soc. 160 (2013) A1653. Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103. J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Mater. Sci. 321 (2008) 651. N.L. Wu, Nanocrystalline oxide supercapacitors, Mater. Chem. Phys. 75 (2002) 6.



References 13

[20] Y.M. Cai, Z.Y. Qin, L. Chen, Effect of electrolytes on electrochemical properties of graphene sheet covered with polypyrrole thin layer, Progress Nat. Sci: Mater. Int. 21 (2011) 460. [21] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 38 (2009) 2520. [22] A.K. Mittal, M.J. Kumar, A brief review of electrode materials for supercapacitor, Nanosci. Nanotech. 13 (2011) 263. [23] B. Rajagopalan, J.S. Chung, Reduced chemically modified graphene oxide for supercapacitor electrode, Nanoscale Res. Lett. 9 (2014) 1. [24] A. Schneuwly, R. Gallay, Properties and applications of supercapacitors from the state-of-the-art to future trends, Proceeding PCIM (2000) 1–10. [25] S.C. Smith, P.K. Sen, B. Kroposki, Advancement of energy storage devices and applications in electrical power system, IEEE Power and Energy Society General Meeting—Conversion and Delivery of Electrical Energy in the 21st Century, 2008, pp. 1–8. [26] W. Lorenz, G. Salie, Reaction steps of the electrochemical phase-boundary reaction, Zeit. Phys. Chern., N.F. 29 (1961) 390–408. [27] J.W. Schultze, F.D. Koppitz, Bond formation in electrosorbates—I correlation between the electrosorption valency and Pauling’s electronegativity for aqueous solutions, Electrochim. Acta 21 (1976) 327–337. [28] B.E. Conway, H.A. Kozlowska, W.B.A. Sharpe, Chemical aspects of specific adsorption and underpotential electrode position in relation to charge transfer, Zeit. Phys. Chem., N.F. 98 (1975) 61. [29] D. Linden (Ed.), Handbook of Batteries, second ed., McGraw-Hill, New York, 1995.



C H A P T E R

2

Fundamentals and energy storage mechanisms—overview O U T L I N E 2.1 Introduction

15

2.2 Fundamentals

16

2.3 Supercapacitors: types 2.3.1 Electrochemical double layer capacitors (EDLCs) 2.3.2 Pseudocapacitors 2.3.3 Hybrid capacitors

18 18 19 20

2.3.3.1 Composite supercapacitors 21 2.3.3.2 Asymmetric supercapacitors 21 2.3.3.3 Battery-type supercapacitors 21

2.4 Electric double layer 2.4.1 Helmholtz model

2.4.2 Gouy-Chapman or diffuse model 2.4.3 Stern model 2.4.4 Grahame model 2.4.5 Bockris-Devanathan-Müller (BDM) model 2.4.6 Trasatti-Buzzanca-Conway model 2.4.7 Marcus model 2.4.8 Electric double layer in supercapacitors

21 21

23 24 26 26 27 27 27

2.5 Pseudocapacitance

29

2.6 Summary and outlook

30

2.1 Introduction Fundamentals in electrodics and electrochemistry of supercapacitors, basic principles governing the overall performance of supercapacitors and their unique characteristics, especially in aspects of charge transfer at interfaces involving electrode with special emphasis on kinetics and energetics are presented and discussed in this chapter, considering the needs of the diverse class of researchers in this field—electrochemical engineers and technologists, material scientists, physicists and specialists in electrochemistry. The charging and discharging of a capacitor involve only electrostatic processes and not any type of Faradaic electrode processes. Therefore, the effect of electrode kinetics will arise practically only while a capacitor electrode ceases, and thereby it can no longer be an ideally polarizable one. Ideally polarizable electrode: it is an electrode characterized by the absence of Materials for Supercapacitor Applications. http://dx.doi.org/10.1016/B978-0-12-819858-2.00002-0 Copyright © 2020 Elsevier Inc. All rights reserved.

15

16

2.  Fundamentals and energy storage mechanisms—overview

net current between the two sides of the electrical double layer, that is, no Faradic current between the electrode surface and the electrolyte. No direct current flows between the electrode and the surrounding electrolyte. In this sense, the electrode/electrolyte interface behaves like a capacitor. This is contrasted with ideally nonpolarizable electrode in which Faradic current pass freely. The equilibrium potential does not change with the passage of current and in this sense, the interface behaves as a resistor. Typical examples of these two types are the platinum electrodes (polarizable) and the silver/silver chloride (nonpolarizable)]. This situation sets in during an overcharge owing either to decomposition of solution or the occurrence of Faradaic self-discharges due to the presence of oxidizable or reducible impurities or even due to the presence of reactive functional groups on carbon surfaces. Therefore, even in case with electrochemical capacitors, either indirect or direct contribution from Faradaic electrode processes can happen due to the following reasons [1–6]: 1. During the decomposition of electrolyte, inevitable occurrence of overcharge or overdischarge of the double-layer capacitors occurs. 2. In carbon-based double-layer capacitors, charge or discharge of though small but significant component of pseudocapacitance occurs. 3. During the self-discharge processes in electrochemical capacitors, open circuit followed by charging takes place. 4. While the governance of the Faradaic mechanisms of charging or discharging of oxide based or conducting polymer based pseudocapacitors or due to adsorption, the kinetics and energetic of the processes are directly involved.

2.2 Fundamentals In conventional capacitors, two conducting electrodes are separated by a dielectric and insulating material. When a voltage is applied to a capacitor, opposite charges kept accumulating at the surface of each electrode. The charges are kept separated by the insulator, thereby the electric field produced let the capacitor to store the energy. This concept is pictorially represented in Fig. 2.1. Capacitance C is defined as the ratio of stored (positive) charge Q to the applied voltage V: (2.1) C = Q/V For a conventional capacitor, C is directly proportional to the surface area A of each electrode and inversely proportional to the distance D between the electrodes. This can be expressed as in Eq. (2.2). (2.2) C = ε 0ε r ⋅ A/D where, ε0 is the dielectric constant, that is the so called “permittivity” of free space and εr is the dielectric constant of the insulating material between the electrodes, and the product of these two factors is proportionality constant. The main attributes of a capacitor are its energy density and power density. In order to measure either of these, the density has to be calculated as a quantity per unit mass or per unit volume. The energy E stored in a capacitor is directly proportional to its capacitance: (2.3) E = 1 2 CV 2 



2.2 Fundamentals

17

FIGURE 2.1  Diagrammatic representation of a conventional capacitor [6a].

It is generally known that the power P is the energy expended per unit time. To determine P for a capacitor, one must consider that capacitors are generally represented as a circuit in series with an external “load” resistance R, as is shown in Fig. 2.1. The internal components of the capacitor, such as electrodes, current collectors, and dielectric material also contribute to the resistance, which is measured cumulatively by a quantity termed as the equivalent series resistance (ESR). The voltage during discharge is determined by these resistances. When measured in terms of impedance (R = ESR), the maximum power Pmax for a capacitor [1–3] is given by Eq. (2.4), showing that how ESR can limit the maximum power of a capacitor. Pmax = V 2 /4 × ESR (2.4)

Conventional capacitors have relatively high-power densities, but relatively low energy densities compared to electrochemical batteries and fuel cells (two other energy conversion devices), and hence a battery can store more total energy than a capacitor, but it cannot de­ liver it faster, implying that its power density is low. Capacitors, on the other hand, store relatively less energy per unit mass or volume, but the stored electrical energy can be discharged rapidly to produce significant power, so their power density is usually high. Supercapacitors are governed by the same basic principles as conventional capacitors, but they incorporate electrodes with higher surface area (A) and thinner dielectrics that decrease the distance D between the electrodes. Thus, from Eqs. (2.2) and (2.3), it can be seen that this leads to an increase in both capacitance and energy. By maintaining the low ESR characteristic of conventional capacitors, supercapacitors also are capable of attaining comparable power densities. Further, supercapacitors have several advantages over batteries and fuel cells, including higher power density, shorter charging times, and longer cycle life and shelf life. A schematic of a supercapacitor illustrating some of the physical features described is shown in Fig. 2.2. 

18

2.  Fundamentals and energy storage mechanisms—overview

FIGURE 2.2  Schematics of an electrochemical double layer capacitor [6a].

2.3  Supercapacitors: types The operational principle of a supercapacitor is based on energy storage and the distribution of the ions coming from the electrolyte with respect to surface area of the electrodes. Based on the energy storage mechanism, supercapacitors are classified into three classes: Electrochemical double-layer capacitors, pseudocapacitors, and hybrid supercapacitors as shown in Scheme 2.1 [6b], and depicted in Fig. 2.3.

2.3.1  Electrochemical double layer capacitors (EDLCs) EDLCs consist of two carbon-based materials as electrodes, an electrolyte, and a separator. EDLCs can store charge either electrostatically or through non-Faradic process, which involves no transfer of charge between electrode and the electrolyte [5, 6]. The principle of energy storage involved by EDLCs is of the electrochemical double layer. When the voltage is applied, there is an accumulation of charge on electrode surfaces, due difference in potential there is an attraction of opposite charges, these result to ions in electrolyte diffusing over the separator and onto pores of the opposite charged electrode. To avoid recombination of ions at electrode, a double layer of charge is formed. The double layer, combined with the increase in specific surface area and decreased distances between electrodes allow EDLCs to attain higher energy density [7]. The storage mechanism in EDLCs permits highly fast energy uptake, delivery, and better power performance. There is no chemical reaction due to non-Faradic process, thereby eliminates swelling observed in active material which batteries demonstrate during charging and discharging processes. The notable differences between EDLCs and batteries are: 1. EDLCs can withstand millions of cycles unlike batteries that can withstand only a few thousands at best.





2.3  Supercapacitors: types

19

SCHEME 2.1  Different classes and subclasses of supercapacitors. Source: Reproduced with permission from Ref. [6].

2. Charge storage mechanism does not involve solvent of the electrolyte; while in Li-ion batteries it contributes to solid electrolyte inter phase when high-potential cathodes are employed or with graphite anodes [8, 9]. However, due to the electrostatic surface charging mechanism, EDLC devices experience a limited energy density, and this is the reason for present focus of research in to increase energy performance and to enhance the temperature range where batteries cannot function. Performance of EDLC can be modulated depending on the choice of the electrolyte.

2.3.2 Pseudocapacitors Compared to EDLCs which store charge electro-statically, pseudocapacitors store charge via Faradic process, which involves the transfer of charge between electrode and electrolyte [10]. When a potential is applied to a pseudocapacitor reduction and oxidation takes place on the electrode material, which involves the passage of charge across the double layer, resulting in Faradic current passing through the supercapacitor cell. The Faradic process involved in pseudocapacitors aids them to attain greater specific capacitance and energy densities than EDLCs. Examples are metal oxides, conducting polymers, etc., which lead to interest in these materials



20

2.  Fundamentals and energy storage mechanisms—overview

FIGURE 2.3  Supercapacitor types. (A) EDLC, (B) pseudocapacitor, and (C) hybrid capacitor. Source: Reproduced with permission from Ref. [4].

but due the Faradic nature, it involves reduction–oxidation reaction just like in the case of batteries; hence they also suffer lack of stability during cycling and low power density [11–13].

2.3.3  Hybrid capacitors EDLCs offer good cyclic stability and good power performance, while pseudocapacitors offer greater specific capacitance. The hybrid system offers a combination of both, that is, by combining the energy source of battery-like electrode, with a power source of capacitor-like electrode in the same cell [14, 15]. With appropriate electrode combination, it is possible to increase the cell voltage, which in turn leads to an improvement in energy and power densities. Various combinations have been tested in the past with both positive and negative electrodes in aqueous and inorganic electrolytes. Generally, the Faradic electrode results in an increase of energy density at the cost of cyclic stability, which is the main drawback of hybrid 



2.4  Electric double layer

21

devices compared to EDLCs, it is imperative to avoid turning a good supercapacitor into an ordinary battery [16]. Currently, researchers focus on the three different types of hybrid supercapacitors, which can be distinguished by their electrode configurations namely, composite, asymmetric, and battery-type. 2.3.3.1  Composite supercapacitors Composite electrodes combine carbon-based materials with either metal oxides or conducting polymer in a single electrode, that is, a single electrode will possess both physical and chemical charge storage mechanisms. Carbon-based materials offer capacitive doublelayer of charge and high specific surface area, which increases the contact between pseudocapacitive materials and electrolyte. Through Faradaic reaction, pseudocapacitive material increases capacitance in composite electrode [6]. Presently, there are two different types of composites: binary and ternary composites. Binary composites involve the use of two different electrode materials, while in the case of ternary, it uses three different electrode materials to form single electrode. 2.3.3.2  Asymmetric supercapacitors Asymmetric hybrids combine non-Faradic and Faradic processes by coupling and EDLC with a pseudocapacitor electrode. They are set up in a way that the carbon material is used as a negative electrode while either metal oxide or conducting polymer as positive electrode [6]. 2.3.3.3  Battery-type supercapacitors Battery-type hybrid combines two different electrodes, like in the case of asymmetric hybrids but in these cases they are made up by combining a supercapacitor electrode with battery electrode. This configuration was set up so as to utilize both properties of supercapacitors and batteries in one cell [6].

2.4  Electric double layer An electric double layer is a structure appearing when a charged object is placed into a liquid (electrolyte). The balancing counter charge for this charged surface will form on the liquid (electrolyte), mainly concentrating near to the surface. There are several theories or models for this interface between a solid and a liquid. It is interesting to witness the historical evolution of the models, incorporating successively more factors, which reflect the interfacial structure. The Figs. 2.4–2.7, pictorially illustrate the early double layer models namely Helmholtz model, Gouy-Chapman model, Stern model, and Grahame model, while Fig. 2.8 represent the later model by Bockris-Devanathan-Müller (known as BDM Model).

2.4.1  Helmholtz model Hermann von Helmholtz was the first one to realize that charged electrodes immersed in electrolyte solutions repel the ions of the same charge while attracting ions of the opposite charge to their surfaces. Two layers of opposite polarity form at the interface between electrode and electrolyte. In 1853, he has demonstrated that that an electrical double layer (DL) is essentially a molecular dielectric and statically stores electric charge. This theory is the 

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2.  Fundamentals and energy storage mechanisms—overview

simplest approximation for modeling the spatial charge distribution at double layer interfaces. The charge of the solid electronic conductor is neutralized by opposite sign ions at a distance from the solid. This is the distance from the surface to the center of the ions. As shown in the Fig. 2.4, the decomposition voltage of the electrolyte in the stored state is linearly dependent on the voltage applied. Here, XH would be the distance of closest approach of the charges, that is, ionic radius, which, for the purpose of calculation, were treated as point charges. By analogy with a capacitor the capacity would be

εε Cd , H = r 0 (2.5) XH where ε r is the relative permittivity, which is assumed not to vary with distance and ε 0the permittivity of vacuum. The decay of the electrostatic potential from φM to φS is linear (Fig. 2.4B) and Cd,H does not vary with the potential applied to the electrode (Fig. 2.4C). This early model predicted a constant differential capacitance independent from the charge density depending on the dielectric constant of the electrolyte solvent and the thickness of the double-layer, and provides a good foundation for the description of the interface,

FIGURE 2.4  The Helmholtz model of the double layer. (A) Rigid arrangement of ions; (B) variation of the

electrostatic potential, ф, with distance x, from the electrode; and (C) variation of Cd with applied potential. Source: Reproduced with permission from Ref. [19].





2.4  Electric double layer

23

but does not consider several important factors including diffusion/mixing of ions in solution, the possibility of adsorption onto the surface and the interaction between solvent dipole moments and the electrode. As of today, this is taken as the simplest theory, which does not adequately explain what occurs in nature [17, 18].

2.4.2  Gouy-Chapman or diffuse model Louis Georges Gouy in 1910 and David Leonard Chapman in 1913 both observed that capacitance was not a constant and that it depended on the applied potential and ionic concentration. Gouy suggests that the equal amount of opposite ionic charge appears in a liquid surrounding a charged solid, but the ions are not rigidly attached to the surface [17]. These ions in solution tend to diffuse into the liquid phase until the counter potential set up by their departure restricts this tendency. The kinetic energy of ions in solution will partially determine the thickness of the diffuse layer. Gouy and Chapman developed theories with significant improvements by introducing a diffuse model of the double layer as can be seen from Fig. 2.5. In this model, the charge distribution of ions as a function of distance from the metal surface allows Maxwell-Boltzmann statistics to be applied. Thus, the electric potential decreases exponentially away from the surface of the fluid bulk.

FIGURE 2.5  The Gouy-Chapman model of the double layer. (A) Arrangement of the ions in a diffuse way; (B)

variation of the electrostatic potential, ф, with distance, X from the electrode, showing effect of ion concentration, с; and (C) variation of Cd with potential, showing the minimum at the point of zero charge Ez. Source: Reproduced with permission from Ref. [19].



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2.  Fundamentals and energy storage mechanisms—overview

This model fails for highly charged double layers [18]. Experimentally, the measured thickness of double layers is greater than the calculated.

2.4.3  Stern model The Gouy-Chapman model makes a better approach to the reality than the Helmholtz model, but still has limited quantitative applications. The Gouy-Chapman model fails for highly charged DLs. It assumes that the ions are point charges and that they can approach the surface with no limits, which is not true. In 1924, Otto Stern suggested combining the Helmholtz model with the Gouy-Chapman model. In Stern’s model, some ions adhere to the electrode as suggested by Helmholtz, giving an internal Stern layer, while some form a GouyChapman diffuse layer. Stern modified the Gouy-Chapman model stating that the ions have a finite size, so limiting their approach to the surface and hence the Stern layer accounts for ions’ finite size and consequently an ion’s closest approach to the electrode is on the order of the ionic radius. The first ions in the Gouy-Chapman model are at the δ distance away from the surface, but the Stern model assumes that there can be specifically surface-adsorbed ions in plane δ, this is known as the Stern layer. Ions are strongly adsorbed by the electrode within this so-called compact layer. In the compact layer, there are specifically adsorbed ions (forming the inner Helmholtz plane), and nonspecifically adsorbed counter-ions (forming the outer Helmholtz plane) (Fig. 2.6) [19].

FIGURE 2.6  The Stern model of the double layer. (A) Arrangement of the ions in a compact and a diffuse layer; (B) variation of the electrostatic potential, ф, with distance, X, from the electrode; and (C) variation of Cd with potential. Source: Reproduced with permission from Ref. [19]. 



2.4  Electric double layer

25

FIGURE 2.7  Schematic representation of Grahame model. (A) Arrangement of ions; (B) variation of the electrostatic potential, ф, with distance, X, from the electrode, according to the applied potential; and (C) variation of Cd with potential. Source: Reproduced with permission from Ref. [19].

FIGURE 2.8  The model of Bockris et al. of the double layer. (A) Arrangement of ions and solvent molecules; (>) represents a water molecule; (B) variation of the electrostatic potential, ф, with distance, x, from the electrode. Source: Reproduced with permission from Ref. [19]. 

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2.  Fundamentals and energy storage mechanisms—overview

In mathematical terms, this is equivalent to two capacitors in series, with capacities CH representing the rigid compact layer and CGC representing the diffuse layer. The smaller of the two capacities determines the observed behavior: 1 1 1 (2.6) = + Cd CH CGC Fig. 2.6C shows the variation of the total capacity with potential. There are two extreme cases: The first one is where, close to Ez, CH » CGC and so Cd ∼ CGC and the other situation far from Ez, CH « CGC and Cd ∼ CH, which satisfies the assumptions of the model. As in the Gouy-Chapman model, the more concentrated the electrolyte the less the importance of the thickness of the diffuse layer and the more rapid the potential drop. At distance XH, there is the transition from the compact to the diffuse layer. The separation plane between the two zones is called the outer Helmholtz plane (OHP): the origin of the inner Helmholtz plane will be discussed later. Comparison between Figs. 2.5 and 2.6 reveals that this model is the best of the three so far, but does not yet explain all the facets of the curves. Summarizing, in order to resolve the shortcomings of the Gouy-Chapman model for the diffuse layer, Stern suggested the combination of both previous models, giving an internal Stern layer (e.g., the Helmholtz layer) and an outer diffuse layer (e.g., the Gouy-Chapman layer). The Stern model has its own limitations, namely, that it effectively treats ions as point charges, assumes all significant interactions in the diffuse layer are Coulombic, and assumes dielectric permittivity to be constant throughout the double layer and that fluid viscosity is constant plane.

2.4.4  Grahame model D.C. Grahame modified Stern in 1947. He proposed that some ionic or uncharged species can penetrate the Stern layer, although the closest approach to the electrode is normally occupied by solvent molecules. This could occur if ions lose their solvation shell as they approach the electrode. He called ions in direct contact with the electrode as “specifically adsorbed ions.” This model proposed the existence of three regions. The inner Helmholtz plane (IHP) passes through the centers of the specifically adsorbed ions. The OHP passes through the centers of solvated ions at the distance of their closest approach to the electrode. Finally, the diffuse layer is the region beyond the OHP (Fig. 2.7). The difference between this and the Stern model is the existence of specific adsorption, the phenomena in which a specifically adsorbed ion loses its solvation, approaching closer to the electrode surface—besides this it can have the same charge as the electrode or the opposite charge, but the bonding is strong. In both the Stern and Grahame models, the potential varies linearly with distance until the OHP and then exponentially in the diffuse layer.

2.4.5  Bockris-Devanathan-Müller (BDM) model In 1963, J. O’M. Bockris, M.A.V. Devanathan, and Klaus Müller proposed the BDM model of the double-layer that included the action of the solvent in the interface. They suggested that the attached molecules of the solvent, such as water, would have a fixed alignment to the electrode surface. This first layer of solvent molecules displays a strong orientation to the





2.4  Electric double layer

27

electric field depending on the charge. This orientation has great influence on the permittivity of the solvent that varies with field strength. The IHP passes through the centers of these molecules. Specifically adsorbed, partially solvated ions appear in this layer. The solvated ions of the electrolyte are outside the IHP. Through the centers of these ions pass the OHP. The diffuse layer is the region beyond the OHP (Fig. 2.8).

2.4.6  Trasatti-Buzzanca-Conway model Further research with double layers on ruthenium dioxide films in 1971 by Sergio Trasatti and Giovanni Buzzanca demonstrated that the electrochemical behavior of these electrodes at low voltages with specific adsorbed ions was like that of capacitors. The specific adsorption of the ions in this region of potential could also involve a partial charge transfer between the ion and the electrode. It was the first step toward understanding pseudocapacitance. Later, between 1975 and 1980 Brian Evans Conway conducted extensive fundamental and development work on ruthenium oxide electrochemical capacitors. In 1991, he described the difference between “Supercapacitor” and “Battery” behavior in electrochemical energy storage. In 1999, he coined the term supercapacitor to explain the increased capacitance by surface redox reactions with Faradaic charge transfer between electrodes and ions. His “supercapacitor” stored electrical charge partially in the Helmholtz double-layer and partially as the result of Faradaic reactions with “pseudocapacitance” charge transfer of electrons and protons between electrode and electrolyte. The working mechanisms of pseudocapacitors are redox reactions, intercalation and electrosorption.

2.4.7  Marcus model The physical and mathematical basics of electron charge transfer absent chemical bonds leading to pseudocapacitance was developed by Rudolph A. Marcus. Marcus theory explains the rates of electron transfer reactions—the rate at which an electron can move from one chemical species to another. It was originally formulated to address outer sphere electron transfer reactions, in which two chemical species change only in their charge, with transfer of electron. For redox reactions, without making or breaking bonds, Marcus theory takes the place of Henry Eyring’s transition state theory, which was derived for reactions with structural changes. Marcus received the Nobel Prize in Chemistry in 1992 for this theory. It has to be mentioned that we have not dealt in depth or in detail with Marcus mathematical model except by indicating the relevance and importance in this context. The aspiring readers may kindly refer for the excellent articles elsewhere.

2.4.8  Electric double layer in supercapacitors Although the models discussed give a satisfactory description of the electrical double layer on plane surfaces, they fall short of describing the real charge distribution in nanoporous electrodes employed in supercapacitors. The peculiarities of ion electrosorption in porous media make the process of charge storage extremely difficult, and there is still a lack of complete understanding of the behavior of ions in nanopores. When a supercapacitor is charged, electrons are forced to go from the positive electrode to the negative electrode 

28

2.  Fundamentals and energy storage mechanisms—overview

through an external circuit. As a consequence, cations within the electrolyte concentrate in the negative electrode and anions in the positive electrode forming an EDL that compensates the external charge unbalance. During the discharge, electrons travel from the negative electrode to the positive electrode through an external circuit, and both kinds of ions in the pores become mixed again until the cell is discharged. Ions do not move in the bulk electrolyte the same way as they do within the pores of an electrode material. The mobility of ions into the pores is greatly influenced by the pore size, which if too small makes the pores inaccessible, not contributing to double layer capacitance [20]. As not all the pores are accessible to the ions, there is no linear relation between the capacitance exhibited by a material and its specific surface area [21–24] measured with a small gas molecule probe, such as N2 or Ar. Various studies suggest that pore size below 0.5 nm is not accessible to hydrated ions [24, 25], and pores smaller than 1 nm can be too small for organic electrolytes [26]. Generally, there is a controversy regarding the effect of pore size on capacitance. Chmiola et al. claimed that pores with sizes below 1 nm greatly contribute to the capacitance [27]. This increase was explained with the distortion of the salvation shell, thus reducing the distance between charges and enhancing the capacitance [28, 29]. On the other hand, constant capacitance in the micropores was measured in organic electrolyte on the basis of a detailed assessment of pore size using complementary adsorption techniques. The decrease in the distance between the electronic and ionic charges is counter balanced by the corresponding decrease in the effective dielectric permittivity inside the pores, which occurs due to gradual ion desolvation [30–32]. Leaving the controversy of capacitance versus pore size behind, it is worth mentioning that industrially important values are also calculated on a volumetric basis. It then becomes clear that too wide pores contain free space, which is not used for capacitive charge storage, but decreases the density of electrodes. This effect is detrimental to volume-based capacitance as well as the existence of narrow electrolyte-inaccessible pores. Thus, tuning pore size is anyway needed to have carbon materials with narrow, short, and electrolyteaccessible pores [27]. Apart from it, there is a general agreement that the power capability of a supercapacitor can be enhanced by the presence of a small amount of mesopores (pores wider than 2 nm) for a rapid supply of electrolyte to the micropore surface where main charge storage takes place [33]. There have been numerous attempts to properly describe the capacitance of carbon materials depending on the pore shape and size and the specific character of their interaction with electrolytes. For mesoporous carbons with cylindrical pores, the traditional model is used [34]: C ε rε 0 (2.7) = A

b ln (b/b − d)

where b is the pore radius and d the distance between the ion and the carbon surface. For micropores, it is assumed that the ions line up in the center of a cylindrical pore, so the capacitance is calculated from [34]: ε rε 0 C (2.8) = A

b ln(b/a 0 )

where a0 is the effective size of the ion. This ionic radius was found to be close to the bare ion size, which means that the ions could be completely desolvated. However, more realistic





2.5 Pseudocapacitance

29

approximation to the pore shape in carbon is a slit, not a cylinder, a sandwich capacitance model was later proposed [35]. Ctot Cs εrε0 (2.9) = = 2 A A b − a0

2.5 Pseudocapacitance Pseudocapacitance is a Faradaic charge storage mechanism based on fast and highly reversible surface or near-surface redox reactions. Importantly, the electrical response of a pseudocapacitive material is ideally the same as the one of a double-layer capacitor. That is, the state of charge changes continuously with the potential, leading to proportionality constant that can be formally considered as capacitance. Some materials can also store significant charge in a double layer, such as functionalized porous carbon, combining thus both capacitive and pseudocapacitive storage mechanisms. The significance of the term and phenomenon referred to as “pseudocapacitance” has not been well understood in the fields of electrochemical capacitor and batteries. Let us therefore first discuss on its origin and significance. Regular double-layer capacitance arises from the potential-dependence of the surface density of charges stored electrostatically (i.e., non-Faradaically) at the interfaces of the capacitor electrodes. On the capacitor electrodes (metals or carbon surfaces), the accumulated charge is a combination of excess or a deficit of conductionband electrons at or in the near-surface region of the interface, together with counterbalancing charge densities of accumulated cations or anions of the electrolyte on the solution side of the double layers at the electrode interfaces. However, a double-layer capacitor device must employ two such double layers, one at each electrode interface, working one against the other on charge or discharge. Pseudocapacitance arises at electrode surfaces where a completely different charge storage mechanism applies. It is Faradaic in origin, involving the passage of charge across the double layer, as in battery charging or discharging, but capacitance arises on account of the special relation that can originate for thermodynamic reasons between the extent of charge acceptance (∆q) and the change of potential (∆V), so that a derivative d(∆q)/ d(∆V) or dq/dV, which is equivalent to a capacitance, can be formulated and experimentally measured by dc, ac, or transient techniques. The capacitance exhibited by such systems is referred to as pseudocapacitance as it originates in a quite different way from that corresponding to classical electrostatic capacitance of the type exhibited (mainly) by double-layer capacitors. It is now known that double-layer carbon capacitors exhibit perhaps 1%–5% of their capacitance as pseudocapacitance due to the Faradaic reactivity of surface (edge) oxygen-functionalities (depending on the conditions of preparation or pretreatment of the carbon material). On the other hand, pseudocapacitor devices, like batteries, always exhibit some electrostatic double-layer capacitance component proportional to their electrochemically accessible interfacial areas, probably about 5%–10%. The pseudocapacitance of the material can be intrinsic or extrinsic [36]. In the first case, materials exhibit pseudocapacitive behavior in a broad range of particle size and morphologies. Extrinsic pseudocapacitance only appears under specific conditions for nanosized material whereas the same material does not show pseudocapacitive behavior in the bulk.



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FIGURE 2.9  Schematic representation of interfaces of various different types of capacitors. (A) Conventional capacitor, (B) electric double-layer capacitor, and (C) asymmetric capacitor. Source: Reproduced with permission from Ref. [4].

Kinetically, pseudocapacitive materials can be distinguished from battery-type materials through electroanalytical experiments, with their kinetics being limited by a surface-related process as opposed to diffusion-controlled reactions governing the electrochemical response of battery electrodes. Different charge storage mechanisms can be distinguished in a pseudocapacitive electrode: under potential deposition, redox reactions of transition metal oxides, intercalation pseudocapacitance [37], and also reversible electrochemical doping and de-doping in conducting polymers. Materials used for building such electrodes are normally carbons, metal oxides, and conducting polymers [38]. Faradic processes occurring together with EDL charge storage increase the specific capacitance of an electrode. The capacitance of a pseudocapacitor can be 10–100 times higher than that of an EDLC. Nevertheless, the power performance of a pseudocapacitor is usually lower than that of EDLCs, due to the slower Faradic processes involved [39]. Electrodes exhibiting pseudocapacitance are more prone to swelling and shrinking on charge/discharge cycling, which can lead to poor mechanical stability and low cycle life [40]. The pictorial representations of interfaces in various types of capacitors are shown in Fig. 2.9.

2.6  Summary and outlook A comparison of supercapacitor with a typical battery like lithium ion is given in Table. 2.1. The advantages and disadvantages of supercapacitor are summarized in Table 2.2. In short, there are three kinds of capacitors and they are classified according to the capacitance value. 1. Electrostatic capacitor (pico farads to low microfarad) 2. Electrolytic capacitor (microfarad) 3. Supercapacitor (farad)



References 31

TABLE 2.1  Comparison between supercapacitor and Li-ion battery. Function

Supercapacitor

General lithium-ion battery

Charging time Cycle life Cell voltage Specific energy (Wh kg−1) Specific power (W kg−1) Service life (industrial) Charge temperature

10 s 1 million or 30,000 h 2.3–2.75 V 5 (Typical value) Up to 10,000 10–15 years −40 to 65°C

Up to 60 min 500 or higher 3.6 V 120–240 1000–3000 5–10 years 0–45°C

TABLE 2.2  Advantages and limitations of supercapacitors. Advantages

• • • • • •

Nearly unlimited cycle life can be cycled millions of times High specific power low resistance facilities high load currents Charges in seconds, no end of charge termination required Simple charging draws only what it needs not subject to overcharge Safe and allows abuse Excellent low temperature charge-discharge performance

Limitations

• • • • •

Low specific energy, only a fraction of a regular battery Linear discharge voltage prevents using the full energy spectrum High self-discharge higher than most of the batteries Low cell voltage requires series connections with voltage balancing High cost per watt

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[37] B.E. Conway, W.G. Pell, Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices, J. Solid State Electrochem. 7 (9) (2003) 637–644, doi: 10.1007/ s10008-003-0395-7. [38] K. Naoi, P. Simon, New materials and new configurations for advanced electrochemical capacitors, Electrochem. Soc. Interf. 17 (1) (2008) 34–37. [39] C.-M. Chuang, C.-W. Huang, H. Teng, J.-M. Ting, Effects of carbon nanotube grafting on the performance of electric double layer capacitors, Energy Fuels 24 (12) (2010) 6476–6482, doi: 10.1021/ef101208x. [40] Z.Z. Zhu, G.C. Wang, M.Q. Sun, X.W. Li, C.Z. Li, Fabrication and electrochemical characterization of polyaniline nanorods modified with sulfonated carbon nanotubes for supercapacitor applications, Electrochim. Acta 56 (3) (2011) 1366–1372, doi: 10.1016/j.electacta.2010.10.070.



C H A P T E R

3

Electrode materials for supercapacitors O U T L I N E 3.1 Introduction

3.2.2.3.2 Carbon-based stretchable and twistable supercapacitors (film-/fibershaped) 67 3.2.2.3.3 Carbon-based ultrafast superca­ pacitors for ac-line filtering 73

37

3.2 Electrode materials 38 3.2.1 Carbon materials in supercapacitors 39 3.2.1.1 Activated carbon 41 3.2.1.2 Mesoporous carbon 41 3.2.1.3 Carbide-derived carbons (CDC) 43 3.2.2 Carbon nanomaterials in supercapacitors 43 3.2.2.1 EDLCs 44 3.2.2.1.1 CNTs in EDLCs 44 3.2.2.1.2 Graphene in EDLCs 45 3.2.2.1.3 Hybrid carbon nanomaterials in EDLCs 47 3.2.2.2 Pseudocapacitors (PCs) 50 3.2.2.2.1 CNTs in PCs 50 3.2.2.2.2 Graphene in PCs 52 3.2.2.2.3 Hybrid carbon nanomaterials in PCs 59 3.2.2.3 Carbon-based hybrid supercapacitors 60 3.2.2.3.1 Carbon-based bendable supercapacitors (film-/fibershaped) 61

3.3 Perspectives on carbon for SC electrodes

78

3.4 Transition metal oxides/hydroxides 79 3.4.1 RuO2 79 3.4.2 RuO2-based composites 83 3.4.2.1 Mixed-oxide composites 83 3.4.2.2 RuO2/carbon composites 86 3.4.2.3 RuO2/polymer composites 87 3.4.3 Manganese dioxide (MnO2) for PCs 88 3.4.3.1 Recent R&D Advancements in MnO2 95 3.4.4 Cobalt oxides and hydroxide for supercapacitors 97 3.4.4.1 Co3O4 97 3.4.4.2 Co(OH)2 100 3.4.5 Nickel oxide/hydroxide (NiO/Ni(OH)2) 102 3.4.5.1 NiO 102 3.4.5.2 Ni(OH)2 104 3.4.6 Nickel cobaltite (NiCo2O4) 104 3.4.7 Tin oxide 107

Materials for Supercapacitor Applications. http://dx.doi.org/10.1016/B978-0-12-819858-2.00003-2 Copyright © 2020 Elsevier Inc. All rights reserved.

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36

3.  Electrode materials for supercapacitors

3.4.8 Vanadium oxides-based materials 107 3.4.8.1 Vanadium pentoxide 108 3.4.8.2 Other elemental metal doped vanadium pentoxide composites 111 3.4.8.3 Other vanadium pentoxide composites 112 3.4.8.4 Vanadium pentoxide/ compound-carbon material composites 112 3.4.8.4.1 Vanadium pentoxide/activated carbon or carbon fiber material composites 112 3.4.8.4.2 Vanadium pentoxide/ carbon nanotubes composites 114 3.4.8.4.3 Vanadium pentoxide/graphene composites 117 3.4.8.5 Vanadium pentoxide/ conducting polymer composites 119 3.4.8.6 Vanadium dioxide 121 3.4.8.7 Vanadium trioxide 124 3.4.8.8 Mixed valence vanadium oxide and its composite 125 3.4.8.9 Nitrides 128 3.4.8.9.1 Vanadium nitride 128 3.4.8.9.2 Vanadium nitride/ compoundcarbon material composites 130 3.4.8.9.3 Vanadium nitride/ titanium nitride composites 132 3.4.8.10 Vanadium sulfide 133 3.4.8.10.1 Vanadium disulfide 133 3.4.8.10.2 Vanadium tetrasulfide 133 3.4.8.10.3 Silver vanadium sulfide 134 3.4.8.10.4 Mixed metal vanadates 134 3.4.8.11 Vanadyl phosphate 136

3.4.9 Iron oxide-based materials 137 3.4.9.1 Influence of preparation routes 138 3.4.9.1.1 Hydrothermal method 140 3.4.9.1.2 Solvothermal method 141 3.4.9.1.3 Electrodeposition method 141 3.4.9.1.4 Spin coating technique 141 3.4.9.1.5 Electrospinning technique 142 3.4.9.1.6 Sol-gel method 142 3.4.9.1.7 Precipitation method 143 3.4.9.1.8 Successive ionic layer adsorption and reaction (SILAR) method 144 3.4.9.2 α-Fe2O3-based composites 144 3.4.9.2.1 α-Fe2O3- carbon composites 144 3.4.9.2.2 α-Fe2O3-conducting polymer composite 149 3.4.9.2.3 α-Fe2O3- metal oxide/hydroxide composite 151 3.4.9.2.4 Ternary nanocomposite 154 3.4.9.3 Cell performance of α-Fe2O3 156 3.4.9.3.1 Symmetric supercapacitor of α-Fe2O3 156 3.4.9.3.2 Asymmetric supercapacitor (ASC) of α-Fe2O3 160 3.5 Perspectives on transition metal oxides for SC electrodes 164 3.6 Conclusions and outlook



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3.1 Introduction

37

3.1 Introduction One of the key challenges faced by this present century is to identify and develop suitable and efficient energy storage devices in order to tap the full potentialities of various renewable energy technologies. In the ongoing pursuit for energy sustainability, supercapacitors are in forefront owing to their superior characteristics, such as high power densities at relatively high energy densities and long cycle life, compared to other storage devices known to date. Supercapacitors are also known as ultracapacitors or electrochemical capacitors, having other advantages compared to batteries. Performance comparison reveals that supercapacitor devices exhibit significantly lower energy storage capability compared to existing commercial batteries. The energy density of commercial supercapacitor cells is limited to 10 Wh kg−1 whereas that of common lead acid batteries reaches 35–40 Wh kg−1, while lithium ion batteries exhibit values higher than 100 Wh kg−1. As supercapacitors are characterized by fast discharge rates and minimum maintenance requirements, these are in demand for frequency regulation applications, apart from their applications in the fields of power electronics, large scale transport systems, energy storage at intermittent power generations, such as wind mills, solar, and tidal renewable technologies. As we have seen in the last chapter, supercapacitors are conventionally classified into two categories: electrochemical double layer capacitors (EDLCs), which are non-Faradaic and pseudocapacitors (PCs), which are Faradaic based on electrochemical redox mechanism of charge storage. The electrochemical interface between an electrode and electrolyte functions like a capacitor and thus the term “double-layer” is commonly employed while referring to the capacitor phenomenon. The rapid response of the interface to the changes in electrode potential and establishment of high reversibility, is utilized in EDLCs. On the other hand, pseudocapacitors are devices based on electrode charge storage phenomenon accompanied by highly reversible Faradaic electrochemical redox surface processes. Hence, the electrode materials requirement for the supercapacitor application varies based on the underpinning mechanisms in the different categories. Materials targeted for EDLCs have primarily been high surface area carbons, while pseudocapacitor research has largely been dominated by oxides, such as those of ruthenium and manganese. A schematic diagram for an EDLC is shown in Fig. 3.1 [1].

FIGURE 3.1  Schematic diagrams of an electrochemical double layer type capacitor showing the charged (left) and discharged (right) states. Source: Reproduced with permission from Ref. [1]. 

38

3.  Electrode materials for supercapacitors

For supercapacitors, in the present days, mostly the electrodes are fabricated from nanoscale materials that have high surface area and high porosity. It can be seen from Fig. 3.1 that charges can be stored and separated at the interface between the conductive solid particles (particles of carbon or metal oxide) and the electrolyte. This interface can be treated as a capacitor with an electrical double-layer capacitance, as expressed in Eq. (3.1): (3.1) C = Aε /4π d where A is area of the electrode surface, which for a supercapacitor, the active surface of the electrode porous layer; ε is the dielectric constant of the medium (electrolyte), which will be equal to 1 for vacuum and larger than 1 for all other materials, including gases; and d is the effective thickness of the electrical double layer. As supercapacitor devices are expected to fill the gap between conventional capacitors and batteries in terms of energy and power performances, and to be commercially viable for large-scale applications, it is essential that they exhibit greater energy densities with simultaneous reduction in the cost. Greater energy densities can be attained by increasing either the electrode capacitance or the electrolyte voltage window. The former is achieved by using high capacitance electrode materials and the latter through use of nonaqueous electrolytes with wider window of electrochemical stability. This has spurred research into a large number of alternative electrode materials including nanocomposites to improve capacitance largely utilizing the faster electronic and ionic transport characteristics of nanoscale systems. However, increasing the voltage window is considered to be a viable approach to improve the energy density on account of the squared dependence of energy density on voltage as well as the absence of issues associated with an increase in RC-time constant on increasing capacitance to achieve higher energy density, which would lead to a drop in rate capability [2]. Electrolyte development would be the subject of the next chapter for discussion in this monograph, while this chapter exclusively deals with materials aspects for electrodes. As indicated earlier, the unresolved issues of supercapacitors to date include low energy density, production cost, low voltage per cell, and high self-discharge. One of the straightforward approaches to overcome the issue of low energy density is to develop new electrode materials. Apparently, electrochemical performances of an electrode material directly rely on factors, such as surface area, electrical conductivity, wetting characteristics of the electrode, and permeability of electrolyte solutions [3]. The electrode material is a key component, which dictates the cell performance of a supercapacitor. Notable efforts have been made in the past years to increase the storage energy density of electrochemical capacitors, as can be witnessed from the numerous published works. The purpose of this chapter is to present and critically analyze the attempts made in search of new materials and approaches for finding supercapacitor electrodes, with a focus on the progress in energy storage capabilities for practical applications.

3.2  Electrode materials The parameters that rely on the type of electrode materials used in supercapacitors are capacitance and charge storage. Advancements in material science have unearthed a number of options that offer great advantages for supercapacitor nanostructures, including those





3.2  Electrode materials

39

based on electrostatic double-layer capacitance, electrochemical pseudocapacitance, and hybrids. The capacitance of electrochemical supercapacitors (ES) mainly depends on the specific surface area of the electrode materials. However, the measured capacitance of various materials does not linearly increase with increasing specific surface area, as not all the specific surface area is electrochemically accessible when the material is in contact with an electrolyte. Thus, for those electrochemically accessible surface area (useful surface area), the term “electrochemical active surface area” may be more appropriate in defining the electrode capacitance behavior. The pore size of the electrode material plays an important role in the electrochemical active surface area. According to Largeot et al., [4] the pore size of electrode materials that yielded maximum double-layer capacitance was closer to the ion size of the electrolyte, while both the larger and smaller pores led to a significant drop in capacitance. However, increasing the pore size can also increase the average distance d between the pore wall and the center of an ion, and hence making the capacitance drop in the materials with larger pores, according to Eq. (3.1) [4]. The porosity relevant to the development of high capacitance is itself not a simple parameter, involving both pore sizes and pore-size distribution for a given overall specific area (m2g−1) of the material. Therefore, ES capacitance strongly relies on the surface area of the electrode accessible to the electrolyte.

3.2.1  Carbon materials in supercapacitors Carbon materials are considered to be prospective electrode materials for industrialization. In the fabrication of supercapacitors, various forms of carbon are the most employed electrode materials due to its high surface area, low cost, availability and well-established electrode production technologies. The storage mechanism used by carbon materials stem from the electrochemical double layer formation at the interface between the electrode and electrolyte. The key factors influencing their electrochemical performance are specific surface area, pore-size distribution, pore shape and structure, electrical conductivity, and surface functionality [5–7]. Among these, specific surface area and pore-size distribution are the two most important factors affecting the performance of carbon materials, as the capacitance mainly depends on the surface area accessible to the electrolyte ions. In the case of carbon materials, high specific surface area elevates the capability for charge accumulation at the interface of electrode and electrolyte. For carbon materials, apart from pore size and high specific surface area characteristics, surface functionalization is also an important criterion, which plays a significant role in improving the specific capacitance. Examples of carbon materials, which are used for electrode fabrication, include activated carbon, carbon aerogels, carbon nanotubes, graphene, etc. This section details on the various carbons employed for supercapacitors, from the most common ones to the newest development. Carbon materials show a nearly rectangular shape cyclic voltammogram in both aqueous and organic electrolytes as illustrated in Fig. 3.2 [8]. As suggested by Conway [9], the carbon for double-layer type supercapacitors must possess three key properties, such as high specific areas in the order of 1000 m2 g−1, significant intra- and interparticle conductivity in porous matrices, and good electrolyte accessibility to the intrapore space of carbon materials. Hence, a fundamental guideline in the process of selecting supercapacitor electrode materials is to obtain a high and accessible specific surface area with excellent electrical conductivity.



40

3.  Electrode materials for supercapacitors

FIGURE 3.2  Cyclic voltammogram of an EDLC cell at 5 mVs−1 in (A) aqueous 6 M KOH and (B) organic 1 M tetraethyl ammonium tetrafluoroborate electrolytes. Source: Reproduced with permission from Ref. [8].

High surface area carbon materials mainly include activated carbon, carbon aerogels, carbon nanotubes (CNTs), templated porous carbons, and carbon nanofibers. The performance of these carbons as ES electrode materials has been described in detail in the literature [10]. However, in most cases [11] the measured specific capacitances of carbon materials in real supercapacitors are less than those stated in the literature, with values in the range of 75– 175 F g−1 for aqueous electrolytes and 40–100 F g−1 for organic electrolytes. In general, carbon materials with larger specific surface areas exhibited higher capability for charge accumulation at the electrode–electrolyte interface. Several approaches were examined to increase the specific surface area, including heat treatment, alkaline treatment, steam, or CO2 activation, and plasma surface treatment with NH3 [12–22] in order to effectively form micropores and defects on the carbon surface. Till today, there is a lack of common understanding or agreement on the influence of optimal pore size on the performance of carbon electrode materials. Certain research reports claim that the pore sizes of either 0.4 or 0.7 nm could be suitable for aqueous electrolytes, while a pore size of around 0.8 nm might be better for organic electrolytes [14, 23]. Further, a match between the pore size and ion size was demonstrated by achieving a maximum capacitance [4, 24]. Surface functionalization has also been considered as an effective means to improve the specific capacitance of carbon materials [25–35]. It is demonstrated that surface functional groups or heteroatoms help the adsorption of ions and thereby improve the hydrophilicity/lipophilicity of the carbon materials, leading to enhanced wettability and facilitate rapid electrolyte ion transport within the micropores. Further, these may induce Faradaic redox reactions [36], leading to ∼5%–10% increase in the total capacitance. The heteroatoms commonly present in a carbon framework are oxygen, nitrogen, boron, and sulfur. Among these, the introduction of nitrogen has been investigated extensively in the literature [30, 34, 37–40]. It has to be noted that the risk of electrolyte decomposition induced by active surface functional groups, especially oxygen-containing acidic groups is also increased, particularly in an organic electrolyte, depending on the concentration of functional groups, the electrode 



3.2  Electrode materials

41

surface area, and the operating ES voltage [32]. In addition, the intrinsic electrical resistance of the material may increase because the bonded heteroatoms possess higher reactivity, resulting in barriers to electron transfer. 3.2.1.1  Activated carbon Activated carbon is the most extensively employed active material for supercapacitor electrodes due to its high surface area and relatively low cost [41–43]. These are derived from carbon-rich organic precursors through heat treatment under inert atmosphere, a process known as carbonization and activation resulting in the formation of pores. Carbonization process produces amorphous carbon through the thermochemical conversion of precursors, while the activation leads to high surface area that is achieved through making a partial control oxidation of the carbon precursor grains by physical or chemical activation [43]. The precursors are obtained from various natural resources, such as wood, coconut shells, fossil fuels and their derivatives, such as pitch, coal, coke, or from synthetic precursors, such as polymers [41, 42]. Physical activation is done at high temperatures under oxidizing atmosphere (CO2, H2O, etc.) [42], while the chemical activation is performed on amorphous carbons previously mixed with chemicals, such as alkalies, carbonates, chlorides, or acids (e.g., KOH, K2CO3, etc.). Any activation process leads to the formation of porous network in the bulk of the carbon particles with high specific surface area (SSA). Nanopores can be classified based on their size, namely into micropores ( 50 nm). The accurate measurement of SSA is not a straightforward one, depending greatly on the calculation method and measurement conditions. Specific surface areas of 3000 m2 g−1 are sometimes reported, but the actually accessible and useable SSA falls in the range of 1000–2000 m2 g−1 [41]. In Table 3.1, we present different precursors and the corresponding Brunauer-Emmett-Teller (BET) surface area values for activated carbons derived from them. Most of the commercially available devices employ activated carbon electrodes and organic electrolytes, exhibiting operable cell voltage of 2.7 V with a specific capacitance of 100– 120 F g−1 and up to 60 F cm−3 [41]. As a promising candidate for mass production, low-cost carbon-rich biochar of red cedar claim to attain a gravimetric capacitance of 115 F g−1 in aqueous electrolyte [45]. Carbon hallow fibers attain 287 F g−1 [46] at 50 mA g−1 with a capacitance retention of ∼86% at 1 A g−1. A specific capacitance of 340 F g−1 was reported for carbon prepared by phosphoric acid activation from a sugarcane bargasse precursor [47]. In aqueous electrolyte, the operating cell voltage is limited to 0.9 V [41] and the specific capacitance reaches 300 F g−1 [42]. Activated carbon powders mixed with carbon blacks and organic binders are used to make thin films to coat current collectors. The pore size distribution in activated carbon power is broad and it is not generally optimized due to difficulty in the activation process. Longer activation time and higher temperatures yield larger average pore size [42]. However, the whole of SSA of the material is not used and hence, a part of it does not contribute to capacitance. 3.2.1.2  Mesoporous carbon High surface area ordered mesostructures are of interest due to their capabilities to deal with high power ratings without notable capacity fading. Micropores offer bottlenecks, which can drastically decrease ion mobility, thereby reducing the power capability of the electrode.



42

3.  Electrode materials for supercapacitors

TABLE 3.1  BET specific surface areas for different carbon precursors. Carbon precursor

Activation medium

SBET m2 g−1

Furfurol

Steam

1040

Coconut shell

KOH

1660

Eucalyptus wood

KOH

2970

Firewood

Steam

1130

Bamboo

KOH

1290

Cellulose

KOH

2460

Potato starch

KOH

2340

Starch

KOH

1510

Sucrose

CO2

2100

Beer lees

KOH

3560

Banana fiber

ZnCl2

1100

Corn grain

KOH

3200

Sugar cane bagasse

ZnCl2

1790

Apricot shell

NaOH

2335

Sunflower seed shell

KOH

2510

Coffee ground

ZnCl2

1020

Wheat straw

KOH

2316

Fish scale

—

2270

Cherry stone

KOH

1300

Rice husk

NaOH

1890

Rice husk

KOH

1390

Adapted with permission from Ref. [44].

As mesopores are not narrow paths to slow down the ion transport, these can maintain the capacitance even at high current densities. Mesoporous carbons can be prepared through various routes—high degree activation, carbonization of precursors composed of one thermosetting component and one thermally unstable component, catalyst assisted activation of carbon precursors with metal oxides or organometallic compounds, or carbonization of cryogels or aerogels. These methods yield mesoporous carbons with broad pore size distribution with considerable amount of microporosity. These can also be made through replication synthesis using hard templates and by self-assembly using soft templates through co-condensation and carbonization. The last two methods are desirable, because pore distribution and pore size can be better controlled [48]. In hard template synthesis, templates function as molds with no significant chemical interactions between templates and carbon precursors, leading to well-defined nanostructures. The soft template generates nanostructures through self-assembly of organic molecules. In this approach, the pore structure is determined by synthesis conditions, such as mixing ratios,





3.2  Electrode materials

43

solvents and temperature. These two methods yield mesoporous carbons with well-defined pore structure and narrow pore size distribution [48]. Mesoporous carbon derived from lignin using Pluronic F127 surfactant subjected to CO2 activation exhibited SSA of 624 m2 g−1 and a gravimetric specific capacitance up to 102 F g−1 [49]. Mesoporous carbon derived from rice husk precursor reaches a BET surface area of 1357 m2 g−1 with a total pore volume of 0.99 mL g−1 with 44.4% mesoporosity [50], exhibiting a specific capacitance of 114 F g−1 at a scan rate of 5 mV s−1 in organic electrolyte. 3.2.1.3  Carbide-derived carbons (CDC) Carbide-derived carbons (CDCs) are prepared by high-temperature extraction of metals from carbides as precursors. The common routes employed for the CDC synthesis are hightemperature chlorination [51, 52] and vacuum decomposition [53]. CDCs are considered as promising candidates for supercapacitors because carbide precursors allow fine-tuning of porous networks [54] and better control over surface functional groups than activated carbons [55]. Porous network in CDCs can be tailored owing to the availability of varied distribution of carbon atoms in carbide precursors as well as by changing the synthesis temperature. For instance, while comparing the titanium and silicon carbide derived carbons prepared by using the same synthesis temperature of 1200°C, leads to narrower pore size distribution and smaller average pore size of SiC-CDC [55]. A common trend of increasing pore size with increasing synthesis temperature is observed, independent of the precursor used [56]. The porous structure often collapses, if the synthesis temperature exceeds 1300°C, while the graphitization occurs at temperatures >1000°C [57]. The properties of CDCs can be enhanced by post-treatment, such as treatment with hydrogen [58]. In supercapacitor applications, it has been observed that the capacitance is governed by the structure of CDCs, while the rate performance depends significantly on carbide precursor. Titanium CDCs exhibit highest gravimetric capacitance up to 220 F g−1 in KOH and 120 F g−1 in organic electrolyte, while SiC-CDC exhibit the highest volumetric capacitance of 126 F cm−3 in KOH and 72 F cm−3 in organic electrolyte [53]. The specific capacitance of TiC-CDC in an organic electrolyte using (CH3CH2)3CH3NBF4 as a salt [59] was found to be 70–90 F cm−3 or 100–130 F g−1 depending on the synthesis conditions. A series of CDCs with tailored porosity prepared in the temperature range 600–1200°C were subjected to study the effect of pore size, and it is infered that the pores 2 nm [60]. Another interesting general trend observed was that the capacitance decreases with increasing synthesis temperature despite the rise in the specific surface area and pore volume, suggesting that the capacitance is influenced mainly by the pore size. Just by tuning synthesis temperatures, CDC can be adapted for higher energy/ power applications [61].

3.2.2  Carbon nanomaterials in supercapacitors The recent developments in the fields of nanoscience and nanotechnology have opened up a new arena for carbon materials by producing novel graphitic carbon nanomaterials with multi-dimensions, such as dimensionless (0D) fullerene, one-dimensional (1D) carbon nanotubes (CNTs) [62–70], and two-dimensional (2D) graphene sheets [71–82]. Fullerene C60 has a football-like structure containing 20 carbon hexagons and 12 carbon pentagons forming a



44

3.  Electrode materials for supercapacitors

cage of truncated icosahedrons. Fullerene C60 is an electron acceptor and hence widely used for charge separation in solar cells. However, due to its small surface area, intractability, and low electrical conductivity these are rarely employed for energy storage applications compared to other carbon nanomaterials. CNTs [83–93], graphene [80, 94–124], mesoporous carbon [10, 19, 42, 125–128] and their hybrids [129–142] have been extensively studied as supercapacitor electrodes due to their excellent electrical conductivity, high specific surface area, electrochemical activity, and the ease with which they can be functionalized into multidimensional and multifunctional structures with excellent electrical and mechanical properties. 3.2.2.1 EDLCs As mentioned earlier, EDLCs store energy through charge separation as in case with traditional capacitors, which lead to double-layer capacitance. But, unlike a traditional capacitor, an EDLC contains two separated charge layers at the interfaces of electrolyte with positive and negative electrodes. The separation between electrical double layers in an EDLC is smaller than that in a conventional capacitor, yielding several orders of magnitude higher specific capacitance for EDLC. In the absence of any involvement of chemical reactions, the transport of ions in the electrolyte solution or electrons through the electrodes is responsible for charge storage, and hence EDLCs can be fully charged or discharged within a short time with a highpower density. Ideally, EDLCs require electrode materials with a high specific surface area and excellent electrical conductivity, which can be found in CNTs and graphene. 3.2.2.1.1  CNTs in EDLCs

CNTs, either in the pure form or by compositing with other electrode materials are ideal ones for fabricating supercapacitor electrodes. The reported specific surface area of pure CNTs is around 120–500 m2 g−1 with the specific capacitance between 2 and 200 F g−1 [83–86]. Using single-walled carbon nanotubes (SWNTs) as the electrode materials, a specific capacitance, power density, and energy density up to 180 F g−1, 20 kW kg−1 and 7 Wh kg−1, respectively, have been reported [87, 88]. By activating the walls and/or tips of the CNTs, an enhancement in specific surface area can be achieved. For instance, Pan et al. [89] have improved the specific surface area of SWNTs from 46.8 to 109.4 m2 g−1 through electrochemical activation, which led to threefolds increase in the specific capacitance. For pristine SWNTs with specific surface area of 1300 m2 g−1, in an organic electrolyte 1 M Et4NBF4/propylene carbonate (to ensure a high voltage of 4 V), an energy density as high as 94 Wh kg−1 (or 47 Wh L−1) and a power density up to 210 kW kg−1 (or 105 kW L−1) has been reported [90]. The intrinsic surface area of CNTs is controlled by the size of tube-diameters. The specific surface area of multiwall carbon nanotubes (MWNTs) with outer diameter of 10–20 nm and inner diameter of 2–5 nm varied from 128 to 411 m2 g−1 with increasing diameters, and showed the highest specific capacitance of 80 F g−1 in 6 M KOH electrolyte [91]. In most cases, EDLCs employing pure CNTs exhibited high-rate capabilities and cyclic stabilities. They exhibited rectangular cyclic voltammograms and symmetric triangular galvanostatic charge-discharge profiles, indicating high charge storage efficiencies. Apart from improving the specific surface area, efforts have been made to improve the electrical conductivity and number of active sites on CNTs. Heteroatom doping has been demonstrated to be an important and efficient method to achieve these requirements. For instance, nitrogen-doped (N-doped) CNTs were synthesized by in situ polymerization of aniline monomers on CNTs, followed by carbonization





3.2  Electrode materials

45

of polyaniline (PANI)-coated CNTs [92]. In this study, the N-doping level was controlled by adjusting the amount of aniline, yielding a highest specific capacitance of 205 F g−1 in 6 M KOH electrolyte—a value higher than 10 F g−1 for the pristine CNTs, at 8.64% (mass percentage) nitrogen doping. Nevertheless, 97.1% of the initial capacitance was maintained after 1000 cycles. Later, Gueon, and Moon prepared N-doped CNT-based spherical particles by emulsion-assisted evaporation of hexadecane, followed by N-doping using melamine [93]. A specific capacitance of 215 F g−1 was achieved at a current density of 0.2 A g−1 which is ∼3 times of enhancement as compared with pristine CNTs. The observed performance improvement was attributed to the combined effect of enhancement in number of active sites with higher electrical conductivity induced by N-doping. Interestingly, N-doped aligned CNT arrays have also been synthesized and systematically tested for their application in supercapacitors [94]. It was found that the supercapacitor performance at a low scan rate was highly dependent on the pyridinic nitrogen content in N-doped CNTs due to net charges induced over the neighboring carbon atoms through protonation of the pyridinic nitrogen. 3.2.2.1.2  Graphene in EDLCs

Having the basic carbon lattice structure similar to CNTs with all carbon atoms exposed at the surface, the single-atom-thick 2D graphene sheets exhibit similar electrical and other properties as CNTs but with larger specific surface area [83]. Graphene sheets have also been extensively studied as electrode materials in ESCs. The availability of graphene oxide (GO) by acid oxidation of graphite [95–97], followed by chemical reduction [80, 95–97], offers lowcost mass production of reduced graphene oxide (rGO), which can directly be used as EDLC electrode materials. In this regard, Stoller et al. used hydrazine hydrate as the reducing reagent to produce rGO from GO [80]. The resultant rGO exhibited a specific capacitance of 135 F g−1 and specific surface area of 705 m2 g−1 [80], which is lower than the theoretic value of 2630 m2 g−1, presumably due to rGO aggregation. To minimize the aggregation of rGO, Chen et al. [98] synthesized graphene with mesoporous structure through thermal exfoliation of rGO at 1050°C, which exhibited a specific capacitance up to 150 F g−1 in 30% KOH aqueous solution. Lv et al. [99] employed microwave irradiation in vacuum to decrease the temperature required for thermal exfoliation, and they decreased the exfoliation temperature down to 200°C with a concomitant increase in specific capacitance up to 264 F g−1. By using microwave radiation to assist the exfoliation process, Zhu et al. [100] also effectively decreased the exfoliation time to as short as 1 min and thus the produced graphene could still exhibit specific capacitance of 191 F g−1 in 5 M KOH. For conventional graphene and rGO electrodes, electrolyte ions can only transfer charges between graphene sheets, which resulted in longer ion-transport path with respect to ions transferring through the graphene sheets (Fig. 3.3). To address this issue, Xu et al. synthesized holey graphene sheets, which allow ions to pass through the holes with a minimized transport path while still maintaining efficient electron-transport [101]. As expected, the hierarchically structured 3D holey graphene electrode exhibited both high gravimetric and volumetric specific capacitances of 298 F g−1 and 212 F cm−3, respectively. In addition, the energy density of a corresponding fully packaged supercapacitor is as high as 35 Wh kg−1 (49 Wh L−1), which is considered to be sufficient to bridge the gap between supercapacitors and batteries. As in case with CNTs, surface activation has been used to improve the specific capacitance of graphene electrodes without a detrimental effect on electrical conductivity. Ruoff



46

3.  Electrode materials for supercapacitors

FIGURE 3.3  Pictorial representations of the graphene and holey graphene foams. (A, B) Initial 3D macroporous (A) graphene foam and (B) holey graphene foam. (C, D) Compressed films of the (C) graphene foam and (D) holey graphene foam. (E, F) Enhanced views of (E) graphene and (F) holey graphene films. The arrows indicate the ion transport pathway. Source: Reproduced with permission from Ref. [101].

et al. obtained a dramatically enhanced specific surface area up to 3100 m2 g−1 by activating exfoliated GO with KOH [102], which is even higher than the theoretically predicted specific surface area of monolayer graphene (2630 m2 g−1) and attributable to the presence of a 3D network containing pores with sizes of 1–10 nm. In another study, the same group reported to employ activated rGO films to form graphene films exhibiting a specific capacitance of 120 F g−1 at high current density 10 A g−1 with corresponding energy density and power density of 26 Wh kg−1 and 500 kW kg−1, respectively [103]. A further improvement in the specific surface area up to 3290 m2 g−1 was realized by designing a mesoporous structure integrated with macroporous scaffolds [104]. As a result, specific capacitance of 174 F g−1 (100 Fcm−3) was achieved with energy and power densities of 74 Wh kg−1 and 338 kW kg−1, respectively. Doping graphene with heteroatoms can also improve its electrical/electrochemical properties for energy storage applications [105]. Indeed, Jeong et al. synthesized N-doped graphene through a simple plasma process [106], and it is found to exhibit a specific capacitance of 280 F g−1, which is 4 times higher than that of the corresponding undoped pristine graphene. This is attributed to the introduction of charge-transferring sites through N-doping, to induce charge modulation and thereby improving electrical conductivity of graphene, leading to higher specific capacitance with an enhanced power and energy densities of 8 × 105 W kg−1 and 48 Wh kg−1 respectively. N-doped graphene has also been synthesized through hydrothermal reduction of GO with nitrogen containing precursors [107]. The resultant 3D Ndoped graphene framework had a low density of 2.1 mg cm−3 with a high specific capacitance of 484 F g−1 in 1 M LiClO4 electrolyte and maintained the capacitance of 415 F g−1 even after 1000 cycles at a high current density of 100 A g−1 [107]. Furthermore, doping graphene with other elements, such as boron, phosphorous, and/ or co-doping with N and P or B and N, has also been demonstrated to significantly enhance their energy-storage performance [108, 109]. Graphene-based self-assembled 3D structures, such as hydrogels and aerogels have lately emerged as electrode materials for supercapacitors due to their high porosity, low density, and excellent adsorption capabilities [110–112].





3.2  Electrode materials

47

The detailed synthetic processes and properties of graphene hydrogels and aerogels have been reviewed in References [110–112]. Briefly, in an aqueous solution of GO, the van der Waals attractions from the basal planes of GO sheets and the electrostatic repulsions from the functional groups of GO sheets are balanced each other to maintain the well-dispersed state of GO sheets. While this balance is lost, gelation of the GO dispersion takes place, leading to the formation of 3D GO hydrogels that can be further reduced or functionalized to produce 3D graphene-based architectures [113–115]. Different techniques have been used to produce graphene hydrogels, such as hydrothermal reduction [113], chemical reduction [115], cross-linking agents (including metal ions [116], biomolecules [117], polymers [118], etc.), sol-gel reaction [119], freeze-drying [120], etc. Similar to hydrogels, graphene aerogels are made by replacing the solution with a gas [121, 122]. There have been numerous reports on graphene aerogels for supercapacitor applications and certain successful efforts have been summarized in Table 3.2 [123a]. The assemblies of graphene hydrogels, aerogels, or organogels [123b] in general exhibit poor overall conductivities. As the graphene-based nanostructured carbon materials often offer low density, in most cases, the volumetric energy densities of carbon-based supercapacitors are low, which hinders their practical application. Yang et al. and Yoon et al. have demonstrated graphene-based highly packed supercapacitors with volumetric energy density of 59.9 Wh L−1 and specific capacitance of 171 F cm−3, respectively [124, 125]. However, still considerable R&D effort is necessary to improve the volumetric energy density. 3.2.2.1.3  Hybrid carbon nanomaterials in EDLCs

Carbon nanomaterials with distinct structures are expected to exhibit synergetic effects toward electrochemical performance. For example, carbon black has been used to separate graphene sheets to produce 3D hybrid materials with minimized aggregation of graphene, leading to a high specific capacitance of 175 F g−1 at 10 mV s−1 scan rate in 6 M KOH electrolyte [130]. In other work, mesoporous carbon spheres were sandwiched between graphene sheets and the resulting 3D structure exhibited a specific capacitance of 171 F g−1 at 10 mV s−1 in 6 M KOH [131]. More interestingly, CNTs were intercalated between graphene sheets to retain the specific surface area of graphene by minimizing its aggregation [132]. The π–π interaction between graphene and CNTs can also improve the electrical conductivity and mechanical strength [133]. Just like GO to disperse CNTs in solvents [131], oxidized CNTs have been used to form composites with graphene [134]. In this direction, Yu and Dai produced hybrid films of CNT and graphene interconnected network with well-defined nanopores [134], which exhibited a specific capacitance of 120 F g−1 in 1 M H2SO4 electrolyte and an almost rectangular cyclic voltammogram even at a 1 V s−1 scan rate. Sun et al. reported other interesting works [135], in which graphene sheets were intercalated between CNTs in aligned CNT fiber. Although the resultant hybrid fiber exhibited a specific capacitance of only 31.5 F g−1, it was higher than that of the pure CNT fiber (5.83 F g−1). By using a modified hydrothermal microreactor, Yu et al. produced a continuous CNT and graphene hybrid fiber with welldefined mesoporous structures [136], which had a specific surface area as high as 396 m2 g−1 with an electrical conductivity of 102 Scm−1. The corresponding fiber-shaped supercapacitor showed a volumetric specific capacitance of 305 F cm−3 at 26.7 mA cm−3 current density and a volumetric energy density of 6.3 mWh cm−3, which is comparable to the energy density of a 4 V–0.5 mAh thin-film lithium-ion battery. Further, a 3D N-doped CNT/graphene network



48

3.  Electrode materials for supercapacitors

TABLE 3.2  Carbon nanomaterials in electrical double-layer capacitors (EDLCs). Specific capacitance (F g−1)

Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

3199 at 5 mV s−1

—

—

70% after 350 cycles

[84]

PEDOT/MWCNT 1 M LiClO4

79

5

11.3

85% after 1,000 cycles

[85]

PTFE/PANI-CNT

30% KOH in H2O

163 at 0.1 A g−1

—

—

—

[86]

C tubes

0.5 M H2SO4

315 at 0.35 V

—

—

—

[89]

SWCNT

1 M Et4NBF4/ propylene carbonate

160 at 4 V

24

17

96.4 % after 1,000 cycles

[90]

MWCNT

6 M KOH

80–135

—

—

—

[91]

N-doped (8.64 wt. %) carbon shell and a CNT-core

6 M KOH

205

—

—

97.1 % after 1,000 cycles

[92]

Spherical particles of Ndoped CNT

1 M H2SO4

215 at 0.2 A g−1

—

—

99% after 1,500 cycles

[93]

rGO

1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6)

348 at 0.2 A g−1

—

—

120% after 3,000 cycles at 10 mVs−1

[95]

Graphene hydrogel

5 M KOH

220 at 1 A g−1

30

5.7

92 % after 2,000 cycles

[96]

rGO by Na2CO3

6 M KOH

228 at 5 mA cm−2

—

—

—

[97]

Graphene sheets

30 wt.% KOH in H2O

150 at 0.1 Ag−1

—

—

100% up to 500 cycles

[98]

Exfoliated rGO

30 wt.% KOH in H2O

264 at 0.1 A g−1

—

—

97% after 100 cycles

[99]

MW assisted rGO

5 M KOH

191 at 0.15 A g−1

—

—

91% at 0.6 A g−1

[100]

Hollow graphene

1-ethyl-3-methylimid- 298 at 1 A g−1 azolium tetrafluoroborate/acetonitrile (EMIMBF4/AN)

1000

35

—

[101]

Exfoliated rGO

1-ethyl-3-methylimid- 166 at 5.7 A g−1, azolium tetrafluo3.5 V roborate/acetonitrile (EMIMBF4/AN)

250

70

97% after 10,000 cycles

[102]

Electrode

Elecrtolyte

Carbon nanotube (CNT) CNT (functionalized by –OH, –COOH)

0.075 M hydroquinone (HQ) into 1 M H2SO4 aqueous

Graphene





49

3.2  Electrode materials

TABLE 3.2  Carbon nanomaterials in electrical double-layer capacitors (EDLCs). (Cont.) Specific capacitance (F g−1)

Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

Electrode

Elecrtolyte

rGO (KOH activated)

Tetraethylammonium tetrafluoroborate (TEABF4) in AN

120

500

26

95% after 2,000 cycles

[103]

Graphene made porous carbon

1-ethyl-3-methylimidazolium bis(trifluoromethyl sulfonyl) imide, [EMIM][TFSI]:AN, 1:1

174 at 4.1 A g−1

338

74

94% after 1,000 cycles

[104]

N-doped graphene

1M Tetraethylammoniumtetrafluoroborate (TEA BF4)

280 at 20 A g−1 6 M KOH

800

48

99.8% after 230, 000 cycles

[105]

N-doped (5.86 at%) graphene hydrogel

6 M KOH

308 at 3 A g−1 current

—

—

92% after 1,200 cycles

[106]

3D N-doped graphene

1 M LiClO4

484 at 1 A g−1, 415 at 100 A g−1

—

—

100% after 1,000 cycles

[107]

B-doped rGO

6 M KOH

200 at 0.1 A g−1

∼10

∼5.5

95% after 4,500 cycles

[108]

N/P doped rGO

6 M KOH

165 at 0.1 A g−1

—

—

80% after 2,000 cycles at 0.5 A g−1

[109]

Graphene hydrogel (hydrothermal)

—

175 at 10 mVs−1

—

—

—

[113]

4.38 at% Ngraphene hydrogel

5 M KOH

131 at 80 A g−1

144

4.5

95.2% after 4,000 cycles at 100 A g−1

[114]

Graphene aerogel (3D printed)

3 M KOH

4.76 at 0.4 A g−1

4.08

0.26

95.5% after 10,000 cycles at 200 mVs−1

[121]

Graphene aerogel

0.5 M H2SO4

325 at 1 A g−1

7

45

98% after 5,000 cycles

[122]

2D microporous covalent triazine-based framework

Solvent-free ionic liquid EMIMBF4

151.3 at 0.1 A g−1

10

42

85% after 10,000 cycles at 10 Ag−1

[123b]

Adapted with permission from Ref. [123].



50

3.  Electrode materials for supercapacitors

was also synthesized through hydrothermal treatment and freeze-drying, followed by carbonizing the GO and pristine CNT mixture in the presence of pyrrole [137]. The resultant hybrid carbon fiber showed high electrochemical performance, especially capacitance retention of 96% after 3000 cycles [136]. In case of self-assembled carbon-composite material [138–143], freestanding 3D graphene hydrogel and carbon nanofiber composite material demonstrated 150.2 F g−1 specific capacitance at 1 A g−1 current with 97.8% capacitance retention after 2000 cycles [138]. Carbon nanofiber and nanotube network was also synthesized from conjugated polymer for electrochemical energy storage [139]. Carbon-based electrical double-layer supercapacitors (EDLCs) are listed in Table 3.2. 3.2.2.2  Pseudocapacitors (PCs) Pseudocapacitors store energy through reversible Faradaic charge transfer, which involves fast and reversible electrochemical redox reactions on the interface between the electrodes and electrolyte. As such, the specific capacitance of a pseudocapacitor is often higher than that of an EDLC, as is the energy density. As the redox reactions occur on the electrode surface, a high specific surface area and high electrical conductivity are essential for electrodes in a high-performance PC. Therefore, carbon nanomaterials, including CNTs, graphene, mesoporous carbon, and their hybrids, have also been used as the substrate to load active materials and/or current collector to ensure high capacitance and fast charge transfer for electrodes in high-performance PCs. 3.2.2.2.1  CNTs in PCs

CNTs have been used in pseudocapacitors either in a functionalized form or composited with other active components, such as conductive polymers and metal oxides. CNTs can be functionalized through chemical or electrochemical methods. The most common way to functionalize CNTs is acid oxidation (e.g., a mixture of concentrated sulfuric acid and nitric acid) to introduce surface carboxyl groups [144]. Through acid oxidation, the specific capacitance of CNTs can be increased by 3.2 times due to the increased hydrophilicity of the electrodes in aqueous electrolytes and the introduction of pseudocapacitance. Treatment of CNTs with NaOH solution at 80°C, followed by ultrasonication in H2SO4/HNO3 solution, can also improve the specific capacitance from 28 F g−1 for the pristine CNTs to 85 F g−1 for the functionalized CNTs [84]. However, the oxidation treatments inevitably induced defects to degrade the CNT structure and reduce the electrical conductivity. Therefore, a delicate balance between the electrode performance and its structural integrity is important for attaining high performance in pseudocapacitors. Conducting polymers with good electrical conductivities and redox activities often exhibit high specific capacitances while these are composited with CNTs. In this regard, Bai et al. increased the energy density of a CNT-based PC by 4 times, up to 11.3 Wh kg−1, by compositing poly (3,4-ethylenedioxythiophene) (PEDOT) homogenously onto the CNT electrode through in situ polymerization [145]. Similarly, polypyrrole (PPy)/CNT composite electrodes have been also synthesized to yield a specific capacitance of 165 F g−1 in 1 M KCl solution [146]. Compared with PEDOT and PPy, PANI possesses a higher theoretical specific capacitance [147], which was confirmed by a high specific capacitance of 501.8 F g−1 reported for flexible PANI/SWNT composite films synthesized through in situ electrochemical polymerization (Fig. 3.4) [148]. Subsequent electrodegradation further increased the specific capacitance to 706.7 F g−1 by forming charge transfer channels via





3.2  Electrode materials

51

FIGURE 3.4  Schematic illustration and SEM image of PANI/SWNT composite for pseudocapacitor and specific capacitance (solid squares) and surface resistances (half-filled circles) for samples SWNT/PANi90 before and after electrochemical degradation for 5, 10, 20, 40 cycles. (The arrows in panel (B) point toward the different y-axis labels.) Source: Reproduced with permission from Ref. [148].

selective dissolution of polycrystalline and off-lying disordered PANIs. As PANI changes its color during charge-discharge process, PANI/CNT composites have been used for high-performance (308.4 F g−1 in PVA/H3PO4) smart supercapacitors with highly reversible chromatic transitions during charge-discharge processes for monitoring the energy-storage status by the PANI color changes [149]. Apart from carbon materials, metal oxides and hydroxides are two classes of active electrode materials used for pseudocapacitors. In spite of their lower electrical conductivity compared to conducting polymers, metal oxides and hydroxides exhibit better electrochemical stability. For metal oxides and hydroxides to be used in high-performance pseudocapacitors, the corresponding metals must exhibit two or more oxidation states, which can co-exist and inter-transfer freely. The examples include RuO2, MnO2, NiO, V2O5, Fe2O3, Co3O4, TiO2, SnO2, Mn3O4, and Ni(OH)2 [150–157]. In the metal oxide-CNT composite electrodes, CNTs provide 

52

3.  Electrode materials for supercapacitors

the high electrical conductivity and large specific surface area for efficient loading of active materials and in addition, effectively restrict the volumetric change of metal oxides, or hydroxides caused by the cyclic charge-discharge processes [155]. As RuO2 has three oxidation states and a wide operational potential window, RuO2/CNTs have become a typical composite electrode material for pseudocapacitors. The specific capacitance of RuO2 with a large surface area can reach up to 1170 F g−1 in 0.5 M H2SO4 electrolyte [157]. Reddy and Ramaprabhu have synthesized RuO2/CNT, TiO2/CNT, SnO2/CNT composites by chemical reduction of corresponding salts to functionalize CNTs and demonstrated the highest capacitance of 160 F g−1 for the TiO2/CNT [156]. On the other hand, MnO2 possesses a high theoretical specific capacitance of 1370 F g−1 [158] and has been electrochemically deposited onto chemical vapor deposition (CVD)-grown CNT arrays to exhibit a specific capacitance of 642 F g−1 in 0.2 M Na2SO4 electrolyte [159]. Notable charge/discharge stability up to 10,000 cycles was reported for MnO2/CNT composite electrodes [160]. Further, flexible and wearable supercapacitors based on MnO2/CNT composite fibers prepared by electrochemical deposition of MnO2 on aligned CNT fibers exhibited a specific capacitance of 3.707 mF cm−2 [161]. 3.2.2.2.2  Graphene in PCs

Graphene with a high specific surface area and high electrical conductivity has been composited with other active materials, such as conductive polymers, metal oxides, and hydroxides, as electrodes for pseudocapacitors [162–168]. For example, PANI/GO composites have been synthesized through in situ polymerization of aniline into PANI on GO [162], and it has been noticed that depending on the mass ratio of PANI to GO, performance of the PANI/ GO electrode varied and the highest specific capacitance of 746 F g−1 has been achieved at a PANI/GO mass ratio of 200/1. Like a pristine PANI electrode, the PANI/GO composite electrode showed very low electrochemical cyclic stability, with a retention rate of only 20% after 500 cycles. Increased electrochemical cyclic stability was achieved by increasing the GO content in the PANI/GO composite and a 73% retention rate was obtained at the PANI:GO ratio of 23:1. By growing vertically aligned PANI nanowires on GO substrate (Fig. 3.5A) [163], the morphology of PANI and the mass ratio of PANI to GO was well controlled to exhibit a specific capacitance as high as 555 F g−1 in 1 M H2SO4 at 0.2 A g−1 with 92% retention rate after 2000 cycles at 1 Ag−1. Yan et al. also synthesized a PANI/graphene composite with PANI nanoparticles (2 nm) uniformly decorated on the graphene flakes (Fig. 3.5B) [164], and found that graphene not only acted as a support material to offer active sites for the nucleation of PANI, but also provided a better electron-transfer path, leading to a specific capacitance of 1046 F g−1 in 6 M KOH at a 1-mV s−1 scan rate, energy density of 39 Wh kg−1 and power density of 70 kW kg−1. To further improve the performance, graphene sheets with more active nucleation sites were prepared by unzipping CNTs to produce graphene nanoribbons with a specific capacitance of 340 Fg−1 and a retention rate of 90% after 4200 charge-discharge cycles [165]. Interestingly, higher specific capacitance of 989 F g−1 in 6 M KOH electrolyte was obtained when cobalt was introduced into PANI/graphene composites through polymerizing aniline in the presence of both cobalt and graphene [166]. 3D PANI/graphene composite hydrogel was also synthesized as a free-standing film to prepare flexible supercapacitors without binders [167, 168], which exhibited excellent energy-storage properties as well as outstanding flexibilities for portable electronic devices. Later, a simple and cost-effective





3.2  Electrode materials

53

FIGURE 3.5  (A) Schematic illustrations to the heterogeneous nucleation on graphene oxide nanosheets and in bulk solution for the growth of PANI nanowires. (B) Schematic illustration for the synthesis of GNS/PANI composite. Source: Part A, Reproduced with permission from Ref. [163]. Copyright American Chemical Society (2010); Part B, Reproduced with permission from Ref. [164].

method was reported to produce graphene/polystyrene sulfonic acid-graft-aniline (Gr/S-gA) nanocomposite through direct exfoliation of graphite using S-g-A as a surfactant [169]. The Gr/S-g-A composite electrode was demonstrated to exhibit a superior specific capacitance of 767 Fg−1 at 0.5 Ag−1 current density in 0.1 M Bu4NPF6/acetonitrile electrolyte with 92% capacitance retention after 5000 cycles. The corresponding supercapacitor showed an energy density of 208.8 Wh kg−1 and a power density of 347.8 W kg−1 [169].



54

3.  Electrode materials for supercapacitors

Similar to PANI, polypyrrole (PPy) has been composited with graphene through a simple route by directly mixing PPy with rGO [170], and the PPy/graphene composite exhibited a specific capacitance of 400 F g−1 in 2 M H2SO4 at a current density of 0.3 Ag−1. PPy/GO composite was also synthesized through in situ chemically polymerizing pyrrole monomers in the presence of FeCl3 to improve the homogeneity of the composite which exhibited a mildly enhanced specific capacitance of 421.42 F g−1 in 0.1 M KCl electrolyte [171]. By substituting FeCl3 with ammonium persulfate dissolved in citric acid, the specific capacitance was further improved to 728 Fg−1 at 0.5 A g−1 current density with a retention rate of 93% after 1000 cycles [172]. PPy was assembled with GO into layer-by-layer composites through the electrostatic interaction [173]. However, the electrochemical performance of PPy/GO composite is not promising due to the poor electrical property of GO. Through electrochemical polymerization of pyrrole monomer and simultaneous electrostatic deposition, Zhao et al. synthesized 3D PPy/graphene composite foam [174] which exhibited a specific capacitance of 350 F g−1 at a current density of 1.5 A g−1 with no change in initial capacitance even after 1000 cycles. In another attempt, PPy/graphene composite films were prepared through electrochemical polymerization of PPy on graphene pre-deposited electrophoretically onto a Ti substrate [175], leading to a specific capacitance of 1510 F g−1 (or 151 mFcm−2, or 151 Fcm−3) in 0.1 M LiClO4 electrolyte at 10 mVs−1 scan rate. This high specific capacitance was attributed to the porous structure, effective utilization of the pores and the large specific surface area for rapid redox reactions during the charge-discharge process in the pseudocapacitor. As in case with CNTs, metal oxides or hydroxides were composited with graphene-based nanomaterials to improve the pseudocapacitance. For instance, MnO2 nanoneedles-GO composite was synthesized via a chemical route by using MnCl2 and KMnO4 in water/isopropyl alcohol mixture in the presence of GO [176]. The as-synthesized MnO2/GO composite exhibited a specific capacitance of 216 F g−1 with the retention of 84.1% after 1000 cycles. By perpendicularly grafting poly(sodium methacrylic acid) polymer brushes onto the GO sheets prior to loading of MnO2 nanoparticles, MnO2/GO composite with uniformly distributed MnO2 on the GO substrate was prepared, which showed an improved specific capacitance of 372 F g−1 in 1 M Li2SO4 at 0.5 A g−1 current density and an increased retention of 92% even after 4000 cycles [177]. Alternatively, MnO2/GO composite with porous structure has been synthesized with the aid of microwave irradiation [178, 179] and exhibited a specific capacitance of 310 Fg−1 at 2-mV s−1 scan rate and 228 F g−1 at a high scan rate of 500 mV s−1. Further, Mn3O4/graphene composite was prepared, and showed a specific capacitance of 271.5 F g−1 at 0.1 A g−1 and 180 F g−1 at even a high current density of 10 A g−1 with no significant decay of capacitance even after 20,000 cycles [180]. A filtration technique has been developed for preparing freestanding Mn3O4/graphene composite film as the electrode [181]. With regard to carbon-supported metal oxide PCs, vanadium phosphates (VOPO4)/graphene composited with vertically aligned porous 3D structure obtained through an ice-templated self-assembly process showed a 527.9 F g−1 gravimetric capacitance at 0.5 A g−1 current density with 85% capacitance retention after 5000 cycles at a 100 mV s−1 scan rate in 6 M KOH electrolyte, which is considered to be a promising supercapacitive performance [182]. As a highly active metal oxide for redox reactions, RuO2 has also been composited with graphene as electrodes in pseudocapacitors. RuO2/graphene composite was synthesized through a sol-gel technique, followed by annealing, to exhibit a specific capacitance of 570 F g−1 in 1M H2SO4 with a retention of 97.9% after 1000 cycles [183]. This method effectively reduced the 



3.2  Electrode materials

55

aggregation of both RuO2 nanoparticles and graphene sheets by separating graphene sheets with the grafted RuO2 nanoparticles. An energy density up to 20.1 Wh kg−1 at 0.1 A g−1 current density was achieved, which remained at 4.3 Wh kg−1 when the power density approached 10 kW kg−1. Cobalt oxide is another metal oxide that is capable of demonstrating rapid and reversible redox reactions during the charge–discharge process in pseudocapacitors. In this direction, Co3O4/graphene composite was synthesized via a chemical reaction of cobalt nitrate with urea in GO suspension under microwave irradiation to exhibit a specific capacitance of 243.2 F g−1 in 6 M KOH [184]. Co3O4/graphene composite has also been synthesized by a hydrothermal method, followed by calcination to improve the electrochemical performance [185]. The ratio of Co3O4 to graphene appears to influence the morphology of the resultant Co3O4/graphene composite. At a 7% mass ratio of graphene, Co3O4 nanoplates homogeneously grew on the graphene sheets, leading to improved specific capacitances of 667.9 and 385.1 F g−1 at the current densities of 1.25 and 12.5 A g−1, respectively. Co(OH)2 has also been composited with graphene to prepare pseudocapacitor electrodes. A sheet-on-sheet structured Co(OH)2/graphene composite was synthesized through a one-step in situ hydrothermal method and exhibited a specific capacitance of 540 F g−1 at a high current density of 10 A g−1 [186]. Increased specific capacitances with charge/discharge cycles were observed for both the Co3O4/graphene and Co(OH)2/graphene composites, attributable to a gradual activation of the active materials wrapped up by graphene during the initial charge–discharge cycling [185]. Apart from Mn, Ru, and Co oxides, oxides of other metals with multiple valences, such as Ni, Fe, Ti, and Zn, have also been investigated as alternative electrode materials that can be composited with graphene to further improve pseudocapacitive performance [187–191]. For instance, hexagonal nanoplates of single-crystal Ni(OH)2 have been grown on lightly oxidized conductive graphene sheets using an in situ synthesis technique [187]. The resultant composite exhibited an ultrahigh specific capacitance of 1335 F g−1 in 1 M KOH at a current density of 2.8 A g−1 with a notably high-rate capability of 953 F g−1 at 45.7 A g−1 and an excellent cycling stability of 94.3% specific capacitance retention even after 3000 cycles. Self-assembled graphene-based structures, such as hydrogels and aerogels, were also extensively employed for pseudocapacitor electrodes [192]. In particular, Zhang et al. recently developed a plasma treatment approach to fabricate 3D N-doped graphene aerogel/Fe3O4 nanostructures, which showed a specific capacitance of 386 F g−1 in 6 M KOH electrolyte with a 97% retention capability after 1000 cycles [193]. In situ growth of active materials on a graphene substrate could exhibit high energy-storage properties because of the intimate interactions and efficient charge transfer between active nanomaterials and the conductive graphene substrate. Among other self-assembled graphene-based 3D structures, graphene hydrogels modified with different oxygen-containing groups using hydroquinones [193], MnO2/graphene hydrogel composites [195, 196], graphene hydrogels modified with 2-aminoanthraquinone [197], RuO2/ reduced graphene-oxide hydrogels [198], free standing polyaniline/reduced graphene-oxide composite hydrogels [151], and single crystalline Fe2O3 nanoparticles directly grown on graphene hydrogels [200] have been studied. Details of their supercapacitive performance have been summarized in Table 3.3. Self-assembled porous graphene networks synthesized from various organic chemicals, which are generally termed as organogels, have also been used as electrode materials for supercapacitors [201–204]. However, more R&D efforts are required for better understanding of the mechanism of charge storage in such organogels for improving their performance as efficient supercapacitor electrodes. Similar to EDLCs, it is highly desirable to 

56

3.  Electrode materials for supercapacitors

TABLE 3.3  Carbon nanomaterials in pseudocapacitors (PCs).

Elecrtolyte

Specific capacitance (F g−1)

Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

Poly(3,4-ethylene dioxy thiophene) or PEDOT/MWCNT

1 M LiClO4

79 at 1 Ag−1

5

11.3

85% after 1000 cycles

[145]

Polypyrrole-coated MWCNT

1 M KCl

165 at 0.5 mAcm−2

—

—

100% after 1000 cycles

[146]

Polyaniline (PANI)/ SWNT

0.5 MH2SO4

706.7 at 5 mVs−1

—

—

—

[148]

PANI/aligned CNT

PVA/H3PO4 308.4 (1:0.85, mass)

—

—

73.4%

[149]

0.5

36.8, 40.2, — 25

[156]

Electrode Carbon nanotube (CNT)

RuO2/MWCNT, TiO2/ 1 M H2SO4 MWCNT, SnO2/ MWCNT

138, 160, 93 at 2 mVs−1

RuO2/CNT

0.5 M H2SO4

1170 at 10 mVs−1 —

—

82% at 400 mVs−1

[157]

MnO2/CNT

0.2 M Na2SO4

642 at 10 mVs−1

—

—

100% up to 700 cycles

[160]

MnO2/CNT

1 M Na2SO4

201 at 1 A g−1

0.6

13.3

100% up to 10, 000 cycles at 1 A g−1

[161]

GO-doped polyaniline 1 M H2SO4 (PANI)

531 at 0.2 A g−1

—

—

—

[162]

PANI nanowire/GO

555 at 0.2 A g−1

—

—

92% after 2000 cycles at 1 A g−1

[163]

Graphene sheet/PANI 6 M KOH

1046 at 1 mVs−1

70

39

—

[164]

PANI nanorods/Graphene nanoribbon

1 M H2SO4

340

3.15, 9.47

7.56, 4.01

90% after 4200 cycles

[165]

Co-PANI/graphene

6 M KOH

989 at 2 mVs−1

1.58

352

79% after 1000 cycles

[166]

3D graphene/PANI hydrogel

6 M KOH

334 at 3 A g−1

—

—

57% after 5000 cycles

[167]

PANI/graphene

1 M H2SO4

375.2 at 0.5 A g−1 1

30.34

90.7% after 500 cycles at 3 A g−1

[168]

208.8

92% after 5000 cycles

[169]

Graphene

1 M H2SO4

Graphene/poly 0.1 M (styrenesulfonic acid- Bu4NPF6/ graft-aniline) acetonitrile

0.35

767 at 0.5 A g−1





57

3.2  Electrode materials

TABLE 3.3  Carbon nanomaterials in pseudocapacitors (PCs). (Cont.) Specific capacitance (F g−1)

Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

Graphene/poly pyrrole 2 M H2SO4 (PPy) nananotubes

400 at 0.3 A g−1

—

—

88% after 200 cycles at 1.5 A/g

[170]

PPy/GO

Electrode

Elecrtolyte

421.4 at 2 mA

—

—

—

[171]

Graphene/PPy nanow- 1 M KCl ires

0.1 M KCl

728 at 0.5 A g−1

—

—

93% after 1000 cycles

[172]

Pillared GO/PPy

2 M H2SO4

510 at 0.3 A g−1

—

—

70% after 1000 cycles at 5A g−1

[173]

PPy/3D graphene foam

3 M NaClO4

350 at 1.5 A g−1

—

—

100% after 1000 cycles

[174]

PPy/Graphene

0.1 M LiClO4

1510 at 10 mVs−1 3

5.7

Up to 25 cycles

[175]

GO/MnO2 needles

1 M Na2SO4

216 at 0.15 A g

—

—

84.1% after 1000 cycles at 0.2 A g−1

[176]

−1

Amorphous MnO2/GO 1 M Li2SO4

372 at 0.5 A g−1

—

—

92% after 4000 cycles at 0.5 A g−1

[177]

MnO2/porous graphene

1 M H2SO4

256 at 0.25 A g−1

24.5

20.8

87.7% after 1000 cycles

[178]

MnO2/graphene

1 M Na2SO4

310 at 2 mVs−1

—

—

88% at 100 mVs−1

[179]

Mn3O4/graphene sheets

6 M KOH

271.5 at 0.1 A g−1 —

—

100% after 20,000 cycles

[180]

Mn3O4/graphene paper Potassium 321.5 at 0.5 A g−1 — polyacrylate (PAAK)/KCl

—

—

[181]

3D VOPO4/graphene

—

85% after 5000 cycles at 100 mVs−1

[182]

6 M KOH

527.9 at 0.5 A g−1 —

RuO2/Graphene sheets 1 M H2SO4

570 at 1 mVs−1

10

20.1

97.9% after 1000 cycles at 1 A g−1

[183]

Co3O4/graphene sheets 6 M KOH

243.2 at 10 mVs−1

—

—

95.6% after 2000 cycles at 200 mVs−1

[184]

Co3O4 nanoplates/Gra- 2 M KOH phene sheets

667.9 at 1.25 A g−1

—

—

81.3% after 1000 cycles

[185] (Continued)



58

3.  Electrode materials for supercapacitors

TABLE 3.3  Carbon nanomaterials in pseudocapacitors (PCs). (Cont.)

Electrode

Elecrtolyte

Specific capacitance (F g−1)

Co(OH)2/graphene sheet layers

1 M KOH

622 at 2 A g−1

15.8

86.6

80% after 10000 cycles at 10 A g−1

[188]

Ni(OH)2/graphene

1 M KOH

1335 at 2.8 A g−1

10

37

100% after 2000 cycles at 28.6 A g−1

[189]

α-Fe2O3 nanotubes/ rGO

0.1 M K2SO4

181 at 3A g−1

—

—

108% after 2000 cycles at 5A g−1

[190]

TiO2/graphene

1 M KOH

84 at 10 mVs−1

—

—

87.5% after 1000 cycles at 2 A g−1

[191]

ZnO/graphene

1 M KOH

62.2

8.1

—

94.9% after 200 cycles

[192]

Fe3O4/N-doped graphene aerogel

6 M KOH

386 at 5 mVs−1

—

—

153% after 1000 cycles

[193]

Graphene hydrogel 1 M H2SO4 functionalized using hydroquinones

441 at 1 A g−1

—

—

86% after 10,000 cycles at 10 A g−1

[194]

MnO2/graphene hydrogel

1 M KOH

445.7 at 0.5 A g−1 6.4

∼18

82.4% after 5000 cycles at 50 mVs−1

[195]

MnO2/rGO hydrogel and aerogel

1 M Na2SO4

242, 131 at 1 Ag−1

0.82

212

89.6% after 1000 cycles

[196]

Graphene hydrogel 1 MH2SO4 modified by 2-amino anthraquinone

258 at 0.3 A g−1

—

—

Slightly increased after 2000 cycles at 10 A g−1

[197]

15% RuO2/rGO hydrogel

1 M H2SO4

345 at 1 A g−1

—

—

100% after 2000 cycles at 1 A g−1

[198]

PANI/graphene hydrogel

1 M H2SO4 and 223.82 and 1 M H2SO4 580.52 at + 0.4 M 0.4 A g−1 Hydroquinone

2.637

13.2

87.5 and 70% af- [199] ter 5000 cycles at 10 Ag−1

Single-crystal Fe3O4/ graphene hydrogel

1 M KOH

908 at 2 A g−1

—

—

69% at 50 A g−1

Graphene organogel

1 M propylene carbonate in tetraethyl ammonium tetrafluoroborate

140 at 1 A g−1

16.3

15.4

64.3% at 30 A g−1 [201]



Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

[200]



59

3.2  Electrode materials

TABLE 3.3  Carbon nanomaterials in pseudocapacitors (PCs). (Cont.) Specific capacitance (F g−1)

Power Energy density density Retention (kW kg−1) (Wh kg−1) capability

References

46.3 mF cm−2 at 0.1 A g−1

—

—

98% after 1000 cycles at 10 A g−1

[202]

Graphene sheet/CNT/ 6 M KOH PANI

1035 at 1 mVs−1

—

—

94% after 1000 cycles at 200 mVs−1

[212]

N-doped porous C/MWCNT (SBET = 1270 m2/g)

1 M H2SO4

262 at 0.5 A g−1

—

—

—

[213]

PANI-graphene/CNT

1 M KCl

271 at 0.3 A g−1

2.7

188.4

82% after 1000 cycles at 2 Ag−1

[214]

Graphene-MnO2/CNT 1 M Na2SO4/ Polyvinyl Pyrrolidone (PVP)

486.6 at 1 A g−1

∼1.7

15

92.8% after 800 cycles at 1 A g−1

[215]

Cobalt chloride carbonate hydroxide nanowire/AC

1 M KOH

1737 at 2.5 mA cm−2

0.1

29.03

85.6% after 1000 cycles at 7.5 mA cm−2

[217]

Patronite (VS4)/ SWNT/rGO

0.5 M K2SO4

558.7 at 1 A g−1

13.85

174.6

97% after 1000 cycles

[218]

PANI/GO/graphene

2 M H2SO4

793.7 at 1A g−1

2.14

50.2

80% after 1000 cycles at 100 mVs−1

[219]

Ni(OH)2-graphene/ CNT stacked layers

2 M KOH

1065 at 22.1 A g−1

8

35

96% after 20,000 cycles at 21.5 A g−1

[220]

Electrode

Elecrtolyte

Polyethyleneimine 6 M KOH induced coagulated GO nanosheets from dispersion Hybrid carbon nanomaterial

Adapted with permission from Ref. [123].

improve the volumetric capacitance of graphene-based PCs for practical applications. In this context, Xu et al. recently reported a PC fabricated using a PANI and graphene composite monolith, which demonstrated 802 F cm−3 volumetric capacitance at 54% PANI loading and 66% of the capacitance was maintained when the current density increased by 100 times [205]. Alternate methods have been employed in supercapacitor formation from mesoporous carbons. These include functional group variation using nitric acid oxidation, [206], doping with metal oxides or nitrogen or others [207–211]. 3.2.2.2.3  Hybrid carbon nanomaterials in PCs

The composites that may be formed by using different selective carbon materials are expected to exhibit synergetic effect. For instance, CNT and graphene have been composited with PANI via in situ polymerization [212] and the resultant PANI/CNT/graphene composite showed notably a high specific capacitance of 1035 F g−1 in 6 M KOH electrolyte at 

60

3.  Electrode materials for supercapacitors

1-mVs−1, which is comparable to that of PANI/graphene (1046 F/g) and higher than that of PANI/CNT (780 F g−1) or pure PANI (780 F g−1). Furthermore, 94% of the initial capacitance was maintained after 1000 cycles, while the corresponding retention rates for PANI/ graphene and PANI/CNT composites were only about 52% and 67%, respectively. Various other ternary composites, such as metal oxide/CNT/graphene, conductive polymer/metal oxide/graphene, N-doped microporous carbon/CNT) were also examined as electrode materials for SCs [213–216]. Particularly, cobalt chloride carbonate hydroxide nanowire arrays (CCCH NWAs) with an average length of 8 mm were synthesized on the surface of a Ni foam via a hydrothermal process, and exhibited specific capacitance of 1737 F g−1 in 1 M KOH at 2.5 mA cm−2, and good cycling stability with a capacitance retention of 87.3% after 2000 cycles at a current density of 7.5 mA cm−2. An asymmetric supercapacitor based on CCCH NWAs as positive and activated carbon as negative electrodes exhibited an energy density of 29.1 Wh kg−1 and a power density of 100 W kg−1 with substantially good stability over a wide voltage range of 0–1.6 V [217]. Patronite (VS4) has also been demonstrated to perform well in combination with SWNT and rGO to show a specific capacitance of 558.7 F g−1 in 0.5 M K2SO4 at 1-A g−1 current density in 0.5 M K2SO4 electrolyte and delivered an energy density of 174.6 Wh kg−1 with a power density of 13.85 kW kg−1 [218]. Further, a combination of both GO and pristine graphene with PANI has led to a specific capacitance of 793.7 F g−1 in 2 M H2SO4 at a 1-A g−1 current density [219]. This symmetric supercapacitor has exhibited an energy density of 50.2 Wh kg−1 at a power density of 2.14 kW kg−1, and the elevation in the performance has been attributable to the synergistic effects of individual components. A graphene/ CNT stacked structure has also been investigated as electrodes for supercapacitors [220,221]. A summary of various carbon materials based pseudocapacitors can be found in Table 3.3. 3.2.2.3  Carbon-based hybrid supercapacitors Hybrid supercapacitors (HSCs) are designed to bridge the gap between ESCs, which have high power but low energy, and batteries which have high energy but low power. Mostly, HSC consists of a capacitive carbon electrode matched with either a pseudocapacitive or a lithiuminsertion electrode (Fig. 3.6) [44, 222–224]. In HSCs, the combination of Faradaic intercalation on cathode and non-Faradaic surface reaction on anode provides a platform to achieve both high energy and power densities without compromising cycling stability and affordability.

FIGURE 3.6  Schematic representation of hybrid supercapacitor (HSC). Source: Reproduced with permission from Ref. 123a.





3.2  Electrode materials

61

The carbon-based materials used for cathodes in HSCs are graphite, CNTs, graphene, activated carbon (AC), 3D mesoporous carbons and different metal oxide or polymer-based carbon composites [225]. 3D graphene/MnO2 composite showed a maximum specific capacitance of 1145 F g−1, which is about 83% of theoretical capacitance at a mass loading of 13% of MnO2 [227]. Attempt has been made to employ the cathode in HSCs fabricated using a Fe3O4 nanoparticle/graphene composite prepared through a simple solvothermal method (Fig. 3.7). The Fe3O4/graphene-based half-cell exhibited a high reversible specific capacity exceeding 1000 mAh g−1 at a current density of 90 mA g−1 with high rate of capability and cycle stability [226]. While this composite was assembled in a Li-ion-based (LiPF6) HSC, exhibited elevated energy densities in the range of 204–65 Wh kg−1 and power densities from 55 to 4600 W kg−1 [227]. A little later, Lim et al. reported HSCs based on a mesoporous Nb2O5/ carbon-composite anode and AC (MSP-20) cathode showing excellent energy and power densities of 74 Wh kg−1 and 18,510 W kg−1 (at 15 Wh kg−1) respectively, with a capacity retention of 90% at 1000 mA g−1 after 1000 cycles in the electrolyte mixture of LiPF6 (1.0 M)/ ethylene carbonate and dimethyl carbonate (1:1volume ratio) [228]. Li et al. reported a hybrid-type supercapacitor based on N-doped AC, which showed high material-level energy densities of 230 Wh kg−1 with a power density of 1747 W kg−1 with a capacity retention of 76.3% after 8000 cycles [229]. 3.2.2.3.1  Carbon-based bendable supercapacitors (film-/fiber-shaped)

Flexible and wearable SCs, either in the shape of a thin film or fiber (coating, fabric/cloth, paper, textile, etc.), are considered as attractive for advanced power sources. Carbon nanomaterials perform as promising electrode materials for flexible supercapacitors (FSCs), due to their large surface area, excellent mechanical and electrical properties, and high electrochemical stability. For instance, Chen et al. produced flexible and transparent supercapacitors based on In2O3 nanowire/CNT heterogeneous films, and observed an increase in specific

FIGURE 3.7  Schematic illustration of the synthesis of the negative electrode material Fe3O4/G nanocomposite and the positive electrode material 3D graphene, together for the configuration of a Li-ion containing organic hybrid supercapacitor. Source: Reproduced with permission from Ref. [227].



62

3.  Electrode materials for supercapacitors

capacitance up to 64 F g−1 with increasing numbers of In2O3 nanowires (up to 0.007 mg) dispersed on the CNT films [230]. In another study, a 2-mm-thick film-based FSC made of MnO2 nanosheet-decorated carbon nanofiber electrodes was demonstrated to exhibit a gravimetric capacitance of 142 F g−1 at a slow scan rate (10 mVs−1) when the electrode was interfaced with PVA-H4SiW12O40·nH2O [231]. Other materials used for the electrode in FSCs include TiO2/MWNT/PEDOT composited carbon fibers [232] and various carbon papers made of fibers, aerogels, and nanotubes [233]. Liu et al. fabricated highly flexible porous films of carbon nanofibers (PCNFs) by an electrospinning technique combined with a Co ion-assistant acid corrosion process [234]. The resultant fibers have high conductivity and outstanding mechanical flexibility, with little change in their resistance under repeated bending, even up to 180°. The P-CNF electrode showed a specific capacitance of 104.5 F g−1 in 0.5 M H2SO4 at 0.2 A g−1 current, effectively improved cycling stability and 94% retention of specific capacitance after 2000 charging/discharging cycles, along with a retention rate of 89.4% capacitance after 500 bending cycles [234]. These outstanding performances are attributed to the high degree of graphitization and the unique hierarchical pore structures of P-CNF [234]. As low processing cost for flexible electrode manufacturing is desirable, Du et al. developed a low-cost, flexible, and high-performance hybrid electrode based on a MnO2 nanotube (NT) and CNT composite film obtained through a vacuum-filtering method [235]. Due to an elongated 1D nanotube morphology, the synergetic effects between pseudocapacitive MnO2NTs and conductive CNTs, the hierarchical porous structure of free-standing film and the high mass loading of MnO2 (4 mg cm−2), the resultant MnO2-NT/CNT electrodes showed excellent mechanical and electrochemical performance with a volumetric capacitance of 5.1 Fcm−3 in polyvinyl alcohol (PVA)/LiCl gel electrolyte, a high energy density of 0.45 mWh cm−3 for the entire FSC volume with retention of capacitance at about 105% of initial capacitance after 6000 cycles due to a self-activation effect. As shown in Fig. 3.8 these SCs can be integrated in wearable electronic devices as flexible power sources that can be employed in watches and light emitting diodes (LEDs). Furthermore, flexible supercapacitors have also been fabricated from conducting polymers with and without compositing with other electrode materials (e.g., CNTs, graphene). PANI, PPy, and PEDOT possess high specific capacitances of 1284, 480, and 210 F g−1, respectively [236]. PPy is extensively employed polymer electrode material for FSCs due to its high environmental stability, excellent redox activity, and easy availability. The electrochemical properties of PPy can be enhanced by compositing with CNTs, graphene or their hybrid/composites. Yesi et al. prepared films of CNT-PPy core-shell composite by growing CNTs directly on carbon cloth (CC) as a skeleton, followed by electropolymerization of PPy on CNTs [237]. The direct fabrication of CNT-PPy on the flexible CC electrode increased the interfacial conductance and ion transport between electrode and electrolyte. The PPy/CNT-CC electrode exhibited 1038-F g−1 gravimetric capacitance per active mass of PPy and up to 486.1 F g−1 per active mass of the PPy/CNT composite, with excellent mechanical flexibility and cycle stability up to 10,000 cycles with 18% capacitance retention The corresponding asymmetric supercapacitor (PPy-CNT-CC/CNT-CC) delivered a maximum power density of 10,962 W kg−1 and energy density of 3.9 Wh kg−1 at 1.4 V potential. It has to be mentioned that for most solid-state supercapacitors based on freestanding graphene materials, specific capacitance ranges from 80 to 135 F g−1 while the corresponding





3.2  Electrode materials

63

FIGURE 3.8  (A) Schematic illustration of the fabrication process of flexible freestanding CNT/MnO2 NT hybrid film. (B) Ragone plots of the flexible solid-state SC device. Inset shows a group of LEDs (consisting of 32 green LEDs) powered by four series-connected SCs. (C) CV curves collected at the same scan rate of 5 mVs−1 under normal, bent and twisted conditions; insets are the digital images under the corresponding test conditions. (D) An electronic watch wrapped around a transparent glass tube demonstrating the flexibility of the SC-integrated watch band. Source: Reproduced with permission from Ref. [235].

theoretical value is ∼550 F g−1. The observed difference is attributed to restacking of graphene sheets, leading to reduced active surface area of the graphene-based electrodes [235]. To overcome the π–π restacking of graphene sheets, various attempts were made to control the electrode structure into, for example, porous 3D graphene hydrogels on Ni foam by CVD or freeze-drying GO. Mitchell et al. have further proposed a strategy to produce hierarchical and flexible nanosheets of NiCo2O4-GO composite on nickel foam by using electrochemical deposition [238], which exhibited a specific capacitance of 1078 F g−1 in 3 M KOH electrolyte at 1 mA discharge current and a relatively poor cyclic stability of almost 45% decay over 500 cycles. In another attempt to prevent the restacking of graphene layers in a 3D graphene electrode, Li et al. fabricated a solid-state asymmetric supercapacitor (ASC) based on flexible electrodes [239]. In this case, the positive electrode was made from densely packed graphene sheets with intercalated Ni(OH)2 nanoplates, showing a gravimetric capacitance of 573 Fg−1 and volumetric capacitance of 655 F cm−3 in 1 M KOH electrolyte at 0.2 A g−1 current density, excellent rate capability, and cycling stability. The negative electrode was fabricated with CNT layers stacked in between highly dense graphene sheets. The asymmetric supercapacitor exhibited 

64

3.  Electrode materials for supercapacitors

an energy density of 18 Wh kg−1 and power density of 850 W kg−1 at 1 A g−1 current density. When the current density increased up to 20 A g−1 the corresponding energy density remained at 6.4 Wh kg−1 with a high-power density of 17 kW kg−1. Bending the device up to 180° did not cause any impact on the electrochemical performance [239]. With the aim to improve the specific capacitance in film-based FSCs, complex and hybrid structures, such as CNT/graphene and Mn3O4 nanoparticles/graphene paper electrodes with a polymer gel electrolyte of potassium polyacrylate (PAAK)/KCl were conceived and specific capacitance of 72.6 F g−1 at 0.5 A g−1 current was obtained [181]. Similar to conventional ESCs, composites of CNT/graphene and conducting polymers have also been widely used in FSCs. Wang et al. synthesized polyacrylic acid/PANI composites enhanced by nitrogen-doped graphene (NG) (NG-PAA/PANI) [240] and demonstrated that the CC electrodes containing 32 wt.% PANI and 1.3 wt.% NG showed a high capacitance of 521 F g−1 in 1 M H2SO4 electrolyte at 0.5 A g−1 (Fig. 3.9). A symmetric supercapacitor fabricated from 20 wt.% PANI-CC electrodes exhibited 4 times higher capacitance of 68 F g−1 at 1 A g−1 than the previously-reported SCs based on flexible PANI-CNT composites. The NG-PAA/PANI electrode retained the full capacitance over large bending angles with an energy density of 5.8 Wh kg−1, a power density of 1.1 kW kg−1, a higher rate capability of 81% at 10 A g−1 and long-term electrochemical stability of 83.2% retention after 2000 cycles [240]. Aphale et al. fabricated freestanding hybrid electrodes by incorporating graphene and CNTs with PPy, which showed a specific capacitance of 453 F g−1 in 1 M H2SO4 at 5 mVs−1 (Fig. 3.10) [241]. Further, the hybrid electrode exhibited an ultrahigh energy density of 62.96 Wh kg−1 at a power density of 566.66 W kg−1. Four such SCs assembled in a series were demonstrated to power a 2.2 V LED.

FIGURE 3.9  Illustration of the process from synthesis to obtaining a NG-PAA/PANI composite coating on CC. PANI is supplied by both the first and second steps. NG doping in step 3 improves the conductivity further and reduces swelling. Source: Reproduced with permission from Ref. [240].





3.2  Electrode materials

65

FIGURE 3.10  Fabrication and characterization of hybrid electrode. (A) Schematics of fabrication process of nanocomposite electrodes. (B) Illustration of hybrid nanocomposite film forming a unique interface where graphene and CNT are embedded in situ during polymerization of PPy. (C, D) Optical image of the freestanding film on graphite substrate with ∼2 × 2 cm area. (E) Cross-sectional SEM micrograph of layered polypyrrole film. (F) Layers of graphene-CNT coated with PPy during polymerization forming a nanocomposite PCG film. Source: Reproduced with permission from Ref. [241].

A wearable, fiber-shaped, and all-solid-state asymmetric FSC was recently fabricated with a 1.5 V operating voltage using ultrathin MnO2 nanosheets on carbon fibers as the positive electrode and graphene on carbon fibers as the negative electrode [242], showing an energy density of 27.2 Wh kg−1 and power density of 979.7 W kg−1. These values are higher than that reported for MnO2-based asymmetric or symmetric supercapacitors, including MnO2 nanotubes/activated graphene (22.5 Wh kg−1 at 146.2 W kg−1) [243], MnO2 nanoflowers/Bi2O3 nanoflowers (11.3 Wh kg−1 at 352.6 W kg−1) [243], graphene foam (GF)-CNT-MnO2/GF-CNTPPy (22.8 Wh kg−1 at 860 W kg−1) [245], 3D graphene-MnO2/3D graphene-MnO2 (6.8 Wh kg−1 at 62 W kg−1) [246], multilayer MnO2-GO/porous carbon (energy density of 46.7 Wh kg−1 at power density of 100 W kg−1) [246], 3D MnO2/graphene hydrogel (21.2 Wh kg−1 at 0.82 kW kg−1) [248], and 2D planar MnO2/graphene (17 Wh kg−1 at 2.52 kW kg−1) [247–249]. The fiber-shaped asymmetric FSC displayed sufficiently good bendability and mechanical stability while connected in parallel and woven into cotton textiles (Fig. 3.11). By integrating this asymmetric FSC with a nanowire-based photodetector into a self-powered nanodevice, the fiber-shaped asymmetric FSC efficiently demonstrated to power a photodetector without the requirement for an external bias [242]. Using a silica capillary column as a hydrothermal micro-reactor as depicted in Fig. 3.12 [250], Yu et al., reported a large-scale method for producing carbon microfibers with a distinct hierarchical structure composed of an interconnected network of CNTs with interposed



66

3.  Electrode materials for supercapacitors

FIGURE 3.11  (A) SEM image of carbon fibers bundles. (B, C) SEM image of MnO2 nanosheet arrays on single carbon fiber at a low and high resolution. (D) TEM of MnO2 nanosheets. (E–G) Photos of two fiber-shaped ASCs connected in parallel under different bending state which woven into a cotton textile. (H) CV curves of two asymmetric fiber supercapacitors in flat and the corresponding bending states with a scan rate of 100 mVs−1. Source: Reproduced  with permission from Ref. [242].



3.2  Electrode materials

67

nitrogen-doped rGO sheets. The resultant carbon fiber electrode showed an electrical conductivity of 102 S cm−1, volumetric capacity of 305 F cm−3 in sulfuric acid (measured at 73.5 mA cm−3 in a three-electrode cell) and 300 F cm−3 in polyvinyl alcohol (PVA)/H3PO4 electrolyte (measured at 26.7 mA cm−3 in a two-electrode cell). The full micro-supercapacitor with PVA/H3PO4 gel electrolyte, free from binder, current collector, and separator, showed a volumetric energy density of 6.3 mWh cm−3, which is comparable to that of 4 V–500 mAh thin-film lithium battery while maintaining a power density two orders of magnitude higher than batteries with a long cycle life. Further, this fiber-based, all-solid-state micro-supercapacitor was interfaced into miniaturized flexible devices to power a TiO2-based UV photodetector and a LED (Fig. 3.12). Another type of cable/wire-shaped flexible SC, as shown in Fig. 3.13, was fabricated on a stainless-steel wire using rGO nanosheets [251]. In the presence of redox additive electrolyte (PVA/H3PO4/Na2MoO4), this flexible SC exhibited a maximum length capacitance and energy density of 18.75 mF cm−2 (areal capacitance of 38.2 mF cm−2) and 2.6 mWh cm−1 (areal energy density of 5.3 mWh cm−2), respectively. The flexibility and stability of this FSC device have been investigated and three serially connected devices were used to light up the green and blue LEDs (Fig. 3.13) [251]. A similar attempt was made to demonstrate an rGO-based wearable SC on Cu wire [252]. For further details one may refer to some earlier reviews on flexible SCs related to MnO2 [253] and carbon materials [236, 244], respectively. 3.2.2.3.2  Carbon-based stretchable and twistable supercapacitors (film-/fiber-shaped)

Stretchable and twistable FSCs are desired for advanced electronics, including polymerbased self-powered sensors, polymer light-emitting diodes, polymer solar cells, and active matrix displays, etc. [244]. In the early reports on stretchable SCs, buckled SWNT/polydimethylsiloxane (PDMS) electrodes have drawn considerable attention, as those showed a strain up to 140% with no change in resistance [254, 255]. The advent and employment of crumpled graphene papers reduced the cost and complexity in the fabrication of stretchable and highperformance electrodes for SCs [255, 256]. The crumpled graphene-paper-based electrode demonstrated high stretchability up to 300% linear strain and 800% aerial strain with a high specific capacitance of 196 F g−1 in H3PO4-PVA electrolyte and reliability up to 1000 stretch/ relax cycles [256]. Kim et al. have reported a delamination-free stretchable supercapacitor, in which all component layers were prepared with a single matrix composed of an ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide, and a polymer, poly(vinylidene fluoridehexafluoropropylene), as an electrolyte and a supporting layer, respectively, in the stretchable supercapacitor [258]. The electrode layer was fabricated by incorporating CNTs in the common (polymer) matrix with all the layers being seamlessly fused into a single unit by dissolving the surface of the composite with acetone. The operational cell voltage was as high as 3 V due to the use of ionic liquid-based gel electrolytes. Specific electrode capacitance and areal cell capacitance were 67.2 F g−1 and 12.7 F cm−2, respectively. The standard deviations of the capacitance were only ±2.1% and ±1.4%, respectively, after 500 cycles of the lateral and radial stretches at 0.5 strain. Polypyrrole (PPy)-coated MnO2 nanoparticles were deposited onto CNT-based textile supercapacitor electrodes, which increases by 38% the electrochemical energy storage of MnO2/CNT-based flexible (13% bend) and stretchable (21% tensile strain) supercapacitors (Fig. 3.14) [259]. A specific capacitance of 461 F g−1 in H3PO4-PVA electrolyte was reported at 0.2-A g−1 current density, which was



68

3.  Electrode materials for supercapacitors

FIGURE 3.12  (A) Schematics of fiber synthesis by injecting a homogeneous solution of acid oxidized SWNTs, GO, and EDA through a pump into a flexible silica capillary column, followed by in situ thermal treatment at 220°C for 6 h before a continuous fiber was pushed into a water reservoir by pressurized nitrogen flow. (B) Pictures of asprepared fibers collected in water. (C) A dry fiber with diameter of ∼50 mm and length ∼0.5 m. (D) Planar structures obtained by bending fibers. (E) Compressed and stretched fiber springs. (F) A knitted textile fabricated from fibers. All scale bars: 0.5 cm. (G) Schematic of a micro-SC constructed using two fiber three-electrodes on polyester (PET) substrate. (H) Capacitance retention after 1000 cycles up to 908 bending angle. Inset: Photo of a bent micro-SC. (I) Current response assembly of the UV photodetector based on TiO2 nanorod array powered by micro-SC. Source: Reproduced with permission from Ref. [250].





3.2  Electrode materials

69

FIGURE 3.13  (A) Schematics of fabrication of rGO-based cable supercapacitor using PVA/H3PO4/Na2MoO4 electrolyte. (B) Galvanostatic charge-discharge curves at different current densities (0.5–4 mA) for the fabricated cable SC. (C) Capacitance variation at different bending states. Inset of (C): Digital images of the different bending states. (D, E) Demonstration of three serially connected devices can power the green and blue LEDs, respectively. Source: Reproduced with permission from Ref. [251]. 

70

3.  Electrode materials for supercapacitors

FIGURE 3.14  (A) Schematic illustration of the fabrication of polpyrrole-MnO2-coated textile supercapacitor. (B) Schematic of bending test performed on polypyrrole-MnO2-coated SC. (C) Cyclic voltammetry of supercapacitor under 13% bending strain. Source: Reproduced with permission from Ref. [259].





3.2  Electrode materials

71

attributed to the delamination prevention of MnO2 nanoparticles by PPy coating. Further, the capacitance retention was 96.2% even after 750,000 bending (13%) cycles [259]. A thin SWNT film and a honeycomb PDMS structure have been utilized as electrode materials and stretchable substrate supports for fabricating stretchable micro-supercapacitors (MSC) [260]. An array of 4 × 4 MSCs showed that the maximum strain in the MSC regions was almost 5 orders of magnitude lower than that the applied strain (of about 150%), and the device capacity remained same even at 150% stretch. Yun et al. designed a stretchable MSC of practical relevance, where a stretchable patterned graphene gas sensor driven by integrated micro-supercapacitor array on the same deformable substrate, as represented in Fig. 3.15 [261]. Patterned MSCs consisting PANI-wrapped multiwalled carbon nanotubes (MWNTs) and an ion-gel electrolyte of poly(ethylene glycol)diacrylate and 1-ethyl-3-methylimidazoliumbis (trifluoro-methylsulfonyl) imide, reported to exhibit excellent electrochemical performance under a uniaxial strain of 50% and a biaxial strain of 40%; their initial performance (capacitance of 6.1 F cm−3 at 5-mA cm−3 current) characteristics were retained even after 1000 cycles of repetitive uniaxial (50%) and biaxial (40%) stretching. Moreover, the patterned graphene sensor successfully detected NO2 gas for longer than 50 min via integration with MSCs using the serpentine interconnections even under uniaxial stretching by 50%. Such a hybrid

FIGURE 3.15  (A) Schematics of biaxial strain ε biaxial. (B) Optical micrograph of MSC array (left) and the strain distribution estimated by FEM analysis (right) under a biaxial strain of 40%. (C) Optical images (left) and results obtained from FEM analysis (right) of a serpentine interconnection in various uniaxial stretching states. The crosssectional view shown is for a strain of 50%. Source: Reproduced with permission from Ref. [261].



72

3.  Electrode materials for supercapacitors

combination of SCs with other practical electronic devices is highly desirable for near-term applications. Further R&D efforts are needed for improving capacitive storage capability in flexible and stretchable supercapacitors. Buckled CNT film was examined as stretchable electrodes in SCs, as shown in Fig. 3.16A–C [262]. A comparative study between PANI composites of buckled CNT with and without nitric acid treatment revealed that acid-treated buckled CNT@PANI electrodes exhibited a higher specific capacitance of 1147.12 mF cm−2 in H3PO4-PVA electrolyte at 10 mVs−1 [262]. This is attributed to the formation of enhanced interfacial bonding between acid-treated CNTs and PANI. The acid-treated electrode also showed an energy density from 31.56 to 50.98 µWh cm−2 with power density changing from 2.294 to 28.404 mW cm−2 at 10–200 mVs−1. The corresponding supercapacitor sustained an omnidirectional strain of 200%. In case of fiber-shaped stretchable and twistable SCs, yarn-based SCs were employed which consist of core-shell structured coiled electrodes with pseudocapacitive CNT-cores and MnO2-shells, as shown in Fig. 3.16D and E [263]. The linear and volumetric capacitances of the coiled yarn were found to be 2.72 mF cm−1 and 34.6 F cm−3, respectively. Interestingly, around 84% of its static capacitance was retained after being reversibly stretched by 37.5% strain, while 96.3% dynamic capacitance was maintained during 20% strain deformation despite the extremely high strain rate of 6%/s. The yarn supercapacitors exhibited 95% or 98.8% capacitance retentions after many stretching/releasing or charge/discharge cycles [263]. Stretchable coaxial fiber-shaped SCs have also been fabricated using CNT sheets wrapped on an elastic

FIGURE 3.16  (A) SEM images of buckled structures formed in CNT films at 50% omnidirectional prestrain. (B) Digital images of buckled CNT film at various stretching states. (C) Specific capacitance variation for CNT/PANI SCs with scan rate. (D) Schematic illustration for the complete solid-state coiled supercapacitor, which comprises two symmetric MnO2/CNT core-shell-coiled electrodes and gel electrolyte. (E) Stress loading/unloading curves of the hybrid MnO2/CNT coiled electrode with tensile strains from 20% to 40%. Source: Part C, Reproduced with permission from Ref. [262]. Copyright of American Chemical Society (2016); Part E, Reproduced with permission from Ref. [263].





3.2  Electrode materials

73

fiber using a polymer gel sandwiched between the two coaxial CNT layers as the electrolyte and separator as well [264, 265]. Transparent SCs have become prominent for various optoelectronic applications. Indeed, transparent energy-storage devices are desirable for automobile/building windows or personal electronics with high esthetic appeal. In this regard, a simple dry press transfer technique has been used to transfer thin SWNT films to transparent substrates to prepare transparent and flexible EDLCs with a transparency of 92% at 550 nm and high transparency over visible light and near infrared (NIR) wavelengths [266]. The resultant films showed an extremely high mass specific capacitance of 178 F g−1, which is 482 F g−1 when calculated per mass of carbon—in PVA/H3PO4 electrolyte and area specific capacitance of 552 µF cm−2 compared to other reported carbon-based flexible and transparent EDLCs [267]. The films were also highly stable in terms of specific capacitance over 10,000 loading cycles [266]. Chen et al. also fabricated graphene-based transparent and flexible SCs [257], which are attractive for various portable electronic devices. Table 3.4 summarizes carbon-based flexible, stretchable, wearable, and transparent supercapacitors. The advantages of using carbon materials for supercapacitor electrodes lay in the facts that they are highly conductive, binder-free and flexible with a large surface area and excellent properties intrinsically associated with different carbon allotropes. Carbon materials are earth-abundant and environmentally friendly compared to metal- or polymer based electrode materials. Through using flexible carbon materials in supercapacitors, it would be possible to reduce the use of metal foils, such as Al as the current collector. It is also possible to eliminate polymer-binders or other conductive additives. Therefore, the use of carbon materials can advance the flexible supercapacitors to be lighter, portable and more easily manufactured. For many practical applications, their efficiencies are still need to be improved by combining flexible carbon materials with pseudocapacitive materials such as metal oxides and polymers. 3.2.2.3.3  Carbon-based ultrafast supercapacitors for ac-line filtering

Alternating current (ac)-line filtering by using ultrafast supercapacitors is essential for domestic power usage for eliminating high-frequency noises, as ac electricity has a frequency of either 50 or 60 Hz. The combination of various nonlinear loads from different electronic devices in domestic requirements, portable electronics, automobiles, and medical appliances often induces higher-order harmonics (>120 Hz) of the basic generating frequency [268]. In order to protect electronic devices from such voltage ripples, aluminum electrolyte capacitors (AECs) are used for ac-line filtering [43, 269, 270]. However, AECs have low specific capacitance, and hence occupy a large space and volume in electronic circuits. In this regard, supercapacitors, possessing specific capacitance of 2–5 orders higher in magnitude than that of AECs, could be used for effective ac-line filtering with highly negligible space or volume requirement in capacitive components [271, 272]. A supercapacitor generally acts like a resistor at 120 Hz after being introduced into transmission lines [270]. The typical resistor-capacitor (RC) time constant for a supercapacitor is around 1 s, associated with the high electrochemical series resistance and microporous structure of supercapacitor electrodes, which is far too long to be useful for the common application of 120-Hz filtering (nearly 8.3 ms period), implying the necessity for smoothing the



74

3.  Electrode materials for supercapacitors

TABLE 3.4  Carbon nanomaterials in flexible, stretchable, wearable, and transparent supercapacitors.

Electrode

Elecrtolyte

Power Energy Specific capaci- density density Retention capa- Refertance (F g−1) (kW kg−1) (Wh kg−1) bility ences

Flexible CNT-graphene/ Potassium 72.6 at 0.5 Ag−1 Mn3O4 graphene polyacrylate (flexibility:bendable, (PAAK)/KCl twistable)

9

22.9

86% after 10,000 [181] cycles at 10 mVs−1

MnO2/carbon nanofiber

PVA-KOH, PVA-H4SiW12O40 nH2O

100, 142 at 5 mVs−1

—

—

60%, 28% at 100 mV s−1

[231]

TiO2/MWCNT

1 M H2SO4

36.8 at 20 mV s−1

—

—

—

[232]

Porous carbon nanofibers (flexibility: no degradation after 100 times bending)

0.5 M H2SO4

104.5 at 0.2A g−1

0.6

0.0322

94% after 2000 cycles at 1 A g−1

[234]

MnO2 nanotune/ PolyvinylalcoCNT (flexibility: no hol (PVA)/ degradation on bend LiCl and twist)

29.3 at 0.5 mA cm−2

13.8 mW cm−3

0.45 mW h 105% after 6000 cm−3 cycles at 1.2 mA cm−2

[235]

CNT(core)/PPy (shell) 0.5 M H2SO4 (flexibility: no degradation after 20 bending cycles

486.1 at 1.25 A g−1

10.96

3.9

82% after 10000 cycles at 8 A g−1

[237]

NiCo2O4-GO (flexibility: no degradation up to 180° bending, complete twisting)

3 M KOH

1078 at 1 mA

—

—

∼58% after 100 cycles at 3 mA

[238]

Ni(OH)2/dense stack of graphene sheets (flexibility: no degradation up to 180° bending)

1 M KOH

573 at 0.2 A g−1

8.5

9

89% after 2000 cycles

[239]

N-doped graphenepolyacrylic acid (PAA)/PANI (flexibility: no degradation up to 135° bending)

1 M H2SO4, H2SO4-PVA

521 at 0.5 A g−1

1.1

5.8

83.2% after 2000 cycles at 1 A g−1

[240]

PPy/CNT-graphene 1 M H2SO4 (flexibility: not demonstrated)

453 at 5 mV s−1

0.56666

0.06296

—

[241]

rGO coated on stainless-steel wire (flexibility: no degradation after 180° bend)

38.2 mF cm−2 at 5.3 µ W  hcm−1 0.5 mA

63.7 µW  cm−2

100% after 2500 cycles

[251]

PVA/H3PO4/ Na2 MoO4 polymer gel





75

3.2  Electrode materials

TABLE 3.4  Carbon nanomaterials in flexible, stretchable, wearable, and transparent supercapacitors. (Cont.)

Electrode

Elecrtolyte

Power Energy Specific capaci- density density Retention capa- Refertance (F g−1) (kW kg−1) (Wh kg−1) bility ences

Flexible and wearable MnO2 sheets/carbon fiber (highly flexible—no degradation on bend)

PVA-LiCl

634.5 at 2.5 A g−1

0.9797

0.0272

95.2% after 3000 cycles at 20 A g−1

[242]

SWNT/N-doped rGO 1 M H2SO4 fiber (flexibility: 97% of capacitance after 1000 times bending at 90°)

305 F cm−3 at 26.7 mA  cm−3

1.085 W  cm−3

6.3 mWh  cm−3

93% after 10000 cycles at 250 mA cm−3

[250]

Graphene aerogel on Cu wire (flexibility: 99% of capacitance after 1000 times bending at 160°

12.5 F cm−1 at 5 mVs−1

—

—

95% after 10000 cycles at 1 A g−1

[252]

Buckled SWNT macro- 1 M Et4NBF4/ film (no degradation propylenafter 30% strain) ecarbonate

54 at 1 A g−1

1

∼3.5

96.3% after 1000 [254] cycles −30% strain at 1 A g−1

SWNT/poly dimethH2SO4-polyviylsiloxane (PDMS) nyl alcohol (sustain 120% strain) (PVA)

53 at 10 A g−1

32

—

100% after 1000 cycles under 120% strain

[255]

Crumpled graphene hydrogel (sustain 300% linear and 800% areal strain)

H3PO4-PVA

166–196 at 1 A g−1

—

30 under 300% axial strain

95% after 1000 stretching cycles with 200% strain

[256]

Double-walled CNT / poly (vinylidenefluoride—hexa fluoro propylene (sustain 50% lateral strain)

1-ethyl67.2 at 2.5 A g−1 3.7 3-methyl imidazolium bis(trifluoro methyl sulfonyl)imide

20.3

97.9% after 500 cycles

[258]

PPy-MnO2/CNT textile (sustain 21% tensile and 13% bending strains)

H3PO4-PVA

[259]

Polyvinylpyrrolidone (PVP)

Flexible/wearable and stretchable

461 at 0.2 A g−1

22.1

31.1

96.2% after 750000 cycles

SWNT/honeycomb H3PO4-PVA PDMS (sustain 150% stretch)

3.3 F cm−3 at 10 Vs−1

—

—

100% after 150% [260] stretch

PANI/MWCNT (sustain 50% uniaxial and 40% biaxial strain)

6.1 F cm−3 at 5 mA cm−3

3 mW  cm−3

3.2 mWh  cm−3

95% up to 1000 cycles

Poly (ethylene glycol) diacrylate/ Et4NBF4

[261]

(Continued) 

76

3.  Electrode materials for supercapacitors

TABLE 3.4  Carbon nanomaterials in flexible, stretchable, wearable, and transparent supercapacitors. (Cont.)

Electrode

Elecrtolyte

Power Energy Specific capaci- density density Retention capa- Refertance (F g−1) (kW kg−1) (Wh kg−1) bility ences

Buckled CNT/PANI H3PO4-PVA (sustain 200% strain)

364.6 at 10 mVs−1

2.34–30.04  30.2–54.6  mW µWh  cm−2 cm−2

96.9% after 20 stretch cycles

[262]

MnO2-CNT (shell)/ PVA-LiCl nylon (core) (sustain 150% strain)

5.4 mF cm−1 (linear), 40.9 mF cm−2 (areal)

66.9 µW  cm−2

2.6 µWh cm−2

87.8% with 17.1% strain for 0–120% strain cycles

[263]

CNT sheets—on elastic H3PO4-PVA fiber (sustain up to 75% strain)

41.4 at 0.1 A g−1 421

0.363

95% after 100 stretch cycles at 75% strain

[264]

PEDOT-PSS/CNT/ Elastic wires (can sustain up to 350% strain)

122.8 at 0.5 A g−1

—

—

106% after 100 stretch cycles with 200% strain

[265]

64 at 0.5 A g−1

7.48

1.29

82.8% after 500 cycles at 0.5 A g−1

[230]

Wrinkled graphene PVA/H3PO4/ (57% transparent at H2O 550 nm, sustain bending and 40% strain)

7.6 at 1 Vs−1

—

—

100% after 100 cycles with bending or 40% strain

[257]

SWNT (92% transparent at 550 nm, flexible up to180°)

178 at 0.53 A g−1 —

—

100% up to 10000 cycles

[266]

H3PO4-PVA

Flexible/stretchable and transparent In2O3 nanowire/ CNT (∼60% transparent at 600 nm, flexible)

1 M LiClO4

PVA/H3PO4

Adapted with permission from Ref. [123].

leftover ac ripples in most of the line-powered electronics [272]. This is mainly because of the fact that unsuitable pore structures of supercapacitor electrodes impede high-rate ion diffusions or their high resistances restrict efficient charge transfer [273]. Thus, the design and fabrication of highly conductive electrodes with optimized micro-/nano-architectures for facile electron/ion transportations can improve the performance for ac-line filtering. High-surface area materials with less inherent porosity have been studied for such applications. Among a variety of other electrode materials, including onion-like carbon [274] and CNTs [275, 276], carbide-derived carbon [277], metal oxides [278], polymers [279], and mesoporous carbons [271], utilized to improve related rate capability for ac-line filtering, graphene-based materials, and graphene/CNT hybrid structures have recently emerged to be promising over conventional carbon materials because of their superior electrical conductivity and high specific surface area [273, 280, 281]. Graphene and porous carbon composites have also been demonstrated to be excellent acline filters, though their energy-storage capabilities still need improvements [282]. Lim et al. demonstrated that the substitutional pyridinic nitrogen dopant sites in carbon nanotubes can 



3.2  Electrode materials

77

selectively initiate the unzipping of CNT side walls at a relatively low electrochemical potential (0.6 V) [281]. The resultant nanostructures consisting of partially zipped and/or unzipped graphene nanoribbons wrapping around carbon nanotube cores maintain the intact 2D crystallinity with well-defined atomic configuration at the unzipped edges (Fig. 3.17). The synergistic interaction between the large surface area and robust electrical connectivity of the unique nano-architecture are responsible for ultrahigh-power supercapacitor

FIGURE 3.17  (A) Schematic illustration of N-dopant-specific unzipping of NCNT. (B) Aberration-corrected TEM images of 2, 8, and 16 h unzipped nanostructures (in 1 M H2SO4 at 0.8 V) (scale bar: 2 nm). (C) AC impedance phase angle versus frequency; vertical dotted line indicates 120-Hz frequency. Source: Reproduced with permission from Ref. [281]. 

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3.  Electrode materials for supercapacitors

performance, which can serve for ac filtering with the record high-rate capability of 85° (very close to that of AECs, 83.9°) of phase angle at 120 Hz. Lin et al. also fabricated a 3D graphene/ CNT carpet (G/CNTC)-based micro-supercapacitor on nickel electrodes [283], which exhibited an 81.5° phase angle at a 120-Hz frequency. However, further R&D effort is needed to realize supercapacitors having both ac-line-filtering capability and excellent charge-storage capability for practical applications.

3.3  Perspectives on carbon for SC electrodes Carbon nanomaterials, including 1D CNTs, 2D graphene, 3D mesoporous carbon, and their composites with conductive polymers or metal oxides, have been widely used as electrodes in supercapacitors, such as EDLCs, PCs, and HSCs. Generally, pure carbon nanomaterials without any functional groups are useful as EDLC-electrodes because of their high specific surface area and excellent electrical conductivity. The advantages of EDLC include its high-rate capability and outstanding cyclic stability (e.g., retention of 95%–100% after 1000–10,000 cycles). The specific capacitance for pure carbon nanomaterials in EDLC has been demonstrated to be in the range of 10–300 F g−1. For carbon-composite nanomaterials, the specific capacitance can be increased by one order of magnitude: generally 100–1000 F g−1. By compositing carbon nanomaterials with other materials having pseudocapacitances (e.g., conducting polymers, metal oxides, or hydroxyls), the energy density can be largely improved, but their rate capability and cyclic stability may decrease to 60%–90% after 1000 cycles. Numerous efforts have been made to improve the electrochemical performance of the supercapacitors based on carbon nanomaterials by improving their specific capacitance, energy density, power density, rate capability, and/or cyclic stability. The design and development of advanced 3D electrode structures and compositing carbon nanomaterials with other active materials have been demonstrated to be effective for highperformance carbon-based SCs. Hybrid supercapacitors can fill the gap between a supercapacitor and a battery by improving both energy and power density in a single electrochemical device. Flexible, stretchable, and even transparent supercapacitors are required for the next generation wearable electronics. Although compositing graphene and CNT with appropriate conducting polymers has been demonstrated to be an effective approach towards SCs with excellent flexibility and strain resistance while retaining their electrochemical performance, conducting polymers often lose their inherent electrical properties in composites. As can be seen from the earlier discussions, tremendous progress has been made in the development of carbon-based supercapacitors, including EDLCs, PCs and HSCs in traditional or flexible, stretchable, and even transparent forms. Compared with batteries, SCs possess a high-power density, short charging time, good discharge/charge cyclability, and broad-temperature-range applicability. These advantages make SCs useful for various potential applications, including hybrid electric vehicles, renewable-energy-storage gadgets, and portable electronics. However, there are still some challenges await to be addressed to further improve the performance of carbon-based electrodes. First, carbon is well known to have a lower specific capacitance as compared to other pseudocapacitive materials, such as metal oxide and conducting polymers. Therefore, the carbon content, nature of heteroatom dopant(s), if any, and its crystallinity or connectivity of a





3.4  Transition metal oxides/hydroxides

79

carbon network within composite electrodes must be well understood in order to obtain high electrode performance. Second, it is also essential to further improve electrolytes and separators for efficient charge storage as well as high-rate capability and cycling stability. Third, SCs with excellent ac-line-filtering capability still need to be developed to bring advanced SC technologies to the marketplace for various practical applications, ranging from self-powered wearable optoelectronics to electrical vehicles. Finally, the energy and power densities of SCs need to be further improved while their weight, volume, and cost to be reduced.

3.4  Transition metal oxides/hydroxides Metal oxides have high specific capacitance and conductivity, and hence considered as suitable for electrode fabrication in high energy and high power supercapacitors. Especially, transition metal oxides have been widely explored for supercapacitor applications due to their layered structure and multiple oxidation states. Inherently, the electrochemical behavior of oxides is pseudo-capacitative in nature due to either highly reversible surface chemical reactions or extremely fast and reversible lattice intercalation. Oxides of a wide variety of transition metals have been explored for this purpose. Metal oxides can provide higher energy density for SCs than conventional carbon materials and better electrochemical stability than polymer materials due to the facts that they not only store energy like electrostatic carbon materials but also exhibit electrochemical Faradaic reactions between electrode materials and ions within appropriate potential windows. The general requirements of metal oxides to suit for SC applications are: (1) the oxide should be electronically conductive, (2) the metal can exist in two or more oxidation states that coexist over a continuous range with no phase changes involving irreversible modifications of a 3D structure, and (3) the protons can freely intercalate into the oxide lattice on reduction (and out of the lattice on oxidation), allowing facile interconversion of O2− ↔ OH−. To date, those investigated include ruthenium oxide, manganese oxide, cobalt oxide, nickel oxide, vanadium oxide, and iron oxide. The simple and effective synthetic routes were developed, the nanomaterials with different morphologies were prepared, and the specific capacity and stability of devices were significantly improved.

3.4.1 RuO2 Among the transition metal oxides, RuOx has been the most extensively studied candidate for supercapacitor applications, especially due to its highest specific capacitance of about 1000 F g−1 among pseudocapacitive materials. Further, it exhibits wide potential window, highly reversible redox reactions, high proton conductivity, good thermal stability, long cycle life, metallic-type conductivity, and high rate capability [284–288]. While RuO2 is used as an electrode material, a series of reversible redox processes occur, resulting in the variation of oxidation state among Ru4+, Ru3+, and Ru2+, accessible within 1.2 V [42, 289], where pseudocapacitance mainly contributes to capacitance. These unique electrochemical features result in quasi-rectangular shape CV curve (Fig. 3.18). The double layer capacitance only contributes to about 10% of the stored charge in RuO2 electrodes, working in parallel with pseudocapacitance [289]. The main factor that limits its applications is only its prohibitively high cost.



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3.  Electrode materials for supercapacitors

FIGURE 3.18  Cyclic voltammogram (CV) of RuO2·xH2O electrodes annealed at 150°C. The voltage scan rate was 2 mVs−1, and the electrolyte was 0.5 M of H2SO4. Source: Reproduced with permission from Ref. [289].

The pseudocapacitive behavior of ruthenium oxide involves different reactions in acidic and alkaline electrolyte solutions, which in turn show different sensitivities toward crystallinity. In acidic electrolyte solutions, a fast-reversible electron transfer and an electro-adsorption of protons onto the surface occurs where ruthenium oxidation states change from (II) to (IV) [290–294]: RuO 2 + xH + + xe − ↔ RuO 2 − x ( OH )x (3.2) The change of x during proton insertion/deinsertion occurs over 1.2 V voltage window and leads to capacitive behavior due to ion adsorption following a Frumkin-type isotherm [294]. Specific capacitances above 600 F g−1 [295] have been achieved, but ruthenium-based aqueous systems are expensive, and their 1 V working voltage window limits their applications to small electronic devices [42]. To work with wider voltage windows, organic electrolytes with proton surrogates (e.g., Li+) are recommended [42]. In alkaline solutions, it has been reported that ruthenium is oxidized to RuO42−, RuO4−, and RuO4 on charging and reduced back to RuO2 on discharging [296, 297]. It has been established that irrespective of the class of electrolyte that is being employed, the following factors influences the electrochemical behavior of Ru oxides. 1. Specific surface area. The pseudocapacitance of RuOx emerges mainly from the surface reactions. Higher specific surface area and the number of metal centers capable of providing multiple redox reactions will lead to higher specific capacitance. Therefore, one of the most effective ways to increase the specific capacitance is to increase the specific surface area of RuOx [289]. Several approaches, such as depositing RuO2 films on substrates with a rough surface, coating a thin RuO2 film on high-surface-area materials, making nanometer-sized oxide electrodes, etc., have been explored to maximize the surface area of RuO2 by creating micropores large enough for ion diffusion [298–303]. For example, hydrous ruthenium oxide (RuO2 xH2O) thin-film electrodes electrodeposited on titanium substrates exhibited high reversible characteristics, excellent cycle stability, and superior power characteristics. The maximum specific capacitance of the RuO2.xH2O film electrode was as high as 786 F g−1 [300]. In yet another study revealed that metal particles which are prepared using the polyol method having small and uniform in size when highly dispersed on a carbon surface, exhibited a redox specific capacitance of 914 F g−1[298]. Furthermore, a specific capacitance as high as 1300 F g−1 was reported for nanotubular arrayed electrodes incorporated with hydrous ruthenium oxide [304]. 2. Extent of hydrated water content in RuOx. As implied by Eq. (3.2), the reversible redox transitions mainly depend on the processes of proton/cation exchange and electron





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3.4  Transition metal oxides/hydroxides

hopping. The quasimetallic conductivity of RuO2 allows facile electron transfer into and through the electrode matrix, so the transfer of cations in the solid phase is critical for a RuO2 electrode. It was reported that cation diffusion in hydrated electrodes could occur via hopping of alkaline ions and H+ ions between H2O and OH− sites, suggesting that the hydrogen atoms were relatively mobile in RuO2 xH2O samples as compared to those in rigid samples [305]. Thus, the hydrated water in RuO2 is expected to enhance the diffusion of cations inside the electrode layer. In fact, hydrous ruthenium oxide is a good proton conductor (H+ diffusion coefficient reaches 10−8–10−12 cm2 s−1) [306–308] and fast ionic conduction via hydrous micropores, mesopores, or the interlayer does lead to an increase in the capacitive behavior [309]. As reported in the literature [310] hydrous ruthenium oxide (RuO2 0.5H2O) showed the capacitance as high as ∼900 F g−1. However, when the water content was decreased to RuO2 0.03H2O, the capacitance dropped to 29 F g−1 and the anhydrous phase displayed a capacitance of only 0.5 F g−1. Sugimoto et al. [311] also demonstrated the importance of hydrous regions (either interparticle or interlayer). These hydrous regions can allow appreciable protonic conduction for highenergy and high-power electrochemical capacitors. The results are shown in Table 3.5. The water content in RuO2 xH2O primarily depends on the preparation process and conditions. Normally, chemically formed RuO2 is redox active only within a fraction of the surface Ru atoms, while electrolytically formed RuO2 can have more hydrated oxide states in which a substantial volume fraction of Ru atoms is redox active. The water content, x, of RuO2 xH2O is typically 0.9 for a room-temperature dried product, and triand dihydrated phases can also be obtained, depending on the preparation conditions [309]. Increasing the annealing temperature can result in a loss of chemically bound water, inhibiting proton intercalation and leading to a decrease in specific capacitance [312, 313]. 3. The crystallinity of RuO2 xH2O. The pseudocapacitance of RuO2 xH2O materials is also closely related to the degree of crystallinity. For a fully crystalline structure, the process of expansion or contraction is difficult, and hence it prevents protons from permeating in the bulk material, leading to a diffusion limitation. As a result, the fast, continuous, and reversible Faradaic reactions are compromised. The capacitance of RuO2 with good crystallinity stems mainly from the surface reaction [314]. In contrast, the redox TABLE 3.5  Specific capacitance obtained by cyclic voltammetry at various scan rates for RuO2 × H2O. Specific capacitance/F (g-RuO2)−1 Scan rate (mV s−1)

RuO2

RuO2 0.03H2O

RuO2 0.5H2O

2

24

124

342

5

22

112

331

20

19

93

316

50

18

82

304

200

16

68

280

500

15

61

256

Reprinted from Ref. [311] with permission from American Chemical Society.



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reaction of an amorphous composite occurs not only on the surface but also in the bulk of the powder, and this is the reason why amorphous composite exhibits far superior performance when compared with crystallized structures [289]. The crystallinity of RuO2 xH2O depends on the synthesis procedure. RuO2 xH2O obtained via a vapor procedure has exhibited better crystallinity and lower specific capacitance than that obtained via a solution procedure [315].   Temperature is another important factor regulating the crystallinity and water content of RuO2 xH2O materials. Although active sites can be obtained at a lower heat treatment temperature, their activities are quite low due to their low conductivity, even though water is still retained in the oxide. Under a higher heat treatment temperature, the number of active sites is decreased due to crystallization and water loss. For instance, when the annealing temperature exceeds 175°C, the specific capacitance will drop rapidly due to the formation of a crystalline phase. A similar trend was also observed by Kim and Popov [316]. A disordered form is essential to attain improved capacitance of RuO2 xH2O. As inferred from the literature, RuO2 xH2O can remain amorphous and hydrous when the temperature is close to the crystallization temperature of ruthenium oxide [289, 299, 317]. The optimal annealing temperature is reported to be ∼150°C [289]. 4. The size of RuO2 xH2O. Smaller the size of particles, two desired characteristics emerge. At first, it shortens the diffusion distance and secondly, it facilitates proton transport in the bulk of ruthenium oxide, with increase in the specific surface area, and enhancement in the electroactive sites. Thus, smaller the particle size leads to higher gravimetric capacitance and utilization efficiency [309, 315, 316]. For example, assuming the particle is spherical (where redox reactions can occur to a depth of 2 nm from the surface of the particle), when the particle diameter is 10 nm only 49% of the total particle volume can contribute to the total capacitance. However, for a particle with a diameter of 3 nm, this value can reach 96%. Therefore, one of the key points is to produce the RuO2 particles in nanosize and maintain high electrical and protonic conduction throughout the particles. For example, a nanometer sized crystalline RuO2, anchored on CNTs using a simple supercritical fluid deposition method, reported to exhibit a specific capacitance up to 900 F g−1, which is close to its theoretical value [318]. By deploying RuO2 into a nanostructure to increase its surface area and paths for diffusion, specific capacitance values as high as 1300 F g−1 were achieved [304, 309, 310, 319].   The presence of ordered nanopores can also lead to better electrolyte diffusion into the electrode material and consequently improve the completion of the redox reaction, resulting in higher pseudocapacitance [320]. The particle size of RuO2 xH2O can be controlled in different ways, depending on the preparation method. By adapting a colloidal method, the particle size of RuO2 xH2O can be altered by the amount of NaHCO3 and the reaction time [316]. With a conventional sol-gel method, the size of RuO2 particles can be adjusted by the RuO2 content in the composites as well as the alkali used in the preparation procedure. It was also reported that RuO2 particles prepared via NaHCO3 titration were smaller than those produced by NaOH titration [321]. According to Ramani et al., [322] varying the deposition temperature and/or pH can control the cluster size in the process of electroless deposition. Using a template method, the particle size of RuO2 xH2O can be controlled by altering the template size. Cui et al. and Lin et al. [323, 324] reported that nanostructured crystalline RuO2 particles prepared using a SiO2





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83

hard template had a narrow size distribution. However, it should be noted that a hard template is often accompanied by a higher pyrolysis temperature, which can lead to well crystallized RuO2 with a small amount of water, and a relatively low specific capacity of 229 F g−1 for 40 wt.% Ru at 10 mV s−1[323]. Therefore, a soft template is preferable for preparing nanosized RuO2 composites, although the particles and pores are not as regular and ordered as in those samples prepared using a hard template. For instance, when the soft template is employed, the annealing temperature can be as low as 100°C, and the specific capacitance of a composite with 50 wt.% RuO2 can reach 642 F g−1[317].   In addition to the soft template method, the polyol process also appears to be a promising alternative for preparing nanosized RuO2. Particles prepared from this process were found to be uniform in size and distribution, and displayed a specific capacitance of 914 F g−1 when the Ru loading was 20 wt.% [298]. Obviously, ruthenium oxide can be brought close to its theoretical capacitance by deploying the RuO2 in a nanostructure to increase its surface area and pathways for diffusion [304, 309, 311, 319]. 5. The choice of electrolytes. The specific capacitance values of RuOx electrodes can be tuned by employing different electrolytes [321]. In some electrolytes, the RuO2 electrode may behave like a double-layer capacitor, regardless of the RuO2 loading in the electrode layer. However, when the electrolyte is altered, RuOx materials can display pseudocapacitance behavior that is dependent on the RuOx content. Furthermore, the concentration of electrolyte can also impact the performance of RuOx electrodes. For example, when KOH electrolyte concentration was higher than 0.5 M, the capacitance increased linearly with increasing KOH concentration, but for concentrations lower than 0.5 M, the capacitance decreased sharply with declining concentration [324]. Therefore, the ionic concentration in electrolytes must suit the needs of both the electrical double layer and the Faradaic reactions.

3.4.2 RuO2-based composites Despite the exceptional properties of ruthenium oxide and its high theoretical capacitances (1360 F g−1 for RuO2 0.5H2O), its high cost is an impediment for its employment in practical applications. Roughly around 90% cost of the supercapacitor arises from the electrode material. Therefore, many researchers [295, 325–330] made attempts to reduce Ru usage and to increase its utilization by two means. At first, through synthesis of mixed-oxide composites by doping base metal oxides, such as MnO2, NiO, etc., into ruthenium oxide [329, 331–336] and fabrication of ruthenium-based composite materials in which particles of hydrous ruthenium oxide are deposited on other low-cost materials (e.g., carbons with different surface textures and conductive polymers) [316, 326, 337–342]. 3.4.2.1  Mixed-oxide composites In the attempt to reduce the cost of precious metal usage, many studies have focused on incorporating RuO2 with cheap metal oxides, such as SnO2, MnO2, NiO, VOx, TiO2, MoO3, WO3, and CaO to form composite oxide electrodes [335, 336, 343, 344]. For instance, a specific capacitance of 710 F g−1 in a KOH electrolyte [343] was reported for a SnO2-RuO2 composite electrode wherein RuO2 was deposited by an incipient wetness precipitation method. Takasu et al. [331] reported that the specific charge of a RuO2 (33%)-VO2 (67%)/Ti electrode was ∼50



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times higher than for a RuO2/Ti electrode. Sugimoto et al. [335] also demonstrated enhanced supercapacitive properties by doping with vanadium, and found that the introduction of V not only extended the electrochemical window of the Ru0.36V0.64O2 electrode from much less than 1.0–1.1 V but also increased the utilization of Ru as well as the electrochemical stability of the electrode [345]. Amorphous Ru1–yCryO2/TiO2 nanotube composites were synthesized by loading various amounts of Ru1–yCryO2 on TiO2 nanotubes via a reduction of aqueous Ru1–yCryO2 with RuCl3 [346]. The results reveal that the 3D nanotube network of TiO2 offered a solid support structure for Ru1–yCryO2, by exposing the active material for electrochemical reactions in addition to simultaneous improvement of its utilization. A maximum specific capacitance of 1272.5 F g−1 was obtained for an optimum loading of Ru1–yCryO2 on TiO2 nanotubes. The large surface area of TiO2 nanotubes not only facilitates the transportation of ions but also enhanced the doublelayer charging [334]. The following are the five key factors which mainly influence the capacitive performance of Ru-based oxide composites [289, 304, 347–351]: (1) the electrochemical kinetics of electron transfer for active materials, (2) “in-particle” electron-hopping resistance within every RuO2 particulate, (3) “between-particle” electron-hopping resistance between particulates, (4) electron-hopping resistance at interface between active materials and current collectors, and (5) barriers to proton diffusion within oxide composites and particulates. Apart from offering an additional pseudocapacitance, the additive metal oxides can simultaneously function in the composite as follows: 1. To facilitate electron and proton conduction in the oxides. One of the functions of the additive metal oxides is to aid in electron and proton conduction. It was reported that the structural support from Co species facilitated electronic conduction within the metal oxide. As a result, at higher potential scan rates (500 mV s−1), a Ru-Co mixed oxide electrode exhibited a superior performance of 570 F g−1 compared to a RuO2 electrode (475 F g−1) [352]. In composites of ruthenium oxide and niobium hydroxide, RuO2 particles can provide electrically conductive pathways and niobium oxide possesses good proton diffusion [353].   Hence, there is an optimization among electrical conduction through interconnected RuO2 grains, proton conduction through the mixed oxide, and thereby realizing an efficient utilization of ruthenium. A higher specific capacitance can be expected, only if the concentrations of mixed metals are moderate. It was reported that the optimum composite consisted of roughly equal amounts of ruthenium and niobium. At the same time, the Ru/Nb oxide composite maintained a high specific capacitance with increasing potential scan rate, and the overall capacitance loss of these mixed oxides was much less significant than for pure RuO2. 2. To enhance the dispersion of ruthenium oxide. The dispersion of ruthenium oxide can be achieved via doping with other metal oxides. As shown in Fig. 3.19, [353] with greater niobium content, the dispersion of ruthenium increases significantly. For example, at 33.3% niobium (1:2 Nb:Ru), the aggregation of RuO2 grains exists in the composite. However, when niobium contents reached 50% (1:1 Nb:Ru) and 66.7% (2:1 Nb:Ru), the ruthenium and niobium oxide species could homogenously disperse throughout the film. Furthermore, ∼95% niobium could result in a film without discernable RuO2 grains. Kuo and Wu [333] have developed an electrochemical capacitor electrode containing highly dispersed RuO2 electroplated on a conductive mesoporous tin oxide substrate. 



3.4  Transition metal oxides/hydroxides

85

FIGURE 3.19  Compositional mapping using TEM-EDS, showing layers of mixed Nb:Ru oxides (500 nm field of view). Source: Reprinted from Ref. [353], with permission from Elsevier.

3. To minimize the RuO2 particle size. Additions of other metal oxides can influence the particle size of RuO2. For instance, niobium can induce a decrease in RuO2 particle sizes, placing more RuO2 in electroactive regions, that is, near grain surfaces [353]. In addition, the incorporation of vanadium diminished the particle size of Ru oxide, giving rise to a higher specific surface area. Consequently, its redox capacitance and surface utilization were dramatically increased [354]. According to Hu et al. [356], the average particle size of RuO2 can be decreased by increasing the Sn content. There are other reports on modifying ruthenium oxide tin oxide and other preparation methods for supercapacitor applications [355–357]. For example, in a series of Ru oxide materials, such as RuO2 xH2O, Ru0.8Sn0.2O2 xH2O, Ru0.6Sn0.4O2 xH2O, and SnO2 xH2O, the mean particle sizes of the primary particulates were 3.0, 2.2, 1.9, and 1.6 nm, respectively, which decreased with increasing Sn content. It was also observed that when RuO2 was electroplated and dispersed onto a highly conductive mesoporous SnO2 matrix, RuO2 existed in the form of particles with sizes  1.5 kW kg−1 [333]. It can be seen from the mentioned roles of doping metal oxides that the capacitive performance (e.g., specific capacitance 

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3.  Electrode materials for supercapacitors

and power characteristics) of binary oxides is mainly influenced by their molar ratio [244]. Liu et al. [317] obtained a capacitance of 642 F g−1 from a composite with a molar ratio of Co:Ru = 1:1 at 150°C. For materials like RuO2 xH2O, Ru0.8Sn0.2O2 xH2O, Ru0.6Sn0.4O2 xH2O, and SnO2 xH2O, the main factor influencing the total specific capacitance is their composition, and a higher specific capacitance of 850 F g−1 is obtainable with a Sn content of 0.2 [356]. 3.4.2.2 RuO2/carbon composites In recent years, composites containing RuO2 and carbon materials (e.g., carbon, carbon aerogels, CNTs, carbon nanofibers, etc.) have been intensively studied as electrode materials for supercapacitors [301, 302, 318, 322, 339, 341, 342, 358–374]. The quantity of RuO2 required in the electrode layer has thereby been reduced significantly, and higher specific capacitances have been achieved, such as 256 F g−1 with 14% Ru in a carbon aerogel, [360] 647 F g−1 with 60% RuO2 in Ketjen black, [256] 628 F g−1 with RuO2/MCNTs in a ratio of 2:1, [302] ∼850 F g−1 for carbon-RuO2 composite electrodes, and 900 F g−1 for RuO2 oxide powder pressed on conductive carbon [310]. It is essential to mention that many investigators utilized a high annealing temperature to obtain RuO2/carbon composites [360]. Obviously, as mentioned earlier, high temperature will lead to higher crystallinity, compromising the utilization of RuO2. Therefore, synthesis conditions need to be optimized. In RuO2/carbon composites, a carbon support mainly plays the following roles: (1) Carbon support can favor the dispersion of amorphous RuO2 xH2O particles, usually 3–15 nm in size [375]. As shown by Kim et al. [301], carboxylated carbon nanotubes can prevent RuO2 particles from agglomerating and contribute to the high dispersion of RuO2 nanoparticles by forming a bond between RuO2 and the surface carboxyl groups of the carbon nanotubes. Kim et al. showed a strong interaction between RuO2 and the surface carboxyl groups of carbon, which contributed to the high dispersion of RuO2 nanoparticles. It was also observed that the highly dispersed RuO2 nanoparticles exhibited increased capacitance, because the protons were able to access the inner part of RuO2. (2) A carbon support can facilitate the transfer of ions and electrons in the electrode layers. By inducing more porosity [375], the carbon support can enable easy penetration of the electrolyte up to the oxide primary particles. The porous morphology of MCNTs allows excellent electrolyte access in three dimensions, reducing the ion intercalation distance to nanometers when a thin layer of RuO2 was coated on CNTs [302]. Besides introducing more porosity to the electrode, the surface functional groups of CNTs may also enable the electrode to easily transport the solvated ions to the electrode–electrolyte interfaces, increasing the Faradaic reaction sites of the RuO2 nanoparticles, and then affecting the pseudo-Faradaic reactions of those nanoparticles [372]. The conductivity of the dispersed MCNTs can also increase the electrical conductivity of the composite [302]. For RuO2/carbon composites, two points should be noted: 1. Carbons with different porosity, crystallinity, and specific surface area will exert different influences on the electrochemical behavior of RuO2. For instance, in case with microporous carbon-derived composites, the specific capacitance remains nearly constant in the range of 0–20 wt.% RuO2. This is because the increase in specific capacitance due to the increased RuO2 content is balanced out by the decrease in specific capacitance owing to





3.4  Transition metal oxides/hydroxides

87

the decrease in the specific surface area of carbon. When the RuO2 content is greater than 20 wt.%, the contribution of RuO2 increases the specific capacitance [315]. In the case of mesoporous carbon-derived composites, their specific capacitance increases linearly with increasing RuO2 content. When carbon nanofibers were used to construct a RuO2 composite, even though their specific surface areas were small, the specific capacitance of the composites also increased with increasing RuO2 content [374]. It was reported that oxide deposited on crystalline carbons is texturally different from oxide deposited on amorphous carbons [374, 375]. The former showed some crystallization and larger particle size (2–4 nm), while the latter was amorphous with a smaller particle size (1–1.5 nm) and had a large specific surface area (450 m2 g−1). Although the RuO2 composites with amorphous carbon exhibited lower conductivity than those with crystalline carbon, their specific capacitances were found to be larger. 2. The particle size of supported RuO2 often increases with increased RuO2 loading, within a range that depends on the type of carbon used. For carbon nanofibers, when RuO2 loading was 0–11 wt.%, [374] the particle size increased from 2 to 4 nm. When microporous and mesoporous carbons were used as supports, and the RuO2 loading was increased from 0 to 15 wt.% on microporous carbon and from 0 to 40 wt.% on mesoporous carbon, the corresponding particle sizes grew from 1.4 to 4 nm and from 1.4 to 1.8 nm, respectively. However, when loading was increased beyond these ranges, the particle size of RuO2 xH2O was found to remain constant (∼4 nm) [315]. Increase in RuO2 particle size is often associated with decreased specific capacitance. Favorably, particle size can be conveniently tuned by adjusting the mass ratio of the carbon precursor [315, 316, 318, 343]. It can be concluded that the addition of carbon to a Ru oxide-based electrode layer may improve the homogeneity of the electrochemical reaction, reduce the ionic resistance of the metal oxide, expand the active sites, increase electrical conductivity, and consequently further increase the power and energy densities of the corresponding supercapacitor [326]. 3.4.2.3 RuO2/polymer composites Lately, RuO2/polymer composites receive attention. For example, hydrous RuO2 particles, electrochemically loaded into poly(3,4-ethylenedioxythiophene)-doped poly(styrene sulfonic acid) exhibited a high specific capacitance of 653 F g−1 [376]. Zang et al. [377] reported a nanostructured polypyrrole/RuO2 composite that achieved a specific capacitance of 302 F g−1. Liu et al. [378] synthesized a RuO2/poly(3,4-ethylenedioxy thiophene) nanotube composite that displayed a high specific capacitance and rapid charging/discharging capability. This RuO2based composite reached a high specific capacitance of 1217 F g−1, and when it was used as the electrode material in a supercapacitor, a power density of 20 kW kg−1 was achieved, maintaining 80% of the maximum energy density (28 Wh kg−1). The power capability and specific capacitance were attributed to the special hollow nanotube structures and thin walls, which allowed ions to readily penetrate into the composite material and access their internal surfaces, and hence to accomplish a short diffusion distance for ion transport. Polymers are claimed to play several roles in RuO2/polymer composites: (1) preventing the aggregation of RuO2 xH2O particles by steric and electrostatic stabilization mechanisms; (2) uniformly distributing RuO2 xH2O particles on the polymer matrix; (3) increasing the active



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surface area of RuO2 xH2O; (4) providing an efficient route to deliver proton species; and (5) improving the adhesion of RuO2 xH2O to the current collector [376]. Unfortunately, the accompanying volume increase caused by the polymer in the RuO2 composite is undesirable for light and compact energy devices, as the density of the polymer is relatively low compared to that of hydrous ruthenium oxide. Another critical problem is swelling and shrinking of the polymer during the supercapacitor cycling process. In summary, when RuO2-based composites are used as electrode materials in supercapacitors for practical applications, they should possess the following properties: (1) nanosized, amorphous, hydrous structures (solid-state proton diffusion), (2) networked particles with good connections (electronic conductivity), (3) networked pores inside electrode layers (proton conduction), (4) high packing density (volumetric energy density), and (5) mechanical stability (cycle stability) [379]. Although amorphous hydrous RuO2 can provide extremely high specific capacitance, its drawbacks, such as relatively high cost and environmental harmfulness, prevent it from being used in the commercialization of supercapacitors [380]. As an alternative approach, researchers have bestowed significant effort into finding cheaper and environmentally friendly materials that exhibit electrochemical behavior similar to that of RuO2. These alternative materials include MnO2, NiO, Fe3O4, and V2O5.

3.4.3  Manganese dioxide (MnO2) for PCs Manganese oxides appear to be an alternative to RuO2 due to their relatively low cost, low toxicity and environmental safety, and theoretical high capacitances up to 1100–1300 Fg−1. Since the early report by Lee and Goodenough in 1999 [381] MnOx has attracted attention and is considered as a promising alternative class of materials for ES applications [382–385]. The capacitance of manganese oxides arises mainly from pseudocapacitance, which is attributed to reversible redox transitions involving the exchange of protons and/or cations with the electrolyte, as well as the transitions between Mn3+/Mn2+, Mn4+/Mn3+, and Mn6+/Mn4+ within the electrode potential window of the electrolyte [386, 387]. The proposed mechanism is expressed in Eq. (3.3) [388–390]: MnOα (OC)β + δ C + + δ e − ↔ MnOα − β (OC)β +δ (3.3) where C+ denotes the protons and alkali metal cations (Li+, Na+, K+) in the electrolyte, and MnOα(OC)β and MnOα-β(OC)β+δ indicate MnO2 nH2O in high and low oxidation states, respectively. Eq. (3.3) suggests that both protons and alkali cations are involved in the redox process, and that the MnOx material must have high ionic and electronic conductivity. Despite the redox nature of the energy storage mechanism, MnOx-based electrodes can also demonstrate typical rectangular-shaped cyclic voltammetry curves (Fig. 3.20 presents an example), analogous to non-Faradaic energy storage mechanisms [391, 392]. As reported in the literature [351, 394, 395], both physical properties (e.g., microstructure and surface morphology) and chemical factors (valence and the hydrous state of the oxide) affect the pseudocapacitive performance of Mn oxides. It was found that the cycle stability of Mn oxides is mainly controlled by their microstructure, while their specific capacitance is governed primarily by their chemical hydrous state [396]. There are various physical and chemical factors, which affect the pseudocapacitance of MnOx electrodes. 



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FIGURE 3.20  Cyclic voltammograms of MnO2-based electrodes in a 1 M Na2SO4 electrolyte at a potential scan rate of 5 mV s−1. (A) MnO2 obtained by using CNT and (B) MnO2 obtained by using mesoporous carbon [393]. Source: Reprinted from Ref. [393], with permission from Elsevier.

1. Crystallinity. Similar to ruthenium oxide, if the crystallinity is too high in MnOx, the protonation or deprotonation reaction will be limited. Although high crystallinity can give rise to higher conductivity, loss of surface area occurs simultaneously. On the other hand, although lower crystallinity can result in a highly porous microstructure, the electrical conductivity of the resulting MnOx will be low. Consequently, there should be a trade-off between electrical conductivity in the solid phase and ionic transport in the pore. With respect to crystallinity-dependent electrical conductivity, the annealing temperature plays an important role in achieving optimal conductivity. When compared with those nonheat treated MnOx, the ones heat treated at 200°C showed a higher specific capacitance at high scan rates, and lower specific capacitance at low scan rates [397]. This phenomenon might be attributable to the fact that films treated at 200°C possess both lower open porosity and lower surface area. At a high scan rate, diffusion of H+ and Na+ ions is limited and some pores and voids become inaccessible. Obviously, excellent pseudocapacitive behavior can be obtained when Mn oxide is treated at an appropriate annealing temperature [384, 398]. 2. Crystal structure. Crystallized MnO2 materials have several crystalline structures, including α-, β-, γ-, and δ-MnO2. Among them, α-, β-, and γ-MnO2 have a tunnel structure (2 × 2) octahedral units for α-MnO2, the relatively large tunnel structured phase; 1 × 1 octahedral units for β-MnO2, the compact and dense phase), and δ-MnO2 has a relatively open layered structure (as shown in Fig. 3.21 by birnessite) [399, 400].   It has been recognized that different preparation conditions can lead to the formation of different MnOx structures. For example, gradual increase in the precursor acidity can lead to a progression from layered δ-MnO2, through the relatively large tunnel structured phase α-MnO2, to a compact and dense β-MnO2. In NaOH or KOH solution, the product is mainly a δ-MnO2 phase. These structural changes in MnO2 give rise to significant changes in electronic and ionic conductivity, affecting the pseudocapacitive behavior of the material [400]. With α-MnO2, it was reported by Hu and Tsou [386] that an amorphous hydrous manganese oxide (α-MnO2 nH2O) fabricated from an MnSO4 5H2O solution via anodic deposition yielded a specific capacitance in the range of 265–320 F g−1 

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FIGURE 3.21  Manganese dioxide structural transitions induced during material synthesis [400] . Source: Reprinted from Ref. [400], with permission from Elsevier.

when measured in a potential range between 0 and 1.0 V using a 0.1 M Na2SO4 aqueous electrolyte solution. Further, amorphous MnO2 made from an Mn(CH3COO)2 solution showed a capacity of 195–275 F g−1 in a 2 M KCl solution and 310 F g−1 in 2 M (NH4)2SO4. However, nanostructured α-MnO2 synthesized via a hydrothermal technique under mild conditions yielded a specific capacitance of 168 F g−1 at a potential scan rate of 1 mV s−1[401]. Single-crystal α-MnO2 prepared by hydrothermal reaction of KMnO4 under acidic conditions exhibited a specific capacitance of only 71.1 F g−1 at a current density of 300 mA g−1[402]. For δ-MnO2 based materials, although one high specific capacitance of 240 F g−1 at a current density of 1 mA cm−2 was reported [403], the majority of materials yielded specific capacitances of only 20–30 F g−1 [391, 404].   The two-dimensional layered structure of δ-MnO2 can facilitate cation intercalation/ deintercalation with little structural rearrangement. Several methods for synthesizing δ-MnO2 have been developed. δ-MnO2 obtained via a hydrothermal synthesis process exhibited a higher specific capacitance than γ-MnO2 and β-MnO2 [400]. δ-MnO2 synthesized through a redox reaction between MnSO4 and KMnO4 in an aqueous medium gave a specific capacitance of 236 F g−1 at 0.5 mA cm−1 in 0.1 mol L−1 Na2SO4 electrolyte [405]. Besides these two methods, δ-MnO2 was obtained via using 



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sonochemical process and exhibited a specific capacitance value exceeding 350 Fg−1 [406,407]. 3. Morphology. The morphology of MnO2 closely relates to the specific surface area and therefore the specific capacitance. Thus, the morphology of MnO2 plays a determinant role in its electrochemical performance. In the literature, prepared manganese oxides have several morphologies, such as nanowires [408–410], nanorods [411], nanobelts [412], flower-like microspheres [400, 413], nanobundles [414], and flower-like nanowhiskers [401]. Depending on the morphology, the obtained material’s specific surface area can range from 20 to 150 m2 g−1. Normally, the morphology of MnOx can be controlled by altering the preparation process or reaction conditions. For example, using a soft template method, various morphologies are obtainable by adjusting the precursors and conditions [415]. Using an anodic deposition method [416], the morphology of manganese oxide prepared from manganese acetate solutions is controllable by varying the deposition current density. In a hydrothermal reaction, the morphology of MnO2 changes by varying the reaction time at a given molar ratio of reactants [417], the solvent, the temperature of the reaction, and so on [418]. In a microemulsion process [419], precise control of particle size and shape can be achieved. Templated electrosynthesis [420], which is widely used as an effective pathway to form one-dimensional porous structures using anodic aluminum oxide and polycarbonate membranes, can also be employed for MnOx synthesis.   Having detailed the several morphologies of MnO2, it is worth discussing their effects on specific capacitance. The first is one-dimensional nanostructured MnOx. In general, for one-dimensional nanostructured materials, short transport/diffusion path lengths for both ions and electrons can be realized, leading to faster kinetics, offering large specific surface areas, leading to the resulting high charge/discharge capacities. For example, nanowires with smaller diameters can offer larger specific surface area for the access of electrolyte ions, more active sites for charge transfer, and short transport/diffusion distance for proton diffusion. This may be the reason why they have a higher capacitance of 350 F g−1 compared with capacitance of nanorods, 243 F g−1[421]. Another example is one-dimensional nanobelt materials. They not only offer large electrode surface area and provide conducting pathways for ions, leading to high capacity and fast kinetics, but also better accommodate large volume changes, resulting in improved cycle performance for cathode materials [422, 423]. The second MnO2 morphology is porous spherical particle materials. According to Donne et al. [400, 445], these are better than platelet particles. The third morphology is a Mn-oxide film with a rod-like structure, prepared through anodic deposition from a 0.01 M Mn acetate solution at various deposition current densities on Au-coated Si substrates, whose maximum capacitance is 185 F g−1 [416]. The fourth is a petal-shaped manganese oxide, which may not be a good choice for ES material because it could reduce the specific surface area, leading to capacitance fading. Further, the more ordered hexagonal NiAs-type crystal structure possesses fewer electrochemically active sites available for fast ionic transport and charge transfer. The fifth morphology is MnO2-pillar layered manganese oxide, which shows good cycling stability because pillaring agents are not favored for significant structural or mesostructural change during the intercalation and deintercalation processes, leading to enhanced stability of the layered structure [424]. Based on the knowledge in the literature, one-dimensional



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nanostructured materials with special morphologies and large surface areas are necessary for potential applications in ES. 4. Thickness of the electrode layer. In general, the specific capacitance decreases with increasing thickness of the electrode layer (or film) due to the low conductivity of MnO2. There are quite a number of publications on this issue. For example, when the deposited loading of nanostructured MnO2 was increased from 50 to 200 mg cm−2 the specific capacitance decreased from 400 to 177 F g−1 [425]. For birnessite-like compounds prepared by electroxidation of acidic aqueous solutions of MnSO4, capacities vary in the range 70–150 F g−1, revealing the strong dependence on the thickness of the electrode layer [426]. Some researchers have also pointed out that the thicker MnO2 films deposited at higher potential are different from the thin films obtained at lower potentials. The difference between their specific capacitance can be as large as 100 F g−1 [427]. For instance, the specific capacitance of a MnO2 layer prepared from the oxidation of MnSO4 decreased from 220 to 50 F g−1 as the MnO2 loading was increased from 100 mg cm−2 to 4 mg cm−2 [428]. Therefore, it is obvious that thicker the film, lower the specific capacitance. The major benefits of thin layers can involve (1) lower series resistance due to shorter transport paths for the diffusion of protons (low concentration polarization of the electrolyte), (2) easy access of the electrolyte to the active surface of manganese dioxide, and (3) higher electronic conductivity. Hence, high specific capacitance and rate capability can be achieved, especially when a thin MnO2 layer is uniformly dispersed on conductive and porous carbonaceous materials with a high surface area. For example, Prasad and Miura [429] reported a thin amorphous MnO2 film obtained by potentiodynamic deposition that exhibited a specific capacitance of 482 F g−1. When this thin layer was used for ES electrodes, high power density and stability were achieved. Most of the reported MnO2 thin layers have exhibited specific capacitances as high as ∼600 F g−1 or more, within a potential window of 0.9–1.2 V in aqueous electrolytes containing KCl, K2SO4, Na2SO4, or KOH [153, 430–435]. A thin MnO2 layer prepared by a sol-gel method (MnO2 loading of ∼4 mg cm−2) showed a specific capacitance of 700 F g−1 in a potential window of 0.9 V [388]. A thin layer of MnO2 electrode prepared by dipcoating colloidal MnO2 exhibited a specific capacitance value close to 700 F g−1 [210]. A specific capacitance as high as 1300 F g−1 was reported with thin MnO2 deposits (on the scale of nanometers) because the reaction kinetics was no longer limited by the electrical conductivity of MnO2 [436]. Besides nearly perfect cyclic performance, the MnO2 thin layer yielded a high specific capacitance of 149 F g−1 even at the high potential scan rate of 500 mV s−1 [437]. Unfortunately, even though this kind of MnO2 thin layer can exhibit an extremely high specific capacitance of 1330 F g−1, the entire electrode can only provide a capacitance of 0.039 F cm2 [419] far from the requirements for practical applications, which need high power and energy. 5. Specific surface area and pore structure. As with RuO2, larger surface area implies more Faradaic active sites and thereby higher pseudocapacitance. Normally, the specific capacitance of a metal oxide material will increase significantly as its surface area increases. For example, a fibrous electrode has a high surface area and contains more active sites for redox reactions. The porous structure of the material can offer more channels for the electrolyte, leading to less electrochemical polarization and therefore minimal dissolution of Mn oxide. Further, high porosity can easily relieve the internal





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stress created during the charging and discharging processes, protecting the electrode from physical damage. Hence, for more fibrous or porous oxide, better cycle stability is attained [396]. It is worth mentioning that both the surface area and the pore-size distribution of MnO2 can be controlled by adjusting the reaction time and the surfactant content in the aqueous phase [438]. 6. Chemical factors (the hydrous state of the oxide and valence). Physically and chemically bound water is believed to be helpful for the transportation of electrolyte ions. Thus, the loss of water content caused by heat treatment will result in poor ion conductivity as well as loss of pseudocapacitance if the heat treatment temperature is high [439, 440]. For instance, it was reported that heat treatment at a temperature up to 200°C could result in the removal of both physically and chemically adsorbed water [441]. It was also suggested that heat treatment can be optimized to possibly enhance the overall pseudocapacitive behavior [433]. The Mn oxidation state is another critical factor affecting the electrochemical performance of Mn oxides. Owing to the Jahn-Teller distortion of the Mn3+O6 octahedron, MnOx involving trivalent Mn shows a rather lower conductivity compared with MnO2 or amorphous manganese oxides involving Mn4+ [399, 442]. Consequently, with different nanostructured manganese oxides synthesized via a precipitation technique using KMnO4 and various alcohols, the presence of Mn3+ results in reduced specific capacitance [385]. It was reported that when γ-MnO2 was transformed to α-Mn2O3 and Mn3O4 by mechanical grinding, the specific capacitance decreased linearly as the amount of γ-MnO2 decreased [442]. Regarding this issue, the literature also shows that MnO2 exhibits much better performance than Mn(OH)2, Mn2O3, and Mn3O4 [386, 443]. The factors mentioned earlier are related to the MnOx preparation processes and conditions. To improve the pseudocapacitive performance of Mn oxides, developing more favorable preparation processes, optimizing the microstructure, crystallinity, and the chemical state of Mn oxides appears to be an important approach. Several typical techniques reported in the literature include thermal decomposition, co-precipitation, sol-gel processes, electrodeposition, mechanical milling processes, and hydrothermal synthesis. Among these, the hydrothermal route has been proven to be an effective and controllable method to produce Mn oxides with various nanostructures, such as nanowhiskers, nanoplates, and nanorods [408, 418, 444]. It has to be noted that the mechanical milling process can lead to a sequential phase transition from γ-MnO2 to the thermodynamically stable α-Mn2O3 and subsequently to Mn3O4, depending on the duration of mechanical grinding [442]. Regarding Mn oxides as ES materials, several challenges, which are listed, must be addressed to employ them in practice: 1. Dissolution problem. Owing to the partial dissolution of MnO2 in the electrolyte during cycling, MnO2 oxide electrodes suffer from capacitance degradation [404, 445, 446]. The dissolution reactions can be expressed as in Eqs. (3.4) or (3.5): 2+ (3.4) Mn 2 O 3(s) + 2H + → Mn (aq) + MnO 2(s) + H 2 O

(or) 2+ 2MnO(OH) → Mn (aq) + MnO 2 (s) + 2OH − (3.5)



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  Several approaches have been adapted to prevent the dissolution of Mn oxides during cycling. For example, new electrolyte salts have been developed to avoid forming acidic species in solution [447, 448]. Another approach is to form a protective shell to the Mn oxide surface. Babakhani and Ivey [449] prepared Mn oxide/CP coaxial core/shell electrode. Mn oxide rods were first synthesized through anodic deposition from Mn-acetate solution, and then were coated using electropolymerization of a conducting polymer to yield coaxial rods. The presence of a conducting polymer suppresses the dissolution of Mn oxide and improves its resistance to failure, leading to both enhanced capacitance and high cycling rate capability [450]. It was demonstrated that this Mn oxide/CP coaxial core/shell electrode could exhibit a specific capacitance of 285 F g−1 with 92% retention after 250 cycles in 0.5 M Na2SO4 at 20 mV s−1, which is much better than uncoated Mn oxide rods [449]. In addition to the above two methods (new electrolytes and material coating), it was reported that inert cobalt oxide also suppressed the dissolution of Mn into electrolyte and stabilized MnOx [451, 452] leading to both high cycling stability and higher power when measured at a high scan rate of 200 mV s−1[440]. However, if the cobalt oxide content was too high, a significant reduction in the specific capacitance was observed. 2. Low surface area and poor electronic conductivity. Another concern is the low surface areas and poor electronic conductivity of MnOx materials. In order to improve these, several approaches have been made in ES material development. For example, a large surface area can be achieved by introducing multilayered film electrodes containing transition metal composite materials [453]. In this fashion, the formed Mn oxides have high electrochemical activities, leading to excellent electrochemical capacitance and long cycling durability. Besides this method, doping other metals into MnOx is a proven effective way to increase the surface area. In a nanostructured nickel-manganese oxide composite, the doping of nickel increased the Mn oxide surface area by about 46% and the specific capacitance by 37% [394, 454]. Increase in the surface areas of MnOx materials can also be realized by introducing Co into MnOx or by adding a surfactant, such as sodium lauryl sulfate or Triton X-100 [391]. In addition, hierarchical hollow nanospheres of MnOx yielded a specific surface area of 253 m2 g−1[455]. To address the issue of enhancing the poor electrical conductivity of manganese oxides, doping with ruthenium [312], nickel [456], and Mo [457] appears to be helpful.   In addition to doping, highly conductive supports, such as active carbon, CNTs, graphite, and conducting polymers have also been utilized to support manganese oxides in an attempt to promote their conductivity. In reality, the presence of conductive supports not only improves the electrical conductivity of MnOx but also increases its active surface area. Using a supporting strategy, MnO2 can be dispersed over a large area, preventing its further growth by agglomeration and ensuring high utilization of the active materials in addition to providing double-layer capacitance. The most important feature of the supporting strategy is that these conductive supports can form a 3D porous conducting network to effectively assist electron transfer and ion transport within MnOx. For instance, a rectangular shape in a cyclic voltammogram [458] and a volumetric specific capacitance of 253 F cm−3 can be obtained from a manganese oxide/carbon composite electrode [459]. Mn oxide/CNT composite electrodes [460] reached a specific capacitance of 415 F g−1 because CNTs were able to provide electronic conductive paths and form a network of





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open mesopores. Li et al. [383] reported that when MnO2 particles were partially coated on the surfaces of MCNTs through a hydrothermal process, a specific capacitance of 550 F g−1 was obtained. The MnO2/C composite electrode also exhibited highly stable performance up to 10,000 cycles [411]. In addition, composites with a single-walled CNT support showed excellent cycling capability even at a high current of 2 A g−1 [461].   In summary, to achieve high energy and power densities for ES, MnOx based electrode materials can be modified through depositing MnOx onto carbon materials with high surface areas, highly ordered mesopores, and high electrical conductivity, which can then yield high specific capacitances. However, it should be noted that the specific capacitance of the composite electrode can be compromised by the low Faradaic reaction activity of the conductive supports. 3. Poor ionic conductivity. The poor ionic conductivity of Mn oxides is another challenge that hinders their practical application in ES. To improve their ionic conductivity, great effort has been bestowed onto nanostructured MnOx materials. For example, multilayered MnO2 with a macropore surface and an interlayer space yielded high ionic conductivity, leading to improved pseudocapacitive behavior with high retention of a rectangular shape during charging/discharging, even at the high potential scan rate of 100 mV s−1 [392]. In addition, a nanoscopic MnO2 phase can minimize solid-state transport distances for ions going into the oxide. For example, Mn oxide nanowire arrays with high aspect ratio and high surface area, prepared on anodic aluminum oxide templates, displayed a specific capacitance of 254 F g−1 [462]. Nanostructured Co- and Ni-doped MnO2 deposited by a potentiodynamic method have also been explored, showing specific capacitances of 621 and 498 F g−1, respectively, at a scan rate of 10 mV s−1; at a high scan rate 200 mV s−1, they exhibited 377 and 307 F g−1, respectively [440]. A uniformly dispersed fibrous manganese oxide electrode with unique nanoporous structure displayed a high specific capacitance of 502 F g−1 and excellent cyclic stability [463], mainly due to the highly porous structure which enhanced electrolyte accessibility, promoting ionic transportation within the electrode, kinetic reversibility, and electrochemical reaction homogeneity. 3.4.3.1  Recent R&D Advancements in MnO2 In recent years, some intensive studies on MnO2 nanostructured and MnO2 composite materials as well as their asymmetric ES have been carried out as outlined. 1. Nanostructured MnO2. Nanostructured MnO2 materials have been intensively investigated [464–466]. Nano-MnO2 can exhibit an electrochemical performance superior to its bulk counterpart due to its higher specific surface area and the short transport/diffusion path lengths of ions and electrons. Some single-crystal α-MnO2 nanotubes [467], which were synthesized by a hydrothermal method without the assistance of templates, surfactants, and heat treatment, could possess a high specific capacitance. α-MnO2 nanowires with a diameter 30–40 nm and mean pore diameter 3.1 nm could yield a specific capacitance of 466 F g−1 as well as high cycling efficiency at a current density of 10 mA cm−2 [468]. α-MnO2 nanorods with a diameter  90%, and the energy density was up to 107 Wh kg−1 at a high-power density of 9.4 kW kg−1 (as shown in Fig. 3.28D). Wee et al. synthesized V2O5 nanofibers (VNF) through a simple electrospinning method, and their application as supercapacitor electrodes are demonstrated [579]. The highest specific capacitance was achieved for VNF annealed at 400°C, which yielded 190 F g−1 in aqueous electrolyte (2 M KCl) and 250 F g−1 in organic electrolyte (1 M LiClO4 in PC) with promising energy density of 5 Wh kg−1 and 78 Wh kg−1 respectively.



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Based on Wee’s idea, Lala et al. prepared V2O5 tubular nanofibers (TNFs) via electrospinning technique using a single spinneret for the first time by controlling the properties of the precursor solution [580]. A partially miscible polymeric solution of vanadium oxytrihydroxide [VO(OH)3] was produced by hydrolysis of vanadyl acetylacetonate in poly (vinylpyrrolidone) (PVP). The phase-separated polymer solution formed the core of the electrospun fibers whereas the VO(OH)3 formed the shell; the core PVP had been removed by controlled heat treatment. The TNFs had an inner diameter 60 nm and wall thickness ±100 nm. The development of asymmetric supercapacitor is also a trend in recent years. Qu et al. prepared V2O5.0.6H2O nanoribbons and investigated their electrochemical behavior in K2SO4 aqueous solution [581]. Results showed for the first time that K+ ions could intercalate/deintercalate reversibly in the V2O5 0.6H2O lattice. An asymmetric supercapacitor with the structure of activated carbon/0.5 MK2SO4/V2O5 0.6H2O was successfully assembled, which could be cycled reversibly in the voltage region of 0–1.8 V. This supercapacitor presented an energy density of 29.0 Wh kg−1 based on the total mass of the active electrode materials, a very good rate behavior with energy density of 20.3 Wh kg−1 at power density of 2 kW kg−1. Later, Lin et al. developed a high-performance asymmetric supercapacitor by using porous vanadium pentoxide (V2O5) nanotubes as positive electrode and activated carbon nanorods as negative electrode in an aqueous 2 M LiNO3 electrolyte [582]. To maximize the energy density of the asymmetric supercapacitor, the initial potentials of working electrodes were tuned to different values (0 V, 0.1 V, 0.2 V, and 0.3 V vs. SCE), and the influence of the electrode potential on the electrochemical properties of the obtained asymmetric supercapacitor has been investigated in depth. The results showed that 0.2 V is the optimal initial electrode potential. At this initial electrode potential, the built V2O5//C asymmetric supercapacitor could be cycled reversibly in the voltage region of 0–1.8 V, and exhibited high energy and power density (46.35 Wh kg−1 at 1.8 kW kg−1 and 18 kW kg−1 at 28.25 Wh kg−1). Further, the supercapacitor showed excellent cycling stability, with nearly 100% specific capacitance retention after 10,000 cycles. Nanowires are considered to have good electrochemical properties. Wang et al. prepared ultrahigh-aspect-ratio V2O5 nanowires, which used [VO(O2)2(OH)2] as the starting material by a template-free hydrothermal route without the addition of organic surfactant or inorganic ions [583]. The results revealed that the peroxovanadium (V) complexes can be easily transformed to V2O5 nanowires by this hydrothermal route. The uniform nanowires had width about 50 nm and length about dozens of micron. The BET analysis showed the V2O5 nanowires had a specific surface area of 25.6 m2 g−1. The synthesized V2O5 nanowires performed a high capacitance of 351 F g−1 when used as supercapacitor electrode in 1 M LiNO3. As spin coating sol-gel is a good method for the preparation of thin film materials, Jeyalakshmi et al. prepared vanadium pentoxide thin films via this method [584]. The films coated on fluorine doped Tin Oxide (FTO) and glass substrates were treated at different temperatures ranging from 250 to 400°C. The vanadium pentoxide films annealed at 300°C for an hour exhibited a maximum specific capacitance of 346 F g−1 at a scan rate of 5 mV s−1. Surface active agents often play an important role in the morphology control of nanomaterials. Nair et al. synthesized V2O5 nanoparticles via an anionic, cationic, and nonionic surfactant assisted hydrothermal method in which ammonium metavanadate (NH4VO3) was used as precursor [585]. In order to further study the effect of different surfactants on the synthesis of V2O5, Qian et al. used three kinds of surfactants, including polyethyleneglycol 6000 (PEG-6000),





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sodium dodecylbenzene sulfonate (SDBS), and Pluronic P-123 (P123), to prepare nanolayered V2O5 H2O through a simple hydrothermal process, and this resulted in different morphologies, including flower-like flakes, linear nanowires, and 3D networks connected with curly bundled nanowires [586]. The electrochemical performance of these powders showed that the nanowires, which are electrodes mediated by PEG-6000, exhibited the highest capacitance of 349 F g−1 at a scan rate of 5 mV s−1 of all the surfactants studied. However, a symmetric P123 electrode comprising curly bundled nanowires with numerous nanopores showed an excellent and stable specific capacitance of 127 F g−1 after 200 cycles. From the earlier discussion, it is evident that V2O5 nanostructures have a variety of forms, Mu et al. synthesized four types of typical morphologies of differently dimensional pure V2O5 nanostructures including nanoflowers, nanoballs, nanowires, and nanorods (Fig. 3.28) through a simple hydrothermal method and compared their electrochemical properties [587]. The morphology of the product depends on the types of solvent and acid employed in the preparation process. Hierarchical nanoflowers and zero-dimensional (0D) nanoballs of V2O5 nanocrystals were obtained while using C2H5OH as solvent. 1D nanowires and nanorods were obtained, if H2O was used to participate in the reaction. Electrochemical tests indicated that the rod-like structure leads to a significant improvement of storage capacity, electrochemical kinetics, and rate capability. The 1D V2O5 nanorods showed the largest specific capacitance of 235 F g−1 at the current density of 1 A g−1 as a supercapacitor electrode in 1 M Na2SO4 electrolyte. 3.4.8.2  Other elemental metal doped vanadium pentoxide composites Doped elemental metal is a subject of interest for enhancing the capacitive behavior of supercapacitor such as maximum current density, good reversibility, good ion storage capacity and cyclic stability. Moreover, to achieve fast Faradic reaction for electron transport in the electrode and ion transport in the solution, doped metal element electrodes are preferred. Jeyalakshmi et al. [588] prepared interesting thin film electrodes of nickel doped vanadium pentoxide with different levels of doping (2.5–10 wt.%) on FTO and glass substrate at 300°C using sol-gel spin coating method. The doping of nickel with β-V2O5 had led to enhanced intercalation and deintercalation of ions. β-V2O5 films with 5 wt.% of Ni exhibited the maximum specific capacitance of 417 F g−1 at a scan rate of 5 mV s−1, with a good cyclic stability. Sn4+ doping could alter the microstructure and the morphology of V2O5. Wang et al. prepared Sn4+ doped V2O5 cathode materials via a sol-gel method [589]. The results showed that the modified cathode material had a mixture of V4+ and V5+. It was a typical mesopore material with pores of 2–4 nm diameters. Symmetrical curves were obtained by cyclic voltammetry (CV) tests performed at different scan rates and voltage ranges. In particular, the CV curve showed more obvious rectangle property and better redox properties when the scanning rate was 5 mV s−1. At the current density of 200 mA g−1, the maximum specific energy, specific power, and coulomb efficiency of the material were 27.25 mA hg−1, 494.87 W kg−1, and 97%, respectively. Due to its porous structures, sodium-doped vanadium oxide is widely used in energy storage systems. Khoo et al. successfully synthesized a nanostructured oxide pseudocapacitor electrode utilizing a sodium-doped vanadium oxide (β-Na0.33V2O5) nanobelt network with a 3D framework crystal structure via mild hydrothermal conditions and heat treatment [590]. A high specific capacitance of 320 F g−1 at 5 mV s−1 scan rate has been achieved. With two sets of redox peaks being identified, corresponding to the half occupancy at M3 and M2 intercalation sites along the tunnel in the β-Na0.33V2O5 crystal lattice. The β-Na0.33V2O5 nanobelt



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electrode was able to deliver a high energy density of 47 Wh kg−1 at a high-power density of 5 kW kg−1. Superior cycling stability, with only 34% degradation in specific capacitance, was observed with β-Na0.33V2O5 nanobelts after 4000 cycles. Chang et al. reported that Na-doped V2O5 was successfully synthesized using an anodic deposition technique in a plating solution containing VOSO4 and NaCH3COO [591]. The Na doping significantly improved the oxide capacitance. The optimum specific capacitance is about 180 F g−1. 3.4.8.3  Other vanadium pentoxide composites In recent years various mixed oxide composites, such as NiO-MnO2, CoO-MnO2 , and aFe2O3/MnO2 core-shell nanowire heterostructure arrays were fabricated to improve the capacitive performance of the materials through intruding synergistic effects into an electrode system. Yang et al. prepared highly ordered mixed V2O5-TiO2 nanotubes via self-organizing anodization of Ti-V alloys with vanadium content of up to 18 wt.% [592]. In the resulting oxide nanotube arrays, the vanadium is electrochemically switchable leading to a specific capacitance up to 220 F g−1 and the energy density of 19.56 Wh kg−1 with perfect reversibility and long-term stability. As a common pseudocapacitance material, MnO2 has been extensively studied. Saravanakumar et al. prepared MnO2 grafted V2O5 nanostructures, which exhibited elevated specific capacitance (450 F g−1 at 0.5 A g−1), good rate capacity (251 F g−1 at 5 A g−1), and provided better cycling stability (retaining 89% of capacitance after 500 cycles) [593]. They developed an asymmetric supercapacitor, which used MnO2 grafted V2O5 and activated carbon (AC) as electrodes and exhibited a specific capacitance of 61 F g−1 with an energy density of 8.5 Wh kg−1. 3.4.8.4 Vanadium pentoxide/compound-carbon material composites Carbon materials, including activated carbon, carbon nanotubes, and graphene are the most frequently used conductive substrates because of their good electronic conductivities, high specific surface areas, and great chemical stabilities. Nevertheless, it is difficult to directly grow metal oxides/hydroxides on carbon materials, because their surfaces are not compatible. To improve their surface compatibility, oxidative treatments of carbon materials are necessary, which could introduce oxygen-containing groups facilitating the growth of metal oxides/hydroxides and numerous structural defects on the surfaces of carbon materials. In order to improve the electrical conductivity and cyclic stability of V2O5, the researchers tried to introduce into the material systems the high conductive carbon materials, such as activated carbon or carbon fiber, carbon nanotubes, graphene in order to prepare the V2O5-based composites [594–597]. 3.4.8.4.1  Vanadium pentoxide/activated carbon or carbon fiber material composites

The activated carbon has been widely used, because of the advantages of simple preparation, low cost, large specific surface area, and good electrical conductivity. Activated carbon, with extremely high surface areas, controllable pore size, narrow pore size distribution, ordered pore structures, and interconnected pore channels, have been studied for applications in electrochemical energy storage. Besides directly being used as electrode materials for EDLCs, activated carbon has also been used as 3D supports for pseudocapacitive materials. Kudo et al. first developed V2O5/carbon composite for supercapacitors [594]. V2O5 sol was obtained by reacting metallic vanadium with hydrogen peroxide solution. Then, acetylene





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black powder was added into the sol with acetone to yield a homogeneous suspension. The sample electrode was constituted of a composite of amorphous V2O5 and carbon, was loaded on a macroporous nickel current collector, and heat-treated at 120°C to obtain a sample electrode. The experimental results verified that the composite electrode with the V2O5/carbon ratio of 0.7 in weight showed 54% of the ideal capacity, 360 mAh g−1 (4.2–2.0 V) based on V2O5, even at a high rate discharge at 54 A g−1 V2O5. In order to further study the effect of sol condition on the electrochemical performance of V2O5/carbon composite cathode, Watanabe et al. used vanadium oxide sol with acetone to develop V2O5/carbon composite cathodes, which could acquire high rate charge/discharge capacity [595]. As flexible supercapacitor has a broad application prospect, 3D nanoarchitectures on flexible current collectors has emerged as an effective strategy for preparing advanced portable and wearable power sources [596]. Li et al. developed a flexible and efficient electrode based on electrospun carbon fibers substrate (ECF) with elaborately designed hierarchical porous V2O5 nanosheets (V2O5-ECF) via a simple solvothermal method. The unique configuration of V2O5-ECF composite film fully enables utilization of the synergistic effects from both high electrochemical performance of V2O5 and excellent conductivity of ECF, endowing the films to be an excellent electrode for flexible and lightweight electrochemical capacitors (ECs). Benefiting from their intriguing structural features, V2O5-ECF and ECF films, directly used as electrodes for flexible asymmetric quasi-solid-state electrochemical capacitors, achieve superior flexibility and reliability, enhanced energy/power density, and outstanding cycling stability. Spray pyrolysis is a valid method to synthesize large quantities of high-purity oxide powders that have homogeneous nanosized crystals. Wang et al. synthesized nanostructured vanadium pentoxide/carbon (V2O5/carbon) composite powders via spray pyrolysis technique, with enhanced specific capacitance [597]. Electrochemical properties were examined by the cyclic voltammetry technique. Following analysis of powders sprayed at different temperatures, composite powders obtained at an optimum temperature of 450°C yielded a maximum specific capacitance of 295 F g−1 in 2 M KCl electrolyte at a 5 mV s−1 scan rate. Electrodeposition is one of the most active fields in the preparation of nanomaterials. Electrodeposition of an ultrathin metaloxide layer on carbon-nanofiber can provide a high surface area and improved conductivity of the electrode. Ghosh et al. developed an ultrathin V2O5 layer that was electrodeposited by cyclic voltammetry on a self-standing carbon-nanofiber paper, which was obtained by stabilization and heat-treatment of an electrospun polyacrylonitrile (PAN)-based nanofiber paper [598]. A very-high capacitance of 1308 F g−1 was obtained in a 2 M KCl electrolyte when the contribution from the 3 nm thick vanadium oxide was considered alone, contributing to over 90% of the total capacitance (214 F g−1) despite the low weight percentage of the V2O5 (15 wt.%). By a simple electrospinning method, Kim et al. prepared the vanadium pentoxide (V2O5)/carbon nanofiber composites (CNFCs) from polyacrylonitrile/V2O5 in N, N-dimethylformamide, and investigated their electrochemical properties as supercapacitor electrodes [599]. Different loadings of V2O5, the microstructures of the CNFCs (e.g., nanometer-size diameters, high specific surface areas, narrow pore size distributions, and tunable porosities) were changed, and the textural parameters significantly affected the electrochemical properties of the composites. The CNFC capacitors delivered the high specific capacitances of 150.0 F g−1 for the CNFCs in an aqueous electrolyte, with promising energy densities of 18.8 Wh kg−1, over a power density range of 400–20,000 W kg−1. Their team has also developed mesopore-enriched activated carbon nanofiber (ACNF) mats, which



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were produced by incorporating vanadium (V) oxide (V2O5) into polyacrylonitrile (PAN) via electrospinning, and their electrochemical properties are investigated as an electrode in supercapacitors [600]. The microstructures of the ACNFs (e.g., nanometer-size diameter, high specific surface area, narrow pore size distribution, and tunable porosity) were changed, and the textural parameters are found to significantly affect the electrochemical properties through different V2O5 loadings and activation process. The V2O5/PAN-based ACNF electrodes with well-balanced micro/mesoporosity having an optimal pore range for effective double layer formation in an organic medium and are expected to be useful electrode material for supercapacitors. Guo et al. found that the electrochemical performance and stability of V2O5 nanowires were significantly improved by coating a thin carbon layer as shell [601]. Their study showed that the V2O5/C core shell nanowires achieve a remarkable areal capacitance of 128.5 F cm−2 at 10 mV s−1 with excellent rate capability. More than 94.4% of the initial capacitance was retained after 10,000 cycles for V2O5/C core shell nanowires, which was higher than the pristine V2O5 nanowires (13.3%). Atomic layer deposition is a method that can be used to coat the surface of the substrate by a single atomic film. Daubert et al. used atomic layer deposition (ALD) to grow V2O5 on the surface of activated carbon materials [602]. The V2O5 ALD process was characterized at various temperatures to confirm saturated ALD growth conditions. Capacitance and electrochemical impedance analysis of subsequently constructed electrochemical capacitors showed improved charge storage for the ALD coated electrodes, but the extent of improvement depended on initial pore structure. The ALD of V2O5 onto mesoporous carbon increased the capacitance by up to 46% after 75 ALD cycles and showed a maximum pseudocapacitance of 540 F g−1 (V2O5) after 25 ALD cycles, while maintaining low electrical resistance, high columbic efficiency, and a high cycle life. A commercial carbon black namely Ketjin black is widely employed for the preparation of electrode materials. Peng et al. prepared V2O5/Ketjin black (VK) nanocomposites with mesoporous mica-like structure via a facile sol-gel method. Through a dip-dry process, the VK nanocomposites were successfully assembled on nickel foams with controllable mass loadings [603]. The as-prepared electrode (VK2) showed high areal capacitance (3.9506 F cm−2 at 5 mA cm−2) and good cycling stability (90% after 8000 cycles) in a LiCl/PVA gel electrolyte. Further, the VK nanocomposite-based all-solid-state symmetric supercapacitor can provide a maximum energy density of 56.83 Wh kg−1. Carbon coating on metal oxide significantly modify the surface chemistry, which provides protection layer to active sites, and improve the electronic conductivity. Recently, Balasubramanian et al. synthesized carbon coated V2O5 with flowery architecture via co-precipitation method followed by thermal treatment [604]. The carbon coated flowery V2O5 exhibited maximum specific capacitance of 417 F g−1 with 100% capacitance retention even after 2000 continuous charge-discharge cycles. 3.4.8.4.2  Vanadium pentoxide/carbon nanotubes composites

Carbon nanotubes (CNT) are one of the most attractive nanostructured carbons with 1D tubular structure, exhibiting outstanding physicochemical properties, such as high electrical conductivity, high mechanical strength, high chemical stability, and high-activated surface areas. In the last two decades, the potential of CNT as electrodes in ECs has been extensively explored. The unique tubular structure of CNT is believed to be able to produce consistent composites and an interconnected conducting network with high porosity for





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enhanced electron transfer as well as electrolyte accessibility. V2O5 can be coupled with CNT by diverse routes, including electrodeposition, in situ precipitation/thermolysis/thermal decomposition, solid-state reaction, etc., Kim et al. first prepared V2O5 xH2O on the CNT film substrate. The V2O5 xH2O/CNT film electrode showed that V2O5 xH2O was heterogeneously nucleated and uniformly deposited on the CNT film substrate [605]. The V2O5 xH2O/CNT film electrode showed not only a high specific capacitance of 1230 F g−1, but also a high rate capability. The maximum specific energy of 851 Wh kg−1 and specific power of 125 kW kg−1 were obtained from the discharging curves of the V2O5 xH2O/CNT film electrode. Hybrid nanocomposites containing CNT have attracted much attention when each constituent component provides different functions for specific applications. Fang developed vanadium oxide/CNT composites [606]. The cross-sectional scanning electron microscope images revealed the presence of CNTs covered with uniformly dispersed vanadium oxides and from the electrochemical studies, it is inferred that the uniform distribution of vanadium pentoxides on CNT support led to a significantly improved capacitive performance, as compared with bare oxide films. As of date, hydrothermal process is considered as the most classic and most commonly used method to prepare nanomaterials. Chen et al. synthesized nanocomposites of interpenetrating CNT and V2O5 nanowires networks via a simple in situ hydrothermal process [607, 608]. These fibrous nanocomposites are hierarchically porous with high surface area and good electric conductivity, which made them excellent candidates for supercapacitors with high energy and power densities. Nanocomposites with a capacitance up to 440 and 200 F g−1 were achieved at current densities of 0.25 and 10 A g−1, respectively. Asymmetric devices based on these nanocomposites in aqueous electrolyte exhibited an excellent charge/ discharge capability. It showed high energy densities of 16 Wh kg−1 at a power density of 75 W kg−1 and 5.5 Wh kg−1 at a high-power density of 3750 W kg−1. Flexible VNW-CNT nanocomposite papers can be used as electrode material for supercapacitor. Perera et al. developed a simple method for preparing freestanding CNT-V2O5 nanowire (VNW) composite paper electrodes without using binders [609]. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode as the cathode. The supercapacitor with CNT-VNW composite paper electrode exhibited a power density of 5.26 kW kg−1 and an energy density of 46.3 Wh kg−1. The VNWs and CNT composite paper electrodes showed improved overall performance with a power density of 8.32 kW kg−1 and an energy density of 65.9 Wh kg−1. Carbon nanotubes are 1D nanomaterials with a huge aspect ratio. By controlled hydrolysis of vanadium alkoxide, Sathiya et al. developed a functionalized CNT that were coated with a 45 nm thin layer of V2O5 [610]. The resulting V2O5/CNT composite had been investigated for electrochemical activity, and the capacity value showed both Faradaic and capacitive (non-Faradaic) contributions. At high rate (1 C), the capacitive behavior dominated the intercalation as 2/3 of the overall capacity value out of 2700 C g−1 is capacitive, while the remaining is due to Li-ion intercalation. The growth of V2O5 on the surface of multi-walled carbon nanotube can improve the specific capacitance. Shakir et al. developed a simple, lowcost, safe and broadly applicable hierarchical bottom-up assembly route for the formation of ultrathin (3 nm) vanadium oxide (V2O5) film on conducting multiwalled carbon nanotube (MWCNT) [611]. The ultrathin V2O5 showed a very high capacitance of 510 F g−1 and possess excellent cyclic stability with negligible decrease in specific capacitance after 5000 cycles.



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The supercapacitor device based on this hierarchical bottom-up assembly exhibited an excellent charge/discharge capability, and energy densities of 16 W h kg−1 at a power density of 800 W kg−1. By printing the active material onto an ITO glass current collector, Yilmaz prepared V2O5/ CNT gel composites for solid-state supercapacitors [612]. V2O5/CNTs at a 0.5:1 wt ratio have been found to attain the highest capacitance among various V2O5/CNT ratios investigated. The composite had hierarchically porous architecture, owing to interpenetration of the CNTs with the V2O5 nanosheet. The V2O5/CNT electrodes exhibited high energy density (1.47 mWh cm−3) and power density (0.27 Wcm−3). The capacitance retention after 5000 cycles was more than 91% at a charge-discharge current density of 5 A g−1. As atomic layer deposition technique allows one to uniformly deposit metal oxides on porous CNT electrodes, Lee et al. used ALD technology to form nanostructured vanadium oxide coatings on the surface of MWCNT electrodes, thus adapting a novel route for the formation of binder-free flexible composite electrode fabric for supercapacitor applications with large thickness, controlled porosity, greatly improved electrical conductivity and cycle stability [613]. Electrochemical measurements revealed stable performance of the selected MWCNTvanadium oxide electrodes and remarkable capacitance of up to 1550 F g−1 per active mass of the vanadium oxide and up to 600 F g−1 per mass of the composite electrode, significantly exceeding specific capacitance of commercially used activated carbons (100–150 F g−1). In view of identifying an efficient and effective methodology to prepare 3D V2O5/CNT composite electrode materials for enhanced supercapacitors, Wu et al. synthesized V2O5/MWCNT core/ shell hybrid aerogels with different MWCNT contents via a facile mixed growth and selfassembly methodology [614]. V2O5 coated MWCNT raised from the in situ growth of V2O5 on the surface of acid-treated MWCNT, incorporated with V2O5 nanofibers from the preferred orientation growth of V2O5 in a one-step sol-gel process. These two kinds of 1 D fibers selfassemble into a 3D monolithic porous hybrid aerogel. Owing to its high specific surface area, favorable electrical conductivity and unique 3D core-shell structures, the lightweight hybrid aerogel (∼30 mg cm−3) exhibited excellent specific capacitance (625 F g−1), high energy density (86.8 Wh kg−1) and outstanding cycle performance (>20,000 cycles). The optimal content of MWCNT in hybrid aerogels for highest-performance supercapacitor is 7.6%. Functionalized carbon nanotube has higher surface area, low resistivity and high stability. Using simplified solution-based approach, Saravanakumar et al. developed V2O5/functionalized multiwall carbon nanotube (f-MWCNT) hybrid nanocomposite [615]. The addition of f-MWCNT with V2O5 significantly improved the surface area and conductivity, which led to high energy and power densities. This nanocomposite showed highest specific capacitance up to 410 F g−1 and 280 F g−1 at current densities of 0.5 and 10 A g−1, respectively. Moreover, this nanocomposite provided excellent energy density (57 Wh kg−1), better rate capacity, and a good retention of capacity (86%) up to 600 cycles of charge/discharge. Further a symmetric supercapacitor was fabricated using V2O5/f-MWCNT nanocomposite as electrodes. It showed a specific capacitance of 64 F g−1 at a current density of 0.5 A g−1. As mentioned earlier, electrodeposition and atomic layer deposition techniques have been used for the deposition of a thin layer of V2O5 on CNTs. However, the high cost of ALD and the corrosive nature of the electrolyte deposition have limited their practical applications. To overcome these defects, Shakir et al. developed a layer-by-layer assembly (LBL) technique in which a graphene layer was alternatively inserted between MWCNT films coated with ultrathin





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(3 nm) V2O5 [616]. The insertion of a conductive spacer of graphene between the MWCNT films coated with V2O5 not only prevents agglomeration between the MWCNT films but also substantially enhances the specific capacitance by 67%, to as high as 2590 F g−1. Furthermore, the LBL assembled multilayer supercapacitor electrodes exhibited an excellent cycling performance of >97% capacitance retention over 5000 cycles and a high energy density of 96 Wh kg−1 at a power density of 800 W kg−1. By a supercritical fluid CO2 adsorption-calcination method, Do et al. fabricated electrochemical capacitor electrodes by depositing an ultra-thin layer of vanadium oxide on highly conducting, large specific surface area (SSA) substrates [617]. The high SSA materials included binder free single walled carbon nanotube activated carbon (SWCNT-AC) composites and the traditional electrodes of activated carbon-carbon black-polymer binder (ACCB-binder). The uptake of the organometallic precursor for the oxide (vanadium (III) acetylacetonate) on the substrates was investigated and related to their specific surface area. Precursor uptake of up to 54.7 wt.% of the initial carbon substrate was achieved. Calcination conditions for converting the precursor to oxide and electrochemical properties of the electrodes were thoroughly investigated. The V2O5 greatly enhanced the overall electrode performances, which showed extremely high specific pseudocapacitance (>1000 F g−1 at 100 mV s−1). 3.4.8.4.3  Vanadium pentoxide/graphene composites

As a newly emerging carbon material, graphene has become an attractive low-cost alternative for CNTs with characteristics of high specific surface area, high electrical conductivity, and good mechanical properties. Graphene, due to its high conductivity (26,000 S cm−1) and high specific surface area (2630 m2 g−1, theoretical value), provides good electron transfer paths and ensures the direct contact of V2O5 with graphene. Graphene is strictly a 2D singleatom-thick planar sheet of sp2 bonded carbon atoms. Due to the strong sp2 bonds between the carbon atoms, graphene exhibits high mechanical strength in the in-plane direction, which offers the potential to built flexible supercapacitors. Chemically derived graphene, such as graphene oxide and reduced graphene oxide, offer advantages in terms of chemical processability to form various composites with V2O5. By solvothermal treatment and a subsequent annealing process, Li et al. first developed V2O5/reduced graphene oxide (rGO) nanocomposites as electrode materials for supercapacitors [618]. In this method, the reduction of graphene oxide has been achieved in a cost effective and environmentally friendly route followed by the formation of rod-like V2O5 nanocrystals on the surface of rGO. Compared to pure V2O5 microspheres, the V2O5/rGO nanocomposites exhibited a higher specific capacitance of 537 F g−1 at a current density of 1 A g−1 in neutral aqueous electrolytes, a higher energy density of 74.58 Wh kg−1 at a power density of 500 W kg−1, and better stability even after 1000 charge/discharge cycles. Balkus et al., [619, 620] developed V2O5 nanowires (VNW)-graphene composite flexible paper electrodes without using binders. The composite electrode showed balanced EDL and pseudocapacitance as well as an energy density of 38.8 Wh kg−1 at a power density of 455 W kg−1. The maximum power density of 3.0 kW kg−1 was delivered at a constant current discharge rate of 5.5 A g−1. The device prepared using VNW-rGO anode showed a specific capacitance of 80 F g−1. They have developed an asymmetric supercapacitor with MnO2 nanorods (MNR) on rGO electrodes and V2O5 nanowire on rGO electrodes as anodes. In addition, the VNWrGO anode and MNR-rGO cathode were used to form a novel hybrid supercapacitor. The



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hybrid supercapacitor exhibited excellent electrochemical performance manifesting the synergistic effect of combining the MNR-rGO electrode and VNW-rGO electrode. This novel hybrid supercapacitor delivered an energy density of 15 Wh kg−1 with a specific capacitance of 36.9 F g−1. All-solid-state thin-film supercapacitors (ASSTFSs) have attracted tremendous attention, due to their high flexibility and safety. Bao et al. developed a nanocomposite electrode combined with pseudocapacitive vanadium pentoxide and highly conductive graphene with ultrathin thickness for the application in ASSTFSs [621]. The novel structure of the nanocomposite could integrate the merits of each component with high conductivity and ultrathiness, by enhancing the electron transfer, shortening the ion diffusion paths with increased electrode–electrolyte contact in ASSTFSs, leading to high electrochemical performance. The freshly fabricated ASSTFS achieved a high areal capacitance of 11,718 mF cm−2, a remarkable energy density of 1.13 mWh cm−2 at a power density of 10.0 mW cm−2 and long term cycling stability for 2000 cycles, demonstrating the superior electrochemical performance and rendering it a promising candidate for portable electronics. Through electrospinning, Thangappan et al. prepared graphene oxide/V2O5 nanofibers [622], and observed a better capacitive behavior with better reversible charging/discharging ability and higher capacitance compared to pristine V2O5 electrodes. As superior electrochemical performance is expected from 2D heterostructures, Nagaraju et al. synthesized electrode materials based on 2D heterostructures of V2O5 nanosheets (V2O5 NS) and rGO electrodes for asymmetric supercapacitor [623]. The 2D V2O5 and rGO/V2O5 NS electrodes showed a specific capacitance of 253 and 635 F g−1, respectively at a current density of 1 A g−1. The capacitance of the heterostructures is almost 2.5 times higher than the 2D V2O5 nanosheets. The corresponding energy density of 39 Wh kg−1 and 79.5 Wh kg−1 were achieved for the two electrodes at a power density of 900 W kg−1 in an asymmetric supercapacitor configuration. As the fabricated freestanding electrode was flexible and demonstrated good mechanical properties, Foo et al. developed V2O5/rGO freestanding electrodes through a facile and low temperature synthetic route, which eliminated the need for current collectors and reduced resistance [624]. The effective exfoliation of rGO allows improved electrolyte ions interaction, achieving high areal capacitance (511.7 mFcm−2) coupled with high mass loadings. The asymmetric flexible device fabricated based on rGO/V2O5-rGO (VGO) consisted of approximately 20 mg of active mass and still could deliver a low equivalent series resistance (ESR) of 3.36 Ω with excellent cycling stability. Wu et al. fabricated a V2O5/graphene hybrid aerogel at ambient pressure through a simple sol-gel method by using commercial V2O5 powder [625]. The V2O5/graphene hybrid aerogel was synthesized through the in situ growth of V2O5 nanofibers on graphene sheets, and the supercapacitors based on V2O5/graphene hybrid aerogel exhibited enhanced specific capacitance (486 F g−1), high energy density (68 Wh kg−1), and outstanding cycle performance. Using a single-step, low-temperature hydrothermal method, Lee et al. synthesized graphene-decorated V2O5 nanobelts (GVNBs) [626]. V2O5 nanobelts (VNBs) were formed on the surface of graphene oxide, a mild oxidant, which also enhanced the conductivity of GVNBs. Electron energy loss spectroscopy (EELS) analysis revealed that rGO was inserted into the layered crystal structure of V2O5 nanobelts, confirming the enhanced conductivity of the nanobelts. The electrochemical energy-storage capacity of GVNBs was investigated for supercapacitor applications. GVNBs having V2O5-rich composite showed superior specific capacitance compared to other composites and pure materials. Moreover, these





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composites showed excellent cyclic stability and capacitance retention of ∼82% even after 5000 cycles. A novel nanohybrid material composed of V2O5 nanofibers (VNFs) and exfoliated graphene were prepared by in situ growth of VNFs onto graphene nanosheets, and explicated as electrode material for supercapacitor applications [627]. The existence of noncovalent interactions between VNFs and graphene surfaces, was confirmed by Raman and Fourier transform infrared (FTIR) spectroscopies. Morphological analysis of the nanohybrid revealed that the VNF layer was uniformly grown on the graphene surfaces, producing high specific surface area and good electronic or ionic conducing path. Compared to pristine VNF, the VNF/graphene nanohybrid exhibited higher specific capacitance of 218 F g−1 at current density of 1 A g−1, higher energy density of 22 Wh kg−1 and power density of 3594 W kg−1. Asymmetric supercapacitor devices were prepared by the Spectracarb 2225 activated carbon cloth and VNF/graphene nanohybrid as positive and negative electrode, respectively. The asymmetric device exhibited capacitance of 279 F g−1 at 1 A g−1, energy density of 37.2 Wh kg−1 and power density of 3743 W kg−1, which were comparable and superior to reported asymmetric devices consisting of carbon material and metal oxide as electrode components. In order to shorten the synthesis time and improve the capacitance properties, Geng et al. synthesized V2O5.nH2O/graphene composites by self-assembly in a graphite oxide solution under hydrothermal condition [628]. They found that the V2O5.1.6H2O nanobelts were in width of 90 nm and length of 1.5 mm. The hybrid capacitance of the composite reached 579 F g−1 at current density of 1 A g−1 and decreased 21% after 5000 cycles at 4 A g−1. Compared with other conventional methods, microwave reduction has the advantages such as fastness, low cost, energy efficiency and homogeneous heating. By a microwave assisted facile route, Ramadoss et al. synthesized reduced graphene oxide/vanadium pentoxide (GV) composites and reported the application of this material for supercapacitor application [629]. The as- fabricated GV composite electrodes displayed outstanding electrochemical performance with a maximum specific capacitance of 250 F g−1 at 5 mV s−1 and excellent cycling stability, with the retention of 95% of the initial capacitance, even after 5000 consecutive cycles. As asymmetric supercapacitors gain great attention owing to their significantly increased energy density, Li et al. achieved in situ growth of V2O5 nanorods on highly conductive graphene sheets and employed as anode materials for asymmetric supercapacitors which exhibited high specific capacitance and excellent rate capability, attributable to the intimate contact between the nanorods and graphene sheets [630]. An asymmetric supercapacitor fabricated based on graphene/V2O5composites and activated carbon exhibited a high energy density of ∼50 Wh kg−1 at a power density of 136.4 W kg−1, with notable cycling stability. 3.4.8.5 Vanadium pentoxide/conducting polymer composites As pseudocapacitive materials, an increasing number of researchers examined the conducting polymer-based composites for their application in supercapacitors. Pentoxide/ poly(3,4-ethylenedioxythiophene) (PEDOT) has the advantages of simple structure, small energy gap, and high electrical conductivity, which is widely used in energy storage materials [631–633]. Mai et al. designed the heterostructured nanomaterial with PEDOT as the shell and MnO2 nanoparticles as the protuberance and synthesized the novel cucumber-like MnO2 nanoparticles enriched V2O5/PEDOT coaxial nanowires [631]. This heterostructured nanomaterial exhibited enhanced electrochemical cycling performance with the reduced capacity



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fading during 200 cycles from 0.557% to 0.173% over V2O5 nanowires at the current density of 100 mA g−1. Subsequently, Reddy et al. prepared poly(3,4-ethylenedioxypyrrole) (PEDOP)/ V2O5 nanobelt hybrid films. The V2O5 nanobelts with a monoclinic structure were grown by a hydrothermal route and a PEDOT layer was coated onto V2O5 nanobelts by electropolymerization to form PEDOT/V2O5 hybrid [632]. The synergistic effects of PEDOP (high electrical conductivity) and V2O5 nanobelts (large surface area) and their ability to store/release charge through reversible Faradaic reactions were reflected in a high specific capacitance of 224 F g−1 delivered by the hybrid, higher by 83% and 69% relative to pristine V2O5 and PEDOP. The hybrid showed an energy density of 223 Wh kg−1 at a power density of 3.8 kW kg−1, and an acceptable cycling performance with 90% capacitance retention after 5000 cycles. Later, Guo et al. postulated a tandem redox reaction strategy for forming layered V2O5 (LVO), conducting polymerpoly(3,4-ethylenedioxythiophene) (PEDOT), and layered MnO2 (LMO) into a sandwich structured LVO/PEDOT/LMO [633]. Asymmetric supercapacitors fabricated by using LVO/PEDOT/LMO cathode and active carbon (AC) anode (LVO/ PEDOT/LMO//AC) using Na2SO4 aqueous electrolyte showed an energy density of 39.2 Wh kg−1 (based on active materials), which was among the highest reported for supercapacitors with neutral aqueous electrolytes. The LVO/PEDOT/LMO//AC supercapacitors also offered high rate capability (21.7 Wh kg−1 at 2.2 kW kg−1) and good cycle stability (93.5% capacitance retention after 3000 cycles). PPy is an ideal electrode material for supercapacitor, which has high electrical conductivity, good environmental stability, reversible electrochemical redox properties and high charge storage capacity [634–638]. Bai et al. used electrochemical co-deposition to form V2O5PPy composite, during which 1D growth of polypyrrole (PPy) was directed [634]. Due to the organic-inorganic synergistic effect, V2O5-PPy composite exhibited good charge-storage properties in a large potential window from −1.4 to +0.6 V versus SCE, with a specific capacitance of 412 F g−1 at 4.5 mA cm−2. A model supercapacitor assembled by using the V2O5-PPy composite as electrode materials displayed a high operating voltage of 2 V and a high energy density of 82 Wh kg−1 (at the power density of 800 W kg−1). It is well known that the continuous 3D network can create channels for better ion transport, and the high degree of pore connectivity in the network enhance the mass transport. Qian et al. synthesized 3D V2O5 network with PPy uniformly decorated onto each nanowire to enhance their pseudocapacitive performance [635]. The PPy shell could enhance the electric conductivity and prevent the dissolution of vanadium. These merits together with the ideal synergy between V2O5 and PPy lead to a high specific capacitance of 448 F g−1, which was 3 times higher than that of the stacked V2O5. The all-solid-state symmetric supercapacitor device assembled by the V2O5/PPy coreshell 3D network exhibited a high energy density (14.2 Wh kg−1) at a power density of 250 W kg−1 and good cyclic stability (capacitance retention of 81% after 1000 cycles). In order to study the optimal ratio of PPy to V2O5, Sun et al. conducted a number of controlled trials and found that the optimal ratio of PPy to V2O5 to be 40% where the sum of the polymer’s electric double-layer (EDL) capacitance and the layered oxide’s subsurface Faradic capacitance could be maximized [636]. By a combined hydrothermal, freeze-drying and nanocasting process, Cao et al. developed a new 3D V2O5/ PPy network, which was built from numerous ultrathin, flexible, and single-crystalline nanoribbons [637]. The unique network can not only provide a high surface area for enhancement of electrolyte/electrode interactions, and reduce the diffusion length of ions, but also





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efficiently maintain high electrical conductivity. As a result, this network exhibited high capacitance, excellent rate capability and good charge discharge stability for energy storage. An asymmetric supercapacitor based on a 3D V2O5/PPy network as the cathode material further delivered high energy density and high-power density. The addition of surfactant improved both the conductivity and yield of the polymer. Qu et al. developed a core-shell structure of PPy grown on V2O5 nanoribbons fabricated using SDB as surfactant [638]. Benefiting from the nanoribbon morphology of V2O5, the improved charge-transfer and polymeric coating effect of PPy, PPy/V2O5 nanocomposite exhibited high energy density, excellent cycling behavior, and good rate capability. Compared with the PEDOT and PPy, only a few reports are found on V2O5/polyaniline composites in the literature. Bai et al. used an electrocodeposition method to synthesize a high performance negative electrode composed of a V2O5 and polyaniline (PANI) composite [639]. Scanning electron microscopy revealed that the composite film was composed of 1D polymer chains. Significantly, the V2O5-PANI composite nanowires exhibited a wide potential window of 1.6 V (between 0.9 and 0.7 V vs. SCE) and a maximum specific capacitance of 443 F g−1 (664.5 mF cm−2). The flexible symmetric supercapacitor assembled with this composite film yielded a maximum energy density of 69.2 Wh kg−1 at a power density of 720 W kg−1, and a maximum power density of 7200 W kg−1 at an energy density of 33.0 Wh kg−1. These values were substantially higher than those of other pure V2O5 or PANI based supercapacitors. Moreover, the assembled symmetric supercapacitor device showed an excellent stability with 92% capacitance retention after 5000 cycles. Polyindole (PIn) is an intrinsically conducting polymer as a pseudocapacitor material because of its superior conductivity, excellent thermal stability, electrochemical reversibility, and storage ability. Zhou et al. fabricated a bamboo-like nanomaterial composed of V2O5/polyindole (V2O5/PIn) decorated onto the activated carbon cloth supercapacitors [640]. The PIn could effectively enhanced the electronic conductivity and prevent the dissolution of vanadium and the activation of carbon cloth with functional groups was conductive to anchoring the V2O5 and improving surface area, which resulted in an enhancement of electrochemical performance and lead to a high specific capacitance of 535.5 F g−1. Moreover, an asymmetric flexible supercapacitor based on V2O5/PIn on activate carbon cloth and reduced graphene oxide on activate carbon cloth exhibited a high energy density (38.7 Wh kg−1) at a power density of 900 W kg−1 and good cyclic stability (capacitance retention of 91.1% after 5000 cycles). 3.4.8.6 Vanadium dioxide Although vanadium pentoxide is one of the most widely used vanadium oxides in the field of electrochemistry, vanadium dioxide is also explored by some researchers. Research on vanadium dioxide showed that it also had great potential in the field of electrochemistry. As an important functional vanadium oxide, vanadium dioxide possesses excellent physical and chemical properties. VO2 has been found to show better performance compared to the well known V2O5. This is due to its higher electronic conductivity arising from a mixedvalence V3+/5+ and structural stability arising from increased edge sharing and the consequent resistance to lattice shearing during cycling. Nevertheless, VO2 do not usually deliver higher specific capacitance behavior than other transition metal oxides (e.g., Co3O4, NiO, MnO2) because of its poor electrical conductivity and the poor structural stability, resulting in limited long-term cycling stability. Shao et al. prepared homogenous hexangular star fruit-like



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vanadium dioxide for the first time by a one-step hydrothermal method [641]. The assembly process of hexangular star fruit-like structure was observed from SEM images (Fig. 3.29A). It exhibited a high-power capability (19 Wh kg−1 at the specific power of 3.4 kW kg−1) and good cycling stability for supercapacitors application. Similar V2O5, a strategy for improving the electrochemical property of VO2 as the supercapacitors material by designing the material structure is necessary. Pan et al. reported a strategy for decreasing the VO2 resistance by nearly 3 orders of magnitude through H2 treatment, which improved its conductivity for supercapacitor application [642]. A specific discharge capacitance of 300 F g−1 and a specific energy density of 17 Wh kg−1 at a rate of 1 A g−1 were demonstrated with long-term cycling stability, which was 4 times higher than the untreated samples. The established method of improving the conductivity of VO2 is to prepare the carbon-based composite material. Liang et al. synthesized a coaxial-structured hybrid material of VO2 and MWCNT by using a facile sol-gel method assisted with freeze-drying process [643]. A few layers of VO2 sheath are firmly coated on the CNTs surfaces (Fig. 3.29B), resulting in the formation of network

FIGURE 3.29  (A) SEM images of hexangular starfruit-like vanadium dioxide; (B) VO2(B)/CNTs composite; (C) 3D graphene/VO2 nanobelt composite hydrogel; (D) VO2/HMB core-shell arrays on GF. Source: Part A, Reprinted with permission from Ref. [641], Copyright 2012, Elsevier; Part B, reprinted with permission from Ref. [643], Copyright 2013, Elsevier; Part C, reprinted with permission from Ref. [645], Copyright 2013, Elsevier; Part D, reprinted with permission from Ref. [648], from The Royal Society of Chemistry.





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morphology with abundant pores and good electric conductivity. This VO2/CNTs composite for the first time was employed as supercapacitor electrode material, demonstrating better specific capacitance and superior rate capability than the individual components. A specific capacitance of 250 F g−1 was obtained in 1 M Na2SO4 solution at a current density of 0.5 A g−1, with a capacitance retention up to 71% when the current density was increased to 10 A g−1. By one-step simultaneous hydrothermal reduction technology, Deng et al. prepared graphene/ VO2 (RG/VO2) hybrid materials with different RG amounts in a mixture of ammonium vanadate, formic acid, and graphite oxide (GO) nanosheets [644]. The hydrothermal treatment performed the reduction of GO into RG and the formation of VO2 particles with star fruit morphology (Fig. 3.29C). The star fruit-like VO2 particles were uniformly embedded in the hole constructed by RG nanosheets, which made the electrode–electrolyte contact better. A high specific capacitance of 225 F g−1 had been achieved for RG(1.0)/VO2 electrode with RG content of 26 wt.% in 0.5 M K2SO4 electrolyte. An asymmetrical electrochemical capacitor was assembled by using RG(1.0)/VO2 as positive electrode and RG as negative electrode, and it can be reversibly charged-discharged at a cell voltage of 1.7 V in 0.5 M K2SO4 electrolyte. The asymmetrical capacitor delivered an energy density of 22.8 Wh kg−1 at a power density of 425 W kg−1. Moreover, the asymmetrical capacitor preserved 81% of its initial capacitance over 1000 cycles at a current density of 5 A g−1. Using commercial V2O5 and graphene oxide as precursors, Wang et al. developed a facile one-step strategy to prepare 3D graphene/VO2 nanobelt composite hydrogels, which can be readily scaled-up for mass production [645]. In the two-electrode configuration, the graphene/VO2 nanobelt composite hydrogel exhibited a specific capacitance of 426 F g−1 at 1 A g−1 in the potential range of 0.6 to 0.6 V, which greatly surpassed that of each individual counterparts (119 F g−1 and 243 F g−1 at 1 A g−1 for VO2 nanobelt and graphene hydrogel, respectively). Without using any toxic organic solvent, Xiao et al. adopted a hydrothermal method assisted with freeze drying process to form composite supercapacitor materials of metastable vanadium dioxide nanobelts on reduced graphene oxide (RG) layers [646]. The initial specific capacitance of the composites reached 290.4 F g−1 at 0.2 A g−1 and maintained 82.3% of the initial value after 1000 cycles at 2 A g−1, 37.9% higher than the pure VO2. To examine the effect of graphene lateral size on the electrochemical performance of a hybrid supercapacitor composed of VO2/GO electrodes, Lee et al. developed a flexible hybrid supercapacitor electrode composed of UGO (ultra large graphene oxide) sheets (average lateral size of 47 ± 22 mm) and VO2 nanobelts [647]. Thermal treatment converts UGO/VO2 to URGO/VO2 (denoted VURGO). The VURGO hybrid electrode showed a specific capacitance of 769 F g−1. Recently, Xia et al. reported that hydrogen molybdenum bronze (HMB) was electrochemically deposited as a homogeneous shell on VO2 nanoflakes grown on graphene foam (GF), forming a GF + VO2/HMB integrated electrode structure (Fig. 3.29D) [648]. Asymmetric supercapacitors based on the GF + VO2/HMB cathode in neutral electrolyte are assembled and exhibited enhanced performance with weaker polarization, higher specific capacitance and better cycling life than the unmodified GF + VO2 electrode. Capacitances of 485 F g−1 (2 A g−1) and 306 F g−1 (32 A g−1) were obtained because of the exceptional 3D porous architecture and conductive network. Various mixed oxides composites can increase the capacitance of active electrode materials. Electrostatic spray deposition can prepare ideal film material. Hu et al. synthesized VO2/ TiO2 nanosponges via electrostatic spray deposition with easily tailored nanoarchitectures



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and composition as binder-free electrodes for supercapacitors [649]. Benefiting from the unique interconnected pore network of the VO2/TiO2 electrodes and the synergistic effect of high capacity VO2 and stable TiO2, the as-formed binder-free VO2/TiO2 electrode exhibited a high capacity of 86.2 mF cm−2 (548 F g−1) and satisfactory cyclability with 84.3% retention after 1000 cycles. 3.4.8.7 Vanadium trioxide V2O3 possesses a 3D V-V framework and its V 3d electrons can itinerate along the V-V chains, leading to metallic behavior. The tunneled structures exist in V2O3 facilitates ion intercalation/deintercalation. Therefore, V2O3 is especially suitable as an electrode material in supercapacitors. However, V2O3 have relatively low electric conductivity compared to RuO2, which decreases the charge transfer rate during charging/discharging process and limits their specific capacitance as electrode materials of supercapacitors. Interestingly, there are only limited reports concerning the synthesis of V3+ based nanostructured materials and their functional applications. It is relatively hard to prepare pure V2O3 nanomaterials, to a certain extent due to their sensitivity to temperature and atmosphere. Liu et al. prepared a novel 3D hierarchical flower-like vanadium sesquioxide (V2O3) nano/microarchitecture (Fig. 3.30A) consisting of numerous nanoflakes via a solvothermal approach followed by an appropriate heating treatment [650]. When used as the cathode material of pseudocapacitors in Li2SO4, the flower-like oxide displayed a very high initial capacitance of 218 F g−1 at a current density of 0.05 A g−1. On this basis, Li et al. developed a micelle-anchoring method for the in situ synthesis of V2O3 nanoflakes-C coreshell composites (Fig. 3.30B) as electrode materials in supercapacitors [651]. Hexadecyltrimethylammonium bromide (CTAB) micelles assembled to solubilize activated carbon and anchor vanadate ions of the precursor, NH4VO3, onto the carbon surface. During drying and calcination, CTAB and NH4VO3 decompose to produce V2O5, which were carbon-thermally reduced to V2O3 in situ. In the as-obtained composites, monodisperse V2O3 nanoflakes stand edge-on in the carbon surface, forming a carbon core with a shell layer of edge-on standing V2O3 nanoflakes. Because of the increased electric conductivity and high specific surface area, V2O3 nanoflakes-C composites exhibited a specific capacitance of 205 F g−1 at 0.05 A g−1 over a potential range of 0.4–0.6 V, which surpassed those of individual counterparts (67 F g−1 and 159 F g−1 at 0.05 A g−1 for activated carbon and bulk V2O3, respectively). FIGURE 3.30  (A) SEM image of flowerlike V2O3 nano/ micromaterial; (B) FESEM images of V2O3 nanoflakes-C composites. Source: Part A, Reprinted with permission from Ref. [650], Copyright 2010, Wiley; Part B, reprinted with permission from Ref. [651] from The Royal Society of Chemistry.





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3.4.8.8  Mixed valence vanadium oxide and its composite The multiple stable oxidation states (III–V) of vanadium in its oxides and typical layered structures enable VOx to have a higher charge storage capability than most of other inexpensive transition-metal oxides. Mixed-valence vanadium oxides are promising electrode materials for supercapacitors, as they have multiple oxidation states (V2+, V3+, V4+, V5+) available for charge storage in a wide range of potential windows. However, the poor electron transport in VOx and the poor cycle stability of VOx both hinder its applications. Core-shell nanocomposite materials have been investigated as electrodes for supercapacitors. Pan et al. dispersed quasi-metallic V2O3 nanocores on graphene sheets for electrical connection of the whole structure, while a naturally formed amorphous VO2 and V2O5 generally termed as called as VOx in this study, thin shell around V2O3 nanocore acts as the active pseudocapacitive material [652]. This high rate was attributed to the largely enhanced conductivity of this unique structure and a possibly facile redox mechanism. Such an EC can provide 1000 kW kg−1 power density at an energy density of 10 Wh kg−1. The synthesis of VOx nH2O using anodic deposition from aqueous VOSO4 solutions was considered as an effective method [653, 654]. Hu et al. used anodic deposition to develop a new type vanadium oxide (VOx nH2O) with porous, 3D network architecture plated at 0.7 V (vs. Ag/AgCl) from 25 mM VOSO4 with 5 mM H2O2 showed capacitive-like behavior at 250 mV s−1 and CS ∼167 F g−1 at 25 mV s−1 in 3 M KCl for pseudocapacitor applications [652]. On the basis of Hu’s work, Huang et al. reported that a 3D porous vanadium oxide was anodically deposited onto graphite substrates denoted as VOx nH2O/G [653]. Through annealing at temperatures up to 350°C, the thermal stability of VOx nH2O preserved its porous morphology and excellent capacitive performances in 3 MKCl at pH of 2.4. A maximal specific capacitance of ca. 150–160 F g−1 measured at 250 mV s−1 was obtained for this porous VOx nH2O annealed between 150 and 250°C. Only 9%–17% loss in specific capacitance was found for the VOx nH2O when the scan rate of the cyclic voltammetry was increased from 25 to 250 mV s−1, demonstrating a typical high power property. The researchers explored several other methods for the preparation of VOx nH2O. Li et al. synthesized VOx nH2O with long cycle life for Li-ion supercapacitors successfully by means of the microwave-assisted hydrothermal synthesis method, which was a faster and energysaving method than the conventional hydrothermal synthesis [655]. Without using any surfactant or capping agent, Cheng et al. prepared 3D interconnected porous vanadium oxide network VOx nH2O via a facile and effective method through controlling solution polarity at room temperature [656]. The experimental results indicated that the microstructure of the as-prepared samples can be effectively tailored by the polarity of reaction solution and the obtained 3D interconnected network delivered a high specific capacitance of 280 F g−1. Lai’s group carried out a systematic study of V6O13 [657, 658]. As shown in Fig. 3.31A, they used thermal decomposing and quenching method to prepare V6O13 sheet with morphology and mixed valence of V(+5) and V(+4) [657]. The products exhibited sheet morphology with an average size of 2 mm and a thickness of ∼200 nm. In the voltage range of −0.2 to +0.8 V (vs. SCE) in 1 M NaNO3 electrolyte, V6O13 electrode exhibited obvious capacitance performance. At the current density of 50 mAg−1, the material delivered a specific capacitance value of 285 F g−1. At the current of 200 mA g−1, the electrode exhibited an initial specific capacitance of 215 and 208 F g−1 after 300 cycles, as shown in Fig. 3.31B. Subsequently, they synthesized hollow flowers-like V6O13 with an average size of 3 mm through a facile solvothermal route in a short 

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FIGURE 3.31  (A) SEM image of V6O13 sheet; (B) Cycle tests of V6O13 electrode at 200 mA g−1 in the potential range of −0.2 to +0.8 V (vs. SCE) in 1 M NaNO3 electrolyte. (a) Cycle number versus specific capacitance and; (b) cycle number versus charge-discharge efficiency. Source: Reprinted from Ref. [657], with permission from Elsevier.

time [658]. Experimental results indicated that hollow flowers-like V6O13 can deliver a capacitance of 417 F g−1 at scan rate of 5 mV s−1. Different from V6O13 and VO2, Li et al. synthesized novel hierarchical vanadium oxide microspheres that contain V6O13 and VO2 forming from hyperbranched growth of nanoribbons via a solvothermal method [659]. The as-prepared hierarchical microspheres have a diameter of 5 mm, in which 400 nm long nanoribbons grow as hyperbranches on the clustering backbones of the nanobelts. These hierarchical microspheres contain 86.2 mass% V6O13 with metallic conductivity and 13.8 mass% VO2. The hierarchical microspheres exhibited a specific capacitance as high as 456 F g−1, with a corresponding volumetric specific capacitance of 3.09 F cm−3, at 0.6 A g−1 in the potential range of 0–1.2 V. The maximal energy density and power density achieved were up to 22.8 W h kg−1 (0.16 mWh cm−3) and 1.2 kW kg−1 (8.14 mW cm−3). To further explore the effect of the valence state of vanadium on the electrochemical performance of VOx by tuning the valence state of vanadium, Yu et al. reported an innovative and effective method to significantly boost the durability and capacitance of VOx [660]. The valence state of vanadium was optimized through a highly facile electrochemical oxidation method. A superior electrochemical performance and an outstanding cyclic stability up to 100,000 cycles were obtained for these electrodes. Zhai et al. first demonstrated the sulfur-doped, oxygen-deficient V6O13−x as an anode electrode for ASCs [661]. This new electrode achieved a benchmark capacitance of 1353 F g−1 (0.72 F cm−2) at a current density of 1.9 A g−1 (1 mA cm−2) in 5 M LiCl solution. The availability of multiple oxidation states in the V6O13−x electrode led to the significant pseudocapacitance in the negative potential window 0 to −1 V versus SCE. Chemical vapor deposition (CVD) is a chemical process that is usually used to produce high quality, high-performance, nanomaterials. Through a CVD technique, Jampani et al. reported the supercapacitance behavior of titanium doped vanadium oxide films grown on vertically aligned carbon nanotubes (VOx:Ti-VACNT) [662]. The capacitance of CVD derived





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titanium doped vanadium oxide-carbon nanotube composites was measured at different scan rates to evaluate the charge storage behavior. In addition, the electrochemical characteristics of the titanium doped vanadium oxide thin films synthesized by the CVD process were compared to substantiate the propitious effect of the carbon nanotubes on the capacitance of the doped vanadium oxide. Considering the overall materials loading with good rate capability and excellent charge retention up to 400 cycles, it can be noted that attractive capacitance values as high as 310 F g−1 were reported. Zhao et al. synthesized hierarchical, micro/nanostructured porous VOx/C composites via a one-step method using phenolic resin as carbon precursor and ammonium metavanadate as source of vanadium oxides [663]. They found that vanadium oxides greatly enhanced the electrochemical performance of the materials, due to the emergence of Faradic capacitance from vanadium oxide nanoparticles. A maximum specific capacitance of 171 F g−1 was obtained from VOx/C composite with vanadium loading of 44 wt.%. Through the hydrothermal process, Fu et al. synthesized graphene/vanadium oxide nanotubes (VOx-NTs) composite in which acetone as solvent and 1-hexadecylamine (HDA) as structure-directing template were used [664]. The composite with VOx-NTs amount of 69.0 wt.% delivered a specific capacitance of 210 F g−1 at a current density of 1 A g−1 in 1 M Na2SO4 aqueous solution, which is nearly twice as that of pristine graphene (128 F g−1) or VOx-NTs (127 F g−1), and exhibited a good performance rate. Hu et al. prepared graphene/vanadium oxide (RG/VOx nH2O) hybrid electrodes with varied amounts of graphene (RG), through a simultaneous one-step hydrothermal-reduction using a suspension of NH4VO3, NH2CSNH2 and graphite oxide (GO) nanosheets [665]. RG/VOx nH2O electrode with 10 wt.% RG content exhibited high specific capacitance of 384 F g−1 at a scan rate of 5 mV s−1 in 0.5 M K2SO4 electrolyte and relatively good cycle stability at a current density of 5 A g−1 (60% capacitance retention after 1000 cycles). In order to investigate the effects of the graphene content and the treatment temperature on the supercapacitive properties of VOx/graphene nanocomposites, Li et al. prepared the vanadium oxides (VOx)/graphene hybrid materials constructed from 2D graphene nanosheets (GNS) and VOx through a simple two-step procedure including solvothermal method and subsequent thermal treatment [666]. The electrochemical properties of as-prepared composites are systematically investigated by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy, which are highly dependent on the content of GNS in composite and the annealing temperature. Further, the VOx-7.4% GO-300 composite electrode exhibited the largest specific capacitance and excellent rate capability among these composites. Wang et al. found that the electrochemical instability of V3O7 was mainly due to the chemical dissolution in aqueous electrolyte and the structure pulverization during charging/discharging cycling [667]. They had demonstrated a novel strategy to address these limitations by replacing the aqueous electrolyte with a neutral pH LiCl/PVA gel electrolyte. The vanadium oxide nanowire pseudocapacitors with gel electrolyte achieved a maximum areal capacitance of 236 mF cm−2 at a current density of 0.2 mA cm−2 and an excellent capacitance retention rate of more than 85% after 5000 cycles. As a kind of chemical deposition methods, electroless deposition is also an effective method for the preparation of nanomaterials. Wu et al. synthesized the vanadium oxide, which consisted of a mixture of amorphous V2O5 and VO2 as a thin film via a simple electroless deposition method [668]. Electrochemical characterizations of the synthesized vanadium oxide showed ideal capacitive behavior with good cycle life.



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3.4.8.9 Nitrides Transition metal nitrides are of particular interest by virtue of their synergic advantages of superior electrical conductivity, excellent environmental durability and high reaction selectivity. VN and TiN are considered as the promising pseudocapacitive candidates for nextgeneration high performance SCs owing to their excellent electrical conductivity and high specific pseudocapacitance [669–672]. However, the capacitance would significantly degrade at high voltage scanning rates/current densities. 3.4.8.9.1  Vanadium nitride

Choi et al. first used a low-temperature route based on a two-step ammonolysis reaction of VCl4 in anhydrous chloroform to synthesize nanocrystalline VN [673, 674]. The nanometersized crystals increase the susceptibility for surface oxidation, while the high surface area of the nitrides provides more redox-reaction sites. The specific capacitance improved with reduced material loading, and the highest specific capacitance of 1340 F g−1 recorded at a scan rate of 2 mV s−1, that decreased to 554 F g−1 while tested at 100 mV s−1, exhibiting a logarithmic trend. In addition, an impressive specific capacitance of 190 F g−1 is obtained at a very high scan rate of 2 V s−1. The semi-logarithmic behavior of the specific capacitance versus scan rate was characteristic of all electrochemical capacitors derived from highly porous powders. The VN obtained at 1000°C, however, exhibited a capacitance of only 58.3 F g−1 at a scan rate of 2 mV s−1. In order to further evaluate the structural characteristics and electrochemical properties of vanadium nitride, which was prepared by temperature-programmed ammonia reduction of V2O5 powder, Glushenkov et al. synthesized vanadium nitride and carried out a systematic performance test [675]. The large volume of pores in VN is represented by the range of 15– 110 nm (Fig. 3.32A). The material has an acceptable rate capability in all electrolytes, showing about 80% of its maximal capacitance at a current load of 1 A g−1 in galvanostatic charging/ discharging experiments. The capacitance of 186 F g−1 was observed in 1 M KOH electrolyte at 1 A g−1. By calcining V2O5 xerogel in a furnace under an anhydrous NH3 atmosphere at 400°C, Zhou et al. synthesized vanadium nitride (VN) powder (Fig. 3.32B). SEM images showed the homogeneous surface of the VN [676]. The CV studies illustrated the existence of fast and reversible redox reactions on the surface of VN electrode. The specific capacitance of VN was 161 F g−1 at 30 mV s−1. Further, the specific capacitance remained 70% of the initial value when the scan rate increased from 30 to 300 mV s−1. Lu et al. developed an effective strategy to stabilize VN nanowire anode without sacrificing its electrochemical performance by using LiCl/PVA gel electrolyte [677]. By suppressing the oxidation reaction and structural pulverization, the VN nanowire electrode exhibited remarkable cycling stability in LiCl/PVA gel electrolyte with capacitance retention of 95.3% after 10,000 cycles. The VN nanowire anode attained a high energy density of 0.61 mWh cm−3 at current density of 0.5 mA cm−2 and a high-power density of 0.85 W cm−3 at current density of 5 mA cm−2. VN thin film electrodes have unique advantages compared with other kind of electrodes, because there is no need to add additives to increase the electronic conductivity, nor binder to improve the mechanical stability. Lucio-Porto et al. prepared thin films of VN with different thickness by D.C. reactive magnetron sputtering. Crystalline films with a preferential growth in the direction (111) were obtained [678]. Thin films with a thickness of 25 nm showed the highest specific capacitance





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FIGURE

3.32 (A) porous VN nanorods; (B) SEM image of VN powders; (C) VN/CNT 3D array; (D) VN-MWCNT composite; (E) MVN@NC NWs; (F) mesoporous coaxial titanium nitride- vanadium nitride fibers. Source: Part A, Reprinted with permission from Ref. [675], Copyright 2010, American Chemical Society; Part B, reprinted with permission from Ref. [676], Copyright 2009, Elsevier; Part C, reprinted with permission from Ref. [684], Copyright 2011, American Chemical Society; Part D, reprinted with permission from Ref. [686], Copyright 2014, Elsevier; Part E, reprinted with permission from Ref. [688], Copyright 2015, Wiley; Part F, reprinted with permission from Ref. [695], from American Chemical Society.

(422 F g−1) in 1 M KOH electrolyte. Using a combination of electrostatic spinning and high temperature calcination in ammonia, Xu et al. prepared 1D vanadium nitride nanofibers [679]. The cross-linked nanofibers composed of nanoparticles construct a facile transport path for charge and electrolyte ion. Moreover, VN nanoparticles encapsulated into carbon prevent grain growth and aggregation, which provided more active sites for electrolyte ion. Owing to this unique structure, vanadium nitride nanofibers exhibited high specific capacitance of 291.5 F g−1 at 0.5 A g−1 and rate capability with a capacitance of 105.1 F g−1 at 6 A g−1. Based on Xu’s electrostatic spinning and high temperature calcination in ammonia, Zhao et al. synthesized porous vanadium nitride (VN) hollow fibers by using low-cost starting materials [680]. The VN hollow fibers with their sidewalls consisted of numerous porous nanoparticles, reported to retain their 1D texture. The electrochemical performance of VN hollow nanofibers was investigated, and the specific capacitance was 115 F g−1 at a current density of 1 A g−1 in 2 M KOH electrolyte.



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Xie et al. synthesized mesocrystal nanosheets (MCNSs) of vanadium nitride (VN) via a confined-growth route from thermally stable layered vanadium bronze, representing the first two-dimensional metallic mesocrystal in inorganic compounds. Benefiting from their singlecrystalline-like long-range electronic connectivity, VN MCNSs delivered an electrical conductivity of 1.44 × 105 S m−1 at room temperature, among the highest values observed for 2D nanosheets [681]. Coupled with their unique pseudocapacitance, VN MCNS-based flexible supercapacitors afford a superior volumetric capacitance of 1937 mF cm−3. Nitride MCNSs are promising candidates the energy storage and conversion fields, due to their intrinsic high conductivity coupled with the reactivity of inorganic lattices. Through direct oxidation of metallic vanadium in vacuo, Bondarchuk et al. synthesized pure oxygen-free vanadium nitride films with thickness from 1–400 nm [682]. These films delivered a surface capacitance of 3 mF cm−2 at a potential scan rate of 3 mV s−1 and 2 mF cm−2 at a potential scan rate of 1 V s−1 in basic (1 M KOH, 1 M LiOH) or neutral (Li2SO4, K2SO4) electrolytes while no redox reactions can be assigned to the charge/discharge process. 3.4.8.9.2  Vanadium nitride/compound-carbon material composites

The poor electrochemical stability of VN causes severe capacitance loss during charging/ discharging cycling process. Eventually, this problem can be solved by the formation of nanocomposites, most favorably with carbon from a concurrent synthesis approach that generates and controls all synthetic components at the same time. From the literature, it can be seen that the presence of high purity CNTs in the active electrode could improve the electrochemical properties of VN in supercapacitors. Through the sol-gel synthesis of organic or inorganic vanadium oxide precursors followed by temperature programmed ammonia reduction, Ghimbeu et al. developed nanostructured vanadium nitride/multiwalled carbon nanotubes (VN/ CNTs) composites for pseudocapacitor applications [683]. Nitrogen adsorption and impedance spectroscopy measurements showed that the incorporation of CNTs during VN synthesis allows VN/CNTs nanocomposites to be obtained with higher porosity, narrower pore size distribution, better conductivity and improved electrochemical properties compared to VN without CNTs. In particular, cyclic voltammetry using three-electrode cells in KOH showed that the contribution of the redox peaks was increased when VN is associated with the carbon nanotubes. Consequently, a capacitance increase has been noted in the two-electrode system. Another important advantage of using VN/CNTs composites was their high capacitance retention (58%) at high current density (30 A g−1) compared with VN (7%). Carbon nanotube arrays are attractive as 3D current collector materials for promoting supercapacitiveness. Zhang et al. demonstrated a simple, direct synthetic methodology to achieve 3D arrays of MWCNT/VN nanostructures, consisting of multiwalled carbon nanotubes covered by nanocrystalline vanadium nitride, firmly anchored to glassy carbon or inconel electrodes (Fig. 3.32C) [684]. These nanostructures demonstrated a notable specific capacitance of 289 F g−1, which was achieved in 1M KOH electrolyte at a scan rate of 20 mV s−1. The well-connected highly electrically conductive structures exhibited remarkable rate capability; at a very high scan rate of 1000 mV s−1 there is less than a 20% drop in the capacitance relative to 20 mV s−1. Binder-free films composed of active materials and CNTs lead to flexible hybrid electrodes, which demonstrated the potential for SCs. Xiao et al., fabricated light weight, thin, and flexible freestanding mesoporous VN nanowires (MVNNs)/CNT hybrid electrodes via a simple vacuum-filtering method, which utilized the synergistic effects of high electrochemical





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performance of MVNNs and the high conductivity and mechanical consolidation of the CNTs [685]. High performance all-solid-state flexible SCs were constructed based on freestanding MVNN/CNT hybrid electrodes with H3PO4/poly (vinyl alcohol) (PVA) as electrolyte. The whole device (including electrodes, separator and electrolyte) was only 15 mg, exhibiting a high volume capacitance of 7.9 F cm−3 and energy and power density of 0.54 mWh cm−3 and 0.4 W cm−3 at a current density of 0.025 A cm−3. Zhitomirsky et al. prepared composite materials, containing fibrous VN nanoparticles and multi-walled carbon nanotubes (MWCNT) (Fig. 3.32D) via a chemical method for application in ECs [686]. They demonstrated for the first time that VN-MWCNT electrodes exhibited good capacitive behavior in 0.5 M Na2SO4 electrolyte in a negative voltage window of 0.9 V. Quartz crystal microbalance studies provide an insight into the mechanism of charge storage. Composite VN-MWCNT materials showed significant improvement in capacitance, compared to individual VN and MWCNT materials. Testing results indicate that VN-MWCNT electrodes exhibited high specific capacitance at high mass loadings in the range of 10–30 mg cm−2, good capacitance retention at scan rates in the range of 2–200 mV s−1 and good cycling stability. The highest specific capacitance of 160 mV s−1 was achieved at a scan rate of 2 mV s−1. Increasing the surface areas of the nanomaterials can improve their capacities. Using simple salts as porogens, Fechler et al. reported to obtain composites of highly porous nitrogen-doped carbons with functional vanadium nitride nanoparticles with tunable surface area, pore size, pore volume, and nanoparticle size [687]. Cesium acetate as a porogen at low concentrations yielded microporous materials with small VN nanoparticles with a surface area of around 1000 m2 g−1, while increasing salt amounts promote small mesopores with bigger nanoparticles and surface areas of up to 2400 m2 g−1. By virtue of their potential applications for flexible supercapacitors, the flexible 3D nano-architectures are highly attractive. Gao et al. developed 3D intertwined nitrogen-doped carbon encapsulated mesoporous vanadium nitride nanowires (MVN@NC NWs) (Fig. 3.32E), which are investigated as thin, lightweight, and self-supported electrodes for flexible supercapacitors (SCs) [688]. The MVN NWs with their abundant active sites accessible to charge storage, and the N-doped carbon shell suppressing electrochemical dissolution of the inner MVN NWs in an alkaline electrolyte, led to excellent capacitive properties. The flexible MVN@NC NWs film electrode delivered a high areal capacitance of 282 mF cm−2 and exhibited excellent long-term stability with 91.8% capacitance retention after 12,000 cycles in a KOH electrolyte. All-solidstate flexible SCs assembled by sandwiching two flexible MVN@NC NWs film electrodes with alkaline poly(vinyl alcohol) (PVA), sodium polyacrylate, and KOH gel electrolyte boasted a high volumetric capacitance of 10.9 F cm−3, an energy density of 0.97 mWh cm−3, and a power density of 2.72 W cm−3 at a current density of 0.051 A cm−3 based on the entire cell. Wang et al. prepared 3D porous VN nanowires-graphene composite as a superior anode for high-performance hybrid supercapacitors [689]. The 3D VN-RGO composite exhibited large Li-ion storage capacity and fast charge/discharge rate within a wide working window from 0.01 to 3 V (vs. Li/Li+), which could potentially boost the operating potential and the energy and power densities of Li-ion hybrid capacitors (LIHCs). By employing such 3D VN-RGO composite and porous carbon nanorods with a high surface area of 3343 m2 g−1 as the anode and cathode, respectively, a novel LIHCs has been fabricated with an ultrahigh energy density of 162 Wh kg−1 at 200 W kg−1, and with 64 Wh kg−1 even at a high power density of 10 kW kg−1. Balamurugan et al. fabricated novel vanadium nitride/nitrogen doped graphene (VN/ NG) composite for stable high-performance anode materials for supercapacitors [690]. The



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VN/NG composite anode material exhibited excellent rate capability, good cycling stability, and superior performance. The NG provided a highly conductive network to boost the charge transport involved during the capacitance generation and also aided the dispersion of nanostructured VN within the NG network. The synergetic VN/NG composite exhibited an ultra-high specific capacitance of 445 F g−1 at 1 A g−1 with a wide operation window (−1.2 to 0 V) and showed an outstanding rate capability (98.66% capacity retention after 10,000 cycles at 10 A g−1). The VN/NG electrode offered a maximum energy density (∼81.73 Wh kg−1) and an ultra-high power density (∼28.82 kW kg−1 at 51.24 Wh kg−1). Through the ammonification process of ionic amphiphilic triblock copolymer micelles/ vanadium-contained ions system in NH3/N2 atmosphere, Liu et al. reported an in situ preparation route to obtain vanadium nitride nanoparticles on porous carbon nanospheres (PCNS/ VNNP) [691]. The prepared PCNS/VNNP material had a wide operating potential of 1.2 V, and a capacitance of 229.7 F g−1. A hybrid supercapacitor device of PCNS/VNNP//NiO exhibited a high energy density 16 Wh kg−1 at the power density of 800 W kg−1. 3.4.8.9.3  Vanadium nitride/titanium nitride composites

Binary metal nitrides have been studied as EC electrodes, exhibiting good capacitive behavior in aqueous electrolytes. Among nitride materials, TiN holds great promise as an electrode material for SCs, due to its superior electrical conductivity and mechanical stability. Incorporating VN and TiN into an efficiently fast mixed (electron and ion) transportation nanocomposites can be expected to deliver the ingredients for efficient charge transportation and electrochemical energy storage [692–694]. By the coaxial electrospinning, and subsequently annealed in the ammonia, Zhou et al. prepared titanium nitride-vanadium nitride fibers of core-shell structures (Fig. 3.32F) for supercapacitor applications [695]. These coreshell (TiN-VN) fibers incorporated mesoporous structure into high electronic conducting transition nitride hybrids, which combined higher specific capacitance of VN and better rate capability of TiN. These hybrids exhibited higher specific capacitance (2 mV s−1, 247.5 F g−1) and better rate capability (50 mV s−1, 160.8 F g−1). On the same year, Dong et al. prepared TiN/VN core-shell composites by a two-step strategy involving the coating of commercial TiN nanoparticles with V2O5.nH2O sols followed by ammonia reduction [696]. The highest specific capacitance of 170 mV s−1 was obtained when scanned at 2 mV s−1 and a promising rate capacity performance is maintained at higher voltage sweep rates. Pang et al. hybridized core-shell-structured metal nitride (TiN/VN) nanowires with a 3D carbon substrate [697]. This hybrid electrode demonstrated high volumetric capacitance and good cycling stability. The high capacitance is attributed to the exceptional electrochemical properties of TiN and VN, as well as the vast surface area and conductive network provided by the microporous 3D carbon structure. The improved stability arises from preventing transformation of TiN into poorly conductive TiO2 by the thin shell of VN. Later, Zhang et al. fabricated ternary vanadium titanium nitride/carbon (VTiN/C) nanofibers through a facile electrospinning strategy and investigated their electrochemical performance [694]. The well-interconnected VTiN/C nanofibers with VTiN nanoparticles embedded into carbon ensured rapid electron/ion transfer and offered a highly ion-accessible surface. Appealingly, the VTiN/C nanofibers exhibited a notably enhanced performance, with a high specific capacitance (430.7 F g−1, 0.5 A g−1) and a good rate capability (141.7 F g−1, 10 A g−1).





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3.4.8.10 Vanadium sulfide As vanadium sulfide exhibit higher electrical conductivity and Li-ion diffusion rate than vanadium oxides, researchers focus on VS2, V3S4, and their composites. 3.4.8.10.1  Vanadium disulfide

As a kind of layered transition-metal dichalcogenides (TMDs), VS2 has been established a new paradigm in the chemistry of nanomaterials especially for nanotubes and fullerene-like nanostructures as well as the graphene analogues in the past decades. Feng et al. developed a new 2D VS2 graphene (Fig. 3.33A) analog with less than five atomic layers and employed directly to assemble the highly c-oriented VS2 thin films with synergic advantages of high conductivity and 2D permeable channels, thereby opening the door to design practical in-plane supercapacitors for the power sources in advanced ultrathin electronics [698]. Electrochemical characterization revealed a considerable specific electric capacitance of 4760 mF cm−2 and an excellent cycling stability even after 1000 cycles for this inplane supercapacitor. 3.4.8.10.2  Vanadium tetrasulfide

Among vanadium sulfides, V3S4 has a unique distorted Ni-As type structure with ordered metal vacancies in alternate metal layers, which enhances the electrical conductivity of V3S4. Zhai et al. demonstrated that V3S4 was an exceptional pseudocapacitive anode material, which serves not only as a good power source (high electrical conductivity) but also as a good energy source (high capacitance) for the assembled AEC device [699]. By coupling the

FIGURE 3.33  (A) SEM image of VS2 nanosheets; (B) V3S4/3DGH electrode; (C) FESEM images of SVS nanospheres anchored by PANI matrix NCs. Source: Part A, Reprinted with permission from Ref. [698]; Part B, reprinted with permission from Ref. [699]; Part C, reprinted with permission from Ref. [700].



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V3S4/3DGH (Fig. 3.33B) anode and MnO2/3DGH cathode, it delivered a remarkable energy density of 7.4 Wh kg−1 (based on the weight of entire device) at the average power density of 3000 W kg−1. 3.4.8.10.3  Silver vanadium sulfide

Diggikar et al. synthesized silver vanadium sulfide (SVS) anchored by PANI matrix nanocomposite (NC) (Fig. 3.33C) via in situ polymerization of aniline [700]. For the preparation of NC, aniline, silver nitrate, ammonium metavanadate and thiourea (TU) were used as precursors. SVS NPs of size 10–20 nm were anchored in the PANI matrix. The capacitance of PANIanchored SVS was 440 F g−1 and that of PANI was 128 F g−1. 3.4.8.10.4  Mixed metal vanadates

Double metal oxides of vanadium as supercapacitor and hydrogen storage material have shown encouraging results. Mixed metal vanadates (MmV) is one of the most important families of nanomaterials with various intriguing properties, such as optical, catalytic, magnetic, LIB material, and supercapacitors. Using a facile and template free method, Butt et al. synthesized novel hierarchical nanospheres (NHNs) of ZnV2O4 (Fig. 3.34A) [701]. The electrochemical measurements were performed in 2 M KOH solution. The measured specific capacitance

FIGURE 3.34  (A) SEM image of ZnV2O4 hierarchical nanospheres; (B) Ni3V2O8/Co3V2O8 nanocomposite; (C) FESEM images of Co3V2O8 nanoplate; (D) FESEM images of Ni3(VO4)2 nanospheres. Source: Part A, Reprinted with permission from Ref. [701], Copyright 2014, American Chemical Society; Part B, reprinted with permission from Ref. [703], Copyright 2014, The Royal Society of Chemistry; Part C, reprinted with permission from Ref. [704], Copyright of Nature; Part D, reprinted with permission from Ref. [707], Copyright of The Royal Society of Chemistry.





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of ZnV2O4 electrode was 360 F g−1 at 1 A g−1 with good stability and retention capacity, which was still 89% after 1000 cycles. Vijayakumar et al. Zn3V2O8 synthesized nanoplatelets using a hydrothermal method [702] and were examined for their potential application in supercapacitors. The Zn3V2O8 nanoplatelets exhibited a maximum specific capacitance of 302 F g−1 at a scan rate of 5 mV s−1. Furthermore, a Zn3V2O8 electrode retained about 98% of its initial specific capacitance after 2000 cycles. In order to compare the electrochemical performance of different mixed metal vanadates, Liu et al. designed and synthesized Ni3V2O8, Co3V2O8, and the Ni3V2O8/Co3V2O8 nanocomposite (Fig. 3.34B) as a new class of high performance electrode material for supercapacitors [703]. Ni3V2O8 and Co3V2O8 showed a structure consisting of nanoflakes and nanoparticles, respectively. The Ni3V2O8/Co3V2O8 nanocomposite was prepared by growing Co3V2O8 nanoparticles on the surface of Ni3V2O8 nanoflakes. The composite inherited the structural characteristics and combines the pseudocapacitance’s benefited of both Ni3V2O8 and Co3V2O8, showing higher specific capacitance than Co3V2O8 and superior rate capability as well as better cycle stability to Ni3V2O8. Zhang et al. described Co3V2O8 thin nanoplates (Fig. 3.34C) as a kind of electrode material for supercapacitors [704]. The electrochemical measurements showed that the obtained Co3V2O8 nanoplate electrode exhibited a good specific capacitance (0.5 A g−1, 739 F g−1) and cycling stability (704 F g−1 retained after 2000 cycles). Using a combination of a hydrothermal strategy and subsequent annealing treatment, Kong’s team synthesized a novel self-supported electrode of nickel vanadate and nickel oxide nanohybrid on nickel foam with excellent pseudocapacitive properties [705]. The electrode had an energy density of 46 Wh kg−1 at a power density of 101 W kg−1, demonstrating the importance and great potential of nickel vanadate in the development of supercapacitors. Among the mixed metal vanadates, sodium-vanadate-doped material is believed to be a promising candidate for supercapacitor. Zhang et al. prepared sodium-vanadate-doped ordered mesoporous carbon foams (V-MCFs) via an evaporation-induced self-assembly strategy [706]. The resultant V-MCFs exhibited highly ordered mesostructure with specific surface areas of 714 m2 g−1 and uniform pore sizes of 4.1 nm. Adapting a template-free hydrothermal method, Kumar et al. synthesized urchin-shape Ni3(VO4)2 hollow nanospheres (Fig. 3.34D) [707]. The as-fabricated porous urchin-shaped Ni3(VO4)2 nanosphere electrode exhibited a specific capacity of 402.8 C g−1 at 1 A g−1 with enhanced rate capability and an excellent capacity retention of 88% after 1000 cycles. An asymmetric supercapacitor was fabricated using Ni3(VO4)2 nanospheres as the cathode and activated carbon as the anode and the electrochemical properties were studied at various scan rates in the potential range of 0.0–1.6 V. The as-fabricated asymmetric supercapacitor (Ni3(VO4)2//AC) achieved a high specific capacity (114 C g−1), energy density (25.3 Wh kg−1), and power density (240 Wkg−1). Moreover, this asymmetric supercapacitor displayed an excellent life cycle with 92% specific capacity retention after 1000 consecutive charge-discharge cycles. Later, Yan et al. reported the synthesis of amorphous aluminum vanadate hierarchical microspheres via a simple hydrothermal approach with polyvinylpyrrolidone as a surfacedirecting agent [708]. The measured specific capacitance of the amorphous aluminum vanadate electrode was 497 F g−1 at 1 A g−1 with good stability and a retention capacity of 89% after 10,000 cycles. In addition, the fabricated asymmetric supercapacitor device delivered better performance with an extended operating voltage window of 1.5 V, excellent cycle



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stability (10,000 cycles, 85% capacitance retention), high energy density (37.2 Wh kg−1 at 1124.4 W kg−1), and high power density (11,250 W kg−1 at 25 Wh kg−1). 3.4.8.11 Vanadyl phosphate As a kind of layered materials with wide application prospect, vanadyl phosphate (VOPO4) has good electrochemical performance. Due to the enhanced inionicity of (V–O) bonds when (PO4)3− anion is introduced, V4+/V5+ redox couple of VOPO4 possesses the higher potential than that for simple vanadium oxide. However, it is seldom used in pseudocapacitors because VOPO4 also has an intrinsic high electrical resistance (∼3.0–107 Ω cm−1) and the layered structure of bulk VOPO4 has limited surface area, which lowers the power density of electrochemical devices. Ultrathin 2D graphene and graphene analog material showed a strong advantage in the construction of a flexible supercapacitor. Wu et al. developed an inorganic graphene analogue, 2D vanadyl phosphate ultrathin nanosheets (Fig. 3.35A) with less than six atomic layers, as a promising material to construct a flexible ultrathin-film pseudocapacitor in all-solid-state [709]. The material showed a high potential plateau of 1.0 V in aqueous solutions, approaching the electrochemical potential window of water (1.23 V). The as-established flexible supercapacitor had a high redox potential (1.0 V) and a high areal capacitance of 8360.5 F cm−2, leading to a high energy density of 1.7 mWh cm−2 and a power density of 5.2 mW cm−2. Subsequently, Lee et al. developed a simple ice-templated self-assembly process which is used to prepare a 3D and vertically porous nanocomposite of layered vanadium phosphates (VOPO4) and graphene nanosheets (Fig. 3.35B) with high surface area and high electrical conductivity [710]. The resulting 3D VOPO4-graphene nanocomposite had a much higher capacitance of 527.9 F g−1 at a current density of 0.5 A g−1, compared with 247 F g−1 of simple 3D VOPO4, with solid cycling stability. It exhibited a wide cell voltage of 1.6 V and a largely enhanced energy density of 108 Wh kg−1. In order to study the influence of different preparation methods on the properties of VOPO4 2H2O, Luo et al. carried out a systematic comparison of VOPO4 2H2O prepared by reflux and hydrothermal methods [711]. They found that the material synthesized by reflux method had better performance in the capacitance than that by

FIGURE 3.35  (A) SEM image of 2D vanadyl phosphate ultrathin nanosheets; (B) SEM image of 3D VOPO4-graphene nanocomposite. Source: Part A, Reprinted with permission from Ref. [709], Copyright 2013, Nature; Part B, reprinted with permission from Ref. [710].





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hydrothermal method, and its specific capacitance was up to 202 mV s−1 at 2 mV s−1. A total of 67.4% of capacitance was maintained for the VOPO4 2H2O synthesized by reflux method when current density changed from 0.2 to 2 A g−1, much higher than those obtained from the hydrothermal-synthesized VOPO4 2H2O supercapacitor of 42.5%. The energy density of VOPO4 2H2O supercapacitor synthesized by reflux method was 18.7 W kg−1 at 290 W kg−1 and maintained 6.2 Wh kg−1 at 1.421 W kg−1, much higher than the hydrothermal-synthesized VOPO4 2H2O supercapacitor. He et al. prepared the VOPO4/GO layered hybrid material by a controllable nanosheet reassemble technology between VOPO4 nanosheets and GO nanosheets at room temperature on the basis of the investigation of VOPO4 nanosheet structural stability, then it was calcinated in a tubular furnace at 400°C for 3 h under N2 atmosphere, GO was successfully converted into RGO while VOPO4.2H2O into VOPO4, and VOPO4/RGO layered hybrid material was obtained [712]. The VOPO4/RGO hybrid electrode with a mass ratio of VOPO4/RGO = 1 exhibited a high specific capacitance of 378 F g−1 at a scan rate of 5 mV s−1 with a good rate capability. In this section, we discussed the application of vanadium-based compounds including vanadium oxide, vanadium nitride, vanadium sulfide, mixed metal vanadates, vanadyl phosphate, and their composite materials in the supercapacitors along with their synthesis methods, microstructure, and electrochemical properties. As a class of pseudocapacitive materials, vanadium-based compounds have a wide range of applications, but their poor electrical conductivity, poor cycling stability, low specific capacitance, and low energy limit its application. In order to overcome these limitations, controlled preparation routes have been adapted to tune the micro/nanostructures of these materials. The development of new classes of vanadium-based composite materials along side with the development of asymmetric supercapacitors will become the focus of future research. The literature reveals that some vanadiumbased compounds with special micro/nanostructure have good surface area, high electrical conductivity, and good cycling stability, but the methods for synthesizing these compounds appear to be complicated and expensive. Therefore, it is necessary to find simple and controllable methods to synthesize vanadium-based pseudocapacitor materials. As in other cases, developing composite materials is an important way to improve the electrical conductivity and the stability of the vanadium-based compounds. At present, vanadium-based materials and electric double layer capacitors (activated carbon, carbon nanotubes, graphene, etc.), other materials, such as conducting polymer (polyaniline, polypyrrole), other metal oxides (manganese oxide, cobalt oxide, etc.) have become a central focus. In addition, high energy and power densities synchronously can be accomplished by constructing asymmetric capacitor. The electrochemical performance of asymmetric supercapacitor, such as high energy density, is obviously better than that of the symmetric supercapacitor. It is promising that such high-rate capability electrodes are of great interest when coupled with a capacitive carbon negative electrode to design asymmetric (hybrid) devices with improved rate capability.

3.4.9  Iron oxide-based materials Generally, iron oxide exists in various crystallographic forms, such as FeO (wustite), αFe2O3 (hematite), γ-Fe2O3 (maghemite), and Fe3O4 (magnetite), based upon the atomic arrangements of Fe3+ and O2− ions. Among various crystallographic forms, hematite (α-Fe2O3) is one of the most stable hexagonal corundum-like structures. In α-Fe2O3, the O2− ions lie



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along (0 0 1) plane of a hexagonal closed-packed lattice, whereas the cation Fe3+ occupies twothirds of the octahedral interstices in (0 0 1) basal planes. This cationic arrangement forms a pair of FeO6 octahedrons with edges shared by three neighboring octahedrons in the same plane. The α-Fe2O3 has been proved as a potential candidate for multifunctional applications, such as photocatalysis, photoelectrochemical water splitting, rechargeable batteries, gas sensors, and electrochemical sensors [713–718]. In this section, we present a detailed discussion over the supercapacitor electrodes based on α-Fe2O3. Generally, the pseudocapacitive performance of Fe2O3 originates from the reversible oxidation/reduction between Fe3+ and Fe2+ [719a]. A major drawback of α-Fe2O3 is its poor electrical conductivity (10−14 S cm−1), which causes high charge transfer resistance between the electrolyte and the electrode. Yet another limitation of α-Fe2O3 is the capacitance decay due to its volume expansion during multiple charge-discharge cycles. Therefore, much efforts has been taken to enhance the number of active sites, reducing the diffusion length of the charge carriers, minimize the volume expansion by reducing the particle size, synthesizing porous, or arrays are discussed in an excellent review [719b] and briefly summarized later. The electrode materials with high surface area possess high Csp. The electrode materials prepared in nanometer size could offer a high surface area which provides more active sites, increased electrode–electrolyte contact per unit mass facilitating an enhanced charge transfer reaction. It also favors a shorter diffusion time and a high charge-discharge rate due to decreased ionic and electronic diffusion distance. Also, they are capable of withstanding the volumetric changes caused due to strain [720]. For instance, α-Fe2O3 nanostructures delivered a high Csp of 300 F g−1 when compared to bulk α-Fe2O3 (40 F g−1) at a current density of 1 A g−1 [721]. The capacitance of α-Fe2O3 in 0.5 M Na2SO3 electrolyte arises due to the redox reaction Fe2+/Fe3+ accompanied by the intercalation of sulfite ions. The electrochemical stability was ∼2 A g−1 and the capacitance retention was ∼73% even after 1000 cycles. 3.4.9.1  Influence of preparation routes Synthesizing electrode materials with appropriate porosity is desirable to attain enhancement in their electrochemical performance. The porous structures would assist in facilitating the fast redox reactions, short diffusion path lengths and high surface area rendering a high Csp with good cycling stability [722]. The CV curves of porous α-Fe2O3 were nearly rectangular and the current response increased with an increase in the scan rate. The Csp of porous α-Fe2O3 was found to be 127, 81, 73, 67, 62 F g−1 at the current densities of 1, 2, 3, 4, and 5 A g−1, respectively. Also, the capacitance was maintained to be 40 F g−1 even at a current density of 14 A g−1 indicating the good power capability of porous α-Fe2O3. It was found that, the α-Fe2O3 porous fibers exhibited an enhanced electrochemical behavior compared to α-Fe2O3 nanograins. The porous fibers are formed due to the fusion of spherical particles that results in the declination of grain boundaries between the particles, which contributes to the supercapacitive performance [723]. The cyclic voltammetric measurements of α-Fe2O3 porous fibers and the nanograins were carried out in the potential range from −0.1 to +0.9 V where the oxidation and the reduction occurred at 0.65 and 0.12 V. The area under the cyclic voltammetric curve was found to be higher for the porous fibers compared with nanograins of α-Fe2O3. A maximum Csp of α-Fe2O3 porous fibers and the αFe2O3 nanograins at a scan rate of 1 mV s−1 was found to be 256 and 102.4 F g−1, respectively.





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In addition, the RCT was found to be smaller for porous fibers (5.4 Ω) compared with nanograins (12.8 Ω), indicating the good electrode kinetics of the α-Fe2O3 porous fiber electrode. The unique porous fibers of α-Fe2O3 structures favored the rapid electron mobility and electrode–electrolyte interactions. The other criteria to enhance the electrochemical performance of the material are the fabrication of high aspect ratio and the arrayed structures. The nanoarrays provide short super highways to ions leading to high charge-discharge rates. Also, uniform channeled structures provide a quick access to the electrolyte [724]. The electrochemical performance of the αFe2O3 nanotube arrays synthesized by simple anodization technique was tested in the aqueous Li2SO4 electrolyte. The α-Fe2O3 nanotube arrays delivered a high specific capacitance (138 F g−1), which arose from the combination of both the electric double layer capacitance and the pseudocapacitance. The high surface area of the nanotube arrays provided a maximum utilization of the active materials. Also, the Csp was slightly decreased with an increase in current density that confirmed the good rate capability of α-Fe2O3 nanotube arrays, along with good long-term stability ∼88.9% even after 500 cycles. Sarkar et al. have synthesized 1D porous α-Fe2O3 nanoribbons using solvothermal method. The 1D structure (nanoribbons, nanorods, and nanotubes) could reduce cyclic degradation against the volumetric changes and mechanical forces. The additional advantage of possessing porous structures in the nanoribbons is that, these acts as ion buffering reservoirs, which provide successive OH− ions for the redox reactions even at high current densities [725]. Followed by this, 1D porous α-Fe2O3 nanorods synthesized by simple wet chemical route have been employed as the electrode material in supercapacitors. The rod-shaped porous structure aided faster Faradaic reactions toward electrolyte and delivered a high specific capacitance ∼308 F g−1 [726]. Similarly, a 10-fold increase in Csp was observed for the α-Fe2O3 nanoflakes when compared to α-Fe2O3 nanorods [727]. The cyclic voltammetric study carried out in the potential range from −0.7 to 0 V at a scan rate of 100 mV s−1 revealed that the current response of α-Fe2O3 nanoflakes was superior compared to α-Fe2O3 nanorods. The nanoflakes possess enhanced specific surface area and a lower charge transfer resistance than the nanorods. This lower charge transfer resistance is attributed to the highly conductive (0001) plane that favors the electronic and the ionic transport. The enhanced performance would be observed for highly crystalline materials when compared with noncrystallized material. A high specific capacitance of 166 F g−1 was obtained for the crystallized α-Fe2O3 prepared at a high reaction temperature of 140°C than noncrystallized material (80.4 F g−1). Another approach is to introduce oxygen vacancies in α-Fe2O3 lattice to enhance its electrochemical performance. The oxygen deficient α-Fe2O3 was obtained by the thermal decomposition of FeOOH under inert atmosphere by Lu et al. [728]. It exhibited a higher Csp than pristine α-Fe2O3 nanorods. By modifying the oxygen content, the donor density becomes high, and consecutively increasing its conductivity and the reactivity of the surface redox process. Till date, numerous synthetic strategies such as hydrothermal [729–733], solvothermal [719], electrodeposition [734–738], spin coating [739], electrospinning [723], sol-gel [721], precipitation [740–742], and successive ionic layer adsorption and reaction (SILAR) [743] have been reported for the preparation of α-Fe2O3. The experimental parameters in the mentioned techniques have a crucial role in altering the electrochemical behavior of α-Fe2O3. In the further sections, the various synthesis techniques and the influence of the parameters on the supercapacitor behavior of α-Fe2O3 are described.



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3.4.9.1.1  Hydrothermal method

Hydrothermal method is a bottom-up approach for synthesizing a variety of nanostructures. The major advantage of this method is its capability to produce a high yield of products with controlled morphology, reproducibility, in addition to its eco-friendliness, facile fabrication and low energy consumption. This method is based on the solubility variation in the heated aqueous solution sealed within Teflon-lined stainless-steel autoclave maintained at an ambient temperature and pressure [744]. Various metal oxides and hydroxides have been synthesized using hydrothermal technique and their use in supercapacitor was reported by Yang et al. [745]. Also, several morphologies of α-Fe2O3, such as, spinous, nano shuttles, red blood cell structure, nanorod, urchin, octahedron, and hexagonal platelets structures have been synthesized using hydrothermal technique [729–733, 746–750]. The various controlling parameters in the hydrothermal method, which affects the morphology and the size of the final product, are precursor concentration, reaction time, reaction temperature, concentration of the precipitating agent and surfactant. The effect of these parameters on the formation and growth mechanism of hematite (α-Fe2O3) nanostructures was investigated by Zhu et al. [751], and it has been found that the size of α-Fe2O3 increases with an increase in the concentration of the precursor. It is well known that, the formation of nanoparticles during wet chemical synthesis involves (1) nucleation of primary crystals and (2) growth of the crystals. As a consequence, the increase in the precursor concentration causes the fusion of the large number of nanocrystals leading to the increased particle size. In addition, the precursor concentration also has its effect on the particle dispersibility. The dispersion of the particle is poor at very high or low concentration. The effect of the precipitating agent concentration on the crystal size is similar to the effect of the precursor concentration. To study the influence of surfactant concentration, different amount of the capping agents (e.g., PVP) was utilized. The selective adsorption of the surfactant onto a particular facet of a growing crystal is called the capping effect. At sufficient surfactant concentrations, these capping agents could completely adsorb on the crystals and prevents the aggregation of the particles. If the concentration of PVP is less, then the capping effect is insufficient which lead to the particle aggregation. The reaction time should be sufficient enough for the complete crystallization of the material. Further, the size of these α-Fe2O3 structures could be controlled by varying water to ligand concentration [752]. In addition, study of the morphological variation of α-Fe2O3 using various additives was investigated by Li et al. [731]. Initially, α-Fe2O3 nanoparticles (α-Fe2O3-P) were synthesized using 0.1 M FeCl3 6H2O dissolved in 30 mL of deionized water along with a carbon fiber paper (CFP). Successively, it was then transferred to a Teflon autoclave and maintained at 180°C for 12 h. A similar procedure was followed with the addition of 0.5 M urea at 100°C for 12 h leads to the formation of Fe2O3 nanorods (Fe2O3-R). Finally, Fe2O3 nanosheets (α-Fe2O3-S) was prepared using 0.1 M FeCl3.6H2O, 0.5 M Urea and 0.1 M C6H8O6 in 30 mL deionized water which was maintained at 180°C for 4 h. All these three materials were annealed at 500°C for 3 h. The Csp of the synthesized Fe2O3-P, Fe2O3-S, and Fe2O3-R at a scan rate of 5 mV s−1 were found to be 235.7, 279.9, and 125.6 F g−1; their corresponding capacitance retention after 10,000 cycles was 93.9%, 98.8%, and 97.5%, respectively. It was found that, the relative contribution of surface capacitance (Cout) and the inner capacitance (Cin) varies with a morphology, which leads to the change in Csp values (Csp = (Cout) + (Cin)). The greater intercalation effect at the inner surface contributes to Csp in the synthesized Fe2O3 nanosheets. 



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3.4.9.1.2  Solvothermal method

Solvothermal method is almost identical to the hydrothermal method where nonaqueous solution is utilized instead of water. The reaction temperature in solvothermal method is much elevated when compared to the hydrothermal method due to the usage of the organic solvents, which possess high boiling point. It is proved to be a versatile technique in the synthesis of uniform sized particles with high dispersibility [725]. One-step template free solvothermal method was employed to synthesize Fe2O3/graphene composite. This technique allows the active reduction of graphene oxide (GO) into rGO along with a fine distribution of quasihexagonal α-Fe2O3 nanoplates on the graphene nanosheets [719]. 3.4.9.1.3  Electrodeposition method

Electrodeposition is an inexpensive and versatile method, in which the film is coated on the substrate that is kept immersed in the electrolyte containing precursors. The thin film coating using electrodeposition is carried out in two ways via potentiostatic and galvanostatic methods by varying the potential and current densities. In this technique, the size, morphology and compositions of the materials are easily controllable [753]. The morphology of Fe2O3 thin film could be modified by varying the anodic current density [734]. In this regard, larger nanorods with small pores are formed at lower current densities due to the growth of independent nuclei, while at higher current densities, thin nanosheets with larger pores are observed due to the higher nucleation rate. The nanosheets facilitated intercalation of the electrolyte ions and provided large pores for the electrolyte percolation that resulted in the increased Csp of the nanosheets when compared with that of the nanorods [734]. The molar concentration dependent supercapacitive behavior of the electrodeposited Fe2O3 thin films was carried out by Lokhande et al. [735]. Upon increasing the precursor concentration, the film thickness was rapidly increasing and a Csp of 540 F g−1 was obtained at a current density of 2.5 A g−1. Nonetheless, the main disadvantage of this method is the dissolution of the active materials into the electrolyte due to the low adhesion of deposits onto the electrode surface. Therefore, decreasing the temperature of the electrodeposition bath would overcome this problem. Thus, the decrease in bath temperature would increase the adhesion by reducing the kinetic energy of the molecules and the gas elevation rate at the electrode [736]. The deposition potential is another important parameter determining the uniform film growth and adherence of the metal oxides with substrate. The Csp was found to be higher in the uniformly deposited films when compared to overgrown films. Lokhande et al. have optimized the deposition potential (1.1 V) of Fe2O3 thin films that produces uniform growth [737]. Also, at this potential a higher Csp was achieved (390 F g−1) when compared with those of other potentials under the study. Apart from this, an appropriate annealing temperature is necessary for the phase stabilization, which affects the electrochemical performance of the material [737, 738]. 3.4.9.1.4  Spin coating technique

Spin coating is a predominant technique employed for the fabrication of thin films, which is based on coating a flat surface with thin liquid film when rotated at constant angular velocity. Thus, the obtained film thickness depends upon the solution concentration, viscosity, drying rate, spinning time, and rotation speed [754]. The films of α-Fe2O3 with two different nanoscopic morphologies, such as nanowires and mesoporous were obtained by varying



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the relative humidity during the aging process [739]. A higher Csp was achieved for α-Fe2O3 nanowires when compared with that of the mesoporous structure due to their larger surface areas and lesser charge transfer resistance. Especially in terms of ionic current flow through the pores, it was found that the large pores in nanowires were less resistive than small pores in the mesoporous structures. Also, the grain boundaries for electron conduction in the nanowires were lesser than those of the mesoporous α-Fe2O3. However, due to poor mechanical strength, the cycling stability of nanowires was slightly lower than the mesoporous structure. 3.4.9.1.5  Electrospinning technique

Electrospinning is a facile technique used for the fabrication of ultrafine fibers of various materials. The principle of electrospinning relies on the asymmetric bending of charged liquid jet when accelerated by a longitudinal electric field. Once the applied field overcomes the surface tension of the liquid, the liquid is ejected into jet and the liquid gets evaporated to form nanofibers with high aspect ratios. For the formation of particular inorganic structures, the respective metal ions are added with polymeric solution and injected from the needle in the presence of the electric field. Then, annealing of these polymeric fibers at desired temperatures would lead to the formation of respective nanofibers [755,756]. Generally, the morphology of the metal oxide obtained from this technique is dependent on the chemical interaction between the metal oxide precursor and the polymer. Binitha et al. [723] employed two types of polymers, such as polyvinyl pyrrolidone (PVP) and polyvinyl acetate (PVAc) to vary the polymer metal oxide precursor interaction. The variation in interactions has led to two different morphologies, such as nanograin and porous fiber. There was a strong interaction between the PVP and the metal oxide colloid that resulted in the formation of porous fiber. While in reverse, PVAc exhibited a weak interaction with the metal oxide colloids and formed nanograin morphology, in the loss of continuous fiber structure. The obtained Csp was found to be 256 and 102 F g−1 at 1 mV s−1 for α-Fe2O3 with porous fiber and the nanograins morphology. This was due to the high aspect ratio and high surface area of the porous fiber. Additionally, this technique is also adapted for the fabrication of composite structures [757, 758]. The V2O5 doped α-Fe2O3 composite in different weight ratios was synthesized using a single-step electrospinning technique by Nie et al. [759]. The TEM image of V2O5 doped α-Fe2O3 composite is displayed in Fig. 3.36A and B. The corresponding fast Fourier transform (FFT) version, energy dispersive X-ray analysis (EDX), high-angle annular dark-field detector in the scanning TEM (HAADF-STEM) pattern, and EDX mapping of Fe, O, and V elements are also given in Fig. 3.36B–D. It is clearly observed that, the composite material is nanotubular in structure and it is highly crystalline. It was evident from the EDX mapping that, the vanadium atoms are uniformly distributed throughout the V2O5 doped α-Fe2O3 composite. Hence, this technique enables homogeneous distribution of atoms in the synthesized composites. 3.4.9.1.6  Sol-gel method

In sol-gel method, initially sol is formed from the hydrolysis and polymerization of the inorganic metal salts. Later, the sol is converted into solid gel upon evaporation of the solvent. α-Fe2O3 based composites in different morphologies were prepared using sol-gel technique [721, 760, 761]. The annealing temperature is found to influence the morphology of the material, thereby affecting the Csp. Shivakumar et al. has synthesized porous α-Fe2O3 nanostructures





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FIGURE 3.36  TEM image of V2O5/Fe2O3 composite (1.0% weight ratio) (A) low and (B) high magnifications. Insets: Corresponding FFT version and an individual nanotube. (C) EDX spectrum (D) HAADF-STEM pattern and EDX mapping of Fe, O, and V elements (indicated in the white box). Source: Reproduced from Ref. [759], with permission of The Royal Society of Chemistry.

by sol-gel technique followed by heating at different temperatures [760]. It was found that the pore volume and the surface area of α-Fe2O3 nanostructures decreased with increasing the temperature. As a result, the Csp was decreased with increasing the preparation temperature. 3.4.9.1.7  Precipitation method

Among various wet chemical synthesis routes, precipitation method is a very simple process with cost effectiveness, easy control over particle size and compositions. The α-Fe2O3 electrode material for supercapacitor was prepared using this simple precipitation technique [740–742, 762, 763]. The solvent used in the precipitation method plays a key role in tuning the morphology and facets during the crystal growth. By varying the solvent concentration, α-Fe2O3 with different crystal facets were achieved by Barik et al. [740]. Here, ethylene glycol (EG) was used as a solvent medium and the ratio of Fe: EG concentrations were varied. At higher concentrations of EG, facets with high surface energy are formed; while at low EG concentration, more stable facets, that is, (012) or (001) were favored. Surprisingly, the Csp of α-Fe2O3 varies with respect to the Fe: EG concentrations and a maximum Csp of 450 F g−1 was achieved for the particular ratio (1:2) of Fe: EG due its high surface area. This technique is also used to synthesize different α-Fe2O3 composite structures [762, 763].



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3.4.9.1.8  Successive ionic layer adsorption and reaction (SILAR) method

The fabrication of thin films in successive ionic layer adsorption and reaction (SILAR) method is carried out by means of ion-by-ion growth mechanism. In this method, the substrate is immersed separately in cationic and anionic precursors followed by rinsing after each immersion. The main advantage of this method is that, the thickness of the film and deposition rate could be controlled with a large-scale production. The optimizable parameters in SILAR method are precursor concentrations, immersion time, and number of immersions, thickness of the film and deposition temperature. Kulal et al. have fabricated α-Fe2O3 thin films by employing SILAR method using ferrous sulfate and sodium hydroxide as precursor [743]. Initially, the hydrolysis of ferrous sulfate at pH∼5 gave ferrous hydroxyl ion which gets adsorbed onto the substrate. Then, successive immersion in the anionic solution (i.e., NaOH) led to the deposition of α-Fe2O3 layer. Here, the reaction time and the adsorption time were kept as 15s and 10s, respectively. Using SILAR method, the fabrication of supercapacitor electrode is possible without binders or additives with a controlled film thickness. In addition, Raut et al. [764] has decorated α-Fe2O3 nanoparticles onto multi-walled CNT’s (MWCNT) through SILAR method and their schematic growth mechanism was illustrated in Fig. 3.37A. Here, the functionalized MWCNT was first deposited on the stainless steel substrate followed by the coating of Fe2O3 nanoparticles and their corresponding morphologies were depicted in Fig. 3.37B(a–c). 3.4.9.2  α-Fe2O3-based composites Combining different materials to form a composite is an alternative approach to minimize the drawbacks of the poor electronic conductivity, low rate capability, and cycling stability of α-Fe2O3. In this regard, α-Fe2O3 based composites provide an additional capacitance through a synergistic effect by enhancing the specific surface area, preventing particle agglomeration, expanding voltage window, improving cycling stability by enhancing conductivity and mechanical stability. The composite materials may be either carbon materials or conducting polymer or metal oxides. Nevertheless, various factors are to be focused while preparing composite structures, such as the molar ratio of the constituents, composition of individual substances to stay away from reverse effects. In this regard, various composites structures of α-Fe2O3, such as carbon nanotubes (CNT) [764–766], graphene/reduced graphene oxide [767–778], graphitic carbon nitrides (g-C3N4) [779], carbon [780], ordered mesoporous carbon [781, 782], polymer [783], metal oxides [784–790], ternary composites containing metal oxide, conducting polymer, and carbon materials [791, 792] has been reported. 3.4.9.2.1  α-Fe2O3-carbon composites

Carbon materials, such as CNT [764–766], graphene/reduced graphene oxide [767–778], g-C3N4 [779], carbon [780], and ordered mesoporous carbon [781,782] have been utilized as conductive carbon composites for α-Fe2O3. These conducting carbon materials are employed as a composite material to enhance the electronic conductivity, rate capability, and the cycling stability of Fe2O3. Initially, Raut et al. have decorated α-Fe2O3 on MWCNT in order to increase their Csp and electrical conductivity [764]. The MWCNT are capable of rendering high electrical conductivity, high mechanical stability and enhanced surface area. The current response and the Csp of α-Fe2O3/MWCNT composites (431 F g−1) were much higher than those of the 



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FIGURE 3.37  (A) Schematic representation for the formation of Fe2O3 nanoparticles onto MWCNT through SILAR method. (B) Corresponding SEM images of (a) MWCNT (b) Fe2O3 (c) Fe2O3/MWCNT thin films. Source: Reproduced from Ref. [764], with permission of The Royal Society of Chemistry.

pristine α-Fe2O3 (187 F g−1) and MWCNT (81 F g−1). The more active sites afforded by α-Fe2O3 nanoparticles and the shortened ionic pathways of porous MWCNT resulted in the enhanced electrochemical performance of Fe2O3/MWCNT composites. Later, CNT sponge was utilized as composite material by Cheng et al. [765]. Here, the α-Fe2O3 nanohorns were uniformly coated on CNT sponge, which acts as a compressible electrode for supercapacitors. The CNT sponge was highly flexible and able to recover its initial shape after being compressed. The CNT/ α-Fe2O3 composite exhibited a Csp of 296.3 F g−1 at a scan rate of 5 mV s−1 which was higher than that of the CNT sponge (80.2 F g−1). This enhanced performance emerged from the combination of both electric double layer capacitance of CNT and pseudocapacitance of Fe2O3. Besides, the effect of strain on the electrochemical performance of CNT/Fe2O3 composite was evaluated to test the compressibility of the electrode. No change in ESR values was observed from the Nyquist plot of CNT/Fe2O3 composite electrode even after an increase in strain. However, there was a slight change in the charge transfer resistance from 1.9 to 2.8 Ω when the strain increased from 0% to 70%. As well, the deformed sponge maintained 90% of its original specific capacitance after 70% of its volume reduction. This property is strongly attributed to the high mechanical strength and open porous structure of the 3D CNT essential for the ion diffusion and minimizing capacitance degradation. Similarly, Fe2O3 grown on the 

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hierarchical graphite foam-carbon nanotube (GFCNT@α-Fe2O3) framework have provided an ultrahigh specific capacitance and long-time cycling stability [766]. The GF-CNT exhibited a rectangular cyclic curve (EDLC) whereas GFCNT@α-Fe2O3 displayed redox peaks due the presence of α-Fe2O3. The oxidation (∼0.7 V) and reduction peak (∼1.1 V) was due to the reaction between Fe3+ and K+ species in the 2 MKOH electrolyte. The specific capacitance of GF-CNT@Fe2O3 was ∼470.5 mF cm−2 at a current density of 20 mA cm−2 and found to be 4 times higher than GF-CNT (∼93.8 mF cm−2). Graphene is considered to be an ideal conducting support for construction of composites due to its extraordinary electrical conductivity. The graphene sheets with wrinkles and folds are capable of providing more reactive sites for altering reaction energetic of graphene resulting in the enhanced electrochemical performance [719, 761, 767–770]. In this regard, Yang et al. has reported the fabrication of α-Fe2O3/ graphene composite using a hydrothermal method combined with slow annealing route and investigated their electrochemical properties [767]. The pseudocapacitance of α-Fe2O3/graphene composite arose due to the oxidation and reduction reaction of Fe3+ and Fe2+/Fe. A high Csp of 343.7 F g−1, high columbic efficiency (∼98.6%) and 95.8% capacitance retention after 5000 cycles was achieved for the α-Fe2O3/graphene composite. Next, the electrochemical performance of self-assembled α-Fe2O3 mesocrystals/graphene composite was studied by Yang et al. [768]. The α-Fe2O3 mesocrystals/graphene composite with a specific surface area of 89.1 m2 g−1 and mesoporous (3–28 nm) structure has facilitated an easy access of the electrochemical site and accelerated the electron transfer. As a result of this contribution, a high Csp of 306.9 F g−1 at 3 A g−1 was achieved. Similarly, α-Fe2O3/ graphene composites prepared by Wu et al. showed a Csp of 264 F g−1 at 2.5 A g−1 with 95.7% capacitance retention after 5000 cycles [769]. Futhermore, the α-Fe2O3 quantum dot decorated on functionalized graphene sheet composite was tested as anode material for supercapacitor by Xia et al. [770]. It offered a large Csp of 347 F g−1 in 1 M Na2SO4 electrolyte and it was larger than α-Fe2O3 quantum dot (200 F g−1) and functionalized graphene sheet (120 F g−1). Also, it was proved that the nitrogen doping into graphene could increase the electrical conductivity of graphene and favorable for providing an increasing number of active and nucleation sites resulting in the enhancement of the electrochemical performance. Zhao et al. have reported the synthesis of nitrogen-doped graphene/Fe2O3 composite and demonstrated its use as electrode material for supercapacitors [771]. The electrical conductivity of nitrogen-doped graphene/Fe2O3 composite was 1.33 times higher than pure graphene/Fe2O3 composite. This was due to the enhancement of the charge carrier mobility occurred from pyrrolic N-doping. Also, the calculated Csp of the nitrogen-doped graphene/Fe2O3 composite (260.1 F g−1) was higher than those of the undoped graphene/Fe2O3 composite (150.4 F g−1). Moreover, the nitrogen doping has decreased the internal resistance and charge transfer resistance of graphene/Fe2O3 composite. Another form of graphene namely graphene aerogel is a 3D porous structure with high surface area of 964 m2 g−1. The ultrathin layers of graphene sheets provided a high conductivity that allows rapid diffusion of the electrolyte ions and faster electron transport. The Csp of 81.3 F g−1 was obtained with a potential window of −0.8 to +0.8 V versus SCE is the widest range of aqueous electrolyte systems [772]. Likewise, graphene hydrogel was proved to offer the positive synergy with Fe2O3 and served as a matrix to maintain Fe2O3 structure. It is a 3D interconnected structure with a strong mechanical strength that provided high rate capability and an excellent cycling performance. As an example, the graphene hydrogel was utilized as





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a composite material to improve the electrochemical performance of Fe2O3 [773]. The cyclic voltammetric curves of graphene hydrogel in the potential range of −1.0 to −0.3 V were found to be rectangular indicating the electric double layer characteristics of graphene hydrogel. The Fe2O3/graphene hydrogel composite has one pair of redox peaks that correspond to the conversion between Fe2+ and Fe3+. The pristine Fe2O3 shows a similar type of redox peaks, but the current response of Fe2O3/graphene hydrogel composite was much higher than pristine Fe2O3. The Csp obtained for graphene hydrogel and Fe2O3/graphene hydrogel were 272 and 908 F g−1, respectively. As a whole, Fe2O3/graphene hydrogel composite provided a high Csp, excellent rate capability and good cycling stability. Lee et al. employed α-Fe2O3-reduced graphene oxide (rGO) composite as a negative electrode in a neutral electrolyte [774]. The achieved Csp of α-Fe2O3-rGO composite (114 F g−1) was 5 times higher than pure α-Fe2O3 nanotubes (22 F g−1) due to the emergence of synergistic effect from the high surface area of α-Fe2O3 nanotubes and 2D conductive pathways of the rGO. Also, α-Fe2O3-rGO composite was found to exhibit an excellent cycling stability and possessed a large negative potential window. Similarly, Fe2O3/r-GO composite prepared by one-pot solvothermal technique displayed a Csp of 1083 F g−1 at 2 A g−1. The Fe2O3 nanoplates grown two-dimensionally on rGO sheet provided an open architecture for faster ionic and electronic transport [719,775]. The α-Fe2O3 nanoplates uniformly distributed on the rGO network through one-pot hydrothermal method was employed as supercapacitor electrode by Quan et al. [775]. The rGO showed a rectangular cyclic voltammetric behavior whereas α-Fe2O3 and α-Fe2O3/rGO exhibited redox behavior due to the reversible reaction between Fe3+/Fe2+. The obtained Csp of α-Fe2O3/rGO were 930.6, 777.99, 450.5, and 231.3 F g−1 at the scan rate of 5, 10, 20, and 50 mV s−1, respectively. Moreover, the achieved Csp of α-Fe2O3/rGO (231.3 F g−1) was higher than α-Fe2O3 (37 F g−1) and rGO (167 F g−1), respectively. As a result, α-Fe2O3/rGO displayed a vertical curve in the low frequency region of the Nyquist plot due to the fast ionic diffusion in the electrolyte and adsorption on the electrode surface. As the morphology of the composite structures is a key factor influencing the supercapacitor performances, in this regard Low et al. have prepared and reported on the electrochemical properties of the different morphologies of α-Fe2O3 based rGO composites, such as nanoparticles (NPrGO), nanorods (NP-rGO), and assembled nanorods (ANR-rGO) [776], and the observed specific capacitance were 138 F g−1, 504 F g−1, and 193 F g−1, respectively. The nanorods provided a favorable path for electrolyte ions transportation and hence possessed highest supercapacitive performance. Similarly, reduced graphene sheets/Fe2O3 nanorod composite synthesized by hydrothermal route was tested as supercapacitor electrode by Yang et al. [777]. The rGO sheets exhibited EDLC behavior and rGO/Fe2O3 nanorod composite exhibited redox peak around −0.8 V due the synergistic contribution from both the counterparts. A maximum Csp of 320 F g−1 at 10 mA cm−2 was obtained for the composite material and it was higher than pure Fe2O3 (36 F g−1) and pure reduced graphene sheets (90 F g−1). Followed by this, Liu et al. [778] has utilized Fe2O3 nanodots/nitrogen-doped graphene as supercapacitor electrode. A high Csp of 274 F g−1 at 1 A g−1 was observed due to the combined double-layer capacitance from nitrogen doped graphene and pseudocapacitance from Fe2O3 nanodots. The combined surface capacitive effect and diffusion-controlled process contributed to the total capacitance of the composite material. Besides, the heteroatom doping of rGO has improved the electrochemical performance of Fe2O3. It is found to offer additional capacitance, enhancing electrochemical stability via improving the interaction between active material and carbon



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supports. Ma et al. [793] has employed a facile technique to prepare Fe2O3 nanoparticle on nitrogen-doped graphene (N-rGO) by utilizing the oxidation ability of graphene oxide for Fe2+ followed by the hydrothermal nitrogen doping to overcome the drawbacks, such as active material aggregation and assemblage. The Fe2O3/N-rGO exhibited a well-defined redox peak at −0.7 V (anodic scan) and −1.1 V (cathodic scan) and it was attributed to the reversible reaction of Fe2+/Fe3+. A maximum nitrogen content of 6.7% was included into graphene which has enhanced the Csp of graphene to 267 F g−1 at 0.5 A g−1. The introduction of N-rGO in Fe2O3 has decreased the charge transfer resistance resulting in the enhancement of Csp. In this sequence, graphitic carbon nitrides (g-C3N4), the most stable allotrope of CN was coupled with α-Fe2O3 and utilized as supercapacitor electrode. The g-C3N4 hold delocalized conjugate π structure that has the property to easily coat over other compounds. Because of its high crystallinity and high nitrogen content, the charge transfer reaction in g-C3N4 is more facilitated when compared with other N-carbon materials. As an example, the g-C3N4/αFe2O3 provided a higher Csp than α-Fe2O3 [779]. There were no redox peaks observed in the potential range from −1.0 to 0 V inferring that the g-C3N4/ α-Fe2O3 exhibited an ideal capacitive behavior in 2.5 M Li2SO4 electrolyte. The calculated Csp of g-C3N4/α-Fe2O3 (167 F g−1) was 2.3 times higher than α-Fe2O3 (72 F g−1) at 1 A g−1. Following this, the carbon coated Fe2O3 was employed as supercapacitor electrode exhibited a Csp of 612 F g−1 at 0.5 A g−1 in 5 M NaOH electrolyte [780]. The charge storage occurs due to the quasi-reversible redox reaction between Fe3+ and the electrolyte. Although, the earlier strategies could improve the electrochemical performance of α-Fe2O3 supercapacitor, other factors such as the weight of the supercapacitor should be considered for commercial applications. The current collector (substrate) used in α-Fe2O3 supercapacitor, such as nickel foam, stainless steel, graphite foil, nickel foil are passive and they deliver 10 times higher mass compared with active material mass. The use of the flexible, lightweight conductive substrate, and binder free substrate could drastically reduce the device mass and contribute to the overall EC performance. Also, the current collector used in supercapacitor plays a major role in the transfer of electric charge of the electrode material. Hence, the desired properties of the current collector are: (1) high electrical conductivity, (2) low interfacial electrical resistance, (3) stable and inertness toward the electrolyte, and (4) thin and flexible texture to minimize mass and allow easy handling [720]. When the electrode material is directly grown on the conductive substrate, the contact resistance between the active material and substrate is reduced. In case of electrode material grown directly on the conductive substrate, there is no need for the conductive agents and binders, which further helps in decreasing the contact resistance and weight of the electrode. Such architectures are capable of enhancing the Csp and cycling stability of pseudocapacitive materials [57]. In this direction, a conductive carbon acted as an electrical conductive substrate (4.102 Ω cm−2) providing 3D porous conducting networks that assist in charge transport process of α-Fe2O3. It also offered an excellent interfacial contact between α-Fe2O3 and conductive carbon [732]. Also, 3D and high electronic conductive nickel foam were used as a binder free substrate by Huang et al. [733]. It exhibited a Csp of 147 F g−1 at 0.36 A g−1 and maintained a capacitance retention of 86% after 1000 cycles. The binder and additive free substrate were capable of preventing the detachment of the active Fe2O3 during repetitive charge-discharge cycling. Ordered mesoporous carbon is considered mostly as an ideal support for metal oxides due to its distinct channel structures, high corrosion resistance, and enlarged surface area





3.4  Transition metal oxides/hydroxides

149

with uniform pore size distribution, easy handling, high electronic conductivity, and ionic mobility. The dispersion of Fe2O3 nanoparticles on mesoporous carbon may possibly benefit the surface utilization of the active material, in turn improving the total charge storage. This was confirmed from the supercapacitive performance of mesoporous carbon/Fe2O3 thin films [781]. The CV curves of the mesoporous carbon/Fe2O3 composite thin films were only mildly rectangular with a shallow redox peak at −0.42 V. As the amount of Fe2O3 in the composite increases from 16 to 30 wt.%, the Csp was increased from 204 to 235 F g−1. The achieved Csp of the mesoporous carbon/Fe2O3 thin films (235 F g−1) was higher than mesoporous carbon film (153 F g−1). In addition, the morphology of mesoporous carbon plays a major role in improving the structural stability and the ionic mobility of the composite structures. Three different symmetries of mesoporous carbon, that is, 1D cylindrical, 2D hexagonal, and 3D bicontinuous was constructed by Hu et al. [782]. The results showed that 2D hexagonal structure was not suitable as a host material due to its poor structural stability and ionic mobility. Wherein, the 1D cylindrical and 3D bicontinuous structures provided better ionic mobility. The 1D cylindrical structure is considered to be the most stable and suitable one for better ionic mobility. The observed capacitance of Fe2O3 loaded on 1D cylindrical, 2D hexagonal, and 3D bicontinuous mesoporous carbon was found to be 677, 587, and 570 F g−1, respectively. Practically, ordered mesoporous carbon can be derived from either soft template or hard template method. The hard template carbon is prepared by using ordered mesoporous carbon or aluminum scaffolds, which offer high pore volume. In turn, soft template carbon prepared through surfactant micelles delivers a stable mesostructure. However, the carbon obtained from the hard template is structurally unstable upon a high loading of the metal oxide. While, in the case of soft template, these structures have smaller pore volume and a lower surface area. In order to overcome these limitations, Hu et al. [794] has employed both hard and soft templates using anodized aluminum oxide and a triblock copolymer F127 to fabricate ordered mesoporous carbon nanowires/Fe2O3 nanoparticles. This dual template ordered mesoporous carbon nanowire has high surface area with large pore volume and maintains its stable structure when loaded with high amount of Fe2O3 nanoparticles. The oxidation and reductions peaks of ordered mesoporous carbon (OMC)/Fe2O3 were observed at −0.3 V and −0.6 V due to the redox reaction from sulfite anions adsorbed on Fe2O3 and Fe2+/Fe3+ transitions. The OMC/Fe2O3 derived from a dual template method delivered a high Csp of 264 F g−1 when compared to soft template (157 F g−1) and hard template (210 F g−1) route. 3.4.9.2.2  α-Fe2O3-conducting polymer composite

Electronically conducting polymers were utilized as composite materials in supercapacitors. Conducting polymers are highly preferable in supercapacitors because of their high conductivity, inexpensiveness, flexibility, variable redox states, and easy synthesis. It was found that, the incorporation of metal oxides into the conducting polymers has enhanced the electrochemical properties [795]. The core-branch honeycomb-like α-Fe2O3@PPy heterostructures was fabricated by utilizing 1D PPy, 2D Fe2O3 nanoflakes, and 3D nickel foam (Fig. 3.38A) [783]. This fabrication employs hydrogen bubble dynamic template, hydrothermal, and electrochemical polymerization techniques. The morphology of fabricated pristine Fe2O3 and Fe2O3@PPy is shown in Fig. 3.38B. A honeycomb-like structure of Fe2O3 was obtained with interconnected network and its magnified image exhibits the presence of numerous nanoparticles [Fig. 3.38B(a–b)]. The SEM image of Fe2O3@PPy shown in Fig. 3.38B(c–d) confirms the



150

3.  Electrode materials for supercapacitors

FIGURE 3.38  (A) Fabrication process of core-branch Fe2O3@PPy heterostructures (B) SEM image of (a–b) Fe2O3 nanoflakes and (c–d) Fe2O3@PPy heterostructures at different magnification (C) (a) STEM-EELS chemical composition maps obtained from the red-rectangle area shown in the STEM micrograph, (b) HAADFSTEM micrograph, (c) Relative compositions of Fe, O, C, and N (d) Formation mechanism of PPy nanoleaves on Fe2O3 nanoflakes (D) (a) Schematics of electrolyte ion and electron transportation path (b) volume change during charge discharge cycles of core-branch Fe2O3@PPy heterostructures. Source: Reproduced from Ref. [783], with permission of Elsevier. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)





3.4  Transition metal oxides/hydroxides

151

complete wrapping of PPy over Fe2O3 nanoflakes. Fig. 3.38C(a–c) confirms the distribution of Fe and O over entire nanoflakes. The formation of PPy nanoleaves on Fe2O3 nanoflakes involves the following carefully controlled reaction steps: (1) immersion of Fe2O3 nanoflakes into aqueous solution of 80 mM pyrrole, 100 mM LiClO4, and 140 mM sodium dodecyl sulfate; (2) electrodeposition of PPy over Fe2O3 nanoflakes at 1.0 V/SCE; (3) produced hydroxyl ions (OH−) oxidizes PPy; and (4) electropolymerization growth of PPy [Fig. 3.38C(d)]. The Csp as high as 1167.8 F g−1 was achieved for Fe2O3@PPy at 1 A g−1 with a high rate capability when compared with pristine Fe2O3 (312.5 F g−1). The honeycomb-like structure in Fe2O3 nanoflakes facilitates a large contact area between active materials and electrolyte, in addition to PPy action as superhighways for charge transport [Fig. 3.38 D(a)] resulting in enhanced EC performance. The presence of PPy over Fe2O3 nanoflakes prevents the loss of the active material during charge-discharge cycles resulting in the enhanced cycling stability [Fig. 3.38D(b)]. Likewise, Ni(OH)2@α-Fe2O3 core shell hybrid nanoarchitectures was synthesized by Tian et al. [787]. It exhibited an excellent rate capability, specific energy (22.8 Wh kg−1), specific power (16.4 kW kg−1), and excellent cycling retention (85.7% after 5000 cycles). Similarly, carbon coated Fe2O3 core shell nanostructures (Fe2O3@C) by direct current carbon arc discharge method exhibited excellent electrochemical properties and long-term stability [780]. Herein, the electrochemical performance of both Fe2O3@C and activated Fe2O3@C was evaluated. The CV curves of Fe2O3@C exhibited weak redox peak, whereas activated Fe2O3@C showed a pronounced redox peak between −1.3 and −0.3 V. The activated Fe2O3@C had its contribution from both double layer capacitance due to carbon coating and pseudocapacitance from Fe2O3. A maximum Csp of 612 F g−1 was achieved for the activated Fe2O3@C at 0.5 A g−1 in 5 M NaOH electrolyte and it was greater than pure Fe2O3@C (418 F g−1). This was due to the formation of micropores due to the activation process that resulted in the low ionic resistance and high-energy storage. There are some reports for α-Fe2O3 hybrid materials utilized as positive electrodes in aqueous electrolytes [788,789]. Herein, Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures (HNs) on Au substrate was fabricated by Singh et al. [788] via twostep method, that is, nanowire fabrication and their controlled oxidation (Fig. 3.39A). The prepared HNs are vertically aligned with a diameter and vertical length of 150 nm and 2 mm, respectively [Fig. 3.39B(a–b)]. 3.4.9.2.3  α-Fe2O3-metal oxide/hydroxide composite

While constructing metal oxide heterostructures, both the materials chosen must be highly redox active. Taking the advantage of multiple oxidation states and layered structure of V2O5, the electrochemical performance of α-Fe2O3 was aimed to improve by fabricating α-Fe2O3/ V2O5 hybrid metal oxide composite by Nie et al. [759]. A pair of redox peaks was observed in the potential range from −0.1 to 0.6 V due the reversible reaction of Fe2+ ↔ Fe3+. When compared with pure α-Fe2O3, there was enhancement in the capacitance, improved cycling stability and perfect reversibility observed for α-Fe2O3/V2O5 composite. Also, α-Fe2O3@NiO heterostructures anchored on carbon cloth was explored as flexible supercapacitor electrode by Jiao et al. [784]. When the n-type semiconductor α-Fe2O3 is combined with NiO, the electrical conductivity gets enhanced leading to superior electrochemical performance. It was also due to the uniform wrapping of NiO nanosheets over α-Fe2O3 nanorods resulting in minimizing the ions diffusion path lengths. The electrochemical performance of α-Fe2O3@ NiO heterostructures was evaluated in 1MLiOH electrolyte. Because of the reaction between



152 3.  Electrode materials for supercapacitors



FIGURE 3.39  (A) (a–c) Schematics for the synthesis of Fe-Ni/Fe2O3-NiO core/shell HNs on Au substrate (d) Sectional view of single HNs (B) (a–b) FESEM images (c) EDAX spectrum (d) TEM image (e) HRTEM image and (f) SAED pattern of Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures. (C) (a) STEM image of individual Fe-Ni/Fe2O3-NiO core/shell HNs. (b–d) EDS line scanning of Ni, Fe, and O, respectively, across the Fe-Ni/Fe2O3-NiO core/ shell HNS indicated in (a). (e–g) EFTEM elemental mapping images of Ni, Fe, and O, respectively, for the Fe-Ni/Fe2O3-NiO core/shell HNS indicated in (a). (h–j) The high resolution XPS spectrum of the Ni2p, Fe2p, and O1s core level, respectively, of Fe-Ni/Fe2O3-NiO core/shell HNs. Source: Reproduced from Ref. [788], with permission of AIP Publishing LLC. (IV) Fe2O3 nanospindle (top) and MnO2@Fe2O3 nanospindles (bottom) before and after successive charge discharge cycles. Reproduced from Ref. [789], with permission of The Royal Society of Chemistry.



3.4  Transition metal oxides/hydroxides

153

Ni2+/Ni3+ and OH−, a pair of redox peaks was observed. In addition, the electrochemical stability was 96.2% due to the dense growth of α-Fe2O3@NiO on carbon cloth. Thus, the NiO sheets covered over α-Fe2O3 prevented from collapsing during continuous charge-discharge cycles (3000 cycles). Moreover, FeS2 nanosheets/Fe2O3 nanosphere heterostructures prepared by one-step hydrothermal method was employed as supercapacitor electrode [785]. Here, the pyrite FeS2 structure possesses a good electrical conductivity (0.03–333 S cm−1) and it was higher than Fe2O3 (∼10−14 S cm−1). As expected, the FeS2/Fe2O3 composite exhibited a good capacitive performance with a Csp of 255 F g−1 and it was superior to that of bare α-Fe2O3 (112 F g−1). Furthermore, different types of hybrid structures, including core-shell, sandwiched structure of carbon and metal oxides were constructed which proved to improve the energy density of supercapacitors. The advantages of these core-shell structures are (1) When redox active shell material is integrated into core architectures, it facilitates charge transfer reaction, (2) core structure would have direct contact with the current collector, facilitating effective electron transfer, and (3) the use of conductive additive and polymer binder could be avoided by constructing the core-shell structures on the substrates [796]. Following this, α-Fe2O3/MnO2 core-shell nanowire heterostructures array was fabricated by Sarkar et al. [786]. It was aimed that, constructed MnO2 shell over porous α-Fe2O3 core would favor fast and reversible redox reactions. In addition, α-Fe2O3 core (10−4–10−5 S cm−1) are highly electrically conductive than MnO2 (10−5–10−6 S cm−1) that provided a path for fast electron transport. As expected, α-Fe2O3/MnO2 core-shell nanowire heterostructures array was found to deliver a high Csp of 838 F g−1 at a scan rate of 2 mV s−1. The pure α-Fe2O3 nanowire exhibited a pair of redox peaks in 1 M KOH electrolyte due to the oxidation and reduction reaction given as: Fe 2 O 3 + M + + e − → α -Fe 2 O 3 M, M + = K+ or H 3 O + (3.10) Similar to α-Fe2O3, the α-Fe2O3/MnO2 core-shell structure exhibited a distinct pair of redox peaks. Here, the charge storage in MnO2 occurs through the surface adsorption of electrolyte cations accompanied by proton incorporation and is given as: MnO 2 + xK + + yH + + ( x + y ) e − → 4 MnOOK x H y (3.11) The α-Fe2O3/MnO2 exhibited capacitive charge storage ∼13 times higher than α-Fe2O3. The presence of Fe, Ni, and O elemental composition is evidenced from Fig. 3.39B(c). TEM, HRTEM, and SAED confirm the formation of core-shell structure and highly crystalline nature [Fig. 3.39(d–f)]. The HNs consists of highly porous shell (Fe2O3-NiO) that facilitates the electrolyte diffusion with faster redox reaction and conductive core (Fe-Ni) nanowires, which pave way for easy transport of electrons to current collector. A thin Fe2O3-NiO shell layer with 25 nm thickness surrounds the core FeNi nanowires (100 nm in diameter) as evident from the STEM micrographs [Fig. 3.39C(a)]. The EDS line scanning illustrates the higher intensity of Fe and Ni at the center and O at the outer edge [Fig. 3.39C(b–d)]. The elemental composition was further confirmed by energy filtered transmission electron microscopy (EFTEM) images. The XPS spectrum confirmed the presence of +2, +3, and −2 oxidation states of Ni, Fe, and O, respectively in Fe-Ni/Fe2O3-NiO core/shell HNs. The electrochemical performance was tested in the potential range from 0 to 0.55 V versus Ag/AgCl in 1 M KOH electrolyte. A high specific capacitance of 1415 F g−1 at 

154

3.  Electrode materials for supercapacitors

2.5 A g−1 with long term cycling stability (95% retention after 3000 cycles) was achieved due to its unique core-shell structure. Another evidence for the capacitance improvement and cycling performance due to the fabrication of core-shell structure was reported in MnO2@Fe2O3 nanospindles [789]. Coreshell MnO2@Fe2O3 nanospindles suffered only 3.84% loss after 1000 cycles, whereas it was 36.0% for pristine Fe2O3. This is because the pristine Fe2O3 was partly damaged during the consecutive charge-discharge cycles [Fig. 3.39D(top)]. However, the twinning of MnO2 shell over Fe2O3 core prevented the dissolution of Fe resulting in the enhanced cycling performance [Fig. 3.39D(bottom)]. Besides, Ti doped Fe2O3@PEDOT core/shell nanorod arrays grown on flexible carbon cloth were utilized as anode material for supercapacitor by Zeng et al. [790]. A quasi-rectangular CV curves were obtained suggesting that the capacitance was mainly contributed via double layer capacitance and partly by the Faradaic reactions. Further, the quantity of Ti in Fe2O3 influenced the electrochemical performance. Initially, the areal capacitance was increased from 91.5 to 163.6 mF cm−2 with an increase in the Ti content but later, when the Ti content was greater than 23.1%, the capacitance was decreased. A maximum areal capacitance of 395.6 mF cm−2 was achieved at 10 mV s−1 and the capacitance retention was ∼96% after 3000 cycles. 3.4.9.2.4  Ternary nanocomposite

The electrochemical performance of the ternary composite arises due to the synergistics among three components namely, metal oxides, conducting polymers, and carbon materials. The electrochemical behavior of ternary composite is superior to binary composite and also its individual components. To ascertain this, a ternary composite based on porous α-Fe2O3 nanorods on PEDOT: PSS, that is, poly(3,4-ethylene dioxythiophene): poly(styrenesulfonate) functionalized graphene was produced through aqueous-based self-assembly process [791]. Aforementioned 3D interconnected layer-by-layer architecture offered highly conductive pathways for the charge transportation and its flexible, lightweight nature were its added advantage. The porous α-Fe2O3 nanorods (HNR) were synthesized by advanced spray-precipitation process form stable homogeneous dispersion in aqueous medium Fig. 3.40A(a), which is capable of interacting with the PSS functional ends. Considering this interaction as an advantage, PEDOT: PSS and liquid crystalline graphene oxide were added into aqueous α-Fe2O3 to form flexible ternary composite [Fig. 3.40A(b)]. The morphology and the elemental mapping analysis suggested a uniform distribution of sulfur and iron, indicating the successful attachment of PEDOT: PSS to rGO with HNR as interlayer material (Fig. 3.40B). The electrochemical performance of the ternary composites and its individual components was studied by using CV and GCD analysis (Fig. 3.40C). The ternary composite exhibited a Csp of 875 F g−1 at 5 mV s−1 with extraordinary capacitance retention of 100% after 5000 charge-discharge cycles. The merits of extraordinary capacitance retention have been due to the (1) increased graphene interlayer gap due to uniform distribution of α-Fe2O3 nanorods, (2) strong π–π interaction in graphene/PEDOT: PSS, and (3) noncovalent interaction between HNR and PEDOT: PSS that allows self-assembled stable architecture. The observed Csp of ternary composite was higher than pristine Fe2O3 (107 F g−1), rGO (96 F g−1), and binary Fe2O3/ rGO composite (224 F g−1), respectively. Similarly, Xia et al. [792] have synthesized graphene/ Fe2O3/polyaniline ternary composite via two-step approach. The graphene sheets were well dispersed with Fe2O3 particles and polyaniline was coated over graphene/Fe2O3 composite.



3.4  Transition metal oxides/hydroxides



FIGURE 3.40  (A) (a–i) Dispersion behavior of highly porous α-Fe2O3 nanorods (HNR) in stable aqueous dispersion (0.25 mg mL−1) after 2 weeks; HRTEM images of HNR in (a-ii) bright field (a-iii) dark field. (b) Self-assembly process of HNR on PEDOT: PSS functionalized GO sheets (bei) Liquid crystalline (LC) GO, (b-ii) PEDOT:PSS functionalized LC GO, (b-iii) HNR attached to GO-PEDOT: PSS sheets in aqueous medium, (b-iv) self-assembled flexible rGO/PEDOT:PSS/HNR (rGPPHNR) composite after reduction. (B) (a) Morphology of rGPPHNR and its elemental mapping images of carbon (C), sulfur (S), oxygen (O) iron (Fe) and (b) Corresponding EDS spectrum (C) (a) CV curves of composites and its individual components at 5 mVs−1, (b) Charge-discharge curves of composites and its individual components at 0.5 A g−1. Source: Reproduced from Ref. [790], with permission of John Wiley and Sons.

155

156

3.  Electrode materials for supercapacitors

The main advantages of graphene/Fe2O3 composite are as follows, (1) the graphene acted as a conducting framework and prevented PANI from the mechanical deformation during multiple charge-discharge cycles, (2) thin film of polyaniline enhanced the surface area of the electrode and hinder from dissolution, aggregation, volume changes of Fe2O3, and (3) Fe2O3 along with graphene matrix increased the rate stability of the ternary composite. Graphene/ Fe2O3 displayed rectangular CV characteristics indicating that the capacitance contribution due to the charge storage occurred at the electrode–electrolyte interface of the host material. However, when polyaniline was introduced into graphene/Fe2O3 sheet, two pairs of redox peaks were observed (−0.7 to −0.57 V and −0.38 to −0.18 V). This was due to the redox activity of polyaniline that attributed to the leucoemeraldine/emeraldine/pernigraniline structural conversions. It was also observed that the current response of graphene/Fe2O3/polyaniline was superior to pure Fe2O3, polyaniline, Fe2O3/graphene, respectively. The graphene/Fe2O3/ polyaniline ternary composite exhibited a pronounced electrochemical performance with a specific capacitance of 638 F g−1 and experienced a minor capacitance loss of only 8% after 5000 cycles. In addition, the electrochemical performance of pure α-Fe2O3 and its various composites are summarized in Tables 3.7 and 3.8. Among all composites, the electrochemical behavior of Fe-Ni/Fe2O3-NiO core/shell hybrid nanostructures has displayed ultrahigh capacitance (1415 F g−1) with long term cycling stability of 95% after 3000 cycles [788]. 3.4.9.3  Cell performance of α-Fe2O3 As mentioned earlier, a supercapacitor cell is normally constructed using two electrodes separated by an ion-permeable separator in an electrolyte. When the two electrodes are identical, it is described as symmetric supercapacitor. The total capacitance achieved in symmetric supercapacitor is one-half of either one-electrode capacitance. Alternatively, when both the electrodes employed are different (positive and negative), then it is called as an ASC. As we know the energy density and power density are the two main parameters that determine the supercapacitor performance. The energy density of the supercapacitor is mainly dependent on the specific capacitance (Csp) achieved by the electrodes and the cell voltage (V) [E = ½ CspV2]. By enhancing the specific capacitance of the electrode material and the cell voltage, the energy density could be efficiently enhanced. The specific capacitance of both the positive and negative electrode should be improved to enhance the energy density. Also, the positive and negative materials employed in ASC must possess high hydrogen and oxygen evolution over potentials leading to larger cell voltage (V). From the literature, it is evidenced that the α-Fe2O3 is an efficient negative (anode) electrode for supercapacitor with pronounced electrochemical performance in the potential range from −1.2 to 0 V. More effective ways to enhance the specific capacitance of α-Fe2O3 were made. The α-Fe2O3 hence formed with a high capacitance coupled with large potential window could be a promising anode material in developing high energy density supercapacitor. 3.4.9.3.1  Symmetric supercapacitor of α-Fe2O3

Symmetric supercapacitors are normally constructed using similar types of positive and negative electrode. Chen et al. have constructed aqueous symmetric supercapacitor using α-Fe2O3 grown on the conductive carbon substrate. It has delivered an energy density of 11 mWh cm−3 with a power density of 1543.7 mW cm−3 using 2 M Li2SO4 with high operating voltage of 2.0 V. The capacitance retention was 83.08% after 5000 cycles at a current density of



TABLE 3.7  Electrochemical parameters of pristine α-Fe2O3 electrodes.

Synthesis method

Electrolyte

Potential window/V

Specific capacitance (F g−1)

Cycle life retention

References

α-Fe2O3

Sol-gel

0.5 M Na2SO3

−0.8 to 0/SCE

300 at 1 A g−1

—

[721]

α-Fe2O3 porous flower Self assembly

0.5 M Na2SO3

−0.8 to 0/SCE

127 at 1 A g

80% after 1,000 cycles

[722]

α-Fe2O3

Electrospinning

1 M LiOH

−0.1 to 0.9/Ag/AgCl 256 at 1 mV s

∼82% after 3,000 cycles [723]

α-Fe2O3

Anodization

1 M Li2SO4

−0.8 to 0/SCE

α-Fe2O3

Solvothermal

α-Fe2O3

Wet chemical

α-Fe2O3 ultra thin nanoflake

Electrochemical route 3 M LiCl

−0.7 to 0/Ag/AgCl

100.6 at 1 mA cm

α-Fe2O3 thin film

SILAR method

1 M Na2SO4

−0.1 to 0/SCE

290 at 5 mV s−1

α-Fe2O3

Two-step approach

3 M LiCl

−0.8 to 0/SCE

89 at 0.5 mA cm

α-Fe2O3

Hydrothermal

1 M LiOH

0.0 to 0.5/SCE

681 mF cm−2 at 1 mA cm−2 76.1% after 6,000 cycles [729]

α-Fe2O3 hollow nanoshuttle

Hydrothermal

1 M KOH

−1.2 to 0/Ag/AgCl

249 at 0.5 A g−1

93.6% after 2,000 cycles [730]

α-Fe2O3 nanosheets

Hydrothermal

2 M LiCl

−0.8 to 0/Ag/AgCl

279.9 at 5 mV s−1

98.8% after 10,000 cycles [731]

α-Fe2O3 /C

Hydrothermal

2 M Li2SO4

−0.1 to 0/Ag/AgCl

1.784 at 2 mA cm−2

—

[732]

Porous α-Fe2O3

Template free hydro- 1 M thermal

−0.8 to 0.2/SCE

147 at 0.36 A g

86% after 1,000 cycles

[733]

α-Fe2O3 TF

Electrodeposition

1 M Li2SO4

−0.8 to −0.1/SCE

116.6 at 25 mV s−1

70% after 500 cycles

[734]

α-Fe2O3 TF

Electrodeposition

1 M KOH

−1.2 to 0/Ag/AgCl

540 at 2.5 A g−1

—

[735]

α-Fe2O3 TF

Electrodeposition

0.25 M Na2SO3 −0.75 to 0.15/Ag/ AgCl

87.4 at 2 mV s

—

[736]

α-Fe2O3 TF

Electrodeposition

1 M KOH

−1.2 to 0/Ag/AgCl

487.07 at 5 mV s−1

—

[737]

α-Fe2O3 TF

Electrodeposition

1 M Li2SO4

−0.9 to −0.1/SCE

173 at 3 A g

—

[738]

α-Fe2O3 TF

Spin coating

0.5 M Na2SO3

−0.8 to 0/Ag/AgCl

365.7 at 3 A g−1

—

[739]

Nano α-Fe2O3

Solvent-mediated precipitation

0.5 M Na2SO4

−1.0 to 0.8/Ag/AgCl 450

88% after 500 cycles

[740]

α-Fe2O3

Precipitation

0.1 M Na2SO4

0 to 0.8/Ag/AgCl

>99% after 500 cycles

[741]

α-Fe2O3 microrods

Chemical treatment

0.5 M Na2SO3

−1.0 to 0.1/Ag/AgCl 346 at 2 mV s−1

88% after 5000cycles

[742]

α-Fe2O3 TF

SILAR method

1 M NaOH

−0.6 to 0.1/SCE

—

[743]

−1

89% after 500 cycles

[724]

1 M KOH

−0.1 to 0.4/Ag/AgCl 145 at 1 A g

−1

96% after 1,600 cycles

[725]

1 M H3PO4

−0.1 to 0.9/Ag/AgCl 308 at 1 A g−1

77% after 1,000 cycles

[726]

138 at 1.3 A g−1

−2

— −2

−1

−1

−1

200 at 5 A g−1 178 at 5 mV s

−1

87.2% after 5,000 cycles [727] —

95.2% after 10,000 cycles [728]

157

Adapted with permission from Ref. [719].

−1

3.4  Transition metal oxides/hydroxides



Material

158

3.  Electrode materials for supercapacitors

TABLE 3.8  Electrochemical parameters of α-Fe2O3 based composites. Material

Synthesis method

Electrolyte

Potential window (V)

CNT@ α-Fe2O3

Hydrothermal

2 M KCl

−1.0 to 0/Ag/ 296.3 at 5 mV s−1 AgCl

60% after [765] 1,000 cycles

Graphene Atomic layer Foam-CNT-αdeposition Fe2O3

2 M KOH

−1.2 to 0/SCE

470.5 mFcm−2 at 20 mAcm−2

95.4% after 50,000 cycles

[766]

α-Fe2O3/G

Hydrothermal with slow annealing

1 M Na2SO4

−1.0 to 0.3/ Ag/AgCl

343.7 at 3 A g−1

95.8% after 50,000 cycles

[767]

α-Fe2O3/G

Hydrothermal

1 M Na2SO4

−1.2 to 0.2/ Ag/AgCl

306.9 at 3 A g−1

92% after [768] 2,000 cycles

Fe2O3/G

One-step chemi- 2 M KOH cal reaction

−1.0 to 0/SCE

264 at 2.5 A g−1

95.7% after [769] 5,000 cycles

Fe2O3 quantum dot/G

Thermal decom- 1 M Na2SO4 position

−1.0 to 0/Ag/ 347 at 10 mV s−1 AgCl

—

N doped G/ Fe2O3

Hydrothermal

1 M Na2SO4

−1.1 to 0.1/ SCE

260.1 at 2 A g−1

82.5% after [771] 1,000 cycles

α-Fe2O3/Gaerogel

Hydrothermal

0.5 M Na2SO4 −0.8 to 0.8/ SCE

81.3 at 1 A g−1

—

[772]

α-Fe2O3/Ghydrogel

Hydrothermal

1 M KOH

−1.05 to 0.3/ Hg/HgO

908 at 2 A g−1

75% after 200 cycles

[773]

α-Fe2O3-rGO

Hydrothermal

1 M Na2SO4

−1.0 to 0/Ag/ 114 at 5 A g−1 AgCl

92% after [774] 2,000 cycles

α-Fe2O3/rGO

Hydrothermal

1 M KOH

−1.2 to −0.20/ 903 at 1 A g−1 Hg/HgO

70% after [775] 1,000 cycles

Fe2O3/rGO nanorods

Hydrothermal followed by base reduction

1 M Na2SO4

−1.0 to 0/Ag/ 504 at 2 mA cm−2 AgCl

—

[776]

Fe2O3/rGO nanorods

Hydrothermal

6 M KOH

−1.0 to 0.2/ SCE

320 at 10 mA cm−2

97% after 500 cycles

[777]

Fe2O3 nanorods @N doped G

Solvothermal

2 M KOH

−1.0 to 0/SCE

274 at 1 A g−1

75.3% after 100,000 cycles

[778]

g-C3N4/α-Fe2O3

Solvothermal

2.5M Li2SO4

−1.0 to 0/SCE

260 at 0.5 A g−1

100% after [779] 1,000 cycles

Activated DC carbon arc Fe2O3@carbon discharge core shell

5 M KOH

−1.3 to −0.3/ Hg/HgO

612 at 0.5 A g−1

90% after 10,000 cycles

[780]

Fe2O3/ordered Self assembly mesoporous C

1M Na2SO3

−1.0 to 0.2/ Ag/AgCl

235 at 0.5 A g−1

95% after 380 cycles

[781]

Fe2O3/ordered Template mesoporous C method

1M Na2SO3

−0.8 to 0/Ag/ 677 at 5 mV s−1 AgCl



Specific capaci- Cycle life tance F g−1 retention

References

[770]

89.8% after [782] 1,000 cycles



159

3.4  Transition metal oxides/hydroxides

TABLE 3.8  Electrochemical parameters of α-Fe2O3 based composites. (Cont.) Material

Synthesis method

Electrolyte

Potential window (V)

Specific capaci- Cycle life tance F g−1 retention

References

−1

Fe2O3@PPy

Template, 0.5M Na2SO4 −1.0 to −0.2/ hydrothermal, SCE electrochemical polymerization

255 at 1 A g

97.1% after [783] 3,000 cycles

α-Fe2O3@NiO

Hydrothermal

1M LiOH

−0.2 to 0.8/ SCE

557 at 1 mA cm−2

96.2% after [784] 3,000 cycles

FeS2 nanosheet/ Hydrothermal Fe2O3 nanosphere

1M Li2SO4

−1.0 to 0/ Ag/ AgCl

α-Fe2O3/MnO2 core shell

Electrochemical deposition

1M KOH

−0.1 to 0.6/ Ag/AgCl

838 at 2 mV s−1

98.5% after [786] 1,000 cycles

Ni(OH)2@αFe2O3 core shell

Thermal oxidation and hydrothermal

1M NaOH

0 to 0.6/Ag/ AgCl

908 at 21.8 A g−1

85.7% after [787] 5,000 cycles

Fe-Ni/Fe2O3Two-step synNiO core shell thesis hybrid

1M KOH

0 to 0.55/Ag/ AgCl

1415 at 2.5 A g−1

95% after [788] 3,000 cycles

MnO2@Fe2O3 core-shell nanospindles

Hydrothermal

0.5M K2SO4

0 to 1.0/SCE

159 at 0.1 A g−1

97.4% after [789] 5,000 cycles

Ti-Fe2O3@ PEDOT core shell

Two-step synthesis

5M LiCl

−0.8 to 0/SCE

311.6 at 1 mA cm−2

>96% after 30,000 cycles

α-Fe2O3/rGO/ PEDOT; PSS

Self assembly

1M KOH

−1.0 to 0/Hg/ 875 at 5 mV s−1 HgO

100% after [791] 5,000 cycles

G/Fe2O3/PANI

Two-step synthesis

6M KOH

−1.0 to 0.1/ Hg/HgO

92% after [792] 5,000 cycles

90% after [785] 5,000 cycles

638 at 1 mV s−1

[790]

Adapted with permission from Ref. [719].

20.0 mA cm−2 [732]. Recently, a solid-state symmetric supercapacitor was constructed using α-Fe2O3/graphene aerogel hybrid electrode by Khattak et al. [797]. For this, Fe2O3/graphene aerogel was synthesized by in situ hydrothermal method. The electrochemical property of Fe2O3/graphene aerogel was initially tested using three-electrode system. Well-defined redox peaks were observed which corresponds to the redox reactions of Fe2+ ↔ Fe3+. A high Csp of 1045.3 F g−1 was attained at the current density of 0.4 A g−1. To explore its potentiality, Fe2O3/graphene aerogel was coated onto the nickel foam with PVA-KOH gel electrolyte sandwiched between the two electrodes to fabricate symmetric supercapacitor. The symmetric device delivered an energy density and power density of 9.8 Wh kg−1 and 90.1 W kg−1 in the cell voltage range of 0–0.8 V. Also, the electrochemical performance of the device tested in various bending conditions (90 and 180°) revealed that they are highly stable and highly flexible. Similarly, Park et al. have assembled solid-state symmetric supercapacitor using



160

3.  Electrode materials for supercapacitors

α-Fe2O3/poly(3,4-ethylenedioxythiophene) (PEDOT) composite structure as anode material [798]. First, the hierarchical core-shell type Fe2O3/PEDOT nanoparticles was prepared by sonochemical, liquid-liquid diffusion-assisted crystallization and vapor deposition polymerization methods. The synthesized material possessed large surface area (376.4 m2 g−1), high conductivity (120 S cm−1) and excellent electrochemical properties. The solid-state symmetric supercapacitor was fabricated with Fe2O3/PEDOT as both positive and negative electrodes using H2SO4-PVA based hydrogel electrolyte. The device worked up to 2.0 V where a high Csp, excellent energy and power density of 252.8 F g−1, 136.3 Wh kg−1, and 10,526 W kg−1 was attained. 3.4.9.3.2  Asymmetric supercapacitor (ASC) of α-Fe2O3

As mentioned earlier, the ASC are constructed using two dissimilar electrodes in an electrolyte with porous separator sandwiched between them. Based on the electrolytes used, they are classified into two categories, namely aqueous ASC and solid-state ASC. The aqueous and solid-state asymmetric supercapacitors based on α-Fe2O3 anodes have been reported [799, 800–809]. On basis of the published literature, the latest developments in the two types of asymmetric supercapacitors are summarized as follows. Recently, Tang et al. have fabricated NiO nanoflakes//Fe2O3 nanowire aqueous ASC [799]. The homogeneous Fe2O3 nanowire grown on carbon fiber paper exhibited a high Csp of 908 F g−1 at 2 A g−1. Thus, the excellent capacitive nature of α-Fe2O3 has great potential for use as an anode material against NiO cathode (1520 F g−1). The electrochemical reaction of ASC is given by: (3.12) At anode: 1 2 Fe 2 O 3 + e − + 3/2 H 2 O → Fe ( OH )2 + OH − At cathode: NiO + OH − ↔ NiOOH + e − (3.13) A higher cell voltage of 1.8 V was achieved with a Csp of 240 F g−1, energy density of 105 Wh kg−1 and power density of 1400 W kg−1. The observed energy density was even higher than Ni-MH batteries [799]. Likewise, a low cost, high performance ASC using ZnCo2O4@ MnO2 core shell nanotube array as positive electrode and a 3D porous α-Fe2O3 as negative electrode were fabricated by Ma et al. [800]. The CV measurement was carried out in the potential range of −0.1 to 0.4 V for ZnCo2O4@MnO2 and −0.8 to 0 V for α-Fe2O3. The assembled device afforded a Csp and maximum energy density of 161 F g−1 and 37.8 Wh kg−1, respectively with an operating window of 1.3 V. Beside, a 2 V aqueous ASC was assembled using Fe2O3 quantum dot decorated on functionalized graphene sheets (FGS) as an anode and MnO2/FGS as cathode material in two-electrode Swagelok cell. The working potential range was −1.0 to 0 V for Fe2O3/FGS and 0 to +1.0 V versus Ag/AgCl for MnO2/FGS, a Csp of 73.2 F g−1 was obtained. The achieved high Csp and rate capability was due to the composite electrode design that offers high surface area and good electrical conductivity. This asymmetric supercapacitor Fe2O3/FGS//MnO2/FGS has delivered an energy density of 50.7 Wh kg−1 and power density of 100W kg−1 with good cycling stability [770]. Similarly, a 1.6 V aqueous asymmetric supercapacitor based on Ni3(PO4)2@grpahene oxide cathode and α-Fe2O3@graphene oxide anode in aqueous KOH electrolyte was fabricated by Li et al. [801]. Negative α-Fe2O3@PANI coreshell nanowires array electrode constructed using electrodeposition technique (Fig. 3.41A)





3.4  Transition metal oxides/hydroxides

161

FIGURE 3.41  (A) Electrode fabrication of α-Fe2O3@PANI core-shell nanowires (B) SEM images of (a) α-Fe2O3

(b) α-Fe2O3@PANI (c) TEM (inset: SAED pattern) (d) HRTEM images of α-Fe2O3@PANI core-shell nanowires (C) (a) CV curves of individual α-Fe2O3@PANI and PANI@carbon cloth in three electrode system at 100 mV s−1 (b) Green LED rope lights powered by α-Fe2O3@PANI//PANI based ASC. Source: Reproduced from Ref. [802], with permission of American Chemical Society.

was tested in aqueous ASC by Lu et al. [802]. The SEM images of α-Fe2O3 with and without coating of PANI were shown in Fig. 3.41B(a–b). The uniform coating of PANI over α-Fe2O3 was confirmed from TEM and HRTEM images [Fig. 3.41B(c–d)]. The SAED pattern inferred the polycrystalline nature of α-Fe2O3. In order to find the optimum operating voltage, the CV analysis for individual α-Fe2O3@PANI and PANI@carbon cloth in three electrode system was performed at a scan rate 100 mV s−1 [Fig. 3.41C(a)]. It was found that, the potential window for the ASC could be as large as 1.5 V. The constructed ASC has delivered a maximum energy density of 0.35 mWh cm−3 at 120.51 mWcm−3. Three serially connected ASC was capable of lighting a green LED [Fig. 3.41C(b)] rope lights which indicate the high power and energy characteristics of α-Fe2O3@PANI/PANI ASC. An anode material using hierarchical graphite foam-carbon nanotube framework coated with a thin layer of α-Fe2O3 (GFCNT@Fe2O3 was designed by Guan et al. [803]. At 20 mA cm−2, GFCNT@Fe2O3 exhibited an areal capacitance of 470.5 mF cm−2 which was 4 times higher than GF-CNT (93.8 mF cm−2).



162

3.  Electrode materials for supercapacitors

To construct ASC, CoMOO4 nano honeycomb on graphite foam (GF-CoMOO4) was selected to pair with GF-CNT@ Fe2O3 anode. For this, the electrodes were packed with electrolyte (KOH) soaked separator using two PET sheets by the Lacor home vacuum pack. A couple of redox peaks were observed in the cell voltage of 0–1.6 V, which corresponds to Fe0 ↔ Fe3+ and Co2+ ↔ Co4+ reactions. The ASC full cell delivered a Csp, energy density and power density of ∼210 F g−1, ∼74.7 Wh kg−1 and 1.4 kW kg−1, respectively. The Fe2O3@Fe2O3 shell nanorod array as anode material for ASC was studied by Tang et al. [804]. The Fe2O3@-Fe2O3 anode and Fe3O4@MnO2 cathode was assembled in a Swagelok cell with nonwoven fabric as a separator using 1 M Na2SO4 electrolyte. The CV curves were rectangular at 5 mV s−1 and it was retained even at a high scan rate of 800 mV s−1 indicating the good capacitive behavior of assembled device. This ASC delivered an energy density and power density of 0.83 mWh cm−3 and 15.6 mW cm−3, respectively. Similarly, an aqueous ASC was fabricated using NiCo2O4/NiO and Fe2O3 electrode by Shanmugavani et al. [805]. The positive NiCo2O4/NiO electrode was synthesized by the microwave assisted reflux method and the negative Fe2O3 electrode was prepared by hydrothermal method. The Fe2O3 electrode in three electrode system has delivered an ultrahigh Csp of 1201 F g−1 at 2 mV s−1. The constructed ASC showed up to 1.6 V with one pair of redox peaks within 0–1.6 V. The fabricated ASC provided a Csp, energy density and power density of 417 F g−1 at 1 mV s−1, 19Wh kg−1, and 157 W kg−1, respectively. An asymmetric supercapacitor containing graphene/porous Fe2O3 nanocomposite as anode and CoNi-layered double hydroxide/carbon nanotube composite as cathode was fabricated by Chen et al. [806]. The graphene/porous Fe2O3 nanocomposite in KOH electrolyte has delivered a Csp of 1095 F g−1 at a current density of 3 A g−1. The charge storage process of Fe2O3 in KOH was due to the diffusion of K+ and e− within Fe2O3 given by: Fe 2 O 3 + 2K + + 2e − ↔ K 2 Fe 2 O 3 (3.14) An ASC was constructed to explore the potential of graphene/porous Fe2O3 nanocomposite using CoNi-LDH/CNT as the positive electrode. The CV curves of assembled ASC demonstrated a high symmetric anodic and cathodic peak with equal integrated area, indicating the low polarizations and good reversibility of both cathode and anode materials. The achieved energy density and power density were 98 Wh kg−1 and 22,826 W kg−1, respectively. The flexible supercapacitor using liquid electrolyte is difficult to employ in practical applications, due to its leakage problem and hence to address this issue, solid-state flexible supercapacitor was constructed using solid-state electrolytes sandwiched between the positive and negative electrodes. The solid-state electrolyte is usually a polymer gel electrolyte such as polyvinyl alcohol (PVA) supported ionic compounds (e.g., KOH and LiCl). The main advantage of the gel electrolyte is to free from leakage issues. They are highly flexible, lightweight, and provide high ionic conductivity (10−4–10−2 S cm−1) [810]. Due to these added merits, they are widely used in wearable electronics and flexible displays. In this regard, they are fabricated in various structures, including sandwich type, wire type and chip type as per the need. The sandwich type flexible supercapacitor is designed using two planar flexible electrodes with gel electrolyte in-between in wire shaped structures to form a flexible wire supercapacitor. Whereas in chip type, the flexible supercapacitor are constructed on the chip which is often fabricated via lithographic techniques. The main factor is that, the folding, bending, twisting of flexible supercapacitor should not affect the Csp of 



3.4  Transition metal oxides/hydroxides

163

the material. A solid-state ASC was fabricated using MnO2 cathode, Tidoped Fe2O3@PEDOT anode and polyvinyl alcohol (PVA)/LiCl electrolyte gel as separator by Zeng et al. [790]. Initially, Ti doped Fe2O3@PEDOT core-shell nanorod array was grown on flexible carbon cloth. Here, PEDOT, that is, poly(3,4-ethylenedioxythiophene) is a stable conducting polymer utilized to prevent the structure degradation during long term cycling. The Ti doping introduced oxygen vacancy that acted as a donor to increase their electrical conductivity. There was no variation in electrochemical performance upon bending to various angles indicating the high flexibility of assembled ASC. A high volumetric capacitance of 2.02 mF cm−3 with energy density of 0.35 mWh cm−3 and power density of 120.51 mW cm−3 was obtained. A similar type of flexible solid-state thin film ASC was designed using α-MnO2 and α-Fe2O3 thin films using PVA-LiClO4 gel electrolyte [807]. The applicability of the ASC as an effective power source for color switchable sunglasses and smart windows was tested by Yang et al. [808]. MnO2 nanowires (NWs) and Fe2O3 nanotubes (NTs) grown on carbon fiber was selected as low cost positive and negative electrode. The schematic diagrams for the synthesis and fabrication of ASC using MnO2 NWs and Fe2O3 NTs is depicted in Fig. 3.42A(a) and (b) and the corresponding SEM images are shown in Fig. 3.42B. The electrochemical performance of the fabricated ASC is shown in Fig. 3.42C(a–e). The assembled ASC exhibited a typical capacitive behavior with high specific capacitance of 91.3 F g−1 at 2 mA cm−2. The tandem ASC was used to power WO3 based electrochromic device and the corresponding transmittance spectra were recorded, as shown in Fig. 3.42C(f). The tandem solid state ASC was proved to feasibly act as a power source for electrochromic devices. The summary of the electrochemical performance of various ASC using Fe2O3 anodes is listed in Table 3.9. Finally, the MnO2 and Fe2O3 thin films directly grown on the stainless steel substrate was employed as a flexible solid state supercapacitor by Gund et al. using the Na2SO4/carboxymethyl cellulose gel as the separator [809]. Both symmetric supercapacitor (MnO2 as positive and negative electrode) and ASC (MnO2 as a positive and Fe2O3 as negative electrode) were fabricated. The electrochemical measurement of MnO2-MnO2 symmetric cell was evaluated in the potential range of 0 to +1.0 V where a Csp of 85 F g−1 was obtained. The operating potential of MnO2-Fe2O3 ASC was 2 V and a maximum Csp of 92 F g−1 was achieved. Between both the devices, ASC provided a maximum Csp and 2-times higher energy density than symmetric supercapacitor. As the interest in the development of high performance electrode material for supercapacitor has intensed particularly for identifying suitable the negative electrodes with high specific capacitance friendliness to enhance the energy density of the supercapacitor, hematite (α-Fe2O3) is proved to be promising negative electrode in supercapacitor. In this section, the recent progress and, wide operating potential, high redox activity, low cost, abundant availability, and eco-development of α-Fe2O3 as negative electrode for supercapacitors has been examined critically and discussed the impact of morphology, surface area, crystallinity, and dispersibility over its electrochemical performance. The strategies to overcome the poor electrical conductivity and capacitance decay due to its volume expansion during multiple charge-discharge cycles were also discussed. An over view of asymmetric cell performance of α-Fe2O3 with various positive electrodes are presented, and it appears that the asymmetric configurations using α-Fe2O3 electrode provides a high energy density that can compete with commercial batteries and hence forms a wide scope in developing cost effective, high energy density devices.



164

3.  Electrode materials for supercapacitors

FIGURE 3.42  (A) Schematics for the (a) synthesis procedure of MnO2 nanowires (NWs) and Fe2O3 nanotubes (NTs) on carbon cloth (b) Assembled ASC (B) SEM images of MnO2 NWs and Fe2O3 NTs (C) (a) CV curves of MnO2 NWs-Fe2O3NTs ASC in different voltage range (b) GCD curves of assembled ASC (c) Specific/volumetric capacitance values from GCD curves vs. current density plot (d) Ragone plot (Inset: LED powered by tandem ASC) (e) Cycling performance of ASC at 2 mA cm−2 under various potentials (f) Transmittance spectra of WO3 electrode in original, colored and bleached states. Source: Reproduced from Ref. [808], with permission of American Chemical Society.

3.5  Perspectives on transition metal oxides for SC electrodes Most studied and promising transition metal oxides (RuO2, MnO2, SnO2, NiO, Co3O4, NiCo2O4, V2O5, and α-Fe2O3) for applications in supercapacitors were selected and critically examined. It can be noted that progress has been made in the past two decades. High specific capacity and excellent stability of nanomaterials have been observed, especially from some





165

3.5  Perspectives on transition metal oxides for SC electrodes

TABLE 3.9  Performance of α-Fe2O3 based electrodes in asymmetric supercapacitors. Cathode

Anode

Specific capacitance (F g−1)

Cell voltage (V)

Power density (kW kg−1)

Energy density (Wh kg−1)

References

ZnCo2O4@MnO2 core shell nanotube array

3D porous α-Fe2O3

161 at 2.5 mA cm−2

1.3

0.648

37.8

[800]

Ni3(PO4)2@GO

α-Fe2O3@GO

189 at 0.25 A g−1 1.6 88% after 1000 cycles

0.200

67.2

[801]

PANI nanorods

α-Fe2O3@ PANI

1.5 2.02 mF cm−3 ∼95.77% retention after 10,000 cycles

120.51 mWh cm−3

0.35 mWh cm−3

[802]

Graphite foam@ CoMoO4

G foam CNT @α-Fe2O3

210

1.4

74.7

[803]

Fe3O4@MnO2

Fe3O4@Fe2O3

2.0 1.49 F cm−3 92% retention after 5000 cycles

15.6 mWh cm−3 0.83 mWh cm−3

[804]

NiCo2O4/NiO

Fe2O3

417, 97% reten- 1.6 tion after 5000 cycles

0.157

19

[805]

CoNi-layered double hydroxide/ CNT

G/α-Fe2O3

252.4 at 0.5 A g−1 1.5–1.7

22.826

98

[806]

α-MnO2 thin film

α-Fe2O3 thin film

145 at 2 mA 2.0 cm−2, 97.1% retention after 3000 cycles

2.1

41

[807]

91.3 at 2 mA cm−2

1.6

0.55 mWh cm−3 0.55 mWh cm−3

[808]

75 at 1.28 A g−1

2.0

1.276

[809]

α-MnO2 nanowires α-Fe2O3 nanotubes MnO2 thin film

α-Fe2O3 thin film

1.6

41.8

Adapted with permission from Ref. [719].

mesoporous materials. Simple synthetic methods that can produce metal oxide nanomaterials with appropriate morphology and suitable for large-scale production are a concern for the development of real supercapacitor devices. Further, attention should also be bestowed on the design and synthesis of composite materials, such as transition metal oxides/metal, and transition metal oxides/active carbon, even on the combining of ionic liquids with mesoporous transition metal oxides. By doing so, the electrochemical performance of transition metal oxide-based pseudocapacitance could be envisioned to be improved substantially attributing to the synergistic effect of individual constituents. Especially, composite materials with 3D structures may provide more efficient and more rapid transportation for ions and electrons and thus result in higher electrochemical performance.



166

3.  Electrode materials for supercapacitors

3.6  Conclusions and outlook To meet the increasing demand for clean, sustainable energy coupled with the advantages of high power density, high efficiency and long life expectancy, electrochemical supercapacitors appear to be promising and one of the major emerging devices for energy storage and power supply. In spite of their feasibility for practical applications in hybrid power sources, back-up power sources, starting power for fuel cells, and burst-power generation in electronic devices, a key challenge for ES is their limited energy density, which has hindered their wider application in the field of energy storage. To address this challenge, research and development efforts has to be fostered to identify or search for new electrode materials with high capacitance and a wide potential window. The desired properties of ES electrode materials include: (1) high specific surface area, thus more active sites, (2) suitable pore-size distribution, pore network, and pore length for facilitating the diffusion of ions at a high rate, (3) low internal electrical resistance for efficient charge transport in the composite electrode, and (4) better electrochemical and mechanical stability for good cycling performance. Regarding porosity of ES materials, nanomicropores are necessary to achieve higher specific surface area, and these micropores have to be ensured to be electrochemically accessible for ions. Therefore, the pore network, the availability and wettability of pores, with dimensions matching the size of solvated anions and cations are crucial for ES electrode materials. The most studied and important ES electrode materials include: carbon-based materials, conductive polymers, and metal oxides. For carbon materials, higher specific surface area and rational pore distribution have been achieved, and in the commercially available ES, although their capacitances (or energy densities) are still low. Regarding conductive polymers, which exhibit high specific capacitances, the major challenges are their swelling and shrinking during charge/recharge, leading to a short ES lifetime. With respect to metal oxides, amorphous structures can possess high specific surface areas and favor the diffusion of ions into the bulk of the materials, and the hydrated water appears to facilitate the ion transport process. In case of RuO2-based materials, in spite of their high capacitances, their prohibitively high cost prevents their practical application in ES. It is worth mentioning that although thin films can provide ultra high surface area, high specific capacitance (>2000 F g−1), and rate capability due to increased electrical conductivity, in a real ES they may not necessarily be ideal electrode materials. This is because, when a film is used to construct a thicker electrode layer, the electrochemical utilization of the material and ionic transport throughout the internal volume of the thicker electrode layer will be limited, becoming a large obstacle to their practical use in ES electrodes. To develop new materials with optimal performance, two important research directions in ES electrode exploration are: (1) Development of composite materials: Regarding the materials for ES electrodes, combining different materials to form composites should be an important approach because the individual substances in the composites can exhibit synergistic effects through minimizing particle size, enhancing specific surface area, inducing porosity, preventing particles from agglomerating, facilitating electron and proton conduction, expanding active sites, extending the potential window, protecting active materials from mechanical degradation, improving cycling stability, and providing additional pseudocapacitance. Consequently, the composites can overcome the limitations of the individual substances and embody the advantages of



References 167

all constituents. High capacities over 1700 F g−1 have been reported on the basis of composite materials. But it is has to be pointed out that the reverse effects may also take place in the process of making composites. Consequently, there should be a compromise among the composition of individual substances and an optimized molar ratio of constituents for every composite material. (2) Development of nanomaterials: Recent trends in ES also involve the development of nanostructured materials, such as nanoaerogels, nanotubes/rods, nanoplates, nanospheres, etc. As nanostructured materials possess high specific surface area, they provide short transport/diffusion path lengths for ions and electrons, leading to faster kinetics, more efficient contact of electrolyte ions, and more electroactive sites for Faradaic energy storage, resulting in high charge/discharge capacities even at high current densities. Further, as material morphology is closely related to the specific surface area and the diffusion of ions in the electrode, and 1D nanostructure materials seem to be very promising for ES application due to their reduced diffusion paths and larger specific surface areas. Despite the research advances, it is clear that the development of supercapacitor electrode materials is extremely vital for the improvement of power quality with the advantage being a faster access to the stored energy up to 106 times charge/discharge cycles.

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[794] J. Hu, M. Noked, E. Gillette, F. Han, Z. Gui, C. Wang, S.B. Lee, Dual-template synthesis of ordered mesoporous carbon/Fe2O3 nanowires: high porosity and structural stability for supercapacitors, J. Mater. Chem. A 3 (2015) 21501–21510, doi: 10.1039/C5TA06372H. [795] M. Mallouki, F.T. Van, C. Sarrazin, P. Simon, B. Daffos, A. De, C. Chevrot, J.F. Fauvarque, Polypyrrole-Fe2O3 nanohybrid materials for electrochemical storage, J. Solid State Electrochem. 11 (2007) 398–406, doi: 10.1007/ s10008-006-0161-8. [796] M.B. Gawande, A. Goswami, T. Asefa, H. Guo, A.V. Biradar, D.L. Peng, R. Zboril, R.S. Varma, Core–shell nanoparticles: synthesis and applications in catalysis and electrocatalysis, Chem. Soc. Rev. 44 (2015) 7540–7590, doi: 10.1039/C5CS00343A. [797] A.M. Khattak, H. Yin, Z.A. Ghazi, B. Liang, A. Iqbal, N.A. Khan, Y. Gao, L. Li, Z. Tang, Three dimensional iron oxide/graphene aerogel hybrids as all-solid-state flexible supercapacitor electrodes, RSC Adv. 6 (2016) 58994–59000, doi: 10.1039/C6RA11106H. [798] J.W. Park, W. Na, J. Jang, Hierarchical core/shell Janus-type α-Fe2O3/PEDOT nanoparticles for high performance flexible energy storage devices, J. Mater. Chem. A 4 (2016) 8263–8271, doi: 10.1039/C6TA01369D. [799] Q. Tang, W. Wang, G. Wang, The perfect matching between the low-cost Fe2O3 nanowire anode and the NiO nanoflake cathode significantly enhances the energy density of asymmetric supercapacitors, J. Mater. Chem. A 3 (2015) 6662–6670, doi: 10.1039/C5TA00328H. [800] W. Ma, H. Nan, Z. Gu, B. Geng, X. Zhang, Superior performance asymmetric supercapacitors based on ZnCo2O4@MnO2 core–shell electrode, J. Mater. Chem. A 3 (2015) 5442–5448, doi: 10.1039/C5TA00012B. [801] J.J. Li, M.C. Liu, L.B. Kong, D. Wang, Y.M. Hu, W. Han, L. Kang, Advanced asymmetric supercapacitors based on Ni3(PO4)2@GO and Fe2O3@GO electrodes with high specific capacitance and high energy density, RSC Adv. 5 (2015) 41721–41728, doi: 10.1039/C5RA06050H. [802] X.F. Lu, X.Y. Chen, W. Zhou, Y.X. Tong, G.R. Li, α-Fe2O3@PANI core–shell nanowire arrays as negative electrodes for asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 14843–14850, doi: 10.1021/ acsami.5b03126. [803] C. Guan, J. Liu, Y. Wang, L. Ma, Z. Fan, Z. Shen, H. Zhang, J. Wang, Iron oxide-decorated carbon for supercapacitor anodes with ultrahigh energy density and outstanding cycling stability, ACS Nano 9 (2015) 5198–5207, doi: 10.1021/acsnano.5b00582. [804] X. Tang, R. Jia, T. Zhai, H. Xia, Hierarchical Fe3O4@Fe2O3 core–shell nanorod arrays as high-performance anodes for asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 27518–27525, doi: 10.1021/ acsami.5b09766. [805] R. Shanmugavani, Kalai Selvan, Microwave assisted reflux synthesis of NiCo2O4/NiO composite: fabrication of high performance asymmetric supercapacitor with Fe2O3, Electrochim. Acta 189 (2016) 283–294, doi: 10.1016/j.electacta.2015.12.043. [806] J. Chen, J. Xu, S. Zhao, N. Zhao, C.P. Wong, Facile and scalable fabrication of three-dimensional Cu(OH)2 nanoporous nanorods for solid-state supercapacitors, Nano Energy 15 (2015) 719–728, doi: 10.1039/C5TA04164C. [807] N.R. Chodanker, D.P. Dubal, G.S. Gund, C.D. Lokhande, Bendable all-solid-state asymmetric supercapacitors based on MnO2 and Fe2O3 thin films, Energy Technol. 3 (2015) 625–631, doi: 10.1002/ente.201402213. [808] P. Yang, Y. Ding, Z. Lin, Z. Chen, Y. Li, P. Qiang, M. Ebrahimi, W. Mai, C.P. Wong, Z.L. Wang, Low-cost highperformance solid-state asymmetric supercapacitors based on MnO2 nanowires and Fe2O3 nanotubes, Nano Lett. 14 (2014) 731–736, doi: 10.1021/nl404008e. [809] G.S. Gund, D.P. Dubal, N.R. Chodankar, J.Y. Cho, P.G. Romero, C. Park, C.D. Lokhande, Low-cost flexible supercapacitors with high-energy density based on nanostructured MnO2 and Fe2O3 thin films directly fabricated onto stainless steel, Sci. Rep. 5 (2015) 12454, doi: 10.1038/srep12454. [810] Y. Wang, J. Zeng, J. Li, X. Cui, A.M. Al-Enizi, L. Zhang, G. Zheng, One-dimensional nanostructures for flexible supercapacitors, J. Mater. Chem. A 3 (2015) 16382–16392, doi: 10.1039/C5TA03467A.



C H A P T E R

4

Electrolyte materials for supercapacitors O U T L I N E 4.1 Introduction

207

cal double-layer capacitors 221 4.3.1.2.2 Alkaline electro­ lytes for pseudoca­ pacitors 221 4.3.1.2.3 Alkaline electro­ lytes for hybrid capacitors 223 4.3.1.3 Neutral electrolytes 223 4.3.1.3.1 Neutral electrolytes for electrical double-layer capacitors 223 4.3.1.3.2 Neutral electrolytes for pseudocapaci­ tors 225 4.3.1.3.3 Neutral electrolytes for hybrid capacitors 228

4.2 Influence of electrolytes on the performance factors of ESs 209 4.2.1 Capacitance 211 4.2.2 Energy density and power density 212 4.2.3 Equivalent series resistance 212 4.2.4 Cycle life 213 4.2.5 Self-discharge rate 213 4.2.6 Thermal stability 213 4.3 Electrolyte materials and compositions for electrochemical supercapacitors 213 4.3.1 Aqueous electrolytes 215 4.3.1.1 Strong acid electrolytes 216 4.3.1.1.1 Acid electrolytes for electrical double-layer capacitors 216 4.3.1.1.2 Acid electrolytes for pseudocapaci­ tors 219 4.3.1.1.3 Acidic electrolytes for hybrid capacitors 220 4.3.1.2 Strong alkaline electrolytes 221 4.3.1.2.1 Alkaline electro­ lytes for electri­

4.3.2 Organic electrolytes

Materials for Supercapacitor Applications. http://dx.doi.org/10.1016/B978-0-12-819858-2.00004-4 Copyright © 2020 Elsevier Inc. All rights reserved.

205

230

4.3.2.1 General composition, properties, and ES performance of organic electrolytes 230 4.3.2.1.1 Organic electro­ lytes for electrical double-layer capacitors 230

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4.3.2.1.2 Organic electro­ lytes for pseudoca­ pacitors 241 4.3.2.1.3 Organic electro­ lytes for hybrid capacitors 241 4.3.2.2 Organic solvents 242 4.3.2.2.1 Single organic solvents for electrolytes 244 4.3.2.2.2 Solvent mixtures for electrolytes 247 4.3.2.3 Conducting salts for electro­ lytes 248 4.3.2.3.1 Effect of conduct­ ing salt on ES performance 248 4.3.2.3.2 Exploration of new conducting salts 250

4.3.4.1.2 Organogel electro­ lytes 270 4.3.4.1.3 IL-based solid-state electrolytes 272 4.3.4.1.4 Environmentally friendly gel polymer electrolytes 273 4.3.4.1.5 Structural electro­ lytes 273 4.3.4.2 Inorganic solid-state electrolytes 273

4.3.5 Redox-active electrolytes

274

4.3.5.1 Redox-active aqueous electrolytes 274 4.3.5.1.1 Redox-active aque­ ous electrolytes for carbon-based ESs 274 4.3.5.1.2 Redox-active aque­ ous electrolytes for pseudocapacitive electrodes 278 4.3.5.2 Redox-active nonaqueous electrolytes 278 4.3.5.3 Redox-active solid electrolytes 279

4.3.3 Ionic liquid-based ES electrolytes 250 4.3.3.1 General composition, properties and ES performance of ionic liquid electrolytes 250 4.3.3.2 Solvent-free ionic liquids 253 4.3.3.2.1 Solvent-free ionic liquids for EDLCs 253 4.3.3.2.2 Solvent-free ionic liquids for pseudo­ capacitors 259 4.3.3.2.3 Solvent-free ionic liquids for hybrid electrochemical capacitors 260 4.3.3.3 Mixtures of ionic liquids and organic solvents 261

4.4 Electrolyte compatibility with inactive components of ESs 4.4.1 Compatibility with current collectors 4.4.2 Binders 4.4.3 Separators

4.3.4 Solid- or quasi-solid-state electrolytes for ESs 263 4.3.4.1 Gel polymer electrolytes 266 4.3.4.1.1 Hydrogel polymer electrolytes  266



280 280 281 282

4.5 Electrolyte performance validation using supercapacitor test cells

283

4.6 Challenges in the development of ES electrolytes

285

4.7 Summary and future research directions

288



4.1 Introduction

207

4.1 Introduction Electrolyte is one of the components and the most influential one in dictating the performance of any electrochemical energy device. It is necessary to enhance the energy densities of the state of the art electrochemical supercapacitors (ES) known to date to an extent of ~10 Wh kg−1 for EDLCs and > 50 Wh kg−1 for both pseudocapacitors and hybrid capacitors, in order to qualify them to meet the demands of high energy density applications, compared to other electrochemical devices, such as batteries and fuel cells. Extensive R&D efforts have been bestowed to enhance the energy density of ESs [1, 2], and to widen the scope of their application. As the energy density (E) of ESs is proportional to the capacitance (C) and the square of the voltage (V), that is, E = 1/2 CV2, increasing either/or both of the capacitance and the cell voltage is a direct means to increase the energy density. This can be accomplished by employing advanced electrode materials with high capacitance, electrolytes (electrolyte salt + solvent) with wide potential windows, and integrated systems/advanced cell architectures with a new and optimized structure. Various developmental attempts that are made to date can be enlisted as follows: (1) increasing the specific capacitance of carbon-based electrodes by incorporating novel carbon structures with high effective specific surface area and high packing densities [3]; (2) developing pseudocapacitors based on pseudocapacitive materials, such as electroactive transition-metal oxides and conducting polymers with high specific capacitance which contribute to pseudocapacitance [4]; (3) enhancing the cell voltage through the development and application of new electrolytes and (4) exploring ESs based on new concepts and novel structures, such as the hybrid or asymmetric capacitors (e.g., lithium-ion capacitors, LICs) [1]. It has to be pointed out that all of these developments are closely related to each other. Although it is relatively simple to fabricate the individual components, such as electrode materials, electrolytes, and structures of the ESs, it is challenging to identify and develop an appropriate material to promote the synergistic effect in addition to its compatibility with other cell components. For instance, the design and preparation of the porous carbon electrode materials with high specific surface area should also consider the matching between the pore structure and size of the electrolyte ions in order to fabricate a high capacitive electrode [5]. During the development of advanced electrolytes for any electrochemical device, it is essential to consider their possible interaction and compatibility with the electrode materials and other electrode components. Regarding the development of electrolytes for ES, widening the potential window of an electrolyte solution, that is, enlarging the cell voltage (V), can effectively increase the energy density as can be seen from the equation of E = 1/2 CV2. It is worth noticing that increasing the cell voltage would be more efficient than increasing the electrode capacitance to achieve an enhancement in the energy density, as the energy density is proportional to the square of the cell voltage. On this ground, developing new electrolytes/solutions with wide potential windows gains higher prominence in the development of new electrode materials. As mentioned, the operating cell voltage of the ESs is largely dependent on the electrochemical stable potential window (ESPW) of the electrolytes, provided the electrode materials are stable in the entire working voltage regime. For example, ESs based on aqueous electrolytes, usually have an operating potential window of about 1.0-1.3 V because the potential window of the aqueous electrolyte is about 1.23 V (the potential window of H2/O2 evolution reactions



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4.  Electrolyte materials for supercapacitors

at 1.0 atm and room temperature), while the ESs based on organic electrolyte and ionic liquid (IL) generally have potential windows of 2.5-2.7 and 3.5-4.0 V, respectively. Further, the interactions between the electrolyte salt and solvent (electrolyte solution components and their concentrations) also play a critical role in establishing other important properties, such as the power density, internal resistance, rate performance, operating temperature range, cycling efficiency, life-time, self-discharge, and toxicity (Table 4.1), which are also important in the practical applications of ESs. Besides the importance of the electrolyte potential window, the interaction between the electrolyte and the electrode materials also plays an important role in the ES performance. For instance, a matching between the size of the electrolyte ion and the pore size of carbon electrode material has a profound influence on the achievable specific capacitance. The pseudocapacitance that emerges from the carbon-based materials and transition metal oxides are also mainly dependent on the nature of the electrolytes [6]. The ionic conductivity of the electrolytes plays a significant role in the internal resistance of ESs especially for organic and IL electrolytes. The viscosity, boiling point, and freezing point of the electrolytes can also directly impact the thermal stability and thereby the operating temperature range of ESs. As observed, the aging and performance failure of ESs are also related to the electrochemical decomposition of the electrolytes. Further, the development of new types of ESs, such as flexible or solid-state ESs and micro-ESs also relies mainly on new electrolytes, especially on solid-state electrolytes. Regarding electrolyte developments for ESs, a large variety of electrolytes, such as aqueous electrolytes, organic electrolytes, IL electrolytes, redox-type electrolytes, and solid or semi-solid electrolytes have been explored and tremendous advancement has been made in the past few decades. For example, the operating potential window of aqueous electrolyte-based ESs has been reported to be greatly enhanced to ~2 V by using neutral aqueous electrolytes [7]. A wide variety of new organic electrolytes with wider operating potential windows and less toxicity compared to the commercial organic electrolytes have been developed for ESs. Regarding the development of IL electrolytes, the operating cell voltage of the corresponding ESs was further increased to 4 V, in spite of the associated issues, such as low ionic conductivity and high viscosity [8]. The exploration for solid or semi-solid electrolytes has led to the invention of flexible or solid-state ESs, which exhibits no leakage issue as in case with the liquid electrolyte-based ESs [9, 10]. Recently, TABLE 4.1  Various performance parameters of ES influences by electrolyte characteristics [20]. Thermal Stability

Cycle life

Ionic conductivity

Boiling point

Stability

Potential window

Freezing point

Ion-electrode material interaction

Ion-electrode material interaction

Salt solubility

Capacitance

ESR

Energy density

Power density

Ion size

Ionic conductivity

Concentration

Matching between ion size and pore size

Ionic mobility

Electrochemical stability

Viscosity





4.2  Influence of electrolytes on the performance factors of ESs

209

redox-type electrolytes have also been examined for ESs due to the additional pseudocapacitance contribution from the redox reaction of the electrolyte at the electrode/electrolyte interface [11]. The desired characteristics of an ideal electrolyte can be enlisted as: (1) wide potential window; (2) high ionic conductivity; (3) high chemical and electrochemical stability; (4) high chemical and electrochemical inertness to ES components, such as electrodes, current collectors, and packaging, etc.; (5) wide operating temperature range; (6) good matching with the electrode materials; (7) low volatility and flammability; (8) environmental friendliness; and (9) low cost. In reality, it is rare to find an electrolyte meeting all of these requirements, and each electrolyte has its own advantages and limitations. This has prompted tremendous research efforts to improve the overall performance of electrolytes for their employment in ESs. In the literature, one can find several excellent reviews focusing on the electrode materials [4, 12-17], while reviews concerning the electrolytes for ESs are rather limited [8-10, 13, 18, 19]. Although several excellent reviews concerning solid polymer electrolytes for ESs do exist, [9, 10, 18, 19], reviews or documents covering the historical developments to latest achievements in this field and providing an insight into the electrolyte development is limited [20]. Hence, it is our intention to provide a panoramic view on the development of various classes of electrolytes and to bring out the logistics with an aim to develop electrolytes suiting to long-term operations of ESs in order to make them viable for their employment in alternate energy systems those operate based on renewable energy sources. In this chapter, the first section deals with the general performance scaling of ESs and their relationship with electrolytes. In the second section, we provide a comprehensive overview of various types of ES electrolytes/solutions, their performance comparison, and the interactions between electrolytes and electrode materials. The influence of electrolytes on the ES performance is discussed to aid an understanding on the performanceinfluencing factors in the design and optimization of electrolytes. Further, the interplay between electrolytes and inactive components of ESs, such as current collectors, binders, and separators is also presented. In the final section, the major challenges and perspectives in ES electrolyte/solution research and development are documented and possible research directions in overcoming the challenges are proposed to facilitate efforts in this area.

4.2  Influence of electrolytes on the performance factors of ESs The properties of the electrolyte-solution system (solvent plus solute salt) employed for electrochemical capacitors impact their electrical behavior in three ways: At first, in a primary manner, it impacts the conductance of the electrolyte in the device and its equivalent series resistance (ESR), and thus its capability for power output. The second is through anion adsorption from the electrolyte, impacting in part the specific double-layer capacitance, especially at potentials positive to the potential-of-zero charge of the carbon electrode material. The third is through the dielectric properties of the solvent, which also determine the specific doublelayer capacitance value and its dependence on electrode potential, as well as the extent of ionization or ion pairing of the solute salt, which influences the conductance.



210

4.  Electrolyte materials for supercapacitors

FIGURE 4.1  Types of ion pairs in an electrolyte solution. (A) Solvated-ion pairs, (B) solvent shared ion pairs, and (C) contact ion pairs. Source: Reproduced with permission from Ref. [21].

The two principal factors involved in conductance are (1) the concentration of free charge carriers, cations, and anions of a given salt or acid solute and (2) the ionic mobilities, or conductance contributions per ion, of the dissociated ions of the electrolyte solute. The factors that in turn determine (1) and (2) are (3) the solubility of the salt in the solvent of choice and (4) the degree of dissociation, α, of the dissolved salt "molecules" into free ions, or conversely, the extent of cation-anion association or pairing of the ions of the dissolved salt or acid. Three stages of ion pairing, depending on the extent of retention of solvation, are illustrated in Fig. 4.1A-C [21]. The degree of dissociation α, determines the fraction of salt moles that are available as free charge carriers of electric current. Usually, there is a dynamic equilibrium between free charge carriers and undissociated salt, the species depending on salt concentration, temperature, and the dielectric constant of the solvent medium. Other factors are (5) the viscosity of the solvent, η, a temperature-dependent property; and (6) long-range electrostatic interactions between the free, dissociated ions, which are determined by the dielectric constant of the solvent. For maximum power performance, the electrical resistance distributed in the porous matrix must be minimized by maximizing the conductance of the invading electrolyte, which provides the basis for doublelayer capacitance or for the Faradaic processes associated with charge or discharge of pseudocapacitance, for example, due to underpotential electrosorption processes or redox reactions. To summarize, the factors, which determine the conductance of electrolyte solutions, are: • Solubility of the salt or acid • Degree of dissociation or extent of cation and anion pairing in solution 



• • • • • • •

4.2  Influence of electrolytes on the performance factors of ESs

211

Dielectric constant of the bulk solvent Electron pair donicity of the solvent molecules Mobility of the free, dissociated ions Viscosity of the solvent Solvation of the free ions and the radii of the solvated ions Temperature coefficient of viscosity and of ion-pairing equilibria Dielectric relaxation time of the solvent

4.2.1 Capacitance The ES can be considered as two capacitors connected in series, as both electrode/electrolyte interfaces represent a capacitor. If the capacitances of the two electrodes are expressed as C1 and C2, respectively, the overall capacitance (CT) can be expressed as: 1/ CT = 1/ C1 + 1/ C2 (4.1) For a symmetric ES, as C1 = C2, and hence the total capacitance (CT) would be half of either one of the electrode capacitance. In case of the asymmetric ES, CT is mainly influenced by the electrode with a smaller capacitance. To evaluate the capacitance of an electrode material, the parameter specific capacitance is often used, which can be expressed either as the mass specific capacitance, which is also termed as gravimetric specific capacitance (Cs), or the volumetric capacitance (CV). Generally, Cs is the most frequently used one to characterize an electrode material, with a unit of Faraday per gram (F g−1) and can be expressed as: Cs = Ci / Wem (4.2) where Wem is the weight of the electrode material in the electrode layer (g), and Ci is its corresponding electrode capacitance (anode or cathode) (F). A comparison between different electrode materials can be made based on their values of Cs. For an ES, its total special capacitance (CTS) should be the total capacitance of the device (CT) normalized by the weight of the device WTM (WTM is the sum weight of all necessary components including the anode, cathode, electrolyte solution, current collector, and others), which can be expressed as: CTS = CT / WTM (4.3) Normally, Eq. (4.3) indicates that the weight of the electrolyte solution can have a negative impact on the capacitance of ES, because it can increase the weight of the entire device. Therefore, lighter weight and smaller volume of electrolyte solution is desired. It can be noted that although Cs in Eq. (4.2) does not contain the weight of the electrolyte, the electrolyte solution can also affect its capacitance value by affecting the capacitance of the electrode [Ci in Eq. (4.2)]. For instance, if the electrolyte solution has better contact (wettability) with the electrode material, the area of the double-layer is increased, leading to a larger electrode capacitance. Therefore, developing electrolytes, which could completely fill the pore areas in the electrode layer, to maximize the material utilization for the generation of capacitance is a critical aspect in improving the ES performance. 

212

4.  Electrolyte materials for supercapacitors

4.2.2  Energy density and power density When an ES is charged, a cell voltage (V) will be built up across the two electrodes. The theoretical (or maximum) energy density of the ES cell (E), and the power density (P) can be expressed as Eqs. (4.4) and (4.5), respectively [16]: E = 1 2 CTSV 2 (4.4) P=

1 ×V2 4 WTS Rcell

(4.5)

where in Eq. (4.5), Rcell is the ESR of the ES cell (Ω). Hence, the energy density [Eq. (4.4)] is generally expressed in the unit Wh kg−1, and the power density [Eq. (4.5)] with the unit W kg−1 [22, 23]. These two equations show that V, CTS, WTS, and Rcell are four important variables in determining the performance of an ES. In order to improve both the energy and power densities of an ES, it is essential to increase the values of both V and CTS and simultaneously reduce the values of both WTS and Rcell. Since both energy and power densities are proportional to the square of the operating voltage, increase in the cell voltage would have a greater contribution to the improvement in the energy and power densities of the ES than with increase in the capacitance or reduction in the resistance. In general, the maximum operating voltage of an ES is strongly dependent on the ESPW or the potential window of the electrolyte.

4.2.3  Equivalent series resistance The ESR is an important parameter for determining the power density of the ES, as indicated by Eq. (5.5), which shows that the power density increases with a decreasing ESR value. Similar to other electrochemical energy storage devices, a high ESR limits the charging/discharging rate, leading to low power density. Therefore, for some pulse power applications, the ESR value of an ES is even more important than the capacitance value. Normally, ESR is the sum of various types of resistances including intrinsic resistance of the electrode material and electrolyte solution, mass transfer resistance of the ions, and contact resistance between the current collector and the electrode [16]. As identified, the resistances of the bulk electrolyte solution and the electrolyte inside the pores of the electrode layer tend to dominate the ESR, especially when nonaqueous electrolytes, such as organic, IL, and solid-state electrolytes are used in the ESs. Therefore, in order to achieve a high-power density in ES, it is necessary to use an electrolyte with high ionic conductivity. However, there is often a trade-off between the ionic conductivity and the operating potential window of the electrolytes. Aqueous electrolytes, such as H2SO4 and KOH have high ionic conductivities, but the operating potential window is small. In contrast, although nonaqueous electrolytes, such as organic and IL can offer the advantage of high operating voltages, their ionic conductivities are generally at least one order of magnitude lower than that of the aqueous electrolytes [24], leading to a higher ESR, and thereby limiting the power density. Therefore, for ESs to achieve both high energy and power densities, it is essential to develop electrolytes with a wide operating voltage and a small ESR (or high ion conductivity).





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

213

4.2.4  Cycle life As cycle life is an essential indicator of the stability of the ES as well as one of the important parameters for measuring the overall performance of the ES. In the stability analysis, the electrode is subjected to continuous charge and discharge cycling in a chosen electrolyte to compare the initial and final capacitance. For example, EDLCs using carbon electrodes generally have a very high cycling stability [22]. However, when pseudocapacitive reactions are introduced, the cyclic stability is generally reduced due to the nonideal electrochemical reversibility arising from the interactions between the electrolyte ions and the electrode materials. Actually, the cycle life of the ES depends on many factors, such as the type of the cell, electrode material, electrolyte, charging/discharging rate, operating voltage, and temperature, which will be discussed in the following sections.

4.2.5  Self-discharge rate Another issue concerning the ES performance is self-discharge rates, which are related to potential losses of a charged electrode over a period of storage time [25, 26]. During the self-discharge process, current leakage leads to a decrease of the cell voltage, which in turn may limit the use of ESs for some applications requiring a fixed amount of energy retention for a relatively long time. Several mechanisms have been postulated to explain the potential change during the self-discharge process [27]. As will be discussed later, the ES self-discharge rate and its mechanism depend on the type of electrolyte, impurities, and residual gases.

4.2.6  Thermal stability As most potential applications for ESs eventualize till date, in the temperature range, 30-70°C [2], expanding the working temperature range of ESs can widen the fields of their applications. For instance, the electronics related to space avionics applications required to operate at temperatures as low as -55°C, while the fuel cell vehicles require a high working temperature for ESs. Hence, the working temperature impacts several properties of ESs, such as the energy and power densities, rate performance, ESR, cycle life, and self-discharge rate. Especially, the temperaturedependent performance of ESs is mainly influenced by the nature of electrolyte, such as its concentration, type of conducting salt, and specific properties of the solvent, such as freezing point, boiling point, and viscosity, etc.; this aspect will be dealt with greater details in the later sections. It can be seen that the primary characteristics of ESs are strongly dependent on the specific electrolytes employed in the system. The following sections provide a focused overview of the developments concerning various types of electrolytes and their associated influences on the operating characteristics and performance of the ESs.

4.3  Electrolyte materials and compositions for electrochemical supercapacitors The electrolyte, which consists of the electrolyte salt + solvent, is one of the key components of ESs, providing ionic conductivity and thus facilitating charge compensation on each electrode in the cell. The electrolyte not only plays a fundamental role in the EDL



214

4.  Electrolyte materials for supercapacitors

formation (in EDLCs) and the reversible redox process for the charge storage (in pseudocapacitors) but also determines the performance of ES (Fig. 4.1). At present, most of the commercial ESs use organic electrolytes with a cell voltage regime 2.5-2.8 V [23]. Most organic electrolyte-based ESs uses the acetonitrile (ACN) as solvent while the others employ propylene carbonate (PC). The electrolyte nature, including: (1) the ion type and size; (2) the ion concentration and solvent; (3) the interaction between the ion and the solvent; (4) the interaction between the electrolyte and the electrode materials; and (5) the potential window, all have an influence on the EDL capacitance and pseudocapacitance, the energy/ power densities and the cycle life of the ES. For example, the ESPW of the electrolyte directly determines the operational cell voltage of the ES, through which both the energy and power densities are affected [Eqs. (4.4) and (4.5)]. The ESR of an ES is directly related to the ion conductivity of the electrolyte and has notable influence on the power density. Furthermore, the interaction between the ion and the solvent, and the interaction between the electrolyte and the electrode material can affect the lifetime and self-discharge of ESs. Various types of electrolytes have been developed as can be witnessed from the literature. As shown in Scheme 4.1, these electrolytes are broadly classified as liquid electrolytes and solid/quasi-solid-state electrolytes. In general, liquid electrolytes can be further grouped into aqueous electrolytes, organic electrolytes, and ILs, while solid or quasi-solid-state electrolytes can be broadly divided into organic electrolytes and inorganic electrolytes. To date, no electrolyte has been identified to have all the desired characteristics indicated earlier. Each electrolyte has its own advantages and disadvantages. For example, ESs using aqueous electrolytes possesses both high conductivity and capacitance, but their working

SCHEME 4.1  Classification of electrolytes for electrochemical supercapacitors. Source: Reproduced with permission from Ref. [20].





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

215

voltage is limited by the narrow decomposition voltage of aqueous electrolytes. Although organic electrolytes and ILs can operate at higher voltages, these normally have lower ionic conductivity. Solid-state electrolytes may avoid the issues concerning the leakage of the liquid electrolytes, but they also suffer from low conductivity. To overcome the drawbacks of targeted electrolytes, tremendous efforts have been devoted to explore new electrolyte materials to improve the overall performance of the ES. In this regard, various approaches have been undertaken, including: (1) development of new and high-performing electrolytes to achieve a wider operating potential window, higher ionic conductivity/viscosity, a wider working temperature range, etc.; (2) exploration of the desirable effects of given electrolytes on the ES properties, such as capacitance, energy and power densities, thermal stability, and the self-discharging process; and (3) establishing a fundamental understanding of the effect of the electrolyte on the ES performance through advanced experimental, modeling, and simulation methods [20].

4.3.1  Aqueous electrolytes In terms of energy density, aqueous electrolytes are of least choice for commercial ES, due to their narrow voltage windows, and this is the main reason for the commercial ESs to opt for organic electrolytes. However, aqueous electrolytes have been investigated extensively in R&D, for example, nearly 80% of the published literature employed aqueous electrolytes for ESs [20]. This is mainly because, aqueous electrolytes are inexpensive and can be easily handled in the laboratory in the absence of special conditions, thus greatly simplifying the fabrication and assembly processes. Organic electrolytes and ILs, on the other hand, require complicated purification procedures under strictly controlled atmosphere to avoid introduction of moisture. Normally, aqueous electrolytes exhibit high conductivity (e.g., ~0.8 S cm2 for 1 M H2SO4 at 25°C), which is at least one order of magnitude higher than that of organic and IL electrolytes [28]. This is beneficial for lowering the ESR, and to promote better power delivery. The selection criteria for aqueous electrolytes generally consider the sizes of bare and hydrated cations and anions and the mobility of ions (Table 4.2), which affect not only the ionic conductivity but also the specific capacitance value. In addition, ESPW and the extent of corrosiveness of electrolyte should also be taken into consideration. In general, aqueous electrolytes can be grouped into acid, alkaline, and neutral solutions in which H2SO4, KOH, and Na2SO4 are representatives and also the most frequently used electrolytes (Scheme 4.1). As mentioned earlier, the main disadvantage of aqueous electrolytes is their relatively narrow ESPW, restricted by the decomposition of water. For example, hydrogen evolution occurs at a negative electrode potential of around 0 V versus SHE, and oxygen evolution at a positive electrode potential of around 1.23 V, the resulting ES has a cell voltage about of 1.23 V [29]. The gas evolution would potentially cause the rupture of the ES cells, threatening the safety and deteriorating the performance. To avoid the gas evolution, the cell voltage of ESs with aqueous electrolytes is generally confined to about 1.0 V. Table 4.3 lists the typical aqueous electrolyte-based ESs and their performance. It can be seen that for acid and alkaline electrolytes, the cell voltages are all limited within 1.3 V, irrespective of the electrode material. For neutral electrolytes, the highest cell voltage reported in Table 4.3 is 2.2 V [7]. Besides, the operating temperature range of ESs with aqueous electrolytes has to be restricted to above the freezing and below the boiling points of water.



216

4.  Electrolyte materials for supercapacitors

TABLE 4.2  The sizes of bare and hydrated ions, and ionic conductivity values. Ion

Bare ion size (Å)

Hydrated ion size (Å)

Ionic conductivity (S cm2 mol−1)

H+

1.15

2.80

350.1

+

Li

0.60

3.82

38.69

Na+

0.95

3.58

50.11

+

K

1.33

3.31

73.5

+ 4

1.48

3.31

73.7

Mg2+

0.72

4.28

106.12

2+

Ca

1.00

4.12

119

Ba2+

1.35

4.04

127.8

Cl

1.81

3.32

76.31

NO −3

2.64

3.35

71.42

2− 4

2.90

3.79

160.0

−

1.76

3.00

198

2.92

3.38

67.3

3− 4

2.23

3.39

207

2− 3

2.66

3.94

138.6

NH

−

SO

OH

ClO PO

CO

2− 4

Reproduced with permission from Ref. [20].

4.3.1.1  Strong acid electrolytes As listed in Table 4.3, acidic electrolytes are the main choice for most of the ES studies. Among different acidic electrolytes, H2SO4 is the most commonly used one for aqueous-based ESs mainly due to its high ionic conductivity (0.8 S cm−1 for 1 M H2SO4 at 25°C). As the conductivity is strongly dependent on the concentration of H2SO4, extensive studies have been made to find the optimum concentrations to achieve the maximum ionic conductivities of the H2SO4 electrolytes at certain temperatures. In general, the ionic conductivity of the electrolyte is decreased when the concentration is too low or too high. As the maximum ionic conductivity of the H2SO4 electrolyte is achieved for 1.0 M concentration at 25°C, most of studies use 1.0 M H2SO4 electrolyte, particularly, for the ESs using carbon-based electrode materials. 4.3.1.1.1  Acid electrolytes for electrical double-layer capacitors

From the literature, it can be inferred that the specific capacitances of EDLCs, obtained in the H2SO4 electrolyte are higher than that in the neutral electrolytes [33, 58-60]. In addition, due to the higher ionic conductivity of H2SO4 compared to neutral electrolytes, the ESR of ESs with H2SO4 is generally lower than the ones realized with the neutral electrolytes [33, 59, 60]. Also, it has been found that there is a relationship between the specific capacitance of the activated carbons and the electrolyte conductivity, that is, the specific capacitance rises with the rise in electrolyte conductivity [58]. This can be understood by considering the ion mobility, which is closely related to the electrolyte conductivity. For EDLCs with a strong





217

4.3  Electrolyte materials and compositions for electrochemical supercapacitors

TABLE 4.3  Aqueous electrolyte-based supercapacitors and their performance. Aqueous electrolyte/concentration (M)

Electrode materials

Specific capacitance (F g−1)

Cell voltage (V)

Energy density (Wh kg−1)

Power density (W kg−1)

Temperature (°C)

References

105 at 4 mV s−1

0.8

4

20

RT

[30]

−1

Strong acid electrolyte H2SO4/2

MMPGC

H2SO4/1

AC fibers

280 at 0.5 A g

0.9

-

-

RT

[31]

H2SO4/1

GQD/3DG composite

268 at 1.25 A g−1

0.8

-

-

RT

[32]

H2SO4/1

Microporous carbon

~100 at 0.2 A g−1

1

~ 3.8

~100

RT

[33]

H2SO4/2

3D heteroatom doped carbon fiber

204.9 at 1 A g−1

1

7.76

~100

RT

[34]

H2SO4/1

P-enriched carbon

220 at 1 A g−1

1.3

16.3

33

-

[35]

H2SO4/1

ANS-rGO

375 at 1.3 A g−1

2.0

213

1328

RT

[36]

H2SO4/0.5

RuO2-graphene

479 at 0.25 A g−1

1.2

20.28

600

-

[37]

H2SO4/1

PANI-grafted rGO

1045.51 at 0.2 A g−1

0.8

8.3

60 000

-

[38]

H2SO4/0.5

PPy thin films

510 at 0.25 mA cm−2

1

133

758

-

[39]

H2SO4/1

Graphene/ mPANI

749 at 0.5 A g−1

0.7

11.3

106.7

-

[40]

H2SO4/1

N and O containing hierarchial porous frameworks

428.1 at 0.5 A g−1

0.8

37.4

197

-

[41]

Strong alkaline electrolyte KOH/6

3D FHPC

294 at 2 mV s−1

1

-

-

-

[42]

KOH/6

Highly porous graphene planes

303 at 0.5 A g−1

1

6.5

~50

-

[43]

KOH/6

p-CNTn/CGBs

202 at 0.325 A g−1

0.9

4.9

150

RT

[44]

KOH/2

Sub-3 nm Co3O4 nanofilms

1400 at 1 A g−1

0.47

-

-

RT

[45]

KOH/2

Porous NiCo2O4 nanotubes

1347.6 at 1 A g−1

0.41

38.5

205

-

[46]

LiOH/1

MnO2 nanoflower

363 at 2 mV s−1

0.6

-

-

-

[47] (Continued)



218

4.  Electrolyte materials for supercapacitors

TABLE 4.3  Aqueous electrolyte-based supercapacitors and their performance. (Cont.) Aqueous electrolyte/concentration (M)

Electrode materials

Specific capacitance (F g−1)

Cell voltage (V)

Energy density (Wh kg−1)

Power density (W kg−1)

Temperature (°C)

References

Neutral electrolyte Na2SO4/1

3D FHPC

-

1.8

15.9

317.5

-

[42]

NaNO3/1

AC

116 at 2 mV s−1

1.6

-

-

RT

[48]

1.8

~7

~40

-

[33]

−1

Na2SO4/0.5

Microporous carbon

~ 60 at 0.2 A g

NaNO3-EG/4

AC

22.3 at 2 mV s−1 (0°C)

2

14-16 (RT)

~500

0-60

[49]

Na2SO4/0.5

Sea weed carbon

123 at 0.2 A g−1

1.6

10.8

-

-

[50]

Na2SO4/0.5

AC

135 at 0.2 A g−1

1.6

~10

-

-

[51]

KCl/1

MnCl2-doped PANI/ SWCNT

546 at 0.5 A g−1

1.6

194.13

~550

RT

[52]

Li2SO4/1

AC

180 at 0.2 A g−1

2.2

-

-

-

[7]

−1

1

~28.4

~70

RT

[53]

Na2SO4/1

Mesoporous MnO2

278.8 at 1 mV s

Li2SO4/1

Mesoporous MnO2

284.24 at 1 mV s−1

1

~28.8

~70

RT

[53]

K2SO4/0.65

Mesoporous MnO2

224.88 at 1 mV s−1

1

~24.1 (0.5 M K2SO4)

~70

RT

[53]

Na2SO3/1

Ordered mesoporous C/ Fe2O3 NP

235 at 0.5 A g−1

1

-

RT

[54]

Na2SO4/0.5

MnO2/CNF

551 at 2 mV s−1 (75°C)

0.85

-

0-75

[55]

Na2SO4/1

Hydrous RuO2

52.66 at 0.625 A g−1

1.6

500

-

[56]

Na2SO4/1

CNFs radially grown graphene sheets

-

1.8

450

-

[57]

Abbreviations: 3D FHPC, 3D flower-like and hierarchical porous carbon material; AC, activated carbon; ANS, 6-amino-4-hydroxy-2naphthalenesulfonic acid; CNFs, carbon nanofibers; EG, ethylene glycol; MMPGC, macro/mesoporous partially graphitized carbon; GQD-3DG composite, graphene quantum dots-3D graphene composites; mPANI, mesoporous PANI film on ultra-thin graphene nanosheets; PANI, polyaniline; p-CNTn/CGBs, porous CNT networks decorated crumpled graphene balls; PPy, polypyrrole; rGO, reduced graphene oxide; RT, room temperature; SWCNTs, single-walled carbon nanotubes; Temp, temperature. Adapted with permission from Ref. [20].

acid electrolyte, such as H2SO4, the reported specific capacitances in the literature published in the past several years are mainly in the range, 100-300 F g−1 [31-33], which are generally higher than those obtained in organic electrolytes. Actually, the major contribution to the specific capacitance is made from the carbon-based electrode materials as reviewed by excellent reviews [12-17], and the electrolyte contribution to the capacitance value should be 



4.3  Electrolyte materials and compositions for electrochemical supercapacitors

219

less than that of electrode materials. As observed, the H2SO4 electrolyte-based EDLCs have higher specific capacitances when compared to those with organic electrolyte-based ones, even while using the same electrode materials. This may reflect the different interactions induced by different electrolytes with the electrode materials. 4.3.1.1.2  Acid electrolytes for pseudocapacitors

Due to low energy density of EDLCs, extensive efforts have been made to increase the value of the energy density by exploring other types of ESs, such as pseudocapacitors. For carbon-based electrode materials, the specific capacitance in the aqueous H2SO4 electrolyte also included some pseudocapacitance contributions besides the electrostatic EDL capacitance [27]. This has been attributed to the fast redox reactions occurred on the particular surface functionalities, such as oxygenated carbon species [27]. The pseudocapacitance could be further enhanced by introducing heteroatoms such as oxygen [61], nitrogen [62, 63] and phosphorous [35, 62] or certain surface functional groups (e.g., anthraquinone [64]) to the carbon surfaces. It should be emphasized that the nature of electrolyte has a strong influence on the pseudocapacitive properties of carbon-based materials because the surface functionalities behave differently with different electrolytes. For example, surface quinone-type functionalities could exhibit pseudocapacitive effects in the presence of acidic aqueous electrolytes (e.g., H2SO4) as the protons are involved in the redox processes as shown in Eq. (4.6) [6, 64]. In alkaline electrolytes, this effect was hardly observed [6, 64].

(4.6)

Therefore, generating favorable surface functionalities on the carbon-based electrode materials using suitable electrolytes appears to be relevant to achieve an optimum ES performance. However, the cycle lives of pseudocapacitors are normally shorter than that of EDLCs, due to the degradation of electrode functional materials in aqueous electrolytes. In contrast, with organic electrolytes, the ES cycle lives are notably longer than those in aqueous electrolytes. This is because; the capacitances of organic electrolytes largely arise from the electrostatic charge separation [65]. The introduction of certain surface functional groups (e.g., phosphorus groups) into the carbon surface, showed some improvement in the stability of electrode materials in aqueous electrolytes even at higher voltages [35]. In addition, the pseudocapacitance could also be derived from other pseudocapacitive materials, such as metal oxides, sulfides, and electrically conductive polymers (ECPs), which have much higher theoretical capacitances than carbon-based materials in aqueous electrolytes [4, 15]. However, these electrode materials are not normally stable in acidic aqueous electrolytes due to their sensitivity to the type and pH of the electrolytes. In this regard, a few of the noncarbon materials could be suitable for the pseudocapacitors in strong acidic electrolytes except RuO2. As 

220

4.  Electrolyte materials for supercapacitors

recognized, RuO2 is one of the most extensively studied pseudocapacitive materials in H2SO4 electrolytes. The capacitance of amorphous RuO2 could give a very high value of ~1000 F g−1 probably due to relatively easy insertion of protons into the amorphous structure [66]. Unfortunately, the high cost and limited sources of Ru have restricted their commercial usage. Some alternative materials, such as α-MoO3 with pseudocapacitive behavior were tested for such strong acidic electrolytes [67]. 4.3.1.1.3  Acidic electrolytes for hybrid capacitors

To increase the energy densities of aqueous electrolyte-based ESs, efforts were made to develop hybrid supercapacitors to widen their cell voltage. As discussed earlier, when a symmetric ES with the same type of electrode materials as both the electrodes in aqueous electrolytes (e.g., H2SO4 or KOH) is employed, the maximum cell voltage is limited by the gas evolution reactions [4]. However, if an ES with asymmetric configurations (the anode materials are different from those of the cathode) is used, the resultant ESs could have a wider working potential window even in aqueous electrolytes [68]. The combination of two different electrodes in an ES can work complementarily in separate potential windows, leading to a higher operating voltage in aqueous electrolytes. For example, the high over potential for H2 evolution at a carbon-based negative electrode and O2 evolution at a battery-like (e.g., PbO2) or pseudocapacitive electrode (e.g., RuO2) could give an ES a working voltage window of aqueous electrolytes beyond the thermodynamic limit of water [68]. To date, several types of asymmetric ESs, such as carbon//PbO2 [69], carbon//RuO2 [70] carbon//ECPs [71], and carbon with different mass or properties in each electrode [65, 72] have been tested in strong acid electrolytes, and demonstrated the feasibility for application. For example, typical energy densities for a carbon//PbO2 hybrid ES with an H2SO4 electrolyte were reported to have a value range of 25-30 Wh kg−1, which was much higher than that for symmetric carbon-based EDLCs with the same H2SO4 electrolyte (3-6 Wh kg−1) [73]. However, the chemical stability of PbO2 in H2SO4 electrolyte remains as a matter of concern. Perret et al. [74] found that the structure of PbO2 nanowires could be changed during the electrochemical potential cycling in 1 M H2SO4, leading to poor cycling stability. To address this issue, they explored an alternate electrolyte composed of methane sulfonic acid (CH3SO3H) and lead methane sulfonate [69]. In this case, the redox process at the PbO2 electrode was changed from a solid/solid couple in the H2SO4 electrolyte to a solid/solvated ion one in the methane sulfonic-based electrolyte [69]: 2+ PbO 2 + 4H + + 2e − ↔ Pb(aq) + 2H 2 O (4.7) 2+ During the discharging process, PbO2 was reduced into Pb(aq) . During the charging pro2+ cess, Pb(aq) was oxidized and PbO2 formed was electrodeposited on the electrode surface. Therefore, sulfation of PbO2 in H2SO4 electrolytes was not a limitation and a much-improved cycle life can be realized for this system. Although the energy densities could be improved, both the power density and cycle life of the aqueous hybrid ESs would be compromised. To overcome this, other types of asymmetric/hybrid ESs with a combination of carbon-based material with pseudocapacitive materials, such as AQ-modified carbon//RuO2 [70] carbon// ECP [75] and/or carbon//carbon (with different surface functionalities), were also explored [65, 72]. It may be noted that, there are other types of acid electrolytes that may be used for ESs, including perchloric acid, hexafluoro silicic acid, and tetrafluoro boric acid. However, a few of them have been studied for the application in ESs due to the concern for safety [65].





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

221

In addition, the self-discharge in the concentrated electrolytes, especially in the presence of contamination (e.g., metal ions) and oxygen is also a concern [76, 77]. 4.3.1.2  Strong alkaline electrolytes From the statistics in the literature [20] and from the data in Table 4.3, one can infer that the alkaline electrolytes are of a class widely used aqueous electrolytes. Unlike the case with the strong acidic electrolytes, some cost-effective metal materials like nickel can be used as the current collectors for ESs. Among various alkaline electrolytes, KOH has been the most extensively used one due to its high ionic conductivity (a maximum value of 0.6 S cm−1 for 6 M at 25°C) although other base electrolytes, such as NaOH and LiOH, have also been investigated. These alkaline electrolytes can be used for carbon-based EDLCs, pseudocapacitors [e.g., Ni(OH)2 and Co3O4] and hybrid ESs. 4.3.1.2.1  Alkaline electrolytes for electrical double-layer capacitors

In the literature, reported values of EDLC specific capacitances and energy densities with an aqueous KOH electrolyte, are generally similar to that reported with an H2SO4 electrolyte. Besides the efforts to use strong acid electrolytes for ESs, considerable effort has also been devoted to improve the ES energy densities using base electrolytes through increasing the capacitance and/or widening the operating voltage window. These developments can be briefly summarized as follows: (1) enhancing the capacitance of carbon-based electrode materials through the introduction of a pseudocapacitance contribution; (2) developing pseudocapacitive materials with high specific capacitance; (3) exploring composite materials that combine the carbon-based materials and pseudocapacitive materials; and (4) increasing the operating voltage window of base electrolytes via the designing of asymmetric ESs. 4.3.1.2.2  Alkaline electrolytes for pseudocapacitors

As we know already, the pseudocapacitance of carbon-based electrode materials is contributed from the carbon surface functionalities, which are closely related to the Faradaic interactions between the ions in the electrolyte and surface functional groups [53]. In this regard, studies found that the KOH electrolyte was beneficial for the nitrogen-doped carbon electrode materials, suggesting that the corresponding pseudocapacitance was strongly dependent on the nature of electrolytes (e.g., type and pH) [78, 79]. Wang et al. [80] reported that co-doping the porous carbons with phosphorus and nitrogen could yield a higher specific capacitance, a wider potential window and an enhanced stability compared to pristine carbon materials, resulting in further improvements in the ES performance. In addition, it was observed that hydrogen could be stored through negative polarization at potentials lower than the thermodynamic value of water reduction [72, 81]. This behavior was also strongly dependent on the type of the electrolyte and favorable in alkaline electrolytes [81]. In alkaline electrolytes, some transition metal oxides (e.g., NiOx [82], CoOx [45], MnO2 [83] and NiCo2O4 [46]), hydroxides (e.g., Ni(OH)2 [84], Co(OH)2 [35]), sulfides (e.g., cobalt sulfide [85]), and nitrides (e.g., VN [86]) have also been explored due to their high theoretical capacitances. It is recognized that the interaction between ions in the electrolytes and the electrode materials plays an important role in the pseudocapacitive behavior of these materials. As discussed, the charge-storage mechanism of pseudocapacitive electrode materials normally involves the adsorption/desorption or insertion/extraction of electrolyte ions into/from the electrode materials [82, 87].



222

4.  Electrolyte materials for supercapacitors

For example, Feng et al. [45] prepared Co3O4 nanofilms, and obtained a specific capacitance as high as 1400 F g−1 in the 2 M KOH electrolyte. Mefford et al. [88] demonstrated the anion intercalation-based charge storage mechanism for LaMnO3 perovskite pseudocapacitive electrodes in the KOH electrolyte, providing a new strategy to obtain high specific capacitance in pseudocapacitive materials. It needs to be noted that the possible electrochemical reaction between the electrode materials and the electrolytes might also have a strong influence on the pseudocapacitive behavior. For example, some metal sulfides, such as CoSx and NiS, showed poor pseudocapacitive performance in KOH electrolytes [89, 90], however, when they were electrochemically transformed into new electroactive phases [e.g., Co(OH)2 and Ni(OH)2] in the KOH electrolytes, a large increase in the pseudocapacitance was observed [89, 90]. Normally, the electrolyte properties, such as ion type, concentration, and the operating temperature can affect the performance of ESs. For example, it was observed that the alkaline electrolyte concentrations can affect the value of ESR [91], specific capacitance [91, 92], as well as the oxygen evolution reaction [91]. A drawback in using a concentrated electrolyte is that it causes corrosion at the electrode substrate surface, which may lead to the delamination of electrode material from the substrate [92]. Therefore, it is necessary to optimize the electrolyte concentration with respect to the overall ES performance. An increase in the electrolyte temperature can usually result in a decrease in ESR and then an increase in the specific capacitance due to the enhanced ion diffusion process in the electrolyte [91, 93]. However, the oxygen evolution reaction at the positive electrode was found to be facilitated with increasing temperature, leading to a decrease in the on-set potential of oxygen evolution [2, 94]. Furthermore, the elevated temperatures were reported to decrease the cycling stability of activated carbons due to the material degradation caused by the surface oxidation in KOH electrolytes [91]. As the intercalation and deintercalation of alkaline electrolyte ions generally involve pseudocapacitive materials, the bare (unsolvated) ionic size is expected to have a pronounced influence on the pseudocapacitive behavior. Normally, the effect of the electrolyte ion type on the ES performance is relatively complicated. For example, some researchers found that the specific capacitance of MnO2 [47, 83] was higher in LiOH electrolyte than that in KOH or NaOH electrolyte. The authors attributed this to the relatively easy intercalation/deintercalation of the Li+ ion due to its smaller ionic radius compared to that of the K+ or Na+ ion. Inamdar et al. [95] found that the specific capacitance of NiO in the NaOH electrolyte was nearly 2 times higher than in the KOH electrolyte, and they attributed this to a higher intercalation rate of Na+ ions into the electrode material surface. However, other comparative studies with different alkaline electrolytes for electrode materials, such as Co2P2O7 [96], MnFe2O4 [97], and Bi2WO6 [98] showed that higher specific capacitances could be achieved in KOH electrolytes than those in the NaOH or LiOH electrolyte. As comparative studies of various alkaline electrolytes have been rather limited, it still remains unclear whether this phenomenon is related to the electrolyte, or the type of electrode material, or the preparation procedure and the resultant material structure. As mentioned earlier, pseudocapacitive materials usually have less cycling stability when compared to those nonpseudocapacitive ones. This instability could be partially caused by the repeated ion intercalation/deintercalation in alkaline electrolytes. Besides that, the dissolution of electrode materials in alkaline electrolytes might also be responsible for the decrease of capacitive performance after long-term charging-discharging cycling [87]. Similar to those ES studies in acidic electrolytes, widening the operation voltage window of ESs with alkaline electrolytes





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

223

is the current research trend to significantly improve the energy density. In symmetric ESs, this can be generally achieved through the modification of electrode materials to increase the electrode stability at wide/large potential windows and/or inhibit the side reaction in the electrolytes [35, 99]. 4.3.1.2.3  Alkaline electrolytes for hybrid capacitors

As per the literature, to improve the energy density, a series of alkaline electrolyte-based asymmetric ESs with wide potential windows have been developed. Generally, for asymmetric ESs, the positive electrodes are different from the negative electrode. The positive electrode is a battery-type one [e.g., Ni(OH)2] [73] or pseudocapacitive one (e.g., RuO2) [100] where charge is stored through Faradaic reactions, and the negative electrode is a carbonbased one where, charge is primarily stored by the EDL [68]. The operating cell voltages of these asymmetric ESs were effectively increased in KOH electrolyte, e.g., 1.7 V for carbon// Ni(OH)2 [101], 1.4-1.6 V for carbon//Co(OH)2 [102], carbon//Co3O4 [103], and carbon// Co9S8 [85], 1.6 V for carbon//Ni3S2 [104], and 1.4 V for carbon//RuO2-TiO2 [100]. Because of larger operating voltage window and usage of a high capacity Faradaic-type electrode, most of these ESs could deliver higher energy densities ranging from 20 to 40 Wh kg−1, with some even as high as 140 Wh kg−1 [101], which was comparable to rechargeable lithium-ion batteries. However, due to the usage of Faradaic-type electrodes, the cycling stabilities of asymmetric ESs are lower than those of the EDLCs. It was reported that there were more than 10% loss in specific capacitance of some asymmetric ESs after a certain number of cycles (1000-5000) [101, 105]. Besides, the asymmetric ESs usually suffer from a slower charging/discharging process compared to EDLCs and their few-second response. 4.3.1.3  Neutral electrolytes Besides the acidic and alkaline electrolytes, neutral electrolytes have also been widely investigated for ESs, as can be seen from Table 4.2. This is due to their advantages, such as larger working potential windows, less corrosion, and greater safety. Typical conducting salts in the neutral electrolytes include Li+ (e.g., LiCl, Li2SO4, and LiClO4), Na+ (e.g., NaCl, Na2SO4, and NaNO3), K+ (e.g., KCl, K2SO4, and KNO3), Ca2+ (Ca(NO3)2), and Mg2+ (e.g., MgSO4) salts. Among various neutral electrolytes, Na2SO4 is the most commonly used and a promising one for many pseudocapacitive materials (especially MnO2-based materials). These neutral electrolytes are mostly used in pseudocapacitors and hybrid ESs although some studies have also focused on EDLCs. 4.3.1.3.1  Neutral electrolytes for electrical double-layer capacitors

Comparative studies reveal that the specific capacitances of EDLCs with neutral electrolytes were lower than those with the H2SO4 electrolyte or the KOH electrolyte [33, 50-60]. Due to the lower ionic conductivities, the ESRs of ESs using neutral electrolytes are generally lower than those using H2SO4 or KOH electrolytes [33, 59, 60]. However, carbon-based ESs with neutral electrolytes could offer larger operating voltages due to increased electrolyte stable potential windows (ESPWs) when compared to both acidic and alkaline aqueous electrolytes [7, 50, 51]. As a neutral electrolyte has lower H+ and OH− concentrations compared to acidic and alkaline electrolytes, a higher overpotential for hydrogen and oxygen evolution reactions can be expected, suggesting an increased ESPW. In this regard, Demarconnay et al.



224

4.  Electrolyte materials for supercapacitors

[51] showed an excellent cycle life with 10,000 charging-discharging cycles at a high cell voltage of 1.6 V for a symmetric active carbon-based ES with a 0.5 M Na2SO4 electrolyte. Using nitrogen and oxygen doping for carbon nanofiber electrodes, Zhao et al. [47] further increased the ES cell voltage to 1.8 V and achieved a high energy density of 29.1 Wh kg−1 at a power density of 450 W kg−1 with 1 M Na2SO4 as an electrolyte. While using a 1 M Li2SO4 electrolyte, Fic et al. [7] obtained an even higher working voltage of 2.2 V for a carbon-based symmetrical ES. Their ES could be cycled for 15,000 cycles without any significant decrease in capacitance. To investigate the degradation of carbon-based ESs under high voltage operation in the Li2SO4 electrolyte, some accelerated ageing tests were performed by Ratajczak et al. [106, 107]. They found that gases (e.g., CO2 and CO) could start to evolve at a cell voltage >1.5 V due to the oxidation of carbon electrode material. It was thus concluded that the carbonbased ESs with aqueous 1 M Li2SO4 could operate safely up to a cell voltage of 1.5 V, which was lower than that reported by other studies [7, 57]. Noticeably, the current collector in this study was stainless steel instead of a gold collector as reported in other studies [7, 57], which may be the reason why the results were different. As the operating voltages obtained for neutral electrolyte-based ESs are remarkably higher than those with KOH and H2SO4 electrolytes (usually 0.8-1 V for carbon-based symmetrical ESs) and neutral electrolytes are generally less corrosive than the strong acidic and alkaline electrolytes. Neutral electrolyte-employed symmetric carbon-based ESs, have been identified as the most promising candidates in terms of producing a lower environmental impact and higher energy density. For neutral electrolytes, obtaining high salt concentration is an important concern. But this is not an issue for acidic and alkaline electrolytes as they can achieve a high concentration (e.g., 6 M for the KOH electrolyte). Normally, highly concentrated electrolytes are generally used for practical ESs to ensure a high performance. However, some salts (e.g., K2SO4) cannot achieve such a high concentration, especially when it is used in lower temperatures. In fact, the effect of a neutral electrolyte on the ES performance is also dependent on the type of electrolyte [7, 108]. To understand the effect of different ions on the performance of carbon-based ESs, some comparative studies for neutral electrolytes with different salts have been carried out. However, some controversial results have been noticed in this area. For example, for alkali metal sulfate electrolytes including Li2SO4, Na2SO4, and K2SO4, several studies found that the ES specific capacitance values followed the order: Li2SO4 > Na2SO4 > K2SO4 [7, 109], while other research did not show such behavior [110]. There are other factors, such as the material preparation methods and measurement conditions, such as voltage scan rate/discharge rate that might also affect the results. In this regard, further work may be needed to clarify the salt effect on the ES performance. As for the ESR, it was found that the ESR increase followed the order of the resistivity of bulk electrolytes, that is, Li2SO4 > Na2SO4 > K2SO4 [109, 110], leading to increasing order of both power density and rate performance: Li2SO4 < Na2SO4 < K2SO4 [110]. Regarding the effect of anions on neutral electrolytes, Chae et al. [109] found that for electrolytes with the same cations and concentration, changing anions from SO42− ions to Cl− ions could increase the specific capacitance due to the smaller size of Cl− ions compared to that of SO 2− ions. Later, a few new types of electrolyte, such as lithium, sodium, and po4 tassium salts of silicotungstic acid (Li-SiW, Na-SiW, and K-SiW), were explored as aqueous neutral electrolytes for EDLCs by Gao et al. [111]. These electrolytes exhibited higher ionic − conductivities compared to their counterpart with Cl−, SO 2− 4 , or NO 3 anions. This is because





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

225

of the larger amounts of dissociated cations and greater anion mobility of the Keggin anions. A cell voltage of 1.5 V was achieved for carbon-based EDLCs with these neutral electrolytes. 4.3.1.3.2  Neutral electrolytes for pseudocapacitors

In neutral electrolytes, MnO2 and V2O5 based electrode materials have shown to be promising pseudocapacitive materials for ESs [112]. So far, MnO2 is the most extensively studied pseudocapacitive material in neutral electrolytes. When MnO2 was used as the electrode material, during the charging-discharging process, the Mn oxidation state could be varied between III and IV, accompanied by surface adsorption/desorption or intercalation/deintercalation of electrolyte cations M+ (e.g., K+, Na+, and Li+) as well as protons (H+). The process can be described by Eq. (4.8) [113]: Mn (IV) O 2 .nH 2 O + δ e − + δ (1- f)H 3 O + + δ fM + ↔ (4.8) (H 3 O)δ (1-f) Mδ f  Mn (III)δ Mn (IV) 1-δ   O 2 .nH 2 O As the electrolyte ions are directly involved in the charge storage process, the nature of the neutral electrolytes is expected to have a significant influence on the pseudocapacitive performance. Various factors of the neutral electrolytes, such as pH, the types of cation and anion species [113], salt concentrations [114-116], additives [117, 118], and solution temperatures, have been found to have an influence on the ES performance. Regarding the cation species, various alkaline metal or alkaline-earth metal cations have different ionic sizes and hydrated ion sizes, and thus have different diffusion coefficients and ionic conductivities, which are expected to have strong influences on both the specific capacitance and ESR of the ESs. However, the dependence of specific capacitance on cation species has not been fully understood due to variations in preparation methods and electrode material structures. Li et al. [53] reported that the specific capacitance values and the corresponding energy and power densities of mesoporous MnO2 oxide-based electrode were in an electrolyte order: Li2SO4 > Na2SO4 > K2SO4. This behavior seems to be related to the order of bare (unsolvated) ion size of these alkali metal ions, that is, Li+ < Na+ < K+ (Table 4.3), indicating that a smaller ionic size is beneficial for increasing the specific capacitance. In contrast, several studies using MnO2 as an electrode material showed that Na salts (e.g., Na2SO4 [114, 115] and NaCl [119]) gave higher specific capacitances than those of Li+ and K+ salts. When KxMnO2.nH2O was used as the electrode material, K2SO4 salts could result in a higher specific capacitance than those of Na2SO4 and Li2SO4 salts [120]. Wen et al. [116] found no obvious change in specific capacitance when three different electrolytes (KClO4, NaClO4, and LiClO4) were used. It should be noted that most studies used mass specific capacitance to compare the ES performances in different electrolytes. However, the mass specific capacitance value of an ES is also dependent on a series of factors, such as the structure (e.g., δ-MnO2 or α-MnO2), morphology, preparation methods, and the amount of the electrode materials. It is therefore not easy to identify the specific effect of cation species on the ES capacitance if mass specific capacitances are used for comparison. For example, different preparation methods can generally lead to electrode materials with different pore structures that may also affect the diffusion path of electrolyte ions and make the whole process complicated. In this regard, further investigations using electrode materials with well-defined surfaces may be beneficial to understand the effect of cation species on the intrinsic capacitance of electrode materials.



226

4.  Electrolyte materials for supercapacitors

Furthermore, as the cation intercalation/deintercalation were involved in MnO2-based electrode materials, the scan rate of CV or charging/discharging current density might also have a pronounced influence on the specific capacitance values [53, 121]. As identified, the ion intercalation/deintercalation are believed to be the rate-determining step during the charging-discharging process, and thus a lower scan rate or charging/discharging rate may favor the intercalation-deintercalation process. In this case, the bare cation size, which is closely associated with the intercalation-deintercalation process, may play a dominant role. For example, the electrolytes containing Li+ ions would be beneficial to ES performance, because the smaller Li+ ion size might favor the intercalation-deintercalation process. Besides alkaline metal ions, the effect of alkaline-earth cations on ES performance has also been studied [115, 122]. For example, Xu et al. [115] reported that the specific capacitance of MnO2 was doubled by replacing univalent cations (i.e., Li+, Na+, and K+) with bivalent cations (i.e., Mg2+, Ca2+, and Ba2+). This observation was explained by the fact that when one bivalent alkaline earth metal cation was intercalated into MnO2, it could balance the valence change of two Mn ions from (IV) to (III), whereas one univalent alkaline metal cation could only balance the valence change of one Mn ion. Regarding the anions, Boisset et al. [123] carried out in-depth research on the effect of anions on the ES performance in neutral aqueous electrolytes containing different lithium salts, in which Birnessite-type and Cryptomelane-type MnO2 electrode materials were used. As shown in Fig. 4.2, 10 different lithium salts based on mineral anions and organic anions are investigated in their study. The basicity and the volume of the anion in each aqueous lithium salt electrolyte were found to be the two key parameters in controlling the electrochemical properties (e.g., oxidation and reaction currents) of MnO2 electrodes (Fig. 4.3). Regarding the temperature effect, unlike carbonaceous materials that were generally stable at a wide temperature range, MnO2 electrode materials were found to undergo some structural change at high temperatures during the charging-discharging process, which could negatively affect the cycling stability [55]. Therefore, the effect of electrolyte temperature on the pseudocapacitive behavior especially the cycling stability of MnO2 based ESs should be considered as one of the important issues. In an effort to improve the ES performance, other strategies, such as adding additives into the neutral electrolytes have also been explored [117]. For example, Komaba et al. [117] found that upon addition of a small amount of Na2HPO4, NaHCO3, or Na2B4O7 into the Na2SO4 electrolyte, the specific capacitance of the birnessite electrode could be increased from 190 F g−1 to 200-230 F g−1 at 1.0 A g−1, and at the same time the cyclability was also significantly improved (>1000 cycles). This was attributed to the formation of a protection layer on the birnessite, which could inhibit Mn dissolution due to the optimized pH buffer provided by the additives (Fig. 4.4). Regarding the ES cycle life in neutral electrolytes, as the charge storage process of most pseudocapacitive materials (e.g., MnO2) involves intercalation/deintercalation of ions from the electrolyte, the material structure change upon a repeated charging-discharging process is considered as an important issue that could strongly affect the cycle life of the electrode materials [120]. Besides MnO2, the effect of electrolytes on other pseudocapacitive metal oxides (e.g., V2O5 [124], Fe3O4 [54], SnO2 [125], ZnO [126], and RuO2 [46]) and ECPs [127, 128] has also been





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

227

FIGURE 4.2  Structure, abbreviation, and Cosmo volume evaluated by Cosmotherm X Interface of studied anion (X) in lithium salt (LiX). Source: Reprinted with permission from Ref. [123].

reported although they have not received much attention as those with MnO2. Actually, the electrolyte effect is strongly dependent on the type of electrode material. For example, although RuO2-based electrodes were normally used in acid electrolytes, it was found that RuO2-based ESs with a neutral aqueous electrolyte (i.e., 1 M Na2SO4) could also achieve a high operating voltage of 1.6 V, and thus a relatively high energy density of 19 Wh kg−1 at a power density of 500 W kg−1 [56]. In addition, the use of neutral electrolytes is also beneficial to various ECP-based ESs due to the mild pH condition [127].



228

4.  Electrolyte materials for supercapacitors

FIGURE 4.3  Variation of the potential limit on: (A) oxidation, Eonset, or (B), in reduction expressed by, i(epc ) as

function of the anion (denoted X−) basicities expressed by their pKa (HX/X−). Source: Reprinted with permission from Ref. [123].

4.3.1.3.3  Neutral electrolytes for hybrid capacitors

Neutral electrolytes have also been extensively used for asymmetric ESs, allowing for a larger operative voltage and thus a higher energy density. As identified, most pseudocapacitive materials (e.g., MnO2) have high specific capacitances, although their potential windows are limited, which restrict the cell voltage and thus the energy density of symmetrical ESs





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

229

FIGURE 4.4  Schematic illustration of electrode/electrolyte structure in aqueous electrolyte solutions (A) without and (B) with buffer action. Source: Reproduced with permission of The Electrochemical Society from Ref. [117].

using these materials. For MnO2-based symmetrical ESs, the cell voltage is about 1 V in most cases. By replacing the negative electrode with other different electrode materials (e.g., activated carbons), which have a complementary potential window to that of MnO2, the cell voltage can be significantly increased through extending to a more negative voltage [68]. Compared with the previously mentioned asymmetric ESs using a battery-type positive electrode [e.g., AC//PbO2 and AC//Ni(OH)2] in the strong acidic or alkaline electrolyte, asymmetric AC//MnO2 ESs in neutral electrolytes have a great advantage of a long cycle life due to the pseudocapacitive behavior of MnO2. As neutral electrolytes were used in the early work on asymmetric AC//MnO2 ESs reported by Hong et al. [129] and Brousse et al. [130], considerable efforts have been made to focus on the development of neutral electrolyte-based asymmetric ESs [131]. To date, various types of negative and positive electrode materials have been explored for asymmetric ESs using neutral aqueous electrolytes (mostly sulfate salt-based electrolytes) [131-136]. These asymmetric ESs could reach an operative cell voltage range of 1.8-2.0 V [131136], which is higher than those reported for the asymmetric ECs with strong acidic or alkaline electrolytes. Because of the increased cell voltage, most of the reported energy densities could achieve high values above 20 Wh kg−1 [131-136], and some reported values could even be as high as 50 Wh kg−1 [134]. Excitingly, some of these reported energy density values can be comparable to or higher than those of the organic electrolyte based EDLCs. Therefore, if other issues (e.g., cycle life and rate performance) could be further improved, these asymmetric ESs with neutral electrolytes should be highly promising alternatives for commercial



230

4.  Electrolyte materials for supercapacitors

organic electrolyte-based EDLCs. It was reported that a very high cell voltage of about 4 V could be achieved in neutral electrolytes, such as Li2SO4 and LiCl by using a battery-type Li negative electrode coupled with an appropriate positive electrode material (e.g., AC and MnO2) [137]. As the metallic Li electrode could not work directly in contact with an aqueous electrolyte, a water-stable multilayered Li negative electrode (a protected Li electrode) might be used [137]. To conclude, using neutral aqueous electrolytes in ESs can not only solve the corrosion issues but also provide a cost effective and environmentally friendly means to increase the operating voltage and thereby the energy density. However, more improvements in the ES performance with neutral electrolytes are still needed to further increase the energy density and cycle life.

4.3.2  Organic electrolytes Although extensive studies have focused on the aqueous electrolyte-based ESs in the academic research, organic electrolyte-based ESs are presently predominant in the commercial market owing to their high operation potential window typically in the range of 2.5-2.8 V. The increased operation cell voltage can provide a significant improvement in both the energy and power densities. Furthermore, using organic electrolytes allows the use of cheaper materials (e.g., Al) for the current collectors and packages. Typical organic electrolytes for the commercial EDLCs consist of the conductive salts [e.g., tetraethylammonium tetrafluoroborate (TEABF4)] dissolved in the ACN or PC solvent. Table 4.4 lists the typical ES systems using different organic electrolytes and their corresponding performances. However, there are other issues that should be considered when using the organic electrolytes for ESs. Compared to ESs using aqueous electrolytes, ESs with organic electrolytes usually have a higher cost, a smaller specific capacitance, a lower conductivity, and safety concerns related to the flammability, volatility, and toxicity. Furthermore, an organic electrolyte requires complicated purification and assembling processes in a strictly controlled environment to remove any residual impurities, such as water that can lead to large performance degradation and serious self-discharge issues [27]. 4.3.2.1  General composition, properties, and ES performance of organic electrolytes Organic electrolytes for ESs typically consist of conducting salts dissolved in organic solvents. Tables 4.5 and 4.6 show some commonly used salts and solvents, respectively, for the organic electrolytes along with their abbreviations for convenient understanding. Similar to aqueous electrolyte-based ESs, the nature of salts and solvents, such as ion size, ion-solvent interaction, conductivity, viscosity, and ESPW, has profound influences on the performance of organic electrolyte-based ESs, which will be discussed further in more detail. In literature, most studies related to organic electrolytes have focused on EDLCs. With advances in the development of new electrode materials and electrolytes in recent years, research on organic electrolytes for pseudocapacitors and hybrid ESs has also received great attention. 4.3.2.1.1  Organic electrolytes for electrical double-layer capacitors

From Tables 4.3 and 4.4, it can be seen that for carbon-based ESs, the specific capacitances obtained in organic electrolytes are normally lower than those in aqueous electrolytes [158-160].





231

4.3  Electrolyte materials and compositions for electrochemical supercapacitors

TABLE 4.4  Organic electrolyte-based supercapacitors and their performance. Electrolyte/concentration (M)

Electrode materials

Specific capacitance (F g−1)

Cell voltage (V)

Energy density (Wh kg−1)

Power density (W kg−1)

Temperature (°C)

References

Carbon-based symmetric ESs 1 M TEABF4/ ACN

Highly porous interconnected C nanosheets

~120-150 at 1 mV s−1

2.7

25

25,00027,000

-

[138]

1 M TEABF4/ PC

Graphene-CNT composites

110 at 1 A g−1

3

34.3

400

-

[139]

1 M TEABF4/ HFIP

AC

110 at 1 mV s−1

-

-

-

-

[140]

0.7 M TEABF4/ ADN

AC

25 at 20 mV s−1

3.75

~28

-

RT

[141]

1.6 M TEAODFB/PC

AC

21.4 at 1 A g−1

2.5

~28 (20°C)

~1000

−40 to 60

[142]

1 M TEMABF4/ (PC-PS 95:5)

Microporous TiC-CDC

100 at 10 mV s−1 (60°C)

2.7

~25-27

~1000

−45 to 60

[143]

1 M SBPBF4/ ACN

Carbon (from Batscap)

109

2.3

-

-

−30 to 60

[144]

1.5 M SBPBF4/ PC

AC

122 at 0.1 A g−1

3.5

52

-

RT

[145]

1 M LiPF6/(ECDEC 1:1)

Heteroatom doped porous carbon flakes

126 at 1 A g−1

3

29

2243

RT

[146]

1 M NaPF6/ (EC-DMCPC-EA 1:1:1:0.5)

Microporous carbide-derived carbon

120 at 1 mV s−1

3.4

~40

~90

−40 to 60

[147]

Pseudocapacitive electrode material-based symmetric ESs 1 M LiPF6/(ECDEC 1:1)

Nanoporous Co3O4graphene composites

424.2 at 1 A g−1

-

-

-

RT

[148]

1 M LiClO4/PC

MoO3 nanosheets

540 at 0.1 mV s−1

-

-

-

-

[149]

0.5 M Bu4NBF4/ACN

Heterostructured poly(3,6-dithien2-yl-9Hcarbazol-9-yl acetic acid)/ TiO2 nanoparticles

462.88 at 2.5 mA cm−2

1.2

89.98

-

RT

[150]

0.5 M LiClO4/ PC

PANI/graphite

~420 at 50 mV s−1

1

-

-

RT

[151] (Continued)



232

4.  Electrolyte materials for supercapacitors

TABLE 4.4  Organic electrolyte-based supercapacitors and their performance. (Cont.) Electrolyte/concentration (M)

Electrode materials

Specific capacitance (F g−1)

Cell voltage (V)

Energy density (Wh kg−1)

Power density (W kg−1)

Temperature (°C)

References

Asymmetric ESs 1 M SBPBF4/ PC

Non-porous activated mesophase C microbeads// AC

-

3.5

~ 47

~100

-

[152]

1.5 M TEMABF4/PC

Non-porous activated mesophase C microbeads// graphitized C

-

4

~ 60

~30

RT

[153]

1 M LiPF6/ECDMC (1:1)

Commercial AC (MSP-20)// mesoporous Nb2O5-C nanocomposite

-

3.5

74

~100

-

[154]

1 M LiPF6/ECDEC-DMC (1:1:1)

Fe3O4-graphene

-

3

147

150

RT

[155]

1 M LiPF6/ECDMC (1:1)

Porous graphitic carbon//Li4Ti5O12

-

3

~55

~110

-

[156]

1 M LiTFSI/ ACN

MnO2 nanorodsrGO//V2O5 NWs-rGO

36.9

2

15.4

436.5

RT

[157]

Note: A//B means A is the negative electrode and B is the positive electrode. Abbreviations: AC, Activated carbon; ADN, adiponitrile; CNT, carbon nanotube; DEC, diethyl carbonate; DMC, dimethyl carbonate; EA, ethyl acetate; EC, ethylene carbonate; HFIP, 1,1,1,3,3,3-hexafluoropropan-2-ol; NWs, nanowires; PANI, polyaniline; PS, 1,3-propylene sulfite; rGO, reduced graphene oxide; RT, room temperature; SBPBF4, spiro-(1,10)-bipyrrolidinium tetrafluoroborate; TEAODFB, tetraethylammonium difluoro(oxalato)borate; TEMABF4, triethylmethylammonium tetrafluoroborate; Temp, temperature; TiC-CDC, titanium carbide derived carbon. Adapted with permission from Ref. [20].

Generally, organic electrolytes have larger solvated ion sizes and lower dielectric constants, which can lead to lower EDL capacitance values. Furthermore, the pseudocapacitance contribution of carbon-based electrode materials is small or negligible in organic electrolytes, such as the TEABF4/CAN [158, 161]. This can be understood by considering the nature of carbon surface functionalities. For example, the pseudocapacitive contribution of AQ-modified carbons requires the participation of proton, but this is not favored in the aprotic organic electrolytes, thus resulting in a lower specific capacitance. As mentioned earlier, the specific





233

4.3  Electrolyte materials and compositions for electrochemical supercapacitors

TABLE 4.5  Summary of structures, abbreviations, physical properties, limiting electrochemical reduction and oxidation potentials of commonly used organic solvents containing 0.65 M TEABF4 at 25°C. Solvent

εT

η (cp) bp (°C) mp (°C)

MW

σ (mS cm−1)

Ered/V, Ered/V, SCE SCE

65

2.5

242

−49

102

10.6

−3.0

3.6

53

3.2

240

−53

116

7.5

−3.0

4.2

42

1.7

204

44

86

14.3

−3.0

5.2

34

2.0

208

−31

100

10.3

−3.0

5.2

(Continued)



234

4.  Electrolyte materials for supercapacitors

TABLE 4.5  Summary of structures, abbreviations, physical properties, limiting electrochemical reduction and oxidation potentials of commonly used organic solvents containing 0.65 M TEABF4 at 25°C.  (Cont.) Solvent

εT

η (cp) bp (°C) mp (°C)

MW

σ (mS cm−1)

Ered/V, Ered/V, SCE SCE

36

0.3

82

−49

41

49.6

−2.8

3.3

26

0.5

97

−93

55

Insoluble

37

5.3

286

−29

94

5.7

−2.8

5.0

30

6.0

295

2

108

4.3

−2.9

5.2

21

0.7

120

−35

71

21.3

−2.7

3.0

36

1.1

165

−57

85

15.8

−2.7

3.1

37

0.8

153

−61

73

22.8

−3.0

1.6





235

4.3  Electrolyte materials and compositions for electrochemical supercapacitors

TABLE 4.5  Summary of structures, abbreviations, physical properties, limiting electrochemical reduction and oxidation potentials of commonly used organic solvents containing 0.65 M TEABF4 at 25°C.  (Cont.) Solvent

εT

η (cp) bp (°C) mp (°C)

MW

σ (mS cm−1)

38

0.9

166

−20

87

15.7

32

1.7

202

−24

99

8.9

78

2.5

270

15

101

10.7

−3.0

1.7

38

1.9

226

8

114

7.0

−3.0

1.2

38

0.6

101

−29

61

33.8

−1.2

2.7

Ered/V, Ered/V, SCE SCE

(Continued)



236

4.  Electrolyte materials for supercapacitors

TABLE 4.5  Summary of structures, abbreviations, physical properties, limiting electrochemical reduction and oxidation potentials of commonly used organic solvents containing 0.65 M TEABF4 at 25°C.  (Cont.) Solvent

εT

η (cp) bp (°C) mp (°C)

MW

σ (mS cm−1)

Ered/V, Ered/V, SCE SCE

28

0.7

−90

75

22.1

−1.3

3.2

43

10.0 287 (30°C)

28

120

2.9

−3.1

3.3

29

11.7

276

6

134

Insoluble

47

2.0

189

19

78

13.9

−2.9

1.5

21

2.2

197

−46

140

8.1

−2.9

3.5

115

Reproduced with permission from Ref. [20].





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

237

capacitance of an EDLC depends not only on the specific surface area but also on the pore size and pore size distribution of the carbon materials [5]. The accessibility of pores is closely related to the properties of an organic electrolyte, such as the sizes of cation and anion species, and the ion-solvent interactions. The presence of pores in a carbon material with very small particle size may increase the specific surface area, but it can also limit the accessibility of electrolyte ions. Especially the larger organic ions cannot easily gain access in the small pores, resulting in a negative effect on specific capacitance. Therefore, it is important to match the pore size of carbon materials with the size of electrolyte ions to maximize the specific capacitance. In this direction, considerable efforts have been devoted to understand the relationship between ion size and the capacitive behavior of carbons with different pore size distributions, and then optimize the matching between pore size and electrolyte ion size [5, 159, 160]. In addition, both the sizes of solvated ions and bare (desolvated) ions should also be considered. As observed, the desolvation or distortion of the ion solvation shell when the ion entered into the pores with a size close to the size of desolvated ion could lead to an anomalous increase in the specific capacitance of EDLCs [5]. Besides the specific capacitance, other properties, such as the ESR, charging/discharging rate and power density should also be considered when optimizing the pore structures and designing the electrolytes. As discussed by Jiang et al. [162], theoretical modeling and simulations of the EDL in organic electrolytes have received increasing attention, which may provide useful guidance for EDLC design. These theoretical approaches using molecular dynamics (MD) simulation, density functional theory (DFT) calculations and Monte Carlo (MC) simulation offered some insight into the solvation of ions in organic solvents [163], the EDL structure and capacitance [163], the effect of carbon pore size [164], and their morphologies [165]. In parallel, a number of experimental approaches on the electrolyte-electrode interactions have also been attempted to gain a deeper understanding of the electrolyte behavior under working conditions [163, 166-171]. In this regard, various instrumental analysis methods, including nuclear magnetic resonance (NMR) [166-169], quartz-crystal microbalance (QCM) [170], in situ Raman microspectrometry [172], and in situ small angle neutron scattering (SANS) [171], were also employed for the performance analysis. With the help of these analytical methods, a fundamental understanding of ion dynamics during the charging-discharging process [167-170], local ion structures at the electrolyte/electrode interface [163, 167], ion electroadsorption [171] and the pore size effect [173] has been greatly advanced. The ESPW of the organic electrolyte is a rather important property that determines the ES operation cell voltage, and subsequently the energy and power densities. In fact, the ESPW of an organic electrolyte depends on several factors, including the type of conducting salts (i.e., cation and anion) [24], solvent [24], and impurities especially the trace amounts of water [174]. The ESPW could be obtained through both experimental [24] and theoretical methods [175, 176]. It should be noted that for fundamental research, the experimental measurements for determining the ESPW are normally carried out on the Pt or glassy carbon electrode, which are quite different from the actual electrodes (e.g., porous carbons) in EDLCs. The obtained EDLC voltage windows are usually lower than those obtained on the



238

4.  Electrolyte materials for supercapacitors

TABLE 4.6  Summary of abbreviations, electrolyte conductivities, and limiting electrochemical reduction and oxidation potentials of some quaternary ammonium or phosphonium tetrafluoroborate conducting salts (0.65 M) in PC at 25°C. Electrolyte

σ (mS cm−1)

Ered/V, SCE

Ered/V, SCE

Methylammonium tetrafluoroborate (Me4N BF4)

2.41

−3.10

3.50

Triethylmethylammonium tetrafluoroborate (Me3EtN BF4)

10.16

−3.00

3.60

Tetrafluoroboric acid dimethyldi ethyl ammonium (Me2Et2N BF4)

10.34

−3.00

3.65

Triethylmethylammonium tetrafluoroborate (MeEt3N BF4)

10.68

−3.00

3.65

Tetraethylammonium tetrafluoroborate (Et4N BF4 or TEA BF4)

10.55

−3.00

3.65

Tetrapropylammonium tetrafluoroborate (Pr4N BF4)

8.72

−3.05

3.65

Methyltributylammonium tetrafluoroborate (MeBu3N BF4)

7.80

-

-

Tetrabutylammonium tetrafluoroborate (Bu4N BF4)

7.23

−3.05

3.65

Tetrahexylammonium tetrafluoroborate (Hex4N BF4)

5.17

−3.10

3.85

Tetramethylammonium tetrafluoroborate (Me4P BF4)

9.21

−3.05

3.60

Tetraethyl phosphonium tetrafluoroborate (Et4P BF4)

10.52

−3.00

3.60

Tetrapropylphosphonium tetrafluoroborate (Pr4P BF4)

8.63

−3.05

3.60

Tetrabutylphosphonium tetrafluoroborate (Bu4P BF4)

7.14

−3.05

3.80

10.36

−3.00

3.65

10.82

−3.00

3.70

10.40

−3.00

3.60

10.20

−3.05

3.65

10.40

−3.05

3.70

10.17

−3.05

3.60





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4.3  Electrolyte materials and compositions for electrochemical supercapacitors

TABLE 4.6  Summary of abbreviations, electrolyte conductivities, and limiting electrochemical reduction and oxidation potentials of some quaternary ammonium or phosphonium tetrafluoroborate conducting salts (0.65 M) in PC at 25°C. (Cont.) Electrolyte

σ (mS cm−1)

Ered/V, SCE

Ered/V, SCE

10.94

−3.00

3.60

9.67

−3.00

3.60

8.78

−3.00

3.60

Abbreviations: Ered, limiting reduction potential; Eox, limiting oxidation potential (glassy carbon electrode was used as a working electrode to measure the Ered and Eox); σ, ionic conductivity. Reproduced with permission from Ref. [24].

Pt or the glassy carbon electrode. Aging and the related failure of organic electrolyte-based EDLCs are also important issues, which need to be addressed. The degradation is mainly due to the following reasons: (1) wide operation cell voltage could accelerate the oxidation of electrode materials. Higher the operation cell voltage, higher the energy density. However, when the cell voltage is higher than the typical values of 2.5-2.8 V, such as above 3 V, the electrode material oxidation may occur. This could lead to gas evolution due to the electrolyte decomposition and carbon electrochemical oxidation [1, 177]. (2) The electrolyte ion intercalation [178] or the electrochemical reaction of the organic electrolytes [177] could also cause the degradation of the ES performance and (3) harsh working conditions (e.g., high peak temperature and working voltage) may cause the degradation of ES performance. Therefore, understanding the aging and failure mechanisms will be beneficial to the development of ESs with wider voltage windows. In addition, from the safety and reliability point of view, it is also important to have a better understanding of the ES aging behavior. In an effort to understand the failure mode, Ishimoto et al. [3, 177] separately analyzed the gases evolved from the positive and negative compartments in an H-type cell after float-tests (Fig. 4.5). It has been observed that charging beyond the limiting voltage of 2.7 V caused serious decomposition of the electrolyte and water, leading to gas evolution. Thus, the capacitance gradually faded and the internal resistance increased with an increase in the applied voltage. The gas evolution has been experimentally analyzed using an H-type cell capable of separately collecting gases from the positive and negative compartments. The gaseous products were separately collected after a float-test, which applies a constant voltage of 3.0, 3.3, or 4.0 V at



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4.  Electrolyte materials for supercapacitors

FIGURE 4.5  Gas evolution from an EDLC cell upon over-voltage application. Source: Reproduced from Ref. [1], with permission of The Royal Society of Chemistry.

an elevated temperature of 60°C for 50 h. In the positive compartment, there are two main gas components: CO2 and CO, which result from the electrochemical oxidation of PC (propylene carbonate) and the surface functional groups. The easily oxidizable functional groups, such as carboxyl groups, can be oxidized at 3.0 V and CO2 gas is released. Other functional groups, such as phenol and ketone, produce CO gas at voltages higher than 3.3 V. More importantly, the surface functional groups in the micropores and mesopores in the activated carbon electrode detach the adsorbed water clusters as they are oxidized into gases (CO or CO2) at more than 3.0 V. This results in the release of substantial amounts of free water into the electrolyte in the positive compartment. In the negative compartment, on the other hand, the evolved gaseous product is mostly H2 at 3.0 V. It is evident that water is electrochemically reduced to generate H2 gas and OH. Due to the formation of OH, the negative compartment becomes very alkaline. At 3.3 V, OH-catalyzed PC hydrolysis occurs in the negative compartment consuming water after a 50 h floating test. Other gases, such as propylene, CO2, ethylene, and CO, can be detected at voltages higher than 4.0 V, indicating the direct electrochemical reduction of PC. Another drawback of organic electrolytes is their lower ionic conductivity when compared to the aqueous electrolyte. For example, the ionic conductivity of the commonly used 1 M TEABF4/ACN electrolyte is 0.06 S cm−1, which is significantly lower than that of the 30 wt.% H2SO4 electrolyte (0.8 S cm−1 at 25°C) [28]. The low conductivity of the organic electrolyte can result in ES higher ESR compared to the aqueous electrolyte based ESs [161], which then limits the maximum power density. As the ESs can work in a wide temperature range, the effect of the electrolyte on their performance at different temperatures should be considered. Kotz et al. [179] reported that the specific capacitance of an EDLC with 1 M TEABF4/ACN did not change obviously while





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

241

that of an EDLC with TEABF4/PC as an electrolyte did change significantly upon decreasing the temperature from 60 to 40°C. Normally, the lower temperature limit of typical ESs with organic electrolytes is 40°C [2, 27]. To widen the temperature range for operation (e.g., spacerated electronics), a number of studies have been devoted to the development of organic electrolytes with the temperature limits lower than 40°C [2, 180]. This could be achieved through the careful design of the salts, solvents, and/or additives, which will be further discussed in the following sections. Regarding the self-discharge of the ESs, the presence of a trace amount of water in the organic electrolyte was identified to be mainly responsible [181]. The self-discharge mechanism was also found to be dependent on the type of organic electrolyte, as reported by Zhang et al. [182]. 4.3.2.1.2  Organic electrolytes for pseudocapacitors

Besides the EDLCs, organic electrolytes are also used for pseudocapacitors with pseudocapacitive materials, such as metal oxides [149], ECPs [151], and composite materials [150] as seen from Table 4.4. To facilitate the ion intercalation/deintercalation, most of the organic electrolytes used for pseudocapacitors contain Li ions due to their smaller bare ion size. As reported, LiClO4 [149] and LiPF6 [154, 183, 184] were typical salts used in these organic electrolytes. The typical organic solvents used in the literature were PC [149], ACN or a mixture of different solvents, such as EC-DEC [148], EC-DMC [154], EC-EMC [183], EC-DMC-EMC [185], and EC-DMC-DEC [184]. Actually, most of these organic electrolytes are widely used in the Li-ion batteries. Regarding ECPs, although they have been considered to be promising pseudocapacitive electrode materials due to their low cost, light weight, and easier processings, mechanical flexibility, and relatively fast ion insertion/extraction (doping/dedoping) process, their cycling stabilities are normally poor, probably caused by trace impurity of water in the electrolytes [151]. In this regard, the choice of appropriate organic electrolytes may be able to minimize the degradation of the ECP materials, thereby improve the cycling stability [151]. 4.3.2.1.3  Organic electrolytes for hybrid capacitors

In order to increase the energy density further, asymmetric ESs with organic electrolytes have been considered. Since the early studies on the asymmetric ESs with the organic electrolytes by Amatucci et al. [186] a number of organic electrolyte based asymmetric ESs, such as graphite//AC (electrolyte: 1.5 M TEMABF4/PC) [153], carbon//TiO2 (1M LiPF6/EC-DMC) [187] carbon//V2O5 (1M LiTFSI/ACN) [188], carbon//Li4Ti5O12 (1M LiPF6/ EC-EMC) [183], and carbon//ECP (1M TEABF4/PC) [189] have been developed. Due to wider operative cell voltages (generally 3-4 V) obtained in the organic electrolyte, these asymmetric ESs were able to deliver energy densities (usually above 30 Wh kg−1) much higher than those reported for aqueous-based asymmetric ESs, although the former ones have lower specific capacitances compared to the later ones. Among these asymmetric ESs, Li-ion capacitors (LICs) have attracted much attention [183, 187, 190]. Typical LICs combine a Li-ion battery-type negative electrode and an EDLC-type positive electrode (e.g., AC) with a Li-containing organic electrolyte, such as LiPF6, LiClO4, and mixtures of two or more solvents (e.g., EC-DMC). There are two types of negative electrodes, that is, carbon-based (mostly graphite) and lithium titanate (Li4Ti5O12) electrodes. LICs using a single solvent are rare. In some cases, additives are used



242

4.  Electrolyte materials for supercapacitors

in the electrolytes [191]. Actually, the compositions of the reported electrolytes for LICs are almost the same as those for LIBs, which have been reviewed elsewhere [192]. Even extensive studies have been focused on the development of electrode materials, while the effect of electrolytes on LIC performance has received relatively limited attention [73, 190]. Unlike the ACN-based EDLCs whose performance was reported to be relatively insensitive to temperatures between −30 and 40°C, the LIC performance (e.g., energy and power densities) with carbonate-based electrolytes was found to degrade severely at low temperature especially below 0°C [190]. Besides the poor low temperature performance, the LICs also suffer from a relatively low rate capability arising from the battery-type negative electrode. Further efforts on both LIC electrode materials and electrolytes are necessary to solve this temperature limitation. 4.3.2.2  Organic solvents

Solvent is a component of the ES electrolyte, playing a critical role in achieving a high performance. For an ideal organic solvent, it should fulfill several criteria including good solvation ability for a chosen conducting salt, high ESPW, low viscosity within the operating temperature range as well as safety (e.g., nonflammability and nontoxic). The main requirements for suitable nonaqueous solutions are: 1. An adequate voltage window of electrochemical stability (i.e., the decomposition voltage of the desired solution) (Fig. 4.6), which should be little larger than the intended operating range of the capacitor device in order to minimize problems arising from adventitious overcharge. Also, it is not just the overall range of operating voltage that is important for solution stability, but the individual positive (anodic) and negative (cathodic) potential limits of stability of the solution relative to some reference electrode potential in the same solution. Furthermore, it is also not just the electrochemical stability of the solvent that is the principal factor but also that of the solute ionic species that must not be discharged or decomposed (e.g., with R4N+ solute salts); this is also a factor of major practical importance. Thus, tetraalkylammonium salts of anions that are difficult to discharge, such as PF6− , BF4− , and AsF6− are preferred solutes. Li+ salts could also be used provided the cathodic limit for Li metal deposition is not reached on charging of the capacitor. 2. Minimum viscosity of the solvent (or solution) in order to maximize ionic mobility and resulting conductance. 3. Maximum solubility of the solute salt to maximize conductance. 4. Minimum ion pairing at given practical solute salt concentrations, again to maximize conductance. 5. Optimum dielectric permittivity or donor number of the solvent to maximize salt solubility and minimize ion pairing. This requirement also determines the solvation of the ions of the solute salt. Other factors are the ionic radii of the solute salt ions and the magnitudes of their charges, often ± Ie, which determine strengths of solvation and of ion pairing or degrees of dissociation of the solute salt. A summary of the various properties involved is given in Table 4.7. All these factors are involved in determining the two leading quantities that determine specific conductance at a given concentration, namely, (1) the mobility of the dissociated ions 



4.3  Electrolyte materials and compositions for electrochemical supercapacitors

243

FIGURE 4.6  Estimated potential ranges (decomposition limits) for aqueous and nonaqueous electrolyte solutions. Source: From A.J. Bard and L.R. Faulkner, Electrochemical Methods, Wiley, New York (1980). Reprinted by permission of John Wiley & Sons. The lower curves illustrate decomposition limits, V+ and V−, in a cyclic yoltammetry experiment; i(dl) = double-layer charging currents and iF-Faradaic decomposition currents; Reproduced with permission from Ref. [21].

and (2) the concentrations of free charge carriers, the cations, and anions, at the experimental salt concentration. A further factor that determines optimization of solution properties for electrochemical capacitors is the temperature coefficient of specific conductance, which is determined by the temperature coefficients of factors (2) to (5) in a complex, interactive way. As mentioned earlier, the most commonly used organic solvents for EDLCs so far are acetonitrile (ACN) and propylene carbonate (PC). As shown in Table 4.7, compared to the PC, ACN has a low viscosity, a lower melting point, and a higher dielectric constant. Normally, ACN-based electrolytes have higher conductivity when compared to those of PC-based electrolytes, which can make a lower ESR and a higher power performance of the resultant EDLCs even at low temperatures. However, the low flash point and toxicity of the ACN are two major drawbacks. Due to its high toxicity, the use of ACN in ESs is prohibited in 

244

4.  Electrolyte materials for supercapacitors

TABLE 4.7  Summary of factors determining electrolyte solution properties. Ion/salt properties

Solvent/solution properties

Ionic radius

Dielectric permittivity of the solvent

Ionic charge

Donicity or electron acceptor numbers

Ionic solvation energy

Solvent viscosity

Ion pairing

Solvent dielectric relaxation times

Solubility of the solute salt

Mixture properties of solvents Melting point of solvent or solution Temperature coefficients of viscosity and dielectric permittivity

Temperature coefficients of ion pairing and solubility

Solvent vapor pressure, as a function of temperature

Thermodynamics of the solvent

Electrochemical stability range Liquid phase temperature range (difference between boiling and freezing points)

Reproduced with permission from Ref. [21].

Japan [193-194]. As PC has a higher flash point and is less toxic than ACN solvents, there has been growing interest in the PC-based electrolyte as an alternative to the ACN-based electrolytes. Due to the lower conductivity of the PC-based electrolyte than the ACN-based electrolyte, both a lower power density and energy efficiency of the PC-based EDLCs could be expected. This problem is serious at low temperatures due to the high viscosity of the PC (Fig. 4.7) [195]. Furthermore, as previously discussed, the operating cell voltages of both the ACN- and PC-based EDLCs are limited to 2.5-2.8 V, and further increasing of the working voltages and thereby increasing the energy densities would be highly desirable. In this aspect, extensive efforts have been devoted to the development of other organic electrolytes for high-performing EDLCs in the last decade. 4.3.2.2.1  Single organic solvents for electrolytes

Regarding the single organic solvent, several studies focused on the g-butyrolactone (GBL) and its associated electrolyte performance in ESs. Comparative studies between the GBLbased electrolyte and both ACN- and PC-based electrolytes were made for their employment in EDLCs [196-198]. For example, Ue et al. [24] reported that the GBL-based electrolyte had a higher oxidative stability than the PC-based electrolyte (both with 0.65 M TEABF4). It was also observed that the lower viscosity and ionic conductivity of the GBL-based electrolyte could result in a lower power performance when compared to the ACN-based electrolyte [196]. Based on the observation made by Ue et al., [24] that butylene carbonate (BC) exhibited a better oxidative stability than the PC, Chiba et al. [199] postulated that the alkyl substituent at the fourth and/ or the fifth position on the five-membered rings of carbonates (Fig. 4.8A) might be able to increase the oxidation resistance. It was found that when the 2,3-BC whose fourth and fifth positions were substituted by methyl groups, withstanding voltage of up to 3.5V (containing SBPBF4 salt) could be observed, which was higher than those of PC solventbased EDLCs (2.5-2.7 V).





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

245

FIGURE 4.7  Ragone plots for ACN- and PC-based electrochemical supercapacitorsat different temperatures. Source: Reprinted from Ref. [195], with permission from Elsevier.

It was also reported that the substitution of C-H bonds by C-F in the solvent molecule could result in a remarkably higher chemical and electrochemical stability [140]. For example, a highly fluorinated solvent, that is, a nonflammable 1,1,1,3,3,3-hexafluoropropan2-ol (HFIP), was investigated by Francke et al. [140]. Due to the higher electrochemical stability, the HFIP-based electrolyte for active carbon (AC) electrodes could be stable up 

246

4.  Electrolyte materials for supercapacitors

FIGURE 4.8  Structure of (A) cyclic carbonates, and (B) cyclic carbonates. Source: Reproduced by permission of The Electrochemical Society from Ref. [199].

to almost 2 V, while the ACN-based electrolyte had been fully degraded at a lower voltage of 1.4 V. However, the HFIP has a small liquid-state range of 3-59°C, which is narrower than those of the ACN and PC, thus limiting its application in a wider temperature range. Suzuki et al. [200] explored fluoroacetonitorile (FAN) as an alternative solvent for EDLCs. The ionic conductivity of a 1 M TEABF4/FAN electrolyte was found to be slightly lower than that of 1 M TEABF4/ACN solution, but much higher than that of a 1 M TEABF4/PC solution. The requirements for further increasing the operation voltage of organic electrolyte-based EDLCs have driven the development of other novel solvent systems such as sulfolanated solvents. For example, sulfolane (SL)- and dimethyl sulfide (DMS)-based electrolytes were reported to have a higher operation voltage than the carbonate-based electrolytes. However, due to the melting point and viscosity of SL solvents (28.5°C compared to those of PC (49°C) and ACN (45°C), their practical usage could be limited [201]. In an effort to decrease the melting point and viscosity of SL-based electrolytes, Chiba et al. [201] explored eight different types of linear sulfones with relatively low molecular

FIGURE 4.9  Structures of sulfones. Source: Reproduced by permission of The Electrochemical Society from Ref. [201].





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

247

weights (Fig. 4.9). It was found that by changing the molecular structure of cyclic SL to a linear structure, the melting point, and viscosity can be significantly decreased. EDLCs with SBPBF4/ethyl isopropyl sulfone (EiPS) showed a higher operating voltage (3.3-3.7 V), leading to around 2 times increase in energy density compared to that with PC-based electrolytes. However, a high viscosity of this electrolyte system (e g., EiPS has a value as high as 5.6 mPa s at 25°C) still needs to be reduced because it is higher than those of both PC (2.5 mPa s at 25°C) and CAN (0.3 mPa s at 25°C). To address the limitation of ACNbased electrolytes, such as the low flash point and relatively low electrochemical stability, adiponitrile (ADN) was also explored as a possible solvent for EDLCs [141]. Brandt et al. [141] reported the use of 0.7 M TEABF4/ADN as the electrolyte for EDLCs. An operative voltage, as high as 3.75 V was achieved for the ADN-based EDLCs due to the high ESPW of the ADN. However, the low solubility limits of the commonly conducting salts, such as TEABF4 in ADN solvent has led to a lower ionic conductivity than that of the ACN-based electrolytes. As can be inferred from this discussion, so far there has been no ideal single solvent that can replace the commercially used solvents (e.g., ACN) in terms of the viscosity, conductivity, and thermal stability, as well as overall ES performance. Although some promising solvents have been proposed to have higher ESPW, they generally cannot compete with CAN-based electrolytes in ES application. 4.3.2.2.2  Solvent mixtures for electrolytes

In an effort to address the issues related to the single solvent systems, solvents based on a combination of different organic solvents or additives have been proposed in the literature. As mentioned earlier, although PC has been regard as a promising alternative for the ACN solvent, it suffers from lower conductivity and viscosity, and poorer thermal stability when compared to the ACN-based electrolytes. To make improvements in viscosity and ionic conductivity, a number of studies have recently focused on the development of PC-based solvent mixtures, such as PC-TMC (TMC, trimethylene carbonate), PC-EC (EC, ethylene carbonate), and PC-FEC (FEC, fluoroethylene carbonate) [202, 203]. In the literature, carbonates (EC, DEC, and DMC [143, 144, 204]) and sulfites [diethyl sulfite (DES) and 1,3-propylene sulfite (PS) [143]] were studied as the additives or co-solvents for the binary PC-based solvents. The addition of DES or PS into the PC solvent could noticeably change both the viscosity and the conductivity of electrolytes, positively affecting the capacitance, time constant, and energy/ power densities of the obtained ESs [143]. Actually, the solvent mixtures have already been widely studied as the battery solvents for LIBs and currently receiving interest in the field of EDLCs. In this regard, binary, ternary, and quaternary solvent mixtures based on the organic carbonates (e.g., PC, EC, DMC, EMC, and DEC) and organic esters [e.g., methyl formate (MF), methyl acetate (MA), and ethyl acetate (EA)] have also been investigated [205]. Depending on the composition, these solvent mixtures had different properties. These properties include the relative macroscopic dielectric permittivity, viscosity, melting temperatures, and dipole moment. Variation in any of these properties makes the solvents with different solubilities of salts and ionic conductivities. It is noted that the use of some organic esters, such as EA and MA may introduce a safety issue due to their low flash point and high volatility. To address this concern, Perricone et al. [206] proposed the addition of organic ester cosolvents with me-



248

4.  Electrolyte materials for supercapacitors

thoxy or fluorinated groups to improve safety when compared to EA. Despite the toxicity of the CAN solvent, the ACN-based mixed solvents are still very attractive and have been developed with the aim of increasing the operating cell voltage and/or to extend the lower temperature limit of the ACN-based EDLCs. Ding et al. [197] found that the addition of GBL into ACN could significantly expand the liquid-state range of ACN (with 1 M TEMABF4) in both positive and negative temperature regime. The oxidative stabilities of the solvents were increased by increasing the GBL content. In an effort to extend the low temperature operating limit of the ACN-based EDLCs to below −40°C, Brandon et al. [180] produced some mixed solvents by integrating low melting cosolvents (e.g., formates, esters, or cyclic ethers) into the ACN solvent. Electrolytes based on these solvents containing 0.75 M TEABF4 were found to enable the EDLCs to charge and discharge at a temperature as low as −75°C. Regarding LICs, Lee et al. [191] found that the addition of 1,3,5 trifluorobenzene (TFB) (3.0 wt.%) into 1 M LiBF4/EC-DEC could increase the high rate performance of AC-based Li batteries. 4.3.2.3  Conducting salts for electrolytes 4.3.2.3.1  Effect of conducting salt on ES performance

A conducting salt dissolved in an organic solvent to form an organic electrolyte can provide charge carriers (i.e., cations and anions) for ES operation. In this regard, the ion concentration and mobility should play an important role in electrolyte ionic conductivity. The conducting salts also have a profound influence on the ESPW, the thermal stability of the organic electrolytes, as well as the ES capacitance. While choosing or developing appropriate salts for certain solvents, several factors, such as solubility, conductivity, stability, safety, and cost should be considered. Due to the good over all properties, TEABF4 is currently the most commonly used salt and has been widely applied in commercial ESs. Besides TEABF4, many other salts have also been studied or developed with the aim of further improving one or several of the following properties of the electrolytes: (1) solubility, (2) conductivity, (3) stability, and (4) temperature performance. Regarding the solubility, ion concentration (salt concentration) in the electrolyte not only affects the ionic conductivity but also the maximum energy density attainable of its associated ESs [207]. Fig. 4.10 shows an example demonstrating that there are optimum ion concentration values for achieving the maximum ionic conductivity of the electrolytes [194, 208]. It can also be seen that TEABF4 has a limited solubility (up to 1 M) in PC whereas other salts show a higher solubility above 2 M. As ESs generally work in a wide temperature range (e.g., from −30 to 70°C) for many applications, the salt solubility at lower temperatures is an important issue. The conductivity of the electrolyte is strongly dependent on a number of factors such as salt concentration, dissociation degree of the dissolved salt to provide free charge carriers (cations and anions), mobility of these dissociated ions, types of solvents (as mentioned earlier), and the temperature. Several aqueous electrolytes for ESs have high dissociation degree values close to 1, while common organic electrolytes have much lower ones. Therefore, the salt solubilities in organic solvents are normally low, leading to low ionic conductivity of the organic electrolytes and high ESR when compared to aqueous electrolytes. Actually, the structure of organic salts (e.g., symmetrical ones or asymmetric ones, and the ionic size of cation and anion) can also affect the dissociation degree and ion mobility, thus playing a critical role





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

249

FIGURE 4.10  Ionic conductivity as a function of quaternary ammonium tetrafluoroborate concentration in PC. Source: Reproduced by permission of The Electrochemical Society of Japan from Ref. [194].

in influencing conductivity of the electrolytes. Ue et al. [24] investigated the conductivities of some common salts in various solvents including PC, GBL, N,N-dimethylformamide (DMF), and CAN [24]. Regarding the cations, the conductivity generally decreases in the following order: TEA+ > Pr4N+ > Bu4N+ > Li+ > Me4N+, while for the anions, the conductivity decreases in the following order: BF4− > PF4− > ClO −4 > CF3 SO 3 , respectively. The conducting salt also plays an important role in the ESPW of organic electrolytes [194, 209]. For example, Xu et al. [209] found that the ESPWs of different cations at an AC electrode followed the order: Pr3MeN+ > Et4N+ > Bu3MeN+ > Et3MeN+ > iPr2MeEtN+ > Me3EtN+ > Bu3MeP+ > − Et3MeP+ while that of the anions at a glassy carbon electrode followed the order of: AsF6 ~ − − BF4 > PF6 > Tf ~ Im (solvent: EC-DMC). As various salts contain different types of cations and anions with different bare and solvated ion sizes, the matching between the pore size of carbon-based electrode materials and the electrolyte ions is essential to achieve a high ES performance. To realize a perfect matching, the partial removal or distortion of the ion solvation shell should be considered when the pore size is close to the bare ion size. Literature reveals that such a matching has a large effect on the capacitance and power performance of EDLCs. Koh et al. [210] employed several salts including TEABF4, TEMABF4, trimethylpropylammonium BF4 (TMPABF4), and diethyldimethylammonium BF4 (DEDMABF4) to investigate the effect of ionic size of different salts on the EDLC specific capacitance. These salts have different lengths of hydrocarbon chains and thus different cation sizes. It was observed that the EDLC specific capacitance could be increased by decreasing the cation size of quaternary ammonium salts. Also, the charged state (valent) of ions was found to have an effect on the capacitance or energy density of EDLCs [211]. Using asymmetric ESs, Yokoyama et al. [212] observed that the capacitance could be strongly affected by the type of anion, and the de− − − scending order was found to be: PF6 > BF4 > ClO 4 (all with TEA cations; solvent is PC-EMC). − Among the investigated anions, it has been inferred that PF6 was the most stable one, contributing to the largest capacitance. 

250

4.  Electrolyte materials for supercapacitors

4.3.2.3.2  Exploration of new conducting salts

To increase the salt solubility in organic solvents (especially PC), some asymmetric tetraalkylammonium salts and cyclic quaternary ammonium salts, such as TEMABF4, 1-ethyl-1-methylpyrrolidinium, (MEPYBF4), and tetramethylenepyrrolidinium (TMPYBF4), have been explored for use in ESs. These salts were found to have much higher solubilities in PC when compared to TEABF4, as shown in Fig. 4.10 [194]. As salts contribute a large part of the cost to the total electrolyte cost, a balance between performance and cost (i.e., salt concentration) should be considered. For example, a spiro-type quaternaryammonium salt called spiro-(1,10)bipyrrolidinium tetrafluoroborate (SBPBF4) received considerable attention due to its smaller cation size, higher mobility, and higher solubility compared to the common TEABF4 [145, 152, 201, 213, 214]. However, SBPBF4 is expensive compared to TEABF4, which may limit its widespread applications. In addition, Kurig et al. [215] tested a series of substituted phosphonium cation-based electrolytes for EDLCs with titanium carbide derived carbon (C(TiC)) as the electrodes. The C(TiC) electrodes were found to be ideally polarizable in a 1 M tetrakis(diethylamino)-phosphonium hexafluorophosphate (TDENPPF6)/ACN electrolyte up to 3.2 V. The EDLC with a 1 M TDENPPF6/ACN electrolyte showed a gravimetric capacitance of 85 F g−1, a characteristic relaxation time of 0.9 s, and a gravimetric energy density of 27 Wh kg−1 when the cell voltage was 3.2 V. No visible loss in the discharging capacitance could be observed after 10,000 charging-discharging cycles. However, the conductivities of these substituted phosphonium cation based electrolytes were much lower than those of TEABF4/ACN and TEMABF4/ACN, which might have some negative effects on the ESR of ESs, rate performance or thermal stability. Regarding the anion, it can be noted that the commonly used anions (e.g., BF4− ) contain halogen. Several authors pointed out that these fluoro-complex salts might be hydrolyzed to generate highly toxic hydrogen fluoride (HF) [216, 217]. To address this issue, they explored bis(oxalato)borates (BOB anion)-based salts, such as TMABOB, ETMABOB, TEMABOB, and TEABOB [217]. Changing bis(oxalato)borate anions to asymmetric difluoro(oxalato) borate anions was found to increase the solubility of the salts [142, 218]. In addition to the organic cations, organic salts based on the inorganic cations, such as Li+ [204, 219, 220], Na+ [147], and Mg2+ [221, 222], have also attracted attention. As mentioned earlier, organic electrolytes based on the lithium salts have been widely used in the pseudocapacitors and hybrid ESs such as LICs due to the small ionic size of Li+. There are also some studies on the application of lithium salt-based organic electrolytes for EDLCs [204, 220]. The use of lithium salts for the ESs is largely inspired by the development of LIBs.

4.3.3  Ionic liquid-based ES electrolytes 4.3.3.1  General composition, properties and ES performance of ionic liquid electrolytes ILs, also known as low temperature or room temperature molten salts, are generally defined as those salts composed solely of ions (cations and anions) with melting points below 100°C [222]. An IL usually consists of a large asymmetric organic cation and an inorganic or an organic anion, and this special combination of certain cations and anions contributes to a low melting point [223,224]. Owing to the unique structures and properties, ILs have received significant interest as alternative electrolytes for ESs. Normally, ILs have several potential 



4.3  Electrolyte materials and compositions for electrochemical supercapacitors

251

FIGURE 4.11  Basic types of ionic liquids: aprotic, protic and zwitterionic types. Source: Reprinted by permission from Macmillan Publishers Ltd., Nature Materials [223].

advantages including high thermal, chemical and electrochemical stability, negligible volatility, and nonflammability (depending on the combination of cations and anions) [8, 213]. Furthermore, the physical and chemical properties of ILs are highly tunable due to their huge variety of combinations of cations and anions [214]. In this sense, ILs are also regarded as "designer solvents" [215]. This property of ILs is highly attractive for ES electrolytes because the electrolyte compositions can be optimized or customized to meet certain requirements of ES performance, such as operative cell voltage, the working temperature range, ESR (related with the ionic conductivity), etc. Based on their composition, ILs can be basically classified as aprotic, protic, and zwitterionic types (Fig. 4.11) [213]. The cited ILs used in ESs so far represent only a very small part of the large family of ILs. In the published literature, the ILs employed for ESs are commonly based on imidazonium, pyrrolidinium, ammonium, sulfonium, phosphonium cations, etc., (Fig. 4.12). Typical anions of ILs are tetrafluoroborate (BF4), hexafluorophosphate (PF6), bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and dicyanamide (DCA) (Fig. 4.12). In general, the imidazolium-based ILs can provide higher ionic conductivity while the pyrrolidinium-based ILs have larger ESPWs [28, 216]. Actually, there exists a trade-off between the IL ionic conductivity and ESPW. As mentioned earlier, the operative cell voltages of commercial organic electrolytes (e.g., ACN and PC) based EDLCs are generally limited to 2.5-2.8 V, and increasing the cell voltage beyond this limit would cause serious electrochemical decomposition of organic solvents. However, many studies using IL-electrolyte-based ESs could give operative cell voltages above 3 V [8, 225-227]. Besides, commercially used organic solvents (e.g., ACN) also face safety issues due to their volatile and flammable nature especially when used at high temperatures. In this regard, the solvent-free ILs may have an advantage in solving the safety problems associated with those organic solvents, making IL-based ESs favorable for hightemperature applications. Unfortunately, there are several main drawbacks with most ILs, such as high viscosity, low ionic conductivity, and high cost, which can limit their practical use in ESs. Even for the [EMIM][BF4] electrolyte, which has a relatively high ionic conductivity among the com

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FIGURE 4.12  Commonly used cations, anions of ILs for ESs, and some typical examples of ILs [20] . 



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mon ILs, its conductivity (14 mS cm−1 at 25°C) is still much lower than that of TEABF4/ACN (59.9 mS cm−1 at 25°C). Further, the viscosities of ILs, such as [EMIM][BF4] and [BMIM][BF4] are 41 cp and 219 cp [228, 229], respectively, which are much higher than that of the organic electrolyte (e.g., 0.3 cp for the ACN organic electrolyte [148]). Both low conductivity and high viscosity of IL-based electrolytes can significantly increase the ESR values of the ILs-based ESs, limiting both the rate and power performance if the loss of the power density due to the increased ESR cannot be buffered by the increase of cell voltage. This issue seemed to be more serious at room temperature and low temperature, as demonstrated by some comparative studies between organic and IL electrolytes [8, 226, 227]. In addition, the specific capacitance values of IL electrolyte-based EDLCs are often lower than those of both aqueous and organic electrolyte-based ones especially at high scan rates or high charging/discharging rates, probably due to the high viscosity of ILs [220, 230, 231]. To improve the performance of IL-based ESs, optimization of IL composition and cell design has been carried out both experimentally and theoretically [232-234], to provide fundamental insights into the electrochemical behavior, structure and corresponding capacitive behavior of the electric double-layer (EDL) at the IL/electrode interface. In this regard, notable achievements have been made, as documented by excellent reviews [162, 234-237]. These reviews discussed the EDL structure and capacitance, influencing factors on the EDL, effects of carbon electrode morphology and pore size on the EDL structure, capacitance and dynamics of IL ions. Interestingly, some cations of the investigated ILs for ESs have a structure similar to that of surfactants [238-239]. Whether these cations tend to aggregate in ILs like surfactants is an important issue since this would be able to affect both the ion mobility and ionic conductivity [238, 239]. In order to understand the aging or failure mechanisms of IL electrolyte-based EDLCs, the electrochemical decomposition of ILs beyond the ESPW has been investigated by using instrumental techniques, such as in situ infrared and electrochemical spectroscopy methods [240], and in situ XPS [241]. The research activities in this field can be briefly summarized as follows: (1) fundamentally understanding the EDL structure and capacitance in ILs, (2) developing ILs for pseudocapacitive electrode material-based ESs with improved charge storage ability, (3) improving the properties of ILs, such as ionic conductivity, viscosity, and ESPW, by modifying the cations or anions or both, and (4) using a mixture of ILs or utilizing organic solvents to improve the overall performance of ILbased electrolytes. 4.3.3.2  Solvent-free ionic liquids 4.3.3.2.1  Solvent-free ionic liquids for EDLCs

As earlier mentioned, the major challenge in developing ILs for EDLCs is to design or choose ILs with a high ionic conductivity, a large ESPW and a wide operational temperature range. Although the [EMIM][BF4] electrolyte has been extensively studied for EDLCs due to its relatively high conductivity, its narrower ESPW compared to other ILs and relatively high melting point (15°C) appears to be the limitations. To overcome these limitations, considerable work has been devoted to the development of other alternative ILs based on the tunable properties of ILs to change the structure of either the anion, or the cation, or both as well as the IL composition itself [234].



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4.3.3.2.1.1 Aprotic ionic liquids. Regarding 1-ethyl-3-methylimidazolium ([EMIM])-based ILs, Sun et al. [242] investigated the performance of AC-based EDLCs with [EMIM][SCN] IL electrolytes (SCN: thiocyanate), which had both lower viscosity and higher ionic conductivity compared to those of [EMIM][BF4] IL. Handa et al. [243] found that the ionic conductivity of [EMIM][FSI] IL (FSI: bis(fluorosulfonyl)imide) was comparable to the TEMABF4/PC organic electrolyte, and the EDLCs with this [EMIM][FSI] IL electrolyte showed a good rate capability similar to that with a TEMABF4/PC organic electrolyte. Pandey et al. [244] demonstrated that 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB]) IL (TCB: tetracyanoborate) could be an electrolyte for AC-based EDLCs as it had a high ionic conductivity (~1.3 × 10−2 S cm−1 at 20°C) and low viscosity (~22 cp). Matsumoto et al. [245] found that the [EMIM] [PO2F2] IL (PO2F2: difluorophosphate) electrolyte could provide a higher specific capacitance when compared to the [EMIM][BF4] IL at a charging voltage of 2.5 V. Unfortunately, the operative cell voltage of the EDLC with a [EMIM][PO2F2] IL electrolyte was below 3 V, lower than that with a [EMIM][BF4] IL electrolyte. Kurig et al. [246] systematically investigated various IL electrolytes with [EMIM]+ cations and different anions for EDLCs using carbon cloth electrodes. The structure and chemical composition of the IL anion were found to have a profound influence on both the ESPW and operative cell voltage (Fig. 4.13). It was concluded that the [EMIM][BF4] and [EMIM][B(CN)4] ILs could give the best overall performance among the investigated ILs. Shi et al. [247] investigated the performance of graphene-based EDLCs with a series of ILs composed of [EMIM]+ cations and different anions including BF4− , NTF2− , DCA − , EtSO −4 , and OAc−. It was found that the hydrogen-bond-accepting ability of these anions was closely related to the viscosity of the ILs. EDLCs with the [EMIM][DCA] IL electrolyte showed

FIGURE 4.13  Power density vs. energy density plots at 2.7 V and at 3.2 V for electrochemical supercapacitors with ILs studied. Source: Reproduced by permission of The Electrochemical Society from Ref. [246].





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FIGURE 4.14  Relationship between the electrochemical window and the energy density of graphene electrodes measured in different IL electrolytes. Source: Reprinted from Ref. [247], with permission from John Wiley and Sons.

the highest specific capacitance and rate capability as well as the smallest resistance due to the lowest viscosity, ion size, and molecular weight among the investigated ILs. However, [EMIM][DCA] IL-based EDLCs had a much smaller ESPW (2.3 V), which was much narrower than that of the [EMIM][BF4] IL-based EDLC (~4 V) (Fig. 4.14). As a result, the latter delivered the highest specific energy density (67 Wh kg−1 at 1 A g−1), which was much higher than the former (20 Wh kg−1). In addition to changing the anion of [EMIM]-based IL electrolytes, other attempts proposed to improve the properties (e.g., ESPW and ionic conductivity) [248-250]. For example, Kong et al. [248] reported that the addition of small amounts of single walled CNTs (e.g., 0.1 and 0.5 wt.%) into the [EMIM][BF4] IL electrolyte could increase the ionic conductivity of the electrolyte. The specific capacitance, energy density and cycling stability of EDLCs with this CNT-added IL electrolyte were all improved. Some studies showed that the addition of Li salt into the IL electrolyte could negatively shift the cathodic potential limit for increasing the cathodic stability [249, 250]. Other imidazolium-based cations besides [EMIM]+, such as 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([BDMIM]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM]), and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C12MIM]), have also been studied [241, 242]. For instance, Bettini et al. [263] found that among the four investigated ILs ([BMIM][NTf2], [C12MIM][NTf2], [EMIM][NTf2] and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BPyr] [NTf2])), [BMIM][NTf2]-based ES could exhibit the highest specific capacitance of 75 F g−1. This high specific capacitance was thought to be related to the shortest alkyl chain of the IL. ILs with other cations (e.g., pyrrolidinium-based [243-245], temperature stable supercapacitors on functionalized carbon nanomaterials or cluster assembled carbon materials in ionic liquids [251, 252] ammonium-based [256-258], sulfonium-based [259], and fluorohydrogenate ILs [269]) have also been extensively studied as potential electrolytes for EDLCs. Regarding the ILs with pyrrolidinium cations, Lazzari et al. [253] employed N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([PYR14][TFSI]) as the electrolyte for EDLCs with mesoporous carbon xerogel electrodes. In order to fully take advantage of the high ESPW of [PYR14] [TFSI] IL, the EDLCs had the asymmetric configuration with a higher carbon loading at the 

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positive electrode than that at the negative one. A maximum cell voltage of 3.7 V was achieved only for that asymmetric EDLC using [PYR14][TFSI] IL as the electrolyte, which was higher than that using [EMIM][TFSI] IL (3.4 V). Both EDLCs delivered a maximum specific energy of ~30 Wh kg−1 at their maximum cell voltage at 60°C. Lazzari et al. [254] tested [PYR12O1][TFSI] IL as the electrolyte for asymmetric EDLCs (AEDLCs) with different carbon loadings on the positive and negative electrodes. It was observed that the [PYR12O1][TFSI] based AEDLC could operate within a wide temperature range from −30 to 60°C at a high operative cell voltage of 3.7 V. A high cycling stability was also observed for these IL-based AEDLCs with a capacitance loss of 2% over 27,000 cycles at 60°C [255]. Such AEDLC could reach a maximum cell voltage of 4 V and could deliver a maximum energy density of 40 Wh kg−1 [255]. Considering that azepanium-based ILs were much cheaper than pyrrolidinium-based ILs, Pohlmann et al. [260] tested two azepanium-based ILs, (N-methyl, N-butyl-azepanium bis(trifluoromethanesulfonyl)imide ([AZP14][TFSI]) and N-methyl, N-hexylazepanium bis(trifluoromethanesulfonyl)imide ([AZP16] [TFSI])), as electrolytes for EDLCs, and compared them with [PYR14][TFSI]. Both of these two IL electrolytes showed operative working voltages up to 3.5 V, which was similar to [PYR14] [TFSI]. However, EDLCs utilizing these two IL electrolytes were found to deliver less energy densities especially at higher current density when compared to that with [PYR14][TFSI] IL as the electrolyte, which was attributed to the larger cation sizes and thus lower accessibilities to the electrode material surfaces. Regarding the ILs with ammonium-based cations, Sato et al. [256] employed N,N-diethyl- N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate ([DEME][BF4]) and [DEME] [TFSI] ILs as electrolytes for the EDLCs, and compared them with both [EMIM][BF4] IL and 1 M TEABF4/PC organic electrolytes. The ESPW of [DEME][BF4] IL (6 V) was obtained on a Pt electrode, which was wider than that of [EMIM][BF4] (4.5 V). The EDLC using [DEME] [BF4] IL as an electrolyte showed a much better high-temperature performance up to150°C. Kim et al. [257] found that [DEME][BF4] and [DEME][TFSI] ILs showed a wider ESPW than those of both the TEABF4/PC organic electrolyte and conventional [EMIM][BF4] IL. The specific capacitance of AC-based electrodes in both ILs could be increased upon increasing the applied voltage from 2.5 to 3.5 V. Senda et al. [258] studied AC-based EDLCs with fluorohydrogenate ILs (FHILs) as electrolytes. Five such FHILs contained different cations, such as 1,3-dimethylimidazolium (DMIM), 1-ethy1-3-methylimidazolium (EMIM), 1-butyl-3-methylimidazolium (BMIM), 1-ethyl-1-methylpyrrolidinium (EMPyr), and 1-methoxymethyl1-methylpyrrolidinium (MOMMPyr), respectively. The specific capacitances obtained using these fluorohydrogenate ILs were all higher than those obtained in [EMIM][BF4] IL or a 1 M TEABF4/PC organic electrolyte at the investigated cell voltage from 1 to 3.2 V. For three imidazolium based FHILs, the maximum specific capacitances were decreased in the following order: [DMIM][(FH)2.3F] (178 F g−1) > [EMIM][(FH)2.3F] (162 F g−1) > [BMIM][(FH)2.3F] (135 F g−1). This was in accordance with the order of increase of cation size. AC-based EDLCs using some FHILs with low melting points had suffcient specific capacitance even at 40°C (e.g., 64 F g−1 for [EMIM][(FH)2.3F]), which was much higher than that of TEABF4/PC (20 F g−1). Regarding the sulfonium-based cations, Anouti et al. [259] studied the trimethylsulfonium bis(trifluorosulfonimide) [Me3S][TFSI] IL electrolyte for AC-based EDLCs. [Me3S][TFSI] IL had an ESPW of about 5 V on a Pt electrode and could operate up to 3 V on AC electrodebased EDLCs. It was also observed the EDLC using this IL as an electrolyte was suitable for high temperature operation, delivering a maximum energy density of 44.1 Wh kg−1 at 80°C.





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FIGURE 4.15  Cyclic voltammograms for different ILs at 5 mV s−1. Source: Reproduced from Ref. [261], with permission of The Royal Society of Chemistry.

Comparative studies on ILs with different types of cations were also carried out [261, 262]. Sillars et al. [261] studied a series of ILs including [EMIM][BF4],1-ethyl-3-methylimidazoliumdicyanamide ([EMIM][N(CN)2]), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl) imide ([DMPIM][TFSI]), and 1-butyl3-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([BMPy][FAP]) as electrolytes for AC-based EDLCs, and compared them with a 1 M TEABF4/PC organic electrolyte. EDLCs using [BMPy][FAP] IL showed the highest operating voltage of 3.5 V (Fig. 4.15). The capacitive performances of EDLCs using these ILs were decreased in the following order: [EMIM][BF4] > [DMPIM][TFSI] > [BMPy][FAP] > [EMIM] [N(CN)2], which was thought to be related to the electrolyte viscosity and ion size. The ESR values were increased in the following order: [EMIM][N(CN)2] < [EMIM][BF4] < [DMPIM] [TFSI] < [BMPy][FAP], which was in accordance with the increasing order of viscosity. EDLCs using [EMIM][BF4] IL could deliver much higher energy and power densities when compared to an EDLC using 1 M TEABF4/PC as an organic electrolyte. Rennie et al. [262] investigated ILs with different types of cations (Fig. 4.16). It was found that the introduction of an ether bond into the cation alkyl side chain could effectively increase the specific capacitance and reduce the ESR of EDLCs (Fig. 4.17). The authors attributed this to the introduction of a small electronegative region to the cation structure through the ether bond, facilitating a denser packing of ions at the electrolyte/electrode interface, resulting in an increased charge amount. Anions of ILs also play an important role in their hydrophilic-hydrophobic properties. The hydrophobicity was found to increase in the following order: CH 3 CO −2 (acetate), NO −3 (nitrate) < Tf−, BF4− < PF6− , TFSI (hydrophobic) [263]. In addition, a small amount of water in ILs is often deleterious to their ESPW [264]. Therefore, hydrophobic anions, such as TFSI and FSI have received increasing attention for their usage in ESs [244, 265].



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FIGURE 4.16  Schematic structure of the constituent ions in ILs. (A) Tf2N, (B) 1-n-butyl-2,3-dimethylimidazolium (BMIM), (C) N-n-butyl-N-methylpiperidinium (BMPi), (D) N-n-butyl-N-methylpyrrolidinium (BMPy), (E) 1-(2-ethoxyethyl)-2,3-dimethyl-1H-imidazol-3-ium (EtO(CH2)2MMIM), (F) butyltrimethylphosphonium (P2225), and (G) (2-methoxyethyl)trimethylphosphonium (P222(201)). Source: Reprinted with permission from Ref. [262] (open access).

FIGURE 4.17  Specific capacitance determined at different rates of constant current discharge between 0 and 2.5 V for cells using phosphoniumbased ILs as electrolytes. Source: Reprinted with permission from Ref. [262] (open access).





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4.3.3.2.1.2  Protic ionic liquids. Protic ILs normally attract limited interest as electrolytes for EDLCs [266-269]. This may be because a much lower operative cell voltage (1.2-2.5 V) can be seen for EDLCs with protic ILs when compared with aprotic ILs. The protic ILs include protic pyrrolidinium nitrate (PyNO3) [266], triethylammonium bis(trifluoromethylsufonyl)imide ([Et3NH][TFSI]) [267, 269], pyrrolidinium bis(trifluoromethanesulfonyl)imide ([Pyrr][TFSI]) [268], and diisopropyl-ethyl-ammonium bis(trifluoromethanesulfonyl)imide([DIPEA][TFSI]) [268]. Protic ILs, however, have some advantages such as being generally easier to synthesize and cheaper compared to aprotic ILs [269, 270]. 4.3.3.2.1.3  Mixture of ionic liquids. As earlier mentioned, the most commonly studied ILs have higher melting points above 0°C, which prevent their use as ES electrolytes for low temperature applications. In order to extend the operative temperature range to lower temperatures significantly below 0°C, the strategy of a mixture of IL mixtures has been explored [265, 271, 272]. For example, Lin et al. [271] developed some eutectic IL mixtures of propylpiperidinium bis[fluorosulfonyl]imide([PIP13][FSI]) and [PYR14][FSI] (1:1by weight or molar ratio). This IL mixture had a liquid-state range lowered to 80°C. Using exohedral nanostructured carbon (nanotubes and onions) as electrodes, the EDLCs using this [PIP13][FSI] + [PYR14][FSI] IL mixture electrolyte could be operable within a significantly expanded temperature range from 50 to 100°C (Fig. 4.18). 4.3.3.2.2  Solvent-free ionic liquids for pseudocapacitors

ILs have also been explored as the electrolytes for pseudocapacitors. Normally, for ES electrode materials, the pseudocapacitance contribution is closely related to surface functionalities, IL's hydrophobicity (the anions) and free water in ILs [266]. In an earlier work, Rochefort

FIGURE 4.18  Normalized capacitance (C/C20°C) for the OLC and VA-CNT electrodes in ([PIP13][FSI])0.5([PYR14]

[FSI])0.5 IL mixture and PC + 1 M TEABF4 electrolytes. Capacitances were calculated at 100 mV s−1, except for the 50°C (1 mV s−1) and 40°C (5 mV s−1) experiments. Source: Reprinted with permission from Ref. [271], Copyright 2011 American Chemical Society.



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et al. [273] provided experimental evidence for pseudocapacitance could be achieved on thermally prepared RuO2 electrodes in a protic IL composed of 2-methylpyridine (α-picoline) and trifluoroacetic acid (TFA) while no obvious pseudocapacitance could be observed in [EMIM] [BF4] IL. It was reported that the shape of cyclic voltammograms and the specific capacitance (83 F g−1) of the RuO2 electrode in a protic IL were similar to those obtained in a 0.1 M H2SO4 aqueous electrolyte. However, the high viscosity and slow proton transfer in the IL electrolyte could limit the charging rate. Mayrand Provencher et al. [274] tested a series of pyridinium-based protic ILs as ES electrolytes, and found that the alkyl chain length of a cation's substituent and the substituent position had an effect on the electrolyte conductivity and viscosity, ESPW, specific capacitance and cycling stability of the RuO2 electrodes. Using the in situ infrared spectroscopy technique, Richey et al. [275] tried to obtain a fundamental understanding of individual cation and anion dynamics of [EMIM][Tf] IL in a working RuO2-based pseudocapacitor. Other less expensive metal oxides than RuO2, such as Mn oxides, have also been investigated as electrode materials for IL electrolyte-based ESs [276-279]. For example, Chang et al. [280] obtained pseudocapacitive performance from a Mn oxide in an aprotic 1-ethyl-3-methylimidazolium-dicyanamide aprotic IL electrolyte (i.e., without involving protons and alkali cations in the electrolyte). Mn oxide showed a specific capacitance of 72 F g−1 with a potential window of ~2 V in the [EMIM][DCA] electrolyte. Using XPS and in situ X-ray absorption spectroscopy (XAS), Chang et al. [281] found that smaller [DCA] anions, instead of [BMP]+ cations, could reversibly insert/desert into the tunnels between MnO6 octahedral units, compensating the Mn3+/Mn4+ valent state variation during the charging-discharging process in the [BMP][DCA] IL electrolyte, as expressed as Eq. (4.9): MnO 2 − x (DCA) 2 x + 2 xe − ↔ MnO 2 − x + 2 xDCA − ( x ≤ 0.5) (4.9)

When using protic ILs rather than aprotic ones, Ruiz et al. [282] found that MnO2 electrodes could exhibit a pseudocapacitive behavior in protic ILs composed of 2-methoxypyridinium and trifluoroacetate, as observed by in situ UV-visible spectroscopy. In addition to the extensively studied MnO2 materials, other pseudocapacitive material-IL electrolyte systems have also been studied, such as Ru-doped Cu oxide in 3-carboxymethyl-1-methylimidazolium bisulfate [CMIM][HSO4] [283], TiO2 in [EMIM][TFSI] [284], Fe2O3 in [EMIM][BF4] [285] porous Ni in [EMIM][BF4] [286], nanocomposites composed of VN and N-doped carbon in [PYR14] [TFSI] and [PYR14][FSI] [287] poly(3-methylthiophene) in [EMIM][TFSI] [288], as well as PAN in [EMIM][BF4] [289] and [EMIM][TFSI] [290]. 4.3.3.2.3  Solvent-free ionic liquids for hybrid electrochemical capacitors

ILs have also been explored as electrolytes for hybrid ESs with asymmetric electrode configurations (e.g., AC//poly(3-methylthiophene) [291, 292], AC//MnO2 [293], and AC//graphene supported Fe2O3 [285]). For example, Balducci et al. [291] used both [BMIM][PF6] and [PYR14][TFSI] ILs as electrolytes for AC//poly(3-methyl-thiophene) (PMeT) hybrid ESs, and observed some enhanced cell voltage, cycling life at high temperatures when compared to that of ESs using an organic electrolyte (e.g.,TEABF4/ PC). The AC//pMeT hybrid ES using [PYR14][TFSI] IL could deliver maximum energy and power densities of 24 Wh kg−1 and 14 kW kg−1, respectively, after the first thousand cycles at 10 mA cm−2 and 60°C. Sun et al. [285] used [EMIM][BF4] IL as an electrolyte for asymmetric ESs with an activated PANI-derived carbon nanorod as a negative electrode and a graphene sheet supported Fe2O3 nanoparticle 



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261

as a positive electrode. This hybrid ES was able to operate within a wide voltage range of 0-4 V, and could deliver a very high maximum energy density of 177 Wh kg−1 and showed a relatively large energy density of 62.4 Wh kg−1 at a high-power density of 8 kW kg−1. As discussed earlier, the cycling stability of a hybrid ES is always an important concern when compared to that of an EDLC. In this regard, Arbizzani et al. [255] reported that although an AC//pMeT hybrid ES using [PYR14][TFSI] IL as an electrolyte could deliver a 30% of higher energy than the carbon-based AEDLC, its cycling stability was lower than the later. The specific capacitance of this hybrid ES was decreased by 50% after 5000 cycles due to the deterioration of pMeT electrode. 4.3.3.3  Mixtures of ionic liquids and organic solvents In order to reduce the viscosity and increase the conductivity of ILs, particularly at low temperatures, mixed solutions containing ILs and organic solvents have been explored as electrolytes for ESs. Similar to the research in solvent-free ILs-based ESs, imidazolium-based ILs seem to be the most extensively studied ones in organic solvents due to their relatively high conductivity among commonly used ILs. In this aspect, McEwen et al. [305] found that the conductivities of IL-based carbonate electrolytes (ILs: [EMIM][PF6] and [EMIM][BF4], both of which contain EMI cations) were roughly 25% greater than that based on the TEABF4 organic electrolyte. Conductivity as high as 27 mS cm−1 was observed in 2 M [EMIM][PF6] plus EC-DMC solvent. In addition, a 2 M [EMIM][PF6]/PC electrolyte could exhibit both a higher specific capacitance and more thermal stability than 1 M TEABF4/PC. Orita et al. [295] studied a series of alkyl functionalized ILs mixed with organic solvents (typically PC) as electrolytes for EDLCs. Two ILs, one composed of imidazolium cations with allyl groups (diallylimidazolium (DAIM)) and TFSA anions, and the other composed of imidazolium with saturated alkyl groups (EMIM) and TFSA anions, were used in the preparation of solvent-based IL electrolytes [294]. EDLC measurements in 1.4 M IL electrolytes showed that the former electrolyte could give higher capacitance and lower resistance within a wider temperature range than the latter. However, the stability of the EDLC using 1.4M [DAIM][BF4]/PC was lower than the one using [EMIM][BF4]/PC. It was also demonstrated that the stability could be improved though the addition of DMC to PC. Lin et al. [296] found that although the bare molecular sizes of [EMIM]+ and [TFSI] were similar, they had different solvated ion sizes in the ACN solvent and an increase in size in the order of: [TFSI] in ACN > [EMI]+ in ACN > [EMI]+ ≍ [TFSI]−. Therefore, the optimization of the pore structure and size distribution of porous carbon with respect to IL size in an organic solvent should consider the solvated ion size, which might be different from that in the pure IL electrolyte. As mentioned in Section 4.3.2.2, the use of ADN as an organic solvent has the advantages of a larger EDLC operative cell voltage and higher flash point than the ACN solvent [141]. However, the solubility of commonly used conducting salt such as TEABF4 in ADN is lower than in ACN, thus limiting its application [141]. The use of IL, such as 1-ethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide ([C2mIM] [TFSI]) as a salt in ADN was tested to address this issue, and the results showed higher salt solubility [297]. It should be pointed out that not all the mixtures of IL and organic solvents provide a benefit to ES performance. For example, Palm et al. [298] reported the addition of an organic solvent (ACN, PC, or GBL) to pure [EMIM][BF4] to compose an electrolyte for EDLCs, and the results showed some negative effects although this mixing strategy could lead to a decreased viscosity and melting point, and increased conductivity when compared to those



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of pure [EMIM][BF4]. Therefore, attention should be paid to not compromising the important advantages of pure IL while adding solvent into the IL for the ES electrolyte. In addition, imidazolium-based ILs mixed with organic solvents have also been explored as electrolytes for both pseudocapacitors and hybrid ESs [293]. For example, Zhang et al. [293] found that the introduction of DMF to [BMIM][PF6] IL increased the capacitance and at the same time decreased the internal resistance of the asymmetric AC//MnO2 ES. This improvement was attributed to the improved electrolyte penetration and ion mobility when compared to pure IL. Due to the relatively high ESPWs, pyrrolidinium-based ILs have also been mixed with organic solvents to form mixed electrolytes for ESs [299-301]. As observed, the addition of organic solvents to the pyrrolidinium-based ILs significantly increased the conductivity and decreased the viscosity of the electrolytes due to the salvation effect provided by the organic solvents [299]. Compared to the conventional TEABF4/PC organic electrolyte, the use of pyrrolidinium-based ILs mixed with PC effectively increased the operative EDLC cell voltage to 3.2-3.5 V [300, 301]. Regarding ammonium-based ILs, Abdallah et al. [302] found the addition of [TMPA][TFSI] IL could reduce both the flammability and volatility of ACN solventbased electrolytes. With respect to sulfonium-based ILs, Brandt et al. [303] reported that the EDLC using an electrolyte containing 3.8 M [Me3S][TFSI] in PC could show an operative cell voltage up to 2.9 V. This EDLC also showed a stable performance at both 20 and 60°C while an EDLC using [Me3S][TFSI]/PC with a lower concentration of 1.9 M did not show a stable performance. This might be due to the different abilities to inhibit the anodic oxidation of the Al current collector in these two different electrolytes. Regarding the phosphonium-based ILs, Frackowiak et al. [304] found that the amount of CAN in trihexyl(tetradecyl) phosphonium bis(trifluoromethylsulfonyl) imide ((C6H13)3(P(C14H29))[Tf2N]) or trihexyl (tetradecyl) phosphonium di cyanamide ((C6H13)3(P(C14H29))[(CN)2N]) could play an important role in both specific energy and power densities of EDLCs [304]. An AC-based EDLC using (C6H13)3(P(C14H29))[Tf2N] IL with 25 wt.% ACN could achieve an operative cell voltage of 3.4 V and had a higher energy density of about 40 Wh kg−1 and better cycling stability when compared to that using conventional ILs and with a different amount of ACN. Regarding piperidinium-based ILs, Lewandowski et al. [305] reported that the ionic conductivity (1.5 mS cm−1 at 25°C) of pure N-methyl N-propyl piperidinium bis(trifluoromethanesulphonyl)

FIGURE 4.19  Chemical structures of cations. (A) DMES (dimethylethylsulfonium), (B) DEMS (diethylethylsulfonium), (C) EMPS (ethylmethylpropylsulfonium), (D) BDMS (buthyldimethylsulfonium), (E) MTT (1-methyltetrahydrothiophenium), (F) ETT (1-ethyltetrahydrothiophenium), (G) PTT (1-propyltetrahydrothiophenium), (H) BTT (1-butyltetrahydrothiophenium), (I) MOT (1-methyl-[1,4]-thioxonium). Source: Reprinted from Ref. [307], with permission from Elsevier.





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imide ([MePrPip][NTf2]) IL could be significantly increased to about 40 mS cm−1 after the addition of 52 wt.% ACN. This mixture electrolyte showed an ESPW of about 3.7 V on AC electrodes. Some comparative studies between different types of ILs mixed with organic solvents have also been reported [300, 306, 307]. For example, Orita et al. [307] investigated various sulfonium- and thiophenium-based ILs (Fig. 4.19) in the PC solvent as EDLC electrolytes. They found that [DEMS][TFSA]/PC had the highest ionic conductivity among the investigated ILs with TFSA anions. The ionic conductivity of [DEMS][BF4]/PC was higher than those of [DEMS][PF6]/PC and [DEMS][TFSA]/PC. The EDLC using the [DEMS][BF4]/PC electrolyte showed a higher specific capacitance than the one using the conventional [EMIM] [BF4]/PC or the [TEMA][BF4]/PC electrolyte. However, the [DEMS][BF4]/PC electrolyte resulted in EDLCs having a shorter lifetime when compared to that in an [EMIM][BF4] electrolyte. Brandt et al. [308] found that EDLCs with a 1.9 M [PYR14][BF4]/ PC electrolyte could show both higher energy and power densities than EDLCs with 1.5 M [PYR14][TFSI]/PC, 3.8 M [Me3S][TFSI]/PC or conventional 1 M TEABF4/PC at all investigated current densities. This was attributed to the larger operative cell voltage (Fig. 4.20). It should be noted that besides aprotic ILs, protic ILs mixed with organic solvents have also been studied although it received less attention when compared to aprotic ILs due to their lower cell operative cell voltage (1.5-2.5 V) [267, 269, 309-311]. Several studies reported that ELDCs using mixture electrolytes containing protic ILs and organic solvents (e.g., [Bu3PH] [BF4] in GBL [309] and [Pyrr][NO3] in GBL [310]) could operate in a wide temperature range from −40°C to 80°C. In addition, theoretical modeling and simulation study has also been carried out to study the EDL structure and capacitance in the mixtures of ILs and organic solvents by using MD simulations [233, 312].

4.3.4  Solid- or quasi-solid-state electrolytes for ESs With the rapid growing demand of power supply for portable electronics, wearable electronics, micro-electronics, printable electronics, and especially flexible electronic devices, solid-state electrolyte based electrochemical energy devices have attracted great interest in recent years. The solid-state electrolytes cannot only serve as the ionic conducting media but also as the electrode separators. The main advantages associated with using solid-state electrolytes are the simplification of packaging and fabrication processes of ESs and free from liquid-leakage and associated issues as handling, spillage, and corrosion, etc., To date, major types of the solid-state electrolytes developed for ESs have been based on polymer electrolytes, and only very limited work has been focused on inorganic solid materials, such as ceramic electrolytes [313, 314]. The polymer-based solid electrolytes for ESs can be further classified into three types: the solid polymer electrolyte (SPEs, also known as dry polymer electrolytes), the gel polymer electrolyte (GPE), and the polyelectrolyte. Due to the presence of a liquid phase in GPEs, these are also termed as quasi-solid-state electrolytes [315, 316]. As shown in Fig. 4.21, the SPE is composed of a polymer (e.g., PEO) and a salt (e.g., LiCl), without any solvents (e.g., water). The ionic conductivity of SPE is provided by the transportation of salt ions through the polymer. In contrast, the GPE consists of a polymer host (e.g., PVA) and an aqueous electrolyte (e.g., H2SO4) or a conducting salt dissolved in a solvent. In this case, the polymer serves as a matrix, which can be swollen by the solvent, and the ions transport in the solvent instead of in the polymer phase, which is different



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FIGURE 4.20  Ragone-like plots of EDLCs containing different electrolytes. The average values of energy and power densities refer to the total AC loading in the cell. Source: Reprinted from Ref. [308], with permission from Elsevier.



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FIGURE 4.21  Schematic diagrams of (A) a dry solid-state polymer electrolyte (e.g., PEO/Li+), (B) a gel polymer electrolyte, and (C) a polyelectrolyte. Source: Reproduced with permission from Ref. [20].

from that of SPE. In the polyelectrolyte, the ionic conductivity is contributed by the charged polymer chains. As identified, each type of these solid-state electrolytes has their own advantages and disadvantages. Normally, GPEs have the highest ionic conductivity among these three types of solid-state electrolytes. Due to the presence of a liquid phase in a GPE, its ionic conductivity is significantly higher than that of the dry SPE. Due to this reason, GPE-based ESs currently dominates the solid electrolyte-based ES products, and studies on ESs using dry SPEs are scarce [317]. However, depending on their composition, GPEs may suffer from relatively poor mechanical strength and a narrow operative temperature range



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particularly when water is used as the solvent. Furthermore, the weak mechanical strength of some GPEs is the main concern, as it may lead to internal short circuits, causing safety issues [19]. Although dry SPEs normally have low ionic conductivities, they have relatively high mechanical strength when compared to GPEs. It should be noted that these solid-state electrolytes for ESs normally have some common disadvantages including the limited contact surface area between solid-state electrolytes and electrode materials especially for the nanoporous materials. This issue could increase the ESR value, reduce the rate performance, and limit the utilization of active electrode materials, resulting in a low specific capacitance of ESs. As reported in the literature, solid electrolytes have been employed in various types of ESs such as EDLCs, pseudocapacitors and hybrid ESs with different kinds of electrode materials. When developing solid-state electrolytes for ESs, the following critical requirements should be considered: (1) high ionic conductivity, (2) high chemical, electrochemical, and thermal stability, and (3) sufficient mechanical strength and dimensional stability. In practice, it is difficult for a solid-sate electrolyte to meet all of these requirements. There is often some trade-off between ionic conductivity and mechanical strength. In this aspect, several reviews have been published on the solid-stated electrolytes for ESs [9, 10, 18, 19, 318]. Therefore, this present section is mainly focused on the new developments. 4.3.4.1  Gel polymer electrolytes As mentioned earlier, GPEs are currently the most extensively studied electrolytes for solid-state ESs due to their high ionic conductivity. A GPE is typically composed of a polymer matrix (host polymer) and a liquid electrolyte (e.g., aqueous electrolyte, organic solvent containing conducting salt and IL). Regarding the host polymer, various polymer matrices have been explored for preparing GPEs, including poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), potassium polyacrylate (PAAK), poly(ethyl oxide) (PEO), poly(methylmethacrylate) (PMMA), poly(ether ether ketone) (PEEK), poly(acrylonitrile)-block-poly(ethylene glycol)block-poly(acrylonitrile) (PAN-b-PEG-b-PAN), and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). When water is used as a plasticizer, the resulting GPE is also called a hydrogel polymer electrolyte, which has a kind of 3D polymeric networks that can trap water in the polymer matrices mainly through surface tension [9]. Besides water, organic solvents, such as PC [319], EC, and DMF [320] or their mixtures (e.g., PC-EC [321], PC-ECDMC [322], and PC-EC [320]) have also been commonly used as the plasticizers in GPEs. The composition ratio between a polymer and a plasticizer normally plays an important role in the degree of plasticization, thus affecting the glass-transition temperature of GPEs [9]. One of the most significant advantages of using solid-state electrolytes including GPEs in ESs is that they allow the development of diverse and bendable structures and tunable shapes for various desired applications. For example, based on the PVA-based hydrogels, various solid-state ESs have been developed, including flexible ESs [323], stretchable ESs [324], flexible micro-ESs [325], printable micro-ESs [326] on chip micro-ESs [327], 3D microESs [328], yarn ESs [329], wire or fiber-shaped ESs [330], transparent ESs [331], ultrathin ESs [332], weaveable ESs [333], paper-like ESs, [334] and integrated ESs with other devices [335]. 4.3.4.1.1  Hydrogel polymer electrolytes

Poly(vinyl alcohol) (PVA), as a linear polymer, has been the most extensively studied polymer matrix till date, among the various kinds of host polymers for hydrogel electrolytes. This





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FIGURE 4.22  Capacitance per area as a function of electrode layer thickness (CNT film) in two different ES electrolytes (1 M H2SO4 aqueous electrolyte and PVA/H3PO4 gel electrolyte). Source: Reprinted with permission from Ref. [337]. Copyright 2009 American Chemical Society.

is mainly due to its ease of preparation, high hydrophilicity, good film-forming properties, nontoxic characteristics, and low cost [336]. PVA is generally mixed with various aqueous solutions, such as strong acid (e.g., H2SO4 and H3PO3), strong alkaline (e.g., KOH) and neutral (e.g., LiCl) electrolytes to form hydrogels. 4.3.4.1.1.1  Hydrogel polymer electrolytes for carbon-based electrodes. When GPEs are used as ES electrolytes, it is inevitable to attain the appropriate designing of the electrode/electrolyte interface for high performance. Studies reveal that GPE-based ESs could suffer from poor rate capabilities due to the limited ion diffusion rate or large interfacial resistance at the electrode/ electrolyte interface [337]. Kaempgen et al. [337] also found that the thickness of the electrode layer played an important role in achieving high capacitance of PVA/H3PO3-based ESs. As shown in Fig. 4.22, in a 1 M H2SO4 aqueous electrolyte, the capacitance increases linearly with increasing thickness of the electrode layer, while a saturation of the capacitance value can be observed when using the PVA/H3PO3 hydrogel as an electrolyte. This cause was attributed to the limited ion penetration into the porous electrodes. In this case, the hydrogel electrolyte may not be the suitable electrolyte for applications requiring high charge storage capabilities. Normally, the matching among the electrode/hydrogel electrolyte interface, the electrode material, and hydrogel electrolyte is very critical in achieving high ES performance. With respect to this, electrode materials with various structures, such as CNTs grown on carbon clothes [338], activated carbon clothes [339], 3D graphene networks [340], porous graphene ribbons [341], and graphene/porous carbon aerogels [342] have been studied using PVAbased hydrogel electrolytes. It was observed that these advanced carbon structures can facilitate the infiltration of a hydrogel electrolyte into the porous electrode, leading to a large ES performance improvement and a high utilization of the active electrode materials. The type of aqueous solutions in a hydrogel electrolyte also has a significantly impact the ES performance due to their different



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ionic conductivities, thermal, and environmental stabilities. Optimization is generally needed through careful selection of the electrolyte solution for the PVA, modifying the PVA and incorporating additives into the hydrogel. Chen et al. [343] prepared six different PVA-based hydrogels using different electrolytes, such as H3PO4, H2SO4, KOH, NaOH, KCl, and NaCl for graphene-based ESs. Based on the electrochemical measurements, they found that the PVA/ H3PO4 hydrogel electrolyte exhibited the best capacitive performance among all the investigated PVA-based hydrogels. Regarding the effect of temperature, PVA-based electrolytes generally suffer fluidity problems at high temperatures [336]. To improve this temperature related performance, Fei et al. [336] prepared a cross-linked PVA/H2SO4 hydrogel as an ES electrolyte, and the fabricated ES showed good cycling stability with 78.3% retention of the initial specific capacitance after 1000 cycles at a high temperature of 70°C. Similarly, various additives or fillers, such as SiO2 [344], TiO2 [345], Sb2O3 [346], graphene oxide (GO) [347], and hydroxyl ethyl cellulose [348] have also been used for PVA-based hydrogels to improve performance. For example, Huang et al. [347] used electrically insulating GO as an ionic conducting promoter in a boron cross-linked PVA/KOH hydrogel for an EDLC electrolyte. It was observed that the ionic conductivity increased with an increasing amount of GO in the low GO content range. However, the ionic conductivity decreased with a further increase in the GO content, probably due to the blockage of ion channel caused by the aggregation of GO sheets (Fig. 4.23).

FIGURE 4.23  Illustration of the ion transport mechanism in GO-PVA and GOB-PVA alkaline gel electrolytes with low and high GO contents. Source: Reprinted from Ref. [347], with permission from Elsevier.





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The specific capacitance of an EDLC using a boron cross-linked PVA/KOH hydrogel containing 20 wt.% GO was found to be 141.8 F g−1 at 0.1 A g−1, which was 29% higher than the specific capacitance using a bare KOH aqueous electrolyte. The environmental stability of hydrogel electrolyte-based ESs is also one of the important factors used in evaluating the ES performance. It was found that the performance of a PVA/KOH hydrogel electrolytebased ES could suffer a significant loss over time (e.g., after 40-67 days of shelf storage) due to the dehydration of the KOH/PVA electrolyte [339]. In order to improve the environmental stability of ESs using alkaline PVA hydrogel electrolytes, Gao et al. [349] explored the replacement for the KOH in the PVA-based hydrogel electrolyte with tetraethylammonium hydroxide (TEAOH), due its higher water retention capability than KOH. The addition of SiO2 into a PVA-silicotungstic acid (SiWA)-H3PO4 hydrogel was also found to stabilize the water content in the hydrogel at relatively low humidity, leading to an ES with improved environmental stability. This was attributed to the interaction between SiO2 and water molecules [344]. 4.3.4.1.1.2  Hydrogel polymer electrolytes for pseudocapacitors and hybrid capacitors. Besides EDCLs, considerable efforts have also been focused on the development of both hydrogel electrolyte based pseudocapacitors and hybrid ESs. As mentioned earlier, the electrochemical behavior of pseudocapacitive electrode materials is strongly dependent on the nature of electrolytes. For ESs using acid PVA-based hydrogels (e.g., PVA/H2SO4 and PVA/H3PO4), typical pseudocapacitive electrode materials explored include transition metal oxides (e.g., RuO2 [350], TiO2 [351], and manganese oxide [332]), NiCo2O4@NiO [355] metal nitrides (e.g., molybdenum nitride [363], sulfides (e.g., MoS2 [330], TiS2 [330], and TaS2 [330]), selenides (e.g., GeSe2 [323] and NbSe2 [330]), and ECPs (e.g., PANI [353], PPy [354], and PEDOT). A detailed account of the all solid pseudocapactive electrochemical capacitors is available in reference [352]. In the case of alkaline PVA-based (mostly PVA/KOH) ESs, the most extensively studied pseudocapacitive electrode material is Ni(OH)2 [84], other electrode materials, such as NiCo2O4 [367], CoMn-layered double hydroxide [356], and MnO2 [357] have also been investigated. For neutral PVA-based hydrogel electrolytes (e.g., PVA/LiCl [358], PVA/Na2SO4 [359], and PVA/NaNO3 [360]), pseudocapacitive materials, such as MnO2 [360], V2O5 [361], ZnO [362], VOPO4 [363], VN [364], and KCu4Se8 [361] were frequently studied as electrode materials. To improve the intercalation/deintercalation performance, Li-containing PVA-based hydrogels (e.g., PVA/LiCl [358, 363], PVA/LiOH [356], PVA/LiNO3 [365]) have also been studied with pseudocapacitive electrode materials. It should be noted that compared to aqueous electrolytes, hydrogels offered greatly enhanced electrochemical stability for some pseudocapacitive electrode materials, particularly with vanadium oxide [358] and VN [364]. Once again, the matching between the electrode surface and the PVA-based hydrogel electrolyte needs to be optimized [363, 366]. To improve the conductivity and the utilization degree of the pseudocapacitive electrode materials in the hydrogel electrolyte, they could be dispersed on high-surface-area carbonaceous supports [332] or combined with carbonaceous materials to make hybrid composites [330, 366]. Hydrogel electrolytes generally have shortcomings similar to that of aqueous electrolytes. For instance, symmetric ESs using hydrogel electrolytes can suffer from limited operative cell voltages and thus low energy density due to the narrow ESPW of the aqueous component in the hydrogel. To increase the operative



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cell voltage, hydrogel electrolyte-based asymmetric ESs using two different electrodes with complementary potential windows have been explored [335, 364]. It was determined that compared to the symmetric PVA hydrogel-based ESs, the operating cell voltage of asymmetric ESs can be greatly improved to a high value ~ 1.8 V [364]. Besides PVA, other polymers have also been studied for use in hydrogels in ESs. For example, poly(acrylate) and poly(acrylic acid) based hydrogels were tested as promising ES electrolytes. These polymers can promote proton conduction as the protons in their side chains can be easily abstracted in an aqueous medium [367-369]. These alternative hydrogel electrolytes have been tested for AC-based EDLCs [370], pseudocapacitors with electrode materials, such as RuO2 [367], MnO2 [369], and ECPs [368], as well as asymmetric ESs [371]. Since the performance of a pseudocapacitive electrode material depends on the properties of the electrolyte, the nature of the polymer host in a hydrogel electrolyte might have an influence on the performance of its associated pseudocapacitors. In this regard, Kim et al. [367] investigated the performance of RuO2-based ESs using various acrylic hydrogel electrolytes, such as poly(acrylic acid) (PAA), potassium polyacrylate (PAAK), and poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPS). The specific capacitance of a RuO2 electrode in different electrolytes was found to decrease in the order: PAMPS/H2O > 1M H2SO4 (aqueous electrolyte) > PAA/H2SO4 > PAAK/H2SO4 > PAMPS/H2SO4. They attributed the highest capacitance of the RuO2 electrode obtained in the PAMPS electrolyte to the most favorable proton accommodation in the side chain groups of PAMPS. In general, hydrogel electrolytes have limited thermal stability. To improve the thermal stability, Shimamoto et al. [372] explored a phosphosilicate gel as the electrolyte for asymmetric ES with an MnO2-CNT composite positive electrode and an AC negative electrode. The resulting ESs could be operated within a wide temperature range from −30 to 100°C. In addition, Nafions (a perfluorosulfonic acid polymer), a well-known proton-conducting polymer, has also been employed as an electrolyte for various kinds of solid-state ESs due to its high ionic conductivity [373, 374]. Several studies showed that ESs using Nafions as an electrolyte could support high potential scan rates [373]. 4.3.4.1.2  Organogel electrolytes

To increase the working cell voltage, GPEs based on organic solvents (plasticizers), known as organogel electrolytes, have been studied as electrolytes for ESs. In these organogel electrolytes, various polymer hosts, such as PEO [322, 375], PMMA [376, 377], polyvinylpyrrolidone (PVP), PEEK [378], and copolymers [320, 379-381], were reported in the literature as having been tested. Copolymers with different constituent units, such as poly(acrylonitrile)-block-poly(ethylene glycol)-block-poly(acrylonitrile) (PANb-PEG-b-PAN), have shown to be the most promising polymeric hosts in organogel electrolytes-based ESs [320, 379]. For example, Hsueh et al. [379] reported a synergistic effect between PAN and PEG when employed as the host for GPE. The ionic conductivity of copolymer-based organogels could be optimized by tuning the AN/EG chain length ratio. A maximal ionic conductivity of 1.1 × 10−2 S cm−1 was obtained, which was much higher than that of the LiClO4/DMF organic liquid electrolyte (1.6 × 10−3 S cm−1). Carbon-based EDLCs using this copolymer based organogel electrolyte could deliver both higher energy and power densities (20 Wh kg−1 at 10 kW kg−1) than those of EDLCs





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FIGURE 4.24  Ragone-like plot for two ESs and two LICs each containing either the liquid electrolyte LP30 or the methacrylate-based GPE. The average energy and power indicated in the figure is referred to the total active mass of the LIC. Source: Reproduced by permission of The Electrochemical Society from Ref. [382].

using organic liquid electrolytes, suggesting a positive synergistic effect between PAN and PEG blocks. Regarding the organic solvent (plasticizer) in the organogel electrolyte, typically investigated systems include PC, EC, DMC, or their mixtures [322, 375], ACN [376, 377], DMSO [380], and DMF [320, 379]. Using these organogel electrolytes, the ES cell voltages were increased up to 2.5-3 V [320, 379-382], which was higher than those of hydrogel electrolyte-based ESs including asymmetric ones. These high cell voltages indicate one of the most significant advantages of organogel electrolyte-based ESs. Due to a direct result of the high cell voltages, most of the reported organogel electrolytebased ESs exhibited relatively high maximum energy density values around 18-25 Wh kg−1 [322, 375, 376, 379]. Interestingly, Schroeder et al. (Fig. 4.24) [382] reported a LIC using the methacrylate-based organogel electrolyte that could be operated at a cell voltage of 4 V, resulting in a further increase in energy density. Regarding the salts used in organogel electrolytes, the ones typically investigated include LiClO4 [320, 379], LiPF6 [381], TBAPF6 [376, 377], NaTFSI [322], etc., Research has revealed that the selection of the salt and the ratio between salt and the host polymer has a great influence on the ionic conductivity of the electrolyte [383]. As Mg is abundant than Li on the earth crust, several studies tested Mg salt in organogel electrolytes [375, 380]. Normally, the organogel electrolytes would suffer from low ionic conductivities. In an effort to overcome this issue, Jain et al. [384] added SiO2 particles into the PVDF-HFP/Mg(ClO4)2-PC electrolytes for EDLCs. The resulting organogel electrolyte with the optimized composition showed a relatively high ionic conductivity ~5.4 × 10−3 S cm−1 at room temperature, and in addition, exhibited good mechanical and dimensional stability [384]. Furthermore, organogel 

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electrolytes were found to have high cycling stability especially at higher ES cell voltages [381]. 4.3.4.1.3  IL-based solid-state electrolytes

Recognizing the large ESPW and high thermal stability of ILs, considerable research efforts have been channeled to learn about the IL-based GPEs, also known as ionogels, which are made by the incorporation of ILs into polymer hosts. Lately, majority of the studies focused on the ionogel electrolyte-based EDLCs with electrode materials such as AC [385], porous carbon [386], and CNT [387]. There were also some studies devoted to ionogel electrolyte-based pseudocapacitors with electrode materials such as MnO2 [388], PEDOT [389] and composite materials (e.g., PMeT with hydrous RuO2 [390]). With ionogel electrolytes, ESs such as flexible ESs [388], stretchable ESs [391], micro-ESs [386], and stretchable microESs [392] were developed. The properties such as the conductivity and ESPW of an ionogel depend on the nature of IL and the host polymer, as well as their interaction. Liew et al. [393] found that with the incorporation of [BMIM][Cl] IL, the glass transition temperature (Tg) of PVA-based polymer electrolytes could be decreased to the sub-ambient temperature range. Regarding the IL as the solvent, the selection or design of an appropriate host polymer is crucial for achieving high ES performance of IL-based GPE. Currently, a variety of host polymers studied for ionogel electrolytes include PVA [393], PEO [394], PMMA [391], poly(ethylene glycol) diacrylate [392], and PVDF-HFP [387, 388, 395]. Using PMMA as a polymeric host, Tamilarasan et al. [391] prepared a highly transparent and stretchable PMMA/[BMIM][TFSI] ionogel with nearly 4-fold stretchability. The assembled PMMA/ [BMIM][TFSI]-based EDLC with graphene electrodes showed a specific capacitance of 83 F g−1, and an energy density of 26.1 Wh kg−1 at a power density of 5 kW kg−1, in terms of mass of total electrode material. In addition, a positive effect of the chitosan host on the performance of ionogel-based EDLCs was reported by Yamagata et al. [396]. Due to the high chemical and thermal stabilities as well as high flexibility of PVDF, it was used as the host for the [BMIM][BF4] IL solvent to form an ionogel for flexible ESs by Yang et al. [397]. They blended amorphous polyvinyl acetate (PVAc) with PVDF as a polymeric matrix, and the resulting ionogel containing a PVDF-PVAc host and 50 wt.% of [BMIM][BF4] exhibited good flexibility, high thermal stability (~300°C), and also had an ionic conductivity of 2.42 × 10−3 S cm−1 at room temperature. AC-based EDLCs using this ionogel electrolyte could be operated at a cell voltage of 3 V, with a specific capacitance of 93.3 F g−1 (in terms of the mass of AC) at 200 mA g−1, and the capacitance retention was ~ 90% after 5000 charge-discharge cycles. As the properties of ILs have a great influence on ES performance, it is important to suitably select and design the IL component in an IL-based polymer electrolyte. To overcome the viscosity issue with common ILs, some low viscosity ILs, such as [EMIM][SCN] [398] and [EMIM][TCB] [387] were explored. However, EDLCs using these ionogel electrolytes could only give some low specific energy densities, such as ~3.5-4.7 Wh kg−1 due to the limited cell voltage [387, 398]. Pandey et al. [399] demonstrated that the addition of Li-salt into the PVDF-HFP/[EMIM][FAP] GPE could markedly improve the specific capacitance of CNT-based EDLCs from about 76 F g−1 (without Li-salt) to about 127 F g−1. Ketabi et al. [394] found that the addition of 10 wt.% SiO2 into PEO/EMIHSO4 could increase the ionic conductivity of the electrolyte from 0.85 (without SiO2) to 2.15 mS cm−1 at room temperature. The EDLC fabricated showed a good capacitive behaviour after 5000 cycles at a relatively





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high scan rate of 1 V s−1. In addition, incorporating plastic crystalline materials [e.g., succinonitrile (SN)] into IL-based GPEs was also studied by Suleman et al. [395] in an effort to increase the mechanical stability of the electrolyte. Preparation methods seem to play an important role in the ES performance of IL-based GPEs [400]. For example, Liu et al. [400] described a self-triggered UV photo polymerization method to prepare ionogel electrolytes made from [BMIM][Cl], chitosan and HEMA in the absence of initiators and crosslinkers. AC-based EDLCs with this ionogel electrolyte could be operated well at a high temperature of 100°C, with a good performance stability for 2000 cycles at 0.5 A g−1 under normal and bent conditions. Interestingly, such ionogel electrolytes not only exhibited a high mechanical strength but also possessed a self-recovering ability, induced by the crosslinking interaction via hydrogen bonds [400]. Some IL-based solid-state electrolytes composed of new polymeric ILs were also explored for ESs [397, 401, 413]. For example, Ayalneh Tiruye et al. [385] studied a binary blend of polymeric IL, that is, poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide ([pDADMA][TFSI]) and [PYR14][TFSI] (4:6 by mass ratio), and used as a solvent. This pyrrolidinium-based IL had a wide ESPW compared with imidazolium-based IL. The AC-based EDLC with this ionogel electrolyte could be operated at a large cell voltage up to 3.5 V, and consequently delivering a high specific capacitance of 32 Wh kg−1. 4.3.4.1.4  Environmentally friendly gel polymer electrolytes

Considering the environmental impact, the use of environmentally friendly materials from renewable sources (e.g., corn starch [402] and chitosan [321, 403]) or biodegradable materials (e.g., poly (epsilon-caprolactone) [404]) as polymer hosts for solid-state electrolytes has also been received increasing attention. However, natural polymers, such as starch films generally suffer from low mechanical properties. To improve the mechanical property of natural polymers, blended materials, such as a blend of chitosan and starch [403], and a blend of chitosan and PEG [321], were developed. 4.3.4.1.5  Structural electrolytes

For application of solid-state ESs in some vibration devices, such as electric vehicles, developing load-bearing solid-state electrolytes (also called structural electrolytes) has attracted attention [405, 406]. For example, Shirshova et al. [405] prepared some IL-epoxy resin composites as structural electrolytes for ESs. The morphology, ionic conductivity and mechanical properties of these kinds of electrolytes were found to be dependent on the weight ratio between the IL and the epoxy resin. The optimized electrolyte, having a composition of 30 wt.% of resin and 70 wt.% of IL can provide a room temperature ionic conductivity of 0.8 mS cm−1 and a Young's modulus of 0.2 GPa. 4.3.4.2  Inorganic solid-state electrolytes Some studies have been devoted to the development of inorganic solid-state electrolytes for ESs [313, 314, 407], although they receive less attention compared to the polymer-based solid-state electrolytes. In general, this kind of inorganic solid-state electrolyte is not bendable with almost nil flexibility. However, they are mechanically robust and thermally stable. Francisco et al. [313] reported the use of a Li2S-P2S5 glass-ceramic electrolyte as both the ion conductor and separator for all-solid-state ESs. This electrolyte was found to have a rela-



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4.  Electrolyte materials for supercapacitors

tively high Li-ion conductivity. CNTs were also incorporated into the electrode to improve the interfacial contact between the solid-state electrolyte and the electrode material. As a result, the assembled ES exhibited a specific capacitance of nearly 10 F g−1. Ulihin et al. [314] presented a composite LiClO4-Al2O3 solid electrolyte (4:6 ratio of LiClO4-Al2O3) for both symmetric and asymmetric ESs. The active electrode materials were based on the mixtures of manganese oxides and lithium-manganese-nickel spinel. It should be noted that the fabricated ESs could be operated at high temperatures in a range of 150-200°C. However, the specific capacitance was low (29 F g−1 at 0.05 A g−1 under 150°C). GO have also been reported as a solid electrolyte for ESs [407, 408]. Interestingly, Zhang et al. [419] studied an all-solid-state sandwiched RGO//GO//RGO supercapacitor with a layering structure, which exhibited a fairly high specific capacitance up to ~0.86 mF cm−2.

4.3.5  Redox-active electrolytes A new strategy has been explored to increase the capacitance of ES by inducing the pseudocapacitive contribution from the redox-active electrolytes. The Faradaic reactions occur in the electrolyte can contribute extra capacitance to the ESs [11, 409]. In this case, the pseudocapacitance is not only emerge from the pseudocapacitive electrode materials but can also be contributed from the electrolyte (i.e., from the reduction/oxidation of the redox mediator in the electrolyte). 4.3.5.1  Redox-active aqueous electrolytes 4.3.5.1.1  Redox-active aqueous electrolytes for carbon-based ESs

A simple and typical example is an iodide/iodine redox pair, which is used for the redoxactive aqueous electrolyte for carbon-based ESs by Lota et al. [11]. The maximum capacitance of the carbon positive electrode in a 1.0 M KI electrolyte could exhibit an extremely high value of 1840 F g−1, due to the pseudocapacitive capacitance contribution from the electrolyte based on the following reactions [11]: 3I −1 ↔ I 3−1 + 2e − (4.10)

2I −1 ↔ I 2 + 2e − (4.11) 2I −31 ↔ 3I 2 + 2e − (4.12)

I 2 + 6H 2 O ↔ 2IO −31 + 12H + + 10e − (4.13) However, such a high capacitance value was observed only at the positive electrode within a very narrow potential range (Fig. 4.25). Two-electrode cell using a KI electrolyte showed a specific capacitance of 125 F g−1 at 50 A g−1. In these reactions [reactions (4.10-4.13)], the type of the alkali metal counter-ion of the iodides was found to play a significant role in the capacitive behavior of the electrode [410]. The capacitance of the positive electrode was decreased with a decreasing van der Waals radius of alkali metal, following the order: RbI





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

275

FIGURE 4.25  Voltammetry curve at 5 mV s−1 scan rate for AC-carbon electrode in 1 mol L−1 KI solution. Potentials of both electrodes are measured separately versus a saturated calomel electrode. Source: Reprinted from Ref. [11], with permission from Elsevier.

(2272 F g−1) > KI (1078 F g−1) > NaI (492 F g−1) > LiI (300 F g−1) (all with 1.0 M concentration) [410]. As the total capacitance of an ES is largely dominated by the electrode with low capacitance, the low capacitance of the negative electrode in the iodide-based electrolyte sets the limitation. In order to increase the total capacitance, Frackowiak et al. [411] used a 1 M KI aqueous electrolyte for the positive electrode and 1 M VOSO4 for the negative electrode. The Faradaic reactions [Eqs. (4.14-4.17)] can occur at the negative electrode in the VOSO4 electrolyte, making a high pseudocapacitance contribution to the negative electrode [411]: VOH 2 + + H + + e − ↔ V 2 + + H 2 O (4.14) 4−

 H 2 V10 O 28  + 54 H + + 30e − ↔ 10 V 2 + + 28 H 2 O (4.15) 4−

 H 2 V10 O 28  + 44 H + + 20e − ↔ 10 VOH 2 + + 18H 2 O (4.16) HV2 O 73 − + 9 H + + 6e − ↔ 2 VO + 5H 2 O (4.17)

To avoid the mixing of electroactive species in the cell, the assembled AC-based ES used a glassy paper and a Nafion 117 membrane to separate two different electrolytes. This EDLC could deliver a high capacitance of 500 F g−1 at 0.5 A g−1 with a cell voltage of 1 V, and also exhibited an energy density of 20 Wh kg−1 with a maximum power density of



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4.  Electrolyte materials for supercapacitors

2 kW kg−1. Heteropoly acids, such as phosphotungstic acid (PWA) and silicotungsticacid (SiWA), were explored as the redox mediators for redox-active electrolytes. These acids could offer high proton conductivities and also multiple rapid electron transfer redox processes [412-414]. Suarez-Guevara et al. [412] found that the use of phosphotungstic acid (H3PW12O40, PW12) as the electrolyte for AC-based ESs could not only provide pseudocapacitance from the electrolyte but also increase the operative cell voltage to 1.6 V due to high overpotential of PW12 toward H2 evolution. Redox-active electrolytes based on the redox pair of metal ions were also investigated for ESs [415, 416]. For example, Mai et al. [415] used Cu2+ reduction by the surface oxygen group on the carbon electrode to enhance the specific capacitance. Electrochemical studies reveal that the electrode

TABLE 4.8  Some typical redox couples related to the redox active electrolyte-based ESs. Capacitance, (F g−1)

Redox mediator Redox reactions

Supporting electrolyte

HQ

1M H2SO4

72 at 2.65 mA 220 at 2.65 cm−2 mA cm−2

IC

1M H2SO4

17 at 41 mA g−1

50 at 41 mA 70% re- [420] g−1 tention after 10,000 cycles

MB

1M H2SO4

5

23



Without re- With redox Cycling Referdox mediator mediator stability ences [417]

88% re- [419] tention after 6,000 cycles



4.3  Electrolyte materials and compositions for electrochemical supercapacitors

277

TABLE 4.8  Some typical redox couples related to the redox active electrolyte-based ESs. (Cont.) Capacitance, (F g−1)

Redox mediator Redox reactions

Supporting electrolyte

SPANI

4M H2SO4

47.5 at 5.22 A g−1

57 at 5.22 A 99% re- [424] g−1 tention after 1,000 cycles

PPD

2M KOH

144.037 at 1 A g−1

605.225 at 1 94.53% [421] A g−1 retention after 4,000 cycles

MPD

2M KOH

36.43 at 0.5 A g−1

78.01 at 0.5 90.68% [422] A g−1 retention after 10,000 cycles

Without re- With redox Cycling Referdox mediator mediator stability ences

Adapted with permission from Ref. [20].

capacitance could be increased by ~10 times in a 0.09 M CuCl2 + 1 M HNO3 electrolyte than in the conventional 1 M H2SO4 electrolyte, due to the pseudocapacitive contribution from the following reaction [415]: (4.18) It should be noted that when using such electrolytes containing metal ions, the ES selfdischarge may be an issue, as mentioned in Section 4.3.1.1. In addition, there have been considerable studies focused on the organic redox mediators as pseudocapacitive sources, including hydroquinone (HQ) [417, 418] methylene blue (MB) [419] indigo carmine (IC) [420], p-phenylenediamine (PPD) [421] m-phenylenediamine (MPD) [422] lignosulfonates [423], sulfonated polyaniline (SPANI) [424] as well as humic acids [425]. Table 4.8 shows 

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4.  Electrolyte materials for supercapacitors

some typical redox-active aqueous electrolytes containing organic redox-active molecules, the corresponding redox processes, and their corresponding ES performance. In general, the addition of these organic redox additives can greatly increase the specific capacitance of the carbon-based ESs by ~2-4 times. It should be noted that ESs using a HQ-based redox-active electrolyte showed a much faster self-discharge process [418]. Chen et al. [418] attributed this faster self-discharge to the migration of a redox-active electrolyte between two electrodes. To inhibit the migration of a redox-active electrolyte and thus to decrease the self-discharge, two strategies were tested, one was to use anion-exchange membrane (e.g., Nafions) as a separator, and the other was to choose a redox-active electrolyte that could be reversibly converted into insoluble species during the charging-discharging process. 4.3.5.1.2  Redox-active aqueous electrolytes for pseudocapacitive electrodes

Redox-active electrolytes have also been used for pseudocapacitors, asymmetric ESs and hybrid ESs. The pseudocapacitive electrode materials were RuO2 [413, 414], MnO2 [93, 426], Co(OH)2 [427], Bi2O3 [428], ECPs [429], etc., A wide variety of redox-active electrolytes, such as KI [428], heteropoly acids [414], HQ [429], PPD [93] and K3Fe(CN)6 [426, 427] were used to make the redox-active electrolytes. Similar to carbon-based ESs, the addition of a redox mediator to the electrolyte could significantly increase the specific capacitance and thereby the energy and power densities of the pseudocapacitive ESs. Regarding the cycling stability, several studies found that the redox-active electrolytes could lead to similar or poorer cycling stabilities especially during the initial cycles [429, 430] when compared to the conventional aqueous electrolytes without redox mediators. It is interesting to note that besides the increase in specific capacitance due to the pseudocapacitance of a redox-active electrolyte, the presence of the redox mediators could also fasten the relatively slow process of Faradic reactions of pseudocapacitive electrode materials by providing the electron buffer source [426, 427]. 4.3.5.2  Redox-active nonaqueous electrolytes To achieve a higher cell voltage and thereby a higher energy density, a number of nonaqueous electrolytes including organic and IL-based electrolytes [431-434] have been studied. In this direction, Ionica-Bousquet et al. [431] found that the introduction of a redoxactive polyfluorododecaborate cluster ions (i.e., [B12FxH12−x]2−) into an organic PC-DMC mixture solvent could effectively offer an additional pseudocapacitance to the total capacitance of the carbon-based ESs. Compared to the TEABF4/PC-DMC organic electrolyte, tetraethylammonium undecafluorododecaborate, the (Et4N)2B12F11H/PC-DMC electrolyte was also able to provide overcharge protection of ESs due to the presence of redox-active anions. Regarding IL-based electrolytes, Tooming et al. [433] demonstrated that the addition of 5 wt.% 1-ethyl-3-methylimidazolium iodide ([EMIM][I]) into [EMIM][BF4] IL could result in nearly 50% increase in the specific capacitance of ES, when compared to a bare [EMIM][BF4] IL electrolyte. Greatly increased specific energy density of 36.7 Wh kg−1 was achieved for the ES using a 5 wt.% [EMIM][I] + [EMIM][BF4] electrolyte. Taniki et al. [432] reported that N-ethyl-N-methylpyrrolidinium fluorohydrogenate (EMPyr(FH)2.3F) IL electrolyte notably contributed extra specific capacitance to ES through the redox reaction of electrolyte.





4.3  Electrolyte materials and compositions for electrochemical supercapacitors

279

FIGURE 4.26  (A) Ragone plots of EDLCs with PVA-H2SO4, PVA-H2SO4-KI, PVA-H2SO4-VOSO4, and PVA-H2SO4KI-VOSO4 GPEs. (B) Cyclic performance of EDLC with PVA-H2SO4-KI-VOSO4 GPE at a current density of 0.5 A g−1; the inset shows the GCD curves of the EDLC at the 1st and 3000th cycles. Source: Reproduced from Ref. [316] with permission of The Royal Society of Chemistry.

4.3.5.3  Redox-active solid electrolytes The strategy of using a redox-mediator has also been studied in solid- or quasi-solid- state electrolyte-based ESs, and some enhanced performances were observed. In this regard, typically investigated redox additives in the solid-state electrolytes were iodides (e.g., NaI [435, 436] and KI [316]), K3Fe(CN)6 [437], Na2MoO4 [438] organic redox mediators (e.g., hydroquinone [439], PPD [440], p-benzenediol [441] and MB [442]), mixture of redox additives (e.g., KI-VOSO4 [316]), and so on. Actually, these redox mediators are almost the same as those used in the liquid electrolytes. Regarding the polymer hosts, the majority of the solid-state electrolytes studied were focused on the GPEs with polymer hosts, such as PVA [316, 438442], PEO [436, 437, 443], and Nafions [435, 444]. As these redox mediators are normally dissolved in the liquid phase of GPE, their effect and the working principle are generally quite similar to those in aqueous or organic electrolytes. For example, these redox-active GPEs showed a much higher ES specific capacitance than those GPEs without redox mediators. Fan et al. [316] reported that an AC-based ES using KI-VOSO4-containing PVA/H2SO4 GPE could exhibit a high specific capacitance of 1232.8 F g−1 and an energy density of 25.4 Wh kg−1, which were approximately eight folds higher than that of the ES without KI-VOSO4. The GPE using mixtures of redox mediators was also found to have higher energy densities than the GPE using a single redox additive (Fig. 4.26). To increase the operation cell voltage, Zhou et al. [444] used the PEO/LiClO4-ACN organogel as an electrolyte for the ESs and investigated two types of redox mediators-NaI/I2 and K3Fe(CN)6/K4Fe(CN)6. ESs containing NaI/I2 and K3Fe(CN)6/K4Fe(CN)6 could deliver the specific energy densities of 49.1 Wh kg−1 at 1.6 kW kg−1 and 33.6 Wh kg−1 at 1.3 kW kg−1, respectively, indicating that the enhancement effect of the NaI/I2 mediator was more pronounced than the K3Fe(CN)6/K4Fe(CN)6 mediator. Further, the introduction of a redox mediator into the GPE can also significantly increase the ionic conductivity, especially for the GPE with a conducting salt dissolved in an organic solvent, such as acetonitrile [435437, 442].



280

4.  Electrolyte materials for supercapacitors

4.4  Electrolyte compatibility with inactive components of ESs Beyond the consideration of the interaction between the electrolyte and the active electrode materials in ESs, it is also necessary to consider the compatibility or the possible interaction between the electrolyte and the inactive components such as current collectors, binders and separators in the development of ESs. Depending on the type and nature of electrolyte, inactive components may have a profound influence on both the performance and durability of the ESs.

4.4.1  Compatibility with current collectors In general, chemical and electrochemical stabilities of the current collector in a certain electrolyte can strongly affect the lifetime and performance (e.g., operating cell voltage) of ESs. In addition, the morphology or structure of the current collector plays an important role in the degree of utilization of the active electrode materials. When a strong acid electrolyte (e.g., 1 M H2SO4) is used in ESs, due to its high corrosive nature, some corrosion-resistant metallic materials such as Au are conventionally used. To reduce the cost of the current collectors, other types of materials, such as indium tin oxide (ITO) [457], coaxial RuO2-ITO nanopillars [445], carbonbased materials [446] and ECPs [447] have been investigated for ES current collectors while employing a strong acid electrolyte. Free-standing electrodes (e.g., carbon based composite papers) have also been developed for ESs, which have the advantage of avoiding the use of additional current collectors [448]. These newly developed current collectors may offer additional benefits. For example, due to its high transparency, the ITO-based current collector is beneficial to the development of transparent ESs [442]. The use of carbon-based or ECP-based current collectors is particularly favorable for use in the flexible ESs [447]. For alkaline electrolyte-based ESs, Ni current collectors are commonly used due to their relatively low cost, and good chemical and electrochemical stability in alkaline electrolytes [449]. To increase the surface area and the utilization of the active electrode materials, the Ni current collectors are often in the form of Ni foam [449]. Furthermore, the use of Ni foam as a current collector may have an additional pseudocapacitive contribution to the total capacitance due to the presence of Ni hydroxide/oxide on the Ni foam, especially when the amount of electrode material is small [450]. Other metallic materials, such as stainless steel [451], and Inconel 600 (a Ni-based alloy) [452] were also investigated as current collectors for alkaline electrolyte-based ESs. Carbonaceous materials, due to their lightweight, good conductivity, high flexibility, and good mechanical strength, have attracted growing attention in recent years. These carbonaceous materials include carbon fabrics [453], CNTs [454], carbon clothes [455], carbon fiber papers [456], ultrathin-graphite foam [457], and so on. Due to their much less corrosiveness, neutral aqueous electrolytes are considered to be the best of choices for ES current collectors. Consequently, a wide variety of materials have been used as current collectors. These materials include Ni [458], Ti [459], stainless steel [106, 107], titanium oxynitride [460], ITO [461] modified TiO2 [462], CNT [463], and so on. Wang et al. [122] demonstrated that the current-collection property of the Ni foam was better than that of a Ti mesh in a 1 M LiAc-1 M MgSO4 mixed electrolyte. In the case of organic electrolyte-based ESs, the majority of the studies use Al as the current collector [147, 464]. A previous study found that the aging of organic electrolyte-based ES also involved the degradation of Al current collectors [464]. Vali et al. [147] reported that the type of conducting salt could greatly affect the electrochemical stability of Al current collectors in an organic solvent. The oxidation of an Al current 



4.4  Electrolyte compatibility with inactive components of ESs

281

collector in a NaFSI-based electrolyte was observed. This condition could lead to much lower electrochemical stability when compared to both NaPF6− and NaClO4−based electrolytes. This condition was indicated by an associated lower cell voltage of 2.5 V compared to those of 3.4 V and 3.2 V for NaPF6 and NaClO4 electrolytes, respectively. With the development of new electrolytes, such as IL and redox-active electrolytes, their compatibility with the ES current collectors should be given more attention. Regarding the IL electrolytes, Kuhnel et al. [465, 466] investigated the anodic stability of some Al current collectors in [PYR14][FSI], [PYR14][TFSI] and [PYR14][FTFSI] IL electrolytes. It was found that Al could be slowly corroded in [FSI]-based [PYR14][FSI] IL while it was much better passivated in [TFSI]-based [PYR14][TFSI] IL [466]. The same team [465] also found that Al current collectors showed excellent anodic corrosion stability in [PYR14][TFSI] IL. Brandt et al. [301] reported that the possible anodic oxidation of Al current collectors could strongly affect the operation cell voltage of EDLCs when IL-organic electrolyte mixtures (i.e., [Me3S][TFSI]-PC) were used. Regarding the redox-active electrolytes, Meller et al. [467] reported that an Au current collector was not suitable for iodide electrolytes due to its reactivity with iodides while stainless steel was a suitable material showing good long-term stability. When solid- or quasi solid-state electrolytes are used for ESs, the compatibility between the electrolyte and the current collector should be optimized with respect to the ES performance. Additional attractive criteria for advanced current collectors such as flexibility, stretchability and transparency need to be considered. In order to avoid potential issues related to current collectors and to simplify the fabrication process, some studies have also focused on the development of free-standing electrodes, which are in general carbon-based materials, without the use of current collectors [468].

4.4.2 Binders Binders are used for the fabrication of electrode layers composed of active material particles or powders such as AC powders, which hold up the structural integrity of the layer and also provide good adhesion between active particles and the current collector. Currently, the commonly used ES binder materials are fluorinated polymeric ones, such as poly (vinylidene fluoride) (PVDF) and polytetrafluoroethylene (PTFE). It is generally observed that the performance of electrodes and their associated ESs can be strongly affected by the property and content of the binders [48, 108, 469]. For example, due to the hydrophobic nature of PTFE, excessive PTFE in an ES electrode layer would make the electrode layer excessively hydrophobic, causing difficulty for electrolyte penetration, if an aqueous electrolyte is used [108]. Therefore, it is important to optimize the binder content in the electrodes [108]. Paul et al. [470] found that the addition of a small amount of PVP (3%) into a PTFE binder could significantly increase the wettability of the electrode materials in an aqueous electrolyte. Timperman et al. [268] compared the effects of two different binders such as PVDF and carboxy methyl cellulose-styrene butadiene rubber (CMC-SBR), on an AC electrode layer. They found that the wettability of the [DIPEA][TFSI] IL electrolyte was higher on PVDF-containing electrodes than on CMC-SBR electrodes, while similar wettability was found for both electrodes in the [Pyrr] [TFSI] IL electrolyte. This difference in electrolyte wettabilities could lead to a difference in electrochemical performance of the electrodes. Liu et al. [471] found that the use of Nafion as a binder was beneficial for increasing the power density of a RuO2 powder-based pseudocapacitor, probably due to its high proton conducting ability. Cao et al. [469] reported that the type



282

4.  Electrolyte materials for supercapacitors

of the binder could also affect the performance of LIC using the LiPF6/EC-DMC electrolyte. The use of PTFE as a binder in a negative electrode layer was found to have a beneficial effect in achieving higher energy and power performance when compared to a PVDF binder [469]. To reduce the environmental concern, growing interest has been found in developing environmentally friendly (e.g., fluorine-free) binders. Various materials, such as PVP [472] polyacrylic acid (PAA) [473], and natural cellulose [474], were tested as alternative binders for ES electrode layers. In addition, some ECPs, such as PPy [475], PEDOT [476] and PANI [477] were also explored as the binders for electrode materials. These binders are normally electrically conductive, which are different from the conventional binders (e.g., PVDF and PTFE) and are electrically insulating. Moreover, they can contribute additional pseudocapacitance to the electrode capacitance. In addition, some work has been devoted to the development of binder-free electrode materials in the literature [415]. If this approach is feasible, some negative effects related to the use of binders, such as an increase in electrode resistance and a reduction of specific surface area may be avoided.

4.4.3 Separators The separator is located between the two electrodes in an ES cell to prevent the contact and electron transfer between the negative and positive electrodes. There are several critical requirements for the separators such as: (1) having a minimal resistance for ion transfer within the electrolyte while having a strong electronic insulating capability; (2) having both high chemical and electrochemical stabilities in the electrolyte; and (3) having good mechanical strength to provide device durability. Based on these requirements, ES separators are usually made from thin and highly porous films or membranes. Commonly used separator materials are cellulose, polymer membranes and glass fibers. As generally realized, the choice of separator materials depends on the type of electrode, working temperature and ES cell voltage. Although cellulose separators can operate well in organic solvents [464], they may suffer from degradation in H2SO4 electrolyte. Bittner et al. [464] found that the trace amount of water in the organic electrolyte (TEABF4/ACN) could play a role in accelerating the ageing process of EDLCs when a cellulose separator was used. When viscous electrolytes (e.g., IL) were used, the ionic conductivity of the electrolyte in the separator could have a considerable effect on the ES performance through changing the internal resistance (ESR) [478]. Furthermore, the properties of the separators, such as chemical composition, thickness, porosity, pore size distribution, and surface morphology were found to have a noticeable influence on several EDLC performance indicators including polarizability limits, specific capacitance, ESR, characteristic time constant, specific energy, and power densities [478]. In addition to EDLCs, the separator could also affect the performance of pseudocapacitors and hybrid ESs. Liu et al. [479] investigated four different separators for a RuO2-based ES with 0.5 M H2SO4 as an electrolyte. Among these four separators, Nafion 115 showed the highest energy density of 31.2 Wh kg−1 at 1 mA cm−2 while the Celgardt separator gave the lowest value of 23.4 Wh kg−1 at the same current density. Cao et al. [469] used a cellulosebased TF40-30 separator for LIC, and found that this separator could give a better discharge capacity retention of LIC than that with a Celgard PP 3501 separator. This was probably due to the lower ohmic resistance of a cellulose based TF40-30 separator. When redox-active electrolytes were used for ESs, selecting the appropriate separator could have a positive effect on inhibiting the self-discharge processes [418, 424]. A few new separator materials, such as GO films [480] and egg-shell membranes [481] have been explored for ESs. For instance, Shulga 



4.5  Electrolyte performance validation using supercapacitor test cells

283

FIGURE 4.27  Schematic diagrams of (A) a three-electrode cell, (B) a two-electrode cell, (C) a sandwiched flexible electrochemical supercapacitor, (D) a fiber electrochemical supercapacitor, and (E) a microelectrochemical supercapacitor. Source: Reproduced with permission from Ref. [20].

et al. [480] demonstrated that GO films could exhibit proton conductivity after being permeated with a H2SO4 electrolyte, indicating that it might be usable as an ES separator.

4.5  Electrolyte performance validation using supercapacitor test cells Characterization and validation of the behavior and performance of electrolytes are vital in the development and optimization of associated ESs. To evaluate the ES performance of an electrolyte, a three-electrode electrochemical cell (Fig. 4.27A) or a two two-electrode cell (Fig. 4.27B) is commonly used. Some specially designed test cells for evaluating an electrolyte 

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and its associated ES performance have also been fabricated, as shown in Fig. 4.27 (C-E) for the sandwiched flexible ES, fiber ES and micro-ES, respectively. Normally, the three-electrode cell does not require the entire ES assembly. It is suitable for fast screening and characterizing of electrode materials and electrolytes as well as for investigating the fundamental electrochemical processes of the electrolyte and electrode material. The two-electrode cell is closer to the real operating conditions of ESs. Because the specific capacitance of a two-electrode cell is contributed by two capacitive interfaces in series, the capacitance values should be lower than that obtained in a three-electrode cell or half when using a symmetric EDLC. Several commonly used electrochemical characterization methods such as cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) have been widely used to characterize, test and diagnose ES performance in the presence of a targeted electrolyte [482-484]. With the help of these electrochemical techniques, a series of important parameters of ESs, such as capacitance, ESR, energy and power densities, as well as cycle life, can be measured to understand ES performance related to the electrolyte. It should be noted that the testing conditions, such as cell configuration, potential (voltage), scan rate in CV, current density in GCD, and voltage range, can affect the measured results (e.g., specific capacitance) of the cell. The specific capacitance achieved at a lower scan rate or current density is commonly higher than that obtained at a higher scan rate or current density. Therefore, to compare the performances of different electrolytes and their associated ESs, the measurement and cell conditions should be controlled under as close to the same conditions as possible. Due to the determining role of electrolyte's ESPWs in the ES performance such as both energy and power densities, it is essential to develop accurate quantification methods to measure the ESPW value of the electrolyte being tested. The conventional method of determining the ESPW value is based on the measurement of the electrolyte's cathodic reduction and anodic oxidation limits by linear sweep voltammetry on the smooth working electrode, such as Pt and GC [24, 198, 485]. As shown in Fig. 4.28A, the oxidative and reductive decomposition of an electrolyte can lead to an abrupt increase of the current densities [485]. The applied negative and positive potentials at which the current densities reach critical values are generally defined as the anodic and cathodic limits of the ESPW. Since, the definition of these limits seems to be somewhat arbitrary, this may bring about some uncertainty for the comparison of ESPW values among different electrolytes. Olson et al. [486] demonstrated a method for defining the ESPW limits that seemed to depend less on experimental parameters, allowing more meaningful comparison of data from different investigators. This method involved linear fits in two approximately linear ranges of the current-potential curve. Actually, the electrodes exposed to the electrolyte in ESs are often highly porous, which can give a much larger capacitive current response than that of non-porous electrodes (Fig. 4.28B). Therefore, several studies were devoted to the development of an ESPW determination method for porous electrodes used in ESs [485, 487]. For example, Xu et al. reported an improved ESPW quantification method for porous electrodes. In their study, the Faradaic (related to electrolyte decomposition) and capacitive current were isolated. Besides the electrochemical methods, other instrumental techniques, such as FTIR and NMR [166] have also been employed to characterize the properties of electrolytes. In some cases, the cells need to be specially designed to meet the requirements of these experiments [166]. Application of





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FIGURE 4.28  (A) Cyclic voltammetry and conventional determination of stability limits of a popular non-aqueous electrolyte for a double layer capacitor: 1.0 M Et3MeNPF6 in EC/DMC (1:1 by wt) on a nonporous working electrode GC. (B) Cyclic voltammetry of the same electrolyte when a porous composite based on M30 activated carbon (95% with 5% PVDF) is used as the working electrode. Scan rate: 5 mV s−1; Li as reference and Pt as counter electrodes. Successive scans were conducted with an interval of 0.25 V between their cut-off potential limits (only sixth and tenth scans are shown). Source: Reprinted from Ref. [485], with permission from Elsevier.

these techniques for investigating the electrolyte has already been discussed in Section 4.3 for specific electrolytes.

4.6  Challenges in the development of ES electrolytes Enlarging the energy/power densities and enhancing the durability (cycle life) of ESs including EDLCs, pseudocapacitors, and hybrid capacitors are always a major challenge in ES development. Electrolytes have been identified as one of the determining components in the ES performance. Their values of the ESPW and ionic conductivity, chemical and thermal stabilities, as well as the operating temperature regime, have significant effects on both ES performance and practical applications. Although great progress has been made in the field of ES electrolytes, there are still a number of major challenges, which hinder to advance this technology and its commercial applications: 1. Low ESPW limits and its consequence on energy density The ES's operating cell voltage is largely determined by the ESPW of the electrolyte, which can affect both the energy and power densities [Eqs. (4.4) and (4.5)]. As the energy density is proportional to the square of the cell voltage, electrolytes with higher ESPW values allow increased cell voltage of the ESs, which can significantly improve the energy density. However, this higher cell voltage often results in the deterioration of other properties of electrolytes. For



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example, although organic electrolyte-based ESs exhibit much higher cell voltages compared with the aqueous electrolyte-based ones, the ionic conductivities of the organic electrolytes are about one magnitude lower than the aqueous electrolytes. Although the use of ILs can further increase the cell voltage of ESs up to 4 V, their ionic conductivities are much lower than those of the organic electrolytes. Similarly, electrolytes with wider ESPWs generally have a lower viscosity. In addition, ESs using organic or IL electrolytes commonly show a smaller specific capacitance value compared to those with aqueous electrolytes. It is therefore, truly challenging to develop ESs with improved energy densities without sacrificing other advantages such as high power densities and durability. 2. High ESR values and their effect on power density Organic and IL-based electrolytes are favorable for increasing the energy density of ESs. However, due to their lower ionic conductivities, the ESR of organic electrolyte-based ESs can be a magnitude higher than the aqueous electrolyte-based ones [161]. The ESR values of IL electrolyte-based ESs are even higher because of the low ionic conductivities and high viscosity. Consequently, the application of electrolytes with higher ESPWs for ESs often leads to increased ESR values and a decreased rate and power performance unless the loss of power density due to the increased ESR can be buffered by an increase of the cell operating voltage. 3. Impurities in electrolyte and their effect on ESPW and self-discharge Impurities, especially trace amounts of water, can markedly decrease the ESPW values of organic or IL electrolytes. Regarding industrial production, organic electrolytes are more expensive due to its water-free purification, which is needed to provide the higher cell voltage without degrading the electrolyte. Water content should be kept below 3-5 ppm in organic electrolytes. The self-discharge rate and its mechanism are also very sensitive to impurities and residual gas in the electrolytes. Studies have identified that the self-discharge process can be accelerated by certain amounts of metal ion contamination, water, oxygen etc., in the electrolytes [76, 77, 181]. 4. Compatibility issues between the electrolyte and electrode materials Compatibility and a close matching between the electrolyte and electrode materials is very important in achieving high-performance ESs. Compared to the extensive research on maximizing the specific capacitance through the matching between the electrolyte ion size and the porous structure of carbon materials, relatively few studies have focused on the rate and power performance of ESs resulting from optimizing the matching between electrolytes and electrode materials. Since high power density is one of the key advantages of ESs, the matching to improve specific capacitance should not sacrifice power performance. In addition, some different results have also been found in the relationship between pore size and the surface-area-normalized capacitance [5, 164, 488, 489]. For example, Chmiola et al. [5] and Largeot et al. [488] both reported that the specific capacitance can increase significantly when the average pore size of the carbon electrode was close to the electrolyte ion size for IL and TEABF4/ACN organic electrolytes (desolvated ion size in the case of organic solvents). However, Centeno et al. [489] studied 28 porous carbons in the TEABF4/ ACN electrolyte and found that the specific capacitance was relatively constant between 0.7 and 15 nm. Furthermore, with the development of various new electrolytes such as electrolytes containing multivalent ions, IL mixtures, IL-organic solvent mixtures and solid-state





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electrolytes, the optimization of matching between the electrode materials and these electrolytes does not seem to be receiving sufficient emphasis. 5. Unfavorable temperature range for ES operation Commonly used organic solvents (e.g., ACN and PC) for ESs have a relatively narrow operating temperature range due to high volatility at high temperatures or low viscosity at low temperatures. Although some new electrolytes, such as ADN- and IL-based ones with higher temperature limits have been explored, they have other issues. For example, the salt solubility in ADN solvent is low and most IL-based ESs suffer from poor performance at low temperatures. 6. High cost of certain electrolytes Decreasing the cost of ESs is critical for their commercialization. The cost of the state of the art ESs is $10-20 W h−1, which is significantly higher than LIBs with the cost of only $1-2 W h−1 [490]. Although aqueous electrolytes are inexpensive, they generally give a very limited energy density especially in the case of symmetrical ESs due to the low ESPW values. ESs using organic or IL electrolytes have higher operating cell voltages and thus exhibit higher energy and power densities. However, the use of organic electrolytes, especially ILs, significantly increases the cost of ESs. For example, the price of [EMIM][BF4] IL is ~ $1328 kg−1, which is much higher than the commercial TEABF4/ACN organic electrolyte (~$345 kg−1 for TEABF4). 7. Lack of a fundamental understanding on electrolyte process in ES performance For new electrolyte material selection and design, as well as optimization of the interaction between the electrode material and the electrolyte, considerable theoretical and in situ experimental advancements have been made. However, in the development of new electrolytes (e.g., ILs and solid-state electrolytes) and new electrode materials, there still exists a challenge for the in-depth fundamental understanding of the charging-discharging mechanisms for these newly developed systems. For example, some controversies exist on the effects of temperature [234] and organic solvent addition [233, 312] on the EDL structure and the capacitance at the IL/electrode interface. DFT studies predicted that the capacitance of nanoporous electrodes in an IL could oscillate with increasing pore size [491], however, direct experimental work is required to confirm this prediction. Besides, there is a lack of a complete understanding of the dynamics of electrolyte ions in more complex electrode structures (e.g., hierarchical porous nanostructures) during the charging-discharging process. Furthermore, compared to the fundamental studies on the EDL charge storage mechanisms, fundamental understanding, such as theoretical modeling for pseudocapacitors and hybrid ESs with certain electrolytes is rather limited [492]. Moreover, the dependence of the electrolyte parameters (e.g., ion type and size) on the pseudocapacitive behavior of electrode materials is not very clear. 8. Suitable standardized characterization methods With the development of many new electrolytes (e.g., solid-state electrolytes), there is an urgent need to establish standardized methods suitable to characterize the performance of electrolytes and their associated ESs. Currently, it is difficult to compare the experimental data from different literatures in the performance scaling, as these are usually obtained under different conditions and hence it



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is not easy to identify the ideal candidate electrolytes from different literature. Further, the ES gravimetric capacitance, energy, and power densities are generally reported based on the mass of active electrode materials. The electrolyte, however, also contributes to the total mass and cost of ESs, a fact that cannot be neglected.

4.7  Summary and future research directions To advance the research and development of electrolytes for ESs, this chapter sets a platform with a comprehensive overview of the developmental trends concerning electrolytes for ESs. Various types of electrolytes explored and reported in the literature are summarized and classified into aqueous, organic, ILs, solid-state or quasi-solid-state, and redox-active electrolytes. The effects of the electrolyte properties, such as ESPW, ionic conductivity, viscosity, and thermal stability on the ES performance, such as the specific capacitance, specific energy and power densities, ESR, rate performance, cycling stability, and temperature performance are critically analyzed. Principles and methods to design and optimize electrolytes for ESs are presented. Especially, the importance of interplay between the electrolytes and electrode materials, including carbon-based and pseudocapacitive electrode materials, is discussed. Additionally, possible interactions between the electrolytes and inactive components, such as current collectors, binders, and separators, are presented. In spite of considerable achievements in this field, several major challenges still exist, such as low energy density, un-optimized matching between newly-developed electrolytes, and electrode materials, the lack of standard methods to evaluate the performance of electrolytes in ESs, a high cost of some electrolytes, and an insufficient fundamental understanding of charge storage mechanisms for some newly developed electrolytes in more promising and complex electrodes. To overcome these challenges, several future research directions are suggested as follows [20]: 1. Improving electrolyte's ESPW values to increase the ES's energy density. Regarding the organic electrolytes, the ESPWs are dependent on the cation and anion of the conducting salt, and the organic solvent including a single solvent or a solvent mixture. Therefore, the improvement of ESPWs could be achieved by exploring new organic solvents, new conducting salt or by optimizing/modifying the commonly used organic electrolytes. For example, electrolytes containing organic solvents such as sulfone-based and AND solvents have shown promising results with higher ESPWs when compared to commercial ACN and PC solvent-based electrolytes. It should be noted that in most cases, an increase in electrolyte ESPWs may lead to the deterioration of other properties such as ionic conductivity and viscosity of electrolytes. To address this issue, all the composition of the electrolyte should be considered. For instance, the combination of a salt with a higher ionic conductivity or solubility and organic solvent(s) with a higher ESPW may provide a promising solution. Besides, the use of ILs as electrolytes for ESs is beneficial for increasing the cell voltage and thus the energy density. The studies on ILs for ESs are still at the infancy, and there are many ILs that have not yet been explored. By rationally designing the cation and anion or using a electrolyte mixture, ILs with higher ESPWs and acceptable ionic conductivities and viscosity may be developed. On the other





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hand, it is difficult for an electrolyte to meet all the requirements (high ESPW, high ionic conductivity, high thermal stability, low viscosity, low cost and environmental friendliness) at the same time. Some tradeoffs may be reasonable in solving the practical problems. The trade-offs mainly depend on the electrolyte and the electrode material as well as the requirements of the ES application. 2. Enhancing the charge capacity by utilizing the pseudocapacitive contribution. Both the electrode materials (pseudocapacitive electrodes) and electrolytes (redoxactive electrolytes) can contribute to the pseudocapacitance. In this case, even aqueous electrolyte based ESs may achieve a significantly higher energy density especially for the hybrid or asymmetric ESs, such as LIC. Aqueous electrolyte-based hybrid ESs are highly promising candidates for high-energy-density ESs, due to their attractive advantages, such as very low cost, high safety, and simplified fabrication procedures, without complicated and strict drying and purification processes. 3. Increasing the purity of the electrolyte. Since residual impurities in electrolytes can affect the ESPW and accelerate the selfdischarge, it is highly desirable to develop a suitable purification process to decrease the amount of impurities in electrolytes. Based on the extensive experience that has already been gained in the field of Li-ion batteries, the purification procedures for organic solvents can be adapted from the Li-ion battery technology. Regarding IL-based electrolytes, special attention is suggested to be paid to ILs with hydrophobic anions. This may allow for a low amount of water in the IL and hence beneficial for the long-term stability of the electrolyte. 4. Decreasing ESR values to further increase the ES's power density. Developing electrolytes with high ionic conductivities and low viscosity is beneficial for decreasing the ESR values. Regarding organic electrolytes, this may be achieved by exploring innovative conducting salts such as SBPBF4, developing mixtures of organic solvents to decrease the viscosity, optimizing the salt-solvent systems, and so on. In the case of IL electrolytes, the design/selection of proper IL's cations and anions can contribute to increasing ionic conductivity and reducing viscosity of electrolytes. For instance, the dicyanamide anion (DCA)-based IL was found to offer a smaller ESR compared to both the tetrafluoroborate (BF4) and bis (trifluoromethylsulfonyl) amide (NTF2) based IL [247]. The modification (e.g., introduction of the ether bond) of commonly used IL's cations and anions could also decrease the ESR of ESs [262]. 5. Optimizing the matching between the electrolyte and electrode materials to improve the overall performance. The matching should not only consider the specific capacitance (energy density) but also take into account the power density of ESs. Regarding the specific capacitance, considering that different results have been reported by relating the pore size and surfacearea-normalized capacitance [5, 488, 489], more fundamental experiments are required to fully understand this relationship. This would provide the basis for guidance to optimize the matching between the pore size of carbonaceous materials and electrolyte ion size. To address this issue, it is necessary to use more reliable methods to characterize the true surface area of the electrode materials [489]. Furthermore, to increase the ES's specific capacitance (energy density) without sacrificing the power density, further research is necessary for the development of electrodes with a special structure such as a hierarchical



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porous structure which could provide a high interfacial area for high capacitance while maintaining efficient ion transport for a high power performance [493, 494]. 6. Increasing the working temperature range for ES operation. Depending on the type of electrolyte, different strategies may be explored to widen the operating temperature range of ESs. Regarding an aqueous electrolyte, some additives (e.g., ethylene glycol) may be explored to decrease the lower temperature limit [495]. In terms of organic electrolytes, the development of an innovative organic solvent mixture is possible to achieve the electrolyte with a larger working temperature range compared to the single solvent-based electrolytes. In the case of ILs, the exploration of eutectic mixture of ILs together with the development of electrode materials may effectively extend the temperature range, especially on the lower temperature limit of IL-based ESs [271]. 7. Further fundamental understanding through both theoretical and experimental investigations. For the design and optimization of new electrolytes, down-selection of electrolytes in terms of improving ESPWs, ionic conductivity, thermal stability, and decreasing viscosity, a better fundamental understanding by both experimental and theoretical work is certainly required. It is necessary to understand the mechanisms of electrolyte ion dynamics, solvation/desolvation (if there is solvent), and the charge storage mechanism in a more realistic electrode structure during the charging-discharging process. This can be achieved using both theoretical modeling (molecular/electronic level) and experimental approaches especially involving in-situ characterization methods, such as NMR and FTIR spectroelectrochemistry. Especially, special attention needs to be bestowed onto the recently developed electrolytes containing multivalent ions, IL mixtures, IL-organic solvent mixtures, and solid-state electrolytes. Furthermore, with the development of pseudocapacitive electrode-electrolyte systems and hybrid ESs, fundamental work on the interaction between the electrolyte and pseudocapacitive or hybrid electrode materials are essential. Such a fundamental understanding will not only provide guidance for choosing or developing innovative electrolytes but also facilitate the development of electrode materials to match specific types of electrolytes. To mitigate the electrolyte degradation, it is also necessary to understand the degradation mechanisms and failure modes. This can be achieved by both experimental and theoretical modeling approaches. For example, a variety of instrumental analysis methods, especially in situ or on-line characterization methods (e.g., on-line DEMS and Raman microscopy), can be used to characterize the electrolytes during lifetime tests. 8. Development of standard methods to evaluate the performance of electrolytes. With widening investigations in this area, it is important to develop suitable and standardized methods to evaluate and compare the performance of different electrolytes and their associated ESs. It is necessary to know the mass and volume of the electrolyte used when characterizing the performance of ESs. Besides, to evaluate the flexible ESs using solid-state electrolytes, a standard specifications and methods are required to evaluate the mechanical properties and the performance under applied stress. Further, to accelerate the development of new electrolytes, higher output and fast screening methods for evaluating the performance of electrolytes are necessary. When a three-electrode cell is used to evaluate the electrolyte performance, it is suggestive to use the electrode (substrates) that is similar to that used in real ESs, to obtain more realistic performance metrics of the electrolytes for ESs.



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C H A P T E R

5

Characterization methods for supercapacitors O U T L I N E 5.1 Introduction 5.2 Evaluation of supercapacitors performance 5.2.1 Overview of test procedures 5.2.2 Electrochemical apparatus 5.2.3 Electrochemical cell 5.2.4 Electrochemical interface: supercapacitors 5.3 Transient techniques 5.3.1 Cyclic voltammetry 5.3.2 Galvanostatic cycling

316 318 319 320 322 323 323 327

5.3.2.1 Constant current charge or discharge 329 5.3.2.2 Constant potential charge or discharge 329 5.3.2.3 Constant power charge or discharge 329 5.3.2.4 Leakage current and self-discharge behavior 330

5.3.3 Stationary technique

330

5.3.3.1 Electrochemical impedance spectroscopy 330 5.3.3.2 Supercapacitor impedance 336

5.4 Key scaling parameters 5.4.1 Capacitance 5.4.2 Evaluation of CT

5.4.3 Evaluation of CS 5.4.4 Major influencing factors

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342 342 342

345 346

5.5 Equivalent series resistance 5.5.1 Evaluation of RESR 5.5.2 Key influencing factors

349 350 351

5.6 Operating voltage, Vo 5.6.1 Evaluation of Vo 5.6.2 Major factors influencing Vo

352 352 353

5.7 Time constants

354

5.8 Power and energy densities 5.8.1 Power density 5.8.2 Energy density

355 355 356

5.9 Leakage and maximum peak currents

359

5.10 Cycle life and capacitance retention rate

359

5.11 Inconsistencies in evaluation of SCs 359 5.11.1 Causes for the inconsistencies 359 5.11.2 Device performance versus material property 360 5.11.3 Rate dependence 361 5.12 Other test procedures

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5.13 Summary

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Materials for Supercapacitor Applications. http://dx.doi.org/10.1016/B978-0-12-819858-2.00005-6 Copyright © 2020 Elsevier Inc. All rights reserved.

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5.1 Introduction Performance of a supercapacitor (SC) can be evaluated using a set of key parameters such as cell capacitance, operating voltage, equivalent series resistance, power density, energy density, and time constant. In order to measure these characteristic parameters with high accuracy and precision, a variety of methods have been proposed and employed both in academia and industry. In this chapter, we present and discuss on the several attempts which were made to identify, address and eliminate various uncertainties stem from the commonly used instruments, key performance factors and parameters (metrics), calculation methods, and the major influencing factors associated with the performance evaluation of supercapacitors from time to time and in the present day practice.

5.2  Evaluation of supercapacitors performance To evaluate the performance of SCs, three essential parameters, cell (total) capacitance CT, operating voltage Vo, and equivalent series resistance RES, are often used to assess their energy and power performance, and considered acceptable for commercial systems for which the materials present in the cell components, fabrication method, and cell design are all fixed. However, the research arena in which there is a continuous pursuit in search for novel materials, more advanced manufacturing processes, and new cell designs, encompassment of additional factors become indispensible. In fact, additional key factors are to be considered to evolve the absolute and perfect depiction and description of the supercapacitors. The capsulization of various inter-relationships between different performance scaling parameters, major influencing factors, and the general test methods employed in the evaluation of SCs are presented in Fig. 5.1 [1].

FIGURE 5.1  The main influencing factors, analytical techniques and key scaling parameters involved in the performance evaluation of SCs [1].





5.2  Evaluation of supercapacitors performance

317

As can be seen, the specific capacitance of super capacitor depends on the active material, the characteristics of the electrode such as thickness, density and the mass of the active material loading. The value of specific capacitance is derived from CV curve and Nyquist plot. The resistance depends on current collector, additives used and the nature of the active material and it is generally derived from IR drop or AC measurements. The operating voltage depends mainly on the energy and power density and the usable extent of these parameters decides the cell performance. Due to the complex and multifarious relationships amidst them, the experimental results are inconsistent even for a typical cell tested in different laboratories, employing different testing methods, and also between the academic institutes and industries. In order to understand and to circumvent the causes for such inconsistencies, certain key issues—for instance, material property versus device performance and the rate dependency of supercapacitor performance etc., have to be carefully identified, analyzed, and addressed [1]; and hence the standardization becomes inevitable. Several attempts were made to standardize the evaluation methods for SC devices. Some national and international bodies including DOD (US Department of Defense), DOE (US Department of Energy), IEC (International Electrochemical Commission), and SAE (Society of Automotive Engineers) have worked intensively toward this objective. The chronological order of standards for supercapacitor evaluation is as follows: • • • • •

1986—Capacitors fixed electrolytic double layer carbon, general specifications—DOD 1994—Electric vehicle capacitor test procedures manual—DOE 2004—Freed in CAR ultra capacitor test manual—DOE 2006—Fixed electric double layer capacitor for use in electronic equipment—IEC 2009—Electric double layer capacitors for use in hybrid vehicles—test methods for electrical characteristics—IEC • 2012—Railway applications—Rolling stock equipment—Capacitors for power electronics—IEC • 2013—Capacitive energy storage devices for automotive applications—SAE Apparently, such efforts mainly focus on specific applications oriented industry, and still there exist a lack of general understanding and knowledge collected/derived from the most recent cutting edge research and to set a more accurate and effective practice for performance evaluation of SCs. In view of the inevitable need for more reliable test methods for new energy storage systems, considering the associated complexities, a few attempts were made in the past as seen from the literature, to clarify and streamline the existing evaluation methods, with the aim of eliminating or at least alleviating such inconsistencies. In this chapter, we present and discuss on the various testing procedures that are employed for both prototype and commercial electrochemical capacitor devices intended for various industrial and vehicle applications. Testing of these devices is normally carried out using DC test procedures similar to the ones used for testing batteries. Mostly, the materials investigations and small laboratory devices involve the application of cyclic voltammetry and AC impedance test procedures. These approaches, in most cases, utilize small currents and limited voltage ranges and/or AC frequencies and are intended primarily to determine the electrochemical characteristics of the materials and the electrodes used in the capacitors. At first, we present the details of the DC test procedures employed for the testing of electrochemical capacitors followed by the discussions on the testing of carbon/carbon and hybrid



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(asymmetric) devices and the relationship between the AC impedance and DC test methods. We present typical data for the capacitance, resistance, energy density, power capability, and cycle life of various types of devices. Finally, the uncertainties in the interpretation of the test results are discussed, especially in comparing the power capabilities of electrochemical capacitors and high-power lithium batteries.

5.2.1  Overview of test procedures When we examine the energy storage systems, there exist certain similarities and differences in the test procedures employed for evaluating the electrochemical capacitors and high-power batteries. It is a usual practice to perform constant current and constant power tests of both types of devices. From the constant current tests, the charge capacity (capacitance (F) and Ah) and resistance of the devices are measured. From the constant power tests, the energy storage characteristics (Whkg−1 vs. Wkg−1—the Ragone curve) are derived. The magnitude of currents and power to be employed in the testing methods are chosen such that the charge and discharge times are compatible with the capabilities of the corresponding devices. In case with capacitors, the testing duration for discharge times are usually in the range of 5–60 s, while for batteries it varies from several minutes to a fraction of an hour. There is also wide difference in the recharge times of these devices. For example, the capacitors can be fully charged within 5–10 s, while the high-power batteries require a minimum of 10–20 min for a complete charge even when the initial charge current is set at a maximum value. In addition to the constant current and constant power tests, the capacitors and batteries are examined using charge/discharge pulses of 5–15 s. For these tests, the current and power levels for the capacitors and high-power batteries are comparable on a normalized basis. For both capacitor and battery systems, test cycles consisting of a sequence of charge and discharge pulses (power density for a specified time) are administered, with the aim to simulate how these devices would be performing in the specific choice of applications [2,3]. A set of assessments to be made on capacitors and batteries are summarized later. Performance characteristics of electrochemical capacitors: • • • • •

Energy density (Whkg−1 vs. Wkg−1) Cell voltage (V) and capacitance (F) Series and parallel resistance (Ω and Ω cm−2) Power density (Wkg−1) for a charge/discharge at 95% efficiency Temperature dependence of resistance and capacitance especially at low temperatures (−20C and lower) • Cycle life for full discharge • Self-discharge at various voltages and temperatures • Calendar life (h) at fixed voltage and high temperature (40–60C) Testing of electrochemical capacitors • Constant current charge/discharge capacitance and resistance for discharge times of 60–5 s. Pulse tests to determine resistance. • Constant power charge/discharge.





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• Determine the Ragone curve for power densities between 100 and atleast 1000 Wkg−1 for the voltage between V rated and 1/2 V rated. Test at increasing Wkg−1 until discharge time is less than 5 s. The charging is often done at constant current with a charge time of at least 30 s. • Sequential charge/discharge step cycling. • Testing done using the PSFUDS (Pulsed Simple Federal Urban Driving Schedule) test cycle with the maximum power step being 500–1500 Wkg−1. From the data, the roundtrip efficiency for charge/discharge is determined. • Tests modules with at least 15–20 cells in series. Various instruments or test modes have been developed from time to time and employed consistently to characterize and to assess the electrochemical performance of SCs. Cyclic voltammetry (CV), constant current charge/discharge (CCCD), and electrochemical impedance spectroscopy (EIS) tests are commonly used. Basically, all such instruments can be employed to measure the three fundamental governing parameters: voltage, current, and time; other scaling parameters (metrics), including the capacitance, equivalent series resistance, operating voltage, and, subsequently, the time constant, energy, and power performance of SCs, can be derived from them. Each of the instruments has its own targeted specific parameters; their designs, applications, and limitations are discussed in the following section.

5.2.2  Electrochemical apparatus State of the art electrochemical work stations are constructed with three major components namely, signal waveform generator, potentiostat/galvanostat, and computer interfacial system. When the users designate all the set up parameters in the computer, it will transfer these instructions to the signal waveform generator and potentiostat/galvanostat block. The latter applies the required signal to the electrochemical cell and the measurement is performed. Each electrochemical method is distinguished by its own waveform. There are transient techniques such as cyclic voltammetry (CV), chronopotentiometry, chronoamperometry, etc., and stationary techniques such as electrochemical impedance spectroscopy (EIS), rotating disk electrode (RDE), etc. To characterize an electrochemical cell, both two-electrode and threeelectrode configurations can be employed. A simple sketch of two- and three-electrodes electrochemical cell configurations are depicted in Fig. 5.2A,B, respectively. The two-electrode cell consists of a working electrode (WE), on which the electrochemical reaction under study takes place and a counter electrode that acts as the other half of the cell. The counter electrode (CE) in the two-electrode set up serves two functions; it completes the circuit allowing charge to flow through the cell, and it also maintains a constant interfacial potential regardless of current. Fulfilling both of these requirements seems to be an unattainable task under most of the conditions. In a two-electrode system, it is difficult and challenging to maintain a constant counter electrode potential while the current is flowing. This fact, along with a lack of compensation for the voltage drop across the solution leads to poor control of the working electrode potential with a two-electrode system. The roles of passing current and maintaining the reference voltage, are better served by two separate electrodes. The applied potential (EA) is measured between the working and counter electrode and the resulting current is measured in the working or counter electrode lead. The three-electrode system alleviates many of the issues associated with the two electrode cell configurations. The 

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FIGURE 5.2  Simple schematic of two electrodes (A) and three electrodes (B) configurations. EA is the applied voltage and WE, CE, and RE are the working and counter and reference electrodes, respectively.

three-electrode system consists of a working electrode, counter electrode, and reference (RE) electrode. The role of reference electrode is to act as a reference in measuring and controlling the working electrode potential, without passing any current. The reference electrode should have a constant electrochemical potential at low current density. Additionally, since the reference electrode passes negligible current, the iR drop between the reference and working electrode is often very small. Thus, with the three electrode system, the reference potential is much more stable, and offers a compensation for iR drop across the solution. This translates into superior control over working electrode potential. The most commonly employed laboratory reference electrodes are the saturated calomel electrode (SCE) and the Ag/AgCl electrode. In the three-electrode configuration, the only role of the counter electrode is to pass all the current which is necessary to balance the current observed at the working electrode. The counter electrode will often switch to extreme potentials in order to accomplish this task.

5.2.3  Electrochemical cell Electrochemical cells can be modeled as a network of passive electrical circuit elements, and such networks are commonly named as equivalent circuits. The equivalent circuit of a simple three-electrode cell is shown in Fig. 5.3. The current generator responds to the chemical reaction at the WE. The capacitor represents the double layer capacitance, while the resis-

FIGURE 5.3  Equivalent circuit of three-electrode cell. 



5.2  Evaluation of supercapacitors performance

321

FIGURE 5.4  Schematic view of potentiostat/galvanostat system.

tor at the counter electrode represents the solution/electrolyte resistance between the counter electrode and the current source. There is another resistor in series with the reference electrode, although the reference electrode does not generally conduct current. Considering ZCE, ZWE, ZRef as the counter electrode impedance, working electrode impedance, and ZRef the reference electrode impedance respectively, and i, i’ as the current passing through each electrical branch (CE to WE and CE to RE), impedance can be represented in terms of Ohm’s law generalization as given in Eq. (5.1), where V is the voltage drop, Z the impedance, and i the current. V (V) = Z(Ω) I (A) (5.1) While performing an electrochemical experiment, the WE potential has to be controlled precisely. In order to achieve the accuracy, at first, it is necessary to bring down i’ to zero. Manufacturers design their PG in such a way, and in these days, the input impedance is as high as 1015 Ω. A PG behaves like an operational amplifier in a potential follower configuration. The measurement set-up for a typical electrochemical three-electrode measurement is shown in Fig. 5.4. The RE is plugged to one input of this amplifier, which is well known to have high input impedance. We have

VO = G (V+ − V− )( V )

(5.2)

where VO is the output voltage, G the amplifier gain, and V+ and V− the two input voltages. Vi (imposed voltage) = V+. As for an ideal operational amplifier G → ∞, we obtain:

Vi = ZWE I ( V )

(5.3)

Such architecture allows following the voltage of the WE without any Ohmic drop through the RE. In this electric circuiting, the current only passes through the CE and the WE. Only the impedance of the WE has to be considered, and this is described since the voltage is measured (or controlled) between the RE and the WE, as shown in Fig. 5.5.

FIGURE 5.5  Impedance of the working electrode. RS is series resistance, ZF is Faradic impedance, and Cdl is double-layer capacitance. 

322

5.  Characterization methods for supercapacitors

FIGURE 5.6  Voltage drop across an electrochemical cell. VWE is the real working electrode voltage and Vi is the imposed (controlled) voltage. Source: Reproduced with permission from Ref. [4].

As shown in this figure, the WE impedance is formed by a series resistance RS, including mainly the bulk electrolyte resistance, Relectrolyte; Cdl and ZF stand for the double-layer capacitance and the Faradic processes, respectively, occurring at the electrode. Relectrolyte has to be minimized as low as possible and the rest of the parameters are to be measured for the study. It has to be mentioned that the value of Vi = VWE + Relectrolytei, and Relectrolyte is mainly influenced by the position of RE in the electrochemical cell, as the distance between WE and RE affects the potential drop as can be seen from Fig. 5.6 [4]. It is not possible to eliminate this contribution and it is not required for consideration of stability, but attention must be paid to the cell design; the RE has to be set as close as possible to the WE and a Luggin capillary could be used for this purpose.

5.2.4  Electrochemical interface: supercapacitors As we know, we can classify supercapacitors mainly into two types, namely electrical double-layer capacitors and pseudo-capacitors. The former implies only electrostatic interactions, while the latter surface Faradic reactions. The impedance of an electrical double-layer capacitor can be described in terms of the equivalent circuit components as shown in Fig. 5.7A. There are no Faradic reactions involved and in a first order such an electrode can be reduced to a simple R-C series equivalent circuit, where Cdl can be simply designated as follows:

Cdl = ε 0ε r

S ( F) d

(5.4)

with S the electrode surface area, d the charge separation corresponding to the outer Helmholtz plane [2], and ε0 and εr the vacuum permittivity and the relative permittivity of the dielectric material, respectively. This equation clearly reveals the reason for why high surface area activated carbons are used as electrical double-layer capacitor-active material. 



5.3  Transient techniques

323

FIGURE 5.7  (A) Equivalent electrical circuit of an electrical double-layer capacitor and (B) equivalent electrical circuit of a pseudo-capacitor.

In the pseudo-capacitor case, as shown in Fig. 5.7B, the Faradic impedance ZF has to be taken into account, which can be expressed by a capacitance CP in series with a charge-transfer resistance RCT [3]. RT (5.5) RCT = (Ω) α nFi0



CP =

zF θ (1 − θ ) ( F) RT gθ (1 − θ ) − 1

(5.6)

where θ is the saturation coverage of the electrochemical sites and g is a repulsion factor that is negative when repulsive forces occur between each site and positive in the opposite case. It has to be mentioned that for a real electrode, owing to its porous nature, diffusion impedance and a geometric distribution have to be considered. This point is discussed further in this chapter.

5.3  Transient techniques Cyclic voltammetry and galvanostatic cycling techniques are often employed to test the electrochemical performance of SCs, as these techniques can monitor and measure the variations in current or in voltage instantly with high precision and efficiency.

5.3.1  Cyclic voltammetry Due to its versatility, it is one of the widely exploited tools in electrochemical analysis; however it is mainly designated for the measurements in laboratory scale test cells, because large devices lead to significantly large magnitudes of current from several hundreds to thousands of amps, which could be technically difficult to handle. At the laboratory or material scale testing it has been aptly employed as it is an accurate technique that enables qualitative



324

5.  Characterization methods for supercapacitors

and pseudo-quantitative studies, kinetic analysis by scanning a wide range of scan rates and for voltage window determinations. In a CV technique, the electric potential is periodically and linearly varied with time between the positive and negative electrodes for two-electrode systems, or between reference and working electrodes for three-electrode configurations. The total charge accumulated at the surface of the electrode can be calculated by integrating the electric current with respect to time. The capacitance can be estimated as the total charge divided by the potential window. The fastness of the potential change in mV s−1 is designated as the sweep rate or scan rate ν, and the range of potential change is named as the potential window or operating potential. During the cathodic and anodic sweepings, the current is instantaneously recorded and there by one can characterize the associated electrochemical reactions. The data are plotted as current (A) vs. potential (V) or at times, as current (A) or potential (V) vs. time (s) [3]. Hence, the principle of this technique is to apply a linear voltage ramp to an electrode (or a device) between two voltage limits and to measure the resulting current. The applied voltage is represented as follows: V (t ) = V0 + vt for V ≤ V1 (5.7) V (t ) = V0 − vt for V ≥ V2 where v is the scan rate (V s−1) and V1 and V2 are the two voltage boundaries. Capacitance is generally measured at different scan rates and used to characterize the performance of energy storage device such as supercapacitor. At lower scan rates, the capacitance values are higher compared to higher scan rates and are closed to ideal shape. Further, the shape of the CV curve is used to deduce the electrochemical processes involved in the charging and discharging of supercapacitor. For instance, charging the supercapacitor from zero potential, the current initially increases and then it decreases upon further increase in the electric potential. Therefore a “hump” is generally observed in the CV curves. In case of supercapacitors, the CV of an ideal capacitor having negligible resistance shows a perfect rectangular shape as can be seen from Fig. 5.8, and it also exhibits scan rate dependent behavior [5]. An ideal capacitor loses no power or energy during charge or discharge, so the equation above can also be used to describe the charge process. An ideal capacitor with no current flow will store energy and charge forever. In reality, the ideal capacitor does not exist, for real capacitors have limitations and imperfections. In case of real capacitors, deviations from rectangular shape are observed as shown in Fig. 5.8 and it is represented as a series combination of internal resistance R and overall capacitance C. Capacitors can only operate within a “voltage window” with both an upper and lower voltage limit. Voltages outside the window can cause electrolyte decomposition damaging the device. Capacitor electrolytes may be aqueous or non-aqueous. While aqueous electrolytes are generally safer and easier to use and handle, capacitors with non-aqueous electrolytes can have a wider voltage window. Fig. 5.9 presents the cyclic voltammograms for carbon materials in both, aqueous and organic electrolytes. Eq. (5.8) is commonly used to describe the electrochemical signature:

i = vCdl 1 − exp ( −t RS Cdl )  ( A ) 

(5.8)



5.3  Transient techniques

325

FIGURE 5.8  A cyclic voltammogram shows the fundamental differences between static capacitance (rectangular) and pseudocapacitance (curved). Typical charge/discharge voltammetry characteristic of an electrochemical capacitor. Source: Reproduced with permission from Ref. [5].

FIGURE 5.9  Cyclic voltammogram of an EDLC cell at 5 mV s−1 in (A) aqueous 6 M KOH and (B) organic 1 M tetraethylammonium tetrafluoroborate electrolytes. Source: Reproduced with permission from Ref. [6].

where Cdl and RS are the double-layer capacitance and the equivalent series resistance which is often reduced to the bulk electrolyte resistance), respectively. From this curve, it is possible to measure the voltage window of an electrode (or a supercapacitor), that is, a signal excluding any irreversible Faradic reactions; electrolyte degradation or electrode oxidation usually restricts the voltage window. Also, by applying Eq. (5.9), the plot Q versus V can be obtained:

Qi =

∫ i dt (C ) Vi



(5.9)

326

5.  Characterization methods for supercapacitors

FIGURE 5.10  Cyclic voltammetry of (A) MnO2 in aqueous K2SO4 electrolyte and (B) RuO2 in aqueous H2SO4 electrolyte. Measurement made at 25°C and at ν = 20 mV s−1

where Qi is the local capacity (C) obtained for V = Vi. The capacitance is usually calculated from the backward step of the cyclic voltammetry, that is, while the supercapacitor or the electrode is discharged. The calculation is made for each Vi between V1 and V2. Since

Q = CV ( C )

(5.10)

The slope of the curve represents the capacitance C(F). If in case, Q-V is not so linear, as such situation often occurs when the material is pseudocapacitive, the capacitance can be calculated in a different way. In Fig. 5.10, the cyclic voltammograms of two well-explored electrochemical systems—MnO2 (Fig. 5.10A) and RuO2 (Fig. 5.10B) are shown in Ref. [4]. In such cases, the capacitance is more voltage dependant than activated-carbon-based electrodes; the accuracy of the linear regression is not as good as expected. To overcome this issue, the expression (5.11) is applied: V2

∫ i dt ( F) C= (5.11) ∫ V dt V1 V2

V1

where [V1;V2] is the voltage window, C the capacitance, i the current, and V the voltage. The calculation is usually made on the backward scan (discharge). Though, cyclic voltammetry is useful to evaluate the cyclability of a supercapacitor or an electrode, and the variation in capacitance on cycling would give an idea of the electrochemical process. However, normally galvanostatic cycling is preferred while performing such experiments. To examine the charge storage mechanisms of SC materials where EDLC and PC types are distinctly different, CV testing with the three-electrode configuration is considered as the most suitable one [7,8]. The test results can first be analyzed by examining the shape of





5.3  Transient techniques

327

the CV curves, as for EDLC and most PC materials, the shape of the resulting CV curves is rather rectangular, whereas for some PC materials, pronounced redox peaks may occur in a highly reversible fashion [9]. Therefore, one cannot differentiate EDLC and PC materials solely by observing the shape of CV curves. A more quantitative and reliable method for interpreting the data from a CV test to extract the contributions from EDL and PC mechanisms separately is by utilizing the knowledge that the instantaneous current induced by the EDL mechanism is proportional to the scan rate, while the semi-infinite diffusion limited cation adsorption/insertion at/near the electrode surface from PC mechanism is proportional to the square root of the scan rate [10–17]. However, this approach is limited in its ability to separate the contribution of surface-redox reactions from EDL mechanisms due to the fact that both of these processes occur roughly in the same time scale. Therefore, more experimental and theoretical studies are required to address this issue. However, CV testing is practically suitable to determine the operating voltage or potential window for SC materials by successive adjustment of the reversal potential in a three-electrode system, and the reversibility of the charge and discharge processes can also be studied simultaneously [10,18]. In addition, the specific capacitance and energy performance of the SC materials can be obtained via integration of the CV curves as discussed in detail later. Similar process can also be carried out for SC devices to derive their total cell capacitance and hence the extent of electrical storage capabilities.

5.3.2  Galvanostatic cycling The galvanostatic charge-discharge (cycling) is a reliable method to evaluate the electrochemical capacitance of materials under controlled current conditions. This technique is different from cyclic voltammetry, because here the current is controlled and the voltage is measured. As this technique can be extended from a laboratory scale to an industrial one, it is widely employed in the battery field. This method is also termed as chronopotentiometry and gives access to different parameters such as capacitance, resistance, and cyclability. The voltage variation is described by Eq. (5.12):

V (t ) = Ri +

t i (V) C

(5.12)

The voltage variation of a supercapacitor is represented in Fig. 5.11: As can be seen from Eq. (5.13), the capacitance of a supercapacitor can be calculated from the slope of the curve; for a pseudo-capacitor, when the V-t tracing is not as linear as it should be, the capacitance can be calculated by integrating the current over the discharge time or charge time: ∂t ( F) ∂V

(5.13)

I ∆t ( F) ∆V

(5.14)



C=I



C=



328

5.  Characterization methods for supercapacitors

where i is the set current, t is the discharge time (or charge time), and V is the voltage window. The series resistance can be deduced from the voltage drop (Vdrop) occurring over the current inversion (i); it is illustrated in the inset of Fig. 5.11 [6]: Vdrop R= (5.15) (Ω) ∆I When the current is inversed or interrupted, the voltage drop is directly linked to the resistance of the cell. Further, by repeating both capacitance and resistance measurements over cycling, it is also possible to monitor the cyclability of supercapacitors (electrical double-layer capacitors and pseudo-capacitors). In industry, constant current tests are performed to determine the key characteristics of devices, using the notation in Fig. 5.11 with the following expressions. The capacitance calculation from the integral of the area obtained during the discharge (i.e., C can be deduced from Idischarge × tdischarge divided by V1–V2), and resistances associated to the cell such as ESR (obtained from V4 divided by Idischarge) and equivalent distributed resistance (EDR, which can be obtained from V3 divided by Idischarge), which represents ESR and the resistance in the pores (part of the discharge with curvature).

FIGURE 5.11  Galvanostatic curve: voltage variation versus time while applying a current through a supercapacitor cell. Inset: Zoom-in of the current inversion region. Source: Reproduced with permission from Ref. [6].





329

5.3  Transient techniques

5.3.2.1  Constant current charge or discharge Charging at constant current, followed by discharge across a known load, is a conventional method employed for testing the batteries, and this test suits fairly well to capacitors. These tests can be done in the following sets of sequences: 1. Charging at constant current followed by abrupt discharge across various load resistors. 2. Charging at constant current followed by a holding period (variable, followed by discharge across a pre-selected load resistance). 3. Charging at various rates and discharging across a fixed load resistor. 4. The charge, q, and energy, E, input and output to the capacitor can be calculated through integration of the voltage time transients according to Eq. (5.16), where q is the charge, V is voltage, RL is the load resistor, and E is energy.

q=

∫ (V / R ) dt L

and E =

∫ (V

2

)

/ RL dt

(5.16)

The figures of merit for the charge and discharge efficiency can be derived by computing the input energy and the realized output energy. The internal resistance (esr) of the capacitor can be determined by applying a square-wave current pulse of appropriate magnitude (to be determined from the size of the capacitor) and measuring the instantaneous voltage decay at the break points. From the magnitude of the internal resistance, one can elucidate the computed energy efficiency, that is, large internal resistances lead to poor energy efficiency. The effective or mean RC time constant is determined from the voltage decay curves by measuring the time to reach Vo/e where Vo is the initial potential before decay. The external resistance of the system (e.g., solution resistance) can be determined by means of the impedance measurements, with data being plotted according to complex-plane analysis. It has to be pointed out that a porous electrode does not have a unique RC time constant owing to the distribution of R and C elements. Charge and discharge and self-discharge characteristics should also be determined as a function of temperature in order to fully understand the characteristics of the device and its operational limits in relation to utilization requirements and ambient conditions. 5.3.2.2  Constant potential charge or discharge In this technique, a potentiostatic (a constant potential) step is applied to the electrode and the time-dependent current transient is recorded. The integral of this transient in time gives the delivered charge. Usually a series of transients is recorded at various potentials in the operating range of the electrode. Various electrical procedures employ digital recording of either potential or current which is advantageous that are set up through a computer or by using devices such as Nicolet digital oscilloscope. 5.3.2.3  Constant power charge or discharge Constant power charge/discharge is a common procedure that is employed in battery testing and is advantageous for evaluating electrochemical capacitors, especially where the output power capability during declining voltage on discharge can be an important practical test parameter.



330

5.  Characterization methods for supercapacitors

5.3.2.4  Leakage current and self-discharge behavior Leakage current or self-discharge behavior is conveniently determined by monitoring the residual current flow that is generally termed as “float current,” when a fully charged capacitor is maintained under constant voltage control. This measurement can be executed using a potentiostat in a two-electrode configuration; the cell is charged at constant voltage (i.e., corresponding to the open-circuit voltage of a fully charged capacitor) and the residual current flow is monitored when the capacitor is fully charged, as indicated by constancy of the current observed beyond 5 RC times. Leakage current can also be measured using power supplies with good voltage control, and shunt resistors are included in the circuit to accurately monitor the current flow. Under such conditions, leakage current is sometimes referred to as the “float current.” Self-discharge behavior can directly be determined by following the voltage decay of a fully charged capacitor over a period of 24 h. There will be a rapid initial drop in voltage from the elimination of any IR drop, but the voltage drop will level off over longer periods of time. The analysis of time-dependent forms of this potential decay (e.g., in log time or a square root of time) can lead to diagnostic information on the mechanism of the self-discharge process. Sarangapani [19] has employed an indirect procedure for following the effects of self-discharge: the energy efficiency on discharge vs. recharge was evaluated after various durations of self-discharge at the open circuit. It is essential to use the potential-measuring system with high input resistance, ideally ∼1010 Ohm or more, in order to ensure that the measuring circuit itself does not draw significant charge from the capacitor that may lead to an anomalous rate of self-discharge. Here also, a digital potential recording system is desirable. Finally, it is best if single electrode measurements can be made against a third reference electrode in the potential-measuring system, as in “single electrode” polarization experiments. Wholecell measurements are less informative as one cannot typically predict which of the two electrodes of a capacitor cell suffer greater self-discharge rate.

5.3.3  Stationary technique 5.3.3.1  Electrochemical impedance spectroscopy The frequency-response characteristics of a capacitor and its equivalent series resistance are important in the evaluation of a capacitor and are dependent upon various factors including the intrinsic nature of the electrode material, pore-size distribution of the high-surface area material used in the fabrication of the electrodes and the engineering parameters used in the formulations of the electrodes, for example, the thickness of the active material of the electrode and the nature of the particle-particle contact, as well as the macropore distribution as determined by the applied pressure. Electrochemical impedance spectroscopy (EIS) offers a convenient path to assess the frequency-response characteristics of a capacitor material, especially the power limiting internal resistance. This electrochemical method allows covering a wide time scale from microseconds to hours as the electrochemical processes are classified depending on their own time constant. Further, such measurements are made under steady-state conditions that allow acquisition times high enough to get accurate measurements. In contrast to the previous techniques, this technique works by first polarizing the cell at a fixed voltage and then applying a small





5.3  Transient techniques

331

FIGURE 5.12  An oscillating perturbation in cell voltage gives an oscillating current response [20].

additional voltage (or occasionally, a current) to perturb the system. The perturbing input oscillates harmonically in time to create an alternating current, as shown in Fig. 5.12 [20]. It means that this technique can be carried out by controlling either the current or the voltage, thus measuring either the voltage or the current. Most widely, the voltage of the system is set as required (VS) and a small-amplitude (in the range of a few millivolts or so) sinusoidal signal is overlaid and measurement is made at several frequencies f, expressed in Hz (f = w/2Π). δv and δi are the amplitudes of the voltage and the current, respectively. However, to get highly reliable measurements, it is to be ensured that stationary conditions are reached before launching the experiment, that is, ∂i/∂t→0. As the signal is small enough, a linear relationship exists between the current and the voltage at each pulsation (w = 2Πf ): V = Zi ( V ) (5.17) where V is the voltage, i the current, and Z the impedance. For a particular amplitude and frequency of applied voltage, the electrochemical cell responds with certain specific amplitude of alternating current at the same frequency. In real systems, the response may be complicated due to the contributions from the components of other frequencies also. In EIS experiments, the frequencies of the applied perturbation have been varied in the range between mHz and kHz. The relative amplitude and time shift (or phase shift) between the input and output signals change with the applied frequency. These factors depend on the rates at which the physical processes in the electrochemical cell respond to the oscillating stimulus. Different frequencies are able to separate different processes that have different time scales. At lower frequencies, there is time for diffusion or slow electrochemical reactions to proceed in response to the alternating polarization of the cell. At higher frequencies, the applied field changes direction faster than the chemistry responds, so the response is dominated by capacitance from the charge and discharge of the double layer. The time-domain response is not the succinct way to interpret these frequency dependent amplitudes and phase shifts; instead, a quantitative term called impedance is used. Like resistance in a static system, impedance is the ratio of voltage to current. However, it uses the real and imaginary parts of a complex number to represent the relation of both amplitude and 

332

5.  Characterization methods for supercapacitors

phase to the input signal and output response. The mathematical tool that relates the impedance to the time-domain response is a Fourier transform, which represents the frequency components of the oscillating signal. To explain the idea of impedance more fully for a simple case, consider expressing V and i using a complex notation, that is:

V (ω ) = δ v exp ( jω t )( V )

(5.18)



i (ω ) = δ i exp  j (ω t + Φ )  ( A )

(5.19)

Hence, the previous expression can be re-written as follows: Z (ω ) =



δv exp ( − jΦ )( Ω ) δi

(5.20)

where Z(w) is also known to be the complex impedance and different definitions can now be offered. The impedance can also be represented as Z (ω ) = ZRe + jZIm ( Ω )



(5.21)

with the impedance modulus as Z (ω ) =

and the phase angle as

δV 2 2 = ZRe + ZIm (Ω) δi

(5.22)

Φ = arctan ( ZIm ZRe )( ° )



(5.23)

where ZRe and ZIm are the real and imaginary parts of Z(w), respectively. This technique is thus convenient for linearization of a complex electrochemical system. Such linearization enables the performance of the usual electrical analysis. It gives the opportunity to find an analogous electrical circuit behaving like the studied electrochemical cell, also called as equivalent circuit, as mentioned before. It helps predict the behavior of a system; moreover, with other physical analysis, it brings clues to the understanding of reaction kinetics. It has to be mentioned that the user must handle modeling tools with care, since many equivalent circuits can fit experimental data and will not be related anymore to the electrochemistry of the system. Before going further, the following table (Table 5.1) shows some impedances of simple ideal electrical components.

TABLE 5.1  Impedances of ideal electrical components. Component

|Z|

ZRe

ZIm

Φ (rad)

Resistance

R

R

0

0

Capacitance Inductance

1 Cω Lw

−1 Cω

0 0

Lw



π 2 −π 2



333

5.3  Transient techniques

One has to exercise caution on the phase angle definition and it is easy get perplexed since many electrochemical impedance spectrometer manufacturers assimilate the phase angle to designate the impedance. Actually, representation is the opposite of the phase angle, as is shown in Eq. (5.24): Z (ω ) = Z exp ( − jΦ ) = Z exp ( jθ )( Ω )



(5.24)

where Φ is the phase angle and θ represents complex number. Electrochemical systems are often more complex and can be modeled with a combination of components presented in the table. Mostly used components are resistances (R) and capacitances (C). The former is readily identified with electrochemical processes and kinetics, the latter is mainly related to charge accumulation at different interfaces involved in an electrochemical cell. Fig. 5.13A represents the Randles equivalent electrical circuit and it is mostly used to describe simple electrochemical reactions. The RS represents the series resistance which is mainly related to the bulk electrolyte resistance, while Cdl is the double-layer capacitance related to a charge accumulation at the electrode/electrolyte interface and RCT is the charge-transfer resistance linked to the exchanged current as defined in the Butler-Volmer equation (in Nernst-like systems, RCT is close to zero); the component W, is the diffusion impedance and is intended to define the polarization of an electrochemical system due to diffusion limitation. For an electrochemical reaction [Eq. (5.25)], the Warburg element can be expressed as

Ox + ne −  Red



ZW =

σ (1 − j )

and

σ=

RT 2

2

n FS 2

 1  D1 2 C Ox

ω1 2

+ Ox

(5.25)

(Ω)

(5.26)

 1 Ω rad1 2 s −1/2 D CRed  12 Red

(

)

(5.27)

where n is the number of the exchanged electron, R the gas constant, T the temperature in Kelvin, F the Faraday constant, S the footprint area, DOx and DRed the diffusion coefficients of Ox and Red, respectively, and COx and CRed the bulk concentration of the electroactive species. Electrochemists commonly use Nyquist plots and Bode plots, which are, respectively,

FIGURE 5.13  (A) Randles’ electrical equivalent circuit. (B) Components ZIm, ZRe and |Z| of EIS plots. 

334

5.  Characterization methods for supercapacitors

ZIm versus ZRe and |Z|-versus frequency, and these components of EIS plots are shown in Fig. 5.13B for clarity. Fig. 5.14A–C represents, respectively, the Nyquist and Bode plots of a Randles equivalent circuit [4]. The left part of the Nyquist plot (low ZRe values) is related to high frequencies (HFs) and the right part (high ZRe values) is related to low frequencies (LFs). At HFs, the RCTCdl (Fig. 5.14) is predominant and exhibits an HF loop in the Nyquist plot, a (−1) slope for the impedance modulus and a peak for the phase angle. At LF, the mass transport impedance is predominant, leading to a slope of (−1) in the Nyquist plot and a slope of (− ½) in the Bode

FIGURE 5.14  (A) Nyquist plot and (B) Bode plots of Randles circuit: (A) Under semi-infinite diffusion limitation; (B) Under finite length diffusion limitation. (C) Under restricted diffusion limitation. Source: Reproduced with permission from Ref. [4].





5.3  Transient techniques

335

FIGURE 5.15  Nyquist plot of (A) ideal electrical double-layer capacitor; (B) ideal pseudo-capacitor. Source: Reproduced with permission from Ref. [4].

impedance plot. Such diffusion impedance is observed for the semi-infinite condition, that is, when the diffusion layer thickness is continuously increasing from the electrode to the bulk of the electrolyte (but the diffusion layer is small enough compared to the electrode dimension). Another specific diffusion condition can be obtained when the diffusion layer has a finite length under specific hydrodynamic conditions, as is the case, for a rotating disk electrode. Fig. 5.15 represents the Nyquist and Bode plots, respectively [4]. At HF, RCT is still observed but an LF loop related to finite length diffusion is observable. The transition between the two loops is particular and leads in the Bode impedance plot, to a slope of (−1/2). The resistance of the LF loop is directly linked to the thickness of the diffusion layer. This type of plot is also encountered for spherical diffusion when the diffusion layer thickness is in the range or larger than the electrode dimension. Finally, let us consider a particular condition in which the diffusion layer is restricted as is the case for a thin electrochemical solution layered between the electrode and another nonreactive surface. As in previous case, similar HF behavior can be observed but the LF impedance is similar to that of a capacitor. A transition slope of (−1) is seen between the RCT loop and the LF vertical line in the Nyquist plot [(− ½) slope at the Bode impedance plot], which means actually that D/2Πf ≫ l (l is the electrochemical solution thickness, D the diffusion coefficient, and f the frequency of the signal). To summarize, the EIS testing, also known as the dielectric spectroscopic testing, measures the impedance of a power cell as a function of frequency by applying a low-amplitude alternative voltage (normally 5 mV) superimposed on a steady-state potential. The resulting data are usually expressed graphically in a Bode plot to demonstrate the cell response between the phase angle and frequency, and in a Nyquist plot to show the imaginary and real parts of the cell impedances on a complex plane. In addition to the frequency response and impedance, EIS has also been used to characterize the charge transfer, mass transport, and charge storage mechanisms, as well as to estimate the capacitance, energy, and power properties. Different equivalent circuits and models have



336

5.  Characterization methods for supercapacitors

been developed to distinguish the contribution of individual structure component in a cell system to the total impedance. When SC devices are tested, the real parts of the complex impedance at selected frequencies are used in literature to represent RES. However, one needs to keep in mind that this RES from an EIS test is often smaller than that derived from the CCCD test, and is therefore limited in describing the power performance of SC devices. For SC materials, EIS testing can be used to study the impedance, specific capacitance, charge transfer, mass transport, and charge storage mechanisms involved by executing similar analysis in a three-electrode system. 5.3.3.2  Supercapacitor impedance Supercapacitor electrodes are unique cases, as mostly the electrode impedance can be modeled by two simple sketches (Fig. 5.15). Although it is a very basic description, the essential picture can be obtained. The first figure (Fig. 5.15A) represents the behavior of an electrical double layer capacitor, which can be easily described by a simple series resistance (RS) and a capacitance (Cdl). The first term is mainly linked to the electrolyte resistance (contact resistances, but current collector resistance could be also included) and the second to the charge accumulation at the electrode/electrolyte interface. Though several theories describe such charge separation but the Helmholtz one is sufficient since electrolyte concentration is well above 0.1  mol l−1; thus, the diffusion layer (Gouy-Chapman layer) can be neglected [21,22]. The second figure (Fig. 5.15B) describes the behavior of a pseudo-capacitor, which is basically described by an electrolyte resistance, a double-layer capacitance in parallel with a pseudocapacitive branch [21,23]. This last branch is modeled by a charge-transfer resistance related to the associated Faradic processes; the capacitance is linked to the charge accumulation at the electrolyte/electrode interface but this differs from the electric double-layer capacitor in that specific sites are related. Indeed, Faradic reactions occur at favorable active sites, whereas it is not the case for purely capacitive charge accumulation. It means that kinetic rate constants exist (charge-transfer resistance) and mass transfer limitation could appear. The last point is not considered here but could be done with the addition of the relevant Warburg impedance to the pseudocapacitive branch. Fig. 5.15A represents the Nyquist plot of an ideal electrical double-layer capacitor. As can be seen, a vertical line is obtained, implying that the capacitance is constant over the entire frequency window studied. An ideal pseudo-capacitor behavior exhibits the electrochemical impedance response shown in (a) of Fig. 5.15B. A first HF loop is related to the charge-transfer resistance and the double-layer capacitance is observed; the LF vertical line is linked to the charge storage on surface electrochemical reaction. This capacitance has been expressed in Eq. (5.6). Actually, real electrode behavior is slightly more complex due to dispersion factors. These factors are mainly due to geometrical aspects such as electrode porosity and electrode roughness but also active site activation energy dispersion, especially for pseudo-capacitors. Such fractal electrodes induce frequency dispersions of the electrical parameters [24]. Resistances and capacitances are no more constant over whole range of the studied frequencies. In early 1960s, the porosity aspect was studied by de Levie by a unique pore model [25]. Later models are more sophisticated as they take into account several parameters such as shape of the pores [26]. The de Levie’s model set the foundation for the influence of porosity on the electrochemical impedance signal.





337

5.3  Transient techniques

Activated carbon can be electrically modeled by a cylindrical pore structure. Along the length of the pore, a transmission line is used to model the mass transport behavior of the electrochemical species, indicating that the path toward each active site is not equivalent and it involves time factor, depending on the tortuosity. To reflect on this aspect, de Levie [25] proposed a transmission line model exhibiting several RC constants. The following equation is proposed to describe the impedance of a porous electrode:

ZP = RP ZE cotanh

(

RP / ZE

ZSC = RS + ZE ( Ω )

) (Ω)

(5.28) (5.29)

ZP is the impedance due to the porosity of the electrode, RP is the ionic resistance of the active material, and ZE is the electrode/electrolyte interface impedance. For the particular case of the electrical double-layer capacitance, ZE can be replaced by 1/jCdlw, which is achieved by eliminating the Faradic resistance. The total impedance of an electrode is given by the expression (5.28) to which the series resistance is added [Eq.(5.29)]. This last term is mainly governed by the bulk electrolyte resistance but contact resistances could be considered as well. ZSC represents the total impedance of an electrode or a device; this impedance is frequency dependant. Nyquist plot distinctly exhibit the three components of the total impedance as shown in Fig. 5.16 [27]; bulk electrolyte resistance (x-intercept at the highest frequency region), interfacial impedance between electrode and bulk solution (semicircle at the middle frequency region), and the impedance that is associated with intra-particle pores (spike at the low frequency region) (Fig.  5.16). The first two terms are mainly dependent on the electrolyte

FIGURE 5.16  Equivalent circuit model for the porous carbon electrodes. Source: Reproduced with permission from Ref. [27].



338

5.  Characterization methods for supercapacitors

solution, while the last one (low frequency tails) is controlled by both electrode materials and electrolytes. Practically, the process for the semicircle can be regarded as a simple resistance at lower frequency than 10 Hz, while the typical EDLC charge/discharge condition corresponds to the frequency of 0.1 Hz. In a simple approach, capacitive electrochemical systems can be represented by a serial connection of equivalent series resistance (Resr) and utilizable capacitance (Cutil), which approximates the capacitor behavior at sufficiently low frequencies. In this approximation, Resr comprises bulk electrolyte resistance, interfacial resistance, and apparent resistance of intra-particle pores (Fig. 5.16). Fig. 5.17A gives an overview of the consequence on the electrode analysis. While r1 and r2 are the radii of the pores, λ1 and λ2 are the penetration length of the electrical signal. Song et al. [28] demonstrated with this figure that the higher the frequency (f1 and f2) or/and the smaller the pore radius, the shallower the penetration of the AC signal. It means that at high frequencies, the external surface and the larger pores could only interact with the electrochemical species. While at low frequency, the electrochemical species could be able to penetrate and have more access in to the whole active electrode surface area. An electrical double layer is formed at the interface between an electrode and an electrolyte at a given potential; while in the absence of Faradaic reactions, smooth and clean surfaces show ideal capacitive behavior, where the double layer capacitance Cd is independent of frequency. The ideal behavior is represented as a vertical line in the Nyquist plot of impedance (curve a in Fig. 5.17B). However, this is applicable only for the liquid mercury electrode and the impedance of most solid electrodes deviates from the purely capacitive behavior. The non-ideality or frequency dispersion has been described widely by a more convenient mathematical tool namely constant phase element (CPE), rendering the electrical dispersions

FIGURE 5.17  (A) Schematic view of AC signal penetration inside a porous electrode. (B) Nyquist plot of impedances considering various frequency dispersions (a) for ideally capacitive behavior of plain electrodes; (b) for the in-a-pore dispersion simulated by the de Levie ; (c) for the in-a-pore and by-PSD dispersion simulation. A part of curve c was fitted with CPE1 at high frequency and CPE2 at low frequency. The increasing directions of frequency f and penetrability α were indicated. Rs on the abscissa is the solution resistance. Source: Reproduced with permission from Ref. [28].





339

5.3  Transient techniques

occurring in a real electrode. The CPE element arise from various origins of frequency dispersion; diffusion in a diffusion-limited system, geometric factors, sluggish processes such as adsorption of anions, surface reconstruction and transformation in adlayer and crystallographic heterogeneity. In Fig. 5.17B, curve b can be described with a CPE1 at high frequencies and with an ideal capacitance at low frequencies, while curve c can be approximated with a CPE1 at high frequencies and roughly with a CPE2 at low frequencies. The main drawback of such a tool is that it is a black box analysis though it is easy to handle. CPE is expressed as ZCPE =



1

Y0 ( jω )

α

(Ω)

(5.30)

where ZCPE is the equivalent impedance, Y0 is the admittance modulus (1/|Z|); w is the signal pulsation (w =2Πf ); and α is the dispersion exponent. Three situations can be expected depending on the value of α, as can be seen from Table 5.2. It can be seen from Fig. 5.18 that CPE can be used to obtain an electrical description of the electrode and the two following figures give an idea of how an electrical double-layer capacitor or a pseudo-capacitor varies with α [4]. As observed, when the frequency dispersion increased (low α), the Nyquist plots are shifted to higher resistances. For describing a real electrode, a linear combination of several CPE could be done: Z=



1

Y0 ( jω )

a0

+

1

Y1 ( jω )

a1

+

1

Y2 ( jω )

a2

(Ω)

(5.31)

For instance, Eq. (5.31) can be used to describe an activated carbon electrode (Fig.  5.18) with α0  =  0, α1  =  0.5, and α2 = 1. EIS is a powerful technique that allows measuring both the equivalent resistance and the capacitance at different frequencies. Also, the frequency analysis allows parting different processes occurring at the electrode. For example, combining galvanostatic cycling with such measurements gives information about resistances and capacitance evolution. Electrode degradation can be due to current collector corrosion, increased electrode resistance, increased contact resistance, electrochemical shuttles, and so on. It can be roughly stated that at HFs, contact resistances, current collector corrosion is mainly concerned; at medium frequencies, electrochemical species accessibility [Eq. (5.28)] throughout the electrode is involved; charge storage, diffusion limitations, and leak resistance (electrochemical shuttles, irreversible electrochemical processes) occur at LFs. Thus, depending

TABLE 5.2  The definition of impedance for resistor, Warburg, and capacitor. α = 0 Impedance

α = 0.5

α = 1

Warburg 1

Capacitor 1 C= jω Y0

Resistor 1 R= Y0

W=



Y0 ( jω )

0.5

340

5.  Characterization methods for supercapacitors

FIGURE 5.18  (A) Nyquist plots of a CPE for different values of α, (B) a pseudo-capacitor including a CPE instead of Cdl. Source: Reproduced with permission from Ref. [4].

on the frequency region mainly implies that the impedance variation on cycling an accurate analysis can be done. The assumptions made with the impedance technique have to be confirmed by using other analytical tools. Though this technique is powerful it is really easy to be lost, as stated before since the main drawback is that many equivalent circuits can be found to describe the same electrical behavior. Fig. 5.19 presents an impedance trace of a supercapacitor electrode composed of an aluminum foil coated with an activated carbon film depending on the current collector surface treatment [4]. Actually, an important contribution to the impedance of such an electrode is the contact impedance between the current collector and the active material [29]. Portet et al. [30] showed that for a carbon-carbon electrical double-layer capacitor, surface treatment on

FIGURE 5.19  Influence of the electrode/current collector interface on the electrode impedance. (A) An aluminum current collector used as-received; (B) etched one; (C) a treated one. Source: Reproduced with permission from Ref. [4]





341

5.3  Transient techniques

an aluminum current collector was needed to improve the power ability. When current collectors are used as received, a large HF loop is observed owing to the thick aluminum oxide layer. By increasing the contact surface and decreasing the oxide layer thickness by acidic etching, the loop is reduced. The HF loop is removed and a decrease in the contact resistance is obtained by filling the etched aluminum foil using acetylene black. As can be seen, the HF loop is no longer observed and the total impedance of the electrode is drastically lowered [30]. Another way to analyze the impedance data is to plot complex capacitance plots. The complex capacitance can be expressed as follows [31]: C = CRe − jCIm ( F )



(5.32)

with

CRe =

− ZIm

ω Z

2

and CIm =

ZRe

ω Z

2

( F)

(5.33)

Such expressions are very convenient to define accurately the knee frequency since a peak is observed on CIm-f plot and CRe =C0/2 for f = fknee. C0 is the LF capacitance that can also be measured by galvanostatic experiments. Fig. 5.20 shows the capacitance plots of an activatedcarbon based electrical double-layer capacitor electrode depending on the type of electrolyte. It compares an acetonitrile (AN)-based electrolyte and a propylene carbonate (PC) electrolyte. The former solvent led to a more conductive electrolyte because of its lower viscosity and higher wettability. As observed here, the lower the fknee, the higher the dispersions. This frequency is very particular since it is the boundary between the full access of the electrode surface and the electrolyte penetration region. Also, it is the frequency of the median charge storage efficiency and can be defined as a merit factor: at higher frequencies the charge storage efficiency is below 50% and at lower frequencies it is superior to 50%. Actually, this

FIGURE 5.20  Evolution of the (A) real part and (B) imaginary part of the capacitance versus frequency for 4 cm2 cells assembled with two electrodes containing 15 mg cm−2 of PICACTIF SC activated carbon in AN and PC with 1.5 and 1.0 M NEt4BF4, respectively. Source: Reproduced with permission from Ref. [31].



342

5.  Characterization methods for supercapacitors

efficiency is the ratio of stored energy/lost energy. Such a representation is complementary to the other and offers directly the charge storage ability of an electrode. Actually, this fknee also defines the bandwidth of the supercapacitor, which needs to be known when it is to be integrated in an electrical circuit. It is possible to see from Fig. 5.20 that it is preferable to use an AN-based electrolyte if high-power applications are aimed at [31]. The goal of this section is to give the main keys to analyze electrical double layer capacitors and pseudocapacitors. The electrochemical techniques exposed in this chapter are the most widely used to characterize supercapacitors and are sufficient to get relevant parameters. The combination of the three techniques allows understanding the electrochemical phenomena that occur.

5.4  Key scaling parameters 5.4.1 Capacitance The total capacitance CT of a supercapacitor implies the extent of electrical charge ∆Q storage under a specific change in voltage ∆V (Eqn. 5.34). This is preferred when specifying the total charge storage abilities of SC devices.

CT =

∆Q ∆V

(5.34)

Now, the specific capacitance can be calculated from Eq. (5.34) by multiplying the denominator (on the right-hand side of the equation) with a suitable parameter (for instance, mass, volume, area, or length) and the corresponding unit of the specific capacitance can be determined. Hence, for the more intrinsic specific capacitance CS, the relevant expression is given in Eq. (5.35), which is in particular employed to determine the charge storage ability of SC materials.

CS =

∆Q ∆V ∏

(5.35)

where Π can be the mass, volume, surface area of the electrode material, or even the size of the electrode, and the resulting specific capacitance CS is often named correspondingly as the gravimetric capacitance (F g−1), volumetric capacitance (F mL−1), normalized capacitance (µF cm−2) and areal capacitance (F cm−2) or linear capacitance (F cm−1). Sometimes, CS is also used to describe device performance, when normalized by the whole cell weight or volume [32]. It has to be noted that although CS is considered as the most important parameter in comparing the charge storage ability of SC materials, it is rarely mentioned by industry, as most of the commercial SCs are based on activated carbon (AC), and its CS is generally considered a constant between 100 F g−1 and 70 F cm−3 in organic electrolyte [33]. However, for scientists searching for new materials, CS is the more informative one to depict the charge storage ability of a given material.

5.4.2  Evaluation of CT The total capacitance CT of a cell can be calculated from the cyclic voltammetry data by mathematically integrating the absolute area under the curve. The mathematical expression





5.4  Key scaling parameters

343

for the calculation of CT from a typical cyclic voltammogram is given in Eq. (5.34). Through the integration of the resulting cyclic voltammograms, the accumulated charge as a function of potential can be obtained. Normally, the entire curve is recommended for consideration [34–38], as shown in Eq. (5.36).

∆Q CT = = ∆V

∫

2V0 v

0

i dt

2V0

(5.36)

However, in practice different segments of the curve, as depicted in Fig.  5.21 [38] have been used in integration [38–42], and such inconsistencies lead to discrepancies in test results [36,37,41,43,44]. It is worth noting that the potential change in Eq. (5.36) ∆V = 2Vo (0 to Vo and back to 0), for there are mistakes made in ∆V values [45–48]. Calculating the capacitance from the constant current charge-discharge curve is a simple and straightforward approach, compared to the calculation from the CV curves. The plot obtained from the CCCD measurement exhibits the variation of potential as a function of time under the influence of an applied constant current. Keen observation of the CCCD curve can provide additional information regarding the potential drop (iR) due to internal resistance, which is otherwise termed as the equivalent series resistance, ESR. One may refer to Fig. 5.30 given in the later section, for a typical iR drop in a commercial supercapacitor obtained from Skeleton Tech. Sometimes, the accurate method is to extrapolate the discharge curve and calculate the net potential drop, which is in most cases greater than the iR drop calculated in conventional method. CCCD technique offers yet another advantage, that is, to obtain the ESR value by calculating the iR (potential) drop during discharge process. The slope of that particular region of CCCD curve would give the ESR value of the supercapacitor device. This along with the discharge time can be helpful in calculating the value of specific power [43]. Since, constant current is used in a CCCD test, Eq. (5.34) can be converted to:

CT =

I ∆t ∆V

FIGURE 5.21  Schematics of a typical CV test result. Source: Reproduced with permission from Ref. [38].



(5.37)

344

5.  Characterization methods for supercapacitors

Here, I is the constant current, ∆t is the charging or discharging time corresponding to the specified potential change ∆V. So, presently the main issue is that the correct time ∆t and ∆V are used in calculation. Often the complete discharging curve is used:

CT =

I dis ∆tV0 − 2V0 V0

(5.38)

Since IR drop is inevitable in CCCD test, one can adjust ∆V so as to exclude the IR drop for more accurate result, that is,

CT =

I dis ∆tV0 − 2V0 V0 − VIR-drop

(5.39)

Similar to CV tests, different segments of the CCCD plots, as illustrated in Fig.  5.22, have been used in computing the cell capacitance [37,42,49]. In this case, as current remains constant, an identical CT value is obtained independent of the segment chosen, as long as the voltage changes linearly with time as shown in Fig.  5.22. In case of hybrid capacitors (HCs), for instance, Ni(OH)2 or PbO2 based ones [33,50,51], which exhibit non-linear curves as depicted in Fig. 5.23, the selection of different regions from the curve can exhibit a large difference in determining ∆V, and thereby impacting CT values [52–55]. In such cases, the use of an appropriate or fixed region, or the selection of suitable potential window, becomes critical and it has to be standardized. Burke and Miller [46] recommend considering Region (iii) or a potential window from Vo to the shoulder voltage based on their study. Besides, one can also adjust ∆V to eliminate the IR drop, to improve the accuracy. The conventional method of deriving capacitance from EIS test is based on the imaginary part of the complex impedance Im(Z) as shown in Eq. (5.40) [56–58].

FIGURE 5.22  An illustration of CCCD test result from EDLCs or PCs with linear potential change over time. Source: Reproduced with permission from Ref. [38].





5.4  Key scaling parameters

345

FIGURE 5.23  An illustration of CCCD test result from HCs with nonlinear potential change over time . Source: Reproduced with permission from Ref. [38].



CTf = −

1 2π fIm ( Z )

(5.40)

where f is the frequency. Normally, this frequency is identified at which the phase angle reaches −45 degrees, [58] or simply as the lowest applied frequency [56]. Another method introduced by Simon et al., helps to determine the extents of energy dissipation of the cell and the stored energy [59]:



− Im ( Z )

Re ( C ) =

ω Z

Im ( C ) =

2

Re ( Z )

ω Z

2

(5.41)

(5.42)

where Z = Re(Z) 2 + Im(Z) 2 is the overall complex impedance, w  =  2πf is angular velocity, Re(Z) is the real part of the complex impedance, and Re(C) and Im(C) are the real and imaginary capacitances, respectively. Im(C) is a term related to the energy dissipation of the cell, and Re(C), calculated at the lowest-frequency applied, indicates the energy stored, thus can be used to represent CT [59].

5.4.3  Evaluation of CS Once the CT is obtained, the corresponding CS can be calculated using Eq. (5.37). This seemingly straight forward step is made complicated by the fact that there is no established standard procedure in determining the base value for Π, be the mass, volume, or other quantity. A mediocre CT value can lead to an excellent CS if a sufficiently small Π is used. For this reason, 

346

5.  Characterization methods for supercapacitors

along with other possible considerations, technical or cost related, although attractive CS results are frequently reported [45–48,52–55], only a few have been successfully transferred to commercial products. In order to ameliorate this issue, it is suggested that both CT and the corresponding CS values, along with Π value, has to be reported explicitly along side. In addition to Π, other factors will be discussed in the next section, including experimental setup, mass loading, electrode thickness, and electrode density, can also dramatically alter the CS value [36,60,61] and hence recommended to be reported as necessary information.

5.4.4  Major influencing factors Studies have demonstrated that significantly different CS values can be obtained for the same SC electrode when different experimental setups are employed for the evaluation. The three major experimental setups, which commonly employed are: symmetric two-electrode, asymmetric two-electrode, and three-electrode configurations. The threeelectrode one is particularly useful in accurately determining the CS for SC materials, and the two-electrode ones are normally used in SC device prototypes or final products. It is worth noting that it is also possible [62] to insert a reference electrode in the two-electrode system to study the detailed potential change in other two electrodes, but this scenario is not discussed here. As can be seen from the literature, the electrochemical characteristics of electrode active materials are evaluated in either a two electrode or a three-electrode system. In case of supercapacitors, the two-electrode cell setup is composed of a pair of closely spaced electrodes containing the electrode active materials, separated by an electrically insulating but an ion permeable separator. Electrochemical performance of a two-electrode cell configuration depends on both the electrodes in a chosen electrolyte. Depending on the type of the electrode active material on each of the electrodes, supercapacitors are grouped into two major categories known as symmetric and asymmetric supercapacitors (Fig. 5.24) [63]. Electrodes in a symmetric device contain the same active materials with the same mass loading (m+ = m−) for both the positive and the negative electrodes. Symmetric supercapacitors are generally made of EDLC-based materials or materials consisting of contributions from both EDLC and Faradaic capacitances. Symmetric supercapacitors based on just Faradaic materials are rarely used since the redox reactions only take place either at the positive voltages or the negative voltages, but not at both. Asymmetric supercapacitors employ two different electrode active materials, that is, one EDLC and another pseudocapacitive, together in the same electrolyte. Supercapacitors with the same type of electrode active material, but different masses of materials (m+ ≠ m−) on each electrode are also considered as asymmetric supercapacitors (Fig. 5.24). The following analysis demonstrates how different setups can lead to different results. For brevity the gravimetric CS is used. The weight of each individual electrode in Fig. 5.24A is assumed to be m, while m1 and m2 are for the two different Electrodes 1 and 2 in Fig. 5.24C. Assigning the single electrode capacitance as CE, the gravimetric CSa in the three-electrode system can be calculated as [64]:

CSa =



CE m

(5.43)



5.4  Key scaling parameters

347

FIGURE 5.24  Schematic illustrations of the symmetric, asymmetric and hybrid configurations. (A) A symmetric supercapacitor constructed by using the same material for both positive and negative electrodes. Symmetric supercapacitors commonly use carbon-based materials. Asymmetric supercapacitors cover a wide range of electrode combinations including (B) two EDLC electrodes with different materials, (C) electrodes of the same material, but with different mass loadings, (D) two pseudocapacitive electrodes with different compositions, (E) one EDLC electrode with a pseudocapacitive electrode, e.g. an activated carbon//MnO2 aqueous supercapacitor, (F) one EDLC electrode with a composite electrode (that can be a composite of carbon-based material with a metal-based species). (G) A hybrid configuration that is fabricated by using a capacitor-type negative electrode and a battery-like positive electrode, and (H) a hybrid architecture, typical of a Li ion capacitor, that is fabricated by using a battery-type negative electrode and an EDLC positive electrode. Source: Reproduced with permission from Ref. [63].

For the symmetric two-electrode system shown in Fig. 5.24A, the cell total capacitance CTb can be obtained using:

1 1 1 = + CTb CE CE

(5.44)

By counting the mass for both electrodes, hence:

CSb =

CTb 1CE = 2m 4m

(5.45)

that is, even with identical SC material, the specific capacitance obtained from the three-electrode system actually quadruples that from the symmetric two-electrode system, that is,

CSa = 4CSb

(5.46)

This relationship has been validated experimentally by Béguin et al. [35]. For the asymmetric two-electrode system in Fig. 5.24C, the cell capacitance CTc:

1 1 1 = + CTc CE1 CE 2 

(5.47)

348

5.  Characterization methods for supercapacitors

By adding the mass for both electrodes, there is: CSc =



CTc 1 CE1CE 2 = m1 + m2 m1 + m2 CE1 + CE 2

(5.48)

In this configuration, the electrode capacitances of the two electrodes are balanced to fully unitize the charge storage ability of the SC material, that is, CE1 = CE2 = CE. By using other active materials, such as pseudocapacitive ones, it is normally accepted that the specific capacitance of Electrode 1 is larger. Conversely if the two electrodes achieve the same capacitance CE, then less mass is needed in Electrode 1, say at a fraction α (0  0 min, ∆I ≅ I (5.55) Combined with the potential change, they observed a smaller RES ≈ 0.93 Ohms at 0 min dwelling time, compared to 1.47 Ohms for cases with dwelling time > 0 min. Till date, the influence of cell size on CCCD results is not clearly understood, though it is generally accepted that the IR drop method works well only for small SCs, and a steady-state voltage drop method has to be employed for large SCs. It has been demonstrated that the steady-state voltage drop ∆V2, rather than the IR drop ∆V1was obtained by the back extrapolation of the potential trace and it was almost 50% larger than ∆V1, thus leading to a nearly 50% increase in RES. The typical example in Fig. 5.29B was reported by Burke et al. [52], and the studies [52,53] reveal that more accurate power performance can be estimated using ∆V2 for large cells, and hence recommended.

5.6  Operating voltage, Vo Operating voltage Vo normally refers to the potential applied to the system or the suitable potential window within which a cell normally operates. In this section, the term is at times interchangeable with cell voltage or rated potential, representing the maximum voltage the cell can endure.

5.6.1  Evaluation of Vo Both CV and CCCD tests can be used to determine Vo of either the SC materials or the devices, however the actual testing of this maximum potential may destroy the cell. An 



5.6  Operating voltage, Vo

353

FIGURE 5.29  (A) A typical CCCD plot for large SCs with IR drop and steady-state voltage drop marked as ∆V1 and ∆V2, and (B) a real case illustration of the discharge part via Skeleton Tech 1600F SC. Source: Reproduced with permission from Ref. [52].

expedient method is usually employed, in which Vo can be achieved by applying a lower starting voltage to the cell, and then progressively increasing the voltage until a spike appears at the boundary of the potential window as seen for an example of an asymmetric MnO2/AC capacitor shown in Fig. 5.30 [45].

5.6.2  Major factors influencing Vo The solvent used in the electrolytes and the configuration of the cells, are the two major factors, which affect Vo in SC devices. In aqueous systems, a cell can usually be charged to 1.0 V, limited by the thermodynamic decomposition potential of water at room temperature;

FIGURE 5.30  An illustration of Vo determination methods using (A) CV and (B) CCCD tests. Source: Reproduced with permission from Ref. [45].



354

5.  Characterization methods for supercapacitors

while Vo in organic solvent electrolyte varies between 2.3 and 2.7 V [84–89]. As both energy and power densities are proportional to Vo2 much effort has been dedicated in developing novel electrolytes that can endure high voltage (>3  V). Room temperature ionic liquids (RTILs) are considered to be the most promising candidates, exhibiting high Vo values between 3.0 and 6.0 V in laboratory tests [90,91]. Furthermore, various mixtures of different RTILs, or RTIL and organic solvents, are also considered as attractive candidates [92,93]. The other factor influencing Vo is the cell configuration. In an asymmetric system, Vo can be increased by using different SC materials so as to introduce additional electrochemical potential difference [44]. In this way, even in aqueous systems, Vo can reach 2.0–2.3 V [44, 94–96], giving rise to much improved energy storage [94,97]. In conclusion, for SC materials, all the three techniques, that is, CV, EIS, and CCCD tests, can be employed with different emphases. However, for SC devices, the most effective and accurate approach is CCCD testing to measure the cell capacitance, equivalent series resistance, and operational voltage [50]. Subsequently, the time constant, energy and power densities, and leakage and maximum current of SC devices can be derived from these three core parameters.

5.7  Time constants The time constant τ, is the factor considered only for SC devices and it can be defined as the product of RES and CT as shown in Eq. (5.56) using the equivalent RC circuit for a SC in Fig. 5.27. A smaller τ reflects a better responsiveness of the device, and for most of commercial SCs, τ normally ranges from 0.5 to 3.6 s [98].

τ = RES CT



(5.56)

Based on the RC circuit model, the voltage of SC device changes by 36.8% at time t = τ, and by 98% at time t = 4 τ, during the charge/discharge processes. Burke [99] has reported that normally τ is fixed around a certain value for SCs manufactured by using the same technology, for example, 0.55 s from Maxwell Technologies, 1.1 s from NessCap and 3.8 s from JSR Micro. Consequently, CT and RES for the same type of SCs are inversely proportional to each other when τ is fixed. An example is shown in Table 5.4 for BCAP SCs from Maxwell Technologies, and the corresponding τ values are calculated by Zhang and Pan [1]. It has to be mentioned that this τ is completely different from “relaxation time constant” τ0, that is found in Refs. [48, 99–101]. The τ0 proposed by Simon et al. is based on EIS tests [62]. By plotting both Re(C) and Im(C) versus frequency as shown in Fig. 5.31 [102], τ0 can be found as marked at the position where the imaginary part of the capacitance reaches its maximum at frequency f0, and it is calculated to be 10 s using Eq. (5.57):

τ 0 = 1/ f 0 (5.57) TABLE 5.4  CT and RES for BCAP SCs from Maxwell Technologies [1]. CT [F]

1

3.3

5

10

25

50

100

310

350

650

1200

1500

2000

3000

RES [mΩ]

700

290

170

75

42

20

15

2.2

3.2

0.8

0.58

0.47

0.35

0.29

τ [s]

0.7

0.96

0.85

0.75

1.05

1.0

1.5

0.68

1.12

0.52

0.7

0.71

0.7

0.87





5.8  Power and energy densities

355

FIGURE 5.31  Dependence of real and imaginary capacitances over frequency for SC with the relaxation time constant τ0 indicated. Source: Reproduced with permission from Ref. [102].

5.8  Power and energy densities Among various performance indicators that are preferred to benchmark various kinds of energy storage and conversion systems for their eventual applications, power density and energy density are the most pertinent ones. Their gravimetric or volumetric (W kg−1 or W L−1) efficacies of power density describes the efficacy in energy uptake/delivery and their energy density in Wh kg−1 or Wh L−1 manifests the amount of electrical energy stored or deliverable. The energy density value of any supercapacitor devices is usually calculated by numerically integrating the discharge curves. The Ragone plot (energy density vs. power density) has been widely employed to evaluate the overall performance of a supercapacitor device. The comparison of supercapacitors with other electrical energy storage (EES) devices is presented in a Ragone plot [103] as shown in Fig. 5.32 [105]. The diagonal time line represents the so-called “characteristic time,” [104] in view of running time of the devices at the rated power. The actual running times of EES devices may vary substantially, depending on the load or discharging rate; the so-called rate dependence. The values are gathered from various sources [103–108].

5.8.1  Power density One of the distinct merits of SC devices is their outstanding power performance. The most generally adapted route for calculating the maximum power density is:

PD =

V02 4 RES ∏

(5.58)

The maximum power delivery can be realized only when the load has the identical resistance as RES, and is often termed as the matched load condition; but in realistic situation, the load resistor often does not match with RES. Therefore, alternate methods are adapted to compute the actual power capacity. Three most widely adapted ones are DOE-Freedom Car, [109] IEC 62576 [110] and the pulse energy efficiency (PEE) [98] methods, and the detail discussions can be found elsewhere [36, 109–112]. Table 5.5 provides the resulting actual power densities with respect to maximum PD based on the different testing procedures and the data 

356

5.  Characterization methods for supercapacitors

FIGURE 5.32  Ragone plot illustrating the performances of specific power versus specific energy for different electrical energy-storage technologies. Times shown in the plot are the discharge time, obtained by dividing the energy density by the power density. Source: Reproduced with permission from Ref. [105].

TABLE 5.5  Power densities obtained from different methods.

Power density

Matched load

USABC

IEC

PPE

PD = V02 / ( 4ΠRES )

50% PD

48% PD

11.25% PD

published by Burke [43,113]. Therefore, although the maximum value of PD is widely chosen for comparison, one has to keep in mind that it does not normally manifest the actual deliv­ erable power density, and one can expect the values lower than PD depending on the particular applications. Though an advanced PD value is beneficial, only limited effort has been bestowed towards this, may be because SCs already exhibit relatively high PD.

5.8.2  Energy density The characteristics of the capacitor can be readily assessed from the time for charging tc, and discharging td as in Fig. 5.33A [63]. The operable time for a capacitor can be deduced from the plot. Further, the equal values of tc and td indicates that the Columbic efficiency of the device is 100% (i.e., qc = qd), with theoretically unlimited cyclability and the operating potential window of the capacitor can be deduced from the Y-axis of the plot. The potential across the plates of the capacitor increases from Vi to a maximum value of Vf; however, to draw further useful information, the variables in both axes are to be considered simultaneously. For instance, the stored and delivered energy of a capacitor can be drawn by integrating the area 



5.8  Power and energy densities

357

FIGURE 5.33  V-t (A) and V-q (B) plots of the output signal of an ideal capacitor charged with a constant current, i, for a definite time, t, and then discharged with the same current. Source: Reproduced with permission from Ref. [63].

FIGURE 5.34  Charge–discharge profiles of two supercapacitors that deviate considerably from an ideal capacitive behavior; representative working diagrams from CCCD tests for HCs. Source: Reproduced with permission from Ref. [63].

under the V-q plot (i.e., the area of the triangle), during the charge and discharge processes (Fig. 5.33B). The slope of the plot is reciprocal of capacitance of the capacitor, 1/C. The instantaneous charging and discharging abilities of a capacitor implies the characteristics of high power energy storage devices, since power is the rate of energy transfer, that is, the amount of energy that is stored or delivered per unit of time as represented by P = E/t. Fig. 5.33B can be considered as the representative working diagram from CCCD test for EDLCs and PCs and Fig. 5.34 for HCs, where the difference in shape is caused due to their distinct charge storage mechanisms. In all cases, the stored electric energy can be obtained from the charge curve, and the deliverable energy from the discharge curve. The ratio of the two yields the energy efficiency of the cell, an indicator of the difference between the two parts of the curve. As indicated in Fig. 5.34, charge-discharge profiles of a hybrid energy storage system can deviate considerably from the one that is expected from an ideal capacitor [63]. Thus, the equation E = 1/2 CV2 does not hold anymore, since it could lead to over/underestimation of the energy that an SC could store/deliver. In charge storage based on the EDLC mechanism, the active materials will decompose, thus the device can strive for unlimited charge/ discharge cycles. At high charge and discharge rates, during which the redox reactions cannot proceed to any great extent, EDLC can provide the required energy. Thus, charge storage 

358

5.  Characterization methods for supercapacitors

based on the EDLC mechanism allows the system to be charged and discharged much faster than batteries. Further, as the equivalent series resistance of EDLCs is negligible, they can offer close to 100% efficiency. Calculations for the stored electrical energy are given below. For EDLCs and PCs with linear charge/discharge curves, the integration of the working diagram direct to the calculation of triangle area as shown in Fig. 5.34A, therefore:

ED =

Q

∫

0

1 V0 Q 2

V0 dq =

(5.59)

Substituting Eq. (5.35) into Eq. (5.59) yields,

ED =

1 CTV02 2Π

(5.60)

dividing by 3600 converts ED in Joule Π−1to watt hour Π−1kk. 1 CTV02 2 ED = 3600 Π



(5.61)

By combining Eqs. (5.56),(5.58), and (5.60), the relationship between PD and ED for EDLCs is given as:

ED = 2 RES CT = 2τ PD

(5.62)

This equation indicates that the energy and maximum power densities are closely coupled by the cell time constant τ = RESCT. Although ED can be increased as in Eqn. (5.60), by improving either the capacitance or operating voltage, elevating the capacitance alone will simultaneously increase the time constant τ, leading to sluggish cell, under the same RES. Boosting the voltage can significantly elevate both PD and ED, while still maintaining the same τ value. Although increasing ED is the major stride in the field of SCs, further attention is required by simultaneously considering the possible associated changes in τ or PD. However, for HCs with nonlinear charge/discharge curves as shown in Fig. 5.34, the integration of the diagram is complicated, depending on the specific shape of the curve, so that:

ED =

∫

Q

0

V dq =

∫

tQ

0

VI dt

(5.63)

Dividing by 3600 and Π, one can still obtain the energy density in watt-hour Π −1,

ED =

∫

Q

0

∫ V dq =

tQ

0

VI dt

3600 Π

(5.64)

That is, Eq. (5.61) is not valid for HCs with nonlinear charge/discharge curves, and for them, Eq. (5.64) has to be used.





5.11  Inconsistencies in evaluation of SCs

359

5.9  Leakage and maximum peak currents For SC devices, an additional yet useful parameter is the leakage current, which is widely used in industries to evaluate the efficiency of the SCs to maintain the rated potential while at rest. Normally, it is reported as the compensating current that is applied to hold a fully charged SC after 72 h. Another similar device parameter is the maximum peak current, which usually appears in the specifications for commercial SCs. It is evaluated by discharging a fully charged SC device from Vo to ½ Vo in 1 s, and calculated as:



I max@1s

1 CTV0 2 = CT RES + 1

(5.65)

5.10  Cycle life and capacitance retention rate Long cycle life of SC devices is one of their outstanding merits, that lead to the so-called “fit-and-forget” benefits, which are highly desirable for certain unique applications. This extensively long cycle life (>1,000,000 cycles) makes the direct measurements difficult. Hence the term “capacitance retention rate” is introduced to indirectly estimate the cycle life of SCs in such cases. It is easily obtained in CCCD test by comparing the capacitance after given thousands of cycles with that of the first cycle. One such notable attempt [114] was made a few years ago by continuously testing SCs for 3.8 years, and the results showed that the capacitance retention rate is decreasing almost linearly with the square root of the number of cycles. Further validations are needed to establish this relationship, but it does provide us a glimpse on the time demanding nature of the direct measurement of cycle life.

5.11  Inconsistencies in evaluation of SCs 5.11.1  Causes for the inconsistencies In order to avoid inconsistencies and to set universal platform for comparison, it is essential to use the same scaling parameters and test methods. The most common five causes are: (1) Differences in instruments or calculation methods; for instance, CV, CCCD or EIS; (2) Differences in experimental setups—three-electrode, and symmetric/asymmetric two-electrode configurations; (3) Differences in electrode fabrication—with varies mass loading, electrode thickness, and density; (4) Differences in base Π used: volume or mass; active material alone or active material with additives/binders; single or two electrodes; with or without electrolyte, or the whole cell; (5) Differences in applied test conditions (rate dependency)—scan rate in CV, charge/discharge current in A g−1, mA cm−2, or mA F−1 for CCCD tests. With the aim to facilitate the understanding, we present our discussions in addition to the recent discussions by Zhang and Pan from their excellent article [1], on the inconsistencies caused by various factors.



360

5.  Characterization methods for supercapacitors

5.11.2  Device performance versus material property Though the difference between the device performance and material property has been indicated in the above sections, it is often overlooked in the literature reports; however, it is a pertinent factor for consideration as it may lead to huge variations in the performance evaluation of any EES system. Such difference has been emphasized by Gallagher et al. [115] in the case of lithium-air batteries. Zhang and Pan [1] have collected and analyzed certain useful information from a few of SC manufacturers such as Maxwell Technologies, WIMA, Nesscap, and Ioxus [85–88] to substantiate this point. From the material front, they have considered only AC-based SCs, as the main SC material in industry is still activated carbon (AC). For SC devices, their energy and power densities are summarized as: ED ranging from 1 to 6 Wh kg−1 or 2 to 8 Wh L−1 ; and PD ranging from 2 to 20 kW kg−1 or 4 to 30 kW L−1. The same devices but for SC materials (AC), with CS of 100 F g−1 and 70 F cm−3 [33] the parameters are summarized as: ED ranging from 15 to 25 Wh kg−1 or 12 to 18 Wh L−1; and PD ranging from 20 to 120 kW kg−1 or 13 to 80 kW L−1. The substantial difference between device performance and material properties can clearly be found here. This is attributed to the fact that only a small portion of the weight (∼ 2.5%–30 wt%) and volume (∼ 10%–50 vol.%,) of SC devices are composed of electrode materials and this may even smaller in case with micro-SCs [61,73,74]. It has to be pointed out that in most of the scientific literature [54,62,116–126], no explicit distinction has been pointed out between the device performance and material properties, and this may lead to inconsistency. It is understood that since in most of the R&D explorations, the main focus is to identify and probe the novel materials for their primary characteristics and the system employed for the study may not be in the form of a well packaged cell, it is not realistic to expect energy and/or power densities for the whole device in a scientific report. Also from engineering point of view, while searching for new cell design developments or more advanced manufacturing processes [14,48,127,128], the different research groups are likely to use rather distinct types cell components such as unique current collectors, separators, packaging materials, and packaging methods [36,47,129,130] and to report all such details for the entire cell in every publications may not be feasible. For convenience, only the specifications of the electrodes are reported for comparison. It is suggested that based on the specific requirement of targeted application, either gravimetric or volumetric value or both can be reported. Also, similar to the determination of CS, the mass or volume of one entire electrode including the conductive additive and polymer adhesive is recommended to be used in the calculation for macro-SCs (CT > 1 mF). Further, the mass loading (mass/area) or packing density (mass/volume) and thickness of the electrode are to be reported explicitly for fair comparison. For micro-SCs (CT