Handbook of Nanocomposite Supercapacitor Materials II: Performance [1st ed.] 9783030523589, 9783030523596

Table of contents :
Front Matter ....Pages i-xxiii
Introduction to Supercapacitors (Prerna Sinha, Kamal K. Kar)....Pages 1-28
Materials for Supercapacitors (Mukesh Kumar, Prerna Sinha, Tanvi Pal, Kamal K. Kar)....Pages 29-70
Characteristics of Supercapacitors (Prerna Sinha, Kamal K. Kar)....Pages 71-87
Transition Metal Oxides as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, Kamal K. Kar)....Pages 89-111
Activated Carbon as Electrode Materials for Supercapacitors (Prerna Sinha, Soma Banerjee, Kamal K. Kar)....Pages 113-144
Transition Metal Oxide/Activated Carbon-Based Composites as Electrode Materials for Supercapacitors (Prerna Sinha, Soma Banerjee, Kamal K. Kar)....Pages 145-178
Carbon Nanofiber as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, Kamal K. Kar)....Pages 179-200
Transition Metal Oxide/Carbon Nanofiber Composites as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, Kamal K. Kar)....Pages 201-227
Carbon Nanotube as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, Kamal K. Kar)....Pages 229-243
Transition Metal Oxide/Carbon Nanotube Composites as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Tanvi Pal, Alekha Tyagi, Kapil Dev Verma, P. K. Manna et al.)....Pages 245-270
Graphene/Reduced Graphene Oxide as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, P. K. Manna, Kamal K. Kar)....Pages 271-296
Transition Metal Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors (Bibekananda De, Prerna Sinha, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi et al.)....Pages 297-331
Conducting Polymers as Electrode Materials for Supercapacitors (Soma Banerjee, Kamal K. Kar)....Pages 333-352
Transition Metal Oxide/Electronically Conducting Polymer Composites as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna et al.)....Pages 353-385
Transition Metal Oxide-/Carbon-/Electronically Conducting Polymer-Based Ternary Composites as Electrode Materials for Supercapacitors (Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna et al.)....Pages 387-434
Recent Trends in Supercapacitor Electrode Materials and Devices (Prerna Sinha, Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna et al.)....Pages 435-461
Applications of Supercapacitors (Ravi Nigam, Kapil Dev Verma, Tanvi Pal, Kamal K. Kar)....Pages 463-481
Back Matter ....Pages 483-496

Citation preview

Springer Series in Materials Science 302

Kamal K. Kar   Editor

Handbook of Nanocomposite Supercapacitor Materials II Performance

Springer Series in Materials Science Volume 302

Series Editors Robert Hull, Center for Materials, Devices, and Integrated Systems, Rensselaer Polytechnic Institute, Troy, NY, USA Chennupati Jagadish, Research School of Physical, Australian National University, Canberra, ACT, Australia Yoshiyuki Kawazoe, Center for Computational Materials, Tohoku University, Sendai, Japan Jamie Kruzic, School of Mechanical & Manufacturing Engineering, UNSW Sydney, Sydney, NSW, Australia Richard M. Osgood, Department of Electrical Engineering, Columbia University, New York, USA Jürgen Parisi, Universität Oldenburg, Oldenburg, Germany Udo W. Pohl, Institute of Solid State Physics, Technical University of Berlin, Berlin, Germany Tae-Yeon Seong, Department of Materials Science & Engineering, Korea University, Seoul, Korea (Republic of) Shin-ichi Uchida, Electronics and Manufacturing, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan Zhiming M. Wang, Institute of Fundamental and Frontier Sciences - Electronic, University of Electronic Science and Technology of China, Chengdu, China

The Springer Series in Materials Science covers the complete spectrum of materials research and technology, including fundamental principles, physical properties, materials theory and design. Recognizing the increasing importance of materials science in future device technologies, the book titles in this series reflect the state-of-the-art in understanding and controlling the structure and properties of all important classes of materials.

More information about this series at http://www.springer.com/series/856

Kamal K. Kar Editor

Handbook of Nanocomposite Supercapacitor Materials II Performance

123

Editor Kamal K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Materials Science Programme Indian Institute of Technology Kanpur Kanpur, Uttar Pradesh, India

ISSN 0933-033X ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-3-030-52358-9 ISBN 978-3-030-52359-6 (eBook) https://doi.org/10.1007/978-3-030-52359-6 © Springer Nature Switzerland AG 2020 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Dedicated to my wife, Sutapa, and my little daughter, Srishtisudha for their loving support and patience, and my mother, late Manjubala, and my father, late Khagendranath

Preface

Supercapacitors are emerging as promising devices for electrochemical energy storage, which plays an important role to harvest renewable electrical energy to meet the demand of global energy consumption. Various types of supercapacitors have been developed such as electrochemical double-layer capacitors (EDLCs), pseudocapacitors (or, redox capacitors), and capacitors. They store charges electrochemically and exhibit high power densities, moderate to high energy densities, high rate capabilities, long life, and safe operation. The electrode materials are the key part of the supercapacitors for energy storage to determine the electrochemical properties, energy storage mechanism, and mechanical properties of the supercapacitor devices. Therefore, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years through the development of the electrode materials and novel device designs. But the performance of devices is still challenging in terms of capacitance, flexibility, cycle life, etc. These deciding factors depend on the characteristics of materials used in the devices. The book “Handbook of Nanocomposite Supercapacitor Materials with a theme of Performance” focuses on the various characteristics of prospective materials. This book provides a comprehensive study on several architectural carbon materials, transition metal oxides, conducting polymers, and their binary and ternary composite electrodes that are using in the current era of supercapacitors. Finally, it highlights the advantages, challenges, applications, and future directions of the supercapacitors. Therefore, this book will provide the readers with a complete and composed idea about the fundamentals of supercapacitors, recent development of electrode materials for supercapacitors, and design of their novel flexible solid-state devices. This book will be useful to the graduate students and researchers from various fields of science and technology, who wish to learn about the recent development of supercapacitor and to select the material for high-performance supercapacitor. Chapter 1 deals with the basic introduction of supercapacitor and how it is different from other electrochemical energy storage devices. A brief history and properties of different components used in supercapacitors have also been

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discussed. A special emphasis is given on the various charge storage mechanisms involved during the charge–discharge process in a supercapacitor. The characteristic features of different types of electrodes, electrolytes, current collectors, separators, etc., used in supercapacitors are discussed in Chap. 2. Chapter 3 emphases the various electrochemical techniques to study the performance of the electrical energy storage system. Various parameters such as capacitance, energy and power density, equivalent series resistance, cyclic stability, etc., are discussed to understand the performance of the supercapacitor device. Chapter 4 covers the electrochemical properties of transition metal oxide-based electrodes for supercapacitor applications in a nutshell. Transition metal oxide (e.g., MnO2, NiO, Co3O4)-based electrodes have grabbed the scientific attention due to the low cost, eco-friendly nature, and high electrochemical properties that extend their usage in the supercapacitor devices. Activated carbon emerges as an attractive form of carbon to be used as electrode material for supercapacitor applications due to the versatility in morphology and surface properties. Chapter 5 discusses the method of synthesis and properties of activated carbon derived from different biowastes intended to be used as electrode material for supercapacitor devices. Transition metal oxide and activated carbon-based composites serve as secondgeneration hybrid electrode material for supercapacitors. The performance of the composite electrode is due to the synergistic contribution of double-layer capacitance and pseudocapacitance of the composite structure. Chapter 6 studies the importance of composite electrode material based on transition metal oxides and activated carbon along with the effect of various transition metal oxides on the electrochemical properties of the activated carbon in combination. Chapter 7 provides a comprehensive overview of carbon nanofiber-based electrode materials for supercapacitor devices with major emphasis on structure, properties, synthesis methodology, and supercapacitive performance such as current and power densities, rate capability, cyclic stability, energy density. Chapter 8 outlines the progress in the field of hybrid supercapacitor electrodes based on different types of transition metal oxides and carbon nanofiber composites. Details on the research studies on RuO2, MnO2, and V2O5 with carbon nanofibers have been included. Other metal oxides such as nickel, cobalt, and iron oxides in combination with CNF have also been overviewed as electrode materials for supercapacitor devices. A short yet inclusive overview of the progress in the field of carbon nanotubebased electrodes for supercapacitor devices is presented in Chap. 9. The development of electrode materials based on single- and multi-walled carbon nanotubes have been described in brief. A combination of carbon nanotube and transition metal oxide as a hybrid composite as the electrode material of supercapacitor devices has been overviewed in Chap. 10. An emphasis on the use of this particular set of materials in view of energy storage devices has been given. The chapter further explores the synthesis, properties, and electrochemical performance of these materials to have a broad scenario of the recent progress in this field.

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Highly conductive graphene is the material of interest for the modern generation supercapacitor devices. Graphene-based materials can be of several architectures, like free-standing particles or dots, one-dimensional fibers or yarns, twodimensional films, and three-dimensional networks. Chapter 11 provides the details of recent progress in the field of graphene-based materials as electrodes for the latest supercapacitor devices. Chapter 12 describes the development of hybrid composites based on transition metal oxides and graphene/reduced graphene oxide as electrode material for the supercapacitor applications. The basics of graphene/reduced graphene oxide and transition metal oxides, i.e., structure, property, and synthesis, have also been included. A comprehensive overview of the electrode materials for supercapacitor devices based on conducting polymers and their nanostructures is discussed in Chap. 13. The utilization of conducting polymers to fabricate efficient modern generation supercapacitor devices has been discussed in brief. Chapter 14 describes the progress in the field of conducting polymers and transition metal oxide-based composites as electrode materials for modern generation energy storage devices such as supercapacitor devices. A brief on the common conducting polymer, their structure, synthesis, and effect of doping on conductivity has also been included. Present days high-performance supercapacitors are being developed utilizing an oxide from the class of transition metal oxides and carbon nanostructures from the carbonaceous material with common conducting polymers as the third component, which are discussed in Chap. 15. A proper selection of material from these three classes produces ternary composites of high specific capacitance, energy density, and rate stability all in one to form promising supercapacitor devices. Chapter 16 provides a comprehensive overview/recent trends of the specific capacitance and cycle life of various electrode materials used in supercapacitors, which are carbon nanofibers, carbon nanotubes, graphene/reduced graphene oxide, activated carbon, transition metal oxides, conducting polymers and their composites. The design and flexibility of electrode material has promoted the supercapacitor to design bendable, lightweight, miniaturized (micro), planar, flow, shape memory, piezoelectric, self-healing, and multifunctional energy storage devices and is discussed in this Chap. 16. The last Chap. 17 provides a brief insight into the commercially available supercapacitors and the applications in various fields like wearable electronics, portable electronic devices, transportation, industrial applications, military, defense, and national security, renewable energy sector, power electronics, communication, artificial intelligence, internet of things, cyber-physical system, soft robotics, complementary metal–oxide–semiconductor (CMOS), very large-scale integration (VLSI), memory, medical and health care, buildings, gas sensors, and futuristic applications. The editor and authors hope that readers from materials science, engineering, and technology will be benefited from the reading of these high-quality review articles related to the characteristics of materials used in supercapacitor. This book

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is not intended to be a collection of all research activities on composites worldwide, as it would be rather challenging to keep up with the pace of progress in this field. The editor would like to acknowledge many material researchers, who have contributed to the contents of the book. The editor would also like to thank all the publishers and authors for permitting us to use their published images and original work. I also take this opportunity to thank Viradasarani, Zachary, and the editorial team of Springer Nature for their helpful advice and guidance. There were lean patches when I felt that one would not be able to take time out and complete the book, but my wife Sutapa, and my little daughter Srishtisudha, played a crucial role to inspire us to complete it. I hope that this book will attract more researchers to this field and that it will form a networking nucleus for the community. Please enjoy the book and please communicate to the editor/authors any comments that you might have about its content. Kanpur, India

Kamal K. Kar

Contents

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2

Introduction to Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . Prerna Sinha and Kamal K. Kar 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Brief Overview on History of Supercapacitors . . . . . . . . . . 1.3 Comparison Among Various Electrochemical Energy Storage Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Batteries Versus Supercapacitors . . . . . . . . . . . . . 1.3.2 Fuel Cells Versus Supercapacitors . . . . . . . . . . . . 1.3.3 Conventional Capacitors Versus Supercapacitors . 1.4 Components of Supercapacitors . . . . . . . . . . . . . . . . . . . . . 1.4.1 Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.4 Current Collector . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Classification of Supercapacitors Based on the Charge Storage Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Electric Double-Layer Capacitor (EDLC) (Capacitive Mechanism) . . . . . . . . . . . . . . . . . . . 1.5.2 Pseudocapacitor (Faradaic Mechanism) . . . . . . . . 1.5.3 Hybrid Supercapacitor (Capacitive and Faradaic Mechanism) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . Mukesh Kumar, Prerna Sinha, Tanvi Pal, and Kamal K. Kar 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Components of Supercapacitor . . . . . . . . . . . . . . . . . 2.2.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Electrolytes . . . . . . . . . . . . . . . . . . . . . . .

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2.2.3 Current Collector . . . . . . . . . . . . . . . . . . . . . . . 2.2.4 Separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Carbon-Based Electrode Material . . . . . . . . . . . . 2.3.2 Transition Metal Oxide-Based Electrode Material 2.3.3 Vanadium Oxide (V2O5) Based Electrode . . . . . 2.3.4 Conducting Polymers . . . . . . . . . . . . . . . . . . . . 2.4 Emerging Electrode Materials . . . . . . . . . . . . . . . . . . . . . 2.4.1 Metal-Organic Framework (MOF) . . . . . . . . . . . 2.4.2 MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

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Characteristics of Supercapacitors . . . . . . . . . . . . . . . . . . Prerna Sinha and Kamal K. Kar 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Supercapacitor Principles . . . . . . . . . . . . . . . . . . . . . 3.2.1 Electric Double-Layer Capacitor (EDLC) . . 3.2.2 Pseudocapacitor . . . . . . . . . . . . . . . . . . . . 3.2.3 Hybrid Supercapacitor . . . . . . . . . . . . . . . . 3.3 Principles and Methods for Experimental Evaluation of Supercapacitor Electrode . . . . . . . . . . . . . . . . . . . 3.3.1 Methods for Experimental Evaluation . . . . 3.3.2 Parameters Evaluation for Supercapacitor . . 3.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Materials: Transition Metal Oxides . . . . . . . . . . . . . . . . . 4.3 Supercapacitive Performance of Ruthenium Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Supercapacitive Performance of Manganese Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 Supercapacitive Performance of Nickel Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Supercapacitive Performance of Cobalt Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.7 Supercapacitive Performance of Other Transition Metal Oxide-Based Electrodes . . . . . . . . . . . . . . . . . . . . . . . . .

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4.8 Transition Metal Oxide-Based Supercapacitor Device . . . . . . . 104 4.9 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5

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Activated Carbon as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prerna Sinha, Soma Banerjee, and Kamal K. Kar 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Historical Background and Importance of Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Preparation of Activated Carbon . . . . . . . . . . . . . . . . . . . . 5.3.1 Carbonization and Activation . . . . . . . . . . . . . . . 5.3.2 Hydrothermal Carbonization . . . . . . . . . . . . . . . . 5.3.3 Microwave-Assisted Activation . . . . . . . . . . . . . . 5.3.4 Surface Modification . . . . . . . . . . . . . . . . . . . . . . 5.4 Materials: Activated Carbon for Supercapacitors . . . . . . . . . 5.5 Charge Storage Mechanisms in Biomass Derived Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5.1 Electric Double-Layer Capacitor . . . . . . . . . . . . . 5.5.2 Pseudocapacitance . . . . . . . . . . . . . . . . . . . . . . . 5.6 Electrochemical Performance of Activated Carbon as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . 5.6.1 Electrochemical Performance of Activated Carbon Synthesized from Different Routes/Methods . . . . . 5.6.2 Electrochemical Performance of Doped Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6.3 Electrochemical Performance of Symmetric Supercapacitor in Organic Electrolyte and Ionic Liquid Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . 5.7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Transition Metal Oxide/Activated Carbon-Based Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . Prerna Sinha, Soma Banerjee, and Kamal K. Kar 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Supercapacitor Electrode Based on Charge Storage Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Electric Double-Layer Capacitor . . . . . . . . . . . 6.2.2 Pseudocapacitance . . . . . . . . . . . . . . . . . . . . . 6.2.3 Hybrid Supercapacitor . . . . . . . . . . . . . . . . . . . 6.3 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Transition Metal Oxides . . . . . . . . . . . . . . . . . 6.3.2 Activated Carbon . . . . . . . . . . . . . . . . . . . . . .

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Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Activated Carbon as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Importance of Activated Carbon and Metal Oxide-Based Composites as Electrode Materials . . . . . . . . . . . . . . . . . . . 6.7 Electrochemical Performance of Composite Electrodes . . . . 6.7.1 Ruthenium Oxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . 6.7.2 Manganese Dioxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . . 6.7.3 Nickel Oxide/Hydroxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . 6.7.4 Titanium Oxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . . . . . . . . . . 6.7.5 Zinc Oxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . . . . . . . . . . 6.7.6 Bismuth Oxide and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . . . . . . . . . . . 6.7.7 Other Metal Oxides and Activated Carbon Composites as Electrode Materials . . . . . . . . . . . . 6.8 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Carbon Nanofiber as Electrode Materials for Supercapacitors . . Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Material: Synthesis, Structure, and Characteristics of Carbon Nanofiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 Carbon Nanofiber Prepared by Catalytic Chemical Vapor Deposition Growth . . . . . . . . . . . . . . . . . . 7.2.2 Carbon Nanofiber Prepared by Electrospinning . . . 7.2.3 Characteristics of Carbon Nanofibers . . . . . . . . . . 7.3 Carbon Nanofiber as Electrode for Supercapacitors . . . . . . . 7.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Transition Metal Oxide/Carbon Nanofiber Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . 201 Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202

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8.2

Materials: Carbon Nanofiber and Transition Metal Oxides . . 8.2.1 Carbon Nanofiber . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Transition Metal Oxides . . . . . . . . . . . . . . . . . . . 8.3 Carbon Nanofiber as Electrode for Supercapacitors . . . . . . . 8.4 Transition Metal Oxides as Electrode for Supercapacitors . . 8.5 Transition Metal Oxides and Carbon Nanofiber Composites as Electrode for Supercapacitors . . . . . . . . . . . . . . . . . . . . . 8.5.1 MnO2-CNF Composites as Electrode for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 RuO2-CNF Composites as Electrode for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 8.5.3 V2O5-CNF Composites as Electrode for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 8.5.4 Other Transition Metal Oxides-CNF Composites as Electrode for Supercapacitors . . . . . . . . . . . . . 8.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

9

Carbon Nanotube as Electrode Materials for Supercapacitors Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Materials: Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . 9.3 Carbon Nanotube as Electrode for Supercapacitors . . . . . 9.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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203 203 204 204 205

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10 Transition Metal Oxide/Carbon Nanotube Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . Bibekananda De, Soma Banerjee, Tanvi Pal, Alekha Tyagi, Kapil Dev Verma, P. K. Manna, and Kamal K. Kar 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Materials: Carbon Nanotubes and Transition Metal Oxides 10.2.1 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Transition Metal Oxides . . . . . . . . . . . . . . . . . . 10.3 Carbon Nanotubes as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Transition Metal Oxides/CNT Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . 10.5.1 Ruthenium Oxide/CNT Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . 10.5.2 Manganese Oxide/CNT Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . .

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10.5.3

Nickel/Cobalt/CNT Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . 10.5.4 Other Metal Oxides/CNT Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . 10.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Graphene/Reduced Graphene Oxide as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, P. K. Manna, and Kamal K. Kar 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Material: Structure and Properties of Graphene/Reduced Graphene Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Synthesis of Graphene/Reduced Graphene Oxide . 11.3 Graphene/Reduced Graphene Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Zero-Dimensional Graphene-Based Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.2 One-Dimensional Graphene-Based Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.3 Two-Dimensional Graphene-Based Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.4 Three-Dimensional Graphene-Based Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Transition Metal Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibekananda De, Prerna Sinha, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna, and Kamal K. Kar 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Materials: Graphene/Reduced Graphene Oxide and Transition Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.1 Synthesis and Characteristics of Graphene/Reduced Graphene Oxide . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.2 Synthesis and Characteristics of Transition Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Graphene/Reduced Graphene Oxide as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4 Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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12.5

Transition Metal Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . 12.5.1 Ruthenium Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 12.5.2 Manganese Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 12.5.3 Cobalt Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 12.5.4 Nickel Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 12.5.5 Bimetallic Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 12.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13 Conducting Polymers as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Soma Banerjee and Kamal K. Kar 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Conducting Polymers in Supercapacitors . . . . . . . . 13.3 Mechanism of Conduction in Conducting Polymers: Effect of Doping . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Conducting Polymer-Based Supercapacitors . . . . . . 13.4.1 Polyaniline-Based Supercapacitors . . . . . . 13.4.2 Polypyrrole-Based Supercapacitors . . . . . . 13.4.3 Polythiophene-Based Supercapacitors . . . . 13.4.4 Nanostructured Conducting Polymers . . . . 13.5 Future Prospects and Challenges . . . . . . . . . . . . . . 13.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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337 338 339 341 343 345 347 349 349

14 Transition Metal Oxide/Electronically Conducting Polymer Composites as Electrode Materials for Supercapacitors . . . . . . . . . 353 Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna, and Kamal K. Kar 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 14.2 Materials: Electronically Conducting Polymers and Transition Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . 355

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14.2.1

Synthesis and Characteristics of Electronically Conducting Polymers . . . . . . . . . . . . . . . . . . . . . 14.2.2 Synthesis and Characteristics of Transition Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 Electronically Conducting Polymers as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4 Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.5 Electronically Conducting Polymer/Transition Metal Oxide Composites as Electrode Materials for Supercapacitors . . . . 14.5.1 Transition Metal Oxide/Polyaniline Composites as Electrode Materials for Supercapacitors . . . . . . 14.5.2 Transition Metal Oxide/Polypyrrole Composites as Electrode Materials for Supercapacitors . . . . . . 14.5.3 Transition Metal Oxide/Poly(3,4Ethylenedioxythiophene)/Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . 14.6 Transition Metal Oxide/Electronically Conducting Polymer Composite-Based Supercapacitor Device . . . . . . . . . . . . . . 14.7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Transition Metal Oxide-/Carbon-/Electronically Conducting Polymer-Based Ternary Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibekananda De, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna, and Kamal K. Kar 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.1 Transition Metal Oxides . . . . . . . . . . . . . . . . . . . 15.2.2 Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . 15.2.3 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . 15.2.4 Carbon Nanofibers . . . . . . . . . . . . . . . . . . . . . . . 15.2.5 Graphene/Reduced Graphene Oxides . . . . . . . . . . 15.2.6 Electronically Conducting Polymers . . . . . . . . . . . 15.3 Transition Metal Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4 Activated Carbon as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.5 Carbon Nanotubes as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.6 Carbon Nanofibers as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.7 Graphene/Reduced Graphene Oxides as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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15.8

Transition Metal Oxide-/Carbon-/Electronically Conducting Polymer-Based Ternary Composites as Electrode Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.8.1 TMO–Carbon–PANI Ternary Composites . . . . . . 15.8.2 TMO–Carbon–PPy Ternary Composites . . . . . . . . 15.8.3 TMO–Carbon–PT Ternary Composites . . . . . . . . 15.9 Ternary Composite-Based Supercapacitor Device . . . . . . . . 15.10 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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16 Recent Trends in Supercapacitor Electrode Materials and Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Prerna Sinha, Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Carbon Nanofibers as Supercapacitor Electrode Materials . . . . 16.3 Carbon Nanotubes as Supercapacitor Electrode Materials . . . . 16.4 Graphene/Reduced Graphene Oxide as Supercapacitor Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.5 Activated Carbon as Supercapacitor Electrode Materials . . . . . 16.6 Transition Metal Oxides as Supercapacitor Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.7 Conducting Polymers as Supercapacitor Electrode Materials . . . 16.8 Transition Metal Oxides and Carbon Composites as Supercapacitor Electrode Materials . . . . . . . . . . . . . . . . . . . 16.9 Conducting Polymers, Transition Metal Oxides, and Carbon Composites as Supercapacitor Electrode Materials . . . . . . . . . . 16.10 Recent Advancement in Supercapacitor Devices . . . . . . . . . . . 16.10.1 Flexible Fiber-Shaped Devices . . . . . . . . . . . . . . . . . 16.10.2 Planar Micro-Supercapacitor Devices . . . . . . . . . . . . 16.10.3 Flow-Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . 16.10.4 Shape Memory Supercapacitors . . . . . . . . . . . . . . . . 16.10.5 Piezoelectric Supercapacitors . . . . . . . . . . . . . . . . . . 16.10.6 Self-Healing Supercapacitors . . . . . . . . . . . . . . . . . . 16.11 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Applications of Supercapacitors . . . . . . . . . . . . . . . Ravi Nigam, Kapil Dev Verma, Tanvi Pal, and Kamal 17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 17.2 Supercapacitor Market . . . . . . . . . . . . . . . . . . 17.3 Applications of Supercapacitors . . . . . . . . . . . 17.3.1 Wearable Electronics . . . . . . . . . . . . 17.3.2 Portable Electronic Devices . . . . . . .

406 406 416 421 425 429 431 435

436 436 437 438 440 441 442 443 446 448 448 450 451 453 454 455 456 457

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17.3.3 17.3.4 17.3.5 17.3.6 17.3.7 17.3.8 17.3.9

Transportation . . . . . . . . . . . . . . . . . . . . . . . . Industrial Applications . . . . . . . . . . . . . . . . . . Military, Defense, and National Security . . . . . Renewable Energy Sector . . . . . . . . . . . . . . . . Power Electronics . . . . . . . . . . . . . . . . . . . . . . Communication . . . . . . . . . . . . . . . . . . . . . . . Artificial Intelligence, Internet of Things, Cyber-Physical System, Soft Robotics . . . . . . . 17.3.10 Complementary Metal–Oxide–Semiconductor (CMOS), Very Large-Scale Integration (VLSI), Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.3.11 Medical and Health care . . . . . . . . . . . . . . . . . 17.3.12 Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17.3.13 Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 17.3.14 Futuristic Applications . . . . . . . . . . . . . . . . . . 17.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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475 475 477 478 479 479 480

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

Editor and Contributors

About the Editor Prof. Kamal K. Kar, Ph.D. Champa Devi Gangwal Institute Chair Professor; Professor, Department of Mechanical Engineering, Materials Science Programme, Indian Institute of Technology Kanpur, India. Prof. Kar pursued higher studies from the Indian Institute of Technology Kharagpur, India, and Iowa State University, USA, before joining as a Lecturer in the Department of Mechanical Engineering and Interdisciplinary Programme in Materials Science at IIT Kanpur in 2001. He was a BOYSCAST Fellow in the Department of Mechanical Engineering, Massachusetts Institute of Technology, USA, in 2003. He is currently holding the Champa Devi Gangwal Chair Professor of the Institute. Before this, he has also held the Umang Gupta Institute Chair Professor (2015–2018) at IIT Kanpur. He is the former Head of the Interdisciplinary Programme in Materials Science from 2011 to 2014, and Founding Chairman of Indian Society for Advancement of Materials and Process Engineering Kanpur Chapter from 2006 to 2011. Professor Kar is an active researcher in the broad areas of nanostructured carbon materials, nanocomposites, functionally graded materials, nano-polymers, and smart materials for structural, energy, water, and biomedical applications. His research works have been recognized through the office of the Department of Science and Technology, (ii) Ministry of Human Resource and Development, National Leather

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Development Programme, Indian Institute of Technology Kanpur, Defence Research and Development Organisation, Indian Space Research Organization, Department of Atomic Energy, Department of Biotechnology, Council of Scientific and Industrial Research, Aeronautical Development Establishment, Aeronautics Research and Development Board, Defence Materials and Stores Research and Development Establishment, Hindustan Aeronautics Limited Kanpur, Danone research and development department of beverages division France, Indian Science Congress Association, Indian National Academy of Engineering, and many more from India. Professor Kar is the Editor-in-Chief of Polymers and Polymeric Composites: A Reference Series published by Springer Nature, and Members in the Editorial Board of SPE Polymers published by Wiley, Advanced Manufacturing: Polymer and Composites Science published by Taylor & Francis Group, International Journal of Plastics Technology published by Springer Nature and many more. Professor Kar has more than 220 papers in international refereed journals, 135 conference papers, 10 books on nanomaterials and their nanocomposites, 3 special issues on polymer composites, 50 review articles/book chapters, and more than 55 national and international patents to his credits; some of these have over 200 citations. He has guided 18 doctoral students and 80 master students so far. Currently, 17 doctoral students, 10 master students, and few visitors are working in his group, Advanced Nanoengineering Materials Laboratory.

Contributors Soma Banerjee Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Bibekananda De Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

Editor and Contributors

xxiii

Kamal K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Mukesh Kumar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India; A.P.J. Abdul Kalam Technical University, Lucknow, India P. K. Manna Indus Institute of Technology and Management, Kanpur, India Ravi Nigam Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Tanvi Pal A.P.J. Abdul Kalam Technical University, Lucknow, India; Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Prerna Sinha Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Alekha Tyagi Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Kapil Dev Verma Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India

Chapter 1

Introduction to Supercapacitors Prerna Sinha and Kamal K. Kar

Abstract The supercapacitor has emerged as a promising electrochemical energy storage device. Its excellent performance, easy handling, and stability have gained remarkable attention. In comparison with batteries, it delivers high-power density and cyclic stability. This is basically due to its charge storage mechanism, where ions get adsorbed at the electrode surface during charging and get released while discharging. This makes it different from batteries, where repeated redox reactions lead to poor stability and low-power density. Supercapacitor works similarly to the conventional capacitor, where two conductors are separated by a dielectric medium. The capacitance arises from the separation of charges at the conductor surface. In supercapacitor, the conductors have been replaced by the porous electrode, which provides efficient surface areas for the adsorption of ions. Also, the separation between two opposite charges is in the nanometer range, which further contributes to high capacitance than the conventional capacitor. Basically, the supercapacitor is classified by two types of charge storage mechanisms, where pure electrostatic, non-Faradic processes are called electric double-layer capacitor (EDLC). The other includes the Faradaic process, where a reversible redox reaction is involved and known as pseudocapacitor. Carbon-based materials are used as EDLC electrode; whereas, metal oxides and conducting polymers are used as pseudocapacitor electrode material. Further improvement in terms of performance is reported by combining both types of charge storage mechanisms called a hybrid supercapacitor. The phenomena of the charge storage mechanisms in supercapacitors have been discussed in detail. Different components of the supercapacitor and their functions have been briefly introduced in this chapter. P. Sinha · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] P. Sinha e-mail: [email protected] K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_1

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1.1 Introduction In recent years, the fast depletion of non-renewable resources and growing concern toward environmental pollution has motivated the researchers to develop sustainable energy technologies, particularly via solar, wind, waves, etc. Extracting energy from renewable sources leads to minimizing the usage of fossil fuels [1]. From an environmental point of view, gases emitted by the burning of fossils pollute our air and the waste product like fly ash dumped in open atmosphere contaminates water and land [2–4]. Due to these severe concerns, there is a need for the sustainable development of energy from renewable sources. Solar, wind, and waves dominate the significant section; other energy sources are generated by biomass, geothermal, hydropower plant, etc. The main issues related to the effective utilization of renewable sources are its intermittency. The sun does not shine at night, and the wind does not blow 24 h a day. To overcome these limitations, there is a need for efficient energy storage systems that can store a significant amount of energy from renewable sources and release when there is a need. During the energy storage process, one type of energy is converted to another form, which can be stored and transported to the different parts of the country. In this vein, the electrochemical energy storage systems, such as batteries, supercapacitors, and fuel cells come into existence for the development of society [5–9]. In electrochemical energy storage systems and conversion devices, electrons and ions are employed for the storage/release during charge/discharge processes. In these systems, electrical energy is stored by two different mechanisms. (i) chemical energy gets converted into electrical energy via Faradaic redox reaction by electroactive material having dissimilar electrode potential, and (ii) electrostatic energy storage, where two electrodes are separated by dielectric or electrolytic medium [10]. Apart from having applications for storing renewable energy, energy storage systems are widely used in automobiles, houses, gadgets, etc. [11, 12]. These systems are now made portable so that they can be easily carried from one place to another based on the utility. For example, these include lithium-ion batteries present in mobile phones, cameras, etc. At present, technology for various energy storage system comes with significant limitations, which have promoted a new era of research to develop high-performance energy storage devices. The research aims to develop novel material in terms of structures and composition or to assemble different energy storage systems to achieve highly efficient energy storage devices [5, 13, 14]. It is important to know the performance of various energy storage devices that have been compared using the Ragone plot as shown in Fig. 1.1 It relates power density (W kg−1 ) to its achievable energy density (Wh kg−1 ). The overall target of the research is to develop high-energy density at a high-power density device [15]. This means that the energy storage device could store a large amount of energy and can deliver them quickly. Batteries and fuel cells are high-energy density devices, which can store a large amount of energy but their energy delivery time is slow. The

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Fig. 1.1 Ragone plot for different electrochemical energy storage devices. Redrawn and reprinted with permission from [21]

conventional capacitors can deliberately release their charge during the requirement but could not store a large amount of energy. In order to achieve high-energy and power density devices, electrochemical capacitors come into existence [16, 17]. As shown in the Ragone plot also, electrochemical capacitors or supercapacitors can bridge the gap between conventional capacitors and batteries. In terms of specific power and energy density, this device covers the gap in several orders of magnitude [18–20]. Supercapacitors can improve battery performance in terms of power density and enhance the capacitor performance with respect to its energy density [22–25]. They have triggered a growing interest due to their high cyclic stability, high-power density, fast charging, good rate capability, etc. [16]. Their applications include load-leveling systems for string renewable energy storage, hybrid electric vehicles, and storing regenerative braking energy [13, 26]. Supercapacitors work very well when it is integrated with batteries or fuel cells. To achieve high-energy density for hybrid electric vehicles, supercapacitors are combined with fuel cells and batteries. This combination improves the energy density while keeping high its high-power density [27–29]. Although, the widespread use of supercapacitor is being limited and extensive research is underway to achieve high-energy density devices without sacrificing its power density and cycle life [30]. The electrochemical capacitor or supercapacitor consists of two porous electrodes immersed in an electrolyte, which are separated by a separator [28]. When potential is applied, electrolytic ions form a bilayer structure at electrode–electrolyte interfaces [31]. The charges are stored at these interfaces. Based on the charge storage mechanism, a supercapacitor can be classified as an electric double-layer capacitor (EDLC) and pseudocapacitor [16, 32]. In EDLC, the charges are stored by the physisorption of the electrolytic ion at the electrode surface [33]. The formation of a bilayer takes place at electrode–electrolyte interfaces. Since only physical process is involved during the charge/discharge process, this type of capacitor shows extremely

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high cycle life, which is much higher than batteries. Electrode materials for EDLC include allotropes of carbon [34–37]. These various carbon materials display high surface area, distribute pore size, and high electronic and ionic conductivity [38–40]. When potential is applied, negatively charged ions from the electrolyte get adsorbed at positively charged carbon electrode and vice versa. This leads to the formation of a bilayer [41]. The fast ion transportation in the electrolyte delivers high-power density [41–43]. As the charge storage mechanism is limited only to the charged surface, EDLC suffers from low-energy density as compared with batteries [44]. In contrast, pseudocapacitance arises from reversible redox reactions, which are Faradaic in nature [6, 16]. It also stores charges by the formation of an electric bilayer. It delivers high capacitance than EDLC, which arises from (i) reversible redox reaction (ii) adsorption/insertion of electrolytic ions at the electrode surface, and (iii) intercalation of ions [8]. Metal oxides and conducting polymers act as pseudocapacitive material. Due to the involvement of chemical processes during charge/discharge, the pseudocapacitive device suffers from poor cyclic stability and power density [8, 41, 45, 46]. To eliminate the restricted capacitance of EDLC and narrow potential window of pseudocapacitor, a hybrid capacitor comes into existence. This type of capacitor is called a second-generation supercapacitor. These include asymmetric supercapacitor, composite electrode for supercapacitor, and battery-type capacitor [47–49]. Various combinations of EDLC and pseudocapacitive materials are mixed to get highenergy and power density device, along with good cycle life [23, 50–53]. The hybrid capacitor provides the synergistic effect of EDLC (which improves conductivity and stability) and pseudocapacitor (which provides capacitance), which increases the overall performance of the device [15, 31, 54, 55]. During the charge storage process, two main factors determine the performance of the device. (i) electrochemical activity and (ii) kinetic feature of electrode and electrolyte ions. Electrochemical activity depends upon the type of electrode material, which participates in the charge storing process. If the material is highly porous or contains a large amount of redox-active site, its electrochemical activity will be higher [32, 54]. This results in a high amount of charge storage, which increases the energy density of the device. So, optimization in structure and surface feature is important to increase the electrochemical activity of the electrode. Enhancement in kinetic feature in ions and electrons transportation increases the rate capability and power density of the devices. The conductivity of the electrode and the type of electrolytic ion decide the kinetic nature [13, 27, 56].

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This chapter deals with the basic overview of the supercapacitor, its history and evolution from Leyden Jar to ultracapacitor, and comparison from different electrochemical energy storage devices to understand the importance of supercapacitor. The discussion is further carried out by describing a different component of the supercapacitor, its function, and contribution toward the performance of supercapacitor. At last, the chapter describes different types of supercapacitors based on the charge storage mechanism. The chapter also introduces a second-generation supercapacitor also called hybrid supercapacitor.

1.2 Brief Overview on History of Supercapacitors The development of a supercapacitor as an energy storage device is the story of the charge storage mechanism. Figure 1.2 shows the historic timeline for the evolution of the supercapacitor. 1746 Leyden jar was the first capacitor, which was invented by German cleric Ewald Georg von Kleist and a Dutch scientist Pieter van Musschenbroek. It consists of two metal foil pieces, water, and a conductive chain inside a glass jar. It produces static electricity during rotation of glass jar. This develops the concept of storing static charges at the interface of the electrode surface and electrolyte [13, 15, 17, 28]. 1853 Helmholtz first studied the electrical charge storage mechanism in a capacitor. He developed the first electric double model using a colloidal solution. The behavior of static electricity was poorly understood before Helmholtz’s explanation [13, 15, 17, 28]. Early twentieth century Gouy, Chapman, Stern, and Grahame described the modern theory for the interaction of electrolytic ions at electrode–electrolyte interfaces forming double layer [13, 15, 17, 28]. 1954 The first electrochemical capacitor got patent by H. I. Becker at General Electric. The patent describes the energy storage device consisting of porous carbon electrodes, which are immersed in aqueous electrolyte. This patent never gets commercialized because of its lower operating potential window [13, 15, 17, 28]. 1969 Robert Rightmire at Standard Oil Co. of Ohio (SOHIO) invented the first nonaqueous electrolyte-based electrochemical capacitor. This patent describes that the device provides a very high working potential window of 3.4–4 V [13, 15, 17, 28]. 1978 SOHIO patent got commercialized with the name of “Supercapacitor” by a Japanese company, Nippon Electric Corporation (NEC). The product got successfully commercialized and found application for backup power for clock chips [13, 15, 17, 28].

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Fig. 1.2 Historic timeline for the development of supercapacitors. Redrawn and reprinted with permission from [17]

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1971 Pseudocapacative material RuO2 was discovered. This was the new class of material other than carbon. Their charge storage mechanism involves Faradaic reactions. Although, the cyclic voltammamogram curves of RuO2 show rectangular behavior, which indicates capacitive behavior. This discovery boosts the charge storage capability of the electrochemical capacitor [13, 15, 17, 28]. 1980 To develop high-performance supercapacitor, ruthenium and tantalum oxide were used as electrode material besides porous carbon. This project was started by Pinnacle Research Institute (PRI), and the capacitor was named PRI Ultracapacitor. Although the performance is good, due to the high cost of electrode material, the price of the device increases. This limits its commercialization and was used for military applications [13, 15, 17, 28]. 1989 US Department of Energy (DOE) promotes long-term research for supercapacitors, which aims to provide a high-energy density device. This project was later collaborated by Maxwell Technologies Inc. to develop a high-performance device. After this approach, large varieties of supercapacitors were introduced such as EDLC, pseudocapacitor, and asymmetric supercapacitor. Each supercapacitor was assembled to address a particular requirement. Many other companies such as ELTON (Russia), CAP-XX (Australia), Nippon Chemicon (Japan), and Nesscap (Korea) have been developing different types of supercapacitors to meet various requirements [13, 15, 17, 28]. Since 2000 The amount of research in the field of supercapacitor has been increased a few times according to the increasing demand for energy storage in society. Supercapacitors have the potential to deliver high-power density, good rate capability, and long cycle life, which makes it an essential field of research. Also, compared to other electrochemical energy storage devices, it is safe to handle. The charge storage mechanism of various supercapacitor devices clearly demands more research in this field [17]. But, to date, research in the field of supercapacitor has dramatically increased. These include thermal self-protection supercapacitor, piezoelectric supercapacitor, electrochromic supercapacitor, metal ion hybrid supercapacitor, self-healing supercapacitor, shape memory supercapacitor, micro-supercapacitor, flow supercapacitor, etc. [14, 27].

1.3 Comparison Among Various Electrochemical Energy Storage Devices Supercapacitor, battery, and fuel cell work on the principle of electrochemical energy conversion, where energy transformation takes place from chemical to electrical energy. Despite of different energy storage systems, they have electrochemical similarities. Figure 1.3 shows the schematic diagram of battery, fuel cell, conventional

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capacitor, and supercapacitor. The energy storage process is carried out at electrode–electrolyte interfaces, where electrons and ions get separated [57]. The electrochemical system consists of two electrodes immersed in the electrolytic solution and separated by a membrane called a separator. Batteries and fuel cells store energy by conversion of chemical energy into electrical energy. At the anode, reactions take place at lower electrode potential than the cathode. For supercapacitors, energy is stored electrostatically. It does not undergo Faradaic reactions. The electrolyte ions get polarized via application of potential and form bilayer at the electrode–electrolyte interface. In comparison, to fuel cell and supercapacitor, batteries have been commercially used in most of the applications. Fuel cells have been used only for space applications. Due to its high cost and poor power and energy performance and poor durability, it cannot act as strong competitors for other devices. Till now, Fig. 1.3 Schematic representation of a a battery (Daniell cell) [58], b a fuel cell [59] , c a conventional capacitor showing two oppositely charged conducting plate separated by dielectric medium, and d electrochemical capacitor (supercapacitor), illustrating double-layer formation at the electrode surface [58]. Redrawn and reprinted with permission from [58]

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Fig. 1.3 (continued)

the supercapacitor has found its limited application as memory protection in electronic gadgets. The modern research and availability of hybrid materials aim to overcome the shortcomings of supercapacitors. Supercapacitor and energy storage devices present a new breed of technology that can store a large amount of energy than conventional capacitors and are able to deliver higher charge/discharge rate capability than fuel cells and batteries [28, 58].

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1.3.1 Batteries Versus Supercapacitors Batteries undergo chemical reactions to store energy. These chemical processes result in varieties of resistive losses, which occur from the movement of sluggish electrons. This upshot shows in heat generation when operated at high power that leads to serious safety issues [17, 57]. Although, it provides high-energy density and have been widely used but lacks power density. This limits its application, where instant power burst is required. The supercapacitor can complement or even replace batteries in some applications. When integrated with batteries, it can provide rapid charging and high-power density having a long cycle life. Supercapacitor delivers high-power density because the charges get stored electrostatically. The cyclic stability of the supercapacitor is much higher than batteries as it does not undergo any chemical changes during charge/discharge processes. Since charge storage phenomena are limited to the electrode surface availability, it delivers poor energy density [5, 17, 58].

1.3.2 Fuel Cells Versus Supercapacitors Fuel cells are the primary energy conversion device that provides uninterrupted electrical energy for power generation. It has a high-energy density and eliminates environment-friendly by-products. It is safe with low-maintenance costs [59]. Fuel cell finds a major limitation because of its high cost and complex operation. It is very sensitive toward contamination, which degrades cell life. Also, power density per unit volume is low, which provides poor efficiency. On the other hand, a supercapacitor provides energy storage by physical adsorption. Generally, the carbon-based electrode material is used, which is safe for the environment. The device fabrication is simple and cheaper than fuel cells. Since no chemical reactions undergo during the charge/discharge process, supercapacitor shows high cycle life and power density [58].

1.3.3 Conventional Capacitors Versus Supercapacitors The conventional capacitor also called electrostatic capacitor has been limited to lowpower application or short-term memory backup supply due to their low capacitance value. An electrostatic capacitor is made up of two conducting metal electrodes separated by non-conducting material, which acts as a dielectric medium [60]. The operating voltage depends on the strength of dielectric material, and the capacitance of the assembly is expressed as [17] follows: C=

Q V

(1.1)

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where C is the capacitance of the device, Q is the amount of charge stored, and V is the operating voltage. Also, the capacitance depends upon the area, A of conducting electrodes separated by distance d, and ε is the relative permittivity. Equation (1.2) can be expressed as follows: C =ε

A d

(1.2)

Equation (1.2) suggests that capacitance increases on increasing the area of the electrode and decreasing the distance of charge separation. The electric double-layer capacitor shares a similar charge storage mechanism with a dielectric capacitor. On the application of voltage, the polarization of electrolytic ions occurs, which acts as dielectric material in supercapacitor. Supercapacitors show greater capacitance as compared to electrostatic capacitors. This is due to the porous behavior of electrode material, which allows a large number of electrolyte ions to get adsorbed at the surfaces. The compact bilayer is called the Helmholtz layer having thickness ~1 nm. At the macroscopic level, according to (1.2), high surface area (A) of the electrode material and atomic range separation d between electronic and electrolyte ion charge at the electrode surface deliver higher capacitance value than the conventional electrostatic capacitor. The high surface area enables a large amount of charge to get stored, which increases its energy density over the electrostatic capacitor. The charge and discharge process is purely electrostatic, no chemical reactions are involved, and it delivers high-power density [22]. A detailed characterization of the conventional capacitor is reported elsewhere [61]; whereas, various features of supercapacitors are reported by Banerjee et al. [62].

1.4 Components of Supercapacitors Supercapacitor consists of four components, each having their role. The four components are (i) (ii) (iii) (iv)

electrode, electrolyte, separator, and current collector.

Electrode stores charge by capacitive or Faradaic mechanism by the interaction of electrolyte ions. Electrolyte provides necessary ions to carry out the charge/discharge process. Separator acts as a barrier, which prevents the device from short circuits. The current collector provides a conducting pathway for the transportation of electrons from the electrode to the external circuit. Each component is discussed briefly as follows.

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1.4.1 Electrode Electrode plays an important role in determining the performance of the cell. It stores charges when potential is applied [63]. It should have high conductivity, which facilitates ease in the transportation of electron toward the external circuit. The essential criteria for the ideal electrode are as follows [48, 54, 64]: • High specific surface area: The electrolytic ions interact with the electrode surface. High surface area exposes more amount of electrode material for electrolyte ion. This enhances the specific capacitance and energy density of the electrode material. • Controlled porosity: It affects the specific capacitance and rate capability of the electrode material. Pore size should be greater than electrolytic ions to get adsorbed at the electrode surface. • High electronic conductivity: It determines the rate capability and power density of the material. High conductivity minimizes the resistance and provides ease in electron to transfer from electrode to current collector. • Surface electroactive sites: Electroactive sites at the surface attract the electrolytic ions and promote pseudocapacitance. Many electroactive species such as oxygen and nitrogen functional group undergo pseudocapacitance and enhances the conductivity of the electrode material. • High thermal and chemical stability: During repeated charge/discharge processes, ion movements are involved, which can increase the temperature of the device. Also, the electrode should be resistant to chemical and corrosion; this improves the stability of the electrode material. • Low cost and environment-friendly: Low cost of electrode reduces the overall price of the supercapacitor device. Electrode with environment-friendly material provides a sustainable approach. Based on the charge storage mechanism in supercapacitors, the electrode material can be classified into two parts: (a) EDLC-type electrode, which stores charges electrostatically at the electrode– electrolyte interface and (b) pseudocapacitive-type electrode, which stores charge by Faradaic processes. Carbon-based materials exhibit EDLC-type behavior, where the non-Faradaic process is involved and charges are stored at the electrode surface. These materials have a high surface area and tunable pore size, which are the ideal material for the EDLC electrode [65]. Nanostructured materials such as carbon nanotube [66], carbon fiber, carbon nanofiber [67], activated carbons [68], and graphene [69] show high conductivity, which minimizes the resistance of the device. Carbon allotropes are corrosion resistance and environment-friendly [31, 70–72]. On the other hand, metal oxide and conducting polymers show a pseudocapacitive-type charge storage mechanism. This process is Faradaic nature, where reversible redox reaction occurs at the electrode. Since metal oxide can exist in a variable oxidation state, it can

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gain/release electrons during the charge/discharge process. Conducting polymer also shows Faradaic behavior by doping and de-doping during charging and discharging. Since the whole material is involved in the charge storage process, it shows extremely high capacitance value that increases the energy density of the material [6, 48, 73]. Various features of metal oxides used as electrode materials for supercapacitors are reported elsewhere [74]. Electrode materials used in supercapacitors have some kind of limitations. • Carbon allotropes have high conductivity, stability, and surface area but lack a high capacitance value. • For metal oxides, it shows high capacitance but fails in its stability. Also, metal oxide except RuO2 shows poor conductivity. • Conducting polymer degrades after a few cycles of the charge/discharge process due to swelling and shrinking. • To eliminate these drawbacks, composite materials have been developed by mixing EDLC and pseudocapacitive material. EDLC electrode provides conductivity and stability, and pseudocapacitive material undergoes redox reaction at its surface. This approach has the potential to provide high power as well as highenergy density along with enhanced rate capability and cyclic stability [15, 48, 75]. A detailed characterization of various electrode materials is reported elsewhere [76].

1.4.2 Electrolyte The electrolyte is one of the essential components of electrochemical energy storage devices. Their physical and chemical properties play an important role in determining the efficiency and performance of the cell. It affects the capacitance, energy and power density, rate capability, cycle life, and safety of the device. It balances the charges between two electrodes. The choice of electrolytes is an important factor as it significantly influences the electrode–electrolyte interfaces. To date, no perfect electrolyte can meet all the requirements for supercapacitor devices. The basic requirements of electrolytes for electrochemical devices are as follows [34, 41]: • Electrolyte conductivity: To achieve a high-performance device, the conductivity of electrolyte should be high. The conductivity of the electrolyte depends upon the (a) mobility of ions, (b) concentration of ions, (c) elementary charge, and (d) magnitude of the valence of mobile ion charges. • Salt effect: Conductivity of electrolyte can be different for different solvents. Also, conductivity varies for salt concentration in the same solvent. The number of free ions determines the ionic conductivity, so optimum conductivity can increase the ionic conductivity of the electrolyte. • Solvent effect: The constituent of the solvent greatly affects the conductivity of the electrolyte. The main properties, which influence the conductivity of the electrolyte, are viscosity and dielectric constant. The dissociation of salt and viscosity

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affect the ionic mobility, which depends upon the nature of the dielectric. So, a suitable solvent for supercapacitor should have low viscosity and high dielectric constant. • Electrochemical stability: It is related to device stability and safety. The electrochemical stability depends upon (a) the interaction of electrolyte with electrode material and (b) constituent elements present in the electrolyte. • Thermal stability: For high-temperature working of the device, the thermal stability of electrolytes is important. Also, during repeated charge/discharge cycles, electrolytes should be stable by the heat release. The thermal stability mainly depends upon the composition of electrolytes such as salt, solvent, and additives. Generally, electrolytes for electrochemical devices can be categorized as (a) aqueous electrolyte, (b) organic electrolyte, and (c) ionic electrolyte. Among all, aqueous electrolyte shows high conductivity and capacitance at low cost. Although its operating voltage window is small, it leads to low-energy density. This is due to the electrolysis of water having a potential difference of 1.23 V. The organic and ionic electrolytes can be operated at a higher potential window, but these electrolytes suffer from lower ionic conductivity. The organic electrolyte is toxic and suffers from handling issues. Ionic electrolytes are expensive and can access comparatively lower electrode surface due to the higher ions size [41]. Various features of electrolyte materials are reported elsewhere [77].

1.4.3 Separator The assembly of a supercapacitor requires a separator, which is used to separate two electrodes from each other forming a barrier. The essential requirement of a separator is to prevent the device from short circuit. In a supercapacitor, it allows smooth transportation of ions without undergoing any chemical changes. So, the selection of separators is an essential requirement for the proper functioning of the device. Some of the essential properties of separator are as follows [78]: • • • •

It should be non-conductive. It should have low ionic resistance with electrolyte ion permeability. It should possess ease in wetting by the electrolyte. It should provide mechanical support to the cell.

Materials such as glass, paper, and ceramics can be used as separators. Polymerbased separators are commonly used in supercapacitors due to their low cost, porous nature, and flexibility [55]. Basic requirements of separator materials are reported elsewhere [79].

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1.4.4 Current Collector The function of the current collector is to transport electrons from the device to the external circuit. It should have high electrical conductivity to promote resistancefree transport of electrons from the electrode to the external circuit. Since varieties of electrode and electrolyte material are used in supercapacitor devices, the current collector should be corrosion resistant. It should also have good mechanical strength as it provides mechanical support to the whole cell. Generally, aluminum, iron, and steel alloys are used as current collector material. To minimize contact resistance, the active material is coated over the current collector. Nowadays, nickel mesh, metal foams, and carbon cloth are used, which provide efficient surface areas for electrode material. This not only reduces the contact resistance but enables uniform dispersion of electrode [55]. A detailed characterization of various current collectors is reported elsewhere [80].

1.5 Classification of Supercapacitors Based on the Charge Storage Mechanism 1.5.1 Electric Double-Layer Capacitor (EDLC) (Capacitive Mechanism) The electric double layer is a capacitive mechanism, which is seen in a device due to the application of electrostatic force. This bilayer formation is observed when the electronic conducting electrode material is immersed in an ion-conductive electrolyte. Arrangement of charges is noticed at the electrode–electrolyte interfaces, which is also called bilayer formation. The capacitance arises from the electrode potential-dependent accumulation of electrostatic charges at the interface. The most significant feature of EDLC is that no charge transfers occur between electrode and electrolyte interfaces [39]. This process in non-Faradaic in nature. The specific capacitance strongly depends upon the accessible surface area of the electrode material and surface properties of the electrode. Generally, the carbon-based material shows EDL-type capacitive mechanism [17, 81, 82]. Figure 1.4 displays the electrical bilayer formation at the electrode surface and electrolytic ions counterbalance the charges to achieve electroneutrality. During the charging process, electrons are transferred from a negative electrode to a positive electrode via an external circuit. During this process, cations (positive ions) move toward the cathode (negative electrode) in the electrolyte, while anions move toward the positive electrode [18, 28]. During the discharge process, the reverse process takes place. No charge transfer occurs during charging and discharging. This shows that the electrolytic ion concentration remains constant. The capacitance arises from the physical adsorption of electrolyte ions at the electrode surface [48]. The whole

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Fig. 1.4 Schematic representation of the electric double-layer capacitor using a porous carbon electrode and illustration of the potential drop at the electrode–electrolyte interface. Redrawn and reprinted with permission from [18]

charging and discharging mechanism of EDLC electrode material can be expressed by the following equations. The two electrode surfaces are expressed as S1 and S2, anions are expressed as A− , cations are expressed as C+ , and electrode–electrolyte interface is depicted as follows [48]. On the positive electrode  charging S1 + A− −−−−→ S1+ A− + e−

(1.3)

 discharging S1+ A− + e− −−−−−→ S1 + A−

(1.4)

On the negative electrode

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 charging S2 + C+ + e− −−−−→ S2− C+

(1.5)

 discharging  S2− C+ −−−−−→ S2 + C+ + e−

(1.6)

The overall electrochemical processes can be summarized as follows:  charging − S1 + S2 + A− + C+ −−−−→ S1+ A− + S2 C+

(1.7)

  discharging − S1+ A− + S2 C+ −−−−−→ S1 + A− + C+

(1.8)

The formation of bilayer at the interface and interaction of electrolytic ions at the electrode surface is explained by several theories and models. Figure 1.5 describes three models, i.e., the Helmholtz models, the Gouy–Chapman model, and the Stern model [70]. • Helmholtz model This model explains the simplest theory for the spatial distribution of charges at the bilayer interfaces. The theory considers a rigid layer of charges, which is formed to

Fig. 1.5 Schematic of a the Helmholtz model, b the Gouy–Chapman model, and c the Stern model, at a positively charged surface (ψ denotes the potential, and ψo is the electrode potential). Redrawn and reprinted with permission from [81]

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counterbalance the charges from the electrode surface. Figure 1.5a shows a schematic of the Helmholtz model, in which distance d is the distance between the layer of two opposite charges. Although this model is the simplest, it does not explain the other phenomena of double-layer formation [81]. • Gouy–Chapman or diffuse model This model explains that ionic charges present in the electrolyte surrounding the charged particles but they are not rigidly attached to the charged surface. This theory contradicts the Helmholtz theory, which explains that electrolytic ions are rigidly attached to a charged surface at distance d. The electrolytic ions diffuse over a distance into the liquid phase forming a diffuse layer. The kinetic energy of the ions partially determines the thickness of the diffuse layer. Gouy–Chapman’s theory for doublelayer formation follows Boltzmann distribution for ions, where ion concentration plays an important role to explain the thickness of the diffuse layer. Figure 1.5b shows a schematic of the Gouy–Chapman model. For a highly charged double layer, this model fails to explain the charge accumulation [81]. • Stern modification of diffuse double layer The Gouy–Chapman model assumes that the ions are point charge and can approach to the charged surface with no limits. Stern modified this assumption by assuming that ions possess finite size. So, ions can approach the charged surface having some distance. The Stern model depicts that there is surface adsorbed ion in the plane having distance δ, where δ is the distance of the first layer of ions from the surface. This layer is called the Stern layer. This layer consists of electrolytic ions, which strongly get adsorbed at the electrode surface resulting in the formation of a compact layer called the inner Helmholtz plane (IHP). After this layer, a poorly adsorbed counter ions form the outer Helmholtz plane (OHP), as shown in Fig. 1.5c. ψ denotes the potential, and ψo is the electrode potential. The Stern model explains a better approach to the reality that the other two models do. The above model provides a satisfactory explanation for the double-layer formation on the plane surfaces. These models have a major shortcoming to describe real charge distribution in a hierarchical pore structure containing pores in multiple sizes and shapes. The pore size greatly influences the mobility of ions, where very small pore size makes it inaccessible for ions to get adsorbed, hence not contributing to the double-layer formation. This suggests that there is no linear relation between the capacitance of the material and its specific surface area. Generally, the surface area of the electrode material is measured by the small gas molecules such as nitrogen, argon, and helium, whose size is smaller than electrolytic ions. So, the process of charge/discharge in an electric double-layer capacitor involves rearrangement of ions at the electrode surface, without undergoing any Faradaic reaction. This makes it highly reversible having extremely large cyclic stability [81].

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1.5.2 Pseudocapacitor (Faradaic Mechanism) Pseudocapacitor stores charge by Faradaic processes. On the application of a potential to the electrode, the material undergoes fast and highly reversible redox reactions. The charge storage mechanism is based on the change in the valance state of the electrode material, which results in electron transfer [17]. The invention of pseudocapacitance behavior leads to a new diverse approach, which enhances the charge accumulation behavior and charge storage capacity of supercapacitors. The electrical response of pseudocapacitive-type material is almost similar to double-layer capacitive material. This means that the state of charge changes continuously with potential, resulting in potential constant, which is considered as capacitance. Pseudocapacitance is used to those types of electrode material, whose electrochemical response is capacitive but charges are getting stored by charge transfer during redox reactions across the bilayer formation. The process undergoes Faradaic behavior, which is due to the fast and reversible surface redox reactions, but capacitance arises from the linear relation between charge storage and potential change. There are various Faradaic mechanisms, which can result in different electrochemical capacitive features. These are as follows and also shown in Fig. 1.6.

Fig. 1.6 Schematic representation of different pseudocapacitive charge storage mechanisms a underpotential deposition, b redox pseudocapacitor, and c ion intercalation pseudocapacitor. Redrawn and reprinted with permission from [6, 17]

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underpotential deposition, redox pseudocapacitor, intercalation pseudocapacitance, reversible doping and de-doping in conducting polymers.

Underpotential deposition has happened when electrode metal cations form a monolayer of adsorbed ions above their redox potential [6]. This process can be described as follows: M + xCz+ + x ze− ↔ C.M

(1.9)

Here, C is the adsorbed atoms, M is the metal, x is the number of adsorbed atoms, and z is the valence of the adsorbed atom. So, zx is the number of transferred electrons. Pseudocapacitance based on redox reactions utilizes the multiple valence states of metal oxides. The electrons are transferred between oxidized and reduced species. The redox reactions can be described as electrochemical adsorption of cations on the surface of oxidized species. These reactions accompany fast and reversible electron transfer across the electrode–electrolyte interfaces. The redox reaction can be explained as follows: Ox + zC+ + ze− ↔ RedCz

(1.10)

Here, C is the surface adsorbed electrolyte cation C+ , and z is the number of transferred electrons. The third type of pseudocapacitance can be observed as the insertion of the ion into layered crystalline materials. Intercalation changes the metal valances to maintain electric neutrality. The intercalation reaction can be described as follows: MA y + xLi+ + xe− ↔ LixMA y

(1.11)

MAy is the layer lattice intercalation host material, and x is the number of transferred electrons. Since batteries also undergo intercalation–deintercalation during the charge/discharge process, there are several characteristics by which pseudocapacitor differs from batteries. Ion insertion in pseudocapacitor undergoes • • • •

fast ion transportation kinetics, high rate capability, short charge time, and long cycle life.

Whereas in batteries, these processes are hindered by chemical reactions and slow diffusion, leading to lower power density. Ion insertion characteristics in pseudocapacitor material can be classified into two categories, i.e., intrinsic and extrinsic materials. The charge storage behavior of the intrinsic pseudocapacitor does not depend on crystalline grain size and morphology. For extrinsic pseudocapacitor,

1 Introduction to Supercapacitors

21

it uses battery-type material, which shows battery-type behavior in bulk, but on decreasing the particle size to nano-level, these materials demonstrate pseudocapacitive behavior. So, depending on the particle size, extrinsic material can exhibit battery type or pseudocapacitive charge storage mechanism [50]. The overall process of ion intercalation can be classified into three parts as follows: (i)

Faradaic contributions from the bulk solid-state diffusion dominated by ion intercalation, (ii) fast ion diffusion dynamics by Faradaic charge transfer process at the surface of active material, and (iii) electrostatic adsorption and desorption of ions depicting non-Faradaic EDLC contribution [17]. The charge storage mechanism of conducting polymers is via a redox reaction. During the oxidation reaction, ions get transferred to the polymer backbone. When the reduction process occurs, ions get released from the polymer backbone into the electrolyte. The capacitance value achieved is much higher than other electrode material, because redox reaction occurs throughout the material, not just concentrates on the surface. Charging and discharging in conducting polymer do not involve any structural changes, and these processes are highly reversible, which increase the rate capability of the device [29]. Depending on the type of ion insertion, conducting polymers can be charged positively or negatively to make electroneutrality. The conductivity of conducting polymer can be altered during reduction and oxidation processes, which generates delocalized n electrons into the polymeric chain. During the oxidation process, polymers get positively charged and termed as p-type, while negatively charged polymers are prepared via reduction reactions and termed as n-type [22, 29, 83, 84]. Equations (1.12) and (1.13) show the oxidation and reduction of conducting polymers [29]. Detailed features of conducting polymers used as electrode material for supercapacitors are reported elsewhere [85] De - doping

Oxidation p - doping

De - doping

Reduction p - doping

(CP)− C+ −−−−−→ CP −−−−−−−−−−→ (CP)+ A− (CP)+ A− −−−−−→ CP −−−−−−−−−−→ (CP)− C+

(1.12) (1.13)

Here, CP is the conducting polymer, C+ is the cation, and A− is the anion. Generally, metal oxides such as RuO2 and MnO2 and conducting polymers such as polyaniline and polypyrrole are used as pseudocapacitive material. Intercalationtype pseudocapacitive material includes LiCoO2 , V2 O5 , etc. Some material such as functionalized porous carbon shows both EDL and pseudocapacitive type of charge storage mechanism. Pseudocapacitive behavior in carbon arises from the presence of heteroatoms such as nitrogen, oxygen, sulfur, and boron functionalities [72, 86]. The combination of both processes increases the specific capacitance of the electrode. Pseudocapacitive material shows very high capacitance value compared to the EDL-type capacitor. This enables them to be used for devices having high-energy

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density. Although, these materials lack cycle stability and material swelling as chemical reactions are involved during the charge/discharge process.

1.5.3 Hybrid Supercapacitor (Capacitive and Faradaic Mechanism) Hybrid supercapacitor combines capacitive and Faradaic types of charge storage mechanisms to achieve high-energy density supercapacitor without compromising its power density, rate capability, and cycle stability. Hybrid supercapacitor consists of a polarizable electrode and non-polarizable, redox electrode to store charges. Carbon-based material provides high conductivity, high surface area, and material stability, and redox-active material, such as metal oxides and electronically conducting polymer, provides a large number of electrons, which increases the capacitance. The contribution of both types of charge storage mechanisms results in a high-performance device having high rate capability, high-energy and power density, and long cycle life [55]. Various combinations of materials are available, which shows a hybrid-type charge storage mechanism as shown in Fig. 1.7. These combinations are classified into three main categories and described in the next section.

1.5.3.1

Asymmetric Supercapacitor

The asymmetric assembly of a supercapacitor is a novel approach to address the challenge of relatively low-energy density by widening the working potential window of the device. It offers a wide range of device configurations. During charge and discharge process, asymmetric supercapacitor takes full advantage of the different working potential window of both electrodes, as shown in Fig. 1.7a. This allows the asymmetric supercapacitor to work in the greater potential window. It improves the overall cell voltage, power, and energy density of the device. This assembly shows both electrostatic and Faradaic types of charge storage mechanisms occurring simultaneously at two different electrodes. For both types of charge storage mechanism, high specific surface area, controlled pore size, and high electronic conductivity are essential criteria to achieve high-performance device [48]. The advantage can be seen in the aqueous electrolyte, where working potential cannot exceed the decomposition voltage of water. The operating voltage in the asymmetric system can exceed beyond 2 V [87, 88]. The energy density of the device can be expressed as follows: ED =

1 CV2 2

(1.14)

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Fig. 1.7 Schematic diagram of hybrid supercapacitor undergoing capacitive and Faradaic types charge storage mechanism. a asymmetric supercapacitor, b composite-type symmetric supercapacitor, and c battery-type supercapacitor

where C is the capacitance of the device, and V is the operating voltage window for the cell. According to (1.14), energy density is proportional to capacitance and square of the voltage, where a twofold increase in voltage increases the energy density to fourfold for the same value of capacitance [17]. The proper design of asymmetric supercapacitors may yield a high-energy density device without compromising its power density and cyclic stability.

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1.5.3.2

P. Sinha and K. K. Kar

Composite Electrode-Based Supercapacitor

The composite electrode shows synergistic characteristics of both EDLC and pseudocapacitive types of electrode material. These materials aim to overcome the shortcoming of the individual type of the electrode material. It is well known that carbon material shows high conductivity and cyclic stability [89]. Also, its rate capability and power density are high because of adsorption and desorption at the electrode surface. Since the charge storage mechanism is limited as it is observed only on the surface, carbon materials show low specific capacitance value leading to low-energy density. On the other hand, metal oxide and conducting polymer are pseudocapacitive material, which undergoes Faradaic redox reaction under the application of potential. These materials deliver very high specific capacitance, which provides high-energy density. Pseudocapacitive material undergoes chemical processes, which results in poor cyclic life. Also, most of the metal oxide has a poor electrical conductivity that increases the equivalent series resistance of the device. RuO2 shows good conductivity, but its material cost is very high. In the case of conducting polymers, stability is the major issue, since this polymer shows shrinking and swelling phenomena after a repeated charge/discharge cycle. To address the above-stated limitations, the composite electrode is reported by using EDLC and pseudocapacitive-type material. This approach finds its way to address the limitations of supercapacitor by increasing energy and power density along with having a large cycle life. Carbon provides mechanical support and stability, whereas metal oxides or conducting polymer undergo Faradaic reactions during the charge/discharge process. Figure 1.7b shows the charge/discharge mechanism of a symmetric composite electrode-based supercapacitor. The electrons from metal oxide get conductive pathways by carbon material, which minimizes charge transfer resistance of the electrode and provides ease in movement of ions and electrons. Composite electrode material can be assembled as a symmetric or asymmetric-type supercapacitor [31, 32, 90].

1.5.3.3

Battery-Type Hybrid Supercapacitor

Battery and supercapacitor-type hybrid electrode material shows a facile approach to obtain the high-energy density of battery and high-power density of supercapacitor. It consists of two different electrodes as shown in Fig. 1.7c. One electrode stores charge by battery-type Faradaic process, and another electrode stores charge by adsorption of ions at the electrode surface. Battery-type electrode stores charge by intercalation of electrolytic ions, which provide high capacitance and high-energy density to the device. By this process, the detrimental effect of current fluctuations on the battery is reduced, and its cycle stability is increased [52, 90].

1 Introduction to Supercapacitors

25

1.6 Concluding Remarks Supercapacitors represent one of the promising electrochemical energy storage systems. It has the potential to bridge the gap between conventional capacitors and batteries. It delivers high-energy density than conventional capacitors and high-power density than batteries. Supercapacitor shows unique electrochemical characteristics of short charge/discharge time, high rate capability, and wide operating temperature range. This is due to its charge storage mechanism, where electrolytic ions are adsorbed at the electrode surface defined as an electric double-layer capacitor This provides high cyclic stability as no chemical reactions are involved. Supercapacitor shows another type of charge storage, where reversible redox reactions are involved, commonly known as pseudocapacitance. This improves the capacitance and energy density of the device. To further enhance the performance of the device, various combinations of Faradaic and non-Faradaic electrode materials are used. It is called a hybrid capacitor. The hybrid capacitor includes an asymmetric capacitor, composite electrode, and battery-type capacitor. This provides a synergetic approach to optimize efficiency. A wide array of material and architecture are available for supercapacitor devices, which are suitable for a particular target application. Yet a significant amount of research is carried out for the development of a supercapacitor having a high-energy density as batteries without compromising other properties. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India, (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 2

Materials for Supercapacitors Mukesh Kumar, Prerna Sinha, Tanvi Pal, and Kamal K. Kar

Abstract Considerable efforts have been given on the advancement of the electrical energy storage system. Supercapacitor-an electrochemical energy storage device gains significant interest over batteries and fuel cells because of its high-power density, excellent cyclic stability, and easy handling. However, the poor energy density of supercapacitors has accelerated the research to explore different types of material for the betterment of the device. Various combinations of materials used for the electrode, electrolyte, separator, and current collector are developed for the supercapacitors to achieve good performance. The choice of electrolyte influences the working electrochemical potential window of the device. Different types of electrolytes such as aqueous, organic, and ionic liquid have been discussed with their merits and demerits. Among all other components of the supercapacitor, the choice of electrode material mainly determines the electrochemical behavior of the device. In this vein, various types of material ranging from carbon-based electric doublelayer capacitor electrode to transition metal oxide and conducting polymer-based pseudocapacitor electrode materials are discussed in detail. Apart from commonly studied material, few recent emerging electrode materials such as metal-organic M. Kumar · P. Sinha · T. Pal · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] M. Kumar e-mail: [email protected] P. Sinha e-mail: [email protected] T. Pal e-mail: [email protected] M. Kumar · T. Pal A.P.J. Abdul, Kalam Technical University, Lucknow 226031, India K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_2

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framework and MXene have been also discussed, considering the potential application soon. It also discusses the various types of current collectors and separators used in supercapacitors.

2.1 Introduction Fossil fuel is the main source of current energy generation material. The extensive use of fossil fuel generates greenhouse gases, i.e., carbon dioxide (CO2 ), methane (CH4 ), nitrous oxide (N2 O), etc., which badly affect the environment. This has forced scientists to think for renewable energy as an alternative energy source [1]. Renewable energy is a promising solution that can fulfill the requirement of energy and power [2]. All renewable energy sources have some inherent limitations such as solar energy produces power in a day, wind energy makes power in a particular region and circumstances. It is not possible to use all generated energy continuously as there is a large fluctuation in the generation of energy from renewable energy sources. In this vein, there must be a need to develop energy storage devices that can store energy from renewable sources and then deliver the energy when there is a requirement from the users. There are different types of energy storage devices among which electrochemical energy storage devices such as the battery, fuel cell, and supercapacitor are sustainable and do not harm the environment [3]. Performance of different types of electrical energy storage devices in term of power density and energy density are shown in Fig. 2.1. This indicates that batteries have higher energy density (lithium-ion battery >180 Wh kg−1 ) Battery, whereas capacitor and supercapacitor have high power density (metal-organic frameworks 800 W kg−1 ) [4]. An electrochemical supercapacitor is preferred over the battery in an application, where high power density is required. Herein, the current chapter describes the material used for the supercapacitor in detail. The supercapacitor is classified into three groups on the basis charge-discharge mechanisms such as electric double-layer capacitor (EDLC), pseudocapacitor, and Fig. 2.1 Ragone plot of energy density and power density in energy storage devices. Redrawn and reprinted with permission from [4]

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Fig. 2.2 Classification of supercapacitor and materials used in supercapacitor [6, 7]

hybrid type capacitor. The classification and type of material used are shown in Fig. 2.2. EDLC stores charge electrostatically by the formation of bilayer at the electrode-electrolyte interface. Pseudocapacitors store more amounts of charge per gram than EDLC due to the involvement of the Faradaic process [5]. The hybrid supercapacitor uses both types of charge storage mechanisms, which provide a synergistic effect. It includes the formation of a composite electrode using EDLC and pseudocapacitive material. Other type includes asymmetric assembly, where two different types of the electrode are used in a single device. Another type of hybrid supercapacitor comprises one electrode of battery type material and another electrode of supercapacitor type. Hybrid assembly helps to increase the energy density without hampering the power density. The supercapacitor is also known as ultra-capacitor because it has the capacitance value in the range of 100–1000 F. The supercapacitor is a promising candidate among other electrochemical energy storage devices due to their high specific power, greater than 10 kW kg−1 , fast charge/discharge cycle (within a second), long cyclic stability greater than 105 cycle [8]. It uses the phenomenon of the conventional capacitor, where two conducting metal plates are separated by a dielectric material. The important characteristics of the capacitor are reported elsewhere [9]. But in a supercapacitor device, the two conducting plates are conducting the porous high surface electrodes immersed in an electrolyte solution. The detailed features of the capacitor to supercapacitor are also reported elsewhere [10]. For a better understanding, Table 2.1 summarizes the difference in terms of material, cost, performance, and charge storage electrode material between conventional capacitor and supercapacitor. It is a fact that the high surface area significantly increases the capacitance value for supercapacitor enabling them to utilize as an energy storage system. However, its specific energy density is 10–50 times less than battery. But the battery has a safety issue during operation due to the resistance of ion, sluggish electron transportation, and dendrite formation at the interfaces, which produce heat [11]. But the commercially available supercapacitor has offered energy density nearly equal to 5 Wh kg−1 , but in lab-scale

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Table 2.1 The difference between capacitor and supercapacitor [9, 10] Capacitor

Supercapacitor

Material

Dielectric material

Separator is used in place of dielectric material. Electric layer formation at the interface of electrode and electrolyte, pseudocapacitive material

Charge storage

In the form of the electric field between two plates

In the form of electric double-layer capacitance, electrochemical pseudocapacitance or both

Electrode material

It is made up of two conducting metal plate

Either of the electrodes is made up of carbon material or one is made up of other materials such as transition metal oxide (TMO), conducting polymer, or composite material

Capacity

Low

High

Energy

) [10, 23].

Fig. 2.4 Schematic diagram of the EDLC interface model, a Helmholtz model, b Gouy–Chapman model and c Stern model. Redrawn and reprinted with permission from [23]

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The Gouy–Chapman model modifies the Helmholtz model, and say that cation and anion of electrolyte are distributed in solution, and form a diffuse layer as depicted in Fig. 2.4b. The Gouy–Chapman model gives a better approximation than the Helmholtz model. But it has some limited applications in reality. Because in this theory, the ion is considered as point charge and assumes that it has no limitation to approach the surface of the electrode. But in reality, it is not like that [10, 23]. Stern modifies the Gouy–Chapman model and states that the ion has a finite size and so it can reach to the surface, but remain at a few distances away from the surface, which is the ionic radius of ion. Stern also assumes that some ion gets adsorbed on the surface of the electrode, which is known as the stern layer. Stern model is the combination of the Helmholtz model and the Gouy–Chapman model. It is formed by the combination of two regions as shown in Fig. 2.4c Inner Helmholtz Layer distribution refers to the closest approach to specially adsorbed ion and Outer Helmholtz Layer is non-specifically adsorbed ion, which is formed by the solvated ion [10, 23]. EDLC electrode is mainly fabricated by carbon-based material such as activated carbon, CNT, graphene, carbon aerogel, 3D nanostructured carbon, etc. Among various carbon structures, activated carbon is mostly used in the fabrication of such electrode because it possesses characteristics of high specific surface area due to the very high porous structure. The presence of a porous structure holds an excellent ability to adsorb ions on the interface between electrode and electrolyte. The effect of pore size strongly dominates the charge adsorption and desorption mechanisms [10]. For the pseudocapacitance electrode, an electrochemical reaction takes place in two ways, one is reversible faradic charge transfer process and the other is chemical redox reaction at the electrode surface or nearby it. This mechanism is associated with the valance state of material as a result of electron transfer. There are mainly three types of faradic processes observed in the pseudocapacitance electrochemical devices. This process includes a chemical redox reaction in the material, which transfers electron through the interface of electrode/electrolyte. This type of supercapacitor involves the processes of electronic and ionic charge migration and the reversible redox reaction [10]. These reactions are illustrated as underpotential deposition, redox pseudocapacitance, and intercalation pseudocapacitance, as shown in Fig. 2.5. Underpotential deposition (Fig. 2.5a) is well known for the monolayer adsorption of a metal ion on the catalytic metal surface. This process can be described as [7] M + xC Z + + x ze− ↔ C.M where C is the absorbed atoms (H or Pd), M is the noble metal (Pt or Au), x is the number of absorbing atom, z is the valance of the absorbing atom, and zx is the number of the transferred electron. A typical example is a deposition of lead (Pb), which occurs on the gold (Au) surface because the interaction of Pb–Au is more than the Pb–Pb interaction in Pb metal lattice [25]. Electrodeposition reaction occurs at less potential than the equilibrium potential of cation reduction [26]. Here, the applied

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Fig. 2.5 Different types of reversible redox mechanisms at electrode/electrolyte interface of pseudocapacitive electrodes: a underpotential deposition, b redox pseudocapacitor, and c intercalation pseudocapacitor Redrawn and reprinted with permission from [24]

voltage should be below the cation reduction potential. Usually, operation potential is in the range of 0.3–0.6 V. Energy density obtained by underpotential deposition is very small, thereby limiting its application as compared to another pseudocapacitor electrode material. On the other hand, the redox pseudocapacitance is a charge storage mechanism, where metal ion is electrochemically adsorbed on or near the surface through the faradaic charge transfer process as shown in Fig. 2.5b. Ruthenium oxide (RO2 ) is the first reported material, which demonstrates such type of redox mechanism [27]. Apart from this material, MnO2 , p-doped conducting polymer, and reduced species (RuO2−z (OH)z ) electrode materials show such behavior. This mechanism is described as [7] Ox + zC+ + ze− ↔ RedCz where C is the surface adsorbed electrolyte cation, C+ (H+ , K+ , Na+ , and many more) and z is the number of transfer electron [7]. The intercalation pseudocapacitance process (Fig. 2.5c) undergoes ion insertion/desertion into layered crystalline electrode material accompanied by faradaic reaction without crystallographic phase change as shown in Fig. 2.5c. This mechanism is described as [7]. MA y + xLi+ + xe− ↔ Lix MA y

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where MAy is the layer-lattice intercalation host material, x is the transfer electron number. This type of pseudocapacitor materials is gaining more interest in the research because of relatively high capacitance, wide potential window, high rate capability. This material has been classified into two types as intrinsic and extrinsic. In intrinsic type, material capacitance does not depend upon the crystallite size and morphology but in the extrinsic electrode material, mechanism of charge storage occurs through diffusion mechanism, where ion size plays an important role. Charge storage in the intercalation type electrode occurs through three main processes [28]: • Faradaic contribution from bulk solid-state diffusion dominant ion intercalation • Faradaic charge transfer process at the surface and/or the surface of active material with fast ion diffusion dynamic • a non-faradic EDLC contribution [10]. TMO, metal hydroxide, metal sulfides, metal nitrides, and conducting polymer are prime material used in the pseudocapacitive type electrode. These materials possess drawbacks like low conductivity and low electrolyte accessible surface area, which results in high resistance or ion\charge transfer resistance that significantly reduces the charge storing capacity, rate capability. It is known that energy density and specific capacitance of pseudocapacitance is higher than EDLC. The highly efficient properties of pseudocapacitive material are the current interest of research. However, material undergoing redox reactions are used as charge storage processes depicting high energy density devices such as battery show low stability during the repeated charge-discharge cycle.

2.2.2 Electrolytes Electrolyte plays an essential role in supercapacitor performance. This electrolyte is categorized in the three types given as follows: (i) liquid, (ii) solid-state or quasi-solid-state electrolyte, and (iii) redox-active electrolyte. This is shown in Fig. 2.6. Aqueous electrolytes deliver high capacitance due to high ionic conductivity but possess low energy density, cyclic stability, and show high leakage problems. There is a need to develop a good packaging system for liquid-based electrolytes. Ionic and organic electrolytes offer to work on the higher voltage but the main demerit is its lower conductivity [8]. Solid-state electrolyte prevents leakage problems, but display low ionic conductivity. Therefore, the selection of electrolytes plays a crucial role in designing high-performance energy storage devices such as batteries and supercapacitors. Temperature coefficient and conductivity are the main characteristics of electrolyte used in a supercapacitor, which determine the equivalent series resistance (ESR) value [8]. Apart from the above-stated characteristics, other important properties such as [29]

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Fig. 2.6 Classification of electrolyte for supercapacitor [30]

• • • • • • •

viscosity, high electrochemical stability, low volatility, wide voltage window, low toxicity, high ionic concentration, and low cost also plays an important role in determining the performance of supercapacitors [8, 30]. The specific conductivity of a solution can be improved to a certain extent by mixing the solvents that are modifying the solvation effect and viscosity of the solution. It is worth mentioning that thermodynamic potential range plays an important role in the thermodynamic stability of electrolyte therefore the non-aqueous solution has higher life than an aqueous solution [30]. The aqueous electrolyte has many advantages compared to the non-aqueous electrolyte but one major drawback associated is its thermodynamic potential window, which is lower than 1.3 V and it is due to the presence of water as a solvent, which is easily electrolyzed and produces gases above 1.3 V [31]. The detailed characteristic features of electrolytes are reported elsewhere [29].

2.2.2.1

Liquid Electrolytes

Aqueous and organic electrolytes are widely used in supercapacitors. For aqueous electrolyte, the thermodynamic potential window is limited to 1 V because the water decomposition potential window is 1.3 V, whereas organic electrolyte has operating potential window in the range of 2.7 V or higher [32]. Although the organic electrolyte has a higher electrochemical working potential window, it shows higher

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resistance than the aqueous electrolyte. The aqueous electrolyte has a higher capacitance value than the organic electrolyte. Since aqueous electrolyte has electrolytic ion with small ionic radius and a high concentration of the ions, which are required to interact with small pore size and also to travel from one side to another side through membrane/separator. Organic electrolytes possess a high potential window, as its degradation potential is high. Again, it is a costly material because the purification of organic electrolytes from water is an expensive and tedious process. For the high performance of organic electrolytes, water content should be low in the range of 3–5 ppm. Generally used organic electrolytes are propylene carbonate (PC), acetonitrile (ACN), tetrahydrofuran (THF), ethylene carbonate in which PC and ACN are the most widely used in supercapacitor application [8, 33]. Acetonitrile (ACN) has a high capability of dissolving the larger amount of salt but it is toxic, whereas PC-based electrolyte is environmentally friendly, high electrochemical potential window, a wide range of working temperatures. Generally, salt used with organic electrolyte is tetraethylammonium tetrafluoroborate, tetramethyl phosphonium tetrafluoroborate, and triethylmethylammonium tetrafluoroborate, etc. Salt in the electrolyte enhances the ion conductivity and stability of the electrochemical potential window for the electrochemical supercapacitor. These electrolytes have asymmetric structure, which leads to lower crystal lattice energy. As a result, these salts are easily soluble in this type of organic electrolyte [34]. Ionic liquids are the main interest as an electrolyte in the supercapacitor, which is a pure form of organic salt containing no solvent. The low-temperature ionic liquid is one type of ionic liquid, whose melting temperature is below 100 °C. If the liquid state of salt is maintained at room temperature, it is known as room temperature ionic liquid (RTIL). It is gaining the main interest in the research of supercapacitor due to some special properties like non-volatile, poor combustible, and heat resistance. Ionic liquid shows potential voltage around 4.5–6 V. These electrolytes have resistance to oxidation and reduction in wide potential voltage. The other thing on which potential cell voltage depends is a counter ion. In RTILs, at least one ion is delocalized form (main aromatic ring) and another ion is of organic. RTIL consists of bulky and asymmetric structures, which prevents to form a stable crystal. Properties such as viscosity, conductivity, and the melting point of an ionic liquid are improvised by the substitution of organic ion or counter ion. The main demerit of this material is that it has a lower conductivity of 10 mS cm−1 even lower than the aqueous electrolyte. Ionic liquid polymer gel electrolytes are developed with the incorporation of ionic liquid in the polymer gel electrolyte, which shows the high conductivity, mechanically strong, electrochemically stable, and temperature stable. Ionic conductivity and mobility of ion of electrolyte are important factors on which the performance of supercapacitor depends.

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2.2.2.2

Solid and Quasi-Solid-State Electrolytes

Polymer electrolytes 3D polymer material has also been explored as a supercapacitor component. Polymer electrolytes can serve the dual purposes of electrolyte and separator. It facilitates ion/mass transfer through a pore [35]. The solid-state electrolyte has higher reliability and wider operation temperature. It eliminates the leakage problem [8]. The ion transportation mechanism in 3D polymer electrolyte takes place below the glass transition temperature. Transportation of ion is ascribed to the direct segmental motion of the polymer chain. These electrolytes should possess the following properties [8, 34–36]: • • • • •

high ionic conductivity at room temperature low electronic conductivity good mechanical properties and stability high chemical and electrochemical stability ease processability of thin film.

Polymer hydrogel electrolytes Polymer hydrogels prepared from a soft and flexible molecular chain such as poly (vinyl alcohol) (PVA), poly (ethylene oxide) (PEO), polyacrylate (PAA), polyacrylonitrile (PAN), polyacrylamide (PAM), poly (methyl methacrylate) (PMMA), etc., are used as gel electrolyte. Apart from the abovestated polymers, the natural polymer is also used to prepare gel electrolytes such as cellulose, chitin, chitosan, etc. These hydrogels are classified into four categories according to electrolytic salt. They are as follows: • • • •

proton-conducting polymer gel electrolyte alkaline gel polymer electrolyte lithium-ion polymer gel electrolyte other polymer gel electrolyte.

Proton conducting polymer gel electrolyte is the most widely used in the fabrication of electrolyte because proton has high mobility so it shows a high fast charge/discharge process. Han et al. have developed a flexible hybrid supercapacitor using zinc and zwitterionic natural polymer hydrogel [37]. This hybrid supercapacitor exhibits high flexibility, good mechanical strength, stable voltage window of 2.4 V, high energy density of 286.6 Wh kg−1 at a power density of 220 W kg−1 , and superior capacity retention of 95.4% after 2000 cycle at 2 A g−1 [37]. Alkaline gel polymer electrolyte (GPE) is a promising candidate because of electrode protection, high chemical stability, leak-proof, and high ionic conductivity. Recently many alkaline GPEs have been developed for the supercapacitor. Few examples are PVA/KOH/H2 O, PEO/KOH, PVA/PAA/KOH [38], and many more. Hu et al. have prepared a double network PVA/KC kappa-carrageenan alkaline gel polymer electrolyte in which KOH serves two purposes as ion conductivity and cross-linker agent. This GPE exhibits excellent tensile strength (2.2 MPa), high ionic conductivity (0.21 S cm−1 ) and stretchability (12.13 mm/mm) [39]. Lithium-ion gel polymer electrolyte is generally used for the lithium-ion battery and supercapacitors. Recently polyvinyl alcohol (PVA)-lithium perchlorate (LiClO4 )

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[40], and PVA/LiCl, PEEK/PVA/LiClO4 [30] lithium-ion gel polymer electrolyte have been studied. Chodankar et al. have synthesized a lithium-ion GPE with different polymer hosts such as PVA, CMC, and PE with the incorporation of lithium salt, LiClO4 [40]. They have reported an acceptable ionic conductivity of 48 mS cm−1 , which is comparable to the liquid electrolyte, long cycle life, high compatibility with the electrode, and mechanical stretchability. PVA/LiClO4 based MnO2 is the best supercapacitor, which exhibits operating potential voltage (1.2 V), specific capacitance of 112 F g−1 , energy density of 15 Wh kg−1 with extended cycling stability up to 2500 cycles [40]. Other gel polymer electrolytes are used to get specific performance of supercapacitor. Different types of plasticizers and fillers are incorporated to enhance the conductivity of electrolyte [41]. Redox-active electrolytes The redox-active electrolyte is the promising electrolyte for the next-generation hybrid supercapacitor. This redox-active electrolyte increases capacitive behavior by the contribution of pseudocapacitance of these electrolytes. The hybrid supercapacitor is comprised of this type of electrolyte, where the capacitive behavior not only occur by electrode/electrolyte interphase contribution but also a bulk electrolyte. Chen et al. have reported that the capacitance of devices increases (36–92%) when the redox-active electrolyte is used with polyaniline coated curved graphene in comparison to non-redox active electrolyte [42]. Iodide/iodine redox pair is studied as an aqueous redox-active pair that only affects the capacitive behavior of the positive electrode, where it is found that counter metal ion has a greater effect on the capacitance of electrode. Capacitive behavior decreases as the van der Waals radius of alkali metal cation decreases. The trend is as RbI (2272 F g−1 ) > KI (1078 F g−1 ) > NaI (492 F g−1 ) > LiI (300 F g−1 ) (all with 1 M concentration) [43]. Iodide/iodine pair redox electrolyte only contributes to enhancing the capacitance of the positive electrode, but not the negative electrode, so the overall contribution is significant. To overcome this problem, Frackowiak et al. have used 1 M KI for the positive electrode and 1 M VOSO4 for the negative electrode and found that the capacitive behavior of the negative electrode increases [44]. To find the high performance hybrid supercapacitor wide varieties of redox-active electrolyte have been studied with different types of electrode materials such as hydroquinone (HQ) [45], methylene blue (MB) [46], indigo carmine (IC) [47], p-phenylenediamine (PPD) [48], m-phenylenediamine (MPD) [49], lignosulfonates [50], sulfonated polyaniline (SPAni) [51], ferricyanide (Fe(CN)46− ) [51], heteropoly acid [30], and, many more [29]. This electrolyte increases the capacitance of the device, but it brings the severe problem of fast self-discharge. This electrolyte accelerates the self-discharge process. To suppress the self-discharge process, the migration of redox-active species between electrodes must be blocked by two methods as [30] • to use the ion-exchange membrane for blocking the migration of active electrolyte. • to choose special active electrolyte material having no shuttle effect. In the shuttle effect, the electrolysis product from one electrode migrates to the other electrode,

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where they deplete the charge storage on the other electrode by inverse electrochemical reaction. For example, when Cu2+ redox-active electrolyte is used, Cu2+ is converted into insoluble Cu and deposited on the electrode and prevents from shuttle effect. Several studies have reported similar and poorer cyclic stability of devices composed of these types of redox-active electrolytes [29, 42].

2.2.3 Current Collector The current collector is another element of the supercapacitor, which enhances the charge transfer through the external circuit. It collects the electrons from the electrode and supply to the output devices and load. For the high performance of supercapacitors, the current collector should have high electrical conductivity. Its main function is to reduce contact resistance. It should have the following characteristics to enhance the performance of supercapacitor [52]: • • • • • •

high electrical and thermal conductivity low contact resistance high chemical and electrochemical stability low corrosion resistance compatible with electrode material light in weight and flexible

In a supercapacitor device, sometimes the current collector acts as the base to grow the electrode material. Generally, the electrode material is grown on the metallic foil, which acts as a substrate for the electrode and as a current collector for the device. The physical and chemical properties of the current collector influence the electrochemical performance of supercapacitors. For high performance, it should have less thickness and low contact resistance. Generally, the material used for current collectors is aluminum foil, copper foil, titanium foil, nickel foam, carbon clothes, stainless steel fiber, etc., which act as a conductor between electrode and external circuit [53]. Foam form of the current collector has low contact resistance than the foil type current collector. The nickel-based current collector is mostly used in the fabrication of supercapacitors. It provides high mechanical strength, low contact resistance, high electrical conductivity, and cheap material. However, nickel-metal acts as redox material and show the capacitive material, which may produce an error in the capacitance calculation of material. From the electrochemical point of view, aluminum is the most suitable current collector for cathode material. Metallic foil has a problematic issue as it can easily get detached from the electrode material. For high performance, it should have less thickness and low contact resistance. Carbonbased material, conducting polymer, etc., are also used as a flexible current collector in which CNT and graphene are grown over it [54–56]. The detailed characteristic features of current collectors are reported elsewhere [52].

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2.2.4 Separator It is one of the main components in the supercapacitor, which prevents the devices from the short-circuit. Although, it does not participate in the electrochemical reaction in devices but acts as a separator to separate the anode and cathode, which only allow ion to pass through it. The component of the device should have the following properties [57, 58]: • • • • • • • •

good electrical insulator minimum resistance to electrolyte ion porous structure ease of wettability by electrolyte high resistance to change in volume (low swelling) good mechanical and chemical stability non-flammable high ionic conductivity.

Ionic conductivity depends upon the porosity of separator and supercapacitor performance depends upon the concentration of ions. Porosity in the separator is another performance defining factor for ionic conductivity and mechanical strength. It is known that the ionic conductivity increases as porosity increases but its mechanical strength decreases. Generally, polymer membrane and hydrogel are used as a separator, which is mainly made from polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), natural and synthetic rubber due to porous, low cost, flexibility, easy processability, high mechanical properties, and high chemical resistance [59, 60]. Among these, PP is the most widely used because of its high wettability. At the time of the short-circuit, the device temperature increases to 100 °C. So, the above-mentioned materials cannot solely sustain that temperature. For this reason, presently separator material is synthesized by blending more than two material or is fabricated with multilayer. This fabrication method improves thermal stability and eliminates shutdown ability. Generally, separator material should have a high melting temperature because at low melting point polymer gets soften after gaining temperature and its pore will get closed, which restricts the ion transfer. Polyolefin material or a mixture of it prepared by the dry and wet process has been used as a separator [61]. The presence of pore in the separator much more depends upon the manufacturing process. Figure 2.7 shows the SEM micrograph of polyolefin synthesized using a dry and wet process [62]. It is quite evident that the pore structure depends upon the fabrication techniques. Apart from the synthesis techniques, filler-reinforced polymer composite enhances the thermal stability and makes it suitable for high thermal stable supercapacitor. Entek Membrane Company has made such a type of separator. Few other companies are M/S Asahi Kasei Chemicals, M/S Celgard LLC, M/S ExxonMobil/Tonen, M/S SK Energy, M/S Ube Industries, etc. These separators are generally made from polyolefin and ceramic-filled polyolefin (PE, PP, and

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Fig. 2.7 SEM images of separator fabricated by a dry, b wet process. Redrawn and reprinted with permission from [62]

PP/PE/PP). The detailed characteristic features of separator are reported elsewhere [58].

2.3 Electrodes 2.3.1 Carbon-Based Electrode Material Carbon-based electrode material shows tuneable properties, which include electrical conductivity, surface area, pore size distribution, and fast electron transfer kinetics with low fabrication cost. Capacitance arises from adsorption and desorption of electrolytic ions at the electrode surface. Greater the surface area of the electrode material higher will be the capacitance. Mostly, in carbon materials surface phenomena are involved during charge storage mechanism, so whole material does not participate to decide the capacitance of the device [63, 64]. Few carbon-based materials with their special characteristics are given in Table 2.2 [65]. Much more detailed characteristics are reported elsewhere [22].

2.3.1.1

Activated Carbon

Activated carbon (AC) is the most widely used electrode material in EDLC supercapacitor because of its high surface area up to 3000 m2 g−1 [66, 67], relatively high electrical conductivity of 10−8 –1010 S m−1 [68] and low cost. These materials can be produced through physical and chemical methods [69, 70]. In the physical method, carbon is pyrolysis at high temperatures from 700 to 1200 °C in the presence of oxidizing gases such as steam, CO2 , and air. Chemical activation is carried out at lower temperatures from 400 to 700 °C with activation agents like phosphoric acid, KOH, NaOH, and zinc chloride. The surface properties of ACs depend

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Table 2.2 Different structures of carbon used in the EDLC supercapacitor [65] Material

Carbon onions

Carbon nanotubes

Graphene

Activated carbon

Carbide derived carbon

Templated carbon

Dimensionality

0D

1D

2D

3D

3D

3D

Conductivity

High

High

High

Low

Moderate

Low

Volumetric capacitance

Low

Low

Moderate

High

High

Low

Cost

High

High

Moderate

Low

Moderate

High

Structure

upon the synthesis techniques. It is composed of a wide range of pores structure, i.e., micropores (50 nm). Capacitance values of the electrode mainly depend upon the surface area of ACs. However, the electrochemical performances not only depends on the surface area but also the pores size distribution, shape, the structure of pores, conductivity, and surface functionality. Excessive activation of carbon leads to large pore volume, which has drawbacks of low material density and conductivity [71]. This leads to low volumetric energy density and loss of power capability. High surface activation energy leads to the agglomeration of electrolyte in the dangling bond position. Mysyk et al. have reported the optimal pore size of EDLC, which should be 0.7 nm for aqueous electrolytes and 0.8 nm for organic [66]. Organic electrolyte contains large ion in size, where the large optimal pore size of the electrode is required as compared to the ion size of aqueous electrolyte. Activated carbon-containing high oxygen functional group but low surface area shows good electrochemical stability. Another researcher finds the relation between optimum pore sizes of the electrode with ionic electrolyte size. The capacitance of the reversible hydrogen electrode (RHE) electrode is optimal when both pore size and ionic electrolyte sizes are equal. The detailed features of activated carbon are reported elsewhere [70].

2.3.1.2

Hierarchical Porous Carbon

Hierarchical porous carbon has been mostly used as advance carbon material for supercapacitor due to tuneable morphology, simple preparation, and porous structure [67]. The pore size in hierarchical porous carbon is categorized into four parts, where each part has its advantage as macroporous pore size >50 nm serves as ionbuffering reservoirs, mesopores with a size of 2–50 nm minimize ion transport resistance, micropores with size 0.7–1 nm accommodate charge and localized graphitic structure enhances the electrical conductivity. Wang et al. have reported the 3D

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hierarchal mesoporous graphitic carbon structure depicts the combination of micropores, mesopores, and macropores [72]. Figure 2.8a shows the SEM micrograph of hierarchical porous graphitic carbon revealing macropores. Figure 2.8b, c show TEM image indicating mesopores and micropores, respectively. Figure 2.8d shows the HRTEM micrograph, which reveals graphitic fringes along with the mesoporous structure. The schematic diagram of a 3D hierarchical porous texture, a 3D selfsupported network of macrospores including the meso-micro porous structure and localized graphitic structure is shown in Fig. 2.8e. Due to its unique structure, it is a promising material for advanced electrode materials. Different pore textures with hierarchical pores: marco, meso, micropores are shown in Fig. 2.8e. All these pores have different electrochemical reactions at each pore area. Mesoporous structure shortens the ion diffusion path for electrolytic ions and lowers the ion transport resistance of the device [72]. Appropriate shape and size of pore play an important role in electrochemical performance. Lower and higher pore sizes of the material decrease the specific capacitance of material due to ion sieving effect. Apart from that, the wider and complex shape of pore restricts the ion transport and declines ion adsorption than the planar pores. This hierarchical porous carbon is obtained from the carbon-rich precursor through high-temperature processing under inert environment and activation resulting in pores formation.

Fig. 2.8 a SEM image of the macroporous cores of the hierarchical porous graphitic carbon material, b TEM image of the mesoporous walls, c TEM image showing the micropores, d high-resolution TEM (HRTEM) image of the localized graphitic mesopore walls, and e schematic representation of the 3D hierarchical porous texture. Redrawn and reprinted with permission from [72]

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2.3.1.3

47

Carbon Aerogel

Carbon aerogel is a low density, and highly porous material (80–98%) composed of carbon nanomaterial in a few cases. It is produced by the sol-gel method by the polycondensation reaction of resorcinol and formaldehyde. The pyrolysis process in the nitrogen atmosphere produces a uniform mesoporous texture having a size ranging from 2 to 50 nm. It also shows high electrical conductivity (25–100 S cm−1 ), extremely high surface area (up to 1100 m2 g−1 ) [73]. A very low density of material leads to the poor volumetric capacitance, which limits its interest in the application.

2.3.1.4

Carbon Nanotube (CNT)

The carbon nanotube is known for its high conductivity and unique mechanical, high thermal conductivity properties that have been studied for EDLC applications [74– 76]. It has been found in a different type of structure such as single-walled CNT and multiwalled CNT [77, 78]. Its excellent tubular structure has high porosity and high electrical conductivity, which make this material a good choice for high-power EDLC electrode. Due to their unique merit, it has been extensively studied in many electrochemical storage devices [79]. A major limitation associated with CNT is its low specific surface area (Cx O//H+

(5.1)

Where >Cx O//H+ represents a proton adsorbed by a carbonyl or quinone-type sites, induced by ion-dipole attraction. This facilitates specific adsorption processes due to the local changes in electronic charge density. During the charging of the negative electrode, strong bonds may be formed among the functional groups and protons due to the process of transfer of electrons across the double layer. >Cx O + H+ + e− ↔ >Cx OH

(5.2)

Again, during the discharge process, the redox reaction gives rise to pseudocapacitance. The presence of redox humps in cyclic voltammetry (CV) experiments shows the existence of these functional groups. The other oxygen functional groups that do not undergo redox reactions are chemically inert and improve the wettability of carbon electrodes. This improves pore accessibility and greater surface utilization. Also, the presence of highly polar oxygen groups such as carboxyl, anhydride, and lactone may hinder the motion of the electrolytic ions. Hence, an increase in

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the resistance and capacitance fading at high current density is quite an expected phenomenon. The nature of nitrogen-containing surface functionalities differs depending on temperature and heat treatment processes. Literature reports that functional groups external to aromatic rings such as amides, aromatic amines, and protonated amides are predominant at low temperature (400–700 °C), while nitrogen groups within aromatic ring structure with a delocalized charge, such as pyrrolic-N, pyridinic-N, aromatic amines, quaternary nitrogen, and protonated pyridine are dominated at a temperature greater than 700 °C. The positively charged quaternary-N and pyridineN-oxide help in electron transport through the carbon matrix, whereas negatively charged pyrrolic-N and pyridinic-N show pseudocapacitive interactions by possible Faradic reactions [35]. Pyridinic N : >C = NH2 + 2OH− ↔ >C = NH + 2e− + H2 O Pyrrolic N:

>C-NH2 + 2OH− ↔ >C-NHOH + 2e− + H2 O

(5.3) (5.4)

The presence of functional groups such as sulphide, sulfone, or sulfonic is attributed to the oxidation-reduction reaction. The possible redox reactions may be as follows [72], Thiol Sulphite

>SH + OH− ↔ >S-OH

(5.5)

>S-OH + OH− ↔ −SO3 H

(5.6)

These re-oxidized groups function as electron shuttles during electrochemical analysis [73].

5.6 Electrochemical Performance of Activated Carbon as Electrode Materials for Supercapacitors The electrochemical testing is performed in three-electrode and two-electrode configurations. Three-electrode system consists of a working electrode as a sample (active electrode material), reference and counter electrodes, which are connected via potentiostat. Two-electrode system is a prototype of the well-assembled supercapacitor. The active material is evenly deposited on both current collectors separated by separator and electrolyte. The full assembly system is packed and sealed. The tests are carried out by applying voltage or current via two current collectors, which act as cathode and anode [74]. The electrochemical performance of the electrode is analyzed by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). Table 5.1 summarizes various char-

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Table 5.1 Analytical techniques for the performance assessment of electrochemical capacitors (ECs) [17] Characterization technique

Equations

Characteristics/remarks

∫ idt V ·μ

Cyclic Voltammetry

C=

Charge/discharge

t C = i V

Output energy

E = ∫ i V dt

• Useful in determining practical output energy and round trip efficiencya

Alternating current (AC) impedance spectroscopy

C=C =

• Evaluated at low frequencies (10 MHz) • Technique also used to evaluate ESR • Phase angle approaching 90 °C at low frequencies indicating ideal capacitance

Energy density

E = 21 CV2

• Characteristics assessment of practical performance

Power density

Pmax =

• Characteristics assessment of practical performance

Coulombic efficiency

η=

IR drop

–

• Ideal behavior results in constant equivalent ± (i.e., rectangular box structure) • Supercapacitive current peaks are notable, repeating deviations • Used in both battery and EC testing, ideal capacitive behavior has been observed with linear slope (i.e., triangular charge/discharge shape)

1 2π f Z 

E t

Charge time discharge time

× 100

• Characteristics to determine efficiency of energy taking and giving process • Voltage loss due to the internal resistance

Notation of the following variables: C capacitance; I current; V potential; μ scan rate; t time (charge/discharge rate); f frequency; Z  imaginary part of impedance; E energy density; P power density a Round trip efficiency; ratio of output to input energy

acterization techniques that are in common practice to measure the variables related to capacitance. Besides determining the performance of the material through capacitance, energy and power densities, the stability testing and self-discharge tendency of the electrode is also the other important aspects to be considered from the commercial perspective [17]. Parameters listed in Table 5.1 are used to evaluate the performance of supercapacitor devices. The area under the cyclic voltammetry loop denotes the amount of charge stored by the supercapacitor. The GCD plot shows various parameters such as capacitance, energy density, and power density by evaluating discharge time. Also, the term coulombic efficiency denotes the ratio of the amount of time required to charge the material to specific potential by the time required to discharge. Coulombic

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efficiency near to 100% implies that the amount of energy taken by the device is equally delivered. The term IR drop symbolizes the potential at which the device is getting discharged after getting fully charged. It explains the presence of internal resistance in the material. The more the IR drops the greater will be the resistance. The overall performance of the device is characterized by energy and power density. In continuation of this, a high amount of energy density is feasible at specific power. From the above formula, energy density depends on capacitance and the operating voltage of the device. So, increasing the operating voltage along with discharge time is beneficial to obtain a high-performance supercapacitor. Carbon material shows an EDLC behavior, where the charges are stored electrostatically at the electrode-electrolyte interface. The capacitance is mainly produced from the adsorption of charge at the exposed electrode surface. Carbon-based material, from traditional activated carbon to advanced nanostructured carbon has been widely used as supercapacitor electrodes. They exhibit excellent chemical stabilityenhancing long cycle life, high surface area with controlled porosity favoring the accumulation of charges, and eminent electronic conductivity. In this prospect, activated carbon is a promising electrode material since the surface area, porosity and electronic conductivity can be tuned easily during synthesis and thereby the overall performance can be tailored as per requirement. From an energy and environmental viewpoint, various efforts have been taken to develop functional carbon materials from biomasses and their by-products using different synthesis methodologies. Earlier, the synthesis of activated carbon takes place from non-renewable petroleum products. Also, the synthesis conditions are harsh, where temperature above 1000 °C has been employed. This whole processing makes it expensive and detrimental to the environment. Various biomasses including agricultural and energy crops, municipal bio-waste have been utilized as the precursor to prepare carbon materials using different synthesis routes and environments. The electrochemical performance of the activated carbon derived from biomass by different mechanisms has been discussed in the next section.

5.6.1 Electrochemical Performance of Activated Carbon Synthesized from Different Routes/Methods Properties of activated carbon can be altered by changing the synthesis condition. Hence, the performance of the electrode depends upon synthesis routes followed to prepare the desired activated carbon. Also, the synthesis mechanism is a point of concern. The high-performance activated carbon can be developed for supercapacitor application by the judicial choice of method of preparation. In this regard, various synthesis methods have been discussed in the following section.

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Electrochemical Performance by Carbonization and Activation

Activated carbon prepared by carbonization and activation leads to the formation of the highly porous carbon structure. Carbonization involves decomposition of biomass into carbonaceous material through pyrolysis. Subsequently, activation is done by mixing the activating agent with the char followed by heating in an inert atmosphere. The presence of activator at high temperature leads to the formation of pores. Researches have been carried out by combining both the steps followed by heating in an inert atmosphere. This process involves the formation of the hierarchical porous structure of activated carbon that widens up its usage as excellent EDLC material. One such example of activated carbon prepared from cassava peel has been observed, where cassava derived activated carbon has been prepared using KOH chemical activation followed by CO2 physical activation [50]. Further to this, surface modification is carried out by various oxidizing agents. GCD curves in 0.5 M H2 SO4 electrolyte show high charge-discharge time with excellent coulombic efficiency indicating that contact between carbon electrodes is good with the electrolyte solution. The highest specific capacitance obtained is 264.08 F g−1 . CV curve shows symmetric rectangular behavior for all the samples inferring good charge-discharge reversibility and high material stability in EDLC during charge and discharge process [63]. Another literature reports different activation strategies for preparing nanoporous carbon using Sunflower seeds as a carbon source. The first strategy is the impregnation of KOH followed by activation. In the second approach, two-step carbonization followed by KOH activation has been employed. CV curves obtained using a sandwiched two-electrode system show near rectangular behavior. In the CV profile of two-step carbonization, the activated sample shows slight distortion than the impregnation-activation process. This is due to the accessibility of large surface areas and more abundance of interior micropore channels. It has also been observed that the impregnation-activation process shows porous texture on a microscopic scale, which makes easy diffusivity of ions. Again, carbon made through the carbonization-activation process is not as highly porous [33]. In another work, various Chinese tea leaves have been used as a precursor for the synthesis of activated carbon by two-step carbonization and KOH activation. The CV and GCD curves reveal that electrochemical performance in 2 M KOH electrolyte has been remarkably enhanced owing to KOH activation. Again, the carbonized sample exhibits a low current density response. The CV curve of all tea leaves derived activated carbon shows ideal rectangular shape with weak redox peak around −0.52/−0.42 and slight polarization on negative and positive potential as shown in Fig. 5.4a. The existence of a redox peak is due to the presence of oxygen-containing functional groups on activated carbon surfaces. Among all, as-synthesized activated carbon samples, Bi luo chun tea leaves (B-AC) show the highest specific capacitance value of 330 F g−1 at a current density of 1 A g-1 . The CV curve of B-AC has still maintained a rectangular shape at a sweep rate of 100 mV s−1 . This is due to the unrestricted motion of electrolyte ions in the pores. GCD plot in Fig. 5.4b also shows linear and symmetrical curves at an increased current density of 1–10 A g-1 , which shows typical characteristics of an ideal capacitor. The EIS measurement of the as-synthesized activated

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Fig. 5.4 a CV curves at different sweep rates, b GCD curves at different current densities for the waste tea-leaves derived activated carbon (B-AC) electrode at different current densities [75], c nyquist plots (inset: magnification of the Nyquist plot), d the frequency response, e ohmic drop associated with the equivalent internal resistance at various current densities for the TFAC-5 K based supercapacitors in 6 M KOH, 1 M Na2 SO4 and EMIM BF4, f ragone plots for Tremella derived activated carbon-based supercapacitors in different electrolytes [30]. Redrawn and reprinted with permission

carbon shows a small semicircle at high frequency followed by a transition to linear behavior at low frequency. The presence of semicircle at the Nyquist plot indicates Faradaic charge transfer at electrode-electrolyte interfaces. The straight lines at lower frequency show diffusion of electrolyte ions into electrode pores and the vertical line close to 90° at low frequency suggests pure capacitive behavior. The cycle life-time

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displays 92% capacitance retention, exhibiting excellent cycle stability, and good supercapacitive properties due to the excellent electrical conductivity [75]. A wide operating potential has been obtained by soya derived heteroatom doped carbon as electrode material. A conventional symmetric supercapacitor device has been assembled. The highest potential window is 0–1.5 V, which is higher than the theoretical value of water decomposition potential (1.23 V). This can be attributed to over potential water splitting in the materials. The soya derived material shows 93% capacitance retention for continuous charge-discharge cycles at a current density of 10 A g−1 . The high specific capacitance, rate capability, and good electrode stability make soya derived activated carbon as promising electrode material for electrochemical energy storage applications [65]. Following the gravimetric capacitance, a study in volumetric capacitance is essential to determine the performance of a supercapacitor. The study in volumetric capacitance has been carried out by using hollow activated carbon microtubes derived from low-cost biomass, willow catkins via a low-temperature pre-carbonization combined with KOH chemical activation. The CV curve shows a near rectangular behavior with the existence of broad pseudo peaks due to the high heteroatoms content in 6 M KOH electrolyte. The as-prepared material exhibits high gravimetric capacitance of 306 F g−1 at 0.1 A g−1 along with an outstanding volumetric capacitance of 303 F cm-3 . The volumetric capacitance mainly depends on pore size distribution with lower pore volume that provides high efficiency in EDL charge storage. The Nyquist plot also shows good capacitive behavior with nearly vertical slope at the low-frequency region. Equivalent series resistance is found to be 0.78  indicating good conductivity. The semicircle shows the resistance of 0.39  indicating good ionic conductivity at electrode-electrolyte interfaces [76]. Tremella, renewable biomass has also been utilized to prepare hierarchical activated carbon through carbonization and KOH activation. The prepared carbon shows an extremely high surface area of 3760 m2 g−1 containing 72% of mesoporous volume. Three electrodes’ electrochemical performance in 6 M KOH electrolyte shows a specific capacitance of 71 F g−1 at 1 A g−1 with an excellent rate capability of 53.5 F g−1 at 30 A g−1 . Two electrodes assembled supercapacitor shows excellent cyclic stability of the electrode with 99% rate capability after 10,000 GCD cycle at 5 A g−1 . Figure 5.4c displays a Nyquist plot at different electrolytes having low resistance. Figure 5.4d represents the frequency response of the electrode. The IR drop in aqueous electrolyte indicates low resistance as compared to the ionic electrolyte (Fig. 5.4e). In ionic liquid electrolyte, the assembled supercapacitor exhibits an energy density of 65.6 Wh kg−1 , which still maintains to be 28 Wh kg−1 at a high power density of 19,700 W kg−1 as shown in Fig. 5.4f [30]. In another work, Qian et al. have prepared human hair derived carbon flakes using two-step carbonizations followed by KOH activation at various temperatures [32]. The obtained CV curve shows a rectangular shape with a small hump at about −0.45 V due to the presence of a Faradaic reaction in 6 M KOH. Although the presence of heteroatom such as pyridinic and pyrrolic nitrogen species induces pseudocapacitance, a major contribution in capacitance is due to EDL formation. The Nyquist plot shows low resistance during ion transportation in aqueous electrolytes due to the short length of

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slopes. GCD curve shows quasi-linear behavior indicating enhancement of graphitization and increment of conductivity. The specific capacitance is 340 F g−1 at a current density of 1 A g−1 , which is retained up to 128 F g−1 at even at a very high current density of 80 A g−1 . This may be due to the presence of heteroatoms, which increases the wettability and polarity of carbon materials. These results indicate that heteroatom doped carbon materials could reach high capacitive behavior in aqueous solution [32]. Activated carbon has also been derived from rice husk via carbonization and KOH activation at various temperatures. The rectangular shape of the CV curve of rice husk activated carbon indicates highly capacitive behavior. This shows that the hindrance of the motion of the ions in the pores is low. Although on increasing the scan rate gradually above 500 mV s−1 to 1 V s−1 the capacitance decreases by 35 and 44% of the initial value. Hence, the charge-discharge rates are substantially limited [77]. Gopiraman et al. have worked on corn residues (corn silk, corn leaves, and corn cobs) to prepare activated carbon [78]. The precursors are pre-carbonized followed by NaOH activation. The CV curves exhibit symmetric near rectangular behavior at a low scan rate indicating that charges are stored by the accumulation of electrolyte ions between electrode-electrolyte interfaces in 1 M H2 SO4 . Again, the rectangular voltammogram has been still maintained at a high scan rate of 100 mV s−1 , inferring low internal resistance, good rate capability, and fast kinetics for the electric double layer formation. The maximum capacitance value is 575 F g−1 at 5 mV s−1 . The high specific capacitance is obtained due to the unique 3D structure with well-balanced hierarchical meso and micropores. In addition to this, a considerable amount of N and O functional groups enhance the surface utilization of active material. The Nyquist plot shows poor electrical resistance and better ion transfer nature. The lower resistance value could be associated with the formation of a high amount of C=C that facilitates the electron mobility and contributes to the charge storage mechanism. Also, a small semicircle and nearly vertical line have been observed indicating less resistance and capacitive behavior. The GCD curves show isosceles triangle shapes with no obvious IR drop, which further indicates small internal resistance, supreme capacitive behavior, and excellent electrochemical reversibility for charge storage and release [78]. Chang et al. have prepared activated porous carbon by costeffective pyrolysis carbonization followed by KOH activation on paulownia flower, easily available biomass [79]. The symmetric supercapacitor has been fabricated in 1 M H2 SO4 showing approximately rectangular loops with a pair of weak redox peaks in the CV curve indicating a synergistic contribution of EDL capacitance along with the slight pseudocapacitive contribution. Ragone plots of the devices show that the device delivers energy densities in the range of 44.5~22.2 Wh kg−1 under the power output range of 247~3781 W kg−1 . The electrical resistances and ion transport behavior are studied through the Nyquist plot showing a contact resistance of 2.91 . The combined effect of high BET surface area and porous texture with well-defined pores along with inherent heteroatom doping deliver high electrochemical performances of this supercapacitor electrode material [79]. Again, Wang et al. have utilized a waste carton box to prepare porous carbon material via a two-step process involving pre-carbonization followed by chemical activation using NaOH-KOH melt [80]. The

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electrochemical performance has been evaluated in a two-electrode system within the potential window of 0–1 V using 6 M KOH electrolyte. The CV curves show quasi-rectangular behavior and GCD shows linear shape both resembling capacitive behaviors. The result shows similar characteristics reported in the literature for activated carbon. Although, carbon prepared from carton box shows well-balanced mesoporous nanostructures, which promote quick charge transfer and smooth diffusion of electrolyte ions in the electrode [80]. Rufford et al. have utilized waste coffee grounds to produce activated carbon via one-step ZnCl2 activation [38]. The CV curve in 1 M H2 SO4 shows symmetrical rectangular behavior. The three-electrode test shows small redox peaks at around 0.3–0.4 V due to the presence of quinone, pyrrolic, and pyridinic nitrogen that are taking part in the pseudo-Faradaic reactions [38]. The literature on activated carbon suggests that carbonization and activation are some of the important synthesis routes to derived high surface area activated carbon. These synthesis routes are simple and easy. Also, by varying parameters such as temperature, amount of activators, and synthesis time, variations in structure and morphology of activated carbon are possible. Because of the performance of activated carbon as an electrode material, the extremely high surface area is required along with high conductivity. This requirement can be achieved by tailoring the carbonization and activation conditions of various biomass precursors.

5.6.1.2

Electrochemical Performance by Hydrothermal Carbonization and Activation

The hydrothermal technique involves the extraction of biochar via hydrolysis and dehydration of biomass followed by efficient chemical activation. The process results in a porous structure with a large number of functional groups attached to the carbon matrix. The presence of functional groups enhances the capacitance by inducing Faradaic redox reaction with the functional groups. Moreover, oxygen and nitrogen functional groups increase the conductivity of the carbon matrix, hence decreasing the internal series resistance. Various literature reports the synthesis of activated carbon using different biomasses by hydrothermal route. Hydrothermal carbonization followed by activation of Auricularia (a class of fungi) has been conducted to prepare activated carbon. The compact architecture with wrinkles and intercalated small particles endows volumetric swelling properties combining with its chemical composition. The CV in 6 M KOH quasi-rectangular shape indicates typical rectangular behavior. The presence of bumps indicates the contribution of heteroatoms via a redox reaction. The sample shows a superior capacitive property of 196 F g−1 at 5 mV s−1 . The galvanostatic charge-discharge cycling exhibits a quasi-symmetric capacitive behavior with a small deviation from linearity demonstrating pseudocapacitive contribution. The resultant carbon delivers both higher energy and power densities. The specific energy density of 23 Wh kg−1 at a power density of about 88 W kg−1 has been reported that has been retained to a value of 17 Wh kg−1 at a power density of 900 W kg−1 [81]. The high-performance carbon electrode material

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has also been derived from recycled jute [82]. The CV curve depicted in Fig. 5.5a shows an ideal characteristic of EDLC behavior showing rectangular nature in 3 M KOH. The shape of the curve has been retained at a high scan rate suggesting a high rate of performance due to the porous structure that facilitates easy and smooth transportation of electrolyte ions preventing distortion at a high scan rate. Figure 5.5b shows that capacitance decreases on increasing the scan rate due to the insufficient time for adsorption and desorption process. GCD plot shows a symmetrical triangular curve, which is retained at high current density suggesting a high rate of performance as shown in Fig. 5.5c, d. The small deviation in charge-discharge characteristics has been observed at low density due to the different rates of adsorption and desorption of electrolyte [83]. The highest energy density is reported to be 21 Wh kg-1 and the highest power density obtained is 1.82 kW kg-1 . The cyclic stability of jute derived carbon has been determined through continuous charge-discharge curves over 5000 cycles. Around 100% capacitance retention has been evidenced after 5000 cycles. Zhang et al. have prepared porous carbon microtubes and flake-based composites from waste bamboo chopsticks via facile hydrothermal treatment [84]. The CV curves exhibit a well-defined rectangle from 5 to 200 mV s−1 suggesting typical EDLC behavior in 6 M KOH electrolyte. The Nyquist plot suggests that at the high-frequency region, the equivalent series resistance is 0.2  whereas, the semicircle impedance loop shows a very low charge transfer resistance of 1.3 . The small ion transport at

Fig. 5.5 a CV curves of carbonized jute at various scan rate, b variation of specific capacitance versus scan rate, c charge-discharge characteristics of carbonized jute and d variation of specific capacitance versus applied current. redrawn and reprinted with permission from [82]

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the low-frequency region suggests that the sample favors access of ions within the electrolyte. The stability of electrode materials has been examined through repeat charge-discharge cycles at 1 A g−1 . It maintains at about 96% of the initial specific capacitance with a slight fluctuation over 1000 cycles indicating that the as-prepared activated carbon shows good material stability [84]. Tian et al. have synthesized bamboo-based by-products using the hydrothermal process [85]. The unique beehivelike nanoporous activated carbon shows almost symmetric rectangular curves with weakly broadened humps even at a high scan rate of 400 mV s−1 . The GCD plot shows linear behavior with symmetric triangular shape at different current densities of 0.1–100 A g−1 . The highest specific capacitance of 301 F g−1 has been achieved at 0.1 A g−1 . The CV and GCD curves indicate the dominance of electric doublelayer capacitance behavior. The charge-discharge plot implies low equivalent series resistance as voltage drop is very less leading to large power delivery. The cyclic stability of the material shows 100 capacitance retention after 20,000 cycles at 1 A g−1 [85]. Hydrothermal route of synthesis is a fast and efficient technique to achieve high performance activated carbon. The synthesis condition allows the attachment of functional groups that are present in the biomass with the carbon matrix. This promotes the full utilization of biomass and biowaste as efficient resource material for preparing functionalized activated carbon. The presence of functional groups contributes to pseudocapacitance and also enhances the conductivity inside the carbon skeleton.

5.6.1.3

Electrochemical Performance by Microwave-Assisted Activation

The microwave-assisted heating conversion uses microwave energy as a heating source. Unlike conventional heating, it requires less time and leads to high-energy savings. Microwave-assisted synthesis of activated carbon leads to significant adsorption properties, high surface area, diverse pore sizes, and induction of heteroatom functionalities. High surface area mesoporous carbon has been synthesized from a renewable by-product rice husk via a one-step ZnCl2 activation process utilizing microwave-assisted heating. Excellent performance of the activated carbon has been observed in 6 M KOH electrolyte. Impedance measurement shows lower contact resistance and hence, high conductivity. The rectangular shape of the CV curves indicates high ion diffusivity in the pores, which promotes bilayer formation. The symmetric shape of the electrode implies a quick diffusion and good charge propagation. The specific capacitance obtained is 233 F g−1 at a current density of 0.05 A g−1 , which drops to 233 F g−1 with the discharge current density of 2.0 A g−1 retaining 95.1% of its initial specific capacitance. The cycle life study shows a slight decrease in specific capacitance even after 1000 charge-discharge cycles. These results indicate that the mesoporous carbon derived from rice husk shows low densities of surface functional groups and good conductivity. The ZnCl2 activation of rice husk is an efficient approach for the synthesis of porous carbon to fabricate a high-performance supercapacitor [86].

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Microwave-assisted activation is a new and fast approach for the preparation of activated carbon. The two major advantages associated with microwave heating is even heating and less preparation time. This process can overcome major limitations associated with conventional heating without compromising the electrode performance.

5.6.2 Electrochemical Performance of Doped Activated Carbon Besides the preparation of activated carbon by different synthesis mechanisms, many studies have been carried out to modify the porous carbon by the addition of foreign atoms on the surface. This enhances the wettability of carbon surface and promotes ease in the conduction of electrons through the carbon surface. Commonly, heteroatoms such as nitrogen and oxygen elements are doped in the carbon structure that enhances the material properties for supercapacitive applications. Depending on the requirement, other heteroatoms can also be added by modifying the synthesis routes. For this reason, nitrogen-containing precursors such as lignin or flow of ammonia gas are employed during pyrolysis or mixing along with activators to prepare nitrogen-doped carbon [87, 88]. For the addition of oxygen, H2 O2 and other oxygen compounds are also added [63, 89, 90]. Some of these approaches are discussed in the following section. Jiang et al. have demonstrated one-step construction of nitrogen-doped porous carbon from Auricularia biomass through the carbonization process using ZnCl2 as an activating agent and NH4 Cl as the nitrogen source [69]. The specific surface area obtained is 1607 m2 g−1 with a high mesopore ratio of about 91% and the nitrogen doping obtained is 4.8 at.%. The electrochemical studies show a specific capacitance of 347 F g−1 at 1 A g−1 in 6 M KOH. The material has also exhibited excellent capacitance retention of 278 F g−1 at 50 A g−1 . No voltage (IR) drop in the GCD curve has been observed indicating small equivalent series resistance (ESR). The CV curve also shows quasi-rectangular nature indicating capacitive behavior, which is maintained up to 500 mV s−1 . At a low scan rate, the small hump is observed suggesting a little contribution from the redox reaction. Both N and O functionalities enhance surface wettability by contributing to pseudocapacitance in aqueous electrolytes. For the real application of the electrode material, the symmetric supercapacitor has been assembled with 1 M H2 SO4 aqueous electrolyte. The CV and GCD curves indicate EDLC behavior with slight distortion due to induced pseudocapacitance [91, 92]. From the Ragone plot, the maximum amount of energy stored by the porous carbon symmetric supercapacitor is found to be 22 Wh kg−1 at a power density of 213 W kg−1 [69]. Other literature reports the modification of coconut shell derived activated carbon surface with nitrogen and oxygen using melamine and urea. To investigate Faradaic interaction on oxygen/nitrogen groups, the same carbon material is kept as both working and counter electrode, Ag/AgCl is used as a reference electrode in 1 M H2 SO4 .

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The analysis shows that nitrogen and oxygen-containing functional groups at pores larger than 10 Å contribute to pseudocapacitive behavior whereas pores between 5 and 6 Å participate in electric double-layer formation. The overall capacitance has been enhanced due to the presence of positively charged quaternary and pyridinicN-oxide nitrogen groups, which improves electron transfer at a high current load. The enhancement in charge storage performance is due to the pyrrolic and pyridinic nitrogen along with quinone oxygen [93]. Li et al. have utilized kelp derived activated carbon prepared by carbonization in the presence of argon and ammonia [92]. The CV curve depicted in Fig. 5.6a shows rectangular nature as well as a triangular GCD profile demonstrating reversible capacitive behavior in 6 M KOH electrolyte. The CV curve maintains its shape even at a high scan rate of 500 mV s 1 implying extraordinary performance at a high rate of operation. EIS of porous carbon shows a small diameter of semicircle implying less charge transfer resistance on the electrode surface. Also, a higher slope at the low-frequency region indicates better pore accessibility of the electrolyte ions. The maximum specific capacitance of 440 F g−1 has been achieved at a current density of 0.5 A g−1 , which is maintained up to 180 F g−1 at a high current density of 150 A g−1 . This high capacitance performance is due to the unique three-dimensional structure with a high surface area that creates an efficient electron path and accessibility of electrolyte ions to access the electrochemically active element. Kelp derived activated carbon shows excellent energy density (as shown in Fig. 5.6b) of 28.8 Wh kg−1 and retained up to 14.4 Wh kg−1 at a maximum power density of 14.4 W kg−1 . The volumetric capacitance is also calculated to evaluate the performance of the electrode material that comes out to 360 F cm−3 . A comparison of the volumetric capacitance of various carbon material is shown in Fig. 5.6c. The cyclic stability shows 95.4% capacitance retention after 15000 cycles under a sweep rate of 50 mV s−1 shown in Fig. 5.6d This indicates the strong architecture of the 3D structure and superior stability of nitrogen or oxygen functional groups. The potential application of as-synthesized carbon material is tested in a two-electrode system using 1 M H2 SO4 as an electrolyte. The CV profile shows a rectangular shape indicating ideal capacitive behavior. The symmetric supercapacitor displays an impressive energy density of 28.8 Wh kg−1 and excellent cyclic stability of 92.3% after 10000 cycles. Kelp derived 3D structural carbon not only exhibits remarkable capacitance and stability but also taking into account of cost-effective and scalable approach towards the development of alternative green energy resources [37]. Doping of activated carbon is one of the essential tools to prepare functionalized activated carbon. Carbon as an element allows the addition of various heteroatoms in a stable state. Concerning the electrochemical performance, functionalized activated carbon enhances the wettability of activated carbon by changing the surface polarity and also participates in a redox reaction to provide extra electrons resulting in pseudocapacitance. It is well known that the charge storage mechanism of the carbon material is by the bilayer formation at the surface of the electrode. So, the performance of activated carbon is limited to its surface area. The addition of these foreign elements improves the electrochemical performance by promoting pseudocapacitance.

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Fig. 5.6 a CV curves of kelp derived activated carbon symmetric capacitor at a scan rate of 50 mV s−1 in different voltage windows; b Ragone plots of kelp derived activated carbon (KCN700) and other carbon-based symmetric supercapacitors [94–98]; c comparison of the volumetric and gravimetric energy densities of different symmetric supercapacitors using carbon-based electrode materials in aqueous electrolytes [95–97, 99–104]; d cycling stability of the symmetric supercapacitor within 10,000 cycles at a scan rate of 50 mV/s and (insets) CV curves of the selected cycles [37]. Redrawn and reprinted with permission

5.6.3 Electrochemical Performance of Symmetric Supercapacitor in Organic Electrolyte and Ionic Liquid Electrolyte Compared to batteries and fuel cells the major limitation suffered by supercapacitor is its low energy density. To overcome this challenge, extensive works have

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been done to increase the energy density of the supercapacitor. Besides, improving the capacitance, increasing the potential widow is one of the major factors to get a high energy density device. This is because energy density is directly proportional to the square of the operating voltage. In this regard, electrolyte plays an important role to resolve the issue of energy density. The choice of electrolyte for a particular electrode depends upon the matching of electrolyte ion size with the pore size of the carbon electrode. Besides this, the ionic conductivity of electrolyte determines the internal resistance of the device [105]. Among the large variety of electrolytes, mainly aqueous electrolytes, organic electrolytes, and ionic liquid electrolytes are used. Aqueous electrolyte possesses high conductivity and specific capacitance but its operating voltage is limited due to the decomposition of the aqueous electrolyte. The aqueous electrolyte is stable in the potential range of 0.6 to 1.0 V in case of symmetric EDLC, whereas operation voltage of organic electrolyte and the ionic liquid electrolyte is around 2.2–2.9 V. Since, the choice of electrolyte determines the maximum operating voltage, it strongly affects the energy density of assembled supercapacitor. The organic electrolyte consists of organic salts [e.g., tetraethylammonium tetrafluoroborate (TEABF4 )] dissolved in the acetonitrile (ACN) or propylene carbonate (PC) solvent. Another example of organic electrolyte used for carbon electrodes is lithium hexafluorophosphate (LiPF6 ) dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC). Organic electrolyte offers very high cycle life along with high operational voltage. Due to their remarkable properties, these electrolytes are commercially used in EDLCs with activated carbon electrodes [20]. Ionic liquids are other class of electrolytes, which are also known as room temperature molten salts. These electrolytes have large asymmetric organic cation and inorganic anion. Few examples of ionic liquid used for carbon electrodes are organic cations such as 1ethyl-3-methylimidazolium (EMIM+ ) with inorganic anion such as tetrafluoroborate (BF4 − ) or bis(trifluoromethanesulfonyl)imide (TFSI− ) [105]. To study the increment in energy density, some of the reported organic, as well as ionic liquid electrolytes for used for the development of activated carbon-based electrode materials are discussed herein. Low dimensional structured activated carbon has been derived from hemp hurd and bust via hydrothermal carbonization followed by KOH chemical activation. Two electrode supercapacitor cell is prepared to test the performance of the electrode in organic electrolyte 1.8 M TEMABF4 /PC. In the low-frequency region, the curve shows high capacitive behavior due to the formation of ionic and electronic charges of the electric double layer on the surface of micropores. The quasi-rectangular shape of the CV curves has been observed suggesting that charges are stored due to the formation of the bilayer. The CV curve indicates a rapid current response to the change of voltage from 0 to 2.5 V and a fast ion transport during charging and discharging. The charge/discharge behavior shows excellent reversibility with linear discharge curves. The overall properties demonstrate excellent capacitive characteristics of the supercapacitor cells [106]. Using the same electrolyte, two electrode assembly for activated carbon prepared from various pollens show specific capacitance of 185 F g−1 in 1 M TEABF4 /ACN and 207 F g−1 in neat EMIM BF4 electrolytes at a current density of 1 A g−1 . Gravimetric energy densities are found to be

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46 Wh kg−1 in 1 M TEABF4 /AN and 88 Wh kg−1 for neat EMIM BF4 electrolyte, respectively. Based on the weight of electrode materials, pollens exhibit good cycle stability [41]. Another organic electrolyte such as 1 M LiPF6 EC/DEC has been used for the assembly of human hair derived activated carbon. At a low scan rate, CV shows near rectangular behavior, which gets deformed with an increase in scan rate due to the inability to access the surface in a short time. The GCD shows a specific capacitance of 107 F g−1 at 2 A g−1 . Also, the Ragone plot shows an excellent specific energy density of 45.33 Wh kg−1 at a current density of 0.1 A g−1 . For practical application, Tian et al. have utilized a two-electrode assembly to study the performance of beehive-like nanoporous activated carbon in a neat ionic liquid electrolyte of EMIM TFSI with a wide potential window of 3.5 V [85]. The CV shows an almost rectangular loop while GCD exhibits symmetric triangular shape indicating the dominance of electric double-layer formation at the electrode-electrolyte interface. The maximum specific capacitance of 146 F g−1 at 0.2 A g−1 has been obtained, which is retained up to 74% till 50 A g−1 . The symmetric supercapacitor exhibits a high energy density of 9.5 Wh kg−1 at a power density of 25 W kg−1 , whereas energy density remains up to 6.1 Wh kg−1 at a power density of 26 kW kg−1 [85]. However, organic electrolytic provides an excellent potential window, which results in the enhancement of energy density. These electrolytes suffer serious limitations. Most of the organic electrolytes are expensive. The handing of organic electrolyte is not easy. Also, these electrolytic compounds suffer from high flammability and potential explosion risk due to high vapor pressure [20]. Nowadays, research has been promoted towards the use of aqueous electrolytes. Aqueous electrolytes are cheap and easy to handle. Since aqueous electrolyte suffers from lower operating potential as theoretical splitting potential of water is around 1.23 V. More input is required to enhance the operating potential of supercapacitor in aqueous electrolyte. Few others activated carbon derived from different precursors i.e., cassava [107], human hair [32], fish gill [108], rice husk [109], bamboo chopsticks [84], etc., are promising materials. Therefore, supercapacitors become an emergent technology to breakthrough in energy storage systems with miscellaneous applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, aerospace, etc., [110, 111]. Table 5.2 summarize the list of biomasses derived activated carbon precursors, the activating agents, and electrochemical properties obtained therein.

5.7 Concluding Remarks Activated carbon offers promising solutions for supercapacitive electrode material, which boosts the amount of energy stored, without compromising with cyclic stability and power capability. A variety of methods and activators as discussed here are available for the development of various nanostructured activated carbon. The inherent heteroatom doping in the carbon matrix further enhances its suitability to be utilized in the supercapacitor electrode application. The flexibility to tune the

Paulownia flower

Rice husk

Waste carton box

Waste coffee grounds

Auricularia

Bamboo chopsticks

Jute

Bamboo by-product

14

15

16

17

18

19

Corn cob residue

8

13

Soya

7

12

Willow Catkins

6

Sunflower seed shell

Rice husk

5

11

Human hair

4

Fish gill

Tremella

3

Cotton stalk

Waste Tea leaves

2

10

Cassava

1

9

Precursor

S. No.

Hydrothermal and Chemical

Hydrothermal

Hydrothermal and Chemical

Hydrothermal

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical

Chemical and Physical

Activation mechanism

KOH

1 M H2 SO4

KOH

KOH

ZnCl2

NaOH–KOH melt

KOH

KOH

KOH

KOH

KOH

NaOH

NaOH

KOH

KOH

KOH

KOH

KOH

KOH and CO2

Activating agent

3 2

EMIM TFSI

3

3

3

2

2

2

2

2

2

3

3

3

6 M KOH

3 M KOH

6 M KOH

6 M KOH

1 M H2 SO4

6 M KOH

6 M KOH

1 M H2 SO4

30 wt% KOH

1 M H2 SO4

6 M KOH

1 M H2 SO4

1 M H2 SO4

3

2

1.5 M TEA-BF4 6 M KOH

2

3

3

3

2

System

6 M KOH

6 M KOH

6 M KOH

2 M KOH

0.5 M H2 SO4

Electrolyte

at 1 A

146 F

at 0.2 A

(continued)

[85]

301 F g−1 at 0.1 A g−1 g−1

[82]

408 F g−1 at 1 mV s−1 g−1

[84]

[38] [96]

at 0.05 A

212 F g−1 at 0.1 A g−1

386 F

g−1 196 F g−1 at 5 mV s−1

g−1

[80]

[79] [109]

at 0.5 A

311 F g−1 at 0.5 A g−1

324.1 F

147 F g−1 at 0.1 A g−1

g−1

[33] g−1

[67] 250 F g−1 at 0.1 A g−1

at 10 mV

[108]

229 F

254 F g−1 at 0.2 A g−1

[78] s−1

[65]

575 F g−1 at 5 mV s−1 g−1

[76]

193 F g−1 at 0.5 A g−1

[77]

[32]

306 F g−1 at 0.1 A g−1

174 F g−1 at 5 mV s−1

367 F g−1 at 5 mV s−1

340 F

g−1

[30]

g−1

at 1 A

[75]

330 F 71 F g−1 at 1 A g−1

[107] g−1

264.08 F g−1 g−1

References

Capacitance

Table 5.2 Comparative electrochemical performance of different supercapacitors electrodes based on biomass-derived activated carbon

5 Activated Carbon as Electrode Materials for Supercapacitors 139

Precursor

Coca Cola

Rice husk

Auricularia

Kelp

Hemp

Human hair

Pollens

S. No.

20

21

22

23

24

25

26

Table 5.2 (continued)

Chemical

Chemical

Hydrothermal and Chemical

Carbonization

Activation and surface modification

Microwave assisted

Hydrothermal and Chemical

Activation mechanism

KOH

KOH

KOH

Ammonia

ZnCl2 & NH4 Cl

ZnCl2

–

Activating agent

2 2

1L of EMIM B4

2

2

3

3

2

3

System

1 M TEABF4 /AN

1 M LiPF6 EC/DEC

1.8 M TEMABF4 /PC

6 M KOH

6 M KOH

6 M KOH

6 M KOH

Electrolyte

at 1 A

[106] [32] [41]

107 F g−1 at 2 A g−1 185 F g−1 at 1 A g−1 207 F g−1 at 1 A g−1

160 F

at 1 A

[37]

[69]

g−1

g−1

440 F g−1 at 0.5 A g−1

347 F

[86]

245 F g−1 at 0.05 A g−1 g−1

[34]

352.7 F g−1 at 1 A g−1

g−1

References

Capacitance

140 P. Sinha et al.

5 Activated Carbon as Electrode Materials for Supercapacitors

141

structure and properties of activated carbon by changing the synthesis condition, opens the window for diverse application. The mechanism offers low-cost elemental doping, which increases the surface wettability during the charge-discharge process. Biomass-derived activated carbon stores charge by the formation of an electric double layer and a small fraction of pseudocapacitance. The synergistic effect of both charge storage mechanisms makes activated carbon as an excellent electrode material for supercapacitor applications. To increase the energy density of the activated carbon material, various approaches can be adopted: (1) to tune the morphology and modify carbon surface with heteroatom species to increase the overall capacitance of the material and (2) to use organic electrolyte, which offers a wide potential window. Also, the possibility of using aqueous electrolytes makes it more economic and environmentally friendly. This chapter elaborates on the theoretical benefits and practical value of activated carbon material as an efficient electrode for supercapacitor applications due to the provisions of high energy and power densities combined with excellent material stability. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 6

Transition Metal Oxide/Activated Carbon-Based Composites as Electrode Materials for Supercapacitors Prerna Sinha, Soma Banerjee, and Kamal K. Kar

Abstract The supercapacitor is a new generation charge storage device, which can satisfy the demand of huge energy and power density. First-generation supercapacitor deals with an electric double-layer capacitor (ELDC) and pseudocapacitor electrode material, which suffers from their limitations of low energy density and stability. To develop a high-performance supercapacitor, a second-generation hybrid supercapacitor device comes into play. Various combinations of assembly and composite electrode materials are designed, which can deliver a high energy and power density along with high rate capability and cyclic stability. Among various hybrid supercapacitors, this chapter deals with the composite electrode materials, which can overcome the limitation of individual components and enable large amounts of charge storage. The composite material is a combination of transition metal oxide serving the role of a pseudocapacitive material and activated carbon as EDLC material. Activated carbon shows good conductivity along high surface area and porosity. The main advantages of using activated carbon are tuneable porosity and low cost; however, capacitance is restricted to the surface of the electrode that is responsible for delivering low capacitance value. Nowadays, activated carbon has been synthesized using various biomasses and biowastes, which further extend the availability of cost-effective starting materials for the synthesis of activated carbon. On the other hand, transition metal oxide shows multiple oxidation states, which participate in Faradaic redox reaction delivering high capacitance however, unable to express high cyclic stability due to poor conductivity. Transition metal oxide and activated carbon composite overcome the limitations of individual components as metal oxide can P. Sinha · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] P. Sinha e-mail: [email protected] S. Banerjee Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_6

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reside in carbon matrix providing electrical conductivity to the electrons generated during redox reaction from the metal oxides. The large specific surface area of activated carbon and Faradaic redox reaction in metal oxide are expected to deliver excellent electrochemical performance in the device.

6.1 Introduction The growing energy demand and depletion of fossil fuel require the development of high-performance and low-cost energy storage and production systems [1, 2]. The evolution of supercapacitors from conventional capacitors has promoted the use of supercapacitors for energy storage applications. Supercapacitor offers much higher power density than the conventional capacitor due to its unique charge storage ability. It can be used as a replacement or complementary with other energy storage systems. Some of the applications, where supercapacitors can replace batteries, are backup power sources for gadgets, energy storage generated from renewable sources like solar or wind [1]. Supercapacitor stores electrical energy due to the formation of electric bilayer at electrode–electrolyte interfaces. It works on the mechanism of adsorption of ions at the electrode surfaces. Hence, higher will be the adsorption of ions, greater will be the capacitance. The special features of capacitors [3] and capacitor to supercapacitor [4] are reported elsewhere. Generally, it is understood that the high surface area materials are technically suitable as electrode material for supercapacitor applications. Till now, carbonaceous materials have been the most suitable electric double-layer capacitor (EDLC) electrode material because of its physicochemical properties, appreciable conductivity, and low cost. Among all forms of carbon, activated carbon (AC) has been widely studied as electrode material. It offers an extremely high surface area, tunable pore size, high electrical conductivity, excellent stability, and low cost. All these properties help to deliver excellent power density of the devices. Besides all these advantages, carbon-based electrode in supercapacitor limits in delivering high energy density. Since the storage of ions takes place mainly at the surfaces, the contribution of the bulk material is almost nominal [5]. Currently, the research is focused to improve the performance and overcome the limitations of the low energy density of the supercapacitors [6]. In continuation of this, another class of materials, metal oxides, and conducting polymer-based composites come into existence, which delivers high specific capacitance compared to conventional carbonaceous material [7]. High capacitance can be achieved for conducting polymers and metal oxides due to the reversible redox reactions also called Faradaic reaction. The capacitance arises from the Faradaic process called pseudocapacitance. Although among pseudocapacitive materials metal oxide shows more stability than conducting polymer [8], the major limitation associated with conducting polymer is its stability over repeated charge–discharge cycles. The polymeric chain swells and shrinks during the charge–discharge process, which decreases the material stability. Despite the theoretical expectation of metal oxides to deliver high specific capacitance and store a large amount of energy, their poor

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cyclic stability and rate capability due to low conductivity limit their performance in practical application. To overcome these inherent limitations of pseudocapacitive material, the formation of composite with conducting material such as AC is expected to be beneficial. A detailed review of metal oxide and conducting polymer composites as electrode material for supercapacitor, their properties, synthesis methodology, and application areas have been included in Chap. 14. Carbon materials exist in various forms from zero-dimension to three-dimensional materials. To achieve high energy and power densities along with good cyclic stability and rate capability research has been focused on the development of secondgeneration hybrid supercapacitors [9]. The second-generation hybrid supercapacitors exhibit capacitive and Faradaic charge storage mechanisms that are capable to achieve high-performance materials. In this regard, various carbonaceous-metal oxide composite electrodes have been developed by combining metal oxides into varieties of nanostructured carbon morphology [10]. Metal oxide and carbon composite combines the energy characteristics of pseudocapacitive material (via fast redox reversible reaction) with cyclic stability and power characteristics of carbon materials (electrostatic fast ion adsorption–desorption at electrode–electrolyte interfaces and robust cycle life), thereby overcoming the limitations of both constituent materials. Concerning dimensionalities of carbon material, three-dimensional materials such as carbon foam, carbon aerogel are combined with various metal oxides. These carbon materials enlarge the total active volume of the material resulting in compact energy storage devices [10]. Two-dimensional carbon structures such as graphene/reduced graphene oxide provide high surface area and high electronic conductivity, which is suitable for fabricating flexible devices [11]. One-dimensional carbon structures such as carbon nanotubes, carbon fibers, etc., facilitate the charge transport mechanism [12]. Zero-dimensional carbon materials such as AC, mesoporous carbon, carbon nanosphere are one of the most versatile classes in carbon family. They offer flexibility for altering the porosity depending on the electrolytes. Following metal oxide composites, AC provides (i) large pore volume that can store a large amount of charge (ii) ion transport channel for the fast delivery of ions and (iii) active pore walls capable to increase the specific capacitance. These above features of AC make it suitable to combine with metal oxides for synthesizing high-performance electrode materials for supercapacitor devices [10].

6.2 Supercapacitor Electrode Based on Charge Storage Mechanisms The electrochemical performance of a supercapacitor depends upon the type of electrode material. Figure 6.1 shows a broad overview of the specific capacitance of various electrode materials. Generally, the properties depend upon microstructure,

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Fig. 6.1 Specific capacitances for both EDLCs and pseudocapacitors including metal oxides and conducting polymers in single or composite electrode materials (redrawn and reprinted with permission from [14])

crystal, and electronic structure of the material used as electrode [13]. The electrode material plays an important role in storing charge and performance of the device. An ideal electrode material should have [10]: • • • • • •

High surface area contributes to the specific capacitance Controlled porosity affecting the surface area and rate capability High electronic conductivity controlling the rate capability and power density Presence of electroactive sites contributing to pseudocapacitance High thermal and chemical stability contributing to cyclic stability Low cost of raw material and preparation.

6.2.1 Electric Double-Layer Capacitor The charge storage mechanism of the EDL capacitor is based on the formation of double electric charge layers. The charge separation mechanism is similar to the conventional capacitor, where two conducting plates are separated by a dielectric medium. In the EDL capacitor, instead of conducting plates, high surface area porous conducting materials are used. In addition to the surface area, the distance between

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the bilayer of electric charges remains low, which further increases the capacitance compared to the conventional capacitor. The enhancement of surface area (< 1000 m2 g−1 ) enables the use of supercapacitor in energy storage applications. Although charges are mainly stored by the adsorption of electrolytic ions at the electrode– electrolyte interfaces, the surface area plays an important role to provide capacitance in the electrode. Figure 6.2a shows the schematic representation of the charge storage mechanism of the EDL capacitor. The more will be the surface, the greater will be the capacitance value [15]. Electrode materials used in the EDL capacitor are generally from carbon family. Carbon portrays as a suitable material for EDLC due to its ability to exist in various forms and structures. In an amorphous state, its degree of graphitization displays

Fig. 6.2 Schematic representation of a carbon-based electric double layer capacitor, b metal oxidebased pseudocapacitor, c carbon- and metal oxide-based asymmetric hybrid supercapacitor and d carbon- and battery type electrode-based hybrid battery type capacitor (redrawn and reprinted with permission from [42, 43])

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a wide variety of micro-texture and it can be engineered from zero-dimensional to three-dimensional structure [16]. This includes amorphous carbon such as AC, mesoporous carbon, carbon aerogel, etc., and carbon nanomaterials such as carbon nanotubes (CNTs) [17], graphene [18], carbon dots [19], etc. Carbon materials are preferred for EDL capacitor electrodes due to the high surface area and electrical conductivity, low cost, ease in handling, widely available precursor, and simple production methodologies [8]. The carbon-based electrode delivers high stability over a wide temperature range. These electrodes are stable in both acidic and basic solutions. By using different methodologies, it is possible to tailor the physicochemical properties such as high surface area and controlled pore size distribution of carbon material. In addition to various properties exhibited by carbon electrodes, the performance can be further enhanced by the addition of foreign atoms at the carbon surface. The presence of surface functionalities boosts the capacitive performance by contributing to pseudocapacitance and increasing the wettability of the carbon surface [20]. In this regard, AC, a type of amorphous carbon is an excellent electrode material for supercapacitor applications [21]. AC exhibits all the carbonaceous properties including chemical inertness and temperature stability [22]. The synthesis technique is simple and can be prepared in bulk. Nowadays, research has been promoted for the use of biomass and biowaste derived carbon precursors for the preparation of AC; this further slashes out the cost issue [23–27]. Depending on the application, the properties can be tailored leading to the high surface area with controlled pore size distribution in the material. AC is also compatible with the addition of heteroatoms in the carbon skeleton, which further helps to increase the performance by doping with redox-active species [22, 28–30]. Despite all the interesting features exhibited by the carbon family, they lack to produce high capacitance. Since the main mechanism of charge storage is the electrostatic interaction of electrolytic ion with the electrode surface, the performance of the electrode material depends on the availability of surface area [1, 31].

6.2.2 Pseudocapacitance Pseudocapacitor electrode material stores charge by the process of the Faradaic redox reaction of the electrode material with the electrolyte. The electrons produced by the redox reaction are transferred across the electrode–electrolyte interfaces, where the transfer of electron takes place between electrode and electrolyte [10]. Figure 6.2b shows a redox reaction undergone by pseudocapacitive electrodes. Unlike the EDL capacitor, where the capacitance remains constant and independent of voltage, for pseudocapacitance electrode materials most of the charge is accumulated at the surface or in the bulk near the surface [32]. The interaction of electrolyte ion with an electrode surface involves Faradaic reactions. Also, the transfer of charge during the Faradaic reaction is voltage-dependent. In general, three types of electrochemical

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processes occur during the charge–discharge process of pseudocapacitive material [10]: 1) adsorption of electrolytic ions at electrode surface 2) redox reaction between electrode and adsorbed electrolytic ions 3) doping and doping with active conducting polymer material The first two processes strongly depend on the surface area, whereas the third process occurs in conducting polymer electrode material. Although it is necessary to have high surface area, the overall capacitance of pseudocapacitive material is less dependent on its surface area. Instead of this, the electrode should show high electronic conductivity to collect electron current [33]. Metal oxides are pseudocapacitive electrode material. It promotes electronconducting reactions due to the presence of oxides. Charging and discharging of charge occurs with proton insertion and liberation. Metal oxides show better reversibility and long-time stability among other pseudocapacitive material and give high capacitance compared to EDLC electrode material. The general requirements of the metal oxides to be used for supercapacitor application are (i) the oxides should have good electrical conducting property (ii) metal should exist in more than one oxidation state having no phase change during the reversible reaction and (iii) protons can easily intercalate into oxide lattice and allow ease in conversion between O2− ↔ OH− [34] Among various metal oxides, transition metal oxides such as ruthenium oxide (RuO2 ), nickel oxide (NiO), manganese dioxide (MnO2 ), cobalt oxide (Co3 O4 ), etc., are examined as pseudocapacitive electrode materials. Transition states of metal oxides at particular voltage give rise to the local or constant charge that depends on the number of redox couples defined in the operational potential [35]. RuO2 shows the best pseudocapacitive material in terms of capacitance and electrical conductivity. Although, high cost limits the use of RuO2 for commercial applications. Due to the high cost, research has been progressed toward the synthesis of mixed metal oxidebased electrodes. The use of metal oxide as a supercapacitor electrode is limited due to the self-discharge, low operational potential window, low electrical conductivity, and low cyclic stability. In this regard, other metal oxides are explored as pseudocapacitive material. Another approach has also been studied, where different forms of composites are formed to minimize the metal oxide loading, hence reducing its cost and increasing electronic conductivity [36]. Conducting polymers are another class of materials that can undergo fast and reversible redox reaction [37]. During oxidation, ions are transferred to the polymer backbone, when reduction occurs ions are realized from polymer chain to the electrolyte. They have high electronic conductivity and operating voltage and can accumulate a high amount of capacitive charge throughout the entire volume of the electrode material [38]. Also, they have environmental stability, high flexibility, low cost, and ease of large-scale production [39]. Some common conducting polymers used as

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supercapacitor electrode materials are polyaniline (PANI), polypyrrole (PPy), polythiophene (PTh), and poly(3,4 ethylenedioxythiophene) (PEDOT). Doping of metal oxides in the conducting polymer further enhances the capacitance value as it leads to an increase in electronic conductivity. These polymers are inexpensive and of good conductivity [35, 40, 41]. The major drawback of conducting polymer as a pseudocapacitive electrode is the stress induced by fast volumetric swelling and shrinkage during the process of intercalation and deintercalation. This results in mechanical degradation and instability in the material.

6.2.3 Hybrid Supercapacitor Hybrid supercapacitors are the latest generation technology also popularly known as the second generation of supercapacitors [44]. Major research in this field is devoted to the improvement of energy density and the exploration of new materials to increase the performance of supercapacitor devices. To achieve a high-performance supercapacitor, an increase in both operating voltage and capacitance of electrode are the key factors. With this motivation, three approaches are proposed (i)

replacing conventional materials with engineered nanostructured carbon or pseudocapacitive materials for high capacitance value (ii) choice of electrolyte to increase operating potential window and (iii) development of hybrid devices [36]. Hybrid supercapacitor includes two different electrodes, where the positive and negative electrodes are made of different materials. The two electrodes are made of EDLC-type electrode (i.e., materials from carbon family) and pseudocapacitive type electrode (i.e., materials from metal oxide and conducting polymer) or batterytype electrodes [45]. Depending on the assembly and type of electrode used, hybrid supercapacitors can be classified in different classes. A few of them are discussed herein.

6.2.3.1

Asymmetric Supercapacitor

An assembly of asymmetric supercapacitor includes two different electrode materials. Generally, carbon-based EDLC material is used as the negative electrode and metal oxide or conducting polymer-based pseudocapacitive electrode acts as positive electrode material. Figure 6.2c shows the schematic representation of asymmetric supercapacitor. Therefore, the asymmetric assembly combines the power and cycling characteristics of carbon materials and energy characteristics of pseudocapacitive materials. This overcomes the limitations of constituent materials. Also, asymmetric assembly allows enlargement of working potential window beyond the thermodynamic limit of 1.2 V in aqueous electrolytes, thereby increasing the energy density [44].

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Lithium-Ion Battery-Type Capacitor

Lithium-ion capacitor is a battery type capacitor, which is characterized as a combination of an EDLC-type electrode and a lithium intercalation compound [46]. These types of capacitors deliver fast charge and discharge processes and high energy density with robust cycle life. The whole assembly consists of two electrodes, where one is made of an EDLC material, and the other consists of Li-ion compounds [47]. Figure 6.2d shows the schematic representation of battery-type capacitor. During the charge–discharge process, adsorption of ions and formation of bilayer occur in the EDLC electrode, whereas Li+ intercalation–deintercalation occurs in the other electrode of the assembly [44].

6.2.3.3

Composite Electrode for Supercapacitor

Both EDLC and pseudocapacitor can be simultaneously combined in a single electrode of the supercapacitor to form a hybrid supercapacitor. Hybrid supercapacitor uses Faradaic and non-Faradaic process to store charge to achieve high energy and power densities while maintaining good cycle life [10]. An innovative way to increase the performance of the supercapacitor is to induce high specific surface area and/or highly reversible redox reactions of the electrode materials. Hence, a combination of porous carbon of high surface area and hydrous transition metal oxides capable to undergo reversible redox reaction is the basic strategy followed for the development of composite electrodes for supercapacitor devices [10, 48, 49].

6.3 Materials 6.3.1 Transition Metal Oxides An important class of material that has achieved tremendous attention in the field of supercapacitor devices is the transition metal oxides (TMOs). TMOs are of prime importance as an electrode material for supercapacitors not only due to an excellent combination of structural, electrical, and mechanical properties but also their capacitive behavior. The unique capacitive behavior of TMOs arises due to the variable oxidation states of these materials exhibiting pseudocapacitance that carbon materials lack in general. TMOs are classified in two major classes, the noble metal oxides, e.g., RuO2 , IrO2 , etc., and base metal oxides, e.g., MnO2 , NiO, Fe3 O4 , etc. The noble metal oxides are of excellent electrochemical performance; however, the major drawback remains high cost. The other class of TMOs-based metal oxides can also be utilized for this purpose since they are of low cost and environment-friendly with the characteristics of moderate electrochemical performance. The TMOs can also be used in the form of thin films for supercapacitor devices. Commonly used

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TMOs used in the form of thin film are RuO2 , MnO2 , NiO, In2 O3 , Co3 O4 , NiFe2 O4 , BiFeO3 , etc. The nanostructure form of TMOs is of recent interest to excel in the performance of the supercapacitor devices. Different types of TMOs such as RuO2 , MnO2 , NiO, etc., have been utilized. In addition to the energy storage devices, TMOs cover diverse application areas such as flexible electronics, catalysis, biomedicine due to attractive combinations of electronic, surface, and biocompatible properties. A detailed review of the types of TMOs, their properties, synthesis methodology, and application areas has been reported elsewhere [50].

6.3.2 Activated Carbon Activated carbon (AC) is a class of carbon family, which delivers a range of physicochemical and structural properties along with a wide range of morphology. The composition of AC is mainly carbon element, which is attached in different hybridization state with other carbon elements. Commercial AC has been prepared from various petroleum products under harsh reaction conditions that limit their wide usage. Following this, various types of biomass-derived activated carbons are now synthesized showing remarkable properties along with cost-effectiveness. Depending on the type of application, the properties of AC can be easily tailored. AC exhibits high surface area, good electrical conductivity along with tunable pore sizes. These properties make it suitable to be used in major applications such as gas absorbent, electrode material for energy storage applications, water purification, etc. Mainly biomassderived AC is prepared via carbonization, and activation processes, where carbonization converts biomass into biochar and activation leads to the development of pore structure and enhancement in surface area of the carbon matrix. During synthesis, various factors are associated resulting in the formation of different morphologies and physicochemical properties in AC. Some of the factors include, (i) type of biomass used as a precursor (ii) temperature of carbonization and activation process (iii) type of activator used for activation (such as KOH, NaOH, ZnCl2 , H3 PO4 , K2 CO3 , etc.) (iv) heating rate, etc. Also, the utilization of biomass for the preparation of AC induces heteroatoms such as nitrogen, oxygen, and sulfur, which increases the wettability and reactivity of AC. Hence, for supercapacitor electrode application, mainly AC with the high surface area along with high conductivity achieves superior electrochemical performance. In addition to this, the presence of heteroatoms enhances the electrochemical properties by inducing pseudocapacitance via a Faradaic reaction. An exclusive overview of the development of AC and its role as the electrode material for supercapacitor devices has been reported elsewhere [51].

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6.4 Transition Metal Oxides as Electrode Materials for Supercapacitors Transition metal oxides (TMOs) promise electrode materials for electrochemical energy storage, as they exhibit higher specific capacity/capacitance and energy density due to their fast and reversible surface redox reactions. They exhibit much higher specific capacitance and energy density compared to EDLCs due to their fast charge–discharge process through fast Faradaic redox reactions occurring at the interface between TMO electrodes and electrolyte. These materials have the potential to overcome the low energy density limitation of electrochemical capacitors and low power density of the battery. Few TMOs like RuO2 , IrO2 , etc., have limited their widespread application in supercapacitors due to their very high cost and toxicity, even though they have high specific capacitance, good electrical conductivity, fast and reversible charge–discharge properties. But, MnO2 , NiO, Co3 O4 , etc., are alternatively considered for supercapacitor electrodes due to their cost-effectiveness, environment-friendly nature, and high electrochemical properties. However, still, their poor electronic conductivity, surface area, and power density are remaining as major disadvantages. Again, the poor cyclic stability of TMO electrodes is a permanent issue in energy storage applications due to their volume change in electrolyte solution during electrochemical analyses. The supercapacitive performance of different transition metal oxides is summarized in Table 6.1, according to the synthesis techniques and structures. A detailed review of the supercapacitive performance of different transition metal oxides has been included in Chapter 4 of this book.

6.5 Activated Carbon as Electrode Materials for Supercapacitors Activated carbon (AC) offers promising solutions for supercapacitive electrode material, which boosts the amount of energy stored, without compromising with cyclic stability and power capability. The flexibility to tune the structure and properties of AC by changing the synthesis condition opens the window for diverse application. Biomass-derived AC stores charge by the formation of an EDL and a small fraction of pseudocapacitance. The synergistic effect of both charge storage mechanisms makes AC as an excellent electrode material for supercapacitor applications. To increase the energy density of the AC material various approaches can be adopted: (1) to tune the morphology and modify carbon surface with heteroatom species to increase the overall capacitance of the material and (2) to use organic electrolyte, which offers a wide potential window. Also, the possibility of using aqueous electrolytes makes it more economic and environmentally friendly. Chapter 5 elaborates on the theoretical benefits and practical value of AC material as an efficient electrode for supercapacitor applications

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Table 6.1 Supercapacitive performance of different transition metal oxides (see Chap. 4 for references) Transition metal oxide

Synthesis technique

Structure

Capacitance (F Cycle g−1 ) life

RuO2

Sacrificial template

Nanotubular

860 at 0.5 A g−1

RuO2

Anodic deposition

Nanotubular array

1300

_

MnO2

Coprecipitation

Powder

1380

_

MnO2

Reduction

Crystalline α-phase

253 at 0.5 mA cm−2

54% after 1000 cycles

MnO2

Microwave heating and ultrasonication

Sphere-network

214

~90% after 5000 cycles

MnO2

Microwave-assisted emulsion

Birnessite-type 1D

277 at 0.2 mA cm−2

_

MnO2

Hydrothermal

Lamellar birnessite-type

242.1 at 2 mA cm−2

108% after 200 cycles

MnO2

Electrochemical deposition Film

410 at 1 mA cm−2

~100% after 10,000 cycles

MnO2

Electrodeposition

Nanowire array

493 at 4 A g−1 ~100% after 10,000 cycles

NiO

Calcination

Crystalline β-Ni(OH)2

696

NiO

Hydrothermal

Porous nanocolumns

390 at 5 A g−1 ~100% after 10,000 cycles

NiO

Annealing

Cubic

167

NiO

Gas/liquid interfacial microwave

Flowerlike hollow nanospheres

770 at 2 A g−1 95% after 1000 cycles

Co3 O4

Sol-gel

Xerogel

291

Stable up to 10,000 cycles

Co3 O4

Solution and thermal

Hexagonal nanosheets

227

_

_

_

_

(continued)

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Table 6.1 (continued) Transition metal oxide

Synthesis technique

Structure

Capacitance (F Cycle g−1 ) life

Co3 O4

Microwave-assisted hydrothermal

Nanorods

456

_

Co3 O4

Chemically depositing

Nanotube

574

95% after 1000 cycles

Co3 O4

Supported technique

Nanowire arrays

746

86% after 500 cycles

Co3 O4

Epoxide addition

Aerogel

623

96% after 1000 cycles

Co3 O4

Self-organization

Brush-like nanowires

1525

94% after 5000 cycles

Co3 O4

Solvothermal and calcination

Nanoflower

1936.7

78.2% after 1000 cycles

SnO2

Electrochemical deposition Amorphous nanostructured 285

88% after 1000 cycles

FeCo2 O4

Hydrothermal

Co3 O4 @NiO Hydrothermal

Nanoflakes array

433 at 0.1 A g−1

Stable up to 2500 cycles

Hierarchical nanowire array

720.71 at 1 A g−1

91.35% after 5000 cycles

due to the provisions of high energy and power densities combined with excellent material stability.

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6.6 Importance of Activated Carbon and Metal Oxide-Based Composites as Electrode Materials Here, the charge storage mechanism is achieved either by the Faradaic or nonFaradaic process. The reversibility adsorption and desorption process at the electrode–electrolyte interface determines the capacitive performance of the electrode material. Carbon is a class of material that exhibits EDL capacitance. Its electrode can be involved in a much more complex design to achieve a high-performance supercapacitor. Some of the well-known carbon material includes carbon nanotubes (CNT), graphene, carbon aerogel, AC, etc., [10, 52, 53]. CNTs and graphene are engineered carbon structure, which can act as excellent electrode material for supercapacitor application. They suffer from the severe drawback that limits their use in commercial applications. The mass production and purification are the major difficulties along with reproducibility concerning homogeneous conduction property. Also, carbon aerogel can be used as a supercapacitor electrode; however, the methodology of preparation is unsafe and expensive that restricts practical application. On the other hand, AC is a low-cost abundant material. It exhibits high surface area, tunable pore size, chemical, and structural stability. The electrons are transported via diffusion or hopping between trap sites among various pore sites [10]. AC may perform as a suitable and cheap electrode material for supercapacitor applications. The other charge storage mechanism includes a Faradaic process, where charges are stored via reversible redox reactions. The materials include metal oxide and conducting polymer. Metal oxides are electroactive species, which on certain voltage undergo redox reaction and deliver high capacitance value as compared to carbonaceous material [10, 54]. However, metal oxides are of poor electrical conductivity that delivers poor rate capability and cycle life. Following this, many studies have aimed to improve capacitance by combining the charge storage mechanism of Faradaic and non-Faradaic material. The capacitance of AC can be enhanced by introducing foreign heteroatoms at the electrode surface, which contribute to pesudocapacitance. Another possibility is the synthesis of AC and metal oxide composite. Among various approaches, integrating different metal oxides and AC shows great potential [8]. Composite of AC and metal oxide displays both capacitive and Faradaic charge storage. This increases the utilization of active material and also improves the mechanical strength and electrical conductivity of the composite material [8]. Metal oxide enhances the pseudocapacitance by a reversible redox reaction. AC shows capacitive response due to the accumulation of charge and formation of an EDL and provides excellent media for electron conduction, hence improving the performance [56]. Figure 6.3 shows the metal oxide and AC composite electrode assembled as a symmetric supercapacitor device. The magnified image explains the electron transport mechanism, where the redox reaction takes place on the surface of the metal oxide. The electron is diffused or conducted by a carbon block to the current collector. The mechanism reveals that the composite electrode generates a large number of charges due to the presence of metal oxide and the carbon

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Fig. 6.3 Schematic representation of metal oxide/activated carbon-based composite electrode for symmetric supercapacitor, the magnified image shows the electron transport pathway for composite electrode (redrawn and reprinted with permission from [55])

skeleton helps in the conduction of electron contributing to a significant reduction in overall resistance of the composite electrode.

6.7 Electrochemical Performance of Composite Electrodes Composites show a synergistic effect of both the constituents [10, 57]. The properties can be enhanced by the following methods (i) (ii) (iii) (iv) (v) (vi) (vii) (viii)

minimizing the particle size of metal oxides improving the specific surface area and controlled porosity of the composite preventing agglomeration of the particles inducing active sites expanding operation potential window improving proton and electron conductivity enhancing cyclic stability and providing the site for pseudocapacitance [8, 10, 58]

The key parameters along with their significance to determine the overall performance of the supercapacitor include: • specific capacitance (normalized by mass, area, or volume of the active electrode material): Signify ability of the material to store charge. • energy density (normalized by mass, area, or volume of the active electrode material): Ability of electrode to store energy. • power density (normalized by mass, area, or volume of the active electrode material): Rate at which energy can be released. • rate capability: signify capacitance retention at high current loading. • cyclic stability: representing durability and stability of the material with repeated charge–discharge cycles.

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The high energy and power densities can again be achieved by increasing the specific capacitance and operating the potential window while reducing equivalent series resistance. For supercapacitors, operating potential mainly depends on the type of electrolyte used. Various transition metal oxides such as RuO2 , NiO, MnO2 , Fe3 O4 , etc., are used to prepare composite with AC. The presence of multiple transition states in metal oxides generates charge within the operating potential window. In addition to this, the overall performance of the metal oxide electrode materials depends on the electrical conductivity and cyclic stability [59]. It is well known that among all forms of carbon, AC possesses high surface area and tunable pore size distribution along with interesting textural morphology. These overall properties of AC ensure proper skeleton to hold metal oxides leading to the formation of homogeneous composite materials. Metal oxide and AC undergo different charge storage mechanism. Hence, the combined effect of both the constituents of the composite generates an effective electrode material for supercapacitor application. The amount of metal oxide and method of fabrication significantly influences the surface area, surface chemistry, and electrochemical performance of the final composites [60, 61]. The electrochemical performance of some commonly used metal oxide/AC composites has been discussed in brief in the next sections.

6.7.1 Ruthenium Oxide and Activated Carbon Composites as Electrode Materials Among various metal oxides, RuO2 shows the best pseudocapacitive behavior. It is a conductive metal oxide having three accessible distinct oxidation states. It involves Faradaic charge transfer reactions. The rapid proton transport and good conductivity contribute to fast and reversible Faradaic reactions yielding extremely high capacitance [62]. The pseudocapacitive nature of RuO2 has been studied for the past 30 years [63]. It is used in the either crystalline or amorphous hydrous form. The amorphous form generally demonstrates extraordinary capacitance, high conductivity, and appreciable electrochemical stability [59, 64]. The potential range of amorphous RuO2 is about 1.35 V, greater than crystalline RuO2 of about 1.05 V measured in aqueous electrolyte [59]. RuO2 delivers about ten times higher capacitance as compared to carbon materials due to surface reactions between Ru and H ions [59, 65]. The redox reaction that governs the charges storage mechanism is depicted as follows: RuO2 + dH+ + de− ↔ RuO2−d · d(OH)d with 0 < d < 2

(6.1)

Literature suggests four main phenomena in RuOx .nH2 O regulating the capacitive performance: (i)

hopping of electrons within RuOx.nH2 O particles

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(ii) hopping of electrons within particles (iii) hopping of electrons between electrode and current collectors, and (iv) diffusion of protons within RuOx.nH2 O particles. Capacitive performance of amorphous RuOx·nH2 O is governed by electron hopping persuades intra-particle electron hopping resistance that can be addressed by the crystallization of RuO2 ; however, this may lead to the enhancement of the diffusion barrier of protons within crystalline RuO2 and subsequently, a loss of active site is possible [36, 59]. The major limitation associated is a high cost that restricts the use of RuO2 for commercial applications. The cost issue of RuO2 has shifted the focus toward the development of RuO2 -based composite electrodes. To minimize the content of RuO2 , foreign elements like metals or carbon can be introduced to improve conductivity. Generally, carbonaceous materials are used to form bulk composite electrodes. AC support on RuO2 composite can serve as excellent electrode material. AC possesses high surface area and good electrical conductivity and all at a low cost. In this regard, Sato et al. have prepared RuO2 /AC composite by dispersing Ru over high surface area AC, which minimizes the amount of Ru in the composite [66]. The composite shows a 20% increase in specific capacitance for 7.1 wt% Ru in AC. Another study of RuO2 /AC carbon composite in 1 M H2 SO4 displays that the increase in Ru content over a certain limit decreases specific capacitance and surface area of the resultant composite. This is due to the blockage induced via large RuO2 particles inside the mesopores of AC. The study reveals that 40 wt% of RuO2 loaded composite provides maximum capacitance with an energy density of 17.6 Wh kg−1 . Further increase in RuO2 leads to poor rate capability [67]. Jang et al. have carried out similar work, where the composite of mesoporous carbon with RuO2 has been prepared [68]. The unloaded mesoporous carbon shows capacitance of 100 F g−1 that rises to 243 F g−1 with 54.3% of RuO2 loading, as presented in Fig. 6.4 [68]. Dandekar et al. have synthesized RuOx (OH)x /AC composite at different RuOx (OH)x loading to optimize pore size distribution [56]. The electrochemical studies show that 9% of RuOx (OH)x gives the best result among other composites in 1 M H2 SO4 . The CV curve shows that current increases with the increase in the scan rate indicating a capacitive response. The presence of porous AC leads to an enhancement in charge Fig. 6.4 Specific capacitance delivered by carbon and ruthenium, and specific capacitance of ruthenium component (CRu sp ) as a function of ruthenium loading (redrawn and reprinted with permission [68])

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storage capability. Also, RuOx (OH)x shifts the cathodic peak toward negative potential and anodic peak toward positive potential. The reversible reduction–oxidation reaction from Ru4+ to Ru3+ at electrode–electrolyte interfaces appears at −0.4 to 0.1 V. At 9% of RuOx (OH)x loading, the capacitance reaches 250 F g−1 . Also, the studies show a linear relationship between the amount of ruthenium loaded in AC and the specific capacitance. EIS measurements and the Nyquist plot provide valuable information on charge transfer resistance and diffusion kinetics. Semicircle indicates the charge transfer process at the electrode–electrolyte interfaces and straight line at the low-frequency region determines the diffusion-controlled kinetics. With an increase in ruthenium content, the diameter of the semicircle increases, which attributes to the resistance imposed by ruthenium oxide. Also, the straight line of the curve at low frequency starts deviating indicating filling up of the pores of the ruthenium oxide particles [56]. Ramani et al. have utilized electroless deposition to prepare composites with variable RuO2 on AC [69]. The results show that specific capacitance increases with RuO2 . The electrochemical studies are carried out in three-electrode configurations. 5 wt% composite shows specific capacitance of 145 F/g and reaches to 260 F/g for 20 wt% RuO2 loading [69]. In another work, Zang et al. have synthesized amorphous hydrous ruthenium oxide/activated carbon (RuO2 ·xH2 O/AC) composites [70]. The result displays that specific capacitance is proportional to the mass of Ru in the electrode. Till 20 wt% Ru, specific capacitance attains 243 F/g. For 35 wt% to 100 wt% RuO2 ·xH2 O, the specific capacitance increases from 350 to 715 F/g [70–72]. The addition of RuO2 leads to drastic enhancement in specific capacitance and energy density; however, a decrease in rate capability and increase in Ohmic drop (equivalent series resistance) have been noted. In addition to this, several literatures have been reported on the synthesis mechanisms of RuO2 . The literature survey on this metal oxide suggests that the optimization loading along with appropriate synthesis techniques is of uttermost importance to achieve high-performance RuO2 /AC composite electrode materials for supercapacitor devices.

6.7.2 Manganese Dioxide and Activated Carbon Composites as Electrode Materials Manganese oxide is one of the most promising transition metal oxides for the next generation supercapacitor electrode material. It is cheap and non-polluting and shows ideal capacitor performance with the provision of safe handling during operation. After RuO2 , MnO2 is the second most studied metal oxide for pseudocapacitor electrode material. Manganese exists in a variety of stable oxide states and crystallizes in different types of crystal structures. Different forms of this oxide and their crystal structures are MnO with rock salt structure, Mn3 O4 having tetragonal spinel [73], Mn2 O3 (bixbyite) having body-centered cubic [74], α-MnO2 (psilomelane) with monoclinic structure [75], β-MnO2 (pyrolusite) with rutile structure [76],

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β-MnO2 (ramsdellite) [77], γ-MnO2 (nsutite) [78], η-MnO2 , δ-MnO2 (phyllomanganate) with birnessite structure [79], ε-MnO2 having defective NiAs structure [80]. A wide variety of crystal forms, morphology, porosity, and texture of manganese oxides plays a decisive role in optimizing the electrochemical performance. The charge storage mechanism in MnO2 occurs via Faradaic reactions on the surface and bulk of the electrode. The surface Faradaic reaction undergoes surface adsorption of the electrolytic cations (C+ =H+ , Na+ , Li+ and K+ ) on MnO2 as follows [80–82]: (MnO2 )surface + C+ + e− ↔ (MnOOC)surface

(6.2)

whereas the bulk Faradaic reaction involves intercalation or deintercalation of the electrolytic cation in bulk MnO2 as follows [80–82]: MnO2 + C+ + e− ↔ MnOOC

(6.3)

Both the above-stated reactions involve the redox process in MnO2 between the oxidation states of III and IV. MnO2 exhibits high specific capacitance. However, it lacks in cyclic stability and rate capability due to poor conductivity. The poor conductivity issue of MnO2 can be addressed via the introduction of conductive and high surface area porous carbon materials in the MnO2 matrix. This may enhance the charge storage capability of the composite electrode by shortening the electron transport distance [36, 80]. Another approach suggests that the poor conductivity of MnO2 is due to thick particles. The deposition of a thin MnO2 layer on the surface of high surface area, porous, and electronically conducting AC can provide good electrochemical properties. In such hybrid electrodes, electronically conducting carbon of high porosity provides diffusion pathways for electrolyte ions and thin MnO2 films experience reduced solid-state transport distance for the progress of electrolytic ions into the oxide electrode [83]. MnO2 /AC composite can be used to overcome this limitation as AC is conducting in nature. In this view, Malak-Polaczyk et al. have prepared a composite with commercial AC and λ-MnO2 [84]. The introduction of λ-MnO2 into the AC matrix generates synergism due to the high surface area of porous carbon skeleton and electroactive properties of λ-MnO2 . This improves the specific capacitance of the overall composites as compared to AC [84]. Again, in a similar study, Zhou et al. have fabricated MnO2 /AC composite to get the synergistic contribution of both materials [85]. The high surface area AC (surface area more than 2800 m2 g−1 ) has been derived from the oil tree shell by ZnCl2 activation. The 3D plot of atomic force microscopy (AFM) shows that MnO2 particles reside on the AC matrix are well connected (Fig. 6.5a). The observation has also been supported by transmission electron microscopy (TEM) analysis (not shown here). The electrochemical studies are carried out in 1 M KOH electrolyte. MnO2 /AC composite reveals the presence of prominent oxidation and reduction peaks in cyclic voltammetry (CV) as shown in Fig. 6.5b. Figure 6.5c shows the galvanostatic charge–discharge curves (GCD) of the composite. The curve displays two humps indicating the occurrence of two redox reactions due to multiple valence states of MnO2 . The maximum capacitance

164 Fig. 6.5 a 3D AFM plot, b CV curves at different scan rate and c GCD plot at different current densities of MnO2 /AC composite (redrawn and reprinted with permission from [85])

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obtained is 1126 F g−1 at a current density of 0.5 A g−1 . This can be due to the large surface area of AC that provides easy dispersion of MnO2 particles and hence increases the conductivity among the metal oxide particles [85]. In other literature, Zhang et al. have synthesized MnO2 /AC composite by chemical precipitation method at different wt% of AC [86]. All composites show uniform dispersion of nanosize MnO2 particles having a high specific surface area, which promotes easy access of the electrolytic ions and shortening the diffusion paths for ions and electrons. Figure 6.6a displays the scanning electron microscopy (SEM) image of MnO2 /AC nanocomposite with 0.4 g of AC (S2 composite in Fig. 6.6). Electrochemical studies of MnO2 /AC composite in 1 M Na2 SO4 depict a distorted CV curve due to the effect of polarization. The specific capacitance of various composites Fig. 6.6 a SEM image of MnO2 /AC nanocomposite synthesized using chemical precipitation and b specific capacitance of composites with 0.6 g (S1), 0.4 g (S2) and 0.2 g (S3) of AC as a function of current density (redrawn and reprinted with permission from [86])

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obtained from the GCD plot reveals that composites with 0.6, 0.4, and 0.2 g of AC have 182.8, 210, and 163.3 F g−1 , respectively, at a current density of 1 A g−1 . The rate capability curve is shown in Fig. 6.6b. Among all the samples, 0.4 g AC loading in MnO2 /AC nanocomposite shows the maximum capacitance indicating optimized MnO2 dispersion in the AC skeleton [86]. Huang et al. have also synthesized bamboo-based AC and MnO2 nanocomposites via the hydrothermal method [87]. The study reveals that the composition, structure, and interfaces of the nanocomposite depend on the temperature and time of reaction. The SEM micrograph reveals uniform and small fragments of MnO2 particles residing on the surface of bamboo-based AC that is interconnected with each other forming highly porous morphology. The formation of a conductive network structure improves the electrical contact with the current collector resulting in an enhancement in electrochemical capacity. The CV curves of bamboo-based AC and MnO2 /AC nanocomposite at a scan rate of 10 mV s−1 show near rectangular and symmetric shape having a large charge storage capacity of the bare AC. This can be due to the synergistic effect among the components of the system, where MnO2 particles are evenly dispersed in highly porous bamboo-derived AC material [87]. For MnO2 /AC in the organic electrolyte, Li et al. have prepared short fiberlike MnO2 nanostructure coated onto activated mesocarbon microbeads [88]. The composite exhibits maximum specific capacitance of 475 F g−1 with an energy density of 106 Wh kg−1 [88]. Again, Wang et al. have fabricated a composite of activated mesocarbon microbeads and Mn3 O4 [89]. The maximum specific capacitance obtained in this study is reported to be 178 F g−1 in LiPF6 (EC + DMC) electrolyte [89]. The literature on MnO2 /AC composite demonstrates that extremely high specific capacitance can be achieved for these composites. The performance of the composite is governed by the synthesis methods, the surface area of the AC, and uniform dispersion of the MnO2 particles into the AC surface. MnO2 /AC composites are capable to fabricate high-performance devices.

6.7.3 Nickel Oxide/Hydroxide and Activated Carbon Composites as Electrode Materials Nickel oxide/hydroxide shows electroactive properties along with good thermal stability and highly electronic conductive behavior. It also increases the rate of Faradaic reactions by providing short diffusion pathways for electrolytic cations toward electroactive surfaces hence, commonly utilized as pseudocapacitor electrode materials [60, 90, 91]. This material possesses both Faradaic charge transfer reaction and non-diffusional charge reaction characteristics along with high bulk electrical conductivity. Theoretical capacitance reported for NiO is 2584 F g−1 within 0.5 V [92]. The major limitation associated with the poor capacitance of NiO is its

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availability in a non-stoichiometric form that decreases the capacitive value. The electrochemical studies reveal that cathodic and anodic peaks of NiO appear at ~ 0.26 and 0.38 versus SCE [92]. During the anodic scan, oxidation occurs, whereas reduction takes place during a cathodic scan. The corresponding redox reaction is as follows [92]: NiO + OH− ↔ NiOOH + e−

(6.4)

The above reaction of NiO to NiOOH shows that the oxidation state of Ni2+ changes to Ni3+ . Literature suggests that the electrochemical surface reactivity of NiO depends on the degree of crystallinity that can be controlled during synthesis [36]. Above 280 °C, crystallinity increases resulting in an increase in capacitance [92]. The main drawbacks of NiO restricting the electrode performance are low porous structure and high resistivity. Unsatisfactory porous structure limits the diffusion of electrolytic ions leading to a slow charge delivery process. This results in low capacitance retention at high current density. The second issue remains the poor conductivity responsible for high resistance, hence decreasing the performance of the electrode [7]. To address these limitations, the synthesis of NiO composite with highly conductive and porous carbon material has been utilized. In this regard, AC can serve the purpose, where its conductivity and porosity can be controlled during synthesis [36]. Yuan et al. have studied the electrochemical behavior of NiO/AC composite at various loadings [93]. The result shows that although 4 wt% loading of NiO in AC decreases surface area from 1332 to 1232 m2 g−1 , the capacitance increases from 175 to 194 F g−1 . This can be attributed to the contribution of pseudocapacitance from the redox reaction of NiO [93]. Also, the deposited nickel acts as an electroactive site for the Faradaic reaction to store more capacitance in an alkaline medium. Study shows that besides NiO, Ni(OH)2 has also been studied as pseudocapacitive material. In this regard, Huang et al. have prepared nickel hydroxide/AC composite by simple chemical precipitation method [94]. The composite has been tested in the threeelectrode cell, where it delivers specific capacitance of 292–314 F g−1 for Ni(OH)2 content of 2–6%, pure AC exhibits specific capacitance of 255 F g−1 . The composite shows good electrochemical performance and high charge–discharge properties. For further increase in Ni(OH)2 loading i.e., 8–10% capacitance decreases to 261–302 F g−1 [94]. Hence, it is worthy to mention that NiO displays various oxidation states that enable it to be used as pseudocapacitive material for supercapacitor devices. The major drawbacks such as poor crystallinity and conductivity can be addressed by the inclusion of AC and the formation of composite electrodes to develop highperformance hybrid supercapacitor devices.

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6.7.4 Titanium Oxide and Activated Carbon Composites as Electrode Materials Titanium oxide (TiO2 ) is an n-type semiconductor and considered as an environmentfriendly metal oxide. It has been utilized for antibacterial applications due to the effective decomposition of harmful pollutants in the air and aqueous systems. Till date, TiO2 has not been explored in the field of supercapacitor electrode material due to poor conductivity. Instead of its poor conductivity, many studies have revealed that TiO2 can enhance homogeneity when mixed with other electrochemical electrode materials [95]. Seo et al. have studied the effect of TiO2 nanoparticles on the AC electrode material in 1 M H2 SO4 [96]. The study reveals that the specific capacitance increases with the increase in TiO2 loading showing a linear relationship between specific capacitance and loading of TiO2 along with specific surface area as shown in Fig. 6.7a. Figure 6.7b displays the CV curve of AC/TiO2 composite exhibiting near rectangular behavior due to electrostatic attraction involved in the electrode during

Fig. 6.7 a Variation of specific surface area and specific capacitance with variable TiO2 content in ACs/TiO2 composite electrodes b cyclic voltammogram of ACs/TiO2 composite electrodes as a function of TiO2 content at 1 mV s−1 in 1 M H2 SO4 c total specific capacitance of ACs/TiO2 composite electrodes as a function of TiO2 content at 1 mV s−1 in 1 M H2 SO4 . [96], and d variation of specific capacitance of the AC/TiO2 supercapacitor electrode during long-term cycling study [34] (redrawn and reprinted with permission)

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the charge–discharge process. It can be also be attributed that the bare AC electrode shows a specific capacitance of 100 F g−1 that has been dramatically improved to 155 F g−1 due to TiO2 dispersion [96]. Although TiO2 itself has low capacitive properties, improvement in the capacitance value of the composite electrode can be credited to the effect of the single-direction polarity of TiO2 nanoparticles. This enables the formation of stable TiO2 colloidal particles reducing the polarization of AC, which dramatically increases the mobility of ions in the pores, hence reducing total resistance of the device. With a further increase in TiO2 loading in the composite, a decrease in polarization has been observed. The composite shows an increase in contact resistance between TiO2 and AC decreasing the capacitive performance of the composite. Figure 6.7c represents the specific capacitance of the AC/TiO2 composite electrode versus TiO2 loading in 1 M H2 SO4 [96]. Based on the above discussion, TiO2 plays an important role in improving the performance of electrodes based on TiO2 and AC composites. TiO2 provides a three-dimensional network structure, which improves the capacitance of the composite. It helps in uniform dispersion of the AC to acquire the complete benefit of using it. In continuation of the study of TiO2 to AC mass ratio, Selavkumar and Bhat have reported nanostructured TiO2 /AC composite in three ratios via microwave-assisted approach [34]. At first, TiO2 nanoparticles are studied in 0.1 M HClO4 at different scan rates. Cathodic and anodic peaks are observed due to the redox reactions as depicted below [34]: Ti4+ O2 + H+ + e− ↔ Ti3+ O(OH)

(6.5)

The reduction of Ti4+ combined with H+ intercalation represents a cathodic reaction, while the release of H+ shows an anodic reaction. In TiO2 /AC composite material, the Nyquist plot displays that charge transfer resistance of TiO2 /AC at a ratio of 1:3 (20 ) is less compared to 1:1 (70 ) and 1:2 (50 ) compositions. The increment in the amount of AC reduces resistance and hence improves the capacitance of the composite. The capacitance reaches to 92 F g−1 due to the synergistic effect of Faradaic and EDLC. The high surface area of TiO2 also provides good support to AC during electrochemical processes [34]. The cyclic stability of the composite is shown in Fig. 6.7d. The study reveals that the addition of carbon in the TiO2 matrix enhances electrochemical stability and can be utilized as electrode material for supercapacitor applications [34].

6.7.5 Zinc Oxide and Activated Carbon Composites as Electrode Materials Zinc oxide (ZnO) exhibits good electrochemical properties and has been widely used as an active material for solar cells, batteries, light-emitting diodes, and gas sensors. It shows good surface conductivity, high electron mobility, and wide bandgap

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contributing to efficient electrochemical properties. ZnO is cheap, easily available, and eco-friendly. However, a major drawback associated with the use of ZnO is the formation of dendritic growth with charge–discharge cycles [36, 97]. The cyclic performance and capacitance of ZnO can be improved by forming composite structures. Selvakumar et al. have prepared a composite of ZnO and commercial AC via physical mixing [98]. The electrochemical performance has been carried out in three-electrode cell configuration using 0.1 M Na2 SO4 electrolyte. The result shows specific capacitance decreases with an increase in ZnO loading. At a scan rate of 50 mV/s specific capacitance reaches 42 F g−1 for 1:3 AC: ZnO ratio, which further increases to 76 F g−1 for 1:1 ratio of AC and ZnO. Hence, removal of carbon lessens the capacitance suggesting that the introduction of the carbon matrix in ZnO reduces the formation of dendritic structure, and an improvement in the capacitance has been observed. This can be attributed to the fact that high surface area carbon electrodes provide pore or voids for cations and anions of the electrolyte and enable metal oxides to reside inside the AC to participate actively in electrochemical activity [98].

6.7.6 Bismuth Oxide and Activated Carbon Composites as Electrode Materials Bismuth oxide (Bi2 O3 ) a class of metal oxide also exhibits good electrochemical properties as well as high cyclic stability when tested in aqueous NaOH solution. The specific capacitance obtained is 98 F g−1 [99]. Wang et al. have prepared AC and Bi2 O3 composite via vacuum impregnation followed by roasting [100]. The TEM micrograph reveals the uniform distribution of Bi2 O3 particles in the AC matrix (not shown here). This facilitates the efficient transport of ions and produces pseudocapacitance. Again, the uniform distribution of Bi2 O3 in AC improves the electrical conductivity of the composite. Electrochemical analysis has been carried out in 6 M KOH electrolyte. The CV shows a pair of redox peaks in composite material indicative of pseudocapacitance behavior. Literature suggests following reaction during the charge–discharge process in Bi2 O3 Bi2 O3 + 2OH− ↔ 2BiO2− + H2 O

(6.6)

BiO2− + e− ↔ BiO2− 2

(6.7)

2− 3BiO2− + 4OH− + Bi0 2 + 2H2 O ↔ 2BiO

(6.8)

Bi0 ↔ Bimet

(6.9)

According to the CV curve as shown in Fig. 6.8a specific capacitance increases

6 Transition Metal Oxide/Activated Carbon-Based … Fig. 6.8 a CVs of AC, Bi2 O3 and AC/Bi2 O3 , b CVs of AC/Bi2 O3 at different sweep rates and c charge–discharge curves of AC/Bi2 O3 at different current densities (redrawn and reprinted with permission from [100])

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from 110.9 to 252.9 F g−1 for AC for AC-Bi2 O3 composite indicating the pseudocapacitive contribution of Bi2 O3 . Figure 6.8b shows the CV curve of the composite electrode at different scan rates. The curve suggests that rate capability decreases with an increase in scan rate. This is because at lower scan rate ions can diffuse in inner active sites and intercalation occurs, whereas at high scan rate ions interact with the surface material. The charge–discharge curve shown in Fig. 6.8c reveals a combination of EDLC and pseudocapacitance charge storage mechanism. The presence of a kink in a charge–discharge curve can be due to the oxidation of untransformed BiO in the reduction process, the composite electrode exhibits specific capacitance of 332.6 F g−1 , which is much higher than bare AC having a capacitance of 106.5 F g−1 . Apart from the enhancement of capacitance in a composite electrode, the percentage mass of Bi2 O3 is another important factor that governs the performance of the electrode. In this work, 24.53% of Bi2 O3 loading in AC delivers the maximum capacitance [100].

6.7.7 Other Metal Oxides and Activated Carbon Composites as Electrode Materials Various metal oxides other than those explained earlier have also the potential to be explored as pseudocapacitive material for supercapacitor devices. They can be used as hybrid electrode materials when integrated with AC. Some of the metal oxides are discussed below in brief. Vanadium oxides (V2 O5 ) can be used as electrode material for supercapacitors. V2 O5 exists in variable oxidation states that shows bulk and surface redox reactions [101]. Lee and Goodenough have studied the electrochemical performance of amorphous V2 O5 in the KCl solution [102]. The specific capacitance obtained is 350 F g−1 [102]. The main limitation of V2 O5 is poor conductivity. Following this, V2 O5 can be well integrated with AC to minimize the limitation of low conductivity [103, 104]. AC can provide voids for V2 O5 particles to disperse easily and the high conductivity of AC facilitates the ion conduction process. Hence, the electronic properties can be suitably changed by forming a composite structure with AC [7]. Cobalt oxide (Co3 O4 ) is deemed to be a suitable alternative owing to the high surface area, controllable shape and structural characteristics, good corrosion stability, strong electron storage ability, and good redox performance [59]. However, it suffers from poor cycle stability due to the large volume expansion and contraction during the charge–discharge processes. The utilization of Co3 O4 for supercapacitor electrode materials is reported in the form of composites. The addition of high surface area material such as AC can provide large sites for Co3 O4 to undergo redox reactions thereby increasing the capacitance of composite. Possible redox reactions occurring during the charge–discharge cycles in cobalt oxides are as follows [7, 95]: Co3 O4 + H2 O + OH− ↔ 3CoOOH + e−

(6.10)

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Cobalt hydroxide (Co(OH)2 )-based material finds application as supercapacitor electrodes due to the layered structure having large interlayer spacing. This enables a high surface area and a fast ion diffusion rate. Two possible reactions occurring during ion insertion/desertion are expressed below [7, 95]: Co(OH)2 + OH− ↔ CoOOH + H2 O + e−

(6.11)

CoOOH + OH− ↔ CoO2 + H2 O + e−

(6.12)

Tin oxide (SnO2 ) shows very low specific capacitance as compared to other metal oxides [105]. Literature suggests the amorphous nanostructures in SnO2 and it delivers specific capacitance of 285 F g−1 at a scan rate of 10 mV s−1 that retains up to 101 F g−1 even at a very high scan rate of 200 mV s−1 [106]. This shows that finely dispersed SnO2 nanoparticles can perform redox reactions. The major limitation associated with SnO2 is a small working potential window [107]. Sugimoto et al. have studied molybdenum oxide (MoO3 )/AC composite for the supercapacitor electrode [108]. The addition of 1.4 wt% of MoO3 on AC increases the capacitance from 132 to 177 F g−1 in 1 M NaOH, whereas in 0.5 M H2 SO4 , the capacitance of the composite is reported to be 176 F g−1 . The pure AC shows a capacitance value of 136 F g−1 [36, 108]. Iron oxides are a class of materials, which are unexplored in the field of charge storage electrode materials. It owes the advantage of low cost, low toxicity, cheap and wide availability. Although the electrochemical performance of this material is not competitive with other established material of the field due to the poor electronic conductivity, in conjunction with AC it can be utilized as the active electrode material of extremely low preparation cost. FeOx coatings in carbon nanofoam show electrochemical active sites when operated in aqueous electrolytes [59]. The above-discussed metal oxides have also been explored for the potential use of electrode materials for supercapacitor applications like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military and aerospace, etc., [109, 110]. The major limitation associated with these oxides includes poor conductivity that can be minimized by the introduction of electronically conducting AC. The formation of composites with AC may yield high-performance hybrid electrode materials for supercapacitor devices. Table 6.2 presents the electrochemical performance of various transition metal oxide/activated carbon composite.

6.8 Concluding Remarks The increasing demands of high energy and power devices have promoted research toward the fabrication of high-performance and long-life devices. Metal oxides provide the source of high specific capacitance, energy density, and surface area.

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Table 6.2 Electrochemical performance of transition metal oxide/activated carbon composites Metal oxide/AC composite

Loading wt%

Electrolyte

Capacitance (F g−1 )

Energy density (Wh kg−1 ) at power density (W kg−1 )

Ref.

RuO2 /AC

7 wt% of RuO2

0.5 M H2 SO4

308

–

[66]

Hydrous RuO2 /AC

40 wt% of RuO2

1 M H2 SO4 407

17.6 at 4000

[67]

Hydrous RuO2 /mesoporous carbon

54.3 wt% of RuO2

2.0 M H2 SO4

243

–

[68]

Hydrous RuOx (OH)x /AC

9 wt% of RuO2

1 M H2 SO4 250

–

[56]

RuO2 /AC

20 wt% of RuO2

1 M H2 SO4 260

–

[69]

λ-MnO2 /carbon

–

1M Na2 SO4

110

MnO2 /activated carbon

–

1 M KOH

1126

24 at 275

[85]

MnO2 /activated carbon

0.4 of AC

1M Na2 SO4

210

9.7 at 60

[86]

MnO2 /activated carbon

–

1M Na2 SO4

221.45

–

[87]

Mn3 O4 /activated carbon

–

1 M LiPF6 (EC + DMC)

183

106 Wh kg−1

[88]

NiO/AC

4 wt% of NiO

6 M KOH

194

–

[93]

Ni(OH)2 /AC

6 wt% Ni(OH)2

6 M KOH

314

–

[94]

TiO2 /AC

10 wt% TiO2

1 M H2 SO4 155

TiO2 /AC

–

0.1 N Na2 SO4

122

–

[34]

ZnO/AC

–

0.1 M Na2 SO4

160

–

[98]

Bi2 O3 /AC

–

6 M KOH

332.6

–

[100]

MoO3 /AC

–

0.5 M H2 SO4

176

–

[108]

[84]

[96]

Again, AC ensures high power density, cyclic stability, and good rate capability at high current density. The flexibility to tailor the morphology of AC provides new pathways to form composite electrodes with various sizes of metal oxides. During the formation of the composites, AC serves as backbone support for the metal oxides; hence, its structure and physicochemical properties determine the properties of the electrode material. Moreover, the content of metal oxide plays a major role

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in increasing or decreasing the performance of the electrolyte. Hence, studies on the optimization of metal oxide content on AC are necessary. The other factors include the particle size of metal oxide and the pore diameter of AC. These two factors play a decisive role in controlling the extent of dispersion inside the composite materials. Hence, transition metal oxide and AC composite materials provide more efficient and rapid transportation for ions and electrons resulting in high electrochemical performance. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 7

Carbon Nanofiber as Electrode Materials for Supercapacitors Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar

Abstract Carbon nanofibers are important one-dimensional carbon-based nanostructured materials. They are generally prepared by electrospinning of polymer precursors and followed by thermal treatment, or by chemical vapor deposition growth through the decomposition of hydrocarbons in the presence of a metal catalyst. They are promising electrode materials for supercapacitors due to their high electrical conductivity, specific surface area, and porosity. They store energy through the electrical double layer capacitor mechanism. Nitrogen doping, activation process, etc., can further improve the specific capacitance values of them. Therefore, this chapter provides decent and updated coverage on the basic structure, properties, and supercapacitor performance of carbon nanofibers at different synthetic approaches. The chapter also describes the supercapacitive performance of carbon nanofibers through different surface and structural modifications. B. De · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] B. De e-mail: [email protected] S. Banerjee · K. D. Verma · T. Pal · K. K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. D. Verma e-mail: [email protected] T. Pal e-mail: [email protected] T. Pal A.P.J. Abdul Kalam Technical University, Lucknow 226031, India P. K. Manna Indus Institute of Technology and Management, Kanpur 209202, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_7

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7.1 Introduction The supercapacitor is an effective energy storage device, which is used to meet the demand for global energy consumption. The high power density, long cycle life, and fast charge–discharge rates render it a promising material in portable and flexible electronics, energy backup systems, hybrid electric vehicles, defense, aerospace, industrial-scale power and energy management systems to transport high power within a very short period [1–3]. Carbon-based materials are most widely considered as electrode materials for battery and supercapacitor devices since they exhibit high power density and excellent cycling stability due to the large specific surface area, excellent electrical conductivity, superior elasticity, high chemical and thermal stability [4]. They store energy by the mechanism of electric double-layer capacitors (EDLCs) through the accumulation of charges at the electrode and electrolyte interfaces since the storage capacity is mainly controlled by the specific surface area along with pore size and electrical conductivity [5–7]. The features of supercapacitors [8] and capacitor to supercapacitor [9] are reported elsewhere. Among different carbon-based materials, carbon nanofiber (CNF) is an important one-dimensional nanostructured material due to the high aspect ratio, surface area, good electrical conductivity, and electronic transport properties facilitating kinetics of the electrochemical reactions for supercapacitor applications. CNF is inexpensive and can be easily fabricated on a massive scale by various methods like electrospinning or vapor growth approach. Activated and nitrogen-doped CNFs have also been employed as the alternative for high-performance supercapacitor electrodes through the electrospun method. Again, the pore size and specific surface area of CNFs can easily be controlled via the gas phase or chemical activation processes [10–12]. Therefore, the CNF-based electrode exhibits a comparable specific capacitance as that of activated carbon-based electrodes. This chapter presents a comprehensive study of the development of electrode materials for supercapacitors based on CNFs, synthesized by several methods using numerous resources.

7.2 Material: Synthesis, Structure, and Characteristics of Carbon Nanofiber Carbon nanofiber (CNF) is a linear, sp2 -based discontinuous filament with a diameter of 50–200 nm and a length of 50–100 µm. According to the growth of graphene layers during preparation, CNF can be classified into three types: – ribbon or tubular CNF, in which the graphene layers are arranged parallel to the growth axis – herringbone CNF, in which the graphene layers are stacked obliquely (like cupstacked or fishbone) to the fiber axis – and platelet CNF, in which the graphene layers are stacked perpendicular to the fiber axis [13–16]. The structures of the three types of CNFs are shown in Fig. 7.1.

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Fig. 7.1 Structures of three types of CNF:a ribbon CNF,b herringbone CNF, andc platelet CNF (redrawn and reprinted with permission from [14])

These types of layering growth mechanism of CNF depend on the geometric facets of the metallic catalyst particle and the gaseous carbon feedstock (hydrocarbon or CO gas) that are introduced during CNF processing. The internal structures of the three types (platelet, herringbone, and ribbon or tubular) of CNFs are shown in highresolution transmission electron microscope (HRTEM) images in Fig. 7.2 [14]. The inherent structure of CNF, in general, depends on the production process employed. It is mainly prepared by two methods, (a) catalytic chemical vapor deposition and (b) electrospinning.

7.2.1 Carbon Nanofiber Prepared by Catalytic Chemical Vapor Deposition Growth In this method, CNFs are synthesized through the catalytic decomposition of certain hydrocarbons in the presence of a metal catalyst. Several types of metals or alloys are used as catalysts (such as iron, nickel, cobalt, vanadium, chromium and) that can dissolve carbon to form metal carbide. Generally, the carbon sources like carbon monoxide, (H2 /CO), methane, ethyne, or ethene are used in the temperature range from 500 to 900 °C [17]. The structures of the CNF are governed by the shapes of metal catalysts. The deposited hydrocarbons are dissolved in the metal particle and precipitated on the metal surface as graphitic carbon. The size of the catalyst particle determines the size of the graphitic structure of CNF. Normally, it is used in the range of 10–100 nm, which determines the outer diameter of the CNF produced [18–20]. The angle of the layering growth strongly affects the properties of CNF. A faceted metal catalyst particle allows the formation of angled layers of graphitic

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Fig. 7.2 HRTEM images ofa platelet,b herringbone, andc ribbon or tubular CNFs (redrawn and reprinted with permission from [14])

Fig. 7.3 Schematic representation of catalytic chemical vapor deposition growth process for CNF (reprinted with permission from [21])

platelets to grow CNF. A simple schematic illustration of the catalytic chemical vapor deposition growth process for CNF is shown in Fig. 7.3 [21]. The use of a spherical metal catalyst results in parallel graphitic layers to the growth axis.

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7.2.2 Carbon Nanofiber Prepared by Electrospinning Electrospinning is another widely used alternative method for the production of CNF. A schematic illustration of the electrospinning process to produce CNF is shown in Fig. 7.4 [22]. Polymer nanofibers are required to be prepared as the precursors to fabricate CNFs by this method. The properties of the finally obtained CNFs are defined by the natures of the polymer solution and the processing parameters. A fine tip needle syringe is used in this process. High voltage is applied to the droplet at the tip of the needle, which causes the solution to spurt out from the needle to a target [21]. A fibrous structure is developed and collected at the target when the surface tension is high enough for the solution to prevent breaking into a fine droplet. Polymeric precursors such as polyacrylonitrile (PAN), cellulose, and pitch are the most frequently used. Also, poly(vinyl alcohol) (PVA), poly(vinylidene fluoride) (PVDF), polyimides (PIs), polybenzimidazole (PBI), phenolic resin, lignin, etc., are also used [23]. PAN is the most desirable and often used due to high carbon yield and strength [24]. The usage of PAN also facilitates high mass production [25]. After the successful formation of the polymer nanofibers, a heat treatment is used to carbonize them to form CNFs. The morphology, crystallinity, purity, porosity, and diameters are governed by the parameters of the heat treatment process and operational conditions, such as atmosphere and temperature. A heterogeneous skin-core structure is observed in this process, in which the graphitic layers are oriented radially along with the fiber axis; however, a randomized granular structure is observed along with the axis of the fiber core [21, 26–29]. The morphology of a CNF after annealing is shown in Fig. 7.5. If we compare both the processes, the chemical vapor deposition generally tends to yield ultra-high modulus CNFs; however, the presence of a significant amount of residual catalyst leads to the generation of relatively lower yield. Along with this, the requirement of expensive equipment is another main disadvantage associated with this process. In contrast, the electrospinning method has the advantage of a “topdown” approach and hence, facilitates the production, assemblage, and alignment of CNFs. Fig. 7.4 Schematic representation of the electrospinning technique for the preparation of CNF (reprinted with permission from [22])

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Fig. 7.5 TEM images of broken edges of carbonized electrospinning CNF witha loosened outermost layer and b several sheath layers at the edge (redrawn and reprinted with permission from [26])

7.2.3 Characteristics of Carbon Nanofibers CNFs have been widely investigated in supercapacitor applications due to the excellent mechanical properties, high electrical and thermal conductivity, and high surface area. They exhibit outstanding tensile strength up to 8.7 GPa on direct measurements of individual fibers, which is similar to the strength of graphite microfibers. They reach up to 600 GPa modulus based on this direct measurement [29, 30]. However, the mechanical properties of CNFs are dependent on the processing methods, fiber diameter, and type of loading used for the mechanical analysis. They also exhibit a very high elastic modulus up to 207 GPa by a three-point bending test method [31]. The modulus of CNFs depends on the thickness of the fiber. The modulus is higher when the outer wall thickness of nanofibers is larger than that of the inner wall and again decreases with the overall wall thickness [31–33].

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Another most important property of CNFs is high electrical conductivity. They are widely used in electronic devices and supercapacitor electrodes due to the high surface area, porosity, and superior conductivity. The reported intrinsic conductivity of highly graphitic vapor-grown carbon fiber shows value up to 5 × 10−5  cm at room temperature, which is near to the resistivity of the graphite [29]. They have also a very high aspect ratio, more than 100, and exhibit extremely high surface area in the range of 1000–2000 m2 g−1 . An exclusive overview of the synthesis of CNF and its properties and applications has been reported elsewhere [34].

7.3 Carbon Nanofiber as Electrode for Supercapacitors The high electrical conductivity and excellent surface area with high porosity of CNFs make them promising materials for supercapacitor electrode. They behave as an EDLC, which stores energy via forming the electrical double-layer surrounding the surface of the electrode. In this operating mechanism, the depleted oppositely charged species store energy at the interface of the electrode and electrolyte. The capacitance value (C) of the EDLC mainly depends on the conductivity and surface area of the electrode, as it is calculated using (7.1), C = εr ε0 A/d

(7.1)

where Er is the relative permittivity of the medium in the electrical double-layer, ε0 is the permittivity of vacuum, A is the specific surface area of the electrode, and d is the effective thickness of the electrical double-layer. Fine control of the properties of CNFs in terms of electrical conductivity, surface area, and pore size can be achieved by using proper processing and carbonization conditions. The specific capacitance as high as 300 F g−1 is observed [35]. A selftemplated solution grown mesoporous CNF exhibits a BET surface area of around 1725 m2 g−1 , and hence, the supercapacitor electrode based on this CNF delivers a specific capacitance value of 280 F g−1 at a current density of 0.5 A g−1 [35]. The electrode also shows excellent rate capability and cyclic stability, whereas the normal CNF prepared by electrospinning from PAN exhibits relatively low surface area ~ 550 m2 g−1 and therefore delivers capacitance only up to 140 F g−1 [10]. The superior supercapacitor performance of the former CNF-based electrode is attributed to the architecture of the CNF and thereby, the synthesis process. In the case of a selftemplate strategy, the mesoporous CNFs are synthesized through a solution-growth process, where ethylene glycol is used as a carbon precursor and Zn(CH3 COO)2 is used to form zinc glycolate template during the subsequent carbonization process [25]. The schematic illustration of the formation process for the mesoporous CNFs along with mesoporous ZnO nanotubes is shown in Fig. 7.6. Figure 7.7 shows that self-templated solution grown CNF (a, b) possesses a more porous structure compared to simple electrospun CNF (c, d). This porous architecture helps to shorten the path for electron transport and electrolyte penetration, as well as increases the

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Fig. 7.6 Schematic illustration of the formation process for the mesoporous CNFs along with mesoporous ZnO nanotubes (reprinted with permission from [35])

Fig. 7.7 TEM images of (a, b) self-templated solution-growth, and (c,d) electrospun CNFs (reprinted with permission from [10, 35])

diffusion rate of the ions through the mesoporous structure [35, 36]. The high surface area also offers good charge accommodation and the ability to handle high current loads. The performance of supercapacitors based on CNF electrodes can be enhanced by increasing their conductivity. The conductivity of CNFs mainly depends on the

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Fig. 7.8 Schematic representation for the fabrication of nitrogen-doped CNFs (reprinted with permission from [37])

carbonization process that found to be increased with an increase in the processing temperature. The addition of conducting polymers or doping of nitrogen is other promising approaches to enhance conductivity [37, 38]. Chen et al. have performed a systematic study on supercapacitor performance of CNF by coating polypyrrole (CNF@Ppy) followed by carbonization to dope nitrogen as shown in Fig. 7.8 [37]. The carbonization process has been optimized from 500 to 1100 °C under a nitrogen atmosphere, and the best performance is observed at 900 °C. The supercapacitor performance of this study is represented in Fig. 7.9. Figure 7.9a shows that nitrogen-doped CNF (N-CNF) carbonized at 900 °C exhibits far better specific capacitance value (202 F g−1 at a current density of 1 A g−1 ) compared to CNF@Ppy and CNF. This enhancement reveals that the coating of polypyrrole only enhances the conductivity of CNF; however, doping of nitrogen and high-temperature carbonization improves both the conductivity and surface area of CNF. Therefore, very low charge transfer resistance of N-CNF (carbonized at 900 °C) is found in electrochemical impedance spectroscopy (EIS) (Fig. 7.9b) that favors increasing the diffusion rate of ions. The enhanced surface area is further confirmed by BET analysis (Fig. 7.10), where it is found that N-CNF carbonized at 900 °C exhibits surface area of 562.51 m2 g−1 . This surface area is far larger than CNF@Ppy (46.75 m2 g−1 ) and CNF (348.12 m2 g−1 ). The coating of polypyrrole increases the conductivity of CNF; however, the surface area is sharply reduced. As a consequence of this, the diffusion rate of the electrolyte has been decreased further. Yan et al. [39] have prepared nitrogen/phosphorus co-doped nonporous CNFs through electrospinning of the solution of polyacrylonitrile and phosphoric acid precursors [40] followed by thermal treatment [39]. The rectangular-shaped CV curves (Fig. 7.11a) depict the EDLC behavior of CNFs-based electrode, and the curves retain the shape even at a high scan rate of 200 mV s−1 that indicates good capacitive response and low equivalent series resistance of the material. GCD curves also reveal a nearly symmetric triangular shape and low ohmic drop even at high current densities as shown in Fig. 7.11b. These N/P co-doped nonporous CNFs show excellent supercapacitive performance with the achievement of maximum specific capacitance of 224.9 F g−1 at a current density of 0.5 A g−1 using 1 M H2 SO4 as the electrolyte. The material also displays excellent cyclic stability with an increase in the specific capacitance from 192.9 to 203 F g−1 after 8000 cycles at a current density of 5 A g−1 as depicted in Fig. 7.11c. Therefore, the rate performance of the electrode is further improved after 8000 cycles as shown in Fig. 7.11d. This increment

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Fig. 7.9 a Specific capacitance values of CNF (carbonized at 900 °C), CNF@Ppy and N-CNF (carbonized at 500–1100 °C); b EIS of CNF, CNF@Ppy, and N-CNF (redrawn and reprinted with permission from [37])

Fig. 7.10 Nitrogen adsorption–desorption isotherms of CNF (carbonized at 900 °C), CNF@Ppy and N-CNF (carbonized at 500–1100 °C) (redrawn and reprinted with permission from [37])

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Fig. 7.11 a CV curves of nitrogen/phosphorus co-doped nonporous CNF-based electrode at different scan rates, b GCD curves of the electrode at different current densities, c cyclic stability of the electrode at a current density of 5 A g−1 , and d plots of specific capacitance values at different current densities of the electrode tested before and after 8000 GCD cycles (redrawn and reprinted with permission from [39])

in specific capacitance and rate performance is ascribed to the active process of the electrode. After long-term performance, the intercalation and de-intercalation of the ions within the electrodes are enhanced resulting in more effective active sites within the materials, thereby improving the specific capacitance. Phosphorus functionalities also block the active oxidation sites to achieve long-term stability. The presence of heteroatoms significantly enhances the supercapacitive performance through the synergetic effect that provides high pseudocapacitance and improved surface wettability strengthening the EDL capacitance of the electrode materials. The supercapacitive property of CNF also varies with the architecture and arrangement of CNFs. A porous CNF paper has been prepared through one-step carbonization of polyacrylonitrile followed by thermal treatment in the range of 700–1000 °C under the CO2 atmosphere. The CNF paper displays a maximum capacitance of 240 F g−1 in the KOH electrolyte [41]. The supercapacitor made of the CNF paper delivers very high power density in both aqueous and organic electrolytes. Another highly ordered mesoporous CNF arrays have been derived from organic resols using a crab shell biological template as schematically represented in Fig. 7.12. The CNF arrays exhibit a large specific surface area (1270 m2 g−1 ) and provide a more favorable path for electrolyte penetration and transportation, with good electronic conductivity [42]. Therefore, the resultant materials display a high specific capacitance of ∼152 F g−1 .

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Fig. 7.12 Schematic representation of the preparation steps of highly ordered mesoporous CNF arrays (redrawn and reprinted with permission from [42])

Fig. 7.13 a Schematic illustration for the preparation of heteroatom doped CNF network from bacterial cellulose, b photograph of bacterial cellulose pellicle and (c, d) SEM micrographs of the CNF at the surface and inner, respectively (redrawn and reprinted with permission from [43])

A supercapacitor of 3D heteroatom-doped carbon nanofiber networks has been designed through pyrolysis of bacterial cellulose immersed in H3 PO4 , NH4 H2 PO4 , and H3 BO3 /H3 PO4 aqueous solution, respectively, as illustrated in Fig. 7.13a [43].

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Fig. 7.14 Electrochemical performance of the heteroatom- doped CNF network-based supercapacitor: a CV curves at different scan rates, b GCD curves at different current densities, c Ragone plot, and d cycling performance up to 4000 GCD cycles at a current density of 2.0 A g−1 (redrawn and reprinted with permission from [43])

SEM micrographs (Fig. 7.13b–d) display the formation of highly porous and ultrafine network nanostructures, where the carbon nanofibers are interconnected with numerous junctions from the water-rich morphology of the original bacterial cellulose pellicle. The porous network structure shows a high specific surface area of 289.90 m2 g−1 . Hence, it displays high electrochemical performance through the fabrication of a two-electrode system based symmetric supercapacitor using 2.0 M aqueous H2 SO4 as the electrolyte. The CV curves (Fig. 7.14a) display quasi-rectangular shapes and retain the shape as the scan rate increases from 50 to 700 mV s−1 indicating good capacitive behavior and high-rate capability. The GCD curves (Fig. 7.14b) of the nanofiber-based supercapacitor reveal a nearly triangular shape without obvious IR drop even at a high current density of 2.0 A g−1 . This indicates the high reversibility of a typical capacitor with a rapid I–V response and with small equivalent series resistance. This is because of the presence of heteroatoms into the 3D carbon fiber matrix that enhances the electrical conductivity and wettability between the electrolyte and electrode materials. Therefore, a high specific capacitance value of about 204.9 F g−1 is found at a current density of 1.0 A g−1 . This high specific capacitance is ascribed from the pseudo-capacitive contribution of the surface functionalities and heteroatoms along with the EDLC. Further, the Ragone plot (Fig. 7.14c) relative to the corresponding energy and power densities of the supercapacitor reveals that it can deliver a high energy density of 7.76 Wh kg−1 and reach a maximum power density of 186.03 kW kg−1 . Moreover, the supercapacitor

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Fig. 7.15 Ragone plot of the nitrogen-doped CNF derived flexible all-solid-state supercapacitor device, inset images display (i) photograph and (ii) schematic diagram of the flexible solid-state supercapacitor (redrawn and reprinted with permission from [44])

shows long-term cyclic stability due to the very stable specific capacitance value up to 4000 GCD cycles at a current density of 2.0 A g−1 (Fig. 7.14d). Further, the nitrogen-doped CNF derived through the hydrothermal reaction of the bacterial cellulose with aqueous ammonia at a mild temperature offers high power flexible supercapacitor using an all-solid-state device [44, 45]. The solid-state supercapacitor is fabricated by integrating the highly flexible porous 3D network of N-doped CNF-based electrodes with poly(vinyl alcohol)–H2 SO4 gel electrolyte as shown in the inset of Fig. 7.15. This flexible supercapacitor delivers a very high power density of 390.53 kW kg−1 with a maximum energy density of 6.07 Wh kg−1 as shown in the Ragone plot (Fig. 7.15). Moreover, the flexible supercapacitor displays excellent cycling stability by retaining ∼ 95.9% of the initial capacitance even after 5000 GCD cycles at a current density of 2 A g−1 . Sometimes polymer blend is also utilized to obtain high surface area porous structured CNFs by electrospinning method for the fabrication of high-performance supercapacitor electrodes. In this process, blends of PAN with other sacrificial polymers (polymethylhydrosiloxane, polyvinylpyrrolidone, polymethylmethacrylate, Nafion, etc.) are utilized to generate a range of phase-separated structures in the fibers. The overall idea is that upon carbonizing at high temperatures, PAN will convert to carbon and the sacrificial polymer will be decomposed out to create intra-fiber pores [46– 48]. Morphology of PAN and Nafion blend-based CNF is shown in Fig. 7.16 after carbonization at 800 °C [48]. A uniform porous structure is formed through the decomposition of Nafion inside the fiber at high temperatures. Therefore, the CNF achieves a specific surface area up to 1600 m2 g−1 with a large fraction of mesopores (2–4 nm). This high surface area is extremely beneficial in supercapacitor applications due to greater electrolyte accessibility. Hence, the specific capacitance of this CNF reaches 210 F g−1 at a high cyclic voltammetry scan rate of 100 mV s−1 . The material also retains 75% of its initial capacitance even at a large current density of 20 A g−1 demonstrating the outstanding capability of power handling.

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Fig. 7.16 SEM micrographs of porous carbon nanofibers formed by electrospinning of PAN and Nafion (redrawn and reprinted with permission from [48])

Chemical activation of CNFs also enhances the supercapacitor performance [10, 49, 50]. The process improves both surface area and conductivity of the CNF electrodes. The addition of 5 wt% ZnCl2 with PAN solution during the electrospinning process increases the specific surface area from 310 to 550 m2 g−1 [7]. The specific surface area of a resole-type phenolic resin-based electrospun CNF increases to 597 from 385 m2 g−1 by incorporation of KOH solution as the activator during electrospinning of the resin solution [38, 50, 51]. This KOH-activated CNF electrode delivers a specific capacitance of 256 F g−1 that again remains far higher than the CNF without activation (196 F g−1 ). The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) analyses of the electrode are shown in Fig. 7.17. The CV area and the discharge time of the CNF enhance with KOH up to 20%. Figure 7.17 shows that the best supercapacitor performance has been obtained for 10% KOHmodified CNF and retained up to 67% at a high current density of 20 A g−1 (Fig. 7.17c, d). Kim et al. have also prepared CNF webs through electrospinning of poly(acrylonitrile) solutions in dimethylformamide followed by stabilization and activation by steam [52]. The activation process enhances the surface area through creating micropores, which facilitates the ion mobility and introduces a high specific capacitance of ~173 F g−1 at a current density of 10 mA g−1 . They have also prepared a similar activated CNF web from polybenzimidazole solutions of dimethyl acetamide using the same procedure and achieved a specific capacitance value up to 178 F g−1 [53]. Again, the CNF web derived through electrospinning of a polybenzimidazole/dimethylacetamide solution followed by steam activation at 700–900 °C delivers similar specific capacitance in the range of 125 to 178 F g−1 , depending on the activation temperature, as plotted in the Fig. 7.18 [54]. Similarly, the activation of polybenzimidazole-derived CNF web using a steam of 30 vol.%, a maximum specific surface area of 1220 m2 g−1 , and a specific capacitance of 202 F g−1 can be achieved at an activation temperature of 800 °C [55].

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Fig. 7.17 Comparative: a CV (at 100 mV s−1 ) and b GCD (at 0.2 A g−1 ) curves of CNF and KOH activated CNFs, c GCD curves of 10% KOH activated CNF at different current densities, and d comparative specific capacitance values of CNF and KOH activated CNFs at different current densities (redrawn and reprinted with permission from [50])

Fig. 7.18 Specific capacitance versus current density of CNF at activation temperatures, a 700, b 750, c 800, and d 850 °C (redrawn and reprinted with permission from [54])

Steam activated another CNF prepared by electrospinning, and thermal treatment of poly(acrylonitrile-co-vinylimidazole) produces a high surface area of 1120 m2 g−1 [56]. Therefore, coin cells assembled two-electrode device using ethylmethylimidazolium bis(trifluoromethylsulfonyl) imide (EMITFSI) as the electrolyte shows classic box-like CV curves of electrochemical double-layer capacitors with a specific capacitances value of 122 F g−1 at a scan rate of 10 mV s−1 using a high working voltage up to 4.0 V as shown in Fig. 7.19a. The supercapacitor also delivers the

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Fig. 7.19 a CV curves at different scan rates and b Ragone plots of coin cells assembled twoelectrode supercapacitor device (redrawn and reprinted with permission from [56])

maximum energy and power densities of 47.4 Wh kg−1 and 7.2 kW kg−1 as shown in Ragone plots (Fig. 7.19b). Liu et al. have utilized a metal etching process to design flexible and freestanding hierarchical CNF films fabricated by electrospinning of PAN [57]. The synthesis procedure with digital photographs and SEM micrographs of the CNF film is shown in Fig. 7.20. The electrospinning nanofibers are carbonized in the presence of cobalt salt to incorporate the metal nanoparticles into the CNFs. The cobalt nanoparticles are removed by acid corrosion to make a porous 3D structure. The freestanding CNF film-based electrode shows excellent flexibility. The electrical resistance of the film remains unaffected at different bending angles as represented in Fig. 7.21a. No crack is generated in the film even after bending for 100 times, and the film recovers to its initial state after releasing the stress as shown in the inset of Fig. 7.21a. This freestanding CNF film-based electrode displays a specific capacitance of 104.5 F g−1 with a high rate capability (retains 56.5% at 10 A g−1 ) and excellent cyclic stability since the capacitance is retained up to ∼ 94% after 2000 cycles. Further, the assembled supercapacitor delivers an energy density of 3.22 Wh kg−1 at a power density of 600 W kg−1 . The supercapacitor also retains ~ 90% of its initial specific capacitance even after 500 bending cycles indicating excellent mechanical and flexible properties as shown in Fig. 7.21b.

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Fig. 7.20 a Schematic illustration of the synthesis of metal- etched CNF film, b digital photographs, and c SEM images of the films (redrawn and reprinted with permission from [57])

Fig. 7.21 a Electrical resistance of the CNF film at different bent angles (inset shows the digital photographs of flexibility), b capacitance retention of the supercapacitor at different bending cycles (inset shows the digital photographs of the supercapacitor for flexibility of the device during releasing and bending operations) (reprinted with permission from [57])

The above-discussed CNF has also been explored for the potential use of electrode materials for supercapacitor applications like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, aerospace, etc., [4, 58, 59]. A summary of the electrochemical performance of different types of CNF-based supercapacitors based on carbon precursors and synthesis techniques are given in Table 7.1.

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Table 7.1 Comparative evaluation of the performance of different types of CNF-based supercapacitor devices Precursor

Technique

Surface Capacitance (F Cycle area g−1 ) life (m2 g−1 )

Zinc glycolate fibers

Self-templated 1725 solution-growth

280 at 0.5 A g−1

91% after 1000 cycles

Ref.

[35]

PAN

Electrospinning

550

140

[10]

CNF and polypyrrole

Carbonization

562.51

202 at 1 A g−1 97% after 3000 cycles

[37]

224.9 at 0.5 A g−1

105% after 8000 cycles

[39]

705

240

~90% after 5000 cycles

[41]

1270

152

95% after 1000 cycles

[42]

[43]

Polyacrylonitrile and phosphoric acid Electrospinning

12.15

Polyacrylonitrile

Carbonization

Organic resols

Crab shell biological template

Bacterial cellulose

Pyrolysis

289.90

204.9 at 1 A g−1

4000 cycles

Bacterial cellulose and ammonia

Hydrothermal reaction

312.48

195.44

∼95.9% [44] after 5000 cycles

PAN and Nafion

Electrospinning 1600

210 at 100 mV s−1

–

[48]

597

256

95% after 1000 cycles

[50]

Electrospinning 1220

202

–

[55]

122 at 10 mV s−1

–

[56]

Resole-type phenolic resin and KOH Electrospinning

Polybenzimidazol and steam

Poly(acrylonitrile-co-vinylimidazole) Electrospinning 1120

7.4 Concluding Remarks Carbon nanofiber (CNF) is one of the important carbon materials for supercapacitor applications due to the high surface area and good electrical conductivity. The structure and surface area of CNF can be modified by using various methods like doping,

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chemical activation, surface etching, etc. Freestanding 3D structure of CNF can easily be formed for supercapacitor electrodes and solid devices. Hence, the proper design of CNF-based electrodes makes it possible to achieve performance like commercial activated carbon-based supercapacitors. However, the large surface area and high electrical conductivity of CNFs only provide high power density and long cycle life as the main mechanism is EDLC, and there is a limit for ion-accessible and electron transport on the surface. Thus, they exhibit very low energy density (4–7 Wh kg−1 ), and also, the attainment of capacitance better than 300 F g−1 is impossible via surface modification or chemically activation processes. Therefore, to improve the energy density of CNF-based supercapacitors, redox-active catalysts like pseudo-capacitive transition metal oxides in combination with CNF have been recently introduced. In these composite systems, CNF provides a large surface area and high electrical conductivity, whereas metal oxides act as redox-active pseudo-capacitors. Therefore, the hybrid supercapacitors simultaneously deliver high energy and power densities with long cycle life and rate capability. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)), for carrying out this research work.

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Chapter 8

Transition Metal Oxide/Carbon Nanofiber Composites as Electrode Materials for Supercapacitors Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar Abstract The supercapacitor is recognized as an important device for nextgeneration energy storage due to its high-power densities. Carbon-based materials are the most widely considered as electrodes for supercapacitors due to their large specific surface area and excellent electrical conductivity. However, they generally suffer from low specific capacitance values and therefore poor energy densities. Recently, transition metal oxides are vastly integrated with carbon materials to design hybrid supercapacitors to improve energy density. Due to the existence of high electrical conductivity and specific surface area, carbon nanofibers are widely used in hybrid supercapacitor electrodes with different transition metal oxides like MnO2 , RuO2 , and V2 O5 . These hybrid supercapacitors simultaneously deliver high energy and power densities with long cycle life and rate capability. Therefore, this chapter is mainly focused on hybrid supercapacitors of transition metal oxides with carbon B. De · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] B. De e-mail: [email protected] S. Banerjee · K. D. Verma · T. Pal · K. K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. D. Verma e-mail: [email protected] T. Pal e-mail: [email protected] T. Pal A.P.J. Abdul Kalam Technical University, Lucknow 226031, India P. K. Manna Indus Institute of Technology and Management, Kanpur 209202, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_8

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nanofiber. The chapter provides decent and updated coverage on the fabrication and structure of different hybrid supercapacitors based on transition metal oxides and carbon nanofiber, and their electrochemical performance.

8.1 Introduction Supercapacitors are the most promising energy storage systems for the nextgeneration wearable electronic devices owing to the transport of high power within a very short period. The applications of supercapacitors include consumer and portable electronics, transportation and vehicles, power backup, biomedical, military, aerospace, etc., [1]. The features of capacitors [2] and capacitor to supercapacitor [3] are reported elsewhere. Generally, carbon-based electrochemical capacitors store energy by the electrical double-layer capacitance (EDLC) mechanism [4, 5] and have low energy density due to the limited electrical charge separation at the interface of the electrode and electrolyte materials. Therefore, transition metal oxide- based pseudocapacitors are the alternative options for the energy storage devices as they exhibit very high specific capacitance and energy density due to the fast and reversible surface faradic redox reactions for charge storage [6–8]. However, these materials suffer from poor electrical conductivity, small surface area, poor rate capability, low power density, and poor cyclic stability due to the volume change in the electrolyte solution. Therefore, current research is mainly focused on the development of hybrid supercapacitors based on carbon and transition metal oxide hybrid nanostructures [9–12]. In such carbon material/metal oxide hybrid supercapacitor electrodes, the carbon materials serve as the physical support for the metal oxides as well as provide the channels for charge transport. The high electronic conductivity of carbon materials is beneficial because of capability and power density at a high charge/discharge current during electrochemical analysis [9, 13]. On the other hand, metal oxides are the main sources to store charge and energy. The electrocatalytic activities of metal oxides contribute high specific capacitance and energy density of the hybrid supercapacitor electrodes. Hence, a synergistic effect is always expected in carbon and transition metal oxide-based hybrid supercapacitors. Among different carbon-based materials, one-dimensional (1D) nanostructured carbon nanofiber (CNF) exhibits superior electrical conductivity and a faster diffusion rate for electrons and ions during the electrochemical process [14–16]. The surface of CNF can easily be modified to achieve high surface area and also easy to build 3D self-standing electrodes for high-performance supercapacitors. Therefore, transition metal oxide and CNFbased composites have been widely investigated in supercapacitor applications [15, 17–19]. Herein, a comprehensive overview of hybrid supercapacitors based on transition metal oxides and CNF has been included to get the scenario of recent research development of this field.

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8.2 Materials: Carbon Nanofiber and Transition Metal Oxides 8.2.1 Carbon Nanofiber CNF has classified as linear and sp2 carbon-based discontinuous filaments with a diameter of ~100 nm and a length of ~100 μm. The integral structure of CNF, in general, depends on the synthesis process. They are mainly prepared by two methods, namely catalytic chemical vapor deposition growth and electrospinning. Attributes of excellent surface area with high porosity and electrical conductivity of CNF endow them a promising material for supercapacitor electrodes. They behave as electrochemical double-layer capacitors (EDLCs) and store energy via forming the electrical double-layer surrounding the surface of the electrode. The energy storage mechanism in EDLC shows that the depleted oppositely charged species store energy at the interface of the electrode and electrolyte. The capacitance (C) of the EDLC mainly depends on the conductivity and surface area of the electrode calculated by (8.1), C = εr ε0 A/d

(8.1)

where Er is the relative permittivity of the medium in electrical double-layer, ε0 is the permittivity of vacuum, A is the specific surface area of the electrode, and d is the effective thickness of the electrical double-layer. Normally, CNF delivers specific capacitance value in the range of 100–150 F g−1 . However, the supercapacitor performance of CNFs can be enhanced by increasing their conductivity, porosity, or surface area. Therefore, fine control of the properties of CNF in terms of electrical conductivity, surface area, and pore size achievable via proper processing and carbonization conditions will make it possible to reach specific capacitance up to 300 F g−1 [14]. However, large surface area and high electrical conductivity of CNF only provide high-power density and long cycle life as the governing mechanism is EDLC. There is a limit for ion-accessibility and electron transport on the surface of the CNF. Hence, they exhibit very low energy density (~ 5 Wh kg−1 ), and enhancement in capacitance more than 300 F g−1 is impossible by surface modification or chemical activation. Therefore, the energy density of CNF-based supercapacitors can be improved by the introduction of redox-active catalysts like pseudocapacitive transition metal oxides in CNF. In these composite systems, CNF provides large surface area and high electrical conductivity, whereas metal oxides act as redox-active pseudocapacitors. Therefore, the hybrid supercapacitors simultaneously deliver high energy and power densities with long cycle life and rate capability. An exclusive overview of the synthesis of CNF, and its properties and applications has been reported elsewhere [20].

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8.2.2 Transition Metal Oxides Transition metal oxides (TMOs) are the oxides of d-block elements in the periodic table, which are characterized by their partially filled d sub-shells. TMOs have attracted the attention of the research community because of the unique and diverse variations in properties such as electrical, magnetic, optical, and electrochemical. They exhibit a wide range of electrical and magnetic properties varying from insulators to conductors and diamagnetism to ferromagnetism, respectively. These variations are a result of the differences in the numbers and arrangements of d-electrons in various TMOs. The novel electrical and surface properties of nanostructured TMOs have envisaged them for many practical device applications including supercapacitors, lithium-ion batteries, non-volatile memory devices, sensors, solar cells, infrared detectors, bio-imaging, etc. It is observed that the preparation methods greatly influence the morphologies and hence the properties and applications of nanostructured TMOs. Electrochemical deposition, hydrothermal, sol–gel method, and solution precipitation include the major techniques employed to prepare unique nanostructures of TMOs. Thorough knowledge of the synthesis methods can assist in the preparation of interesting nanostructures including nanoneedles, nanocubes, nanorods, nanowires, etc. Further, an in-depth understanding of the characteristics of TMOs including the physical (size, shape, and color), physiochemical (surface area and pore size distributions), chemical, electronic, magnetic, optical, thermoelectric, electrochemical, etc., can help in a more precise applicationoriented selection. The ability to tune the electrical, chemical, and surface properties by modification in morphologies and sizes can help in designing devices with tunable desired properties. This has greatly helped in enhancing the industrial importance of TMOs. For commercialization, it is important to look into the environment and the economic aspects. The environment-friendly nature and cheap cost of TMOs have further assisted the applications in areas like nanoelectronic and energy conversion devices, gas sensors, optoelectronic devices, catalytic, and biomedical applications. An exclusive overview of the synthesis of TMOs, and its properties and applications has been reported elsewhere [21].

8.3 Carbon Nanofiber as Electrode for Supercapacitors Carbon nanofiber (CNF) is one of the important carbon materials for supercapacitor applications due to the high surface area and good electrical conductivity. The structure and surface area of CNF can also be modified by various methods like doping, chemical activation, and surface etching. Freestanding 3D structure of CNF can easily be formed for supercapacitor electrodes and solid devices. Thus, by proper design of CNF-based electrodes, it can achieve performance like commercial activated carbon-based supercapacitors. A summarized supercapacitive performance of different types of CNF according to the carbon precursors and synthesis techniques is

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given in Table 8.1. An exclusive overview of the performance of CNF as an electrode for supercapacitor has been included in Chap. 7. However, the large surface area and high electrical conductivity of CNF only provide high-power density and long cycle life as the main mechanism is EDLC and there is a limit for ion-accessible and electron transport on the surface. Thus, they exhibit very low energy density (4–7 Wh kg−1 ), and also enhancement of capacitance value more than 300 F g−1 is impossible by their surface modification or chemically activation processes. Therefore, to improve the energy density of CNF-based supercapacitors recently introduces redox active catalysts like pseudocapacitive transition metal oxides with CNF. In these composite systems, CNF provides large surface areas and high electrical conductivity, whereas metal oxides act as redox active pseudocapacitors. Therefore, the hybrid supercapacitors simultaneously deliver high energy and power densities with long cycle life and rate capability.

8.4 Transition Metal Oxides as Electrode for Supercapacitors The supercapacitive performance of different TMOs depends on their nature, approach, morphology, and architecture. TMOs exhibit much higher specific capacitance and energy density compared to EDLCs due to their fast charge-discharge process through fast Faradaic redox reactions occurring at the interface between TMO electrodes and electrolytes. These materials have the potential to overcome the low energy density limitation of electrochemical capacitors and low power density of the battery. Therefore, the pseudocapacitive TMOs bridge the gap between existing EDLCs and batteries to form units of intermediate specific energy and power densities. However, few TMOs like RuO2 , and IrO2 have limited their widespread application in supercapacitors due to their very high cost and toxicity, even though they have high specific capacitance, good electrical conductivity, fast and reversible charge-discharge properties. Therefore, MnO2 , NiO, Co3 O4 , etc., are alternatively considered for supercapacitor electrodes due to their cost effectiveness, environmentfriendly nature, and high electrochemical properties. However, still, their poor electronic conductivity, surface area, and power density are remaining as major disadvantages. Again, the poor cyclic stability of TMO electrodes is a permanent issue in energy storage applications due to their volume change in electrolyte solution during electrochemical analyses. Therefore, several approaches and synthetic methods are utilized to achieve desire morphologies and architectures of TMOs to fabricate highperformance electrode materials for supercapacitor applications. Currently, intensive research has been going on hybrid supercapacitors utilizing both pseudocapacitive TMOs and high surface area conducting carbon-based EDLCs for the development of a diverse range of next-generation energy storage devices. These hybrid supercapacitors offer a promising avenue for designing smart multifunctional electrodes by integrating the advantages of each constituent. The supercapacitive performance

Hydrothermal reaction

Electrospinning

Electrospinning

Electrospinning

Electrospinning

PAN and Nafion

Resole-type phenolic resin and KOH

Polybenzimidazol and steam

Poly(acrylonitrile-co-vinylimidazole)

Carbonization

Polyacrylonitrile

Bacterial cellulose and ammonia

Electrospinning

Polyacrylonitrile and phosphoric acid

Crab-shell biological template

Carbonization

CNF and polypyrrole

Pyrolysis

Electrospinning

PAN

Bacterial cellulose

Self-templated solution-growth

Zinc glycolate fibers

Organic resols

Technique

Precursor

1120

1220

597

1600

312.48

289.90

1270

705

12.15

562.51

550

1725

Surface area (m2 g−1 )

122 at 10 mV

202 s−1

–

–

95% after 1000 cycles

– 256

210 at 100 mV s−1

4000 cycles ∼95.9% after 5000 cycles

205 at 1 A g−1 195.0

95% after 1000 cycles

152

~90% after 5000 cycles

105% after 8000 cycles

240

97% after 3000 cycles

225 at 0.5 A g−1

91% after 1000 cycles

Cycle life

202 at 1 A g−1

140

280 at 0.5 A

g−1

Capacitance (F g−1 )

Table 8.1 Comparative supercapacitive performance of different types of CNF (see references in Chap. 7 for numerical values of surface area, capacitance, cycle life, etc.)

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of different transition metal oxides is summarized in Table 8.2, according to the synthesis techniques and structures. An exclusive overview of the performance of transition metal oxides as an electrode for supercapacitor has been included in Chap. 4.

8.5 Transition Metal Oxides and Carbon Nanofiber Composites as Electrode for Supercapacitors In transition metal oxide-based pseudocapacitors, the capacitance is originated from the reversible redox reactions between active electrode materials with the electrolyte. The accumulation of electrons at the electrode is a Faradaic process, where the electrons produced through the redox reactions are transported across the electrolyte– electrode interface. The theoretical capacitance of metal oxide is calculated using (8.2) as follows C = n F/mV

(8.2)

where n is the mean number of the electrons transferred in the redox reaction, F is the Faraday constant, m is the molar mass of the metal oxide, and V is the operating voltage window [9, 22, 23]. Therefore, pseudocapacitors exhibit much higher capacitance and energy density compared to EDLC. In CNF-based hybrid supercapacitors transition metal oxides such as MnO2 , RuO2 , ZnO, V2 O5 , Nio, and Co3 O4 are commonly employed. Among them, MnO2 is the most widely used material for CNF-based hybrid supercapacitor electrodes. The details of the fabrication process, characterization, and supercapacitor performance of transition metal oxide-CNF composites are discussed in the next section.

8.5.1 MnO2 -CNF Composites as Electrode for Supercapacitors Among different transition metal oxides, MnO2 is one of the most promising pseudocapacitor materials because of high theoretical capacity (1370 F g−1 ), wide operating potential window in the mild electrolyte, nontoxicity, low cost and natural abundance [24, 25]. However, in practice, MnO2 -based electrode material shows limited specific capacitance (low energy density), limited structural stability, long-term cyclability, and low rate-capacity. The underlined reasons are poor electrical conductivity, low surface area, and high rate of dissolution during the electrochemical performance. Therefore, carbon materials including CNF are added with MnO2 to form hybrid electrode materials of improved electrical conductivity, surface area, and stability.

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Table 8.2 A summarized supercapacitive performance of different transition metal oxides (see references in Chap. 4 for numerical values of surface area, capacitance, cycle life, etc.) Transition metal oxide

Synthesis technique

Structure

Capacitance (F Cycle g−1 ) life

RuO2

Sacrificial template

Nanotubular

860 at 0.5 A g−1

RuO2

Anodic deposition

Nanotubular array

1300

_

MnO2

Coprecipitation

Powder

1380

_

MnO2

Reduction

Crystalline α-phase

253 at 0.5 mA cm−2

54% after 1000 cycles

MnO2

Microwave heating and ultrasonication

Sphere-network

214

~90% after 5000 cycles

MnO2

Microwave-assisted emulsion

Birnessite-type 1D

277 at 0.2 mA cm−2

_

MnO2

Hydrothermal

Lamellar birnessite-type

242 at 2 mA cm−2

108% after 200 cycles

MnO2

Electrochemical deposition Film

410 at 1 mA cm−2

~100% after 10,000 cycles

MnO2

Electrodeposition

Nanowire array

493 at 4 A g−1 ~100% after 10,000 cycles

NiO

Calcination

Crystalline β-Ni(OH)2

696

NiO

Hydrothermal

Porous nanocolumns

390 at 5 A g−1 ~100% after 10,000 cycles

NiO

Annealing

Cubic

167

NiO

Gas/liquid interfacial microwave

Flowerlike hollow nanospheres

770 at 2 A g−1 95% after 1000 cycles

Co3 O4

Sol–gel

Xerogel

291

Stable up to 10,000 cycles

Co3 O4

Solution and thermal

Hexagonal nanosheets

227

_

_

_

_

(continued)

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Table 8.2 (continued) Transition metal oxide

Synthesis technique

Structure

Capacitance (F Cycle g−1 ) life

Co3 O4

Microwave-assisted hydrothermal

Nanorods

456

_

Co3 O4

Chemically depositing

Nanotube

574

95% after 1000 cycles

Co3 O4

Supported technique

Nanowire arrays

746

86% after 500 cycles

Co3 O4

Epoxide addition

Aerogel

623

96% after 1000 cycles

Co3 O4

Self-organization

Brush-like nanowires

1525

94% after 5000 cycles

Co3 O4

Solvothermal and calcination

Nanoflower

1937

78.2% after 1000 cycles

SnO2

Electrochemical deposition Amorphous nanostructured 285

88% after 1000 cycles

FeCo2 O4

Hydrothermal

Stable up to 2500 cycles

Co3 O4 @NiO Hydrothermal

Nanoflakes array

433 at 0.1 A g−1

Hierarchical nanowire array

721 at 1 A g−1 91.35% after 5000 cycles

Different fabrication methods, e.g., vapor deposition growth and electrospinning are utilized to obtain MnO2 -CNF composites. Liu et al. have fabricated a hybrid supercapacitor of MnO2 -CNF through electrochemical deposition of MnO2 on a vertically aligned vapor grown carbon nanofiber array [26]. The resulted multifunctional core–shell nanostructure is shown in Fig. 8.1. In this fabrication process, at first, a Cr layer (thickness ~ 100 nm) is coated on the Si substrate to form the conductive film, and then a Ni layer (thickness ~ 22 nm) is deposited as the catalyst. Second, the CNF arrays are grown by a plasma-enhanced chemical vapor deposition method. Third, to remove the catalyst samples are treated

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Fig. 8.1 a Schematic representation of fabrication process for MnO2 coated CNF, b SEM image of CNF, c SEM and d TEM images of MnO2 coated CNF, e schematic illustration of the uniform MnO2 coating on the sidewall of the cup-stacking graphitic structured CNF (reprinted with permission from [26])

in a 1.0 M HNO3 solution [26]. Finally, a thin layer of MnO2 is deposited on the surface of CNF using an electrochemical method. The complete fabrication process is shown as a schematic illustration in Fig. 8.1a. The SEM image of the vapor grown CNFs is shown in Fig. 8.1b, where the average diameter of the CNFs is found to be ∼ 150 nm. Figure 8.1c shows the SEM image after the deposition of MnO2 on acid-treated CNF arrays at −0.30 V (vs. Ag/AgCl saturated KCl)) from 0.05 M manganese (II) acetate for 20 min [26]. A thin layer of MnO2 uniformly wraps around each CNF. However, it is difficult to determine the deposition by SEM analysis, as the coating is so thin and uniform. Therefore, it is further examined under TEM for in-depth analysis as shown in Fig. 8.1d. The image reveals that the thickness of MnO2 deposited is about 5–10 nm. A schematic illustration of uniform MnO2 coating on the sidewall of the cup-stacking graphitic structured CNF is also represented in Fig. 8.1e. This unique core–shell nanostructure offers a highly conductive and robust

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CNF core for reliable electrical connection to the MnO2 shell, which favors the fastredox reaction kinetics and easy access of the electrolyte to a large volume of active electrode materials. These factors enhance the charge transfer rate and total specific capacitance [26]. Therefore, the electrode delivers a specific capacitance up to 365 F g−1 in 0.10 M Na2 SO4 aqueous solution. The electrode also demonstrates high performance with a maximum specific energy of 32.5 Wh kg−1 and specific power of about 6.216 kW kg−1 . Zhi et al. have fabricated a highly conductive CNF/MnO2 coaxial cables, where individual electrospun CNF is coated with an ultrathin hierarchical MnO2 layer [27]. The CNF is synthesized by electrospinning a PAN precursor containing iron acetylacetonate that enhances the specific surface area and electronic conductivity of CNF to improve the specific capacitance and rate capability of CNF/MnO2 composite electrode. The hierarchical MnO2 is coated on the carbonized CNF by immersing the CNF sheets into the KMnO4 solution. The morphologies of the CNFs and composites are shown in Fig. 8.2. The SEM images of as-synthesized CNFs containing iron acetylacetonate (AAI) before and after acid treatment are shown in Fig. 8.2a, b, respectively. The carbonized nanofibers are treated with 1 M HCl solution to remove the residual surface iron as well as to enhance the surface area. The nanofibers are found to be 200 nm in diameter and are quite uniform. Some small iron oxide (Fe3 O4 ) nanoparticles are decorated on the surface of the nanofibers (Fig. 8.2a), which are obtained from the decomposition of iron acetylacetonate in the carbonization process. However, after treating with HCl, the CNF surface becomes clean as the particles are etched away shown in Fig. 8.2b. This process enhances the surface area from 28 to 102 m2 g−1 of CNF after acid treatment. Figure 8.2c, d show the SEM images of MnO2 coated CNF at the ratios of CNF: KMnO4 of 4:1 and 1:2, respectively. The results evident that squama-like MnO2 textures have appeared on the CNF surface. As the amount of KMnO4 increases, the MnO2 shell homogeneously covers the entire CNF surface. The diameter of CNF also increases with an increase in the MnO2 coverage [27]. The core–shell structure of the nanohybrid is further examined under TEM as shown in Fig. 8.2e, f. Examination reveals that a porous MnO2 shell has been formed on the surface of the CNF. The supercapacitor performance of iron acetylacetonate-carbon nanofibermanganese oxide (AAI-CNF@MnO2 ) electrode has been measured by cyclic voltammetry (CV) analysis as shown in Fig. 8.3. The AAI-CNF/MnO2 electrode shows a specific capacitance of 311 F g−1 at a scan rate of 2 mV s−1 . Figure 8.3a shows the CV curves of the AAI-CNF/MnO2 (at different CNF: KMNO4 ratios, 1 = 4:1, 2 = 2:1, 3 = 1:1 and 4 = 1:2) electrodes at a scan rate of 100 mV s−1 . The electrodes 1 and 2 show nearly symmetrical box-like shape indicating a good capacitor characteristic. The electrode 3 exhibits the highest specific current density with a moderately disordered rectangular shape. When the mass loading of MnO2 increases to 59% in electrode 4, the gravimetric current density decreases and the shape is further disordered [27]. The CNF@MnO2 electrodes in the absence of AAI exhibit different trends and show very small specific current density as shown in Fig. 8.3b. In this case, an increase in the amount of MnO2 severely deviates the CV curves from the ideal capacitor behavior and becomes of “diamond” shape. Figure 8.3c, d show the comparison

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Fig. 8.2 SEM images of AAI-CNF a before and b after acid treatment; SEM images of MnO2 -CNF composites at CNF: KMnO4 ratios of c 4:1 and d 1:2; TEM images of e CNF@MnO2 composite and f porous MnO2 shell (reprinted with permission from [27])

of specific capacitance of electrodes at different scan rates with and without AAI. The optimized electrode material also achieves energy density up to 80.2 Wh kg−1 and power density up to 57.7 kW kg−1 that are similar to a pseudosupercapacitors. Coaxial CNFs/MnO2 nanocomposites-based freestanding electrodes have also been prepared by using in situ redox deposition and electrospinning [28]. First, CNFs are prepared by electrospinning technique using PAN, which serves as an electrically

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Fig. 8.3 CV curves of the electrodes at a scan rate of 100 mV s−1 for a AAI-CNF@MnO2 and b CNF@MnO2 ; Specific capacitance of the electrodes as a function of scan rate for c AAICNF@MnO2 and d CNF@MnO2 (redrawn and reprinted with permission from [27])

conducting core having a conductivity of 1–10 S cm−1 to support electro-active nanoMnO2 . MnO2 deposits on CNFs through in situ redox reaction between permanganate ions and CNFs using a simply modified dip-coating method. A schematic illustration of the fabrication procedure of the nanocomposites is shown in Fig. 8.4. In the redox reaction, permanganate ions (MnO4 − ) are reduced to MnO2 by oxidizing carbon in acidic aqueous solution at room temperature and deposited on the smooth surface of CNFs. The composite forms a porous network structure, where amorphous MnO2 nanowhiskers are distributed uniformly throughout CNFs. The supercapacitor performance through CV analysis of the composite is shown in Fig. 8.5. The CV curves of the composites at different scan rates are shown in Fig. 8.5a, which exhibits ideal rectangular profiles and mirror symmetry suggesting a fast and reversible reaction with excellent capacitive behavior. The specific capacitance of the optimized composite is 539 F g−1 at a low scan rate of 2 mV s−1 and also retains a high value of 213 F g−1 at a higher scan rate of 100 mV s−1 . The rate capability of the composite is shown in Fig. 8.5b. The decrease in specific capacitance at a high scan rate is due to the reduced diffusion time as a result of the insertion/extraction of ions into the birnessite-type MnO2 matrix. The unique coaxial nanostructure provides good electrochemical performance of the composite. The

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Fig. 8.4 Schematic illustration of the fabrication of CNFs/MnO2 nanocomposites (redrawn and reprinted with permission from [28])

Fig. 8.5 a CV curves of the composite at different scan rates, b specific capacitance versus scan rates of CNF and composite, c cyclic stability of the composite up to 1500 cycles at 100 mV s−1 scan rate (redrawn and reprinted with permission from [28])

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porous CNFs network and interconnectivity enable fast ions transport and continuous charge transfer pathways, while the uniform nano-MnO2 layer offers short ion diffusion pathways for both ions and electrons. Figure 8.5c shows the cycling performance of the composite electrode measured by the CV method at a scan rate of 100 mV s−1 for 1500 cycles. The inset CV curves of Fig. 8.5c are almost overlapping with each other and the specific capacitance of the electrode only loses ∼ 6% after 1500 cycles indicating excellent long-term cyclic stability. The composite electrode also exhibits very high energy and power densities, 62.6 Wh/kg at 0.45 kW/kg, and 20.9 Wh/kg at 13.5 kW/kg, respectively. Yang et al. have reported ultrafine manganese oxide decorated carbon nanofibers (MnOn -CNF) as electrode materials, fabricated through pyrolysis of Mn, Zncontaining metal-organic framework fibers [29]. The resulted composite electrode exhibits a high capacitance of 179 F g−1 per mass of the composite electrode, and a remarkable capacitance of 18,290 F g−1 per active mass of the manganese (IV) oxide. A maximum energy density of 19.7 Wh kg−1 at the current density of 0.25 A g−1 has been reported. The composite electrode is also highly stable and can be cycled up to 5000 times with high capacitance retention of 98% during electrochemical analysis.

8.5.2 RuO2 -CNF Composites as Electrode for Supercapacitors Besides MnO2 , CNFs are also utilized with RuO2 , NiCo2 O4 , V2 O5 , ZnO, etc., for the development of high-performance hybrid supercapacitor electrodes. Among the metal oxides, RuO2 is the most prominent material for supercapacitor applications because of high specific capacitance, good electrical conductivity, and reversible charge–-discharge properties. RuO2 exhibits the highest theoretical specific capacitance value of 1450 F g−1 [30]. Therefore, several times it has been utilized as hybrid supercapacitors with CNFs. Lee et al. [31] have used hydrous amorphous RuO2 (RuO2 .xH2 O) with vapor grown CNF (VGCF) [32] for supercapacitor electrode material. The nanocomposite (VGCF/RuO2 ·xH2 O) has been fabricated through annealing the homogeneous mixture of VGCF in the ruthenium ethoxide solution and casted on a Pt electrode (1 cm × 1 cm). The electrochemical analysis of the electrode is performed through CV using 1.0 M H2 SO4 as the electrolyte, in the voltage range of 0–1.0 V at different scan rates as shown in Fig. 8.6. The composite electrode shows excellent redox transitions even at a very high scan rate of 1000 mV s−1 as displayed in Fig. 8.6d compared to pristine RuO2 ·xH2 O. From a control experiment, the specific capacitance value of the pristine VGCF is found to be only 1.9 F g−1 [31]. Therefore, the redox transition and the high current value observed for the composite material are mainly coming from the Faradaic redox transitions of RuO2 ·xH2 O in the composite. The calculated specific capacitance values of RuO2 ·xH2 O and composite electrodes are plotted in Fig. 8.7a [31]. At a small scan rate of 10 mV s−1 , the specific capacitance values are found to be 410 and 1017 F g−1 for RuO2 ·xH2 O and

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Fig. 8.6 CV curves of pristine RuO2 ·xH2 O and VGCF/RuO2 ·xH2 O nanocomposite electrodes at scan rates of a 100, b 300, c 500, and d 1000 mV s−1 (redrawn and reprinted with permission from [31])

Fig. 8.7 a Rate capability and b cyclic stability of pristine RuO2 .xH2 O and the nanocompositebased electrodes (reprinted with permission from [31])

the composite, respectively. The capacitance values are remained up to 258 and 824 F g−1 , respectively, at a very high scan rate of 1000 mV s−1 . So, the composite material exhibits two to three times of higher specific capacitance value (based on Ru weight) compared to the pristine RuO2 ·xH2 O. The composite electrode shows excellent rate capability through capacitance retention of 81% at 1000 mV s−1 compared to the capacitance value at 10 mV s−1 . The composite electrode also exhibits outstanding cyclic stability up to 97% capacitance retention after 10,000 cycles at a scan rate of 300 mV s−1 as shown in Fig. 8.7b. The pristine RuO2 also retains 90% capacitance after the same number of cycles at the same scan rate. Such high electrochemical performance is due to the highly open morphology of the composite material and

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Fig. 8.8 TEM image of RuO2 .xH2 O-CNF nanocomposite (reprinted with permission from [34])

high electronically conducting support of CNF that provides favorable proton and electron transportation through the RuO2 ·xH2 O matrix. RuO2 /CNF nanocomposite-based supercapacitor electrodes are also fabricated by grafting RuO2 nanoparticles on CNF through hydrothermal deposition [33]. The presence of CNFs leads to reduced contact resistance among the RuO2 nanoparticles and provides a network for fast electron transport enhancing the electrochemical performance of the composite electrode. The improvement is proportional to the amount of RuO2 content, and it is up to 638% at a high scan rate of 200 mV s−1 . The RuO2 grafted CNF also shows 99% of capacitance retention after 1000 cycles at a high scan rate of 500 mV s−1 . Again, the composite electrode exhibits a very short relaxation time of 0.17 s, which is desirable for a high rate of charge and discharge. Pico et al. have designed a supercapacitor electrode based on hydrous RuO2 (RuO2 .xH2 O) impregnated CNF [34]. Figure 8.8 shows the morphology of composite, where very fine particles (2–4 nm) of RuO2 particles are distributed inside the CNF. The composite electrode shows a specific capacitance of nearly 440 F g−1 , which is close to bare RuO2 ·xH2 O (460 F g−1 ). Though RuO2 is the most prominent material for supercapacitors, however, the commercialization of RuO2 based supercapacitors is still under critical observation because of the rareness of Ru in nature and high cost.

8.5.3 V2 O5 -CNF Composites as Electrode for Supercapacitors As one of the most promising candidates for supercapacitor electrode, vanadium pentoxide (V2 O5 ) has great potential because of low cost, natural abundance, multiple stable oxidation states (V2+ to V5+ ), and ease of synthesis [35–38]. The electrochemical properties of V2 O5 strongly depend on the preparation condition, morphology, particle size, and measurement techniques [39, 40]. However, poor electronic conductivity and high dissolution in liquid electrolyte determine their practical application

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in high-performance supercapacitor electrodes. Therefore, in several reports, CNFs are utilized to improve its electrochemical performance in the supercapacitor electrode. A CNF/V2 O5 composite electrode has been prepared through electrospinning of PAN/V2 O5 composite in DMF that delivered promising energy densities of 18.8 Wh kg−1 over a power density range of 400–20,000 W kg−1 with the additional benefit of high stability [41]. The electrochemical properties of composite electrodes with varying 5–20 wt% of V2 O5 are studied in two-electrode capacitors using the aqueous electrolyte. Figure 8.9a shows the CV curves of the composites with varying amounts of V2 O5 at a scan rate of 25 mV s−1 . The study reveals that the composites exhibit quasi-rectangular voltammogram shapes of perfect EDLC and the induced current of the CV curves increases with an increase in the amount of V2 O5 . Figure 8.9b shows the variation of specific capacitance values of the composites at different current densities (1–20 mA cm−2 ). The composite electrode with 20 wt% V2 O5 shows three times higher specific capacitance compared to pristine CNF. The composite also delivers energy densities in the range of 18.8–13.8 Wh kg−1 and the power density in the range from 400 to 20,000 W kg−1 . The study reveals that the surface area and electrical conductivity of the composite electrodes are increased with V2 O5 . The improvement in energy densities and capacitance is attributed to the porous morphology of the composite that interacts with the electrolytes. These interconnected mesopores provide a good charge propagation and the ability for high current loads. This morphology is also beneficial for providing quick pathways for electrolyte transportation. An ultrathin layer of V2 O5 is electrodeposited on heat-treated electrospun PANbased CNF paper by Ghosh et al. for high-performance pseudocapacitors as represented in Fig. 8.10a [42]. The deposited thickness of V2 O5 depends on the number of cycles during CV analysis. Figure 8.10b shows a TEM image for 20 cycles of V2 O5 deposited CNF in CV analysis. The study reveals that ultrathin films of V2 O5 of the thickness of 10 nm have been deposited on the surface of CNF. The CV curves of the composite electrode are shown in Fig. 8.10c. It reveals that the curves are not perfectly rectangular and a broad oxidation band from 0.0 to 0.6 V is found that indicates the

Fig. 8.9 a CV curves of the CNF/V2 O5 composites with the variation of the amount of V2 O5 (5, 10, and 20 wt%) at the same scan rate of 25 mV s−1 , and b specific capacitance values of these composites at different current densities (redrawn and reprinted with permission from [41])

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Fig. 8.10 a Schematic representation of V2 O5 -coated CNF electrode, b TEM image of V2 O5 coated CNF, c CV curves of the electrode at different scan rates (2–50 mV s−1 ) (redrawn and reprinted with permission from [42])

contributions of both pseudocapacitance and EDLC. The specific capacitance value is found to be 214 F g−1 at a scan rate of 2 mV s−1 . The high capacitance value of the composite electrode can be attributed to the synergistic effect of the large external surface area of CNF and the maximum number of active sites for the ease in redox reaction of the ultrathin V2 O5 layer. Again, a V2 O5 /CNF composite-based flexible and efficient electrode has been demonstrated by Li et al. through solvothermal growth of hierarchical porous V2 O5 nanosheets on electrospun PAN-based CNF followed by thermal treatment [43]. The growth of porous V2 O5 nanosheets on CNFs is shown in Fig. 8.11. The FESEM images (Fig. 8.11a, b) reveal that the V2 O5 layer is constructed by cross-linked porous subunit sheets with open space in between playing a crucial role during electrochemical performance through penetration of the electrolyte. Therefore, the composite exhibits a high specific capacitance of 408 F g−1 at a current density of 1 A g−1 with 75% retention of capacitance at a very high current density of 20 A g−1 as well. The assembled device formed using V2 O5 /CNF as a cathode, CNF as an anode, and the corresponding LED powered by the device are shown in Fig. 8.11c, d, respectively. The hybrid supercapacitor device delivers outstanding cycling stability (10.7% decay in specific capacitance after 10,000 cycles), high energy of 22.3 Wh kg−1 at a power density of 1500 W kg−1 , and excellent mechanical flexibility. The excellent supercapacitor performance attributes to the synergistic effects from both high electrochemical performance of V2 O5 and excellent conductivity of ECF.

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Fig. 8.11 FESEM images of a PAN-based electrospun CNF, b V2 O5 grown CNF (inset shows the view of large-area); photographs of c assembled flexible supercapacitor device and d LED powered by a device (reprinted with permission from [43])

8.5.4 Other Transition Metal Oxides-CNF Composites as Electrode for Supercapacitors Recently, the researcher introduces a battery-type mixed transition metal oxide, nickel cobaltite (NiCo2 O4 )-based supercapacitor electrodes of low cost and environmentally friendliness [44–46]. This battery-type electrodes possess much better electronic conductivity, at least two orders of magnitude higher than those of nickel and cobalt oxides and hence shows comparatively better electrochemical activity. It also offers better redox reactions and is a potentially cost-effective alternative to RuO2 . However, this type of electrode is comparatively less used in supercapacitor electrodes with CNF. Figure 8.12 displays a one-dimensional hierarchical hybrid nanostructure composed of NiCo2 O4 on CNF synthesized through facile solution methods followed by a simple thermal treatment [47]. The hybrid supercapacitor electrode shows a high capacitance of 902 F g−1 at a current density of 2 A g−1 with a good cyclic performance by retaining ~ 97% capacitance after 2400 cycles. Lei et al. have decorated nanostructured NiCo2 O4 into electrospun CNF of PAN/lignin by the solvothermal process as shown in Fig. 8.13, and utilized as flexible high-performance electrodes for hybrid supercapacitor applications [48]. NiCo2 O4 decorated CNF with PAN and lignin ratio of 80:20 wt% (CNF1) shows nanosheet

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Fig. 8.12 TEM images of hierarchical NiCo2 O4 /CNF composite (inset of A is an enlarged view of corresponding FESEM image) (reprinted with permission from [47])

Fig. 8.13 Schematic representation of the synthesis of NiCo2 O4 @CNF composites (redrawn and reprinted with permission from [48])

morphology, whereas with PAN and lignin ratio 50:50 wt% (CNF2) shows hierarchical nanoneedle morphology represented in Fig. 8.13. Therefore, NiCo2 O4 @CNF1 delivers the highest specific capacitance of 1757 F g−1 at a current density of 2 mA cm−2 with an excellent cyclability of ∼ 138% capacitance retention after 5000 GCD cycles at a current density of 7 mA cm−2 . The solid-state asymmetric

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Fig. 8.14 Schematic representation of the preparation of Ni(OH)2 /CNF hybrid membranes (reprinted with permission from [49])

supercapacitor device designed from NiCo2 O4 @CNF1 as cathode and N-doped rGO as anode displays a specific capacitance of 134.3 F g−1 at a current density of 1 A g−1 . The asymmetric supercapacitor also delivers a maximum energy density of 47.75 Wh kg−1 at a power density of 799.53 W kg−1 (Fig. 8.14). Nickel oxides and hydroxides are also considered as promising materials for supercapacitor applications due to the high theoretical capacitance (>2000 F g−1 ), environmentally benign nature, low cost and easy preparation process, well-defined redox behavior and high redox activity [49, 50]. Therefore, Zhang et al. have fabricated flexible hybrid membranes as electrode materials for supercapacitors through anchoring ultrathin Ni(OH)2 nanoplatelets vertically and uniformly on the electrospun CNF of poly(amic acid) (PAA) by chemical bath deposition (CBD), schematically illustrated in Fig. 8.14 [49]. Figure 8.15a reveals the growth of Ni(OH)2 nanoplatelets on interconnected CNFs to form 3D hierarchical nanostructures. Figure 8.15a also shows that Ni(OH)2 nanoplatelets are vertically and uniformly grown on the CNF. As a result, the hybrid membranes greatly improve electrochemical performance compared to their counterparts. It shows a high specific capacitance of 2523 F g−1 based on the mass of Ni(OH)2 (which is 701 F g−1 based on the total mass) at a scan rate of 5 mV s−1 with a long cycle life by retaining of 83% of its initial capacitance after 1000 cycles [49]. The significant improvement in the performance of the hybrid membranes is due to the synergistic effects of 1D carbon nanofibers and 2D Ni(OH)2 nanoplatelets. This provides a higher specific surface area and more exposure of active sites for efficient electrochemical interactions with the electrolyte as represented in Fig. 8.15c [49]. Additionally, the overall 3D hierarchical macroporous hybrid architectures significantly shorten the ion diffusion path length, improve the contact area, and ensure the ion/electron diffusion with low resistance. The highly

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Fig. 8.15 a SEM and b TEM images of Ni(OH)2 /CNF hybrid membranes, c schematic illustration of the ion diffusion and electron transportation in the hybrid membranes (reprinted with permission from [49])

conductive CNF also acts as a conductive core to provide efficient electron transport for fast Faradaic redox reactions of the Ni(OH)2 [49]. A new type of CNF-based electrode material decorated with dilute NiO particles (NiO/CNF) has been fabricated by Yang et al. through direct pyrolysis of Ni, Zn-containing metal-organic framework fibers [50]. The combination of CNF with evenly dispersed NiO particles and a successful modulation of conductivity and porosity of the final composites reaches an excellent capacitance of 14,926 F g−1 contributing to over 35% of the total capacitance (234 F g−1 ). The researcher also encapsulates Co3 O4 nanoparticles into electrospun CNF for high-performance supercapacitor [51]. The Co3 O4 /CNF hybrid electrode delivers a remarkable capacitance of 586 F g−1 at a current density of 1 A g−1 . Therefore, supercapacitors are used in several applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, and aerospace [1, 52]. A summary of the supercapacitive performance of different types of transition metal oxide-CNF composites is given in Table 8.3.

Synthesis technique

Electrochemical deposition

Coating

In situ redox deposition and electrospinning

Pyrolysis

Annealing

Hydrothermal

Chemical deposition

Electrospinning

Heat-treated electrospun

Solvothermal

solution method and thermal treatment

Solvothermal

Chemical bath deposition

Pyrolysis

Electrospinning

Composite

MnO2 -CNF

MnO2 -CNF

MnO2 -CNF

MnOn -CNF

RuO2 -CNF

RuO2 -CNF

RuO2 -CNF

V2 O5 -CNF

V2 O5 -CNF

V2 O5 -CNF

NiCo2 O4 -CNF

NiCo2 O4 -CNF

Ni(OH)2 -CNF

NiO-CNF

Co3 O4 -CNF

Nanoparticles encapsulated nanofiber

Particle decorated nanofibers

Nanoplatelets deposited nanofibers 3D network

Hierarchical nanoneedle

Hierarchical 1D

hierarchical nanosheets on nanofiber

Ultrathin layer on nanofiber

Particle decorated nanofibers

Particle impregnated fiber

Nanoparticle grafted nanofiber

Amorphous particles on nanofiber

Particle decorated nanofibers

Coaxial

Coaxial cables

Core–shell nanostructure

Structure

586

14,926

2523

1757

902

408

214

150

440

218

1017

18,290

90% after 5000 cycles 74% after 2000 cycles

[51]

[50]

[49]

[48]

∼138% after 5000 cycles 83% after 1000 cycles

[47]

[43]

[42]

[41]

[34]

[33]

[31]

[29]

[28]

[27]

[26]

Ref.

~97% after 2400 cycles

~89% after 10,000 cycles

80% after 1000 cycles

~82% after 100 cycles

93% after 10,000 cycles

99% after 1000 cycles

97% after 10,000 cycles

98% after 5000 cycles

∼94% after 1500 cycles

s−1

539 at 2 mV

~97% after 1000 cycles

89% after 500 cycles

Cycle life

311 at 2 mV s−1

365

Capacitance (F g−1 )

Table 8.3 Supercapacitive performance of different types of transition metal oxide-CNF composites

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8.6 Concluding Remarks This chapter has summarized the development of transition metal oxide (TMO) and CNF-based composites for supercapacitor electrodes. Generally, TMO has very low electronic conductivity and thus, composites with carbon nanostructure are the best option to tune the properties. The intimate contact of the nanostructured carbon with the current collector has also minimized the interfacial resistance. In metal oxideCNF composite electrodes, the metal oxide contributes high specific capacitance and energy density, whereas CNF supports good rate capability and high-power density. CNF can facilitate the charge transport in the supercapacitor electrode. The high conductivity of CNF supports the redox activities of the TMO, which enhances the pseudocapacitance and improves the rate capability. However, they are of relatively smaller specific surface area as compared to the graphene or CNT. The intimate contact and the surface area at the carbon-metal oxide interface are the key factors that affect the charge transferability of the composite electrodes. Hence, a need to develop CNFs with more surface area and high conductivity is the main focus of recent studies in this field. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)), for carrying out this research work.

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Chapter 9

Carbon Nanotube as Electrode Materials for Supercapacitors Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar

Abstract High-performance supercapacitors are promising candidates for future energy storage devices for alternative power sources. Carbon nanotubes (CNTs) are considered as potential electrode materials for supercapacitors due to their superior electrical conductivity, high electrochemical stability, good mechanical properties, high specific surface area, and so on. Both single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) have been considered for electrochemical supercapacitor electrodes due to their unique properties like novel structure, narrow distribution of size in the nanometer range, highly accessible surface area, low resistivity, and high stability. The specific capacitance of CNTs mainly originated through the electric double-layer capacitor (EDLC) mechanism. Therefore, the supercapacitive performance of CNTs mainly depends on the physical properties, such as specific surface area, electrical conductivity, which are related to the synthesis and post-treatment B. De · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] B. De e-mail: [email protected] S. Banerjee · K. D. Verma · T. Pal · K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. D. Verma e-mail: [email protected] T. Pal e-mail: [email protected] T. Pal A.P.J. Abdul, Kalam Technical University, Lucknow 226031, India P. K. Manna Indus Institute of Technology and Management, Kanpur 209202, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_9

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methods of them. This chapter mainly emphasizes on the recent progress and development of the use of CNTs as supercapacitor electrodes through fabrication and post-treatment techniques.

9.1 Introduction Carbon-based materials are strongly considered as electrode materials in electrochemical energy conversion devices due to their unique properties, including a large specific surface area, high conductivity, excellent mechanical flexibility, and high chemical and thermal stability [1, 2]. Supercapacitors are the most promising devices to store electrical energy for the next-generation electronics owing to the transport of high power within a very short period. The special features of capacitors [3] and capacitor to supercapacitor [4] are reported elsewhere. These supercapacitors are used in the consumer and portable electronics, transportation and vehicles, power backup, biomedical, military, aerospace, etc. [5]. Carbon-based materials act as electric double-layer capacitors (EDLCs), where the capacitance is originated from the accumulation of charges at the electrode and electrolyte interfaces. Therefore, the capacitance is mainly controlled by the specific surface area along with pore size and electrical conductivity. Among the different carbon materials, carbon nanotubes (CNTs) are of particular interest for the supercapacitor electrodes due to the unique tubular porous structures and superior electrical conductivity, which favor fast ion and electron transportation [1, 2]. Therefore, they are usually considered as the choice of the high-power electrode material. CNTs are classified as single-walled carbon nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs), and both of which have been widely explored as supercapacitor electrode materials. However, the capacitance and energy density values of the CNTs are quite low due to their relatively small specific surface area. The electrochemical performance of CNTs is affected by various factors, like specific surface area, pore size, pore size distribution, electrical conductivity, etc. Therefore, by optimizing these factors the supercapacitive performance can be improved. Several syntheses and post-treatments methods have been proposed for this purpose. Hence, this chapter provides a decent and updated coverage of the recent progress and development of carbon nanotube (CNT)-based supercapacitor electrodes.

9.2 Materials: Carbon Nanotubes CNTs can be defined as a graphite sheet rolled up into a tube (defined as single-walled carbon nanotube, SWCNT), or with additional graphene sheets rolled up around the core of an SWCNT (defined as multi-walled carbon nanotube, MWCNT). These CNTs have diameters in the range of fractions of nanometers to tens of nanometers and lengths up to several centimeters, and both the ends are normally capped by

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fullerene-like structures. CNT has been discovered by Iijima through an arc-discharge method similar to that used for preparing fullerenes [6, 7]. CNT is mainly synthesized by three main methods: arc-discharge, laser ablation, and chemical vapor deposition (CVD) [7–11]. The first two are preliminary methods used to produce CNT, where solid-state carbon precursors have been employed. These methods lead to the formation of nearly perfect nanotube structures with large amounts of by-products [12–14]. The laser ablation process is not amenable for scale-up, whereas the arc-discharge process has been used for large-scale production of CNTs. However, the purity of the latter process is rather modest with the majority of the impurity being amorphous carbon. CVD has been widely used to grow CNTs of high quality. In this method, CO or hydrocarbon gases are used as sources of carbon atoms with a metal catalyst particle that serves as a seed to nucleate the growth of the CNTs. By positioning the catalyst seed in arrayed fashions, organized patterns of nanotube structures can be produced on surfaces, comprising both SWCNTs and MWCNTs [15–18]. In the CVD method, the source is heated to 800–1000 °C using a transition metal catalyst to promote the nanotube growth of CNTs. This method is amenable for nanotube growth on patterned surfaces and therefore suitable for the fabrication of electronic devices, field emitters, sensors, and other electronics, where controlled growth over masked areas is needed for further processing. More recently, plasma-enhanced CVD (PECVD) has been introduced to produce vertically aligned CNTs [19–22]. An SWCNT is a cylindrical nanostructure, formed through a rolling-up of a graphene sheet into a tubular shell that is made up of benzene-type hexagonal rings of carbon atoms, as shown in Fig. 9.1. In contrast, MWCNT is made of stacks of several layers of graphene cylinders that are concentrically nested like rings of a tree trunk having an interlayer spacing of 3.4 [23]. SWCNT exhibits unique electronic properties and can be metallic or semiconducting depending on chirality. This allows the formation of semiconductor-semiconductor and semiconductor-metal junctions that are useful in device fabrication. SWCNT also exhibits extraordinary mechanical properties. The Young’s modulus of individual SWCNT has been estimated to be around 1 TPa and the yield strength can be as large as 120 GPa [14, 24, 25].

9.3 Carbon Nanotube as Electrode for Supercapacitors SWCNT and MWCNT have attracted great attention as an electrode for supercapacitor devices due to the beneficial pore structural nature with superior electrical, mechanical and thermal properties. They are usually utilized as a high-power electrode material due to the good electrical conductivity with the readily accessible surface area. Further, the high mechanical resilience and open tubular network of CNTs make them good support for active materials. Electrodes based on pure CNT exhibit both the characteristics of EDLC and pseudo-capacitance. Broad Faradaic peaks are observed in the MWCNT-based electrodes during a CV scan in various aqueous electrolytes. This is due to the presence of different surface functional groups and/or the impurities in CNT that take part in the redox reaction [26]. The highest

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rolled up graphene

SWCNT

rolled up two layered graphene

DWCNT

rolled up Four layered graphene

MWCNT

Fig. 9.1 Graphical representation for SWCNT, DWCNT, and MWCNT formed through rolling-up graphene sheets into the tubular shell. Drawn by Nanotube Modeler, © JCrystalSoft, 2005-2018

specific capacitance of the pure CNT-based electrode is found to be around 100 F g−1 [27, 28]. Niu et al. are the first to report the development of CNT-based supercapacitor electrode [29]. The electrodes composed of a sheet of CNT have been prepared through catalytic growth of CNTs of high purity and narrow diameter distribution. The morphology of the electrode depicts that the electrode consists of randomly entangled and cross-linked CNTs with diameters of ~80 Å. Different functional groups are introduced on the surface of MWCNT through functionalizing with nitric acid. This functionalized MWCNT exhibits a specific surface area of 430 m2 g−1 , with a gravimetric capacitance of 102 F g−1 and an energy density of 0.5 Wh kg−1 using 38% sulfuric acid as the electrolyte. The structural properties such as diameter, length, and pore size play an important role in the EDLC. The presence of catalysts, other impurities, and functional groups affect the pseudo-capacitance of CNTs. The single-cell device also delivers a power density >8000 W kg−1 , as shown in Fig. 9.2.

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Fig. 9.2 Ragone plot of the single-cell capacitor of the nanotube electrode (the power is average power over the 0.5 or 1 V window. Redrawn and reprinted with permission from [29])

In EDLC the amount of accumulated electrical charge by the electrostatic attraction depends on the electrode/electrolyte interface area. The higher surface area of the electrode leads to higher capacitance as the area can be fully accessed by the charge carriers. However, the high capacitance not only depends on the high surface area but also governed by the pore size, size distribution, and conductivity of the material. Therefore, a high capacitance can be achieved by the optimization of all of these factors [30–32]. Frackowiak et al. have systematically investigated the effects of structures, diameters, micro-texture, and elemental composition of CNT on capacitance [33, 34]. The capacitance increases with the increase of the specific surface area as shown in Table 9.1. The CNTs with closed tips and graphitized carbon layers show the smallest value. The mesopore volume of the material affects the limited diffusion of ions and the active surface for the formation of the electrical double layer [27]. The most efficient CNTs for the collection of charges are those with numerous edge planes, either due to herringbone morphology (A900Co/Si) or due to amorphous carbon coating (A700Co/Si) as given in Table 9.1. The straight and rigid CNTs of large diameter (P800Al) exhibit quite moderate performance despite the relatively high specific surface area. The effect of oxygen content on capacitance has also been represented in Table 9.1 [33]. The hydrophobic character of the CNTs also affects interactions with the solvated ions. The CV curve of MWCNT obtained by decomposition of acetylene at 700 °C (A700Co/Si) is shown in Fig. 9.3a, which Table 9.1 Capacitance value of the CNTs with respect to BET specific surface area, mesopore volume, and percentage of oxygen (reprinted with permission from [33]) Type of CNTs

Vmeso (cm3 STP g−1 )

SBET (m2 g−1 )

Oxygen (mass%)

Capacitance (F g−1 )

A700Co/Si

435

411

10.8

80 62

A900Co/Si

381

396

4.6

A600Co/NaY

269

128

0.8

4

P800/Al

643

311

1.7 g cm−3 ) and Au wire (ca. 20 g cm−3 ), respectively [66]. A wired-shaped supercapacitor fabricated using these graphene fibers, 3-D graphene and H2 SO4 -PVA gel polyelectrolyte, maintains good fiber shape and excellent flexibility as shown in Fig. 11.7a–e. The electrochemical performance of the supercapacitor is measured by CV and GCD analyses as shown in Fig. 11.7f–i. The quasi-rectangular shape of CV curves confirms the formation of an efficient EDLC and good charge propagations between the electrodes. Almost overlapped CV curves are observed for the fiber supercapacitor at straight and bending conditions, retain the shape again at straight. This is also the case for GCD curves with a typical triangular shape, indicating the high flexibility and electrochemical stability of the supercapacitor fiber.

Fig. 11.7 a Schematic illustration of a wire-shaped supercapacitor fabricated from graphene fibers (GF) and 3-D graphenes, b, c photos of fiber supercapacitor in the free and bending state, d, e SEM images of the fiber supercapacitor under different magnifications, f, g CV and GCD curves of the fiber supercapacitor in straight and bending conditions, and h, i capacitance stability of fiber supercapacitor at straight-bending cycles (redrawn and reprinted with permission from [66])

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Fig. 11.8 a–c SEM images of cross-sectional and side of two-ply yarn supercapacitor, d two intact coaxial fibers are woven with cotton fibers and e its optical macroscopic image, f GCD curves of the cloth supercapacitor (1 represents initial cloth supercapacitor without bending, and 2, 3, 4 show cloth supercapacitor with bending angles of 180° along three directions) (redrawn and reprinted with permission from [68])

Huang et al. have also fabricated flexible and all-solid-state graphene fiber supercapacitors from wet-spun graphene fibers [67]. The supercapacitor delivers a high areal capacitance of 3.3 mF cm−2 with good stability. Almost no changes are observed after 5000 GCD cycles and bending cycles. Again, graphene yarns possess a high electrical conductivity with a great surface area due to the 3-D interpenetrating porous networks of graphene. Therefore, the solid-state supercapacitors made of graphene fibers or yarns using solid or gel electrolytes possess highly compressible and stretchable mechanical performance with a high areal capacitance [68]. An assembled two-ply yarn supercapacitor (shown in Fig. 11.8a–c) has been fabricated by the coaxial wet-spinning assembly method. A continuously spin polyelectrolytewrapped graphene/carbon nanotube core-sheath fiber has been formed that exhibits an ultra-high capacitance of 177 mF cm−2 with an energy density of 3.84 μWh cm−2 . The coaxial fibers are flexible and robust enough to be intertwined, knotted, and woven with cotton fibers as shown in Fig. 11.8d, e. The interwoven cloth supercapacitor shows a capacitance of 28 mF at the current of 10 μA (Fig. 11.8f) and remains unchanged under the bending angle of 180° along with the three directions. However, the restacking of the graphene sheets dramatically lowers the surface area of graphene sheets, although they are light in weight, highly flexible, and electrically conductive.

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11.3.3 Two-Dimensional Graphene-Based Supercapacitors Two-dimensional graphene is considered as one of the most promising materials for the supercapacitor applications due to the unique structural features and property, like large surface area ensuring an extensive transport platform for the electrolytes [19], high conductivity allowing a low diffusion resistance thereby enhanced power and energy density, and superior mechanical property helping the graphene sheets to assemble easily into free-standing films with robust mechanical stability [7]. Among the graphene-based 2-D materials, graphene papers have attracted much attention in supercapacitor applications due to the tunable electrical properties, lightweight, thickness, and structural flexibility [19]. Therefore, substantial research efforts have been devoted to exploring novel graphene-based films and papers through several processing methods including layer-by-layer deposition, interfacial self-assembly, spin-coating, Langmuir-Blodgett, and vacuum filtration [69–73]. However, the interplanar π–π interactions and van der Waals forces between the graphene layers force to aggregate and restack the individual graphene sheets during the fabrication process, which reduces the surface area of the graphene films. These result in a decrease in the electrochemical performance due to poor diffusion of the electrolyte ions. Therefore, several strategies have been developed to prevent the restacking of graphene sheets to enhance the surface area and transport of the electrolyte ions. To overcome this issue, a flexible pillared-type graphene paper is designed by incorporating small amounts of carbon black nanoparticles as spacers between the graphene sheets [74] as shown in Fig. 11.9a. The restacking between individual graphene sheets is reduced by the addition of carbon black. This strategy results in a significant improvement in electrochemical performance by facilitating the ion transport process. The pillared graphene paper electrode delivers a specific capacitance of 138 F g−1 at a scan rate of 10 mV s−1 using the aqueous electrolyte [19]. It shows only 3.9% degradation of initial specific capacitance after 2000 cycles at a current density of 10 A g−1 as shown in Fig. 11.9b. The electrode also exhibits a maximum energy density of 26 Wh kg−1 using an organic electrolyte at a potential window of 3 V shown in the Ragone plot in Fig. 11.9c. The electrode displays a maximum power density of 5 kW kg−1 in an aqueous electrolyte. Again, Nafion-functionalized reduced graphene oxide film (Fig. 11.10a) using a supramolecular assembly approach prevents the restacking of graphene sheets with the improvement of interfacial wettability between the electrodes and electrolyte [68]. The wrinkled and curved surfaces of the film are shown in the SEM image, Fig. 11.10b. The interconnected functionalized graphene networks provide continuous and fast transport pathways for ions. The supercapacitor made from this graphene film delivers a specific capacitance of ~119 F g−1 at a current density of 1 A g−1 , with 90% retention of capacitance at 30 A g−1 , as shown in Fig. 11.10c. About two times higher capacitance has been obtained in comparison with reduced graphene oxide without Nafion.

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Fig. 11.9 a Schematic representation of the flexible pillared-type graphene paper, b cyclic stability of the pillared graphene paper in both aqueous and organic electrolytes through GCD tests at a current density of 10 A g−1 at room temperature, c Ragone plot of energy density against power density of the pillared graphene paper in both aqueous and organic electrolytes (redrawn and reprinted with permission from [74])

Water is also utilized as a simple, bioinspired approach to effectively prevent the restacking of graphene sheets [76]. The formation of a unique graphene-water hybrid structure is attributed to the balance between repulsive interactions and intersheet π–π attractions among the solvated graphene layers. This solvated graphene film-based supercapacitor exhibits a high specific capacitance (215 F g−1 ) in an aqueous electrolyte. It maintains a capacitance of 157 F g−1 even when operated at an ultrafast charge/discharge rate of 1080 A g−1 . The supercapacitor can deliver a maximum power density of 414 kW kg−1 and retains more than 97% of the initial capacitance after 10,000 cycles under a high current density of 100 A g−1 [19].

11.3.4 Three-Dimensional Graphene-Based Supercapacitors The 2-D graphene’s tendency toward aggregation and restacking considerably affects the electrochemical performance of the supercapacitor electrodes by decreasing the

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Fig. 11.10 a Photograph and b SEM image of Nafion-functionalized reduced graphene oxide film, c specific capacitances at different current densities of reduced graphene oxide films with and without Nafion functionalization (redrawn reprinted with permission from [75])

ion-accessible surface area and restraining the ion and electron transport because of the narrow conduction channels. Therefore, a wide range of graphene-based threedimensional (3-D) porous materials, like aerogel, hydrogel, porous films, sponges, etc., have been widely investigated to overcome these limitations by providing the additional ion-accessible surface area for charge storage and also to facilitate diffusion of ions within the structure [77–82]. These 3-D graphene-based materials consist of micro-, meso-, and macro-pores in their network structures as shown in Fig. 11.11, which provides a high surface area, lightweight, and fast ion/electron transport. Recently, several strategies have been developed to synthesis 3-D graphene-based materials like self-assembly, template-assisted preparation or direct deposition, etc. In the self-assembly method, at first gelation of the GO dispersion occurs once the force balance is broken. During the gelation process, GO sheets partially overlap to form GO hydrogels with 3-D architectures. Finally, the reduction of GO hydrogels generates 3-D rGO networks. The gelation of dispersed GO can be triggered in several ways such as the addition of cross-linkers [83], changing the pH of the GO dispersion [84], ultrasonication of the GO dispersion [85, 86], hydrothermal or solvothermal treatment [87], etc. In the case of a template-assisted method, graphene is directly grown on 3-D templates using the CVD method. This method produces

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Fig. 11.11 3-D graphene prepared by using templates of a NiCl2 ·6H2 O and b Ni foam (reprinted with permission from [99])

much more controlled morphologies and properties. Generally, the graphene 3-D network is grown on Ni foam, which acts as both the template and catalyst [88, 89]. 3-D graphene is also obtained alternatively through the assembly of GO sheets onto 3D templates followed by the reduction of GO to rGO. Different assembly techniques have been developed, such as refluxing in an autoclave [90, 91], template-assisted freeze-drying [92], electrophoretic deposition [93], dip-coating [94], etc. In the direct deposition strategy, the graphene sheets are vertically grown on conducting substrate like, Au, stainless steel, etc., and are connected with each other to form 3-D porous architecture [95–97]. Currently, graphene-based 3-D materials have been proven as promising candidates for supercapacitors due to the porous structure (Fig. 11.11) with a high specific surface area (>1000 m2 g−1 ) [98, 99]. This unique porous structure improves the accessibility of the electrolyte to the surface of the electrode and provides electrically conductive channels enhancing the electrochemical performances of supercapacitors [100]. Therefore, 3-D graphene-based materials with the above-mentioned structures, such as hydrogels, aerogels, sponges, and porous films, have been extensively explored in the area of supercapacitors. The graphene-based hydrogel in the solid-state supercapacitor device delivers a high specific capacitance of 186 F g−1 at a current density of 1 A g−1 . Based on the thickness of electrodes with high mass loading, this graphene hydrogel film achieves a superior areal specific capacitance of 372 mF cm−2 [19]. The specific capacitance of the supercapacitor is enhanced up to 412 F g−1 at a current density of 1 A g−1 with 74% capacitance retention at 20 A g−1 by the functionalization of graphene of this hydrogel. Again, another symmetrical supercapacitor constructed from 3-D porous graphene of the extremely high surface area (3100 m2 g−1 ) has been obtained by chemical activation of the microwave exfoliated graphene oxides. The material exhibits a specific capacitance of 150 F g−1 at a current density of 0.8 A g−1 [98]. Xu et al. have fabricated the high-performance solid-state flexible supercapacitors using a 3-D graphene hydrogel schematically represented in Fig. 11.12 [77]. The highly interconnected 3-D network structure of graphene hydrogel is shown in SEM

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Fig. 11.12 a Schematic diagram and b photographs of the fabrication process of flexible solid-state supercapacitors based on graphene hydrogel films, c low- and d high-magnification SEM images of the graphene hydrogel before pressing, e low- and f high-magnification SEM images of the graphene hydrogel film after pressing (reprinted with permission from [77])

images of Fig. 11.12. This 3-D structure exhibits exceptional electrical conductivity, surface area, and mechanical robustness to make flexible supercapacitor devices. The flexible supercapacitor devices of 120 μm thickness of graphene hydrogel film exhibit a high gravimetric specific capacitance of 186 F g−1 and an extraordinary areal capacitance of 372 mF cm−2 with a low leakage of current (10.6 μA). The flexible supercapacitor is also of excellent cycling stability and extraordinary mechanical flexibility. The CV curves shown in Fig. 11.13a reveal the same capacitive behavior at various bending angles. The device also suffers from only 8.5% decay in specific capacitance after 10,000 GCD cycles at a high current density of 10 A g−1 under a 150° bending angle with Coulombic efficiency of 98.8–100% as shown in Fig. 11.13b. Xu et al. have also synthesized functionalized graphene hydrogels (FGHs) by a single-step chemical reduction of graphene oxide (GO) using hydroquinones as the reducing and functionalizing agent [101]. The free-standing hydrogel is converted into lightweight aerogel through freeze-drying as shown in Fig. 11.14a. The interconnected 3-D macro-porous network structure with ultrathin layers of stacked graphene sheets is shown in SEM images (Fig. 11.14b, c). This hierarchical network structure shows a high specific surface area of ~297 m2 g−1 with a pore volume of ~0.95 cm3 g−1 with the pore sizes in the range of 2–70 nm. Therefore, these FGHs perform as ideal electrode materials for flexible solid-state supercapacitor devices as they have excellent mechanical and electrical robustness with highly interconnected porous 3-D network structure. These FGHs produce a solid-state highly flexible and robust supercapacitor using an H2 SO4 -PVA gel electrolyte as shown in Fig. 11.14d, e. The FGHs-based supercapacitor shows a high specific capacitance value of 441 F g−1 at a current density of 1 A g−1 , which is more than two times higher compared to

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Fig. 11.13 a CV curves of the flexible solid-state device at a scan rate of 10 mV s−1 at different bending angles, b cycling stability of the device by GCD analysis at a current density of 10 A g−1 (reprinted with permission from [77])

that of GH-based supercapacitor (211 F g−1 ) as shown in Fig. 11.15. The CV curves in Fig. 11.15a reveal that the GHs-based supercapacitor shows a typical rectangular shape of pure EDLC behavior. On the other hand, the FGHs show a pair of Faradaic peaks in the potential range of 0.1–0.3 V, due to the reversible redox reaction of hydroquinone molecules (hydroquinone ↔ quinone) confirming the simultaneous existence of pseudocapacitance with EDLC. The significant contribution of pseudocapacitance in the FGHs-based supercapacitor is also clear in the GCD curves in Fig. 11.15b. Close scrutiny of the GCD curve reveals a deviation from the ideal triangle shape of GHs-based supercapacitor in the potential range of 0–0.35 V. The high rate capability of the supercapacitor is shown in Fig. 11.15c. Further, the FGHsbased supercapacitor exhibits excellent electrochemical stability by retaining 86% of its initial capacitance after 10,000 GCD cycles at a high current density of 10 A g−1 as shown in Fig. 11.15d. A very small change in the CV curves before and after 10,000 GCD cycles is also observed in the inset of Fig. 11.15d. This high stability is due to the presence of non-covalent interactions between hydroquinone and graphene, which

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Fig. 11.14 a Photographs of aqueous dispersed GO precursors and the prepared FGH and aerogel, b low and c high-magnification SEM images of the freeze-dried FGHs, d digital photograph of FGH-based flexible solid-state supercapacitor, and e schematic representation of the solid-state supercapacitor (reprinted with permission from [101])

are strong enough to sustain a long cycle life. Again, the presence of a few oxygen functionalities on graphene sheets of FGHs persists the reduction of hydroquinone. A nitrogen-doped graphene hydrogel is also produced for the development of ultrafast supercapacitor [102], synthesized through the hydrothermal process using organic amine and graphene oxide as precursors, as shown in Fig. 11.16. The organic amine is not only used for nitrogen doping but also acts as an important modifier to control the stacking of graphene sheets in the 3-D network structures. The supercapacitor made of the nitrogen-doped graphene shows the ultrafast charge/discharge rate with a medium energy density and very high-power density. The supercapacitor delivers the specific capacitance and power density of 190 F g−1 and 245 kW kg−1 at a constant current density of 10.0 A g−1 . Again, it shows the specific capacitance and power density of 114 F g−1 and 205 kW kg−1 at a high current density of 185.0 A g−1 . Even, the specific capacitance is retained up to 109 F g−1 and a power density of 173 kW kg−1 at a very high current density of 250.0 A g−1 . In addition, the supercapacitor retains 95% of its initial capacitance at a current density of 100.0 A g−1 after 4000 GCD cycles. A summarized supercapacitive performance of these different types of graphenebased materials is given in Table 11.1. Therefore, these supercapacitors are used

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Fig. 11.15 a CV curves of FGH- and GH-based supercapacitors at a scan rate of 5 mV s−1 , b GCD curves of the supercapacitors at a current density of 1 A g−1 , c specific capacitances of the supercapacitors at different current densities, and d cycling stability of the FGH-based supercapacitor at a current density of 10 A g−1 (inset shows the CV curves of the supercapacitor at a scan rate of 5 mV s−1 after 1st and 10,000th GCD cycles (reprinted with permission from [101])

Fig. 11.16 a Illustration for the synthesis of nitrogen-doped graphene hydrogel with the enhancement of the distance between graphene layers by ethylene diamine, and b the possible reaction pathways for nitrogen doping (redrawn and reprinted with permission from [102])

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Table 11.1 Summarized supercapacitive performance of different types of graphene/reduced graphene oxide-based materials Structure

Surface area (m2 g−1 )

Capacitance

Cycle life

References

Particle

696

210 F g−1 at 1 mV s−1

95.6% after 1000 cycles

[60]

Quantum dot

–

140 μF cm−2 at 1 mV s−1

~98% after 5000 cycles

[62]

Quantum dot

1502

236 F g−1

–

[65]

Fiber

–

1.7 mF cm−2

–

[66]

Fiber

–

3.3 mF

cm−2

~100% after 5000 cycles

[67]

Yarn

–

177 mF cm−2

~100% after 2000 cycles

[68]

Pillared-type paper

–

138 F g−1 at 10 mV s−1

96% after 2000 cycles

[74]

Film

–

~119 F g−1 at 1 A g−1

>90% after 1000 cycles

[75]

Sheet

–

215 F g−1

97% after 10,000 cycles

[76]

3-D porous

3100

150 F g−1 at 0.8 A g−1

97% after 10,000 cycles

[98]

Hydrogel

~414

186 F g−1

~92% after 10,000 cycles

[77]

Hydrogel

~297

441 F g−1 at 1 A 86% after 10,000 g−1 cycles

[100]

Hydrogel

–

190 F g−1

[101]

95.2% after 4000 cycles

in several applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, and aerospace, etc. [1, 103].

11.4 Concluding Remarks Graphene/reduced graphene oxide-based materials are promising candidates for supercapacitors due to the large surface area, high conductivity, lightweight, good chemical stability, and superior mechanical flexibility. Graphene is an ideal material to conduct heat and electricity. It also has a very high theoretical specific surface area close to activated carbon. Hence, graphene-based electrodes behave as EDLCs and exhibit high power density and excellent cycling stability. However, the interesting properties and electrochemical performance of the graphene materials mainly depend

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on the synthetic methods as well as the architectures/dimensions. Therefore, considerable efforts have been made on several architectures and dimensions of graphene to be utilized as excellent electrode materials for supercapacitors. Graphene-based materials are made into several architectures, like free-standing particles or dots, onedimensional fibers or yarns, two-dimensional films, and three-dimensional networks for utilization as perfect supercapacitor electrodes. This chapter mainly gives attention to the recent development of graphene-based materials for supercapacitor electrodes based on their dimensions from zero to three dimensions. However, in the case of lower dimension (0-D and 1-D) and 2-D graphene materials suffer from moderate rate stability and power density due to the propensity toward aggregation and restacking of graphene sheets. This in effect reduces the surface area and effective diffusion of the electrolyte ions. Therefore, interconnected graphene networks with tunable 3-D porous structures are the better options to improve the rate and power performance, though mechanical strength has to be compromised. The 3-D graphenebased materials consist of several micro-, meso-, and macro-pores in their network structures, which provide high surface area, lightweight, and fast ion/electron transport. The porous architecture of these materials improves the accessibility of electrolyte on the surface of the electrode and also provides electrically conductive channels enhancing the electrochemical performances of the supercapacitor. Although the more effective surface area of graphene can be obtained by the construction of several 3-D architectures, the specific capacitance values are still far from the theoretical capacitance value of graphene (550 F g−1 ). They suffer from the low energy density and low specific capacitance due to the graphene’s propensity toward aggregation and restacking. The tendency of restacking reduces the ion-accessible surfaces and limiting ion and electron transport due to the narrower channels. Therefore, recently hybrid of graphene-based and pseudocapacitive materials are emerging as promising candidates for supercapacitor applications to achieve higher specific capacitance and energy density, with better rate capability, power density, and long cycling life. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 12

Transition Metal Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors Bibekananda De, Prerna Sinha, Soma Banerjee, Tanvi Pal, Kapil Dev Verma, Alekha Tyagi, P. K. Manna, and Kamal K. Kar Abstract Graphene-based materials have been extensively used as electrode materials for supercapacitor applications due to their extraordinarily high electrical conductivity and large surface area. However, they suffer from the low energy density and specific capacitance because of the graphene’s propensity toward aggregation and restacking, which reducing the ion-accessible surfaces and limiting ion and electron transport. Therefore, to enhance electrochemical performance for highperformance supercapacitor, pseudocapacitive transition metal oxides are integrated with graphene-based materials. Currently, these hybrid supercapacitors have been attracted much attention due to the combination of rapid charge–discharge and long B. De · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] B. De e-mail: [email protected] P. Sinha · S. Banerjee · T. Pal · K. D. Verma · A. Tyagi · K. K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] T. Pal e-mail: [email protected] K. D. Verma e-mail: [email protected] A. Tyagi e-mail: [email protected] T. Pal A.P.J. Abdul Kalam Technical University, Lucknow 226031, India P. K. Manna Indus Institute of Technology and Management, Kanpur 209202, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_12

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cycle life for energy storage in modern electronic devices. In these hybrid materials, the emphasis is given to synergistic effects between graphene/reduced graphene oxide and metal oxides, which results in high energy and power densities along with high specific capacitance. This chapter is mainly focused on hybrid supercapacitors of transition metal oxides with graphene-based materials. The chapter provides decent and updated coverage on the synthesis, structure, properties, and supercapacitor performance of graphene and transition metal oxide-based composite materials.

12.1 Introduction Supercapacitors represent the promising energy storage devices for the next generation of electronics as they can transport high power within a very short period. They are potentially used in different portable electronics, memory backup systems as well as hybrid electric vehicles, industrial-scale power and energy management, and so on [1–5]. The fundamentals of capacitors [6] and specific features of the capacitor to supercapacitor [7] are reported elsewhere. Graphene-based materials are most extensively explored as electrode materials for supercapacitors and batteries as they possess high- power density and excellent cycling stability [8–11]. The large specific surface area and excellent electrical conductivity with superior elasticity and high chemical stability make them a material of choice for the intended applications [12–16]. The characteristics of graphene are reported elsewhere [17]. It is also a potentially low-cost material with a theoretical capacitance of ~550 F g−1 and capable to achieve the highest limit of capacitance among all carbon-based materials [18]. Generally, graphene-based materials store energy through the mechanism of electric double-layer capacitors (EDLC), where the capacitance is originated from the accumulation of charges at the electrode and electrolyte interfaces. Their capacity is mainly controlled by the specific surface area along with pore size and electrical conductivity [8, 10]. Although the graphene-based electrodes exhibit high- power density and cycle life, they suffer from the low energy density and specific capacitance because of the propensity toward aggregation and re-stacking. Therefore, recently pseudocapacitors (transition metal oxides (TMOs) and conducting polymers) are introduced with graphene/reduced graphene oxide materials to improve the capacitance and energy density. The pseudocapacitance is originated by involving reversible multi-electron redox reactions that help to transfer Faradic charges between the electrolyte and electrode [10, 19–21]. Pseudocapacitors generally exhibit superior specific capacitance and energy density as compared to EDLC. However, the poor electrical conductivity and stability of pseudocapacitive materials lead to insufficient electrochemical performance (mainly low-power density) and limited cycle life. Therefore, the formation of a hybrid supercapacitor of TMOs with graphene materials has attracted great attention due to the synergistic effects. This particular material combination improves the electrochemical performance by combining the redox reaction of metal oxide with the high surface area and conductivity of carbon-based materials [22].

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Hence, nanohybrids of graphene-based materials and TMOs are extensively used in the current era of supercapacitors and achieve high energy and power densities with excellent rate capability and cycle life. Therefore, this chapter presents a comprehensive account of synthesis, characterization, and supercapacitor applications of graphene and TMOs-based hybrids.

12.2 Materials: Graphene/Reduced Graphene Oxide and Transition Metal Oxides 12.2.1 Synthesis and Characteristics of Graphene/Reduced Graphene Oxide Graphene is an allotrope of carbon in the form of a two-dimensional monolayer sheet of sp2 -hybridized carbon atoms arranged in a honeycombed network [13]. Geim and coworkers have discovered graphene by peeling single layer from graphite in 2004 [12]. Ideally, graphene is a monolayer material; however, two or more layers of graphene are extensively studied with equal interest, and the number of layers is strongly dependent on the method of synthesis. The electronic properties of graphene mainly depend on the electronic structure. Graphene is inherently a semimetal or zero-gap semiconductor. The electronic band structure of graphene combines semiconducting and metallic characteristics [14, 15]. The unique electronic properties of graphene originate from the charge carriers in graphene are defined by a Dirac-like equation, since the conduction and valence bands meet at the Dirac points. Graphene shows remarkable electron mobility at room temperature of >15,000 cm2 V−1 s−1 [23]. Hole and electron mobilities are also expected to be nearly identical. The mobility is nearly independent at temperature 10–100 K, which implies the dominance of defect scattering [24]. The corresponding resistivity of graphene sheets is 10−6  cm, which is less than the resistivity of silver. Few-layer graphenes, as well as nanographite particles, exhibit semiconducting behavior in the temperature range of 100–300 K [9]. The resistivity of graphene increases abruptly below 50 K and decreases distinctly when the graphene is heated to high temperatures. Therefore, graphene prepared through the exfoliation of graphite displays semiconducting properties and is predicted to be half-metallic [17]. Graphene is also an ideal material for heat conduction in the plane due to a repeating structure. The regular structure of graphene allows the movement of phonons over the material without inhibition at any point along the surface. Graphene can conduct heat through the in-plane and inter-planes. The in-plane conductivity of an ideal monolayered graphene sheet is around 5000 W m−1 K−1 [4, 25], whereas the cross-plane conductivity remains quite low (6 W m−1 K−1 ) because of the weak inter-plane van der Waals forces. The specific heat capacity of graphene is estimated to be ~2.6 μ J g−1 K−1 at 5 K.

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Single-layer graphene has also a large theoretical specific surface area of around 2630 m2 g−1 , which is close to the activated carbon [26, 27]. However, the Brunauer– Emmett–Teller (BET) surface area of few-layer graphene is varied in the range of 270–1550 m2 g−1 [14]. Graphene is one of the strongest materials with an intrinsic tensile strength of ~130 GPa as well as Young’s modulus of ~1 TPa (150,000,000 psi), though it is a very lightweight material (0.77 mg m−2 ) [28]. Graphene is relatively brittle with a relatively low fracture toughness of about 4 MPa m1/2 [29]. However, it shows the ability to distribute the force from an impact, which is ten times greater than of steel per unit weight. The repeating sp2 hybridized backbone of graphene allows for flexibility, as there is rotation around some of the bonds. Regarding the elasticity of graphene, the spring constant has been found between 1 and 5 N m−1 with Young’s modulus of 0.5 TPa. However, all these properties of the graphene mainly depend on the synthesis methods, as the structural perfectness and number of few-layer graphene are varied with the synthesis procedure. True graphene is monolayer one atomic layer thick and typically exists as a film. In 2004, Geim and Novoselov have discovered graphene through the peeling of single layer from graphite by mechanical exfoliation [13]. This leads to an explosion of interest in the study of graphene, and a huge work has been carried out for the synthesis of graphene for the possible application in the field of supercapacitor devices. Numerous approaches are developed to produce singlelayer and few-layer graphene, like mechanical cleavage [15], epitaxial growth [30], chemical vapor deposition (CVD) [31–33], chemical method [34–37], etc. Chemical reduction of graphite oxide is one of the most conventional procedures to synthesize large quantities of graphene/reduced graphene oxide [38, 39]. Graphite oxide is normally synthesized through the oxidation of graphite using strong oxidizing agents like concentrated sulfuric acid, nitric acid, potassium permanganate, etc., based on Brodie, Staudenmaier and Hummers methods, respectively [40–43]. Another widely investigated approach to prepare graphene/reduced graphene oxide is the reduction of graphene oxide via sonication of graphite oxide solution. The reducing agents mainly used are NaBH4 , hydrazine, hydroquinone, hydroxylamine, ascorbic acid, glucose, etc. [44–49]. The exfoliation, as well as reduction of graphite oxide, is now established as a primary cost-effective technique to yield reduced graphene oxide in bulk. Exfoliation of graphene intercalated compounds through rapid heating or sonication is another important approach to synthesize single to few-layer graphene [50, 51]. Graphene is also synthesized through a bottom-up approach, where nano/micrographene and graphene-based materials are synthesized from organic precursors like polycyclic aromatic hydrocarbons. The method can precisely control the production of molecular graphene, nanographene, and integrated macro-graphene with welldefined structures and high processability. However, this approach suffers from the drawback of low productivity [52–54]. There are other methods to produce graphene, such as electrochemical exfoliation of graphite [55, 56], electron beam irradiation of PMMA nanofibers [57], arc discharge of graphite [58–60]. Studies on the synthesis of graphene/reduced graphene oxides, and its properties and applications are reported elsewhere [61, 62].

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12.2.2 Synthesis and Characteristics of Transition Metal Oxides Transition metal oxides (TMOs) represent a versatile group of compounds that include the oxides of d-block elements. Nanostructured TMOs are gaining worldwide attention due to their excellent and novel properties like ferromagnetism, photoluminescence, semiconducting, superconductivity, catalytic activity, etc. These properties are the result of characteristics including the varying number and arrangement of unpaired electrons in d-orbital, tunable energy bandgap, and the high surface area and controllable pore size distribution. It is observed that the properties of TMOs can be modulated by explicit control over the size and structure. This fine control over the electrical, magnetic, optical, and surface properties is achieved by molecular level engineered preparation of nanostructured TMOs. A wide range of interesting morphologies including nanocubes, nanoneedles, nanoflowers, nanospheres, nanooctahedra, nanotubes and nanorods like TMOs (FeOx , MnOx , RuOx , CoOx , WOx , NiO, TiO2 , etc.) have been reported in recent literature. The obtained nanostructures are the result of the employed synthesis strategies and used metal precursors. Techniques used for TMOs preparation constitute electrochemical deposition, physical vapor deposition (PVD), chemical vapor deposition (CVD), hydrothermal, sol-gel method, inert atmosphere pyrolysis, and solution precipitation. The prepared nanostructured TMOs possess exciting and variable electrical, magnetic, catalytic, thermoelectric, surface, and optical properties. A detailed understanding of the fabrication methods, obtained dimensions/morphologies, and their influence on the resulting physical and chemical properties can help in designing application-specific nanostructures. This is leading to an exploration of TMOs nanostructures in the fields like nanoelectronics, energy storage, optoelectronics, homogeneous and heterogeneous catalysis, and bio-medicine. A considerable improvement in these regimes has fueled the development of sensors, FET transistor, nanolasers, solar cells, supercapacitors, batteries, drug delivery, and bio-imaging by making use of TMOs-based materials. The commercialization of devices utilizing these materials is further favorable due to the cheap cost, easy availability, and eco-friendly nature of TMOs. An exclusive overview of the synthesis of TMOs, and their properties and applications have been reported elsewhere [63].

12.3 Graphene/Reduced Graphene Oxide as Electrode Materials for Supercapacitors Graphene is a promising material for supercapacitor application due to the large tunable surface area, excellent electrical conductivity, high chemical and thermal stability with excellent mechanical behavior [64, 65]. Graphene-based electrodes behave as EDLCs and display high-power density and excellent cycling stability.

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Table 12.1 Summarized supercapacitive performance of different architectures of graphene materials [66] Structure

Surface area (m2 g−1 )

Capacitance

Cycle life

References

0D particles

696

210 F g−1 at 1 mV s−1

95.6% after 1000 cycles

[66]

0D quantum dot

–

140 μF cm−2 at 1 mV s−1

~98% after 5000 cycles

[66]

0D quantum dot

1502

236 F g−1

–

[66]

1D fiber

–

3.3 mF cm−2

~100% after 5000 cycles

[66]

1D yarn

–

177 mF cm−2

~100% after 2000 cycles

[66]

2D paper

–

138 F g−1 at 10 mV s−1

96% after 2000 cycles

[66]

2D film

–

~119 F g−1 at 1 A >90% after 1000 g−1 cycles

[66]

2D sheet

–

215 F g−1

97% after 10,000 cycles

[66]

3D porous

3100

150 F g−1 at 0.8 A g−1

97% after 10,000 cycles

[66]

3D hydrogel

~414

186 F g−1

~92% after 10,000 cycles

[66]

3D hydrogel

~297

441 F g−1 at 1 A g−1

86% after 10,000 cycles

[66]

Graphene can also be obtained into several architectures, like free-standing zerodimensional (0D) particles or dots, one-dimensional (1D) fibers or yarns, twodimensional (2D) sheets or thin films. and three-dimensional (3D) porous materials. All these types are widely used in supercapacitor application and their performances are summarized in Table 12.1. However, the interplanar π –π interactions and van der Waals forces between the graphene layers force to aggregate and restack the individual graphene sheets during the fabrication process reducing the surface area of graphene films. This results in a decrease in electrochemical performance due to the poor diffusion of electrolyte ions. Several strategies have been established to inhibit the re-stacking of graphene sheets, enhance the surface area, and transport of the electrolyte ions. A large number of graphene-based 3D porous materials, like aerogel, hydrogel, porous films, sponges, etc., have been widely considered to overcome these limitations. These materials provide the additional ion-accessible large surface area for charge storage and facilitate the process of ion diffusion inside the structure [67–71]. The graphene-based 3D materials have been demonstrated as a promising material for supercapacitors due to the porous structure with the high specific surface area (>1000 m2 g−1 ). This unique porous structure improves the accessibility of electrolyte to the surface of the

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electrode and provides electrically conductive channels, which enhance the electrochemical performances of the supercapacitor. Although a huge effective surface area of graphene can be achieved through the construction of 3D structures, however, the specific capacitances are still far away from the theoretical capacitance of graphene (550 F g−1 ). Therefore, to design ideal electrode materials for supercapacitors with high specific capacitance, energy density, power density with better rate capability, and long cycling life, the researcher introduces graphene-based composite materials with redox-active pseudocapacitive TMOs. An exclusive overview of the performance of graphene/reduced graphene oxide as electrode material for supercapacitor has been reported elsewhere [66].

12.4 Transition Metal Oxides as Electrode Materials for Supercapacitors Inorganic compounds are one of the excellent classes of materials, which are widely used in almost every application. Among them, TMOs, a part of the inorganic family plays an essential role in delivering remarkable properties. These properties are mainly explored because of the exciting phenomenon exhibits by TMOs. In simple terms, the study of oxygen bonding with different transition metals is carried out to produce excellent properties. The main attraction of d-block metal oxide is its variable oxidation states, which arise due to the incomplete d subshell. This allows them to exhibit multiple types of phenomena when interacts with a physical condition such as light, electric field. The formation of oxide compounds, along with the type of transition metal determines its electronic properties ranging from insulators to conductors. Transition metal oxide can be scaled down to the nano level to achieve some of the excellent material properties which its bulk material cannot meet. At the nanoscale, these oxides deliver remarkable morphology, which is widely utilized in the field of photocatalysis, electrodes for energy storage systems. Along with nanoscale morphology, they have a high surface to volume ratio which provides active sites during chemical processes. The ease and flexibility of tuning the TMOs enable them to be widely used in extensive applications. TMOs exhibit much higher specific capacitance and energy density compared to EDLCs due to their fast charge–discharge process through fast Faradaic redox reactions occurring at the interface between TMO electrodes and electrolytes. These materials have the potential to overcome the low energy density limitation of the electrochemical capacitor and low-power density of the battery. Therefore, the pseudocapacitive TMOs bridge the gap between existing EDLCs and batteries to form units of intermediate specific energy and power densities. However, few TMOs like RuO2 , IrO2 , etc., have limited their widespread application in supercapacitors due to the high cost and toxicity, even though they have high specific capacitance, good electrical conductivity, fast and reversible charge–discharge properties. Therefore, MnO2 , NiO, Co3 O4 , etc., are alternatively considered for supercapacitor electrodes

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due to their cost-effectiveness, environment-friendly nature, and high electrochemical properties. However, still, their poor electronic conductivity, surface area, and power density are the major disadvantages. Again, the poor cyclic stability of TMO electrodes is a permanent issue in energy storage applications due to their volume change in electrolyte solution during electrochemical analyses. Therefore, several approaches and synthetic methods are utilized to achieve desire morphologies and architectures of TMOs to fabricate high-performance electrode materials for supercapacitor applications. The supercapacitive performance of different TMOs is summarized in Table 12.2, according to the synthesis techniques and structures. An exclusive Table 12.2 A summarized supercapacitive performance of different TMOs [72] Transition metal oxide

Synthesis technique

Structure

Capacitance (F Cycle g−1 ) life

References

RuO2

Anodic deposition

Nanotubular array

1300

–

[72]

MnO2

Coprecipitation

Powder

1380

–

[72]

~100% after 10,000 cycles

[72]

–

[72]

g−1

MnO2

Electrodeposition Nanowire array

493 at 4 A

NiO

Calcination

Crystalline β-Ni(OH)2

696

NiO

Gas/liquid interfacial microwave

Flower-like hollow nanospheres

770 at 2 A g−1 95% after 1000 cycles

[72]

Co3 O4

Self-organization Brush-like nanowires

1525

94% after 5000 cycles

[72]

Co3 O4

Solvothermal and Nanoflower calcination

1937

78.2% after 1000 cycles

[72]

SnO2

Electrochemical deposition

Amorphous nanostructured

285

88% after 1000 cycles

[72]

FeCo2 O4

Hydrothermal

Nanoflakes array

433 at 0.1 A g−1

Stable up to 2500 cycles

[72]

Co3 O4 @NiO Hydrothermal

Hierarchical nanowire 721 at 1 A g−1 91.35% [72] array after 5000 cycles

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overview of the performance of TMOs as an electrode for supercapacitor has been reported elsewhere [72]. Currently, intensive research has been going on hybrid supercapacitors utilizing both pseudocapacitive TMOs and high surface area conducting carbon-based EDLCs for the development of a diverse range of next-generation energy storage devices. These hybrid supercapacitors offer a promising avenue for designing smart multifunctional electrodes by integrating the advantages of each constituent.

12.5 Transition Metal Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors Because of the EDLC energy storage mechanism, the electrochemical performance of pure graphene is usually limited by the electroactive surface area, the pore size distribution, and the transport resistance of the electrolyte ions [18]. Supercapacitors based on graphene-based materials can achieve a capacitance up to 550 F g−1 provided the full surface area has been utilized. However, the actual gravimetric specific capacitance is observed to be lower than 300 F g−1 leading to very low energy density [18]. Therefore, to increase the energy densities, some electrocatalysts like redox-active pseudocapacitive materials are needed. They have much higher capacitance than graphene and have attracted great interest in hybrid supercapacitors with carbonaceous materials. The most extensively explored pseudocapacitor electrode materials are TMOs like RuO2 , MnO2 , NiO, Co3 O4 , Ni(OH)2 , Fe3 O4 , etc. The main functions of graphene and TMOs in these hybrids are 1. graphene acts as a stabilizer or support to uniformly decorate metal oxides with well-defined shapes, sizes, and structures 2. metal oxides prevent the re-stacking of graphene sheets 3. graphene acts as a 2D conductive template or builds a 3D conductive porous network to improve the poor electrical conductivity and charge transfer pathways of metal oxides 4. graphene prevents the volume change, as well as an agglomeration of metal oxides and 5. oxygen-containing groups of reduced graphene oxide ensures good bonding, electrical contacts, and interfacial interactions between graphene and metal oxides [73]. Generally, wet-chemical strategies, such as chemical in situ deposition, hydrothermal or solvothermal synthesis, sol-gel processes, etc., are widely used to fabricate graphene–metal oxide composites. A dispersed solution of suspended graphene oxide or reduced graphene oxide is used in these processes, which acts as a 2D precursor for an integrated support network for discrete metal nanoparticles (Fig. 12.1a). The presence of hydrophilic oxygen-containing functional groups such

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Fig. 12.1 a A general strategy for the fabrication of graphene–metal oxide composite, b schematic of structural models of graphene–metal oxide composites: (i) anchored, (ii) wrapped, (iii) encapsulated and (iv) sandwich-like (reprinted with permission from [73])

as epoxides, hydroxides, and carboxylic groups on the surface of graphene oxide or reduced graphene oxide helps to disperse. Such a well-dispersed solution acts as a good template suspension to chemically react with metal ions generated from the precursors of inorganic and organic metal salts [73]. After the final annealing process, graphene or reduced graphene oxide stabilizes the as-prepared metal oxide through anchoring, wrapping, encapsulating, and leads to the formation of the sandwichlike structure as shown in Fig. 12.1b. Here, graphene or reduced graphene oxide acts as good support to nucleate and stabilize the metal oxide nanoparticles. Special importance is given to the significant role of graphene to suppress the accumulation and control the size of metal oxide nanoparticles. This provides a facile and useful approach to achieve a uniform distribution of metal oxide nanoparticles on graphene sheets with controlled structure, shape, and size. Figures 12.2a, b show the SEM images of graphene-RuO2 composite and RuO2 in absence of graphene [74]. In the presence of graphene, tiny-sized (5–20 nm) RuO2 nanoparticles are obtained homogeneously anchored on the surface of graphene. In the absence of graphene, the as-prepared RuO2 particles tend to aggregate spontaneously and form larger particles with a size of 100 nm to 10 μm. A similar effect has been observed for graphene-Co3 O4 composite as shown in Figs. 12.2c, d [75]. The presence of functional groups on graphene oxide or reduced graphene oxide strongly impacts on the shape, size, and distribution of metal oxide particles on the graphene. On the other hand, the presence of metal oxide nanoparticles on both sides of graphene sheets can work as a nanospacer to prevent the stacking of graphene sheets [73]. Generally, reduced graphene oxide or graphene usually suffers from agglomeration and re-stacking due to van der Waals interactions between the adjacent sheets [76]. This leads to a great loss of the actual surface area, and therefore, inferior electrochemical performance is observed violating the theoretical prediction. The loading of metal oxide particles inhibits the agglomeration as

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Fig. 12.2 a TEM image of graphene-RuO2 composite and b SEM image of RuO2 particles in absence of graphene under the same synthesis conditions (reprinted with permission from [74]), c TEM image graphene-Co3 O4 composite and d SEM image of Co3 O4 in absence of graphene under the same synthesis conditions (reprinted with permission from [75])

well as re-stacking of graphene sheets and successively enhances the available electrochemically active surface area of graphene along with the formation of a flexible porous structure to enhance capacitance of the final composites. During the electrochemical performance, graphene as a conductive carbon material in hybrid electrodes is expected to build a 3D conductive network among the metal oxide particles and also suppresses the volume change of metal oxide electrodes [73]. This improves the cycle life and rate capability along with the power density of the supercapacitor. The metal oxides simultaneously enhance the specific capacitance along with the energy density of the supercapacitor through a redox reaction.

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12.5.1 Ruthenium Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors RuO2 has been proven as excellent electrode material for supercapacitor applications. The capacitance achieved is due to reversible faradaic reactions, which shows the pseudocapacitive charge storage mechanism. Among other metal oxides, RuO2 shows superior electrochemical properties due to the excellent electrical conductivity and reversible charge–discharge features. The main disadvantage associated with RuO2 is its high cost, which limits its commercial application as an electrode material. Also, RuO2 forms big agglomerates, which results in an incomplete and slower redox reaction, hence significantly degrade the electrochemical performance. To utilize the maximum benefit of RuO2 , the formation of a carbon-based composite can be used. The presence of RuO2 and carbon-based composite reduces RuO2 loading, enhances the conductivity of ions, and prevents RuO2 agglomeration. In this vein, graphene, one atom planar sheet of carbon atoms, shows remarkable physical and chemical properties. Graphene is an excellent electrode material for an electric double-layer capacitor. The resultant RuO2 and graphene or graphene oxide composite also helps to deliver a high energy and power density by increasing the working potential window. In this regard, a combination of sol-gel and low-temperature annealing processes is adopted to synthesize hydrous RuO2 and graphene sheet composite. The preparation method yields well-separated graphene sheets with RuO2 particles. The capacitance of 570 F g−1 at a scan rate of 1 mV s−1 has been achieved at 38.3 wt% of Ru loading. Figure 12.3a shows that the composite electrode shows the highest capacitance among the individual electrodes. The composite shows a high energy density of 20.1 Wh kg−1 shown in Fig. 12.3b. The high electrochemical stability of

Fig. 12.3 a The Csp of the as-prepared graphene sheet (GS), RuO2 , and RuO2 /graphene composite (ROGSC) (Ru, 38.3 wt%) electrodes as a function of scan rate and b Ragone plot for the as-prepared graphene sheet (GS), RuO2 , and RuO2 /graphene composite (ROGSC) (Ru, 38.3 wt%) electrodes supercapacitors (redrawn and reprinted with permission from [74])

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Fig. 12.4 Schematic of solution-phase assembly of graphene and RuO2 nanosheets for RuO2 /graphene hybrid material (redrawn and reprinted with permission from [77])

RuO2 graphene composite is due to the synergistic effect of physical adsorption and electrochemical Faradic reaction [74]. The main issue related to the electrode material is the agglomeration of particles, which eventually decreases the surface area of the material. To address this issue, Deng et al. have synthesized RuO2 and graphene composite using graphene and RuO2 nanosheets by the solution-phase assembly as described in Fig. 12.4 [77]. Both the nanosheets maintain the high structural stability of hybrid material. The electrochemical analysis in the three-electrode system using 0.5 M H2 SO4 shows a specific capacitance of 497 F g−1 at 40% RuO2 mass loading. The high capacitance indicates the effective utilization of the nanosheets, which contributes to double-layer adsorption and pseudocapacitance [77]. Another literature reports the sol-gel synthesis and low-temperature annealing of RuO2 and graphene composite. HRTEM micrograph shows the well-dispersed RuO2 nanoparticles over graphene sheets. The electrochemical studies show near rectangular CV loops, and the charge–discharge curve shows a specific capacitance of 375 F g−1 . The enhancement in specific capacitance is due to the spacer effect of RuO2 nanoparticles along with graphene, which increases the surface area of the electrode for the accessibility of electrolytic ions. High chemical and thermodynamic stability of RuO2 and graphene show 90% capacitance retention after 6000 consecutive charge–discharge cycles [78]. Another work demonstrates a one-pot hydrothermal synthesis of RuO2 and reduced graphene oxide (RGO) composite using poly (diallyl dimethylammonium chloride) (PDDA) stabilizer. The RGO provides the pinning of RuO2 nanoparticles to prevent its agglomeration and facilitates electrolyte infiltration, which helps in the ion transport redox process. The specific capacitance achieved is 540 F g−1 at 27.5% RuO2 loading under 5 mV s−1 scan rate [79]. Hwang et al. have been demonstrated the one-step process for the preparation of RuO2 and graphene composite [80]. The as-synthesis nanocomposite shows a threedimensional laser scribed graphene framework with RuO2 nanoparticles. The highly porous graphene serves as an excellent substrate for RuO2 nanoparticles to reside and interact with electrolytic ions. This promotes fast ion diffusion, which increases the overall performance of the composite electrode. The hybrid electrode delivers a specific capacitance of 1139 F g−1 in 1 M H2 SO4 aqueous electrolyte. The assynthesized composite electrode exhibits stable capacitive behavior, which retains even at a high scan rate of 100 mV s−1 as shown in Fig. 12.5a. The charge–discharge

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Fig. 12.5 Evaluation of the electrochemical performance of RuO2 and graphene composite electrodes in a three-electrode setup, a cyclic voltammetry curves of RuO2 and graphene composite electrode at scan rates of 20, 30, 50, 70, and 100 mV s−1 , b charge/discharge curves of an LSG/RuO2 electrode at current densities of 30, 45, 60, 75, 90, 120, 150, and 300 A g−1 (redrawn and reprinted with permission from [80])

curve shows a triangular shape at high current densities indicating excellent capacitive performance as shown in Fig. 12.5b. RuO2 shows very good capacitance retention due to rapid internal electron and ion transport. Also, asymmetric assembled supercapacitor delivers 55.3 Wh kg−1 of energy density, indicating excellent electrode material for supercapacitor [80]. RuO2 serves as excellent pseudocapacitive electrode material for supercapacitor. It has high electrical conductivity, high-temperature stability, chemical inertness, and its morphology can be tailored to achieve the high surface of the material. Although RuO2 shows excellent electrochemical properties, its high cost limits the application as electrode material. To address this issue, two approaches have been used, (i) study for alternate and cheap metal oxides and (ii) formation of the composite using other supercapacitor electrode material. This reduces the Ru metal loading, thereby reducing its cost. On the other hand, another metal oxide has also been explored, which can deliver superior or comparable properties.

12.5.2 Manganese Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors Among the different TMOs, MnO2 , a traditional pseudocapacitive material, has received great attention as a promising supercapacitor electrode material due to costeffectiveness, natural abundance, and environmentally friendly nature with enormous theoretical specific capacitance (~1370 F g−1 ) [18, 81]. However, the poor conductivity of MnO2 often leads to high internal resistance in the electrodes, and thereby, poor electrochemical performance is observed. Thus, the formation of composite

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materials in combination with graphene-based highly conductive materials has demonstrated to be an effective approach to improve the performance of MnO2 . Therefore, several approaches have been investigated to fabricate graphene-MnO2 composites for high-performance supercapacitor electrodes. Li et al. have fabricated graphene-MnO2 composites through a simple hydrothermal approach, where composite materials show a specific capacitance of 211.5 F g−1 using CV at a scan rate of 2 mV s−1 with capacitance retention about 75% after 1000 GCD cycles using 1 M Na2 SO4 as an electrolyte [82]. A film-type electrode of graphene-MnO2 hybrid is further developed by Choi et al. for a high-performance supercapacitor application [83]. The 3D macroporous hybrid film is fabricated by building chemically modified 3D graphene using polystyrene colloidal particles as a sacrificial template followed by the deposition of the MnO2 thin layer as shown in Fig. 12.6a. The 3D microporous structure is shown in SEM images in Fig. 12.6b, c. This porous architecture provides a large surface area assisting the fast transportation of ions within the electrode with the conservation of sufficient electronic conductivity. Therefore, the composite electrode shows excellent electrochemical properties. The hybrid electrode reaches a specific capacitance of 389 F g−1 in 1 M Na2 SO4 electrolyte and retains 97.7% capacitance upon an increase in current to 35 A g−1 . The asymmetrically assembled supercapacitor device with chemically modified graphene and the hybrid electrodes delivers remarkable cell performance. A specific energy density of 44 Wh kg−1 and a power density of 25 kW kg−1 along with a good cycle life showing 95% capacitance retention after 1000 cycles have been reported. Yu et al. have demonstrated the high-performance electrochemical capacitors using graphene/MnO2 -based nanostructured textile materials designed through solution-processing [84]. In this report, the solution exfoliated graphene nanosheets

Fig. 12.6 a Schematic illustration to fabricate 3D macroporous films of chemically modified graphene-MnO2 hybrid, b low-magnification and c high-magnification cross-sectional SEM images of the hybrid (reprinted with permission from [83])

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of around 5 nm thickness are coated on three-dimensional, porous textiles. Later, pseudocapacitive MnO2 nanomaterials are electrodeposited to form the hybrid graphene-MnO2 -based textile electrode. The schematic and morphology of the material are shown in Fig. 12.7. The electrode achieves high electrochemical performance and exhibits a specific capacitance of 315 F g−1 . Further, the asymmetric supercapacitor device is fabricated using graphene-MnO2 hybrid textile as the positive electrode and single-walled carbon nanotube-based textile as the negative electrode in an aqueous solution of Na2 SO4 as the electrolyte. The supercapacitor device delivers a maximum power density of 110 kW kg−1 with an energy density of 12.5 Wh kg−1 . An outstanding cycling life with ~95% retention of its initial capacitance after 5000 cycles has also been noticed. The electrochemical analyses of the asymmetric supercapacitor device are shown in Fig. 12.8. The high electrochemical performance of the nanostructured graphene/MnO2 textiles-based supercapacitor is realized from the unique characteristics of the 3D porous architecture of the polyester-based textiles allowing conformal coating of the graphene sheets followed by loading of MnO2 . This facilitates easy access of the ions from an electrolyte to the electrode surface. The coated graphene nanosheets of the high surface area provide conductive paths for the transport of deposited MnO2 particles allowing excellent interfacial interactions between MnO2 particles and graphene sheets for fast electron transportation. The nanoflower like architectures of electrodeposited MnO2 offers huge electrochemical active surface areas for transfer of charge and reduces the path length of ion diffusion during the GCD process.

Fig. 12.7 a Schematic illustration for preparing hybrid graphene-MnO2 -nanostructured textiles electrodes, b, c SEM images of hybrid graphene-MnO2 -nanostructured textiles (Inset: highmagnification SEM image showing the nanoflower structure of electrodeposited MnO2 particles) (reprinted with permission from [84])

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Fig. 12.8 a Schematic representation for the asymmetric supercapacitor based on graphene-MnO2 textile as positive electrode and CNT-textile as negative electrode, b GCD curves of the asymmetric device at different current densities, c Ragone plot of energy density versus power density of the device, and d cycling performance of the device (reprinted with permission from [84])

A graphene-MnO2 3D hybrid network is designed through electrochemical deposition of MnO2 by applying a mass loading of 9.8 mg cm−2 into 3D graphene using Ni foam-based template [85]. The flexible nanocomposite electrode shown in Fig. 12.9a exhibits an aerial capacitance of 1.42 F cm−2 at a CV potential scan rate of 2 mV s−1 with desired cycling stability. Further, for the real application, a supercapacitor device is fabricated through assembling two pieces of the 3D graphene-MnO2 composite network. The assembled supercapacitor device is lightweight (2500 F g−1 ),

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Fig. 12.13 Electrochemical performance of the all-solid-state symmetric supercapacitor device, a CV curves at various bending angles, b GCD curves at various current densities, c specific capacitance versus current densities, and d cycling performance at a current density of 20 A g−1 (reprinted with permission from [97])

environment friendliness, and cost-effective [87, 88]. However, the low surface area and poor electrical conductivity deteriorate the specific capacitance of pure nickel oxide electrodes in the ranges of 50–350 F g−1 , which is far from the theoretical value. Therefore, the formation of NiO-graphene composites, where NiO is dispersed uniformly over the graphene sheet, provides a very high surface area and increased electrical conductivity. These composite materials are considered as potential electrode material for supercapacitors application. Cao et al. have fabricated grapheneNiO composite electrodes through electrochemical deposition of nickel oxide (NiO) onto the 3D graphene networks for supercapacitors [100]. The unique 3D porous structure with a large specific surface area of this material allows rapid access of the electrolyte ions to the NiO surfaces. The composite electrode exhibits a specific capacitance of 816 F g−1 at a scan rate of 5 mV s−1 . The electrode also shows a specific capacitance of 573 F g−1 at a higher scan rate of 40 mV s−1 from CV analysis using aqueous KOH as the electrolyte represented in Fig. 12.14a, b. From the GCD analysis, the composite electrode shows a specific capacitance of 745 F g−1 , at a discharge current density of 1.4 A g−1 as shown in Fig. 12.14c. Most importantly, in the cycling performance of the composite electrode, there is no obvious capacitance drop over 2000 cycles, which demonstrates the excellent electrochemical stability of

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Fig. 12.14 Electrochemical performance of NiO-graphene composites; a cyclic voltammetry curves measured at different scan rates, b specific capacitance as a function of scan rate, c galvanostatic discharge curves measured at different current densities and d cycling performance measured at a scan rate of 80 mV s−1 (reprinted with permission from [100])

the NiO-graphene composite. The cycling performance of the composite electrode is shown in Fig. 12.14d at a scan rate of 80 mV s−1 . The specific capacitance initially increases to 15% during the first 200 cycles, which is possibly due to the activation process that allows the trapped ions to gradually diffuse out. Wang et al. have also synthesized Ni(OH)2 nanoplates on graphene sheets through the hydrothermal method (Fig. 12.15a) [101]. The hybrid electrode material exhibits a very high specific capacitance of ~1335 F g−1 at a current density of 2.8 A g−1 and retains a high specific capacitance of ~953 F g−1 at a very high current density of 45.7 A g−1 shown in Fig. 12.15b. This high specific capacitance and remarkable rate capability are mainly due to direct growth of Ni(OH)2 nanoplates on graphene sheets imparting intimate interactions and efficient charge transport between the active nanomaterials and conducting graphene network. A simple physical mixture of pre-synthesized Ni(OH)2 nanoplates and graphene sheets are also characterized for comparison and shows lower specific capacitance. The electrode material exhibits excellent cyclic stability as no capacitance drop has been observed over 2000 cycles. The composite electrode also delivers a high-energy density of ~37 Wh kg−1 at a high-power density of ~10 kW kg−1 . Wu et al. have fabricated a 3D NiO/ultrathin derived graphene (UDG) hybrid on commercial Ni foam (NF) to make a binder-free pseudocapacitor electrode.

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Fig. 12.15 a SEM image of Ni(OH)2 nanoplates grown on graphene sheets, b specific capacitance of the composite electrode at various discharge current densities (reprinted with permission from [101])

Fig. 12.16 Fabrication scheme of the 3D NiO/ultrathin derived graphene hybrid on commercial Ni foam, its morphology (SEM images) and CV analysis at different scan rates (reprinted with permission from [102])

The NiO nanoflakes are in situ grown by a chemical bath deposition (CBD) technique on the free-standing 3D UDG/NF scaffold [102] as shown in Fig. 12.16. The 3D UDG/NF scaffold is prepared by a simple nano-casting process consisting of hydrothermal reaction and subsequently thermal transformation. SEM images depicted in Fig. 12.16 reveal the morphology of urchin-like NiO formed through the CBD method, which consists of uniform nanoflakes. This hybrid 3D network is utilized as a free-standing binder-free electrode for supercapacitor, where the UDG substrate plays an important role to improve the electron transfer rate and decrease the ohmic resistance of the electrolyte diffusion in the electrode during the redox

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reactions. The CV curves at different scan rates (Fig. 12.16) display a slightly positive shift of the oxidation peak potential and a negative shift of the reduction peak potential, which is due to the electric polarization and irreversible reactions at a higher scan rate. The electrode exhibits a specific capacitance of 425 F g−1 at a current density of 2 A g−1 . The hybrid electrodes also possess good cyclic stability and about 21% loss in specific capacitance after 2000 cycles at a current density of 10 A g−1 has been reported. A hierarchical flower-like NiO/reduced graphene oxide composite (Fig. 12.17) is prepared by reduction reaction using hydrogen gas as the reducing agent [103]. The hybrid electrode shows a specific capacitance of 428 F g−1 at a discharge current density of 0.38 A g−1 using a 6.0 M KOH electrolyte. Additionally, the hybrid capacitor exhibits high cyclic stability and retains 90% of its initial capacitance after 5000 cycles (Fig. 12.17). A nanoporous free-standing Ni(OH)2 /ultrathin-graphite foam (UGF) composite electrode is designed by the formation of Ni(OH)2 thin film on the surface of UGF via a hydrothermal reaction [104]. The highly conductive 3D composite network facilitates electron transport and shortens ion diffusion paths, and favors the rapid migration of electrolyte ions. An asymmetric supercapacitor device is fabricated using the composite as the positive electrode and activated microwave exfoliated graphite oxide as the negative electrode. The composite electrode delivers the maximum gravimetric energy and power densities of 13.4 Wh kg−1 and 85.0 kW kg−1 . This asymmetric supercapacitor has a long cycle life since it retains capacitance of 63.2% after 10,000 cycles. Fig. 12.17 Cyclic stability of the rGO/NiO hybrid-based capacitor at a current density of 0.38 A g−1 (inset shows the hierarchical flower-like morphology of the composite) (reprinted with permission from [103])

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12.5.5 Bimetallic Oxide/Graphene/Reduced Graphene Oxide Composites as Electrode Materials for Supercapacitors Currently, nickel-cobalt oxide (NiCo2 O4 ) has been perceived as a promising candidate for high-performance supercapacitor due to spinel structure with at least two orders of magnitude higher conductivity compared to monometallic nickel or cobalt oxides. Li et al. have fabricated a well-controlled layered NiCo2 O4 /RGO nanocomposite through the layer-by-layer assembly of exfoliated Co-Ni LDH and GO nanosheets followed by freeze-drying and annealing treatment as shown in Fig. 12.18a [105]. The composite structure exhibits a large surface area and increased electronic conductivity with more electroactive sites. This enhances the electrochemical activity toward the fast reversible redox reactions with long-term stability for supercapacitors. The composite electrode delivers an ultra-high specific capacity of 1388 F g−1 at a current density of 0.5 A g−1 . An outstanding rate capability with a specific capacity of 840 F g−1 at a high current density of 30 A g−1 has also been reported. The composite electrode has further characteristics of ultra-long cycle life with 90.2% retention of capacity after 20,000 cycles at a current density of 5 A g−1 . Most importantly, an asymmetric supercapacitor device can be fabricated using NiCo2 O4 /RGO composite as a positive electrode along with activated carbon (AC) as a negative electrode. The schematic reorientation is shown in Fig. 12.18b. The device delivers a high-energy density of 57 Wh kg−1 at a power density of 375 W kg−1

Fig. 12.18 a Schematic of the fabrication of layered NiCo2 O4 /RGO composite, b schematic illustration of the assembled NiCo2 O4 /RGO//AC asymmetric supercapacitor and c Ragone plot of the asymmetric supercapacitor device (the inset shows a photograph of a LED lighted by two supercapacitor devices connected in a series) (reprinted with permission from [105])

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in a working potential of 0–1.5 V (Fig. 12.18c). Further, LED bulbs can light by connecting two devices in a series as shown in the inset picture of Fig. 12.18c. Another high-performance asymmetric electrochemical supercapacitor has been fabricated by using a graphene-NiCo2 O4 nanocomposite as a positive electrode and commercial AC as a negative electrode. The asymmetric supercapacitor delivers an energy density in the range of 19.5 Wh kg−1 with an operational voltage of 1.4 V and capable to retain an energy density of 7.6 Wh kg−1 at a high-power density of about 5600 W kg−1 [21, 106]. Cyclic stability of the cell increases to 116% of its original capacitance after the first 1600 cycles due to a progressive activation of the electrode and maintains 102% of the capacitance after 10,000 cycles. The composite positive electrode also shows significantly higher specific capacitance (618 F g−1 ) compared to graphene-Co3 O4 (340 F g−1 ) and graphene-NiO (375 F g−1 ) nanocomposites synthesized under identical conditions. Nguyen and Shim have also decorated graphene sheets and NiCo2 O4 nanoparticles on conducting nickel foam through two-step electrodeposition followed by a thermal transformation [107], as shown in Fig. 12.19. This 3D nickel foam supported graphene and NiCo2 O4 electrode exhibits very high specific capacitance of 1950 F g−1 at a high current density of 7.5 A g−1 , owing to the rapid electron and ion transport, large electroactive surface area, and excellent structural stability. The 3D composite electrodes possess excellent stability under repeated GCD cycles and retain 92.8% of its initial capacity after 10,000 cycles at a current density of 3 A g−1 . A flower-like NiCo2 O4 /3D graphene foam hybrid is prepared through chemical vapor deposition (CVD) followed by electrodeposition [108]. The hybrid electrode delivers a maximum specific capacitance of 1402 F g−1 at a current density of 1 A g−1 . The nanohybrid-based supercapacitor possesses long cycle stability and retains 76.6% specific capacitance after 5000 cycles at a current density of 5 A g−1 . This high supercapacitive performance is ascribed to the synergistic effects of the high electrical conductivity and large surface area of 3D graphene foam along with the catalytic

Fig. 12.19 Schematic illustration of the fabrication of NF/G/NiCo2 O4 electrode with morphology (reprinted with permission from [107])

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Fig. 12.20 FESEM images of NiCo2 O4 /3D graphene foam hybrid, a before and b after 5000 GCD cycles (reprinted with permission from [108])

activity of the flower-like NiCo2 O4 . FESEM images of the electrode (Fig. 12.20) depict that the flower-like structures are not damaged after 5000 GCD cycles, which further suggest long-term stability of the electrode material. Very recently few other spinel-type mix TMOs are also utilized for highperformance hybrid supercapacitor electrodes with graphene-based materials [109, 110]. These binary metal–oxide nanomaterials have controlled size and shape with unique physicochemical properties because of the presence of multiple oxidation states. They also have a large surface area and good confinement probability of the electrons. Spinel MnCo2 O4 /NG 2D/2D hetero-nanostructures synthesized by a facile polyol method [109] show a specific capacitance of 1170 F g−1 at a current density of 1 Ag−1 with an excellent rate capability. The hybrid electrode has a long cycle of life since it retains 85.9% of specific capacitance after 10,000 GCD cycles. In another work, a solid-state asymmetric supercapacitor derived from nickel vanadium oxide (Ni3 V2 O8 ) and iron vanadium oxide (Fe2 VO4 ) nanoparticles anchored with nitrogen-doped graphene (NG) has been fabricated that delivers very high energy density [110]. The composite electrode materials are synthesized by a cost-effective hydrothermal technique. For the solid-state asymmetric supercapacitor, the hybrid Ni3 V2 O8 /NG is used as positive, where hybrid Fe2 VO4 /NG is used as negative electrode materials, as shown in Fig. 12.21. In an electrochemical analysis of the Ni3 V2 O8 /NG electrode exhibits an excellent specific capacitance of ~1898 F g−1 whereas, the Fe2 VO4 /NG electrode exhibits the maximum specific capacitance of ~590 F g−1 separately in 3 electrode test at a current density of 1 A g−1 . The assembled asymmetric supercapacitor device delivers a very high energy density of ~77.2 Wh kg−1 at a power density of 863 W kg−1 and retains a rate performance of about ~53 Wh kg−1 at a power density of 22.186 kW kg−1 . The supercapacitor device also shows an excellent cycling performance with high capacitance retention of ~83.3% after 20,000 consecutive GCD cycles. A summarized supercapacitive performance of different graphene–transition metal oxide-based composites are tabulated in Table 12.3. Therefore, these supercapacitors are used in several applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, and aerospace, etc., [2, 15, 18, 81, 111].

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Fig. 12.21 Schematic illustration for Ni3 V2 O8 /NG and Fe2 VO4 /NG hybrid electrodes and the schematic of assembled asymmetric supercapacitor device (reprinted with permission from [110])

12.6 Concluding Remarks Graphene-based materials are promising candidates for EDLCs owing to the huge surface area, high conductivity, low weight, and superior mechanical flexibility. However, they suffer from low energy density and low specific capacitance due to graphene’s propensity toward aggregation and restacking, which reduce the ionaccessible surfaces and limit ion and electron transport due to the narrower transport channels. On the other hand, transition metal oxide-based pseudocapacitive materials exhibit high specific capacitance and energy density due to fast and reversible surface redox reactions for charge storage. However, the underlined limitations remain poor electrical conductivity, low surface area, and large volume change in electrolyte solution during electrochemical analyses, which limit the power density and cyclic stability. Therefore, the fabrication of composites of graphene with TMOs is an effective way to enhance performance by combining the advantages of both the materials. In these composites, graphene serves some major roles such as ideal support for the growth of very small metal oxide nanoparticles of high surface area and well-defined structures, a conductive template to build a 3D interconnected conductive porous network to improve the electrical conductivity and charge transport, and finally to suppress the volume change and particle agglomeration of metal oxides during electrochemical analysis. On the other hand, the metal oxides present in the composites, efficiently suppress the re-stacking of graphene and enhance the energy density and specific capacitance of the electrode materials. Therefore, the integration of metal oxides with graphene in a composite ensures complete utilization of each active component and therefore achieves an excellent electrochemical performance like high energy and power densities along with long cycle life as a result of synergistic effects among the components.

Electrodeposition

CVD and electrodeposition

Graphene-NiCo2 O4

Electrochemical deposition

Graphene-NiO

Graphene-NiCo2 O4

Hydrothermal

Graphene-Co3 O4

Chemical treatment and sonication

Hydrothermal

Graphene-Co3 O4

Layer-by-layer assembly

Hydrothermal

Graphene-Co3 O4

Graphene-NiCo2 O4

Hydrothermal

Graphene-Co3 O4

Graphene-NiCo2 O4

Microwave-assisted method

Graphene-Co3 O4

Chemical reduction

Sol-gel

Graphene-RuO2

Graphene-NiO

Electrodeposition

Graphene-MnO2

Hydrothermal

Electrodeposition

Graphene-MnO2

Chemical bath deposition

Deposition

Graphene-MnO2

Graphene-NiO

Hydrothermal

Graphene-MnO2

Graphene-Ni(OH)2

Approach

Composite

Flower-like

3D porous

Nanowires

Layer

Hierarchical flower-like

Urchin

Nanoplates on sheets

3D porous

nanoparticles on vertically aligned sheets

Needles

3D network nanowires

Particle distributed sheet

Particle distributed sheet

Film

3D network

Textile

3D microporous

3D porous

Architecture

1402

1950

618

1388

428

425

~1335

816

580

157.7

768

472

243.2

233

465

315

389

211.5

Capacitance (F g−1 )

Table 12.3 Supercapacitive performance of different types of graphene and transition metal oxide composites Cycle life (%)

76.6% after 5000 cycles

92.8% after 10,000 cycles

102% after 10,000 cycles

90.2% after 20,000 cycles

90% after 5000 cycles

79% after 2000 cycles

100% after 2000 cycles

115% after 2000 cycles

86.2% after 20,000 cycles

70% after 4000 cycles

143% after 1000 cycles

95.6% after 1000 cycles

~95.6% after 2000 cycles

95% after 2000 cycles

81% after 5000 cycles

~95% after 5000 cycles

95% after 1000 cycles

75% after 1000 cycles

References

(continued)

[108]

[107]

[106]

[105]

[103]

[102]

[101]

[100]

[97]

[96]

[94]

[93]

[92]

[86]

[85]

[84]

[83]

[82]

326 B. De et al.

Approach

Polyol method

Hydrothermal

Hydrothermal

Composite

Graphene-MnCo2 O4

Graphene-Ni3 V2 O8

Graphene-Fe2 VO4

Table 12.3 (continued) 1170

Capacitance (F g−1 )

Nanoparticle anchored sheets 590

Nanoparticle anchored sheets 1898

2D/2D

Architecture

Cycle life (%)

~84.9% after 10,000 cycles

~87.1% after 10,000 cycles

85.9% after 10,000 cycles

References

[110]

[110]

[109]

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Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 13

Conducting Polymers as Electrode Materials for Supercapacitors Soma Banerjee and Kamal K. Kar

Abstract The fast-growing modern world endorses the demand for alternative nonconventional energy production and storage devices. In this respect, supercapacitors are the devices of research interest. Electrode materials of a supercapacitor device which is an essential part have been studied long by the scientific community to fabricate one with maximum performance. Different materials such as carbonaceous, metal oxides, conducting polymers and their combination have been utilized to fabricate the device. Many conducting polymers are studied for supercapacitor devices such as polypyrrole, polythiophene, polyaniline, and PEDOT. Conducting polymers possess tuneable morphological features, fast doping, and de-doping ability and charge-discharge kinetics, and each of them makes them suitable to be utilized as an electrode material for supercapacitor devices. However, the inherent drawback of low specific capacitance limits their sole use as the electrode material. A combination of conducting polymers in the form of composite with metal oxide and carbonis found to be beneficial to attend the desired properties of an electrode material for supercapacitor devices. This chapter provides only a brief overview of the types of conducting polymer-based electrodes of a supercapacitor device.

S. Banerjee · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] S. Banerjee e-mail: [email protected] K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_13

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13.1 Introduction In recent days, the need for alternative clean energy sources and storage devices is of uttermost importance to the research communities all over the world [1, 2]. Modern devices, supercapacitors, batteries, solar cells, fuel cells, etc., are the ray of light in the upcoming days [3, 4]. Supercapacitors are the modern energy storage devices designed to bridge the gap between a conventional battery and a capacitor having an advantage of fast charging and intermediate specific energy. Supercapacitors at the early stage were constructed from the high surface area carbon-based materials and continuously have been upgraded to meet the demand of the latest applications to provide thousands of Farads known as super or ultracapacitors. It can be discussed as of three types based on its structural construction. A supercapacitor composed of two capacitors connected in series with a conducting liquid in between is referred to as an electrochemical double-layer capacitor (EDLC) [5] . Here, the electrochemical performance of the device is dependent on the formation of the electrochemical double layer, and the capacitance in these cases is reserved in the form of a build-up charge at the interfacial region of the electrical double layer to charge the balance at the surface of the carbon material. In the case of pseudocapacitors, the capacitance is due to the storage of charge in the bulk of redox materials as a consequence of redox reactions. The major difference that lies between the EDLC and pseudocapacitor is that not only the interface but also the bulk of the materials contributes to the capacitance, and hence, a pseudocapacitor stores a greater amount of charge as compared to EDLC. The fast redox reactions act like a capacitor and hence named as pseudocapacitor. A hybrid supercapacitor is a combination of the above two supercapacitors typically designed to have enhanced characteristics compared to the individual components. In hybrid supercapacitors, one half of the supercapacitor plays the role of an EDLC, whereas the other half acts like a pseudocapacitor/battery. Hybrid supercapacitors are characterized by much higher energy and power densities as compared to EDLC and pseudocapacitors. Figure 13.1 represents the schematic of the three supercapacitors. The characteristics of capacitors [7] and capacitor to supercapacitor [8] are reported elsewhere.

13.2 Conducting Polymers in Supercapacitors Conducting polymers are made of conjugated double bonds formed by chemical or electrochemical oxidation of the monomer to form the polymers [9]. Oxidation proceeds both in the monomer and the polymer leading to the insertion of dopant or counterions in the polymer backbone, e.g., insertion of chloride when iron chloride has been used as an oxidant. The dopant level depends on the possibility to introduce the dopants or polarons inside the polymer structure as close as possible. Many conducting polymers are studied for supercapacitor devices such as polypyrrole, polythiophene, and polyaniline. Figure 13.2 shows the chemical structures of some

13 Conducting Polymers as Electrode Materials for Supercapacitors

Fig. 13.1 Schematic representation of three different types of supercapacitors a electrical double-layer capacitor, b pseudocapacitor, and c hybrid supercapacitor (redrawn and reprinted with permission from [6])

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N H

N H

n

S

n

Polypyrrole (PPy)

Polyaniline (PANI)

n

Polythiophene (PT)

S

n

n

Poly (p-phenylene) (PPP)

N

n Poly (p-phenylene sulfide) H (PPS) Poly (o-toludine) (POT)

n

n

trans-polyacetylene

O

O

cis- polyacetylene

S

n

Poly (p-phenylene vinylene) (PPV)

n

poly(3,4-ethylenedioxythiophene) (PEDOT)

Fig. 13.2 Chemical structures of common conducting polymers

commonly used conducting polymers for supercapacitor devices. Carbon-containing supercapacitors exhibit high power but low specific energy [10]. On the contrary, conducting polymer-based supercapacitors can reduce selfdischarge with the additional benefit of increased energy storage due to the associated redox reactions undergoing in the bulk of the material. However, the major difficulty remains the slower ion diffusion through the bulk of the electrode material responsible for the low rate of charge-discharge or lower power. However, conducting polymerbased supercapacitors still supposed to have potential in the field of supercapacitor due to better kinetics compared to all the pseudocapacitive materials in current use [10]. Again, conducting polymers are attractive due to the high charge density and low cost compared to conventional metal oxides. Polyaniline, for example, exhibits a current density of 140 mA h g−1 , which is comparable to metal oxide [11]. Hence, conducting polymers can make it possible to fabricate supercapacitor devices of low equivalent series resistance with high energy and power densities [12, 13] . Conducting polymer-based supercapacitors are of low cycle ability compared to double-layer supercapacitors since they begin to deteriorate as a result of the change in physical structure caused by doping and de-doping of counterions [14]. The conducting polymer-based supercapacitors may produce higher specific energies by insertion of counterions and increasing the doping levels. However, the insertion of a high amount of dopants causes an increment in cost with accompanying change in volume. This change in volume again leads to a detrimental effect in the mechanical

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strength of the electrodes under prolonged operative cycles. The study reveals that conducting polymer-based electrodes in combination with ionic liquid electrolytes are found to exhibit greater cycle life in actuators [15].

13.3 Mechanism of Conduction in Conducting Polymers: Effect of Doping Conducting polymers are catching the attention of the research communities due to the excellent intrinsic conductivity ranging from a few S cm−1 to 500 S cm−1 depending on the doping [16]. Table 13.1 summarizes the conductivities of common conducting polymers with doping types. They are also characterized by low band gaps of 1–3 eV in comparison with the common polymers [17]. The plastic properties (low melting point) of them make it easy to fabricate thin films for practical purposes [16]. Again, conducting polymers are of suitable morphological features, fast doping, and de-doping ability and charge-discharge kinetics, all of those contributing to their potential as an electrode material for supercapacitor devices [18, 19]. Conducting polymers are p-doped or n-doped by insertion of anions or cations as counterions during oxidation or reduction, respectively. The process of oxidation and reduction can be represented by the following equations as follows. The reverse reactions of the two represent the process of discharge.   p-doping : C p → C p n+ A− n + ne−

(13.1)

  n-doping : C p + ne− → C + n C p n−

(13.2)

Table 13.1 Effect of doping on conductivity (reproduced from [20, 21])

Conducting polymer

Conductivity (S cm−1 )a Doping type

Polyacetylene (PA)

200–1000

n, p

Polyaniline (PANI)

5

n, p

Polypyrrole (PPY)

40–200

p

Polythiophene (PT)

10–100

p

Polyparaphenylene sulfide (PPS)

3–300

p

Poly(3,40.4–400 ethylenedioxythiophe) (PEDOT)

n, p

Polyparaphenylene (PPP)

n, p

aS

Siemens

500

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Fig. 13.3 Schematic representation of P-type and N-type doping in conducting polymers (concept has been adopted from [29])

A supercapacitor device made solely of conducting polymers can be of three types of configurations, type I (symmetric), type II (asymmetric), and type III (symmetric) [22–24]. Type I symmetric is made of both the electrodes with p-dopable conducting polymers, whereas type II asymmetric is composed of two different types of pdopable conducting polymers having electroactivity range. Type III symmetric types use the conducting polymers of the same kind for both the electrodes of p-doping for the positive electrode and n-doping for negative electrodes. Another class, asymmetric or hybrid devices are also fabricated, where positive electrodes are made of conducting polymers and negative electrodes are composed of carbon or lithium materials [25, 26]. Among all, a type III device made of entirely conducting polymers is the most attractive one since theoretically, they can be of high conductivity as a result of charged species present in the doped state for both the electrode s made of p- and n-doped polymers [27]. Figure 13.3 represents the mechanism of doping (pand n-dope) in a conducting polymer film. The detailed characteristics of conducting polymers are reported elsewhere [28].

13.4 Conducting Polymer-Based Supercapacitors Soon after the discovery of polyacetylene, rapid growth in the field of conducting polymers has been evidenced [30]. The most common conducting polymers used in

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the energy storage devices are polypyrrole, polyaniline, polythiophene, poly (3,4ethylenedioxythiophene) (PEDOT), etc., [31, 32]. Conducting polymers have characteristics such as good electrical and optical properties along with ease in synthesis approach, which help in the expansion of these materials in various applications zones. The conductivity in conducting polymers arises due to the presence of conjugated structures as evidenced in the chemical structures of them. The π electrons in the conjugated double bonds take part in delocalization leading to the metallic character. However, these systems are not so stable due to the possible bond alteration leading to energy gaps in the electronic spectrum [30]. The energy gap has been reduced through the introduction of the dopant counterions and hence improving the conductivity of the conducting polymers. The dopant ions convey charge in the form of excess electrons and neutralize the unstable polymer backbone in the oxidized form either by donating or accepting electrons [33]. The conducting polymers can be doped with p- or n-type dopants such as small ions (Cl− , Br− , NO3 − ) or large dopants such as peptides or polymers [20, 34].

13.4.1 Polyaniline-Based Supercapacitors Polyaniline (PANI) has many advantages such as high environmental stability, structural diversity, low cost, and ability to shift from conducting to the resistive state by doping or de-doping [35]. PANI may exist in different chemical forms depending on the oxidation level. The fully oxidized, half-oxidized, and fully reduced forms named as pernigraniline, emeraldine base, and leucoemeraldine, respectively [36]. Among these, the emeraldine base is the most stable and conductive one. In practice, PANI is used as a mixture of all of these states with a relatively higher amount of emeraldine base to maximize the performance. The general synthesis approach of PANI is chemical or electrochemical oxidation of monomer aniline [37]. The chemical polymerization routes result in different morphologies such as nanotubes, nanorods, nanospheres, and nanofibers, through precise control over the reaction parameters [38]. However, electrochemical polymerization is much faster and environment friendly since free from the use of oxidants and additives. PANI is one of the most common conducting polymers used in energy storage and conversion devices such as supercapacitors, batteries, and fuel cells [39–41]. PANI ensures desirable properties in view of supercapacitor applications, high electroactivity, high doping level, good specific capacitance, and excellent stability [14]. Again, the manageable electrical conductivity and ease of processability make them a material of choice [42]. PANI when used in supercapacitor devices stores charges via redox reaction since it transits between the oxidation states. Specific capacitance as high as 950 F g−1 has been achieved through the entire volume of PANI greater than other conducting polymers, where the charge has been stored on the surface only [43]. PANI is reported to exhibit a wide range of capacity from 44 to 270 mA h g−1 [44]. The deviation in the capacity depends on several factors such as synthesis

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methodology, morphology, electrode thickness, types, and concentration of binders and additives. PANI is the conducting polymer having the most variable specific capacitance. PANI has been doped with LiPF6 to prepare electrode material for polymer-based redox supercapacitor devices [45]. The electrode has been fabricated by slurry coating on a charge collector. The redox supercapacitor utilizing polymeric separator reveals a specific capacitance of 100 F g−1 at the initial stage and retains 70 F g−1 after 5000 cycles. The polymer electrolyte, on the other hand, exhibits an initial value of 80 F g−1 and retains 60 F g−1 after 5000 cycles of operation (Fig. 13.4). Although PANI shows high specific capacitance, the process involves swelling, shrinkage, and crack development in the polymer during the doping or de-doping with the counterions, which ultimately lower the cycle stability of the supercapacitor devices. Again, other associated challenges remain the problem of over-oxidation leading to a reduction in the working potential of the electrode. This opens up the possibility of the development of polymer composites using PANI as the matrix material and carbonaceous or metal oxides as the filling material to develop composite designs [12, 13]. Recently, an interesting approach to overcome this issue is to fabricate hybrid composites made of PANI and graphene [46, 47]. Graphene as a component provides excellent conductivity coupled with mechanical properties and improved cyclic stability, whereas PANI serves the role of high pseudocapacitance Fig. 13.4 a Charge-discharge cycle and b cycle life at current density 2 mA cm−2 by polymer electrolyte and Et4 NBF4 in acetonitrile (redrawn and reprinted with permission from [45])

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[48]. Chaps. 14 and 15 provide the details of conducting polymer-based composites recently used for supercapacitor devices.

13.4.2 Polypyrrole-Based Supercapacitors Doped polypyrrole (PPY) has been extensively used for energy storage devices due to the several advantages such as ease of preparation and electrical conductivity. PPY can be easily synthesized in large amounts at room temperatures with several solvents and also in water. The conductivity can be varied depending on the doping type and concentration. The rigid, hard, and crystalline nature of PPY makes it difficult to be processed further, once it is synthesized due to crystalline nature. PPY has been extensively used as electrode materials for supercapacitor devices due to the ease of electrochemical processing [49]. However, PPY is not able to be doped with n-doped materials, and hence, its application is limited to cathode material of the supercapacitor devices. Again, the high density of PPY makes it possible to achieve high capacitance per unit volume of the material, e.g., 400–500 F cm−3 [50]. However, one dark side of the dense growth of PPY is that the access of the dopants is limited only to the interior site of the polymer. This, in turn, reduces the yield based on capacitance per gram especially for the electrodes with thick PPY coating [51]. PPY, in general, is doped with singly charged anions such as Cl− , SO3 − , and ClO4 − , often with multicharged ions like sulfate leading to crosslinking in the polymer backbone [52]. PPY has been utilized both as to type I and type II supercapacitor devices. PPY, when combined with poly(3-methyl thiophene), acts like a type II supercapacitor [22]. The type I device is reported to exhibit discharge capacitance of 8–15 mF cm−2 that is quite similar to type II. The voltage range improves from 0.5–1 to 1.2 V as the device has been modified from type I to type II. Another study reports the development of PPY based solid-state supercapacitor using PVA as the polymer electrolyte [53]. The solid-state design exhibits capacitance as high as 84 F g−1 and stable up to 1000 cycles with a specific energy of 12 Wh kg−1 . PPY electrodes are developed for redox supercapacitor devices via electrodeposition of PPY on Ti foil through cyclic voltammetry [54]. A maximum specific capacitance of 480 ± 50 F g−1 has been evidenced at a scan rate of 10 mV s−1 . The specific capacitance of the supercapacitor has also been determined as a function of specific mass by deposition of PPY of various specific masses (Fig. 13.5). The specific capacitance is found to be increased at first with a specific mass and then decreases once the maximum value has been reached. The increase at the initial stage can be attributed to the improved porosity that eases the electrolytes to penetrate the mesopores of the electrode material. The mesoporous morphology is helpful in creating channels to allow easy access to the electrolytes. On the later stage, an overgrowth of the PPY leads to the blockage in the channels leading to a notable decrease in specific capacitance at a very high specific mass.

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Fig. 13.5 Specific capacitance as a function of scan rate and specific mass for polypyrrole electrodes (redrawn and reprinted with permission from [54])

Recently, nanostructures of PPY have attracted great attention in the field of supercapacitors due to the advantage of high surface area, high conductivities, etc., which are favorable to improve the contact between the electrode and electrolyte material of the device [55, 56]. The nanostructured designs are suitable for the fabrication of nanodevices due to the ease of fixing the structures to the collector plates [57]. However, the use of PPY in this aspect is still limited as a result of low surface area. The development of mesoporous morphology is beneficial for high surface area and controlled morphological features [58]. PPY nanowires with ordered mesoporous structures are suitable to be used as a novel electrode material. The development of mesoporous structures in the nanowires improves the accessibility of the active sites, and hence, fast diffusion of the reactants and products may commence therein [59]. Yin et al. have synthesized PPY nanowires having ordered mesoporous structures via template-guided chemical polymerization approach [60]. The mesoporous nanowires exhibit a high aspect ratio and surface area with large interconnected mesopores. The electrodes developed therein obtain specific capacitance of 453 F g−1 at 0.25 A g−1 . The sulfur/PPY nanowires show a large discharge capacity of 1601 mA h g−1 at an early stage, which is retained to 1014 mA h g−1 after 100th test cycles. The electrochemical performance of nanostructured PPY electrode-based supercapacitor devices has been improved due to the large surface area and interconnected network structures, which help to supply more and more active sites for the electrochemical reactions and smoothen the process of mass transfer.

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13.4.3 Polythiophene-Based Supercapacitors Polythiophenes and derivatives are capable to be either p-doped or n-doped. The mass-specific capacitance of the n-doped form is much lower compared to the pdoped form. In general, for thiophene derivatives, the conductivity in the n-doped form is quite lower. This limits the use of n-doped material as the anode of the supercapacitor only. Most of the thiophene-based derivatives are stable in air or moisture in p-doped and n-doped forms [17]. N-doped polythiophenes are of low stability in oxygen and water with low conductivity compared to p-doped form [25]. Hence, the n-doped thiophenes are of low cycle life and high discharge. i.e., they convert back to neutral form and easily get oxidized. To resolve this issue, polythiophene derivatives having low bandgap are at less negative potential [23, 61]. This can be done by substitution at the third position of the polythiophene ring with either, phenyl, ethyl, or alkoxy groups [62]. Another way out is to introduce the electron-withdrawing groups to the substituents. Another interesting strategy to find the solutions for the n-doped polymers is the use of carbon-based materials as the electrode in an asymmetric supercapacitor device. The p-doped polymer is to be used as the positive electrode material. This improves the cycle life of the supercapacitor device and can be operated at least up to 10,000 times with an excellent specific power compared to carbon-carbon supercapacitors [25]. Poly (3,4-ethylenedioxythiophene) (PEDOT) is one of the successful derivatives of polythiophene utilized in many applications due to the high electrical conductivity along with good chemical stability. PEDOT can be p-doped and n-doped. However, the research with PEDOT is quite new, and it is a growing conducting polymer for the researcher of the interdisciplinary field. PEDOT exhibits a high potential range of 1.4 V; however, the specific capacitance remains low due to the high molecular weight and low doping level [63]. This polythiophene derivative is of the low bandgap of 1–3 eV and is of excellent conductivity (300–500 S cm−1 ) [17]. PEDOT has added advantages of fast electrochemical characteristics due to the high charge mobility, good thermal, and chemical stability [17, 61]. The reason behind this excellent kinetics is a high surface area in combination with high electrical conductivity [64]. This polymer possesses excellent film-forming ability with long cycle life; however, on the other hand, the specific capacitance remains low 90 F g−1 due to the large molecular weight and low level of doping [63]. The charge storage capacity of PEDOT depends on the surface doping and dedoping process inside the polymer matrix guided by diffusion of counterions, surface area, and electrical conductivity of the PEDOT electrodes. Several chemical and electrochemical methods have been adopted to enhance the capacitance of the PEDOT made electrodes [65, 66]. Different strategies have been adopted for the development of simple polymerization methods to construct supercapacitors of PEDOT nanostructures with outstanding conductivity and electrochemical stability [67, 68]. In a recent study, Rajesh et al. have developed a new approach for the growth of PEDOT in a template-free method directly on carbon fiber cloths [69]. SEM images reveal the deposition of PEDOT over the surface of the carbon cloth indicating a

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Fig. 13.6 Cycling stability of CFC/PEDOT device up to 12,000 cycles and SEM images of hydrothermally polymerized PEDOT nanostructures (redrawn and reprinted with permission from [69])

three-dimensional dendritic growth of PEDOT. The galvanostatic charge-discharge behavior for the first and 12,000 cycles is displayed as well indicating cyclic stability with 86% retention of the initial value after 12,000 cycles (Fig. 13.6). The dense coating of PEDOT nanostructures eases rapid penetration of the electrolytes permitting rapid electron transfer and long cycle life. Again, the porous nature helps in the formation of the channels for the passage of the electrolyte ions, and electrostatic stress gets relieved at the electrolyte/electrode interface. Vapor-phase deposition is one of the widely accepted approach since the process helps in the retention of high intrinsic conductivity. Vapor-phase polymerization provides strong adhesion between the polymer and current collector. However, this method involves high temperatures, post-treatment for the removal of the template, high cost, sophisticated machineries, and complex methodologies. PEDOT may achieve the highest conductivity of about 4500 S cm−1 among the different conducting polymers only when deposited from the vapor-phase process [70]. Instead of the above advantages of the vapor-phase process, most of the time, symmetric supercapacitors are prepared by electrochemical deposition of PEDOT. Symmetric supercapacitors are in general prepared by the electrochemical process due to the enhanced electrical contact between the polymer and current collector [65]. D’Arcy et al. have developed nanofibrillar PEDOT for supercapacitors by vapor-induced polymerization [67]. SEM images of freestanding PEDOT film exhibit a topography of wrinkles densely coated with vertically aligned one-dimensional nanoribbons of PEDOT (Fig. 13.7). A high-performance supercapacitor device has been made of the sealed plastic cell containing two symmetric PEDOT coated carbon fiber as current collectors. The cyclic voltammetry study shows ideal capacitance behavior

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Fig. 13.7 Supercapacitor structures, cyclic voltammograms, and SEM images of evaporative vaporphase polymerized PEDOT (redrawn and reprinted with permission from [67])

at different scan rates having a rectangular nature. A specific capacitance as high as 175 F g−1 at 5 mV s−1 for 0–1 V has been reported.

13.4.4 Nanostructured Conducting Polymers Among all the conducting polymers, PANI is the most widely explored materials for supercapacitor devices. PANI possesses theoretical capacitance of 200 F g−1 ; however, achieving this much of capacitance is difficult due to the limited reactive sites available for the access of the electrolytes. Hence, various studies have been conducted to prepare PANI based nanostructures [71–73]. The preparation of a well-controlled nanomorphology of PANI can achieve the desired supercapacitor performance. The electrochemical behavior of PANI is strongly dependant on the shape and morphology of the materials at the nanoscale. Park et al. have developed PANI nanostructures of different morphologies such as nanospheres, nanofibers, and nanorods and have shown the dependence of morphology on the electrochemical performance [74]. The specific capacitance is in the order of nanospheres 0). The angle α is considered a determining parameter for the physicochemical characteristics of nanofibers. Also, CNFs lack the hollow cavity, a distinguishable characteristic from the CNTs. The CNFs are composed by stacking of curved graphene layers that result in assembled cones- or “cups”-like structure giving rise to fishbone or bamboo-like CNFs, respectively [15, 24]. CNFs exhibit varied diameters ranging from few nanometers (nm) to several hundreds of nanometers with large aspect ratios (~100) resulting in stable structures with an excellent electrically conductive network. The lower crystallinity, defect, and functionality-rich composition of CNFs assist in charge storage. The CNFs possess a high surface area (~200 m2 g−1 ), which assists in unveiling the edge and basal graphitic planes. Further, the hierarchical porous morphology assists in the amplification of surface availability. This is particularly helpful in enhancing exposure to active sites and assists in applications like energy storage and conversion. The exposed edge sites and unsaturated bondings within the graphitic framework result in increased reactivity in CNF as compared to the CNT. Among the plethora of synthesis techniques reported, the bottom-up approach-based catalytic chemical vapor deposition (CVD) and plasma-enhanced (PE-) CVD offer meticulous fabrication with fine control over the shape, size, and alignment of CNFs. Also, electrospinning offers a cost-effective and commercially viable alternative for CNF synthesis with tunable characteristics [24]. Parameters including the polymer precursor concentration, heating rate, temperature, and duration determine the dimensions and quality of electrospun fibers. Also, the mechanical properties, namely the tensile strength (~0.3–8 GPa) and Young’s modulus (~228– 724 GPa) of CNF, are inferior to the CNTs owing to the inefficient networking of the graphene layers. However, the significant thermal retention of strength and modulus in CNFs is also worth mentioning. Due to the above-mentioned physiochemical features, CNFs have found applications as gas storage materials, catalyst materials in energy storage and conversion devices (fuel cells, Li-ion batteries, and supercapacitors), and composite additives to enhance the electrical and mechanical properties. The production expense of CNFs is ~50 $ kg−1 , synthesized in a fixed-bed reactor [24]. For the sake of large-scale industrial utility, it is desirable to further reduce the cost by embracing more efficient synthesis measures. An overview of the synthesis of CNFs, and their properties and applications has been reported elsewhere [24].

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15.2.5 Graphene/Reduced Graphene Oxides Graphene is the term coined for 2-D monolayers of sp2 -hybridized carbon atoms organized in an extended honeycomb lattice. The carbon–carbon bond length of 1.42 Å is observed with a π-electron cloud above and below the plane of the graphene sheet. It is quite exciting to acknowledge that 3 million graphene monolayers stack together to give rise to 1-mm-thick graphite crystal, the most common allotrope of carbon. Graphene is the fundamental unit for 0-D fullerene, 1-D CNT/CNF, and 3-D graphite. Single-layer graphene exhibits exceptional electrical (charge carrier mobility and concentration ~250,000 cm2 V−1 s−1 and 1012 cm−2 ), mechanical (stiffness ~1 TPa), and thermal (conductivity = 5000 W m−1 K−1 ) characteristics and is very lightweight (0.77 mg m−2 ) [25]. These impressive properties result from the confinement of π-electrons in 2-D layers and can be tuned by controlling the number of layers, defects, and the edge roughness [25]. The electronic properties are determined by the number of layers present. When layers approach ~10, electronic characteristics are similar to that of bulk graphite [25]. The synthesis of graphene is explored using chemical/mechanical exfoliation of graphite, epitaxial growth, and CVD synthesis. The top-down mechanical exfoliation is the best approach by far to obtain high-quality graphene. But, obtaining a commercially viable yield with ease of fabrication is still a challenge using the above-mentioned techniques. Further, production cost, time, and quality are additional parameters of concern in determining the utility of a fabrication technique. Therefore, as an alternative economical and scalable approach, the graphite oxide is explored as the starting material owing to the larger interlayer separations (6.8 Å) in comparison with graphite (3.35 Å), which facilitates the exfoliation to produce graphene oxide when subjected to low-power ultrasonication in an aqueous medium. The chemical reduction of obtained graphene oxide results in conductive graphene sheets, at least theoretically. The hydroxyl, epoxy, and carbonyl functionalities on the basal planes and edges of graphene oxide constitute 2000 m2 g−1 ), high mechanical strength, and good thermal stability. High processability and low production cost of CNTs make it suitable for commercial supercapacitor applications. The pore size should reside in mesopores (in between

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Table 15.3 Comparative electrochemical performance of different supercapacitor electrodes based on the biomass-derived activated carbon (Chap. 5) [28] S.

Precursor

Activation mechanism

Activating agent

Electrolyte

System Capacitance Ref.

1

Cassava

Chemical and physical

KOH and CO2

0.5 M H2 SO4

2

264.08 F g−1

[28]

2

Waste tea leaves

Chemical

KOH

2 M KOH

3

330 F g−1 at 1 A g−1

[28]

3

Tremella

Chemical

KOH

6 M KOH

3

71 F g−1 at 1 A g−1

[28]

4

Human hair

Chemical

KOH

6 M KOH

3

340 F g−1 at 1 A g−1

[28]

5

Rice husk

Chemical

KOH

6 M KOH

2

367 F g−1 [28] at 5 mV s−1

1.5 M TEA-BF4

2

174 F g−1 [28] at 5 mV s−1

no.

6

Willow catkins

Chemical

KOH

6 M KOH

3

306 F g−1 [28] at 0.1 A g−1

7

Soya

Chemical

NaOH

1 M H2 SO4

3

193 F g−1 [28] at 0.5 A g−1

8

Corn cob residue

Chemical

NaOH

1 M H2 SO4

3

575 F g−1 [28] at 5 mV s−1

9

Fish gill

Chemical

KOH

6 M KOH

3

229 F g−1 at 10 mV s−1

10

Cotton stalk

Chemical

KOH

1 M H2 SO4

2

254 F g−1 [28] at 0.2 A g−1

11

Sunflower seed shell

Chemical

KOH

30 wt% KOH

2

250 F g−1 [28] at 0.1 A g−1

12

Paulownia flower

Chemical

KOH

1 M H2 SO4

2

324.1 F g−1 [28] at 0.5 A g−1

13

Rice husk

Chemical

KOH

6 M KOH

2

147 F g−1 [28] at 0.1 A g−1

14

Waste carton box

Chemical

NaOH–KOH 6 M KOH melt

2

311 F g−1 [28] at 0.5 A g−1

15

Waste coffee grounds

Chemical

ZnCl2

1 M H2 SO4

2

386 F g−1 at 0.05 A g−1

16

Auricularia Hydrothermal

KOH

6 M KOH

3

196 F g−1 [28] at 5 mV s−1

17

Bamboo chopsticks

Hydrothermal and KOH chemical

6 M KOH

3

212 F g−1 [28] at 0.1 A g−1

18

Jute

Hydrothermal

3 M KOH

3

408 F g−1 [28] at 1 mV s−1

1 M H2 SO4

[28]

[28]

(continued)

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Table 15.3 (continued) S.

Precursor no.

19

Activation mechanism

Activating agent

Bamboo Hydrothermal and KOH by-product chemical

Electrolyte

System Capacitance Ref.

6 M KOH

3

301 F g−1 [28] at 0.1 A g−1

EMIM TFSI

2

146 F g−1 [28] at 0.2 A g−1

20

Coca-Cola

Hydrothermal and – chemical

6 M KOH

3

352.7 F g−1 [28] at 1 A g−1

21

Rice husk

Microwave assisted

ZnCl2

6 M KOH

2

245 F g−1 at 0.05 A g−1

[28]

22

Auricularia Activation and surface modification

ZnCl2 and NH4 Cl

6 M KOH

3

347 F g−1 at 1 A g−1

[28]

23

Kelp

Carbonization

Ammonia

6 M KOH

3

440 F g−1 [28] at 0.5 A g−1

24

Hemp

Hydrothermal and KOH chemical

1.8 M 2 TEMABF4 /PC

160 F g−1 at 1 A g−1

[28]

25

Human hair

Chemical

KOH

1 M LiPF6 EC/DEC

2

107 F g−1 at 2 A g−1

[28]

26

Pollens

Chemical

KOH

1 M TEABF4 /AN

2

185 F g−1 at 1 A g−1

[28]

1L of EMIM B4

2

207 F g−1 at 1 A g−1

[28]

micropores (500 Å) for enhanced electrolyte accessibility). This helps in effective and fast ion migrations and successful formation of EDLs. Mesoporous structure and entangled network of CNT enable easy access on electrolyte ions, which makes it a promising candidate as an electrode material for an EDLC. Multiwalled carbon nanotube (MWCNT)- and single-walled carbon nanotube (SWCNT)-based electrode materials also show pseudocapacitive nature (redox response) in the cyclic voltammetry (CV) curve because of the functional groups and impurities. Impurities present on the CNTs reduce the EDLC capacitance. For example, impurities like carbonaceous or metallic impurities present on the wall of the CNT hinder the electrolyte ion movement, which reduces the effective surface adsorption sites. Several physical and chemical purification methods are present, which increases the CNT performance in many orders. Oxidation method for purification using the chemical method, photo-oxidation method, oxygen plasma, and gas-phase treatment methods has gained attention. The oxidation method also enhances the chemical reactivity by adding oxygen-containing functional groups on the graphitic surface. The oxygen-containing functional groups like carboxyl and hydroxyl groups increase the capacitance value due to increased hydrophilicity of CNTs. Tunable functionalities over the surface of the CNTs have a great advantage of tuning the surface functionalities according to the need. For improvement of the

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Table 15.4 Capacitance value of the CNTs with respect to BET-specific surface area, mesopore volume, and percentage of oxygen (reprinted with permission from [20]; see reference Chap. 29) Type of CNTs

Vmeso (cm3 STP g−1 )

S BET (m2 g−1 )

Oxygen (mass%)

Capacitance (F g−1 )

Ref.

A700Co/Si

435

411

10.8

80

[29]

A900Co/Si

381

396

4.6

62

[29]

A600Co/NaY

269

128

0.8

4

[29]

P800/Al

643

311

90% after 1000 cycles

[31]

Sheet

_

215 F g−1

97% after 10,000 cycles

[31]

3-D porous

3100

150 F g−1 at 0.8 A g−1

97% after 10,000 cycles

[31]

Hydrogel

~414

186 F g−1

~92% after 10,000 cycles

[31]

Hydrogel

~297

441 F g−1 at 1 A g−1

86% after 10,000 cycles

[31]

Hydrogel

_

190 F g−1

95.2% after 4000 cycles

[31]

18]. However, electrode materials prepared using solely PANI deliver very low energy density due to the low potential window, and it shows very poor stability due to the material swelling and shrinkage during the electrochemical analyses. Therefore, the formation of ternary composites simultaneously resolves both the problems, as TMOs enhance the energy density and carbon materials improve the stability. Hence, several strategies have been followed to fabricate TMO–carbon–PANI ternary composites. Most of the cases, they follow two steps fabrication processes, wherein the first step TMO-/carbon-based nanohybrids are prepared followed by the fabrication of polymer nanocomposites using aniline in the second step. Tan et al. have reported TiO2 /activated carbon/PANI ternary composites, where activated carbon (AC)/TiO2 nanowires are prepared through the sonochemical–hydrothermal method followed by fabrication of the composites with PANI using in situ polymerization route [32]. TEM morphology of TiO2 and SEM morphology of TiO2 /AC/PANI composites are shown in Fig. 15.2a, b. The SEM image in Fig. 15.2b of ternary composite shows the nanometer-scale, porous network layer with a homogeneous coating of PANI on the

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Fig. 15.2 TEM image of a TiO2 nanowires and b SEM image of TiO2 /AC/PANI ternary composite; d CV and e GCD curves of AC/TiO2 and TiO2 /AC/PANI electrodes (redrawn and reprinted with permission from [32])

outer surface of AC/TiO2 nanohybrids. Therefore, the structure provides high chemical activity and surface area. In this structure, AC acts as the main supporting material and TiO2 nanowires provide interconnection between AC and PANI particles, which is also beneficial for the mechanical strength of the ternary composite. The electrochemical performance of the ternary composite electrode material is evaluated by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses using 1.0 M LiClO4 in propylene carbonate as the electrolyte. The CV curves in Fig. 15.2c show that AC/TiO2 electrode remains similar to the CV curves of AC, which is close to the ideal rectangular shape or EDLC behavior. On the other hand, the ternary composite electrode shows two pairs of redox peaks, which are obtained by the chemical state of PANI that results in the redox reaction and indicate the pseudocapacitance besides the EDLC. From the GCD analysis, the ternary composite shows similar initial specific capacitance (286 F g−1 ) compared to the AC/TiO2 (292 F g−1 ) at a current density of 1 A g−1 . However, after 2000 continuous GCD cycles, the former retains the value up to 230 F g−1 , whereas the latter shows only 124 F g−1 , as displayed in Fig. 15.2d. Similarly, Yan et al. have fabricated PANI/mesoporous carbon/MnO2 ternary composite, where mesoporous carbon/MnO2 binary composite has been prepared by a chemical oxidation process and subsequently ternary composite is made through

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Fig. 15.3 a SEM image of PANI/mesoporous carbon/MnO2 ternary composite; b GCD, c CV, and d Nyquist plots of PANI binary and ternary composite electrodes (redrawn and reprinted with permission from [33])

in situ chemical polymerization of aniline on mesoporous carbon/MnO2 composite [33]. PANI-deposited ternary composite shows a rough surface with uniform nanolayer morphology as shown in the SEM image in Fig. 15.3a. Here, PANI chains are adsorbed uniformly on the surface of mesoporous carbon/MnO2 composite and stabilized due to the interaction between the quinoid ring of PANI and the binary composite. Therefore, the electrode of ternary composite shows superior supercapacitor properties compared to the binary composite and pure PANI-based electrodes, as shown in the GCD and CV analyses in Fig. 15.3b, c. Electrochemical impedance spectroscopy (EIS) measurement also shows that the diameter of the semicircle in Nyquist plots (Fig. 15.3d) representing the charge-transfer resistance (Rct ) is much smaller for the ternary composite electrode compared to pure PANI and binary composite electrodes. The results depict ternary composite leads to faster electron transport and faster charge-transfer in the electrochemical system. Further, the ternary composite exhibits better specific capacitance with superior rate capability and cyclic stability compared to the binary composite. Again, carbon nanotubes (CNTs) are one of the most favorable electrode materials for supercapacitors due to the large accessible surface area, high conductivity, superior mechanical and chemical stability [34, 35]. However, the specific capacitance of pristine CNTs is very poor. Therefore, incorporating highly capacitive TMO and PANI enhance the supercapacitive performance while retaining the high cycling

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Fig. 15.4 SEM images for a TiO2 , b PANI, and c PANI/CNTs/TiO2 ternary composite; d comparative CV curves of TiO2 , PANI ternary composite at a scan rate of 10 mV s−1 ; and e cyclic stability for the ternary composite (redrawn and reprinted with permission from [36])

stability of CNTs and the high capacitance of TMO and PANI [36, 37]. Singu et al. have prepared a ternary composite of PANI/CNTs/TiO2 by using by in situ chemical polymerization of aniline in the presence of MWCNTs and TiO2 [36]. The SEM images displayed in Fig. 15.4a, b of pure TiO2 and PANI indicate nano sphere and nanofibrous like morphologies. The SEM image of the ternary composite in Fig. 15.4c shows that nanofibrous PANI grows uniformly on CNTs with homogeneously distributed TiO2 nanospheres, as the polymerization of aniline takes place preferentially and continuously on the surface of CNT. Therefore, CNT not only provides high conductivity and surface area but also acts as the rigid support for the stability of the material. The formation of the conductive network structure of high surface area by synergistic effects between all the components is favorable because of ion diffusion and migration during electrochemical analysis. The comparative CV curves of PANI, CNTs, and the ternary composite at a scan rate of 10 mV s−1 using 1 M H2 SO4 as electrolyte are shown in Fig. 15.4d. The area under the CV curves clearly depicts the higher capacitance for the ternary composite compared to the pure PANI and CNTs. The specific capacitance obtained from the CV curves of the ternary composite, PANI, and CNTs is 390, 360 and 18 F g−1 , respectively, at the scan rate of 10 mV s−1 . The maximum specific capacitance obtained for the ternary composite is 525 F g−1 at a scan rate of 1 mV s−1 . Figure 15.4e shows the electrochemical stability of the PANI/CNTs/TiO2 ternary composite by continuous GCD measurements for 6000 cycles at a current density of 4.0 A g−1 . The composite displays 67% retention of its initial capacitance, which is of high electrochemical stability for a pseudocapacitive material.

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Ternary coaxial structures of multiwalled carbon nanotube (MWCNT)/PANI/MnO2 are also designed through wet chemical methods for the supercapacitor electrode [38]. The composite is fabricated in two steps. In the first step, MWCNT/PANI binary coaxial structures are prepared using chemical polymerization of PANI in the presence of MWCNT. In the second step, ternary composite is formed by attaching MnO2 nanoflakes in a chemical oxidation process. The composite shows PANI coating on MWCNT followed by the growth of MnO2 nanoflakes. HRTEM image in Fig. 15.5a confirms the coaxial structure of ternary composite, where three distinct regions (I, II, and III) are prominent representing the MWCNT, PANI layer, and MnO2 nanoflakes, respectively. The porous surface morphology of the electrode material prepared from this ternary composite is shown in the SEM micrograph (Fig. 15.5b). This electrode provides a large interaction area between the MnO2 nanoflakes and electrolyte, which improves the electrochemical performance and decreases the contact resistance between MnO2 and PANI layercoated MWCNTs. Therefore, the electrode delivers a high specific capacitance of 330 F g−1 and a volumetric capacitance of 296 F cm−3 . The electrode material

Fig. 15.5 Morphology and electrochemical properties of coaxial MWCNT/PANI/MnO2 ternary composite: a HRTEM and b SEM; c CV curves and d Nyquist plots at 1st, 100th, and 1000th cycles at a scan rate of 20 mV s−1 (redrawn and reprinted with permission from [38])

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also shows high cyclic stability tested in a CV at a scan rate of 20 mV s−1 for 1000 cycles, as shown in Fig. 15.5c. The ternary composite electrode retains a high specific capacitance, and up to 77% of its initial value has been retained after 1000 cycles. EIS measurement also shows high conductivity of the active material due to the large interconnected area between MnO2 nanoflakes and conducting PANI coating on the surface of MWCNTs. Hence, the electrode material displays a very low equivalent series resistance (7.95 ) in the Nyquist plot, shown in Fig. 15.5d. The value increases to 8.65  after 100th cycles, and it remains the same even after the 1000th cycles (Fig. 15.5d). Recently, graphene is most widely used as the electrode material for EDLCs because of its unique structure with outstanding intrinsic properties, mainly extraordinarily high electrical conductivity and large surface area. However, graphene’s tendency toward aggregation and restacking significantly affect the electrochemical performance by reducing the ion-accessible surfaces and limiting ion and electron transport due to narrower channels. Also, as it is an EDLC material, it has a limited specific capacitance value of ~550 F g−1 for a single sheet or when the entire surface area is fully utilized [39–41]. Therefore, the integration of MOs and CPs with graphene significantly enhances the electrochemical performance after the formation of ternary composites through effectively synergizing the properties of the components. Pan et al. have designed supercapacitor electrodes from ternary hierarchical nanocomposites of MnO2 -coated 1-D PANI nanowires with 2-D graphene sheets [42]. The formation of PANI@MnO2 /graphene ternary composite with FESEM morphology is schematically shown in Fig. 15.6. FE-SEM image of the ternary composite shows the hierarchical porous structure. The electrochemical performances of the electrodes derived from graphene, PANI, PANI/graphene, and PANI@MnO2 /graphene ternary composite are evaluated through CV and GCD analysis using 1 M Na2 SO4 electrolyte. GCD curves in Fig. 15.7a reveal that the ternary composite electrode exhibits the highest capacitance compared to the other electrodes and indicates excellent electrochemical performance. The specific capacitance of PANI@MnO2 /graphene ternary composite is found to be 875.2 F g−1 , which is much higher than that of graphene (148.1 F g−1 ), PANI (174.8 F g−1 ), and PANI/graphene (335.4 F g−1 ) at the same current density of 0.2 A g−1 . The specific capacitance values at different current densities are shown in Fig. 15.7b, indicating the robustness of the electrode materials. Further, the good stability of the electrode derived from the ternary composite is observed from the cycling stability test (Fig. 15.7c) using continuous GCD cycles at a current density of 4 A g−1 for 1000 cycles. Reduced graphene oxide (rGO)/MnO2 /PANI ternary composites have been also fabricated through in situ chemical oxidative polymerization of aniline on MnO2 decorated rGO sheets (rGO/MnO2 ), where rGO/MnO2 binary composite is prepared using rGO treated with KMnO4 in a water–ethylene glycol system followed by hydrothermal reaction [43]. The entire fabrication process with their morphologies is schematically shown in Fig. 15.8a. The TEM images (Fig. 15.8a) reveal that MnO2 nanoparticles are successfully deposited on the rGO sheet and PANI nanofibers are homogeneously and compactly coated on the sheet of rGO–MnO2 . SEM image

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Fig. 15.6 Schematic representation of the step-by-step formation of PANI@MnO2 /graphene ternary composite with their FE-SEM morphologies (scale bar 100 nm) (redrawn and reprinted with permission from [42])

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Fig. 15.7 Electrochemical performance of graphene, PANI, PANI/graphene, and PANI@MnO2 /graphene ternary composite electrodes, a GCD curves at a current density of 0.2 A g−1 , b specific capacitance versus current density plots, and c cyclic performance of PANI@MnO2 /graphene ternary composite electrode using GCD cycles at a current density of 4 A g− 1 (reprinted with permission from [42])

further confirms the homogeneous coating of PANI nanofibers on rGO and the formation of the porous structure of the ternary composite. Therefore, the ternary composite electrode exhibits much better electrochemical performance compared to binary and GO-based electrodes, as shown in CV (Fig. 15.8b) and GCD (Fig. 15.8c) analyses using 1.0 M aqueous H2 SO4 electrolyte. The area under the CV curves and discharge time in GCD curves reveal the highest supercapacitive performance of the ternary composite electrode among the three electrodes. The CV curves (Fig. 15.8b) at the scan rate of 20 mV s−1 clearly show that GO and rGO/MnO2 electrodes are almost ideally rectangular, which is of typical EDLC behavior. On the other hand, the CV curve at the same scan rate for composite electrode indicates the energy storage is composed of an EDLC and Faradaic pseudocapacitance (as a pair of redox peaks are observed in the range of −0.4 to 0.8 V), where the Faradaic pseudocapacitance is dominant. GCD curves (Fig. 15.8b) at a constant current density of 10 mA cm−2 also show the same results. The specific capacitance obtained from the GCD curves is 1.5, 54, and 395 F g−1 for GO, rGO/MnO2 , and rGO/MnO2 /PANI, respectively. Further, the solely PANI-based electrode materials commonly suffer from poor cycling stability during the charge–discharge process since the redox

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Fig. 15.8 a Schematic representation for the step-by-step formation of rGO/MnO2 /PANI ternary composite with their morphologies, and comparative b CV and c GCD curves of GO, rGO/MnO2 , and rGO/MnO2 /PANI electrodes (redrawn and reprinted with permission from [43])

sites of the polymer backbone are not sufficiently stable and undergo swelling or volume shrinkage during the electrochemical process [44]. However, the formation of rGO/MnO2 /PANI ternary composite electrode largely improves the cyclic stability, and 92% of its initial specific capacitance is retained after 1200 continuous CV scans at a high scan rate of 100 mV s−1 . A terribly poor cyclic stability is observed for the PANI electrode, where only 31% of its initial capacitance has been retained after 1200 continuous CV cycles at the same scan rate. Other MOs like TiO2 [45], SnO2 [46], MoO3 [47], Fe2 O3 [48], etc., are also utilized in PANI- and graphene-based ternary composites for high-performance supercapacitor electrodes. The well-designed structures of these ternary nanocomposites with a good combination and synergistic effect among the components provide high electrochemical performance. Graphene sheets act as the conducting frameworks for sustaining PANI and MOs to separate and disperse them well in the ternary composites [48]. Graphene sheets also enhance stability by restricting the volume changes of the composites during electrochemical performance [46, 49]. The formation of PANI on the surface of graphene/MOs enhances the surface area and conductivity,

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and restricts the dissolution or aggregation of MOs inside the composites [48]. On the other hand, MOs enhance the specific capacitance and rate stability of the ternary composites.

15.8.2 TMO–Carbon–PPy Ternary Composites PPy is another important CP used in supercapacitor electrodes due to the high capacitance per unit volume (400–500 F cm−3 ) since PPy possesses greater density and offers a better degree of flexibility in electrochemical processing compared to the other CPs [50, 51]. Therefore, PPy is also significantly used in TMO/carbon/CP ternary composites to develop high-performance supercapacitor electrodes. Lim et al. have prepared a PPy/graphene/manganese oxide (MnOx ) ternary nanocomposite using potentiostatic polymerization, where at first graphene oxide is electrodeposited with a pyrrole monomer to form a PPy/graphene nanocomposite film followed by direct growth of MnOx particles along the PPy and conducting graphene potentiostatically by the addition of MnSO4 in the deposition solution [52]. The morphological study shows that PPy exhibits a typical granule-like surface (Fig. 15.9a). The PPy/graphene binary composite shows a condensed network structure of fiber-like morphology with no clear distinction of a single component (Fig. 15.9b). However, the formation of PPy/graphene/MnOx ternary composite achieves a porous 3-D structure (Fig. 15.9c, d), as MnOx acts as a spacer material to prevent the restacking of graphene sheets. This 3-D porous network structure of the ternary composite facilitates electrolyte penetration to achieve high electrochemical performance, which is reflected from the CV and GCD analyses. Figure 15.9e represents the CV curves of PPy, PPy/graphene, and PPy/graphene/MnOx ternary composite electrodes at a scan rate of 1 mV s−1 . The ternary composite electrode exhibits a high output current in the CV curve compared to the PPy and PPy/graphene electrodes, which indicates higher charge storage in the ternary nanocomposite. A similar trend is found from the GCD curves as shown in Fig. 15.9f. The specific capacitance values calculated from the CV curves are found to be 320.6, 255.1, and 118.4 F g−1 for the PPy/graphene/MnOx , PPy/graphene, and PPy electrodes, respectively. The ternary composite electrode also shows high cyclic stability with only 7% decay of its initial capacitance after 1000 continuous GCD cycles. Wang et al. have rationally synthesized hierarchical MnO2 /PPy@carbon nanofiber (CNF) triaxial nano-cables by nanocoating of MnO2 /PPy on CNF through in situ interfacial redox reaction, as schematically shown in Fig. 15.10a [53]. TEM image (Fig. 15.10b) shows a MnO2 /PPy nano-shell of ~20 nm deposited on the surface of the CNF, where MnO2 particles are embedded at the molecular-level dispersion into PPy since MnO2 particles are embedded in the PPy matrix with no distinct boundary between them. The high magnification SEM image reveals a conformal coating of MnO2 /PPy deposits on the CNFs, constructing a hierarchical triaxial configuration. The electrochemical performance of the free-standing electrodes of MnO2 /PPy@CNF, MnO2 @CNF, and PPy@CNF is measured through CV, GCD, and

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Fig. 15.9 FE-SEM images of a PPy, b PPy/graphene, c and d PPy/graphene/MnOx ternary composite (d is cross-sectional FE-SEM image); e CV and f GCD curves of PPy, PPy/graphene, PPy/graphene/MnOx electrodes (reprinted with permission from [52])

EIS analyses using 2 M KCl as the mild electrolyte. The comparative CV curves of the electrodes at a scan rate of 2 mV s−1 are shown in Fig. 15.10c, which displays good rectangular and symmetric shapes that demonstrate fast reversible Faradaic reactions and ideal capacitive behavior. The leveled current densities are observed from the CV curves, and the MnO2 /PPy@CNF electrode possesses much higher than those of the other two, which represent better supercapacitive performance of the ternary composite electrode. Therefore, the specific capacitance of the ternary composite electrode obtained from CV curves at the scan rate of 2 mV s−1 is 705 F g−1 . In contrast, MnO2 @CNF and PPy@CNF electrodes display the values of 469 and 458 F g−1 , respectively, at the same scan rate. The plots of specific capacitance versus scan rates are shown in Fig. 15.10d, where the specific capacitance values are gradually decreased with an increase in scan rate due to the small diffusion time as the electrolyte ions are not fully accessible to the interior surfaces of the active materials for charge storage at a high scan rate. However, the specific capacitance of the ternary

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Fig. 15.10 a Schematic illustration of the fabrication of MnO2 /PPy@CNF ternary composites, b TEM image of the ternary composite, comparative c CV, d rate capability, e EIS, and f cyclic stability test for MnO2 /PPy@CNF, MnO2 @CNF, and PPy@CNF electrodes (reprinted with permission from [53])

composite electrode at a high scan rate of 100 mV s−1 remains 376 F g−1 , while those for MnO2 @CNF and PPy@CNF electrodes decrease promptly to 82 and 178 F g−1 . The high specific capacitance with superior rate capability of the ternary composite compared to those of binary composites is because of the coating of MnO2 /PPy nanohybrid on the CNFs. The coating offers a strong synergistic effect among the

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components. Again, the simultaneous formation of conducting PPy and MnO2 leads to mutual combination on the nanoscale level, and the incorporation of MnO2 in the PPy matrix also minimizes electron shuttling along the PPy, which contributes to the overall conductivity [38, 54–56]. This results in effective electron transport to the CNF cores and offers a continuous conductive network for fast charge storage and transfer throughout the electrode. Further, the MnO2 /PPy nano-shell with an intimate electrical connection to the CNF provides a much shorter pathway to diffuse and transport ions and electrons. The interwoven porous structure also substantially reduces the dead volume through effective access of the electrolyte ions. The Nyquist plots in Fig. 15.10e of the EIS spectra also show a smaller internal resistance (3.5 ) and charge-transfer resistance (2.7 ) for the ternary composites compared to the MnO2 @CNF (4.5/5.4 ) and PPy@CNF (3.7/4.4 ). The more vertical shape of the plot at low frequencies for the ternary composite again represents better capacitive behavior with lower diffusion resistance. The symmetrical features of GCD curves further suggest good capacitive behavior and the curves follow the same trend with CV curves. The MnO2 /PPy@CNF, MnO2 @CNF, and PPy@CNF electrodes display specific capacitance values of 698, 453, and 437 F g−1 , respectively, from GCD analysis at a current density of 1 A g−1 . Furthermore, the ternary composite electrode exhibits high cyclic stability by retaining 91% of its initial capacitance after 2000 continuous GCD cycles at a current density of 12 mA cm−2 . However, MnO2 @CNF and PPy@CNF electrodes show higher (14 and 22%, respectively) decay of initial capacitance at the same test (Fig. 15.10f) due to the dissolution of the active materials and mechanical faults of the electrodes. Again, in the case of ternary composite, PPy stabilizes the MnO2 particles and keeps them mechanically adhered to the CNFs. This strong structural integrity prevents the dissolution and agglomeration of the active material during the cycling tests. The morphology of the ternary composite after the cycling tests is well maintained as the hierarchical triaxial structure (shown in the inset of Fig. 15.10f). Han et al. have also prepared sandwich-structured hierarchical MnO2 /PPy/rGO nano-sheet ternary composite through co-assembly of MnO2 /GO and PPy/GO into layer-by-layer architecture followed by reduction of GO as schematically illustrated in Fig. 15.11a [57]. The ternary composite with layered structure is formed by assembling two kinds of surface-deposited rGO nano-sheets to form the sandwiched structure. The surface morphology of the ternary composite is shown in Fig. 15.11b, where MnO2 and PPy particles are deposited on the surface of the rGO sheets. The layered structure of the composite is shown in cross-sectional SEM images (Fig. 15.11c). The crinkled rGO nano-sheets are well exfoliated by the deposited MnO2 and PPy particles, which helps to enhance the electrical conductivity of the composites and also prevents MnO2 particles from the aggregation and electrical dissolution. The electrochemical performance through CV and GCD measurements of the ternary composite, single components, or the binary composite electrodes are shown in Fig. 15.11d, e, respectively. Similar to the other ternary composite electrodes, here also MnO2 /PPy/rGO shows better supercapacitive performance compared to the single components or binary composite electrodes due to the synergetic contribution of both conducting materials and the conductive network of the sandwiched

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Fig. 15.11 a Schematic illustration for the preparation, and b SEM and c cross-sectional SEM images of MnO2 /PPy/rGO ternary composite; comparative d CV and e GCD curves of the ternary composite and its component electrodes (redrawn and reprinted with permission from [57])

structure. The ternary composite electrode delivers a high specific capacitance of 404 F g−1 at a current density of 0.25 A g−1 and also retains 91% of the initial capacitance after 5000 continuous GCD cycles at a current density of 4 A g−1 . Similarly, SnO2 [58], Fe2 O3 [59], ZrO2 [60], etc., MOs are also utilized with PPy and rGo to design the ternary composites for the development of high-performance supercapacitor electrodes. Alves et al. have revealed that the maximum capacitance can be achieved for PPy or its binary composite with rGO about 213.7 and 171.1 F g−1 at a scan rate of 5 mV s−1 [60]. These values increase to 341 F g−1 by the incorporation of ZrO2 nanoparticles in the PPy/rGO binary composite system. The presence of ZrO2 nanoparticles into the ternary composite not only enhances the

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pseudocapacitance in the system but also helps to prevent rGO nano-sheets from restacking, which consequently improves the electrolyte access to the active sites of the electrodes [61–64].

15.8.3 TMO–Carbon–PT Ternary Composites Further, PT is another important CP that is highly promising pseudocapacitive materials for high energy and power density supercapacitor applications [65–67]. Among the different PT derivatives, PEDOT has grown rapidly due to the extremely desirable properties such as it is electron-rich and consequently has a low oxidation potential [68], together with a wide potential window (1.2–1.5 V) and high capacitance value [69–71]. It also has a low band gap of 1–3 eV and is highly conducting in the p-doped form (300–500 S cm−1 ) with good thermal and chemical stability, and high charge mobility, which results in fast electrochemical kinetics [72]. Therefore, PEDOT in composite materials effectively improves the conductivity and electrochemical performance, and also acts as a protective layer to prevent the architectures from destruction or degradation [66]. Additionally, in ternary composites PEDOT acts as an excellent dispersant material to stabilize the composite suspension, and to facilitate the design of electrode film, PEDOT also offers good interparticle connectivity between the MOs and carbon-based EDLC [73–75]. Therefore, PEDOT is mainly utilized in ternary composites to bridge the carbon materials and MOs to improve the electrical conductivity and significantly enhance the electrochemical performance of the composite electrodes [74]. Hou et al. have designed a MnO2 /CNT/PEDOT–PSS ternary composite film to achieve high electrochemical performance for supercapacitor electrodes [73]. In this approach at first MnO2 , nanospheres are directly grown on functionalized MWCNTs followed by the addition of PEDOT–PSS dispersant to stabilize the composite suspension. PEDOT–PSS facilitates the fabrication of the electrode film and offers good interparticle connectivity between MnO2 and CNTs. The fabrication of the ternary composite is schematically shown in Fig. 15.12a. The morphology of the functionalized CNTs and MnO2 /CNT hierarchical network are shown in Fig. 15.12b, c. Unique hierarchical architecture with strong interparticle interaction of the ternary composite is shown in the TEM image (Fig. 15.12d). The image also shows that the MnO2 nanospheres are grown uniformly like a “crumpled paper ball.” SEM micrograph in Fig. 15.12e of the ternary composite further reveals that MnO2 shows “wormhole-like” architecture of lots of small pores (size 2–50 nm). These mesopores provide high surface areas, which enable effective electrolyte transport and active-site accessibility. The MnO2 nanospheres are also intertwined with both highly conductive CNTs and PEDOT–PSS, which facilitates efficient electron transport. The electrochemical performance of the electrodes is evaluated through CV and GCD analyses using 1 M Na2 SO4 as the electrolyte as shown in Fig. 15.13. Comparative CV curves (Fig. 15.13a) at a scan rate of 5 mV s−1 reveal that severely distorted shape is observed for pure MnO2 film due to poor intrinsic electrical conductivity. However, MnO2 /PEDOT–PSS binary composite displays an

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Fig. 15.12 a Schematic illustration for the fabrication of MnO2 /CNT/PEDOT–PSS ternary composite; TEM images of b functionalized CNT, c MnO2 /CNT and d MnO2 /CNT/PEDOT–PSS; e SEM image of the ternary composite (redrawn and reprinted with permission from [73])

improved current compared to pure MnO2 electrode at the same scan rate, indicating conductive PEDOT–PSS assists the electron transport in the electrode. However, the CV curve of the ternary composite film shows the nearly rectangular shape and displays a much higher current value compared to pure MnO2 and MnO2 /PEDOT– PSS electrodes. The comparative GCD curves (Fig. 15.13b) at a current density of 5 mA cm−2 also follow the same trend. The ternary composite displays ideal capacitive behavior with very sharp responses and small internal resistance (IR) drop. It achieves a high specific capacitance of 200 F g−1 at the current density of 5 mA cm−2 , whereas MnO2 and MnO2 /PEDOT–PSS exhibit lower specific capacitance of 129 and 132 F g−1 , respectively. The ternary composite also shows high rate capability

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Fig. 15.13 Comparative: a CV curves, b GCD curves, and c plots of rate capability for MnO2 , MnO2 /PEDOT–PSS, and MnO2 /CNT/PEDOT–PSS electrodes; d cyclic stability of MnO2 /CNT/PEDOT–PSS ternary composite electrode (redrawn and reprinted with permission from [73])

as it preserved 85% of its specific capacitance (from 200 to 168 F g−1 ) when the current density increases from 5 to 25 mA cm−2 , as shown in Fig. 15.13c. However, the specific capacitance of MnO2 (129 F g−1 ) and MnO2 /PEDOT–PSS (132 F g−1 ) is sharply decreased to 20–37 F g−1 , respectively, as the current density increases from 5 to 25 mA cm−2 . The rate capability of the ternary composite improves due to the short diffusion path of the ions, high surface area, and increased electrical conductivity through the synergetic contribution of each component. The high surface area and porous network structure also allow a higher rate of solution infiltration and facilitate the ion insertion/extraction and electron transport in the electrode film. On the other hand, the severer aggregation, lower conductivity, and poor mechanical stability of MnO2 and MnO2 /PEDOT–PSS composite increase the ion diffusion and electron transport resistance, which compromise their electrochemical performances [73]. The cyclic stability test of the ternary composite film further suggests the synergetic interaction among the components, which significantly improves the electrical properties and mechanical stability of the electrode. The electrode displays less than 1% decay of its initial specific capacitance after 1000 GCD cycles as shown in Fig. 15.13d. Lv et al. have designed 3-D porous ternary composite electrodes of carbon fabricaligned CNT (CF-ACNT)/MnO2 /PEDOT through electrodeposition of MnO2 and PEDOT on CF-ACNT [76]. The fabrication process of the ternary composites is shown schematically in Fig. 15.14a, where at first CF-ACNT hybrids are prepared by chemical vapor deposition (CVD) method followed by electrochemical deposition of MnO2 and PEDOT on CF-ACNT hybrids. The morphological analyses

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Fig. 15.14 a Schematic illustration for the fabrication of 3-D porous CF-ACNT/MnO2 /PEDOT ternary composite and its b SEM and c TEM micrographs (redrawn and reprinted with permission from [76])

observed in Fig. 15.14b, c for the ternary composite reveal that MnO2 petallike nano-sheets are deposited on the CF-ACNT surface and PEDOT is uniformly encapsulated and interconnected with MnO2 nano-sheets and CF-ACNTs. Therefore, the CF-ACNT/MnO2 /PEDOT electrode delivers high electrochemical performance due to the synergetic effect of each component. The major responsible factors boosting the performance are unique porous 3-D structures, the interconnectivity of CF-ACNTs, ultrathin MnO2 nano-sheets, and improvement in conductivity due to PEDOT. The high areal capacitance of 1.3 F cm−2 at a scan rate of 0.1 mV s−1 with an excellent rate capability has been observed. The electrode also retains over 95% of its initial charge after 1000 cycles. Further, Yan et al. have prepared a MnO2 /rGO/PEDOT–PSS ternary composite by two-step processes, where the MnO2 /rGO binary nanohybrids are obtained by hydrothermal reaction followed by dispersing them in PEDOT–PSS solution in water using ultrasonication [77]. The electrode designed from this ternary composite material exhibits a high specific capacitance of 169.1 F g−1 at a current density of 0.2 A g−1 , with good cycle stability of 66.2% of capacitance retention after 2000 GCD cycles at a current density of 2 A g−1 . Wang et al. have also designed a graphene/SnO2 /PEDOT ternary composite for the supercapacitor electrode using chemical polymerization of EDOT in the presence of graphene/SnO2 composite [78]. The well-designed ternary

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nanostructure of graphene/SnO2 /PEDOT displays excellent electrochemical performance with enhanced capacitance and energy density in both acidic and neutral electrolytes, compared to the binary composites. The electrode shows maximum specific capacitance values of 184 and 180 F g−1 in 1 M H2 SO4 and 1 M Na2 SO4 electrolytes, respectively, obtained from CV analysis at a scan rate of 1 mV s−1 . The electrode delivers an energy density of 22 Wh kg−1 at a power density of 238.3 W kg−1 and retains 17.1 Wh kg−1 at a high power density of 5803.3 W kg−1 in acidic electrolyte. While using a neutral electrolyte, the electrode achieves an energy density of 23.4 Wh kg−1 at a power density of 253 W kg−1 and maintains up to 10.2 Wh kg−1 at a high power density of 3684 W kg−1 . More interestingly, the electrode displays outstanding cyclic stability in an acidic electrolyte by retaining the specific capacitance of ~100% after 5000 GCD cycles at a current density of 1 A g−1 . It also shows 70% cyclic stability in the neutral electrolyte using the same test.

15.9 Ternary Composite-Based Supercapacitor Device Construction of a supercapacitor device is the most important part of its practical application as an energy storage device in different electronics. Fabrication of ternary composites using CPs has many advantages to construct supercapacitor devices due to flexibility, high conductivity, and easy processability to form solid films [10]. Usually, supercapacitor devices are fabricated using either two symmetric or asymmetric electrodes separating through a separator soaked with electrolyte solution followed by sandwiching them with two conductive current collectors [79–82]. Symmetric assembly implies that both the cathode and anode electrodes are made of the same active materials; i.e., both are TMO–carbon–CP ternary composite electrodes, yielding a total capacitance of half the value of every single electrode [79, 80]. On the other hand, the asymmetric supercapacitor devices are designed by assembling two electrodes made of TMO–carbon–CP ternary composites (cathodes or positive electrodes) and carbon-based EDLC materials (anodes or negative electrodes) [81, 82]. The asymmetric or hybrid supercapacitors offer high operation potential window due to the use of two different active materials with different potential windows and also achieve high energy and power densities. The schematic illustrations for the fabrication of symmetric and asymmetric devices are shown in Fig. 15.15. Wang et al. have designed a symmetric supercapacitor using nitrogen-doped graphene/NiFe2 O4 /PANI ternary nanocomposite electrodes, which deliver a high energy density of 23.2 Wh kg−1 at a power density of 27.7 W kg−1 and retain the energy density up to 9.6 Wh kg−1 at a higher power density of 1445.8 W kg−1 in 1 M aqueous KOH electrolyte [83]. The supercapacitor also exhibits excellent cycling stability by retaining its initials specific capacitance approximately 90% after 10,000 GCD cycles at a high current density 5 A g−1 . Again, Sivakumar et al. have fabricated a symmetric supercapacitor cell by assembling two pieces of CNT/PPy/MnO2 ternary composite electrodes [84]. The assembled device shows a specific capacitance of 149 F g−1 at a current loading of 1.0 mA cm−2 [84, 85]. On the other hand,

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Fig. 15.15 Schematic representations for the fabrication of a symmetric [81] and b asymmetric supercapacitor devices using MO–carbon–CP ternary composites [82] (redrawn and reprinted with permission from [81, 82])

Sankar and Selvan have designed a hybrid or asymmetric supercapacitor device using MnFe2 O4 /graphene/PANI ternary composite and activated carbon (AC) electrodes [86]. The hybrid supercapacitor shows an ideal capacitor behavior with good electrochemical reversibility, as observed from CV and GCD analyses (Fig. 15.16a, b) at a potential window of 1.6 V. It displays a specific capacitance of 40.33 F g−1 at a scan rate of 5 mV s−1 and also maintains the rectangular shape at a higher scan rate of 100 mV s−1 . The GCD analysis shows symmetrical charge and discharge behavior with no IR or potential drop at initial discharge time [86, 87]. It provides a specific capacitance of 48.5 F g−1 at a current density of 0.5 mA cm−2 . The fabricated hybrid supercapacitor device delivers the maximum energy density of 17 Wh kg−1 , as obtained from the Ragone plot in Fig. 15.16c. Also, the hybrid supercapacitor exhibits excellent cyclic stability and retains 100% of initial specific capacitance after 5000 continuous GCD cycles at a current density of 5 mA cm−2 as shown in Fig. 15.16d. This excellent electrochemical performance of the device is attributed to the high surface area, low internal resistance, reduction of diffusion path length and contact resistance between the ternary composite electrode and electrolyte, large active sites, high diffusive coefficient, lower diffusive resistance, and synergistic effect among the individual constituents of the hybrid supercapacitor.

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Fig. 15.16 a CV curves, b GCD curves, c Ragone plot (inset: schematic of device structure), and d cyclic stability of the hybrid supercapacitor device using MnFe2 O4 /graphene/PANI ternary composite and AC electrodes (reprinted with permission from [86])

Fig. 15.17 Schematic representation for the synthesis of MWCNT/Ni(OH)2 /PEDOT–PSS ternary composite (reprinted with permission from [88])

Similarly, Jiang et al. have designed a coaxial ternary hybrid material comprising amorphous Ni(OH)2 deposited on MWCNT wrapped with PEDOT– PSS, schematically represented in Fig. 15.17 to fabricate an asymmetric supercapacitor device [88]. The asymmetric supercapacitor device is fabricated using the MWCNT/Ni(OH)2 /PEDOT–PSS as the positive electrode and rGO/CNT hybrid as the negative electrode, and these are coated on two pieces of carbon cloth current

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collector followed by sandwiching with a membrane separator in 1 M aqueous KOH electrolyte. The device shows excellent specific capacitance of 159.4 F g−1 at a CV scan rate of 5 mV s−1 and 179.8 F g−1 at a GCD current density of 1 A g−1 . This demonstrates a high rate capability and retains the specific capacitance values up to 102.3 F g−1 when the CV scan rate increases to 150 mV s−1 . Again, the specific capacitance is retained up to 9.6 F g−1 when current density increases to 20 A g−1 . Figure 15.18a shows that the supercapacitor device also delivers a high specific energy density of 58.5 Wh kg−1 at a power density of 780 W kg−1 and retains to

Fig. 15.18 a Ragone plot of the supercapacitor device (inset: photographs of two asymmetric supercapacitors connected in series to light up two LED indicators) and b cyclic stability of the asymmetric supercapacitor up to 30,000 GCD cycles at a current density of 10 A g−1 with a potential range of 0–1.5 V (insets: first and last ten GCD profiles during cyclic stability test) (reprinted with permission from [88])

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30.8 Wh kg−1 at a high power density of 11.5 kW kg−1 . For practical use as an efficient power source, a demonstration is shown in the inset of Fig. 15.18a, where two supercapacitors are connected in series to light up two light-emitting diode (LED) indicators of green (3.0–3.2 V, 20 mA) and red (2.0–2.2 V, 20 mA) after charging about 9 min to 3 V in 8 s at the current density of 20 A g−1 . Further, the supercapacitor device displays excellent cyclic stability, as shown in Fig. 15.18b. The device retains about 90% of its initial specific capacitance after 5000 GCD cycles at a high current density of 10 A g−1 and even maintains 86% after 30,000 GCD cycles. Insets of Fig. 15.18b show the initial ten cycles and the last ten cycles of the GCD profiles during cyclic stability test, which display the minor changes in GCD curves after 30,000 cycles. The composites are using carbon nanotubes [23, 56, 85, 89], carbon nanofiber [24], activated carbon [24, 75, 87, 90], doped transition metal oxides [21, 91], graphene [25], etc., as the electrodes [92] for supercapacitor devices are quite promising due to ease of fabrication, high flexibility, and supercapacitive performance. Therefore, these supercapacitors are getting used in several applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, and aerospace, etc [3, 5, 93]. The supercapacitive performance with fabrication techniques and morphologies of these ternary composites are given in Table 15.7.

15.10 Concluding Remarks TMO–carbon–CP ternary composites represent the promising materials for supercapacitor electrodes, as the single components or binary composites always cannot satisfy the requirement of the high-performance supercapacitor. Therefore, many approaches have been employed to fabricate ternary composites by combining all the three types of electroactive materials for high-performance supercapacitor application. The formation of ternary composites using three types of capacitive materials composed of a high surface area carbon, a strong pseudocapacitance metal oxide, and elastic, pseudocapacitive CP offers good electrical conductivity, high specific surface area, better electrochemical accessibility of the redox sites, and better processibility to manufacture solid films by utilizing the advantages of each of the components. The reported ternary composites are fabricated by introducing CP as the third component to the binary components of TMO and carbon-based composites. PANI, PPy, and PEDOT are the most used CP for the supercapacitor application. This chapter represents a comprehensive account of fabrication, characterization, and electrochemical performance of TMO–carbon–CP ternary composites by PANI, PPy, and PEDOT and their supercapacitor performance. The designing of these ternary composites by choosing proper combination induces synergistic effects among the components generating high electrochemical performance. Here, carbon-based EDLC materials act as the conducting substrate for sustaining PCs and TMOs to separate and disperse them well in the ternary composites. They also enhance stability by restricting the

Fabrication technique

Sonochemical–hydrothermal and in situ polymerization

Chemical oxidation and polymerization

In situ chemical polymerization

Wet chemical

In situ polymerization and chemical deposition

In situ chemical oxidative polymerization

Potentiostatic polymerization

In situ interfacial redox reaction

Co-assembling and reduction

Deposition

Electrodeposition

Hydrothermal and ultrasonication

Chemical polymerization

Ternary composite

TiO2 /activated carbon/PANI

PANI/mesoporous carbon/MnO2

PANI/CNTs/TiO2

MWCNT/PANI/MnO2

PANI@MnO2 /graphene

rGO/MnO2 /PANI

PPy/graphene/MnOx

MnO2 /PPy@CNF

MnO2 /PPy/rGO

MnO2 /CNT/PEDOT–PSS

CF-aligned CNT/MnO2 /PEDOT

MnO2 /rGO/PEDOT–PSS

Graphene/SnO2 /PEDOT

330

525

695

286

Capacitance (F g−1 )

Nanoparticles-deposited sheets

Nano-sheet array

3-D porous

Nanospheres-deposited nanotube

Sandwich nano-sheets

Hierarchical triaxial nano-cables

3-D porous network

Particles deposited on sheets and fibers

184

169.1

1.3 F cm−2

200

404

705

320.6

395

Hierarchical nanowires with sheets 875.2

Nanoflakes on nanotubes

Nanofibrous

Mesoporous nano-layer

Porous network layer

Morphology

~100% after 5000 cycles

66.2% after 2000 cycles

95% after 1000 cycles

99% after 1000 cycles

91% after 5000 cycles

91% after 2000 cycles

93% after 1000 cycles

92% after 1200 cycles

~100% after 1000 cycles

77% after 1000 cycles

67% after 6000 cycles

88% after 1000 cycles

80.4% after 2000 cycles

Cycle life

[78]

[77]

[76]

[73]

[57]

[53]

[52]

[43]

[42]

[38]

[36]

[33]

[32]

Ref.

Table 15.7 Supercapacitive performance of different reported MO–carbon–CP-based ternary composites with fabrication techniques and morphologies

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volume changes of the composites during the electrochemical performance. The formation of CPs with carbon/TMO binary composite enhances the surface area and conductivity, and restricts the dissolution or aggregation of TMOs inside the composites. They also act as an excellent dispersant material to stabilize the composite suspension and facilitate the designing of the electrode films. This, in turn, offers good interparticle connectivity between the TMOs and carbon-based EDLC. On the other hand, TMOs enhance the specific capacitance, energy density, and the rate stability of the ternary composites. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)), for carrying out this research work.

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Chapter 16

Recent Trends in Supercapacitor Electrode Materials and Devices Prerna Sinha, Bibekananda De, Soma Banerjee, Kapil Dev Verma, Tanvi Pal, P. K. Manna, and Kamal K. Kar

Abstract Supercapacitors are the most promising energy storage devices that bridge the gap between capacitors and batteries. They can reach energy density close to the batteries, and power density close to the conventional capacitors. Several types of researches have been carried out in the field of supercapacitors for the development of promising electrodes, electrolytes, separators, current collectors as well as device fabrications to get a breakthrough in energy storage systems with diverse applications. This chapter provides the trend of supercapacitor electrodes made of carbon nanofibers, carbon nanotubes, graphene/reduced graphene oxide, activated carbon, transition metal oxides, conducting polymers and their composites

P. Sinha · S. Banerjee · K. D. Verma · T. Pal · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] P. Sinha e-mail: [email protected] S. Banerjee e-mail: [email protected] K. D. Verma e-mail: [email protected] T. Pal e-mail: [email protected] B. De · K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] T. Pal A.P.J. Abdul, Kalam Technical University, Lucknow 226031, India P. K. Manna Indus Institute of Technology and Management, Kanpur 209202, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_16

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concerning specific capacitance, and cycle life. The design and flexibility of electrode material has promoted the supercapacitor to design bendable, lightweight, miniaturized (micro), planar, flow, shape memory, piezoelectric, self-healing, and multifunctional energy storage devices, which suggest the near future demand of supercapacitor over a wide range of applications. The chapter is also ended with several concluding marks.

16.1 Introduction The supercapacitor is a promising energy storage device that bridges the gap between capacitors and batteries. Its energy density is close to the batteries, and power density is comparable to the conventional capacitors. The characteristics of capacitors [1] and features to understand the fact of the capacitor vs supercapacitor [2] are reported elsewhere. Several types of researches have been carried out in the field of supercapacitor for the development of promising electrodes, electrolytes, separators, and current collectors, as well as device fabrications, to get the breakthrough in energy storage systems with diverse applications such as different portable electronics, memory backup systems as well as hybrid electric vehicles, industrial-scale power and energy management, and so on [3].

16.2 Carbon Nanofibers as Supercapacitor Electrode Materials The electrochemical properties of supercapacitors are mainly controlled by the nature of electrode materials. The specific capacitance of the electric double-layer capacitor (EDLC) electrodes depends on the surface area of the electrodes and its electrical properties. Among the different carbon-based materials, carbon nanofiber (CNF) is an important 1D nanostructured material due to the high aspect ratio, surface area, good electrical conductivity, and electronic transport properties facilitating kinetics of the electrochemical reactions for supercapacitor applications. Carbon nanofibers (CNFs) are sp2 -based linear, noncontinuous filaments with graphene layers arranged as stacked cones, cups, or plates. The capacitance values of different carbon materials vary with the synthesis approach, architecture, or type of materials. Like, the capacitance value of CNFs changes with the preparative techniques due to the change in surface area and conductivity. The general characteristics of CNF are reported elsewhere [4]. It shows a promising electrode material for supercapacitors [5]. A trend of capacitance value along with the cycle life of different CNFs prepared by several approaches is shown in Fig. 16.1. The carbonization process of CNF increases conductivity. Therefore, high-temperature carbonization-based CNF displays a high specific capacitance of 240 F g−1 [6, 7]. However, the solution growth CNF using

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mesoporous metal nanotubes has shown a very high surface area that helps to reduce the path for electron transport and electrolyte penetration and increases the diffusion rate of the ions through the porous structure of CNF [8]. This type of CNFs is capable to achieve very high specific capacitance of 280 F g−1 . The cycle life of CNF-based electrodes in a supercapacitor is varied from 90 to 100%.

16.3 Carbon Nanotubes as Supercapacitor Electrode Materials Among the different carbon materials, carbon nanotubes (CNTs) are of particular interest for the supercapacitor electrodes due to the unique tubular porous structures and superior electrical conductivity, which favor fast ion and electron transportation. These are composed of carbon atoms linked in hexagonal shapes, with each carbon atom covalently bonded to three other carbon atoms. The specific features of CNTs are reported elsewhere [9]. Therefore, they are usually considered as the choice of the high-power electrode material. Similarly, the capacitance value of carbon nanotubes (CNTs) is also governed by types of materials, as shown in Fig. 16.2. In the case of CNTs, single-walled carbon nanotube (SWCNT) exhibits higher specific capacitance compared to multi-walled carbon nanotube (MWCNT) due to the higher surface area per unit mass of the material [10, 11]. Again, the specific capacitance of SWCNT varies with the architecture of the tube, e.g., assembled sheets of fiber type SWCNT show higher value compared to the film type as fiber has more accessible surface area [12]. Detailed electrochemical performance of CNT-based electrode materials for supercapacitors is reported elsewhere [13]. The SWCNT electrode having an

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ideal specific surface area (1300 m2 g−1 ) shows a high specific capacitance (160 F g−1 ) at an operating potential of 4 V. The supercapacitor maintains durable full charge–discharge cyclability with a high energy density (94 Wh kg−1 , 47 Wh L−1 ) and power density (210 kW kg−1 , 105 kW L−1 ). It also displays an impressive long cycle life as the capacitance degradation is limited to 2% over 10,000 cycles [14].

16.4 Graphene/Reduced Graphene Oxide as Supercapacitor Electrode Materials Graphene/reduced graphene oxide is an allotrope of carbon in the form of a twodimensional monolayer sheet of sp2 -hybridized carbon atoms arranged in a honeycombed network. It finds applications as electrode material for supercapacitor devices due to the tunable surface area, high electrical properties, mechanical, chemical, and thermal stability. The general characteristics of graphene/reduced graphene oxide are reported elsewhere [15]. This graphene-based electrode material is blessed with the excellent power density and long cycle stability. The provisions of different architectures of graphene further make it a promising material for energy storage devices. Graphene-based electrodes are of different types based on the nanoarchitectures, e.g., zero-, one-, two- and three-dimensional graphene-based supercapacitors. However, the zero- and one-dimensional graphene-based electrodes suffer from relatively low

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rate stability and power density as a result of stacking and aggregation tendencies of graphene material. This, in turn, reduces the performance of the device due to the minimization in effective surface area and diffusion of the electrolyte ions. The formation of three-dimensional porous networks is in a recent research trend with the target to achieve a high rate and power performance with a minimum compromise to its mechanical properties. Figure 16.3 displays the bar chart for specific capacitance and cycle life of different types of graphene-based materials. The detailed electrochemical performance of graphene/reduced graphene oxide as electrode material for supercapacitors is reported elsewhere [16]. The formation of 3D type hydrogel displays the highest specific capacitance due to the presence of interconnected porous structure, which facilitates the diffusion of ions into the pores and transport of electrons throughout the entire graphene framework [17, 18]. The hydrogel-based supercapacitor shows a high specific capacitance value of 441 F g−1 at a current density of 1 A g−1 , which is more than two times higher compared to that of nonfunctionalized hydrogel-based supercapacitor (211 F g−1 ) [18]. The high rate capability of this supercapacitor is also reported. Further, the functionalized hydrogel-based supercapacitor exhibits excellent electrochemical stability by retaining 86% of its initial capacitance after 10,000 GCD cycles at a high current density of 10 A g−1 [18].

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16.5 Activated Carbon as Supercapacitor Electrode Materials Activated carbon is another form of carbon processed to have unique properties, e.g., tunable porosity, lightweight, electrical conductivity, chemical inertness, etc., due to the wide range of morphologies. It is a disorganized form of graphite. Due to diverse properties, activated carbon has been widely used in many applications, such as environment, petrochemical, chemical, automobile, food industries, etc., besides being used as electrode materials. Its general characteristics are reported elsewhere [19]. This activated carbon remains one of the other exciting carbon material as electrodes for supercapacitor devices and much more reported elsewhere [20]. This activated carbon can store energy with high cycle life and power capability. The nanostructured activated carbon with heteroatom doping improves the supercapacitive performance via improved surface wettability during the process of charging and discharging. The preparation of activated carbon from biowastes has received the attention of the scientific community due to the possible generation of various morphologies and surface textures. Figure 16.4 shows the trend for specific capacitance and cycle life of activated carbon derived from various biomasses. The specific capacitance of human hair derived activated carbon is 340 F g−1 at a current density of 1 A g−1 , which is retained up to 128 F g−1 at even at a very high current density of 80 A g−1 [21].

Fig. 16.4 Bar chart for specific capacitance and cycle life of activated carbon-based electrodes derived from various biomasses (see Chap. 5 for references of numerical values of specific capacitance and cycle life) [20]

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16.6 Transition Metal Oxides as Supercapacitor Electrode Materials Nanostructured transition metal oxides (TMOs) are a fascinating and diverse set of compounds. They exhibit full range variation in terms of electrical, magnetic, thermal stability, and chemical bonding behaviors. For instance, the electrical properties vary from insulating to semiconducting to conducting and a shift from diamagnetism to paramagnetism to ferromagnetism is observed in magnetic properties. In addition, different types of chemical bonding including the ionic, covalent, and metallic are readily observed in different TMOs compounds. Also, the easy availability, cheap, and negligible environmental concerns further support the advancement of industrial applications. The general characteristics of TMOs are reported elsewhere [22]. The specific capacitance values of the pseudocapacitance materials like TMOs are controlled by the reversible redox reactions along with the surface area and conductivity. Therefore, their capacitances are varied with the nature as well as the architecture of the materials, as shown in Fig. 16.5. The supercapacitive performance of different TMOs is reported elsewhere [23]. Among the several TMOs, cobalt oxide (Co3 O4 ) is the most suitable and ideal electrode material for supercapacitors because of very high theoretical capacitance (3560 F g−1 ) [24]. The pseudocapacitance originates from very fast reversible redox reactions involving the exchange of protons 1936

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and/or cations. Again, the formations of 3D porous architectures of Co3 O4 electrode materials further help in excellent mass transport property and large surface area per unit volume. The Co3 O4 nanoflower exhibits very high specific capacitance as shown in Fig. 16.5 [25]. This type of structures facilitates the diffusion of electrolyte into the inner region and shortening the diffusion path length for the electrolyte ions [26].

16.7 Conducting Polymers as Supercapacitor Electrode Materials Carbon-based electrodes display high power however, the specific energy of these materials remains low. On contrary to this, the conducting polymer-based supercapacitors exhibit low self-discharge property and high energy density due to the possible redox reactions in the material. However, the major drawback remains a slow rate of diffusion of ions through the electrode material leading to low chargedischarge or in other words reduced power output. Despite this issue, conducting polymer-based supercapacitors have potential in the field of supercapacitor devices since they exhibit better kinetics in comparison to all other pseudocapacitor materials. The general features of conducting polymers (CPs) are reported elsewhere [27]. The low cyclic stability of the conducting polymers in comparison to the EDLC is due to doping and de-doping of counter ions leading to a massive change in the physical structure of the material. A bar diagram (Fig. 16.6] is represented to have a broad overview of specific capacitance and cycle life of different conducting polymers. But the supercapacitive performance of various CPs is reported elsewhere [28]. PPY Fig. 16.6 Bar chart for specific capacitance and cycle life of different types of conducting polymeric materials (see Chap. 13 for references of numerical values of specific capacitance and cycle life) [28]

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electrode shows a maximum specific capacitance of 480 ± 50 F g−1 at a scan rate of 10 mV s−1 [29]. Other PANI nanowhisker electrode shows a specific capacitance of 470 F g−1 [30]. But the PANI nanowire graphene-based electrode shows a very good performance, which is 555 F g−1 [31].

16.8 Transition Metal Oxides and Carbon Composites as Supercapacitor Electrode Materials However, as carbon-based materials suffer from low capacitance value, and pseudocapacitors exhibit low surface area and poor conductivity, the fabrications of composites of carbon-based materials and pseudocapacitors improve the overall performance of the electrode materials. Figure 16.7 shows the variation of specific capacitance with their cycle life for different types of composite electrode materials prepared from activated carbon and various TMOs. The supercapacitive performance of different composites made by activated carbon and TMOs are reported elsewhere [32]. Bi2 O3 shows the best results followed by TiO2 (Fig. 16.7). The composite electrode exhibits a specific capacitance of 332.6 F g−1 , which is much higher than bare AC having a capacitance of 106.5 F g−1 [33]. Apart from the enhancement of capacitance in a composite electrode, the percentage mass of Bi2 O3 is another important factor that governs the performance of the electrode. Here, 24.53% of Bi2 O3 loading in AC delivers the maximum capacitance [33].

Fig. 16.7 Bar chart for specific capacitance and cycle life of activated carbon-based composite electrodes with different types of TMOs (see Chap. 6 for references of numerical values of specific capacitance and cycle life) [32]

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Figure 16.8 shows the variation of specific capacitance with their cycle life for different types of composite electrode materials prepared from CNF and various TMOs. The detailed supercapacitive performance of CNF and TMOs composites are described elsewhere [34]. Ni- and Co-based TMOs have very high theoretical capacitance due to the fast reversible redox reactions. Therefore, composites of these materials with carbon display high capacitance. Again, the capacitance can be further improved by the formation of composites with high surface area nano architectures to generate 3D hierarchical structures. Recently, the composites of the mixed transition metal oxide of Ni- and Co-, nickel cobaltite (NiCo2 O4 ) deliver the highest specific capacitance with most of the carbon materials [35–38], since they have much better electronic conductivity (two orders of magnitude higher compared to nickel and cobalt oxides) and also offer better redox reactions. The electrode is made through anchoring ultrathin Ni(OH)2 nanoplatelets vertically and uniformly on the electrospun CNF of poly(amic acid) (PAA). It shows a high specific capacitance of 2523 F g−1 based on the mass of Ni(OH)2 (which is 701 F g−1 based on the total mass) at a scan rate of 5 mV s−1 with a long cycle life by retaining of 83% of its initial capacitance after 1000 cycles [37]. The significant improvement in the performance of the hybrid membranes is due to the synergistic effects of 1D carbon nanofibers and 2D Ni(OH)2 nanoplatelets. Whereas NiCo2 O4 -decorated CNF with PAN and lignin ratio of 80:20 wt% delivers the highest specific capacitance of 1757 F g−1 at a current density of 2 mA cm−2 with an excellent cyclability of ~138% capacitance retention after 5000 GCD cycles at a current density of 7 mA cm−2 [38]. Figure 16.9 shows the variation of specific capacitance with their cycle life for

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different types of composite electrode materials prepared from CNT and various TMOs. The detailed supercapacitive performance of CNF and TMOs composites are reported elsewhere [39]. NiCo-based CNT electrode delivers the best results followed by NiO/RuO2 /MnO2 (Fig. 16.9). The NiCo2 O4 -SWCNT nanocomposite material exhibits a very high specific capacitance of 1642 F g−1 with excellent cyclic stability. About 94.1% retention of capacitance after 2000 cycles at high mass loading has been evidenced [40]. Figure 16.10 shows the variation of specific capacitance with their cycle life for different types of composite electrode materials prepared from graphene/reduced graphene oxide and various TMOs. The selected supercapacitive performance of CNF and TMOs composites are reported elsewhere [41]. Nickel-cobalt-oxidegraphene shows the best performance followed by nickel-vanadium-oxide-graphene composite. The 3D nickel foam supported graphene and NiCo2 O4 electrode exhibit very high specific capacitance of 1950 F g−1 at a high current density of 7.5 A g−1 , owing to the rapid electron and ion transport, large electroactive surface area, and excellent structural stability [35]. The 3D composite electrodes possess excellent stability under repeated GCD cycles and retain 92.8% of its initial capacity after 10000 cycles at a current density of 3 A g−1 . The other Ni3 V2 O8 /NG electrode exhibits an excellent specific capacitance of ~1898 F g−1 at a current density of 1 A g−1 [42]. The device also shows an excellent cycling performance with high capacitance retention of ~83.3% after 20,000 consecutive GCD cycles.

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16.9 Conducting Polymers, Transition Metal Oxides, and Carbon Composites as Supercapacitor Electrode Materials On the other hand, for conducting polymers (CPs) based composites, the specific capacitance is mainly controlled by the formation of architectures, as CPs also contribute to the electrical conductivity and pseudocapacitance. Therefore, formation 3D hierarchical architectures of core-shell materials display the highest specific capacitance among the CP based composites [43–45]. The detailed supercapacitive performance of CPs and TMOs composites are reported elsewhere [46]. Figure 16.11 shows the variation of specific capacitance and cycle life of different CP-TMO composite electrodes. An electrode based on CoO and polypyrrole (PPY), where well-aligned CoO nanowire array has been grown on 3-D nickel foam and polypyrrole (PPY) uniformly immobilized onto or firmly anchored to each nanowire surface to boost the pseudocapacitive performance shows a high specific capacitance of 2223 F g−1 at a current density of 1 mA cm−2 with good rate capability and cycling stability (99.8% capacitance retention after 2000 cycles) [43]. For other composites, the specific capacitance varies from 305 to 826 F g−1 . Figure 16.12 shows the variation of specific capacitance and cycle life of different ternary composites made of CP, transition metal oxide, and carbon. The detailed supercapacitive performance of CPs, carbon, and TMOs composites are reported elsewhere [47]. Polyaniline (PANI)-manganese-oxide-graphene

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composite shows the best results followed by polypyrrole (PPY)-manganese-oxideCNF and PANI-manganese-oxide-carbon composites. The specific capacitance of PANI@MnO2 /graphene ternary composite is found to be 875.2 F g−1 , which is much higher than that of graphene (148.1 F g−1 ), PANI (174.8 F g−1 ) and PANI/graphene (335.4 F g−1 ) at the same current density of 0.2 A g−1 [48]. The PPYmanganese-oxide-CNF and PANI-manganese-oxide-carbon composite electrode show the capacitance of 705 and 695 F g−1 , respectively.

16.10 Recent Advancement in Supercapacitor Devices 16.10.1 Flexible Fiber-Shaped Devices Flexible fiber shaped micro-supercapacitor can be assembled in three types of configuration. This includes (a) two parallel fiber, each fiber representing electrode, (b) two twisted fiber, and (c) coaxial fiber [49, 50]. Parallel fiber is assembled by placing two or more fibers on a planar substrate as shown in Fig. 16.13a. Twisted fiber configuration is assembled by twisting two fiber electrodes on each other separated by solid-state electrolyte, depicted in Fig. 16.13b. For coaxial fiber, assembly is obtained by layer by layer coating of fiber electrode, solid-state electrolyte, and an outer electrode layer, as shown in Fig. 16.13c [45, 51]. Fu et al. have designed fiber shaped parallel supercapacitor by a simple dip-coating method. A spacer wire is being utilized for the separation of two electrodes [52]. The maximum capacitance of 19.5 mF cm−2 has been achieved, along with energy density and power density of 2.7 μWh cm−2 and 9.07 mW cm−2 , respectively. This fabrication technique provides low-cost, high stability, and large industrial production [52]. Harrison et al. have fabricated coaxial fiber shaped supercapacitor using ink-gel activated carbon [53]. The assembled device shows three active layer coating, where the first and third layers act as the positive and negative electrodes, and the second layer is PVA-H3 PO4 -H2 O known as gel electrolyte, which also acts as a separator. The capacitance achieved is 3.18 mF cm−2 [53]. To make a flexible fiber supercapacitor, flexibility is the major criteria. It means that material should not undergo any performance degradation when bent or compressed. In this regard, carbon fiber is a widely used material for supercapacitors. It acts as a substrate for electrode material, which also provides mechanical support and current collector for flexible supercapacitor [54]. However, pristine carbon fiber displays low surface area and poor porosity. So, activated carbon fiber has been utilized as fiber to construct flexible supercapacitor, this approach show increase in wearability and flexibility [55]. The volumetric capacitance obtained is 14.2 F cm−3 , which is maintained nearly 100%, while bending from 0 to 180°. It also maintains

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Fig. 16.13 Schematic illustration of fiber-shaped micro-supercapacitors with different configurations, a parallel electrodes, b twisted electrodes, and c coaxial electrodes (redrawn and reprinted with permission from [50])

an energy density of 3 mWh cm−1 at a high-power density of 220 mW cm−1 . Other carbon materials such as CNT and graphene are widely studied electrode for fabricating fiber shaped supercapacitor [56–58]. CNT and graphene show high mechanical strength, appreciable electrical, and thermal properties along with high surface area. CNT-based coaxial structure exhibits volumetric capacitance of 32.09 F cm−3 . This value is much higher than the twisted CNT fiber configuration [56]. Also, graphenebased coaxial fiber shaped supercapacitor shows lower resistance as compared to twisted structure [59]. Apart from the carbon-based electrode, conducting polymer and metal oxide material have been utilized to obtain high capacitance value. Also, based on device configuration, the performance can be enhanced by designing the asymmetric assembly. Here, two different electrode materials are utilized to achieve high voltage window and high energy density. Mixed transition metal oxide such as NiCo2 O4 and CoFe2 O4 , and p-doped conducting polymer are commonly used positive electrodes, which store charges by the Faradaic charge transfer process. Carbon-based material is used as a negative electrode, where capacitive charges are stored by the adsorption of ions at the electrode surface. Asymmetric assembly can extend the operating potential window up to 1.4–2 V [60–67].

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16.10.2 Planar Micro-Supercapacitor Devices Planar micro-supercapacitor emerges as a recent advancement in the micro-energy storage device with relatively small volume and high electrochemical properties. The active electrode material includes carbon-based material mainly carbon nanotubes and graphene, conducting polymer and composite material. The primary feature of the planar micro-supercapacitors is its fabrication techniques [51]. The device consists of a thin-film electrode assembled in micron level size [68, 69]. The various methods for the fabrication of supercapacitor include, (a) (b) (c) (d) (e)

laser patterning on a thin film of active material, printing of actives material on the current collector, mask-assisted current collector protection method, spray coating, and photolithography [51].

Some of the fabrication techniques used for micro-supercapacitor are discussed as follows: Photolithography: It is a common technique used to fabricated micron level devices. Kurra et al. have fabricated micro-supercapacitor using reduced graphene oxide composite with pseudocapacitive material [70]. The device fabrication utilizes a well-established strategy, which can be widely implemented. The as-assembled device shows good cyclic stability during electrochemical performance [70]. The composite electrode material includes PEDOT (poly(3,4ethylenedioxythiophene)/Au. The fabricated device delivers volumetric capacitance of 59 F cm−3 and areal capacitance of 9 mF cm−2 in 1 M H2 SO4 electrolyte. The corresponding energy density of 7.7 mWh cm−3 is reported [70]. Laser writing and etching: In the laser writing approach, a laser is irradiated on graphene oxide films, where sp3 carbon atom changes its binding state to sp2 atom photothermally [71]. So, C=O of graphene oxide changes to the C-O bond of reduced graphene oxide. The synthesis of graphene by laser is called laser-induced graphene (LIG). Figure 16.14a shows the fabrication process of the planar supercapacitor via laser ablation. SEM of as-assembled micro-supercapacitor (top view) is shown in Figs. 16.14b, c. The clean fringes can be seen, which act as an electrode. Figure 16.14d provides the schematic dimensions of the designed interdigitated electrodes of the planar micro-supercapacitors [72]. El-Kady et al. have fabricated LIG-MnO2 composite based micro-supercapacitor using laser direct writing [73]. The device exhibits extremely high volumetric capacitance of 1100 F cm−3 with energy density between 22 and 42 Wh L−1 [73]. Laser-based another process includes laser etching. This is a simple process, where excess material is removed to form a pattern of the active electrode material. The process is illustrated in Fig. 16.14. Jianga et al. have fabricated MXene based micro-supercapacitors by laser etching [74]. The obtained capacitance is 23 mF cm−2 along with 95% capacitance retention after 10,000 charge–discharge cycles [74]. Printing: Printing technique shows great potential for the preparation of versatile and scalable micron level energy storage devices. It mainly aims to design flexible and

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Fig. 16.14 a Schematic diagram showing the fabrication process of planar micro-supercapacitors using the laser etching technique, b and c top view SEM images of the interdigitated microelectrodes, d a diagram showing the dimensions of the designed interdigitated electrodes of the planar micro-supercapacitors (redrawn and reprinted with permission from [72])

wearable electronics as different types of printing inks can be used [75]. Yang et al. have prepared a micro-supercapacitor using a printing technique [76]. PVA/H3 PO4 is used as a solid electrolyte and MnO2 /carbon composite as electrode material. The device demonstrates capacitance of 7.04 mF cm−2 at a current density of 20 μA cm−2 with 80% capacitance retention after 1000 cycles [76]. However, the study in the field of micro-supercapacitor is still immature as some vital issues need to be addressed, before its practical application. Microsupercapacitor fails to maintain a stable output voltage. Also, the obtained output voltage and the current value are not remarkable. Widening of output potential window is out most important for application purposes. In addition to this, advancement in micro-supercapacitor can be extended for other features such as stretchability, hydrophobicity, self-healing, etc., [77].

16.10.3 Flow-Supercapacitors For large-scale energy storage devices, the flow-supercapacitor is the most suitable candidate. The major application includes the storage of energy in the power system. In this device configuration, energy density depends upon the volume and concentration of electrolyte, whereas tuneable power density can be achieved by altering the number of cells. In 2012, a demonstration shows that supercapacitors can be scaled up to store a large amount of energy, which can be efficiently used for power grid energy storage applications. The storage of a large amount of energy is made possible by the utilization of flowable carbon slurry in the electrolyte. Figure 16.15 shows the

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Fig. 16.15 Schematic illustration of the flow-supercapacitor (redrawn and reprinted with permission from [51])

schematic illustration of the flow-supercapacitor [51]. Here, two redox electrolytes are stored in two primary tanks. During the charging process, the slurry electrode is pumped through a polarized flow-cell. Here, the formation of the electric double layer at the surface of carbon particle stores the capacitive charges. When the carbon particles are charged, co-ions and counter ions present at the carbon surface change the suspension to neutral state [51]. The flow-supercapacitor holds an advantage over solid supercapacitor and flowbatteries, in terms of high-power density, long cycle life, and comparable energy density the flow-batteries. So, it can be potentially used for power grid applications [51]. However, this assembly of supercapacitor contains two disadvantage such as (a) it contains non-uniform flow profile that results in charge redistribution and (b) charge leakage outside the electroactive region increases charge inefficiencies. So, a synergistic optimization between suspension electrode design and flow properties plays an important role to minimize polarization issues and enhance the effective use of active material present in the suspension electrode [78]. Suspension electrode is the main component of flow-supercapacitor, as it stores electrochemical active species, electrolyte, and conductive additive. The first assembled flow-supercapacitor includes spherical activated carbon and non-spherical titanium carbide [79]. Spherical carbon slurry improves the percolation through particle network, increases the active surface area, and promotes the diffusion of ions through the porous structure. This improves the capacitance and rate capability of the device [80]. Graphene and its composite material have also been explored in the flow-supercapacitor assembly. The capacitance of around 1.08 F at a potential window of 2 V is obtained with an input slurry of 24 mL [81]. The energy density of 0.064 Wh L−1 is obtained at a power density of 0.013 W L−1 . The value for the graphene-based flow-supercapacitor

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is better than graphite slurry. The increase in value is due to the size of graphene, which falls in the nanometre range. It is also studied that composite of reduced graphene oxide and carbon sphere deliver high capacitance of 200 F g−1 at very low resistance [82]. Although, optimization is a necessary study to obtain the best performance as a higher amount of reduced graphene oxide high viscosity, which can result in poor gravimetric capacitance and poor rate capability. Some others suggest the introduction of redox-active species such as hydroquinone and p-phenylenediamine into carbon slurry exhibit 50% higher capacitance than pristine carbon slurry [83– 85]. Another approach to increase the performance of the flow-supercapacitor is to assemble in an asymmetric configuration. For a normal supercapacitor device, it is well-established that asymmetric assembly improves the working potential of the device. In this regard, the potential window of 1.6–1. V is achieved by combining MnO2 -carbon suspension as the positive electrode and activated carbon suspension as a negative electrode [86, 87]. This configuration delivers an energy density of 10 Wh kg−1 , much higher than the symmetric assembly.

16.10.4 Shape Memory Supercapacitors Shape memory supercapacitors can be utilized to recover devices back to their initial shape and size after irreversible deformation. The shape memory effect arises from the change in reversible crystalline phases known as martensite to the austenite phase. On the application of heat, shape-memory alloy returns to its original shape eventually removing plastic deformation. Nickel-titanium (NiTi) alloy are such types of shape memory alloy [51]. For the supercapacitor application, Liu et al. have fabricated graphene-coated TiNi alloy at the negative electrode and MnO2 /Ni composite as a positive electrode separated by gel electrolyte [88]. The device delivers a specific capacitance of 53.8 F g−1 at a current density of 0.5 A g−1 . Although, bent device regains its shape at a relatively low speed of 550 s. Apart from shape memory alloys, few polymers also exhibit shape memory properties. The polymer can be extended to hundreds of percent, above its glass transition temperature or melting temperature. These shape memory polymers can memorize several shapes and respond according to stimuli such as temperature, light, electromagnetic waves, etc. Deng et al. have designed a CNT sheet coated shape memory polymer as an electrode of shape memory supercapacitor [89]. Figure 16.16 shows a schematic representation of the fiber shaped shape memory supercapacitor. The fabric can be reversibly transformed into flexural or elongated states and recovered to the original shape [89]. The electrochemical studies show no obvious decrease in performance after 500 cycles of deformation and recovery [89]. The capacitance of 42.4 mF cm−1 is obtained, which is very low as compared to another conventional supercapacitor. The introduction of various pseudocapacitive material and their composite may enhance the electrochemical performance of shape memory supercapacitor [51].

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Fig. 16.16 Schematic representation of the fiber shaped shape memory supercapacitor (redrawn and reprinted with permission from [89])

16.10.5 Piezoelectric Supercapacitors Piezoelectric-based supercapacitor uses mechanical force to charge itself by storing it as electrochemical energy. Piezoelectric supercapacitor can be designed by using piezoelectric film instead of a conventional separator. A piezoelectric separator acts as the core of the piezoelectric supercapacitor. The piezoelectric separator uses a well polarized PVDF film placed between the two electrodes. Piezoelectricity arises from the polar β crystalline phase, here C-F dipoles align in the same direction that induces intrinsic charges [51, 90]. The potential is generated due to the mechanical strain along with the PVDF film. This drives the electrolytic ion toward the positive and negative electrode. This results in the formation of bilayer or pseudocapacitance at the electrode-electrolyte interface as shown in Fig. 16.17. In this regard, Song et al. have reported the assembly of the piezoelectric supercapacitor, by using carbon cloth as an electrode and polarized PVDF film as separator coated with H2 SO4 /PVA gel electrolyte [91]. The obtained specific capacitance of 357.6 F m−2 at a current density of 8 A m−2 . The rate capability of 59.84% is achieved even at a high current density of 200 A m−2 . During charging, the compressive force with an average frequency of 4.5 Hz is applied. To confirm that the result obtain is due to PVDF film, another supercapacitor with conventional separator is designed. However, the supercapacitor without PVDF film fails to store charge on the application of the compression force. This confirms the importance of piezoelectric film as a separator for the piezoelectric supercapacitor [91].

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Fig. 16.17 Schematic illustration of the working principle of piezoelectric supercapacitors (redrawn and reprinted with permission from [51])

16.10.6 Self-Healing Supercapacitors Conventional supercapacitors or fiber-based supercapacitors suffer from major limitations during accidental mechanical damage. This reduces the lifetime of the device. To restore the capacitor configuration during damage, a self-healing supercapacitor came into existence. The fabrication of self-healing supercapacitors requires the integration of self-healing materials [51]. The concept for designing the device includes the development of wrapping of wire type electrode with self-healing polymer [92, 93]. As electrode material, CNT acts as suitable self-healing material, since its electrical properties are easily favorable to get self-healed. When the broken parts are brought together, the formation of a hydrogen bond takes place between the two sections. Also, in CNT electrical properties are restored by van der Waal’s forces [92]. The first self-healing supercapacitor had been designed by adding CNT films onto the supramolecular network and TiO2 nanostructure. This acts as a substrate for CNT films. PVP-H2 SO4 gel is used as electrolyte and separator both. After cutting, the lateral movement of self-healing supramolecular composition forces the CNT layer into contact, where device conductivity gets restored. Here, PVP gel electrolyte, also show self-adhering properties. Although, upon several cuts, the electrochemical performance gradually decreases due to the increase in resistance [94]. To improve the specific capacitance of the device, PANI has been incorporated into CNT structure through electro-polymerization. To enhance the self-healing properties, the introduction of magnetic material based electrode is used, such as Fe3 O4 /PPy with polyurethane [51, 93]. The presence of magnetic forces helps to attract the broken yarn fibers, enabling the efficient recovery of the device (Fig. 16.18).

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Fig. 16.18 Schematic illustration of a manufacturing process of magnetic-assisted self-healable supercapacitor and b supercapacitor self-healing process. The magnetic force (as shown in inset) helps to reconnect the broken fiber electrode when they are brought together (redrawn and reprinted with permission from [93])

Because of the promising performance, these supercapacitors are getting used in several applications, like portable and wearable electronics, smart clothes, transportation and vehicles, power backup systems, implantable bioelectronics, military, and aerospace, etc., [3, 50, 51, 78, 95, 96].

16.11 Concluding Remarks Although supercapacitors possess several advantages over batteries and fuel cells such as high-power density with long cycle life, wide-ranging temperature operation, environmental friendliness, etc., still they exhibit some disadvantages like low energy density compared to a battery, low operating potential window, high cost,

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etc. The main disadvantage of supercapacitors remains low energy density, which is still unresolved. Still, supercapacitors suffer from limited energy density when compared to lithium-ion batteries. To achieve high energy density, comparatively larger supercapacitors are needed, which in turn increases the size of the assembled system as well as the overall cost of the device. Another challenge faced by the supercapacitor industry is the huge cost associated with the raw materials as well as costs related to manufacturing. The high cost of electrode-active materials impedes supercapacitor technology. Though low-cost carbon materials such as activated carbons (~$ 15/kg) are available as electrode-active materials for supercapacitors however, performance is still not adequate for fulfilling the requirements. On the other hand, CNTs and graphene are highly efficient electrode-active materials however their cost remains very high (~$ 5000/kg, for multi-walled CNTs and ~$ 100,000/kg, for single-walled CNTs). Several composites using CP, TMO, and carbon have been made to improve the performance of supercapacitors. An electrode based on CoO and polypyrrole (PPY) shows a high specific capacitance of 2223 F g−1 at a current density of 1 mA cm−2 . Another electrode made of graphene and NiCo2 O4 exhibits a very high specific capacitance of 1950 F g−1 at a high current density of 7.5 A g−1 . A good performance is noticed for the electrode made of Ni(OH)2 nanoplatelets and CNF of poly(amic acid) (PAA). It shows a high specific capacitance of 2523 F g−1 based on the mass of Ni(OH) at a scan rate of 5 mV s−1 . Not only the electrodeactive material, the electrolyte separator membrane, and electrolyte can also boost the manufacturing cost. Therefore, extensive efforts should be given to solve these challenges for the utilization of supercapacitor devices in advanced electronics. To address the major issue of energy density, extensive research is to be carried on for the development of high-performance electrode materials, electrolytes, and substrates at low cost such that the wheel of modern generation energy storage devices can be propelled in no time. Apart from the advancement in the field of conventional design for supercapacitor, the design in the field of supercapacitor has extended its research to fabricate micro-supercapacitor, fiber supercapacitor, multifunctional supercapacitor, flow supercapacitor, etc. The simple charge storage phenomena of electrochemical supercapacitor have made it possible to be a highly promising candidate for high-performance energy storage device. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Chapter 17

Applications of Supercapacitors Ravi Nigam, Kapil Dev Verma, Tanvi Pal, and Kamal K. Kar

Abstract Supercapacitors have attracted a lot of attention because of their unique quality of fast charging and discharging capability, high-power density, and long service life. Easy fabrication and lightweight make supercapacitors suitable for a wide range of applications as electrical energy storage devices. Supercapacitors are taking place of batteries and conventional capacitors in many applications. They possess higher power density than batteries and fuel cells and higher energy density than conventional capacitors. In most applications, hybrid battery/supercapacitor energy storage systems are used to utilizing the higher rate capability, better cyclability, and it also extends the battery life. Supercapacitors have been designed in various ways like flexible, foldable, self-charging, electrochromic, microbial, etc. They are used in portable electronics, backup power supply, smart grid, memory backup, hybrid vehicles, transportation, and wearable electronic fields. This chapter provides a brief insight into the commercially available supercapacitors and the applications in various fields.

17.1 Introduction In the present day, non-renewable energy sources are not sufficient to fulfill all power demands. There is a need for moving toward renewable energy sources. But there are two problems in the use of renewable energy, which are low power quality and instability. One of the solutions to these problems is to use storage systems. Commonly R. Nigam · K. D. Verma · T. Pal · K. K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India T. Pal A.P.J. Abdul Kalam Technical University, Lucknow 226031, India K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] © Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6_17

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Fig. 17.1 Forecast of global supercapacitor market (Source IDTechEX report 2015), inset figure: supercapacitor market value by region (2018), share % (Source Future Market Insights, 2018)

used electrical energy storage systems are battery, fuel cell, and conventional capacitor. But the supercapacitors have a higher power density than batteries and better energy density than conventional capacitors. Another advantage of the supercapacitor is a fast charging process. The charging time of supercapacitors varies from a few seconds to minutes. The characteristics of capacitors [1] and features to understand the fact of the capacitor versus supercapacitor [2] are reported elsewhere. Supercapacitors are used in several areas, i.e., energy storage, transportation, electronics, health care, military and defense technologies, and power electronics [3–5]. The revenue of the global supercapacitor market is projected in Fig. 17.1 at a compound annual growth rate of 19.8% (2018–2028). The inset figure shows the growth rate of selected countries in the global market.

17.2 Supercapacitor Market Global supercapacitor market revenue is growing at a remarkable rate. According to the supercapacitor’s growth rate forecast, the global compound growth rate will be more than 20% [6]. The supercapacitor market has a wide application field and bright future, which attracts the new companies to join in the electrochemical supercapacitor industry. Market driving forces for electrochemical supercapacitors applications are high potential window, long cycle life, environmentally friendly, low cost, etc. More than 80 supercapacitor manufacturing companies are present today [7]. Among these companies, Maxwell and Panasonic are the market leaders. Table 17.1 shows a list of a few manufacturers of supercapacitors, just as a reference, to understand the global markets.

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Table 17.1 List of supercapacitor manufacturing companies with their few product specifications [8, 9] Company

Device

Country

Voltage range (V)

Capacitance (F)

AVX

BestCap

USA

3.5–12

0.022–0.56

Copper

PowerStor

USA

2.5–5

0.47–50

ELNA

Dynacap

USA

2.5–6.8

0.033–100

Eaton corporation

XL60

USA

0–2.7

3000

EPCOS

Ultracapacitor

USA

2.3–2.5

5–5000

Evans

Capattery

USA

5.5–11

0.01–1.5

Ioxus

iCAP

USA

0–2.85

1200

Kold Ban

Kapower

USA

12

1000

Maxwell (Tesla owned from 2019)

BoostCap

USA

2.5

1.60–2600

VINATech

Hy-Cap NEO

USA

0–3

60

NEC

Supercapacitor

Japan

3.5–12

0.01–6.50

Panasonic

Gold capacitor

Japan

2.30–5.5

0.10–2000

LSMtron

LSUC

Korea

0–2.8

3000

NessCap

EDLC

Korea

2.7

10–5000

Arvio

SIRIUS capacitor module

Australia

44–54

–

Cap-XX

Supercapacitor

Australia

2.25–4.5

0.09–2.80

ESMA

capacitor modules

Russia

12–52

100–8000

Aowei

UCR series

China

0–2.7

3000

CRRC

CRRC

China

0–3

12,000

Supreme power solutions (SPS)

SPSCAP

China

0–2.7

360

NAWA technology

NAWACAP

France

0–2.7

250

Skeleton technologies

SkelCap

Estonia

0–2.85

3200

Yunasko

Yunasko

UK

0–2.7

3000

17.3 Applications of Supercapacitors Due to the fast charge storage capability and high cycle life, the supercapacitor has been used in various applications. The supercapacitor has advantages of highpower density, long cycle life, wide operating temperature range, and environmentally friendly. It is used in various fields like wearable electronics, hybrid vehicles, industrial applications, medical applications, military and defense applications, flashlight, renewable energy storage, artificial intelligence, etc. [10], and few of these are mentioned in the forthcoming sections.

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17.3.1 Wearable Electronics Conventional batteries are bulky in size and limit its usage in flexible, foldable, portable, and wearable electronics. To overcome these problems, recently microsupercapacitors and flexible supercapacitors [11–13] have been developed, which can provide instantaneous power to wearable electronics or biosensors. In wearable electronics, the supercapacitor is integrated with clothes, hand bands, goggles, etc. But the flexibility leads to strain and consequently wears it, which leads to degradation of the supercapacitor. Self-healing, shape memory, thermally chargeable, and electrochromic supercapacitors have also been developed, which further lead to the incorporation of supercapacitors in wearable electronics [14, 15]. Figure 17.2 shows a classic example of a flexible supercapacitor. In Fig. 17.2a, flexible mesh of the aligned electrode is integrated into the cloth, and in Fig. 17.2b, the wearable electronic-based power supply and storage system are imbedded in fabrics, which is a fabric-based integrated energy device for wearable activity monitors. It generates energy using

Fig. 17.2 a Supercapacitor embroidered on a cotton t-shirt [15] and b wearable electronic-based power supply and storage system in fabrics [16] (redrawn and reprinted with permission from [15, 16])

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Fig. 17.3 Carbon fiber electrode knitted and woven on a garment a 3D simulated model, b, c zoom-in of the shirt with a textile supercapacitor electrode, embedded as part of a long sleeve t-shirt (redrawn and reprinted with permission from [17])

human activity and stores in an integrated supercapacitor. Figure 17.3 shows the carbon fiber electrode knitted and woven on the modeled garment. Here, the electrode is made by screen printing an activated carbon paint onto a custom knitted carbon fiber cloth that acts as the current collector and uses a solid electrolyte.

17.3.2 Portable Electronic Devices Lightweight electrical energy storage devices are preferred for portable electronic devices. Arrays of supercapacitors alone or with batteries are used to provide energy backup to portable electronic devices like mobile phones, laptops, remote controllers, screwdrivers, portable speakers, and wireless handheld scanners [18]. The hybrid system of supercapacitors and batteries provides mixed benefits like the fast charging of supercapacitors and large energy density of batteries. Photo-supercapacitors have also been developed using the energy capture and storage simultaneously [19]. Transparent supercapacitors have the potential to be used in transparent electronic devices. It is flexible and based on ultrathin graphene [20, 21] and Au/graphene composite electrodes. CAP-XX has worked with Nokia for using supercapacitors in camera flashes. Printed supercapacitors can be used as a power source in intelligent packaging. Paper Battery Co. has worked on the integration of thin supercapacitors with batteries. Figure 17.4 shows supercapacitor applications in different portable devices. A wearable wristband as shown in Fig. 17.4a possesses flexible supercapacitor cells that are connected in series and giving power to five LEDs. Figure 17.4b shows a self-power pack system. It has two supercapacitor cells that are connected with a

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Fig. 17.4 Applications of supercapacitors in portable devices, a wristband, b LEDs, c portable fans, and d prosthetic hand (redrawn and reprinted with permission from [22])

solar cell and it is powering 84 LEDs for more than one minute. The supercapacitor is powering a motor, which is attached to a portable fan shown in Fig. 17.4c, d supercapacitor is covering the prosthetic hand to powering the fingers [22].

17.3.3 Transportation The supply of fossil fuels is dwindling and also it contributes to pollution. There has been an ever-increasing focus on alternative options like electric vehicles/trains [23]. Supercapacitors complement the properties of batteries as used in automobiles like hybrid vehicles. They lead to battery safety, long life, and more acceleration. They efficiently use the kinetic energy of the regenerative braking to get a charge in a few seconds to minutes. During power demand in hybrid vehicles, supercapacitors fulfill the power need. Lamborghini has started using a supercapacitor in its car. Tesla has also paid a lot of money to US-based supercapacitor manufacturer Maxwell technologies. The Chinese company, Sunwin has introduced electric buses with supercapacitors, which get charge instantaneously at the bus stop. CRRC Zhuzhou Locomotive Co. (China) has developed a rail vehicle that uses a supercapacitor as an energy source. It takes 30 s to get charge and runs for 3–5 km based

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Fig. 17.5 Electric vehicle with hybrid energy system (redrawn and reprinted with permission from [24])

on operating conditions. It has 85% braking energy efficiency and energy storage capacity of 9500 farads. Figure 17.5 shows an electric vehicle with real localization of each element including the hybrid energy storage system. Here supercapacitors associated with electric vehicles solve the slow dynamic problem of proton electrolyte membrane fuel cell (PEMFC) [24].

17.3.4 Industrial Applications Supercapacitors are used in industries along with batteries. These provide power for lifting operations and automated guided vehicles. Regenerative braking is being used for energy storage in supercapacitors. Easy to operate, fast charging, long cycle life, and low maintenance cost of the supercapacitors are economical for industrial applications. Forklifts, shovel trucks, excavators, agricultural machinery, cranes, etc. deploy supercapacitors [25]. The battery performance is also improved involving run-time, reliability, and lifetime. Supercapacitors due to more power density get the work done in less time and limit the usage of the number of batteries. Ioxus Inc. has provisions of supercapacitor usage in industry. Figure 17.6 shows an example of the hybrid battery supercapacitor-based commercial CESAB B516 electric forklift. The use of a hybrid storage system improves performances of forklift like durability, availability, and range [26].

17.3.5 Military, Defense, and National Security Supercapacitors are used in military applications that need enormous power. They are used to provide power to laser weapons, railgun, and munitions [27]. These are also used in power and data backup for communications, avionics, military robots, etc.

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Fig. 17.6 Hybrid battery supercapacitor-based commercial CESAB B516 electric forklift (reprinted with permission from [26])

Fig. 17.7 Integration of energy storage, communication, and energy harvesting devices in one garment (redrawn and reprinted with permission from [17])

Supercapacitor also helps in making smart garments, which can do malfunction like energy storage, communication, and energy harvesting. The Tecate group is devoted to the use of supercapacitors in military applications. Figure 17.7 shows the integration of energy storage, communication device, and energy harvesting device in one garment. Body moment charges the supercapacitor device, which is made as a textile with seamless knitting and further this energy is used for communication. Supercapacitors can withstand harsh environments and high-temperature range. FastCAP is designing supercapacitors for deep space missions, satellites, and planetary balloons. Supercapacitors reduce the load and volume of the energy storage system and make them applicable in aerospace applications and unmanned aerial vehicles.

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17.3.6 Renewable Energy Sector The ability to withstand harsh environments, high maintainability, and ability to provide high power makes supercapacitors suitable for renewable energy integration. Renewable energy systems (wind energy, solar energy, tidal energy, etc.) have power fluctuations due to the irregular, intermittency, and cyclical pattern, which can be smoothened by the use of storage systems like supercapacitors with battery in a hybrid system. This is due to the quick response time of supercapacitors. This system has reduced battery cost, longer lifetime, and improved efficiency. Wind turbine pitch control and emergency response are important areas for the application of supercapacitors. This also leads to the incorporation of supercapacitors in solar energy harvesting, wave energy, and electric grid regulation. Skeleton Technologies is providing a solution to the grid and renewable energy operation. Kilowatt Labs produce Sirius energy storage, which is a supercapacitor-based energy storage system and Centauri energy server to manage distributed energy. Renewable energy with supercapacitors is used in lightening systems due to low environmental impact and a wide temperature range of operation. The use of a supercapacitor along with battery or fuel cell improves the dynamic load-leveling capacity of a hybrid energy storage system. The integration of supercapacitors with a solar system can be used as the charging station for public transport. Renewable energy (solar energy) sources can be used to power the electric vehicle with the help of a supercapacitor. Figure 17.8 is an example of a hybrid energy storage system use in the regenerative braking system.

Fig. 17.8 Hybrid storage system application in renewable energy (redrawn and reprinted with permission from [28])

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17.3.7 Power Electronics Generally, the supercapacitors are used with batteries in high-power applications like the starting of motors. These are non-hazardous, and integration with batteries, it increases the battery life. They are small in size and operate in wide temperature ranges. Low cost, long cycle life, and highly efficient allow them to be used in uninterruptible power supplies (UPS). Riello UPS uses a supercapacitor instead of conventional batteries for electrical energy storage. Inventlab LLC is also providing ATX UPS with supercapacitors. The electric energy storage and delivery characteristics of battery and supercapacitors are different. DC-DC converters are being developed to synchronize the power flow with an efficient energy management system [29]. Supercapacitors can be used in AC line filtering for signal stabilization by smoothening arbitrary waves due to the high specific capacitance, frequency adaptability, and ease of fabrication [30]. Supercapacitor electrical cables have also been developed recently [31]. Also, supercapacitors have captured a vast field of electronic devices (Fig. 17.9). Figure 17.10 is a schematic of supercapacitor-based power failure detection and notification system. It consists of a D-type latch (U1), a supercapacitor, and three diodes. When there is a power failure, the output of the D-type latch (it is an electronic device that can be used to store one bit of information) becomes low. The signal is used as a power failure notification signal. Figure 17.11 shows a supercapacitor-based power failure detection and notification system plugged into a laptop computer equipped with a solid-state disk. Supercapacitor supplies power to the solid-state disk in absence of power supply. There is no power fluctuation, which validated the stability of a supercapacitor-based power failure detection and notification system.

Fig. 17.9 Several applications of supercapacitor as power source for portable electronic devices (redrawn and reprinted with permission from [32])

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Fig. 17.10 Schematic of supercapacitor-based power failure detection and notification system (reprinted with permission from [33], Copyright (c) 2016 IEICE)

Fig. 17.11 Supercapacitor-based power failure detection and notification system plugged into a laptop computer equipped with a solid-state disk (reprinted with permission from [33], Copyright (c) 2016 IEICE)

17.3.8 Communication The telecom industry uses batteries for a cell tower in case of power failure. There are sometimes power glitches, which are removed by using supercapacitors. This protects the telecom hardware or equipment. It reduces the cost of the industry and increases the life of the battery. The supercapacitors have also been used in radio frequency identification tag RFID system for wireless power transfer and data communication [34]. The wireless charging system has also been integrated with supercapacitors. Graphene/carbon-based supercapacitors [35–37] have been demonstrated to be used in broadband optical modulators for communication, information technology, and optoelectronic applications [38]. VINATech supercapacitors have applications in communication equipment. Figure 17.12 demonstrates the trend in

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Fig. 17.12 Improvement in portable electronic devices from 1983 to 2018 (reprinted with permission from [39])

portable electronic devices from 1983 to 2018. Communication and other electronic devices ranging from the computer, laptop, digital camera, tablets to wearable electronics are developing dramatically. Fast growths of electronic devices are closely related to the new energy storage technologies [39].

17.3.9 Artificial Intelligence, Internet of Things, Cyber-Physical System, Soft Robotics The artificial neural network has been used to accurately predict the usage of supercapacitors [40]. Connected devices use small, portable, and smart sensors. Supercapacitors provide high power and a large number of cycles before degrading, making them appropriate for usage in providing energy to several devices. On-chip microsupercapacitors can be used in these applications [41]. VINATech has made supercapacitors for the Internet of things. Cyber-physical systems decentralize the computational process and require higher power at the point of application. Supercapacitors with a long life cycle and peak power supply capabilities serve as an energy buffer for these systems [42]. It has also been demonstrated to provide resilient microgrid against communication outages and cybersecurity threats by their contribution to

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Fig. 17.13 Self-powered electronic skins for robotic hand (reprinted with permission from [46])

fault current through a control scheme [43]. Flexible and compressible supercapacitors are being used in soft robotics [44]. Figure 17.13 demonstrates self-powered electronic skins for a robotic hand. The graphene touch sensor [45] is grabbing the softball using tactile feedback [46].

17.3.10 Complementary Metal–Oxide–Semiconductor (CMOS), Very Large-Scale Integration (VLSI), Memory Batteries during operation get heated up, which degrades their performance and lifetime. Supercapacitors are used in a hybrid architecture with batteries to insert idle periods and thermally management of the batteries. Supercapacitors have very low resistance and a high number of discharging and charging cycle [47]. The size of integrated chips is decreasing, but batteries storage systems are still bulky. Now supercapacitors have been presented to resolve these issues, which are complementary metal– oxide–semiconductor, also known as complementary-symmetry metal–oxide–semiconductor(CMOS) compatible and can be used in integrated circuit [48, 49]. Solidstate drives and data servers use supercapacitors as power backup to prevent data loss while writing. Magnetoelectric supercapacitors have applications in data storage and spintronics [50]. Smoltek supercapacitors (SmolCAP™ is Smoltek’s unique capacitor) are CMOS compatible. Figure 17.14 is an example of a backup power solution with the help of a supercapacitor. It represents the schematic of backup power for the solid-state disk (SSD) with a supercapacitor. The supercapacitor is placed inside the solid-state disk and power supplied to the solid-state disk charges the supercapacitor.

17.3.11 Medical and Health care Biological supercapacitors have been developed that can be used for supplying power to various medical implants as shown in Fig. 17.15 [32]. The unconventional super-

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Fig. 17.14 Supercapacitor as power backup for memory (reprinted with permission from [33])

Fig. 17.15 Several applications of supercapacitor in the field of power electronics used for medical devices for implantation in the human body (redrawn and reprinted with permission from [32])

capacitors with flexible, macro and micro, stretchable, compressible, and fiber properties can be used in artificial skin, medical wearables, and sensors [51]. Smart and soft contact lenses have been developed and trial on humans conducted. These lenses detect biomarkers for the diseases. These are continuously used in human eyes. Supercapacitors are printed on a substrate, which is integrated with a wireless charging system without affecting the vision [52]. Human motion energy harvesting is possible by piezoelectric sensors, which convert pressure to electrical energy used to charge supercapacitors [53]. Self-charging supercapacitors have also been developed by utilizing piezoelectric separators. Microbial supercapacitors can also be used [32]. Multifunctional actuators integrated with supercapacitors can be used in artificial muscles [54]. Supercapacitor with a triboelectric nanogenerator has been

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Fig. 17.16 Wearable glucose sensor with integrated micro-supercapacitor (redrawn and reprinted with permission from [56])

developed, which acts as a biomimetic pressure sensor having vibration counting and frequency computing capabilities [55]. These are also used in medical equipment like in sensors and contact lenses. CAP-XX supercapacitor has applications in peak power support for drug delivery systems. Figure 17.16 shows a wearable glucose sensor with an integrated micro-supercapacitor. An integrated micro-supercapacitor keeps the sensor reliable with excellent stability and ultrafast response speed [56].

17.3.12 Buildings Elevators in the building consume a lot of power due to lifting operations. The inrush current can be supplied by supercapacitors. As a result, it makes elevators as energy-efficient and prevent from voltage drop. The regenerated energy can be stored in the supercapacitors, which fulfill the transient power demands and power smoothing. Supercapacitors can also be coupled with a fuel cell for reliability and power backup in case of electricity failure [57]. These can also be used in smart electricity meter to power the clock and microcomputer. Due to long life, low cost and instant current supply supercapacitor increased the efficiency of the equipment. They protect the battery from failure [58]. Smart meters can intrude user privacy by revealing the consumption pattern to the utility provider. Supercapacitors with a control circuit can be used to insert noise randomly to protect the privacy and they are less costly and light in weight [59]. Electrochromic supercapacitors can be used in smart windows with energy storage and smart homes [60]. GTCAP supercapacitors have potential applications in smart electricity meter, smart water meter, and many household appliances and utilities.

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17.3.13 Gas Sensors Figure 17.17 shows a wearable gas sensor attached with a gas concentration analyzer driven by the supercapacitor. Figure 17.17a demonstrates a complete working process of supercapacitor integrated system and a phone is used to know the concentration value/curve of the gas. It exhibits a real-time gas concentration analysis in the

Fig. 17.17 Wearable gas sensor attached with gas concentration analyzer driven by supercapacitor (reprinted with permission from [61])

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unknown gas concentration (no gas, low, middle, and high concentrations) Similarly, Fig. 17.17b illustrates the wearable feasibility of the sensor device on a wrist. It is a digital photograph of the wearable supercapacitor array-gas sensor and printed circuit board. Figure 17.17c demonstrates the circuit diagram of the sensor. It includes the name of the main components used in the printed circuit board [61].

17.3.14 Futuristic Applications Several efforts are being taken to increase the energy density of supercapacitors, which will make them preferable instead of batteries. Quantum dots host qubits, which are fundamental to quantum computers. Quantum supercapacitors may be incorporated in quantum computers. Supercapacitors can also be used in the power supply management of quantum computers. Nanobots can be powered by nanosupercapacitors with biodegradable or wireless control features. Supercapacitors may be made of materials absorbing electromagnetic waves and hence useful in stealth applications. Improvements in supercapacitor ubiquitous influence the industrial society.

17.4 Concluding Remarks The supercapacitor is a relatively new emerging electrical storage device, which has captured a vast field of application. Now supercapacitor is used from consumer electronics to military and industrial applications. Although supercapacitors have many advantages like high-power density and high cycle life, still they have some drawbacks like the low operating voltage, low energy density, and high cost in comparison with batteries. By overcoming the low energy density drawback supercapacitors will replace the batteries completely. Supercapacitors are used as backup power sources in consumer electronics, smart toys, digital cameras, wireless devices, computers, and telecommunications, which require low voltage and low power performance. They are used in space, military, and electric vehicles load-leveling applications, which have requirements of high-power density, high voltage, and high reliability. They are also used in transportation applications (electric vehicles, hybrid vehicles, and automotive subsystems), which have requirements of medium power and medium voltage. The supercapacitor has a great future, and it has several new possibilities for application. Acknowledgements The authors acknowledge the financial support provided by the Department of Science and Technology, India (DST/TMD/MES/2K16/37(G)) for carrying out this research work.

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Index

A Acceptor, 357, 358 Acetonitrile, 39 Acid treatment, 211, 212 Activated carbon, 12, 35, 44, 45, 47, 61, 65, 113–124, 126–131, 133–139, 141, 145, 146, 154, 155, 158–160, 162, 166, 168–170, 172–174, 180, 198, 241, 246, 250, 258, 264, 275, 292, 300, 322, 375, 376, 378–380, 388, 389, 391–394, 398, 400, 401, 403, 407, 426, 429, 430, 440, 443, 448, 452, 453, 457 Activation, 113, 115–120, 127, 129–131, 133, 134, 137, 139, 140, 154, 163, 179, 180, 193, 194, 198, 205 Activator, 118, 127, 131, 134, 138 Active material, 75, 80, 81 Active sites, 33, 34, 64, 159, 161, 172, 173 Actuators, 337, 476 Adsorption, 1, 4, 10, 15, 20, 21, 24, 33–35, 44, 46, 50, 54, 57, 72, 73, 90, 114, 120, 123, 126, 132, 133, 146, 147, 149, 151, 153, 158, 163 Aerogel, 302 Aerospace, 180, 196, 456, 470 Agglomeration, 159, 253 Aggregation, 297, 298, 325, 412, 416, 419, 423, 431 Agricultural, 115, 116, 126 Alignment, 183 Alkaline, 167 Amorphous, 114, 116, 121, 149, 150, 157, 160–162, 172, 173, 209, 213, 215, 224 Amorphous carbon, 231, 233, 247

Aniline, 357, 366, 368, 407, 409, 410, 412 Annealing, 92, 100, 101, 107, 183, 208, 215, 224, 234, 261, 263, 276, 306, 308, 309, 322, 381 Anodic peak, 76 Application, 435, 436, 438, 440, 441, 451– 454, 456 Aqueous, 152, 155, 160, 168, 170, 173 Aqueous electrolytes, 5, 14, 22, 37–39, 45, 47, 56 Arc-discharge, 231, 247 Architecture, 302, 304, 311, 312, 326, 327, 364, 366, 376, 380, 381, 398, 404, 419, 421 Array, 189, 190, 208–210 Artificial intelligence, 465, 474 Aspect ratio, 180, 185, 342 Asymmetric, 105, 149, 152, 338, 343 Asymmetric supercapacitor, 4, 7, 22, 23, 56, 74, 84, 85 Auricularia, 131, 134, 139, 140 Austenite, 453

B Bamboo, 132, 133, 138, 139, 166 Bandgap, 169, 337, 355, 356, 358, 359 Band theory, 358 Batteries, 1–4, 7–10, 20, 21, 23–25, 29–32, 34, 37, 40, 52, 59, 60, 65, 74–77, 90, 109, 146, 149, 152, 153, 155, 169, 334, 339, 435, 436, 452, 456, 457, 463, 464, 466–473, 475, 477, 479 Battery-supercapacitor hybrid, 72–75 Battery type capacitor, 4, 25 Bending, 105

© Springer Nature Switzerland AG 2020 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II, Springer Series in Materials Science 302, https://doi.org/10.1007/978-3-030-52359-6

483

484 Benzene, 231, 247 Bilayer, 31, 273 Bilayer structure, 3 Binary composites, 388, 389, 406, 408, 409, 412, 416, 418–422, 425, 429, 431 Binder, 340 Biochar, 116, 131 Bioelectronics, 456 Biomass, 2, 113–123, 126, 127, 129–131, 133, 134, 138, 139, 141 Biomedical, 355 Biowaste, 113, 133 Bipolaron, 358, 359 Bismuth oxide, 170–172, 174 Bode phase, 78, 79 Bonding, 303, 305, 358 Bottom-up, 277, 279, 300 Brillouin zone, 274 Brunauer-Emmett-Teller (BET), 233, 275, 281, 300 Building, 477

C Calcination, 99, 107, 108 Capacitance, 1, 4, 10–15, 18, 19, 21–25, 31, 32, 34, 37, 39, 41, 42, 44–47, 49, 51– 62, 64, 65, 71–74, 76–78, 80–84, 89– 93, 95, 96, 101, 104, 105, 107, 108, 115, 121, 123–127, 129–135, 137– 141, 145, 146, 149–152, 155–163, 166, 167, 169, 170, 172–174, 185, 189, 192, 195–198, 202, 203, 205– 209, 215–225, 230, 232–236, 241, 249, 251, 253, 254, 256, 258, 259, 261–264, 272, 279–293, 298, 302– 305, 307–319, 321, 323, 324, 326, 327, 333, 334, 339–345, 347–349, 354, 361, 362, 365–369, 373, 375– 382, 388, 389, 399–407, 410–412, 415, 416, 419–421, 424, 425, 430, 436–441, 443–453, 457 Capacitive behavior, 77, 78 Capacitor, 90, 109 Capacity, 32, 37, 40, 53, 55, 64, 65 Carbon, 1, 4, 5, 7, 12, 13, 15, 16, 21, 24, 29, 30, 32, 33, 35, 42, 44–54, 57, 59–62, 64, 65, 113–121, 123, 124, 126, 127, 129–138, 141, 146, 147, 149, 150, 152–155, 158, 160, 161, 163, 167, 169, 170, 173, 174, 181, 183, 185, 191, 192, 196, 197, 201–205, 207, 213, 225, 229–231, 233, 236, 241,

Index 245–247, 250–252, 254, 261, 262, 266, 267, 273–275, 277, 279, 282– 284, 292, 299, 307, 308, 312, 316, 317, 333, 334, 338, 343, 344, 347, 348, 354, 375, 380, 435–438, 440, 444, 446, 448, 450–454, 457 Carbonaceous, 113, 114, 119, 127, 146, 147, 150, 158, 161, 236, 246, 333, 340 Carbon aerogel, 35, 47, 52 Carbon fiber, 12, 101, 105, 343, 344, 347, 388, 448 Carbonization, 113, 116, 118–120, 127, 129–131, 134, 135, 137, 140, 436 Carbonize, 183, 211 Carbon materials, 436, 437, 444, 449, 457 Carbon Nanofibers (CNFs), 48, 49, 59, 65, 179–198, 201–204, 207, 209, 211, 215, 222, 435, 436 Carbon Nanotube (CNTs), 33, 45, 47, 48, 52, 91, 229–238, 240, 241, 245–247, 249, 250, 252–267, 275, 313, 388, 389, 392, 395, 396, 400, 402, 403, 409–411, 421–424, 427, 428, 430, 435, 437, 438, 450, 457 Carbon slurry, 451–453 Carboxylic, 238 Carton box, 130, 131, 139 Cassava peel, 127 Catalyst, 179, 181–183, 198, 203, 205, 209, 231, 232, 234, 236, 247, 249, 287 Catalytic, 91, 181, 182, 203, 204 Cathode, 341 Cathodic, 357, 360, 369 Cathodic peak, 76 Cations, 442 Carbon Nanofibers (CNFs), 444 Cellulose, 183, 190–192, 197 Cell voltage, 82, 83 Channels, 303, 325 Characterization, 89, 90, 231, 233, 252, 272, 299, 380, 388, 389, 431 Charge, 31, 34–37, 42, 45, 47, 50, 51, 54, 55, 59, 71–77, 81–85 Charge balance, 85, 334 Charge collector, 340 Charge/discharge, 30–32, 34, 37, 40, 48, 54, 59, 90, 91, 106, 109 Charge/discharge plateau, 77 Charge/discharge processes, 2–4, 10–13, 20, 22, 24 Charge mobility, 343 Charge separation, 388

Index Charge storage, 31, 32, 34, 36, 37, 42, 44, 54, 55, 60, 65, 92, 93, 95, 97, 120, 121, 123, 129, 130, 135, 141, 202, 388, 395, 400, 404, 416, 419 Charge storage mechanism, 1, 3–5, 7, 11, 12, 15, 19, 21–25, 73–76, 82 Charge time, 83 Charge transfer, 160, 162, 166, 169 Charge transfer resistance, 79, 81 Charging, 1, 3, 10, 13, 15, 16, 21 Charging time, 115, 234, 235 Chemical activation, 203, 204 Chemical energy, 2, 8 Chemical polymerization, 339, 342, 346 Chemical process, 74, 82 Chemical reactions, 10, 11, 20, 22, 25 Chemical stability, 12, 14, 33, 40, 43, 48, 343 Chemical vapor deposition, 179, 181–183, 203, 209, 231, 247, 249, 258, 277, 300, 301, 316, 323, 326 Chirality, 231, 247 Co-assembling, 430 Coating, 173, 210, 224, 233, 237, 240, 284, 287, 312, 340, 341, 344, 373, 397, 407, 411, 412, 414, 416, 418 Coaxial, 211–213, 224, 257, 261, 411, 427 Coaxial fiber, 448, 449 Cobalt oxide, 151, 154–157, 172 Coffee ground, 131, 139 Collectors, 101, 104, 105 Colloidal, 169 Communication, 469, 470, 473, 474 Complementary Metal–Oxide–Semiconductor (CMOS), 475 Composite, 31–33, 43, 47, 49, 53, 56–61, 91, 104, 145–148, 151, 153, 158–170, 172–175, 333, 340, 341, 347, 348, 435, 443–448, 450–453, 457 Composite electrode, 4, 24, 25, 31, 51, 52, 56, 60, 74, 443, 446, 448 Composition, 154, 166, 169, 233, 234, 249 Condensation, 118 Conducting metal plates, 31, 32 Conducting Polymers (CPs), 1, 4, 12, 13, 20–22, 24, 29, 32, 36, 37, 40, 42, 51–53, 57, 59, 60, 65, 72–74, 187, 298, 333, 334, 336–341, 343–345, 347–349, 353–364, 366, 368, 370, 373, 375, 380, 382, 387–389, 397, 404, 406, 412, 416, 421, 425, 426, 428–431, 435, 442, 446 Conduction band, 358, 359

485 Conductivity, 4, 12–15, 21, 22, 24, 90, 91, 93, 95, 103, 104, 109, 113–115, 119, 120, 126, 129–131, 133, 137, 145– 148, 150–152, 154, 155, 158–161, 163, 165–170, 172, 173, 184–187, 189, 193, 202, 203, 205, 211, 213, 217, 219, 220, 223, 225, 230, 233, 246, 249, 250, 254, 256, 263, 264, 266, 271, 272, 275, 278, 279, 281, 283, 284, 288, 292, 298, 299, 304, 308, 310, 311, 322, 325, 337–344, 353–359, 361, 363, 369, 370, 373, 375, 382, 389, 391, 396, 397, 403, 404, 409, 410, 412, 415, 419, 421, 423–425, 431, 436, 437, 440, 441, 443, 444, 446, 455 Conductors, 90 Configuration, 416 Conjugated bond, 354 Conjugated double bonds, 334, 339 Contact, 342, 344 Conventional capacitor, 1, 3, 8–11, 25, 31 Conversion devices, 2, 10 Co3 O4 , 89, 91, 101–103, 105–109, 246, 250–252, 261–263, 267, 303–307, 315–317, 323, 326, 365, 376, 381, 441, 442 Corn silk, 130 Corrosion, 12, 15, 355 Corrosion stability, 172 Cotton stalk, 122, 139 Coulombic efficiency, 71, 72, 83, 85 Counter cation, 359 Counter electrode, 75 Counterions, 334, 336, 337, 339, 340, 343 Crack, 340 Crosslinking, 341 Crystalline, 93, 95, 99, 107, 114, 341 Crystallinity, 95, 99, 167 Current, 71, 75–77, 80, 84, 85 Current collector, 11, 12, 15, 29, 30, 32, 42, 52, 55, 65, 73, 78, 83, 124, 234, 235, 237, 257, 258, 435, 436, 448, 450 Current density, 75, 79, 81, 93, 95–97, 99, 102, 105, 106, 164–167, 171, 174, 185, 187, 189, 191–194, 211, 215, 218–223, 234–237, 240, 246, 250, 254, 256, 264, 265, 280, 284, 285, 287–291, 310, 313–324, 362, 367– 369, 371–373, 375, 378–380, 408, 410, 412, 414, 417, 419, 420, 423, 425–429 Current fluctuations, 24

486

Index

Current leakage, 83 Cyber-physical systems, 474 Cycle life, 114, 126, 128, 133, 137, 337, 340, 343, 344, 436–447, 452, 456 Cyclic stability, 1, 3, 4, 10, 13, 18, 23–25, 29, 31, 32, 37, 42, 49, 51–53, 57, 60, 64, 65, 71, 72, 74, 77, 82, 90, 92, 95, 97, 105, 109, 145, 147, 148, 151, 155, 159, 160, 163, 169, 170, 174, 185, 187, 189, 192, 195, 202, 205, 214– 216, 236, 238, 246, 250, 256, 259, 261, 263, 265, 280, 285, 304, 315, 319, 321, 323, 325, 340, 344, 347, 348, 362, 363, 366, 367, 369, 372, 376–378, 380, 388, 389, 398, 409, 410, 412, 415, 416, 418, 419, 423, 425, 427–429 Cyclic voltammamogram, 7 Cyclic voltammetry, 71, 72, 75, 76, 80–82, 84, 85, 92, 123–125, 127–138, 187, 189, 191–195, 211, 341, 344, 357, 367–371, 373, 374, 376, 377, 379, 402, 408–412, 414–420, 422, 423, 425–427

262, 264, 284, 286, 293, 302, 309, 312, 320, 321, 336, 342, 343, 348, 360, 362, 375, 410, 417, 419, 423, 427, 437, 439, 442, 452 Diffusion layer, 51, 59 Diffusion rate, 186, 187, 202 Dip-coating, 213 Dirac points, 274, 299 Discharge, 32, 40, 60, 116, 120, 123–127, 129–133, 137, 141 Discharge capacitance, 341 Discharge state, 81 Discharge time, 80, 83, 414, 426 Dispersion, 165, 166, 169, 175, 253, 254 Dissolution, 207, 217 3D nanostructured carbon, 35, 52 Dopant, 334, 336, 339, 341, 359, 361 Dope, 187, 222 Doping, 13, 20, 150–152, 358–361, 363, 366, 369, 375, 380, 391, 397, 404, 440, 442 Double layer, 90, 92, 149, 179, 180, 185, 194, 203, 233, 257–259, 280, 298, 308, 309, 353, 354, 388, 400, 404

D Decomposition, 116, 127, 129, 137, 168, 179, 181, 192, 211, 277 De-doping, 333, 336, 337, 339, 340, 343 Defect, 274, 277, 299, 358 Defense, 464, 465, 469 Degenerate, 359 Degradation, 238, 241 Dehydration, 118, 119, 131 De-intercalation, 74 Delocalization, 339 Dendrite, 31 Dendritic, 344 Dendritic growth, 170 Deposition, 341, 343, 344 Derivative, 343, 355, 361, 397, 406, 421 Desorption, 33–35, 44, 57, 147, 158 Device, 71–78, 80–85, 90, 91, 93, 104–106, 109, 145–148, 152–154, 158, 162, 166, 167, 169, 172, 173, 333, 334, 336–345, 347–349, 436, 438–440, 442, 445, 448–453, 455, 457 Diameter distribution, 232, 237 Dielectric, 1, 2, 8, 10, 11, 13, 14, 31, 32, 121 Diffusion, 20, 21, 71, 73, 74, 79, 93, 102, 113, 128, 131, 133, 158, 161–163, 165–167, 173, 233, 236, 237, 252,

E Eco-friendly, 170 Edge planes, 233 Effective surface area, 439 Efficiency, 10, 13, 25, 119, 125–127, 129 Elastic, 184 Elasticity, 180, 275, 298, 300 Electrical conductivity, 33, 42, 44, 45, 47, 49, 51, 52, 57, 59, 61, 62, 91, 92, 109, 179, 180, 185, 191, 197, 198, 201– 205, 207, 215, 218, 229–231, 241, 245, 246, 249, 250, 257, 261, 263, 266, 267, 271, 272, 278, 279, 281, 283, 288, 297, 298, 301, 303, 305, 308, 310, 314, 315, 318, 323, 325, 339, 341, 343, 354, 358, 359, 361, 362, 364, 380, 388, 389, 397, 400, 404, 406, 412, 419, 421–423, 429 Electrical contact, 305 Electrical double layer capacitor, 179, 202, 229, 271, 278 Electrical energy, 2, 7, 8, 10 Electrical energy storage devices, 30, 52 Electrical properties, 436, 438, 441, 455 Electric double layer, 114, 115, 121, 123, 129, 130, 135, 137, 138, 141, 452 Electric double layer capacitor, 15, 34, 73, 74, 76, 84

Index Electric vehicle, 115, 468, 469, 471, 479 Electroactive, 2, 12, 92, 101, 148, 158, 163, 166, 167 Electro-active material, 73 Electroactive site, 114 Electroactivity, 338, 339 Electrocatalyst, 305 Electrocatalytic, 202 Electrochemical, 1–5, 7, 8, 13, 14, 17, 19, 20, 25, 29–33, 35, 38–40, 42, 43, 45– 49, 51–57, 89–92, 94–96, 99, 105– 109, 120, 121, 124–127, 129–131, 133–136, 138, 139, 146, 147, 150, 153–157, 159–163, 165–170, 172– 175, 180, 187, 191, 194, 196, 202– 205, 207–210, 213, 215–220, 222, 224, 229, 230, 238, 240, 241, 245, 246, 249–259, 261–267, 271, 272, 278, 281, 282, 284, 285, 287, 289, 292, 293, 297, 298, 300–313, 316– 319, 322–326, 334, 339, 341–348, 354, 355, 357, 359–366, 369–371, 373, 375, 376, 379, 380, 388, 389, 391, 396–401, 403, 404, 406–412, 414–416, 419, 421–427, 429, 431, 436, 437, 439, 450, 452–455, 457 Electrochemical activity, 4 Electrochemical capacitors, 3, 5, 7, 8 Electrochemical double layer capacitor, 90, 334 Electrochemical energy, 29–31, 49 Electrochemical energy storage systems, 2, 25 Electrochemical impedance spectroscopy, 71, 72, 75, 77, 78, 81, 85 Electrochromic, 355, 463, 466, 477 Electrode, 29, 31–37, 40–42, 44–47, 49, 51– 57, 59–62, 64, 65, 71–76, 78–85, 145–155, 158–163, 166–170, 172– 174, 333, 337, 338, 340–343, 347, 349, 435–457 Electrode-electrolyte interfaces, 3, 5, 8, 12, 13, 15, 16, 20, 31, 34, 36, 72, 73, 80 Electrode material, 89–92, 94–97, 99, 101, 102, 104, 109, 113–115, 120, 124, 126, 129–131, 133–138, 141, 179, 180, 189, 191, 196, 205, 207, 211, 212, 215, 222, 223, 229–231, 245, 246, 249–252, 254, 255, 257, 258, 261, 264, 266, 272, 279, 282, 288, 293, 297, 298, 301, 303–305, 308– 310, 315–319, 322, 324, 325, 333, 336, 337, 340–343, 348, 353, 360,

487 361, 363, 364, 368, 370, 373, 387, 389, 390, 397, 398, 400, 402–404, 406–409, 411, 412, 414, 436–438, 440–446, 449, 457 Electrodeposition, 304, 323, 326, 341, 364– 367, 373, 381, 399, 423, 430 Electrolyte, 3–5, 8, 11–16, 18, 20, 21, 29, 31, 32, 34–43, 45, 48, 51, 53, 56, 57, 59, 62, 64, 65, 71–73, 75, 78, 81– 84, 90–97, 100, 102–105, 109, 113– 115, 120, 121, 123, 124, 126–141, 146, 147, 149–152, 155, 158, 160, 162, 163, 166, 170, 173–175, 180, 185, 187, 189, 191, 192, 194, 202, 203, 205, 207, 211, 215, 217–219, 222, 230–233, 236–240, 246, 249, 250, 255, 256, 258, 261, 272, 279, 280, 283–285, 287, 288, 293, 298, 302–305, 309, 311, 312, 315–318, 320, 321, 325, 337, 341, 342, 344, 345, 353, 357, 360, 363, 366, 369– 371, 376, 378, 379, 388, 389, 398, 400–404, 406, 408, 410–412, 414, 416, 417, 419, 421, 422, 425–427, 435–437, 439, 442, 448, 450–455, 457 Electrolyte ion, 33, 43, 47, 51, 61, 64 Electrolytic ion, 3–5, 11, 12, 15, 17, 18, 24, 25, 74, 80, 81 Electrolytic medium, 2 Electronic, 180, 185, 189, 196, 230, 231, 241, 246, 247, 257, 265, 272–277, 292, 298, 299, 303, 304, 311, 322, 324, 354, 355, 358, 360, 363, 373, 375, 380, 382, 388, 389, 396, 397, 403, 406, 425, 429, 436, 451, 456, 457 Electronic conductivity, 12, 22 Electronic gadgets, 9 Electrons, 2, 4, 8, 10–13, 15, 19–22, 24 Electron transfer, 34, 35, 44, 54, 121, 135 Electron transport, 437 Electron withdrawing group, 343 Electrophoretic deposition, 237, 279, 287 Electrospinning, 179–181, 183–185, 187, 192–195, 197, 203, 206, 209, 211, 212, 218, 224 Electrostatic, 34, 72–75, 233, 252, 344 Electrostatic energy, 2 Electrostatic separation, 114 Elevators, 477 Emeraldine base, 339

488 Energy, 29–33, 39, 43, 45, 47, 49, 50, 60, 62, 64, 65, 89–91, 104, 105, 109, 179, 180, 185, 191, 192, 195, 198, 201–205, 211, 215, 219, 463, 464, 466–472, 474, 476, 477, 479 Energy density, 2–4, 7, 10–14, 22–25, 29– 32, 36, 37, 40, 41, 45, 47–49, 51–54, 60, 65, 72–74, 78, 82, 89, 90, 105, 106, 109, 114, 115, 125, 126, 129– 132, 135–138, 141, 201–203, 205, 207, 212, 215, 218, 222, 225, 230, 232, 234, 241, 245, 246, 249, 250, 254, 256–259, 261, 262, 264, 266, 272, 281, 283–285, 290, 293, 297, 298, 303, 305, 307, 308, 310–314, 317, 319, 322–325, 353, 354, 363, 364, 366, 369, 370, 375, 376, 378, 380, 382, 388, 389, 397, 400, 404, 406, 407, 425–427, 431, 435, 436, 438, 442, 448–453, 456, 457 Energy sources, 334 Energy storage, 1–3, 5, 7–10, 13, 25, 146, 147, 149, 154, 155, 354, 355, 359, 360, 363, 364, 375, 380, 387, 392, 395–398, 400, 414, 425 Energy storage devices, 30, 37, 48, 52, 53, 61, 64, 435, 436, 438, 450, 451, 457 Epitaxial, 276, 300 Equivalent, 125, 128, 129, 132–134 Equivalent series resistance, 77, 79, 81, 82, 85, 187, 191, 403, 412 Ethylene carbonate, 39 Ethylene glycol, 185 Excitation, 359 Exfoliation, 273–275, 277, 278, 299, 300 Extrinsic electrode, 20 F Fabrication, 89, 90, 105, 187, 191, 192, 202, 207, 209, 210, 213, 214, 230, 231, 238–240, 247, 249, 252, 256, 257, 259, 261, 262, 284, 288, 301, 302, 306, 317, 320, 322, 323, 325, 364, 366, 374, 375, 377, 378, 380, 381, 387, 388, 395–398, 407, 412, 418, 421, 422, 424–426, 429, 430, 431, 435, 436, 443, 448, 450, 451, 455 Faradaic, 90–94, 101, 102, 106, 145–147, 150, 153–155, 158, 160, 163, 166, 167, 169, 231, 234, 246, 250, 252, 289, 389, 414, 449 Faradaic process, 1, 12, 19, 24, 31 Faradaic reaction, 73

Index Faradic, 115, 124, 128, 129, 131, 134, 202 Fe2 O3 , 415, 420 Fe3 O4 , 305 Feedstock, 181 Fermi, 273, 274 Fiber, 437, 448, 449, 453–457 Fiber axis, 180, 183 Fiber core, 183 Fiber-shaped device, 448 Field emitters, 231, 247 Filament, 203 Film, 195, 196, 208, 209, 218, 236–239, 250, 253, 258, 271, 275, 277, 278, 284, 286–288, 292, 293, 300, 302, 311, 313, 314, 321, 326, 361, 362, 366, 367, 375, 376, 380, 381, 389, 407, 416, 421–423, 425, 429, 431, 437, 450, 454, 455 Fish gill, 138, 139 Flexibility, 272, 275, 281, 282, 284, 288, 292, 300, 303, 313, 314, 317, 325, 361, 375, 380, 416, 425, 429 Flexible, 104, 448, 450 Flexible electronics, 463 Flow supercapacitor, 457 Forward scan, 75 Fossil fuel, 2, 30, 114, 116 Fracture toughness, 300 Framework, 439 Frequency, 77–81, 85 Fuel cells, 2, 3, 7–10, 29, 30, 72, 334, 339, 463, 464, 469, 471, 477 Fullerene, 231, 247, 273 Functional groups, 115, 119, 120, 123, 124, 127, 130, 131, 133, 135, 231, 232, 234, 249, 253, 280, 305, 306, 355 Functionality, 21, 32, 45, 47, 189, 191 G Galvanostatic charge-discharge, 71, 75–77, 80–83, 85, 187, 189, 191–194 Gasification, 117, 118 Gas phase, 180 Gel electrolyte, 39, 40, 60, 192 Gel polymer electrolyte, 40, 41 Gouy–Chapman model, 17, 18, 34 Graphene, 33, 35, 41, 42, 45, 49–52, 58, 59, 65, 91, 180, 225, 230–232, 246–248, 261, 271, 297–300, 302, 303, 305– 321, 323–326, 388, 389, 392, 395– 397, 400, 404, 406, 407, 412–417, 425–428, 430, 435, 436, 438, 439, 445, 446, 448–450, 452, 453, 457

Index Graphene oxide, 261, 396, 397, 404, 406, 407, 412, 416 Graphite, 273–279, 299, 300, 321 Graphite slurry, 453 Graphitic, 181–183, 185, 210 Graphitization, 130, 276 Green energy, 115, 135 Growth, 338, 341, 343, 344, 346, 347, 349

H Healthcare, 464, 475 Heat treatment, 183, 219, 220, 224, 234, 235 Helmholtz layer, 11 Helmholtz model, 34, 35 Hemp, 137, 140 Herringbone, 180–182 Heteroatom, 21, 49, 52, 113, 115, 119–121, 123, 129–131, 133–135, 138, 141, 150, 154, 155, 158, 189–191, 358 Hierarchical, 113, 116, 120, 127, 129, 130, 209, 211, 219–222, 224, 234, 235, 250, 251, 261, 264, 265, 288, 304, 321, 326, 363, 365, 376, 395, 398, 399, 412, 416, 419, 421, 430 Hierarchical porous carbon, 45, 46 Hierarchical structure, 234 High-performance device, 7, 13, 22 Homogeneity, 168 Homogeneous, 158, 160 Honeycomb, 273, 274, 299 Human hair, 129, 138–140 Hybrid, 105, 109, 145, 147, 149, 152, 153, 163, 167, 172, 173, 180, 198, 201– 203, 205, 207, 209, 215, 219, 220, 222, 223, 241, 246, 251, 252, 256, 257, 259, 264–267, 272, 285, 293, 297–299, 305, 307, 309, 311–314, 316, 317, 319–321, 323–325, 354, 357, 364, 387, 388, 398, 424–427 Hybrid electric vehicles, 3, 436 Hybrid supercapacitor, 31, 40, 41, 65, 72–75, 334, 335 Hydrocarbon, 179, 181, 231, 247, 277, 278, 300 Hydrogel, 302, 439 Hydrolysis, 263, 267 Hydrophobic, 233, 278 Hydroquinone, 453 Hydrothermal, 92, 95, 99, 101, 107, 108, 119, 131–133, 137, 139, 140, 204, 206, 208, 209, 217, 224, 249, 251, 256, 258, 266, 301, 304, 305, 309,

489 311, 315–317, 319–321, 324, 326, 327, 365, 381, 391, 392, 399, 401, 402, 405, 407, 412, 425, 430 Hysteresis, 274

I Ideal supercapacitor, 76 Imaginary capacitance, 79, 80 Impedance, 77, 78, 124, 125, 132, 133 Impedance spectra, 78 Impedance spectroscopy, 187 Impregnation, 170 Impurities, 231, 232, 247, 249 Inductance, 77 Industry, 464, 469, 473 Insertion, 4, 20, 21, 213 Insulators, 91, 358–360 Intercalate, 118, 131, 300 Intercalation, 4, 19–21, 24, 74, 95, 189, 360 Intercalation pseudocapacitance, 35, 36 Interconnected, 166, 342 Interconnection, 408 Interface, 113–115, 120, 121, 126, 128–130, 138, 180, 185, 202, 203, 205, 207, 225, 230, 233, 237, 250, 257, 272, 276, 280, 298, 303, 334, 344, 353, 360, 363, 388 Interlayer spacing, 231, 247 Internal resistance, 76 Internet of Things, 474 Intrinsic electrode, 20 Ionic conductivity, 4, 13, 14, 81 Ionic electrolyte, 14 Ionic liquid, 29, 39, 53, 65, 337, 361 Ions, 1, 2, 4, 8, 11, 13–15, 18, 20, 21, 24, 186, 187, 189, 193, 198, 202–205, 213, 215, 222, 223 Ion transportation, 4, 20 IR drop, 72, 79, 81 Iron oxide, 102, 104, 173, 211 Irradiation, 300, 357

K Kelp, 135, 136, 140 Kinetics, 4, 18, 20, 180, 211, 333, 336, 337, 343, 361, 373, 421

L Laser ablation, 231, 247 Laser etching, 450, 451 Lattice, 118, 253

490 Layer, 334, 336 Leucoemeraldine, 339 Leyden Jar, 5 Life cycle, 272 Lightweight, 355 Linear charge discharge curve, 77, 83 Lithium-ion battery, 153, 457 Lithography, 276 Loading, 151, 159, 161, 162, 166–170, 172, 174

M Macrofilm, 238, 239 Macropores, 45, 46 Magnetic material, 455 Manganese dioxide, 151, 153–156, 160, 162–166, 174 Manganese oxide, 56 Martensite, 453 Mass density, 246, 267 Mass ratio, 85 Materials, 436, 437, 439–444, 446, 455, 457 Matrix, 115, 118, 120, 121, 124, 131, 133, 138, 191, 213, 217, 340, 343 Mechanical, 184, 195, 219, 229–231, 240, 241, 245–247, 249, 259, 267, 272, 273, 275, 276, 278, 283, 284, 288, 292, 293, 300, 301, 314, 325, 362, 392, 395, 396, 403, 404, 409, 419, 423 Mechanical exfoliation, 49, 50 Mechanical strength, 337, 400, 408 Mechanism, 114, 118, 120, 121, 123, 126, 130, 134, 135, 139–141, 179–181, 185, 198, 202, 203, 205 Medical, 465, 475–477 Memory, 463, 466, 475, 476 Mesocarbon, 166 Mesopore, 45, 46, 117, 118, 120, 134, 161, 341, 342 Mesoporous, 96, 98, 101, 185, 186, 189, 190, 233, 235, 246, 252, 402, 403, 408, 409, 430, 437 Metal, 179, 181, 182, 195, 196, 198, 201– 205, 207–209, 215, 220, 223–225 Metal hydroxide, 37, 52 Metallic, 90, 91, 93, 273, 299 Metal nanotubes, 437 Metal-organic framework, 30, 52, 61, 215, 223 Metal oxide, 1, 4, 12, 13, 21, 22, 24, 29, 32, 53, 56, 57, 60, 61, 72–74, 145–149,

Index 151–153, 155–160, 162, 165, 168, 170, 172–175, 333, 336, 340, 347, 348 Microfiber, 184 Micropore, 45, 46, 51, 117, 127, 130, 137 Microscopy, 163, 165 Microsphere, 119 Microstructure, 147 Microtube, 129, 132 Microwave, 95, 100, 101, 107, 108, 119, 133, 134, 140, 156, 157, 169 Military, 464, 465, 469, 470, 479 Mirror symmetry, 213 Mixed transition metal oxide, 444, 449 Mixing, 170 MnO2 , 89, 91, 94–98, 104, 105, 107, 109, 201, 205, 207–215, 224, 246, 250– 252, 255–260, 266, 267, 303–305, 310–314, 326, 364–376, 378, 381 Mobility, 121, 130, 169, 274, 299, 359, 361, 396, 421 Modulus, 183, 184 Molar mass, 207 Molecular structure, 355, 356 Momentum, 274 Mono-layer, 273, 299 Monomer, 355, 357, 368, 397, 416 MoO3 , 415 Morphology, 90–95, 99, 101–103, 105, 106, 109, 113, 115, 116, 121, 122, 131, 141, 147, 154, 155, 160, 163, 166, 174, 183, 191, 192, 204, 205, 211, 216–218, 221, 232, 233, 236, 237, 240, 249–251, 253, 256, 259, 262– 264, 266, 267 Multifunctional, 109 Multi-Walled Carbon Nanotubes (MWCNT), 230, 247–249, 252, 253, 261 Municipal waste, 116 MXenes, 30, 62 N Nanoarchitecture, 92, 235, 263 Nanocomposite, 212–217 Nanocrystalline, 253 Nanodevice, 342 Nanoelectronics, 276 Nanofibers, 211, 224, 339, 345, 346, 348, 395, 403, 412, 414, 416, 428 Nanofibrillar, 344 Nanoflake, 105, 108, 251, 256, 261, 267, 399, 411, 412, 430

Index Nanoflowers, 95, 102, 103, 108, 442 Nanofoam, 173 Nanohybrid, 211 Nanometer, 229, 230, 247, 279 Nanoparticle, 168, 169, 173, 412, 420, 430 Nanoplate, 99 Nanoplatelets, 222, 224, 444, 457 Nanopowders, 348 Nanorods, 93, 95, 101, 108, 339, 345, 348 Nanoscale, 93 Nanosheets, 101, 107, 208, 219, 220, 224 Nanoslice, 99 Nanosphere, 100, 107, 339, 345, 346 Nanostructure, 95, 99, 191, 202, 204, 209, 210, 213, 220, 222, 224, 225, 231, 249, 264, 301, 324, 342–347, 354, 357, 359, 360, 362, 364, 365, 368, 375, 390, 391, 398, 399, 425 Nanotube, 93, 95, 101, 108, 185, 186, 229– 231, 233, 236, 245–249, 254, 273, 279, 283, 301, 312, 339, 346, 347 Nanotubular, 251, 252 Nano-urchins, 95 Nanowire, 95, 96, 98, 101, 103, 105–108, 362, 365, 366, 373–376, 381, 399, 407, 408, 412, 430 N-doped, 337, 338, 341, 343 N-doping, 358–360 Negatively charged, 4, 21 Network, 190–192, 208, 215, 217, 224, 252, 266, 395, 402, 407, 410, 416, 419– 421, 423, 430 Network structures, 213, 342 Nickel cobaltite, 444 Nickel foam, 42, 56, 234 Nickel oxide, 30, 56, 151, 153–157, 160, 166, 167, 174 Ni foam, 234, 235, 287, 313, 319, 320, 376 NiO, 89, 91, 97, 99, 100, 107, 109, 246, 249– 252, 261–263, 267, 301, 303–305, 317–321, 323, 326 Ni(OH)2 , 304, 305, 317, 319–321, 326 Non-degenerate, 359 Non-Faradaic mechanism, 73 Non-Faradaic process, 12 Nyquist plot, 78, 79, 128–130, 132, 236

O Ohmic drop, 187 One-dimensional, 202, 220, 271, 278, 281, 293, 302 Optical, 355, 360, 375, 376, 378

491 Organic electrolyte, 14, 37–39, 45, 51 Oxidation, 151, 162, 163, 167, 172, 189, 217, 218, 234, 250, 266, 277, 300, 303, 321, 324, 334, 337, 339, 347, 355, 357–361, 363, 369, 371, 373, 397, 402, 408, 411, 421, 430 Oxidation peak, 84 Oxidation states, 145, 151, 153, 160, 163, 167, 172 Oxides, 90–92, 94, 97, 101, 102, 104, 107 Oxygen content, 233 P Parallel fiber, 448 Particle, 211, 217, 223, 224 Particle-particle interaction, 78 Particle size, 159, 175 Path length, 354, 359, 362, 380, 426, 442 Paulownia flower, 130, 139 P-doped, 337, 343 P-doping, 358–360 PEDOT, 361, 373–375, 378, 381, 406, 421, 422, 424, 426, 430, 431 Penetration, 185, 189, 219, 416, 437 Performance, 1–5, 7, 8, 12, 13, 25, 29–33, 37–39, 41–43, 45–48, 51, 52, 54–57, 65, 71–75, 78, 82–85, 113, 114, 120, 124–127, 129–135, 137–139 Permeable, 104 Permittivity, 185, 203 Pernigraniline, 339 Phonon, 274, 275, 299 Photochemical, 355, 357 Photochromic, 355 Physicochemical properties, 146, 150, 154, 174 Physisorption, 3 Piezoelectric, 436, 454, 455, 476 Piezoelectric supercapacitor, 454 Plasma, 231, 247 Plasma enhanced, 209 Plastic deformation, 453 Platelet, 180–182, 222, 224 Polarity, 169 Polarizable electrode, 77 Polarization, 11, 127, 165, 169 Polaron, 334, 358, 359 Polyacetylene, 337, 338, 355, 356, 359 Poly(acrylonitrile), 206 Poly(acrylonitrile) fiber, 183 Polyaniline, 21, 41, 53, 59, 333, 334, 336, 337, 339, 355–357, 361–364, 366– 370, 375, 379, 381, 397, 406, 446

492 Polyhedral, 261 Polymer, 146–148, 151, 152, 179, 183, 192, 238, 272, 298, 334, 336–341, 343, 344, 354, 355, 357–359, 361, 371, 373, 395, 397, 403, 407, 415 Polymer composites, 340, 347, 348 Polymer electrolyte, 340 Polymeric chain, 355 Polymerization, 343, 344, 347, 355, 357, 366, 368, 370, 371, 373, 381, 397, 407, 409–412, 416, 425, 430 Poly(p-phenylenevinylene), 355 Polypyrrole, 53, 59, 187, 197, 333, 334, 337, 339, 341, 342, 355–357, 359, 361, 362, 370–373, 375–377, 381, 397, 405, 406, 416, 417, 419–421, 426, 429–431 Polystyrene sulfonate, 406, 421–423, 425, 427, 428, 430 Polythiophene, 333, 334, 337, 339, 343, 355, 356, 359, 361, 397, 406 Pore, 79, 80 Pore size, 4, 12, 18, 22, 33, 35, 39, 44–47, 52, 53, 57, 61, 78, 113, 121, 129, 133, 137, 146, 154, 158, 180, 185, 203, 204, 230, 232, 248, 249, 272, 288, 298, 301, 305, 372 Pore size distribution, 150, 160, 230, 233 Pore volume, 147 Porosity, 33, 43, 47, 48, 57, 61, 62, 145, 147, 148, 159, 163, 167, 203, 223, 341 Porous, 91, 93, 99, 102, 107, 114–117, 120, 123, 127, 130–135, 230, 234–237, 250, 256–258, 261, 262, 266, 267, 278, 283, 286–288, 292, 293, 302, 305, 307, 309, 311, 312, 317, 318, 325, 326, 344, 366, 369, 371, 373– 375, 381, 395, 398, 404, 407, 411, 412, 414, 416, 419, 423, 424, 430, 437, 439, 442, 452 Porous carbon potential drop, 72 Porous electrode, 1, 3 Porous structure, 185, 192 Portable devices, 467, 468 Positive charged, 4, 17, 21 Potential, 29, 30, 34–39, 41, 51, 60, 61, 65, 75–81, 83–85, 89, 91, 93, 94, 97, 99, 101, 105, 109, 229, 240, 245, 246, 250, 261, 264, 271, 272, 281, 284, 289, 303, 308, 313, 315, 317, 318, 321, 323, 357, 360, 361, 363, 364, 366, 367, 371, 373, 375, 379, 381,

Index 389, 392, 397, 398, 400, 406, 407, 421, 425, 426, 429 Potential ramp, 75 Potential range, 343 Potentiostatic, 416, 430 Power, 180, 192, 196, 201–203, 205, 211, 212, 215, 218, 219, 222, 223, 225, 229–233, 236, 240, 241, 249, 254, 257, 264, 265, 271, 272, 281, 284, 285, 290, 292, 293, 298, 324, 336, 347, 349, 354, 376, 380, 382, 388, 396, 428, 435, 436, 438–440, 442, 448, 449, 451, 452, 456 Power backup, 456 Power density, 1–4, 7, 10–13, 20, 22–25, 30– 32, 40, 47, 49, 52, 53, 60, 65, 91, 105, 106, 109, 114, 120, 125, 126, 129, 131, 132, 134, 135, 138, 141, 180, 189, 191, 192, 195, 198, 202, 203, 205, 212, 218, 219, 222, 225, 232, 234, 237, 239, 241, 245, 246, 250, 252, 254, 256–258, 261, 264, 267, 271, 272, 278, 281, 284, 285, 290, 292, 293, 298, 299, 301, 303, 304, 307, 308, 311–314, 317, 319, 321– 325, 334, 336, 354, 363, 366, 369, 370, 375, 376, 378–380, 388, 389, 398, 400, 403, 404, 421, 425–428, 435, 436, 438, 439, 448, 449, 451, 452 Power electronics, 464, 472, 476 Practical evaluation, 83 Precursor, 92, 101, 114–116, 120, 121, 126, 127, 130, 131, 134, 138–140, 150, 154, 179, 183, 185, 187, 196, 197, 204, 206, 211, 231, 247, 249, 252, 253, 264, 277, 290, 300, 301, 305, 306, 375, 376 Printing, 450, 451 Processability, 339 Processing, 181, 183–185, 187, 203 Protic, 361 Protons, 441 Pseudocapacitance, 4, 12, 19, 20, 25, 113, 123, 129, 133–135, 141, 146, 148, 150, 153–155, 158, 159, 167, 170, 172, 189, 219, 231, 232, 249, 252, 257, 272, 280, 289, 298, 309, 364, 369, 389, 400, 408, 414, 421, 429 Pseudocapacitor, 1, 3, 4, 7, 19, 20, 29–31, 36, 37, 72–74, 76, 77, 83, 90, 91, 94, 334, 335, 347, 442, 443 Purity, 183, 231, 232, 247

Index PVDF film, 454 Pyrolysis, 116, 118, 127, 130, 134, 206, 215, 223, 224, 249, 261, 267 Pyrrole, 357, 373

Q Quasi-solid-state electrolyte, 37, 40 Quinoid, 409

R Radical, 357, 359 Ragone plot, 2, 3, 105, 106, 191, 192, 195, 254, 255, 264, 265 Rate capability, 3, 4, 7, 9, 12, 13, 20–22, 24, 25, 37, 49, 52, 58, 60, 145, 147, 148, 158, 159, 161–163, 166, 172, 174, 185, 191, 195, 198, 201–203, 205, 211, 213, 216, 225, 299, 303, 307, 316, 319, 322, 324, 368, 369, 375, 378, 379, 388, 389, 398, 404, 409, 418, 422, 423, 424, 428 Reactivity, 154, 167 Real capacitance, 76–78 Rectangular, 187, 191, 211, 213, 218 Rectangular shape, 408, 422, 426 Redox, 91, 93, 95, 97, 101, 198, 202, 205, 207, 211–213, 215, 219, 220, 222– 225 Redox active, 203 Redox active sites, 4 Redox activity, 33, 75 Redox pseudocapacitance, 35, 36 Redox reaction, 1, 2, 13, 19–21, 24, 89–93, 95, 97, 101, 106, 145, 146, 150, 151, 153, 155, 158, 160, 163, 167, 169, 172, 173, 231, 246, 249, 250, 272, 280, 289, 298, 303, 307, 308, 321, 322, 325, 334, 336, 339, 353, 354, 363, 371, 373, 378, 380, 388, 397, 408, 416, 430, 441, 442, 444 Redox site, 389, 406, 415, 429 Reduced graphene oxide, 49, 52 Reduction, 151, 156, 159, 162, 163, 167, 169, 172, 337, 340 Reduction peak, 75 Reference electrode, 75 Regenerative braking energy, 3 Regenerative energy, 468, 469, 471 Relaxation time, 80, 81 Renewable energy, 2, 3, 30, 463, 465, 471 Resilience, 249, 252

493 Resistance, 31, 37, 39, 42, 43, 45–47, 51, 53, 56, 71, 72, 76–79, 81, 93, 101, 120, 124–126, 128–135, 137, 159– 162, 167, 169, 187, 195, 196, 217, 222, 225, 235–237, 246, 252, 258, 266, 284, 305, 310, 320, 355, 360, 370, 371, 390, 397, 403, 409, 411, 419, 423, 426, 449, 453, 455 Resistive losses, 10 Resistivity, 167 Resol, 189, 197 Response, 187, 191 Restacking, 297, 298, 302, 325 Retention, 105, 129, 132–135 Reverse scan, 75 Reversible redox reactions, 1, 4, 12, 19, 25, 441, 444 Rice husk, 130, 133, 138–140 RuO2 , 89, 91–94, 101–103, 107, 109, 201, 205, 207, 208, 215–217, 220, 224, 246, 250–255, 266, 303–310, 314, 326 Ruthenium oxide, 7, 36, 54, 151, 153–156, 160–162, 174 S Salt, 75 Salt effect, 13 Scanning Electron Microscopy (SEM), 96, 98, 100, 103, 190, 191, 193, 195, 196, 210–212, 223, 237, 238, 240, 257, 260, 282–284, 286–288, 290, 306, 307, 311, 312, 320, 363, 366–368, 371–376, 407–413, 416, 419–422, 424 Scan rate, 93, 94, 99, 104, 106, 187, 189, 191, 192, 194, 195, 211–219, 222, 236, 237, 239, 256, 258–260, 279, 280, 284, 289, 291, 308–311, 313– 316, 318–321, 368–371, 374, 379, 410–412, 414–417, 420, 421, 422, 424–427 Scattering, 274, 299 Second generation supercapacitor, 4, 5 Self-activation, 118 Self-adhering, 455 Self-discharge, 336, 354, 380 Self-discharge rate, 83 Self-healing, 436, 451, 455, 456, 466 Self-healing supercapacitor, 455 Semi-circle, 236 Semiconductor, 91, 168, 231, 247, 273, 274, 299, 355, 358, 359

494 Sensors, 231, 247, 249 Separator, 3, 8, 11, 14, 29, 30, 32, 39, 40, 43, 44, 65, 73, 83, 104, 124, 435, 436, 448, 454, 455, 457 Shape-memory, 436, 453, 454, 466 Shape memory supercapacitor, 453 Short fiber, 166 Shrinkage, 340, 407 Shrinking, 13, 24, 74 Single-layer, 273, 275, 299, 300 Single-Walled Carbon Nanotubes (SWCNT), 230, 247–249, 263, 437 Sluggish electron, 31 Smart meter, 477 SnO2 , 246, 251, 399, 415, 420, 425, 430 Soft robotics, 475 Solar cells, 334 Sol-gel, 107, 301, 305, 308, 309, 326 Soliton, 358, 359 Solvent, 13, 14, 38, 39, 61, 75 Solvothermal, 95, 101, 108, 209, 219, 220, 224 Sonication, 300, 326 Sonochemical, 407, 430 Soya, 121, 122, 129, 139 Specific capacitance, 91–97, 99–106, 109, 146–148, 155, 159–163, 165–170, 172, 173, 179, 180, 185, 187–189, 191–195, 201–203, 205, 207, 211– 219, 221, 222, 225, 229, 232, 234– 240, 245, 249, 250, 252–261, 263– 267, 272, 279–282, 284–288, 290, 291, 293, 297, 298, 303, 305, 307, 309–312, 315–321, 323–325, 361– 364, 366–369, 371, 373, 375, 379, 388, 389, 397, 398, 400, 404, 406, 408–412, 414–417, 419, 420, 423– 428, 431, 436–448, 453–455, 457 Specific energy, 334, 336, 341, 347 Specific masses, 341, 342 Specific power, 343 Specific surface area, 113–116, 118, 134 Spintronics, 475 Spray deposition, 238 Stability, 114, 115, 125–127, 129, 132, 133, 135, 136, 138, 141, 207, 218, 219, 339, 340, 343, 344 Stack, 231, 259 Stern layer, 18, 35 Stern model, 34, 35 Storage capacity, 180 Storage devices, 180, 202, 205, 333, 334, 339, 341, 346

Index Storage systems, 90, 435, 436 Strain, 238, 239 Stress, 195 Structure, 33–35, 39, 43, 45–48, 51–54, 56, 57, 59, 61, 62, 437, 439, 442, 444, 449, 452, 455 Sublimation, 276 Substrate, 238, 239, 274, 276, 277, 287, 309, 320 Sunflower seed, 127, 139 Supercapacitor, 1–15, 19, 21–25, 29–35, 37– 45, 47–49, 51–54, 56, 57, 60–62, 64, 65, 71–73, 75–85, 89–93, 95, 97, 99, 101–106, 109, 113–115, 119, 120, 124–126, 128–130, 133–139, 141, 145–147, 149–155, 158–160, 162, 167–169, 172, 173, 179, 180, 184– 187, 189–198, 201–205, 207, 209, 211–213, 215, 217–220, 222, 223, 225, 229–232, 234–241, 245, 246, 249–267, 271, 272, 278–285, 287– 293, 297–299, 301–305, 307, 308, 310–318, 320–324, 333–345, 347– 349, 353, 354, 359–364, 366, 370, 371, 373, 375–382, 387–389, 391, 392, 395, 397, 398, 400, 401, 403, 404, 406, 409, 411, 412, 415, 416, 421, 425–429, 431, 435–443, 441, 442, 446, 448–457, 463–479 Supercapacitor device, 180, 192, 195, 197, 219, 220, 222, 231, 238, 239, 246, 256–258, 261, 262, 264, 265, 287, 288, 300, 311–318, 321, 322, 324, 325, 375, 377, 397, 398, 400, 403– 405, 425–429 Supercapacitor performance, 203, 207, 211, 213, 219 Support, 161, 169, 174 Supramolecular, 284, 455 Surface area, 1, 4, 11–13, 15, 18, 22, 31, 33–35, 37, 44, 45, 47–49, 51, 52, 56, 57, 59, 61, 90–93, 95, 100–102, 109, 145–151, 153–155, 158–161, 163, 165–170, 172, 173, 179, 180, 184– 187, 189, 191–194, 197, 198, 201– 208, 211, 218, 219, 222, 225, 229– 234, 237, 240, 241, 245, 246, 248– 251, 253, 256, 257, 261–263, 266, 267, 271, 272, 275, 277–279, 281, 283, 284, 286–288, 292, 293, 297, 298, 300–307, 309, 311, 312, 315, 318, 322–325, 334, 342, 343, 347, 353, 354, 359, 360, 362, 363, 366,

Index 369–371, 373, 380, 382, 388, 389, 391–393, 395, 398, 400, 403–410, 412, 415, 422, 423, 427, 429, 431, 436–439, 441–445, 448, 449, 452 Surface tension, 183 Surface texture, 113 Sustainable energy, 2, 114 Swelling, 13, 22, 24, 74, 340, 407, 415 Symmetric, 127, 129–131, 133–138, 158, 159, 166, 187, 191, 211, 256, 259, 263, 338, 344 Symmetric configuration, 72, 74 Symmetric device, 104, 105 Symmetric supercapacitor, 60, 80 Synergetic, 189 Synergism, 250 Synergistic, 72, 74, 130, 141, 155, 159, 163, 166, 169, 202, 219, 222 Synergistic effect, 4, 410, 415, 418, 426, 431, 444 Synthesis, 93, 95, 97, 99, 101, 106–108, 229, 241, 246, 247, 249, 251, 256, 271– 273, 275–278, 286, 291, 298–301, 304, 305, 307, 309, 317, 354, 355, 357, 359, 360, 364, 365, 368, 388, 389, 391, 392, 395–400, 404, 427, 436, 450

T Tea leave, 127, 139 Technology, 457 Temperature, 453, 456 Template, 185, 189, 197, 257, 342–344, 346 Tensile strain, 238, 239 Tensile strength, 275, 300 Ternary composites, 388, 389, 406–431 Texture, 121, 127, 130 Thermal stability, 14, 33, 43, 56, 83, 272, 278, 301, 438, 441 Thermogram, 116 Thickness, 203, 209, 210, 218 Thin film, 92, 93, 104, 105 Thiophene, 341, 343, 357, 406 Three-dimensional, 271, 278, 286, 293, 302, 309, 312 Tin oxide, 157, 173 TiO2 , 390, 407, 408, 410, 415, 430 Titanium carbide, 452 Titanium oxide, 168, 169, 174 Toxicity, 155, 173 Transition metal, 231, 241, 245–248, 250– 252, 261, 263, 264, 266, 267, 272,

495 277, 297, 298, 301, 303–305, 313, 324–326, 387, 388, 390, 397, 399, 404, 406, 428 Transition metal oxides, 29, 32, 51, 53, 60, 89, 90, 102, 104, 106–108, 201–205, 207–209, 220, 223–225, 299, 301, 303, 305, 353–355, 359, 360, 363– 365, 370, 373, 375, 380–382, 387– 391, 397–399, 404, 406, 407, 409, 410, 416, 421, 425, 428, 429, 431, 435, 441, 443, 446 Transmission Electron Microscope (TEM), 96, 98, 100, 184, 186, 210–212, 217– 219, 221, 223, 234, 235, 237, 238, 253, 257, 258, 263–265, 281, 307, 363, 366, 367, 373, 375, 376, 407, 408, 412, 416, 418, 421, 422, 424 Transport, 90, 91, 93, 102, 180, 185, 198, 202, 203, 205, 215, 217, 223, 225, 230, 235, 240, 257, 261, 274, 284, 286, 293, 297, 298, 302, 305, 309, 310, 312, 319, 321, 323, 325, 354, 359, 360, 362, 366, 373, 375, 380, 388, 409, 412, 419, 422, 423 Transportation, 456, 463, 464, 468, 479 Tremella, 122, 128, 129, 139 Tubular, 180–182 Tubular network, 231, 249 Twisted fiber, 448 Two-dimensional, 271, 273, 278, 284, 293, 299, 302

U Ultracapacitor, 5, 7, 31 Ultrasonication, 396, 424, 430 Underpotential deposition, 35, 36

V Valence band, 358, 359 Vanadium oxide (V2 O5 ), 59, 172, 201, 207, 215, 217–220, 224 Van der Waal, 275, 276, 284, 299, 302, 306 Vapor growth, 180 Vapour phase polymerization, 344 Vertically aligned, 209 Viscosity, 453 Very Large-Scale Integration (VLSI), 475 Voltage, 71, 74–77, 80–83, 183, 194, 207, 215 Volume shrinkage, 415

496 W Warburg impedance, 78, 79 Water decomposition, 38 Wearable electronic, 463, 465, 466, 474 Wet chemical, 411, 430 Willow catkin, 129, 139 Wireless, 467, 473, 476, 479 Working electrode, 75, 76 Working potential, 340, 453 Working potential window, 76, 84

Index Y Yield strength, 231, 247 Young’s modulus, 231, 247, 275, 300

Z Zero-dimensional, 271, 278, 302 Zero-gap, 273, 274, 299 Zinc oxide, 169, 170, 174