Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems [1 ed.] 0128195525, 9780128195529

Table of contents :
Cover
Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems
Copyright
Contributors
1 - Basic principles in energy conversion and storage
1. Introduction
2. Lithium batteries
2.1 Battery principle and basics
3. Supercapacitors
3.1 Working principle of a supercapacitor
4. Dye-sensitized solar cells
4.1 Major components of dye-sensitized solar cells
4.2 Working principles of dye-sensitized solar cells
5. Hydrogen production by photocatalytic water splitting
6. Fuel cell
7. Conclusions
Acknowledgments
References
2 - Low-dimensional carbon-based nanomaterials for energy conversion and storage applications
1. Introduction
2. Synthetic aspects of carbon-based nanomaterials
2.1 Synthesis of carbon nanodots
2.2 Graphene preparation
2.3 Synthesizing graphene quantum dots
3. Energy characteristics of carbon nanodots
4. Potential properties of graphene
5. Carbon nanotubes in renewable energy applications
5.1 Hydrogen storage
5.2 Solar cells
5.3 Energy conversion using carbon nanotubes
5.4 Carbon nanotubes in energy storage
5.4.1 Batteries
5.4.2 Supercapacitors
6. Applications of carbon nanodots in energy conversion and storage
6.1 Application in supercapacitors
6.2 Application in Li-ion batteries
6.3 Application in solar cells
6.4 Application in light-emitting diodes
7. Applications of graphene in energy conversion and storage
7.1 Solar cells
7.2 Battery
7.3 Fuel cells
7.4 Supercapacitors
7.5 Hydrogen storage devices
8. Applications of graphene quantum dots related to energy conversion and storage
8.1 Supercapacitors
8.2 Batteries
8.3 Photovoltaic cells/solar cells
8.4 Fuel cells
9. Summary and future aspects
Acknowledgment
References
3 - Nanostructured bifunctional electrocatalyst support materials for unitized regenerative fuel cells
1. Introduction
2. Unitized regenerative fuel cell system
3. Role of electrocatalysts and electrocatalyst support materials
4. Types of electrocatalysts support materials and their performance in a URFC
4.1 Carbon structures as electrocatalyst supports
4.2 Unsupported and IrO2-supported electrocatalysts
4.3 Ti-based compounds as electrocatalyst supports
4.4 Sb-doped SnO2 and SiO2-SO3H electrocatalyst support
5. Concluding remarks
Acknowledgments
References
4 - Polymeric nanomaterials in fuel cell applications
1. Introduction
2. Polymeric nanomaterials in microbial fuel cells
3. Polymeric nanomaterials in hydrogen fuel cells
4. Polymeric nanomaterials in direct methanol fuel cells
5. Conclusions and future directions
References
5 - Nanocarbon: lost cost materials for perovskite solar cells
1. Introduction
2. Inorganic perovskite layers
3. Carbon materials for low-cost perovskite solar cells
4. Hole transport membrane
5. HTM-free cells
6. Electron transport membranes
7. ETM-free cells
8. Fullerene and its derivatives
9. Graphene and its derivatives
10. Conductive carbon
11. Embedment C-PSCs
11.1 Carbon nanoparticles
12. Conclusion and future prospects
References
6 - Recent advances in synthesis, surface chemistry of cesium lead-free halide perovskite nanocrystals and their potential appl ...
1. Perovskites-an introduction
2. Introduction of cesium lead halide perovskite nanocrystals
3. Emergence of cesium lead-free halide perovskite nanomaterials
4. General approaches in synthesis of cesium metal halide perovskite NCs and surface chemistry of binary solvent mixture
5. Development in synthesis and surface chemistry of cesium lead-free halide perovskite nanocrystals
5.1 Cesium tin halide perovskite nanocrystals (CsSnX3, Cs2SnX6 where X=Cl, Br, I)
5.2 Cesium bismuth halide perovskite nanocrystals (Cs3Bi2X9 X=Cl, Br, I)
5.3 Cesium lead-free double perovskite nanocrystals (Cs2AgMX6 where M=Bi, Sb, In and X=Cl, Br, I)
5.4 Cesium stibium halide perovskite nanocrystals (Cs3Sb2X9 where X=Cl, Br, I)
6. Miscellaneous
7. Applications of cesium lead-free halide perovskite nanocrystals
8. Conclusion and future perspectives
ACKNOWLEDGMENTS
References
7 - Hierarchically nanostructured functional materials for artificial photosynthesis
1. Introduction
2. Different types of hierarchical nanomaterials for artificial photosynthesis
2.1 Fiber-like hierarchical nanomaterials
2.2 Carbon dioxide reduction into CH4 and CO using GaN nanowire
2.3 Hierarchical nanobox-based nanomaterials
2.4 Hierarchical ZnO-based hollow nanostructures
2.5 Titanium oxide nanotubes as a photoreduction material
2.6 Silver nanowire as an artificial photocatalyst
2.7 Hierarchical based metal organic nanoflowers and nanorods
3. Application of hierarchal photocatalytic nanomaterials
3.1 Water splitting
3.2 Hierarchical nanomaterials for chemical fuels
3.3 Hierarchal structures in biofuel cells as light-harvesting systems
3.4 Hierarchical nanostructures for the production of biohydrogen
3.5 Homogeneous artificial photosynthesis system
3.5.1 Heterogeneous artificial photosynthesis system
3.6 Photosensitizers in artificial photosynthesis
3.6.1 Photosensitizers in water oxidation
3.6.2 Photoelectrochemical water splitting
4. Concluding remarks and future prospects
Acknowledgments
References
8 - New-generation titania-based catalysts for photocatalytic hydrogen generation
1. Introduction
2. Basic principle of photoelectrochemical water splitting
3. Material selection for photoelectrochemical water splitting
4. TiO2 photocatalyst for photoelectrochemical water splitting
4.1 TiO2 nanotube arrays and anodization method
4.2 The four synthesis generation of TiO2 nanotubes
5. Formation mechanism of TiO2 nanotube arrays
5.1 Mechanism of formation of TiO2 nanotubes
6. Tuning the photocatalytic activity of TiO2 into the visible light region
7. WO3-incorporated TiO2 photocatalyst
8. Preparation of WO3-TiO2 photocatalyst
9. Water photoelectrolysis using WO3-TiO2 photocatalyst
10. Conclusions
Acknowledgment
References
9 - Graphitic carbon nitride-based nanocomposite materials for photocatalytic hydrogen generation
1. Introduction
2. Road map of g-C3N4 as efficient photocatalyst for photocatalytic hydrogen generation
2.1 Electronic structure and physicochemical properties of g-C3N4 photocatalyst
3. Synthesis methods of g-C3N4 photocatalyst
3.1 Thermal heating of carbon-rich polymers
3.2 Template-based method
3.3 Sol-gel method
4. Design of various structures of g-C3N4 for photocatalytic hydrogen generation
4.1 Bulk g-C3N4 for photocatalytic hydrogen generation
4.2 g-C3N4 nanosheets for photocatalytic hydrogen generation
4.3 Porous g-C3N4 for photocatalytic hydrogen generation
4.4 g-C3N4 nanotubes for photocatalytic hydrogen generation
5. Composites of g-C3N4 for improved photocatalytic hydrogen generation
5.1 Metal/g-C3N4 composites for efficient hydrogen generation
5.2 Metal oxide/g-C3N4 composites for efficient hydrogen generation
5.3 Metal sulfide/g-C3N4 composites for efficient hydrogen generation
5.4 Metal organic framework/g-C3N4 composites for efficient hydrogen generation
5.5 Carbon-based/g-C3N4 composites for efficient hydrogen generation
6. Conclusion and outlook
Acknowledgments
References
10 - Nanostructured materials for photocatalytic energy conversion
1. Introduction
2. Hydrogen production from sunlight converting techniques
2.1 Photovoltaic technology
2.2 Wet-chemical photosynthesis
3. Photoelectrolysis for the generation of hydrogen by TiO2 nanohybrid
4. Evaluation of hydrogen by photoelectrochemical activity using nanomaterials
5. Carbon nanotube/TiO2 nanocomposite for hydrogen production
6. Induced photocatalysis over Fe2O3 for production of hydrogen from water splitting
7. Evolution of hydrogen from water photocatalytic splitting using graphene/TiO2
8. Visible light photocatalytic hydrogen production by Ti3C2 MXene cocatalyst with metal sulfide
9. Hydrogen gas for transportation and sustainable power generation
10. Tungsten-doped Ni-Zn nanoferrites for the recovery time for hydrogen gas sensing application
11. Summary
Acknowledgments
References
11 - Graphene-based composite materials for flexible supercapacitors
1. Introduction
2. Energy storage devices: an overview
3. Flexible supercapacitors: device structure and fabrication
4. Graphene composite materials-based flexible supercapacitor devices
4.1 Pure graphene-based flexible electrode materials for electrical double-layer capacitor
4.2 Graphene with conducting additives as composite material for flexible supercapacitor devices
4.3 Graphene with metal oxides as composite material for flexible supercapacitor devices
5. Concluding remarks and future perspectives
References
12 - Present status of biomass-derived carbon-based composites for supercapacitor application
1. Introduction
2. Fundamentals of supercapacitor: an overview
3. Carbon materials for supercapacitor electrodes
3.1 Activated carbon
3.2 Porous carbon
3.3 Carbon aerogel/carbon hydrogel
3.4 Graphene
3.5 Carbon nanotube
3.6 Fullerene
4. Synthesis of biomass-derived porous carbon electrodes
4.1 Activation
4.2 Carbonization
4.2.1 Pyrolysis
4.2.2 Hydrothermal carbonization
4.2.3 Ionothermal carbonization
4.2.4 Molten salt carbonization
5. Types of biomass precursors
5.1 Plant biomass
5.2 Animal-based biomass
5.3 Fruit-based biomass
5.4 Microorganism-based biomass
6. Structural specification of biomass-derived porous carbon
6.1 Sphere-like structure
6.2 Tube-like structure
6.3 Fiber-like structure
6.4 Sheet-like structure
7. Natural polymer-derived porous carbon
7.1 Cellulose-derived porous carbon
7.2 Alginate-derived porous carbon
7.3 Lignin-derived porous carbon
7.4 Starch-derived porous carbon
7.5 Chitin-derived porous carbon
7.6 Gelatin-derived porous carbon
8. Application of biomass-derived porous carbons in supercapacitor technology
9. Conclusion and prospective
Acknowledgments
References
13 - 2D materials-based flexible supercapacitors for high energy storage devices
1. Introduction
2. Supercapacitors
3. Cell design
4. Three-electrode system
5. Two-electrode system
6. Calculations
6.1 Cyclic voltammetry
7. Galvanostatic charge-discharge
8. Energy and power densities
9. Two-dimensional materials
10. Synthesis method
11. Graphene-based flexible energy storage devices
12. Transition metal dichalcogenide-based flexible energy storage devices
13. Hybrid-based flexible energy storage devices
14. Recent developments
15. Future opportunities and challenges
16. Summary
Acknowledgments
References
14 - Nanostructured transition metal sulfide/selenide anodes for high-performance sodium-ion batteries
1. Introduction
2. Working principle of sodium-ion batteries
3. Active components of sodium-ion batteries
3.1 Cathodes
3.2 Electrolytes used in sodium-ion batteries
3.3 Anodes
3.4 Transition metal oxides
3.5 Metal sulfides
3.6 Transition metal selenides
4. Summary
Acknowledgments
References
15 - Emerging anode and cathode functional materials for lithium-ion batteries
1. Introduction
2. Electrochemistry of lithium-ion batteries
3. Anode
3.1 Graphene-based materials
3.2 Silicon
3.3 Silicon-based composites
3.4 Transition metals-based materials
3.5 Transition metal dichalcogenides
3.6 Tin oxide materials
3.7 Other functional materials
4. Cathode
4.1 LiFePO4 and its nanocomposites
4.2 Lithium cobalt oxide
4.3 Lithium manganese oxide
4.4 Lithium-rich NCM materials
4.5 3D graphene/organic nanocomposite
4.6 Other nanomaterials
5. Conclusions
List of abbreviations
Acknowledgments
References
16 - Transition metal-based nitrides for energy applications
1. Introduction
2. Mechanism involved in electrochemical water splitting and a short preview
2.1 Overpotential (η) and Tafel slope
2.2 Stability
3. Why transition metal nitrides are important in electrochemical water splitting?
4. Metal nitrides in electrochemical water splitting
4.1 Monometallic nitrides for electrocatalytic water splitting
4.2 Bimetallic nitrides for electrocatalytic water splitting
4.3 Trimetallic nitrides for electrocatalytic water splitting
5. Conclusion
Acknowledgments
References
Index
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Back Cover

Citation preview

NANOSTRUCTURED, FUNCTIONAL, AND FLEXIBLE MATERIALS FOR ENERGY CONVERSION AND STORAGE SYSTEMS

Edited by

ALAGARSAMY PANDIKUMAR PERUMAL RAMESHKUMAR

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2020 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-819552-9 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Christina Gifford Editorial Project Manager: Gabriela D. Capille Production Project Manager: Nirmala Arumugam Cover Designer: Mark Rogers Typeset by TNQ Technologies

Contributors Rafael Abargues UMDO, Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia, Spain Sourav Acharya Department of Chemistry, IIT(ISM) Dhanbad, Dhanbad, Jharkhand, India Somasundaram Anbu Anjugam Vandarkuzhali National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India A. Arulraj Graphene and Advanced 2D Materials Research Group (GAMRG), School of Science and Technology, Sunway University, Selangor, Malaysia Saravana Vadivu Arunachalam Department of Chemistry, School of Advanced Sciences, Kalasalingam Academy of Research and Education, Virudhunagar, Tamil Nadu, India Norfatehah Basiron School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia R. Joseph Bensingh Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India Ganesh Chandra Nayak Department of Chemistry, IIT(ISM) Dhanbad, Dhanbad, Jharkhand, India Myong Yong Choi Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, Gyeongnam, Republic of Korea Shrabani De Department of Chemistry, IIT(ISM) Dhanbad, Dhanbad, Jharkhand, India Duraisami Dhamodharan CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China Nidhin Divakaran CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China

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xii

Contributors

Kingshuk Dutta Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India Chandra Sekhar Espenti Department of Chemistry, Rajeev Gandhi Memorial College of Engineering and Technology, Kurnool, Andhra Pradesh, India A. Gowrisankar Department of Chemistry, Bharathiar University, Coimbatore, Tamil Nadu, India Vasanth Rajendiran Jothi Department of Chemical Engineering, Hanyang University, Seongdong-gu, Seoul, South Korea Ho-Young Jung Department of Environment & Energy Engineering, Chonnam National University, Gwangju, Republic of Korea; Center for Energy Storage System, Chonnam National University, Gwangju, Republic of Korea M. Abdul Kader Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India Manoj B. Kale CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China K. Karthick Academy of Scientific and Innovative Research (AcSIR), CSIR- Campus, New Delhi, India; Materials Electrochemistry (ME) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India K. Karuppasamy Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, South Korea Hyun-Seok Kim Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, South Korea Vijay S. Kumbhar Department of Energy Chemical Engineering, School of Nano & Materials Science and Engineering, Kyungpook National University, Sangju, Gyeonsang-daero, Republic of Korea Subrata Kundu Materials Electrochemistry (ME) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India

Contributors

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N. Lakshmana Reddy Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa, Andhra Pradesh, India; Department of Energy Chemical Engineering, School of Nano & Materials Science and Engineering, Kyungpook National University, Sangju, Gyeonsang-daero, Republic of Korea Seung Jun Lee Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, Gyeongnam, Republic of Korea Kiyoung Lee Department of Energy Chemical Engineering, School of Nano & Materials Science and Engineering, Kyungpook National University, Sangju, Gyeonsang-daero, Republic of Korea N. Malarvizhi Department of Chemistry, Guru Nanak College, Chennai, Tamil Nadu, India Juan P. Martínez-Pastor UMDO, Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia, Spain Suhail Mubarak CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China A. Murali School for Advanced Research in Polymers (SARP)-ARSTPS, Central Institute of Plastics Engineering & Technology (CIPET), Chennai, Tamil Nadu, India A. Nichelson Department of Physics, National Engineering College, K.R. Nagar, Kovilpatti, Tamil Nadu, India S.T. Nishanthi Electrochemical Power Sources Division, CSIR- Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India Alagarsamy Pandikumar Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India S. Priya Department of Plant Biology & Biotechnology, Loyola College, Chennai, Tamil Nadu, India Alagar Ramar Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China Pedro J. Rodríguez-Cant o UMDO, Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia, Spain

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Contributors

T. Sadhasivam Department of Environment & Energy Engineering, Chonnam National University, Gwangju, Republic of Korea; Center for Energy Storage System, Chonnam National University, Gwangju, Republic of Korea Khairul Arifah Saharudin School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia Sumanta Sahoo Department of Chemistry, IIT(ISM) Dhanbad, Dhanbad, Jharkhand, India M. Sakar Centre for Nano and Material Sciences, Jain University, Bangalore, Karnataka, India S. Sam Sankar Academy of Scientific and Innovative Research (AcSIR), CSIR- Campus, New Delhi, India; Materials Electrochemistry (ME) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India T. Saravanakumar Department of Nanoscience and Technology, Anna University Regional Campus, Coimbatore, Tamil Nadu, India M. Selvaraj Department of Chemistry, Guru Nanak College, Chennai, Tamil Nadu, India T. Selvaraju Department of Chemistry, Bharathiar University, Coimbatore, Tamil Nadu, India T. Senthil Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India M.V. Shankar Nanocatalysis and Solar Fuels Research Laboratory, Department of Materials Science & Nanotechnology, Yogi Vemana University, Kadapa, Andhra Pradesh, India Paramasivam Shanmugam Department of Chemistry, St. Joseph University, Dimapur, Nagaland, India Subramanian Singaravadivel Department of Chemistry, SSM Institute of Engineering and Technology, Dindigul, Tamil Nadu, India Gandhi Sivaraman Department of Chemistry, Gandhigram Rural Institute eDeemed to be University, Gandhigram, Tamil Nadu, India Ananthakumar Soosaimanickam UMDO, Instituto de Ciencia de los Materiales, Universidad de Valencia, Valencia, Spain

Contributors

xv

Srimala Sreekantan School of Materials & Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal, Pulau Pinang, Malaysia Tammineni Venkata Surendra Department of Chemistry, Rajeev Gandhi Memorial College of Engineering and Technology, Kurnool, Andhra Pradesh, India Waqas Hassan Tanveer Research Centre for Carbon Solutions, Heriot-Watt University, Edinburgh, United Kingdom; Department of Mechanical Engineering, School of Mechanical and Manufacturing Engineering, National University of Sciences and Technology (NUST), Islamabad, Pakistan Jayaraman Theerthagiri Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, Gyeongnam, Republic of Korea; Centre of Excellence for Energy Research, Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology (Deemed to be University), Chennai, Tamil Nadu, India Dhanasekaran Vikraman Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, South Korea Fu-Ming Wang Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan, Republic of China Lai Chin Wei Nanotechnology & Catalysis Research Centre, Institute for Advanced Studies, University of Malaya, Kuala Lumpur, Malaysia Lixin Wu CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China Sung-Chul Yi Department of Chemical Engineering, Hanyang University, Seongdong-gu, Seoul, South Korea; Department of Hydrogen and Fuel Cell Technology, Hanyang University, Seoul, South Korea

CHAPTER 1

Basic principles in energy conversion and storage Jayaraman Theerthagiri1, 2, a, Seung Jun Lee1, a, Paramasivam Shanmugam3, Myong Yong Choi1 1

Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju, Gyeongnam, Republic of Korea; 2Centre of Excellence for Energy Research, Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology (Deemed to be University), Chennai, Tamil Nadu, India; 3Department of Chemistry, St. Joseph University, Dimapur, Nagaland, India

1. Introduction Energy is indisputably one of the foremost issues of modern society and plays an important role in economic growth. In the current energy scenario, researchers have particularly focused on the production and consumption of energy. However, energy consumption increase is inevitable because of the growing population and economic growth in developing countries. Nevertheless, the tremendous exploitation of fossil fuels as nonrenewable sources has raised concerns about the lack of energy resources and CO2 emissions that deteriorate the environment [1e3]. To overcome these issues, an affordable, clean, and renewable energy resource, which can be an alternative to fossil fuels, is urgently required. As an important step toward the utilization of renewable energy resources, energy storage and conversion devices have attracted global attention. Highly efficient electrochemical energy storage and conversion devices with minimal toxicity, low cost, and flexibility in energy utilization are considered to meet the ever-expanding energy demand in electric vehicles (EV), consumer electronics, and miniaturized devices. The performance of the electrochemical energy storage and conversion devices is closely associated with physicochemical properties of materials utilized. For example, materials with limited electrochemical active surface sites and bulk materials with slow diffusion cannot be utilized in energy devices such as batteries and supercapacitors. Also, the low electrocatalytic behavior of energy materials is not suitable for making highly efficient

a

These authors contributed equally to this work.

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00001-4

© 2020 Elsevier Inc. All rights reserved.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

dye-sensitized solar cells (DSSCs) and fuel cells [4e6]. On the other hand, nanomaterials with exceptional surface structures have gained immense interest because of their unique electrical and mechanical properties, and their surface properties also play a vital role in the electrochemical behavior. The advent of nanotechnology offers a new platform for the design and fabrication of energy materials with nanosized structures. Thus, the performance of electrochemical energy storage and conversion devices has highly benefitted from the field of nanotechnology. This chapter outlines the specific features, basic landscape, general components, and performance evaluation of various electrochemical energy storage and conversion devices, such as batteries, supercapacitors, DSSCs, photocatalytic hydrogen production via water splitting, and fuel cells. Also, nanostructured materials in energy storage and conversion technologies are emphasized.

2. Lithium batteries Lithium-ion batteries (LIBs) are the most promising candidates for portable electronics and EV applications. It was first developed in Japan by Asahi Kasei Company in 1991. The first rechargeable LIB made up of LiCoO2 cathode and graphite anode was commercialized by Sony [7]. Currently, available LIBs in the market possess high energy density and good performance, as lithium is the lightest metal and most electropositive metallic element (
3.04 V vs. standard hydrogen electrode) and therefore enables an electrochemical storage device with high energy densities [8]. Moreover, LIBs can undergo more than 1000 charge/discharge cycles and can be manufactured in various sizes and dimensions. The maintenance of LIBs is quite simple compared with the other battery technologies, such as leadeacid, NaeNiCl2, and NieMH batteries. The LIB technology has been widely used in electronic devices and has recently been introduced to the hybrid EV market as a suitable candidate to power electric cars [9]. Still, researchers have been focusing on electrodes, electrolyte materials, and designs of this technology to decrease the cost, improve the cycling life, and increase the safety.

2.1 Battery principle and basics A LIB is a type of rechargeable energy storage device that converts stored chemical energy into electrical energy by means of chemical reactions of lithium. The simplest unit of LIBs called electrochemical cell consists of

Basic principles in energy conversion and storage

3

three key components: cathode, anode, and electrolyte. Faradaic redox reactions take place at a lower electrode potential called the anode (negative electrode) and a more positive electrode called the cathode. The first working rechargeable LIBs consisted of LiCoO2 as the positive electrode and graphite as the negative electrode [10]. Fig. 1.1 shows the schematic diagram of the LIB design. In the LIBs, Liþ ions are transferred between the cathode and anode during charge and discharge processes. The LIB design shown in Fig. 1.1 is an example of “rocking chair” battery [11], where Liþ ions are deintercalated from the layered lithium cobalt oxide during the charging process (oxidation of LiCoO2). The deintercalated Liþ ions from LiCoO2 migrate to the graphite to form LiC6. During the discharge process, the reverse reaction of Liþ ions is intercalated to LixCoO2. The electrochemical reactions at the positive and negative electrode during the charge/discharge process are given below [12]: Positive : LiMO2 #Li1
x MO2 þ xLiþ þ xe


Figure 1.1 Schematic representation of Li-ion battery design using a LiCoO2 cathode and graphite anode [13]. Copyright (2020) American Chemical Society.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Negative : C þ xLiþ þ xe
#Lix C Overall : LiMO2 þ C#Li1
x MO2 þ Lix C One of the three main components in LIBs is the cathode/positive electrode. The capacity and voltage of the LIBs depend on the cathode material, which is the limiting factor of the device. For example, the theoretical capacity of LiFePO4 is limited to 170 mAh g
1 based on the one Liþ transfer, although the theoretical capacity of the anode material, graphite, is 372 mAh g
1. Electrolytes are used to transport Liþ ions from one electrode to another electrode. They can be divided into two types: (1) liquid electrolyte and (2) solid electrolyte. The liquid electrolytes, such as NaPF6 in ethylene carbonate/propylene carbonate, require separator to prevent short circuits. However, solid electrolytes do not need any additional separator as they can function as both a separator and an electrolyte. In addition, electrolytes should have high ionic conductivity and broad operating voltage range without any decomposition [13].

3. Supercapacitors A supercapacitor is an electrochemical energy storage device, which can be used to store and deliver charge by reversible adsorption and desorption of ions at the interface between the electrode material and electrolyte. Supercapacitors are also called ultracapacitors or electrochemical capacitors. They can withstand many charge and discharge cycles compared to rechargeable batteries. Supercapacitors have many advantages, such as high power density and specific capacitance (SC), long life cycle, ecofriendliness, and flexibility of working temperature [14,15]. In addition, they can rapidly charge with quick power conveyance and are competent to replace conventional capacitors. Also, supercapacitors can act like bridges and decrease the gap among capacitors, batteries, or fuel cells. The operating voltage range of a standard capacitor is very high, but for supercapacitors, it is between 2.5 and 2.7 V. The electrochemical supercapacitors are classified into three categories based on the charge storage mechanism: (1) electrochemical double-layer capacitors (EDLCs), (2) pseudocapacitors, and (3) hybrid capacitors. EDLCs consist of two electrodes and an electrolyte. The two electrodes are separated by a separator, and the electrolyte is the combination of negative and positive ions dissolved in suitable solvents. In pseudocapacitors, energy is stored faradaically by means of charge transfer between the electrolyte and

Basic principles in energy conversion and storage

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electrode. The charge transfer may occur via redox reactions and electrosorption. Thus, the Faradaic process can achieve high SC and energy density compared with non-Faradaic process of EDLCs. Hybrid capacitors are a combination of EDLCs (non-Faradaic) and pseudocapacitors (Faradaic). Types of supercapacitors and the corresponding materials are shown in Fig. 1.2.

3.1 Working principle of a supercapacitor A current collector attracts oppositely charged ions when voltage is applied, and the ions from the electrolyte are collected on both the current collector surfaces and then the charge is built. Supercapacitors comprise current collectors (conducting metal plates), electrodes, an electrolyte, and a separator. The structures of supercapacitors vary from standard capacitors to batteries. The utilization of activated carbon increases the surface area, thereby increasing the capacitance value. Moreover, electrolyte with a low internal resistance increases power density. These two features lead to the ability to quickly store and release energy in supercapacitors.

Figure 1.2 Types of supercapacitors.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

4. Dye-sensitized solar cells The renewable energy device of solar cells converts solar energy (sunlight) into electrical energy and potentially can solve the growing energy demand. Currently, solar lightebased technologies are gaining recognition because of its various advantages, such as low toxicity and noise. Among various solar cells, DSSCs have been receiving considerable interest because of their low manufacturing cost, simple fabrication, ecofriendliness, excellent stability, and relatively high power conversion efficiency. Since the mid1980s, Gratzel’s group at EPFL (Switzerland) has been the main driving force for the development of DSSCs. In 1991, they invented a DSSC with a conversion efficiency of 7.1% based on low-cost nanoporous TiO2 particles together with newly developed ruthenium dyes, which opened a new way in the area of DSSCs (aka Gratzel cells) by providing better stability and enhanced efficiency [16].

4.1 Major components of dye-sensitized solar cells A DSSC contains four main components: photoanode (photoelectrode), counter electrode, dye sensitizer, and electrolyte. A schematic of a typical DSSC is shown in Fig. 1.3. Specific functions of all four components are

Figure 1.3 Schematic of a dye-sensitized solar cell [23]. Copyright (2020) Elsevier.

Basic principles in energy conversion and storage

7

crucial to accomplish a maximum power conversion efficiency and lifetime of the device. The photoanode is an important part of a DSSC. In general, photoanodes prepared by a nonporous layer of a semiconductor metal oxide, such as TiO2, ZnO, SiO2, and Nb2O5, are coated on a transparent conducting (indium- or fluorine-doped tin oxide) glass substrate, which largely enhances the surface area of photoanode and allows more dye molecules to be adsorbed, thereby improving the light absorption efficiency. The dyesensitized semiconductor oxide collects the photoinduced electron from the lowest unoccupied molecular orbital of the dye sensitizer, providing electron mobility to the conducting substrate. The dye molecules’ (sensitizer) function in a typical DSSC is, with the absorption of maximum solar light, to produce electrons that are consequently injected into the conduction band (CB) of the nonporous oxide electrode on which the dye molecules are attached. Generally, ruthenium-based polypyridyl complexes (N719 and N3 dye) are used as high-performance dye molecules in the fabrication of DSSCs. Another important component of DSSCs is redox electrolytes. The function of the electrolyte is to regenerate the oxidized form of dye molecules and completion of an electric circuit by transporting positive charges to the counter electrode. Also, the stability of the fabricated DSSC device is closely associated with the nature of the redox electrolytes. To recover oxidized dye molecules, the potential of the redox electrolyte should be marginally situated at a more negative level than the higher unoccupied molecular orbital of the dye molecules. Numerous redox mediators are utilized in the DSSC electrolyte, for





example, I
/I
3 , SCN /SCN3 , Br /Br3 , and Co(II)/Co(III). I /I3 is the most commonly utilized among redox mediators in electrolytes. In DSSCs, the counter electrode comprises an electrocatalyst coated on the conducting substrate. The function of the counter electrode is to inject electrons into the redox electrolytes to electrocatalyze the triiodide into iodide ions. A good counter electrode should possess high electrocatalytic
behavior toward the reduction of I
3 into I ions, low resistance to obtain a high fill factor, and be available at low cost. Generally, platinum is used as a conventional electrocatalytic material because of its high catalytic reduction performance [6,16]. However, its large-scale applications are limited because of its high cost and scarcity; thus, these limitations fueled various studies to develop a substitute material for the platinum-based counter electrodes and decrease their cost while maintaining the efficiency of DSSCs.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

4.2 Working principles of dye-sensitized solar cells Upon the illumination of solar light onto the fabricated DSSC device, dye molecules are excited from the ground state to the excited state. The excited dye molecules inject its electron to the CB of the semiconductor metal oxideebased photoanode. Then, the oxidized form of dye molecules is regenerated by I
of the electrolyte, and I
is converted to I
3 ions.

Simultaneously, I3 is reduced to I because of electron gain from the counter electrode. The cycle is completed by the flow of injected electrons from the photoelectrode to the counter electrode via an external circuit. The performance of a DSSC device can be assessed by the fill factor (FF) and a power conversion efficiency (h) using the following equations [6]: FF ¼ hð%Þ ¼

Vmax  Jmax Voc  Jsc

Pout Voc  Isc  FF ¼  100 Pin Pin

(1.1) (1.2)

where Vmax is the maximum voltage, Jmax is the maximum current density, Voc is the open circuit voltage, Jsc is the short circuit current, and Pin and Pout are the power input and output from solar light, respectively.

5. Hydrogen production by photocatalytic water splitting Hydrogen is a kind of energy source with zero greenhouse gas emission and thus has attracted much attention as an alternative to fossil fuels, providing ecofriendliness, sustainability, and clean energy [17,18]. Hydrogen is mainly obtained from the decomposition of fossil fuels (natural gas, oil, and coal). However, the consumption of fossil fuels has generated a substantial quantity of CO2 emission and is the main reason for global warming. It is critically needed to supplant fossil fuels with advanced alternate energy sources. Hence, hydrogen production via water splitting has gained interest because water is naturally abundant, carbon-free, and environmentally friendly [19]. Water splitting via photocatalytic reaction is initiated by the absorption of photons by a semiconductor photocatalyst, which can be photoexcited by the absorption of photons with equal or higher energy than its band gap energy. Then, the photoinduced holes and electrons are directly involved in the splitting of water. The production of hydrogen by water splitting using TiO2 photocatalysts under UV light illumination was first reported by

Basic principles in energy conversion and storage

9

Fujishima and Honda in the 1970s, where they utilized a photoelectrochemical cell comprising TiO2 and Pt electrodes under an electric potential [20]. The chemical process involved in photocatalytic water splitting is demonstrated in Fig. 1.4. Mostly, the water splitting process via photocatalysis takes place in several reactions: (1) Intrinsic ionization of photocatalysts by the absorption of photons over the band gap, generating holes in valence band (VB) and electrons in CB of catalysts; Photocatalyst þ 2hv / 2hþ þ 2e
(2) Water molecules oxidized by photogenerated holes; 2H2O þ 4hþ / 4Hþ þ O2 (3) Hydrogen ion reduction by photogenerated electrons; 2Hþ þ 2e
/ H2 The overall reaction of water splitting is 2H2O / 2H2 þ O2 During the water splitting reaction, the photoinduced electrons can reduce Hþ into H2 only when the CB potential is more negative than the

Figure 1.4 Chemical process involved in the photocatalytic water splitting [24]. Copyright (2020) Royal Society of Chemistry.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

0 V of hydrogen evolution, whereas holes can oxidize water molecules into O2 only when the VB is more positive than þ1.23 V of oxygen evolution. To achieve highly efficient hydrogen production, it is critical to develop semiconductor materials with appropriate band energy gaps as a catalyst for water splitting to produce hydrogen under visible light illumination. Further effort was put into developing high-performance photocatalysts for hydrogen production, but they are still struggling with low activity because of the recombination of charge carriers. The photocatalytic materials are often tuned with other cocatalyst, which encourages the separation of charge by developing a new semiconductor photocatalyst/cocatalyst interfaces, subsequently improving the photoinduced charge carrier lifetime and photocatalytic activity.

6. Fuel cell Fuel cell is an electrochemical device that converts chemical energy (hydrogen, methanol, or natural gas) and pure oxygen or air into electricity, water, and heat. In some cases, by-products of carbon dioxide and low molecular weight hydrocarbons may be produced [21]. Fuel cells can continuously produce electricity as long as fuel and oxidant are supplied. Fuel cells are suitable choices for applications in buildings because of their high electrical efficiency, environmental compatibility, f lexibility, and quiet operation. Currently, fuel cells are recognized in transportation and various markets (booming power sector, fuel cellebased vehicles, portability, and stationary applications, etc.) as a better choice compared to combustionbased conventional generators [22]. A typical fuel cell comprises a cathode, an anode, and an electrolyte [1,21]. The choice of electrocatalysts utilized in electrode materials relies on the operating temperature. The types of electrochemical reactions that take place in the fuel cell device are associated with electrolyte type. Based on the fuel and electrolyte utilized, the fuel cell is categorized into different types, as shown in Fig. 1.5. Alkaline fuel cell, polymeric electrolyte membrane fuel cell, direct methanol fuel cell, and phosphoric acid fuel cell have a low operating temperature range from 60 to 220 C, whereas solid oxide fuel cells can operate at high temperatures from 800 to 1000 C (Fig. 1.5). Upon constant supply of fuel at the anode side and oxidant at the cathode side of the device, the fuel (hydrogen) at the anode side is decomposed into hydrogen ions and electrons (H2 / 2Hþ þ 2e
). Thus, the produced hydrogen ions move to the cathode part through an

Basic principles in energy conversion and storage

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Figure 1.5 Different types of fuel cells operating at various temperatures [25]. Copyright (2020) Elsevier.

electrolyte, and the electrons flow through the external circuit to the cathode. Subsequently, the reached ions and electrons at the cathode are combined with an oxidant to produce water (1/2O2 þ 2Hþ þ 2e
/ H2O). In contrast to the water electrolysis, the anode is negative and the cathode is positive in fuel cells. Besides, the produced electricity by a fuel cell mostly depends on the electrocatalysts utilized in the fabrication of electrodes. Nevertheless, some drawbacks of fuel cells include high fabrication cost, low stability, and limitation of fuels in the market. To overcome these disadvantages, researchers are working to advance fuel cells by using alternative materials with low cost and excellent stability, in addition to focusing on the challenges related to the production of fuel (hydrogen) and consumption. The choice of electrocatalysts for electrode fabrication may involve unique surface structures at the nanoscale, high electrocatalytic activity, and improved mass transport at the electrode surface.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

7. Conclusions In this chapter, fundamental considerations of energy conversion and storage devices are summarized to solve challenges related to the utilization of nonrenewable fossil fuel energy sources (coal, gas, and oil), such as increasing CO2 emission because of human activities and global warming. Energy conversion and storage devices that can convert or store energy in various forms are being improved by various advanced nanomaterials. Currently, the field of nanotechnology has opened new avenues for novel energy conversion and storage devices. We discussed basic working principles, components, and analysis methods of these technological devices, including batteries, supercapacitors, DSSCs, hydrogen production via water splitting, and fuel cells. Energy production from renewable energy sources requires storing energy in the device for utilization on an as-needed basis. Designing new integrated technologies for both energy conversion and storage needs much consideration for the management and control of electrical grids.

Acknowledgments This work was supported by the Korea Basic Science Institute (KBSI) National Research Facilities and Equipment Center (NFEC) grant funded by the Korea government (Ministry of Education) (No. 2019R1A6C1010042). The authors Prof. M. Y. Choi and Dr. J. Theerthagiri acknowledge the financial support from National Research Foundation of Korea (NRF), (NRF-2019H1D3A1A01071209, NRF-2017M2B2A9A02049940). Also, Dr. J. Theerthagiri thankfully acknowledges the financial support from Indian Space Research Organization (Respond program grant No. ISRO/RES/3/792/18-19), India.

References [1] J. Theerthagiri, J. Madhavan, S.J. Lee, M. Ashokkumar, B.G. Pollet, Sonoelectrochemistry for energy and environmental applications, Ultrasonics Sonochemistry 63 (2020) 104960. [2] J. Theerthagiri, S. Sunitha, R.A. Senthil, P. Nithyadharseni, A. Madan kumar, A. Prabhakarn, T. Maiyalagan, H.S. Kim, A review on ZnO nanostructured materials: energy, environmental and biological applications, Nanotechnology 30 (2019) 39200. [3] J. Theerthagiri, A.P. Murthy, V. Elakkiya, S. Chandrasekaran, P. Nithyadharseni, Z. Khan, R.A. Senthil, R. Shanker, M. Raghavender, P. Kuppusami, J. Madhavan, M. Ashokkumar, Recent development on carbon based heterostructures for their applications in energy and environment: a review, Journal of Industrial and Engineering Chemistry 64 (2018) 16e59.

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[4] J. Liu, J. Wang, C. Xu, H. Jiang, C. Li, L. Zhang, J. Lin, Z.X. Shen, Advanced energy storage devices: basic principles, analytical methods, and rational materials design, Advancement of Science 5 (2017) 1700322. [5] J. Theerthagiri, K. Karuppasamy, G. Durai, A.H. Sarwar Rana, P. Arunachalam, K. Sangeetha, P. Kuppusami, H.S. Kim, Recent advances in metal chalcogenides (MX; X ¼ S, Se) nanostructures for electrochemical supercapacitor applications: a brief review, Nanomaterials 8 (2018) 256. [6] J. Theerthagiri, R.A. Senthil, J. Madhavan, T. Maiyalagan, Review on recent progress in non platinum counter electrode materials for dye-sensitized solar cells, ChemElectroChem 2 (2015) 928e945. [7] M.S. Whittingham, R.F. Savinelli, T. Zawodzinski, Introdeuction: batteries and fuel cells, Chemical Reviews 104 (2004) 4243e4886. [8] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359e367. [9] A. Yoshino, K. Sanechika, T. Nakajima, Secondary Battery, United States Patent 4668595, 1987. [10] K. Mizushima, P. Jones, P. Wiseman, J. Goodenough, LixCoO2 (0 < x  1): a new cathode material for batteries of high energy density, Solid State Ionics 3e4 (1981) 171e174. [11] A.J. Salkind, J.J. Kelly, A.G. Cennone, in: D. Linden (Ed.), Hand Book of Batteries, McGraw Hill, New York, 1995. [12] K. Ozawa, Lithium-ion rechargeable batteries with LiCoO2 and carbon electrodes: the LiCoO2/C system, Solid State Ionics 69 (1994) 212e221. [13] K. Xu, Nonaqueous liquid electrolytes for lithium-based rechargeable batteries, Chemical Reviews 104 (2004) 4303e4418. [14] J. Theerthagiri, G. Durai, K. Karuppasamy, P. Arunachalam, V. Elakkiya, P. Kuppusami, T. Maiyalagan, H.S. Kim, Recent Advances in 2-D nanostructured metal nitrides, carbides and phosphides electrodes for electrochemical supercapacitors e a brief review, Journal of Industrial and Engineering Chemistry 67 (2018) 12e27. [15] J. Li, J. Qiao, K. Lian, Hydroxide ion conducting polymer electrolytes and their applications insolid supercapacitors: a review, Energy Storage Materials 24 (2020) 6e217. [16] B.O. Regan, M. Gratzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films, Nature 353 (1991) 737e740. [17] J. Theerthagiri, E. Cardoso, G. Fortunato, G. Casagrande, B. Senthilkumar, J. Madhavan, G. Maia, Highly electroactive Ni pyrophosphate/Pt catalyst towards hydrogen evolution reaction, ACS Applied Materials and Interfaces 11 (2019) 4969e4982. [18] N. Fajrina, M. Tahir, A critical review in strategies to improve photocatalytic water splitting towards hydrogen production, International Journal of Hydrogen Energy 44 (2019) 540e577. [19] A.P. Murthy, D. Govindarajan, J. Theerthagiri, J. Madhavan, K. Parasuraman, Metaldoped molybdenum nitride films for enhanced hydrogen evolution in near-neutral strongly buffered aerobic media, Electrochimica Acta 283 (2018) 1525e1533. [20] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37e38. [21] P.E. Dodds, I. Staffell, A.D. Hawkes, F. Li, P. Grunewald, W. McDowall, P. Ekins, Hydrogen and fuel cell technologies for heating: a review, International Journal of Hydrogen Energy 40 (2015) 2065e2083.

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[22] K. Amarsingh Bhabu, J. Theerthagiri, J. Madhavan, T. Balu, G. Muralidharan, T.R. Rajasekaran, Superior oxide ion conductivity of novel acceptor doped cerium oxide electrolytes for IT-SOFC applications, Journal of Physical Chemistry C 120 (2016) 18452e18461. [23] A. Pandikumar, S.P. Lim, S. Jayabal, N.M. Huang, H.N. Lim, R. Ramaraj, Titania@gold plasmonic nanoarchitectures: an ideal photoanodefor dye-sensitized solar cells, Renewable and Sustainable Energy Reviews 60 (2016) 408e420. [24] J. Ran, J. Zhang, J. Yu, M. Jaroniec, S.Z. Qiao, Earth-abundant cocatalysts for semiconductor-based photocatalytic water splitting, Chemical Society Reviews 43 (2014) 7787e7812. [25] N. Seselj, C. Engelbrekt, J. Zhang, Graphene-supported platinum catalysts for fuel cells, Science Bulletin 60 (2015) 864e876.

CHAPTER 2

Low-dimensional carbon-based nanomaterials for energy conversion and storage applications T. Senthil1, Nidhin Divakaran2, Manoj B. Kale2, Suhail Mubarak2, Duraisami Dhamodharan2, Lixin Wu2, R. Joseph Bensingh1, M. Abdul Kader1, Kingshuk Dutta1 1

Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India; 2CAS Key Laboratory of Design and Assembly of Functional Nanostructure, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China

1. Introduction The US department of energy estimates that in 15 years from now the global energy demand will increase by about 20% (Fig. 2.1A). The use of fossil fuels has created a concern over its effect on climate change, which led to the search for renewable and sustainable energy resources. It is important to develop renewable energy generation and storage systems to counter the problem of climate change and meet the requirement of increasing energy consumption [1]. The world population is expected to be around 9 billion by 2050, and as a result the energy demand is going to increase to 13 trillion watts by 2050. The natural oil resources may be able to fulfill the need of energy for 60 years, natural gas for 60 years, and coal for 200 years [2]. Currently, the world’s electricity need is fulfilled by fossil fuel- and nuclearpowered plants, which are causing environmental pollution because of green gas effects. Therefore, new energy generation techniques have been developed, which are nonpolluting and easy to store, transfer, and use whenever needed. The conventional energy resources can be replaced and/ or accompanied by solar power, wind energy, and water energy, but these energy resources need efficient conversion and storage devices. The storage devices, such as batteries and supercapacitors, can be useful to store the excess energy and use it whenever needed. Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00002-6

© 2020 Elsevier Inc. All rights reserved.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.1 (A) The past and projected energy consumption on a global scale; (B) schematic setup of the carbon nanodots (CNDs) possessing various structures. (A) Based on data from the US Energy Information Administration, 2011 (https://voer.edu.vn/ m/world-energy-use/99605200#import-auto-id1764068, accessed on July 10, 2019). (B) Reproduced from C. Hu, M. Li, J. Qiu, Y.-P. Sun, Design and fabrication of carbon dots for energy conversion and storage, Chemical Society Reviews 48 (2019) 2315e2337, https:// doi.org/10.1039/c8cs00750k with permission from The Royal Society of Chemistry.)

Carbon has played a very important role in the development of modern science and technology. It originated from the discovery of fullerene and carbon nanotubes (CNTs) to the highly efficient graphene in the mid 2000s. The constant urge of inclusive properties of nanofillers propelled the search of the carbon-based materials having luminescent property. It was until the discovery of the carbon nanodots (CNDs) in the year 2006. CNTs have been discovered in 1991 by Iijima [3]. They have high surface area, better electrical conductivity, high mechanical strength, and corrosion resistance [4e6]. CNTs are made up of covalently bonded carbon atoms in hexagonal

Low-dimensional carbon-based nanomaterials for energy conversion

17

shape, just like in graphene. The CNTs are classified into single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT). The SWCNTs are just like single-folded sheets of graphene, having inner diameter of about 0.4e3 nm and few micrometers long. The MWCNTs are concentric folded sheets with varying diameters of different sheets each ranging from 1.4 to 25 nm, and the intersheets distance is about 0.4 nm [7e9]. The CNTs can be prepared by various methods, such as arc-discharge method [10], spray pyrolysis [11], chemical vapor deposition (CVD) [12], electrocatalytic method, and laser ablation. CNTs are as conductive as copper and have the capacity to transfer more current. The high electrical conductivity is the result of transfer of electrons through the sidewalls. The CNTs are mostly limited to the laboratory research for energy storage application because of their low dispersion, poor electrochemical performance, and low specific capacitance [13,14]. But the surface functionalization of CNTs can enhance the dispersion, electrochemical performance, and specific capacitance. The CNTs have been researched for various applications, such as hydrogen storage [15], fuel cells [16], anode material for Li-ion batteries [17], drug delivery system [18], and energy conversion and storage [19]. The CNTs have high mechanical and charge transfer properties, which are favorable in the application of energy conversion. Also, the surface of CNTs can be functionalized using surface functional groups to enhance their electrical and dispersion properties [20]. The precursor carbon material to synthesize CNTs is cheap and abundant, so the use of CNTs in storage devices makes the product economically cheap. CNDs are also known by the name of carbon quantum dots (QDs) or carbon dots (CDs) [21]. The discovery added a laurel in the ever-blooming applications of the carboniferous materials with CNDs, being one of the proponents. It came under the category of the nanoparticles with the dimension less than 10 nm. The discovery of CNDs transformed the arch type of carbon being the material that is unable to emit light as it is black in color [22]. The revelation of the CNDs could be traced back to the research on fluorescent materials derived from CNTs (SWCNTs) in the year 2004. These fluorescent materials were coined as CNDs by Sun et al., who also devised the synthesis route to develop CNDs with augmented fluorescent properties by surface passivation [23]. The invention of the CNDs created ripples of resonance in the field of science and technology with pros in the form of chemical stability, water solubility, low toxicity, high luminescence, and less photobleaching [24e27]. CNDs are often analogous to the semiconductor QDs and their fascinating properties make them pompous

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

to that of conventional luminous QDs and luminous organic dyes [28,29]. The subsequent survey in the application of CNDs in the application point of view has opened the gates for their commercial use in various fields, such as optoelectronics, photovoltaic devices, photocatalysis, biosensing, and bioimaging devices [30e38]. The structural perspective of the CNDs can be elaborated in terms of a carbogenic core comprising of crystalline and amorphous fragment with functional groups attached on the surface. There seems to be a similarity to that of the graphene structure with the researchers suggesting the presence of crystalline sp2 carbon segments. The core CNDs seem to have possessed the amorphous carbon structure, graphene/graphitic structure, or the diamond-like structure. The presence of defects within the CNDs structure appears to be more than that of the graphene quantum dots (GQDs). It also holds poorer crystallinity than GQDs. Nevertheless, GQDs could be considered as one of the CNDs as they both have similar arrangement of oxygen-terminated functional groups onto their surface [39]. Fig. 2.1B displays the schematic setup of the structure of the CNDs with various configurations. The molecular orbital theory encapsulates the electronic structure of the CNDs. The CNDs display n / p and p / n transition because of the presence of difference in the energy levels. The p states indicate the sp2 hybridization in the core, while the n states specify the oxygen functional group attached to the sp2-hybridized carbon. The functional group could be carbonyl, amine, amides, and thiol [40]. CNDs can act as hydrophobic or hydrophilic, depending on the structure of the surface. The fluorescence characteristics of the CNDs could be depicted on the basis of the synthesis procedure to develop it. It depends on the synthesis methods whether they will be capable of emitting fluorescence at different wavelength. The fluorescence is commonly tunable with it, having the ability of emitting blue, green, or red light, independent of the excitation wavelength. The arrangement of honeycomb crystal lattice structure of sp2-bonded carbon atoms is called graphene. It has number of applications in various sectors because of its high surface area, unique electronic quality, and high mechanical and thermal properties [41,42]. The adjacent bonding between the carbon atoms and the delocalization of bonding electrons throughout the lattice structure may be the key reason for the attractive properties of graphene [43]. Graphene is a combination of many polycyclic aromatic hydrocarbons, such as anthracene, pyrene, naphthalene, etc. [44]. The first successful graphene production was done by Andre Geim and his fellow

Low-dimensional carbon-based nanomaterials for energy conversion

19

workers, through the approach of simple scotch tape method [45]. Graphene can be synthesized by different kinds of physical and chemical process; however, there are very few notable methods, such as chemical exfoliation of natural graphite [46e48], thermal reduction of graphene oxide (GO) [49], chemical [50], micromechanical [51], and CVD [52,53]. Among this many methods, chemical reduction of GO is the most common synthesis method in the research laboratories. In addition, many carbonbased materials have been derived by chemical modification of pristine graphene, such as acetylenic chainemodified graphene (graphdiyne and graphyne), fluorine-modified graphene (fluorographene), hydrogenated graphene (graphane), and oxidized graphene (GO). Graphene has attracted many researchers because of its excellent application potentials. The implementation of graphene in the battery, electronics, and semiconductors applications is growing rapidly, and the large-scale synthesis methods approach is still developing. Single-layer graphene is electrically conductive, optically transparent, and flexible, which leads to the key reason for its implementation in the electronics and electrical industries. Graphene is the firmest material, with high mechanical and thermal properties, compared with steel [54]. Conversely, graphene has very good elastic properties, with a large elastic modulus, compared with natural rubber [54]. The high electrical conductivity, high strength, less weight, and high elastic nature of graphene are the key reasons for its implementation in the electrical and electronics industries [55]. Apart from this, graphene showed better performance with the various composite materials owing to its better properties and applications [56e60]. At present, the application of graphene materials became much needed source for many industries, such as aerospace, chemical, biomedical, energy, etc. In this chapter, we have mainly focused on the synthesis of graphene and its outstanding properties and applications in energy sector. In the past many years, there has been an enduring zeal to convert graphene to zero-dimensional (0D) GQDs and to study the new phenomena from GQDs allied with quantum confinement and edge effects [61e64]. GQDs, defined as graphene dots of smaller than 100 nm in size and less than 10 layers in thickness, set for a new type of QDs with unique properties coupled with both graphene and QDs [62,65e68]. GQDs are in general either formed from graphene-based starting materials or synthesized with an unambiguous structure by solution wet chemistry; therefore, they evidently possess graphene lattices inside the dots apart from the dot size [23,69,70]. Besides, GQDs have several tremendous characteristics, such as

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

high surface area, larger diameter, improved surface grafting using the pep conjugated system or surface groups, and other particular physical properties because of the structure of graphene [71,72]. As a significance of their simple structure, as well as health concerns and biological vulnerability of QDs, GQDs are at the core of significant research efforts to build up low-toxicity, eco-friendly substitutes that have the attractive performance characteristics of QDs [62,73,74]. In fact, the extraordinary optoelectronic properties, biocompatibility, and well dispersion in various solvents led to GQDs showing vivid promise for integration into devices of bioimaging, light emitting applications, solar cells, batteries, supercapacitors, fuel cells, and others [22,62,73,75e77].

2. Synthetic aspects of carbon-based nanomaterials 2.1 Synthesis of carbon nanodots The discovery of the fluorescent CNDs was accidently done by Xu et al., while purifying the SWCNTs using arc-discharged soot. Thereafter, many synthetic routes have been proposed to synthesize CNDs, which comprise of the fundamental preparation of nanomaterials like top-down approach and the bottom-up approach. Fig. 2.2A shows the schematic setup of the preparation technique adopted to fabricate CNDs. The top-down approach for the synthesis of the nanomaterials includes the breaking/disorganization of bulk carbon nanomaterials with higher dimensions than CNDs, for example, CNTs or graphite. There are many methods to synthesize the CNDs, which comprise of laser ablation, electrochemical technique, microwave synthesis, and hydrothermal method. Generally, the CNDs synthesized are not promptly fluorescent. They can be altered into highly fluorescent CDs by reducing the reactivity on to their surface with various polar moieties after synthesis [78]. Bottom-up synthesis usually involves the carbonization of different molecular precursors, such as citric acid or sucrose [79]. The carbonization of these compounds at low melting points can be carried out at low temperature to manufacture CNDs. The alternate route of mixing the carbon sources with other precursors, such as urea and thiourea, can be adopted to clod the structure of CNDs [80].

2.2 Graphene preparation Graphene preparations have faced a lot of developments, and many different methods have been approached. CVD, arc discharge, intercalation

Low-dimensional carbon-based nanomaterials for energy conversion

(A)

Top-down approach

21

Bottom-up approach

Carbon dot (CD)

Carbon resources

Organic molecules

(B) Graphene

Copper substrate Graphene growth

(b) OCVD graphene growth

Cu surface

Copper substrate Oxidation

Copper oxide

Reduction of Cu oxide

Graphene growth

(c) Surface coverage (%)

(a) CVD graphene growth

100 80 60 40 CVD growth (top surface) CVD growth (bottom surface) OCVD growth (top surface) OCVD growth (bottom surface)

20 0 0

40

80

120

160

200

Growth time (min)

Figure 2.2 (A) Schematic setup of preparation technique of carbon nanodots (CNDs); (B) Schematic illustration of graphene growth mechanism of (a) regular chemical vapor deposition (CVD) method, (b) oxidative chemical vapor deposition (OCVD) method, and (c) growth kinetics of CVD and OCVD methods. (A) Reproduced from B. De, N. Karak, Recent progress in carbon dot-metal based nanohybrids for photochemical and electrochemical applications, Journal of Materials Chemistry A 5 (2017) 1826e1859, https://doi. org/10.1039/C6TA10220D with permission from The Royal Society of Chemistry. (B) Reproduced from R.J. Chang, C.H. Lee, M.K. Lee, C.W. Chen, C.Y. Wen, Effects of surface oxidation of Cu substrates on the growth kinetics of graphene by chemical vapor deposition, Nanoscale 9 (2017) 2324e2329, https://doi.org/10.1039/C6NR09341H with permission from The Royal Society of Chemistry.)

in graphite, electrochemical, chemical, micromechanical exfoliation, epitaxial growth, and unzipping of CNTs were few notable synthetic approaches of graphene. Graphene is obtained by reduction of GO through chemical methods. Chemical methods started with oxidation of natural graphite to obtain GO, followed by reduction to graphene with the assistance of strong reducing agent [82]. Electrophoretic deposition (EPD) is another technique to obtain graphene sheets (GSs) [83,84]. Guo et al. synthesized graphene through arc-discharge techniques, with the assistance of atmospheric hydrogen and graphite rods [85]. Florescu et al. performed laser pyrolysis methods to obtain graphene with the assist of vapor particles

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

[86]. Plasma-enhanced CVD and thermal CVD methods are fruitful preparation methods for graphene. Among all the graphene syntheses, CVD techniques are mostly considered because of excellent crystal structure formation and its outstanding applications in many industries. The detailed graphene growth in the CVD route is depicted in Fig. 2.2B.

2.3 Synthesizing graphene quantum dots The approach for synthesizing GQDs with tunable properties can be generally classified into two methods: top-down and bottom-up methods (Fig. 2.3A) [62,73,75,77,87]. Top-down strategy refers to the cutting of GSs into GQDs. The method consists of nanolithography technique, hydrothermal cutting of GSs, solvothermal cutting of GSs, chemical ablation, electrochemical oxidation, and oxygen plasma treatment, where GQDs are formed or “broken off” from larger GSs. Bottom-up methods involve the synthesis of those graphene moieties including an assured number of conjugated carbon atoms by stepwise reactions of molecular precursors; for example, the cage opening of fullerene or solution chemistry methods during which the GQDs are formed from molecular precursors. Typically, these GQDs have surfaces rich in carboxylic acid functionalities, which can be used to bind surface passivation reagents [88e94] (Fig. 2.3B).

3. Energy characteristics of carbon nanodots The environmentally friendly nature and the imminent photoluminescence property of the CNDs have prompted the researchers to explore the use of CNDs in energy storage devises. They pose as an excellent candidate for photovoltaic cell and solar cells application, in addition to the other energy storageebased applications. The prominent property of the CNDs to possess high optical absorptivity at the visible light range has enhanced its scope for a possible alternate sensitizer in dye-sensitized solar cell (DSSC) [30,95]. Carbon has the tendency to act as a conducting medium for transferring electrons because of the p-electron network. This allows them to act as a bridge for the charge carrier sources [96e100]. The other important property of the CNDs is its photoluminescence. CNDs synthesized from different routes emit different color, including ultraviolet, red, blue, green, yellow, and near-infrared region. The fundamental concept behind the luminescence is the tussle within the CNDs between defect state emission and intrinsic state emission. The photoluminescence of the CNDs also depends on the excitation wavelength, pH, solvent, and size of the nanodots. The luminescence occurs because of the excitons of carbon, quantum confinement

Low-dimensional carbon-based nanomaterials for energy conversion

23

Figure 2.3 (A) Schematic diagram of the top-down and bottom-up methods for synthesizing graphene quantum dots (GQDs); (B) Preparation and structural characterization of OH-GQDs: (a) synthetic procedure, (b) X-ray diffraction pattern, (c) Raman spectrum, (d) Fourier-transform infrared spectrum, (e) survey X-ray photoelectron spectroscopy spectrum, (f) high-resolution C 1s spectrum, (g) high-resolution O 1s spectrum, (h) atomic force microscope image (scale bar: 500 nm), (i) transmission electron microscopy image (scale bar: 20 nm), and (j) high-resolution transmission electron microscopy image (scale bar: 1 nm). (A) Reproduced from S. Zhu, Y. Song, J. Wang, H. Wan, Y. Zhang, Y. Ning, B. Yang, Photoluminescence mechanism in graphene quantum dots: quantum confinement effect and surface/edge state, Nano Today 13 (2017) 10e14, https://doi.org/10.1016/j.nantod.2016.12.006 with permission from Elsevier. (B) Reproduced from L. Wang, Y. Wang, T. Xu, H. Liao, C. Yao, Y. Liu, Z. Li, Z. Chen, D. Pan, L. Sun, M. Wu, Gram-scale synthesis of single-crystalline graphene quantum dots with superior optical properties, Nature Communications 5 (2014) 5357, https://doi.org/10. 1038/ncomms6357 with permission from Springer Nature.)

effect, aromatic structures, oxygen-containing groups, zigzag sites, and defects within edges. The CNDs also possess the property of electrochemical luminescence reported by Chi et al. This property was observed while applying scanning potential to graphite rods during the preparation of CNDs [101]. Fig. 2.4A exhibits the electroluminescence mechanisms existing in the CNDs. The above properties contribute to the versatility of the CNDs in the energy storageebased devices.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.3 Cont’d.

4. Potential properties of graphene Graphene has high young’s modulus of 1 TPa and excellent carrier mobility of 200,000 cm2 V
1s
1 [66]. The surface area of single graphene layer is 2630 m2g
1 [102]. The electrical conductivity of the graphene is very similar to the copper materials. The density of the graphene is much lesser than copper materials, which is more convenient to make use of graphene materials in many applications [103]. Thermal conductivity of graphene is five times better than copper [103], which leads to the graphene materials able to act as semiconductors and many electrical circuits. Graphene density is much lesser than steel materials but more than 50-times stronger than steel. This leads to far better application of graphene over other materials.

Low-dimensional carbon-based nanomaterials for energy conversion

Figure 2.4 (A) Electroluminescence mechanism in the carbon nanodots (CNDs); (B) (a) Synthesis and hydrogen adsorption study of metaloxide/multi-walled carbon nanotubes (MWCNTs) composite. The hydrogen storage capacity of MgO/MWCNT increased threefold compared to pristine MWCNTs, and (b and c) field emission scanning electron microscopy images of pristine MWCNTs and manganese oxide/MWCNTs. (A) Reprinted with permission from L. Zheng, Y. Chi, Y. Dong, J. Lin, B. Wang, Electrochemiluminescence of water-soluble carbon nanocrystals released electrochemically from graphite, Journal of the American Chemical Society 131 (2009) 4564e4565, https://doi.org/10.1021/ ja809073f. © 2009 American Chemical Society. (B) Reproduced from S.u. Rather. Hydrogen uptake of manganese oxide-multiwalled carbon nanotube composites. International Journal of Hydrogen Energy 44 (2019) 325e331, https://doi.org/10.1016/j.ijhydene.2018.03.009 with permission from the Hydrogen Energy Publications LLC and Elsevier.)

25

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.4 Cont’d.

Low-dimensional carbon-based nanomaterials for energy conversion

27

5. Carbon nanotubes in renewable energy applications 5.1 Hydrogen storage In next 50 years, the world is going to face two major problems that are environmental and energy. The problem regarding energy is crucial as it can be the reason for environmental pollution because of the burning of fossil fuels and carbon emission. Hydrogen is the safest replacement to conventional energy generation resources as it generates water after combustion [104]. The hydrogen holds more energy per kilograms than petrol and diesel. But as it is in gaseous form in natural environmental conditions, it is difficult to store. MWCNTs are suitable material for hydrogen storage because it has high surface area, cage structure, and high conductivity [105e108]. Verdinelli et al. [109] studied molecular hydrogen storage on Rudecorated SWCNTs using spin-polarized density functional theory. They found out that the SWCNTs can absorb up to five Ru atoms without clustering. The Ru atoms get located at the hollow space, and further adsorption takes place near axial sites with adsorption energy of 2.51 eV per atom. Furthermore, molecular hydrogen adsorption study showed that the Ru/SWCNTs can bear four hydrogen atoms with adsorption energy of about
0.93 eV per H2. Rather et al. [110] used simple in situ reduction method to functionalize MWCNTs with manganese oxide (MgO) (Fig. 2.4B). The hydrogen adsorption uptake of virgin MWCNTs increased three times on addition of MgO in the range of 0.26%e0.94%. The MgO/ MWCNTs also showed cyclic stability upon successive adsorptione desorption cycles. There are various studies carried out to investigate and improve the hydrogen storage capacities of CNTs [111e114].

5.2 Solar cells Solar energy is freely available and in abundant amount. The photon energy from the sun is absorbed by the solar panels and converted into electricity. But the low efficiency of the conversion of light energy into electricity limits the wide use of solar cells. It is well known that carbon materials are extensively used in solar thermal transportation application because of their high solar absorbance capacity and thermal heat transfer capacity [115]. Kilic et al. [116] used the CNT/ZnO nanowires in DSSC as photoanodes, and the hybrid structure of CNT and ZnO showed excellent work function alignment, increased surface area, and enhanced optoelectronic properties. The modified DSSC showed 20% increase in the solar cell efficiency (h ¼ 5.55%) in a cell fabricated without CNT layer. Sakali

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

et al. [117] fabricated the CNTs with the new poly(acrylonitrile)-based gel polymer electrolyte (GPE) to enhance the efficiency of DSSC. The maximum energy conversion efficiency achieved was 8.87% with addition of 11 wt% CNTs. Li et al. [118] studied the perovskite solar cells using a mesoscopic TiO2/Al2O3 structure as a framework along with the SWCNT-doped graphite/carbon black counter electrode material. The CH3NH3PbI3-based solar cell could achieve the efficiency of 14.7% under the illumination of AM 1.5G. The SWCNT’s excellent hole conductivity helped to improve the charge collection compared to the non-SWCNT device. Jeon et al. [119] designed the perovskite solar cell without using indium but by using direct and dry transferred aerosol SWCNTs. They resolved the hydrophobicity and doping incapability by just varying the wettability of poly(3,4-ethylenedioxythiophene) (PEDOT):poly (styrenesulfonate) (PSS), MoO3 thermal doping, and HNO3 (aq.) doping with different dilutions. The 35 v/v% diluted HNO3-doped SWCNT-based device gave the highest PCE of 6.32%, which is about 70% of the indium tin oxideebased device. Although there have been many new techniques developed to design and enhance the efficiency of solar cells, still lot of work needs to be pursued to make laboratory research products commercially available.

5.3 Energy conversion using carbon nanotubes Fuel cells produce electrical energy by electrochemical reduction of oxygen and produce water as by-product. The research and use of fuel cells are in trend because of high efficiency in energy production and almost no pollution. Gong et al. [120] reported the use of the inexpensive vertically aligned nitrogen-doped CNTs as an electrode with enhanced electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than Pt for oxygen reduction alkaline fuel cells. They observed for 0.1 M KOH, a steady-state output potential of
80 mV and current density of 4.1 mAcm
2 at
0.22 V, which was higher than the PteC electrode. The reason for such an enhancement is that the electronaccepting nitrogen atoms imparted the high positive charge density on adjacent carbon atoms in the conjugated CNTs. This effect, along with the vertical alignment, provided four electron ways for the oxygen reduction on VACNTs with excellent performance. Tian et al. [121] prepared proton exchange membrane (PEM) fuel cells using very low loading of Pt in the membrane electrode assembly, using vertically aligned carbon nanotube (VACNT) as a catalyst support. The PEM fuel cells depicted an enhanced

Low-dimensional carbon-based nanomaterials for energy conversion

29

performance even with the Pt loading as low as 35 mgcm
2, compared to commercial Pt catalyst with 400 mgcm
2 loading. Amode et al. [122] used CVD-grown VACNTs as a cathode material in single-chambered microbial fuel cells (MFCs). The MnO2 catalyst was deposited on the cathode side to improve the oxygen reduction. They could achieve a twofold increase in the output voltage and power density when compared to the inert stainless steel electrodes, with power density of 24 mWm
2.

5.4 Carbon nanotubes in energy storage 5.4.1 Batteries The Li-ion batteries involve the intercalation and deintercalation of Li-ion in the anode or cathode, depending on the charging and discharging. While charging and discharging of the batteries number of times, the conventional electrode materials expand and contract leading to early damage. The life and efficiency of electrode materials can be increased by using carbon material, like CNTs. Cui et al. [123] fabricated heavy metal-free Si anode material by incorporating CNT network (Fig. 2.5A). This network worked as mechanical support as well as electrical conductor, while Si worked as high-capacity anode materials for Li-ion battery. The fabricated sheet has the lowest resistance of 30 Ohmsq
1 and specific charge as high as 2000 mAhg
1 with good cycling rate (Fig. 2.9C). Xia et al. [124] used a facile synthesis method to prepare the three-dimensional (3D) hierarchy structure of MnO2/CNT nanocomposite. The nanoflaky MnO2/CNT nanocomposite as an anode material for Li-ion battery exhibits the reversible capacity of 801 mAhg
1 for the first 20 cycles. The enhancement in the performance was attributed to the unique hierarchy structure, which allowed the fast Li-ion and electron charge transport. Li et al. [125] prepared the 3D structure of LiFePO4 nanoparticles by using MWCNTs. They could achieve an initial discharge capacity to 155 mAhg
1 at C/10 rate and 146 mAhg
1 at 1c rate because of the network structure. Li et al. [126] grew VACNT on the w3-mm GS by using CVD. The synthesized structure further tested as an anode material for Li-ion battery and also as counter electrode for DSSCs. Guo et al. [127] synthesized sulfurimpregnated disordered CNTs as cathode materials for Li-ion batteries. The CNTs could improve the cyclability and coulombic efficiency of the sulfur batteries. Furthermore, the electrochemical characterization revealed stabilized mechanism of sulfur owing to heat treatment of carbon.

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

30

(A) (b) Capacity (mAh/g)

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Figure 2.5 (A) (a) Photograph of a free-standing carbon nanotube CNT-Si film; (b and c) charge (red [dark black in print version]) and discharge (green [gray in print version]) capacity and coulombic efficiency (blue [light black in print version]) versus cycle number for a half cell using free-standing CNT-Si films as the working electrode cycled between 1 and 0.01 V, where (b) uses a single-layer, free-standing film and (c) uses two layers of free-standing films; and (d) voltage profile of the cell in (b); (B) (a) Schematic of the paper supercapacitor with the CNTs electrodes, (b) transmission electron microscopy image of the CNTs, (c) scanning electron microscopy image of CNTs film surface, (d) cyclic voltammetry at 20 and 150 mVs
1, and (e) galvanostatic chargedischarge results. (A) Reprinted with permission from L.F. Cui, L. Hu, J.W. Choi, Y. Cui, Light-weight free-standing carbon nanotube-silicon films for anodes of lithium ion batteries, ACS Nano 4 (2010) 3671e3678, https://doi.org/10.1021/nn100619m. © 2010 American Chemical Society. (B) Reproduced under Creative Commons Attribution License (CC BY). L.F. Aval, M. Ghoranneviss and G.B. Pour, High-performance supercapacitors based on the carbon nanotubes, graphene and graphite nanoparticles electrodes, Heliyon 4 (2018) e00862, https://doi.org/10.1016/j.heliyon.2018.e00862.)

5.4.2 Supercapacitors A supercapacitor’s working mechanism includes energy storage on the surface of conducting electrodes via reversible ion. Presently, supercapacitors have low capacitance and energy density because of scattered structure and low electrochemical performance of electrodes [128]. Aval et al. [129] fabricated paper supercapacitors using CNTs (Fig. 2.5B),

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Figure 2.5 Cont’d.

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Low-dimensional carbon-based nanomaterials for energy conversion

PVA+H3PO4 BaTiO3 Paper BaTiO3 PVA+H3PO4 CNTs

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32

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

graphite nanoparticles, and graphene electrodes. They used polyvinyl alcohol/phosphoric acid as gel electrolyte and BaTiO3 as barrier film. The CNT-based paper supercapacitors showed the highest specific capacitance of 411 Fg
1, compared to the graphite nanoparticles and graphene electrodes. Furthermore, the series resistance of the graphite nanoparticles, graphene, and CNTs was 210, 96, and 101 U, respectively. Kaempgen et al. [130] fabricated supercapacitors using printable aqueous gel electrolyte and organic liquid electrolyte. The SWCNT networks sprayed on sheets worked as both electrodes and charge collectors. The supercapacitors showed very high energy and power densities of 6 Whkg
1 for both electrolytes and 23 and 70 kWkg
1 for aqueous gel and organic electrolyte, respectively. Chen et al. [131] reported fabrication of supercapacitors having over 100 mm thick film electrodes of CNT/V2O5, along with organic electrolyte. The organic electrolyte helped to achieve high initial cell potential. The supercapacitor had energy density of about 40 Whkg
1 at a power density of 210 Wkg
1, with excellent cyclic stability. Tahir et al. [132] fabricated microsupercapacitors (MSCs) based on PEDOT-coated MWCNT network electrodes by photolithography, along with the electrochemical codeposition on microcurrent collectors. They reported that the fabricated MSC exhibited a maximum specific capacitance of 20.6 mFcm
2, a high energy density of 2.82 mWhcm
2 and a maximum power density of 18.55 Wcm
3 at an energy density of 8.1 mWhcm
3. Avasthi et al. [133] performed CVD growth of VACNT forest with millimeter height on Si/SiO2 substrate and engineered its microstructure and wettability for remarkable supercapacitor performance. A simple method to alter the microstructure of VACNT forest by adding small amount of ethanol in KOH electrolyte was demonstrated. The modified electrolyte facilitated the formation of pores and channels in CNT forest and led to increased active surface area. Microstructure-engineered CNT forest was further coated conformally with 3 nm of TiO2 using atomic layer deposition. The developed VACNT-TiO2 hybrid showed 102-fold increase in energy density, 20-fold increase in specific capacitance, and 13fold increase in power density along with high capacitive retention as compared to bare VACNT in KOH (Fig. 2.6A). This improved performance was correlated to formation of microchannels that enabled more accessible surface area for electrolytic ions. The demonstrated simple electrolyte engineering approach with increased energy density of aligned CNT-TiO2 hybrid is relevant for portable energy storage applications.

Low-dimensional carbon-based nanomaterials for energy conversion

33

Figure 2.6 (A) (a) Optical image of inch scale chemical vapor deposition (CVD) growth of vertically aligned carbon nanotube (VACNT) on Si/ SiO2 substrate, (b) high contact angle of water droplet on superhydrophobic VACNT surface, (c) cross-sectional field emission scanning electron microscopy image of VACNT, (d) high-resolution transmission electron microscopy image of carbon nanotube (CNT) walls and hollow tube, and (e) Raman spectra of VACNT showing structural quality of CVD-grown VACNT; (B) Increment in the specific capacitance of the supercapacitors due to the presence of the CNDs within the electrodes. (A) Reproduced from P. Avasthi, V. Balakrishnan, Tuning the wettability of vertically aligned CNT-TiO2 hybrid electrodes for enhanced supercapacitor performance, Advanced Materials Interfaces 6 (2019) 1801842, https://doi.org/10.1002/admi.201801842 with permission from Wiley. (B) Reproduced from J.S. Wei, C. Ding, P. Zhang, H. Ding, X.Q. Niu, Y.Y. Ma, C. Li, Y.G. Wang, H.M. Xiong, Robust negative electrode materials derived from carbon dots and porous hydrogels for high-performance hybrid supercapacitors, Advanced Materials 31 (2019) 1806197, https://doi.org/10.1002/adma.201806197 with permission from Wiley.)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.6 Cont’d.

Low-dimensional carbon-based nanomaterials for energy conversion

35

6. Applications of carbon nanodots in energy conversion and storage 6.1 Application in supercapacitors The CNDs were observed to be a superior material in supercapacitors as compared to the CNTs. Hu et al. synthesized the CND-doped CNTs, which resulted in the formation of micropores between the CNDs and CNTdddenoted as the electric double-cylinder model [134]. This configuration resulted in an augmented capacitance and stability. There were similar hybrid structures in the form of CNDs/graphene and CNDs/aerogel [135,136]. The use of the CDs has enabled surface area increase and proper diffusion of ions within the supercapacitors. Liu et al. devised the first CND-based supercapacitor by using electrodeposition method on gold electrodes. The fabricated devise displayed superior power response and fabulous rate capability with very short relaxation time constant in the aqueous and ion liquid electrolytes. This procedure was also adopted in constructing polyaniline (PAni)-based supercapacitance. Here, CNDs were electrodeposited on the aligned PAni fibers [137]. The devices comprising of PAni and CNDs as the positive and negative materials displayed high rate capability and short relaxation time of 115.9 ms. Higher energy density is vital for a superior supercapacitance. The pseudocapacitors related to redox reactions exhibit better energy density. Wei et al. framed hybrid supercapacitors with high power and long life spans, where carbon negative electrodes have high specific capacitance because of the presence of CNDs [138]. It deviates from the stereotypical method of modifying the pseudocapacitive groups on the carbon materials by depositing electron-rich regions on the electrode surface for absorbing cations. This is possible by electrodepositing CDs-hydrogel composites on the electrodes. This has ultimately resulted in excellent specific energy densities prompting the device to exhibit superior supercapacitance. Fig. 2.6B shows the increment in specific capacitance because of the presence of the CNDs in the electrodes.

6.2 Application in Li-ion batteries Zhu et al. synthesized the first ever CD-based Li-ion batteries by assimilating CuO/Cu/C-dots triaxial nanowire arrays [139]. The novel material implementing CDs assisted in the enhancement of surface conductivity and the nanowire array stability. The triaxial nanowire structure exhibited high coulombic efficiency and higher retention capability. Jing et al. devised CD-coated Mn3O4 composite by green electrochemical method [140].

36

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

This composite was used as an anode material for Li-ion batteries and it provided excellent electrochemical performances and ameliorated discharge capacity as compared with that of pure Mn3O4. The constant urge for high energy and dominant power densityebased batteries have enticed the use of LieCO2 batteries as the possible alternate of the Li-ion batteries. However, they have the disadvantage of getting decomposed during the discharging process leading to poor cyclability and charge overpotential. Jin et al. employed the use of CNDs supported by holey graphene as a cathode catalyst in the LieCO2 batteries [141]. This prevented the decomposition of the electrodes during discharging and exhibited excellent long-term stability with up to 235 cycles at the current density of 0.1 Ag
1. This inferred a high catalytic activity of CNDs in assisting the performance enhancement of the batteries. Another work has been reported where Tong et al. used the coating of CNDs onto the surface of vanadium oxide interwoven nanowires and utilized them as a cathode materials in Li-ion batteries, which showed capacities of 420 mAhg
1 at the current density rate of 0.3C [142].

6.3 Application in solar cells The propitious properties of CNDs possessing high mobility, high specific area, and tunable bandgap propel it as a potential candidate for application in solar cells. CDs have attracted researchers as a potential photosensitizer in the solar cells. Ma et al. inferred from the maneuvering of CNDs in RhBeTiO2 system that the CNDs act as a bridge between the RhB molecules and the TiO2 [143]. It also augmented the electrochemical efficiency by as much as seven times. The CNDs could pose in various roles from sensitizers and transporting layers to photoabsorption agents. For example, CNDs/Si nanowire array-based devices exhibited a conversion efficiency of 9.1%, which can be compared with that of Si-based hybrid solar cell [144]. This could be attributed to the special energy band structure of the CNDs, which acted as an electron blocking layer [145]. The nanosized CNDs play a very important role in the device performance based on the open circuit voltage (VOC) and short circuit current (JSC), with the VOC increasing and the JSC decreasing with the decrease in size of the CNDs. This could be ascribed to the quantum size effect, which results in the increase of the heterojunction barrier with the decrease in the size, and the hole transportation enhances, which amplifies the VOC and the JSC decreases. For example, Li et al. devised a heterojunction solar cell with the composite layer of poly(3-hexylthiopene) (P3HT) incorporated

Low-dimensional carbon-based nanomaterials for energy conversion

37

with CNDs. The CNDs boosted the photoconversion efficiency by 1.28% and also resulted in high VOC and conversion efficiency [97]. CDs are also used as charge transport materials or modifiers to enhance the performance of the organic solar cells. For instance, nitrogen/sulfur co-doped CNDs were proved to be an efficient modifier in organic solar cells. The assimilation of the CNDs decreased the surface energy and roughness of the ZnO layers, thereby facilitating the transport of photoelectric carriers [146]. In the DSSC, CND hybrids are regarded as an alternate to Pt. Yan et al. constructed the first DSSC using CNDs with very limited efficiency and low current density [147]. DSSCs, amalgamated with dyes and CNDs as co-sensitizers, were developed and high conversion efficiency was obtained [148]. CND-doped PAni manifested its dominance as counter electrodes for DSSCs [149]. Perovskite-based solar cells have appeared to be a major breakthrough in the field of solar cells because of their low cost and high efficiency. CNDs have acted as a potential prospect as superfast electron tunnels in the perovskite solar cells. For example, the efficiency of perovskite solar cells was amplified from 8.81% to 10.15% because of the incorporation of the CDs between the perovskite and mesoporous TiO2 [150] (Fig. 2.7A).

6.4 Application in light-emitting diodes The recent economic low-cost requirement of the electricity has enabled the optimum use of light-emitting diodes (LEDs) in daily lives. LEDs appended with phosphor are the versatile fabrication approach for LED devices. Chen et al. initially developed the CND-based phosphors to build LED devices [151]. The CNDs displayed strong photoluminescence as compared with the conventional materials. Before using the CNDs as a photoluminescence, proper care must be taken to avoid the solid-state quenching. This can be done by modifying the CNDs and dispersing them in a bulk matrix. The blue and red emissive CNDs mustered into mesoporous alumina displayed excellent photoluminescence and thermal stability [152]. Some luminescent films have also been synthesized by dispersing CNDs within the poly(methyl methacrylate) matrix, appended with the InGaN chips to obtain the LED with a correlated color temperature of 5080.4K with no profane degradation [153]. The conversion of the light of the LED into blue was possible by coating the CND/ clay-carbon nanofiber films [154]. White LEDs are considered to be a revolution in the lighting devices with several applications. It is possible to increase the color rendering index of the white LEDs by controlling the

38 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.7 (A) Current densityevoltage curves for the perovskite-based solar cells. The red line represents the nanodots incorporated mesoporous TiO2. The black line represents the mesoporous TiO2 without nanodots; (B) Schematic representation of the nitrogen doping process of reduced graphene oxide (a) and the bulk heterojunction solar cell using the N-doped graphene/P3HT:PCBM active layer (b). (A) Reprinted with permission from Z. Zhu, J. Ma, Z. Wang, C. Mu, Z. Fan, L. Du, Y. Bai, L. Fan, H. Yan, D.L. Phillips, S. Yang, Efficiency enhancement of perovskite solar cells through fast electron extraction: the role of graphene quantum dots, Journal of the American Chemical Society 136 (2014) 3760e3763, https://doi.org/10.1021/ja4132246. © 2014 American Chemical Society. (B) Reproduced from G.H. Jun, S.H. Jin, B. Lee, B.H. Kim, W.-S. Chae, S.H. Hong, S. Jeon, Enhanced conduction and charge-selectivity by N-doped graphene flakes in the active layer of bulk-heterojunction organic solar cells, Energy and Environmental Science 6 (2013) 3000e3006, https://doi.org/10.1039/C3EE40963E with permission from The Royal Society of Chemistry.)

Low-dimensional carbon-based nanomaterials for energy conversion

Figure 2.7 Cont’d.

39

40

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

ratio of two different color-emissive CNDs. For example, blue-, green-, and red-emissive CNDs were mixed together into an epoxy resin, and by optimizing the ratio of the three CNDs, pure white LED devices were obtained [155]. Recently, redeblueegreen CND films using blue-emissive CNDs as a phosphor, dispersant, and curing agent were fabricated [156].

7. Applications of graphene in energy conversion and storage Graphene applications are remarkable in many industries, such as aerospace, chemical, biomedical, energy, electrical, and electronics sectors. In addition, the applications of graphene and its derivatives in the telecommunication sector are outstanding because of their excellent physical and chemical properties. There are many research works that reported development of graphene-based chips for electrical and electronics sectors. In the energy industry, the effect of graphene is much more remarkable. The detailed applications of graphene in the energy industries have been discussed in the following section.

7.1 Solar cells The solar cell devices are utilized for conversion of natural energy (sunlight) into electrical energy [157]. Solar cells consist of indium tin oxide and protective glass layers. These cells are designed in the form of sandwich structure, with organic layers between different charges collecting devices in that one side should be transparent and other side may be aluminum or lithium fluoride coated. The fact that the cost of indium is high and it is a rare material leads to solar cells to be much more expensive in future. However, the replacement of graphene instead of indium became more economically feasible for the solar cells production industries. In addition, indium tin oxide’s conductivity and flexibility are very less compared with graphene, which attracts more interests on the replacement of indium by graphene in solar cells production. Some researchers reported that graphene layers show efficient photoelectron conversion [158e160]. Yue et al. reported that the graphene-modified molybdenum sulfide composites exhibited better power efficiency than Pt electrode [161]. There is an advancement of solar cells called organic photovoltaic cells, which has its own advantages compared with traditional solar cells. Nevertheless, replacement of indium tin oxide by graphene made remarkable revolutions in the solar cells industries. Many published reports reveal that the

Low-dimensional carbon-based nanomaterials for energy conversion

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implementation of GSs as a counter electrode in the solar cells showed better power conversion efficiency [162,163]. In addition, graphene nanosheets produce better transportation of elections and exciton dissociation in the solar cells [164e166]. Applications of solar cells are remarkable because of the low-cost process involved in obtaining large quantity of outcome. Nevertheless, further production cost reduction is been targeted with assistance of graphene nanosheets and its related derivatives. This is because the revolution of graphene in solar industries will become remarkable in the future. The graphene fabrication route in solar cells has been presented in Fig. 2.7B.

7.2 Battery Batteries are much needed devise for automobiles, electronic equipment, ships, and aircrafts. One of the best battery storage devises are Li-ion batteries [167]. There are many recent researches on the progress of rechargeable batteries to achieve higher capacity and portable size for friendly use. According to literature reports, incorporation of pristine graphene nanosheets within Li-ion batteries does not lead to much impressive efficiency. On the other hand, the specific energy of Li-carbon batteries is 370 mAhg
1 because the six carbon molecules are able to form intercalation with only one Li-molecule [168]. Chemically modified graphene-based nanocomposites are able to show better battery efficiency compared to neat graphene nanosheets. Choi et al. reported that the graphene-modified batteries showed negligible degradation after 700 cycles [169]. It has been reported by many researchers that graphene-based batteries demonstrate excellent performance [170e172]. This indicates that graphene is capable to make notable advancement in the batteries production. Based on the properties, graphene can be utilized in different kinds of batteries, as represented in Fig. 2.8A.

7.3 Fuel cells Fuel cells are able to convert chemical energy into electrical energy with the assistance of oxygen and a fuel (such as hydrogen, alcohol, etc.) [173]. The major key issue in the fuel cell process is selection of proper electrode materials, due to the lack of availability of Pt and its cost expensiveness. Many research reports are available to understand the effects of carbonbased materials for fuel cell process [174,175]. Graphene has notable physicochemical properties and high surface area, which leads to the replacement of Pt in the fuel cell process [176]. Some reports suggest that

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 2.8 (A) A schematic illustration of the applications of graphene based on its unique properties; (B) The overall graphene applications in the field of supercapacitors. (A) Reproduced from N. Mahmood, C. Zhang, H. Yin, Y. Hou, Graphenebased nanocomposites for energy storage and conversion in lithium batteries, supercapacitors and fuel cells, Journal of Materials Chemistry A 2 (2014) 15e32, https://doi.org/ 10.1039/C3TA13033A with permission from the Royal Society of Chemistry. (B) Reproduced from W. Yang, M. Ni, X. Ren, Y. Tian, N. Li, Y. Su, X. Zhang, Graphene in supercapacitor applications, Current Opinion in Colloid and Interface Science 20 (2015) 416e428, https:// doi.org/10.1016/j.cocis.2015.10.009 with permission from Elsevier.)

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graphene-modified fuel cells are showing better performance in the oxygen reduction reaction (ORR) on the electrochemical surface [177e179]. Utilization of graphene in fuel cells led to enhanced platinumegraphene interaction and better electrochemical surface morphology, which leads to the formation of more catalytically active cites during fuel cell process [180,181].

7.4 Supercapacitors Supercapacitors are excellent energy storage devices and are considered as replacement of Li-ion batteries. The high power density and the fast chargingedischarging ability of supercapacitors have made it more attractive toward many industries, such as automobiles, aerospace, and telecommunication [182]. Supercapacitors are able to deliver high power under low energy storage conditions. Supercapacitors are commonly divided into two types such as pseudocapacitors and double-layer electrical capacitors. Carbon-related materials are mostly used as electrode in the double-layer electrical capacitors, owing to their excellent physicochemical properties [183]. In the case of pseudocapacitors, conductive polymers are used as electrodes because of their high surface area. Conversely, graphene-based polymeric nanocomposites are also utilized as electrode materials in the pseudo and double-layer electrical capacitors because of their extraordinary physicochemical properties and higher surface area [184,185]. Earlier research reports confirm that graphene-containing supercapacitors demonstrate excellent performances in the form of power density [186]. Graphene is set to take very important role in the supercapacitor application in near future. Overall application of graphene in the field of supercapacitors has been represented in Fig. 2.8B.

7.5 Hydrogen storage devices Hydrogen is a very important resource for the next-generation electricity sources. Hydrogen can react with atmospheric oxygen to generate energy, with the release of water as by-product. For this reason, hydrogen-based fuel cells are considered as clean energy generating devices. Conversely, hydrogen storage and transport of energy are the key issues under the current scenario. In this case, graphene is a very promising material that has high surface area, low weight, and higher physicochemical properties, which could take part in the hydrogen storage process. Hydrogen reacts with graphene surfaces and makes bonding or forms a deposition on the surfaces. This bonding or adsorption process needs very high pressure and

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low temperature to establish the stability of hydrogen storage. Durgun et al. reported that the chemical functionalized GSs show attractive hydrogen storage efficiency [187]. Many research works introduced graphene in hydrogen storage process and revealed better storage capacity and safe transportation [164e166]. The key problem of hydrogen storage is finding the perfect atmosphere for the chemisorptions/adsorption process without assistance of higher pressure and temperature. However, implementation of graphene in the hydrogen storage process will overcome this issue [188]. While incorporating the graphene materials into the hydrogen storage process, the hydrogen will take part in the combination of CeH binding; these hydrogen bindings are highly stable with graphene, which leads to safe transportation of hydrogen energy with no dispersion.

8. Applications of graphene quantum dots related to energy conversion and storage GQDs promise utilization in a wide range of applications in energy conversion and storage on account of their large surface area, copious active sites, potential to facilitate charge transfer and transport, tunable optical properties, and ability to intimately integrate with other nanomaterials. Electrochemical energy storage and conversion applications, such as electrochemical capacitors, Li-ion batteries, fuel cells, and solar cells, have attracted much attention because of the strong demand in our society. Many researchers are trying to find energy-related and environmentally friendly products using GQDs that have unique properties such as high transparency, large surface area, and strong and PL intensity. Therefore, it is meaningful to introduce energy-related and environmental applications of GQDs.

8.1 Supercapacitors Supercapacitance property of GQD depends on their large specific surface area, high pseudocapacitance originating from edges, defects, functional groups, and dopants [62,73]. Mondal et al. synthesized GQD-doped PAni composites by chemical oxidation of aniline, which showed an excellent specific capacitance value of 1044 Fg
1 at a current density of 1 Ag
1, as well as moderate cyclic stability with a retention of life time of 80.1% after 3000 cycles [189]. Hassan et al. report a facile ultrasonic approach with chemical activation using KOH to prepare activated GQDs (aGQDs) enriched with both free and bound edges. Compared to GQDs, the aGQDs they synthesized had enhanced BET surface area by a factor of

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about six, the photoluminescence intensity by about four and half times, and electrocapacitance by a factor of about two. The specific capacitance calculated from the discharge curves of the GQD film on the GCE was 236 Fg
1, which was greater than that of FLGs (172 Fg
1), GQDs (108 Fg
1), and FLGs (63 Fg
1) [190]. The electrochemical assembly of the GQDs on HACNTs proceeded smoothly and led to the formation of a uniform film on the surface of the HACNTs, thus transforming 0D nanodots into a 3D aligned composite structure that was subsequently applied in the energy storage devices. Supercapacitors fabricated from GQD/CNT composite film exhibited a high capacitance of 44 mFcm
2, representing a more than 200% improvement over that of bare HACNT electrode [191]. Supercapacitors fabricated from GQD-3DG composite electrodes, with 10 h GQD deposition, exhibited a high capacitance of 268 Fg
1, representing a more than 90% improvement over that of bare 3DG electrodes (136 Fg
1). Considering the convenience of the electrodeposition of GQDs, the current method could also be used in other well-defined electrode materials, such as CNTs, carbon aerogels, and so on, to further boost the performance of the supercapacitors [136]. GQDs/ MnO2 heterostructural electrode not only can enlarge the operating potential window from 0 to 1.3 V but also can improve the specific capacitance to 1170 Fg
1. The extended potential window and improved specific capacitance of GQDs/MnO2 heterostructural electrodes can be attributed to the built-in electric field in heterostructures [192]. Zhang et al. demonstrated a novel top-down strategy to obtain GQDs through sonication crush and strong acid cutting to break the CeC bonds of GO. Besides, the abundant edge and defects sets, as active sites, can promote ion diffusion in the interior of the material. They exhibited the characteristic virtues of an SC value of 307.6 Fg
1 at a scan rate of 5 mVs
1, a high energy density of 41.2 Whkg
1, and an excellent capacitance retention after 5000 cycles [193]. Upon fabricating GQD-based MSCs and studying their electrochemical properties, a new application of GQDs as an electroactive material for supercapacitors was identified. The electrochemical tests showed that as-made GQDs//GQDs symmetric micro supercapacitor has superior rate capability with the scan rate up to 1000 Vs
1, excellent power response with a small RC time constant (103.6 ms), high area specific capacitor (468.1 mFcm
2), and outstanding cycle stability in 0.5 M Na2SO4 aqueous solution. In addition, another kind of GQDs//MnO2 asymmetric MSC was also built. Compared with GQDs//GQDs symmetric MSC in aqueous electrolyte, GQDs//MnO2 symmetric MSC displayed two times

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higher specific capacitance (1107.4 mFcm
2) and energy density (0.154 mWhcm
2) [194]. Lee et al. fabricated new transparent and flexible MSCs using chelated graphene and GQDs by a simple EPD method. Through a chelate formation between graphene and GQDs with metal ions, the GQD materials were strongly adhered on an interdigitated pattern of graphene (ipG-GQDs), and its resulting porous ipG-GQDs film was used as the active material in the MSCs. Amazingly, these supercapacitor devices showed high transparency (92.97% at 550 nm), high energy storage (9.09 mFcm
2), short relaxation time (8.55 ms), stable cycle retention (around 100% for 10,000 cycles), and high stability even under severe bending angle 45 degrees with 10,000 cycles [195].

8.2 Batteries In Li- or Na-ion batteries, GQDs can help to relieve the volume expansion, accelerate Liþ/Naþ diffusion and electron transfer, enhance the interfacial double layer, and improve electrochemical properties. Fig. 2.9 shows the GQD in the field of batteries. Guo et al. developed GQDs doped into MoS2 nanosheets via a solvothermal method for high-performance anode materials. The obtained GQDs/MoS2 remarkably improved electrochemical Li storage properties than pristine MoS2, such as high reversible capacity (1099 mAhg
1 at 100 mAg
1), good cyclic stability, and excellent rate performance (660 mAhg
1 at 5000 mAg
1) [196]. Chen et al. fabricated a GOQDdecorated PAVM polymer framework that minimized both the ion-solvent clusters and degree of Liþ ion solvation to facilitate transport of Liþ ions in GPEs and transfer at the electrode surface. The high-polarity nitrile groups and GOQD acceptors on PAVM immobilized PF
6 ions; hence, formation of ion-solvent clusters was suppressed and the number of Liþ ions coordinated with the solvent was reduced. Smaller GOQDs were þ more effective in immobilizing PF
ion 6 ions and promoting the Li mobility of the GPEs because of the superior distribution of GOQDs over the polymer chains [197]. For a multifaceted design of silicon anode for high-performance Li-ion batteries, at first silicon nanoparticles (Si) were encapsulated in 3D interconnected networks of multiple graphene aerogel (MGA-n, inner shell). Then, MGA-n/Si was embedded in the binder layer (outside shell) composed of tryptophan-functionalized GQDs (Trp-GQDs) and sodium alginate. As a result, Trp-GQDs@MGA-n/Si electrode exhibits excellent electrochemical performance for Li-ion batteries. The specific capacity is 1427 mAhg
1 at 100 mAg
1 and 637 mAhg
1 at

(A)

(B)

(C) (D)

(E)

Figure 2.9 Graphene quantum dots (GQDs) for batteries: (A) GQD/MoS2 hybrid for Li-ion battery. (B) Ion-solvent clusters/GOQD for Li-ion batteries. (C) VO2/graphene arrays coated with GQDs as the cathode for both Li-ion and Na-ion batteries. (D) Enhanced lithium storage performance of CuO nanowires by coating of GQDs. (E) GQD/manganese vanadate composites as Li-ion battery anodes. (A) Reproduced from J. Guo, H. Zhu, Y. Sun, L. Tang, X. Zhang, Boosting the lithium storage performance of MoS2 with graphene quantum dots, Journal of Materials Chemistry A 4 (2016) 4783e4789, https://doi.org/10.1039/C6TA00592F with permission from the Royal Society of Chemistry. (B) Reproduced from Y.M. Chen, S.T. Hsu, Y.H. Tseng, T.F. Yeh, S.S. Hou, J.S. Jan, Y.L. Lee, H. Teng, Minimization of ion-solvent clusters in gel electrolytes containing graphene oxide quantum dots for lithium-ion batteries, Small 14 (2018) 1703571, https://doi.org/10.1002/smll.201703571 with permission from Wiley. (C) Reprinted with permission from D. Chao, C. Zhu, X. Xia, J. Liu, X. Zhang, J. Wang, P. Liang, J. Lin, H. Zhang, Z.X. Shen, H.J. Fan, Graphene quantum dots coated VO2 arrays for highly durable electrodes for Li and Na ion batteries, Nano Letters 15 (2015) 565-573, https://doi.org/10.1021/nl504038s. © 2015 American Chemical Society. (D) Reproduced from C. Zhu, D. Chao, J. Sun, I.M. Bacho, Z. Fan, C.F. Ng, X. Xia, H. Huang, H. Zhang, Z.X. Shen, G. Ding, H.J. Fan, Enhanced lithium storage performance of CuO nanowires by coating of graphene quantum dots, Advanced Materials Interfaces 2 (2015) 1400499, https://doi.org/10.1002/admi.201400499 with permission from Wiley. (E) Reproduced from Y. Ji, J. Hu, J. Biskupek, U. Kaiser, Y.F. Song, C. Streb, Polyoxometalate-based bottom-up fabrication of graphene quantum dot/manganese vanadate composites as lithium ion battery anodes, Chemistry e A European Journal 23 (2017) 16637e16643, https://doi.org/10.1002/chem.201703851 with permission from Wiley.)

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4200 mAg
1. The capacity retention is more than 93.3% after 100 cycles at 100 mAg
1 with a high columbic efficiency of about 99.8%. Such a multifaceted design can also be used for fabrication of other large-volumechange electrodes for Li-ion batteries [198]. Chao et al. design a new type of binder-free cathode by bottom-up growth of biface VO2 arrays directly on a graphene network for both high-performance Li-ion and Na-ion battery cathodes. More importantly, GQDs are coated on to the VO2 surfaces as highly efficient surface “sensitizers” and protection toward further boost of the electrochemical properties. The integrated electrodes deliver a Na storage capacity of 306 mAhg
1 at 100 mAg
1 and a capacity of more than 110 mAhg
1 after 1500 cycles at 18 Ag
1. This result of Na-ion battery may pave the way to next-generation post-Li batteries [139]. A novel CuO/Cu/GQD triaxial nanowire electrode has been developed for their application in Li-ion storage. Owing to the unique Cu and GQD double-layer enhancement, the CCG triaxial nanowire electrodes show high capacity retention in first 100 cycles and nearly no capacity decay afterward until 1000 cycles. A high rate capability recovery is achieved even after cycled at 30 C. Furthermore, the achieved high initial coulombic efficiency (c. 87%) can be ascribed to a synergistic contribution from the Cu and GQD layers [199]. The LTO/N,S-GQDs anode exhibited remarkably enhanced electrochemical performance for Li-ion battery. The specific discharge capacity was 254.2 mAhg
1 at 0.1 C and 126.5 mAhg
1 at 10 C. The capacity remained 96.9% after at least 2000 cycles at 2 C. The battery performance is significantly better than that of pure LTO electrode and LTO/graphene electrode [200].

8.3 Photovoltaic cells/solar cells GQDs have served as sensitizers in GQD/semiconductor solar cells, electron/hole transport layer materials in organic or perovskite solar cells, and the active layer additive in donor/acceptor blends for DSSC. The GODs in photovoltaic devices/solar cells have been presented in Fig. 2.10. In the GQD/semiconductor heterojunction-based solar cell, GQDs favor carrier separation, light absorptions, charge carrier extraction, and down-conversion property [145,201]. A solar cell fabricated by kinetic spray of GQDs onto crystalline Si showed a high power conversion efficiency of 15.3% partly because of a UV photon absorbed by GQD can generate two electrons at the heterojunction [201]. For organic or perovskite solar cells, GQDs with electron-donating (or electron-withdrawing) functional groups serve well in the electron or hole extraction layer by

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Figure 2.10 Graphene quantum dots (GQDs) for photovoltaic devices/solar cells: (A) crystalline Si/GQDs heterojunction solar cells. (B) GQD layers with energy-down-shift effect on crystalline Si solar cells. (C) GQDs/Si heterojunction solar cells. (D) GQD-modified organic/Si hybrid device. (A) Reprinted with permission from P. Gao, K. Ding, Y. Wang, K. Ruan, S. Diao, Q. Zhang, B. Sun, J. Jie, Crystalline Si/graphene quantum dots heterojunction solar cells, Journal of Physical Chemistry C 118 (2014) 5164e5171, https://doi.org/10.1021/jp412591k. © 2014 American Chemical Society. (B) Reprinted with permission from K.D. Lee, M.J. Park, D.Y. Kim, S.M. Kim, B. Kang, S. Kim, H. Kim, H.S. Lee, Y. Kang, S.S. Yoon, B.H. Hong, D. Kim, Graphene quantum dot layers with energy-down-shift effect on crystalline-silicon solar cells, ACS Applied Materials and Interfaces 7 (2015) 19043e19049, https://doi.org/10.1021/acsami.5b03672. © 2015 American Chemical Society. (C) Reproduced from S. Diao, X. Zhang, Z. Shao, K. Ding, J. Jie, X. Zhang. 12.35% efficient graphene quantum dots/silicon heterojunction solar cells using graphene transparent electrode. Nano Energy 31 (2017) 359e366. https://doi.org/10.1016/j.nanoen.2016.11.051 with permission from Elsevier. (D) Reprinted with permission from M.L. Tsai, D.S. Tsai, L. Tang, L.J. Chen, S.P. Lau, J.H. He, Omnidirectional harvesting of weak light using a graphene quantum dot-modified organic/silicon hybrid device, ACS Nano 11 (2017) 4564e4570, https://doi.org/10.1021/acsnano.6b08567. © 2017 American Chemical Society.)

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decreasing (or increasing) the work function [202e205]. Being sandwiched between perovskite and TiO2 cathode, the electron-donating phenylenediamine-functionalized GQDs with narrow bandgap enhanced electron extraction and consequently increased the power conversion efficiency by 15.1% compared to the solar cell without GQDs. When being coated between the polymer active layer and the ITO anode, the electronwithdrawing COOH-functionalized GQDs enhanced hole extraction and thus improved the power conversion efficiency by 50% [205]. In DSSC, GQDs promote light absorption, charge separation, and exciton dissociation. When GQDs are bonded into the active layer by poly(ethylene glycol) (PEG) or P3HT as the donor and PCBM as the acceptor, the solar cell exhibited efficiency improvement compared to the solar cell without GQDs [150,206,207]. A new type of solar cells based on the c-Si/GQDs heterojunction was developed by Gao et al. The GQDs also served as an electron blocking layer to further prevent the carrier recombination at the anode. An optimum power conversion efficiency of 6.63% was obtained by tuning the GQDs size and layer thickness [201]. Tsai et al. investigated photovoltaic behavior using outdoor weather-dependent and indoor weak light conditions to demonstrate the superior PCE of the hybrid cells in all weak light conditions via an enhanced fill factor because of efficient hole transport (i.e., electron blocking) of the PEDOT:PSS layer, as compared to all-inorganic Si solar cells [203]. Kim et al. reported the synthesis of reduction-controlled GQDs and their application to bulk heterojunction (BHJ) solar cells with enhanced power conversion efficiency. The device prepared by embedding the GOQDs in the BHJ layer showed a considerable enhancement of absorptivity and thereby an increase of current density (Jsc). On the other hand, the excitation band varies with reduction time as the functional groups were removed [208]. GQDs in PEDOT:PSS had achieved an efficiency of 13.22% in Si/PEDOT:PSS hybrid solar cells. After introducing GQDs into PEDOT:PSS, the short circuit current and the fill factor of rear contact optimized hybrid cells were increased from 32.11 mAcm
2 to 36.26 mAcm
2 and 62.85% to 63.87%, respectively [209]. Ding et al. demonstrated that tetraalkylammonium-functionalized GQDs can be used as CILs in PSCs for improved device efficiency. The periphery COO
$(CH3)4 Nþ groups in GQD-TMA form interfacial dipole with metal cathode to decrease the work function [210]. Zhu et al. reported a significant power conversion efficiency improvement of perovskite solar cells from 8.81% to 10.15% due to insertion of an ultrathin GQDs layer between perovskite and TiO2. A strong quenching of perovskite

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photoluminescence was observed at w760 nm upon the addition of the GQDs, which is pronouncedly correlated with the increase of the IPCE and the APCE of the respective cells [206]. Novac et al. developed improved P3HT:PCBM solar cells through the addition of GQDs functionalized with various molecular weights of PEG. Functionalization with low molecular weight PEG was shown to significantly improve the GQD interaction with the active layer, resulting in improved PCE over both reference cells and cells produced with pristine GQDs. The mechanism of enhancement was shown to be faster than P3HT exciton dissociation in GQD-containing cells, which resulted in an absorption improvement in the visible range because of fewer bound charges remaining in P3HT [211].

8.4 Fuel cells GQDs are useful for fuel cells as they help demonstrate excellent electrocatalytic performance in the ORR. GQDs are hybridized with other nanomaterials, such as C3N4 [212], CNTs [213], graphene [214,215], and reduced graphene oxide (rGO) [216,217], to realize synergistic effects of catalytic materials for ORR. B,N-GQDs [218] and N-GQDs [219] have been employed as ORR catalysts based on their catalytically active dopant sites. As a new metal-free catalyst, the prepared s-s-g-C3N4@GQD nanohybrids exhibited remarkably enhanced catalytic activity in the ORR far better than the original s-g-C3N4 and GQDs, which is even comparable to those of well-developed graphene-based materials [212]. A novel metalfree electrode composed of GQDs and MWCNTs exhibited a significant synergistic effect on enhanced catalytic activity for ORR [213]. The N-GQDs/G performed four-electron pathway resulting from the excellent structural properties of N-GQDs, such as abundant pyridinic-N, rich edges, and crystal structure. The N-GQDs/G also showed high electrochemical stability and resistance to methanol crossover [214]. Hybrid of B,N-GQD and graphene combined the catalytic abilities of the former and the high conductivity of the latter, leading to excellent ORR performancedfar superior to commercial Pt/C [215]. MoS2-rGO 3D framework decorated with N-GQDs has been used for direct methanol fuel cells with excellent ORR performance, stability, and methanol tolerance [220]. Because of abundant active sites (edges, defects, and interfaces) and fast charge transfer between GQDs and graphene nanoribbons (GNRs), GQD-GNR hybrid demonstrated ultrahigh performance for ORR with high selectivity and stability, as well as higher limiting current density and lower overpotential than those of Pt [221].

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9. Summary and future aspects The limited availability of conventional natural energy resources and their adverse effect on the environment led to research and developments of alternative energy sources. Carbon nanomaterials, such as graphene and CNTs, have become the center spot of research because of their unique structural, surface, electrical, and mechanical properties. The CNTs and conducting polymer composites are considered as promising materials for solar cells application. The VACNTs can be an alternative to Pt electrode as an inexpensive material in fuel cells because of their high electrocatalytic activity and high cyclic stability. CNTs also play a vital role in enhancing the charge storage capacity in Li-ion batteries. The CNTs’ high surface area, good charge storage capacity, and better electron conductivity is helpful in improving the storage capacity and life of Li-ion batteries. However, the low dispensability and low specific capacitance limit the CNT applications in many areas. From literature, we can realize that the CNTs dispersion and specific capacitance could be improved by the surface functionalization. Furthermore, supercapacitors possess high power density, but they are lower than batteries. CNTs play a vital role in controlling the capacitance of supercapacitors. The capacitance of supercapacitors is also dependent on synthesis of CNTs, surface functional groups, surface area, and pore size. In addition, we have furnished the survey on the application of the CNDs in the energy conversion and the energy storageebased devices. The exciting outputs of the research based on CNDs have further strengthened their role in future multidisciplinary applications. The main challenge lies in unearthing a synthetic route to fabricate good quality CNDs. This could further assist in improving the efficiency of the energy-based devices. There is a constant urge in sorting out the issues in synthesizing CNDs for specific applications, as some impurities exist in it. The solution lies in controlled synthesis of the CNDs and also development of some in situ techniques to prepare them. The adverse effects of the luminescent impurities present in the CNDs samples must be taken into consideration before using them in the energybased devices. CNDs possess the unique combinational characteristics of optical properties of semiconductor QDs and electrical properties of carbon materials, thereby acting as an excellent sensitizer in the solar cells. By controlling the size, surface functional groups, and dopant concentration, the wide spectrum in enhancement of the solar cell efficiency could be obtained. The surface modification of the CNDs and improved charge diffusion between the CNDs and catalysts will be instrumental in the light harvesting

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characteristics in the near future. The large specific area and sufficient defects have enabled the use of the CNDs in conversion from electrical or solar energy to chemical energy. They are also considered to be a possible alternate for Pt as a counter electrode. Their imminent optical characteristics have accredited them to be used as a phosphor in the LEDs. Overall, the potential applications of the CNDs have not been exploited completely. Therefore, further extensive research has to be done and comparative study with other nanomaterials has to be carried out. On the other hand, graphene nanosheets are promising materials for energy storage applications owing to their excellent charge transport and higher physiochemical properties. Effects of graphene incorporation in the fuel cells, supercapacitors, batteries, and solar cells are remarkable. Graphene and its derivatives have been implemented in many fields of applications. Graphene utilization in the energy storage led to our environment being secure, safe, and clean. The implementation of graphene-based materials in the energy sector has made a serious impact in the automobile, telecommunication, and aerospace industries. There are many researchers still putting their effort to develop the storage capacity and charge transportation with no dispersion. Future developments will make Li-ion batteries, supercapacitor, and solar cells as core part of energy storage and conversion systems. These devices will play an important role in providing clean energy in various fields. The research direction should be to make these devices more economically affordable and available for general purpose use.

Acknowledgment Nidhin Divakaran, Manoj B. Kale, Suhail Mubarak and Duraisami Dhamodharan contributed equally to this chapter.

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CHAPTER 3

Nanostructured bifunctional electrocatalyst support materials for unitized regenerative fuel cells T. Sadhasivam1, 2, Ho-Young Jung1, 2 1

Department of Environment & Energy Engineering, Chonnam National University, Gwangju, Republic of Korea; 2Center for Energy Storage System, Chonnam National University, Gwangju, Republic of Korea

1. Introduction In current times, the search for alternative and green energy systems has gained significant attention because of fossil fuel depletion and environmental pollution issues. Fossil fuels significantly emit greenhouse gases during energy conversion, which is a major cause of air pollution and risk to the ozone layer [1e5]. Moreover, worldwide, the energy demand is steadily increasing because of the increased energy utilization with rapid population growth and industrialization [6e8]. Since the past several decades, there have been noteworthy attempts to produce energy from renewable energy sources, such as solar, wind, hydro, bio, geothermal, waste, and tidal, as alternative and sustainable energy sources [9e11]. To utilize the energy derived from renewable sources, different types of electrochemical energy storage and conversion systems have been developed by various approaches [12e17]. Compared to various devices, the attention paid to fuel cell technologies is greater because of the relatively high energy density [18e20]. In this regard, numerous types of fuel cell systems have been invented and developed to overcome energy-related issues. Some examples are proton exchange membrane or polymer electrolyte membrane (PEMFC) [12,18e20], direct methanol [21,22], solid oxide [23,24], phosphoric acid [25,26], formic acid [27,28], molten carbonate [29,30], direct ethanol [31,32], alkaline [33,34], enzymatic [35,36], microbial [37,38], and direct ethylene glycol fuel cells [39,40]. In general, PEMFCs use hydrogen gas as the energy carrier [41,42]. Here, electricity is generated by the splitting of hydrogen molecules into protons and electrons Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00003-8

© 2020 Elsevier Inc. All rights reserved.

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at the anode. The generated electrons can be transferred and used via an external circuit. The complete process of the oxygen reduction reaction (ORR) occurs at the positive electrode, where water molecules are produced as a by-product [43,44]. In PEMFCs, energy is generated without any harmful emission or dangerous by-products. In addition, PEMFCs have numerous advantages such as wide applicability, and high energy density. However, the reaction in a PEMFC system is irreversible. Hence, a hydrogen infrastructure is required for PEMFC technologies. Moreover, the production of the energy carrier, hydrogen, from fossil fuels, is achieved by a reforming process, which can cause issues like greenhouse gas emission [45,46]. To generate hydrogen by a relatively greener route, electrochemical water splitting techniques (hydrogen evolution reaction [HER] and the oxygen evolution reaction [OER]) are prominent [47,48]. In this connection, the advanced developed form of PEMFCs is known as a unitized regenerative fuel cell (URFC) system [49e57].

2. Unitized regenerative fuel cell system The most significant advantage of a URFC system is that the round-trip energy conversion (by fuel cell and water electrolysis mode) in a single unit cell system without any hazardous emissions [49,50,58,59]. A URFC system possesses numerous advantages such as the following: (i) the most abundant and inexpensive source of water molecules is used to produce the energy carrier, hydrogen, (ii) the energy from renewable energy sources such as solar/wind, is used by the water electrolyzer process, (iii) a hydrogen infrastructure is not required, (iv) it is one of the prominent green and environment-friendly energy systems in which no harmful emission occurs throughout energy conversion, (v) the specific energy density is considerably high, approximately 3660 Wh Kg
1 and 400e1000 Wh Kg
1 for the theoretical and packaged system, respectively, and (vi) it is a light-weight, highly stable, renewable, and sustainable energy system [60e63]. In view of the above promising advantages, the URFC is being used in numerous applications such as space applications, solar rechargeable aircrafts, and zeroemission vehicles [60,63,64]. The fuel cell mode and water electrolysis mode in a single unit cell device make up the URFC and represent a significant development from PEMFCs. Fig. 3.1 presents the schematic of a URFC device with fuel cell and water electrolysis modes [65]. As shown in the figure, the energy derived from renewable energy sources, like solar, can be used to conduct

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Figure 3.1 Schematic representation of unitized regenerative fuel cell process. (Reproduced with permission from Ref. T. Sadhasivam, G. Palanisamy, S.H. Roh, M.D. Kurkuri, S.C. Kim, H.Y. Jung, Electro-analytical performance of bifunctional electrocatalyst materials in unitized regenerative fuel cell system, International Journal of Hydrogen Energy 43 (39) (2018) 18169e18184. Copyright 2018, Elsevier (License number: 4640210179208).)

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electrochemical water splitting. In this process, hydrogen gas is produced and can be stored as a gas, liquid, or solid hydride. The stored hydrogen can be utilized in the conventional PEMFC mode to generate electricity. In a URFC, the fuel cell and water electrolysis modes correspond to the processes of the ORR and OER, respectively [65]. In the OER, water splitting occurs at the anode (positive electrode) by an external energy supply. Here, the water molecules split on the anode surface into oxygen atoms, protons, and electrons. At the positive electrode, the oxygen atoms combine to form oxygen molecules, releasing as oxygen gas. Concurrently, the electrons and protons approach the cathode (negative electrode) via an external electric circuit and a polymer electrolyte membrane, respectively. At the cathode, the HER occurs by the combination of the protons and electrons [65,66]. In this process, high-purity hydrogen gas is produced, which can be externally stored. In the ORR, the reverse reaction of the OER occurs in the same URFC unit cell device. Initially, the produced hydrogen gas is distributed on the anode (negative electrode). At the anode, the hydrogen molecules are split into protons and electrons. Then, the electrons move toward the cathode (positive electrode) via the external circuit, where they can be used as electrical energy. Concurrently, the protons move owing to the cation exchange properties of the polymer electrolyte membrane. At the cathode, oxygen gas is supplied to complete the ORR, where oxygen, protons, and electrons react and form water as a by-product. The electrochemical reactions at the positive and negative electrodes occurring in the water electrolysis and fuel cell modes are shown below [49,51,53,65]. Fuel cell mode reaction (oxygen reduction reaction): At the negative electrode 2H2 (anode) At the positive electrode 4Hþ þ 4e
þ O2 (cathode) Overall reaction 2H2 þ O2 Water electrolyzer mode reaction (oxygen evolution At the positive electrode (anode) At the negative electrode (cathode) Overall reaction

/

4Hþ þ 4e


/

2H2O

/ 2H2O reaction):

2H2O

/

4Hþ þ 4e
þ O2

4Hþ þ 4e


/

2H2

2H2O

/

2H2 þ O2

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Figure 3.2 Digital photograph image of unitized regenerative fuel cell test station.

The URFC test station, components, and stack arrangement of a URFC unit cell are shown in Figs. 3.2 and 3.3. In a URFC system, the middle component is a membrane electrode assembly (MEA). An electrically insulating polymer membrane is sandwiched between positive and negative electrodes. From the middle of the device, the components are arranged as follows: polymer electrolyte membrane, electrode (positive electrode), gas diffusion layer, bipolar plate, and endplate. The same arrangement is repeated on the other side, except that the positive electrode is replaced by the negative electrode.

3. Role of electrocatalysts and electrocatalyst support materials In URFC system, the electrodes (positive and negative) are one of the most important components, where the electrochemical redox reactions occur

74 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 3.3 Digital photograph images of (A and B) unitized regenerative fuel cell stack and its components: (C) current collector, (D) bipolar plate, (E) membrane and electrode assembly, and (F) endplate.

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during water electrolysis mode and fuel cell mode operations [49,55,56,60,65]. During the fuel cell mode, the negative and positive electrodes function as the anode and cathode, respectively. The hydrogen molecules are oxidized by the anode, so that they dissociate as hydrogen ions and electrons. The protons and electrons transfer to the cathode via the membrane and external electric circuit, respectively. At the cathode, the ORR occurs by the combination of oxygen molecules, protons, and electrons. A reaction opposite to that in the fuel cell mode occurs during the water electrolysis mode. In the water electrolysis mode, the positive and negative electrodes function as the anode and cathode, respectively, where the OER and HER occur. In this process, the oxidation reaction occurs with the reaction of the water molecules at the positive electrode; here, the water molecules are split into oxygen ions, protons, and electrons, and oxygen gas is produced on the anode surface. Concurrently, the protons and electrons approach the cathode, where they recombine to form hydrogen gas. For enhancing the electrochemical reaction kinetics during the water electrolysis and fuel cell modes, various types of approaches and developments, specifically the modification of the electrodes, have been developed [49,55,65]. In a URFC, the electrodes are composed of electrocatalysts and their support materials (electrocatalyst support). Generally, platinum (Pt) metal is considered an efficient electrocatalyst material, specifically as the negative electrode in the fuel cell mode in the URFC systems. However, the performance of only Pt electrocatalyst as the positive electrode is poor in the electrolysis mode because of less activity and oxide layer formation [65,67,68]. To enhance the performance and overcome the issue, different types of platinum group metals (PGMs) and their compounds have been studied as capable electrocatalyst materials. Moreover, binary and ternary alloys and/or composites of PGMs and their oxides have shown promising advantages and are considered as efficient electrocatalyst materials for URFC applications [65]. Another important component of a URFC electrode is the electrocatalyst support material, which plays a prominent role in enhancing the electrode reaction kinetics and performances. A material possessing the following requirements and properties can be considered as an efficient electrocatalyst support for URFC applications [56,57,62,65,69]: (i) A high surface area to increase the electrochemical active surface area, on which small-sized catalyst particles can become homogeneously dispersed on. The amount of catalyst loading can be lowered by increasing the surface area of the support material.

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(ii) Good electrical conductivity. (iii) High durability and stability (corrosion resistant) at high anodic potentials (>1.5 V). (iv) Ability to prevent the detachment of the electrocatalyst from the catalyst layer, which leads to dissolution in the electrolyte and/or migration. (v) Prohibition of the aggregation of the electrocatalysts. (vi) Control of the wettability of the catalyst layer. (vii) Low production cost of the support material and easy synthesis process.

4. Types of electrocatalysts support materials and their performance in a URFC 4.1 Carbon structures as electrocatalyst supports In PEMFCs, carbon is normally used as an electrocatalyst support material [62,70]. Yim et al. [64] reported various types of bifunctional properties of PGMs and their oxides as electrocatalysts for URFC applications. The asreceived and as-prepared electrocatalysts were Pt black, PtIr, PtRu, PtIrOx, PtRuOx, and PtRuIr. To form an electrode, an as-prepared electrocatalyst slurry was coated on carbon paper. Fig. 3.4A,B presents the fuel cell and water electrolysis mode performances of different electrodes in the URFC system. As shown in the figures, in the fuel cell mode, the order of the performance is Pt black > PtIr > PtRuOx > PtRu w PtRuIr > PtIrOx. However, in the water electrolysis mode, a similar performance trend is not exhibited; under identical conditions, the order of the performances of the different electrocatalysts is PtIrwPtIrOx > PtRu> PtRuIr > PtRuOx w Pt black. As expected, in the fuel cell mode operation, Pt black shows excellent electrocatalytic performance. On addition of other elements with the Pt electrocatalyst, the fuel cell mode performance decreases. However, in the electrolysis mode, the efficient fuel cell mode catalyst, i.e., Pt, does not exhibit excellent performance. In the water electrolysis mode, the introduction of Ir and IrOx with Pt results in an improved performance. The round-trip energy conversion efficiencies of Pt black, PtIr, PtRu, PtIrOx, PtRuOx, and PtRuIr are 48, 53, 48, 49, 44, and 46, respectively, at a current density of 200 mA cm
2. Based on the round-trip energy conversion, it can be concluded that PtIr and PtIrOx show considerable performances. In addition, the electrocatalyst performance was tuned by

(B)

1.1 Pt black Ptlr (50:50, w) PtlrOx (50:50, w) PtRu (50:50, a) PtRuOx (50:50, w) PtRulr (50:45:5, a)

1.0

Cell voltage (V)

0.9 0.8

2.1 Pt black Ptlr (50:50, w) PtlrOx (50:50, w) PtRu (50:50, a) PtRuOx (50:50, w) PtRulr (50:45:5, w)

2.0 1.9

Cell voltage (V)

(A)

0.7 0.6

1.8 1.7 1.6

0.5

1.5

0.4

1.4

0.3 0

200 400 600 800 1000 1200 1400

0

Current density (mA/cm )

(C)

(D)

1.1

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Pt (0.5 mg/cm2)

500

Pt (1.0 mg/cm2) Pt (0.5 mg/cm2)

0.8 0.7 0.6

1.8 1.7 1.6 1.5

0.4

1.4

0.3 0

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(F)

1.1 Pt (4.0 mg/cm2) Pt (2.0 mg/cm2) Pt (1.0 mg/cm2)

1.0 0.9

100

200

300

400

500

Current density (mA/cm2)

Current density (mA/cm2) 2.1

Pt (4.0 mg/cm2) Pt (2.0 mg/cm2) Pt (1.0 mg/cm2)

2.0 1.9

Cell voltage (V)

Cell voltage (V)

400

1.9

0.5

(E)

300

Pt (4.0 mg/cm2)

2.0

Cell voltage (V)

Cell voltage (V)

Pt (1.0 mg/cm2)

200

2.1

Pt (4.0 mg/cm2)

1.0

100

Current density (mA/cm2)

2

0.8 0.7 0.6

1.8 1.7 1.6

0.5

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0.4 0.3

1.4 0

200 400 600 800 1000 1200 1400

Current density (mA/cm2)

0

100

200

300

400

500

Current density (mA/cm2)

Figure 3.4 (A) Fuel cell mode and (B) water electrolysis mode performance of various electrocatalysts. The effect of Pt black catalyst loading amount in hydrogen electrode: (C) fuel cell mode and (D) water electrolysis mode. The effect of PtIr catalyst loading amount in oxygen electrode: (E) fuel cell mode and (F) water electrolysis mode. (Reproduced with permission from Ref. S.D. Yim, G.G. Park, Y.J. Sohn, W.Y. Lee, Y.G. Yoon, T.H. Yang, S. Um, S.P. Yu, C.S. Kim, Optimization of PtIr electrocatalyst for PEM URFC, International Journal of Hydrogen Energy 30 (12) (2005) 1345e1350. Copyright 2005, Elsevier (License number: 4640211215767).)

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varying the concentration of Ir or IrOx added to the Pt electrocatalyst. The evaluated URFC performance based on the catalyst loading amount is displayed in Fig. 3.4CeF. The optimized content of the catalyst loading on the carbon supports for the hydrogen and oxygen electrodes is 1 mg cm
2 of Pt black and 2 mg cm
2 of PtIr (99:1), respectively. Because of the carbon, the catalyst loading cannot be minimized; therefore, the URFC performance is considerably decreased to 1 mg cm
2 for the PtIr (99:1) electrocatalyst. The catalyst loading amount can be decreased by replacing carbon [64]. Besides the above problem, the major disadvantage of carbon supports is the high anodic corrosion occurring during the water electrolysis mode [50,64,67,70,71]. To overcome these issues, various types of electrocatalyst materials were investigated to replace carbon in a URFC system. The effect of graphitization in carbon (graphitized carbon) was evaluated to increase its stability in a URFC unit cell operation. Pai et al. [72] developed graphitized carbon by thermal treatment at 2250 C and used it as a support material for Pt with montmorillonite being used in ultrasonic mixed processes. The Raman spectra revealed an increase in the degree of graphitization in the carbon material after the heat treatment process, as exhibited in Fig. 3.5A. Compared to a common carbon support, graphitized carbon attains a lower ID/IG value (from 1.43 to 0.81), which evidently confirms the increment in graphitization. The hydrophobic nature is considerably increased in a graphitized carbon support than in the normal carbon support and can effectively increase the corrosion resistance in a graphitized carbon support. Fig. 3.5B,C exhibits the scanning electron microscopy (SEM) images of the studied electrocatalysts with a carbon support and graphitized carbon support, respectively. Achieving a uniform dispersion of the small-sized catalyst on a carbon support was difficult, and the catalyst particles could easily aggregate. By using graphitized carbon with a dispersing agent, its self-aggregation can be controlled by the dispersing agent, and a fine dispersion of catalyst particles can be achieved on the support. Fig. 3.5D presents the cyclic voltammetry (CV) curves of the different concentrations of Pt/graphitized carbon electrodes. Among the different ratios, 20 wt% Pt/graphitized carbon shows the highest specific charge transfer in the regions of H-desorption and -adsorption. Fig. 3.5E shows the cyclic performance of a URFC unit cell with an MEA including carbon or graphitized carbon as the support. In a round-trip energy conversion, graphitized carbon exhibits more efficient and stable performances than the normal carbon support. The energy conversion efficiency of the carbon support is drastically decreased from 37.2% to w28% at

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Figure 3.5 (A) Raman spectra analysis of carbon and graphitized carbon. Scanning electron microscopy and STEM images of (B) carbon black and (C) graphitized carbon with dispersing agent. (D) Cyclic voltammetry analysis of different ratio of Pt/graphitized carbon in 1 M H2SO4 solution at 50 mV s
1. (E) Unitized regenerative fuel cell unit cell cyclic stability performance of carbon and graphitized carbonsupported electrocatalysts. (Reproduced with permission from Ref. Y.H. Pai, C.W. Tseng, Preparation and characterization of bifunctional graphitized carbon-supported Pt composite electrode for unitized regenerative fuel cell, Journal of Power Sources 202 (2012) 28e34. Copyright 2012, Elsevier (License number: 4640221011810).)

Nanostructured bifunctional electrocatalyst support materials

1200

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

100 mA cm
1 after two cycles, with the graphitized carbon exhibiting a stable performance. By using graphitized carbon, the corrosion and degradation of the electrode layer can be suppressed and the URFC performance can be increased [72]. In another approach, the dimensional effect of the graphene structure was studied as a catalyst support for URFC applications [73]. Graphene structures have been considered as an efficient electrocatalyst support materials because of the electrical conductivity and large surface area of graphene [73e75]. However, a serious issue which occurred was during the catalyst preparation, which was related to the restacking of graphene layers because of Van der Waals forces. To avoid this issue, Kim et al. developed a crumpled reduced graphene oxide (rGO) as a three-dimensional (3D) support for PteIr alloy (PteIr/rGO_P600) electrocatalysts, first, by the spray pyrolysis method and, second, by the heat treatment process, as shown in Fig. 3.6A [73]. As seen from the X-ray diffraction (XRD) spectra in Fig. 3.6B, the crystallinity of the electrocatalysts is considerably increased after the heat treatment process. The SEM and transmission electron microscopy (TEM) images (Fig. 3.6CeE) of PteIr/rGO_P600 show that the graphene oxide (GO) sheets shrink and become compressed as submicrometer-sized crumpled balls. PteIr alloy nanoparticles (mean particle diameter of 4.5  1.6 nm) are homogeneously dispersed on the crumpled rGO sheets throughout the layer; this can provide a significant electrochemical activity during the unit cell operation. The CV and electrochemical surface area (ESA) curves of the different electrodes are shown in Fig. 3.6F,G. Compared to the ESA of the conventional Pt/C electrode, that of PteIr/rGO_P600 is considerably higher (1.4 times) whereas that of Pt/C and PteIr/rGO_P600 is 57.3 and 79.7 m2 g
1, respectively. The higher ESA of PteIr/rGO_P600 results in a higher ORR efficiency than that of the commercial Pt/C electrocatalyst, as shown in Fig. 3.6H. In addition, it shows a remarkable OER performance, which is higher than and comparable to those of Pt/C and Ir electrocatalysts, respectively. For the OER, the high performances are mainly attributed to PteIr/ rGO_P600 owing to its high crystalline behavior with numerous active planes present on the surface. These planes can facilitate the oxidation of water, deprotonation, release of oxygen, and reoxidation of water [73,76]. To examine the stability of the electrodes, the potential cycle test of Pt/C and PteIr/rGO_P600 was performed in 0.1 M HClO4 in the potential range of 0.059e1.259 at 50 mV s
1. As shown in Fig. 3.6J, the ESA of the commercial Pt/C electrocatalyst is considerably decreased with increasing

Nanostructured bifunctional electrocatalyst support materials

81

Figure 3.6 (A) Synthesis procedure of Pt-Ir/rGO electrocatalyst. (B) X-ray diffraction spectra of heat-treated and noneheat-treated electrocatalysts. (CeE) Scanning electron microscopy and transmission electron microscopy images of Pt-Ir/rGO_P600. (F) Cyclic voltammetry, (G) electrochemical surface area (ESA), (H) oxygen reduction reaction, and (I) oxygen evolution reaction polarization curves of Pt/C, heattreated, and noneheat-treated Pt-Ir/rGO electrocatalysts. (J) Cycle versus normalized ESA of Pt/C and Pt-Ir/rGO_P600 electrocatalysts. (Reproduced with permission from Ref. I.G. Kim, I.W. Nah, I.H. Oh, S. Park, Crumpled rGO-supported Pt-Ir bifunctional catalyst prepared by spray pyrolysis for unitized regenerative fuel cells, Journal of Power Sources 364 (2017) 215e225. Copyright 2017, Elsevier (License number: 4640251272598).)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 3.6 Cont’d.

Nanostructured bifunctional electrocatalyst support materials

83

cycle number. Contrastingly, the PteIr/rGO_P600 electrode exhibits a stable performance with a slight performance degradation after 4500 cycles [73].

4.2 Unsupported and IrO2-supported electrocatalysts Bifunctional oxygen electrodes without an electrocatalyst support have been developed to avoid the support materialerelated issues. For URFC applications, Jung et al. developed PtIr electrocatalysts in different ratios without support materials [77]. The catalyst layer was prepared by coating the catalyst dispersion solution on a gas diffusion layer and then drying. The MEA was formed by hot-pressing the electrode on to the polymer electrolyte membrane at 140 C. Among the different Ir black content with Pt, the Pt:Ir ratio of 85:15 (Pt85Ir15) showed a considerable electrochemical active surface area, which was comparable to that of the Pt black catalyst (Pt100Ir0). Fig. 3.7A,B presents the ORR and OER performances of the unsupported electrocatalysts. The PtIr catalysts provide mass transfer and kinetic regions with similar characteristics as those by the Pt electrocatalyst. However, the ORR performances of the PteIr catalysts decrease with increasing Ir content because of less activity. Regarding the OER performances, the introduction of Ir in Pt causes a significant performance enhancement, as exhibited in Fig. 3.7B. Fig. 3.7C,D presents the URFC unit cell performances of the water electrolysis mode and fuel cell modes. In the electrolysis mode, the unsupported PtIr electrodes show better performances than the Pt electrode because of the efficient oxygen evolution reactivity. Moreover, Pt85Ir15 exhibits a comparable fuel cell mode operation to the Pt electrode. To further understand the effect and advantage of the Ir content in Pt, the round-trip energy conversation efficiency was determined. It was 44%, 49%, 47%, and 25% for Pt:Ir(100:0), Pt:Ir(85:15), Pt:Ir(70:30), and Pt:Ir(40:60), respectively. The introduction of Ir with Pt catalyst without support materials led to considerable URFC performances [77]. Thus, for a URFC system, Ir and IrO2 can be considered as an efficient electrocatalyst and support material. More interestingly, owing to its efficient OER catalytic properties, IrO2 can be considered as one of the best support materials for Pt electrocatalysts. Over several years, different synthesis techniques have been established to develop various structured IrO2 support materials. The developed IrO2supported electrodes for URFCs include Pt nanoparticles on IrO2 via chemical deposition, highly dispersed Pt nanoparticles on the IrO2 surface

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Figure 3.7 (A) Oxygen reduction reaction, (B) oxygen evolution reaction, (C) water electrolysis mode, and (D) fuel cell mode of different ratio of unsupported electrocatalysts. (E, F) Transmission electron microscopy images; particle size distribution of (G) Pt and (H) IrO2 of Pt-IrO2 electrocatalysts. (I) Unitized regenerative fuel cell unit cell performance of different synthesis method of Pt-IrO2 electrocatalysts. (AeD) Reproduced with permission from H.Y. Jung, S.Park, B.N. Popov, Electrochemical studies of an unsupported PtIr electrocatalyst as a bifunctional oxygen electrode in a unitized regenerative fuel cell, Journal of Power Sources 191 (2) (2009) 357e361. Copyright 2009, Elsevier (License number: 4640501100910). (EeI) Reproduced with permission from J.C. Cruz, V. Baglio, S. Siracusano, R. Ornelas, L.G. Arriaga, V.Antonucci, A.S. Aricò, Nanosized Pt/IrO2 electrocatalyst prepared by modified polyol method for application as dual function oxygen electrode in unitized regenerative fuel cells, International Journal of Hydrogen Energy 37 (7) (2012) 5508e5517. Copyright 2012, Elsevier (License number: 4640501241368).)

by the Adams method, nanosized Pt/IrO2 and Pt/Ir(IrO2) by a modified ultrasonic and microwave-assisted polyol method, and porous IrO2 formed by the Adams fusion method followed by template removal [78e82]. In the TEM images in Fig. 3.7E,F, small-sized Pt clusters are observed to be successfully dispersed on the rutile tetragonal phase of IrO2, with a ratio of 50:50 [80]. The particle sizes of the as-prepared Pt and IrO2 are in the range of 3e6.5 nm and 5e9 nm, respectively, as presented in Fig. 3.7G,H, and the mean particle sizes are 5 nm for Pt and 7 nm for IrO2. Fig. 3.7I displays the water electrolysis and fuel cell mode performances of the mechanically

Nanostructured bifunctional electrocatalyst support materials

Figure 3.7 Cont’d.

85

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

mixed Pt/IrO2 and as-prepared Pt/IrO2 by a modified polyol method. In the water electrolysis mode, both the electrodes show comparable performances. However, in the fuel cell mode, the as-prepared Pt/IrO2 exhibits a better performance than the mixed catalysts. This performance enhancement is mainly achieved by the efficient dispersion of Pt on the IrO2 surface, which is inferior in mixing. Kong et al. [81,82] developed Ir(IrO2) and porous IrO2 electrocatalyst supports for a URFC system. Fig. 3.8AeD shows the TEM images for different concentrations (2:8 and 3:7) of Ir:IrO2 electrocatalyst support materials. For both the ratios, the as-shown crystalline texture of the electrocatalyst materials is homogeneously dispersed, and nanosized Pt and Ir particles are embedded on the surface of the IrO2 support. The mean particle sizes of the Pt electrocatalyst with Ir:IrO2 (2:8) and Ir:IrO2 (3:7) are 2.64 and 2.87 nm, respectively. The ORR and OER polarization curves for various concentrations of the electrocatalyst support are shown in Fig. 3.8E,F. Among the various concentration of electrocatalyst support, the Pt electrocatalyst having a high ESA (24.74 m2 g
1) with a 3:7 ratio of Ir:IrO2 exhibits efficient performances for both the ORR (21.71 mA mg
1 at 0.85 V) and OER (42.35 mA mg
1 at 1.55 V) [81]. In another approach, a porous IrO2 support was developed, and its performance evaluation was compared to that of commercial IrO2 [82]. The developed porous-structured support material could realize a higher surface area than commercial IrO2, with the specific surface areas of the porous and conventional IrO2 being 222 m2 g
1 (with average pore size of 4.3 nm) and 42.7 m2 g
1, respectively. As seen in the TEM image displayed in Fig. 3.8G, numerous Pt nanoparticles are deposited on the surface and in the pores of the IrO2 support, so that the Pt particles are introduced in both the internal and external surfaces. The determined ESAs were 9.9, 26.9, and 31.8 m2 g
1 for the Pt, Pt with commercial IrO2, and Pt with porous IrO2 electrocatalysts, respectively. A large ESA can provide efficient active sites to the Pt electrocatalyst. As derived from the ORR polarization curves shown in Fig. 3.8I, the kinetic currents determined at 0.85 V are 13.3, 15.6, and 35.9 mA mg
1 for the Pt, Pt with commercial IrO2, and Pt with porous IrO2 electrocatalysts, respectively. The ORR performance of Pt with porous IrO2 is 2.3 times higher than that of Pt with commercial IrO2. Moreover, the OER performance of the Pt/porous IrO2 electrocatalyst is considerably better than that of Pt/commercial IrO2, with the corresponding measured current densities being 22.6 and 29 mA mg
1 at 1.55 V, as shown in Fig. 3.8J. The high surface area of the porous IrO2 along with a

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Figure 3.8 Transmission electron microscopy (TEM) images of (A and B) Pt/Ir2(IrO2)8 and (C and D) Pt/Ir3(IrO2)7 electrocatalysts. (E) Oxygen reduction reaction (ORR) and (F) oxygen evolution reaction (OER) polarization curves of different ratio of Ir and IrO2 support with Pt electrocatalysts. TEM images of Pt electrocatalyst with (G) commercial and (H) porous IrO2 electrocatalysts. (I) ORR and (J) OER polarization curves of Pt, IrO2, Pt-commercial IrO2, and Pt-porous IrO2 electrocatalysts. (AeF) Reproduced with permission from Ref. F.D. Kong, S. Zhang, G.P. Yin, N. Zhang, Z.B. Wang, C.Y. Du, Preparation of Pt/Irx(IrO2)10
x bifunctional oxygen catalyst for unitized regenerative fuel cell, Journal of Power Sources 210 (2012) 321e326. Copyright 2012, Elsevier (License number: 4640530668823). (GeJ) Reproduced with permission from F.D. Kong, S. Zhang, G.P. Yin, N. Zhang, Z.B. Wang, C.Y. Du, Pt/porous-IrO2 nanocomposite as promising electrocatalyst for unitized regenerative fuel cell, Electrochemistry Communications 14 (1) (2012) 63e66. Copyright 2011, Elsevier (License number: 4640530799597).)

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Pore

Nanostructured bifunctional electrocatalyst support materials

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high degree of Pt dispersion facilitates and enhances the OER and ORR performances of the Pt electrocatalyst with the porous IrO2 support [82].

4.3 Ti-based compounds as electrocatalyst supports Ti-based compounds, specifically titanium oxide (TiO2), titanium carbide (TiC), titanium nitride (TiN), and titanium carbonitride (TiCN), have been studied as alternative and efficient electrocatalyst support materials for URFC applications [62,69,83]. Huang et al. [62] developed a bifunctional electrocatalyst (Pt and Ir) dispersed on a TiO2 support by the modified reduction method . As shown in the TEM images in Fig. 3.9AeC, the asprepared TiO2 support is 7e15 nm with a specific surface area of 250 m2 g
1. More interestingly, the small-sized Pt (average particle size 4.2 nm) and Ir (average particle size 2 nm) are homogeneously dispersed on the TiO2 support surface. The ORR and OER polarization curves of Pt/ TiO2 and Ir/TiO2 are shown in Fig. 3.9D,E, respectively. Pt/TiO2 exhibits a significantly improved ORR performance than Ir/TiO2. By contrast, Ir/ TiO2 shows a considerable OER performance than the Pt/TiO2 electrocatalyst. In view of the above, the combination of Pt and Ir is being considered an efficient bifunctional electrocatalyst. The effect of the homogeneous dispersion of PteIr on the TiO2 support was studied in the water electrolysis and fuel cell mode operations, as shown in Fig. 3.9F,G. In the fuel cell mode operation, PteIr on TiO2 displays a considerable performance enhancement compared to the PteIr black electrode with corresponding current densities at 0.6 V of 1.38 and 0.74 A cm
2. In addition, a higher OER performance is achieved by PteIr on TiO2 than that of the PteIr black electrocatalyst. The lower performance of PteIr black is mainly attributed to the aggregation of the electrocatalyst. The round-trip energy conversion efficiency of PteIr on TiO2 is considerably higher than that of PteIr black, as illustrated in Fig. 3.9H. At 1 A cm
2 of current density, the round-trip energy conversion efficiencies of the PteIr on TiO2 and PteIr black electrocatalysts are 42.2% and 29.8%, respectively. In this case, the higher performance of the PteIr on TiO2 electrocatalyst is mainly ascribed to the uniform dispersion of the Pt and Ir electrocatalysts on the surface of the TiO2 support [62]. Moreover, the increased ESA of the electrocatalyst can enhance the performance of the URFC unit cell. Roca-Ayats et al. [69] prepared and investigated a (3:1) ratio of PtIr on TiC, TiCN, and TiN support materials. As shown in Fig. 3.10AeC, smallsized PtIr electrocatalysts of size 2.8  0.6, 2.6  0.7, and 2.6  0.7 are

90 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 3.9 Transmission electron microscopy images of (A) TiO2 support, (B) Pt/TiO2, and (C) Ir/TiO2 electrocatalysts. (D) Oxygen reduction reaction and (E) oxygen evolution reaction analyses of TiO2 support with Pt and Ir electrocatalysts. (F) Fuel cell mode, (G) electrolysis mode, and (H) round-trip energy conversion efficiency of Pt-Ir black and Pt-Ir/TiO2 electrocatalysts. (Reproduced with permission from S.Y. Huang, P. Ganesan, H.Y. Jung, B. N. Popov, Development of supported bifunctional oxygen electrocatalysts and corrosion-resistant gas diffusion layer for unitized regenerative fuel cell applications, Journal of Power Sources 198 (2012) 23e29. Copyright 2011, Elsevier (License number: 4640540749702).)

Nanostructured bifunctional electrocatalyst support materials

Figure 3.9 Cont’d.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

deposited on the TiC, TiCN, and TiN supports, respectively. It appears that the nanosized PtIr particles can effectively distribute on the surfaces of the Ti-based support materials. The ORR and OER polarization curves of PtIr/TiC, PtIr/TiCN, and PtIr/TiN are shown in Fig. 3.10D,E. During the ORR process, the catalytic performance of PtIr on the Ti compound supports follows the order: PtIr/TiCN > PtIr/TiN and PtIr/TiC. A similar trend is observed for the OER performance. Typically, TiC and TiN have high thermal and melting points, high corrosion resistance, and high electrical conductivity [69,84]. These Ti-based compounds exhibit corrosion resistance at anodic potentials. Based on the above characteristics, Tibased compounds, specifically TiCN, can be considered as efficient support materials for electrocatalysts [57,69]. Another interesting electrocatalyst support is TiC, which can show stable performances chemically and electrochemically [83,85]. More recently, Kús et al. [83] developed new approaches with TiC support, such as co-sputtered PteIr/TiC and sandwichlike-structured Ir/TiC/Pt as a thin film by magnetron sputtering. The schematics shown in Fig. 3.10F,G display the physical appearances of catalyst-coated membranes of cosputtered PteIr/TiC and sandwich-likestructured Ir/TiC/Pt. As shown in the SEM cross-sectional images in Fig. 3.10I and K, the thickness of the TiC support layer is in the range of 200e300 nm. The co-sputtered PteIr/TiC is comprised of an approximately 100-nm-thick PteIr electrocatalyst deposited cosputtered on the TiC support. Alternatively, 50-nm-thick Ir and Pt are separately deposited on the top and bottom of the TiO2 support to form a sandwich-like structure. In the fuel cell and water electrolysis modes, as presented in Fig. 3.10L,M, the sandwich-like-structured Ir/TiC/Pt shows a considerable performance enhancement than the co-sputtered PteIr/TiC. In both the cases, the Pt catalyst layer approaches the membrane in the sandwich-likestructured Ir/TiC/Pt, which is the major reason for the performance change [83].

4.4 Sb-doped SnO2 and SiO2eSO3H electrocatalyst support Cruz et al. [86] developed an Sb-doped SnO2 as a support material for the bifunctional PteIrO2 electrocatalysts. Sb-doped SnO2 as a support possessed significant advantages because of its surface properties and high stability in an acidic medium [86e90]. More importantly, Sb-doped SnO2 showed a lower electrical resistance than the typical Ebonex [86,87]. The specific surface area of Sb-doped SnO2 was 115.17 m2 g
1 with a pore volume of

Figure 3.10 Transmission electron microscopy images of Pt3Ir electrocatalyst on (A) TiC, (B) TiCN, and (C) TiN support materials. (D) Oxygen reduction reaction and (E) oxygen evolution reaction performances of Pt3Ir/TiC, Pt3Ir/TiCN, Pt3Ir/TiN electrocatalysts. Schematic representation of (F) cosputtered and (G) sandwich-like electrocatalysts. Scanning electron microscopy surface morphology and cross-sectional analysis of (H and I) cosputtered and (J and K) sandwich-like electrocatalysts. (L) Fuel cellmodeand(M)waterelectrolysismodeoperationofcosputteredandsandwich-likeelectrocatalysts.(AeE) Reproduced with permission from M. Roca-Ayats, G. García, J.L. Galante, M.A. Peña, M.V. Martínez-Huerta, Electrocatalytic stability of Ti based-supported Pt3Ir nanoparticles for unitized regenerative fuel cells, International Journal of Hydrogen Energy 39 (10) (2014) 5477e5484. Copyright 2014, Elsevier (License number: 4640580472043). (FeM) Reproduced with permission from P. Kús, A. Ostroverkh, I. Khalakhan, R. Fiala, Y. Kosto, B. Smíd, Y. Lobko, Y. Yakovlev, J. Nováková, I. Matolínová, V. Matolín, Magnetron sputtered thin-film vertically segmented Pt-Ir catalyst supported on TiC for anode side of proton exchange membrane unitized regenerative fuel cells, International Journal of Hydrogen Energy 44 (31) (2019) 16087e16098. Copyright 2011, Elsevier (License number: 4640580582409).)

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Figure 3.10 Cont’d.

Nanostructured bifunctional electrocatalyst support materials

95

0.272 cm3 g
1, which was beneficial to load a high amount of the electrocatalyst on the surface. As shown in Fig. 3.11A,B, the average size of the spherical-structured Sb-doped SnO2 is 4.4 nm. The Sb-doped SnO2-supported PteIrO2 electrocatalysts show a considerable electrochemical behavior in the water electrolysis and fuel cell modes, as presented in Fig. 3.11D [86]. In another approach, Roh et al. [63] developed sulfonated silica (SiO2
SO3H) as a support material for Pt electrocatalyst to replace the conventional carbon support. As shown in Fig. 3.11E, the small-sized Pt nanoparticles are effectually deposited on SiO2
SO3H without aggregation. The ESAs of Pt black and PteSiO2
SO3H are 23.02 and 22.96 m2 g
1, respectively. In this case, the ESA of the electrocatalyst is not much affected by the SiO2
SO3H support. When using carbon as a support material for the electrocatalysts, the water electrolysis and fuel cell mode performances are significantly decreased after a cycle, as shown in Fig. 3.11G,H. The performance degradation was mainly caused by the stability issue on the carbon support. The carbon corrosion was significantly high at high anodic potentials. By using SiO2
SO3H as a support instead of the carbon support for the Pt electrocatalyst, the cyclic performance was considerably enhanced (as shown in Fig. 3.11K,L), and a higher round-trip energy conversion efficiency was achieved. In the first cycle, the round-trip energy conversion efficiencies of Pt/C, PteSiO2, and PteSiO2
SO3H were 45.32%, 44.60%, and 46.13% at 0.2 A cm
2, respectively. The energy conversion efficiency of Pt/C was significantly affected after one cycle, whereas a considerable performance was attained by PteSiO2
SO3H under identical conditions. The introduction of the sulfonated functional group (
SO3H) in SiO2 improves the water electrolysis and fuel cell mode performances, as shown in Fig. 3.11I,J. The eSO3H functional group in SiO2 improves the proton transportation on the catalyst layer during the water electrolysis and fuel cell mode operations, as presented in Fig. 3.11F. This phenomenon can enhance the overall URFC unit cell performance because of the relatively easier proton transportation and lower ohmic resistance [63].

5. Concluding remarks For the near future, a URFC system can be considered as an efficient and green energy system because of its high energy density and round-trip energy conversion without any harmful emission. The performance of a URFC system also depends on the effect and stability of the electrocatalyst support materials. In this chapter, we have presented in detail the effects of different

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Figure 3.11 (A) Transmission electron microscopy and (B) particle size distribution analysis of Sb-doped SnO2. (C) Impedance and (D) unitized regenerative fuel cell unit cell performances of Pt-IrO2/Sb-doped SnO2 electrocatalysts. (E) Scanning electron microscopy images of SiO2-SO3H supported Pt electrocatalyst. (F) Reaction mechanism of SiO2-SO3H support. (G) Fuel cell mode and (H) water electrolysis mode performance of Pt/C electrocatalysts. Comparative (I) fuel cell mode and (J) water electrolysis mode performance of Pt/ SiO2 and Pt/ SiO2-SO3H electrocatalysts. (K) Fuel cell mode and (L) water electrolysis mode performance of Pt/ SiO2-SO3H electrocatalysts. (AeD) Reproduced with permission from J.C. Cruz, S. Rivas, D. Beltran, Y. Meas, R. Ornelas, G. Osorio-Monreal, L. Ortiz-Frade, J. Ledesma-García, L.G. Arriaga, Synthesis and evaluation of ATO as a support for PteIrO2 in a unitized regenerative fuel cell, International Journal of Hydrogen Energy 37 (18) (2012) 13522e13528. Copyright 2012, Elsevier (License number: 4640591395139). (EeL) Reproduced with permission from S.H. Roh, T. Sadhasivam, H. Kim, J.H. Park, H.Y. Jung, Carbon free SiO2eSO3H supported Pt bifunctional electrocatalyst for unitized regenerative fuel cells, International Journal of Hydrogen Energy 41 (45) (2016). 20650e20659. Copyright 201, Elsevier (License number: 4640600032351).)

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structured and sized electrocatalyst support materials for URFC applications. The modified carbon structure of graphitized carbon and 3D crumpled rGO as support materials for electrocatalysts revealed considerable stability during the URFC operation. The high OER activity of nanosized and porousstructured IrO2 supports showed improved electrocatalytic performances and enhanced cyclic performances. Ti-based compounds such as TiO2, TiC, and TiCN could effectively improve the URFC unit cell performances because of their stable behavior chemically and electrochemically. Nanosized Sb-doped SnO2 and SiO2eSO3H electrocatalyst support materials also showed considerable performances in URFC operations. Besides carbon structures, Ti-based compounds, IrO2, and numerous types of electrocatalyst support materials were developed by a progressive approach. However, most of the studies were performed for ORR and OER performances without URFC unit cell operations. To optimize the efficient electrocatalyst support materials, the support material performance should be evaluated in a URFC unit cell system. To utilize a URFC in a real device application, electrocatalyst support materials with a low cost, high specific surface area, high electrochemical active surface area, high electrical conductivity, and high corrosion resistance should be developed in a progressive manner. Therefore, the evolution of porous-structured materials and different dimensional and structured properties of materials, alloys, doping materials, functionalized materials, and nanocomposites in a URFC device will be relevant.

Acknowledgments This study was financially supported by Chonnam National University (Grant number: 2018e3274).

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CHAPTER 4

Polymeric nanomaterials in fuel cell applications Kingshuk Dutta

Advanced Research School for Technology and Product Simulation (ARSTPS), School for Advanced Research in Polymers (SARP), Central Institute of Plastics Engineering and Technology (CIPET), Chennai, Tamil Nadu, India

1. Introduction Research and developmental works in the field of proton exchange membrane fuel cells (PEMFCs) have been an integral part of activities in the alternative, renewable, and sustainable energy sector for several decades now [1e5]. This is because it has long been believed that these fuel cell device technologies will play a pivotal role in providing clean energy in future, which will in turn reduce stress on the ever-depleting natural fossil fuel reserves and will help reduce the environmental pollution caused by usage of these fossil fuels [6e8]. Accordingly, research activities on development of PEMFCs, which consist of hydrogen fuel cells (HFCs), direct alcohol (methanol and ethanol) fuel cells and bio(microbial) fuel cells, have attracted both public and private funding in substantial quantities, and have progressed to different levels of developments. For instance, HFCs have reached the most advanced stage of development among the PEMFCs and are already getting commercialized in small scale in many parts of the world. The major usage of HFCs has been in the transportation sector [9,10], followed by portable electronic and electrical applications [11,12]. Direct alcohol fuel cell, mainly the direct methanol fuel cells (DMFCs), comes second in terms of development index and is now at the stage of getting commercialized (small-scale), mainly to power portable handheld electronic devices [13,14]. Microbial fuel cell (MFC) technology is comparatively a new technology and is still largely in the laboratory-scale developmental stage [15,16]. The most critical problem of these device technologies has been with the catalytic reactions [17e20]. First, the reactions at both the electrodes (cathode and anode) are very sluggish, leading to high startup times and low device efficiency; second, catalyst poisoning by reaction by-products reduces the catalyst life; third, generation of cross/mixed potential when Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00004-X

© 2020 Elsevier Inc. All rights reserved.

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using liquid fuels reduces the overall device efficiency; and lastly and most importantly (from commercialization viewpoint), very high cost, scarce availability, and low durability of the generally used state-of-the-art Pt and Pt-based transition metal catalysts increase the device cost [21e24]. These factors have been seriously contributing as hindrance to the desired largescale commercialization of these device technologies. Therefore, it was urgently required to look into and explore new forms of materials within the catalyst system that can eliminate or significantly reduce the performance hindering and cost factors involved. This necessitated the use of nanoparticles, including polymeric ones, mainly as catalyst supports to reduce usage and maximize efficiency of costly Pt and Pt-based catalysts. The most important aspects of using polymeric nanoparticles as catalyst supporting matrices in comparison to their micro- and macro counterparts include (a) higher surface area, (b) better and more uniform dispersion of metal catalyst particles, (c) higher interaction with the deposited metal particles, (d) increased reaction rates, (e) increased conductivity of the generate electrons, and (f) better interfacial properties [25e28]. This chapter is dedicated to the discussion and analysis of polymeric nanoparticles in PEMFCs and will provide up-to-date information on the research and developmental activities in this domain. It should be noted in this respect that the discussions will be limited to those reports that contain the use of polymeric nanoparticles, aligning with the theme of the chapter. This chapter will not include information on polymeric nanocomposites that contain polymers (in nonenanodimensions) and nonpolymeric (mainly inorganic/ceramic) nanoparticles. Readers interested in polymeric nanocomposites in fuel cell applications are advised to refer to these published literatures [29e31].

2. Polymeric nanomaterials in microbial fuel cells MFCs, a category of biofuel cells, are devices that run on waste materials, typically wastewater, sewage, animal excretion, etc., and convert the chemical energy contained in the organic matter present in these wastes to electrical energy via oxidation of these organic matter by microorganisms (either naturally occurring or designed), especially bacteria [15,26]. A schematic representation of a typical MFC has been presented in Fig. 4.1A [32]. Although MFCs are relatively new in the field of fuel cells, they hold an important place owing to their multifaceted utility that includes electricity

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Figure 4.1 (A) A schematic representation of a typical microbial fuel cell (MFC) [32]. (B) Output power density and (C) polarization curve of a dual-chambered MFC using different anodes. CC, Unmodified carbon cloth; PAni-TA, PAni-HCl, and PAni-H2SO4, carbon cloth modified with tartaric acidedoped PAni, HCl-doped PAni, and H2SO4doped PAni, respectively [33]. (Reproduced with permission from Elsevier.)

generation from wastes, wastewater treatment, sensing of wastes, etc. [15]. Polymeric nanomaterials have started to find some application in MFCs, mainly at the anode as coatings and/or modifiers. It has been well-established now that use of nanostructured catalyst supports results in better dispersion of the deposited metal catalyst particles, better interaction between the support material and the deposited catalyst, and presentation of higher electrochemically active surface area of the catalyst particles [34]. For example, Mehdinia et al. [35] utilized nanostructures of polyaniline (PAni) as a coating over the glassy carbon anode of a dual-chambered MFC and observed an improved power output (i.e., 8.2 mWcm 2  10 2 mWcm 2) in comparison to the glassy carbon anode coated with microstructures of PAni (i.e., 3.1 mWcm 2  10 2 mWcm 2). In this study, Pt rod was used as the

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cathode, while Escherichia coli served as the biocatalytic microorganism. The higher electrochemical surface area provided by the nanomorphology over the micromorphology was inferred to be the reason behind the observed higher electrocatalytic activity. On a more advanced note, Qiao et al. [36] have reported synthesis of a composite of titanium dioxide and nanostructured PAni and have utilized this composite as the anode of a dualchambered MFC. The MFC consisted of a Nafion-117 membrane as the solid electrolyte, a carbon cloth as the cathode, and E. coli as the biocatalyst. The authors observed a current density at 0.41 V of 3650 mAm 2 and a corresponding high power density of 1495 mWm 2 with this MFC, which also demonstrated an open circuit potential of 0.88 V. It was further noted that 30 wt% of incorporated PAni within the composite produced the best result. On the other hand, Liao et al. [33] synthesized PAni nanowire network doped with tartaric acid on carbon cloth and applied it as the anode of a dual-chambered MFC. The authors determined that compared to the unmodified carbon cloth as the anode, the modified anode demonstrated an approximately four times higher power generation. It was also noted that compared to inorganic acid doping, the organic acidedoped anode exhibited better performance. Fig. 4.1B,C clearly presents the difference of performance level upon using different inorganic acidedoped anodes, unmodified carbon cloth anode, and the modified anode prepared in this work. Apart from PAni, another aromatic conjugated conducting polymer, polypyrrole (PPy), has also found use as an anode modifier in MFCs. In a typical work, Zou et al. [37] used PPy nanofibers as a coating on carbon fiber anodes at an optimized loading of 3 mgcm 2. The authors used this anode in a single-chambered photosynthetic MFC and obtained a current density of 46.6 mAm 2 and a corresponding power density of 5.9 mWm 2. These results were attributed to the high electron transport rate between the photosynthetic biofilm and the anode owing to the presence of the conduction polymer PPy. In a unique approach, Zhao et al. [38] synthesized a cost-effective nanotube membrane composed of PPy and employed this flexible membrane as the anode of a dualchambered MFC. Fig. 4.2I presents images of the flexible membrane and electron micrographs of both the fabricated membrane and the synthesized nanotubes. The MFC was constructed using the fabricated anode, a Nafion-117 membrane and Shewanella oneidensis as the biocatalyst. It was observed that the nanotube membrane anode was able to generate a power density which was more than six times higher (i.e., 612 mWm 2 at a

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Figure 4.2 (I) (A and B): Photos of the flexible polypyrrole (PPy) nanotube membrane, (C and D): surface and cross-sectional scanning electron micrographs, respectively, of the flexible paper-like membrane, and (E and F): transmission electron micrographs of the PPy nanotubes recorded under two different magnifications [38]. (II) (A) Polarization and (B) power density curves obtained from the microbial fuel cell constructed with the PPy nanotube membrane [38]. (Reproduced with permission from Wiley.)

2.1 Am 2 current density) than that generated by a normal carbon paper anode (i.e., 92 mWm 2 at a 0.45 Am 2 current density) (Fig. 4.2II).

3. Polymeric nanomaterials in hydrogen fuel cells As already mentioned in the introductory comments, HFC technology is presently the most advanced technology within the PEMFC domain. An HFC device uses hydrogen as a fuel, which upon oxidation at the anode

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generates protons and electrons. These charged moieties take different routes to reach the cathodedwhile the protons get transported through the proton exchange membrane, the electrons traverse through the externally connected conducting wire. Upon reaching the cathode, the protons and the electrons combine in presence of air/pure oxygen to form water as the end product. A schematic illustration of an HFC has been depicted in Fig. 4.3A. Use of polymeric nanomaterials in HFCs has been illustrated in many published reports. The primary reason behind the use of these nanomaterials is the same as for the MFCs and the DMFCs [40]. However, in contrast to MFCs, polymeric nanomaterials in HFCs have been used as a cathode catalyst support. For instance, Hu et al. [41] have utilized PAni nanofibers as nitrogen and carbon precursors for Fe/carbonized PAni cathode catalyst having nanoworm morphology. Carbonization was conducted because it is a known process to increase the conductivity of PAni nanofibers when done at high temperature (w900e1100 C). The optimum loading of Fe was determined to be 3 wt%, demonstrating the highest onset potential of 0.905 V versus Reversible Hydrogen Electrode (RHE) for the oxygen reduction reaction (ORR). In a similar work, Wang et al. [42] synthesized carbonized PAni nanofibers, followed by carboxylation and deposition of Pt, as a cathode catalyst system. The carbonization, carboxylation, and Pt deposition steps have been schematically illustrated in Fig. 4.4. It was realized that the Pt/carboxylated carbonized PAni nanofibers catalyst system demonstrated the best results in terms of open circuit potential and power and current densities (Table 4.1). Although hightemperature carbonization results in enhanced conductivity of PAni nanofibers, this process also renders the nanofibers hydrophobic enough to support loaded Pt. Therefore, to increase the surface hydrophilicity of the nanofibers, Wu et al. [44] conducted sulfonation of the PAni nanofibers. The incorporation of sulfonic acid groups via grafting onto the nanofibers led to substantial increase in the Pt metal loading efficiency of the nanofibers. This invariable led to an enhanced single cell performance of 1393.7 mAcm 2 maximum current density and 414.0 mWcm 2 maximum power density. This obtained result can be compared with that obtained for the carbonized PAni nanofibers and carboxylic acid modified carbonized PAni nanofibers presented in Table 4.1. In a contrasting approach, Gavrilov et al. [45] carried out chemical treatment of the surface of carbonized nanoPAni with nitric acid, hydrogen peroxide, and sodium hydroxide, followed by deposition of Pt metal particles. It was noted that the surface treatment

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Figure 4.3 (A) A schematic representation of a typical hydrogen fuel cell (HFC) [32]. (B) HFC performance using the fabricated Pt/polypyrrole oxygen reduction reaction catalyst. Anode catalyst loading: 0.5 mg Pt per cm2; cathode catalyst loading: given in figure; reactant flow rates: 150 mL min 1; relative humidity: 100% [39]. (Reproduced with permission from Elsevier.)

resulted in better dispersion of the catalyst nanoparticles and small mean Pt particle size (w5.1 nm). This, in turn, produced an enhanced power density of w34% in comparison to the untreated Pt/nano-PAni and a higher costly Pt metal utilization in comparison to the commercial Pt/C catalyst system. The superior results observed were attributed to better interaction between

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Figure 4.4 A schematic depiction of the morphological variation of PAni nanofibers after carbonization, carboxylation, and Pt deposition [42]. (Reproduced with permission from Elsevier.) Table 4.1 Maximum voltage, maximum power density, and maximum current density obtained upon using PAni nanofibers, carbonized PAni nanofibers, carboxylated carbonized PAni nanofibers, and the state-of-the-art commercial Vulcan XC-72 carbon [42]. Pt/carboxylated carbonized PAni Pt/Vulcan Pt/PAni Pt/carbonized XC-72 Parameters nanofibers PAni nanofibers nanofibers

Maximum open circuit potential Maximum current density (mAcm 2) Maximum power density (mWcm 2)

0.70

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the functionalized nano-PAni surface, obtained after chemical treatment, and the deposited Pt nanoparticles. Nafion-117 was used as the membrane electrolyte in this study. In a very preliminary analysis, it has also been

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found that PAni nanofibers can also behave as a sensor to detect the presence of carbon monoxide, which is the most critical catalyst poison in fuel cells, apart from their abilities to act as catalyst supporting matrices [46]. Getting inspired by the results demonstrated by functional groupe containing carbonized nano-PAni, researchers have also tried their hands at nano-PPy. In a typical work, Huang et al. [39] demonstrated the efficacy of PPy as a support matrix for deposited Pt catalyst particles for fabrication of the cathode of an HFC. The Pt/PPy ORR catalyst, when operated in an HFC, produced a satisfactory cell performance, as presented in Fig. 4.3B. In another work, Sapurina et al. [43] performed carbonization of PPy nanotubes deposited with Pd, Rh, Pt, and Ru. The metal nanoparticlesdeposited carbonized PPy nanotubes are expected to function as potential catalyst systems owing to their high surface area and interaction sites.

4. Polymeric nanomaterials in direct methanol fuel cells A DMFC is a device that utilizes the chemical energy stored in methanol and converts it into electrical energy [47]. In brief, the fuel methanol gets oxidized in the anode in presence of the anode catalyst, generating protons and electrons. These charged moieties, upon reaching the cathode in a manner identical to that explained for the HFCs, combine with oxygen and form water as the end product. A schematic illustration of a typical DMFC device has been presented in Fig. 4.5A. Polymeric nanomaterials have so far found the most use in DMFC device technology in terms of the number of published reports [7,27]. These nanomaterials have been mainly used in fabricating the anode catalyst system of DMFCs, where methanol oxidation reaction (MOR) takes place. For example, Chen et al. [49] found out that Pt nanocatalyst supported on PAni nanofibers of diameter w60 nm can serve as a very efficient catalyst system for executing MOR at a higher rate and lower catalyst poisoning effect compared to the pristine Pt/C catalyst system, owing to the high electrochemical surface area of the nanofibers that supported welldispersed, uniform, and smaller Pt nanoparticles. Adopting a similar approach, Guo et al. [50] showed that Pd/Pt and Pt nanoparticles of ultrahigh density can be readily adsorbed and grown on high surface area PAni nanofibers. Again, Zhiani et al. [51] modified Pt/C catalyst system by electropolymerized PAni nanofibers. The modified Pt/C/PAni nanofibers catalyst system demonstrated reduced catalyst poisoning, higher electrocatalytic activity toward methanol oxidation, higher mechanical strength,

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Figure 4.5 (A) A schematic representation of a typical direct methanol fuel cell (DMFC) [32]. (B) DMFC single cell performance using different anode catalyst systems at 60 C [48]. (A) Reproduced with permission from Elsevier; (B) Reproduced with permission from Wiley.)

higher electronic conductivity, and stability of the catalytic performance for a longer period of time in comparison to the Pt/C catalyst system. Apart from nanofibers, other nanomorphologies of PAni have also been employed. For instance, Huang et al. [52] have synthesized a variety of nano- and microstructures of PAni, namely nanotubes, nanofibers, hollow microspheres, and submicron spheres, as catalyst supports and have deposited Pt on these supports to determine their electrocatalytic efficiency toward MOR. The observation made by the authors was that by virtue of the high surface area of nanofibers compared to other synthesized

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morphologies, the Pt/PAni nanofibers catalyst system exhibited the best efficiency and performance. In fact, both the performance and the stability of Pt/PAni nanofibers catalyst system were even better than the conventionally used Pt/Vulcan XC-72 catalyst systems. Rajesh et al. [53] synthesized aligned nanotubes of PAni on carbon cloth electrode and deposited Pt nanoparticles on the synthesized PAni nanotubes. Utilizing this electrode for MOR, the authors observed a maximum current density of 80 mAcm 2 at a Pt loading of 80 mgcm 2. The alignment of the nanotubes into arrays was found to be crucial, as the nonaligned nanotubes of PAni on carbon cloth could produce only 26 mAcm 2 of current at the same Pt loading. In addition, the aligned nanotube catalyst support was successful in demonstrating better performance and stability in comparison to the commercial Vulcan XC-72R support. The next popular morphology after nanofibrillar is the nanowire. Yang et al. [54] fabricated PAni nanowires after doping the PAni with poly(acrylic acid-co-maleic acid) and then embedded Pt nanoparticles within this nanowire matrix. This fabricated anode electrocatalyst exhibited superior current density and reduced onset potential compared to the Pt/ undoped PAni nanowire catalyst system. It was inferred that the carboxylic acid group present in the doped PAni nanowires led to stabilized Pt ions within the matrix, resulting in uniform dispersion and distribution of Pt nanoparticles. Liu et al. [55] synthesized PAni nanowires potentiostatically and galvanostatically, followed by deposition of Pt nanocatalytic particles on the synthesized nanowires. The authors noted that the galvanostatically synthesized PAni nanowires were much superior to the potentiostatically synthesized ones, and as a result deposited Pt on the former one exhibited much higher value of current density (i.e., 24.7 mAcm 2mg 1 at 0.68 V) compared to the latter (i.e., 5.5 mAcm 2mg 1) and also the bulk Pt (i.e., 7.5 mAcm 2mg 1). There are reported works on other morphologies as well. In a typical study, Das et al. [48] synthesized PAni nanowhiskers using a template-free surfactantassisted chemical etching process and deposited Pt-Ru catalyst nanoparticles on the synthesized nanowhiskers via reduction by sodium borohydride. It was observed that the PAni nanowhiskers-supported Pt-Ru achieved higher performance level (i.e., current density: 175.43 mAcm 2 at 0.2 V, maximum power density: 35.09 mWcm 2) compared to the PAni nanofibers-supported Pt-Ru (i.e., current density: 150.41 mAcm 2 at 0.2 V, maximum power density: 30.08 mWcm 2) as well as the conventionally used Pt-Ru/Vulcan carbon catalyst system (i.e., current density: 120.32 mAcm 2 at 0.2 V,

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maximum power density: 24.06 mWcm 2). The DMFC single cell performances of the different catalyst systems have been presented in Fig. 4.5B. PAni nanowhiskers dispersed phase, embedded in poly(vinylidene fluoride-co-hexafluoro propylene) continuous matrix phase, has also been utilized in the fabrication of nanocomposite polymer electrolyte membrane for DMFCs [56]. The authors achieved significant reduction of methanol crossover across the fabricated membrane and much enhanced membrane selectivity in comparison to the state-of-the-art Nafion-117 membrane electrolyte. Researchers have also employed certain modifications and PAni to make it more appropriate as a highly efficient anode catalyst support. Das et al. [57] adopted the sulfonation route. They incorporated sulfonic acid group within PAni by using chlorosulfonic acid. Incorporation of sulfonate group resulted in stronger interaction with the deposited Ni catalyst particles and enhanced the dispersion, distribution, and retention of the particles. This, in turn, led to availability of higher electrochemical surface area of the catalyst particles for MOR. As a result, the electrocatalytic performance was found to be high and stable, along with a low catalyst poisoning. On the other hand, Eris et al. [58] fabricated a composite of nano-PAni and reduced graphene oxide to support the deposited Pt nanoparticles as a catalyst for MOR. With this nanocomposite catalyst system, the MOR current was found to reach 47.2 mA, which was retained to its 68.4% value after 100 cycles. Wu et al. [59] designed a copolymer of PAni in the form of poly(aniline-co-metanilic acid) and employed its porous nanowire structure to arrest deposited Pt nanoparticles in a uniform manner. The presence of sulfonic acid group within the structure of the copolymer was found to be responsible in strongly attaching and holding deposited Pt particles and ensuring their uniform and homogeneous distribution. As a result of this, the Pt/copolymer anode catalyst demonstrated higher methanol oxidation current density and performance stability compared to Pt/PAni catalyst system. As found in the cases of other two types of fuel cells described above, PPy is another conducting polymer that has found use in DMFCs mainly as anode catalyst support matrix. In a typical study, Zhao et al. [60] polymerized nanoparticles of PPy on Vulcan XC-72 support. They doped the polymer with naphthalene sulfonic acid for ensuring better interaction with the deposited Pt nanoparticles of size w3e4 nm. As a result, it was observed that 40 wt% of Pt could be loaded on the PPy nanoparticles/ Vulcan XC-73 catalyst support. This synthesized catalyst system produced higher CO tolerance and a higher and more stable performance even after

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500 cycles, compared to the commercial Pt/C catalyst system containing 40 wt% of Pt. The higher conductivity of PPy, both in terms of protonic and electronic conductivities, and the better interaction with Pt particles by the dopant were the probable reasons behind the enhanced performance of the synthesized catalyst system. Choi et al. [61] synthesized PPy nanocapsules, possessing spherical shape and hollow morphology, by using a spherical nanosilica template for effectively acting as a support matrix for depositing Pt-Ru nanocatalyst. This Pt-Ru/PPy nanocapsules catalyst system exhibited high CO tolerance and could easily oxidize CO because of Pt-Ru catalyst’s better accessibility to the CO molecules. Rajesh et al. [62] showed that nanotubules of PPy can also be a potential nanomorphology to play the role of a catalyst support for the deposition and incorporation of Pt nanoparticles. The authors adopted two different processes to synthesized PPy nanotubules, namely templated synthesis that produced aligned nanotubules and nontemplated synthesis that generated randomly oriented nanotubules. It was observed that the templated PPy nanotubules, owing to their aligned nature, showed higher effective loading (i.e., increase in performance with loading) of Pt nanoparticles of up to 140 mgcm 2 (producing 302.5 mAcm 2 of current density with an increasing trend) while the random nanotubules could only effectively load 80 mgcm 2 of Pt (generating 119.4 mAcm 2 of current density with a saturation at 120.4 mAcm 2). Nanofiber is a common morphology that researchers adopt because of the high electrochemical surface area it provides. Ghosh et al. [63] took advantage of this positive attribute of radiolytically synthesized PPy nanofibers to radiolytically deposit Pt, Pt-Pd, and Pt-Pd-Au nanoparticles. The nanofiber synthesis and catalyst deposition processes have been schematically illustrated in Fig. 4.6. The morphological structures of the fabricated catalyst systems have been shown in Fig. 4.7. It was found that among the three metallic catalysts synthesized, the ternary alloy catalyst demonstrated the best performance in terms of costly Pt loading, rate of methanol oxidation, performance stability, and tolerance toward in situ generated catalyst-poisoning intermediate species. The alloy catalysts were also realized to be superior to the commercial Pt/C catalyst system in oxidizing methanol at a stable rate. This enhanced result was attributed to the stronger interaction between the PPy matrix and the deposited catalyst particles as well as the availability of higher number of catalytic sites in the order ternary alloy > binary alloy > single metal. Liu et al. [64] synthesized unique intercalated Pt/PPy nanofibers catalyst system with the assistance of

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(A) (NH4)2S2O8 α β O

S

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CTAB based lamellar structure

ys

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Figure 4.6 Schematic illustration of the radiolytically (A) synthesized polypyrrole (PPy) nanofibers and (B) deposited metallic nanocatalysts on the nanofibers [63]. (Reproduced from S. Ghosh, S. Bera, S. Bysakh, R.N. Basu, Conducting polymer nanofibersupported Pt alloys: Unprecedented materials for methanol oxidation with enhanced electrocatalytic performance and stability, Sustainable Energy and Fuels 1 (2017) 1148e1161, https://doi.org/10.1039/C7SE00126F with permission from The Royal Society of Chemistry.)

polyvinylpyrrolidone, composed of Pt nanoparticles with a size of w3.5 nm (Fig. 4.8A). This nanocatalyst system showed much better methanol oxidation and oxygen reduction capabilities, along with higher tolerance toward CO, compared to commercial Pt/C catalyst system, owing to factors such as steric effect and interfacial interaction (leading to transfer of charge) between metallic catalyst and the nanofibrous support matrix. The most commonly used morphology of PPy used in DMFC is the nanowire morphology. The advantage of nanowires over nanofibers can be in terms of even higher surface area and the ability to form aligned arrays for better holding and retention of the deposited metal particles. For example, Xia et al. [66] synthesized PPy nanowires with vertical alignment directly on Nafion membrane modified with Pd. This designed cathode system, obtained after spraying Pt/C onto the nanowire with a Pt loading of

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Figure 4.7 (I) The morphology and structure of Pt66Pd34/Ppy: (A) HAADF-STEM image of the catalyst composite. Inset: high magnification transmission electron microscopy (TEM) image of the composite. (B) Bright field TEM image of the corresponding area of Pt66Pd34/Ppy. (C and D) HAADF-STEM-EDS mapping images of Pt66Pd34/Ppy. (II) The morphology and structure of Pt24Pd26Au50/Ppy: (A) HAADF-STEM image of the catalyst composite. (BeD) HAADF-STEM-EDS mapping images of Pt24Pd26Au50/Ppy [63]. (Reproduced from S. Ghosh, S. Bera, S. Bysakh, R.N. Basu, Conducting polymer nanofiber-supported Pt alloys: Unprecedented materials for methanol oxidation with enhanced electrocatalytic performance and stability, Sustainable Energy and Fuels 1 (2017) 1148e1161, https://doi.org/10.1039/C7SE00126F with permission from The Royal Society of Chemistry.)

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Figure 4.8 (A) Schematic illustration of the mechanism involved in the synthesis of Pt/PPy nanofibers catalyst system, having intercalated structure, with the assistance of polyvinylpyrrolidone [64]. (B) (I) A schematic of the fabrication process of the Pt/C-containing PPy nanowire grown of Pd-modified Nafion-115 membrane and (II) DMFC cell performance obtained using the designed cathode catalyst. DMFC, direct methanol fuel cell; PPy, polypyrrole [65]. (Reproduced from (A) Y. Liu, N. Lu, S. Poyraz, X. Wang, Y. Yu, J. Scott, J. Smith, M.J. Kim, X. Zhang, Onepot formation of multifunctional Pt-conducting polymer intercalated nanostructures, Nanoscale 5 (2013) 3872-3879, 10.1039/C3NR00595J with permission from The Royal Society of Chemistry. (B) Z. Xia, S. Wang, Y. Li, L. Jiang, H. Sun, S. Zhu, D.S. Su, G. Sun, Vertically oriented polypyrrole nanowire arrays on Pd-plated Nafion® membrane and its application in direct methanol fuel cells, Journal of Materials Chemistry A 1 (2013) 491e494, 10.1039/c2ta00914e with permission from The Royal Society of Chemistry.)

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0.5 mgcm 2, produced a maximum power density of w100 mWcm 2. The performance was found to decrease with decreasing order of alignment, which is a very important observation that proves the superiority of aligned uniform structures over random orientations. The fabrication process of the catalyst system and the DMFC cell performance obtained have been presented in Fig. 4.8B [65]. In a different approach, Wu et al. [67] reported growth of network of PPy nanowire on carbon paper anode, followed by loading of Pt-Ru electrocatalyst at 1:1 ratio by weight. Application of this anode in a passive DMFC resulted in generation of 43.5 mWcm 2 of peak power density at 25 C, 2 mgcm 2 of catalyst loading, and 4 M concentration of the methanol fuel. This result was found to be superior to that obtained for Pt-Ru/C catalyst system under similar operating conditions, i.e., 33.9 mWcm 2 of peak power density. The authors noted that even upon reducing the catalyst loading to half of this value (i.e., 1 mgcm 2) on the modified support material, the DMFC performance was found to be even higher (i.e., 34.3 mWcm 2 of peak power density) than the fully loaded Pt-Ru/C anode catalyst system. This observation is very important because it resulted in reduction of the costly Pt usage to produce a better result. Ma et al. [68] showed that PPy nanowires can be converted to carbon nitride nanofibers upon calcination at a temperature of 800 C. The inherent nitrogen atoms (w10 at% concentration) present within these nanofibers matrix ensured easy incorporation and dispersion of Pt catalyst particles. This unique catalyst system synthesized demonstrated high and stable electrocatalysis for methanol oxidation owing to its very high electrochemical surface area. The authors noted two important observations: (a) in terms of the catalytic performance, the increasing order of activity was Pt/Vulcan XC-72 < Pt/carbon nitride nanofibers < Pt/PPy, and (b) in terms of CO tolerance, the increasing order of stability was Pt/PPy < Pt/carbon nitride nanofibers < Pt/Vulcan XC-72. From these two observations, the authors inferred that Pt/carbon nitride nanofibers have the best optimized properties (i.e., balance between catalytic performance and CO tolerance) among the three studied catalyst systems. Another approach to modify PPy for making it more efficient as an anode catalyst support is to incorporate sulfonic acid group within its chemical structure. Accordingly, Das et al. [69] have performed sulfonation of PPy nanoparticles by chlorosulfonic acid and deposited Ni nanocatalyst on the sulfonated support matrix. The sulfonic acid group was found to be responsible for effecting more uniform and homogeneous dispersion of the smaller Ni nanoparticles on to the matrix material, owing to establishment

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of stronger interaction between this charged functional group and the Ni atoms, in comparison to the Vulcan carbon and the unsulfonated PPy matrices. This Ni/sulfonated PPy catalyst system demonstrated a current density at 0.2 V of 144.5 mAcm 2 and a peak power density of 28.9 mWcm 2, which were higher than that the values of 119.7 mAcm 2 at 0.2 V and 23.9 mWcm 2 produced by Pt/Vulcan carbon. Das et al. [70] later showed that Ni-Ag (80/20 wt/wt) binary electrocatalyst deposited on sulfonated PPy can demonstrate even higher DMFC performance (i.e., values of 179.5 mAcm 2 at 0.2 V and 35.9 mWcm 2) owing to the availability of higher number of active catalytic sites in binary metal catalyst. Researchers have also explored other polymers with different nanostructures to achieve better performance compared to the commercial catalyst. For example, Rajesh et al. [71] fabricated template-assisted poly(3-methyl) thiophene nanocones with incorporated Pt nanoparticles and Zhou et al. [72] electrosynthesized poly(3,4-ethylenedioxythiophene) nanoflowers with incorporated Pt nanocatalyst. Both of these reports noted significant catalytic performance toward methanol oxidation.

5. Conclusions and future directions This chapter has summarized the use of polymer nanomaterials in PEMFCs, which includes MFCs, HFCs, and DMFCs. Although there are many published reports in this area of research, most of them are mainly concerned with the various synthetic schemes and limited up to the chemical and physical analyses of the developed materials and electrochemical analyses of fuel oxidation. Full demonstrations in actual fuel cells have very rarely been reported in the literature. However, the results reported so far are enthusiastic and hold extreme potential to minimize the use and efficiency of the high cost commercial Pt-based electrocatalysts. The nanostructures have all demonstrated high electrochemical surface area, good interaction with the deposited metal catalyst, uniform and homogeneous dispersion and distribution of the catalyst nanoparticles, good holding and retention capability of the costly catalyst, high electrocatalytic efficiency, high electronic and proton conductivities, and high CO tolerance. These are very important results, as these fulfill our main objectives of overcoming the critical performance-hampering factors and hindrances toward widespread commercialization of PEMFCs. A comparative performance evaluation of different polymeric nanostructures reported in this chapter has been presented in Table 4.2. However, the developed nanostructured

Table 4.2 A comparative performance evaluation of different polymeric nanostructures reported in this chapter. Fuel cell performance Co-component/ catalyst deposited/electrode Power density Nanostructured polymeric material Used as Current density (mWm 2) materials used

References

Microbial fuel cells

Polyaniline (PAni) nanogranules PAni nanogranules Polypyrrole (PPy) nanotube PPy nanofibers PAni nanowire

e

Anode

60 mAm

TiO2 e Carbon fibers Tartaric acid/carbon cloth

Anode Anode Anode Anode

3650 mAm 2100 mAm 46.6 mAm 1600 mAm

2

820 mWm 2 2 2 2

2

[34] [35] [38] [37] [36]

660 mWm 2 307.4 mWm

[45] [41]

Hydrogen fuel cells

PPy nanogranules Carboxylated carbonized PAni nanofibers Sulfonated carbonized PAni nanofibers

Pt Pt

Cathode Anode and cathode Anode and cathode

Pt

555 mAm 2 970.6 mAm 1393.7 mAm

2

2

414 mWm

2

2

[42]

Direct methanol fuel cells

Ni-Ag Pt-Ru Pt-Ru Pt Ni Ni Pt-Ru/carbon paper Pt-C/Pd Pt-C

Anode Anode Anode Anode Anode Anode Anode Anode Cathode

179.5 mAm 2 150.41 mAm 2 175.43 mAm 2 e 135 mAm 2 144.5 mAm 2 e e e

35.9 mWm 2 30.08 mWcm 2 35.09 mWm 2 22 mWm 2 27 mWm 2 28.9 mWm 2 43.5 mWm 2 104 mWm 2 98.2 mWm 2

[70] [55] [55] [51] [57] [69] [67] [66] [65]

123

Sulfonated PPy nanogranules PAni nanofibers PAni nanowhiskers PAni nanofibers Sulfonated PAni nanogranules Sulfonated PPy nanogranules PPy nanowires PPy nanowires PPy nanowires

Polymeric nanomaterials in fuel cell applications

1495 mWm 2 612 mWm 2 5.9 mWm 2 490 mWm 2

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polymer-based catalyst systems should very immediately be tested in actual fuel cell setups to realize the practical potentials of these systems and to calculate the total material and operation cost. This will enable realization of the performance-to-cost ratio of these systems in comparison to the commercial Pt/C catalyst system. In this way, we can move few steps forward toward commercialization goals. In addition, more conducting polymeric nanostructures should be developed, as almost all the studies so far have been based on only PAni and PPy. Moreover, steps should be taken to analyze the efficiency of the developed catalyst systems toward oxidation of other types of fuels within the domain of PEMFCs, such as ethanol [73] and other alcohols and organic compounds.

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CHAPTER 5

Nanocarbon: lost cost materials for perovskite solar cells Somasundaram Anbu Anjugam Vandarkuzhali1, Subramanian Singaravadivel2, Alagarsamy Pandikumar3, Gandhi Sivaraman4 1

National Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai, Tamil Nadu, India; 2Department of Chemistry, SSM Institute of Engineering and Technology, Dindigul, Tamil Nadu, India; 3Functional Materials Division, CSIR-Central Electrochemical Research Institute, Karaikudi, Tamil Nadu, India; 4Department of Chemistry, Gandhigram Rural Institute eDeemed to be University, Gandhigram, Tamil Nadu, India

1. Introduction Perovskite solar cells (PSCs), as a new player in the photovoltaic filed, exhibit rapid development with an original efficiency of 3.81% in 2009, a promising efficiency of over 20% in 2014 and a record reported power conversion efficiency (PCE) of 24.2% [1e19]. In recent years, PSCs have shown great promise for their solution process and high efficiency, which have overtaken organic and quantum dot (QD) solar cells in a short time span in PCE, vigorously catching up with crystalline silicon solar cells [4e8]. Traditional PSCs possess a typical layer-by-layer structure (front electrode of transparent conductive oxide [TCO] [mainly FTO and indium-doped tin oxide] on the rigid or flexible substrate, electron transport layer, perovskite light absorber layer, hole-transporting material [HTM], and back electrode) [10e13]. PSCs evolved from DSSCs and are a new class of PV technology that have great potential to replace current commercial solar cells. A typical mesoscopic PSC is constructed using a TCO film (usually fluorine-doped tin oxide [FTO]), a thin, dense compact semiconducting layer that prevents short circuits, a mesoporous nanocrystalline semiconducting oxide layer, a light harvesting perovskite layer, a HTM, and a metal electrode (Au or Ag) [6]. The entire working principles of PSCs have not been satisfactorily explained and can be different depending on the exact PSC structure. It is accepted that in the case of typical PSCs: upon illumination, the perovskite layer is excited, producing an electronehole pair. The charge carriers can then diffuse to an interface where the electrons are injected into Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00005-1

© 2020 Elsevier Inc. All rights reserved.

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the conduction band of the semiconducting material while the holes are transported to the valence band of the HTM. Finally, the electrons and holes are then collected by the conductive electrodes. Organometal trihalide PSCs have attracted wide attention because such a solution processebased photovoltaic device has obtained comparable PCEs with traditional commercial solar cells in a short time span of its development. The special and advantageous optoelectronic properties of the hybrid materials (ABX3: A ¼ CH3NH3, HC(NH2)2, Cs; B ¼ Pb, Sn; X ¼ Cl, Br, I) that account for the rapid growth in PCE include highmobility [20e23], long-balanced carrier diffusion length and low exciton binding energy [24,25]. Perovskite-based solar cells have attracted great attention from the PV research community due to their extraordinary lightharvesting characteristics. Over the past 2 years, the implementation of organiceinorganic lead halide perovskite-based light absorbers into solidstate solar cells has brought breakthroughs in low-cost PVs. The general structure of PSCs has remained similar with improvements in efficiency related to the implementation a solid-state HTM and the mixing of the perovskite within the mesoporous metal oxide layer. Benefiting from the low cost, high efficiency, and solution process, PSCs have become a promising candidate for next-generation photovoltaic technology. It is well known that most of the high-performance PSCs (PCE>20%) are fabricated with organic HTMs and noble metal electrodes (Au or Ag, need to be deposited by vacuum thermal evaporation), where adoption of the highly expensive organic HTMs and noble metal electrode as back contacts significantly increases the cost of raw materials and fabrication equipment [5].

2. Inorganic perovskite layers Perovskite layers are divided into polycrystalline thin films and QD films according to their morphologies. The polycrystalline thin films, which are usually used in PSCs, can be fabricated by solution-processing or vacuumprocessing technique. The QD film can be fabricated by solutionprocessing technique using QD solution. The fabrication of inorganic perovskite films is similar to those of organiceinorganic hybrid perovskite films. The adoption of different fabrication processes commonly depends on the limitation of the perovskite composition. Under the main framework of these fabrication steps, precursor engineering, solvent engineering,

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133

crystallization engineering, and posttreatment can be utilized to improve the film quality. One-step Solution Deposition: Namely, stoichiometric amounts of PbX2 (SnX2) and CsX are dissolved together in polar organic solvent such as dimethylformamide or dimethyl sulfoxide to form a perovskite precursor solution. Then the perovskite film can be prepared by directly spin coating the prepared precursor on the substrates, and followed by an annealing process, as shown in Fig. 5.3A. For tin-based perovskite, additional SnF2, SnCl2, or SnI2 are normally added to the precursor to reduce tin vacancies and avoid the fast oxidation of tin from Sn2þ to Sn4þ [26e28]. Two-step Deposition: As shown in Fig. 5.1, two-step method involves a first deposition of the PbX2 layer, followed by a reaction with CsX via dip coating or spin coating to form the perovskite film. Finally, the perovskite film is annealed to obtain the desired crystal phase. This method is commonly used to fabricate the CsPbBr3 and CsPbIBr2 films because both the PbBr2 and CsBr (CsI) have limited solubility in the same organic solvent [29e31]. Vacuum Processing Technique: Vacuum deposition enables the researcher to study the perovskite more deeply by controlling the thickness precisely and producing PSCs with more repeatable PCE because of less handmade uncertainty (Fig. 5.1B). Vacuum deposition was commonly adopted at the beginning stage of inorganic PSCs, especially CsPbI3 PSCs, because it is a good method to guarantee a high-quality perovskite film. The vacuum deposition approaches include coevaporation [32], sequential evaporation [33],

Figure 5.1 (A) One-step deposition and two-step deposition of solution processing technique. (B) Vacuum processing technique. (Reproduced with permission from American Chemical Society.)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

double-layer evaporation [34], and alternative evaporation of PbI2 and CsI followed by a high-annealing temperature of above 300 C [35]. PSCs have several advantages such as remarkably high efficiency along with a simple and low cost synthesis. However, they also suffer from several drawbacks, namely (i) use of expensive, rare materials, (ii) high-temperature processing of n-type TiO2 layer, (iii) relatively slow electron transport between the perovskite and TiO2, and (iv) lack of long-term stability. At present, there are still lack of studies on large-area (e.g., module level) and flexible inorganic PSCs. The currently used vacuum deposition and solution-processing methods are both suitable for large-scale fabrication of perovskite films. Though high temperature (w300 C) annealing is typically required for preparing CsPbI3-xBrx perovskite films, promising progress has been made toward low-temperature ( Cs2AgSbCl6 [67]. Still more studies in this area are required to identify the influence of acid/amine equilibria on the structural and optical properties of different class of silver bismuth double perovskite nanomaterials. One of the problems persisting with the synthesis of silver-based double perovskite NCs is ionization of Ag atoms in the reaction medium. The nanocubes (w10 nm size) synthesized by Dhal et al. are decorated with the Ag(0) nanocrystallites on their surface illustrating that optimized conditions have to be developed to control the effect of Agþ ions during the synthesis of double perovskite NCs. This observation is similar to the CsPbX3 NCs in which the formation of Pb(0) atoms on the surface of the NCs was observed [136]. Formation of these Ag(0) atoms is speculated owing to the reduction ability of the OAm used in the reaction. This could be avoided through the addition of HCl and HBr solution to expedite the ionization of Ag atoms. Furthermore, Cs2AgSbCl6 NCs showed a less stability under ambient conditions and also in the presence of electron beam, whereas Cs2AgInCl6 NCs showed higher stability which reveals that the nature of metal ions is additionally playing a key role in imparting stability to the NCs. Despite of lower PLQY achieved in this method, later, Liu et al. have experimentally demonstrated that doping bismuth (Bi) with Cs2AgInCl6 leads to 11.4%, indicating that altering the electronic structure of double perovskite with doping of suitable metal ions will be one of the potential method for future improvements in the optical properties [137]. The crystal structure, experimental arrangement for synthesis, structural, optical, and morphological analysis of the synthesized Cs2AgInCl6 NCs are given in Fig. 6.13 AeI. This PLQY is still lower than the one achieved by Yang et al. earlier through the modifications of composition of Bi2þ and In3þ ions [131]. The lifetime of the Bi-doped Cs2AgInCl6 NCs in this case was found to be much higher (w1633 ns) than the undoped NCs (105/cm). Unlike Sn2þ ions, Sb3þ ions are resistant against adverse surface process, e.g., oxidation and substitution of Sb3þ ions in place of Pb2þ ions make considerable effect on the metal-halide [MX6]n octahedra which stimulate the formation of 2D assembly [139]. This kind of formation of 2D structure under [MX6] unit influence was also observed in Rb3Bi2I9 [71]. Based on this, Pal et al. synthesized quantum-confined nanoplatelets (180 C) and nonequantum-confined nanorods (230 C) of Cs3Sb2I9 by simply varying the temperature [139] (Fig. 6.14A). The nanoplatelets had thickness w1.5 nm, and the nanorods possessed the diameter w46 nm and length w655 nm (Fig. 6.14B and C). Here, octanoic acid (OTAc), OAm, and 1ODE were used together with SbI3 to prepare the stibium precursor for the reaction. Although a very good passivation through these ligands is commonly expected, the experimentally obtained low lifetime values of nanoplatelets and nanorods (1e1.5 ns of major contributions) clearly reveal about the possible role of defect density factor over the ligands. Subsequently, Zhang et al. reported on the synthesis of highly luminescent Cs3Sb3X9 QDs with the PLQY 46% (lem ¼ 410 nm) by a room temperature LARP method (Fig. 6.14D and E) [72]. Surprisingly, this higher PLQY was achieved by controlling the crystallization process during the synthesis through optimizing the precursors and solvent ratio. This higher PLQY assures the possibility of further PLQY enhancement by reducing the surface defects through additional surface engineering methods. The synthesized QDs were also halide-exchanged and tunable PL emission was achieved between 370 and 560 nm. Recently, Pradhan et al. have achieved monodispersed nanowires and nanorods of Cs3Sb2Cl9 by simply varying the source [140]. The authors synthesized nanowires of Cs3Sb2Cl9 in presence of hexadecylamine hydrochloride (HDA-HCl) (T ¼ 170 C), and similar conditions with hexadecylamine (HDA) alone produced nanorods with different dimensions (T ¼ 160 C). The size and aspect ratio of the synthesized nanorods were tuned by adjusting the amount of SbCl3. Furthermore, different phases of the NCs (trigonal and orthorhombic) were achieved by simply varying the reaction temperature. In this case, the observed low PL quantum yield (4%) value clearly explores that improvement of PLQY of these NCs using suitable surface ligands and posttreatment processes would be an interesting subject of study for future improvements in optical properties through suppressing defects.

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Figure 6.14 (A) Schematic diagram of formation of Cs3Sb2I9 nanoplates and nanorods by hot-injection method; (B, C) TEM images of the Cs3Sb2I9 nanoplates and nanorods. (D) Schematic representation of room temperature synthesis of Cs3Sb2I9 NCs, and (E) optical image of the synthesized Cs3Sb2I9 NCs under normal and UV light (365 nm). (B and C) Reprinted with permission from Ref. J. Pal, S. Manna, A. Mondal, S. Das, K. V. Adarsh, A. Nag, Colloidal synthesis and photophysics of M3Sb2I9 (M¼Cs and Rb) nanocrystals: lead-free perovskites, Angewandte Chemie International Edition 56 (2017) 14187e14191. Copyright©2017 Jhon Wiley and Sons. (E) Reprinted with permission from Ref. J. Zhang, Y. Yang, H. Deng, U. Farooq, X. Yang, J. Khan, J. Tang, H. Song, High quantum yield blue emission from lead-free inorganic antimony halide perovskite colloidal quantum dots, ACS Nano 11 (2017) 9294e9302. Copyright©2017 American Chemical Society.)

6. Miscellaneous Other than the above-discussed family of lead-free perovskite NCs, recently emerged cesium palladium perovskite NCs (Cs2PdBr6), cesium germanium iodide (CsGeI3) NCs, and cesium copper halide perovskite (Cs2CuX4, X ¼ Cl, Br, I) NCs also attracted considerable attention. Zhou et al. prepared highly stable cubic Cs2PdBr6 NCs through room

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

temperature solvent-assisted method through a two-step procedure [141]. Firstly, microcrystals of Cs2PdBr6 particles were synthesized through the oxidizing HBr-DMSO adduct, which finally transformed as Cs2PdBr6 NCs with single unit cell thickness through solventeantisolvent interaction process. Here, synthesizing Cs2PdBr6 NCs through traditional hotinjection method was failed because of the strong coordinating ability of OA and OAm with Pd2þ ions. The authors further investigated the influence of various antisolvents with different polarity values on the stability of the NCs and found that along with the size variation, high polarity and strongly coordinating solvents were not able to precipitate NCs in the reaction medium. Besides, low polarity (less than 4.5) and weak coordinating solvents such as propanoic acid, butanol, and chloroform were found to be efficient in producing NCs through precipitation. The NCs were exhibited with broad absorption edge at w600 nm and PL maxima at w730 nm. A successful halide-exchange process has also been carried out with the Cs2PdBr6 NCs film in the presence of TMS-I in propionic acid to convert it as Cs2PdI6 NCs film. This is illustrating the specific role of solvents and their polarity in synthesizing lead-free perovskite NCs for solution-processed deposition processes. The first report on the synthesis of CsGeX3 (X ¼ Cl, Br, I) quantum rods was reported by Chen et al. by means of solvothermal method [142]. In their procedure, the precursors GeX2, 1-ODE, Cs-oleate, HCl, and TOPO together with OA/OAm were loaded in an autoclave which consisted DIEN as solvent. At 180 C for 6 h, formation of quantum rods with the average diameter of 5 nm took place in solution which was separated out after multiple washing and centrifugation processes. The TEM and EDX analysis of the synthesized quantum rods are given in Fig. 6.15AeE. Interestingly, the synthesized CsGeX3 quantum rods had a strong excitonic peak with 90 nm shift in the absorption onset of their halide counterparts. Moreover, tunable PL emission spectra in the region 607e696 nm were achieved with extremely narrow FWHM 25 nm. With 24.2 ns as lifetime, the synthesized CsGeI3 quantum rods showed their potential use to fabricate solar cell which delivered highest performance 4.94%. The IeV curves and IeP CE curves of the solar cells fabricated using CsGeX3 quantum rods are given in Fig. 6.15F and G. The formation mechanism of these quantum rods in this case was not discussed, although it could be concluded as due to the influence of amine medium under high pressure. In advance of this, synthesis of CsGeX3 NCs using hot-injection method was demonstrated subsequently. Wu and coworkers synthesized

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Figure 6.15 (A) TEM image of CsGeI3 quantum rods and their selected magnified TEM image (inset); (B, C, D, E) elemental mapping analysis of CsGeI3 quantum rods; (F) IeV and (G) IPCE curves of the fabricated solar cell devices using CsGeX3 quantum rods. (G) Reproduced from L-J. Chen, Synthesis and optical properties of lead-free cesium germanium halide perovskite quantum rods. RSC Advances 8 (2018) 18396e18399 with permission from The Royal Society of Chemistry.)

CsGeI3 nanocubes using GeI2 as the precursor of germanium together with OAm, OA, and 1-ODE through hot-injection method [143]. Here, the reaction was carried out at 180 C and even after two times centrifugation process, the resultant product consisted of CsI NCs with CsGeI3 nanocubes. Thus, suitable solvents, ligands, and potential precursors that are able to dissolve the precursors/by-products will still be expected to contribute phase-phase, highly crystalline CsGeX3 NCs. Interestingly, a narrow PL spectrum was realized at the maxima 804 nm. This is illustrating the requirement of improvement in the wet chemistry of synthesizing CsGeX3

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

NCs. Also, the authors noticed that similar to the CsPbX3 NCs [144], reduction of Ge2þ to Ge0 took place under the electron beam irradiation of CsGeX3 NCs. It is expected that efforts in exploring possible ways to conver the CsGeX3 nanomaterials for optoelectronic applications would be of great interest for further investigation. Use of copper (Cu) in place of lead will not only reduce the toxicity but also explore the utilization of earthy abundant elements in the perovskite structure. Copper-doped CsPbX3 NCs, specifically CsPb0.93Cu0.07Br3 QDs, have shown impressive PLQY 95% with excellent optical properties [145]. Synthesis of Cs2CuX4 perovskite QDs was firstly demonstrated using improved LARP method at room temperature [146]. For instance, when the dissolved precursors of CsBr/CuBr2 in DMSO/DMF are dropped in to the mixture of OTAm/octane/OA, the crystallization takes place at the interface. As a result, quasi-spherical shaped orthorhombic Cs2CuBr4 QDs with the absorption onset at 360 nm were formed, and the optical properties were found to be tunable through varying halides. The PL, UVvisible spectra, TEM image, and size distribution of the prepared QDs are given in Fig. 6.16 (AeD). Along with this, it was found that the ratio of the precursors was also an another important factor in tuning the optical properties of the synthesized QDs. Importantly, the Cs2CuCl4 QDs synthesized through this method delivered a high PLQY 51.8%, predicting their promising use for future devices. Very recently, synthesis of Cs2CuCl4 and Cs2CuCl3 NCs has been achieved through hot-injection method, which could produce different morphologies (nanorods, nanoplatelets, and nanowires) with different sizes (6e300 nm) by varying the ratio of OAm and OA [147]. All these morphologies delivered a quite broad emission spectra with maximum wavelength at 525 nm, possibly because of the Cu(II) defects. Still, because of the electronic configuration of copper, analysis on the deficiency/excess of copper in the perovskite structure may deliver more interesting findings. The overall literature data about the solvents, reaction conditions, shape, and optical properties of CsLFHP NCs discussed in this chapter are collectively presented in Table 6.1.

7. Applications of cesium lead-free halide perovskite nanocrystals Applications of CsLFHP NCs for the optoelectronics are quite limited because of their poor luminescent properties. This is in contradictory with

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Figure 6.16 (A, B) PL and absorption spectra of Cs2CuBr4 QDs with various ratio of CsBr:CuBr2; (C, D) TEM image and size histogram of the synthesized Cs2CuBr4 QDs [Ref. 146]. (C, D) Reproduced from Ref. P. Yang, G. Liu, B. Liu, X. Liu, Y. Lou, J. Chen, Y. Zhao, All-inorganic Cs2CuX4 (X¼Cl, Br, and Br/I) perovskite quantum dots with blue-green luminescence, Chemical Communications 54 (2018) 11638e11641 with permission from The Royal Society of Chemistry.)

the excellent results rendered by CsLHP NCs in several promising areas. Despite this, the photoresponse property of lead-free perovskite nanostructure has been studied in recent years. Ghosh et al. fabricated photodetectors based on the different morphologies of Cs2SnI6 nanostructures through spin-coating method on the patterned ITO substrate [110]. Metallic silver was deposited on the top of this active layer. As mentioned earlier, under a white light irradiation, the device fabricated using Cs2SnI6 nanorods showed high photocurrent gain value over other morphologies. Following this, using CsSnX3 nanostructure array, Han et al. fabricated first NIR photodetector through a CVD technique [105]. To prevent the phase transition, the fabricated CsSnX3 nanostructures were covered with polymethyl methacrylate polymer. The photodetector fabricated using CsSnI3 nanowire array showed increased photocurrent from 1.85 to

1

2

3

Lead-free perovskite compound

Precursors, solvents, and synthesis conditions

CsSnX3 (X ¼ Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I) CsSnX3 (X ¼ Cl, Br, I)

Cs2CO3, SnCl2, SnBr2, SnI2, TOP, OAm, OA, 170 C Cs2CO3, SnCl2, SnBr2, SnI2, ODA, TOPO, HCl 1-ODE, MgBr2.6H2O, OAmBr, tin 2ethylhexanoate, Cs2CO3, OAm, OA, 120 C, 1 min OAm, OA, 1-ODE, Cs2CO3, SnBr2, TBPO, 170 C Cs2CO3, OA, OTA, SnI2, OAm, TOP, 135 C, 60 min CsX, SnX2, mica substrate, 220e350 C (SnX2), 610e670 C(CsX), 7 min

CsSnBr3

4

CsSnI3

5

CsSnX3 (X ¼ Cl, Br, I)

Method of synthesis

Shape

Size

PLQY

Ref.

Hot-injection method

Nanocrystals

7.6e9.9 nm

0.14%

[100]

Solvothermal method

Nanorods

5 nm (diameter)

NA

[102]

Hot-injection method

Nanocages

80e150 nm 20 nm (average shell thickness)

2.1% (unmodified) 1.8% (modified)

[101]

Nanocubes

NA

NA

Hot-injection method

Nanoplates

3.8 nm (height)

NA

[104]

CVD

Nanowires

Longer than 30 mm (length)

NA

[105]

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Table 6.1 Synthesis parameters, solvents, ligands, size, shape, and photoluminescent quantum yield (PLQY) values of CsLFHP NCs discussed in this chapter.

6

Cs2SnCl6

Cs2CO3, SnCl2, OLA, 1-ODE, OAm, TOP, 30 min

Hot-injection method

Cs2SnI6

1-ODE, Cs2CO3, SnI4, 80e220 C

Hot-injection method

8

Cs2SnI6

Cs2CO3, 1-ODE, SnI4, OA, OAm, 220 C, 1e60 min

Hot-injection method

9

Cs2SnI6

Cs2CO3, 1-ODE, OA, OAm, SnI4, toluene, hexane, isopropanol, TMAOH, 220 C, 30 s, 30 min, 60 min

Reverse hotinjection method

10

Cs2SnI6

Cs2CO3, SnI4, 1-ODE, OA, n-OTAm, hexane, 210 C, 1 min

Hot-injection method

Spherical QDs (1 min) nanorods, nanowires, nanobelts, nanoplatelets QDs nanocubes Nanosheets Nanorods

Nearly spherical Nanoplatelets

400e500 nm (edge length) 300e500 nm

NA

50e100 (dia)

NA

12  2. 8 nm (80 C) 38  4.1 nm (220 C) (Average diameter) w 2.5 nm 30 nm (ave. width), 3e8 nm (thickness) 8 nm (thickness)

NA

[112]

0.48% (QDs) 0.11% 0.08% 0.054% 0.046%

[107]

NA

[110]

NA

[109]

3  1 nm 30  3 nm w500e600 nm (length), 100e150 nm (width) 400  50 nm (length), 20  5 nm (width) 10e15 nm (average particle size) several hundred nanometers (lateral size)

[86]

NA

207

Continued

Recent advances in synthesis, surface chemistry of cesium

7

Octahdedral (200 C) Nanoplates (180 C) Whiskers (200 C) Quasispherical

Lead-free perovskite compound

Precursors, solvents, and synthesis conditions

11

Cs3Sb2I9

Cs2CO3, 1-ODE, OA, OAm, SbI3, 180oC and 230 C

12

Cs3Sb2X9 (X ¼ Cl, Br, I)

13

Cs3Sb2Cl9

14

Cs3Bi2X9

Method of synthesis

Shape

Size

PLQY

Ref.

Hot-injection method

Nanoplatlets (180 C)

length ¼ w27 nm, width ¼ w14 nm, thickness ¼ w1.5 nm Length ¼ w655 nm, diameter ¼ w46 nm 3.07  0.7 nm (Cs3Sb2Br9 QDs)

NA

[139]

11% (Cs3Sb2Cl9) 46% (Cs3Sb2Br9) 23% (Cs3Sb2I9)

[72]

3.4  0.6mm (length) w20  6 nm (diameter) 165  12 nm (length) 20  5 nm (diameter) 18 nm (average diameter) (Cs3Bi2I9)

NA

[140]

0.017%

[118]

Cs2CO3, 1-ODE, SbCl3, SbBr3, SbI3, CsCl, CsBr, CsI, n-OTAm, OAm, OA, DMF, DMSO, n-octane, toluene SbCl3, OA, 1-ODE, OAm, HCl, Cs2CO3, HDA, acetone, toluene, t-butanol

Room temperature (m-LARP)

Cs2CO3, BiBr3, BiCl3, BiI3, OA, OAm, 1-ODE, toluene, 100 C

Hot-injection method

Hot-injection method

Nanorods (230 C) NA

Nanowires

Nanorods

Hexagonal NCs

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Table 6.1 Synthesis parameters, solvents, ligands, size, shape, and photoluminescent quantum yield (PLQY) values of CsLFHP NCs discussed in this chapter.dcont’d

Cs3Bi2X9 (X ¼ Cl, Br, I)

BiBr3, BiI3, BiCl3, CsI, CsBr, CsCl, OA, OAm, TOPO, 1,2-EDT, DMSO, isopropanol

Room temperature

Quasispherical

6 nm (Cs3Bi2Br9 NCs, average diameter)

16

Cs3Bi2X9

CsBr, DMSO, BiBr3, OAmBr, ethyl acetate, DMF, octane, OA

Room temperature

Quasispherical QDs

3.5 nm (average dia) (Cs3Bi2Br9 QDs) 3.15 nm (Cs3Bi2Cl9 QDs) 3.86 nm (Cs3Bi2I9 QDs) 3.88 nm  0.67 nm (Cs3Bi2Br9 QDs)

17

Cs3Bi2X9 (X ¼ Cl, Br, I)

CsBr, CsCl, CsI, BiCl3, BiBr3, BiI3, OAm, OA, DMSO, toluene, ethanol

Room temperature

NA

18

Cs3Bi2X9 (X ¼ Cl, Br, I)

Cs2CO3, 1-ODE, OA, OAm, BiCl3, BiBr3, BiI3, TOP, toluene

Hot-injection method

Quasispherical Long rectangular

w40 nm

0.2% (ligand-free Cs3Bi2Br9 NCs) 4.5%(Cs3Bi2Br9 NCs with OA) 0.09% (ligand-free Cs3Bi2Cl9 NCs) 0.08% (ligand-free Cs3Bi2(Br0.5I0.5)9 NCs) 0.3% (Cs3Bi2(Br0.5I0.5)9 NCs with OA) 22%

[120]

[122]

62% 2.3% 19.4% (Cs3Bi2Br9 QDs) 26.4% (Cs3Bi2Cl9 QDs) 0.018 (Cs3Bi2I9 QDs) NA

[70]

[80]

209

Continued

Recent advances in synthesis, surface chemistry of cesium

15

Lead-free perovskite compound

Precursors, solvents, and synthesis conditions

19

Cs3Bi2I9

20

Cs3Bi2Br9

Cs2CO3, 1-ODE, OA, OAm, BiI3, isopropanol, 180 C, 10 s Cs2CO3, Bi2O3, OA, TBABr, HBr, RT

21

Cs3Bi2Br9

22

Cs3Bi2Br9

23

Eu3þ:Cs3Bi2Br9

Cs2Co3, BiBr3, DMSO, OA, OAm, toluene Cs2CO3, BiBr3, OA, OAm, 1-ODE, 180 C, hexane, 60 s CsBr, BiBr3, GBL, TBAmBr, OAm, OA, acetonitrile, toluene, ethanol, RT CsBr, BiBr3, HBr, OAmBr, DMSO, OA, ethanol, 90 C, 10 min

Method of synthesis

Shape

Size

PLQY

Ref.

Hot-injection method

Quasispherical

5  1 nm (average diameter)

w1 or less

[71]

Weak-polar solvents at room temperature LARP

Quasispherical

10.44  1.29 nm

29.6%

[124]

Hot-injection method

Parallelogram shaped nanoplatelets

Improved LARP

Quasispherical

modified LARP

Spherical

(average size) 4.3% w60e250 nm (length)

0.54%

[126]

w9 nm (thickness) 4.80  1.24 nm (average diameter)

37%

[119]

18.1% w42.4%

[127]

w3.33 nm (undoped Cs3Bi2Br9 QDs) w2.94 (1.7% mol Eur3þ) w2.75 (4.7% mol Eu3þ)

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Table 6.1 Synthesis parameters, solvents, ligands, size, shape, and photoluminescent quantum yield (PLQY) values of CsLFHP NCs discussed in this chapter.dcont’d

w2.59 (8.6% mol of Eu3þ) w2.46 (11.3% mol of Eu3þ) 24

Cs3Sb2I9

Cs3Sb2Br9

26

Cs3Sb2Cl9

27

Cs2AgBiX6 (X ¼ Cl, Br, I)

Hot-injection method

SbBr3, CsBr, DMF (or) DMSO, octane, OA, RT Cs2CO3, SbCl3, OA, HDA.HCl, HDA, 1-ODE, 170 C and 160 C

modified LARP

QDs

Hot-injection method

Nanowires (HDA.HCl, 170 C) Nanorods (HDA)

Hot-injection method

Nanocubes

Nanorods (230 C)

27  3 nm (ave. length) 14  3 nm (breath) 655  20 nm (average length) 46  4 nm (dia) 3.07  0.6 nm 3.4  0.6 mm (long) w20  6 nm (dia) 0.2 mmol SbCl3 165  12 nm (average length) 20  5 nm (dia) 0.12 mmol SbCl3 290  12 nm (length) 20  5 nm (width) w8 nm (edge length)

NA

[139]

NA

20%e46%

[72]

4% NA

[140]

NA

[69]

211

Cs(OAc), Ag(OAc), Bi(OAc)3, 1-ODE, OA, TMSX (X ¼ Cl, Br, I), 140 C, 10e60 s

Nanoplatelets (180 C)

Recent advances in synthesis, surface chemistry of cesium

25

Cs2CO3, SbI3, OAm, OTAc, 1ODE, 180oC and 230 C, 1 min

Continued

Lead-free perovskite compound

Precursors, solvents, and synthesis conditions

28

Cs2AgBiBr6

29

Cs2AgBiX6 (X ¼ Cl, Br, Ij)

30

Cs2AgBiX6 (X ¼ Cl, Br, I)

1-ODE, OA, OAm, AgNO3, BiBr3, HBr, CHCl3, noctane, DMF, acetone, ethylacetate, 200 C Cs2CO3, 1-ODE, OA, OAm, BiBr3, AgBr, CsAc, BiAc, AgAc, TMS-Br, HCl, 200 C, 5 min BiBr3, BiI3, BiCl3, CsBr, CsI, CsCl, AgBr, AgI, AgCl, HBr, OA, DMSO, isopropanol

31

Cs2AgBiX6 (X ¼ Cl, Br)

Cs(OAc), Bi(OAc)2, Ag(OAc), Yb(C2H3O2)3.H2O, Mn(OAc)2, TMSCl, TMS-Br, 1ODE, OA, OAm, 145 C

Method of synthesis

Shape

Size

PLQY

Ref.

Hot-injection method

Nanocubes

9.5 nm (average size)

NA

[117]

Hot-injection method

Nanocubes

8e15 nm (side length)

NA

[129]

Room temperature (antisolvent recrystallization method)

Quasispherical shape

5.0 nm (average diameter)

[131]

Hot-injection method

Nanocubes

Edge length 8.5  0.5 nm (Cs2AgBiCl6 NCs) w8.0  0.6 nm (Yb:Cs2AgBiCl6 NCs) 8.5  0.6 nm

6.7% (Cs2AgBiCl6 NCs) 0.7% (Cs2AgBiBr6 NCs) 450 nm), resulting from the effective separation of the photogenerated charge carriers originated from the synergistic effect of g-C3N4 and ZnO. Krishnan and co-workers [67] prepared 2D N-doped ZnO/g-C3N4 nanosheet heterojunctions and explored their photocatalytic HER property. The optimal photocatalytic HER performance was achieved with 30 wt% of g-C3N4 nanosheets (18,836 mmol h1 g1) as shown in Fig. 9.11A. The ultrahigh HER yield was mainly because of the presence of 2De2D interfacial contact. This face-to-face contact surface was beneficial for the effective separation of the photogenerated charge carriers in space, thus facilitating H2 production (Fig. 9.11B). In addition, many combinations of metal oxides/g-C3N4 have been investigated for photocatalytic H2 generation, viz., WO3/g-C3N4 [68], Bi2O3/g-C3N4 [69], Cu2O/g-C3N71 4 , etc.

310 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 9.11 (A) H2 generation activity of N-ZnO, g-C3N4, and its composites under solar light irradiation. (B) Schematic diagram of photocatalytic H2 generation over 2De2D heterojunction of N-ZnO and g-C3N4 nanosheets under simulated light irradiation using Na2SNa2SO3 as sacrificial reagent. (Reprinted with permission from S. Kumar, et al., Two dimensional N-doped ZnO-graphitic carbon nitride nanosheets heterojunctions with enhanced photocatalytic hydrogen evolution, International Journal of Hydrogen Energy 43 (2017) 3988e4002, Copy right 2018 Elsevier.)

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Thus, the metal oxide/g-C3N4 is also considered as important strategy for improved photocatalytic activities of g-C3N4-based photocatalysts.

5.3 Metal sulfide/g-C3N4 composites for efficient hydrogen generation Recently, the exploitation of g-C3N4 hybridized with a low band gap metal sulfide semiconductor photocatalyst has attracted more attention [70]. CdS possesses a band gap of 2.4 eV, ensuring the absorption of visible light up to 520 nm. The highly matched energy levels of g-C3N4 and CdS, being beneficial for the construction of heterostructures, will ease the charge migration transfer upon light irradiation [71]. Yu and co-workers [72] synthesized a heterostructured g-C3N4/CdS photocatalyst with homogeneous dispersion and strong coupling interface through a simple in situ high-temperature self-transformation strategy employing melamineCdS composite as the precursor. When the amount of g-C3N4 was 1 wt%, the obtained g-C3N4/CdS exhibited the highest H2 evolution rate (5303 mol h1 g1), which was 2.5-fold higher than the pure CdS. In the g-C3N4/CdS composites, the g-C3N4 worked as an effective hole migration co-catalyst to boost the fast migration of the photogenerated holes from the CdS surface, resulting in the highly efficient separation of the photogenerated charges in CdS. Apart from the CdS modification, NiS/g-C3N4 composites have received attention. Hong et al. [12] reported deposition of NiS nanoparticles on the g-C3N4 nanosheets via hydrothermal treatment. Here, NiS can serve as an active co-catalyst site for the storage of photogenerated electrons from the g-C3N4. This NiS/g-C3N4 photocatalyst exhibited high H2 evolution rate (482 mmol h1 g1), exceeding 241-fold to the pure gC3N4. In another study, the Li’s group [73] reported the H2 evolution rate of the NiS/g-C3N4 composites as high as 992 mmol h1 g1 and Bahnemann’s group [74] reported 3429 mmol h1 g1 for NiS/g-C3N4 composites. The NiS/g-C3N4 composite with the highest H2 evolution rate was reported by Zhao et al. [75], which reached 16,400 mmol h1 g1, c. 2500-fold higher than pure g-C3N4. Recently, 2D MoS2 has become an ideal co-catalyst because of its low cost, robustness, and high activity [76]. MoS2 has a similar structure to graphite, and it possesses a layered crystal structure consisting of S-Mo-S “sandwiches” hanged together via van der Waals force. The fact that gC3N4 and MoS2 possess analogous layered structures should minimize the lattice mismatch and boost the planar growth of MoS2 slabs along with

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

g-C3N4 surface [77]. Hou et al. [78] reported the fabrication of earthabundant 2D/2D layered MoS2/g-C3N4 heterojunction through an impregnation sulfidation approach, which can raise the accessible area around the planar interface of the MoS2 and g-C3N4 layers and reduce the barriers for electron migration through the co-catalyst, thus further facilitating fast electron transfer across the interface by the electron tunneling effect. Moreover, the intrinsic band structures of g-C3N4 and MoS2 provided the possibility of the directional migration of the photogenerated electrons from g-C3N4 to MoS2. Thus, the photocatalytic HER property of g-C3N4 under visible light was considerably enhanced via growing hin-layered MoS2 on g-C3N4 surface (Fig. 9.12A). Other metal sulfide semiconductors have also been employed to modify g-C3N4. For example, Hou and co-workers [79] prepared 2D/2D WS2/gC3N4 heterojunction via loading thin-layered WS2 on g-C3N4 surface to form intact junctions to improve the electron tunneling effect. The prepared WS2/g-C3N4 displayed an improved photocatalytic property, and the H2 evolution rate reached the optimum when the loading amount of WS2 was about 0.3 at% (Fig. 9.12B). Jing et al. [80] reported synthesis of different types of SnS2 nanostructures coupled with g-C3N4 for stable visible light photocatalytic H2 generation (Fig. 9.12C). Hao et al. [81] reported an integrated design and synthesis of ZnS/g-C3N4 heterostructure with affluent zinc vacancy defects on ZnS surface to emphasize the synergistic promotion on charge separation (Fig. 9.12D). The prepared ZnS/g-C3N4 heterojunction displayed a high photocatalytic H2 evolution rate (713.68 mmol h1 g1), overtopping 30-fold to the pure g-C3N4. Hence, modification of g-C3N4 with metal sulfides semiconductor materials has greatly investigated for improved H2 production activities.

5.4 Metal organic framework/g-C3N4 composites for efficient hydrogen generation Metal organic framework (MOF) is a new type of crystalline organice inorganic complexes with uniform pore structures, high stability and tunable metrics, and one kind of potential low band gap materials [82]. Unlike many other materials, MOFs benefit from a high surface area, controllable pore size, and tunable light harvesting capacity [83]. Till date, much attention has been paid in integration of MOFs with g-C3N4 because of their structural and species diversities, controllable cavities and porosity, and high specific surface areas [84]. Yuan and co-workers [85] prepared a novel highly active composite photocatalyst UiO-66/g-C3N4

Figure 9.12 Photocatalytic H2 generation on metal sulfides/g-C3N4 composite. (A) MoS2/g-C3N4 act as co-catalyst for efficient charge carrier trapping for H2 generation. (B) WS2/g-C3N4 for photocatalytic H2 generation. (C) SnS2/g-C3N4 composite for H2 generation. (D) ZnS2/g-C3N4 heterojunction composite for photocatalytic H2 generation. (A) Reprinted with permission from Y. Hou, et al., Layered nanojunctions for hydrogen-evolution catalysis, Angewandte Chemie International Edition 52 (2013) 3621e3625. Copy right 2013 Wiley & Sons. (B) Reprinted with permission from Y., Hou, Y. Zhu, Y. Xu, X. Wang, Photocatalytic hydrogen production over carbon nitride loaded with WS2 as cocatalyst under visible light, Applied Catalysis B: Environmental 156e157 (2014) 122e127. Copy right 2014 Elsevier. (C) Reprinted with permission from L. Jing, et al., Different morphologies of SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation, ACS Sustainable Chemistry & Engineering 6 (2018) 5132e5141 . Copy right 2018 American Chemical Society. (D) Reprinted with permission from X. Hao, et al., Zn-vacancy mediated electron-hole separation in ZnS/g-C3N4 heterojunction for efficient visible-light photocatalytic hydrogen production, Applied Catalysis B: Environmental 229 (2018) 41e51. Copy right 2018 Elsevier.)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

by hybridizing Zr-containing MOFs (UiO-66) and g-C3N4 via a thermal annealing process. Through coupling UiO-66 with g-C3N4, the photocatalytic HER property was greatly improved. The optimal H2 evolution rate was up to 14,110 mmol h1 g1, and it is 17-folds over the pure g-C3N4 (800 mmol h1 g1). The improvement was because of the efficient interfacial charge migration from photoexcited g-C3N4 to UiO66. Besides, the high content of g-C3N4 endowed the UiO-66/g-C3N4 composites with the “quasi-polymeric nature,” which was different from the reported heterostructures. Thus, MOF/g-C3N3 combination is also an important choice of improving the photocatalytic performance.

5.5 Carbon-based/g-C3N4 composites for efficient hydrogen generation Carbon-based materials, such as carbon nanotubes, graphite, graphene, carbon quantum dots etc., have been showing growing attention because of their outstanding optical and electronic performances. When these types of materials coupled with g-C3N4, as effective electron-transfer materials to transfer the photogenerated electrons in g-C3N4 and reduce H2O to H2 because of their suitable LUMO position, which was lower than g-C3N4 but higher than the potential position of the normal hydrogen electrode according to theoretical calculations. Liu et al. [86] reported carbon quantum dots/g-C3N4 composites as photocatalyst for HER application. The as-developed carbon quantum dots/g-C3N4 composite photocatalyst exhibited high quantum efficiency (16%, for l ¼ 420  20 nm) and overall solar energy conversion efficiency (>2.0%). Moreover, the prepared CDs/ g-C3N4 composite photocatalyst maintained a high rate of H2 and O2 generation (for l > 420 nm) with moderate stability after 200 cycles and uses over 200 days. In addition, the introduction of multiwalled carbon nanotubes (MWCNTs) may change the g-C3N4 band gap and facilitate visible light harvesting, thus further improving photocatalytic performance. Wang and co-workers fabricated various MWCNTs/g-C3N4 composites via direct thermal polymerization of cyanamide with appropriate amounts of MWCNTs, which provided a good thermal stability for the obtained composites which showed an enhanced photocatalytic property when compared to the pure MWCNTs and g-C3N4. Also, Wu et al. [87] reported conductive carbon black/g-C3N4 composites with carbon black nanoparticles uniformly dispersed on g-C3N4 surface using simple one-step molten salt route. The carbon black nanoparticles acted as conductive pathways to effectively facilitate the separation and transfer of the

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315

photoinduced charge carriers, thus enhancing the photocatalytic property under visible light irradiation, and showed an improved photocatalytic activity 4.2-fold higher H2 evolution rate (689 mmol h1 g1) than the pristine g-C3N4 intercalation compound (163 mmol h1 g1). Finally, the most recent literature survey on various g-C3N4-based composite photocatalysts for HER application has been tabulated and summarized as the best combinations in Table 9.1. Here, the nonemetaldoped metal oxide modification (N-doped ZnO/g-C3N4) performed highest H2 generation rate of 18,836 mmol h1 g1cat and metal sulphideebased g-C3N4 (NiS/g-C3N4) also results in improved performance about 16,400 mmol h1 g1cat. Also, noble metalebased g-C3N4 (Au/g-C3N4) also performed accountable rate of H2 production of 8870 mmol h1 g1cat. But high cost of noble metals greatly limits development for practical applications. Thus, it was finally concluded that transition metalemodified g-C3N4 can be the best choice to improve the photocatalytic activity for practical applications.

6. Conclusion and outlook In summary, we studied that the g-C3N4-based composites have provided a great number of opportunities and challenges in the field of photocatalysis for clean energy production. We believe that this chapter will be able to provide comprehensive guide to reasonably design and develop novel g-C3N4-based photocatalysts with high activity, stability, visible light utilization efficiency, and low cost for visible light photocatalytic HER application. Among the various types of materials coupled up with g-C3N4, only few showed positive results and improved the photocatalytic activity in a countable range. For example, as summarized in Table 9.1, none metal-doped metal oxide modification (N-doped ZnO/g-C3N4) performed highest H2 generation rate of 18,836 mmol h1 g1cat and metal sulphideebased g-C3N4 (NiS/g-C3N4) also results in improved performance about 16,400 mmol h1 g1cat. In addition, noble metale based g-C3N4 (Au/g-C3N4) also performed accountable rate of H2 production of 8870 mmol h1 g1cat. But high cost of noble metals greatly limits development for practical applications. Thus, it was finally concluded that transition metalemodified g-C3N4 can be the best choice to improve the photocatalytic activity for practical applications. Although many attempts have been made with earth-abundant metals to improve the HER performance of g-C3N4, it was still low to reach the practical efficiencies.

Table 9.1 Comparison of highly efficient g-C3N4-based composite photocatalysts for hydrogen evolution reaction application.

References

4318 8870

[88] [62]

2014 995 326 960 1250 560 18,836

[89] [90] [91] [92] [93] [63] [67]

316

H2 evolution rate (mmol. h1 g1)

Photocatalyst

Metal/g-C3N4

Ni/g-C3N4 Au/g-C3N4

e e

Water:TEOA Water:TEOA

Ag/g-C3N4 Pd/g-C3N4 AuPd/g-C3N4 PtCo/g-C3N4 PdAg/g-C3N4 TiO2/g-C3N4 N-doped ZnO/g-C3N4 SnO2-ZnO/ g-C3N4 WO3/g-C3N4 Cu2O/ g-C3N4 CoO/g-C3N4 CeO2/ g-C3N4 CdS/g-C3N4 NiS/g-C3N4 MoS2/gC3N4 WS2/g-C3N4 ZnS/g-C3N4

e e

Water:TEOA Water:TEOA Water:TEOA Water:TEOA Water:TEOA Water:MeOH Water:Na2S þ Na2SO3

300 W: Xe (420 nm) 125 W visible light Hg 300 W: Xe (420 nm) 300 W: Xe (400 nm) 300 W: Xe (400 nm) 300 W: Xe (400 nm) 300 W: Xe (400 nm) 500 W: Xe (400 nm) 300 W: Xe (400 nm)

Water:glycerol

300 W: Xe (400 nm)

2735

[94]

Pt Pt

Water:TEOA Water:TEOA mixture

300 W: Xe (420 nm) 300 W: Xe (420 nm)

110 241

[95] [96]

Pt Pt

Water:TEOA Water:TEOA

300 W: Xe (400 nm) 300 W: Xe (420 nm)

651 1100

[97] [98]

Pt e e

Water:Na2S þ Na2SO3 Water:TEOA Water:lacticacid

350 W: Xe (420 nm) 300 W: Xe (400 nm) 300 W: Xe (420 nm)

5303 16,400 1030

[72] [75] [78]

e e

Water:lacticacid Water:Na2S þ Na2SO3

300 W: Xe (420 nm) 300 W: Xe (420 nm)

125 714

[79] [81]

Bimetal/g-C3N4 Metal oxide/ g-C3N4

Metal sulphide/ g-C3N4

e

Reaction medium

Light source

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Composite type

Cocatalyst

Metal phosphide/ g-C3N4 Carbon/g-C3N4

e Pt e

Water:TEOA Water:TEOA Water:TEOA

300 W: Xe (420 nm) 300 W: Xe (320 nm) 300 W: Xe (420 nm)

475 201.5 1643

[99] [100] [101]

Pt e Pt

Water:MeOH Water:TEOA Water:TEOA

350 W: Xe (400 nm) 300 W: Xe (420 nm) 300 W: Xe (420 nm)

451 715 2322

[102] [103] [104]

Pt

Water:TEOA

300 W: Xe (420 nm)

1760

[105]

Pt

Water:TEOA

300 W: Xe (420 nm)

470

[106]

Pt

Water:TEOA

350 W: Xe (420 nm)

1080

[107]

Pt

Water:oxalic acid

250 W: Xe (420 nm)

440

[108]

Pt

Water: ascorbic acid

300 W: Xe (420 nm)

109

[85]

e

Water:TEOA

300 W: Xe (420 nm)

905

[109]

e

Water:TEOA

350 W: Xe (420 nm)

152

[110]

Graphitic carbon nitrideebased nanocomposite materials

Other semiconductor/ g-C3N4

Ni2P/g-C3N4 CoP/g-C3N4 NiCoP/gC3N4 GO/g-C3N4 rGO/g-C3N4 Carbon quantum dots/ g-C3N4 Carbon nanodots/ g-C3N4 MWCNTs/ g-C3N4 Carbon fiber/ g-C3N4 SrTiO3/ g-C3N4 UiO-66/ g-C3N4 g-C3N4/ MIL-53(Fe) Ni(OH)2/ g-C3N4

317

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Thus, we strongly suggest premodification of g-C3N4 before coupling with transition metals and metal oxides will give good results which can be comparable with that of the practical efficiency. For example, premodification-like synthesis of porous (micro-, mesoporous) g-C3N4 can result in higher activities. Modifications of the porous g-C3N4 with transition metal/metal oxides can be the best choice of improving the HER performance of the mesoporous g-C3N4. Therefore, with more research findings in g-C3N4-based photocatalysts for HER application obtained from various fields, the bottleneck and issues will be fully addressed. There is no doubt that the emerging g-C3N4-based photocatalysts will be regarded as the “holy grail” for achieving highly efficient photocatalytic H2 generation under visible light in the near future.

Acknowledgments Authors acknowledge the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2019R111A3A01041454) and Ministry of New and Renewable Energy (MNRE), funded by Government of India (Grant Application No. 103/227/ 2014-NT).

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[85] R. Wang, et al., Quasi-polymeric metaleorganic framework UiO-66/g-C3N4 heterojunctions for enhanced photocatalytic hydrogen evolution under visible light irradiation, Advanced Materials Interfaces 2 (2015) 1500037. [86] J. Liu, et al., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway, Science 347 (2015) 970e974. [87] Z. Wu, H. Gao, S. Yan, Z. Zou, Synthesis of carbon black/carbon nitride intercalation compound composite for efficient hydrogen production, Dalton Transactions 43 (2014) 12013e12017. [88] L. Kong, et al., Light-assisted rapid preparation of a Ni/g-C3N4 magnetic composite for robust photocatalytic H2 evolution from water, Journal of Materials Chemistry A 4 (2016) 9998e10007. [89] J. Qin, et al., Improving the photocatalytic hydrogen production of Ag/g-C3N4 nanocomposites by dye-sensitization under visible light irradiation, Nanoscale 8 (2016) 2249e2259. [90] S. Bai, et al., Two-dimensional g-C3N4: an ideal platform for examining facet selectivity of metal co-catalysts in photocatalysis, Chemical Communications 50 (2014) 6094e6097. [91] C. Han, L. Wu, L. Ge, Y. Li, Z. Zhao, AuPd bimetallic nanoparticles decorated graphitic carbon nitride for highly efficient reduction of water to H2 under visible light irradiation, Carbon N. Y. 92 (2015) 31e40. [92] C. Han, et al., Novel PtCo alloy nanoparticle decorated 2D g-C3N4 nanosheets with enhanced photocatalytic activity for H2 evolution under visible light irradiation, Journal of Materials Chemistry A 3 (2015) 23274e23282. [93] I. Majeed, et al., PdeAg decorated g-C3N4 as an efficient photocatalyst for hydrogen production from water under direct solar light irradiation, Catalysis Science and Technology 8 (2018) 1183e1193. [94] S.V.P. Vattikuti, P.A.K. Reddy, J. Shim, C. Byon, Visible-light-Driven photocatalytic activity of SnO2eZnO quantum dots anchored on g-C3N4 nanosheets for photocatalytic pollutant degradation and H2 production, ACS Omega 3 (2018) 7587e7602. [95] H. Katsumata, Y. Tachi, T. Suzuki, S. Kaneco, Z-scheme photocatalytic hydrogen production over WO3/g-C3N4 composite photocatalysts, RSC Advances 4 (2014) 21405e21409. [96] J. Chen, et al., In-situ reduction synthesis of nano-sized Cu2O particles modifying g-C3N4 for enhanced photocatalytic hydrogen production, Applied Catalysis B: Environmental 152e153 (2014) 335e341. [97] Z. Mao, et al., Novel g-C3N4/CoO nanocomposites with significantly enhanced visible-light photocatalytic activity for H2 evolution, ACS Applied Materials and Interfaces 9 (2017) 12427e12435. [98] W. Zou, et al., Crystal-plane-dependent metal oxide-support interaction in CeO2/gC3N4 for photocatalytic hydrogen evolution, Applied Catalysis B: Environmental 238 (2018) 111e118. [99] D. Zeng, et al., Toward noble-metal-free visible-light-driven photocatalytic hydrogen evolution: monodisperse sube15nm Ni2P nanoparticles anchored on porous g-C3N4 nanosheets to engineer 0D-2D heterojunction interfaces, Applied Catalysis B: Environmental 221 (2018) 47e55. [100] X.-J. Sun, et al., ZIF-derived CoP as a cocatalyst for enhanced photocatalytic H2 production activity of g-C3N4, Sustainable Energy and Fuels 2 (2018) 1356e1361. [101] L. Bi, et al., Enhanced photocatalytic hydrogen evolution of NiCoP/g-C3N4 with improved separation efficiency and charge transfer efficiency, ChemSusChem 11 (2018) 276e284.

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[102] Q. Xiang, J. Yu, M. Jaroniec, Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites, Journal of Physical Chemistry C 115 (2011) 7355e7363. [103] J. Wan, et al., A facile dissolution strategy facilitated by H2SO4 to fabricate a 2D metal-free g-C3N4/rGO heterojunction for efficient photocatalytic H2 production, International Journal of Hydrogen Energy 43 (2018) 7007e7019. [104] K. Li, F.-Y. Su, W.-D. Zhang, Modification of g-C3N4 nanosheets by carbon quantum dots for highly efficient photocatalytic generation of hydrogen, Applications of Surface Science 375 (2016) 110e117. [105] Q. Liu, T. Chen, Y. Guo, Z. Zhang, X. Fang, Ultrathin g-C3N4 nanosheets coupled with carbon nanodots as 2D/0D composites for efficient photocatalytic H2 evolution, Applied Catalysis B: Environmental 193 (2016) 248e258. [106] Y. Chen, et al., Origin of the enhanced visible-light photocatalytic activity of CNT modified g-C3N4 for H2 production, Physical Chemistry Chemical Physics 16 (2014) 8106e8113. [107] J. Zhang, F. Huang, Enhanced visible light photocatalytic H2 production activity of g-C3N4 via carbon fiber, Applications of Surface Science 358 (2015) 287e295. [108] X. Xu, G. Liu, C. Randorn, J.T.S. Irvine, g-C3N4 coated SrTiO3 as an efficient photocatalyst for H2 production in aqueous solution under visible light irradiation, International Journal of Hydrogen Energy 36 (2011) 13501e13507. [109] C. Bai, et al., Fabrication of noble-metal-free g-C3N4-MIL-53(Fe) composite for enhanced photocatalytic H2-generation performance, Applied Organometallic Chemistry 32 (2018) e4597. [110] J. Yu, S. Wang, B. Cheng, Z. Lin, F. Huang, Noble metal-free Ni(OH)2eg-C3N4 composite photocatalyst with enhanced visible-light photocatalytic H2-production activity, Catalysis Science and Technology 3 (2013) 1782e1789. [111] S. Yang, et al., Exfoliated graphitic carbon nitride nanosheets as efficient catalysts for hydrogen evolution under visible light, Advanced Materials 25 (2013) 2452e2456.

CHAPTER 10

Nanostructured materials for photocatalytic energy conversion Tammineni Venkata Surendra1, Chandra Sekhar Espenti1, Saravana Vadivu Arunachalam2 1

Department of Chemistry, Rajeev Gandhi Memorial College of Engineering and Technology, Kurnool, Andhra Pradesh, India; 2Department of Chemistry, School of Advanced Sciences, Kalasalingam Academy of Research and Education, Virudhunagar, Tamil Nadu, India

1. Introduction In recent years, researchers focused on the study of energy resource as clean and renewable energy for the entire world. The fossil fuels are used in all fields of life for a long time and the combustion of fossil fuels produces dangerous CO and CO2. Burning of fossil fuels is not environmentally friendly and hence searching for an eco-friendly alternative source is most important [1]. Nowadays, environmental pollution and energy crises have become major problems because of the rapid economic and industrial development [2,3]. The hydrogen (H2) is considered as one of the major clean and renewable resources and it can play a significant role in various studies [4,5]. The production of hydrogen without releasing harmful products is very easy by photocatalytic water splitting [6]. The photochemical reaction is a chemical reaction used in the production of hydrogen induced by existing catalyst and irradiation source. Semiconductor materials such as TiO2, ZnO, Fe2O3, CdS, etc., were used as the catalyst for the photocatalytic reaction because of the long-time thermodynamic stability, strong oxidizing power, and nontoxicity, respectively. The three different important parameters such as absorption of photons prevent recombination and consuming of the excited electrons should contain for the photocatalyst [7]. The electrons easily transfer from valence band (VB) to conduction band (CB) by the energy of absorption photons. The electrons at CB prevent the recombination of electrons and holes by the acceptance of species [8]. The active sites are generated on the surface of the material because of the inhibition of recombination of photogenerated electrons and holes. This process can create the abilities to semiconductors to production of hydrogen from water splitting [9]. The various techniques involved in Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00010-5

© 2020 Elsevier Inc. All rights reserved.

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hydrogen production such as sunlight converting techniques and photolytic water splitting using light energy are discussed in this chapter.

2. Hydrogen production from sunlight converting techniques 2.1 Photovoltaic technology The photovoltaic technique is one of the important techniques to convert sunlight into electricity and this technique is viable for generating electrical power on a large scale. Collection of sunlight and inducement of charge separation for electronehole pair generations are the major steps in the generation of electricity from solar energy using photovoltaics. The charge separation can occur by semiconductors which are prepared by the silicon and these can exhibit causes the migration of excitons to a p-n junction. The process of charge separation creates the electromotive force which helps in supplying current. Mainly, photo-electricity to -electricity conversion processed when the p-n junction is electrically connected to an external load and this can generate power through the flow of current via load [10]. Also, electrical energy converts into some form of chemical energy like hydrogen fuel which can easily store and transport. Crystalline silicon is a good photomaterial which is majorly used in the photovoltaics but the process with silicon is very expensive. Alternately, the absorbance of sunlight was executed by the other techniques such as dye-sensitized solar cells and organic photovoltaics because of cheap material utilization [11]. The process of the electrical current generation and flow was depicted in Fig. 10.1.

Figure 10.1 Evaluation of photovoltaics.

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327

The thin-film photovoltaics have been generated to reduce the cost of expense in the preparation of photovoltaics. The silicon-based photoactive material consumption was reduced in thin-film photovoltaics to control the cost effect. Especially, thin-film solar cells fabricated from the utilization of cheap materials such as glass and plastics with the thickness range of 1e2 mm. The production of hydrogen from solar cells was improved by the fabrication of semiconductors with GaAs, CdTe-CuInSe2, and amorphous/ polycrystalline silicon [12].

2.2 Wet-chemical photosynthesis Furthermore, artificial or wet-chemical photosynthesis process was developed for the conversion of sunlight as hydrogen. This process was developed for generating efficient and stable energy conversion using earth-abundant substances. The natural biological systems generate the photosynthetic process for the generation of solar fuels by chemical reactions. The process of photosynthesis is a part of the wet-chemical process which is useful for the transformation of carbon dioxide into organic compounds and chemical fuels such as hydrogen from sunlight [13]. The process of generation of hydrogen from wet-chemical or artificial photosynthesis is shown in Fig. 10.2.

Figure 10.2 Generation of hydrogen by wet-chemical photosynthesis.

3. Photoelectrolysis for the generation of hydrogen by TiO2 nanohybrid The photoelectrolysis is the process of the combination of both photovoltaics and wet-chemical photosynthesis. The photoelectrolysis is an important process to produce chemical fuel such as hydrogen by water splitting through solar energy. In this process, the electrochemical process is

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assisted for generating chemical energy from sunlight. This process is adopted by the utilization of photoelectrode and it helps in the conversion of the sunlight energy by water splitting. Hydrogen is the major fuel playing an important role in the process of green energy development and because it is in the form of clean energy, it is utilized in the fuel cells [14]. Generally, steam reforming, which makes consumption of fossil fuel for production of carbon dioxide process, produces the hydrogen primarily. Moreover, photoelectrolysis is the process mainly generates the hydrogen without realizing any by-products by splitting of water. However, the yield of hydrogen production from this process is less than photovoltaics due to the low performance of photoelectrodes [15]. In the 1970s, Honda and Fujishima carried out the water splitting technique by the photoelectrochemical cell which contains a photoanode and a photocathode [16]. The cathodic and anodic reactions occur in the process of water splitting by using TiO2 as photoelectrode. TiO2 acts as a semiconductor, and it can generate the electron and holes when TiO2 is irradiated by UV illumination. The electrode potential of TiO2 maintains the process of production of hydrogen by photogenerated electrons and holes which can oxidize the water to form hydrogen [17]. The natural sunlight considered as visible light can generate the hydrogen by the irradiation of TiO2 photoelectrode, in addition to it being more feasible than the UV light. The recent research completely focused on the photoelectrodes, which can be irradiated under visible light for water splitting by the anodicecathodic reactions in the photoelectrochemical cell [18]. Also, it was recognized as best practice for the generation of hydrogen under visible irradiation. Especially, rapid charge transfer at the semiconductor interface, long-time stability, and production of a wide range of solar spectrum efficiently are the major requirements for the photochemical cells [19]. Accordingly, photoelectrochemical cells were prepared with the p-type and n-type semiconductors which possess various band gaps and surface-bound electrocatalysts. Nanomaterials contain properties such as band structural modification, quantum dot sensitization, plasmonic association, and the domination of crystal facets which are predominant factors for the photoelectric cells [20]. Fig. 10.3 explains the mechanism of photoelectrolysis process for the splitting of water molecule and hydrogen generation.

4. Evaluation of hydrogen by photoelectrochemical activity using nanomaterials The conversion of one molecule of H2O into H2 and 1/2 O2 results in the change in free energy (DG) as 237.2 kJ mol
1. The electrochemical process

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329

Figure 10.3 Generation of hydrogen by photoelectrolysis.

which makes water splitting is carried by two electrodes such as anode and cathode. The semiconductor material used for the preparation of photoactive electrode undergoes irradiation by light for the water splitting [21]. The photocathode cannot undergo irradiation by light as it acts as counter electrode. The light can be absorbed by the photoactive material which is coated on the anode and makes the reaction faster in the form of splitting the water molecule. The process of irradiation of anode by the light can produce an electrode potential more than 1.23 V. This potential is useful for the oxidation of water molecule as O2 and protons followed by the H2 at the cathode. This process mainly depends on the photoactive materials prepared by the metal and metal oxide, which are also called as nanomaterials or nanohybrids. However, using the metal and metal oxide nanomaterials on photoanode creates the more energy band gap, which makes the electrons excite from VB to CB. The holes will remain in the VB and photogenerated electrons pass to the cathode through the external wire [22]. The electrons react with protons at the cathode for the generation of H2, and holes at photoanode produce the O2 by oxidation of H2O (Fig. 10.4). Mainly, some semiconductor materials cannot oxidize the water molecule but can reduce the protons, whereas some semiconductors such as Fe2O3 easily oxidize the water molecule but cannot reduce the protons. Reduction of protons is very important in the generation of hydrogen; therefore, reduction of protons using semiconductors can increase by applying external bias. Moreover, applying external bias helps in the creation of photocurrent in the photochemical cell. The semiconductor materials are similar photocatalytic material for the generation of hydrogen. The RuO2 and Pt nanohybrids are considered as effective photocatalytic agents in the evolution of hydrogen. The main factor in the selection of photocatalyst is electrochemical stability or resistance of photocorrosion and most metals are thermodynamically

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Figure 10.4 Schematic electrochemical activity.

representation

of

hydrogen

evaluation

by

photo-

unstable. The unstable materials oxidized the photogenerated holes themselves instead of water. The pH plays a major role in undesired photodecomposition process in photoactive materials. Accordingly, TiO2 and SnO2 nanomaterials are favorable photoactive materials due to the high stability over various factors such as pH values in an aqueous environment and oxygen stoichiometry [23]. In the other hands, electron transfer prevented through the interface between photocatalysts in some nonoxide semiconductors because of the formation of oxide films in its own surface. Hence, the development of photocatalyst with more stability on photocorrosion is very important in hydrogen generation [24].

5. Carbon nanotube/TiO2 nanocomposite for hydrogen production Abdulrazza and Hussein in 2018 suggested the sonochemical/hydratione dehydration technique for the synthesis of single-walled carbon nanotube/ TiO2 (SWCNT/TiO2) and multiwalled carbon nanotube/TiO2 (MWCNT/ TiO2) nanocomposites [25]. The two nanocomposites were fabricated under the ultrasonication for 7 h by the dispersion of TiO2 on the SWCNTs and MWCNTs. The two nanocomposites were further washed and dried at 45 C using vacuum evaporator and hot air oven for 12 h at 90 C. Formation of both

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SWCNT/TiO2 and MWCNT/TiO2 nanocomposites was further analyzed by X-ray and Raman spectroscopy for the confirmation of morphology and identification of metals in the fabricated composites. The size of the composites was determined using Scherrer’s equation, and the size of nanocomposites was found to decrease gradually while increasing carbon nanotube (CNT) ratios [7]. Accordingly, the size of SWCNT/TiO2 nanocomposite was less than that of MWCNT/TiO2 nanocomposite and is influenced by a number of layers in MWCNT/TiO2 [26]. The hydrogen was produced using xenon lamp (Osram XBO) with a 1000-Watt UV-B light at the 240e1000 nm wavelength as the source of energy and light source was switched on 15 min before irradiation. However, light source gets stability for the lamp at 40 mW cm
2 intensity. The quartz disc-equipped double-jacket reactor vessel was arranged for light penetration. Also, argon gas was purged throughout the suspension before irradiation. The reactor cooled down to room temperature using Land Nds. Uni Hancooler system. The evaluated hydrogen was analyzed using gas chromatography (Shimadzu GC e 8A) equipped with a thermal conductivity detector (TCD) and a Carboxen 1000-packed column. The hydrogen production from photocatalytic reactions was carried out using 65 mg of SWCNT/TiO2 and MWCNT/TiO2 nanocomposites. The schematic representation for hydrogen production system was depicted in Fig. 10.5. Mainly, TiO2 without any CNT did not show any activity on

Figure 10.5 Schematic representation of hydrogen production system.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

producing hydrogen under dark or illuminated conditions. The hydrogen production mainly depends on the electron transfer from TiO2 to CNTs [27]. The binary matrix contains active site, it can stimulate under UV light, and it can help in the movement of electrons from TiO2 surface to CNTs graphitic network by excitation. The synergistic effect between TiO2 and CNTs increases photocatalytic activity. The composites create N-P type heterojunction which acts as an ideal bridge for withdrawing and transferring the electrons from TiO2 to CNT network.

6. Induced photocatalysis over Fe2O3 for production of hydrogen from water splitting Gondal et al. [28] in 2004 reported the production of hydrogen and oxygen from water splitting by laser-induced photocatalysis over Fe2O3. For the first time, the splitting of water as hydrogen and oxygen was progressed under irradiation of string laser beam at 355 nm by the pure Fe2O3 and electron capture agents such as Fe3þ, Agþ, Alþ3, and Liþ. The evaluation of hydrogen was carried out with the help of laser photocatalytic reactor made of a special Pyrex cell with 35 mm diameter and 120 mm length [29e31]. The optical grade quartz windows were fabricated in the reactor for transmission of UV and visible laser beam. The cell set in the reactor contained three ports and a rubber septum for sampling. The authors prepared a solution of Fe2O3 by dissolving 0.3 g of Fe2O3 in 60 mL of water. The electron capturing agents were prepared by dissolving the stoichiometric amounts of metal nitrates in 1000 cm3 of double distilled water and these metal solutions were considered as solvents. The production of H2 and O2 was measured by gas chromatography and TCD in the presence of argon. The evolved H2 and O2 gases were analyzed by removing 100 mL of the gas sample from the dead volume of the photocatalytic reactor at regular intervals. The oxygen meter was utilized for the optimization of parameters of water splitting [32]. The rate of H2 and O2 production was measured by the data collected from the gas chromatograph. The schematic illustration to produce H2 and O2 by laser-induced photocatalytic water splitting was demonstrated in Fig. 10.6. The incident laser energy and concentration of the catalyst such as Fe2O3 plays a major role in the production of hydrogen. The evaluation of the hydrogen strongly depended on the incident photon flux. Mainly, increase in H2 and O2 evaluation was observed at 50e100 mJ laser energy

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333

Figure 10.6 Laser-induced photocatalytic water splitting for H2 and O2 production.

and it was increased to 500% at 100 mJ laser energy. Furthermore, the yield of those gases remained constant at 100e200 mJ laser energy. The huge increase in H2 and O2 yields at 100 mJ laser energy is due to the creation of increase in electronehole pairs with incident laser photon flux. It plays a major role in the water splitting process [33,34]. The generation of electronehole pairs increases with increasing incident laser energy at 100e200 mJ, but the production of H2 and O2 remains constant because of the recombination of the electronehole process. The catalyst concentration can affect the production of H2 and O2. The increase in catalyst results from the increase in H2 and O2 yields. According to their reduction potentials, the electron capturing groups such as Fe3þ, Agþ, Alþ3, and Liþ were used as electronehole recombination inhibitors in the production of H2 and O2 by the photocatalytic process. The reducing potentials of these ions are more positive than the potentials of the semiconductors, and these can act as electron scavengers; hence, these are called as electron capturing agents [28]. The ions which contain negative reduction potentials cannot capture the electrons of conduction band directly and these cannot act as electron capturing agents [35]. The effect of Fe3þ and Agþ on the yield of H2 and O2 depending on the suitability of the reduction potentiality of conduction band electron capture processes of the ions. The regenerative behavior of the Fe3þ/Fe2þ couple is through the reaction resulting in the sustained increase in the production of H2 and O2.

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Fe3þ þ ecb
/Fe2þ

(10.1)

Fe3þ þ hþ /Fe3þ

(10.2)

The plausible mechanism for the H2 and O2 production over Fe2O3 by induced laser photocatalytic effect is given below: a
Fe2 O3 þ hyðlaserÞ/a
Fe2 O3 ðecb
; hvb
Þ

(10.3)

hvbþ þH2 OðadsÞ / H2 Oþ /HO$ þ Hþ

(10.4)

HO• þ Hþ /H2 O2 /H2 O þ 1=2O2

(10.5)

ecb
þ Hþ /H•

(10.6)

•/H2

H• þ H •

þ

(10.7)

HO þ H /H2 O ðrecombinationÞ

(10.8)

H2 þ 1=2 O2 /H2 O ðrecombinationÞ

(10.9)

7. Evolution of hydrogen from water photocatalytic splitting using graphene/TiO2 In 2010, Zhang et al. [36] studied the evolution of hydrogen from photocatalytic water splitting using fabricated graphene/TiO2 nanocomposite. The solegel method was utilized for the fabrication of a series of TiO2 and graphene sheets (GSs) from the tetrabutyl titanite (TBT) and graphite oxide (GO) precursors. The synthesis of TiO2/GSs nanocomposites was synthesized in different steps using the starting materials. Initially, GO was prepared by Hummer’s method using natural graphite. Furthermore, GSs were made up from synthesized 0.117 g of GO by dissolving into 200 mL of ethanol with sonication for 30 min [37]. The reductant sodium borohydride was used as a reducing agent to form GSs from GO at room temperature. Mainly, the TiO2/GS nanocomposites were synthesized by the sonication of GSs in 50 mL of ethanol followed by the dropwise addition of 10 mL of TBT. The reaction mixture was stirred mechanically for 2 days and extended for another 2 days by the addition of 5 mL acetic acid glacial and 2 mL deionized water. Furthermore, the sol was obtained from precursor and dried for 10 h at 80 C. Finally, the TiO2/ GS nanocomposites were prepared from the precursor of TiO2/GSs by thermal annealing in the nitrogen atmosphere at 450 C for 2 h [38,39].

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The obtained photocatalysts samples were calculated in nitrogen atmospheres with different weight percentages of TiO2/GSs-N2. The fabricated TiO2/GS nanocomposites were characterized using various analytical experiments such as X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, diffuse reflectance spectroscopy, and UV-vis spectroscopy. The synthesized TiO2/GS nanocomposites were further used for water photocatalytic splitting under UV-vis light by calcination in air. The photocatalytic effect of TiO2/GS nanocomposites initially increased with increasing GS content and then decreased. Accordingly, the evolution of hydrogen was enhanced to cal. 17.2 mmol within 2 h with 8.6 mmol h
1 by the TiO2/5wt%/GSs. The enhanced hydrogen with 5 wt% of GSs is higher than that of P25 (4.5 mmol h
1). The excellent thermal conductivity and large surface areas of GSs were the major reasons for the increase in the hydrogen evolution by photocatalytic effect [40,41]. These specifications of GSs were useful for the easy transportation of generated electrons to the surface of composites. Hence, the recombination of holes and electrons can inhibit. Afterward, according to the obtained results, the photocatalytic effect in the evolution of hydrogen was decreased because of the increase in GSs more than 10 wt%. The more content of GSs in the composite may lead to the recombination of generated electronehole pairs. Hence, optimal concentration of GSs should be considered for the preparation of TiO2/GS nanocomposites. The authors suggested that 5 wt% of GSs is a suitable concentration for the fabrication of TiO2/GS nanocomposites and for enhancement of the photocatalytic effect on hydrogen evolution. The photocatalytic hydrogen evolution also depends on the effect of the calcination atmosphere. Experimentally, the photocatalytic hydrogen evolution using nanocomposites was studied under the calcination of air and nitrogen atmospheres. As a result, TiO2/GSs-N2, which means calcination under nitrogen atmosphere, shows higher photocatalytic activity than the TiO2/GSs-air. Mainly, oxygen vacancies existed during the calcination process and these oxygen vacancies can trap the electrons. Thus, calcination under nitrogen atmosphere favors to oxygen in the lattice of TiO2, leading to the formation of oxygen vacancies, whereas calcination under air leads to reaction with free carbon, which is available on the surface of TiO2 with oxygen in the air [42]. Hence, the photocatalytic effect in hydrogen evolution from water splitting is enhanced by the calcination under nitrogen atmosphere than that of calcination under air. The authors demonstrated that

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graphene, as well as a nitrogen atmosphere, is the very promising candidate for the development of high-performance photocatalysts [42].

8. Visible light photocatalytic hydrogen production by Ti3C2 MXene cocatalyst with metal sulfide Ran J et al. [43] 2017 reported the cocatalyst of Ti3C2 MXene (titanium carbide) on metal sulfide for enhancing photocatalytic hydrogen production under the visible light. The photocatalytic water splitting and the production of sustainable solar hydrogen require highly active and stable earth-abundant cocatalyst. Authors synthesized the Ti3C2-incorporated CdS (cadmium sulfide) material. Initially, Ti3C2-E was obtained from the Ti3AlC2 in HF solution through the ultrasonication process with deionized water. The Ti3C2 nanoparticles were incorporated in CdS by the hydrothermal process and fabricated it as Ti3C2/CdS nanocomposites. The synthesized nanocomposites further investigated the various characterization techniques for the confirmation of Ti3C2/CdS nanocomposites formation. Moreover, the synthesized material was used as a photocatalytic agent under the visible light for enhancing hydrogen evolution. The photocatalytic hydrogen production was carried out at room temperature and atmospheric pressure in Pyrex flask which contains opening seals and silicon rubber septum. The photocatalytic reaction processes under the visible light made up of xenon arc lamp (300 W) with an ultraviolet cutoff filter. The synthesized Ti3C2/CdS (20 mL) photocatalyst was mixed with 80 mL of an aqueous solution which contains 20 mL of lactic acid under constant stirring. The impurities and dissolved air present in the reaction mixture were suspended by purged reaction mixture under argon for 30 min. The purging of the reaction mixture under argon makes reaction mixture stable in anaerobic condition [44]. The hydrogen content was measured using gas chromatography. The Ti3C2 MXene as a highly active photocatalytic agent and of low cost promotes cocatalyst easily for production of hydrogen. The highly active cocatalyst can show two important characteristics such as rapid extraction of photoinduced electrons from a photocatalyst on its surface and catalysis of the hydrogen evolution on its surface by using photoinduced electrons [45]. Generally, the hydrogen evolution reaction (HER) depends on the three important state diagrams such as initial state Hþ þ e
, an intermediate absorbed H*, and final product 1/2 H2. The major indicator of HER activity of photocatalysts is an intermediate state of the Gibbs free energy DGH*. Also, all

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photocatalysts show the DGH* value HER activity as zero [46]. In addition, the active cocatalyst should contain appropriate electronic bands and excellent electrical conductivity to extract the photoinduced electrons and to deliver them on its surface. The aforementioned characteristics of Ti3C2 appear in the Ti3C2 cocatalyst and hence it was used as an active photocatalyst for the evolution of hydrogen [47]. The mechanism involved in the production of hydrogen through the photocatalytic effect of Ti3C2/CdS was explained below. Firstly, the authors prepared different cocatalysts such as Ti3C2-E, Ti3C2-5000, and Ti3C2 NPs and determined the surface areas of respected samples. The CdS was added in the same concentration under the identical thermal condition to all three nanomaterials and determined surface area respectively [48]. The addition of the CdS to Ti3C2-E, Ti3C2-5000, and Ti3C2 NPs enhanced their surface areas and increased the photocatalytic activities. The enhancing of photocatalytic activities may be because of the smaller size and larger number of the active site of Ti3C2 and it makes stronger coupling with CdS. In other hand, functionalities also influenced the photocatalytic activity of cocatalyst Ti3C2/CdS [49]. The immobilized positive charges are available in the VB (valance band) of CdS and the space charge layer was formed at the interface of Ti3C2/CdS. Also, the covalent band (CB) and VB were bent upward in the CdS. The Schottky junction is formed because of the VB and CB upward between the CdS and Ti3C2. The valence electrons available in the VB exited to the CB because of the irradiation under the visible light at lmax 420 nm [50]. The decrease in the space charge layer thickness in nanosized CdS also reduces the “upward” bending of the CB and VB for CdS. Thus, photoinduced electrons in the CB migrate to the upward of the VB and reach to the Fermi level of Ti3C2. The Schottky junction can easily capture the photoinduced electrons without impeding the electron transfer from CdS and Ti3C2 [51]. The transferred photoinduced electrons to Ti3C2 can bind tightly to its surface because of the excellent metallic conductivity of Ti3C2. Finally, the HER capacity of Ti3C2 provides the ability of photoinduced electrons to reduce protons of aqueous solutions efficiently. This overall process leads to the evolution of hydrogen in the presence of visible light using the photocatalytic effect of Ti3C2/CdS nanocomposites. The plausible mechanism for hydrogen production using Ti3C2/CdS nanocomposites under visible light was depicted in Fig. 10.7.

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Figure 10.7 Visible lighteenhanced photocatalytic production of hydrogen from Ti3C2/CdS.

9. Hydrogen gas for transportation and sustainable power generation Nabil and Dawood in 2019 developed an application of water produced oxyhydrogen gas (OHG) for the generation of power with cleaner production. Initially, the OHG was produced by the electrolytic dissociation process of water molecules. Generally, the OHG contains various characteristics such as high flammability and burning velocity. Hence, the OHG was used as an additive for the generation of power by generators with environmental quality. The OHG generators should prepare for the generation of power and are generally made up of metal plates and stainless steel for good electrical and thermal conductivity. Moreover, the plates of the generator are connected to the DC source with positive and negative terminals. Two plates connected to cathodes and anodes were forming a closed compartment with the aid of good sealing material such as rubber gasket. Correspondingly, the existence of water and gas can occur because of the fixing of rubber gasket inside of the generator. The water level equalizes the small holes of plates and permits electron flow. This process creates small friction under the voltage drop and it leads to heat generation. In the other hand, the plates contain large holes at the upper side for the flowing of gas to vent. These holes are useful for the contact of water and gas by increasing the surface area. The produced OHG was applied in two different types of engines, i.e., 150 and 1300 CC, and the results gave very good reduction in consumption of fuel and the reduction ranges recorded as 14.8% and 16.3%. The reducing capacity of CO is noticed, that is, 33% and 24.5% in each engine; finally, the reduction percentage of hydrocarbons is listed as 27.4% and 21% in the exhaust gases temperature for 150 and 1300 CC engines, respectively [52]. The different oxyhydrogen generators were depicted in Fig. 10.8.

Nanostructured materials for photocatalytic energy conversion

Figure 10.8 Various oxyhydrogen gas generators with construction materials. (Reproduced from Nabil and Dawood in 2019 Elsevier 2019.)

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10. Tungsten-doped Ni-Zn nanoferrites for the recovery time for hydrogen gas sensing application Noble metals such as palladium (Pd) and platinum (Pt) are best examples for sensing applications because these types of metals reduce the adsorption activation energy and hence improve the sensor performance. For the hydrogen gas sensing application, Many researchers were focused on the development of Ni-Zn nanoferrites with doping of tungsten. In this process, authors used a series of tungsten-doped Ni-Zn nanoferrites which were prepared by penetration of metal oxide pellet-type resistive sensor for the gas sensor application which are selective for the H2 gas. In the process of fabrication of nanocomposite materials, authors used the conducting polymer polyvinyl alcohol, which improves the sensing nature of the nanomaterials or nanocomposites. Researchers tested these composite ferrites between 80 and 300 C at 1000 mg L
1 of H2 gas and they measured that the huge change in the resistance was observed even at low temperatures also [53].

11. Summary In summary, in this chapter, the authors have provided complete details about the fabrication of semiconductors which can possess various applications. The semiconductors are gaining more importance in research toward photocatalytic driving of hydrogen production by water splitting. The photocatalysis is the important energy conversion technique as sunlight is converted into chemical energy. The authors mainly focused on the novel semiconductor materials used in the photocatalytic water splitting for hydrogen production. The main reason behind the hydrogen production is the property of semiconductors to inhibit the recombination of valence electron pair and holes. The mechanism involved the photocatalytic water splitting to produce hydrogen by semiconductors was explained in detail. In future prospects, these nanocomposite materials are applied in the hydrogen sensors used in the wastewater treatment. These fabricated nanosemiconductors are used to decompose the organic dyes from the various industrial effluents.

Acknowledgments Dr. T.V. Surendra and Dr. Chandra Sekhar Espenti express gratitude to RGM College of Engineering and Technology, Nandyal, for giving opportunity to continue research in good manner. Dr. S.V. Arunachalam is grateful to the Department of Chemistry, School of Advanced Sciences, Kalasalingam Academy of Research and Education, Tamil Nadu, for providing facilities to continue the upgraded research work.

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CHAPTER 11

Graphene-based composite materials for flexible supercapacitors A. Gowrisankar1, T. Saravanakumar2, T. Selvaraju1 1

Department of Chemistry, Bharathiar University, Coimbatore, Tamil Nadu, India; 2Department of Nanoscience and Technology, Anna University Regional Campus, Coimbatore, Tamil Nadu, India

1. Introduction Energy is an essential ingredient for human life on earth. Presently, there is a huge demand for energy in terms of domestic and industrial operations. Since industrial revolution, in around 1760, the major proposition of energy is derived from a dense, energy-rich resource collectively known as fossil fuels [1]. It is a nonrenewable resource which requires billions of years to be formed from the remains of living organisms but is being used up at a faster rate and cannot be replenished, i.e., remains exhausted [2]. Also, burning fossil fuels produces excessive greenhouse gas emission and induces man-made climatic changes. Relying on these resources for energy generation is therefore unsustainable, thereby needing to find more renewable and sustainable ways of generating energy. Renewable energy resources (RES) are the sources of power that quickly replenish and generate clean energy without the release of greenhouse gas emissions and can be used frequently. In contrast, the nonrenewable resources such as coal, natural gas, petrochemicals, and nuclear energy possess wide limitations. In last two years, the statistics refers that the energy generations from RES are very fast, economic, and inexhaustible. In addition, the International Renewable Energy Agency (IRENA) has announced that approximately half a million jobs were created in the last year and a year-on-year rise of 5.3%, reaching a total head count of 10.3 million worldwide (Fig. 11.1A) [3]. Moreover, the RES have become a pillar of the global energy system, which in turn grows global economy. As a result, RES could be a suitable power source to meet the energy demand because of its low maintenance and operational features [4e6].

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00011-7

© 2020 Elsevier Inc. All rights reserved.

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Figure 11.1 (A) Growing number of jobs created in the renewable energy resources sector. (B) Global supercapacitor market 2014e25. (A) IRENA jobs database; (B) IDTechEx report.)

2. Energy storage devices: an overview In the coin, the one side is to promote the key technologies in the development of renewable energy resources, and the other side is the keen development in the field of energy storages. In recent days, efficient, portable, and miniaturized devices’ power requirements are in demand. As a result, tremendous efforts have been made in the development of lithium-ion batteries and supercapacitors (SCs). SCs are also called as electrochemical capacitors or ultracapacitors, where an energy storage device lies between secondary batteries and traditional capacitors. This is due to the longer cycle life (>10,000 cycles), fast dynamics of charge

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propagation, better environmental friendliness, and maintenance than secondary batteries. SCs have gained much attention over the past decades [7e11]. As per IDTechEx report, the global SC market is expected to reach 8.3 billion dollars by 2025 (Fig. 11.1B) [12]. According to the energy storage mechanism, there are three types of SCs: electrical doublelayer capacitors (EDLCs), pseudocapacitors (PCs), and hybrid capacitors. The EDLC stores energy on the electrode surface via fast charge adsorption/desorption, while the PCs store energy through surface faradaic reactions between the electrolyte ions and electrode materials. Carbon materials, including activated carbon, carbon nanotubes (CNTs), and graphene, have been extensively used in the fabrication of EDLC electrodes. Fig. 11.2 shows the schematic representation of the charge storage mechanism of EDLC-type SCs. Promising pseudocapacitive materials include transition metal oxides, chalcogenides, hydroxides, and conductive polymers [13]. In comparison, PC dominates over EDLC because of its high faradaic constant. On the other hand, low life cycles are observed for PCs. It is surpassed by hybrid capacitor, which means the combination of the EDLC and the PC where combination of metal oxide and carbon or utilization of lithium-based metal oxide has succeeded the limitations into advantageous in the improvement of life cycle and in the power storages. In the current electronic era, most of the devices are simplified depending on the requirements. Furthermore, the fundamental aspects in the simplification of device materials are long life, good mechanical strength, portability, and compactness. These are the major standards incorporated in the design of flexible SCs [14,15].

Figure 11.2 Schematic representation of the charge storage mechanism in electrical double-layer capacitorebased supercapacitors.

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3. Flexible supercapacitors: device structure and fabrication Because of the popularization of commercialized pocket electronic devices, such as wearable or foldable electronics, electronic papers, mobile electronic devices, and smart products, the modern world is strongly demanding flexible, inexpensive, lightweight, and environment-friendly energy storage devices [16]. In comparison, the flexible SCs could be a promising candidate because of its superior physical flexibility that is highly required [17]. Thus, the free-standing or self-standing flexible electrode materials show an excellent volumetric capacitance, power, and energy density of portable SC devices without any current collectors, conductive additives, and binder materials [18]. Furthermore, the configuration of flexible electrode materials is directly synthesized on substrate surface, which results in the increase of energy density, whereas the conventional SCs require additional binders, resulting in low energy density and mechanical durability [19]. The fabrication steps of flexible SC electrode devices are summarized in Fig. 11.3 [20]. Firstly, the electrode material needs to be coated on the flexible or free-standing substrate such as graphene sheets, stainless steel, nickel foam, and carbon fiber paper. Secondly, the polymer gel electrolytes such as PVA/H2SO4, PVA/H3PO4, and PVA/KOH are dropped on top of the electrode material surface or dipping the electrode materialecoated substrate into the polymer gel electrolyte. These polymer gel electrolytes possess high ionic conductivity, wider range of operating voltage, and superior flexibility and act as an ion-permeable membrane and absorbent electrolyte for maintaining the wettability. Moreover, solid gel electrolytes provide supporting structure to allow deformation test without suffering an unwanted short circuit between the electrodes [21]. Furthermore, the semisolid gel electrolyte offers easy ion transport in minimal distance between the electrode/electrolyte/electrode interfaces to enhance the charge storage properties [22,23]. As a result, the structural architecture of the as-prepared flexible SC is made of lightweight device with more simplified to moveable electronics.

4. Graphene composite materialsebased flexible supercapacitor devices To date, numerous works have been demonstrated in the preparation and fabrication of flexible supercapacitor devices (FSDs) based on carbon

Graphene-based composite materials for flexible supercapacitors

Figure 11.3 Schematic representation of the fabrication of flexible supercapacitor.

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allotropes, especially zero-dimensional (0D) and one-dimensional (1D) CNTs, two-dimensional (2D) graphene, three-dimensional (3D) graphite, and carbon-based composite materials, etc. Among the carbon allotropes, CNT and graphene-based electrode materials are the most widely used in the SC field because of its simple processing, nontoxicity, and high chemical stability [24]. Owing to its unique 2D structure, graphene, a 2D flat monolayer of sp2-hybridized carbon bonded in a hexagonal lattice, is the parent of all the graphitic carbons [25e27]. The 0D fullerene, 1D CNT, and 3D graphite or diamond can be formed by wrapping, rolling, and stacking of a graphene sheet, respectively [28]. Moreover, graphenebased composites materials are suitable electrode materials because of its eminent physical properties such as good mechanical strength (w1 TPa), high electrical conductivity (w106 S cm
1), high specific surface area (2630 m2 g
1), as well as good chemical stability, thermal conductivity (5300 W m
1 K
1), and mechanical flexibility for FSDs [29e32]. Besides, the graphene based SC device delivers a theoretical electrical double-layer capacitance of w550 Fg
1, which remains as higher capacitance than all other carbon materials [33]. On the other hand, metal oxides/hydroxides essentially hold a high charge storage capacity when compared with carbon-based materials because of an enhanced faradaic reaction at the electrode/electrolyte interface for enriching the electrochemical energy storages and at the same time with the limitations such as lack of cyclic stability and being quite expensive in nature. For instance, Fe-doped Sr(OH)2 [34], ZnCo2O4 [35], CoMoO4 [36], and CuSbSe2 [37], etc., are flexible electrode materials displaying promising electrochemical performances toward energy storage property with the above-mentioned limitations. Alternatively, Al, Ti, Cu, and Ni foams are effectively used as metallic substrate to construct the flexible and bendable electrodes with good cycling stability. Besides, stainless steel is one among the flexible electrode substrates which has excellent conductivity for direct fabrication of the electroactive materials. But the metal-supported flexible electrodes are less transparent and less stretchable in nature. As a result, flexible plastic substrates are tried to be used in the touch screen displays but are limited because of its low electrical conductivity. Thus, the significant parameters in the construction of high-performance FSDs are the kind of materials and fabrication methodologies. Indeed, graphene has great advantages with respect to flexibility and versatility in regulating structural and compositional properties where synergistic effect between the graphene layers and nanostructures of transition metal oxides/hydroxides with

Graphene-based composite materials for flexible supercapacitors

351

multiple redox states or any conductive additives would enrich the electrochemical properties and improve the flexible properties. The 3D nanostructure of the highly porous graphene serves as a backbone in the infiltration of the electrolyte ions into the electrode materials. In flexible SCs, the highly conducting and flexible carbon substrate or metal substrate serves as both electrode material and current collector [38,39]. Different fabrication techniques such as electropolymerization or electrodeposition, chemical vapor deposition (CVD), filtration, printing, evaporation, direct coating, and dip coating are used to form carbon networks by van der Waals interaction or hydrogen bonding at graphene or graphene sheet surface [20]. Fig. 11.4 displays graphene composite materialsebased electrodes coated using different techniques and attempted to fabricate flexible devices [40e50].

4.1 Pure graphene-based flexible electrode materials for electrical double-layer capacitor Control of structure and morphology are the two key parameters for carbon-based electrodes for effective utilization of electrolyte ions in EDLCs [51]. At this juncture, a direct fabrication of pure graphene or reduced multilayer graphene oxide (RMGO) filmsebased flexible electrodes are discussed using pristine prepared from CVD method or from chemical reduction of graphene oxide (GO) using organic reducing agents [52] where the GO films are obtained by layer-by-layer (LBL) assembly. The 2D in-plane individually designed two identical flexible graphene electrodes or RMGO-coated electrodes are separated by acidic polymer gel electrolyte (PVA/H2SO4). Thus, a combined flexible device can be designed with ultrathin, flexible, and optically transparent features. Moreover, the electrochemical performances derived from cyclic voltammetric (CV) curves showed that pure graphene has delivered a normalized capacitance of 80 mF cm
2. In the case of stacked geometry, part of the regions is inaccessible to the electrolyte ions, and thereby the electrochemical surface area remains partially utilized. Alternatively, the capacitance of the RMGO-based FSDs show five times higher (390 mF
2) capacitance. It is because of the presence of multiple interlayers in the RMGO, which facilitates counterions intercalation. Meanwhile, the technique involving direct laser reduction of graphite oxide films to graphene was introduced elsewhere [43]. Primarily, the 2D in-plane geometry offers higher electrochemical surface area for accessing electrolyte ions. As a result, the possibilities of miniaturization of the device

352

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 11.4 Graphene composite materials based flexible supercapacitors.

Graphene-based composite materials for flexible supercapacitors

353

thickness could be achieved. The schematic illustration of the stacked geometry, 2D in-plane geometry design, and the detailed depiction of the fabrication of SC device are shown in Fig. 11.5. Firstly, the dispersed GO solution is coated on flexible substrate and irradiated the GO film with infrared laser inside an inexpensive commercially available LightScribe CD/DVD optical drive. The resulting laser-scribed GO film (LSG) showed an excellent conductivity (1738 S/m) as opposed to 10e100 S/m for activated carbons. Also, LSG shows superior mechanical strength with w1% change in electrical resistivity after 1000 bending cycles. In addition, the exfoliated LSG sheet can produce a stacked layer structure of LSG and would prevent the agglomeration effectively. Therefore, the counterion intercalation and deintercalation process do easily access the electrode’s surface. It is firmly possible to choose LS gas flexible EDLC-based SC without the need of any binders or conductive additives. Here, the fabricated LSG device showed perfect rectangular CV curves over a wide range of scan rates (100e10,000 mV/s) in 1 M H2SO4 electrolyte. The LSG device possessed an areal capacitance of 4.04 mF cm
2 at 1 Ag
1 and maintained capacitance of 1.84 mF cm
2 even after high current density at

Figure 11.5 Schematic depiction of the stacked geometry (A), 2D in-plane geometry design for the fabrication of supercapacitor devices (B), and fabrication of laser-scribed graphene (LSG)ebased electrochemical capacitors (C).

354

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

1000 Ag
1. Furthermore, the fabricated LSG device would deliver an excellent cycling stability (96.5%) of its initial capacitance after 10,000 charge discharge cycles. As a result, the LSG device exhibits high power and energy densities compared to commercially available activated carbon, aluminum electrolytic capacitors, and a lithium thin-film battery. The fabricated LSG device can be easily extended to flexible thin-film SCs, which holds various promising structural and hybrid designs for highpower miniaturized devices. On the other hand, a vertically oriented “graphene forest (GF)” as flexible SC electrode material has been investigated [53] where the GF electrodes are fabricated using plasma-enhanced CVD process. The morphological images shown in Fig. 11.6AeC confirmed the vertically grown GF structures. Generally, the close stacking of reduced graphene oxide (rGO) sheets can be exfoliated by inserting any spacers such as carbon black nanostructures, carbon spheres, or CNT or using ultrasonication. The main advantage of vertical graphene grown by plasma deposition allowed easy access of electrolyte ions without any spacers. In addition, concentrated nitric acid is used, as dopant induces the defects in the vertically grown graphene which is confirmed by the increase of ID/IG ratio in Raman spectroscopy (Fig. 11.6D). In the absence of any binders, the GF electrode fabrications are succeeded directly on quartz substrate using PVA solution. Then, the sandwich arrangement of two identical GF/PVA//PVA/GF electrodes is formed and shown in Fig. 11.6E, where copper foil is used as the current collector to enhance the performance of capacitor. To ascertain the use of current collector and positive effects of doping, four different types of capacitors are discussed: pristine GF-EDLC (PC), doped GF-EDLC (DC), PC with current collector (PC-C), and DC with current collector (DC-C) with the areal capacitance obtained from the charge discharge curve as 1.67, 2.21, 2.30, and 2.45 mF cm
2, respectively. It is understood that the doped capacitor with current collector shows slightly higher capacitance. However, the capacitance values of DC-C are fairly close to that of DC capacitors, i.e., 2.21 mF cm
2 for DC versus 2.45 mF cm
2 for DC-C. These findings conclude that GF-EDLCebased capacitors do not need any of current collectors and show that the advantageous effect of doping is sufficient for capacitors. Besides, the deformation tests are performed for the DC and DC-C capacitors. Upon repeated bending, the device shows a significant loss in the capacitance, which is a critical factor in the power supply of any wearable device. From Fig. 11.6F, the DC-C exhibits an improved

Graphene-based composite materials for flexible supercapacitors

355

Figure 11.6 Morphological images of graphene forest (GF) (AeC), Raman spectra of GF before and after HNO3 doping (D), and schematic illustration of fabrication of flexible GF electrode and that of electrical double-layer capacitor (E). Flexibility tests of DC and DC-C: Capacitance with respect to the bending radius (F), by the times of bending at a bending radius of 10 mm (G), and the bending limitation studies of DC-C capacitors (H, I).

356

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

capacitance for the bending radii of 12.95, 11.13, and 7.59 mm. Furthermore, it is to state that no loss in the capacitance up to 60,000 bending and an extreme reduction in the capacitance were observed after 80,000 bending of DC capacitors at bending radius of 10 mm (Fig. 11.6G). Moreover, the DC-C capacitor shows no decay in the capacitance even after 100,000 times of bending at bending radius of 10 mm. This outstanding bending stability is attributed by the nature of GF in which each graphene stands its own rather than connected by network of GF. In addition, the folding test is also performed to investigate the bending limitations of DC-C capacitors which show no further loss in the capacitance during the charging and discharging process (Fig. 11.6H). The bending and flattening study of DC-C capacitor was fitted in the finger joint of a glove to understand the dynamic operation situation. Both the folding and bending tests show excellent flexibility of the GF-based EDLC without any marginal decrease in the capacitance for DC-C (Fig. 11.6I). Table 11.1 summarizes the comparison of various graphene-based flexible electrode materials.

4.2 Graphene with conducting additives as composite material for flexible supercapacitor devices Graphene-conducting polymer has received great interest [54,55]. A chemically converted graphene (CCG) and polyaniline nanofibers (PANI-NFs) were prepared using Hummer’s method and interfacial polymerization method, respectively. The stable dispersion of graphene sheets offers more electrochemical surface area rather than aggregated graphene sheets. The prepared PANI-NFs possess positively charged in the emeraldine salt form. On the other hand, CCG shows negatively charge which is due to the existence of residual carboxylic groups. As a result, the positively charged PANI-NFs form a stable composite dispersion with negatively charged CCG by electrostatic interaction. Thus, the basic form of CCG and the acidic form of PANI-NF combine to generate a high salt concentration. To get a stable dispersion, the PANI-NFs is dialyzed to remove the excess unreacted ions and subsequently mixed with CCG colloid to minimize aggregation. As a result, stable mixture of dispersed composite CCG/(PANI-NFs þ CCG) (rg) in ammonia solution (pH ¼ 9) is achieved. The mechanical property of the G-PNFs is mainly dependent on the CCG weight content in the mixed dispersion. Suppose the rg level is too lower than 20%, the prepared composite would be fragile in nature. To obtain a high-quality composite film, the rg level has to be increased above 30% (labeled as G-CNF30). As rg level is

Table 11.1 Summary of the reported graphene compositeebased flexible supercapacitor devices.

S.No.

Electrode material and technique

Current collector

Specific capacitance
1

Current density
1

Electrolyte

Mass loading (cm2)

References

1 M KCl

e

[67]

237 Fg

e

372 Fg
1

0.4 mgcm
2

1M Na2SO4

e

[68]

Ni foam

1.42 Fcm
2

2 mVs
1

0.4 mg

[69]

4

Graphene/carbon black hybrid film (vacuum filtration method)

112 Fg
1

5 mVs
1

1.5 mg

[50]

5

rGO/Single wall carbon nanotubes (spray coating technique) rGO fibers (Scalable nonliquid crystal spinning method) MnO2 nanowire/graphene hybrid fibers (wet-spinning method) Carbon nanofibers@PPy@ graphene film (electrospun and electrochemical method)

Au-coated polyethylene terephthalate (PET) film Au-coated PET film

0.5 M Na2SO4, pH ¼ 10 PVA/ H2SO4

42.2 F/g

1 Ag
1

PVA/ H3PO4

e

[70]

e

185 Fg
1

0.2 Ag
1

PVA/ H2SO4

e

[71]

e

66.1 Fcm
3

60 mAcm
3

PVA/ H3PO4

e

[72]

Aluminum foil

836.8 mFcm
2

2 mVs
1

PVA/ H3PO4

e

[73]

2

3

6

7

8

1 Ag

357

e

Graphene-based composite materials for flexible supercapacitors

Graphene/PPy film (pulsed electrodeposition) Graphene/MnO2/CNT (vacuum filtration technique) 3D graphene/MnO2 (chemical vapor deposition)

1

Continued

9 10

11 12

3D graphene/graphite paper (chemical vapor deposition) Carbon nanofibers yarn@ PPy@graphene (electrospun and electrochemical method) Graphene/PANI composite Anthraquinone/graphene composite (hydrothermal method and vacuum filtration method)

Current collector

Specific capacitance
1

Current density

References

PVA/ H2SO4 PVA/ H3PO4

e

[74]

e

[75]

PVA/ H2SO4 PVA/ H2SO4

e

[76]

0.5 mg

[77]

Electrolyte
2

e

260 Fg

Copper foil

336.2 Fg
1

2 mV/s

Stainless steel fabric Nylon 66 filter paper

1506.6 mFcm
2

6 mAcm
2

122.7 mFcm
2

0.1 mAcm
2

15.6 mFcm

Mass loading (cm2)

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

S.No.

Electrode material and technique

358

Table 11.1 Summary of the reported graphene compositeebased flexible supercapacitor devices.dcont'd

Graphene-based composite materials for flexible supercapacitors

359

increased above 40%, the composite film shrunk dramatically because of the aggregation of CCG. Fig. 11.7 shows the uniform sandwich arrangement of PANI-NFs between CCG layers at G-PNFs composite-based flexible SC. Furthermore, the overall mechanical strength and conductivity properties (5.5  102 S m
1) have been greatly improved in the composite material than those of the CCG or pure PANI-NFs films. Moreover, the assembled symmetric SC made of CCG, PANI-NFs, or G-CNF30 as two electrodes separated using filter paper soaked in 1 M H2SO4 electrolyte where flexible carbon-based material is used as current collector. Thus, the components are assembled into a layer structure, sandwiched between two plastic sheets. The obtained symmetric SC device has delivered the specific capacitance of 210 Fg
1 at 0.3 Ag
1 current density. The calculated capacitance retention is 94% (197 Fg
1) maintained as the discharging current density increased from 0.3 to 3 Ag
1. The G-CNF30 thin film shows the improved electrochemical double-layer capacitance by forming porous structure of PANI-NF with CCG colloidal dispersion, which is comparatively better than those of pure CCG or PANI-NFsebased electrode materials. Thus, G-CNF30 thin film plays a dynamic role in the fabrication of FSDs such as roll-up displays, LED screens, and so on. Polypyrrole (PPy) is considered as a promising conductive additive to enhance the electrical, mechanical, and flexible property of the electrode material. Combination of PPy with carbon-based material would enhance the cyclic stability and increase the mechanical flexibility [56]. As a result, a free-standing 3D PPy@rGO hydrogel (PPy@rGOH) prepared by the combined hydrothermal treatment of GO to assemble hydrogel followed by in situ electropolymerization of PPy on the surface of graphene [57] are discussed. For comparison, PPy@rGOH-x films are prepared by varying the time of electropolymerization process (x ¼ 0, 10, 20, and 30 s). The interconnected porous structure of the graphene and presence of PPy can be clearly seen in the scanning electron microscopy images shown in Fig. 11.8AeD. At this juncture, PPy is served as separate material to avert the aggregation of graphene layers. Upon increasing the time for electropolymerization, the excessive PPy would block the interconnected graphene structure. In addition, BET studies have confirmed that the PPy@rGOH-20s composite has larger surface area of 231.2 m2 g
1 than other composites. Furthermore, PPy@rGOH-20s composite allows PPy to intercalate the graphene layer, which effectively suppresses the stacking of graphene layers and increases the transportation of electrolyte ions.

360 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 11.7 Cross-sectional scanning electron microscopy image of G-PNF30 composite film (A, B), pure layered structure of chemically converted graphene (C), PANI-NF (D), and digital photograph of a flexible G-PNF30 film (E).

(A)

(B)

(C)

(D)

87.1%

80

flat

60 40

bent

twist

5.0

j /Ag-1

Capacitance Retention (%)

(F)

100

2.5 0.0 -2.5

20

-5.5 0.0

0

0.4

0.8

E/ V

1.2

1.6

0.0

0.4

0.8

E/ V

1.2

2000

1.6

0.0

4000

0.4

0.8

E/ V

1.2

1.6

6000

Cycle numbers Figure 11.8 Scanning electron microscopy images of PPy@rGOH-0s (A), PPy@rGOH-10s (B), PPy@rGOH-20s (C), and PPy@rGOH-30s (D). Influence of mechanical bending on cycling stability (E) and the corresponding inset shows CV curves at a constant scan rate of 40 mV s
1 under different states. Red LED powered by flexible asymmetric supercapacitor device photograph (F).

Graphene-based composite materials for flexible supercapacitors

(E)

361

362

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

On the contrary, long electropolymerization time (>20 s) leads to aggregation because of the presence of excessive PPy on the surface of graphene. Moreover, the electrochemical measurements show that the PPy@rGOH-20s composite electrode shows flexibility, conductivity, and high electrochemical stability compared with other electrodes prepared at 0, 10, and 30 s of electropolymerization process. Interestingly, PPy@rGOH-20s exhibits an excellent specific capacitance of 340 Fg
1 at a current density of 1 Ag
1 and superior cycling stability of 87.4% capacitance retention after 10,000 cycles in 1 M KNO3 electrolyte. Noticeably, tuning the time corresponding to PPy electropolymerization on graphene gels has adversely affected the electrochemical performance of the composites. To construct a flexible asymmetric supercapacitor (FASC), rGOH is employed as a negative electrode and PPy@rGOH-20s as a positive electrode with a filter paper immersed in PVA/KNO3 gel electrolyte for 30 min. Finally, the assembled FASC is solidified at room temperature and examined by its electrochemical properties. The observed CV curves show a perfect quasi-rectangular shape at low scan rate. From charge discharge curve, the calculated specific capacitance is 131.8 Fg
1 at a current density of 1 Ag
1 and exhibits remarkable energy density of 46.9 Whkg
1 at a power density of 0.8 kW kg
1. Moreover, the FASC device has indicated an excellent cyclic stability as 94.57% after 10,000 cycles at 4 Ag
1. The practical bendability studies of various states in CV are shown in Fig. 11.8E. Even after 6000 cycles in all the three states (flat, bent, and twist), still the initial capacitance remains as 87.1%, which confirms the high mechanical flexibility of as-fabricated FASC. As a result, the electrochemical property of PPy@rGOH-20s is mainly because of the synergistic effect between PPy and rGOH, which provides a higher interconnected porous network to insertion/extraction of electrolyte ions. Also, it is a free-standing electrode and does not require any binders. For the practical applications, the red LED bulb powered by two devices in series has connected for 30 s (Fig. 11.8F). As a result, it is possible to conclude that the PPy@rGOH-20s composite electrode could be one of the ideal candidates for flexible SC applications. Besides, many graphene composites with conducting polymers including poly(3,4ethylenedioxythiophene) [58,59], poly(3-(4-fluorophenyl)thiophene), and poly(3-methyl thiophene) have been widely investigated in the field of flexible SC applications [60].

Graphene-based composite materials for flexible supercapacitors

363

4.3 Graphene with metal oxides as composite material for flexible supercapacitor devices Commonly, the metal oxideebased SCs exhibit a large energy and power densities. Transition metal oxides such as RuO2, NiO, Co3O4, Mo2O3, V2O5, and MnO2 possess multiple oxidation states that enable rich redox reactions and exhibit higher capacitance performances in SC applications [61]. Furthermore, hybridizing graphene with metal oxides has delivered high electrochemical performances because of good electronic conductivity and mechanical stability with improvement in the flexibility and cyclic stability [62]. A stretchable, planar-type hybrid nanostructure of d-MnO2/graphene nanosheet composite material has been developed for the potential applications of wearable and portable electronic devices. Briefly, the 2D nanostructure of d-MnO2/graphene prepared by adopting vacuum filtration technique has produced controllable thickness films. Initially, the as-prepared hybrid thin film is rolled up with glass rod on the flexible polyethylene terephthalate substrate. Then, by mechanical pressing, hybrid films are perfectly coated with the help of cellulose acetate membrane followed by scraping the dashed line as shown in Fig. 11.9A, where the array of parallel lines of thin films is produced. Furthermore, two columns of gold current collectors are placed on either side and thermally evaporated. Later, PVA/H3PO4 polymer gel electrolyte is filled between two working electrodes and, finally, the constructed 2D planar SC device is successfully explored in electrochemical measurements. The as-fabricated flexible device has delivered an excellent specific capacitance of 267 Fg
1 at current density of 0.2 Ag
1 and also exhibited good rate capability and cyclic stability with good capacitance retention of 92% after 7000 cycles. More importantly, a slight difference in specific capacitance of the planar SC is observed when tested under different bending states (Fig. 11.9B). Notably, the device has no structural destruction while performing bendability test. The digital photographs of flatted, folded, and rolled devices are shown in Fig. 11.9CeE. Moreover, the availability of Mn atoms on the graphene sheets serves as the chemical active sites for the redox reaction during charge/discharge process. Therefore, the bendability tests of the planar SC device do not affect its capacitance. As a result, the planar-based SC device can be a promising direction for building high-performance flexible SC electrode materials [63]. Besides, graphene would be used to fabricate flexible and binder-free

364 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 11.9 Schematic representation of flexible planar supercapacitors fabrication (A), currentevoltage curves under bendability test (B), and demonstration of the different bending states (flat [C], fold [D], and rolled [E]).

Graphene-based composite materials for flexible supercapacitors

365

SC electrodes [64]. In addition, binder-free and self-standing MoS2/rGO papers such as composites have been prepared by vacuum filtration technique. Recently, MoS2 earned widespread attention because of their high conductivity and specific capacity where MoS2 nanospheres are obtained by adopting hydrothermal method with the help of ZnS nanoballs as templates [65,66]. Afterward, MoS2/rGO paper-like composite would be achieved without any binder and relatively exhibits a good specific area of 585.4 m2 g
1. The density functional theory studies have confirmed that the as-prepared composite consists of micropores (1e2 nm) and mesopores (2.5e9 nm), which provides sufficient space for energy storage. Moreover, MoS2 nanospheres are located on the rGO surface or among the rGO nanosheets to form layer-by-layer structure through van der Waals interactions between eS and eOH, eCOOH functional groups of GO. The fabricated symmetrical devices such as rGO paper device, flat MoS2/rGO paper device, and bended MoS2/rGO paper device are shown in Fig. 11.10AeE. To test the efficiency, the rGO device possesses poor discharge specific capacitance of 90 Fg
1, and the calculated columbic efficiency of rGO paper is 67%. After insertion of MoS2 nanospheres into rGO nanosheets, the flat MoS2/rGO paper device showed an enhanced specific capacitance of 323 Fg
1 at 0.2 Ag
1 current density. In addition, 72.6% of columbic efficiency has been achieved for flat MoS2/rGO paper device. With the bending MoS2/rGO paper device, the specific capacitance is slightly decreased (227 Fg
1 at 0.2 Ag
1) because of the lack of contact between electrode material and separator. As a result, 12% of columbic efficiency has decreased as compared to flat MoS2/rGO paper device. From the Ragone plot shown in Fig. 11.10F, the maximum energy density of the flat device with self-standing electrode is observed as 44.9 Wh kg
1 at 0.2 Ag
1 corresponding to the power density of 401 W kg
1 [66]. The energy and power density of the flat MoS2/rGO exceeds in comparison to the other SC devices such as pure MoS2 [78], RuO2/rGO//RuO2/rGO [79], Fe2O3/rGO//Fe2O3/rGO [80], MoS2/WS2/rGO//rGO asymmetric device [81], and MoS2/rGO//rGO asymmetric device [82]. As a result, a flexible paper-like MoS2/rGO electrode material could be one of the lightweight, wearable, and flexible material for practical SC application.

366 Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 11.10 The digital photographs of reduced graphene oxide (rGO) paper (A), MoS2/rGO paper (B), cross-sectional view of fieldemission scanning electron microscope image of MoS2/rGO paper (C), digital photographs of bended (D) and flat device (E), and the comparative Ragone plots of symmetric devices (F).

Graphene-based composite materials for flexible supercapacitors

367

5. Concluding remarks and future perspectives Flexible and wearable SC devices are one of the major challenges and essential innovations in the current research. Despite the impressive progress made in the electrochemical performances of pure graphene or its composites with different conducing polymers and metal oxides, still challenges are yet to be addressed in the enhancement of electrochemical stability and commercialization of the graphene-based flexible smart devices as flexible and wearable supercapacitors (FWSCs). Numerous efforts have taken to improve the flexibility property of graphene with or without flexible substrates. As a result, solid gel electrolytes with good mechanical flexibility are urgently needed to overcome the barrier in the fabrication of FWSCs. Furthermore, the energy and power densities are still need to be enhanced for the real-time applications of GO-based devices. At present, researchers are aiming to develop a new freestanding, cost-effective, and binder-free graphene nanocomposite-based electrode material to excel in the domain of ultimate electrochemical performances in flexible energy storage devices. Although an enormous number of complications and tasks exist, the graphene composites would play a significant role in the development of flexible SCs. Tuning the dopant properties and enhancing the interlayer distance of graphene sheets with conducting polymers or metal oxides/hydroxides nanostructures or metal chalcogenides is a potential way to produce high power density SCs. On the other hand, the technical parameters such as bending, twisting, and rolling properties of the electrode material are to be addressed to retain the electrochemical capacitances. It can be overcome by designing fascinating nanostructures and well-regulated porosity of functionalized in-planar graphene sheets, 3D graphene hydrogels composites, generating flexible graphene nanofibers by electrospun method, or the combination of two methods could significantly improve the flexibility of the SC devices. The graphenebased flexible devices made of organic and inorganic composite materials have wider perspectives in the commercialization and open many avenues for job opportunities in near future.

References [1] A.K. Chandrakar, Energy Resources: Indian Scenario An Assignment On, 2015. [2] N.K. Sharma, N. Tyagi, V. Rana, S., Photovoltaic, renewable energy scenario in India, A Current Status 14 (2019) 150e154.

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CHAPTER 12

Present status of biomassderived carbon-based composites for supercapacitor application Shrabani De, Sourav Acharya, Sumanta Sahoo, Ganesh Chandra Nayak Department of Chemistry, IIT(ISM) Dhanbad, Dhanbad, Jharkhand, India

1. Introduction Supercapacitors (SCs) are those elite classes of electrochemical energy storage (EES) systems, which have the ability to solve the future energy crisis and reduce the pollution [1e10]. Rapid depletion of crude oil, natural gas, and coal enforced the scientists to think about alternating renewable energy sources. On the other hand, the use of fossil fuels causes enormous environmental pollution. In this circumstance, the energy sources such as solar, wind, biomass, and hydropower have been considered as the valuable assists of energy supply. However, the applications of these energy sources are limited because of the low conversion rate, high cost, and minute energy output. The next generation of these renewable energy sources needs to enhance these properties. However, the integration of these renewable energy sources to the active power systems demands EES devices with high energy and power densities. Among different EES systems, SCs and batteries are the most popular ones. However, SCs are generally considered the bridge between the batteries and capacitors. Even these two EES systems can be assembled to develop hybrid electric vehicles with zero emission, which is an excellent solution for one of the major problems of today’s world, i.e., global warming. However, with respect to the charge storage mechanism, these two storage devices are working differently. In case of batteries, the charge storage process is chemical and it has high energy density but limited power density. But, SCs are renowned for its high power density. Moreover, the charge storage mechanism is also found to be different from the batteries. Based on the charge storage mechanism, Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00012-9

© 2020 Elsevier Inc. All rights reserved.

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SCs are classified into main two classesdelectric double-layer capacitor (EDLC) and pseudocapacitor. However, a combination of these two produces hybrid capacitor/composite capacitor. A detailed discussion on the charge storage mechanism, classification of SCs, and the SC electrodes is included in the next section of the book chapter. In general, SC cells have three componentsdelectrode, electrolyte, and separator. Among these, electrodes are the most important one. The intrinsic properties of the electrodes significantly control the performance of the SCs. From the early stage of SCs, carbon-based materials have been frequently used as the electrodes. However, with drastic development in SC technology, the fabrication of carbon-based SC electrodes from different natural resources has drawn huge attention of the researches because of its cost-effectiveness. In this regard, biomass-derived carbon materials are considered as one of the major components of advanced SC electrodes. In general, biomass materials are broadly explored as a source of advanced carbons because of their high abundance, renewable nature, low cost, and eco-friendly properties. This chapter will focus on the utilization of different biomass-derived precursors for the synthesis of carbonaceous materials to design advanced SCs. The carbon materials have been successfully synthesized from various biomass resources including municipal solid waste, sewage, animal residues, industrial residues, forestry crops and residues, agricultural crops and residues, etc. Mainly, this chapter deals with the “waste to wealth” approach for the development of advanced SCs.

2. Fundamentals of supercapacitor: an overview As discussed in the previous section, SCs fulfill the gap between the simple capacitor and batteries. These three EES devices have some similarities as well as some differences. The capacitors are the simplest energy storage devices. The better versions of capacitors are the SCs. But, like capacitors, SCs do not use dielectric materials. When the dielectric medium was replaced by the ionic conductive medium (i.e., electrolyte), the capacitors were called as the electrolytic capacitors. The resulting capacitance was found to be in mF for physical capacitors and in mF for electrolytic capacitors. But, the capacitance of SCs is found to be in F range, which is far better than the capacitors. SCs are generally consisted of two electrodes, separated by ion-permeable separators and electrolyte (connects the electrodes through ion transfer). The EDLC stores charges through the formation of Helmholtz double layer at the electrodeeelectrolyte interface;

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on the other hand, pseudocapacitors store charges by the Faradaic charge transfer through redox reactions. Like SCs, batteries are also configured by two electrodes separated by electrolyte. However, batteries usually store chemical energy inside the electroactive materials and then convert the chemical energy into unswerving current electricity through either redox reactions or intercalation (in case of Li-ion batteries). The hybrid between the batteries and SCs forms hybrid energy storage systems, which demonstrates the intermediate electrochemical properties between the EDLC and Faradaic processes. The schematic representation of the charge storage mechanism for these four types of energy storage systems is shown in Fig. 12.1AeD.

3. Carbon materials for supercapacitor electrodes Various biomass-derived carbons have been utilized as SC electrodes including activated carbon, porous materials, aerogel or hydrogel, graphene, carbon nanotube (CNT), and fullerene. The limiting factors such as high surface area and favorable pore size of electrode materials for a specific electrolyte control the efficiency in terms of energy density and power

Figure 12.1 Different electrochemical energy storage systems: schematic representation of (A) electric double-layer capacitor (EDLC), (B) a battery, (C) a pseudocapacitor, and (D) a hybrid system that comprises an EDLC and an intercalative charge storage process. (Copyright from A. Noori, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi, Towards establishing standard performance metrics for batteries, supercapacitors and beyond, Chemical Society Reviews 48(5) (2019) 1272e1341.)

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density of SC. Porous materials facilitate charge storage and transport of ions while high surface area is useful to store energy on the surface. Large surface area and availability of carbonaceous materials in different forms and dimensions make them an attractive option in SC application. The surface chemistry of carbon can easily be tuned and they have excellent conductivity, good stability, high surface area, low cost, favorable pore size distribution, and efficient compatibility with various materials in forming composites [11,12].

3.1 Activated carbon Activated carbon (AC) is the most extensively studied carbon material with highly porous structure, large surface area, light weight, and excellent electrical conductivity for SC electrodes. After carbonization process, the carbon material usually becomes less porous and has low surface area. To enhance the surface area, activation process is carried out in inert atmosphere at high temperature around 600e800
C. Activated carbon is mainly prepared through activation of carbon either physically or chemically [13]. To ensure the complete development of porosity of activated carbon, activation process including oxidizing gases such as carbon dioxide, oxygen, or steam is required along with the heat supply. The characteristics of activated carbon mainly depend on the types of precursor taken for activation. Along with the raw materials used for the synthesis process, some other parameters including temperature, conditions applied, and time span of heat treatment are also very crucial for the chemical and physical properties of activated carbon. The characteristics such as pore size distribution, surface area, and pore volume are very much important in SC application. Precursors are opted in such a way that the activated carbon possesses tunable morphology as well as high carbon content for the application in SC electrode [14]. Having excellent stability against electrolyte, activated carbon can store energy through electrochemical double-layer formation which leads to the generation of high power density. Besides having highly porous structure and a large surface area, activated carbons can store high amount of charge on its surface. Krummacher and his coworkers proposed different solvents including 2-methylglutaronitrile (2-MGN), 3-cyano-propionic acid methyl ester (CPAME), and adiponitrile (ADN) to replace acetonitrile as efficient and new electrolyte [15]. It allowed the generation of EDLCs above 3 V potential as well as sufficient cyclic stability at room temperature. Fig. 12.2

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Figure 12.2 Comparative study of efficiency, resistance, capacitance, and impedance spectra of electric double-layer capacitors with new electrolytes (with Et4NBF4 as supporting electrolyte) at 3.1 V during float test at room temperature. 2-MGN, 2-methylglutaronitrile; ACN, acetonitrile; ADN, adiponitrile; CPAME, 3-cyano-propionic acid methyl ester. (Copyright from J. Krummacher, C. Schütter, L. Hess, A. Balducci, Non-aqueous electrolytes for electrochemical capacitors, Current Opinion in Electrochemistry 9 (2018) 64e69.)

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depicts the EDLC behavior with CPAME electrolyte with enhanced performance at a potential 3.1 V after 480 h floating test. Also, the extended stability of SCs is confirmed from electrochemical impedance spectra. On the other hand, pore size and distribution of pores are closely related to the characteristics of organic electrolyte including size of cations, anions, and solventeion interaction.

3.2 Porous carbon Porous carbons (PCs) are versatile materials with significant industrial applications. The major properties of PCs are huge surface area and hierarchical porosity. Pores can be categorized into three classes such as macropores, mesopores, and micropores. According to IUPAC, macroporous materials have pore diameter larger than 50 nm, mesoporous materials have pore diameter smaller than 50 nm and higher than 2 nm, and microporous materials have pore diameter lower than 2 nm [16]. For SC application, different types of pores present in a material should be interconnected to each other for the ion diffusion. PCs can be synthesized by different strategies such as hard template methods, soft template methods, and template-free methods. A variety of biomass has been used as precursor for PC synthesis such as plant-derived, animal-derived, fungi, and sewage sludge materials. The charge storage mechanism in PC materials can be explained by three directions such as pore width and capacitance relationship, effect of surface functionalization on the inner wall of micropores, and influence of pore distribution on overall performance. Recently, Forse et al. used in situ diffusion nuclear magnetic resonance spectroscopy to study ioneelectrode interaction [17]. They established the fact that the pores with larger than 2 nm diameter are capable in ion diffusion and exhibited excellent rate capability performance. Zhang et al. reported a high energy density and large capacitance SC electrode using PC derived from Chinese date [18]. The PC consisted of multilevel pores and self-doped with oxygen and nitrogen exhibited ultrahigh energy density of 51.3 Wh/kg and high specific capacitance of 518 F/g at 0.5 A/g.

3.3 Carbon aerogel/carbon hydrogel Aerogels are a special class of materials having foam-like structure with interconnected pores. Kistler first introduced the aerogels where the liquid part of the gel was replaced by air [19]. He exhibited that the liquid phase

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can be entirely removed at a certain critical point without interfering the structure. Among large variety of aerogels, carbon-based (CNT, graphene, and nanodiamond) aerogels having high porosity, huge surface area, good permeability, and high electrical conductivity become an efficient candidate for electrode material in SC applications. On the other hand, hydrogels are the water-based gel having three-dimensional (3D) cross-linked polymer network, can easily consume water and swell up without dissolving. They have the properties like tunable mechanical, thermodynamic, transport properties, ionic conductivity, electrical conductivity, and flexibility which meet the versatile approach toward the application in energy field [20]. For example, cellulose having a porous structure acts as an efficient material when introduced with graphene aerogel. Zhang et al. reported the specific capacitance of cellulose graphene aerogel of 300 F/g at a scan rate of 5 mV/s [21].

3.4 Graphene Graphene is a two-dimensional (2D) carbon material with sp2-hybridized densely packed carbon atoms. It is a one-layer thick hexagonal structure that has appeared as a widely studied novel class of materials with excellent potential for SC applications [22e24]. Associating with a unique combination of properties such as superior electron mobility (2.53  105 cm2/Vs) [25], highly adjustable nanostructure, electrical conductivity (w106 S/m) [26], corrosion resistance in aqueous electrolyte, excellent thermal conductivity (5000 W/m.K) [27], huge theoretical specific surface area (SSA) above 2630 m2/g [28], and large cyclic stability, the application of graphene as an excellent SC electrode has emerged as the focus of scientific research in the field of energy storage devices [29]. In 2004, a stable monolayer graphene was first synthesized using mechanical stripping process, and the researchers were awarded the Nobel Prize in Physics for this discovery in 2010 [30]. Single-layer graphene holds honeycomb structure, and its lattice is completely conjugated with alternative single and double bonds with delocalized p-electron cloud over the sp2-hybridized carbon atoms. Graphene is the slimmest nanomaterial in the world with a stable monolayer structure holding thickness of only 0.334 nm [30]. To tune the electrochemical properties of graphene, heteroatom doping with graphene has been reported and introduced in energy storage devices. Doped elements with different electronegativities enhance the permeability of graphene and introduce Faradaic reactions which increase

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the specific capacitance of the electrode material [31,32]. In the recent studies, reduced graphene oxide (rGO) has been activated electrochemically to amplify the specific capacitance of graphene-based electrodes. During the activation process, ions become intercalated into the graphene sheets which enlarge the electrolyte-accessible surface area of rGO and become beneficial for SC application [33,34].

3.5 Carbon nanotube CNT is a layer of graphite rolled up in a cylindrical form. The conceptual recognition of CNT occurred around 1950s [35] and 1970s [36] before the notability in 1990s by Iijima [37]. Single-walled CNTs and multiwalled CNTs are differentiated by the array of graphite cylinders present [38]. The diameter of each CNT remains in between few nanometers to few tens of nanometers, but the length of CNT can be around several tens of microns. The major techniques used to prepare CNTs are CVD, arc discharge method, and laser ablation method [39]. The electrical conductivity of individual CNT lies in between 104dand 107 S/m [40]. The surface area of CNT is around 1300 m2/g, which is crucial in generating large electrodeeelectrolyte interface and storing energy through EDLC mechanism [7]. Fekri Aval et al. reported a symmetric CNT-based SC electrode which showed higher specific capacitance of 411 F/g than graphene and graphite nanoparticles [41].

3.6 Fullerene The buckminsterfullerene (C60) was discovered in 1960 by Kroto et al. in honor of the famous architect Buckminster Fuller [42]. Fullerene structure consists of closed carbon cages made up of 12 pentagons and a few hexagons which increase with increasing size of the fullerene ([(2n/2)d10], n ¼ number of carbon atoms) [43]. Fullerene is one of the zerodimensional allotropes of carbon having desirable physical and chemical properties with high mechanical strength, excellent electron-accepting property, and less biotoxicity [44]. The electrochemical property of fullerene was enhanced by combining fullerene C60 whisker with polyaniline emeraldine base [45]. Low electrical resistance, large specific capacitance of 813 F/g at 1 A/g current density, and specific capacitance retention of 85.2% after 1500 cycles were obtained

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by applying the material as an electrode in SC. Recently, Kim et al. proposed a Fe-Nielayered double hydroxide (LDH) incorporated in fullerene (C60) and explored its electrochemical performance as hybrid SC electrode [46]. The synthesized C60eFe-Ni-LDH nanohybrid exhibited better performance such as high specific capacitance and cyclic stability than graphene-Fe-Ni-LDH nanohybrid.

4. Synthesis of biomass-derived porous carbon electrodes The PCs because of their high surface area can store a high amount of charge by EDLC mechanism, thus giving a high specific capacitance for use as SC electrode. Synthesis procedure for preparing electrodes for energy storage devices should be cost-effective, environment-friendly, and easy, so that a sustainable product can be developed. In this regard, the PCs with variable heteroatom doping, a high surface area, and a suitable morphology can have good charge storage characteristics, a high specific capacitance, eco-friendly nature, a good cycle life, and compatibility with different electrolytes, i.e., aqueous, organic, and ionic liquids (ILs). In combination to the above advantages, if the synthetic procedure can be made ecofriendly and be cost-effective, then a low-cost high-capacity SC device can be obtained. Here, the biomass-derived PCs come into view. Biomass-derived carbons are synthesized from various natural products, with high carbon content, like egg shell membrane, coconut shell, lignin, cellulose, orange peel, egg white, mushrooms, chitin, human hair, bacterial cellulose, different kinds of plants, agricultural waste products, etc., and can be broadly categorized into four categories: plant-based biomass, fruit-based biomass, animal-based biomass, and microorganism-based biomass [47e55]. Apart from the choice of above precursors, the process adopted for synthesis of the PCs also plays an important role in determining their morphology. However, it is very important to stop overoxidation of the carbons as it leads to collapse of its pores, thus leading to increased interface resistance [56].

4.1 Activation This is an important step to induce mesoporous and microporous nature as well as to get a good pore volume and pore width in the carbons. This can be done in two ways by using chemicals (chemical activation) or by not using chemicals (physical activation).

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Physical activation also known as pyrolysis has the advantage of not using corrosive chemical agents which has to be washed after activation. It occurs in steps; first is the conversion of the biomass precursors into carbon by pyrolysis at temperatures between 400 and 1000
C and second is the development of porosity of the surface by gasification at around 700e1200
C in oxidizing atmosphere created by gases such as carbon dioxide, steam, air, and many others [57e59]. The different oxidizers used in the gasification step tend to produce tar-like products in the pores of the carbons during heating. The porous nature of the carbons obtained by physical activation is due to the removal of these tar-like products as well as the removal of volatile impurities. Even ammonia used during pyrolysis has shown to act as both reducing and activating agent to form highly porous N-doped (around 10.3%) carbons at relatively low temperature of 550
C which gave higher capacitance than normal carbons [60]. Another activation technique widely used is the chemical activation. Chemical activation is generally a one-step process and has many advantages over physical activation such as better control over development of pores and pore sizes, thus yielding a high surface area PC, and it is more energy efficient because of its requirement of less temperature for pyrolysis [60]. In this activation, the biomass is often pretreated with different chemicals such as strong acids (H2SO4, HNO3), weak acids (H3PO4, H3BO3), salts (ZnCl2), and strong bases (NaOH, KOH) [61e67]. The mixture is then heated in inert atmosphere at temperatures between 400
C and 900
C. Among all the above chemicals, the most commonly used activating agent for SC electrode material preparation is KOH as it yields carbons with high specific surface area (SSA) along with development of higher pore volume. KOH activation occurs in different stages at different temperatures. Dehydration of KOH and reaction of carbon precursor with water occur around 400
C, while there is formation of K2CO3 and reaction of CO around 450e650
C identified by the evolution of CO and CO2. Finally, the generation of pores occurs at around 700
C by reaction with K2CO3 and CO2 and also because of the reduction of K2O to K by carbon [67]. After heat treatment, the residual KOH and the salts formed during the activation process are removed through repeated washing with dilute HCl (0.1 M) until pH ¼ 7 is obtained. The various forms of activation (physical and chemical) lead to the development of dangling bonds on the surface of the carbon, which ultimately lead to the creation of oxygen functionality through the creation of free radicals. These oxygen functionalities store charge through different faradaic reactions by different mechanisms [13]:

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4.2 Carbonization It is the conversion of biomass, a waste product, into valuable carbon products through decomposition at high pressure and temperature. Composition of the biomass includes major elements such as carbon (maximum 60 wt%), oxygen (maximum 40 wt%), and hydrogen (maximum 6 wt%) and minor components such as sulfur, nitrogen, and chlorine (maximum 1 wt%). Thus, carbonization helps in producing high-quality oxygen-functionalized carbon with little heteroatom doping through the decomposition of the carbonaceous material under the applied conditions. 4.2.1 Pyrolysis It is the heating of the carbonaceous material in inert atmosphere at temperatures of 700e900
C to get meso/micro-PCs. This type of treatment can produce PCs with high SSA as shown by Luo et al. from filter paper in ammonia atmosphere (1973.3 m2/g) [60], Bichat et al. from brown seaweed in CO2 atmosphere (1480 m2/g) [68], Subramanian et al. from banana fibers through ZnCl2 activation (1097 m2/g), etc. It is important to note here that the choice of activating agent along with carbonization strongly affects the SSA of the PC obtained and thus subsequently has an effect on the specific capacitance of the electrode material. 4.2.2 Hydrothermal carbonization This is a highly efficient, energy saving, and sustainable way of carbonization of biomass to produce high surface area PCs. In it, the carbonaceous precursor is converted to PCs through thermally treating in presence of water at 150
Ce300
C (low-temperature hydrothermal carbonization [HTC]) and 300
Ce800
C (high-temperature HTC) [69,70]. The mechanism of HTC has been reported to be containing the following steps: (1) hydrolysis, (2) dehydration, (3) decarboxylation, (4) polymerization, and (5) aromatization. Jain et al. reported PCs with BET surface area of 2440 and 1121 m2/g from hydrothermal treatment of coconut shells with ZnCl2 and H2O2, respectively, at 275
C [71]. 4.2.3 Ionothermal carbonization This is a single-step method which uses salt mixtures, as template/solvent for carbonization and ILs, during solvothermal treatment to get high SSA PCs. The ILs are specially chosen because of their low melting point, chemical and thermal stability, negligible vapor pressure, and high ionic and thermal conductivity. In a study, Lin et al. used iron-based IL 1-butyl-

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3-methylimidazolium tetrachloroferrate [Bmim][FeCl4] to develop PC from fructose with BET surface area of 1200 m2/g in one-step ionothermal method [72]. The IL served as template, solvent, and catalyst for producing high mesoporous and microporous PC and could also be reused after recovery. [Bmim][FeCl4] was also used by Liu et al. to synthesize N-doped PC with BET surface area of 2532 m2/g with good mesoporous volume from jujun grass through solvothermal treatment at 180
C [73]. They also compared their results with HTC of the same biomass and found ionothermal carbonization to be more effective. Inspired by the use of [Bmim] [FeCl4], Zhang et al. in their study studied the effects of using different nonmetallic ILs during carbonization of glucose, cellulose, and sugarcane bagasse [74]. A high yield of heteroatom-doped carbon was obtained with maximum BET surface area of 627 m2/g at 200
C. 4.2.4 Molten salt carbonization In this method, the biomass precursor is dipped into salt or salt mixture which have been melted by heating to their melting point and then carbonized at temperatures above 400
C in inert atmosphere. After cooling, the salts are removed from the carbonized product by repeatedly washing with HCl and distilled water. The molten salt helps by cracking the large molecules of the biomass. Many salt and salt mixtures have been used to derive PCs from biomass such as Na2CO3eK2CO3 (818 m2/g) from firwood, ZnCl2 (1582 m2/g) from chitosan of shrimp shells, and CaCl2 (550 m2/g) and Na2CO3eK2CO3 (436 m2/g) from boiled coffee beans [75e77].

5. Types of biomass precursors The biochar content of the biomass, the development of microstructure, and heteroatom doping of the PCs depend on the chemical structure of the biomass and its elemental composition. Hence, it is very important to investigate the characteristics of the precursors at elemental and molecular level to get optimized specific capacitance for practical applications. Based on their origin, the biomass precursors are divided into four categories:

5.1 Plant biomass Chemical compositions of the plant-based biomass differ for different plant species as well as different organs of the plant. The main contents of the plant species and other parts of the plant are lignin, cellulose, hemicellulose,

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and extractives. These are the main parts that are converted to carbon during the carbonization process. Cellulose is a polysaccharide that forms the backbone of the cell wall of plants, whereas lignin is an organic polymer with cross-linked structure to provide rigidity to the plants and acts as a support tissue. Hemicellulose is a highly branched heteropolymer with short chains of sugar unit often associated with cellulose in terrestrial plants. The extractives are inorganic or organic compounds associated with lignocellulosic tissue which may be phenolic, aliphatic, acyclic, terpenoids, fatty acids, calcium, magnesium, potassium, etc. All the above components of the plant-based biomass are converted to biomass-derived PC during thermal carbonization. The actual yield of carbon from the biomass depends on the relative presence of cellulose, lignin, and hemicellulose, although the carbon fraction among the other elements in them is relatively high (maximum 60%). It was found that lignin, being the most thermally stable, was the major contributor to the PCs, whereas cellulose and hemicellulose, being less thermally stable than lignin, could only contribute moderate carbon content [78,79]. Another important aspect of the carbons is their elemental composition, i.e., the content of nitrogen, oxygen, sulfur, etc., which could affect the yield, porous structure, and the conductivity of the synthesized PCs. It was observed from different studies that higher presence of oxygen leads to lesser carbon yield with more defects and less crystallinity because of the formation of more volatile components while the higher presence of nitrogen can induce better electrochemical properties to the PC [78,80,81]. Hence, it can be concluded that higher lignin content and nitrogen presence along with lower cellulosic content and oxygen presence are crucial for producing high-degree PCs with good porosity, carbon yield, controllable defects, and good specific capacitance.

5.2 Animal-based biomass Animal-based biomass mainly contains either chitin or keratin as main constituent. Chitin is a promising biomass for synthesis of PCs with high nitrogen content available in abundance. Unlike cellulose, chitin is more thermally and chemically stable because of its intensive intermolecular hydrogen bonding and cross-linking networks with other natural polymers. Arbia et al. combined different studies showing different animals, such as insects, molluscs, crustaceans, etc., which could act as a primary source for extraction of chitin and hence can provide economic, green, and recyclable ways of producing biomass-derived PCs [82]. Before using the chitin from

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the biomass, various chemical and mechanical processes have to be used such as mechanical grinding, chemical demineralization, and deprotonization for extraction of chitin [83]. Besides chitin containing biomass, some other precursors from animals such as sloughs, hairs, horns, claws, and hooves of animals have been used to derive PCs. The basic units of these are fibrous structural proteins called keratin. Keratin exists in two types: a-keratin and b-keratin which primarily consists of amino acids. Keratin, like chitin, has good chemical and thermal stability because of a large number of intermolecular bonding. Because of the high intermolecular bonding within its structure, keratin has sufficient stability and shows high carbon yield during thermal carbonization.

5.3 Fruit-based biomass The fruit-based biomasses like their plant-based counterparts have different chemical compositions for different plant species and different parts of the plant. The major components of the fruit-based biomass are carbohydrates, crude fibers, lipids, crude proteins, and ash [84,85]. Liu et al. in their study found that the content of lipids and proteins in pulp and peels of the fruitbased biomass are maximum. This higher percentage of lipids and crude proteins does not contribute much to the carbon yield as they degrade at
relatively lower temperature of around 300 to CO2, H2O, NH3, olefins, and esters [86]. However, this high concentration of lipids and proteins assists in synthesizing phosphorous- and nitrogen-doped PCs. The main contents of the crude fibers in fruit are cellulose, lignin, and hemicellulose. Among the fibers, the fruit biomasses are generally rich in cellulose than lignin; however, the content of the fibers is not as high as for the plant biomass and thus is responsible for lower yield of carbon and graphitization [87].

5.4 Microorganism-based biomass The microorganisms that are involved in the production of PCs are fungi and bacteria especially their cellulose part. Fungi, along with mushrooms and yeasts, are considered good biomass precursors because of their high natural abundance and faster growth rate. Fungi contain the same components like plant- or fruit-based biomass, such as carbohydrates, proteins, fiber, fats, and ash, but differ greatly from them in their actual structural form and the elemental composition of the constituents. The carbohydrates of the microorganism-based biomass contain chitin rather than sucrose or starch as for plant-based biomass and thus are more thermally stable and

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produce higher yield of carbon [88]. The crude fibers of carbon are mainly composed of cellulose and thus can add to the yield of carbon, while the other components such as crude protein, ash, and crude fat decompose quickly during carbonization and do not contribute much toward the carbon yield. The nitrogen content of fungi is very high, and so they can be used to produce N-doped biomass-derived PCs. Bacterial cellulose has also been used to derive carbons. Strains such as Aerobacter, Acetobacter, Achromobacter, Agrobacterium, Alacaligenes, Azotobacter, Pseudomonas, Rhizobium, and Sarcina have been widely used to derive bacterial cellulose. They have been used to produce different doped carbons by mixing with activating agents before thermal carbonization [89]. Often they are also combined with conducting polymers such as polypyrrole and polyaniline before carbonization to get N-doping.

6. Structural specification of biomass-derived porous carbon Structural diversity is an inherent property of carbon materials. Biomassderived carbon materials are no exception. It is well-known that the electrochemical properties are highly dependent on morphology. On the other hand, morphology-controlled materials are highly acceptable in the fabrication of energy storage devices. Biomass-derived carbon materials are available in different microstructures such as spheres, tubular, fiber-like, sheets, rod-like, graphene-like, and many others [90]. In this section, we will discuss about the electrochemical performance of biomass-derived carbon materials of specific morphologies.

6.1 Sphere-like structure Nanofibrous microspheres of N-doped carbon materials were synthesized from the chitin resources (waste seafood) [91]. The microspheres were found to be highly elastic and mechanically strong. Owing to its highly ordered 3D nanoarchitecture, the biomass-derived carbon achieved high surface area of 1000 m2/g. As a result, the carbon microspheres displayed high rate capability and desirable energy density of 58.7 Wh/kg in 1-ethyl3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM TFSI) electrolyte. Jin et al. produced the hierarchically porous microspheres of activated carbon from different spores, i.e., Lycopodium clavatum, Ganoderma lucidum, and Lycopodium annotinum spores through cost-effective green synthetic route [92]. The activated carbon microspheres achieved ultrahigh

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surface area of 3053 m2/g. Owing to the enhanced surface area and spherelike morphology, the carbon material exhibited maximum capacitance of 300 F/g and the cycling stability of 93.8% after 10,000 cycles. In a recent study, Wei et al. produced 3D interconnected carbon nanorings from batata leaves and stalks, which displayed superior electrochemical performance in both acidic and alkaline electrolytes [93]. Specifically, the SC electrode exhibited the specific capacitance of 532.5 F/g and cycling stability of 91.7% after 10,000 cycles in 1M H2SO4 electrolyte, whereas in 6M KOH electrolyte, it displayed the capacitance of 350 F/g at 1 A/g current density and the cycling stability of 91.1% after 10,000 cycles. All these studies clearly demonstrate the enhanced electrochemical performance of biomassderived sphere-like carbon materials in various electrolytes.

6.2 Tube-like structure Xie et al. synthesized hierarchical PC (HPC) microtubes from willow catkins (catkins of willow tree), which exhibited promising electrochemical properties after activation by KOH [94]. Owing to the porous structure and high surface area (1775.7 m2/g), the tube-like carbon material exhibited the specific capacitance of 292 F/g at the current density of 1 A/g in 6M KOH electrolyte. In addition, the corresponding symmetric device based on the electrode displayed the energy density of 37.9 Wh/kg and cycling stability of 90.6% after 4000 cycles in LiPF6 electrolyte. In another report, these willow catkinederived carbon materials were combined with MnO2 with little modification to enhance the capacitance [95]. In this case, the authors used the reagent K4Fe(CN)6 for activation and graphitization of the precursor material. The composite electrode demonstrated maximum capacitance of 550.8 F/g at the current density of 2 A/g and capacitance retention of 89.6% after 5000 charge/discharge cycles. In another report, Qu et al. prepared macro-PC tubes through the carbonization of eggplant [96]. The authors further developed hybrid SC electrode by growing Co(OH)(CO3)0.5 nanocone arrays on these macro-PC tubes, which displayed good rate capability (capacitance fading of 44% even at a high current density of 40 A/g).

6.3 Fiber-like structure A facile approach to synthesize fiber-like PC from lotus seedpods through two-step pathway was reported by Liu et al. [97]. The fibrous carbon material exhibited BET surface area of 1813 m2/g with average pore diameter of 3.3 nm. The reported specific capacitance of the PC was 402 F/g at

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0.5 A/g current density with outstanding cyclic stability of 95.4% after 10,000 cycles in 6 M KOH electrolyte. Also, the symmetric SC assembled with this carbon fiber exhibited energy density of 12.5 Wh/kg at 260 W/kg power density. Recently, a meso/microporous network like carbon nanofiber as binder-free electrode material was derived from bacterial cellulose [98]. The synthesized material showed 3D interconnected structure with hierarchical porosity and surface area of 624 m2/g. The electrode material displayed a specific capacitance of 302 F/g at 0.5 A/g current density with excellent rate capability with 98% capacitance retention after 5000 cycles in 6 M KOH. The assembled device displayed high energy density of 6.9 Wh/ kg with 128.3 W/kg power density. Hu et al. reported a ramie-derived carbon fiber coated with MnO2/poly(3,4-ethylenedioxythiophene) core shell as a flexible high-performance SC [99]. This hybrid electrode exhibited specific capacitance of 922 F/g at 1 A/g current density with sufficient rate capability and high surface area of 101.7 m2/g. The flexible symmetric SC device displayed huge energy density of 19.17 Wh/kg with 500 W/kg power density as well as good cyclic stability with 83% capacitance retention after 10,000 cycles.

6.4 Sheet-like structure Nano-PC nanosheets were prepared by Park et al. from waste coffee grounds through a simple one-step top-down method [100]. Waste coffee beans mixed with KOH were pyrolyzed at 800
C under N2 flow to synthesize carbon nanosheets without any additional treatment. The nanosheets, containing different redox-active heteroatoms such as sulfur, nitrogen, and oxygen, showed very large surface area of 1960.1 m2/g. This material acted as an efficient electrode material for SC with specific capacitance of 438.5 F/g at 2 mV/s scan rate and exhibited 81.9% capacitance retention after 2000 cycles in 6 M NaOH. A 3D PC nanosheet was synthesized from plant waste by activation with KOH and carbonizing at 700
C under Ar flow [51]. The fabricated nanosheet provided huge specific capacitance of 470 F/g and 310 F/g in three-electrode system and twoelectrode system, respectively, at 1 A/g current density. In addition, it showed an impressive rate capability as well as high cyclic stability (98% after 50,000 cycles) in 6M KOH electrolyte. Also, at 15 kW/kg power density, the material exhibited 25.4 Wh/kg energy density in twoelectrode electrochemical analysis. In another study, N-doped PC nanosheets were fabricated from eucalyptus leaves by simply activating with

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KHCO3 and subsequent pyrolysis at 850
C under Ar flow [101]. The material showed a specific capacitance of 312 F/g at 0.5 A/g in 1 M H2SO4. This porous nanosheet also exhibited very high surface area of 2133 m2/g and 97.7% capacitance retention after 15,000 cycles. The morphological study of different types of biomass-derived carbon materials is shown in Fig. 12.3.

7. Natural polymerederived porous carbon Natural polymers are the most abundant biomass available in our surroundings. Biomass-derived PC is prepared by converting natural products such as food, microorganism, animal, and plant-based waste materials through artificial method such as high-temperature carbonization and activation. Because of their easy availability and abundant nature, PC obtained from the natural polymers is cheaper and can be produced in bulk scale. Table 12.1 summarizes the electrochemical performance of different natural polymerederived carbon materials.

7.1 Cellulose-derived porous carbon Cellulose is the most abundant natural polymer which is mainly present in the primary cell wall of green plant. It is made up with a linear chain of Dglucose. The crucial properties of cellulose fibers such as flexibility, mechanical strength, and huge surface area large scope of chemical modification make it an attracting option as electrode material for SC application [102,103]. CMC exhibited cross-linked porous wrinkled surface. After pyrolysis, the surface area became disorganized for CA where an interconnected 3D porous network was observed for activated carbon aerogel samples [103]. Cellulose can be classified according to the origin into two categories: cellulose isolated from lignocellulose and commercial cellulose for precursor of PC. PC was synthesized at elevated temperature through carbonization and activation via physical or chemical process. Although having very high surface area, PC showed lower specific capacitance than HPC because of having fewer amounts of active sites, slow ion diffusion rate, and low wettability.

7.2 Alginate-derived porous carbon Other varieties of natural polysaccharide acquired by artificial extraction except cellulose have been hugely utilized as precursor for PC because of their low cost, availability, and stability. Alginate is one of the naturally

Figure 12.3 (A) (a, b) Scanning electron micrographs (SEMs) of hydrothermal carbon obtained from spruce hydrolysis product, (c, d) corncob hydrolysis product, and (e, f) glucose at 200
C and after 24 h; (B) SEM images of (a) Carbon microtubes (CMT), (b) Graphitic carbon microtubes (PGCMT), (c) PGCMT/MnO2 under low magnification, and (d) PGCMT/MnO2 under high magnification; (C) (a) SEM image of origin Bacterial Cellulose (BC) membrane, (b) transmission electron micrograph (TEM), and (c, d) highresolution TEM (HRTEM) images of Carbon Nanofiber-Bacterial Cellulose (CN-BC). (D) (a) SEM micrograph highlighting the interconnected 2D structure of sample CNS800, (b) TEM micrograph highlighting the structure of CNS-800, (c) HRTEM micrograph highlighting the porous and partially ordered structure of CNS-800, and (d) ADF TEM micrograph and Electron Energy Loss Spectroscopy (EELS) thickness profile (inset) of CNS-800. (A) Copyright from C. Falco, J.M. Sieben, N. Brun, M. Sevilla, T. Van der Mauelen, E. Morallón, D. Cazorla-Amorós, M.M. Titirici, Hydrothermal carbons from hemicellulosederived aqueous hydrolysis products as electrode materials for supercapacitors, ChemSusChem 6(2) (2013) 374e382; (B) Copyright from X. Zhang, K. Zhang, H. Li, Q. Cao, L.e. Jin, P. Li, Porous graphitic carbon microtubes derived from willow catkins as a substrate of MnO2 for supercapacitors, Journal of Power Sources 344 (2017) 176e184; (C) Copyright from X. Hao, J. Wang, B. Ding, Y. Wang, Z. Chang, H. Dou, X. Zhang, Bacterial-cellulosederived interconnected meso-microporous carbon nanofiber networks as binder-free electrodes for high-performance supercapacitors, Journal of Power Sources 352 (2017) 34e41; (D) Copyright from H. Wang, Z. Xu, A. Kohandehghan, Z. Li, K. Cui, X. Tan, T.J. Stephenson, C.K. King’Ondu, C.M. Holt, B.C. Olsen, Interconnected carbon nanosheets derived from hemp for ultrafast supercapacitors with high energy, ACS Nano 7(6) (2013) 5131e5141.)

Table 12.1 Alginate, lignin, starch, chitin, and gelatin-derived porous carbon and their electrochemical performance in SC application.

APCA

6M KOH

1811

188 at 1 A/ g

KOH, 750
C under N2

HPC

6M NaOH

3077

270 at 0.1 A/g

KOH, 700
C under NH3

PC

6M KOH

1002.6

440 at 0.5 A/g

KOH, Na2SO3, 950
C under N2 and NH3 KOH, heat treatment

N, S-doped wood carbon

4M KOH

1438

704 at 0.2 A/g

LHPC

1M H2SO4

907

165 at 0.05 A/g

KOH, 800
C under N2

HPC

6M KOH

3775

286.7 at 0.2 A/g

Mg(NO3)2, 800
C under N2

LCNFs

6M KOH

1140

248 at 0.2 A/g

Alginate




Lignin

Electrode material

KOH, 800 C under N2

Cyclic stability

93.2% after 10,000 cycles 90.8% after 10,000 cycles 92.3% after 10,000 cycles 122% after 10,000 cycles 97.3% after 5000 cycles 97% after 1000 cycles 97% after 1000 cycles

Energy density (Wh/kg)

Power density (W/kg)

Reference

10.4

w700

[105]

e

e

[106]

28.8

w100

[107]

15.2

w100

[108]

5.7

15

[109]

e

e

[110]

e

e

[111]

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Specific capacitance (F/g)

Preparation method

392

Electrolyte

Surface area (m2/g)

Natural polymer

PC

EMIMBF4

2207

224 at 0.1 A/g

KOH, 800
C under N2

N-doped PC

KOH/ PVA

3130

306 at 0.1 A/g

Hydrothermal, 900
C under N2

MCMSs// RGO

1M KOH

456

245 at 1 A/ g

Hydrothermal, K2CO3, carbonization

BiOCl// AC

6M KOH

KOH, urea, Fe2O3, 800
C under N2

N-PGCNS

1M TEABF4/ AN

2129.8

337.6 at 0.5 A/g

Hydrothermal, H3PO4, 800
C under N2

PC

6M KOH

1239

144 at 0.625 A/g

KOH, 650
C under N2

PC

1M TEABF4/ AN

1367.87 272 at 1 A/g

124 at 0.5 A/g

92

e

[112]

17

e

[113]

21.5

759

[114]

17.2

250.9

[115]

27.5

270

[116]

19.9

311

[117]

25.9

249.6

[118]

393

80% after 5000 cycles 100% after 1000 cycles w100% after 4000 cycles 82% after 3000 cycles 87.6% after 5000 cycles 99% after 5000 cycles 97% after 10,000 cycles

Present status of biomass-derived carbon-based composites for supercapacitor application

Starch

KOH, 800
C under NH3

Continued

Specific capacitance (F/g)

1M H2SO4

1363

192 at 0.5 A/g

HCMPANI

1M H2SO4

1450

76 at 0.2 A/g

N-doped PC

6M KOH

260.6

284 at 0.5 A/g

PC composited with graphene oxide N-doped PC

6M KOH

2252

306 at 0.5 A/g

6M KOH

1346

362 at 1 A/g

Preparation method

Electrode material

Electrolyte

Chitin

NaOH, urea, 800
C under Ar

Carbon nanosphere

LiOH/KOH/ urea/H2O, 800
C in inert atmosphere NH4Cl, 650
C under N2 KOH, 800
C

Gelatin

Adsorption on MAR, 750
C under N2

Energy density (Wh/kg)

Power density (W/kg)

Reference

95% after 10,000 cycles 90.6% after 10,000 cycles 96.9% after 5000 cycles 92% after 5000 cycles

5.1

2364.9

[119]

8.9

1644

[120]

18.33

500

[121]

7.43

263.5

[122]

92.9% after 5000 cycles

9.13

254

[123]

Cyclic stability

AC, activated carbon; AN, acetonitrile; APCA, activated hierarchical macro-meso-microporous carbon aerogel; CHPC, chitosan-based hierarchically porous carbon; EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate; HCM-PANI, hierarchically porous carbon microsphere-polyaniline; LCNFs, lignin-based carbon nanofiber films; LHPC, lignin-derived hierarchical porous carbon; MAR, macroporous adsorption resin; MCMSs, uniform and monodispersed carbon microspheres; N-PGCNSs, N-doped porous graphitized carbon sheets; TEABF4, tetraethylammonium tetrafluoroborate.

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Surface area (m2/g)

Natural polymer

394

Table 12.1 Alginate, lignin, starch, chitin, and gelatin-derived porous carbon and their electrochemical performance in SC application.dcont’d

Present status of biomass-derived carbon-based composites for supercapacitor application

395

available polysaccharide made up of covalently bonded guluronate and mannuronate. Alginate is extensively available in the cell walls of brown algae. An egg boxelike structure has been formed because of the chelation between the metal ions and alginate followed by gel formation [104]. Macro-, meso-, and microporous 3D HPC aerogel had been prepared by calcinating the egg box structure at 800
C.

7.3 Lignin-derived porous carbon Among the natural aromatic polymers, lignin is the most abundant one. It is the second abundant natural polymer obtained from plant and being produced around 50 million tons per year. Recently, lignin has become one of the most promising sources of carbon in the field of energy application regarding the cost and environment issues. Lignin-derived PC nanocomposites, having outstanding electrochemical performance, hugely introduced into the SC applications. However, potential oxidation and condensation reaction during the separation process makes it relatively tough to separate lignin. Also, lignin separated by hydrolysis or solubilizing plant parts limits its vast application because of the lack of purity and mass production [124].

7.4 Starch-derived porous carbon A common polymeric carbohydrate starch, made up with glucose molecules bonded through glycosidic bonds, present mainly in human diet, is becoming a potential candidate for carbon source. Starch-derived PC has been widely used in energy field because it is cost-effective, easily available, sustainable, and biodegradable. As a precursor, starch is relatively a pure one and the resulted products are associated with less impurity and high yield [115]. Hong et al. reported synthesis of activated carbon from sweet potato starch and fabricated a asymmetric SC device using activated carbon as anode and BiOCl as cathode materials (shown in Fig. 12.4) [115]. Some of the starch-derived PC nanocomposites reported for SC application are summarized in Table 12.2.

7.5 Chitin-derived porous carbon Chitin is mostly obtained from marine biomass waste such as shells of crab or shrimp with sufficient nitrogen content (w6.9 wt%). It consists of a stiff chain of b-(1,4)-linked-acetamido-2-deoxy-D-glucopyranose. N-doped PC can easily be synthesized by pyrolysis of chitin and can be used as

396

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 12.4 Schematic elastration of synthesis of activated carbon from starch and fabrication of supercapacitor device. (Copyright from W. Hong, L. Wang, K. Liu, X. Han, Y. Zhou, P. Gao, R. Ding, E. Liu, Asymmetric supercapacitor constructed by self-assembled camellia-like BiOCl and activated carbon microspheres derived from sweet potato starch, Journal of Alloys and Compounds 746 (2018) 292e300.)

highly efficient SC. Because of the stiff chains of chitin, PC derived from chitin exhibited sufficient retention of morphology. But, direct carbonization of chitin does not provide tuned morphology and porosity. To obtain large specific surface area, porosity, and better electrochemical activity, chitin aerogel and nanofiber were used as the precursor for Ndoped PC [119]. A series of electrochemical performance of chitinderived PC are summarized in Table 12.2.

7.6 Gelatin-derived porous carbon Gelatin is a sustainable natural polymer synthesized by partial irreversible hydrolyzation of collagen obtained from the bone, skin, and the connecting tissues of animals. It is a noncrystalline transparent mixture of biomass resource which consists of sufficient amount of eNH3 groups that confirm good wettability after pyrolization. Thus, N-doped PC can easily be

Cyclic stability

Reference

[128]

6M KOH

e e 90% after 3000 cycles 82% after 10,000 cycles 99.5% after 5000 cycles 100% after 1000 cycles e

6M KOH

e

[131]

Banana fiber Broad bean

KOH ZnCl2 KOH

686 1097 655.4

5 mV/s 5 mV/s 0.5 A/g

66 86 202

1M Na2SO4 1M Na2SO4 6M KOH

Coffee bean

ZnCl2

742

0.05 A/g

180

1M H2SO4

Coconut shell

FeCl3 þ ZnCl2

1874

1 A/g

268

6M KOH

Rice husk

H3PO4

2009

50 mA/g

176

6M KOH

Rotten carrot Corn syrup

ZnCl2

1253

e

135.5

Self-physical

1364

0.2 A/g

168

[125] [125] [126]

[127]

[129]

[130]

397

Continued

Present status of biomass-derived carbon-based composites for supercapacitor application

Table 12.2 Comparative table for biomass-derived carbon-based supercapacitor electrodes. Current BET density (A/g)/ surface scan rate (mV/ Capacitance area Activation s) (F/g) Electrolyte Biomass process (agent) (m2/g)

398

Table 12.2 Comparative table for biomass-derived carbon-based supercapacitor electrodes.dcont’d Current density (A/g)/ scan rate (mV/ s)

Capacitance (F/g)

Electrolyte

Cyclic stability

Reference

Cassava peel waste Food waste Cotton stalk

KOH

1352

e

153

0.5M H2SO4

e

[132]

H3PO4

535.8

10 mV/s

85

6M KOH

e

[133]

H3PO4

1481

0.5 A/g

114

1M Et4NBF4

[134]

Cow dung

KOH

1500 e2000

0.1 A/g

124

1 M Et4NBF4/AN

Pine cone

KOH

3950

1 A/g

142

Sewage sludge

KOH

2839

5 mV/s

340

1M LiPF6 in ethylene carbonate/dimethyl carbonate (1:1 v/v) 1M Na2SO4

Waste cumin plant

H3PO4

1468

1.5 mA/cm2

155

1M H2SO4

95.3% after 500 cycles 85% after 1000 cycles 90% after 20,000 cycles 90% after 20,000 cycles 96.9% after 5000 cycles

Biomass

[135]

[136]

[137]

[138]

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Activation process (agent)

BET surface area (m2/g)

Self-physical

1214.25

10 mV/s

51

EMIMBF4

KOH

2841

1 A/g

330

2M KOH

Waste bagasse

KOH

3151

1 A/g

413

6M KOH

Ginkgo shells

KOH

1775

2 mV/s

365

6M KOH

Woody biochar

HNO3

317

0.2 A/g

115

0.5M H2SO4

Natural silk

KOH

2494

0.1 A/g

242

EMIMBF4

AN, acetonirile; EMIMBF4, 1-ethyl-3-methylimidazolium tetrafluoroborate; Et4NBF4, tetraethylammonium tetrafluoroborate.

70% after 2000 cycles 92% after 2000 cycles 93.4% after 10,000 cycles 92.6% after 5000 cycles 100% after 5000 cycles 91% after 10,000 cycles

[139]

[140]

[141]

[142]

[143]

[144]

Present status of biomass-derived carbon-based composites for supercapacitor application

Waste compact discs Waste tea leaves

399

400

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

fabricated from the activation of gelatin. As an abundant and renewable reservoir, gelatin has been hugely used in pharmaceutical, imaging, manufacturing, and food industries [123]. As it is an efficient source of N-doped PC, it becomes an efficient candidate for SC applications (Table 12.2). Jia et al. reported an N-doped PC derived from gelatin adsorbed on macroporous adsorption resin (NCGM) [123]. The threeelectrode electrochemical performance with capacitive behavior, chargee discharge process, along with the cyclic life of this N-doped carbon, is shown in Fig. 12.5.

8. Application of biomass-derived porous carbons in supercapacitor technology As discussed before, biomass-derived carbon materials are extensively utilized as the SC electrodes because of their high porosity, large surface area, and good conductive nature. The previous sections have also demonstrated the electrochemical performance of several biomass-derived carbon materials. This chapter deals with the comparison of supercapacitive performance of biomass-based electrodes and their composites. A comparative study of different biomass-based electrodes is shown in Table 12.2. From the table, it can be concluded that most of activation process for biomassderived carbon materials have been performed with KOH treatment. As compared with ZnCl2, KOH treatment is more beneficial for the activation to enhance the surface area as well as specific capacitance. Apart from these chemicals, H3PO4 has also been used as the activating agent. For electrochemical test, KOH has been majorly used as the electrolyte (aqueous). It is obvious that the electrode materials show better electrochemical performance in aqueous electrolytes as compared with organic one, which can be attributed to the smaller size of ions for aqueous electrolytes. In a recent study, Ghosh et al. compared the electrochemical performance of four different hard carbons, which were synthesized from KOHactivated banana stemederived carbon, phosphoric acidetreated banana stemederived carbon, corncob-derived hard carbon, and potato starche derived hard carbons [145]. Among these, KOH-derived hard carbon showed the highest capacitance of 479.23 F/g. The authors claimed that the better electrochemical performance of KOH-treated electrode was attributed to its high porosity, the presence of pseudocapacitance due to oxygen functionalities (introduced by the KOH treatment), and large surface area. In another work, Durairaj et al. synthesized AC from the laboratory tissue

Present status of biomass-derived carbon-based composites for supercapacitor application

401

Figure 12.5 Electrochemical performance of NCGM-based electrode using a three-electrode test system: (A, B) Cyclic voltammetry curves at scan rates from 5 to 100 mV/s, (C, D) galvanostatic chargeedischarge profiles at current densities from 1 A/g to 30 A/g, (E) specific capacitance as a function of current density, and (F) cycle testing at 8.0 A/g current loading for 5000 cycles. (Reproduced from H. Jia, H. Zhang, S. Wan, J. Sun, X. Xie, L. Sun, Preparation of nitrogen-doped porous carbon via adsorption-doping for highly efficient energy storage, Journal of Power Sources 433 (2019) 226712.)

402

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

paper and hardboard waste [146]. Furthermore, the AC was used to adsorb methylene blue dye. The dye-adsorbed AC was used as the SC electrode. It has been observed that the dye-adsorbed tissue paperederived electrode exhibited higher capacitance (260 F/g) than the hardboard wasteederived electrode (155 F/g) at the constant current density of 0.5 A/g. Overall, these electrodes served as the instant solution for environmental remediation and future energy storage devices. Flexible SC electrode was further developed by carbonizing the recycled jute [147]. The KOH-activated carbon electrode demonstrated the capacitance of 408 F/g and no capacity fading after 5000 cycles. Even the SC device based on this electrode displayed 60% enhancement in charge storage ability with the increase of temperature from 5 to 75
C. Following the research trend on biomass, human urine was utilized for the synthesis of heteroatom-doped carbon materials [148]. With the removal of mineral salts, the urine-derived carbon became highly porous and displayed high BET surface area of 1040.5 m2/g. The electrode showed maximum capacitance of 166 F/g at 0.5 A/g current density and capacitance decay of only 1.7% after 5000 cycles. Fu et al. synthesized multihierarchical carbon materials from crab shell [149]. After the activation of the carbonized material, the electrode displayed the capacitance of 322.5 F/g at the current density of 1 A/g and cycling stability of 99% after 10,000 cycles. Furthermore, a hybrid SC based on this crabderived carbon and SrFe12O19 achieved the capacitance of 690.4 F/g at 1 A/g current density and the capacitance retention of 94.5% after 10,000 cycles. All these studies clearly demonstrate that the biomass-derived carbon materials are highly desirable candidates for the development of advanced SC electrodes. Inspired by these findings, the researchers have synthesized composites based on these carbon materials for further enhancement of electrochemical performance. A composite electrode based on crab-derived carbon and pseudocapacitive CoFe2O4 exhibited the enhanced capacitance of 701.8 F/g at the current density of 1 A/g and cyclic stability of 90.9% after 10,000 cycles [150]. The synthesis process of crab-derived carbon materials and the SC application of the composite are schematically shown in Fig. 12.6. Likewise, in another study, banana peelederived 3D PC was integrated with MnO2 [151]. To make the electrode porous, the authors employed freeze-drying technique. The detailed synthetic process is schematically shown in Fig. 12.7A. Although this hybrid electrode showed moderate capacitance of 139.6 F/g, it displayed high capacitance retention of 92.3% after 1000 cycles. It is obvious that the addition of metal oxides with carbon

Present status of biomass-derived carbon-based composites for supercapacitor application

403

Figure 12.6 (A) Graphical illustration of the synthesis process of crab-derived carbon (CDC) and (B) the evolution from crab waste to CDC and its applications in supercapacitors. (Copyright from M. Fu, W. Chen, J. Ding, X. Zhu, Q. Liu, Biomass waste derived multi-hierarchical porous carbon combined with CoFe2O4 as advanced electrode materials for supercapacitors, Journal of Alloys and Compounds 782 (2019) 952e960.)

materials generally enhances the electrochemical properties of carbon materials. For both the above cases, the same trend has been followed. In particular, the addition of MnO2 enhanced the cycling stability of banana peelederived carbon from 89.5% to 92.3%, which can be evident from Fig. 12.7B along with the enhancement of other electrochemical properties. In another study, the egg yolk was used to synthesize heteroatom-doped carbon materials. The resultant P, N, and O-tridoped PC was further

404

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Figure 12.7 (A) Schematic illustration for the preparation of the biomass-derived porous carbon (BPC) and the petal-like nanosheets hierarchical MnO2/BPC composites electrode; (B) (a) CV curves of MnO2 and the MnO2/BPC composites electrode at a scan rate of 5 mV/s, (b) CV curves of the MnO2/BPC composites electrode at different scan rates (5, 10, 20, and 30 mV/s), (c) charge/discharge curves of MnO2 and the MnO2/BPC composites electrode at 1 A/g, (d) charge/discharge curves of the MnO2/BPC composites electrode at different current densities (0.3, 0.5, 1, 2, 5, and 10 A/g), (e) specific capacitance of MnO2 and the MnO2/BPC composites electrode at different current densities calculated from charge/discharge curves, (f) cycling performance of MnO2 and the MnO2/BPC composites electrode at a current density of 1 A/g. (Copyright from G. Yang, S.-J. Park, MnO2 and biomass-derived 3D porous carbon composites electrodes for high performance supercapacitor applications, Journal of Alloys and Compounds 741 (2018) 360e367.)

Present status of biomass-derived carbon-based composites for supercapacitor application

405

combined with MnO2 through hydrothermal process using KMnO4 as the precursor [152]. The synthesized hybrid electrode displayed the specific capacitance of 341 F/g at 1 A/g and no capacitance fading up to 15,000 cycles in 6M KOH electrolyte. In another study, Kim et al. synthesized hybrid electrode by combining crab-derived PC with Co3O4 [153]. In this case, the activation process was performed by CO2 treatment. After activation, the electrode displayed high BET surface area of 2430 m2/g. Owing to its high porosity and enhanced surface area, the electrode exhibited high capacitance of 508 F/g at 1 A/g current density and cycling stability of 95% after 10,000 cycles. Therefore, it can be observed that the crab-derived carbon showed higher capacitance with MnO2 as compared to Co3O4; but, the stability was found to be higher for the Co3O4-based electrode than the MnO2-based electrode. However, it’s difficult to compare these two electrodes as the activation processes were different in these two cases. The above studies indicate the enhanced electrochemical performance of biomass-derived carbon-based composite electrodes. The activation process is necessary for the enhancement of the porosity, surface area, and electrochemical performance. In addition, the hierarchical structure of the carbon materials is favorable for easy electron and charge transport during electrochemical test.

9. Conclusion and prospective As discussed in this chapter, biomass-derived PC materials have grown considerable research interest in energy-related fields. They have some attractive features such as high surface area with well-distributed pores, high electrical conductivity, and impressive accessibility of ions of electrolyte. The activation and carbonization processes lead to the enhancement of surface area as well as heteroatom (N, O, and S) doping leads to the enhanced electrochemical performance through higher EDLC and some faradaic contributions. Modern research on SCs has been devoted to the development of devices with high energy density. However, the PCs often suffer from limitations like collapsing of pores leading to inadequate utilization of specific surface area which also reduces the electrolyte interaction. Also, it has been seen that the various morphologies and structures such as sheet, tube, fiber, and sphere are unable to be reproduced while upscaling the materials at an industrial level. Hence, there is a need to design efficient bulk production method for development of PCs with specific morphologies. Moreover, the PCs offer limited contribution

406

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

toward pseudocapacitance, which is an important requirement for efficient electrode material. Most importantly, the above features should be introduced maintaining the cost-effectiveness and environmental friendliness of the device. In addition, the assessment of the materials should always be verified in two-electrode system, the configuration for commercial devices, while the three-electrode study should only serve as a guide. In this aspect, there is still ample scope for the researchers to develop high-performance SC devices from biomass, which will reduce the cost of future energy storage devices. The “waste to wealth” approach is beneficial for both environmental remediation and future energy crisis. It is expected that the commercialization of these biomass-derived carbon materialebased SCs will open a new door for the energy sectors in near future.

Acknowledgments Sumanta Sahoo acknowledges DST-SERB, India, for the National Postdoctoral Fellowship (NPDF File No.: PDF/2017/000328).

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[129] M.-b. Wu, L.-y. Li, J. Liu, Y. Li, P.-p. Ai, W.-t. Wu, J.-t. Zheng, Template-free preparation of mesoporous carbon from rice husks for use in supercapacitors, New Carbon Materials 30 (5) (2015) 471e475. [130] S. Ahmed, A. Ahmed, M. Rafat, Supercapacitor performance of activated carbon derived from rotten carrot in aqueous, organic and ionic liquid based electrolytes, Journal of Saudi Chemical Society 22 (8) (2018) 993e1002. [131] W. Cao, F. Yang, Supercapacitors from high fructose corn syrup-derived activated carbons, Materials today energy 9 (2018) 406e415. [132] A.E. Ismanto, S. Wang, F.E. Soetaredjo, S. Ismadji, Preparation of capacitor’s electrode from cassava peel waste, Bioresource Technology 101 (10) (2010) 3534e3540. [133] C.-K. Sim, S.R. Majid, N.Z. Mahmood, Electrochemical performance of activated carbon derived from treated food-waste, International Journal of Electrochemical Science 10 (10157) (2015) e10172. [134] M. Chen, X. Kang, T. Wumaier, J. Dou, B. Gao, Y. Han, G. Xu, Z. Liu, L. Zhang, Preparation of activated carbon from cotton stalk and its application in supercapacitor, Journal of Solid State Electrochemistry 17 (4) (2013) 1005e1012. [135] D. Bhattacharjya, J.-S. Yu, Activated carbon made from cow dung as electrode material for electrochemical double layer capacitor, Journal of Power Sources 262 (2014) 224e231. [136] K. Karthikeyan, S. Amaresh, S.N. Lee, X. Sun, V. Aravindan, Y.G. Lee, Y.S. Lee, Construction of high-energy-density supercapacitors from pine-cone-derived highsurface-area carbons, ChemSusChem 7 (5) (2014) 1435e1442. [137] H. Feng, M. Zheng, H. Dong, Y. Xiao, H. Hu, Z. Sun, C. Long, Y. Cai, X. Zhao, H. Zhang, Three-dimensional honeycomb-like hierarchically structured carbon for high-performance supercapacitors derived from high-ash-content sewage sludge, Journal of Materials Chemistry A 3 (29) (2015) 15225e15234. [138] I.I.G. Inal, S.M. Holmes, E. Yagmur, N. Ermumcu, A. Banford, Z. Aktas, The supercapacitor performance of hierarchical porous activated carbon electrodes synthesised from demineralised (waste) cumin plant by microwave pretreatment, Journal of Industrial and Engineering Chemistry 61 (2018) 124e132. [139] R. Farzana, R. Rajarao, B.R. Bhat, V. Sahajwalla, Performance of an activated carbon supercapacitor electrode synthesised from waste Compact Discs (CDs), Journal of Industrial and Engineering Chemistry 65 (2018) 387e396. [140] C. Peng, X.-b. Yan, R.-t. Wang, J.-w. Lang, Y.-j. Ou, Q.-j. Xue, Promising activated carbons derived from waste tea-leaves and their application in high performance supercapacitors electrodes, Electrochimica Acta 87 (2013) 401e408. [141] P. Yu, Y. Liang, H. Dong, H. Hu, S. Liu, L. Peng, M. Zheng, Y. Xiao, Y. Liu, Rational synthesis of highly porous carbon from waste bagasse for advanced supercapacitor application, ACS Sustainable Chemistry and Engineering 6 (11) (2018) 15325e15332. [142] L. Jiang, J. Yan, L. Hao, R. Xue, G. Sun, B. Yi, High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors, Carbon 56 (2013) 146e154. [143] J. Jiang, L. Zhang, X. Wang, N. Holm, K. Rajagopalan, F. Chen, S. Ma, Highly ordered macroporous woody biochar with ultra-high carbon content as supercapacitor electrodes, Electrochimica Acta 113 (2013) 481e489. [144] J. Hou, C. Cao, F. Idrees, X. Ma, Hierarchical porous nitrogen-doped carbon nanosheets derived from silk for ultrahigh-capacity battery anodes and supercapacitors, ACS Nano 9 (3) (2015) 2556e2564. [145] S. Ghosh, R. Santhosh, S. Jeniffer, V. Raghavan, G. Jacob, K. Nanaji, P. Kollu, S.K. Jeong, A.N. Grace, Natural biomass derived hard carbon and activated carbons as electrochemical supercapacitor electrodes, Scientific Reports 9 (1) (2019) 1e15.

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[146] A. Durairaj, T. Sakthivel, S. Ramanathan, A. Obadiah, S. Vasanthkumar, Conversion of laboratory paper waste into useful activated carbon: a potential supercapacitor material and a good adsorbent for organic pollutant and heavy metals, Cellulose 26 (5) (2019) 3313e3324. [147] C. Zequine, C. Ranaweera, Z. Wang, P.R. Dvornic, P. Kahol, S. Singh, P. Tripathi, O. Srivastava, S. Singh, B.K. Gupta, High-performance flexible supercapacitors obtained via recycled jute: bio-waste to energy storage approach, Scientific Reports 7 (1) (2017) 1174. [148] F. Razmjooei, K. Singh, T.H. Kang, N. Chaudhari, J. Yuan, J.-S. Yu, Urine to highly porous heteroatom-doped carbons for supercapacitor: a value added journey for human waste, Scientific Reports 7 (1) (2017) 10910. [149] M. Fu, W. Chen, X. Zhu, B. Yang, Q. Liu, Crab shell derived multi-hierarchical carbon materials as a typical recycling of waste for high performance supercapacitors, Carbon 141 (2019) 748e757. [150] M. Fu, W. Chen, J. Ding, X. Zhu, Q. Liu, Biomass waste derived multi-hierarchical porous carbon combined with CoFe2O4 as advanced electrode materials for supercapacitors, Journal of Alloys and Compounds 782 (2019) 952e960. [151] G. Yang, S.-J. Park, MnO2 and biomass-derived 3D porous carbon composites electrodes for high performance supercapacitor applications, Journal of Alloys and Compounds 741 (2018) 360e367. [152] C. Feng, D. Chen, Y. Tang, S. Chen, Y. Liu, C. Zhu, W. Xie, L. Ye, Q. Zhang, P. Qian, Incorporation of MnO2 into egg yolk derived P, N, O-tridoped carbon for supercapacitors with excellent cycling stability, International Journal of Electrochemical Science 14 (2019) 8284e8295. [153] H.S. Kim, M.S. Kang, W.C. Yoo, Co3O4 nanocrystals on crab shell-derived carbon nanofibers (Co3O4@ CSCNs) for high-performance supercapacitors, Bulletin of the Korean Chemical Society 39 (3) (2018) 327e334.

CHAPTER 13

2D materialsebased flexible supercapacitors for high energy storage devices A. Arulraj1, S.T. Nishanthi2 1

Graphene and Advanced 2D Materials Research Group (GAMRG), School of Science and Technology, Sunway University, Selangor, Malaysia; 2Electrochemical Power Sources Division, CSIR- Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India

1. Introduction Recent scenario in the field of market focuses on developing modern equipment with ease of fabrication and flexibility to significantly enhance the consumer’s daily use. Specifically in electronics and automobile industries, consumers are more interested with the flexible and wearable devices as compared with conventional rigid one. Because, for both cases, it mainly needs power to run, this can be stored using energy storage device. Currently the manufacturing industries utilize rigid energy storage devices such as coin cells and batteries which are not compatible, thereby resulting in poor user experience, physical discomfort, etc. [1,2]. Hence to sort out the need, they have been a hot spot in both industry and academia. A myriad study has been carried out to investigate flexible energy storage (FES) devices and emerged with a solution of flexible supercapacitors (FSCs). The nature of two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and transition metal oxides (TMO) possessing versatile properties with high mechanical stability and roust compact degree of flexibility attracted renewed interest for application as electrode materials in energy storage devices [3,4]. Since the 2D materials mimic the nature of layered sheet like structure which helps in fast ion intercalation in supercapacitors (SCs) thereby boosting the performance of the devices. Besides its layered structure exhibiting the planar geometrics that are directly compatible to planar device configuration, the extensibility of 2D layered structure also craters to achieve an ion migration channel which is more beneficial for electrochemical performances in ultrathin flexible electrode. Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00013-0

© 2020 Elsevier Inc. All rights reserved.

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2. Supercapacitors SCs also known as electrochemical capacitors are state-of-the-art superfast rechargeable electrochemical energy storage devices bridging gap between batteries and fuel cells which store energy via two operating mechanisms: (1) Pseudocapacitance, a Faradaic process originated from the reductione oxidation (redox) reaction of the electrode material with an electrolyte [5,6]. The accumulations of electrons produced by redox reaction are transferred across the electrolyteeelectrode interface. (2) Electrochemical double-layer capacitance (EDLC), a non-Faradaic process resulting from the electrical double layer surrounding the surface of electrode [7,8]. In this, the depletion of the oppositely charged species stores the energy at the interface of the electrode and the electrolyte, respectively.

3. Cell design For characterizing SCs, two different configurations of testing apparatus are employed: (i) three-electrode and (ii) two-electrode systems. Threeelectrode configurations mainly focus on screening electrode materials with minimal amounts of active material, whereas the two-electrode systems resemble an architect of fully assembled cells, which evaluates the performance of a cell under less-than-ideal conditions.

4. Three-electrode system A typical three-electrode system consists of a working, reference, and counter electrode, altogether connected to a potentiostat. The potentiostat plays a role in controlling the electrode potential by recording the change in electrode current with potential. First, the working electrode is normally prepared by coating the active material onto the surface of a stable electrode (glassy carbon, mesh, conducting substrate or platinum metal). The active material is dispersed in a selected solvent (e.g., water, ethanol, or isopropanol) until an ink with a uniform dispersion is acquired. A desired amount of ink is dropped onto the surface of prepolished electrodes. Often, a small amount of polymeric binder (e.g., Nafion) is incorporated following the ink deposition to prevent the ink from diffusing away into the electrolyte. A reference electrode establishes a base potential in the threeelectrode system by acquiring a fixed potential. There are many different types of reference electrodes with discrete fixed potentials. A few common reference electrodes are normal hydrogen electrode, silver chloride

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electrode (Ag/AgCl), and saturated calomel electrode. Finally, a counter electrode (also known as an auxiliary electrode) balances the reaction that is occurring in working electrode by adjusting its potential. For this purpose, highly conductive yet inert materials such as platinum meshes or graphitic rods are employed as the reference electrodes.

5. Two-electrode system In a two-electrode system, there exist two active electrodes: a cathode and an anode. A separator is placed between these electrodes to prevent from short circuits. Metal plates as current collectors adjoin the two electrodes and their separator. A metal casing with three screws applying pressure evenly encloses the cell. Before testing, the cell is submerged in a vessel filled with electrolyte and then it is dried in a vacuum oven to remove any trapped air. The anode and cathode are synthesized by creating inks/pastes from the active material(s). The inks are subsequently pasted or sprayed onto the stable electrode sheets (e.g., carbon papers or carbon fibers) and trimmed to specified dimensions. To measure the potential of individual electrodes, a reference electrode can be installed in the cell; however, this may give extra design challenges.

6. Calculations 6.1 Cyclic voltammetry A qualitative and quantitative data of active materials in the working electrode can be accessed from cyclic voltammogram (CV) by means of an electrochemical phenomenon. In this technique, the two predefined potentials linearly sweep back and forth between potentials applied to the working electrode and reference electrode. The potential window is limited by the operating stability of an electrolyte. Scanning the potential range yields a time-dependent current, and plotting the obtained current (I) against the scanned potential (E) repels a CV curve for capacitance diagnosis R idV C¼ 2Vs DV where !idV is the integrated area under CV curve, Vs the potential scan rate, and DV the potential range. The specific capacitance (Cs) is obtained by dividing the capacitance by mass of the active materials Cs ¼

C m

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where C is the calculated capacitance and m the mass of active material. Moreover, potential scan rates have significant effects on measured capacitances. At lower scan rates (e.g., 0.005 mV s1), CV curves exhibit near ideal capacitive behavior with a rectangular-shaped curve; as the scan rate increases, the ideal rectangular curve gets distorted. By increasing it to extreme conditions, electrochemical kinetics cannot afford with potential change, which tends to decline the performance of capacitance. This kind of consequence can be commonly observed in pseudocapacitive materials.

7. Galvanostatic chargeedischarge An alternative method used to measure the capacitance of the active materials is galvanostatic chargeedischarge (GCD). It measures potential with respect to time by applying a constant current density (e.g., A/g). Generally, the working electrode is charged to a preset potential and then discharge process is monitored to assess the capacitance. In the case of EDLC materials, charge and discharge will be in linear, while in pseudocapacitive materials, it is nonlinear because of the redox reactions. Because of this discrepancy, each type of material has its unique equation to calculate its capacitance. For EDLC, the slope of the discharging section is utilized, which gives C¼

I dV =dt

where C is the capacitance of the material, I the applied current, and dV/dt the slope of the discharging GCD curve. For pseudocapacitive material, an altered form of the equation without the slope is employed C¼

ðDtÞI DV

where Dt is the total discharge time, I the applied current, and DV the potential difference at the discharging phase.

8. Energy and power densities Energy and power densities of SCs are imperative in diagnosing its deliverable performance for real-life applications. It can be determined from

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both CV and GCD techniques. Specific energy density (ED) and power density (PD) can be expressed as 1 2 Cs ðDV Þ ðWh = kgÞ 2 2 ðDV Þ PD ¼ ðW = kgÞ 4mRESR

ED ¼

where Cs is the specific capacitance from CV or GCD, DV the potential range, m the mass of the active material, and RESR the equivalent series resistance.

9. Two-dimensional materials As discussed in the introduction part, there are several materials available for SC applications. Among them, 2D materials (graphene, MoS2, WS2, etc.) are most charming because of their high porosity and thickness in confineddimension, which tends to high electrocatalytic activity. Moreover, owing to its fascinating mechanical integrity originating from layer-by-layer stacking of atom in 2D lattice plane to form a thin-film architecture, which provides possibilities for more active electrochemical sites, the large overlapping of area between each sheet/layer has great physicochemical properties to remain unaffected during blending or folding. These combined properties of 2D materials provide a platform to fabricate ultrahigh FSCs [11e14]. A scheme of 2D materials illustrating its properties, configuration, and flexible electrode is given in Figs. 13.1 and 13.2 [14]. Since the discovery of the exotic properties of graphene, 2D layered materials such as TMDs [15], TMO [16], and other 2D compounds have attracted renewed interest in application as energy storage and conversion devices.

10. Synthesis method A numerous reliable method has been pursued for fabricating 2D nanomaterials with suitable structure and optimal physicochemical properties. In general, not only the 2D nanomaterials but also all the nanomaterials can be synthesized by either of the two approaches: (i) top-down or (ii) bottom-up [17e19]. In the top-down approaches, three main techniques namely mechanical exfoliations [20e22], liquid exfoliation [23,24], and ion/moleculesintercalation assisted exfoliation [25,26] are used to exfoliate single or few

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Figure 13.1 Schematic illustration of (A) three-electrode and (B) two-electrode configurations setup. (Reproduced from I.M. Apetrei, C. Apetrei, Voltammetric determination of melatonin using a graphene-based sensor in pharmaceutical products, International Journal of Nanomedicine 11 (2016) 1859e1866. and K.C. Tsay, L. Zhang, J.Zhang, Effects of electrode layer composition/thickness and electrolyte concentration on both specific capacitance and energy density of supercapacitor, Electrochimica Acta 60 (2012) 428e436, Elsevier 2012.)

layer/sheets from the bulk stacked layers. Mechanical exfoliation also referred as Scotch-tape method is the most common strategy to extract single/few layers from the bulk crystals surface [27,28]. As no chemical reactions are involved, the obtained 2D materials remain to be their pristine structure and intrinsic properties that are suitable for the structureeproperty relationship investigation. However, the main constrain in this approach is the time; it consumes more time to extract as well as the yield will be very less, which limits in producing larger quantity. To overcome the limitations as well as synthesis 2D materials in large quantity, liquid exfoliation [13,29]

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Figure 13.2 The scheme illustrates the structure design of different types of twodimensional (2D) nanomaterials which will lead to the optimal performance of flexible supercapacitors. (Reproduced with the permission from X. Peng, L. Peng, C. Wu, Y. Xie, Two dimensional nanomaterials for flexible supercapacitors, Chemical Society Reviews 43 (2014) 3303e3323.)

and ion/molecule-intercalation assisted exfoliation [30e32] methods are introduced. The liquid exfoliation process utilizes an external stimulation forces, i.e., ultrasonication in solvents. A good matching of surface tension between solvents and produced nanosheets is critical for efficient exfoliation [23,33]. The most commonly used solvents for exfoliations are N-methylpyrrolidone and N,N-dimethylformamide. But this method also suffers from low yield and less control over the dimension of nanosheets. The ion/molecule-intercalation assisted exfoliation (Fig. 13.3) refers to intercalation of ions or molecules into the interlayer spacing of layer in the bulk form to enlarge interlayer distances and weaken the van der Waals interaction between layers. By means of this, exfoliation of layers from bulk takes place easily under ultrasonication in solvents [34].

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Figure 13.3 Schematic illustration of ion/molecules-intercalation assisted exfoliation [19].

Commonly used intercalators include organometallic compounds such as butyl lithium [35,36] or small molecules such as H2O [26,32] or NH3 [16]. Intercalating ions (e.g., Liþ and Hþ) into layered compounds can also be achieved via electrochemical routes, and the amount of intercalated ion can be monitored and controlled [25,30,36]. For fabrication of layered graphene, a graphite oxide is used as precursor that may be synthesized by Hummers’ method [37]. For graphene exfoliation, bulk graphite is first oxidized into graphite oxide by strong oxidizing agents and then exfoliated into graphene oxide (GO) sheets under ultrasonication [38]. The produced GO with abundant oxygen containing functional groups is an insulating material. Removing the functional groups to form graphene (e.g., reduced graphene oxide, rGO) can be achieved via chemical reduction [39], thermal annealing [40], or electrochemical reduction [41]. Direct synthesis of 2D materials relies on bottom-up strategy using various precursors by chemical reaction routes, mainly including chemical vapor deposition (CVD) [42,43] and wet chemical synthesis [44]. A highly crystalline with flexible size and thickness of various 2D materials (graphene, TMDs, etc.) can be synthesized from CVD method [42,45,46]. Its limitation lies in involving complicated and expensive procedure which requires high vacuum and temperature and a specific solid substrate to support the growth of 2D materials. One of the most common bottom-up approaches is wet-chemical synthesis, which includes self-assembly, hydro/ solvothermal, template-assisted, colloidal, etc., which is also used to prepare 2D materials. These approaches have been demonstrated effective fabrication of materials from its precursors via chemical reactions in solution [47e49]. Moreover, these can be used to realize high reaction yield for mass production at relatively low cost with desired size, shape, and morphology by tuning the reaction parameters (Fig. 13.4).

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Figure 13.4 Schematic illustration of the wet-chemical synthesis of two-dimensional nanosheets: (A) ordered mesoporous graphene nanosheets; (B) single- and multilayer transition metal sulphide sheets. (A) Reproduced with the permission from Y. Fang, Y. Lv, R. Che, H. Wu, X. Zhang, D. Gu, G. Zheng, D. Zhao, Two-dimensional mesoporous carbon nanosheets and their derived graphene nanosheets: synthesis and efficient lithium ion storage, Journal of American Chemical Society 135 (2013) 1524e1530; (B) Reproduced with the permission from D. Yoo, M. Kim, S. Jeong, J. Han, J. Cheon, Chemical synthetic strategy for single-layer transition-metal chalcogenides, Journal of American Chemical Society 136 (2014) 14670e14673.)

11. Graphene-based flexible energy storage devices Graphene, one of the most representative defect-free carbon allotropes discovered by Novoselov and Geim in 2004 [27], is a leading runner in the field of 2D materials attracted by many researchers because of its outstanding chemical/physical properties. Recently, increasing research efforts have been devoted to the functionalization of graphene through chemical modification or its derivatives with organic and inorganic

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molecules [50] resulting in synthesizing large number of useful graphene and graphene-based composites. Graphene has high surface area of w2630 m2 g1 (theoretical) [51] and superior mechanical strength, thereby employed as FES devices from its discovery. El-Kady et al. [52] reported deposition of graphene on flexible substrate using direct infrared laser radiation on DVD optical drive. In this method the restacking of graphene sheets is avoided and best electrochemical performances have been achieved with excellent cyclic stability. Z. Niu et al. [53] reported a “leavening” process to transform compact graphene paper to porous graphene film and built FSCs using free-standing rGO foams as both current collectors and electrodes. They used a filtrationassembled graphene paper as dough and hydrazine vapor as foaming and reducing agent. In this chapter, the authors concluded that the hydrazine vaporeinduced gaseous species (such as H2O and CO2) were responsible for the formation of the porous structure and achieved performance of 110 F g1 under 2e3 cm bending (Fig. 13.5). Similarly, Choi et al. [54] reported graphene-based FSCs by easy assembly of functionalized reduced graphene oxide (f-rGO) as working electrode and used solvent-cast Nafion electrolyte membranes (as electrolyte and separator) . In this work, the author reported that Nafion acts as electrochemical binder as well as separator, by means of which the RCT of f-rGO was lower than that of rGO, which was attributed to close contact between the electrode and the electrolyte, and thus the enhanced electrochemical performance displayed. As a consequence, the f-rGO-based SCs showed higher specific capacitance (118.5 F g1 at 1 A g1) and rate capability (retention rate 90% at 30 A g1) compared to all-solid-state graphene SCs (62.3 F g1 at 1 A g1 and 48% retention at 30 A g1). Besides, the f-rGO-SCs clearly exhibited more capacitive and less resistive behavior compared to the rGO-SCs, and the facilitated ion diffusion at the electrical double layer is proven by the fourfold faster relaxation of the f-rGO-SCs than that of the rGO-SCs and the greater capacitive behavior of the former at the low-frequency region where the ion diffusion occurs.

12. Transition metal dichalcogenideebased flexible energy storage devices TMDs belong to 2D compounds with general formula MX2, where M is the transition metal atoms (Mo, W, etc.) and X is the chalcogen (S or Se). Here, the metal atom belonging to fourth, fifth, or sixth group is sandwiched between chalcogen by means of strong covalent bond, and the layers are held together by means of Van der Waals interactions forces

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Figure 13.5 (A) Schematic representation of leavening process to prepare rGO foam; (B) Schematic diagram; (C) Optical image of the flexible rGO foam supercapacitor; (D, E, F, G) CV curves, Charge and discharge curves, Nyquist impedance plot and CV of the rGO foam supercapacitor before bending (l ¼ 3 cm) and while bent (l ¼ 2 cm). (Reproduced with the permission from Z. Niu, J. Chen, H.H. Hng, J. Ma, X. Chen, A leavening strategy to prepare reduced graphene oxide foams, Advanced Materials 24 (2012) 4144e4150.)

[55e57]. This kind of layered structure can be easily exfoliated into a single sheet of layer which repels in enriching the capacitance behavior by increasing its contact area with electrolyte [58,59]. Moreover, inherent electrochemistry of TMDs offers large oxidative and reductive current signals upon potential scanning [57,60]. Because of its combined properties of 2D TMDs, researchers made more efforts on developing FSCs using this (especially MoS2, WS2, TiS2, etc.). MoS2, a representative of 2D TMDs family, has attracted much attention in SCs owing to its unique structure, abundance, and high

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capacitance [61,62]. MoS2 exhibits two crystalline phase structure (i) 2H phase (semiconductor) with triangular prism structure, which demonstrates poor rate capability, and (ii) 1T phase (metal) with octahedral structure dominating single layer of sheets, which provides high rate capability [25,58,63]. The large surface area in MoS2 provides more active sites for charge storage in EDLC and it also offers pseudocapacitance because of the wide range of oxidation state of Mo atom in MoS2 [61]. In this regard, M. S. Javed et al. hydrothermally synthesized MoS2 on carbon cloth for the first time and reported a high specific capacitance of 368 F g1 at scan rate of 5 mV s1. The fabricated MoS2-based FSCs deliver long-term stability with 3.5% reduction in capacitance after 5000 cycles. The capacitance behavior appears to be almost same for different bending angles [64].

13. Hybrid-based flexible energy storage devices B. Wang et al. developed intercalated graphene/MoS2 hybrid fiber using one-step hydrothermal synthesis and its schematic is given in Fig. 13.6. This intercalated nanostructure exhibits large ion-accessible surface areas and high active material contents up to 33.98 wt% and achieved high specific capacitance of 368 F cm3 [65]. Y. Lv et al. [66] reported new type of all-solid-state flexible hybrid SCs comprising of (cellulose nanofibers (CNFs)/(MoS2)/(rGO)) aerogel film as an electrode material and charge collector using supercritical CO2-assisted method (Fig. 13.6). Owing to its porous structure and remarkable electrolyte absorption properties, the SCs exhibit 916.42 F g1 at a scan rate of 2 mV s1 with retention of more than 98% even after 5000 chargee discharge cycles. In addition, the areal capacitance, areal power density, and energy density of the SCs are reported with 458.2 mF cm2, 8.56 mW cm2 (4.3 kW kg1), and 45.7 mWh cm2 (22.8 Wh kg1), respectively [66]. Similarly, F. Clerici reported rapid one-pot synthesis of MoS2-decorated laser-induced graphene (MoS2-LIG) on polyimide foils. The schematic of laser writing process and its morphology is given in Fig. 13.6. Owing to its morphology, it allows high-rate transportation of electrolyte ions and electrons throughout the electrode network while the in situ decoration with MoS2 flakes permits the comprehensive utilization of pseudo- and double-layer capacitance, resulting in excellent electrochemical performances [67].

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Figure 13.6 (A and B) Schematic representation of graphene/MoS2 fabrication with its CV performance; (C and D) fabrication of cellulose nanofiber/ MoS2/rGO hybrid electrodes by supercritical CO2-assisted method with its electrochemical performance [66]; (E and F) scheme of fabrication MoS2decorated laser-induced graphene with its field-emission scanning electron microscope images and its applications. (A and B) Reproduced with the permission from B. Wang, Q. Wu, H. Sun, J. Zhang, J. Ren, Y. Luo, M. Wang, H. Peng, An intercalated graphene/ (molybdenum disulfide) hybrid fiber for capacitive energy storage, Journal of Materials Chemistry A 5 (2017) 925e930; (C,D,E and F) Reproduced with the permission from F. Clerici, M. Fontana, S. Bianco, M. Serrapede, F. Perrucci, S. Ferrero, E. Tresso, A. Lamberti, In situ MoS2-decoration of laser induced graphene as flexible supercapacitor electrodes, ACS Applied Materials Interfaces 8 (2016) 10459e10465.)

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14. Recent developments The change in generation and the need of modern equipment always stipulate the researchers to move forward in finding interesting devices and replacing the benchmark of their own or other’s. The latest development in electrode materials for FSCs also gives rise to researchers to trend of searching new alternative materials with both superior electrochemical performance and excellent mechanical properties. To an interesting fact in addition to that of developing novel active materials, researchers started working on configuration design of the devices and emerged with a concept called “planar SCs.” FSCs work at different folding/bending states and it requires the ions to transport smoothly under mechanical strain effects without degradation. In the case of planar configuration, an additional problem of short circuit may takes place, so all the facts have to be considered and required to push the performance of FSCs to higher level. Planar SCs have proven to be an alternative option by optimizing ion transport mechanism with 2D migration channels which travel short distance by eliminating the need of separator. It also reduces the thickness of layers/sheets in vertical direction rather expanding along intrinsic 2D horizontal plane making compact device with efficient charging/discharging process. Sun et al. reported fabrication of graphene-based flexible planar micro-SCs using microextrusion technique. In this report, the graphene was printed on flexible substrate in the layer-by-layer form as shown in Fig. 13.7. The laminated graphene films and polyvinyl alcoholeH2SO4 gel serve as the interdigitated microelectrodes and electrolyte, respectively. The resultant flexible micro-SCs exhibit high capacitive performance with excellent flexibility and cycling stability. Such device promises potential applications in flexible electronics and lab-on-a-chip systems [68].

15. Future opportunities and challenges There is no doubt that synergetic improvements have been achieved in the fabricating inexpensive, lightweight, flexible, and environmentally friendly energy storage devices. In this regard, integration of 2D materials (graphene, MoS2, WS2, and its composites) synergizes its advantageous features with effective conducting component in many energy storage devices because of its high electrical conductivity. However, there remain some constraints such as high reversible capacity and low coulombic efficiency

2D materialsebased flexible supercapacitors for high energy storage devices

GO

431

5

Programme d Patterning

Reduction using HI

Thickness (µm)

Substrate

4 3 2 1 0

Electrolyte Coating PVA-H2SO4gel electrolyte

0 rGO

1

2 3 No. of Layers

4

Figure 13.7 Schematic illustration of device fabrication; field-emission scanning electron microscope images and its corresponding capacitance behavior of ultrathin planar flexible supercapacitors. (Reproduced with the permission G. Sun, J. An, C. K. Chua, H. Pang, J. Zhang, P. Chen, Layer-by-layer printing of laminated graphene-based interdigitated microelectrodes for flexible planar micro-supercapacitors, Electrochemistry Communications 51 (2015) 33e36, copyrights Elsevier.)

because of its high surface area in 2D materials. These high surface areas also aggravate unwanted side reactions during cycling, which includes decomposition of an electrolyte, particularly at the anode side. The decomposition products of an electrolyte tend to dissolve during charging, which will deteriorate the electrolyte and result in a poor life cycle. In addition to that, the decomposition may produce harmful gases which potentially cause safety issues. 2D morphology of the electrodes may also lead to a low tap density, which limits the volumetric energy densities of the systems. Finally the thickness in the layered materials plays key role in determining the energy density (i.e., increase in thickness of film leads to decrease in energy density value). To ensure that the inventory of inherently flexible electrode, an outside box thinking is needed in seeking appropriate fabrication protocols.

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16. Summary In summary, this chapter provides an overview of the 2D materials (graphene, MoS2, WS2, hybrid, etc.) fabrication and their recent applications in FES devices. FSCs are receiving tremendous attention in research because of its increasing need for flexible/wearable energy storage devices. Recent progress made on 2D materials based FSCs, with a special focus on the fabrication methods; electrode configuration, planar micro-SCs, and their electrochemical performance evaluation are briefly discussed. Although recent developments in 2D materials for FSCs appear extremely promising, there remain some challenges such as mechanical flexibility, large volume of production, rational design of electrode structures, and controlled intercalations reaction between electrode/electrolyte interfaces, etc., which limits its applications in transferring on technology in pilot scale. To overcome all the challenges and inventory of inherently flexible electrode, an outside box thinking is needed.

Acknowledgments Dr S.T. Nishanthi acknowledges start-up grant and the Director, CSIR-CECRI, Karaikudi. Dr. A Arulraj expresses sincere gratitude to his Ph.D. supervisor Dr G. Senguttuvan, Professor and Head, Department of Physics, UCE-BIT Campus, Tiruchirappalli, for constant support and encouragement.

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CHAPTER 14

Nanostructured transition metal sulfide/selenide anodes for high-performance sodium-ion batteries K. Karuppasamy1, Vasanth Rajendiran Jothi2, A. Nichelson3, Dhanasekaran Vikraman1, Waqas Hassan Tanveer4, 5, Hyun-Seok Kim1, Sung-Chul Yi2, 6 1

Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, South Korea; Department of Chemical Engineering, Hanyang University, Seongdong-gu, Seoul, South Korea; Department of Physics, National Engineering College, K.R. Nagar, Kovilpatti, Tamil Nadu, India; 4 Research Centre for Carbon Solutions, Heriot-Watt University, Edinburgh, United Kingdom; 5 Department of Mechanical Engineering, School of Mechanical and Manufacturing Engineering, National University of Sciences and Technology (NUST), Islamabad, Pakistan; 6Department of Hydrogen and Fuel Cell Technology, Hanyang University, Seoul, South Korea 2 3

1. Introduction In the evolution of different types of rechargeable batteries, only lithiumion battery (LIB) is commercially successful, which is now being operated in almost every portable devices [1]. The performances of LIB are investigated extensively in its constituents such as anode [2,3], electrolyte [4e10] and cathode [11e15]. But its low power density, poor cycle life, and safety issues of lithium-based batteries made scientists to pursuit for an alternative material which might have almost the similar ability as lithium in storing and dispensing energy. On the other hand, sodium lies just beneath lithium, in the same period, which is ingrained with some indistinguishable chemical properties, including electrochemical reactivity and ionicity. Furthermore, the comparable electrochemical performances and synthetic protocols make the sodium-ion batteries (SIBs) successful development into batteries like its lithium ion counterpart. With the 3 decades of research experience with LIBs, researchers inclined their studies toward SIBs in the past decade which can store electricity in the same industrial standard as LIBs. The SIBs share a similar operation mechanism like LIBs, so with this logic a quick evolution happened in the material investigation of SIBs. The abundance of sodium is another reason for the widespread investigations on SIBs nowadays. Furthermore, the prime advantages of SIBs over LIBs are its Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00014-2

© 2020 Elsevier Inc. All rights reserved.

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fast charging capacity and cycling behavior, i.e., the SIBs can charge until 4C and the cycling behavior is very easy compared with the LIBs because SIBs can discharge the battery until 0 V, making it free from leakage of electric current; thus these batteries are quite safe while storing or shipping. However, the common problems that we have not yet been able to resolve are the size of battery (slightly larger than LIBs housing the same volt) and it drains out charge faster than the LIBs, which unfortunately makes them pretty ordinary as charge carriers. In spite of those noticeable issues, the technology of emerging SIBs as an alternative for the expensive LIBs has been greeted by the electronics and automobiles industries in common in the recent years. Because of such extensive application, and in the perspective of the safety issue of energy storage devices, there are substantial efforts by research communities to overcome the problems they are facing now. One of the most inspiring approaches to strengthen safety as well as power and energy densities for SIBs is to identify long-term stable and cost-effective materials. In this chapter, we describe the recent developments and trends of anodes of metal chalcogenides in the field of SIBs to identify several similar energy systems for harvesting energy to future scientists. The content of this chapter are further subdivided into (a) working principles of SIBs; (b) components of SIBs; (c) anodes for SIBs; and (d) metal sulfide and selenide anodes for SIBs applications.

2. Working principle of sodium-ion batteries The alkali elements in group I of the periodic table (group IA) are highly reactive and have the tendency to lose their valence electron to become cations with charge which aids alkali atoms to create ionic bonds with other elements. Furthermore, the alkali elements in group I are analogous to each other, so the working mechanism of rocking chair SIBs [16,17] is exactly similar to LIBs as shown in Fig. 14.1A. It should be noted that clear picture of the sodium transport properties is highly essential in developing novel electrode materials with specific structures (Fig. 14.1B). At very first in sodium storage electrodes, it was presumed that because of the larger radius of Naþ iond97 p.m., the transport of Naþ ions would be sluggish. However, on a longer run, it was realized that the Na transport properties are highly connected to the crystal morphology (A-O coordination, octahedral or prismatic). Ceder et al. demonstrated that in case of layered structure, the Naþ ion migration barrier could be lesser than that of Liþ ion [18e20].

Nanostructured transition metal sulfide/selenide anodes for high-performance

Figure 14.1 (A) Earth abundance elements ; (B) various components of sodium-ion batteries (SIBs); (C) various cathode employed in SIBs; (D) different anodes employed in SIBs; (E) different electrolytes used so far in SIBs; (F) different binders used so far in SIBs [21]. (A) Taken from G. Haxel, Rare Earth Elements: Critical Resources for High Technology, US Department of the Interior, US Geological Survey, 2002. Copyright 2019 The Royal Society of Chemistry.)

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Figure 14.1 Cont’d.

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The sodium-based layered electrode materials are classified into two groups as O3 type or P2 type, where the sodium ion occupies the octahedral and prismatic sites, respectively, as illustrated in Fig. 14.1C [21]. O3-type NaFeO2 is surprisingly electrochemically active. For instance, the reaction mechanism with O3-type NaMnO2 and graphitic carbon electrode materials are given below (Eqs. (14.1)e(14.6)). During charging process, Cathode: NaMnO2 / Na1x MnO2 þ xNaþ þ xe þ



(14.1)

Anode: C þ xNa þ xe /Nax C

(14.2)

Overall: C þ NaMnO2 /Na1x MnO2 þ Nax C

(14.3)

whereas during discharge Cathode: Na1x MnO2 þ xNaþ þ xe / NaMnO2

(14.4)

Anode: Nax C / C þ xNaþ þ xe

(14.5)

Overall: Na1x MnO2 þ Nax C/C þ NaMnO2

(14.6)

3. Active components of sodium-ion batteries 3.1 Cathodes Of the components of SIBs, cathode plays a crucial role in the electrochemical performance because they are considered to be one of the significant factors that decide the overall cost of a battery. A good cathode material can be recognized by its overall performance with respect to rate capability, capacity retention, cycle life, and specific capacity. In the recent times, widely investigated cathode materials are the transition metal oxides (TMO) in the form of layered oxides and tunnel oxides, olivine-phased NaFePO4, NASICON Na3V2(PO4)3, pyrophosphates, fluorophosphates, sulfates, organic compounds, and metal hexacyanometalates as shown in Fig. 14.1C. Among different cathode materials studied earlier for SIBs, the materials include TMO, NaFePO4, NaMnO2, and Na4Fe(CN)6, provided excellent electrochemical and cycling performances at ambient temperature and discussed in brief as follows. For instance, the cathode material of layered TMO provided the highest specific capacity of 600 mAh g1 at ambient temperature, which is threefold higher than that of the earlier reported specific capacities: 180e220 mAh g1 for NaFePO4 adding to these features; this material can also be

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operated between the potential range of 2.7 and 3.2 V (vs. Naþ/Na). Furthermore, the TMO’s feasibility in the structural network displayed a very good cycling stability even after 100 cycles [22]. On the other hand, the amorphous layered NaFePO4 and pteridine derivatives are predicted as encouraging cathode materials for SIBs because of their outstanding electrochemical properties [23,24]. Likewise, layered Na0.6MnO2 material prepared by solegel process resulted in maximum specific capacity of 140 Ah kg1 at low current density rate in the potential region of 3.8 and 2.0 V [25]. Similarly, when Prussian blue crystals using Na4Fe(CN)6 (as iron source precursor) were investigated as cathode material, an excellent structural stability with good ion storage capacity was achieved as result of low crystal water content and less vacancy inertial sites for Fe(CN)6 [26].

3.2 Electrolytes used in sodium-ion batteries One of the conceivable ways to enhance the overall battery performance is by designing a better electrolyte with high ionic conductivity, chemical and thermal stabilities, and excellent electrochemical properties because it forms a major part accountable for the shelf-life and realistically achievable performance (Fig. 14.1E). From the footprints of the LIB electrolyte (LiClO4 or LiPF6) followed the developments of SIB electrolytes such as nonaqueous electrolytes (NaClO4 or NaPF6, which is synonymous to the one employed in LIB), polymer electrolyte, and ionic liquidebased electrolytes. From past one decade, extensive research efforts have been committed to develop a better electrolyte and at present only the aforementioned three types are commercially successful [20,27]. Among the nonaqueous electrolytes such as BF4, ClO4, PF6, CF3SO3(Tf), and [N(CF3SO2)2] TFSI of sodium salt, Tf is widely accepted because it is free from the following issues related with other anions such as ClO4 (strong oxidant), BF4 (less conductive), PF6 (chemically unstable), and TFSI (readily undergoes corrosion). Polymer electrolytes in particular gel type which constitutes a salt, ionic liquid, binder (Fig. 14.1F), and polymer host have a number of advantages over nonaqueous electrolytes such as enhanced mechanical stability, better anode compatibility without dendrite layer formation, etc. For example, a gel polymer electrolyte prepared by solvent cast technique used sodium triflate (NaCF3SO3) salt, 1-ethyl-3-methylimidazolium trifluoromethane sulfonate (EMITf) ionic liquid, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) as polymer host. The observed findings revealed the highest ionic conductivity of 5.74  104 S cm1 at room temperature [28]. Because

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a number of electrolytes in both types (nonaqueous, gel electrolytes) have been investigated in the past, our ultimate focus here is to provide detailed investigations on SIB’s anode which is discussed below.

3.3 Anodes To construct SIBs with great efficiency, anode materials should be coupled with appropriate cathode materials during which several factors have to be taken into account such as the relevant specific capacity, states of charge or discharge, working potential range, etc. Nevertheless, the majority of research studies only target on the development of single-electrode materials while overlooking the significance of their coordinated performance in full batteries. Generally, in LIBs, pure graphite material has been attempted as anodes, but when employed in SIBs, pure graphite leads to form only NaC64, which does not favor in intercalation process because of its unreliable thermodynamic condition. Safety and cycling are the major characteristics of anodes in metal ion batteries. Graphite as anode in SIBs is a failure and as a result researchers moved on to hard carbon anodes and organic anodes. Like graphite, the naturally available cellulose could be used as a cheap carbon source for large-scale production, which can also act as a soft substrate for tin anodes. The different anode materials for SIBs are pictorially represented in Fig. 14.1D.

3.4 Transition metal oxides The possible utility of TMOs for the negative electrode has not been much reported in SIBs. An alternate for carbon electrodes in SIBs was first discussed by Doeff et al. Yinzhu Jiang et al. did a series of studies on TMO as anodes for SIBs for the first time. The sodium intercalation and deintercalation was verified by the reversible conversion reaction. A reversible capacity of 386 mAh g1 at 100 mA g1 was achieved over 200 cycles for Fe2O3 anode, and a sustainable capacity of 233 mAhg1 was obtained at a large current density of 5 A g1 [29]. NiCo2O4 spinel oxide as electrode material in sodium and sodium-ion cells in SIBs was examined by Alcantra et al. [30]. Hui Xong et al. without the application of binders and additives directly have grown amorphous titanium dioxide nanotube (TiO2NT) on current collectors as anode material to use for SIBs. When TiO2NT anode was paired with Na1.0Li0.2Ni0.25Mn0.75Od cathode, the resulting SIB exhibited good rate capability with a reversible capacity of 150 mAh g1 in 15 cycles and found that electrochemical cycling with sodium ions can be supported only on amorphous nanotubes with large diameter (>80 nm) [31].

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Despite several reports on positive electrode materials being proposed, developing appropriate negative electrode materials for SIB is still in research. Recently, P2eNa0.66[Li0.22Ti0.78]O2, a layered type of material, which is capable of showing only 0.77% change in volume during sodium insertion/extraction, was proposed as the negative electrode. The electrode material exhibited 0.75 V average storage voltage with 1  1010 cm2 s1 Naþ diffusion coefficient and also has a long cycle life because of the zerostrain characteristics [32]. A similar type anode Na2Ti3O7 was studied by Premkumar and his coworkers. It provided a good intercalation of sodium ion in low voltage potential of 0.3 V. This material exhibits a capacity of 200 mAh g1. This low potential reversible reaction of sodium was the first ever reported. Some more improvements may lead to a different direction of research in the development of room-temperature high-performing Na ion cells [33]. Among P2-type layered oxides, P2eNa0.6[Cr0.6Ti0.4]O2 is capable of intercalating strongly between charge ordering transition metal layer and the layer of alkali metal to form Naþ/Naþ ion transport kinetics. Metals such as Cr3þ and Ti4þ having similar ionic radii and different redox potentials were chosen to prevent Naþ/vacancy ordering. A symmetric SIB using the same P2eNa0.6[Cr0.6Ti0.4]O2 electrode exhibited 75% of the initial capacity at 12C rate. Wang et al. by shattering the charge ordering in the transition metal layer prevented Naþ/vacancy and demonstrated an easy approach to develop disordered electrode materials with long cycle life and high power density [34].

3.5 Metal sulfides Although there are various anode materials for SIBs, the sulfide-based anode materials are performing well enough in the past one decade [35,36]. Scrutinizing better performance anode materials are recently paid more attention and it becomes a huge task for progressing the SIBs. Till date, only a few layered transition metal sulfides such as (MSx; M ¼ Mo, W, Sn, Zr, and V) were employed as anodes for SIBs. The reason for choosing such layered metals is because of the fact that their redox variabilities and structural peculiarities lead to better electrochemical and cycling stabilities than other transition metal atoms, which could make them as a potential candidates for sodium anode materials [37,38]. However, the common disadvantages that we come across while using metal sulfides alone as anode are in the context of sodium ion storage abilities. In brief, the sodium ion storage mechanism in transition metal sulfides is based

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on the conversion reaction to form Na2S but if the transition metal may take up Na which leads to happen both the conversion and alloying reactions. There are plenty of inherent factors based on such complexity which are provided below: (A) decomposition into multiphase due to mass transport and kinetic factors in terms of reactivity [39], (B) while conversion and alloying reactions, large volume change occurs, which in turn leads to agglomeration of electrode material, thereby detaching the electrode from current collector resulted in poor cycling performances [40], (C) the electronic conductivities of transition metal sulfides are very low which reflects in poor cycling and rate performances [41]. The better way to overcome the above said factors to improve the storage of sodium in SIBs is to design different types of nanostructures to reduce the size of the transition metal sulfides and achieve definite morphology to reduce the transport length of metal ions. Some of the few examples are listed below. For instance, the iron sulfide Fe3S4, a transition metal sulfide material have employed as an anode material for a rechargeable SIBs in which the Fe3S4 could be used as primary host material for storage of sodium as it was evident through conversion mechanism. It is further noticed that the quantum-sized FeSx compound produced by conversion reaction obstructed the constrains in its kinetics and thermodynamic chemical conversion to achieve greater rate and cycling capabilities. From the obtained findings, the as-prepared Fe3S4 anode offered the maximum reversible specific capacity of 548 mA h g1 at 0.2 A g1 and possessed excellent capacity retention even after 3500 cycles at 20 A g1 [42]. Likewise, WS2 nanowires (WS2 NWs) have been synthesized and reported by Liu et al. The WS2 NWs were synthesized by facile solvothermal process having the diameter and extended interlayer spacing of 25 and 0.83 nm, respectively. The as-prepared WS2 NW anodes provided two various potential windows including 0.01e2.5 V and 0.5e3 V, respectively, at ambient temperature. Furthermore, the cycling stability analysis revealed that the prepared electrode comprised an excellent capacity of 605.3 mAh g1 at 100 mA g1. However, the irreversible conversion reaction occurred in the voltage window between 0.01 and 2.5 V. On comparing its performance in different potential window, at 0.5e3 V range, they showed the reversible intercalation and the NW framework is established well. Henceforth, it was confirmed that the WS2 interlayers are progressively elongated and exfoliate while repeated cycling tests which in turn facilitates charge carrier sites and provided an open channel paths for the Naþ intercalation, electronic and ionic diffusion

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which shows WS2 NWs are the most potential electrode material for SIBs at ambient temperature [43]. In a similar manner, WS2-CNT-reduced graphene oxide (RGO) aerogel composite was prepared by facile solvothermal approach using N,N0 dimethylformamide as solvent. The synthesis procedure involved four major steps such as mixing of starting materials, solvothermal growth, freeze dry, and postannealing process. The prepared aerogel possessed an excellent ordered three-dimensional (3D) microchannel scaffold structure. Owing to this unique 3D microchannel morphology, the aerogels provided a pathway for the electronic transportation and also for ionic conductive channels, thereby leading to enhance the electrochemical performances in both LIBs and SIBs. The maximum capacity found for aerogel was 749 mAhg1 at 100 mAg1 with the coulombic efficiency of 53.4% for LIBs. On the other hand, it delivered the highest capacity of 311.4 mAhg1 at 100 mAg1 for SIBs. The outstanding electrochemical performance is owing to the fact that a synergistic effect caused between the CNT-RGO scaffold and WS2 nanosheets and also the coherent design of 3D ordered nanostructure. The observed outcome from different characterization analyses confirmed that the CNT-RGO aerogel could act as a platform for transition metal sulfide (i.e., WS2) to improve the efficiency of the cell for energy storage device applications [44]. A cost-effective hierarchical free-standing MoS2 nanosheet was grown vertically on the carbon paper and has been used as a potential anode for SIBs in the recent years. Initially, the carbon paper was derived from waste material paper towel. The microstructure of the electrode enables the fast electron migration and provided enough interaction between electrode and electrolyte interfaces. The prepared self-standing MoS2@carbon paper anode provided a better reversible capacity and excellent rate capability with retention properties for 50 consecutive cycles. They further observed that while insertioneconversion process, the 2HeMoS2 was converted into 1T-MoS2, which is evident through in situ Raman analysis, which in turn favors the electrochemical performance and storage for sodium in SIBs [45]. Similarly, a simplistic way of microwave-assisted MoS2/RGO composites synthesis was achieved by Mo and S precursor solutions followed by annealing at 800 C for 2 h under H2/N2 atmosphere. The obtained MoS2-RGO composites were subjected to fabricate SIB coin cells, which displayed the highest reversible capacity of about 305 mAh g1 at 100 mA g1 after 50 consecutive cycles. The excellent rate performance suggested that MoS2-RGO might be a well-suited candidate as an anode material for

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rechargeable SIBs [46]. Furthermore, the other studies such as on MoS2/ graphene composites may also help to understand the heterointerface and the electrochemical performance and sodium ion storage in SIBs [47]. The different micro-nanostructures of cobalt sulfide (CoS2) such as primary octahedron (P-CoS2), hollow architecture octahedron (HeCoS2), and nanooctahedron (O-CoS2) were systematically prepared via solvothermal route by Xue Liu et al. [48] to improve the storage of sodium ions in SIBs. It is interesting to observe that the investigated nanostructured CoS2 involved in conversion reaction results in pulverization of CoS2 at charged state. The fabricated H-CoS2 incorporated SIB cell decreased the specific capacity from 690 to 240 mAhg1 at 1 Ag1 when the potential difference plateau changes from 1.0e3.0 to 0.1e3.0 V. Its corresponding insertion and conversion reactions happened at 1.4 and 0.6 V, respectively, and the cell reaction Eqs. (14.7), (14.8) is described below for better understanding, CoS2 þ xNaþ þ xe /Nax CoS2

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The capacity fading was further analyzed through high-resolution transmission electron microscope (HR-TEM) analysis and the cells were subjected to TEM analysis before and sodiation. The internal structures of the H-CoS2 for the 1st and 10th cycles were obtained through HR-TEM analysis and it is displayed in Fig. 14.2A and B, which indicates that the intercalation and deintercalation of sodium above 1 V provided H-CoS2 nanoparticles with the average size of 10 nm and below 0.1 V provided the fine nanoparticle structure with size of 1e4 nm. Among the different CoS2 nanostructures prepared, the H-CoS2 exhibited the maximum capacity of 411 mAhg1 as shown in Fig. 14.2C and D and discussed above [48]. Similarly, Shadike et al. [49] reported the CoS2/multi-walled carbon nanotube (MWCNT) composites as anode for SIBs. The anchoring of CoS2 nanoparticles on the surface of MWCNT was confirmed by HR-TEM analysis, which consisted of agglomerated nanoparticles having the size of 150e200 nm. Furthermore, the prepared composites were subjected into HR-TEM after sodiation which provides nanograins with the size of about 5 nm which distributed evenly on the CNT matrix. Owing to its excellent microstructure, it offered 690 mAhg1 along with the energy density of 1027 WhKg1, which in turn suggests that the CoS2/ MWCNT composite could be the promising anode for SIBs.

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An alternative type of CoS2-based anode for high-performance LIBs and SIBs was synthesized through the bottom-up approach which involved two steps as shown in Fig. 14.3A: (A) first step involves the polymerization of polydopamine (PDA) layer over the Ni-Co coordination polymer (NiCoCP) surfaces, which leads to form a bimetallic NiCoCP@PDA coreeshell nanostructured cubes (Fig. 14.3B and C); (B) second step involves the sulfurization of initial product at high temperature, which results in biactive NiS2@CoS2 nanocrystals wrapped into nitrogen-incorporated carbon coreeshell nanocubes and its corresponding morphology structure is provided in Fig. 14.3BeN. During electrochemical and cycling tests, the bicontinuous carbon encapsulated NiS2@CoS2 nanocrystals exhibited outstanding storage capacities for lithium/sodium ions, which resulted in very good capacities and cycling stabilities at ambient temperature. The

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Figure 14.3 (A) Schematic preparation of NiCoCP@PDA nanocubes; (B, C) SEM and TEM image of NiCoCP nanocubes; (D, E) SEM and TEM image of NiCoCP@PDA nanocubes; (F, N) SEM, HR-TEM, and elemental mapping images of NiCoCP@PDA@C nanocubes; (O) rate capability curve of NiCoCP@PDA@C nanocubes at the current densities from 0.1 to 5 Ag-1; (P) first 10 consecutive charge discharge cycles at 100 mAg1 for NiCoCP@PDA@C nanocubes [50]. PDA, polydopamine. (Copyright 2019 Elsevier.)

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improved electrochemical performance is explained by its porous nature, especially the mesoporous structures of double outer layered carbon frameworks tend to decrease the diffusion length for electrons and ions, protect the volume expansion during intercalationedeintercalation process, and finally maintain the structural integrity for the internal components [50]. An ultrafine new approach applied for synthesizing SnS2/RGO was proposed by Zhang et al., in which 6e7 layers of SnS2 were anchored on the 7e8 layers of RGO to form a specific plate on sheet structure. The observed experimental findings indicated that the capacity of SnS2/RGO was fourfolds (649 mAhg1) higher than that of SnS2 (178 mAhg1) at 100 mAg1 owing to the excellent dispersing ability and conducting behavior of RGO. Furthermore, the SnS2/RGO composite anode maintained 61% of its initial capacity over 1000 cycles and also sustained the current density of 12.8 A g1 at ambient temperature. Furthermore, the charging process might be finished within 1.3 mins and yet delivering the capacity of 337 mAh g1, which could make SnS2/RGO as a better anode material for SIBs [51]. The other transition metal sulfides such as ZrS2, MoS2, VS2, TiS2, NbS2, and CrS2 are predicted using first-principle computational calculations. M ¼ Zr, Ti, Nb, and Mo are preferred as anodes for SIBs. Using firstprinciple computational calculations, a variety of transition metal sulfides were studied and checked their possibility of sodium adsorption and phase transition in the potential window between 0.49 and 0.95 V. Among the different sulfides studied, especially the sulfides of Ti and Nb are expected to keep the similar configurational phase upon sodiation. The migration barriers of sodium ions were found to be 0.22 and 0.07 eV with voltage window of 0.49e0.95 V and theoretical capacities of 260e339 mAh g1. Among the latter four screened TMDs, in particular, TiS2 and NbS2 are anticipated to maintain the same configurational phase upon sodiation (favorable kinetics) with Na ion migration barriers of 0.22 and 0.07 eV, respectively, suggesting it could be favorable for high-power energy storage applications [21,52,53]. Despite the nanostructured metal sulfides as suitable electrode materials for SIBs, they lack in some of the important properties like very poor cycling stability, volume expansion during chargeedischarge process, and dissolution of discharge products. A new material nickel sulfide (NiSx) was proposed and its electrochemical behaviors could be strikingly enriched by confining the NiSx nanoparticles in an infiltrating conductive carbon nanotube network, and an ultrathin layer of carbon was coated to stabilize

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the structure. The carbon layer overawed the barrier owing to effects of both the volume expansion and dissolution of product materials. The excellent cycling stability was observed for consecutive 200 cycles and the maximum capacity was found to be 400%) during the lithiationedelithiation process and formation of the SEI layer at low potential. The LieSi alloy forms and then shrinks when the lithium ions are released. After a few cycles of discharging and charging, the structure of the Si begins to crumble away and can no longer hold Liþ ions effectively, so bulk Si breaks and drastically fades capacity [23]. To overcome these problems, two strategies were used, which are designing the nanoscale hierarchical structures and using the composite material. It was found that the volume change could be buffered by downsizing the Si particle to nanosize. Mazouzi et al. reported that the most stable reversible capacity of 960 mAh/g after 700 cycles can be achieved in Si particleebased anodes using 100 nm diameter Si nanoparticles [24]. Kim et al. synthesized Si nanoparticles with various sizes of 5, 10, and 20 nm. The results showed that 10 nm-sized Si nanoparticles displayed the highest capacity retention among all samples [25]. Ma et al. reported the synthesis of nest-like silicon nanospheres, their highly reversible lithium storage, and excellent high-rate capability. Nestlike Si nanospheres were prepared via a solvothermal route from the reaction of sodium silicide and ammonium bromide with the mixture of pyridine and dimethoxyethane as the solvent. The nest-like Si nanospheres displayed an initial specific capacity of 3052 mAh/g at the current density of 2000 mA/g. In addition, after cycling up to 48 cycles at 2000 mA/g, the electrode made up of the nest-like Si nanospheres retained 1095 mAh/g. This result suggests that the as-prepared nest-like Si nanospheres are promising candidates as the anode materials of rechargeable LIBs [26]. Si nanowires (NWs) prepared by a vaporeliquidesolid state synthesis method was reported by Chan et al., and a highly reversible first discharge capacity was 3124 mAh/g, indicating a coulombic efficiency of 73% and maintained

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a discharge capacity retention of 75% up to 10 cycles. There was sufficient electrical contact between the current collector and every NW so that there would be more active material contributed to the capacity. As there were no binders or conductive carbon added, which add extra weight and lower the overall specific capacity of the battery [27], Cui et al. produced crystalline-amorphous coreshell Si NWs by growing directly on stainless steel current collectors by a simple one-step synthesis. The crystalline Si cores function as a stable mechanical support and an efficient electrical conducting pathway while amorphous shells store Liþ ions. These coreshell nanowires have displayed a high charge storage capacity (w1000 mAh/g) with w90% capacity retention over 100 cycles [28]. Doublewalled Si nanotubes (NTs) were prepared by adding a solid silicon oxide outer wall to the silicon nanotube using an electrospun nanofiber template, in which the nanotube undergoes expansion and contraction while the outer layer protects the nanotube from the electrolytic solution by forming a thin stable SEI layer. The result is a LIB with extended useful life and increased energy storage without increasing battery size. The surfaceclamed Si NT electrodes exhibited a discharge capacity of 1000 mAh/g at 12 C current rate after 6000 electrochemical cycles while retaining the more 85% of its initial capacity [29].

3.3 Silicon-based composites Composite Si nanoparticleebased anodes exploit a matrix that does not undergo large volume expansion. They can buffer the significant expansion of Si while maintaining the structural integrity of the electrode and enhance their stability by reducing Si aggregation or electrochemical sintering [30]. The Si-based composite electrodes consist of (i) Si, (ii) conductive additive such as carbon, graphite, carbon nanotube (CNT), and graphene, and (iii) binder. Yi et al. reported a microsized SieC composite anode consisting of interconnected Si and carbon nanoscale building blocks. The SieC composite demonstrated a reversible capacity of 1459 mAh/g after 200 cycles at 1 A/g current rate with a capacity retention of 97.8% which could be seen from Fig. 15.2. It is notable that the nanoscale size of primary particles and the organized carbon and Si networks retained internal electrical contact and continued the cycling stability [31]. Zhang et al. reported a ternary Sibased composite Si@C/GF in which Si nanoparticles were coated on a thin carbon layer by pyrolysis of phenolic resin and encapsulated in a graphene framework (GF). The Si@C/GF electrode showed a discharge capacity of 650 mAh/g at a current density of 1 A/g after 200 cycles. The improved electrochemical performance of the Si@C/GF electrode has been

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recognized to the GF, which provided an elastic and robust 3D structure to buffer the large volume changes of Si. In addition, the pyrolytic carbon retained good contact with both the GF and Si, maintained electrode integrity, and buffered the huge volume change of Si [32]. Jimenez et al. fabricated a novel architecture in the silicon anodeebased batteries. They introduced a carbon layer with a thickness of 5 nm between the Si layers as Si/C/Si (70/5/70 nm). The interlayer carbon with 5 nm surface significantly improves the electrochemical performance of Si thin film anodes in terms of an enhanced coulombic efficiency and capacity retention. It was also shown that the surface layer of carbon can effectively protect the highly reactive Si surface from the bulk electrolyte. The interlayer of amorphous carbon was also able to control the volume expansion and to enhance the mechanical stability of Si during lithium uptake/ extraction, thus leading to a better integrity, and lowered thickness increase of the electrode. In the merit of these features, the Si/C/Si thin-film anodebased LIBs exhibited a capacity retention of 87% after 150 cycles [33].

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3.4 Transition metalsebased materials Xia et al. fabricated Co3O4/NiO coreeshell nanowire arrays on conductive substrates by the hydrothermal method followed by chemical bath deposition for the NiO coating [34]. Both the nano-structural feature and the combination of materials result in a high specific capacitance of 853 F/g at 2 A/g after 6000 cycles and an excellent cycling stability. Self-assembled porous NiO-coated ZnO nanorod electrodes were prepared on a stainless steel substrate by hydrolysis of aqueous nickel chloride in a hexagonal ZnO nanorod template, followed by heat treatment to convert Ni(OH)2 to NiO [35]. The thin porous NiO shell acts as a buffer layer, which mainly reacts with the lithium ion and permits the ions to reach the ZnO core. This coating also helped to provide structural integrity and enhance the electron conductivity of the electrode. The effects were clearly perceived by significant improvement of the reversible capacity value 1100 mAh/g for NiO-coated ZnO nanorod arrays, as compared with ZnO nanorod arrays with 700 mAh/g and 1 C current rate up to 15 cycles. Liao et al., reported a high-performance 3D electrodes for LIBs consist of TiO2eC/MnO2 coreedouble-shell nanowire arrays, as reported [36]. The complex structure was fabricated on flexible Ti-foil by layer-by-layer deposition with soaking, followed by a glucose-assisted hydrothermal method and calcination for carbon coating and finally via redox reaction with KMnO4 solution to deposit MnO2 nanoparticles on the surface. The 3D electrode exhibits improved electrochemical performance with a higher dischargee charge capacity, superior rate capability, and longer cycling lifetime.

3.5 Transition metal dichalcogenides Layered transition metal dichalcogenides (2D) are found to exhibit excellent lithium ion storage capacity because of their layered structure in the crystal structure. Depending upon the lithiation potential values, metal sulfides MS2 of M ¼ Fe, Ti, Co, Ni, and Cu are considered as cathode material, whereas M ¼ Mo, W, Ga, Nb, and Ta are known to be used as anode materials in LIBs [37]. Nanocomposites of MoS2 with carbonaceous nanomaterials seem to be promising materials for LIBs, and they exhibited reversible capacity values between 800 and 1000 mAh/g with good cycle stability [38e40]. WS2 nanotubes were synthesized by sintering amorphous WS3 at high temperature under flowing hydrogen as reported by Wang et al., and the nanotubular structure allowing easy Liþ ion intercalation through the open ends into the 4.6 nm diameter inner core and into intertubular sites. The WS2 nanotube layers exhibited an initial high capacity value of w900 mAh/g [41]. As in the case of MoS2, WS2 also

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showed excellent electrochemical behavior with carbon nanomaterials. For example, Rao et al., reported about the synthesis of WS2 nanosheet supported on reduced graphene oxide (RGO) through a hydrothermal synthesis route. The WS2eRGO composite system showed a reversible capacity value in the range 400e450 mAh/g over 50 cycles with good cycle stability [42]. Jang et al., synthesized ultrathin ZrS2 nanodiscs with various diameter and thickness by surfactant-assisted chemical method and proved to be a highly functional host material for the lithium intercalation process. The ZrS2 nanodiscs acquire a very high surface area and showed excellent nanoscale size effects, enhancing the discharge capacity by 230% and stability for 20 nm nanodiscs in comparison with bulk ZrS2 [43]. Nanoconfined SnS in 3D interconnected macroporous carbon (3D SnS/C) has been produced using silica opals as template following a carbonization and sulfuration route by Xue et al. It delivers a high specific capacity of 869 mAh/g at 1 A/g after 1000 cycles and the rate performance is also excellent (550 mAh/g at 3 A/g). The outstanding electrochemical performance of the 3D SnS/C is ascribed to its 3D porous carbon interconnected structure and nanoconfined SnS nanoparticles distributing broadly in carbon matrix, which not only improve the conductivity, but also keep the structure integrity, and as a result of enhancing the cycling stability of the material [44].

3.6 Tin oxide materials Hollow nanostructured anode materials contribute to higher capacity and higher rate capability compared to their bulk materials. This can be ascribed to their high surface area, shortened path length for Liþ ion diffusion, and more freedom for volume change which could reduce the over potential and allow good reaction kinetics at the electrode surface [45]. Archer et al., developed new functional nano architecture and coaxial SnO2@carbon hollow spheres for highly reversible lithium storage. It has been synthesized by three steps. In step 1, silica nanospheres (about 240e250 nm in diameter) are coated with uniform SnO2 double-shells. In step 2, these core/shell silica@SnO2 nanospheres are coated with glucose-derived carbon-rich polysaccharide (GCP) by a simple hydrothermal approach. After carbonization, the silica nanotemplates are removed in the final step 3, to produce SnO2@carbon coaxial hollow nanospheres. These hollow nanocomposites with double-shelled architecture deliver a stable capacity of about 210 mAh/g at a high rate of 4.8 C (1 C denoted as 625 mAh/g). After more than 200 cycles at a rate of 0.32 C, a stable high capacity of 500 mAh/g can be continued. These advantages are achieved by structural

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integrity and enhanced weight fraction of the electrochemically active component (SnO2) in the designed composite anode [46].

3.7 Other functional materials Germanium-based compounds are recognized as one of the most promising candidates because of their extremely high theoretical capacity. A facile TEOA-assisted technology was presented to fabricate Co-doped Zn2GeO hollow microspheres. The shell of the hollow microspheres was constructed by the self-assembly of uniform 1D single-crystalline nanorods with a length and diameter of about 2 mm and 100 nm, respectively. When used as an anode material for LIBs, the optimized Co-doped Zn2GeO hollow microspheres deliver a high discharge capacity of 1419 mAh/g and a high charge capacity of 1063 mAh/g for the first cycle, corresponding to a very high initial coulombic efficiency of 75%. A high capacity of 882 mAh/g at 1.0 A/g after 100 dischargeecharge cycles was maintained and a capacity of 464 mAh/g can be retained even at a high current density of 5.0 A/g. The remarkable electrochemical lithium storage performance can be a result of the synergistic effect of the hierarchical hollow structure and unique chemical composition. The hierarchical hollow structure allowed for easy diffusion of electrolytes and shortened the pathway of Li transport during repeated Li extraction/insertion [47]. Very recently, a low bandgap polymer having high conductivity, polyisothianaphthene exhibited as an active material for Li ion storage. As an anode, polyisothianapthene shows a specific capacity of 730 mAh/g at 0.1 C charge/discharge rate in the second cycle. The capability of accepting both Liþ and PF 6 ions and high reversibility in terms of bipolar electrochemical reactions indicates that it can be a promising bipolar organic material for use in LIBs [48]. There are many nanostructured anode materials which showed a descent specific capacity and good cycle stability which are nitrogen-doped carbon-coated tin oxide hollow nanofibers, and the morphology is shown in Fig. 15.3 [49], Hybrid porous bamboo-like CNTs embedding ultrasmall LiCrTiO4 nanoparticles [50], self-assembled ZnO on graphene oxide [51], a-Fe2O3 nanorod arrays on reduced graphene oxide [52], SnCoS4/graphene composites [53], porous, cobalt vanadate (Co3V2O8) microsphere [54], hierarchical hybrid nanostructure of ordered mesoporous carbon supported Ni3V2O8 composites [55], ZnMnO3 nanotube arrays [56], hybrid vertically aligned Li4Ti5O12 nanowire arrays on freestanding ultrathin graphite [57], hierarchical porous TiNb2O7 nanotubes [58], ZnOe NiOeCo3O4 hybrid nanoflakes [59], and ZnSb2O6 nanoparticle [60].

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Figure 15.3 SEM images of (A) as-electrospun polymeric SnOx-based nanofibers, (B) SnO2 hollow nanofibers calcinated at 600 C, (C) conductive polymer Ppy-coated SnO2/ hollow nanofibers and (D) SnO2/ nitrogen-doped carbon coated hollow nanofibers annealed at 600 C in nitrogen gas. The inset images display an enlarged view for each sample [49].

4. Cathode The term cathode designates positive electrode in discharge cycle. Cathode materials are usually intercalation compounds and serve as hosts in which Li ions can be reversibly intercalated into and extracted out from. The amount of electrical energy per mass or volume that a LIB can provide is a function of the cell’s voltage and capacity, which currently are primarily dependent on the cathode material. As a result, the increasing demands for lighter and thinner LIBs with higher capacity continue to motivate researchers on cathode materials with superior properties to the current state-of-the-art. Until the introduction of LiFePO4, LiCoO2, LiMn2O4, and LiNi1/3Co1/ 3Mn1/3O2 (NCM) cathode materials, the research was very slow over the last two decades [61,62]. The cathode materials currently used in LIBs suffer from low energy density and poor electronic conductivity. However, the nanotechnology lifted up the growth of research on cathode materials. In order to get high energy density batteries, not only the voltage plateau should be high but also should be inexpensive viz. cathode materials need

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to be composed of highly abundant elements. Let us see the effect of some important nanostructured cathode materials in improving the electrochemical performance of LIBs.

4.1 LiFePO4 and its nanocomposites Goodenough et al., introduced the olivine LiFePO4 as cathode material for LIBs [63]. LiFePO4 possess many good features such as high theoretical capacity (170 mAh/g), suitable operating voltage (3.4 V vs. Liþ/Li), good cycling stability, low toxicity, low cost, safety, and good thermal stability [64,65]. However, the poor electronic conductivity (109 to 1010 S/cm2), low diffusion of Liþ ions across the boundary of LiFePO4/FePO4 leads to increase in impedance and low practical capacity which limits the practical use of LiFePO4 cathode in electrical and hybrid electrical vehicles [66,67]. To enhance the electrochemical performance of LiFePO4 cathodes toward LIBs, graphene has been used for making graphene/LiFePO4 nanocomposites to use the features of graphene such as high surface area (2630 m2/g), superior charge carrier mobility (2.5 cm2/Vs  105 cm2/Vs), thermal conductivity (5000 W m/K), mechanical strength (Young’s Modulus 1 TPa), and structural flexibility [68,69]. The unique sp2 hybridized carbon network of graphene gives the chemical stability and has been employed as auspicious material in LIBs. In the context of surface modification, preparation of LiFePO4/graphene composite by simple solution mixing can also be included which displayed a capacity decay rate of less than 15% when cycled at 10 C charging and 20 C discharging rate for 1000 cycles [70]. Ding et al. synthesized a graphene/LiFePO4 composite through a coprecipitation method, which exhibited enhanced electrochemical performance compared with the pristine LiFePO4 [71]. Sun et al. thoroughly compared the impact of stacked graphene and unfolded graphene used in a composite. Interestingly, they found that the unfolded graphene enabled good dispersion of LiFePO4 and restricted the LiFePO4 particle size at the nanoscale which led to great electrochemical performance, indicating the advantages of 3D graphene compared to the planar graphene sheets [72]. On the basis of the enhancement mechanism of 3D graphene, they further prepared a 3D hierarchical graphene/LiFePO4 composite cathode with a porous structure through a facile template-free solegel method. The 3D graphene integrated with LiFePO4 creates an effective electronically conducting network, resulting in a reversible capacity of 146 mAh/g at

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17 mA/g over 100 cycles and also 3D graphene serves as a buffer to accommodate the structure change during lithiationedelithiation. From these outcomes, it can be seen that with a 3D graphene structure, the electron transfer and electrolyte diffusion can be significantly improved, which indicate a auspicious direction for high energy density electrode designs [73]. Nitrogen-doped 3D graphene (NG) has also been a great interest when it combines with the LiFePO4 (LFP). A nanocomposite consisting of (010) facet-oriented LiFePO4 nanoplatelets wrapped in a nitrogen-doped 3D graphene aerogel has been reported with a BET surface area of 199 m2/g. The nitrogen-doped graphene aerogel combined with its interconnected porous 3D networks affords pathways for rapid electron transfer and ion transport. The LFP/NG composite displayed a high discharge capacity of 178 mAh/g at 0.1 C and demonstrated a capacity retention of 95% after 300 cycles at 1 C. The 3D grapheneebased nanocomposites show a high rate capability (78 mAh/g at 100 C) accompanied by long life cycling stability. The unique open porous microsphere structure, the firm contact and interaction between the LFP nanoplates, and NG nanosheets resulted in superior charge transportation and structure stability of LFP/NG composites [74]. Not only that, the graphene-based LiFePO4 nanocomposites with an attractive 3D structure have also been proved as promising for fabricating high-performance flexible batteries [75]. Carbon coating on LiFePO4 is believed to be giving good thermal stability and electrical conductivity during the battery testing at high temperature (60 C). The carbon coating significantly enriches the electronic conductivity of LiFePO4 which helps for high cycling rate and cycle life in chargeedischarge process, Fig. 15.4 [76]. Not only that but it also has reduced the side reactions on the cathode i.e., reduces the dissolution of iron to the electrolyte [77].

4.2 Lithium cobalt oxide LiCoO2 is still the leading cathode material for LIBs for mobile electronics, owing to its high energy density, low self-discharge, excellent cycle life, and easy synthesis, although the high cost and thermal instability have caused significant concern. Currently, the major drawback of LiCoO2 is its fast capacity decay at high current rates or above 4.4 V (vs. Li/Liþ). It could be achieved by preparation of nanostructured LiCoO2 and coating with other nanomaterials [78,79]. A high-performance LiCoO2 cathode was successively fabricated by a solgel coating of Al2O3 to the LiCoO2 particle surfaces and subsequent heat treatment at 600 C for 3 h. Compared with

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the bare LiCoO2, the Al2O3-coated LiCoO2 cathode exhibits no decrease in its original specific capacity of 174 mAh/g (vs. lithium metal) and excellent capacity retention (97% of its initial capacity) between 4.4 and 2.75 V after 50 cycles. This is because the high concentration of Al atoms at the particle surface region leads to the enhancement of structural stability of LiCoO2 during cycling, which originates from the disappearance of the phase transition from a hexagonal to monoclinic phase [80]. Chemical etching concurrently modifies pristine to layer-by-layer 3D LiCoO2 with a Co3O4 coating. The layered morphology affects the high electrochemical performance by reducing the structure instability of the LiCoO2 during cycling. The capacity of the layered 3D-LiCoO2 is approximately 2.2 times higher than that of bare LiCoO2 at the 7 C rate. Also, 3D-LiCoO2 exhibits much improved average working voltages at higher C rates compared with the bare one. These results indicate that the layer-by-layer 3D-LiCoO2 has a larger surface contact area with the electrolytes, which can facilitate fast Li-ion transport into the structure [81]. A thick and dense flake-like LiCoO2 with exposed {010} active facets was synthesized using Co(OH)2 nanoflake as a self-sacrificial template obtained from a simple coprecipitation method. When operated at a high cutoff voltage up to 4.5 V, the flake-like LiCoO2 exhibited a reversible discharge capacity as high as 179, 176, 168, 116, and 96 mAh/g at 25 C under the current rate of 0.1, 0.5, 1, 5, and 10 C, respectively. When charge/ discharge cycling at 55 C, a high specific capacity of 148 mAh/g (w88% retention) can be retained after 100 cycles under 1 C, demonstrating excellent cycling and thermal stability. Such superior high-voltage electrochemical performances of the flake-like LiCoO2 arise from the exposed {010} active facets which provide a preferential crystallographic orientation for Li-ion migration, while the micrometer-sized secondary particles agglomerated by submicron primary LiCoO2 flakes provide the electrode with better structural integrity [82].

4.3 Lithium manganese oxide Spinel lithium manganese oxide (LiMn2O4) has been considered as an alternative cathode material for commercial LiCoO2 in batteries, because of its abundant and cheaper resources, environmental benignity, and better safety. However, LiMn2O4 suffers from severe capacity fading because of Mn dissolution into the electrolyte and JahneTeller distortion, and the low intrinsic conductivity [83]. Composites of LiMn2O4 nanoparticles in a 3Dgraphene matrix have been prepared to address the low electronic

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conductivity issue of the Li Mn2O4 cathode. The 3D-graphene/LiMn2O4 electrode shows higher discharge capacity and rate capability than the pristine LiMn2O4 electrode, which is attributed to the high surface area of LiMn2O4 nanoparticles and good electronic conductivity of graphene [84]. In addition to the LiMn2O4 nanoparticles, extensive efforts have also been devoted to developing other LiMn2O4 nanostructures. Hosono et al. synthesized high-quality single crystalline LiMn2O4 nanowires by using Na0.44MnO2 nanowires as a self-template. The LiMn2O4 nanowire electrode exhibited excellent rate capability and cycling stability viz. the average discharge capacity at the current rate of 10 C was around 105 mAh/g and those at 60 and 150 C were around 100 and 78 mAh/g, respectively. These large capacities at high current rates arise from the nanowire morphology and the high quality of the single crystal which can shorten the diffusion lengths of both the lithium and electrons [85]. A nanochain of LiMn2O4 was prepared by a solegel method using an aqueous solution of metal salts containing starch. The nanochain LiMn2O4 cathode showed reversible capacity of 100 mAh/g at 100 mA/g (about 1 C) and 58 mAh/g even at a charge rate of 20 C. In addition, when the cathode is charged at 1 C, 70 mAh/g (70% capacity at 1 C) can be achieved even at a discharge rate of 50 C, with a cut-off voltage of 3.0 V [86]. Sun et al. demonstrated a facile template-free route to synthesize nanoporous LiMn2O4 nanosheets composed of single-crystalline LiMn2O4 nanorods with exposed {111} facets via an in situ lithiation of ultrathin MnO2 nanosheets. Almost 100% of the initial capacity can be retained after 500 cycles at a 1 C discharge rate using the nanoporous nanosheets as a cathode, whereas at a discharge rate of 25 C, the capacity retention is about 86% of the initial capacity after 500 cycles [87].

4.4 Lithium-rich NCM materials Layer-structured ternary cathodes have been considered as other promising cathode materials. Among the various investigated cathodes, the LiNi1_x_yCoxMnyO2 series such as LiNi1/3Co1/3Mn1/3O2 (NCM) integrates the advantages of LiCoO2, LiNiO2, and LiMnO2. NCM possesses a high operating voltage, high specific capacity, cyclic stability, and structural stability [88]. However, the poor electronic conductivity of NCM results in low electrochemical performance and limits its practical applications. The use of graphene to form 3D composite materials has been demonstrated to improve the capacity, rate capability, and cyclic stability of

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NCM cathodes. It has been confirmed that the 3D graphene network greatly decreases the resistance of batteries, especially the charge transfer resistance [89]. It has been shown that the NCM/graphene composites exhibit high discharge capacity (188 mAh/g), good rate capability, and good cycling performance compared with pure NCM [90]. Very recently, hollow corn-like LiNi0.8Co0.1Mn0.1O2 material was synthesized from hollow hierarchical precursor that derives from hydrothermal method by Zhang et al. The 3D corn-like architecture cathode delivered a high-rate capacity of 186.94 mAh/g, and the capacity retention was 85.71% after 100 cycles at 200 mA/g. Particularly, this unique cathode architecture was stable, tolerated high-rate charging process, and kept the original structure intact after fast Liþ extraction and diffusion, thus possessing the advantages present in both nanostructures and microstructures [91].

4.5 3D graphene/organic nanocomposite Current inorganic electrode materials are primarily based on costly transition metals, which will become one of the great hurdles for the large-scale application of LIBs. Hence, it is important to develop low-cost electrode materials to meet the demands of the growing electric vehicle industry and other energy storage applications [78,79]. Organic materials have the advantages of abundant, low-cost, chemical diversity, and tunable redox properties. So the organic materials become electrode materials for LIB applications [64]. Organic materials follow different mechanism in lithium ion storage, viz. chemical bond reactions [92,93]. To overcome the problems associated with organic materials such as severe dissolution and poor charge transfer, composites have been prepared with 3D graphene. For example, Sun et al. reported a 3D graphene/acid-treated multiwalled carbon nanotubesesupported 1,5-diaminoanthraquinone organic nanocomposites synthesized by an organic solvent displacement method and combining with a solvothermal reaction. The nanocomposites exhibit a high discharge capacity of 289 mAh/g at 30 mA/g and retain 122 mAh/g at an extreme current density of 10 A/g. Particularly, excellent cycling stability is obtained with only 14.8% capacity loss after 2000 cycles at a current density of 1 A/g [94]. The cyclic voltammetry, electrochemical behavior, and the electrochemical stability before and after the testing can be found in Fig. 15.5. A 3D graphene conductive network-based polyimide cathode has been fabricated on a flexible substrate, which exhibits a capacity

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Figure 15.5 The electrochemical performance of pure PDAA and oGCTF@PDAA with polymerization time of 2.7 h. (A) CV curves at a scan rate of 0.1 mV/s in a potential window from 1.5 to 3.5 V (vs. Li/Liþ) (first cycle). (B) The specific capacity based on total mass and only PDAA mass for oGCTF@PDAA as a function of various PDAA contents. (C) The discharge and charge profiles at a current density of 30 mA/g. (D) Rate capability at various current densities. (E) Cycling performance at 1 A/g (the coulombic efficiency for oGCTF@PDAA). (F) Nyquist plots before and after the cycle [94].

of 175 mAh/g and high rate performance. After 150 cycles at 0.5 C, 82% of the initial capacity was retained [95]. So it can be understood that 3D graphene/organic composites possess another fascinating feature which could be used for fabricating flexible batteries.

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4.6 Other nanomaterials Some new nanomaterials were reported other than the above-listed nanomaterials for their good electrochemical behavior in LIBs. For example, Kravchyk et al. reported a simple colloidal synthesis of highly uniform and highly crystalline NaFeF3 nanoplates by thermal decomposition of a single-source precursor, NaFe(hexafluoroacetylacetone)3, in high boiling point solvents and in the presence of long alkyl chain ligands. The solvent/ligands ratio was found to be a primary factor for obtaining phasepure nanomaterials. The NaFeF3 nanoplates are capable of delivering high initial capacity of 183 mAh/g at a current density of 0.2 A/g (w1 C) for lithium ion storage. At least 50% of these capacities were retained after 200 cycles. This work overcomes the low electronic conductivity issue of NaFeF3 and affirms that such perovskite-based metal fluorides are promising low-cost electrode materials for LIBs [96]. Layered Li2MoO3/C composite was successfully synthesized with the simple addition of acetylene black by Kumakura et al. As the electronic conductivity of the composite is enhanced to more than 1000 times higher than that of bare Li2MoO3, the Li2MoO3/C composite delivers approximately 230 mAh/g of initial discharge capacity in a voltage range of 1.5e4.3 V, while carbon-free Li2MoO3 shows only 110 mAh/g of initial discharge capacity [97]. Fabrication of FeS2/C nanotube arrays with the help of sacrificial Co2(OH)2CO3 nanowires template and glucose carbonization was reported by Pan et al. Self-supported FeS2/C nanotubes consist of interconnected nanoburrs of 5e20 nm and showed enhanced cycling life and noticeable high-rate capability with capacities ranging from 735 mAh/g at 0.25 C to 482 mAh/g at 1.5 C, superior to those FeS2 counterparts [98]. Li et al. demonstrated a facile synthesis of hybrid materials by in situ polymerization of a conducting polymer of polypyrrole (PPy) on NiS-carbon nanofiber (CNF) films to form high-performance freestanding LIB cathodes. Using the freestanding NiSPPy-CNF hybrid film with a high NiS loading of 5.5 mg cm2 as a cathode, we demonstrate a high discharge capacity of 635 mAh/g at 0.1 A/g and exceptional areal capacity of 3.03 mAh/cm2 at 0.7 mA/cm2 with long cycling life over 700 cycles [99]. Some of the selected nanostructured functional materials used as anode and cathode in LIBs are listed in Table 15.1.

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3D hierarchical MnO/N-GSC/GR heterostructure 3D porous MoS2-reduced graphene oxide 3D nanoconfined SnS/macroporous carbon Co-doped Zn2GeO hollow microspheres SnCoS4/graphene composite ZnOeNiOeCo3O4 hybrid nanoflakes 3D hierarchical graphene/LiFePO4 composite Hollow corn-like LiNi0.8Co0.1Mn0.1O2 NaFeF3 nanoplates Polypyrrole-NiS-carbon nanofiber film N-doped graphene-VO2(B) 3D flower hybrid

Anode Anode Anode Anode Anode Anode Cathode Cathode Cathode Cathode Cathode

812 800 869 1419 1396 1060 146 186 183 635 418

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[20] [40] [44] [47] [53] [59] [73] [91] [96] [99] [100]

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Table 15.1 Electrochemical Performances of some selected nanostructured functional materials used in LIBs.

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5. Conclusions The nanostructured Si electrodes exhibited great improvement; however, future research is still necessary. Mn-based and polyanion-based cathodic materials with nanometer-scale features were reviewed, covering layered and spinel structure of Mn-based materials as well as lithiated metal phosphates and other promising polyanion-based materials. Enhanced performances of lithium ion rechargeable cells such as higher capacity, improved rate capability, and sustained capacity retention for longer cycles were achieved with various nanostructures such as nanoparticles, nanotubes, nanorods, and nanoplates, as well as the secondary structure of the primary nanounits. Useful featured structures to overcome low electronic conductivity, slow solid-state diffusion of lithium ions, and slow kinetics on introduction of lithium ions from electrolyte to electrode (or vice versa) were designed by using nanoshaped precursors, hard or soft templates, or agents to limit the growth of particles with the minute control of synthetic conditions such as heating temperature and environments, the choice of solvent, and pH. The Li-ion technology is currently the best performing technology for energy storage based on batteries. Yet, there is a large scope for developing new materials to enrich the future batteries with better energy storage, faster charge and discharge, and higher safety targeting many uses from smartphones to electric bicycle and cars.

List of abbreviations 1D 2D DWSiNT FT-IR GF HRTEM LIB MWCNT NCM NS NT NW oGCTF PDAA PPy SEI Si

One dimensional Two dimensional Double-walled silicon nanotube Fourier transform infrared spectroscopy Graphene framework High-resolution transmission electron microscopy Lithium-ion battery Multi-walled carbon nanotube Nickel cobalt manganese Nanosheet Nanotube Nanowire Graphene nanosheet/acid treated MWCNT organic foam Poly(1,5-diaminoanthraquinone) Polypyrrole Solid electrolyte interphase Silicon

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Acknowledgments Alagar Ramar and Fu-Ming Wang acknowledge the Ministry of Science and Technology, Taiwan, for the financial support.

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CHAPTER 16

Transition metalebased nitrides for energy applications K. Karthick1, 2, #, S. Sam Sankar1, 2, #, Subrata Kundu2, * 1

Academy of Scientific and Innovative Research (AcSIR), CSIR- Campus, New Delhi, India; 2Materials Electrochemistry (ME) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, Tamil Nadu, India

1. Introduction The world’s energy requirements as of now fulfilled to the maximum extent with the carbon-based fuels [1]. These carbon-based fuels are finite and also do emit CO2 to the environment which in turn results in global warming [2]. Therefore, people do research on developing sustainable energy systems by energy conversion and technology for the eco-friendly environment [2,3]. Considering alternates of carbon fuels, hydrogen is one of the promising energy sources that can nullify the demands of energy in future without harming the environment as it is carbon neutral technology [4]. This is possible with the appliance of electricity from the renewable energy sources such as solar, wind, and tidal to the splitting of water into hydrogen and oxygen electrochemically [4]. The produced hydrogen and oxygen can be adapted for the fuel cell and automobile engines for the exploitation of energy in time. This cycle of energy supply starting from renewable energy supply to fuel cell with hydrogen by electrochemical means gives the complete carbon neutral technology which is greener than others [5]. Electrochemical splitting of water consists of two fundamental reactions namely oxidation and reduction. Here, in this case, water into oxygen (oxygen evolution reaction [OER]) at anode and hydrogen (hydrogen evolution reaction [HER]) at cathode occurs [6e12]. The theoretical potential for water to split into components of oxygen and hydrogen is 1.23 V. However, in real terms, the thermodynamic and kinetic barriers tend to increase the required potentials to be more than the theoretical potential [13,14]. This additional potential

#

Both the authors contributed equally. * Corresponding author

Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems ISBN 978-0-12-819552-9 https://doi.org/10.1016/B978-0-12-819552-9.00016-6

© 2020 Elsevier Inc. All rights reserved.

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required for both OER and HER is called as overpotential. The catalysts should take minimum input of overpotential for both OER and HER for viable hydrogen production [7]. For replacing noble metals, transition metalebased catalysts, and their hydroxides [15e23], oxides [24e34], sulfides [35e40], selenides [29,41e45], tellurides [46,47], and phosphides [48e51] had been found to show enormous activities. The activities of these transition metalebased catalysts are ascribed to the favorable electronic configuration of them that give optimum bond energy for the hydroxide ion intercalation and the oxygen cleavage by electronic repulsion. Also, the morphology, electronic structure, charge transfer kinetics, and anion counterparts (S, Se, and Te) can influence the activity and stability [35]. Moreover, the actual phase formed during OER/HER also determines the nature of activity and stability of the electrodes [52]. Therefore, it is the electrocatalyst which is highly abundant, easy to prepare, less cost, less carcinogenic, and highly active and stable one is required in universal pH conditions. However, as the sluggish kinetics in OER with four proton-coupled electron transfer retards the overall efficiency of electrolysis, the commercialization becomes difficult [6,7]. In case of HER, it is the two electron transfer and just the adsorption of proton and desorption of proton show lesser cathodic overpotentials [6,7]. Recently, considering transition metalebased catalysts, the layered double hydroxides (LDH) had shown high activity for OER in alkaline conditions [19,53,54]. This layered structures of M2þ/M3þ hydroxides with stacked nature allow ease of electrolyte interaction with high proportion and ensure high electronic conductivity. Based on this, there are many reports highlighting the transition metalebased LDHs such as Ni-Fe [55], Ni-Co [56], Ni-V [57], Co-Fe [58], and many for the better OER activity and stability. For HER, the chalcogenides got huge activities and however could not compete with the state-of-the-art Pt catalyst. Iron group metals (Fe, Co, and Ni) as sulfides, selenides, and tellurides could give enhanced activities, and based on this, transition metalebased sulfides are studied for electrocatalytic HER activity [35]. Currently, the activities of them are fine-tuned with surface engineering, doping of nanoparticles (NPs), stoichiometric variations, and by other means for the enhanced electrochemical water splitting (EWS) applications [59]. This is done to make the catalysts as bifunctional to both OER and HER and hence the single electrode can act as both anode and cathode. Researchers work on searching effective catalysts that is bifunctional with less required voltage for the splitting of water.

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In the evaluation of different electrocatalysts for OER and HER, recently metal nitrides have also been found to be advanced one for the efficient hydrogen production. In general, in metal nitrides, the nitrogen atoms are coordinated to the metal surface through covalent interactions to make them as metallic nature [60e62]. This metallic nature ensure high electrical conductivity and low corrosion resistance. Moreover, the density of states near Fermi level shows high electron density and hence in accordance with the metallic nature and ensures high conductivity [63,64]. Moreover, the stoichiometric ratios in transition metal nitrides (TMNs) and their changes affect activity by creating more pores. These pores ensure high electrolyte interaction and easy diffusion of formed hydrogen and oxygen [64]. This chapter focuses on mono-, bi-, and trimetallic nitrides for EWS in terms of stoichiometry, porous nature, electrical conductivity, and stability for the sustainable production of hydrogen.

2. Mechanism involved in electrochemical water splitting and a short preview EWS is one of the fast and purest ways of production of hydrogen. As the free energy change associated with the splitting of water is highly positive with a value of þ237 kJ mol1, the conversion of stable H2O into O2 and H2 becomes a bottleneck. With a stable octet configuration of H2O, the energy required is more to split it and hence the enthalpy of formation of H2 and O2 from is highly positive with a value of þ286 kJ mol1 [65e69]. Although entropy is getting increased, it is relatively small and the overall free energy change is ultimately highly positive. Therefore, to split water into hydrogen and oxygen, high input of energy is required, and if the energy is given by electric input, then it is called EWS. According to the Nernst equation (DG ¼ nFE0), the free energy change (DG) is related to the potential and here the þ237 kJ mol1 (DG) equals to 1.23 V (E0) (Eq. 16.1). This is the equilibrium potential of H2O which has been derived theoretically [65e69]. Considering EWS, it consists 1 H2 O / H2 þ O2 2

ðEeq ¼ 1.23 VÞ

(16.1)

of OER at anode and HER at cathode. The designing of electrodes as anodes and cathodes with high efficiency to split water is inevitable to make it commercial applications. The activity of the catalyst can be inferred from

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the overpotential required at a defined current density. The Tafel slope derives the charge transfer kinetics from the overpotential versus current density. In case of stability, chronoamperometry or chronopotentiometry gives information on stability for a prolonged period of time.

2.1 Overpotential (h) and Tafel slope In case of H2O molecule, it is in equilibrium at 1.23 V where both forward and backward reactions are at the same rate. To say simply, the OER occurs at 1.23 V and HER occurs at 0 V. To make it irreversible, additional energy is required either cathodically or anodically. Moreover, pure H2O with very less conducting nature cannot be used straightaway for splitting via electricity. Therefore, highly conducting acidic (0.5 M H2SO4) and basic (1 M KOH) solutions are preferred for easy generation of H2. OER involves four protons and electron-coupled reaction with sluggish kinetics and hence the overpotential required becomes huge. In case of HER, the mechanism is based on hydrogen adsorption and desorption and hence the kinetics is facile compared to OER [70]. The overpotential can generally be termed as the additional potential required from equilibrium potential that arises because of thermodynamic and kinetic hindrances. As the overall efficiency of the system depends on both anode and cathode, the finding of catalysts with lesser applied overpotentials is required for both anode and cathode for high-scale production of hydrogen. The inherent kinetics of the electrodeeelectrolyte system can be identified with the Tafel slope and gives a clue on electron transfer. Tafel slope generally relates the overpotential applied with the observed current density. From the ButlereVolmer equation, the current observed with respect to the applied potentials depends on charge transfer coefficient (a). In case of HER, if the Tafel slope value is 30 mV dec1, then it follows chemical desorption method (hydrogen adsorption followed by chemical desorption) which is found for Pt catalyst [71]. If the exposed surface area is lesser, then it is VolmereHeyvrosky mechanism (hydrogen adsorption followed by electrochemical desorption). For OER, if the Tafel slope value is near 30 mV dec1, then it follows four electron transfers across the electrodeeelectrolyte interface. If it is higher, then the electron transfer rate is lesser than four. Accordingly, the Tafel slope value gives a clue about the facile kinetics of charge for both OER and HER over the electrode surface.

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2.2 Stability The stability of the catalysts for both OER and HER is verified with potential sweeping and potentiostatic techniques. For potential sweeping, it is monitored with cyclic voltammetry at high scan rates for more than 500 cycles. This ensures the high rate of OER and HER at the respective anode and cathode and because of which the electrode stability can be analyzed. At such high scan rates, we can relate the stable nature of electrode by monitoring the current density from 1st cycle to 500th cycle [72]. The change in overpotential at a defined current density before and after the cycling study relates the stability of an electrode. In another way, constant potential or current is given to relate the activity over a long period of time to see whether the given catalyst shows constant current or potential, respectively. In one case, potential is fixed and current is monitored with respect to time (usually 12e24 h) to see if there is any degradation in current (chronoamperometry). In another case, current is fixed and potential is monitored to see any degradation in potential (chronopotentiometry) [21].

3. Why transition metal nitrides are important in electrochemical water splitting? For the electrocatalyst to give high efficiency, the energy barrier should be lesser. Compared to noble metals, the kinetic barrier to effect the HER and OER is too facile in case of TMNs. This is possible because of successful incorporation of N into the metallic structure that gives enormous conductivity, synergistic enhancements with respect to different metal ions, exposes more active sites, and also ensures fast diffusion of the products [63,64,73]. These parameters together ensure that TMNs can act as an effective electrocatalyst for both OER and HER. In general, the bonding between metal and nitrogen can be of covalent, ionic, and metallic in nature. The covalent bonding between metal and nitrogen show high hardness, and the ionic bonding shows contraction of metal d-banding and thus enables fast electron transfer kinetics. Considering metallic bond, it shows less corrosion resistance and high electrical conductivity [74e76]. Moreover, the presence of N and its stoichiometry to the TMNs also do have more influence in terms of conductivity. In general, the increase in number of Ns shows increase in activity with lesser resistance. The electron-donating nature of N with M also shows high conductivity

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Figure 16.1 Transition metal nitrides as catalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER).

nature for ease of electrolyte interaction. In addition to this, the presence of mono-, bi-, and trimetallic nitrides will have difference in activities because of synergistic enhancements, metal d-band density, and conductivity. Therefore, by varying the metals and their composites, it will be easy to formulate catalysts that are efficient for both OER and HER in universal pH conditions (Fig. 16.1). By means of method of synthesis of TMNs (nitridation of TMNs, solvothermal method, thermal decomposition of polymeric materials, and ammonolysis of metal precursors) composites, stoichiometry of Ns, porosity, morphology, and by others, we can develop catalysts derived from TMNs for viable EWS [64].

4. Metal nitrides in electrochemical water splitting From the above discussions, it is clear that finding of TMNs that can be bifunctional in nature for EWS is inevitable for large-scale hydrogen production. This area focuses on synthesis, characterization, and evaluation of electrocatalytic activities for OER and HER by mono-, bi-, and trimetallic nitrides.

4.1 Monometallic nitrides for electrocatalytic water splitting TMNs are highly demandable owing to their unique properties, wherein nitrogen can be incorporated to the interstitials of the metal host lattice. There it can covalently bond to the metal atoms [60]. So, the formation of this metalenitrogen bond would provide the electron-donating character from the metal d-band. These features can give the better conductivities and stabilities which lead to the high electrocatalytic activity for water splitting reactions [60].

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Figure 16.2 Schematic representation of Co5.47N NP@N-PC synthesis. (Reproduced with permission [60] and copyrighted 2018, American Chemical Society.)

For instance, Chen et al. reported the in situ encapsulated binary and nonstoichiometric cobalt nitride by using simple self-template route (Fig. 16.2), and the synthesized cobalt nitride (Co5.47N) NPs have been incorporated on the three-dimensional (3D) N-doped porous carbon (Co5.47N NP@N-PC) polyhedral and this can be confirmed by the transmission electron microscopy (TEM) analysis [60]. Fig. 16.3A and B is the low-magnification TEM image of the polyhedron-like composites which can uniformly dispersed over their rough surfaces. In case of Fig. 16.3C, polyhedron was composed with numerous nanocrystals which are clearly embedded on the carbon matrix. Fig. 16.3D and E revealed that the carbon matrix is mainly comprised with a carbon matrix and the observed lattice fringe with a d-spacing value of 0.207 nm, which corresponds to (111) plane of Co5.47N. Fig. 16.3F is the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the materials and the color mapping results also confirmed that the homogeneous distribution of cobalt, nitrogen, and carbon in Fig. 16.3EeI, respectively. Furthermore, the resultant material has been subjected as an electrocatalyst for water splitting reaction in alkaline condition where at 10 mA cm2 current density, Co5.47N NP@N-PC had shown the overpotentials of 149 and 248 mV for HER and OER, respectively, with high charge transfer kinetics, stability, and low charge transfer resistance (Fig. 16.4AeD). XPS analysis confirmed the hydroxide/oxide phase formed during OER showing the easy formation of active phase from nitrides (Fig. 16.4E). In the case of electrolyzer, Co5.47N NP@N-PC electrodes could act as both the cathode and anode

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Figure 16.3 (AeC) Transmission electron microscopy (TEM) images of transition metal nitrides and (D, E) HRTEM of Co5.47N NP@N-PC; (F) HAADF-STEM image and corresponding (GeI) elemental color mapping images showing homogeneous distribution of elements of Co, N, and C. (Reproduced with permission [60] and copyrighted 2018, American Chemical Society.)

in alkaline electrolyte solution where it has shown the cell voltage of only 1.62 V at a current density of 10 mA cm2 (Fig. 16.4F). This excellent electrocatalytic activity is mainly attributed to the inherent conductivity and the hierarchically porous structure with metallic nature from MeN. In addition to that, Shalom et al. have reported an efficient way to modify the surface of Ni foams with a Ni3N layer that was achieved by using supramolecular complexes such as cyanuric acid, melamine, and barbituric acid [63]. However, a rough Ni3N layer was formed at the surface of the Ni foam with high porous nature from the foam that could deliver high activity. The synthesized Ni3N/Ni foam exhibited a very low overpotential (w50 mV) with high current density and excellent stability for HER in alkaline solution. In addition to that, it has also shown

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Figure 16.4 (A) Polarization curves of Co5.47N NP@N-PC, Co NP@N-PC, and IrO2@C samples for oxygen evolution reaction (OER), (B) Tafel slopes, (C) chronoamperometry of Co5.47N NP@N-PC, Co NP@N-PC, and Pt@C, (D) EIS of Co5.47N NP@N-PC and Co NP@N-PC; (E) XPS spectra of O 1s in Co5.47N NP@N-PC after chronoamperometry; and (F) overall water splitting of Co5.47N NP@N-PC and (Pt@C)k(IrO2@C). (Reproduced with permission [60] and copyrighted 2018, American Chemical Society.)

enhanced activity in the OER and oxygen reduction reaction. These enhancements were owing to the facile formation of the Ni(OH)2 layer on the nitride layers. This leaded to the formation of Ni3N/Ni (OH)2 catalyst, so the resultant overpotentials associated with nickel hydroxide layer formation on the catalyst surface decreased. These results confirmed that the monometallic nitrides are one of the promising catalysts for EWS [60,63].

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

4.2 Bimetallic nitrides for electrocatalytic water splitting Bimetallic nitrides are another important finding in the material chemistry for extended catalytic applications. They have shown better catalytic activities than the monometallic counterparts which is attributed to the contraction of metal d-bands with increased activity. The coordination of this bimetallic combo could give the extended catalytically active sites and give the improvised electrical conductivity with low electrical resistance which are highly beneficial for electrocatalytic applications [76e78]. In these aspects, Zhang et al. have reported that ironenickel nitride nanostructure on surface redox etching on the Ni foam surfaces (FeNi3N/NF) by in situ growth method [78]. The schematic representation of the synthesis process is given in Fig. 16.5. The synthesized materials are subjected as an electrocatalyst for water splitting reactions. While Ni foam surface was partially etched by the Fe(III) ion, there the slow release of Ni ions with Fe3þ ion which leaded to the formation of NiFe(OH)x nanosheets was shown. To form the FeNi3N/NF, NiFe(OH)x nanosheets on nickel foam were subjected to thermal ammonolysis [78]. The formed FeNi3N/NF had been subjected for water splitting in alkaline condition where it had shown high activities in both OER and HER with overpotentials of 202 and 75 mV at 10 mA cm2 current density and corresponding Tafel slope values are 40 and 98 mV dec1, respectively (Fig. 16.6). These results showed the excellent activity of bimetallic nitrides with high charge transfer kinetics with the very low observed overpotentials. Macroscopic view FeCI2, SCT

Calcination

90°C, 6h

500°C, NH3

V vs. NHE 0.771 Fe3+/Fe2+

2+

Fe

Fe3+ Ni2+

0.000 NHE 2+/ -0.257 Ni Ni 2+/ -0.440 Fe Fe

In-situ growth process

NiFe hydroxide on Ni foam Microcosmic view

FeNi3N/NF

Figure 16.5 Schematic illustration of the synthesis of FeNi3N/NF. (Reproduced with permission [78] and copyrighted 2016, American Chemical Society.)

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Figure 16.6 Polarization curves for (A) oxygen evolution reaction (OER) and (B) hydrogen evolution reaction (HER) measured at a scan rate of 5 mV s-1. (Reproduced with permission [78] and copyrighted 2016, American Chemical Society.)

They have been taken as bifunctional electrode and the overall water splitting reaction with them as anode and cathode, as FeNi3N/NFjjFeNi3N/ NF electrolyzer has shown the cell voltage of 1.62 V at a current density of 10 mA cm2 without iR compensation in 1.0 M KOH. This enables the conclusion of efficient activities of bimetallic nitrides in alkaline conditions for both OER and HER [79]. Also, it showed extremely good durability, and it is reflecting in more than 400 h of consistent galvanostatic electrolysis. The corresponding plots have been given as Fig. 16.7A and B. The outstanding electrocatalytic performance of FeNi3N/NF is because of their

Figure 16.7 (A) Polarization curve of water electrolysis for FeNi3N/NFjjFeNi3N/NF with a scan rate of 5 mV s1. (B) Chronopotentiometric curve of FeNi3N/NFjjFeNi3N/NF with a constant current density of 10 mA cm2 at room temperature. (Reproduced with permission [78] and copyrighted 2016, American Chemical Society.)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

-o-

H2 O

NH3

+o-

H2

623 K Nanoparticles stacked porous Co3Fe DH nanowires

O2

Co3FeNx nanowires

excellent OER and HER performance

high electron conductivity poor electron conductivity Limited active sites

high specfic area

expose more active sites

grain boundary, defects

Figure 16.8 Schematic illustration of NSP-Co3FeNx nanowires by annealing with NH3. HER, hydrogen evolution reaction; OER, oxygen evolution reaction. (Reproduced with permission [73] and copyrighted 2016, The Royal Society of Chemistry.)

intrinsic metallic character and unique electronic structure. In addition to this, Wang et al. reported bimetallic nickelecobalt nitride nanosheets which were grown on the nickel foam. Here, the method of formation of TMNs is by facile electrodeposition method followed by a one-step ammonia annealing process [76]. The method of formation has been showed here as a pictorial representation as Fig. 16.8. This way of electrodeposition method can influence the growth of nitrides over the metal precursor significantly. With respect to the potential applied, concentration, run time, and the substrate, the resultant growth of TMNs and their electronic density, porosity, and morphological features together can result in enormous OER and HER activity. After this, the subsequent thermal annealing process ends up in rich formation of TMNs with exposed area [76]. Based on this strategy, intentional incorporation of different transition metals and their growth on external substrates such as Ni foam, Cu foam, carbon cloth, and fluorinated tin oxide can facilitate the activity and stability in both OER and HER. To tune the electronic and physical property of the electrodes, the 3D interconnected porous structure is highly preferred, as the electrolyte can be interacted with full of facility and hence the activity can be a contemporary one. It has lots of advantages such as enlarged surface area and more active sites and facilitates the facile charge transfer process and gas diffusion process (O2 and H2). Furthermore, here, in this case, the obtained NiCo2N electrodes were subjected for overall water splitting reactions, where it has shown a lower overpotential of 290 and 180 mV achieved at a current density of 10 mA cm2 in the case of OER and HER, respectively, in alkaline condition [76].

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In case of alkaline electrolyzer, when kept as both anode and cathode, they showed the cell voltage of 1.7 V at 10 mA cm2 current density. The observed HER polarization curves, Tafel plots, Nyquist plot, and stability test showed the requirements of low overpotential, high charge transfer rate, less resistivity with enlarged stability of bi-metallic nitrides.They have found that this enhancement in the catalytic performance was mainly because of the synergistic effect of a bimetallic structure which could further boost up the electrocatalytic activity. In addition to that, Wang et al. have reported the NP-stacked porous Co3FeNx nanowires by a simple nitridation reaction [73]. Fig. 16.9A shows the electrocatalytic OER activity of bimetallic Co3FeNx nanowires and others such as NF-NH3, Co3Fe DH and Co3Fe DH-Ar and among all the catalysts analyzed for OER, the Co3FeNx showed superior activity by requiring a very low overpotential of 222 mV at 20 mA cm-2 current density. The other catalysts required overpotentials in the range of 240-360 mV. Moreover, at high current densities like 100 mA cm-2, the Co3FeNx required overpotential of 253 mV which too lesser

Figure 16.9 (A) Polarization study. (B) Corresponding Tafel plots. (C) Nyquist plots. (D) Cycling stability study for 2000 cycles. (Reproduced with permission [73] and copyrighted 2016, The Royal Society of Chemistry.)

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

than that of NF-NH3 (473 mV higher) at the same current density. The corresponding Tafel slopes extracted from the polarization curves showed the high rate charge transfer kinetics of bimetallic nitrides with a very low Tafel slope value of 46 mV/dec (Fig. 16.9B). Moreover, very less charge transfer resistance was observed for nitrides compared to the double hydroxides as can be seen from Fig. 16.9C. The cycling stability further assured the less degradation in activity even after 2000 cycles as clearly seen from Fig. 16.9D. Here, Co3Fe double hydroxide precursors were grown on nickel foam by a hydrothermal approach and then the obtained NP-stacked porous Co3FeNx nanowires on the nickel foam were subsequently annealed at 350 C under ammonia condition. The obtained Co3FeNx electrocatalyst was subjected to the water splitting reactions in alkaline condition [73]. However, it had shown a lower overpotential of 222 mV at 20 mA cm2 and 23 mV at 10 mA cm2 current density for both OER and HER, respectively. This method is simple and with the NP-stacked bimetallic nitrides, they achieved unprecedented activity for both OER and HER. When subjected for overall water splitting reactions by keeping them as both anode and cathode, they just required a very low overpotential of 1.539 V at a current density of 10 mA cm2 in alkaline medium [73]. The corresponding linear sweep voltammetry curves and the stability after the cycling study of overall water splitting for NSP-Co3FeNX have been given as Fig. 16.10 and proved the high-scale activity and stability of NSPCo3FeNX for EWS.

Figure 16.10 (A) Linear sweep voltammetry curves and (B) cycling study of overall water splitting for NSP-Co3FeNX in a two-electrode system. (Reproduced with permission [73] and copyrighted 2016, The Royal Society of Chemistry.)

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These discussions on bimetallic nitrides show the effective activities of them toward OER and HER and surface engineering can be done to further extend the activity and stability. Recently, researchers also focused on trimetallic nitrides, which have been discussed below.

4.3 Trimetallic nitrides for electrocatalytic water splitting Just ahead to the bimetallic nitrides, trimetallic nitrides also have tremendous attention owing to their hierarchical porous feature, uniform incorporation of element in the metal lattices, conductivity, less resistivity, and higher stability nature. Hence, it is highly desirable to focus on the synthesis tri-metallic electrocatalysts which are highly active, low cost and robustly stable during electrochemical studies. To address this issue, Zhu et al. have synthesized shape-controlled NieFeeMo trimetal nitride NTs by weak alkaline-etching process [75]. Here, they have initially prepared MoO3 nanorods followed by uniform incorporation of the metal ions. And the resultant material was utilized as an electrocatalyst for overall water splitting in alkaline condition. Owing to its exceptional features, the optimized NieFeeMo trimetal nitride nanotubes have shown the overpotential of 55 and 228 mV at the current density of 10 mA cm2 in both HER and OER, respectively. And as a two-electrode electrolyzer, it has shown a cell voltage of 1.513 V at a current density of 10 mA cm2. The observed activity can be fine-tuned with respect to the method of growth and the metal precursors and the substrate used [75]. These exceptional activities have been achieved from its uniform incorporation of metals in the lattices and extraordinary stability. From the mono-, bi-, and trimetallic nitrides for both OER and HER, we can see the increased activity compared to the oxide and hydroxide counterparts, which is because of the increased electronic conductivity, metallic nature, metal d-band electron density, and less corrosion resistance. In general, we could see that the TMNs are chemically stable in different pH conditions. Some of the metal nitrides and their electrochemical performance in both HER and OER reactions comprised are given in Tables 16.1 and 16.2. The tables show the OER and HER activity of different metal nitrides in different electrolytes. From Tables 16.1 and 16.2, it is clear that the activity of metallic nitrides can be varied with respect to the nature of nitrides and metal precursors. By optimizing these, the activity and stability of them can be modified accordingly. Even at near-neutral conditions, the prepared TMNs were able to give higher activity by requiring lesser overpotentials. Therefore, we can formulate TMNs for EWS in universal pH conditions in future.

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Nanostructured, Functional, and Flexible Materials for Energy Conversion and Storage Systems

Table 16.1 Comparison of transition metal nitrideebased catalysts in hydrogen evolution reaction. Over potential @ 10 mA Tafel slope S.No Catalyst Electrolyte cm2 [mV] [mV dec1] References

1

Co3FeNx

2

Ni3FeN

3

Ni3FeN nanoparticles FeNi3N/Ni foam Co0.6Mo1.4N2

4 5 6

Ni3N/Ni foam

1M KOH 1M KOH 1M KOH 1M KOH 0.1 M HClO4 1M KOH

23

94

[73]

45

75

[79]

158

42

[77]

75

98

[78]

190

e

[80]

150

120

[63]

Table 16.2 Comparison of transition metal nitrideebased catalysts in oxygen evolution reaction. Over potential Tafel slope @ 10 mA cm2 (mV dec1) S.No Catalyst Electrolyte (mV) References

1

Co2N

2

Co3N

3

Co4N nanowires Ni3N nanoparticles Ni3N/Nifoam Ni3N@NiCi NA/CC Ni3FeN nanoparticles Ni3N NA/ CC

4 5 6 7 8

1M KOH 1M KOH 1M KOH 1M KOH 1M KOH 1M KHCO3 1M KOH 1M KHCO3

390

72

[81]

370

80

[81]

257

44

[82]

430

64

[77]

370 ̴

65

[63]

400 @ 20 mA cm2 280

143

[74]

46

[77]

540 @ 20 mA cm2

162

[74]

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509

5. Conclusion A finding of efficient catalyst for EWS with betterment in activity and stability is unavoidable when we consider commercial hydrogen production in large scale. In an arena of replacing noble metal catalysts, the role of TMNs has been investigated recently for efficient splitting of water and showed overwhelming activities. The TMNs as electrocatalysts had showed high charge transport compared to hydroxides/oxides of metal counterparts. This was attributed to the enriched metallic nature of TMNs, conductivity, less corrosive nature, and high chemical stability in different pH conditions. This chapter highlighted the role of different TMNs such as mono-, bi-, and trimetallic in EWS. The synthetic methods followed and the substrates used can facilitate activity with respect to the exposed active surface area of TMNs. The enormous activity of TMNs is because of metallic nature of it from MeN bond with high density of electron that ensures facile transfer of electrons at electrode/electrolyte interface. Moreover, the stoichiometric ratios and the corresponding defects from it will have more accessible sites for the electrolyte to interact with the electrode surface. This particular chapter focused on the fundamental aspects of EWS and the recent developments on catalysts from TMNs. Also, mono-, bi-, and trimetallic nitrides and their synthetic approaches such as hydrothermal, thermal annealing, ammonolysis, and other methods were investigated. The prepared TMNs and their electrocatalytic activities and external substrates used and the corresponding OER/HER activities at different pH values were analyzed in detail. In future, these kinds of different TMNs can be prepared with various metal precursors from the deepest knowledge gained from this chapter for EWS and other energy-related applications.

Acknowledgments This chapter is written by K. Karthick, Sam Sankar, and Subrata Kundu. Authors thank the continuous support from CSIR-CECRI, Karaikudi, India. K. Karthick and Sam Sankar acknowledge UGC, New Delhi, India, for SRF awards.

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Index Note: ‘Page numbers followed by “f ” indicate figures and “t” indicates tables’.

A

B

Activated carbon (AC), 376e378 Active components, 441e456 Alginate-derived porous carbon, 390e395 Animal-based biomass, 385e386 Anodes, 443, 467e474 “Artificial leaf ”, 244e245 Artificial photocatalyst, silver nanowire, 238e241 Artificial photosynthesis, 229e230, 249 hierarchal photocatalytic nanomaterials biofuel cells, light-harvesting systems, 247e249 biohydrogen production, 249e250 chemical fuels, 246e247 homogeneous artificial photosynthesis system, 250 photosensitizers in artificial photosynthesis, 250e251 water splitting, 244e246 hierarchical nanomaterials for, 230e244 carbon dioxide reduction, 232e234 fiber-like hierarchical nanomaterials, 230e232 hierarchical based metal organic nanoflowers and nanorods, 241e244 hierarchical nanobox-based nanomaterials, 234e235 hierarchical ZnO-based hollow nanostructures, 235e237 silver nanowire as artificial photocatalyst, 238e241 titanium oxide nanotubes, photoreduction material, 237e238

Batteries, 41, 46e48 basics, 2e4 principle, 2e4 Bimetallic nitrides, 502e507, 503f Binary solvent mixture, 166e174 Biohydrogen production, 249e250 Bismuth halides, 182e183 Bottom-up methods, 22

C Calcium titanate (CaTiO3) compound, 158e160 Carbon aerogel, 378e379 Carbon-based composites, 314e315 Carbon-based materials, 16e17 Carbon-based nanomaterials, 20e22 Carbon containing materials, 295f Carbon dioxide reduction, 232e234 Carbon dots (CDs), 17e18 Carbon hydrogel, 378e379 Carbonization, 383e384 Carbon materials, 375e381 for low-cost perovskite solar cells, 134e135 Carbon nanodots energy characteristics of, 22e23 synthesis of, 20 Carbon nanodots applications light-emitting diodes (LEDs), 37e40 li-ion batteries, 35e36 solar cells, 36e37 supercapacitors application in, 35 Carbon nanoparticles (CNPs), 148e151 Carbon nanotubes (CNTs), 16e17, 52e53, 330e332, 346e347, 380 energy conversion using, 28e29 energy storage, 29e32 batteries, 29

517

518

Index

Carbon nanotubes (CNTs) (Continued) supercapacitors, 30e32 Carbon quantum dots (QDs), 17e18 Carbon-rich polymers thermal heating, 298e299, 298f Carbon structures, 76e83 Cathodes, 441e442, 475e483 Cell design, 418 Cellulose-derived porous carbon, 390 Cesium bismuth halide perovskite nanocrystals, 182e189 Cesium lead-free double perovskite nanocrystals, 191e199 Cesium lead-free halide perovskite nanocrystals, 174e200, 204e218, 206te216t cesium bismuth halide perovskite nanocrystals, 182e189 cesium lead-free double perovskite nanocrystals, 191e199 cesium stibium halide perovskite nanocrystals, 200 cesium tin halide perovskite nanocrystals, 174e182 Cesium lead-free halide perovskite (CsLFHP) nanomaterials, 160e166 Cesium metal halide perovskite NCs, 166e174 Cesium stibium halide perovskite nanocrystals, 200 Cesium tin halide perovskite nanocrystals, 174e182 Chemical fuels, 246e247 Chemically converted graphene (CCG), 356e359 Chitin-derived porous carbon, 395e396 CNDs, 17e18, 52e53 structural perspective of, 18 Conductive carbon, 147e148 Conventional energy resources, 15 Conventional PSCs, 135 Covalent band (CB), 337 Cyclic voltammetric (CV), 351e354, 419e420

D 3D graphene/organic nanocomposite, 481e482 Direct methanol fuel cells (DMFCs), 105, 113e122 Dye-sensitized solar cells (DSSCs), 1e2, 6e8, 6f, 22e23, 250 major components of, 6e7 working principles of, 8

E Effective artificial photosynthetic material, 230e232 Electrical double-layer capacitors (EDLCs), 346e347, 373e374 Electrocatalysts carbon structures as, 76e83 and electrocatalyst support materials, 73e76 Ti-based compounds as, 89e92 Electrochemical double-layer capacitance (EDLC), 418 Electrochemical energy storage (EES) systems, 1e2, 373e374 Electrochemical supercapacitors, 4e5 Electrochemical water splitting (EWS), 495e497 bimetallic nitrides, 502e507, 503f mechanism, 495e497 metal nitrides, 497e507 monometallic nitrides, 498e501 trimetallic nitrides, 507, 508t Electrochemistry, 466e467, 467f Electrolytes, 442e443 Electron transport membranes, 138 Electrophoretic deposition (EPD), 20e22 Embedment C-PSCs, 148e151 Energy consumption, 1 Energy conversion carbon nanotubes, 28e29 efficiency, 78e80 and storage, 44e51 carbon nanodots in, 35e40 and storage, graphene in, 40e44

Index

Energy storage, 29e32 devices, 346e347 ETM-free cells, 138e141

F Fabricated symmetrical devices, 365 Fe2O, 332e334 Fiber-like hierarchical nanomaterials, 230e232 Fiber-like structure, 388e389 Flexible and wearable supercapacitors (FWSCs), 367 Flexible supercapacitor devices (FSDs), 357te358t carbon nanotubes (CNTs), 346e347 chemically converted graphene (CCG), 356e359 conducting additives, 356e362 cyclic voltammetric (CV), 351e354 device structure, 348 electrical double-layer capacitors (EDLCs), 346e347 energy storage devices, 346e347 fabricated symmetrical devices, 365 fabrication, 348 future perspectives, 367 graphene composite materials, 348e365, 349f graphene forest (GF), 354e356, 355f graphene oxide (GO), 351e354 International Renewable Energy Agency (IRENA), 345 laser-scribed GO film (LSG), 351e354 lithium-ion batteries, 346e347 polypyrrole (PPy), 359e362 pseudocapacitors (PCs), 346e347 pure graphene-based flexible electrode materials, 351e356, 352f reduced multilayer graphene oxide (RMGO), 351e354 renewable energy resources (RES), 345 supercapacitors (SCs), 346e347 Fossil fuels, 15, 69e70 Fruit-based biomass, 386 Fuel cells, 10e11, 41e43, 51 Fullerene, 141e143, 380e381

519

Fulleropyrrolidinium iodide (FPI), 142e143 Functional building block material, 157e158

G Galvanostatic chargeedischarge, 420 Gaseous hydrogen, 293e294 Gelatin-derived porous carbon, 396e400 Goldschmidt factor, 158e160 Graphene, 18e19, 334e336, 379e380 derivatives, 143e146 energy conversion and storage, 40e44 potential properties of, 24 Graphene-based flexible electrode materials, 351e356, 352f Graphene-based flexible energy storage devices, 425e426 Graphene-based materials, 468e469 Graphene-based polymeric nanocomposites, 43 Graphene composite materials, 348e365, 349f Graphene forest (GF), 354e356, 355f Graphene oxide (GO), 351e354 Graphene preparation, 20e22 Graphene quantum dot (GQD), 18e20, 44e51, 49f, 144 batteries, 46e48 fuel cells, 51 photovoltaic cells/solar cells, 48e51 supercapacitors, 44e46 Graphitic carbon nitride carbon containing materials, 295f carbon-rich polymers thermal heating, 298e299, 298f composites, 303e315 carbon-based composites, 314e315 metal composites, 306e308, 308f metal organic framework (MOF), 312e314 metal oxide, 308e311, 310f metal sulfide, 311e312, 313f electronic structure, 296e298 gaseous hydrogen, 293e294

520

Index

Graphitic carbon nitride (Continued) hydrogen evolution reaction (HER), 293e294 nitrogen containing materials, 295f oxygen, 293e294 photocatalytic hydrogen generation design, 301e303 bulk g-C3N4, 301 g-C3N4 nanosheets, 301e303 nanotubes g-C3N4, 303 porous g-C3N4, 303 photocatalytic hydrogen generation photocatalyst, 294e298 photocorrosion, 293e294 physicochemical properties, 296e298 self-oxidation, 293e294 solegel method, 300e301 synthesis methods, 298e301 template-based method, 299e300 X-ray crystallography, 294e296

H Heat treatment process, 78e80 Heterogeneous artificial photosynthesis system, 250 Hexagonal layered two-dimensional (2D) metal selenide, 453e456 Hierarchal photocatalytic nanomaterials biofuel cells, light-harvesting systems, 247e249 biohydrogen production, 249e250 chemical fuels, 246e247 homogeneous artificial photosynthesis system, 250 photosensitizers in artificial photosynthesis, 250e251 photoelectrochemical water splitting, 251 photosensitizers in water oxidation, 251 water splitting, 244e246 Hierarchical based metal organic nanoflowers and nanorods, 241e244 Hierarchical nanobox-based nanomaterials, 234e235 Hierarchical nanomaterials, for artificial photosynthesis, 230e244

carbon dioxide reduction, 232e234 fiber-like hierarchical nanomaterials, 230e232 hierarchical based metal organic nanoflowers and nanorods, 241e244 hierarchical nanobox-based nanomaterials, 234e235 hierarchical ZnO-based hollow nanostructures, 235e237 silver nanowire as artificial photocatalyst, 238e241 titanium oxide nanotubes as photoreduction material, 237e238 Hierarchical ZnO-based hollow nanostructures, 235e237 High energy storage devices, flexible supercapacitors cell design, 418 challenges, 430e431 cyclic voltammetry, 419e420 electrochemical double-layer capacitance (EDLC), 418 energy and power densities, 420e421 future opportunities, 430e431 galvanostatic chargeedischarge, 420 graphene-based flexible energy storage devices, 425e426 hybrid-based flexible energy storage devices, 428, 429f pseudocapacitance, 418 supercapacitors, 418 synthesis method, 421e424, 424fe425f three-electrode system, 418e419 transition metal dichalcogenide, 426e428 two-dimensional materials, 421 two-electrode system, 419 Hole transport membrane, 135e137 Homogeneous artificial photosynthesis system, 250 Honeycomb crystal lattice structure, 18e19 Hot-injection method, 168e170 HTM-free cells, 137e138 Hybrid-based flexible energy storage devices, 428, 429f

Index

Hydrogen evolution reaction (HER), 293e294, 336e337, 493e494 Hydrogen fuel cells (HFCs), 105 polymeric nanomaterials in, 109e113 Hydrogen production by photocatalytic water splitting, 8e10 Hydrogen storage devices, 27, 43e44 Hydrothermal carbonization, 383 Hydrothermal synthesis, 273e275

I Induced photocatalysis, 332e334 Inorganic perovskite layers, 132e134 International Renewable Energy Agency (IRENA), 345 Intrinsic ionization of photocatalysts, 9 Ionothermal carbonization, 383e384 IrO2-supported electrocatalysts, 83e89

L Laser-induced photocatalytic water splitting, 333f Laser-scribed GO film (LSG), 351e354 Layered double hydroxides (LDH), 494 Lead-free perovskites, 163e164, 217e218 LiFePO4, 476e477 Ligand-assisted reprecipitation method (LARP), 166e168 Light-emitting diodes (LEDs), 37e40 application in, 37e40 Light-harvesting systems, 247e249 Lignin-derived porous carbon, 395 Li-ion batteries, 35e36 Liquid electrolytes, 4 Lithium cobalt oxide, 477e479, 478f Lithium-ion batteries (LIB), 2e4, 346e347 anode, 467e474 cathode, 475e483 3D graphene/organic nanocomposite, 481e482 electrochemistry, 466e467, 467f functional materials, 474 graphene-based materials, 468e469 LiFePO4, 476e477 lithium cobalt oxide, 477e479, 478f

521

lithium manganese oxide, 479e480 lithium-rich NCM materials, 480e481 nanoscience and nanotechnology, 466 other nanomaterials, 483, 484t silicon-based composites, 469e471 tin oxide materials, 473e474 transition metal dichalcogenides, 472e473 transition metals, 472 Lithium manganese oxide, 479e480 Lithium-rich NCM materials, 480e481

M Membrane electrode assembly (MEA), 73 Metal composites, 306e308, 308f Metal halide perovskites, 158e160 Metal nitrides, 497e507 Metal organic framework (MOF), 241, 312e314 Metal oxide, 308e311, 310f Metal sulfides, 311e312, 313f, 336e337, 444e451, 454te455t Microbial fuel cell (MFC), 105 polymeric nanomaterials in, 106e109 Microorganism-based biomass, 386e387 Molten salt carbonization, 384 Monometallic nitrides, 498e501 Multi-walled carbon nanotube (MWCNT), 16e17, 27, 447

N Nanocarbon carbon materials for low-cost perovskite solar cells, 134e135 conductive carbon, 147e148 electron transport membranes, 138 embedment C-PSCs, 148e151 ETM-free cells, 138e141 fullerene and its derivatives, 141e143 graphene and its derivatives, 143e146 hole transport membrane, 135e137 HTM-free cells, 137e138 inorganic perovskite layers, 132e134 perovskite solar cells (PSCs), 131 Nanocomposites, 335e336 Nanofiber, 117e118

522

Index

Nanostructured bifunctional electrocatalyst, 69e70 carbon structures as electrocatalyst supports, 76e83 electrocatalysts and electrocatalyst support materials, 73e76 Sb-doped SnO2 and SiO2eSO3H electrocatalyst support, 92e95 Ti-based compounds as electrocatalyst supports, 89e92 unitized regenerative fuel cell system (URFC), 70e73, 73f unsupported and IrO2-supported electrocatalysts, 83e89 Nanostructured materials photocatalytic energy conversion carbon nanotube, 330e332 covalent band (CB), 337 Fe2O, 332e334 graphene, 334e336 hydrogen evolution reaction (HER), 336e337 induced photocatalysis, 332e334 laser-induced photocatalytic water splitting, 333f metal sulfide, 336e337 nanocomposites, 335e336 oxyhydrogen gas (OHG), 338, 339f photoelectrochemical activity, 328e330 photoelectrolysis, 327e328, 329f sunlight converting techniques. See Sunlight converting techniques Ti3C2 MXene cocatalyst, 336e337 TiO2 nanocomposite, 330e332 tungsten-doped Ni-Zn nanoferrites, 340 visible light photocatalytic hydrogen production, 336e337 water photocatalytic splitting, 334e336 water splitting, 332e334 Natural photosynthesis, 249 Natural polymer alginate-derived porous carbon, 390e395 cellulose-derived porous carbon, 390

chitin-derived porous carbon, 395e396 gelatin-derived porous carbon, 396e400 lignin-derived porous carbon, 395 starch-derived porous carbon, 395 Near-infrared (NIR) region, 157e158 Nitrogen containing materials, 295f Noble metals, 494 Nonefluoride-based anodization, 264e265

O One-step Solution Deposition, 133 Optoelectronics, 217e218 Organic and quantum dot (QD), 131 Organometal trihalide PSCs, 132 Oxygen, 293e294 Oxygen evolution reaction (OER), 493e494 Oxygen reduction reaction (ORR), 69e70 Oxyhydrogen gas (OHG), 338, 339f

P Perovskite derivatives, 178 Perovskites, 157e160 Perovskite solar cells (PSCs), 131 Photocatalytic hydrogen generation, 257e258 photoelectrochemical water splitting basic principle of, 258e259 material selection for, 260e261 TiO2 photocatalyst for, 261e265 TiO2 nanotubes arrays and anodization method, 262e263 formation mechanism of, 265e267 four synthesis generation of, 263e265 photocatalytic activity of, 267e269 WO3-incorporated TiO2 photocatalyst, 272e273, 278t, 280te282t preparation of, 273e279 water photoelectrolysis using, 279e285, 284te285t Photocatalytic water splitting, 8e9 hydrogen production by, 8e10

Index

Photocorrosion, 293e294 Photoelectrochemical activity, 257e258, 328e330 Photoelectrochemical water splitting, 251 basic principle of, 258e259 material selection for, 260e261 TiO2 photocatalyst for, 261e265 Photoelectrolysis, 327e328, 329f Photoluminescent quantum yield (PLQY), 160e162, 197e198 Photoreduction material, 237e238 Photosensitizers, 245 in artificial photosynthesis, 250e251 photoelectrochemical water splitting, 251 photosensitizers in water oxidation, 251 in water oxidation, 251 Photovoltaic cells/solar cells, 48e51 Plant biomass, 384e385 Plasma-enhanced CVD methods, 20e22 Platinum group metals (PGMs), 73e76 Polydopamine (PDA) layer, 447 Polymeric nanomaterials in fuel cell applications (PEMFCs), 105e106 in direct methanol fuel cells, 113e122 in hydrogen fuel cells, 109e113 in microbial fuel cells, 106e109 Polypyrrole (PPy), 359e362 Porous carbons (PCs), 378 Potential properties of graphene, 24 Power conversion efficiency (PCE), 131 Proton exchange membrane (PEM) fuel cells, 28e29 Pseudocapacitance, 418 Pseudocapacitors (PCs), 346e347 Pure inorganic metal-halide perovskite nanomaterials, 160 Pyrolysis, 383

Q Quantum confinement, 191e193 Quantum dots (QDs), 247e249

523

R Reduced multilayer graphene oxide (RMGO), 351e354 Renewable energy applications, 27e32 Renewable energy resources (RES), 345

S Sb-doped SnO2 electrocatalyst support, 92e95 Self-assembled nanostructures, 230e232 Self-oxidation, 293e294 Silicon, 469e471 Silver nanowire as artificial photocatalyst, 238e241 Single-walled carbon nanotube (SWCNT), 16e17 SiO2eSO3H electrocatalyst support, 92e95 Sodium-ion batteries (SIB) active components, 441e456 anodes, 443 cathodes, 441e442 electrolytes, 442e443 hexagonal layered two-dimensional (2D) metal selenide, 453e456 metal sulfides, 444e451, 454te455t multi-walled carbon nanotube (MWCNT) composites, 447 polydopamine (PDA) layer, 447 principle, 438e441, 439f transition metal oxides, 443e444 transition metal selenides, 451e456 Solar cells, 27e28, 40e41 application in, 36e37 Solar fuel system, 244e245 Solegel method, 275e277, 276t, 300e301 Stability, 497 Starch-derived porous carbon, 395 Structured TiO2 nanofibers, 230e232 Sunlight converting techniques photovoltaic technology, 326e327, 326f wet-chemical photosynthesis, 327, 327f Supercapacitors (SCs), 4e5, 5f, 43e46, 346e347 activated carbon (AC), 376e378

524

Index

Supercapacitors (SCs) (Continued) application in, 35 biomass-derived porous carbon electrodes, 381e384 activation, 381e382 application, 400e405, 401f, 403f carbonization, 383e384 fiber-like structure, 388e389 hydrothermal carbonization, 383 ionothermal carbonization, 383e384 molten salt carbonization, 384 natural polymer, 390e400 pyrolysis, 383 sheet-like structure, 389e390 sphere-like structure, 387e388 structural specification, 387e390 tube-like structure, 388 biomass precursors, 384e387 animal-based biomass, 385e386 fruit-based biomass, 386 microorganism-based biomass, 386e387 plant biomass, 384e385 types, 384e387 carbon aerogel, 378e379 carbon hydrogel, 378e379 carbon materials, 375e381 carbon nanotube, 380 definition, 373e374 electric double-layer capacitor (EDLC), 373e374 electrochemical energy storage (EES) systems, 373e374 fullerene, 380e381 fundamentals, 374e375 graphene, 379e380 porous carbons (PCs), 378 working principle of, 5 Surface chemistry, binary solvent mixture, 166e174 Synthesis methods, 298e301, 421e424, 424fe425f Synthesizing graphene quantum dots, 22

T Tafel slope, 496 Template-based method, 299e300

Thermal CVD methods, 20e22 Three-electrode system, 418e419 Ti-based compounds as electrocatalyst supports, 89e92 Ti3C2 MXene cocatalyst, 336e337 Tin oxide materials, 473e474 TiO2 nanocomposite, 330e332 TiO2 nanotubes arrays and anodization method, 262e263 formation mechanism of, 265e267 four synthesis generation of, 263e265 photocatalytic activity of, 267e269 TiO2 photocatalyst, 261e265 Titania-based hierarchal nanocatalysts, 237 Titanium oxide nanotubes as photoreduction material, 237e238 Top-down strategy, 22 Traditional PSCs, 131 Traditional semiconductor nanomaterials, 194 Transition metalebased nitrides electrochemical water splitting, 495e497 bimetallic nitrides, 502e507, 503f mechanism, 495e497 metal nitrides, 497e507 monometallic nitrides, 498e501 trimetallic nitrides, 507, 508t hydrogen evolution reaction (HER), 493e494 layered double hydroxides (LDH), 348 noble metals, 494 oxygen evolution reaction (OER), 493e494 stability, 497 tafel slope, 496 transition metal nitrides (TMNs), 495, 497e498 Transition metal dichalcogenides, 426e428, 472e473 Transition metal nitrides (TMNs), 497e498 Transition metal oxides, 443e444 Transition metals, 472 Transition metal selenides, 451e456

Index

Triethanolamine (TEOA), 241 Trimetallic nitrides, 507, 508t Trimethyl silyl chloride (TMS-Cl) precursor, 194e197 Tungsten-doped Ni-Zn nanoferrites, 340 Two-dimensional materials, 421 Two-electrode system, 419 Two-step Deposition, 133

U Unitized regenerative fuel cell system (URFC), 70e73, 73f

V Vacuum Processing Technique, 133e134 Visible light photocatalytic hydrogen production, 336e337

525

W Water electrolysis modes, 70e72 Water oxidation, 251 Water photocatalytic splitting, 334e336 Water splitting, 8e9, 244e246, 332e334 Weak polar solvents at room temperature (WPRT), 186e187 WO3-incorporated TiO2 photocatalyst, 272e273, 278t, 280te282t preparation of, 273e279 water photoelectrolysis using, 279e285, 284te285t

X X-ray crystallography, 294e296 X-ray diffraction (XRD) spectra, 80e83