Handbook of Nanocomposite Supercapacitor Materials IV: Next-Generation Supercapacitors 3031237005, 9783031237003

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Springer Series in Materials Science 331

Kamal K. Kar   Editor

Handbook of Nanocomposite Supercapacitor Materials IV Next-Generation Supercapacitors

Springer Series in Materials Science Volume 331

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

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

Kamal K. Kar Editor

Handbook of Nanocomposite Supercapacitor Materials IV Next-Generation Supercapacitors

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

ISSN 0933-033X ISSN 2196-2812 (electronic) Springer Series in Materials Science ISBN 978-3-031-23700-3 ISBN 978-3-031-23701-0 (eBook) https://doi.org/10.1007/978-3-031-23701-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

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

Preface

The global energy scene, one of the world’s largest and most diversified fields, is in a state of flux. These include the moving consumption away from non-renewable energy sources, rapid deployment of major renewable energy technologies and deep decline in their costs, and a growing shift toward electricity in energy use across the globe. This power and energy system is experiencing significant changes and challenges due to transitioning from traditional power and energy networks to smart power/energy grids. As long as the energy consumption is intended to be more economical and more environment-friendly, electrochemical energy production is under serious consideration as an alternative energy/power source. In other words, a large amount of electricity can be generated from natural sources like solar, wind, and tidal energy. It is imperative to stock the produced energy since man has constrained control over these natural wonders. Batteries, fuel cells, and supercapacitors belong to the same energy storage devices, ubiquitous in our daily lives. But the supercapacitor is a step-up device in the field of energy storage. It has a lot of research and development scope in design, parts fabrication, and energy storage mechanisms. Various types of supercapacitors have been developed, such as electrochemical double-layer capacitors (EDLCs), pseudocapacitors (redox capacitors), and capacitors. They store charges electrochemically and exhibit high power densities, moderate-to-high energy densities, high rate capabilities, long life, and safe operation. The electrode, electrolyte, separator, and current collectors are the critical parts of the supercapacitors for energy storage to determine the electrochemical properties, energy storage mechanism, and other properties of the supercapacitor devices. Volume I, i.e., characteristics for the book series Handbook of Nanocomposite Supercapacitor, emphasizes the features of the capacitor, i.e., fundamental aspects; capacitor to supercapacitor; characteristics of transition metal oxides, activated carbons, graphene/reduced graphene oxide, carbon nanotubes, carbon nanofibers, and conducting polymers; characteristics of electrode materials, electrolytes, separators, and current collectors; and applications of supercapacitors. Volume II, i.e., performance for the book series Handbook of Nanocomposite Supercapacitor, discusses the electrochemical properties of transition metal oxide-based electrode, activated carbon-based electrode, composite electrode based vii

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on transition metal oxides and activated carbon, carbon nanofiber-based electrode, composite electrode based on different types of transition metal oxides and carbon nanofibers, carbon nanotube-based electrodes, combination of carbon nanotube and transition metal oxide as a hybrid electrode, graphene-based electrodes, hybrid composites based on transition metal oxides and graphene/reduced graphene oxide, electrode materials based on conducting polymers and their nanostructures, composites of conducting polymers and transition metal oxide, and comprehensive overview/recent trends of the specific capacitance and cycle life of various electrode materials used in supercapacitors, which are carbon nanofibers, carbon nanotubes, graphene/reduced graphene oxide, activated carbon, transition metal oxides, conducting polymers, and their composites. Finally, it highlights the advantages, challenges, applications, and future directions of these materials. The performance of devices is still challenging in terms of capacitance, flexibility, cycle life, etc. These deciding factors depend on the characteristics of the materials used in the devices. The key objective is to select the right materials with new technologies and developments for the electrodes, electrolytes, separators, and current collectors, which are the essential components of supercapacitors with an aim to enhance the performance of supercapacitors. Volume III, i.e., material selection for the book series Handbook of Nanocomposite Supercapacitor emphasizes a comprehensive study on the fundamentals of supercapacitors, recent development of supercapacitors, material selection for electrodes, electrolytes, separators, and current collectors using Ashby chart, market trend of supercapacitors, and applications of supercapacitors. Many significant breakthroughs have been reported in recent years through the development of these materials and novel device designs. Volume IV, i.e., next generation for the book series Handbook of Nanocomposite Supercapacitor, emphasizes micro-supercapacitors, shape memory supercapacitors, self-healing supercapacitors, high mass loading solid-state supercapacitors, magnetoelectric supercapacitors, atomic-layer-deposited electrodes for supercapacitor, additive manufacturing/3D printing of supercapacitor, etc. In this book, next-generation supercapacitors, Chap. 1 discusses the fundamentals of supercapacitors, the charge storage mechanism of supercapacitors, electrochemical cell configuration (i.e., three-electrode and two-electrode systems), electrochemical measurement techniques for supercapacitor (i.e., cyclic voltammetry, constant current charge-discharge, electrochemical impedance spectroscopy, and electrochemical methods for determining the contribution of various charge storage mechanisms. The supercapacitor has four essential components: electrode, electrolyte, current collector, and separator, in which electrode material selection is the most important factor for the charge storage mechanism. Different types of electrode materials like carbon-based electrode material, transition metal oxides, transition metal dichalcogenides, and conducting polymers are used in supercapacitor applications. Chapter 2 discusses the properties of electrode materials, nanomaterials as electrode materials (i.e., zero-dimensional nanoparticles, one-dimensional nanostructures, twodimensional nanosheets, three-dimensional porous architectures), carbon materials

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(i.e., activated carbon, carbon aerogels, carbon nanotubes, carbon nanofibers, carbon nanodot, graphene, graphene oxide, reduced graphene oxide, fullerenes), transition metal dichalcogenides (i.e., molybdenum di-sulfide, tungsten di-sulfide, cobalt disulfide, tin di-sulfide, titanium di-sulfide, zirconium di-sulfide, vanadium di-sulfide, molybdenum di-selenide, vanadium di-selenide, tungsten di-selenide, nickel diselenide), transition metal oxides (i.e., ruthenium oxide, manganese dioxide, nickel oxide, nickel hydroxide, iron oxides, cobalt oxide, cobalt hydroxide, vanadium oxide, tin oxide, iridium oxide, titanium oxide, zinc oxide, molybdenum oxide, tungsten oxide), and spinel-based nanostructured materials. MXenes have been recognized as front-runners in energy storage, thanks to their abundant surface functional groups, large electrochemically active surface area, redox activity, and metallic conductivity. MXenes display extraordinarily higher volumetric capacitance, making them a considerable contender in portable electronic devices. Chapter 3 discusses the current advances, achievements, and challenges in MXene-based supercapacitors, including important synthetic aspects of MXenes along with their physical and chemical characteristics. The laser provides a single-step, low-cost, and fast processing of materials to form and integrate interdigitated electrodes for micro-supercapacitors. Chapter 4 illustrates the different parameters of the laser, laser-based processes, effect of laser– matter interactions, different materials synthesized from the laser, and the formation of micro-supercapacitors with performance details. The performance deteriorates with an increase in the size of devices due to the internal resistance from non-active materials such as binders and additives, heating issues, and the high cost of production. To address these challenges, the designer develops electrode structures such as self-standing architectures, mesh-type electrodes, and fractal designs that can be viable solutions to enhance the performance of large-scale energy storage devices. Chapter 5 discusses the challenges in scalableenergy storage devices (i.e., degradation in performance, cost-effectiveness, heating issues, voltage imbalance, etc.), ways to address challenges for large-scale supercapacitors (i.e., geometry/electrode structure, cost-effectiveness by using industrial waste, device architecture, voltage stabilization), and various fabrication techniques (i.e., printed supercapacitors, additive nanomanufacturing, electrode production, electrolyte production, material processing, and optimization). Numerous developments have been made in supercapacitors’ three-dimensional (3D) printing. In a consistently changing technological landscape, it is important to understand how 3D printing could evolve in the future in supercapacitor technology. Chapter 6 describes the main 3D printing technologies and the relevant materials used to make supercapacitors. The chapter also discusses the prospects of 3D printingbased development of supercapacitors in the future. Atomic layer deposition (ALD) is considered an efficient technique for depositing various kinds of films with excellent uniformity and conformity. This makes ALD an attractive choice for designing high-performance supercapacitor electrode materials possessing fast charge transfer kinetics, improved energy, and power delivery with better cycling and rate performances. Chapter 7 presents the recent advances in the use of ALD to design electrodes for supercapacitors. In addition, the present

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challenges and potential opportunities for future exploration of ALD to achieve desired electrochemical performance of next-generation supercapacitors are also pointed out. Binder restricts the electrode material’s performance by increasing the contact resistance and preventing electrolytes from utilizing the whole area of the electrode. A binder-free supercapacitor is a new approach for improving the performance of supercapacitors by growing or depositing the active material on the conducting substrate. Binder-free electrode material can be fabricated by physical, thermal, and electrical methods. Chapter 8 discusses different fabrication methods (i.e., electrospinning, vacuum filtration, physical vapor deposition, thermal treatment, hydrothermal treatment, chemical bath deposition, chemical vapor deposition, atomic layer deposition, electroplating, anodization, electrophoretic deposition, etc.) and performances of binder-free electrodes. High mass loading electrode materials are a commercial requirement of supercapacitor fabrication. At least 30% of the device weight should be posed by active electrode material to stable performance of electrochemical energy storage supercapacitor devices. Commercial-level supercapacitors require high mass loading greater than 10 mg cm−2 or a film thickness of 150–200 µm. High mass loading supercapacitor performance decreases because of the tortuous path of ion diffusion. Chapter 9 discusses the effect of mass loading, selective electrode materials (i.e., carbon, metal oxide, conducting polymer, MXenes, metal-organic framework, interconnected conducting porous network structure, aerogel, doping, surface modification, etc.), and electrochemical performance. In polymer-in-salt-electrolytes, the ion transport is decoupled from the polymersegmental motion. Hence, faster and better-targeted ion transport is achieved. Special polymer hosts are required to hold salt concentration above the required threshold value for ion-cluster formation. Crosslinked starch seems to be an excellent host to hold sufficient salt in dissociated form along with flexible morphology required for commercial application and has high conductivity (>0.01 S/cm) and a wide electrochemical stability window (>2.5 V). Chapter 10 focuses on starch-based electrolytes and their performances. Over the last few decades, transition metal oxides have been the most used materials in pseudocapacitors. The magnetic nature of these materials originates from the effective spin interaction of the materials, which can be in short- or long-range order. Fe-based materials are mostly used for magnetic applications and have also been used as pseudocapacitor electrodes. There are other magnetic transition electrodes that are being used in supercapacitors. These include iron oxide, nickel oxide, cobalt oxide, copper oxide, manganese oxide, etc. Chapter 11 deals with understanding the effect of the external magnetic field on the performance of supercapacitors fabricated using magnetically responsive materials, i.e., magnetoelectric supercapacitors. Further, a simple theoretical model is also provided chapter to explain the experimental data. A new theory was required because the conventional models used to explain the supercapacitive behavior do not have any terms which consider the possibility of changing magnetic fields and their impact on electrochemical behavior.

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The advancement in the technology application of micro-electronic gadgets has seen an upsurge. The progress of micro-scale devices is significantly dependent on the development of micro-scale energy storage devices with outstanding charge storage properties. In this Chap. 12, the various device architecture designs and the state of the art of it have been discussed. Further, the different device preparation methods have been discussed, outlining their advantages and disadvantages. This is followed by a short and precise discussion about the patterning, and micro-supercapacitor systems developed recently. This chapter also discusses works reporting the various applications of micro-supercapacitors in different fields. This intelligent technology using shape memory is a primary requirement of flexible and wearable electronics. NiTi alloy and shape memory polymer are used to assemble a smart energy storage device with property shape recovery. Shape memory properties bring the device electrochemical stability, high performance, and long cycle life. Chapter 13 discusses shape memory alloys, shape memory polymers, shape memory characterization techniques, and electrochemical performances. With the advancement of current wearable electronic gadgets, a flexible and self-healing supercapacitor is required. Flexible supercapacitors can often endure bending, and stretching stains, so mechanical damage or micro-cracks can degrade the electrochemical performance of supercapacitors. Intrinsic and extrinsic selfhealing mechanisms are used during repair. Since self-healing supercapacitors are developing rapidly, still these are in their infancy because of many limitations like high cost and lower performance. Chapter 14 discusses various fabrication methods of self-healing electrode material and self-healing electrolyte materials with their electrochemical performances in supercapacitors. The world is utilizing optics through optical fibers, data communication, processing, and fabrication of high-resolution or precise instruments. Chapter 15 deals with the applications of optics-based devices, fabrication of optical chips, transmission systems, and infrastructure, along with computational and governance requirements. Laser-based on-chip micro-supercapacitors and energy storage management facilities for clean, renewable energy are also discussed. The use of supercapacitors is increasing in the electronics field due to their properties and sustainability. Controlling this e-waste generation by supercapacitors should be considered seriously to overcome the upcoming problem of e-waste management. Recycling supercapacitors is cost-effective and beneficial for the environment because it keeps dangerous elements out after the device has completely degraded. Chapter 16 discusses the recycling of ruthenium oxide-based supercapacitors. Sonication, chemical separation, and thermal decomposition methods were discussed. The electrochemical performance of the supercapacitor based on recycled RuO2 material was also reported. Therefore, this book will provide the readers with a complete and composed idea about the fundamentals of supercapacitors, the recent development of electrode materials for supercapacitors, and the design of their novel flexible solid-state devices. This book will be useful to graduate students and researchers from various fields of science and technology, who wish to learn about the recent development of supercapacitors and select the right material for high-performance supercapacitors.

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The editor and authors hope that readers from materials science, engineering, and technology will be benefited from reading these high-quality review articles related to the characteristics of materials and their selections used in supercapacitors. This book is not intended to be a collection of all research activities on composites worldwide, as it would be rather challenging to keep up with the pace of progress in this field. The editor would like to acknowledge many material researchers, who have contributed to the contents of the book. The editor would also like to thank all the publishers and authors for permitting us to use their published images and original work. I also take this opportunity to thank Viradasarani, Zachary, Viradasarani Natarajan, Adelheid Duhm, and the editorial team of Springer Nature for their helpful advice and guidance. There were lean patches when I felt I would not be able to take time out and complete the book, but my wife, Sutapa, and my little daughter, Srishtisudha, played a crucial role in inspiring me to complete it. I hope that this book will attract more researchers to this field and that it will form a networking nucleus for the community. Please enjoy the book, and please communicate to the editor/authors any comments that you might have about its content. Kanpur, Uttar Pradesh, India

Prof. Kamal K. Kar [email protected]

Contents

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2

Introduction to Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chirag Mevada and Mausumi Mukhopadhyay 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Fundamentals of Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 The Charge Storage Mechanism of Supercapacitors . . . . . . . . . . . 1.4 Electrochemical Cell Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.1 Three Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Two Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Electrochemical Measurement Techniques for Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Cyclic Voltammetry (CV) . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.2 Constant Current Charge–Discharge (CCCD) . . . . . . . . . 1.5.3 Electrochemical Impedance Spectroscopy (EIS) . . . . . . . 1.6 Electrochemical Methods for Determining the Contribution of Various Charge Storage Mechanisms . . . . . . . . . . . . . . . . . . . . . 1.6.1 Trasatti Method (Voltammetric Charge Dependence on Scan Rate) . . . . . . . . . . . . . . . . . . . . . . . . . 1.6.2 Dunn Method (Current Dependence on Scan Rate from the CV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Traditional Electrode Materials for Supercapacitor Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Saheli Bera, Kapil Dev Verma, and Kamal K. Kar 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1 Properties of Electrode Materials . . . . . . . . . . . . . . . . . . . . 2.2.2 Nanomaterials as Electrode Materials . . . . . . . . . . . . . . . . 2.3 Materials for Electrodes of Supercapacitors . . . . . . . . . . . . . . . . . . 2.3.1 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Transition Metal Dechalcogenide (TMD) . . . . . . . . . . . . .

1 1 2 6 8 8 9 11 11 12 13 14 14 15 17 19 19 21 21 21 25 25 37

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Contents

2.3.3 Transition Metal Oxide (TMO) . . . . . . . . . . . . . . . . . . . . . 2.3.4 Spinel-Based Nanostructured Materials . . . . . . . . . . . . . . 2.3.5 Spinel-Type Oxides (MMoO4 (M=Fe, Ni, Co)) . . . . . . . . 2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

45 56 57 58 58

Emerging 2D Materials for Supercapacitors: MXenes . . . . . . . . . . . . Shagufi Naz Ansari, Mohit Saraf, and Shaikh M. Mobin 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Synthetic Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 HF Etching Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Alkali Etching Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Molten Salt Etching Method . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Acid/fluoride Salt or Hydrofluoride Etching . . . . . . . . . . 3.2.5 Electrochemical Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Structure and Properties of MXenes . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 MXenes in Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 MXenes as Supercapacitor Electrode Materials . . . . . . . . 3.4.2 MXene-Based Composites as Supercapacitor Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Progress of MXenes-Based Supercapacitor Devices . . . . . . . . . . . 3.6 Conclusions and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Laser as a Tool for Fabrication of Supercapacitor Electrodes . . . . . . Ravi Nigam, Rajesh Kumar, and Kamal K. Kar 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Laser Technology in Energy Electrodes Design . . . . . . . . . . . . . . . 4.3 Processing of Laser in Carbon Materials . . . . . . . . . . . . . . . . . . . . . 4.3.1 Cutting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Ablation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.4 Laser Writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.5 Laser Printing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.6 Defect Creation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Laser-Assisted Modification of Carbon Materials . . . . . . . . . . . . . 4.4.1 Carbonization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Transformation of Graphite to Graphene . . . . . . . . . . . . . 4.4.3 Non-crystalline Carbon to Graphene . . . . . . . . . . . . . . . . . 4.4.4 Laser-Induced Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.5 Reduction of Graphene Oxide . . . . . . . . . . . . . . . . . . . . . . 4.5 Laser-Derived Material in Supercapacitor . . . . . . . . . . . . . . . . . . . . 4.5.1 Electrochemical Double-Layer Capacitors . . . . . . . . . . . . 4.5.2 Pseudocapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Hybrid Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.6 Direct Laser-Based Fabrication of Micro-supercapacitor . . . . . . .

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66 67 67 69 70 72 73 73 74 75 76 82 82 84

89 90 92 92 94 94 95 97 99 99 100 102 102 103 104 106 106 107 108 109

Contents

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4.7 Conclusions and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . 116 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 5

6

Scalable Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Snehraj Gaur, Ajay B. Urgunde, Gaurav Bahuguna, S. Kiruthika, and Ritu Gupta 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Challenges in Scalable Energy Storage Devices . . . . . . . . . . . . . . . 5.2.1 Degradation in Performance . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Cost-Effectiveness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Heating Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.4 Voltage Imbalance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Ways to Address Challenges for Large-Scale Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Geometry/Electrode Structure . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Cost-Effectiveness by Using Industrial Waste . . . . . . . . . 5.3.3 Device Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Voltage Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.1 Printed Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.2 Additive Nanomanufacturing (ANM) . . . . . . . . . . . . . . . . 5.4.3 Electrode and Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.4 Material Processing and Optimization . . . . . . . . . . . . . . . 5.5 Testing of Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.6 Conclusions and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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3D Printed Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Naga S. Korivi and Vijaya Rangari 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Printing Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Fused Deposition Modeling . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Direct Ink Writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Printable Materials for Supercapacitors . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Electrolyte Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Device Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Recent Progress in 3D Printing of Supercapacitors . . . . . . . . . . . . 6.5.1 FDM Printed Supercapacitors . . . . . . . . . . . . . . . . . . . . . . 6.5.2 DIW Printed 3D Supercapacitors . . . . . . . . . . . . . . . . . . . . 6.6 Technology Considerations, Challenges, and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.1 Choice of Printing Method . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.3 Use of Non-3D Printing Methods to Fabricate Device Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

143

124 125 125 126 126 127 127 127 128 130 132 132 132 134 135 138 139 139 140

143 145 145 145 147 147 148 148 149 150 151 155 155 157 158

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6.6.4 Post-processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6.5 Device Design and Electrode Architecture . . . . . . . . . . . . 6.6.6 Sustainability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

159 161 161 162 163

Atomic Layer Deposited Supercapacitor Electrodes . . . . . . . . . . . . . . Mohd Zahid Ansari, Soo-Hyun Kim, Arpan Dhara, and Dip K. Nandi 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Fundamentals of ALD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 ALD-Grown Electrodes for Supercapacitors . . . . . . . . . . . . . . . . . . 7.3.1 ALD Coating on Carbonaceous Scaffolds . . . . . . . . . . . . 7.3.2 ALD Coating on Non-carbonaceous Three-Dimensional Scaffolds . . . . . . . . . . . . . . . . . . . . . . . 7.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167

Binder-Free Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kapil Dev Verma and Kamal K. Kar 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Fabrication Strategies of Binder-Free Electrode . . . . . . . . . . . . . . . 8.2.1 Physical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Chemical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Electrical Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

195

High Mass Loading Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mukesh Kumar and Kamal K. Kar 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Effect of Mass Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Materials for High Mass Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.1 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.2 Transition Metal Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3 Conducting Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.4 Emerging Electrode Materials . . . . . . . . . . . . . . . . . . . . . . 9.4 Electrode Materials Synthesis Techniques . . . . . . . . . . . . . . . . . . . . 9.4.1 Interconnected Conducting Porous Network Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2 Aerogel Synthesis Techniques . . . . . . . . . . . . . . . . . . . . . . 9.4.3 Doping and Surface Modification . . . . . . . . . . . . . . . . . . . 9.5 Electrochemical Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Summary and Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225

167 169 172 173 182 192 192

195 196 197 205 216 220 220

226 227 229 229 230 230 230 231 232 232 233 233 237 241

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10 Flexible-High-Conducting Polymer-In-Salt-Electrolyte (PISE) Membranes: A Reality Due to Crosslinked-Starch Polymer Host . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neelam Srivastava 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Starch-Based Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Starch as a Host Matrix for Polymer-Electrolytes . . . . . . 10.2.2 Reasons Why Starch Has not Been as Popular as It Deserves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Possible Approaches to Rectify the Problems with Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Success Story of Crosslinked Starch-Based PISEs . . . . . 10.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Magneto-Electric Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ananya Chowdhury, Sudipta Biswas, Abyaya Dhar, Joyanti Halder, Debabrata Mandal, Poornachandra Sekhar Burada, and Amreesh Chandra 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Synthesis of Magnetic Transition Metal Oxides . . . . . . . . . . . . . . . 11.2.1 Synthesis of Fe2 O3 Nanoleaflets . . . . . . . . . . . . . . . . . . . . 11.2.2 Synthesis of Fe2 O3 Rod-Like Structures . . . . . . . . . . . . . . 11.2.3 Synthesis of Fe2 O3 Nanospheres . . . . . . . . . . . . . . . . . . . . 11.3 Magnetic Electrolyte or Effect of the External Magnetic Field on the Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3.1 Introduction of Magneto-Electric Effect (MEE) . . . . . . . 11.3.2 Effect of Magnetic Field on the Electrochemical Performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.4 Origin of Magnetic Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5 Explanation of MEE in Supercapacitor . . . . . . . . . . . . . . . . . . . . . . 11.5.1 Magnetic Nature of the Material Used as an Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.5.2 Effect of the Lorentz Force on the Material . . . . . . . . . . . 11.5.3 Domains Arrangement of the Electrode Material . . . . . . 11.5.4 Effect of the Lorentz Force on the Electrolyte . . . . . . . . . 11.5.5 Magneto-Hydrodynamic Effect of the Electrolyte . . . . . . 11.6 Theoretical Interpretation of Magneto-Electric Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.1 Existing General Theories . . . . . . . . . . . . . . . . . . . . . . . . . . 11.6.2 Solution of Diffusion Equation . . . . . . . . . . . . . . . . . . . . . 11.6.3 Diffusion-Related Explanation of Magnetic Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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247 248 252 252 253 253 254 259 260 265

265 266 266 267 268 268 269 269 273 275 275 276 277 278 279 282 282 284 290 292 293

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12 Advancement in the Micro-supercapacitors: Synthesis, Design, and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mandira Majumder and Abha Misra 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2 Device Architecture Designing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Brief Introduction to the Reaction Mechanism . . . . . . . . . . . . . . . . 12.4 Device Fabrication Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.1 Screen Printing for Electrode Fabrication . . . . . . . . . . . . . 12.4.2 Inkjet Printing for Micro-electrode Fabrication . . . . . . . . 12.4.3 Lithography for Micro-electrode Fabrication . . . . . . . . . . 12.4.4 Laser Scribing for Micro-electrode Fabrication . . . . . . . . 12.4.5 Mask-Assisted Filtering for Micro-electrode Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.5 Patterning of Micro-electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 Micro-supercapacitor Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7 Application of MSCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7.1 Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.7.2 Integration with Various Types of Sensors . . . . . . . . . . . . 12.7.3 Medical Assistant Examination . . . . . . . . . . . . . . . . . . . . . 12.7.4 Alternating Current (AC) Line Filtering . . . . . . . . . . . . . . 12.8 Evaluation of Various Parameters of Supercapacitors . . . . . . . . . . 12.8.1 Necessary Details About the System to be Reported . . . 12.8.2 Single Electrode Capacitance . . . . . . . . . . . . . . . . . . . . . . . 12.8.3 Difference of Capacitance in Three-Electrode and Two-Electrode System . . . . . . . . . . . . . . . . . . . . . . . . . 12.8.4 Operating Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.8.5 Micro- and Macro-Supercapacitors . . . . . . . . . . . . . . . . . . 12.8.6 Cycling Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.8.7 Energy, Power, and Ragone Plot . . . . . . . . . . . . . . . . . . . . 12.8.8 Coulombic Efficiency—Coulombic Efficiency is the Factor that Determines the Rate Capability of the Supercapacitor Device . . . . . . . . . . . . . . . . . . . . . . . 12.8.9 Determining the Percent of Diffusion Controlled and Surface Capacitance . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.9 Conclusions and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Shape Memory Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mukesh Kumar, Manas K. Ghorai, and Kamal K. Kar 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2 Shape Memory Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.1 High-Temperature Shape Memory Alloy . . . . . . . . . . . . . 13.2.2 Magnetic Shape Memory Alloy . . . . . . . . . . . . . . . . . . . . . 13.2.3 Phenomena of Transformation of Alloy . . . . . . . . . . . . . . 13.3 Shape Memory Polymer (SMP) . . . . . . . . . . . . . . . . . . . . . . . . . . . .

295 295 298 299 300 300 302 304 305 306 308 310 312 312 314 315 317 318 318 319 319 320 321 321 322

323 323 324 326 331 332 333 334 334 335 337

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13.3.1 Shape Memory Principle of Polymer . . . . . . . . . . . . . . . . 13.3.2 Classification of SMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.4 Shape Memory Characterization Techniques . . . . . . . . . . . . . . . . . 13.5 Design and Architecture of Structural Shape Memory Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.5.1 1D Yarn/Fiber Type Supercapacitor . . . . . . . . . . . . . . . . . 13.5.2 Planar and 2D Shape Memory Supercapacitor . . . . . . . . . 13.6 Electrochemical Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.7 Summary and Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

338 338 341

14 Self-healing Supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kapil Dev Verma and Kamal K. Kar 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Self-healing Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.1 Intrinsic Self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2.2 Extrinsic Self-healing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 Self-healing Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3.1 Self-healing Electrode for Supercapacitor Devices . . . . . 14.3.2 Self-healing Electrolyte for Supercapacitor Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

357

15 Optical Revolution with Sustainable Energy Framework . . . . . . . . . . Ravi Nigam and Kamal K. Kar 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2 Optics-Based Infrastructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.1 Optical Chips . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.2 Optical Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.2.3 Integration of Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . 15.2.4 Fabrication Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Sustainable Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.1 Implementation Policy Aspects . . . . . . . . . . . . . . . . . . . . . 15.4 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

379

16 Recycling of Supercapacitor Materials . . . . . . . . . . . . . . . . . . . . . . . . . . Harish Trivedi, Kapil Dev Verma, and Kamal K. Kar 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2 Methodology of Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.2.1 Important Steps in Recycling . . . . . . . . . . . . . . . . . . . . . . . 16.2.2 Processes Involved in Recycling Supercapacitors Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3 Recycling of Different Materials Used in Supercapacitor . . . . . . .

393

342 342 345 345 349 351

357 359 359 360 361 361 368 373 375

379 380 380 381 383 384 385 387 389 389

393 394 395 395 396

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16.3.1 Nanotubes and Organic Nanocrystals Materials Recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.3.2 Graphene Electrode Materials Recycling from the Decayed Supercapacitor . . . . . . . . . . . . . . . . . . . 16.4 Recycling of RuO2 from Decayed Supercapacitor . . . . . . . . . . . . . 16.4.1 Pseudocapacitance of RuO2 . . . . . . . . . . . . . . . . . . . . . . . . 16.4.2 Material and Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.3 Steps Involved in Recycling RuO2 . . . . . . . . . . . . . . . . . . . 16.4.4 Characterization of Extracted RuO2 via XRD . . . . . . . . . 16.4.5 Electrochemical Characterization of RuO2 -Based Hybrid Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.4.6 The Percentage Recovery of RuO2 . . . . . . . . . . . . . . . . . . 16.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

397 398 400 401 401 401 403 403 407 408 408

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413

Editor and Contributors

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

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Editor and Contributors

Kanpur, Defence Research and Development Organisation, Indian Space Research Organization, Department of Atomic Energy, Department of Biotechnology, Council of Scientific and Industrial Research, Aeronautical Development Establishment, Aeronautics Research and Development Board, Defence Materials and Stores Research and Development Establishment, Hindustan Aeronautics Limited, Kanpur, Danone Research and Development Department of Beverages Division, France, Indian Science Congress Association, Indian National Academy of Engineering, and many more from India. Professor Kar is the Editor-in-Chief of Polymers and Polymeric Composites: A Reference Series published by Springer Nature, Member of the Editorial Board of SPE Polymers published by Wiley, Advanced Manufacturing: Polymer and Composites Science published by Taylor & Francis Group, and International Journal of Plastics Technology published by Springer Nature and many more. Professor Kar has more than 300 papers in international refereed journals, 135 conference papers, 12 books on nanomaterials and their nanocomposites, three special issues on polymer composites, 100+ review articles/book chapters, and more than 65 national and international patents to his credits; some of these have over 200 citations. He has guided 35 doctoral students and 100 master students so far. Currently, seven doctoral students, two master students, and few visitors are working in his group, Advanced Nanoengineering Materials Laboratory.

Contributors Mohd Zahid Ansari School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea; Department of Chemistry, Indian Institute of Technology Indore, Simrol, Indore, India; Department of Chemistry, School of Engineering, Presidency University, Bangalore, India

Editor and Contributors

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Shagufi Naz Ansari Department of Chemistry, Indian Institute of Technology Indore, Simrol, Indore, India; Department of Chemistry, School of Engineering, Presidency University, Bangalore, India Gaurav Bahuguna Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India Saheli Bera Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Sudipta Biswas Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Poornachandra Sekhar Burada Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Amreesh Chandra Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Ananya Chowdhury Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Abyaya Dhar Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Arpan Dhara Department of Solid State Sciences, COCOON Research Group, Ghent University, Ghent, Belgium Snehraj Gaur Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India Manas K. Ghorai Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur, India Ritu Gupta Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India Joyanti Halder Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Kamal K. Kar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India; Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur, India Soo-Hyun Kim School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea S. Kiruthika Department of Physics, School of Electrical and Electronics Engineering, SASTRA Deemed to be University, Thanjavur, Tamil Nadu, India

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Naga S. Korivi Department of Electrical and Computer Engineering, Tuskegee University, Tuskegee, AL, USA Mukesh Kumar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Rajesh Kumar Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Mandira Majumder Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka, India Debabrata Mandal Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal, India Chirag Mevada Department of Chemical Engineering, S. V. National Institute of Technology, Surat, Gujarat, India Abha Misra Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore, Karnataka, India Shaikh M. Mobin Department of Chemistry, Indian Institute of Technology Indore, Simrol, Indore, India; Department of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Indore, India; Department Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Simrol, Indore, India; Center for Electric Vehicle and Intelligent Transport Systems, Indian Institute of Technology Indore, Simrol, Indore, India Mausumi Mukhopadhyay Department of Chemical Engineering, S. V. National Institute of Technology, Surat, Gujarat, India Dip K. Nandi School of Materials Science and Engineering, Yeungnam University, Gyeongsan, Gyeongbuk, Republic of Korea; Lumileds Singapore Pte. Ltd., Yishun, Singapore Ravi Nigam Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Vijaya Rangari Department of Materials Science and Engineering, Tuskegee University, Tuskegee, AL, USA Mohit Saraf Department of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Indore, India; A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA Neelam Srivastava Department of Physics (MMV), Banaras Hindu University, Varanasi, India

Editor and Contributors

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Harish Trivedi Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India Ajay B. Urgunde Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India Kapil Dev Verma Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur, India

Chapter 1

Introduction to Supercapacitors Chirag Mevada and Mausumi Mukhopadhyay

Abstract Supercapacitors (SCs) are the essential module of uninterruptible power supplies, hybrid electric vehicles, laptops, video cameras, cellphones, wearable devices, etc. SCs are primarily categorized as electrical double-layer capacitors and pseudocapacitors according to their charge storage mechanism. Various nanostructured carbon, transition metal oxides, conducting polymers, MXenes, and metal– organic frameworks based on electroactive materials are extensively studied for practical application. Moreover, electroanalytical techniques such as cyclic voltammetry (CV), constant current charge–discharge (CCCD), and electrochemical impedance spectroscopy (EIS) are used to evaluate the performance parameters like operating potential window, specific/areal/volumetric capacitance, equivalent series resistance, time constant, energy density, and power density of the assembled device/cell. Furthermore, the contribution of different charge storage mechanisms like the capacitive and diffusion-limited processes is estimated via several electrochemical methods such as CV recorded at different scan rates to obtain the relationship between voltammetric current and scan rate, a voltammetric charge and scan rate, and step potential electrochemical spectroscopy. Additionally, the key performance metrics such as mass loading, capacitance, potential window, cycle stability, leakage current, dwelling time, equivalent series resistance, time constant, device configuration and energy, and power densities of SCs need to study carefully for practical application.

1.1 Introduction Nowadays, renewable energy sources like solar, wind, and tidal are used to generate electricity. These resources need highly efficient energy storage devices to provide reliable, steady, and economically viable energy supplies from these reserves. Because of this, major efforts have been made to develop high-performance energy storage devices. Batteries and electrochemical capacitors are a prime area of interest C. Mevada · M. Mukhopadhyay (B) Department of Chemical Engineering, S. V. National Institute of Technology, Surat, Gujarat 395007, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_1

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in the field of high-performance electrical energy storage devices [1]. The charge– discharge processes of batteries generate thermochemical heat as well as reduce the cycle life due to continuous reversible redox reactions. In contrast, supercapacitors or electrochemical capacitors, or ultracapacitors are delivering excellent advantages like safe usage, fast charging–discharging, and superior cycle stability (> 100,000 cycles) compared to batteries [2]. Supercapacitors are mainly classified into two categories which are electrochemical double-layer capacitors (EDLCs), and pseudocapacitors (PCs). EDLCs use reversible ion adsorption at the interface between electrode and electrolyte to store energy therefore the key property of ELDCs includes the high specific surface area (SSA). Nanosized carbon materials are chosen as EDLCs materials which provide high SSA and good electronic conductivity. EDLCs provide high cycle stability and power densities which are characterized by rectangular cyclic voltammetry (CV) and triangular galvanostatic charge–discharge (GCD). PCs utilized faradic reactions to store energy at the electrode surface by changing its oxidation state during charging and discharging processes [3]. The fundaments and charge storage mechanism of the supercapacitor are explained in detail in the forthcoming section.

1.2 Fundamentals of Supercapacitor The charge storage mechanism of the supercapacitor is easily understood when it is compared with the conventional capacitors. Conventional capacitors such as dielectric capacitors and electrolytic capacitors are widely used in electronic devices. The schematic illustration of conventional capacitors is displayed in Fig. 1.1. As displayed in Fig. 1.1a, the dielectric material (e.g., mica) is placed in between the two conducting plates. When the power is supplied to dielectric capacitors, the charge is stored due to an equal amount of positive charge (Q+ ) and negative charge (Q− ) accumulating on both conducting plates. On the contrary, the electrolytic capacitors (Fig. 1.1b) utilize a liquid electrolyte instead of a dielectric medium, where the charge storage is accomplished via the accumulation of cations (positive ions) of electrolyte at the interface between the negative current collector and electrolyte, and an equal amount of anions (negative) are assembled at the in the interface between the positive–negative current collector and electrolyte [4]. The charge density of electrolytic capacitors is more in comparison to dielectric capacitors due to the high mobility of the electrolytic ions. Therefore, the electrolytic capacitor capacitance is generally in the range of millifarads (mF), whereas the dielectric capacitors capacitance exhibit microfarads (µF). The amount of electrical charge storage (Q) in the conventional capacitors is proportional to the applied voltage (V ) between the positive and negative conducting plates [1, 4]. Hence, the fundamental relationship between Q and ΔV is given as Eq. 1.1. Q = CV

(1.1)

1 Introduction to Supercapacitors

3

Fig. 1.1 Conventional capacitors: a dielectric capacitors and b electrolytic capacitors

C=

ε0 εr A d

(1.2)

where Q: stored charge in (coulombs), V: applied voltage between two terminals (volts), C: capacitance (mF or µF), ε0 : vacuum permittivity (8.854 × 10−12 Fm−1 ), εr : relative permittivity of the dielectric medium, A: area of the plate (m2 ), d: distance between two plates (m) or thickness of the dielectric. The charge storage (Q) and applied voltage (V ) both are time-dependent parameters so the mathematically differentiating form of Eq. 1.1 with respect to time, dV dC dV dQ =C +V =C dt dt dt dt

(1.3)

di dQ = dt dt

(1.4)

On the left side,

Further modifying Eq. 1.4 based on the charge–discharge curve; dQ =i dt

(1.5)

dV dt

(1.6)

Thus, Eq. 1.3 is simplified as: i =C

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If the applied voltage varies linearly with time, V = V0 + Δt

(1.7)

where V 0 : initial voltage which is equal to zero, Δ: scan rate or sweep rate (speed of the potential change, mVs−1 ) and t is the time. Then, dV =Δ dt

(1.8)

Substituting them into Eq. 1.6 yields, i = CΔ

(1.9)

Equation 1.9 signify that the current (i) passing through a capacitor is a strong function of scan rate (Δ) and more importantly, it is independent of the applied voltage (V ). Additionally, the plot of the current versus voltage (i vs. V ) for various scan rates yields a rectangular shape which is known as a cyclic voltammogram (CV) (Fig. 1.2a). CV is the electroanalytical technique that is used to justify the capacitive behavior of electrode material or device. The voltage in the three-electrode configuration is referred to as the electrode potential. Thus, the plot of applied potential (V ) against the charging–discharging time (t) at a constant current gives a triangular shape of the curve (V vs. t) as displayed in Fig. 1.2b. This technique is widely known as constant current charge–discharge (CCCD) or galvanostatic charging–discharging (GCD) which is a reliable and accurate method for estimating the capacitance and ohmic drop (IR drop) of the capacitor electrode or device [5]. Both electrochemical measurements (CV and CCCD) methods are discussed in more detail in the forthcoming section. Furthermore, the amount of energy stored and delivered by the capacitor can be evaluated from the CCCD curves of the device. The triangle area of the working diagram of CCCD curves shown in Fig. 1.2b is utilized to evaluate the energy store [1].

Fig. 1.2 a CV curves at various scan rates and b CCCD curves at various current densities

1 Introduction to Supercapacitors

5

∫Q E=

V dQ

(1.10)

0

Now, substituting Eq. 1.1 into Eq. 1.10 gives, ∫Q E=

Q2 QV CV 2 Q dQ = = = C 2C 2 2

(1.11)

0

where E is the energy density of the device (volumetrically: Wh L−1 or gravimetrically: Wh kg−1 ) which demonstrates the amount of energy stored in the device during charging and similarly the power density (P) (W L−1 or W kg−1 ) from the device can be obtained by dividing E by the time needed to fully discharge of device excluding ohmic resistance (iR drop) [3]. Thus, P=

E Δt

(1.12)

However, the maximum power output of the device is evaluated using the shortest discharging time. The current transfer via circuit is given by I = V /R, where R is the internal resistance or also referred to as equivalent series resistance (ESR). When the power source is connected to a load, R = RL + ESR. The power transmitted from the source to the load is given by P = iV = i2 RL [4]. Thus, ( P=

V RL + E S R

)2 RL

(1.13)

The maximum power (Pmax ) output can be reached when RL = ESR. Hence, Eq. 1.13 can be converted to: ( Pmax =

V ESR + ESR

)2 ESR =

V2 4E S R

(1.14)

Pmax is a function of applied voltage and ESR but is independent of the capacitance of the device. However, the energy density is the strong function of the capacitance of the device [6]. Additionally, the shortest discharge time (t min ) can also evaluate by putting Eq. 1.14 into Eq. 1.12, tmin =

4ESR CV 2 CV 2 × = = 2CESR 2Pmax 2 V2

(1.15)

Based on the above mention Eq. 1.15, the discharge time or time constant (τ ) of the device is the ratio of energy density to power density which can be simply

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obtained by the product of capacitance and ESR [4]. It is a very useful term for determining the response of supercapacitor.

1.3 The Charge Storage Mechanism of Supercapacitors As discussed in the above section that the capacitance of conventional capacitors are ranging from µF to mF, whereas the supercapacitors are normally rated at 100– 1000 F. Though, there is much difference in capacitance value but the fundamental governing equations of supercapacitors for calculating capacitance, power density, and energy density are still similar to conventional capacitors. According to the charge storage mechanism, SCs are mainly classified as EDLCs and PCs. As displayed in Fig. 1.3a, ELDCs store charge at the interface which is formed by electrolyte ions on the surface of the electroactive materials (EMs), therefore, the nanostructured porous carbon-based materials with good electrical conductivity and high specific surface area offer good electrochemical properties [2]. The first electric double layer (EDL) model was attributed to the Helmholtz model (1879) which describes that two compact layers of counter-ions are formed at electrode-electrolyte which behave like the conventional parallel plate-type capacitor and are referred to as Helmholtz double layers (Fig. 1.3b). Thereafter, Gouy–Chapman (1910) considers the Boltzmann transport of ions near a charged surface and introduced a term called diffuse double layer (DDL) comprises counterions between the electrode surface and bulk electrolyte. Later, the Gouy–Chapman–Stern model (1924) was developed a model to get a more realistic picture of EDL (Fig. 1.3b) by combining the Helmholtz and the Gouy–Chapman concepts [7]. The theory describes that the EDL consists of three distinct layers: (i) the inner Helmholtz layer (IHL), (ii) the outer Helmholtz layer (OHL), and (iii) a double diffuse layer (DDL) which is near to bulk electrolyte solution. The surface potential is linearly decayed from IHP to OHP with different gradients and when the potential approaches the DDL, it deviates from the linearity and remains plateau throughout the electrolyte. So, the overall EDL capacitance is represented by the following Eq. 1.16. 1 1 1 1 1 1 = + + = + Cdl CIHL COHL CDDL CStern CDDL

(1.16)

where CStern and CDDL are the capacitance of the Stern plane and diffusive layer. Though the well-established models are available for EDL, they fail to explain the charge storage mechanism in the porous surface of the EM due to the complete different movement of electrolyte ions in the porous medium [1]. Additionally, carbon material has various porous morphology such as cylindrical, spherical, and slit types, subject to its synthesis methods; therefore, it is difficult to define charge distribution in porous materials. Fast faradic reactions are responsible to store electrical energy in pseudocapacitive electrodes, and their cyclic voltammetry shape is displayed as a quasi-rectangular shape due to the overlap of the redox peaks happening at very

1 Introduction to Supercapacitors

7

Fig. 1.3 a Charge storage mechanism in EDLC, b Gouy–Chapman–Stern model for EDLC, various types of redox mechanisms in PCs c redox pseudocapacitance, d intercalation pseudocapacitance, e underpotential deposition, and f doping pseudocapacitance. Source Reproduced with permission from Ref. [9]

similar potentials (i.e., differential capacity remains fairly constant within the operational voltage window) [3]. In contrast, the cyclic voltammetry curve of batteries electrodes materials exhibits oxidation and reduction peaks with completely different electrochemical signatures than supercapacitors [8]. Based on the charge storage mechanism, PCs are also classified into four categories: (i) redox pseudocapacitance, (ii) intercalation pseudocapacitance, (iii) underpotential deposition-based pseudocapacitance, and (iv) doping pseudocapacitance [1]. When electrolyte ions are electrochemically adsorbed onto or near the EM surface, the( faradic reactions are responsible for charge storage)in redox pseudocapacitors e.g., RuOx (OH) y + δ H + + δe− ↔ RuOx−δ (OH) y+δ (Fig. 1.3c) [10]. Reversible intercalation or deintercalation of electrolyte ions from the layers of EMs without phase transformations store charge storage in the intercalation pseudocapacitance (e.g., Nb2 O5 + xLi+ + xe− ↔ Lix Nb2 O5 ) (Fig. 1.3d) [11]. The phenomenon called underpotential deposition is observed when the metal ions form an adsorbed monolayer at a surface of a substitute metal well above their redox voltage and (e.g., Pt+H+ +e− ↔ Pt−Hads ) (Fig. ) ( 1.3d) [12]. Lastly, the reversible insertion de-insertion in conductive polymers PPyn+ · δ A− + δe− ↔ PPy + δ A− (Fig. 1.3f) refers to doping pseudocapacitance [13].

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Furthermore, the mass loading and coating thickness of EM are crucial parameters as far as electrochemical performance (charge storage) is concerned. Many researchers have reported low mass loading of EM (≤ 1 mg cm−2 ), and their results demonstrated excellent capacitance of synthesized electrodes or devices. However, the capacitance found for such electrodes may not reveal the factual electrochemical performance of the devices since the small amount of EM loading results in a small areal capacitance. Therefore, it is desirable that the mass loading of EM should be more than 10 mg cm−2 [14]. For high mass loading EM-based electrode, the electrolyte ions have to diffuse through a long distance to access all of the electrode surfaces which enhances the resistance to the flow of electrolytes and thereby the capacitance of electrode decreases considerably with increasing mass loading of EM [15]. Hence, it is advisable to synthesize nanocomposites with low agglomeration and high conductive connections that allow the building of electrodes with high mass loadings of EM which offers good capacitance and low impedance [16]. As displayed in Fig. 1.4, the high mass loading electrodes composed of hierarchical nanostructure with a good conductive network offer excellent advantages such as low tortuosity path for electrolyte ions, high electronic conductivity, and excellent mechanical strength. However, the synthesis of such electrodes required a novel fabrication method with a clear understanding of the dynamics of the electrolyte ions at the interface or electrode surface during the charging–discharging process [17].

1.4 Electrochemical Cell Configuration The electrochemical performance of the electrode is mainly characterized using the three-electrode and two-electrode configurations. Specifically, the three-electrode system is generally used to determine the electrochemical properties of supercapacitor electrode materials, while the two-electrode configuration is applied to test the prototype device or final symmetrical or asymmetrical device [3].

1.4.1 Three Electrode System The three-electrode configuration comprises the working electrode (WE: EM coated on current collectors such as glassy carbon, nickel foam or foil, etc.), reference electrode (RE: Ag/AgCl, Hg/HgO and saturated calomel electrode (SCE), etc.) and counter electrode (CE: inert material such as Platinum, gold, graphite, etc.) as displayed in Fig. 1.5a [18]. In a three-electrode configuration, RE allows the measurement of the potential of the working electrode without passing a current through it, whereas CE helps to pass current. For example, if oxidation occurs at the WE, reduction using the same magnitude of the current is continued at the CE, and therefore there is no net current flow between WE and RE allowing to track changes

1 Introduction to Supercapacitors

9

Fig. 1.4 Charge-transport mechanism a hierarchical nanostructures with conductive connections for high mass loading electrodes, ASC based on b low mass loading EMs and c conventional high mass loading EMs (Source Reproduced with permission from Ref. [9])

in WE potential precisely. Additionally, the total surface area of the CE must be higher than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical process under investigation. Hence, the high surface area coil and plate-shaped platinum are normally utilized as CE [19]. Three electrode measurement primarily gives information about the specific capacitance, voltage window, and impedance of the WE. Additionally, it is necessary to evaluate the electrochemical properties of WE before using the same electrode for the fabrication of a device.

1.4.2 Two Electrode System Two electrode configuration (Fig. 1.5b) comprises a positive electrode and negative electrode. A separator (cellulose, polymer membranes, glass fibers, etc.) is used to avoid direct contact between positive and negative electrodes. An ideal separator should be thermally and mechanically stable, have minimum resistance for ion transfer, chemically/electrochemically inert, and most importantly it must be thin and highly porous [2]. For a solid-state device, the ionic conducting solid or quasi-solid electrolyte is used. A sandwich-type method is utilized to fabricate the

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Fig. 1.5 a Three electrode configuration, various types of two-electrode configuration b platetype supercapacitors, c flexible supercapacitors, d cable-type supercapacitors, and e coil-type supercapacitors

two-electrode type quasi solid-state device where two electrodes and separators were initially submerged in a beaker content electrolyte. Then, two electrodes and separators were taken out of the electrolyte and hung to remove the excess electrolyte from the surfaces of the electrodes. Finally, the electrodes with the separator were pressed firmly and dried in a vacuum oven at a desired temperature to form a device. As displayed in Fig. 1.5c–g, various structures such as plate, flexible, cable, coil, and coin-shaped devices are constructed to study the electrochemical performance in the two-electrode configuration [20]. Additionally, the notations for the electrochemical cell are given as negative electrode||positive electrode like activated carbon||RuO2 . Supercapacitors are categorized as symmetric and asymmetric based both on their mass loading and EMs. Hence, if the supercapacitors are assembled using the same mass loading (mpositive = mnegative ) and EMs then it’s called symmetric supercapacitor. In contrast, asymmetric supercapacitors (ASCs) are either assembled using the same or different EMs on both current collectors, but with distinctive mass loadings (mpositive /= mnegative ). Moreover, it is advisable to adjust the mass loading of EMs for the charge equality (Q+ = Q− ) with Eq. 1.17 [1] Q + = Q − ⇒ m + C+ ΔV+ = m − C− ΔV− ⇒

m+ C− ΔV− = m− C+ ΔV+

(1.17)

where m+ and m− are the mass loading (in mg cm−2 ) of positive and negative electrodes, and C + and C − , ΔV + and ΔV − are the corresponding capacitances and voltage windows of the positive and negative electrodes, respectively. The mass loading of the EMs of each current collector needs to be accommodated according to the electrolyte ions size to obtain good electrochemical performance of the device. It is important to emphasize that the charge storage capacity deprives when the EMs with lower and higher capacitance will overcharge/over-discharge and not fully charge/discharge

1 Introduction to Supercapacitors

11

respectively. Therefore, it is crucial to optimize the mass loading of EMs to achieve the superior electrochemical performance of ASCs [21].

1.5 Electrochemical Measurement Techniques for Supercapacitor Cyclic voltammetry (CV), constant current charge–discharge (CCCD), and electrochemical impedance spectroscopy (EIS) are employed to test the electrochemical properties of electrodes and devices.

1.5.1 Cyclic Voltammetry (CV) CV is used to determine stable operating voltage or potential window and capacitance of the electrode material. In CV, the voltage is supplied between reference and working electrode for three-electrode systems or between positive and negative for two electrode systems, and the cathodic and anodic current are recorded which are used to characterize the electrochemical reactions. The speed of voltage changed during CV measurement is called a scan rate or sweep rate (mVs−1 ) and the range of voltage change is known as voltage window or potential window (ΔV ). As far as the supercapacitor electrode performance is concerned, the EDLCs EMs are characterized by a perfectly rectangular CV, whereas the PCs exhibit a quasirectangular-shaped CV curve due to faradaic reactions. The following equations are used to calculate the capacitance of the electrode material from the CV curves [3]. 1 CS = m × υ × ΔV 1 CA = A × υ × ΔV 1 CV = V × υ × ΔV

∫V f idV

(1.18)

idV

(1.19)

idV

(1.20)

vi

∫V f vi

∫V f vi

where C S , C A , and C V are the corresponding capacitance based on mass, area, and ∫V volume of EM, vi f idV : integrated area under the CV curve, υ: scan rate (mVs−1 ), ΔV : voltage window (V), m: an active mass of electrode (g), A: active area of an electrode (cm2 ), V: an active volume of the electrode (cm3 ). For three-electrode systems,

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single electrode mass, area, and volume are considered and for two-electrode configuration, both electrode mass, area, and volume are taken into account to calculate the corresponding capacitance. An increase in the scan rate (e.g., 2–200 mVs−1 ) of the CV would results in the reduction of the capacitance because the electrolyte ions get less time to penetrate deeply into pores which produce a low charge and capacitance for the electrode or device [22].

1.5.2 Constant Current Charge–Discharge (CCCD) Constant current charge–discharge (CCCD) or galvanostatic charging–discharging (GCD) is a more reliable technique to evaluate the capacitance, rate capability, and ohmic resistance (iR drop) offered by the working electrode or device [1]. The working electrode is charged to the desired voltage (e.g., 0–2 V) and then discharges to 0 V. To evaluate capacitance, the discharge time is monitored without considering iR drop and a dwelling period. Selection of the charging–discharging current is very crucial to compare the obtained results with the existing literature data. The capacitance retention rate is also found by comparing the capacitance obtained after thousands of cycles with that of the first cycle. All core three electrochemical performance parameters such as capacitance, iR drop, and peak voltage (V 0 ) help to determine the energy and power density of devices. The following equations are used to calculate the capacitance of the electrode material from the CCCD curves [3]. Cs =

I × Δt m × ΔV

(1.21)

CA =

I × Δt A × ΔV

(1.22)

CV =

I × Δt V × ΔV

(1.23)

where I: applied constant current (A) and Δt : discharge time after excluding iR drop. The ohmic resistance or iR drop helps to determine the equivalent series resistance (ESR) which is an important parameter to evaluate the energy efficiency of the device. A device or supercapacitor can be identified as a system consisting of a capacitor in a series arrangement with a resistor. The resistance offered by this resistor is known as ESR which can be found at the start of the discharging process in CCCD measurements [4]. By relating the iR drop of CCCD curves with Ohm’s law, ESR can be easily calculated by the following Eq. 1.24. ESR =

ΔV ΔI

(1.24)

1 Introduction to Supercapacitors

13

where ΔV and ΔI are the correspondings to the change in the voltage and current of the IR drop, respectively.

1.5.3 Electrochemical Impedance Spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) or dielectric spectroscopic measurement is a frequency domain method that is used to study the impedance of the device by supplying a low-amplitude alternative voltage superimposed on a stable potential. The results of the measurements yield the Bode and Nyquist plots that display the relationship between the phase angle and frequency and the imaginary and real parts of the devices impedances, respectively. As displayed in Fig. 1.6, the Nyquist plot consists of three regions: (i) intercept at the real axis gives ohmic resistance, (ii) semicircle in the high-frequency region gives the charge transfer resistance (resistance due to electron transfer at electrode/electrolyte interface, and (iii) linear part near low frequencies demonstrates diffusion resistance (Warburg resistance) [23]. The impedance characteristic of the device is oscillating between a pure resistor (phase angle ~ 0°) and a pure capacitor (phase angle ~ 90°). As the frequency is inversely proportional to the capacitor, the electrode or device near the high frequencies region behaves as a pure resistor that induces near-zero impedance for a capacitor [24]. The intermediate frequencies regime is influenced by the physical parameters such as EM thickness on the current collector, porosity and morphology of the EM, etc. At low frequencies, the diffusion of electrolyte ions occurs because the high frequencies restrict the diffusion of ions owing to shorten time scale. Various equivalent circuits and models have been established which are fitting with the experimental data that help to understand the charge storage mechanisms of the device. Similarly, the Bode plot is also used to understand single components of the circuit, and it gives a direct reading of the resistance of the material under study at low frequencies [3]. Fig. 1.6 The Nyquist plot of supercapacitors and batteries

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The ESR value obtained using the Nyquist plot of the EIS measurement near the real part of the complex impedance at 1000 Hz is generally smaller than the ESR calculated from the CCCD test.

1.6 Electrochemical Methods for Determining the Contribution of Various Charge Storage Mechanisms As aforementioned, the rectangular or quasi-rectangular CV or triangular CCCCD curves indicate the capacitive behavior of the electrode materials. However, the ASCs sometimes displayed the peak-shaped CV so it would be difficult to differentiate between battery-like and supercapacitive behaviors by only visualizing the obtained CV or CCCD curves [1]. Therefore, it is important to quantify whether the contributions to the total capacitance of the electrode are from diffusion-controlled or capacitive charge storage processes for the supercapacitor application. Three differentiation methods are widely used to evaluate the contributions of diffusive and capacitive charge storage processes (i) the Trasatti method, (ii) the Dunn method, and (iii) Step potential electrochemical spectroscopy analysis [25].

1.6.1 Trasatti Method (Voltammetric Charge Dependence on Scan Rate) Trasatti studied the dependency of the total measured volumetric charge Q(v) which is derived from the integration of the internal area of the CV curve, on the scan rate (V). The measured volumetric charge Q(v) can be divided into two fractions: (a) capacitive charge storage (Qc ), which comes from the electro-active sites of the electrode, such as pores, cracks, and grain borders, and (b) diffusive charge storage (Qd ) which is originated from the outer surface of the electro-active material. The charge storage processes in Trasatti analysis can be represented by the following equations [26]: β Qv = Qc + √ ν

(1.25)

√ 1 1 = +β ν Qv Qt

(1.26)

where Q(v) : the total measured voltammetric charge derived from the integral area of the CV curves, Qt ; the total charge (Qc + Qd ), and β and ν are the constant and scan rate (mVs−1 ). At higher scan rates of CV (i.e., ν → ∞, Eq. 1.26), only the outer surface of the EM is accessible for the electrolyte ions which excludes the diffusive charge-storage process. Hence, extrapolating the plot of Q(v) versus 1/υ −1/2 yields

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Fig. 1.7 Trasatti analysis: a plot of Q versus υ −1/2 , b the plot of the reciprocal of Q versus υ 1/2 , Dunn analysis: c plot i/ν 1/2 versus ν 1/2 and d capacitive and diffusive capacitance contribution to charge storage

the capacitive charge storage (Qc ) at the intercept (Fig. 1.7a). On the other hand, when the scan rate approaches 0 (i.e., ν → 0, Eq. 1.26), the diffusive charge-storage processes dominate which could access all electrochemical active sites of the EM. Hence, the extrapolation of the plot of 1/Q(v) versus υ 1/2 gives the total amount of charge Qt at intercept (Fig. 1.7b). Thus, the diffusive charge storage (Qd ) is obtained by subtracting the capacitive charge storage from the total charge. The disadvantage associated with the Trasatti method is that it overestimates the diffusional, capacitive, and total capacitances due to its concept of extrapolation to either zero or infinite scan rate. However, the method was found more suitable when the diffusional, capacitive, and total capacitances are derived at low scan rates [25].

1.6.2 Dunn Method (Current Dependence on Scan Rate from the CV) To quantify the contribution of diffusive charge storage and capacitive charge storage using the Dunn method, the current density [i(V )] is recorded from a CV curve at a fixed potential which is conveyed as a combination of the two terms as per the following equation [27]:

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i (V ) = k1 υ + k2 υ 0.5

(1.27)

where i(V ) is the current at a given potential and k 1 and k 2 are scan rate-independent constants. The term k1 υ and k2 υ 0.5 represents the capacitive charge storage and diffusive charge storage, respectively. By dividing Eq. 1.27 through the υ 0.5 from both sides gives, i (V ) = k1 υ 0.5 + k2 υ 0.5

(1.28)

The linear fitting of plot i(V )/V0.5 versus V0.5 yields the slope k 1 and intercept k 2 for various recorded i(V ) from CV curves at a series of scan rates (Fig. 1.7c). Hence, the obtained results help to differentiate the diffusive and capacitive charge storage at the specified potential V and a chosen scan rate, v (Fig. 1.7d). The Dunn method provides accurate measurements of diffusion-limited capacitance over a wide range of scan rates. Although, both electrochemical methods occasionally give nonlinear behavior for the wide range of scan rates which may not be applied to evaluate the contribution of diffusive and capacitive charge storage. Therefore, the step potential electrochemical spectroscopy (SPECS) is also used to differentiate the diffusive and capacitive charge storage of the electrode. In the SPECS (chronoamperometry and chronocoulometry) method, the current is recorded as a function of time after applying the sudden potential step change (potential of the electrode changes from the equilibrium state (V 0 ) to the final potential (V f ). The series of equal magnitude potential steps are applied to EMs within the specified voltage window. After the individually potential step, the EMs are allowed adequate time to attain quasiequilibrium. This slow approach allows the EMs to store the highest charge storage capability [28]. Additionally, it also enables the separation of the charge storage mechanism such as capacitive and diffusion control charge storage processes. The work reported by Forghani and Donne [25] showed the comparison of specific capacitance obtained from CV measurement with the voltammetric charge dependence on the scan rate, and current dependence on scan rate from the CV and SPECS methods [25]. CV data are collected using electrolytic manganese dioxide (EMD) as electrode material in 0.5 M K2 SO4 electrolyte using a voltage window of 0–0.8 V. Their finding revealed that the specific capacitance obtained using CV measurements shows good agreement with the Dunn and SPECS method, whereas Trassati analysis exhibited the maximum specific capacitance. Hence, the Dunn and SPECS methods are a good model for determining the contribution of diffusional, capacitive, and total capacitances over the full range of scan rates (0.1–100 mVs−1 ) of CV. Supercapacitors are excellent energy storage devices but the commercialization of the same due to low energy density is still considered the biggest challenge for the scientific community. Presently, numerous potential developments in terms of synthesizing EM, electrolyte, separator, current collector, and designing and fabrication of supercapacitors are underway around the globe intending to achieve the high energy density without forfeiting power density [29–31]. Considering the recent

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innovation happening in the world, it is worth mentioning that supercapacitors will be key energy storage alongside batteries that help to switch from fossil-based energy to renewable energy that offers clean and efficient energy storage.

References 1. A. Noori, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi, Chem. Soc. Rev. 48, 1272 (2019) 2. C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, Chem. Soc. Rev. 44, 7484 (2015) 3. S. Zhang, N. Pan, Adv. Energy Mater. 5, 1 (2015) 4. G.Z. Chen, Prog. Nat. Sci. Mater. Int. 23, 245 (2013) 5. J. Xie, P. Yang, Y. Wang, T. Qi, Y. Lei, C.M. Li, J. Power Sources 401, 213 (2018) 6. A. Burke, M. Miller, Electrochim. Acta 55, 7538 (2010) 7. H. Wang, L. Pilon, J. Phys. Chem. C 115, 16711 (2011) 8. M.R. Lukatskaya, B. Dunn, Y. Gogotsi, Nat. Commun. 7 (2016) 9. C. Mevada, M. Mukhopadhyay, Ind. Eng. Chem. Res. 60, 1096 (2021) 10. X. Wu, W. Xiong, Y. Chen, D. Lan, X. Pu, Y. Zeng, H. Gao, J. Chen, H. Tong, Z. Zhu 294, 88 (2015) 11. J.W. Kim, V. Augustyn, B. Dunn, Adv. Energy Mater. 2, 141 (2012) 12. V. Augustyn, P. Simon, B. Dunn, Energy Environ. Sci. 7, 1597 (2014) 13. L.Z. Fan, J. Maier, Electrochem. Commun. 8, 937 (2006) 14. Y. Gogotsi, P. Simon, Science 334, 917 (2011) 15. C. Mevada, M. Mukhopadhyay, J. Energy Storage 31, 101587 (2020) 16. C. Mevada, M. Mukhopadhyay, J. Energy Storage 33 (2021). 17. R. Chen, M. Yu, R.P. Sahu, I.K. Puri, I. Zhitomirsky, Adv. Energy Mater. 10, 1 (2020) 18. B.K. Kim, S. Sy, A. Yu, J. Zhang, Handb. Clean Energy Syst. 1 (2015) 19. N. Elgrishi, K.J. Rountree, B.D. McCarthy, E.S. Rountree, T.T. Eisenhart, J.L. Dempsey, J. Chem. Educ. 95, 197 (2018) 20. G. Zhang, X. Xiao, B. Li, P. Gu, H. Xue, H. Pang, J. Mater. Chem. A 5, 8155 (2017) 21. B. Andres, A.C. Engström, N. Blomquist, S. Forsberg, C. Dahlström, H. Olin, PLoS ONE 11, 1 (2016) 22. C. Mevada, M. Mukhopadhyay, J. Energy Storage 30, 101453 (2020) 23. C. Mevada, P.S. Chandran, M. Mukhopadhyay, J. Energy Storage 28, 101197 (2020) 24. T.C. Girija, M.V. Sangaranarayanan, J. Power Sources 156, 705 (2006) 25. M. Forghani, S.W. Donne, J. Electrochem. Soc. 165, A664 (2018) 26. C. Mevada, M. Mukhopadhyay, Mater. Chem. Phys. 245, 122784 (2020) 27. H.S. Kim, J.B. Cook, H. Lin, J.S. Ko, S.H. Tolbert, V. Ozolins, B. Dunn, Nat. Mater. 16, 454 (2017) 28. M.F. Dupont, S.W. Donne, J. Electrochem. Soc. 162, A5096 (2015) 29. K.K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials I Characteristics (Springer, 2020). https://doi.org/10.1007/978-3-030-52359-6 30. K.K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials II Performance (Springer, 2020). https://doi.org/10.1007/978-3-030-52359-6 31. K.K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials III Selection (Springer, 2021). https://doi.org/10.1007/978-3-030-68364-1

Chapter 2

Traditional Electrode Materials for Supercapacitor Applications Saheli Bera, Kapil Dev Verma, and Kamal K. Kar

Abstract A supercapacitor is a type of electrochemical energy storage device that holds charge both electrostatically and electrochemically. High power density, high energy density, long cycle life, and wide operating temperature range properties make supercapacitor different form other energy storage devices. The supercapacitor has four essential components: electrode, electrolyte, current collector, and separator. In which electrode material selection is the most important factor for the charge storage mechanism. Different types of electrode materials like carbonbased electrode material, transition metal oxides, transition metal dichalcogenides, and conducting polymers are used in supercapacitor applications. Electrode material should have high electrical conductivity, high surface area, lightweight, and low cost for high performance. This article will discuss different electrode materials with their electrochemical performances in supercapacitors.

2.1 Introduction As civilization has entered the hi-tech century, due to a steeper increment of consumption of energy in various fields from daily needs to industrialization, the demand for electricity is growing every day [1]. To achieve technologies for high-efficiency energy conversion and storage, ample observation has been carried out on rechargeable lithium batteries, supercapacitors, fuel cells, and photocatalytic water splitting [2]. Conventional capacitors, batteries, and fuel cells are the three types of energy storage devices that may be distinguished from one another [3]. The supercapacitor has the potential to be a promising solution to the challenges that will be faced in the future by energy storage technology. It has significantly higher capacitance S. Bera · K. D. Verma · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_2

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and energy density than conventional capacitors, which operate within lower voltage limits, and it has a higher power density than batteries and fuel cells [4, 5]. A supercapacitor with infinite charge–discharge cycles at a high rate, roughly 500,000– 1,000,000 cycles, has a long lifespan [6]. Because of its excellent cyclic ability, it does not require any further maintenance. Even when running at rates exceeding 1 kW kg−1 , the energy loss as a form of heat energy throughout each cycle is relatively minimal and easily eliminated (i.e., simple heat control technique), making supercapacitors highly efficient (about 95%) [7]. The electrochemical capacitors work well at both high and low temperatures, with charge–discharge performance ranging from –40 to 70°C [8]. This is especially helpful in military service applications, which require dependable energy storage to run proprietary electronic equipment in all kinds of weather conditions, including extreme natural disasters. This is especially useful because military service applications require reliable energy storage to run proprietary electronic equipment [9, 10]. This energy storage system is safer and more environmentally friendly than others [11]. Along with many advantages, the supercapacitor has some limitations. It has lower specific energy (5 Wh Kg−1 ) than a typical battery (more than 50 Wh Kg−1 ). The self-discharging rate of an electrochemical supercapacitor is relatively high (approximately 10–40% each day) [12, 13]. The cost of raw materials for supercapacitor electrode fabrication, as well as the cost of commercial manufacture, remains a barrier to ultra-capacitor industrialization. Low cyclic stability is a problem with some supercapacitors. A supercapacitor is made up of four parts: two electrodes, an electrolyte medium, a separator, and current collectors. To conduct the electron to the metallic current collector, the electrodes of a good supercapacitor must have good electrical conductivity. It should have a larger active site, excellent adsorption–desorption characteristics, and strong redox activity. The electrode materials determine the supercapacitor’s qualities and features [14–17]. As the electrolyte’s dielectric constant rises, the capacitance rises with it. There are several forms of electrolytes, including solidstate or quasi-solid-state redox-active electrolytes, aqueous electrolytes, organic electrolytes, and ionic liquids [18–21]. Electrolytes ought to have characteristics such as a broad voltage window, a high breakdown voltage, electrochemical stability, high ionic concentration, high electrical conductivity, low viscosity, low volatility with minimum toxicity, and a reasonable cost [22, 23]. The separator separates the two electrodes, preventing a short-circuit and allowing ions to pass through. So, the separator for a supercapacitor must be chemically and electrochemically inert and thermally stable for the working temperature range [24–26]. The current collectors are responsible for providing support for the active materials, transporting electrons that are moving between the active materials and the external circuit, and dissipating the internal thermal heat of the electrode layer [27–29]. The electrochemical window is determined by LUMO and HOMO, which are the lowest occupied molecular orbital and the highest occupied molecular orbital, respectively. The chemical potentials of the current collectors in both the anode and cathode should be in the electrochemical window [30] to prevent the current collectors from being corroded by electrochemical processes [31–36].

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The primary distinction between the two types of supercapacitors depends on how the device stores energy; Faradic and non-Faradic supercapacitor; there is a third type, called hybrid supercapacitor, which possesses both the characteristics of the prior two. One type of electrochemical capacitor is known as the electric double-layer capacitor. This type of capacitor is characterized by the buildup of electric charges on the electrode and electrolyte interface, resulting in the formation of an electric double layer [37], a complete non-Faradic process. In pseudocapacitors, electrical energy is stored here by the Faradic process, which involves charge transfer between electrode and electrolyte via electro-sorption, reduction-oxidation, or intercalation processes [37–39]. The charging and discharging process is quite similar to the batteries [40, 41] and lacks cyclic stability compared to electric double-layer capacitors [37]. The third type is the hybrid of these two above supercapacitors [42, 43]. Because of the fact that the capacitance and charge storage of a supercapacitor are highly reliant on the active surface area of the electrode material. The development of novel materials that have high capacitance and increased performance in contrast to existing electrode materials is the essential step that has to be taken in order to overcome the limits of supercapacitors [44]. The Various kinds of electrode materials that may be found in supercapacitors will be covered in the next portion of this article.

2.2 Electrode Materials There are many types of materials used for preparing electrodes of a supercapacitor. Carbon materials are among the most widely utilized materials due to the high specific surface area of these materials [14, 45, 46] conducting polymers [47–51], metal oxides (e.g., ruthenium oxide (RuO2 ), iridium oxide (IrO2 ), manganese oxide (MnO2 ), nickel oxide (NiO), cobalt oxide (Co2 O3 ), tin oxide (SnO2 ), vanadium oxide (V2 O5 ), and many more) are also used.

2.2.1 Properties of Electrode Materials Table 2.1 provides the lists of the characteristics and performance of carbon materials, metal oxides, and composites built of metal oxide–carbon supercapacitor electrodes.

2.2.2 Nanomaterials as Electrode Materials When the system goes down to the nanoscale (having one or more dimensions in the 1–100 nm range), drastic change in properties has been observed. So, the properties of a material can be manipulated and tuned as required on the nanoscale. In energy storage systems also, nanostructured materials, such as nanosphere, and nanotubes,

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Table 2.1 Characteristics and performance of different electrode materials for supercapacitor application [52–54] Electrode material

Carbon

Metal oxide

Metal oxide carbon composite

Surface area

High

Generally low

Generally controlled by carbon support

Pore size distribution

Various ways to tailor

Difficult to tailor

Carbon support tailoring options

Specific capacitance

Low

High

High

Conductivity

High

Low

Tunable depending on a carbon support

Rate capability

High

Low

Good

Stability

Good

Poor

Good

Cost

Low

High

Moderate

or nanofiber are high in recent demands because, in the nanostructure, there are unbelievably high surface areas in the same volume. So, there are two main attributes in nanomaterials or nano-systems, extremely high surface-to-volume ratio and quantum confinement. Quantum confinement only occurs when the dimension ranges between 1 and 5–10 nm, and it depends on the material too. Quantum confinement indicates the confinement of electrons in one or more directions. Lower-dimensional nanostructure materials based on carbon appear to have a lot of potential for usage in supercapacitors, not only because of the inexpensive cost of mass manufacture but also because of the low inherent toxicity and multifunctional surface functionalization of these materials [55–57]. Activated carbon, aerogel, graphene, carbon nanotubes, carbon nanofiber, and carbon nanodot are examples of materials that are frequently utilized for electrodes.

2.2.2.1

Zero-Dimensional Carbon Nanoparticles

Zero-dimensional nanoparticles imply nanoparticles that have all three dimensions on the nanometer scale can be called carbon nanodots which can be a nano-cube (where length, width, and height all are in the nanometer range) or hollow or solid nanosphere (which has the diameter in nanometer). Zero-dimensional carbon nanoparticles have a higher surface area than other low-dimensional carbon nanoparticles with high quantum yield. It includes ultrafine activated carbon (AC), mesoporous carbon nanospheres, carbon quantum dots, and graphene quantum dots [58]. They have a high specific area (3000 m2 g−1 ) with an ultra-small size, leading to more active edge sites per unit mass. To support the metal oxides in supercapacitor electrodes, tailoring of pore content and size distribution is needed, and carbon nanodots provide the freedom of tunability of porosity according to different electrolytes.

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2.2.2.2

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One-Dimensional Carbon Nanotubes and Nanofibers

Nanostructures with only one dimension are those that take the form of a fiber and have good electrical characteristics in one direction. (i.e., along the dimension which is not in nanoscale) make kinetics easier for electrochemical reactions. Onedimensional carbon nanostructures are comprised of things like carbon nanotubes (CNTs), carbon nanofibers (CNFs), and carbon nano-coils, among other things. One-dimensional carbon structures like nanorod, nanopillar, nanowire, nanotube, and nanofiber make the charge transport easier in the supercapacitor electrode. The redox activities of metal oxides benefit substantially from the high conductivity of 1D carbon nanostructures, which boosts the pseudocapacitance and rate capability. However, the surface-to-volume ratio is smaller than that of zero-dimensional carbon nanoparticles [59–61].

2.2.2.3

Two-Dimensional Nanosheets

Two-dimensional nanostructures are described as sheet-shaped thin layers being a minimum of one atomic layer thick with a high surface-to-volume ratio. As these surface atoms are different from internal ones, it leads to a drastic change in the behavior and properties of that nanostructure than bulk ones. When it comes to twodimensional carbon nanostructures, graphene, graphene oxide (GO), and reduced graphene oxide (rGO) are the materials that are most commonly utilized and considered to be the most important, possess large specific surface areas, which makes them a good contender for supercapacitor electrodes. Graphene and its derivatives show excellent electronic conductivity and outstanding mechanical strength. Therefore, two-dimensional carbon nanostructures are attractive for flexible energy storage devices, especially supercapacitor electrodes, because of their high aspect ratio, electronic conductivity, and mechanical strength.

2.2.2.4

Three-Dimensional Carbon Nanostructures

Three-dimensional carbon nanostructures are created by putting together lowdimensional carbon nanomaterials while keeping the benefits of the building blocks, which start from one parent structure and gradually grow into a more complicated hierarchical nanostructure. This results in an increase in the overall active volume of the supercapacitor electrodes while simultaneously lowering the overall footprint of the device. Now, these three-dimensional structures are not confined to the nanometer dimension anymore. This can contain the dispersion of nanoparticles in colloids, nano-powders in bulk, bundles of nanowires, nanofibers or nanotubes, and multi-nanolayer. As a consequence of the usage of carbon nanofoams or sponges with a large specific surface area, a broad region of electrolyte–electrode interface and a continuous electron transport channel as supercapacitor electrodes, energy storage

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devices are becoming more compact. Figure 2.1 demonstrates a diagrammatic illustration of the usual geometries of self-supported homogeneous and heterogeneous nanoelectrodes for supercapacitors.

Fig. 2.1 A schematic representation of the typical geometries of self-supported homogeneous and heterogeneous nanoelectrodes for supercapacitors is presented here (reprinted with permission [62])

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2.3 Materials for Electrodes of Supercapacitors 2.3.1 Carbon Materials The capacitance of electric double-layer capacitors, also known as EDLCs, comes from the accumulation of charges at the electrode and electrolyte interfaces. These capacitors are made of carbon-based materials. Controllable characteristics of carbon-based electrode materials include their electrical conductivity, surface area, pore size distribution, and quick electron transfer kinetics. Carbon-based electrode materials also have low production costs. Capacitance is caused when electrolytic ions are adsorbed and desorbed at the electrode surface. The capacitance may be controlled by the electrode’s specific surface area, pore size, and electrical conductivity. The capacitance increases as the surface area of the electrode material increases. Since the process of charge storage involves surface phenomena in the vast majority of carbon materials, it follows that the material as a whole does not govern the device’s capacitance [49, 50, 63]. Carbon materials have a wide working temperature range and are abundant, low cost, easy to process, non-toxic, have a greater specific surface area, strong electrical conductivity, and chemical stability [35, 64, 65], which makes them considered as promising electrode materials for industrialization. Carbon-based electrochemical capacitors show good rectangular shapes in cyclic voltammetry (CV) curves and triangular symmetrical distribution in galvanostatic charge-discharge (GCD) profile which implies good capacitive properties. Because carbon materials store charges primarily in an electrochemical double-layer formed at the interface between the electrode and the electrolyte and not in the bulk of the capacitive material, the capacitance of an EDLC is primarily dependent on the surface area of the electrode materials that is accessible by the ions of the electrolyte. This is the case because an EDLC’s capacitance is directly proportional to the surface area of the electrode materials that is accessible by the ions of the electrolyte. Along with this, porosity plays a significant role in increasing surface area, so the shape and size of the pore distribution also are great factors in ultimate supercapacitor performance. High surface area carbon materials include activated carbon, carbon aerogels, carbon nanotubes (CNTs), templated porous carbons, and carbon nanofibers. In the literature, the performance of these carbons as electrode materials has been well discussed. [66]. However, the carbon-based supercapacitor has lower specific capacitance and energy density than transition metal oxides [67]. The specific capacitances of carbon materials in actual supercapacitors are lower than those in the literature. For example, the capacitance for aqueous electrolytes ranges from 75 to 175 F g−1 , whereas the capacitance for organic electrolytes ranges from 40 to 100 F g−1 . Carbon materials with large specific surface areas have a greater capacity to accumulate charge at the electrode/electrolyte contact in general. [39]. The carbon network’s specific surface area can be increased by creating effective micropores and flaws through various processes, including heat treatment, alkaline treatment, steam or CO2 activation, plasma surface treatment with NH3 , and so on. Some of these

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processes include heat treatment, alkaline treatment, steam or CO2 activation, and plasma surface treatment with NH3 .[68–76]. Specific capacitance is not always perfectly proportional to the specific surface area. This is because not all the electrode layer micropores are necessarily accessible to the electrolyte ions. The impact of appropriate pore size on carbon electrode material performance is yet unknown. Pore widths of 0.4 or 0.7 nm for aqueous electrolytes and 0.8 nm for organic electrolytes are suitable, as determined by a number of investigations [69, 77]. In recent papers [78, 79], by obtaining a maximum capacitance, it was demonstrated that there is a correlation between the pore size and the ion size. In addition to having a high specific surface area and having pores of a suitable size, surface functionalization has also been thought to be an effective technique to increase the specific capacitance of carbon materials [80]. It is possible for surface functional groups or heteroatoms to assist in the adsorption of ions, so increasing the hydrophilicity of carbon materials and making it easier for electrolyte ions to flow quickly through micropores. This is made possible by the presence of surface heteroatoms. Simultaneously, the presence of functional groups on the surface of carbon materials may result in Faradaic redox processes [81], resulting in an increase of 5–10% in overall capacitance. Redox peaks clearly prove pseudocapacitance which comes from the oxidation and reduction reactions. The CV curve of an activated glassy carbon electrode is depicted in Fig. 2.2. Here, the scan rate is 100 mV s−1 Electrolytes are 3 M H2 SO4 (aq.) and 1 M tetraethylammonium tetrafluoroborate (TEABF4 ) in acetonitrile [81]. Redox peak shows the contribution of function group in aqueous electrolyte, and the organic electrolyte is the best material to approach the rectangular form desired for a perfect supercapacitor. The capacitance of the electrochemical capacitor can be increased by adding heteroatoms on the surface or putting different functional groups on the surface of the carbon material. As the heteroatoms bonded with carbon, their higher reactivity causes a barrier in electron movement, leading to higher electrical resistance. Oxygen, nitrogen, boron, and sulfur are the most common heteroatoms found in the carbon framework. The introduction of nitrogen is the one that has received the most significant attention in the literature. In an organic electrolyte, the risk of electrolyte dissolution caused by active surface functional groups, particularly oxygen-containing acidic groups, can be increased depending on the concentration of functional groups, the electrode surface area, and the operating voltage. However, this risk can be mitigated by increasing the electrode surface area [82]. In short, it would appear that high surface area carbon materials that have adequate porous architectures are the best materials to use for supercapacitors in terms of the specific power and cycle life of the materials. Unfortunately, because of the contact resistance between carbon particles, their high resistivity causes high internal series resistance, resulting in poor supercapacitor performance [45]. Additionally, the surface area of carbon materials that is inaccessible to electrolyte ions is a factor that works against the capacitance performance of carbon materials [83]. This hinders the capacitance performance of carbon materials, resulting in capacitance values that

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Fig. 2.2 CV curve of activated glassy carbon electrodes (reprinted with permission [81])

are typically only 40–160 F g−1 for activated carbon and carbon aerogels, respectively [84], and 10–35 F g−1 for CNTs [85]. These values are highly influenced by the nanotubes’ micro-texture, numerous flaws, micropore volume, and contamination. The creation of carbon electrodes with increased specific surface area, rational pore distribution, mild surface modification, improved electrical conductivity, and stability to achieve maximum capacitance are thought to be future research areas in carbon materials. Different allotropes of carbon have gained attention as a supercapacitor electrode material because of its unique combination of chemical and physical properties. Both strong conductivity and a high surface area range (from less than one to more than two thousand square meters per gram) are necessary features for the fabrication of supercapacitor electrodes; moreover, these properties must be able to be adjusted and optimized. These kinds of operations are still the focus of a significant amount of investigation. Before reviewing the findings of the study, it is important to carefully cultivate the structural morphology and chemical reactions of the carbon material in order to have a clear concept about the carbon material that will be used for the electrode material of the supercapacitor.

2.3.1.1

Activated Carbon

Activated carbon has an extraordinarily large internal surface area and highly accessible pore volume, which is responsible for its adsorption properties, and it can be manipulated as required. By structure, it is a three-dimensional twisted network of defective layers of carbon, basically an amorphous, which are cross-linked by aliphatic bridging groups [86]. Activated carbon possesses a very large internal

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surface area, which makes it highly suitable for ion adsorption during supercapacitor charging. Powdered activated carbon (PAC) has a finer particle size of around 44 microns, allowing for quicker adsorption although it is harder to manage in permanent adsorption beds. Granulated activated carbon (GAC) has 0.6–4.0 mm granules and is harder and more abrasion resistant than PAC. It is also more expensive and simpler to renew than PAC. Although fibrous-activated carbon fibers are expensive, they have the benefit of being able to be readily molded into the shape of the adsorption system. Additionally, they provide very little hydrodynamic resistance to flow [87, 88]. Activated carbon has low electrical conductivity and high volumetric capacitance. It is an amphoteric substance, meaning it may be positively or negatively charged depending on the pH of the solution. Activated carbon has tunable pore sizes and hierarchical pore structure with a very large internal surface area (up to 1500 m2 /g) with different morphology, which makes it highly suitable for adsorption. Its high gravimetric and volumetric capacitance than any other carbon material makes it suitable for supercapacitor application. Figure 2.3 shows that this is an example of how activated carbon can work as a supercapacitor (electrode material: activated carbon that was cut by a laser, laser scribed activated carbon (LSAC), electrolyte: 1.0 M tetraethylammonium tetrafluoroborate (TEABF4 ) in acetonitrile (ACN), (a) CV curves of supercapacitors before (black) and after (gray) laser treatment, where scan rate is 50 mV s−1 (Fig. 2.3a), Nyquist plots of the LSAC supercapacitor (gray), before (black), and a supercapacitor over a frequency range of 0.2 MHz to 0.1 Hz (Fig. 2.3b) [89]. At 50 mV s−1 , the LSAC exhibits an improved capacitance with a virtually textbook rectangular CV curve. The findings reveal that the LSAC electrodes exhibit decreased equivalent series resistance (ESR), with values as low as 1.66 Ω. A variety of applications are there, but the most popular ones are electrode materials of energy storage material, methane and hydrogen storage, fuel storage, gas purification, chemical purification, and mercury scrubbing.

2.3.1.2

Carbon Aerogels

Aerogels are non-fluid colloidal linked porous networks made up of loosely packed covalently bound particles that are synthesized into lightweight 3D nanostructures. Carbon aerogels, carbon nanotube aerogels, and graphene aerogels are all examples of aerogels. It has a large surface area (surface areas can be from 400 to 1000 m2 g−1 ). It has an air-pockets inside a porous solid network and is particularly porous (over 50%, with pore diameter under 100 nm). Aerogel is the lightest synthetic solid (the maximum portion is full of air). Aerogel nanostructures’ low electrical conductivity prevents sales growth in energy storage and conversion systems [90]. Because infrared radiation (which transmits heat) flows through them, they are good thermal insulators but poor radiative insulators. It is optically clear, but in the infrared spectrum, it is exceedingly “black,” reflecting just 0.3% of the energy between 250 nm and 14.3 m. Aerogels are hydrophilic by nature, and absorbing moisture causes structural changes such as contraction and deterioration. However, degradation may be avoided by chemically treating them to make them hydrophobic. Even if a break reaches the

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Fig. 2.3 This is an example of supercapacitive performance of activated carbon (electrode material: laser scribed activated carbon (LSAC), electrolyte: 1.0 m tetraethylammonium tetrafluoroborate (TEABF4 ) in acetonitrile (ACN)); a CV curves of supercapacitors before (black) and after (gray) laser treatment, where scan rate is of 50 mV s−1 and b Nyquist plots of the LSAC supercapacitor (gray) before (black) and supercapacitor over a frequency range of 0.2 MHz to 0.1 Hz (redrawn and reprinted with permission [89])

surface, aerogels with only an exterior hydrophobic layer deteriorate quicker than those with hydrophobic interiors. Aerogel is chemically inert in most cases. It is a chemical adsorber and has catalytic nature [91]. In building high-performance energy storage systems, an open porous structure improves ionic mobility and electrolyte diffusion. Lawrence Livermore National Laboratory was the first to attain a specific capacitance 19.2 F g−1 having a power density of 1.2 W cm−2 [92]. The specific capacitance values were increased from 20 to 82 F g−1 using Ru-doped carbon aerogel as an electrode. The capacity gain was determined to be attributed to Ru nanoparticles rather than the large surface area. [93]. Zapata et al. used lignin as a precursor and reached an excellent specific surface area 1243 m2 g−1 and got a specific capacitance 234.2 F g−1 when the current density is 0.124 A g−1 [94]. Carbon aerogel supercapacitors have capacitance densities of 104 F g−1 and 77 F cm−3 and can store thousands of farads. For a limited specific surface area, they have a little energy density. By altering the synthesis parameters, carbon aerogel porosity and surface area may be adjusted across a wide range. The high costs of commercial production are one of the key challenges. CV curve with varying ruthenium/carbon (R/C) ratios is depicted in Fig. 2.4, and the following electrode materials are listed: Carbon aerogels have a scan rate of 1 mV s−1 and KOH as its electrolyte. At 1500 ruthenium/carbon (R/C) ratio, it shows 110 F g−1 specific capacitance [95]. The CV curves are very symmetric, rectangular in shape, and the in that potential range, the charging process is reversible [95]. Figure 2.5 depicts a CV curve of the same sample taken at a number of various scan speeds of 10 mV s−1 . The charge and discharge curves of the sample measured at a current of 5 mA. The supercapacitor’s maximum capacitance reached 28 F g−1 .

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Fig. 2.4 CV curves having different R/C (ruthenium/carbon) ratios (electrode material: carbon aerogels, scan rate: 1 m Vs−1 , and electrolyte: KOH) (redrawn and reprinted with permission [95])

Fig. 2.5 Charge/discharge curves of carbon aerogel-based supercapacitor at various currents 5 mA (redrawn and reprinted with permission [95])

Carbon aerogels have potential use as electrode materials in a variety of electrochemical devices, including batteries and supercapacitors. Carbon nanotube aerogels are another electrically conducting material that might be used in battery and supercapacitor electrodes [96]. Carbon aerogel nano-catalysts can be used as electrode materials in electrocatalysis and as a support for catalysts in proton-exchange membrane fuel cells. These are highly “black” in the infrared, which makes them

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ideal for solar collectors. Ulker et al. looked into aerogel-based drug delivery devices [97].

2.3.1.3

Carbon Nanotubes (CNT)

Carbon nanotube (CNT) has remarkably high electrical conductivity (ohmic conductivity) (lowest resistance ∼22 kΩ) [98–100], which shows metallic or semiconducting nature. The bandgap can vary from zero to 2 eV. CNT diameter and chirality influence their electrical characteristics. As the diameter increases, the energy gap decreases, and properties roam from semiconductor behavior to good conductor. CNTs have the highest conductivity along their axis and the lowest in the perpendicular direction, making them one-dimensional conductors. The magneto-resistant effect occurs when resistance reduces with an increasing magnetic field applied across the CNT at low temperatures. It has a high strength-to-weight ratio and a low density [101]. There are many configurations of carbon nanotubes, including single-walled, double-walled, and multiwalled CNT [102]. Carbon nanotubes are one-dimensional, and tubular, which have widely accessible porous-specific surface areas. Because of its nanostructure and the strength of its carbon-atom bonds, it exhibits tremendous tensile strength (almost 200 times that of steel) [103]. CNT has Young’s modulus of 1800 GPa. CNTs are excellent thermal conductors with excellent thermal stability, with thermal conductivity more than double that of diamonds. This strong thermal conductivity is concentrated along the tube’s axis while it is low in the perpendicular direction. Carbon nanotubes have absorption and photoluminescence (fluorescence) properties with exceptional chemical stability. It possesses excellent ion and electron transport properties, as well as electrolyte accessibility and electrochemical stability [104]. Carbon nanotubes substantially modify porosity and give mechanical strength and conductivity when utilized in a composite material with activated carbon. In aqueous electrolyte solutions, supercapacitors with CNT-activated carbon composite electrodes were able to withstand 50,000 cycles with low capacitance fading, but identical systems without CNTs showed capacitance fading even after 30,000 cycles [105]. A high-power electrode material is the carbon nanotube. However, it has low volumetric and gravimetric capacitance and high expense. The CV curves are displayed in Fig. 2.6a. Electrode material: activated mesh carbon microbead (AMCMBs)/CNTs compound scan speeds are 10, 40, 80, and 160 mV s−1 ; (b) GCD curve with a constant specific current of 0.5 A g−1 (Fig. 2.6b). As determined from the GCD curve, the specific capacitance (Cs) of a single electrode was 243 F g−1 [106]. Figure 2.7 shows CV curve of supercapacitor device. Material for the anode electrode is nanotubes of carbon with several walls, and 0.5 M H2 SO4 electrolyte is used for the supercapacitor. Commercial CNTs (20–40 nm in diameter) have a specific capacitance of around 23 F g−1 [107]. CNT-based supercapacitors find use in a variety of industries, including those dealing with consumer and portable electronic systems, transportation and vehicle

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Fig. 2.6 a CV curves (electrode material: activated mesh carbon microbead (AMCMBs)/CNTs compound scan rates are 10, 40, 80, and 160 mV s−1 ); and b GCD curve with a constant specific current of 0.5 A g−1 (reprinted with permission [106])

Fig. 2.7 CV (electrode material: multiwalled carbon nanotubes, and electrolyte: 0.5 M H2 SO4 , and scan rate: 50 mV s−1 ) (redrawn and reprinted with permission [107])

parts, power backup, biomedical services, military services, and aerospace applications [104], photo-detectors and light-emitting diodes (LEDs). Bolometers and optoelectronic memory devices have been created using ensembles of single-walled carbon nanotubes. All carbon nanotubes are strong heat conductors along the tube axis, displaying a “ballistic conduction” feature, but they show good insulating behavior on the lateral side.

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Carbon Nanofibers (CNFs)

The carbon nanofiber (CNF) is a one-dimensional linear structure that is sp2 based and has a diameter of 50–200 nm and a length of 50–100 m [108]. It has a large specific surface area and sufficient porosity. Carbon nanofiber possesses a rather good electrical conductivity for its size. The electrical two-layer capacitor technology allows them to store energy. Young’s modulus may range from 228 to 724 GPa, while tensile strength can range from 1.5 to 7 GPa. It has a thermal conductivity of 2000 W/mK, and its coefficient of thermal expansion is −1.0 (10–6 C−1 ). It is appropriate for supercapacitor application as electrode material due to its high aspect ratio, ease of preparation, low cost, ease of manufacture on a large scale, and precision control of characteristics. Carbon nanofiber is utilized in supercapacitor electrode materials, anode material for rechargeable lithium-ion batteries, scanning probe microscope (SPM) tips, electrical and optical nano-devices, sensors, energy devices, tissue engineering, water treatment, and filtration, drug delivery, among other applications.

2.3.1.5

Carbon Nanodot

Carbon nanodot is a zero-dimensional nanostructure with all dimensions in the nanoscale range and it has the highest surface-to-volume ratio of any low-dimensional nanostructure. The electrical conductivity of carbon nanodots is quite high. Their dense electronic structure includes quantum effects, including strong and controllable fluorescence throughout a broad range of electromagnetic spectra and increased inherent capacitance. It has increased quantum yields as well as significant UV– visible absorption. [109]. Carbon nanodots have a multiphoton excitation capability and are easily soluble in water. It is chemically stable, inert, strong, and biocompatible with minimal toxicity, making it environmentally beneficial. Despite having a low volumetric capacitance, carbon nanodots can be used as electrode material for supercapacitors due to their huge specific surface area and high level of stability. Figure 2.8 displays the electrochemical performance of carbon quantum dots (CQDs800), which were used to create a layer of carbon structure for use as high-density electrode material in supercapacitors ((a) Curves of charge and discharge for galvanostatic systems (Fig. 2.8a) and (b) Nyquist plot, Fig. 2.8b). A magnified curve in the high-frequency band may be seen in the inset. The discharge curve at 0.5 A g−1 yielded a specific capacitance of 106 F g−1 [110]. Photocatalysis, optoelectronics, photovoltaics, drug delivery, sensors, bioimaging, and other disciplines all benefit from it.

2.3.1.6

Graphene

Graphene is one of the most widely used carbon compounds, with several uses in a variety of disciplines due to its adaptability. It is a single-atom-thick layer of

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Fig. 2.8 Electrochemical performance of carbon quantum dots (CQDs-800 ), which made a layer carbon structure for high-density electrode material for supercapacitor a GCD curves and b Nyquist plot (Inset: a magnified curve in the high-frequency range) (redrawn and reprinted with permission [110])

sp2 -bonded carbon atoms, which makes a honeycomb-like shape with sp2 -bonded carbon atoms densely packed with an oxygen molecule. It has a large specific surface area since it is a two-dimensional substance (up to 2675 m2 g−1 ) [111]. The enormous uniform arrangement of carbon atoms connected by covalent bonds makes graphene very strong. The fact that charge carriers in graphene are characterized by a Dirac-like equation rather than the standard Schrodinger ¨ equation gives it unique electrical features [112]. Its electrical conductivity is high (approximately 2 × 102 S m−1 ), which makes it suitable for supercapacitor applications. It has a tensile strength of 130 GPa and Young’s modulus of 1000 GPa. It has 2600 m2 g−1 specific surface area. Another remarkable property of graphene is its thermal conductivity, which ranges from 1800 to 5400 Wm−1 K−1 . It has a reasonably high melting point as well. Its oxidation temperature is 450 °C. It is permeable to protons but impenetrable to liquids and gases. It does not have such good rate stability and power density because of the aggregation restacking of graphene sheets which leads to lowering the surface area of the electrolyte ions and their effective diffusion. Threedimensional graphene has a large surface area, is lightweight, and transports ions and electrons quickly. Due to the shorter channels, graphene’s proclivity for aggregation and restacking decreases the ion-accessible surfaces and inhibits ion and electron transport [113–116]. Because every single atom is exposed to a chemical reaction from both sides, graphene is the most reactive chemical form of carbon. It burns at 350 °C, which is quite low. When compared to CNT, it contains the largest ratio of edgy carbon. Oxygen and nitrogen-containing functional groups are frequently added. The electrical conductivity of graphene is extremely high, and its surface area is quite variable. It is a lightweight material with magically favorable chemical, thermal, and mechanical properties. It has a modest to moderate volumetric capacitance. Commercial graphene manufacturing is difficult to fabricate and has significant expenses. Specific capacitance values are still considerably below the

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Fig. 2.9 Graphene–graphene supercapacitor CV curve (scan rate: 10 and 20 mV s−1 ) (redrawn and reprinted with permission [117])

predicted graphene (550 F g−1 ) value. Due to graphene’s proclivity for aggregation and restacking nature, it has a poor energy density and specific capacitance in electrochemical applications [104]. In supercapacitor applications, a mixture of graphene-based and pseudocapacitive materials is developing as a viable contender for increased specific capacitance and energy density, superior rate capability, power density, and long cycle life. Figure 2.9 presents the CV for two different scan rates, 10 and 20 mV s−1 . Graphene provides an approximately rectangular CV curve [117]. Applications for graphene include the portable and wearable electronic industry, smart apparel, transportation systems and autos, power backup devices, implantable bioelectronics sectors, and military services, and it is utilized to make thermal interface materials for electronics in composites [118]. Small quantities of a graphene derivative added to a thermoplastic or thermoset resin can increase the underlying resin’s mechanical stability and durability. It is a viable choice for conductive transparent electrodes (e.g., touch screens) and electromagnetic interference shielding because of its great electrical conductivity mixed with its mechanical and transparent qualities.

2.3.1.7

Graphene Oxide (GO)

Graphene oxide (GO) is formed by oxidizing graphene layers, yet it preserves its characteristic hexagonal shape despite the addition of oxygen-containing functional groups. The carbon basal plane has hydroxyl and epoxy groups, whereas the edges have carboxyl and carbonyl groups. With Young’s modulus of 207.6 GPa [119] and fracture strength of 120 MPa, graphene oxide possesses electrically semiconducting to insulating characteristics (poor electrical conductivity) and high mechanical strength. It is reasonably priced.

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Fig. 2.10 a GCD curve of rGO nanocomposites electrode material at 1 and 0.5 A g−1 current density, and b EIS of rGO-based supercapacitor with 6 M KOH electrolyte (redrawn and reprinted with permission [122])

2.3.1.8

Reduced Graphene Oxide (rGO)

In comparison to graphene oxide, the carbon-to-oxygen ratio in reduced graphene oxide (rGO) structures is substantially greater, nearly nil oxygen content. It has a very large surface area. Reduced graphene oxide has almost as good an electrical conductivity as pure graphene (but it cannot imitate pristine graphene). It has better electrical conductivity than GO (6300 S cm−1 ) and mechanical strength. The depletion of oxygen-containing molecules causes its hydrophobic property. The conductive carbon network facilitates ion transmission, and electron exchange results in good electrical conductivity. Furthermore, its high specific surface area and cyclic stability make it an ideal material for supercapacitor electrodes [120]. Figure 2.10 shows a supercapacitor that is lightweight, highly flexible, and based on cellulose fibers, and rGO nanocomposite electrode has been devised and put together. The specific capacitance of the supercapacitor was measured to be 255 F g−1 when it was scanned at a rate of 10 mV s−1 in a solution containing 6 M KOH [121]. Figure 2.10a shows the GCD curve of rGO nanocomposite electrode material at 1 A g−1 and 0.5 A g−1 current density. Figure 2.10b shows the electrochemical impedance spectroscopy (EIS) of rGO-based supercapacitor with 6 M KOH electrolyte. The equivalent series resistance of rGO-based supercapacitor is 7 Ω. It is also used in hole transport and electron blocking layer in solar cells, an effective interfacial layer (IFL) in organic photovoltaics, gas sensing applications, biosensors, and biomedical applications.

2.3.1.9

Fullerene

Fullerene is a molecule made up of carbon atoms that are joined by single and double bonds to create a closed or partially closed mesh with five to seven atoms

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fused rings. The molecule can take different shapes and sizes, like a hollow sphere, ellipsoid, and many more. There are different structures of bucky ball according to the number of carbon atoms in the molecule. DFT research revealed that this material has an exceptional bandgap and quantum capacitance. [123]. When the scan rate is 1 mV s−1 , fullerene C76 as a supercapacitor electrode produces specific capacitances of 171 F g−1 (positive potential window) and 142 F g−1 (negative potential window) in neutral Na2 SO4 electrolyte. In the positive potential window, a pseudocapacitance performance of 65% has been observed in the Na2 SO4 electrolyte. As a positive electrode in the acidic electrolyte (here H2 SO4 ) having a potential window of 1 V, the bucky ball demonstrates a maximum capacitive performance of 257 F g−1 at a scan rate of 1 mV s−1 [124].

2.3.2 Transition Metal Dechalcogenide (TMD) Figure 2.11 depicts the structural properties of the 2D-general MX2 (M = transition metal and X = S, Se) ((a) The top view of a 2D-TMD, and (b) the side view of a bilayer 2D-TMD).

2.3.2.1

Molybdenum Disulfide (MoS2 )

Molybdenum disulfide has a two-dimensional structure like graphene. Lattice constants of MoS2 are seen in Fig. 2.12. The first diagram depicts MoS2 from the side, while the second shows the top perspective. The yellow and blue spheres represent the S and Mo atoms of MoS2 , respectively [126].

Fig. 2.11 2D-general MX2 ’s structural characteristics a 2D-TMD top view, and b bilayer 2D-TMD side view (reprinted with permission [125])

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Fig. 2.12 Lattice constants of molybdenum disulfide (MoS2 ) (reprinted with permission [126])

Molybdenum disulfide (MoS2 ) is a semiconductor with a 1.2–1.8 eV indirect bandgap (direct monolayer). Weak Vander Wall bonds allow sheets to glide freely over each other. With energy above the bandgap, a single monolayer of MoS2 may absorb 10% of incoming light. There is a 1000-fold increase in photoluminescence intensity compared to a bulk crystal. It is diamagnetic and has poor electrical conductivity. Its low friction and robustness make it can be used as a dry lubricant. It is chemically unreactive and dilute acids and oxygen have no effect on it. It functions as a host for the creation of intercalation compounds, making it useful as a cathode material in batteries. Due to a greater bandgap and volume change during cycling, it has low electrochemical conductivity. The intrinsic conductivity of Sun et al. lowering the bandgap from 1.8 to 1.3 eV and raising the interlayer spacing up to 9.8 eV, allowed for quicker electrolyte diffusion in hydrothermally produced oxygenintegrated MoS2 (O-MoS2 ) hollow microspheres with a customizable inner structure. Because of these enhancements, O-MoS2 now has a specific capacitance of 744.2 F g−1 at a current density of 1 A g−1 and capacitance retention of more than 77% after 10,000 cycles [127]. After integrating the area under the CV curve, one can determine the Csp of all of the samples (bare MoS2 (i.e., MoS2 without vapor grown carbon fiber (VGCNF), Mo precursor with VGCNF at ratio 6:1 (MPR25-1), Mo precursor and VGCNF ratio 2:1 (MPR25-2), Mo precursor and VGCNF ratio 1:1 (MPR25-3), MPR25-1 with slightly different preparation than previous one (MPR25-1-2), GO with same experimental condition as MoS2 but without any VGCNF (MGO). As can be seen in Fig. 2.13, the area of the MPR25-1 sample is the largest in comparison to

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Fig. 2.13 CV curves of bare MoS2 and composites at 5 mV s−1 (redrawn and reprinted with permission [128])

that of the other samples. MPR25-1 provides the highest specific capacitance of 248 F g−1 at 5 mV s−1 scan rate. It can be used in high-vacuum applications instead of graphite; however, it has a lower maximum working temperature than graphite. The bandgap of MoS2 makes it favorable for optoelectronic applications. This material is used in field-effect transistors, photodetectors, solar cells, chemical sensors, supercapacitor electrodes, and valleytronic devices.

2.3.2.2

Tungsten Disulfide (WS2 )

Tungsten disulfide (WS2 ) can be prepared as nanosheets, nanoplates, and nanofilms well dispersed on carbon fiber cloth (CFC). Stacks of triple layers are formed when each tungsten (W) atom is encased inside two layers of sulfur (S) atoms. Each of these layers has a hexagonal lattice structure. Strong ionic-covalent connections connect the tungsten and sulfur atoms. Bulk WS2 has a dark gray hexagonal crystal structure with layers. It also has a large surface area. But, it faces stacking problems and limits its performance as a supercapacitor. It has a good bandgap, high optical absorption coefficient, and high carrier mobility. It also has good intrinsic conductivity. Quantum confinement and interactions between the layers will cause indirect-to-direct bandgap transitions to happen when the thickness of WS2 is reduced to a single layer. Because the valence and conduction band states at K point (energy band of WS2 ) are only connected to the transition metal state and aren’t affected by interactions between layers, the direct bandgap at K stays the same as the thickness of WS2 changes. This material is chemically irritant. Because of the enormous surface area of the stacked sheet-like structure enabling facile electrolyte insertion/extraction, tungsten

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Fig. 2.14 a CV curve of WS2 /rGO, WS2 , and rGO electrode material at 10 mV s−1 and b GCD curve of WS2 /rGO, WS2 , and rGO electrode at 2 A g−1 current density (redrawn and reprinted with permission [131])

disulfide possesses a high specific capacitance. It has an oxidation state range of + 2 to + 6. It has diverse applications, such as nanoelectronics, optoelectronics, and valleytronics. This material is used in energy storage devices (lithium-ion batteries, sodium-ion batteries, and solar cells) and water-splitting applications. Taiwan Semiconductor Manufacturing Company (TSMC) is investigating the use of WS2 as a channel material in field-effect transistors. It is also used in gas-sensing devices, hydrogen evolution reactions, photodetectors, photocatalysis, electrocatalysis, laser saturable absorbers (SAs) [SAs for fiber lasers, solid-state lasers], and lubrication purposes. Nanosheets of WS2 , their quantum dots, and their composition with other materials have shown great impact in enhanced catalytic activity for hydrogen evolution, photocatalytic activity, and flexible and highly stable memristive devices [129, 130]. For the preparation of WS2 /rGO nanosheets as an electroactive material, a simple molten salt technique is used for the supercapacitors device, which displays a high specific capacitance of 2508 F g−1 at 1 mV s−1 . Figure 2.14 shows (a) the CV curves of WS2 /rGO, WS2 , and rGO electrode materials at 10 mV s−1 and (b) GCD curve of WS2 /rGO, WS2 , and rGO electrodes at 2 A g−1 current density.

2.3.2.3

Cobalt Disulfide (CoS2 )

The black structure model of cobalt disulfide (CoS2 ) shows that an octahedron comprises six S atoms around a Co atom, and a tetrahedron comprises three Co atoms and one S atom around an S atom. For a current density of 1 A g−1 , cobalt disulfide has a specific capacitance of 236.5 F g−1 . It is semiconducting in nature and insoluble in water. This material is a metallic ferromagnet, and its nanoparticles can give rise to ferromagnetic ordering at 122. It displays a large magneto resistance effect (6.5%) just below Curie temperature (T C ). The redox activity of this substance is high and exhibits numerous valence states. After 2000 charge–discharge cycles, the cobalt disulfide electrodes experienced a capacitance loss of about 7.4%. Thermal

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Fig. 2.15 CoS2 /CoS heterojunction electrocatalytic performance by CV curve. Here, the catalyst is deposited on glassy carbon in the solution of a 4 mM NaI + 0.5 M NaCl and b 0.06 M Na2 S + 0.02 M S + 0.5 M NaCl, where the scan rate is 50 mV s−1 (redrawn and reprinted with permission [133])

breakdown and sulfidation were used to create CoS2 ellipsoids with tube-like cavities [127]. The Co2+ /Co3+ redox pair produced typical pseudocapacitive behavior when CV was measured on CoS2 ellipsoids. At a current density of 0.5 A g−1 , the GCD curves were symmetrical, indicating good capacitive behavior, and its pseudocapacitive nature gives a decent specific capacitance of 1040 F g−1 [132]. CoS2 can be employed as a catalyst in the hydro desulfurization process, which is a molybdenumbased industrial process. When combined with molybdenum, cobalt sulfides are used as catalysts in the industrial process of hydride sulfurization, which is widely used in refineries. It is also used as an electrocatalyst. The CV curve in Fig. 2.15 shows how well the CoS2 /CoS heterojunction works as an electrocatalyst. Here, the catalyst is deposited on glassy carbon in a solution of (a) 4 mM NaI + 0.5 M NaCl (Fig. 2.15a), and (b) 0.06 M Na2 S + 0.02 M S + 0.5 M NaCl (Fig. 2.15b), where the scan rate is 50 mV s−1 .

2.3.2.4

Tin Disulfide (SnS2 )

Two layers of hexagonally closed sulfur anions with sandwiched tin cations are octahedrally coordinated with the sulfur atoms of their six closest neighbors. Each sulfur atom is nested at the apex of a tin atom’s triangle, and weak Van der Waals interactions hold the neighboring S–Sn–S layers together. The optical characteristics of tin disulfide (SnS2 ) nanoparticles are intriguing. It has a layered n-type semiconductor possessing a bulk bandgap of 2.18 eV (Bohr radius 2.2 nm) and a bulk bandgap of 2.18 eV. It has excellent electrochemical properties. It has a specific capacitance of 524.5 F g−1 and a power density of 12.3 W kg−1 at a current density of 0.08 A g−1 [134]. Tin disulfide has wide applications in photodetectors, solar cells, lithium/sodium-ion batteries, field-effect transistors, optoelectronics, photoluminescent materials, photocatalysts, and sensors. Figure 2.16 shows the GCD curve of

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Fig. 2.16 GCD curve of SnS2 -based electrode material at various current densities (redrawn and reprinted with permission [134])

SnS2 -based electrode material at various current density GCD curves of SnS2 -based electrode materials at various current densities [134].

2.3.2.5

Titanium Disulfide (TiS2 )

Titanium disulfide (TiS2 ) is another transition metal dichalcogenide that has a twodimensional nanostructure. Under room temperature, TiS2 performs as a semiconductor, but it behaves as a semimetal at high pressure of 8 GPa [135, 136]. This is the lightest and cheapest chalcogenide. The specific capacitance of two-dimensional TiS2 nanocrystal-based electrodes was 320 F g−1 [137]. When tested in a 6 M KOH electrolyte, TiS2 nano-disc electrode-based supercapacitors had a specific capacitance of 70 F g−1 . This substance is utilized as a cathode in rechargeable batteries [138]. Figure 2.17 displays a pure TiS2 structure. Fig. 2.17 Pristine TiS2 structure (redrawn and reprinted with permission [139])

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Zirconium Disulfide (ZrS2 )

Zirconium disulfide (ZrS2 ) has a two-dimensional structure and will have a layered structure when in the stack. This is a low-cost, thermodynamically stable, ecologically friendly material with great sensitivity. ZrS2 monolayers can display n-type transport with relatively high mobility. It has an indirect bandgap in bulk, but in mono- or few-layer forms, it becomes a direct gap semiconductor. The bandgap of the ZrS2 monolayer bandgap may be adjusted from zero to 2.47 eV under symmetrical strain. Thermoelectric properties, electrical conductivity, and the strain-driven indirect-to-direct band gap transition may all be improved using single-layer ZrS2 . At room temperature, it possesses more carrier mobility and a higher current density than MoS2 . Due to its higher carrier mobility at room temperature and greater current density than MoS2 , ZrS2 is a preferred material for optoelectronic applications such as low-power devices, Schottky solar cells, field-effect transistors, photodetectors, thermoelectric devices, catalytic hydrogen production, and many others. The electron mobility of two-dimensional ZrS2 is 1200 cm2 V−1 s−1 , which is more than three times that of the commonly studied MoS2 (340 cm2 V−1 s−1 ). ZrS2 field-effect transistors feature increased electronic sensitivity and superior semiconducting characteristics. Tunneling field-effect transistors (TFETs) based on ZrS2 can have current sheet density as high as 800 Am−1 (100 times greater than MoS2 ), allowing for a broader range of applications in low-power systems.

2.3.2.7

Vanadium Disulfide (VS2 )

A layer of vanadium atoms is sandwiched between two layers of sulfur atoms in monolayer vanadium disulfide (VS2 ). There are two crystalline phases (T and H) of VS2 . T and H are both impacted by variations in temperature and layer counts in the VS2 . Vanadium disulfide is metallic and conductive. The emerging density of both V3d and S-3p states in the monolayer and bulk forms of VS2 demonstrates the metallic property at the Fermi level. Monolayer band structures have a band that crosses the Fermi level, and bulk VS2 indicates a zero bandgap. The emerging density of both V3d and S-3p states in the monolayer and bulk forms of VS2 demonstrates the metallic property at the Fermi level. This substance can improve electron transport during cycle operations as an anode in ion batteries. In lithium-ion batteries, vanadium disulfide is a reliable electrode material. The presence of many flaws (both in-plane and out-of-plane faults) in VS2 nanoplates increases the aspect ratio. It opens up more active sites for redox reactions, allowing them to offer a substantial-specific capacitance of 2200 F g−1 [140].

2.3.2.8

Molybdenum Diselenide (MoSe2 )

The crystal structure of molybdenum diselenide (MoSe2 ) is hexagonal just, similar to graphene connected with a two-dimensional Vander Waal bond. It has very high

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Fig. 2.18 CV curve of MoS2 and MoS2 –Ni electrode-based supercapacitor with 6 M KOH (reprinted with permission [142])

inherent electrical conductivity. Selenide transition metal dichalcogenides are more electrically conductive than sulfide TMDs in monolayer, which has a direct band gap of 1.8 eV (ideal for use as switching nanodevices). For a quicker reversible redox reaction, molybdenum diselenide has a large surface area and more active sites. In galvanostatic charge discharge testing, at a current density of 5 A g−1 , a MoSe2 nanosheet electrode-based supercapacitor retained around 75% of its capacitance after 10,000 cycles. Hierarchical MoSe2 spheres were interlaced together to form linked channels for quicker charge transfer, delivering a specific capacitance of 243 F g−1 at a current density of 0.5 A g−1 , and a rate capability of 60% even at 15 A g−1 [141]. A hydrothermally created MoSe2 microsphere made of two-dimensional nanosheets demonstrated a specific capacitance of 272 F g−1 at a current density of 1 A g−1 . MoSe2 nanosheets produced using a hydrothermal process on a Ni-foam substrate showed good electrochemical performance, with a specific capacitance of 1114.3 F g−1 and capacitance retention of around 104.7% after 1500 cycles. Figure 2.18 shows the CV curve of MoSe2 and MoSe2 -Ni electrode-based supercapacitor with 6 M KOH [142]. It involves extending the use of flexible 2D graphene and materials with graphene-like properties into semiconducting and transparent conducting applications. It is a component of photovoltaic, memory, transistors, and photodetectors.

2.3.2.9

Vanadium Diselenide (VSe2 )

Vanadium diselenide (VSe2 ) has a sandwiched layer structure, with weak Van der Waals forces that allow the layers to slide on top of one another and separate layers from bulk crystals (or powder), resulting in dimensional reduction that can be accomplished by mechanical or chemical exfoliation. VSe2 is paramagnetic in its natural state. It condenses most typically in the 1T form, which is metallic and has no optical bandgap and it shows magnetic properties and a reduced work function near

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the border of monolayer VSe2 . Strong room temperature ferromagnetism in VSe2 monolayers made on Van der Waals substrates like graphite has been observed. Its composition is semimetal. It is unsuitable for optoelectronic applications as its conduction band (CB) and valence band (VB) overlap. Under acute conditions, VSe2 can be decomposed chemically by itself. The metallic characteristics of vanadium diselenide lead to its potential for use as electrodes, moisture-detection sensors, and electrocatalysts in supercapacitors and lithium-ion and sodium-ion batteries. It has wide applications in photocatalysts and energy storage devices.

2.3.2.10

Tungten Diselenide (WSe2 )

Tungsten diselenide (WSe2 ) has a hexagonal crystalline structure. It has a layered structure via Van der Waals interactions. This material is semiconducting in nature. It has wide applications in solar cells, photonics, and photoelectrodes. Mechanically exfoliated monolayers of WSe2 can be exfoliated using mechanical methods. For its light-emitting properties, i.e., LED, it is used as a transparent photovoltaic material.

2.3.2.11

Nickel Diselenide (NiSe2 )

Nickel diselenide (NiSe2 ) nanoparticles can have different morphology in their nanostructure. It has an adjustable electrical arrangement and numerous oxidation states. When the scan rate is 2 mV s−1 , NiSe2 nanoparticles provide a maximum specific capacitance of 75 F g−1 , indicated by their quasi-rectangular-shaped CV curves. It can be said that both EDLC and pseudocapacitance store charges. They also showed good cycling stability, with 94% capacitance remaining after 5000 cycles at 100 mV s−1 [143]. Single-crystal NiSe2 cubes (as truncated cubes with smooth surfaces and edge lengths ranging from 100 to 400 nm) had an excellent specific capacitance of 1044 F g−1 at a current density of 3 A g−1 and a 60% rate capability at a higher current density of 30 A g−1 [144]. Figure 2.19 shows the GCD curve of NiSe2 electrode material at different current densities [144]. The fact that they changed from cubes to agglomerated particles after 2000 cycles may be managed by creating a composite. This caused them to lose 33% of their initial capacitance.

2.3.3 Transition Metal Oxide (TMO) Due to their quick and reversible surface faradaic reaction, transition metal oxides (TMOs) are reliable and promising electrode materials for supercapacitor application, as they can provide high specific capacitance and energy density [145, 146].

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Fig. 2.19 GCD curve of NiSe2 electrode material at different current densities (reprinted with permission [144])

2.3.3.1

Ruthenium Oxide (RuO2 )

Among the transition metal oxides selected as the electrode material for supercapacitors, ruthenium oxide has received the most research interest. It has a large surface area, which is crucial for a material used as a supercapacitor electrode. The gravimetric capacitance will increase with lowering the particle size as it increases the aspect ratio. The Faradaic charge transfer method gives it a higher specific capacitance (in the range of 750–2000 F g−1 ) [147]. Because of surface interactions between Ru and H ions, ruthenium oxide (RuO2 ) has a tenfold greater capacitance than carbon materials. It possesses three unique oxidation states that are accessible, as well as strong electrical conductivity, higher rate capability [148], high redox activity, and high stability. The aggregation of RuO2 particles during the charge/discharge process hinders RuO2 ’s high electrochemical performance. This material is extremely durable in various chemical conditions and highly resistant to chemical degradation. Ruthenium dioxide is the best pseudocapacitor electrode material among TMOs, which can enhance any carbon-based electrodes used in EDLC because of its wide potential window, highly reversible redox processes, three distinct oxidation states accessible in under 1.2 V, extremely high specific capacitance, and reasonably high conductivity. RuO2 -based ternary hybrid composites are famous for highly efficient supercapacitors with very high specific capacitance values. The preparation of this material is fast too. But, it is expensive and has environmental toxicity [149]. This material is popular for electrode material in energy storage devices like supercapacitor. Binary and Ternary Oxide Materials Bimetallic oxides prepared with RuO2

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Attempts were made largely to minimize the loading of pricey RuO2 . However, this resulted in a considerable drop in material conductivity, lowering the composite electrodes’ capacitive performance. High interfacial charge transfer resistances at heterogeneous surfaces between various redox-active systems and the current collector, poor conductivity, poor binding ability to the current collector, nanoparticle aggregation effects, and dissolution of the active material into the electrolyte through chemical/electrochemical reactions are some of the known probable causes that act as an insurmountable barrier to superior electrochemical performance. Ruthenium oxide-titanium oxide, (Ru–Ti)Ox Aerogels made of ruthenium oxide (RuO2 ) and titanium oxide (TiO2 ) have higher porosity and better crystallinity. Better crystallinity reduces the material’s specific capacitance [150]. Asymmetric supercapacitors using a high aspect ratio RuO2 /TiO2 nanotube composite displayed a very high gravimetric capacitance of 1263 F g−1 [151]. Hydrous ruthenium oxide (RuO2 · · · xH2 O) and TiO2 nano-flowers, nanotubes can be used in electroactive materials for nourishing quicker electron transport [151–153]. Ruthenium oxide and titanium oxide TiO2 /RuO2 nanocomposites The electrochemical performance of core–shell TiO2 /RuO2 nanocomposites (with a RuO2 loading of 60% by weight) is excellent ~ 990 F g−1 [154]. Ruthenium oxide and Hafnium oxide (RuO2 and HfO2 ) Ruthenium oxide (RuO2 ) and hafnium oxide (HfO2 ) were combined in 1:1 mol percent in the most basic thermally decomposed mixed oxide composites, which possessed a 789.3 F g−1 specific capacitance [155]. Ruthenium oxide and Manganese oxide (MnO2 and RuO2 ) The nanocomposite formed by co-precipitating ruthenium oxide (RuO2 ) and manganese oxide (MnO2 ) nanoparticles with 60 wt% MnO2 has a high gravimetric capacitance of 438 F g−1 [156]. Ruthenium oxide and Manganese oxide (RuO2 –Mn3 O4 ) composite nanofibermats Porous ruthenium oxide and manganese oxide (RuO2 –Mn3 O4 ) composite nanofiber-mats with a cross-linking morphology have a gravimetric capacitance of 293 F g−1 and are strongly conductive. [157]. Ruthenium oxide and Tin oxide (SnO2 /RuO2 ) nanocomposite films The specific capacitance of ruthenium oxide and tin oxide (SnO2 /RuO2 ) nanocomposite films with adjustable thickness and homogenous shape generated by the CVD technique was 150 F g−1 [158]. The CV of SnO2 –RuO2 composite files for various composition samples at a scan rate of 20 mV s−1 with a 0.5 M H2 SO4 electrolyte is shown in Fig. 2.20 [158]. Mixed oxide (Pb2 Ru2 O6.5 )

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Fig. 2.20 CV of SnO2 –RuO2 composite files for different composition samples at 20 mV s−1 scan rate and 0.5 M H2 SO4 electrolyte (reprinted with permission [158])

Using a high-temperature solid-state process, the Park group was able to produce a mixed oxide nanocomposite (Pb2 Ru2 O6.5 ) with a capacitance of 160 F g−1 [159].

2.3.3.2

Manganese Dioxide (MnO2 )

Manganese dioxide (MnO2 ) has a high surface area. Crystallized MnO2 materials have several crystalline structures. Low conductivity, high theoretical specific capacity (1100–1300 F g−1 ), and prolonged working potential make it an ideal faradaic material. Its actual capacitance and rate capabilities are limited by its weak ionic (1013 S cm−1 ) and electrical (105 –106 S cm−1 ) conductivities. The protonation (or deprotonation) process in manganese oxide (MnOx ) will be hindered if the crystallinity is too high, while decreased crystallinity might result in a very porous microstructure with low electrical conductivity. Due to MnO2 ’s poor conductivity, specific capacitance reduces with increasing electrode layer (or film) thickness. This material possesses photocatalytic and pseudocapacitive properties [160]. This is highly stable in various chemical environments. The transportation of electrolyte ions is helpful by the water bound inside either chemically or physically. Therefore, heat treatment in this material will lead to poor electrical conductivity and pseudocapacitance. In neutral aqueous media, MnO2 operates in aqueous solution electrolytes and experiences a pseudocapacitive charge-storage mechanism, where just a thin coating of MnO2 is electrochemically active and participates in the reduction-oxidation procedure. Because of their poor conductivity, MnO2 electrodes may have restricted capacity and power density. In compared to other transition metal oxides, it is cost-effective and has low toxicity. Figure 2.21 displays the performance of supercapacitors ((a) CV at 10 mV s−1 scan rate, (b) Nyquist diagram with several samples). MnO2 deposited on graphene/activated carbon, GN/AC/MnO2 -1200 s electrodes have much higher area-specific capacitances than other electrodes (520 mF

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Fig. 2.21 Supercapacitor performance a CV plot; scan rate: 10 mV s−1 and b Nyquist plot for different samples (redrawn and reprinted with permission [161])

cm−2 at a 10 mV s−1 scan rate). The GN/AC/MnO2 -1500 s electrode exhibited a high resistance (Rs = 21.11) [161]. Oxides of manganese are employed in supercapacitors, batteries, and fuel cells, as well as photocatalysis and water purification.

2.3.3.3

Nickel Oxide (NiO)

Nickel oxide (NiO) has a high surface area. It is highly electronic conductive, having high theoretical specific capacitance ~(2584 F g−1 ) [162] from the Faradaic charge transfer process and good thermal stability. This material is extremely stable in a variety of chemical conditions. It is non-toxic and also resistant to chemical agents’ destruction. It has distinct redox activity with great theoretical capacitance (2584 F g−1 ), large surface area, low manufacturing costs, and enough availability in nature. NiO-microflower is shown as a supercapacitor electrode in Fig. 2.22. CV was performed on a variety of NiO-based electrodes (Fig. 2.22a). The electrolyte was 1 M KOH, and the scan rate was 10 mV s−1 . The system consists of three electrodes. (b) Nyquist plots of two-electrode supercapacitor cells composed of a variety of materials based on NiO (Fig. 2.22b). The Ni–NiO-5 min sample demonstrates the maximum specific capacitance, which comes in at 964.9 F g−1 [163]. Electrodes made of nickel oxide can be employed in energy storage systems (active electrode material for supercapacitor in alkaline electrolytes).

2.3.3.4

Nickel Hydroxide (Ni(OH)2 )

The surface area of nickel hydroxide (Ni(OH2 ) is considerable, like that of other transition metal oxides. Examples of nickel oxide or hydroxide nanostructures with a higher surface area, better mass transportation, and a consistent supply of ions for Faradic reactions at high current densities are nanocolumns, nanosheets, nanoflakes, nano-rings, hierarchical porous nanoflowers, nano/microspheres, porous nano-wall

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Fig. 2.22 NiO-microflower as supercapacitor electrode, a CV for different NiO-based electrodes (electrolyte: 1 M KOH, scan rate: 10 mV s−1 , and three-electrode system). and b Nyquist plots of two-electrode supercapacitor cells made of various NiO-based materials (redrawn and reprinted with permission [163])

Fig. 2.23 a GCD curves and b Nyquist plots of the EIS for CNRG and CNRG/Ni(OH)2 composite (redrawn and reprinted with permission [164])

arrays, and hollow nanospheres. Its electrical conductivity value is smaller than that of other transition metal oxides, and it is chemically stable and resistant to chemical degradation. It has high specific capacitance and low cost of raw materials but poor cycle performance in electrochemical test. GCD and Nyquist plots of the EIS for g-C3 N4 /rGO (CNRG), and CNRG/Ni(OH)2 composite are shown in Fig. 2.23. This substance is employed in the oxidation of methane to synthesis gas (syngas). .

2.3.3.5

Iron Oxides (Fe2 O3 and Fe3 O4 )

Oxides of iron have poor electronic conductivity than other transition metal oxides. FeO is ferrous oxide, and Fe2 O3 is called ferric oxide, commonly known as rust.

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Now, the compound of ferrous and ferric oxide, Fe3 O4, is called ferrous ferric oxide. Iron oxide reacts with water or water vapor and generates iron hydroxide. Fe2 O3 + H2 O → Fe(OH)3 Sulphuric acid interacts with iron (III) oxide to create iron (III) sulfate and water. Fe2 O3 + H2 SO4 → Fe2 (SO4 )3 + H2 O Iron oxide is harmful to the human body when inhaled. Ferrous oxide is highly reactive, even flammable, and can combust spontaneously in the air. This material is plentiful, inexpensive, and has a low environmental effect. For supercapacitors, iron oxide can be employed as an electrode material. Ferrous oxide is used in the pharmaceutical, paint, cosmetic, and plastic industries (as pigment, dye, and ink).

2.3.3.6

Cobalt Oxide (Co3 O4 )

Cobalt oxide (Co3 O4 ) has a high surface area like other transition metal oxides. This material possesses a spinel structure having both Co2+ and Co3+ ions in tetrahedral 8a and octahedral 16d sites of the cubic closed-packed lattice of the oxygen ions [165]. It can have different morphologies like microspheres, nanowires, nanorods, nanotubes, nanosheets, and thin films. There is a family of cobalt oxides, which includes cobalt (II) oxide CoO, cobalt (III) oxide Co2 O3, and cobalt oxide (II, III) Co3 O4 . The latter one is being discussed here. This material has theoretically high specific capacitance (3560 F g−1 ). It is an antiferromagnetic p-type semiconductor. It has distinguishable magnetic, optical, and electrochemical properties which make it a promising material for device application. When nano-cobalt oxide is heated to 900 °C (1652°F) in a hydrogen flame, it transforms into metal cobalt. Cobalt oxide can cause irritation in the eyes, skin, and respiratory tract infection. It is harmful if inhaled or swallowed. It is very toxic to the environment and aquatic life. It has exceptionally good redox properties [166]. It has practical availability and cost-effectiveness. Co3 O4 nanowire arrays constructed on nickel foam achieved a maximum specific capacitance of 746 F g−1 at a current density of 5 mA cm−2 [167]. Single-crystal Co3 O4 nanorods showed a high specific capacitance of 456 F g−1 when the current density was 1 A g−1 . Figure 2.24 shows the GCD curve of Co3 O4 nano-rods-based electrode at 1 A g−1 current density [168]. Cobalt hydroxide was chemically deposited in anodic aluminum oxide templates and then thermally annealed to produce Co3 O4 nanotubes with an outer wall diameter of 300 nm and a wall thickness of 50 nm [169]. It is used to convert CO to synthesis gas (syngas) and methane to synthesis gas (syngas). It is utilized in several applications, including heterogeneous catalysts, solid-state sensors, gas sensors, and energy storage devices like supercapacitors and lithium-ion batteries [170–172].

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Fig. 2.24 GCD curve of Co3 O4 nano-rods-based electrode at 1 A g−1 current density (reprinted with permission [168])

2.3.3.7

Cobalt Hydroxide (Co(OH)2 )

This material has a large surface area because of its layered structure and wide interlayer spacing. Strong bases and cobalt (II) hydroxide (Co(OH)2 ) interact to form cobalt (II) salts and the dark blue cobaltate (II) anions [Co(OH)4 ]2 and [Co(OH)6 ]4 , respectively [173]. It is strongly insoluble in water. It has a greater specific capacitance than cobalt oxide (Co3 O4 ) and has a faster ion insertion/desertion rate. However, it is only suitable at modest potential levels. Its easy production might possibly replace the use of high-cost, ecologically unfriendly RuO2 . It may be found in abundance in nature. In physical chemistry, it is one of the most commonly utilized elements. It is utilized for CO oxidation, base catalysis, and other hydroxide applications.

2.3.3.8

Vanadium Oxide (V2 O5 )

Because of the Faradaic charge, transfer method it possesses a high theoretical specific capacitance and good catalytic activity, low conductivity than other TMOs. It has very modest acute toxicity to humans. It can cause severe allergy to human eyes, skin, and mucus membranes and can cause harm by inhalation, ingestion, and absorption by the skin. Vanadium pentoxide (V2 O5 ) has a unique layered structure [174] with different oxidation states (V2+ to V5+ ) and a wide potential window [175]. When the scan rate is 10 mV s−1 in cyclic voltammetry analysis, the specific capacitance of the graphene oxide/vanadium pentoxide (GO/V2 O5 ) nanofibers becomes 453.82 F g−1 , which is far better than the individual GO and V2 O5 electrodes. Because only the outside surface of the active electrode is utilized during charging and discharging, the specific capacitance of GO/V2 O5 nanofibers drops as the scan rate rises. This is owing to the limitation of ion incorporation into the active electrode. The enhanced

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Fig. 2.25 GV curves of the materials which have different molar ratio of vanadly acetylacetonate (VO(acac)2 )/styrene monomer (SM)/acrylonitrile (AN) as 1:0.25:10, 1:0.5:10, 1:0.75:10, and 1:1:10 1:2:10 named as VMCNFs-0.25, VMCNFs-0.5, VMCNFs-0.75, VMCNFs-1, VMCNFs-2, and vanadium-doped CNF at 500 °C (VCNF-500), and they are represented by a, b, c, d, e, f curves, respectively, with a current density of 0.5 A g−1 (reprinted with permission [177])

capacitance may be caused by graphene oxide’s high surface area and strong conductivity. [176]. Amorphous V2 O5 doped with multichannel CNFs (VMCNFs) has the largest specific capacitance (739 F g−1 at 0.5 A g−1 ). GV curves of VMCNFs0.25, VMCNFs-0.5, VMCNFs-0.75, VMCNFs-1, VMCNFs-2, and VCNF-500 with a current density of 0.5 A g−1 are shown in Fig. 2.25 [177]. Large power plants, such as wind farms, employ vanadium redox batteries. It is also employed as a catalyst, gas sensor, and detector material in bolometers and microbolometer arrays for thermal imaging.

2.3.3.9

Tin Oxide (SnO2 )

Tin oxide is an amorphous nanostructure. It has two main characteristics which make it different from other materials. It has a variable valance state, and there are oxygen vacancy defects. There are inherent acidic and redox characteristics. It has a lower specific capacitance than other metal oxides. It has weak cycle stability and electronic conductivity. The specific capacitance of the electrochemically deposited amorphous nanostructured SnO2 was 285 F g−1 at a low scan rate of 10 mV s−1 and 101 F g−1 at a high scan rate of 200 mV s−1 [178]. Charge/discharge charting for current densities of (4, 8.5, and 12.6) A g−1 is shown in Fig. 2.26a, and Fig. 2.26b shows EIS. It is also used in gas sensors, catalysis, electrocatalysis, battery materials, solar energy conversion, antistatic coating, transparent conductive electrodes, and electrochromic devices.

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Fig. 2.26 a CV for current densities of (4, 8.5, and 12.6) A g−1 and b EIS (redrawn and reprinted with permission [179])

2.3.3.10

Iridium Oxide (IrO2 )

Iridium oxide (IrO2 ) has a tetragonal rutile structure with having a high surface area. It is an electrically conductive material. Single crystals of iridium oxide have the lowest room temperature resistivity, which is as low as 32 µΩ cm. It is chemically toxic in nature. Large Faradaic pseudocapacitance results in a high specific capacitance. It possesses strong electrical conductivity, reversible charge–discharge capabilities, and high-power densities, but it is too costly. The specific capacitance of hydrous IrO2 was close to 550 F g−1 [180]. It is used in ferroelectric capacitors as thin-film electrodes.

2.3.3.11

Titanium Oxide (TiO2 )

Titanium oxide (TiO2 ) has three types of crystalline structure, among which two are found in minerals anatase and rutile. The anatase phase is found at lower temperatures, and above 600 °C, rutile phase starts to form. As the anatase phase has a lower surface Gibbs free energy than the rutile phase, so, at lower synthesis temperature, TiO2 anatase phase nanoparticle tends to form. Another one is brookite. In the anatase phase, TiO2 has a tetragonal structure and space group I41/amd; the rutile phase has a tetragonal structure as well, but the space group is P42/mnm, whereas the brookite phase has an orthorhombic structure with a Pbca space group. The agglomerated microstructure, porosity, and particle size all influence the physical characteristics of TiO2 . Surface particles are considerably more reactive than interior particles due to the dangling connections. It is an n-type semiconductor that has poor electrical conductivity than other TMOs. It has a broad and tunable bandgap (3.2 eV for anatase). It is a metal oxide that is harmless to the environment. It has the ability to break down dangerous contaminants in the air and water [181]. It is a dielectric that exhibits Faradic capacitance. The fundamental drawback of a supercapacitor based on TiO2 is the low specific stored energy. Porosity enhances its charge–discharge

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behavior and intensifies the electrochemical performance. It is utilized for antibacterial applications. It is widely used as a pigment because of its white color, such in cosmetics (e.g., sunscreen and UV absorption pigments), and paint. It has applications in photocatalysts, different sensors, solar cells, fuel cells, pollution control, waste management, and glass coating materials that can be cleaned themselves.

2.3.3.12

Zinc Oxide (ZnO)

Zinc oxide (ZnO) has mainly two types of structural form; the most common and stable one is hexagonal wurtzite, and another one is cubic zinc blende structure and does not have any inversion symmetry. For both the structures, zinc and oxide centers are tetragonal. Nanostructures of ZnO can have different morphologies like nanowire, nanorod, nanobelt, nanoflower, nanodots, etc. Zinc oxide appears as a white powder. It belongs to the II–IV semiconductor group and has a broadband gap. This has piezoelectric properties in its wurtzite structure. Its piezoelectric tensor is the highest among the II–IV semiconductor group. The piezoelectric coefficient of zinc blend ZnO is far smaller than a wurtzite ZnO. It has photoluminescence properties at room temperature. It is water-insoluble yet soluble in a variety of acids. It is non-toxic and has antibacterial properties. It has good surface conductivity and high electron mobility. It is a low-cost, easily available, and eco-friendly material. For its wide band gap, it is used in laser diodes and light-emitting diodes (LEDs). It has many piezoelectric applications. Solar cells, batteries, light-emitting diodes, and gas sensors all use it as an active material. It is also used in catalysis and biomolecular systems. ZnO is a popular lithium-ion battery anode material for its low cost, biocompatibility, and eco-friendly nature [182].

2.3.3.13

Molybdenum Oxide (MoOx )

Molybdenum dioxide (MoO2 ) crystal has a distorted rutile structure where the octahedral is distorted, and the molybdenum atoms are off-center. As a result, there are short and long Mo–Mo distances and Mo–Mo bonding. It has a very high melting point of 1075 K. It has high stability for use as a supercapacitor electrode. MoO2 has a possible application as an anode of Li-ion batteries. Molybdenum trioxide (MoO3 ) is popular as an industrial catalyst. MoO3 nanobelts have layered structures [183]. It is used for high-performance supercapacitors.

2.3.3.14

Tungsten Oxide (WO3 ) Nanoparticles

Tungsten trioxide (WO3 ) has a temperature-dependent crystal structure, i.e., it possesses tetragonal crystal structure when the temperature is above 740 °C, but the structure is orthorhombic between 330 and 740 °C temperature, and it becomes monoclinic between 17 and 330 °C temperature; it also has triclinic structure between

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− 50 and 17 °C and below − 50 °C too. Monoclinic WO3 is the best on the basis of prevalence. It can have a monoclinic structure with nanoplates morphology. To create pure tungsten metal, WO3 interacts vigorously with several elements from the periodic table, including rare-earth elements, iron, copper, aluminum, manganese, zinc, chromium, molybdenum, carbon, hydrogen, and silver. If it reacts with gold or platinum, then tungsten trioxide will be reduced into tungsten dioxide. It is used in photocatalysis, sensor, gas sensors, electrochromic devices, X-ray screen phosphors, fireproofing fabrics, and smart windows.

2.3.4 Spinel-Based Nanostructured Materials Spinel-based nanostructured materials gain attention for making electrodes of supercapacitor. Spinel materials have a typical chemical formula AB2 O4 . Compared to the individual oxides of A and B, the mixed bi-metallic oxides in spinel materials often exhibit better electrochemical activity, electrical conductivity, and redox reactions. Metal−cobalt oxide (MCo2 O4 , M=Ni, Mn, Zn) The octahedral sites are occupied by M metal, and Co is spread throughout both octahedral and tetrahedral sites. Nickel−cobalt oxide (NiCo2 O4 ), manganese−cobalt oxide (MnCo2 O4 ), copper−cobalt oxide (CuCo2 O4 ), and zinc−cobalt oxide (ZnCo2 O4 ) are all derived from cobalt oxide (Co3 O4 ) [184]. The metal–cobalt oxide has more electrical conductivity than cobalt oxide or the other metal’s (M) oxide. The metal cobalt oxide has ultrahigh-specific capacitance and rate capabilities than individual oxides. They take advantage of both metal ions, which enrich the redox reaction from both the other metal and cobalt ions. At the time of charging and discharging the following redox reaction takes place [184]. Co3 O4 + OH− + H2 O ↔ 3CoOOH + e−

2.3.4.1

Nickel–cobalt Oxide (NiCo2 O4 )

Nickel–cobalt oxide (NiCo2 O4 ) processes spinel structure where cobalt is spread throughout both octahedral and tetrahedral sites, with nickel occupying the octahedral sites [185]. Its structure contains solid-state redox couplings Co3+ /Co2+ and Ni3+ /Ni2+ , which provide significant catalytic activity. Basically, a nickel atom is replacing one cobalt atom from Co3 O4, where the nickel atom’s grain size is comparable with the cobalt atom’s grain size with a few changes in the crystal structure, which makes the differences in electrochemical structure [186]. NiCo2 O4 is at least two orders of magnitude more conductive and electrochemically active than typical transition metal oxides [187, 188]. It exhibits excellent electrocatalytic activities. It is also available at low cost, with abundant resources, and is environmentally

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(b)

Fig. 2.27 a BET plotting of specific surface area and specific capacitance for nickel oxide, cobalt oxide, and nickel cobaltite aerogels with variations in calcination temperature, as well as b specific capacitance vs cycle number (redrawn and reprinted with permission [189])

friendly with low toxicity. The material is environmentally friendly. It has ultrahighspecific capacitance and rate capabilities, high electrochemical activity, low cost, and many abundant resources. They take advantage of both metal ions. In Fig. 2.27a, specific surface area and specific capacitance for nickel oxide, cobalt oxide, and nickel cobaltite aerogels are plotted using Brunauer–Emmett–Teller (BET) method with varying calcination temperatures and specific capacitance is shown as a function of cycle number in Fig. 2.27b. Both the change in composition and the drop in surface area led to a large loss in the specific capacitance, which went from 719 to 420 F g−1 throughout the course of the experiment. In energy storage applications, this material is an effective negative electrode of sodium-ion or lithium-ion battery [190, 191], responding record for this reference. It shows unusual ferromagnetic properties. It can have different morphologies like nanoparticles, nanowires, and nanospheres [192].

2.3.5 Spinel-Type Oxides (MMoO4 (M=Fe, Ni, Co)) Its nanowire array topology provides a wide response surface area, quick ion and electron transport, and structural stability [193]. It has ultrahigh-specific capacitance and rate capabilities. It will have strong electrochemical energy storage qualities, as well as noteworthy pseudocapacitive nature with a great capacitance and favorable cycle stability [194]. Anode material for Li-ion batteries is made of this substance.

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2.4 Conclusion The supercapacitor is a promising device to fulfill the need for high-performance energy storage applications. Although all four parts of a supercapacitor are equally important, the performance of a supercapacitor mainly depends on the electrode. So, choosing a suitable electrode material is very important to construct a supercapacitor device as per the requirement, and other components like electrolytes, separator, and current collector can be chosen according to the electrode material. There are carbon materials that store charges by accumulating the charges at the interface of the electrode and electrolyte by forming an electric double layer. On the other hand, metal oxides store charges by reduction-oxidation reactions. Ruthenium oxide and manganese oxides are highly studied metal oxides, whereas there are many more other metal oxides and transition metal dichalcogenides which have excellent properties to be electrodes of a supercapacitor but are yet to be explored vivaciously. Composites of different materials discussed above can be used instead of pristine ones, which can enhance the performance of the supercapacitor.

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Chapter 3

Emerging 2D Materials for Supercapacitors: MXenes Shagufi Naz Ansari, Mohit Saraf, and Shaikh M. Mobin

Abstract MXenes are two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides that display layered structure, rich surface chemistry, superior hydrophilicity, and intrinsic electronic conductivity. Since the very first report on MXenes (Ti3 C2 Tx ) in 2011, the focus on MXenes has increased exponentially due to their favorable properties for a diverse range of applications including energy storage, thanks to their high conductivity, redox activity, and electrochemically active surface. In this chapter, we have targeted the current advances, achievements, and challenges in MXenes research on supercapacitors in a concise manner. In the beginning, an overview of various synthetic approaches for 2D MXenes are presented, and then, their structural aspects and properties are summarized. Moreover, MXenes and their composites-based supercapacitors are discussed highlighting their potential in such energy storage devices. Finally, few challenges and perspectives are provided to encourage the further improvement of MXenes in supercapacitors.

S. N. Ansari · S. M. Mobin (B) Department of Chemistry, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India e-mail: [email protected] M. Saraf · S. M. Mobin Department of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India S. M. Mobin Department Biosciences and Biomedical Engineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India Center for Electric Vehicle and Intelligent Transport Systems, Indian Institute of Technology Indore, Khandwa Road, Simrol, Indore 453552, India M. Saraf A. J. Drexel Nanomaterials Institute and Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USA S. N. Ansari Department of Chemistry, School of Engineering, Presidency University, Bangalore 560064, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_3

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Graphical Abstract

3.1 Introduction Renewable/clean energy sources are getting huge consideration due to the deterioration of the environment because of excessive consumption of fossil fuels [1]. The shortage of energy sources has been spectacularly addressed by advanced energy storage devices [2]. Rechargeable batteries and supercapacitors are the most employed electrochemical energy storage devices which significantly encourage sustainability. Batteries have high energy densities but suffer from inadequate power density and poor cyclic stabilities [3]. On the other hand, supercapacitors have been progressively getting extensive attention due to their prompt power supply, elongated cycling life, and exceptional rate performance [4]. At present, supercapacitors find applications in industrial energy/power management systems, consumer electronics, and memory backup systems [4]. In electric double-layer capacitors (EDLCs), the

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charge is stored in the electric double layers of the supercapacitors that are generated at the electrolyte/electrode interface via the process of adsorption and desorption of ions [5]. A potential alternative to these EDLCs is pseudocapacitors that store more charge using rapid, surface-controlled redox reactions, through a redox capacitance mechanism [6]. The performance and efficiency of the electrode materials depend on some crucial properties such as large surface area, well-organized pores, good corrosion resistance, temperature stability, cost-effectiveness, and convenient processing [7]. Nevertheless, common limitations associated with conventional electrode materials such as porous carbons or metal oxides are their small volumetric capacitance, low electronic conductivity, poor structure stability, and unsatisfactory rate capability. These drawbacks severely limit their use in practical applicability [4– 7]. Therefore, the selection of appropriate materials for electrode fabrication and optimizing their design strategies are the key challenges to build efficient supercapacitors [8–11]. MXenes are the 2D-layered transition metal carbides/nitrides first reported by Gogotsi et al. in [12]. They are represented by a general formula Mn+1 Xn Tx (where M symbolizes transition metal, X represents carbon or nitrogen, and Tx signifies surface functional groups, e.g., –F, –OH, –O, –Cl) [13]. MXenes possess striking physical and chemical properties because of which they have gained intensive research attention. They demonstrate high electrical conductivity and mechanical robustness, high negative zeta-potential, further aiding stability to its colloidal solutions in water, and proficient absorption of electromagnetic waves [14]. Their unique 2D structure with functionalized oxide-like surfaces makes MXenes hydrophilic allowing them to react with various species [15]. Thanks to their excellent conductivity, electrochemically active surfaces, high charge transferability due to the presence of transition metals with variable oxidation numbers, and unique stacked arrangement, MXenes offer fascinating electrochemical properties in different electrolytes, thereby being an attractive contender in energy storage applications, particularly for supercapacitors [13, 16, 17].

3.2 Synthetic Strategies Since the discovery of the first Ti3 C2 Tx MXene, many new synthetic methods have been explored for the synthesis of novel MXenes or their derivatives where MXenes’ morphology, quality, surface termination, and other properties are significantly driven by synthesis conditions (Fig. 3.1).

3.2.1 HF Etching Method The first MXene (Ti3 C2 Tx ) constructed was an accordion-like multilayered arrangement, prepared by immersing a Ti3 AlC2 powder in HF (50% concentration) for 2 h

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Fig. 3.1 Different etching processes to produce MXenes from MAX phases

[8]. Since then, numerous MXenes structures have been reported by HF etching method employing diverse HF concentrations and etching times, such as Ti2 CTx , TiNbC, Ta4 C3 , Ti3 CNx, and (V0.5 , Cr0.5 )3 C2 [9–11]. Taking Ti3 C2 Tx as an illustration, preferential etching of the “M–A” bond in Mn+1 AXn phases can be defined as follows [12]: Ti3 AlC2 (s) + 3HF(aq.) = Ti3 C2 (s) + AlF3 (aq.) + 3/2H2 (g)

(3.1)

Ti3 C2 (s) + 2HF(aq.) = Ti3 C2 F2 (s) + H2 (g)

(3.2)

Ti3 C2 (s) + 2H2 O(aq.) = Ti3 C2 (OH)2 (s) + H2 (g)

(3.3)

Ti3 C2 (OH)2 (s) = Ti3 C2 O2 (s) + H2 (g)

(3.4)

The etching process is explained in Fig. 3.2, where the layers of transition metal carbides or nitrides (Mn+1 Xn ) are incorporated within the layers of chemically active pure A-group elements to form the precursor MAX phases (A is groups 13 and 14 element) [18]. Generally, MXenes are obtained by etching reactive A-layers from these MAX phases [19]. After the removal of A-layer, the remaining MX layers are impulsively switched with surface functional groups, such as –F, –OH, –O, or –H

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with a common formula Mn+1 Xn Tx [20]. Aqueous hydrofluoric acid (HF) of various concentrations has been furthermost used for the selective etching of the MAX phases to prepare MXenes. Direct use of aqueous HF reagent or its in situ formation requires considerable safety and environmental concerns due to its hazardous nature for causing systemic toxicity in the human body and fatality [21]. MAX phases are mostly manufactured by the high-temperature sintering and ball-milling method. OH-terminated MXenes tend to convert into O-terminated MXenes releasing H2 gas, as established by density functional theory (DFT) calculations. Diluted HF (6 M) etched-MXenes retain additional open interlayer spaces and provide more mobility for ions in the interlayers to reach active sites and convey better capacitance. However, monolayer MXene that is similar to graphene can be obtained by the mechanical exfoliation of the multilayered etched-MXene or by the chemical intercalation delamination method. Many techniques are used for exfoliation processes such as scotch-tape exfoliation method, ultrasonication technique, or simple mechanical exfoliation [22, 23]. Ultrasonication technique for exfoliation generates shredded MXenes with more defects, whereas high-quality MXene is manufactured with low defects in case of the scotch-tape exfoliation method [22, 23]. Various organic molecules can be employed for chemical intercalation such as urea, tetrabutylammonium hydroxide (TBAOH), dimethyl sulfoxide (DMSO), N,N-dimethylformamide, isopropylamine (i-PrA), hydrazine, choline, and n-butylamine to weaken the interlayer assembly for delamination of MXenes into separate sheets [24–26]. Besides delamination, the chemical composition and structure of the MXene get influenced by the organic molecules, further affecting the electrochemical behavior of the MXene. Interestingly, different precursors can be used to prepare similar MXene structures as confirmed by different spectroscopy techniques with slight differences in surface chemical composition.

3.2.2 Alkali Etching Method Since HF is environmentally harmful, there was an urgent need for new fluorine-free methods of MXenes synthesis. Noteworthy, efforts were dedicated to fluorine-free alkali-etching methods, knowing the fact that –F groups do not have a positive influence on the electrochemical properties of MXenes [27, 28]. There was the frequent use of inorganic alkalies in the etching of Ti3 AlC2 because of its strong affinity with the amphoteric element Al [29]. However, MXene preparation via alkali attack can be a struggling process due to some (oxide) hydroxide layers formation on the surface of Ti3 AlC2 [30]. In one report, a core–shell MAX@K2 Ti8 O17 composite was formed when KOH (2 M) was utilized for Ti3 SiC2 etching using hydrothermal reaction at 200 °C [31]. There are reports where NaOH-assisted hydrothermal process has been employed to etch Ti3 AlC2 to prepare Ti3 C2 Tx (T = OH, O) [32]. The reaction between the alkali and MAX phase will experience qualitative changes by increasing the concentration of alkali and temperature to a certain extent. In 2018, Zhang and coworkers reported selective exclusion of Al from Ti3 AlC2 effectively in

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Fig. 3.2 Illustrative pathway to obtain 2D MXene single flakes by etching process: Selective etching of precursor MAX phase layers is followed by exfoliation into single flakes. SEM images of a Ti3 AlC2 particles, b multilayers of Ti3 C2 Tz after Al layer etching. c TEM image of overlying single layers of Ti3 C2 Tz [18]. Copyright (2019) Elsevier

a NaOH solution (concentration 27.5 M) at 270 °C forming OH and O-terminated multilayer Ti3 C2 Tx with a purity of 92 wt% [32]. The resulting Ti3 C2 Tx film electrode displayed a gravimetric capacitance of 314 F g−1 in 1 M H2 SO4 . The Al layers in Ti3 AlC2 attacked by OH– get oxidized into Al (oxide) hydroxides and further dissolute in alkali (Fig. 3.3). The Al atoms present on the outer surface of Ti3 AlC2 are more exposed to oxidation and dissolution in NaOH forming soluble Al(OH)4– . Furthermore, the inner Al atoms oxidize as well if this etching process continues. The corresponding microstructures of MXene had a typical stack-like structure with multilayers. The fluorine-free characteristics delivered additional – OH and – O groups, further increasing the whole performance of supercapacitors. It is relatively efficient to use the concentrated alkali for etching the MAX phase to obtain F-free terminated products with a high hydrophilic nature, although the highly concentrated alkali and high temperature have certain limits in the preparation of large-scale MXenes. Furthermore, the produced MXenes are multilayered, which need to be intercalated and delaminated additionally to yield single-layer MXene nanosheets.

3.2.3 Molten Salt Etching Method This is another emerging technique to prepare MXenes. Nitride MXenes can be easily synthesized by molten salt etching. For the first time, Ti4 AlN3 was etched through the mixed molten salts of NaF, LiF, and KF for 0.5 h at 550 °C to produce Ti4 N3 Tx nitride MXenes [33, 34]. It is relatively challenging to prepare nitride-based MXenes by any method other than the molten salt etching method. The fluoride-free

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Fig. 3.3 a–c Representation of the reaction between NaOH and Ti3 AlC2 water solution in various conditions [32]. Copyright (2018) Wiley–VCH

molten salt etching approach can also be utilized for the MAX phase etching. In 2019, Huang et al. employed a mixed system of ZnCl2 /NaCl/KCl molten salt to etch Ti3 AlC2 , Ti2 AlC, Ti2 AlN, and V2 AlC MAX phases under a nitrogen-protected environment [34]. Here, the role of NaCl and KCl was to develop the molten salt bath, which further reduces the melting point of the eutectic system, while the ZnCl2 acts as an etchant for the MAX phase etching, further transforming the weakly linked Al atoms into Al3+ . The Zn atoms in reduced form consequently produce a new Zn-MAX phase, i.e., Ti3 ZnC2 (Fig. 3.4). Furthermore, the excessive ZnCl2 produces MXenes through the etching of the Zn atoms present in the interlayer of the Zn-MAX phase. Ti3 AlC2 is taken as an example to describe the etching process [34]: Ti3 AlC2 + 1.5ZnCl2 = Ti3 ZnC2 + 0.5Zn + AlCl3

(3.5)

Ti3 ZnC2 + ZnCl2 = TiC2 Cl2 + 2Zn

(3.6)

Here, the final products are considerably influenced by the proportion of ZnCl2 and the Al-enclosing MAX phase. As the procedure is non-aqueous and fluoride-free, the surface of MXene is occupied by –Cl groups, in place of the –O, –OH, and –F groups. It was also established that melted halides having superior electrochemical redox potentials could certainly etch the MAX phases showing inferior electrochemical redox potentials of the elements located at the A-site.

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Fig. 3.4 Replacement reaction among the MAX phase and late transition metal halides producing Cl-terminated MXenes and Zn-based MAX phases [34]. Copyright (2019) American Chemical Society

3.2.4 Acid/fluoride Salt or Hydrofluoride Etching To avoid the direct consumption of HF, in situ HF-forming approach look more favorable than the conventional HF methods, comprising low energy consumption, less chemical risk, and simple operation throughout the etching process. Ghidiu et al. reported the etching of the Ti3 AlC2 employing HCl/LiF solution for the first time at 40 °C. Ti3 C2 Tx conductive clay produced by this method displayed strong plasticity (Fig. 3.5) [35]. Outstanding flexibility, great toughness, and good hydrophilicity are exhibited by the rolled, free-standing MXene clay with a retained conductivity equivalent to 1500 S cm–1 . The fluoride salts can be varied to control the MXenes interlayer’s space toward achieving the requirements of anticipated applications. Liu et al. incorporated HCl in diverse fluoride salts, i.e., NaF, LiF, KF, and NH4 F for the preparation of a mixed solution system to etch the Ti3 AlC2 [36]. The HCl/NH4 F mixture attained 24 h as the minimum etching time to etch the Ti3 AlC2 phase completely into Ti3 C2 Tx at the lowest temperature of 30 °C. The variations in the composition of the etchant mixture could have a significant effect on the physicochemical properties and morphologies of the etched MXenes. The MXene with a large lateral dimension can be fashioned into shredded slices, and the one having a small lateral size can be made into powdered form through grinding. Various surface functionalities can be incorporated into MXenes through the in situ HF etching process, such as –F, –OH, and –O. The produced MXenes are generally accompanied by intercalated water molecules, thus resulting in an extended drying time [36, 37].

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Fig. 3.5 a (Step 1) Etching of MAX phase in a solution of fluoride salt and acid, (step 2) followed by its washing with water for the eradication of reaction products to adjust the pH toward neutral. (Step 3) Resultant sediment resembles like clay; it can form flexible and free-standing films through rolling, else can be molded and dried out for the procurement of conducting entities of anticipated size and shape. b Presenting the morphology of the rolled Ti3 C2 Tx “clay” film through digital photo. c Conductivity of the Ti3 C2 Tx “clay” film [35]. Copyright (2021) Wiley–VCH

3.2.5 Electrochemical Etching In an electrochemical etching process, a certain voltage is applied to remove the Al atomic layer selectively, where the MAX phase is used as an electrode. The electrochemical etching process is quite beneficial for the etching of the carbide-derived carbon from the MAX phase in an electrolytic system of HF, HCl, or NaCl. Electrochemical etching allows the breaking of the M–A bond, further removing the A-layer in the MAX phase under a voltage range between 0 and 2.5 V [38]. Additionally, M-layer can be further removed through a gradual upsurge in voltage subsequently forming the amorphous carbon materials. Ti3 SiC2 , Ti3 AlC2, and Ti2 AlC MAX phases can be successfully converted into carbide-derived carbon within various electrolytes, which can be established by the Raman spectrum [38]. Therefore, selective elimination of A-atoms can be accomplished by evaluating suitable etching time along with regulating the etching potential, further permitting an accurate control of synthesized MXenes. As the MAX phase is generally employed in the composition of the working electrode, the etching is primarily recognized on the surface of the electrode [39]. The electrode composed of the MAX phase can be recycled many times. The electrochemical etching method consumes low energy, exhibiting a safe and green synthetic approach. However, it is still challenging to overcome the presence of the carbide-derived carbon layer besides the insufficient yield.

3.3 Structure and Properties of MXenes It is witnessed experimentally that MXenes are imperfect in their structural aspects. The existence of Ti vacancies in Ti3 C2 Tx MXene can be validated through electron energy loss spectroscopy (EELS) and STEM techniques of characterization [40, 41]. These vacancies are initiated by the HF etching and are usually created on the sublayers of two surfaces. Further, the concentration of the etchant can regulate the

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extent of Ti vacancy [40, 41]. Superior electrical conductivities and greater volumetric capacitance are displayed by the 2D Mo4/3 C sheets having ordered metal divacancies in comparison to their counterpart, Mo2 C, devoid of any vacancies [42]. It can be concluded that the surface properties of MXenes can be certainly controlled for energy storage through the existence of point defects. Usually, MXenes inherit the point defects present in their MAX phases. Since the MAX phases universally embrace carbon vacancies, it can be anticipated that the corresponding MXenes acquire the intrinsic “defect” of their host. Consequently, the flexibility and electronic conduction of MXenes also get enhanced, further stimulating their applications in flexible electronic devices [40]. Pristine MXenes are always metallic. However, the conductivity of some MXenes was found to be improved when terminated with surface functional groups. For instance, the metallic character is exhibited by the pristine Ti2 C; however, Ti2 CO2 attains semiconducting nature as their d band is elevated above the Fermi level. In the case of F-functionalized MXenes, i.e., Ti2 CF2 persists metallic nature due to the location of its Fermi energy at the d band of Ti layers. The first explored metallic MXene-Ti3 C2 Tx is the potential contender for supercapacitor electrode material due to its high conductivity [43]. The Mo-containing MXenes comparatively exhibit semiconductor-like transport features that are further appropriate for transistors. However, the 2D Mo4/3 C sheets comprising well-arranged metal divacancies can be used as a supercapacitor electrode material due to their superior electrical conductivities [44]. MXenes have been studied for their mechanical properties through theoretical calculations concerning material composition, surface thickness, and terminations. The flexibility and elasticity of MXenes can be reflected by the in-plane stiffness (C) and out-of-plane rigidity (D). The nature of M element, the thickness of MX, and the surface functional groups highly influence the C and D of MXenes [41].

3.4 MXenes in Supercapacitors Highly conductive MXenes are frequently explored in energy storage applications due to the fast transmission of electrons [45]. Two-dimensional lamellar-structured MXenes retain the charge storage ability through the intercalation of the metal ions, impulsively or electrochemically. Diverse cations, e.g., Li+ , Na+ , K+ , Mg2+ , Ca2+ , Al3+ , NH4+ , and Cs+ can be reversibly perturbed into MXenes, inhabiting electrochemically active sites on the surface of MXenes, further contributing to fast pseudocapacitive charge storage process in acid aqueous electrolyte and non-aqueous electrolyte [46]. Initially, ions get adsorbed continuously on the adsorption localities present at the particle’s edges, rapidly to assure specific charge storage, and then get accommodated in the particle’s interior with deep-adsorption sites having higher activation energy [47]. Therefore, MXenes display an elevated amount of capacitance in comparison to all EDLC carbon materials. In general, MXenes are believed to be superior to conventional pseudocapacitive materials (such as conducting polymers

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and metal oxides) in terms of their excellent cycle life showing negligible decay in initial capacitance [48].

3.4.1 MXenes as Supercapacitor Electrode Materials Generally, the voids of Ti3 C2 Tx MXene get contracted via smaller cations and higher charges, whereas the interlayer spaces in MXene are expanded by the bigger cations with smaller charges. Finally, there will be an upsurge in the interlayer distance if contraction rules overexpansion; else, there will be a contraction in interlayer space. This change in structure throughout the ion intercalation/deintercalation progressions is reversible, moreover complemented through softening/stiffening of electrodes prepared by MXene [49]. MXenes show significant ion-accessible electrochemical surface areas due to their layered assembly. The high density shown by MXene electrodes (around 3 ~ 4 g cm−3 ) is beneficial for achieving high volumetric capacitance [17]. So far, exceptional conductivity values are shown by most of MXenes against additional 2D materials, e.g., graphene and MoS2 . The first report on MXene employed as electrode materials for supercapacitance was available in 2013 achieving a conspicuous volumetric capacitance of 350 F cm−3 in KOH electrolyte [28]. Here, Ti3 C2 Tx was synthesized as multilayered MXene via HF etching of Ti3 AlC2 MAX phase. Further, the acquired etched phase was delaminated by DMSO, yielding flakes of MXene having a few-layered structure. The reported capacitance was superior to that of graphene-based materials (200 ~ 350 F cm−3 ) and conventional EDLC carbons (60 ~ 200 F cm−3 ) [50, 51]. Further, the capacitive performance of MXene was improved considerably in H2 SO4 electrolyte by employing LiF/HCl mixture as an etching solution. For example, free-standing and flexible films of Ti3 C2 “clay” prepared by LiF/HCl-etching revealed a high gravimetric capacitance of 245 F g−1 and an exceptional volumetric capacitance as 900 F cm−3 at 2 mV s−1 and in the potential range of − 0.35 ~ 0.2 V. An excellent cycling performance was observed showing a minor loss in initial capacitance at 10 A g−1 after undergoing 10,000 cycles [35, 52]. The approach of fluoride-free etching leads to the progression of environmentally sustainable and safer MXene synthesis, along with enhancing the electrochemical performance. Multilayer Ti3 C2 Tx MXenes synthesized via alkalisupported hydrothermal technique were drop-casted into a 52 µm thick film and 1.63 g cm−3 density, displaying a volumetric capacitance of 511 F cm−3 and a gravimetric capacitance of 314 F g−1 in H2 SO4 electrolyte at 2 mV s−1 [32, 53]. Multiple oxidation states acquired by vanadium help vanadium-based MXenes to attain enhanced capacitance. A delaminated V2 C film electrode prepared by HF etching could accomplish a high gravimetric capacitance equivalent to 487 F g−1 using H2 SO4 as an electrolyte. Subsequently, MXenes can be made more stable by increasing n in Mn+1 Xn Tx [54]. For this purpose, V4 C3 Tx was examined with a multilayered structure prepared through HF etching revealing large spacing between the interlayers (~0.466 nm) and high thermal stability. However, the applied capacitance shown by V4 C3 Tx was even lower than V2 C, i.e., ~209 F g−1 at the scan

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rate of 2 mV s−1 [55]. Further, niobium carbides were explored by preparing a freestanding film by etching via HF and TMAOH delamination. This niobium carbides’ film revealed a huge spacing between the interlayers equivalent to 1.77 nm and volumetric capacitance of 1075 F cm−3 at the scan rate of 5 mV s−1 using 1 M H2 SO4 as electrolyte [56]. The use of transition metal alloys at the position of M has further expanded the MXene family to virtual countless numbers. For example, Mo2 TiC2 Tx , a doubleM MXene, was prepared through the HF etching which displayed an ordered and stable structure [17]. Density functional theory (DFT) calculations are utilized to predict and combine these alloys with C and/or N for MXene synthesis. The prepared Mo2 TiC2 Tx displayed relatively different electrochemical behaviors than Ti3 C2 Tx despite being isostructural with it. Mo2 TiC2 Tx exhibited two diverse voltage plateaus throughout the lithiation in a non-aqueous electrolyte, i.e., below 0.6 V and at 1.6 V. Additionally, more rectangular CV profiles were observed than Ti3 C2 Tx [17]. This behavior specified the surface Mo layer’s controlled properties of the MXenes with binary M elements. Even though MXenes have been recognized as an auspicious contender for supercapacitors, their real-world capacitances are far away from theoretical standards. For instance, MXene Ti3 C2 O0.85 (OH)0.06 F0.26 revealed the maximum theoretical capacity up to about ∼615 C g−1 as measured by Faraday’s law; however, experimentally using MXene clay as electrodes displayed merely 135 C g−1 or ∼245 F g−1 of capacitance at 0.55 V in 3 M H2 SO4 as electrolyte [9, 57]. Restacking of MXene nanosheets and horizontal aggregation is the principal cause for the large inconsistency among the experimental and theoretical capacitance values. The adjacent layers of MXenes involve strong van der Waals interactions, which restrict the availability of electrolyte ions in the interlayers and limit the usage of the entire 2D MXene surface [58].

3.4.2 MXene-Based Composites as Supercapacitor Electrode Materials There was a need to combine Ti3 C2 Tx MXenes with various other types of materials to prepare hybrids, such as polymers, metal compounds, and carbon-based materials for the improvement of the mechanical properties, and further increase the electrochemical performances. The composite materials based on MXenes have encouraged their growth enthusiastically for the applications in energy storage. Carbon materials, such as graphene, CNTs, and polymers derived from carbons, express controllable porosity, large specific surface area, high conductivity, non-toxic nature, and admirable chemical stability further contributing enormously as supercapacitor electrodes [59–63].

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MXene/Metal Oxides or Metal Sulfides

Ti3 C2 TX MXene retains outstanding electrical conductivity; however, comparatively low capacitance is shown in comparison to transition metal compounds. On the other hand, transition metal-based oxides/sulfides present large values of specific capacitance theoretically, e.g., MnO2 (1370 F g−1 ), MoS2 (811 F g−1 ), and RuO2 (720 F g−1 ) [54–66]. However, their electrochemical performance is hindered by their small electrical conductivities. The transition metal compounds and Ti3 C2 Tx MXene can be integrated to be employed as supercapacitor electrodes to terminate each other’s drawbacks and associate their advantages. The insertion of metal compounds in MXene can act as spacers to control the restacking of MXene layers along with preventing the structural collapse of metal sulfide and oxide assemblies. Zhao et al. reported Ti3 C2 Tx /Co3 O4 composite using alternating filtration method which generated comparatively ordered sandwich-like structures superimposed on each other [67]. MnO2 is an abundantly available classic pseudocapacitive transition metal oxide displaying low toxicity, low cost, and high theoretical capacitance of 1370 F g−1 . Wang et al. synthesized a heterostructure of Ti3 C2 Tx /1 T-MoS2 via magneto-hydrothermal method at 210 °C and preserved it under a magnetic field having the strength of 9 T for 18 h (Fig. 3.6a) [68]. An all-solid-state symmetric supercapacitor was prepared by Ti3 C2 Tx /1 T-MoS2 hybrid electrode using PVA/H2 SO4 as an electrolyte, which was further utilized to drive an electronic watch (Fig. 3.6b), by three devices connected in series. The maximum capacitance value of 347 mF cm−2 was presented by the flexile supercapacitor at 2 mA cm−2 showing up to 91.1% retention of initial capacitance after 20,000 cycles (Fig. 3.6c). The multiple oxidation states shown by transition metal elements support them to achieve high capacitance values through redox reactions. Yu and coworkers hybridized Ti3 C2 Tx and ultra-long MnO2 nanowires to fabricate highly flexible and conductive Ti3 C2 Tx /MnO2 composite paper [69]. The MnO2 nanowires suppressed the restacking in 2D Ti3 C2 Tx MXene flakes by acting as a superfluous active material. Significantly high volumetric capacitance equivalent to 1025 F cm−3 was exhibited by the prepared free-standing film electrode. Further, an areal capacitance of 205 mF cm−2 was displayed at the current density of 0.2 mA cm−2 . A highly flexible all-solidstate (ASS) supercapacitor assembly was prepared by employing Ti3 C2 Tx /MnO2 as a hybrid electrode using PVA/LiCl as electrolyte. The prepared supercapacitor revealed an unnoticeable decay in performance when bent up to 120°. Although hybrid supercapacitor electrodes fabricated from MXene/transition metal compounds are deficient in good mechanical strength due to the high stiffness and less flexibility of transition metal compounds, insights into controlled morphology and surface chemistry variations are still needed to be studied.

3.4.2.2

MXene/Graphene, rGO, CNTs

MXene-based film electrodes generally face problems like poor ionic transport, which can be fixed by combining it with graphene, a widely studied 2D carbon

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Fig. 3.6 a Schematic illustration of the preparation of 3D heterostructural Ti3 C2 Tx /1 T-MoS2 MXene and 2H-MoS2 /Ti3 C2 MXene. b Stable cycle life of the all-solid-state supercapacitors showing retention of initial capacitance with the inset showing galvanostatic cycling data. c Representation of an electronic watch driven by three all-solid-state supercapacitors (1 T-MoS2 /Ti3 C2 ) joined in series 68. Copyright (2020) Wiley–VCH

nanomaterial, through the enlargement of the interlayer spacing. Graphene, graphene oxide (GO), and rGO can be easily employed to fabricate supercapacitor electrodes [70–72]. Specifically, the flexible supercapacitor electrodes can be prepared due to the high stability and flexibility of these materials. Xu et al. utilized Ti3 C2 Tx and rGO nanosheets and prepared a self-supporting flexible film of Ti3 C2 Tx /rGO as a supercapacitor electrode (Fig. 3.7) [73]. In 6 M KOH, this electrode provides a superior specific capacitance of 405 F g−1 over rGO films thanks to the hierarchical matrix and synergistic effect of Ti3 C2 Tx and rGO. GO’s surface is abundantly allocated with functional groups like –O and –OH which consequences in displaying significant mechanical and energy storage performance of the supercapacitor electrodes prepared by the combination of GO and Ti3 C2 Tx . Fan et al. introduced rGO into Ti3 C2 Tx flakes, where holes were present on the rGO plane. A porous composite film was fabricated, which effectively restricted the “face-to-face” restacking and aggregation tendency of MXene nanosheets [74]. The prepared free-standing and flexible film presented commendable electrical conductivity along with showing nanoporous networks. A volumetric capacitance of 1445 F cm−3 was displayed at a 2 mV s−1 voltage window in a three-electrode arrangement in presence of 3 M H2 SO4 aqueous solution as electrolyte. Carbon nanotubes (CNTs) have also been extensively explored with MXenes to be utilized as supercapacitor electrodes. CNTs can be integrated with MXenes to upsurge the electrochemical performance by increasing the specific surface area, enabling smooth ion diffusion, tuning the interlayer spacing, and enhancing the electrical conductivity. In a study, cetyltrimethylammonium bromide (CTAB) was embedded on single-walled carbon nanotubes (SWCNTs), providing it with positive charges and further combined with negatively charged Ti3 C2 Tx nanosheets. Thanks to electrostatic interaction, self-assembly of Ti3 C2 Tx /SWCNT composite electrode was fabricated which delivered a capacitance of 220 mF cm−2 (314 F

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Fig. 3.7 a Representation of the fabrication route for porous rGO/Ti3 C2 Tx films. b Plots relating the gravimetric capacitances of rGO films, Ti3 C2 Tx powders, RGM-3, and hybrid rGO/Ti3 C2 Tx powders at various current densities. c Profile showing the Nyquist plots of the/Ti3 C2 Tx / rGO films. An aqueous solution of 6 M KOH was used as an electrolyte [73]. Copyright (2020) Royal Society of Chemistry

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cm−3 ) at 2 mV s−1 in 1 M KOH electrolyte [75]. The obtained performance was greater than that of the original Ti3 C2 Tx electrode. The introduction of SWCNTs expanded the interlayer spacing and improved the accessible proficiency of ions through acting as spacers which resulted in an amplified capacitance. Using the chemical vapor deposition (CVD) method, CNTs can be in situ grown homogeneously on the surface of Ti3 C2 Tx MXene. This approach can terminate the problem of restacking of Ti3 C2 Tx sheets; alongside, in situ grown CNTs can perform as intraparticle and interparticle charge collectors. In 2020, Li et al. fabricated a hybrid electrode, Ti3 C2 Tx /CNT, where multiwall carbon nanotubes (MWCNTs) were in situ grown on a Ti3 C2 Tx MXene sheet-coated carbon cloth. The binder-free approach was explored due to the presence of strong interactions between the Ti3 C2 Tx MXenes and MWCNTs in a well-designed 3D interconnected structure, further presenting superior mechanical and physicochemical stability [76]. An areal capacitance equivalent to 114.58 mF cm−2 was achieved at 1 mA cm−2 retaining an initial capacitance of 118% at 5 mA cm−2 after crossing 16,000 cycles. Recently in 2021, Yang et al. developed an effective and prompt self-assembly method to synthesize 3D cross-linked porous MXene/graphene composite induced by an in situ sacrificial metallic zinc template [77]. The prepared self-assembled 3D porous architecture can efficiently obstruct the MXene layers’ oxidation showing no obvious fluctuations in electrical conductivity under ambient conditions even after two months. Plentiful electrochemically active sites accessible to electrolyte ions and exceptional electrical conductivity are ensured by this technique. Specific capacitance equivalent to 393 F g–1 was attained at 2 mV s–1 , showing an incomparable rate performance of 32.7% at 10 V s–1 , and spectacular cycling stability with a negligible degradation after 30,000 cycles.

3.4.2.3

MXene/Conducting Polymers

Comprehensive research has been expounded for conductive polymers such as polyaniline (PANI), polypyrrole (PPy), and poly(3,4-ethylenedioxythiophene) (PEDOT), for electrochemical applications, owing to their exceptional flexibility, large specific theoretical capacity, significant electrochemical activity, and lightweight [15, 21, 60–72, 78]. These polymer molecules can be incorporated into the MXene layers to amplify the spacing of the interlayers of MXene, which in turn can increase the mechanical flexibility and augment the electrochemical capacitance. The conductive polymers offer their capacity to the entire capacitance through redox reactions by any of the following reactions [79]: ( ) Cp + ne− + nC+ ↔ Cpn+ C+ n (n - doping)

(3.7)

( ) Cp + nA− ↔ Cpn+ A− n + ne− (p - doping)

(3.8)

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where Cp represents the conductive polymer, and C+ and A− symbolize the cation and the anion, respectively. However, throughout the charge and discharge progressions, polymers usually undergo structural pulverization which can be resolved through its blending with MXenes. Ti3 C2 Tx MXenes can act as the framework stabilizer for the hybrid electrode by increasing the cycle stability of the system. In 2018, a report came on Ti3 C2 Tx MXene@PANI composite electrode fabrication having graphene encapsulations. In this method, Ti3 C2 Tx nanosheets and graphene were assembled through electrostatic interactions where PANI chains were intercalated into it (Fig. 3.8 a-f). The prepared electrode Ti3 C2 Tx MXene@PANI displayed a volumetric capacitance of 1143 F g−3 (gravimetric capacitance of 635 F cm−1 ) at a current density of 1 A g−1 (Fig. 3.8g,h). About 97.54% of initial capacitance was retained after 10,000 cycles showing highly stable cycle life in 1 M H2 SO4 as an electrolyte [15]. For the preparation of the large-scale synthesis of the MXene/conductive polymers’ composite, a colloidal suspension of MXenes is prepared and mixed with the organic monomers, consequently growing the polymers on the surface of MXenes [21]. This method is advantageous in terms of simple fabrication practice, without any additional oxidants. However, in the presence of oxidants, some MXenes can be oxidized during the in situ polymerization of monomers, which requires the development of oxidation-free methods.

Fig. 3.8 Representation of MXene Ti2 CTx , MP, and GMP in a, c, e schematic diagram and b, d, f SEM images. g, h GCD profile and specific capacitance values of the MXene, MP, and GMP at 1 A g–1 and different current densities, respectively [15]. Copyright (2018) American Chemical Society

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3.5 Progress of MXenes-Based Supercapacitor Devices Energy storage devices made of wire-type supercapacitors are significant in the preparation of wearable and smart microelectronic devices. Yarn-based supercapacitors can be fabricated by using fiber-shaped substrates, e.g., silver-plated nylon fibers and stainless steel wires. They show great flexibility and high retention of capacitance. Ti3 C2 Tx MXene can be loaded on A4 printing paper to fabricate planar microsupercapacitors, thanks to laser machining technology [79]. Due to their hydrophilic nature, MXenes can be used to prepare conductive and electroactive inks, which can be printed, written, stamped, or dip-coated on various substrates for the fabrication of planar electrodes. These electrodes can be made into as-stamped supercapacitors with excellent flexibility to achieve a high retention rate of initial capacitance under diverse bending positions. Similarly, the cellulose yarns can be coated via Ti3 C2 Tx MXene layers and further knitted or woven into fabrics as supercapacitor electrodes (Fig. 3.9) [79, 80]. Three-dimensional porous or open assembly of MXene can function as ion-buffering chambers, which can increase electrolyte ion transportation. Assembling asymmetric supercapacitors is a promising approach to attaining high energy density. Reduced graphene oxide can be used along with Ti3 C2 Tx for the assembly of asymmetric supercapacitors. Symmetric or asymmetric supercapacitors based on MXene can be considered to design micro-supercapacitors (MSCs) comprising superior power and energy densities. These MSCs can be utilized to power micromechanical set-ups, microsensors, or wearable electronics that consume less power due to the presence of more pseudocapacitance nature of MXenes. Various techniques can be used to make MSCs from MXenes, thanks to their excellent mechanical flexibility and ease of processing. The interdigital geometry of MXene inks can also be defined by the inkjet printing technology. MSCs based on MXenes can also be fabricated on A4 paper. The lightweight and free-standing composite of MXene/bacterial cellulose paper can be interestingly organized into a stretchable, twistable, and bendable all-solid-state MSC array via an easy laser-cutting Kirigami patterning procedure. MSCs are a favorable applicant for energy storage solutions related to miniaturized electronics. There is a need for more insights into the MSCs’ evolution and device optimization as it is still merely in its initial stages.

3.6 Conclusions and Outlook MXenes are certainly promising 2D materials for energy storage devices. Their remarkable high-conductivity and electrochemically active surfaces make them suitable candidates for supercapacitors. However, the emerging field of MXenes requires continuous efforts, and several key issues should be addressed such as the development of a safe, environment-friendly, and scalable synthesis process of MXenes (with and without non-Al MAX phases), controlling the surface chemistry of MXenes, exploring other MXenes beyond Ti3 C2 Tx , understanding charge transport and ionic

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Fig. 3.9 a Graphic showing yarn-based supercapacitor device for energy textile configuration. Demonstrations of providing power to b a digital timer and a digital watch through the energy textile alignment. c Model representation of cellulose-based yarns coated by knitted MXene. FESEM images of d original cotton fibers, e cotton fibers coated with MXene flakes, f surface of pristine cotton yarn, g pristine cotton yarn coated with MXene flakes [79, 80]. Copyright (2018) Wiley–VCH, Copyright (2019) Wiley–VCH

transport mechanism, and investigating charge storage mechanism of MXenes in different electrolytes, etc. [78]. Furthermore, MXenes can be assembled with various other materials in the form of heterostructures or composites, which allow synergistically enhanced properties of materials while eliminating the associated shortcomings. In this respect, various attempts have been made, and continuous efforts are required to maximize the properties. Moreover, computational programs, artificial intelligence, and machine learning can play an important role in predicting their properties which can be experimentally verified. In conclusion, the future of MXenes is very bright and several hidden prospects are expected which can play a major role in the advancement of materials’ sciences and electrochemical energy storage areas. Acknowledgements S.M.M. acknowledges SERB-DST (Project CRG/2020/001769), CSIR, New Delhi, India [Project 01(2935)/18/ER-II], BRNS (Project 58/14/17/2020-BRNS), and IIT Indore for financial support. M.S. expresses gratitude to the Ministry of Human Resource Development, New Delhi, India, for a Teaching Assistantship Fellowship during his Ph.D. and the United States−India Educational Foundation, New Delhi, India, for the Fulbright-Nehru Postdoctoral Fellowship (Award 2558/FNPDR/2020). S.N.A thanks the Ministry of Human Resource Development, New Delhi, India, for a Teaching Assistantship Fellowship during her Ph.D.

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Chapter 4

Laser as a Tool for Fabrication of Supercapacitor Electrodes Ravi Nigam, Rajesh Kumar, and Kamal K. Kar

Abstract Micro-supercapacitors have high power density, long lifetime, quick charge, and applications in wearable devices and portable electronics. The functioning of micro-supercapacitors is mainly dependent on the charge storage mechanism in electrodes. A laser is a promising tool to pattern supercapacitor electrodes by photothermal, photochemical, etc. The laser provides a fast, efficient, cheap, and low defect fabrication of supercapacitor. The different laser parameters that affect the final device are discussed in the article. The various processes associated with lasers and their effects on materials are elaborated. Different materials exhibit electric double-layer behavior, pseudocapacitance, or hybrid nature. The progress in laser-derived materials in each type of charge storage mechanism and the final direct laser-based fabrication of supercapacitors focusing on recent areas are mentioned in detail.

4.1 Introduction Currently, energy security is essential to any national policy worldwide to promote the economy and development. Fossil fuels and other polluting energy sources are utilized to fulfill domestic and industrial requirements. These fossil fuels related energy sources are depleting due to ever-increasing consumption by the population on the earth planet. The other drawbacks of these conventional energy sources are their harmful impact on the environment and human health. Renewable energy sources like sunlight, wind, and tidal energy have extensively gained importance in recent times [1–4]. These natural renewable energy sources are intermittent and need to be stored to fulfill human energy requirements. R. Nigam · R. Kumar · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. K. Kar Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_4

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Energy storage can be done through an electrochemical process via Faradaic and non-Faradaic processes [5, 6]. Non-capacitive Faradaic process occurs in batteries. However, capacitive Faradaic or non-Faradaic processes are responsible individually or together in supercapacitors. The Faradaic process involves the transfer of charge instantaneously, and the charge is not stored locally at the electrode–electrolyte interface [7]. Due to the electrostatic forces, the non-Faradaic process stores the charges at the electrode–electrolyte interface. An electric double layer is formed at the interface between electrodes and electrolyte [8]. Supercapacitors are energy storage devices that store charges by electrochemical interactions. It is classified into electric double-layer supercapacitors owing to the non-Faradaic process, pseudocapacitors due to the Faradaic process, and hybrid supercapacitors involving both types of mechanism [9]. Supercapacitors have four main components, i.e., electrodes, electrolyte, separator, and current collector [10–17]. Electrodes are the most critical component for energy storage in supercapacitors by Faradaic or non-Faradaic processes. Various types of electrodes have been fabricated using carbon-based materials, conducting polymers, transition metal oxides, and MXenes [18–32]. Carbon-based materials are of various types, primarily activated carbon, graphene derivatives, carbon nanotubes, carbon nanofibers, etc., used as electrodes that are environmental-friendly and low cost [33–46]. Activated carbon has been synthesized from biomass, leading to doped nitrogen, oxygen, sulfur, and phosphorous groups. The effect of doping in activated carbon provides higher capacitance due to pseudocapacitance and the existence of an electric double layer. Graphene-based materials in 0D, 1D, 2D, and 3D are excellent candidates for supercapacitors. These contain good electrochemical performance due to the high surface area, stability, and conductivity. Doped conducting polymers have high electrical conductivity, high operating potential windows, and redox activity. Polyaniline, polythiophene, and polypyrrole are a few conducting polymers [47]. Transition metal oxides exhibit pseudocapacitance due to multiple oxidation states. Supercapacitors are of various sizes for various applications. Micron-sized supercapacitors are needed for on-chip supercapacitors and small devices [48–50]. A laser is a powerful tool for synthesizing very small interdigitated electrodes for supercapacitors. The article initially discusses the various laser technologies used for fabricating electrodes and different processes that can be performed with laser techniques. Both the modification and deriving of the materials directly from the laser are discussed in detail. Finally, the laser-based fabrication of a complete supercapacitor is discussed.

4.2 Laser Technology in Energy Electrodes Design Laser parameters are important factors in the electrode design due to the breaking of bonds via energy transfer. The laser-based technology is environmental-friendly, low temperature, easy and faster processing, energy saving, no further use of catalyst or chemicals, better reproducibility, high productivity, negligible contamination, energy savings, scalability, and good fabrication control [51].

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The different laser parameters which can be adjusted for optimum electrodes design are: 1. Laser wavelength: Lasers are coherent energy sources with fixed light emission frequency. The frequency is directly related to the laser energy, which is needed for various processes and electrodes design [52]. A high energetic laser beam can remove the material or break chemical bonds based on penetration depth. Infrared lasers have less energy and cause localized heating. Localized heating also depends on the thermal conductivity of the material. 2. Laser power: Laser power is the ratio of energy transferred to the substrate per unit of time. The laser power is defined based on the frequency in two ways, viz., average laser power spread over a period of time or instantaneous power applied to the substrate at a particular instance [53]. 3. Pulse width: Pulse width determines the amount of time interaction between the laser beam and the substrate. It is determined by the laser production process and can vary from milliseconds to femtoseconds [54]. Continuous lasers can be assumed to have infinite width as there is no reduction in pulse profile. 4. Fluence: Fluence is defined as the ratio between the laser energy per unit of surface area. This is a valuable parameter to determine the nature of laser–material interaction. This can be varied by varying the laser energy profile and the laser beam area by changing the laser focus equipment parameters [55]. 5. Beam energy profile: The distribution of laser photons intensity over an area is defined as the beam energy profile determined statistically [56]. It can be constant over a cross-sectional area, as in the case of excimer laser known as flat-top beam profile. Gaussian-shaped beam profile has the irradiance decreasing while moving away from the center according to the Gaussian equation. In contrast, flat-top beam profiles have an irradiance constant over a given area. Figure 4.1 shows three-beam energy profiles, viz., circle, ring, and star, and corresponding patterns. 6. Beam focusing: Lenses are used to concentrate the laser beams at a particular point which is also helpful for making miniaturized patterns. The lenses can be used alone or in combination with multiple lenses [57]. The depth of focus and spot size are two conflicting parameters, and optimum beam focusing should be done [58]. 7. Writing speed: The laser writing speed is dependent on the relative motion between the laser source and substrate. This governs the interaction time between laser and matter and the corresponding pattern or final product formed [59]. 8. Local environment: The local environment between the point of contact between laser and matter can be air, vacuum, and variable pressure and temperature adjusted by the user. The effect of the local environment on the final product can be due to chemical reactions, diffusion of particles, and heating or melting processes [60].

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Fig. 4.1 Laser beam profile [61], redrawn and reprinted (adapted) with permission from Ref. [61], Copyright (2015), Elsevier B. V. All rights reserved

4.3 Processing of Laser in Carbon Materials Laser parameters substantially impact the laser–matter interaction and final product formed. Different energy lasers can lead to laser cutting, etching, or ablation. The transformation of materials can be done by laser writing, laser printing involves charge transfer, and defects can also be induced in the materials leading to drastic failure too. The laser–matter interaction leads to different processes discussed in the following sections.

4.3.1 Cutting Laser cutting is an efficient process as it does not involve mechanical wear, tear and inflexibility. It involves the removal of material by vaporization. Nanosecond laser technology is used for cutting in battery technology [62]. Laser technology is reliable because it has high energy concentration, contact-free, low noise level, fast processing, and a very narrow heat-affected zone. The laser power can be modified for drawing patterns with optimization and applicability to all materials. It is also a cheaper method when compared to material loss and defects in mechanical processing. Laser cutting involves the absorption of laser energy and evaporation of the substrate. At the evaporation interface, a kinetic Knudsen thin layer is formed.

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The laser cutting speed is an important parameter and should not be below a critical value for pattern formation. Laser cutting speed can be increased with direct proportionality to laser power. The laser cutting quality can be determined by electron microscopy and measuring frazzling width and clearance width. A defect of laser cutting is burr along the cutting kerf [63]. Figure 4.2a shows the variations of graphitic conversion and laser cutting as a function of laser power and scanning speed, whereas Fig. 4.2b shows graphitic conversion of polyimide film, and Fig. 4.2c shows the schematic of laser cutting. Figure 4.2d, e show the microscopic features of graphitic transformation, followed by Fig. 4.2f representing an optical image of a slot produced by laser cutting. The heat transfer during the laser cutting process depends on thermal diffusion length, and the surface temperature rises directly influenced by laser pulse duration and laser pulse repetition. A short pulse duration minimizes the heat transfer losses due to the less interaction times. Lower pulse repetition results in higher thermal conduction, whereas higher pulse repetition increases the local temperature of the substrate. A higher surface temperature results in cutting and an increase in the efficiency of the pattern formation. The ultrafast laser technology is the most effective laser cutting tool and decreases defects, including material losses [64].

Fig. 4.2 a Variations in laser-film interaction-based processes depending on laser power and scanning speed, b schematic of graphitic conversion, c laser cutting, d, e images of graphitic conversion, and f image after laser cutting [65], redrawn and reprinted (adapted) with permission from Ref. [65], Copyright (2018), Nature Publishing Group

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4.3.2 Etching Laser etching, marking, and engraving are different terms used interchangeably. Laser marking is a process that utilizes a low-power laser beam, which alters the property of the substrate. The substrate is heated, which changes its color due to oxidation, and laser marking is completed. Laser engraving uses a slightly higher laser power beam which vaporizes the substrate; the material is removed, resulting in a pattern formation visible to the eyes. Laser etching is a subset of laser engraving where the depth of the cavity is less. The laser beam melts the material, forming a pattern but of lower depth. The laser etching is 10–4 m in depth. Laser engraving has 10–4 to 5 × 10–3 m depth. Deep laser engraving is greater than 5 × 10–3 m depth. The laser pattern formation has been done for text writing, logos, stamps, serial number, barcodes, and other industrial processes. This is a quick, durable, and high-quality process [66]. The timeline for laser etching is given as follows: 1917 Albert Einstein publishes that electrons can emit light of specific wavelength. 1951 Charles Hard Townes initiates a precursor to laser known as maser. 1957 Gordon Gould conceives laser. 1960 Construction of first laser. The first continuous laser beam is also generated. 1962 Pulsed laser beams of high power were generated. 1964 Nd:YAG, CO2 laser was invented. 1978 First laser engraver was used to make patterns on wood. 1996 Software systems used in laser machines. Figure 4.3a shows the schematic of the laser beam movement path and corresponding pattern formation. Figure 4.3b represents the depth of removed material in a stainless steel workpiece due to irradiation by 14 successive laser beam pulses. Laser etching is a permanent process that can be used in various metals and non-metals. Laser etching can generate black, white, and different shades of gray. The black pattern appears when the light is absorbed due to the surface roughness, whereas it appears white when all the light is reflected back from the surface.

4.3.3 Ablation Laser ablation is an environmental-friendly process of removing material using laser energy. The material gets evaporated or sublimated by low-energy laser, whereas a higher-energy laser pulse results in plasma formation. The laser penetrates a surface depending on its wavelength and refractive index of the material. The high electric field associated with a laser creates a free electron due to higher electric force. A heating effect is created due to the collisions of free electrons with atoms of the material, and the result is the removal of material. The removal of material due to heating has been deployed to create nanoparticles and other products. The laser pulse

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Fig. 4.3 a Schematic of laser engraving, and b optical microscope image of laser engraving depth [67], redrawn and reprinted (adapted) with permission from Ref. [67], Copyright (2018), MDPI

width plays an important role in the final product of laser ablation. The interaction between laser and electrons in energy bands is determined by the time to deposit energy in the electronic states, referred to as pulse laser width. Multiple phenomena like carrier–carrier, carrier–phonon, and phonon–phonon interactions lead to the redistribution of deposited energy in the electronic states. The time taken to achieve final thermal energy distribution, which may be Fermi–Dirac or Bose–Einstein for electrons and phonons, respectively, is known as energy relaxation time [68]. Figure 4.4 shows the schematic of laser ablation where laser irradiation on a liquid medium has created a plume consisting of vapors, ions, clusters, and atoms. Laser energy must be above a threshold level to cause ablation. The ablation time is determined by mass transport. The heavier particles or metals where the energy is quickly distributed have a higher ablation time. Hydrodynamic and acoustic processes also determine the ablation time. The thermal ablation mechanism occurs when the laser ablation time exceeds the energy relaxation time. During heating, a solid–liquid twophase system is formed. However, in the case of short pulses like picosecond and femtosecond lasers, the heat-affected zones are reduced. Laser ablation is used in various diverse applications. It is used in surgery to remove tissues without causing thermal and mechanical damages. It is used in mass spectroscopy to create organic molecules or ions in the gas phase. Pulse layer deposition is an important and effective technique for synthesizing thin films [69].

4.3.4 Laser Writing Direct laser writing is also known as multiphoton lithography or direct laser lithography. Two-photon absorptions are found to change the solubility of the resist drastically. Nonlinear optical processes are involved in laser writing, but masks are unnecessary. Laser direct writing allows the addition, removal, or modification of material

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Fig. 4.4 Schematic of laser ablation process [70], redrawn and reprinted (adapted) with permission from Ref. [70], Copyright (2017), Elsevier B. V. All rights reserved

from a distance without any contact. Material processing can be done with a resolution from millimeters to micron size. The laser direct writing system consists of a laser source, laser pathway, and substrate–laser translation system. The laser is focused through a lens on the substrate material. The laser–matter photochemical interaction leads to the writing of the pattern like polymerization of polymer which happens due to laser interaction, and laser writing is completed. This process occurs at high laser intensity and high specific resolution, limiting the beam to a smaller area. The different photochemical and photophysical processes involved in direct laser writing are photoreductions, spatially confined polymerization, melting of nanoparticles, or ablation. Three-dimensional structures can also be made using direct laser writing [71]. Figure 4.5 shows the laser writing of free-standing graphite oxide paper. Laser irradiation results in the photoreduction of graphite oxide films. Reduced graphite oxide patterns are represented by black contrast, and unmodified films are shown by lighter contrast. Direct laser writing generally involves an ultrafast laser with high lase intensity and short pulse length into a small volume known as “voxel,” which is dipped in photosensitive material. The laser writing approach has been utilized for covalent 2D patterning of graphene, resulting in high functionalization. Direct laser writing using the two-photon polymerization approach has been used to create structured polymers which are adaptable, accessible, and low cost [72]. Laser writing has disadvantages that it depends on laser wavelength, which affects pattern height and morphology and is limited by the material to the pattern. This limitation can be overcome by using a nanoparticle-based solution and irradiating with a laser to form hierarchical structures by assembling nanoparticles [73]. This is known as versatile direct laser writing. Laser writing has been used to produce graphene quantum dots from graphite by photothermal gasification and recrystallization mechanism [74]. Direct laser writing has been deployed to make a microfluidic platform for cardiac tissue engineering [75].

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Fig. 4.5 Laser writing of graphite oxide in different patterns [76], reprinted (adapted) with permission from Ref. [76], Copyright (2019), Elsevier B.V

4.3.5 Laser Printing Laser printing is a high-quality digital printing process that uses an electrostatic mechanism. The different processes involved in laser printing are as follows: 1. Image processing: The picture is converted into digital format and stored in printer memory. 2. Cleaning: The cleaning of the printer is done to remove any residue particles or charges from previous printing processes. 3. Conditioning: The laser printer is warmed up, and it can now transfer the static charges from the primary charge roller to the paper and organic photoconductor cylindrical drum. This is a molecular-level process, and the drum is coated with negative charges as it is completely rolled in one revolution.

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4. Exposing: The charged drum is exposed to the laser beam, which decreases and reverses the surface charge to positive at the point of interaction. Electrons on the drum form a latent image to be printed. 5. Developing: Toner, which is negatively charged, is applied to the latent image on the drum. The toner consists of plastic particles, colored pigments, fumed silica, and control agents. Silica is used to prevent the agglomeration of toner. Control agents are zinc, iron, and chromium, which retain the negative charge on the toner particles. 6. Transferring: The second corona wire or transfer roller positively charges the paper. The toner is heated up, and a negatively charged latent image is transferred to the paper. 7. Fusing: Fusing involves the application of heat and pressure, which melts the toner and permanently bonds the image on the paper. Figure 4.6 shows the different parts of a laser printer. Laser printing is of high quality, good speed, minimal noise, reliability, and low cost on a long-term basis process. The disadvantages are bulky size. Repairs are a complex, costly, and environmental hazard of particles [78]. The history of laser printing is as follows: 1971 Gary Starkweather used a xerox machine to create a scanned laser output terminal. 1976 IBM 3800 is the first application of laser printer. 1977 Xerox 9700 is another commercially available laser printer. 1979 Cannon develops a commercial desktop laser printer. 1981 Laser printer Xerox Star 8010 is made for office use. 1984 HP LaserJet printer for mass-market sales is introduced. 1985 Apple introduced laser writer, which makes content printing independent of printer brand and resolution.

Fig. 4.6 Different components of a laser printer [77], redrawn and reprinted (adapted) with permission from Ref. [77], Copyright (2011), Elsevier B. V. All rights reserved

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Laser-induced forward transfer technique for digital transfer of graphene has also been demonstrated. CVD-grown graphene on quartz/Ni substrate is ejected by laser and deposited on Si/SiO2 substrate [79]. The laser heats the material, and restructuring of the material takes place. This phenomenon has been used in laser printing on plasmonic nanostructures and meta-surfaces [80].

4.3.6 Defect Creation Laser is used to create defects for applications. Still, the exposure of optically active materials to intense laser can cause damage to the material by melting, pitting, softening, bending, cracking, vaporization, or shattering. Laser-induced defects can be in bulk on the material’s surface or both. Laser has been used to create single and connected defects in liquid crystals due to associated electric fields. These defects can be moved within the material, and these defects occur at the site of exposure and are easily excitable. The centrosymmetric and line defects were found to be associated with the photogenerated electric field. The point defects were found to be due to the local modulation of the effective refractive index [81]. Bulk defects in ZnO and TiO2 have been reported by laser due to isochoric melting. The phase change in TiO2 is found to be accompanied by both bulk and surface defects [82]. Figure 4.7 shows the shear stress images of 300 mm silicon-on-insulator (SOI) wafers before annealing and subsequently after the first and second laser annealing steps. The defects are evolved due to laser annealing because of stress distribution. Selective laser melting has three defects porosity, incomplete fusion holes, and cracks. Pores are formed due to the low density of materials that dissolve in the molten matter, and gases get trapped. On cooling, this leads to the formation of pores. Lak of fusion is due to incomplete melting of different materials or a part or region of the material. Cracks are formed due to the rapid melting and solidification of the material [83]. Nitrogen-vacancy color center defects have been introduced by laser processing in the diamond. These have applications in quantum computing and technologies [84]. The surface density of defects in graphene depends on laser fluence and the number of pulses. A Stone–Wales defect dominates at lower laser fluence and less number of pulses, whereas single carbon vacancy, multivacancy, and divacancy are observed at higher laser fluence and more number of pulses [85].

4.4 Laser-Assisted Modification of Carbon Materials Laser–matter interactions lead to changes in the bonds and can lead to carbonization, conversion of graphite to graphene due to removal of layers, or crystallization or graphene formation. Laser-induced graphene and reduced graphene oxide with different porosities can also be formed. The modification of carbon materials by laser and the mechanisms involved are mentioned in the following sections.

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Fig. 4.7 Shear stress images before, after the first step, and second step laser annealing of SOI wafers [86], redrawn and reprinted (adapted) with permission from Ref. [86], Copyright (2008), Springer

4.4.1 Carbonization Carbonization is a process in which plastic bonds are broken due to the energy of a laser leading to the release of bonded carbon. The discoloration varies from gray color to blue-gray to black. Carbonization always results in a darker color. Carbonization is done in plastic and organic materials. Polyimide was carbonized by laser in 1991, which significantly increased the conductivity. The laser energy photothermally breaks the C=O, C–O, and C–N bonds in polyimide, leading to porous and amorphous carbonaceous structures. Carbonization also forms tetrahedral carbon (sp3 ) as in diamond-like carbon structure [87]. Polymers can be carbonized when they reach threshold energy for pyrolization process [88, 89]. Low laser power is required for carbonization as threshold energy is reached easily with low laser speed, whereas higher power is required for higher laser speed. A decrease in mass is also associated with the carbonization process as gases are also released [90]. Figure 4.8 shows the laser-patterned black carbon conducting paths printed on PET substrate. The conducting pattern is used to light an LED. The direct laser writing carbonization process can be done at ambient temperature and does not require any ink. This process has been demonstrated in lignin, wood, food, and cellulose paper [92]. The carbonization process depends on the laser type as the transmittance of the polymer, or any other material varies with different laser wavelengths. Carbonization in the Kapton tube is found to be due to the thermal energy associated with plume formation. Laser carbonization takes place in milliseconds in the case of organic materials. This suppresses the oxidation and other reactions from the atmosphere [93]. Laser carbonization is a surface phenomenon with

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Fig. 4.8 Laser-carbonized conducting carbon pattern used for LED [91], reprinted (adapted) with permission from Ref. [91], Copyright (2020), Wiley–VCH

the depth of penetration governed by the Beer–Lambert law [94]. Carbonization with high laser power is more electrically conducting, porosity due to the release of gases, but fragile carbon films are formed. The carbonized films have been transferred to PDMS for stability and stretchability, which can be utilized as strain gauge sensors. The organic materials can be pre-treated to prevent any volatility when exposed to the laser. Laser scribing has been used to prepare MXene-based hierarchical nanofibers. The β phase of electrospun dehydrofluorinated MXene-poly(vinylidene fluoride) (PVDF) nanofibers is destroyed and transformed into hexagonal sp2 -hybridized carbon structure following laser carbonization.

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4.4.2 Transformation of Graphite to Graphene The femtosecond laser has been shown to exfoliate graphene nanoflakes from graphite. The exfoliation occurs when the repulsive forces due to laser shock are greater than or equal to the intrinsic attractive forces. The exfoliation occurs when the laser is absorbed in the topmost layer of graphite, and the remaining bulk acts as a heat sink. Laser exfoliation of graphite to form graphene has been done in various media like DMF. Laser-induced intercalation followed by exfoliation of graphite in liquid nitrogen has also been reported. The liquid gas seeps in between carbon layers and reduces lattice vibration due to cryogenic temperature. The laser irradiation increases the energy, and the gas bursts out. The expansion of gas results in the exfoliation of graphite to form a few layers of graphene. Graphene nanoribbons have also been synthesized by laser irradiation of CNTs. The multi-walled CNTs unzip longitudinally on the application of laser forming graphene nanoribbons due to local heat defect sites [95]. Laser exfoliation of highly ordered pyrolytic graphite (HOPG) has been done to form free-standing 2D few layers of graphene. The final product of laser irradiation of HOPG was amorphous carbon, few-layer graphene, and thin graphite films due to the photothermal process [96]. Selective photothermolysis is used in biological systems to destroy cancer cells, and the remaining material remains unaffected or not damaged. This technique was used to exfoliate graphene from graphite and iron base. Graphite readily absorbs laser compared to the iron base, which is used for domino exfoliation of graphene from spheroidal graphite of cast iron under argon atmosphere [97]. Nitrogen-doped graphene quantum dots have also been prepared from graphite flakes as carbon sources using pulse laser deposition in liquid [98].

4.4.3 Non-crystalline Carbon to Graphene Pulse laser deposition is an important fabrication method to synthesize graphene from non-crystalline carbon. Bilayer and multilayer graphene have been formed on nickel catalyst substrates by pulsed laser deposition. The mechanism involves adsorption of carbon atoms on the catalyst surface, precipitation, segregation, and recrystallization. After laser irradiation, the carbon atoms attain high energy and diffuse in metal catalyst thin films. The segregation of carbon from carbon-metal solution occurs on cooling, and a thin layer of graphene is formed on the carbon– metal solid solution surface. Ruthenium, platinum, copper, cobalt, and iron can also catalyze graphene growth [99]. The number of graphene layers is determined by laser ablation time, and the crystallinity depends on the surface temperature of the metal catalyst surface. The cooling rate and laser energy also define the properties of synthesized graphene. The low cooling rate allows the diffusion of carbon in the metal, but a higher cooling rate saturates the carbon at the surface, leading to precipitation of the graphene. The advantage of the pulsed laser deposition method

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is that it is a lower temperature process when compared to other fabrication methods like chemical vapor deposition.

4.4.4 Laser-Induced Graphene Laser-induced graphene is the 3D porous material formed by direct laser writing on carbon materials. This 3D porous material has a high surface area in addition to the chemical, physical, and electronic properties of graphene. Polyimide was found to form laser-induced graphene by CO2 laser irradiation in an ambient atmosphere. Laser-induced graphene has been found to have five- and seven-membered rings instead of the honeycomb structure. This higher energy state of laser-induced graphene is possible due to the rapid heating and cooling of the substrate material. There is no annealing to form the six-membered rings. The properties of laserinduced graphene can be modified by changing laser parameters, substrate composition, or laser environment. The surface wettability of laser-induced graphene can be changed by varying the composition of carbon, nitrogen, and oxygen atoms, which is achieved by varying laser power and scan speed. Boron-doped laser-induced graphene can be produced by changing the substrate composition with boric acid. The porosity is dependent on the gas releasing process, which is accelerated by high laser power and results in higher porosity. Above threshold power, the structural failure of laser-induced graphene may occur [100, 101]. Figure 4.9a shows the laser-induced graphene, Figure 4.9b–d show the laserinduced graphene fibers at different magnifications, Fig. 4.9e shows laser-induced graphene at the interface of laser-induced graphene fibers and polyimide, and Fig. 4.9f shows the “R”-shaped pattern formed by laser-induced graphene fibers. Various carbon materials have been found to form laser-induced graphenes, such as polyetherimide, sulfonated poly (ether ketone), polysulfone, polyethersulfone, phenolic resin, and its composites, wood, cross-linked polystyrene, and epoxy resin. Initially, a focused laser with single scribing or multiple lasing using a focused laser on the substrate was used to form laser-induced graphene. The substrate was first converted to amorphous carbon, which subsequently transforms into graphene. Another method by using single lasing but defocusing the laser can also produce laser-induced graphene. A focused laser beam is conical, but different spot sizes are produced at the same point when defocused, and multiple lasing occurs due to overlapping. Materials with lignin and cellulose like bread, muslin clothes, wood, cotton paper, and cardboard have been converted to laser-induced graphene. The volatility in cellulose-rich materials has to be suppressed using flame retardants [102, 103]. The laser irradiation energy can be tuned to form different laser-induced graphene morphologies like graphene fibers. The irradiation energy increases and changes the graphene layers into fibers and droplets. Laser-induced graphene has been found to be biocompatible and has no toxic effect [104].

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Fig. 4.9 a Laser-induced graphene, b, c, d laser-induced graphene fibers at different magnifications, e laser-induced graphene at the interface of fibers and polyimide, and f R pattern formed by fibers [105], redrawn and reprinted (adapted) with permission from Ref. [105], Copyright (2018), Elsevier B. V. All rights reserved

4.4.5 Reduction of Graphene Oxide Laser reduction of graphene oxide has several advantages. It can be used selectively and localized phenomena like sub-micron feature size, chemical agent-free process, no additional heating requirement, convenience, and adaptability flexibility. The laser reduction can be photothermal, photochemical, or a combination of both processes. The photothermal process involves the removal of oxygen groups from graphene oxide due to the thermal mechanism by laser. It is followed by the conversion of carbon to sp2 graphene structure. The photochemical process involves the direct breaking of C–O or C=O bonds. Laser power and scanning speed are important parameters to tune the properties of reduced graphene oxide. Low laser power facilitates the conversion of sp3 carbon to sp2 carbon, whereas at higher power, the

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conversion of carbon is accompanied by the removal of oxygen atoms. C–O bonds are broken at higher laser power compared to C=O bonds [106]. An intensity-modulated line beam of a CO2 laser in a nitrogen environment has been used for high throughput reduction of graphene oxide paper to swollen and porous graphene [107]. Figure 4.10a shows the schematic of laser reduction of the graphene oxide film. Figure 4.10b reveals SEM image of laser-treated graphene oxide, and Fig. 4.10c shows the enlarged SEM image of laser-reduced graphene oxide and graphene oxide. Femtosecond laser plasmonic lithography is a recent technique for photothermal reduction of graphene oxide on a silicon substrate at high speed and large area micro/nanomanufacturing. Plasma waves are controlled, which are lossy and reversible, and their energy distribution is utilized to generate regular patterns. The plasma waves are generated in graphene oxide due to the changes in the dielectric permittivity at the surface and bulk due to the laser gradient. These waves interfere with incident light, and modulation of light energy distribution takes place, which reduces graphene oxide, and patterns are generated [108]. The gold particles are found to enhance the photoreduction when a femtosecond laser is used [109]. Picosecond laser technology has been used to form non-agglomerate and porousreduced graphene oxide in a liquid nitrogen environment, which results in a more ordered structure, defect-free, and crack-free morphology [110]. Pre-fabricated graphene oxide-based electronic devices have also been reported, which result in enhanced conductivity due to fewer defects and less oxygen sites [111]. Laserinduced reduction of graphite oxide to form graphene-like features has also been

Fig. 4.10 a Schematic of laser reduction of graphene oxide, b SEM image of laser irradiation, and c enlarged SEM image [106]. Redrawn and reprinted (adapted) with permission from Ref. [106]. Copyright (2019), Elsevier B. V. All rights reserved

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achieved. The laser induces electron–hole plasmas, which get trapped due to the strain associated with oxygen groups. The exciton trapping and hole localization result in material removal. Electron–phonon coupling occurs in graphite oxide, which causes rapid local heating and reduction of graphite oxide by breaking bonds. CO and CO2 are released in the process [112].

4.5 Laser-Derived Material in Supercapacitor Carbon-based materials generally show electrochemical double-layer capacitance. Transition metal oxides and conducting polymers are known to show pseudocapacitance. Hybrid supercapacitors show both types of behavior. These materials can be produced with the help of a laser. Different types of laser-derived materials in each type of supercapacitor in current literature are discussed in the following sections.

4.5.1 Electrochemical Double-Layer Capacitors Electrochemical double-layer capacitors generally consist of carbon-based materials which form electric double layers and give rise to capacitance. Lignin was deposited on carbon cloth, and direct laser writing was done to form 3D porous graphene, which also bonded with carbon. Carbon cloth acts as a good substrate for bonding, and also, it is a good current collector and flexible. The lignin-carbon cloth electrode shows a quasi-rectangular C–V curve representing excellent electric double-layer capacitance. The GCD curves are quasi-rectangular, which means good capacitive behavior and reversibility during charging and discharging. The areal capacitance of the electrode was determined around 573 mF cm−2 at 1 mA cm−2 [113]. Laserassisted aluminum microgrid vertically aligned CNT (VACNT) electrodes have been prepared. Laser etching is done to increase the contact strength between VACNT and substrate and also active mass loading. At various scan rates, the CV and GCD curves exhibited an electric double energy storage mechanism. These electrodes have shown an areal capacitance of 1.3 F cm−2 at 13 mA cm−2 [40]. Laser-reduced graphene oxide is also an important material for the electrochemical double-layer capacitor. Two laser beam interference with masking techniques have been used for graphene oxide photoreduction and hierarchical structuring. The planar supercapacitor with an interdigital pattern was fabricated using a mask with a two-beam laser interference process. This supercapacitor has an areal capacitance of 3.97 mF cm−2 at 10 mV s−1 [114]. Laser-induced graphene shows electric doublelayer capacitance, but the specific capacitance varies from laser-reduced graphene oxide and depends on the type of laser, i.e., CO2 or UV laser. This is due to the differences in the product formed by the two processes, as the content of oxygen has an effect on equivalent series resistance. The degree of recrystallization and porosity

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also depends on the types of laser and parameters and different processes involved in laser-based fabrication.

4.5.2 Pseudocapacitors Pseudocapacitors involve an instantaneous charge transfer mechanism and report high capacitance. Transition metal oxides show pseudocapacitance due to valence states and can be produced by pulsed laser deposition. Conducting polymers also exhibit pseudocapacitance due to charge transfer in branches. Different types of laser-based polymers and transition metal oxides used in pseudocapacitance are mentioned here.

4.5.2.1

Conducting Polymer-Based Pseudocapacitors

Laser-assisted carving method has been used to prepare paper-based supercapacitors coated with polypyrrole. The CO2 laser was used to form patterns on polypyrrole, which act as in-plane electrodes for supercapacitor. The planar supercapacitor has an areal capacitance of 13.9 mF cm−2 , which is higher than the sandwichtype device displaying 12.3 mF cm−2 [115]. Polymerization of pyrrole-3-carboxylic acid and patterning has been achieved through laser in an aqueous solution. The laser polymerization depends on the substrate material, materials in aqueous solution and their composition, and laser parameters like laser power [116]. Red mud nanoparticles prepared by mechanical milling have been decorated on laser-induced graphene on a polyimide substrate. The supercapacitor exhibited pseudocapacitance and areal capacitance of 203 mF cm−2 [117]. Laser scribing has been used to directly form polycarbonate laser-induced graphene nanocomposite from a polycarbonate/polyetherimide blend. These have highly DC electrically conductive material like 20–400 S m−1 [118]. Laser-reduced graphene oxide/polyaniline nanofibers’ composite has been synthesized, which has applications in supercapacitor. Polyaniline increases the surface area directly related to good capacitance and pseudocapacitive material and also prevents restacking of graphene sheets during supercapacitor fabrication and charging/discharging. The graphene oxide is reduced by laser, and polyaniline is electrodeposited. The gravimetric or specific capacitance of electrochemical capacitors is 442 F g−1 at a current density of 0.18 A g−1 [119]. Polyaniline nanofiber hydrogel can also be directly used as electrodes without using a binder and has good mechanical properties. The hydrogel coated on PET film can be patterned and photothermally welded using a laser for supercapacitor application [120]. Direct metal laser sintering has been done, creating 3D metal scaffolds co-electrodeposited by transition metal oxides and conducting polymers with applications in energy storage devices [121].

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Metal Oxides-Based Pseudocapacitors

Polyimide substrate has been CO2 laser irradiated to form graphene-interdigitated electrodes pattern. MnO2 is deposited on the laser-induced graphene forming a composite via self-limiting process and contains pseudocapacitive properties. The supercapacitor formed has a volumetric capacitance of 1.7 F cm−3 and an energy density of 0.15 mWh cm−3 at a scan rate of 10 mV s−1 [122]. Cobalt-doped nickel hydroxide nanosheets have been prepared by utilizing laser-induced cobalt colloid in the hybrid medium of nickel ions and sodium thiosulfate. The mechanism of laser interaction with cobalt substrate is that the bulk cobalt is converted into plasma and cobalt clusters. These ultrafine cobalt clusters are reactive and metastable, which get doped into nickel hydroxide lattice. Sodium thiosulfate prevents the growth of cobalt clusters and acts as an evocating agent facilitating doping. Cobalt-doped nickel oxide is formed on the further thermal decomposition of cobalt-doped nickel hydroxide in air. The Co-doped Ni(OH)2 has a specific capacitance of 1421 F g−1 at a current density of 6 A g−1 and a cycle life of 76% after 1000 cycles [123]. Pulse laser deposition of porous nickel oxide films has been done for application in pseudocapacitive energy storage. The nickel targets are irradiated with laser, which lead to the ablation of nickel atoms and ions and their reaction with oxygen to form nickel oxide thin film. The electrode based on NiO thin film is porous and has a specific capacitance of 835 F g−1 . A cycle life of up to 1000 cycles with no degradation is achieved [124]. CO2 laser has been utilized to convert metal–organic framework MOF-74(Ni) to nickel nanoparticles in porous carbon having a specific capacitance of 925 F g−1 [125]. Co3 O4 nanocrystals were synthesized by laser ablation of a cobalt target in the aqueous medium. ITO-coated PET is used as a substrate for dispersing cobalt oxide nanocrystals and fabrication of a supercapacitor. The pseudocapacitor has a specific capacitance of 177 F g−1 at a scan rate of 1 mV s−1 and a 100% retention rate after 2 × 104 cycles [126]. Mo-gelatin hydrogel has been coated on a polyimide substrate and irradiated with a laser to form interdigitated pattern acting as a supercapacitor. This supercapacitor can function at a wide temperature range from −50 to 300 °C in electrolyte solution [127].

4.5.3 Hybrid Supercapacitors An in-plane hybrid micro-supercapacitor has been made with anode as Fe3 O4 nanoparticle-supported porous laser-induced graphene and cathode as laser-induced graphene. Polyimide substrate was converted into laser-induced graphene, and the Fe3 O4 nanoparticles were deposited on the graphene by multiple laser overlapping scans and annealing. Fe3 O4 nanoparticles are obtained by laser, which oxidizes ferric chloride. The areal capacitance was shown to be 719.28 mF cm−2 at 2 mV s−1 and cycle life retention of 74% after 900 cycles. The mechanical flexibility is 94% after 2000 bending cycles [128]. Reactive inverse matrix-assisted pulsed laser evaporation has been used to form surface layers of bimetallic oxide nanoparticles, reduced

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graphene oxide, and multi-walled carbon nanotubes that act as electrodes for supercapacitors. The dispersion of graphene oxide platelets, multi-walled carbon nanotubes, nickel oxide, and zinc acetate was irradiated by Nd:YAG laser, which reduced graphene oxide and formed a surface with a combination of materials. The volumetric capacitance is found to be 40 F cm−3 at a 10 mV s−1 scan rate, energy density of 1.5 mW cm−3 , and power density of 12 W cm−3 at a current density of 4 mA cm−3 [129]. Cobalt/reduced graphene oxide composites have been prepared using a single laser scribing process. PET films were used as a substrate, and cobalt/graphene oxide was deposited on it, on which laser irradiation reduces graphene oxide and forms a cobalt-based composite. The micro-supercapacitors made from the electrodes of this composite have a specific capacitance of 2.27 F cm−3 [130]. A composite of laser-scribed graphene and LiNi1/3 Mn1/3 Co1/3 O2 (LSG/NMC) has been synthesized for hybrid supercapacitors. This composite has a capacitance of 141.5 F g−1 , and after 1000 cycles, there is 98.1% capacitance retention at a current density of 5 A g−1 . The supercapacitor with this composite acting as cathode and H2 T12 O25 anodes with AlPO4 /carbon hybrid coating layer exhibit an energy density of 123.5 Wh kg−1 , power density of 14,074.8 W kg−1 , and cycle life of 20,000 cycles with 94.6% stability [131]. Another similar supercapacitor having only laser-scribed graphene as cathode and AlPO4 /carbon hybrid-coated H2 T12 O25 has been designed. It has an energy density of 70.8 Wh kg−1 and a power density of 5191.9 W kg−1 .

4.6 Direct Laser-Based Fabrication of Micro-supercapacitor The polyimide sheet is directly irradiated with a laser to form porous interdigitated electrodes for a flexible micro-supercapacitor. The laser irradiation carbonizes the polyimide substrate and forms conducting patterns that act as electrodes. Polyvinyl alcohol–phosphoric acid gel solid electrolyte is used in the micro-supercapacitor. The specific capacitance is 800 μF cm−2 at a scan rate of 10 mV s−1 [132]. The micro-supercapacitors have been developed using Kirigami constructs, making the device highly stretchable and flexible. The CO2 laser is used to convert polyimide into graphitic features, and also cutting is done for pattern formation when the heat generated by the laser is above a threshold level. The electrolyte solution consists of polyvinyl alcohol, phosphoric acid, and deionized water. The areal capacitance of supercapacitor varies from 1.125 mF cm−2 (500 mV s−1 scan rate) and 3.654 mF cm−2 (20 mV s−1 scan rate). The energy density is 0.32 μWh cm−2 , and the power density is 11.4 μW cm−2 . There is a negligible difference in the capacitance during the elongation of laser-formed constructs to 382.5% of the initial length [65, 133]. A micro-supercapacitor with manganese dioxide nanowire arrays has been developed. The laser-induced hydrothermal process was used to synthesize nanowire arrays for application as electrodes in a supercapacitor. MnO2 nanoparticles were

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dispersed on gold/titanium-coated glass substrate, which was placed on potassium permanganate solution in deionized water in a polydimethylsiloxane chamber. Laser photothermal reaction leads to nanowire arrays on MnO2 nanoparticles seeded glass substrate. A comb pattern is formed by laser ablation of a metal layer of gold/titaniumcoated glass substrate. Polyvinyl alcohol and lithium perchlorate gel electrolyte was used in the supercapacitor. The areal capacitance is 227 mF cm−2 at a current density of 1 mA cm−2 and a cycle life of 3000 cycles with 84% stable capacitance retention. It has an energy density of 30.9 μWh cm−2 and a power density of 1112.2 μW cm−2 [134]. Polyimide sheets have been irradiated with a laser to form laser-induced graphene foam patterned as interdigitated electrodes. ZnP nanosheets have been spray-coated on them to form hybrid electrodes for supercapacitors. PVA/KCl was used as a polymeric gel electrolyte in the supercapacitor. These hybrid electrodes have a good capacity varying between 1425 F g−1 (1 A g−1 ) and 926 F g−1 (30 A g−1 ). The cycle life is 5000 cycles with a capacity retention of 68.5% in Na2 SO4 aqueous solution electrolyte [135]. Boron carbon nitride-based electrodes for micro-supercapacitors have been developed using laser patterning. A mixture with 80 wt% boron carbon nitride, 15 wt% carbon, and 5 wt% of polyvinylidene fluoride was prepared. N-methyl-2-pyrrolidone was used for the homogeneous viscosity of the mixture. The mixture was coated on ITO/PET substrate, and laser patterning was done to form interdigitated electrodes. PVA/H2 SO4 gel electrolyte was used for the device. The micro-supercapacitor has a specific capacitance of 72 mF cm−2 at a current density of 0.15 mA cm−2 . The cyclic stability has been measured for 8 × 104 cycles with no degradation and high flexibility of bending angle 150° for 1500 cycles [136]. The deposition of MnO2 nanosheets has been done on a 3D Si/C hybrid structure using the hydrothermal method. This hybrid material is deposited on glass with glue, and the connected electrodes were divided into anode and cathode using the laser scribing technique. LiCl/PVA was prepared for usage as a gel electrolyte. The Si/C/MnO2 micro-supercapacitor have a specific capacitance of 29.45 mF cm−2 , a power density of 117.82 μW cm−2 , and an energy density of around 2.62 μWh cm−2 at 10 mV s−1 [137]. Figure 4.11 shows a schematic of different steps in the preparation of graphenebased planar micro-supercapacitors on an alumina ceramic substrate by the direct laser writing process. Laser-induced graphene has been prepared by laser irradiation of polyimide to form electrodes for micro-supercapacitors by various groups. PVA/H3 PO4 gel electrolyte layer has been prepared and deposited on the electrodes. A novelty has been reported in device assembly by laser carving of patterns on the deposited solid electrolyte, converting the gel into conductive carbon materials. These high-voltage micro-supercapacitors were cut from polyimide substrate, and the final device consisted of nine micro-supercapacitors in series connection. The potential window is measured to be 10 V and areal capacitance of 244 μF cm−2 . The capacitance retention is 90% after 104 cycles and performs well even after bending at various angles [138]. Spatially shaped femtosecond laser, which can produce directly desired patterns by various beam shapes due to phase modulation, has been used to prepare laser-induced graphene/MnO2 micro-supercapacitors. This is like a photonic stamp instead of direct laser writing and can be used to produce more than 3 × 104

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Fig. 4.11 a Schematic of different processes involved in a direct laser patterning of a supercapacitor. Optical images containing b 12 interdigitated electrodes and c 18 interdigitated electrodes [141], redrawn and reprinted (adapted) with permission from Ref. [141], Copyright (2020), Springer

micro-supercapacitors within 10 min. A single micro-supercapacitor has an energy density of 0.23 Wh cm−3 and a specific capacitance of 128 mF cm−2 [139]. Laser cutting has been done to form interdigital patterns on a nickel foil coated with MXene on both sides. The areal capacitance is 52 mF cm−2 . The micro-supercapacitors can be fabricated in series or parallel connections through laser cutting [140]. Table 4.1 shows the laser parameters to fabricate interdigitated micro-supercapacitors and its electrochemical analysis. Direct laser patterning of MXene films deposited on the paper substrate has been done to synthesize flexible micro-supercapacitors. Ti3 C2 Tx MXene inks were prepared and spray-coated on paper. Laser scribing was done through a 355 nm ultraviolet laser to form interdigitated electrodes, and polyvinyl alcohol and H2 SO4 gel solid electrolytes were placed over the interdigitated electrodes. The width of the interdigitated electrodes is 0.76 mm, the separation between neighboring electrodes is around 224 μm, and the thickness of the deposited MXene layer is 7.9 μm. The supercapacitor is encapsulated with polydimethylsiloxane. The areal capacitance is 23.4 mF cm−2 with 81.4% rate capability. The device has a good flexible performance by retaining 92.4% capacitance after 5000 cycles. The volumetric energy density was found to be 1.48 mWh cm−3 at a power density of 189.9 mW cm−3 [142]. Doping of graphene leads to enhanced capacitance due to the incorporation of pseudocapacitance. Laser engraving has been done to prepare 3D oxygen and sulfur co-doped graphene electrodes having porosity and wettability for enhanced

PVA/H2 SO4

10.6 μm wavelength

Polyimide films

CO2 laser

1.0 M H2 SO4 and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4 )

Laser wavelength was 10.6 μm, and pulse duration was around 14 μs. The beam size was ~ 120 μm. Laser power had 0.6 W increment varying from 2.4 to 5.4 W

1.0 M Na2 SO4 and 1.0 M tetraethylammonium tetrafluoroborate (TEABF4 ) in anhydrous acetonitrile

Power = 2.4 W; scanning speed, 30%; wavelength = 10.6 μm

CO2 laser

Hydrated graphite oxide films; 22 μm in thickness

Polyimide and CO2 laser polyetherimide films

0.1 M Na2 SO4

9 mF cm−2 (0.02 mA cm−2 )

> 4 mF cm−2

Thick laser-induced graphene layer (∼25 μm)

Spacing = 300 μm

1 mF cm−2 (aqueous), A rectangle with 2.5 mF cm−2 dimensions 2 cm × (organic) 5 mm having five 3.5 mm × 1.5 mm equally distributed branches; concentric circles patterns, in-plane, and sandwich structures

Pattern features Area is 1.14 cm2 (length = 19 mm and width = 6 mm) Each electrode length is 5 mm length, reduced width is 100 μm, and unreduced spacing of 400 μm between electrodes

Capacitance 0.2 μF mg−1 , 0.9 μF cm−2 , and 4.2 mF cm−3

Laser parameters/wavelength Electrolyte

Power = 244 mW at 2.5 mm s−1 scan speed with laser pulse frequency around 0.9 kHz. Laser fluence is 2.5 × 10–4 J mm−2

Laser

Nd:YVO4 pulsed laser

Substrate/thickness

Graphite oxide film (diameter is 35 mm and thickness is 2.2 μm)

Table 4.1 Laser parameters for interdigitated micro-supercapacitors and its features with electrochemical analysis References

(continued)

[149]

[148]

[147]

[146]

112 R. Nigam et al.

355 nm wavelength

Nd:YVO4 UV laser

NaOH, Na2 SO4 , and KCl electrolytes

2.40, 2.23, and 1.62 μF cm−2

Electrode length is 5 mm and 180 μm wide, separation of unreduced GO is 290 μm

(continued)

[152]

Electrode length = [132] 0.5 cm. The separation between two electrodes was ∼90 μm

[151]

4.0 μm-thick graphite oxide

800 μF cm−2 at 10 mV s−1

Electrode width and length were 300 μm and 5 mm. Thickness was ~ 7 μm with 100 μm gap

Pulsed Wavelength = 522 nm, pulse Polyvinyl femtosecond lasers duration around ∼500 fs at alcohol–phosphoric acid repetition rate of 1 MHz (PVA–H3 PO4 )

9.05 mF cm−2 , 12.92 mF cm−3 (0.05 mA cm−2 )

Polyimide sheet (Kapton® HN, 125 μm thickness)

PVA–H2 SO4 (1 M) gel electrolyte

Power range is 4.8–5.2 W, wavelength is 10.6 μm, and resolution is 1000 pulse per inch

CO2 laser

Mixture of mushroom-derived carbon, acetylene black, and PVDF deposited on indium tin oxide/polyethylene terephthalate (ITO/PET) substrate

References

16.5mF cm−2 at Twelve interdigitated [150] 0.05 mA cm−2 current electrodes having density length = 5 mm, width = 1 mm, and a spacing ∼300 μm between two neighboring microelectrodes

Pattern features

Capacitance

10.6 μm wavelength, laser PVA and H2 SO4 power = 4.8 W when the laser scan rate was ∼8.9 cm/s

Laser

Laser parameters/wavelength Electrolyte

Substrate/thickness

Poly(amic acid) CO2 laser solution with H3 BO3

Table 4.1 (continued)

4 Laser as a Tool for Fabrication of Supercapacitor Electrodes 113

Nd:YVO4 UV laser

CO2 laser

Universal X-660 laser cutter

Lignin/polyvinyl alcohol (PVA) composite film

MoS2 film on Si/SiO2 wafer

Metal–organic CO2 laser framework (MOF-199@ZIF-67)

Laser

Substrate/thickness

10 μm-thick GO paper

Table 4.1 (continued)

Power 2.4 W (25–30%), scanning speed = 20–25%

1 M NaOH

The widths of the finger electrodes are 1 mm, and the lengths of LSG finger electrodes are 5 mm

8 mF cm−2 when scan Finger electrodes: rate is 10 mV s−1 4.5 mm length by 820 μm width with 200 μm spacing

25.1 mF cm−2

1 M H2 SO4 /PVA gel

10.6 μm

Electrode length = 5 mm and width = 100 μm. The spacing of unreduced GO was 400 μm

9.3 μF cm−2 and 13.8 μF cm−2

Length of one electrode is ~ 1 mm, the width of one electrode is ~ 5 mm, and the spacing between two adjacent electrodes is 300 μm

Pattern features

Capacitance

8.1 mF cm−2 at 1 mV s−1

KOH and NaCl

Maximum laser power = 60 1 M H2 SO4 watts, Maximum scan speed = 1.78 m s−1 . Diameter of the laser spot size = 120 μm

355 nm wavelength

Laser parameters/wavelength Electrolyte

[155]

[154]

[153]

[76]

References

114 R. Nigam et al.

4 Laser as a Tool for Fabrication of Supercapacitor Electrodes

115

electrochemical performance in supercapacitor. Initially, laser-induced graphenebased electrodes are prepared on polyimide sheets and subsequently coated with sodium lignosulfonate. Duplicate laser scribing is done to form interdigitated electrodes. The thickness of electrodes was around 65 μm for different samples. The areal capacitance is 53.2 mF cm−2 , the optimal energy density is 4.73 μWh cm−2 , and the maximum power density is 1.6 mW cm−2 . The capacitance retention is also 81.3% after 8000 cycles [143]. Laser writing of fluorinated polyimides to produce laser-induced graphene also leads to enhanced electrochemical performance due to the large specific surface area of around 1126 m2 g−1 . The fluorine groups lead to the release of additional gases like CHF2 , CH2 F, and COF2 during photothermal reactions, which results in porosity and enhanced area. Figure 4.12 is a schematic of enhanced microporous graphene from laser-patterned fluorinated polyimides. The areal capacitance of micropatterned electrode in the H2 SO4 aqueous electrolyte is 110 mF cm−2 . The micro-supercapacitors based on them have a high working potential of 3 V, energy density of around 0.01 mWh cm−2 , and power density of 0.58 mW cm−2 [144]. Laser printing has been used to print sacrificial patterns on PET substrate. The platinum thin films are sputtered over it, which is followed by the removal of the toner sacrificial patterns. This leads to the formation of interdigitated fingers over which MnO2 is electrodeposited. PVA/LiCl is used as a gel electrolyte. This device

Fig. 4.12 a Schematic of laser-prepared highly microporous graphene due to the presence of fluorene groups b Laser-prepared 3D porous graphene for polyimides [144], reprinted (adapted) with permission from Ref. [144]. Copyright (2021), ACS Publications

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has a high surface area and large number of reactive sites due to nanobranches of MnO2 , which results in good electrochemical performance. The specific capacitance is found to be 615.14 F cm−3 , power density of 11.677 W cm−3 , and energy density around 54.679 mWh cm−3 . The micro-supercapacitors have excellent flexibility with negligible degradation in capacitance at 180° bending angle [145].

4.7 Conclusions and Future Perspectives Laser technology is an important tool for modifying and patterning carbon-based materials through photothermal, photochemical, or combined mechanisms. The laser parameters like power, intensity, and scan time influence the photon–matter interactions and final product formation. Various laser-associated processes like cutting, engraving, etching, ablation, or writing are used interchangeably but have slight important differences between them. The laser can directly form conductive patterns on insulting materials like polymers through carbonization or other processes. These properties have been used in various applications, including micro-supercapacitors by patterning conducting interdigitated electrodes. Micro-supercapacitors have utilization in sensors or wearable devices with flexibility. The device assembly of supercapacitors has been directly done through laser assistance for forming series or parallel combinations. This will be very useful in forming direct integration on micro-supercapacitors on chips like on-chip micro-supercapacitor. Another futuristic application of laser-based micro-supercapacitors is coupling energy storage mechanisms with trapped states in materials, introducing quantum electronic and computing devices. The power bursts through micro-supercapacitors can be utilized alone or combined with batteries in defense, security, and civilian applications.

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Chapter 5

Scalable Supercapacitors Snehraj Gaur, Ajay B. Urgunde, Gaurav Bahuguna, S. Kiruthika, and Ritu Gupta

Abstract In the past few decades, energy-storage technology has evolved rapidly as dependence on renewable energy sources have increased due to drastic changes in energy demands. A supercapacitor finds many applications that need high peak power and energy boosts, such as wireless sensor networks, regenerative braking in vehicles, IoT applications, RF transmissions, backup power supply, transport sector, energy harvesting systems, industrial and consumer electronics. Though the lab-scale supercapacitors perform well, there is considerable scope of improvement for commercially scalable supercapacitors. Low-cost, simple-processing, and highperformance material provides a possible solution for large-scale industrial efficient energy storage systems that can bridge the gap between lab-based energy storage technologies and large-scale commercial applications. The performance deteriorates with an increase in the size of devices due to the internal resistances from non-active materials such as binders and additives, and heating issues. To address these challenges, designer electrode structures such as self-standing architectures, mesh-type electrodes, and fractal design can be viable solutions to enhance the performance of large-scale energy storage devices. Industrial byproducts in the form of waste can be recycled and processed to synthesize cost-effective electrode materials. In addition, the fabrication of electrodes by printing techniques and additive nanomanufacturing has gained significant scientific attention as they are cost-effective and economical for the production of energy storage devices. Printing techniques such as inkjet, micro-gravure, and 3D printing possess the merit of easy manufacturing steps to produce scalable supercapacitors.

S. Gaur · A. B. Urgunde · G. Bahuguna · R. Gupta (B) Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, Rajasthan, India e-mail: [email protected] S. Kiruthika (B) Department of Physics, School of Electrical and Electronics Engineering, SASTRA Deemed to be University, Thanjavur, Tamil Nadu, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_5

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5.1 Introduction With the industrialization and increase in the population, the energy demand has increased proportionally. Energy storage devices are needed to facilitate renewable energy sources as alternatives by collecting energy, energy storage, and efficient use of stored energy [1]. The current supercapacitor market includes applications for the transport sector, industrial/commercial electronics, consumer electronics, clean technology, energy harvesting systems for devices that are based on IoT, etc. For commercial supercapacitors, the voltage range is 2.3–3.0 V with the highest energy density of 7.5 Wh kg−1 for symmetric cells, while asymmetric cells achieve 8.8 Wh kg−1 and 37 Wh kg−1 in the case of hybrid cells [2]. A wide operating temperature range up to −60 °C is one of the key features that makes them advantageous over batteries [3]. Applications of supercapacitors include peak-power load in RF transmissions, load leveling in electric-hybrid vehicles, starting of engines, and uninterruptable power supply (UPS) installations. Supercapacitors find applications in fields that need high peak power or energy boost. Because of their high power density, supercapacitors are used in cars and railway transportation with regenerative braking systems. One of the typical ways to use supercapacitors in transportation applications is to charge them from waste energy while braking and discharge energy while accelerating. For example, Mazda i-ELOOP claims fuel saving of ~10% while using supercapacitortype regenerative braking [2]. Furthermore, supercapacitors are also used to deliver auxiliary accelerative power to start the main engine in vehicles. The frequent stopand-go driving conditions of buses can utilize supercapacitors saving fuel resources. For example, Capabuses with two-roof mounted pantographs are developed for flash charging by the supercapacitor-based energy storage system at bus stops, providing sufficient energy for driving between two adjacent stops [2]. Though remarkably high performances have been achieved in supercapacitor technology at lab scale in terms of high energy density and power density, with excellent charge–discharge rates, more advancement is needed to bring these technologies from the lab to practical, real-world applications on a large scale [4]. Despite numerous efforts in nanotechnology for energy storage devices, improving the device performance with reduced manufacturing cost at a large scale remains a significant technological challenge. One of the ways to address this issue is by manufacturing highefficiency supercapacitors in high volume following cost-effective techniques. The scaling of supercapacitors can be achieved either by parallel and series connections of high-performance supercapacitors or by fabricating large-area printed electrodes for supercapacitors [5]. Therefore, low-cost, simple-processing and device architecture and high-performance material provide a possible solution for large-scale industrialefficient energy storage systems that will bridge the gap between lab-based energy storage technologies and commercial applications by enhancing the performance at large scale.

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5.2 Challenges in Scalable Energy Storage Devices Several challenges are associated with the scalability of the energy storage devices, which majorly include heating of the device, high cost, and performance limitations. In the following sections, the associated challenges are described briefly.

5.2.1 Degradation in Performance One of the critical hindrances in scalability to high volume is reducing cycling stability and energy storage performance due to undesired electrochemical reactions and accumulation of byproducts that increase the internal resistances. The scalability is affected by various performance factors such as energy density, power density, gravimetric capacitance, and they differ much from the lab-scale devices. The capacitance is inversely proportional to the distance between the electrodes and directly proportional to the electrode area [6]. Eventually, by increasing the area of electrodes, the capacitance of a supercapacitor should increase. In contrast, the performances of supercapacitors decline drastically with an increase in the area. As the scalability increases, more inevitable components and binder/additives, current collectors, separator, and cell assembly add to the mass and volume of the supercapacitor [7]. Therefore, reporting the power and energy density per unit weight of active material may not give a realistic picture of energy storage performance at the large-scale assembly because the weight of other device components must also be considered [4]. One of the ways to solve this issue is to use self-supported binder-free electrode materials. High surface area binder-free electrodes can enhance electron adsorption/transfer and enable a preferable connection between the active materials and current collectors. Various approaches like Chemical Vapour Deposition (CVD), vacuum filtration, hydrothermal/solvothermal growth, electrospinning, electrophoretic, and electrochemical deposition, aerogel production are adopted for binder-free electrode fabrication in energy storage applications [8]. However, laboratory-scale devices have achieved high specific capacitance owing to the porous nature of the materials. Since these materials have low packing density and large pore volume, the use of a large amount of electrolyte eventually adds to the device’s mass without enhancing capacitance [9]. There have been efforts in the literature to decrease the “dead volume” of the electrolyte by decreasing the macropores while maintaining the mesopores for high capacity with low electrolyte usage [10]. The essential factor for scaling supercapacitors without compromising the performance is to develop high specific surface area materials with many active sites without degradation of electrochemical performance, along with a significantly lower cost of fabrication.

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5.2.2 Cost-Effectiveness One of the limiting factors for the production of supercapacitors is cost-effectiveness, which includes the cost of raw materials and processing. Some transition metal oxides, such as ruthenium oxide, are well known for their exceptionally high theoretical and experimental capacity and low electrochemical series resistance. However, its scalability for widespread application is limited by the soaring cost [11]. Even the comparatively economic transition metal oxide-based supercapacitors are inherently expensive, almost 100 times higher than the carbon nanomaterials, which restrict their production at large scale [12]. Therefore, cost-effective materials from industrial waste have been studied, which provide competitive cost-per kilo-watt but suffer in terms of performance, such as low conductivity and absolute capacitance. Some of the waste materials such as plastic waste, cotton, rice husk, coconut shell, and other agricultural waste have been explored, which are cheap as raw materials but have cost burden processing due to complicated processing steps. Therefore, large-scale, cost-effective production of raw materials and their processing is crucial for scalable supercapacitors.

5.2.3 Heating Issues Most commercial supercapacitors are cylindrically manufactured by a rolling jelly consisting of current collectors made of Aluminium foil with electroactive electrode materials on both sides. These electrodes are separated by a porous paper separator and height adjustable according to desired capacitance value. This trend was addressed by Maxwell Technologies, the world’s leading manufacturer. Despite easy automation of this manufacturing technique which makes it cheaper to use, the technology suffers a major heating drawback. The heat generation in supercapacitors is due to the resistive losses, i.e., Joule heating. The heat generated must be evacuated through conduction for better performance. The parasitic electrochemical reactions of electrolytes are enhanced at higher temperatures, resulting in decreased capacitance with aging and increased internal resistance [13]. Therefore, even with high efficiencies and low-power losses, the temperature remains the limiting factor for maximum manageable power. The pouch supercapacitors have been introduced in the market with improved thermal conductivities. These are manufactured by stacking the electrodes, current collectors, and separators sealed in a plastic bag. In stackedtype or pouch supercapacitors, the highest temperature is at the center position, and the heat radiation is better at this position leading to good heat dissipation [14]. Although designed pouch supercapacitors offer improved thermal conductivities, they are usually more expensive than cylindrical ones [2].

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5.2.4 Voltage Imbalance When supercapacitors are stacked and coupled in series, the leakage current might cause overvoltage and damage to some of the supercapacitors. Impurity ions in the lattice can penetrate the electrodes during charging/discharging, thus contributing to leakage current/self-discharge and other performance degrading parameters. Extremely clean electrode materials are the key to low self-discharge supercapacitors, requiring highly sophisticated and costly synthesis techniques [15]. As a result, an economical alternative to safeguard the stack from damage is using a balancing circuit. The balancing circuits are commonly employed to bypass the charging current to prevent overvoltage on each capacitor terminal. The major issue with interconnected supercapacitors is the voltage difference generated across individual supercapacitors, thus deteriorating the overall supercapacitor performance. This voltage mismatch arises due to the variations in the electrode area increasing the equivalent series resistance (ESR), self-discharge rate, and electrolysis of the electrolyte in the supercapacitor, ultimately leading to the degradation in the performance of the supercapacitor.

5.3 Ways to Address Challenges for Large-Scale Supercapacitors The challenges with the scalability can be approached in several ways, out of which electrode structure, cost-effectiveness, and device architecture are elaborated below.

5.3.1 Geometry/Electrode Structure Self-standing architectures play a crucial role in scalable supercapacitors as the contribution of binder is negligible, which acts as a non-active material and inhibit the scaling of capacitance for large-scale fabrication. The electrode material is used directly on the conductive surface, enhancing the gravimetric energy density in supercapacitors [16]. The performance of supercapacitors is enhanced in the 3Dnanostructured printed electrodes because of the high surface area available for electrolyte diffusion, direct contact of the active material to the current collector, and 1D transport of electrons. The possible nanostructures for self-supported electrodes include nanowires, nanorods, and nanotubes with a thin film, the thickness of several microns. Few methods to prepare 3D self-supported electrodes include the hard template method [17], lithographic techniques [18], solution-based techniques such as electrodeposition [19], hydrothermal techniques [20], and anodization [21]. Other strategies include employing mesh-type electrode structures with a high specific surface area than plane plate supercapacitors. The mesh electrodes offer

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efficient ion-diffusion pathways with better flexibility. Stainless steel mesh electrodes have the advantage of low-cost, lightweight, excellent mechanical strength, high bending endurance, flexibility, stability, etc. Further, the pseudocapacitive active material such as MnO2 ink can be easily dip-coated on the electrodes, contributing to simplistic manufacturing steps. Moreover, they also demonstrate high current density, high specific areal capacitance, better flexibility, and stable electrochemical performance even after severe mechanical deformations (bending and twisting) due to their open pore structure and larger specific surface area. Based on the performance and quality of chosen pseudocapacitive material, mesh-based supercapacitors show comparable performance to graphene-based supercapacitors. Due to the high integration level of mesh, fabrication of large-area supercapacitors is easier, facile, and fully compatible with industrial processing [22]. Micro-supercapacitors (MSCs) have a total footprint area in millimeters or centimeters, but they usually consist of a thin film of thickness 90% over 1200 cycles

DIW/ [30]

PEDOT:PSS, carbon nanotubes (PVA–H2 SO4 )

730 mF/cm2

74.7% after 14,000 cycles

FDM/ [24]

Graphene, molybdenum sulfide (PVA–H2 SO4 )

4.15 mF/cm2

90% after 10,000 cycles

>99% after 2000 cycles

(continued)

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Table 6.1 (continued) Method

Electrode (Electrolyte)

Capacitance

Cyclic stability

DIW/ [39]

MXene or Ti3 C2 Tx (PVA–H2 SO4 )

2.1 F/cm2

>90% after 10,000 cycles

DIW/ [29]

Single-walled carbon nanotubes (PVA–Lithium chloride)

1.94 F/cm2

95.6% after 5000 cycles

DIW/ [43]

Carbon aerogel (Tetraethylammonium tetrafluoroborate in 1:1 acetonitrile/methyl formate)*

148.6 F/g

–

DIW/ [23]

Graphene aerogels, manganese dioxide (Lithium chloride)*

18.74 F/cm2

92.9% after 20,000 cycles

*

Aqueous electrolyte

behavior, sufficient viscosity, shear elastic modulus, and shear yield strength to allow for accurate deposition.

6.6.2 Materials Analysis of available literature on 3D printed supercapacitors indicates that one of the main challenges is the limited device capacitance [18, 21, 26]. This is an aspect that is largely influenced by the materials used for the electrode, and to a lesser extent, for the electrolyte. Therefore, it is critical to consider the materials aspect. While most of the 3D printed supercapacitors developed till now have employed graphenebased electrode materials, the recent forays into MXene-based materials and activated carbon clearly show the promise of these materials. These materials will continue to be investigated further. It can also be anticipated that newer electrode materials will emerge in the future and find applications in 3D printed supercapacitors. While activated carbon materials typically offer enhanced surface area through porosity, graphitic materials are generally considered to offer high conductivity compared to porous carbon. Ideally, carbon-based material for supercapacitor electrodes should simultaneously provide large specific surface area and high conductivity to ensure sufficient space for charge storage and fast electron transport, respectively [45]. One can see the application of hybrid carbon-based materials that offer a combination of porosity and conductivity for 3D printed supercapacitor electrodes. Such materials are already being developed and applied for energy storage applications [46]. Based on technology trends, it is expected that the next generation of lightweight and high-capacity energy storage devices will be based on 3D printed solid-state devices comprised of 3D electrode architecture and solid electrolyte [47]. In this context, there will be growing interest in solid electrolytes, compared to aqueous electrolytes. There will be exploration of solid electrolytes that allow for operational voltage windows that are wider than currently possible with existing solid electrolytes. Lastly, whether it is electrodes or solid electrolytes, the formulation of printable inks will continue

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to be an important aspect. Printability will have to be balanced with supercapacitor performance. This can be expected to influence the development of new electrode and electrolyte materials. While DIW can offer considerable flexibility in terms of materials that can be printed, the FDM approach needs further development of printable materials.

6.6.3 Use of Non-3D Printing Methods to Fabricate Device Components The development of 3D printed supercapacitors till now has primarily been based on the fabrication of the electrodes by 3D printing methods. In most cases, nonadditive manufacturing methods have been employed to fabricate the remaining key components of the supercapacitor such as solid electrolyte and current collectors. Table 6.2 summarizes the use of non-3D printing methods used to fabricate key components of supercapacitors with 3D printed electrodes. As can be seen, most of the 3D printed supercapacitors reported till date involve the solution casting of solid electrolyte layers. In some reports on 3D printed supercapacitors, the current collectors were fabricated using non-3D printing methods such as sputtering. The use of non-3D printing methods can counteract the inherent advantages of 3D printing and limit the prospects of industrialization of 3D printing of supercapacitors. For these reasons, it is desirable to 3D print more components of the supercapacitor, and where possible, the entire device itself. This can be done in two ways: one, by the development of materials, and more specifically printable inks for different components of supercapacitors, and two, by being able to print different materials in an integrated manner, which may not be possible by one 3D printing technology. Many of existing 3D printing technologies only allow one material to be printed at one time. These limitations are being addressed by the recent development of multi-material 3D printing platforms which are comprised of multiple additive manufacturing technologies [48, 49]. These printers can combine materials which conventional 3D printers are unable to print simultaneously. Such printing platforms allow for the printing of a wide range of materials with disparate functional and rheological characteristics. In the context of supercapacitors, multi-material 3D printers can allow for the printing of entire devices. There are already some efforts in this direction. There have been a few recent reports of fully 3D printed supercapacitors wherein the electrodes, electrolyte and current collectors were 3D printed. Fully 3D printed supercapacitors have been Table 6.2 Non-3D printing methods used to fabricate parts for 3D printed supercapacitors Component

Fabrication method

Reference(s)

Electrolyte

Solution casting/drop casting

[18–22, 25, 30, 34, 36, 37, 39]

Current collectors

Sputtering

[19, 22, 34, 36]

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reported wherein a single 3D printing technique (DIW) was used to print a variety of printable inks to make electrodes, electrolyte, current collectors, and support substrate [42]. These efforts were possible due to the development of different types of printable inks with different rheological properties to work with one printing method (DIW). It is anticipated that future work on 3D printed supercapacitors will be along these lines. Fully 3D printed supercapacitors have also been made by integrating two printing techniques (FDM and DIW) on one printer [40]. In this case, DIW was used to print the electrodes, electrolyte, and current collectors, while the FDM method was used to print the outer device packaging. With the advent of multi-material 3D printing platforms, it is anticipated that future developments will increasingly involve fully 3D printed supercapacitors. In the development of 3D printed solid-state supercapacitors, the typical approach has been to fabricate freestanding layers of PVA-based solid electrolyte by solution casting and subsequently assemble it with 3D printed electrodes. In the case of interdigitated or in-plane electrodes, the typical approach has been to solution cast a PVA electrolyte on 3D printed electrodes. It should be noted that PVA-based electrolytes are typical dried at room temperature, often overnight or longer. This could be a potential hindrance in the integration of electrolyte fabrication in the 3D printing manufacturing of supercapacitors. The recent emergence of UV curable polymer gel electrolytes can help in this regard [50, 51]. UV curable electrolytes can be formed into layers in minutes with the assistance of UV light. It would be possible to perform in situ UV curing of such electrolytes during 3D printing. Due to recent technological advances, 3D printing heads are now available integrated with UV light sources to facilitate in situ curing.

6.6.4 Post-processing Post-processing is defined as any additional process steps performed on 3D printed parts. Post-processing is common in 3D printing, regardless of the application area [52–54]. A review of published reports on 3D printed supercapacitors made by FDM and DIW shows that post-processing is widely prevalent, primarily on printed electrodes. Table 6.3 summarizes the typical post-processing steps done on 3D printed supercapacitor electrodes. The post-processing on 3D printed supercapacitors is seen to be broadly influenced by the electrode materials used, the desired electrode functionality, and by the requirements of the intended electrode architecture. In terms of electrode materials, the material being printed may need further processing to either achieve its intended properties, modify it functionally, or to optimize its energy performance. To illustrate, when printing graphene oxide electrodes, a common postprocessing step involves the reduction of graphene oxide. In the context of electrode structure or architecture, post-processing can involve the physical modification (e.g., removal of material to create or enhance porosity or 3D structure) of printed electrodes to achieve the desired functionality. Regardless of the reasons, the use of

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post-processing can pose limitations. Post-processing can negate the inherent advantages of 3D printing, mainly the short manufacturing lead times. Post-processing can increase the overall complexity of the manufacturing process, driving up costs of manufacturing supercapacitors by 3D printing. Consequently, post-processing is seen as a major obstacle to the industrial advancement of 3D printing [55, 56]. While it is desirable to avoid post-processing altogether when developing 3D printed supercapacitors, it may not be possible to do so. Considering the past and present technological trends, it is fair to conclude that there will continue to be some extent of post-processing in future developments on 3D printed supercapacitors. It will be useful to investigate printable materials that enable electrode development with minimal or no post-processing. It will also be useful to use 3D printing approaches that allow for simultaneously printing a wide range of disparate materials (e.g., polymers, metals, and carbon). In this context, the recent emergence of multi-material 3D printers with capabilities to perform different types of 3D printing using one printer is a promising step. Table 6.3 Typical post-processing steps done on 3D printed supercapacitor electrodes Electrode material

Post-processing steps on printed electrode

References

Graphene, MoSx

Solvent treatment for 4 h for electrode activation, electrodeposition of MoSx

[24]

RGO, PPy

In situ electrochemical polymerization of polypyrrole/reduced graphene oxide

[22]

Graphene aerogel, MnO2

Freeze drying, annealing in nitrogen gas, electrodeposition of MnO2

[23]

Carbon aerogel

Freeze drying, carbonization, chemical etching to remove filler, activation chemical, followed by drying and annealing

[43]

PEDOT:PSS, carbon nanotubes

Treatment in liquid nitrogen, freeze drying in [30] vacuum

Single-walled CNTs

Treatment in nitric acid to remove surfactant and improve conductivity

[29]

rGO

Solidification at −80 °C, freeze drying, followed by reduction of graphene oxide in hydrothermal reactor

[36]

MXene (Ti3 C2 Tx )

Freeze drying

[39]

GO, graphene nanoplatelets

Gelation, supercritical drying, carbonization, [20] chemical etching

HGO, cobalt oxide

Thermal reduction at 600 °C in hydrogen/argon ambient

[25]

GO, nickel oxide (NiO)

Liquid nitrogen treatment, freeze drying, thermal reduction at 700 °C in hydrogen/argon ambient, electrodeposition of NiO

[37]

GO

Chemical reduction of graphene oxide

[18]

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6.6.5 Device Design and Electrode Architecture The device design is one of the major factors influencing the overall performance of the supercapacitor. As described in the preceding sections in this chapter, 3D printed supercapacitors have been made with in-plane or interdigitated electrodes, and the sandwich-type structure. The choice of the device design must be made by considering a few factors, namely—energy, power, and space requirements related to the intended application, the printable electrode material, pattern resolution needs if any, among others. From a practical perspective, 3D printed supercapacitors with in-plane or interdigitated electrodes may find uses in chip-scale applications needing on-chip energy. Sandwich-type devices may find wider range of applications and may not need high resolution 3D printers for their fabrication. The other aspect to consider is the electrode architecture, which can also influence the device performance. Some 3D printed supercapacitors have employed a porous 3D electrode with a scaffoldtype structure [20, 23]. Porosity is considered a useful attribute and can contribute to an enhanced 3D structure. Porosity in the printed electrodes can be tailored by controlling printing parameters including print pattern, extrusion pressure, infill, among others. Porosity can also be incorporated at the smaller scales in the electrode material itself, in the form of porous carbon [28].

6.6.6 Sustainability In addition to the various fabrication advantages, 3D printing offers the means of sustainable manufacturing. 3D printing methods involve minimal wastage of materials while enabling environmentally friendly manufacturing. In the context of 3D printed supercapacitors, any discussion of the sustainability aspect should involve a discussion on the use of sustainable materials. Over the past decade, there have been numerous developmental efforts on sustainable electrode materials for supercapacitors made by non-3D printing methods. Various carbon-based electrode materials have been synthesized from different types of waste materials. These carbon materials, including activated carbon, have been tailored to exhibit desirable attributes such as porosity, surface area, conductivity, among others. Activated carbon has been synthesized from natural and bio-waste materials such as peanut shells, onion peels, corn husk, and neem plant bio-waste [4, 7, 57, 58]. Other types of waste materials such as used ink toners and waste office paper have also been used to generate carbon for supercapacitors [6, 59]. There have also been efforts to generate carbon and graphene-based materials from plastic waste for supercapacitor electrodes [60]. A literature search will yield hundreds of refereed publications on the generation of carbon and activated carbon from waste materials. With such large-scale activity in the production of sustainable electrode materials, it would be natural to expect that such materials find wide application in 3D printed supercapacitors. However, this has not been the case till date. A survey of the published reports on FDM

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and DIW printing of supercapacitors shows very little work on the use of sustainable electrode materials, especially those synthesized from waste. However, recent efforts in this area have been encouraging. There has been a recent report on the use of activated carbon derived from packaging waste as electrode material in 3D printed supercapacitors [28]. The work by Idrees et al. demonstrates that it is possible to integrate sustainable materials with sustainable manufacturing methods like 3D printing. Clearly, there is scope for increased efforts on similar lines in the future. In the context of sustainable manufacturing, it is also relevant to consider materials that have been made by green synthesis. There have been recent reports of supercapacitors developed by non-3D printing methods using green synthesized materials, more specifically electrode materials [61, 62]. It is reasonable to expect that such materials will find applications in 3D printed supercapacitors in the future. Ultimately, any future developments in sustainable materials for supercapacitors must consider the aspects of materials selection and performance which will directly influence the characteristics of the supercapacitor [63–65].

6.7 Conclusions In this chapter, an overview was provided on the emerging area of 3D printed supercapacitors. A fundamental description was provided by the key 3D printing methods used to make supercapacitors, along with a discussion of device architectures, electrode, and electrolyte materials. Related developments in printable materials and 3D printed supercapacitors were presented. A detailed discussion was presented on key technological considerations and the future outlook in the area of 3D printed supercapacitors. Key areas of technological considerations were identified including materials, electrode architecture, sustainability, post-processing, and the use of non3D printing methods to make device components. Post-processing was identified as one of the major challenges to the industrialization prospects of 3D printing of supercapacitors. It was concluded that post-processing on 3D printed supercapacitors was mainly influenced by the electrode materials used, the desired electrode functionality, and by the requirements of the intended electrode architecture. It was concluded that multi-material 3D printing platforms can help minimize post-processing. This chapter also discussed how multi-material printing can address the issue of non3D printing methods being used for supercapacitor components such as electrolyte and current collectors. Recent emergence of multi-material 3D printers and their demonstrated use in fully 3D printed supercapacitor development support these conclusions. On the aspect of sustainability, there is need to employ electrode materials synthesized from natural and waste products in 3D printed supercapacitors. To further complement the sustainability attribute of 3D printing, electrolyte materials generated through green synthesis can be investigated for 3D printed supercapacitors. Finally, it is anticipated that with advances in electrode architecture design and materials development can create new avenues and application areas for 3D printed supercapacitors in the future.

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Acknowledgements The authors received financial support from National Science Foundation’s Division of Materials Research (DMR) under the grant NSF-PREM #1827690.

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Chapter 7

Atomic Layer Deposited Supercapacitor Electrodes Mohd Zahid Ansari, Soo-Hyun Kim, Arpan Dhara, and Dip K. Nandi

Abstract Atomic layer deposition (ALD), an advanced and modified version of chemical vapor deposition (CVD), has become one of the state-of-the-art technologies for depositing high quality thin films with precise thickness control over a variety of planar as well as complex three-dimensional structures. In contrast to CVD, the thin film growth via ALD involves chemical reactions between the precursor (in gaseous phase) and surface species in a self-limiting fashion, which allows extremely uniform deposition of desired materials on virtually any substrate feature. Although the materials presently being used in energy storage devices are performing close to their theoretical limits in bulk form, the recent developments in nanotechnology allow for extracting novel properties from their nanosized forms. This poses ALD as an ideal technique for designing high-performance supercapacitor (SC) electrode materials possessing fast charge transfer kinetics and improved energy and power delivery with better cycling and rate performances. This chapter presents a summary of the recent advances on the use of ALD to design SCs with desirable structures and the ensuing properties. In addition, the present challenges and potential opportunities for future exploration of ALD to achieve desired electrochemical performance of next generation SCs are also pointed out.

7.1 Introduction The technological advancements and ubiquitous commercialization of portable consumer electronic devices have accelerated the demand for energy storage systems M. Z. Ansari · S.-H. Kim · D. K. Nandi (B) School of Materials Science and Engineering, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 38541, Republic of Korea e-mail: [email protected] A. Dhara Department of Solid State Sciences, COCOON Research Group, Ghent University, Ghent, Belgium D. K. Nandi Lumileds Singapore Pte. Ltd., Yishun 768925, Singapore © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_7

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simultaneously that should offer high energy and power density with fast charging rates. The prominent energy storage devices presently in commercial use are batteries and electrostatic capacitors, lying at the extreme levels of energy and power density scales, respectively (Fig. 7.1). Therefore, neither batteries nor capacitors are sufficient for modern day energy and power dense applications. Supercapacitors (SCs), as bridging devices between batteries and capacitors, are emerging energy storage technology having high power capacities, faster charging/discharging rates, prolonged cyclic lifespan, higher coulombic efficiency, and high-temperature tolerance. Nevertheless, the lower energy density than batteries limits the commercial development of SCs [1–3]. A typical SC stores charge either through the formation of electric double-layer or in the form of faradaic reactions, resulting in two broad categories of SCs as— electric double-layer capacitors (EDLCs) and pseudocapacitors, respectively. While EDLCs operate by accumulating electrostatic charges on carbon-based electrodes, pseudocapacitors rely on fast and reversible redox reactions occurring at the electrode surface in contact with the electrolyte. Consequently, the performance of both types of SCs can be enhanced by increasing the electrode surface area, reducing ion transfer length, and improving the conductivity of electrode materials as we have also seen in the last chapters. Moreover, the electrode materials requirement for the supercapacitor application varies based on the underpinning mechanisms in the different categories. In this regard, designing the electrode materials at nanoscale has emerged as the most widely accepted strategy to improve all the parameters which are vital in view of such applications [1–7]. As a result, a wide array of high porosity three-dimensional (3D) nanostructures (e.g., interconnected networks of nanowires, nanotubes, etc.) have been pursued as active materials for high-performance SCs. However, the performance of such devices may get deteriorated due to the following problems: (i) large surface area could enhance the active material dissolution due to the development of surface cavities; (ii) grain boundaries and lattice mismatch between the deposited layer and substrate could aggravate the carrier scattering and led to poor charge transfer Fig. 7.1 Ragone plot showing the typical energy and power limits of different types of energy storage devices [3]

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dynamics. This leads to an urgent need to develop a growth strategy which can deposit materials with high conformality over the substrates having complex morphologies and modifying novel electroactive materials on the atomic scale to attain synergistic design-performance behavior, whereas wet chemical routes take longer process time and laborious steps, and hard to control the materials properties [3–7]. The conventional thin film deposition routes including physical vapor deposition (PVD) and chemical vapor deposition (CVD) face significant challenge to achieve high quality, ultrathin (3.5 V, which was >20% more than bare AC electrodes [21]. The Al2 O3 coated AC obtained after 20 ALD cycles when operated under 4 V, could retain 84% capacitance after 1000 cycles,

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in comparison with bare AC retaining only 48% [22]. In this structure, the thin oxide coating increased the separation between the AC surface and coions, thereby modifying the Stern layer, which resulted in improved cycling performance and energy density [21, 22]. The same group further demonstrated a combined method of oxygen plasma and Al2 O3 ALD on AC electrodes to decrease the impedance and enhance the cycling stability of SCs [23]. The oxygen plasma treatment allowed fine-tuning of the defect structure, thereby controlling the microstructure of the AC electrode. The AC electrode with increased defects and ALD passivation demonstrated outstanding rate performance (40.6 and 20.1 F g−1 at 5 and 100 mA cm−2 , respectively) and cyclic life (~90% capacitance retention over 5000 cycles). This excellent behavior was attributed to the synergy between high capacity of plasma-enhanced AC electrode and high cyclic retention of ALD oxide protection, which gave rise to high energy as well as power density [23]. Thus, ALD oxide coating is beneficial for increasing the energy density as well as operating potential of AC-based SCs. Among the different carbonaceous materials, free-standing carbon nanotubes (CNTs) thin films have become the most frequently utilized ALD substrate for developing power dense SCs owing to their highly porous and excellent electric and thermal conduction nature [17]. The charge storage in CNTs occurs both on the surface as well as the defects. As a result, for a given surface area, the electroactivity of CNT-based electrodes can be enhanced by defect engineering. In this regard, Ready et al. [25] recently investigated the effects of different functionalization and graphenation processes on enhancing the charge storage properties of CNTs. For this purpose, the authors performed three different modifications on CNTs forests: (i) graphenation via plasma enhanced chemical vapor deposition, (ii) pseudocapacitive TiO2 coating via ALD, and (iii) graphenation followed by pseudocapacitive deposition. Graphenation resulted in increase in the surface area of the electrodes, while functionalization with TiO2 coating introduced pseudocapacitive charge storage behavior. As a result, the electrodes modified with graphenation delivered an energy density of 26.0 W h kg−1 , the functionalized electrodes exhibited 39.4 W h kg−1 , and the electrodes modified with both the treatments offered 63.4 W h kg−1 , in comparison with 2.6 W h kg−1 presented by untreated CNTs, indicating the potential of various functionalization treatments to enhance the performance of CNT-based electrodes [25]. Ready et al. [26] decorated vertically aligned multi-walled CNTs with titanium oxide (TiO2 ) via ALD to introduce pseudocapacitance charge storage properties to the electrodes. The resulting TiO2 /MWCNT electrodes combining both faradaic and non-faradaic charge storage mechanisms yielded specific capacitances of 73 F g−1 and 1364 F g−1 (based one total electrode and TiO2 mass, respectively) with high energy and power densities of 14.0 ± 4.1 W s g−1 and 24.8 ± 4.8 W g−1 [26]. Moreover, TiO2 coated graphene composites obtained for 50 and 100 ALD cycles demonstrated specific capacitances of 75 F g−1 and 84 F g−1 at 10 mV s−1 , respectively [27]. The authors also observed no significant performance degradation when the electrode mass loading was increased to 3–4 times, indicating suitability of their synthesis approach to fabricate advanced graphene-based nanocomposite electrodes for SCs.

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Fig. 7.5 Fabrication process for 3D MSC: a Si oxidation, b metal deposition, c growth of vertically aligned CNT, and d pseudocapacitive layering [28]

Recently, Brown et al. [28] demonstrated the use of ALD coatings in a full-cell 3D microsupercapacitors (MSC) by depositing conformal pseudocapacitive TiO2 and TiN coatings on CVD-grown vertically aligned CNTs (Fig. 7.5), resulting in ALD TiN-TiO2 -VACNT hybrid structure. This hybrid TiO2 served dual roles as pseudocapacitive coating to enhance charge storage capacity and protective layer between TiN and CNTs to avoid short circuiting (Fig. 7.6). Besides, the formation of titanium oxynitride (TiON) at TiO2 /TiN interface further enhanced the electrochemical performance. As a result, the optimized ALD TiN (100 ALD cycles)-TiO2 (100 ALD cycles)-VACNT device offered over 74 times higher specific capacitance (5.18 mF cm−2 ) than bare VACNT device (0.07 mF cm−2 ) [28]. Parsons et al. [29] presented fundamental investigations of how the surface chemistry (i.e., porosity and surface area) of the substrate affected the ALD nucleation, layer conformity, and the ensuing electrochemistry, such as pseudocapacitance, power density, and prolonged cycling stability. For this purpose, the authors used two different types of commercially available ACs: G60 (mesoporous, lower surface area) and Supra (microporous, larger surface area). Both the carbons were acid treated in 15 M HNO3 prior to V2 O5 ALD at 150 °C using vanadium triisopropoxide and water as precursors. Although both the carbons demonstrated improved charge storage properties after ALD coating, the degree of improvement strongly depended upon the initial pore structure. The V2 O5 coated G60 electrode initially showed a 46% improvement in the specific capacitance after 75 ALD cycles, however, further increase in ALD cycles resulted in decreased capacitance values. This was attributed to the filling of the mesopores with V2 O5 , resulting in reduced accessible surface area. Whereas ALD of V2 O5 on Supra (pore diameter 972% coulombic efficiency. The authors also assembled flexible asymmetric SCs using the Ni3 C/CNT composite and activated carbon (AC) as cathode and anode, respectively, and PVA/KOH as solid-state polymer electrolyte (Ni3 C/CNT//AC). The asymmetric device exhibited very high energy density (57 W h kg−1 ) at 1.26 kW kg−1 power density, and conversely a large power density (12.8 kW kg−1 ) with an energy density of 35 W h kg−1 . In addition, the asymmetric SC retained its excellent performance even under bending and twisting conditions and maintained 98.6% of its initial capacitance after 50,000 charge–discharge cycles conducted at 16 A g−1 current rate. Finally, the practical suitability of the device was tested by glowing commercial LEDs with a combined voltage range of 2.2–3.2 V. For a 60 s charging, a series combination of two asymmetric SCs maintained the LED glow for ~10 min, indicating the excellent energy and power delivery of the device [38]. Zhong et al. [39] reported ALD-assisted template synthesis approach to prepare monolayer titanium carbide (TiC) hollow sphere arrays on conducting graphite paper substrate. First, monolayer ALD-TiO2 hollow spheres were homogeneously coated with glucose using a sacrificial polystyrene sphere template, followed by carbothermal reaction which converted TiO2 into TiC hollow spheres. Owing to the porous hollow structure and high electric conductivity, the TiC hollow sphere arrays performed excellently as high-temperature (65 °C) organic SC electrodes with high capacitance, superiors long term cycling (98% capacitance maintained after 75,000 cycles at 12 A g−1 ), and good high-rate (296/239 F g−1 at 12/96 A g−1 ) performances. The enhanced high-temperature electrochemical performance of TiC hollow spheres was attributed to—(i) high chemical stability of TiC, which stabilized the active material against oxidation; (ii) the hollow architecture which absorbed the stress generated during charge/discharge, thereby maintaining the structural integrity; and (iii) good electric conductivity, shortened ionic diffusion paths, and robust connections with the substrate, which not only retained good contact between the active material and the electrolyte but also the reaction kinetics and ion transport [39]. Kao et al. [40] coated titanium nitride (TiN) via ALD on high-aspect ratio vertically aligned porous CNT forest and demonstrated its electrochemical performance. The resulting ALD TiN-CNT electrode showed 2000 times higher capacitance than flat-shaped TiN electrode and over 500% more capacitance (81 mF cm−2 ) than bare CNT forest electrodes (14 mF cm−2 ). This enhancement in specific capacitance was owed to the enhanced oxygen vacancies on TiN surface after exposure to ambient environment. However, the electrode exhibited limited cycling stability (~10% capacitance fading over 350 cycles), which needs to be improved [40].

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7.3.2 ALD Coating on Non-carbonaceous Three-Dimensional Scaffolds 7.3.2.1

Metal Oxides

Because of large surface area and interconnectivity, ultrathin TiO2 nanomembranes deposited over Ni foam via 100 ALD cycles demonstrated a specific capacitance of 2332 F g−1 at 1 A g−1 with an energy of 81 W h kg−1 [41]. Zhuiykov et al. [42] employed a facile two-step ALD followed by post-deposition annealing at 380 °C to functionalize two-dimensional (2D) WO3 films with TiO2 nanoparticles (TiO2 NPWO3 ) for SC electrode materials. The obtained hybrid demonstrated a pseudocapacitive charge storage along with faster charge diffusion processes than 2D WO3 electrode due to electronic modification at the TiO2 -WO3 heterojunction. As a result, the TiO2 NP-WO3 hybrid electrode showed a ~1.5 times increase in the specific capacitance values in comparison with pure 2D WO3 electrode [42]. Xiong et al. [43] integrated ALD and electrodeposition to fabricate ALD-TiO2 nanotube/electrodeposited MnO2 -C nano flake core/shell arrays (TiO2 /MnO2 -C). The highly porous architecture and good electric conductivity of the TiO2 /MnO2 -C hybrid ensured fast charge diffusion and enhanced electrochemical performance with high specific capacitance (880 F g−1 at 2.5 A g−1 ), better high-rate behavior (735 F g−1 at 30 A g−1 ), and stable prolonged cyclic performance (94.3% capacitance retained after 20,000 cycles). Full asymmetric SC cell comprising TiO2 /MnO2 -C electrode revealed energy density from 18.1 to 14.1 W h kg−1 at power density ranging from 1.7 to 20 kW kg−1 , higher than the corresponding TiO2 /MnO2 (12.2 to 9.6 W h kg−1 ) [35]. Similarly, another metal oxide, i.e., ZnO, was deposited on polyurethane sponge to obtain ZnO nanomembranes as electrode materials for electrochemical SCs (Fig. 7.9) [44]. The charge storage performance of the developed ZnO nanomembranes was analyzed using different aqueous electrolytes (i.e., KOH, KCl, and Na2 SO4 ). Due to improved ionic adsorption/desorption and extraction/insertion in the electrode, the ZnO nanomembrane (100 ALD cycles) with 6 M KOH offered specific capacitance of 846 F g−1 and powered 180-min flashing of a red LED [44]. Apart from using as active materials, ZnO nanowires (NWs) offering large surface area, attractive mechanical, and chemical properties, are also the most frequently used nanostructured scaffolds for coating active materials for futuristic energy storage devices. However, NiO coated ZnO NWs-based SCs suffer from insufficient electroactivity due to poor electrical conductivity of NiO coupled with large interfacial resistance and deficit redox activity. For enhancing the specific capacitance of ZnO NWs-based NiO SCs, Ren et al. [45] prepared ZnO NWs/Ni-NiO core–shell hybrid structure for SC applications. The authors first prepared ZnO NWs by hydrothermal treatment of ZnO seed layer, followed by depositing ultrathin Ni layer on ZnO NWs via a plasma enhanced ALD process. Owing to the high reactivity of underlying ZnO NWs and self-limiting reactions of ALD, first few Ni layers were found to oxidize into NiO. This NiO imparted additional faradaic redox activity to the hybrid

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Fig. 7.9 a Schematic of the fabrication procedure of ZnO NMs electrode and corresponding supercapacitor. b–d SEM images of ZnO NMs with b 50, c 100, and d 200 ALD cycles. The red circles in b represent the existence of the holes in ZnO NMs, e Cycle performance of the electrode prepared from ZnO NMs with 100 ALD cycles., and f Potential stability of the supercapacitor made by ZnO NMs with 100 ALD cycles [44]

structure and the Ni coating enhanced electrical transport characteristics of the electrode. As a result, the core–shell ZnO NWs/Ni-NiO hybrid with 30 nm thick Ni-NiO shell presented an outstanding specific capacitance of ~2440 F g−1 at 10 mV s−1 with high-rate capacity (retaining 80.5% at 100 mV s−1 ) and good cycling stability (retained 86.7% after 750 cycles at 10 A g−1 ) [45].

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Kim et al. [46] combined hydrothermal and ALD processes to design a freestanding core/shell type NiO/Co3 O4 hybrid on Ni foam for SC electrodes. First, cobalt nanocone structures were hydrothermally grown on Ni foam followed by calcination to yield cobalt oxide (Co3 O4 ). In the second step, ultrathin NiO shell was deposited over the Co3 O4 nanocones via an advanced ALD technique to obtain the NiO/Co3 O4 @NF hybrid electrode material. The electrode comprising a 5 nm thick NiO coating exhibited the highest specific capacity of 1242 C g−1 (2760 F g−1 ) at a current rate of 2 A g−1 (as compared to 1045.8 C g−1 or 2324 F g−1 for uncoated Co3 O4 @NF electrode). Furthermore, the 5 nm NiO/Co3 O4 @NF hybrid displayed ~77% rate capability (959.8 C g−1 or 2133 F g−1 ) at 10 A g−1 (versus 46% for Co3 O4 @NF) and retained 95.5% after 12,000 cycles. The 5 nm NiO/Co3 O4 @NF structure exhibited lower charge transfer resistance resulting in better interaction between the core and shell, which in turn facilitated faster charge transport. An asymmetric SC comprising 5 nm NiO/Co3 O4 @NF (cathode) || AC (anode) achieved an energy density of 81.45 W h kg−1 at 4268 W kg−1 with superior cyclic stability. In addition, two asymmetric SCs in series could energize commercial LED (2.0 V) for >5 min on a 5 min charge. This hybrid electrode not only afforded the synergistic effects from both core Co3 O4 and ultrathin NiO shell, but also benefitted from efficient stabilization of NiO shell which preserved the Co3 O4 structure without compromising the electrochemical performance [46]. Thompson et al. [47] fabricated solid-state on-chip SCs comprising ruthenium oxide (RuO2 ) coated silicon NWs using an IC compatible moderate temperature (290 °C) ALD process (Fig. 7.11). First, high-surface-area silicon NWs arrays were prepared via metal-assisted anodic etching (MAAE), followed by ALD coating with RuOx . The resulting Si NW@400 RuOx (400 ALD cycles) electrode, presented a specific capacitance of 19 mF cm−2 at 5 mV s−1 in a neutral Na2 SO4 electrolyte (Fig. 7.10). Finally, symmetric solid-state full SC cell using the Si NW@400 RuOx electrodes and a polymer-based solid electrolyte delivered a specific capacitance of 6.5 mF cm−2 at 2 mV s−1 along with a cyclic retention of 92% after 10,000 cycles at 0.4 mA cm−2 . The device delivered high specific energies (0.4 mW h cm−2 and 0.32 mW h cm−2 ) without sacrificing the power densities (0.03 mW cm−2 and 0.17 mW cm−2 ). Further, the specific capacitance of the electrode could be easily adjusted by tuning the aspect ratios of the NWs [47]. Bimetallic oxides offer considerably improved capacitance values owing to the synergistic effects occurring between the constituting elements and thus are a major research area in recent times. The performance of these oxides can be further increased by decorating them over highly porous and conducting substrates. For instance, Co–Ni bimetallic oxides (CoOx -NiO) deposited over Ti3 C2 TX MXene nanosheets by ALD were recently demonstrated as attractive pseudocapacitive materials for SC electrodes (Fig. 7.12) [48]. The uniform distribution of CoOx -NiO nanoparticles on Ti3 C2 TX surface endowed the hybrid electrode more active sites, and the synergy between CoOx and NiO greatly improved the electrochemical performance. The CoOx -NiO/Ti3 C2 TX electrode obtained after 90 ALD cycles demonstrated the highest specific capacitance

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Fig. 7.10 TEM images of a SiNW1-150cyc sample, b SiNW2-400cyc sample, c schematic of the symmetric solid-state on-chip supercapacitors based on SiNW-RuOx electrodes, and d The specific capacitance versus cycle number for the solid-state supercapacitor device based on SiNW2 -400cyc electrodes at a discharge current density of 0.4 mAcm−2 [47]

of 1960 F g−1 at 1 A g−1 (~20 times higher than bare Ti3 C2 TX electrode), good rate capability (87.3% retention from 1 to 18 A g−1 ) and retained 90.2% of the initial capacitance after 8000 charge/discharge cycles [48]. The ternary nickel cobaltite (NiCo2 O4 ) has gained high recognition due to its high theoretical capacity, low cost, and environmentally benign characteristics. Further, the presence of both nickel and cobalt ions render it with higher electroactivity, better redox reactivity, and much improved electrical conductivity (an improvement of over two orders of magnitude) in comparison with the NiO and Co3 O4 counterparts. Nevertheless, the rate capability of NiCo2 O4 is not satisfactory and needs to be improved. In this direction, Alshareef et al. [6] developed core–shell NiCo2 O4 @TiN nanostructures on carbon fiber by depositing a thin and conformal TiN layer via ALD on NiCo2 O4 nanofiber arrays, demonstrating much improved electric conductivity, mechanical property, and rate behavior. Consequently, symmetric all-solid-state SC employing core–shell NiCo2 O4 @TiN hybrid as electrodes offered a considerable stack power density of 58.205 mW cm−3 at a stack energy density of 0.061 mW h

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Fig. 7.11 a Schematic diagram for the preparation of xCoOx -NiO/Ti3 C2 Tx , b SEM images of multilayer Ti3 C2 Tx , and c Cycle stability of xCoOx -NiO/Ti3 C2 Tx at a current density of 5 A g−1 [48]

cm−3 and stable cycling with retaining 70% after 20,000 cycles at 10 mA cm−2 . The enhancements in electrochemical performance of the hybrid electrode were ascribed to the conformal ultrathin TiN layer (~8 nm, 300 cycles) which not only acted as a mechanical buffer to absorb cycling induced volume fluctuations of the underlying NiCo2 O4 core but also facilitated charge transport at the electrode/electrolyte interfaces. In addition to this, TiN also contributed pseudocapacitance through faradaic reactions at its oxidized surface layer [6]. To stabilize the cycling performance of NiCo2 O4 -based electrodes, Chodankar et al. [49] deposited an ultrathin NiO layer on NiCo2 O4 NW arrays via ALD, resulting in a high specific capacitance of 2439 F g−1 with a remarkable cyclic performance (retaining 94.2% capacitance over 20,000 cycles). To simultaneously enhance the capacity and cycling stability of NiCo2 O4 , Kavinkumar et al. [50] recently presented a novel strategy to encapsulate core–shell NiCo2 O4 /MoO2 heteronanostructure by ALD-grown NiO nanolayer (denoted as NiCo2 O4 /MoO2 @ALD-NiO) for high-performance SC electrodes (Fig. 7.13a). The

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Fig. 7.12 a Synthesis procedure for NiCo2 O4 /MoO2 @ALD-NiO heteronanostructure. b Rate and c Capacitance retention of NiCo2 O4 /MoO2 @ALD-NiO heteronanostructure [50]

nanolayer endowed high flexibility to the heteronanostructure, and accommodated the cycling induced volume fluctuations, thereby retaining the physical integrity of the encapsulated structure. The encapsulating NiO layer resulted in electronic modulation at its interface with the shell layer of the heteronanostructure, thus providing a large increase in capacity (450 C g−1 ) (Fig. 7.13b). Moreover, the ALD-NiO layer was found to reduce the capacity fade from >10% to below 3% over 20,000 cycles (Fig. 7.13c). Finally, an asymmetric SC was assembled with NiCo2 O4 /MoO2 @ALDNiO (positive) || B-RGO@ALD-Fe2 O3 (anode) configuration, which delivered an energy density of 136 W h kg−1 at a power density of 1800 W kg−1 . Furthermore, a series combination of two asymmetric SCs (charged up to 1.8 V) provided enough energy to light a commercial blue LED for several minutes. These results project nanolayer-encapsulated core–shell type electrodes as potential SC electrodes which can match the energy density of even Li-ion batteries with a power density matching that of typical capacitors [50].

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Fig. 7.13 a Cross-sectional view TEM image of MoNx @NF, b crystalline and amorphous regions identified in the film by the HRTEM analysis, c CV of bare NF and MoNx @NF at a scan rate of 20 mV/s, (inset, CVs of the composite at different scan rates), and d cycling performances with corresponding coulombic efficiency, (inset, last few charge–discharge cycles) [51]

7.3.2.2

Other Transition Metal Compounds (Nitrides, Sulfides) and Single Element (Pt) to Fabricate SC Electrode

The excellent properties of conformality and pinhole-free coverage, render ALD suitable for direct deposition of active materials over nanostructured templates other than carbon materials for SC electrode fabrication. For instance, Nandi et al. [51] directly deposited molybdenum nitride (MoNx ) on 3D Ni foam (NF) using a low temperature (250 °C) ALD process using Mo(CO)6 and NH3 to produce MoNx @NF composite electrode (Fig. 7.14). The NF facilitated the redox reactions in composite, resulting in high areal capacity of 130 mC cm−2 at 2 mA cm−2 with good highrate capability (retaining ~85% from 2 to 10 mA cm−2 ) and cyclic life (~115% capacitance after 8000 cycles) at ~100% coulombic efficiency [51]. By utilizing the same technique, Ansari et al. [52] reported a novel low temperature (70–200 °C) ALD process to deposit high quality tin nitride (SnNx ) thin films on NF using tetrakis(dimethylamino) tin and ammonia as the precursors. As free-standing electrodes for electrochemical SCs, the SnNx -coated NF (SnNx @NF) displayed nearly eightfold improvement in charge storage capacity vis-a-vis bare NF electrode and stable cycling with ~92% retention for 3000 cycles at a coulombic efficiency of >97%. The enhanced behavior was owed to the distinct structure of SnNx @NF

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Fig. 7.14 Schematic diagrams of a ALD process of MoS2 and b the film deposition on the flat SS-foil and 3D Ni foam [53]

where 3D NF enhanced the electrical conduction and offered large surface area to facilitate fast and reversible redox reactions, while SnNx presented additional redox contribution [52]. Molybdenum disulfide (MoS2 ), being layered tradition metal sulfide, offers both pseudocapacitive as well as double-layer charge storage mechanisms, resulting in high theoretical capacitance values. In addition, it also exhibits good electrical and ionic conductivities, making it an attractive material for next generation energy dense SC applications. Nandi et al. [53] deposited MoS2 over Ni foam via ALD as illustrated in Figs. 7.15 and 7.16, resulting in a high areal capacitance (3400 mF cm−2 at 3 mA cm−2 ) with high-rate ability (2760 cm−2 at 18 mA cm−2 ) and stable cycling of 82% for over 4500 charge/discharge cycles at ~95% coulombic efficiency. The optimum number of ALD cycles was also found out for achieving maximum capacitance for such a MoS2 @3D Ni foam electrode. Similarly, another sulfide, phase dependent SnSx film was deposited over 3D Ni foam for the first time by ALD (Fig. 7.16). The double-layer capacitance with the composite electrode of SnSx @NF grown at 160 °C, which has SnS2 dominant phase is higher than that of SnS dominant phase at 180 °C (SnSx @NF-180), while pseudocapacitive Faradaic reactions are evident for both SnSx @NF electrodes. Further, the optimal thickness of ALD-SnSx thin film is found to be 60 nm for the composite electrode of SnSx @NF grown at 160 °C by controlling the number of ALD cycles to yield an aerial capacitance of 805.5 mF cm−2 at 0.5 mA cm−2 and outstanding cyclic stability for 5000 charge/discharge cycles [54]. Li et al. [55] employed ALD to fabricate novel hierarchical ZnO@ZnS core– shell NWs arrays on Ni foam (ZnO@ZnS/Ni) as binder free electrodes for highperformance SCs. In brief, first ZnO NWs arrays were hydrothermally grown on Ni foam, followed by ALD depositions of ZnS nanothin films. ZnO NWs/Ni not only uniformly held the ZnS nanofilm, but also offered large active area, resulting in high capacitance. As a result, the ZnO@ZnS/Ni hierarchical electrode exhibited large specific capacitance of 227.0 F g−1 at 1.28 A g−1 and retained 87.3% capacitance at 12.80 A g−1 . Moreover, the hierarchical electrode demonstrated ultra-stable cycling

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Fig. 7.15 a, b Cross-section TEM images of ALD-grown MoS2 @Ni foam using 500 ALD cycles, electrochemical performance of c CV curves with different cycle number and d cyclic stability of the electrode prepared by 400 ALD cycles [53]

with no apparent capacitance loss after 20,000 cycles at 12.80 A g−1 [55]. Wang et al. [56] employed ALD to successfully deposit conformal thin films of cobalt sulfide (Co9 S8 ) on 3D Ni foam using bis(N,N' -diisopropylacetamidinato)cobalt (II) and H2 S as coreactants within a temperature window from 80 to 165 °C. The highly stoichiometric polycrystalline Co9 S8 films with low surface roughness and high conductivity displayed good electrochemical performance with high specific capacitance of 1645 F g−1 at 3 A g−1 and retained 1309 F g−1 at 45 A g−1 (corresponding to 80% retention). Further, the electrode exhibited excellent cyclic stability with 94.4% capacitance retention after 2000 cycles performed at 45 A g−1 at 99.4% coulombic efficiency [56]. Apart from synthesizing and functionalizing active materials, ALD has also been explored to fabricate efficient current collectors for SC applications. The use of ALD results in significant reductions in material requirements, thereby allowing the use of even rare and expensive metals in device fabrication. For instance, to efficiently utilize Pt, Lei et al. [57] developed an economic ALD technique with a low N2 filling step, to synthesize regular Pt nanotube (NT) arrays on pre-patterned

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Fig. 7.16 a Cross-sectional view TEM image of the SnSx @NF-160 electrode grown by 500 ALD cycles, and b cycling performances with corresponding coulombic efficiency, (Inset figure shows charge/discharge curves of the 1st and 5000th cycles) [54]

alumina nanoporous templates. In comparison with the conventional Pt-ALD, this process required much fewer ALD cycles and shorter precursor exposure time mainly due to the low N2 filling step, thus reducing the ALD cycle number by half and precursor exposure time by 10%. The authors further investigated the Pt NT array as a current collector for SCs by electrodepositing MnO2 over the Pt NT arrays to obtain core/shell Pt/MnO2 nanotubes. Thus obtained core/shell electrode exhibited high gravimetric and areal capacitances (810 F g−1 and 74 mF cm−2 at 5 mV s−1 scan rate) with superior rate behavior (68% capacitance retained from 2 to 100 A g−1 )

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and excellent cycling stability (no apparent capacitance fading after 8000 cycles at random current rates from 2 to 100 A g−1 ). The good performance of the Pt/MnO2 electrode was ascribed to the well-defined Pt NT arrays which provided a large active surface area. Moreover, the inner tube voids facilitated fast and reversible faradaic reactions and shorted the ion diffusion length. Therefore, Pt NT arrays appear to be excellent candidates as nanostructured current collectors for high-performance SCs. Thus, developed strategy can also be extended to fabricate nanostructures of other noble metals, since these materials have similar ALD reactions to those of Pt [57]. For more details, materials ranging from carbon-based electric double-layer capacitor electrodes to transition metal oxides and conducting polymers are discussed in details [58–60].

7.4 Conclusions ALD is an emerging thin film preparation technique to fabricate nanostructured materials for high-performance supercapacitors. The most common strategy involves decorating high capacitance pseudocapacitive materials via ALD over conducting substrates such as carbon materials or 3D porous metallic scaffolds. Benefitting from the unique merits of excellent conformality, precise thickness and stoichiometric control, ALD deposited materials maintain strong chemical bond with the conducting substrates, resulting in superior mechanical stability of the electrode architecture during charge–discharge cycling by effectively buffering the volumetric changes of active materials. Additionally, ALD enables rapid ionic diffusion at the interface, thereby enhancing the charge storage and rate performance of the SC electrodes. In this chapter, we summarized the various performance characteristics of SC electrode materials, synthesized by ALD. The existing literature shows that metal oxides are one the highest explored category of materials by ALD in this regard. However, growing attempts are also made with ALD-grown metal sulfides, nitrides, carbides, and with single element as well, to come up with an efficient electrode design for SCs. By enabling ALD parameters, it has become one of the most promising technique for enhancing the performance of SCs.

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Chapter 8

Binder-Free Supercapacitors Kapil Dev Verma and Kamal K. Kar

Abstract Binder restricts electrode material’s performance by increasing the contact resistance and preventing electrolytes from utilizing the whole surface area of the electrode. A binder-free supercapacitor is a new approach for improving the performance of supercapacitors by growing or depositing the active material on the conducting substrate. It will also increase the flexibility and energy density of the supercapacitor device because dead mass has been removed. Binder-free electrode material can be fabricated by physical, thermal, and electrical methods. This article discusses different fabrication methods and performances of binder-free electrodes for supercapacitor applications.

8.1 Introduction Energy demand is increasing day by day. Renewable energy is the only hope for the future because fossil fuel resources are limited. Renewable energy storage in electrical energy form is a big challenge because batteries have a short life cycle, low charging–discharging rate, and small operating temperature range [1]. Supercapacitors, with or without batteries, are economical and provide high efficiency [2]. But the supercapacitors have an increased life cycle, fast charging–discharging rate, and high operating temperature range. These are used in bust mode power delivery, regenerative braking, and hybrid energy storage devices. It is made of the current collector, binder, electrode, electrolyte, and separator [3–5]. The binder-free supercapacitor is one of the scopes for increasing the performance of supercapacitors. Electrode design is an essential factor in the electrochemical performance of supercapacitors. Conventional electrodes are prepared by mixing a K. D. Verma · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_8

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slurry of active material, binder, and conductive carbon [2]. This electrode material slurry is deposited over the current collector [6]. But binder is an electrochemically inactive material that hinders the electron transfer from the electrode to the current collector and ion transfer from the electrolyte to the electrode material. Binder is like a dead mass in electrode material and reduces the energy density of the supercapacitor. With the increasing demand for supercapacitors with high specific capacitance, energy density, and smaller size, it has been essential to reduce the inactive material and increase the active material in supercapacitor devices. Electrically insulating binders like polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC), etc., will decrease the conductivity of the electrode and increase the equivalent series resistance (ESR). As a result, supercapacitor performance will decrease [7]. There are several advantages of binder-free supercapacitors. Due to the absence of an electrically insulating binder, charge transfer resistance between electrode and substrate will decrease; ion penetration and diffusion of electrolyte will improve because of electrode material’s completely accessible porous structure and surface area. Moreover, active material will more uniformly get distributed over the surface of the substrate. To fabricate binder-free electrode conductive substrate, active material and adhesion sites are required. For fabrication of binder-free supercapacitor, physical, chemical, and electrical methods are available, which provide advantages of high porosity, low contact resistance, improved ionic conductivity, uniform active material distribution, and increased flexibility of supercapacitor (Fig. 8.1) [8].

8.2 Fabrication Strategies of Binder-Free Electrode Binder-free electrodes can be fabricated with or without using conducting substrate. In the case of the substrate-free method, nanostructured electrode materials are combined with a one-dimensional carbon framework, two-dimensional films, and three-dimensional foams to form free-standing electrodes. Commonly used substrate-free methods for fabrication of binder-free electrodes are graphene-based [9–14] free-standing electrodes, carbon nanotubes-(CNTs) [15–19] based freestanding electrodes, and carbon nanofiber-(CNFs) [20–24] based free-standing electrodes. In the substrate-assisted method, the electrode material is deposited over the metal foils [25–27]. Commonly used substrates for the fabrication of binderfree electrodes are stainless steel mesh-based substrate, MXene-based substrate, and metal/carbon foam-based substrates [28, 29]. A large number of adhesion sites, active material, and conducting substrates are required to fabricate binder-free electrodes. Physical, chemical, and electrical are three methods for manufacturing binder-free electrodes. Figure 8.2 shows different physical, chemical, and electrical methods for fabricating binder-free electrodes.

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Fig. 8.1 Requirements, fabrication strategies, advantages, and future directions of the binder-free electrode (redrawn and reprinted with permission from [8]

8.2.1 Physical Methods In a physical method for binder-free electrode fabrication, the active material is physically deposited over the surface of the current collector (substrate). Primarily used physical methods for binder-free electrode fabrication are electrospinning, vacuum filtration, and physical vapor deposition.

8.2.1.1

Electrospinning

The electrospinning process is used for 1D nanostructured fiber production with the help of electrostatic force. In this process, a suitable solution filled in a syringe, a needle, a pump, a high voltage source, and a grounded collector will be required.

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Fig. 8.2 Fabrication methods of binder-free electrodes for supercapacitor application

To start the electrospinning process, a solution filled in the syringe is pumped at a constant flow rate, and then the solution will move into the needle and form a droplet. After applying the high voltage between the syringe and grounded collected, an electric field is generated, and an electrostatic force acts on the liquid droplet [30]. This liquid meniscus is reshaped into a conical-shaped structure when this electrostatic force becomes higher than the droplet’s surface tension. A liquid jet is injected toward the grounded collector, which will form solid fibers. Figure 8.3a, b shows horizontal and vertical electrospinning setups [30]. Figure 8.3c shows a dual-core Si/C–C coaxial electrospinning setup [31]. For fabrication of nanofibers in electrospinning, homogeneous and stable solution of polymer, aqueous or organic electrolyte [32–34], and active nanoparticles (electrode material) play an essential role [35]. The solution should have minimum viscosity for homogenous fiber structure, and optimum concentration and solvent should have a lower evaporation rate for solidification after leaving the syringe. Apart from these intrinsic properties of the solvent, flow rate, temperature, voltage, distance between needle and collector, and environmental humidity also affect the morphology of fibers [6].

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Fig. 8.3 Electrospinning arrangement of a horizontal set up, b vertical setup [10], and c dual nozzle coaxial setup [31] (Redrawn and reprinted with permission from [10, 31])

Fabrication of Electrospinning Method-Based Supercapacitors Jayawickramage et al. fabricated carbon nanofibers using lignin blended with polyacrylonitrile (PAN). Activated carbon nanofibers (ACNF) were synthesized by electrospinning, subsequently thermal stabilization, carbonization, and CO2 activation of carbonized mats. The electrospinning process occurs at 24 °C and 65% humidity. Obtained fiber mat could easily peel off from the collector. Activated carbon nanofibers (ACNF) mats show 2370 m2 g−1 surface area [36]. Han et al. fabricated CNF/MnO2 composite by electrospinning of polyimide (PI) with polyvinyl pyrrolidone (PVP), subsequently carbonization of PI/PVP fiber for fabrication of CNTs and coating with MnO2 [37]. Kundu et al. fabricated binder-free NiO nanofibers on nickel foam (NiO-NFs/Ni) electrode for supercapacitor using electrospinning method followed by heat treatment [38]. Gopalakrishnan et al. synthesized bubbled surface carbon nanofibers (BCNFs) using mesoporous KIT-6 silica template by electrospinning method. KIT-6 was mixed with PAN-DMF at different wt% and stirred for 24 h to get the viscous and homogeneous suspension for electrospinning over an aluminum collector [39]. Wang et al. fabricated electrospun carbon nanofibers (ECNFs) using PAN (polyacrylonitrile) and enzymatic hydrolysis lignin by electrospinning method followed by stabilization in air and carbonization process. Carbonized carbon

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Fig. 8.4 a Schematic of fabrication of PCNFs and Co3 V2 O8 -Ni3 V2 O8 thin layer @PCNFs based asymmetric supercapacitor using electrospinning method and thermal treatments [41] and b Schematic demonstration of the fabrication of the fiber mat using PBI/6FDD immiscible polymer solution by electrospinning method [42] (redrawn and reprinted with permission from [41, 42])

nanofibers show a 675 m2 g−1 specific surface area [40]. Hosseini et al. fabricated binder-free electrodes using porous carbon nanofibers (PCNFs) substrate with a thin layer of Co3 V2 O8 -Ni3 V2 O8 nanostructures [41]. Figure 8.4a shows the fabrication of PCNFs using PAN/TEOS nanofibers by electrospinning method and subsequently carbonization of PAN/TEOS. After coating PCNFs using cobalt, cobalt–nickel, and cobalt–nickel-vanadium salt, PCNFs and Co3 V2 O8 -Ni3 V2 O8 thin layer @PCNFs electrode-based asymmetric supercapacitor were fabricated [41]. Abeykoon et al. fabricated free-standing phase-separated nanofibers mats by electrospinning of PBI/6FDD blend precursor dissolved in N,N-dimethylacetamide (DMAc) followed by carbonization and activation with CO2 . Figure 8.4b shows the fabrication of phase-separated nanofibers mats by electrospinning of PBI/6FDD blend precursor [42].

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Performance of Electrospinning Method-Based Supercapacitors Activated carbon nanofibers (ACNF) show 2370 m2 g−1 surface area along with 128 F g−1 specific capacitance and 59 Wh kg−1 energy density in coin cell supercapacitor [36]. CNF/MnO2 composite electrode provides 456 F g−1 specific capacitance at 10 mV s−1 scan rate along with ~ 17.8 Wh kg−1 energy density [37]. NiONFs/Ni gives 737 F g−1 specific capacitance at 2 A g−1 current density [38]. Bubbled surface carbon nanofibers (BCNFs) exhibit 593 m2 g−1 specific surface area along with 287 F g−1 specific capacitance at 0.4 A g−1 current density [39]. Carbonized electrospun carbon nanofibers (ECNFs) showed 593 m2 g−1 specific surface area and 216.8 F g−1 specific capacitance at 1 A g−1 current density [40]. Co3 V2 O8 Ni3 V2 O8 TLs@PCNFs positive electrode and PCNFs negative electrode materialbased supercapacitor was showing 59.7 Wh kg−1 energy density and 1.97 kW kg−1 power density at 5 A g−1 current density [41]. PBI/6FDD-based coin supercapacitor provides 142 F g−1 specific capacitance at 10 mV s−1 scan rate [42]. Table 8.1 shows the electrospinning method-based binder-free electrode material-based supercapacitor’s performance.

8.2.1.2

Vacuum Filtration

The vacuum filtration process is a facile, rapid, economical, and efficient method for the fabrication of binder-free electrodes with uniform thickness and homogenous structure. In the vacuum filtration method, the solid active material is separated from liquid with the help of vacuum pressure, and as a result, dense solid is stacked over subtract.

Fabrication of Vacuum Filtration Method-Based Supercapacitors Yuan et al. fabricated free-standing and binder-free electrodes through the vacuum filtration technique (Fig. 8.5a). Na4 Mn9 O18 (NMO) and GO composite was filtered through the filter membrane under reduced pressure, and after a few minutes of washing and drying, it gets peeled off from the membrane. To prepare a freestanding hybrid electrode (NMO-RGO), this composite was heated at 220 °C for 2 h in the atmosphere [43]. Xu et al. provided the fabrication method of AC/rGO composite using the vacuum filtration technique (Fig. 8.5b). Graphene oxide (GO) aqueous dispersion was mixed with activated carbon (AC) dispersion. Activated carbon contains a high specific surface area and hierarchical porous structure [44– 50]. Then some quantity of hydrazine hydrate was added for chemically reduced graphene oxide (rGO). After stirring this mixture, the flask was put into a water bath (~95 °C) for 3 h. Then this mixture solution was vacuum filtered with the help of an organic microscope membrane and got free-standing binder-free AC/rGO film.

17.8Wh kg−1 and 320 Wkg−1 power 22.7 Whkg−1 and 1.25 kWkg−1 power density

67.5 W h kg−1

737 Fg−1 at 2 Ag−1 287 F g−1 at 0.4 A g−1 216.8 F g−1 at 1 A g−1 –

59.7 Wh kg−1 and 1.97 kWkg−1 power density

456 F g−1 at 10 mV s−1

1731 at 1 Ag−1 142 F g−1 at 10 mV s−1

−0.2–0.8 V

0–1 V −1–0 V −1–0.6 V −2–2 V

Electro-spinning

Electro-spinning

Electro-spinning

Electro-spinning

Electro-spinning

Electro-spinning

Core shell type CNF/MnO2

NiO-NFs/Ni

BCNFs

Electrospun carbon nanofibers

Co3 V2 O8 -Ni3 V2 O8 TLs@PCNFs

PBI/6FDD

CNF-Carbon nanofibers, BCNFs-Bubbled surface carbon nanofibers, PCNFs-porous carbon nanofibers

–

59 Wh kg−1 and 15 kW kg−1 power density

128 F g−1 at 10 mV s−1

−2–2 V

Electro-spinning

Polyacrylonitrile-ligninblends carbon fiber –Al

0–0.45 V

Energy density

Specific capacitance/Capacity

Potential window

Synthesis method

Material and substrate

Table 8.1 Electrochemical performances of electro-spun binder-free electrode material-based supercapacitor

92.8% (100 cycles)

85.5 (3000 cycles)

88.8 (2000 cycles)

90.6 (500 cycles)

95.8% (2400 cycles)

95.8% (3000 cycles)

75% (1000 cycles)

Cycle stability % (cycles)

[42]

[41]

[40]

[39]

[38]

[37]

[36]

Refs.

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Fig. 8.5 a Schematic of vacuum filtration method for synthesis of Na4 Mn9 O18 /reduced graphene oxide composite [43] and b activated carbon and rGO composite film-based electrode material fabrication via vacuum filtration method [51] (redrawn and reprinted with permission from [43, 51]).

Performance of Vacuum Filtration Method-Based Supercapacitors Free-standing NMO-RGO composite as a cathode and zinc metal at an anode in an aqueous electrolyte provided a reversible discharge capacity of 83 mAh g−1 at 0.1 A g−1 current density [43]. This composite film of the electrode-(AC/rGO)based light and environmentally friendly supercapacitor delivered 207 F g−1 specific capacitance at 0.2 mA cm−2 current density and 85% capacitance retention after 10,000 charge– discharge cycles [51]. Table 8.2 shows the electrochemical performance of vacuum filtration method-based binder-free electrode-based supercapacitors.

8.2.1.3

Physical Vapor Deposition (PVD)

In the physical vapor deposition technique, the material is deposited over the surface of conducting substrate in a vacuum chamber. Here, three steps take place. First, condense phase material gets vaporized, then this vapor gets condensed over the substrate, and finally, nucleation and development of the film. The most promising methods for physical vapor deposition are thermal evaporation, magnetron sputtering,

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Table 8.2 Electrochemical performances of vacuum filtered binder-free electrode material-based supercapacitors Material and substrate

Synthesis Potential method window

Specific Energy density capacitance/Capacity

NMO-RGO Vacuum filtration

1.08–1.85 V 83 mAh g−1 at 0.1 A − g−1

AC: rGO

0–1 V

Vacuum filtration

207 F g −1 at 0.2 mA cm−2

Cycle Refs. stability % (cycles) ~30% (10,000 cycles)

[43]

16.2 µW h cm−2 85% (10,000 cycles)

[51]

ion beam deposition, and pulse laser deposition (PLD). Through the physical vapor deposition technique, active materials were uniformly distributed over the substrate surface [52].

Fabrication of Physical Vapor Deposition Method-Based Supercapacitors Ataherian et al. fabricated binder-free supercapacitor electrodes by the growth of manganese dioxides (MnOx –MnO, Mn2 O3 , MnO2, and mixed MnO/Mn3 O4 ) nanostructures on carbon nanotubes (CNT) using pulsed laser deposition (PLD) under vacuum and various oxygen partial pressure [53]. Yang reported manganese oxide thin film on the silicon wafer and stainless steel substrates using PLD at various substrate temperatures and oxygen pressure for supercapacitor application [54]. With efficient use of substrate temperature and oxygen pressure in PLD methods, Mn2 O3 , Mn3 O4, and MnOx pure crystalline phase has been synthesized [54]. Liu et al. fabricated a binder-free electrode of nickel–cobalt phosphide (NiCoP) films decorated with size-controlled Ag quantum dots by magnetron sputtering (Ag/NiCoP). Because of good ohmic contact between the interface of Ag quantum dots and nickel–cobalt phosphide (NiCoP), this self-supported electrode exhibits ultrahigh electrochemical performance [55]. Figure 8.6 shows the fabrication schematic of Ag/NiCoP selfstanding electrode using the magnetron sputtering method. First, pretreated nickel foam was immersed in 3 mmol Ni(NO3 )2 ·6H2 O, 3 mmol Co(NO3 )2 ·6H2 O, and 12 mmol hexamethylenetetramine (HMT) dissolved in 60 mL of distilled water for nickel–cobalt hydroxide layer (NiCo-LDHs). NiCoP was fabricated by phosphorylation route using NaH2 PO2 .H2 O. At the last, silver quantum dots dispersion takes place using the magnetron sputtering method [55].

Performance of Physical Vapor Deposition Method-Based Supercapacitors The MnO2 -CNT-based binder-free electrode shows 549 F g−1 specific capacitance at 10 mV s−1 scan rate and 95% capacity retention after 10,000 cycles [22]. Ag/NiCoP binder-free electrode provides 3050 F g−1 specific capacitance at 1 A g−1 current

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Fig. 8.6 Fabrication of Ag/NiCoP binder-free electrode with hydrothermal and magnetron sputtering methods (redrawn and reprinted with permission from [55])

Table 8.3 Synthesis methods and electrochemical performances of binder-free electrode material Material and substrate

Synthesis method

Potential window

Specific Energy capacitance/Capacity density

Cycle Refs. stability % (cycles)

MnO2 -CNT Pulsed laser deposition

0–1 V

549 F g−1 at 10 mV s−1

–

95% (10,000 cycles)

[53]

Mn2 O3 film Pulsed laser deposition

−0.1–0.9 V 210 F g−1 at 1 mV s−1

–

96.7% (from 8 to 30 cycles)

[54]

Ag/NiCoP

0–0.4 V

Magnetron sputtering

050 F g−1 at 1 A g−1 0.254 mW h 75% cm−2 and (4000 cycles) 1.88 mW cm−2 power density

[55]

density with 75% capacity retention after 4000 cycles [55]. Mn2 O3 film thin fabricated by PLD provided 210 F g−1 specific capacitance at 1 mV s−1 current density with 96.7% capacity retention from 8 to 30 cycles [54]. Table 8.3 shows the electrochemical performance of the physical vapor deposition method-based binder-free electrode materials.

8.2.2 Chemical Methods In the chemical method, the active material is deposited over the conductive substrate using chemical treatment methods. Commonly used chemical methods are thermal,

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hydrothermal, chemical bath deposition, chemical vapor deposition, and atomic layer deposition.

8.2.2.1

Thermal Treatment

Thermal treatment of active material creates a self-supporting backbone by immobilizing the active materials. The thermal treatment method changes the physical and chemical properties of active material. Through thermal treatment, graphene oxide converts into reduced graphene oxide, inorganic salt will convert into transition metal oxides, and polymer will form a carbon-based self-supporting structure.

Fabrication of Thermal Treatment-Based Supercapacitors In Fig. 8.7, rGO is used as the carbon black (CB) binder and reduced graphene oxide (rGO) nanocomposite through the thermal treatment process. After applying the high temperature to CB/GO composite, there forms high electrically conductive nanocomposite (CB/rGO). Because of the rGO binder, there is free access to the pore for the electrolyte ions. Figure 8.7a shows binder-free electrode material using CB/rGO nanocomposite and noncovalent bonding of CB/rGO with the current collector. Figure 8.7b, c shows an FE-SEM image of thermally treated carbon black and graphene oxide (CB/GO_TT). Figure 8.7d demonstrates the schematic of CB/rGO nanocomposite formation. First, carbon black and graphene oxide dispersion was stirred with H2 O/TX100 solution. Then through thermal treatment, CB/rGO composite was deposited over the surface of the rGO current collector. For supercapacitor device assembly, CB/rGO composite was used for symmetric supercapacitor with TEMA-TFB in ACN electrolyte.

Performance of Thermal Treatment-Based Supercapacitors Carbon black/rGO-rGO paper-based supercapacitor provides 8.8 Wh kg−1 energy density with a high specific power of 32.1 kW kg−1 at 1 A g−1 current density [56]. Table 8.4 shows the electrochemical performance of thermal treatment-based binder-free electrode material-based supercapacitors.

8.2.2.2

Hydrothermal Treatment

Hydrothermal is a precise, facile, and versatile method for preparing binder-free electrode material. It provides eco-friendly, inexpensive, well-defined crystalline morphology, highly pure, uniform pose size distribution for material fabrication, and manageable nanoscale material. At high pressure and temperature, material solubility gets increased in water. Metals ions form a supersaturated solution with solvent at

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Fig. 8.7 a Carbon black (CB) and reduced graphene oxide-(rGO)based composite with noncovalent interaction (NCl) with the current collector, b, c FE-SEM images of thermally treated carbon black and graphene oxide (CB/GO_TT), d Scheme of fabrication of CB/rGO nanocomposite and electrochemical testing of CB/rGO-based supercapacitor device (redrawn and reprinted with permission from [56])

Table 8.4 Electrochemical performance of thermal treatment method based binder-free electrode material Material and substrate

Synthesis Potential method window

Specific Energy density capacitance/Capacity

Refs.

Thermal −2.7–2.7 V 31.1 F g−1 at 1 A g−1 8.8 Wh kg−1 and [56] Carbon black 32.1 kW kg−1 power /rGO-rGO paper treatment density

high temperature and pressure, forming crystal growth at the nucleation point at the substrate. Hydrothermal treatment has attracted vast interest in the field of energy storage [57, 58].

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Fig. 8.8 a Nio-PANI on carbon cloth-based electrode material preparation using hydrothermal and electrodeposition methods [59], b CoMn2 O4 and MnS nanoparticles growing on Ni foam using hydrothermal treatment [60], c preparation of CuCo2 S4 hollow nanoneedles array on Ni foam using hydrothermal method [61] (Redrawn and reprinted with permission from [59–61])

Fabrication of Hydrothermal Treatment-Based Supercapacitors Razali et al. have fabricated a binder-free composite electrode consisting of carbon cloth-nickel oxide-polyaniline (EC-NiP) for supercapacitor application by growing NiO over the carbon cloth through hydrothermal method followed by the electrodeposition of PANI, then fabricated PVA/H2 SO4 electrolyte-based symmetric supercapacitor (Fig. 8.8a) [59]. Figure 8.8b shows core shell-like electrode preparation via two steps hydrothermal method. First, CoMn2 O4 nanowires were deposited over the Ni foam, then MnS nanoparticles were built-up over the CoMn2 O4 via hydrothermal treatments [60]. In Fig. 8.8c, Moosavifard et al. have fabricated a CuCo2 S4 hollow nanoneedles array on Ni foam-based binder-free electrode material using the template-free hydrothermal technique. First, copper-cobalt (CuCo) nanoneedles grow over the surface of nickel foam using the hydrothermal method. These nanoneedles get converted into hollow nanoneedles through the Kirkendall effect using the hydrothermal method [61].

Performance of Hydrothermal Treatment-Based Supercapacitors Symmetric supercapacitor using PVA/0.5 M H2 SO4 electrolyte provided 193 F g−1 specific capacitance at 0.5 A g−1 current density and 72% capacity retention after

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4500 charge–discharge cycles [28]. CoMn2 O4 @MnS-Ni foam binder-free electrode material provided 213 mA h g−1 specific capacity at 2 A g−1 current density and 91% capacity retention after 5000 charge–discharge cycles [60]. The reported value of electrode CuCo2 S4 -Ni foam is 2163 F g−1 at 6 mA cm−2 scan rate [61]. Table 8.5 shows the electrochemical performance of hydrothermal treatment-based binder-free electrode material-based supercapacitors.

8.2.2.3

Chemical Bath Deposition

Chemical bath deposition (CBD) is the process of homogenous thin film deposition on the substrate through a chemical reaction using heterogeneous nucleation of ions of the aqueous precursor solution. It can occur at low temperature, ambient pressure, and low cost. CBD can be one of the most efficient methods for fabricating binderfree electrode material by directly nucleating and growing electrode material on the current collector by adjusting the pH and temperature of the chemical reaction. There are many advantages of the CBD method compared to conventional coating approaches. It will reduce the charge transfer resistance between the electrode and current collector, evenly distribute active material on the current collector, control the growth of electrode material, and make it easy to do mass production [62].

Fabrication of Chemical Bath Deposition-Based Supercapacitors In Fig. 8.9a, Heo et al. have synthesized a template and binder-free 1D bimetallic hydrogen phosphate (Cox Nix (HPO4 ) electrode through the CBD method for supercapacitor application. Co(NO3 )2 .6H2 O and Ni(NO3 )2 .6H2 O were dissolved in water, then Na2 HPO4 dissolved in water was gradually added to it under a stirrer. Through the chemical bath deposition method pale pink precipitate of CoNi(HPO4 ) was deposited over nickel foam [63]. Figure 8.9b demonstrates the deposition of NiCo2 S4 on Ni foam by chemical bath deposition and calcination for the binder-free electrode. Nickel acetate, cobalt acetate, and thiourea were used as the precursor for NiCo2 S4 preparation. First, nickel foam gets dipped into the solution for chemical bath deposition and then calcinated at 350 ˚C for 3 h [62].

Performance of Chemical Bath Deposition Method-Based Supercapacitors Asymmetric supercapacitor based on (Co0.75 Ni0.25 (HPO4 ) and activated carbon was showing 182.5 F g−1 specific capacitance at 0.5 A g−1 current density along with 64.88 Wh kg−1 energy density at 800 W kg−1 power density. (Co0.75 Ni0.25 (HPO4 ) electrode exhibits 94.8% capacity retention after 5000 charge–discharge cycles [63]. NiCo2 S4 and activated carbon-based hybrid capacitor shows 198.6 C g−1 capacity and 29.1 Wh kg−1 energy density at 1 A g−1 current density with a 2.6 V potential

Synthesis method

Hydrothermal treatment

Hydrothermal treatment

Hydrothermal treatment

Material and substrate

NiO-PANI-carbon cloth

CoMn2 O4 @MnS-Ni foam

CuCo2 S4 -Ni foam

213 mA h Ag−1 2163 F g−1 at 6 mA cm−2

0–0.55 V −0.1–0.55 V

at 2

44 Wh kg−1

–

21.63 mWh kg−1

192.31 at 0.5 A g−1

−0.2–0.7 V g−1

Energy density

Specific capacitance/Capacity

Potential window

96% (3000 cycles)

91% (5000 cycles)

72% (4500 cycles)

Cycle stability % (cycles)

Table 8.5 Electrochemical performances of hydrothermal treatment based binder-free electrode material for supercapacitor applications

[61]

[60]

[59]

Refs.

210 K. D. Verma and K. K. Kar

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Fig. 8.9 a Cox Nix (HPO4 ) binder-free electrode material fabrication using chemical bath deposition method [63] and b deposition of NiCo2 S4 on Ni foam using chemical bath deposition and calcination methods (redrawn and reprinted with permission from [62])

window [62]. Table 8.6 shows the electrochemical performance of chemical bath deposition-based binder-free electrode material-based supercapacitors.

8.2.2.4

Chemical Vapor Deposition (CVD)

Chemical vapor deposition is a chemical reaction method in which high vapor pressure gaseous substance deposits on the hot substrate surface in a vacuum. The

Chemical bath deposition

Chemical bath deposition

NiCo2 S4 and AC

Hydrogen phosphate (Co0.75 Ni0.25 (HPO4 )/AC 0–1.6 V

0–2.6 V

Synthesis method Potential window

Material and substrate Energy density

29.1 Wh kg−1 and 13 kW kg−1 power density 64.88 Wh kg−1

Specific capacitance/Capacity 198.6 C g−1 at 1 A g−1 182.5 F g−1 at 0.5 A g−1

Table 8.6 Electrochemical performances of chemical bath deposition method based binder-free electrode materials

95% (5000 cycles)

52% (1500 cycles)

Cycle stability % (cycles)

[63]

[62]

Refs.

212 K. D. Verma and K. K. Kar

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CVD method is widely used to forma dense, stable, and uniform thin film on threedimensional substrates and nanowires with the help of catalysts [64, 65]. It is a threestep method for directly growing active material on the current collector surface. First, gaseous substances will diffuse and adsorb on the substrate surface then, this will do a chemical reaction with the active site of the surface and form a uniform thin coating, and at last, exhaustion of generated gases will take place [16]. The main parameters which influence the size, morphology, and defects of the coating are temperature, pressure, gas flow rate, and distance between substrate and source.

Fabrication of Chemical Vapor Deposition-Based Supercapacitors Hsieh et al. have fabricated the binder-free electrode material using CoMn2 O4 (CMO) coated carbon nanotube (CNT), which is grown on stainless steel mesh using chemical vapor deposition, electrodeposition, and calcination process (Fig. 8.10a). First, CNT was grown on stainless steel mesh via the CVD method using C2 H2 precursor gas at 700 °C then galvanostatic electrodeposition was carried out to get the CMO nanosheets on CNTs [66, 67]. Figure 8.10b shows showing fabrication of electrodes using carbon nanofibers (CNF) grown on nickel foam using the CVD method. First, nickel foam was placed in a quartz tube for chemical vapor deposition using acetylene C2 H2 as a carbon source. CNF decorated Ni-f-based binder-free electrode was housed in stainless steel split cell with 6 M KOH electrolyte for supercapacitor application [68].

Performance of Chemical Vapor Deposition Method-Based Supercapacitors Further, CMO/CNT/SSM and AC/SSM-based asymmetric supercapacitors were fabricated, which showed 133 F g−1 specific capacitance at 0.5 A g−1 current density and 47.39 W kg−1 energy density with 400 Wh kg−1 power density [66]. CNF on Ni-f-based binder-free supercapacitor was showing 142 ± 7 mF cm−2 areal capacitance at 10 mA cm−2 current density [68]. Table 8.7 shows the electrochemical performance of chemical vapor deposition-based binder-free electrode material-based supercapacitors.

8.2.2.5

Atomic Layer Deposition (ALD)

Atomic layer deposition is a layer-by-layer thin film deposition technique. It is similar to the chemical vapor deposition technique. It consists of a minimum of two different and reactive precursor gasses in which the first gas will be deposited over the surface of the current collector. After forming a uniform coating on the substrate, the other gas will form the next layer over the first layer.

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Fig. 8.10 a fabrication of CoMn2 O4 (CMO) coated carbon nanotube (CNT) grown on stainless steel mess-based binder-free electrode for asymmetric supercapacitor [66]. b Fabrication of binderfree electrode composed of carbon nanofibers (CNF) grown on nickel foam (Ni-f) [68] (redrawn and reprinted with permission from [66, 68])

Fabrication of Atomic Layer Deposition Method-Based Supercapacitors Nandi et al. have fabricated bind-free electrodes through atomic layer deposition of MoS2 using Mo(CO)6 and H2 S plasma on 2D stainless steel foil (SS foil) and 3D nickel foam (Fig. 8.11). Atomic layer deposition provides a uniform coating on stainless steel and nickel foam. Because of the three-dimensional structure, nickel foam will contain higher mass loading [69].

Performance of Atomic Layer Deposition Method-Based Supercapacitors MoS2 @3D-Ni foam-based binder-free supercapacitor electrode was showing 3400 mF cm−2 areal capacitance at 3 mA cm−2 current density along with 82% capacitance retention after 4500 charge–discharge cycles [69]. Table 8.8 shows the electrochemical performance of atomic layer deposition-based binder-free electrode material-based supercapacitors.

Synthesis method

Chemical vapor deposition

Chemical vapor deposition

Material and substrate

CoMn2 O4 -CNT/AC

CNF/Ni-f 0–0.9 V

0–1.6 V

Potential window

Energy density 47.39 W kg−1 62 mWhm−2 at 82 W m−2 power density

Specific capacitance/Capacity 133 F g−1 at 0.5 A g−1 142 ± 7 mF cm−2 at 10 mA cm−2

Table 8.7 Electrochemical performances of chemical vapor deposition method based binder-free electrode material

100% (10,000 cycles)

90% (1000 cycles)

Cycle stability % (cycles)

[68]

[66]

Refs.

8 Binder-Free Supercapacitors 215

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Fig. 8.11 MoS2 thin film growth on 2D stainless steel and 3D nickel foam using atomic layer deposition technique (redrawn and reprinted with permission from [69])

Table 8.8 Electrochemical performances of atomic layer deposition method based binder-free electrode material Material and substrate

Synthesis method

Potential window

Specific capacitance/Capacity

Cycle stability % (cycles)

Ref.

MoS2 @3D-Ni foam

Atomic layer deposition

0.0–0.6 V

3400 mF cm−2 at 3 mA cm−2

82% (4500 cycles)

[69]

8.2.3 Electrical Methods In an electrical method, the electrode material is deposited over the current collector using electrical treatment methods. Commonly used electrical methods are electroplating, anodization, and electrophoretic deposition.

8.2.3.1

Electroplating

Electroplating is a metal coating process in which metal ions transfer from the positive electrode to the substrate surface via electrolyte solution after applying the electric current. The process used in electroplating is known as electrodeposition. It is used to form a nano-rage structure and coating of active material on the current collector for binder-free supercapacitors. Electroplating time and current density during the process will decide the thickness of the coating on the current collector [70]. This method provides low contact resistance between the electrode and current collector and a protective interface between electrode and electrolyte.

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Fig. 8.12 Illustration of binder-free electrode (MnO2 / ZnCo2 O4 /G/Ni foam) using hydrothermal and electroplating methods (redrawn and reprinted with permission from [71])

Fabrication of Electroplating Method-Based Supercapacitors Cong et al. have shown the fabrication of binder-free supercapacitors using hydrothermal and electroplating methods (Fig. 8.12). Nickel foam was used as a substrate for supercapacitors. First, graphene (G) was dropped over the nickel substrate with the help of the hydrothermal method, then ZnCo2 O4 precursor was deposited over the graphene substrate. After the annealing process for 4 h, the electroplating technique was used to depose MnO2 over the surface of ZnCo2 O4 substrate [71].

Performance of Electroplating Method-Based Supercapacitors This binder-free electrode (MnO2 /ZnCo2 O4 /G/Ni foam) can provide 131.78 F g−1 specific capacitance at 0.5 A g−1 current density and 46.85 Wh kg−1 energy density and 166.67 W kg−1 power density along with 91% capacitance retention after 5000 cycles [71]. Table 8.9 shows the electrochemical performance of electroplating-based binder-free electrode material-based supercapacitors.

8.2.3.2

Anodization

Anodization is a well-accepted electrochemical process for modifying the oxide layer on the metal surface. Anodization is a fast, simple, and economical method that

218 Table 8.9 material

K. D. Verma and K. K. Kar Electrochemical performances of electroplating method based binder-free electrode

Material and substrate Synthesis method

Potential Specific Energy Cycle Ref. window capacitance/Capacity density stability % (cycles)

MnO2 /ZnCo2 O4 /G/Ni Electroplating 0–1.6 V foam

Table 8.10 material

131.78 F g−1 at 0.5 A g−1

46.85 Wh kg−1

91% (5000 cycles)

[71]

Electrochemical performances of anodization method based binder-free electrode

Material and substrate

Synthesis method

Potential window

Specific capacitance/Capacity

Cycle stability % (cycles)

Refs.

Nickel compound

Anodization

−0.3–0.8 V

167 mF cm−2 at 500 mV s−1

115% (4500 [73] cycles)

NiO–Ni foam

Anodization

0.05–0.45 V

4.74 F cm−2 at 4 mA cm−2

85.4% (1000 [74] cycles)

provides controllable nanostructure oxide film, large surface area, and low electrical resistivity. These properties can provide high capacitance and rate capability for binder-free supercapacitor applications [72]. Aluminum, titanium, iron, niobium, zirconium, and tantalum are some common substrates on which anodization has been done.

Fabrication of Anodization Method-Based Supercapacitors Zhang et al. demonstrated a one-step anodization method, hierarchical nickel compound (HNC) film with a large surface area and interconnected a nanoscale pore which provides high electrical conductivity [73]. Yang et al. have reported a one-step anodization process to fabricate binder-free electrodes by growing porous NiO nanosheets on nickel substrate using molten KOH electrolyte [74].

Performance of Anodization Method-Based Supercapacitors HNC electrode demonstrates 167 mF cm−2 areal capacitance at 500 mV s−1 scan rate along with 115% capacitance retention after 4500 charge–discharge cycles [73]. NiO supported on nickel foam electrode provides 4.74 F cm−2 areal capacitance at 4 mA cm−2 current density [74]. Table 8.10 shows the electrochemical performance of anodization method-based binder-free electrode material-based supercapacitors.

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Fig. 8.13 Fabrication of binder-free electrode by deposition of MXene (Ti3 C2 Tx ) on nickel foam (NF) using EPD method (redrawn and reprinted with permission from [77])

8.2.3.3

Electrophoretic Deposition

Electrophoretic deposition (EPD) is a widely used material processing technique used for surface coating and thin-film fabrication. Here, colloidal charge particles dispersed into a liquid solution will migrate toward the conductive substrate under an applied electric field [75]. The EPD method provides homogenous, mass loading adjustable film, economical, green, and controllable operation to fabricate binderfree electrodes. The film thickness of active material is controlled by electrolyte solution, deposition time, and applied voltage [15, 75, 76].

Fabrication of Electrophoretic Deposition Method-Based Supercapacitors Xu et al. have fabricated a binder-free electrode by Ti3 C2 Tx deposition over the nickel foam (NF) using the electrophoretic deposition (EPD) method (Fig. 8.13). First, MXene was synthesized by HF-etching of MAX phase (Ti3 AlC2 ) and ultrasonication. Then dispersed MXene (Ti3 C2 Tx ) was deposited on nickel foam using the EPD technique [77].

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Table 8.11 Electrochemical performances of electrophoretic deposition method based binder-free electrode material Material and substrate

Synthesis method

Potential window

Specific capacitance/Capacity

Cycle stability % (cycles)

Ti3 C2 Tx film

Electrophoretic deposition

−0.75–0.25 V

140 F g−1 at 5 mV s−1 100% (10,000 cycles)

Ref.

[77]

Performance of Electrophoretic Deposition Method Based Supercapacitor This binder-free electrode can deliver 140 F g−1 specific capacitance at 5 mV s−1 scan rate with 100% capacity retention after 10,000 charge–discharge cycles [77]. Table 8.11 shows the electrochemical performance of electrophoretic method-based binder-free electrode material-based supercapacitors.

8.3 Conclusions The binder-free electrode material is one of the scopes to increase the performance of supercapacitors because this will reduce the contact resistance between the electrode and current collector and provide a higher effective surface area for charge storage in the active material. There are three types of fabrication strategies for binderfree supercapacitors which are physical, chemical, and electrical methods. Electrospinning, vacuum filtration, and physical vapor deposition techniques are physical methods. Thermal, hydrothermal, chemical bath deposition, chemical vapor deposition, and atomic layer deposition come under chemical methods. Electroplating, anodization, and electrophoretic deposition techniques work as electrical methods.

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Chapter 9

High Mass Loading Supercapacitors Mukesh Kumar and Kamal K. Kar

Abstract The demand for energy storage devices is growing daily in stationaries and mobile applications. Commercial supercapacitors require a high (at least 30%) active electrode material mass of the whole device to fulfill particular applications’ energy and power density demands. Therefore, high mass loading supercapacitors have greatly interested in researching high-specific capacitance electrodes for the commercial application of high-energy storage devices. High mass loading in supercapacitor fabrication is challenging because of the sluggishness of electrical and ion migration kinetics. The commercial level supercapacitor requires high mass loading greater than 10 mg cm−2 or film thickness of 150–200 μm. These are scaleable production parameters. On the other hand, the strategies adopted to lower the production cost reduce the supercapacitor’s performance. To overcome the adverse effects, researchers and scientists take many synthesis approaches to increase the pore distribution, the proper pore size for electrolytes, and improve the active surface area of electrode materials. Chemical vapor deposition, layer by layer deposition, aerogel, and hydrogel methods are used to create an advanced porous structure that provides an easy ion diffusion path. Apart from doping heteroatoms, intercalation in a layered structure and surface medication increase the active surface area, which controls the electrochemical performance of high mass loading supercapacitors. But the ion diffusion in electrode materials largely depends on the proper pore size, which quickly provides the path.

M. Kumar · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_9

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9.1 Introduction Energy storage devices are primary requirement to derive the power system in stationary and mobile applications such as wearable electronic devices, power grid, electromotive vehicle (EV), renewable energy, household, rocket propulsion, automotive, flexible wearable, Internet of things (IoT). Various energy storage devices have been developed to fulfill the requirement, i.e., mechanical energy storage systems (flywheels), and electrochemical energy storage devices like batteries and supercapacitors, etc. [1]. Among these devices, supercapacitors play an essential role in all sectors. They have unique characteristics with high power density than batteries. But the supercapacitors have low energy density. Hence, these devices are bridging the gap between capacitors and batteries. Supercapacitors are classified into three categories based on electrode materials and charge storage mechanisms: [2–4] 1. EDLC (ion adsorption). 2. Pseudocapacitance (surface redox reaction). 3. A hybrid supercapacitor (above mentioned both mechanisms). Generally, the energy in supercapacitors is stored through ion adsorption nearsurface in an electric double-layer capacitor (EDLC), faradic reversible electron transfer reactions in pseudocapacitance, and a combination of the charge storage process used in hybrid supercapacitors [5]. There are different types of materials as electrodes, i.e., carbons [6–8] (which have low charge capacitance), conducting polymers [9, 10], and transition metal oxide and hydroxide as pseudocapacitive material [11–14]. This will be discussed in the next segment. EDLC electrode material has high electrical conductivity but low specific capacitance, but pseudocapacitance material has high specific capacitance but somewhat low electrical conductivity. Both the materials separately do not fulfill the energy and power density demand. Therefore, composite, advanced, or hybrid materials are being developed to meet the high-energy and power density of supercapacitors. Still, the production of this device to the industrial level needs some improvement in energy density that should be fulfilled. For commercial-level supercapacitors, production requires scaleable production techniques and low production costs. For commercial production and supercapacitor feasibility, the electrode’s active mass should be >10 mg cm−2 or film thickness of 100–200 μm. Generally, it is observed that laboratory-level supercapacitor devices usually are prepared by lower mass loading ( C2 H3 O2 > Cl

− − − − > Br− > NO− 3 > ClO3 > I > ClO4 > SCN

+ + + 2+ NH+ > Ca2+ > guanidinium 4 > K > Na > Li > Mg

The initial members of the series have dominating salting-out [98] behavior and on moving toward the right side, the salting-in behavior starts dominating. Salts having cations and anions from salting-in properties results in good solutions, and they help other solutes to be accepted by the solvents. Salting-in and salting-out behavior also [92, 98] affect the transparency and mechanical behavior of final product. Three cations, namely, NH4 , Na, and Mg have been selected to study the nature of synthesized electrolytes. The photographs of the membranes, shown in Fig. 10.5, indicate that Mg-based materials are smooth, and they are found to be more flexible also. Nabased materials also have good flexible morphology, but the NH4 -based electrolytes are comparatively brittle. Hence, further studies have been focused on Na [86–88, 93, 94, 96] and Mg [89, 90] salts. A variety of sodium and magnesium salts have been used with different starches (arrowroot, rice, potato, wheat, and corn starches) and all resulted in smooth, homogeneous, and flexible electrolyte membranes with good electrochemical properties. Table 10.2 gives the summary of different starch-salt combinations already investigated or under-investigated in our lab. PISE synthesis has been tried up to 1:5 Starch:Salt ratio. Although the electrolyte remained homogeneous and no crystallization or inhomogeneity is observed in the system, but all the materials irrespective of the type of starch and salt, were in the high viscous liquid state. Till 1:3 starch: salt ratio good quality free-standing membranes were obtained for all the starch and salt combinations. Mg(ClO4 )2 with potato starch (1:3 starch:

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Fig. 10.5 Photograph of crosslinked starch-based PISE systems a Mg-salt based, b Na-salt based, and c NH4 -salt based

Table 10.2 Starch-salt systems studied by our group which resulted in good flexible PISE Starches

Salts (already investigated)

Salts (under-investigated)

Potato starch

NaI, NaSCN, NaClO4 , NH4 I, Mg(ClO4 )2 , MgCl2

Li-salts

Arrowroot starch

NaI, NaClO4 , Mg(ClO4 )2

Li-salts, MgCl2

Corn starch

NaI, NaClO4 , Mg(ClO4 )2

Li-salts, MgCl2

Rice starch

NaI, NaClO4

Li-salts, Mg(ClO4 )2 , MgCl2

Wheat starch

NaI

Li-salts, Mg(ClO4 )2 , MgCl2 , NaClO4

Fig. 10.6 Photographs depicting the transparency and flexibility of crosslinked starch-based PISEs

salt ratio) was the highest conducting (0.18 S/cm) combination with 3.2 V ESW. The material is very flexible and soft. Figure 10.6c indicates that material can very easily be folded many times. All the starch: salt combinations given in Table 10.2 resulted in conductivity >0.01 S/cm, ESW > 2.2 V and ion relaxation time in micro sec range. All have similar kinds of flexibility in the PISE range as shown in Fig. 10.6.

10.2.4.3

Effect of Ambient on the Electrochemical Properties

For stable device performance, its component should have ambient independent electrochemical behavior. The conductivity of synthesized electrolytes has been studied

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Fig. 10.7 The variation of conductivity with a temperature and b humidity variation for a typical crosslinked starch based PISEs

in the normal ambient temperature variation (up to 100 °C) [89, 93] and for humidity variation (%RH varying from 25 to 99%). Figure 10.7a and b indicate the variation of conductivity with temperature and relative humidity for a typical crosslinked starchbased PISEs. All the PISE systems behave similarly with very small variations. As indicated, the variation of conductivity is very nominal which confirms that synthesized material may be a potential PISE to reach at the commercial level if explored more systematically, especially targeting the morphology details.

10.3 Conclusion Scientists have established the importance of PISEs in the 1990s, but the unavailability of a suitable host, which can accommodate a large amount of salt required for ion-cluster formation throughout the matrix and keep the mechanical properties good enough for device fabrication, is still the biggest challenge. Literature indicates that still efforts are being made to achieve even conductivity ~10−4 S/cm with good mechanical properties. Till date to achieve PISEs, special efforts have been made to synthesize PISEs. First, the salt or salt combination is identified in the molten state and a suitable polymer is recognized. Many times some additives/fillers/plasticizers, if required, are also added to improve the electrochemical and/or mechanical properties. LiTFSI and LiFSI are found to be the most suitable salts for PISE synthesis, but both are very costly. Since in PISEs a larger salt amount is required hence the synthesized PISEs are not suitable on the economical ground also. For commercial use of any material being economical is the most important factor. In comparison to all these synthetic polymer host-based studies, the crosslinked starch seems to be an excellent host for PISE synthesis. It is an economical renewable polymer, and it accepts all kinds of salt in the normal state. The synthesis process neither requires any sophisticated instrument nor any complicated chemical procedure is involved hence the developed synthesis protocol is economically viable. These materials seem to

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inherently have the benefits of WIPSE also, because of high moisture content. The electrochemical properties are exceptionally good. If the structural investigations are carried out in greater detail, then a suitable guideline can be sketched for the selection of starch and salt. These electrolyte systems have all the favorable properties to be used at the commercial level. Acknowledgements The author is thankful to University Grant Commission (New Delhi) for supporting the project entitled “Synthesis & Electrical Characterization of Starch-based Electrolyte Systems” through project sanction no 42-814/2013 (SR) dated 22.03.2016. and to BHU-Varanasi for providing an “Incentive grant to senior faculties” under IoE Scheme (Year 2021–2022) to carryout crosslinked starch-based electrolytes work. Author is thankful to Ms. Dipti Yadav (Ph.D. scholar) for helping in manuscript preparation.

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Chapter 11

Magneto-Electric Supercapacitors Ananya Chowdhury, Sudipta Biswas, Abyaya Dhar, Joyanti Halder, Debabrata Mandal, Poornachandra Sekhar Burada, and Amreesh Chandra

Abstract This chapter deals with understanding the effect of external magnetic field on the performance of supercapacitors fabricated using magnetically responsive materials, i.e. magneto-electric supercapacitors. Further, a simple theoretical model is also provided to explain the experimental data. A new theoretical model is required because the conventional models used to explain the supercapacitive behaviour do not have any terms, which consider the possibility of changing magnetic fields and its impact on electrochemical behaviour.

11.1 Introduction Supercapacitors, as energy storage devices, are being investigated for last many decades. It is only over the last few years that magnetic supercapacitors have started emerging as useful devices for various applications [1–4]. These devices can be easily operated near equipments, which are known to generate magnetic field of varying strengths. Few examples of such equipments are: generators, solenoids, permanent magnets, etc. Nanoscale magnetic materials can also act as a source for magnetic field. The field generated by such materials may be weak but leads to a gradient magnetic field, which can directly affect the transfer mechanisms within the nanosized paramagnetic or ferromagnetic molecules. Hence, the last few years have seen a revisiting of works on supercapacitors and batteries, that were using magnetic metal oxides as electrodes [5–7]. Results actually indicate that, in presence of a magnetic field, there can be an appreciable enhancement in the current due to a convection flow of the electrolyte ions near the electrodes. The modified electrode-electrolyte interface (EEI) leads to a reduction in the interfacial contact resistance and suppression of the Nernst layer dimension. Hence, modulated electrochemical response characteristics can be expected. A. Chowdhury · S. Biswas · A. Dhar · J. Halder · D. Mandal · P. S. Burada · A. Chandra (B) Department of Physics, School of Nanoscience and Technology, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_11

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Transition metal oxides, over the last few decades, are the most used materials in pseudocapacitors. The magnetic nature in these materials originates from the effective spin interaction of the materials, which can be short or long-range ordering [1]. Fe-based materials are mostly used for magnetic applications and have been also used as pseudocapacitor electrodes. There are other magnetic transition electrodes, which are being used in supercapacitors. These include: Fe2 O3 , NiO, Co3 O4 , CuO, Mn3 O4 , etc. This chapter deals with such magnetic supercapacitors, their fabrication processes, and the origin of magnetic field-dependent behaviour in them.

11.2 Synthesis of Magnetic Transition Metal Oxides It is well known and already shown in other chapters that morphologies of the electrode materials play an important role in deciding the electrochemical performance. Mostly, solid morphologies of various nanoparticles have been utilized, but a recent trend indicates that appreciable performance enhancement can be achieved by using hierarchical and porous morphologies [8–10]. The morphology of the synthesized materials depends upon the synthesis protocols. Most of the synthesis protocols for obtaining nanomaterials involve top-down or bottom-up approaches. For obtaining hierarchical structures, with high surface area, bottom-up approaches are more common. For solid structures, both strategies are utilized. Therefore, a few of the synthesis protocols used to obtain solid, hollow, and porous magnetic electrode nanomaterials are discussed below.

11.2.1 Synthesis of Fe2 O3 Nanoleaflets Figure 11.1a shows the scanning electron micrographs (SEM) of Fe2 O3 , with leafletlike solid morphology. Forced hydrolysis and reflux condensation of Fe3+ can be used for the preparation of such particles in the presence of (NH2 )2 CO (urea). 0.25 M 50 mL of FeCl3 and 1 M 50 mL of (NH2 )2 CO can be used as the precursor. The solution is initially prepared in deionized (DI) water, under continuous stirring for 45 min. In the next step, transfer the reaction mixture to a round bottom flask and reflux at 100 °C for 12 h. A yellow-coloured precipitate would be obtained, which needs to be washed with DI water and ethanol (three times each) in order to get rid of the unreacted impurities. The obtained precipitate should be dried at 80 °C, under vacuum, for 15 h. Following calcination in air at 300 °C for 2 h, the desired phase and morphology of Fe2 O3 can be obtained [9].

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Fig. 11.1 SEM micrographs of a nanoleaflets, b solid microrods, c porous microrods, d porous nanorods, e solid sphere and f hollow sphere for Fe2 O3

11.2.2 Synthesis of Fe2 O3 Rod-Like Structures Let us discuss a few more synthesis protocols to obtain different types of Fe2 O3 nanoparticles. For example, let us start with the protocol to obtain solid microrods (MR) of Fe2 O3 . Start with a solution of 0.4 M FeSO4 .7H2 O in water. To this, add an equal volume of 0.4 M oxalic acid solution, stir the solution at RT for 30 min. This would lead to an iron(II) oxalate complex, indicated by a colour change of the solution from colourless to yellow. Following centrifugation and washing with DI water and ethanol, leave the precipitate to dry overnight at 45 °C. The dried product

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can be calcined at 400 °C, with a slow ramp, for 12 h, leading to the formation of Fe2 O3 . This can be seen from SEM depicted in Fig. 11.1b. A slight change in this protocol can lead to the formation of porous microrods (PMR). Instead of mixing at room temperature, heat the mixture of iron(II) and iron(III) salt at 70 °C for 2 h. In addition, a mixed solution of oxalic acid and sodium dihydrogen phosphate should also be used as they would facilitate the wet chemical etching of the porous microrods. The SEM micrograph of the porous particles can be seen in Fig. 11.1c, d.

11.2.3 Synthesis of Fe2 O3 Nanospheres Similarly, conventional solid to fashionable hollow morphologies can also be synthesized. For the formation of solid Fe2 O3 spheres, take a solution of 3:2 ratio by weight of Fe(III) salt and sodium acetate in 20 mL ethanol. Stir this solution at room temperature till the solute dissolves completely. Thereafter, dropwise add 2 mL of DI water and stir continuously for another 30 min. Transfer the prepared solution to a Teflonlined stainless-steel autoclave and heat it at 200 °C for 1 h before allowing it to naturally cool down to room temperature. The precipitate can be centrifuged and washed using the strategies discussed earlier. For the formation of hollow spheres, the entire process is to be kept the same, except for the reaction duration, which needs to be increased to 4 h. There are many published reports, which can be referred to for understanding the synthesis routes utilized to obtain other magnetic transition metal oxides such as MnO2 , Mn3 O4 , NiO, NiCo2 O4 , Co3 O4 , etc. [5, 8, 11–13].

11.3 Magnetic Electrolyte or Effect of the External Magnetic Field on the Electrolyte Both the experimental and theoretical calculations show the importance of understanding the behaviour of electrolytes under varying magnetic fields. There are studies, which have shown that magnetic field can actually destroy the aggregate structure of the water molecule and cause enhancement in the activity of water. This clearly means that the dynamics of anions and cations present in the solution can be modulated, which would lead to changes in mass transfer within the aqueous solutions [14]. In the context of supercapacitors, this would mean that the charge storage would be affected under magnetic field. In addition, the nature of electrolyte, i.e. acidic, alkaline or neutral, would also decide the extent of affect because the nature of mobile ions would be different in these electrolytes [15]. Additionally, the magnetic convection that is observed at the electrode-electrolyte interfaces will also vary as a function of electrolyte concentration. The damping force in the magnetic convection

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is associated with electrolyte conductivity while the driving force is attributed to the Lorentz force.

11.3.1 Introduction of Magneto-Electric Effect (MEE) The magneto-electric effect (MME) is the physical phenomenon that causes the introduction of electric polarization as a function of the applied external magnetic field. Another aspect of this is the appearance of the magnetic polarization due to the external electric field. Magneto-electric effect is generally measured by measuring the induced electric field generated on the application of the magnetic field [16]. A magnetic free energy Gm in transition magnetic oxides is associated with a material when it is used under an influence of a magnetic field. Considering paramagnetic or diamagnetic components that take part in the reaction or phase change, the change in magnetic free energy can be written as [17]: ∆G m = −

1 ∆χ B 2 2μ0

(11.1)

where ∆χ is the change in susceptibility per reaction or phase change and μ0 is the permeability of a vacuum. Even at a field as high as B = 10 T, the value of ∆G m is of the order of 1 J mol−1 . This value is much smaller that the thermal energy at room temperature, which is approximately 2.5 kJ mol−1 . But, for materials with ferromagnetic components, the value is significantly larger than the thermal energy. Here, the change in magnetization ∆M is given by: ∆G m = −∆M B

(11.2)

This was first exemplified by the MFE on the chemical equilibrium of ferromagnetic metal hydride-hydrogen reactions. Applying magnetic fields to the ferromagnetic LaCo5 Hx and hydrogen system induces an increase in the equilibrium hydrogen pressure, which is a measure of the chemical equilibrium [18]. The MFE on a phase change has also been observed in the magnetic field-induced martensitic transformations in ferrous alloys.

11.3.2 Effect of Magnetic Field on the Electrochemical Performances The effect of magnetic field on lithium batteries has been investigated for some time now [19–33]. The origin of magnetic field-induced modulations has been linked to: magnetic force, magnetization, magneto-hydrodynamic (MHD) effect [1], spin effect, and nuclear magnetic resonance [6, 34, 35]. The modulation in structure

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of the electrode materials and full device, under magnetic field, can be investigated with techniques like electron spin probe magnetometry, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), X-ray absorption spectroscopy, etc. [36, 37]. But such studies remained ignored in supercapacitors till recent years. Few years back, initial papers on magnetic field-induced capacitance increment in graphene and other magnetic material based on carbon composites started getting published [38, 39]. Viswanathan and Shetty investigated the magnetic field-dependent electrochemical performance of nickel hydroxide-based nanocomposite, which was antiferromagnetic in nature [40]. The energy storage activity of nickel hydroxide was enhanced by fabricating reduced graphene oxide and polyaniline composite, which further showed increment in the storage capability due to the alteration in the conductivity and antiferromagnetic nature of the parent material under external magnetic field. Authors reported 69.4% increment in the specific capacitance of the two-electrode system, with an energy and power density of 0.3349 W h kg−1 and 17.57 W kg−1 on the application of a small external magnetic field of 625 μT. According to the authors, the antiferromagnetic nature of the material might be altered to ferromagnetic behaviour by the application of magnetic field because of the change in the band gap. This change in the magnetic behaviour was supposed to affect the corresponding electrochemical performance, resulting in an appreciable enhancement in the specific capacitance. Another study by Fite et al. observed appreciable increase in the specific capacitance values in nitrogen-doped graphene oxide/magnetite with polyaniline or carbon dots-based systems, under external magnetic field [41]. Investigating the electrochemical performance of Fe2 O3 based ternary composite under external magnetic field, authors reported nearly twofold increment in the electrochemical activity. The generation of magnetic convection and the changing concentration of the electrolyte at the electrode/electrolyte interface, because of the application of the external magnetic field, was supposed to be the reason behind this performance modification. The Lorentz force induces a magnetic convection, which in turn alters the electrode resistance, electrical conductivity, and charge-transfer resistance. This results in an enhanced double-layer capacitance under an external magnetic field. Another study by Ahmed et al. discussed the manner in which external magnetic field can affect the electrochemical performance of exfoliated graphene-based magnetic composites with conductive polymer and carbon dots [42]. But major interest in magnetic supercapacitors was generated when very high change was observed in supercapacitors based on Fe2 O3 , Mn3 O4 , MnO2 , Co3 O4 , etc., which were operated under magnetic field1,2 . As expected, ferromagnetic Fe-oxidesbased device have shown maximum modulation under magnetic field. The electrochemical cyclic voltammetric and charge–discharge data of Fe2 O3 , under magnetic field, is shown in Fig. 11.2a, b, respectively. The data was collected under an optimized voltage window of −0.85 to +0.1 V. You can clearly see the change in the profiles as the magnetic field strength increases. The change was observed only up to a certain value. Along with the redox peaks associated with the electrode–electrolyte interaction, a change in area of the CV curves was also observed. The values of specific capacitance were found to vary from 88 to 29 F g−1 , for scan rates from 5 to

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200 mV s−1 , respectively. Such response characteristics are linked with the muchreduced time, that becomes available for the ions to intercalate or de-intercalate. Therefore, the electrochemical behaviour is mostly dominated by the ion adsorption/desorption occurring at the electrode surface. Additionally, Fe2 O3 is a known pseudocapacitive material. Therefore, at higher scan rates, Faradaic contributions are suppressed. The CD curves also clearly depict good capacitive behaviour of the material with excellent Coulombic efficiency of 96% at 1 A g−1 . Similar to the earlier case, the CD data can also be collected under the effect of magnetic field, varying at fixed increments. The values of specific capacitance with scan rate and current densities, under external magnetic field, are depicted in Fig. 11.2c, d, respectively. Clearly, magnetic field can lead to large change in the specific capacitance. A feature that is worth mentioning and understanding is that the Coulombic efficiencies remain nearly same even after the application of magnetic field. This means that the electrode materials were not degrading as a function of applied field and the changes in electrochemical performance were happening mostly due to the changes near the SEI. Similar behaviour has also been observed in other materials. This can be seen from Fig. 11.3, which shows magnetic field-dependent electrochemical behaviour of NiCo2 O4 , NiO, Co3 O4 , NaFePO4 , NaMnPO4 , etc. For each of these materials, area 2

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under the CV curves increases with the application of 50 Gauss external magnetic fields. That indicates the enhancement of the electrochemical performance under the action of the external magnetic fields.

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11.4 Origin of Magnetic Field The magnetic field effects (MFEs) can be classified into time–space properties of fields (steady or time varying, and homogeneous or gradient), as shown in Table 11.1. If a magnetic effect occurs in a much smaller region, compared to the special variation of an applied gradient field, it is regarded as a homogeneous field. The effect of magnetic field can be pulsed or steady type, depending upon the field that is applied. Table 11.2 shows some of the observed magnetic phenomena that have been explained using some of the MFE explained in Table 11.1. According to the quantum theory, any particle which possesses a spin S is accompanied by a magnetic moment. This magnetic moment interacts with magnetic field B by the Zeeman effect [43]. This interaction is expressed by the Hamiltonian of an electron, which is given by: H = −gμ B Sz B

(11.3)

Table 11.1 Magnetic field effects for steady and time varying fields Effect

Magnetic field Steady field

1. Homogeneous field

Quantum effect (Zeeman effect) Magneto-thermodynamic effect Magnetic torque Lorentz force

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1. Gradient field 2. Alternating field 3. High-frequency field

Magnetic force (Faraday force) Eddy current Energy injection

Table 11.2 Various magnetic phenomena due to MFE Mechanism

Effect

Quantum effect

1. Magnetic fluorescence quenching 2. MFE on photoreaction

Magneto-thermodynamic effect

1. Magnetic field-induced martensitic transformation 2. MFE on chemical equilibrium 3. MFE on electrode potential

Magneto-hydrodynamic effect (MHD)

1. MHD effects in electrolysis 2. Magnetic adsorption 3. MFE on the deposition of metal leaves

Magnetic alignment

1. Magnetic alignment of crystalline polymers 2. MFE on the refractive index of water 3. Magnetic alignment of fibrin

Magnetic force

1. Moses effect 2. Magnetic Archimedes effect 3. Diamagnetic levitation

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where g is the g-factor (g = 2 for the electron spin) and μ B is the Bohr magneton. The z-component of the spin is Sz = −1/2 and 1/2 when the z-axis is along the magnetic field. Consequently, the state of the electron spin split into two energy levels in the presence of magnetic field. In chemical reactions, it is common to find radicals with unpaired electrons. These radicals can be then affected by a magnetic field, influencing their kinetics or reaction yield. The electron pairs can be either in singlet state (S = 0) or in a triplet state (S = 1). The energy levels of the singlet state do not split in presence of a magnetic field but for the triplet state the energy level will split into 3 (Sz = −1, 0, 1). The final yield of a chemical reaction depends upon the state, singlet or triplet, at the early stage of the reaction. As a consequence, the yield of the final products varies with the strength of magnetic fields. This is called the radical pair mechanism of the MFE [44]. When a magnetic material is exposed to a gradient magnetic field, the material will then experience a magnetic force, called the Faradaic force, expressed as: Fz = V M

χ ∂B ∂B =V B ∂z μ0 ∂z

(11.4)

where V is the material’s volume and M is the magnetization. The second equality differentiates between paramagnetic or diamagnetic materials, as the value of χ is different. This force is very weak for usual diamagnetic substances in ordinary fields. However, if the term ∂∂zB is enhanced with the help of some high field magnets (up to 20 T or more), then the diamagnetic substances, such as water, organic substances, can experience appreciable force. Also, the free surface of water is deformed when it is exposed to a magnetic field strength of several Tesla. This effect is called the Moses effect [45]. The diamagnetic levitation and the Moses effect can be enhanced by using a counterparamagnetic liquid. This phenomenon is called the magnetic Archimedes effect, which is analogous to the usual Archimedes effect. The magnetic free energy of a magnetic body, including the paramagnetic and diamagnetic bodies, can be expressed as: G m = −V M · B = −V M Bcosθ

(11.5)

where θ is the angle between the magnetization M and the field B. As a consequence, the body undergoes a magnetic torque T as: T =−

∂G m = V M Dsinθ ∂θ

(11.6)

Here, the torque acts around the axis perpendicular to both M and B. This torque is quite weak for a single diamagnetic molecule. But the magnetic torque becomes capable of overcoming the thermal disturbance if a molecular assembly is formed with N = 104 –105 molecules. Consequently, the molecular assemblies are aligned

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by magnetic fields and show anisotropy in macroscopic properties. This magnetic alignment was first indicated in the biological material of fibrin fibres and later in a variety of substances such as crystalline polymers and organic gels.

11.5 Explanation of MEE in Supercapacitor With the reference from the previous section, it can be said that the change in performance for the supercapacitor under magnetic field can be attributed to: (i) (ii) (iii) (iv) (v)

Magnetic nature of the material used as the electrode. Effect of the Lorentz force on the material. Domains arrangement in the electrode material. Effect of the Lorentz force on the electrolyte. Magneto-hydrodynamic effect in the electrolyte.

11.5.1 Magnetic Nature of the Material Used as an Electrode If the surface states change with magnetic field, then the I–V characteristics or conductivity will also show modifications. For example, I–V characteristics of Fe2 O3 nanoleaflets, under applied magnetic field, are shown in Fig. 11.4a. The pattern shows the typical non-ohmic behaviour of Fe2 O3 nanoleaflets under no magnetic field as well as in the presence of magnetic field. Figure 11.4a shows the increase in the current with magnetic field being increased from 0 to 50 gauss, before showing saturation. The values of conductivity were calculated from the I–V curves, using the equation: σ =

Vt I wL

(11.7)

where σ, w, L, t, V and I denote conductivity, material width, material length, material thickness, voltage and current, respectively. The values are shown in Fig. 11.4b. This clearly showed that the conductivity was changing as a function of magnetic field. Hence, change in charge collection during electrochemical performance of a supercapacitor can be envisaged (see Fig. 11.4c, d).

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Fig. 11.4 a I–V profiles b conductivity c Voltage drop and d ESR values of Fe2 O3 nanoleaflets under increasing external magnetic fields

11.5.2 Effect of the Lorentz Force on the Material The Lorentz force acting on the moving charges/ions can modulate the charge density on the electrode, leading to variation in electronic states, etc. As a result, the supercapacitor functioning may be affected by the external magnetic field. The Lorentz force acting on a charged particle is represented as: − → − → − → → FL = q E + q(− v × B)

(11.8)

− → − → → where q, E , − v , and B represent charge, an applied electric field, the velocity of the charge, and magnetic field, respectively. Initially, when only the potential is applied, − → the component q E will contribute. In this case, the redox activities at the electrode are responsible for the specific capacitance variation [1]. Following the magnetic field application, the charged particles moving in the − → channels (inside the electrode materials), which make an angle with B will experi− → ence FL . The specific capacitance value of the material will improve because of this additive force. The intercalation of the charged particles will enhance owing to an accelerated motion along the respective channels. With increasing magnetic fields,

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Fig. 11.5 Experimental arrangement for magnetic field dependent measurement

random magnetic domains will also start to align along the direction of the magnetic − → − → field. Beyond a value of B , the alignment will saturate, and the effect FL will decrease − → → because the term (− v × B ) will become nearly zero. As a result, saturation in the specific capacitance will occur. Moreover, reduced magnetoresistance and improved − → charge collection of the ferromagnetic materials at increasing B are supposed to lead to further improvement in specific capacitance values. The measurements can be performed using the experimental arrangement shown in Fig. 11.5.

11.5.3 Domains Arrangement of the Electrode Material The observed results can be further understood by postulating a convoluted mechanism owing to various reasons, which can affect the charge transfer and related electrochemical process occurring at the electrode-electrolyte interface (EEI). The possible reasons can be: (1) Directed diffusion of solvated cations in the presence of magnetic field. (2) Change in EDLC characteristics at the electrode-electrolyte interface because of reduction in the Nernst layer. (3) Reduction in charge-transfer resistance as a result of the generated Lorentz force. (4) Modulation in the spin energy states of electrons taking part in the reactions.

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11.5.4 Effect of the Lorentz Force on the Electrolyte Electrolyte ions involving in the charge storage in a supercapacitor is another important parameter, other than the electrode materials. Some of the previous works already established the fact that the external magnetic field has a direct impact on the movement of paramagnetic ions in the electrolyte solution. This behaviour was found to be attributed to enhancement in the kinetic energy of the ions due to the magnetic attraction force [1]. As a result, the effective flux of the ions on the electrode increases. In the presence of an external magnetic field, both the electrical and magnetic components synergistically contribute in the total generated force, i.e. Lorentz force, as shown in Eq. (11.8). Various phenomena that can impact electrolyte performance under magnetic field are shown in Fig. 11.6.

Fig. 11.6 Schematic of mechanism at EEI

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11.5.5 Magneto-Hydrodynamic Effect of the Electrolyte The magneto-hydrodynamic effect (MHD) demonstrates the motion of a conducting fluid flowing under an external magnetic field. The interaction between electromagnetism and the motion of the liquid is dealt by this effect, provided the liquid has conductive nature. The force becomes more dominant physical force for fluid with high volumetric flow rates and in large current densities. MHD finds its implementation in a wide range of applications starting from micro- to macro-scale hydrodynamics, MHD actuators, MHD sensors, etc. [46–48]. In an electrolyte, the changes in the charge distribution in the presence of external magnetic field found originates because of the magneto-hydrodynamic (MHD) effect. Moreover, it has been found that MHD increases the limiting current as a result of the reduced Nernst layer because of magnetically simulated convection. Additionally, MHD affects the electrochemical phenomena in three ways: change in the diffusion current, decrease in the charge-transfer resistance, and influencing the mechanism occurring at the electric double-layer interface [1]. In the absence of the external magnetic field, the electrode-electrolyte interface (EEI layer) shows normal EDLC behaviour, with zero polarization current. After application of magnetic field, the electrode potential is lowered because of the movement of maximum solvated cations towards the electrode surface. If the magnetic field is in a parallel direction with the current lines, F L = 0, the local domains within the material generate a local gradient of the magnetic field, which can increase the limiting current due to the MHD effect. This applied magnetic field or the change of magnetic susceptibility (χ ) within the electrolyte further generates the paramagnetic forces.33 Due to χ gradient, the paramagnetic ions (solvated K+ in the previously discussed case) follow the magnetic field direction. Although this effect appears to be very small, its consequence cannot be neglected for high concentration of charge carrier, where the kinetics of this behaviour simulates a large alteration in the overall performance.33 The MHD effect introduces reduction in the dimension of Nernst layer and corresponding decrement in the charge-transfer resistance at the electrode-electrolyte interface. On the other hand, under external magnetic field, the magnetic dipole moment of charge carrier becomes distinguishable, forming degenerated energy level at the electrical double layer.23,34 Further, the microscopic structure of water changes in the magnetic field, resulting enhancement in the dielectric constant of the electrolyte at the EEI. As a synergistic result of all these factors, the size of the Nernst layer reduces, enhancing the double-layer capacitance. Moreover, the degeneracy of the electronic levels leads to enhancement in the electron energy, which take part in the reaction at the electrode, contributing to the overall increase in specific capacitance.35 This explanation is supported by the equivalent series resistance (ESR) calculation, which are mostly found to be reduced with the increment in the applied magnetic field intensity. The conductivity values in the magnetic field ranging from 0 to 50 gauss were also found in accordance with the current explanation. A schematic of the mechanism occurring at EEI is shown in Fig. 11.6.

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After a particular magnetic field ion diffusion, polarization effect and the values of charge-transfer resistance show saturation. This can be attributed to the limitation of the charge holding capacity of the electrode and self-repulsion of the overaccumulated charges at the electrode. These effects were reflected in the saturated values of specific capacitance and conductivity at 50 gauss. Abrupt voltage drops can also be found at the start of the discharge cycle in every CD curves. The voltage drop was less at lower current densities and vice-versa. This can occur because, at lower current densities, ions encounter lowered resistance for intercalation–deintercalation. The average equivalent resistance can be calculated using the equation: RESR =

VIR 2I

(11.9)

where V IR is the voltage drop in volts at the starting of the discharge cycle and I is the corresponding current in amperes.36 Fig. 11.4c shows the variation of voltage drop with current densities in different magnetic fields. The voltage drop was found to be lower at higher magnetic fields. The values of ESR, obtained using Eq. (11.5), are shown in quantitative form in Fig. 11.4d. At higher magnetic field ESR value get reduced from 9.6 to 6.7 Ω/cm2 . The area taken in the calculations was the total coated area, i.e. 1 cm2 . The results reconfirmed the proposed hypothesis. The field at which the saturation is observed, for example 50 gauss for Fe2 O3 nanoleaflets, can be termed as par magnetic field (PMF). As per our analysis, every material should have different PMF value depending upon the nature of material, its magnetic property, size and shape and the available surface sites for adsorption and desorption of ions. The magneto-hydrodynamic (MHD) effect is a physical phenomenon that defines the fluid motion under an external magnetic field. The MHD is explained by combining electromagnetic induction and dynamics [49]. This mainly comes into the consideration for a material, which can behave as a conductor and can be modified by the external magnetic fields when there is relative motion. This creates a dominant physical force in conductive fluids with high volumetric flow rates, and in large current carriers. The magnitude and the direction of the MHD effect are − → affected by the magnitude and direction of the fluid flow ( V ) and the direction of the −→ external magnetic field ( Bext ). This will lead to variance in the induced Hall Effect voltages. Consider the four Maxwell’s equations [46, 50]: → ρe − → − ∇ · E = ε0

(11.10)

→ − → − ∇ · B =0

(11.11)

− → ∂B → − → − ∇ × E =− ∂t

(11.12)

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281

( − →) ∂E → − → − → − ∇ × B = μ0 J + ε0 ∂t

(11.13)

Assuming that the medium is highly conductive, the displacement current becomes negligible as the propagation of electric field is inhibited. Thus, the displacement current term can be neglected from the Eq. (11.4). In order to define this phenomenon in terms of the external magnetic field, Ohm’s law must be employed and also modified to include induction term: J = σE

(11.14)

− → − → − → − → J = σ( E + V × B )

(11.15)

In such a situation where a current is allowed to flow across the medium, an additional force is present (the Lorentz force) due to the interaction between the external magnetic field and that of the conductor [51]. − → − → − → F = J × B − → F =

(

) 1 − → − → → − (∇ × B ) × B μ0

(11.16) (11.17)

This should be applied to fluid flow and MHDs. For this, consider the Navier– Stokes equation: ρ

) ( − ∂→ v −−→ → → +− v · ∇− v = −∇ p + μv∇ 2 v + f E M ∂t

(11.18)

On exposure to the magnetic field, the total force can be written as a summation of Lorentz, magnetophoretic (occurring in ferrous materials), and electrostatic forces, i.e. → −→ − → −−→ − f E M = f L + f∇ B + f E

(11.19)

During the consideration of most applications of the MHD effect, the applied magnetic field is uniform and static, especially during MRI, and magnetophoretic and electrostatic forces become nullified. This simplification allows for the only remaining force, i.e. the Lorentz force to be substituted into the Navier–Stokes equation. This describes the flow of a conductive fluid when subjected to an external magnetic field. ρ

) ( − ( ) ∂→ v 1 − → − → → − → → (∇ × B ) × B +− v · ∇− v = −∇ p + μv∇ 2 v + ∂t μ0

(11.20)

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While comparing the Lorenz force against the fraction force and the inertial force, Lorenz force does not benefit from miniaturization, and a larger magnetic field flux density may be needed for pronounced MHD effects [52]. Thus, final equation becomes: − → − → 0 = −∇ p + μv∇ 2 v + J × B

(11.21)

This equation defines the MHD effects.

11.6 Theoretical Interpretation of Magneto-Electric Supercapacitors The theoretical investigation of magnetic field-induced variations in supercapacitors remained unexplored till recent times. Theoretical formulations proposed in the Gouy Chapman or the Stern models are the most used concepts to explain the origin of supercapacitance. But none of these models considered the presence of a magnetic field or its effects. Further, none of these mathematical formulations considered the →|| − → ||− term B or | B |. It is not always easy to work with background processes in a controlled-potential mode. The surface boundary condition in a current-controlled experiment is based on known currents or fluxes (i.e. concentration gradients) at the electrode surface. However, in presence of a magnetic field, an additional magneto-convection flux arises due to the Lorenz force exerted by the magnetic field.

11.6.1 Existing General Theories The standard current-controlled theories are based on the mathematics of SemiInfinite Linear Diffusion. The theory considers the simple electron-transfer reaction where the flux of the ionic substance J at given location x and time t can be manifested according to the Fick’s law [53]: J = −D

∂C ∂x

(11.22)

where in case of a planar working electrode and an unstirred solution are assumed, with only species O initially present at a concentration C0∗ . The electrode reaction +ne → R, in a current control experiment, where semiinfinite linear diffusion applies the experiment began at a potential (E) at which no current flows; and at t = 0, E takes a constant value anywhere on the reduction wave. Further, it is assumed that here that charge-transfer kinetics are very rapid. Therefore,

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a Nernstian relationship [Eq. (11.23)] links the surface electrode potential with the concentration of the ions as: ( ) C0 RT (11.23) ln E = E0 + nF CR where E 0 is the reference potential of the electrochemical system and n is the number of electrons passed by the electrode when the reaction takes place. The terms F, R, T represent the Faraday number, universal gas constant and the temperature of the system, respectively. The concentration C (for both O and R species) are governed from the diffusion equation derived from Eq. (11.22) as: ∂ 2 C O (x, t) ∂C O (x, t) = DO and ∂t ∂x2 ∂ 2 C R (x, t) ∂C R (x, t) = DR ∂t ∂x2

(11.24)

Supported by the boundary condition, C O (x, 0) = C O∗ ; C R (0, t) = 0

(11.25a)

lim C O (x, t) = C O∗ ; lim C R (x, t) = 0

(11.25b)

x→∞

x→∞

and the flux balance as, ( ( ) ) ∂C O (x, t) ∂C R (x, t) + DR =0 DO ∂t ∂t x=0 x=0

(11.26)

The initial condition (11.25a), just expresses the homogeneity of the solution prior to the start of the experiment at t = 0. The semi-infinite condition (11.25b), states that locations far away from the electrode are unaffected by the experiment. The third condition describes the state of the electrode surface following the potential transfer. Since the applied current i(t) is known previously, and the flux at the electrode surface is also known, according to the Eq. (11.24), we can obtain: ( DO

∂C ∂x

) = x=0

i (t) nF A

(11.27)

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11.6.2 Solution of Diffusion Equation The above problem can be solved using the Laplace transformation method [53], where the species concentration C(x, t) is defined as: ∼s)e−st ds, C(x, t) = ∫ C(x,

(11.28)

∼s) is the concentration of ions/charges in Laplace space. In literature, where C(x, definition used in Eq. (11.24) has been used to solve the Laplace concentration with: ∗ ∼s) = C0 + B(s)e−(s/D)1/2 x C(x, s

(11.29)

Further, one can express the current conservation relation [in Eq. (11.27)] into the Laplace space: (

∂ C˜ D ∂x

) = i (s)/(n F A)

(11.30)

x=0

where i (s) is the Laplace transformation of the current. For the present system of interest, the current i(t) is described like: i (t) = i + Stc (−2i )

(11.31)

where Stc is a step function defined as: Stc = 1 t > tc and Stc = 0 t ≤ tc The above current function i (t) corresponds to a situation at which the electrochemical cell experiences a forward current i till the charging time tc , i.e. the time required to achieve the maximum voltage of the window E max . Beyond that, a reverse current −i is applied to them which discharges the system to the minimum potential of the window E min . The Laplace transformation of the current profile in Eq. (11.31) is given by: ) ( i (s) = (i /s) 1 − 2e−tc s

(11.32)

Now, with a straightforward substitution, the unknown coefficient in Eq. (11.29) can be obtained, which express the concentration in Laplace space as: ∗ ∼s) = C0 − C(x, s

(

)

i (s) n F A(s D)

e−(s/D)

1/2

1 2

x

(11.33)

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285

The inverse Laplace transformation of Eq. (11.32) would yield the concentration of the species. However, to evaluate the potential, one needs the concentration across the reference electrode (which is at position x = 0). This can be written like: C(0, t) = C0∗ −

2i 1 2

n F AD π

1 2

( )) ( 1 1 t 2 − 2Stc (t − tc ) 2

(11.34)

The above variation of the species is valid for the incoming electrons that are oxidized at the electrode surface (i.e. the O species) with a finite initial concentration C O∗ . However, for the reduced species (i.e. R species with C0∗ = 0), the variation could be determined from flux conservation equation using similar approximations: C R (0, t) = C0∗

1 2

⎞ ( )) ( 1 1 t 2 − 2Stc t − tc ) 2 ⎠ 1

⎛

2i D0 ⎝ 1 1 2 D R n F AD02 C0∗ π 2

(11.35)

Further, a Nernstian relationship [in Eq. (11.23)] which links the surface electrode potential to the species concentration yields the time variation of the electrode potential as: (

E = E0 −

RT D0 ln 2n F DR

)

)⎞ ( 1 1 2 − 2S (t − t ) 2 1 − i α t tc c RT ⎝ ) ⎠, ( 1 + ln 1 nF i α t 2 − 2Stc (t − tc ) 2 ⎛

(11.36)

where α = n F AC ∗2√ D √π is the constant that depends on the system parameter. 0 0 Because of one set of convection and the nonlinearity of diffusion, problems develop over long experimental times in both controlled-current and controlled-potential approaches. Therefore, the above equation is valid for the rapid charging and discharging of the electrode where the diffusive process dominates. The effect of Lorentz force in presence of a magnetic field applied in a perpendicular direction to the electric field would be proportional to the term v B, which is comparable to the effect of the potential gradient (i.e. ∂∂∅x ) in the Nernst-Planck equation [54]. To theoretically explain the changes, the flux of incoming ions in the Eq. (11.22) is modified in presence of a magnetic field will have to be written as: J = −D

nF ∂C − DCv B ∂x RT

(11.37)

where B is strength of the applied magnetic field and v is the corresponding velocity of the charged species. The modified flux current in Eq. (11.37) pertains to the change in concentration C with time as: ∂ 2C ∂C n F Dv B ∂C =D 2 + ∂t ∂x RT ∂ x

(11.38)

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The modified diffusion equation can be solved by adaption of a similar approach ∼s), as the previous section. Subsequently, from Eq. (11.38), the concentration C(x, i.e. the concentration of ions/charges in Laplace space will be governed by: ( ) n Fv B D ∂ C˜ ∂ 2 C˜ D 2 + − s C˜ = − C0∗ ∂x RT ∂x

(11.39)

For this particular case, the roots of the complementary functions are often complicated and follow a mathematical structure ⎛ ⎞ ) /( ) ( ) ( n Fv B 2 s⎠ 1 ⎝ n Fv B ± +4 m= − 2 RT RT D

(11.40)

Note that we are interested in the concentration of the ions/charges as a function of time at a fixed position. Correspondingly, the “s” dependence of the concentration C˜ plays a pivotal role. Thus, in the limit that the applied magnetic field is weak, the “s” dependence of the roots of the above equation is approximated as ≃ ±(S/D)1/2 . ∼s) follows the same structure as in Eq. (11.29). Therefore, C(x, However, in presence of magnetic field B, at the position of the electrode i.e. at x = 0, the net flux and the current are modified as: ) ( n Fv B D ∂C |x=0 + C|x=0 = i(t)/n F A (11.41) D ∂x RT By taking the indicated derivative of the concentration and substitution of i (s) [as ∼s) as: in Eq. (11.32)] would yield the final solution of the C(x, ∗ ( ) i −tc s −(s/D)1/2 x ∼s) = C0 − e C(x, 1 3 1 − 2e s n F AD 2 s 2 C ∗ n Fv B D 1/2 −(s/D)1/2 x + 0 e RT s 3/2

(11.42)

Accordingly, an inverse transformation of the above equation would yield the concentration of initially approaching ions at the reference electrode (at x = 0): C(0, t) = C0∗ − +

2i 1 2

n F AD π 1/2

( ) (t 1/2 − 2Stc t − tc )1/2

2C0∗ n Fv B D 1/2 1/2 t RT π 1/2

(11.43)

Here, in presence of a magnetic field, one can use the above-mentioned general expression [Eq. (11.43)] to derive the concentration profiles for C0 (0, t) with C0∗ as the initial concentration of the ions/charges and C R (0, t) from the modified flux

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conservation equation at the reference electrode: ) ( ∂C0 ∂CR n Fv B D0 |x=0 + C0 |x=0 + D R |x=0 D0 ∂x RT ∂x ) ( n Fv B D R C R |x=0 = 0 + RT

(11.44)

Therefore, the final form of the oxidized (O species) and reduced (R species) concentrations are given by: ( ( 1 ) ) 1 1 C0 (0, t) = C0∗ 1 − iα t 2 − 2Stc (t − tc ) 2 + Bγ t 2 C R (0, t) =

(

D0 DR

) 21 ( )) ( 1 1 1 − i α t 2 − 2Stc (t − tc ) 2

)

( where α =

C0∗

2 1 n F AC0∗ D02

1

π2

( and γ =

1

2n Fv B D02 RT π

1 2

(11.45)

(11.46)

) are the constant system parameters.

Substitution of analytical expression of C0 (0, t), C R (0, t) yields the time variation of the electrode potential as: ) ( D0 RT ln 2n F DR ( 1 ) ⎛ ⎞ 1 2 RT ⎝ 1 − i α t 2 − 2Stc (t − tc ) + Bγ ⎠ ) ( 1 + ln 1 nF i α t 2 − 2Stc (t − tc ) 2

E = E0 −

(11.47)

One can clearly see that Eq. (11.47) represents the analytical time dependence of the electrode potential E for the current-controlled techniques in presence of magnetic field. In the limit B → 0, the above equation reduces to the standard response of the electrode potential, with current reversal techniques as mentioned in previous section. During charging (t ≤ t c ), the step function is taken as Stc = 0. Now, under the 1 expanded into premise of the assumption that (iα − Bγ )t 2 < 1, the numerator can ( be1 ) 1 2 2 suitable powers of t viz., t and t. In the denominator, the term ln i αt corresponds to the time variation of the resultant ionic species “R” across the electrode. However, without the loss of generality, we may safely assume that over the time, the developed electrode potential is essentially dominated by the initial ionic/charge species “O”, which are on the electrode surface. Subsequently, one can neglect the ) ( accumulating 1 term ln i αt 2 . Therefore, using a second-order approximation, the final expression for electrode potential during the charging period can be reduced to:

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E ≈ E0 −

) ( ( ) RT D0 t RT 1 − ln (i α − Bγ )t 2 + (i α − Bγ )2 2n F DR nF 2

(11.48a)

During discharging we have Stc = 1. Therefore, under similar assumptions, the second-order approximated expression for the electrode potential during discharging could be obtained as ) ( D0 RT ln E ≈ E0 − 2n F DR ( ) ( RT iα 1 1 −(iα − Bγ ) t 2 − 2 + (t − tc ) 2 nF i α − Bγ )2 ) 2( iα 1 1 (i α − Bγ ) − t2 −2 (11.48b) (t − tc ) 2 2 iα − Bγ To compare the approximated analytical expressions mentioned in Eqs. (11.26), (11.27), parameterize the above equations such that electrode potential during charging and discharging can be, respectively, written as: 1

E = a c + bc t 2 + cc t ( 1 ) 1 1 2 1 E = a d + bd (t 2 − ed (t − tc ) 2 ) + cd t 2 − ed (t − tc ) 2

(11.49) (11.50)

where the time dependency is denoted through the coefficients a c , bc , cc and a d , bd , cd , ed for charging and discharging time, respectively. Figure 11.7 depicts the best-fitted theoretical expression for Fe2 O3 nanoleaflets, at a fixed current density of 1 A g−1 , in presence and absence of magnetic field. The line curve, simulated using the theoretical equations, is able to successfully match the experimental values, with relative errors (R.E) in the range 0–5%. It was observed that, for higher current density, a third-order approximation of the logarithmic polynomial may yield more accurate results. However, for the considered current density in the present work, the second-order approximation is good enough to illustrate the comparison of the charging-discharging behaviour. In a current controlled experiment, it is an arduous task to derive an exact theoretical expression for the specific capacitance value. However, from charge discharging profiles, the specific capacitance can be calculated using the equation [55]: CC D =

I.dt m.(V − I R)

(11.51)

Here, typically the voltage window V for charging-discharging is kept constant, while I R is the internal voltage drop in the circuit. Therefore, by plugging the maximum charging voltage E max in Eq. (11.49), the time required to achieve the maximum value can be evaluated. Remember, at this stage, a c , bc , cc are known. This is identified

11 Magneto-Electric Supercapacitors 0.2

0.2

(a)

(b)

0 Gauss

0

10 Gauss

0

Potential (V)

Potential (V)

289

-0.2 -0.4

-0.2

-0.4

-0.6

-0.6

1 M KOH

-0.8 0

40

80

120

-0.8

160

1 M KOH 0

50

0.2

0.2

30 Gauss

-0.2 -0.4

50

100

150

200

50 Gauss

-0.2 -0.4

-0.6

1 M KOH 0

(d)

0

Potential (V)

Potential (V)

0

-0.8

200

Time (s)

(c)

-0.6

150

100

Time (s)

250

-0.8

1 M KOH 0

50

100

Time (s)

150

200

250

Time (s)

Fig. 11.7 The experimental and theoretical comparison of the CD data of Fe2 O3 at 1 A g−1 current density for a 0, b 10, c 30 and d 50 Gauss magnetic field, respectively. The experimental data and the corresponding theoretical curves are identified by points and lines, respectively

as the charging time of the system i.e. tc . Similarly, from the discharging curve [Eq. (11.50)], using tc and the minimum potential E min , the unknown coefficients a d , bd , cd , ed followed by the time (t f ) required to achieve E min can be calculated. Subsequently, from the analytical values of tc , t f , the discharging time dt = t f − tc can be evaluated. Consequently, on using the prescribed voltage window and specific current density, we obtain the specific capacitance of the electrolyte. This gives us the analytical value of the specific capacitance. The comparison of the C D values obtained from the experimental and theoretical charging and discharging behaviours are provided in Table 11.3. Table 11.3 Comparison of specific capacitance ( ( −1 )) Cd Fg at 1 A g−1 current density

Magnetic field Specific capacitance (F g−1 ) (Gauss)

Relative error (%)

Experimental Theoretical 0

86.05

89.67

4.20

10

104.60

103.79

0.77

30

132.01

130.93

0.81

50

133.33

131.71

1.21

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The analytical expressions for the charging and the discharging curves reveal that the magnetic convection term (Bγ ) is negatively correlated with the drift (iα)(provided by the )applied(current. Therefore, ) the coefficients follow the identity bc , cc , bd , cd , ed B=0 > bc , cc , bd , cd , ed B/=0 . The magnetic convection term effectively reduces the rate of “increase” (or “decrease”) of the electrode potential during charging (or discharging) cycle. Consequently, the time required to achieve the upper (or lower) voltage limit E max (or E min ) increases. As a result, the net discharging time (dt) and the specific capacitance (Cd ) of the electrolyte increase if all the other parameters are kept constant. Nevertheless, at higher magnetic fields, some of the electrons will get deflected towards the edge of the electrode because of the drift velocity provided by the applied magnetic field in the direction perpendicular to it [49]. Thus, a charge accumulation will develop at the edge of the electrode and develop cite-oriented potential δV . This small potential will affect the voltage window and decreases it by Vm = V − δV . Even though the discharging time dt increases, the system would not be able to exploit the complete voltage window range. Hence, the capacitance would not increase further and attain a saturation value.

11.6.3 Diffusion-Related Explanation of Magnetic Supercapacitors A gradual increment in diffusion coefficient with the rising intensity of the external magnetic fields can be attributed to the MHD effect as stated before. When the magnetic field is applied, it induces a magnetically stimulated convection at the electrolyte ions, making the ions more diffusive near the SEI. This fact shortens the thickness of the Nernst layer and increases the charge storage capability. A correlation between experiment and theory supports the fact that higher diffusion coefficients of the electrolyte ions are the reason behind the enhancement in the specific capacitance of the electrode. The diffusive behaviour of electrolyte ions can be greatly affected by the application of external magnetic fields. This may be due to the reduced thickness of the Nernst layer near the vicinity of the electrode surface. Let us briefly understand of the calculation of the diffusion coefficients. To calculate diffusion coefficients Eqs. (11.48–11.50) of the previous sections can be taken into account. Consider, E denotes the electrode surface’s potential for both charging and discharging. The terms E 0 , i, B, D O and D R are the electrochemical system’s reference potential, current density, magnetic field intensity, and initial and final diffusion coefficients, respectively. F, R, and T are the Faraday number, universal gas constant and system temperature, respectively, whereas α, γ are the constant parameters of the system, which take the following values: ⎛ α=⎝

⎞ 2 1

n F AC0∗ D02 π 2

1

⎠ and

⎞ 1 2 2n Fv D 0 ⎠ ⎝ ⎛

γ =

1

RT π 2

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At time t = 0, A, D0 and C0∗ denote the electrode area, the diffusion coefficient and the ion concentration. In the expression of γ , the numerator includes the velocity of ionic species. As the sign of velocity is the opposite for the charging and discharging of the supercapacitor, it remains the opposite for γ also. Therefore, the comparison of Eqs. 11.48a, b and 11.49, 11.50, respectively gives the following relations: RT RT (iα − Bγ ), c1c = − (i α − Bγ )2 for charging nF 2n F

(11.52)

RT RT (i α + Bγ ), c1d = − (iα + Bγ )2 for discharging nF 2n F

(11.53)

b1c = − b1d = − Therefore,

2

c1c = (iα − Bγ ) b1c

2

c1d = (i α + Bγ ) b1d

and

Solving these two equations, one can get the value of i α, and from that value, the diffusion coefficients can be evaluated using the following expression: ⎞

⎛ iα = ⎝

2i 1

n F AC0∗ D02 π 2

1

⎠

(11.54)

where n is the number of electrons passing through the electrode during the reaction, and I is the applied current density. Assuming the diffusion coefficient values remain the same for charging and discharging, the values can be obtained for various morphologies and magnetic fields. Figure 11.8 shows the variation of the diffusion coefficients with the increase of magnetic field intensity. For each of the NiO, Co3 O4 , NiCo2 O4 and Fe2 O3 , diffusion coefficient of the electrolyte ion shows an increasing trend. This also confirms the convection flow of the electrolyte ions at the EEI. Similarly, the validity of this theoretical model can be seen from its use to fit the experimentally observed data for other metal oxides or complex structures (Fig. 11.9). The theoretical and experimental curve agrees well for each case, which confirms the validity of the model [8].

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Fig. 11.8 The diffusion coefficients with the variation of magnetic field for a NiO, b Co3 O4 , c NiCo2 O4 and d Fe2 O3 (three rod-like morphology)

11.7 Summary A new theoretical model is developed to explain the electrochemical performance of magnetic supercapacitors. Pseudocapacitors fabricated using magnetic metal oxides show large change when they are made to operate near magnetic field. This opens new areas of application for such energy storage devices. EDLCs do not show any change when they are operated under magnetic field.

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Fig. 11.9 The experimental and theoretical comparison of the CD data at 50 Gauss magnetic field of a NiO, b Co3 O4 , c NiCo2 O4 and d NaMnPO4

References 1. V. Sharma, S. Biswas, A. Chandra, Adv. Energy Mater. 8, 1800573 (2018) 2. K.K. Kar, Handbook of Nanocomposite Supercapacitor Materials II, vol II (Springer, 2020) 3. K.K. Kar, Handbook of Nanocomposite Supercapacitor Materials II, vol. III, 313 (Springer, 2021) 4. K.K. Kar, Handbook of Nanocomposite Supercapacitor Materials II, vol. I, 300 (Springer, 2020) 5. M.A. Akhtar, A. Chowdhury, A. Chandra, J. Phys. D: Appl. Phys 52, 155501 (2019) 6. Z. Zeng, Y. Liu, W. Zhang, H. Chevva, J. Wei, J. Power Sources 358, 22 (2017) 7. M. Singh, A. Sahoo, K.L. Yadav, Y. Sharma, ACS Appl Mater Interfaces 12, 49530 (2020) 8. A. Chowdhury, S. Biswas, V. Sharma, J. Halder, A. Dhar, B. Sundaram, B. Dubey, P.S. Burada, A. Chandra, Electrochim. Acta 397, 139252 (2021) 9. A. Chowdhury, A. Dhar, S. Biswas, V. Sharma, P.S. Burada, A. Chandra, J. Phys. Chem. C 124, 26613 (2020) 10. D. Mandal, S. Biswas, A. Chowdhury, A. Chandra, Mater. Adv. (2022) 11. D. Majumdar, ChemElectroChem 8, 291 (2020) 12. X. Yan, X. Tong, J. Wang, C. Gong, M. Zhang, L. Liang, Mater. Lett. 95, 1 (2013) 13. P. Haldar, S. Biswas, V. Sharma, A. Chandra, J. Electrochem. Soc. 165, A3230 (2018) 14. L. Zhang, Z. Zeng, D.-W. Wang, Y. Zuo, J. Chen, X. Yan, Cell Rep Physical Science 2, 100455 (2021) 15. D.O. Derecha, Y.B. Skirta, I.V. Gerasimchuk, J. Phys. Chem. B 118, 14648 (2014)

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Chapter 12

Advancement in the Micro-supercapacitors: Synthesis, Design, and Applications Mandira Majumder and Abha Misra

Abstract With the advancement in the technology, application of microelectronic gadget has seen an upsurge. The progress of the microscale devices is significantly dependent on the development of microscale energy storage devices with outstanding charge storage properties. Supercapacitors have long cycle life and higher power density as compared to the rechargeable batteries. These are reasons why it is tempting to integrate the modern electronic gadgets with micro-supercapacitors as the energy storing mode. Despite the tremendous research dedicated in this field, there are still some challenges faced and needs more research to further improve the physical as well as the electrochemical properties of the micro-supercapacitor devices. In this chapter, we have discussed about the various device architecture designs and the state of art of it. Further different device preparation methods have been discussed with outlining their advantages and the disadvantages. This is following by a short and precise discussion about the patterning and micro-supercapacitor systems which have been developed recently. We have also discussed in detail various works reporting the various applications of MSCs in different fields. Lastly, the chapter has concluded on a note of the future direction of research assumed in this field. The architecture design of the micro-supercapacitors (MSCs) and brief description of the reaction mechanism have been provided. This is followed by the device preparation methods. We have also discussed in brief the device patterning and various systems integrating MSCs. Finally, discussion about applications and future developments have been discussed.

12.1 Introduction As a result of tremendous development in the field of advanced technology, the utilization of electronic gadgets is growing at extreme rate. For the betterment of mankind’s living standards, numerous number of smart, flexible, and microsized M. Majumder · A. Misra (B) Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, Karnataka, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_12

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electronic gadgets have seen their emergence gradually in the industry, including smart robots, health trackers, folding mobile phones, micro-sensors, and other miniaturized systems. Together with the combination of these microelectronic microsized devices and smart independent modules in different applications (Fig. 12.1), emergence of miniaturized smart energy storage devices compatible with the various miniaturized smart electronic gadgets is becoming highly urgent [1]. Currently, transportable devices are traditionally implemented with micro-supercapacitors (MSCs) and micro-batteries as source of power. In a typical way, these are possible to be implemented with miniaturized electronic devices to supply the requisite power and energy. Particularly, MSCs have created growing interest in some specific regime which needs rapid rate of charge–discharge and long lifetime. For example, in sensor and biomedical fields, micro-batteries are challenged by replacement attributed to their short life span. In addition to the environmental and economic problems created, in various instances including driving implantable biochips, the replacement of the micro battery may also require a surgery. Unlike micro-batteries, MSCs are characterized with ultralong cycle life accompanied by a negligible capacitance attenuation. Attributed to the long cycle life the MSCs are not required to be replaced frequently which can effectively mitigate the problem of frequent replacement of the energy storage device implanted in the microelectronic system [2]. Furthermore, microbatteries are incompatible where large power is needed. Even though it is believed that the high power can be extracted from the parallel and/or series combination of batteries, this will eventually increase the volume of the device. Clearly, this is not acceptable in case of designing of microelectronics. MSCs, unlike the microbatteries, are able to deliver way more larger power density within a little volume without any extra combination. This is quite apt for integrated flexible systems which require large power density [3]. Moreover, implementation of substrate with considerably high flexibility offers planar MSCs which can deliver remarkably good mechanical properties. Hence, MSCs are more apt and showcase larger practical applicability as compared to the micro-batteries for these particular fields of applications [4]. Presently, there are two different architectures of MSCs: (a) the traditional sandwich like structures and (b) interdigital structures with the electrodes on the same plane (Fig. 12.1). Generally, the coplanar interdigitated electrode design is expected to offer better improved rate performance and higher power density as compared to the device having sandwich architecture. This is attributed to its unique architecture which is largely responsible for shortening the distance of electrolytic ion diffusion providing a larger active surface area [5]. In this context, it is essential to highlight the influence of the structure of the device on MSCs’ electrochemical performance. These days, attributed to increased studies on MSCs, and as a consequence, progress on gadgets with high performance has emerged largely. Nevertheless, one significant challenge is still the low energy density restricting the application in practical fields. There are two primary reasons limiting the development MSCs with high performance. Electrode material designing for the energy storage devices can play a pivotal role in determining the performance of the device. A high number of studies have implied that the performance of the energy storage can be improved significantly by rational design of the electrode materials [6]. In order to design high performance

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MSCs, the selection of electrode materials proves to be of great importance; further, the intrinsic characteristics of the various electrode materials are vital. Various kinds of materials possess distinct characteristics; however, high active surface area, high conductivity, and short ion diffusion paths are the common necessities to attain satisfactory electrochemical performance. Apart from the materials used as electrodes, the MSC’s performance can also be improved through optimization of the architecture of different portions of the device (i.e., electrolyte, electrode, collector, and substrate). The attainment of this target is possible only with proper selection of electrode material and improved device manufacturing approaches. Currently, various miniaturized machining techniques are implemented to design MSCs with planar architecture. And the design strategies of electrolytes and electrodes resembling fingers of MSCs are intimately associated with the applicability of the MSCs in electronic components [7, 8]. Therefore, in this book chapter, the structural design and fabrication methods along with the applications of advanced MSCs have been discussed. Also, different electrode materials and the associated optimal design have also been summarized. In a nutshell, this book chapter is initiated with outlining the various design concepts of MSCs’ topology, followed by the introduction of the various fabrication techniques

Fig. 12.1 Schematic diagram illustrating various types of micro-supercapacitors (MSCs), their components, and various applications. Reproduced with permission [9] (Copyright © 2020, Elsevier)

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to design MSCs. At last various kinds of active materials, such as conducting polymers and metal-based materials, carbon-based materials and the associated design in optimized form are reviewed. Finally, this chapter addresses the note of applications of MSCs and future prospects associated with its further development.

12.2 Device Architecture Designing At present, the MSCs can be classified into two types according to their structure: interdigital-structure MSCs and traditional sandwich-structure MSCs. The traditional sandwich MSC comprises a perpendicular structure with two working electrodes and electrolyte in between the electrodes. The performance of the supercapacitor is significantly influenced by the separator’s thickness and the thickness of the electrode. For improving the energy storage performance, an efficient approach is to maximize the active materials’ mass loading. However, as a result of the limitations of its structure, the electrode mass loading can only be increased by enhancing the thickness of the electrode. As a result the pathway associated with the ion diffusion will increase in turn causing low lower power density [10, 11]. On the other hand, in case of the MSCs having in-plane interdigital-electrode design, there exists an insulated gap in between the fingers. This type of architecture possesses some advantages, as discussed. Firstly, there is no use of separators any longer, which results in higher diffusion efficiency for the ions. At the second place, attributed to the distinct finger electrode architecture, the passage of electrolytic ions generally takes place in plane of the MSC components. Therefore, increasing the electrode thickness can result in the improvement in the energy density and capacitance without disturbing the power density. On the third place, the combining different components in plane has the advantage for application of the integrated system and industrial production on a large scale [10]. Overall, the interdigital structure can satisfy the requirements associated with the development socially and lifestyle of mankind more aptly. Hence, a substantial attention has been given in the research and development of MSCs showcasing interdigitated architecture. The electrochemical properties of an MSC with interdigitated structure primarily are influenced by the width of the electrode, its thickness, and the space between two neighboring fingers, which has been confirmed by a few works [12, 13]. For instance, Li et al. [14] reported that at the similar various parameters, the current density is associated with the total area of the device got enhanced as the finger size was doubled. However, as only the area of the electrode was taken into consideration, there was a fall in the areal capacitance as the width of the fingers increased to a value more than 500 μm. This could be attributed to increase in the diffusion length for the electrolytic ions instead of improving the conductivity with the increase of the finger width beyond a limit. Nevertheless, with the enhancement in the thickness of the fingers, there becomes a perpetual increase in the areal capacitance. Therefore, a rational designing of a structure of device, the higher areal capacitance of value ∼0.7 mF cm−2 has been reported. Furthermore, Chih and co-workers [11, 15] reported an all-solid-state MSC exhibiting outstanding electrochemical performance

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with the help of optimizing the various parameters of the device. Specially, in association with the electrode distance, width, size, etc., a solid-state MSC was reported to exhibit high volumetric (77.3 mF cm−3 ) and areal (7.7 mF cm−2 ) capacitances. The results are explained as: the power density decreases with an increase in the insulated gap as a result of bigger space between the electrode reducing the ion diffusion efficiency. In case of extremely small electrode finger width, there will be insufficient active area of electrode materials; on the other hand, more width will lead to increase in the diffusion length. There might be drop in the capacitance value as a result of all these characteristics. Hence, for achieving improved performance, an optimized electrode width and gap between the electrode fingers need to be chosen. Normally, an increase in the ratio of electrode width to distance between electrodes (W e /W g ) is supposed to significantly decrease the resistance (ESR) and also increase the energy and power density [8, 10] Additionally, for obtaining larger flexibility that would satisfy the necessities of design of devices those can be worn and smart systems, MSCs having improved mechanical characteristics have been developed. Jiao and coworkers [16] could design an MSC arrays with all-solid-state components (MSCAs) showcasing remarkably good mechanical properties. The fabrication was influenced by the synthesized papers comprising of composite of MXene and bacterial cellulose and then by applying a facile laser-cutting approach. The so-fabricated MSCAs were noted with an areal capacitance attaining a value of 111.5 mF cm−2 along with appreciable bending, twisting, and tensile properties. The outstanding tensile characteristics allow MSCAs to alter according to a varied range of applications. In other words, influenced by the aforementioned elaborately designed MSCs exhibiting outstanding electrochemical performances, along with the relation between the structural parameters and the performances, it is believed that the MSCs design is obliging for bringing improvement in their electrochemical performances and broaden their applications. And the discussed designs are adjustable and optimizable with respect to various intrinsic parameters including electrode thickness, width, gap width, etc., as well as the overall design of MSCs [9].

12.3 Brief Introduction to the Reaction Mechanism MSCs are classified into different classes, according to the choice of electrolyte, the energy storage mechanism of the electrodes, and the electrode materials’ nature. According to the energy storage mechanisms, MSCs are classified into two types: (1) electric double-layer capacitors (EDLCs), where double-layer structure formed as a result of the charge separation at the electrolyte and electrode interface leading to the storage of charge. At the time of charging, the two-electrode surface becomes negatively or positively charged as a result of influence from the external electric field. As a result the electrodes attract oppositely charged electrolytic ions and scattering of the electric field, the oppositely charged layers develop mutual attraction resulting in the stability of the electric double-layer. In this process, the energy is stored. At the time of discharging process, electrons pass through the load from negative to the

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positive electrode, leading to release of the energy causing a current in the external circuit [17]. EDLCs are mainly comprised of porous carbon electrode material credited to their high surface area which largely influence the value of EDL capacitance. (2) Pseudo-supercapacitors, store charge by fast and reversible adsorption/desorption or redox reactions. At the time of charging, the ions will accumulate at the electrodeelectrolyte interface as a result of applied electric field; this is followed by the highly reversible reaction occurring in the electrode resulting in the storage of energy. While the occurrence of the reaction, the ions are discharged from the electrode, releasing the stored energy through the external circuit. Normally attributed to the access of the electrolytic ions into the near surface of the electrode material and not only the surface, the value of capacitance should be 10–100 folds more for pseudocapacitor as compared to the EDLCs [18]. Metal compounds, conducting polymers, MXenes, and metal–organic frameworks are examples of some typical materials of pseudocapacitive origin [9].

12.4 Device Fabrication Techniques Planar electrochemical devices although reduce the gap between electrodes for faster ion transport, but require a precise fabrication tools to control device parameters. The electrochemical performance of the fabricated device is influenced severely by the designing of the fingers with various architecture and method. This implied that the fabrication approach itself can largely influence the device performance, which might be attributed to reasons stated herewith. At first, for a particular type of electrode material, a little variation in the method of fabrication will result in the formation of different microstructures, which will affect the properties of the device properties in terms of the ability to transfer electrons and ions. Secondly, it is generally known that the charge storage performance of MSCs is significantly controlled by both the electrodes’ thickness and the distance between the pair of electrodes. The above parameters can be tuned through altering the fabrication approaches, leading to improved electrochemical performance. Hence, in this section, various methods of electrode fabrication for MSCs previously reported are reviewed. The disadvantages and advantages of each approach are compared and analyzed to outline apt process for the manufacture of the high-performance MSCs.

12.4.1 Screen Printing for Electrode Fabrication Screen printing is a widely accepted technique for printing electronics. This is an approach where ink is implemented for printing the particular electrode designs on the substrates employing a screen. It comes with several advantages including rapidity,

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simplicity, and cost-effectiveness, and avails a number of choices of electrode materials to be printed. All these properties make this approach very suitable for the electrode fabrication for MSCs. Hence, this approach is very much effective for the manufacturing of the MSCs, specifically when scaling up is necessary [19]. The traditional approach of screen printing comprises printing active current collector and electrode materials separately. For instance, as shown in Fig. 12.2a, Chih and co-workers [15] demonstrated an all-screen-printable method for the production of solid-state flexible MSCs. Following screen printing of the composite of graphene/carbon nanotube (CNTs) composite electrode active material, there occurred a screen printing of an extra Ag current collector over the electrode layer. Lastly, employing 2 M electrolyte comprising of H3 PO4 /PVA, the so-obtained MSC with finger length 0.5 mm and distance between two-electrode finger of 0.8 mm, could reach an areal capacitance of 7.7 mF cm−2 together with a volumetric capacitance of 77.3 F cm−3 at the scan rate of 5 mV s−1 , associated with outstanding cyclic life (>99% capacity retention following 15,000 cycles). All these improvements in the electrochemical performance were attributed to the amalgamation of carbon nanotube and graphene (Fig. 12.2b). In due course, Liu et al. [20] efficaciously designed a flexible MSC by employing screen printing of Au current collector at first followed by the printing of Ag and polypyrrole (Fig. 12.2c). After injecting polymer electrolyte, the fabricated MSC exhibited outstanding electrochemical performance associated with large energy density corresponding to a value of 4.33 × 10−3 mWh cm−2 and appreciable cyclic life accompanied by good mechanical flexibility. MXene materials can also be printed by implementing screen printing method. For example, Yu and co-workers [21] fabricated crumpled nanosheets of N-doped MXene employing a melamine formaldehyde as a template. As a result of the N doping and crumpled architecture improved redox activity, conductivity, and the areal capacitance exhibited by the MSCs reached a value of 70.1 mF cm−2 and excellent mechanical strength. For another instance, by procuring multilayered MXene and the unelected precursor into aqueous inks, Abdolhosseinzadeh and co-workers [22] effectively designed a paperbased MSC, as shown in Fig. 12.2d. This approach enabled the resulting MSCs to exhibit significantly superior areal capacitance corresponding to the value of 158 mF cm−2 . In case of the screen printing, in a very small time, production of numerous batch is considered to be its greatest credit which is difficult to achieve for the other production methods. Furthermore, the step-by-step printing of electrolytes as the electrode materials possible as a result of screen printing provides a path to fabricate all-solid-state MSCs. Nevertheless, it is noticeable that for this fabrication approach and for the electrode materials ink is challenged by low resolution which is required to be addressed. The multilayers formed due to the screen printing allows attaining high surface area of the electrodes.

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Fig. 12.2 Fabrication approaches of: a Preparation of the MSCs with screen-printable properties; b SEM images corresponding to the hybrid graphene/CNTs electrodes; c designing of the inkbased silver polypyrrole composite-based MSCs; d direct screen printing of MXene; e designing of the flexible MSCs comprising of MnO2 porous nanofiber-like electrodes; f digital picture of fully printed MSCs integrated on a large scale on Kapton. CV plots corresponding to the arrays of MSCs at g various scan rates and h various cycles. a, b Reproduced with permission [15]. (Copyright © 2019, Royal Society of Chemistry). c Reproduced with permission [20] (Copyright © 2017, John Wiley and Sons). d Reproduced with permission [22] (Copyright © 2020, John Wiley and Sons). e Reproduced with permission [26] (Copyright © 2019, John Wiley and Sons). f–h Reproduced with permission [14] (Copyright © 2017, American Chemical Society)

12.4.2 Inkjet Printing for Micro-electrode Fabrication Utilisation of inkjet printing in supercapacitor was a breakthrough in large scale manufacturing. This is an approach of fabrication of electrode where the surface of the substrate is sprayed with ink to design different patterns. Unlike the other approaches, it exhibits the benefits including feasibility of manufacturing at the room temperature, large precision, as well as the scope for scaled up mass production. Furthermore, in this approach direct printing of the electrode material onto the substrate is also possible, which makes it apt for the designing of thin films functionalized by various moieties for applications in electrochemical energy storage devices, solar cells, etc. [23] In context with the designing of electrode material through inkjet printing leads to the procurement of ink with proper fluidity/viscosity. Generally, in the inkjet printing method, the ink is prepared and is directly printed on the substrate

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to get the electrodes for MSCs. For instance, Wang and co-workers [24] synthesized prepared an excellent and flexible MSC by implementing the process of ink printing comprising of three-dimensional (3D) metal-ion-doped MnO2 nanosheets. As a result of the introduction of the MnO2, the performance of the device increased dramatically. Additionally, Pang and co-workers [25] designed asymmetric MSCs comprising of planar K2 Co3 (P2 O7 )2 ·2H2 O nanomaterials and graphene nanosheets acting as anodes and cathodes, respectively, by inkjet printing. And the fabricated MSC showed a large volume capacitance corresponding to the value of 6.0 F cm−3 . Apart from conventional inkjet printing approach comprising of direct implementation of the active material ink, other approaches integrating other active material growing like electrochemical deposition along with the inkjet printing have also been reported. For example, Cheng and co-workers [26] initially procured interdigitated silver electrodes on the substrate of PET by employing inkjet printing. Following this electrode structures comprising of porous nanofibers were developed by electrochemical deposition of MnO2 on the Ag electrodes, as shown in Fig. 12.2e. Aiding from the structure of the electrode, the designed MSC exhibited a large areal capacitance and a capacitance retention of 86.8% following 1000th cycles of bending to an extent of 180°. Large-scale MSC arrays can also be fabricated by inkjet printing approach rapidly and in a facile way. For example, Li and co-workers [14] reported the fabrication of an MSC through inkjet printing approach, and the integrated device solved the drawback of low voltage that generally is detected in a single device. Thus, the MSC arrays were more practical from the point of view of application. To be more specific they designed MSCs comprising of graphene implementing a facile full-inkjet printing approach. The MSC comprising of optimized electrode width, along with different parameters, the fabricated MSCs were able to achieve a large areal capacitance reaching a value corresponding to 0.7 mF cm−2 . More importantly, >100 devices (Fig. 12.2f) could be connected efficiently acting as power banks leading to the fabrication of MSC with operating potential of 12 V (Fig. 12.2g). These could maintain their performance up to eight months or more (Fig. 12.2h), affirming an excellent electrochemical property. Although, inkjet printing fabrication efficiency is smaller than that of screen printing, the advantage of inkjet printing is its large resolution. In addition to the high resolution printing also provides a wide selection window of electrode materials and substrates. This factor paves the path for opportunities associated with fabrication of high-performance flexible MSCs. Furthermore, high-energy density and operating potential windows are possible to be achieved with the help of largescale printing and integrating MSCs on flexible substrates. With better quality of ink and appropriate tunning of the thickness of the electrode and the distance between them, the electrochemical performance of the fabricated MSCs is possible to be improved significantly.

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12.4.3 Lithography for Micro-electrode Fabrication This process implements photoresist for pattern designing, which is not only able to reach large resolution of the electrodes. It can also produce various plane patterns complex in nature. Hence, it is frequently employed to offer templates on purpose of the fabrication of electrode materials of MSCs [27]. For instance, as shown in Fig. 12.3a, Sun and co-workers [28] designed carbon nanotube/carbon current collectors through carbonization process through converting photoresist on carbon nanotube into amorphous carbon. This was followed by of the growth of MnO2 nanostructures on the previously grown electrode material, and hence, an in-plane MSC exhibiting a large stack capacitance of 20.4 F cm−3 could be. Furthermore, the implementation of photolithography together with pyrolysis-reduction presents the possibility for bringing about efficient MSCs, according to the report by Hong and co-workers [29]. They fabricated porous carbon/tin quantum dots, which played the role of both as the current collector and as the electrode active material. Importantly, the tin quantum dots showed large electrical conductivity and allowed the designed MSC to offer a large areal-specific capacitance reaching a value of 5.79 mF cm−2 as compared to that of C-MSC. Furthermore, the capacitance retention reached a value of 93.3% following 5000 charge–discharge cycles. The mentioned work is also a demonstration about the involvement of more than only photolithography approach for the fabrication of large-performance MSCs as shown in the report by Tian and co-workers [30], as shown in Fig. 12.3b. They reported the fabrication of the interdigital patterns comprising of layer of Ti/Au amalgamation on SiO2 substrate by employing physical vapor deposition (PVD) along with lithography. This was followed by the deposition of the graphene oxide intermediary current collector layers on through a drop casting and drying approach. The topmost polyaniline layer was deposited by the method of electropolymerizing. The ingenious integration of these electrode designing approaches allowed as fabricated MSCs to showcase a large capacitance corresponding to a value of 271.1 F cm−3 , energy density of 24.1 mWh cm−3 together with a good rate capability. To summarize, photolithography projects itself as a promising approach for the designing of various types of MSCs exhibiting good charge storage performance. Attributed to the large resolution and improved process of operation, lithography is broadly employed implemented in microscaled integrated systems for reaching large accuracy. However, due to various restrictions related to the technology, it is necessary to combine with other techniques to improve the entire manufacturing process. In addition, the template (photoresist) should be eliminated at high temperature or in buffer solution. All these factors result in the efficiency reduction and limit the possibility of large-scale production. Furthermore, the pollution and the high cost of the photoresist reagent are also responsible for its limited practical application. For all these reasons, the development of photolithography approach is still at its infancy.

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Fig. 12.3 Laser scribing and photolithography techniques for designing MSCs. a Schematic illustration of construction approach associated with SWNT/carbon/MnO2 -based MSC grown on a soft substrate; b MSCs designed with holey graphene/polyaniline hetero-structures; c designing of LIG films implementing laser scribing for MSCs; d top-view; and e cross-section SEM images of LIG films; f synthesis of RuO2 -based flexible MSCs. a Reproduced with permission [28] (Copyright © 2016, Elsevier). b Reproduced with permission [30] (Copyright © 2016, Springer Nature). c–e Reproduced with permission [31] (Copyright © 2019, John Wiley and Sons). f Reproduced with permission [32] (Copyright © 2019, John Wiley and Sons)

12.4.4 Laser Scribing for Micro-electrode Fabrication This technique employs high-energy beam of laser to enlighten the targeted piece surface for melting it locally and gasify the enlightened portion, for scribing. Attributed to its simple approach, high precision, and speed, laser scribing has garnered much attention and has been more applied for the manufacturing of MSC [33]. For instance, Peng et al. [34] tested the designing of a B-doped porous graphene using laser induction, from the sheets of polyimide immersed in boric acid. The sofabricated device showed a high areal capacitance reaching a value of 16.5 mF cm−2 . Likewise, Shi et al. [31] reported a cost effective, facile, one-step and scalable fabrication of micropatterns by implementing laser-induced graphene (LIG) (Fig. 12.3c). The fabrication along with the patterning of LIG films could synchronously be achieved by laser scribing process, as shown in Fig. 12.3d, e. This factor was quite beneficial for MSCs. At the time of applying a polymer gel of PVA/H3 PO4 , the

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associated LIG-MSCs exhibited an appreciable areal capacitance value of 0.62 mF cm−2 associated with a good cycle stability accompanied by negligible capacitance loss following 10,000 cycles. Furthermore, this approach could also prevent the complexity and expensive conventional manufacturing process, and proved to be a better approach for applications of MSCs. Furthermore, Li et al. [35] reported flexible MSCs accompanied by charge storage performance through implementation of laser induction accompanied by technology of electrodeposition. This type of integration approach offers a reference toward the preparation of MSCs. Figure 12.3f illustrates another work, in which Brousse et al. [32] fabricated a flexible MSCs consisting of ruthenium oxide (RuO2 ) on platinum foil employing a simple approach consisting of laser writing of a bilayered film. As a result of the pillar-like architecture of the electrodes, the fabricated MSC showed a high value of capacitances corresponding to 27 mF cm−2 or 540 F cm−3 in 1 M H2 SO4 electrolyte, together with presenting an appreciable cycle performance. In short, unlike from photolithography, laser scribing prevents the implementation of complex approaches and implementation of additional templates. Additionally, laser scribing can reach direct obtaining of graphene from the conversion of commercial polymer films and other active materials, such as metal oxides. Moreover, by implementing laser scribing, it is also possible to attain fair tensile properties of the obtained MSCs, as has been demonstrated in the work of Jiao and co-workers [16]. Hence, scribing through laser has been implemented broadly in the manufacturing of various types of MSCs devices and has attracted high expectations for the manufacturing of MSCs.

12.4.5 Mask-Assisted Filtering for Micro-electrode Fabrication Mask-assisted filtering employs an approach for the procurement of MSCs or microbatteries through vacuum filtration in liquid form accompanied by the template assistance. This approach is facile concise and cost effective unlike other methods [36]. Recently, Huang and co-workers [37] reported a simple and effective method for the purpose of exfoliating the multilayer-MXene with the help of mild water freezing-and-thawing (FAT) method, as a consequence massive FAT-MXene flakes were possible to procure the following four cycles of this approach. FAT-MXene when tested as on-chip all-MXene MSC fabricated through template-assisted filtering (Fig. 12.4a), the so-fabricated MSC exhibited high volumetric capacitance of 591 F cm−3 and an areal capacitance of 23.6 mF cm−2 when optimal electrode thickness has been implemented, as shown in Fig. 12.4b. It is a suitable method to design asymmetrical MSCs (AMSCs) implementing mask-assisted filtering. For instance, as shown in Fig. 12.4c, Qin and co-workers [38] initially grown current collectors of graphene on the mask, followed by the addition of solution of MnO2 nanosheets and porous nanosheets on opposite sides of the mask. Further with facile deposition of

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gel electrolyte with the help of a facile mask-assisted deposition method, all-solidstate AMSC was procured. The as-prepared AMSC showed an appreciable energy density of 21.6 mWh cm−3 , good cycling stability, and a wide working potential of 2 V (Fig. 12.4d). Apart from the methods discussed previously, there are a few more ways which can be employed to fabricated MSCs. For instance, using a low-cost stamping, scalable strategy, Zhang and co-workers [39] fabricated flexible MSCs containing 2D MXene. They initially implemented 3D printing technology to receive in the required shapes the templates. This was followed by MXene ink printing on the substrate (Fig. 12.4e, f). This MSC exhibited a capacitance of 61 mF cm−2 at 25 μA cm−2 (Fig. 12.4g). Significantly, the strategy of stamping reported in this work provides a route for fabrication of flexible MSCs. In a nutshell, the automatic scalpel technology offers

Fig. 12.4 a The mask-assisted fabrication of all-MXene MSC; b cyclic voltammetric curves at various scan rates and the volumetric capacitances associated with the MSCs with various thicknesses; c fabrication of VN//MnO2 -AMSCs involving mask-assisted filtration; d cyclic voltammetric plots obtained at different scan rates; e fabrication technique for all-MXene-based MSCs; f digital pictures of the MSCs; g cyclic voltammetric plots obtained for the as-stamped MSCs. a, b Reproduced with permission [37] (Copyright © 2020, John Wiley and Sons). c, d Reproduced with permission [38] (Copyright © 2019, Elsevier). e–g Reproduced with permission [39] (Copyright © 2018, John Wiley and Sons)

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the possibilities for the fabrication of MSCs from coating technique-yielded films. Even though there exists a wide variety of well-established methods resulting in the fabrication of MSCs, all of them is yet having the scope for more betterment.

12.5 Patterning of Micro-electrodes Even though electrode material plays an important role in determining the properties of a micro-supercapacitor (MSC) but the shape of the electrode also important determining factor of the device performance. Lately, the research group of Alshareef [40] has reported the fabrication of various types of fractal electrochemical MSCs. In their work, the effect of the shape of the MSC electrode on its electrochemical properties was studied and reported. Sputtered anhydrous RuO2 was implemented as the thin-film electrode prototypes, and as shown in Fig. 12.5a–f, MSCs with fractal designs were fabricated [40]. For fractal design geometry, the performance of the MSC with respect to energy density, areal capacitance, and volumetric capacitance significantly got augmented as compared to the conventional MSCs with interdigitated electrode pattern. Particularly, there was a 32% hike in the energy density of Moore-designed MSCs as compared to that of traditional interdigital structures, both implementing thin-film RuO2 corresponding to the same power density [40]. An energy density of 23.2 mWh cm−3 corresponding to the power density of 769 mW cm−3 was exhibited by the Moore-designed MSC. When noted at the similar value of power density, the value of energy density was as low as 17.5 mWh cm−3 for the interdigital-designed MSC [40]. According to the study, it is realized that as a result of edging effects existing in the electrodes with fractal design, the growth of electrical lines of force is also responsible for introducing additional capacitance and energy density. The electric field was simulated with COMSOL simulation tool, the obtained images, and their conclusion of strengthening of the electrical field was confirmed by the results. The research group of Yang [41] also investigated the fractal designs in the work related to bucky paper-based MSC in the year 2019. According to them design comprising of level-three fractal-electrode MSC was more efficient than that of the conventional interdigital-electrode design. Additionally, the applications of the MSCs can also be altered according to the shapes of the electrode. Fabrication of fiber-shaped MSCs is an apt way to procure wearable and flexible MSCs. In this context, a large attention has been dedicated to the wire-shaped energy storage devices. For instance, the research group of Schulz [42] fabricated a wire-shaped microelectrode and implemented it in a MSC in the year 2007. The growth of wearable micro-devices, which are able to convert mechanical, [43] light, [44] and heat, [45] energy into electricity, is main triggering fact for the development of wire-shaped energy storage devices. In the year 2013, the research group of Peng [46] reported the fabrication of a wire-shaped MSC. Two MWCNT–PANI robust composite fibers comprised of the wire-like electrode, which was twisted around each other, as shown in Fig. 12.5g, h.

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Fig. 12.5 3D images corresponding to a interdigital, b fractal Hilbert, c Peano, and d fractal Moore electrode. A schematic corresponding to the edging effect in e conventional IDE and f fractal-electrode designs. Reproduced with permission [40]. (Copyright © 2017, The WILEY– VCH). g Schematic and h SEM image of a micro-supercapacitor wire comprising of MWCNT– PANI composite fibers. Reproduced with permission [43] (Copyright © 2013, The Royal Society of Chemistry)

These MSC electrodes exhibited a specific capacitance of 274 F g−1 or 263 mF cm−3 . In the year 2015, according to report for fabrication of all-solidstate MSCs, titanium wire was also used along with the second electrode comprising of CNT fiber or sheet. The fabricated MSC exhibited a capacitance corresponding to the value of 1.84 mF cm−2 when tested with the CNT sheet electrode as the other electrode, with the single strand of CNT yarn as the second electrode the so-designed MSC exhibited a capacitance three times larger as compared to the MSC with CNT sheet as the other electrode [44]. Recently in the year 2018, Cao and co-workers [47] applied metal–organic framework (MOF)/graphene oxide fibers as precursor for the synthesis of fibers comprising of composite of porous metal oxide/reduced graphene oxide. These electrodes comprising of composite fibers showed improved electrochemical performance and exhibited a large potential for application in micro-energy storage. The previously discussed works confirm the influence of the shape of the electrode in the electrochemical performance of the associated MSC and also their

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applications. It has been seen that the fractal electrodes are very apt in improving the electrochemical performance of the MSCs. Additionally, the electrodes in wire shape are apt to be implemented as the electrode material in MSCs [48].

12.6 Micro-supercapacitor Systems With the growing requirement of the efficient flexible and stretchable electronics, there has been a tremendous growth in the development of wearable electronic devices. Various studies recently have been conducted on the fabrication of different types of stretchable devices, including epidermal electronics126, soft surgical tools [49], wearable photovoltaics [50], sensitive robotic skin [51], and inorganic or organic light-emitting diodes (LEDs) [52]. Attributed to larger power density, better cyclic stability, and improved safety as compared to the batteries, MSCs possess immense potential of being implemented as energy storage devices for LEDs and sensors. Nevertheless, though a great deal of drawbacks has been mitigated, in spite of this there exists some problem till now. Less current, low output voltage, and small discharge time prove to be the largest impediments hindering the application of MSCs. On a positive side, some measures have been undertaken to refrain from these drawbacks as seen in some recent research associated with MSCs. One of the solutions is arrangement of MSCs array. In accordance with the capacitor theory, there will be an increase in the output potential when the capacitors will be connected in series together with the reducing the capacitance. On the other hand, there will be a hike in the value of capacitance for a parallel connection between the capacitors with no other effect. That implies that the output voltage and the capacitance both can be significantly improved by combining several MSCs in parallel. In the year 2014 the research group of Jeong Sook Ha group [53] arranged such a 4S + 4P array of the MSCs array, as shown in Fig. 12.6a, b. The output voltage and the discharge time associated with this array were almost threefold larger as compared to a single MSC (Fig. 12.6c–f). The output voltage was noted to be 3.0 V indicating that the MSCs array would be able to power several LEDs or power an array of LED. The research group of Ha [53] also tried to design stretchable device comprising of two types of MSCs arrays. The first one applied a stretchable MSC array and was grown on a flexible substrate made up of polymer. Under different mechanical distortions including twisting, uniaxial strain, and bending a negligible change was noted in electrochemical performance for this MSC. The other set of array was composed of a MSCs which were biaxially stretchable. The power and energy density delivered by this MSCs array were noted to be 32 W cm−3 and 25 mWh cm−3 , respectively. At the uniaxial stretching of 100% and biaxial stretching of 50%, there was no significant visible change in the electrochemical performance of the MSC [54]. After the designing of MSC, the research group of Ha fabricated a high-performance, encapsulated, and stretchable stacked planar MSCs’ array. This was employed as an waterproof wearable energy storage device [55]. According to the report, they parallelly connected five MSCs

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Fig. 12.6 a Schematic representing the three micro-supercapacitors (MSCs) connected in series on a planar substrate and two pairs of three serially connected MSCs connected on both sides of the stretchable substrate; b the circuit diagram of 4S + 4P array (the working circuit). Reproduced with permission [53]. Copyright (2014) American Chemical Society. Electrochemical characteristics of single and array MSCs; c CV curves of a single MSC at different scan rates varying; d galvanostatic charge/discharge curves; e CV curves of the 4S + 4P array of MSCs at various scan rates (the inset circuit shows the 4S + 4P array of MSCs); f galvanostatic charge/discharge curves of the 4S + 4P array of MSCs at various current densities. Reproduced with permission [49] (Copyright © 2014, American Chemical Society)

and then this MSC array was associated with a micro-LED. The complete device comprises five parallelly linked stacked MSCs; a switch and a μ-LED were kept in a thin Eco flex film. As a result, even after keeping the arrangement in water for 4 days, there was no observable change in the electrochemical performance of the device, and the LED could be lit without much visible reduction in the brightness even with distortions including stretching and bending [55]. The other solution is developing a micro-system with MSC containing an energy storage device, a micro-power source, (like an MSC) associated with a working device. The power source is meant to provide continuous output, which is however not the case as in reality a micro-power source

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is associated with significant fluctuation of output. The MSCs are very effective in reducing the fluctuations, hence making the integrated systems successful and efficient. The research group of Yihua Gao designed a flexible integrated system comprising of an wireless charging coil, a micro-supercapacitor, associated with a photodetector in the year 2016 [56]. In this system, an wireless power transmitter was responsible for powering up the wireless charging coil, the energy from the wireless charging coil was stored temporarily in the MSC. This stored energy was used to power the photodetector in a stable manner [56]. According to the report, the work showed high efficiency and large high sensitivity [48].

12.7 Application of MSCs The continuous progress in the field of microelectronic systems necessitates the development in the field of micro-structured energy storage devices [57]. MSCs satisfy very well the increasing demands of largely integrated and flexible electronics credited to its microstructure, lightweight, outstanding charge/discharge rate, together with great extent of flexibility. Up to date, MSCs have already been successfully integrated in various types of sensors, various advanced wearable devices, and so on [58]. Furthermore, MSCs can play crucial role in the area of medicine attributed to the scope of choosing of several classes of electrode materials. The applications of MSCs in the various fields will be discussed in detail in this section, emphasizing on their significant roles in making these devices function efficiently.

12.7.1 Energy Storage Nowadays, the continued growth of various types of small sensors and wearable electronic devices have tremendously triggered the need for associated power supply modules [59]. However, currently, the main challenge is developing power supply modules with long life cycle and appreciable stability. The limited stability of the micro-batteries and reliance on the solar cells for proper working are the facts which are limiting the use of non-conventional energy resource. In this situation, integrating energy converting devices (e.g., triboelectric nano-generator) with MSCs proves to be a striking solution to achieve the requisite. For instance, Luo and co-workers [60] applied a facile technology implementing laser engraving for integration of MSCs with tribiologic nano-generator, i.e., TNG to design a system driven by its own. The system is able get charged by basic movement and exhibited outstanding continuous ability to discharge. However, it generally took a while for charging MSCs in systems alike for the provision of practical application. To mitigate this drawback, Zhang and co-workers [61] adapted an approach by implementing dual devices for energy harvesting, e.g., electromagnetic generators and TNG, for the purpose of mechanical energy (Fig. 12.6a), which will play a major role in enhancing the rate of charging

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of MSCs (Fig. 12.7a). The entire process was combined in a bracelet which was wearable in the wrist. In the device, the MSCs were possible be charged to 2 V by wrist swaying occurring while various day-to-day action, through which it is possible to carry out the working of various sensors. Furthermore, MXene-based MSCs also exhibit remarkable potential for application systems driven by self. As shown in Fig. 12.7b, Jiang and co-workers [62] developed a highly dense power module charging automatically by combining the TNG and MXene-based MSC. That kind of integrated device is possible to be safely worn on human skin and the integrated device can transform the mechanical energy produced by the human body movement into energy and store it in the MSC. It is noteworthy that the MSC could be further applied to drive various wearable electronic sensors. In conclusion, attaching flexible MSCs as power source together with energy harvesting devices is very effective in providing the power continuously without any interruption for microscale integrated systems on a long-term basis and has no influence of the external conditions. This is effective in enhancing the practical application possibility of wearable electronic devices.

Fig. 12.7 a The designing approach of energy generating bracelet; b schematic demonstration of the self-charging device which is wearable; c designing of the MSC-sensor integrated system; d schematic representing porous CNT-PDMS elastomer-based all-in-one sensing patch; e schematic representing self-powered integrated device. a Reproduced with permission [61] (Copyright © 2019, John Wiley and Sons). b Reproduced with permission [62] (Copyright © 2018, Elsevier). c Reproduced with permission [65] (Copyright © 2020, John Wiley and Sons). d Reproduced with permission [66] (Copyright © 2018, Elsevier). e Reproduced with permission [69] (Copyright © 2017, John Wiley and Sons)

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12.7.2 Integration with Various Types of Sensors As has been discussed in the previous section, MSCs are possible to be implemented to power different kinds of sensors, and in several fields their integration can be implemented [63]. For instance, MSCs integrated with gas sensor can be applied in monitoring the change of the gases present in the surrounding in real-time. This can play a significant role in controlling the quality of the environment and health of human beings. An MSC integrated gas sensor system was reported by Yun and co-workers [64], in which graphene-based gas sensor was used. The electrodes of the MSCs comprised of polyaniline-wrapped MWCNTs. With the help of the lucid design of the substrate, different devices are possible to be successfully integrated with each other, and the resulting device exhibited excellent tensile property with not much visible change in the electrochemical performance at the uniaxial strain of 50%. Furthermore, this system could be implemented to monitor NO2 for >50 min. However, it was noted that non-similar cells, implemented with unsimilar active materials, were responsible for not only causing complexity in the synthesis of material but also reduce the compatibility between the various modules. For this reason, it is significantly necessary to fabricate multifunctional materials which will be able to work for more than one type of integrated devices. For instance, Qin and coworkers [65] investigated 2D hierarchically ordered dual-mesoporous polypyrrole and graphene nanosheets implemented as dual-functional materials for application in NH3 sensor and MSC both (Fig. 12.7c). Attributed to the distribution of the same kind of material, the system of MSC and integrated sensor showed high compatibility and responded to the 10–40 ppm amount of NH3 after being charged for only 100 s. More significantly, the device still provided a good response value for NH3 detection when bended at large angles. In addition to the gas sensor, Song and co-workers [66] investigated an assemblage of the piezoelectric sensors (PRSs) associated with MSC, and the integrated device was able to detect pressure in real-time. As shown in Fig. 12.7d, by implementing the approach of solution evaporation, an all-in-one sensing patch consisting of PRS as functional part and MSC comprising of porous elastomer comprising of CNT polydimethylsiloxane for energy storage was integrated. It is noteworthy the sensor and the MSC worked well after integration. The PRS showed high sensitivity of value corresponding to 0.51 kPa−1 and a wide range of detection, while the MSC exhibited remarkable areal capacitance. Furthermore, the designed patch was compatible with human skin and could be easily attached for detecting body conditions. It could also be implemented as a 3D torch, which could extract its parameters and detect the signals for identification of an individual and so on. Apart from this, the integrated system of MSCs could also act as strain sensor. Yun and co-workers fabricated self-powered sensors [67] by applying solar cells, MSCs, and strain sensors together. The designed device showed outstanding tensile characteristics and was able to monitor the change in the external strain. Furthermore, by the combination of UV sensors and MSCs, it is possible to detect UV radiation. For instance, Kim and co-workers [68] designed an all-solid-state flexible MSCs array comprising of graphene foam/MWCNT-COOH/MnOx composite as the

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electrode material. After being integrated with SnO2 nanowire-based UV sensors, the whole system was able to monitor the UV-induced photocurrent with response time of more than 10 min. Furthermore, the sensor system became self-powered as it was combined with solar cells. Guo and co-workers [69] reported an approach to accommodate MSCs and nanowire-based UV sensor in Ni circuit on a piece of paper. The so-formed system of MSC-UV sensor showed good self-powering and sensing abilities. Furthermore, MSC was connected to a solar cell to prepare self-powered gas sensor, as shown in Fig. 12.7e. Cai and co-workers [70] designed a self-powered UV-light detection device by integrating photodetector based on ZnO nanoparticles and a MSC designed by laser writing on PI films and a solar panel. The electrode of the MSC was also fabricated through one-step laser direct writing. The entire system had rapid response ability and large on/off ratio. In a nutshell, micro-sensor integrated systems are garnering more attention in monitoring, gas in environmental surrounding, and so on. Generally, these systems must show the characteristics including flexibility to adjust in different application scenarios, like sticking to human skin, etc. Furthermore, the development in the field of sensor integrated system requires the prop up and support from the MSCs exhibiting large power density, larger flexibility, fast charging rate, and response time.

12.7.3 Medical Assistant Examination The high rate progress in the painless diagnosis necessitates introduction of the miniaturized medical devices. Attributed to the lack of certainty associated with the pathological characteristics of the generally used biocompatible materials, such as discontinuous electrode elements associated with variable and complex surface strain, novel energy devices which can be degraded are challenged by unpredictable performance and risks involved in usage for diagnosis process. In this scenario, MSCs with good flexibility, robust selectivity of electrode materials project themselves as apt for application in medical field requiring additional energy. For instance, inspired by the charge storage properties of the natural food, Gao and co-workers [71] intended an MSC (EMSC) which was edible comprising of food constituents with the help of template-imprinting strategy, as shown in Fig. 12.8a. In this design, the implemented electrolyte and electrode materials were edible and so were appropriate to be degraded or absorbed by the human body. The designed EMSC showed outstanding flexibility and mechanical stability, which enabled its integration in various objects having different shapes, including fruits and human skin. As shown in Fig. 12.8b, this microdevice was also possible to be coiled and put inside a shell of medical capsule to deliver power for capsule endoscopy in an environment having gastric juice. The apt characteristics of MSCs made it possible the monitoring of real-time vivo and opened the possibilities for other biological applications. Another type of significant wearable electronic device meant for personal health is attachable and flexible smart system with an ability to monitor body fluid, and predict status of health in real-time

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with the help of composition monitoring of body fluids. Attributed to its flexibility and selectivity toward the substrate the planar MSCs prove to be apt to satisfy this demand. For instance, Lu and co-workers [72] fabricated a system that could monitor the perspiration by integration MSC comprising of NiCo2 O4 − . The so-fabricated MSCs exhibited an energy density corresponding to a value of 0.64 μWcm−2 . Moreover, the system exhibited outstanding performance related to monitoring and ions and glucose detection ability in sweat. As a matter of more interest, the fabricated system was possible to combine with wireless sensor technology for attaining a real-time sweat monitoring with much accuracy. The device could send the result and other related information to the associated mobile phone meant for assessment of health. In short, by implementing flexible MSCs, it is possible to refrain from frequent replacement problem which is a common problem as MSCs generally have a short life span. Furthermore, the MSCs made up from biodegradable materials are harmless to any type of living being. In a nutshell, the development and implementation of MSCs will also augment the progress in the area of medicine.

Fig. 12.8 a The schematic illustration of fabrication of edible MSC (EMSC); b structural schematic of commercial gastroscope with power supply from the EMSC; c schematic diagram showing the fabrication approach of high-frequency MSC; d circuit diagram and digital picture of a low-pass filtering circuit. An electrolytic capacitor of 0.47 mF has been represented for comparison; e a LED array powered through flexible power filtering circuit employed with AC input. a, b Reproduced with permission [71] (Copyright © 2020, Royal Society of Chemistry). c–e Reproduced with permission [75] (Copyright © 2019, Elsevier)

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12.7.4 Alternating Current (AC) Line Filtering Aluminum electrolytic capacitors (AECs) are widely implemented for applications in filtering AC line. Nevertheless, AECs usually own large volume and hence small energy density are not suitable for the fabrication of devices on-chip [73]. As compared to the AECs, MSCs are smaller in dimension, and can be integrated in a facile way as well as deliver high power density, making them appropriate for AC line filtering. Yang and co-workers [74] designed a coordination polymer framework based on azulene-bridges and implemented it as electrode material for MSC using a layer-by-layer approach. The fabricated MSC was able to deliver a specific capacitance corresponding to a value of 34.1 F cm−3 at 50 mV s−1 and could attain −73° of phase angle at the frequency of 120 Hz exhibiting a small resistance-capacitance constant of ~0.83 ms. However, at the current situation, inadequate rate performance is a persisting challenge limiting the implementation of MSCs for AC line filtering. To mitigate this problem, 2D materials including graphene and MXene are recognized as apt candidates attributed to the presence of several ionic channels and rapid redox reactions occurring at the surface. For instance, Xu and co-workers [75] tested a high-frequency MSC integrated with MWCNTs and 2D pseudocapacitive MXene, as shown in Fig. 12.8c. Since, MXene can deliver large value of capacitance while, MWCNT provided pathways rapid transport of ions as well as support in the interlayers, the associated MSC showed large power density as well as an improved frequency response than the commercially applied tantalum-based capacitors operated at the frequency of 120 Hz. Moreover, the MSC exhibited better electrochemical performance when measured as both a relaxation oscillator circuit and a lowpass filter circuit as compared to the supercapacitors in terms of size and function (Fig. 12.8d). And as shown in Fig. 12.8e, the designed integrated device could power up array of LEDs successfully with AC voltage. Similarly, Jiang and co-workers [76] also integrated an MSC on the basis of solution processable 2D MXene nanosheets for application in filtering AC line. With the help of optimizing of the electrode thickness, dimension of the flake, and gap between the fingers of the electrode, the so-obtained MSC could exhibit a volumetric capacitance corresponding to the value of 30 F cm−3 at the frequency of 120 Hz associated with a relaxation time constant of τ0 = 0.45 ms. This value was improved as compared to the electrolytic capacitors with value τ0 = 0.8 ms. The results implied that the integrated device could successfully filter 120 Hz ripples. In a nutshell, as compared to the AECs, MSCs have tremendous potential for application in AC line filtering with unique benefits and can be implemented in electronic devices and Internet of Things (IoTs) [9].

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12.8 Evaluation of Various Parameters of Supercapacitors In a past few years, the number of publications in context of electrochemical capacitors (ECs) has enhanced tremendously, concerning a diverse research community from fields of material science, electrochemistry, and engineering [77]. Though the contributions from such different communities with varied perspectives are responsible for the advancement of EC technology and science, however this diversity has also caused some inconsistencies and confusion regarding the associated scientific literature as a result of broad range of implemented device configurations, active and inactive materials, and electrochemical testing approaches. Unfortunately, the assessment of the various electrochemical properties of ECimplemented active materials often becomes challenging as the primary experimental procedures and parameters are not adequately discussed in the scholarly articles. Furthermore, sometimes the electrochemical performance of a single electrode is extrapolated to represent the performance of the device like EC comprising of double electrodes. Inconsistent report regarding the key performance parameters restricts the comparison of data originating from different research groups and laboratories on otherwise related devices and materials [77]. In this section, we present various perspective identified by various articles for correct evaluation of the electrochemical performance of devices and the associated electrode materials.

12.8.1 Necessary Details About the System to be Reported As a novel (electrode active) material is reported, the synthesis approach of the material should be stated in sufficient detail so that it can be reproduced by other researchers with information about the synthesis from the article. Basic characteristics of the material are also required to be reported such as crystallite/particle/film morphology, crystal structure). Any kind of fabrication of electrode employing the synthesized materials is required to be discussed adequately, including details about content ratio of the conductive agent if any, active material, and binder(s) involved electrode fabrication. Additionally, a report should include various information including electrode preparation parameters including mass loading per electrode, electrode area, etc. This information is crucial for the researchers to evidently assess the electrochemical performance of the full device as well as the electrode active material. This becomes important particularly at the time of drawing comparing between the results from other research laboratories or commercial products. At the time of reporting a new electrolyte, the content of the studied electrolyte should be precisely stated in the units of Molarity. The viscosity and the ionic conductivity of the discussed electrolyte at ambient temperature are required to be reported. The working potential at the time of measuring the cyclic stability is also a vital performance parameter. Linear-sweep voltammetry is taken as the most appropriate electrochemical testing more for such assessment, where the temperature conditions

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and sweep scan rate must be noted. Furthermore, observation about the density, boiling point, thermal stability, and flash point of the studied electrolyte must be provided.

12.8.2 Single Electrode Capacitance Capacitance is considered as the capacity of an electrode to store electrostatic electrical energy, and it is denoted in the unit Farad (F). For assigning a meaningful amount of capacitance to an electrode, the stored electric charge on the active material is essential to be reported showing a linear dependency on potential (or voltage) on an applied operating window as tested in testing conditions which are galvanostatic. Electrode materials displaying such kind of capacitive behavior are activated carbons (AC), storing charge essentially in double layers which are electrostatic and produced as a result of charge separation, pseudocapacitive metal oxides, such as RuO2 , stores charge by the means of fast and reversible redox reaction [78]. The cyclic voltammetry plots associated with these materials should exhibit rectangular nature on application of scan rate and voltage, and at very small scan rate the CV plot can even appear to be distorted [79]. The value of the capacitance can be expressed in three different ways viz: F cm−3 (volumetric capacitance), F cm−2 (areal capacitance), and F g−1 (gravimetric capacitance). The volumetric capacitance has a significant role to play as the electrode material volume essential to deliver a particular amount of capacitance is vital for fabrication of micro-supercapacitors. During comparing values of various capacitance together with rate-influenced response corresponding to other electrode materials, serious care should be taken to choose similar mass loading/thickness for all electrodes/devices which are compared. For some cases, battery-type materials including Ni(OH)2 are studied as electrochemical capacitor electrode material. For the materials which exhibit well-defined redox active peak when tested with cyclic voltammetric technique which is recognized as exhibit typically irreversible redox behavior (non-electrostatic behavior), the capacitance values should not be assigned. In place of capacitance value of capacity in units of C g−1 or mAh g−1 , should be assigned.

12.8.3 Difference of Capacitance in Three-Electrode and Two-Electrode System A two-electrode system comprising both the electrodes and the working electrodes resembles a supercapacitor device more closely. Where the capacitance measured in a three-electrode system is often overestimated. This can be explained by the following discussion:

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In two-electrode system, the two electrodes are considered to be connected in series (through the electrolyte). So the total capacitance of the entire system is 1/C = 1/C1 + 1/C1 1/C = 2/C1 C = C1 /2 Now the specific capacitance of the whole system (C sp ) is C/2 m. Where ‘m’ is the mass of active material in single electrode And the specific capacitance of the C 1sp = C 1 /m. So, Csp = C1 /4m Csp = C1sp /4 C1sp = 4Csp Hence, this shows that if we determine the specific capacitance of a single electrode, the specific capacitance is four times the specific capacitance of the two-electrode system.

12.8.4 Operating Voltage For the electrodes of capacitive origin, the operative voltage is generally influenced by the electrochemical stability of the electrolyte implemented. The measurement of the operative voltage of an EC device never follows a standard operating procedure; nevertheless, it is suggested that the electrochemical potential window for the electrodes to be assessed and reported. For instance, the operating voltage should be assessed by implementing voltammetry with gradually increasing the limits of the potential configured with a three-electrode half-cell, before measuring in a full device comprising of a two-electrode system. The highest limit is demarcated by setting a critical value for the Coulombic efficiency (e.g., 99%) for the value less than which the charging and discharging process is not considered to be effective [80]. For the pseudocapacitive materials, the effective working potential is dependent in the potential range through which the faradaic charge-storage process happens. Such info is very much required to design an effective two-electrode cell, in which the mass loading of the negative and the positive electrodes much be fixed to balance the

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total charge contained, which should be followed the determination of the operating voltage of the full device.

12.8.5 Micro- and Macro-Supercapacitors It is important to draw some boundaries between traditional macroscale ECs and MSCs in the context of expressing some specific performance metrics. The dimension of the traditional ECs lies between few cm3 and up to one dm3 which are mainly constructed for applications covering consumer electronics to transportation. For the ultimate user, the volume and weight of macro-ECs are the primary parameters affecting the choice of products for a specific application, thus volumetric/gravimetric capacitance performance is the most crucial. The mass loading of the active material should also be on the order of 5–10 mg cm−2 to attain effective quantity of stored energy [80]. It is comparatively simple to assess gravimetric values, while the volumetric characteristics are more challenging to investigate as the electrode volume is influenced by the process of fabrication and the packaging of prototype cells. However, the instances when it can be assessed in an easy way at the level of electrode, volumetric capacitance is considered to be more appropriate to be reported. Micro-supercapacitors comprising of thin-film electrodes can be arranged in several configurations [80], typically at the scale of cm2 or mm2 for expressing. MSCs are fabricated with a purpose to replace or complement the microbatteries for applications in power sensors and microelectronic gadgets. In those kind of applications, area-normalized energy (Wh cm−2 ), power densities (W cm−2 ), and capacitance (F cm−2 ) are the most significant performance parameters. Device and electrode thickness (taking in consideration current collectors, electrolyte, and packaging) are better to report where applicable. The micro-supercapacitor thickness should be restricted to few hundreds of μm, which has influenced the fabrication of interdigitated electrodes [80].

12.8.6 Cycling Stability Extended cycle life and the much larger power densities are the two significant factors those differentiate ESs from the batteries. Hence, cycling stability should be considered with utter significance when assessing new materials or device configurations for ECs. Generally, the cyclic stability is assessed with the help of galvanostatic charge/discharge process. A preliminary idea about the material can be got by testing the electrode material in a three-electrode configuration while cyclic stability tested in a two-electrode configuration gives a more realistic scenario. At the time of reporting cycling stability, the voltage ranges and current densities applied during the charge/discharge measurement should be clearly mentioned. In case of the devices originating from the EDLC type materials like carbons exhibiting large-surface-area,

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a minimum of 10,000 cycles is required to be reported. For the device containing electrode material exhibiting pseudocapacitance 5.000 cycles is sufficient. Another approach is ‘float test’ [81], which is implemented to measure the cycling stability, in which the device is kept at a specific voltage for a finite give time along with periodical through charge/discharge or cyclic voltammetry or impedance measurements. It is noteworthy that two aforementioned measurements help to get distinct data, and hence, it is more useful to perform both the tests for obtaining adequate insight into the stability of the concerned EC devices.

12.8.7 Energy, Power, and Ragone Plot It is noteworthy that energy and power density should be evaluated for (both for mass and volume normalized) and must only be calculated for device or in twoelectrode configuration, and not for a single electrode implementing a three-electrode configuration. The energy and power densities can be calculated from the galvanostatic charging and discharging time. Generally, discharging time is considered for calculation of the energy and power densities. Following equations are used to calculate energy density, E d (Wh kg−1 /Wh cm−1 /Wh L−1 ), and power density (W kg−1 /W cm−1 /W L−1 ): E d = 1/2C V 2

(12.1)

Pd = E/∆td

(12.2)

where C is the capacitance of the device, V is the working potential, and ∆t d is the discharging time. Furthermore, the kind of calculated power and energy, for instance, average or maximum, should be clearly reported. For obtaining a realistic value of the energy of a device, it is better to determine the integral of the galvanostatic discharge curve (average energy). Subsequently, the average power can be calculated from the average energy. As implied in many works the energy and power density values calculated remain far from the values of power and energy obtained for a lab cell [82]. Values derived for a single electrode are four times greater than the real value to be obtained for the entire device. The Ragone plot comprises the plot of the energy versus the corresponding power density of at least four different current densities (to correctly assess the device performance) on a log scale.

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12.8.8 Coulombic Efficiency—Coulombic Efficiency is the Factor that Determines the Rate Capability of the Supercapacitor Device For a supercapacitors, the specific/volumetric power and energy are considered to be significant properties along with other parameters, including self-discharge current, cycle life, as well as Coulombic efficiency. The Coulombic efficiency (η) is defined as the ratio of discharging time and charging at the same current density and is calculated by the following Eq. (12.3): η = td /tc

(12.3)

where t d is the discharging time and the t c is charging time. The Coulombic efficiency is usually employed for evaluating charge storage efficiency and the cycle stability of the devices and the electrode material by drawing a comparison between the first and the last cycle.

12.8.9 Determining the Percent of Diffusion Controlled and Surface Capacitance The redox peaks obtained from the cyclic voltammetry plots associated with an electrode material indicate the contribution of faradaic capacitance. Actually, the charges are contributed both by diffusion-related charge components (pseudocapacitance) and surface-controlled (EDL). Hence, the total charge stored (qT ) is actually a combination of charge contributed by the ‘inner (qi )’ less accessible surface as well as the ‘outer (qo )’ more exposed and accessible surface of the electrode material. The component of these charges stored can be calculated distinctly using [Eqs. (12.4) and (12.5)] [83, 84] The surface charge (qo ) is of the outer electrochemical surface interaction origin associated with the electrolyte and electrode active material. Unlike this, the inner charge (qi ) is contributed mainly by the dislocations, grain boundary voids, cracks, pores, crevices, etc. which are present in the inner portion of the electrode material. The presence of the inner diffusion-controlled interactions for charge storage depends on the scan rate (v) and becomes less effective with increasing the scan rate. Equations (12.4) and (12.5) representing these behaviors are [84]: (√ ) v

(12.4)

( √ ) q(v) = qo + const' 1/ v

(12.5)

1/qv = 1/qT + const

The total charge (qT ) coming from both the inner and the outer charge storage components are possible to be evaluated by extending the linear region of the graph

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of ‘q(v)−1 ’ versus ‘scan rate1/2 ’ which gives y-intercept with ‘zero’ value and for the x-axis value, i.e., the ‘scan rate’ goes to zero. At this point, the complete participation of the outer as well as the inner charge storing components is assumed [84]. On the other hand, ‘qo ’ is obtained with the help of extrapolation of the ‘q(v)’ versus ‘scan rate−1/2 ’ plot, where the scan rate tends to be infinite and the charge storage driven by diffusion is completely seized [81]. Hence, the contribution of the ‘qi ’ is possible to be easily calculated by subtracting ‘qo ’ from ‘qT ’.

12.9 Conclusions and Future Perspectives In conclusion, this chapter outlines the development in the research field that has been attained in designing the architecture, manufacturing methods, electrode materials fabrication for MSCs, and their significant application arena. In the past few decades, the research corresponding to the MSCs has achieved significant progress. Our insights and views regarding the primary developments and the persisting challenges associated with the MSCs are outlined. (1) With regard to the device designing structure, unlike the traditional sandwich designing, the device architecture with interdigital structure exhibits some palpable advantages. For instance, this device designing approach results in shortening of the ion diffusion distance, along with generating the possibility to associate MSCs with the other devise. It is noteworthy that the mechanical properties of the MSCs with interdigitated electrodes exhibit significant accomplishments. For meeting the needs of the complex application situations, flexible MSCs exhibiting outstanding tensile, bending, and torsional characteristics have been investigated perpetually. These appreciable mechanical characteristics originates from the flexibility of substrates. Presently, PET, PI, paper, etc., are some of the commonly used flexible substrate. Nevertheless, the flexible nature of the MSC devices received from these flexible substrates feels to be a little in adequate for keeping pace with the speedy progress made in the case of the flexible integrated systems. Hence, for meeting the perpetually increasing standards associated with the wearable electronic gadgets and smart integrated systems, people put ever increasing requirements in context with the mechanical properties of flexible MSC devices. Even though designing more flexible, lightweight substrates projects itself as an efficient method to achieve large flexibility in the MSCs, the process may be time taking and effortful. As an alternate way, the microstructure patterns of electrode materials can be made with superstructure shapes with the help of chopping the electrode material films and substrates directly through templates and lasers. Recently, arrays of MSCs with flexible possessing superstructures of Kirigami have been fabricated employing laser-cutting active material films technique and which exhibit outstanding tensile and flexible characteristics. Nevertheless, a few reports exist regarding flexible MSCs possessing different superstructures, including

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origami, etc. Therefore, enhancing the mechanical characteristics associated with the flexible MSCs will pave the path for accommodating applications with various specific requirements. (2) In the context of manufacturing of device, the technologies associated with the micro-machining of MSCs have seen significant progress. It is noteworthy that each developing manufacturing approach has its own pros and cons. Specifically, screen printing method is facile and largely efficient attributed to the possibility of synchronous printing of electrode materials and electrolyte to make all-printed solid-state MSCs. This also enables the scaling up of the manufacturing process of the MSC devices on an industrial scale. Nevertheless, screen printing is required to procure a device pattern through a mask, and the associated resolution is limited. Furthermore, it is also a mandate to procure an ink having the expected fluidity to refrain from the screen jam. As compared to the screen printing, inkjet printing refrains from the implementation of masks, but is unable to mitigate the problem of jamming. Unlike these electrode fabrication processes, photolithography is able to produce MSCs exhibiting large resolution and precision in the nano dimensions. However, it needs a multi-step process for manufacturing together with an ultraclean working environment, which severely restricts its progress in further. Laser scribing as novel micromachining approach with facile methods and flexible operation projects a great potential in the context of MSCs fabrication. This is attributed to the ability to cut the materials or the substrates with a large precision besides creating electrode materials through laser irradiating. In case of the method of mask-assisted filtering, in spite of the necessity of the pre-fabricated mask with the appropriate patterns, it should get a place in MSCs fabrication methods by virtue of its strong universality and simple operation. Apart from these, there are technologies including 3D printing which is able to 3D electrode with large precision exhibiting excellent electrochemical performance. 3D printing has a great scope to develop as the key method for production of on-chip MSCs in future. (3) In relation to the electrode materials, the high performance is decided by the realization of large energy storage capacitance in MSCs. Various 2D materials such as MXene, transition metals, and graphene have proved to be acting as possible electrode materials attributed to their 2D open structures offering large surface area for charge storage and adequate ion diffusion channels. Furthermore, there are some conducting polymers, such as Ppy, PANI, PIN, and some metal-based compounds like RuO2 which also projects adequate charge storage properties. It is noteworthy that achieving high value of energy density is always an aim to be reached by MSCs devices. In this context, designing active materials showing large conductivity and several ion diffusion channels by implementing heteroatoms or pores have proven to be quite efficient. Furthermore, the developed technologies related to the machining technologies on a microscale, such as laser scribing, screen printing, offer easy incorporation of porous or heterostructures into electrode materials. In addition to intrinsically modifying the electrodes, with the help of combining the various material to reach charge storage both by EDL formation and reversible faradaic reaction, a large value

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of energy density as well as capacitance can be reached. The resulting hybrid MSCs can operate in wider potential window and attain large energy density. However, the pursuit of planar MSCs with high performance is still on, and there still exist challenges for meeting the increasing demands. (4) A large part of functional energy harvesters are still in the primary state of exhibition of concept and experimental research. Now the vital thing is to improve the MSCs compatibility associated with the energy harvesting devices to augment the efficiency of the energy conversion. Additionally, MSCs can also be integrated together with various types of sensors and hence have exhibit functions influenced by the nature of the sensors, including pressure sensors, gas sensors, and so on. For this kind of assembled unit, a basic topic is to match the MSCs’ degree, and sensor and to improve the sensitivity of the sensor. Furthermore, reducing the entire system’s volume for satisfying the requirements of various scenarios is challenging. For implementing within the medical equipments, it is very significant to design non-toxic and safe MSC materials which can be easily decomposed or absorbed. For the filtering of AC line, the charge storage properties can be further improved by enhancing the rate capability and improving the capacitance. It is very crucial that there is a wide scope for applications associated with the MSCs [9].

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Chapter 13

Shape Memory Supercapacitors Mukesh Kumar, Manas K. Ghorai, and Kamal K. Kar

Abstract Nowadays, the increasing demand for energy storage devices and highpower density compared to batteries makes promising supercapacitor candidates for commercial application. Shape memory properties are seamlessly integrated with the supercapacitor to fulfill the stable energy requirement of flexible devices. Considerable research gained attention to developing shape memory supercapacitors in electrochemical energy devices. Different shape memory materials have been used to assemble the device and study the electrochemical performance, cyclic stability, etc. We have reviewed and explained shape memory materials types, such as shape memory alloy (SMA) and shape memory polymer (SMP). Both types of material have unique intrinsic shape memory properties such as strain recovery (Rr ), shape fixity (Rf ), and recovery time. Mainly heat-triggered shape memory material is used in the application, and its transition temperature largely depends on the material and material composition. The flexibility of the shape memory device depends on the design, architecture, materials, etc.; wire-shaped and planar devices have been studied extensively. Symmetric and asymmetric shape memory supercapacitors have been reviewed. Apart from this principle behind shape memory properties, the design aspect and electrochemical performance of recent advancements in SMSC have been reported.

M. Kumar · K. K. Kar (B) Advanced Nanoengineering Materials Laboratory, Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India e-mail: [email protected] M. K. Ghorai Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India K. K. Kar Advanced Nanoengineering Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 K. K. Kar (ed.), Handbook of Nanocomposite Supercapacitor Materials IV, Springer Series in Materials Science 331, https://doi.org/10.1007/978-3-031-23701-0_13

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13.1 Introduction The global demand for energy storage devices is increasing day by day. There are different energy storage devices; the supercapacitor is one promising energy storage device because of its high-power density, high cycle life, and high-performance stability [1, 2]. Therefore supercapacitors are ideal for applications where power density is the primary requirement [3, 4]. But supercapacitor is not till now beat batteries in energy density categories. The battery has a high energy density in comparison with a supercapacitor. Energy density is directly proportional to capacitance and the square of the potential window. So energy density can be improved significantly, increasing the potential window [5]. Supercapacitors are classified into three categories, i.e., electric double-layer supercapacitors (EDLC), pseudocapacitance supercapacitors, and hybrid supercapacitors according to charge storage mechanisms [2, 6–11]. Carbon-based materials are generally used to fabricate EDLC supercapacitors. These electrode materials possess a high surface area for adsorption and desorption of the electrolytic ion [12– 16], even though carbon-based electrodes have low specific capacitance. Pseuocapacitance supercapacitors are made of transition metal oxide, and conducting polymers [17–19] possess high specific capacitance than carbon materials. One type of electrode material is not enough to fulfill the demand for supercapacitor performance. High energy and power density are required for commercial applications, fulfilled by a hybrid electrode material combination of EDLC and pseudocapacitance or two different varieties of electrodes. The literature review observed that researchers and scientists are trying to enhance energy density by using asymmetric electrode and battery-type hybrid electrode supercapacitors [20]. In recent years, supercapacitor has gained more attention due to their high-power density, fast-charge discharge rate, high capacitive performance, and high cycle life [4, 21–24]. Above-mentioned performance largely depends on structural stability, such as flexibility [24, 25], stretchability [26, 27], shape recoverability, and selfheal ability [28, 29]. With the increasing demand for energy storage devices in the wearable electronics field, scientists and researchers are also trying to develop flexible devices with high electrochemical performance [30–32]. Flexible supercapacitors are fabricated by assembling a flexible electrode with electrolyte and separator. A flexible electrode is generally manufactured by coating an active electrode material such as carbon, transition metal oxide, and conducting polymer [6, 33–35]. The flexible supercapacitor has unavoidable deformation by the mechanical shearing force that leads to mechanical damage and generates defects in the material. Regular deformation and damage in material structure alter supercapacitor performance, structural stability, and electrochemical performance. Recently, many studies have been carried out to make energy storage devices flexible, stretchable, and mechanically stable. Such devices’ performance was not reduced after several deformation cycles [36]. Therefore, it is necessary to solve the problem mentioned above irreversibility properties of a material. An intelligent shape memory supercapacitors come into the light to resolve the irreversibility properties. Now, shape memory material was introduced

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in supercapacitors’ flexible electrode active material to maintain the electrochemical performance and structural integrity. Shape memory supercapacitors (SMSCs) are novel and intelligent materials that remember their shape. The deformed sample gained its original form through reversible phase transformation by the external stimuli of temperature, light, pH, IR, etc. [37–41]. SMSC regains its shape when heating above the phase transition temperature. This fascinating feature makes suitable material to make an intelligent supercapacitor device. There are mainly two categories of shape memory material alloy (SMA) and shape memory polymer (SMP). These materials are used to fabricate the flexible electrode in which shape memory material acts as a substrate on which active electrode material is grown. Shape memory supercapacitors are intelligent energy storage devices that restore their shape after deformation by coming in contact with heat or other external stimuli and keep their shape and performance unaltered. SMSC has potential applications like energy storage devices, biomedical field, space structure, structural components, aerospace application [42], flexible wearable devices [43], transportation sector [44], and textile [45]. Herein, we have described the shape memory materials, recent development in novel shape memory materials, shape transformation principle of shape memory materials, design and architecture of 1D wire/yarn, and 2D planar SMSC, and effect of shape memory on the electrochemical performance and recent advancement in the SMPC.

13.2 Shape Memory Alloy Shape memory materials are being used in almost every application. Nowadays, these materials have gained attention in energy storage devices. Shape memory properties of materials fix that deformation occurs by mechanical action such as shear, bending, and twisting. Mechanical deformation occurs and influences the electrochemical performance of devices. This limitation is overcome by developing advanced material that recovers its original form by stimuli of heat, light, pH value, and magnetic field [46, 47]. Shape memory polymer and alloy can remember that regain their original shape. Shape memory alloy is at the forefront of advanced functional material. When heat is applied to shape memory alloy, it reverses its initial phase from austenite to martensite [48]. After discovering Ni-Ti alloy in 1963, shape memory gets attention in the commercial application. Buehler and co-workers discovered it when using it in the heat shielding application [47]. They found that they possess good mechanical properties compared to all other materials and have unique shape memory characteristics. It is also called NiTiNOL in honor of the discoverer of the Naval Ordinance factory. Ni-Ti shape memory alloy was studied with the Co and Fe and observed that transformation temperature decreases [49, 50]. Ni-Ti-Nb shaped memory alloy synthesis and analyzed the fatigue test. It is observed that it was easier to handle due

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to large hysteresis. High-temperature SMA (HTSMA) such as Ti-Pd, Ti-Pt, and TiAu have reversible phase transition temperatures greater than 100 °C [51]. Ternary compound SMA Ni-Ti-Cu does not change the transition temperature but improve the fatigue life, the most critical dynamic motion instrument. Iron and copper-based shape memory alloy are also used in commercial applications but not as much as Ni-Ti alloy due to their intrinsic properties such as brittleness, low stability, and poor thermomechanical performance [52, 53]. Ni-Ti alloys are the preferred shape memory material for commercial use in aerospace applications, biomedical implants, and energy storage devices. Ni-Ti shape memory alloy has a phase transition temperature below 40 °C. Material will regain its shape after deformation in the elastic region. Subsequently, under constant strain cooling, the temperature below the T f < T trans forms a semicrystalline state permanent deformation due to the immobilization of the chain. After that, upon unloading, minor recovery of its shape occurs but did not get its original condition, upon heating the above, the T rans recovers its form. This is an entropy-driven process. SMA’s transition temperature mainly depends on Ni and Ti’s composition. When the 50% Ni and Ti austenitic phase transition temperature (Af ) shows 120, and if decreasing, Ni atomic % does not change the Af temperature. Still, if Ni atomic % increases above 50%, Af temperature drops and reaches −40 for 51 atomic % of Ni [54]. Apart from Ni-Ti shape memory, other shape memory alloys are widely used in applications.

13.2.1 High-Temperature Shape Memory Alloy High-temperature SMA has gained in demand in high-temperature applications. Many researchers are extensively studied to develop the high-temperature SMA with a ternary atom in the Ni-Ti alloy. High-temperature SMA is used when phase transition (shape recovery and shape fixity) occure at temperature greater than 100 °C. High-Temperature alloys are only Ni-Ti-Hf, Ti-Ni-Pd, Ni-Ti-Zr Ti-Ni-Pt, Ni-Ti-V, and Cu-Al-Mn-Ni, and alloys are useful at 100–300 °C [55–60]. High-temperature SMA has limitations due to manufacturability, limited flexibility, and poor fatigue resistance at a lower temperature.

13.2.2 Magnetic Shape Memory Alloy Magnetic shape memory alloy (MSMA) is also known as ferromagnetic SMA, which is actuated by a magnetic field’s higher frequency (up to 1 kHz). MSMA is brittle, stiff, and only operable at a lower temperature. This type of shape memory is not suitable for higher force, and high-temperature applications—so many researchers are concentrating their research on overcoming its limitation and improving its

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properties. Some magnetic SMA materials are Ni–Mn–Ga, Fe–Pd, and Ni–Mn–Al [61, 62].

13.2.3 Phenomena of Transformation of Alloy SMA has two phases with two different crystal structures and properties. These two phases are called austenite (high-temperature) and martensite (low-temperature). The austenite phase is stable at high temperatures, and the martensite phase is stable at a lower temperature, as shown in Fig. 13.1. Phase transformation of SMA from austenite to martensite and vice versa depends on heating and cooling. When the alloy is cooled in the absence of applied load, phase transformation from austenite to martensite is known as forwarding transformation. Upon heating, the phase transformation from martensite to austenite (parent phase) is called a reversible phase transformation [63]. From Fig. 13.1, SMA transforms into a detwinned martensite state if mechanical stress is applied to the twinned martensite state. This detwinned martensite is the only microscopic change retained when the load is removed. Further, this phase is heated above Af; the detwinned martensite phase transforms into the austenite phase. If it is cooling below M f temperature, it gains twinned martensite structure associated with no shape change. Ni-Ti alloys are the most extensively studied SMA. It has the following characteristics: corrosion resistance, biocompatible, and lower to higher phase transformation temperature. Shape memory alloy has two distant properties: shape recovery and superelasticity. The shape memory effect or pseudo-elastic effect can be classified into three categories. These effects find in alloy and polymers shape memory material too. Fig. 13.1 Phase transfer principle of shape memory alloy with load versus temperature (reprinted with permission from [63])

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Fig. 13.2 Schematics diagram of a 1 W-SME and b 2 W shape memory effect (redrawn and reprinted with permission from [64])

13.2.3.1

One-Way Shape Memory Effect

One-way shape memory retains its deformed shape after removing load and regaining its original condition upon heating above austenite temperature. Figure 13.2a shows the one-way shape memory effect process.

13.2.3.2

Two-Way Shape Memory Effect or Reversible SME

In addition to the one-way shape memory effect, two-way shape memory can remember their shape at high and low temperatures. However, it is less used in application due to half of its recovery strain than one-way shape memory alloy and requires a trained person to operate. It is also called a reversible phase-shifting phenomenon without any change in stress. Figure 13.2b shows the two-way shape memory effect process.

13.2.3.3

Pseudo-elastic and Super-Elastics

Shape memory alloy reverts to its original shape after loading between Af and Md without any need for thermal activation [61]. These two unusual behaviors make it different from all available intelligent materials. Ni-Ti shape memory alloy recovers its original shape from it’s deformed shape above the transition temperature. Shape recoverability of materials occurs due to phase change on applying stimulus. Phase changes occur from martensite to the austenite phase and vice versa on thermal conditions. Ni-Ti shape memory alloy has high mechanical and electrical conductivity than polyurethane shape memory polymer; therefore, it is suitable as a substrate and current collector for the supercapacitor electrode. Huang et al. were the first ones who developed the wire shape memory supercapacitor. Ni-Ti wire was used as a current collector, and MnO2 and PPy were used as electrode materials. Electrode materials are deposited on the Ni-Ti wire’s surface, and a thin layer of H3 PO4 -PVA gel electrolyte by dip-coating method

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fabricates wire shape memory supercapacitor [65]. SMSC exhibits high mechanical properties, good flexibility, and electrical properties. The specific capacitance of SMSC is noted as 192.2 F g−1 at current density 1A g−1 , 90% capacitance retention after a series of deformations such as bending, twisting, and knotting, and high mechanical stability (92% capacitance retention after 1400 bending cycle). Such a shape memory supercapacitor exhibits nearly the same electrochemical performance after many of the shapes recovery processes [66].

13.3 Shape Memory Polymer (SMP) The shape memory material is intelligent and recovers its shape with exposure to external stimuli. Thermally-induced SMPs have been widely studied and used in commercial applications. Generally, most polymer structures are semicrystalline and composed of a cross-linked network of crystalline and amorphous phases such as PET, PE, PTFE PP, and many more [67, 68]. Almost SMPs are semicrystalline materials. Shape recovery and shape fixing in SMP are the microstructural transformation. Advantages of shape memory polymer [69] Low density Low cost Large recoverable strain Controllable stimulus method Flexible manufacturability An intrinsic disadvantage of SMP Low thermal conductivity Low mechanical strength Lower recovery stress Shorter cycle life, Longer recovery time Reduction in shape memory after several thermomechanical cycles compared to SMA.

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Fig. 13.3 Schematics diagram of shape memory polymer mechanism (redrawn and reprinted with permission from [70])

13.3.1 Shape Memory Principle of Polymer Generally, shape memory polymer is composed of crystalline and amorphous phase networks. Crystalline structure is associated with melting temperature when polymer changes from solid to the liquid phase. The amorphous phase is related to glass transition temperature Tg. Heating above Tg, the amorphous structure of SMP changes from a solid state to a rubbery state. In SMP, crystalline structure is a permanent phase associated with T perm. If thermal transition temperature is below melting temperature, it acts as a permanent structure. The amorphous spiral structure is the temporary phase associated with T trans . Above this temperature, the chain starts to uncoil, prefers segmental motion, and is oriented by losing entropy in the most favored stage. These elastic coil network structures regain their shape when it is cool down. This phenomenon is known as the shape memory effect. There are three types of shape memory effects, which describe later section. Figure 13.3 shows the shape memory phenomena; above the Trans, Tg amorphous starts to uncoil; therefore, it behaves like a flexible substrate. The load is applied to the chain deformed to the most favorable oriented structure by losing entropy. Further cooling adequately under stress below Trans, its system gets fixed into a deformed shape. When the deformed shape is heated above the Trans, it recovers its original state by gaining entropy. The shape memory effect is mainly known as an entropy-driven process.

13.3.2 Classification of SMP 13.3.2.1

Physically Cross-Linked Polymer

Polyurethane’s Tg varies from −30 to 100 °C, depending on the urethane ingredients such as diisocyanate, polyol, chain extender, and molecular weight [71]. Hard and soft segments can precisely control the optimal temperature of the shape memory effect and shape memory properties. Hard element maintains the shape. The soft

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Fig. 13.4 Shape memory recovery with a hard segment of polyurethane (redrawn and reprinted with permission from [72])

part absorbed the external stress by unfolding and elongating the molecular chain. If stress exceeds and breaks the rigid segment shape, memory is lost. The hard segment concentration should be greater than 20% than soft segment to gain good shape memory properties. The shape recovery graph with % of the hard segment is shown in Fig. 13.4, which indicates that increasing the hard segment content and shape memory recovery properties enhance [72]. The physically cross-linked polymer is prone to creep or irreversible deformation during shape memory recovery.

13.3.2.2

Chemically Cross-Linked SMP

Chemically cross-linked polymer networks are synthesized by polymerization with the multifunctional chemical cross-linker. Rosseau et al. synthesize a sematic C liquid crystal elastomer (LCE) [73] that scheme is shown in Fig. 13.5. This shape memory elastomer shows recovery behavior at a low temperature of 37 °C; therefore, body temperature triggered recovery possible. The cross-linked polymer also improved mechanical properties such as tensile strength and creep. Polyurethane (PU) is a widely used commercial shape memory polymer. Chemically cross-linked PU synthesized by an excess of a diisocyanate, cross-linkers (more than three functional groups) such as glycerin and trimethylolpropane. Such SMP has intrinsic disadvantages: poor thermal conductivity, low mechanical properties, reduced shape recovery after the thermos-mechanical cycle, and high thermal expansion. The incorporation of conducting filler generally shores low thermal conductivity problems. Many researchers have tried to improve the mechanical and shape recovery properties. The thermal expansion coefficient of SMP is one or two orders more in magnitude than SMA. Inorganic fillers incorporation reduces the coefficient of thermal expansion.

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Fig. 13.5 Schematics representation of synthesis route of symmetric liquid crystal (redrawn and reprinted with permission from [73])

13.3.2.3

Epoxy-Based Shape Memory

Structural-based applications require high mechanical properties, chemical resistance, and thermal stability; therefore, epoxy-based shape memory material is in demand for commercial application. Shape memory properties are triggered by temperature, and here, Tg depends on the curing agent percent. Liu et al. prepared a series of epoxy-based shape memory properties with varying content of aromatic curing agent DDM (4,4' -diaminodiphenyl methane) [74]. Hence, they found that Tg shape memory properties depend upon the curing % and content of the curing agent. They also reported that Tg value varies from 45 to 145 °C, respectively, curing % 50–100. Higher Tg temperatures take less time to recover the original shape than lower Tg temperatures. All samples prepared using the above composition proportion take several seconds to regain their shape, indicating they are suitable for shape memory application. Some structural applications required high strain failure mechanical properties. Epoxy resin possesses high mechanical stability but low strain failure. Researchers use the aliphatic curing agent to improve strain failure with aromatic/aliphatic epoxy resin [75]. The incorporation of nanoparticles can enhance the shape memory properties of the material. Lin et al. reported that composite PVA–SCF (short carbon fiber) decreases Tg with increasing the SCF. Shape recovery time decreases with an increase in the fiber content [76]. Recently, polyaniline and polyurethane have gained attention in the shape memory supercapacitor. Polyurethane is a widely used shape memory polymer because of its ease of processing and low cost. Polyurethane and polyaniline are high-temperature shape memory polymers; their transition temperature is nearly 80 °C. This intrinsic

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disadvantage of SMP is usually improved by preparing a blend or composite. Gu et al. developed the low-temperature triggered PU-CNT nanocomposite, which behaves with enhanced mechanical strength and shape recovery properties [77]. Usually, shape memory polymer has a low conductivity of less than 0.03 W/mK [78]. Many researchers have reported many approaches in the literature to improve the thermal conductivity of shape memory polymer. Thermal conductivity is increased by incorporating conductivity inorganic additives [79]. Usually, thermal conductivity can be enhanced by the incorporation of alumina (AlN), boron nitride (BN), silicon carbide (SiC), etc. The polymer blend is usually used to focus on fulfilling the two target. 1. Polymer blend to improve shape memory properties such as triggered temperature, mechanical properties, and shape recovery time and shape 2. Develop a new material in which one polymer phase has shape-fixing properties, and the other stage has the attributes of shape reversible [78]. Generally, polymer blend techniques tune the polymer properties according to the requirement for application. Polymer blend techniques improve the mechanical, shape memory triggered temperature, recovery, and shape memory properties. Chatterjee et al. developed a shaper memory polymer blend from the ethylene propylene diene terpolymer (EPDM) rubber and ethylene oceten copolymer (ECO) and studied the shape memory behavior at 60 °C [80]. They observed that polymer blends showed superior mechanical behavior in shape recovery and shape fixity, increasing the EPDM content. Jeong et al. developed a miscible blend of SMPU and epoxy resin and observed tunable transition temperature [81]. Many researchers have tried to create a novel type of shape memory polymer by blending the amorphous and crystalline polymer [82], crystalline and crystalline polymer, elastomer, crystalline, amorphous polymer [69, 83–86]. In crystalline and amorphous polymer blends, the crystalline polymer acts as a cross-linked fixing structure, and the amorphous segment serves as a switching element [82]. These blending techniques gain commercial attention. This method is facile as compared to developing a cross-linked shape memory blend.

13.4 Shape Memory Characterization Techniques Shape memory characterization is done by cyclic thermomechanical tests, DMTA, and cyclic tensile testing [53, 87]. The parameter required to quantify the shape memory properties is strain recovery (Rr ), shape fixity (Rf ), and recovery time. Strain recovery tells how much the extent of material recovery is permanent. Strain recovery was calculated in two ways. One is calculated ‘per cycle’ on the previous cycle, and the other is the strain recovery N cycle compared to the original permanent shape. Rr (N ) =

εm − εp (N ) εm − εp (N − 1)

(13.1)

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Rr, tot (N ) =

εm − εp (N ) εm

(13.2)

where Em is the maximum strain applied to the sample, and Ep (N) is the residual strain in permanent shape after N cycle. Another property. Shape fixity (Rf ): This indicates the extent to which material retains in temporary shape after removing stress, which means the ability of the material to fix the deformation. RF (N ) =

εu (N ) − εp (N − 1) εm − εp (N − 1)

(13.3)

where, Eu (N) is the strain retained in the sample in a stress-free state after N cycle. All these parameters are required to characterize the shape memory properties of the material.

13.5 Design and Architecture of Structural Shape Memory Supercapacitor Recently, flexible supercapacitors are being used in portable and lightweight energy storage devices such as wearable electronics, flexible displays, health tracking devices, and flexible mobile. These flexible devices require high mechanical strength, flexibility, stretchability, bendability, high cycle life, energy density, and power density. All these properties and performance depend on the structural design and architecture. Mechanical deformation by the shear action and vibration leads to unstacking the supercapacitor component, affecting the electrochemical performance or destroying the electrochemical performance. Many works of literature have recently tried to design and architecture a flexible shape memory supercapacitor to overcome the problem mentioned earlier. On the structural design, it is classified into 1D yarn/fiber type and 2D planar. In 1D yarn/fiber-shaped supercapacitor packed with two fiber electrodes in parallel, twisted in one yarn or assembled in the core–shell structure as shown in Fig. 13.6a–d. 2D planar supercapacitors are formed by two parallel plate electrodes separated by solid or gel electrolytes, as shown in Fig. 13.7a–c.

13.5.1 1D Yarn/Fiber Type Supercapacitor Shape memory supercapacitors are designed in different structural forms by changing material arrangement architecture. 1D supercapacitor shape memory supercapacitor

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Fig. 13.6 Schematics diagram of a design and architecture of assembly of 1D wire/yarn shape supercapacitor schematics diagram of shape memory supercapacitor [89], b assembly of co-axial TPU, CNT, and PVA gel electrolyte wire shape memory supercapacitor and resulting textile deformed and elongated shape and recovered shape [88], c assembly of Ni-Ti and stainless yarn electrode with solid polymer gel electrolyte (H3 PO4 -PVA) [65], d assembly of supercapacitor with Ni-Ti coated with porous carbon and graphene electrolyte and ionogel electrolyte. e FESEM image of the cross-section of a device [66] (redrawn and reprinted with permission from [65, 66, 88, 89])

has gained excellent attention in wearable and portable electronic devices. Its high flexibility and deformation recoverability make them a promising candidate for nextgeneration supercapacitors compared to the conventional supercapacitor [66]. Deng et al. first designed the fiber-shaped shape memory supercapacitor by wrapping the CNT sheet on the shape memory PU fiber and assembling it with the PVA gel electrolyte [88]. Figure 13.6b shows that the deformed shape regains in the original condition as the temperature exceeds the T trans. This type of woven fabric is used

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Fig. 13.7 a Schematics diagram of planar supercapacitor [89], b schematic diagram of assembling an asymmetric shape memory supercapacitor (SMASC) [43], c schematics diagram of the assembly of 2D planar symmetric shape memory supercapacitor from TPU-PCL as substrate, CNT as active electrode material, and PAA-PEO gel electrolyte [26] (redrawn and reprinted with permission from [26, 43, 89])

in flexible electronic devices. The co-axial design of the shape memory supercapacitor provides high mechanical and electrochemical performance stability. Both these parameters are deciding factors for supercapacitor application for long cycle life. Huang et al. fabricated a 1D supercapacitor with deposition of active electrode material MnO2 on NiTi/steel yarn which acts as a substrate and current collector [65]. Meanwhile, a thin layer of PVA is coated; subsequently, both the electrodes are coated with H3 PO4 -PVA gel electrode-electrolyte by dip-coating process. These electrodes were then twisted and covered with another thin gel electrolyte layer, as shown in Fig. 13.6c [65]. However, the fabricated supercapacitor device with NTA and steeliness yarn has flexibility and electrochemical performance limitations. To improve devices’ flexibility, Shi et al. manufactured supercapacitor with NTA coated with porous carbon dodecahedra as one electrode and graphene fiber as another electrode. Carbon dodecahedra derived from metal–organic frameworks exhibit high porosity and a large surface area of 1620 m2 g−1 . These two electrodes assembled with ionogel electrolyte P(PVDF)-EMIMBF4, as shown in Fig. 13.6d. This fabricated device has

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Table 13.1 Comparison between a shape memory alloy and polymer SMA

SMP

Mechanical properties

NiTi annealed 700–1100 MPa NiTi has not annealed 1300–2000 MPa Cu-based 800 MPa [69]

5–100 MPa [69]

Recovery stress

800 MPa

2.01–0.1 MPa

Recovery strain

7%

400% [90]

Thermal conductivity

High

Low 0.3 W/mK [78] but SMPC 1.115 W/mK [69]

Transition temperature

35–300

−30 to 322 [91, 92]

Phase transformation

Austenite to martensite

Glass transition

Density

6–7

gm/cm3

1 gm/cm3 [90]

high flexibility, recoverability, and high electrochemical performance [66]. Electrochemical performance is shown in Table 13.1. Figure 13.6e shows the FESEM cross-section image of the assembly of flexible wire shape supercapacitor.

13.5.2 Planar and 2D Shape Memory Supercapacitor Nowadays, the flexible health band and smartwatches are gaining popularity. Liu et al. fabricated planar shape memory flexible asymmetric supercapacitor using reduced graphene oxide coated on the NiTi alloy negative electrode. MnO2 coated ultrathin Ni foil as a negative electrode with an aqueous and ion-based gel electrolyte separator [43].

13.6 Electrochemical Performance Electrochemical performance parameters are the main interest of researchers and scientists in designing the supercapacitor. Nowadays, researchers are trying to improve energy density of devices and electrochemical performance by using a new approach. For high performance, the selection and development of materials are primary necessities. A single type of material no longer has the potential to fulfill the requirements. Therefore carbon, conducting polymer, and transition metal oxide nanocomposites are prepared for electrode [11, 17, 93–95, 95–100]. These materials should possess a high active surface area so that the supercapacitor stores and delivers high power density, energy density, and long cycle life.

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Cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS) techniques are used to characterize the electrochemical performance of shape memory supercapacitor. The electrochemical performance of a few supercapacitors is shown in Table 13.2. Aside from electrochemical performance, supercapacitors should possess brilliant flexibility and shape recoverability. Ma et al. fabricated a symmetric supercapacitor based on shape memory polyurethane polymer as the base substrate for active electrode materials (MnO2 /MWCNT) deposition. H2 SO4 /PVA gel sandwiched between the hybrid films. The hybrid film had the stretchability to 30% strain without crack. The areal capacitance of shape controllable and recoverable shape device is 17.4 mF cm−2 at 10 mV s−1 that makes these smart supercapacitor devices suitable for wearable device applications. Their electrochemical performance is also characterized by different bending and deformation modes, even though their performance remains constant [101]. Huang et al. fabricated a yarn-type SMSC and analyzed electrochemical performance using the two electrode methods. Figure 13.8a shows a unique rectangular shape CV curve of SMSC at different scan rates ranging from 10 to 200 mvs−1 ; increasing the scan rate large enclosed area they got indicates well the capacitive nature. Figure 13.8b shows that charging-discharging (GCD) profiles are symmetric, indicating no potential drop and a specific capacitance of 198.2 F g−1 at a current density of 1 A g−1 . Corresponding areal capacitance is 75.8 mF cm−2 . Figure 13.8c shows the capacitive retention after a series of deformations such as bending, twisting, knotting, and release. 90% capacitive retention after three repetitions of the previous mention deformation process. Figure 13.8d shows the fatigue stability and specific capacitive retention, and 1400 bending test was performed and found no significant deterioration in the particular capacitance in CV curve [65]. Regular deformation and bending affect the electrochemical performance of supercapacitors. This effect was eliminated by using the shape memory supercapacitors. Figure 13.9 shows the effect of shape recovery on electrochemical performance. Figure 13.9a shows CV curve before and after shape recovery; it is depicted that 96% of capacitance retention after the shape recovery. CV curves remain rectangular after and before recovery without significant deviation. Figure 13.9b shows that SEM image of SMSC after recovery, and no crack is found; therefore, their capacitive retains after shape recovery. Figure 13.9c shows the relation between configuration restoration and capacitance retention in SMSC. The author prepared a SMSC deformed to nearly 90°, then it heated up to above transition temperature (As > 35 °C) shape memory started to recover its original shape. They have calculated 15 times the deformation and recovery process and found the capacitive retention through GCD and found that as the configuration restoration decreases capacitive retention, it decreases. Figure 13.9d impedance spectra have a similar curve. Spike at lower frequency and arc at the higher frequency indicated, respectively, capacitive and electrical resistance in nature. From Fig. 13.9d inset graph, it is concluded that after the multiple deforming and restoration cycle, Rct remains constant. Rs solution resistance slightly increases from 50 to 58 Ω., which indicates the growth of defects such as rift and wrinkle, which lead to low electronic and ionic conductivity [65].

PVA/SWCNT fiber

37 F g−1

Polyethylenimine (PEI) solution (1.0 318.7 F g−1 wt%) and MWCNT solution (0.9 wt%) dispersed with sodium dodecyl sulfate (SDS)

Carbon nanotube (CNT)-coated poly(acrylic acid)–poly(ethylene polyurethane–poly(3-caprolactone) oxide) (PAA–PEO) hydrogel (PU–PCL)

H2 SO4 /poly(vinyl alcohol) (PVA) gel 17.4 mF cm−2 at 10 mV s−1

MWCNT/MnO2 /SMPU

427 F/cm3

3.4 Wh kg−1

NA

198.2 F g−1 at a current density of 1 NA A g−1 areal capacitance of 75.8 mF cm−2

H3 PO4 –PVA gel electrolyte

NiTi wire/stainless yarn

NA

202 W kg−1

NA

NA

1080 mW/cm3

Energy density Power density 8.9 Wh/cm3

Specific capacity 7.1 F/cm3

Electrolyte

Ionic liquid P(PVDF)-EMIMBF4

Electrode material

Porous carbon dodecahedra coated on NTA wire (c-NTAw) electrode and graphene-fiber electrode

Table 13.2 Electrochemical comparison of shape memory supercapacitors

[102]

[26]

[101]

[65]

[66]

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Fig. 13.8 Electrochemical performance of SMSC. a CV curves at different scan rates. b GCD curves at different current densities. c Capacitance retention to varying types of deformations. d CV curves at different types of bending (180° bending angles) cycles (redrawn and reprinted with permission from [65])

Shi et al. also fabricated a wire-shaped asymmetric supercapacitor. Ni-Ti alloy wire is coated with porous carbon dodecahedra and graphene for electrode preparation and coated with ionogel electrolyte on the substrate. And it is found that its energy density and capacitance remain unaltered before and after shape recovery [66]. Intelligent energy storage textile: Huang et al. developed smart textile energy storage devices with NiTi SMA with magnetic properties of shape memory, triggering the shape of the body temperature [65]. Figure 13.10a shows the fabrication process of an intelligent energy storage device by weaving traditional yarn and shape memory type yarn. Ni-Ti wire is soft and flexible above Af temperature, so easy can effortlessly be weaved in the textile. In addition to the fabrication process, the shape recovery step is also shown in Fig. 13.10a. This shape memory energy storage device is heated above Af temperature; the deformed shape gets recovered to its original curl shape. This intelligent textile curls the clothes when coming into contact with body temperature. Textile

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Fig. 13.9 a Electrochemical measurement CV curves of SMSC before and after the shape memory process, b SEM image of electrolyte coated NiTi shape memory electrode after shape recovery (higher resolution image inset image). c Specific capacitance retention % and the shape recovery ratio of the SMSC original state and after recovery state. d Nyquist plots the original condition and after recovery state for different numbers of cycles (redrawn and reprinted with permission from [65])

recover in curl original shape after getting transition temperature. Figures 13.10b, c show the capacitance of the number of devices when connected in parallel and series. Capacitance increases with an increasing number of devices in parallel combination but decreases with an increasing number of devices in series combination.

13.7 Summary and Perspective Smart shape memory material gains attention in energy storage devices. Shape memory materials are used as a substrate to design the supercapacitor component. Active electrode material coated on the substrate to fabricate the supercapacitor electrode, which possesses shape memory properties. Proper shape memory material selection is an essential and challenging criterion for manufacturing or developing smart shape memory supercapacitors. For every application, a different shape recovery transition temperature is required. The transition temperature of shape

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Fig. 13.10 a Schematic diagram of shape memory textile supercapacitor device. Inset is the asprepared textile. b Capacitances in parallel. c Capacitances in series (redrawn and reprinted with permission from [65])

memory materials varies in a wide range from –35 to 330 °C. It depends on SMM composition. Shape memory polymer and shape memory alloy are two different classes of shape memory material in which Ni-Ti alloy and polyurethane are primarily used in the application. Ni-Ti alloy has characteristic like high electrical conductivity, but polyurethane is nonconducting. Conductivity can be introduced in polyurethane by incorporating fillers like CNT, CNF, reduced graphene, shape memory alloy, and shape memory polymers have different types of limitations in their performance. Fiber shape memory supercapacitor has high structural stability and flexibility. NTA has low flexibility and electrochemical performance, poor wearable comfort, low volumetric energy density (