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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
This unique book provides an in-depth and systematic description of an integrated approach for innovative functionalized nanomaterials, interfaces, and sustainable supercapacitor fabrication platforms.
The requirement for energy-storing devices that can handle the necessary power for modern day electronic systems and the miniaturization of electronic devices, has sparked the evolution of energy-storing devices in their most portable forms. Integration of mini- or micro-powering devices with tiny electronic devices has led to the simultaneous evolution of nanomaterials and, correspondingly, nanotechnology. The nanotechnology evolution has provided the control and ability to restructure matter at the atomic and molecular levels on a scale of l-100 nm. Nanotechnology primarily aims to create materials, devices, and systems that exhibit fundamentally new properties and functions. As such, nanotechnology and functionalized nanomaterials have proven to be the ultimate frontier in the production of novel materials that have manufacturing longevity and cost-efficiency.
The integration of nanotechnology to produce functionalized nanomaterials and energy storage from electrochemical principles has established a new platform for science and technology. The integration of two technologies does not compromise their fundamentals and principles, but instead results in novel and high-performance supercapacitors.
This book consists of 11 chapters that review state-of-the-art technologies detailing:
- the developments in flexible fabric-type energy storage devices as well as hybrid fabrics for energy storage and harvesting in flexible wearable electronics;
- the role of electrolytes in the development of sustainable supercapacitors and the performance optimizations associated with them;
- green supercapacitors as sustainable energy storage devices;
- the materials used in sustainable supercapacitors, such as novel transition metal oxides, metal-organic frameworks, conductive polymers, and biomass-based, as well as their composites (binary and ternary);
- a discussion on the significance of material selection, emphasizing the properties and characteristics required for sustainable electrode materials;
- how supercapacitors, ultracapacitors, and electrostatic double-layer capacitors (EDLC) offer a more significant transient response, power density, low weight, low volume, and low internal resistance, making them suitable for several applications;
- how sustainable supercapacitors have steadily gained traction due to their potential for non-invasive health monitoring.
Audience
The book is ideal for a broad audience working in the fields of electrochemical sensors, analytical chemistry, chemistry and chemical engineering, materials science, nanotechnology, energy, environment, green chemistry, sustainability, electrical and electronic engineering, solid-state physics, surface science, device engineering and technology, etc. It will also be an invaluable reference source for libraries in universities and industrial institutions, government and independent institutes, individual research groups, and scientists working in supercapacitors.
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Table of Contents
Cover
Table of contents
Series Page
Title Page
Copyright Page
Preface
1 Flexible Sustainable Supercapacitors
1.1 Introduction
1.2 Flexible Electrodes
1.3 Electrode Materials
1.4 Modifying Techniques to Enhance Electrochemical Performance
1.5 Flexible Supercapacitors
1.6 Sustainable Supercapacitors
1.7 Conclusions
References
Notes
2 Role of Electrolytes in Sustainable Supercapacitors
2.1 Introduction
2.2 Parameters Characterizing Sustainable Supercapacitors and Their Interactions with Electrolytes
2.3 Different Types of Electrolytes Used in Sustainable Supercapacitors
2.4 Di?culties Associated with Electrolytes in a Sustainable Supercapacitor
2.5 Potential Research Avenues for Resolving the Problems with Electrolytes
2.6 Conclusion
References
Notes
3 Green Supercapacitors
3.1 Introduction
3.2 History of Supercapacitors
3.3 Supercapacitors
3.4 Advantages of Supercapacitors
3.5 Disadvantages of Supercapacitors
3.6 Applications of Supercapacitors
3.7 Classification of Supercapacitors
3.8 Importance of Supercapacitors in Our Everyday Life
3.9 The Future of Supercapacitors
3.10 Comparison of Supercapacitor versus Battery
3.11 Role of Metal–Organic Framework in Supercapacitors
3.12 Eco-Friendly Supercapacitors
3.13 Conclusions
References
Notes
4 Materials for Sustainable Supercapacitors
4.1 Introduction to Supercapacitors and Sustainability
4.2 Fundamentals of Supercapacitors
4.3 Sustainable and Eco-Friendly Materials for Supercapacitors
4.4 Advancements in Electrode Materials
4.5 Challenges and Future Perspectives
4.6 Conclusions
References
Notes
5 Role of Material Selection and Fabrication Approach in the Performance of Sustainable Supercapacitors
5.1 Introduction
5.2 Electrode Materials for Supercapacitors
5.3 Fabrication Techniques for Supercapacitors
5.4 Conclusion
References
Notes
6 Electronics and Communication Applications
6.1 Introduction
6.2 Fundamentals of SCs
6.3 Environmental Impact
6.4 Technological Aspects for SCs
6.5 Role of SCs in the Electronics Sector
6.6 Future Prospects of SCs in the Electronics Sector
6.7 Role of SCs in the Communications Sector
6.8 Future Prospects of SCs in the Communications Sector
6.9 Summary and Conclusion
References
Notes
7 Energy Storage Breakthroughs: Supercapacitors in Healthcare Applications
7.1 Introduction
7.2 Supercapacitors
7.3 Material Selection for Bio-Compatible Supercapacitor
7.4 External Power Supply for Health Monitoring
7.5 Bio-Based Supercapacitor Integration
7.6 Charging Strategy
7.7 Challenges and Future Prospects
7.8 Conclusion
Acknowledgements
References
Notes
8 Recent Trends in the Development of Sustainable Supercapacitors
8.1 Introduction
8.2 Recent Trends in Electrode Materials
8.3 Role of Different Electrolytes in the Field of Sustainable SCs
8.4 Recent Trends in the Synthesis Mechanism for Sustainable SCs
8.5 Green Synthesis of the Sustainable SCs
8.6 Conclusion and Future Prospects of Sustainable SCs
References
Notes
9 Cyclic Stability and Capacitance Retention of MXene-Based Supercapacitors
9.1 Introduction
9.2 Cyclic Stability and Capacitance/Capacity Retention of Supercapacitors and Batteries
9.3 Challenges, Limitations, and Future Prospects of MXene-Based Energy Storage Devices
9.4 Potential Future Directions
9.5 Conclusions
Acknowledgments
References
Notes
10 Current Status of Sustainable Supercapacitors
10.1 Introduction
10.2 Supercapacitors
10.3 Necessity of Supercapacitors
10.4 Electrostatic Double-Layer Capacitors
10.5 Hybrid-Based Supercapacitors
10.6 Pseudo Capacitors
10.7 Green Supercapacitors
10.8 Current Challenges of Supercapacitors
10.9 Future Scope of Supercapacitors
10.10 Conclusions
References
Notes
11 Future Perspective of Sustainable Supercapacitors
11.1 Introduction
11.2 Research Motivation and Objectives of the Sustainable SCs
11.3 The Challenges for Sustainable SCs
11.4 Technical Aspect
11.5 Application-Level Aspects for Sustainable SCs
11.6 Future Perspectives and Challenges for the Sustainable SCs
11.7 Conclusion
References
Notes
Index
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1
History and advancement of supercapacitors.
Figure 1.2
(a) Schematic illustration of the structure and (b) the det...
Figure 1.3
Schematic illustration of the hydrogels.
Figure 1.4
Schematic illustrates the fabrication of the all-in-one fl...
Figure 1.5
Schematic diagrams of (a) all-solid-state flexible superca...
Figure 1.6
Schematic illustration of each layer of the flexible supercap...
Figure 1.7
(a) Optical images of self-healing graphene/BA/PVA organo-hyd...
Figure 1.8
GCD and CV curves of the 3D flexible electrode at various...
Figure 1.9
Schematic illustration of hierarchical porous 3D Ni3N-CoN/NC...
Figure 1.10
(a) Self-healing of PNB, (b) homogeneity mixture identificati...
Figure 1.11
(a) Structure diagram of the interdigitated micro-supercapaci...
Figure 1.12
Schematic representation of the sustainable production technolog...
Figure 1.13
Porous carbon particle synthesizing procedure.
Figure 1.14
SEM images of (a) AFP, (b) AFK-2, (c) AFPK-2, and (d) AFPK-4.
Figure 1.15
(a) AR husk-derived porous carbon synthesis procedure, (b) KOH...
Figure 1.16
Synthesis route to prepare the nitrogen-doped activated carbon...
Figure 1.17
Schematic illustrations of the synthesis procedure of CFSPC.
Figure 1.18
(a–c) Graphical representation of the synthesis procedure...
Figure 1.19
Schematic diagram of the preparation process of porous carbons...
Figure 1.20
Synthesis road map of N, O co-doped hierarchical porous carbon...
Figure 1.21
Schematic working mechanism of supercapacitor.
Figure 1.22
Synthesis procedure of C/FeSi and CoNiSi obtained from BLs for...
Chapter 2
Figure 2.1
Classifications of supercapacitors and mechanism to store energy...
Figure 2.2
Data trends of electrode and electrolyte publication devices from...
Figure 2.3
Effect of electrolytes in supercapacitor performance.
Figure 2.4
Classification of electrolytes.
Figure 2.5
Basic types of ionic liquid.
Chapter 3
Figure 3.1
Supercapacitor hybrid bus.
Figure 3.2
Classification of Supercapacitors [21].
Figure 3.3
Supercapacitor electrode materials from agricultural corncob waste...
Chapter 4
Figure 4.1
Schematic diagrams of a supercapacitor [7].
Figure 4.2
Criteria to evaluate the performance of an energy storage system.
Figure 4.3
Important unique considerations for picking the right energy storage...
Figure 4.4
Diagram of the Helmholtz double layer on a liquid–solid...
Figure 4.5
Schematic diagrams of a capacitor showing (a) the charged (left...
Figure 4.6
Schematic representations of different modes of POPs as electrode...
Chapter 5
Figure 5.1
Mechanism of ELDC.
Figure 5.2
Mechanism of a pseudocapacitor.
Figure 5.3
(a) Scanning electron microscope (SEM) depiction of the CMG particle...
Figure 5.4
(A) Illustration of the CNT construction. (B) Different types of tu...
Chapter 6
Figure 6.1
The schematic of the SCs with their applicability and usefulness...
Figure 6.2
The fundamental aspects of the SCs in the various fields.
Figure 6.3
Structure of SCs.
Figure 6.4
Schematic for the classification of the SCs.
Figure 6.5
Schematic for the electrode materials and their role in SCs.
Figure 6.6
Schematic of the technical aspects of the SCs.
Figure 6.7
Schematic for the role of SCs in the electronics sector.
Chapter 7
Figure 7.1
Schematic illustration of supercapacitors as energy sources in...
Figure 7.2
Biocompatible supercapacitor, mechanical property, energy harv...
Figure 7.3
(a) Basic required properties for biocompatible and implantable...
Figure 7.4
(a) Laser-induced graphene-based flexible supercapacitor fabric...
Figure 7.5
(a) Stretchable supercapacitor fabrication techniques and real...
Figure 7.6
Advanced manufacturing technique for the fabrication of bioco...
Figure 7.7
Hydrogel-based biocompatible supercapacitor, fabrication tec...
Figure 7.8
Mechanical of electrochemical and bicompatability properties...
Figure 7.9
Electrochemical performance analysis and the self-life analysis...
Figure 7.10
Bio-based electrode materials. (a) Wood-based composite for SC e...
Figure 7.11
Bio-based electrolyte for supercapacitor application. (a) Flour...
Figure 7.12
Bio-based supercapacitor encapsulation. (a) PCL-based supercapa...
Figure 7.13
Implantable supercapacitors. (a) MnO
2
and carbon-ba...
Figure 7.14
External rechargeable supercapacitors for health monitoring and...
Chapter 8
Figure 8.1
The classification of the supercapacitors based on the electrode...
Figure 8.2
The different types of electrolytes based on aqueous, non-aqueous,....
Figure 8.3
The different types of aqueous electrolytes...
Figure 8.4
Schematic for the synthesis of nanomaterial...
Figure 8.5
Schematic for the synthesis of nanomaterial...
Figure 8.6
Schematic for the synthesis of nanomaterial...
Figure 8.7
Schematic for the synthesis of nanomaterial...
Figure 8.8
Schematic for the synthesis of nanomaterial...
Chapter 9
Figure 9.1
The periodic table of elements, focusing on elements relevant to...
Figure 9.2
(a) CVs of Ti
3
C
2
T
x
-MXene and N...
Figure 9.3
Supercapacitor and battery charging/discharging characteristics...
Figure 9.4
(a) Cyclic stability of Ti3C2Tx MXene-based supercapacitors at ...
Figure 9.5
Capacitance retention of the Co-MXene//AC ASCs at 5 A g...
Figure 9.6
(a) Plot shows the data for both electrolytes’ power...
Figure 9.7
Cycle life of ZHMSC. Copyright from Ref. [54].
Chapter 10
Figure 10.1
Applications of supercapacitors.
Figure 10.2
Hybrid vehicle [28].
Figure 10.3
Schematic representation of supercapacitors that can be...
Figure 10.4
Eco-friendly vehicles [34].
Figure 10.5
Activated carbon generated from garlic roots and electrode...
Figure 10.6
Paper-based supercapacitors made from seaweed [41].
Figure 10.7
Carbon yarn-based supercapacitors [46].
Chapter 11
Figure 11.1
Classification of the sustainable SCs based on the...
Figure 11.2
Ragone plot of the different energy storage devices.
Figure 11.3
The challenges for the sustainable SC field.
Figure 11.4
Role of the electrode materials in the field of susta...
Figure 11.5
Role of the electrolytes in the field of sustainable SCs.
Figure 11.6
The schematic of the mechanism for symmetric and asym...
Figure 11.7
The various technical aspects to overcome the drawbac...
Figure 11.8
The several application aspects for sustainable SCs.
List of Tables
Chapter 4
Table 4.1
Details on lead acid batteries, capacitors, and supercapacitors...
Chapter 6
Table 6.1
Classification of ESDs based on their nature and life cycles.
Chapter 9
Table 9.1
Performance comparison of supercapacitors and batteries...
Table 9.2
The evaluation of other cutting-edge 2D materials and p...
Table 9.3
Supercapacitor MXene-based electrodes and their electro...
Guide
Cover
Table of Contents
Series Page
Title Page
Copyright
Preface
Begin Reading
Index
End User License Agreement
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Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106
Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])
Sustainable Supercapacitors
Next-Generation of Green Energy Storage Devices
Edited by
Basheer Ahamed
Dept. of Physics, BSA Cresent Institute of Science and Technology, Vandalur, Chennai, India
and
Chaudhery Mustansar Hussain
Dept. of Chemistry & Environmental Sciences at New Jersey Institute of Technology, Newark, New Jersey, USA
This edition first published 2025 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2025 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-394-23787-6
Front cover images courtesy of Wikimedia CommonsCover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
Printed in the USA
10 9 8 7 6 5 4 3 2 1
Preface
The evolution and expansion of electronic products and devices have led to rapid development in modern science and technology. As this expansion increases briskly, so does the requirement for energy-storing devices that can handle the necessary power for their functioning. The miniaturization of electronic devices has sparked the evolution of energy-storing devices in their most portable forms. Integration of min- or micro-powering devices with tiny electronic devices has led to the simultaneous evolution of nanomaterials and, correspondingly, nanotechnology. Thus, procuring nanomaterials has emerged as an essential branch in synthetic chemistry. In recent years, various techniques have emerged that improve the composition control, shape, and size of nanomaterials in order to reach their greatest potential for dedicated applications.
The nanotechnology evolution has provided the control and ability to restructure matter at the atomic and molecular levels on a scale of l-100 nm. Nanotechnology also involves exploiting different properties and phenomena at that scale, in contrast to the properties exploited by single atoms or molecules of bulk behavior. Nanotechnology primarily aims to create materials, devices, and systems that exhibit fundamentally new properties and functions. As such, nanotechnology and functionalized nanomaterials have proven to be the ultimate frontier in the production of novel materials that have manufacturing longevity and cost-efficiency.
The integration of nanotechnology to produce functionalized nanomaterials and energy storage from electrochemical principles has established a new platform for science and technology. The integration of two technologies does not compromise their fundamentals and principles, but instead results in novel and high-performance supercapacitors.
This book consists of 11 chapters that review state-of-the-art technologies.
Chapter 1 deals with the developments in flexible fabric-type energy storage devices, as well as hybrid fabrics for energy storage and harvesting in flexible wearable electronics.
Chapter 2 elaborates on the role of electrolytes in the development of sustainable supercapacitors and the performance optimizations associated with them. The interaction between electrodes and electrolytes is fundamental to supercapacitor performance, as outlined in this chapter. It also discusses factors such as mobility, diffusion coefficient, ionic conductivity, ion solvation, thermal stability, viscosity, ionic size, electrochemical stability, dielectric constant, and dispersion interaction that affect the overall device performance.
A vast amount of contemporary research is dedicated to developing green supercapacitors as sustainable energy storage devices. Energy storage system that are based on green supercapacitors allow the storage of electricity during lean hours at a relatively cheaper value and later delivery. Chapter 3 highlights the recent developments in this area.
With escalating international demands for cost-effective and clean energy storage systems, supercapacitors have attracted more attention than ever. The strategies, synthesis methods, applications, and electrochemical studies supply the basis for the development of sustainable supercapacitors. The materials used in sustainable supercapacitors, such as novel transition metal oxides, metal-organic frameworks, conductive polymers, and biomass-based, as well as their composites (binary and ternary), are explored in Chapter 4, which also highlights the different materials used to develop and fabricate sustainable supercapacitors.
Chapter 5 focuses on the role of material selection and fabrication approach in achieving optimal performance and sustainability in supercapacitors. This section includes a detailed discussion on the significance of material selection, emphasizing the properties and characteristics required for sustainable electrode materials.
Battery technologies are widely used technology, but they offer several disadvantages like weight, volume, large internal resistance, poor power density, and poor transient response. On the other hand, advancements in material and technology have made increased the promise of supercapacitors, ultracapacitors, and electrostatic double-layer capacitors (EDLC). Chapter 6 explores how these solutions offer a more significant transient response, power density, low weight, low volume, and low internal resistance, making them suitable for several applications.
Chapter 7 explains how sustainable supercapacitors have steadily gained traction due to their potential for non-invasive health monitoring, while chapters 8, 9, and 10 outline the current state of sustainable supercapacitors. These chapters provide a thorough introduction to the various electrolytes and electrode materials used in the most recent supercapacitor technologies. The current challenges of the fabrication and utilization of SCs are critically examined and potential solutions are analyzed in this section. Plus, the performance inconsistencies that arise from the improper use and extreme diversity of performance evaluation and reporting methods are highlighted.
Chapter 11 discusses the development opportunities and the challenges faced by supercapacitors, such as technical problems, establishing electrical parameter models, consistency testing, and establishing industry standards.
Overall, this is a timely handbook for researchers and scientists in pursuit of new advancements in supercapacitor technology. The handbook is ideal for a broad audience working in the fields of electrochemical sensors, analytical chemistry, chemistry and chemical engineering, materials science, nanotechnology, energy, environment, green chemistry, sustainability, electrical and electronic engineering, solid-state physics, surface science, device engineering and technology, etc.
Furthermore, this book will be an invaluable reference source for libraries in universities and industrial institutions, government and independent institutes, individual research groups, and scientists working in supercapacitors. We are tremendously grateful to all the authors for their outstanding enthusiastic efforts in making this book possible. Finally, our special thanks to Martin Scrivener and Laura Mohr of Scrivener Publishing for their continued support and help during this project.
The Editors
August 2024
1Flexible Sustainable Supercapacitors
S. Siva Shalini, R. Balamurugan, I. Ajin and A. Chandra Bose*
Nanomaterials Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamil Nadu, India
Abstract
The intense growth in renewable energy sources and the global drive towards meeting the net-zero target have encouraged the development of long-cycle life- sustainable electrochemical energy storage devices. Despite many ongoing research and advancements, challenges still remain in commercial applications such as boosting power and energy density, improving scalable fabrication techniques, developing safety, increasing cycling lifetime, and securing comfortable wearing experiences. For wearable and flexible applications that demand high capacitance, high power ratings, long life cycles, stretchability, portability, and durability, flexible supercapacitors are a very promising development. Applications for smart textiles include communications devices, thermoregulated clothing, heat storage, thermoelectric energy harvesting, electroluminescence applications, and energy storage. For the above-mentioned applications, sustainable production of the power source is crucial. There are many ways that energy storage can be accomplished. Supercapacitors offer better performance in addition to a wide range of applications, including energy harvesting devices, hybrid cars, and smartphone components. In this chapter, we systematically summarize the developments in flexible fabric-type energy storage devices, as well as their hybrid fabrics for energy storage and harvesting in flexible wearable electronics.
Keywords: Flexible electrodes, sustainability, energy storage, biomass derived, portable hybrid supercapacitors, superior electrochemical performance
1.1 Introduction
The development of energy storage and conversion devices using fossil fuels (coal, oil, gas, etc.) holds certain drawbacks such as skin diseases, excess pollution, etc. Recently, with the continuous increase in the world’s population and the endless modernization of technologies, an intelligence era has arrived [1]. The history and advancements of supercapacitor are shown in Figure 1.1. Right now, wearable/portable electronics have obtained a fast growth in various fields such as sports, environmental, biomedical, and electronics applications [2]. One of the upcoming technologies with a significant potential influence in textile electronics, which offers wearable, flexible, and comfortable electrical systems [3, 4]. Their primary requirements are flexible energy conversion and storage systems that exhibit superior mechanical durability, high safety, low weight, and outstanding electrochemical performance [5]. Flexible supercapacitors are a desirable option because of their strong flexibility, increased safety, and nearly constant performance under a range of mechanical deformations [6]. However, they are required to possess a high energy density, long cycle life, and an excellent electrical conductivity [7].
Figure 1.1 History and advancement of supercapacitors.
The components of a flexible supercapacitor (FSC) usually include a flexible electrode with superior electrochemical qualities, a separator in a flexible assembly, and an electrolyte that is compatible [8]. In this chapter, we systematically summarized starting from the electrode materials to the assembly of flexible supercapacitors, with a focus on various types of electrode materials for flexible supercapacitors, their improving electrochemical performance, fabrication of the flexible electrode, and frequently used electrolytes [9].
1.2 Flexible Electrodes
The performance of flexible supercapacitors is significantly determined by the electrode. In particular, the flexible supercapacitors are better in overall performance, including their lifetime, power density, and specific capacitance [10]. The materials used for the electrodes affect the energy density and flexibility. Numerous nanomaterials, including conductive polymers, metal compounds, and carbon materials have played a key role in the development of flexible, smart, and self-sustaining FSCs [11]. The electrode materials for FSCs are categorized based on the energy storage mechanisms. The pseudocapacitor includes transition metal compounds, and conductive polymers have unique benefits and drawbacks to the use of flexible supercapacitors, and the electric double layer capacitor (EDLC) uses carbonaceous materials [12].
1.3 Electrode Materials
EDLCs uses carbonaceous electrode materials that consist of graphene, carbon nanotubes, activated carbon, and carbon nanofibers [13]. They serve as a viable candidate for constructing flexible supercapacitor electrodes due to their excellent stability, electrical conductivity, huge surface area, and strong mechanical performance [14]. However, their usage is constrained by the very low specific capacitance of these carbonaceous materials reduces the energy density. Surface modification techniques like heteroatom doping, surface activation, and exfoliation were utilized to significantly increase the capacitance [15]. Transition metal compounds are used as a pseudocapacitive electrode material because they are affordable, have a high theoretical specific capacitance, easy to handle, and chemically stable. These compounds include transition metal hydroxides, carbides, oxides, sulfides, and nitrides [16]. However, most transition metal oxides have short cycle lives and limited rate capabilities due to their poor reversibility and poor electrical conductivity. The Mn+1XnTx MXenes have hydrophilic surfaces, a high surface-to-volume ratio, and strong electrical conductivity. X can be carbon or nitrogen, and Tx is the surface termination. The electrochemical capacities can be improved by adding quantum dots to nanostructured transition metal oxide [17].
Conductive polymers such as polypyrrole (PPy), polythiophene, poly-aniline (PANI), and their derivatives are an excellent choice for pseudo-capacitors. Certain benefits of pseudocapacitive materials include their remarkable environmental stability, ease of synthesis, and electrical conductivity [18]. In this regard, the specific capacities of PPy, poly (3,4-ethylenedioxythiophene), and PANI are 480 F g-1, 210 F g-1, and 1284 F g-1, respectively. The polymerization procedure, dopant concentration, structural morphology, and ionic diffusion length all have an impact on conductive polymers specific capacitance [19]. However, the doping and dedoping processes that occur during the charge and discharge course lead the conductive polymers to expand and contract, which causes mechanical deterioration and poor cycling stability [20]. In addition, recently developed innovative electrode materials include POMs (polyoxometalates), MOF (metal-organic framework), and BP (black phosphorus) [21]. Owing to its porous structure, significant surface area, and abundance of active redox sites that contain metal, MOF is one of them that is attracting the most attention as a supercapacitor electrode material [22].
1.4 Modifying Techniques to Enhance Electrochemical Performance
To ensure rate performance and high capacity, the ideal electrode materials for flexible supercapacitors need to have a large ion accessible surface area and rapid electron transfer kinetics [23]. Therefore, flexible supercapacitor electrode materials can achieve superior electrochemical performance by increasing their surface area, mass loading, reducing the path length for the electrolyte ion diffusion, and improving the ion transport [24]. Many strategies, like building diverse electrode material microstructures and creating composites with a well-balanced composition, are used to improve the electrochemical performance of the flexible electrode materials [25].
1.5 Flexible Supercapacitors
Xiaojing Lv et al. fabricated a flexible laterally configured electrochromic supercapacitor through a fully solution process. Figure 1.2a schematically illustrates the structure of flexible electrodes, and Figure 1.2b shows the detailed step-by-step fabrication process of the flexible laterally configured electrochromic supercapacitor device. For transparency and flexibility, polyethylene plastic film (PET) was used as a substrate. The supercapacitor’s current collectors need high electrical conductivity. However, PET is a resistive material. For inducing high electrical conductivity, the conducting layer of the AgNWs/Aa-PDA complex and PEDOT:PSS was produced using spraying in the sequence method. On the above conducting layer, poly (3,4-propylenedioxythiophene) (magenta electrochromic polymer) and the gel electrolyte PMMA/PC/LiBF4/BMIMOTF/ACN were cast sequentially to fabricate the asymmetric device PProDOT/PEDOT:PSS. It demonstrated strong cyclic stability, retaining 85.65% of its initial areal capacitance even after 1000 cycles of mechanical deformation, and a maximum areal capacitance of 2.15 mF/cm2 at a current density of 0.05 mA/ cm2. Therefore, the electrochromic supercapacitor device with a flexible laterally configuration shows the potential for use in wearable and portable displays like smart watches and electronic paper [26].
Figure 1.2 (a) Schematic illustration of the structure and (b) the detailed fabrication process of the flexible laterally configured electrochromic supercapacitor device.
Qing Xin et al. optimized the ionic conductivity of flexible supercapacitor’s electrolytes and electrode material with glycerol-Mo (Gly-Mo). Figure 1.3 shows a schematic illustration of the hydrogel with partially substituting water molecules and Gly-Mo. The electrolytic solution resistance of the hydrogel was optimized by increasing the concentration of Gly-Mo and investigating the solution resistance of the hydrogel at various temperatures (–40, 50 °C, and room temperature). The result indicates that an 8-mL substitution of Gly-Mo in 10 mL of hydrogel (PVA/(Gly-Mo)8) provided low solution resistance at a wide range of temperatures from –40 to 50 °C. The ionic conductivity of the hydrogels was examined at a wide range of temperatures for various concentrations of Gly-Mo. PVA/(Gly-Mo)8 provides an excellent ionic conductivity in extremely low temperature and high temperature. The flexibility of the hydrogel was investigated at room temperature and –20 °C. By increasing the concentration of Gly-Mo, predominantly hydrogel’s flexibility increases drastically. The supercapacitor’s specific capacitance retention rate was 64.2 % after 50 charge–discharge cycles at –10 °C, indicating that it is still effective. The above results show that the organohydrogel PVA/(Gly-Mo)8 is suitable for flexible supercapacitors due to its good flexibility and conductivity at a wide temperature range of – 40 to 50 °C [27].
Shuzhen Cui et al. fabricated a flexible and hydrophilic hydrogel film-based all-in-one supercapacitor through a simple polymerization method. At first, through the polymerization process, PAM (polyacrylamide)/LiCl hydrogel film was fabricated. Then, polypyrrole (PPy) nanocomposites were grown onto the two-sided surfaces of the PAM/LiCl hydrogel via the polymerization method. Figure 1.4 demonstrates the fabrication process of an all-in-one flexible supercapacitor. Mechanical properties of the PPy/ PAM/LiCl hydrogel were examined by stretching, twisting, knotting, folding, compressing, and bending the device, which is shown in Figure 1.4. Additionally, the all-in-one flexible supercapacitor demonstrated exceptional cycling stability, an appealing capacitive performance, and the ability to prevent electrolyte and electrode delamination under deformations. The all-in-one supercapacitor is found to have high ionic conductivity (29 mS cm-1), high specific capacitance of 229.8 mF cm-2 (0.8 mA cm-2), and excellent flexible stretching up to 650 % of the original length. The successful coating of electrode materials onto hydrophilic hydrogel electrolytes offers remarkable opportunities for the creation of a flexible all-in-one supercapacitor, thereby promoting the advancement of wearable electronics [28].
Figure 1.3 Schematic illustration of the hydrogels.
Figure 1.4 Schematic illustrates the fabrication of the all-in-one flexible supercapacitor, flexibility of the device, its electrochemical performance, and its cross-sectional SEM image.
Soomin Suh et al. demonstrated an ultrafast flexible supercapacitor. Figure 1.5a represents the scheme of an as-fabricated flexible PEDOT:PSS supercapacitor consisting of the electropolymerized PEDOT:PSS (EPPEDOT:PSS) thin-film electrodes on Au/Ti/Ti-foil current collectors, along with a PTFE separator permeated with a sulfuric acid-polyvinyl alcohol (H2SO4-PVA) polymer gel electrolyte. Figure 1.5b demonstrates the electropolymerization experimental setup’s scheme. The current response and mass loading on the current collector with respect to time are represented in Figure 1.5c,d, respectively. The EP-PEDOT:PSS-based flexible SC demonstrated high-performance EDLC behaviors, such as triangle-shaped galvanostatic charge/discharge (GCD) curves and rectangular cyclic voltammetry (CV) curves up to 300 V s-1. It also demonstrated an excellent cyclic stability (~95 % capacitance retention over 50,000 cycles) and a high rate capability (greater than 90 % capacitance retention from 1 to 10 mA cm-2), and offers remarkable mechanical adaptability, with bending radii between 4.0 and 0.6 mm, and resilience, maintaining approximately 94 % of its capacitance after 1,000 bending cycles [29].
Figure 1.5 Schematic diagrams of (a) all-solid-state flexible supercapacitor, and (b) three-electrode cell configuration for electropolymerization; (c) electropolymerization chronoamperometric current response at a constant applied voltage; (d) mass loading on current collector during electropolymerization.
Bhagya Dharmasiri et al. fabricated a flexible carbon fiber-based structural supercapacitor. In this, the electrical conductivity and pseudocapacitive activity of the carbon fiber (CF) were improved by the surface functionalization of CF with a covalently grafted layer of redox-active poly(o-phenylenediamine). The electrical conductivity of the solid electrolyte was optimized by incorporating SIL (solvated ionic liquid) with epoxy resin (RIM935 +RIMH936). Increasing the SIL concentration in solid electrolytes increases the electrical conductivity and flexibility but reduces the tensile strength. Seventy percent of SIL mix produced the optimized electrical conductivity with the required tensile strength. Using this electrical conductivity-optimized solid electrolyte and surface-functionalized CF with glass fibers as a separator, they fabricated the flexible supercapacitor. Its structural prototype is illustrated in Figure 1.6. The device’s flexibility was examined at different bending angles, and it exhibited an increased specific capacitance at the bending angles of 90° and 135°. It also exhibited a maximum specific capacitance of 1439 mF g-1 at 0.5 mA g-1, a maximum energy density of 181.79 mW h kg-1, and a maximum power density of 6.18 W kg-1 [30].
Hengyu Zheng et al. used a wide temperature-tolerant and self-healing flexible supercapacitor with a ternary network organo-hydrogel electrolyte. A self-healing ternary network electrolyte was fabricated by freeze-drying method using polyvinyl alcohol (PVA), graphene, dimethyl sulfoxide, boric acid (BA), and sulfuric acid. Its self-healing capability was tested at room temperature, as shown in Figure 1.7a. Ten seconds after cutting, it joined together, and its mechanical stability was retained after 5 min. Its microscopical images (Figure 1.7b) showed that after the self-healing process, some cracks are found in graphene/BA/PVA organo-hydrogel. After self-healing, its temperature sustainability was examined, as shown in Figure 1.7c. Its electrical conductivity was investigated before and after the self-healing process by lighting LED, as shown in Figure 1.7d. This self-healing organo-hydrogel was used as an electrolyte and polyaniline fibers coated on carbon cloth were used as electrodes to fabricate the flexible symmetric supercapacitors. It exhibited a specific capacitance of 237.8 F g-1 and 152 F g-1 at 65 °C and –65 °C, respectively [31].
Figure 1.6 Schematic illustration of each layer of the flexible supercapacitor.
Figure 1.7 (a) Optical images of self-healing graphene/BA/PVA organo-hydrogel, (b) optical microscopic images, (c) self-healed organo-hydrogel at various temperatures, and (d) optical images of blue LED lit after cutting and self-healing of organo-hydrogel.
Hanie Kazari et al. fabricated 3D flexible binder-less supercapacitor electrodes with ruthenium metal on a free-standing multi-walled carbon nanotube forest through plasma-enhanced atomic layer deposition. The fabricated 3D flexible binder-less supercapacitor electrode’s bending capability was investigated through GCD and CV curve analysis at various bending angles, as shown in Figure 1.8. It provided an excellent charge storage properties at all investigated bending states. The areal capacitance of free-standing multi-walled carbon nanotubes exhibited greater than 400 % after depositing ruthenium on the surface through plasma-enhanced atomic layer deposition. This is due to the core electrode material being made up of electrically conductive material and the surface material being made up of electrochemically active material. It exhibited a maximum areal capacitance of 1834.5 mF cm-2 (specific capacitance of 393 F g-1). It provided a capacitance retention of 92 % after 1000 CV cycles [32].
Figure 1.8 GCD and CV curves of the 3D flexible electrode at various bending angles (a, b) 180°, (c, d) 80°, (e, f) 60°, and (g, h) spiral.
Jiawei Guo et al. fabricated a hierarchical porous 3D Ni3N-CoN/ NC heterojunction nanosheet with nitrogen vacancies to create a high- performance flexible supercapacitor. For high-performance flexible electrode fabrication, carbon cloth was used as a flexible and electrically conductive current collector. On the carbon cloth, porous nickel cobalt metal-organic framework (MOF) nanosheets were grown using the solvothermal method. Then, to improve the electrochemical activity of MOF, it is annealed in the ammonia environment and produced hierarchical porous 3D Ni3N-CoN/NC heterojunction nanosheets with nitrogen vacancies. Due to the base material, MOF’s porous nature and nanosheetlike structure, resultant nitrides provided a rapid ion diffusion path with the support of nitrogen vacancies, as shown in Figure 1.9. It provided a maximum specific capacity of 468.3 mA h g-1 at 3 A g-1 current density. With this porous Ni3N-CoN/NC acting as an anode, and AC/CC acting as a cathode with PVA/KOH electrolyte, they fabricated a flexible high- performance supercapacitor (Ni3N-CoN/NC/CC//AC/CC). It exhibits the highest energy density of 0.2144 mWh cm-2 and a maximum power density of 80 mW cm-2. It also exhibited excellent cycling stability over 1,500 charge–discharge cycles [33].
Figure 1.9 Schematic illustration of hierarchical porous 3D Ni3N-CoN/NC heterojunction nanosheets with nitrogen vacancies.
Serkan Demirel et al. fabricated a flexible supercapacitor using stretchable and self-healable PVA–Nickel–Borax (PNB) matrix electrodes. In this, PNB electrodes were fabricated through the solution self-curing method. Its self-healing ability was tested, as shown in Figure 1.10a. It confirmed that after 8 min of disjoining, PNB was completely rejoined together. Homogeneity of the as-fabricated electrode was attained through proper solidification of the mixture. It is confirmed by finding the homogeneity of the mixture with different solidification percentages of the mixture, as shown in Figure 1.10b. The stretching ability of the PNB matrix was demonstrated, as shown in Figure 1.10c. In this, dI/dV analysis was carried out for a more precise determination of redox peak values of PNB samples. Using these PNB electrodes, the fabricated devices provided a maximum specific capacitance of 88.95 F g-1 with an energy density of 49.42 W h kg-1 at a power density of 18.75 kW kg-1 [34].
Figure 1.10 (a) Self-healing of PNB, (b) homogeneity mixture identification, and (c) stretchablility of PNB.
Figure 1.11 (a) Structure diagram of the interdigitated micro-supercapacitors, optical images of (b) appearance size and (c) flexure flexibility test.
Shaoyi Lyu et al. fabricated a flexible supercapacitor using polymer– inorganic hybrid nanocomposite aerogel film electrodes. The flexible electrode was manufactured using cellulose nanofibers (CNFs), MXene, and carboxylic single-walled carbon nanotubes (SWCNT). First, the homogeneous mixture of all three materials was dispersed well in aqueous to prepare hydrogels. Then, the hydrogels were dried using the supercritical point drying method to produce aerogels. Due to the advantages of all three precursor materials, it provided an excellent electrical conductivity, flexibility, and porosity. Using this aerogel, the interdigitated micro-supercapacitors are fabricated, as shown in Figure 1.11a. Figure 1.11b displays the dimensions of the fabricated micro-supercapacitor. Figure 1.11c demonstrates the flexibility of the supercapacitor. It exhibited a maximum areal capacitance of 746.68 mF cm-2 with an energy density of 9.04 W h cm-2. It provided an outstanding cycle stability over 10,000 cycles with a capacity retention rate of 91.23 % [35].
1.6 Sustainable Supercapacitors
Activated carbon is the perfect negative electrode material for supercapacitor applications because of its special physical and electrochemical properties, which include high electrical conductivity, large specific surface area (SSA), flexible porous architectures, and high chemical stability. Furthermore, a remarkable idea directly connects green chemistry and sustainability, and synthesizes the activated carbon from waste biomass. Biomass-derived activated carbon has various benefits, including capacity for industrial production, renewability, abundance, and environmental friendliness [36–39]. Because they are easily repeatable and inexpensive, biomass and its derivatives have been widely used to create porous carbon materials [40–42]. Using mixed egg yolk/white and rice waste as precarbonization catalysts, a two-step technique of pre-carbonization and pyrolytic activation was used to create an in situ nitrogen–oxygen co-doped porous carbon for supercapacitor applications [43]. Activated carbons from red pepper industrial waste (RPW) were used to create green and sustainable supercapacitor electrodes utilizing traditional chemical activation with ZnCl2 at varied carbonization and activation temperatures [44]. Using coconut husk as a green and natural resource was used to create reduced graphene oxide (rGO) for supercapacitor (SC) applications [45]. Cotton-based activated carbon fiber was prepared by a urea-enhanced low-temperature hydrothermal carbonization and activation method used for supercapacitor applications [46]. Spent tea wastes were used to create porous carbon for supercapacitor applications [47]. Resin-based carbon materials were used for creating honeycomb-like carbon nanosheets [48]. Cashew nut shells [49], dry almond leaves [50], pineapple peel [51], purple leaf plum kernel shell [52], pine needles [53], Abutilon theophrasti steam [54], Sterculia foetida fruit shell [55], and subbituminous coal feedstock [56] were used for making sustainable electrode materials.
Zhu et al. use the renewable lignin as a precursor for the coproduction of porous carbon and calcium chloride. Homogeneously mixing the CaO with the lignin precursor to create the porous carbons, CaO acts as a hard template for pore creation. The combined sustainable production of lignin-derived porous carbon (LPC) and CaCl2 was obtained through a simple preparation method. The method of preparation is shown in Figure 1.12. The lignin precursor was mixed with CaO, and carbonization was carried out; after carbonization, it was sonicated in an HCl solution, followed by filtration. The filter residue was dried to obtain the LPC; the filtrate was collected in a Petri dish and dried to get CaCl2 powder. This was a cheap technique for producing LPC, which was used in supercapacitors; the templating agent CaO was recycled as CaCl2. The porous carbon obtained in this process has rich pores and a high SSA. These pores create the most abundant sites for electrochemical reaction, reduce the ion mass transfer distance, and improve the rate performance capacity of the hybrid supercapacitors. The obtained LPC carbon was assembled with a zinc ion hybrid capacitor, which has high cycle stability, better rate performance, and high specific capacitance [57].
Figure 1.12 Schematic representation of the sustainable production technology for the synthesis of LPC and CaCl2.
Kitamoto et al. use a kraft lignin carbon precursor obtained from the kraft pulp for supercapacitor applications. The cross-linked kraft lignin was dissolved in double-distilled water with a NaOH activating agent to form a homogeneous suspension. Then, the above mixture was spray-dried to form a lignin/NaOH composite particle. Finally, it was carbonized, followed by acid wash to get the porous carbon particles. The synthesis route is shown in Figure 1.13. The amount of NaOH plays a significant role in the morphology of the kraft lignin. In the absence of NaOH and a low mass ratio of NaOH, the surface of kraft lignin was in perfect spherical shape. Gradually increasing the activating agent, the morphology changes from spherical to non-spherical with the development of many holes on the surface. At a 1:1 mass ratio of lignin/NaOH composite, the high amount of NaOH decomposes the carbon walls and completely changes the regular porous structure into hollow, non-spherical morphology. Thus, the high activating agent (NaOH) containing composite has a higher surface area than the low NaOH-containing composites. Even though it has a high SSA, the value of gravimetric specific capacitance, volumetric specific capacitance, and electrode density was high for the low SSA lignin/NaOH composite. The low-mass ratio NaOH/lignin composite has more spherical particles and was tightly bound to each other. As the NaOH/lignin mass ratio increases, voids develop between the particles. Hence, high specific capacity was due to the composite’s high packing density [58].
Wang et al. fabricated the symmetric supercapacitor device using discarded aramid fibers (AF). AF was dipped into the H3PO4 solution, followed by activation in a N2 environment, and the sample was noted as AFP. Above, AFP was further submerged into a KOH solution with carbon mass/alkali ratios varying between 2 and 5, followed by activation in a N2 environment. The sample was noted as AFPK-n, where n represents the alkali mass ratio. The sample’s morphology changes during the different KOH activations, as shown in Figure 1.14a–d. AFP has a smooth surface. While increasing the alkali mass ratio, the streak grooves along the axial direction on the surface, and there are many mesoporous structures on the surface of the AFPK-4 sample. Figure 1.14d AFPK-4 has a lot of streaks on the surface. The SSA of all the samples was measured using BET characterization; AFPK-4 exhibits the highest SSA. It has been demonstrated that efficient charge storage and quick ion diffusion benefit from large SSA and hierarchical pore structures. The as-assembled aqueous symmetric supercapacitor based on AFPK-4 had an excellent energy density of 25.26 W h kg-1 at a power density of 450 W kg-1 [59].
Figure 1.13 Porous carbon particle synthesizing procedure.
Figure 1.14 SEM images of (a) AFP, (b) AFK-2, (c) AFPK-2, and (d) AFPK-4.
Mehdi et al. produce activated carbon (AC) materials using bio-char generated by pyrolyzing date seeds (DS) and activating them with H2SO4. For getting activated carbon, the initial step was pyrolysis of DS at 600 °C (DSBC 600 °C), followed by the carbonization process at 700, 800, and 900 °C temperatures; corresponding samples were represented as DSAC 700 °C, DSAC 800 °C, and DSAC 900 °C, respectively. The carbon content in DSBC 600 °C, DSAC 700 °C, DSAC 800 °C, and DSAC 900 °C was 62.86%, 68.83%, 74.56%, and 71.3%, respectively. From Raman spectra, the sample with the lowest (ID/IG) value, the DSAC-700, likely underwent more graphitization and developed ordered graphene layers. It suggests that the sample has more conductivity and better electrolyte ion mobility, resulting in an increased electrochemical performance. From the SEM image, the sample DSBC 600 °C has bulk particles with less porosity before activation. All the samples showed a three-dimensional structure; however, only DSAC-700 had macro- and mesopores. The performance of supercapacitors was enhanced by the existence of macro- and mesopores, which offer plenty of space for charge storage, electrolyte access, and pathways for quick charge transfer [60].
Narayanan et al. proposed a novel technique for producing agricultural waste-based electrode materials for supercapacitor applications. This work obtained activated carbon from the areca nut (AR) husk using KOH as the activating agent. The synthesis procedure was described in Figure 1.15a. Figure 1.15b illustrates the carbonization followed by the KOH activation of the samples; KOH activation created the pores on the surface of the activated carbon material. The activation was carried out in three different temperatures, 600, 700, and 800 °C, and the samples are denoted as AR-600, AR-700, and AR-800, respectively. Figure 1.15c displays the XRD curve of three samples; two peaks at 23.7° and 43.5° correspond to the (002) and (100) planes respectively of the amorphous natured carbon materials. Figure 1.15d–f indicates the FESEM images of AR-600, AR-700, and AR-800. As the temperature increases from 600 °C to 800 °C, the pores in the material surface increased gradually, and the sample activated at 800 °C had more pores than the other samples. Further, the samples surface area and pore volume are measured using BET analysis, and AR-800 has more pore volume and surface area than the other samples. Porous materials with large surface area produce more active sites for establishing electrical double layers, increasing the pore size and capacitance [61].
Figure 1.15 (a) AR husk-derived porous carbon synthesis procedure, (b) KOH activation process, (c) XRD patterns of AR carbon, (d–f) FESEM images of activated carbon at different temperatures 600 °C, 700 °C, and 800 °C, respectively.
Shaku et al. used marula nutshell as the sustainable precursor for super-capacitor electrode material. Figure 1.16 represents the synthesis procedure of nitrogen-doped KOH-activated carbon. Before activation, the sample had a smooth agglomerated surface; after activation, the surface had a lot of holes; these holes provide the path for the electrolyte ions during charge storage. The sample activated using a 1:1 ratio of carbon material and activating agent gives a larger surface area than the sample activated with a high proportion of activating agents. Excess KOH destroys the carbon structure rather than increasing the surface area. The sample activated using a 1:1 ratio of carbon material and activating agent was further doped by nitrogen, creating some new micropores that facilitate the charge storage, improving the electron transfer.
The nitrogen doping increased the sample’s surface area from 1266 to 1427 m2 g-1. This increase in surface area shows that the melamine decomposition, which covers the surface with C and N atoms but does not fill the holes created by the KOH reaction with carbon, caused the surface to become more porous. Before nitrogen doping, the device fabricated using the as-prepared sample exhibits the energy density of 12.5 W h kg-1 in 2.5 M KNO3 electrolyte with a 1.8 V potential window. After nitrogen doping, the device can provide an energy density of 17.2 W h kg-1 [62].
Figure 1.16 Synthesis route to prepare the nitrogen-doped activated carbon material.
Arjunan and Kim used a single-step direct-carbonization procedure to create porous carbon material from the renewable source of Cassia fistula flower petals. The synthesis procedure is shown in Figure 1.17. The activation method used here was the physical activation method; the resultant sample was denoted as Cassia fistula flower-petal-derived sheetlike porous carbon (CFSPC). FE-SEM image of CFSPC has a sheetlike morphology with large cavities. Due to the fast ion diffusion kinetics, this sheetlike irregular architecture is enabled, and the electrolyte ions are entirely accessible to the adsorption sites. From BET analysis, the specific surface was about 556 m2 g-1 for CFSPC. This number was relatively high without any chemical activation of the biomass precursor. The pore size significantly influences the electrochemical performance of an active electrode. The hierarchical pore structure of the CFSPC was the combination of the microporous and mesoporous framework. Fast interfacial ion-transport kinetics and an efficient ion-accessible surface area for charge storage are made possible by the hierarchical porosity of the CFSPC. Device fabricated using CFSPC demonstrated a high specific energy and power of 48.2 W h kg-1 and 687 W kg-1, respectively, and a good cycle stability (95.3% for at least 10,000 charge–discharge cycles) [63].
Figure 1.17 Schematic illustrations of the synthesis procedure of CFSPC.
Zhu et al. developed a green and effective method of catalytic activation caused by trace KOH to produce activated carbons (AC) from oaks for electrochemical capacitor applications to utilize oak resources more effectively. The synthesis process of AC from oaks is shown in Figure 1.18a–c. Figure 1.18d depicts the tracheid-coated cytoderm of oak cells. The xylem conducting tissue in plants, which includes the tracheid, was crucial for transporting water and nutrients to plants. In addition to this, hemicellulose, cellulose, and lignin were the constituents of the biomass cytoderm. These cytoderm constituents were the main carbon sources during the carbonization process.
Figure 1.18 (a–c) Graphical representation of the synthesis procedure for the AC from oaks, SEM images of (d) raw oaks, (e, f) activated carbon using KOH.
According to Figure 1.18e, f, the generated carbon had an uneven shape and ranges in size from 5 to 10 μm. The entire smashing process was responsible for the homogeneous particle size, which was advantageous for making carbon-based electrodes. To create a logical hierarchical porous structure, trace KOH can restrict mesopore growth and promote micropore development. The resulting sample had a large pore volume of 0.448 cm3 g-1 and a high SSA of 985 m2 g-1. As a result, the electrochemical performances of the sample were enhanced. The resulting sample exhibits outstanding cycling performance with 97% retention over 10,000 cycles at 2 A g-1 and a high specific capacitance of 157.5 F g-1 at a current density of 1 A g-1 [64].
Wei et al. had been utilizing mantis shrimp (MS) shell waste as a precursor, and a simple self-template coupled dual-hydroxide (NaOH/KOH) activation approach successfully produced the honeycomb-like hierarchical porous carbons (HPCs). The synthesis process is shown in Figure 1.19; in the same procedure, the carbonization was carried out at three different temperatures, 700, 800, and 900 °C, and the samples are denoted as MSHPC700, MSHPC800, and MSHPC900, respectively. MS shells are mostly made of chitin fibers, minerals, colors, and proteins, which are put together in a particular combination. The activation temperature strongly influences surface functional groups, SSA, pore size distribution, and microstructure of as-obtained carbon products, which was a crucial component of preparing porous carbon materials. These factors significantly impact the electrochemical performance of carbon-based energy storage devices. Among the three samples, the MSHPC800 electrodes had the longest discharge times, attesting to their highest specific capacitance values. The hierarchical pore structure provides numerous paths for the transfer of electrolyte ions and can contribute to electrolyte penetration [65].
Figure 1.19 Schematic diagram of the preparation process of porous carbons from MS shells.
Qiu et al. developed a simple and effective approach to convert discarded bamboo shavings into petal-like hierarchical porous carbon electrode materials with an extremely high SSA. The combined catalyzed hydrothermal pretreatment method and co-activation by KOH/melamine was performed to obtain N, O co-doped biomass. Figure 1.20 displays a process flow diagram for synthesizing N, O co-doped hierarchical porous carbon. In this synthesis, melamine was employed as an N-element dopant and a pore modifier for porous carbon materials, while KOH primarily functions as a porogenic agent. The intercross-linked porous network structure of bamboo shavings-based porous carbon results in a multidimensional transport path for electrolyte ions and dramatically lowers the ion diffusion resistance. The hydrophilicity and accessible SSA of carbon-based materials can be greatly increased by the presence of N- and O-containing functional groups. Additionally, by participating in redox processes with the electrolyte on the materials surfaces, these functional groups can contribute to a portion of the pseudocapacitance. The resultant porous carbon materials exhibit great qualities such as smooth ion diffusion channels, a lot of charge storage space, and significant pseudocapacitance because of the excellent hierarchical porous structure, exceptional SSA, and abundant N and O doping [66].
Figure 1.20 Synthesis road map of N, O co-doped hierarchical porous carbon.
Cevik et al. reported fabricating highly ion-conducting, partly cross-linked hydrogels with sodium carboxymethyl cellulose (C) intercalated at varied fractions by Hibiscus sabdariffa (H). Both C and H served as the ion source to create stable blends, allowing H3O+ and Na+ ions from the blend to migrate where no external salt was added. The construction of activated carbon-based supercapacitors with considerably better energy storage capacity than EDLC-based devices was made possible using highly safe biopolymer hydrogels. Carbon composite electrodes with conductive additives and activated carbon were employed to create symmetric super-capacitors. The bio-polymer hydrogel electrolyte (PHE) plays a dual role such as an electrolyte and a separator in fabricating highly flexible super-capacitors, which leads to a highly effective device. The PHE provides a relatively peaceful atmosphere that allows for an excellent ion transport. In the supercapacitor charge–discharge phenomenon, the polarization of the electrolyte molecules and the development of a double layer capacitor are depicted in Figure 1.21. The supercapacitor electrodes were supplied with sodium ions (Na+) from the C and hydronium (H3O+) ions probably derived from H organic acids. H intercalates in the polymer structure through hydrogen bonding to create a negatively charged region throughout the electrolyte. During the charge–discharge process, unbound organic acids contribute to the environment as negatively charged anions [67].
Zhang et al. made the hybrid supercapacitor (HSC) device using the amorphous carbon/iron silicate (C/FeSi) anode and cobalt-nickel silicate (CoNiSi) cathode materials that are naturally obtained from bamboo leaves (BLs). CoNiSi cathode C/FeSi anode and materials made from natural BLs are prepared and it was shown in Figure 1.22. The morphological change and the sample’s structural evolution happened during calcination. After two hours of calcination, the bamboo leaves’ 3D-organized porous constructions are created. C/FeSi was an exceptionally well-known electrode material because of its significant surface area and remarkable porous properties. The C/FeSi and CoNiSi electrodes developed here have better electrochemical characteristics than current silicates-based electrodes. The assembled C/FeSi//CoNiSi HSC device achieves an excellent areal specific capacitance of 492 mF cm-2 (96 F g-1) at 4 mA cm-2, an outstanding energy density of 3.35 W h m-2 (6.55 W h kg-1) at a power density of 3.5 W m-2 (68.36 W kg-1), and a remarkable cycle stability of 76% retention after 5000 cycles. These materials perform well because of the 3D hierarchical architectures of C/FeSi and CoNiSi, which facilitate electron transport and electrolyte movability [68].
Figure 1.21 Schematic working mechanism of supercapacitor.
Figure 1.22 Synthesis procedure of C/FeSi and CoNiSi obtained from BLs for use as anode and cathode materials in a high-performance HSC device.
1.7 Conclusions
Flexibility and sophisticated features are prerequisites for modern wearable and portable electronics. Undoubtedly, the energy storage device is the crucial and core unit in portable/wearable electronics, and supercapacitors are the most suitable ones in this field. To meet these requirements, flexible, smart, and self-sustainable supercapacitors, with suitable electrode material, have received extensive attention and made remarkable progress in the last decade. In this chapter, we provide an in-depth analysis of the design of flexible, intelligent, and self-sustaining SCs for wearable and portable electronics. Developing electrodes in a variety of forms, such as fiber, yarn, fabric, paper, foam, hydrogel, aerogel, etc., could result in high flexibility. These fundamental research studies are crucial for the development of smart materials, appropriate device configurations, and innovative working mechanisms, all of which are key components of smart supercapacitors. As different green energy-harvesting devices have been developed over the past five years, numerous researchers have combined them with supercapacitors into a single, compact configuration to create all-in-one, self-sustaining supercapacitors. They have the capacity to continuously power wearable and portable electronics by simultaneously converting and storing ambient energy as electricity. Naturally, demand for wearable and portable electronics will rise as artificial intelligence and the “internet of things” advance. As a result, flexible, smart, and self-sustaining SCs gained a lot of attention and have produced some interesting outcomes.
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Energy Storage
