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ISBN 978-1-119-71145-2
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20.4.3 Polythiophene and its Derivatives-Based Flexible Supercapacitors 621
Preface
The tremendous demand for energy for miniaturized portable and wearable electronic devices has inspired intense research on lightweight, flexible energy storage devices for commercial applications such as smartwatches, mobile phones, flexible displays, electronic skin and implantable medical devices. The speedy progress in flexible electronics has sparked wide-ranging endeavors in exploring coordinating power sources as flexible supercapacitor devices. Flexible supercapacitors are flexible, wearable devices that deliver high-power density, high specific capacitance, fast charge/ discharge processes, long cycle life, low cost, and environmental friendliness. They hold enormous potential to meet the rapidly expanding market for portable and wearable electronics. Designing flexible supercapacitors requires essential architectures such as electrodes, electrolytes, and substrate materials that become robust, flexible, and durable under mechanical deformations without sacrificing the electrochemical performance. These flexible supercapacitors are promising energy technologies that can supplement or even substitute batteries in portable and flexible electronics; however, research and development (R&D) studies need to be conducted for their large-scale commercialization. Therefore, awareness and knowledge of flexible supercapacitors is crucial for advanced energy research.
This book presents a comprehensive overview of flexible supercapacitors using engineering nanoarchitectures mediated by functional nanomaterials and polymers as electrodes, electrolytes, separators, etc., for advanced energy applications. Various aspects of flexible supercapacitors, including capacitor electrochemistry, evaluating parameters, operating conditions, characterization techniques, different types of electrodes, electrolytes, and flexible substrates are covered. Since it is probably the first book of its type to systematically describe the recent developments and progress in flexible supercapacitor technology, it will help readers understand fundamental issues and solve problems. This book is the result of the commitment of top researchers with various backgrounds and expertise in the flexible power sources field. Those working in science, research, industry, or academia
Preface
will benefit from the information archived herein relating to the fields of flexible power sources, solid-state electrochemistry, advanced energy storage material science, energy, electronics, advanced materials, and wearable science. It will be a very helpful reference source for generating innovative ideas in the field of energy storage material for wearable/flexible industry applications and also useful in resolving current industry issues. A summary of the information included in the 21 chapters is given below.
Chapter 1 discusses the types of electrode materials and the role they play in the high performance of flexible supercapacitors. Device preparation is described as well as the integration of flexible supercapacitors in various applications.
Chapter 2 highlights flexible fiber-shaped electrodes for flexible supercapacitors. Supercapacitors have an incredible impact on electrochemical devices in energy storage systems. To meet the rapid consumer demand for wearable and portable devices a new class of energy devices employ flexible fibrous electrodes/supercapacitors. These fiber-shaped flexible electrodes have garnered great attention for use in miniaturized microscale devices and the modern textile industry.
Chapter 3 discusses recent developments in graphene-based flexible supercapacitors, the structural morphology of flexible graphene-based electrodes and methods used to fabricate them, and the electrochemical performance of the devices.
Chapter 4 mainly discusses the preparation of polymer-based electrode materials. Also highlighted are the various prominent characterization techniques to elucidate the intercorrelation between physicochemical and performance properties of polymer-based electrode materials. The new reinforced polymer-based electrode materials for flexible supercapacitor applications are also discussed.
Chapter 5 thoroughly reviews the energy storage system and types of capacitor modeling. The structure, types of flexible supercapacitors and industrial applications are introduced.
Chapter 6 discusses the types of electrolytes for flexible supercapacitors and their salient features. Various electrolytes such as polyethylene glycol, polyvinylidene fluoride, ionic liquid and redox-active materials-based electrolytes are discussed along with their effect on the performance of flexible supercapacitors.
Chapter 7 discusses the preparation and properties of carbon-derived composite materials such as CNT-conducting polymer, CNT-metal oxide, activated carbon-conducting polymer, and activated carbon-metal oxide. The main focus of this chapter is to provide an overview of the latest progress in the development of flexible supercapacitors beyond graphene.
Chapter 8 highlights the various synthesis processes for making biomassderived electrode materials, their recent developments and the associated challenges for the near future. After a brief general introduction, the chapter moves on to discuss various electrode materials used for flexible supercapacitors; biomass-derived carbon materials and their different activation processes like physical, chemical and other activations; and carbonization processes using the hydrothermal method, pyrolysis method, etc. The possible incorporation of biomass-based electrodes in flexible supercapacitors and the challenges for using biomass-derived materials in the near future are also discussed in detail.
Chapter 9 portrays the importance and applicability of conducting polymer electrolytes, especially in flexible supercapacitors. The components of supercapacitors and their configurations are discussed in detail along with the role of conducting polymer-based electrolytes and their significance in the performance of flexible supercapacitors. The essential enhancing parameters of such electrolytes, including their consequences and electrochemical activity, are also elaborated.
Chapter 10 discusses the various inorganic electrode materials used in flexible supercapacitors. These flexible inorganic-based electrodes have great potential in the field of stretchable, lightweight and intrinsic fast charging and discharging performance.
Chapter 11 focuses on different new generation materials used for flexible supercapacitor electrodes. Also, in order to predict future trends, the direction towards developing new materials exhibiting superior electrochemical performance and their feasibility in practical applications are discussed.
Chapter 12 briefly describes flexible supercapacitors and their flexible components with a concise outline of innovative cell designs. Additionally, there is an overview of the principle behind the energy-storage mechanism and the anode and cathode materials used for asymmetric supercapacitors.
Chapter 13 provides detailed insights into the gradual development and latest accomplishments achieved with aqueous electrolyte-based flexible supercapacitors. Advantages of low production costs, eco-friendliness, non-flammability, and many other attractive factors have motivated scientists to design these smart devices to meet the high rising energy demands of modern society.
Chapter 14 presents systematic evaluations of different kinds of micro-supercapacitor configurations, possible strategies of fabrication, and state-of-the-art electrode materials. Discussions on designing asymmetric micro-supercapacitors and the influence of electrolytes on enhancing charge-storage properties are also provided. Finally, the challenges of current technologies and possible solutions are highlighted.
Chapter 15 discusses the categories of supercapacitors and their mode of action. Different types of nanomaterials, including metallic, non-metallic
and graphene-based hybrid, are discussed in detail for their self-healable properties to modify the electrodes in supercapacitors. The major focus is given to those nanomaterials that increase the self-healing properties of supercapacitors with enhanced capacitance.
Chapter 16 discusses the recent advancements for the fabrication of flexible and stretchable electrode supercapacitors using metal oxides, 2D materials, carbon, conductive polymers, and various hybrid nanocomposites. Moreover, possible applications of flexible/stretchable supercapacitors using these electrode materials, along with upcoming opportunities and challenges in this emerging field, are also discussed.
Chapter 17 discusses the classification of flexible supercapacitors and various superconducting materials. Additionally, different fabrication methods, namely, electrochemical deposition, chemical bath deposition (CBD), inkjet printing spray deposition, sol-gel technique, and direct writing method are discussed in detail.
Chapter 18 deals with the fundamental aspects of flexible supercapacitors with naturally inspired electrodes for energy storage systems. The mechanisms and principle behind energy storage in supercapacitors along with its essential parameters are presented. The use of common and naturally occurring materials and their electrochemical behavior is also discussed.
Chapter 19 focuses on advances in the field of high-performance ionic liquid electrolytes for flexible supercapacitors. After a brief discussion of the fundamentals, developments in the field of ionic liquids are presented. Design perspectives like electrolyte-electrode hybridization, challenges in encapsulation and mechanical stability are also presented.
Chapter 20 describes various types of conducting polymer-based flexible supercapacitors. Special emphasis is given to the fabrication methods employed for flexible supercapacitor devices. The different electrolytes, which play a significant role in flexible supercapacitors, are also discussed. The chapter concludes with perspectives on flexible supercapacitors.
Inamuddin, Mohd Imran Ahamed, Rajender Boddula and Tariq Altalhi
March 2021
1 Electrodes for Flexible Integrated Supercapacitors
Sajid ur Rehman1,2 and Hong Bi1*
1School of Chemistry and Chemical Engineering, Anhui University, Hefei, China
2High Magnetic Field Laboratory, Key Laboratory of High Magnetic Field and Ion Beam Physical Biology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, China
Abstract
Supercapacitor, as a new type of energy storage device lying in-between battery and traditional capacitor, owns many advantages such as fast charge and discharge time, high power density, environmental-friendly and long cycle life. It has become one of the hot research topics in the field of energy storage. Electrode materials play a vital role in flexible supercapacitors, the common electrode materials include carbon materials, conducting polymers and transition metal oxides. In order to exploit flexible high-performance supercapacitors, new highperformance electrode materials need to be developed. Metal oxides are promising supercapacitor electrode materials due to their low cost, good chemical stability, high theoretical specific capacitance and environmental friendliness. However, cycling stability and rate performance of metal oxides based supercapacitor still can’t meet the requirements of practical applications. Therefore, the research on electrode materials are not limited to single-component material, and nanocomposites can synergistically enhance the intrinsic properties of each component to exhibit more outstanding electrochemical properties. In this chapter, we discuss the electrode materials for flexible supercapacitors in detail and also describes the device preparation as well as the integration of the flexible supercapacitors in various applications.
In today’s world, coal, oil, natural gas and other traditional nonrenewable fossil energy are gradually exhausted, as the demand and consumption of energy are increasing day by day, which have been difficult to maintain for the sustainable development of human society and economy. Supercapacitor, also known as an electrochemical capacitor, is a new type of energy storage device between the battery and traditional capacitor, which is based on the principle of electric double-layer capacitance (EDLC) or pseudocapacitance. Because of its fast charge and discharge rate, high power density, environment-friendly and long cycle life, it has attracted increasing attention [1].
Supercapacitor is mainly composed of current collector, electrode active material, diaphragm and electrolyte (see Figure 1.1) [2]. Among them, electrode materials play an important role in improving electrochemical performance. The collector usually has good conductivity, does not react with the electrolyte, can exist stably in it, and has little contribution to the specific capacity of the capacitor. Different metal materials, such as nickel foam and aluminum foam, are used according to the electrolyte. An ideal electrode material should have the characteristics of large specific surface area, good conductivity, unique porous structure, high catalytic activity, good chemical stability and low manufacturing cost [2, 3]. Supercapacitors are commonly categorized into three sets based on the mechanism of charge storage: (1) EDLCs that store charge statically at interface of carbon electrode with large specific surface area; (2) Faraday pseudocapacitors that store electric energy electrochemically through electron transfer during reversible redox reaction, usually based on metal oxides and conducting polymer; (3) hybrid supercapacitor composed of special hybrid electrode or asymmetric electrode, which has significant carbon double-layer
capacitance and pseudocapacitance of conducting polymer or transition metal oxide [4, 5].
Starting from the theory of interface double electric layer proposed by Helmholtz, a German physicist, EDLCs began to be developed gradually. When two electrodes are inserted into the electrolyte, the positive and negative ions in the electrolyte will move towards the two poles rapidly under the action of the electric field, and attach to the electrode surface, forming a compact double electric layer [6]. As shown in Figure 1.1 [7], in the charging process, the applied electric field releases electrons, and the direction of electrons is from the negative electrode to the positive electrode. At this time, the ions present in the electrolyte will transport towards the respective electrodes respectively, so as to adsorb on the electrode surface and form a stable voltage. In the process of discharge, electrons flow through the conductor to generate current, at this time, the anion and cation on the electrode surface will be released into the electrolyte. In this process, the electrode material will not react with the electrolyte, only the adsorption and desorption of anions and cations in the electrolyte on the electrode surface [8, 9].
Hybrid supercapacitor (HSC) includes a composite symmetrical supercapacitor, battery supercapacitor mixer and asymmetric supercapacitor (ASCs). The structure of ASCs is shown in Figure 1.5 [10], which is usually assembled by two different materials as anode and cathode. In the process of charging and discharging, oxidation–reduction (Faraday) reaction usually takes place at the positive electrode, while adsorption and desorption of the negative electrode mainly take place at the double electric layer. In fact, EDLC can achieve fast and stable charge storage but provide relatively low specific capacitance, while the pseudocapacitor can obtain high specific capacitance but has poor multiplier performance and low cycle stability. The hybrid supercapacitor, which combines the advantages of EDLC and pseudocapacitor, has become a new hotspot in capacitor research area. It can achieve high energy and power density as well as good cycle stability in one device. However, the performance of all these supercapacitors depends on the properties of their active materials, the fabrication of electrodes, the selection of electrolytes and the geometry of devices [11, 12].
Throughout the development of supercapacitors, electrode materials have always played an important role in the electrochemical performance. Generally, the ideal electrode material should have the characteristics of large specific surface area, good conductivity, unique porous structure, high mechanical strength, good chemical stability and low manufacturing cost. Among them, carbon materials, metal oxides, conductive polymers and other electrode materials are the main research objects.
1.2 Electrode Materials for Flexible Supercapacitors
1.2.1
Carbon Materials
As electrode material of flexible supercapacitors (FSCs), carbon-based materials are beneficial due to their low cost, large specific surface area, stable electrochemical performance, better electrical and thermal conductivity, and mature synthesis process. At present, the commonly used carbon materials include activated carbon [14], carbon nanotubes [15], graphene [16], and carbon aerogel [17]. Their specific surface area, pore size and distribution, conductivity and heteroatom doping have certain effects on the electrochemical performance of [18].
1.2.1.1 Activated Carbon
Activated carbon is the first electrode material used in supercapacitors. Due to its advantages of low price, wide source of raw materials and stable physical and chemical properties, it has been widely used in commercial supercapacitors. So far, it still has a broad market [19]. After activated by KOH, the specific surface area of activated carbon can reach as high as 2,000 m2 g–1. However, the specific surface area of the active carbon electrode material is not directly proportional to the specific capacitance. The main reason is that the activated carbon has not been fully explored in terms of specific surface area, and the diameters of various electrolyte ions are required to be different for different pore diameters in the activated carbon, so some micropores do not play the role of storing electric charge, resulting in the available effective specific surface area becoming smaller, affecting its electrochemical performance [20]. As shown in Figure 1.2 [21], when the porous carbon has large pores (>50 nm), the electrode surface can rapidly adsorb electrolyte ions. Because the pore size is large, its specific surface area is reduced, resulting in a small effective adsorption area and poor capacitance performance. When the pore is a mesopore (2–50 nm), the inner surface of the pore can also rapidly adsorb electrolyte ions, and the mesopore also results in a large specific surface area and specific surface area. When the porous carbon is microporous (<2 nm), the size of ions is larger than that of the pore size, so, it cannot enter into the inner part of the pore, and the ion adsorption is reduced, resulting in the reduction of effective adsorption area. Therefore, in order to improve the electrochemical properties of the materials, in addition to improving the specific surface area of the activated carbon, the doping of heteroatoms, pore size control and the addition of surface functional groups
DecreasingPoreSize
Figure 1.2 Schematic illustration of charge adsorption in porous carbon materials with different sizes in double-layer capacitors [24]. Copyright 2016. Reproduced with permission from John Wiley & Sons.
can also be used. Zhou et al. [22] reported N-doped porous carbon with pore size classification by activating m-aminophenol formaldehyde resin with KOH, which has a high specific surface area of 1,847.5 m2 g−1 and thus a specific capacitance of 114 F g−1. Bleda Martine et al. [23] obtained oxygen-containing functional groups on the activated carbon through HNO3 peroxidation and subsequent heat treatment in N2 atmosphere, which not only improved the wettability of the surface of the activated carbon to the electrolyte but also generated additional pseudocapacitance to improve the specific capacitance.
1.2.1.2 Carbon Nanotubes
Carbon nanotubes (CNTs) are a kind of tubular carbon material made of single or multi-layer graphite curled. Its structure is very perfect, with seamless porous structure connected by hexagonal carbon atoms [25]. CNTs can be categorized into single-walled carbon nanotubes (SWCNTs, single layer) and multi-walled carbon nanotubes (MWCNTs, two or more layers). Single-walled carbon nanotubes have a higher specific surface area,
Nanoporous Carbon Supercapacitors
Figure 1.3 (a, b) HRTEM images of multi-walled carbon nanotube (MWNTs) synthesized at different temperatures, (c–d) CV curves at 20 mV s−1 rate in 1 M H2SO4 aqueous electrolyte, respectively [30]. Copyright 2016. Reproduced with permission from John Wiley & Sons.
but it is more difficult to prepare and purify. CNTs have excellent physical and chemical properties. Due to its unique hollow porous structure, large specific surface area, and good conductivity, it is considered to be an ideal electrode material for supercapacitors [26–29]. As shown in Figures 1.3(a, b) carbon nanotubes can form a network structure when they are entangled with each other. Most of the pore diameter is more than 2 nm, which is conducive to the penetration of electrolyte ions. Therefore, their specific surface area utilization ratio is high. Popvo et al. [30] have synthesized the MWCNTs at different temperatures and study the influence on supercapacitance properties. As shown in Figures 1.3(c, d), they found an increase in double-layer capacitance because of the larger surface area as well as the improvement in pseudocapacitance owing to the larger oxygenated groups grown on the exterior of nanotubes.
1.2.1.3 Graphene
Graphene is a kind of two-dimensional crystal plane material [31, 32] which is composed of sp2-hybridized carbon atoms tightly stacked and connected, in which the covalent bond between carbon atoms is formed,
presenting a hexagonal ring honeycomb shape. It has a unique twodimensional (2D) structure and many attractive characteristics and is widely used in electrochemical energy storage devices, as shown in Figure 1.4 [33]. Graphene is one of the allotropes of carbon, and it is also the basic unit of other dimensional carbon materials. A single layer of graphene has only one carbon atom thickness (0.335 nm).
Graphene has a large specific surface area, better electrical and thermal conductivity, excellent mechanical strength and chemical stability. Its surface is easy to show a three-dimensional (3D) fold structure, which is conducive to the transmission of electrons on the surface and the diffusion of ions in the material. It has great potential to apply it to electrode materials of supercapacitors. In fact, graphene itself is easy to aggregate, making its specific surface area far away from the theoretical value, thus limiting its electrochemical performance [34–36]. Therefore, it is very important to modify the surface of graphene or composite it with other materials. Si et al. [37] combined Pt particles with graphene, which made Pt nanoparticles deposit on graphene sheets. Pt played a role of separation, prevented the aggregation of graphene sheets face to face, mechanically peeled off graphene effectively. As a result, the embedded Pt@graphene had a highly expanded layered structure and retained the characteristics of 2D graphene hexagonal carbon network with large specific surface area. The specific surface area of Pt@graphene composite was 862 m2 g−1, and the specific capacitance of Pt@graphene composite was increased to 269 F g−1. Zhu et al. [38] reported that porous graphene oxide prepared by KOH chemical activation has a specific surface area of up to 3,100 m2 g−1, accompanied with high conductivity and low hydrogen and oxygen content, and the sp2-bonded carbon has a continuous highly curved three-dimensional
network, forming a hole with a width of 0.6–5 nm. Using organic and ionic liquid electrolytes, the double electrode supercapacitor made of this kind of carbon can obtain 3.5 V working voltage and 167 F g−1 specific capacitance (5.7 A g−1 current density), and the energy density can reach 70 Wh kg−1. In addition, the electrochemical properties of graphene can also be improved by combining graphene with other pseudocapacitor materials (such as transition metal oxides (NiO, MnO2, etc.) or conducting polymers (polyaniline, polypyrrole, etc.).
1.2.1.4
Carbon Aerogels
Carbon aerogel is a lightweight, porous, amorphous three-dimensional carbon nanostructured material. Because of its large specific surface area, abundant mesoporous and wide range of density variation, it is considered as an ideal electrode material for supercapacitors. In 1989, Pekala used resorcinol and formaldehyde as raw materials and sodium carbonate as catalyst. It is found that the carbon aerogel has the specific surface area of 400–800 m2 g−1 and the ultrafine pore size (<100 nm), and the specific capacitance of the 5 mol L−1 KOH solution is 45 F g−1. Subsequently, the research on carbon aerogels in supercapacitors has attracted more and more attention. Lin et al. [39] synthesized a series of carbon aerogels with a bimodal (microporous and mesoporous) structure by using iron-based ionic liquids as a solvent ionic thermal carbonization method and pore-forming agent. It has a high specific surface area up to 1,200 m2 g−1 and pore volume of 0.8 cm3 g−1, and a specific capacitance reaching as high as 245 F g−1. Carbon aerogels have certain advantages in the application of supercapacitors, but their shortcomings have seriously restricted their industrialization, such as expensive raw materials, long-time synthetic process, high equipment cost and difficulty to achieve a large-scale production.
1.2.1.5
Graphene Hydrogel
The effective specific area of graphene is greatly reduced due to the stacking and agglomeration of graphene layers. Therefore, the researchers have envisaged the connection and integration of 2D structure and designed various kinds of graphene, such as graphene hydrogel, aerogel, foam and sponge, to develop and utilize the properties of graphene in various 3D network structures. Its preparation methods are various, mainly including
self-assembly, template oriented, new 3D printing and ultrasonic-assisted technology. Although the structures and properties of these 3D graphene materials are different, they all have the common characteristics of high specific surface area and porosity, low bulk density, high conductivity and so on. Therefore, they have been widely studied and applied in adsorption, catalysis, sensing, energy storage and conversion, biomedicine and other fields [40].
Graphene hydrogel is a 3D solid structure cross-linked by 2D graphene, which can be prepared by freeze-drying or supercritical drying to remove moisture. It has both the intrinsic properties of graphene nanosheet and 3D porous material and shows better performance than the graphene nanosheet in the electrochemical application. Graphene hydrogel has interconnected porous structure, large specific surface area, low mass density and strong mechanical properties, making it widely used in electrode materials of the supercapacitor. As a high volume capacitor material, the large pores interconnected in the frame become unimpeded ion transmission channels, which is conducive to shortening the diffusion distance from the external electrolyte to the internal surface, thereby enhancing the ion transmission; while the graphene sheet in the frame is conducive to promoting the electron transmission on the electrode surface [12, 39]. Huang et al. [41] reported the elastic carbon aerogels and graphene as 3-D Matrix for supercapacitance properties. The schematic diagram as illustrated in Figures 1.5(a, b) demonstrates the interconnected macropores which exhibit high elasticity, improved surface area along with charge-transfer efficiency owing to the excessive interconnections between graphene flakes and carbon nanofiber ribs, which reveals prominent capacitive performance as supercapacitor electrode as shown in Figures 1.5(d–e). Xu et al. [42], using hydroquinone as both reductive and functionalized molecules, synthesized functional graphene hydrogels through a simple one-step reduction method, showing excellent electrochemical properties. Graphene hydrogel as a negative electrode can effectively prevent the aggregation of graphene nanosheets, provide large active surface area, and promote the transmission of electrolyte ions. Gao et al. [43] used graphene hydrogels with 3D interconnected pores as negative electrodes, and vertically aligned MnO2 nanosheets loaded on nickel foam as positive electrodes, successfully produced asymmetrical supercapacitors with high energy and power density. The potential window could reach 2.0 V, and the energy density achieved 23.2 wh kg−1 while the power density was 1.0 kW kg−1.
