Handbook of Nanocomposite Supercapacitor Materials II

The supercapacitor has emerged as a promising electrochemical energy storage device. Its excellent performance, easy handling, and stability have gained remarkable attention. In comparison with batteries, it delivers high-power density and cyclic stability. This is basically due to its charge storage mechanism, where ions get adsorbed at the electrode surface during charging and get released while discharging. This makes it different from batteries, where repeated redox reactions lead to poor stability and low-power density. Supercapacitor works similarly to the conventional capacitor, where two conductors are separated by a dielectric medium. The capacitance arises from the separation of charges at the conductor surface. In supercapacitor, the conductors have been replaced by the porous electrode, which provides efficient surface areas for the adsorption of ions. Also, the separation between two opposite charges is in the nanometer range, which further contributes to high capacitance than the conventional capacitor. Basically, the supercapacitor is classified by two types of charge storage mechanisms, where pure electrostatic, non-Faradic processes are called electric double-layer capacitor (EDLC). The other includes the Faradaic process, where a reversible redox reaction is involved and known as pseudocapacitor. Carbon-based materials are used as EDLC electrode; whereas, metal oxides and conducting polymers are used as pseudocapacitor electrode material. Further improvement in terms of performance is reported by combining both types of charge storage mechanisms called a hybrid supercapacitor. The phenomena of the charge storage mechanisms in supercapacitors have been discussed in detail. Different components of the supercapacitor and their functions have been briefly introduced in this chapter.

Considerable efforts have been given on the advancement of the electrical energy storage system. Supercapacitor-an electrochemical energy storage device gains significant interest over batteries and fuel cells because of its high-power density, excellent cyclic stability, and easy handling. However, the poor energy density of supercapacitors has accelerated the research to explore different types of material for the betterment of the device. Various combinations of materials used for the electrode, electrolyte, separator, and current collector are developed for the supercapacitors to achieve good performance. The choice of electrolyte influences the working electrochemical potential window of the device. Different types of electrolytes such as aqueous, organic, and ionic liquid have been discussed with their merits and demerits. Among all other components of the supercapacitor, the choice of electrode material mainly determines the electrochemical behavior of the device. In this vein, various types of material ranging from carbon-based electric double-layer capacitor electrode to transition metal oxide and conducting polymer-based pseudocapacitor electrode materials are discussed in detail. Apart from commonly studied material, few recent emerging electrode materials such as metal-organic framework and MXene have been also discussed, considering the potential application soon. It also discusses the various types of current collectors and separators used in supercapacitors.

Supercapacitors are electrical energy storage system, which aims to deliver high-performance electrochemical properties. In order to meet the current demand for modern electronics, specialized testing techniques are required. In this context, electrochemical testing techniques provide an essential platform to study the performance of a different kind of energy storage device. The main electrochemical testing techniques include cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). These techniques provide necessary information about the diffusion mechanism of electrolyte ions at the electrode surface, occurrence of redox peak at a certain voltage, time taken to store charge at constant current, etc. These studies are used to derive comparable parameters like capacitance, energy and power density, induced resistance, cyclic stability, coulombic efficiency, etc. Thus, to study and compare the mechanism of different energy storage devices, electrochemical characterizations are one of the necessary tools.

Transition metal oxides are promising electrode materials for electrochemical energy storage, as they exhibit higher specific capacity/capacitance and energy density due to their fast and reversible surface redox reactions. This chapter provides a comprehensive study on fabrication, structural characterizations, electrochemical properties, and supercapacitor performance of different types of transition metal oxide-based electrode materials to fully exploit the potential of them in supercapacitor applications. A comprehensive overview of supercapacitive performance of different types of transition metal oxide-based electrodes such as RuO₂, MnO₂, NiO, and Co₃O₄ has been included in this chapter to understand the recent progress in this field.

Activated carbon is one of the most versatile materials used as an electrode material for supercapacitor applications. The preparation of activated carbon from various biomasses has attracted the attention of the scientific community in recent days. The synthesis of activated carbon from biowaste exhibits varieties of morphologies and surface textures. Carbonization and activation are the main steps for the synthesis of activated carbon. Due to the tuneable pore sizes and high specific surface area as compared to other carbonaceous material, activated carbon has been widely used as electrode material for supercapacitor applications. The high surface area, hierarchical pore structure, and different morphology enable the formation of a bilayer of ions at the electrode-electrolyte interfaces. Again, the inherent doping of heteroatoms from biomass additionally contributes via pseudocapacitance. The presence of oxygen, nitrogen, and sulfur functionalities promotes the diffusion of ions, enhances the conductivity and wettability at the carbon surface. This also helps to improve the overall performance of the activated carbon to be utilized as electrode material for supercapacitor applications.

The supercapacitor is a new generation charge storage device, which can satisfy the demand of huge energy and power density. First-generation supercapacitor deals with an electric double-layer capacitor (ELDC) and pseudocapacitor electrode material, which suffers from their limitations of low energy density and stability. To develop a high-performance supercapacitor, a second-generation hybrid supercapacitor device comes into play. Various combinations of assembly and composite electrode materials are designed, which can deliver a high energy and power density along with high rate capability and cyclic stability. Among various hybrid supercapacitors, this chapter deals with the composite electrode materials, which can overcome the limitation of individual components and enable large amounts of charge storage. The composite material is a combination of transition metal oxide serving the role of a pseudocapacitive material and activated carbon as EDLC material. Activated carbon shows good conductivity along high surface area and porosity. The main advantages of using activated carbon are tuneable porosity and low cost; however, capacitance is restricted to the surface of the electrode that is responsible for delivering low capacitance value. Nowadays, activated carbon has been synthesized using various biomasses and biowastes, which further extend the availability of cost-effective starting materials for the synthesis of activated carbon. On the other hand, transition metal oxide shows multiple oxidation states, which participate in Faradaic redox reaction delivering high capacitance however, unable to express high cyclic stability due to poor conductivity. Transition metal oxide and activated carbon composite overcome the limitations of individual components as metal oxide can reside in carbon matrix providing electrical conductivity to the electrons generated during redox reaction from the metal oxides. The large specific surface area of activated carbon and Faradaic redox reaction in metal oxide are expected to deliver excellent electrochemical performance in the device.

Carbon nanofibers are important one-dimensional carbon-based nanostructured materials. They are generally prepared by electrospinning of polymer precursors and followed by thermal treatment, or by chemical vapor deposition growth through the decomposition of hydrocarbons in the presence of a metal catalyst. They are promising electrode materials for supercapacitors due to their high electrical conductivity, specific surface area, and porosity. They store energy through the electrical double layer capacitor mechanism. Nitrogen doping, activation process, etc., can further improve the specific capacitance values of them. Therefore, this chapter provides decent and updated coverage on the basic structure, properties, and supercapacitor performance of carbon nanofibers at different synthetic approaches. The chapter also describes the supercapacitive performance of carbon nanofibers through different surface and structural modifications.

The supercapacitor is recognized as an important device for next-generation energy storage due to its high-power densities. Carbon-based materials are the most widely considered as electrodes for supercapacitors due to their large specific surface area and excellent electrical conductivity. However, they generally suffer from low specific capacitance values and therefore poor energy densities. Recently, transition metal oxides are vastly integrated with carbon materials to design hybrid supercapacitors to improve energy density. Due to the existence of high electrical conductivity and specific surface area, carbon nanofibers are widely used in hybrid supercapacitor electrodes with different transition metal oxides like MnO₂, RuO₂, and V₂O₅. These hybrid supercapacitors simultaneously deliver high energy and power densities with long cycle life and rate capability. Therefore, this chapter is mainly focused on hybrid supercapacitors of transition metal oxides with carbon nanofiber. The chapter provides decent and updated coverage on the fabrication and structure of different hybrid supercapacitors based on transition metal oxides and carbon nanofiber, and their electrochemical performance.

High-performance supercapacitors are promising candidates for future energy storage devices for alternative power sources. Carbon nanotubes (CNTs) are considered as potential electrode materials for supercapacitors due to their superior electrical conductivity, high electrochemical stability, good mechanical properties, high specific surface area, and so on. Both single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) have been considered for electrochemical supercapacitor electrodes due to their unique properties like novel structure, narrow distribution of size in the nanometer range, highly accessible surface area, low resistivity, and high stability. The specific capacitance of CNTs mainly originated through the electric double-layer capacitor (EDLC) mechanism. Therefore, the supercapacitive performance of CNTs mainly depends on the physical properties, such as specific surface area, electrical conductivity, which are related to the synthesis and post-treatment methods of them. This chapter mainly emphasizes on the recent progress and development of the use of CNTs as supercapacitor electrodes through fabrication and post-treatment techniques.

Carbon nanotubes (CNTs) are considered as potential electrode material for electrochemical supercapacitors for the past few decades and attracting increasing attention owing to their unique structure with narrow distribution of size, highly accessible surface area, superior electrical conductivity, excellent mechanical properties, and high stability. However, the electrodes composed of pure CNTs have quite low energy density and specific capacitance. Therefore, the integration of redox-active pseudocapacitive materials with CNTs is the best solution to get the ideal supercapacitor electrodes by the synergistic effect of both materials. Recently, transition metal oxides and CNTs composite materials have been widely investigated and demonstrated impressive energy and power densities values and maintain long-term stability in supercapacitors. Therefore, a detailed analysis of CNTs-transition metal oxides-based-nanostructured materials as supercapacitor electrodes have been critically reviewed in this chapter. The chapter provides decent and updated coverage on the synthesis, structure, properties, and supercapacitor performance of CNT and transition metal oxide composites.

Owing to the unique structure and outstanding intrinsic properties, such as extraordinarily high electrical conductivity and large surface area, graphene-based materials have great potential for supercapacitor applications. Graphene-based electrodes act as electrical double-layer capacitors and possess high power density and excellent cycling stability. Extensive studies have been done as well as going on in the rationalization of their structures at varying scales and dimensions, effective and low-cost synthesis techniques, design and architectures, as well as their electrochemical performance in energy storage applications. Several architectures of graphene materials like zero-dimensional particles or quantum dots, one-dimensional fibers or yarns, two-dimensional films, and three-dimensional forms are employed in supercapacitor electrodes. Therefore, this chapter provides decent and updated coverage of graphene-based materials for supercapacitor applications. The chapter mainly focuses on the synthesis, structure, properties, and supercapacitor performance of graphene-based materials according to the architectures.

Graphene-based materials have been extensively used as electrode materials for supercapacitor applications due to their extraordinarily high electrical conductivity and large surface area. However, they suffer from the low energy density and specific capacitance because of the graphene’s propensity toward aggregation and restacking, which reducing the ion-accessible surfaces and limiting ion and electron transport. Therefore, to enhance electrochemical performance for high-performance supercapacitor, pseudocapacitive transition metal oxides are integrated with graphene-based materials. Currently, these hybrid supercapacitors have been attracted much attention due to the combination of rapid charge–discharge and long cycle life for energy storage in modern electronic devices. In these hybrid materials, the emphasis is given to synergistic effects between graphene/reduced graphene oxide and metal oxides, which results in high energy and power densities along with high specific capacitance. This chapter is mainly focused on hybrid supercapacitors of transition metal oxides with graphene-based materials. The chapter provides decent and updated coverage on the synthesis, structure, properties, and supercapacitor performance of graphene and transition metal oxide-based composite materials.

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

Transition metal oxide-based pseudocapacitors fundamentally involve fast charge–discharge processes due to the fast-Faradaic redox reactions occurring at the interface between the active material and electrolyte. Thus, they exhibit high specific capacity and energy density compared to electrical double-layer capacitors. However, their poor conductivity and low surface area restricted their advanced application in supercapacitors. Therefore, to improve conductivity and surface area without losing pseudocapacity, a synergistic effect of transition metal oxides and electronically conducting polymers has been recognized to design composite electrode materials for supercapacitor. The formation of transition metal oxide-conducting polymer composite electrodes can achieve better electrical conductivity and electrochemical accessibility of redox sites for high-performance supercapacitors. Therefore, this chapter provides decent and updated coverage on synthesis, structure, properties, and supercapacitor performance of conducting polymers and their composites of transition metal oxides.

The supercapacitor is an effective energy storage device, which is used to meet the demand for global energy consumption. But the design and fabrication of an ideal electrode material are the most challenging issues to achieve high-performance supercapacitors. The use of single electroactive materials like carbon materials, transition metal oxides, and electronically conducting polymers as an electrode in supercapacitors suffers from several demerits. Thus, different types of hybrid materials are utilized to achieve high-performance supercapacitor electrodes of high specific capacitance, energy density, power density, rate capability, and cycle life with low cost and environmentally friendly. Even the use of two components in binary composites cannot fulfill all the requirements of a high‐performance supercapacitor. Therefore, numerous strategies have been employed to design ternary composites by merging all three types of electroactive materials for the fabrication of ideal high‐performance supercapacitors. Hence, this chapter focuses on the recent progress of supercapacitors based on ternary composites of transition metal oxides, carbon-based materials, and conducting polymers. The chapter also provides a comprehensive study of the synthesis, structural characterizations, electrochemical properties of transition metal oxides, carbon-based materials, and conducting polymers.

Supercapacitors are the most promising energy storage devices that bridge the gap between capacitors and batteries. They can reach energy density close to the batteries, and power density close to the conventional capacitors. Several types of researches have been carried out in the field of supercapacitors for the development of promising electrodes, electrolytes, separators, current collectors as well as device fabrications to get a breakthrough in energy storage systems with diverse applications. This chapter provides the trend of supercapacitor electrodes made of carbon nanofibers, carbon nanotubes, graphene/reduced graphene oxide, activated carbon, transition metal oxides, conducting polymers and their composites concerning specific capacitance, and cycle life. The design and flexibility of electrode material has promoted the supercapacitor to design bendable, lightweight, miniaturized (micro), planar, flow, shape memory, piezoelectric, self-healing, and multifunctional energy storage devices, which suggest the near future demand of supercapacitor over a wide range of applications. The chapter is also ended with several concluding marks.

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