Chapter 8 - Simple Parallel-Plate Capacitors to High–Energy Density Future Supercapacitors: A Materials Review

Abstract

This chapter focuses on the generational emergence of capacitors pertaining to their application in energy storage. Three generations of capacitors (electrostatic, electrolytic, and electrochemical, commonly known as supercapacitors) are discussed. Electrostatic capacitors are conventional parallel-plate capacitors. Examples of commercially available electrostatic capacitors are ceramic and film. A brief overview of nanostructure-based electrostatic capacitors that exhibit high capacitance using high dielectric oxide nanolaminates conformally coated over high–surface area electrodes is provided. The second generation of capacitors is electrolytic capacitors, which are commercialized on a large scale. Some electrode materials are aluminum, tantalum, and niobium, in which a solid or liquid electrolyte separates two symmetrical electrodes. The third generation of capacitors is electric double layer, in which the electric charge stored in the metal–electrolyte interface is exploited to construct a storage device. These capacitors, commonly known as supercapacitors, provide a longer cycle life (∼100,000) compared with batteries that provide 300–500 cycles. The supercapacitors deliver higher power bursts than do batteries and store more energy than conventional capacitors. Although the energy density of most commercially available supercapacitors is much higher (on the order of 10Whkg−1) than conventional dielectric capacitors, it is significantly lower than for batteries and fuel cells. With the rapid development of renewable energy sources such as solar cells, there is an urgent need for devices such as supercapacitors with a high energy density. Thus, a tremendous amount of research is under way seeking to increase the energy density of supercapacitors without sacrificing their high-power capabilities. In addition to traditional commercially available electrostatic and electrolytic capacitors, this chapter reviews significant advances made in electrochemical capacitors, with a discussion of various electrolytes and commonly used electrode materials such as RuO2 and MnO2. The chapter concludes with a brief discussion on reports of emerging electrodes such as metal organic frameworks and mixed transition metal oxides such as NiCo2O4.