Supercapacitor pseudocapacitor

An electric double-layer capacitor, also known as supercapacitor, pseudocapacitor, electric double layer capacitor (EDLC), supercapacitor or ultracapacitor is an electrochemical capacitor with relatively high energy density. Compared to conventional capacitors the energy density is typically on the order of thousands of times greater than an electrolytic capacitor. In comparison with conventional batteries or fuel cells, EDLCs have lower energy density but a much higher power density. 

Recently supercapacitors are attracting much attention as new power sources complementary to secondary batteries. The term supercapacitors is used for both electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The EDLCs are based on the double-layer capacitance at carbon electrodes of high specific areas, while the pseudocapacitors are based on the pseudocapacitance of the films of redox oxides (Ru02, Ir02, etc.) or redox polymers (polypyrrole, polythiophene, etc.). 

In the second type of supercapacitor, sometimes termed pseudocapacitors, redox capacitors or electrochemical capacitors, the non-Faradaic doublelayer charging process is accompanied by charge transfer. This Faradaic process must be characterized by extremely fast kinetics in order to allow device operation with high current density discharge pulses. 

Depending on the charge storage mechanism, one must distinguish between the electrical doublelayer capacitors (EDLC) and the pseudocapacitors. The principles and properties of both types of supercapacitors will be further described. 

There are two types of supercapacitors the first one involves the charging and discharging of the electrical double-layer interface, while in the second one, the charge-discharge is accompanied by -> electron transfer, yet with no phase changes, as is the case in batteries. The latter is also referred to as a pseudocapacitor. The -> capacitance of such a supercapacitor is C = Qi + Cg, where Cdi represents the double-layer capacitance and Cg represents the pseudocapacitance. 

Typically, a supercapacitor is composed of two electrodes dipped in an electrolyte solution with a suitable separator. It is generally accepted that the energy storage mechanism of supercapacitors can be classified into electrical double layer capacitors (EDLCs) and pseudocapacitors (Fig. 6.1A) (Jost et al., 2014). In EDLCs, the charge storage is based on a reversible ion adsorption from an electrolyte onto electrodes with high specific surface areas to form a double layer structure. The capacitance comes from the pure electrostatic... 

At present, a great number of research works on development and improvement of electrochemical supercapacitors (ECSCs) are published. As pointed out above, ECSCs are subdivided into EDLCs, in which the energy of EDL recharging is used pseudocapacitors (PsCs), in which pseudocapacitance of fast quasireversible faradaic... 

Electrodes of various types are used in hybrid (asymmetric) supercapacitors (HSCs). For example, one of the electrodes is highly dispersed carbon, that is, a double-layer electrode, and the other electrode is a battery one or one of the electrodes is carbon and the other one is a pseudocapacitor, for example, based on electron-conducting polymer (ECP). The main advantage of HSCs as compared EDLCs is an increase in energy density because of the wider potential window. The main fault of HSCs, meanwhile, as compared to electric double-layer capacitors (EDLCs), is a decrease in cyclability following the limitations posed by the nondouble-layer electrode. 

As follows from these tables, symmetrical double-layer supercapacitors based on C/C-type carbon electrodes and, in limited quantities, some hybrid C/NiOOH-type ECSCs are produced at present. In the case of pseudocapacitors based on electron-conducting polymers, there are as yet only laboratory prototypes. 

FIGURE 1.3 Schematic diagram for (a) electrostatic capacitor, (b) electric double-layer capacitor, (c) pseudocapacitor, and (d) hybrid capacitor. (Zhong, C. et al. 2015. A review of electrolyte materials and compositions for electrochemical supercapacitors. Chemical Society Reviews 44 7484-7539. Reproduced by permission of The Royal Society of Chemistry.)... 

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