Capacitors and Supercapacitors

FIGURE 10.1 CVs for NiO attached to a graphite plate in contact with 0.10 M NaOH. Potential scan rate of (a) 50, (b) 5 mV/sec. 

The specific capacitance, C,p (F/g), can be evaluated from CVs as the quotient between the box current, 1, and the product of the potential scan rate and the mass of the composite layer, m. 

Analogously, the specific capacitance can be calculated from charge/discharge curves at a given current rate by using the relationship  

FIGURE 10.2 Charge/discharge curves at different current rates for mesoporous NiO (obtained using supramolecular template) in contact with 0.10 M KOH. (From Xu et al., 2007. J. Solid State Electrochem. 11, 372-377, with permission.) 

FIGURE 10.3 Nyquist plot in the KP - 10 Hz frequency range for Ru(bpy)-PA-functionalized polyacetylene on graphite in contact with 0.10 M H2SO4. Applied potentials of (a) +1.0 (b) 0.0 and (c) -1.0 V vs. AgCl/Ag. Material courtesy of Prof. Hermenegildo Garcia, Polytechnical University of Valencia. 

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There are several directions in which electrolytic capacitors and supercapacitors (ECSCs) will be developed in the nearest future and in midterm according to our forecasts. 

FIGURE 9.2 A comparison of discharge rates in batteries, capacitors, and supercapacitors. (Adapted from M. McKubre, An evaluation of supercapacitor technology. Office of Naval Research Grand Challenges Workshop, SRI, 1999. With permission.)... 

During capacitor and supercapacitor charging and discharging, only the ions move in and out of a thin layer. 

Electrochemical capacitors and supercapacitors have also received increasing attention both for energy storage and also due to the fact that such carbon-supported electrode capacitors play an important role in PEM fuel cell operation [9]. 

Lekakou, C. et al. 2011. Carbon-based fibrous EDLC capacitors and supercapacitors. Journal of Nanotechnology, 1-8. 

In electrochemical capacitors (or supercapacitors), energy may not be delivered via redox reactions and, thus the use of the terms anode and cathode may not be appropriate but are in common usage. By orientation of electrolyte ions at the electrolyte/electrolyte interface, so-called electrical double layers (EDLs) are formed and released, which results in a parallel movement of electrons in the external wire, that is, in the energy-delivering process. 

Types of capacitors and mode of energy storage after Ref.206 Reprinted from B.E. Conway, Electrochemical supercapacitors. Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publishers, New York (1999). Copyright 1999 with permission from Kluwer Academic Pubhshers. 

Kotz R, Bartschi M, Biichi F, Gallay R, Dietrich P. HY-POWER—a fuel cell car boosted with supercapacitors. Proceedings of the 12th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices, Deerfield Beach, FL, 2002. 

The double-layer capacitor is one of the electrochemical capacitors showing intermediate performances between conventional capacitors and rechargeable batteries from the viewpoint of energy and power densities. Although the terms supercapacitor and ultracapacitor are often used for double-layer capacitors, in a sense that they have higher capacitance than conventional capacitors (ceramic, film, aluminum electrolytic, or tantalum electrolytic capacitors), these terms are not to be used because they are the trademarks of certain companies products. 

Electrochemical capacitors based on carbon materials can be divided into doublelayer capacitors, where only a pure electrostatic interaction between ions and charged electrode surfaces occur, and supercapacitors, based on the occurrence of faradaic pseudocapacitance reactions (see Chapter 10). It is generally assumed that the charge is mainly stored, in porous carbons, in the double layer at the electrode/electrolyte... 

Electrochemical capacitors, also called supercapacitors, are very attractive electricity sources because of their high power, very long durability, and intermediate energy between the classical dielectric capacitors and batteries. The performance of a typical electrochemical capacitor is based on the accumulation of charges in the electrical double layer without faradaic reactions (no electron transfer The electrons involved in double layer charging are the delocalized conduction-band electrons of the electrode material. As shown in Fig. 23.9, an electrochemical capacitor contains one positive electrode with electron deficiency and the second one with electron excess (negative). The capacitance C of one electrode due to a pure electrostatic attraction of ions is proportional to the surface area S of the electrode-electrolyte interface, according to the formula (23.3) ... 

Another approach to consistent power delivery from renewable sources is to consider the use of supercapacitors placed between the renewable power source and the electrolyzer. These may or may not be viable, and we have not yet researched this possibility. Supercapacitors are basically a cross between capacitor and battery technology. They use electrodes, and a liquid or organic electrolyte, but they store energy by static charge rather than by electrochemical means. They can be cycled millions of times, and have a recharge time of seconds. Supercapacitors might also be viable to enhance peak load performance on the fuel cell end. 

Figure 5.6 Schematic drawings of standard capacitor (a) and supercapacitor (b).

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