ABSTRACT
Faradaic capacitors store charge by Faradaic redox reaction on the electrode surface also penetrating the electrode. These capacitors can be divided into those involving underpotential deposition, redox pseudocapacitance, and intercalation pseudocapacitance, while hybrid supercapacitors combine both double layer and Faradaic charge storage mechanisms. A high-rate discharge capability is one of the most important performances of supercapacitors in the application of electrode battery. As disadvantages with respect to batteries, one can mention that supercapacitors possess relatively low energy density (from 1/5 to 1/10 than a conventional electrochemical battery), low cell voltages, and high self-discharge rates. Fuel cells provide advantages with respect to other energy conversion systems by virtue of their high efficiency, environmental safety, and possibility to recover exhaust heat. Nickel metal-hydride batteries batteries derived from nickel-cadmium ones replaced cadmium by hydrogen absorbed in a metal alloy as active anodic material, whereas NiOOH remains as the active cathode material.
