The role and utilization of pseudocapacitance for energy storage by supercapacitors

The principle of utilizing the non-Faradaic double-layer capacitance of electrode interfaces as a means of storing electrical energy was suggested and utilized in technologies initiated some 37 years ago. However, only over the last ten years has major interest been manifested in commercial development of this possibility in so-called 'supercapacitors' or 'ultracapacitors' based on the large double-layer capacitance achievable at high-area, carbon powder electrodes. In parallel with the utilization of double-layer capacitance is the possibility of use of the large pseudocapacitance that is associated with e.g. electrosorption of H or metal adatoms (underpotential deposition) and especially some redox processes. Such pseudocapacitance arises when, for thermodynamic reasons, the charge q required for progression of an electrode process,e.g. electrosorption or conversion of an oxidized species to a corresponding reduced species in liquid or solid solution, is a continuous function of potential, V; then the derivative dq/dV corresponds to a capacitance but one of a Faradaic kind. This behavior is different from that with an ideal battery where, according to the Nernst equation, V is invariant with state-of-charge measured by q. Various experimental examples are shown and characterized, especially that for RuO2 and other transition metal oxides. Additionally, electroactive polymers such as polyaniline exhibit analogous pseudocapacitative behavior.