From pseudocapacitive redox to intermediary adsorption in oxygen evolution reaction

The mechanism of oxygen evolution reaction (OER) is somehow related to that of the pseudocapacitance of metal oxide electrocatalysts in potentials lower than the OER standard potential. Although pseudocapacitance of metal oxide electrocatalysts is usually studied in the investigations of OER, this dependence is not profoundly inspected. During the pseudocapacitive behavior, some high valence cations are formed over a broad range of potential in an underpotential region before the formal redox potential. This is due to the exceptional activities of some sites, which are capable of further oxidation at the OER potential to form active sites for the OER electrocatalysis. Therefore, a well-defined pseudocapacitance is somehow a requirement for the OER performance. Almost all OER electrocatalysts reported in the literature display considerable pseudocapacitive behavior at the lower potentials. The corresponding pseudocapacitance explains the difference between the OER activities of IrO₂ and RuO₂. Here, the focus is on noble metals as the well-defined OER electrocatalysts, but the reasoning can be extended to other metal oxides. The electrochemical behaviors including the pseudocapacitance of almost all inexpensive OER electrocatalysts such as cobalt oxide, nickel oxide, etc. are similar to that of IrO₂ as described here. Moreover, despite the high cost, noble metals still have the practical potentials for OER, subject to further enhancement in the OER efficiency or as additives in inexpensive electrocatalysts.