Oxygen Evolution at Platinum Electrodes in Alkaline Solutions: II . Mechanism of the Reaction

Abstract

Kinetic data of the oxygen evolution reaction at Pt electrodes in alkaline solutions have shown two types of behavior, a Tafel slope of 60 mV/decade of current density at low applied current densities and a slope of 120 mV/decade at high current densities. At low current densities, and rate of the reaction was found to be independent of the thickness of the underlying Pt oxide film, while the electrode potential has a −120 mV dependence on solution pH. At high current densities, the rate is strongly dependent on film thickness and exhibits a −180 mV dependence on pH. A mechanism of the oxygen evolution reaction which is consistent with this data is presented. It involves a rate‐determining first electron transfer step at high current densities. At low current densities, the chemical step following the now rapid first electron transfer step is rate limiting. The −180 mV pH dependence observed at high current densities implies a fractional reaction order of with respect to the OH− ion. This has been explained in terms of a dual barrier model of the metal oxide film/solution interface. According to this model, the rates across each barrier are pH dependent. The fractional reaction order is due to the existence of these two dependences and is primarily related to the dependence of the potential difference across the outer Helmholtz layer on pH.