CHARACTERIZATION OF ELECTROCATALYSIS IN THE OXYGEN EVOLUTION REACTION AT PLATINUM BY EVALUATION OF BEHAVIOR OF SURFACE INTERMEDIATE STATES AT THE OXIDE FILM

The anodic 02 evolution reaction (OER) at noble-metal anodes provides an important case of electrocatalysis where the electrode surface on which the reaction proceeds depends on potential, through the state of the surface of a thin oxide film developed upon it. An essential but little examined aspect of electrocatalysis is the behavior of the kinetically involved intermediates of the reaction at the electrode's surface in relation to the potential dependence of the electrode reaction rate, characterized by the Tafel slope. Digitally recorded potential relaxation transients following current interruption are used, as in our previous studies on cathodic H2 evolution, to evaluate the pseudocapacitance (CJ of intermediates generated in the steady state of the OER at Pt and hence the potential dependence of coverage by the kinetically significant intermediate states in the reaction. In the case of the OER proceeding as it always does on an oxidized surface of the metal, the intermediate states are not only OH and 0 species but probably also two or more oxidation states of Pt ions in the oxide film acting as a mediator couple in the 02 evolution kinetics. The involvement of such a couple may influence electron transfer through the oxide film. The derived is found to be logarithmically related to potential over two distinguishable ranges, corresponding to the observed changes of the Tafel slope of the OER. The slopes of these log C,, vs potential plots are, however, unexpectedly small, a result that may originate from strong lateral interaction effects or partial charge transfer in the deposition of the intermediates. © 1990, American Chemical Society. All rights reserved.