A Study of the Anomalous pH Dependence of the Oxygen Evolution Reaction at Platinum Electrodes in Acid Solutions

Steady‐state relationships have been determined for the oxygen evolution reaction at platinum electrodes in acid solutions of various pH's. At all pH's, Tafel slopes close to have been obtained after initially cathodically pretreating the electrode, polarizing it at a high anodic current density for a particular length of time, and then measuring the relationships at current densities lower than that used in the initial anodic electrode pretreatment. The order of the reaction with respect to hydrogen ions is then found to be one‐half. In other experiments, a rotating Pt ring disk electrode was used in solutions of various pH's in order to separate the currents due to Pt oxide film growth and those due to the oxygen evolution reaction and to accurately determine the charge density utilized for film growth. From this, it could be seen that the potential, at any constant charge density and pH, increases 120 mV for a tenfold increase of the current density, and that at a given current density and charge density, the potential decreases 60 mV as the pH increases one unit. These results confirm that the unusual fractional reaction order with respect to hydrogen ions is still obtained when the thickness of the oxide film remains constant. Further, it is reported here that the dependence of the oxygen evolution rates on pH can be attributed entirely to the dependence on the pH of the potential difference across the outer Helmholtz layer at the oxide film/solution interface. This simultaneous study of oxide film growth and the oxygen evolution reaction has led to a model for the potential distribution across the metal/oxide film/IHL/OHL/solution interface during the oxygen evolution reaction and to the conclusion that a fast quasi‐equilibrium process exists across the outer Helmholtz layer.