Anodic Oxide Films as Barriers to Charge Transfer in O 2 Evolution at Pt in Acid Solutions

The kinetics of oxygen evolution reaction at platinum anodes in acid solutions with different p H's is examined, and possible reaction mechanisms are discussed. The activity of the reaction at initially prereduced electrodes brought to a constant potential, or kept at a constant current density, decreases with time. Only at the electrodes initially subjected for a given time either to a high potential, or to a high anodic current density, it is possible to observe in "run down," or subsequent "run up" experiments a linear relationship for over several decades of current density with a slope close to 115 mV. The positions, but not the slopes of Tafel lines, are affected by time or conditions of pretreatments of electrodes (various or ). The decrease in the activity is attributed to the increase in average thickness of anodically formed surface oxide films. Only when it is assured that the film thickness remains practically unaltered during the measurements, meaningful kinetic parameters for the oxygen evolution reaction are obtained. The reaction order with respect to H + is then found to be negative and fractional according to the over‐all rate equation A model is suggested according to which a surface oxide film forms a barrier to charge transfer in series with the double‐layer barrier. The fractional reaction order is accounted for with this model. A chemical step that follows a charge transfer step is rate determining. It is suggested that oxygen atoms in the surface oxide films participate in the oxygen evolution reaction.