The role of surface films in the kinetics of oxygen evolution at Pd + Au alloy electrodes

Abstract

The kinetics of anodic oxygen evolution have been studied on Pd, Au and Pd + Au alloys in ultra-pure sulphuric acid and potassium hydroxide solutions and mechanisms of the processes involved are proposed. The role of barrier-layer films in determining the kinetic behaviour is demonstrated and supported by self-consistent interpretations of the Tafel slopes and the e.m.f. decay behaviour observed on open-circuit. Transient effects, characteristic of barrier-layer films, are observed when changes of the polarization current are made. At Au, and the gold-rich alloys in alkaline solution, a transition region in the currentpotential relation is observed which probably corresponds to a limiting high coverage of the electrochemically active surface with adsorbed reaction intermediates. The transition region corresponds to an onset of passive behaviour. In acid solutions, only the lower Tafel region is observed which has the same Tafel constants b and i 0 as those for the lower Tafel line in alkaline solutions indicating that the discharge process involves water rather than OH - ions, irrespective of pH. The relation of exchange currents to composition of the alloys is considered.