Oxygen reduction at Pt-Au and Pd-Au alloy electrodes in acid solution

Abstract

The kinetics of oxygen reduction is examined at Pt, Pd, Au, three Pt-Au and four Pd-Au alloy electrodes in acid solution. At Pt, Pd, Pt-rich Pt-Au, and Pd-rich Pd-Au alloys, Tafel relationships with ∂V/∂ ln i ≈ -RT/F are observed in the range of current densities from ∼10−4 to∼10−6A/ cm2. At gold and gold-rich alloys the slope is close to −2 RT/F. The change of the slope from -RT/F to −2RT/F ocuurs at alloy compositions with more than 50 at-% Au. The exchange current density for the alloys rich in transition metals changes exponentially with alloy composition. It decreases as Au content in alloys increases.It is suggested that the change in the mechanism, as indicated by the change of Tafel slope from -RT/F to −2RT/F, is related to the electronic structure of these alloys. At about 60 at-% or more of Au in these alloys, there are no longer unpaired d-electrons, and the mechanism for the oxygen reduction changes. The linear change in log (activity) at a given potential with atomic per cent of Au in Pd-rich alloys is accounted for in terms of the change of heat of adsorption of reaction intermediates with alloy composition.