Oxygen Reduction on Sodium Tungsten Bronze Electrodes and the Effects of Adsorbed Platinum

Abstract

The Tafel slopes for oxygen reduction on sodium tungsten bronze electrodes are changed from -4RT/F to -2RT/F by adsorption of platinum from solution. Adsorbed platinum reduces the time to attain steady-state conditions but does not catalyse the reaction. Photo-excitation experiments show that the flat band potential is a function of C_H+. Results are interpreted in terms of a thin surface layer of low donor density which is non-degenerate in the potential range of oxygen reduction. It is shown that, without adsorbed platinum, changes in the equilibrium potential difference across the surface layer equal those across the Helmholtz layer due to the presence of hydrogen donors in the lattice and results in -4RT/F Tafel slopes. It is proposed that adsorbed platinum forms surface states pinning the Fermi level at the surface and causing changes in the potential difference to occur entirely across the Helmholtz layer; this results in -2RT/F slopes.