Electrocatalytic activity and surface properties of tungsten bronzes

Abstract

The electrocatalytic activities of sodium tungsten bronzes, including high purity crystals, platinum doped crystals, and platinum plated crystals, have been measured for oxygen reduction in acid solution. In addition, a survey of the electrocatalytic activities and general electrochemical properties of other alkali tungsten bronzes, thalium tungsten bronze, and tungsten trioxide were investigated and compared to sodium tungsten bronze. All measurements were done on single crystals. Pure sodium tungsten bronzes and WO/sub 3/ have a slight catalytic activity for oxygen reduction. The exchange current density is approximately 10/sup -14/ A/cm/sup 2/. Doping the cubic sodium tungsten bronze with up to 800 ppM of platinum slightly increased the catalytic activity of the crystals, but the effect was noticeable only at very low current densities. Platinum preelectrolysis of the solution was shown to contaminate the crystal surface with significant amounts of platinum. For the platinum plated bronze crystals, no synergistic effect between the platinum and the bronze was observed for oxygen reduction. However, different platinum plating methods gave more than an order of magnitude difference in catalytic activity, with the same amount of platinum. The platinum was deposited on the bronze surface in different forms by the different plating methods. One possible form of highly dispersed platinum on a bronze surface is the formation of a platinum tungsten bronze, Pt/sub x/WO/sub 3/. Hydrogen tungsten bronze is formed in the surface layers of all the bronzes at potentials below +0.2 V (NHE), and the reaction is completely reversible. One possible form of highly dispersed platinum on a bronze surface is the formation of a platinum tungsten bronze, Pt/sub x/WO/sub 3/. Hydrogen tungsten bronze is formed in the surface layers of all the bronzes at potentials below +0.2 V (NHE), and the reaction is completely reversible.