Electrocatalysis of Oxygen Reduction by Sodium Tungsten Bronze

When a metallic crystal of the nonstoichiometric compound sodium tungsten bronze, , was used as an oxygen electrode in acidic solution, sodium was lost from the crystal by anodic dissolution, leaving a thin semiconducting layer on the surface. The composition change in the surface layer corresponded to a decrease in x from ∼0.7 to ∼0.25 for a crystal free of intentional impurities, and from ∼0.7 to ∼0.05 for a crystal containing traces of platinum. The difference may result from more rapid hole injection in the latter case, associated with the enhanced rate of oxygen reduction observed. The response of the bronze electrode to light revealed a bandgap of ∼2 eV and a flatband potential of 0.5–0.6V vs. RHE for the semiconducting layer, both apparently independent of platinum content. Donor concentrations in the surface layer determined from (Mott‐Schottky) plots were in good agreement with sodium concentrations determined by ion probe mass spectroscopy.