Electrocatalytic Activity of Cubic Sodium Tungsten Bronze: I. Effects of Platinum Doping, Anodization, and Platinum Pre‐Electrolysis of the Electrolyte

Abstract

The electrocatalytic activity of cubic sodium, tungsten bronzes, including high purity and platinum‐doped crystals, has been measured for oxygen reduction before and after anodization in both fresh and platinum pre‐electrolyzed acid solutions. The effects of platinum doping, anodization, and platinum pre‐electrolysis have been separately studied for current densities between 10−8 and 10−3 A/cm2. A previously unreported break in the Tafel curve at was observed for all clean undoped bronze samples. Below this break the Tafel slope was −110 mV/decade while above it the Tafel slope was −200 mV/decade. Anodization of a pure bronze electrode failed to produce any change in the Tafel curve. Platinum pre‐electrolysis of an anodized, undoped bronze produced a general increase in activity for all current densities. The break in the Tafel curve increased to while the Tafel slopes above and below this current density were unchanged. Platinum doping to 130 ppm increased the electrode activity at current densities below, even when measured in platinum‐free solutions before anodization of the electrode. The increased activity of an unanodized electrode in fresh solutions could be accounted for by the surface platinum concentration expected from the bulk platinum doping. Anodization of a doped bronze electrode increased the limiting current from even in fresh solutions while subsequent platinum pre‐electrolysis increased the limiting current to . Auger analysis of the anodized, platinum‐doped electrode exposed to the platinum pre‐electrolyzed solution revealed platinum on the surface of the bronze electrode at a coverage sufficient to account for the increased electrode activity below . A small but reproducible increase in activity was found at high current densities near 10−3 A/cm2 after either a pure or platinum‐doped bronze electrode was exposed to a platinum pre‐electrolyzed solution that lead to the research reported in the following paper (II).