Combined electrochemical/surface science investigations of Pt/Cr alloy electrodes

Abstract

Chromium addition improves the performance of carbon-supported Pt electrodes for oxygen reduction in phosphoric acid fuel cells. To clarify the role of chromium and its chemical nature at the electrode surface, we have performed a combined electrochemical/surface science investigation of a series of bulk PtxCr1−x alloys (0≤x≤1). In this paper we report the surface characterization of the starting electrodes by XPS, electrochemical results from cyclic voltammetry in 85% phosphoric acid, and postelectrochemical surface characterization. For Cr contents less than 40%, the electrodes were quite stable up to +1.6 V versus dynamic hydrogen electrode (DHE). The surface Cr was largely oxidized to Cr+3 for surfaces at open circuit and those exposed at potentials <+1.4 V. For intermediate Cr levels, Cr was leached from the surface region by +1.5 V, leaving a porous Pt electrode with increased electrochemical hydrogen adsorption capacity. For Pt0.2 Cr0.8, treatments at +1.4 V and above led to the appearance of Pt+4 and Cr+6 species, apparently stabilized in a porous phosphate overlayer up to 50 Å thick. The Pt electrochemical hydrogen adsorption capacity was simultaneously increased by a factor of 15.