Effect of Preparation Conditions of Pt Alloys on Their Electronic, Structural, and Electrocatalytic Activities for Oxygen Reduction-XRD, XAS, and Electrochemical Studies

The effect of different alloying conditions (alloying temperature, annealing period) on the electrocatalytic activities for the oxygen reduction reaction (ORR) by three carbon-supported Pt alloy electrocatalysts (WCr, WCo, m i ) was investigated and correlated with electronic and structural parameters determined by in-situ XAS. The results indicate that all the Pt alloys show enhanced ORR activities relative to a W C electrocatalyst. However, the electrocatalytic activity and activation energy for ORR in the case of Pt/Ni and Pt/Co alloys show marked effect due to different alloying conditions. This was in contrast to W C r alloy, where both parameters remained unchanged over the range of alloying conditions. Those electrochemical results were correlated with those obtained from in-situ X-ray absorption spectroscopic (XAS) investigations, which provided information on the electronic (Pt 5d-orbital vacancy, from the X-ray absorption near-edge structure) and geometric (Pt-Pt bond distances, from the extended X-ray absorption fine structure) factors. In-situ XAS results indicate that the supported alloys possess higher Pt Sd-orbital vacancies and shorter Pt-Pt bond distances. In addition, the XAS results showed that alloying inhibited chemisorption of oxygenated species (OH) on the Pt at potentials above 0.8 V vs RHE. Correlation of electrocatalytic activities and activation energies for ORR with parameters obtained from in-situ XAS studies indicates that, in the case of mi and Pt/Co alloys, higher alloying temperature and longer annealing periods result in higher Pt 5d-orbital vacancies with the geometric parameters remaining unchanged. The W C r alloy on the other hand revealed no dependence of either the Pt d-orbital vacancies or the geometric parameters on alloying temperature. These observations indicate that the dependence of electrocatalytic activities and activation energy for Pt/Co and Pt/Ni alloys on the thermal history and the absence of such an effect in the W C r alloy could be related to the differences in the Pt Sd-orbital vacancies.