Surface Platinum Electrooxidation in the Presence of Oxygen

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Abstract

Understanding the influences of Pt surface oxides is crucial for elucidating the oxygen reduction reaction (ORR) in fuel cells, particularly with an eye toward the design of next-generation Pt-based electrocatalysts with improved activity. Although gaseous O2 is always present during the ORR, the majority of previous Pt surface oxide studies were conducted in the absence of O2 due to experimental limitations. In this study, multiple in situ techniques were applied to study the ORR on platinum in the presence of O2. The thin channel flow Pt electrode and the electrochemical quartz microbalance (EQCM) techniques on Pt polycrystalline electrodes suggest that the influence of O2 in the electrolyte on mass change and charge transfer is negligible. The oxide formation followed a logarithm-growth behavior initiating as early as 0.2 s after potential steps. This suggests that no slow conversion of oxide species (e.g., between OHads and Oads) takes place. Despite the negligible effect of O2 on the measured oxide coverage, steady-state X-ray absorption spectroscopy (XAS) measurements conducted on dispersed Pt nanoparticles suggested that, only when under O2-sparging, place exchange between adsorbed oxide(s) and the Pt surface layer(s) initiated at potentials as low as 0.75 V. This is significantly lower than that observed in an O2-free electrolyte (>1.1 V). The effects of O2 on adsorbed oxide species illustrate the complexities of applying model systems to the real world in order to arrive at a comprehensive description of both the oxygen reduction and the Pt dissolution processes.

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