Rate enhancements in structural transformations of Pt-Co and Pt-Ni bimetallic cathode catalysts in polymer electrolyte fuel cells studied by in situ time-resolved X-ray absorption fine structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt₃Co/C, and Pt₃Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mg_metal cm^-2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt₃M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt-O bond formation/breaking, and Pt-Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt-O bond breaking, and Pt-Pt bond re-forming in the reductive process from 1.0 to 0.4 V.