Increased Oxygen Coverage at Pt−Fe Alloy Cathode for the Enhanced Oxygen Reduction Reaction Studied by EC−XPS

Direct observation of adsorbed species relating to the oxygen reduction reaction (ORR) on cathode catalyst surfaces is very important for an elucidation of the ORR and finding a clue of higher-performance catalysts for polymer electrolyte fuel cells. We first identified quantitatively oxygen-containing species adsorbed on electrodes of pure Pt and a Pt skin layer formed on a Pt−Fe alloy surface in N 2 - and O 2 -saturated 0.1 M HF solution by X-ray photoelectron spectroscopy combined with an electrochemical cell (EC−XPS). Three types of species were distinguished with binding energies at 529.6, 530.5, 531.1, and 532.4 eV, the first two of which were assigned to atomic oxygen (O ad ), hydroxyl (OH ad ), and the others were water molecules (H 2 O ad ), respectively. We found distinctive potential dependences of their coverage in the solution with/without O 2 between both catalysts. The Pt skin layer exhibited a higher affinity to O ad but less to H 2 O compared to pure Pt, particularly in the O 2 -saturated solution. The higher coverage of O ad was ascribed to the higher ORR activity at Pt skin/Pt−Fe alloy electrode than that at pure Pt. It was also found that the coverage of H 2 O ad was considerably lowered at the Pt skin layer in both N 2 - and O 2 -saturated solution, which can be correlated to preferential adsorption of O 2 and the increased coverage of O ad at the Pt skin layer.