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 N2- and O2-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 (Oad), hydroxyl (OHad), and the others were water molecules (H2Oad), respectively. We found distinctive potential dependences of their coverage in the solution with/without O2 between both catalysts. The Pt skin layer exhibited a higher affinity to Oad but less to H2O compared to pure Pt, particularly in the O2-saturated solution. The higher coverage of Oad 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 H2Oad was considerably lowered at the Pt skin layer in both N2- and O2-saturated solution, which can be correlated to preferential adsorption of O2 and the increased coverage of Oad at the Pt skin layer.