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 (OHad), and the others were water molecules (H2Oad), 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 Oad but less to H2O compared to pure Pt, particularly in the O-2-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 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.