Elucidating the Hydrogen Selectivity of Pt/TiO_x/C as a Fuel-Cell Catalyst by Operando Near-Ambient-Pressure XPS

The long-term stability of proton exchange membrane fuel cells (PEMFCs) faces significant challenges, particularly during start-up and shut-down events, which lead to degradation of the cathode catalyst through the oxidation of its carbon support. To improve catalyst durability, an anode catalyst with a high selectivity toward the hydrogen oxidation/evolution reaction rather than the oxygen reduction reaction is necessary. Pt/TiO_x/C (x < 2) catalysts have been reported to provide excellent hydrogen selectivity due to its strong metal-support interaction (SMSI) between Pt particles and TiO_x support. To further elucidate the SMSI-induced effect of the catalyst, this study employs near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) at BESSY II with an upgraded operando cell, optimized for the use of membrane electrode assemblies (MEAs) for the first time. The electrochemical behavior of the operando cell is fully consistent with PEMFC measurements for both the standard Pt/C and investigated Pt/TiO_x/C catalysts. With NAP-XPS, the SMSI-induced effect is observed through a significant suppression of Pt oxidation at high potentials for Pt/TiO_x/C. A precise quantification of the oxidation charge from both electrochemical and NAP-XPS data evidently shows partial Pt oxidation for Pt/TiO_x/C, clearly originating from Pt deposited on carbon instead of TiO_x, as demonstrated by transmission electron microscopy. Nevertheless, the results reveal that barely any oxidation is expected for SMSI-based catalysts such as pure Pt/TiO_x/C. Cracks in the bilayer graphene used as an X-ray transparent window in the operando setup likely explain the lower absolute values in Pt oxidation obtained from NAP-XPS compared with the values from electrochemistry, still allowing valuable insights into the catalyst behavior.