Identification of Catalyst Structure during the Hydrogen Oxidation Reaction in an Operating PEM Fuel Cell

Palladium is among the most active catalysts for the hydrogen oxidation reaction (HOR) and is thus a potential candidate for replacing platinum in fuel cell catalysis. At the same time, it is well-known to absorb large amounts of hydrogen, forming a bulk hydride phase. In several electrochemical studies conducted in liquid electrolytes and temperatures between 60 and 20 °C, the hydrogen from the hydride phase was observed to desorb at potentials positive of ∼32 to 50 mV vs the reversible hydrogen electrode (RHE). Here, we present operando spectroscopic studies in a fuel cell configuration. We first validate our experimental setup by comparing the potential dependence of hydrogen absorption into a Pd/C catalyst under nitrogen determined both by electrochemical means and by operando X-ray absorption spectroscopy (XAS) at various temperatures between 20 and 100 °C. Subsequently, we investigate the structure of the Pd/C catalyst during the HOR in a fuel cell operating at 80 °C in a H₂-pump configuration. Our results unequivocally show that, in contrast to rotating-disk-electrode (RDE) data reported in the literature, the hydride phase is maintained during the HOR in a fuel cell anode environment. The discrepancy between our results and previously published data is explained in terms of the vastly different mass-transport limitations in a fuel cell and in a conventional liquid electrolyte based electrochemical cell and highlights the importance of investigating catalyst structure in a representative reaction environment.