Composition-Structure-Activity Relationships for Palladium-Alloyed Nanocatalysts in Oxygen Reduction Reaction: An Ex-Situ/In-Situ High Energy X-ray Diffraction Study

Understanding how the composition and atomic-scale structure of a nanocatalyst changes when it is operated under realistic oxygen reduction reaction (ORR) conditions is essential for enabling the design and preparation of active and robust catalysts in proton exchange membrane fuel cells (PEMFCs). This report describes a study of palladium-alloyed electrocatalysts (PdNi) with different bimetallic compositions, aiming at establishing the relationship between catalyst's composition, atomic structure, and activity for ORR taking place at the cathode of an operating PEMFC. Ex-situ and in-situ synchrotron high-energy X-ray diffraction (HE-XRD) coupled to atomic pair distribution function (PDF) analysis are employed to probe the structural evolution of the catalysts under PEMFC operation conditions. The study reveals an intriguing composition-activity synergy manifested by its strong dependence on the fuel cell operation induced leaching process of base metals from the catalysts. In particular, the synergy sustains during electrochemical potential cycling in the ORR operation potential window. The alloy with Pd:Ni ratio of 50:50 atomic ratio is shown to exhibit the highest possible surface Pd-Pd and Pd-Ni coordination numbers, near which an activity is observed. The analysis of the Ni-leaching process in terms of atomic-scale structure evolution sheds further light on the activity-composition-structure correlation. The results not only show a sustainable alloy characteristic upon leaching of Ni consistent with catalytic synergy but also reveal a persistent fluctuation pattern of interatomic distances along with an atomic-level reconstruction under the ORR and fuel cell operation conditions. The understanding of this type of interatomic distance fluctuation in the catalysts in correlation with the base metal leaching and realloying mechanisms under the electrocatalytic operation conditions may have important implications in the design and preparation of catalysts with controlled activity and stability.