Structure-Activity Relationship in PtCo L10 Ordered Phase ORR Catalysts: Pt-Rich Shell Having Anisotropic Lattice Distortion Revealed by PDF and XAS Analysis.

Ordered intermetallic PtCo alloys are promising candidates for next-generation low-Pt catalysts in proton exchange membrane fuel cells (PEMFCs) due to their high activity and stability originating from ligand and strain effects. However, the influence of the ordered phase on the surface structure, especially after Pt-rich shell formation, remains poorly understood. In this study, we systematically investigated the structural and electrochemical properties of PtCo catalysts with varying degrees of ordering, prepared by controlling the annealing temperature and time. We combined X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and pair distribution function (PDF) analysis to elucidate the correlation between the ordering degree, the atomic structure, and the electrochemical performance. For the first time, our detailed X-ray total scattering measurements revealed the true structural characteristics of PtCo alloys, indicating that the phase types and their relative contents vary significantly with the ordering degree. The ordered PtCo catalysts develop a Pt-rich surface layer with anisotropic strain, featuring contracted Pt-Pt distances along the surface and elongated distances across the surface, which likely contributes to its enhanced ORR activity and stability compared to their disordered counterparts. The electrochemical studies and PDF analysis suggested that the ordering transition occurs concurrently with particle growth, leading to an abrupt increase in the ORR activity at 350 °C before forming a long-range ordered phase, suggesting that local structural changes at the particle surface play a crucial role in enhancing the ORR activity. Further, XAS studies confirm lesser Pt-oxidation and Pt-OH formation for ordered structures than disordered ones. We believe that our findings provide new insights into the relationship between the ordering degree, particle growth, and catalytic properties of PtCo catalysts, offering guidance for the design of high-performance electrocatalysts with optimized surface structures.