Thermal Facet Healing of Concave Octahedral Pt–Ni Nanoparticles Imaged in Situ at the Atomic Scale: Implications for the Rational Synthesis of Durable High-Performance ORR Electrocatalysts

We performed in situ transmission electron microscopy of phase-segregated octahedral Pt–Ni alloy fuel cell nanocatalysts under thermal annealing to study their morphological stability and surface compositional evolution. The pristine octahedral Pt–Ni nanoparticles (NPs) showed Pt-rich corners/edges and slightly concave Ni-rich {111} facets. Time-resolved image series unequivocally revealed that upon annealing up to 500 °C, the Pt-rich surface atoms at the corners/edges diffused onto and subsequently covered the concave Ni-rich {111} surfaces, leading to perfectly flat Pt-rich {111} surfaces with Ni-rich subsurface layers. This was further corroborated by in situ aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy. Our results propose a feasible approach to construct shaped Pt alloy nanoparticles with Pt-rich {111} surfaces and Ni-rich subsurface layers that are expected to be catalytically active and stable for the oxygen reduction reaction, thus providing important implications for rational synthesis of durably highly active shaped Pt alloy fuel cell electrocatalysts.