Transformation of carbon-supported Pt-Ni octahedral electrocatalysts into cubes: toward stable electrocatalysis

Octahedral Pt-Ni catalyst nanoparticles (NPs) are predicted to exhibit high activity for the oxygen reduction reaction. However, until now this class of catalysts has been limited by its long-term performance, as a result of compositional and morphological instabilities of the NPs. In situ transmission electron microscopy (TEM) is a powerful technique for understanding morphological and compositional evolution under controlled conditions. It is of great importance to study the evolution of the morphology and elemental distribution in bimetallic NPs and their interaction with the support in reducing and oxidizing treatments at the atomic scale for the rational design of catalysts. Here, we use in situ TEM to follow dynamic changes in the NP morphology, faceting and elemental segregation under working conditions in previously unreported Pt-Ni core-shell octahedral structures. We follow changes in the Pt-Ni catalyst from a segregated structure to an alloyed shell configuration and then a more spherical structure as a function of temperature under reducing conditions. Exposure to an oxidizing environment then leads to oxidation of the C support, while the spherical NPs undergo a cycle of transformations into cubic NPs followed by the reaction to spherical NPs. The formation of the cubic NPs results from CO formation during C oxidation, before it is finally oxidized to CO₂. Our observations may pave the way towards the design of optimized structure-stability electrocatalysts and highlight the importance of TEM visualization of degradation and transformation pathways in bimetallic Pt-Ni NPs under reducing and oxidizing conditions.