Revealing the Role of Surface Composition on the Particle Mobility and Coalescence of Carbon-Supported Pt Alloy Fuel Cell Catalysts by In Situ Heating (S)TEM

Thermal annealing is an indispensable process during the preparation and structural ordering of Pt alloy fuel cell catalysts, which exhibit superior electrocatalytic activities as compared to Pt catalysts and thus enable decreased Pt usage. However, thermal annealing usually induces detrimental particle sintering, which greatly offsets the performance enhancement. Although the mechanisms of particle sintering of monometallic Pt catalysts have been well studied, knowledge on the key factors controlling the particle sintering of Pt alloy catalysts is still very poor. Herein, we perform in situ heating (scanning) transmission electron microscopy of carbon-supported low-Pt alloy catalysts (PtFe3, PtCo3, and PtNi3) and reveal that the surface composition plays a key role in both the particle mobility and the coalescence process of the supported low-Pt nanoparticles (NPs) under high temperatures. A surface enrichment of the less-noble transition metals not only induces a faster particle coalescence due to enhanced surface diffusion, but also causes a higher mobility of the NPs on the carbon support due to a strong chemical interaction between the less-noble transition metals and the carbon support. In contrast, the Pt richer surface results in a lower NP mobility as well as slower surface diffusion across contact NPs, which contributes to a higher antisintering capability. Our results suggest that controlling the surface composition, for example, by engineering the elemental growth kinetics during nanoparticle synthesis, is critical for controlling the particle sintering of Pt alloy catalysts during thermal annealing.