Carbon-supported low-Pt ordered intermetallic nanoparticulate catalysts (PtM 3 , M = Fe, Co, and Ni) are explored in order to enhance the oxygen reduction reaction (ORR) activity while achieving a high stability compared to previously reported Pt-richer ordered intermetallics (Pt 3 M and PtM) and low-Pt disordered alloy catalysts. Upon high-temperature thermal annealing, ordered PtCo 3 intermetallic nanoparticles are successfully prepared with minimum particle sintering. In contrast, the PtFe 3 catalyst, despite the formation of ordered structure, suffers from obvious particle sintering and detrimental metal-support interaction, while the PtNi 3 catalyst shows no structural ordering transition at all but significant particle sintering. The ordered PtCo 3 catalyst exhibits durably thin Pt shells with a uniform thickness below 0.6 nm (corresponding to 2-3 Pt atomic layers) and a high Co content inside the nanoparticles after 10 000 potential cycling, leading to a durably compressive Pt surface and thereby both high activity (fivefold vs a commercial Pt catalyst and 1.7-fold vs an ordered PtCo intermetallic catalyst) and high durability (5 mV loss in half-wave potential and 9% drop in mass activity). These results provide a new strategy toward highly active and durable ORR electrocatalysts by rational development of low-Pt ordered intermetallics.

Metal dissolution and redeposition are considered to be the most important degradation mechanism for Pt-based fuel cell electrocatalysts. Understanding key factors mitigating the dissolution and migration under realistic proton-exchange-membrane (PEM) fuel cells is crucial for improving their performance and durability. Using ordered and disordered PtCo electrocatalysts, we address how structural ordering and particle size can affect the dissolution of Co/Pt and their redeposition into the membrane upon catalyst accelerated durability test in PEM fuel cells by statistical scanning transmission electron microscopy (STEM) and spectroscopic analysis. Consistent with the improved performance and durability, we observe that both Co and Pt dissolution were mitigated in the ordered PtCo catalyst compared to the disordered one. The suppressed Pt dissolution was evidenced from the relieved particle coarsening and significantly suppressed Pt redeposition/migration in the membrane after the durability test. Moreover, we reveal an optimum particle size range between 2–5 nm for ordered PtCo catalysts, which favors the highest structural ordering degree and hence the highest retention of Co. These results provide a rationale for implementing ordered Pt intermetallic electrocatalysts in PEM fuel cells and further particle size optimization for improved durability.

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 (PtFe 3 , PtCo 3 , and PtNi 3 ) 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.