Impact of Heat Treatment on the Electrochemical Properties of Carbon-Supported Octahedral Pt-Ni Nanoparticles

Thermal annealing is commonly used to remove surface contaminants and redistribute elements in alloys. In this study, Pt-Ni alloy nanoparticles supported on carbon black are selected as a model catalyst to understand the relationship between the annealing conditions (temperature and atmosphere) and the electrocatalytic performance for oxygen reduction, hydrogen evolution, and ethanol oxidation reactions. The impacts of thermal treatment temperature and atmosphere on structures, compositions, and in turn electrocatalytic activities are systematically studied. Interestingly, an ultrathin carbon layer can be formed on the nanoparticle surface by heat treatment in Ar atmosphere at temperatures higher than 350 degrees C, which significantly decreases its activity toward oxygen reduction and ethanol oxidation reactions. This carbon coating, however, is absent in other atmospheres including N-2, air, 7% H-2/Ar, and vacuum. Aberration-corrected scanning transmission electron microscopic characterizations with atomic-level resolutions confirm the formation of a Ni-enriched surface on Pt-Ni/C after treatment in Ar, which plays a critical role in catalyzing the growth of stable carbon layers from the surrounding carbons. Further density functional theory calculation results suggest that the absence of a carbon layer in N-2 may originate from the stable N-C bond formed during heat treatment and passivation effect of adsorbed N-2. It illustrates different effects of inert gases on carbon layer formation by combining experimental and computational approaches. These results may shed light on the proper design of postheat treatment protocols for carbon-supported catalysts and may also provide a feasible method to coat carbon layers on nanoparticle surfaces for various energy storage and conversion applications.