Hydrocarbothermal flow synthesis of carbon-supported small and dense high-entropy alloy nanoparticles as electrocatalysts.

Supported small and dense high-entropy-alloy nanoparticles (HEA-NPs) are promising functional materials for many applications. However, their synthesis remains a grand challenge because the extreme heating typically required to raise the entropic contribution to the formation of a solid solution unavoidably causes the sintering of HEA-NPs. Herein, we present a one-step continuous-flow spray pyrolysis strategy to synthesize multicomponent (from quinary to denary) HEA-NPs with an average size of <2 nanometers and metal loadings of ~30 wt% uniformly dispersed on various carbon substrates, including graphene and carbon black. In addition, this flow-type synthesis is featured with notable advantages in facile gram scalability and reproducible production when compared with the existing methods. We identify a hydrocarbothermal synthesis mechanism that in situ generates H via the reaction between carbon and water in the aerosol droplets to realize the complete metal reduction and alloying at a reduced temperature. The relatively mild reaction condition, combined with the short heating duration and the strong metal-support interaction, enables the simultaneous achievements of small size and high loading in HEA-NPs. The quinary FeCoNiCuPt HEA-NPs are demonstrated as highly efficient electrocatalysts toward the oxygen reduction and hydrogen evolution reactions.