Size‐adjustable High‐Entropy Alloy Nanoparticles as an Efficient Platform for Electrocatalysis

Abstract The high entropy alloy (HEA) possesses distinctive thermal stability and electronic characteristics, which exhibits substantial potential for diverse applications in electrocatalytic reactions. The nanosize of HEA also has a significant impact on its catalytic performance. However, accurately controlling nanosize and synthesizing small HEA nanomaterials remains a challenge, especially for the ultrasmall HEA nanoparticles. Herein, we firstly calculate and illustrate the impact of size on the electronic structure of HEA as well as the adsorption energies of crucial intermediates involved in typical electrocatalytic processes, such as the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), CO 2 electroreduction (CO 2 RR) and NO 3 − electroreduction (NO 3 RR). Under the guidance of theoretical calculations, we synthesize a range of ultrasmall PtRuPdCoNi HEA nanoparticles with adjustable sizes (1.7, 2.3, 3.0, and 3.9 nm) using a one‐step spatially confined approach, without any further treatment. Experimentally, the smaller size of HEAs is more favorable for the HER and ORR performances, aligning well with theoretical predictions. Specifically, HEA nanoparticles sized at 1.7 nm (HEA‐1.7) endows a 16 mV overpotential at current density of 10 mA cm −2 , yielding a mass activity of 31.9 A mg NM −1 of noble metal in HER, significantly outperforming commercial Pt/C catalyst. This strategy can also be easily applicable to other reduction reactions (e.g. CO 2 , NO 3 − ) attributed to the richness of metal components and size adjustability, presenting a promising platform for various electrocatalytic applications as advanced catalysts.