Octahedral Nanocrystals of Ru‐Doped PtFeNiCuW/CNTs High‐Entropy Alloy: High Performance Toward pH‐Universal Hydrogen Evolution Reaction

Abstract Integrating high‐entropy philosophy and nanocrystal‐specific orientation into a single catalyst represents a promising strategy in development of high‐performance catalysts. Nonetheless, shape‐controlled synthesis of high‐entropy alloy (HEA) nanocrystals is challenging owing to the distinct redox potentials and growth dynamics of metal elements. Herein, a one‐pot co‐reduction method is developed to fabricate ruthenium (Ru)‐doped PtFeNiCuW octahedral HEA nanocrystals onto carbon nanotubes (Ru–PtFeNiCuW/CNTs). It is demonstrated that Ru dopants and W(CO)6 promote the concurrent reduction and growth of other metal precursors to obtain higher yield and larger size of HEA nanocrystals, despite low Ru content in Ru–PtFeNiCuW/CNTs. As an electrocatalyst toward hydrogen evolution reaction (HER), Ru–PtFeNiCuW/CNTs exhibits low overpotentials of 9, 16, and 34 mV at a current density of 10 mA cm−2 and Tafel slopes of 19.2, 27.9, and 23.1 mV dec−1 in acidic, alkaline, and neutral electrolytes, respectively. As a cathodic catalyst, Ru–PtFeNiCuW/CNTs operates for up to 1500 and 1200 h in acidic and alkaline electrolyte, respectively, at a current density of 50 mA cm−2 in a two‐electrode system for full water splitting. Theoretical calculations reveal accelerated kinetics of H2O dissociation on W sites and *H desorption on hollow Cu–Cu–Cu and Cu–Cu–Pt sites.