Atomically Mixed High‐Entropy‐Alloy Nanoframes with 3D Subnanometer‐Thick Electrocatalytic Surfaces

Abstract High‐entropy‐alloy (HEA) nanocrystals, characterized by multicomponent solid solutions and synergistic effects, hold great potential in catalysis. However, synthesizing HEA hollow nanocrystals with 3D architectures and controlled surface atomic arrangements to enhance catalytic activity and durability remains challenging. Highly active and durable electrocatalysts are presented, derived from Pd@HEA core‐shell nanocrystals featuring a few HEA atomic layers containing five platinum‐group metals, synthesized via a Fe(III)‐based wet etching strategy. The controlled etching process transformed Pd@HEA core‐shell nanocubes enclosed by {100} facets into Pd@HEA porous nanocubes, HEA cubic nanocages, and eventually, HEA cubic nanoframes composed of subnanometer‐thick ridges dominated by {110} facets, vacancies, and step atoms. Electron microscopy and synchrotron X‐ray absorption spectroscopy revealed the randomly mixed coordination environments of the constituent elements, underscoring the excellent atomic mixing within the nanoframes. These nanoframes demonstrated a 9.72‐fold higher acidic hydrogen evolution reaction (HER) specific activity at an overpotential of −0.1 V than commercial Pt/C catalysts, remarkable durability after 15 000 potential cycles, and competitive performance as cathode catalysts for practical applications in proton exchange membrane water electrolyzers. Density functional theory calculations attributed the superior HER performance to {110}‐enclosed atomically mixed surfaces and low‐coordination sites, optimizing hydrogen adsorption free energy (ΔG _H* ) near the ideal value of 0.