Synthesis of 4H-phase high-entropy alloys for electrocatalysis.

High-entropy alloy (HEA) nanomaterials are promising catalysts for proton exchange membrane water electrolysers (PEMWE), yet their crystalline structures have typically been restricted to thermodynamically stable phases. Here, using Au nanomaterials with distinct crystal phases as templates, we synthesize and stabilize Au@HEA core-shell nanostructures through a general and robust wet-chemical method in which the HEA is composed of up to ten metallic elements (Ir, Pt, Ni, Fe, Co, Rh, Pd, Ru, Cu and Mn). Phase-dependent water electrolysis is demonstrated as a proof-of-concept application. The hexagonal close-packed 4H-Au@4H-IrPtNiFeCo catalyst exhibits superior activity and stability for the acidic hydrogen evolution reaction, oxygen evolution reaction and overall water electrolysis compared with the conventional face-centred cubic IrPtNiFeCo catalyst. In a PEMWE at 60 °C, the 4H-Au@4H-IrPtNiFeCo catalyst achieves 3,000 mA cm-2 at only 1.90 V and maintains stable operation for over 1,200 h at 1,000 and 2,000 mA cm-2, with degradation rates of ~6.3 and ~15.7 µV h-1, respectively. This work offers a strategy for designing highly efficient and stable HEA catalysts with tailored phases for future practical water electrolysis.