A Fractal-Tip Cu₃Ni/NiMoO₄ Heterostructure for Efficient Hydrogen Evolution via an Accelerated Volmer–Tafel Mechanism

Achieving hydrogen evolution at industrial current densities requires optimized kinetics involving electron transfer, catalytic reactions, and mass transport. Here we report a Cu3Ni alloy and amorphous NiMoO4 heterostructure featuring a Romanesco-like fractal-tip architecture grown on a copper mesh. This cathode exhibits outstanding catalytic activity, requiring low overpotentials of 144.0 and 122.5 mV to reach 1 A cm-2 in 1 M KOH and 100 mA cm-2 in 1 M PBS, respectively, along with a Tafel slope as low as 27.2 mV dec-1 in alkaline media. It also maintains long-term durability over 3000 h at 1 A cm-2 with negligible degradation. At the Cu3Ni/NiMoO4 interface, the distinct chemical environments of Ni atoms result in hydrogen affinity in Cu3Ni and oxygen affinity in NiMoO4, promoting water dissociation (H*-*OH). Experimental results and theoretical calculations reveal that the interface lowers the energy barrier for water dissociation and increases H* coverage, shifting the reaction mechanism from the Volmer-Heyrovsky pathway to the more efficient Volmer-Tafel route. Furthermore, the fractal-tip structure boosts HER kinetics by amplifying local electric fields, concentrating protons, and accelerating bubble release. This work provides a rational design strategy for improving hydrogen evolution kinetics through combined structural and interfacial optimization.