A Fractal-Tip Cu<sub>3</sub>Ni/NiMoO<sub>4</sub> Heterostructure for Efficient Hydrogen Evolution via an Accelerated Volmer-Tafel Mechanism.

Abstract

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.