Reducing the pH dependence of hydrogen evolution kinetics via surface reactivity diversity in medium-entropy alloys†

Abstract

The water dissociation step of the hydrogen evolution reaction is a well-known pH-dependent process, which makes sustainable hydrogen production suffer from sluggish kinetics. Herein, we demonstrate a surface reactivity diversity approach to reduce the pH dependence of HER kinetics in medium-entropy alloys. Grand canonical potential based calculation, CO-oxidation and potential of zero charge results showed that shifts in the Fermi level in neutral electrolytes lead to stronger M–H bonding (M = Ni, Pt, etc.) compared to those in basic solutions. These pH-dependent binding energies disrupt the optimized adsorption strength of advanced alkaline HER catalysts. By introducing a combination of a high surface reactivity metal (Mo) and a low surface reactivity metal (Cu/Zn) into Ni alloys, this surface reactivity diversity approach can significantly accelerate HER kinetics and allows for favorable adsorption of hydrogen and hydroxyl species at different pH levels. The resulting NiCuMo medium-entropy alloy exhibited impressive HER performance, with an overpotential of 63 mV at a current density of 100 mA cm⁻² in alkaline electrolyte and 115 mV in neutral electrolyte. The intrinsic neutral HER activity of this NiCuMo is 3.65 times that of the benchmark alkaline HER catalyst. Furthermore, the NiCuMo-based membrane electrode assembly water electrolyzer can be stably operated for at least 200 h at a larger current density of 1.5 A cm⁻². This surface reactivity diversity approach presents a promising design framework for less pH-dependent electrocatalysis.