Hierarchical Mixed-Phase Fe–Ni–S Nanoparticle/TiO2 Heterostructure As Catalyst For Alkaline Overall Water Splitting

Abstract

Transition metal sulfides are widely recognized as efficient electrocatalysts for the hydrogen evolution reaction, owing to their high metallic conductivity. However, their application in alkaline electrolytes is often constrained by sluggish water dissociation kinetics. Here we report a nanoscale hierarchical mixed-phase Fe–Ni–S/TiO2/NF heterostructure fabricated via a two-step hydrothermal method. The block-like TiO2 nanorod array functions as a structural framework that prevents agglomeration of mixed-phase Fe–Ni–S nanoparticles and ensures high nanoparticle dispersion of active sites. Electrochemical measurements show that the mixed-phase Fe–Ni–S/TiO2/NF electrode exhibits excellent bifunctional catalytic performance. Specifically, it requires an overpotential of only 29 mV to achieve a current density of 10 mA·cm–2 for hydrogen evolution in 1.0 M KOH, substantially outperforming pure mixed-phase Fe–Ni–S/NF. During oxygen evolution, the electrode undergoes rapid surface reconstruction, contributing to enhanced catalytic activity. An alkaline electrolyzer incorporating this catalyst delivers a current density of 50 mA·cm–2 at a low cell voltage of 1.8 V and demonstrates robust long-term stability. Experimental characterization and density functional theory calculations reveal that the enhanced performance originates from strong electronic interactions at the mixed-phase Fe–Ni–S/TiO2 interface, which modulate the electronic structure of active sites. The hydrophilic TiO2 component lowers the energy barrier for water dissociation, while the mixed-phase Fe–Ni–S surface promotes optimal hydrogen adsorption and desorption, together accelerating the overall water splitting kinetics.