Construction of hierarchical FeNi₃@(Fe, Ni)S₂ core-shell heterojunctions for advanced oxygen evolution

The investigation of earth-abundant electrocatalysts for efficient water electrolysis is of central importance in renewable energy system, which is currently impeded by the large overpotential of oxygen evolution reaction (OER). NiFe sulfides show promising OER activity but are troubled by their low intrinsic conductivities. Herein, we demonstrate the construction of the porous core-shell heterojunctions of FeNi₃@(Fe, Ni)S₂ with tunable shell thickness via the reduction of hierarchical NiFe(OH)_x nanosheets followed by a partial sulfidization. The conductive FeNi₃ core provides the highway for electron transport, and the (Fe, Ni)S₂ shell offers the exposed surface for in situ generation of S-doped NiFe-oxyhydroxides with high intrinsic OER activity, which is supported by the combined experimental and theoretical studies. In addition, the porous hierarchical morphology favors the electrolyte access and O₂ liberation. Consequently, the optimized catalyst achieves an excellent OER performance with a low overpotential of 288 mV at 100 mA·cm^-2, a small Tafel slope of 48 mV·dec^-1, and a high OER durability for at least 1, 200 h at 200 mA·cm^-2. This study provides an effective way to explore the advanced earth-abundant OER electrocatalysts by constructing the heterojunctions between metal and corresponding metal-compounds via the convenient post treatment, such as nitridation and sulfidization.