Surface evolution of an amorphous iron selenide-bridged nickel selenide (NiSe₂) heterointerface enabling robust...

Constructing highly active and robust bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) is crucial for overall water splitting. Herein, a hierarchical heterostructure is designed via a simple two-step electrodeposition process, in which an amorphous iron selenide (FeSe_x) layer bridges the catalytically active NiSe₂ layer and the NF substrate (FeSe_x-NiSe₂/NF). The surface oxidation of NiSe₂ results in the incorporation of SeO_x species. The resulting NiSe₂ + SeO₄ surface provides effective active sites, thereby kinetically favoring the HER process. During OER, the surface NiSe₂ reconstructs into a γ-NiOOH phase, which serves as the catalytically active phase for OER. As a consequence, the FeSe_x-NiSe₂/NF electrode requires only 146 mV (HER) and 340 mV (OER) to deliver a current density of 500 mA cm^-2, while maintaining stable operation at this current density for 100 h. These findings offer insights into the superior catalytic performance of the hierarchical FeSe_x-NiSe₂/NF heterostructure, which is expected to foster the rational development of bifunctional electrocatalysts.