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

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 (FeSex) layer bridges the catalytically active NiSe2 layer and the NF substrate (FeSex-NiSe2/NF). The surface oxidation of NiSe2 results in the incorporation of SeOx 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 FeSex-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 FeSex-NiSe2/NF heterostructure, which is expected to foster the rational development of bifunctional electrocatalysts.