Sulfur Vacancy‐Engineered Co ₉ S ₈ ‐Ni ₃ S ₄ Heterostructure as a Hydrogen Spillover Catalyst for Efficient Alkaline Water Splitting

Abstract Developing highly efficient and robust catalysts based on earth‐abundant materials for electrochemical water splitting remains a great challenge. Herein, we report the synthesis of a well‐defined hydrogen spillover electrocatalyst, i.e., sulfur vacancy‐enriched Co 9 S 8 ‐Ni 3 S 4 hollow heterostructure, via a self‐sacrificial template strategy. The introduction of sulfur vacancies greatly decreases the work function of Ni 3 S 4 , thereby narrowing the work function difference (Δϕ) with Co 9 S 8 . The reduced electron density at their interface facilities the hydrogen species (H * ) transfer to trigger hydrogen spillover. Density functional theory (DFT) calculations reveal that H 2 O molecules preferentially adsorb and dissociate at Co sites of Co 9 S 8 to generate active H * intermediates, which subsequently migrate to Ni sites of Ni 3 S 4 domains for H 2 formation. The hydrogen spillover mechanism is strongly supported by experimental characterizations, including pH‐dependent kinetics, in‐situ Raman and electrochemical impedance analysis. Benefiting from these synergistic effects, the titled catalyst exhibited excellent electrocatalytic activity for alkaline hydrogen evolution reaction, requiring only 83 mV to achieve 10 mA cm 2 , along with remarkable durability, showing no detectable degradation even at 1 A cm 2 for 100 h. This work deepens the fundamental understanding of hydrogen spillover mechanism and offers a practical strategy for developing highly active and durable catalysts for water splitting.