Sulfur Vacancy‐Engineered Co9S8‐Ni3S4 Heterostructure as a Hydrogen Spillover Catalyst for Efficient Alkaline Water Splitting

A sulfur vacancy‐engineered Co9S8‐Ni3S4 hollow heterostructure is rationally designed to trigger hydrogen spillover, where vacancy‐mediated interfacial modulation enables efficient H migration and accelerates HER kinetics. 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 Co9S8‐Ni3S4 hollow heterostructure, via a self‐sacrificial template strategy. The introduction of sulfur vacancies greatly decreases the work function of Ni3S4, thereby narrowing the work function difference (Δϕ) with Co9S8. The reduced electron density at their interface facilities the hydrogen species (H*) transfer to trigger hydrogen spillover. Density functional theory (DFT) calculations reveal that H2O molecules preferentially adsorb and dissociate at Co sites of Co9S8 to generate active H* intermediates, which subsequently migrate to Ni sites of Ni3S4 domains for H2 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 cm2, along with remarkable durability, showing no detectable degradation even at 1 A cm2 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. A sulfur vacancy-engineered Co 9 S 8 -Ni 3 S 4 hollow heterostructure is rationally designed to trigger hydrogen spillover, where vacancy-mediated interfacial modulation enables efficient H migration and accelerates HER kinetics. 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. Advanced Science, Volume 12, Issue 48, December 29, 2025.