Oxygen doping-triggered electron redistribution in cobalt-rich sulfide for efficient electrocatalytic water splitting

Cobalt-rich sulfide (Co₉S₈) holds great promise as an electrocatalyst for water splitting, but its performance for hydrogen evolution reaction (HER) in alkaline and neutral media is limited by sluggish water dissociation kinetics. Herein, we find that moderate oxygen doping within Co₉S₈, preferentially at the interstitial sites, triggers significant electron redistribution via Co-O-S bridges, which decreases the local electron density of Co and S sites. This treatment enhances H₂O adsorption and dissociation at the Co-sites and optimizes H* adsorption/desorption at the S-sites, notably on the high-index (311) facet, thus accelerating the water dissociation kinetics. The oxygen-doped Co₉S₈ catalyst, dominated by the (311) crystal plane, demonstrates remarkable HER activity and stability in alkaline solution, with a low overpotential of 142 mV at 10 mA cm^-2 and a Tafel slope of 96 mV dec^-1, outperforming most Co₉S₈-based catalysts. Under neutral condition, it exhibits a low overpotential of 264 mV at 10 mA cm^-2. Further applied in an anion exchange membrane water electrolyzer, it reaches 150mA cm^-2 at 1.70 V, surpassing the commercial Pt/C (134 mA cm^-2). This oxygen doping-triggered electron redistribution strategy paves new ways for developing highly efficient transition metal-based electrocatalysts for sustainable energy applications.