Sulfur Modulation of Tungsten Frameworks Enables Robust Cobalt Tungsten Centers for Bifunctional Electrocatalytic Water Splitting

The development of highly active, durable, and earth-abundant bifunctional electrocatalysts is critical for advancing alkaline water electrolysis toward sustainable hydrogen production. Herein, we demonstrate the rational design of sulfur-modified cobalt tungstate (CoWSO4) as an efficient bifunctional electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Sulfur incorporation into the CoWO4 lattice induces favorable electronic redistribution, lattice distortion, and defect formation while preserving the crystalline tungstate framework. Comprehensive structural and spectroscopic analyses confirm uniform sulfur incorporation, enhanced electronic modulation, and increased active-site exposure. Electrochemical evaluations reveal that CoWSO4 exhibits significantly improved catalytic activity compared to pristine CoWO4, delivering low overpotentials of 124 mV at 10 mA cm–2 for HER and 307 mA cm–2 at 25 mA cm–2 for OER, along with reduced Tafel slopes (150 mV dec–1 for HER and 67 mV dec–1 for OER) and lower charge-transfer resistance. The enhanced performance is attributed to sulfur-induced electronic tuning of Co active centers, improved conductivity, and optimized adsorption–desorption energetics of key reaction intermediates. When employed as both the anode and cathode in a symmetric two-electrode electrolyzer, the CoWSO4∥CoWSO4 device achieves a low cell voltage of 1.76 V at 25 mA cm–2, near-unity Faradaic efficiency, and excellent long-term stability under alkaline conditions. This study elucidates the structure–activity relationship of heteroatom-engineered tungstates and highlights sulfur modulation as an effective strategy for designing cost-effective, robust, and scalable bifunctional electrocatalysts for green hydrogen production.