Cation/Anion Dual-Vacancy Pair Modulated Atomically-Thin Se x -Co 3 S 4 Nanosheets with Extremely High Water Oxidation Performance in Ultralow-Concentration Alkaline Solutions.

The density functional theory calculation results reveal that the adjacent defect concentration and electronic spin state can effectively activate the Co III sites in the atomically thin nanosheets, facilitating the thermodynamic transformation of *O to *OOH, thus offering ultrahigh charge transfer properties and efficiently stabilizing the phase. This undoubtedly evidences that, for metal sulfides, the atom-scale cation/anion vacancy pair and surface electronic spin state can play a great role in enhancing the oxygen evolution reaction. Inspired by the theoretical prediction, interconnected selenium (Se) wired ultrathin Co 3 S 4 (Se x -Co 3 S 4 ) nanosheets with Co/S (Se) dual-vacancies (Se 1.0 -Co 3 S 4 -V S/Se -V Co ) pairs are constructed by a simple approach. As an efficient sulfur host material, in an ultralow-concentration KOH solution (0.1 m), Se 1.0 -Co 3 S 4 -V S/Se -V Co presents outstanding durability up to 165 h and a low overpotential of 289.5 mV at 10 mA cm -2 , which outperform the commercial Co 3 S 4 nanosheets (NSs) and RuO 2 . Moreover, the turnover frequency of Se 1.0 -Co 3 S 4 -V S/Se -V Co is 0.00965 s -1 at an overpotential of 0.39 V, which is 5.7 times that of Co 3 S 4 NSs, and 5.8 times that of commercial RuO 2 . The finding offers a rational design strategy to create the multi-defect structure in catalysts toward high-efficiency water electrolysis.