Bimetallic sulfides feature low-cost, excellent conductivity, and tunable electronics, exhibiting promising applications in energy storage and conversion. Bimetallic sulfides with sulfur vacancies are synthesized and named NiFeS-NF-x (x means the added volume of H2O2 during the hydrothermal process). NiFeS-NF-150 delivers a low overpotential of 171 mV for OER as well as a low overall water-splitting potential of 1.541 V at 10 mA cm−2 in an alkaline electrolyte. The excellent intrinsic activity of NiFeS-NF-150 can be ascribed to its ideal Ni2+/Ni3+ ratio and moderate eg electron filling of Ni. The optimized electron-spin state of NiFeS-NF-150 balances the adsorption of *O2 and the hydrogenation of *OH, declining the energy barrier of the rate-determining step and accelerating OER. Moreover, NiFeS-NF-150//NiFeS-NF-150 indicates excellent stability at 10 mA cm−2 operating for 36 h and nearly 100 % Faradaic efficiency. The excellent electrocatalytic properties compared with other reported Ni-based catalysis indicate that this composition is a promising catalyst for water splitting.
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