Sulfur-Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x-Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co−S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x-Vo-S to exhibit much superior OER activity. FeCoO_x-Vo-S exhibits a mass activity of 2440.0 A g⁻¹ at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec⁻¹, indicative of its excellent charge transfer rate. When FeCoO_x-Vo-S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm⁻² and 406.0 mA cm⁻² at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.