Modulating surface sulfur residues on CoFe (oxy)hydroxides via H2O2 oxidation for enhanced oxygen evolution.

Transition metal sulfides can be controllably transformed into (oxy)hydroxides, offering a promising route to efficient OER electrocatalysts. However, understanding how residual sulfur tunes intrinsic activity remains a challenge. Herein, a mild H2O2-controlled oxidation strategy is developed to tailor surface sulfur residues on cobalt-iron sulfides, yielding high-performance sulfur-doped (oxy)hydroxide catalysts. By tuning the oxidation time, a non-monotonic relationship between residual sulfur content and OER activity is observed. The optimal catalyst reduces the overpotential by 41 mV to 243 mV at 10 mA cm-2 and operates stably for 120 h. Mechanistic studies reveal that an appropriate residual sulfur level enhances the OER kinetics by downshifting the d-band center to optimize intermediate adsorption, while simultaneously lowering the work function and increasing carrier density to accelerate interfacial charge transfer. Excessive sulfur removal degrades performance. This work offers a new perspective for designing OER electrocatalysts through rational regulation of surface residual species.