Surface sulfur-modified NiO nanoparticles with enhanced oxygen evolution reaction kinetics and periodic electrochemical durability evaluation

Developing efficient and durable oxygen evolution reaction (OER) electrocatalysts is essential for sustainable hydrogen production through water electrolysis. Herein, pristine NiO nanoparticles and NiO–S composites containing 10 wt% (NiO@S1) and 20 wt% sulfur (NiO@S2) were prepared through hydrothermal synthesis, solid-state mixing, and thermal treatment. XRD refinement confirmed that the cubic NiO structure was retained after sulfur addition, while FTIR, EDS, XPS, and EPR analyses revealed sulfur-related surface interactions, electronic redistribution, and defect-rich environments. The optimized NiO@S1 electrode exhibited the best OER activity in 1 M KOH, requiring an overpotential of 330 mV to reach 50 mA cm −2 and displaying an apparent Tafel slope of 138.7 mV dec −1 . It also provided the largest electrochemically active surface area of 125 cm 2 and the lowest charge-transfer resistance of 0.40 Ω. Periodic LSV and EIS measurements conducted at 1-h intervals during 25 h chronoamperometry demonstrated better activity retention and smaller resistance growth for NiO@S1 than for NiO and NiO@S2. Moreover, NiO@S1 sustained OER operation for 225 h under strongly alkaline operating conditions with limited morphological deterioration. These findings demonstrate that moderate sulfur loading produces favorable surface and interfacial characteristics, whereas excessive sulfur loading obstructs active sites and compromises structural and electrochemical stability.