Defect engineering of cobalt microspheres by S doping and electrochemical oxidation as efficient bifunctional and durable electrocatalysts for water splitting at high current densities

Herein, we perform a facile one-step deep eutectic solvent electrodeposition approach to fabricate a series of S-doping highly porous Co-S films by in situ grown on Ni foam (Co-S/NF), which exhibits a superior catalytic activity and robust nature for both oxygen and hydrogen evolution reaction (OER and HER) in alkaline media. The doping of S is evidenced to induce a structural and size transition on the Co deposited layer, which leads to a significant increase on the electrochemical surface area and more accessible active sites for enhancing the catalytic activity. The formation of oxygen deficiencies-rich Co-O/OH species with a phase transformation from Co-S species is noticed during OER. Benefiting from the merits of in situ growth, the Co-S films obtained at an optimal S doping level (denoted as Co-S-50/NF) can robustly drive high current densities of 100 and 500 mA cm−2 with low overpotentials of 322 and 368 mV for OER, and 124 and 155 mV for HER. The water electrolyzer assembled with Co-S-50/NF requires only 1.79 and 2.08 V to reach 100 and 500 mA cm−2 in 1.0 M KOH, and 1.72 V to achieve 500 mA cm−2 in 30 wt% KOH at 353 K for over 150 h. •Self-supported highly porous Co-S films grown on Ni foam (Co-S/NF) are fabricated.•The Co-S/NF exhibits strong structural integrity with abundant active sites.•The Co-S/NF serves as efficient and durable catalyst for full water splitting.•Mechanism of Co, S-doping on enhancing the catalytic activity of Co-S/NF is proposed.