A low-cost and efficient route for large-scale synthesis of NiCoS_x nanosheets with abundant sulfur vacancies towards quasi-industrial electrocatalytic oxygen evolution

Transition-metal sulfides (TMS) have piqued a great deal of interest due to their unprecious nature and high intrinsic catalytic activity for water splitting. In this work, a low-cost and efficient route was developed, which included electrodeposition to prepare Ni-Co layered double hydroxide (NiCo-LDH) followed by ion exchange to form nickel cobalt sulfide (NiCoS_x). Electrochemical reduction was used to modulate sulfur vacancies in order to produce sulfur vacancies-rich NiCoS_x with nanosheet arrays on -three-dimensional nickel foam (NiCoS_x-0.4/NF) with a large area of more than 250 cm². Combining data from experiments and density functional theoretical (DFT) calculations reveals that engineered sulfur vacancies change the electronic structure, electron transfer property, and surface electron density of NiCoS_x, significantly improving the free energy of water adsorption and boosting electrocatalytic activity. The developed NiCoS_x-0.4/NF has long-term stability of more than 300 h at 500 mA cm⁻² in 1 M KOH at ambient temperature and only needs a 289 mV overpotential at 100 mA cm⁻². Remarkably, the synthesized electrocatalyst rich in sulfur vacancies, exhibits exceptional performance with a high current density of up to 1.9 A cm⁻² and 1 A cm⁻² in 6 M KOH and leads to overpotentials of 286 mV at 80 °C and 358 mV at 60 °C, respectively. The catalyst's practicability under quasi-industrial conditions (60 °C, 6 M KOH) is further demonstrated by its long-term stability for 220 h with only a 3.9 % potential increase at 500 mA cm⁻².