Design of polymetallic sulfide NiS₂@Co₄S₃@FeS as bifunctional catalyst for high efficiency seawater splitting

The shortage of freshwater resources in the world today has limited the development of water splitting, and our eyes have turned to the abundant seawater. The development of relatively low-toxicity and high-efficiency catalysts is the most important area in seawater electrolysis. In this paper, the preparation of NiS₂@Co₄S₃@FeS via a hydrothermal method on nickel foam has been studied for the first time. In the process of vulcanization, Fe will first generate FeS by virtue of its high affinity for vulcanization. Once Fe is vulcanized, the residual sulfur will be used to generate NiS₂, while the vulcanization of Co requires a higher sulfur concentration and reaction temperature; thus, Co₄S₃ will be generated last. NiS₂@Co₄S₃@FeS is confirmed to have excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic properties in alkaline seawater. Its unique structure allows it to expose more reaction centres, and the synergies between the multiple metals optimize the charge distribution of the material and accelerate the OER and HER kinetics. NiS₂@Co₄S₃@FeS requires overpotentials of only 122 mV and 68 mV for the OER and HER when reaching 10 mA cm^-2, which is superior to most catalysts reported to date for seawater electrolysis, and the material displays acceptable stability. In an electrolytic cell composed of both positive and negative electrodes, when the current density is 10 mA cm^-2, the NiS₂@Co₄S₃@FeS material displays a low overpotential of only 357 mV for seawater splitting. Density functional theory shows that the FeS electrode has the optimum Gibbs free energy of H to accelerate reaction kinetics, and the synergistic catalysis of the NiS₂, Co₄S₃ and FeS materials promotes the hydrogen production activity of the NiS₂@Co₄S₃@FeS electrode. This work proposes a novel idea for designing environmentally friendly seawater splitting catalysts.