Enhanced Activities in Alkaline Hydrogen and Oxygen Evolution Reactions on MoS₂ Electrocatalysts by In-Plane Sulfur Defects Coupled with Transition Metal Doping

2D transition metal disulfides (TMDs) are promising and cost-effective alternatives to noble-metal-based catalysts for hydrogen production. Activation of the inert basal plane of TMDs is crucial to improving the catalytic efficiency. Herein, introduction of in-plane sulfur vacancies (S_v) and 3d transition metal dopants in concert activates the basal planes of MoS₂ (M-S_v-MoS₂) to achieve high activities in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Acetate introducing mild wet chemical etching removes surface S atoms facilitating subsequent cation exchange between the exposed Mo atoms and targeted metal ions in solution. Density-functional theory calculation demonstrates that the exposed 3d transition metal dopants in MoS₂ basal planes serve as multifunctional active centers, which not only reduce ΔG_H* but also accelerate water oxidation. As a result, the optimal Ni-S_v-MoS₂ and Co-S_v-MoS₂ electrocatalysts show excellent stability and alkaline HER and OER characteristics such as low overpotentials of 101 and 190 mV at 10 mA cm⁻², respectively. The results reveal a strategy to activate the inert MoS₂ basal planes by defect and doping co-engineering and the technique can be extended to other types of TMDs for high-efficiency electrocatalysis beyond water splitting.