Strain and defect engineered monolayer Ni-MoS2 for pH-universal hydrogen evolution catalysis

Abstract

The sustainable production of H₂ fuel via the hydrogen evolution reaction (HER) using low-cost catalysts to replace expensive noble metals is highly desired. Here using first-principles calculations, we design a delicate monolayer transition metal compound, Ni-MoS₂, consisting of orderly interlaced Ni and Mo metal ions, and investigate its HER catalytic performance. The Gibbs free energy, ΔG_H, which is the best descriptor for the HER, is calculated and optimized with respect to strain and S vacancies. Remarkably, the calculated ΔG_H is found to be ∼0 eV at the biaxial strain of 11%−12%, which is superior to MoS₂, for which straining alone is insufficient to achieve the optimal performance. We further reveal that along with straining, the bandgap is reduced and a semiconductor-to-metal transition is induced, leading to an enhancement in the charge transfer and HER performance. Moreover, ΔG_H ≈ 0 eV is achieved at the S vacancy concentration of only ∼2.5%, which is in strong contrast to ∼12.5% required for MoS₂. We further show that the defective Ni-MoS₂ is able to enhance the conductivity, which leads to the reduction of ΔG_H. Two remarkable HER mechanisms and alkaline HER kinetics have been demonstrated in this study: perfect Ni-MoS₂ prefers the Volmer–Heyrovsky mechanism in the strain state, whereas the Volmer–Tafel mechanism is more preferred for the defective Ni-MoS₂. The kinetic energy barrier of the alkaline HER is reduced, revealing that Ni-MoS₂ promotes the rate-determining water dissociation step. The present work suggests that the designed monolayer Ni-MoS₂ compound significantly outperforms MoS₂ in terms of HER activity, and thus is promising for low-cost, pH-universal and high-performance HER applications.