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

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, ΔGH, which is the best descriptor for the HER, is calculated and optimized with respect to strain and S vacancies. Remarkably, the calculated ΔGH 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, ΔGH ≈ 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 ΔGH. 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.