  By a combination of the first-principles calculations and climbing image
nudged elastic band method (ciNEB) we investigate structure stabilities and
hydrogen evolution reaction (HER) performance of monolayer 2H-CrS2. The results
suggest the free energy for the Volmer reaction in the monolayer 2H-CrS2 with S
vacancy is 0.07 eV, comparable with Pt-based catalyst, and HER on the surface
of the monolayer is prone to the Volmer-Heyrovsky mechanism with no energy
barrier. We propose that high HER performance stems from the reduction of the
energy level of d-band center. Additionally, the S vacancy leads to defect
states in the middle of electronic bandgap and the reduction of potential
barrier between the S atom layer and the vacuum, which is conducive to improve
HER performance.
