First-Principles Studies On The Hydrogen Evolution Reaction Activity Of MoX₂（X=S,O）

Molybdenum disulfide（MoS₂）is one of the most widely studied catalysts for hydrogen evolution reaction（HER）.Recent studies proved that the HER catalysis performace of MoS₂could be dramatically enhanced by obtaining chemically exfoliatied 1T’-MoS₂ from 2H-MoS₂,or by creating sulfur vacancies（V_S）of 2H-MoS₂.The previous calculations about HER mechanism generally neglected water environment and the effects of interfacial electric field,which were unable to describe the HER process under the real electrochemistry conditions.Based on the first-principles caculations,this paper puts forward the lowest electrochemistry principle,which is used for the screening of interfacial electric double layer structures,and we construct the Pourbaix diagrams of 1T’-MoS₂/H₂O and MoS₂-V_S/H₂O interfacial electric double layer models.Using the screened interfacial models,the HER dynamic mechanisms are investigated by the transition state caculations.Besides,this paper also analyzes the intrinsic mechanism of the increase HER activity for molybdenum dioxide（MoO₂）after Nickel-doped.Our specific research results are as following:1.Under the condition of standard hydrogen electrode（SHE）,the 1T’-MoS₂/H₂O electric double layer features a high hydrogen coverage of ca.37.5%on the catalysis surface,and a relatively low hydronium concentration of no more than ca.1.8%in the water layer,which is conducive to the Tafel reaction for proton desorption.2.The HER mechanism on 1T’-MoS₂ basal plane is the diffusion-assisted Volmer-Tafel mechanism,whose rate-limited step is Tafel process,and the calculated barrier of Tafel reaction is 0.91 e V,which is in good agrement with the 0.87 e V estimated from experiments.3.Under the experimental condition of HER,only half sulfur vacancies of MoS₂-V_Scontribute to HER,whereas the other half are occupied by oxygen atoms or hydroxyls.4.Near the equlibrium potential,protons adsorbe in the sulfur vacancies by water splitting process,and disorbe by Heyrovsky manner.The rate-limited step is Heyrovsky reaction,with a barrier about 0.81 e V,corresponds to a turn-over frequency（TOF）of 0.15 s^-1,which lies exactly on the experimentally determined TOF range of 0.05～0.16 s^-1.5.Ni-doped leads to electron-deficiency on neighboring oxygen atoms,because of the stronger electronegativity of Ni than that of Mo.This change promotes the increase of the hydrogen coverage and enhances the HER activity of MoO₂.