Reaction Mechanism with Thermodynamic Structural Screening for Electrochemical Hydrogen Evolution on Monolayer 1T′ Phase MoS₂

We report on a density functional theory (DFT) calculation to determine the reaction pathway and barrier of the hydrogen evolution reaction (HER) at the interface of monolayer 1T′ phase MoS2 and water. By screening the interfacial structures with the lowest chemical potential of protons and electrons, key structural characteristics that are important for prediction of reaction mechanism are identified. Under typical reaction conditions of HER, the catalyst surface features a high proton coverage of ca. 37% while the aqueous solution has a relatively low hydronium concentration of no more than ca. 1.8%. This contrast leads to proton desorption from the catalyst surface through a diffusion-assisted Tafel manner, rather than the Heyrovsky manner assumed previously. The result is supported by the agreement of the calculated reaction barrier and surface coverage with those of experimental estimate. In prediction of catalytic activity, hydrogen adsorption energies of reaction intermediates are widely used as the thermodynamic descriptor, while reaction barriers usually serve as kinetic parameters. We suggest that both thermodynamic and kinetic description toward HER should be performed on the premise that the lowest chemical potential of protons and electrons is obtained.