Potential and Support-Dependent Hydrogen Evolution Reaction Activation Energies on Sulfur Vacancies of MoS2 from GC-DFT

In this work, we present a detailed mechanistic study of HER at the sulfur vacancy Vs. We evaluate the Volmer, Tafel, and Heyrovsky transition states for the different possible reaction steps, considering the activation energy as a function of electrochemical potential. The results show that the Volmer and Heyrovsky steps depend on the electrochemical value and the activation energies decrease for more negative potential values, while this is not the case for the Tafel step, where the activation energy is essentially constant. From the activation energy values at -0.2 V, it can be concluded that to release H2 at Vs, we follow two Volmer steps and then a Heyrovsky step, since they have the lowest activation energies compared to the others. Heyrovsky is the rate-determining step. In addition, we investigate for the first time the effect of the support on the conductivity of MoS2 and the HER activity of sulfur vacancies. Our results show that copper, gold and graphite supports have no effect on the barrier energies of all steps of the HER mechanism.