The Reaction Mechanism with Free Energy Barriers for Electrochemical Dihydrogen Evolution on MoS_2

Abstract

We report density functional theory (M06L) calculations including Poisson–Boltzmann solvation to determine the reaction pathways and barriers for the hydrogen evolution reaction (HER) on MoS_2, using both a periodic two-dimensional slab and a Mo_(10)S_(21) cluster model. We find that the HER mechanism involves protonation of the electron rich molybdenum hydride site (Volmer–Heyrovsky mechanism), leading to a calculated free energy barrier of 17.9 kcal/mol, in good agreement with the barrier of 19.9 kcal/mol estimated from the experimental turnover frequency. Hydronium protonation of the hydride on the Mo site is 21.3 kcal/mol more favorable than protonation of the hydrogen on the S site because the electrons localized on the Mo–H bond are readily transferred to form dihydrogen with hydronium. We predict the Volmer–Tafel mechanism in which hydrogen atoms bound to molybdenum and sulfur sites recombine to form H_2 has a barrier of 22.6 kcal/mol. Starting with hydrogen atoms on adjacent sulfur atoms, the Volmer–Tafel mechanism goes instead through the M–H + S–H pathway. In discussions of metal chalcogenide HER catalysis, the S–H bond energy has been proposed as the critical parameter. However, we find that the sulfur–hydrogen species is not an important intermediate since the free energy of this species does not play a direct role in determining the effective activation barrier. Rather we suggest that the kinetic barrier should be used as a descriptor for reactivity, rather than the equilibrium thermodynamics. This is supported by the agreement between the calculated barrier and the experimental turnover frequency. These results suggest that to design a more reactive catalyst from edge exposed MoS2, one should focus on lowering the reaction barrier between the metal hydride and a proton from the hydronium in solution.

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