Role of Hyper-Reduced States in Hydrogen Evolution Reaction at Sulfur Vacancy in MoS₂

Abstract

Using the multiscale simulation combining ab initio calculations and kinetic Monte Carlo (KMC) simulations, we theoretically investigate the hydrogen evolution reaction (HER) on the sulfur vacancy of a MoS 2 monolayer. Unlike metal catalysts, the protonation step and the charging step proceed independently in semiconducting MoS 2 . Interestingly, the barrier for hydrogen evolution decreases when the vacancy site is hyper-reduced with extra electrons. The turnover frequency and polarization curve obtained from the KMC simulation agree well with extant experimental data, and the major HER paths underscore the role of hyper-reduced states, particularly when the overpotential is applied. The strain effect is also simulated, and it is found that the tensile strain enhances HER by reducing the energy cost of hyper-reduced states. The estimated reduction in the overpotential agrees favorably with the experiment while the hydrogen binding energy alone cannot account for it, suggesting that the full-blown KMC simulation should be used to accurately predict the variation of HER performance under various conditions. By uncovering the nature of the catalytic reaction at the sulfur vacancy of MoS 2 and revealing a design principle in which the facile formation of hyper-reduced states plays an important role, the present work will pave the way for developing HER catalysts that may replace Pt.

Vacancy in Ultrathin 2D Nanomaterials toward Sustainable Energy Application

“…Gray line: corresponding path at U = 0.) Reproduced with permission . Copyright 2018, American Chemical Society.…”

Unveiling Electrochemical Reaction Pathways of CO2 Reduction to CN Species at S‐Vacancies of MoS2

“…This assumption would be valid for conventional metallic catalysts, in which electronic wave functions are delocalized over numerous atomic sites and the localization of electrons around the active site requires an incoming proton . In contrast, the sub‐gap states associated with V S in MoS 2 are inherently localized and, therefore, the two processes can occur independently …”

“…To determine q i U , we first checked the availability of sub‐gap states for M i and evaluated the charging energy of M i for all of possible charge states q , that is, G ( M i , q )− G ( M i ,0)− q × eU . Then, we selected the q leading to the lowest charging energy at a given U as q i U . If the charge state is the same between successive intermediates, that is, q i U − q i+1 U =0, Δ G i → i +1 linearly depends on U , as found in metallic catalysts.…”

“…Effects of solvation were taken into account by the implicit solvation model, which approximates solvent molecules with a dielectric continuum with parameters of 78.4 (dielectric constant), 0.6 Å (cavity width), 0.525 meV Å −2 (surface tension), and 0.0025 Å −3 (cutoff density). These parameters were reported to accurately reproduce the experimental solvation energies of a water molecule and hydronium ion in the literature . The vibrational entropy and zero‐point energies were considered in the free energy of adsorbed intermediates within the harmonic approximations .…”

“…The free energy of a H 2 O molecule in aqueous electrolyte was calculated by assuming the equilibrium with its gas phase at room temperature, as in previous works . For the calculations of charged systems, we did not consider corrections to the total energy because they were reported to be negligible owing to the high dielectric constant of water . Indeed, we found that the Markov and Payne scheme, which had been modified for two‐dimensional systems in Ref.…”