Computational study of hydrogen evolution reaction on defective two-dimensional transition metal dichalcogenides

Abstract

Hydrogen is a strong contender for a next-generation clean energy source that may replace the current fossil fuels. However, the low-cost and clean production of hydrogen source is a critical issue. In this regard, the hydrogen production by splitting water, the abundant resource on earth, may resolve many of these problems, particularly if it is driven by the solar energy. To split water using solar energy or electricity, catalysts are necessary to reduce the large overpotential during the hydrogen and oxygen evolution. For several decades, Pt is known to be the best catalyst for hydrogen evolution reaction in water splitting, but the material is very expensive and so might not be suitable for large scale applications. As such, numerous studies searched for alternative catalysts that have potential to replace Pt. Especially, transition metal dichalcogenides (TMDs) are receiving much attention as a new class of two-dimensional catalysts for hydrogen evolution reaction (HER). Despite extensive efforts to find highly efficient catalytic TMD systems, strong candidates to replace Pt have not been suggested yet. In TMD catalysts such as MoS2, edges are believed to be the active sites, but their limited density is a problem. Recently, it was found that the basal plane of MoS2 can also be active for HER by introducing sulfur vacancies in addition to strain.1 Herein, we try to identify the mechanism of HER on sulfur vacancy sites by means of kinetic Monte Carlo simulation using the energetics of first-principles calculations. We find that HER at sulfur vacancy site of MoS2 are dominated by the Volmer-Heyrovsky mechanism, and the vacancy site can be electrically charged, thereby lowering the energy barrier of rate-limiting Heyrovsky step. In addition, when a tensile strain is applied to MoS2, it leads to reaction paths with lower energy barriers in more negatively charged states. We also try to find new defective TMD catalyst for HER by means of computational screening based on the density functional theory. We explore the HER efficiencies of basal planes and anion vacancy sites for various TMD materials by using the hydrogen binding free energy as the descriptor for the HER efficiency. We find hydrogen binding energy is varied by the concentration of vacancy and discover good TMD candidates which are expected to show high HER performance in proper vacancy concentrations. We suggest ZrSe2 and ZrTe2 for low vacancy concentration, and MoSe2, MoTe2, WSe2, ReTe2, MoS2 and ReSe2 when intermediate or high concentration of vacancy is accessible. We expect that these materials could compete with Pt as HER catalyst even without strain engineering. In addition, through multiple linear regression, we identify that the formation energies and the electronic structures of anion vacancies strongly affect the hydrogen adsorption energy.