Unveiling the Role of 2D Monolayer Mn-doped MoS2 Material: Toward an Efficient Electrocatalyst for H2 Evolution Reaction

Two-dimensional (2D) monolayer pristine MoS2 transition metal dichalcogenides (TMD) is the most studied material because of its promising aspects as nonprecious electrocatalyst for hydrogen evolution reaction (HER). Previous studies have shown that the basal planes of the 2D MoS2 are catalytically inert and hence, they cannot be used directly in desired applications such as electrochemical HER in industries. Here, we have thoroughly studied the defect-engineered Mn-doped 2D monolayer MoS2 (Mn-MoS2) material where Mn was doped in the pristine MoS2 to activate the inert basal planes. Using density functional theory (DFT) method, we performed rigorous inspection of electronic structures and properties of the 2D monolayer Mn-MoS2 to be a promising alternative to noble metal free catalysts for the effective HER. Periodic 2D slab of the monolayer Mn-MoS2 was created to study the electronic properties (such as band gap, band structures and total density of states (DOS)) and the reaction pathways occurring on the surface of the material. The detailed HER mechanism has been explored by creating the Mn1Mo9S21 molecular cluster model system using M06-L DFT method including solvation effects to determine the reaction barriers and kinetics. Our study reveals that the 2D Mn-MoS2 follows the most favorable Volmer-Heyrovsky reaction mechanism with very low energy barriers during the H2 evolution. It was found that the change of free energy barrier (∆G) during the Heyrovsky reaction is about 11.80 - 12.60 kcal.mol-1 (in in solvent phase), indicating an exceptional electrocatalyst for HER. The Tafel slope is lower in the case of 2D monolayer Mn-MoS2 material due to the overlap of the s-orbital of the hydrogen and d-orbitals of the Mn atoms appeared in the HOMO and LUMO transition states (TS1 and TS2) of both the Volmer and Heyrovsky reaction steps. The better stabilization of the atomic orbitals in the HER rate-limiting step Heyrovsky TS2 is a key for reducing the reaction barrier, thus the overall catalysis indicating a better electrocatalytic performance for H2 evolution. This study is focused on designing low cost and efficient electrocatalysts for HER by using earth abundant transition metal dichalcogenides (TMDs) and decreasing the activation energy barriers by scrutinizing the kinetics of the reaction for reactivity.