  Recently, 2D JTMDs with asymmetric electronic structures are inviting an
intense research interest in modern science and technology. Using the first
principles-based periodic hybrid dispersion-corrected Density Functional Theory
(DFT-D) method, we have investigated the equilibrium structure, geometry, and
electronic properties of the 2D monolayer MoSSe JTMD with the electrocatalytic
activities for the H2 evolution reaction (HER). We have performed non-periodic
quantum mechanical DFT computations to find out the most favorable HER pathway
on the exposed surfaces of the 2D Janus MoSSe material i.e., on the Mo-edges
and S- or Se-edges. To explore the electrocatalytic HER mechanism, reaction
pathways and barriers, we have considered a cluster model system Mo10S12Se9 to
illustrate the Mo-edges and S- or Se-edges of the 2D monolayer MoSSe material.
The present study reveals that the Volmer-Heyrovsky reaction mechanism is
thermodynamically favorable reaction pathway to evolute H2 at the Se-terminated
Mo-edges. It was found that the change of free energy barrier during the
Heyrovsky reaction at the Se-terminated Mo-edges is about 3.93-7.10 kcal.mol-1
(in both the gas and the solvent phases), indicating an exceptional
electrocatalyst for HER with the lowest activation barriers. This study showed
that the Tafel slope (m) is lower in the case of 2D Janus MoSSe material due to
the overlap of the s-orbital of the hydrogen and d-orbitals of the Mo atoms
appeared in the HOMO and LUMO transition state TS1 of the H-migration reaction
step. The better stabilization of the atomic orbitals in the HER rate-limiting
step i.e., H-migration TS1 reaction step (in the solvent phase) is a key for
reducing the reaction barrier, thus the overall catalysis indicating a better
electrocatalytic performance for H2 evolution.
