  Earth-abundant two-dimensional (2D) pristine transition metal dichalcogenides
(TMDs) have emerged as a superlative class of materials for several
applications in electronic devices, energy storage devices, gas sensing, etc.,
and they have recently attracted great attention, owing to their good catalytic
activity and excellent stability toward electrochemical H2 Evolution Reaction
(HER). Each individual layer of the TMDs consists of three atomic layers in
which the transition metal is sandwiched by two chalcogens. To activate the
inert basal plane of the pristine 2D TMDs, it is needed to create some defects
or doping of some heteroatoms in the pristine TMDs. Phase engineering
techniques have been used to activate the basal plane of the 2D TMDs. In this
article, we have computationally developed 2D monolayer Mn-MoS$_2$ material and
its application in HER. Stable S terminated edge of the MoS$_2$ shows low
catalytic activity due to its inert basal plane, so to exploit these edges for
improved performance we doped Mn in the pristine MoS$_2$ material. Using
trailblazing and state of the art first principles-based density functional
theory we performed methodical and rigorous inspection of electronic structures
and properties of monolayer Mn doped MoS$_2$ to be a promising alternative to
noble metal-based catalysts for HER. Periodic 2D slab of Mn-MoS$_2$ was created
to study the electronic properties and the reaction pathway occurring on the
surface of the material has been delved into by creating Mn$_1$Mo$_9$S$_{21}$
molecular cluster model. Our study reveals that the 2D Mn-MoS$_2$ monolayer
based catalyst follows the Volmer-Heyrovsky reaction with very low energy
barriers during the HER mechanism. This study is focused on designing a low
cost and efficient electrocatalyst for HER by using earth abundant TMDs and
lowering the activation barriers by scrutinizing the kinetics of the reaction
for reactivity.
