  Transition metal dichalcogenides are cheap and earth-abundant candidates for
the replacement of precious metals as catalyst materials. Experimental
measurements of the hydrogen evolution reaction (HER), for example, have
demonstrated significant electrocatalytic activity of MoS2 but there is large
variation depending on preparation method. In order to gain information about
the mechanism and active sites for HER, we have carried out calculations of the
reaction and activation energy for HER at the transition metal doped basal
plane of MoS2 under electrochemical conditions, i.e. including applied
electrode potential and solvent effects. The calculations are based on
identifying the relevant saddle points on the energy surface obtained from
density functional theory within the generalized gradient approximation, and
the information on energetics is used to construct voltage-dependent volcano
plots. Doping with 3d-metal atoms as well as Pt is found to enhance hydrogen
adsorption onto the basal plane by introducing electronic states within the
band gap, and in some cases (Co, Ni, Cu, Pt) significant local symmetry
breaking. The Volmer-Heyrovsky mechanism is found to be most likely and the
associated energetics show considerable dopant and voltage-dependence. While
the binding free energy of hydrogen can be tuned to be seemingly favorable for
HER, the calculated activation energy turns out to be significant, at least 0.7
eV at a voltage of -0.5 V vs. SHE, indicating low catalytic activity of the
doped basal plane. This suggests that other sites are responsible for the
experimental activity, possibly edges or basal plane defects.
