  Designing electrocatalysts for HER in alkaline conditions to overcome the
sluggish kinetics associated with the additional water dissociation step is a
recognized challenge in promoting the hydrogen economy. To this end, delicately
tuning the atomic-scale structure and surface composition of nanoparticles is a
common strategy and, specifically, making use of hybrid structures, can produce
synergistic effects that lead to highly active catalysts. Here, we present a
core-shell catalyst of Ag@MoS$_2$ that shows promising results towards the
hydrogen evolution reaction (HER) in both 0.5 M H2SO4 and 0.5 M KOH. In this
hybrid structure, the MoS$_2$ shell is strained and defective, and charge
transfer occurs between the conductive core and the shell, contributing to the
electrocatalytic activity. The shelling process results in a large fraction of
Ag$_2$S in the cores, and adjusting the relative fractions of Ag, Ag$_2$S, and
MoS$_2$ leads to improved catalytic activity and fast charge-transfer kinetics.
We suggest that the enhancement of alkaline HER is associated with a
cooperative effect of the interfaces, where the Ag(I) sites in Ag$_2$S drive
the water dissociation step, and the formed hydrogen subsequently recombines on
the defective MoS$_2$ shell. This study demonstrates the benefits of hybrid
structures as functional nanomaterials and provides a scheme to activate
MoS$_2$ for HER in alkaline conditions.
