  Hydrogen is a crucial source of green energy and has been extensively studied
for its potential usage in fuel cells. The advent of two-dimensional crystals
(2DCs) has taken hydrogen research to new heights, enabling it to tunnel
through layers of 2DCs or be transported within voids between the layers, as
demonstrated in recent experiments by Geim's group. In this study, we
investigate how the composition and stacking of transition-metal dichalcogenide
(TMDC) layers influence the transport and self-diffusion coefficients (D) of
hydrogen atoms using well-tempered metadynamics simulations. Our findings show
that modifying either the transition metal or the chalcogen atoms significantly
affects the free energy barriers (Delta F) and, consequently, the
self-diffusion of hydrogen atoms between the 2DC layers. In the Hh polytype (2H
stacking), MoSe2 exhibits the lowest Delta F, while WS2 has the highest,
resulting in the largest D for the former system. Additionally, hydrogen atoms
inside the RhM (or 3R) polytype encounter more than twice lower energy barriers
and, thus, much higher diffusivity compared to those within the most stable Hh
stacking. These findings are particularly significant when investigating
twisted layers or homo- or heterostructures, as different stacking areas may
dominate over others, potentially leading to directional transport and
interesting materials for ion or atom sieving.
