Hydrogen Transport Between Layers of Transition Metal-Dichalcogenides

TL;DR

This study uses simulations to explore how the composition and stacking of transition metal dichalcogenide layers influence hydrogen transport and diffusion, revealing significant effects of atomic makeup and stacking on energy barriers and diffusivity.

Contribution

It provides new insights into how layer composition and stacking order affect hydrogen diffusion in TMDCs, informing future design of layered materials for energy applications.

Findings

MoSe2 has the lowest energy barrier for hydrogen transport in 2H stacking.

WS2 exhibits the highest energy barrier, resulting in lower hydrogen diffusivity.

RhM (3R) stacking significantly lowers energy barriers, enhancing hydrogen diffusion.

Abstract

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.