Hydrogen adsorption on transition-metal dichalcogenide monolayers: energetics and desorption thermodynamics from first principles

Abstract

Understanding how electronic structure, crystal phase, and lattice asymmetry control molecular hydrogen adsorption in two-dimensional materials is essential for designing reversible hydrogen-storage platforms. Here, we present a systematic first-principles investigation of H2 adsorption on 1T and 2H transition-metal dichalcogenides (MX2, M = Ni, Pd, Pt; X = S, Se) and their Janus counterparts (MSSe). Adsorption energies, coverage effects, ab initio molecular dynamics, and Arrhenius-based desorption kinetics are analyzed to identify the governing adsorption mechanisms and finite-temperature behavior. The calculations reveal three distinct adsorption regimes across the investigated monolayers. Ni- and Pd-based systems predominantly exhibit weak molecular physisorption characterized by small adsorption energies and high reversibility, whereas Pt-containing compounds display substantially stronger interactions and, in selected configurations, partial or complete hydrogen dissociation. The adsorption behavior is shown to depend not only on the density of metal d states near the Fermi level, but also on crystal phase, surface symmetry, molecular orientation, and local orbital hybridization. Janus monolayers introduce an additional polarity-driven asymmetry between the two surfaces, modifying the adsorption energetics and molecular confinement.Coverage-dependent calculations demonstrate that increasing hydrogen loading systematically weakens the average adsorption energy due to intermolecular repulsion and progressive saturation of favorable adsorption regions. Finite-temperature AIMD simulations further indicate that molecular adsorption remains dynamically stable at room temperature over the simulated timescale, with no evidence of irreversible surface degradation or spontaneous hydride formation.Among the investigated systems, Janus PdSSe exhibits adsorption energies and desorption kinetics compatible with reversible molecular hydrogen adsorption near ambient conditions while simultaneously achieving high gravimetric storage capacity at elevated coverage. These results provide insight into the interplay between electronic structure, structural asymmetry, and adsorption thermodynamics in transition-metal dichalcogenide monolayers and highlight Janus TMDs as promising platforms for tunable molecular hydrogen adsorption.