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Hydrogen storage in pristine and Janus transition-metal dichalcogenide monolayers: electronic origins, coverage effects, and finite-temperature stability

Here, we present a systematic first-principles study of hydrogen adsorption on pristine and Janus MX2 and MSSe monolayers (M = Ni, Pd, Pt; X = S, Se), combining density-functional theory (DFT) calculations with finite-temperature ab initio molecular dynamics simulations (AIMD). Orbital-resolved electronic-structure analysis reveals that hydrogen binding strength is controlled primarily by the contribution of metal d states near the Fermi level, which increases from Pd and Pt to Ni. Chalcogen substitution from S to Se further modulates adsorption by enhancing surface polarizability. Structure plays a decisive role: metallic 1T phases promote stronger single-molecule adsorption due to enhanced electronic screening, whereas the more open coordination of the 2H phase provides greater configurational freedom at high hydrogen coverage. Janus functionalization introduces chemically inequivalent S- and Se-terminated surfaces, leading to side-dependent adsorption energies, hydrogen-layer thicknesses, and spatial distributions. Hydrogen uptake remains dominated by molecular physisorption across all systems. Finite-temperature AIMD simulations at 300 K demonstrate that hydrogen remains molecular even at high loadings, with no dissociation or irreversible chemisorption. 2H-NiSSe and 2H-PdSSe exhibit the most favorable combination of moderate adsorption energies, stable multilayer hydrogen configurations, and robust thermal stability, whereas Pt-based Janus systems display strong confinement and reduced reversibility. These results establish clear design principles for hydrogen storage in TMDs, showing that optimal performance arises from intermediate physisorption rather than maximal binding strength and highlighting Janus Ni- and Pd-based systems as promising platforms for reversible molecular hydrogen storage.