Molecular insights into H2 storage in coal: A critical assessment of cushion gases and coal rank

The underground storage of hydrogen (H 2 ) in coal seams presents a promising pathway for large-scale energy storage, yet the influence of coal rank and cushion gases on storage performance remains poorly understood, particularly under varying temperature and pressure conditions. To address this gap, we conducted a systematic comparative study of H 2 geo-storage behavior across four distinct coal ranks, ranging from high-volatile bituminous B (HvbB) to semi-anthracite (Sa), using grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Our results reveal that the adsorption affinity for H 2 follows the order HvbB < Mvb < Sa < Lvb, with low-volatile bituminous (Lvb) coal exhibiting the strongest uptake, while the competitive adsorption capacity consistently ranks as CO 2 > CH 4 > H 2 across all ranks. Notably, the preferential adsorption sites for CO 2 , CH 4 , and H 2 are found to be coal-rank-independent, implying that cushion gases with stronger affinity can effectively occupy the favorable sites for H 2 , thereby influencing storage efficiency. The adsorption selectivity of CO 2 over H 2 ( S CO2/H2 ) increases with H 2 mole fraction but decreases with rising pressure, whereas CH 4 consistently shows lower selectivity than CO 2 , suggesting CH 4 as a more favorable cushion gas. Interestingly, both selectivity coefficients exhibit a non-monotonic temperature dependence, decreasing from 298 K to 338 K and then increasing up to 358 K, pinpointing 338 K as the optimal temperature for H 2 storage. MD simulations indicate that extracted H 2 molecules tend to migrate toward the pore center, while CH 4 and CO 2 remain preferentially adsorbed on micropore surfaces or within coal matrix interiors, even when their molecular counts are lower than that of H 2 . The desorption activation energy (Δ E ) increases with coal rank and follows the order CO 2 > CH 4 > H 2 , further supporting the practical feasibility of H 2 extraction using CH 4 or CO 2 as cushion gases. Collectively, these findings provide the first multi-rank comparative insight into H 2 storage mechanisms in coal seams, offering quantitative guidelines for cushion gas selection and operational condition optimization in future field applications.