The effect of H2 occupancy modes in small and large cages of H2-tetrahydrofuran hydrates on the hydrates' stability and H2 storage capacity

Hydrogen storage as hydrates is one of the most environmentally benign approaches to store hydrogen as it requires only water and traces of promoters. However, the scalability of storing hydrogen via hydrate formation is hindered by the limited understanding of the structure, dynamics and energetics of hydrogen and promoters in the hydrate cages. In this study, molecular dynamics simulation configurations with different occupancy modes of H2 and tetrahydrofuran (THF) in the hydrate cages are investigated under the following scenarios: (i) two H2 molecules occupying the small cages, (ii) occupancy of H2 molecules in the THF-free large cages, and (iii) co-occupancy of H2 and THF in one large cage. Exploring these scenarios reveals the impact of occupancy modes on the dynamic motion of guest and water molecules and on the hydrate structure stability. The results show that the occupancy of two H2 molecules in the small cages reduced the stability of the hydrate structure, triggered the inter-cage hopping of H2 molecules through pentagonal faces, and increased the probability of hydrogen bond formation between THF and cage H2O molecules. The thermodynamic stability of hydrate cages is increased when the THF-free large cages are occupied by H2 molecules and the tetrahedral feature of H2 distribution in the large cages is enhanced when the number of H2 in one cage increased from two to three. When the large cages are co-occupied by H2 and THF, the inter-cage migration of H2 originating from large cages demonstrated two different features, i.e., ballistic motion (MSD proportional to t2) due to the tunneling migration behavior in the initial stage and diffusive motion in the late stage. The ballistic migration of H2 molecules is more favorable for achieving a higher hydrogen storage capacity. The decay rate of THF orientation is reduced when the interaction between THF and H2O molecules is stronger. The mechanistic insights provided by this study are crucial to advancing hydrogen storage as hydrates for a sustainable energy future.