Atomistic–Geometric–Continuum Relations of Hydrogen Structure II Hydrates under Triaxial Strains

Gas hydrates are polyhedral structures formed by a hydrogen-bonded water network. The cage-like structure of the network makes these materials ideal candidates for large-scale energy storage. Hydrogen is viewed as a clean energy source, but its storage remains challenging because of its low density, requiring large storage volumes. Given their composition, hydrates offer a green platform for hydrogen storage. However, the high pressure and low temperature required for hydrate formation make it challenging and costly to explore and characterize their material properties. In this study, we will use a first-principles approach to investigate the stability limits of sII hydrates under triaxial stresses for various compositions of hydrogen in sII hydrates. Additionally, the nonuniform structure of hydrates at the lattice scale is explored. The geometric features are used to better understand the compensatory mechanisms under stress, including cage integrity, polyhedral face deformation, and bond lengths and angles of water oxygen atoms within and beyond stability limits. The main findings indicate that the tensile stability limit decreases with increasing hydrogen concentration in both small and large cages of the structure, whereas the compressive strength increases. The large cages are more susceptible to changes under triaxial stresses because of their hexagonal faces. The OH covalent and hydrogen bond lengths exhibit opposite behaviors, either increasing or decreasing, and the changes in bond angles are more sensitive to triaxial stresses under compression than to the bond length changes. The provided quantitative structure–property relationships enhance our evolving understanding of the intermolecular and intramolecular interactions within the water-based network under triaxial stresses and contribute to the technological advancement of green hydrogen storage materials.