Probing the pathway of H2-THF and H2-DIOX sII hydrates formation: Implication on hydrate-based H2 storage

Hydrogen (H2) is recognized as a promising energy carrier for the future world, owing to its high energy density and zero carbon emissions. H2 storage in the form of solidified hydrates represents an emerging economic-viable and eco-friendly technology for large-scale application. Thermodynamic hydrate promoters (THPs) enhanced H2 hydrate formation at mild pressure conditions by forming sII or sH hydrates. However, the formation pathway and promotion mechanism of H2-THP sII hydrates are yet to be elucidated. In this study, the composition of H2-THF/H2-DIOX sII hydrates was analyzed in both 5.56 mol% THF and DIOX systems by high-pressure μ-DSC. The well-designed kinetic experiments coupled with morphology observation were conducted to reveal the key stage for H2-THF/H2-DIOX hydrates formation at pressures from 8.3 MPa to 18.3 MPa. Moreover, Raman spectroscopy was employed to validate the proposed formation pathway and the cage occupancy ratio of H2 and THP molecules. Both pure THP hydrates and H2-THP hydrates were identified by high-pressure μ-DSC. Formation of H2-THP sII hydrates requires a specific condition that both H2 and THP molecules simultaneously occupy the small and large cages of sII hydrates for 5.56 mol% THP. Based on the Raman spectroscopy, the ratio of H2 molecules in the 512 small cages to THP molecules in the 51264 large cages increased with pressure. The experimental results contribute to a fundamental understanding of the role of THP in promoting H2-THP hydrate formation. The findings guide the adoption of effective THPs with optimal concentrations and contact patterns for hydrate-based H2 storage technology.