Hydrogen Storage in Double Structure Hydrates with SF₆ and TBAB Presence

Hydrogen (H₂) is a promising clean energy source for high energy density and clean combustion products. Hydrogen storage and transport are bottlenecks of the utilization of hydrogen energy. Gas hydrates could store hydrogen molecules at moderate temperatures and pressures, offering a technology of safe, cheap hydrogen storage. However, low hydrogen storage is a major problem in developing large-scale hydrate-based hydrogen storage. In this study, in order to increase the hydrogen storage capacity, we use Sulphur hexafluoride (SF₆), which can form sII hydrates, and tetrabutylammonium bromide (TBAB), which forms semiclathrate hydrates, to construct a double structure hydrate storing hydrogen. The experimental temperature was set at 274 K, and the H₂ pressure was around 20 MPa. The concentration of TBAB solution ranged from 0 wt% to 40 wt%. The highest H₂ storage appeared at 5 wt% of TBAB with 20 wt% SF₆. The capacity of H₂ reached 32 V/V (about 0.324 wt%), which is an increase of 15% compared with no TBAB added. This is due to the fact that in the early stage of hydrate formation, TBAB first form semiclathrate hydrate with water, which increases the nucleation site of sII hydrate. The coupling structure of sII and semiclathrate hydrates increases the hydrogen storage. However, as the TBAB increases, it is easy to form larger particles of TBAB hydrates. This process expends a large number of water molecules, which decrease the formation of sII hydrates, and lower the H₂ storage. Raman spectra of samples with TBAB revealed that the Raman characteristic peaks of SF₆ and H₂ increased towards higher wave numbers compared with samples without TBAB. This showed that the introduction of TBAB increased the binding energy of the hydrate cages, enhancing the hydrogen hydrate stability.