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Tetrabutylammonium bromide (TBAB) semi-clathrate hydrate possesses a unique clathrate structure for capturing and sequestering small-molecule gases, such as CH 4 , H 2 and, CO 2 and the advantage of phase change energy storage. Elucidating the diversified reactions and determining the optimal phase change characteristics of TBAB hydrate is crucial to discovering and overcoming specific limitations hindering the large-scale applications. In this study, in situ macroscopic visualization, three-dimensional reconstruction via X-ray Computed Tomography, and differential scanning calorimeter measurements were performed to discover, verify, and analyze the distinct characteristics of TBAB hydrate phase change, including phase separation, three-dimensional structure, and thermal effects. The optimal concentration of TBAB hydrate energy storage was found and verified experimentally for the first time. A peculiar liquid layer (liquid-liquid phase separation) caused by hydrate-solution phase separation was observed after multiple formation-dissociation cycles. The liquid layer separated a region that optimized the hydrate phase change. The solutions with different initial concentrations could evolve an optimized phase-change region of the same concentration (37 wt%) via multiple cycles. The region of the same concentration was shown to be the result of optimal aggregation of molecules. In proving the particularity of the concentration in the optimized region, TBAB hydrate dissociation exhibit thermal effects that can be easily ignored (TBAB molecules ionization and ionic bond synthesis), which affected the TBAB hydrate latent heat measurement. Finally, the optimal concentration of TBAB hydrate phase change was determined as 37 ± 1 wt%, and the characteristics of the concentration, including the volume fraction, complete conversion of water molecules and TBAB molecules, and latent heat, were confirmed by the liquid layer. The experimental results obtained for the first time in this study are important guidance for TBAB hydrate in the field of gas capture and energy storage. [Display omitted] • Solid-liquid phase separation leads to liquid-liquid phase separation. • Guest molecule optimum aggregation hypothesis of memory effect was proposed. • Changes in ionic bonds play an important role in TBAB hydrate phase change. • Hydrate volume fraction could be up 99% by XCT 3D reconstruction. • 37 wt% was considered to be the optimal concentration. [ABSTRACT FROM AUTHOR]

In gas capture/storage and energy storage, tetrabutylammonium bromide (TBAB) hydrates have been regarded as superior carriers owing to their unique molecular selectivity and high latent heat. Hydrate conversion amount is a significant parameter in evaluating gas selection and cold storage characteristics. This study proposes a method for calculating hydrate conversion amount proposed based on the correlation between electrical conductivity and solution concentration. The correlation index (R2) between electrical conductivity and TBAB solution concentration was above 0.99 at concentrations less than below 5 wt%. The conversion amounts of different hydrate types were obtained using a unique hybrid calculation method. The differences in hydrate conversion amount, conversion ratio, and memory effect were compared at different mass fractions. The results indicated that the hydrate morphology and accumulation patterns were influenced by the TBAB solution ionization degree. Regular hexagonal TBAB hydrate crystals were observed at a low ionization degree. The maximum conversion ratio of TBAB hydrate reached 96% (1 °C/30 wt%). The variation trend of TBAB hydrate phase equilibrium temperature was consistent with that of the electrical conductivity of the solution. The investigation of the memory effect revealed that the phase change process of TBAB hydrate is more predictable in high-concentration initial solutions. [Display omitted] • Regular hexagonal hydrate was observed for the first time. • Insufficient ionization of TBAB slowed hydrate conversion. • TBAB hydrate phase equilibrium temperature was consistent with ionization degree. • Temperature, induction factor, and residual solution volume affected hydrate conversion. [ABSTRACT FROM AUTHOR]