Microscopic Insights into Hydrate Cage Decomposition and CO2 Release of CO2+ TBAB Double Hydrate in Cold Energy Discharge

Semiclathrate tetrabutylammonium bromide (TBAB) hydrate can incorporate gas CO₂ to form CO₂ + TBAB double hydrate, which is an ideal gas and energy storage material due to its high energy density and favorable phase change conditions for refrigeration applications. In this study, the dissociation behaviors and underlying mechanism of this CO₂ + TBAB double hydrate at the microscale for energy release are studied through molecular dynamic simulations under thermal-driven conditions (283 K-301 K). The results indicate that two distinct hydrate cages (CO₂ 5¹² cages and TBAB semiclathrate cages) decompose in a stepwise manner during dissociation and exhibit distinct features. For the TBAB semiclathrate cage, the TBA⁺ ions initially adsorb on the hydrate surface as cages collapse and ultimately diffuse to the liquid phase. This phenomenon is attributed to the strong interaction between TBA⁺ ions and the hydrate cages strengthens the stability of the TBAB semiclathrate cage and empty the 5¹² cage after CO₂ release. For the CO₂ 5¹² cage, the cages deform and decompose upon the release of the encaged CO₂ molecules. The temperature dependence on the dissociation rate of the CO₂ + TBAB double hydrate could be described using the Arrhenius formula, with the activation energy determined to be 61.53 kJ mol^-1. A reduction in the 5¹² cage occupancy greatly diminishes hydrate stability and promotes the hydrate dissociation rate. Furthermore, the regulation of the CO₂ molecule on hydrate dissociation is realized through affecting the reformation of the 5¹² cage. The increase of the CO₂ molecule concentration in the liquid phase shows an inhibitory effect on the release of the CO₂ molecule from the hydrate cage because the possibility of reformation of the 5¹² cage at the hydrate surface increases, providing microscopic insights into the dissociation behavior of hydrate induced by depressurization at the macroscopic scale.