Microscopic insights into the synergic promotion of hydrate formation by tetrabutylammonium bromide and CO2 for gas and energy storage by molecular dynamics

Abstract

CO₂ clathrate hydrate has emerged as a promising carbon and energy storage materials due to its high gas storage capacity (160–180 V V⁻¹) and latent heat (374 kJ kg⁻¹). The tetrabutylammonium bromide (TBAB) is used to thermodynamically promote CO₂ hydrate formation through forming double CO₂ + TBAB hydrate whose molecular structure combines characteristics of both clathrate hydrate and semi-clathrate hydrate. Molecular dynamic simulations are employed to attain microscopic insights into the hydrate growth and CO₂ encaging mechanisms of double CO₂ + TBAB hydrate. The results indicate that small 5¹² cages initially form on the surface of TBAB semi-clathrate hydrate, simultaneously encaging CO₂ molecules. Notably, these encaged CO₂ molecules can further migrate into inner empty 5¹² cages to enhance CO₂ absorption. The double hydrate formation is regulated by the two distinct types of guest molecules (CO₂/TBAB) through influencing the formation and stability of hydrate cages. Compared to pure CO₂ clathrate hydrate, the TBAB semi-clathrate cage provides 5¹² cages that can encage CO₂ molecule under milder conditions. Additionally, the presence of CO₂ promotes the formation of 5¹² cage, thereby enhancing the hydrate growth rate. Thus, TBAB and CO₂ molecules show synergic promoting effects on the growth of double CO₂ + TBAB hydrate. The results clarify the correlation between cage occupancy ratio at micro-scale and cold storage capacity of CO₂ + TBAB hydrate at macro-scale. Specifically, a higher 5¹² cage occupancy ratio leads to larger latent heat and cold storage capacity. The results corroborate the macroscopic properties observed experimentally regarding CO₂ absorption and cold storage performance of double CO₂ + TBAB hydrate where the latent heat increases from 284.8 kJ kg⁻¹ to 298.3 kJ kg⁻¹ as cage occupancy ratio rises from 0.62 to 0.72. These findings can assist in devising optimal production strategies for cold energy storage and CO₂ absorption.