Effects of guest molecular occupancy and electric field on thermal conductivity of CO2 hydrates.

Abstract

CO2 hydrate technology plays a pivotal role in carbon dioxide capture/storage, gas separation, and natural gas recovery from natural gas hydrates, while simultaneously serving as a cost-effective phase-change material for thermal energy storage. The thermal transport characteristics of CO2 hydrates are of particular importance in these promising applications. Here, the role of CO2 molecular occupancy and external electric fields on the thermal conductivity (κ) of sI-type CO2 hydrates is explored using equilibrium molecular dynamics simulations. Results reveal that increasing CO2 occupancy in large 51262 cages enhances κ by up to 27.2%, while small 512 cages contribute minimally (<1%). The water framework dominates heat transport (>90%), with CO2@51262 and CO2@512 cages contributing ∼17%-18% and <1%, respectively, mediated by synergistic host-guest interactions. External electric fields reduce κ by around 4%-5% due to enhanced low-frequency phonon localization in CO2 and intensified anharmonic scattering. Phonon analyses, including phonon density of states, phonon lifetime, phonon participation ratio, and spectral energy density, reveal that CO2 occupancy suppresses water lattice vibrations, while electric fields redistribute phonon modes, reducing delocalization. This work advances the fundamental understanding of thermal transport in hydrate systems.