Molecular Insights into Hybrid CH4 Physisorption-Hydrate Formation in Spiral Halloysite Nanotubes: Implications for Energy Storage

A microscopic insight into hybrid CH4 physisorption-hydrate formation in halloysite nanotubes (HNTs) is vital for understanding the solidification storage of natural gas in the HNTs and developing energy storage technology. Herein, large-scale microsecond classical molecular dynamics simulations are conducted to investigate CH4 storage in the HNTs via the adsorption-hydration hybrid (AHH) method to reveal the effect of gas-water ratio. The simulation results indicate that the HNTs are excellent nanomaterials for CH4 storage via the adsorption-hydration hybrid method. The CH4 physisorption and hydrate formation inside and outside of the HNTs are profoundly influenced by the surface properties of the HNTs and the kinetic characteristics of CH4/H2O molecules. The outer surfaces of the HNTs exhibit relative hydrophobicity and adsorb CH4 molecules to form nanobubbles. Moreover, CH4 molecules adsorbed on the outer surface are tightly trapped between the hydrate solids and the outer surface, inhibiting their kinetic behavior and favoring CH4 storage via physisorption. The inner surface of the HNTs exhibits extreme hydrophilicity and strongly adsorbs H2O molecules; thus, CH4 hydrate can form inside of the HNTs. It is more difficult for CH4 and H2O molecules inside of the HNTs to convert into hydrates than for those outside of the HNTs. A moderate gas-water ratio is advantageous for CH4 physisorption and hydrate formation, whereas excessively high or low gas-water ratios are unfavorable for efficient CH4 storage. These insights can help to develop an efficient CH4 solidification storage technology.