Theoretical Study on Hydrogen Storage and Promoter Effect of Binary Clathrate Hydrates

Although clathrate hydrates(CHs) can be used as potential hydrogen storage materials for large-scale industrial applications due to their high energy storage density and environmental friendliness, major gaps in the understanding of the structures and diffusion of hydrogen gas in CHs remain. Here, we theoretically explored the hydrogen storage structure and properties of type I CHs using density functional theory(DFT) calculations. The results show that up to two H2 molecules can be occupied in the 512 cages of the CHs, and the CH4 and C2H6 promoters have no significant effect on its structure and properties. However, inclusion of the N2 and CO2 promoters in the CHs cages could considerably change the structures and properties including the number of H2 molecules occupied in the 512 cages increased to three, stability increase, the host-guest electrostatic and hydrogen bonding interactions enhance, and the H-H stretching blue-shifts of the H2 molecules. Moreover, when hydrogen occupies up to three H2 molecules in the 512 cages of N2 and CO2 CHs, calculated energy barriers for one H2 molecule migration through the pentagonal surface are close to twice that of pure hydrogen CHs, effectively hindering the diffusion of hydrogen between the cages.