Stability, Vibrations, and Diffusion of Hydrogen Gas in Clathrate Hydrates: Insights from Ab Initio Calculations on Condensed-Phase Crystalline Structures

Despite relevance to potential use as environmentally friendly hydrogen storage materials, major gaps in the understanding of the structures and diffusion of hydrogen gas in clathrate hydrates remain. Here, we apply a general ab initio computational method that can predict the Gibbs free energies and thus the optimum configurations of the hydrogen clathrate hydrates. Using this with second-order Moller-Plesset perturbation theory, we obtain occupancies of up to eight H-2 molecules in the large 5(12)6(4) cage, six in the medium 5(12)6(2) cage, and two in the small 5(12) cage of the clathrates, but the optimum number of H-2 molecules that these types of cages prefer to accommodate are two, two, and one, respectively. The simulated infrared and Raman spectra of the encaged H-2 molecules agree with recent experimental observations. Strongly coupled vibrational modes appear when three H-2 molecules occupy the 5(12)6(2) cage of the type I clathrate hydrate and five H-2 molecules occupy the 5(12)6(4) cage of the type II clathrate hydrate, respectively. Moreover, when the occupancies of the 5(12)6(2) cage in the type I clathrate and the 5(12)6(4) cage in the type II clathrate are up to three and seven H-2 molecules, respectively, both calculated energy barriers for one H-2 molecule migration through the hexagonal face are around 0.04 eV, which is in excellent agreement with the experiment. These findings provide important new insights into characterizing the hydrogen cage occupancy and a refreshing perspective of clathrate hydrates as hydrogen storage media.