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 Møller-Plesset perturbation theory, we obtain occupancies of up to eight H₂ molecules in the large 5¹²6⁴ cage, six in the medium 5¹²6² cage, and two in the small 5¹² cage of the clathrates, but the optimum number of H₂ molecules that these types of cages prefer to accommodate are two, two, and one, respectively. The simulated infrared and Raman spectra of the encaged H₂ molecules agree with recent experimental observations. Strongly coupled vibrational modes appear when three H₂ molecules occupy the 5¹²6² cage of the type I clathrate hydrate and five H₂ molecules occupy the 5¹²6⁴ cage of the type II clathrate hydrate, respectively. Moreover, when the occupancies of the 5¹²6² cage in the type I clathrate and the 5¹²6⁴ cage in the type II clathrate are up to three and seven H₂ molecules, respectively, both calculated energy barriers for one H₂ 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.