Atomistic Investigation of the occupancy limits and stability of hydrogen hydrates as a hydrogen storage medium

Abstract

Gas hydrates are nonstoichiometric inclusion compounds which occur via a hydrogen bonded water network. This network forms cages that encapsulates gas molecules such as hydrogen which are stabilized by weak van der Waals forces. This unique cage-guest interaction offers the potential for a safe, economical, and green medium for hydrogen storage. In this study, we use Density Functional Theory (DFT) to investigate the storage capacity of hydrates which primarily depends on the hydrogen occupancy in the small and large cages of the lattice structure. Energy analysis of the system with various occupancies shows that DFT is indicating occupancy limits beyond what has been previously reported and that the maximum occupancy in small and large cages is interdependent. The maximum hydrogen occupancy is 9 in large cages with an occupancy of 1 and 2 in the small cages and a maximum of 8 with an occupancy of 3 or 4 in the small cages. The occupancy limit mechanism is the breakage of hydrogen bonds due to cage volume expansion, the diffusion of hydrogen through pentagonal and hexagonal faces in small and large cages, and computational convergence.