Physical Storage Using Nanostructured and Porous Materials - Hydrogen Generation, Storage, and Utilization - page 132

Region I

4

Region II Region III

3

2

0 1 2 3 4 5 6

Concentration of THF (mol%)

: H 2
: THF

Region I Region II Region III

5 12 5

5 12 6 4

5 12 6 4

FIGURE 7.9 H 2 gas content as a function of THF concentration, and a schematic diagram of H 2 dis-
tribution in the cages of THF+H 2 hydrate. (H 2 gas content is calculated from g of H 2 per g of hydrate,
and expressed as wt%.) In region III, H 2 molecules are only stored in small cages, while in region II,
both small and large cages can store H 2 molecules. At the highly dilute THF concentrations of region I,
H 2 molecules can still be stored in both cages, but extreme pressures (∼2 kbar) are required to form the
hydrates. Pure H 2 clathrate (2H 2 ) 2 ·(4H 2 )·17H 2 O would have a 5.002 wt% H2 content. Source : Reproduced
with permission from Lee et al. [50]. (See color insert.)

computational study based on first principles electronic structure calcula-
tions of the pentagonal dodecahedron, (H 2 O) 20 , (D-cage) and tetrakaideca-
hedron, (H 2 O) 24 , (T-cage) building blocks of structure I (sI) hydrate lattice
suggest that these can accommodate up to a maximum of 5 and 7 guest
hydrogen molecules, respectively [54]. For the pure hydrogen hydrate, Born-
Oppenheimer molecular dynamics (BOMD) simulations of periodic (sI)
hydrate lattices indicate that the guest molecules are released into the vapor
phase via the hexagonal faces of the larger T-cages. The presence of methane
in the larger T-cages was found to block this release, therefore suggesting
possible means for stabilizing these coated clathrate hydrates and the poten-
tial enhancement of their hydrogen storage capacity.