Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite

Abstract

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.6•10 Ϫ10 to 1.8•10 Ϫ9 m 2 /s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.

Effect of cation distribution on self-diffusion of molecular hydrogen in Na3Al3Si3O12 sodalite: A molecular dynamics study

“…[20][21] We have found in previous studies that it is essential to model the framework in a fully flexible manner in order to accurately model the activated diffusion process. 4,12 Alternatively, one could decide to keep the framework rigid; however, our calculations have indicated that this would artificially increase the energy barrier for diffusion by ϳ20% in the case of NaAlSi-SOD, due to the total loss of six-ring flexibility. Furthermore, it would mask any important influences of cation distribution on six-ring flexibility and, thus, diffusion rates ͑vide infra͒.…”

“…4 All hops that happened within 1 ps of each other were checked manually. A few rapid recrossing events were observed ͑see Table IV͒; however, these were not counted as a hopping event.…”

“…5 ͑gray line͒. 4 Since in all-Si-SOD all six-ring windows are accessible, the diffusion coefficient can be calculated with Eq. ͑1͒ with dimϭ3.…”

“…In previous work, 4 the diffusion of molecular hydrogen in all-Si-SOD was modeled in order to investigate the feasibility of sodalite as hydrogen encapsulating storage material. Although the modeled diffusion rate was promisingly high, all silica sodalite cannot be used in practice, because of the organic template around which the sodalite structure is built and which cannot be removed fully 5,6 without destroying the structure.…”

Molecular hydrogen confined within nanoporous framework materials: Comparison of density functional and classical force-field descriptions

“…The accuracy of such centrosymmetric H 2 -H 2 potentials is also evidenced by their use in the setup of high level quantum mechanical calculations of H 2 cluster systems, 11,12 the calculation of the properties of bulk dense H 2 phases, 2,3 and also for larger scale classical molecular dynamics calculations. [13][14][15] More recently the use of density functional theory ͑DFT͒ has been widespread for estimating the properties of H 2 in confined systems. [15][16][17][18][19][20][21] In such studies the binuclear aspect of the H 2 molecule is explicit and the H 2 -H 2 interaction is provided in an ab initio electronic manner albeit indirectly via the choice of functional.…”