NERSC Initiative for Scientific Exploration (NISE) 2009 Awards

Accommodation of H₂ into Hydrate Lattices for Hydrogen Storage Devices

Sotiris Xantheas, Pacific Northwest National Laboratory

Sponsoring NERSC Project: Chemical reactivity, solvation, and multi-component heterogeneous processes in aqueous environments (mp329), Principal Investigator: Bruce Garrett, Pacific Northwest National Laboratory

Hydrate crystal structures are water scaffolds held together by hydrogen bonds that can act as host lattices by trapping guest molecules such as H2, CO2, Cl2, Br2, SO2, H2S, and CH4 within their cages via weak van der Waals interactions. The potential importance of clathrate hydrates (i.e., the combination of the host lattice and the guest molecules) as inclusion compounds relevant to renewable energy (i.e., molecular hydrogen storage devices) has recently been emphasized.

All of the natural gases that form clathrate hydrates do so in the following three crystal structures: sI (cubic), sII (cubic), and sH (hexagonal). These 3D lattices are formed by a combination of the following building blocks (cages): (i) the pentagonal dodecahedron (D-cage), consisting of 20 water molecules forming 12 pentagonal faces; (ii) the tetrakaidecahedron (T-cage), consisting of 24 water molecules forming 12 pentagonal and two hexagonal faces; (iii) the hexakaidecahedron (H-cage), consisting of 28 water molecules forming 12 pentagonal and four hexagonal faces; (iv) the irregular dodecahedron, consisting of 20 water molecules forming three tetragonal, six pentagonal, and three hexagonal faces and (v) the icosahedron, consisting of 36 water molecules that form 12 pentagonal and eight hexagonal faces.

The unit cell of the (sI) hydrate comprises two units of the D- and six units of the T-cages. The unit cell of the (sII) hydrate comprises 16 units of the D- and eight units of the H-cages. The (sII) hydrate has been shown to meet current U.S. Department of Energy’s target densities for an on-board hydrogen storage system.

A bottleneck in the modeling of those constituent cages and corresponding 3-D hydrate networks lies with the existence of a plethora [(3/2)^N] of possible isomers based on the position of the hydrogen atoms that satisfy the Bernal-Fowler rules. We have previously [see Kirov, Fanourgakis, and Xantheas, Chem. Phys. Lett. Frontiers article 461, 180 (2008)] outlined an approach to scan all possible networks of the constituent cages (30,026 for the D-cage, 3,043,836 for the T-cage) using a discrete model and subsequently refine them using levels of electronic structure theory of increasing accuracy (DFT, MP2). We have subsequently used the identified low-energy networks of the D- and T-cages to construct periodic networks of the sI hydrate [see Yoo, Kirov and Xantheas, J. Amer. Chem. Soc. Communication to the Editor, 131, 7564 (2009)].

We propose to use high-level electronic structure calculations to study the accommodation of H2 inside the D- and T-cages and subsequently into the extended periodic hydrate lattices. Previous results obtained at the DFT level of theory are in disagreement as to the amount of H2 that can be accommodated inside those cages. We plan to refine those calculations at a more accurate level of theory [MP2 and possibly CCSD(T)]. Recent developments at the Oak Ridge National Laboratory in the NWChem suite of electronic structure programs coupled with analogous advances in adapting parallel tools (Global Arrays) suggest the efficient scaling of that code to an excess of 100,000 processors for CCSD(T) calculations enabling the study of systems as large as the D- and T-cages with even a triple zeta quality basis set.