Hydrogen and ammonia both have great potential as carbon-neutral energy carriers for the future. A hydrogen-based energy infrastructure would be more environmentally friendly by preventing carbon dioxide emission and a reduced dependence on the limited non-renewable energy sources, e.g., petroleum, coal and natural gas. However, there are still some major challenges waiting to be addressed concerning the production, storage and the everyday use of hydrogen and ammonia.
Storing hydrogen as a gas or liquid is possible but not efficient in terms of energy capacity and cost for on-board applications, due to the requirement of high pressures and cryogenic temperatures, respectively. Alternatively, hydrogen can also be stored with high capacity in the condensed phase. In particular, metal hydrides, carbon nanotubes, metal-organic frameworks, metal borohydrides and metal ammines have been studied extensively for solid state hydrogen storage.
Details of absorption and desorption mechanisms of NH3/H2 in different storage mediums are based on the crystal structure. This point becomes more delicate if the crystal structure is unknown, as in the case of the low temperature structure of Mg(NH3)6Cl2. Therefore, a new crystal structure prediction method based on Simulated Annealing (SA) is implemented and first applied to Mg(NH3)nCl2 with n=6,2,1. In metal ammines, hydrogen bonds between NH3′s hydrogens and chlorine atoms are important to stabilize the metal complex. This fact is exploited in the SA search to construct crystal structures by maximizing the number of hydrogen bonds within a (2×2×2) cut-through lattice using only several bond length constraints. SA optimizations found all the experimentally known structures and predicted the C2/m structure for the uncharacterized low temperature phase of Mg(NH3)6Cl2.
|Low temperature structures of Mg(NH3)6Cl2 complexes||High temperature structures of Mg(NH3)6Cl2 complexes|
Then the SA method applied to one of the promising metal borohydride, Mg(BH4)2, which stores 14.9 % wt of hydrogen. These SA optimizations successfully yielded previously proposed I4m2 and F222 symmetry structures of Mg(BH4)2. Further optimizations the Density Functional Theory (DFT) level indicated that the ground state structure of Mg(BH4)2 is the one with I4m2 symmetry.
In the last decade, LiBH4 has been proposed as a promising hydrogen storage medium due to its high gravimetric (18.5 % wt hydrogen) and volumetric (121 kg H/m3) hydrogen density. Although a considerable amount of papers have been published on LiBH4, a clear theoretical structure determination seems to suffer from a lack of methodological approach.Therefore, the potential energy surface of LiBH4 was investigated by the SA method and DFT calculations. A new stable orthogonal structure with Pnma symmetry was found, which is 9.66 kJ/mol lower in energy than the proposed Pnma structure. For the high temperature structure, a new orthorhombic P2/c structure was proposed, which is 21.26 kJ/mol over the ground-state energy and showed no lattice instability.