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Boron-nitrogen based materials for hydrogen storage

Final Activity Report Summary - BN HYDROGEN STORAGE (Boron-nitrogen based materials for hydrogen storage)

We have performed computational studies of materials for hydrogen storage based on boron and nitrogen. In particular, we have explored a new concept of hierarchical hydrogen storage. We have in mind materials with more than one level of hydrogen storage, e.g. hydrogen clathrates of ammonia borane NH3BH3 (AB). There would be two levels of hydrogen storage in this material: (i) physisorbed H2 and (ii) hydrogen chemically bound in ammonia borane. The advantages of these materials would be: (i) the fast kinetics of release and uptake of physisorbed hydrogen and (ii) very high gravimetric and volumetric overall hydrogen density. Ammonia borane is a highly polar molecule, in which hydrogens connected to the nitrogen atom are positively charged (protic) and hydrogens connected to the boron atom and negatively charged (hydridic). As a result, molecules of AB interact with each other through the so-called dihydrogen bonds, in which protic hydrogens, H(N), interact with hydridic hydrogens, H(B).

We identified three types of cages built from AB that could be used to construct extended systems. These are truncated octahedron, truncated cuboctahedron or great rhombicuboctahedron, and truncated icosidodecahedron or great rhombicosidodecahedron. We formulated structural rules for the molecular crystal of AB and clathrates thereof. These rules are similar to the Bernal-Fowler rules ('ice rules') for water and ice. We analysed which structures could be feasible for clathrates of AB. We performed a rigorous analysis of the uniform space filling tessellations of 3D space based on the three fundamental cages of AB. The screening of proposed periodic structures has been performed on the basis of their stability determined at the density functional level of theory (DFT). The cantitruncated cubic honeycomb structure was found to be the most stable. Its increase in ground state electronic energy per one AB molecule with respect to the AB molecular crystal is only 0.32 kcal/mol. The same parameter calculated for the structure II of hydrates, in form of which hydrogen hydrates crystallize, with all H(O) hydrogens placed according to the Bernal-Fowler rules is 0.18 kcal/mol, which is an increase in the ground state electronic energy per one water molecule with respect to the ordinary ice Ih. The difference in stability of the water and AB clathrates with respect to the corresponding molecular crystal is very small, even smaller than the anticipated accuracy of the DFT predictions.

We suggest that the AB clathrates would most probably exist as the cantitruncated cubic honeycomb structure, providing the guest molecules are small enough to fit in the cages involved. Finally, we determined the capacity of the proposed clathrate structure to store molecular hydrogen. Our results indicate that the hydrogen capacity of the most stable clathrate structure of AB would be 21 wt%, 19 wt% chemically bound in AB and 2 wt% of H2 physisorbed in the cages of AB.