Hierarchical storage of hydrogen in clathrates of ammonia borane

We have developed a concept of hierarchical hydrogen storage. It is illustrated by clathrates built from ammonia borane and loaded with molecular hydrogen. These new hypothetical materials would have two levels of hydrogen storage: (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 hydrogen density. We developed a construction principle for clathrates of ammonia borane, performed electronic structure calculations for isolated cages and for periodic structures built from these cages. Finally, we performed MD simulations for these clathrates loaded with hydrogen to determine their stability as a function of temperature and pressure. The nature of dihydrogen bonds between ammonia borane molecules and of guest-host interactions was analyzed within the framework of symmetry adapted perturbation theory of intermolecular forces.