Hydrogen Storage in light complex Hydrides – structural studies

The most important unsolved problem for the introduction of the Hydrogen Economy is efficient and safe storage of hydrogen. Alanates, compounds based on the AlH4 unit, are among the most promising metal hydrides for reversible hydrogen storage. The storage capacity is large, e.g. NaAlH4 can release 5.6 wt% hydrogen below 200 C. Work during the last years has revealed that Ti additives improve the kinetics of NaAlH4 and also make re-hydrogenation possible. In order to improve the understanding of the effect of additives and absorption/desorption processes in general, detailed structural studies are very important. Crystal structures of MAlD4, (M=Li, Na, K) Li3AlD6, Mg(AlH4)2 and mixed alanates, like Na2LiAlD6, have been determined from high resolution powder neutron and X-ray diffraction. To understand the nature of additives high-resolution synchrotron X-ray and neutron diffraction experiments have been carried out. NaAlH4 added with Ticompounds shows no sign of solid solution of Ti into neither Na nor Al positions. However, samples being cycled indication the presence of an Al1-xTix alloy. In-situ desorption experiments (both synchrotron X-ray and neutron diffraction) have been important for detailed studies of the desorption processes. LiAlD4 has been shown to decompose completely to LiD, Al and D2 at 127 C, releasing 7.9 wt% hydrogen. Addition of VCl3 by ball milling significantly increases the reaction rate. Recent synchrotron X-ray in-situ experiments will be presented.