Speaker
Description
Fast and efficient hydrogen storage is one of the key components for the use of hydrogen in a sustainable energy economy. Reactive Hydride composites have been considered for some time as potential solid state storage systems, among others also the amide based mixture Mg(NH$_2$)$_2$ + 2 LiH. The kinetic performance of the hydrogen exchange reaction in this system is significantly enhanced by the addition of LiBH$_4$ [Gizer et al. Inter. J. Hydrogen Energy 44, 11920-11929 (2019)] and the subsequent formation of the amide-borohydride compound Li$_4$(BH$_4$)(NH$_2$)$_3$. Here, we present a study of the structure and of the anion motion of in Li$_4$(BH$_4$)(NH$_2$)$_3$ investigated with synchrotron radiation powder X-ray diffraction (SR-PXD) and quasielastic neutron scattering (QENS) at temperatures close to operating condition. SR-PXD confirms the recrystallization of Li$_4$(BH$_4$)(NH$_2$)$_3$ into the $\alpha$-phase during cooling from the melt. The QENS measurements prove a long-range diffusive motion of hydrogen containing species at 514 K with the diffusion coefficient $D \sim 10^{-6}\,\frac{\mathrm{cm}^2}{\mathrm{s}}$. At temperatures below 514 K, localized rotational motions were observed which have been attributed to (BH$_4^-$ tetrahedra units undergoing rotations mainly around $C_3$ axes. The results will be discussed in the context of the improved hydrogen exchange reaction that is observed in Mg(NH$_2$)$_2$ + 2 LiH with LiBH$_4$ additions.
