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 activation energy for this thermally activated process is found to be $E_a = $ 15.5$\pm$0.9 and 17.4$\pm$0.9$\mathrm{\frac{kJ}{mol}}$ for the two instrumental resolutions utilized in the QENS measurements, respectively, corresponding to observation times of 55 and 14 ps. 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.
