Hydrogenation properties of lithium and sodium hydride – closo -borate, $\mathrm{[B_{10}H_{10}]^{2−}$ and $[B_{12}H_{12}]^{2−}}$ , composites

Steffen R. H. Jensen, Mark Paskevicius, Bjarne R. S. Hansen, Anders S. Jakobsen, Kasper T. Møller, James L. White, Mark D. Allendorf, Vitalie Stavila, Jørgen Skibsted, Torben Jensen
The hydrogen absorption properties of metal closo-borate/metal hydride composites, M$_2$B$_{10}$H$_{10}$–8MH and M$_2$B$_{12}$H$_{12}$–10MH, M = Li or Na, are studied under high hydrogen pressures to understand the formation mechanism of metal borohydrides. The hydrogen storage properties of the composites have been investigated by in situ synchrotron radiation powder X-ray diffraction at p(H$_2$) = 400 bar and by ex situ hydrogen absorption measurements at p(H$_2$) = 526 to 998 bar. The in
more » ... experiments reveal the formation of crystalline intermediates before metal borohydrides (MBH$_4$) are formed. On the contrary, the M$_2$B$_{12}$H$_{12}$–10MH (M = Li and Na) systems show no formation of the metal borohydride at T = 400 °C and p(H$_2$) = 537 to 970 bar. 11B MAS NMR of the M$_2$B$_{10}$H$_{10}$–8MH composites reveal that the molar ratio of LiBH4 or NaBH$_4$ and the remaining B species is 1 : 0.63 and 1 : 0.21, respectively. Solution and solid-state 11B NMR spectra reveal new intermediates with a B : H ratio close to 1 : 1. Our results indicate that the M$_2$B$_{10}$H$_{10}$ (M = Li, Na) salts display a higher reactivity towards hydrogen in the presence of metal hydrides compared to the corresponding [B$_{12}$H$_{12}$]$^{2-}$ composites, which represents an important step towards understanding the factors that determine the stability and reversibility of high hydrogen capacity metal borohydrides for hydrogen storage.
doi:10.3204/pubdb-2019-00240 fatcat:ddgxp6rsjfgqronpvdkoavtyxe