In situ X-ray Raman spectroscopy study of the hydrogen sorption properties of lithium borohydride nanocomposites

Abstract

Nanoconfined alkali metal borohydrides are promising materials for reversible hydrogen storage applications, but the characterization of hydrogen sorption in these materials is difficult. Here we show that with in situ X-ray Raman spectroscopy (XRS) we can track the relative amounts of intermediates and final products formed during de- and re-hydrogenation of nanoconfined lithium borohydride (LiBH₄) and therefore we can possibly identify the de- and re-hydrogenation pathways. In the XRS of nanoconfined LiBH₄ at different points in the de- and re-hydrogenation, we identified phases that lead to the conclusion that de- and re-hydrogenation pathways in nanoconfined LiBH₄ are different from bulk LiBH₄: intercalated lithium (LiC_x), boron and lithium hydride were formed during de-hydrogenation, but as well Li₂B₁₂H₁₂ was observed indicating that there is possibly some bulk LiBH₄ present in the nanoconfined sample LiBH₄–C as prepared. Surprisingly, XRS revealed that the de-hydrogenated products of the LiBH₄–C nanocomposites can be partially rehydrogenated to about 90% of Li₂B₁₂H₁₂ and 2–5% of LiBH₄ at a mild condition of 1 bar H₂ and 350 °C. This suggests that re-hydrogenation occurs via the formation of Li₂B₁₂H₁₂. Our results show that XRS is an elegant technique that can be used for in and ex situ study of the hydrogen sorption properties of nanoconfined and bulk light-weight metal hydrides in energy storage applications.