Reaction Mechanisms in the Li3AlH6/LiBH4 and Al/LiBH4 Systems for Reversible Hydrogen Storage. Part 2: Solid-State NMR Studies

In Part 1, the promising hydrogen storage properties of the combined systems of Li3AlH6/LiBH4 and Al/LiBH4, exhibiting the favorable formation of AlB2 during dehydrogenation, were presented based on TGA and XRD analyses. The present Part 2 describes the characterization of the intermediate and final products of the dehydrogenation and rehydrogenation of the above systems by multinuclear solid-state NMR characterization. This work has also verified that the presence of Al resulted in the re-formation of LiBH4 occurring at a much lower temperature and H-2 pressure, under which conditions the dehydrogenation product from LiBH4 alone does not show any degree of rehydrogenation. NMR studies mainly identified various reaction intermediates for LiBH4 dehydrogenation/rehydrotrnation reactions. Unlike the XRD studies, the AlB2 formation, in particular, could not be unambiguously confirmed by NMR. Al-27 NMR showed that aluminum was mainly involved in various Li-Al alloy formations. The catalytic role of Al in the LiBH4 hydrogen storage reactivity could be achieved by a reversible cycle of the Al + LiH <-> LiAl + 1/2H(2) reaction.