Hydrogen Storage Capacity Loss in a LiBH4-Al Composite

A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake cycles for the reactive composite LiBH4-Al (2:3) is presented. Furthermore, the influence of a titanium boride, TiB2, additive is investigated. The study combines information from multiple techniques: in situ synchrotron radiation powder X-ray diffraction, Sieverts measurements, simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. The decomposition of LiBH4-Al results in the formation of LiAl, AlB2, and Li2B12H12 via several reactions and intermediate compounds. The TiB2 additive appears to have a limited effect on the decomposition pathway of the samples, but seems to facilitate formation of intermediate species at lower temperatures compared to the sample without additive. Solid solutions of LixAl1-xB2 or Al1-xB2 are observed during decomposition and from Rietveld refinement the composition of the solid solution is estimated to be Li0.22Al0.78B2. The intercalation of Li in the AlB2 structure is further investigated by B-11 and Al-27 MAS NMR spectra of the LiH-AlB2 and AlB2 samples (presented in Supporting Information). Hydrogen release and uptake for LiBH4-Al reveals a significant loss in the hydrogen storage capacity, that is, after four cycles a capacity of about 45% remains, and after 10 cycles, the capacity is degraded to approximately 15% of the theoretically available hydrogen content. This capacity loss may be due to the formation of Li2B12H12, as observed by B-11 MAS NMR and Raman spectroscopy. Formation of Li2B12H12 has previously been observed during the decomposition of LiBH4, but it has not been reported earlier in the LiBH4-Al (2:3) system.