Hydrogen reversibility of LiBH4–MgH2–Al composites

Abstract

The detailed mechanism of hydrogen release in LiBH₄–MgH₂–Al composites of molar ratios 4 : 1 : 1 and 4 : 1 : 5 are investigated during multiple cycles of hydrogen release and uptake. This study combines information from several methods, i.e., in situ synchrotron radiation powder X-ray diffraction, ¹¹B magic-angle spinning (MAS) NMR, Sievert's measurements, Fourier transform infrared spectroscopy and simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy. The composites of LiBH₄–MgH₂–Al are compared with the behavior of the LiBH₄–Al and LiBH₄–MgH₂ systems. The decomposition pathway of the LiBH₄–MgH₂–Al system is different for the two investigated molar ratios, although it ultimately results in the formation of LiAl, Mg_xAl_1−xB₂ and Li₂B₁₂H₁₂ in both cases. For the 4 : 1 : 1-molar ratio, Mg_0.9Al_0.1 and Mg₁₇Al₁₂ are observed as intermediates. However, only Mg is observed as an intermediate in the 4 : 1 : 5-sample, which may be due to an earlier formation of Mg_xAl_1−xB₂, reflecting the complex chemistry of Al–Mg phases. Hydrogen release and uptake reveals a decrease in the hydrogen storage capacity upon cycling. This loss reflects the formation of Li₂B₁₂H₁₂ as observed by ¹¹B NMR and infrared spectroscopy for the cycled samples. Furthermore, it is shown that the Li₂B₁₂H₁₂ formation can be limited significantly by applying moderate hydrogen partial pressure during decomposition.