Reactivity of magnesium borohydride – Metal hydride composites, $\mathrm{γ-Mg(BH_{4})_{2}-MH_{x}, M = Li, Na, Mg, Ca}$

The reactivity and thermal decomposition of γ-Mg(BH$_4$)$_2$-MH$_x$, M = Li, Na, Mg, and Ca composites has been examined with the objective of studying the hydrogen storage capability of the composites. The samples were prepared by manual grinding $γ$-Mg(BH$_4$)$_2$ with a metal hydride to obtain homogenous mixtures. In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) and simultaneous thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry was performed to analyse the decomposition mechanism, whereas solid-state $^{11}$B nuclear magnetic resonance spectroscopy and SR-PXD was used to investigate the decomposition products. Interestingly, substitution reactions take place between magnesium borohydride and lithium, sodium and calcium hydride forming the more stable metal borohydrides, M(BH$_4$)$_x$, M = Li, Na or Ca. The composite $γ$-Mg(BH$_4$)$_2$-LiH has a hydrogen release at T ∼380–420 °C, which indicates the formation of amorphous LiBH$_4$ during decomposition. For the composites γ-Mg(BH$_4$)$_2$-NaH, formation of crystalline NaBH$_4$ is observed by SR-PXD from T = 150–450 °C, and hydrogen release ascribed to NaBH$_4$ is observed in MS data at T = 460–480 °C. $γ$-Mg(BH$_4$-MgH$_2$ composite decomposes as the individual constituents. $β$-Ca(BH$_4$ is formed at T = 175–375 °C in the composites of $γ$-Mg(BH$_4$-CaH$_2$. Bragg diffraction from CaB$_6$ at T > 370 °C is detected by SR-PXD for $γ$-Mg(BH$_4$-CaH$_2$ (1:0.5) but not for samples richer in CaH2. Release of diborane was not observed for any of the magnesium borohydride metal hydride composites