Reactivity of magnesium borohydride – Metal hydride composites, γ-Mg(BH4)2-MHx, M = Li, Na, Mg, Ca

Elisabeth Grube, Steffen R.H. Jensen, Ulla Gro Nielsen, Torben R. Jensen*

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The reactivity and thermal decomposition of γ-Mg(BH4)2-MHx, 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(BH4)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 11B 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(BH4)x, M = Li, Na or Ca. The composite γ-Mg(BH4)2-LiH has a hydrogen release at T ∼380–420 °C, which indicates the formation of amorphous LiBH4 during decomposition. For the composites γ-Mg(BH4)2-NaH, formation of crystalline NaBH4 is observed by SR-PXD from T = 150–450 °C, and hydrogen release ascribed to NaBH4 is observed in MS data at T = 460–480 °C. γ-Mg(BH4)2-MgH2 composite decomposes as the individual constituents. β-Ca(BH4)2 is formed at T = 175–375 °C in the composites of γ-Mg(BH4)2-CaH2. Bragg diffraction from CaB6 at T > 370 °C is detected by SR-PXD for γ-Mg(BH4)2-CaH2 (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.

TidsskriftJournal of Alloys and Compounds
Udgave nummerJanuary
Sider (fra-til)1155-1163
StatusUdgivet - 5. jan. 2019

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