Tuning hydrogen storage properties and reactivity: Investigation of the LiBH4–NaAlH4 system Original Research Article

Tuning the hydrogen storage properties of complex metal hydrides is of vast interest. Here, we investigate the hydrogen release and uptake pathways for a reactive hydride composite, LiBH4−NaAlH4 utilizing in situ synchrotron radiation powder X-ray diffraction experiments. Sodium alanate transforms to sodium borohydride via a metathesis reaction during ball milling or by heating at T∼95 °C. NaBH4 decomposes at ∼340 °C in dynamic vacuum, apparently directly to solid amorphous boron and hydrogen and sodium gas and the latter two elements are lost from the sample. Under other conditions, T=400 °C and p(H2)=∼1 bar, NaBH4 only partly decomposes to B and NaH. On the other hand, formation of LiAl is facilitated by dynamic vacuum conditions, which gives access to the full hydrogen contents in the LiBH4−NaAlH4 system. Formation of AlB2 is observed (T∼450 °C) and other phases, possibly AlBx or Al1−xLixB2, were observed for the more Li-rich samples. This may open new routes to the stabilization of boron in the solid state in the dehydrogenated state, which is a challenging and important issue for hydrogen storage systems based on borohydrides.