Hydrogen storage properties and mechanisms of a Mg(BH4)2·2NH3-NaAlH4 combination system

The Mg(BH4)2·2NH3-xNaAlH4 (x = 0-4) combination systems were prepared by ball milling, and the reversible hydrogen storage behavior and its mechanisms were investigated and discussed. Combining NaAlH4 with Mg(BH4)2·2NH3 significantly reduces the operating dehydrogenation temperatures and effectively suppresses the emission of NH3 by-products. The dehydrogenation onset temperature of the Mg(BH4)2·2NH3-2NaAlH4 system is lowered to ca. 70 °C, which is much lower than the onset temperatures of either Mg(BH4)2·2NH3 or NaAlH4. In addition, ammonia emission from Mg(BH4)2·2NH3 is thoroughly suppressed by the addition of NaAlH4, leading to approximately 11.3 wt% hydrogen released upon heating to 570 °C. Further investigations revealed that at the initial heating stage, Mg(BH4)2·2NH3 first reacts with NaAlH4 to produce NaBH4, Al3Mg2, Mg, Al0.95Mg0.05, BN, Na and AlN along with the release of hydrogen. Further increasing the operation temperature gives rise to a chemical reaction between NaBH4, AlN and Mg that liberates all of the hydrogen and yields the resultant products of MgAlB4, BN, Na and Al3Mg2. The dehydrogenated products can take up ∼3.5 wt% of hydrogen at 450 °C and 100 bar of hydrogen pressure, exhibiting a partial reversibility for hydrogen storage.