Hydrogen storage properties and mechanisms of the Mg(BH4)2–NaAlH4 system

Hydrogen storage properties and mechanisms of the combined Mg(BH4)2–NaAlH4 system were investigated systematically. It was found that during ball milling, the Mg(BH4)2–xNaAlH4 combination converted readily to the mixture of NaBH4 and Mg(AlH4)2 with a metathesis reaction. The post-milled samples exhibited an apparent discrepancy in the hydrogen desorption behavior with respect to the pristine Mg(BH4)2 and NaAlH4. Approximately 9.1 wt% of hydrogen was released from the Mg(BH4)2–2NaAlH4 composite milled for 24 h with an onset temperature of 101 °C, which is lowered by 105 and 139 °C than that of NaAlH4 and Mg(BH4)2, respectively. At initial heating stage, Mg(AlH4)2 decomposed first to produce MgH2 and Al with hydrogen release. Further elevating operation temperatures gave rise to the reaction between MgH2 and Al and the self-decomposition of MgH2 to release more hydrogen and form the Al0.9Mg0.1 solid solution and Mg. Finally, NaBH4 reacted with Mg and partial Al0.9Mg0.1 to liberate all of hydrogen and yield the resultant products of MgAlB4, Al3Mg2 and Na. The dehydrogenated sample could take up ∼6.5 wt% of hydrogen at 400 °C and 100 atm of hydrogen pressure through a more complicated reaction process. The hydrogenated products consisted of NaBH4, MgH2 and Al, indicating that the presence of Mg(AlH4)2 is significantly favorable for reversible hydrogen storage in NaBH4 at moderate temperature and hydrogen pressure.

► A reactive composite system of Mg(BH4)2–NaAlH4 is designed and prepared.
► A metathesis reaction occurs readily between Mg(BH4)2 and NaAlH4 upon ball milling.
► The post-milled Mg(BH4)2–xNaAlH4 samples exhibit a three-step dehydrogenation.
► 6.5 wt% of hydrogen is stored reversibly in the post-milled Mg(BH4)2–2NaAlH4 sample.