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

Abstract

Abstract Hydrogen storage properties and mechanisms of the combined Mg(BH 4 ) 2 –NaAlH 4 system were investigated systematically. It was found that during ball milling, the Mg(BH 4 ) 2 –xNaAlH 4 combination converted readily to the mixture of NaBH 4 and Mg(AlH 4 ) 2 with a metathesis reaction. The post-milled samples exhibited an apparent discrepancy in the hydrogen desorption behavior with respect to the pristine Mg(BH 4 ) 2 and NaAlH 4 . Approximately 9.1 wt% of hydrogen was released from the Mg(BH 4 ) 2 –2NaAlH 4 composite milled for 24 h with an onset temperature of 101 °C, which is lowered by 105 and 139 °C than that of NaAlH 4 and Mg(BH 4 ) 2 , respectively. At initial heating stage, Mg(AlH 4 ) 2 decomposed first to produce MgH 2 and Al with hydrogen release. Further elevating operation temperatures gave rise to the reaction between MgH 2 and Al and the self-decomposition of MgH 2 to release more hydrogen and form the Al 0.9 Mg 0.1 solid solution and Mg. Finally, NaBH 4 reacted with Mg and partial Al 0.9 Mg 0.1 to liberate all of hydrogen and yield the resultant products of MgAlB 4 , Al 3 Mg 2 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 NaBH 4 , MgH 2 and Al, indicating that the presence of Mg(AlH 4 ) 2 is significantly favorable for reversible hydrogen storage in NaBH 4 at moderate temperature and hydrogen pressure.