The hydrogen storage properties and reaction mechanism of the NaAlH4Â +Â Ca(BH4)2 composite system

The hydrogen storage properties and reaction mechanism of the combined NaAlH4 + Ca(BH4)2 (2:1) composite system was investigated in the present study. Analyses show that after 6 h of milling, the NaAlH4 + Ca(BH4)2 combination fully converted to the mixture of Ca(AlH4)2 + NaBH4, and a metathesis reaction occurred between the hydrides. Four major dehydrogenation stages were observed in the system, which corresponds to the decomposition of Ca(AlH4)2, CaAlH5, CaH2 and NaBH4, respectively. The onset desorption temperature of the composite system is reduced to 125 °C, which is much lower than a unary component of NaAlH4 and Ca(BH4)2. The de/rehydrogenation kinetics of the composite system had improve at a higher temperature. From the Kissinger plot, the apparent activation energies for the decomposition of CaAlH5 and NaBH4 in the composite system were reduced to 142.9 and 146.5 kJ/mol, respectively. It is believed that the formation of AlCa, AlB and CaB alloys during the dehydrogenation process is responsible for the distinct reduction in the onset desorption temperature and kinetics enhancement of the 2NaAlH4 + Ca(BH4)2 composite system.