The influence of ball-milling time on the dehydrogenation properties of the NaAlH4-MgH2 composite

The recently developed NaAlH4-MgH2 composite shows improved hydrogen-storage properties compared to MgH2 and NaAlH4. However, the dehydrogenation reaction rates are still too limited, hampering practical applications. Mechanical ball milling is broadly used to improve the dehydrogenation reaction rates of hydrides. Therefore, the hydrogen-storage properties of the NaAlH4-MgH2 (1:1) composite have been investigated as a function of ball-milling time. Expectedly, elongated milling led to a faster dehydrogenation rates. New insights of the structural transformation pathways of the decomposition reaction are provided. A number of Al-Mg alloys, including the only reported Al12Mg17, seem to participate in the dehydrogenation. Thereby, complex alloying process of Al with Mg or MgH2 has been proposed. Our data indicate the possibility that hydrides Na3AlH6 and NaMgH3, which are the intermediate products of the dehydrogenation, coexist. The study shed a light on the complexity of the decomposition pathways of hydride mixtures in which the key role play alloys.