Direct synthesis of Mg(AlH4)2 and CaAlH5 crystalline compounds by ball milling and their potential as hydrogen storage materials

Mg(AlH4)2 and CaAlH5 were synthesized by direct ball milling of AlH3 and MgH2 or AlH3 and CaH2 hydrides. The XRD profiles indicated crystalline compounds. Several ball-milling conditions were studied and the optimum parameters were found. Among these, the key parameter is the pause used to cool down the milling device, which allows reducing the temperature rise during milling. Thus, the maximum yield of complex hydrides was obtained by minimizing the desorbed alane amount. The decomposition properties were studied and were in agreement with those reported for different synthesis methods. Mg(AlH4)2 with a good hydrogen capacity and a decomposition reaction enthalpy close to 0 kJ/mol H2 can be a candidate for one-way storage systems. As for CaAlH5, it might be suitable for reversible hydrogen storage thanks to its dehydrogenation reaction enthalpy (26 kJ/mol H2). However, rather high activation energy values were evaluated for both compounds (119 and 161 kJ/mol, respectively).