Hydrogen storage in destabilized borohydride materials

Several new destabilized borohydride systems were prepared by mechanochemical synthesis and characterized to determine their suitability for hydrogen storage. The mixtures included: Mg(BH4)2/Ca(BH4)2; Mg(BH4)2/CaH2/3NaH; and Mg(BH4)2/CaH2; systems as well as a double cation hydride MnLi(BH4)3. Temperature programmed desorption, TPD, analyses showed that the desorption temperature of Mg(BH4)2 can be lowered by ball milling it with Ca(BH4)2. The resulting mixture absorbed and released hydrogen with the pressure composition temperature, PCT, isotherm displaying a well-defined plateau region. The other two systems; Mg(BH4)2/CaH2 and Mg(BH4)2/CaH2/NaH, can also absorb and release hydrogen. The desorption enthalpies are all in the 84–88 kJ/mol range. These systems, however, are only partially reversible and lose some of their hydrogen-holding capacity after the initial desorption. A plausible explanation for this is that the mechanisms involve the formation of a (B12H12)−2-containing intermediate which has a high kinetic barrier to re-hydrogenation. TPD analysis also showed that the double cation material, MnLi(BH4)3 can release hydrogen in the range of 130 °C but the process is irreversible. A Kissinger analysis of the first decomposition step in the differential thermal analysis, DTA, data showed that the activation energies for all the Mg(BH4)2-based borohydrides range from 115 to 167 kJ/mol.