Improvement in the Hydrogen Storage Properties of MgH2 by Adding NaAlH4

Milled MgH2, MgH2-10NaAlH4, MgH2-30NaAlH4, MgH2-50NaAlH4, and MgH2-2Ni-10NaAlH4 samples were prepared by milling in a planetary ball mill in hydrogen atmosphere (reactive mechanical milling, RMM). Decomposition temperatures of milled MgH2, NaAlH4, MgH2-10NaAlH4, and MgH2-30NaAlH4 were examined in a Sieverts-type hydrogen absorption and release apparatus, in which the hydrogen pressures were kept nearly constant during hydrogen absorption or release. As the content of NaAlH4 in the sample increased, the temperature at the highest peak in the ratio of increase in released hydrogen quantity to increase in temperature versus temperature curve decreased. Hydriding in 12 bar hydrogen and dehydriding in 1.0 bar hydrogen at 593 K of MgH2-30NaAlH4 are performed by the reversible reactions MgH2 ⇔ Mg + H2 and 17MgH2 + 12Al ⇔ Mg17Al12 + 17H2. MgH2-30NaAlH4 was the best Mg-based composite among Mg-based alloys in which an oxide, a halide, a fluoride, or a complex hydride was added, with a high hydrogen absorption rate for 2.5 min (2.20 wt% H/min) and a large effective hydrogen storage capacity (7.42 wt% H).