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).