Abstract
Nanocrystalline powders of MgH2 was synthesized under a high hydrogen gas pressure (50 bar), using reactive ball milling technique operated at ambient temperature under 50 bar of a hydrogen gas atmosphere. The MgH2 powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250°C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics, three different types of nanocatalysts (metallic Ni, Nb2O5 and (Ni)x/(Nb2O5)y) were utilized with different weight percentages and independently ball milled with the as-synthesized MgH2 powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results have shown that the as-prepared nanocomposite MgH2/5Ni/5Nb2O5 powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). Moreover, this nanocomposite system shows excellent hydrogenation/dehydrogenization behavior indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt.% H2 at 250°C.
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