Superior doping agent of 1.25Ni/3.75Nb2O5 composite nanopowders for improving the hydrogenation/dehydrogenation kinetics of MgH2

Abstract

Reactive ball milling technique, using a high-energy ball mill was employed for preparing nanocrystalline β-MgH2 powders containing a metastable γ-MgH2 phase. The as-prepared MgH2 powders obtained after 200 h of ball milling time under 50 bar of a hydrogen gas atmosphere revealed nanostructured characterizations with an average crystallite size of 5 nm in diameter. The hydrogenation and consequent dehydrogenation kinetics of this end-product were very slow, indexed by the long time required for absorbing and disrobing 5.5 wt% H2 at 250 °C within 500 s and 2500 s, respectively. In the present study, the effect of doping the as-prepared MgH2 powders with a small mass fraction (5 wt%) of a mixed 1.25Ni/3.75Nb2O5 powders on the kinetics behavior of MgH2 was investigated. The results shows that our new ternary system of 95 wt% MgH2/5 wt% (1.25Ni/3.75Nb2O5) nanocomposite, obtained after ball milling for 50 h under 50 bar of a hydrogen gas atmosphere, possessed superior hydrogenation/dehydrogenation behavior, characterized by low values of activation energy for β-phase (129.058 kJ/mol) and the short time required to uptake (90 s–500 s) and release (250 s–500 s) about 5.5 wt% H2 at an applied temperature of 250 °C.