Effect of mechanically-induced solid-state doping time on the morphology and hydrogenation cyclability of MgH2/7 Mn3.6Ti2.4 nanocomposite powders

Nanocrystalline MgH2 powders were prepared by reactive ball milling of pure Mg powders under 50 bar of a hydrogen gas atmosphere, using a high energy ball mill operated at room temperature. The end-product of MgH2 powders obtained after 200 h of a continuous ball milling time composed of γ and β phases. The end-product was doped with 7 wt% of Mn3.6Ti2.4 powders and then mechanically milled under a hydrogen gas atmosphere for 50 h, using a high energy ball mill for different ball milling time. This end product coexisted with Fe and Cr contamination contents of 2.16 and 0.74 wt%, respectively. The effect of the ball milling time on the morphological characterizations, thermal stability and hydrogenation/dehydrogenation properties of MgH2/7 wt% Mn3.6Ti2.4 powders were investigated. The powders obtained after 50 h of milling had spherical-like morphology and homogeneously with uniform composition close to the starting nominal composition. Moreover, this binary nanocomposite system possessed superior hydrogenation/dehydrogenation kinetics at 275 °C, as suggested by the short time required to absorb and desorb 5.3 wt% H2 within 2 and 8 min, respectively. At this temperature, the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 1000 complete cycles within 1400 h. However, a minor degradation (∼0.3-0.4 wt% H2) in the hydrogen storage capacity was observed between 410 h and 1400 h of the cycle-life-time. This slight degradation took place due to the grain growth came off in the Mg/Mn3.6Ti2.4 grains.