Hydrogen Storage Behavior and Performance of Multiple Cold-Rolled MgH₂/Nb₂O₅ Nanocomposite Powders

The global interest in MgH₂ is due to the natural availability of Mg and its capacity to retain hydrogen at a concentration of up to 7.60 wt.%. Despite its appealing characteristics and ease of production on an industrial scale at ambient temperature using the reactive ball milling (RBM) technique, MgH₂ is a highly stable chemical with sluggish hydrogenation and dehydrogenation rates below 300^◦C. Among the different methods used to improve the hydrogenation/dehydrogenation kinetic behavior of MgH₂, mechanical treatment and/or catalysis are regarded to be the most effective methods. The purpose of this research was to explore the effectiveness of several cold rolling (CR) stages on the hydrogenation properties of recycled magnesium rods, as well as the effect of the resulting RBM on the final product. For this process, the as-received waste Mg-rods were firstly cold-rolled 200 times and then remilled under H₂ atmosphere for 100 h. The as-RBM powders were then cold-rolled for 100 passes and then ball-milled with 10 and 15 wt.% Nb₂O₅ powders for 50 h. The results showed that when the materials were subjected to different types of defects (dislocations, stacking faults, and twining) generated by CR and RBM, their gas absorption/desorption kinetics were improved. This was indexed by their ability to achieve a long cycle lifetime at lower temperatures when compared with the as-received materials. The powders were further improved in terms of kinetics and decomposition temperature upon RBM with Nb₂O₅ for 50 h. The nanocomposite MgH₂/10 wt.% and 15 wt.% Nb₂O₅ exhibit good hydrogen storage capabilities at a comparatively low temperature (225^◦C) with a long cycle life that extended from 110 h to 170 h, without serious degradation in storage capacity and kinetics.