Correlation between hydrogen storage properties and structural characteristics in mechanically milled magnesium hydride MgH₂

In this work, we clarified the correlation between hydrogen storage and crystallographic properties in nanostructural magnesium hydride MgH₂ prepared by mechanical milling under hydrogen gaseous atmosphere. At the early stage within 2h milling, the amount of desorbed hydrogen decreases ∼16% from 7.3 to 6.1wt.% and the onset temperature of dehydrogenation decreases by 70K from 670K, while both the powder size and the crystallite size in powder decrease with increasing the milling time down to 1μm and 15nm, respectively, and the lattice strain of 0.3% is rapidly introduced. At the middle stage with longer milling time than 2h, however, the crystallite size hardly change, but the lattice strain is once released at 2-5h milling and again increases for longer milling time than 5h. On the other hand, the amount of desorbed hydrogen suddenly increases from 2 to 5h, and again decreases with a little increase of lattice strain during 5-80h milling. At the final stage, the hydrogen capacity and desorption temperature reach to saturation of, respectively, 6.5wt.% and 600K, whereas the crystallite size and lattice strain reach to saturation of ∼7nm and 0.2%, respectively. The results obtained indicate that the reduction of crystallite size as well as the introduction of lattice strain in MgH₂ during milling gives rise to the decrease in hydrogen storage capacity.