Study of the activation process of Mg-based hydrogen storage materials modified by graphite and other carbonaceous compounds

A nanocomposite (Mg-V)(nano) made of 90 wt% Mg and 10 wt% V was prepared by high-energy ball-milling during 40 h. The activation characteristics of (Mg-V)(nano). are rather poor, the hydrogen content [H] reaching 4 wt% after more than 100 h (t(4w%)) following the initial exposure of the material to H-2. Adding 9 wt% graphite to (Mg-V)(nano) and resuming, the milling operation for 30 min leads to the formation Of (Mg-V)(nano)/G, which exhibits a t(4w%) value of only 10 min. The addition of more than 9 wt% graphite to (Mg-V)(nano) does not lead to any significant reduction of the t(4w%) value. However, extending the milling period with graphite over 30 min leads to a steady increase in t(4wt%) and, thus, to a deterioration of the activation characteristics. Comparison of the behavior of graphite with other C-based compounds revealed that perylene (C20H12) and pentacene (C22H14), which are made of linked benzene rings, and thus have a 2D structure similar to that of the graphene sheet, are as effective as graphite in improving the activation characteristics of (Mg-V)(nano). A structural investigation of (Mg-V)(nano)/G as a function of the milling time through both C 1s core-level x-ray photoelectron spectroscopy and C K edge x-ray absorption near-edge spectroscopy has shown that the integrity of graphite is progressively lost as the milling period is extended over 30 min. On the basis of these results, it is hypothesized that the adsorption of graphene layer on freshly created Mg surfaces and the formation of highly reactive C species during milling prevents the re-formation of the surface oxide layer responsible for the poor activation characteristics of untreated (Mg-V)(nano).