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

Abstract

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 (t4wt%) following the initial exposure of the material to H2. 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 t4wt% 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 t4wt% value. However, extending the milling period with graphite over 30 min leads to a steady increase in t4wt% 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

Effect of activated carbons derived from different precursors on the hydrogen sorption properties of magnesium

“…The activated carbons probably also played a role of dispersing agent and prevent oxidation of magnesium. This effect was proved in previous investigation from the authors [8] and in other published results in the literature [6,7,11,12]. In Fig.…”

“…A large number of studies on composite materials based on magnesium with different additives, obtained by high-energy ball milling, were performed during the past years. It was established that some additives such as 3d metals, different oxides, some intermetallic compounds and carbon containing materials improve the hydrogen sorption properties of magnesium [1][2][3][4][5][6][7][8].…”

Effect of activated alloys on hydrogen discharge kinetics of MgH2 nanocrystals

“…It was believed that carbon would favour a more efficient catalyst particle distribution. Moreover, recent studies have demonstrated that co-milling metal hydrides with carbon improves hydrogen sorption kinetics [10][11][12][13][14][15][16], suggesting that carbon plays a catalytic role. Zr 7 Ni 10 was chosen for the experiment, among the Zr x Ni y catalysts investigated in this work, because this particular composition is easier to activate, thus easier to turn into a fine powdery form.…”

Mg-based compounds for hydrogen and energy storage

“…The introduction of small amounts of additives (typically transition metals and transition metal oxides, but also carbons [6,7]) has also improved the kinetic rates of absorption and desorption significantly [8]. However, reducing the equilibrium temperature of desorption relies on lowering the thermodynamic enthalpy, and one of the more promising pathways for this is the formation of magnesium alloys with other metals.…”