Abstract
Because of its low density, storage of hydrogen in the gaseous and liquids states possess technical and economic challenges. One practical solution for utilizing hydrogen in vehicles with proton-exchange fuel cells membranes is storing hydrogen in metal hydrides. Magnesium hydride (MgH2) remains the best hydrogen storage material due to its high hydrogen capacity and low cost of production. Due to its high activation energy and poor hydrogen sorption/desorption kinetics at moderate temperatures, the pure form of MgH2 is usually mechanically treated by high-energy ball mills and catalyzed with different types of catalysts. These steps are necessary for destabilizing MgH2 to enhance its kinetics behaviors. In the present work, we used a small mole fractions (5 wt.%) of metallic glassy of Zr70Ni20Pd10 powders as a new enhancement agent to improve its hydrogenation/dehydrogenation behaviors of MgH2. This short-range ordered material led to lower the decomposition temperature of MgH2 and its activation energy by about 121 °C and 51 kJ/mol, respectively. Complete hydrogenation/dehydrogenation processes were successfully achieved to charge/discharge about 6 wt.%H2 at 100 °C/200 °C within 1.18 min/3.8 min, respectively. In addition, this new nanocomposite system shows high performance of achieving continuous 100 hydrogen charging/discharging cycles without degradation.
Highlights
Hydrogen is an energy carrier holds tremendous promise as a new clean energy option in future energy systems[1]
This mechanical treatment regime has been successfully achieved by subjecting the powders to a long-term of a high-energy ball milling runs[16] through a mechanically-induced cyclic phase transformations[17]
Since the hydrogen diffusion along grain boundaries is much faster than diffusion in side grains[19], the hydrogenation/dehydrogenation kinetics of MgH2 are outstandingly improved upon producing such fine nanostructured grains, containing a large number of grain boundaries
Summary
The analysis of these new Bragg-peaks indicated the formation of polycrystalline mixture of γ-MgH2 and β-MgH2 phases with orthorhombic and tetragonal structures, respectively The Bragg-diffraction peaks related to MgH2 (γand βphases) show significant broadening (Fig. 1(e)), indicating the effect of RBM time on grain refining and formation of nanocrystallites Those Bragg peaks shown, which are related to fcc-MgO phase, came from the oxidation of the powder surfaces during preparation the XRD sample outside the helium-atmosphere glove box. The HRTEM image taken near the edge of MgH2/5 wt.% amorphous Zr70Ni30Pd10 composite particle obtained after 50 h of RBM time is shown in Fig. 3(e) together with the corresponding NBDP (Fig. 3(f)). The composite powders obtained after this stage of milling consisted of continuous amorphous matrix (maze-like morphology shown in Fig. 3(e)) hosting ultrafine nanoclusters (~4 nm in diameter) of order-structure
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