Magnesium hydride and its compounds have a high hydrogen storage capacity and are inexpensive, and thus have been considered as one of the most promising hydrogen storage materials for on-board applications.
Nanocrystalline Mg-20 wt % Ti−Cr−V x (x = 0.4, 0.6, 0.8; Ti/Cr = 2:3) composites were synthesized by reactive ball milling of Mg and Ti−Cr−V x powders for 8 h under 5.0 MPa H 2 at room temperature. The morphology, crystal structure, and isothermal hydrogen ab-/desorption properties of the materials were investigated. The as-synthesized nanocomposites comprise β-MgH 2 , γ-MgH 2 , and Ti−Cr−V x −H y (1.88 < y < 1.92) as confirmed by X-ray diffraction analysis. High-resolution transmission electron microscopy images show that both β-MgH 2 and γ-MgH 2 co-existed in a single nanocrystalline. Differential scanning calorimetry results reveal that Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders possess the lowest hydrogen desorption temperature at 224 °C with the apparent activation energy of 76.32 kJ/mol for dehydrogenation. The nanocrystalline Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders exhibit excellent dehydrogenation kinetics. They can desorb 5.67 wt % H 2 in 20 min at 270 °C under 0.01 MPa H 2 pressure, compared with only 0.10 wt % H 2 for a pure Mg sample under the same preparation and measurement conditions. More remarkably, the presence of γ-MgH 2 is found to lead to a significant reduction of reaction enthalpy from 74.8 to 54.16 kJ/(mol•H 2 ) for the Mg-20 wt % Ti 0.16 Cr 0.24 V 0.6 powders. The thermodynamic destabilization and superior hydrogen storage properties of the nanocomposite can be attributed to the synergistic desorption effect co-existing in the nanocomposite γ-MgH 2 /β-MgH 2 /Ti 0.16 Cr 0.24 V 0.6 H y phases as well as the high content of γ-MgH 2 (35.4 wt %). Furthermore, cycling results show that the metastable γ-MgH 2 phase still exists after several cycles at selected temperatures ranging from 225 to 255 °C, which is important for improving the thermodynamic properties of the composite, whereas the content of γ-MgH 2 decreased with the increasing de-/absorption cycles. However, this sheds light on the ways to further improve the thermodynamic properties of magnesium-based composites.
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