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A metastable phase of magnesium hydride was prepared and investigated for solid state hydrogen storage. The magnesium hydride has a high theoretical hydrogen storage capacity (7.6 wt.%) but also a high dehydrogenation temperature of more than 300˚C due to its high enthalpy. Its metastable phase γ-MgH2 showed the possibility to overcome this challenge. In this project, magnesium nickel alloy was used for the catalyst effect of nickel. The samples were hydrogenated at 3.7 MPa & 350˚C under hydrogen atmosphere. Then the original α-MgH2 was converted to γ-MgH2 through ultra-high-pressure compression (4 GPa). 2.7 wt.% of γ-MgH2 was detected by X-Ray Diffraction (XRD) analysis and refinement, indicating phase transformation happened at ultra-high pressure. The dehydrogenation properties were revealed with Sieverts instrument and XRD. The samples were heated to different temperatures (100ºC, 200ºC, 300ºC and 400ºC) under vacuum with a discontinuous heating process. γ-MgH2 was found to start dehydrogenation at ~80˚C and reach a near fully decomposition at 200˚C (only 0.1 wt.% remained). The weight loss of γ-MgH2 due to dehydrogenation was calculated to be 0.8 wt.% at 100˚C and 4.5 wt.% at 200˚C. The dehydrogenation temperature of γ-MgH2 is much lower than 400˚C for the original α-MgH2. Also, the change of other phases under ultra-high-pressure compression in the samples were revealed and discussed in this project. Compression under different parameters to achieve a higher content of γ-MgH2 and a cycle test were also carried out. These properties revealed the potential and excellent properties of γ-MgH2 as a future hydrogen storage material. |
