Hydrogen Storage Properties Of Mg-Li-Al Systems

The development of hydrogen storage materials with light weight and high capacity ismost important for the application of hydrogen energy. Based on the review of hydrogenstorage technology and the development of Mg-based hydrogen storage materials, twolight-weight elements of Li and Al were selected to add into the Mg-based hydrogenstorage materials in this thesis. The phase structure, hydrogenation behavior, activationproperty as well as the de/hydrogenation kinetic and thermodynamic properties of theas-cast and ball-milled Mg-Li-Al alloys and the MgH2-LiH-Al composites were studied bymeans of X-ray diffraction (XRD), Sievert-type apparatus and Rietveld refinement.Moreover, the effect of Li and Al addition on the hydrogen storage properties of Mg-basedhydrogen storage materials was discussed.The Mg4-xLixAl (x=0,1,2) alloys were synthesized by induction melting the mixtures ofMg, Al and Mg-Li alloy. It was found that the as-cast Mg4-xLixAl (x=0,1,2) alloys can beactivated completely after two-four de/hydrogenation cycles at623K under3MPa initialhydrogen pressure. After activation, the Mg3LiAl alloy can absorb3.6wt.%of hydrogenwithin2h. In the as-cast Mg4-xLixAl (x=0,1,2) alloys, the element Li can combine withMg to form Mg(Li) solid solution. This phase can reversibly store hydrogen at623K under3MPa initial hydrogen pressure. In comparison, the de/hydrogenation cycle stabilities ofthe as-cast Mg4-xLixAl (x=1,2) alloys are better than that of the as-cast Mg4Al alloy,indicating that the addition of Li is beneficial for the improvement of the cycle stability.The as-cast Mg-Li-Al alloys were subjected for ball milling. The results show that thephase structure of the Mg-Li-Al alloys was changed during the milling process. For theMg2Li2Al alloys, the phase component changes from Mg(Li)+Li3Mg7+AlLi intoLi2MgAl+Li3Mg7. For the Mg2LiAl alloys, the phase component changes from Li3Mg7+AlLi into Li2MgAl. In comparison, the hydrogen storage properties of the ball-milledMg-Li-Al alloys are better than those of the as-cast alloys. For the ball-milled Mg3LiAlalloys,5.7wt.%of hydrogen can be absorbed within2h at623K under3MPa initialhydrogen pressure, and the hydrogen amount absorbed within5min can reach90%of themaximum value. The ball milled Mg-Li-Al alloys also have better de/hydrogenation cyclestabilities, and no obvious degradation in hydrogen storage amount were observed after20 cycles. The P-C-T study indicates that three plateaus exist in the P-C-T curve for theball-milled Mg2Li2Al or Mg3LiAl alloy, corresponding to the de/hydrogenation processesof the phases Mg(Li), Al12Mg17and Al3Mg2, respectively.The MgH2-LiH-Al composites were constructed by ball milling the metal hydridesMgH2and LiH with Al for40h. It was found that only one dehydrogenation process isneeded for activating the MgH2-LiH-Al composites. After activation, the phases reversibilyabsorbing hydrogen for the MgH2-LiH-Al composites are Mg(Li), Al12Mg17and Al3Mg2.The de/hydrogenation kinetic properties of the MgH2-LiH-Al composites are better thanthose of the ball-milled Mg-Li-Al alloys. For the3MgH2+LiH+Al composite,95%of themaximum hydrogenation amount can be obsorbed within5min at623K under3MPainitial hydrogen pressure, and the dehydrogenation process can be finished within20min.The capacity retention rate of the MgH2-LiH-Al composites was above85%after20de/hydrogenation cycles.