Phase Structural Analysis And Hydrogen Storage Properties Of Serveral Mg-based Hydrogen Storage Systems

Magnesium is promising for hydrogen storage due to its high hydrogen capacity of 7.6 wt%, abundant natural resources, low cost and good environmental compatibility. However, relatively high hydriding/dehydriding reaction temperature and sluggish kinetics hindered utilization of Mg as a hydrogen store. By adding lanthanum to form Mg3La alloy, better hydrogen absorption kinetic properties can be archieved. Besides, nano-structure induced by ball milling favors thermodynamic and kinetic properties of hydrogen storage materials.In Chapter 2, alchohol assited ball milling was employed for controlling grain size and microstrain of Mg3La alloy systematically. Change of hydrogen storage properties and the corresponding underlying mechanisms of nanocrystalline Mg3La were studied. Hydrogenation of Mg3La for the first time leads to disproportion of the alloy phase and formation of LaH3 and Mg phases. Mg is further hydrogenated to MgH2 which results in an in situ formed LaH3/MgH2 nanocomposite, which can also be modified by the ball milling process. Hydrogen storage properties were measured by Sivert method. Due to insufficient refinement of the microstructure, significant change on hydriding or dehydriding thermodynamics was not observed. However, hydrogen sorption kinetics was greatly improved. Desorption activation energy can be lowed to 73(6) kJ/mol for sample milled for 1h, which is approximately half of the value of crystalline Mg. In the nanocomposite, keeping the Mg/MgH2 phase at a low grain size level of 25~40 nm and the presence of fine LaH3 phase benefits both hydriding and dehydriding kinetics of Mg. Longer milling time is not favorable for stability of the nanocomposite and leads to higher desorption activation energy and fast decay of cyclic hydrogen storage capacity were observed. Thus addition of alloying elements besides ball milling can be further considered.In Chapter 3, variation of structure and hydrogen storage properties for Mg3La phase in Mg3LaNi0.1, Mg3LaNi0.1Mn0.1 and Mg3LaNi0.1Al0.1 alloys were studied. Powder X-ray diffraction and Rietveld refinement as major methods were used to reveal location and status of the added Ni, Mn and Al. In as-cast samples of the three alloys, Mg3La is a dominant phase in which Ni, Mn and Al dissolve. During hydrogenation and disproportion of Mg3La, such dissolved elements precipitate as dispersed metal nanoparticles at interfaces between LaH3 and MgH2, by which hydrogen storage kinetics is improved with respect to the Mg3La alloy, as extra nucleation sites and H diffusion paths, and catalytic effect of Ni on both dissocation and recombination of H2 molecules, are expected. Stability of the nanocomposite is also imporved because the metal nanoparticles can to some extent stop growth of the LaH3 and Mg/MgH2 phases. The above work was done in a known structure system. Meanwhile, exploration of other Mg-based hydride systems is also very important for developing better hydrogen storage materials. Currently high pressured synthesized immiscible Mg-TM (transition metals) hydride structures are being focused.In Chapter 4, crystal structure of high pressure synthesized hydrides MgZr2Hx and MgNb2Hx were studied by experiments and theoretical calculations. Problems of previously reported monoclinic structure were dicussed. A new trigonal structure is proposed to better describe such hydrides. Space group of the MgZr2Hx structure is No.166 3?, and the unit cell parameters in hexagonal axes are a =3.3592(2)? and c = 25.131(3) ?. The triognal structure was further applied in refining synchrotron data of MgNb2Hx successfully, with a unit cell of a = 3.2901(9) ? and c = 23.09(1) ?. Hydrogen atom positions in the trigonal MgZr2Hx structure are studied by the first principles calculation. A hydride phase MgZr2H6 having the lowest formation enthalpy was found and its unit cell dimensions and metallic atom positions fit very well with the experimental results. The space group 3? was also confirmed by the relaxed structure. Stability of the MgZr2H6 structure was further dicussed by chemical potential equilibrium diagram, elastic mechanical and phonon calculations.