dc.description.abstract | Mg-based hydrogen storage alloys are attractive materials for hydrogen storage because due to their high hydrogen storage capacity, light density and low cost. Unfortunately, most studies indicate that it is difficult to produce Mg2Ni alloy with the accurately desirable composition by conventional melting methods because of the large differences in melting points and vapor pressures between Mg(649℃)and Ni(1455℃). Therefore, an innovative method, Isothermal Evaporation Casting Process (IECP), is developed to produce Mg2Ni alloy for mass production in this study. In the past, high vapor pressure of Mg was considered as a disadvantage for producing pure Mg2Ni alloy. However, this characteristic was used to develop a refinement procedure to separate primary Mg2Ni alloy from Mg/Mg2Ni eutectic matrix. Characteristics of as-cast specimens measured by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and electron probe X-ray microanalyzer (EPMA) reveal that mass production of Mg2Ni alloy was successfully fabricated by IECP.
A series experiments in hydrogenation properties of as-prepared Mg2Ni are also investigated. It is found that he well-activated Mg2Ni alloy achieves 3.58wt.% at 300℃ corresponding to the theoretical hydrogen storage capacity of Mg2NiH4 hydride. In addition, to research the modification of ternary Mg-Ni based alloys, the Mg2Cu1-xNix (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) alloys are also fabricated by IECP. The XRD analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg2Cu with face-centered-orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg2Cu and Mg2Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 300 ℃ under 50 atm H2. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH2, MgCu2 and Mg2NiH4 while the discharged samples contained ternary alloys of Mg-Cu-Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg-Cu and Mg-Ni alloys.
The model of constriction phenomenon induced by 3rd additives is proposed to explain that the hydrogen diffusion process may have on the lattice expansion behavior, as opposed to that of the reactions taking place on the solute atom and multi-hydrides, is an important factor to consider in determining the kinetic parameters of hydrogen movement in metal lattice. Following this model, the correlation of the plateau in PCI and the microstructure of hydrogen storage alloy may be identified clearly.
Finally, this study demonstrated the feasibility of a novel Mg vapor deposition treatment on Ni foam to synthesize a Ni-Mg texture-like structure as a new type of hydrogen absorber. Energy dispersive spectrometry (EDS) yielded an estimative value of the weight percent ratio of Ni and Mg of 71.8 and 20.5 in as-prepared Ni-Mg texture-like structure. The microstructural changes were also characterized by XRD and the formed hydride tetragonal-MgH2 was confirmed. The unique combination of large surface area of catalyst (Ni) and hydrogen acceptor (Mg) reduced the hydrogenation and dehydrogenation temperatures and performed the capability of reversible hydrogen storage capacity up to 0.72 wt.% H2 at 25℃. Ni-Mg texture-like structure achieved significant hydriding-dehydriding performances at lower temperature than traditional Mg-based hydrogen storage alloys. A possible hydrogen storage mechanism was also discussed where the catalytic Ni foam with large surface area was shown to be a vital factor in improving hydriding and dehydriding kinetics.
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