| In order to optimize the thermodynamics and dynamics performance of complex metal hydride Mg(AlHU)2 in possession of high capacity, the domestic and foreign scholars over the recent years has conducted a lot of research work on the structural analysis, preparation methods, catalyst selection and so forth. Although a certain researching results has been obtained, there are still many deficiencies need to be solved to meet the needs of pratical application, such as higher hydrogen de/absorption temperature, slower reaction kinetics and partial reversibility. Therefore, in view of the above problems, methods of nano and catalysis modification were applied in this paper to improve the hydrogen storage properties of Mg(AlH4)2-And then, many modern methods of material analysis including X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), transmission electron microscopy (TEM), differential scanning calorimetry and thermogravimetric analysis (DSC and TG), mass spectrometry (MS) and Sievert’s hydro/dehydrogenation were employed to detect and evaluate the structure, morphology, and hydrogenation/dehydrogenation performances of the hydrogen storage material. A transformational mathematical model based on the desorption behavior of this hydrogen storage material were selected for the interpretation of its reaction mechanism.First, a method of preparing solvent-free nano Mg(AlH4)2 with high efficiency was developed. LiCl as buffer matrix, the solvent-free nano Mg(AlH4)2 was produced successfully through a solid-state ion exchange reaction triggered by high energy mechanical ball milling. SEM and TEM photoes menifested the particle size being 2-7 nm. Hydrogen de/absorption test results showed the excellent kinetics of nano Mg(AlH4)2:Releasing hydrogen started from 80℃ and the first hydrogen evolution reaction step was finished just within 30 min at a constant temperature of 120 ℃. Its dehydrogenation activation energy was also reduced from 123.6 kJ/mol to 105.3 kJ/mol. Moreover, MgH2 as the carrier of partialy reversible de-and rehydrogenated Mg(AlH4)2, showed stable high temperature cycle performance in xLiCl-Mg(AlH4)2 system and the peak temperature of the rehydrogenated MgH2 was hysteresis of less than 2 ℃. Then the corresponding electron microscope photo showed constant particle size as well. By fitting the dynamic model indicated that in the temperature range of 105 ℃～120℃ (tested in this study), hydrogen releasing in nano Mg(AlH4)2 was controlled by the interface reaction among Mg(AlH4)2 hydride and product phase of MgH2 and Al.Secondly, hydrogen storage performance of nano Mg(AlH4)2 added by NbF5 was studied and the specific catalytic mechanism was analyzed as well. Study found that 0.08 mol NbF5 doping can make Mg(AlH4)2 decompose at room temperature and 66% H2 of the total capacity can be emitted at a constant temperature of 95 ℃, if increasing the temperature to 100℃, the hydrogen releasing rate can be doubled. By calculation of activation energy, the first step energy barrier of the mixed system was only 82.04 kJ/mol, and energy barrier of the second step was reduced drastically, and from 183.04 kJ/mol to 101.22 kJ/mol. And as a result, the hydrogen releasing temperature of MgH2, products of Mg(AlH4)2 decomposing in the first step was greatly in advance, and without the appearance of the incubator period. In addition, by screening of hydrogen reaction model, the the reaction speed control step of 0.08NbF5-Mg(AlH4)2 was transformed from interface reaction to the spread of the 3 D. So to further improve its kinetics behavior, it should further accelerate the diffusion of H2 in the bulk phase. |
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