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Improving the Hydrogen Storage Properties of Metal Complex Hydrides by Nano-composite and Catalysis

Author: SunTai
Tutor: ZhuMin；YaoXiangDong
School: South China University of Technology
Course: Materials Processing Engineering
Keywords: Hydrogen storage materials Metal complex hydrides Doping catalysis Synergistic effect Nano confinement
CLC: TG139.7
Type: PhD thesis
Year: 2010
Downloads: 141
Quote: 2
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Abstract


Hydrogen storage materials, especially complex metal hydrides have attracted world-wide research interests. This doctoral dissertation has intensely reviewed the development history and the current research status of complex metal hydrides. Based on the progressive research in recent decades, NaAlH4, LiAlH4 and LiBH4 were selected as research targets. Novel techniques such as nano-composite, catalysis, destabilization and nano-confinement were implemented to improve the hydrogen storage properties of these materials. Meanwhile, sample preparation parameters, micro-structure and mechanism of dehydrogenation have also been discussed.Doping with different kinds of rare-earth metals chlorides by high energy ball milling is an effective way to enhance the dehydrogenation kinetics of NaAlH4. The catalytic effect of these chlorides on the dehydrogenation kinetics of NaAlH4 in our investigation is in the following descending order: SmCl3>CeCl3>TiCl3>NdCl3>GdCl3>LaCl3>ErCl3. The result of doping NaAlH4 with LaCl3 shows that the dehydrogenation kinetics of NaAlH4 improves with the increase of ball-milling time and the doping amount of catalyst. LaCl3 is effective in promoting the 2-stage decomposition of NaAlH4. In the first stage, LaCl3 reacts with a part of NaAlH4 and catalyze the dehydrogenation process by forming hydrides of La; in the second stage, the La hydride reduces the concentration of Al in the whole system by forming La3Al11 alloy and promotes the decomposition of Li3AlH6.The dehydrogenation kinetics of LiAlH4 can be greatly enhanced by optimizing ball-milling time, ball to powder ratios, doping amount and doping process of NiCl2. Ball milling can reduce the crystallite size of LiAlH4 and hence improve its dehydrogenation performance. The dehydrogenation performance of LiAlH4 doped with NiCl2 is 5 times better than the pristine LiAlH4. Different doping amounts and doping processes of NiCl2 lead to different dehydrogenation behavior of LiAlH4. The sample prepared by mixing ball milling has very fine crystallite size and homogenous distribution of catalyst. The dehydrogenation efficiency is 35% higher than the pristine LiAlH4.Our research shows that doping LiBH4 with activated hydride of Mg3La alloy （H-Mg3La） and TiCl3 together can greatly enhance the dehydrogenation kinetics of LiBH4. In addition, the LiBH4-Mg3La-TiCl3 system preserved fast dehydrogenation kinetics and stabilized hydrogen storage capacity in re-/dehydrogenation cycles. A synergistic effect of H-Mg3La alloy and TiCl3 on the improvement of the dehydrogenation of LiBH4 has been revealed by comparing independent performance of adding hydrogenated Mg3La and TiCl3 with adding the combination of the two. The dehydrogenation activation energy of this system is 52.6 kJ/mol, which is much lower than that of pristine LiBH4.Herein we report that ultra low hydrogen release temperature was achieved in SBA-15 confined LiBH4 system by incipient impregnatsion and freeze drying methods. The LiBH4 particles were successful loaded into nanopores of SBA-15 due to strong capillary action. To the best of our knowledge, this is the first report to show that the main hydrogen releasing peak of LiBH4 is dramatically reduced to a temperature lower than 100（?）C. Fast hydrogen released from LiBH4 with large amount of 9 wt.% is achieved at 105（?）C. The fine LiBH4 grains provide extremely large specific surface and many defects. These structure features make atoms on the crystal boundary highly activated. The enhancement of the diffusion of H atoms overcomes the limitations set by the reacting kinetics between LiBH4 and the scaffold material. This amplified diffusion significant lowers the reacting temperature.To further enhance the efficiency of catalysts on the hydrogen storage performance of LiBH4, activated carbon was used as a scaffold to prepare supported catalysts. Results show that the onset and peak decomposition temperatures of LiBH4 doped with activated carbon supported TiCl3 is 12（?）C and 33（?）C, which are lower than those of pristine LiBH4. The dehydrogenation performance of LiBH4 doped with activated carbon supported TiCl3 is similar to （or even better than） samples doped with large amount of catalyst by traditional doping method, while the loading amount of supported TiCl3 is at least 5 times less. The activated carbon supported nano Ni particles, which was prepared by thermal reduction under protection of inert gas, can raise the dehydrogenation efficiency of LiBH4 by 40% and reduce the activation energy of dehydrogenation of LiBH4 to 88 kJ/mol from 156 kJ/mol. The onset decomposition temperature of LiBH4 doped with activated carbon supported nano Ni particles and ball-milled for 10 hours is 42（?）C. However, systems doped with these two kinds of supported catalysts do not show enough hydrogen storage amount stability during re-/dehydrogenation cycles and still suffer from serious hydrogen capacity degradation.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > The alloy learn with a variety of properties of alloys > Other special nature of the alloy > Hydrogen storage alloy
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