Improving The Hydrogen Storage Properties Of Metal Complex Hydrides By Nano-composite And Catalysis

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.