| Based on the review of the research and development of the hydrogen storage materials of the light metal aluminium hydrides,Na2LiAlH6 was selected as the study object of this work for developing novel material with high hydrogen capacity.By means of XRD,XPS,TPD,FTIR and hydrogen storage performance measurements, the reaction process during the hydrogen absorption/desorption of the Na2LiAlH6 and the effects of the catalysts on hydrogen storage performances were systematically investigated.The study includes the following three parts:the formation and hydrogen storage mechanisms of the mixed alanate Na2LiAlH6,the effects of catalysts addition on the hydrogen storage properties of Na2LiAlH6,and the hydrogen storage performances of the Na2LiAlH6/Mg(NH2)2 combined system.Na2LiAlH6 was synthesized by ball-milling a mixture of NaH and LiAlH4 at a molar ratio of 2:1.XRD examinations revealed that NaH and LiAlH4 were readily converted to LiH and NaAlH4 in the initial ball-milling process and then they reacts with the excessive NaH to form Na2LiAlH6.Dehydrogenation/hydrogenation experiments revealed approximately 6.7 wt%of hydrogen was reversibly stored in Na2LiAlH6 through a three-step reaction.The thermodynamic and kinetic investigations showed that the enthalpy change and the apparent activation energy are 63.8 kJ/mol-H2 and 173 kJ/mol for the first-step hydrogen desorption reaction, respectively,indicating a relatively stable thermodynamics and high kinetic barrier for the decomposition of Na2LiAlH6.In-depth kinetic investigations showed the first-step dehydrogenation of Na2LiAlH6 could be well interpreted with a nucleation and growth model(JMA equation),and a diffusion-controlled growth with decreasing nucleation rate was determined.Based on the above results,several Ti-based catalysts(Ti,TiH2,TiSi2,TiF3,TiF4) were employed to improve the hydrogen storage performances of Na2LiAlH6,and the hydrogen absorption/desorption performances and mechanisms were systematically studied.It was found that a significant enhancement in the dehydrogenation/hydrogenation kinetics of Na2LiAlH6 was achieved by adding 5 wt% TiF4.The starting temperature for dehydrogenation of the TiF4-added sample was lowered by 50℃relative to the pristine sample.Further investigations revealed that the addition of TiF4 in Na2LiAlH6 not only enhanced the dehydrogenation kinetics, but also altered the dehydrogenation thermodynamics.The apparent activation energy and the desorption enthaipy change of the 5 wt%TiF4-added Na2LiAlH6 were calculated to be ca.143.6 kJ/mol and 57.3 kJ/mol-H2,both lower than those of the pristine sample,respectively.It is believed that both Ti and F are playing important roles in the dehydrogenation/hydrogenation process of the TiF4-added Na2LiAlH6 sample due to the formation of Ti-Al clusters from the combination of reduced Ti atoms with Al atoms which facilitated the dissociation and recombination of molecular hydrogen,the substitution of F- for H- that can lower the hydrogen desorption enthalpy change,and the formation of LiF which served as the nucleation center for the LiH and Al,and lead to the nucleation morphology of the dehydrogenated products changed from the decreasing nucleation rate mode to the site saturation mode.In order to further improve the hydrogen storage performances of Na2LiAlH6,a novel amide-hydride combined system was prepared by ball milling a mixture of Na2LiAlH6-Mg(NH2)2 in a molar ratio of 1:1.5.The hydrogen storage performances of the Na2LiAlH6-1.5Mg(NH2)2 system were systematically investigated by a series of dehydrogenation/hydrogenation evaluation and structure analyses.It was found that a total 5.08 wt%of hydrogen,equivalent to 8.65 moles of H atoms,was released from the Na2LiAlH6-1.5Mg(NH2)2 combined system.Further investigations revealed that the variable milling treatments resulted in the different dehydrogenation reaction pathways due to the attraction of Al and N caused by the energetic milling.For the sample milled at 500 rpm,the energetic milling facilitates the establishment of Coulombic donor-acceptor attraction between Al and N,such attraction weakens the Al-H bonding and expels hydrogen.While the sample milled at 100 rpm,the input energy of ball milling only enabled the mixture to mix homogenously rather than the formation of Al-N bonding,in the following heating process,the self-decomposition of Na2LiAlH6 first took place to release H2,and then the product LiH reacted with Mg(NH2)2 to desorb more hydrogen.DSC examinations indicated that the hydrogen desorption in the Na2LiAlH6-Mg(NH2)2 system should be thermodynamically reversible.However,the hydrogen uptake experiment indicated that only - 4 moles of H atoms could be recharged at 250℃and 115 bar in the Na2LiAlH6-Mg(NH2)2 system perhaps due to the formation of AlN and Mg3N2 after dehydrogenation.
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