Preparation And Hydrogen Absorption/Desorption Property Of LiBH₄-MgH₂ Composite System

The dehydrogenation of LiBH4 was difficult to occur and it was hard to make the product B gained by decomposition independently to be dehydrogenated due to the thermodynamic stability although the LiBH4 was regarded as one of the most promising hydrogen storage material. The thermodynamic stability of L1BH4 and the dehydriding temperature were decreased due to the reaction between Mg and LiBH4. In this case, the rehydrogenation was easier after the MgB2+LiH were produced in. mutual coupling of hydrogen by the reaction between Mg and LiBH4. Two units of LiBH4-MgH2 system could form mutual coupling when they meet the certain requirements. And thus, the thermodynamic of the hydriding or dehydriding was improved. It could be considered that the key question in the hydrogen reversibility of LiBH4-MgH2 system was the questions of the reaction pathway of LiBH4-MgH2 system.In this paper, the author studied the difference of the absorbing- desorbing properties between the in-situ ball milling and the ex-situ ball milling and explored influence of the preparation procedures on the mutual coupling between LiBH4 and MgH2 simultaneously. Secondly, the author also explored the effect of TiB2 and TiF3 on the dehydrogenation kinetics system. Third, in this paper, the author designed the constant pressure TPD experiment system that the environment hydrogen pressure could be infinitely and variably controlled to study the influence of the dehydrogenation pathway of LiBH4-MgH2 system by the environment pressure simultaneously to study the effect of the dehydrogenation pathway of the system by TiF3.In the compare between in-situ synthesis and ex-situ synthesis, the reactant particles grow up sharply in the hydriding/dehydriding cycle of the composite of ex-situ synthesis. Which decreased the activity and contact of LiBH4 and MgH2 thus causing the mutual coupling difficult to form. What is more, it made difficulties for the transfer because of the composition segregation, the increase of transfer path and the rise of diffusion barrier. Therefore, the composite system was keeping in decoupling state after the secondary dehydrogenation and the dehydriding kinetic and the capacity of the dehydrogenation was decreased significantly. The composite mat erials compounded by in-situ milling have better cycle stability. It might be the reactants form the interaction of small particles with good coupling relationship in the high-energy ball milling process that the composite of LiBH4-MgH2 formed by the hydrogenation was the small particles of mutual dispersion and hold a good coupling relationship. In the study about the effects of the dehydriding kinetic of the composite system by TiB2 and TiF3, the author found that the activation energy of TiB2 was more obviously decreased in the 1 st step of dehydrogenation activation and the activation energy of TiF3 was more obviously decreased in the 2nd step of dehydrogenation activation. Because MgB2 was formed from the reaction of LiBH4 and MgH2 coupled with dehydrogenation in the second step of the dehydrogenation process. Therefore, adding TiF3 could promote the formation of MgB2 and provide a more favorable condition for rehydrogennation. During the study about the effects of the re/dehydriding of the composite by the hydrogen pressure, the author found that the hydrogen pressure would affect the reaction path of the re/dehydriding and the befitting hydrogen pressure could improve the properties of the rehydriding of LiBH4-MgH2 system. In addition, LiBH4-MgH2 system was more likely to arise the dehydrogenation in a favor mode of the coupling after adding TiF3. The coupling hydrogen between LiBH4 and MgH2 occurred after the dehydrogenation of MgH2. The dehydrogenation of LiBH4 and MgH2 was inhibited obviously and the precondition of them was abrogated in the condition of the high hydrogen pressure. However, the low hydrogen pressure availed the dehydrogenation of LiBH4 and MgH2 but made the particles of Mg was produced easily and grow up thus blocking the coupling between Mg and LiBH4. The medium hydrogen pressure in favor of the coupling of dehydrogenation may because the dehydrogenation of MgH2 was slowed down, the particles of Mg produced was smaller and the temperature of the dehydrogenation of MgH2 was higher than the condition of low pressure, which satisfying the temperature conditions of the combined dehydrogenation between Mg and LiBH4. TiF3 catalyst not only significantly improved dehydriding kinetics in the second step, but also made LiBH4 and MgH2 easy to arise coupling dehydrogenation while TiF3 also decreased the rehydriding capacity. Therefore, it was necessary to reduce the amount of catalyst and reduce the degree of catalyst, improve the dispersion of the catalyst particles in the matrix and improve the coupling of catalyst and matrix.