| Hydrogen has attracted a lot of attention as an ideal secondary energy, One of the key steps for the application of hydrogen energy on board is the development of appropriate hydrogen storage materials that have high gravimetric and volumetric hydrogen storage densities, and perfect hydrogenation/dehydrogenation kinetics and reversibility. Hydrogen storage with solid materials is now regarded as a more safe and effective way. Lithium borohydride (LiBH4) is a potential hydrogen storage material due to its large theoretical hydrogen capacity (18.5 wt.%). Unfortunately, LiBH4 is thermodynamically stable and the conditions for forming LiBH4 from its dehydrogenated products are rigorous. Therefore, novel strategies and methods must be found to improve the hydrogen storage properties of LiBH4. In a general view, reactant destabilization is the most promising approach, and its employment has given rise to a number of reactive hydride composites (RHCs) that show remarkable property advantages over pristine LiBH4. In this thesis, MgH2 and Al, AIH3, Li3AIH6 are selected to improve hydrogen storage properties of LiBH4. The dehydrogenation/rehydrogenation properties of LiBH4-MgH2-Al, LiBH4-MgH2-AlH3 and LiBH4-MgH2-Li3AlH6 composites were studied via powder X-ray diffraction (XRD), Temperature Programmed Dehydrogenation (TPD), differential scanning calorimetry (DSC), thermogravimetry (TG), and mass spectral analysis (MS).First of all, Al were introduced into the LiBH4-MgH2 binary system for the purpose of improving the hydrogen sorption properties of the composite. Thermogravimetric analysis revealed that the composite starts hydrogen desorption at about 340℃ and desorbs 7.5 wt% of hydrogen below 520℃, and the onset dehydriding temperature of LiBH4 in the composite is 391℃. The onset dehydriding temperature of MgH2 and LiBH4 in the composite is reduced by 17℃ and 14℃ respectively when mixed with Al. The results of hydrogen desorption cycles show that the first cycle discharge capacity of the composite is 7.5 wt% hydrogen, the total amount of hydrogen capacity reduce with the increase of cycling times. The destabilization of LiBH4-MgH2-AlH3 is believed to be attributed to the formation of Mg-Al-B alloys.Secondly, organometallically prepared AIH3 were introduced into the LiBH4-MgH2 binary system for the purpose of improving the hydrogen sorption properties of the composite. Thermogravimetric analysis revealed that the composite starts hydrogen desorption at about 114℃ and desorbs 10.42 wt% of hydrogen below 520℃. The onset dehydriding temperature of AIH3, MgH2 and LiBH4 in the composite is reduced by 24℃,62℃ and 49℃ respectively when mixed with AIH3. The peak dehydriding temperature of MgH2 and LiBH4 in the composite is reduced by 85.3℃ and 17.9℃. The rehydriding properties of LiBH4-MgH2-AlH3 are much better than that of LiBH4-MgH2-Al respectively. The destabilization of LiBH4-MgH2-AlH3 is believed to be attributed to the formation of Mg-Al-B alloys. AIH3 shows better effect on destabilizing 2LiBH4-MgH2 than the as-received Al for the fact that Al* formed in situ from the decomposition of AIH3 is oxide-free on the particle surfaces, which effectively increases the chemical activity of Al*.Thirdly, prepared Li3AlH6 were introduced into the LiBH4-MgH2 binary system. Thermogravimetric analysis revealed that the composite starts hydrogen desorption at about 169.7℃ and desorbs 10.02 wt% of hydrogen below 520 ℃. The onset dehydriding temperature of MgH2 and LiBH4 in the composite is reduced by 67.4℃ and 48.7℃ respectively when mixed with Li3AIH6. The peak dehydriding temperature of MgH2 and LiBH4 in the composite is reduced by 84.7℃ and 19.3℃ The destabilization of LiBH4-MgH2-LiAlH6 is believed to be attributed to the formation of Mg-Al-B alloys. Li3AlH6 shows better effect on destabilizing 2LiBH4-MgH2 than the as-received Al for the fact that Al* formed in situ from the decomposition of Li3AIH6 is oxide-free on the particle surfaces, which effectively increases the chemical activity of Al*. |
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