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Investigation on the Preparation and Dehydrogenation Properties of Zn(BH4)2Hydrogen Storage Material

Author: YangBaoGang
Tutor: LiSongLin
School: Central South University
Course: Materials Engineering
Keywords: Hydrogen storage materials Zinc borohydride Mechanicalmilling Hydrogenation properties Catalysis mechanism
CLC: TB34
Type: Master's thesis
Year: 2013
Downloads: 9
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Abstract


Hydrogen energy is known as a new kind of green energy in the21st century. It is a precondition to achieve large-scale application of hydrogen before developing a sort of material with high hydrogen storage density that can absorb and desorb hydrogen in low or medium temperature.Zn(BH4)2is a promising hydrogen storage material due to its low dehydrogenation temperature and high hydrogen storage capacity. In this thesis, Zn(BH4)2was synthesized by chemical method and mechanical milling using NaBH4and ZnCl2as raw powders, and the chemical structures of Zn(BH4)2were characterized. Afterwards, the dehydrogenation reaction mechanism of Zn(BH4)2and the impact of additives on the hydrogen desorption properties of Zn(BH4)2were investigated. Meanwhile, the catalytic mechanism of multi-walled carbon nanotubes (MWCNTs) was expounded.Finally, the hydrogen storage properties of Zn(BH4)2-LiNH2composite system were explored. The main conclusions are as follows:Zn(BH4)2can be prepared by both chemical and mechanical milling methods and the synthetic process follows the substitution reaction: ZnCl2+2NaBH4→Zn(BH4)2+2NaCl. Zn(BH4)2can be obtained after2h high energy ball milling using NaBH4and ZnCl2. Mechanical milling has advantages like short reaction time and simple synthetic conditions. Meanwhile, we confirmed the XRD peaks of Zn(BH4)2located in2θ=22°,25～30°and35～40°. The dehydrogenation reaction of Zn(BH4)2occurred at50～150℃, and the main desorption reaction is as follow: Zn(BH4)2→Zn+B2H6+H2. It is noted that boron element existing in the form of borane in the dehydrogenation process of Zn(BH4)2leads to be irreversible of the system.The effect of additives (Co, Cr, CeO2, TiH2and MWCNTs) on the dehydrogenation properties of Zn(BH4)2was studied. MWCNTs have the best catalytic effects due to the fact that Zn(BH4)2dopped with5wt.%MWCNTs decompose at50-125℃, which is25℃lower than that of the undopped sample. Dopped with5wt%Co can partly improve the thermal stability of Zn(BH4)2, but the most of thermal decomposition reaction still takes place above100℃. In addition, Zn(BH4)2dopped with5wt.%CeO2, TiH2or Cr has no any improvement on dehydrogenation properties. However, Zn(BH4)2dopped with5wt.%MWCNTs can only lower the dehydrogenation temperature and do not change the dehydrogenation reaction. The catalytic effect of MWCNTs can be explained as following: first, carbon nanotubes have a high surface energy which facilitates the lowering of dehydrogenation reaction enthalpy. Second, part of the valence electron of carbon nanotubes may be involved in the Zn(BH4)2dehydrogenation reaction. Third, MWCNTs can penetrate in the Zn(BH4)2particles that can act as tunnels for the diffusion of hydrogen from the inside of the Zn(BH4)2+5wt.%MWCNTs composite to surface.Fourth, the positive impact of MWCNTs additives on the hydrogen properties of Zn(BH4)2partly resulted from the anti-sticking effect of MWCNTs.Zn(BH4)2-LiNH2composite system melts at116℃and decomposes at193℃respectively. The composite released0.015mol/g gas at150℃. At200℃, the amount of gas capacity in Zn(BH4)2-LiNH2composite system increased to0.018mol/g. It is found that enhancing the dehydrogenation temperature of Zn(BH4)2-LiNH2has limited impact on its dehydrogenation dynamics. Zn(BHt)2-LiNH2composite system is irreversible under the condition of150℃and0.1MPa H2. There are thirty-six figures, five tables and ninety-three references in this thesis.

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