A Study On The Hydrogen Storage Properties Of LiBH₄ And Al/Aluminum Compounds

Hydrogen as an energy vector is taking more and more important action in energy systems because of its non-polluting characteristics recently. 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 reversibility. Lithium borohydride (LiBH4) is a potential hydrogen storage material due to its large theoretical hydrogen capacity (18.36 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 this thesis, Al, LiAlH4 and Li3AIH6 are selected to improve hydrogen storage properties of LiBH4. The dehydrogenation/rehydrogenation properties of LiBH4-Al, LiBH4-LiAlH4 and LiBH4-Li3AlH6 composites were studied via scanning electron micrograph (SEM), thermogravimetry (TG), differential scanning calorimetry (DSC), mass spectral analysis (MS), powder X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR).Activated Al powder decomposed from AIH3 was ball-milled with LiBH4 to form LiBH4-Al* composites (Al* stands for activated Al powder). Compared with LiBH4-Al composites (Al stands for purchased Al powder), LiBH4-Al* composites show better dehydrogenation properties. When they are heated from room temperature to 600℃, the total weight loss for LiBH4-Al* composites and LiBH4-Al composites are 6.2 wt.% and 5.5 wt.%, respectively. The activation energy of dehydrogenation reaction of LiBH4-Al* composites is smaller than that of LiBH4-Al composites. The dehydrogenated composite of LiBH4-Al* can re-absorb 5.5 wt.% hydrogen under 8 MPa at 400℃.After ball milled, the particle size of powders of LiBH4-LiAlH4 composites is in the range from 30um to 10um. And LiBH4 and LiAlH4 didn’t react with each other during the ball-milling. LiBH4-LiAlH4 composites (molar ratio:1:0.5,1:1,1:2) released 9.6 wt.%,8.7 wt.%.10.2 wt.%hydrogen respectively when they are heated to 500℃. The dehydrogenation of LiBH4-LiAlH4 composite is a four-step hydrogen releasing reaction. And the third step is the main dehydrogenation of LiBH4-LiAlH4 composite, of which the activation energy varied from 131.6 to 204.7 kJ/mol (smaller than that of UBH4-AI composite). The result shows that activated Al formed from LiAlH4 decomposing can accelerate hydrogen releasing of LiBH4. Addition of CeF3 and AIF3 took an opposite effect on the dehydrogenation of LiBH4-LiAlH4 composite: CeF3 can promote the first and second steps of dehydrogenation of LiBH4-LiAlH4 composite, while AIF3 can restrain the process. The dehydrogenated composite of LiBH4-LiAIH4 can re-absorb 5.6 wt.% hydrogen under 8 MPa at 400℃.LiBH4-Li3AlH6 composites (molar ratio:1:0.5,1:1,1:2) released 8.6 wt.%.8.5 wt.%,7.1 wt.% hydrogen respectively when they are heated to 500℃. The dehydrogenation of LiBH4-Li3AlH6 composite is a three-step process, and an intermediate product was formed in second dehydrogenation step. The reaction activation energy of the second dehydrogenation step of LiBH4-Li3AlH6 composite was valued 158.7-201.3 kJ/mol (smaller than that of LiBH4-Al composite). The result shows that Li3AlH6 can turn to activated Al which can facilitate the dehydrogenation reaction of LiBH4. The dehydrogenated composite of LiBH4-Li3AlH6 can re-absorb 4.9 wt.% hydrogen under 8 MPa at 400℃.