Effects Of Lithium Borate/carbon Materials On Hydrogen Storage Properties Of LiBH₄

Among the lightweight hydrogen storage materials, Li BH4 is one of the most promising solid state hydrogen storage materials owing to its high gracimetric hydrogen density and volumetric hydrogen density. However, it is thermodynamically stable,requires harsh hydrogen/dehydrogen conditions and shows slow de/rehydrogenation kinetics, which severely hinders the practical utilization of Li BH4. In this thesis, the additives that lithium borate, lithium metaborate and carbon material were ball milled with Li BH4 to prepared Li3BO3/Li BH4 composite, Li BO2/Li BH4 composite and carbon composite/Li BH4. The hydrogen properties of composites were studied and discussed.The as-prepared Li3BO3 additive exhibited mesoporous structure and provides active sites which could catalyze the hydrogen absorption and desorption reaction of Li BH4,thereby significantly improving the hydrogen adsorption/desorption characteristics of hydrogen storage composites. The Li3BO3/Li BH4 composite with a mass ratio of 4:2 has the best hydrogen ab/desorption and and reversibility properties, its initial desorption temperature of 105 °C and cycling hydrogen storage capacity of 2.75 wt%. The desorption activation energy of Li BH4 with Li BO2 is 77.8 k J/mol, lowering 111.6 k J/mol than that of pure Li BH4, and the dehydrogenation kinetics is greatly accelerated. Meanwhile, due to the formation of Li3BO3, the composite also possesses excellent cycling performance. The Li BO2/Li BH4 composite with a stoichiometric ratio of 1:1 shows a better desorption performance. At 350 °C, the composite realeases 3.05 wt% hydrogen, while Li BH4–20wt% Li BO2 composite and pure Li BH4 only realease 1.77 wt% and 0.39 wt.% hydrogen,respectively.Carbon material additive, exhibiting a hierarchical frame, reacts with Li BH4 to form Li BO2 and Li3BO3 during hydrogen desorption process, which contributes to the superior hydrogen storage performance than Li3BO3/Li BH4 and Li BO2/Li BH4 composites. The composite starts to release hydrogen at 75 °C, lowering 235 °C than that of pure Li BH4.The composite absorbs 5.7 wt.% hydrogen and after Three cycles, de/rehydrogen reaction becomes balanced with reversible hydrogen storage capacity of 3.94 wt.%, whichperforms superior hydrogen absorption and cycling characteristics. Through multi-cycle hydrogen ab/desorption, the featured structure of carbon material additive transforms from hierarchical frame to porous honeycomb and is conducive to the hydrogen desorption and absorption, apparently improving the kinetics of Li BH4.