In-situ Syntheses Of High-effective Catalysts And Their Action Mechanisms For LiBH₄ Hydrogen Storage Material

Faced with increasing pressure of energy crisis and environmental pollution,the development of new energy has a very important strategic significance.Hydrogen,in consideration of its abundant amount,high mass density and environmentally friendly feature,is an excellent alternative energy carrier.The main problem of hydrogen energy utilization is how to store and transport hydrogen efficiently and safely.Solid-state hydrogen storage material is the most practical hydrogen storage technology in the future.Although various methods have been used to improve the hydrogen storage performance of LiBH₄,the hydrogen absorption and desorption temperatures of LiBH₄are still too high,the hydrogen absorption conditions are harsh,and the reversibility is poor,which hinder its practical development.In order to further improve the hydrogen storage properties of LiBH₄,based on the review of the research progress,the present work in-situ introduced high-effective catalysts and systematically studied the influence of the catalysts on the hydrogen storage performance of LiBH₄ and their reaction mechanism.This work obtained various liquid metal organic compounds which act as precursors and introduced high-efficiency and well diffused catalysts into LiBH₄ via ball milling and subsequent heat treatment.The introduced catalysts are controllable and have bi-catalytic effects on the hydrogen absorption and desorption performance of LiBH₄,which results in lower hydrogen absorption and desorption temperatures,improved hydrogen storage kinetics,enhanced reversibility and cyclic performance.This work reveals the mechanisms of the improved performances and provides novel idea for designing high capacity hydrogen storage materials.In this thesis,LiBH₄-0.06TiO system was successfully prepared by introducing Ti?OEt?₄ into LiBH₄ by means of ball milling and thereafter heat treatment under 230°C for 30 min.The initial dehydrogenation temperature and peak temperature of LiBH₄-0.06TiO system are 240°C and 340°C,respectively,which are 140°C and 90°C lower than those of pristine LiBH₄.The LiBH₄-0.06TiO system can rapidly release 9 wt%of H₂ after dwelling at 400°C for 20 min.The apparent activation energy of dehydrogenation is 114.6 kJ/mol,which is 36%lower than pristine LiBH₄.LiBH₄-0.06TiO system can absorb hydrogen from 260°C,and the hydrogen absorption capacity reaches 9 wt%after dwelling at 500°C for 3 h,which is much higher than pristine LiBH₄.The capacity retention of LiBH₄-0.06TiO system is 74.4%after 10rounds of hydrogen absorption and desorption cycles,indicating significantly improved hydrogen absorption properties compared with pristine LiBH₄.The formation of Li₃BO₃ and TiH₂ in the dehydrogenation process catalyze the subsequent hydrogen absorption and desorption process.By adding Nb?OEt?₅ into LiBH₄ vai ball milling and thereafter the heat treatment and the hydrogenation treatment,well dispered NbH and Li₃BO₃ were introduced in-situ into LiBH₄.The onset and peak dehydrogenation temperatures of the optimized LiBH₄-0.04?Li₃BO₃+NbH?system were reduced to 190°C and 340°C,respectively,which were 190°C and 90°C lower than that of pristine LiBH₄.The composite system can absorb 7.9 wt%H₂ in 15 min at 500°C and with 50 bar of H₂.After 30 cycles of hydrogen absorption and desorption,the hydrogen desorption capacity of the composite system is still 7.2 wt%and the capacity retention is as high as 91%which is the best long term cyclic stability to date.In-situ introduced Li₃BO₃ and NbH have bi-catalytic effects for the hydrogen absorption and desorption performances of LiBH₄.Apparent activation energy was apparently reduced and thus improved dehydrogenation kinetics was obtained.Mg?OEt?₂,Al?OEt?₃,Si?OEt?₄ and NH₄VO₃ were selected as precursor and NH₄VO₃ was finally determined as precursor.Li₃BO₃ and active metal species V could be successfully in-situ introduced into LiBH₄ after adding NH₄VO₃ followed by heat treatment and hydrogenation treatment.The onset and peak dehydrogenation temperatures of the optimized LiBH₄-0.06?Li₃BO₃+V?system are 220°C and 370°C,respectively,which are 160°C and 60°C lower than those of pristine LiBH₄.The apparent activation energy of the composite system is 97.3 kJ/mol and thus the dehysdrogenation kinetics is obviously improved.The capacity retention of LiBH₄-0.06?Li₃BO₃+V?system after five cycles of hydrogen absorption and desorption is87.5%,which is much higher than that of pristine LiBH₄.As catalysts,Li₃BO₃ and V can remain stable and have high-efficiency catalytic effects.The effect of different addition of commercial Nb₂O₅ on hydrogen storage performance of LiBH₄ was investigated.In the process of dehydrogenation,LiBH₄ and Nb₂O₅ will react to generate NbO firstly and the remaining LiBH₄ can further react with NbO to form elemental Nb.LiBH₄+0.03Nb₂O₅ has optimum hydrogen absorption and desorption performances among LiBH₄+xNb₂O₅ systems.In order to further improve the properties of LiBH₄+0.03Nb₂O₅,nano-Nb₂O₅ and carbon doped Nb₂O₅@AB materials were prepared.The onset and peak hydrogen desorption temperature for LiBH₄+0.03Nb₂O₅@AB system are 200°C and 340°C,respectively and the apparent activation energy is reduced to 89.7 kJ/mol.In terms of hydrogen absorption performance,LiBH₄+0.03n-Nb₂O₅ system achieved the capacity retention of 60.8%after five cycles of hydrogen absorption and desorption,presenting the best cyclic stability.It is concluded that via introducing catalysts by different means,the amount of the highly thermodynamically stable Li₂B₁₂H₁₂ phase in the hydrogenation products of LiBH₄ is significantly reduced,which results in the improved hydrogen storage performance of LiBH₄.This perspective can provide theoretical guidance for the further investigation on the hydrogen storage performance of LiBH₄.