輕質儲氫材料的改性研究
發(fā)布時間：2019-06-22 12:46
【摘要】：為了達到車載燃料電池應用對氫源的要求,亟需一種高體積和重量能量密度的輕質儲氫材料,為此國內外開展了大量的研究工作。在輕質儲氫材料中,硼氫化鋰(LiBH4)因其具有高達18.4wt.%的理論儲氫容量被認為具有廣闊的應用前景。然而,其脫氫溫度高且易釋放雜質氣體、脫/加氫動力學差和脫氫產(chǎn)物再加氫困難等問題制約了LiBH4的商業(yè)應用。針對這些問題,本論文通過陽離子取代、陰離子取代、催化摻雜和失穩(wěn)體系構建等方法逐步改善了LiBH4的儲氫性能,取得了一些創(chuàng)新性的研究成果：首先采用陽離子取代來改善LiBH4的脫氫性能。球磨LiBH4和MnCl2制備了陽離子取代的LiMn(BH4)3/2LiCl復合體系,研究了Mn離子取代對體系脫氫熱力學和動力學性能的影響,闡明了其脫氫控速步驟,進一步通過添加Ti基摻雜劑改善了復合體系的脫氫動力學性能。其次,在陽離子取代基礎上,采用陰離子取代來進一步改善LiBH4的脫氫性能。球磨LiBH4和MnF2制備了陰/陽離子取代的LiMn(BH4-xFx)3/2LiF復合體系,測試了復合體系的儲氫性能。通過與LiMn(BH4)3/2LiCl復合體系的儲氫性能進行對比,闡明了陰離子取代對儲氫性能和動力學控速機制的影響。然后,針對LiMn(BH4-xFx)3/2LiF復合體系的脫氫過程中存在的雜質氣體問題,采用摻雜改性法優(yōu)化了復合體系的脫氫性能。通過球磨在LiMn(BH4-xFx)3/2LiF復合體系中摻雜一定量的LiNH2,摻雜后能夠顯著地抑制復合體系脫氫過程中雜質氣體B2H6的生成,同時進一步降低了體系的脫氫溫度。最后,針對LiMn(BH4-xFx)3/2LiF復合體系脫氫產(chǎn)物B單質加氫困難的問題,采用與MgH2構建失穩(wěn)體系來調制脫氫反應的產(chǎn)物相。該失穩(wěn)體系在惰性氣體背壓下能夠穩(wěn)定地生成MgB2,且不存在明顯的成核孕育期,闡明了該失穩(wěn)體系中MgB2的形成機制。這一結果顯著改善了硼氫化物／金屬氫化物失穩(wěn)體系脫氫過程金屬硼化物成核期過長的問題。本論文的研究開闊了對配位金屬氫化物性能進行系統(tǒng)調制的思路,為配位氫化物的進一步開發(fā)應用奠定了理論基礎。論文的主要研究結果如下：(1) LiMn(BH4)3/2LiCl復合體系的物相結構是由非晶態(tài)的LiMn(BH4)3和晶態(tài)的LiCl組成。其儲氫性能測試結果表明,LiCl復合體系的初始脫氫溫度從純LiBH4的400℃降低至135℃,在135-190℃區(qū)間內失重為7.0wt.%。氣體成分分析表明釋放的氣體中H2占93.2 mol%, B2H6占6.8 mo1%。LiMn(BH4)3/2LiCl復合體系的脫氫反應激活能Ea=114kJ/mol,脫氫過程受到三維界面的遷移擴散控制。Ti基摻雜劑對LiMn(BH4)3體系脫氫性能改善的效果各異,具體表現(xiàn)在：TiN、TiC或TiO2不能有效催化LiMn(BH4)3/2LiCl復合體系的分解反應；而TiF3摻雜的LiMn(BH4)3/2LiCl復合體系的初始脫氫溫度進一步降低至125℃,其脫氫反應激活能也由未摻雜前的114 kJ/mol下降為104 kJ/mol,推測原因為在球磨摻雜過程中TiF3與復合體系在局部形成了常溫脫氫的Ti(BH4)3。(2)使用MnF2替代MnCl2與LiBH4進行球磨,制備了LiMn(BH4-xFx)3/2LiF復合體系。與LiMn(BH4)3/2LiCl復合體系相比,LiMn(BH4-xFx)3/2LiF復合體系不僅發(fā)生了陽離子Mn2+和Li+之間的取代,同時發(fā)生了陰離子F-和H-之間的取代,且F-離子取代后LiMn(BH4-xFx)3/2LiF復合體系的初始脫氫溫度進一步從135℃下降至120℃,脫氫激活能Ea從114 kJ/mol降低至92 kJ/mol。 LiMn(BH4-xFx)3/2LiF復合體系失重為7.0wt.%。氣體成分分析表明釋放的氣體中H2占94.8 mol%,B2H6占5.2 mo1%,抑制了部分B2H6的產(chǎn)生。脫氫反應由三維界面的遷移擴散轉變?yōu)橐痪S形核長大機制控制。脫氫性能和機制的改變均由F-對H-的部分取代造成B-H鍵失穩(wěn)所引起。(3)系統(tǒng)考察了LiNH2對LiMn(BH4-xFx)3/2LiF復合體系脫氫性能的影響。摻雜LiNH2會促進LiMn(BH4-xFx)3/2LiF復合體系中硼氫化物與MnF2的置換反應,使得復合體系中的MnF2相逐漸轉變?yōu)長iF相并形成非晶態(tài)的LiMn(BH4-xFx)3。相比于未摻雜的LiMn(BH4-xFx)3/2LiF復合體系,摻雜2.5、5、7.5、10和15wt.%LiNH2的復合物的起始脫氫溫度分別下降至101、96、94、78和77℃,失重分別為6.8、5.0、4.8、6.5和9.3wt.%。質譜分析表明大于5wt.%的LiNH2即可顯著抑制LiMn(BH4-xFx)3/2LiF復合體系脫氫過程中所生成的B2H6,但過多的LiNH2也會生成NH3,影響釋放氫氣的純度。實驗證明,添加5 wt.%LiNH2可以獲得純氫氣的釋放,此時摻雜后的復合物在100-140℃分解,放出約5.0 wt.%純氫氣,脫氫反應的激活能Ea為91.0 kJ/molc具體改善機制為[BH4]-基團中Hδ-和[NH2]-中H5-結合致使H2在低溫下逸出,類似地,抑制B2H6產(chǎn)生的機制為[BH4]-基團中Bδ+被[NH2]-中Nδ-固定,阻斷了游離態(tài)的BH3或者BH3帶電基團的產(chǎn)生和進一步結合。(4)研究了MgH2與LiMn(BH4-xFx)3/2LiF構建的失穩(wěn)體系的脫氫性能。升溫實驗表明,復合體系中LiMn(BH4-xFx)3/2LiF在120-160℃分解,失重為4.9Wt.%; MgH2在350～500℃分解,失重為2.1 wt.%。隨后的物相分析顯示,LiMn(BH4-xFx)3/2LiF分解產(chǎn)生的B伴隨著MgH2的分解最終轉化為MgB2。該體系能在惰性氣體中生成MgB2,且不存在明顯的成核孕育期,明顯改善了MgH2-LiBH4體系中MgB2形成條件苛刻且孕育期長等問題。改善得益于：首先伴隨MgH2分解,Mg原子逸出表面能量不斷降低,可動性增強,進而加速了與B形成MgB2的成核動力學；其次,Mn元素附著于MgB2晶核周圍,加速了Mg原子在MgB2晶格中的擴散,進而促進了MgB2晶核生長過程。
[Abstract]:In order to meet the requirements of the on-board fuel cell application to the hydrogen source, a light hydrogen storage material with high volume and energy density is needed, and a great deal of research has been carried out at home and abroad. In the light hydrogen storage material, lithium borohydride (LiBH4) has a capacity of up to 18.4% by weight. The theoretical hydrogen storage capacity of% is considered to have a wide application prospect. However, that problem of high dehydrogenation temperature and easy release of the impurity gas, the dehydro-kinetic difference and the re-hydrogenation of the dehydrogenation product, and the like, has restricted the commercial application of the LiBH4. In view of these problems, this paper has gradually improved the hydrogen storage performance of LiBH4 by the method of cation substitution, anion substitution, catalytic doping and instability system, and has obtained some innovative research results: firstly, the cation substitution is adopted to improve the dehydrogenation performance of the LiBH4. LiMn (BH4)3/ 2LiCl composite system with cation substitution was prepared by ball-milling LiBH4 and MnCl2, and the effect of the substitution of Mn ions on the thermodynamic and kinetic properties of the system was studied. Second, on the basis of cation substitution, anion substitution is used to further improve the dehydrogenation performance of LiBH4. LiMn (BH4-xcarbon)3/ 2LiF composite system with negative/ cationic substitution was prepared by ball-milling LiBH4 and MnF2, and the hydrogen storage performance of the composite system was tested. The hydrogen storage performance of LiMn (BH4)3/2 LiCl composite system is compared, and the effect of the anion substitution on the hydrogen storage performance and the dynamic speed control mechanism is clarified. In the process of dehydrogenation of LiMn (BH4-xn)3/ 2LiF composite system, the dehydrogenation performance of the composite system was optimized by the doping modification method. By ball-milling, a certain amount of LiNH2 is doped in the LiMn (BH4-xn)3/ 2LiF composite system, and after doping, the generation of the impurity gas B2H6 in the dehydrogenation process of the composite system can be remarkably inhibited, and meanwhile, the dehydrogenation temperature of the system is further reduced. In the end, the product phase of the dehydrogenation reaction was prepared by the formation of the instability system with MgH2, in the light of the difficult problem of the simple substance hydrogenation of the dehydrogenation product B of the LiMn (BH4-xn)3/2 LiF composite system. MgB2 can be stably generated under the back pressure of inert gas, and no obvious nucleation period is present, and the formation mechanism of MgB2 in the unstable system is illustrated. This result significantly improves the problem of the too long nucleation period of the metal boride during the dehydrogenation process of the borohydride/ metal hydride unstable system. The research of this thesis is open to the idea of system modulation for coordination metal hydride performance, which lays a theoretical foundation for the further development and application of coordination hydride. The main results of this paper are as follows: (1) The phase structure of LiMn (BH4)3/2 LiCl composite system is composed of amorphous LiMn (BH4)3 and crystalline LiCl. The results of the hydrogen storage performance test show that the initial dehydrogenation temperature of the LiCl composite system is reduced from 400 鈩，
本文編號：2504593
[Abstract]:In order to meet the requirements of the on-board fuel cell application to the hydrogen source, a light hydrogen storage material with high volume and energy density is needed, and a great deal of research has been carried out at home and abroad. In the light hydrogen storage material, lithium borohydride (LiBH4) has a capacity of up to 18.4% by weight. The theoretical hydrogen storage capacity of% is considered to have a wide application prospect. However, that problem of high dehydrogenation temperature and easy release of the impurity gas, the dehydro-kinetic difference and the re-hydrogenation of the dehydrogenation product, and the like, has restricted the commercial application of the LiBH4. In view of these problems, this paper has gradually improved the hydrogen storage performance of LiBH4 by the method of cation substitution, anion substitution, catalytic doping and instability system, and has obtained some innovative research results: firstly, the cation substitution is adopted to improve the dehydrogenation performance of the LiBH4. LiMn (BH4)3/ 2LiCl composite system with cation substitution was prepared by ball-milling LiBH4 and MnCl2, and the effect of the substitution of Mn ions on the thermodynamic and kinetic properties of the system was studied. Second, on the basis of cation substitution, anion substitution is used to further improve the dehydrogenation performance of LiBH4. LiMn (BH4-xcarbon)3/ 2LiF composite system with negative/ cationic substitution was prepared by ball-milling LiBH4 and MnF2, and the hydrogen storage performance of the composite system was tested. The hydrogen storage performance of LiMn (BH4)3/2 LiCl composite system is compared, and the effect of the anion substitution on the hydrogen storage performance and the dynamic speed control mechanism is clarified. In the process of dehydrogenation of LiMn (BH4-xn)3/ 2LiF composite system, the dehydrogenation performance of the composite system was optimized by the doping modification method. By ball-milling, a certain amount of LiNH2 is doped in the LiMn (BH4-xn)3/ 2LiF composite system, and after doping, the generation of the impurity gas B2H6 in the dehydrogenation process of the composite system can be remarkably inhibited, and meanwhile, the dehydrogenation temperature of the system is further reduced. In the end, the product phase of the dehydrogenation reaction was prepared by the formation of the instability system with MgH2, in the light of the difficult problem of the simple substance hydrogenation of the dehydrogenation product B of the LiMn (BH4-xn)3/2 LiF composite system. MgB2 can be stably generated under the back pressure of inert gas, and no obvious nucleation period is present, and the formation mechanism of MgB2 in the unstable system is illustrated. This result significantly improves the problem of the too long nucleation period of the metal boride during the dehydrogenation process of the borohydride/ metal hydride unstable system. The research of this thesis is open to the idea of system modulation for coordination metal hydride performance, which lays a theoretical foundation for the further development and application of coordination hydride. The main results of this paper are as follows: (1) The phase structure of LiMn (BH4)3/2 LiCl composite system is composed of amorphous LiMn (BH4)3 and crystalline LiCl. The results of the hydrogen storage performance test show that the initial dehydrogenation temperature of the LiCl composite system is reduced from 400 鈩，
本文編號：2504593
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