轻质储氢材料的改性研究
发布时间：2019-06-22 12:46
【摘要】：为了达到车载燃料电池应用对氢源的要求,亟需一种高体积和重量能量密度的轻质储氢材料,为此国内外开展了大量的研究工作。在轻质储氢材料中,硼氢化锂(LiBH4)因其具有高达18.4wt.%的理论储氢容量被认为具有广阔的应用前景。然而,其脱氢温度高且易释放杂质气体、脱/加氢动力学差和脱氢产物再加氢困难等问题制约了LiBH4的商业应用。针对这些问题,本论文通过阳离子取代、阴离子取代、催化掺杂和失稳体系构建等方法逐步改善了LiBH4的储氢性能,取得了一些创新性的研究成果：首先采用阳离子取代来改善LiBH4的脱氢性能。球磨LiBH4和MnCl2制备了阳离子取代的LiMn(BH4)3/2LiCl复合体系,研究了Mn离子取代对体系脱氢热力学和动力学性能的影响,阐明了其脱氢控速步骤,进一步通过添加Ti基掺杂剂改善了复合体系的脱氢动力学性能。其次,在阳离子取代基础上,采用阴离子取代来进一步改善LiBH4的脱氢性能。球磨LiBH4和MnF2制备了阴/阳离子取代的LiMn(BH4-xFx)3/2LiF复合体系,测试了复合体系的储氢性能。通过与LiMn(BH4)3/2LiCl复合体系的储氢性能进行对比,阐明了阴离子取代对储氢性能和动力学控速机制的影响。然后,针对LiMn(BH4-xFx)3/2LiF复合体系的脱氢过程中存在的杂质气体问题,采用掺杂改性法优化了复合体系的脱氢性能。通过球磨在LiMn(BH4-xFx)3/2LiF复合体系中掺杂一定量的LiNH2,掺杂后能够显著地抑制复合体系脱氢过程中杂质气体B2H6的生成,同时进一步降低了体系的脱氢温度。最后,针对LiMn(BH4-xFx)3/2LiF复合体系脱氢产物B单质加氢困难的问题,采用与MgH2构建失稳体系来调制脱氢反应的产物相。该失稳体系在惰性气体背压下能够稳定地生成MgB2,且不存在明显的成核孕育期,阐明了该失稳体系中MgB2的形成机制。这一结果显著改善了硼氢化物／金属氢化物失稳体系脱氢过程金属硼化物成核期过长的问题。本论文的研究开阔了对配位金属氢化物性能进行系统调制的思路,为配位氢化物的进一步开发应用奠定了理论基础。论文的主要研究结果如下：(1) LiMn(BH4)3/2LiCl复合体系的物相结构是由非晶态的LiMn(BH4)3和晶态的LiCl组成。其储氢性能测试结果表明,LiCl复合体系的初始脱氢温度从纯LiBH4的400℃降低至135℃,在135-190℃区间内失重为7.0wt.%。气体成分分析表明释放的气体中H2占93.2 mol%, B2H6占6.8 mo1%。LiMn(BH4)3/2LiCl复合体系的脱氢反应激活能Ea=114kJ/mol,脱氢过程受到三维界面的迁移扩散控制。Ti基掺杂剂对LiMn(BH4)3体系脱氢性能改善的效果各异,具体表现在：TiN、TiC或TiO2不能有效催化LiMn(BH4)3/2LiCl复合体系的分解反应；而TiF3掺杂的LiMn(BH4)3/2LiCl复合体系的初始脱氢温度进一步降低至125℃,其脱氢反应激活能也由未掺杂前的114 kJ/mol下降为104 kJ/mol,推测原因为在球磨掺杂过程中TiF3与复合体系在局部形成了常温脱氢的Ti(BH4)3。(2)使用MnF2替代MnCl2与LiBH4进行球磨,制备了LiMn(BH4-xFx)3/2LiF复合体系。与LiMn(BH4)3/2LiCl复合体系相比,LiMn(BH4-xFx)3/2LiF复合体系不仅发生了阳离子Mn2+和Li+之间的取代,同时发生了阴离子F-和H-之间的取代,且F-离子取代后LiMn(BH4-xFx)3/2LiF复合体系的初始脱氢温度进一步从135℃下降至120℃,脱氢激活能Ea从114 kJ/mol降低至92 kJ/mol。 LiMn(BH4-xFx)3/2LiF复合体系失重为7.0wt.%。气体成分分析表明释放的气体中H2占94.8 mol%,B2H6占5.2 mo1%,抑制了部分B2H6的产生。脱氢反应由三维界面的迁移扩散转变为一维形核长大机制控制。脱氢性能和机制的改变均由F-对H-的部分取代造成B-H键失稳所引起。(3)系统考察了LiNH2对LiMn(BH4-xFx)3/2LiF复合体系脱氢性能的影响。掺杂LiNH2会促进LiMn(BH4-xFx)3/2LiF复合体系中硼氢化物与MnF2的置换反应,使得复合体系中的MnF2相逐渐转变为LiF相并形成非晶态的LiMn(BH4-xFx)3。相比于未掺杂的LiMn(BH4-xFx)3/2LiF复合体系,掺杂2.5、5、7.5、10和15wt.%LiNH2的复合物的起始脱氢温度分别下降至101、96、94、78和77℃,失重分别为6.8、5.0、4.8、6.5和9.3wt.%。质谱分析表明大于5wt.%的LiNH2即可显著抑制LiMn(BH4-xFx)3/2LiF复合体系脱氢过程中所生成的B2H6,但过多的LiNH2也会生成NH3,影响释放氢气的纯度。实验证明,添加5 wt.%LiNH2可以获得纯氢气的释放,此时掺杂后的复合物在100-140℃分解,放出约5.0 wt.%纯氢气,脱氢反应的激活能Ea为91.0 kJ/molc具体改善机制为[BH4]-基团中Hδ-和[NH2]-中H5-结合致使H2在低温下逸出,类似地,抑制B2H6产生的机制为[BH4]-基团中Bδ+被[NH2]-中Nδ-固定,阻断了游离态的BH3或者BH3带电基团的产生和进一步结合。(4)研究了MgH2与LiMn(BH4-xFx)3/2LiF构建的失稳体系的脱氢性能。升温实验表明,复合体系中LiMn(BH4-xFx)3/2LiF在120-160℃分解,失重为4.9Wt.%; MgH2在350～500℃分解,失重为2.1 wt.%。随后的物相分析显示,LiMn(BH4-xFx)3/2LiF分解产生的B伴随着MgH2的分解最终转化为MgB2。该体系能在惰性气体中生成MgB2,且不存在明显的成核孕育期,明显改善了MgH2-LiBH4体系中MgB2形成条件苛刻且孕育期长等问题。改善得益于：首先伴随MgH2分解,Mg原子逸出表面能量不断降低,可动性增强,进而加速了与B形成MgB2的成核动力学；其次,Mn元素附着于MgB2晶核周围,加速了Mg原子在MgB2晶格中的扩散,进而促进了MgB2晶核生长过程。
[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，
本文编号：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，
本文编号：2504593
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