氮化硼负载铌基添加剂复合掺杂硼氢化锂体系的吸放氢性能及其协同改性机理

发布时间：2018-07-15 16:42

【摘要】：氢能作为一种极具发展潜力的理想清洁能源而广受关注,而安全、高效和低成本的储氢技术是目前氢能开发应用亟需解决瓶颈技术。硼氢化锂(LiBH4)因具有18.5 wt%的质量储氢密度和121 kg H2/m3的体积储氢密度,成为目前高容量储氢材料的研究热点之一。然而,LiBH4热力学稳定性较高,吸放氢动力学性能较差,可逆吸氢的条件过于苛刻,严重阻碍了其实用化的进程。本文在全面综述国内外LiBH4储氢材料的研究进展的基础上,研究了六方型氮化硼(h-BN)掺杂、h-BN分别负载NbCl5和NbH掺杂对LiBH4的储氢性能的影响规律,并探索揭示了其改性机理。将h-BN作为添加剂掺杂至LiBH4体系以改善其吸放氢性能,详细研究了h-BN添加量为5mol%、15mol%、30mol%和50 mol%的h-BN掺杂LiBH4体系的放氢动力学性能。结果表明：当h-BN添加量为30 mol%时,h-BN掺杂LiBH4体系具有最佳综合储氢性能,其起始放氢温度和放氢反应活化能分别由未掺杂LiBH4样品的280℃和198.31 kJ/mol降低至180℃和155.80 kJ/mol;样品在400℃下完成放氢反应所需时间由未掺杂LiBH4样品的6000分钟减少至120分钟。经过30 mol% h-BN掺杂改性后,LiBH4能够在400℃和10 MPa氢压条件下实现再吸氢,且循环放氢性能相对稳定,第三次放氢容量可达到6 wt%。进一步分析认为：h-BN表面N原子上“孤电子对”对LiBH4起到一定的反应失稳作用,因此h-BN表面的LiBH4率先分解,其分解产物则成为后续LiBH4分解反应的形核中心,降低了LiBH4的分解温度。为了进一步改善LiBH4的储氢性能,采用NbCl5/h-BN复合掺杂LiBH4,并对比分析NbCl5/h-BN复合掺杂LiBH4体系、NbCl5与h-BN单独掺杂LiBH4体系的储氢性能。研究发现：NbCl5/h-BN复合掺杂LiBH4储氢性能比NbCl5或h-BN单独掺杂LiBH4体系具有更好的放氢动力学性能。其中,1 mol% NbCl5+30 mol% h-BN复合掺杂改性的LiBH4样品在20分钟内能够释放出12.19 wt%,为同样条件下1 mol% NbCl5和30 mol% h-BN单独掺杂LiBH4样品所能释放氢气量的17.41和2.61倍。经1 mol% NbCl5+30 mol% h-BN复合掺杂后,LiBH4放氢峰值温度降低了100℃,放氢反应活化能降低122.75 kJ/mol,循环放氢性能得到明显改善。进一步分析认为：NbCl5/h-BN复合掺杂LiBH4时,在h-BN表面NbCl5与LiBH4发生反应原位生成的纳米NbH,起到进一步催化改性LiBH4储氢性能的作用。本文进一步采用机械球磨化学法研究制备出h-BN负载5-20 nm的NbH复合型催化剂,并将其用于催化改性LiBH4储氢性能。通过添加1 mol% 3NbH@h-BN催化剂,LiBH4的放氢峰值降低到380℃,并且LiBH4能够在30分钟完成放氢反应,其反应速率是未掺杂的LiBH4的200倍,LiBH4的循环放氢性能也得到了进一步改善,第三次循环放氢容量可达8 wt%。基于微观结构分析和放氢动力学模型计算结果,我们认为其催化改性机理：NbH@h-BN催化剂在LiBH4放氢反应过程起到了异相形核的作用,使LiBH4放氢反应避免了临界晶核形核功,一定程度上降低LiBH4的放氢温度；纳米NbH负载在h-BN表面,形成稳定纳米结构,能够为LiBH4放氢反应形核提供足够多的形核中心,从而加快LiBH4放氢反应速率。
[Abstract]:Hydrogen energy is widely concerned as an ideal clean energy source with great potential. Safety, high efficiency and low cost hydrogen storage technology is an urgent bottleneck in the development and application of hydrogen energy. Lithium borohydride (LiBH4) has become the current high capacity hydrogen storage material because of the mass hydrogen storage density of 18.5 wt% and the storage hydrogen density of the body of 121 kg H2/m3 However, the thermodynamic stability of LiBH4 is high, the kinetic properties of hydrogen absorption and desorption are poor, the conditions for reversible hydrogen absorption are too harsh, which seriously impede the process of application. On the basis of a comprehensive review of the research progress of LiBH4 hydrogen storage materials at home and abroad, this paper studies the six square boron nitride (h-BN) doping, h-BN load NbCl5 and Nb respectively. The effect of H doping on the hydrogen storage properties of LiBH4 was investigated and its modification mechanism was explored. H-BN was doped into the LiBH4 system to improve the performance of hydrogen absorption and desorption. The hydrokinetics of h-BN addition of 5mol%, 15mol%, 30mol% and 50 mol% h-BN doped LiBH4 system was studied in detail. The results showed that the addition of h-BN was 30. At%, the h-BN doped LiBH4 system has the best comprehensive hydrogen storage performance. The initial hydrogen storage temperature and the activation energy of the hydrogen release reaction are reduced to 180 and 155.80 kJ/mol, respectively, from the 280 and 198.31 kJ/mol of the undoped LiBH4 samples. The time required for the completion of the hydrogen release reaction at 400 centigrade is reduced from 6000 minutes of the undoped LiBH4 sample to 120 minutes. After the doping of mol% h-BN, LiBH4 can re absorb hydrogen at 400 and 10 MPa hydrogen pressure, and the performance of the cyclic hydrogen release is relatively stable, and the capacity of the third discharge can reach 6 wt%.. Further analysis shows that the "solitary electron pair" on the N atom on the h-BN surface has a certain reaction instability to LiBH4, so the LiBH4 of the h-BN surface is the first to decompose. The decomposition product becomes the nucleation center of the subsequent LiBH4 decomposition reaction and reduces the decomposition temperature of LiBH4. In order to further improve the hydrogen storage performance of LiBH4, the NbCl5/h-BN compound doping LiBH4 is adopted and the hydrogen storage properties of NbCl5/h-BN complex doping LiBH4 system are compared and analyzed. The hydrogen storage performance of NbCl5 and h-BN alone doped LiBH4 system is studied. The hydrogen storage performance of Co doped LiBH4 is better than that of NbCl5 or h-BN doped LiBH4 system. Among them, 1 mol% NbCl5+30 mol% h-BN doped LiBH4 samples can release 12.19 wt% in 20 minutes, which is 17. of the amount of hydrogen that can be released by the same condition under the same condition. 41 and 2.61 times. After 1 mol% NbCl5+30 mol% h-BN complex doping, the peak temperature of the LiBH4 dehydrogenation is reduced by 100, and the activation energy of the reaction is reduced by 122.75 kJ/mol, and the performance of the cyclic hydrogen release is obviously improved. Further analysis shows that when NbCl5/h-BN is mixed with LiBH4, the nano NbH, which occurs in the h-BN surface NbCl5 and LiBH4, begins. Further catalyze the hydrogen storage performance of modified LiBH4. This paper further uses mechanical ball milling chemical method to prepare a NbH composite catalyst with h-BN load 5-20 nm, and uses it to catalyze the hydrogen storage performance of the modified LiBH4. By adding 1 mol% 3NbH@h-BN catalyst, the peak dehydrogenation peak of LiBH4 is reduced to 380 C, and LiBH4 can finish in 30 minutes. The reaction rate is 200 times as high as that of the undoped LiBH4, and the performance of the cyclic hydrogen release of LiBH4 has been further improved. The third cycle discharge capacity is up to 8 wt%. based on the microstructure analysis and the calculation of the kinetic model of the hydrogen release. We think that the catalytic modification of the catalyst is that the NbH@h-BN catalyst is in the process of the LiBH4 dehydrogenation process. By the effect of the heterogeneous nucleation, the LiBH4 dehydrogenation reaction avoids the critical nucleation work and reduces the hydrogen release temperature of LiBH4 to a certain extent. The nano NbH load on the surface of h-BN forms a stable nanostructure, which can provide enough nucleation Center for the nucleation of the LiBH4 reactor and accelerate the reaction rate of the LiBH4.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB34

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