固体氧化物阴极表面氧还原反应过程研究

发布时间：2018-03-05 03:02

本文选题：固体氧化物燃料电池　切入点：复相阴极　出处：《中国科学技术大学》2017年博士论文　论文类型：学位论文

【摘要】：固体氧化物燃料电池(SOFC)是一种能够将化学能直接转化为电能的高效、环保的能源转换装置。在其商业化应用的发展进程中,运行条件的中低温化成为了 SOFC技术的必然趋势,但是阴极反应的极化损失严重制约了 SOFC在中低温下的性能提升,因此,设计构建高性能的阴极和深入理解阴极的反应机理势在必行。本论文以典型的混合导体阴极La_(0.6)Sr_(0.4)Co_(0.2)Fe_(0.8)O_(3-δ)(LSCF)为研究对象,通过电导弛豫方法表征离子导电相Sm_xCe_(1-x)O_(2-δ)(SDC)的加入对于阴极表面氧还原反应过程的增强作用,重点研究三相线处的反应过程和对于整体反应的贡献率,确定氧还原反应的反应机理和速控步骤。第一章主要介绍了 SOFC的运行原理和阴极的常用材料,重点阐述了复相材料中的性能协同增强现象,以及三相线对于阴极氧还原反应的重要作用,随后揭示了氧还原反应可能的反应机理步骤,最后对本论文的主要表征手段电导弛豫方法进行了全面的介绍,包括其测试过程、理论方法、影响因素等等。为了实现对复相材料以及三相线处氧还原过程的表征,首先必须从理论上获得相应的反应动力学参数的表达式。第二章的工作以LSCF-SDC复相材料为研究对象,通过把表面交换系数具象化为反应量与反应速率的形式,基于传统的单相材料的表面交换系数,提出了有关复相材料表面以及三相线处氧还原反应动力学参数的理论推导方法,并且定义了三相线对于整体反应的贡献因子。此外,基于电导弛豫测试的氧分压梯度和计算的氧还原反应速率,提出了表面氧还原过程对应的等效电势差、交换电流密度以及极化阻抗的计算方法。第二章的工作为后续的氧还原反应动力学参数表征和反应机理研究提供了理论方法和基础。能够定量表征并区分两相界面和三相线处的反应过程对于研究复相阴极的表面协同效果和氧还原反应机理都是必不可少的,因此第三章的工作主要基于前面提出的理论方法,通过电导弛豫测试表征了 LSCF-SDC复相材料的表面氧还原反应过程,并分别计算得到了两相界面与三相线处的氧还原反应量和反应速率。结果表明加入SDC后样品的表面交换系数增大了 5倍,证明LSCF与SDC之间存在表面协同增强作用。结合样品表面微结构的定量统计结果显示,在复相材料的表面氧还原反应过程中超过70%的氧都是通过三相线进入氧化物体相中的。进一步的结果表明三相线处的反应速率并不是完全由三相线的密度决定,因为LSCF的颗粒大小也会影响三相线的反应速率,这可能与表明吸附的氧物种在LSCF表面的迁移过程有关,基于实验结果发现氧在LSCF表面的最大迁移距离约为1.5 μm。为了定量表征离子导电相电导率对于三相线处氧还原反应的影响,第四章测试了 LSCF-Sm_xCe_(1-x)O_(2-δ)复相材料的氧弛豫过程。通过上述理论方法计算得到的三相线处的贡献因子随着测试温度的升高而降低,说明三相线在低温时的作用更加明显。结合三相线密度等参数的统计结果,发现三相线处的表面反应速率随着Sm_xCe_(1-x)O_(2-δ)的电导率增大而提升,在较低测试温度时,甚至表现为线性增长。经过基元反应的动力学参数的理论推导证明当氧融入过程是唯一的速控步骤时,三相线的反应速率常数会随着Sm_xCe_(1-x)O_(2-δ)的电导率线性增加。因此,氧的融入过程是三相线处氧还原反应的重要速控步骤。由于三相线处的氧还原反应过程不仅与三相线的密度相关,还会受到两相表面微结构的影响,因此第五章主要通过调控LSCF-SDC复相样品中的SDC相的晶粒尺寸来研究其表面形貌对于三相线处氧还原反应过程的影响。结果表明随着SDC初始粉体的热处理温度的升高,复相样品中的SDC粒径逐渐增大,而统计得到的三相线密度则逐渐减小,相应的表面氧还原反应速率也随之降低。但是单位三相线长度的反应速率反而有所增加,表明三相线的实际反应效率随着SDC晶粒尺寸的增加而提升。这可能是因为在三相线处富集的吸附氧离子会溢流到SDC表面,进而与SDC表面的活性氧空位相结合,这样氧融入反应的活性位点就会由三相线向SDC表面扩展,从而提升了单位三相线的反应效率。结果表明吸附氧离子在SDC表面的迁移距离可能不小于0.42 μm。为了直接表征多孔样品的表面氧还原反应过程,必须保证气体在反应容器内的切换过程和样品孔内的扩散过程足够快。考虑到传统的测试方法很难做到,在第六章的工作中设计了真空-电导弛豫测试装置并用于表征了多孔LSCF样品的表面氧还原反应过程。结果发现测得的多孔样品的氧还原反应同时受到表面交换过程和气体在孔内的Knudsen扩散过程的影响。在此基础上,提出了多孔样品的特征厚度的概念,可以用于比较表面交换过程和Knudsen扩散过程的相对快慢。之后推导得到了包含上述两个过程的双参数拟合函数,可以分离得到多孔样品的氧还原反应动力学参数。拟合结果显示多孔样品的氧还原反应速率随着烧结温度的升高和造孔剂含量的减少而降低,而且其表面交换系数的表观活化能(49-70 kJ mol-1)明显低于致密样品(约110 kJ mol-1)。基于多孔LSCF样品在不同氧分压梯度和SDC浸渍量等条件下的实验结果,表明氧的吸附过程是多孔样品氧还原反应的重要速控步骤。
[Abstract]:Solid oxide fuel cell (SOFC) is a way to convert chemical energy directly into electrical energy efficient, environmentally friendly energy conversion device. In the process of the development of the commercial application in low temperature operating conditions has become the inevitable trend of SOFC technology, but loss of cathode reaction seriously restricts the promotion the performance of SOFC in low temperature therefore, imperative reaction mechanism design construction of high performance cathode and understanding the cathode. In this paper, the typical mixed conductor cathode La_ (0.6) Sr_ (0.4) Co_ (0.2) Fe_ (0.8) O_ (3- 8) (LSCF) as the research object, through the conductivity relaxation characterization of ion conducting phase Sm_xCe_ (1-x) O_ (2- 8) (SDC) enhancement to the cathode surface oxygen reduction reaction process, the reaction process focuses on the three-phase contact line and the overall response rate of contribution, to determine the reaction mechanism and the rate controlling step of oxygen reduction reaction The first chapter mainly introduces the operating principle of commonly used materials and the cathode of SOFC, focuses on the performance of the composite material of the synergistic phenomenon, as well as the important role of three-phase line for the cathodic oxygen reduction reaction, then reveals the possible reaction mechanism of oxygen reduction reaction steps, finally the main characterization methods of the conductivity relaxation relaxation methods are introduced, including the test process, theoretical methods, influencing factors and so on. In order to realize the multiphase materials and the three-phase line of the oxygen reduction process characterization, expression must first obtain the kinetic parameters from the corresponding theory. The second chapter work with LSCF-SDC composite as the research object, through the the surface exchange coefficient for concrete reaction and reaction rate in the form of the traditional single-phase materials surface exchange coefficient based on the proposed composite surface and three The theoretical derivation method of phase at the oxygen reduction reaction kinetic parameters, and the definition of the three phase for the contribution factor of the overall reaction. In addition, the electrical conductivity relaxation test of oxygen partial pressure gradient and the calculation of oxygen reduction reaction rate based on the equivalent potential of surface oxygen reduction process corresponding to the exchange current density and polarization impedance calculation method the work of the second chapter. For the oxygen reduction kinetics and reaction mechanism of the reaction parameters provides a theoretical basis and the methods. To quantitatively characterize and distinguish the reaction process and the three-phase line at the interface of the surface to study the synergistic effect of composite cathode and oxygen reduction reaction mechanism is essential, so the work of the third chapter mainly based on the methods proposed above, the conductivity relaxation test to characterize the reaction process of oxygen on the surface of LSCF-SDC composites and reduction, respectively Is the three phase of the two phase interface and the oxygen reduction reaction and the reaction rate. The results showed that after adding SDC sample surface exchange coefficient increased by 5 times, to prove the existence of surface synergistic effect between LSCF and SDC. Combined with the quantitative statistical results of sample surface micro structure, on the surface of the composite materials of oxygen reduction reaction in the process of more than 70% of the oxygen is through the three phase object into the oxidation phase. Further results show that the three-phase line at the reaction rate is not entirely by the three phase determines the density, because the reaction rate of LSCF particle size will also affect the three phase, which may be related to that of oxygen species adsorbed on the migration process of LSCF the surface of the experimental results found that the oxygen in the maximum migration distance from the LSCF surface is about 1.5 m. for quantitative characterization of ion conductive phase conductivity influence on the three-phase contact line based on oxygen reduction reaction, The fourth chapter tested LSCF-Sm_xCe_ (1-x) O_ (2- 8) oxygen relaxation process of composite materials. The contribution factor at the three phase obtained by the theoretical calculation method decreases with the increase of temperature, indicating the three-phase line when the low temperature effect is more apparent. Combined with statistical results of three-phase line density parameter that, the three-phase contact line surface reaction rate as Sm_xCe_ (1-x) O_ (2- 8) the conductivity increases at low temperature, test, even for linear growth. Theoretical derivation by kinetic parameters of reactions that when oxygen is incorporated into the process is the only rate controlling step, reaction rate constant of three-phase line with Sm_xCe_ (1-x) O_ (2- 8) to increase the conductivity of linear. Therefore, in the process of oxygen is an important speed three-phase line oxygen reduction reaction control steps. The reaction of oxygen reduction at the three-phase line not only with the three phase. The degree of correlation is also affected by the two surface micro structure, so the fifth chapter mainly through the regulation of LSCF-SDC composite samples of the SDC phase and the grain size of the surface topography effects on reaction process of three phase line oxygen reduction. The results showed that with the increase of initial SDC powder heat treatment temperature, multiphase samples SDC particle size increases, and the statistics of three phase density is gradually decreased, the corresponding surface oxygen reduction reaction rate is reduced. But the reaction rate per unit length of the three phase but increased, showed that the actual anti three phase should enhance with the increased efficiency of the grain size of SDC. This may be because the adsorption of oxygen ion enrichment in the three-phase line will overflow to the surface of SDC, and combined with the activity of oxygen vacancies on the surface of SDC, such as reaction sites of oxygen into the three phase will be extended from SDC to the surface, thus To enhance the reaction efficiency of unit three phase. The results show that the adsorption of oxygen ions in the migration distance of SDC surface may not be less than 0.42 M. in order to direct characterization of porous sample surface oxygen reduction reaction, must ensure that the switching process and the sample gas diffusion hole in the reaction vessel in the process of fast enough. Considering the traditional test method it is difficult to do, in the work of the sixth chapter in the design of the vacuum - conductivity relaxation test device and used to characterize the reaction process of oxygen on the surface of porous LSCF sample reduction. The results showed that the porous samples measured the oxygen reduction reaction is also affected by the surface exchange process and gas hole in Knudsen diffusion process. On the basis of put forward the concept, the characteristics of thickness of porous samples, can be used to compare relative speed of surface exchange process and Knudsen diffusion process is derived. After the two Double parameter fitting function, can be obtained from the porous samples of oxygen reduction reaction kinetic parameters. The fitting results show that the porous samples of oxygen reduction reaction rate decreased with the decrease and increase the content of pore forming agent and sintering temperature, and the surface exchange coefficient of apparent activation energy (49-70 kJ mol-1) was significantly lower than that of dense samples (about 110 kJ mol-1). The experimental results of porous LSCF sample pressure gradient and SDC impregnation conditions under different oxygen based on that the oxygen adsorption velocity of porous samples is an important oxygen reduction reaction controlling step.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TM911.4

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