【作者】 王运龙；

【导师】 夏长荣；

【作者基本信息】 中国科学技术大学 ， 材料学， 2014， 博士

【摘要】 固体氧化物燃料电池(SOFC)是一种高效、清洁的能源转换装置。其输出性能与材料选择、结构设计有着密切的关系。本论文针对特定的微结构和材料设计,提出定量表征固体氧化物电极的表面反应过程和协同作用的实验方法,讨论表面反应与结构力学稳定性的关系,并提出微观力学参数的测定新方法。提出了阴极反应的一个控制步骤,并从理论和实验上进行了验证。针对阴极的氧气还原反应主导电池极化电阻的问题,论文第二章首先探讨了阴极的表面反应。通常认为,阴极反应是由发生在电极表面的一系列的氧气还原基元步骤组成的。为了表征电解质对阴极性能的影响,本工作假定氧跨越电解质-电极界面的步骤同样作为阴极反应的一个步骤。在此基础上,通过动力学可以推导出阴极极化(Rp)与环境氧分压(pO2)和电解质电导率(σ)的关系,简单表示为：Rp∝σlPo2n。其中l和n为不同限速步骤对应的控制参数。在实验上,采用SmxCe1-xO2-δ作为电解质材料,并通过改变掺杂量(x)对电解质电导率进行调控。以La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)为阴极,采用交流阻抗法测量不同电解质对应的界面极化阻抗。通过分析阻抗谱,高频部分可以拟合成一个Warburg元件。拟合结果显示高频极化阻抗随着电解质电阻率的增大而线性增大。因此,对比理论结果可知高频极化阻抗对应氧跨越电解质-电极界面的限速步骤。提出用电导弛豫法(ECR)研究阳极的表面反应过程,给出了一种测试三相界面反应速率的实验方法。论文第三章探讨了在还原性气氛下氧化铈基材料的表面还原过程,并且通过表面反应常数表征氧化铈基材料的催化氧化燃料的反应能力。对于Gd0.1Ce0.9O2-δ,采用ECR的测量结果与文献中采用重量弛豫法的测量结果基本相同,说明ECR可以用作氧化铈基材料的表面反应过程的测量。当测量试样的扩散尺寸约为0.3mm时,电导弛豫过程主要由表面反应步骤决定,与体相传输步骤基本无关。对于不同元素(La, Y, Sm和Gd)及不同含量(0～30%mol)掺杂的氧化铈材料,Sm0.2Ce0.802-δ(SDC)具有最高的表面反应速率常数。因此,SDC最适合作为阳极中的电解质组分。此外,采用同样的方法研究了表面晶界密度对表面反应的影响。当温度低于700℃时,晶界的表面反应速率明显大于晶内,所以可以通过降低烧结温度提高晶界密度的方法提升SOFC性能。论文第四章采用同样的方法,探讨了在还原性气氛下金属Pt或Au表面修饰的SDC的表面电化学反应过程。当在SDC表面引入金属Pt的颗粒时,表面反应速率得到明显提高。作为对比,当在SDC表面采用同样的方法引入金属Au的颗粒时,表面反应速率未得到提高。由于Au的存在降低了SDC衬底的反应面积,弛豫平衡时间稍有增长。另外,通过定量分析金属增强的表面反应常数与表面微结构的关系,可以确定金属Pt表面修饰的SDC的表面电化学反应,主要发生在Pt-SDC的交界线处。提出并验证了复合材料体系的ECR方法,实现复合材料的表面反应过程的定量表征。论文第五章采用ECR方法研究双相复合材料的表面反应过程。选择不同组分配比的Sr2Fe1.5Mo0.5O6-δ-Sm0.2Ce0.8O1.9(SFM-SDC)为研究对象。实验结果表明,SDC的加入可以明显增强SFM的表面反应动力学过程。在理论分析中,这种增强作用表示为SFM与SDC的协同反应过程。当测试氧分压在0.01到1atm之间突然增大时,氧从环境气氛进入到氧化物,表面反应为阴极的氧气还原过程。协同反应量对总反应量的贡献可高达92%。通过定量分析协同反应速率与表面微结构的关系,发现氧气在SFM-SDC复相表面的氧气还原反应,不仅仅集中在SFM-SDC的交界线处,协同反应可以向SDC表面扩展。此外,协同效应可以通过表观表面反应常数简单计算。表观表面反应常数与单相SFM的反应常数不仅表示了协同反应速率的大小,还可以反映出协同反应的贡献率。当测试气氛从湿润的H2/Ar(60：40)切换到湿润的H2或者从CO/CO2(1:1)切换到CO/CO2(2:1)时,氧从氧化物中脱出,表面反应为阳极的燃料氧化过程,表面协同贡献率可达70%。同样,SFM与SDC之间的协同反应作用可以通过对比复相材料与单相材料的弛豫曲线获得。随着SDC含量的增大,通过协同反应路径脱出氧的含量增大。通过定量分析初始协同反应速率与表面微结构的关系可以确定SFM-SDC复相表面电化学反应的限速步骤。当测试气氛从湿润的H2/Ar(60：40)切换到湿润的H2时,表面协同反应速率与SFM-SDC界线线性相关,说明H2在SFM-SDC界线处的反应为限速步骤。当测试气氛从CO/CO2(1:1)切换到CO/CO2(2:1)时,反应速率与SDC的颗粒大小相关,说明带电氧物种在SDC表面的迁移步骤为限速步骤。提出并验证了表面反应与结构力学稳定性的关系,并提出微观断裂力学参数的测定新方法。论文第六章讨论了表面反应与断裂力学行为的关系。在给定的试样与一定的温度条件下,结合菲克第二定律、材料的化学膨胀行为和氧传输行为,可以从理论上获得试样的表面反应对试样力学分布和变化的影响。由于较小的结构尺寸对应着较小的扩散距离,表面反应产生的最大表面应力较小,结构稳定性较高。通过表面修饰可以提高表面反应常数,但是,较大的表面反应常数对应着较大的表面应力,结构稳定性降低。综合分析,表面最大应力由力学模数(ω)的大小决定。较大的ω对应着较大的表面最大应力。实验上,运用电导率的测量反应LSCM的结构稳定性。多孔LSCM在气氛变换的过程中具有良好的稳定性。通过Ni的表面修饰,虽然LSCM的表面反应性能提高,但是结构稳定性下降。此外,为了建立电导率与微观断裂的定量关系,本章的最后提出了一种测量颗粒间的微观断裂力学参数的新方法。选择YSZ-Al2O3多孔复合材料为研究对象,通过测量YSZ-A12O3多孔复合材料在温度循环过程中的电导率变化可以从统计意义上获得YSZ颗粒间的断裂概率。结合YSZ和A1203热膨胀系数差别产生的热应力及Weibull分布或正态分布即可计算得到YSZ微观断裂力学参数。更多还原

【Abstract】 The solid oxide fuel cell (SOFC) is an energy conversion device that can efficiently convert the chemical energy in fuels to electricity. The performance is sensitive to the materials selection and structure design. In this thesis, efforts are made on the surface reactionprocess and synergistic effect between solid oxides under the decided microstructure and materials, quantificationally. Furthermore, the relationship between surface reaction and mechanical behavior as well as the measurement of mechanical property in micro-scale are also discussed.A new elementary step for theoxygen electrochemical reduction is proposed in theory and experimentally confirmed. Oxygen electrochemical reduction at the cathode is studied owing to itsstrong contribution to the polarization losses of SOFC. In chapter two, an elementary step of oxygen vacancy transport across the electrode-electrolyte interface is proposed to demonstrate the electrolyte effect on electrode performance besides a series of elementary steps occurring on the electrode surface. According to the reaction kinetics, the electrode interfacial polarization resistance, Rp, can be theoretically related to the electrolyte conductivity, σ, with a general formula, Rp∝σ1Po2n, where pO2is the oxygen partial pressure at the cathode,l and n are the controlling parameters corresponding to various elementary steps occurred at the electrode-electrolyte interface as well as on the electrode. The oxygen vacancy transport step is experimentally confirmed by analyzing the electrochemical impedance spectra of symmetric cells of porous La0.6Sr0.4Co0.2Fe0.8O3electrodes on samaria-doped ceria electrolytes with different conductivities as a result of various dopant contents. The high frequency resistance, which can be fitted to a Warburg-type element, increases linearly with the electrolyte resistivity, clearly demonstrating that this process corresponds to the transport of oxygen vacancy at the electrode-electrolyte interface, from the electrolyte to the electrode.Electrical conductivity relaxation (ECR) technique is proposed to study the surface process of anode and measure the rate at three-phase boundary (TPB). In chapter three, the surface process of doped ceria reduction, i.e. the chemical surface exchange process in reduced atmospheres is studied to characterize their catalytic activity for fuel oxidation. The oxygen surface reaction coefficient of Gd0.1Ce0.902-δ is comparable to that obtained by thermogravimetric measurement, demonstrating the feasibility of ECR method. Usually, when the smallest diffusion thickness of the samples is as low as0.3mm, the ECR process is limited by the surface exchange step and almost independent on the bulk oxygen ion diffusion kinetics. Among various materials of R1.2Ce0.8O2-δ (R=Y, Gd, Sm, La) and SmxCe1-xO2-δ (x=0,0.05,0.1,0.2,0.3), Sm0.2Ce0.802-δ (SDC) exhibits the highest surface exchange coefficient, thus should be promise as the anode component. Moreover, it is found that, at temperature below700℃, surface exchange kinetics at the grain boundary is significantly faster than on the grain, suggesting additional advantage of developing SOFCs by low-temperature sintering. In chapter four, the electrochemistry performance of SDC surface-modified by the metal Pt or Au is studied using the similar method. By introducing Pt particles to the surface, the surface exchange kinetics can be remarkably improved. When Au is also deposited as a contrast, the re-equilibration time slightly increased contrast to SDC substrate caused by the decrease of exchange surface. Moreover, the increased catalytic surface exchange coefficient is linked to the surface microstructure, suggesting that the active site of metal support interaction is the Pt-SDC boundary.ECR techniqueis proposed to study the surface process of composites, quantificationally. In chapter five, the oxygen reaction kinetics of Sr2Fe1.5Mo0.5O6-δ-Sm0.2Ce0.8O1.9(SFM-SDC) dual-phase composites has been investigated as a function of SDC phase volume fraction. It is shown that the surface reaction kinetics of SFM could generally be enhanced by SDC. The enhancement is theoretically analyzed to quantitatively reveal the synergic effect between SFM and SDC on surface reaction. When the oxygen partial pressure is step increased in the range from0.01to1atm, the oxygen incorporation reaction take place at the surface of composites like the oxygen electrochemical reduction in cathode. The synergic effect contributes up to92%of the total amount of oxygen that is incorporated. The synergic rate is affected by the composition as well as the surface microstructure, suggesting the synergic reaction occurs on SDC surface rather than at SFM-SDC boundaries. Moreover, the synergic contribution and rate can be easily calculated with the apparent oxygen surface reaction coefficients (ka) and oxygen surface exchange coefficient for pure SFM (k). When the atmosphere is changed from humidified H2/Ar (60:40) to pure H2, as well as from CO/CO2(1:1) to CO/CO2(2:1), oxygen release from oxide like the anode reaction, the oxygen content of released from SDC and SFM through the synergic route can be calculated from the relaxation curves. The synergic effect contributes as high as70%of the total amount of oxygen that releases form SDC. With the increase of SDC content, the release oxygen through the interaction route increases. Furthermore, the rate of extra oxygen released from the oxides is also calculated. The initial release rates of the interaction between SDC-SFM are related to the surface microstructure parameters. When the atmosphere changes from humidified H2/Ar (60:40) to pure H2, the surface synergic rate is affected by the TPB length, linearly. So, the electriferous oxygen species reacted with the fuel is the rate-limiting step. When the atmosphere is abruptly changed from CO/CO2(1:1) to CO/CO2(2:1), the rate is affected by the size of SDC. That is, the electriferous oxygen species transfer is the rate-limiting step.The relationship between surface reaction and mechanical behavior as well as the measurement of mechanical property in micro-scale are discussed in chapter six. A novel method is presented to detect the mechanical stresses by combining the Fick’s second law, oxygen surface exchange and oxygen-ion diffusion properties. The surface tensile stress is weak for the smallstructural dimensions due to the short diffusion length. When the surface exchange kinetics is increased by means such as surface modification, the improved surface exchange rate may result in largemechanical stress and the stress loading rate, and consequently, reduce the mechanical stability. A new mechanicalmodulus (ω) is introduced to predict the stress, and larger co means higher mechanical stress. The predictionis experimentally confirmed with (La0.75Sr0.25)0.95Cr0.5Mn0.5O3-δ (LSCM) samples, where the fracture is related to its conductivity. It is found that porous LSCM has excellent stability while fractures are observed with Ni impregnated porous LSCM. Furthermore, a novel method is presented to determine the relationship between micro-fracture mechanics and conductivity, quantificationally. By the measurement of the conductivity change of YSZ-Al2O3composites in thermal cycles, the fracture between YSZ particles caused by thermal stress can be statistically "counted" using the Weibull or normal distribution. And then the parameters in fracture statistics distributions can be calculated with a statistical principle. The method offers a possible way to understand the fracture in microscale.更多还原