【作者】 吴天植；

【导师】 夏长荣； 彭冉冉；

【作者基本信息】 中国科学技术大学 ， 材料学， 2010， 博士

【摘要】 以质子导体作为电解质的固体氧化物燃料电池(H-SOFC)由于具有高的能量转换效率和燃料利用率,低的传导活化能,高的离子迁移数以及高的EMF值,因而受到了人们的重视。但是,同氧离子导体型燃料电池(O-SOFC)相比,对H-SOFC的研究还只是刚刚起步。目前,H-SOFC的研究主要侧重于发展具有高的质子传导率和稳定性的新电解质材料,而对于其电极材料和电极反应的研究少有报道。研究发现,H-SOFC的阴极材料对电池性能有着重要的影响,其表现存在着许多与O-SOFC相悖的现象。因此,探索H-SOFC中的阴极反应机理,寻找合适的阴极材料,改善其阴极的微结构,对于H-SOFC的发展具有重要的意义。在本论文中,我们的研究主要围绕着降低H-SOFC的阴极极化电阻,提高H-SOFC的电池工作性能,探索H-SOFC的阴极反应机理等问题展开。主要内容包括：1)探索适用于质子型固体氧化物燃料电池(H-SOFC)的阴极材料,降低电极极化阻抗,提高电池性能；2)利用浸渍技术,通过对阴极制备工艺的改进和微结构的优化,以期得到理想的H-SOFC阴极微结构；3)研究阴极在H-SOFC运行中的电化学过程和机理,确定质子的传输过程和通道,分析影响H-SOFC中阴极表现的反应控制步骤,并从H-SOFC的电极理论模型和实验两方面探索电极的极化与电极的组成、微结构等的关系,探索提高H-SOFC阴极性能的重点所在。在第一章中,主要介绍了H-SOFC的研究背景、研究意义以及H-SOFC的各种关键材料和发展趋势,并初步分析了可能会影响H-SOFC工作性能的各项因素。考虑到H-SOFC的阴极反应所造成的高极化电阻,我们确立了本论文的研究目标,即寻找适宜于H-SOFC的理想阴极材料,探索H-SOFC的阴极反应机理,并寻找提高其阴极性能的可能途径。在第二章中,我们系统地介绍了当分别使用电子导体,氧离子-电子混合导体,质子-电子混合导体作为阴极材料的时候,H-SOFC中电极反应的步骤及其与不同的电极材料之间的关系,并认为其电极中空气-电解质-阴极三相界面是反应的主要发生区域。由于氧离子-电子导体是目前应用最广的一种阴极材料,因此,我们以Sm0.5Sr0.5CoO3-BaCe0.8Sm0.2O2.9 (SSC-BCS)复合阴极为例,初步判定了当使用氧离子-电子导体作为H-SOFC的阴极材料时,H-SOFC阴极反应中可能存在的反应速率控制步骤,并初步建立了对其氧还原过程的认知。本论文的三至四章则通过研究相应的对称电池和单电池的表现,研究了当使用Sm0.5Sr0.5CoO3-BaCe0.8Sm0.2O2.9复合阴极材料作为H-SOFC阴极时,其阴极组分和微结构对电池性能的影响,并通过浸渍等制备技术对其阴极的微结构进行改第三章中,我们以具有优良的质子传导性能的BaCe0.8Sm0.2O2.9 (BCS)作为电解质,以Ni-BCS为阳极,Sm0.5Sr0.5CoO3-BaCe0.8Sm0.2O2.9 (SSC-BCS)作为复合阴极制备单电池,探索不同组分之间比例对单电池电极性能的影响。得到的结果如下：1)电极极化阻抗随着SSC含量的增加而减小,当SSC含量为60wt.%的时候,即该复合电极中SSC:BCS体积比为1：1的时候,该类复合阴极表现出了最小的电极极化,在工作温度为600℃时,为0.67Ωcm2。最小的电极极化阻抗源于其所具有的最大的三相界面长度。2)当电解质层厚度为70μm时,在工作温度为700℃下,以湿润氢气作为燃料气,环境空气作为氧化气,单电池的界面极化阻抗为0.21Ωcm2,占单电池全部阻抗的25%(电解质厚度进一步降低后,电极极化阻抗所占比例还将提高),最大输出功率密度为0.24W／cm2。这些结果表明,SSC-BCS复合阴极是一种良好的H-SOFC阴极材料,并且其中的三相界面长度对电极的性能有明显的影响,在后继的工作中,我们将通过改进电极的微结构,以增加电极中三相界面的长度。在第四章中,为了改善阴极的微结构,我们以离子浸渍法为手段,制备以质子导体材料—BCS—为阴极骨架,以氧离子-电子混合导体材料—SSC—为浸渍材料的纳米复合阴极。我们通过优化可能影响该复合阴极性能的一系列参数,例如SSC的浸渍量,热处理温度以及电极骨架的烧结温度等,得到的结果如下：1)在BCS骨架烧结温度为1100℃,SSC浸渍物的热处理温度为800℃,SSC浸渍量为55wt.%时,得到了具有最低电极极化电阻的复合电极：在工作温度为600℃的条件下,该电极的界面极化阻抗值仅为0.21 Qcm2,约为相同SSC含量下,以丝网印刷法制备的阴极的界面极化阻抗的1／3。2)我们以对称电池的阻抗谱表现进行分析,并发现：在对称电池的测量结果中,其阻抗谱图均可分解为高频和低频两个弧段,其峰值所对应的频率分别为1kHz和30Hz,这两个弧段表明,在该电极反应中,至少存在有两个反应速率控制步骤。通过进一步的分析,我们认为这两个反应速率控制步骤对应的可能是质子在电极-电解质界面上的传导过程,以及电极表明氧的吸附、解离以及传导过程。3)我们将这种复合阴极应用到阳极支撑的单电池上,并对其单电池性质进行分析,并发现在单电池的阻抗谱中,除了与对称电池中所对应的高频和低频弧段外,在更低频率的部分还出现了一个新的弧段,其对应的峰值频率为1Hz,这个新的弧段对应的可能是单电池阳极一侧的反应。4)当单电池中阳极,电解质,阴极的厚度分别为500μm,70μm,100μm的时候,在工作温度为700℃,并以湿润氢气作为燃料气,环境空气作为氧化气的条件下,其阳极,电解质,阴极对应的阻抗大小依次为0.021,0.68和0.055Ωcm2,最大输出功率密度为307 mWcm-2,比使用机械混合-丝网印刷法制备的阴极,其电池性能提高了近30%。由此,我们认为,在H-SOFC的阴极中,引入纳米颗粒并使其附着在微米级骨架上,得到的新的电极微结构可以有效地降低其阴极的电极极化阻抗,并提高单电池的工作性能。在第五章中,为了进一步降低H-SOFC的阴极极化阻抗,我们以在低温下具有更高的氧离子迁移数的Ag作为H-SOFC的阴极材料,使用离子浸渍法,得到以BCS为阴极骨架的复合阴极,以期得到更适宜于低温下工作的电极材料,并研究了电子导体在作为H-SOFC电极材料时候的表现。具体的结果如下：1)在该类阴极中,浸渍物的热处理温度对电极中粒子的形貌影响显著,并因此影响了电极的性能。当热处理温度为400℃,Ag浸渍量为0.40mgcm-2,测量温度为600℃的时候,我们在对称电池中得到了该类阴极中最小的电极极化阻抗,为0.11Ωcm2。2)在与SSC浸渍阴极的对比中,我们发现,当两者具有相同的浸渍量(57Vol.%)的时候,热处理温度为400℃,Ag浸渍量为0.46 mgcm-2的对称电池,在600℃的测量温度下其电极极化阻抗为0.13Ωcm2,仅仅是SSC浸渍阴极的一半左右。3)通过对其阻抗谱的拟合,得到的SSC浸渍阴极的高频弧段和低频弧段所对应的阻抗值分别为0.08和0.16Ωcm2,而Ag浸渍阴极所得到的对应阻抗值分别为0.06和0.07Ωcm2。由于同SSC相比,Ag具有更高的氧扩散系数,从而促进了氧离子传递到三相界面这个反应过程。4)当使用热处理温度为400℃,Ag浸渍量为0.46 mgcm-2的复合阴极作为单电池的电极时,在工作温度为600℃,并以湿润氢气作为燃料气,环境空气作为氧化气的条件下,其最大输出功率密度为283m Wcm-2,比使用SSC浸渍阴极的单电池性能提高了17%。考虑到Ag粒子在高温下的稳定性,我们认为,对H-SOFC来说,Ag浸渍复合阴极是一种适宜于低温(≤600℃)工作条件下的电极材料。更多还原

【Abstract】 Protons conducting solid oxide fuel cells (H-SOFC) have attracted much attention because of their unique characters, such as their great efficiency in fuel utilization, low activation energies for proton conduction, high ionic transferring numbers and high electromotive force (EMF). Compared with oxygen-ion conducting SOFCs(O-SOFCs), the theories on H-SOFCs are just inchoate. Lots of efforts are made to search for suitable proton conductors with high proton conductivity and long-term stability, and few studies on electrode materials and reaction mechanisms have been reported. However, water is formed at cathode in H-SOFCs which makes the cathode reaction more complex compared with those of O-SOFCs. Such distinguished characteristic of cathode reactions calls for intensive study on their reaction mechanisms and might lead to some special demands on the cathode materials, as well as microstructure.This p.h. D thesis aims to lower the cathode polarization resistances, improve the performances of cells, and explore cathode reaction mechanisms for H-SOFCs. The main contents are summarized as follows:1) developing novel cathodes materials for H-SOFCs to reduce polarization resistances and to improve cell performance; 2) applying ion impregnation technique to optimize the electrode microstructures; 3) exploring cathode reaction mechanisms both by theoretical models and by experiments, analyzing the proton transfer processes and the rate limiting reactions, discussing the relationship between the cathode polarization resistances and the components, as well as the electrode microstructures. The purpose of all this work is to find how to improve the performance of H-SOFC cathodes.Chapter 1:the background and significance of H-SOFCs, key component materials, and development, are generally introduced. The effect factors of H-SOFCs’ performance are also briefly reviewed. Since cathode performance restrict the electrode performance of H-SOFCs with thin-film electrolyte, this thesis aims to develop appropriate cathode materials, optimize cathode microstructure, and study the cathode reaction mechanisms.Chapter 2:the specific steps of cathode reactions are briefly introduced to understand the oxygen reduction process during cathodes of H-SOFCs, with respect to different conducting species in cathode materials, such as electron conductor, electron-oxygen ion mixed conductor and electron-proton mixed conductor, respectively. Since the electron-oxygen ion mixed conductors are the most potential cathodes, rate limiting reactions of such cathodes in H-SOFCs has been studied with Sm0.5Sr0.5CoO3-δ-BaCe0.8Sm0.2O2.9 composite cathodes.In chapters 3 and 4, electrochemical performance of composite cathodes consisted of Sm0.5Sr0.5CoO3-δ(SSC) and BaCe0.8Sm0.2O3-δ(BCS) are investigated for H-SOFCs by symmetrical cells and single cells, and the systematic study on the effect of its composition and microstructures on its performance has been carried out.Chapter 3:SSC-BCS cathodes are fabricated by screen printing process with BCS as the electrolytes and Ni-BCS as the anodes. The polarization resistances of such cathodes are investigated as function of SSC content in composite cathodes. Details are as follows:1) Interfacial polarization resistances of the composite decrease first and then increase with SSC content. The minimum polarization resistance,0.67Ωcm2 at 600℃, is reached with 60 wt.% SSC-BCS cathode, which corresponds to 50 Vol.% SSC.2) With electrolyte 70μm in thickness, maximum powder density of 0.24 W/cm2 is achieved with 60 wt.% SSC-BCS cathode using humidified hydrogen as the fuel and ambient air as the oxidant. The polarization resistance is 0.21Ωcm2 at 700℃, which was about 25% of the total cell resistance.The low interfacial polarization resistance suggests that SSC-BCS composite cathodes are suitable cathode for H-SOFCs. and that the composition of SSC-BCS cathode has a great effect on its electrode performance due to its impact on TPBs length. Therefore, enlarging the length of TPBs by microstructure optimizations could help to improve the electrode performance.Chapter 4, ion impregnation technique is adopted to fabricate composite cathodes with nano microstructure, by which nano-sized Sm0.5Sr0.5CoO3-δ(SSC) particles are deposited onto the inner face of porous BaCe0.8Sm0.2O2.9 (BCS) backbone. The main achievements are summarized as follows:1) The electro-performance of the composite cathodes is investigated as function of fabricating conditions, such as the SSC-loading, the SSC firing temperatures and the backbone sintering temperatures. The lowest polarization resistance, about 0.21 Qcm2 at 600℃, is achieved with BCS backbone sintered at 1100℃, SSC layer fired at 800℃, and SSC loading of 55 wt.%, which is only 1/3 of that prepared by screen-printing. 2) Impedance spectra of the composite cathodes consisted of two depressed arcs with peak frequency of 1 kHz and 30 Hz, respectively, which might correspond to the reaction of proton and the dissociative adsorption and diffusion of oxygen, respectively.3) There is an additional arc peaking at 1Hz in the Nyquist plots of a single cell, which should correspond to the anode reactions.4) With anode, cathode and electrolyte about 500μm,100μm and 70μm in thickness, the simulated anode, cathode and bulk resistances of cells are 0.021,0.055 and 0.68Ωcm2 at 700℃, respectively, and the maximum power density is 307mWcm-2 at 700℃, about 30% improved compared with the cell with printed cathode.Chapter 5, Ag-BaCe0.8Sm0.2O2.9 (BCS) composite cathodes are fabricated by ion impregnation technique in this work to intensively reduce the polarization resistance. The performances of H-SOFCs with electron conductor as cathode are studied. The main achievements are summarized as follows:1) The polarization resistances are greatly affected by firing temperature of impregnated Ag particles. The minimum polarization resistance of symmetric cells reaches 0.11Ωcm2 at 600℃with Ag loading of 0.40mgcm-2 and tired at 400℃.2) With the same volume ratio of Ag and SSC (57Vol.%), the polarization resistance reaches 0.13Ωcm2 at 600℃with Ag based composite cathode, half of that with the SSC impregnated cathode.3) The simulated high and low frequency resistances are 0.08 and 0.16Ωcm2 for SSC impregnated cathodes, and 0.06 and 0.07 Qcm2 for Ag impregnated cathodes, respectively, suggesting that the reduction of low frequency resistances is the main reason for the decrease of polarization resistances in Ag impregnated cathode, in consistence with the high oxygen diffusion coefficient of Ag.4) With 0.46 mgcm-2 Ag impregnated cathode fired at 400℃, the maximum power densities of single cells is 283 mWcm-2 at 600℃with humidified hydrogen as the fuel and ambient air as the oxidant, about 17% improved compared with SSC impregnated cathode.Consider the long-term test of Ag-impregnated cathodes, Ag impregnated cathode is a promising cathode for fuel cells operating at temperature lower than 600℃.更多还原