【作者】 洪涛；

【导师】 夏长荣；

【作者基本信息】 中国科学技术大学 ， 材料学， 2015， 博士

【摘要】 固体氧化物燃料电池(SOFC)是清洁高效的能源利用装置。它的效率主要制约于阴极的氧还原反应。本文以典型的阴极催化剂La1-xSrxCo1-yFeyO3-δ(LSCF)为对象,研究氧化物对其催化性能即阴极氧还原速率的影响规律,包括掺杂氧化铈、碱土元素的氧化物和过渡元素氧化物等,以增强氧还原动力学,提高中温SOFC的输出性能。第一章阐述了SOFC电解质,阳极,阴极等关键材料的种类和性质。重点介绍阴极材料及结构,特别是用浸渍法制备的含纳米粒子的阴极。介绍了纳米结构阴极的长期稳定性,分析了纳米结构阴极的制备过程和纳米颗粒生长的理论模型,同时针对不同种类的阴极材料,分析了其纳米结构的电化学性能和稳定性。第二章发现,掺杂氧化铈(SDC, SmxCe1-xO2-δ)能够明显增强LSCF上的氧还原动力学,SDC粒子可将LSCF的表面交换系数Kchem提高5倍。K-chem随着SDC粒子的负载量增加到最大值后降低,表明SDC的增强效应与SDC, LSCF和气相的三相线相关。K-chem与SDC的组成相关,在同样的负载量下,K-chem随着x先增加后降低,当x=0.2时达到最大值；进一步分析发现,Kchem与SDC的离子电导率6线性相关,即随着6的增加而增加。结果表明,SDC的自由氧空位促进了氧在LSCF表面的还原反应动力学。此外,氧还原反应的活化能基本不变,表明SDC没有改变LSCF表面的氧还原机理。第三章发现碱土金属化合物的氧还原反应的增强作用。首先,BaCO3纳米颗粒显著地提高了LSCF、LSF (La0.8Sr0.2FeO3-δ)和LSM (La0.8Sr0.2MnO3-δ)等阴极催化剂的表面氧还原反应速率。在BaCO3粒子的作用下,LSF和LSCF的Kchem都有一个数量级的增大。进一步分析发现,LSF电极的低频阻抗减小约数量级,例如,700℃时从2.54Ωcm2减小到0.26Ωcm2。Kchem的增大与低频阻抗的减少,说明BaCO3主要是提高了氧的吸附解离等表面过程。以LSF阴极,YSZ(Y0.15Zr0.85O2-δ)电解质,Ni-YSZ阳极构成的单电池,阴极浸渍BaCO3纳米颗粒后,800℃下的峰值功率输出从0.3Wcm-2增大到0.53Wcm-2。此外,BaCO3纳米颗粒提高了电极的稳定性,在7000C下、380h的测试过程中,纯LSF电极的极化阻抗增长约90%,即从2.96Qcm2增大到5.48Ωcm2,而在LSF电极中浸渍了BaCO3纳米颗粒后,界面极化阻抗基本维持在0.84Ωcm2。同时,微结构分析表明,在热处理过程中,BaCO3纳米颗粒的形貌未发生任何变化。其次,作为LSF的协同催化剂,CaO粒子展现出了特殊的性质。表面浸渍CaO粒子后,LSF的Kchem增大约10倍,但是,其界面极化阻抗没有减少,反而增大。进一步分析发现,代表氧离子传输的高频阻抗明显增大,而与表面过程相关的低频阻抗减小。如果LSF电极中的氧离子传输分为体相和表面路径,而CaO粒子阻碍了02-的表面传输。使用离子导体SDC与LSF混合,形成LSF-SDC复合阴极,解决了氧离子传输问题。750℃C下,以YSZ为电解质,浸渍3.01wt%CaO后,LSF-SDC复合电极的极化阻抗从0.83Ωcm2减小到0.42Ωcm2。浸渍CaO粒子后,单电池的输出功率密度也增加,800℃C从0.45Wcm-2增大到0.58Wcm-2。最后,SrO的作用与BaCO3、CaO均不相同,在La0.8Sr0.2FeO3-δ(LSF113)催化剂表面浸渍SrO,会形成La0.8Sr1.2FeO4-δ(LSF214), LSF214是一种An+1Bn03n+1结构的Rullesden-Popper(RP)相,由于其独特的钙钛矿层和盐岩层排布,具有较高浓度的氧空位和电化学催化活性。因此,浸渍后构成具有核壳结构的催化剂(LSF214壳-LSF113核)。与纯相LSF113相比较,这种核壳结构的Kchem提高接近15倍,并进一步降低界面极化电阻。XPS分析表明,LSF214颗粒只形成在LSF113颗粒的外表面。通过浸渍-反应途径,形成LSF214-LSF113核壳结构,是构建异质结构电极的可能方法。第四章初步研究了过渡金属元素(Cr、Mn、Fe、Ni、Zn、Co、Cu等)氧化物对LSCF的催化的影响。电导弛豫结果表明,Cr的氧化物降低了LSCF表面的氧还原反应速率,造成阴极中毒。其余氧化物均提高了反应速率,其中Cu的增强效应最大,Kchem提高约20倍。利用真空蒸镀技术,在LSCF表面蒸镀不同含量的CuO,同时利用SEM确定CuO的表面覆盖率θCuO以及单位LSCF面积上的三相线长度LTPB,研究Kchem与CuO蒸镀量和微结构的关系。CuO增强的表面交换系数KCuO与θCuO和LTPB均不成严格的线性关系,说明CuO增强作用可能同时发生在CuO的表面和CuO-LSCF-气相的三相线上。在测试温度范围内,CuO对整个氧还原反应的贡献率λCuO,随着温度的升高而增大。800℃C下,LTPB为1.14cm-1时,λCuO为30%,而当LTPB为增大到14.7cm-1时,CuO的贡献率接近80%。在CuO浸渍的LSCF电极中,其极化阻抗明显降低,650℃C下浸渍8.5wt%CuO后,极化阻抗从0.76Ωcm2减小到0.51Ωcm2。阻抗谱分析表明,低频阻抗明显减少。在以SDC为电解质的单电池中,使用CuO浸渍的LSCF阴极,650℃C输出功率密度从0.55Wcm-2增大到0.75Wcm-2。更多还原

【Abstract】 Solid oxide fuel cells (SOFCs) are clean electrochemical energy conversion devices with high efficiency. Its efficiency is mainly restricted by the oxygen reduction reaction in cathode. In this thesis, typical cathode catalytic materials La1-xSrxCo1-yFeyO3-δ (LSCF) is studied to investigate the influence on catalytic activity or oxygen reduction kinetic by oxides, including doped ceria, alkaline earth metal oxides and transition metal oxides, to enchance oxygen reduction reaction rate and improve immediate-temperature SOFC power output.Chapter1brifely introduces the class and properity of SOFC key materials including electrolyte, anode and cathode. Cathode materials and structure especially the nano structure cathode made by infiltrating are introduced detailed. The long term stability of nano structure cathode is introduced as well as the fabrication process and the nano particles growth model. For different kinds of cathode materials, the electrochemical performance and stability of their nano structure are analysised.In chapter2, doped ceria (SDC, SmxCe1-xO2-δ) could greatly enhance the oxygen reduction kinetic in LSCF surface and SDC particles improve the oxygen surface exchange coefficient (Kchem) of LSCF by5times. Kchem value increase with SDC particles loading weight to the maximum and then decrease which suggests that the enhancing performance is related with the three phase boundary (TPB) where SDC, LSCF and gas phase count together. Kchem also relates with the SDC composition and under the same loading, Kchem increases with x reaching the maximum at x=0.2then goes decrease. Further research indicates that Kchem is related with the ionic conductivity (σ) of SDC which increase with σ value. The result suggests that it is the free oxygen vacancy in SDC promotes the oxygen reduction kinetic in LSCF surface. And the oxygen reduction reaction activation energy has no obvious change which imply that SDC doesn’t change the oxygen reduction mechanism in LSCF surface..In chapter3, the synergistic effect in oxygen reduction reaction by alkaline earth metal oxides is studied. Firstly, BaCO3nanoparticles could greatly improve the oxygen reduction rate in LSCF, LSM (La0.8Sr0.2MnO3-δ) and LSF (La0.8Sr0.2FeO3-δ(cathodes. After infiltrated with BaCO3, the Kchem value of LSF and LSCF has a10times increase. Further analysis reveals that the low frequency resistance of LSF electrode decreases by an order of magnitude. For example, at700℃it reduces form 2.54Ωcm2to0.26Ωcm2. The increase of Kchem and decrease of low frequency resistance indicate that BaCO3accelerate the oxygen surface process including adsorption and dissociation. And at800℃, the power output of single cell with LSF cathode, YSZ (Y0.15Zr0.85O2-δ) electrolyte and Ni-YSZ anode increase from0.3Wcm-2to0.53Wcm-2when LSF cathode is infiltrated BaCO3. BaCO3nanoparticles also could improve the cathode long term stability. At700℃, the resistance of bare LSF electrode has a90%increase which increase from2.96Ωcm2to5.48Ωcm2in380h long term test while the resistance of LSF infiltrated with BaCO3stay in0.84Ωcm2. And the microstructure analysis implys that BaCO3nanoparticle morphology has no change in thermal treatment.CaO particles have special property when used as synergistic catalytic in LSF electrode. It could increase the Kchem value of LSF by an order of magnitude, but the polarization resistance of LSF increased after infiltrated with CaO. AC impedance analysis indicates that the high frequency resistance associated with oxygen ionic transport increases and low frequency resistance regards oxygen surface process decreases. Oxygen ionic could transports along LSF bulk and surface, so it must be the surface path is blocked by CaO. The application of ionic conductor SDC in LSF to form LSF-SDC composite cathode resolves the problem. At750℃, the resistance of LSF-SDC electrode in YSZ electrolyte decrease from0.83Ωcm2to0.42Ωcm2after infiltrated with3.01wt%CaO. At800℃, in anode supported single cell with YSZ electrolyte using CaO infiltrated LSF, the power density increase from0.45Wcm-2to0.58Wcm-2.At last, SrO has different working mechanism with BaCO3and CaO. It could react with La0.8Sr0.2FeO3-δ (LSF113) electrode surface to form La0.8Sr1.2FeO4-δ (LSF214). LSF214is a Rullesden-Popper (RP) phase with An+1BnO3n+1structure that has high oxygen vacancy ratio and electrochemical catalytic activity due to its perovskite layer and rock salt layer displace. So the core-shell structure (LSF214shell and LSF113core) catalyst is fabricated after infiltration. The core-shell structure could improve the Kchem value by about15times and further to reduce the polarization resistance. XPS analysis reveals that LSF214nanoparticles only form in the out surface of LSF113electrode backbone. The LSF312-LSF113core-shell structure cathode formed by infiltrating provides a new way to fabricate heterostructured electrode.In chapter4, the catalytic activity effect of transition metal (Cr, Mn, Fe, Co, Ni, Cu and Zn) oxide in LSCF is studied. Electrochemcial conductivity relaxation results show that Cr oxide reduce the oxygen reduction kinetic in LSCF surface and cause cathode poisoned. While the other oxides all could accelerate the oxygen reduction reaction and CuO has the best enhacement which improve Kchem value by30times. In further research, CuO is deposited to LSCF surface by vapor sputter and the CuO surface coverage θCuO and per area TPB length LTPB in LSCF surface are indentified by SEM analysis to analysis the relationship between Kchem value and CuO loading as well as structure. The enhanced surface exchange coefficient by CuO, Kcuo doesn’t go through a liner relationship with neither θCuO nor LTPB.It suggests that the CuO enhanced oxygen reduction reaction maybe occur in CuO surface and the three phase boundary where CuO, LSCF and gas phase counted. In the test temperature range, the reaction contribution of CuO, λCuO increases with temperature. At800℃, λCuO is30%when LTPB is1.14cm-1and increase to80%when LTPB increase14.7cm-1. In CuO infiltrated LSCF electrode, the polarization resistance is reduced that at650℃, the polarization resistance is0.51Ωcm2with8.5w5%CuO compared to0.76Ωcm2for bare LSCF electrode. AC impedance analysis also indicates that it is the reduction associated with low frequency resistance. In single cell with SDC electrolyte, the power density increase from0.55W cm-2to0.75Wcm-2with CuO infiltrated LSCF electrode.更多还原