【作者】 周文龙；

【导师】 姜雪宁；

【作者基本信息】 大连理工大学 ， 凝聚态物理， 2016， 硕士

【摘要】 固体氧化物燃料电池(SOFC)是一种将储存在燃料里的化学能直接转变为电能的固态反应装置,是21世纪极具应用前景的能源技术。传统燃料电池工作温度较高(～1000℃),导致其运行成本高、材料兼容性差等一系列问题。将燃料电池的工作温度降低到600-800℃中温范围内,是SOFC的重要发展趋势。SOFC工作温度的降低提高了电池系统的工作稳定性与不同材料间的兼容性,同时降低运行成本,但随工作温度的降低,电池性能变差,其中最主要的原因是阴极极化阻抗随工作温度降低迅速增大。因此,寻找在中温范围内仍然具有高电化学催化活性的SOFC阴极材料具有重要研究意义。具有混合离子-电子电导率(MIEC)的LnBaCo2O5+δ (Ln=Lanthanide)系列双层钙钛矿氧化物是新型中温SOFC阴极材料,其中,PrBaCo2O5+δ (PBCO)氧化物具有合成工艺简单、电化学催化活性高等优点,得到人们的广泛关注与研究。从目前研究现状看,PBCO阴极材料在700℃以上高温区具有很高的电化学催化活性,但700℃以下低温区阻抗过高,不能满足SOFC对阴极阻抗的要求(ASR<0.15Ω·cm2),同时,PBCO的热膨胀系数(TEC)过大,与常用电解质材料存在热失配问题,容易导致SOFC高温运行过程中的结构开裂。为了进一步提高PBCO的氧还原催化活性及其与电解质材料的TEC匹配,人们采取不同的途径对PBCO进行改性研究,包括A位(或B位)离子掺杂以及PBCO-GDC、PBCO-SDC等复合阴极的制备等。为进一步优化SOFC阴极材料PBCO的综合性能,本工作利用溶胶-凝胶(sol-gel)方法制备了PrBaCoCuO5+δ-CuO (PBCoCu-CuO)双相复合阴极材料,并对它的结构、性能进行了研究。利用X射线衍射(XRD)、热膨胀分析仪分别对其相结构和热膨胀行为进行表征分析；采用直流四电极方法、交流阻抗谱技术和扫描电镜(SEM)对其电学性能、电化学性能及电极的微观形貌进行测试和分析,并对交流阻抗谱结果进行等效电路拟合,分析了PBCoCu-CuO双相复合阴极的电化学反应机制,并与PBCO单相阴极的结果进行了对比讨论。XRD结果表明,该复合材料由层状钙钛矿结构氧化物PBCoCu主要相和少量CuO相构成,与PBCO相比,PBCoCu-CuO晶格发生膨胀；PBCoCu-CuO的TEC与原始样品PBCO目比下降约21%,有助于提高SOFC的结构稳定性；PBCoCu-CuO电导率低于PBCO氧化物,但是满足SOFC对阴极电导率要求(>100S·cm-1)；交流阻抗谱测量结果表明, PBCoCu-CuO复合阴极的极化阻抗为0.1 Ω ·mc2 (600℃),0.042Ω·m2 (650℃),0.019Ω·cm2 (700℃)> 0.010Ω·cm2 (750℃),远远小于PBCO阴极极化阻抗,证明该复合阴极的氧还原催化活性显著增强,是一种很有应用前景的中温SOFC阴极材料；电化学反应机制分析结果表明：PBCoCu-CuO阴极反应过程主要包括以下反应子过程：高频下氧离子体扩散过程、中频下氧的表面吸附与解离过程以及低频下空气在阴极多孔结构内的体扩散过程,并且这些反应子过程对应的阻抗均小于PBCO单相阴极对应阻抗。B位Cu2+掺杂以及P型半导体CuO相的引入促进了阴极反应,使PBCoCu-CuO阴极电化学催化活性显著增强,极化阻抗大幅降低。更多还原

【Abstract】 Solid oxide fuel cells (SOFCs) are all-solid electrochemical devices that directly convert chemical energy stored in the fuel and oxidizer into electrical energy with high efficiency and low emission of pollutants. Traditional SOFCs based on yttria stabilized zirconia (YSZ) as electrolyte operate at high temperatures (800℃-1000℃), which can result in a series of problems such as the electrode sintering, interface reaction between cell components, and high cost of materials and manufacture. With the commercialization of SOFCs, reducing the operation temperature to the intermediate-temperature range of 600℃-800℃ is an important development trend. Reducing temperature can bring several benefits such as extending the materials’ selection, prolonging lifetime and reducing manufacture cost. However, performance of SOFCs degrades with the decreasing temperatures mainly because of the rapid increasing polarization resistances of the cathode. Therefore, it is significant to develop new cathode materials with high catalytic activity (low polarization resistance) at the intermediate-temperature range of 600℃-800℃.Mixed ionic and electronic conducting (MIEC) LnBaCo2Os+δ (Ln=Lanthanide) oxides with double-layered perovskite structures are new cathode materials of IT-SOFCs, among which PrBaCo2O5+δ (PBCO) has received much attention due to its easy synthesis and high ORR catalytic reactivity. The current availble results in literature have shown that PBCO has very high ORR catalytic reactivity at temperatures above 700℃, but its polarization resistances at the temperatures below 700℃ are still too large to meet the requirement of SOFC cathode (ASR<0.15= Ω·cm2); besides, PBCO has a high thermal-expansion coefficient (TEC) that mismatches with TEC of the commonly used electrolyte materials, which can cause structural cracking of SOFC during its high-temperature operating. To further improve ORR catalytic reactivity of PBCO and its TEC matching with electrolyte, various strategies have been adopted in property modification of PBCO, such as A-site and/or B site doping and fabrication of composite materials like PBCO-GDC and PBCO-SDC.In this work, to further improve the overall performance of PBCO cathode, a composite material of PrBaCoCuO5+δ-CuO (PBCoCu-CuO) was synthesized by sol-gel method and its structure and properties were studied. Its phase structure was characterized by XRD measurement, and its thermal-expansion behavior and TEC value was measured at 20-950℃ in air by Thermal expansion analyzer. DC conductivities of PBCO were measured by four-electrode method at 50-800℃ in air, and its electrochemical performance was characterized by AC impedance spectra measurement at various temperatures and oxygen partial pressures. Equivalent circuit models were adopted for fitting of the impedance spectra results amd the cathode reaction mechanism was analyzed based on the fitting results. The corresponding results of PBCO single phase cathode were also provided for comparison with the PBCoCu-CuO composite cathode. The XRD results have indicated that the composite material is composed of the dominant phase of PBCoCu with a double-layered perovskite structure and a minor phase of CuO; the PBCoCu oxide shows a slight structural expansion as compared to PBCO. The TEC value of PBCoCu-CuO composite material decreases by～21% as compared to TEC of PBCO, indicating that structural stability of SOFC can be improved by using PBCoCu-CuO as the cathode. Conductivities of PBCoCu-CuO are lower than the values of PBCO but are all above 100S·cm-1 at the measurement temperature and meet the requirement of SOFC cathode. ASR values of PBCoCu-CuO range from 0.10Ω·cm2(600℃)、 0.042Ω·cm2(650℃h 0.019Ω·cm2(700℃)、0.010Ω·cm2 (750℃), which are significantly lower than the results of PBCO and some other related cathode materials and demonstrate the high ORR catalytic reactivities of the composite cathode. The fitting results of impedance spectra results have demonstrated that the reaction process of the PBCoCu-CuO cathode is composed of three elementary reaction steps:oxygen ionic diffusion process in the bulk the cathode (high-frequency process), oxygen adsorption and dissociation on the cathode surface (medium-frequency process) and air diffusion process through the porous cathode (low-frequency process). The resistances from the above elementary steps are all lower for the PBCoCu-CuO composite cathode than the PBCO cathode. Reaction of the PBCoCu-CuO composite cathode was significantly promoted by the B-site Cu2+-doping and existence of the CuO phase, which resulted in the enhanced ORR catalytic reactivity characterized by low polarization resistances.更多还原