【作者】 聂丽芳；

【导师】 张玉军； Meilin Liu；

【作者基本信息】 山东大学 ， 材料学， 2010， 博士

【摘要】 固体氧化物燃料电池(SOFC)以其高的能量转化率,全固态组件以及燃料多样性等优点成为近年来燃料电池领域研究的热点。研究和开发在中低温(600-800℃)工作的固体氧化物燃料电池(Intermediate temperature solid oxide fuel cells, IT-SOFCs),可以扩大组元材料的选择范围并且有效地提高电池长期运行的稳定性,是目前固体氧化物燃料电池的发展方向。但是,工作温度的降低使电解质的欧姆阻抗迅速增加,同时电极的界面阻抗增大,通过采用阳极支撑的电池结构以及电解质薄膜化技术可以解决电解质欧姆阻抗随温度降低而升高的问题。因此,降低中低温下电极极化过电位,尤其是氧在阴极的还原反应所导致的极化损失,是提高IT-SOFCs性能的一种有效方法。目前可以降低阴极极化阻抗的途径主要有以下几种：(1)使用在中低温温度下具有更高催化活性的离子-电子混合导体；(2)添加第二相氧离子传导材料制备复合电极;(3)通过优化阴极的微观结构,使极化损失达到最小。在本研究中,采用流延成型工艺制备La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)阴极并通过微观结构的优化,提高了LSCF阴极在中低温下的电化学性能；使用溶液浸渗工艺对LSCF多孔阴极表面改性,从而提高了电池长期运行的稳定性和可靠性。LSCF由于其高的离子-电子混合导电性成为近年来IT-SOFC阴极材料研究的热点。在本论文中,研究了不同造孔剂对LSCF阴极微观形貌和电化学性能的影响,通过不同的合成方法制备不同粒径的LSCF粉体,在此基础上,采用流延成型工艺制备了颗粒粒径、气孔大小以及气孔率呈梯度变化的三层LSCF阴极。研究表明,当内层(与电解质接触层)石墨含量为10wt%,中间层石墨和淀粉含量分别为15wt%,外层淀粉含量为35wt%,烧结温度为1050℃时,LSCF梯度多孔阴极在800-650℃的界面极化阻抗分别为0.053,0.11,0.27,0.65Ωcm2,低于文献中报道的采用不同工艺制备的单一结构LSCF阴极；以加湿的氢气为燃料,梯度LSCF为阴极的Ni-YSZ/YSZ/SDC/LSCF单电池在800和700℃操作温度下的最大功率密度分别为1.0和0.58 Wcm-2；SEM扫描图显示LSCF梯度阴极与SDC过渡层及YSZ电解质之间烧结良好,且三层LSCF阴极之间也没有明显裂纹出现。对梯度LSCF阴极的交流阻抗谱分析发现,通过微观结构的优化能够改善阴极／电解质界面,并能促进氧在阴极内部的传输和吸附,从而进一步提高阴极材料的电化学反应活性。虽然LSCF阴极在中低温下显示出良好的反应活性,然而与传统阴极材料La0.85Sr0.15MnO3-δ(LSM)相比,其长期稳定性并不理想。在本论文的第二部分,采用浸渗工艺对LSCF多孔阴极进行表面修饰,以提高LSCF阴极的电化学性能和长期稳定性。本论文首先以离子传导氧化物Sm0.2Ce0.8O2-δ(SDC)作为浸渗材料,采用一次浸渗工艺制备了LSCF-SDC复合电极。XRD图谱表明LSCF-SDC复合电极中没有其它物相生成；SEM显示新生成的SDC颗粒粒径为20-80nm,在LSCF表面分散均匀且具有较小的粒径分布。当浸渗量为10μL,浸渗浓度为0.25mol／L时,LSCF-SDC复合电极的界面极化阻抗最小,在800-650℃分别为0.041,0.074,0.17和0.44Ω-cm2,比纯LSCF阴极的界面极化阻抗降低约～50%。浸渗溶液浓度从0增加至0.35mol／L时,欧姆阻抗在800和650℃时均没有明显变化,LSCF-SDC复合电极性能的提高归因于LSCF表面反应活性区域的增加。以加湿的氢气为燃料, SDC浸渗LSCF为阴极的阳极支撑的薄膜电解质单电池在800℃和750℃操作温度下的输出功率密度分别为1.11和0.85 Wcm-2。100小时测试表明,SDC浸渗LSCF阴极后的电池稳定性有所提高。这说明采用SDC氧离子导体浸渗,不仅能够提高LSCF阴极电化学性能,也有助于稳定性能的提高。本论文的第三部分以LSM为浸渗剂,制备了LSCF-LSM复合阴极,研究了极化条件下LSCF-LSM阴极的电化学性能和长期稳定性,并对LSCF阴极的衰减机理和LSM浸渗LSCF (LSCF-LSM)阴极的增强机理进行了初步探讨。700℃时纯LSCF阴极和LSCF-LSM阴极在不同极化偏压下的交流阻抗谱表明,在开路状态下,LSCF-LSM阴极的界面极化阻抗为0.35Ωcm2,高于纯LSCF的阻抗0.24Ωcm2。这是由于LSM为纯电子导体,与具有高离子-电子混合导电性的LSCF阴极相比,具有较少的TPB反应活性区域,因此电化学活性较差。继续增加阴极极化过电位,LSCF-LSM阴极的界面阻抗迅速下降,而LSCF阴极的界面阻抗降低缓慢。当阴极极化过电位为80mV时,纯LSCF阴极的界面极化阻抗为0.21Ωcm2,而LSM浸渗后LSCF阴极的界面极化阻抗降低至0.18Ωcm2。LSCF-LSM/SDC/YSZ/NiO-YSZ电池则表现出良好的稳定性能,且电池输出电压(加载电流为400mA／cm2)在测试时间内一直增长,120小时测试后的输出电压为0.753V；经过120小时放电后,电池在700℃的最大输出功率达到0.40／Wcm2,比电池放电测试前的功率高27%。采用无线电频率磁电管溅射工艺分别制备了LSCF以及LSM涂覆的LSCF电池模型,并对两者在相同极化条件下的电化学性能进行了研究。结果表明,LSM涂覆LSCF阴极具有更好的表面催化活性,在一定程度上增强了LSCF阴极表面的氧交换反应,提高了LSCF在极化条件下的电化学催化性能。拉曼光谱分析发现,对于LSM涂覆LSCF阴极在极化测试前后并没有发现波数偏移现象,说明LSM涂覆能够抑制LSCF中Sr的迁移,从而提高LSCF阴极在工作状态下的长期稳定性。更多还原

【Abstract】 Solid oxide fuel cells (SOFCs) have attracted much attention due to their high energy efficiency, modularity, and excellent fuel capability. To study and develop intermediate temperature solid oxide fuel cells (IT-SOFCs), which operate at 600-800℃, is the developmental direction of SOFCs in order to widen the selectivity of component materials and improve the reliability of SOFC system. On the other hand, the overall electrochemical performance of SOFC will decrease as temperature drops. This is mainly due to the increased polarization resistances for electrode reactions and the decreased electrolyte conductivity at lower temperature.The resistive contribution of the electrolyte can be reduced by decreasing the thickness of electrolyte to the microscale on an anode-supported structure. Thus, the overall performance loss is increasingly dominated by the polarization loss of the electrode reactions, and especially the cathode one. There are several approaches which can be employed to decrease the cathodic polarization resistance:(1) using mixed ionic-electronic conductor materials with higher catalytic activity at lower temperature; (2) adding an ionically conducting second phase to form a composite cathode; (3) optimizing the microstructure of the cathode, such as the pore size, porosity and grain size. During recent years, many research efforts have been made to develop alternative cathodes or composite with high performance but few studies have been conducted to optimize properties by microstructure design.La0.6Sr0.4Co0.2Fe0.8O3-δperovskite oxide has been extensively investigated and utilized as a cathode material for IT-SOFCs due to its high ionic and electronic conductivity at lower temperatures. In this thesis, the effects of pore formers on microstructure and electrochemical performance of LSCF were studied firstly, then, LSCF powders with different particle sizes were synthesized using different synthesis methods. Based on these results, the 3-layer LSCF cathodes with graded grain size, pore size and porosity were fabricated by low-cost tapecating technique. It was showed that when graphite content for inner layer is 10wt% and corn starch content for outer layer is 35wt%, the graded LSCF fired at 1050℃exhibited the best performance. The lowest interfacial resistances at 800-650℃were 0.053,0.11,0.27 and 0.65Ω·cm2, respectively, which were much lower than that reported in literatures. The maximum power density of Ni-YSZ| YSZ| SDC| LSCF cell with graded LSCF cathode is 1.0 and 0.58 Wcm-2 at 800 and 700℃, respectively. SEM micrographs showed that 3-layer porous structure was good bonded to the electrolyte support and no cracking can be found among different layers. Impedance spectra analysis showed that both charge transfer reactions and the oxygen mass transport were enhanced for graded porous LSCF cathode.While cells with (La, Sr)(Co, Fe)O3-δcathodes offer significantly higher power densities than those with La(Sr)MnO3 cathodes, long-term stability of LSCF cathodes seem to be a concern. Thus, the following part in this thesis would focus on modification of LSCF surface by infiltration process to improve the electrochemical performance and long term stability. In first part, LSCF-Sm0.2Ce0.8O1.95 (SDC) composite cathode was fabricated by infiltration using SDC as infiltrant. XRD spectra showed that the phase formation temperature of SDC was 900℃and no other phase was found in SDC infiltrated LSCF cathode at this temperature. The diameters of new formed SDC nanoparticles were around 20-80nm, which are distributed uniformly on the surface of LSCF backbone with narrow size distribution. The SDC particle size can be controlled by changing concentration of infiltrated solution. Impedance analysis indicates that the SDC infiltration has dramatically reduced the polarization of LSCF cathode, reaching the lowest interfacial resistances of 0.074, and 0.44Ω·cm2 at 750 and 650℃, respectively, which was more than 50% improvement as compared to measurement on the blank LSCF cathode. Meanwhile, ohmic resistance of the symmetrical cell at 800 and 650℃did not show ant increase when the concentration of infiltrant increasing from 0 to 0.35mol/L. The dramatic decrease in the electrode polarization resistance is mainly attributed to the extension of triple phase boundary (TPB) reaction area. The maximum power density of anode-supported cell with SDC infiltrated LSCF cathode was about 1.11 and 0.85 Wcm-2 using H2+3%H2O as fuel. Furthermore, the infiltrated cell showed improved long-term stability within the time range of testing although the detailed mechanism is yet to be determined. These results indicated that the potential promise of ionic conductor infiltration as a method for enhancing the electrocatalytic activities and long-term stability of MIEC cathodes such as LSCF.The last part in this thesis studied La0.85Sr0.1503-δ(LSM) infiltrated LSCF cathodes. The electrochemical performance of LSCF-LSM electrodes and long term stability was investigated under cathodic polarization. At last, the mechanism of LSCF degradation and LSM enhanced LSCF cathode stability was analyzed. The resistance impedance spectra of blank LSCF and LSM infiltrated LSCF cathode under different overpotentials at 700℃showed that under open circuit, the interfacial resistance of LSM infiltrated LSCF cathodes was 0.35Ω·cm2, higher than that of blank LSCF cathodes 0.24Ω·cm2. This was because LSM was pure electronic conductor with less TPB length comparing with mix-conducting LSCF materials. While continuing increasing the cathodic overpotentials, the interfacial resistance of LSM infiltrated LSCF cathodes decreased dramatically. In contrast, the interfacial resistance of LSCF cathodes decreased slowly at the same conditions. The interfacial resistance of LSM infiltrated LSCF cathodes was 0.18Ω·cm2 comparing that of 0.21Ω·cm2 for blank LSCF cathodes. The cell Ni-YSZ| YSZ| SDC| LSCF-LSM showed good stability during 120h test and the output voltage (at 400 mA/cm2 constant current) was increasing all the time. The output voltage was 0.753V operation and the maximum power density was 0.401 Wcm-2 at 700℃after 120h operation, implying about 27% improvement.A new electrode prepared by radio frequency (RF) sputtering is designed to study the electrochemical reactions on the air-exposed surface. It was found that LSM thin film enhanced surface activity for oxygen reduction reaction and the electrochemical performance was improved for LSM coated LSCF cathode under polarization conditions. Raman spectroscopy showed that no obvious main peaks shift were experienced for LSM coated LSCF cathode before and after polarization, representing superior stability of this new cathode.更多还原