Fabrication And Characterization Of Impregnation Cathode For Intermediate-temperature Solid Oxide Fuel Cells

As the problems of the energy and environment becoming more and more prominent, the scientists are willing to develop a more convenient, high efficient and environmental energy for sustainable development. Solid Oxide Fuel Cells (SOFCs) is a clean and high efficient energy type which has attracted increasing attention in recent years. In the last decade some SOFC stack has been established for commercial demonstration in a rapid development. But the SOFC operating at high temperature has low stability and high cost. It is currently not economically competitive with traditional electrical generation technology. It is believed that the stability and cost can be improved by reducing the operation temperature. However, the ohmic resistance and interfacial polarization resistance (especially the interfacial polarization resistance of cathode) will rapidly increase with the reduction in operation temperature. Decreasing the thickness of the electrolyte, developing novel electrolyte with higher ionic conductivity are two approaches for decreasing the ohmic resistance. Optimizing the microstructure of the electrode and developing novel electrode are the approaches for decreasing the interfacial polarization resistance. This thesis closely focuses on reducing cathode polarization resistance. Themicrostructure of the cathodes can be optimized by introducing ion-impregnation method into the cathode fabrication. And the cathodes show high catalytical activity either.In chapter 1, the SOFC principle and key component materials are briefly reviewed. The cathode materials and cathode reactions are primarily introduced here. And the research purpose is proposed on this basis.In chapter 2, High performance Sm0.5Sr0.5CoO3-Sm0.2Ce0.8O1.9 composite cathodes are fabricated by ion-impregnation method. The preparation process consists of two steps: Firstly dense electrolyte substrate and porous SDC backbones are formed by high temperature firing. The substrate and backbones are tightly bonded, forming a continuous path for oxygen ion conduction. Secondly SSC nanoparticles are prepared on the porous backbones by ion-impregnation method. The nanoparticles less than 100nm are well connected, forming a continuous path for electron conduction. The effect of the electrode thickness, the amount of SSC loading, and the firing process is investigated on the interfacial polarization resistance using symmetrical cells. The efficient cathode thickness is 173?m due to the continuous path for oxygen ion conduction. It is much high than the conventional LSM-SDC cathode. The interfacial polarization resistance of the cathode shows the lowest value when SSC loading is about 43 wt%. Moreover, the co-firing process can improve the interfacial performance and reduce the cost of preparation. Single cells with optimized microstructure show a high performance. The power density of the single cells are 437, 574, 660 mW/cm2 at 600, 650, and 700oC, respectively. It is much higher than those reported for single cells with SSC-based cathodes and zirconia-based electrolytes.In the previous chapter, it is clearly show that nano-structured cathode can be prepared by ion-impregnation method. In chapter 3, we propose an alternative impregnation method to fabricate the (La0.85Sr0.15)0.9MnO3-Sm0.2Ce0.8O1.9 composite cathode with nano-structured ion-conducting phase and electron conducting phase. The impregnation process is employed by repeating the following impregnation period: impregnating one times of SDC solution and fired at 800oC/1h; impregnating one times of LSM solution and fired at 800oC/1h. XRD pattern shows the LSM phase can be formed by fired at 800oC for 1h when glycine is added in the impregnation solution. SEM shows LSM and SDC nanoparticles are formed on the inner surface of the backbones. TPB are greatly increased in this cathode, and the symmetrical cells perform lower interfacial polarization resistance than the single phase impregnation. It is 0.93Ωcm2 at 600oC and 0.63Ωcm2 at 650oC. The single cell fabricated by alternative impregnation shows the performance of 348mWcm-2 at 700oC.