Preparation And Characterization For Intermediate Temperature Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is a high efficiency and clean energy conversion device. According to used electrolytes, SOFCs technologies were mainly based on oxygen-ion conductors and proton conductors. From the viewpoint of cost reduction and long-term stability, it is necessary to lower the operating temperature to intermediate temperature. So far, doped ceria oxides are being extensively studied as a promising candidate solid electrolyte for intermediate temperature SOFCs due to high oxide-ion conductivity. Compared with oxygen-ion conductors, proton conductors produce water vapor at the cathode side, which helps to increase the efficiency and improve the EMF. Both oxygen-ion conductors and proton conductor had their own specialty. In this research, this two types cell were studied by optimizing powder synthesis method and cell fabrication, to enhance the cell performance.Based on the development and status of these two types of SOFCs, powders synthesis methods and fuel fabrication technics were optimization. The research aims to lower electrolyte ohmic resistance, to develop polymer assistant combustion synthesis method. The cell performance was improved by thin electrolyte film obtained from dry-pressing method, the assembly of electrode and electrolyte, and the optimization of cell microstructure.Based on investigating and analyzing of SOFC's development and status, the key materials and structure types of SOFC were summarized. Reducing fabrication and operation costs while maintaining high performance is a major consideration for SOFC. This research aims to search for new materials, developing new fabrication approaches and optimizing technics. In chapter 2 and 4, Ce_0.8Sm_0.2O_1.9 (SDC) and BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3-δ (BCZYZ) powders were prepared and characterized. In chapter 3 and 5, the cell prepared by dry-pressing method based oxide-ionics conducting electrolyte and proton-conducting electrolyte were tested.1. Powder synthesisThe powder properties affect the cell preparation techinics and the cell performance. Therefore, one of the emphases of this research was to prepare the powder required in the experiments. Our lab had developed many methods to prepare the powders with excellent properties. For the first time, Ce_0.8Sm_0.2O_1.9 ( SDC ) powders were synthesized by polymer assistant combustion method using methylcellulose (MC) and PVA as the fuel. SDC powders were formed due to polymer chelation and gel properties. Compared with conventional sol-gel, this method is simple, does not need to control the solution pH. The formation mechanism is discussed. The cell parameters and crystal sizes were calculated using the peak positions determined from the XRD patterns, and it was found that stoichiometric SDC powder could be obtained only when stoichiometric fuel contents were used. The relative density is 96% when the SDC pellet was sintered at 1400℃for 5h. The results showed that the conductivity activity energy is 1.17 eV and the conductivity at 800℃is 0.0898 S·cm^-1 with MC as fuel. While, with PVA fuel, a lower synthesis temperature of 350℃is demonstrated and a dense sintering body is obtained at 1300℃and the conductivity at 800℃is 0.0861 S·cm^-1. Higher sintering temperature is not favorable to increasing the sintering density, and the conductivity at 800℃is lowered to 0.0619 S·cm^-1, which present the properties of the powder with high sintering activity.The application and development of proton conducting electrolyte is later than that of oxide-ionics conducting electrolyte. Among the proton conducting electrolyte, BaCeO₃-based material have the highest conductivity, unfortunately, high sintering temperature. The recent research Zn element can assist lowering the sintering temperature to 1325℃by solid-state reaction. Obviously, solid-state reaction is not fit for lowering sintering temperature with combustion synthesis method. Based on the development of soft chemical synthesis method, in this thesis, BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3-δ (BCZYZ) was prepared by citrate combustion synthesis method. The XRD results showed that 1000℃was sufficient for the formation of pure BCZYZ powders. It was found that BCZYZ powders exhibited high stability under 100%CO₂ atmosphere and high sintering activity and could be dense at 1150℃, which was lower than conventional solid-state reaction. The EDX results showed that there is a uniform distribution of all the elements. 2. Performance of the cell with oxide-ionic conducing electrolyteUsing SDC powders prepared by PVA combustion synthesis method, anode-supported cells were assembled with general Sm_0.5Sr_0.5CoO_3-δ (SSC)-SDC and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) -SDC with high catalytic activity as the composite cathodes, and were tested 550℃to 650℃. The maximum power densities were 719 mW/cm²,370 mW/cm²,178mW/cm² at 650℃, 600℃, 550℃with SSC-SDC cathode, while with BSCF-SDC the values were 1018 mW/cm²,646 mW/cm²,288mW/cm², correspondingly. AC impedance spectra results showed that the interfacial polarization resistance with BSCF cathode was lower than that using SSC cathode, which might be the main reason that this cell performance was higher than that of SSC cathode.Using SDC powders obtained from PVA combustion synthesis method as the cathode interlayer of YSZ electrolyte, BSCF cathode is successfully applied to YSZ electrolyte-based SOFCs. SDC interlayer prevents the reaction between YSZ and BSCF. The interfacial polarization of the cell with BSCF cathode is 0.60, 1.06 and 1.64Ωcm², which is lower than the values of 1.73, 1.95 and 3.72Ωcm² with LSM cathode. The maximum power densities of 1049 m W cm^-2,669 m W cm^-2, 304 m W cm^-2 were higher than the that of 595, 444, 215 mW cm^-2, correspondingly, which exhibited the characterization of BSCF cathode is of high catalytic activity.3. Performance of the cell with proton conducing electrolyteFor the first time, the cell was assembled with BCZYZ thin electrolyte prepared by dry-pressing method and sintered at 1250℃, and the electrolyte was controlled at 10-15μm. The studies of the cell performance with BSCF cathode and La_0.6Sr_0.4CoO_3-δ (LSC) cathode due to their high catalytic activity and high electron conduction, the results showed that the maximum power densities were 322 mW/cm², 280 mW/cm²,172 mW/cm² at 650℃, 600℃, 550℃with BSCF cathode, while with LSC cathode the values were 544 mW/cm² , 465 mW/cm² , 345mW/cm², correspondingly, higher than that of BSCF cathode. Using the values obtained from AC impedance spectra, the estimated conductivity were 0.00584 S/cm at700℃. These researches provide the probability by using proton conducting material as the electrolyte at intermediate temperature.In this thesis, the main researches were focused on the powders synthesis and cell preparation technics. First, SDC powders with high oxide-ionics conduction and BCZYZ powders with high proton conduction were prepared by combustion synthesis method. And then the cells were assembled using optimized electrode material and microstructure. By all the optimization, theoretical cell performance was obtained.