Preparation And Characterization Of Key Materials For Intermediate Temperature Solid Oxide Fuel Cells By Novel Wet Chemistry Methods

Solid oxide fuel cell (SOFC) is one of the most promising electric power conversion systems with high conversion efficiency and low environmental impact. A traditional SOFC based on yttria stabilization zirconia(YSZ) electrolytes has to operates at temperature above 800 ℃, because of the relative low oxygen ion conductivity of YSZ at temperature below 800 ℃ . Such a high operating temperature results not only in the critical selection of cell materials, but also in the high cost of preparation and operation of the cell. To realize a commercial SOFC, it is desirable to reduce its operating temperature to intermediate temperature range (550-800℃), while still keep the power output obtained at high temperature. To reduce the operating temperature of a SOFC, the key is to decrease ohmic resistance of the electrolyte and to increase the catalytic activity of electrodes, which relys on the development of novel materials with high-performance.At present, SOFC cathode materials focus on perovskite-type oxides (ABO3) doped with rare earth elements at both A and B sites. These oxides often have mixed electronic and oxygen ionic conductivity, among which has drawn much attention because of the high electrical conductivity and catalytic activity of doped samarium cobaltite. Powders for SOFC cathodes generally were synthesized by solid-state reaction method, co-precipitation method, glycine-nitrate method, and so on. Although studied widely, these methods still exist some defects, for instance inhomogeneous ingredient, complex processing and relatively high cost. In the third and fourth chapter of this thesis, perovskite strontium-doped samarium cobaltite (Sm_1-xSr_xCoO₃, SSC) powders were prepared by gel-casting process and buffer-solution process, respectively. Effects of strontium dopant amount on the phase composition, microstructure, particle size and sinterability of the synthesized powders were studied, and properties of porous sintered samples were also characterized. The results show that perovskite phase can formed at the lowest temperature of 1000℃ and particle size of the calcined powders is about 100nm by gel-casting process, while perovskite phase can formed at a relative low temperature 800℃ by buffer-solution process and powders size is about 30nm. The samples sintered at 1100℃ have perfect porosity and reasonable pore size distribution. Furthermore, doping of strontium obviously improves the electrical conductivity of SSC materials. A conductivity of 2700S/cm at around 500℃ wasobtained for Smo.5Sro.4Co03 samples prepared from the buffer-solution SSC powders.To lower the operating temperature of a SOFC based on YSZ electrolytes to intermediate temperature, there are mainly two routes. One is to reduce the thickness of electrolyte layer to decrease the inner ohmic resistance of the electrolytes. The other is to select proper doping elements to increase the ionic conductivity of zirconia in intermediate temperature range. There have been many publications on Sm doped ceria electrolytes for IT-SOFC. But there is still a lack of publications on Sm doped zirconia lectrolytes. Therefore, in the fifth chapter of this thesis, zirconia stabilized with 4-12mol% samaria (4-12SSZ) powders were synthesized by a buffer-solution process. The properties of the powder, including its phase composition, particle size, as well as its sinterability, electrical conductivity were characterized. It was shown that a single cubic-phase forms at a relatively low calcination temperature of 600 °C when the amount of samaria dopant exceeds 8mol%. The powders have a regular particle shape with an average particle size of 10-15 nm. The relative density of the sintered samples increases as the sintering temperature and the amount of samaria dopants increases. A relative density of 95% was obtained for the 12SSZ sample sintered at 1500°C. The electrical conductivity increases with temperature linearly in the range of 500-800°C. The 12SSZ sample sintered at 1500°C has ionic conductivity of 0.038 S/cm and conductive activation energy of 0.725eV at 800°C in air.The interconnect is also one of the key components in a SOFC system. It can connect anodes and cathodes to one another when single cells are stacked together in series. At present, candidate materials for SOFC interconnect can be divided into two types, doped ACrO3 (A=La, Y, Nd and Sm) ceramic materials and oxidation-resistant metallic alloys, such as stainless steels. In ACrO3 (A=La, Y, Nd and Sm) perovskite system, many studies have been focused on pure and doped LaCrO3 and YCrO3. Little attention has been given to the sintering and electrical conductivity of NdCrO3 and SmCrO3. In the sixth chapter of this thesis, powders for Smi.xCaxCrO3 interconnect were prepared by the gel-casting process. The phase composition, particle size and sinterability of the powders were characterized. The sinterability of Smi.xCaxCrO3 powders was also studied by adding CaF2 sintering aids. The results show that the formation of perovskite phase is influenced by the amounts of Ca dopants, calcination temperature and calcination time. When x>0.3, perovskite phase was obtained as the dried gels were calcined at 1000°C for 3h. The samples sintered at 1550°C only have a relative density of 80%. Byadding 5wt% and 8wt% CaF2 into Smo.7Cao.3Cr03 powders, the relative densities of the sintered samples are increased to 84% and 90%, respectively.