Development Of Novel Cathode Materials For Intermediate Temperature Solid Oxide Fuel Cells

A solid oxide fuel cell (SOFC) is a novel energy conversion device with all solid-state structure. Recently development of intermediate temperature solid oxide fuel cells (IT-SOFC) operated below 800℃is one of the hot research subjects. Lowering the operating temperature to intermediate range not only reduces the over all system cost, but also increases the conversion efficiency of electric energy and the open-circuit voltage. To realize the intermediate temperature process, not only new electrolyte materials with high electrical conductivity and steady performance at 500-800℃, but also high-performance cathodes compatible with the electrolytes are needed to develop.Sm0.2Ce0.8O1.9 (SDC) is a good electrolyte material for intermediate temperature solid oxide fuel cells. Owing to the characteristics of the starting powders having a significant effect on the performance of the electrolytes, in this dissertation, gelcasting and GNP methods were employed to prepare SDC powders respectively. Morphology and sinterability of the powders were analysized. It is shown that SDC powders prepared by the two methods both have nano-meter size with near spherical particle shapes, and the particle sizes are about 70nm and 50nm, respectively. Grain sizes are 13.60nm and 14.79nm respectively calculated from Scherrer Formula. Sintered SDC samples prepared from SDC powders by two methods have dense microstructure with over 96% relative density when sintered at 1400℃. SDC Powder prepared by the GNP method has a high specific area, good sintering activity, and is easy to densify, but difficult to collect. Meanwhile, the gelcasting method has special advantages of simple processing, and can produce superfine particle.Recently, ABO3 type perovskite composite oxides with mixed ionic and electric conductivity (MIEC) have drawn much attention in using as cathode materials of SOFC. Among which, La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) and Sm0.5Sr0.5CoO3-δ(SSC) materials are of big interests. But the application of LSCF is limited due to its higher thermal expansion coefficient than the SDC electrolyte. In this study, La0.6Sr0.4-xCaxCo0.2Fe0.8O3-δ(LSCCF) materials with different Ca dosages were prepared by the gelcasting method to improve the thermal matches between the cathode and electrolyte materials of SOFC. On the other hand, because the addition of electrolyte composite into the cathode material can effectively enlarge the redox area, and can improve the thermal matches between cathode and electrolyte materials, LSCF-SDC composite cathode materials were prepared by mechanically mixing LSCF and SDC powders. The sintered property, microstructure, electrical conductivity and the thermal expansion coefficient (TEC), interfacial resistance of the LSCCF and LSCF-SDC cathode materials were investigated. The results show that sintering properties of the materials are improved by both Ca doping and SDC addition, and porous microstructure is obtained at lower temperature. TEC of LSCF material can be decreased by both Ca doping and SDC addition. The TEC value is 13.05×10-6·K-1 when Ca dosage is 0.2, and the value is 14.95×10-6·K-1 when SDC addition is 30%. Ca doping decreases electrical conductivity of LSCF significantly. When Ca dosage is 0.2, electrical conductivity of LSCCF material is 593.8 S·cm-1 tested at 800℃, which is much lower than that of LSCF materials. However, the addition of SDC has little effect on the electrical conductivity of LSCF-SDC composite cathode materials. This illustrates that LSCF-SDC composite materials have better prospects than LSCCF in using as cathode materials for intermediate temperature solid oxide fuel cells.Recently, Sm0.5Sr0.5CoO3 (SSC) materials have also attracted much attention in using as cathode materials of SOFC with SDC electrolyte. However thermal expansion coefficient (TEC) of SSC is much higher than SDC electrolyte material. To match the TEC between SSC and SDC materials, a gelcasting method was employed to fabricate Sm0.5Sr0.5Co1-xFexO3 (SSCF) powders with different Fe dosages. Porous microstructure was obtained when the SSCF powders were sintered at temperature from 1100℃to 1200℃, relative density of the sintered SSCF samples increases as the sintering temperature increases, and the porosity decreases inversely; Electrical conductivity and thermal expansion coefficient were significantly decreased by Fe dosages. Specially, the TEC value decreases as Fe dosage increases. A typical TEC value of 16.4×10-6·K-1 was obtained for Sm0.5Sr0.5Co0.2Fe0.8O3 samples at 800℃, which is much lower than that of SSC material (21.4×10-6·K-1). However, the polarization resistance of the SSCF materials increases as Fe dosage increases. Maximum polarization resistance of 0.42 ?·cm2 was obtained with Fe dosage of 0.4. Besides, SSCF materials show good oxygen reduction activity at 700-800℃.