| Summary: | 碩士 === 義守大學 === 材料科學與工程學系碩士班 === 98 === The traditional Solid Oxide Fuel Cells need to operate at high temperature(~1000℃), however, a long-term high temperature operation will lead to many problems. Not only the cost of electrode material is relatively high, but also the selectivity of materials is limited. Therefore, the higher efficient and lower operation temperature of solid oxide fuel cell become important topics. Because the polarization loss between electrode and electrolyte will increase with decreasing operation temperature, the polarization impedance mainly comes from the interface of the cathode and the electrolyte, it is expected that electrode which possess an unique microstructure can effectively reduce the polarization impedance.
La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF) has a high ion and electron conductivity in intermediate temperatures(600~800℃) as well as a high compatibility with ceria electrolytes, so it is a good canditate of SOFC cathode materials. In order to further improve the performance of LSCF electrode, the second phase material with higher ion conductivity is often added to cathode. For example, the addition of SDC(Sm dopped Ceria) or GDC(Gd dopped Ceria) not only can extend the three phase boundary (TPB) area but also can improve the overall cell efficiency. This research will on the fabrication of focus the LSCF cathode and CYO eletrolyte by Pechini process and coprecipitation process, a LSCF-CYO composite powder are prepared by mixing precursor solution and oxide powders. For comparsion, a traditional ball milling method also is followed.
The results indicate that two phases in LSCF-xCYO composite cathode do not mutually react after sintering at 1150℃ for 2 hours. In the conductivity analysis, the all samples show a maximum value at 600℃, the conductivity of CYO(U)-LSCF series samples exhibit the highest value. The possible reason is related to the grain size and microstructure of composie powders. By the hot etching of composite electrodes, EDS and line scan analysis can distinguish LSCF phase from CYO phase, the phase distribution is obtained. In the alternating-current impedance analysis, the LSCF-60CYO-P composite cathodes exhibit the lowest polarization impedance, the possible reason could be that composite cathode has a higher density. The influence of composition and processing on polarization impedance and conductivity is not similar, this is possibly related to the three phase boundaries and microstructure of composite cathode.
The above results show that the combination of LSCF-60CYO-P composite cathode with the CYO-P electrolyte has the best performance as compared with others. The polarization resistance of LSCF-60CYO-P at 700℃ approximately is 1 Ω and is lower than other electrodes, thus, it is clean that such processing way can improve, the cathode polarization impedance effectively, and the CYO-P electrolyte also has a higher sinterability than that of CYO-U. The densier electrolyte also can reduce the overall impedance.
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