Study On K, Sr, Sm, Nd Doped LaMnO₃-based SOFC Cathode Materials

Fuel cells are a new type of energy transforming devices which can convert chemicalenergy contained in fuel directly to electric energy. And widespread attention from countriesall over the world has been paid to Solid Oxide Fuel Cell (SOFC) for its advantageouscharacteristics such as complete solid structure, higher energy efficiency, and extensiveadaptability to several kinds of fuel gas. At present, SOFC runs commonly at about 1000℃.So it makes great sense to develop new pattern of electrode materials which have higher ionand electrical conductivity at mesothermal temperature to raise the energy use efficiency.In the present work, K, Sr, Sm doped LaMnO₃-based composite oxide powders aresynthesized with citric acid sol-gel method and effect of synthesizing conditions on theparticle size and crystallography property is analyzed. Bulk samples have been obtainedthrough compaction and then sintering at high temperature. Influence of different sinteringconditions on the properties of bulk specimens is also expounded. Conductivity and activationenergy of bulk samples are analyzed through AC impedance measuring between 300 to 700℃and their relations to composition or temperature are illuminated. La_1-xNd_xMnO₃ powder isprepared by direct roasting microemulsion method and the conductivity of theLa_1-xNd_xMnO₃ compact specimen without binders is measured at different temperature.The result shows that, fine sol and gel can be prepared under the condition of C/Mⁿ⁺=1.4,pH=9 and composite oxide powder be synthesized by roasting the xerogel at 800℃for 4h.The structure of the composite oxide is distorted based on LaMnO₃ structure because of thedoping elements. The particle size of the powder is about 50 to 150nm and the distribution ofthe particle agglomerates are D₅₀=0.55μm, D₉₇=6.91μm. The powder sample is compactedunder the pressure of 13MPa. SEM analyzing results of samples sintered at differenttemperature show that the crystalline size of the materials can be controlled for propergrowing while the green compact specimens sintered at 1100℃for 1h, and that bulk sampleswith fine micromorphological features can be prepared. Heating rate of the sintering processmust be slowed down when the temperature is between 800 and 1000℃because of the specimen's conspicuous contraction. For La_1-xK_xSr_xMnO₃ series, the conductivity can not beincreased by the K-doping and even decrease when the doping ratio x equals 0.25. ForLa_1-x-ySm_xSr_yMnO₃ (doping ratio y=0, 0.05, 0.10, 0.15; x=0.10) and La_1-x-ySm_xSr_yMnO₃(doping ratio x=0, 0.05, 0.10, 0.15; y=0.10), the conductivity climbs the highest when thedoping ratio equals 0.05. Between 300 to 700℃, the conductivity of K-doped sample stayapproximate unchanged with the increase of K doping ratio, and that of Sm-doped samplerises along with the increasing of Sm doping ratio. Between 400～720℃, the conductivity ofthe La_1-xNd_xMnO₃ compact specimen without binders rises with the increase oftemperature. The highest conductivity of the samples appears when x=0.05, and theconductivity rises relatively more quickly with the increase of the temperature whenx=0.15.