| One of the major goals in solid oxide fuel cell （SOFC） research is to reduce theoperating temperature to intermediate temperature range （500-700℃） in order toimprove the compatibility of the component materials, prolong the lifetime anddecrease the operating costs. However, the reduced working temperature generallyleads to increase of the cathode polarization resistance resulting in drastic decrease inthe cell performance. Development of cathode materials with high catalytic activityfor oxygen reduction and low polarization resistance, therefore, is critical forintermediate-temperature solid oxide fuel cells （IT-SOFCs）. In this thesis, systematicinvestigations on the physical properties and electrochemical performance ofDyBaCo2O5+δ，DyBaCo2O5+δdoped with different elements, SrCo0.8Ga0.2O3-δandPr0.6Sr0.4Co0.9Nb0.1O3-δmaterials with perovskite structure are discussed in order todevelop novel cobalt-based cathode materials suitable for IT-SOFCs.The DyBaCo2O5+δcathode materials synthesized by EDTA-citric acid （EC）sol-gel method and ethanol supercritical drying assisted EDTA-citric acid method（SCEC）, respectively, are investigated as potential cathodes for IT-SOFC. The SEMmorphology images show that the powders synthesized via EC method are sphericalwhereas that obtained by SCEC method have sponge-like shape consisted ofsmall-size and uniform particles. The peak power density of the SOFC withDyBaCo2O5+δ（SCEC） as cathode materials reaches529mW cm-2at650℃, while itis436mW cm-2for that with DyBaCo2O5+δ（EC） cathode, indicating that synthesismethod has important effect on the cathode performance of DyBaCo2O5+δmaterials,and SCEC is preferable than EC to prepare high performance DyBaCo2O5+δ. Aimed to improve the electrochemical performance of DyBaCo2O5+δ, dopedmaterials of Dy1.10BaCo2O5+δ, DyBa0.5Sr0.5Co2O5+δand DyBaCoCuO5+δoxidepowders were prepared via EC method by introducing Dy, Sr and Cu ions intoDyBaCo2O5+δ, respectively, and the properties of thermal expansion, conductivity,surface state as well as electrochemical performance are comparatively evaluated. Theresults demonstrated that all the synthesized materials have an orthorhombicperovskite structure and a good chemical compatibility with electrolyte. Theconcentration of adsorption oxygen on the surface of all these doped materials ishigher than that of DyBaCo2O5+δ. The average thermal expansion coefficients, in thetemperature region of30-800℃, of Dy1.10BaCo2O5+δ, DyBa0.5Sr0.5Co2O5+δandDyBaCoCuO5+δare15.9×10-6,18.2×10-6and15.8×10-6K-1, respectively. Theconductivity of Dy1.10BaCo2O5+δand DyBaCoCuO5+δis lower than that ofDyBaCo2O5+δ, whereas that of DyBa0.5Sr0.5Co2O5+δis higher. Among the threedoped materials, DyBa0.5Sr0.5Co2O5+δhas the highest conductivity and minimumpolarization resistance, implying that partial substitution of Sr for Ba in DyBaCo2O5+δis an efficient strategy to reduce the polarization resistance and improve the oxygenreduction activity of cathode.The SrCo1-xGaxO3-δoxide powders are synthesized by EC method and evaluatedas cathode materials for IT-SOFCs. The results show that all the samples havebrownmillerite structure. The thermal expansion coefficient, electrical conductivityand the amount of adsorbed oxygen on the surface of the cathode materials decreasegradually with increase in Ga content. The single-cell with SrCo0.8Ga0.2O3-δ, whichhas the lowest area specific resistance among the studied samples, as cathode achievesa maximum power density of484mW cm-2at650℃.The PrxSr1-xCo0.9Nb0.1O3-δpowders are synthesized by traditionalsolid-state reaction method and the physical and electrochemical characteristics areinvestigated. The results show that all the samples have pervoskite structure. Theelectrical conductivity and area specific resistance of the obtained materials decreasewith increase in Pr content. The single-cell with Pr0.6Sr0.4Co0.9Nb0.1O3-δ, which hasthe lowest area specific resistance among the studied samples, as cathode achieves a maximum power density of1009mW cm-2at650℃. |
