Synthesis and characterisation of the perovskite-related cuprate phases YSr2Cu2MO7+y(M=Co, Fe) for potential use as solid oxide fuel cell cathode materials

In this paper we report the synthesis and characterisation of the perovskite cuprate phases YSr2Cu2MO7+y (M = Co, Fe) in order to examine their potential for use as cathode materials in solid oxide fuel cells (SOFCs). Both samples showed conductivities of approximate to 10 S cm(-1) at 900 degrees C and were also shown to be stable at this temperature in N-2. For YSr2Cu2FeO7+x, semiconducting behaviour was observed up to approximate to 550 degrees C, with a decrease in conductivity at higher temperatures, attributed to oxygen loss reducing the charge carrier concentration. In the case of YSr2Cu2CoO7+y, semiconducting behaviour was observed over the range of temperatures studied, although a small but significant steep increase in conductivity was observed above 800 degrees C. High temperature X-ray diffraction studies of this particular phase showed that this increase in conductivity coincided with an orthorhombic-tetragonal structural transition, accompanied by a significant reduction in cell volume. In addition to measurements in air, conductivities were also measured with varying p(O-2) (0.2 - 10(-5) atm) at 900 degrees C, and these data showed significant hysteresis between measurements on reducing and re-oxidising, suggesting poor oxide ion transport, poor oxygen surface exchange kinetics, or significant structural changes on varying p(O-2). Chemical compatibility studies of these phases with SOFC electrolytes at temperatures between 900 and 1000 degrees C showed reaction in all cases. In the case of CeO2 based electrolytes, the reaction led to the formation of the "fluorite-block'' phases, (Y/Ce)(2)Sr2Cu3-xMxO9+y (M = Co, Fe), and samples of these were subsequently prepared and the conductivities measured. Similar hysteresis between conductivity measurements on reducing and re-oxidising were also observed for these samples.