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 ≈10 S cm−1 at 900 °C and were also shown to be stable at this temperature in N2. For YSr2Cu2FeO7+x, semiconducting behaviour was observed up to ≈550 °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 °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(O2)
(0.2–10−5 atm) at 900 °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(O2). Chemical compatibility studies of these phases with SOFC electrolytes at temperatures between 900 and 1000 °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)2Sr2Cu3−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.