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

Abstract

In this paper we report the synthesis and characterisation of the perovskite cuprate phases YSr₂Cu₂MO_7+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 YSr₂Cu₂FeO_7+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 YSr₂Cu₂CoO_7+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)₂Sr₂Cu_3−xM_xO_9+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.