THERMO-CHEMICAL STABILITY ASPECTS OF MIXED IONICELECTRONIC CONDUCTING (MIEC) CERAMIC MEMBRANE MATERIALS

Cubic perovskite BaxSr1-xCoyFe1-yO3-δ (BSCF), with x = 0.5 and y = 0.8, is one of the oxygen permeable mixed ionic-electronic conducting (MIEC) membrane materials having the highest oxygen permeation flux reported. The material has potential for hightemperature electrochemical applications such as oxygen separation membrane and cathode for Solid Oxide Fuel Cells (SOFCs). However, its thermochemical instability is of great concern. BSCF exhibits a non-linear expansion with temperature and oxygen partial pressure, pO2 due to oxidation, spin transition of cobalt, thermal reduction and precipitation of hexagonal phase. While its cubic perovskite structure is thermodynamically stable above 850 °C, the material is susceptible to degradation by high-temperature creep and kinetic demixing/decomposition. At intermediate temperatures below 650 °C, oxygen permeation flux is relatively low. At temperatures above 650 °C, precipitation of the hexagonal phase affects the oxygen permeation flux exhibited by the cubic perovskite phase. The need for stabilization of the cubic perovskite phase by doping/co-doping can therefore not be overstated. Cation interdiffusion and interface reactions are also of great concern for application in solid oxide fuel cells. Studies indicate that cation interdiffusion and interface reactions occur both during the high temperature sintering and during SOFC operations. Therefore, its occurrence cannot be completed ruled out even for intermediate temperature SOFCs. Clear evidence on the occurrence of cation interdiffusion and interface reactions was demonstrated using cobalt oxide (CoO) and La2NiO4+δ as diffusion couples. Methods that can be used to control the occurrence of cation interdiffusion and subsequent interface reactions are discussed.