Oxygen Transport in the SOFC Cathode

Abstract

The impedance of the La0.75Sr0.2MnO3-cathode/electrolyte interface for cathodes with different porosity is measured. The impedance spectra are fitted using a developed model of the oxygen transport at this interface. After the measurements, the cathode is removed from the electrolyte. The contact area and the three-phase boundary length (TPBL) at the interface are estimated from SEM images of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of oxygen vacancies at the interface at 950°C is high enough to use the whole La0.75Sr0.2MnO3/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2F/TPBL ≤ 2 μm. The impact of the surface diffusion of oxygen species on polarization resistance at operation temperatures <900°C is discussed. The polarization resistance and the morphology of composite cathodes made from La0.75Sr0.2MnO3/YSZ and yttria- or scandia-stabilized zirconia powders (3YSZ, 8YSZ, 10ScSZ) are investigated by impedance spectroscopy at 800–950°C. The polarization (interfacial) resistance decreases gradually with addition of electrolyte powder in the uLSM cathode material independent of the electrolyte powder used. The interfacial resistance of the uLSM/3YSZ, uLSM/8YSZ, and uLSM/10ScSZ composite cathodes is almost the same. The interaction between uLSM and doped zirconia particles is discussed on the basis of the interfacial resistance, activation energies, and high-frequency impedance.