Investigating the Relationship between Operating Conditions and SOFC Cathode Degradation

High polarization losses associated with the oxygen reduction reaction (ORR) that occurs at the cathode and degradation of cathode materials under operating conditions remain serious issues for widespread implementation of solid oxide fuel cells (SOFC). Performance of the cathode is determined by the microstructural/interfacial relationship between the electrode and electrolyte, and the inherent materials properties. Rates of degradation depend significantly on the operating temperature, applied-potential, and environmental conditions, such as the presence of unwanted oxygen-containing compounds, namely H₂O and CO₂. Common degradation mechanisms can be categorized into three main groups: sintering, poisoning, and secondary phase formation. In this study we explore the effects of operating conditions on the above degradation mechanisms for two common cathode materials, (La_0.8Sr_0.2)_0.95MnO_3±δ (LSM) and (La_0.6Sr_0.4) (Co_0.2Fe_0.8)O_3-δ (LSCF) using a multifaceted approach. Three-electrode cells have been aged under various temperatures, applied-potentials, oxygen partial pressures and contaminant conditions in order to observe changes through electrochemical impedance spectroscopy (EIS). In-situ EIS is a powerful tool that allows us to identify changes in conductivity for the reaction steps comprising the overall ORR. We have developed a powerful strategy for identifying the steps in the ORR that contribute to the overall impedance spectra, and how that contribution changes as a function of operating conditions and aging time. Our EIS results indicate a strong correlation between blocking effects caused by CO₂ and H₂O and the operating temperature for LSCF, while similar experiments performed on LSM show significantly less effect caused by the presence of these contaminants. Using EIS to de-convolute the overall cathode polarization helps us identify the mechanisms by which degradation occurs. In addition, focused ion beam-scanning electron microscopy (FIB-SEM) ex-situ analysis of the three-electrode cells has been performed to identify changes in the microstructure and composition of the electrodes, which are directly correlated to changes in the EIS response, and reveal specific regions where degradation has occured. Further, to investigate the fundamental kinetics of these materials, we have used a variety of gas-phase isotope exchange experiments on fresh and aged powder samples of the same cathode compositions to study the change in materials properties. Using impedance spectroscopy, supported by FIB-SEM and isotope exchange, we elucidate how changes in the conductivity, microstructure and kinetics of SOFC cathodes relate to their degradation.