Factors Governing the Performance and Stability of Solid Oxide Fuel Cell Cathodes Prepared by Infiltration

Infiltration method, developed at the University of Pennsylvania, is a unique analytical platform for investigating the effect of material properties and electrode microstructure on the performance of solid oxide fuel cell (SOFC) electrodes. During cell fabrication by infiltration, the ion-conducting electrolyte phase is sintered first, followed by the addition of the catalytically active perovskite phase into the pores of the electrolyte. The use of separate sintering steps for the electrolyte and the active phase gives one a high degree of control over the microstructure of both phases, unattainable with traditional fabrication methods. In this thesis, the infiltration approach has been used to conduct a systematic investigation into the factors that govern the performance and stability of solid oxide fuel cell cathodes. As a result, a number of microstructural and material properties, crucial for obtaining high electrode activity, were identified. In particular, the effect of varying the ionic conductivity of the porous electrolyte, the specific surface area of the perovskite as well as the specific surface area of the porous electrolyte, and the effect of solid-state reactions between the two phases were studied and were found to significantly affect performance. The experimental findings agreed well with the predictions of a mathematical model that was developed to describe the electrochemical characteristics of SOFC composite cathodes. Both theoretical and experimental evidence suggests the performance of SOFC cathodes prepared by infiltration is limited by slow oxygen adsorption on the perovskite surface. The chemical composition of the perovskite surface therefore plays an important role in determining the overall performance of the electrode. The last chapter of this thesis introduces a novel method that may allow one to characterize the active sites on the perovskite surface under SOFC cathode operating conditions (600-700°C, ambient air atmosphere, polarization), unattainable with traditional surface characterization techniques