Measurement and Modeling of Material and Microstructural Factors Governing Performance of Solid Oxide Fuel Cell Cathodes

The mixed conductor La1-xSr xCoO3-delta was studied in this work for application in intermediate temperature SOFCs. Operating SOFCs in an intermediate temperature range (500-700 °C) allows for the use of less expensive materials in the stack assembly. However, efficiency is also reduced with operating temperature as a result of overpotential losses at the cathode due to the high activation energy of the oxygen reduction reaction. The rate-limiting processes of MIEC SOFC cathode materials have small nonlinearities arising from non-ideal thermodynamic behavior and the surface exchange reaction.;The porous microstructure typical of SOFC electrodes is a complex system in which rate-limiting phenomena are highly convoluted and difficult to uniquely distinguish and assess. The use of simplified, dense thin film geometries allows the rate-limiting kinetic reaction and vacancy defect thermodynamics to be examined independent of transport limitations and microstructural effects. To gain insight on the rate-limiting phenomena in lower Sr content materials (Sr=0.2), we studied dense thin films of La0.8Sr0.2CoO 3-delta (LSC-82) of 45 and 90 nm thickness grown epitaxially on (001) oriented single crystal Yttria-stabilized Zirconia. The films were strained in-plane and compressed normal to the substrate with larger relaxed unit cell volumes relative to bulk as measured by X-ray diffraction (XRD). Depth profile Secondary Ion Mass Spectrometry (SIMS) revealed a compositional La/Sr gradient across the film thickness with enhanced Sr2+ at the gas-exposed surface. Enhanced surface concentration of Sr2+ was also observed with Auger Electron Spectroscopy (AES). The capacitance of the thin films as determined by Electrochemical Impedance Spectroscopy (EIS) was greatly enhanced relative to the bulk material. The enhanced capacitance is explained in terms of a two-layer model, in which an enhanced Sr content surface layer results in a much greater concentration of oxygen vacancies than the underlying bulk material of nominal Sr composition. The capacitance resulting from the enhanced oxygen storage capacity of the enriched surface layer dominates the capacitive behavior of the entire thin film at low temperatures and high oxygen partial pressures. Nonlinear Electrochemical Impedance Spectroscopy was used to distinguish the governing rate-limiting surface exchange reaction from the underlying thermodynamic behavior by probing the full nonlinear response of the system. The nonlinear response was consistent with a dissociative adsorption rate-limiting reaction pathway when the unique thermodynamic state of the surface was accounted for in the description of the rate law.;Models of varying geometric complexity were developed to investigate under what circumstances precise microstructural details are important and when simplifications may be made. The linear and nonlinear response was simulated using a simplified representation of the 3D microstructure, as pseudo-particles of cylinders and necked spheres, as well as an actual 3D electrode microstructure obtained from FIB-SEM reconstruction. Not surprisingly, it was found that the microstructure had varying degrees of influence depending upon the utilization length of the electrode. When the utilization length was large compared to the dimensions of individual microstructural features, the electrochemical response was accurately accounted for by a 1D model that included the volume-averaged microstructural properties of surface area, porosity and tortuosity. At the opposite extreme, when utilization lengths are less than the dimensions of individual particles, the triple phase boundary length could be used to accurately account for the effect of the dimensionality of the microstructure on the electrochemical response. Between these two regimes exists an intermediate range where the utilization length is on the order of individual particle dimensions. In this intermediate regime, the linear and nonlinear responses were very sensitive to the precise microstructural details of the electrode. However, it is only the near interfacial microstructure that plays a role; a simplified 1D macrohomogeneous description is adequate to describe the remainder of the electrode. While the results of this theoretical analysis motivate the need for precise microstructural details accessible through the FIB-SEM microstructural reconstruction technique, it has been shown that it is not necessary to simulate the entire 3D microstructure to accurately compute the electrode response. (Abstract shortened by UMI.)...