FUNDAMENTAL UNDERSTANDING OF SOFC CATHODE DURABILITY; A KINETICS AND CATALYSIS STUDY
Wachsman, Eric D
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Solid oxide fuel cells (SOFC) have been demonstrated as great prospects for electrochemical conversion of fuels, providing both high efficiency and high power density. Understanding the fundamentals of the oxygen reduction reaction (ORR) mechanisms is necessary to further improve cathode performance. Two different testing systems, gas phase isotopic oxygen exchange and electrical conductivity relaxation, were built to study the kinetics of cathode powders and bulk samples, respectively. A robust strategy was established to extract kinetic parameters from transient response curves for a variety of materials and conditions using numerical solutions. In-situ gas phase isotopic oxygen exchange, which provides real-time information about cathode surface kinetics, was used to determine the ORR mechanisms and the interactions of other gaseous species with the solid surface for two cathode materials: La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and (La0.8Sr0.2)0.95MnO3±x (LSM). LSCF has a faster dissociation reaction than LSM, and the limiting step is the surface exchange. Additionally, LSM likely contains different vacancy concentrations in the near surface region and in the bulk. A mathematic model is further established to unify surface exchange rates from different experiments and link solid-state diffusion to surface heterogeneous catalysis. In addition, the long-term durability of these materials is a major challenge. A novel technique called isotope saturated temperature programmed exchange (ISTPX) has been developed to determine the temperature and PO2 range that is preferable for the exchange of water and CO2 on LSM and LSCF. The presence of CO2 and water indicates blocking effects on the LSCF surface from 300°C to 600°C, possibly resulting in two separate degradation mechanisms. On the other hand, CO2 and water exchange with LSM through homoexchange mechanism with a relatively minor impact. Based on isotope exchange results, surface modified LSCF cathodes were fabricated. The surface modification of LSCF through Mn ion implantation enhances the chemical surface exchange coefficient (kchem) from 4.4x10-4 cm/s to 1.9x10-3 cm/s at 800°C. The aims of this study are to increase knowledge and information about the ORR. The results allow us to further investigate the ORR mechanisms as well as to engineer new cathode materials/structures that can improve cathode performance and durability.