Characterization and Modeling of La{1-x}Sr{x}CoO{3-delta} Solid Oxide Fuel Cell Cathodes Using Nonlinear Electrochemical Impedance Techniques

Abstract

Solid Oxide Fuel Cells (SOFCs) provide a highly efficient means of chemical to electrical energy conversion, and as such they represent a desirable bridge between current heavy reliance on fossil fuel energy and expanded roles of clean alternative energy sources. There are however significant challenges preventing the wide spread commercialization of SOFCs. Of primary concern is the high costs associated with high temperature operation, and reduced performance in the desired intermediate temperature regime. Mixed ionic and electronic conducting materials, such as La_1-xSr_xCoO_3-δ, offer increased performance for oxygen reduction due to their ability to transport oxygen ions and extend the active region of the electrode beyond the electrode/electrolyte interface. This work primarily explores two aspects of SOFC cathode performance (using the mixed conductor La_1-xSr_xCoO_3-δ as a model electrode material) the affects of inhomogeneous cation compositions on the surface exchange rate and the effects of humidity on electrode performance and behavior. Inhomogeneous compositions are explored by studying the electrochemical response of well characterized highly-crystalline, thin-film, microelectrodes using both linear and nonlinear electrochemical spectroscopy. A dual surface/dual bulk compositional model is used to characterize the main affects of both lateral (parallel to the surface) and axial (perpendicular to the surface) compositional variations. Humidity effects are also studied using linear and nonlinear electrochemical impedance spectroscopy, but on porous electrodes, for which a 1-dimensional, dual transport, altered surface thermodynamic model is used to characterize the observed effects. Finally a new electrochemical technique called Simultaneous Chemical and Electrochemical Impedance Spectroscopy (SCEIS) is proposed which uses dual perturbations to elicit more detailed information about currently indistinguishable co-limiting processes occurring within mixed-conducting porous electrodes. Initial experimental work on assessing the feasibility of such an experiment is reported.