(768e) Nonlinear Analysis of Rate Controlling Mechanisms of Solid Oxide Fuel Cell Cathodes

Most potential losses in intermediate temperature (500-700 C) solid oxide fuel cells stem from the cathode. To engineer better SOFC cathodes, the processes limiting performance must be identified and linked to specific material properties and morphological features. Transition metal perovskite oxides, such as La_xSr_1-xCoO_3-δ are a promising class of materials that possess good electronic and ionic conductivity and catalyze the oxygen reduction reaction in the target temperature range.

In order to gain insight on the physical processes governing cell performance, we have used nonlinear electrochemical impedance spectroscopy (NLEIS) to probe the nonlinear harmonic current-voltage characteristics of a La_xSr_1-xCoO_3-δ electrode. Harmonic spectra acquired over a range of conditions were compared to a theoretical model that considers the relative roles of material thermodynamics, surface exchange reaction and ion diffusion within a cylindrical electrode particle. For a Strontium dopant content of 0.4, a comparison of the theoretical and experimental nonlinearities suggests that the cell behavior is not consistent with a purely bulk-diffusion limited mechanism. This result is contrary to linear impedance observations and suggests that the nonlinear behavior could be influenced by an alternative surface diffusion mechanism or material thermodynamics.