| Mixed ionic and electronic conducting materials (MIECs) comprise the active catalyst in solid oxide fuel cell cathodes, ion transport membranes, and electrically-driven air separation devices. Of particular interest is the mechanism of oxygen reduction and transport pathways in these materials. Understanding these processes is difficult owing to the coupled nature of transport and kinetics in MIECs, and to the inherent limitations of linearized (near equilibrium) measurement techniques. This work presents the development of a nonlinear harmonic measurement technique to investigate these processes involving detection of higher order (nonlinear) harmonics generated by moderate amplitude AC electrochemical perturbations, and comparison of the nonlinear behavior to mathematical models of the physical phenomena governing these materials. Employing Fast Fourier Transforms, this method isolates the linear and nonlinear harmonics to specific frequency bands, and was first validated by application to the rotating disk electrode system, where the measured current harmonics were shown to agree well with theory.; A dense thin film La0.6Sr0.4CoO3-delta (LSC-64) electrode with a thickness of 900 nm, was fabricated to isolate kinetic phenomena. By comparing experimentally measured nonlinear harmonics on the LSC-64 thin film at high temperatures (> 600°C) to predictions from models that adopt various oxygen reduction scenarios, we identified oxygen dissociative adsorption as the rate limiting step in this reaction. This means that oxygen physically adsorbs onto electrode surface, and the reaction is limited by the availability of a second vacancy adjacent to the unstable physically adsorbed intermediate. The detected harmonics were very small (<1% of linear response), and were naturally isolated to appropriate frequency bands with this harmonic technique. This nonlinear behavior would be difficult to distinguish from drift or noise with other techniques. LSC-64 and LSC-82 electrodes with more complex porous microstructures were also examined at high temperatures (> 600°C) to probe transport phenomena. However, disagreement between experiment and the response predicted from a one-dimensional macrohomogeneous porous electrode model was observed. This disagreement is likely due to limitations of the one-dimensional model, or a co-limiting step involved in the oxygen reduction reaction. |
