Development of Nonlinear Harmonic Techniques for Studies of Solid Oxide Fuel Cell Cathodes

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AbstractProposal Title: Development of Nonlinear Harmonic Techniques for Studies of Solid Oxide Fuel Cell CathodesProposal Number: CTS- 0412076Principal Investigator: Stuart AdlerInstitution: University of WashingtonA widespread technique for analyzing fuel cell electrodes and other electrochemical devices is electrochemical impedance spectroscopy (EIS), which seeks to separate and identify physical mechanisms via time scale. While EIS has proven useful, it can be challenging to interpret the impedance of complex systems such as fuel cell electrodes due to wide dispersion and/or overlap among physical processes in the frequency domain, as well as inherent ambiguity in interpreting linearized, low-amplitude response (often described by equivalent circuits) uniquely in terms of physical mechanisms. Upon linearization, many models predict similar or identical impedance behavior, and thus do not provide mechanism-distinguishing characteristics to compare (or fit) to experimental data.This project will develop advanced transient electrochemical techniques that potentially provide much higher resolution than EIS in identifying and quantifying rate-limiting phenomena in solid-oxide fuel cell (SOFC) cathodes. The general approach, which has been called nonlinear EIS (NLEIS) and electrochemical frequency modulation (EFM), involves the measurement of nonlinear 2nd and higher order voltage harmonics resulting from moderate amplitude current perturbations. By the excitation of nonlinear responses and interactions, these techniques potentially offer improved identification of physical steps via nonlinearity and extent of coupling, as well as timescale. These techniques will be developed for SOFC electrodes by studying comprehensively characterized cathodes of well-defined geometry and composition. In order to aid the interpretation of nonlinear harmonic data, mechanistic models will be developed and/or extended for the electrode reaction into the time domain, allowing identification of "fingerprint" harmonic behavior corresponding to specific geometries and reaction mechanisms.