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The extreme shape factors inherent in characterizing thin film electrolytes can present a challenge to quantitative interpretation of impedance spectra. Here, the impedance of a thin film ceramic electrolyte with surface microelectrodes is modeled via direct numerical solution of current conservation. Faradaic and non-faradaic currents at the electrode-electrolyte interface are modeled phenomenologically using a formulation based on the Butler-Volmer equation. The model is able to reproduce complex, experimentally obtained impedance spectra for Pt/YSZ and Pt/GDC cells using only four adjustable, physically intuitive parameters: electrolyte conductivity, permittivity, exchange current density, and double layer capacitance. Equivalent circuit models typically used to fit these spectra instead require six or more adjustable parameters with ambiguous physical meaning. Notably, the model described here is able to capture a heretofore unexplained intermediate frequency arc seen in the experimental results. A parametric study enables the mechanism of the intermediate frequency feature to be identified as a spreading resistance in the electrolyte that vanishes at high frequencies due to low-impedance dielectric transport of current across the electrode-electrolyte interface. The fitting results are validated by comparison of the parameter values with literature reports.

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Numerical modeling has been used to investigate the resistive behavior of a novel hybrid polymer electrolyte fuel cell design that utilizes a porous wicking layer to eliminate flooding. The resistance of the porous layer is shown to depend upon current and to increase nonlinearly with layer thickness. The relationship between conductivity and thickness was found to be related to the physical size of the reaction zone in the porous layer, and the scaling of the reaction zone with current was investigated. The porous layer resistance was also found to increase with temperature in most cases due to a decrease in charge carrier concentration at elevated temperatures.