Interpreting impedance spectra of thin-film electrochemical cells: a two-dimensional numerical modeling study

Hertz, Joshua L.Thin film solid state electrochemical systems have numerous applications in clean/alternative energy technologies including fuel cells, batteries, advanced solar cells, and other devices. However, enhancing the performance of such devices is often hindered by an incomplete understanding of the underlying electrochemistry. Here, two-dimensional continuum numerical modeling was used to simulate the electrochemistry of thin film cells with a goal of aiding the researcher in interpreting experimental results for such cells—particularly electrochemical impedance spectroscopy (EIS). EIS is often useful in understanding the electrochemical processes governing a cell’s overall performance, but EIS spectra can be misleading and difficult to interpret. ☐ Three related models were developed to clarify various aspects of the thin film cell behavior. First, the effect of grain boundary heterogeneity in polycrystalline electrolytes was studied by simulating grain boundaries with randomly assigned conductivity and/or permittivity according to pre-chosen distributions. The study revealed that EIS is capable of characterizing the mean grain boundary conductivity within 30% using a simple equivalent circuit model, and estimating the variation between individual grain boundaries is also possible in some cases. ☐ In the second part of the study, the electrolyte model was extended to also include a phenomenological description of the electrode reaction based on the Butler-Volmer equation. The model was validated by fitting previously reported impedance spectra obtained for Pt/YSZ and Pt/GDC cells and was able to reproduce all salient features. Further, the model was able to capture a previously unexplained intermediate frequency arc seen in the experimental results. A parametric study enabled 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. ☐ In the last part of the work, a physicochemical model based on the Poisson-Nernst-Planck equations was developed for the Pt/YSZ interface. The model included adsorption/desorption, diffusion, and charge transfer, and was able to produce qualitatively realistic impedance spectra matching those obtained from the phenomenological model. A parametric study demonstrated that the physical processes considered may all influence the overall impedance, and the idea of a single limiting step at the Pt/YSZ interface is inaccurate. Practical upper bounds on the reaction rate constants were established based on the model results. With enhanced computing power, necessitated by the computational cost of full-scale modeling, the model will be of great use in better interpreting the experimentally obtained impedance spectra.University of Delaware, Department of Mechanical EngineeringPh.D