Effect of Non-Uniform Electrode Microstructure in Gas Diffusion Impedance

Gas diffusion resistance represents a significant contribution to anode overpotential in anode-supported solid oxide fuel cells (SOFC). Gas diffusion limitations are often detected through the analysis of impedance spectra. Typically, equivalent circuits are adopted to fit the measured impedance data. However, equivalent circuits do not account for the non-uniform microstructure of SOFC anodes, which commonly consist of a fine functional layer and a thicker and coarser supporting layer.

In this study, the impedance response of gas diffusion phenomena in anodes with non-uniform microstructure is simulated by solving the dusty-gas model in frequency domain. The microstructural parameters are evaluated from the tomographic reconstruction of the two layers, as published in the literature. Simulations indicate that the non-uniform electrode microstructure affects the gas diffusion dynamics, thus producing an additional feature in the Nyquist plot at high frequency. Such an additional feature may be erroneously attributed to other phenomena (e.g., charge transfer processes) when phenomenological equivalent circuits are superimposed to fit the data.

The study shows that the gas diffusion fingerprint in impedance spectra can substantially differ from the expected finite-length Warburg response in electrodes with non-uniform microstructure. The paper highlights the potential misinterpretations that can arise when interpreting impedance spectra with phenomenological models, especially when the coupling between microstructural characteristics and electrode dynamics are not properly taken into account.