Effects of Inadequate Current Collection on Impedance of Solid State Electrodes

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Impedance measurements of SOEC, SOFC, and other high temperature solid state electrodes often employ a current collector to insure a laterally homogeneous current density. During recent measurements of H2O and CO2 reduction on rare-earth doped ceria electrodes, we found that inadequate current collection leads to distortion of the impedance response. This distortion not only involves quantitative inaccuracies due to reduced active area, but also changes in the shape and frequency response of the electrodes. In order to better understand these results, we have developed a general model, based on frequency-resolved finite element analysis, for the impacts of finite current collection on impedance. This model predicts a number of surprising results, including severe distortions particularly in the high frequency portion of the spectrum, appearance of apparent Warburg features when none is present in the actual electrode response, shifts in characteristic resistance and frequency of the electrodes, and misattribution of electrolyte resistance to electrode response. This model may help explain a number of seemingly anomalous impedance features often reported by authors.