Impedance Spectra of Composite Electrodes

This section presents two examples of composite cathodes for SOFC. For these systems, the most commonly used cathode material and solid electrolyte are LSM and YSZ, respectively. Electrode impedances appear over a small space scale, characteristic of atomic dimensions, and as a result their spectra differ from those of bulk materials in several ways listed below. 

Their properties are dependent on both the electrolyte and the electrode material and on the microstructure (grain size, porosity, etc). 

The associated capacitances are of the order of 10 Fcm or more when they arise from processes other than charge storage at the double layer. 

Their impedances are in general nonlinear, i.e. dependent on the ampUtude of the applied ac signal and the dc bias. Measurements are performed at small signal amplitudes in order to stay within the linear regime. 

Electrode reactions usually involve a gas phase and, therefore, electrochemical impedances will depend on the gas composition and transport within this phase. 

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