AC Impedance in Characterization of SOFC and Interpretation of a Low Frequency Inductive Loop

AC Impedance has been performed on small (2 cm²) button cells at various conditions, which were fabricated with anode and cathode reference electrodes to facilitate separate testing of anode and cathode. The impedance behavior of anode, cathode and the full button cell are analyzed to evaluate the propriety for separation anodic and cathodic losses. Equivalent circuit models are derived from the impedance data and validated by changing cell temperature, current densities and reactant concentrations. The degree to which electrode kinetics and mass transfer limit cell output may be evaluated from the fitted resistances, capacitances, inductances and their values of the circuit elements. A small but significant inductive loop at low frequency was found in the impedance diagram. An equivalent circuit model of possible electrode response is derived to provide explanation for the inductive loop. The presented work is useful for performing stack maintenance and diagnostics and for designing process controls and power conditioning circuitry.