Characterisation of composite SOFC cathodes using electrochemical impedance spectroscopy

Porous composite cathodes for solid oxide fuel cells (SOFCs) made from a mixture of lanthanum strontium manganite (LSM) and yttria stabilized zirconia (YSZ) are investigated using electrochemical impedance spectroscopy (EIS) at temperatures between 850° and 1000 °C. The aim of the investigations is to determine the total polarization resistance and to identify the mechanism of the oxygen reduction process. A minimum of two overlapping arcs can be distinguished in the impedance spectra. It is, however, difficult to correlate these arcs to specific elementary processes in the oxygen reduction process, as the measured impedance is very sensitive to test circumstances. The standard resistor settings (i.e., current measurement range) on the measurement equipment has been found to affect the measured impedance. Furthermore, the electrode performance is influenced by the previous electrochemical history of the specific electrode. Finally, the reproducibility of electrode characterization as evaluated from obtained impedance spectra is examined. The consequence of these problems is discussed. Two cell geometries are compared: a symmetrical two electrode geometry suitable for open circuit voltage measurements with good reproducibility and a three-electrode geometry for polarized measurement with a less convincing reproducibility.