Study of Reliability of SOFC Cathode Impedances at Applied Overpotentials

In case of measuring separately the behavior of SOFC electrodes the concern of the reliability of impedance results was highlighted by many authors [1-3], especially when overpotentials are applied. A flat thin electrolyte-supported cell in a three-electrode configuration is often investigated, where the RE is deposited in a ring-shape around the WE, being a very convenient geometry from an experimental point of view. The problem arises when the RE is placed in an electrolyte region, which is not equipotential and inaccuracy becomes particularly pronounced when WE and CE are not aligned or have different area specific resistances. In this work LSM cathodes were investigated on a YSZ electrolyte-supported cell, using an electrolyte 3 mm thick. Two types of cells were prepared: the first (CellA) according to the geometric requirements suggested in literature [4]: little WE (diameter 3 mm) aligned to the CE and with equal Rp and time constant; RE co-planar around the WE and placed at a distance greater than three- electrolyte thickness from the WE; the second one (CellB) equal to CellA but with a larger WE (diameter 8 mm). Impedance measurements were carried out both in two- and three-electrode configuration, at OCV and under applied overpotentials (Fig. 1). A preliminary comparison between the results extracted from CellB both at two- and three- electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV conditions. On the other side, when an overpotential is applied, the impedance spectra extracted from CellA and CellB are comparable only at little overpotentials (η < 0.2 V).