On Impedance Measurements Suitability of Electrolyte Supported SOFCs

The suitability of impedance measurements in SOFCs is an important concern, especially in case of measuring separately the behaviour of one of the electrode when an overvoltage is applied. In this case a thin electrolyte-supported cell with the RE coplanar with the WE is experimentally convenient, but many authors highlighted that incorrect results can be obtained if an inappropriate geometric configuration is used. In this work LSM cathodes were investigated in 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: little WE (diameter 3 mm) aligned to the CE and with equal Rpol and time constant; RE co-planar around the WE and placed at a distance greater than three-electrolyte thicknesses from the WE; the second one (CellB) equal to CellA but with a bigger WE (diameter 8 mm). Impedance measurements were carried out both in two- and three-electrode configuration, at OCV and under applied overpotentials. A preliminary comparison between the results extracted from CellB at two- and threeelectrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV and small overpotentials. On the other side, when overpotential higher than 0,2 V is applied, a comparison between the impedance spectra from CellA and CellB gives quite different results. The investigation here presented considers also an experimental approach useful for the comprehension of the main phenomena governing the kinetic of the process.