On Impedance Measurements Suitability of Electrolyte Supported SOFCs

The suitability of electrochemical impedance spectroscopy measurements (EIS) 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 cathodic overpotentials (OCV to 400 mV). A preliminary comparison between the results extracted from CellB at two- and three- electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV and small overpotentials (up to 150 mV). On the other side, when an overpotential over 150 mV is applied, a comparison between CellA and CellB gives quite different results. This study considers also an experimental approach useful for the comprehension of the main phenomena governing the kinetic of the oxygen reduction process at an LSM electrode, in view of a mechanistic model under study for the simulation of impedance spectra from a mathematically reconstructed electrode.