Experimental approach for the study of SOFC cathodes

The suitability of impedance measurements in Solid Oxide Fuel Cells (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 (Reference Electrode) coplanar with the WE (Working Electrode) 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 ((La0.8Sr0.2)MnO3-x) were investigated in a Yttria-stabilised Zirconia (YSZ) electrolyte-supported cell, using an electrolyte 3 mm thick. Two types of cells were prepared: the first (Cell1) according to the geometric requirements suggested in literature: little WE (diameter 3 mm) aligned to the CE (Counter Electrode) and with equal Rpol(polarisation resistance) 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 (Cell2) equal to Cell1 but with a bigger WE (diameter 8 mm). Impedance measurements were carried out both in two-and three-electrode configuration, at OCV (Open Circuit Voltage) and under applied overpotentials. A preliminary comparison between the results extracted from Cell2 at two- and three-electrodes confirms that a thick electrolyte allows extracting suitable three-electrode impedance results in case of OCV and small overpotentials. On the other side, when an overpotential over 0.2 V is applied, a comparison between Cell1 and Cell2 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.