Identification and modelling of the oxygen gas diffusion impedance in SOFC porous electrodes: application to Pr2NiO4+δ

An in-depth analysis of the very low frequency impedance arcs observed when measuring efficient solid oxide fuel cell electrodes by electrochemical impedance spectroscopy (EIS) is reported in this paper. The study was carried out on Pr2NiO4+delta//Ce0.8Gd0.2O2- (delta)//3 mol.% Y2O3-ZrO2 symmetrical half-cell. In the temperature range 500-900 degrees C, three impedance arcs related to O-2 molecular diffusion were distinguished from the EIS measurements. Based on theoretical calculations using the Adler-Lane-Steele (ALS) and Dusty gas models, the arc at highest frequencies was ascribed to the diffusion of O-2 in the porous structures of the electrode and collecting gold grid. It obeys the ALS model, i.e. a parallel R//C impedance with a capacitance coming mainly from the solid phase. The second arc at medium frequencies was ascribed to the diffusion of O-2 in the porous structure of the ceramic part used to maintain the gold grid. It follows the Dusty gas model, i.e. a Warburg impedance with relaxation time depending on the gas phase properties. Finally, the third one at lowest frequencies was ascribed to the "gas conversion" phenomenon, coming from a difference in the local pO(2) above the active sites of the working and counter electrodes. This gas conversion impedance largely increases when clogging the channels of the gas distribution system. (C) 2015 Elsevier Ltd. All rights reserved.