Study of the electrode polarization resistance of cobaltites with high Ba content as cathode for IT-SOFC

Abstract

The phase relationships and the electrochemical behavior of cobaltites with high content of Ba, such as, La1-xBaxCoO3-δ with x = 0.5, 0.7 and 1.0, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) and BaCo0.7Fe0.2Nb0.1O3-δ (BCFN), were studied by XRD and impedance spectroscopy varying the electrode configurations from a single porous layer to a graded-cathode using Ce0.9Gd0.1O1.95 (GDC) as electrolyte. Regardless of the electrode material, symmetrical cells with graded-cathodes displayed the lowest polarization resistance (Rp) values. The minimum Rp values were found to be 0.036 and 0.039 Ω cm2 at 600°C, in air, for La0.5Ba0.5CoO3-δ (LBC) and BSCF, respectively. The impedance spectra reveals mainly two processes, one at intermediate frequency (IF) in the range (0.1 ≤ f ≤ 5 × 103 Hz) and another at low frequency (LF) in the range (1 ≤ f ≤ 3 Hz). The variation of the parameter n (obtained from the log(RIF) vs. log(pO2) plots) in relation with the electrode configuration, indicates the rate-limiting process goes from a mixture of the ion transfer at the electrode/electrolyte interface and the charge transfer at the electrode surface to a mixture between this last process and the dissociation of the oxygen molecule.