Study of La0.6Sr0.4Co1-xFexO3-δ (x = 0.2 & 0.8) Electrochemical Response as SOFC Cathodes and Its Relation with Microstructure

Abstract

This work presents the study of the O2-Reduction Reaction (ORR) by electrochemical impedance spectroscopy of La0.6Sr0.4Co0.8Fe0.2O3-δ and La0.6Sr0.4Co0.2Fe0.8O3-δ cathodes as a function of temperature and pO2. The combination of the impedance data, modeled with a Transmission Line Model, with the microstructural data obtained by FIB-SEM tomography, allowed to obtain and compare the chemical diffusion coefficients (Dchem), O2 equilibrium molar exchange rates (R0) and the oxygen surface exchange rates (kchem) for both compounds. The obtained values were, at 700°C in air, Dchem = 5.4.10-7 cm2 .s-1 and kchem = 1.4.10-6 cm. s-1 for La0.6Sr0.4Co0.2Fe0.8O3-δ, while Dchem = 2.6.10-6 cm2 .s-1 and kchem = 3.1.10-6 cm. s-1 were obtained for La0.6Sr0.4Co0.8Fe0.2O3-δ. The detailed analysis of these parameters as a function of pO2 (10-4 < pO2 ≤ 1) and temperature (500°C ≤ T ≤ 700°C) by means of the Adler-Lane-Steele model, adapted to a finite length porous electrode, allowed identifying the O-ion diffusion and surface exchange as processes co-limiting the ORR. From this analysis, a predominantly surface limited ORR was found for La0.6Sr0.4Co0.2Fe0.8O3-δ, changing to a more bulk limited ORR for La0.6Sr0.4Co0.8Fe0.2O3-δ, which has higher oxygen-vacancy concentration.