Oxygen Surface Exchange and Bulk Diffusion Coefficients Evaluated from Porous Mixed Ionic-Electronic Conducting Cathodes

Abstract

(i) Oxygen Surface Exchange kδ and (ii) Bulk Diffusion Coefficients Dδ of a porous La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) cathode are determined by electrochemical impedance measurements on anode supported solid oxide fuel cells. The cathode polarization resistance of anode supported cells was separated by the method of distribution of relaxation times (DRT) followed by a CNLS fitting approach using a reasonable equivalent circuit consisting of four physical related elements. The values for kδ and Dδ were determined for cathodes fresh from the start of the cell operation up to 700h and compared to literature data on bulk samples. At an operation temperature of T = 750°C the cathode polarization resistance changed substantially with time and ended with an overall increase of 310%. The analysis of the cathode polarization resistance which was described by a Gerischer element revealed a rather constant value for kδ but a distinct decrease for Dδ.