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

摘要

(i) Oxygen Surface Exchange k~δ and (ii) Bulk Diffusion Coefficients Ds of a porous La_0.58Sr_0.4Co_0.2Fe_0.8O_3-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 complex nonlinear least square (CNLS) fitting approach using a reasonable equivalent circuit consisting of four physical related elements. The values for k~δ and D~δ 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℃ 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 k~δ but a distinct decrease for D~δ.