The chemical oxygen surface exchange and bulk diffusion coefficient determined by impedance spectroscopy of porous La0.58Sr0.4Co0.2Fe0.8O3 − δ (LSCF) cathodes

Abstract

A method is evaluated that determines the chemical oxygen surface exchange kδ and chemical bulk diffusion coefficient Dδ of mixed ionic–electronic conducting La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) by using electrochemical impedance spectroscopy. Each measured spectrum contains the Gerischer impedance, which represents the polarization characteristics of a porous LSCF cathode structure. Firstly, it was separated from the impedance data by a well-established equivalent circuit model. Second, the specific values for kδ and Dδ were calculated from the Gerischer impedance using the ALS (Adler, Lane, Steele) model. Third, the corresponding microstructure parameters, porosity, surface area and tortuosity, were quantified by focused ion beam (FIB) tomography. This allows a consideration of the actual sample characteristics. This approach was applied, for the first time, to follow the time- and temperature-dependent course of kδ and Dδ values for porous LSCF cathode structures, from the very beginning of cell operation up to several hundred hours. The microscopic structure characteristics remain constant, as indicated by scanning electron microscope analysis and proven by FIB tomography before and after measurements. At T=600°C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both kδ and Dδ. At T=750°C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for kδ but a distinct decrease for Dδ. For T=900°C the cathode polarization resistance increased only marginal, and so kδ and Dδ remained constant.