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

A method is evaluated that determines the chemical oxygen surface exchange k(delta) and chemical bulk diffusion coefficient D-delta of mixed ionic-electronic conducting La0.58Sr0.4Co0.2Fe0.8O3 (-) (delta) (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 k delta and D-delta 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 k(delta) and D-delta values for porous ISCF 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 degrees C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both k(delta) and D-delta. At T = 750 degrees C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for k(delta) but a distinct decrease for D-delta. For T = 900 degrees C the cathode polarization resistance increased only marginal, and so k(delta) and D-delta remained constant (C) 2014 Elsevier B.V. All rights reserved.