The chemical oxygen surface exchange and bulk diffusion coefficient determined by impedance spectroscopy of porous La sub(0.58)Sr sub(0.4)Co sub(0.2)Fe sub(0.8)O sub(3 - delta ) (LSCF) cathodes

A method is evaluated that determines the chemical oxygen surface exchange k super( delta ) and chemical bulk diffusion co-efficient D super( delta ) of mixed ionic-electronic conducting La sub(0.58)Sr sub(0.4)Co sub(0.2)Fe sub(0.8)O sub(3- delta ) (LSCF) by using electrochemical impedance spectroscopy. Each measured spectrum contains the Genscher 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 super( delta ) and D super( delta ) were calculated from the Genscher 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 super( delta ) and D super( delta ) 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 [degrees]C the cathode polarization resistance changed substantially with time, which is associated with a strong decrease of both k super( delta ) and D super( delta ). At T = 750 [degrees]C the cathode polarization resistance changed nonlinearly resulting in a rather constant value for k super( delta ) but a distinct decrease for D super( delta ). For T = 900 [degrees]C the cathode polarization resistance increased only marginal, and so k super( delta ) and D super( delta ) remained constant.