In this study, two mixed electronic ionic conducting (MIEC) cathode materials,
La0.68Sr0.3FeO3−δ
and
La0.68Sr0.3Co0.2Fe0.8normalO3−δ
, are characterized by electrochemical impedance spectroscopy. The chemical surface and diffusion coefficients of both cathode materials are obtained directly from impedance measurements performed on full anode-supported solid oxide fuel cells. This is done by a combined distribution of relaxation times and an equivalent circuit impedance analysis method, which allows a high resolved identification and deconvolution of each single polarization mechanisms contributing to the overall loss of the cell.
Polarization losses within the electrodes are determined both by material composition and microstructure. Analyzing and modelling of electrode-microstructure can help to understand and improve electrodes. In this initial study the use of a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) for the reconstruction of a high performance LSCF-cathode will be illustrated. Opportunities that arise from this technology for microstructure modelling and possible sources of error will be discussed. From the obtained reconstruction data the calculation of microstructural parameters like surface area, volume/porosity fraction or tortuosity is possible. Such parameters can be used to calculate cathode performance via microstructure models found in literature [1-4]. However, it would be desirable to use the reconstructed microstructure directly in a model in order to investigate the interaction of microstructure and performance more accurately. A three-dimensional (3D) finite element method (FEM) model [5] is presented that allows the analysis and prediction of cathode performance. The model has already been validated, and we will show how to overcome simplifications concerning the microstructure by an extension of the model enabling a direct use of 3D FIB/SEM-data.
Mixed electronic/ionic conducting (MIEC) cathode materials are qualified for the application in intermediate-temperature SOFCs. The area specific resistance of MIEC cathodes is likewise determined by chemical composition and microstructure. A three-dimensional finite element method model, which is capable to analyze and predict the performance of MIEC cathodes, was developed. Representations for the actual cathode microstructure are automatically generated according to characteristic measures like cathode thickness, composition, porosity and average particle size. The model calculates the spatial distributions of the oxygen activity, the diffusive oxygen flux in the pores as well as the ionic current density inside the MIEC-cathode and inside the electrolyte material. Based on these values the area specific resistance of the cathode and the penetration depth of the ionic current can be evaluated. This paper will analyze the effect of surface-exchange kO and bulk-diffusion coefficient DO as well as the influence of microstructure properties on the cathode performance.
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