A series of six cathodes Sr5 ,5La0 55MnO, (SLM) on yttria-stabilized zirconia with different morphology of the electrode/electrolyte interface were characterized by ac impedance and dc polarization measurements. It is found that the electrode kinetics at elevated temperature (945°C) are governed by two serial processes. An activation process can be identified to occur at high cathodic overpotential, whereas a transport process competes with charge-transfer at comparatively low overpotential. Attention is drawn to the profound change in the electrocatalytic properties of Sr0 ,5La0 55MnO3 upon current passage and its influence in elucidation of the interfacial kinetics.

The interface microstructure of the state-of-the-art cathode material for solid oxide fuel cells, SrLa1_1MnO3 (SLM), was investigated with respect to its electrochemical performance. The interface microstructure was characterized by grain size and coverage of SLM on the electrolyte surface. Variation of the grain size was obtained by using three different sintering temperatures, whereas variation of the coverage was obtained by using two powders with a different morphology. This resulted in a set of six cathode/electrolyte samples with different combinations of grain size and SLM coverage at the interface. The cathode overpotential, as a measure for the electrochemical performance, could not be related to the length of the three-phase boundary. Based on the constriction resistance occurring in the electrolyte a model was developed which provides an estimate for the width of the active three-phase boundary zone.This zone is most likely to extend outside the cathode particle across the zirconia surface. The width calculated in this way was found to vary in the range of 0.03 to 0.07 p.m for the different electrode microstructures. It is argued that the actual values may be smaller by one or two orders of magnitude. InfroductionFor solid oxide fuel cells (SOFC5) SrLa1_MnO3 is still the most widely used and favored cathode material. The performance of the SLM electrode is strongly determined by its interface microstructure.12 Despite a lot of work carried out on SLM cathodes, there is not much knowledge available from the literature concerning (semi-) quantitative relations between the interface microstructure and electrochemical performance. A better understanding of this topic may lead to a useful tool for the optimization of SOFC cathode electrodes. Therefore, in this study a series of six different cathodes is investigated. The microstructure is varied by: (i) preparing cathodes from as-received powder and from the powder obtained after milling, and (ii) choosing three different sintering temperatures for cathodes prepared from both unmilled and milled powders. Particular attention is paid to the characterization of the microstructure at the interface in terms of particle contact diameter, total contact surface area between the SLM particles and yttria-stabilized zirconia (YSZ), and the three-phase boundary area (TPB). The relation between the microstructure and the electrode kinetics is the subject of Part II of this study.4Experimental Sample preparation-Cathode powder was commercially obtained with composition Sr0 15La085MnO3. The chemical composition of the SLM powders was checked by inductively coupled atomic emission spectroscopy (ICP-AES). X-ray diffraction was carried out to ensure the phase purity of the powder. A Guinier camera with Cu K12 radiation was used. To vary the microstructure the SLM powder batch was divided into two portions. One portion was attrition milled for 5 h, the other one remained unmilled. The particle size distribution of both powders was measured by a light scattering tech...

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To characterize electrical losses of fuel cells or batteries, impedance spectroscopy (IS) or current interruption (CI) can be used. Analysis and parameter determination of impedance data is widely used. The system under study is usually represented by an equivalent circuit from which the system parameters can be determined. However, the analysis of current‐interruption data is often carried out with too simple circuits, e.g., using pure exponential behavior, because analysis in the time domain (CI) is often much more awkward than analysis in the frequency domain (IS). A comparative study has been carried out on the analysis of a relatively ideal electrical circuit, containing a pure capacitor, and a more realistic circuit where the capacitor is replaced by a constant‐phase element. Equations describing the response in the frequency and time domain are presented. Emphasis is put upon the analysis of circuits containing a constant‐phase element because impedance measurements clearly indicate the presence of such an element in many types of experiments, and because there is only limited literature available describing the behavior in the time domain.