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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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This study demonstrates the significant impact of Cr on the electronic conductivity of a LaNi 0.6 Fe 0.4 O 3 (LNF) porous cathode layer at 800 • C. Vapor transport of Cr-species, originating from a porous metallic foam, and subsequent reaction with LNF, results in a decrease of the electronic conductivity of the LNFlayer. Cr has been detected throughout the entire cross-section of a 16 m thick LNF layer, while Ni, besides its compositional distribution in the LNF layer, has also been found in enriched spots forming Ni-rich metal oxide crystals. Transmission electron microscopy revealed that Cr is gradually incorporated into the LNF-grains, while Ni is proportionally expelled. Electron diffraction performed in the center of a sliced grain showed the initial rhombohedral crystal structure of LNF, whereas diffraction performed close to the edge of the grain revealed the orthorhombic perovskite crystal structure, indicating a Cr-enriched perovskite phase. Progressive Cr deposition and penetration into the LNF grains and necks explains the electronic conductivity deterioration. The impact of Cr-poisoning on the electronic conductivity of the LNF porous layer is considerably smaller at 600 • C than at 800 • C.