Browsing by Author "Maier, Joachim (Prof.)"

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    Oxygen reduction kinetics on mixed conducting SOFC model cathodes
    (2006) Baumann, Frank Stephan; Maier, Joachim (Prof.)
    The kinetics of the oxygen reduction reaction at the surface of mixed conducting solid oxide fuel cell (SOFC) cathodes is one of the main limiting factors to the performance of these promising systems. For "realistic" porous electrodes, however, it is usually very difficult to separate the influence of different resistive processes. Therefore, a suitable, geometrically well-defined model system was used in this work to enable an unambiguous distinction of individual electrochemical processes by means of impedance spectroscopy. The electrochemical measurements were performed on dense thin film microelectrodes, prepared by PLD and photolithography, of mixed conducting perovskite-type materials. The first part of the thesis consists of an extensive impedance spectroscopic investigation of La0.6Sr0.4Co0.8Fe0.2O3 (LSCF) microelectrodes. An equivalent circuit was identified that describes the electrochemical properties of the model electrodes appropriately and enables an unambiguous interpretation of the measured impedance spectra. Hence, the dependencies of individual electrochemical processes such as the surface exchange reaction on a wide range of experimental parameters including temperature, dc bias and oxygen partial pressure could be studied. As a result, a comprehensive set of experimental data has been obtained, which was previously not available for a mixed conducting model system. In the course of the experiments on the dc bias dependence of the electrochemical processes a new and surprising effect was discovered: It could be shown that a short but strong dc polarisation of a LSCF microelectrode at high temperature improves its electrochemical performance with respect to the oxygen reduction reaction drastically. The electrochemical resistance associated with the oxygen surface exchange reaction, initially the dominant contribution to the total electrode resistance, can be reduced by two orders of magnitude. This "activation" of the electrode is permanent in that sense that it remains after the polarisation voltage is switched off. The second part of the work was devoted to the study of this "electrochemical activation effect" by a number of different methods such as impedance spectroscopy, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS) and atomic force microscopy (AFM). By XPS and SIMS it was shown that the strong cathodic polarisation of the microelectrode alters the composition of the near-surface region substantially, i.e. Sr and Co are enriched at the expense of La and Fe. In the third and final part of the thesis, the effect of compositional variations on both A- and B-sites of the perovskite lattice has been investigated. Starting from the reference material La0.6Sr0.4Co0.8Fe0.2O3 the Co/Fe ratio was varied in four steps, and both Co and Fe were replaced by Mn to obtain the standard SOFC cathode material LSM for comparison. Further, the effect of a 10% A-site deficiency was studied, and the element La was replaced by Sm and Ba. It was found, for example, that the variation of the Co/Fe ratio between 0 and 1 does not change the surface resistance at high temperatures significantly, whereas a substitution of La by Sm and especially by Ba enhances the catalytic activity of the surface towards the oxygen reduction reaction considerably. The electrochemical activation effect, on the other hand, is most pronounced for the Co-rich materials of the La0.6Sr0.4Co1-yFeyO3 family, much weaker for the Fe-rich compositions, and virtually not existent for Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF), which is extremely active already in the as-prepared state.