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000863796 1001_ $$0P:(DE-HGF)0$$aErning, J. W.$$b0$$eCorresponding author
000863796 245__ $$aUntersuchungen zur Sauerstoffreduktion an Kathoden für Hochtemperatur-Brennstoffzellen
000863796 260__ $$aJülich$$bForschungszentrum Jülich, Zentralbibliothek, Verlag$$c1998
000863796 300__ $$a145 p.
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000863796 4900_ $$aBerichte des Forschungszentrums Jülich$$v3561
000863796 520__ $$aLanthan-Strontium-Manganite perowskites are the most widespread materials in use for Solid Oxide Fuel Cell cathodes. The electrode reaction taking place, i.e. the reduction of oxygen supplied by air, was investigated by electrochemical means to obtain further knowledge about the electrode processes. The high activation energy of this reaction (200 kJ/mol), preventing lower operation temperatures of the SOFC, was the starting point for the investigation. Quasi steady state current voltage measurements and impedance spectroscopy were perfouned in a three electrode configuration. The electrodes were of circular shape with a diameter of 10 mm. The preparation was made by screen printing as well as Wet Powder Spraying onto plates made of Yttria-stabilized Zirconia. Perowskite powders of varying chemical and stoichiometric composition were used. To obtain higher power densities and, more important, lower apparent activation energies, catalytic layers were added at the interface electrode/electrolyte. Additionally, a less complex system, a model electrode/electrolyte setup made from single-crystal YSZ as electrolyte and gold in liquid and solid state as electrode was developed to create a better defined system. This setup was used to investigate the behaviour of the electrode/electrolyte interface. Reliable, reproducible results could be obtained using either setup. The experimental conditions i.e. oxygen partial pressure, temperature and overpotential were varied in order to determine the kinetic properties of the electrodes. Apparent activation energies, pre-exponential factors, apparent charge-transfer coefficients and electrochemical orders of reaction were calculated from the current-voltage data in order to propose possible reaction steps. The catalytic layer made of palladium lowered the apparent activation energy to about 138 kJ/mol, but lowered the apparent pre-exponential factor as well, thus resulting in current densities one order of magnitude higher than without catalyst. By using a mixture of platinum and palladium, the current densities obtained were even higher, caused by a higher pre-exponential factor. Several electrodes showed a charge-transfer reaction determined behaviour for small cathodic overpotentials (<100 mV). For these potentials the behaviour of the electrodes with additional catalytic layers was dominated by the influence of the catalyst. The apparent electrochemical reaction orders for intermediate temperatures were calculated in the region between 0.4 and 0.8 giving evidence for dissociative adsorption of oxygen. The analysis of the charge-transfer coefficient a and its temperature dependence showed negative values for the entropic part $\alpha^{s}_{c}$. Impedance data gave further evidence for the proposed reaction steps but it was not possible to correlate the time constants with singular reaction steps. All results indicate a complex reaction mechanism involving several rate-determining steps. The use of the model electrode/electrolyte setup made it possible to isolate several reaction steps which are depending on the geometry of the electrode. The combination of all results gave evidence for the formulation of possible reaction mechanisms which were verified by using a complex simulation program which simultaneously fits current-voltage and impedance measurements using a model based on the kinetic analysis of an assumed reaction mechanism. The activation energies computed by the simulation program for single reaction steps are similar to those calculated from specific potential regions for the current potential measurements. Thus the assumption of potential regimes in which specific, different reactions are rate-determining is affirmed.
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