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L5.07 - Analyzing Electrochemical Properties of Mixed Conducting SOFC Anode Materials by a Novel Impedance Spectroscopic Method and Tracer Diffusion 
Date/Time:
April 23, 2014   4:00pm - 4:15pm
 
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Mixed-conducting SOFC anodes may overcome some problems of the currently most used Nickel-YSZ cermets, such as carbon deposition, sulfur poisoning and stability in oxidizing atmosphere. Mechanistic studies on geometrically well-defined model anodes are, however, scarce but crucial for a better understanding of the reaction mechanism and the origin of the polarization resistance. The electronic p-type conductivity of acceptor-doped mixed conductors decreases dramatically in reducing atmosphere. This leads to a complex interplay of charge transport and electrochemical processes that contribute to the electrode impedance, even for geometrically well-defined thin film electrodes. In this contribution, a novel electrode design with applied metallic thin film current collectors was employed to study all relevant charge transport and electrochemical processes of Ce0.8Gd0.2O1.9-δ (GDC) and SrTi0.7Fe0.3O3-δ (STFO) thin films. These films were deposited on single crystalline YSZ substrates by pulsed laser deposition and microelectrodes were obtained from the films by photolithography. Two separated but interdigitating current collectors were applied to each microelectrode, which allows two different electric connection modes for impedance measurements: In one mode, the impedance between the two current collectors on one microelectrode is measured (in-plane measurement). In the other mode, both current collectors are at equal potential, and the impedance is measured against a macroscopic counter-electrode. This second mode is similar to conventional electrochemical impedance spectroscopy. Both measurement modes can be described by transmission line-type equivalent circuits. The impedance spectra of both measurement modes can be fitted in one run with a single parameter set, and all relevant resistive and capacitive processes can be quantified. Electronic and ionic conductivity as well as the area specific resistance of the surface reaction, resistive contributions from the MIEC/electrolyte interface and the chemical capacitance can be determined on one and the same electrode. The local polarization of the MIEC film and therefore the rate of electrically driven oxygen exchange decreases with the distance from the current collector due to limited electronic or ionic conductivity. This could be visualized by bias-driven tracer experiments with 18O enriched water and SIMS analysis and further validates the interpretation of the impedance measurements.
 


 
 
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