Materials Development for Solid Oxide Fuel Cells Using Qualitative Models
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Materials Development for Solid Oxide Fuel Cells Using Qualitative Models Klaus Schmid1 , Volker Krebs1 , Albert Kr¨ ugel2 , Ellen Ivers-Tiff´ee2 , Sven Sch¨afer2 1 Institut f¨ ur Regelungs- und Steuerungssysteme, Universit¨at Karlsruhe (TH), Kaiserstraße 12, D-76131 Karlsruhe, Germany 2 Institut f¨ ur Werkstoffe der Elektrotechnik, Universit¨at Karlsruhe (TH), Kaiserstraße 12, D-76131 Karlsruhe, Germany ABSTRACT Solid Oxide Fuel Cells (SOFC) are high temperature energy converters. When an SOFC is operated for the first time, a substantial increase in cell performance is observed which is caused by microstructural changes at the cathode/electrolyte interface. To optimize the resulting formation of the interface, a dynamic model is required that represents the relations between materials compositions, operating conditions, electric current, and microstructure. Building a model based on chemical reaction equations fails because of the high complexity of the interface reactions. Therefore, this contribution presents an interdisciplinary approach to modeling in materials development by applying computational intelligence techniques. Qualitative models are used to formalize the expert knowledge about the irreversible materials changes at the cathode/electrolyte interface. Fuzzy if-then rules represent the dynamic behavior of the microstructural formation. The resulting model enables the application of simulations instead of time-consuming experiments and thus allows the systematic optimization of the startup process. INTRODUCTION SOFCs produce electricity by direct oxidation of hydrogen or hydrocarbon fuels. In contrast to fuel cells with liquid electrolytes, the all-ceramic SOFC requires high operating temperatures (800-1000 ◦ C) to provide a sufficient ionic conductivity of the ceramic electrolyte material. High temperature operation enables a high catalytic activity of the electrode/electrolyte interfaces. Therefore, the electrochemical energy conversion proceeds at high electrical efficiency and almost zero emissions. SOFCs have been suggested for distributed stationary electrical power generation as well as for use in automotive auxiliary power units. At present, most SOFC systems use a standard composition of materials: yttria-stabilized zirconia (8YSZ) as the solid electrolyte, strontium-doped lanthanum manganite as the cathode, and a composite of the electrolyte material and nickel, Ni/8YSZ cermet, as the anode. Current SOFC materials research activities focus on the further improvement of the cell’s electrical performance, the increase of long term stability, and the reduction of the operating temperature. On the one hand, new materials are being developed. These alternative materials improve cell performance, but they also pose new problems [1]. Therefore, on the other hand, the electrode/electrolyte interfaces of cells with standard materials are investigated. An optimization of the interface microstructures allows to increase the cell performance as well, because the electrochemical losses during operation ar
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