Model Development for Gadolinia-doped Ceria-based Anodes in Solid Oxide Fuel Cells
Jackson, Greg S
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Intermediate temperature (500 &ndash 700 °C) solid oxide fuel cells (IT&ndashSOFCs) with gadolinia&ndashdoped ceria (GDC) electrolytes have significant commercial potential due to reduced materials costs for seals and interconnect and improved performance with high oxide&ndashion conductivity at these temperatures. As an SOFC anode component in the reducing anode environments, GDC offers enhanced catalytic activity and tends to suppress carbon deposition in composite Ni/GDC anodes. The current study investigates relevant kinetics on GDC anodes for IT&ndashSOFC applications. Simultaneous electrochemical characterization and X&ndashray photoelectron spectroscopy of thin&ndashfilm Ni/GDC and Au/GDC electrochemical cells provide a basis for understanding pathways for H<sub>2<sub> and CO electrochemical oxidation as well as H<sub>2<sub>O splitting on GDC and GDC composite electrodes. Differences in electrochemical performance of Ni/GDC and Au/GDC electrodes at temperatures below 650 °C reveal limitations of GDC surfaces in promoting electrooxidation under conditions of low polaron (electron) mobility. These results also suggest the role of the metal in promoting hydrogen spillover to facilitate change transfer reactions at the Ni/GDC interface. Variation in OH<super>-<super> concentration at the metal/GDC interface with operating temperature, effective oxygen partial pressure, and electric bias provides valuable insight into the nature of electrochemical and other heterogeneous reactions in IT&ndashSOFC anodes. A detailed kinetic model for the GDC surface reactions and Ni/GDC charge&ndashtransfer reactions of H<sub>2<sub> oxidation and H<sub>2<sub>O electrolysis is developed based on electrochemical characterization and spectroscopic analysis of GDC surface electrochemistry. The thermodynamically consistent kinetic model is able to capture the observed chemical and electrochemical processes on the thin&ndashfilm Ni/GDC electrode. A full three&ndashdimensional IT&ndashSOFC stack model is developed with simplified kinetics to evaluate GDC&ndashbased anode performance with H<sub>2<sub> and methane&ndashderived fuels. The stack model explores the effects of operating condition on performance of stacks with GDC electrolytes and Ni/GDC anodes. The parametric study results of stack model provide essential information for optimizing performance of IT&ndashSOFCs stack and guiding IT&ndashSOFC design. Temperature distribution in non&ndashisothermal model result suggests that internal CH<sub>4<sub> reforming can be used as an effective thermal management strategy to maintain high current densities and cell voltages and to lower risk to thermo&ndashmechanical degradation.