Model Development for Gadolinia- Doped Ceria-based Anodes in Solid Oxide Fuel Cells

Title of Document: MODEL DEVELOPMENT FOR GADOLINIADOPED CERIA-BASED ANODES IN SOLID OXIDE FUEL CELLS Lei Wang, Doctor of Philosophy, 2014 Directed By: Professor Gregory S. Jackson, Department of Mechanical Engineering Intermediate temperature (500 700 °C) solid oxide fuel cells (IT-SOFCs) with gadolinia-doped ceria (GDC) electrolytes have significant commercial potential due to reduced materials costs for seals and interconnect and improved performance with high oxide-ion 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-SOFC applications. Simultaneous electrochemical characterization and X-ray photoelectron spectroscopy of thin-film Ni/GDC and Au/GDC electrochemical cells provide a basis for understanding pathways for H2 and CO electrochemical oxidation as well as H2O 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 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 ITSOFC anodes. A detailed kinetic model for the GDC surface reactions and Ni/GDC charge-transfer reactions of H2 oxidation and H2O 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-film Ni/GDC electrode. A full threedimensional IT-SOFC stack model is developed with simplified kinetics to evaluate GDC-based anode performance with H2 and methane-derived 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-SOFCs stack and guiding ITSOFC design. Temperature distribution in non-isothermal model result suggests that internal CH4 reforming can be used as an effective thermal management strategy to maintain high current densities and cell voltages and to lower risk to thermomechanical degradation. MODEL DEVELOPMENT FOR GADOLINIA-DOPED CERIA-BASED ANODES IN SOLID OXDE FUEL CELLS