CERAMIC AND COMPOSITE ANODES FOR HYDROCARBON-FUELED INTERMEDIATE TEMPERATURE SOLID OXIDE FUEL CELLS

Abstract

The operation of solid oxide fuel cells (SOFCs) using hydrocarbon fuels at temperatures from 500-650 °C has been studied in this dissertation, with a focus on tailoring the fuel electrode (anode) to overcome challenges at these lower operating temperatures. The parameter space for SOFC operation on hydrogen and hydrocarbon fuels is calculated using thermodynamic methods to find equilibrium conditions. The interrelation between parameters that determine cell efficiency and stability, such as the Nernst open circuit voltage, fuel utilization, and tendency to form solid carbon as a reaction product are explored. Methane fueling is found to have a more consistent voltage than hydrogen at high fuel utilizations at temperatures below 650 °C, which correlates with increased theoretical efficiency. Reformed methane and jet fuel compositions that prevent carbon deposition are identified. SOFCs with nickel - gadolinia-doped ceria (GDC) cermet anodes are fabricated and studied using hydrogen and reformed hydrocarbon fuels to determine the kinetic limitations to SOFC operation at 650 °C and below. Operating SOFCs on reformed hydrocarbons gives comparable performance to using hydrogen as a fuel, with as little as a 5% decrease in maximum power density (MPD). Stable performance of 220 mW/cm2 while fueling with reformed jet fuel at 550 °C is observed. Porous ceramic anodes fabricated from GDC are developed to prevent the mechanical damage caused by volume changes of Ni metal in conventional, randomly-mixed cermet anodes. The mechanical properties of the porous ceramics are extensively characterized, and the electrical properties of the scaffold anode are measured after infiltrating with metals. The porous ceramics attain strengths of >100 MPa, and have similar conductance to Ni-GDC cermet anodes but with ~70% less metal. SOFCs using these anodes have a MPD within 15% of cermet anode cells in H2, but show stable operation at 120 mW/cm2 at 600 °C on methane for >72 hours while cermet anodes are destroyed by solid carbon formation. A mixed-conducting, single-phase material based on BaCeO3 was developed for possible use as a metal-free anode. The stability of the material in CO2 was improved through Zr and Nb doping on the B-site.