Continuum modeling of mixed conductors : a study of ceria

Abstract

In this thesis we have derived a new way to analyze the impedance response of mixed conducting materials for use in solid oxide fuel cells (SOFCs), with the main focus on anodic materials, in particular cerium oxides. First we have analyzed the impact of mixed conductivity coupled to electrocatalytic behavior in the linear time-independent domain for a thick ceria sample. We have derived that, for a promising fuel cell material, Samarium Doped Ceria, chemical reactions are the determining component of the polarization resistance. As a second step we have extended the previous model to the time-dependent case, where we focused on single harmonic excitation, the impedance spectroscopy conditions. We extended the model to the case where some input diffusivities are spatially nonuniform. For instance we considered the case where diffusivities change significantly in the vicinity of the electrocatalytic region. As a third and final step we use to model to capture the two dimensional behavior of mixed conducting thin films, where the electronic motion from one side of the sample to the other is impeded. Such conditions are similar to those encountered in fuel cells where an electrolyte conducting exclusively oxygen ions is placed between the anode and the cathode. The framework developed was also extended to study a popular cathodic material, Lanthanum Manganite. The model is used to give unprecedented insight in SOFC polarization resistance analysis of mixed conductors. It helps elucidate rigorously rate determining steps and to address the interplay of diffusion with diffusion losses. Electrochemical surface losses dominate for most experimental conditions of Samarium Doped Ceria and they are shown to be strongly dependent on geometry.