Quantifying the Role of Cerium Oxide as a Catalyst in Solid Oxide Fuel Cell Anodes

Abstract

Solid Oxide Fuel Cells (SOFCs) are an important electrochemical power conversion device, due largely to their high efficiencies and ability to directly oxidize a variety of fuels, including hydrogen, carbon monoxide, and light hydrocarbons. Conventional Ni-based SOFC anodes are prone to failure due to carbon deposition or unwanted metal oxidation. For this reason, ceria (CeO2) is being explored to replace or supplement Ni in SOFC anodes. CeO2, a mixed ionic-electronic conductor (MIEC), has been shown to improve SOFC anodes' resistance to carbon deposition and sulfur poisoning. Optimization of ceria-based anodes has proven difficult due to the unknown role of ceria during SOFC operation. The electrochemical mechanisms and reaction rates needed to describe fuel oxidation on ceria anodes are not well understood, and thus it is not clear how to model the coupling between electrochemistry and mass transport in complex SOFC geometries containing ceria. Both Ce4+ and Ce3+ are present during fuel cell operation, and the ionic and electronic conductivities are determined by the abundance of Ce3+. The in situ spatial distribution of valence states, then, is expected to have a major impact on ceria's role in SOFC anodes.

This work aims to describe the fundamental role of ceria in SOFC anodes by building a numerical SOFC model for the electrochemical oxidation of small molecules. Porous-media SOFC models are developed and validated against experimental data, correcting previous errors in transport equations. The thermodynamic and kinetic parameters for such a model are obtained from experimental measurements on thin-film ceria anodes, including electrochemical measurements and novel in situ X-ray photoelectron spectroscopy measurements. Fitting thin-film MIEC model results against experimental data leads to the identification and estimation of several key parameters in the proposed H2 oxidation mechanism, with results demonstrating the importance of charge transfer, bulk oxide diffusion and adsorption reactions at the electrode surface. This provides a basis for modeling porous media composite SOFC electrodes with distributed electrochemistry as demonstrated in this work.