Kinetic Modeling and Simulated Performance of CO and Syngas Electro-Oxidation in Ni-YSZ Solid-Oxide Fuel Cells

The use of solid-oxide fuel cells for the conversion of gasified solid carbon is receiving a great deal of interest, and they have long been considered for the conversion of hydrocarbon reformates.  Regardless of the original fuel source, the major species involved in current-producing electrochemical reactions at three-phase interfaces are likely to be hydrogen and carbon monoxide.  One or both of these species may be present and participating in charge transfer and reforming depending on operating conditions, catalyst selection, system design, and so on.  While the elementary steps of H₂ electrochemical conversion in Ni-YSZ anodes have been known for almost a decade, the reaction kinetics are only now coming to light.  The steps governing electro-oxidation of CO, on the other hand, are not yet agreed upon.  It is important that we understand the conversion pathways of H₂ and CO individually, so that SOFC systems reliant on one of these species can be better understood and optimized.  In this work, a mathematical and computational model is developed around the framework of a reduced mechanism describing electrochemical oxidation of CO and syngas mixtures on Ni-YSZ anodes.  We analyze the most likely species to exist on the catalyst and electrolyte surfaces, the interactions between them, and different charge-transfer pathways in the context of steady-state and transient models.  The most likely mechanisms are subsequently included in a homogenized porous-electrode model to compare simulated performance and polarization characteristics to measured results in SOFC button-cell experiments using CO-CO₂ and H₂-H₂O-CO-CO₂ mixtures.