Competition between bulk and surface pathways in mixed ionic electronic conducting oxygen electrodes

Two parallel pathways have been identified for oxygen reduction and evolution on mixed ionic electronic conductors (MIECs). Charge transfer may proceed by reaction at the triple-phase boundary to consume or produce oxygen adsorbed on the surface of the MIEC. Alternatively, charge transfer may proceed by reaction of oxygen with oxygen vacancies and electronic holes from the bulk of the MIEC with subsequent exchange of vacancies with the purely ionic conducting electrolyte. A continuum mass-transport model is developed describing the competition between these two charge-transfer pathways. Key to the model is treatment of the boundary condition at the MIEC-electrolyte interface. The absolute potential across this interface is used to relate the surface overpotential of each pathway in terms of the other and terms arising from the concentration overpotentials. The current attributable to each of the two pathways can be determined. Results show that the two pathways have different dependence on cell overpotential and competition for reagents creates complex mass-transport dependencies. One pathway may dominate at low overpotentials and the other at high overpotentials. The dependence of current on oxygen partial pressure is shown to transit from +1/2 power dependence to zero or negative power dependence as pressure increases, or to transit from zero or negative to +1/2 power depending on which pathway dominates. (C) 2003 The Electrochemical Society.