Mechanistic Insights into Oxygen Exchange in Mixed Ionic-Electronic Conductors: A Current-Voltage Study

The oxygen incorporation reaction at solid/gas interfaces is central to many critical processes in solid-state electrochemistry, including those in electrolytic and thermochemical water splitting, fuel cells, oxygen storage for emissions control, and permeation membranes. To understand the reaction pathway and identify the rate-determining step, near-equilibrium measurements have been employed to quantify the exchange coefficients as a function of oxygen partial pressure and temperature. However, because the exchange coefficient contains contributions from both forward and reverse reaction rate constants and depends on the oxygen gas partial pressure (pO2) and oxygen fugacity (pO2,eff) in the solid, unique and definitive mechanistic assessment has been challenging. In this study, we addressed this challenge by measuring the current-voltage curve at far-from-equilibrium conditions for several mixed ionic-electronic conductors, including the (La,Sr)FeO3-delta family, (La,Sr)(Fe,Co)O3-delta, Sr(Ti,Fe)O3-delta, (La,Sr)MnO3-delta, and (Pr,Ce)O2-delta, which are commonly used as cathode materials for solid-oxide fuel cells. We isolated the driving forces acting in the gaseous and solid phases by independently controlling pO2 and pO2,eff, respectively, and a kinetics model was successfully applied. Key parameters, including the reaction orders with respect to pO2 and pO2,eff, were extracted and correlated to the reaction pathway. Further analysis showed that the vacancy formation energy is a possible descriptor for the reaction pathways.