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 ( p O 2 ) and oxygen fugacity ( p O 2,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)FeO 3−δ family, (La,Sr)(Fe,Co)O 3−δ, Sr(Ti,Fe)O 3−δ, (La,Sr)MnO 3−δ, and (Pr,Ce)O 2−δ, 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 p O 2 and p O 2,eff, respectively, and a kinetics model was successfully applied. Key parameters, including the reaction orders with respect to p O 2 and p O 2,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.