Mechanistic\nInsights into Oxygen Exchange in Mixed\nIonic-Electronic Conductors: A Current–Voltage Study

The oxygen incorporation reaction\nat solid/gas interfaces\nis central\nto many critical processes in solid-state electrochemistry, including\nthose in electrolytic and thermochemical water splitting, fuel cells,\noxygen storage for emissions control, and permeation membranes. To\nunderstand the reaction pathway and identify the rate-determining\nstep, near-equilibrium measurements have been employed to quantify\nthe exchange coefficients as a function of oxygen partial pressure\nand temperature. However, because the exchange coefficient contains\ncontributions from both forward and reverse reaction rate constants\nand depends on the oxygen gas partial pressure (<i>p</i>O<sub>2</sub>) and oxygen fugacity (<i>p</i>O<sub>2,eff</sub>) in the solid, unique and definitive mechanistic assessment has\nbeen challenging. In this study, we addressed this challenge by measuring\nthe current–voltage curve at far-from-equilibrium conditions\nfor several mixed ionic–electronic conductors, including the\n(La,Sr)FeO<sub>3−δ</sub> family, (La,Sr)(Fe,Co)O<sub>3−δ</sub>, Sr(Ti,Fe)O<sub>3−δ</sub>, (La,Sr)MnO<sub>3−δ</sub>, and (Pr,Ce)O<sub>2−δ</sub>,\nwhich are commonly used as cathode materials for solid-oxide fuel\ncells. We isolated the driving forces acting in the gaseous and solid\nphases by independently controlling <i>p</i>O<sub>2</sub> and <i>p</i>O<sub>2,eff</sub>, respectively, and a kinetics\nmodel was successfully applied. Key parameters, including the reaction\norders with respect to <i>p</i>O<sub>2</sub> and <i>p</i>O<sub>2,eff</sub>, were extracted and correlated to the\nreaction pathway. Further analysis showed that the vacancy formation\nenergy is a possible descriptor for the reaction pathways.