The Current-Voltage Characteristics and Partial Pressure Dependence of Defect Controlled Electrochemical Reactions on Mixed Conducting Oxides

Electrochemical reactions at solid|gas interfaces of mixed ionic electronic conductors (MIEC), such as oxygen reduction or evolution, differ substantially from usual electrochemical reactions in aqueous solutions. Overpotentials do not directly translate to electrostatic surface potentials but act mainly by changing the concentration of point defects in the MIEC. This has severe consequences for the mechanistic interpretation of current voltage curves of MIEC electrodes. In this contribution it is shown how overpotential dependent defect concentrations affect the current-voltage curves of oxygen reduction and oxygen evolution at MIEC surfaces. Exemplarily, quantitative current-voltage curves are deduced from the known defect chemical data set (Brouwer diagram) of La0.6Sr0.4FeO3 − δ(LSF). Various curve shapes result, from Tafel-like exponential relations to essentially voltage independent limiting currents. Tafel slopes have a very different meaning compared to charge transfer limited reactions at metal electrode interfaces. It is shown how mechanistic information can be obtained from the difference of anodic and cathodic Tafel slopes or by comparing exchange current densities and ac resistances. Moreover, partial pressure dependences of anodic and cathodic currents are deduced, showing that exponents of power laws often do not indicate whether atomic or molecular oxygen species are involved in the rate limiting step.