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Abstract
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.
Highlights
The Current-Voltage Characteristics and Partial Pressure Dependence of Defect Controlled Electrochemical Reactions on Mixed Conducting Oxides
Several differences exist between such solid|gas reactions and usual aqueous electrochemical reactions at metal electrodes, and the currentvoltage characteristics of the solid|gas reactions often cannot be described by standard electrochemical models such as Butler-Volmer’s equation.[47]
Knowing the defect chemistry of the mixed ionic electronic conductor (MIEC) electrode and the interrelationship between defect concentrations, oxygen partial pressure and electrode polarization is vital for understanding the kinetics of such reactions
Summary
To cite this article: Alexander Schmid and Jürgen Fleig 2019 J. For electrochemical reactions at MIEC|gas interfaces, an applied overpotential acts mainly upon the oxygen chemical potential in the MIEC electrode, and thereby on the MIEC defect concentrations.[37,43,49] The overpotential might affect the potential step at the MIEC|gas interface, These considerations clearly show that the interpretation of currentvoltage curves and their partial pressure dependencies requires novel concepts beyond standard models known from metal|liquid interfaces. A quantitative approach to tackle such situations has been introduced recently,[48] and it was shown how the true, i.e. mechanistically meaningful, reaction orders of defects and gas species can be determined from combined voltage and partial pressure variations In this contribution, we extend these considerations by modeling current-voltage curves for the known defect chemical data set of a specific material (La0.6Sr0.4FeO3−δ) and different possible reaction mechanisms. This opens new ways to draw mechanistic conclusions from empirically measured current-voltage curves and partial pressure dependences
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