(Invited) Towards an Improved Understanding of Electrochemical Oxygen Exchange Reactions on Mixed Conducting Oxides
Authors: Fleig Juergen , Rupp Ghislain M , Nenning Andreas , Schmid Alexander 
Published: February 2020

It is generally agreed that the oxygen reduction reaction on oxide surfaces is strongly affected by defects, and the same is true for the counter reaction of oxygen evolution from an oxide. Relevant defects include the “standard” point defects also present in the bulk, i.e. oxygen vacancies and electrons or holes, but also defects within the ideal termination of an oxide, e.g. cation segregates, and higher dimensional defects such as dislocations or grain boundaries.

However, it is far from trivial to clearly identify the role of specific defects on the oxygen reduction or release reaction. Often, one neither knows exact defect concentrations nor can one vary those in a defined manner without affecting the entity of all defects. This also hampers development of theoretical models describing the current voltage characteristics of oxygen reduction or evolution, i.e. oxygen exchange, at gas/solid interfaces.

In this contribution, two types of experiments are employed to shed further light on the role of defects in oxygen exchange of mixed conducting oxides such as Sr doped LaCoO3 (LSC) and LaFeO3 (LSF).

i) The first approach makes use of the fact that both oxygen partial pressure p(O2) and overpotential affect the oxygen chemical potential and thus defect concentrations in oxides. However, the oxygen reduction or evolution kinetics may still be affected very differently by overpotential and p(O2), since only the latter also changes the concentration of reacting molecules. By properly combining oxygen partial pressure changes and overpotential, we could tune the chemical potentials of LSF and gas phase independently and thus we extracted “true” reaction orders of O2 and defects in the oxygen reduction and evolution reaction. Additional bias dependent three point impedance measurements revealed the chemical capacitance of LSF and this was used for calculating the absolute bulk defect concentration in dependence of the oxygen chemical potential. This analysis also revealed that defect interaction strongly affects the thermodynamic data of LSF and a new defect chemical model including non-dilute defects is suggested.

ii) The second type of experiments deals with the effect of certain cations in the topmost layer of LSC. It is already known that Sr segregation to the surface of mixed conducting oxides may reduce their activity towards oxygen reduction. A kind of SrO termination layer seems to be unavoidable on LSC at higher temperatures but still additional degradation may take place and reasons are not fully understood. A novel type of experiment was thus developed, with impedance spectroscopic analysis of samples inside a PLD (pulsed laser deposition) chamber (in-situ impedance PLD). In this manner, the surface of an LSC electrode in an YSZ based electrochemical cell could be decorated by very tiny amounts of other oxides (SrO, Co3O4, La2O3) and the effect on oxygen exchange kinetics could be detected immediately by impedance measurements. Already small fractions of monolayers of Sr strongly modified the polarization resistance, thus indicating that only a small number of very active sites was present on LSC and those became deactivated by Sr. Co, on the other hand, can significantly activate the LSC electrode surface.