Solid oxide fuel cells (SOFCs) recently attract attentions as one of promising high-efficiency energy-conversion devices. For enhancement of the cell performance or low temperature operation, oxygen reduction kinetics on the cathode should be improved. In a mixed ionic-electronic conducting (MIEC) cathode, such as (La,Sr)CoO_3-d and (La,Sr)(Co,Fe)O_3-d, reaction area for the oxygen reduction reaction is not limited to only the electrode/electrolyte/gas interface (the triple phase boundary) but also distributes to the oxide surfaces within a certain distance from the electrode/electrolyte interface, i.e. within an effective reaction area. However, there are no reports which directly observe such a reaction distribution by experimental manners, as far as the authors know.

For experimental and quantitative observation of the reaction distribution in an MIEC SOFC electrode, a patterned thin film electrode, which is a kind of the columnar electrode simplifying a porous electrode, was used in this work [1]. As a typical MEIC electrode material, La_0.6Sr_0.4CoO_3-d was chosen and deposited on a Ce_0.9Gd_0.1O_1.95 electrolyte. In an MEIC cathode, it is believed the oxygen chemical potential decreases in the area where the electrochemical oxygen reduction reaction takes place. Thus, reaction distribution can be understood if the distribution of oxygen potential is revealed. In this work, operando micro X-ray absorption spectroscopy (XAS) technique was applied to evaluate the oxygen potential was evaluated as a function of the distance from the electrode/electrolyte interface. In this operando technique, X-ray absorption spectra could be obtained with a spatial resolution higher than 1 mm at elevated temperature (~1073 K) while controlling atmospheric condition and passing the electronic current through the cell. The effective reaction area could be estimated, for instance, 20 mm from the electrode/electrolyte interface in the case of the La_0.6Sr_0.4CoO_3-d patterned thin film electrode having a thickness of 400 nm at 973 K under 1 bar of p(O₂) and -220 mV of cathodic overpotential. The effective reaction area tended to expand with decreasing ambient p(O₂) and increasing cathodic overpotential.

References

[1] K. Amezawa, Y. Fujimaki, T. Nakamura, K. D. Bagarinao, K. Yamaji, K. Nitta, Y. Terada, F. Iguchi, K. Yashiro, H. Yugami, T. Kawada, Electrochem. Soc. Trans., 66(2), 129 (2015).