Chromium poisoning is one of main causes of the degradation of SOFC cathodes. Cr-poisoning is caused by the deposition of Cr-containing oxides through reactions between the cathode material and the Cr vapor, such as CrO₃ and CrO₂(OH)₂, coming from constitutional metals of the SOFC system. SrCrO₄ and Cr₂O₃ are known as such oxides. Since Cr ions in the vapors and Cr₂O₃ are hexa- and tri-valent, respectively, the formation of Cr₂O₃ requires the reduction of Cr ions. On the other hand, in this case of the SrCrO₄ deposition, the reduction of Cr ions is not needed. So far, it was pointed out that the type of the Cr deposition and their amounts significantly depends on the electrode material and SOFC operation conditions. But yet the mechanism of Cr-poisoning is not well-clarified.

In this study, in order to investigate the mechanism of Cr-poisoning systematically and quantitatively, the patterned thin film electrode proposed by our group was applied [1]. This model electrode was a kind of a columnar electrode which simplified the complicated microstructure of the practical porous electrode. The electrode contacted with the electrolyte only at the limited and defined area, and the reaction current was distributed simply as a function of the distance from the interface. Because of such a simple structure of the model electrode, it was expected that the influence of the polarization on the Cr-poisoning can be separately evaluated.

La_0.6Sr_0.4Co_3-δ (LSC), which is a typical mixed ionic and electronic conducting oxide, was chosen as a model SOFC cathode. The LSC patterned thin film electrode was fabricated on a Ce_0.9Gd_0.1O_1.95 (GDC) electrolyte by means of photolithography and pulsed laser deposition. One-week degradation tests were carried out at 973 K under p(O₂) = 1 bar and p(H₂O) = 0.019 bar. The wet O₂ gas with or without Cr-vapor was flowed to the patterned electrode under cathodic overpotential of -190 mV. When Cr-vapor was introduced, the gas was preliminarily passed through a column of Cr₂O₃ powder kept at 973 K. After the degradation tests, the patterned electrode was analyzed by using SEM/EDS, SIMS, EPMA , and STEM/EDS/SAED.

From SEM observation, many deposits were observed on the electrode surface after the degradation test with Cr-vapor. Elementary mapping by EPMA revealed that Cr preferentially deposited near the electrode/electrolyte interface together with Sr. The area where Cr deposits were observed corresponded to the effective electrode-reaction area evaluated by our previous works using operando micro X-ray absorption spectroscopy [1]. On the other hand, in the positions far from the electrode/electrolyte interface, where the electrode reaction did not take place, Cr deposits were rarely found. These results clearly demonstrated that the cathodic polarization accelerated the Cr-poisoning. From the selected area electron diffraction, the deposits were identified as SrCrO₄. After the degradation test without Cr-vapor, Sr segregation was observed near the electrode/electrolyte interface. These indicated that the acceleration of the Cr-poisoning by the cathodic polarization was originally due to the Sr segregation followed by the SrCrO4 formation through the chemical reaction with Cr-vapor.

References

(1) K. Amezawa, et al., Electrochem. Soc. Trans., 66 (2), 129-135 (2015).