Investigation of Cr-Poisoning in SOFC Cathode By Using the Patterned Thin Film Electrode

Abstract

1. Introduction Cr-poisoning is one of main causes of the degradation of SOFC cathodes. It is considered the Cr-poisoning is caused by the deposition of Cr-containing oxides through reactions with the Cr vapor, such as CrO3 and CrO2(OH)2, coming from constitutional materials of the SOFC system. One of the most-known oxides causing the Cr-poisoning is SrCrO4 1. Since Cr ion is hexa-valent both in the vapors and SrCrO4, the formation of SrCrO4 does not accompany the reduction of Cr ion. On the other hand, oxides containing tri-valent Cr, for instance Cr2O3, is also likely to deposit. In this case, the reduction of Cr ion from hexa- to tri-valence is required, and thus the deposition of the oxides is expected to be promoted by the oxygen reduction reaction (ORR) in the SOFC cathode. In typical SOFC porous cathodes, the ORR generally progresses in a very limited area within several micrometers from the electrode/electrolyte interface 2,3. Therefore, it is not easy to detect influences of the ORR on the Cr-poisoning. Moreover, in porous electrodes, the concentration of the Cr vapor may be different depending on position of the electrode. Such an inhomogeneous concentration of the Cr vapor makes the accurate understanding of Cr-poisoning phenomena further difficult. In this study, for systematic and quantitative investigation of Cr-poisoning phenomena, the patterned thin film electrode proposed by our group was applied 4. This electrode is a kind of a columnar electrode modeling the porous electrode. The electrode contacts with the electrolyte only at the limited area (henceforth called as the gate), and the reaction current can be evaluated as a function of the distance from the gate. In addition, the Cr vapor can be homogeneously supplied onto the electrode regardless of electrode position. By taking these advantages of the model electrode, it is expected that the influence of the ORR on the Cr-poisoning can be separately evaluated. In this study, Ls0.6Sr0.4CoO3-δ (LSC) was chosen as a typical SOFC cathode material. 2. Experimental The La0.6Sr0.4CoO3-δ (LSC) patterned thin film electrode was fabricated on a Ce0.9Gd0.1O1.95 (GDC) electrolyte by combining techniques of photolithography and pulsed laser deposition. The patterned electrode had the gates with approximately 1 μm of width at every 300 μm. The electrode thickness was about 800 nm in this study. Porous Pt and mixture of LSC and Pt were used as a reference and a counter electrode, respectively. A Cr-poisoning test was carried out at 973 K under p(O2) = 1 bar and p(H2O) = 0.019 bar. First, 210 mV of cathodic bias was applied, and the polarization was kept until the observed current became stable. Then, wet O2 gas, which was preliminarily passed through a column of Cr2O3 powder at 973K, was introduced to the patterned electrode. During the Cr-poisoning test, electrochemical impedance spectroscopy measurements were repeated to evaluate the degradation of area specific resistance. After the Cr-poisoning test, the patterned electrode was analyzed by using SEM/EDS, SIMS, STEM/EDS and EPMA. 3. Results and Discussion From SEM observation, many deposits were observed on the electrode surface after the Cr-poisoning test. Elementary line scans by EPMA were carried out on the electrode surface. Results for Cr, La, Sr and Co are presented in Fig. 1. These results clearly shows that Cr preferentially deposited near the gate together with Sr. According to our previous works using operando micro X-ray absorption spectroscopy and electrochemical impedance spectroscopy measurements, it was suggested that the electrode reaction in the LSC patterned thin film electrode having the thickness of 400 nm effectively progresses in the area within 30 μm from the gate at the same temperature and p(O2) without Cr vapor 4. By considering the difference of the electrode thickness, the effective reaction area in the electrode used in this work would be approximately 40 μm from the gate, if the change in the effective reaction area due to the Cr-poisoning was not significant. This value is very close to the area where Cr deposits were observed, as shown in Fig. 1. This fact demonstrated that there exists a Cr-poisoning mode promoted by the SOFC cathode reaction.