Chromium oxide vapor species that influence cathode degradation were studied and the effect of a cathode second layer that suppresses chromium diffusion to the cathode was investigated in an SOFC using an alloy separator. PCrO2(OH)2 has the highest value at high PH2O condition and PCrO3 becomes highest at low PH2O condition (∼10-4 atm). The cathode degradation rate seemed to be influenced by the total vapor pressure of all gaseous species of chromium. The PH2O dependence of the cell life indicated that cathode degradation can be reduced by decreasing PH2O so far as that PCrO2(OH)2 is equal to PCrO3. Chromium diffusion to the cathode was suppressed by adopting a cathode second layer that contains La2O3. This effect seemed to be caused by the reaction of La2O3 with the chromium vapor which decreased the chromium vapor pressure in the cathode. The life of the cell with the cathode second layer was extended by decreasing the cathode polarization and the chromium vapor pressure from the alloy. Almost all La2O3 in the cathode second layer seemed to react with the chromium vapor and contribute to the increase in cell life.
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