Chromium poisoning and degradation at (_La,Sr) MnO₃ and (La,Sr) FeO₃ cathodes for solid oxide fuel cells

The degradation of cathode performances by chromium poisoning was studied at two cathodes [(La_0.8 Sr_0.2)_0.95 MnO ₃ (LSM) and La_0.8 Sr_0.2 FeO₃] on the Ce_0.8 Gd_0.2 O₂ electrolyte. Cathodic currents were decreased by the supply of Cr vapors under a constant overvoltage (-0.3 V vs reference electrode) at 1073 K up to 300 h. The polarization resistances increased with time to the power of 1/2 and 1/4 in (La_0.8 Sr _0.2) 0.95 MnO₃ and La_0.8 Sr_0.2 FeO₃, respectively. The distributions and the amounts of Cr in the porous cathodes were measured by secondary-ion mass spectrometry: A maximum concentration of Cr was about 1000 ppm at the LSM/ Ce_0.8 Gd _0.2 O₂ contact interfaces, while the surface concentration was about 100 ppm. In La_0.8 Sr_0.2 FeO₃, the concentration of Cr is almost constant at values of 1000-1700 ppm inside the cathode. Among the supplied Cr vapors, about 7-14% of Cr atoms were reacted and condensed in the porous (La_0.8 Sr_0.2) 0.95 MnO ₃, while 13-21% of Cr atoms were reacted in La_0.8 Sr _0.2 FeO₃. Chemical reactivity and electrode reaction mechanism affect the Cr poisoning and degradation of cathode performance.