Chromium Poisoning of La1-xSrxMnO3+/-..delta.. Cathodes and Electrochemical Validation of Chromium Getters in Intermediate Temperature-Solid Oxide Fuel Cells

The effect of chromium poisoning on electrochemical performance degradation has been studied at intermediate temperatures (550-650 degrees C) using La1-xSrxMnO3+/-d (LSM) cathode and an electrochemical cell configuration air/LSM//YSZ//Pt/air. A rapid increase in polarization resistance is observed in the presence of gaseous chromium species containing humid air (3% H2O) at both 550 degrees C and 650 degrees C. Cross-sectional elemental mapping and high-resolution transmission electron microscopy of the LSM/YSZ interface show the presence of crystalline Cr2O3 and (Mn,Cr)3O4 phases indicating reduction of gaseous hexavalent-chromium species and deposition of solid reaction products. Subsequent experiments, performed under the above exposure conditions but in the presence of a chromium getter, showed stable polarization resistance. The electrochemical performance of half-cells in Cr-containing humid air atmosphere and in the presence of the chromium getter also remained stable during 100-h tests at 550 and 650 degrees C, respectively. The post-test structural and chemical analysis of the electrode, electrode-electrolyte interface and getter by scanning- transmission electron microscopy, energy dispersive X-ray and Raman spectroscopy showed that the cathode bulk and cathode- electrolyte interface remained free of chromium deposits whereas chromium species deposited near the air inlet region of the porous getter as crystalline SrCrO4.