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Sintering Effects on Proton Conductivity of Ta-Doped Ba2(CaNb)2O6 and its Reactivity with SOFC Cathodes

The effect of preparation temperature on electrical conductivity of Ba2(Ca 0.75 Nb 0.59 Ta 0.66 )O 6-δ , Ba2(Ca 0.75 Nb 0.66 Ta 0.59 )O 6-δ , and Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ was investigated in air and humidified N 2 and O 2 . Powder X-ray diffraction (PXRD) showed the formation of double-perovskite-type structure. Among the compounds investigated, Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ showed the highest proton conductivity of 3.7 and 5.3 X 10 -4 S/cm at 550°C in wet N 2 , respectively, for a 1400 and 1500°C sintered sample, while no change in conductivity was observed in air. PXRD showed that Ba 2 (Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ is stable against chemical reaction with La 0.8 Sr 0.2 MnO 3 (LSM) and Sm 0.5 Sr 0.5 CoO 3 (SSC) electrodes at 800 and 1000°C. Chemical compatibility was further confirmed by energy-dispersive X-ray (EDX) analysis. The ac impedance employing Pt, LSM, and SSC electrodes on Ba2(Ca 0.79 Nb 0.66 Ta 0.55 )O 6-δ showed that the area specific polarization resistance (ASPR) decreased in wet atmospheres compared to that of air. Unlike the oxide ion system, the ASPR was found to be much higher for the presently investigated proton system, suggesting that proton conductivity at the electrolyte and electrode interfaces or water effusion through microspores appears to control the ASPR. Among the electrodes tested, based on ac impedance studies, the LSM appears to be a better electrode compared to SSC for Ta-doped Ba 3 Ca 1+x Nb 2-x O 9-δ electrolyte.