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 Ba ₂(Ca_0.75Nb_0.59Ta_0.66)O _6-δ, Ba₂(Ca_0.75Nb_0.66Ta _0.59)O_6-δ, and Ba₂(Ca _0.79Nb_0.66Ta_0.55)O_6-δ was investigated in air and humidified N₂ and O₂. Powder X-ray diffraction (PXRD) showed the formation of double-perovskite-type structure. Among the compounds investigated, Ba₂(Ca_0.79Nb _0.66Ta_0.55)O_6-δ showed the highest proton conductivity of 3.7 and 5.3 × 10^-4 S/cm at 550°C in wet N₂, respectively, for a 1400 and 1500°C sintered sample, while no change in conductivity was observed in air. PXRD showed that Ba ₂(Ca_0.79Nb_0.66Ta_0.55)O _6-δ is stable against chemical reaction with La _0.8Sr_0.2MnO₃ (LSM) and Sm_0.5Sr _0.5CoO₃ (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 _0.79Nb_0.66Ta_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 Ba₃Ca_1+xNb _2-xO_9-δ electrolyte.