Strontium doping effect on phase homogeneity and conductivity of Ba1−xSrxCe0.6Zr0.2Y0.2O3−δ proton-conducting oxides

Ba1−xSrxCe0.6Zr0.2Y0.2O3−δ (0.0 ≤ x ≤ 1.0) proton-conducting oxides have been prepared using a citrate-EDTA complexing sol–gel method. In this study, the relationship between the Sr doping content and microstructure, chemical stability against CO2, and conductivity of the sintered Ba1−xSrxCe0.6Zr0.2Y0.2O3−δ pallets are systematically investigated using XRD, SEM, micro-Raman spectroscopy, and dc two-probe measurements. All sintered Ba1−xSrxCe0.6Zr0.2Y0.2O3−δ oxides exhibit excellent chemical stability after being exposed to the CO2 ambient at 600 °C for a long duration; nevertheless, their microstructures and conductivities are very sensitive to the Sr doping amount. The Sr incorporation is found to apparently suppress the formation of CeO2-like second phase, and enhance the grain growth in sintered oxides. Among all sintered samples, the Ba0.8Sr0.2Ce0.6Zr0.2Y0.2O3−δ pallet has the highest conductivity, 0.009 S/cm at 800 °C. This result can be attributed to the competition between the elimination of CeO2- or (Zr,Ce,Y)O2-like phase inhomogeneity and enhanced grain growth in sintered oxides, both of which adversely influence the ionic conductivity. This work demonstrates that Ba1−xSrxCe0.6Zr0.2Y0.2O3−δ would be a promising electrolyte for H+-SOFC applications if the Sr doping is well controlled.