Study On Doping Modification And Electrical Properties Of Perovskite-type Bamo₃and Composite Electrolytes

Conventional solid electrolyte of8mol.%Y2O3-ZrO2(8YSZ) can not meet the requirement of industrial applications in the intermediate temperature range of773to1073K, which makes it quite necessary to explore novel intermediate temperature electrolytes for solid oxide fuel cells (SOFCs). In recent years, doped BaZrO3and BaCeO3with a perovskite-type crystal structure have attracted great attention. However, the electrical conductivity of doped BaZrO3is poor, and the chemical stability of doped BaCeO3is not satisfactory. These two issues to be solved become quite important. In the present work, BaMO3and its composite electrolytes doped with different cations have been fabricated to obtain a better understanding on the relationships between microstructure and electrical properties or chemical stability so that the above-mentioned two issues can be improved.BaMO3and its composite electrolytes doped with different cations have been pressureless-sintered by the solid state reaction method at1973K. The crystal structures were investigated by means of XRD, SEM, EDX, Raman and FT-IR. The resistances in both air and different H2O-H2atmospheres were investigated by complex impedance spectroscopy, and the electrical conductivity, pre-exponential factor and activation energy were then calculated. In addition, optimized BaMO3and composite electrolytes with good electrical properties were tested under the conditions of water vapor and CO2to evaluate their chemical stability, respectively.The electrical conductivities of pure BaZrO3and BaCeO3ceramics are very low, which are not suitable for the application as potential solid electrolytes. At1073K, the measured electrical conductivities of BaZrO3in both4%H2O-H2and air are1.00×10–4S·cm–1and1.43×10–4S·cm–1, respectively. At identical temperature level of1073K, the measured conductivities of BaCeO3in both4%H2O-H2and air are1.56×10–3S·cm–1and4.91×10–3S·cm–1, respectively.BaZr1–xDyxO3–δ ceramic has a cubic structure. The lattice parameters, cell volume and grain size of BaZr1–xDyxO3–δ ceramic increase with increasing the content of Dy2O3. However, BaCe0.85Ln0.15O3–δ (Ln=Gd, Y, Yb) exhibits an orthorhombic structure. The lattice parameters, cell volume and grain size of BaCe0.85Ln0.15O3–δ ceramic decrease with decreasing the ionic radius of Ln3+. However, doping of In2O3and TiO2with low melting points into BaCe0.80–yZryIn0.05Y0.15O3–δ-Y2O3and BaCe0.80–yZryTi0.05Y0.15O3–δ-Y2O3increases the bulk density of composite electrolytes, and generates the defect association in the main phase of perovskite crystal.The effective doping concentration and grain boundary resistance have an important influence on the electrical conductivity of BaZr1–xDyxO3–δ ceramics. BaZr0.90Dy0.10O3–δ has the highest electrical conductivity. At1073K, the measured conductivities of BaZr0.90Dy0.10O3–δ ceramic in4%H2O-H2and air atmospheres are7.90×10–3S·cm–1and7.31×10–3S·cm–1, respectively. At773K, the measured electrical conductivities of both BaCe0.85Y0.15O3–δ and BaCe0.85Gd0.15O3–δ ceramics exceed0.01S·cm–1in both4%H2O-H2and air. At1073K, the conductivities of BaCe0.85Y0.15O3–δ ceramic in both4%H2O-H2and air are4.47×10–2S·cm–1and4.51×10–2S·cm–1, respectively.At773K，the electrical conductivity of BaCe0.85Y0.15O3–δ-Y2O3in4%H2O-H2is1.32×10–2S·cm–1, and the one in air is1.29×10–2S·cm–1. At1073K, the conductivities of BaCe0.85Y0.15O3–δ-Y2O3in both4%H2O-H2and air are the same,4.72×10–2S·cm–1. Doping Y3+and Gd3+at B-site could enhance the content of oxygen vacancies and improve significantly electrical conductivity of BaMO3and its composites. Doping of trivalent In3+at Ce4+site generates more oxygen vacancies, however, excess oxygen vacancies generate small amounts of defect association rather than increasing the effective doping concentration. The measured conductivity of BaCe0.80–yZryIn0.05Y0.15O3–δ-Y2O3composite is lower than that of BaCe0.85–xZrxY0.15O3–δ-Y2O3composite. Doping of Ti4+with a small ionic radius into BaCe0.80–yZryTi0.05Y0.15O3–δ-Y2O3also generates the defect association. At1073K, the conductivities of BaCe0.80Zr0.05Y0.15O3–δ-Y2O3composite in both4%H2O-H2and air are4.23×10–2S·cm–1and4.16×10–2S·cm–1, respectively.Ba(OH)28H2O, CeO2and BaCO3phases are formed on the surface of BaCe0.85Y0.15O3–δ ceramic after exposure in the boiling water vapour. With increasing the exposure durationin water vapor, the surface of BaCe0.85Y0.15O3–δ ceramic is well covered with needle-like BaCO3products. The BaCe0.85–xZrxY0.15O3–δ-Y2O3composite has a much better chemical stability in water vapour than the BaCe0.85Y0.15O3–δ ceramic. Incorporating with the second phase Y2O3or doping with Zr4+cations restrains from the formation of Ba(OH)28H2O and BaCO3products, and improve the chemical stability of doped BaCe0.85Y0.15O3–δ electrolyte in water vapour. A lot of BaCO3rods are found on the surface of BaCe0.85Y0.15O3–δ ceramic after exposure in CO2atmosphere at temperatures of873and1073K. The BaCe0.85–xZrxY0.15O3–δ-Y2O3composite exhibits a much better chemical stability in CO2than BaCe0.85Y0.15O3–δ ceramic. Incorporating with the second phase Y2O3or doping with Zr4+cations restrains from the formation of BaCO3, and enhance the chemical stability of BaCe0.85–xZrxY0.15O3–δ-Y2O3in CO2atmosphere at elevated temperatures.