Characterization of Terbium and Samarium Co-Doped Ceria Films as SOFC Electrolyte Prepared by Using Ultrasonic Spray Pyrolysis Method

Abstract

Because of the high ionic conductivity at intermediate temperatures (500–700 °C), doped ceria oxides are of special interest as the promising electrolyte materials for the intermediate temperature solid oxide fuel cells (IT-SOFC). However, main drawback of the ceria based solid electrolytes is the partial electronic conductivity at reducing conditions resulting in an internal partial short-circuiting of the cell and decreased total efficiency for SOFC. Tb, considered an electron trap, was found to suppress the partial electronic conductivity for microcrystalline ceria electrolyte. The aim of this work is to study the influence of Tb and Sm dopant concentration ratio on the microstructural and electrical properties of Ce0.9Sm0.1-xTbxO2-δ films. Tb and Sm cations binary co-doped ceria films were deposited using the ultrasonic atomizing spray pyrolysis method. The crack-free homogenous films with different dopant concentration were deposited and thereafter annealed at fixed temperatures 900, 1200 and 1300 °C, respectively. It was demonstrated that several microstructural parameters of oxide films are controlled by the sintering temperature. The Ce0.9Sm0.1-xTbxO2-δ films formed were analyzed using X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscopy, atomic force microscopy and four probe dc technique at different pO2 and T values. Based on the SEM analysis the average thickness of Ce0.9Sm0.1-xTbxO2-δ films were 700 nm. The XRD patterns for the Ce0.9Sm0.1-xTbxO2-δ films annealed at 1200 °C indicated the high degree of crystallinity. Tb dopant ions influence the microstructural properties like median diameter of grain, microstrain, lattice parameter and electrical properties like activation energies of ionic and electronic conductivity of the Ce0.9Sm0.1-xTbxO2-δ film. Significantly higher microstrain value for smallest Tb dopant concentration and strong dependence of electrical properties on that was observed. The activation energy of ionic conductivity was lowered in that case, while the activation energy of electronic conductivity was increased. The lower bound of electrolytic domain for Ce0.9Sm0.09Tb0.01O2-δ film was lowered by one order of magnitude toward lower oxygen partial pressures. Further increase of Tb amount increased the activation energy of ionic conductivity, shifting the lower bound of electrolytic domain toward higher oxygen partial pressures.