Ionic Conductivities and Microstructures of Ytterbium-Doped Ceria

Abstract

The ionic conductivities and microstructures of Ce 1−x Yb x O 2−x/2 ͑x = 0.10, 0.15, 0.20, and 0.25͒ with average grain sizes in a range of 0.2-3.7 m were studied systematically. Nanosized domains were confirmed through detailed studies of transmission electron microscopy. The relationships of the domains, doping concentration, and grain size were determined, and their effects on the ionic conductivities were examined. It was concluded that the domains were formed via the segregation of Yb cations so that they could trap the oxygen vacancies and lower the conductivity. Furthermore, the development of the domains, which depends on the doping concentration and grain size, can affect the doping concentration and grain-size dependencies of the conductivity.Ceria doped with rare-earth elements has attracted much attention as an important candidate electrolyte material for intermediatetemperature solid oxide fuel cells ͑SOFCs͒. 1-3 The ionic conductivity of doped ceria is greatly influenced by doping concentration. It was expected that the ceria with higher doping concentration could exhibit higher conductivity. However, the concentration of dopant cations that corresponds to the maximum conductivity is usually about 10-20 atom %, which is much lower than the solubility of the dopants in ceria. 2-5 Some investigators suggested that this phenomenon was attributed to the clustering of dopant cations and their associated oxygen vacancies, 3,5,6 through which the mobility of oxygen vacancies can be decreased. With increasing the doping concentration, the density of the clusters increases, leading to a decrease in conductivity.In addition to the doping concentration, the grain size also influences the conduction behavior. This influence was originally studied in stabilized zirconia. 7-11 Verkerk et al. found that the grainboundary conductivity of Y-stabilized zirconia increased linearly with the grain size increasing from 0.3 m to 2-4 m and was constant for larger grain size, while the bulk conductivity was nearly independent of the grain size. 9 This phenomenon was explained by the space-charge model, 12 in which it was suggested that spacecharge layers were formed by the segregation of Y cations and the depletion of oxygen vacancies around the grain boundaries. Because the space-charge layers can increase the grain-boundary resistivity, the increase in the density of the grain boundaries with decreasing grain size can cause a decrease in the total conductivity. The similar grain-size dependency of conductivity was also observed in doped ceria, 4,13,14 which was also in good agreement with the space-charge model. 15 However, some exceptions were found as well. In 20 atom % Gd-doped ceria, the conductivity could increase as the grain size decreased from 3 to 0.7 m. 16 Recently, in 10-25 atom % Y-doped ceria, Ou et al. also observed that the conductivity increased with decreasing grain size when the grain size was smaller than 200-500 nm. 17 It is difficult to explain this phenomenon by the space-charge model.In recent years...

Oxygen-vacancy ordering in lanthanide-doped ceria: Dopant-type dependence and structure model

“…Though the measured conductivity includes contributions of both the grain boundaries and the bulk, it is believed that the decrease in conductivity is mainly due to the change in bulk conductivity because the grain size, and thereby the density of grain boundary, is similar in different samples. Specifically, as suggested in previous studies, [8][9][10][11][12] this decrease can be explained by the enhanced interaction between the oxygen vacancies and the segregated dopant cations in some regions named microdomains or nanosized domains. In addition, the oxygen-vacancy ordering may also affect the ionic conductivity.…”

“…17 In this work, the EELS study of oxygen-vacancy ordering was performed at room temperature, while the operating temperature of SOFCs based on doped ceria electrolyte, and thereby the measurement temperature for the ionic conduction in doped ceria, is around 500°C. Nevertheless, the segregation of the dopant 10,12 and the local ordering of oxygen vacancies may also exist at the measurement temperature ͑ϳ500°C͒ and have influence on the electrical conductivity because the microstructural rearrangement in doped ceria is difficult at temperatures much lower than 1000°C. 29 To quantitatively clarify the impacts of the ordering on ionic conduction and its dependences on dopant type and doping level, the development of oxygenvacancy ordering with increasing doping level was estimated by the enhancement of peak B and calculated as Table I, in which the data of Sm-, Dy-and Yb-doped ceria are the same as those reported in Ref.…”

“…[2][3][4] However, in heavily doped ceria, nanosized domains can form and give negative impacts on the ionic conduction. [8][9][10][11] These domains have higher dopant concentrations than the matrix and could provide deep traps for the oxygen vacancies. 10,12 As a result of the domain formation, the advantage of the dopant having an optimum radius disappears with increasing the doping level.…”

“…[8][9][10][11] These domains have higher dopant concentrations than the matrix and could provide deep traps for the oxygen vacancies. 10,12 As a result of the domain formation, the advantage of the dopant having an optimum radius disappears with increasing the doping level. As shown in a comparative study of Gd-and Y-doped ceria, 13 conductivities in the order of GdϾ Y and GdϽ Y were observed, respectively, at doping concentrations lower and higher than 20-25 at.…”

“…Because of the small size, the immature structure, and the lattice coherence with the fluorite-structured matrix, 10,11 the direct analysis on these domains is difficult and their crystal structure remains unclear. Despite this, it has been suspected that the negative impacts of domains on ionic conduction are partly related to its ordered structure, which may involve the local ordering of oxygen vacancies.…”