Ionic and Electronic Conductivity of Nanostructured, Samaria-Doped Ceria

Abstract

The ionic and electronic conductivities of samaria doped ceria electrolytes, Ce 0.85 Sm 0.15 O 1.925−δ , with nanometric grain size have been evaluated. Nanostructured bulk specimens were obtained using a combination of high specific-surface-area starting materials and suitable sintering profiles under conventional, pressureless conditions. Bulk specimens with relatively high density (≥92% of theoretical density) and low medium grain size (as small as 33 nm) were achieved. Electrical A.C. impedance spectra were recorded over wide temperature (150 to 650 • C) and oxygen partial pressure ranges (0.21 to 10 −31 atm). Under all measurement conditions the total conductivity decreased monotonically with decreasing grain size. In both the electrolytic and mixed conducting regimes this behavior is attributed to the high number density of high resistance grain boundaries. The results suggest a possible variation in effective grain boundary width with grain size, as well as a possible variation in specific grain boundary resistance with decreasing oxygen partial pressure. No evidence appears for either enhanced reducibility or enhanced electronic conductivity upon nanostructuring. The transport and redox properties of ceria have been studied extensively due to the suitability of this material to a wide range of applications including fuel cells (as both electrolyte 1, 2 and anode component [2][3][4][5] ) and catalytic convertors. 6 In recent years there have been observations of 'non-trivial' size effects in ceria in which nanostructuring results in dramatically enhanced conductivity, particularly for grain sizes of <30 nm.7 Concomitant with this change in transport properties is a dramatic increase in reducibility, manifest as a decrease in the standard enthalpy of the reduction reaction. 8 Although the details of the observations differ between authors, the grain boundary behavior of undoped ceria free of intergranular impurity phases is relatively well-explained in terms of a space-charge model. 7,9 In brief, due to inherent differences in the chemical bonding environment at grain boundaries and in the bulk, the grain boundary interface or core displays a charge imbalance between anionic and cationic species leading to an interfacial space-charge potential, φ 0 . This potential, in turn, causes a redistribution of mobile species in the near vicinity of the core, i.e., the space charge region. Inferred values of φ 0 in ceria are ∼0.3 to 0.7 V, and, being positive in sign, imply that vacancies are depleted, whereas electron concentrations, resulting from the Ce 4+ /Ce 3+ equilibrium, are enhanced in the space charge region. The width of the space-charge zone (the effective grain boundary thickness) typically falls between 1.2 and 6 nm. For microcrystalline materials, the consequence is that the grain boundaries serve to block the motion of the predominately mobile oxygen vacancies as they traverse the depletion region and migrate from one grain to the next. For nanocrystalline materials, because the volume fraction of gr...

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Effect of Al- or Ga-additive on ionic conductivity of thin-film Gd-doped ceria

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