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The electrical properties of SrCe 0.95 Eu 0.05 O 3Ϫ␦ were studied as a function of both oxygen partial pressure and water vapor partial pressure in the temperature range of 500-800°C, and the partial conductivities of protons, holes, and oxygen vacancies were calculated from the proposed defect model. P-type conduction was dominant in an oxidative atmosphere. It was shown that, in a wet atmosphere, SrCe 0.95 Eu 0.05 O 3Ϫ␦ is a mixed conductor of protons, holes, and oxygen ions. A conduction transition from protons to holes and/or oxygen ions was found as temperature increased. The calculated activation energy of oxygen ion transport was 0.67 eV; standard solution enthalpy of water dissolution was Ϫ164 kJ/mole, which is larger than that of SrCe 0.95 Yb 0.05 O 3Ϫ␦ . The effect of dopants on the conduction mechanism is explained by a hopping mechanism.

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The effect of defect structure on the electrical conductivity for Eu-doped SrCeO3 was studied in a reducing range of PH2, PH2O, PO2. A defect model was derived and verified by computational simulation against both electrical conductivity data and Brouwer-type defect equilibria. At higher temperatures, part of the conductivity was due to oxygen vacancies and electrons. At low temperature, protonic conduction dominated. The measured total conductivity is proportional to PO2−1/8 in the temperature range of 650-750°C, and to PO2−1/6 at 800°C in wet atmospheres, because of a transition from ionic- to electronic-dominant conduction. The results agree with a derived defect model based on the equilibrium equation for the valence change of Eu ions between trivalent and divalent. © 2003 The Electrochemical Society. All rights reserved.

Sr2+-doped cerium pyrophosphate (Ce1–x Sr x P2O7, x = 0.05, 0.1, 0.15, and 0.2) electrolytes are synthesized by digesting cerium oxide and strontium nitrate with 85% H3PO4. The ionic conductivity behavior of Ce1–x Sr x P2O7-sintered pellets is analyzed by electrochemical impedance spectroscopy in a dry and humid air atmosphere. The time-dependent variation of ionic conductivity during humidification process and the variation of conductivity with temperature are studied. In humid atmosphere in the 90–230 °C range, the variation of ionic conductivity is explained on the basis of the combined effect of ionic mobility and the availability of charge carriers in the samples. The ionic conductivity shows dependence on dopant concentration and water vapor pressure (pH2O). In dry air, conductivity of Ce1–x Sr x P2O7 is very low, with Ce0.9Sr0.1P2O7 showing a maximum conductivity of 4.3 × 10–6 S/cm at 430 °C. Among various Ce1–x Sr x P2O7 samples, Ce0.9Sr0.1P2O7 shows the highest conductivity in humid air (pH2O = 0.12 atm) with a maximum conductivity of 6.3 × 10–3 S/cm at 90 °C, and the conductivity of Ce0.9Sr0.1P2O7 was 3.5 × 10–3 S/cm at 190 °C and pH2O = 0.12 atm. On the basis of XRD results, the stability of Ce1–x Sr x P2O7 in a humid atmosphere is analyzed.

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The proton conducting electrolyte cerium pyrophosphate was synthesized by digesting cerium oxide with 85% H 3 PO 4 . The crystal structure and phase stability of the material were analyzed by X-ray diffraction and thermogravimetric analysis/differential thermal analysis (TGA/DTA), and the microstructure was analyzed by scanning electron microscopy (SEM). The electric conductivity behavior of CeP 2 O 7 sintered pellets was analyzed by electrochemical impedance spectroscopy (EIS) and modulus spectroscopy. The activation energies for migration (E ω ) and dc conduction (E σ ) calculations indicates that the charge carriers responsible for conductivity and relaxation are same and the concentration of charge carries is independent of temperature. The variation of conductivity with temperature was studied in dry and humid atmosphere for the possible application as electrolyte in proton conducting ceramic electrolyte fuel cells (PCFCs) in the temperature range of 100-220 • C, which showed that the conductivity of CeP 2 O 7 was mainly due to the incorporation of water and the maximum conductivity was found to be 2.1 × 10 −4 S cm −1 at 175 • C and P H2O = 0.06 atm. The presence of water in CeP 2 O 7 matrix increases the number of jump sites and facilitates the hopping of protons, leading to an increase in the conductivity.