ALBERT

Notes: The influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr0.85Y0.15O3–δ was readily sintered to above 93% of theoretical density at 1300°C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO2 showed ZnO-modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H2O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ∼0.9 at 600°C. The combination of electrical and chemical properties and good sinterability render ZnO-modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications.

Notes: The AC impedance response of mixed ionic and electronic conductors (MIECs) is derived from first principles and quantitatively compared with experimental data. While the approach is not entirely new, the derivation is provided in a unified and comprehensive manner. Using Sm0.15Ce0.85O1.925–δ with Pt electrodes as a model system, a broad spectrum of electrical and thermodynamic properties is extracted solely from the measurement of impedance spectra over wide oxygen partial pressure and temperature ranges. Here, the oxygen partial pressure was varied from air [po2=0.21 atm] to H2 [po2=10−31 atm], and the temperature was varied from 500° to 650°C. It was essential for this analysis that the material under investigation exhibit, under some conditions, purely ionic behavior and, under others, mixed conducting behavior. The transition from ionic to mixed conducting behavior is recognizable not only from the oxygen partial pressure dependence of the total conductivity but also directly from the shape of the impedance spectra. Within the electrolytic regime, the impedance spectra (presented in Nyquist form) take the shape of simple, depressed arcs, whereas within the mixed conducting regime (under reducing conditions), the spectra exhibit the features associated with a half tear-drop-shaped element. Parameters derived from quantitative fitting of the impedance spectra include the concentration of free electron carriers, the mobilities and activation energies for both ion and electron transport, the electrolytic domain boundary, and the entropy and enthalpy of reduction. In addition, the electrochemical behavior of O2 and H2 at the Pt∣ceria interface has been characterized from these measurements. Under oxidizing conditions, the data suggest an oxygen electrochemical reaction that is rate limited by the dissociated adsorption/diffusion of oxygen species on the Pt electrode, similar to Pt∣YSZ (yttria-stabilized zirconia). Under reducing conditions, the inverse of the electrode resistivity obeys a 〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE00740:JACE_00740_mu1" location="equation/JACE_00740_mu1.gif"/〉 dependence, with an activation energy that is similar to that measured for the electronic conductivity. These results suggest that ceria is electrochemically active for hydrogen electro-oxidation and that the reaction is limited by the rate of removal of electrons from the ceria surface.