251 83 367 05 Doped ceria and ceria based solid oxide solutions show a unique combination of oxygen ion mobility, electronic conductivity, and high catalytic activity for redox reactions. In this work, the minority conductivity of electrons has been measured directly as a function of the composition of ceria-praseodymia based solid solutions in order to maximize the electronic conductivity without depressing the oxygen ion mobility. The influence of Co as well as the Gd/Pr dopant ratio on the electronic conductivity of ceria-praseodymia pellets was studied for the compositions Ce 0.8 Gd 0.2Àx Pr x O 2Àd (0.05 x 0.15) with and without an additional Co content of 0.02 with respect to the formula. The Hebb-Wagner polarization technique was used with ion-blocking microcontacts. In the temperature range 700-800 8C, the presence of high amounts of praseodymium increases the p-type conductivity by a factor of more than 10 for oxygen partial pressures higher than 10 À10 bar. Co-doped ceria-gadolinia-praseodymia solid solutions showed a further increase of the electronic conductivities in a partial pressure range where the Co-free materials showed the minimum of the electronic conductivities. It is assumed that the effect of the additional cobalt doping is due to electronic short circuits along the grain boundaries via segregated CoO.

The ternary solid solution CeO 2 -ZrO 2 is known for its superior performance as an oxygen storage catalyst in exhaust gas catalysis (e.g. TWC), although the defect chemical background of these outstanding properties is not fully understood quantitatively. Here, a comprehensive experimental study is reported regarding defects and defect-related transport properties of cubic stabilized single crystalline (Ce x Zr 1Àx ) 0.8 Y 0.2 O 1.9Àd (0 r x r 1) solid solutions as a model system for CeO 2 -ZrO 2 . The constant fraction of yttria was chosen in order to fix a defined concentration of oxygen vacancies and to stabilize the cubic fluorite-type lattice for all Ce/Zr ratios. Measurements of the total electrical conductivity, the partial electronic conductivity, the ionic transference number and the non-stoichiometry (oxygen deficiency, oxygen storage capacity) were performed in the oxygen partial pressure range À25 o lg pO 2 /bar o 0 and for temperatures between 500 1C and 750 1C. The total conductivity at low pO 2 is dominated by electronic transport.A strong deviation from the widely accepted ideal solution based point defect model was observed.An extended point defect model was developed using defect activities rather than concentrations in order to describe the point defect reactions in CeO 2 -ZrO 2 -Y 2 O 3 properly. It served to obtain good quantitative agreement with the measured data. By a combination of values for non-stoichiometries and for electronic conductivities, the electron mobility could be calculated as a function of pO 2 , ranging between 10 À2 cm 2 V À1 s À1 and 10 À5 cm 2 V À1 s À1. Finally, the origin of the high oxygen storage capacity and superior catalytic promotion performance at a specific ratio of n(Ce)/n(Zr) E 1 was attributed to two main factors: (1) a strongly enhanced electronic conductivity in the high and medium pO 2 range qualifies the material to be a good mixed conductor, which is essential for a fast oxygen exchange and (2) the equilibrium constant for the reduction exhibits a maximum, which means that the reduction is thermodynamically most favoured just at this composition.

ZnO was studied as dopant or additive for CeO2 in the concentration range x = 0.00 to x = 0.95 referred to (CeO2)1-x(ZnO)x. From XRD, the solubility limit of ZnO in the bulk of CeO2was estimated as 3 ± 0.7 mol% at room temperature. The total conductivity was measured for all compositions in air by impedance spectroscopy. The contributions from bulk and grain boundaries were evaluated separately. Additionally, the electronic conductivities were measured using a Hebb-Wagner technique with microelectrodes. Up to the solubility limit, the total conductivity of the bulk in air increases almost linearly with the ZnO content from 300°C to 800°C, the same holds for the p-type electronic conductivity (in the range 600°C–800°C). Hence, zinc ions act as acceptors on cerium sites, zinc doped ceria is a moderate mixed conductor. In the two-phase domain from 3 mol% up to 20 mol%, the total conductivity increases slightly and the partial electronic conductivity dependence remains nearly constant. Starting at 30 mol% ZnO, the grain boundary conduction rises steeply and the electronic conductivity predominates the total conductivity for this and higher ZnO contents reaching 0.03 S/cm at 800°C in air for a sample with the composition x = 0.95 mol%.

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