Transport properties of mixed ionic and electronic conductors - from bulk to nanostructure

Ceria-based materials exhibit mixed ionic and electronic conductivity due to the redox-activity of the cation (Ce4+/Ce3+) and the oxygen ion mobility in the fluorite-type lattice, which intrinsically tends to form a high concentration of Anti-Frenkel defects. Both electrons and ions migrate by an activated hopping mechanism with activation barriers of 0.4 eV for the hopping process of small polarons, 0.8 eV and up to 1.6 eV for the migration and the extrinsic formation and migration of oxygen vacancies, respectively. This leads to a p(O2)-sensitive electrical conductivity, which can be dominated by each process depending on temperature and oxygen activity. Moreover, the material is quite tolerant regarding the substitution of cations. By choosing the type and range of substitution, electrical properties can be adjusted in different ways. Therefore, ceria and its substituted analogues qualify for various applications as solid electrolytes in oxygen membranes, electrode material in solid oxide fuel cells (SOFCs) and in combination with the high oxygen storage capacity for support material in heterogeneous catalysis. The defect chemistry of ceria is already extensively investigated in literature. Thus the material is an ideal model system to study interface effects, in particular the concentration and type of electronic and ionic defects as well as their transport properties in the vicinity of interfaces, complementing the established defect chemical models for bulk material. Within this work we compare the solid solution of ceria and praseodymia (Ce1-xPrxO2-δ) with the solid solution of ceria and zirconia (Ce1-xZrxO2-δ). It is already known, that due to the redox-activity of Pr-ions the combination with praseodymia can lead to an additional transport of polarons, increasing the electronic contribution to electrical conductivity. In contrast, the combination with isovalent zirconia results in an increase of the so-called reducibility of ceria due to the size mismatch of the cations. To gain a deeper understanding of the role of these substitutions on electrochemical transport processes at interfaces, highly ordered mesoporous thin films of Ce1-xPrxO2-δ (CPO) and Ce1-xZrxO2-δ (CZO) have been investigated. The mesoporous samples have been synthesized by a sol-gel process using evaporation induced self-assembly, resulting in a regular pore structure surrounded by a closed packed, interconnected 3D architecture of nanocrystallites. The structural properties were analyzed by SEM, WAXD, XRD, XPS and Raman spectroscopy, confirming the successful synthesis of a mesoporous material of high structural quality, where the surface dominates over the bulk behaviour. The electrical properties were investigated as a function of temperature and oxygen partial pressure using electrochemical impedance spectroscopy. The comparison of the results with previous results of single crystalline samples elucidates the effect of the continuous pore structure on electrical transport properties. The CZO thin films show an unusual p(O2)-dependence at high oxygen partial pressures, which cannot be explained by standard defect chemical models. Furthermore, both mesoporous samples reveal a conductivity plateau under strongly reducing conditions, which is discussed in terms of hopping statistics and defect-defect interaction