Impact of a high surface area on the defect chemistry of mesoporous ceriaWednesday (26.09.2018) 11:00 - 11:15 S1/03 - 223 Part of:
Ceria-based solid solutions are typical mixed electronic and ionic conductors (MIECs) due to the redox activity of the redox couple Ce3+/Ce4+. The combination of good ionic and electron-ic conductivities with high oxygen storage capacities (OSC) qualifies the material for various applications, e.g. as solid electrolytes in oxygen membranes or as electrode material in solid oxide fuel cells. For these applications, a detailed knowledge of the surface properties is nec-essary, particularly the concentration and type of electronic and ionic defects, as the solid gas interface determines the oxygen kinetics, and thus, the catalytic activity of oxide ceramics. However, studies on surface transport properties are scarce, mainly because of difficulties in preparing high quality thin films, where the surface dominates the bulk behavior. Ideal model systems to investigate the influence of the surface and the surrounding gas atmosphere on the defect chemistry in ceramic oxides are mesoporous materials, which exhibit a high surface area due to their regular pore structure surrounded by a closed packed, interconnected 3D architec-ture of nanocrystallites. Here, we present the investigations of the electrical properties of mesoporous CexZr1-xO2 (CZO) and CexPr1-xO2 (CPO) thin films prepared from common salt precursors and an amphiphilic diblock copolymer by using an evaporation induced self-assembly process. The structural properties of the thin films prepared were analyzed via SEM, WAXD, XRD, XPS and Raman spectroscopy, confirming the successful synthesis of a meso-porous material of high structural quality with a surface area of about 150 m²/g. The tempera-ture-dependence of the electrical conductivity of the thin films was investigated for varying oxygen partial pressures using electrochemical impedance spectroscopy. Comparing the results with transport properties of single crystals reveals the profound effect of the mesoporous morphology on the 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 thin films reveal a conductivity plateau under strongly reducing conditions, which is attributed to a dominant electronic conductivity with restricted hopping sites.