Keramische Protonenleiter mit perowskitartiger Struktur

Abstract

Some aliovalently doped perovskites like SrCe_0_._9_5Yb_0_._0_5O_3_-_#alpha# or BaCe_0_._9_0Nd_0_._0_5O_3_-_#alpha# dissolve significant amounts of water and become proton conducting. These ceramics are promising electrolytes in solid oxide fuel cells (SOFC), where hydrogen containing fuel is directly converted into electricity. This thesis efforts for a fundamental comprehension of the defect properties and the mechanism of the ionic transport. It contains the results of a number of independent solid state physical and electrochemical studies. For the first time measurements of magnetic properties, neutron diffraction and quasielastic neutron scattering are presented. Several new materials, pure and doped strontium-cerates, -zirconates and mixed creates-zirconates, were prepared and their morphology and composition thoroughly characterized. The incorporation of protons was investigated in a broad spectrum of gas phases. Measurements of the magnetic properties were performed at low temperatures (1.8 K<T<300 K). They gave the oxidation states and the concentrations of the different dopant-ions. The crystal structures of the ceramics were obtained by means of X-ray and neutron diffraction, which excellently complemented one another. The proton conductivity or diffusivity, respectively, was studied by impedance spectroscopy (IS) and quasielastic neutron scattering (QENS). While the former method yielded a macroscopic proton diffusion coefficient, the latter method permitted to elucidate the microscopic proton diffusion mechanism. To describe the complex mechanism a two-state model was derived. It explains the diffusion consisting of a sequence of free diffusion and trapping/escape events with the Yb"3"+-ions acting as trapping centers. The model enables the calculation of the spatial and temporal parameters. The self-diffusion coefficient obtained with QENS agrees with the conductivity diffusion coefficient obtained with in-situ-IS considering the transport numbers. The transport of protons was proven by the cell voltages of concentration cells. Furthermore, mixed conduction was observed. In dependence of temperature and composition of the gas phase, protons, oxygen ions and (defect-)electrons determined the charge transport. The total conductivity of the ceramics was divided in the contribution from the bulk and the grain-boundary. The specific conductivity was fairly independent of the nature of the dopant-ion but depends on its concentration. The mixed perovskites show promising properties. The electrode/electrolyte interface was studied by means of current-potential curves and impedance spectroscopy under potential control. In oxygen-rich atmosphere the reaction at the interface consists at least of two processes, which show an apparent activation energy of179 refs.SIGLEAvailable from TIB Hannover: RA 831(3121) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman