Oxygen Diffusion Studies in Mixed Ionic Electronic Conducting Compounds

Cobalt-based oxides with perovskite-related structures are important candidate cathode materials for the next generation solid oxide fuel cells (SOFCs) because of their good catalytic properties and high values of electronic and ionic conductivity. The aim of the work described in the thesis is to investigate oxygen transport in Sr3YCo4O10.5 (SYC, the so-called 314 phase) and double perovskites LnBaCoFeO5+δ (Ln = La, Pr, Sm and Gd) with combination of theoretical and experimental methods. The oxygen transport properties of Sr3YCo4O10.5 were studied by three different techniques. Molecular dynamics simulations (MD) predicted an isotropic character of oxygen diffusion with an activation energy of 1.56 eV in the temperature range of 1000 – 1400 K. Values of tracer oxygen diffusion, D*, and the surface exchange coefficient, k*, were measured as a function of temperature (500 – 900 °C) with isotope exchange depth profiling combined with secondary ion mass spectrometry (IEDP/SIMS). Chemical oxygen diffusion coefficients, Dchem, and chemical surface exchange rates, kchem, were obtained from electrical conductivity relaxation experiments (ECR). Good agreement of the results obtained by different techniques was confirmed after application of thermodynamic factors to the ECR data. Electrochemical performance of symmetrical cells with Ce0.9Gd0.1O1.95 (GDC) as electrolyte (SYC/GDC/SYC) in the temperature range 620 – 770 °C was evaluated. Double perovskites LnBaCoFeO5+δ (Ln = La, Pr, Sm and Gd) were studied by combination of ECR, thermogravimetric analysis (TGA) and dilatometry. It was found that size of rare earth element (REE) has strong influence on electrochemical and mechanical properties of the samples. In case of La and Pr-containing compounds, fast degradation of the surface kinetics during ECR experiments at high oxygen partial pressure was associated with surface segregation. ECR measurements at low pO2 switches were performed for La, Pr and Gd-containing samples to avoid large deviation from chemical equilibrium during experiments. For SmBaCoFeO5+δ it was found that a linear model for the exchange kinetics works in a larger pO2(1)/pO2(2) interval. The effect of A-cation ordering in LaBaCoFeO5+δ on oxygen transport properties was investigated. Oxygen diffusion was found to be slower in ordered structure, due to the influence of ordering in A-cation sublattice on anion ordering.