Atomistic simulation studies of oxygen ion conducting metal oxides for fuel cell applications.

This thesis describes the application of computer simulation techniques to the detailed study of the defect chemistry and ion transport properties of oxygen ion conducting metal oxides. Attention is focused on two materials; stabilised zirconia (ZrO2), a conventional solid electrolyte with important applications within solid oxide fuel cells, and perovskite-structured LaGaO3, a promising new oxygen ion conductor. First, a wide-variety of low-valent metal ions are substituted into zirconia, and the energetics of solution investigated. Favourable dopants (on energetic grounds) have been calculated which include CaO, Y2O3 and Gd2O3, in agreement with observation. Dopant-vacancy clusters are also examined, and the results reveal significant relaxation around dopant ions. These simulations are extended to examining the topical area of Nb/Y co-doping. Oxygen ion diffusion in yttria-stabilised zirconia is studied by application of Molecular Dynamics (MD) techniques; our results support models in which diffusion is mediated by oxygen vacancies, with calculated diffusion coefficients and activation energies in accord with tracer diffusion studies. The results from the first reported computational study of the LaGaO3-based oxygen ion conductor are then presented. We consider a range of cation dopant substitutions with oxygen vacancy compensation. Favourable acceptor-dopants are predicted to be Sr at La and Mg at Ga, in agreement with conductivity studies. The pathway for oxygen vacancy migration is found to be along the GaO6 octahedron edge with a curved trajectory. Hole formation from an oxidation process is calculated to be relatively unfavourable, which is compatible with experimental findings that show predominantly ionic conduction in doped LaGaO3. Furthermore, consideration of water incorporation suggests that proton conduction will not be significant in this material. Finally, shell model MD is used to examine oxygen transport in doped LaGaO3 with varying concentrations of Sr and Mg dopants. Oxygen diffusion coefficients are calculated over a wide temperature range, with useful information on the atomistic mechanism for oxygen ion migration illustrated in the form of trajectory plots