Computational modelling of SmCoO3-based cathode materials for solid oxide fuel cells

This thesis presents the results of a computational study of bulk SmCoO3-based perovskites for use as Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC) cathode. Using both Density Functional Theory with on-site Coulomb corrections (DFT+U), with correction applied to the transition metal d-electrons, and Molecular Dynamics (MD), the properties of this material, and the effects of an array of dopants on these, is investigated, all in relation to cathode efficiency. Firstly, a bulk characterisation using DFT+U of SmCoO3 is conducted. Two crystal structures of SmCoO3 are modelled; cubic and orthorhombic, and both crystal structures are semiconductors at 0 K. The experimentally observed semiconductor-to-metal transition is then investigated by studying different magnetic orders at different temperatures, with C-type antiferromagnetic ordering in the cubic structure being identified as responsible for this transition. Importantly, the spin transition is directly linked to changes in the Co-O bond lengths and distortions in the CoO6-polyhedra. Secondly, the oxygen and metal vacancy formation in SmCoO3 is investigated, as these can directly influence the IT-SOFC cathode efficiency. To put the SmCoO3 results into context, a comparison with LaMnO3 is performed, as this is the current state of the art SOFC cathode parent-material. LaMnO3, and its doped form La1-xSrxMnO3-d (LSM), are the benchmarks for this study, as the aim of this thesis is to identify a material with more favourable cathode properties than LSM. It is shown that oxygen vacancies strongly alter the electronic and magnetic structure of SmCoO3, but barely affect LaMnO3. The intrinsic concentration of oxygen vacancies in both SmCoO3 and LaMnO3 is very low by virtue of the high oxygen vacancy formation energy. Oxygen vacancies are typically induced by doping in these materials. Studying the cation vacancy shows that the formation of cation vacancies is less energetically favourable than oxygen vacancies (typically more than 3 eV higher in energy), and ...