| dc.description.abstract | In recent years it has become clear that a move to cleaner, more efficient energy generation is required. Solid oxide fuel cells (SOFCs) are a promising technology for electricity generation as they can overcome combustion efficiency limitations and can utilise a range of fuels, although there are a number of challenges currently associated with them. High temperatures are required for operation of current generation SOFCs, leading to high costs and accelerated performance degradation. Recently, the development of more efficient SOFCs which operate at intermediate temperature, in the region of 600?1000 K, has been investigated. The cathode, at which the oxygen reduction reaction takes place, currently requires a high operating temperature due to poor catalytic activity for the reaction at lower temperature, thus development of improved materials for intermediate temperature SOFCs is of great importance.
The structure and electronic properties of LaMnO3, which has been widely used in high temperature SOFCs, are modelled, examining the performance of density functional theory (DFT) and post-DFT methods. DFT fails to model the structural and electronic properties of LaMnO3 correctly, and although hybrid DFT can give an improved description of the structure, it does not correctly model features in the electronic density of states. The PBEsol + U functional is used to model the low index surfaces, and oxygen vacancy formation in the bulk and at these surfaces are examined. Nudged elastic band calculations are used to determine the activation energy for oxygen migration in bulk LaMnO3. The introduction of alkaline earth defects to bulk LaMnO3 in investigated, considering both the site selectivity and the charge compensation mechanism. Larger dopant cations (Ca, Sr, Ba) are found to have lower formation energies on the La site, while the smaller Mg cation shows preference for the Mn site. For all defects, charge compensation by hole formation is preferred over oxygen vacancy formation.
Secondly, the layered Ruddlesden-Popper oxide La2NiO4 is examined. As the structure can accommodate both oxygen vacancies and interstitials, both are modelled in the bulk and at the low index surfaces, to establish the dominant oxygen defects. As interstitial defects are found to be dominant, nudged elastic band calculations are used to determine the mechanism and
activation energy for oxygen interstitial migration, with the lowest activation energy significantly lower than was predicted in LaMnO3. Sr and Fe defects are introduced to both the La and Ni site, to determine their site selectivity, with Sr defects having lower formation energies on the La site, and Fe defects on the Ni site. Under SOFC operating conditions, doping with Sr is compensated by the formation of delocalised, unbound holes, while doping with Fe is compensated by the formation of oxygen interstitials.
Finally, LaCrO3 is investigated as a potential mixed ionic and electronic conductor. The origin of the charge carriers in LaCrO3 and Sr-doped LaCrO3 is established, by calculating the thermodynamic transition levels, allowing identification of defects with low formation energies and shallow transition levels. The activation energy for oxygen migration in bulk LaCrO3 is
found to be significantly higher than in either La2NiO4 and LaMnO3, therefore La2NiO4 and LaMnO3 are likely to be more suitable for SOFC applications. | en |
