Computational study of FeNbO4-based fuel electrode for Solid Oxygen Cells

This thesis presents the results of a computational study of the bulk and surface of FeNbO4 used as fuel electrode of solid oxygen cells (SOCs). Using density functional theory with D2 long-range dispersion correction and on-site Coulomb interactions (DFT+U–D2), we have investigated a number of properties of this material.
First, the antiferromagnetic structure of the monoclinic FeNbO4 (m-FeNbO4) observed experimentally is examined to be the most stable one. The dissociation of water at the pristine surfaces without introduced defects are simulated to reveal its mechanism as the cathode of solid oxygen electrolysis cells (SOECs). Secondly, the dissociation of hydrogen and the formation of water on the surfaces of orthorhombic FeNbO4 (o- FeNbO4) is investigated. For the surface reactions, the hydrogen prefers to dissociate in the oxygen-oxygen sites whereas the migration of the hydrogen from the metal-oxygen sites are more energetically favourable. Thirdly, we have analysed the probabilities of inequivalent configurations of the o-FeNbO4 to explain why the cations are distributed disorderedly. The oxygen diffusion in the stoichiometric and non-stoichiometric structures is investigated and it shows that the diffusion in the oxygen-deficient structure is much easier than in the perfect structure and the energy barriers are affected by the surrounding oxygen type. Finally, the effect of the first-row transition metal dopants on the stoichiometric and non-stoichiometric structures are investigated. The simulations show that substituting Fe cations with dopants in the stoichiometric structure is more energetically favourable than Nb cations which leads to significant volume expansion, while the introduction of oxygen vacancies can lower the doping energies for the Nb sites. The electron conduction is strengthened especially for the Ti and V dopants into the Fe sites in all structures.
In this way, the findings presented in this thesis provide a theoretical insight into the bulk and surface properties of the FeNbO4 materials.