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

• Main Author: Olsson, E. I. K.
• Other Authors: de Leeuw, N. H.
• Published: University College London (University of London) 2017
• Subjects: 540
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.746785

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 would thus not markedly influence the performance of the cathode. Thirdly, substitutional doping of Ca2+, Sr2+, and Ba2+ on the Sm-site in SmCoO3 is investigated. DFT+U calculations are employed to investigate dopant configurations at two different dopant concentrations: x=0.25 and 0.50, with two different charge compensation mechanisms; oxygen vacancies, and electronic holes. Comparing hole, and oxygen vacancy formation energies, we conclude that oxygen vacancy charge compensation is the preferred mechanism to maintain electroneutrality of the system. Furthermore, the increase in electronic conduction is not a direct consequence of the appearance of electron holes, but a result of the distortion of the Co-O bonds. Finally, MD is employed to model ionic conduction and thermal expansion coefficients (TEC). All dopants at both concentrations show high ionic conduction comparable, but too high TEC to match IT-SOFC electrolytes, with Ca2+-doping showing the combined most preferable properties. Fourthly, Co-site doping with Fe3+, Mn3+, Ni3+, and Cu3+ is investigated, as this has been shown to reduce TEC in similar materials. Again, doping introduces distortion into the system, inducing different electron occupations of the Co d-orbitals, thus altering the electronic and magnetic structure. From these calculations, the 0 K electronic conductivity (σe) is obtained, with SmMnxCo1-xO3 showing the highest σe, and SmFexCo1-xO3 lowest. From calculations of the oxygen vacancy formation energy, no direct impact on the ionic conductivity is expected from Co-site doping. Mn3+-doping show the lowest TEC at low x. Thus, the subject of our final study is Sm0.75Ca0.25MnxCo1-xO2.88 x=0.125, and 0.25. With the effect of Mn-doping at both x being negligible around the valence band maximum, and mainly being observed 2 eV below the Fermi level and 1 eV above in the conduction band, the electronic conduction is mediated by the Co d-bands. This, together with the presence of an oxygen vacancy, results in lower electronic conduction than observed in Chapter 6, but sufficiently high for IT-SOFC purposes. The limiting factor for IT-SOFC cathodes, and the factor making LSM unsuitable as a cathode material at lowered operating temperatures, is the ionic conduction. Ionic conduction and oxygen diffusion calculations show that Sm0.75Ca0.25MnxCo1-xO2.88 are good oxygen conductors, with higher ionic conduction than LSM. x=0.25 shows the lowest TEC, and is thus concluded to be the most favorable Mn-doping concentration. Thus, from this work, we present Sm0.75Ca0.25Mn0.25Co0.75O2.88 as an IT-SOFC cathode material that offers significant performance benefits when compared to LSM.