Acceptor-doped barium zirconate: Oxidation, hydration and space-charge formation

Licentiatavhandling, 2015

The current production and use of fossil fuels is not sustainable and new technologies are needed to become more independent of these fuels. The hydrogen economy, with the fuel cell as an efficient converter of chemical to electrical energy, is a desirable alternative. For this to become a reality, new materials with tailored properties are required. The effort to find suitable materials could be helped if the different mechanisms that govern the desired properties are well understood. The work done in this thesis has been an effort to address two problems concerning the ionic and electronic conductivity in perovskite oxides, which is a class of materials suitable for the use in solid oxide fuel cells (SOFCs) and related technologies. The research is theoretical and combines atomistic simulations and thermodynamic modeling. The first study is more general and deals with how accurately electronic structure-based methods can treat oxidation and hydration of these materials. Acceptor-doped BaZrO3 is chosen as a model system. Three methods have been considered: Density functional theory with PBE (a GGA functional) and PBE0 (a hybrid functional), and the GW method. While the hydration reaction is well described by all methods the oxidation reaction is found to differ in a qualitative manner. The reaction is found to be exothermic with PBE and endothermic by the two others. The latter scenario is more consistent with experimental data on reaction enthalpies, carrier concentrations and electrical conductivities. The second problem is more specific and concerns the proton conductivity in acceptor-doped BaZrO3, which has great potential as electrolyte material in proton-conducting SOFCs due to the combination of high chemical stability and high bulk proton conductivity. The problem lies in the grain boundaries, which have a deteriorating effect on the total proton conductivity due to the presence of space-charges at these interfaces. The mechanism behind space-charge formation is found to be segregation of charged oxygen vacancies and protons to the grain boundaries, where protons give the largest contribution under fuel cell operating conditions.