Defect chemistry of mixed conducting perovskites. Interplay of protonic defects, oxygen vacancies and electron holes

Mixed conducting oxides are key functional materials for clean energy technologies, and their chemical and electrical properties need to be carefully tuned to optimize the device performance. They can be employed as cathodes for Protonic Ceramic Fuel Cells (PCFCs) which work at intermediate temperatures (400-600 °C), bridging the gap between polymer membrane and solid oxide fuel cells. A cathode with mixed hole, oxygen vacancy, and proton conductivity extends the reactive zone for the oxygen reduction to water beyond the triple phase boundary, making the whole cathode surface an active electrocatalyst. The defect chemistry (concentrations and mobilities of point defects) of such materials with three charge carriers is complex, and some of the desired properties for a PCFC cathode materials are in mutual conflict (i.e., proton uptake, electronic conductivity, catalytic activity, long-term chemical stability). The present thesis deals with fundamental questions such as the relation between proton uptake and structural and electronic properties, or point defect interactions. It also aims at extending the experimental data basis by varying the cation stoichiometry in BaFeO${}_{3-\delta }$-based perovskites. Protons are incorporated into BaFeO${}_{3-\delta }$-based perovskites via hydration at expense of oxygen vacancies (acid-base reaction), or by hydrogen uptake at expense of holes (redox reaction). Their concentration can be determined by thermogravimetric analysis (TGA). Doping has strong effects on the materials properties. Acceptor dopants directly affect defect concentrations via the electroneutrality condition, increasing the oxygen vacancy and/or hole concentration. In addition, oversized B-site dopants such as Zn${}^{2+}$, Mg${}^{2+}$, Ca${}^{2+}$ and Y${}^{3+}$ can lead to local distortions, which furthermore affect the electronic structure. This thesis comprises three groups of materials, they are investigated with a combination of methods to obtain a comprehensive understanding of their defect chemistry.