About the Proton Conductivity of BaFeO2.5+δ Epitaxial Thin Films in the Intermediate Temperature Range

Reduction of the operating temperature to an intermediate temperature range between 350 °C and 650 °C is a necessity to further increase the competitiveness of Solid Oxide Fuel/Electrolysis Cells (SOFC/SOECs) with existing energy conversion technologies. By lowering the operating temperature several high-temperature-related issues can be eliminated leading to lower costs. Motivated by the goal of lower operating temperatures the application of proton-conducting oxides has become an active and broad area of research. The incorporation of these proton-conducting materials entails problems such as ohmic resistances in the electrolyte as well as polarization resistances at the air electrode. To lower the air electrode polarizations, materials, which can be used as effective electrode catalysts on the air electrode, are required to conduct protons and electrons at the same time. Therefore, this thesis focuses on a thorough investigation of the proton conduction in the promising material system BaFeO2.5+δ (BFO). The experiments are conducted on expitaxially grown BaFeO2.5+δ thin films, deposited by pulsed laser deposition on (001)- and (111)-oriented Nb:SrTiO3 substrates. To monitor changes occurring to the thin films, they are examined before and after the electrochemical characterization: This investigation includes the analysis of structural and microstructural information by X-ray diffraction and scanning electron microscopy. Additionally, Mößbauer spectroscopy is used to determine the local coordination and oxidation state of Fe throughout the complete film. For the purpose of accounting for changes to the surface composition the films are furthermore examined using X-ray photoelectron spectroscopy. For the characterization of the conductive properties Electrochemical Impedance Spectroscopy (EIS) is used, yielding a measurable protonic contribution. This protonic contribution can successfully be separated from the total conductivity by comparing measurements in wet and dry atmospheres (Ar or air, respectively). Thereby, the bulk proton conductivity of BFO can be estimated for the first time between 200 °C and 300 °C (3.6 × 10−6 S cm−1 at 300 °C). At temperatures above 300 °C the influence of oxidizing measurement atmosphere and water loss reveals a strong dependence on the conductivity. For the goal of reducing the operating temperature to the intermediate temperature range it is not only important to find and employ well-suited electrode catalysts but furthermore essential to reduce ohmic resistances in the electrolyte. For this reason the search for possible deposition techniques, which enable the deposition of the respective proton conductors according to the required attributes, is subject of research efforts. Based on these grounds two deposition methods, which to date have not been used to synthesize Y-doped BaZrO3 (one of the most promising proton conductors) thin films, were investigated in order to assess their potential of depositing high-quality films. In detail Laser-Assisted Chemical Vapor Deposition (LA-CVD) and Aerosol-Assisted Chemical Vapor Deposition (AA-CVD) are used for this purpose. The deposition parameters of the films are varied aiming to obtain stoichiometric smooth films with perovskite type structure. Both deposition methods present problems with relation to the required film attributes. The identified obstacles, for which solutions/workarounds are suggested, are based on thermodynamical as well as experimental facts