|Title:||Electrochemical characterization of (La,Sr,Ca)2Co2-xMnxO6-δ as cathode materials for solid oxide fuel cells||Language:||English||Authors:||Viernstein, Alexander||Qualification level:||Diploma||Keywords:||Solid Oxide Fuel Cells; Conductivity||Advisor:||Fleig, Jürgen||Assisting Advisor:||Kubicek, Markus
|Issue Date:||2017||Number of Pages:||83||Qualification level:||Diploma||Abstract:||
Intermediate temperature fuel cells, with a working temperature between 500 °C and 700 °C, suffer from the slow reduction kinetics of oxygen at the cathode side which is responsible for most of the resistive losses. The aim of this work was the investigation of the electrical properties, oxygen exchange kinetics and diffusion processes of 18O tracer ions in advanced cathode materials, such as perovskites with the generalized structure (La,Ca,Sr)2Co2-xMnxO6-¿. First, targets materials were synthesized by a Pechini-type method. Subsequently thin films of different compositions were grown by pulsed laser deposition (PLD) under various p(O2) on three different single crystal substrates, since the background pressure in the PLD chamber and the used substrate have a massive impact on the thin film structure and properties. For ionic conductivity determination, some of the samples were microstcructured by UV-lithography and impedance spectroscopy was performed. The electrical properties of bulk and thin film samples were investigated by van der Pauw method. Keeping the Co/Mn ratio 1:1, a higher conductivity for Ca and Sr doped samples is observed due to p-type doping. Moreover, by changing the relative amount of Co without A-site doping conductivity can also be enhanced. All produced and investigated bulk samples possess a lower electrical conductivity and higher activation energy than La0.8Sr0.2MnO3 (LSM). For applicable cathode materials, a good oxygen diffusion is crucial. Hence, La2CoMnO6 thin film were investigated by performing 18O exchange experiments and ToF-SIMS depth profiling. With these techniques, the diffusion coefficient D* and the surface exchange coefficient k* were determined. In La2CoMnO6 samples, grain boundary diffusion plays a vital role. Diffusion coefficients of the grain boundaries are one to two orders of magnitude higher than the bulk diffusion coefficients and stay constant when p(O2) in the PLD-chamber during thin film growth is increased. Additionally, a high p(O2) leads to an additional diffusion process and is most likely related to the dislocation concentration. For the determination of the ionic area specific resistance of La2CoMnO6 impedance spectroscopy was carried out. Activation energies below 550 °C indicate a triple phase boundary path of oxygen reduction. If the samples were heated for the first time above 500 °C irreversible changes took place and the activation energies and resistivity rose tremendously in further measurements.
|Library ID:||AC13773719||Organisation:||E164 - Institut für Chemische Technologien und Analytik||Publication Type:||Thesis
|Appears in Collections:||Thesis|
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checked on Feb 21, 2021
checked on Feb 21, 2021
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