Oxygen reduction oin perowskite-type electrodes

In this project the electrochemical oxygen exchange taking place at perovskite-type electrodes on solid electrolytes is in the focus of research. This reaction is highly relevant from a fundamental point of view (model reaction in solid state electrochemistry) but also due to its importance in solid oxide fuel cells (SOFCs), gas sensors and other electroceramic devices. Active reaction sites of the electrochemical oxidation/reduction are either on the electrode surface or at the three phase boundary where oxygen, electrolyte and electrode meet. The effect of surface chemistry (e.g. surface cation composition, segregation), surface and bulk crystal structure and microstructure on the reaction kinetics is largely unknown and degradation or activation phenomena of electrodes are also not well understood. It is the goal of this project to investigate and quantify structure-property relations of perovskite-type electrodes and to better understand the elementary mechanisms and the functional role of the surface qualities governing the kinetics of the electrochemical oxygen exchange. The following key questions will be considered: i) What is the role played by surface composition, crystal structure and microstructure of perovskite-type electrodes in the kinetics of electrochemical oxygen reduction? ii) What are the reasons for degradation and activation of perovskite-type electrodes? iii) What are the reaction rates at three phase boundaries? iv) What is the exact reaction mechanism of oxygen reduction / oxide ion oxidation? In order to contribute to the solution of these questions, defined thin film microelectrodes ((La,Sr)CoO 3-d , (La,Sr)MnO3 on yttria stabilized zirconia) will be prepared by pulsed laser deposition and lithography and their electrochemical properties will be investigated by impedance spectroscopy and I-V measurements. The surfaces will be systematically varied by different preparation conditions, thermal and voltage treatment and by ion beam etching for "in-situ" creation of new surfaces and three phase boundaries. Chemical and structural information will become available from HRTEM, EELS, TOF-SIMS, XRD and AFM studies performed by collaboration partners. Reaction kinetics at three phase boundaries will be investigated via additional micro-structuring of microelectrodes, extrapolation of data under anodic polarization and by specially designed multilayer microelectrodes which allow electrochemical oxygen reduction only close to three phase boundaries. Additional mechanistic information will be obtained from p(O2 ) and voltage dependent experiments. These studies should allow us to significantly improve the understanding of electrochemical oxygen reduction on perovskite-type electrodes.

Solid oxide fuel cells (SOFCs) are electrochemical devices which directly transfer chemical energy, stored in a fuel, to electrical energy, without the need for conventional combustion. Therefore very high theoretical efficiencies can be reached. Real efficiencies are already quite good but still significantly below the theoretical ones. Moreover, degradation of the cell performance is a permanent issue. Both aspects are often attributed to processes taking place at the cathode of SOFCs, where oxygen is reduced. Usually mixed conducting perovskite-type oxides are employed as cathodes and the question remains, why these materials behave non-ideal. It was the main goal of this project to investigate the oxygen reduction kinetics and its degradation for oxide cathodes, with special emphasis on the role played by surfaces and grain boundaries. Mainly two materials (LaCoO3 and LaMnO3) were studied and for each of them substantial progress could be achieved in the scientific understanding of electrochemical properties. For LaCoO3 it was shown by four different methods that Sr segregation to the surface plays a major role in its electrochemical degradation. This was not only measured qualitatively, Sr segregation kinetics and Sr amounts could even be quantified. Among others the far-reaching conclusions became possible by developing a chemical etching based new tool for surface analysis. For LaMnO3 the importance of two different reaction pathways could be shown. Depending on temperature, electrode geometry and microstructure, the reaction at the three phase boundary or ion transport through the oxide electrode was shown to be important, the latter mainly proceeding via grain boundaries. Again development of a novel set-up, allowing very reliable measurements on microelectrodes, was essential in this respect.