Degradation von Ni-Cermet-Anoden in keramischen Hochtemperaturbrennstoffzellen
- Degradation of Ni-Cermet-anodes in solid oxide fuel cells (SOFC)
Iwanschitz, Boris; Schütze, Michael (Thesis advisor)
Aachen : Publikationsserver der RWTH Aachen University (2012)
Dissertation / PhD Thesis
Aachen, Techn. Hochsch., Diss., 2012
The proof of lifetimes of Solid Oxide Fuel Cell (SOFC) components has become a major issue in SOFC materials research. The Nickel-Cermet-Anode is considered to be a key component, because it is exposed to tough working conditions such as high temperatures, reducing and oxidizing atmospheres, high water vapour contents, mechanical stresses etc. The aim of this work was to experimentally investigate the microstructural degradation of Ni-Cermet-Anodes and to correlate the microstructural parameters to the cell performance. The experimental results serve to validate an electrode model to predict the lifetime of a Ni-Cermet-Anode. The development of such models will become more and more important for the future, because the lifetime of SOFC components is continuously increasing. Lifetime predictions can therefore accelerate the development of new materials and reduce the costs. The major topics considered in this work were (1) the electrochemical characterization of different Ni-Cermet-Anodes under different operating conditions and (2) the quantitative analysis of microstructures to describe the degradation phenomena. It was shown that redox-cycles have a major impact on both the degradation of the microstructure and the cell performance. Both, the microstructural degradation and the decrease in cell performance could be lowered by reducing the operating temperature. For the cells with Ni/YSZ-Anodes the cell performance was clearly influenced by the microstructure and the phase composition. In contrast, cells with Ni/CGO-Anodes showed a different degradation behaviour. Microstructures of aged anodes were investigated by quantitative analysis of scanning electron microscope images and energy dispersive X-ray mappings. The most obvious microstructural changes were the Ni-agglomeration, the destruction of the ceramic backbone and the increase of the porosity. The temperature dependency of the Ni-agglomeration could be described with an Arrhenius-type equation. Furthermore, water vapour was shown to play a significant role for the Ni agglomeration mechanism. It could be seen, that not only the water vapour concentration but also the amount of water influences the Ni agglomeration. Material transport may occur over a surface diffusion process, where the nickel hydroxide is involved. The Ni-agglomeration in humidified reducing atmosphere was observed over 2'000 hours and described by a t1/4 growth law (surface Ostwald-ripening).