Mechanismen der chrombasierten Degradation von metallgestützten Festoxid-Brennstoffzellen

The aim of the present work is the investigation of the chromium degradation of metal supported solid oxide fuel cells (MSC). The starting point is the MSC concept of Plansee SE which utilizes a La$_{0.58}$Sr$_{0.40}$Co$_{0.20}$Fe$_{0.80}$O$_{3-\delta}$ (LSCF) cathode. From the results of three work packages, conclusions are drawn how chromium degradation influences a MSC cathode and if there is a possibility to prevent this interaction. Using thin-film samples, strontium segregation, a key process of chromium degradation, is systematically investigated. For the first time a quantitative comparison between LSCF and La$_{0.58}$Sr$_{0.40}$Co$_{1.00}$O$_{3-\delta}$ (LSC) cathode material using ICP-MS is conducted. The combination of imaging (SEM) and spectroscopic (XPS) methods gives strong evidence that the strontium segregation depends on the thermal treatment of the sample and that it is stronger on cobalt rich LSC compared to LSCF. Moreover, methods for accelerated testing have been developed to poison samples with porous cathode layers reproducibly. Such methods enable a quick comparison of different cathodes before using them in a stack test. While the poisoning via gas phases proved to be poorly reproducible, the desired effect could be achieved with solid state poisoning. It can be shown that both poisoning techniques have the same influence on the impedance spectra of poisoned cells. The third work package deals with the chromium related degradation on stack level and its dependence on the oxygen partial pressure. Depending on the operation conditions, the deposition of a chromium containing phase can be triggered at the interface between the LSCF cathode and the gadolinium doped ceria diffusion barrier when using an LSCF cathode. This must be prevented. The combined results of all three work packages allow the following conclusions: (i) a low operation temperature is helpful to slow down the strontium segregation. (ii) with the solid state poisoning, a method for systematic and reproducible poisoning of single cells is available. With this, different stages of chromium poisoning can be simulated in short time.(iii) a future MSC stack design must be geared to avoid gradients in oxygen partial pressurethroughout the cathode layer. Otherwise, not only the chromium degradation but also the intrinsic degradation of the LSCF cathode is enhanced. (iv) the most effective way to protect the cathode from chromium poisoning is the use of a dense interconnect coating.