Ni-YSZ cermets have been used as anode materials in SOFCs for more than 20 years. Despite this fact, the major cause of degradation within the Ni-YSZ anode, namely Ni sintering / coarsening, is still not fully understood. Even if microstructural studies of anodes in tested cells are of technological relevance, it is difficult to identify the effect from isolated parameters such as temperature, fuel gas composition and polarization. Model studies of high temperature aged Ni-YSZ cermets are generally performed in atmospheres containing relatively low concentrations of H2O. In this work, the microstructural degradation in both electrochemically longterm tested cells and high-temperature aged model materials are studied. Since Ni particle sintering / coarsening is attributed to be the major cause of anode degradation, this subject attains the primary focus. A large part of the work is focused on improving microstructural techniques and shows that the application of low acceleration voltages (≤ 1 kV) in a FE-SEM makes it possible to obtain two useful types of contrast between the phases in Ni-YSZ composites. By changing between the ordinary lateral SE detector and the inlens detector, using similar microscope settings, two very different sample characteristics are probed: 1) The difference in secondary emission coefficient, δ, between the percolating and non-percolating Ni is maximized in the low-voltage range due to a high δ for the former and the suppression of δ by a positive charge for the latter. This difference yields a contrast between the two phases which is picked up by an inlens secondary electron detector. 2) The difference in backscatter coefficient, η, between Ni and YSZ is shown to increase with decreasing voltage. The contrast is illustrated in images collected by the normal secondary detector since parts of the secondary signals are generated by backscattered electrons. High temperature aging experiments of model Ni-YSZ anode cermets show that Ni sintering / coarsening is significantly increased, not only at higher temperatures, but also when the concentration of H2O in the reducing atmosphere is increased. It is proposed that the mobility of Ni is facilitated by the formation of Ni- OH complexes which are capable of segregating on the Ni particles surface, on the YSZ surface and via gas phase. This is a mechanism which has previously been reported in the context of Ni steam-reforming catalysis. In the context of electrochemically tested and technologically relevant cells, the majority of the microstructural work is performed on a cell tested at 850°C under relatively severe conditions for 17,500 hours. It is demonstrated that the major Ni rearrangements take place at the interface between anode and electrolyte. It is also shown that the degree of Ni sintering is dependent on the position along the fuel gas flow. The sintering is found to be most severe close to the fuel gas outlet. This difference in Ni sintering along the fuel gas flow is attributed to the increasing concentration of H2O at high fuel utilizations. Two-dimensional microstructural analyses of the anode from the cell tested for 17,500 hours indicate a general increase of the Ni volume-fraction close to the electrolyte interface. From three-dimensional reconstructions using FIB-SEM it can be concluded that Ni must have segregated from the outer parts of the anode in order to yield the measured Ni content close to the electrolyte. It is also concluded that the Ni segregation has taken place on the length scale of several micrometers in the anode of this long-term tested cell.
Brændselsceller og brint; Risø-PhD-32(EN); Risø-PhD-32; Risø-PhD-0032