• Thydén, Karl Tor Sune

Abstract

Ni-YSZ cermets have been used as anode materials in SOFCs for more than 20years. Despite this fact, the major cause of degradation within the Ni-YSZ anode,namely Ni sintering / coarsening, is still not fully understood. Even if microstructuralstudies of anodes in tested cells are of technological relevance, it is difficult toidentify the effect from isolated parameters such as temperature, fuel gas compositionand polarization. Model studies of high temperature aged Ni-YSZ cermets aregenerally performed in atmospheres containing relatively low concentrations of H2O.In this work, the microstructural degradation in both electrochemically longtermtested cells and high-temperature aged model materials are studied. Since Niparticle 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 onimproving microstructural techniques and shows that the application of lowacceleration voltages (≤ 1 kV) in a FE-SEM makes it possible to obtain two usefultypes of contrast between the phases in Ni-YSZ composites. By changing between theordinary lateral SE detector and the inlens detector, using similar microscope settings,two very different sample characteristics are probed: 1) The difference in secondaryemission coefficient, δ, between the percolating and non-percolating Ni is maximizedin the low-voltage range due to a high δ for the former and the suppression of δ by apositive charge for the latter. This difference yields a contrast between the two phaseswhich is picked up by an inlens secondary electron detector. 2) The difference inbackscatter coefficient, η, between Ni and YSZ is shown to increase with decreasingvoltage. The contrast is illustrated in images collected by the normal secondarydetector since parts of the secondary signals are generated by backscattered electrons.High temperature aging experiments of model Ni-YSZ anode cermets showthat Ni sintering / coarsening is significantly increased, not only at highertemperatures, but also when the concentration of H2O in the reducing atmosphere isincreased. 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 theYSZ surface and via gas phase. This is a mechanism which has previously beenreported 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 Nirearrangements take place at the interface between anode and electrolyte. It is alsoshown that the degree of Ni sintering is dependent on the position along the fuel gasflow. The sintering is found to be most severe close to the fuel gas outlet. Thisdifference in Ni sintering along the fuel gas flow is attributed to the increasingconcentration of H2O at high fuel utilizations.Two-dimensional microstructural analyses of the anode from the cell tested for17,500 hours indicate a general increase of the Ni volume-fraction close to theelectrolyte interface. From three-dimensional reconstructions using FIB-SEM it canbe concluded that Ni must have segregated from the outer parts of the anode in orderto yield the measured Ni content close to the electrolyte. It is also concluded that theNi segregation has taken place on the length scale of several micrometers in the anodeof this long-term tested cell.

Topics

• impedance spectroscopy
• surface
• phase
• mobility
• experiment
• composite
• two-dimensional
• aging
• gas phase
• sintering
• aging
• field-emission scanning electron microscopy