Ageing of Materials at Inlet and Outlet Fuel Manifolds in a SOFC Stack

Abstract

The ageing behaviour of glass-ceramic seal materials and of alloys is a key element for a durable performance of SOFC stacks. Post-experiment analyses performed on a stack operated for more than 4000h at 750°C and at high fuel utilization (80%) reveal significant differences in ageing behaviour of such materials when comparing fuel inlet and fuel outlet areas.Two samples were cut from the fuel inlet and outlet manifolds of an operated SOFC stack. The glass-ceramic sealing material and the alloy were exposed to a dual atmosphere (external air and internal fuel stream). The fuel composition was of dry H2/N2 (60:40 vol. %) at the inlet and of H2O/H2/N2 (48:12:40 vol. %) at the outlet. The stack was operated in co-flow configuration, with a large excess of air. The paper is focused on the sealant, on the structural steel and on the interfaces between both materials. The following figure schematically describes the samples and the zones of interest.Both samples were mounted in epoxy-resin, polished and observed by optical microscopy and SEM-EDX. It was found that the interfaces between the seal and the alloy were significantly affected by the difference in fuel composition, with more severe degradation at the fuel outlet side. Further, an increased porosity of the sealant was observed on the same location, while the inlet sample remained less affected. Finally, the sealant exposed to the outgoing air was significantly polluted by chromium generated in the stack and transported by the air stream.These investigations therefore reveal that the long-term evolution of the sealing and of the metallic support depends on the local conditions in the stack, as well as on additional interactions with other degradation phenomena. Acknowledgement: The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207. Figure 1