Improved durability and reduced system complexity of solid oxide fuel cell systems

Abstract

Solid oxide fuel cells (SOFCs) show great potential for clean and efficient power generation applications. However, their high cost is preventing their market entry. This dissertation focuses on solutions to increase the durability of SOFCs and to reduce the complexity of SOFC systems to drive their cost down. Chromium poisoning of the cathode is a major issue limiting the durability of SOFCs. This issue is addressed by the development of a protective manganesecobalt spinel coating for steel interconnects. Coated interconnects were characterised in SOFC relevant conditions and the results showed that the coating fulfilled its main requirements, which are: limitation of chromium transport from the interconnect to the cathode, protection against oxidation of the steel and low and stable area-specific resistance. Evidence was found that another source of chromium is the balance-of-plant (BoP) components upstream of the cathode, an issue which did not receive much attention in the literature. Therefore, a method for measuring chromium evaporation from BoP components was developed and validated on a stainless steel pipe. SOFC systems based on natural gas commonly include a fuel processing subsystem for fuel steam reforming. The need for an external water source can be eliminating by recycling the steam-rich anode off-gas. Investigations were performed on a pre-reformer with a precious metal catalyst and it was found that adding an anode off-gas recycling loop had no detrimental effect on the activity of the catalyst and carbon formation could be avoided. Additionally, results showed the possibility to generate the hydrogen-containing gas needed to prevent the reoxidation of the anode catalyst during heat-up phase. The results permitted the implementation of an anode off-gas recycling loop in a 10 kW SOFC system. Additionally, the system was heated up without supplying any premixed hydrogencontaining gas, which enables to reduce the complexity of the system. Finally, the durability of a stack can be improved by seal solutions with limited material interactions. A hybrid seal solution was developed by coating a compressible core with glass layers. The developed seal reduced the leak rate compared to a purely compressible seal. Material interactions were studied with a post-experimental investigation of an SOFC stack. Interactions were limited with the exception of evidence of increased oxidation at the steel/seal/air interface. Overall, the solution was found to be promising and the obtained results led to the commercialisation of the developed seal solution by Flexitallic Ltd (UK).