Evaluation and scale-up of intermediate temperature (700 C) SOFC technology (IT-SOFC technology
Evaluation and scale-up of intermediate temperature (700 degree Celsius) solid oxide fuel cell technology
At the outset of the project there were no suitable commercial supplies of gadolinia-doped ceria (CGO) electrolyte and strontium doped lanthanum cobalt ferrate (LSCF) cathode powders. Such powders which were available were of low performance and only available in sub kilogram quantities. Rhodia have been successful in developing a CGO powder which, when sintered gave a ceramic with high ionic and low electronic conductivity. The processing of the powder was subsequently scaled-up to multi-kilogram batch size, and supplies of powder transferred to ECN and CERAM as required. In a similar way, LSCF powder has been developed and its manufacture scaled-up to multi-kilogram batch size. The development of PEN (Positive-Electrolyte-Negative) structures has seen the scale-up of both tape-casting and viscous plastic processing (VPP) from 5x5cm and 3.5cm diameter components to 10x10 and 12.5cm diameter. Close control over processing and firing schedules has enabled the enhancement of performance and long-term stability of PEN's with ECN cells in particular showing low degradation over long periods of operation. GASTEC have assessed a range of catalyst materials for their efficiency and effectiveness in operation at intermediate temperature with both methane and natural gas. The best of these was manufactured by Rhodia and used in a redesigned stack reformer during the demonstrator trial. One of the prime reasons for investigating intermediate temperature systems was the potential for use of cheaper ferritic stainless steels as system components particularly as interconnects. Although ferritic alloys had the potential for this application, at the start of the project no data was available on their performance in fuel and oxidant streams or on the effects of alloy composition. Similarly contacts and sealing systems had been developed for high temperature systems and no data was available on their suitability for ITSOFC. Two closely related alloys based on 17% Cr ferritic stainless steel with stabilising additives were shown to be stable during long-term corrosion tests at both Siemens and Sulzer. The corrosion of selected alloys with ceramic coatings was shown to be palliative and result in high conductivity interfaces. Characterisation of tested alloys showed chemical changes consistent with the behaviour observed. The demonstrator test was carried-out in a Sulzer HEXIS system using the specially designed reformer containing the developed catalyst, interconnect plates incorporating design modifications for intermediate temperature operation, and ECN optimised PEN's. This 3-year development and evaluation of materials and fabrication processes for ITSOFC has resulted in a successful demonstration of the components developed. A 120 mm 5-cell stack was operated over 2000 hours at high fuel utilisation using steam reformed CH4 at temperatures between 630 degree Celsius to 675 degree Celsius. Cost effective materials were largely used resulting in a 45% reduction of costs compared to state of the art SOFC stacks. The demonstration of a large stack was, however, only partially successful due to the inherent thermo-mechanical weakness of the key component, the CGO electrolyte.