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REAL-SOFC Résumé de rapport

Project ID: 502612
Financé au titre de: FP6-SUSTDEV
Pays: Germany

Final Report Summary - REAL-SOFC (Realising Reliable, Durable, Energy Efficient and Cost Effective SOFC Systems)

The project addressed the extension of the lifetime of solid oxide fuel cells (SOFC) stacks to several 10 000 hours, not only under steady state operation, but also under cycling (thermal and redox) conditions and with a variety of fuel impurities and compositions (sulphur and cokeing tolerance). The main objective of this project was to solve the generic problems of ageing with planar SOFC. This included gaining understanding of degradation processes, finding solutions to reduce ageing and producing improved materials that would then be included in components and tested in stacks. Furthermore, the design of cost effective materials, low cost components and optimised manufacturing processes had to be taken into consideration. In close co-operation between industry and research institutions, the following steps had to be accomplished:
- improved understanding of ageing in planar SOFC stacks considering all modes of operation, including long-term testing over 10 000 hrs., thermal cycling up to 100 cycles, and the influences of fuel composition;
- adaptation of materials and protective coatings in order to reduce ageing to well below 0,5 %/1000 hrs, as for instance necessary for stationary SOFC applications;
- manufacturing of improved components under commercial conditions and subsequent characterization in long-term and cycling tests.

The topics that were covered in the project included the following essential properties of SOFC materials:
- high power density (obtaining a margin for lowering the operating temperature whilst maintaining power output) of SOFC cells obtained by high performance cathodes and
- resistance of anodes against redox cycling, fuel gas impurities and coking (working towards the aim of operation with biogeneous fuels, unprocessed natural gas and reformates)
- low chromium emission, high conductivity and low scale growth with interconnect steels (including protective and contact layers applied on the interconnects) and
- well matched properties of glass sealants to steel and cell, i.e. good adherence and good thermal expansion coefficient match.

Furthermore, the topic of low cost materials was addressed through the introduction of standardised commercial powders and development of low-cost processing methods. The project also included activities in the area of standardisation of test procedures for achieving comparability of testing results and a common standard of quantification of degradation, and dissemination and training that contribute towards developing the network of human resources necessary in Europe for commercialisation of SOFC technology and for raising public awareness. Materials and components of two subsequent waves of improvements, termed 'Generation 2' and '3', with subsequently improved operating behaviour (as far as long-term stable operation is concerned) constituted the project main outcome.

Following suggestions from the review panel the individual goals of work tasks were integrated into a more concise project programme and streamlined towards the main focus of reduction of ageing processes. The mission objectives of the project and the relevant actions can be summarised in the following tasks:
- joint testing resource / comparability of results: standardisation of testing procedures;
- quantifying degradation phenomena: testing of components under various conditions under comparable conditions;
- understanding degradation phenomena: modelling of degradation mechanisms;
- improvement of long-term performance through lowering of operating temperature: improved cathodes coupled with improved interlayers or the use of new electrolyte materials;
- chromium tolerant cathodes: use of alternative cathode materials;
- improvement of anode stability under redox cycling conditions: alternative anodes and microstructure modification;
sulphur tolerant anode: anode material modifications;
- coking tolerant anode: anode material surface treatment;
- low-chromium emitting interconnect steels: characterisation and identification of suitable steels;
protective layers: optimised materials.

Other work packages that contributed the overall goals of the project even though they did not directly address the topic of reducing degradation were:
- screening of materials and development of new materials processing for achieving higher performance and lower costs;
- supplying components for testing;
- building a common understanding of SOFC specifications between research and industry community and defining interfaces between cells / stacks / SOFC systems, thus providing important system specifications to cell and materials suppliers;
- analysing environmental implications of the materials used in the project in order to ensure that no problems of environmental impact or workplace hazards prevent industrialisation of results;
- supporting the building of human resources and public awareness in the field of SOFC.

The main outcome of the second project year was as follows:
- finalisation of state-of-the-art characterisation of components;
- good performance at steady state conditions was found as opposed to partly severe degradation under transient conditions (cyling) and high current loads;
- qualification of commercially available powders for interlayers, cathodes and anodes was concluded;
- anode development towards redox stability proved difficult in view of retaining mechanical stability; all qualified materials (SrTi, Ti-Ceria, LSMC) showed low stress tolerance and thus could not be used in anode supported cells;
- electrolyte layer process development was of little success and will be terminated. Solely the PVD application of films rendered helpful results in providing dense, very thin layers. SCF cathodes have been extensively optimised with regard to stochiometry and processing and now yield high performance results;
- new cathode materials were characterised but up to now have not yet reached the status of being sufficiently understood in order to be included in cells for testing;
- characterisation of interconnect steels and protective layers has resulted in the conclusion that the Plansee and Thyssen steels IT11 and CroFer22APU perform best in the three vital categories oxide scale growth, adhesion and electrical conductivity. However, protective coatings are required in order to drastically reduce chromium evaporation;
- cells and stacks for testing were largely produced and delivered to plan. Quality assurance criteria for stacks were introduced and items with lower than desired performance excluded from testing;
- the scheme of distributing test items to several testing laboratories and comparing results has led to a better understanding of the dependence of test results on test rig layout, adherence to testing procedures and adequate handling of components;
- the materials inventory of two stack types has been concluded with the - little surprising - result that from the point of view of the current designs and weight contributions steel makes up the major material.

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