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METSAPP Report Summary

Project reference: 278257
Funded under: FP7-JTI

Periodic Report Summary 2 - METSAPP (Metal supported SOFC technology for stationary and mobile applications)

Project Context and Objectives:
Most SOFC demonstrations with ceramic cells in real system operation have until now revealed problems regarding reliability issues. Attention to reliability and robustness has especially been paid in connection with SOFC technology for mobile application. Modelling studies as well as recent practical experience has shown how up-scaling of cells and stacks to larger more industrially relevant sizes generally leads to lower reliability in real system operation and intolerance towards system abuse and operation failures. The aim of the METSAPP project is to develop novel cells and stacks based on a robust, reliable and up-scale-able metal supported technology for stationary as well as mobile applications with the following primary six objectives:

1. Robust metal-supported cell design, ASRcell < 0.5 ohmcm2, 650ºC
2. Cell optimized and up-scaled to > 300 cm2 footprint
3. Improved durability for stationary applications, degradation < 0.25%/kh
4. Modular, up-scaled stack design, stack ASRstack < 0.6 ohmcm2, 650ºC
5. Robustness of 1-3 kW stack verified
6. Cost effectiveness, industrially relevance, up-scale-ability illustrated.

Due to the ductility and mechanical strength of the metal support, the metal supported cells have an advantage compared to traditional anode supported cells in terms of robustness. Also, the established experience indicates that long term durability of anode supported cell and stack components requires a reduction in the operation temperature to 600 – 700 ºC, which is readily feasible by using the metal supported SOFCs.

Because the overall target of the METSAPP project is to develop novel low cost, durable, robust and reliable metal based stacks that match the requirements defined by stationary applications, the long term stability of the novel materials and components in question represent a huge challenge. The knowledge gained from previous project (METSOFC, grant. No. 211940) highlighted the need for further improvements of the cell components in order to fulfil the durability requirements in stationary applications (such as: Distributed generation, and combined heat and power), where a lifetime of 20.000 to 60.000 hours is required. To improve the longevity of metal supported SOFC aiming at stationary applications, the effects of corrosion on the metal-support and the anode has to be further investigated by combining mathematical oxidation models with test, characterisation and screening of new materials. This includes improvements in understanding of how micro-structural parameters such as porosity or tortuosity change over time due to corrosion. Improvements in reliability and durability include fundamental modelling of physical material parameters such as creep and deformation as function of mechanical and thermo-mechanical load during operation. Feedback from modelling and test of cells and stack is used to improve the materials in the cell development and stack development work to obtain the durability targets. Due to the limited timeframe of the project this approach has to rely upon development and implementation of accelerated test protocols and extended simulation methods. Key development issues in the METSAPP project include:

• Metal powder development for robust metal supported cells
• Development of novel anode designs and nano-structured coatings
• Integration of high performance and stable cathodes
• Integration of developed components to full cells
• Component and cell manufacturing for testing and stacking
• Development of novel stack concepts for metal supported cells
• Development of coatings for ferritic stainless steel interconnects to provide high oxidation resistance and low Cr evaporation in SOFC relevant atmospheres.
• Development of advanced modelling tools and improved models to investigate the loss and degradation mechanism in cells and stacks
• Models to understand corrosion behaviour

The base case cell design of the METSAPP project is based on a multi-layered structure obtainable by cost effective ceramic processing techniques such as tape casting of different layers, lamination, co-sintering and infiltration. The innovative concept, which ensures that the number of different manufacturing steps is minimized, includes an unconventional half-cell design, where the major material and durability problems associated by the use of Ni-YSZ anodes are circumvented by the use of an alternative anode structure. The cell design is based on porous and highly electronically conducting layers (FeCr-based metal support and FeCr-ScYSZ-based anode cermet backbone layer) into which electro-catalytically active materials are infiltrated after sintering.

Project Results:
Key parameters limiting the long term stability of the metal supported cell components have been identified and are used for further development and optimization of the performance and robustness of the anode layer in the metal supported cells.
More than 200 12x12 cm2 reproducible cells based on a FeCr-ScYSZ anode were produced in the project, demonstrating the up scalability and reproducibility of the cell manufacturing process. In addition, significant improvement of the performance and reproducibility of the cell manufacturing based on a cermet anode were obtained
In parallel with the cermet based anodes, efforts are being made to develop new anode designs and materials that are corrosion resistant. This was done by either using improved corrosion protective coatings on base case cermet anodes or developing new anode materials. The most promising results are the development of a new perovskite material La0.49Sr0.31Fe0.03Ni0.03Ti0.94O3. Laboratory scale production of small batches allowed demonstrating that this perovskite exsolved metal nano-particles in reducing conditions, and the metal particles formed in such a manner are most likely FeNi alloys, see Figure 3. In addition, compatibility tests with other cell component materials showed no detrimental interaction with YSZ and FeCr metal particles, moreover, exsolution of metal nano particles from perovskite and chromium oxide layer formation on FeCr metal particles resulted in relatively good adhesion with the metal particles. Figure 3 shows also the resulted microstructure when sintering the developed perovskite material and the metal powder together. Promising electrochemical and corrosion resistance behaviour was observed in a cell with a perovskite/FeCr anode. However, further work is needed to tune the composition and to secure the supply of large batches of good quality perovskite powder before successful integration in to the cells.
To further reduce the cost of current state of the art SOFC stack concepts, METSAPP is introducing thin sheet metallic interconnects continuously PVD pre-coated with functional protective coatings. The Co/Ce coating developed within the project drastically reduced the oxide scale thickness and improved ASR on the air side, while, Ce coatings improved the corrosion resistance on the fuel side.

Modelling and computer simulations represent some important activities in METSAPP and major progresses were achieved. In particular, in order to assess the failure mode degradation, a corrosion model has been developed and implemented. The model describes the growth of the oxide and the change of the pore volume, which in turn influences the diffusion in the microstructure. Also an accelerated stack test procedures defined and the test program needs to be validated.

The project has embarked on a radical new stack design more suitable for metal supported cells than the base case stack design, which was originally developed for anode supported cells. This new design includes stack sealing strategy, geometrical and material designs, as well as procedures for high temperature stack conditioning. This approach is solving a large number of the challenges identified during the first project year. Significant progress was achieved and with the implementation of the new stack design for the metal supported cells, and in particular the successful development of the homogenized model, which makes it possible to model and optimize stack design changes. It was believed that the objectives for testing 10-25 cell stacks with optimal compression forces, capability of internal reforming, and tested for 1000h were soon to be achieved. This work was stopped due to the closure of Topsoe Fuel Cell A/S (TOFC) in charge of stack assembling and testing.

Potential Impact:
To conclude, the main obstacle for the introduction of SOFC technology into the market is currently the high cost per kW. Broken down to the level of a SOFC layer, it is mainly the cell, the processing of the cell and the resulting waste parts. Currently, the main advantage of metal supported cells is the complete change in the way the cell is implemented in the stack. This allows pursuing a manufacturing strategy, which should reduce manufacturing cost substantially. A second advantage is the fact that a METSAPP cell does not need to be reduced before operation. The stack is almost fully operational after assembly. While these advantages are highly promising, the technology needs further development to convert the actual level of development into a product. This is METSAPP aim: to create new and improved cell and stack conceptual solutions in a field of many different competing SOFC concepts, where a breakthrough towards sustainable commercialization still remains to be seen. By combining new ingenious solutions on metal supported cell and stack design with development of new materials, the outcome of the project is expected to improve electrodes operating at lower operation temperatures. Tailored cell and stack components will furthermore improve the performance in regards to the identified failure mechanisms for electrochemical stability, thermo-mechanical issues and red-ox resistance.

As a consequence of TOFC closure, the project is being re-defined with the accession of possible new beneficiaries, in order to insure the exploitation of the results in different markets. The potential impact and use of the project will be defined in collaboration with the new beneficiaries.

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Dahlkilde, Soren (Project Controller)
Tel.: +45 5180 1579
Fax: +45 4677 5858
Record Number: 187826 / Last updated on: 2016-08-24