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Low Cost Interconnects with highly improved Contact Strength for SOC Applications

Periodic Reporting for period 1 - LOWCOST-IC (Low Cost Interconnects with highly improved Contact Strength for SOC Applications)

Reporting period: 2019-01-01 to 2020-06-30

The solid oxide Cell technologies (SOC), i.e. the solid oxide fuel cell technology (SOFC) and solid oxide electrolysis technology (SOEC), are highly energetically efficient technologies. The high efficiency makes them strong alternatives to other fuel cell and electrolysis cell technologies. Besides no noble metals or other scarce resources are used in the SOCs.

The SOEC technology can be used to convert renewable electricity and water into hydrogen but also CO2 into CO, which are base chemicals for the chemical industry, needed for the steel production and to produce e.g. transportation fuels. Synthetic green fuels will inevitable in a future society, for aviation and heavy transportation as batteries or hydrogen will be to heavy or have too low volumetric energy density. In comparison to alternative electrolysis technologies (alkaline (AEC) or proton exchange membrane (PEMEC) based technologies), the SOEC technology is far more efficient and thus operating cost saving, especially because it can utilize waste heat from downstream processes to increase the overall plant efficiency.

The SOFC technology can be used to convert fuel, preferably renewably made, into electricity. An initial use case is micro combined heat and power plants, but some of the identified promising markets are: power supply of data centres, large scale energy storage and in the transportation sector. The reason for the latter is again the very high efficiency as compared to competing proton exchange membrane fuel cell PEMFC technologies (low and high temp.) and their versatility towards fuels. Where the PEMFC requires PURE hydrogen then SOFC can handle various carbonaceous fuels and ammonia, and with the right handling even without pre-reforming. This make them ideal for e.g. shipping, where liquid fuels should be converted to electricity with high efficiency.

A very important factor for the competitiveness of the technologies is their capital cost. The main factors playing in on the cost is the lifetime of the cell stacks and the raw material cost.

The LOWCOST-IC project has the objectives of increasing the robustness of solid oxide fuel cell stacks while decreasing the cost of the steel components, i.e. the so-called interconnects. It brings together a consortium covering the entire value chain from the interconnect production to their final use in solid oxide cell stacks. To achieve the goals, a novel production route is tested together with a number of proposed material and stack manufacturing and design improvements.
In the first part of the project the focus has been on testing the feasibility of the new proposed materials in the laboratory, testing of new processing methods and setting up analysis tools.

One of the new materials used in the project is a pre-coated chromium rich ferritic steel from Sandvik (SAN), Sanergy® HT 441. The thin coating is deposited by physical vapour deposition (PVD). It was investigated whether the coating sustained the chosen hydroforming method conducted by Borit (BOR). No change of corrosion protection could be observed by Chalmers University (CU) after the shaping. It could thus be concluded that the proposed cost-effective roll-to-roll production of interconnects is feasible.

With respect to processing, the hydroforming by BOR was shown to be able to easily reproduce Sunfire’s (SUN) interconnect shape using a 0.3 mm steel plate, whereas some deviations on the curvatures were present when using a stiffer 0.5 mm steel plate. To mitigate this limitation to the degree of freedom in the design process, a novel shape of interconnect ribs has been proposed by Forschung Zentrum Jülich (FZJ).

Besides the standard materials from Sandvik also another steel substrate, K44M, from Aperam (APE) and different coating solutions are investigated. Comparing the different steels, it was concluded by CU that coated K44M and AISI 441 offered similar performance in the lab, as the far more costly state-of-the-art steel, Crofer 22 APU. A replacement of this would induce a significant steel cost reduction (> 80 % reduction).

To join the interconnect with the ceramic cells, a so-called contact layer is needed, and the interface here is critical as loss of contact will lead to failure of the stack. Here a novel contact layer material from Technical University of Denmark (DTU) show a very good performance in terms of high robustness (>200 % tough) and low area specific resistance (~15mOhm.cm2).

It was shown that the very fast drop on demand printing technology (40 m/min) could be used to print ceramic inks on the interconnect tops by Techno Italia (TI). This will save both material and make fast large-scale production feasible.

For the interconnect design process, a new very fast multiphysics stack model, which includes the local failure at the interconnect / air electrode interface, was developed by DTU. This reduces the time for a single computation of all physics relevant for a full stack from at least 40 hours to 15 minutes, making iterative design studies feasible. The flow distribution in this was verified using a detailed flow simulation by FZJ. The cost analysis was initiated by AVL, with preliminary results showing that it is possible to reduce the cost below 5 € per interconnect with the different proposed changes to the processing and materials.

Stacks with Sanergy® HT 441 were produced with the existing interconnect shaping methods and tested. For high temperature operation (850C) at SUN, it was shown that Sanergy 441 HT corroded faster than the rather expensive Crofer 22 APU steel. The reason will be investigated in the second half of the project. The stack test of SOLIDpower’s (SOL) stack at FZJ at 750C are still ongoing.
The project has thus at this stage already has shown a larger number of improvements over state of the start in terms of possible material as described.

For the remaining part of the project, following activities will be undertaken:
- Design new optimized interconnect flow geometry, which minimizes thermal gradients and hereby thermal stresses
- Shape new geometry with hydroforming and test in SUN stacks and thus demonstrate the processing route advocated in the project
- Test new steel and coatings in stacks and in lab
- Further develop the novel contact layers to obtain a compromise between chemical stability and mechanical robustness
- Finish techno-economic analysis to determine the benefit from the many technical improvements
- Show that the novel contact layers can be printed by DoD printing as well
- Build several stacks to investigate the materials and components in the relatively complex environment of an operating SOC stack
- Investigate the difference between lab and stack tests of the corrosion performance of AISI 441 at 850 C.

If successful it is expected that manufacturers gradually will exchange their standard solutions with the most promising from the project to minimize cost in particular. More robust materials also mean that future stack designs can be designed more compact, with less stress adsorbing contact layers and interconnect features, also leading to lower cost. The higher robustness will also be advantageous for more dynamic operation in transportation. Finally, the modelling tools developed will also assist in designing future stacks, as the speed moves the computations away from university owned computer clusters to the R&D departments at the companies.