ThyssenKrupp VDM produces materials for promising fuel cell technology
Cooperation with Jülich Research Center
Fuel cells, the innovative energy source of the future, deliver clean energy in the form of heat and electricity at high levels of efficiency. The best-known type is the polymer fuel cell, which runs on pure hydrogen and oxygen from the air. But there are other ways, as shown by developments by Jülich Research Center into the solid-oxide fuel cell (SOFC) which uses fuels such as diesel, gasoline or methanol. The advantage of this technology is clear: A large-scale hydrogen infrastructure is not yet in place, whereas the fuels for SOFCs are available everywhere. Key materials for the production of these promising solid-oxide fuel cells are supplied by ThyssenKrupp VDM, a subsidiary of ThyssenKrupp Stainless AG operating in the field of high-performance materials.
One fundamental difference between the hydrogen fuel cell and the solid-oxide fuel cell lies in their operating temperatures: While the polymer fuel cell reaches temperatures of 80 to 100 degrees Celsius, the temperatures in the SOFC can climb to 900 degrees. The hydrogen-rich gas is recovered from the fuel at high temperatures. The conditions prevailing in the fuel cell require special materials: One of these is the ferritic chromium steel Crofer 22 APU (“Auxiliary Power Unit”) supplied by ThyssenKrupp VDM. The composition of the alloy was optimized by ThyssenKrupp VDM as part of the “ZEUS II” research program carried out in collaboration with Jülich Research Center. Other partners in the project, supported by the Federal Ministry for Education and Research, included the companies BMW, Liebherr and ElringKlinger.
The material is used in the fuel cell for the so-called interconnectors, steel plates which combine the individual cells into a powerful fuel cell “stack”. The list of requirements that the material for the interconnectors has to fulfill is long: It has to be electrically conductive, corrosion-resistant, mechanically stable and strong, easy to process and not have any negative effects on the cell. Crofer 22 APU is adapted to these requirements. “The material has unique features,” says Frank Scheide, responsible sales manager at ThyssenKrupp VDM in Werdohl: “Its name has already become a kind of generic term.” Another important factor for wide use in fuel cells is price. Crofer 22 APU costs less than other materials suitable for the SOFC. “We have to keep material costs and thus fuel cell production costs low. Ultimately it’s a question of reducing system costs,” explains Dr. Robert Steinberger-Wilckens from Jülich Research Center. “Crofer 22 APU is easy to process and provides high conductivity and corrosion resistance, so it has the right property combination.” Another advantage lies in the material’s thermal expansion, which corresponds to that of the ceramic used for the cells. This prevents mechanical stresses between the two materials which could damage the ceramic.
The current developments are pushing up demand for suitable materials for the interconnectors. “We’ve moved from the 100 kilogram range into the ton range – demand has multiplied in the last two years,” says Scheide. However, Crofer 22 APU is not the only material from ThyssenKrupp used in the SOFC. High-temperature nickel alloys play a role in other parts of the SOFC such as heat exchangers, reformers and bipolar plates.
The aim is for the fuel cell to be used as an auxiliary power unit in many applications, from stationary use in buildings, mini and micro combined heat and power plants, to mobile use in cars, ships and submarines. This kind of fuel cell is already operating for demonstration purposes in the areas of domestic energy and on-board power supply units. BMW for example is examining the use of the SOFC as an engine-independent power supply unit in its vehicles. The small, distributed power plants are on the march: They are highly energy-efficient, supply heat and electricity, and produce harmless emissions – all the properties required for a successful future technology.