Innovation at ThyssenKrupp VDM GmbH: the company is now able, uniquely, to supply a metallic material which is suitable for the volume production of solid oxide fuel cells (SOFCs). Crofer 22 APU, an iron-chromium alloy, for the first time fulfils the previously conflicting requirements of good high-temperature stability, high electrical conductivity and low coefficient of expansion. The material concept - developed jointly with ThyssenKrupp Nirosta GmbH and Forschungszentrum Jülich - clears the way for optimizing the cost and design of high-temperature fuel cells.

The SOFC is a fuel cell variant which works at temperatures up to 900 degrees. The starting materials for energy conversion can be hydrogen, natural gas or reformed gasoline. In cars, SOFCs can be used for example in auxiliary power units (APU) to supply additional energy. A research project into this is currently being carried out with the involvement of BMW AG. The aim is to use an SOFC plant as a supplement for battery and dynamo.

The APU mainly consists of an SOFC stack, made up of several interconnected fuel cells. Each cell contains an anode, cathode and ceramic zirconium dioxide as the electrolyte to start the power generation process. The most important components of the SOFC stack are the interconnector plates between the cells. As well as making the electrical connection they also supply the cells with fuel gas and air via gas passages. The interconnector plates are made of Crofer 22 APU.

Before the development of Crofer 22 APU, no metallic material was able to meet the requirements of SOFC technology. The main reason is the high operating temperature of the fuel cells. Normally, metallic materials achieve their long-term stability at such temperatures by means of high chromium content. Chromium also limits the thermal expansion of the alloy, giving it a similar coefficient of expansion to the surrounding ceramic electrolyte. However, at temperatures of 900 degrees, the chromium is released and destroys the electrolyte.

While the ceramic materials used previously for the interconnector plates do not have the problem of chromium evaporation, they are unsuitable for economic volume production due to the complex production requirements involved for example in milling the passages for the gas supply.

In Crofer 22 APU, ThyssenKrupp VDM offers the first alloy which resolves the conflict between high thermal stability, low chromium evaporation and low expansion. Critical to this is its 0.08 percent content of lanthan, an element from the rare earth series. Together with the other alloying elements titanium and manganese, the lanthan ensures that a protective coating of chromium manganese oxide forms on the surface of the interconnector plates as soon as the fuel cell becomes active. This so-called chromium-manganese spinel prevents chromium evaporation and at the same time assures the necessary electrical conductivity of the interconnector plates.

The manufacture of Crofer 22 APU requires extreme precision at every stage of the production process. In the melting furnace, where several tons of the alloy are produced in one charge, minimum amounts of alloying elements have to be added in precise quantities. During subsequent hot rolling it is important that the temperature window is observed precisely. In terms of fabrication, however, Crofer 22 APU is just as easy to handle as a standard stainless steel. It can be formed without problems, given ribs or other surface structures and machined using conventional production methods and equipment.

Contact:
ThyssenKrupp Steel AG
Dietmar Stamm
Tel.: +49 203 / 52 - 2 62 67
Fax: +49 203 / 52 - 2 57 07
e-mail: dietmar.stamm@tks.thyssenkrupp.com

Bernd Overmaat
Tel.: +49 203 / 52 - 4 51 85
Fax: +49 203 / 52 - 2 57 07
e-mail: bernd.overmaat@tks.thyssenkrupp.com