Publication date: 31-October-2003 
Source:Forschungszentrum Jülich

Gas cells use hydrogen for the production of electricity. Because of their high efficiency high temperature (SOFC) gas cells particularly much are promising. But the high temperatures of 900 degrees Celsius make substantial demands against the used material, which could be fulfilled so far only insufficiently. Scientists of the research center Juelich developed a metal alloy, which is particularly suitable for the standard employment in high temperature gas cells. With ThyssenKrupp the research center found a competent development partner for the industrial magnate conversion now. The steel concern manufactures the metal alloy under the trade name "Crofer 22 APU". BMW tests the material already in prototypes: Gas cells with the new material are to relieve the battery and the generator as zusatzaggregat in the car. In addition for example the enterprise of a condition air conditioning system would be possible with switched off engine.

The Juelicher team around Dr. Willem Josef Quadakkers of Institut for materials and procedures of energy engineering 2 (  IWV 2 , institute leader Professor Lorenz sing-hoarse) developed a special metal alloy: It makes for the first time the series production possible of the interconnectors in such a way specified, which represent the central element of the high temperature gas cells. These interconnectors connect small gas cell units, which are hooked up with their assistance to an efficient large unit - a stack in such a way specified -. In addition they make the electrical connection between the units and supply them via gaskanaele with the gaseous fuel.

So far it was not possible to make the interconnectors economically and with the desired electrical characteristics of metallic alloys. "the requirements of the material are partially contradictory. The problem is to find the correct compromise ", explains Willem Quadakkers. To its team now succeeded. The material "Crofer 22 APU" possesses the combination of characteristics, specific to application in gas cells, and avoids among other things a crucial disadvantage of past heatproof materials.

So that the metal alloys exhibit just as small thermal expansion with operatings temperature by 900 degrees Celsius as the adjacent ceramic materials, a high chrome portion is added to them. Thus one extends also the life span of the interconnectors, acts however at the same time a serious disadvantage in: At the high temperatures that is evaporated chrome partly and penetrates the cathode up to the boundary surface between cathode and electrolyte. Thus will the efficiency of the gas cell is drastically lowered.

The alloy of the Juelicher team solves this problem by a kind "self sealing". As soon as the gas cell becomes active, a protective layer from chrome manganous oxide on the surface of the interconnectors, which prevents chrome evaporation, forms. Besides this oxide coating possesses the demanded high electrical conductivity and does not burst not from the surface. Decisive for it among other things an admixture of the chemical element is lanthan. Further alloying elements are manganese and titanium.

To be used the High Tech material can among other things in the automobile industry. Thus development engineers of the BMW Group already work on a SOFC gas cell with interconnectors from Crofer 22 APU. The abbreviation "APU" in the name of the material, which stands for "Auxiliary power unit", points already on the use of the gas cells - as zusatzaggregat for the discharge of battery and generator in the car.

The gas cells need half of the fuel, which is used for the production of the same quantity of electricity only about, if the generation of current takes the "detour" over engine, generator and battery. Thus the carbon dioxide output of the vehicle is reduced clearly -, even if the gas cells are operated like planned first with gasoline. The hydrogen is then won over a Reformer from the gasoline.

Picture left: Microstructure of the Juelicher of material in the optical microscope (picture in the JPG format to the Download , approx. 500KB)
Picture right: Transverse cross section of the Juelicher of material in the scanning electron microscope (dark layer = protecting oxide coating after 6000 hours at air with 800 degrees Celsius)