Great Progress in Processing Fuel Cells

A solid oxide fuel cell (SOFC) produces electricity directly from oxidizing fuel at very high temperatures, typically between 500 and 1,000°C. At these temperatures, SOFCs do not require expensive platinum catalyst material, as is currently necessary for lower temperature fuel cells. Further advantages of this fuel cells are high efficiency, long-term stability, fuel flexibility, and low emissions.

Between the SOFC’s metal parts is a gas-tight and electrically insulating joint. The two interconnectors are connected both to the anode and cathode, but do not have an electrical connection to each other as this would lead to a short circuit. The sealant separates the fuel gas and oxygen in order to maintain fuel cell performance and to prevent the formation of oxyhydrogen.

For insulating and sealing, glass sealing is the generally used technique today. A glass paste is applied to the individual layers of the SOFC stack with a dispenser. The layers are then stacked on each other and joined in a furnace at temperatures between 850 and 950°C for several hours. Here the glass sealant crystallizes into a glass ceramic.

All layers must be tight after joining, otherwise the entire SOFC stack is unusable. To avoid this problem, D. Faidel, W. Behr, S. Groß, and U. Reisgen employed the laser-assisted glass sealing technique in a research study at the Forschungszentrum Jülich in Germany. They developed technology and manufacturing process concepts for the production of stationary and mobile fuel cell stacks including novel glass solders as joining and insulation medium.

The researchers recorded high-speed videos to analyze the melt dynamics for laser joining processes with glass. They used this information for the temperature-adapted laser power control during the process for the optimal heating of all joining parts.

First results on applications of different laser sources as joining tool in production of fuel cells show the potential of the laser as a flexible tool for manufacturing as well as repairing of SOFC stacks. Defects can be heated and sealed selectively, without putting too much of a load on the rest of the component.

Selective heating with a laser beam enables new process variants that cannot be implemented in a furnace. Also components made of alumina or silicium can be joined to produce high vacuum tight and electrical isolated bonds. But the different properties of the materials used and particularly the thermal expansion coefficients must be considered when joining two metal components with a glass sealant.

To better understand the entire process the German researchers have to analyse transmission, reflection and absorption at the transformation temperature of the glass sealant. Further data have to be gained regarding the interaction between glass sealant and laser radiation during the remelting process.

Source: Martin Grolms, MaterialsViews.com