A single fuel cell only produces a voltage of about 1 volt. To obtain usable voltages it is necessary to connect the cells serially in a so-called stack. The cells are connected by metallic interconnects. The interconnects are needed to ensure electrical contact between the cells and at the same time keep fuel gas and air separated.
At Risø the stacking activities are focused on development of components and small scale stacks, while the industrial stack development takes place at the Danish company Topsoe Fuel Cell A/S in a close collaboration between Risø and Topsoe.
Schematics of a fuel cell stack. The spacer layers are needed to have a free volume for the flow of fuel and air; the spacers can also be integrated in the interconnect. The sealing is only shown for the lower cell. Other flow configurations, where fuel and air flow in parallel or opposite directions, are also possible.
Interconnects
The high operating temperatures make stringent demands on the corrosion resistance of the stacks. Corrosion increases drastically with increasing temperature. Our first cell stacks with electrolyte supported cells used ceramic interconnects, as metals corrode too fast at the operating temperature of 1000 °C. With our anode supported cells, having an operating temperature of around 850 °C, it became possible to use metallic interconnects. However, corrosion is still a problem for the system lifetime, and we are investigating new alloys and coatings for the interconnects to increase their corrosion resistance.
Sealing
To avoid mixing of fuel gas and air, the cells are sealed along the edges. This is done using special glass ceramic seals. We are working on the development and characterization of new sealing materials. In addition we investigate different sealing designs.
Stacks
At Risø a number of stacking activities take place which will lead to a demonstration of a small scale robust, high performance stack at 750 °C. The stack will be tolerant to thermal cycling. The development takes place at both the component, system and process level.
Three cell test stack at Risø
The industrial stack development takes place at Topsoe Fuel Cell in close collaboration with Risø. In 2003 the fundamental stack technology was demonstrated in a small stack of 5 cells and a larger stack of 75 cells; the latter with an electrical power of 1.1 kilowatt. These demonstrations will provide a basis for the development of larger stack systems.
Topsoe Fuel Cell stack with 75 cells
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