A solid oxide cell consists of a number of thin ceramic (and possibly metal) layers. One of the layers must be thick enough to give sufficient structural strength for the cell to be handled; this layer, which the cell is often named after, is called a support.
First Generation Cells
Our first generation of fuel cells were electrolyte supported. The development of the first generation (1G) cell started in 1989. The comparatively thick electrolyte has a large internal resistance which only at high temperatures decreases sufficiently. Consequently the 1G cell had an operating temperature of 1000 °C. In 1995 we tested a 500 watt 1G cell stak, but it became increasingly clear that an alternative design was necessary for the internal resistance of the cells to be brought down.
Schematics of the 1G cell. The anode consists of a mixture of nickel and yttria stabilized zirconia (YSZ), the electrolyte is YSZ and the cathode is a mixture of lanthan strontium
manganite (LSM) and YSZ. The cell is supported by the electrolyte
Second Generation Cells
The development of an anode supported cell started in the late 1990s. Here the support consist of a 0.3 mm thick layer of nickel and YSZ, on top of which a thin, active anode layer of the same materials are put. The manufacturing costs for this cell is significantly lower than for the 1G cell, and the cell internal resistance is lower which made it possible to lower the operating temperature to around 850 °C. The 2G cell is our most mature cell in terms of development, and it is routinely produced in large quantities on our pre-pilot plant.
Schematics of the 2G cell. The cell is supported by the anode
2G cell produced at Risø. The active area (the dark region) is 12 cm×12 cm.
The thickness is less than 1 mm.
2.5G Cell
In parallel with the ongoing 2G cell development and production we work on more fundamental improvements to the cells. The so-called 2.5G cell is based on the same type of support, anode and electrolyte as the 2G cells, but has an improved cathode. This will make it possible to lower the operating temperature to approx. 700 °C. The 2.5G cell is in the process of being transferred from laboratory scale production to the pre-pilot plant.
Schematics of the 2.5G cell. The standard cathode is replaced by an improved, composite cathode consisting of CGO (gadolinium doped ceria)
and LSCF (lanthanide strontium iron cobalt oxide)
Third Generation Cell
The 3G or metal supported cell is at a more experimental stage. Here the anode support is replaced by a porous layer of ferritic stainless steel (approx. 78% iron and 22% chromium). The expected advantages are a significant decrease in cost and an increased robustness in case of fuel supply failure or operating stop (redox and temperature cycles). At the same time the operating temperature is reduced to 550-650 °C which have been shown to be an optimal temperature range. In addition to the completely new support, all active components will be improved. The cathode will be that used for the 2.5G cell.
Schematics of the 3G cell. The cell is constructed on top of a support consisting of iron and chromium. The anode is made of Ni+YSZ or lanthanide doped strontium titanate (LST).
The cathode is identical to that of the 2.5 cell. However, CGO may possibly be replaced by scandium stabilized zirconia (SSZ).
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