Solid Oxide Fuel Cells (SOFC)

Solid Oxide Fuel Cells ,whether flat (planar) or in a tubular formation, work on the same principles. SOFCs operate in a slightly different manner from polymer exchange membrane fuel cells. In general, oxygen ions, which are negatively charged, are conducted through the solid oxide electrolyte from the cathode to the anode. This is the reverse direction of what occurs in a polymer exchange membrane fuel cell. While proton conducting SOFCs are under development, they are still in their experimental stages.

Function and Reactions of SOFCs

SOFCs have four layers as opposed to three. The reaction process actually occurs in the reverse direction of other fuel cells. Oxygen is injected into the cathode, where it is reduced, which means it gains electrons. The now negatively charged oxygen ions are able to cross the solid oxide electrolyte. Oxygen, in its unionized form is not able to cross the electrolyte barrier.

Upon reaching the anode, the oxygen ions are able to oxidize a fuel, usually hydrogen, for the transfer of electrons. The products of this part of the reaction are water and electrons. At this point, the anode is negatively charged just as it would be in other fuel cells. Electrons, because they are unable to travel through a solid oxide electrolyte, are forced to travel through an external circuit to return to the cathode. Once at the cathode, the electrons are available to reduce more oxygen.

The electrolyte is a thin, but dense layer of ceramic that is permeable to oxygen ions at high temperatures. The electrolyte is usually made of zirconia stabilized with yttria, zirconia stabilized with scandia, or ceria doped with gadolinium. It is the permeability of the electrolyte to oxygen that dictates the need for high temperatures in these fuel cells.

Benefits of SOFCs

Solid oxide fuel cells have the tremendous advantage of being available in multiple geometries. The ability to change the geometry of the cell can have tremendous impact on the efficiency of the overall process. In addition, SOFCs can be produced to accommodate novel applications and unique materials. There is currently a great deal of interest in wave-shaped SOFCs, which are more efficient than planar and tubular formations.

Drawbacks and Future Research of SOFCs

SOFCs, while not susceptible to carbon monoxide poisoning, are susceptible to sulfur poisoning. Sulfur is a common contaminant of fossil fuels and must be removed either through extra milk processing or by absorbance within the fuel cell. Research and development time is being invested into SOFCs that can run on natural gas. These would make excellent sources for auxiliary power units, especially given the fact that a natural gas infrastructure already exists.