A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from fuel. Fuel cells are characterized by their electrolyte material and, as the name implies, the SOFC has a solid oxide, or ceramic, electrolyte. Ceramic fuel cells operate at much higher temperatures than polymer based ones.
Solid oxide fuel cells are intended mainly for stationary applications with an output from 100 W to 2 MW. They work at very high temperatures, typically between 700 and 1,000°C. Their off-gases can be used to fire a secondary gas turbine to improve electrical efficiency. This enables efficiency to reach as much as 90% in these hybrid systems, called combined heat and power (CHP) device. In these cells, oxygen ions are transferred through a solid oxide electrolyte material at high temperature to react with hydrogen on the anode side.
Due to the high operating temperature of SOFC's, they have no need for expensive catalyst, which is the case of proton-exchange fuel cells (platinum) and most other types of low temperature fuel cells. This means that SOFC's do not get poisoned by carbon monoxide and this makes them highly fuel-flexible. Solid oxide fuel cells have so far been operated on methane, propane, butane, fermentation gas, gasified biomass and paint fumes. However, sulfur components present in the fuel must be removed before entering the cell, but this can easily be done by an activated carbon bed or a zinc absorbent.
Thermal expansion demands a uniform and slow heating process at startup. Typically, 8 hours or more are to be expected. Micro-tubular geometries promise much faster start up times, typically on the order of minutes.
Unlike most other types of fuel cells, SOFC's can have multiple geometries. The planar geometry is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte is sandwiched in between the electrodes. SOFC's can also be made in tubular geometries where either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal and separate the fuel from the air compared to the planar design. The performance of the planar design is currently better than the performance of the tubular design however, because the planar design has a lower resistance compared to the tubular design. Other geometries of SOFC's include modified planar cells (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are highly promising, because they share the advantages of both planar cells (low resistance) and tubular cells (easier sealing).