Solid Oxide Fuel Cell (SOFC)

The Solid Oxide Fuel Cell (SOFC) is currently the highest-temperature fuel cell in development and can be operated over a wide temperature range from 600ºC–1000ºC allowing a number of fuels to be used. To operate at such high temperatures, the electrolyte is a thin, solid ceramic material (solid oxide) that is conductive to oxygen ions (O^2-). The SOFC has been in development since the late 1950s and has two configurations that are being investigate-planar (flat panel) and tubular.

As a solid electrolyte, it is impervious to gas cross-over from one electrode to another when liquid electrolytes usually consist of the electrolyte contained in some porous supporting structure. The charge carrier in the SOFC is the oxygen ion (O^2-). At the cathode, the oxygen molecules from the air are split into oxygen ions with the addition of four electrons. The oxygen ions are conducted through the electrolyte and combine with hydrogen at the anode, releasing four electrons. The electrons travel an external circuit providing electric power and producing by-product heat.

Anode Reaction:	2 H2 + 2 O2- => 2 H2O + 4 e-
Cathode Reaction:	O2 + 4 e- => 2 O2-
Overall Cell Reaction:	2 H2 + O2 => 2 H2O

The operating efficiency in generating electricity is among the highest of the fuel cells at about 60%. Furthermore, the high operating temperature allows cogeneration applications to create high-pressure steam that can be used in many applications. Combining a high-temperature fuel cell with a turbine into a hybrid fuel cell further increases the overall efficiency of generating electricity with a potential of an efficiency of more than 70%.

SOFCs operate at extremely high temperatures (600ºC–1000ºC) resulting in a significant time required to reach operating temperature and responding slowly to changes in electricity demand. It is therefore considered to be a leading candidate for high-power applications including industrial and large-scale central-electricity generating-stations.

The very high operating temperature of the SOFC has both advantages and disadvantages. The high temperature enables them to tolerate relatively impure fuels, such as those obtained from the gasification of coal or gasses from industrial process and other sources. However, the high temperatures require more expensive materials of construction.