Solid oxide fuel cells, or SOFC, are intended mainly for stationary applications with an output of 1 kW and larger (power plants). They work at very high temperatures (some at 1000ºC), and their off-gases can be used to fire a secondary gas turbine to improve electrical efficiency. Efficiency could reach as much as 70% in these hybrid systems. 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). 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 in active coal bed or by a zinc absorbent.
Since SOFC are made of ceramic materials, they tend to be brittle; they are therefore unsuited as drivetrains in mobile applications. Furthermore, thermal expansion demands a uniform and slow heating process at startup, that will cause very long startup times: typically, 8 hours or more are to be expected, although micro-tubular geometries promise much faster start up times. Research is going now in the direction of lower-temperature SOFC (600ºC) in order to decrease the materials cost, which will enable the use of metallic materials with better mechanical properties and thermal conductivity. Research is also going on in reducing start-up time to be able to implement SOFC's in mobile applications. Due to their fuel flexibility they may run on partially reformed diesel, and this makes SOFC's interesting as auxiliary power units (APU) in refrigerated trucks.
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.