The electrolyte in the solid oxide fuel cell is a solid, non-porous metal oxide, usually zirconia (ZrO₂) stabilised with 8-10 mole% yttria (Y₂O₃). Ionic conduction in the electrolyte is assured by oxygen ion (O^2-) transport. By using a solid as the electrolyte, the system is more stable and safe than the MCFC. No leakage occurs and the cell can be cast into multiple shapes, such as tubular, planar or monolithic. A normal operating temperature is around 1000°C, but it is desired to develop cells operating at a reduced temperature as low as 650°C. This would of course decrease the conductivity of the presently used solid electrolyte material.  

As in the MCFC, no precious metals are needed in the electrodes due to the fast kinetics at these temperatures. Typically, the anode is of ceramic materials based on cobalt or nickel  (Co-ZrO₂ or Ni-ZrO₂ cermet), and the cathode is either yttria-stabilised LaSrMnO₃ or lanthanide-based perovskite. The electrode reactions are shown below:

Anode oxidation of hydrogen:

H₂ + O^2- H₂O + 2e^-

Cathode reduction of oxygen:

½O₂ + 2e^- O^2-

Total SOFC reaction:

H₂ + ½O₂ H₂O

At the present operating conditions, internal reforming of fossil fuels is possible. The higher temperature compared to the MCFC reduces the sulphur problem from the reforming process to a minimum. SOFC's can handle about 5 times the amount of sulphur, which means that gases produced from coal can be used. Internal partial reforming of the incoming fuel is also possible. This leads to a more homogeneous temperature distribution in the cells. 

There are presently three main cell designs: tubular, developed by Siemens Westinghouse, a heat-exchange integrated system by Sulzer Hexis and the ordinary planar concept. The advantages of the non-planar cells are easier sealing techniques and a better heat exchange concept. In addition to the general high-temperature problem of material degradation, both aspects represent challenges for the planar development.

Sulzer-Hexis Heat-Exchange Intergrated System SOFC.

Siemens-Westinghouse tubular SOFC.

Planar SOFC stack.

The high temperature of the SOFC has its drawbacks. Thermal expansion mismatches among materials occurs, and sealing between cells is difficult in the flat-plate configuration. It also places severe constraints on materials selection and results in difficult fabrication processes for interconnections (bipolar plates) and seals. The interconnections must be electrically conductive, seal the anode and cathode gas chambers and withstand the high temperature. Suitable sealants are even more difficult to develop. Most common now are glass materials. Due to the high operating temperature, the start-up and shut-down procedures are very time-consuming. SOFC and MCFC are therefore not suited for dynamic operation. Combined with heat turbines, the same overall efficiency as in the MCFC can be reached.

Solid oxide fuel cells are most suited for stationary applications. Siemens Westinghouse is, together with Sulzer-Hexis, one of the leading actors. Even though the SOFC is the fuel cell with the longest continuous development period, starting in the late 1950's, several years before the AFC, there are still many material issues to be solved. The development of low-cost materials (especially interconnections) with high durability at high temperatures is the key technological challenge.

Copyright © 2004-2005 Fuel Cell Norway ANS