Solid Fuel Cells (SFC)

Solid fuel cells do not contain liquid components, other than the fuel that is burned. They are generally composed of four layers, three of which are a ceramic composite that contains the anode, electrolyte, and cathode. A single ceramic layer is usually only a few millimeters thick, which allows several hundred to be stacked together to create a large fuel cell.

General Operation of SFCs

All solid fuel cells run at high temperatures, usually between 600 C and 1100 C. At these temperatures, the solid electrolyte becomes ionically active, allowing only certain ions through.

In the case of solid oxide fuel cells, it is actually oxygen that travels through the electrolyte layer. This process is backward to that of most fuel cells, with oxygen traveling from the cathode to the anode. The rest of the operation of these fuel cells is typical of any fuel cell. The other types of solid fuel cells, protonic and direct carbon, are identical in operation to normal temperature fuel cells with the exception that they only work properly at high temperatures.

Benefits of SFCs

The major benefit to these types of fuel cells is the reduced or absent need for a catalyst. Catalysts facilitate reactions, helping them to proceed faster. This is very useful at low temperatures, but is often not necessary at high temperatures. By eliminating the catalyst, solid fuel cells remove the expense associated with PEMFCs and other normal temperature fuel cells. They also avoid the problem of carbon monoxide poisoning.

Because solid fuel cells run at temperatures high enough to reform smaller hydrocarbons like methanol and ethanol, there is the added efficiency and reduced complexity of having fuel forming and usage occur in a single step.

Drawbacks of SFCs

The solid fuel cell’s major strength is also its major weakness. High operating temperatures result in a lag between startup and full operational ability that can be as long as several hours. This makes SFCs impractical for use in transportation and as emergency back-up generators. There is currently a great deal of research being invested into of lower temperature SOFCs, which should be able to operate at temperatures below 600° C. Lower temperatures would also provide for the use of different, less expensive materials.

The other drawback to solid fuel cells is their relative complexity in terms of heat generating equipment. Steam is necessary to reach the high temperatures SFCs require, meaning that energy must be expended to heat water, which detracts from the overall efficiency of these fuel cells.

SFC Subtypes

Name

Fuel

Operating Characteristics

Solid Oxide

Oxygen and Hydrogen

500 – 1100 C with production capacity of less than 100 MW

Protonic Ceramic

Hydrogen

700 C

Direct Carbon

Fossil Fuels

700 – 850 C