A fuel cell works like a battery but does not run down or need recharging. It will produce electricity and heat as long as fuel is supplied. A fuel cell consists of two electrodes-a negative electrode (or anode) and a positive electrode (or cathode)-sandwiched around an electrolyte. Fuel is fed to the anode, and oxygen is fed to the cathode. Activated by a catalyst, hydrogen atoms separate into protons and electrons, which take different paths to the cathode. The electrons go through an external circuit, creating a flow of electricity. The protons migrate through the electrolyte to the cathode, where they reunite with oxygen and the electrons to produce water and heat. Fuel cells can be used to power vehicles or to provide electricity and heat to buildings.
The primary fuel cell technologies under development are:
- Phosphoric Acid Fuel Cells
A phosphoric acid fuel cell (PAFC) consists of an anode and a cathode made of a finely dispersed platinum catalyst on carbon paper, and a silicon carbide matrix that holds the phosphoric acid electrolyte. This is the most commercially developed type of fuel cell and is being used in hotels, hospitals, and office buildings. The phosphoric acid fuel cell can also be used in large vehicles, such as buses.
- Proton-Exchange Membrane Fuel Cells
The proton-exchange membrane (PEM) fuel cell uses a fluorocarbon ion exchange with a polymeric membrane as the electrolyte. The PEM cell appears to be more adaptable to automobile use than the PAFC type of cell. These cells operate at relatively low temperatures and can vary their output to meet shifting power demands. These cells are the best candidates for light-duty vehicles, for buildings, and much smaller applications.
- Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFC) currently under development use a thin layer of zirconium oxide as a solid ceramic electrolyte, and include a lanthanum manganate cathode and a nickel-zirconia anode. This is a promising option for high-powered applications, such as industrial uses or central electricity generating stations.
- Direct-Methanol Fuel Cells
A relatively new member of the fuel-cell family, the direct-methanol fuel cell (DMFC) is similar to the PEM cell in that it uses a polymer membrane as an electrolyte. However, a catalyst on the DMFC anode draws hydrogen from liquid methanol, eliminating the need for a fuel reformer.
- Molten Carbonate Fuel Cells
The molten carbonate fuel cell uses a molten carbonate salt as the electrolyte. It has the potential to be fueled with coal-derived fuel gases or natural gas.
- Alkaline Fuel Cells
The alkaline fuel cell uses an alkaline electrolyte such as potassium hydroxide. Originally used by NASA on space missions, it is now finding applications in hydrogen-powered vehicles.
- Regenerative or Reversible Fuel Cells
This special class of fuel cells produces electricity from hydrogen andoxygen, but can be reversed and powered with electricity to produce hydrogen and oxygen.