Fuel cells are characterized by the type of electrolytes used to separate the fuel cell electrodes. Since different materials are electrolytic at different temperature levels, fuel cells are classified as low temperature, medium temperature and high temperature.
There are five primary types of fuel cells:
- Alkaline Fuel Cells (low temperature)
- Proton Exchange Membrane Fuel Cells (low temperature)
- Phosphoric Acid Fuel Cells (medium temperature)
- Molten Carbonate Fuel Cells (high temperature)
- Solid Oxide Fuel Cells (high temperature)
Each type of fuel cell has its own inherent strengths and weaknesses that make them more suitable for specific markets and applications.
For more information about fuel cells, see The Big Deal With Fuel Cells.
Multiple fuel cells are connected together to form a fuel cell stack.
Alkaline Fuel Cells
Alkaline fuel cells (AFCs) are among the most efficient type of fuel cells, reaching up to 60% efficiency (up to 87% combined heat and power). They were developed in 1959 by Francis Thomas Bacon. Using an alkaline electrolyte such as potassium hydroxide (KOH) in water and cathodes usually made with platinum, AFC Alkaline fuel cells also offer virtually instant operation without pre-heating, even at sub-zero temperatures. AFC were used by the NASA and MIR space programs to produce electricity and drinking water.
The alkaline fuel cell is comprised of a pair of porous electrodes—a positively charged cathode and a negatively charged anode—separated by an alkaline electrolyte or membrane electrolyte. Air, containing oxygen, is fed to the cathode gas chamber where it reacts with water to produce four OH- ions and four positive charges. The OH- ions, attracted by the anode, pass from the cathode through the KOH electrolyte. Hydrogen, fed to the anode, reacts with OH- ions to form molecules of water and negative charges.
The electrons are attracted by the positive charge on the cathode and are forced through an external circuit as an electric current. The reaction produces useable heat and water as a byproduct.
The alkaline fuel cell electrochemical process.