Types of Fuel Cells

Fuel cells are generally categorized by their electrolyte — the material sandwiched between the two electrodes. This material's characteristics determine the optimal operating temperature and the fuel used to generate electricity. Each comes with its particular set of benefits and shortcomings.

Fuel Cell Type	Electrolyte	Anode Gas	Cathode Gas	Temperature	Efficiency
Proton Exchange Membrane (PEM)	solid polymer membrane	hydrogen	pure or atmospheric oxygen	75°C (180°F)	35–60%
Alkaline (AFC)	potassium hydroxide	hydrogen	pure oxygen	below 80°C	50–70%
Direct Methanol (DMFC)	solid polymer membrane	methanol solution in water	atmospheric oxygen	75°C (180°F)	35–40%
Phosphoric Acid (PAFC)	Phosphorous	hydrogen	atmospheric oxygen	210°C (400°F)	35–50%
Molten Carbonate (MCFC)	Alkali- Carbonates	hydrogen, methane	atmospheric oxygen	650°C (1200°F)	40–55%
Solid Oxide (SOFC)	Ceramic Oxide	hydrogen, methane	atmospheric oxygen	800–1000°C (1500–1800°F)	45–60%

More commonly known as the proton exchange membrane fuel cell or PEM, this is one of the most promising fuel cell types for widespread use. PEMs are exceptionally responsive to varying loads (such as driving) and are increasingly cheap to manufacture. The PEM fuel cell uses an advanced plastic electrolyte (typically Nafion) to shuttle protons from the anode to the cathode. The PEM's solid electolyte is much easier to handle and use than a liquid counterpart, and its low operating temperature allow a quick startup.

A thin platinum catalyst chemically activates the reactions at the electrodes. In the past, the platinum has made these devices prohibitively expensive, but new application technologies have dramatically thinned the platinum layer, allowing these devices to deliver electricity for less than $3000/kW. PEM fuel cells are best suited for 1kW to 100kW applications.

Widely used by the space program, this device was developed by NASA to power the Gemini missions and subsequent Space Shuttle operations. AFCs are very efficient, and discharge only pure water. However, these devices require very pure hydrogen and oxygen, and the electrolyte, alkaline potassium hydroxide, is exceptionally expensive. Since most fuel processing produces some carbon dioxide, which poisons the alkaline catalyst, AFCs will find only niche markets.

This configuration has been commercially available since 1992. The PAFC has potential for use in small stationary power-generation systems. They are known for their high reliability, quite operation, and high efficiency — over 80 percent conversion efficiency as a co-generation device. They run at a medium temperature range and can run on impure hydrogen.

MCFCs use a carbonate-salt-impregnated ceramic matrix as an electrolyte. Because MCFCs operate at 800°F, they are best suited to large stationary applications. Yet they potentially have the most to gain, as they operate at 85 percent efficiency with cogeneration. Many MCFCs are currently undergoing real-world testing, and they are expected to become marketable around 2004. They will be especially useful in hospitals, hotels, or other industrial applications that require electricity and heating (or cooling) around the clock.

These fuel cells are best suited for large-scale stationary power generators that could provide electricity for factories or towns. SOFCs use a prefabricated ceramic sandwich between electrodes. Like MCFCs, they operate at higher temperatures (about 1000°F) and make excellent co-generation devices for industrial applications where high temperature steam is required. These should be commercially competitive in the 2005 to 2007 timeframe.