fuel cell technologies

There are generally five types of fuel cell technologies:
1. Alkaline fuel cells (AFC)
2. Phosphoric acid fuel cells (PAFC)
3. Proton exchange membrane fuel cells (PEMFC)
4. Molten carbonate fuel cells (MCFC)
5. Solid oxide fuel cells (SOFC)

Alkaline fuel cells (AFC)
Alkaline fuel cells use compressed hydrogen and oxygen to produce electricity. They normally operate at 70-90ºC, with 300-5000 W of power at about 25-30% system efficiency.

The Apollo astronauts used alkaline fuel cells to provide both electricity and drinking water. However, pure hydrogen fuel was used.

More information on AFCs may be found here.

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Phosphoric acid fuel cells (PAFC)
Phosphoric acid fuel cells use phosphoric acid as the electrolyte to produce electricity. Types of fuel that PAFCs can use include anaerobic digester gas, natural gas, gasoline, etc. They operate at 190-215ºC, generating up to 200 kW of power at about 35-40% efficiency.

The internal parts of the fuel cell must be able to withstand the corrosive acid.

More information on PAFCs may be found here.

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Proton exchange membrane fuel cells (PEMFC)
Proton exchange membrane fuel cells (also known as polymer electrolyte fuel cells) use a polymer-based electrolyte, typically in a thin, permeable sheet. This membrane must not leak or crack, while a platinum catalyst must also be coated on both sides of the membrane.

The operating temperature is about 70-90ºC, with outputs of between 1W and 20 kW of power. System efficiency is about 30-35%. The low temperature makes PEMFCs suitable for use in homes and cars. The fuel (typically hydrogen) must also be highly purified. Natural gas can also be reformed to produce hydrogen for fuel cells.

More information on PEMFCs may be found here.

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Molten carbonate fuel cells (MCFC)
Molten carbonate fuel cells use high-temperature carbonates (of sodium or magnesium) as the electrolyte for generating electricity. Hence, the normal operating temperature is around 600-650ºC, producing 250 kW - 2 MW of power at about 45-50% efficiency.

Waste heat produced by the reaction can be utilized to maximize system efficiency. This also means that MCFCs would be too hot for home applications.

More information on MCFCs may be found here.

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Solid oxide fuel cells (SOFC)
Solid oxide fuel cells use a hard, ceramic compound of metallic oxides (of calcium or zirconium) as the electrolyte. They normally operate at about 700-1,000ºC, producing up to 100 kW of power. System efficiency is around 50-55%. At such temperatures, reformers are not necessary for producing hydrogen from fuels, eg natural gas.

Waste heat from SOFCs can also be recovered for use in other applications, eg making more electricity. However, SOFCs are large in size, hence limiting its applications.

More information on SOFCs may be found here.