Alkaline fuel cells (AFC): First used in the Gemini-Apollo space program to produce drinking water and electrical energy; operate on compressed hydrogen; generally use a solution of potassium hydroxide (KOH) in water as electrolyte; output of alkaline fuel cell ranges from 300 W to 5 kW.

Direct-methanol fuel cells (DMFC): Use methanol instead of hydrogen; operating temperatures in the same range as PEM fuel cells — 50 to 100°C (122 to 212°F); transportation-industry focus.

Moltencarbonate fuel cells (MCFC): Use a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix; units with output up to 2 MW have been constructed, and designs exist for units up to 100 MW; nickel electrode-catalysts of molten carbonate fuel cells are inexpensive compared to those used in other cells, but high temperatures limit the materials and safe uses of MCFCs.

Phosphoric-acid fuel cells (PAFC): Use phosphoric acid as the electrolyte; efficiency ranges from 40 to 80% and operating temperature is 150 to 200°C (about 300 to 400°F); existing phosphoric-acid cells have outputs up to 200 kW; and 11-MW units have been tested.

Proton-exchange-membrane fuel cells (PEM): The most common type of fuel cell being developed for transportation use; react quickly to changes in electrical demand and will not leak or corrode; use inexpensive manufacturing materials (plastic membrane).

Regenerative fuel cells (RFC): Separate water into hydrogen and oxygen by a solar-powered electrolyzer; hydrogen and oxygen are fed into regenerative fuel cells, generating electricity, heat, and water; water is then recirculated back to the electrolyzer of the regenerative fuel cell and the process repeats.

Solid-oxide fuel cells (SOFC): Use a hard ceramic compound of metal (like calcium or zirconium) oxides (chemically, O2) as electrolyte; output for solid oxide fuel cells is up to 100 kW; reformer is not required to extract hydrogen from the fuel due to high temperature.