Types of Fuel Cells
Alkaline fuel cells - AFC
Alkaline fuel cells were first used in the Gemini-Apollo space program to produce drinking water and electrical energy.
Operate on compressed hydrogen.
Alkaline fuel cells generally use a solution of potassium hydroxide (chemically, KOH) in water as their electrolyte.
Output of alkaline fuel cell ranges from 300 watts (W) to 5 kilowatts (kW).
Direct methanol fuel cells - DMFC
Direct methanol fuel cells use methanol instead of hydrogen.
Operating temperatures of direct methanol fuel cells are in the same range as PEM fuel cells - 50 to 100°C (122 to 212°F).
Direct methanol fuel cells are being considered for use in the transportation industry.
Molten carbonate fuel cells - MCFC
Molten carbonate fuel cells use a liquid solution of lithium, sodium, and/or potassium carbonates soaked in a matrix.
Units with output up to 2 megawatts (MW) have been constructed, and designs exist for units up to 100 MW.
The nickel electrode-catalysts of molten carbonate fuel cells are inexpensive compared to those used in other cells, but the high temperature also limits the materials and safe uses of MCFCs.
Phosphoric acid fuel cells - PAFC
Phosphoric acid fuel cells use phosphoric acid as the electrolyte to make electricity.
Efficiency ranges from 40 to 80 percent 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
PEM fuel cells are the most common type of fuel cell being developed for transportation use.
They operate at the one kW per liter of volumetric powered level at a temperature under 100°C (212 °F)
PEM fuel cells react quickly to changes in electrical demand and will not leak or corrode.
PEM fuel cells use inexpensive manufacturing materials (plastic membrane).
Regenerative fuel cells - RFC
Regenerative fuel cells separate water into hydrogen and oxygen by a solar-powered electrolyser.
Hydrogen and oxygen are fed into regenerative fuel cells, generating electricity, heat and water.
Water is then recirculated back to the electrolyser of the regenerative fuel cell and the process repeats.
Solid oxide fuel cells - SOFC
Solid oxide fuel cells 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.