A fuel cell is a solid-state electrochemical power conversion device that directly converts the chemical energy of a fuel into electrical energy in a constant temperature process. It is analogous to a battery, but possesses the advantage of being constantly recharged with fresh reactant. In contrast to batteries, the reactants for fuel cells are stored outside the cell. They are fed to the cell only when power generation is required. Therefore, a fuel cell does not consume materials that form an integral part of, or are stored within, its own structure.

A fuel cell is made up of an electrolyte and two electrodes. They also have an anode and a cathode, both of which are electronic conductors. The electrolyte which separates the 2 electrodes acts as an ionic conductor and does not allow electron flow through it.

Six main types of fuel cells exist; these differ in their electrolyte, operating temperature and application, as shown in the table below.

Low Temperature FC High Temperature FC
Fuel Cell Type Alkaline Proton Exchange Membrane Direct Methanol Phosphoric Acid Molten Carbonate Solid Oxide
Operating Temp. (ºC) 50-200 50-100 60-120 180-220 ~650 500-1000
Efficiency (HHV) ~35-60% ~40-60% ~40-60% ~40% ~50% ~45-65%
Applications Military, Space (Apollo, Shuttle) Portable power, Transportation, Backup power, Small distributed generation Portable power Distributed generation (Transportation) Electric utility, Large distributed generation Electric utility, Auxiliary power, Large distributed generation

Fuel cells can run on a variety of fuels. The choice of fuel depends on customer requirement and available local fuel infrastructure. Other influencing factors are fuel cell type, system cost and complexity. Common fuels include hydrogen, methanol, ethanol, methane, carbon monoxide and hydrides. Hydrogen can fuel any fuel cell.