Fuel Cells

Fuel cells are not constrained by the maximum Carnot cycle efficiency as combustion engines are. Consequently, they can have very high efficiencies in converting chemical energy to electrical energy. See the talk page for a discussion.

In the archetypal example of a hydrogen/oxygen proton-exchange membrane (or "polymer electrolyte") fuel cell (PEMFC), a proton-conducting polymer membrane separates the anode and cathode sides. Each side has an electrode, typically carbon paper coated with platinum catalyst.

On the anode side, hydrogen diffuses to the anode catalyst where it dissociates into protons and electrons. The protons are conducted through the membrane to the cathode, but the electrons are forced to travel in an external circuit (supplying power) because the membrane is electrically insulating.

On the cathode catalyst, oxygen molecules react with the electrons (which have travelled through the external circuit) and protons to form water.

In this example, the only waste product is water vapor and/or liquid water.

Fuel cells cannot store energy like a battery, but in some applications, like stand-alone power plants based on discontinuous sources (solar, wind power), they are combined with electrolyzers and storage systems to form an energy storage system. The round-trip efficiency (electricity to hydrogen and back to electricity) of such plants is between 30 and 40%.

In addition to pure hydrogen, researchers have used other, hydrogen-carrying fuels for fuel cells, including diesel, methanol and chemical hydrides.

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Efficiency

A fuel cell typically converts the chemical energy of its fuel into electricity with an efficiency of about 50%. (The rest of the energy is converted