Most commonly refers to a relatively new type of energy storage device that produces energy using a chemical reaction that combines hydrogen and oxygen to produce water. The reaction produces heat which is harnessed for the generation of kinetic energy for vehicles or electric energy for private or commercial use.

Fuel cells are modular devices, so they can be replaced and combined like batteries. These cells range in size from small enough to fit in a hiker's backpack to large enough to provide commercial electric energy. Fuel cells are most commonly used at this time as energy sources for commercial fleet vehicles such as city buses.

Natural gas and methanol have been used in fuel cells, and the removable fuel tanks in racing cars are also referred to as fuel cells, but newer designs use hydrogen storage and. Fuel conversion efficiency can be as high as 90% when the hot water produced by the reaction is used for commercial or residential heating systems. Relative fuel costs for these devices were quite high at the turn of the century, but cost reductions from production increases are likely to make these devices common in a wide range of implementations by the end of decade one.

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