A cell in which one or both of the reactants are not permanently contained in the cell, but are continuously supplied from a source external to the cell and the reaction products continuously removed. Unlike the metal anodes typically used in batteries, the fuels in a fuel cell are usually gas or liquid, with oxygen as the oxidant. The hydrogen / oxygen fuel cell is the most common. In this fuel cell, hydrogen is oxidized at the anode:

Hydrogen/oxygen fuel cell systems have high power-to-weight and volume ratios, and usable reaction product (water). They can function for long periods as long as fuel is supplied and therefore the energy density cannot be measured.