An electric cell in which the chemical energy from the oxidation of a gas fuel is converted directly to electrical energy in a continuous process.

In the hydrogen and oxygen fuel cell, hydrogen and oxygen gas are bubbled into separate compartments connected by a porous disk through which an Electrolyte, such as aqueous potassium hydroxide (KOH), can pass.

Inert graphite electrodes, mixed with a catalyst such as platinum, are dipped into each compartment. When the two electrodes are electrically connected, an oxidation and reduction reaction takes place in the cell hydrogen gas is oxidized to form water at the anode; electrons are liberated in this process and flow through the external circuit to the cathode, where the electrons combine with the oxygen gas and reduce it.

Fuel cells have been used to generate electricity in spacecraft.