fuel cell, electric cell in which the chemical energy from the oxidation of a gas fuel is converted directly to electrical energy in a continuous process (see oxidation and reduction). The efficiency of conversion from chemical to electrical energy in a fuel cell is between 65% and 80%, nearly twice that of the usual indirect method of conversion in which fuels are used to heat steam to turn a turbine connected to an electric generator. The earliest fuel cell, in which hydrogen and oxygen were combined to form water, was constructed in 1829 by the Englishman William Grove. 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 move. Inert graphite electrodes, mixed with a catalyst such as platinum, are dipped into each compartment. When the two electrodes are connected by a wire, the combination of electrodes, wire, and electrolyte form a complete circuit, and an oxidation-reduction reaction takes place in the cell: hydrogen gas is oxidized to form water at the anode, or hydrogen electrode; electrons are liberated in this process and flow through the wire to the cathode, or oxygen electrode; and at the cathode the electrons combine with the oxygen gas and reduce it. The modern hydrogen-oxygen cell, operating at about 250°C and a pressure of 50 atmospheres, gives a maximum voltage of about 1 volt. Fuel cells have been used to generate electricity in space flights.