An electrical cell that converts the intrinsic chemical free energy of a fuel directly into direct-current electrical energy in a continuous catalytic process. As in the classical definition of catalysis, the fuel cell itself should not undergo change; that is, unlike the electrodes of a battery, its electrodes ideally remain unchanged. For most fuel–oxidant combinations, the available free energy of combustion is somewhat less than the heat of combustion. In a typical thermal power conversion process, the heat of combustion of the fuel is turned into electrical work via a Carnot heat-engine cycle coupled with a rotating electrical generator. Since the Carnot conversion rarely proceeds at an efficiency exceeding 40% because of heat source and sink temperature limitations, the efficiency of conversion in a fuel cell can be greater than in a heat engine, especially in small devices.