Publisher Summary This chapter describes fuel cells that convert chemical energy into electric energy. All fuel cell types are in principle capable of operation in both directions, although a particular cell may have been optimized for a particular process. The basic ingredients of an electrochemical device are two electrodes, sometimes called anode and cathode, and an intermediate electrolyte layer capable of transferring positive ions from the negative to the positive electrode or negative ions in the opposite direction, while a corresponding flow of electrons in an external circuit from the negative to the positive electrode provides the desired power. Use has been made of solid electrodes and fluid electrolytes and fluid electrodes such as in high-temperature batteries and solid electrolytes such as ion-conducting semiconductors. The difference in electric potential between the electrodes corresponds to an energy difference for each electron. The total number of electrons that could traverse the external circuit may be expressed as the product of the number of moles of electrons and Avogadro's constant. Hydrogen gas is led to the negative electrode, which may consist of a porous material, allowing H + ions to diffuse into the electrolyte, while the electrons enter the electrode material and may flow through the external circuit. Gaseous oxygen (or oxygen-containing air) is similarly led to the positive electrode, where a more complex reaction takes place. Also at the positive electrode, the reaction rate can be stimulated by a catalyst. Instead of the porous material electrodes, which allow direct contact between the input gases and the electrolyte, membranes can be used like those found in biological material, membranes that allow H+ to diffuse through but not H2.