A fuel cell is a device capable of providing electrical energy from an electrochemical reaction, typically between two or more reactants. Generally, a fuel cell includes two electrodes, called an anode and a cathode, and an electrolyte disposed between the electrodes. The anode contains an anode catalyst, and the cathode contains a cathode catalyst. The electrolyte, such as a solid membrane electrolyte, is typically ionically conducting but electronically non-conducting.
During operation of the fuel cell, the reactants are introduced to the appropriate electrodes. At the anode, a reactant interacts with the anode catalyst and forms reaction intermediates, such as ions and electrons. The ionic reaction intermediates can flow from the anode, through the electrolyte, and to the cathode. The electrons, however, flow from the anode to the cathode through an external electrical conductor connecting the anode and the cathode. As electrons flow through the external electrical conductor, the electron flow can be used to provide energy. At the cathode, the cathode catalyst interacts with another reactant, the ions formed at the anode, and the electrons to complete the fuel cell reaction.
For example, in one type of fuel cell, sometimes called a hydrogen fuel cell, the anode reactant is hydrogen gas, and the cathode reactant is oxygen (e.g., from air). At the anode, hydrogen gas is oxidized to form protons; at the cathode, oxygen gas is reduced and combines with protons to form water:Anode Reaction: H2→2H++2e−  (1)Cathode Reaction: ½O2+2H++2e−→H2O  (2)Overall Reaction: H2+½O2→H2O  (3)
As shown in Equation (1), oxidation of hydrogen produces protons and electrons. The protons flow from the anode, through the electrolyte, and to the cathode. The electrons flow from the anode to the cathode through an external electrical conductor, which can provide electrical energy. At the cathode, the protons and the electrons react with oxygen to form water (Equation 2). Equation 3 shows the overall fuel cell reaction, whereby hydrogen and oxygen are converted into water.