A fuel cell is supplied with a fuel from an external source, such as hydrogen, and with oxygen or air. The fuel is oxidised electrochemically inside the cell, and this produces a potential difference. Unlike a rechargeable battery, a fuel cell does not need to be recharged: it keeps working as long as fuel and oxygen are supplied.
The overall reaction in a hydrogen fuel cell is the oxidation of hydrogen to produce water: 2H2 + O2 → 2H2O. The only product is water, so no carbon dioxide is released when the cell is used.
Hydrogen fuel cells offer a potential alternative to rechargeable cells and batteries. Their advantages are that they do not need recharging, they give only water as a product and they can be refuelled quickly. Their disadvantages are that hydrogen is a flammable gas that is difficult to store, it needs a supply of hydrogen, which is often made using energy from fossil fuels, and fuel cells are expensive to build.
Higher tier only: the half equations for the electrodes in a hydrogen fuel cell are 2H2 + 4OH- → 4H2O + 4e- at one electrode and O2 + 2H2O + 4e- → 4OH- at the other (in an alkaline electrolyte). Under acidic conditions they are 2H2 → 4H+ + 4e- and O2 + 4H+ + 4e- → 2H2O.