A fuel cell is an electrochemical energy converter. It converts chemical energy into electrical energy by two separated electrochemical reactions. In a hydrogen-fuelled polymer electrolyte membrane fuel cell (PEMFC), hydrogen is oxidised to protons and electrons at the anode. Protons migrate through the membrane electrolyte to the cathode. As the membrane is an electric insulator, electrons are forced to flow in an external electric circuit. At the cathode, oxygen reacts with protons to produce water, which is the only waste product from a hydrogen-operated PEMFC, see the figure below.

Hydrogen is oxidised at the anode to protons:

H₂ 2H⁺ + 2e^-

while oxygen is reduced at the cathode:

½O₂ + 2H⁺ + 2e^- H₂O

Total cell reaction:

H₂ + ½O₂ H₂O 

From hydrogen and oxygen we obtain water, heat and power. There are other fuels, electrolytes and charge-transferring ions for the other fuel cell types - but the principle is the same. 

The driving force in a fuel cell is nature's affinity for lower (chemical) free energy. Hydrogen and oxygen are unstable in each other's presence and spontaneously form water in a redox reaction. The product (water) has a lower free energy than the reactants (hydrogen and oxygen), see the figure below, and is therefore preferred by the system (nature). By forcing this reduction and oxidation to take place on different sides of an electrolyte, the energy difference between the reactants and the products can be converted to electricity.   

The maximum operating voltage of a single fuel cell is less than 1 V. To be of any practical use many cells are operated together. Mostly this is done by connecting several cells in series to form a so-called stack. By varying the number and size of the cells, the fuel cell system can be adjusted to suit almost all kinds of requirements.

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