A fuel cell is an electrochemical energy
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.
oxidised at the anode to protons:
H22H+ + 2e-
is reduced at the cathode:
+ 2H+ + 2e-
H2 + ½O2
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
The driving force in a fuel
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.