8.3 Fuel Cells to Generate Electricity

If renewable energy is to be stored as hydrogen, it is necessary to have a means of reconverting this hydrogen to electricity. This is the role of the fuel cell. Fuel cells are essentially water electrolyzers working in reverse; a fuel gas (usually hydrogen) is fed to the negative electrode and oxygen (or air) to the positive electrode; for electrode terminology, see Box 8.2 presented above. An electrochemical reaction takes place - fuel is oxidized, oxygen is reduced, water is formed - and a voltage is generated across the cell. A direct current may then be drawn. The electrode at which the water is formed depends upon the nature of the electrolyte and the ion-conducting species. In LT fuel cells, if the hydrogen ion is the conducting species, then water is formed at the oxygen electrode; conversely, if the hydroxide ion (or the oxide ion) is the conducting species, then the water is formed at the hydrogen electrode. In HT fuel cells, water is formed at the hydrogen electrode.

A fuel cell differs from a conventional battery in that the reactants are gaseous and are stored outside the cell. Therefore, the capacity of the device is limited only by the size of the fuel and oxidant supply and not by the cell design. For this reason, fuel cells are rated by their power output (kW) rather than by their capacity (kWh).

The fuel cell was invented in 1839 by Sir William Grove and therefore...