`*` Fuel cells are another means by which chemical energy may be converted into electrical energy.

`*` The main disadvantage of a primary cell is that it can deliver current for a short period only.

`*` This is due to the fact that the quantity of oxidising agent and reducing agent is limited.

`*` But the energy can be obtained indefinitely from a fuel cell as long as the outside supply of fuel is maintained.

`text(One of the examples is the hydrogen-oxygen fuel cell.)`

`*` The cell consists of three compartments separated by a porous electrode.

`*` Hydrogen gas is introduced into one compartment and oxygen gas is fed into another compartment.

`*` These gases then diffuse slowly through the electrodes and react with an electrolyte that is in the central compartment.

`*` The electrodes are made of porous carbon and the electrolyte is a resin containing concentrated aqueous sodium hydroxide solu­tion.

`*` Hydrogen is oxidised at anode and oxygen is reduced at cathode.

`*` This type of cells are used in space-crafts. Fuel cells are efficient and pollution free.

`*` The overall cell reaction produces water. The reactions which occur are : See fig.