Batteries, fuel cells and corrosion

A battery is just one or more galvanic cells packed for use. There are two kinds.

Primary cells run their reaction once; when it is finished, the cell is dead and cannot be recharged.

• The dry cell (Leclanché cell) in torches and clocks has a zinc container as the anode and a carbon rod with MnO₂ as the cathode, in a paste of NH₄Cl and ZnCl₂. It gives about 1.5 V.
• The mercury cell in hearing aids and watches keeps a steady voltage of about 1.35 V for its whole life.

Secondary cells can be recharged: pass a current through them the other way and the reaction runs backwards.

• The lead storage battery in cars and inverters has a lead anode and a grid of lead dioxide as the cathode, in sulphuric acid. While it gives current: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. Charging turns the lead sulphate back into Pb and PbO₂.
• The nickel-cadmium cell lasts longer but costs more.

So using a battery is a galvanic cell at work, and charging it is electrolysis.

A fuel cell is a galvanic cell that is fed continuously with fuel, so it never runs down. In the hydrogen-oxygen fuel cell, hydrogen and oxygen are bubbled through porous electrodes into a solution, and they combine to give water and electricity. It was used in the Apollo space missions, where the water was drunk by the astronauts. Fuel cells turn about 70% of the fuel's energy into electricity, far more than a power station burning fuel, and they cause no pollution. Methane and methanol can be used as fuels too.

Corrosion is a metal being slowly eaten away; the rusting of iron is the commonest case. It is a small electrochemical cell on the metal's surface. At one spot iron is oxidised (this spot acts as the anode): Fe → Fe²⁺ + 2e⁻. The electrons travel through the metal to another spot (the cathode), where oxygen is reduced in the presence of H⁺ from water. The Fe²⁺ is then oxidised further to rust, Fe₂O₃·xH₂O. So rusting needs both water and oxygen.

To prevent it: cover the metal with paint, oil or grease; coat it with a more reactive metal such as zinc (galvanising), which corrodes first; or join it to a block of a more reactive metal that is eaten away instead (sacrificial protection).