How they work
Structure
The structure of an aluminium/air battery is very simple. A piece of aluminium is immersed in an electrolyte near a porous electrode. This porous electrode has air on one side, and the electrolyte on the other. This is shown in the diagram below.

The electrolyte can be common salt (NaCl) solution or an alkali solution such as potassium hydroxide. Sea water can be used. The choice of electrolyte is fairly flexible, but only special alloys of aluminium can be used. With ordinary alloys the aluminium immediately becomes coated with a protective layer, or simply dissolves as aluminium oxide, giving off hydrogen gas.

You can purchase the special aluminium alloy and porous electrode materials from Electro-Chem-Technic. Using these materials it is quite simple to make a battery for yourself. Our own aluminium/air cell could be bought for initial experiments.

The Chemistry
The aluminium reacts with OH- ions to form aluminium hydroxide and release three electrons. The OH- ions are present either because the electrolyte is an alkali solution, or because they are produced at the other electrode (see below). The reaction is :-

These electrons form the electric current produced by the battery. The equation above shows why aluminium/air cells are so good. The valency of aluminium is three, so three electrons are released. Since three electrons are released for each aluminium atom we get a lot of electricity from our battery!

At the porous electrode the water in the electrolyte reacts with oxygen from the air, and absorbs the electrons produced at the aluminium electrode.

Cations are formed, and so this electrode is called the Cathode. It attracts negative electrons, and so is the Positive terminal of the battery. (Yes, the cathode is the positive electrode!)

The electrons produced at the aluminium electrode (the anode) pass round the external circuit connected to the battery, and both the above reactions carry on until the aluminium is used up, or the circuit is broken. The overall reaction is:-

With salt water electrolyte the open circuit voltage of the cell is about 1.2 volts, but the normal operating voltage is about 0.7 or 0.8 volts. With KOH solution the voltages are about 0.5 volts higher.

If the cathode is covered (e.g. with water) so the oxygen can't get to it anymore, a slightly different reaction occurs. Electrons are still absorbed, but oxygen is not used, and hydrogen gas is evolved:-

Electrons are still absorbed, but the voltage produced by the cell is far lower, typically about 0.5 volts.
Source(s):
http://www.ectechnic.co.uk/ALUMAIR.HTML

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