Aluminium/air Batteries
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.
Applications
The energy density of the aluminium/air battery is excellent, even better
than the Lithium battery, yet it is not greatly used in practice. The main
reason for this is the side reactions that take place between the electolyte and
the aluminium. These involve the corrosion of the aluminium and the production
of small amounts of hydrogen gas. This begins as soon as the aluminium
is in contact with the electrolyte. The reactions are very slow, but
in the time a typical battery might spend in storage before use much
harm will have been done. In other words, if the battery is stored
with its electrolyte it has a very short shelf life. However, there
are applications where the electrolyte can be stored separately, and
added when the power is needed. This type of battery is usually called
a reserve battery, and is the one market where the
aluminium/air battery has had some success.
Large aluminium/air batteries
are used as back-up power supplies in many telephone exchanges. When not is use
the electrolyte is stored in a tank outside the battery. When there is a power cut
it is automatically released into the battery, which starts up. Compared
to lead/acid batteries they store about 5 times as much energy in a
given volume, and can be recharged by replacing the aluminium
electrodes, which in a well designed battery need not be too
difficult. Such batteries are produced by
Alupower, who have their
own web site.
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