Eco Sustainable Village

The following technologies can be considered to power up small appliances. Recommended for student projects. Note: We do not sell any product, however we provide consultation services only. Admin.

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.

A remarkable small fuel cell, designed for demonstrating and explaining fuel cells, and for a wide range of chemistry experiments. The cell can be used with a range of fuels, such as methanol or ethanol. With sodium borohydride as the fuel its power is very impressive.

A very important feature of our mini fuel cell is that a very wide range of fuels can be used, and no hydrogen is needed. It is very easy to use – indeed it has been specifically designed for use by children and students. When using methanol or ethanol as the fuel it produces enough current to show the principle of fuel cell operation. If, NaBH₄ (sodium tetrahydridoborate) is used as the fuel, then much greater power is obtained, and the performance exceeds similar size PEM fuel cells. For its size, it is very powerful, and it will easily drive small electric motors. Through our DC/DC converter it can drive small radios, tape players and the like.

The mini fuel cell is very simple and easy to use. It consists of two components, a red cathode, and a beaker-like anode. These are shown, partly separated, below.

An electrolyte connects the two electrodes. The electrolyte is about 65 cm³ of potassium hydroxide solution. The concentration of the KOH should be 1M, but this is not critical, and 4M can be used if a higher performance is required.

The fuel is mixed with the electrolyte. The cathode is then inserted into the beaker-like anode, and the fuel cell is set up, as in the diagram below.

The best performance is obtained using NaBH₄ (sodium tetrahydridoborate). This is not a well-known chemical, but is not particularly expensive or hazardous. We sell small quantities of the fuel for a few dollars, and this is enough for well over 100 hours use! From its formula, it can be seen that it contains four atoms of hydrogen per molecule, which makes it a very good and convenient "hydrogen store". Using this fuel the performance of this very lost-cost fuel cell matches that of any other low temperature, ambient pressure, fuel cell, including any PEM fuel cell.

The cell can be used with other fuels, a very wide range of fuels. These include:-

	Methanol CH3OH, is more readily available, and gives enough current to show the principle of a fuel cell.
	Methanal (formaldehyde) or methanoic (formic) acid.
	Ethanol C2H5OH
	Any ethanol containing beverage!
	Almost any alcohol, or alcohol containing fluid, such as automobile windscreen washer fluid.

The performance of the cell varies with temperature and other factors, but typical indicative figures are:-

Clearly the performance with NaBH₄ is vastly superior, but nevertheless the liquid fuels, which all give similar performance, do give enough current and voltage to illustrate fuel cell operation, and are usually more readily available. Only tiny quantities of NaBH₄ are used, and it is not expensive. Typically just 20 mg (0.02 cm³) will run the cell for one hour!

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Last modified: 04/08/06