The aluminium forms the negative electrode of the cell, and it typically starts as a plate about 1cm thick. As the reaction proceeds the electrode becomes smaller and smaller. The positive electrode is typically a porous structure, consisting of a metal mesh onto which is pressed a layer of catalysed carbon. A thin layer of PTFE gives it the necessary porosity to let the oxygen in, but prevent the liquid electrolyte getting out. The electrolyte is an alkaline solution, usually potassium hydroxide.

The battery is recharged by replacing the used negative electrodes. The electrolyte will normally also be replenished, as it will be contaminated with the aluminium hydroxide. The essential characteristics of the aluminium air battery are shown in Table III. The big drawback of the aluminium air battery is its extremely low specific power.

..................Table III Nominal battery parameters for aluminium air batteries

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The zinc air battery
The zinc air battery is similar in many ways to the aluminium air battery but it has a much better overall performance, particularly with regard to specific power which is nearly ten times that of the aluminium air battery, making it suitable for use in road vehicles. The structure is similar, with a porous positive electrode at which oxygen reacts with the electrolyte. The electrolyte is a liquid alkaline solution. The negative electrode is solid zinc.

The energy from the battery is obtained by combining zinc with the oxygen in the air and forming zinc oxide. Alternatively, depending on the state of the electrodes and electrolyte, zinc hydroxide may be formed, as for the aluminium-air cell. The process is normally irreversible. The general characteristics of the battery are shown in Table IV. A few manufacturers have claimed to produce electrically rechargeable zinc-air batteries, but the number of cycles is usually quite small. The more normal way of recharging is as for the aluminium air cell, which is by replacing the negative electrodes.

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