| specific energy | 1300 (practical), 6000/8000 (theoretical) W·h/kg[1] |
|---|---|
| energy density | N/A |
| specific power | 200 W/kg |
| Nominal cell voltage | 1.2 V |
Aluminium–air batteries or Al–air batteries produce electricity from the reaction of oxygen in the air with aluminium. They have one of the highest energy densities of all batteries, but they are not widely used because of previous problems with cost, shelf-life, start-up time, and byproduct removal, which have restricted their use to mainly military applications. An electric vehicle with aluminium batteries has potential for ten to fifteen times the range of lead–acid batteries with a far smaller total weight,[1] at the cost of substantially increased system complexity.
Aluminium–air batteries are primary cells, i.e., non-rechargeable. Once the aluminium anode is consumed by its reaction with atmospheric oxygen at a cathode immersed in a water-based electrolyte to form hydrated aluminium oxide, the battery will no longer produce electricity. However, it may be possible to mechanically recharge the battery with new aluminium anodes made from recycling the hydrated aluminium oxide. Such recycling will be essential if aluminium–air batteries are to be widely adopted.
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The anode oxidation half-reaction is Al + 3OH− → Al(OH)3 + 3e− −1.66 V.
The cathode reduction half-reaction is O2 + 2H2O + 4e− → 4OH− +0.40 V.
The total reaction is 4Al + 3O2 + 6H2O → 4Al(OH)3 + 2.71 V.
About 1.2 volts potential difference is created by these reactions, and is achievable in practice when potassium hydroxide is used as the electrolyte. Saltwater electrolyte achieves approximately 0.7 volts per cell.
Aluminium as a "fuel" for vehicles has been studied by Yang and Knickle.[1] They concluded the following:
The Al/air battery system can generate enough energy and power for driving ranges and acceleration similar to gasoline powered cars...the cost of aluminium as an anode can be as low as US$ 1.1/kg as long as the reaction product is recycled. The total fuel efficiency during the cycle process in Al/air electric vehicles (EVs) can be 15% (present stage) or 20% (projected), comparable to that of internal combustion engine vehicles (ICEs) (13%). The design battery energy density is 1300 Wh/kg (present) or 2000 Wh/kg (projected). The cost of battery system chosen to evaluate is US$ 30/kW (present) or US$ 29/kW (projected). Al/air EVs life-cycle analysis was conducted and compared to lead/acid and nickel metal hydride (NiMH) EVs. Only the Al/air EVs can be projected to have a travel range comparable to ICEs. From this analysis, Al/air EVs are the most promising candidates compared to ICEs in terms of travel range, purchase price, fuel cost, and life-cycle cost.
There are some technical problems still to solve, however, in order to make Al–air batteries suitable for powering electric vehicles. Anodes made of pure aluminium are corroded by the electrolyte, so the aluminium is usually alloyed with tin or other elements. The hydrated alumina that is created by the cell reaction forms a gel-like substance at the anode and reduces the electricity output. This is an issue being addressed in the development work on Al–air cells. For example, additives that form the alumina as a powder rather than a gel have been developed. Also, alloys have been found to form less of the gel than pure aluminium.
Modern air cathodes consist of a reactive layer of carbon with a nickel-grid current collector, a catalyst (e.g., cobalt), and a porous hydrophobic PTFE film that prevents electrolyte leakage. The oxygen in the air passes through the PTFE then reacts with the water to create hydroxide ions. These cathodes work well but they can be expensive.
Traditional Al–air batteries had a limited shelf life[2] because the aluminium reacted with the electrolyte and produced hydrogen when the battery was not in use – although this is no longer the case with modern designs. The problem can be avoided by storing the electrolyte in a tank outside the battery and transferring it to the battery when it is required for use.
These batteries can be used as reserve batteries in telephone exchanges, as a backup power source. Al–air batteries could be used to power laptop computers and cell phones and are being developed for such use.
Different types of aluminium batteries had been investigated:
| Book: Aluminium | |
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