Aluminium–air batteries (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 problems with high anode cost and byproduct removal when using traditional electrolytes. This has restricted their use to mainly military applications. However, an electric vehicle with aluminium batteries has the potential for up to eight times the range of a lithium-ion battery with a significantly lower total weight.
|Specific energy||1300 (practical), 6000/8000 (theoretical) W·h/kg|
|Specific power||200 W/kg|
|Nominal cell voltage||1.2 V|
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 is possible to mechanically recharge the battery with new aluminium anodes made from recycling the hydrated aluminium oxide. Such recycling would be essential if aluminium–air batteries are to be widely adopted.
Aluminium-powered vehicles have been under discussion for some decades. Hybridisation mitigates the costs, and in 1989 road tests of a hybridised aluminium–air/lead–acid battery in an electric vehicle were reported. An aluminium-powered plug-in hybrid minivan was demonstrated in Ontario in 1990.
In March 2013, Phinergy released a video demonstration of an electric car using aluminium–air cells driven 330 km using a special cathode and potassium hydroxide. On May 27, 2013, the Israeli channel 10 evening news broadcast showed a car with Phinergy battery in the back, claiming 2,000 kilometres (1,200 mi) range before replacement of the aluminum anodes is necessary.
The cathode reduction half-reaction is O
2 + 2H
2O + 4e− → 4OH−
The total reaction is 4Al + 3O
2 + 6H
2O → 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. They concluded:
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.
Technical problems remain to be solved to make Al–air batteries suitable for 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.
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 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, for example, as reserve batteries in telephone exchanges and as backup power sources. 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 have been investigated:
- Aluminium–chlorine battery was patented by United States Air Force in the 1970s and designed mostly for military applications. They use aluminium anodes and chlorine on graphite substrate cathodes. Required elevated temperatures to be operational.
- Aluminium–sulfur battery worked on by American researchers with great claims, although it seems that they are still far from mass production. Rechargeable aluminium–sulfur battery was first demonstrated at University of Maryland in 2016.
- Al–Fe–O, Al–Cu–O and Al–Fe–OH batteries were proposed by some researchers for military hybrid vehicles. Corresponding practical energy densities claimed are 455, 440, and 380 Wh/kg
- Al–MnO manganese-dioxide battery using acidic electrolyte. Produces a high voltage of 1.9 volts. Another variation uses a base (potassium hydroxide) as the anolyte and sulfuric acid as the catholyte. The two parts being separated by a slightly permeable film to avoid mixing of the electrolyte in both half cells. This configuration gives a high voltage of 2.6–2.85 volts.
- Al–glass system. As reported in an Italian patent by Baiocchi, in the interface between common silica glass and aluminium foil (no other components are required) at a temperature near the melting point of the metal, an electric voltage is generated with an electric current passing through when the system is closed onto a resistive load. The phenomenon was first observed by Baiocchi, and after Dell'Era et Al. began the study and the characterization of this electrochemical system.
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- Gao, Tao (2016). "A Rechargeable Al/S Battery with an Ionic-Liquid Electrolyte". Angewandte Chemie International Edition. 55 (34): 9898–9901. doi:10.1002/anie.201603531. PMID 27417442.
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