Aluminium battery
Encyclopedia
Aluminium–air batteries or Al–air batteries produce electricity from the reaction of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 in the air with aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

. 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, at the cost of substantially increased system complexity.

Aluminium–air batteries are primary cell
Primary cell
A primary cell is any kind of battery in which the electrochemical reaction is not reversible, rendering the cell non-rechargeable. A common example of a primary cell is the disposable battery. Unlike a secondary cell, the reaction cannot be reversed by running a current into the cell; the chemical...

s, 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
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...

, 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.

Electrochemistry

The anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....

 oxidation half-reaction
Half-reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...

 is Al + 3OH → Al(OH)3 + 3e −1.66 V.

The cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...

 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
Potassium hydroxide
Potassium hydroxide is an inorganic compound with the formula KOH, commonly called caustic potash.Along with sodium hydroxide , this colorless solid is a prototypical strong base. It has many industrial and niche applications. Most applications exploit its reactivity toward acids and its corrosive...

 is used as the electrolyte. Saltwater electrolyte achieves approximately 0.7 volts per cell.

Issues

Aluminium as a "fuel" for vehicles has been studied by Yang and Knickle. They concluded the following:
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
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

 with a nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-grid current collector, a catalyst (e.g., cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

), 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 as reserve batteries
Reserve battery
A reserve battery, also called stand-by battery, is a primary battery where part is isolated until the battery needs to be used. When long storage is required, reserve batteries are often used, since the active chemicals of the cell are segregated until needed, thus reducing self-discharge...

 in telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

s, 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.

Aluminium based batteries

Different types of aluminium batteries had been investigated:
  • Aluminium-chlorine battery was patented by United States Air Force
    United States Air Force
    The United States Air Force is the aerial warfare service branch of the United States Armed Forces and one of the American uniformed services. Initially part of the United States Army, the USAF was formed as a separate branch of the military on September 18, 1947 under the National Security Act of...

    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 batteries worked on by American researchers with great claims, although it seems that they are still far from mass production. It is unknown as to whether they are rechargeable.
  • 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

External links

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