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June 16, 2022

Aluminum-Air PowerStream Battery Chemistry FAQ

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Aluminum / Air Cells

Although, to our way of thinking, the metal/air batteries are strictly primary, cells have been designed to have the metal replaceable. These are called mechanically rechargeable batteries. Aluminum/air is an example of such a cell. Aluminum is attractive for such cells because it is highly reactive, the aluminum oxide protective layer is dissolved by hydroxide electrolytes, and it has a nice, high voltage. The overall chemical reaction is:

Location Half Cell reactions Voltage
Anode Al + 4 OH-—> Al(OH)4- + 3e -2.35
Cathode 3/4 O2 + 3/2 H2O + 3e—> 3OH- 0.40
Overall Al + 3/2 HO + 3/4 O2 —> Al(OH)3 2.75 V

As I mentioned above, alkali (chiefly potassium hydroxide) electrolytes are used, but so also are neutral salt solutions. The alkali cell has some problem with the air electrode, because the hydroxide ion makes a gel in the porous electrode, polarizing it. The typical aluminum hydroxide gel is a problem on either electrode because it sucks up a lot of water. Using a concentrated caustic solution prevents this, but is very reactive with the aluminum electrode, producing hydrogen gas. Another way to prevent the gel formation is to seed the electrolyte with aluminum trihydroxide crystals. These act to convert the aluminum hydroxide to aluminum trihydroxide as the crystals grow. To prevent hydrogen gas evolution tin and zinc have been used as corrosion inhibitors. A number of additives are used to control the reactions. A disadvantage of the alkaline electrolyte is that it reacts with atmospheric carbon dioxide.

Aluminum / air cells have also been made for marine applications. These are "rechargeable" by replacing the seawater electrolyte until the aluminum is exhausted, then replacing the aluminum. Some cells that are open to seawater have also been researched. Since salt water solutions tend to passivate the aluminum, pumping the electrolyte back and forth along the cell surface has been successful. For those cells that don't need to use ocean water, an electrolyte of KCL and KF solutions is used.

Air electrodes of Teflon-bonded carbon are used without a catalyst.

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