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June 16, 2022

Zinc-air PowerStream Battery Chemistry FAQ

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Zinc/Air Cells

Anode: Amalgamated zinc powder and electrolyte

Cathode: Oxygen (O2)

Electrolyte: Potassium hydroxide (KOH)

Applications: Hearing aids, pagers, electric vehicles

The zinc air cell fits into the alkaline cell category because of its electrolyte. It also acts as a partial fuel cell because it uses the O2 from air as the cathode. This cell is interesting technology, even aside from the question "how do you use air for an electrode?" Actually, oxygen is let in to the cathode through a hole in the battery and is reduced on a carbon surface.

A number of battery chemistries involve a metal oxide and zinc. The metal oxide reduces, the zinc becomes oxidized, and electric current results. A familiar example is the old mercury oxide/zinc batteries used for hearing aids. If you leave out the metal oxide you could double the capacity per unit volume (roughly), but where would you get the oxygen? Right!

First let's look at the electrochemical reactions and find that the open cell voltage should be 1.65 volts:

Location Half Cell reactions Voltage
Anode Zn2+ + 2OH- —> Zn(OH)2 1.25
Cathode 1/2 O2 + H2O + 2e —> 2 OH- 0.4
Overall 2Zn +O2 +2H2O —> 2Zn(OH)2 1.65

The electrolyte is an alkali hydroxide in 20-40% weight solution with water. One disadvantage is that since these hydroxides are hygroscopic, they will pick up or lose water from the air depending on the air humidity. Both too little and too much humidity reduces the life of the cell. At 30% KOH concentration the electrolyte at its highest conductivity, and is also in equilibrium with atmospheric water vapor when the air is at 50% humidity. Selective membranes can help prevent water exchange. Oxygen from the air dissolves in the electrolyte through a porous, hydrophobic electrode—a carbon-polymer or metal-polymer composite.

Since there is no need to carry around the cathode, the energy density of these batteries can be quite high, between 220–300 Wh/kg (compared to 99–123 Wh/kg with a HgO cathode), although the power density remains low. However, the use of potassium or sodium hydroxides as the electrolyte is a problem, since these can react with carbon dioxide in the air to form alkali carbonates. For this reason large zinc air batteries usually contain a higher volume of CO2 absorbing material (calcium oxide flake) than battery components. This can cancel out the huge increase in energy density gained by using the air electrode.

Since the density of zinc oxide is lower than that of metalic zinc (5.47 g/cc versus 7.14 g/cc) the anode space must have some empty space to accomodate the reacted product ZnO, otherwise the battery would burst during discharge.

This cell has the additional benefits of being environmentally friendly at a relatively low cost.

These batteries can last indefinitely before they are activated by exposing them to air, after which they have a short shelf life. For this reason (as well as the high energy density) most zinc-air batteries are used in hearing aids. There is a company promoting them for use in electric vehicles also because they are environmentally friendly and cost relatively little. The idea is to have refueling stations where the zinc oxide waste can be replaced by fresh zinc pellets.

For discharge curves of hearing-aid style zinc-air batteries click here. discharge of zinc air coin and button cells

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