| Zinc/Air Cells
Anode: Amalgamated zinc powder and electrolyte
Cathode: Oxygen (O2)
Electrolyte: Potassium hydroxide (KOH)
Applications: Hearing aids, pagers, electric
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:
||Half Cell reactions
||Zn2+ + 2OH- >
||1/2 O2 + H2O + 2e >
||2Zn +O2 +2H2O >
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 electrodea
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 220300 Wh/kg
(compared to 99123 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
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.