Zinc-air battery

Zinc-air batteries, also called "zinc-air fuel cells" are a non-rechargeable electro-chemical battery powered by the oxidation of zinc with oxygen from the air. These batteries have very high energy densities and are relatively inexpensive to produce. These batteries are used in hearing aids and in experimental electric vehicles. They may be an important part of a future zinc economy.

Particles of zinc are mixed with an electrolyte (usually potassium hydroxide solution); water and oxygen from the air react at the cathode and form hydroxyls which migrate into the zinc paste and form zincate (Zn(OH)_{4^{2-), at which point electrons are released that travel to the cathode. The zincate decays into zinc oxide and water is released back into the system. The water and hydroxyls from the anode are recycled at the cathode; thus the water only serves as a catalyst. The reactions produce voltage levels max of 1.65 volts, but this is reduced to 1.4 - 1.35 V by reducing air flow into the cell, this is usually done for hearing aid batteries to reduce the rate of water drying out.}}

Zinc-air fuel cell usually refers to a zinc-air battery where zinc fuel is replenished and zinc oxide waste is removed continuously; this is accomplished by pushing zinc electrolyte paste or pellets into an anode chamber, waste zinc oxide is pumped out and into a waste tank or bladder inside the fuel tank, fresh zinc paste or pellets is taken from the fuel tank. The zinc oxide waste is pumped out at a refueling station and sent to a recycling plant. Alternatively a zinc-air fuel cell may refer to an electro-chemical system where zinc is used as a co-reactant to assist the reformation of hydrocarbon fuels on an anode of a fuel cell.

Zinc-air batteries have properties of fuel cells as well as batteries: the zinc is the fuel; the rate of the reaction can be controlled by controlling the air flow; and used zinc/electrolyte paste can be removed from the cell and replaced with fresh paste. Research is being conducted in powering electric vehicles with zinc-air batteries.

Reactions of Zinc-Air "Battery"

Anode: Zn + 4OH^{- → Zn(OH)_{4^{2- + 2e^{- (E₀=1.25 V)}}}}

Fluid: Zn(OH)_{4^{2- → ZnO + H_{2O + 2OH^-}}}

Cathode: O_{2 + 2H_{2O + 4e^{- → 4HO^{- (E₀=0.4 V)}}}}

Overall: 2Zn + O_{2 → 2ZnO (E₀=1.65 V)}

Alternatively the reaction is stated without use of zincate, but this is inaccurate:

Anode: Zn + 2OH^{- → Zn(OH)_{2 + 2e^{- (E₀=1.25 V)}}}

Cathode: O_{2 + 2H_{2O + 4e^{- → 4OH^{- (E₀=0.4 V)}}}}

Overall: 2Zn + O_{2 + 2H_{2O → 2Zn(OH)_{2 (E₀=1.65 V)}}}

Properties of Zinc-Air Battery

Zinc-air batteries have very high energy density (110 to 200 W·h/kg or 400 to 720 kJ/kg)
Zinc-air batteries put out continuous energy as they dissipate their energy, and the voltage does not drop until the battery is over 80-85% depleted.
Zinc-air batteries have very long shelf lives, as long as they are sealed (no oxygen is let in)
Zinc-air batteries have a very high self-discharge rate when exposed to air, as the zinc will spontaneously react with oxygen, and the water catalyst in the battery will tend to dry out.
- To prevent self-discharging the battery has to be resealed when not in use. Moisture in the battery can be maintained with use of a humidified environment.
- Zinc-air batteries must not be over saturated with water, though. Avoid immersing in water!
Zinc-air batteries use cheap materials and can be produced in mass quantities inexpensively.
Zinc-air batteries are not electrically rechargeable, but the zinc can be recycled or “mechanically recharged” in which the zinc oxide from the used batteries can be smelted back into zinc metal (and thus reduced) and remixed with recycled electrolyte.

References

Electric batteries | electrochemistry | Zinc

Zink-Luft-Batterie | 锌空电池

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Reactions of Zinc-Air "Battery"

Properties of Zinc-Air Battery

References