Definitions

air cell

Zinc-air battery

Zinc-air batteries (non-rechargeable), and zinc-air fuel cells, (mechanically-rechargeable) are electro-chemical batteries powered by the oxidation of zinc with oxygen from the air. These batteries have high energy densities and are relatively inexpensive to produce. They 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)42-), at which point electrons are released and 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, so the water serves only as a catalyst. The reactions produce a maximum voltage level 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.

The term zinc-air fuel cell usually refers to a zinc-air battery in which 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 into a waste tank or bladder inside the fuel tank, and fresh zinc paste or pellets are taken from the fuel tank. The zinc oxide waste is pumped out at a refuelling station and sent to a recycling plant. Alternatively, this term may refer to an electro-chemical system in which 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.

Reaction formulas

Here are the chemical equations for the zinc-air cell:

Anode: Zn + 4OH → Zn(OH)42– + 2e (E0 = –1.25 V)

Fluid: Zn(OH)42– → ZnO + H2O + 2OH

Cathode: O2 + 2H2O + 4e → 4OH (E0 = 0.4 V)

Overall: 2Zn + O2 → 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: O2 + 2H2O + 4e → 4OH (E0 = 0.4 V)

Overall: 2Zn + O2 + 2H2O → 2Zn(OH)2 (E0 = 1.65 V)

Properties

  • High specific energy: up to 370 W·h/kg.
  • Terminal voltage does not drop until 80–85% depletion.
  • Very long shelf lives when sealed to exclude oxygen.
  • A very high self-discharge rate when exposed to air due to spontaneous zinc oxidation.
    • The battery must be resealed when not in use. The electrolyte can be maintained in a humidified environment.
    • Must not be over saturated or immersed in water.
  • Zinc is cheap; mass production is inexpensive.
  • Not electrically rechargeable, but recycling can reduce the zinc oxide back to zinc metal for use in new batteries.

Zinc as energy currency

Metallic zinc could be used as an alternative to hydrogen as an energy transfer medium (a fuel). The zinc would be either used in a zinc-air battery or used to generate hydrogen by electrolysis near the point of use.

A major disadvantage is that zinc is not liquid, and cannot be pumped as a fuel. But it may be pumped as pellets. Fuel cells using it (the zinc-air "battery" is considered a primary cell and is non-rechargeable) would have to empty the "spent" zinc and be refueled quickly. The spent zinc-oxide would be reduced at a local facility into zinc.

Hydrogen generated from zinc and water could be burned in conventional internal combustion engines, although this would provide a far less powerful engine than a hydrocarbon-powered engine; a better alternative would be the use of high efficiency electric motors to exploit the power produced by a zinc-air battery and drive the vehicle.

Zinc has a number of advantages over hydrogen as an energy-carrier. Zinc-air cell batteries are already efficient enough for practical use in vehicles. Pure Zinc is non-toxic (commercially available zinc is often contaminated by other metals e.g. toxic lead) and substantially easier to store than hydrogen, and can be processed by water-based electrochemistry. The price of zinc in 2008 is $1/lb . Since India and China began rapidly industrializing at the end of 1990s the price of major metals such as nickel, copper, steel, aluminum and zinc has soared.

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References

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