Zinc-air battery

Zinc-air batteries, also called “zinc-air fuel cells,“ are non-rechargeable 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)₄^2-), 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 refueling 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.

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Reaction formulas

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

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

Fluid: Zn(OH)₄^2– → 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

• Zinc-air batteries have high specific energy for a battery (110 to 200 W·h/kg or 400 to 720 kJ/kg) Table 1.2 in Handbook of Batteries says Zinc-Air button energy density is 370 Wh/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) until they are activated for use.
• 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. Immersion in water should be avoided.
• 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;” the zinc oxide from the used batteries is melted back into zinc metal (and thus reduced) and remixed with recycled electrolyte.

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. ^[1] The spent zinc 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. Zinc is non-toxic and substantially easier to store than hydrogen, and can be processed by water-based electrochemistry. The single greatest problem with using zinc as a fuel is its price. 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. As India and China continue to industrialize, the price of all major metals will rise considerably, and perhaps top the high of $50,000 USD / ton that nickel reached in May 2007.

See also

• Hydrogen technologies
• Aluminium-air battery

References

• Zinc-air powered buses
• Military uses of Zinc-air Batteries
• Zinc-Air Batteries for UAVs and MAVs (includes half-cell reactions)
• Incorrect Zinc-air reaction
• Zinc-air fuel cell
• Procedure to MAKE a simple Zinc-air fuel cell as a science fair project.
• A European company developing RECHARGEABLE Zinc-air batteries
• Duracell technical bulletin (suppliers of zinc-air hearing aid batteries)
• Overview of batteries
• Electric Vehicle division
• Power Air Corporation