Zinc–air battery
| Specific energy | 470 (practical),1370 (theoretical) Wh/kg[1][2] |
|---|---|
| Energy density | 1480-9780 Wh/L[citation needed] |
| Specific power | 100 W/kg[3][4] |
| Nominal cell voltage | 1.65 V |
Zinc-air batteries (non-rechargeable), and zinc-air fuel cells, (mechanically-rechargeable) are electro-chemical batteries powered by oxidizing 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 film cameras that previously used mercury batteries.
In operation, a mass of zinc particles forms a porous anode, which is saturated with an electrolyte. Oxygen from the air react at the cathode and form hydroxyl ions which migrate into the zinc paste and form zincate (Zn(OH)2−
4), releasing electrons to travel to the cathode. The zincate decays into zinc oxide and water returns to the electrolyte. The water and hydroxyls from the anode are recycled at the cathode, so the water is not consumed. The reactions produce a theoretical 1.65 volts, but this is reduced to 1.4–1.35 V in practical cells.
Zinc-air batteries have properties of fuel cells as well as batteries: the zinc is the fuel, the reaction rate can be controlled by varying the air flow, and oxidized zinc/electrolyte paste can be replaced with fresh paste. Research is being conducted in powering electric vehicles with zinc-air batteries.
History
The effect of oxygen was known early in the 19th century when wet-cell Leclanche batteries were observed to benefit from atmospheric oxygen diffusing into the carbon cathode current collector. In 1878 a porous platinized carbon air electrode was found to work as well as the manganese dioxide of the Leclanche cell. Commercial products were not made on this principle until 1932 when George W. Heise and Erwin A. Schumacher of the National Carbon Company built cells [5] with the carbon electrodes treated with wax to prevent flooding. This type is still used for large zinc-air cells for navigation aids and railways. However, the current capacity is low and the cells are bulky.
Large primary zinc-air cells such as the Thomas A. Edison Industries Carbonaire type were used for railway signalling, remote communication sites, and navigation buoys.These were long-duration, low-rate applications. Development in the 1970s of thin electrodes based on fuel-cell research allowed application to small button and prismatic primary cells for hearing aids, pagers, and medical devices, especially cardiac telemetry.[6]
Reaction formulas
Here are the chemical equations for the zinc-air cell[2]:
- 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)
Properties
Zinc-air batteries have significant properties that make them ideal for certain applications. Because one of the battery reactants is supplied by the air, a cell can use more zinc in the anode than a cell that must also contain, for example, manganese dioxide. This gives a high capacity for a given weight.
Storage and operating life
The cells have long shelf life; even miniature button cells can be stored for up to 3 years at room temperature with little loss of capacity, since oxygen is excluded from the cell by a tape barrier. Industrial cells stored in a dry state have an indefinite storage life. Materials for the cell are inexpensive.
The operating life of a zinc-air cell is a critical function of its interaction with its environment. Hot or dry conditions will lead to loss of water from the electrolyte. Because the potassium hydroxide electrolyte is deliquescent, in humid conditions excess water will be accumulated in the cell, causing the cathode to be flooded and lose its active properties. Potassium hydroxide also reacts with carbon dioxide in the air and formation of carbonates will eventually reduce the conductivity of the electrolyte. Miniature cells have high self-discharge once opened to air; ideally the cell's capacity should be consumed within a few weeks.[6]
Electrical discharge properties
Because the cathode does not change properties during discharge, the terminal voltage is quite stable until the cell is nearly completely exhausted.
Power capacity is a function of several variables: area of the cathode, air availability, porosity, and the catalytic value of the cathode surface. Entry of oxygen into the cell must be balanced against loss of water from the electrolyte; cathode membranes are coated with hydrophobic material (Teflon) to limit water loss. Water loss is faster if ambient humidity is low; if enough water is lost the cell will fail. Button cells have a limited current drain; for example an IEC PR44 cell has a capacity of 600 mAh but a maximum current of only 22 mA. Pulse load currents can be much higher since some oxygen is stored in the cell between pulses.[6]
Capacity of primary cells is reduced at low temperatures, but the effect is small for low drains. A cell may deliver 80% of its capacity if discharged at a 300 hour rate and 0 C, but only 20% of capacity if discharged at a 50 hour rate at the same temperature. Lower temperature also reduces cell voltage.
Zinc-air batteries cannot be used in a sealed battery holder since some air must come in; the oxygen in 1 litre of air is required for every ampere-hour of capacity used.
Primary cells
Large zinc-air batteries, with capacities up to 2000 ampere hours per cell, are used to power navigation instruments and marker lights, oceanographic experiments, and railway signals.
Primary zinc-air cells are made in button format to about 1 Ah. Prismatic shapes for portable devices are manufactured with capacities between 5 and 30 Ah. Hybrid cells have manganese dioxide added to the cathodes to allow high peak currents. Button cells are highly effective, but it is difficult to extend the same construction to larger sizes due to air diffusion performance, heat dissipation, and leakage problems. Prismatic and cylindrical cell designs address these problems. Stacking multiple prismatic cells requires air channels to be built into the battery and may require a fan to force air through the stack.[6]
Safety and environment
The vent holes allow any pressure build-up to be released. Some hydrogen could evolve if zinc corrodes, and manufacturers caution against hydrogen build-up in enclosed areas. A short-circuited cell gives relatively low current. Deep discharge below 0.5 V/cell may result in electrode leakage; there is little useful capacity below 0.9 V/cell.
Some types used mercury amalgam for the zinc, about 1% of the weight of a button cell, to prevent zinc corrosion. Newer types have no added mercury. Zinc is relatively low in toxicity. In newer designs that use no mercury for the zinc anode, the discharged cell can be discarded or recycled without special handling.[6]
In United States waters, environmental regulations now require proper disposal of primary batteries removed from navigation aids. Formerly, discarded zinc-air primary batteries were dropped into the water around buoys, which allowed mercury in the cells to escape to the environment.[7]
Secondary rechargeable cells
Rechargeable zinc-air cells are a difficult design problem since deposition of the zinc from the water-based electrolyte must be closely controlled. The problems are dendrite formation, non-uniform dissolving of zinc, and the limited solubility in electrolytes. Electrically reversing the reaction at a bifunctional air cathode, to liberate oxygen from the reaction products of discharge, is difficult; membranes tested to date have low overall efficiency. The required charging voltage is much higher than the discharge voltage, giving cycle energy efficiency as low as 50%. If charge and discharge functions are provided by separate uni-functional cathodes, the size, weight, and complexity of the cell are increased.[6] A satisfactory electrically recharged system would be attractive because of potentially low cost of materials and high specific energy. Electrically rechargeable configurations are yet to be brought to market[8]
Mechanically recharged cells
Rechargeable systems may use mechanical replacement of the anode and electrolyte, essentially operating as a refurbishable primary cell, or may use zinc powder or other methods to replenish the reactants. Mechanically-recharged systems were investigated for military electronics uses in the 1960s because of the high energy density and easy recharging possible. However, primary lithium batteries offered better high discharge rates and easier handling.
Mechanical recharging systems have been researched for decades for use in electric vehicles. Some approaches use a large zinc-air battery to maintain charge on a high-rate battery used for peak loads during acceleration. Zinc granules would be the reactant. The vehicle would exchange used electrolyte and depleted zinc for fresh reactants at a service station to recharge.
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.
Vehicle propulsion
Metallic zinc could be used as an alternative fuel for vehicles, either be used in a zinc-air battery [9] or to generate hydrogen near the point of use. The zinc-air rechargeable battery has relatively high cell voltage. Zinc is widely distributed and low cost, and has low weight for the power produced. These factors have motivated considerable interest in development of a zinc-air battery for vehicles. Gulf General Atomic demonstrated a 20 kW battery in a vehicle, and General Motors conducted tests in the 1970s, but neither project led to a commercial product.[10]
Solid zinc cannot be moved with the convenience of a liquid. An alternative is to form pellets of a small-enough size to be pumped. Fuel cells using it would have to be able to quickly replace "spent" zinc with fresh zinc.[11] The spent material could be reduced to ionic zinc at a local facility. The zinc-air "battery" cell is a primary cell (non-rechargeable); recycling is required to reclaim the zinc, and much more energy is required to reclaim the zinc than is usable in a vehicle.
See also
References
- ^ power one: Hearing Aid Batteries
- ^ a b Duracell: Zinc-air Technical Bulletin
- ^ greencarcongress: zincair_hybrid
- ^ thermoanalytics: battery types
- ^ US Patent Air-depolarized primary battery Heise - February, 1933 no. 1899615
- ^ a b c d e f David Linden, Thomas B. Reddy (ed). Handbook Of Batteries 3rd Edition, McGraw-Hill, New York, 2002 ISBN 0-07-135978-8, chapter 13 and chapter 38
- ^ http://www.uscg.mil/directives/ci/16000-16999/CI_16478_10.pdf retrieved 2010 Jan 18
- ^ http://www.revolttechnology.com/
- ^ J. Noring et al, Mechanically refuelable zinc-air electric vehicle cells in Proceedings of the Symposium on Batteries and Fuel Cells for Stationary and Electric Vehicle Applications Volumes 93-98 of Proceedings (Electrochemical Society), The Electrochemical Society, 1993 ISBN 1566770556 page 235-236
- ^ C. A. C. Sequeira Environmental oriented electrochemistry Elsevier, 1994 ISBN 044489456X, pages 216-217
- ^ Science & Technology Review
External links
- 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.
- ReVolt Technology developing RECHARGEABLE Zinc-air batteries
- Duracell technical bulletin (suppliers of zinc-air hearing aid batteries)
- Overview of batteries
- Electric Vehicle division
- Revolt Introduction
- Metal Air Batteries
Further reading
- Heise, G. W. and Schumacher, E. A., An Air-Depolarized Primary Cell with Caustic Alkali Electrolyte, Transactions of the Electrochemical Society, Vol. 62, Page 363, 1932.