For the Nu Metal/Hard Rock band, see Dry Cell or Dry Cell (band).

A dry cell, also known as a Leclanché cell or a zinc-carbon battery, is a form of primary electrochemical cell that supplieselectrical energy at small currents. Dry cells range in size between large flashlight batteries and tiny watch batteries but the basic construction is the same: a zinc cup lined with paper filled with an electrolyte paste with a graphite (carbon) rod in the center terminated with a metal cathode at the top.

Usage

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Because it was compact and reliable, the dry cell was commonly used to power portable electronic devices such as radios and flashlights until it was replaced by alkaline cells which solve many of the dry cell's shortcomings. Everyday use of the term “battery” usually is a reference to either the dry or the alkaline cell.

Chemistry

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Like all electrochemical cells, the dry cell gets its electrical energy from an internal chemical reaction which takes the form of two half-cell reactions. The electrolyte in the cell consists of ammonium chloride, manganese(IV) oxide, finely granulated carbon and an inert filler which is usually starch. The ammonia from the ammonium ions forms the complex ion Zn(NH₃)₄²⁺ with the Zn²⁺ preventing buildup of Zn ions which would result in reduction of the potential of the cell.

In standard electrochemical cell notation the dry cell looks like:

Zn(s)|ZnCl₂(aq), NH₄Cl(aq)|MnO(OH)(s)|MnO₂(s)|graphite

With a cell potential of approximately 1.5 volts and the following half-cell reactions

Oxidation

At the anode (-):

Zn(s) → Zn²⁺(aq) + 2 e^-

followed by Zn²⁺(aq) + 2 NH₃(g) → ^*2+(aq)

Reduction

At the cathode (+):

MnO₂(s) + H₂O(l) + e^- → MnO(OH)(s) + OH^-(aq)

followed by NH₄⁺(aq) + OH^-(aq) → H₂O(l) +NH₃(g)

Gas absorption

The reaction is further complicated by the production of two gaseous products in the reduction of ammonium ions:

2 NH₄⁺(aq) + 2 e^- → 2 NH₃(g) + H₂(g)

These products must be absorbed within the cell if gas pressure is not to build up. Two further reactions within the paste electrolyte absorb the gas. Zinc chloride reacts with ammonia to form solid zinc ammonium chloride and manganese dioxide reacts with hydrogen to form solid dimanganese trioxide and water.

ZnCl₂(aq) + 2 NH₃(g) → Zn(NH₃)₂Cl₂(s)

2 MnO₂(s) + H₂(g) → Mn₂O₃(s) + H₂O(l)

The cell initially produces about 1.5 volts, but this decreases as the reaction goes on because the moist paste is not completely mobile, so the electrolyte near the electrodes can be completely reacted, inhibiting further reaction, while un-reacted electrolyte is left in the body of the cell.

Practical considerations

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Dry cells have several advantages over wet cells such as a lead acid battery:

• They can be made very compact
• Like wet cells, they can be combined in series to get a higher voltage
• They do not require liquid water or sulfuric acid (H₂SO₄), so they are more easily contained and transported and are lighter weight
• The chemicals involved are relatively safe
• Dry cells are fairly inexpensive to produce.

However, there are also several disadvantages to the dry cell:

• As a primary cell, dry cells cannot be recharged and must be discarded after their potential has dropped below a useful level.
• As mentioned earlier, the paste electrolyte is not completely mobile, so the reagents cannot be used completely.
• The zinc anode of the battery is also its outer case. As the reaction goes on, the anode is consumed and becomes thinner and thinner, until eventually the cell starts to leak. The leaked water and electrolyte make the outside of the cell sticky, and can damage the electronic device that the cell is powering.
• Shelf life is the greatest disadvantage of the dry cell, however. Even when not in use, the zinc electrode reacts with and is corroded by the electrolyte, limiting the shelf life of the cell to at most 1.5 years. Its potential decreases as well, especially in cold conditions.

Because of these shortcomings, dry cells have now been replaced by alkaline cells, which are similar to dry cells except that they use an alkaline electrolyte which the zinc electrode doesn't readily react with when energy is not being drawn from the cell.

History

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An early dry cell was invented in 1802 by Johann Ritter.

The dry cell as we know it today was invented by Georges Leclanché in 1866. His design used a positive electrode consisting of a mixture of manganese dioxide and carbon in a porous pot. This and a zinc rod which served as the negative electrode were then immersed in an ammonium chloride solution. This became known as Leclanché's "wet" cell. Leclanché original design was prone to breakage but was improved by later engineers.

J.A. Thiebaut patented the first cell combining both the negative electrode and porous pot into a zinc cup in 1881; but Carl Gassner is credited with producing the first commercially successful dry cell in 1888 (patent 1887).

In 1909 the tungsten filament provided the impetus to use the dry cell to power the first battery-operated flashlights.

See also

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• Zinc-carbon battery (much more complicated and detailed, perhaps even clearer, explanation of chemical process inside a dry cell)
• Wet cell
• Georges Leclanché
• Galvanic cell
• Electrochemical cell

References

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Dry cell chemisty

• Chemsoc Timeline, Dry Cell Battery ^*
• HyperPhysics, Batteries ^*
• University of Hawaii, The "Dry-Cell" Battery ^*

History

• "Georges Leclanché" biography at Corrosion Doctors
• "Johann Ritter" biography at Corrosion Doctors
• "Galvanic Battery" at Today in Science

Electric batteries

乾電池 | Dry cell | 乾電池