Leclanché cell

A 1919 illustration of a Leclanche cell

The Leclanché cell is a battery invented and patented by the French scientist Georges Leclanché in 1866. The battery contained a conducting solution (electrolyte) of ammonium chloride, a cathode (positive terminal) of carbon, a depolarizer of manganese dioxide, and an anode (negative terminal) of zinc. The chemistry of this cell was later successfully adapted to manufacture dry cells.

History

In 1866, Georges Leclanché invented a battery that consisted of a zinc anode and a manganese dioxide cathode wrapped in a porous material, dipped in a jar of ammonium chloride solution. The manganese dioxide cathode had a little carbon mixed into it as well, which improved conductivity and absorption.[1] It provided a voltage of 1.4 volts.[2] This cell achieved very quick success in telegraphy, signalling and electric bell work.

The dry cell form was used to power early telephones—usually from an adjacent wooden box affixed to the wall—before telephones could draw power from the telephone line itself. The Leclanché cell could not provide a sustained current for very long. In lengthy conversations, the battery would run down, rendering the conversation inaudible.[3] This was because certain chemical reactions in the cell increased the internal resistance and, thus, lowered the voltage. These reactions reversed themselves when the battery was left idle, so it was good only for intermittent use.[4]

Construction

The original form of the cell used a porous pot. This gave it a relatively high internal resistance and various modifications were made to reduce it. These included the "Agglomerate block cell" and the "Sack cell".

Porous pot cell
In Leclanché's original cell the depolarizer, which consisted of crushed manganese dioxide, was packed into a pot, and a carbon rod was inserted to act as the cathode. The anode, which was a zinc rod, was then immersed along with the pot in a solution of ammonium chloride. The liquid solution acted as the electrolyte, permeating through the porous pot to make contact with the cathode.
Agglomerate block cell
In 1871 Leclanché dispensed with the porous pot and replaced it with a pair of "agglomerate blocks", attached to the carbon plate by rubber bands. These blocks were made by mixing the manganese dioxide with binding agents and pressing the mixture into moulds.
Sack cell
In this cell the porous pot was replaced by a wrapping of canvas or sacking. In addition, the zinc rod was replaced by a zinc cylinder to give a larger surface area. It had a lower internal resistance than either of the above (porous and agglomerate).

Chemistry

The chemical process which produces electricity in a Leclanché cell begins when zinc atoms on the surface of the anode oxidize, i.e. they give up both their valence electrons to become positively charged ions. As the zinc ions move away from the anode, leaving their electrons on its surface, the anode becomes more negatively charged than the cathode. When the cell is connected in an external electrical circuit, the excess electrons on the zinc anode flow through the circuit to the carbon rod, the movement of electrons forming an electric current.

After passing through the whole circuit, when the electrons enter the cathode (Carbon rod), they combine with manganese dioxide(MnO2) and water(H2O), which react with each other to produce manganese oxide(Mn2O3) and negatively charged hydroxide ions. This is accompanied by a secondary reaction in which the negative hydroxide ions react with positive ammonium ions in the ammonium chloride electrolyte to produce molecules of ammonia and water.

Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) → ZnCl2 + Mn2O3(s) + 2 NH3(aq) + H2O

Alternately, the reaction proceeds further, the hydroxide ions reacting also with the manganese oxide to form manganese hydroxide.

Zn(s) + 2 MnO2(s) + 2 NH4Cl(aq) + 2H2O(l) → ZnCl2 + 2Mn(OH)2(s) + 2 NH3(aq)

Reaction occurring at electrodes in the cell:

At cathode: 2NH4+(aq)+2MnO2(s)+2e- → 2MnO(OH)+2NH3

At anode: Zn → Zn2+ + 2e-

Uses

The electromotive force (emf) produced by a Leclanche cell is 1.4 volts, with a resistance of several ohms where a porous pot is used.[5] It saw extensive usage in telegraphy, signaling, electric bells and similar applications where intermittent current was required and it was desirable that a battery should require little maintenance.

The Leclanché battery (or wet cell as it was referred to) was the forerunner of the modern zinc-carbon battery (a dry cell). The addition of zinc chloride to the electrolyte paste raised the e.m.f. to 1.5 volts. Later developments dispensed with the ammonium chloride completely, giving a cell that could endure more sustained discharge without its internal resistance rising as quickly (the zinc chloride cell).

See also

References

  1. ↑ Zinc-Carbon Batteries, Molecular Expressions. Last accessed Jan 9, 2007
  2. ↑ The Boy Electrician by J.W. Simms M.I.E.E. (Page 61)
  3. ↑ Battery Facts. "Leclanché Cell". Retrieved 2007-01-09.
  4. ↑ James B. Calvert. "The Electromagnetic Telegraph". Retrieved 2007-01-12.
  5. ↑ The Boy Electrician by J.W. Simms M.I.E.E (page 61)

Bibliography

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