Georges Leclanché invented and patented his battery, the Leclanché cell, in 1866. The battery contained a conducting solution (electrolyte) of ammonium chloride, a cathode (negative terminal) of carbon, a depolarizer of manganese dioxide, and an anode (positive terminal) of zinc. The chemistry of this cell was later successfully adapted to manufacture of dry cells.
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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".
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.
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.
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).
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 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 electrical current.
When the electrons enter the rod, they combine with manganese dioxide and water, which react with each other to produce manganese oxide 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)3(s) + 2 NH3(aq)
The electromotive force (emf) produced by a Leclanche cell is typically around 1.5 volts with a resistance of several ohms where a porous pot is used. 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 dry cell zinc-carbon battery.