TheLeclanché cell is abattery invented and patented by the French scientistGeorges Leclanché in 1866.[1][2][3] The battery contained a conducting solution (electrolyte) ofammonium chloride, acathode (positive terminal) ofcarbon, adepolarizer ofmanganese dioxide (oxidizer), and ananode (negative terminal) ofzinc (reductant).[4][5] The chemistry of this cell was later successfully adapted to manufacture adry cell.
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In 1866,Georges Leclanché invented a battery that consisted of a zinc anode and amanganese dioxide cathode wrapped in a porous material, dipped in a jar ofammonium chloride solution. The manganese dioxide cathode had a little carbon mixed into it as well, which improved conductivity and absorption.[6] It provided a voltage of 1.4 volts.[7] This cell achieved very quick success in telegraphy, signalling and electric bell work.
Thedry 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.[8] This is because certain chemical reactions in the cell increase itsinternal resistance and, thus, lower its voltage. These reactions reverse themselves when the battery is left idle, making it good for many short periods of use with idle time between them, but not long periods of use.[9]
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 the resistance. These included the "Agglomerate block cell" and the "Sack cell". Leclanché first, andCarl Gassner later, both strived to transform the original wet cell into a more portable and more efficientdry cell.
The redox reaction in a Leclanché cell involves the two following half-reactions:
The chemical process which produces electricity in a Leclanché cell begins whenzinc atoms on the surface of the anodeoxidize, i.e. they give up both theirvalence electrons to become positively charged Zn2+ions. As the Zn2+ 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 externalelectrical circuit, the excess electrons on the zinc anode flow through the circuit to thecarbon rod, the movement of electrons forming anelectric current. Thepotential difference in charge over the anode and cathode is equal to the difference of the two half-reaction potentials, producing a theoreticalvoltage of 1.99v of potential energy across the terminals. A variety of factors, such asinternal resistance, lower this output value to the 1.4 volts measured from these cells in practice.
As the current travels around the circuit, when the electrons enter the cathode (carbon rod), they combine withmanganese dioxide (MnO2) and water (H2O), which react with each other to producemanganese oxide (Mn2O3) and negatively chargedhydroxide ions. This is accompanied by a secondary acid-base reaction in which the hydroxide ions (OH–) accept a proton (H+) from theammonium ions present in theammonium chlorideelectrolyte to produce molecules ofammonia and water.[10]
or if one also considers the hydration of the Mn2O3(s)sesquioxide into Mn(III) oxy-hydroxide:
Alternately, the reduction reaction of Mn(IV) can proceed further, forming Mn(II) hydroxide.
Theelectromotive force (e.m.f.) produced by a Leclanche cell is 1.4volts, with aresistance of severalohms where a porous pot is used.[7] It saw extensive usage intelegraphy,signaling,electric bells and similar applications where intermittent current was required and it was desirable that a battery should require little maintenance.
The Leclanché batterywet cell was the forerunner of the modernzinc–carbon battery (adry cell). The addition ofzinc chloride to the electrolyte paste raises the e.m.f. to 1.5 volts. Later developments dispensed with the ammonium chloride completely, giving a cell that can endure more sustained discharge without its internal resistance rising as quickly (the zinc chloride cell).