Zinc–carbon battery

Zinc-carbon batteries of different sizes.

A Zinc-carbon battery, also called a Leclanché (IPA pronunciation: [lɛklɑːnˈʃei]) cell, is packaged in a zinc can that serves as both a container and anode. The cathode is a mixture of manganese dioxide and carbon powder. The electrolyte is a mixture of zinc chloride and ammonium chloride dissolved in water. Carbon-zinc batteries are the least expensive primary batteries and thus a popular choice by manufacturers when devices are sold with batteries included. They can be used in remote controls, flashlights, toys, or transistor radios. Zinc Carbon batteries are usually labeled as "General Purpose" or "(Super) Heavy Duty" batteries.

As a primary cell

A Zinc-carbon dry cell is described as a primary cell because as the cell is discharged, it is not intended to be recharged and must be discarded. Partially discharged Zinc-carbon cells can be "rejuvenated" by applying a reverse current to them. "Battery Rejuvenators" were once marketed for this purpose. Such devices reduced the build up of hydrogen gas bubbles on the cathode, restoring cell performance. However the effects of such devices were only temporary and prone to cause the cell to leak or burst. [1]

Mechanism

The container of the dry cell, which also serves as one of the electrodes, is made of zinc. The container is lined with porous paper bag which separates the metal from the materials within the cell. A carbon (graphite) rod is placed at the center and used as the other electrode. The space between the carbon rod and the zinc container is filled with a moist mixture of ammonium chloride, manganese dioxide, zinc chloride and carbon powder.

Cross-section of a zinc-carbon battery.

In a dry cell, the outer zinc container is the anode (-). The zinc is oxidised according to the following half-equation.

$\mathrm {Zn} (s)\rightarrow \mathrm {Zn^{2+}} (aq)+2e^{-}$

A graphite rod surrounded by a paste containing manganese(IV) oxide is the cathode(+). The cathode reaction is as follows:

$2\mathrm {MnO_{2}} (s)+2\mathrm {H^{+}} (aq)+2e^{-}\rightarrow \mathrm {Mn_{2}O_{3}} (s)+\mathrm {H_{2}O} (l)$ or

$2\mathrm {NH_{4}^{+}} (aq)+2e^{-}\rightarrow 2\mathrm {NH_{3}} (g)+\mathrm {H_{2}} (g)$

In this half-reaction, the manganese is reduced from an oxidation state of (+4) to (+3). The manganese(IV) oxide paste also contains ammonium chloride and zinc chloride which act as the electrolyte for the cell. The ammonium ions also provide the hydrogen ions needed for the cathode process.

$\mathrm {NH_{4}^{+}} (aq)\leftrightarrow \mathrm {NH_{3}} (aq)+\mathrm {H^{+}} (aq)$

The hydrogen gas is oxidised by manganese(IV) oxide to water, while the ammonia gas is absorbed as a ligand by zinc chloride. Manganese(IV) oxide is reduced to manganese(III) oxide.

$2\mathrm {MnO_{2}} (s)+\mathrm {H_{2}} (g)\rightarrow \mathrm {Mn_{2}O_{3}} (s)+\mathrm {H_{2}O} (l)$

As a ligand, the ammonia reacts with zinc chloride to form a solid.

$\mathrm {ZnCl_{2}} (aq)+\mathrm {NH_{3}} (g)\rightarrow \mathrm {Zn(NH_{3})_{2}Cl_{2}} (s)$

The overall reaction in a dry cell can be represented in several ways, ignoring the complex formation, such as the following:

$\mathrm {Zn} (s)+2\mathrm {MnO_{2}} (s)+2\mathrm {NH_{4}^{+}} (aq)\rightarrow \mathrm {Zn^{2+}} (aq)+\mathrm {Mn_{2}O_{3}} (s)+\mathrm {H_{2}O} (l)+2\mathrm {NH_{3}} (aq)$ or

$\mathrm {Zn} (s)+2\mathrm {MnO_{2}} (s)+2\mathrm {H^{+}} (aq)\rightarrow \mathrm {Zn^{2+}} (aq)+\mathrm {Mn_{2}O_{3}} (s)+\mathrm {H_{2}O} (l)$

The battery has an e.m.f. of about 1.5 V. Although the ammonium ions reacts at the cathode, the hydrogen gas formed there can be oxidised by the manganese(IV) oxide.

$2\mathrm {NH_{4}^{+}} (aq)+2e^{-}\rightarrow 2\mathrm {NH_{3}} (aq)+\mathrm {H_{2}} (g)$

$2\mathrm {MnO_{2}} (s)+\mathrm {H_{2}} (g)\rightarrow \mathrm {Mn_{2}O_{3}} (s)+\mathrm {H_{2}O} (l)$

This shows that the hydrogen formed at the cathode is removed by manganese(IV) oxide. Otherwise, the hydrogen will adhere to the carbon electrode increases the internal resistance of the battery. This causes the e.m.f. to drop. This decrease of cell e.m.f. caused by the formation of gas around an electrode separated from the electrolyte. This also increases internal resistance and reduces e.m.f.. The carbon powder in the electrolyte is there to increase the conductivity of the electrolyte.

Leakage

Hydrogen bubbles covering up the graphite rod increases the internal resistance of a zinc-carbon cell.

When the dry cell has been used for a certain time, the zinc container becomes thinner because zinc metal is oxidised to zinc ions. Therefore Zinc Chloride Solution leaks out the battery. Moreover, the ammonium ions are also used up and the cell will no longer function. The old dry cell is not leakproof. It becomes very sticky as the paste leaks through the holes in the zinc case. The service life of the battery is short, with a shelf life of around 1.5 years.

Furthermore, the zinc casing in the dry cell gets thinner slowly, even when the cell is not being used. It is because the ammonium chloride inside the battery is acidic, producing hydrogen ions in water.

$Zn(s)+2H^{+}(aq)\rightarrow Zn^{2+}(aq)+H_{2}(g)$

References

Eveready: Carbon Zinc Application Notes