Dry Cell Battery

Learning Objective

• Discuss the operational components of a dry cell battery and their principal benefits

Key Points

• A battery contains electrochemical cells that can store chemical energy to be converted to electrical energy.
• A dry-cell battery stores energy in an immobilized electrolyte paste, which minimizes the need for water.
• Common examples of dry-cell batteries include zinc-carbon batteries and alkaline batteries.

Terms

• electrolyteA substance that, in solution or when molten, ionizes and conducts electricity.
• cathodeThe electrode of an electrochemical cell at which reduction occurs.
• anodeThe electrode of an electrochemical cell at which oxidation occurs.

Defining a Dry Cell

In electricity, a battery is a device consisting of one or more electrochemical cells that convert stored chemical energy into electrical energy. The dry cell is one of many general types of electrochemical cells.

A dry cell has the electrolyte immobilized as a paste, with only enough moisture in it to allow current to flow. Unlike a wet cell, a dry cell can operate in any orientation without spilling, as it contains no free liquid. This versatility makes it suitable for portable equipment. By comparison, the first wet-cell batteries were typically fragile glass containers with lead rods hanging from an open top. They, therefore, needed careful handling to avoid spillage. The development of the dry-cell battery allowed for a major advance in battery safety and portability.

A common dry-cell battery is the zinc-carbon battery, which uses a cell that is sometimes called the Leclanché cell. The cell is made up of an outer zinc container, which acts as the anode. The cathode is a central carbon rod, surrounded by a mixture of carbon and manganese(IV) dioxide (MnO2). The electrolyte is a paste of ammonium chloride (NH4Cl). A fibrous fabric separates the two electrodes, and a brass pin in the center of the cell conducts electricity to the outside circuit.

Chemical reactions occur in every part of the battery to allow for energy storage; the reactions can be described using balanced chemical equations that delineate the electron flow. The paste of ammonium chloride reacts according to the following half-reaction:

$2NH_4 (aq) + 2e^- \rightarrow 2NH_3 (g) + H_2 (g)$

The manganese(IV) oxide in the cell removes the hydrogen produced by the ammonium chloride, according to the following reaction:

$2MnO_2 (s) + H_2 (g) \rightarrow Mn_2O_3 (s) + H_2O (l)$

The combined result of these two reactions takes place at the cathode. Adding these two reactions together, we get:

$2NH_4 (aq) + 2MnO_2 (s) +2e^ \rightarrow Mn_2O_3 (s) + 2NH_3 (g) + H_2O (l)$

Finally, the anode half-reaction is as follows:

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

Therefore, the overall equation for the cell is:

$Zn (s) + 2MnO_2 (s) + 2NH_4 (aq) \rightarrow Mn_2O_3 (s) + H_2O (l) + Zn_2 + 2NH_3 (g)$

The potential for the above reaction is 1.50 V.

Another example of a dry-cell battery is the alkaline battery. Alkaline batteries are almost the same as zinc-carbon batteries, except that the electrolyte used is potassium hydroxide (KOH) rather than ammonium chloride. In some more modern types of so-called “high-power” batteries that have a much lower capacity than standard alkaline batteries, the ammonium chloride is replaced by zinc chloride.