DRY CELL: STRUCTURE AND FUNCTION - CHEMISTRY - 2025

A dry cell is a battery whose electrolytic medium consists of a paste and not a solution. Said paste, however, has a certain level of humidity, and for these reasons it is not strictly dry.

The little amount of water is enough for the ions to move and, consequently, the flow of electrons inside the cell.

Source: Emilian Robert Vicol via Flickr.

Its enormous advantage over the first wet batteries is that as it is an electrolytic paste, its content cannot be spilled; which was the case with wet batteries, which were more dangerous and delicate than their dry counterparts. Given the impossibility of spillage, the dry cell finds use in many portable and mobile devices.

In the upper image there is a dry zinc-carbon battery. More exactly, it is a modern version of the Georges Leclanché stack. Of all, it is the most common and perhaps the simplest.

These devices represent an energetic convenience due to the fact that they have chemical energy in the pocket that can be transformed into electricity; and in this way, not depending on power outlets or the energy supplied by large power plants and their vast network of towers and cables.

Dry cell structure

What is the structure of a dry cell? In the image you can see its cover, which is nothing more than a polymeric film, steel, and the two terminals whose insulating washers protrude from the front.

However, this is only its external appearance; Inside it lie its most important parts, which guarantee its proper functioning.

Each dry cell will have its own characteristics, but only the zinc-carbon cell will be considered, of which a general structure can be outlined for all other batteries.

A battery is understood as the union of two or more batteries, and the latter are voltaic cells, as will be explained in a future section.

Electrodes

Source: Wikipedia

The upper image shows the internal structure of a zinc-carbon battery. No matter what the voltaic cell is, there should always be (usually) two electrodes: one from which electrons are given off, and one from which receives them.

Electrodes are electrically conductive materials, and for there to be current, both must have different electronegativities.

For example, zinc, a white tin that encloses the battery, is where the electrons leave for the electrical circuit (device) where it is connected.

On the other hand, in the whole medium is the graphitic carbon electrode; also immersed in a paste composed of NH ₄ Cl, ZnCl ₂ and MnO ₂.

This electrode is the one that receives the electrons, and note that it has the symbol '+', which means that it is the positive terminal of the battery.

Terminals

As seen above the graphite rod in the image, there is the positive electrical terminal; and below, the inner zinc can from which electrons flow, the negative terminal.

That is why batteries are marked '+' or '-' to indicate the correct way to connect them to the device and thus allow it to turn on.

Sand and wax

Although not shown, the paste is protected by a cushioning sand and a wax seal that prevents it from spilling or coming into contact with the steel under minor mechanical impacts or agitation.

Functioning

How does a dry cell work? To begin with, it is a voltaic cell, that is, it generates electricity from chemical reactions. Therefore, inside cells redox reactions occur, where the species gain or lose electrons.

The electrodes serve as a surface that facilitates and allows the development of these reactions. Depending on their charges, oxidation or reduction of the species can occur.

To better understand this, only the chemical aspects of the zinc-carbon battery will be explained.

Zinc electrode oxidation

As soon as the electronic device is turned on, the battery will release electrons by oxidizing the zinc electrode. This can be represented by the following chemical equation:

Zn => Zn ²⁺ + 2e ^-

If there is a lot of Zn ²⁺ surrounding the metal, a positive charge bias will occur, so there will be no further oxidation. Therefore, the Zn ²⁺ must diffuse through the paste towards the cathode, where the electrons will enter back.

Once the electrons have activated the artifact, they return to the other electrode: the graphite one, to find some chemical species “waiting” for it.

Ammonium chloride reduction

As previously mentioned, there are NH ₄ Cl and MnO ₂ in the pasta, substances that make its pH acidic. As soon as the electrons enter, the following reactions will occur:

2NH ₄ ⁺ + 2e ^- => 2NH ₃ + H ₂

The two products, ammonia and molecular hydrogen, NH ₃ and H ₂, are gases, and therefore can "swell" the battery if they do not undergo other transformations; such as the following two:

Zn ²⁺ + 4NH ₃ => ²⁺

H ₂ + 2MnO ₂ => 2MnO (OH)

Note that the ammonia was reduced (gained electrons) to become NH ₃. These gases were then neutralized by the other components of the paste.

The ²⁺ complex facilitates the diffusion of the Zn ²⁺ ions towards the cathode and thus prevents the cell from “stalling”.

The external circuit of the device functions as a bridge for the electrons; otherwise there would never be a direct connection between the zinc can and the graphite electrode. In the image of the structure, this circuit would represent the black cable.

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Dry cells come in many variants, sizes, and working voltages. Some of them are not rechargeable (primary voltaic cells), while others are (secondary voltaic cells).

The zinc-carbon battery has a working voltage of 1.5V. Their shapes change based on their electrodes and the composition of their electrolytes.

There will come a point where all the electrolyte has reacted, and no matter how much the zinc oxidizes there will be no species that receive the electrons and promote their release.

Furthermore, it may be the case where the gases formed are no longer neutralized and remain exerting pressure inside the cells.

Zinc-carbon batteries, and other batteries that are not rechargeable, must be recycled; since its components, especially nickel-cadmium components, are harmful to the environment by polluting soils and waters.

References

1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
2. Whitten, Davis, Peck & Stanley. (2008). Chemistry. (8th ed.). CENGAGE Learning.
3. The "Dry-Cell" Battery. Recovered from: makahiki.kcc.hawaii.edu
4. Hoffman S. (December 10, 2014). What is a dry cell battery? Recovered from: upsbatterycenter.com
5. Weed, Geoffrey. (April 24, 2017). How Do Dry Cell Batteries Work? Sciencing. Recovered from: sciencing.com
6. Woodford, Chris. (2016) Batteries. Recovered from: explainthatstuff.com.