The functional elements of a dry cell battery are the negative terminal (a zinc can which encloses the battery materials), the positive terminal (the carbon rod and carbon and manganese dioxide mixture that surrounds the rod), and an electrolyte paste between the two terminals. The electrolytic paste facilitates a chemical reaction involving the constituents of both terminals; this reaction causes a current to flow through a conductor that connects the positive and negative terminals.
In a dry cell the zinc casing serves as the anode and is consumed in the anodic electrode reaction Zn(s) --> Zn2+ + 2e-; the zinc ion dissolves in the moist ZnCl2-NH4Cl electrolyte. A carbon rod serves as the cathode, but it is chemically inert. The cathode electrode reaction, which consumes MnO2, is best written as:
[Mn4+ + 2O2-] + H2O + e- --> [Mn3+ + O2- + OH-] + OH-
Where the square brackets indicate the species present in the solid phase at the cathode. The cathode reaction actually occurs within the solid structure; the carbon rod serves only to transfer electrons from the external circuit.
The dry cell has a potential difference of about 1. 25 V; the zinc electrode is negative. It is a good source of electrical power and the materials of construction are relatively cheap. The cell voltage during discharge falls off rather badly and the dry cell is not a good source of power when a constant voltage is needed.
They are called dry cell because they electrolyte is a paste rather than a liquid.
Types of dry cells:
1. Primary cells - They are not rechargeable. The cell will not function once their chemicals are used up and the cells have to be thrown away. Examples are zinc-carbon cell, alkaline manganese cell and silver oxide cell.
2. Secondary cells - They are rechargeable (can be recharged) and can be used again. Example: nickel-cadmium cell.
Note: The lead-acid accumulator (car battery) used in cars is also a secondary cell but it is not considered as a type of dry cell.
How to make a dry cell
Dry cells are one of the most commonly used household objects. We use dry cells in watches, torches, transistors, walkmans and even the remote controls of our TVs.