The Primary Cell
All galvanic cells consist essentially of a negative electrode and a positive electrode in an electrolytic solution. The solution contains ions (electrically charged atoms or groups of atoms) that promote chemical changes in the electrode materials. When a cell is connected to an electrical circuit, electrons flow from the negative electrode through the circuit into the positive electrode. The flow of electrons, or electric current, is produced by a difference in potential between the electrodes. The difference in potential is measured in volts and is generally referred to as voltage. It is caused by an electromotive force (emf) resulting from the relative strength with which atoms of each electrode attract electrons. The atoms of the positive electrode exert a stronger attraction for electrons than do the atoms of the negative electrode. When a current is established between the two electrodes, chemical reactions at each electrode proceed spontaneously, supplying free electrons to the circuit from the negative electrode and, at the same time, taking up free electrons from the circuit at the positive electrode.
In chemical terms, the type of reaction that occurs at the negative electrode is called oxidation and the type of reaction that occurs at the positive electrode is called reduction. Together, they constitute an oxidation-reduction, or redox, reaction. The oxidized material loses electrons and the material that is reduced gains them. These processes can best be illustrated by using the zinc-mercury cell as an example. In this cell, zinc atoms make up the negative electrode and mercuric oxide molecules the positive electrode. When the two electrodes are connected electrically, the zinc is oxidized and the mercuric oxide reduced.
A simplified description of the cell's operation begins with the oxidation of an atom of zinc, which loses its two outer electrons. The resulting positive zinc ion combines with a negative oxygen ion in the electrolyte. The two free electrons left in the negative electrode enter the external circuit and pass to the positive electrode. There they combine with a molecule of mercuric oxide, reducing it. The mercury breaks away from the oxygen as a neutral metal atom and the oxygen enters the electrolyte as a negative oxygen ion. The overall chemical reaction of the cell during discharge can be expressed as Zn+HgO—ZnO+Hg.
The reactions in a cell continue until the materials in the cell are used up. In general, the larger a particular type of cell is, the more electrical energy it can deliver. However, such effects as the accumulation of waste products from the chemical reactions can decrease a cell's performance. Commercial cells are designed to lessen these effects, which are known collectively as polarization.
Most commercial primary cells use zinc as the negative electrode, a metallic oxide as the positive electrode, and an acidic or alkaline solution formed by water and an electrolyte as the electrolytic solution. Most primary cells today are dry cells—the electrolytic solution is in the form of a moist paste or is held in absorbent materials so that it is not free to flow.
Graphite or carbon black is often mixed with the metallic oxide to improve the conductivity of the positive electrode. The zinc is usually amalgamated—that is, alloyed with mercury. The amalgamation helps prevent any decomposition of water and the associated formation of hydrogen gas. The accumulation of hydrogen gas around the negative electrode is a chief cause of polarization. Between the electrodes there is a porous barrier of paper or other material, called the separator. The separator allows ions in the electrolyte to pass through it but keeps material of one electrode from reacting directly with the material of the other electrode.
Typical voltages for fresh commercial primary cells range from 1.0 to 1.5 volts. Batteries with higher voltages generally contain several cells connected in series—that is, one after the other in a circuit. A 9-volt battery, for example, will contain six cells of 1.5 volts connected in series. The voltage of a cell tends to decrease with amount of use and the strength of current drawn from the cell. It can also be affected by temperature. Over time, the materials in a cell slowly deteriorate, even if the cell is not used. The time during which an unused cell can still perform adequately is referred to as the cell's shelf life.
Primary cells are manufactured in a large variety of shapes and sizes. For consumer use, common shapes and sizes include cylindrical D (flashlight) and AA (penlight) cells and smaller button-shaped cells of various sizes. A discussion of typical examples of major kinds of primary cells follows.
was the original dry cell and is the most common type of primary battery used today. In the typical carbon-zinc cell, a small can made of zinc serves as both the cell's container and the negative electrode. The inside of the zinc can is lined with the separator and filled with a mixture of manganese dioxide, which serves as the positive electrode. The separator and manganese dioxide are saturated with the electrolytic solution—ammonium chloride, zinc chloride, and water. This solution is slightly acidic. A carbon rod is placed in the center of the cell within the manganese dioxide to improve the electrical conductivity of the cell. The rod makes electrical contact with a metal plate that is insulated from the zinc can and forms the positive terminal. The voltage of a carbon-zinc cell tends to drop as the cell is being used and to recover between periods of use.
Cell is very similar to the ordinary carbon-zinc cell. The major difference is that its electrolyte consists chiefly of zinc chloride.
Cell contains electrode materials similar to those used in the carbon-zinc cell. Unlike the carbon-zinc cell, it uses an alkaline electrolyte—potassium hydroxide. In a typical alkaline cell, the zinc is in the form of a fine powder and lies in the center of the cell surrounded by a layer of manganese dioxide. The separator lies between the materials. The entire cell is encased in a steel container.
Cell provides a relatively large amount of energy per unit volume. For this reason small cells, such as button cells, are commonly mercury cells. In mercury button cells, the positive electrode, consisting of mercuric oxide, forms a layer at the bottom of the cell. The negative electrode, consisting of powdered zinc, forms a layer at the top. The electrode materials and the separator are infused with an alkaline electrolyte.
Cell is ordinarily in the form of a button cell that is very similar to the mercury button cell. However, the positive electrode is composed of silver oxide instead of mercuric oxide.
Lithium cells produce voltages higher than other primary cells, but they are relatively difficult and expensive to manufacture because lithium is very reactive chemically. The electrolyte usually consists of organic salts in a nonaqueous solution.
All lithium cells contain a negative electrode composed of lithium, but the composition of the positive electrode varies with different types. The most common lithium cells used for consumer products have a positive electrode composed of manganese dioxide.
once widely used for telegraphic equipment, is historically important but is little used today. In this cell, the positive electrode is made of copper, the negative electrode of zinc. A separate electrolyte is used for each electrode: copper sulfate for the copper electrode and zinc sulfate for the zinc electrode. The two electrolytes are separated by a porous barrier.
is not used as a source of power, but to provide a standard voltage for calibrating electrical instruments. A Weston Standard Cell uses mercury as the positive electrode and cadmium as the negative electrode with an electrolyte of cadmium sulfate. It produces 1.018636 volts at 68° F. (20° C.).