Today, galvanic cells are one of the most common chemical current sources. Despite their shortcomings, they are actively used in electrical engineering and are constantly being improved.
Operating principle
The simplest example of the operation of a galvanic cell looks like this. Two plates are immersed in a glass jar with an aqueous solution of sulfuric acid: one is copper, the other is zinc. They become the positive and negative poles of the element. If you connect these poles with a conductor, you get a simple electrical circuit. Inside the cell, current will flow from the zinc plate, which has a negative charge, to the copper, positively charged. In the external circuit, the movement of charged particles will occur in the opposite direction.
Under the influence of current, hydrogen ions and the acid residue of sulfuric acid will move in different directions. Hydrogen will give its charges to the copper plate, and the acid residue to zinc. So on the clamps of the element voltage will be maintained. At the same time, hydrogen bubbles will settle on the surface of the copper plate, which will weaken the action of the galvanic cell. Hydrogen creates an additional voltage with the metal of the plate, which is called the electromotive force of polarization. The direction of charge of this EMF is opposite to the direction of charge of the EMF of a galvanic cell. Bubbles themselves create additional resistance in the element.
The element we examined is a classic example. In reality, such galvanic cells are simply not used due to the large polarization. To prevent it from happening, in the manufacture of elements, a special substance is introduced into their composition that absorbs hydrogen atoms, which is called a depolarizer. As a rule, these are preparations containing oxygen or chlorine.
Advantages and disadvantages of modern galvanic cells
Modern galvanic cells are made of different materials. The most common and familiar type is coal-zinc cells used in finger batteries. Relative cheapness can be attributed to their pluses, a short shelf life and low power are cons.
A more convenient option is alkaline cells. They are also called manganese-zinc. Here, the electrolyte is not a dry substance such as coal, but an alkaline solution. When discharged, such elements practically do not emit gas, so that they can be made airtight. The shelf life of such elements is higher than coal-zinc.
Mercury elements are similar in design to alkaline. Mercury oxide is used here. Such current sources are used, for example, for medical equipment. Their advantages are resistance to high temperatures (up to +50, and in some models up to +70 ˚), stable voltage, high mechanical strength. The disadvantage is the toxic properties of mercury, due to which the spent elements must be handled very carefully and sent for recycling.
In some elements, silver oxide is used to make cathodes, but because of the high cost of the metal, their use is economically disadvantageous. More common are lithium anode cells. They also have a high cost, but have the highest voltage among all the considered types of galvanic cells.
Another type of galvanic cell is the concentration of galvanic cells. In them, the process of particle motion can proceed with and without ion transfer. The first type is an element in which two identical electrodes are immersed in electrolyte solutions of different concentrations, separated by a semipermeable partition. EMF in such elements arises due to the fact that ions are transferred to a solution with a lower concentration. In the elements of the second type, the electrodes are made of different metals, and the concentration is leveled due to the chemical processes that occur on each of the electrodes. The electromotive force of these elements is higher than that of the elements of the first type.