Branchiostegalmembrane Function How does the cell work

Galvanic cells - principle

What are galvanic cells

In everyday use, a galvanic cell is understood to be the combination of two half-cells (for metals: redox pairs) that are in an electrolyte solution. The electrodes can use a common electrolyte, different electrolytes (one per half-cell) or a connection (ion conductor) with another electrolyte.

Due to their structure, galvanic cells are (mobile) electrical DC voltage sources. So they “supply” us with electrical energy. Without galvanic cells (batteries, rechargeable batteries), everyday life could no longer be imagined (e.g. for operating cell phones).

The simplest combination of a galvanic cell is when two metal rods are immersed in a common electrolyte solution. A simple galvanic cell is a combination of two half-cells, each half-cell consisting of a metal (rod) that is immersed in a metal salt solution of the same metal (e.g. zinc rod in zinc sulfate solution).

Principle of a galvanic cell

The principle of a galvanic cell is based on the fact that different metals have different tendencies to release electrons (electron pressure or the tendency to dissolve). The potential (difference) of a galvanic cell is greater, the more different the individual elements are in their endeavors to emit or absorb electrons.

If you were to immerse two metal rods in an electrolyte solution, a potential would develop (electrochemical double layer). An equilibrium is soon established between the two phases of the electrode (metal rod / electrolyte solution). The metal rod releases metal ions into the electrolyte solution, the electrons remain on the metal rod. From the electrolyte solution (enriched by the metal ions), metal ions are again transferred to the metal rod and discharged there (=> equilibrium). If you now connect both electrodes to each other via an electrical conductor, the electrons can be transferred from one metal rod to the other metal rod. Therefore, this electrode reaction continues until the electrode has dissolved (at least in theory). This is the reason why the individual half cells in a galvanic cell are also spatially separated from one another.

The two electrodes in a galvanic cell are now connected by an electrical conductor. We would now measure a potential difference, but only a short-term flow of current. This will now be explained using a zinc / copper cell. One half-cell consists of a zinc rod and a solution of zinc sulfate, the other half-cell consists of a copper rod in a solution of copper sulfate solution. Both metal rods are conductively connected to one another. Since zinc is less noble than copper, the electron pressure / tendency of zinc to dissolve is greater. This means that the number of zinc ions in the zinc / zinc sulfate half-cell is greater than the number of copper ions in the copper / copper sulfate solution.

Due to the increasing concentration of zinc ions in the zinc sulfate solution, this solution has a strong positive charge. Due to the Coulomb interactions, it becomes more and more difficult for further zinc ions (from the zinc rod to release electrons) to go into solution (positively charged particles repel each other). Therefore, the individual half-cells, which are spatially separated from one another, are connected via a so-called salt bridge (also known as an ion bridge). It is an electrolyte that connects the two electrode spaces with one another. This salt bridge causes an ion exchange to take place between the two electrode spaces. This prevents the electrolyte solution in an electrode chamber from being overcharged and inhibiting the reactions.

Short notation of a galvanic cell
A galvanic cell consists of two half-cells (two electrode spaces) that are separated from one another by a salt bridge or a membrane (diaphragm). In the abbreviated form, the salt bridge is represented by the double vertical line II. To the right and left of this double line, the two half-cells of the galvanic cell are shown in brackets. As a rule, the so-called anode half-cell is usually written to the left of the double line.

Types of galvanic cells

    • Primary cells: also known as batteries. The cell can be discharged once. The discharge is irreversible and can no longer be charged electrically. This is because the redox reactions are irreversible, since when the galvanic element is “operating”, the substances that supply the electrical energy (“electrons”) are consumed. In the Daniell element, for example, the zinc (metal) dissolves through the electrode reactions.
    • Secondary cells: also known as a battery. The cell can be recharged after a discharge. The chemical processes in the cell are reversible. The "redox reactions" that take place in the electrode spaces during discharge can be reversed when electrical energy is supplied (= charging the cell), i.e. the discharge reaction now runs in the opposite direction (= electrolysis)
  • Tertiary cells: In this cell type, the chemical energy carrier is not stored in the cell, but made continuously available from outside the cell.

The combination of two identical metals (same normal potential) in the same but differently concentrated ion solution also results in a galvanic cell (Nernst equation). The electrode with the lower concentration has the lower potential and is therefore the negative pole of the cell. This galvanic cell is also known as the concentration element