When zinc sulfate is dissolved in water, doubly positive zinc ions \( \text{Zn}^{2+} \) and doubly negative sulfate ions \( \text{SO}_4^{2-} \) are produced. If a rod of pure zinc is placed in this solution, zinc ions leave the rod and enter the liquid. As the rod becomes slightly negatively charged, it gives some of its electrons back to the zinc ions, producing elemental zinc. At the interface between the rod and the liquid, there is thus a constant exchange of ions and electrons.
If a copper rod is placed in the same solution, some of the zinc ions deposit on this rod as elemental zinc. In this process, electrons are released from the copper rod, causing it to become slightly positively charged. If the two rods are connected with a conductor, an electric current can be measured between them. The electrons flow from the negatively charged zinc electrode to the positively charged copper electrode. Since the electron flow is opposite to that in electrolysis, in galvanic cells the positive pole is called the cathode and the negative pole is called the anode.
A setup consisting of two electrodes made of different materials and an electrolyte between them is called a galvanic cell. The electric potential of the individual electrodes cannot be measured directly, but the potential difference—the electric voltage—between the electrodes can.
While current flows between the electrodes, more and more zinc atoms deposit on the copper electrode. At the same time, additional zinc ions enter the solution, so that the zinc rod gradually dissolves. Once the copper electrode is fully covered with zinc, no more zinc ions are available, or the zinc rod is completely dissolved, the current flow stops—that is, the battery is discharged.
To increase the capacity of such a system, there are two main options:
- The electrolyte can be replaced by copper sulfate solution, providing more copper ions that can deposit on the copper electrode.
- The two electrodes can be placed in separate containers: the zinc electrode in zinc sulfate solution and the copper electrode in copper sulfate solution.
In order for a current to flow, ion exchange of the negatively charged ions must still be ensured. For this purpose, a salt bridge (or ion bridge) is used. In the simplest case, this consists of a strip of paper soaked in saltwater, with its ends immersed in each solution. It is important that zinc ions cannot directly migrate into the other container, as this would short-circuit the cell.