A galvanic cell is a type of electrochemical cell that converts chemical energy into electrical energy through a spontaneous redox reaction. This device is composed of two electrodes of dissimilar metals, immersed in separate electrolytic solutions and connected through a salt bridge or porous disk, allowing the flow of ions and maintaining charge balance. It is named after Luigi Galvani, whose experiments with electricity and biological tissues were fundamental to the development of electrochemistry.
The term voltaic pile is sometimes used interchangeably. However, the voltaic pile is a type of galvanic cell. It was designed by Alessandro Volta, and is a precursor device based on the same electrochemical principle. His invention represented a major advance by providing the first source of direct electric current.
Parts of a galvanic cell
Before explaining how a galvanic cell works, we must know a little about its parts:
Here is a detailed description of each part of a galvanic cell:
Anode
The anode is the electrode where oxidation occurs, that is, the loss of electrons.
In a galvanic cell, the anode acts as the negative pole, as electrons are released from it and travel through the external circuit to the cathode. The anode is usually made of a more reactive metal that tends to lose electrons easily, such as zinc in the Daniell cell.
As the reaction proceeds, the anode atoms dissolve into the solution as ions.
Cathode
The cathode is the electrode where reduction, i.e. the gain of electrons, occurs. It is the positive pole because it attracts electrons coming from the anode. At this electrode, the ions in the solution combine with the electrons arriving through the external circuit, forming neutral atoms or molecules.
In the case of the Daniell cell, the cathode is made of copper, and copper ions in solution are deposited on its surface by accepting electrons.
Electrolyte
The electrolyte is the ionic solution in which the electrodes are immersed and which allows the movement of ions to maintain the balance of charges in the cell.
This solution is composed of dissolved salts, acids or bases that facilitate ionic conduction.
In the Daniell cell, the zinc sulphate solution (
) surrounds the anode, while the copper sulphate solution (
) surrounds the cathode.
Salt bridge or porous membrane
The salt bridge is a tube or membrane filled with a solution of inert salts, such as potassium chloride (
) or sodium sulfate (
), which allows the circulation of ions between the electrode solutions. Its function is to prevent the accumulation of electrical charge in each half-cell and to maintain electrical neutrality in the cell. Without the salt bridge, the reaction would stop quickly due to the separation of charges.
External circuit
It is the conductive path along which electrons travel from the anode to the cathode. This circuit can be composed of metal cables and elements such as resistors, light bulbs or motors that use the generated electrical energy. In this path, the current flows in the opposite direction to the flow of electrons, from the cathode to the anode, according to the convention of electric current.
How does a galvanic cell work?
A galvanic cell works by an oxidation-reduction (redox) reaction that converts chemical energy into electrical energy.
This cell consists of two electrodes, the anode and the cathode , immersed in electrolyte solutions. At the anode, oxidation occurs , where a metal loses electrons and dissolves in the electrolyte as ions . These electrons travel through an external circuit to the cathode, where reduction occurs , i.e. the electrons are captured by metal ions in the electrolyte, transforming them into atoms of the corresponding metal.
The flow of electrons through the external circuit is what generates an electric current, which can be used to power devices. For the internal circuit of the cell to be complete, ions move between the two electrodes through a salt bridge or porous partition , allowing the balance of charges to be maintained in the solutions.
Thus, the chemical energy stored in the electrodes is continuously converted into electrical energy as long as the redox reaction lasts and there are materials available to react.
Relationship with electrolysis
The reverse process of a galvanic cell is electrolysis, where an external current forces a non-spontaneous reaction. In both cases, the flow of electrons occurs through the external circuit, while positive ions flow into the electrolyte, completing the electrical circuit.
In short, in a galvanic cell, electrons move from the anode (where oxidation occurs) to the cathode (where reduction occurs), generating an electric current from an oxidation-reduction (redox) chemical reaction. This principle is the basis for the operation of batteries used in many electronic devices.
The voltaic pile
The voltaic cell is a specific type of galvanic cell, invented by Alessandro Volta, which employs the same principle of converting chemical to electrical energy, but with a defined structure. Instead of using a single electrolyte, the voltaic cell employs two different metals, such as zinc and copper, immersed in separate electrolytic solutions.
The flow of electrons between the two electrodes, through an external conductor, generates a constant electric current.
The main difference between the voltaic pile and other galvanic cells is their design and their ability to generate a stable source of electrical energy. The voltaic pile was the first to offer a continuous source of electricity, allowing for advances in electrical technology and early battery research.
Types of cells
We distinguish three types of galvanic cells:
Concentration cell
A concentration cell is a primary (non-rechargeable) cell that uses two galvanic half cells with the same chemical species but with different concentrations.
For example, such a cell may consist of two copper electrodes immersed in two solutions containing copper sulphate (
The two solutions have different concentrations and the electrodes are separated by a porous partition or a salt bridge.
The battery will discharge when the electrolyte concentration in the two half cells is the same.
Electrolytic cell
An electrolytic cell consists of two electrodes immersed in a tank containing an electrolyte. The electrolyte usually consists of two electrolyte solutions that can exchange ions across a salt bridge or porous partition.
An oxidation reaction occurs at the anode. On the other hand, a reduction reaction occurs at the cathode. The result is that a redox reaction occurs in the cell that uses external electrical energy to produce it.
The signs of the poles are reversed with respect to a galvanic cell. In an electrolytic cell the anode is the positive pole, while the cathode is the negative pole.
Electrochemical cell
Electrochemical batteries are composed of two half-elements, also called half-cells.
These semi-elements are kept separated by a semi-permeable membrane or are contained in separate containers connected by a salt bridge. By connecting the semi-elements, one semi-element releases electrons through the oxidation reaction. In turn, these electrons are transferred to the other to give rise to the reduction reaction.