Electrochemical cells are devices for turning chemical energy into electrical energy or, alternatively, changing electrical energy into chemical energy. The first type of cell is known as a voltaic, or galvanic, cell, while the second type is an electrolytic cell. The voltaic cell with which you are probably most familiar is the battery. Batteries consist of one or more cells connected to each other. Electrolytic cells are less common in everyday life, although they are important in many industrial operations, as in the electroplating of metals.
Cells that obtain electrical energy from chemical reactions were discovered more than two centuries ago. Italian anatomist Luigi Galvani (1737–1798) first observed this effect in 1771. He noticed that the muscles of a dead frog twitched when the frog was being dissected. Galvani thought the twitching was the result of "animal electricity" that remained in the frog. Although his explanation was incorrect, credit for his observation of the effect is acknowledged in the name galvanic cell, which is sometimes used for devices of this kind.
The correct explanation for the twitching of dead frog muscles was provided by Italian physicist Alessandro Volta (1745–1827) two decades later. Volta was able to prove that the twitching was caused by an electric current that was produced when two different metals touched the animal's bloodstream at the same time. Because of Volta's contribution to this field of science, electricity-generating electrochemical cells are also called voltaic cells.
Voltaic cells contain three main components: two different metals, a solution into which the two metals are immersed, and an external circuit (such as a wire) that connects the two metals to each other.
Anode: The electrode in an electrochemical cell at which electrons are given up to a reaction.
Cathode: The electrode in an electrochemical cell at which electrons are taken up from a reaction.
Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.
Electrolysis: The process by which an electrical current causes a chemical change, usually the breakdown of some substance.
When a metal is immersed in a solution, such as a water solution of sulfuric acid, the metal tends to lose electrons. Each metal has a greater or lesser tendency to lose electrons compared to other metals. For example, imagine that a strip of copper metal and a strip of zinc metal are both immersed in a solution of sulfuric acid. In this case, the zinc metal has a greater tendency to lose electrons than does the copper metal.
Nothing happens if you immerse two separate metals in a solution because any electrons lost by either metal have no place to go. But by attaching a wire across the top of the two metal strips, electrons are able to travel from the metal that loses them most easily (zinc in this case) across the wire to the metal that loses them less easily (copper in this case). You can observe this effect if you connect an electrical meter to the wire joining the two metals. When the metals are immersed into the sulfuric acid solution, the needle on the electrical meter jumps, indicating that an electrical current is flowing from one metal to the other.
Instead of connecting an electrical meter to the wire, you could insert an appliance that operates on electricity. For example, if a lightbulb is attached to the wire joining the two metal strips, it begins to glow. Electrons produced at the zinc strip travel through the wire and the lightbulb, causing it to light up.
Various factors determine the amount of electric current produced by a voltaic cell. The most important of these is the choice of metals used in the cell. Two metals with nearly equal tendencies to lose electrons will produce only a small current. Two metals with very different tendencies to lose electrons will produce a much larger current. Chemists have invented a measure of the tendency of various substances to lose electrons in a voltaic cell. They call that tendency the standard electrode potential for the substance. A metal with one of the highest standard electrode potentials is potassium metal, whose standard electrode potential is 2.92 volts. In comparison, a metal with a very low standard electrode potential is iron, with a value of 0.04 volt.
An electrolytic cell is just the reverse of a voltaic cell. Rather than producing electricity by means of chemical reactions, an electrolytic cell uses electrical energy to make chemical reactions happen.
An electrolytic cell also consists of two metals immersed in a solution connected by means of an external wire. In this case, however, the external wire is hooked to some source of electrical energy, such as a battery. Electrons flow out of the battery through the wire and into one of the two metals. These two metals are known as electrodes. The electrode on which electrons accumulate is the cathode, and the electrode from which electrons are removed is the anode.
The solution in an electrolytic cell is one that can be broken apart by means of an electric current. A common example is the electrolysis of water. When an electric current flows into water, it causes water molecules to break apart, forming atoms of hydrogen and oxygen:
2 H 2 O → 2 H 2 + O 2
Hydrogen gas is given off at one electrode and oxygen gas at the other electrode. The electrolysis of water is, in fact, one method for making hydrogen and oxygen gas for commercial and industrial applications. Another common use for electrolytic cells is in electroplating, in which one metal is deposited on the surface of a second metal.