Electrochemical Cells

More active metals give up electrons to less active metals. If metals of different activities are placed in an electrolyte, electrons will be transferred from the more active metal to the less active metal. If a circuit of wires connects the two metals, the electrons will flow through these wires and can do work. This is how an electrochemical cell works.

An electrochemical cell uses chemical changes to produce electricity. If several electrochemical cells are combined, they are called a battery of electrochemical cells, or just a battery.

When an electrochemical cell uses a liquid as an electrolyte, it is called a wet cell. If the electrochemical cell uses a paste as an electrolyte, it is called a dry cell. A automobile battery is an example of a battery of wet cells. A flashlight battery is an example of a battery of dry cells.

The places where electrons leave or return to the electrochemical cell are called the electrodes. Dry cell batteries commonly use zinc and carbon as electrodes. Electrodes are conductors which carry electrons into or out of a cell. The zinc is in the form of a can. The can holds the electrolyte paste, which has a carbon rod in the center. The electrolyte in the paste of most dry cells is potassium hydroxide. Since potassium hydroxide is a base (an alkaline substance), such cells are known as alkaline batteries. However, acids can also be used as electrolytes for dry cells.

In the dry cell, the zinc gives up electrons which enter the paste electrolyte. The electrons then leave the dry cell through the bottom of the can. The bottom of the can is commonly called the negative terminal or the cathode. The electrons then flow through a circuit and do work. The electrons return to the dry cell by way of the positive terminal (anode) and enter the carbon rod where they are stored for reuse. A dry cell can produce a maximum of 1.5 volts of electricity. A six volt flashlight battery is composed of four dry cells. Most dry cells are not rechargeable. However, some are. One type that is rechargeable is the nickel cadmium battery or NiCad battery.

The most common type of wet cell battery is the automobile battery. This battery of wet cells uses the metals lead and lead dioxide. The electrolyte is sulfuric acid mixed with water. The lead gives up electrons to the lead dioxide. The electrons leave the battery by way of the negative terminal (cathode), go through the circuits of the car to do work, and return by way of the positive terminal (anode). Each wet cell produces a maximum of two volts of electricity. Since the typical car battery is a 12 volt battery, such a battery is composed of six wet cells.

A car battery is recharged by the car's generator or alternator. The generator or alternator generates a an electric current that flows in the opposite direction of the flow of electrons in the battery. This reverses the flow of electrons and places electrons back on the lead. Such a battery can be recharged for years before it wears out.

The car battery primarily produces electric current only to run the car's starter. Once the car engine is running the alternator or generator takes over and supplies the electric current to run all the features of a car which require electric current. The only time the battery supplies all of this electric current is when these features are on and the car's engine is not running, or if the generator or alternator is not functioning.

Another chemical source of electricity is the fuel cell. The fuel cell differs from wet and dry cells in that some of the materials that react are supplied from outside of the cell. In a fuel cell, hydrogen and oxygen are supplied from outside of the cell. At one electrode, hydrogen is oxidized, at the other oxygen is reduced. Therefore, there is an electron flow from the hydrogen to the oxygen. Water is the by-product of this chemical reaction. The draw back to the fuel cell is that it produces small amounts of electricity. In addition, hydrogen gas is very explosive and dangerous to transport and use. Oxygen gas also has certain hazards associated with it, especially that of fire.

Electric current can be used to breakdown many compounds. The breakdown of any compound by the use of electric current is called electrolysis. For example, bauxite is a compound containing both aluminum and oxygen. Electrolysis is used to separate the aluminum from the oxygen, so that aluminum can be made into useful materials. An electric current is also used to separate water into hydrogen gas and oxygen gas.

Electric current can also be used to deposit a metal on a conducting metal. This technique is called electroplating. Electroplating is used to plate cheaper metals with metals which are resistant to corrosion such as: gold, silver, nickel and chromium.

In electroplating, a battery supplies electrons to the cheaper metal which is to be plated. The metal which is to be plated on to the cheaper metal is in a salt solution which contains positive ions. These positive ions are attracted to the negatively charged electrons causing this metal to bind to the cheaper metal and plate it. Much jewelry is gold or silver plated. Nails are plated with zinc to galvanize them. Screws and door knobs are often plated with brass (an alloy of copper and zinc). Automobile bumpers are something plated with chromium.

Electrochemical Cells Study Sheet