An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by the scientist Michael Faraday from the Greek words elektron (meaning amber, from which the word electricity is derived) and hodos, a way.
Anode and cathode in electrochemical cells
An electrode in an electrochemical cell is referred to as either an anode or a cathode, words that were also coined by Faraday. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs, and the cathode as the electrode at which electrons enter the cell and reduction occurs. Each electrode may become either the anode or the cathode depending on the voltage applied to the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.
A primary cell
is a special type of electrochemical cell in which the reaction cannot be reversed, and the identities of the anode and cathode are therefore fixed. The anode is always the negative electrode. The cell can be discharged but not recharged.
A secondary cell
, for example a rechargeable battery
, is one in which the reaction is reversible. When the cell is being charged, the anode becomes the positive (+) electrode and the cathode the negative (−). This is also the case in an electrolytic cell
. When the cell is being discharged, it behaves like a primary or voltaic cell
, with the anode as the negative electrode and the cathode as the positive. The chemical change, which converts chemical energy into electrical energy, is reversible. This cell can be recharged by simply passing electrons in the opposite direction, so it is also called a storage or accumulator cell.
Other anodes and cathodes
In a vacuum tube
or a semiconductor
having polarity (diodes
, electrolytic capacitors
) the anode is the positive (+) electrode and the cathode the negative (−). The electrons enter the device through the cathode and exit the device through the anode. Many devices have other electrodes to control operation, e.g., base, gate, control grid.
In a three-electrode cell, a counter electrode, also called an auxiliary electrode, is used only to make a connection to the electrolyte so that a current can be applied to the working electrode. The counter electrode is usually made of an inert material, such as a noble metal or graphite, to keep it from dissolving.
In arc welding
an electrode is used to conduct current through a workpiece to fuse two pieces together. Depending upon the process, the electrode is either consumable, in the case of gas metal arc welding
or shielded metal arc welding
, or non-consumable, such as in gas tungsten arc welding
. For a direct current system the weld rod or stick may be a cathode for a filling type weld or an anode for other welding processes. For an alternating current arc welder the welding electrode would not be considered an anode or cathode.
Alternating current electrodes
For electrical systems which use alternating current
the electrodes are the connections from the circuitry to the object to be acted upon by the electrical current but are not designated anode or cathode since the direction of flow of the electrons changes periodically
, usually many times per second
Uses of electrodes
Electric currents are run through nonmetal objects to alter them in numerous ways and to measure conductivity for numerous purposes. Examples include: