An electrode is a 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, whence the word electricity is derived) and hodos, a way [1].

Anode vs. 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 defined as the electrode at which oxidation occurs, and the cathode is defined as the electrode at which reduction occurs. Each electrode may become either the anode or the cathode depending on the type of reaction occurring in the 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. It 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.

Other uses of anode and cathode

In a vacuum tube or a semiconductor having polarity (diodes, electrolytic capacitors) the anode is the positive (+) electrode and the cathode the negative (-).

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 MIG welding, or non-consumable, such as in TIG welding.

Types of electrode

• Electrodes for medical purposes, such as EEG, EKG, ECT, defibrillator
• Electrodes for execution by the electric chair
• etc.

Related topics

• battery
• redox
• cathodic protection
• Galvanic cell
• anion vs. cation
• electron vs. hole
• electrolyte
• electron microscope

References

Michael Faraday, "On Electrical Decomposition", Philosophical Transactions of the Royal Society, 1834 (in which Faraday coins the words electrode, anode, cathode, anion, cation, electrolyte, electrolyze).