An electrode is a solid electric conductor that carries electric current into non-metallic solids, or liquids, or gases, or plasmas, or vacuums. Electrodes are typically good electric conductors, but they need not be metals.
In an electrochemical cell, reduction and oxidation reactions take place at the electrodes. The electrode at which reduction takes places is called the cathode. Oxidation takes place at the anode.
Whether an electrode operates as a cathode or anode depends on the direction the cell is operating in.
If a cell is switched from operating galvanically (i.e. outputting energy like a battery) to electrolysis (energy is input to the cell) then its cathode will become its anode and vice versa.
Examples of typical materials used for electrodes in analytical chemistry are amorphous carbon, gold, and platinum. Glass electrodes are often used in pH measurements; in this application the glass is chemically doped to be selective to hydrogen ions.
Batteries contain a variety of electrodes, depending on the battery type.
- Lead-acid batteries are based on lead electrodes.
- Zinc-carbon batteries are made with zinc and amorphous carbon electrodes.
- Lithium polymer batteries have electrodes made of a solid polymer matrix within which lithium ions can move and act as charge carriers.
Electrical energy can be used to convert salts and ores to metals.
In the Hall-Heroult process to extract aluminum metal from aluminum oxide the anode and cathode are both made of graphite.
Sodium metal is produced by electrolysis using a carbon anode and an iron cathode.