Cathodes

In chemistry and electrochemistry, a cathode is the electrode at which reduction occurs. The term comes from the Greek kathodos, meaning “way down.” The cathode’s role depends on the device: in galvanic (voltaic) cells, the cathode is the positive electrode where reduction takes place; in electrolytic cells, the cathode is the negative electrode, also the site of reduction, driven by external voltage. In both cases electrons flow toward the cathode from the anode through the external circuit.

In electrical devices, the cathode also appears in components such as diodes and vacuum tubes. In diodes, the cathode is the electrode that accepts electrons; the anode is the electrode that donates electrons, with conventional current flowing from the anode to the cathode and electrons moving in the opposite direction. In vacuum tubes and cathode-ray tubes, the cathode is the electrode heated to emit electrons (thermionic emission); some devices use field emission or photocathodes.

Materials and applications vary by use. Thermionic cathodes often use heated metals such as tungsten or nickel; photocathodes employ photoemissive materials like cesium-based compounds. Cold cathodes rely on field emission or other mechanisms. Applications include energy storage and electrochemical production: galvanic cells generate electricity, while electrolytic cells drive chemical reactions using external power. In electroplating, the object to be plated is the cathode, where metal ions are reduced and deposited.