Anode
In a galvanic cell the anode is the wire or plate having excess negative charge as a result of the oxidation reaction.
[5] However, the definition of anode and cathode is different for electrical devices such as diodes and vacuum tubes where the electrode naming is fixed and does not depend on the actual charge flow (current).
[5] The polarity of voltage on an anode with respect to an associated cathode varies depending on the device type and on its operating mode.
In the following examples, the anode is negative in a device that provides power, and positive in a device that consumes power: In a discharging battery or galvanic cell (diagram on left), the anode is the negative terminal: it is where conventional current flows into the cell.
[citation needed] In a recharging battery, or an electrolytic cell, the anode is the positive terminal imposed by an external source of potential difference.
In a diode, the anode is the terminal represented by the tail of the arrow symbol (flat side of the triangle), where conventional current flows into the device.
Previously, as related in the first reference cited above, Faraday had used the more straightforward term "eisode" (the doorway where the current enters).
Therefore, "eisode" would have become inappropriate, whereas "anode" meaning 'East electrode' would have remained correct with respect to the unchanged direction of the actual phenomenon underlying the current, then unknown but, he thought, unambiguously defined by the magnetic reference.
In retrospect the name change was unfortunate, not only because the Greek roots alone do not reveal the anode's function any more, but more importantly because as we now know, the Earth's magnetic field direction on which the "anode" term is based is subject to reversals whereas the current direction convention on which the "eisode" term was based has no reason to change in the future.
[citation needed] In electrochemistry, the anode is where oxidation occurs and is the positive polarity contact in an electrolytic cell.
[8] At the anode, anions (negative ions) are forced by the electrical potential to react chemically and give off electrons (oxidation) which then flow up and into the driving circuit.
Historically, when non-reactive anodes were desired for electrolysis, graphite (called plumbago in Faraday's time) or platinum were chosen.
Platinum erodes very slowly compared to other materials, and graphite crumbles and can produce carbon dioxide in aqueous solutions but otherwise does not participate in the reaction.
[citation needed] In a battery or galvanic cell, the anode is the negative electrode from which electrons flow out towards the external part of the circuit.
Internally the positively charged cations are flowing away from the anode (even though it is negative and therefore would be expected to attract them, this is due to electrode potential relative to the electrolyte solution being different for the anode and cathode metal/electrolyte systems); but, external to the cell in the circuit, electrons are being pushed out through the negative contact and thus through the circuit by the voltage potential as would be expected.
Though from an electrochemical viewpoint incorrect, it does resolve the problem of which electrode is the anode in a secondary (or rechargeable) cell.
In a tube, the anode is a charged positive plate that collects the electrons emitted by the cathode through electric attraction.
Sacrificial anodes are particularly needed for systems where a static charge is generated by the action of flowing liquids, such as pipelines and watercraft.
In 1824 to reduce the impact of this destructive electrolytic action on ships hulls, their fastenings and underwater equipment, the scientist-engineer Humphry Davy developed the first and still most widely used marine electrolysis protection system.
