Electrode

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An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit, like an electrolyte, semiconductor, vacuum, air or some other gas. Electrodes are often made of metal, but conductive non-metallic materials are also used.

General

An electrode in an electrochemical cell is referred to as either an anode or a cathode. The anode is now defined as the electrode at which electrons leave the cell and oxidation occurs (indicated by a minus symbol, "−"), and the cathode as the electrode at which electrons enter the cell and reduction occurs (indicated by a plus symbol, "+"). Each electrode may become either the anode or the cathode depending on the direction of current through the cell. A bipolar electrode is an electrode that functions as the anode of one cell and the cathode of another cell.[1]

Metallic electrodes

Copper

Copper is used as cathode in copper deposition from solutions. It has poor resistance to oxidation, and has limited use. Copper however, shows good resistance to molten alkali, making it suitable for use in a Castner electrochemical cell.

Gold

Gold, being largely unreactive, is a good choice when doing electrochemistry, despite its high price. Unfortunately, it does not have good resistance in the presence of hypochlorites, as such it cannot be used safely in the production of chlorates and perchlorates. Gold has been known to form a gold-sodium alloy in contact with molten NaOH above 800 °C.[2]

Iridium

Iridium has slightly better chemical resistance than platinum, but due to its price and rarity it's rarely encountered. Iridium is used in car spark plugs, as it has resists oxidation at high temperatures. There is little data on the general performance of iridium electrodes.

Mercury

Mercury has been used in the past as cathode in the Castner-Kellner process, but today its use has been discontinued due to mercury's toxicity.

Nickel

Nickel is commonly used in zinc plating and zinc electrowinning. It can be used in Castner cells, but is unsuitable for the production of chlorates.

Platinum

Platinum shows excellent resistance to corrosion and can be used as electrodes in chlorate-alkali cells. Unfortunately it will get easily poisoned by sulfur oxides.

Pure platinum electrodes are rarely used as platinum is very expensive, instead platinum deposited on titanium electrodes are more common (and cheaper).

Silver

Metallic silver can be used in various electrochemical experiments, however it will quickly passivate in a chlorate cell, due to the oxidation of silver metal and the formation of silver chloride. Oxidation also appears in solutions of sulfuric acid.

Titanium

Titanium has good chemical resistance, and can be used in many electrochemical processes. Has relative poor performance in the presence of chloride ions.

Non-metallic electrodes

Glassy carbon

Glassy carbon shows good chemical resistance, will rapidly break down in oxidizing processes.

Graphite

Graphite electrodes are commonly used in the electrolysis of water. Graphite, like glassy carbon, cannot handle strong oxidizing processes, and is unsuitable for making oxochlorine salts (hypochlorite/chlorate).

Lead dioxide

Lead(IV) oxide is commonly used in lead batteries, as it can be easily regenerated. It can be used in the production of chlorates, and while it's not as good as platinum, it can be easily regenerated and it's much cheaper. Beta-PbO2 is more attractive for this purpose than the alpha form because it has relatively lower resistivity, good corrosion resistance even in low-pH medium, and a high overvoltage for the evolution of oxygen in sulfuric acid and nitric acid-based electrolytes.

Lead dioxide electrodes can be either deposited on metallic lead or graphite.

Manganese dioxide

Manganese dioxide electrodes, MnO2 deposited on titanium metal, can be used in chlorate or HHO cells. While it will break down after repeated uses, it can easily regenerated, by applying manganese(II) nitrate on the electrode and applying heat until it decomposes to MnO2. At low current intensities, Ti-MnO2 electrodes can last for weeks.

Other electrodes

Mixed metal oxide

Mixed metal oxide (MMO) electrodes have an oxide coating over an inert metal or graphite core. The oxides of a precious metal (Ru, Ir, Pt) are used for catalyzing an electrolysis reaction. Titanium oxides are used for inertness, electrode corrosion protection, and lower cost. The core metals used are titanium (most common), zirconium, niobium, or tantalum.

Availability

Electrodes made of graphite, or some metals (nickel e.g.) can be bought from hardware and technical stores. Noble metal electrodes can be purchased online or from lab suppliers or from eBay and Amazon.

Watch out for fake electrodes, especially if you buy from eBay!

DIY electrode

A simple electrode can be made by connecting a piece of metal using a wired crocodile clip to a power source. To make a circuit, you will need to use another piece of conductive material to serve as either cathode or anode to complete the circuit.

For corrosion resistant anodes, you can make PbO2 or MnO2 deposited on graphite or titanium. NurdRage made a tutorial on Ti-MnO2 electrodes.

Projects

  • Water electrolysis (hydrogen and oxygen generation)
  • Make copper(II) hydroxide and oxide
  • Make sodium and potassium hydroxide
  • Make hypochlorites, chlorates, perchlorates
  • Isolate reactive metals from their molten salt
  • Make a battery
  • Make ozone and nitrogen dioxide

See also

References

  1. ↑ https://en.wikipedia.org/wiki/Electrode
  2. ↑ http://moltensalt.org/references/static/downloads/pdf/ORNL-2048.pdf

Relevant Sciencemadness threads

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