Electrolysis is the process of using an external electric current to drive a non-spontaneous chemical reaction, typically the decomposition of a compound into its constituent elements or ions. It requires an electrolyte (a substance containing mobile ions), two electrodes connected to a power source, and a conducting medium. The technique converts electrical energy into chemical change and is fundamental to metal extraction, surface finishing and chemical manufacture.

Basic principles and components

In an electrolytic cell the positive electrode is the anode and the negative electrode is the cathode. Oxidation occurs at the anode and reduction at the cathode. Ionic species in the electrolyte move toward electrodes of opposite charge, where electrons are supplied or removed by the external circuit. Quantitative aspects are governed by Faraday's laws of electrolysis, which relate the amount of substance transformed to the total electric charge passed.

Methods and characteristics

Practical electrolysis varies with electrolyte composition, electrode materials, temperature and applied voltage. Cells can be simple laboratory set-ups for demonstration, or complex industrial installations with membranes, agitation and controlled current densities. Typical outcomes include metal deposition, gas evolution (for example hydrogen and oxygen from water), and production of acids, bases or chlorine from salt solutions.

History and development

Early investigations into electric decomposition began after the development of voltaic piles around 1800. Subsequent work established the connection between electric charge and chemical change, culminating in systematic laws and practical industrial processes in the 19th and 20th centuries. These advances enabled large-scale extraction and refining methods that are still important today.

Major applications and examples

  • Extraction of reactive metals: Electrolysis is the principal method for obtaining highly reactive metals that cannot be displaced by chemical reduction, for example aluminium produced from molten oxide using electrolytic reduction.
  • Chlor-alkali process: Electrolysis of brine produces chlorine, hydrogen and sodium hydroxide—chemicals central to many industries.
  • Electroplating and electrorefining: Electrochemical deposition is used to coat surfaces for corrosion resistance, aesthetics, or to purify metals such as copper.
  • Water splitting: Electrolytic production of hydrogen from water is a focus for energy storage and low-carbon fuel research.

Distinctions, limitations and safety

Electrolysis is distinct from galvanic (voltaic) cells, which produce electrical energy from spontaneous redox reactions. Electrolysis requires an external power source and often high temperatures or corrosive media. Industrial electrolysis can be energy intensive and may generate hazardous byproducts, so process design emphasizes energy efficiency, appropriate materials and emissions control.

Further reading and resources