The electrolysis of water is the splitting of water (H₂O) into its components hydrogen (H₂) and oxygen (O₂) using an electric current. It is the central reaction behind “green” hydrogen, behind water ionisers, and behind most devices that produce hydrogen water. This article explains the reaction equation, what happens at the anode and cathode, Faraday’s law, and the main processes — in plain language but chemically correct.
What is the electrolysis of water?
In electrolysis, electrical energy is used to force a chemical reaction that would not occur on its own. In the case of water, the current breaks the H₂O molecules apart: hydrogen gas forms at one electrode, oxygen gas at the other. It is therefore the reverse of the “oxyhydrogen reaction”, in which hydrogen and oxygen burn to form water.
The reaction equation
The overall reaction is:
Two molecules of water yield two molecules of hydrogen and one molecule of oxygen. The volume ratio of H₂ to O₂ is therefore 2:1.
Anode, cathode and electrolyte
The reaction splits between the two electrodes. Which half-reaction occurs depends on whether the solution is acidic or alkaline:
| Electrode | Process | Half-reaction (alkaline) |
|---|---|---|
| Cathode (−) | Reduction → H₂ forms | 4 H₂O + 4 e⁻ → 2 H₂ + 4 OH⁻ |
| Anode (+) | Oxidation → O₂ forms | 4 OH⁻ → O₂ + 2 H₂O + 4 e⁻ |
Pure water barely conducts electricity. That is why an electrolyte is needed — usually an acid (e.g. sulphuric acid), a base (e.g. potassium hydroxide, KOH) or dissolved mineral salts. In drinking-water ionisers, the naturally present minerals play this role.
How much voltage is needed?
Thermodynamically, a theoretical 1.23 volts is enough to split water. In practice, because of so-called overpotential, around 1.5 to 2.0 volts is required. This theoretical minimum can be calculated directly from the standard potentials of the half-reactions — and this is exactly where the Nernst equation comes in, showing how potentials shift with concentration and temperature.
PEM vs. alkaline electrolysis
Three processes dominate the technology:
- Alkaline electrolysis: classic, robust, uses a potassium hydroxide solution as the electrolyte. Cheap and proven, but somewhat slower.
- PEM electrolysis (Proton Exchange Membrane / SPE): uses a solid polymer membrane. Compact, fast-reacting, delivers very pure H₂. This technology is found in most hydrogen-water generators.
- High-temperature (solid oxide) electrolysis: operates at several hundred degrees and achieves high efficiencies, mainly in industry.
Faraday’s law
The amount of gas produced can be calculated precisely. Faraday’s first law states: the amount of substance converted is proportional to the charge that has flowed.
where Q = charge (coulombs), M = molar mass, n = number of electrons transferred, F = Faraday constant (96,485 C/mol). Simply put: double the current over the same time → double the amount of gas.
Efficiency
Modern electrolysers reach efficiencies of around 70–80%. The remainder is lost mostly as heat. This efficiency is decisive for evaluating “green” hydrogen, because it determines how much electricity is needed per kilogram of hydrogen.
Applications — from industry to the drinking glass
- Green hydrogen: an energy carrier for industry, mobility and storage when the electricity comes from renewable sources.
- Hydrogen-water generators: enrich drinking water with H₂ (see hydrogen water).
- Water ionisers: separate water into an alkaline and an acidic stream — the principle behind Kangen water.
- Laboratory and industry: supplying pure oxygen and hydrogen.
Frequently asked questions (FAQ)
What is produced by the electrolysis of water?
Hydrogen gas (H₂) forms at the cathode and oxygen gas (O₂) at the anode, in a 2:1 volume ratio.
Why does pure water barely conduct electricity?
Pure water contains too few free-moving ions. Only an electrolyte (acid, base or dissolved salts) makes it conductive enough for efficient electrolysis.
How much voltage does water electrolysis need?
Theoretically 1.23 V is sufficient. In practice, because of overpotentials, 1.5 to 2.0 V is usually required.
Is the electrolysis of water dangerous?
Hydrogen and oxygen together form a flammable mixture (oxyhydrogen). Drinking-water generators produce only tiny, harmless amounts; laboratories and industry follow appropriate safety measures.
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