Water Electrolysis

Process for the electrochemical decomposition of water in a divided electrolytic cell by electrolysis.

The result is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water.

The electrolysis of one mole of water produces a mole of hydrogen gas and a half-mole of oxygen gas.

Balancing the above molecular formulae gives: 2H2O ➜ 2H2 + O2

Pure water is a fairly good insulator and so a large potential will have to be applied

It is normal to add a water soluble electrolyte to increase the conductivity.

In an electrolyte the water is separated into hydrogen ions H+ and hydroxide ions OH- as below:

2H2O → 2H+ + 2OH-

A voltage applied to the electrodes results in electrons given to the hydrogen ions at the cathode (-ve), producing hydrogen by a reduction reaction:

2H+ + 2e- →? H2

At the anode (+ve) the electrons are removed from the hydroxide ions, producing oxygen and water by an oxidation reaction:

2OH- H2O + 1/2O2↑ + 2e-

The ↑ indicates the hydrogen gas released at the cathode and the oxygen gas released at the anode.

Energy consumed per standard volume of hydrogen is 11.7 MJm-3 for a process that is 100% efficient. In reality the process is between 60% and 90% efficient depending on the electrolyte used and the exact process.


See also: Alkaline Water Electrolysis, Electrolysis, Fuel Cell, Hydrogen, Oxygen, Water.

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Subjects: Electrochemistry