Electrolysis of Water

A look at the electrolysis of water. This is the process for the electrochemical decomposition of water in a divided electrolytic cell by electrolysis. Electrolysis is the process of changing the chemical structure of a compound using electrical energy.

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

electrolysis of water
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 or 142 MJ/kg (39.2kWh/kg) for a process that is 100% efficient. In reality the process is between 70% and 80% efficient depending on the electrolyte used and the exact process. Hence 50 to 55kWh of electricity is used to produce 1kg of hydrogen gas.

Further Reading:

  • Fuel Cell – most commonly cited example is the hydrogen fuel cell, in which hydrogen and oxygen are combined, producing electric current and water.
  • Water – the most extraordinary material

Electrodes

The electrodes are a critical part in the electrolysis of water and so worth looking at in detail. They move the electrons into or out of a solution by conduction. The positive electrode is called the anode and the negative electrode is called the cathode. For the electrolysis of water both electrodes are normally made from platinum.

Active Material – the material in the electrodes that takes part in the electrochemical reactions which store-deliver the electrical energy.

Activation Overpotential – the overpotential which results from the restrictions imposed by the kinetics of charge transfer at an electrode.

Geometric Electrode Area – the surface area of an electrode calculated from its geometrical dimensions.

Irreversible Electrode – an electrode with an irreversible electrode reaction.

Standard Electrode Potential – The reversible potential of an electrode with all the active materials in their standard states.

True Electrode Area – the surface area of an electrode taking into consideration the surface roughness. For a perfectly smooth electrode, it is equal to the geometric electrode area. For most electrodes the area is larger than the geometric area. The ratio of the two defines the roughness factor.

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