Hydrogen Electrolysis: What Is Water Splitting?

Hydrogen electrolysis is the process of spitting water into Hydrogen gas and Oxygen gas. The Hydrogen gas can then be used as fuel either by being burnt in an engine or reacted in a fuel cell. This is the process that so-called water powered cars rely on for their energy. No car can use water as a fuel, but a car can be made to run only on Hydrogen, meaning that its exhaust gas will be just water vapour.

The Basics Of Electrolysis

As with all electrolysis, an electric current is pumped into a circuit. Instead of a circuit that is completely made of wires, the reactions of the chemicals in the solution being electrolysed both provide and consume the electrons. This makes the equivalent of a complete wire loop and allows current to be supplied to the liquid continuously. The basic setup for electrolysis of water is shown to the right. This setup is fundamental to all electrolysis reactions.

Adding Labels

We can increase our knowledge of this process by adding some correct labels to the previus diagram. The rods inserted in the water are called electrodes. The one where electrons come out is called the anode and the one where the electrons go in is called the cathode. The reactions that occur at these electrodes are different and this difference is caused by the removal or supply of electrons.

As is the case for other oxidation reduction reaction, the reaction where electrons are removed is called an oxidation reaction, and the reaction where electrons are supplied is called a reduction reaction. Using this knowledge, we can add more detail to the previous diagram:

The ANODE REACTION turns water into Oxygen gas, protons (H+) and electrons. This is shown below. The CATHODE REACTION consumes electrons and turns water into Hydrogen gas and hydroxide ions. This is also shown below.

The Overall Reaction

Those two reactions have a lot of parts to them, so how is it that when electrolyzed, water produces just H2 and O2, and not all these H+, OH- and e- things as well?

We get the overall reaction for the hydrogen electrolysis of water by adding the two electrode reactions together. The, just like with a maths sum, we can cancel things that are the same on both sides. That gets rid of the electrons (e-). The H+ and OH- ions react together to form water again. We can then cancel 4 waters from each side of the reaction. These steps are shown below: