Electrolysis

Your aluminium bike frame, the shiny chrome on the tap, the green hydrogen from the news: the same trick is behind all of them. You send electric current through a liquid and force chemical compounds to split into their components. This trick is called electrolysis.

Electricity as a tool

In electrolysis, two electrodes sit in a conductive liquid and are connected to a power source. Pure water barely conducts the current, so for water electrolysis you stir in a little baking soda. The current pulls the charged particles apart: positive ions move to the negative electrode, negative ions to the positive one. With water, hydrogen (H₂) collects at the negative electrode and oxygen (O₂) at the positive one. Two water molecules turn into two H₂ and one O₂, which is why by volume twice as much hydrogen gas rises as oxygen gas. The compound H₂O turns back into the pure elements.

Combustion in reverse

When hydrogen burns, it combines with oxygen to form water and releases a lot of energy. Electrolysis does exactly the reverse: it splits water back into hydrogen and oxygen. To do that, it has to put back in the energy that was released during combustion. Electrolysis is therefore endothermic: without a constant supply of energy from a power source, it stops immediately.

What electrolysis is used for

Aluminium occurs in nature only firmly bound in ore. Only electrolysis in a melt hotter than 900 degrees separates the pure metal from it, and it devours huge amounts of electricity. That is why many aluminium plants have their own power station next door. In copper plating and chrome plating, the current deposits a wafer-thin metal layer onto an object, for protection or decoration. And when the electricity comes from wind or sun, water electrolysis delivers green hydrogen, an energy store for times without wind and sun.