UCL Department of Chemical Engineering



Water Electrolysis
Water Electrolysis means the splitting of water into its constituents – hydrogen and oxygen. Hydrogen can then be used for electricity production in fuel cells or for many chemical production processes. Traditionally, alkaline electrolysis has been used for large scale hydrogen production. However, in recent years a technology called PEM water electrolysis is gaining popularity. PEM stands for polymer electrolyte membrane or alternatively for proton exchange membrane. While alkaline electrolysis used big amounts of caustic liquids like potassium hydroxide as an electrolyte, PEM electrolysis uses a very thin polymer membrane as the solid electrolyte. In the future, this will make it possible to produce hydrogen cheaper and more effectively than with alkaline electrolysis.


Parts of a PEM Electrolyser
In a PEM electrolyser, water flows in through the flow channels. On the anode side, it flows through the liquid-gas diffusion layer (LGDL) from there and reaches the catalyst layer (CL). The CL is a very thin layer of catalytically active material; mostly iridium ruthenium oxide on the anode side and platinum on carbon on the cathode side. On the anode the water reacts to oxygen, protons and electrons. The protons are then transported through the polymer membrane to the cathode side. Here, they react on the catalyst layer to form hydrogen. The gases diffuse through the LGDL back to the flow channels, where they mix with the water stream and are taken out of the electrolyser cell.

Electrolysis and renewable energy
Energy from renewable sources (Wind, Solar…) is intrinsically intermittent. Depending on the season or time of the day more or less energy is produced. To make sure the supply of energy is secure and stable energy needs to be stored when an excess is produced and later fed back into the grid when needed. Water Electrolysis offers this grid stabilization. When a surplus of energy is available, e.g. during a sunny summer day, a part of the energy is used to produce hydrogen. This hydrogen can then easily be stored in tanks. Whenever more energy is needed, e.g. on a cloudy day, hydrogen is taken from tanks and fuel cells are used to release the energy stored in the hydrogen.