The PEM electrolyser design (polymer electrolyte membrane or proton exchange membrane) successfully addresses some of the issues with the alkaline electrolyser. Joule losses are virtually eliminated by ensuring the electrode separation is measured in microns rather than millimetres. The trick is to use a special plastic film sandwiched between the two electrodes that allows nothing through but protons. Deionised water is fed to a porous anode (the side connected to the positive side of the DC voltage source). Electrons are stripped off leaving positive hydrogen ions, protons. These readily migrate across the membrane to the cathode where the power source supplies the electrons needed to produce free hydrogen gas.
The half reactions are:
Anode
H2O(l) → O2(g) + 4H+(aq) + 4e-;
Cathode
4H+(aq) + 4e- → 4H2 (g).
Though very effective, there are some technical difficulties associated with the physical construction of a PEM electrolyser: water must be trapped on the anode side and at the same time the oxygen produced at that electrode needs to be separated out; for the electrodes to work effectively, a surface catalyst such as platinum or iridium is required; getting the water to the anode side without increasing the gap is tricky – a porous electrode is required, usually titanium (the cathode must let gas through and can be porous carbon paper). The diagram on this web page illustrates it well.
A good way of understanding the challenges is to design, construct and test a small electrolyser as a student project. The parts can be mostly built using a 3-D printer (knowledge of AutoCAD Inventor, Solidworks or similar is required). An active area of 1 cm2 would be adequate, though you can try miniaturising further, perhaps building a stack into the envelope of a ‘C’ or ‘D’ cell. You will need to stamp out gaskets from a rubber sheet and fashion metal electrical connection tabs from thin copper sheeting. Bolts running through the assembly in each corner will ensure an adequate seal. The diagram above shows the most basic working design, which you can modify for higher performance. The membrane and electrodes should be purchased from the Fuel Cell Store and typical prices for diffusion layer, catalyst and nafion PEM sandwich is £3/cm2. Have a look at this site for information about what is available. Note that fuel cell and electrolyser membrane electrode assemblies are very similar and that a PEM electrolyser can also work in reverse as a fuel cell.
Investigations