Both split water into hydrogen and oxygen with electricity. An alkaline electrolyser uses a liquid potassium hydroxide solution and a porous separator, and is the older and cheaper design. A PEM electrolyser uses a solid polymer membrane, starts and stops faster, and costs more because it needs precious metal catalysts.
What both machines are doing
Pass a current through water and it splits. Hydrogen collects at one electrode, oxygen at the other. That reaction has been understood for two centuries. Everything that separates one electrolyser from another is engineering around it: what carries the charge between the electrodes, what stops the two gases mixing, and what catalyst lets the reaction proceed without wasting energy as heat.
The gases must be kept apart. Hydrogen and oxygen mixed together are hazardous, and a separator that leaks contaminates both streams. This is the job the two designs approach differently.
Alkaline
The older design, and the one with the longest industrial record.
The electrodes sit in a strongly basic solution, typically potassium hydroxide, which carries the charge. A porous diaphragm between them lets ions through while keeping the two gas streams separate.
Its advantages follow from that chemistry. The basic environment lets nickel based catalysts survive, so no precious metals are needed. The components are conventional and widely manufactured. Capital cost per unit of capacity is lower, and the technology has decades of operating history behind it.
Its constraints follow from the same place. A liquid electrolyte has to be circulated, monitored and eventually replaced. The porous diaphragm is less effective at low load, because gas crossover rises when the unit is running gently, which sets a minimum operating point. And the whole assembly is slower to bring up from cold and slower to change output.
PEM
Proton exchange membrane. Here the electrolyte is a solid polymer sheet rather than a liquid.
That single change has a number of consequences. The membrane is a much better barrier, so the unit can run at low load without the two gas streams meeting. It responds quickly, so output can follow a changing power supply within seconds. It is more compact for the same capacity. And the hydrogen comes out at higher purity and often at higher pressure, which reduces downstream compression.
The cost is in the catalyst. The surface of the membrane is acidic, and nickel does not last there, so platinum group metals are used. That is most of the price difference, and it is why PEM units are chosen when their responsiveness is worth paying for rather than as a default.
Why responsiveness matters here
On a site where the electrolyser runs on surplus renewable generation, the power supply is not flat. It rises through the morning, falls in the evening, and disappears on a still overcast day.
An electrolyser that must be held at a minimum load will need power bought in to stay there. One that can ramp down and idle can simply follow what is available.
This is what makes an electrolyser useful as a flexible load rather than just as a consumer of electricity. Generation that would otherwise be curtailed, produced but not taken by the grid, becomes a product that can be stored and sold.
Whether that argues for PEM depends on whether the site has battery storage. Storage smooths the supply, which lets a cheaper alkaline unit run steadily. Without storage, the responsiveness of PEM is doing work that has to be done somehow.
The questions that actually decide it
Four, in roughly this order.
How steady is the power going in, once storage and any grid connection are accounted for. A steady feed points to alkaline.
How many hours a year will it run. Utilisation drives the economics more than the capital cost does, because an expensive machine running most of the year can beat a cheap one running a third of it.
What is the water source, and what does treating it cost. This is routinely left until late and it is a genuine constraint on a dry site.
What pressure and purity does the buyer need. If the offtake requires high pressure, the pressure PEM delivers directly may offset its capital cost against compression that would otherwise be needed.
Where this fits
Green hydrogen is one of the income lines on an integrated site, and its role there is specific: it absorbs surplus at the moment the surplus exists. The choice of electrolyser follows from the site’s generation profile rather than being decided in advance.
See also electrolyser, green hydrogen and the fuller article on what green hydrogen is.
Related questions
- Which one suits a solar or wind powered site?
- It depends on how the electrolyser is fed. If it runs on a steady supply, drawn from storage or from the grid, alkaline is usually the economical choice. If it runs directly on variable output, following the sun and wind through the day, PEM handles the ramping and the frequent stops more comfortably. Many sites resolve this with storage rather than with the membrane choice.
- Why does PEM cost more?
- Its catalysts use platinum group metals, principally platinum and iridium, because the acidic environment at the membrane surface corrodes cheaper materials. Alkaline electrolysers work in a strongly basic solution where nickel based catalysts survive, and nickel is far more abundant.
- What is the water actually used for?
- It is the raw material. The hydrogen and the oxygen both come out of the water molecule. It must be purified first, because dissolved salts and minerals foul the membrane or the electrodes, so a site needs a water treatment step ahead of the electrolyser and a plan for where that water comes from.
- Is the hydrogen different depending on which one made it?
- No. The molecule is identical. What differs is the equipment, the cost, and how the unit behaves when the power supply varies. The colour names attached to hydrogen describe the energy used to make it, not the gas.