AEM Electrolysis: The Complete Guide to AEM Electrolysers

AEM electrolysis (anion exchange membrane water electrolysis) produces green hydrogen with a solid polymer membrane in an alkaline environment. It aims to combine the compact, dynamic cell design of PEM with the abundant materials of classic alkaline electrolysis, without scarce precious metals and without PFAS. This guide explains how an AEM electrolyser works, how it compares with PEM and alkaline, which components matter, what drives the levelised cost of hydrogen, and how CENmat builds the whole chain from materials to stacks and systems.

How AEM electrolysis works

Every water electrolyser does the same job: it uses electricity to split water into hydrogen and oxygen. The technologies differ in which ion travels through the cell, and in what separates the two gases.

In an AEM electrolyser, the separator is a thin, dense anion exchange membrane. It conducts negatively charged hydroxide ions (OH⁻) and blocks gases. A single cell works like this:

1. At the cathode, water takes up electrons and is reduced to hydrogen gas and hydroxide ions (hydrogen evolution reaction, HER).
2. Through the membrane, hydroxide ions move from the cathode side to the anode side.
3. At the anode, hydroxide ions release electrons and form oxygen gas and water (oxygen evolution reaction, OER).

Because the cell is alkaline, the electrodes do not have to survive the strongly acidic conditions of a PEM cell. That is what allows catalysts free of precious metals and critical raw materials, and membranes made without PFAS. AEM systems typically run on pure water or a dilute alkaline electrolyte, far less aggressive than the concentrated lye of classic alkaline plants.

Many cells are stacked in series between bipolar plates to form a stack. The stack, together with power electronics, water treatment, gas separation, drying and controls (the balance of plant), makes up an AEM electrolyser system.

AEM vs PEM vs alkaline electrolysis

The three low-temperature technologies each solve a different problem. A short summary:

• Alkaline is mature and runs on abundant materials, with a porous diaphragm in concentrated lye. It needs more active area per megawatt and is less suited to fast load changes.
• PEM is compact and dynamic thanks to a thin solid membrane, but the acidic environment requires scarce precious metals at the electrodes and PFAS-based membranes and ionomers.
• AEM uses a solid membrane like PEM, in an alkaline environment like alkaline electrolysis. The goal is PEM-like compactness without PEM's materials constraints.

The honest weakness of AEM has been durability and scale. Both are now moving fast, as the results below show. For a deeper look at materials, read alkaline vs PEM vs AEM electrolysis: the materials question. For procurement, see PEM vs AEM: a buyer's guide.

The components of an AEM electrolyser

Anion exchange membrane

The membrane carries hydroxide ions, separates hydrogen from oxygen and sets much of the cell's resistance and lifetime. What matters: ionic conductivity, mechanical and chemical stability in alkaline conditions, swelling, gas crossover and how the membrane is assembled. Because AEM is alkaline, the membrane can be PFAS-free. CENmat's AionFLX™ family is.

Ionomer

The ionomer is the ion-conducting binder inside the catalyst layers. It connects the catalyst particles to the membrane so hydroxide ions can reach the reaction sites. A membrane and ionomer from the same material family bond and swell compatibly, which helps both performance and durability. See AionFLX™ ionomer and ionomer and membrane innovation for AEM.

Catalysts and electrodes

The anode drives the oxygen evolution reaction, the slower and more demanding half of the cell. The cathode drives hydrogen evolution. In AEM, both can be built without precious metals or critical raw materials. Electrode quality depends on catalyst activity, how evenly the layer is applied, and how well it adheres over thousands of hours. CENmat's OXYGN (anode) and H2GN (cathode) are CRM-free and spray-printed in the same process.

CCM or CCS

There are two main ways to put the catalyst layer in the cell. In a catalyst-coated membrane (CCM), the layer sits directly on the membrane. In a catalyst-coated substrate (CCS), it is applied to the porous transport layer instead. Each route affects interfaces, handling and manufacturing. We compare them in CCS vs CCM in AEM electrolysis: lessons from NEWELY. Buyers can find the specs worth requesting in membrane electrode assemblies: specs buyers should request.

Stack

The stack combines many cells with bipolar plates, transport layers, seals and end plates. Stack design decides flow distribution, compression, thermal behaviour and how well single-cell results carry over at scale. Results at a few square centimetres say little about an industrial stack, which is why active area and stack-level validation matter.

System

The system adds power electronics, water and electrolyte management, gas–liquid separation, hydrogen drying and purification, and safety and control. For operators, the system determines efficiency at the plant boundary, availability and maintenance. See electrolyser stack lifetime: what operators should track and water quality requirements for AEM and PEM.

How to evaluate an AEM electrolyser or materials supplier

Because AEM is younger than PEM and alkaline, datasheets are not yet easy to compare. Before you shortlist a supplier, ask for:

• Durability with conditions. Hours of operation, current density, temperature, electrolyte and the degradation rate in µV/h, not only start-of-life polarisation curves.
• Scale of the result. Cell active area, and whether single-cell results were reproduced in a short stack.
• Materials declaration. Whether membrane, ionomer and electrodes are PFAS-free and free of critical raw materials, and what the seals and balance of plant contain.
• Manufacturing route. How membranes and electrodes are produced, in which formats, and how reproducible they are from batch to batch.
• A path to the next step. Paid samples, short stacks and pilots, with clear formats, volumes and lead times.

More on this in how to evaluate AEM electrolyser vendors and AEM electrolyser components: sourcing for EU auctions.

Where AEM electrolysers fit

AEM's strengths matter most where compact design, flexible operation and uncritical materials come together:

• On-site industrial hydrogen. Sites that want to make hydrogen for process heat, protective atmospheres or chemical processes benefit from compact stacks and a mild electrolyte. Background: green hydrogen for industrial heat and on-site electrolysis vs delivered hydrogen.
• Coupling with wind and solar. Where power fluctuates, a separator that keeps gases apart at part load is an advantage. See dynamic operation of AEM electrolysers.
• Projects with materials requirements. Funding programmes and auctions increasingly ask about the origin and criticality of materials. A cell without precious metals, critical raw materials and PFAS makes that documentation simpler.
• Modular scale-up. From a short stack for evaluation to a system, capacity can grow step by step with the same materials at every stage.

CENmat: one chain from materials to stacks and systems

Most AEM players buy membranes from one supplier, catalysts from another and integrate them in their own stack. Every interface between them is a qualification risk. CENmat develops AEM electrolysis from material to system, so membrane, ionomer, electrodes and stack are designed to work together.

The scale-up runs through the EU-funded HYScale project, which CENmat coordinates. The story from short stack to system is told in AEM electrolyser stack: from 6 kW to 100 kW in HYScale, and the large-area cell results in HYScale D3.1. Efficiency data for the 100 kW system will be published after acceptance testing.

For OEMs, this means you can buy at the level you need: membranes and ionomer to qualify in your own design, a matched set of membrane, ionomer and electrodes, or short stacks for evaluation and pilots. Start with the AEM qualification pack or the datasheets.

CENmat also works on PEM, where our contribution is ultra-low iridium catalysts. Our AEM vs PEM calculator shows what that means for a project.

AEM electrolysis and the levelised cost of hydrogen (LCOH)

The levelised cost of hydrogen is the total lifetime cost of a plant divided by the hydrogen it produces. For most projects, three levers dominate:

• Electricity price and efficiency. Power is usually the largest cost item. Every millivolt of cell voltage turns into kilowatt-hours per kilogram over the life of the plant.
• Utilisation. A plant that follows variable renewables must run well at part load and through frequent changes.
• CAPEX and stack lifetime. Investment cost per kilowatt, and how often stacks need replacing.

AEM addresses the CAPEX and lifetime levers through its materials. Without precious metals and PFAS, material costs are less exposed to scarce, volatile or regulated inputs. Low degradation rates stretch the time between stack replacements. The HYScale value proposition: a system CAPEX of 400 €/kW and hydrogen below 3 €/kg at a production volume of more than 10 MW per year.

Your own numbers matter more than any average. Use our LCOH calculator to model electricity price, full-load hours, CAPEX and stack lifetime for your project, and read building a bankable electrolyser business case and total cost of ownership for electrolysers.

FAQ: AEM electrolysis

What is AEM electrolysis?

AEM electrolysis is water electrolysis with an anion exchange membrane that conducts hydroxide ions in an alkaline environment. It produces hydrogen at the cathode and oxygen at the anode, like PEM and alkaline electrolysis.

What is the difference between AEM and PEM electrolysis?

Both use a thin solid membrane. PEM conducts protons in an acidic environment, which requires precious metals and PFAS-based membranes. AEM conducts hydroxide ions in an alkaline environment, so it needs neither.

What is the difference between AEM and alkaline electrolysis?

Classic alkaline electrolysis uses a porous diaphragm in concentrated lye. AEM uses a dense membrane with pure water or dilute electrolyte, which allows a more compact cell and lower gas crossover.

Is AEM electrolysis ready for industrial use?

AEM is an emerging technology that is scaling up. CENmat has validated a 6 kW short stack with 400 cm² cells, and its 100 kW HYScale system with 900 cm² cells has completed production and is in commissioning.

How long do AEM membranes last?

CENmat's AionFLX™ Plus has run for more than 8,000 h at a degradation rate below 15 µV/h in a long-term test (50 °C, 1 M KOH).

Are AEM electrolysers PFAS-free?

AEM membranes and ionomers can be PFAS-free, and CENmat's AionFLX™ materials are. Seals, gaskets and balance-of-plant parts should be checked separately.

How do I estimate the cost of hydrogen from an AEM electrolyser?

Use the LCOH calculator with your electricity price, operating hours, CAPEX and stack lifetime.