AEM water electrolysis explained: what changes when the membrane is anionic

Green hydrogen has one production route, and that route is water electrolysis. Which is why the limits of today's electrolysis technologies are not an academic question. They set the floor under the cost of clean hydrogen.

Two technologies carry the commercial market, and each is held back by the same structure that makes it work. AEM exists because of the gap between them.

The trade-off in electrolysis today

Alkaline electrolysis has been in industrial use since the 1800s. It runs liquid KOH between electrodes separated by a porous diaphragm and works with nickel-based catalysts rather than noble metals. That is why it is inexpensive to build, and why most installed electrolysis capacity in the world is alkaline.

The limits come from the same design. Response is slow. Gas crossover is structural, because a porous diaphragm separates the two half-cells but does not seal them, and the mixing gets worse at low load, exactly where a renewable-coupled system spends much of its time. That is a safety limit before it is an efficiency limit. Pressurised hydrogen is difficult for the same reason: suppressing crossover means a thicker diaphragm, and a thicker diaphragm means lower conductivity.

PEM replaced the porous diaphragm with a solid polymer membrane and moved the chemistry into an acidic environment. The result is fast and couples well to variable renewables. The acidity is also the bill, because acidic operation requires noble metal catalysts, principally iridium on the oxygen side.

Nobody knows the final scope of that restriction. But a technology whose core material sits inside an open regulatory question carries a risk unrelated to its performance.

SourcesIEA Global Hydrogen Review 2025, chapter 3: alkaline 60% of installed electrolysis capacity · International Platinum Group Metals Association, White Paper on Iridium (2022) · Hydrogen Europe, Position Paper on PFAS (2023) (industry association)

What the anionic membrane changes

AEM keeps PEM's solid polymer membrane and moves it back into an alkaline environment. In one sentence: the membrane conducts hydroxide ions rather than protons. The technology is written both ways in the literature, anion exchange membrane water electrolysis and AEMWE, and they mean the same thing.

• The catalyst leaves the noble metal market. In alkaline conditions nickel-iron catalysts are stable and active enough for oxygen evolution, so iridium is not required.
• The separator is solid, so crossover is far lower. High-purity hydrogen without a downstream purifier, and pressurised operation without fighting the separator.
• The membrane can be hydrocarbon-based, which sits outside the PFAS scope entirely. Worth stating carefully: some commercial AEM membranes still use partially fluorinated backbones for durability. It is an option the chemistry allows, not a property every AEM product has.
• Response is fast, because the solid membrane gives AEM the dynamic behaviour that makes PEM attractive for renewable coupling.

The OER is the limiting step of the complete electrolysis process due to its lower kinetics and higher overpotential than the HER

That single change moves four things at once.

SourceMolecules 26(21):6326 (2021), section 3.3.1: nickel, nickel alloys and cobalt oxides as stable OER catalysts in alkaline media

What AEM has not solved

AEM is not a finished technology, and it is worth being direct about where it is not.

Membrane durability over long operation is the open problem. Anion exchange membranes can lose conductivity over time through chemical degradation of the cationic groups that carry hydroxide. Improving this is where most of the field's material work sits, and it is done through membrane chemistry and operating conditions together rather than either alone.

The second problem is manufacturability. Reaching a performance number in a single lab cell and reproducing it across hundreds of stacks are different problems with different failure modes. MEA fabrication and stack assembly decide which of the two a customer actually receives.

Both are real. Neither is a reason to dismiss the technology, and the honest way to discuss them is with data rather than adjectives.

SourcesIRENA, Green Hydrogen Cost Reduction (2020): AEM status and degradation mechanisms as of 2020 · U.S. DOE Technical Targets for Liquid Alkaline Electrolysis and PEM Electrolysis: reference degradation rates in mV per 1,000 h

AWE, PEM and AEM compared

Read the last row first. Every technology on this table has an open risk, and the useful question is what kind of risk it is.

The limits of AWE and PEM are structural. They follow from the porous separator and from the acidity, and no amount of engineering removes them. AEM's are maturity problems, which is a different category, and the last decade has moved them substantially.

Where the technology actually stands

General claims about AEM maturity are easy to make and hard to check, so here are ours with the conditions attached.

A second run answers the durability question from the materials side. A HydroXpand grown nickel-iron anode ran 6,358 hours in a 16 cm² two-cell short stack at 0.625 A cm⁻², degrading at 14.7 µV/h per cell on a full-run linear fit with a PiperION® 80 µm membrane. In the same campaign, changing only the membrane gave 1,234 hours, which is the honest way to read any single durability number: it belongs to the cell, not to one component.

Separately, the same stack reaches 90.0% efficiency on a higher heating value basis at beginning of life, measured at 50 A in 0.1 M KOH at 50 °C and ambient pressure. On the hydrogen side it runs up to 10 barg without a separate compressor. The anode is NiFe-LDH grown on nickel foam, with no iridium and no titanium.

Gas crossover is the safety number, and it is the one worth seeing plotted rather than quoted. Hydrogen in oxygen rises as the load drops, which is exactly the condition a renewable-coupled system spends time in, so a single figure at rated load does not describe it.

The safety number, plotted

Across 0 to 6 barg and 40 to 100% of rated load, every one of those 49 points sits below our 2% control limit, and the curves rise as the load falls. Oxygen in the hydrogen stream is a trace, and a purity question for the dryer rather than a safety limit.

We publish test conditions with every number because a number without its conditions cannot be compared to anything. Two thousand hours at low current in a warm cell and two thousand hours at rated current are not the same result, and a specification sheet that gives you only the duration has told you very little.

One thing the durability figure does not tell you is product lifetime. We do not convert it into one.

Why we build the whole chain

AEM performance is decided at the boundaries between material, electrode and stack, not inside any one of them. A membrane that performs well in a half-cell can behave differently once it is under compression against a specific electrode structure, and an electrode optimised in isolation can be the wrong choice for the flow field it ends up in.

Assembling purchased membranes and purchased electrodes into a stack makes it difficult to improve efficiency, durability and cost at the same time, because each change lands in somebody else's design and comes back as a constraint.

So we develop the catalysts and electrodes, design the cell, build the stack, and build the system and control software that run it. Three KAIST PhDs founded the company in December 2023. More than 70 customers in over 10 countries now use our electrodes, stacks and systems, from university laboratories evaluating their own materials to companies producing hydrogen on site.

The stack in those measurements is a product. The HXS-2 is a 23-cell 2 kW AEM water electrolyzer stack rated at 480 NL/h (1 kg/day), available to order and shipping four weeks from PO acceptance, and a five-cell configuration is available for evaluation.