Alkaline, PEM or AEM: how to choose an electrolysis technology

Most technology comparisons in electrolysis end with a winner. That is the wrong shape for the decision, because the three commercial technologies are not competing on one axis. Each is limited by something different, and the right choice is the one whose limit does not bind your project.

This is how we would work through it, including the cases where the answer is not our own technology.

Four questions that come before any supplier comparison

The first question is about the power profile, and it matters more than nameplate efficiency. A flat grid connection running at base load and a solar array with a daily profile are different engineering problems. What decides the outcome is ramp rate and how far the system can be turned down before it has to be shut down rather than reduced.

The second is about the outlet. Hydrogen leaves an electrolyzer at whatever pressure and purity the stack can deliver, and anything beyond that is a compressor and a purifier. That is capital and parasitic load you either avoid inside the stack or add outside it.

The third, scale, usually decides more than the technology choice does. One multi-megawatt train and twenty small units delivering the same output have different economics, different lead times, and different consequences when one of them fails.

The fourth is exposure. Iridium supply and the PFAS restriction proposal are risks outside your project's control. On a ten to twenty year horizon, an open regulatory question about a core material is a line item rather than a footnote.

SourcesInternational Platinum Group Metals Association, White Paper on Iridium (2022) · Hydrogen Europe, Position Paper on PFAS (2023) (industry association)

What each technology is actually chosen for

This allows the intermixing of the produced gases (hydrogen and oxygen – H2 and O2) that are dissolved in the electrolyte, limiting lower power-operating range

We build AEM, so treat the last column as an interested party's summary and check it. The useful test is not whether a supplier praises their own technology, but whether they will state its limits in writing.

SourcesIRENA, Green Hydrogen Cost Reduction (2020): stack costs and operating ranges by technology · U.S. DOE Technical Targets for Liquid Alkaline Electrolysis and PEM Electrolysis: 2022 capital cost status

What actually sets the cost of the hydrogen

Capital cost is easy to compare and is usually the smaller half of the answer. It covers the stack and its catalyst, the balance of plant, which is the rectifier, gas separation, drying and water treatment, and then footprint, installation and permitting. Downstream compression and purification belong in this column too, and they are the items most often left out of a comparison because they sit outside the quoted scope.

The running cost is where the hydrogen price is actually set: the electricity price multiplied by the hours you run, divided by the system efficiency at the load you actually operate at, plus the stack replacement interval and maintenance.

If your plant spends most of its hours at 30% load because that is what the solar profile gives you, the efficiency and the crossover behaviour at 30% load are what set your cost and your safety case.

Where we would answer honestly that it is not us

• A flat, cheap, large industrial load with no interest in pressure or footprint. Alkaline is likely to be the lowest total cost, and no argument about generations changes that.
• A multi-megawatt single train delivered next year with a fifteen-year operating record behind the specific model. AEM suppliers cannot offer that today, ourselves included.

AEM is the right answer when the project needs dynamic operation, pressurised and pure hydrogen at small or mid scale, and a supply chain that avoids iridium and needs little platinum-group metal (our HXP-an anode is PGM-free; the HXP-ca cathode carries 0.2 mg of PGM per cm²), and when the buyer is willing to look at measured data rather than accept a category claim.

What to ask, whichever technology you choose

These six apply to every supplier in every technology, and the answers are more informative than the comparison table at the front of any brochure.

For our own products we publish them. The HXS-2 stack has more than 3,000 hours of cumulative operation, and its long-run data is published with the test conditions in our durability note. Measured at 50 A in 0.1 M KOH at 50 °C and ambient pressure, the same stack reaches 90.0% efficiency on a higher heating value basis at beginning of life, and runs up to 10 barg on the hydrogen side without a separate compressor.

The HXS-2 is available to order and ships four weeks from PO acceptance; the HXP-an anode and HXP-ca cathode ship in two weeks.