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Glossary · Energy industry & energy transition

AEM electrolysis

AEM electrolysis Anion exchange membrane Electrolysis hydrogen Electrolyser power-to-X

AEM electrolysis is the production of hydrogen by water electrolysis using an anion-conducting polymer membrane. AEM stands for anion exchange membrane. Under its intended operating conditions, the membrane primarily conducts negatively charged hydroxide ions and separates the electrode compartments. The development goal is a compact electrolyser that operates in an alkaline environment using less critical and less expensive materials. This does not, however, guarantee that it is free of precious metals or automatically offer a cost advantage over other processes.

How the cell works

A DC power source drives the reaction: hydrogen is produced at the cathode and oxygen at the anode. Hydroxide ions carry charge through the membrane to the anode. The power supply delivers electrons to the cathode and removes them from the anode. The membrane therefore does not conduct electrons. Many cells connected in series form the stack; water treatment, power supply and gas treatment complete the system.

  • Cathode: 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻.

  • Anode: 2 OH⁻ → ½ O₂ + H₂O + 2 e⁻.

  • Overall reaction: H₂O → H₂ + ½ O₂. Hydroxide ions are formed and consumed within the cell; they are not an additional reaction product.

The membrane can be understood as a selective partition: it permits the necessary ion transport while the product gases are removed from different sides. This should not be taken to mean that it is a completely gas-tight barrier. For a specific installation, gas purity, permissible operating states and gas stream monitoring must therefore be assessed alongside the quantity of hydrogen produced.

Comparison with alkaline and PEM electrolysis

Conventional alkaline electrolysis uses a liquid alkaline electrolyte and usually a porous diaphragm. PEM electrolysis, by contrast, uses a proton-conducting membrane; typical catalysts contain platinum and iridium respectively. AEM systems use a hydroxide-ion-conducting membrane and can use nickel-based catalysts, for example. Durability, pure-water operation and any remaining precious-metal content are important development issues.

The common description of AEM as combining the advantages of both processes is an aspiration, not proof of performance. A compact cell design does not in itself prove a long service life. An inexpensive catalyst does not prove low plant costs. Nor does a fast electrochemical reaction prove that a complete plant starting from cold is ready for operation equally quickly. Comparisons must therefore examine specific systems under the same boundary conditions, rather than merely the three technology labels.

In particular, the comparison should not lead to the conclusion that alkaline plants are inherently inflexible or that AEM plants are inherently more dynamic. For procurement, guaranteed minimum load, load change rate, start-up time, pressure control and gas quality are more informative than a blanket ranking. It must also be clear whether a figure applies to a single cell, the stack or the operational plant.

Water management, materials and ageing

AEM electrolysis is not synonymous with operation without alkaline additives. Pure-water-fed designs are possible, but place particular demands on ion transport and the stability of the ion-conducting polymers in the electrodes. A research paper published in 2025 investigates protective interphases at the anode ionomer for this purpose. Such materials advances are important, but do not replace evidence of plant availability over many years.

A distinction must also be made between water supplied from outside and the liquid circulating within the plant. A requirement to supply treated water alone says nothing about whether alkaline electrolyte is present in the internal circuit. For operation, the manual should therefore clearly specify water quality, electrolyte composition, replenishment procedures and maintenance conditions.

Carbon dioxide can form carbonate or bicarbonate in an alkaline environment. This can change the ions present and the conditions under which they are transported. Whether this results in a significant performance loss depends on the system concerned. Claims about CO₂ tolerance therefore require specific test conditions; neither general immunity nor a particular failure rate can be inferred from the term AEM.

“Free of platinum-group metals” also does not mean “free of raw material dependencies”. In a project assessment, the membrane, catalysts, coatings, seals and electrical components belong on the materials list. What matters is which materials are actually used, how much is needed and whether spare parts are available. Material savings in the stack and security of supply for the complete plant are separate assessment tasks.

Power, hydrogen quantity and efficiency

Electrical power is stated in kilowatts or megawatts, whereas hydrogen production is stated in kilograms per hour, for example. Kilograms alone denote a quantity, not a production rate. Kilowatt-hours of electricity per kilogram of hydrogen is a suitable measure of energy consumption. For normal cubic metres, the reference conditions used must be known. A distinction must also be made between the electrical power of the stack, the power consumption of the device and the power connection requirement of the entire plant.

A specific manufacturer example illustrates the order of magnitude, not the general state of all AEM systems: in revision 03 of its data sheet for the air-cooled EL 4.1, Enapter specifies power consumption of 2.4 kW at beginning of life and production of up to 1.0785 kg in 24 hours. The stated outlet pressure is up to 35 bar gauge. The document also contains feedwater requirements and pressure-dependent gas purity figures.

These manufacturer figures yield approximately 53.4 kWh/kg by calculation: 2.4 kW × 24 h divided by 1.0785 kg. This conversion is not an independent measurement or a guarantee for a year of operation. It applies only where the operating conditions of the input figures correspond. A project energy balance must also account for all electricity consumption outside the device boundary, such as water treatment, further compression or external cooling.

An efficiency figure is comparable only once its reference basis is established. This includes, in particular, the lower or higher heating value of hydrogen, the electrical system boundary, and pressure, purity and operating conditions. A high value from a new laboratory cell answers a different question from the measured annual consumption of a plant including downtime and auxiliary equipment. Tender specifications should therefore preferably require clearly defined consumption and production guarantees.

Economics and reliable operating data

Hydrogen costs require electricity supply, investment, financing, maintenance, component replacement and usable production to be considered together. As a simple example, an assumed 54 kWh/kg at 0.05 euro/kWh gives electricity costs alone of 2.70 euros per kilogram. At 0.10 euro/kWh, the figure is 5.40 euros. Both values are freely chosen assumptions for calculation, not market price forecasts or complete hydrogen costs.

Utilisation also affects the result: if the same annual fixed costs are spread over fewer kilograms, their share per kilogram rises. Full-load hours describe the ratio between annual energy used and a defined rated power; they are not simply equivalent to hours switched on. A plant can run for many hours and still have low energy utilisation because it frequently operates at part load.

When dealing with technical trade-offs, a combined assessment is more useful than individual record values. A higher current density is advantageous only if the cell area saved outweighs additional losses and replacement costs. Likewise, frequent load changes should not automatically be considered either harmful or inconsequential. Operating data are needed for the planned load profile, including start-stop cycles, downtime and maintenance.

AEM is not confined to the laboratory: the EL 4.1 mentioned above is a documented commercial device. Conversely, the availability of a product does not prove that all AEM designs are equivalent to alternatives that have operated for many years. For an investment decision, verifiable references, guarantees, spare parts supply and contractual treatment of declining performance matter more than a general statement about “market maturity”.

Relevance to the electricity system and climate protection

As a component of Power-to-X, an AEM electrolyser can convert electrical energy into a chemical energy carrier. Whether this makes economic and system-wide sense also depends on customers, electricity supply and infrastructure. For the European hydrogen market, ACER emphasises electricity costs, appropriate grid incentives, flexible operation and infrastructure development geared to actual demand.

Technical flexibility and operation that benefits the grid are not the same thing. A low wholesale electricity price alone does not indicate whether spare grid capacity is available at a particular connection point. Similarly, a contractual requirement for a steady hydrogen supply can constrain electrical operation. A project should therefore assess whether storage, a flexible customer or additional supply options can decouple electricity consumption from immediate hydrogen delivery.

The climate benefit does not follow from the membrane type either. The source and accounting of electricity must be assessed, as must the production process or application actually displaced. If hydrogen is to be produced, transported and later converted back into electricity or heat, this entire pathway must be compared with direct electricity use. AEM expands the technical toolkit, but does not eliminate the losses from an additional conversion.

Legal framework in the European Union

As of 11 November 2026: Commission Delegated Regulation (EU) 2023/1184 is particularly relevant to recognising electricity used in the production of renewable fuels of non-biological origin as fully renewable. The consolidated version shown as current on EUR-Lex is dated 10 June 2024. Different requirements apply depending on how electricity is sourced; for regular grid electricity supply, these include additionality and temporal and geographical correlation. Exceptions and transitional rules must be observed. The provisions are not specific to AEM and do not replace an assessment of the safety, permitting or grid connection requirements of a specific plant.

Sources

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Edited by eCMS on 11.11.2026 06:52

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