O-rings for electrolyzers

Where are O-rings and seals located in an electrolyzer?

Different types of seals are used in an electrolyzer. In the stack itself, these are often gaskets, frame seals or overmolded seals. These seals are located between cell frames, bipolar plates, separator plates, membranes, manifolds and end plates. They must seal evenly over a large surface area while maintaining the correct contact pressure.

Classic O-rings are mainly found in round, discrete and serviceable interfaces. Think of electrolyzer BOP, low pressure valves, filter housings, pumps, measurement ports, sensors, drain and vent lines, service connections and maintenance covers. This is exactly where O-rings are practical, easy to standardize and relatively simple to replace.

The difference between stack sealing and BOP sealing is important. A stack gasket is often designed as part of the stack architecture. An O-ring in a BOP module usually works in a classic groove, with clear requirements for squeeze, stretch, groove fill, hardness and installation. Treating both situations as the same sealing question creates a risk of leakage, compression loss or material degradation.

PEM, alkaline and AEM: different sealing conditions

In alkaline electrolyzers, material selection is mainly driven by resistance to KOH, NaOH, water, elevated temperature and gas-liquid interfaces. A peroxide-cured EPDM compound is often a logical starting candidate here, because EPDM can perform strongly in water, steam and alkaline environments. PTFE or FFKM can be interesting when chemical inertness, purity or longer service life weigh more heavily. Standard FKM/Viton is not an automatic choice for alkaline stack positions, because many FKM types can be sensitive to strong alkalis, amines and steam-like conditions.

In PEM electrolyzers, the emphasis is different. The installation works with ultrapure water, but acidic and oxidative conditions can occur locally, especially at the membrane and oxygen side. Oxidative stability, acid stability, low extractables and control of gas crossover then become important. FKM, FFKM and PTFE are often logical shortlist materials. EPDM can be functional in certain PEM interfaces, but must be explicitly validated for chemical stability, permeation, leaching and ageing.

AEM electrolyzers are technically positioned between alkaline and PEM. They combine membrane technology with an alkaline operating principle. Because AEM technology is still strongly developing, material advice must remain cautious. The right O-ring or gasket depends on pH, water quality, membrane chemistry, temperature, pressure profile and service life requirements.

Main risks in electrolyzer seals

An electrolyzer seal rarely fails due to a single cause. Often, it is a combination of chemistry, gas-tightness, loss of compression and installation.

Permeation is an important risk. Hydrogen is a small molecule and can diffuse through polymers. Gas can also leak along the contact surface when the contact pressure, surface quality or groove fill is incorrect. Material permeation and interface leakage must therefore be assessed separately.

Gas crossover is especially critical in stack and manifold zones. A seal must reliably keep H2 and O2 separated. Even a small leak can not only cause efficiency loss, but also create safety risks.

Compression set and stress relaxation determine whether the O-ring still has sufficient elastic recovery after a longer period of time. A material may appear chemically suitable, but can still fail if the sealing force is lost due to long-term compression, temperature or swelling.

Pressure cycles, start-stop operation and rapid depressurization also deserve attention. At higher pressure or in downstream components, rapid gas decompression can cause damage, especially when hydrogen dissolves in the elastomer and wants to expand too quickly during pressure drop. Backup rings, harder compounds and controlled groove clearances may then be necessary.

Material selection for O-rings in electrolyzers

Material selection must always be assessed per position. A peroxide-cured EPDM compound can be a strong candidate for alkaline electrolyzers because of its combination of resistance to water, steam and alkalis. For PEM electrolyzers, FKM, FFKM and PTFE may be more logical when the oxygen side, oxidative conditions, high purity or chemical stability are leading.

HNBR is especially interesting in mechanically loaded BOP positions, such as valves, connectors and certain pump or measurement interfaces. NBR is cost-efficient, but usually has too little chemical and thermal margin in electrolyzer stacks. Silicone or VMQ is rarely the first choice at H2/O2 interfaces, because gas permeation and mechanical strength can become critical.

PTFE is chemically very strong, but does not behave like an elastomer. The material requires a well-designed sealing surface, sufficient contact pressure and attention to creep or cold flow. FFKM is the premium option for high chemical uncertainty, higher temperature or high downtime costs, but is not automatically necessary in every electrolyzer position.

Design, groove and installation

A good compound can still leak when the groove design is incorrect. For O-rings in electrolyzers, squeeze, stretch, groove fill, gap size, surface quality and tolerances are at least as important as the material. View our guide to O-ring grooves.

ISO 3601 is the logical basis for O-ring sizes, groove design, surface quality and backup rings. At higher pressure or with larger gaps, PTFE backup rings may be needed to limit extrusion. In the case of chemical swelling or thermal expansion, the groove must retain sufficient space; otherwise, the contact stress increases and the ring can deform or become damaged.

Installation deserves the same attention. Sharp edges, contamination, incorrect lubrication, twisted O-rings or excessive stretch can cause leakage before the electrolyzer is put into operation. Especially in high-purity PEM systems, cleanliness, extractables, leachables and traceability of the compound used must also be considered.

Standards and documentation

In addition to material data, documentation remains important. ISO 3601 helps with dimensions, groove design and quality acceptance of O-rings. For gas permeation, leak testing, ageing and compression set, additional ASTM and ISO test methods are relevant. In practice, an electrolyzer project therefore requires more than a catalogue item: you need compound data, batch traceability, test conditions and clear agreements on replacement or maintenance. Especially in hydrogen production, where safety, efficiency and uptime come together, this documentation prevents discussions afterwards.

Testing and validation

For electrolyzer seals, the availability of a standard O-ring is usually not the bottleneck. The real question is whether the O-ring has been validated for the exact position.

A practical validation package includes at least material testing, media compatibility and assembly testing. Think of hardness, tensile strength, elongation, compression set, stress relaxation, chemical immersion in KOH/NaOH, DI water or coolant, volume change, hardness change, H2/O2 or helium permeation, leak testing, pressure decay, thermal aging, thermal cycling and pressure cycling.

At higher pressure or with rapid pressure changes, additional RGD testing is advisable. For PEM or high-purity applications, extractables, leachables, cleanliness, batch traceability and change control should be included in the assessment.

Read more about common causes of failure in our O-ring failure analysis.

Need advice on O-rings in an electrolyzer?

Are you working on a seal for a PEM, alkaline or AEM electrolyzer? Share the seal position, medium, temperature, pressure, groove size and validation requirements with our team. We will then help you think through a suitable O-ring, compound or sealing solution.

Relevant data includes: electrolyzer type, component position, medium, minimum and maximum temperature, pressure profile, static or dynamic use, desired service life, leakage requirement, existing groove size, contact pressure, purity requirements and desired test data.

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