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30.06.2026 | Story

Sealing requirements along the H2 value chain?

Renewable Energies Energy Generation

The requirements for sealing solutions are defined by the media and temperatures involved in hydrogen production and processing, transport, storage, and use. This spectrum covers a broad range: from different electrolysis processes, compression, purification, transport and storage in gaseous, liquid form (cryogenic) or derivatives such as methanol or ammonia, all the way to the end use in fuel cell systems (FC - fuel cell) or hydrogen internal combustion engines (HICE).

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What are the sealing requirements along the H2 value chain?

Production: PEM, AEM, AEL, and SOEC electrolyzers

Operating parameters and risks

Permeation

H2 permeation causes hydrogen losses and creates a safety risk. Low-permeability material formulations are effective in preventing this. They include CIIR (chlorobutyl rubber), HNBR (hydrogenated nitrile butadiene rubber), EPDM (ethylene propylene diene rubber), and FKM (fluoroelastomer). In this context, it is important to simulate and validate the materialspecific permeation coefficients of the seals for each application.

Explosive decompression (RGD)

RGD-resistant materials are required wherever rapid pressure changes occur, for example in tank or compressor environments. These include EPDM, FVMQ (fluorosilicone rubber), and PVMQ (phenyl methyl silicone), among others. The materials used here must have a proven RGD resistance up to ≥ 700 bar.

Media resistance

PEM electrolysis requires seals to withstand dilute sulfuric acid and continuous exposure to O2. In addition, highly pure materials must be used to ensure that no metal ions leach out and cause ionic contamination.

AEL electrolysis requires seals to withstand a 20-40 percent potassium hydroxide solution. The conventional temperature range (standard AEL) involves operating temperatures of 60–80 °C, an elevated temperature range (modern/optimized AEL) involves operations at 80–90 °C, and high-temperature alkaline electrolysis (advanced AEL) reaches operating temperatures of 100–120 °C. In research and development projects, the operating temperatures can even reach 150 °C. In addition, the oxygen pressure in this application is 35–40 bar. This means that the sealing materials are exposed to strong oxidation with the resulting accelerated material aging. EPDM has proven to be reliable in this area.

Thermal resistance

The sealing materials must also meet the strictest requirements in terms of temperature resistance. Environmental conditions range from <20 to 80 °C during the cold start of a PEM electrolyzer to >800 °C in SOEC applications. Furthermore, tempering processes must be optimized to prevent what is known as fuel cell poisoning. Seal geometry is another fundamental factor in ensuring perfect performance. The seals must pass compression set tests that take all load cases into account, including thermal cycles.

Material and design recommendations

PEM-Stacks

  • FKM is ideal for O2 pressure resistance.
  • EPDM/FKM compounds are used in overmolded frames (bonded systems) for reliable assembly and sealing.
  • Good to know: There are integrated stack frame seals with an L/C cross section that improve barrier performance under operating pressure. During assembly, the cross section has an innovative L-shape. When the electrolyzer starts operating, its internal pressure rises and activates the seal, which then takes on a C-shape. The increases the seal’s contact pressure and turns it into a reliable barrier for gases and liquids.

 

AEM-Stacks

  • Material purity and membrane compatibility are the priorities here.
  • EPDM has proven effective in the alkaline environment of AEM stacks.

 

AEL-Stacks

  • PTFE (polytetrafluoroethylene) and ePTFE seals (expanded PTFE) are resistant to the challenging continuous contact with oxygen and potassium hydroxide.
  • Under suitable framework conditions, EPDM is a sustainable alternative.

Processing: Compression, purification, and conditioning

Components and parameter spaces

Reciprocating compressors (H2)

Reciprocating compressors operate in pressure ranges of 50–500 bar and 350–1000 bar. Small units are designed for a service life of more than 1,000 hours, whereas large units are expected to provide full performance for a period between 8,000 and 24,000 hours. The sealing systems used here (piston rings, rod seals, packings) must be low-wear and absolutely leak-tight and can already function without using PFAS.

Scroll compressors (compressors and vacuum pumps)

Scroll compressors and scroll vacuum pumps operate in pressure ranges of 1–2 bar and a temperature range of -35 to 110 °C. These traditionally use scroll seals (tip seals) made of special PFTE. However, there is a trend towards finding alternative PFAS-free materials.

Tribology and wear

Comprehensive test methods are required to minimize friction-driven interactions between surfaces in compressors, purification systems, and H2 conditioning processes. These methods are used to test
and optimize coefficients of friction and linear wear under realistic conditions. In these applications, fiberreinforced thermoplastics such as PEEK (polyether ether ketone) and PPS compounds (high-performance thermoplastics) offer low friction and therefore low wear.

Transport and storage: Gas, LH2, and refueling stations

Components and parameter spaces

Gas and high-pressure storage

These elements of the H2 value chain are subject to the requirements in UN Regulation R134 with regard to the safety-related properties of hydrogen- and fuel-cell-powered vehicles (HFCVs) and their components. The applications involve O-rings, back-up rings, and valve tappets made of EPDM, FKM, and NBR (acrylonitrile butadiene rubber) that have proven resistance to RGD (rapid gas decompression).

H2 liquid gas transport at -253 °C (cryogenic)

It is crucial for the materials and coating systems used here to remain ductile even at very low temperatures. For this reason, magnet systems and couplings require hydrogen-compatible surfaces that resist embrittlement as effectively as possible.

Refueling stations and refueling

Hydrogen refueling stations and refueling operations involve rapidly changing pressure profiles. RGD-safe elastomers that provide low permeation are used here as well. Regular maintenance intervals should be specified to ensure smooth system operation.

Utilization: fuel cells and hydrogen internal combustion engines (H2 ICE)

Components and parameter spaces

Fuel cells – AFC, PEMFC, PAFC, DMFC, MCFC, SOFC

The six types of hydrogen fuel cells listed above are currently used worldwide. Their differences lie in the gases used, the types of electrolytes, the operating temperature, and their performance capacities. They can be adapted to different requirements. The operating temperatures for LT-PEMFC are 60–80 °C and for HT-PEMFC they are 120–180 °C. PAFC operates at 160–220 °C, MCFC at 620–650 °C, and SOFC at 800–1000 °C. Stack seals made of EPDM, FCPO, FKM, and LSR are used in these applications. It is important to select the materials according to the following properties: their compression set, permeation, and media resistance.

Good to know: For optimum service life and performance of the fuel cells, integrated seals at the interface between the bipolar plate and the gas diffusion layer improve the removal of liquid water from the reaction zones.

Hydrogen internal combustion engine (H2-ICE)

The key advantages of manufacturing H2 internal combustion engines are the robustness of the technology and the rapid market introduction. All the same, the following challenges must be resolved to ensure that these drive system operate safely: NH3 outgassing, the use of lubrication systems, and abnormal combustion. It is crucial to consider the prevailing temperature range and interaction of all the above-mentioned media when selecting the appropriate sealing systems.

Which seals are required and where?

  • Basic functions and installation points in the stack

    Frame seals and flat seals are installed between the cell frames and the bipolar plates. They are used for gas and electrolyte sealing, electrical insulation, pressure retention, and to ensure the purity of the socalled H2/O2 separation. The seals are typically manufactured as molded elastomer parts – for example from EPDM or FKM. At this time, flat seals made of PTFE and ePTFE are also used, particularly in alkaline environments.

    Sub-seals, edge seals, and seals on the substrate provide sealing near the edges of the membrane electrode assembly (MEA), as well as on plastic frames and metal plates. Numerous variants are available for these seals: seals on plastic frames or on bipolar plates, and loose seals that are edge-bonded, overmolded, or press-fitted. O-rings and extruded sealing profiles are also used here. The selection always depends on the automation level, available installation space, surface pressure, and the specified service life of the components used.

    Special high-pressure seals (PEM) are also used in PEM stacks. These seals can be groove-clamped and provided with retention features, for example. They have an L-to-C activating cross section that ensures secure positioning and pressure activation for increasing stack pressures.

    Technology-dependent selection criteria

    In PEM electrolysis, the pH value is 4–6, the oxygen side is oxidative, the temperature range is 60–120 °C and there are pressures of up to 35 bar. The use of FKM is recommended here due to its low H2 permeation as well as its O2oxidation resistance and temperature stability on the stack side. PTFE is normally used in areas with low bolt load and high purity requirements. During the installation, it is important to ensure the utmost cleanliness to minimize ion leaching.

    The requirements placed on sealing solutions in AEL and AEM electrolysis are defined by potassium hydroxide solutions of 3–30 percent, temperatures of 60–150 °C, and pressures of up to 35 bar. The prevalent material groups here are EPDM because of its KOH resistance and PTFE/ePTFE because of its chemical resistance. If necessary, electrical insulation can be optimized by using additional frames or suitable sealing materials.

    Key design recommendations

    • Balance the clamping force against the internal pressure. Sealing grooves provide a force bypass and reduce overcompression and extrusion compared with flat seals that are flush with the flange face.
    • Profiled elastomer cross sections significantly reduce the required bolt load – unlike so-called slab geometry.
    • Perform a finite element analysis (FEA) and tests to verify the tolerance and pressure resistance.
    • Consider seals early on and plan for them before defining the bipolar plate tool. The geometry, suitable material, purity, possible leaching, and the potential degree of automation all have to be decided on.
    • Good to know: Standard parts are rarely sufficient – application-specific compounds and designs are generally required.

  • Compressors

    Packing seals made of graphite-filled PTFE are used in reciprocating and diaphragm compressors. To reduce leakage at rods and shafts, low-contact labyrinth seals are installed. PEEK back-up rings are used as extrusion protection devices at high pressures.

    In turbo and centrifugal compressors, contactless dry gas seals have proven effective due to their low leakage rates and long service life.

    As a general rule, the combination of materials and design in sealing solutions is decisive for the performance of the compression processes.

    Pumps and pump peripherals

    Mechanical seals that correspond to the relevant process media are used in these applications. In addition, gland packings made of graphite are required for rotating units. The seals withstand water and coolants as well as potassium hydroxide solution and CO2-containing streams. Graphite packings and laminates have also proven effective at elevated temperatures and pressures.

    Storage and transport

    In addition to permeation, temperatures between -40 and +85 °C and pressures of up to 105 MPa define the operating parameters for compressed gaseous hydrogen. Depending on the medium and temperature, cover seals, O-rings, and O-ring/back-up ring combinations made of EPDM, FKM, and HNBR are installed here. These applications also employ springenergized PTFE lip seals and metal seals.

    Liquid hydrogen (LH2) is stored and transported in cryogenic form at -253 °C. Elastomers are not suitable materials in this temperature range. Metal and plastic seals made of PTFE are preferred.

    Pipe and fitting interfaces must be sealed to be ultratight, highly pure, and resistant to high pressure. Stainless steel and nickel alloys are utilized here in the form of VCR® fittings and metal face seals. Cone and thread fittings are also ideal for very high pressures. They provide metallic sealing and have low diffusion.

  • Sealing solutions for Power-to-X processes (PtX)

    Syntheses and media

    In the various processes for the production of ammonia (NH3), methanol, e-fuels, and methane through methanation (CO/CO2), the conditions sometimes involve high temperatures and pressures as well as corrosive environments. This is where static high-performance flat gaskets are used. The highly corrosive process sections require the use of PTFE-based sealing materials. Spiral-wound seals, camprofile serrated gaskets, and flexible graphite in the form of graphite laminates have also proven effective here.

    Material selection

    The process chemistry is the main determining factor for the PtX relevance of the materials used in these applications. Graphite is chemically stable even at high temperatures. PTFE and modified PTFE (PTFE/mod) offer universal chemical resistance as well as good creep and set levels in specialty grades.

  • Requirements and standards

    ISO 19880-1:2020 defines the minimum requirements for the design, operation, inspection, and service modalities of hydrogen refueling stations equipped with H35 and H70 dispensing nozzles. At these stations, vehicles can be refueled within a few minutes utilizing pre-cooling down to -40 °C.

    Valves and safety elements for H70 dispensing nozzles

    ISO 19880-3 defines the applicable testing and safety requirements for check valves, overflow valves, shutoff valves, and safety valves, as well as breakaway couplings. The sealing concepts in these applications often incorporate metallic sealing elements and RGDresistant high-performance elastomers.

    Dispenser hoses and couplings

    ISO 19880-5 and ANSI/CSA HGV 4.2 define the internationally applicable safety requirements for highpressure hoses with low permeation that are used at -40 °C. Metal-sealing interfaces are preferred here.

Is there a summary table for applications and typical use cases along with suitable material classes and seal designs?

There is now – and it addresses the question of which seal designs optimize the safety, durability, and efficiency across the entire hydrogen process chain.

The dynamic evolution of technologies for hydrogen generation has been accom panied by a sharply rising learning curve for materials and seal designs. Due to outstanding achievements in materials research and specialized H2 seal designs, many established materials have evolved into highly safe precision sealing solu tions. Advances in production and testing methods are making them increasingly sustainable, durable, and powerful. Which combinations are especially recom mended?

H₂ Applications – Sealing Solutions
Application Typical operating conditions Material class and sealing solution
PEM electrolyzer – stack (O₂ side) pH 4–6 (pure water, possibly slightly acidic),
T < 60–150 °C, p up to ~35 bar
high oxidation and cleanliness requirements 		FKM as a stack seal (O₂ resistance, low H₂ permeation)
Seal on plastic or metal frame/plate
Seal on bipolar plate (overmolding)
PEM electrolyzer – high-pressure seal high internal pressures, secure groove position,
activation under load 		Innovative seal profiles (L-shape) with retention features (press-in-place – PIP)
or overmolded onto a carrier
AEL and AEM electrolyzer – stack (KOH environment) 3–30 wt.% KOH
T < 60–150 °C
p up to ~35 bar
electrical insulation depending on system design 		EPDM seals with retention features (PIP) or overmolded onto a carrier
produced by injection molding or extrusion
PTFE flat seals or films with low creep tendency
seal on plastic or metal frame (depending on automation)
Electrolyzer – peripheral and flange connections low bolt load, purity,
electrical insulation, variable T/P 		EPDM and FKM O-rings
PTFE flange seals
CGH₂ storage and transport (tanks and pipelines) T ≈ -50…+85 °C
p up to ~105 MPa
permeation to be minimized 		O-rings (EPDM/FKM/HNBR depending on medium and temperature)
depending on application, with plastic back-up rings (PEEK, etc.)
Ultra-high-purity and high-pressure connections UHP requirements (clean, low diffusion)
vacuum (pressure, very low leakage) 		VCR® metal face seal (stainless steel/nickel alloys)
metal-to-metal sealing for UHP lines and components
H₂ compressors – reciprocating and diaphragm high pressure, dynamic movement,
wear, leakage minimization 		packing seals (PTFE-filled, graphite)
labyrinth seals, PEEK back-ups for extrusion protection
H₂ compressors – centrifugal continuous rotation, high speed,
low-energy sealing 		dry gas seals (non-contact, thin gas film)
very low leakage, low wear
Pumps and valves in PtX processes corrosive media, high T/P,
changing loads 		mechanical seals, graphite packings,
static spiral-wound seals/camprofile
serrated gaskets, PTFE depending on medium/T
PtX syntheses (NH₃, methanol, e‑fuels) aggressive chemicals
thermal cycles, high pressures 		graphite laminates/spiral-wound/camprofile serrated gaskets
modified PTFE (chemical resistance, set behavior)
Threads and screwed connections (peripherals) Up to 100 % H₂
reliable leakage protection at pipe threads 		hydrogen-ready thread sealants (e.g. LOCTITE 55/567/577)
Material behavior – selection guidance Permeation (H₂ diffuses strongly)
RGD for rapid pressure changes 		FKM tends to have lower H₂ permeation than EPDM/NBR
HNBR/FKM/FFKM compounds show RGD resistance
profile cross sections/sealing grooves reduce assembly force and extrusion

The Hydrogen Rainbow

All hydrogen is not the same. Although it is colorless in its natural form, we differentiate it into seperate color classes based on its mode of production. Whether it is green, yellow or gray, we explain what it stands for.

Which materials are suitable for each type of seal?

The material selection always follows the medium–temperature–pressure cycle and takes the service life and purity into account. The following rule-based guidance does not claim to be exhaustive.

Elastomers for O-rings, frame seals, molded seals, and profilese

EPDM: resistant to KOH and water, good low-temperature flexibility; suitable for AEL/AEM and for peripherals in the electrolysis environment; has higher H2 permeation than FKM. Caution is required in direct contact with oxygen.

FKM: withstands O2 oxidation and high temperatures; resistant to deionized water (DI water) and acidic environments; suitable for PEM stacks and peripheral valves and seats; low H2 permeation. RGD-resistant materials are available for applications with pressure fluctuations.

HNBR/NBR: good mechanical strength; widely used for gases; oil-resistant. RGD-resistant materials are available for applications with pressure fluctuations.

FFKM: maximum chemical resistance; suitable for critical PtX process steps and for high temperatures.

Fluoropolymers and thermoplastics for flat seals, sealing lips, and back-ups

PTFE, ePTFE, and modified PTFE: nearly universal chemical resistance; prone to creep, should be placed in main load area; low permeation. This makes them ideal for KOH, CO2, and flange seals in stacks and fittings.

PEEK: ideally suited for back-up rings and spring energizers. These thermoplastics offer high rigidity and temperature resistance. At the same time, they provide good extrusion protection in high-pressure applications.

    Which seal geometries are needed where in hydrogen technology?

  • Here the selection depends on the degree of assembly automation, the production volumes, surface pressure, and tolerance conditions. Sealing profiles for frames and bipolar plates, PIP seals, O-rings, and extruded profiles are usually installed here. Profiled elastomer cross sections reduce the assembly effort. In PEM electrolysis, high-pressure geometries with retention features and pressure-activating cross sections provide optimum performance.
  • In these applications, the focus is on spiral-wound seals and camprofile serrated gaskets made of graphite laminate. In addition, PTFE flat seals are used for high temperatures and pressures as well as for corrosive media.
  • Metallic sealing profiles with high dimensional accuracy ensure optimum sealing performance here. VCR® metal seals and RTJ, C-ring, and E-ring seals deliver minimal leakage and permeation rates.
  • Contact-free dry gas seal profiles are proven sealing solutions for this type of compressor.
  • These applications use packings, labyrinth seals, and PEEK back-ups.

  • Spring-energized PTFE lip seals are prevalent in the high-pressure and low-temperature environments of these applications. They are supplemented by EPDM, FKM, and HNBR profiles with back-ups.

  • Metallic PTFE-seated valves that comply with ISO 19880-3 are suitable for these application areas, as are hose systems designed for use at temperatures as low as -40 °C.

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New production methods for seals in the hydrogen and electrolyzers sector

Experimental curiosity is accelerating the development of forward-looking solutions, especially in the energy sector. This inventive spirit also fuels continuous advancement in materials, manufacturing processes and sealing solutions at Freudenberg Sealing Technologies. Because one thing is clear: Seals are the decisive factors that enable efficiency, safety, and predictable life cycle costs in hydrogen systems. They are the technology drivers that make it possible to achieve the targeted cost and scaling effects.

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Standards and Regulations

When it comes to hydrogen technologies, the standards landscape is currently undergoing dynamic development. Many standards are being revised or newly developed at this time. In general, global norms and standardization are essential for the hydrogen economy to ensure the integrity and performance of sealing systems under widely varying pressure and temperature conditions.

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Renewable Energies Energy Generation
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