Hydrogen seals: Why elastomers are becoming a safety factor

The smallest component with the greatest responsibility

Electrolysers, pipelines, fuel cell lorries: when people talk about hydrogen, they are usually referring to large-scale installations. However, their safety often depends on a comparatively small component made of elastomer or high-performance plastic – more specifically, the seal in a tank, valve, compressor or stack. For raw material manufacturers, compounders and processors, this makes it an exciting growth area – albeit one with its own set of rules. A German-Japanese research project demonstrates just how seriously the industry now takes this issue. Within the SusSeal4H2 consortium, the Federal Institute for Materials Research and Testing (BAM), Kyushu University, Freudenberg Technology Innovation and the Japanese seal manufacturer NOK Corporation are investigating sustainable, hydrogen-compatible sealing materials for safe H₂ supply networks. And in May 2026, the partners finally presented their findings at a symposium in Berlin.

NOK made a very specific contribution here: as part of the project, the seal manufacturer compiled, amongst other things, an overview of the properties of various rubber and plastic materials for hydrogen infrastructure. This is intended to make it easier for users to select materials and manufacturing processes in future. This is because, in the run-up to the 2050 climate neutrality target set by the EU and Japan, the hydrogen supply chain at every stage – production, storage, transport and use – requires seals that can withstand high pressures and extremely low temperatures.

Permeation, swelling, decompression: three paths to failure

But what actually makes hydrogen so challenging? “As the smallest molecule, it can easily penetrate the polymer structure and trigger various damage mechanisms there,” explains Dr Géraldine Theiler from BAM’s Energy Carrier Components Division. Added to this is the pressure: in some applications, such as at hydrogen refuelling stations, hydrogen operates at pressures of up to 100 MPa, or 1,000 bar.

Specifically, Theiler identifies three effects that put a seal under strain:

• Permeation: Hydrogen diffuses rapidly through many elastomers. Depending on the material, component geometry and operating conditions, this can reduce the sealing effect and lead to leaks.
• Swelling: When hydrogen molecules become trapped between the polymer chains, the free volume and mechanical properties – such as hardness and elasticity – change.
• Explosive decompression (Rapid Gas Decompression, RGD): Under high pressure, hydrogen dissolves in the elastomer. “If the pressure is reduced rapidly, gas bubbles form in the material; their expansion can lead to microcracks, blistering and, ultimately, seal failure,” reports Theiler.

A BAM study from 2024 also demonstrates that there is no such thing as a universal material. The researchers stored EPDM, HNBR and FKM for up to 100 days at 150 °C in 50 bar of hydrogen. In the experiment, whilst FKM allowed the least hydrogen to pass through, compressed FKM O-rings cracked after 100 days – the authors attribute this to decompression. EPDM, on the other hand, showed the greatest stability when aged in hydrogen, but was significantly more permeable. However, these findings apply only to the selected test conditions and the mixtures investigated – no general conclusion about the material classes can be drawn from them. What is therefore required are materials that combine low permeability, high resistance to decompression and stable mechanical and tribological properties.

The only barrier to the environment

But why is so much at stake with seals in the first place? “In many applications, they are the only barrier between the hydrogen and the environment,” says Theiler – and not just in the mobility sector. At the same time, seals are exposed to high mechanical loads, pressure fluctuations and significant temperature differences. Even localised damage can therefore impair the sealing function and bring entire components to a standstill. Such scenarios are therefore a key consideration in risk analyses. A possible consequence in an emergency: the plant is shut down and the seal replaced – resulting in maintenance costs and downtime.

However, seals subjected to dynamic loads – such as those in compressors, valves and refuelling systems – are particularly critical. This is precisely where tribology comes into play: the study of friction and wear. Theiler’s team has gained an important insight in this regard: Friction and wear behaviour in hydrogen cannot always be deduced from experiments conducted in air. “Materials that exhibit very good friction and wear behaviour under ambient conditions can behave quite differently in a hydrogen atmosphere or at elevated pressure.” According to Theiler, this is due to complex interactions between gas absorption, fillers and additives, as well as the processes taking place at the friction surface. Added to this is a second problem: results from different laboratories are often only comparable to a limited extent. Different testing and evaluation methods can lead to significant discrepancies. “That is why the industry needs harmonised test procedures,” says Theiler. After all, uniform standards form the basis for reliable material selection and accelerate the development of safe, long-lasting systems.

Sustainable and hydrogen-compatible – not a contradiction

Alongside safety, the project also focused on sustainability. Established sealing materials for hydrogen applications include not only EPDM and HNBR but also fluorinated polymers such as FKM and PTFE – one a fluoroelastomer, the other a fluoroplastic, each with its own unique property profile. Although both are considered high-performance materials, they are under pressure due to their carbon footprint and the planned EU-wide restrictions on PFAS – even though the scope and possible exemptions from this regulation have not yet been finalised. SusSeal4H2 has therefore specifically tested alternative polymers – statically and dynamically, in cryogenic conditions and in high-pressure hydrogen. The aim: a significantly lower carbon footprint. And the results are certainly positive.

The project thus demonstrates a promising path towards PTFE-free, high-performance and more sustainable sealing materials. In addition, NOK contributed a life cycle assessment (LCA), which identified the main sources of CO₂ in the manufacturing process. Whether an alternative is actually more climate-friendly can only be reliably assessed through such a life-cycle analysis. At the same time, however, the manufacturer highlighted a structural gap: whilst the Japanese tyre industry is already well advanced in recycling and biomass, there are still technical and legal hurdles to overcome when it comes to seals. Furthermore, the example of Freudenberg Sealing Technologies, the sealing division of the Freudenberg Group, demonstrates that hydrogen seals are by no means merely a laboratory concept any longer. (Project partner Freudenberg Technology Innovation is Freudenberg’s central research unit.) According to the company, it has industrialised several production process chains ready for series production for electrolyser stacks, peripheral equipment and H₂ infrastructure. A high-pressure-resistant seal design for PEM stacks has even already been validated for series production. In addition, Freudenberg’s product range also includes stack seals for fuel cells, as well as static and dynamic seals for valves, compressors and pumps. For manufacturers, the manufacturing processes behind these are of particular interest. Freudenberg injects seals directly onto metallic bipolar plates as well as onto metal and thermoplastic frames. This not only reduces the number of components but also increases the level of automation. The aim in each case is to achieve tight tolerances and low creep – a prerequisite for stack service lives of many tens of thousands of hours.

Despite these advances, questions remain – particularly in applications involving dynamic conditions or extremely low temperatures. This is also reflected in the agenda for the SusSeal4H2 symposium: damage analyses of compressor piston rings, the tribological behaviour of seals in cryogenic and high-pressure hydrogen, and the challenges of liquid hydrogen infrastructure. This is because liquid hydrogen is stored at around −253 °C – a temperature at which conventional elastomers have long since become brittle. To address such issues, BAM relies on realistic component testing and real-world laboratories. There, the safety and reliability of hydrogen technologies are assessed under practical operating conditions.

Will the seal become the bottleneck?

The question remains: could the seal, of all things, slow down the ramp-up? Theiler considers sealing materials to be among the crucial components for the safety of future hydrogen infrastructure. Their failure could not only reduce a system’s performance, “but also have safety-related consequences”.

To ensure it never comes to that, she believes two things are needed above all:

• Uniform testing standards for existing and new materials, enabling robust assessment and comparability.
• Digital monitoring of systems and components to detect damage at an early stage and enhance operational safety.

For the rubber and plastics industry, this ultimately also presents an opportunity. Those who supply materials with low permeation, high decompression resistance and a small carbon footprint – and demonstrate their suitability using recognised methods – will occupy a key position in the energy transition.  This places particular demands on the raw materials sector. Until now, raw materials manufacturers have focused their environmental investments primarily on tyres – specialist rubbers for hydrogen infrastructure have been left behind, as became clear during the discussion at the symposium. Polymers and compounds specifically designed for hydrogen applications and fluorine-free formulations could close precisely this gap.