Sealing solutions for hydrogen systems are in high demand, and that should come as no surprise. According to Statista, the world has spent close to $600 billion U.S. dollars in direct investments in hydrogen projects involving key systems such as fuel cells, storage, electrolyzers, transportation, and industrial use.
There is, however, a major challenge with hydrogen: while it has excellent potential as a clean energy carrier, its physical properties make it difficult to seal.
This blog post looks at how selecting the right sealing material is one of the most consequential decisions in hydrogen system design.
What Makes Sealing Solutions for Hydrogen Systems So Challenging
There are several reasons that seals for hydrogen systems can be demanding, beginning with molecular permeation.
Molecular Permeation
The first issue related to hydrogen sealing is molecular permeation. Hydrogen is the smallest molecule (~2.9 angstroms) currently known, and because of that, hydrogen can easily permeate through many materials. This has serious consequences for hydrogen systems, including pressure loss, ignition risk, and fugitive emissions, all of which can be very dangerous. And this eliminates most standard elastomers used for seals from consideration.
Hydrogen Embrittlement
Another problem with sealing solutions for hydrogen systems is hydrogen embrittlement. Atomic hydrogen can diffuse into the metal lattice, thereby reducing ductility and increasing brittleness. Diffusion, in turn, leads to significant problems with fatigue cracking when cyclic pressure loadings are present and has a major impact on the choice of materials for lip material.
Temperature Extremes
Another challenge related to sealing hydrogen systems lies in the temperature range. Hydrogen is a liquid at -423°F (-253°C), the temperature at which most elastomers become brittle and lose their sealing force. However, cryogenic temperatures are not all that involved. High temperatures arise with reformers, fuel cell stacks, and hydrogen combustion, where temperatures can easily exceed 392°F (200°C). Very few materials can maintain consistent properties across this full range of temperature.
Rapid Gas Decompression
Rapid Gas Decompression (RGD) can dissolve into elastomers under pressure. When rapid depressurization occurs, the absorbed gas will rapidly expand internally and lead to cracking, blistering, and catastrophic seal failure. This problem is especially relevant in fueling station dispensers, where pressures can cycle between 350 and 700 bar.
Purity
There can also be chemical purity requirements: fuel cells and electrolyzers are sensitive to contamination from seal extractables. In addition, standard seal compounds often contain plasticizers, fillers, or cure residues that degrade membrane performance. Because of this, the seal materials chosen must be specifiable as non-contaminating or, where applicable, FDA/USP-compliant.
Optimal Polymer Materials for Hydrogen Service
Three specific engineering polymers work extremely well in hydrogen-related applications. These materials are PTFE, PEEK, and UHMW-PE.
PTFE
The most common choice for hydrogen sealing is PTFE, or polytetrafluoroethylene. This material is known for its near-zero permeability, excellent chemical inertness, and a 200°C to +260°C service range. In addition, it is available in a non-contaminating grade with no extractables or plasticizers, making it an excellent choice for fuel cell and electrolyzer environments. PTFE also has an extremely low coefficient of friction, which extends its dynamic seal life in applications with compressors and actuators. It is also self-lubricating, making it an ideal option for applications where lubricants cannot be used. However, PTFE does have a limitation to consider: it tends to cold-flow under sustained compressive loads.
There are also filled grades of PTFE that can improve key properties. These fillers include glass fiber, carbon, graphite, and PEEK-filled PTFE grades. These can increase stiffness and improve creep resistance at the cost of a slight reduction in its chemical purity. Filled grades are recommended for static seals under high sustained loads, while virgin PTFE works best for purity-critical dynamic applications.
PEEK
PEEK, which stands for polyetheretherketone, is another engineering plastic that works extremely well in sealing solutions for hydrogen systems. It offers excellent compressive strength, making it suitable for high-pressure applications. Another benefit of PEEK is its excellent chemical resistance. It also exhibits good resistance to hydrogen permeation and maintains tight tolerances when subject to thermal cycling. In addition, it possesses a low coefficient of friction (though not as low as PTFE). The most common applications for peek include backup rings (BURs), valve seats, and structural seal components.
UHMW-PE
UHMW-PE, or ultra-high-molecular-weight polyethylene, is one of the very few polymers that can retain their ductility at liquid hydrogen temperature. In fact, it is the standard choice for liquid LH2 seal applications in areas such as liquefaction and aerospace. UHMW-PE has a very low coefficient of friction and excellent impact toughness. It is limited at elevated temperatures, however, and should be viewed as a cryogenic-specific material in hydrogen systems.
Conclusion
From cryogenic LH2 transform to PEM electrolysis, engineering polymers are available to provide excellent seal performance in hydrogen applications. These materials can address issues such as molecular permeation, hydrogen embrittlement, extreme temperatures, rapid gas decompression, and purity. To learn more, contact Advanced EMC and allow them to put their 100+ years of combined expertise to work for you.
