The crucial role of PTFE seals in electrolysers

Test stack with 100 seals (Image source: Teadit)

Hydrogen production by means of electrolysis has become a key technology in the energy transition. Choosing the right seals is crucial to ensure that electrolysers operate reliably, efficiently and safely. They prevent leaks, ensure the separation of gas chambers and contribute significantly to the service life of the entire system.

Polytetrafluoroethylene (PTFE) has established itself as the preferred sealing material. It impresses with its high chemical resistance, mechanical stability, and excellent electrical insulation properties. In alkaline electrolysers, PTFE seals achieve service lives of up to three decades without significant material degradation. Alternative materials have not yet been able to demonstrate comparable long-term stability under extreme operating conditions. Particularly relevant here are structured PTFE variants, which exhibit increased dimensional stability and a lower tendency to creep due to the addition of fillers, as well as expanded PTFE with its characteristic microporous structure, which enables high compressibility and elasticity. These properties make it particularly suitable for sealing uneven surfaces.

Hydraulic press with 4 seals and 3 metal plates between the seals. (Image source: Teadit)

Different types of electrolysers have different requirements. While alkaline systems work with KOH or NaOH electrolytes and therefore require high chemical resistance, PEM electrolysers with proton exchange membranes require very thin seals with low screw preload. Structured PTFE has proven itself here, as it seals reliably despite its low seal thickness. In alkaline systems, on the other hand, expanded PTFE is preferred because it retains its sealing function over the long term, even under high pressure and at elevated temperatures. In both cases, the seals must not only be chemically compatible, but also ensure mechanical stability, temperature and pressure resistance, and electrical insulation.

Mechanical stresses and creep behavior

To investigate the actual behavior of seals in stacks, a special test bench was developed that simulates operation with up to one hundred seals simultaneously. The results show that conventional individual tests do not adequately reflect the actual behavior in the stack. Creep and relaxation behavior is much more pronounced when many seals are involved. The greatest stress losses were observed in the first hours of operation, which is why retightening, or the use of disc springs is necessary to keep the preload stable over time. Based on the results, models such as the viscoelastic Burger model could be adapted to enable the prediction of stress losses and support the design of live-loading systems.

Results of material flow for different materials and seal thicknesses. (Image source: Teadit)

In addition to mechanical properties, chemical resistance is also of central importance. Tests with rPTFE-B2 in potassium hydroxide solution at elevated temperatures showed optical changes such as darkening and slight adjustments in compressibility, but no significant loss of mass or strength. rPTFE-F remained completely stable, while expanded PTFE is generally considered to be chemically resistant. These results illustrate that the choice of fillers is crucial for the behavior of seals in electrolysis operations.

Safety and installation during operation

Another critical aspect is installation. With several hundred seals in the stack, compression leads to considerable time expenditure and potential problems due to intrusion, i.e., the penetration of sealing material into flow channels. Pre-sealed ePTFE variants have proven to be particularly advantageous in this regard. They reduce installation time by up to seventy percent while minimizing material intrusion without compromising tightness. In this way, they make an important contribution to increasing operational efficiency.

Safety also remains of central importance. Leaks within the stack, for example due to hydrogen crossover, pose a considerable risk. Even small amounts of hydrogen in the oxygen chamber can cause catalytic combustion and hot spots. Therefore, the strictest requirements apply to tightness, with the permissible leakage rate usually only between 0.01 and 0.1 percent of total production. Tests with helium as the test medium confirmed that PTFE-based seals reliably meet these requirements as long as the surface pressure in the stack is kept constant.

In summary, it can be said that choosing the right seal is crucial for the efficiency, safety, and service life of electrolysers. PTFE-based solutions have established themselves as the standard. Structured PTFE is particularly suitable for PEM systems, while expanded PTFE is ideal for alkaline electrolysers. Further developments such as pre-sealed ePTFE-RGD further improve ease of installation and operational safety. It is crucial for the future that seals are not only evaluated based on individual material characteristics but are also tested in realistic conditions in the stack. This is the only way to develop reliable and durable solutions that support a sustainable and safe hydrogen economy.