Silicone Seals in PEM Hydrogen Fuel Cells
Proton exchange membrane (PEM) fuel cells for automotive, stationary power, and hydrogen vehicle applications require sealing solutions that can withstand simultaneous exposure to humidified hydrogen, oxygen, hot water and steam, and the acidic environment (pH 2–4) of the membrane electrode assembly (MEA). The seal system — bipolar plate gaskets, cell frame seals, and end plate seals — must maintain near-zero gas crossover leakage for a hydrogen vehicle lifetime of 5,000–8,000 operating hours, while tolerating approximately 10,000–20,000 on-off thermal cycles between ambient and 80–95 °C.
Standard VMQ (polydimethylsiloxane-based) silicone rubber satisfies most of these requirements well. Its temperature range (−55 °C to +200 °C), elastic compliance, low compression set, and chemical resistance to water, steam, and dilute acids make it the dominant seal material for cells operating at standard conditions (<85 °C, <3 bar hydrogen). For high-pressure hydrogen storage applications and seal segments in direct contact with unhumidified hydrogen gas, fluorosilicone (FVMQ) provides a substantially lower gas permeability and superior resistance to hydrocarbon contamination from ancillary system components.
Key Sealing Requirements by Stack Location
| Seal Location | Primary Medium | Temperature | Recommended Grade | Critical Property |
|---|---|---|---|---|
| Bipolar plate perimeter | H₂ / Air / Coolant | 70–90 °C | VMQ HTV or LSR | Low compression set, dimensional precision |
| MEA frame / subgasket | Humidified H₂ / O₂ | 70–90 °C | LSR 40–50A | Thin cross-section, <100 µm variation |
| End plate compression seal | Coolant (deionized water) | 60–85 °C | VMQ HTV | Chemical purity (no extractables) |
| High-pressure H₂ injector | Dry H₂, 350–700 bar | Ambient–60 °C | FVMQ | H₂ permeability, swell resistance |
| Balance-of-plant (air hose) | Pressurized humid air | 80–120 °C | VMQ HTV | Cost, temperature |
Compression Set and Long-Term Sealing Force
Compression set is the single most critical material parameter for fuel cell gaskets. A bipolar plate stack is pre-loaded to a defined clamp force (typically 1–5 kN per cell for automotive stacks) at assembly. As the elastomer relaxes under sustained compression, the sealing force drops. When the sealing force falls below the critical threshold for the operating differential pressure, hydrogen crossover leakage begins — an irreversible degradation event.
Compression set requirements for automotive PEM fuel cell seals: <10% after 1,000 h at 90 °C (ASTM D395 Method B, 25% compression). Platinum-cured LSR reliably achieves 5–8% under these conditions; peroxide-cured HTV requires post-cure at 200 °C × 4 h to reach equivalent performance. The post-cure step is not optional — it removes peroxide cure byproducts that would otherwise continue to outgas into the MEA and contaminate the platinum catalyst in the electrode, causing irreversible activity loss.
Fluorosilicone for Hydrogen-Contact Seals
Fluorosilicone (FVMQ, based on 3,3,3-trifluoropropyl methylsiloxane) is specified for seal elements in direct contact with compressed hydrogen gas at pressure >70 bar, or in applications where hydrocarbon contamination from compressor lubricants or system materials must be avoided. The rationale for FVMQ over VMQ in these positions is threefold:
Gas permeability: H₂ permeability of FVMQ is 30–50% lower than VMQ at equivalent hardness, because the trifluoropropyl groups create a denser polymer matrix with a higher energy barrier to small-molecule diffusion. In a 700-bar compressed hydrogen storage seal, this permeability difference translates to a factor-of-2 reduction in steady-state hydrogen loss — critical for range and safety certification.
Swell in organic contaminants: even small amounts of lubricant oil vapor entering a standard VMQ seal cause swelling and loss of sealing geometry. FVMQ resists hydrocarbon swell (volume swell <5% in ASTM Reference Fuel C), maintaining seal cross-section dimensions in balance-of-plant environments contaminated with compressor oil aerosol.
Chemical compatibility: the perfluoroalkyl side chain provides FVMQ with resistance to the degradation products of membrane ionomer (sulfonic acid fragments) and glycol coolant that can contact seals at the cell periphery.
ZEMSIL FL-series fluorosilicone is available as HTV gum for compounding or as preformulated sheet for die-cut gasket fabrication. Standard hardness range: Shore 40A–70A. Compression set (ASTM D395B, 22 h/175 °C, 25% deflection) is typically 12–18%, higher than VMQ — a design trade-off that must be compensated with increased initial compression in the joint design.
Extractables, Purity, and Catalyst Compatibility
PEM fuel cell membrane electrode assemblies are highly sensitive to ionic contaminants. Platinum catalyst activity in both cathode and anode electrodes is poisoned by sulfur compounds (from accelerator residues in peroxide-cured silicone), transition metal ions (from cure catalyst residue), and halide ions (from acetoxy-cure silicone byproducts). The preferred cure system for fuel cell seals is addition cure (platinum-catalyzed hydrosilylation) because:
- No ionic byproducts are generated
- No sulfur-containing accelerators are used
- Extractables in deionized water after post-cure are <50 ppm total dissolved solids
ZEMSIL SB-LSR and SB-HTV grades use platinum catalyst systems and are formulated without sulfur-based vulcanization accelerators. Ion chromatography testing of aqueous extracts from post-cured material shows sulfate, chloride, and fluoride all <5 ppm — within the specification required by automotive Tier 1 suppliers for PEM fuel cell seal approval.
Related Reading
Gasket and O-Ring Applications · Electrical Insulation · Low-Temperature Flexibility · ZEMSIL FL Fluorosilicone · ZEMSIL SB-LSR Liquid Silicone Rubber