Blog
Rubber Seals for Hydrogen Fuel Cells
May
The rubber gaskets inside a hydrogen fuel cell or electrolyzer stack are among the most technically demanding sealing applications in the emerging clean energy supply chain. They must simultaneously seal hydrogen and oxygen on opposite sides of an ion exchange membrane at pressures up to 350 bar in high-pressure electrolyzers, resist the acidic ionomer chemistry of the membrane itself, maintain extremely low gas crossover permeability to prevent explosive hydrogen-oxygen mixing, survive tens of thousands of wet-dry thermal cycles, and do all of this without leaching any compound that would poison the platinum catalyst that makes the electrochemical reaction work. Standard catalog compounds cover none of these requirements adequately. Getting the elastomer selection correct in a fuel cell or electrolyzer is not a routine engineering decision.
Why Hydrogen System Sealing Is a Distinct Engineering Problem
Hydrogen is the smallest molecule in chemistry. At the atomic scale, it permeates through materials that contain larger molecules effectively. The hydrogen permeability of a rubber compound, measured in cm³·cm/(cm²·s·cmHg), is a governing design parameter for fuel cell and electrolyzer gaskets in a way that permeability is never a concern in conventional industrial sealing. A gasket material with high hydrogen permeability allows gas crossover through the gasket body from the hydrogen side to the oxygen side of the stack. In a PEM fuel cell, hydrogen-oxygen mixing above the lower flammability limit at a hot catalyst surface creates a safety hazard. In an electrolyzer, hydrogen crossover into the oxygen stream creates a gas purity problem that reduces efficiency and imposes safety limits on operating pressure.
The gasket must also resist the internal chemical environment of the stack, which is significantly more aggressive than the temperature numbers suggest. PEM fuel cell membranes are perfluorosulfonic acid (PFSA) ionomers operating at 60-90°C and producing a strongly acidic, high-humidity internal environment (effective pH approximately 1-2 at the membrane surface). The oxygen reduction reaction at the cathode produces hydrogen peroxide as an intermediate product, and Fenton-type reactions in the presence of metal ion contaminants can produce hydroxyl radicals that are highly aggressive toward organic polymer materials. Any elastomer used in contact with the PEM internal environment must resist sustained exposure to hot dilute sulfuric acid, hydrogen peroxide, and radical oxidative attack.
The catalyst poisoning constraint is equally critical and almost entirely absent from general elastomer selection literature. PEM fuel cells and electrolyzers use platinum group metal (PGM) catalysts, typically platinum or platinum-ruthenium alloys at loadings measured in milligrams per square centimeter. These catalysts are extraordinarily sensitive to contamination by sulfur compounds, halide ions, organic compounds, and metal ions. A rubber compound that leaches sulfur-containing accelerator residues, plasticizer molecules, or ionic contaminants into the stack internal environment will progressively poison the catalyst, reducing cell voltage and current density over time in a degradation signature that is difficult to distinguish from membrane degradation or catalyst sintering without careful diagnostic testing.
The consequence of catalyst poisoning is expensive: PGM catalyst loadings represent a significant fraction of fuel cell stack cost, and replacing contaminated catalyst layers requires stack disassembly. A gasket compound that passes mechanical qualification but degrades cell performance through catalyst poisoning may not cause a visible leak failure for years while simultaneously destroying the stack’s electrochemical performance. This is why the extractables and leachables validation requirement for fuel cell gaskets is at least as stringent as for pharmaceutical applications, despite the very different service environment.
Two Electrochemical Technologies, Two Different Sealing Problems
Hydrogen fuel cells and electrolyzers come in multiple technology variants with fundamentally different internal environments. The two that are commercially dominant today, and that present the most relevant sealing engineering problems for rubber compound selection, are PEM (proton exchange membrane) and alkaline systems. They require different elastomers because their internal environments are chemically opposite.
Uses a solid perfluorosulfonic acid (PFSA) membrane as the electrolyte. Operating temperature 60-90°C for fuel cells; up to 80°C for PEM electrolyzers. Internal environment: strongly acidic (effective pH 1-2), high relative humidity approaching 100% during operation, hydrogen on one side, oxygen on the other. Pressure: atmospheric to 50 bar for PEM electrolyzers, up to 350 bar for high-pressure configurations. Primary sealing challenge: acid and radical oxidation resistance combined with catalyst contamination avoidance and very low gas crossover permeability. This is the most demanding sealing environment in commercial hydrogen technology.
Uses an aqueous potassium hydroxide (KOH) solution at 20-30% concentration as the electrolyte, or a solid alkaline anion exchange membrane (AEM) in newer designs. Operating temperature 60-80°C for alkaline electrolyzers. Internal environment: strongly alkaline (pH 13-14 for liquid KOH systems), aqueous with dissolved hydrogen and oxygen, lower pressure than PEM electrolyzers (typically below 30 bar). Primary sealing challenge: concentrated hot alkali resistance. The material compatibility profile is almost opposite to PEM: compounds that resist PEM acid environments may be attacked by KOH, and vice versa. NBR, which has poor resistance to the PEM acid environment, has better resistance to dilute alkaline conditions.
PEM Fuel Cell and Electrolyzer Sealing Zones
The membrane electrode assembly (MEA) frame gasket is the most critical sealing component in a PEM stack. It seals the hydrogen and oxygen flow channels on opposite sides of the bipolar plate, prevents gas crossover around the membrane periphery, and provides electrical isolation between adjacent bipolar plates. It operates in continuous contact with the acidic ionomer membrane and the humid, oxidizing internal stack environment.
Why EPDM has become the standard PEM gasket compound: EPDM’s fully saturated backbone provides good resistance to the dilute sulfuric acid and hydrogen peroxide chemistry of the PEM internal environment. Critically, EPDM does not leach sulfur compounds (provided sulfur-cure systems are avoided and peroxide cure is used throughout), does not contain halides that would poison the platinum catalyst, and does not produce ionic extractables that would increase membrane ionic resistance. Peroxide-cured EPDM formulated specifically for PEM fuel cell service has demonstrated thousands of hours of stable operation without measurable catalyst degradation in validated stack programs.
The formulation matters as much as the polymer: Not all EPDM compounds are suitable for PEM fuel cell gasket service, even though EPDM is generally the correct polymer choice. Standard industrial EPDM compounds contain antidegradants, processing aids, and curative residues that can leach into the stack internal environment and cause catalyst poisoning. PEM-grade EPDM formulations use peroxide cure exclusively, restrict the permitted ingredient list to compounds confirmed non-catalytically active, and are validated against extractables requirements specific to the fuel cell application. Specifying “EPDM per ASTM D2000 BA type” is not sufficient for PEM fuel cell service; the formulation must be specifically validated for the application.
FFKM for demanding PEM applications: Where operating conditions are more aggressive (higher operating temperature, higher oxygen partial pressure at the cathode, higher current density producing more radical intermediates), FFKM provides the universal chemical resistance that eliminates material compatibility as a design variable. FFKM’s essentially zero extractables profile makes catalyst poisoning through gasket leachables a non-issue. The significant cost premium is accepted in high-value PEM stack programs where stack replacement cost dwarfs the gasket material cost.
Why silicone is not used in PEM fuel cells: Silicone rubber appears attractive for PEM fuel cell gaskets due to its temperature range and low extractables in clean water applications. It fails in practice because silicone leaches siloxane oligomers into the stack internal environment. Siloxane compounds adsorb onto platinum catalyst surfaces and form a silica layer under the oxidizing conditions at the cathode, permanently poisoning the catalyst active sites. This is the same mechanism that makes silicone incompatible with fuel cell and electrolyzer sealing despite its excellent performance in food, pharmaceutical, and electronic applications. Silicone is eliminated from PEM fuel cell gasket consideration regardless of its other properties.
The end plates of a PEM fuel cell or electrolyzer stack seal the hydrogen and oxygen inlet and outlet manifolds, coolant connections, and the structural perimeter of the stack assembly. End plate gaskets are exposed to the same internal chemistry as the MEA frame gaskets at the process-side face, and to ambient atmosphere at the external face. They operate under sustained bolt compression from the stack clamping load.
Peroxide-cured PEM-grade EPDM is the standard specification for PEM stack end plate gaskets, for the same catalyst compatibility reasons that make it the MEA frame gasket standard. Compression set is the life-limiting property for end plate gaskets: the stack clamping bolt load maintains uniform contact pressure across the MEA active area, which is critical for uniform current distribution. As end plate gaskets accumulate compression set over stack operating life, the clamping load decreases, contact pressure becomes non-uniform, and stack performance degrades through increased contact resistance and non-uniform gas distribution. Specifying peroxide cure and requesting extended compression set data at 80°C (per ASTM D395 Method B, 168-hour duration rather than the standard 22-hour test) identifies compounds with the long-term compression set stability required for fuel cell stack service.
For high-pressure PEM electrolyzer end plate gaskets operating above 100 bar, FKM is preferred over EPDM for its superior compression resistance and dimensional stability under sustained high compression load. The acid resistance of FKM (particularly bisphenol-cured and peroxide-cured grades) is adequate for PEM electrolyzer internal chemistry, though the catalyst compatibility must still be validated for the specific FKM formulation and cure system before specifying for catalyst-adjacent locations.
PEM fuel cells and electrolyzers require active thermal management to maintain the membrane at optimal operating temperature (60-80°C) and to remove the heat generated by electrochemical inefficiency. The coolant loop circulates deionized water or a deionized water-glycol mixture through cooling channels in the bipolar plates or through a separate cooling circuit adjacent to the stack.
Deionized water imposes a specific constraint: standard cooling system materials and compounds are selected for corrosion inhibitor-dosed glycol coolants. PEM fuel cell cooling circuits use deionized water (resistivity typically greater than 1 MΩ·cm) because ionic contamination of the coolant can provide an electrical leakage path between bipolar plates. Coolant loop gasket compounds must not leach ionic species into the deionized water, which rules out compounds with soluble metal salt crosslinker residues and compounds with ionic surfactant processing aids.
Peroxide-cured EPDM meeting the ionic leachables requirements for PEM coolant service is the standard specification. The same PEM-grade EPDM compound used for the stack gaskets is typically qualified for the coolant manifold seals as well, simplifying the supply chain. Standard commercial EPDM compounds with sulfur-accelerator cure systems should not be used for PEM coolant loop seals due to the risk of sulfate ion leaching into the deionized coolant, which would degrade the coolant’s dielectric properties and potentially contaminate the stack internal environment if the coolant makes contact with the membrane assembly.
High-pressure PEM electrolyzers that produce hydrogen at 100-350 bar for direct pipeline injection or compressed storage eliminate the need for a separate mechanical compression stage, significantly reducing system cost. The pressure rating of the gasket sealing system is the governing engineering constraint for these applications, and it changes the compound selection logic significantly from atmospheric-pressure stacks.
At pressures above 100 bar, EPDM’s relatively low modulus and creep resistance under sustained compression become limiting. The gasket must resist both the compressive load from the stack clamping bolts and the pressurized hydrogen and oxygen trying to find a leakage path through or around the gasket body. FKM provides substantially better compression resistance, lower creep rate under sustained load, and better dimensional stability at elevated pressure than EPDM. The catalyst compatibility of the specific FKM compound must be validated for the high-pressure operating environment, as the increased hydrogen crossover driving force at high pressure makes gas permeability an even more critical gasket property.
For the highest-pressure electrolyzer applications (200-350 bar), FFKM is the appropriate specification. At these pressures, the consequences of gasket extrusion, creep, or permeation failure are severe: hydrogen at 350 bar released from a failed stack gasket creates an immediate safety hazard. FFKM’s combination of the highest modulus among elastomers, lowest gas permeability, universal chemical resistance, and zero catalyst contamination risk makes it the specification of choice for safety-critical high-pressure hydrogen sealing despite its cost. Freudenberg has published work on FFKM overmolded gaskets for high-pressure PEM electrolyzer stack plates specifically for this application.
Alkaline Electrolyzer Sealing: A Different Chemistry, Different Compounds
Alkaline water electrolyzers (AWE) use a 20-30% potassium hydroxide (KOH) or sodium hydroxide (NaOH) solution as the electrolyte at 60-90°C. The strongly alkaline, high-temperature aqueous environment requires an elastomer with good resistance to concentrated hot alkali, which is a materially different requirement from PEM acid resistance.
EPDM for Alkaline KOH Service
EPDM has good resistance to concentrated aqueous alkali including KOH and NaOH at the concentrations and temperatures used in alkaline electrolyzers. Its fully saturated backbone resists the saponification (base-catalyzed hydrolysis) that attacks ester-containing polymers in alkaline environments. Peroxide-cured EPDM is the standard gasket compound for alkaline electrolyzer diaphragm and stack sealing in conventional liquid electrolyte designs.
The catalyst poisoning constraint is less critical in alkaline systems than in PEM systems because the nickel-based catalysts used in alkaline electrolyzers are less sensitive to organic contamination than platinum group metals. This allows a broader range of EPDM compound formulations to be considered for alkaline service than for PEM service, though ionic leachables into the KOH electrolyte should still be minimized to avoid electrolyte conductivity degradation over time.
FKM for High-Temperature Alkaline Service
At the elevated temperature end of alkaline electrolyzer service (above 80°C), and for pressurized alkaline systems operating above atmospheric pressure, FKM provides better compression resistance than EPDM. However, FKM’s alkali resistance requires careful compound selection: bisphenol AF-cured FKM (Type B) has adequate resistance to dilute KOH, but concentrated KOH at elevated temperature attacks standard FKM cure system residues in some formulations. Peroxide-cured FKM grades generally provide better resistance to concentrated alkaline environments than bisphenol-cured grades. Validate the specific FKM compound against 25-30% KOH at 80-90°C before specifying for alkaline electrolyzer service, using ASTM D471 immersion testing in the actual KOH concentration and at the service temperature.
Anion Exchange Membrane (AEM) Electrolyzers
AEM electrolyzers use a solid alkaline anion exchange membrane rather than liquid KOH electrolyte, operating at intermediate conditions between PEM and conventional alkaline systems. The membrane is typically a quaternary ammonium functionalized polymer that is less thermally stable than PFSA PEM membranes, operating at 40-60°C with dilute KOH or carbonate electrolyte. The sealing requirements for AEM electrolyzers are still being developed as the technology matures commercially. Current practice borrows from both PEM and alkaline approaches, with peroxide-cured EPDM as the default gasket material for early commercial AEM stacks, pending longer-term validation data.
Elastomer Selection Summary for Hydrogen Systems
| Application | Technology | Recommended compound | Key constraint | Avoid |
|---|---|---|---|---|
| MEA frame gasket (bipolar plate seal) | PEM fuel cell / electrolyzer | Peroxide EPDM (PEM-grade, validated) | Catalyst contamination avoidance, acid/radical resistance, low H2 permeability | Silicone (siloxane poisoning), sulfur-cured EPDM (sulfate leaching), standard NBR (acid attack) |
| Stack end plate and manifold gaskets | PEM fuel cell / electrolyzer | Peroxide EPDM (PEM-grade) | Compression set at 80°C over stack life, ionic leachables into deionized coolant | Silicone (siloxane contamination risk), sulfur-cured compounds |
| High-pressure electrolyzer seals (100-350 bar) | PEM electrolyzer | FKM or FFKM | Compression resistance and creep at high pressure, H2 permeability, catalyst compatibility | EPDM (insufficient compression resistance above 100 bar) |
| Coolant loop seals (deionized water circuit) | PEM fuel cell / electrolyzer | Peroxide EPDM (PEM-grade) | Ionic leachables into deionized water coolant (resistivity maintenance) | Sulfur-cured EPDM (sulfate ion leaching), standard NBR or neoprene |
| Stack gaskets (liquid KOH electrolyte) | Alkaline electrolyzer | Peroxide EPDM | Concentrated hot KOH resistance (20-30% at 60-90°C), low ionic leachables | Standard FKM without alkali validation, nitrile (KOH attack over time) |
| High-temperature / high-pressure alkaline seals | Alkaline electrolyzer | Peroxide-cured FKM (validated for KOH) | Compression resistance, KOH compatibility of cure system residues | Bisphenol AF-cured FKM: validate against concentrated KOH before specifying |
| Balance-of-plant hydrogen supply seals | All hydrogen systems | EPDM or FKM (pressure dependent) | Hydrogen permeability, temperature, compatibility with specific process fluid | Standard catalog compounds: validate H2 permeability for the specific operating pressure |
Compression Set, Gas Permeability, and the Hydrogen System Service Life Problem
Hydrogen fuel cell and electrolyzer stacks are designed for service lives of 20,000-80,000 hours in stationary power applications and 5,000-8,000 hours in vehicle fuel cell applications. These are substantially longer design life requirements than most industrial sealing applications, and they create a compression set validation challenge that standard ASTM D395 22-hour tests cannot address directly.
A PEM fuel cell gasket compressed in a stack operating continuously at 80°C for 40,000 hours is experiencing approximately 1,667 days of sustained thermal compression. The compression set accumulated over this period determines whether the stack maintains acceptable sealing contact pressure at end of life. Stack rebuilds driven by gasket compression set failure before the membrane and catalyst layers reach end of life represent a significant operational cost that the compound specification is responsible for preventing.
Hydrogen Permeability as a Gasket Design Parameter
The hydrogen permeability of the gasket material determines the rate of gas crossover through the gasket body from the hydrogen side to the oxygen side of the stack. In PEM fuel cells, hydrogen concentrations above 4% in the cathode oxygen stream create a flammability risk. In PEM electrolyzers, hydrogen contamination of the product oxygen stream above defined limits (typically 2% for safety) imposes a maximum allowable operating pressure that limits the stack’s compression ratio and efficiency.
Rubber compounds vary significantly in hydrogen permeability. EPDM has lower hydrogen permeability than silicone or natural rubber, which is one of the reasons it is the preferred PEM gasket compound beyond its acid resistance. FKM has even lower hydrogen permeability than EPDM, which contributes to its selection for high-pressure electrolyzer sealing where the driving force for permeation is highest. FFKM provides the lowest hydrogen permeability of any common elastomer. When specifying gasket compounds for hydrogen systems, request hydrogen permeability data (cm³·cm/(cm²·s·cmHg) at operating temperature) from the compound supplier and confirm the calculated crossover rate is acceptable for the stack design before finalizing the specification.