Ion exchange membrane
The ion exchange membrane is the thinnest possible film that effectively separates the two media partners H2 and O2 from each other, but should allow ions to pass through as easily as possible (internal resistance). However, a thin film is mechanically unstable and can have excessive porosity. In many cases, the basic structure is made of ePTFE, which is either supported or reinforced.
The catalyst coating on the anode and cathode side is based on a carbon mixture with tiny amounts of elements from the platinum group (e.g. 0.1 mg/cm² Pt).
Damage mechanisms in PEMFCs
1. Membran-Degradation:
The polymer membrane (PEM) is susceptible to chemical and mechanical degradation processes, which can lead to a decrease in proton conductivity and cell performance.
Causes:
- Oxidative degradation reactions caused by free radicals
- Hydrolysis by water
- Mechanical stress due to pressure and temperature changes
- Ermüdung durch Quellverhalten bei wiederholtem Austrocknen
2. Corrosion:
Corrosion can attack various metal components in the PEMFC, such as the bipolar plates and the current collectors. This can lead to a loss of electrical conductivity, damage to the catalysts in the cells and structural damage.
Causes:
- Contact with acidic media
- Electrochemical corrosion
3. Water management
Inadequate water management can lead to flooding or drying out of the proton exchange membranes. This can impair cell performance and service life.
Causes:
- Imbalance between water production and consumption
- Insufficient gas diffusion
4. Temperature gradients
Inhomogeneous temperature distributions in the PEMFC can lead to thermal stresses and strains. Due to the relatively low efficiency of converting chemical energy into electricity, a fuel cell stack heats up under load. This can damage the membrane and other components.
Causes:
- Uneven distribution of heat sources
- Insufficient cooling
5. Catalyst degradation
The catalysts on the anode and cathode play an important role in the electrochemical reactions in the PEMFC. They can be "poisoned" by various factors. This results in reduced cell performance or even total failure
Causes:
- Poisoning by sulphur compounds and carbon monoxide
- Poisoning by cations of various metal salts
- Washed out components and degradation products of seals, films or adhesives.
- Sintering at high temperatures (reduced contact surface)
- Ostwald maturation, in which small catalyst particles grow together to form larger particles
- Impurities in the air flow
Measures
Specific measures can be taken against all of these influencing factors. Humidity is regulated by humidifiers, the thermal load on the stack is reduced by suitable thermal management and corrosion damage to metallic bipolar plates is avoided by applying corrosion-resistant coatings. The use of high-performance polymers also ensures a particularly long service life for the membrane.
In order to reduce the costs of a fuel cell stack, considerable efforts have also been made in recent years to reduce the content of catalyst metals. In principle, the particle size is being reduced further and further, which increases the usable reactive surface area. However, these catalysts are very sensitive to certain compounds that accumulate and reduce reactivity. This means a loss of performance and possibly even extensive failure.
Reduction in cell performance due to catalyst poisoning
Poisoning of the CCM is very easily caused by leached components in the MEA. The MEA (membrane electrode assembly) is the overall unit of a cell consisting of the bipolar plate, gas diffusion layer, MPL, the CCM and the subframe (if one is used). Each of these material layers can contain components that can be washed out by the constant material flow (hydrogen, air, water).
In a PEM fuel cell (PEM electrolysis cell), polymer compounds are exposed to particular stresses. This is particularly true in combination with polar media such as water. The high operating temperature of almost 100 °C and the strong polarity of the water molecule stress the plastic used through hydrolysis. This can lead to the degradation of the polymer compounds and the formation of harmful degradation products.
Hydrolysis-stable products for sealing and subframes from CMC
To meet these challenges, it is important to use hydrolysis-stable products for sealing (protection against crossover and leakage of the stack) and the sub-frame. With its product CMC 61145, CMC Klebetechnik offers a thin, electrically insulating adhesive film that has been specially developed for such applications.
Stability of the special adhesive
Extensive tests prove the stability of the special adhesive.
The special adhesive used for the adhesive coating has proven to be very stable in extensive tests. The tests included the following conditions:
- Storage under high hydrolysis stress
- Storage in solvents (DMFC)
- Storage in deionized water with a very low pH value
- The aim of the tests was to determine the extent to which the adhesive strength degrades and how intensively ions and other degradation products are washed out. The results of the tests showed that the special adhesive from CMC Klebetechnik has a very high resistance to hydrolytic stress.
Advantages
Advantages of the adhesive used and the CMC 61145 adhesive film:
- Thin and electrically insulating
- Specially developed for applications with high temperatures and polar media
- Very high hydrolysis-stable adhesive strength
- Resistant to solvents and deionized water with a low pH value
- Good adhesion to the ion exchange membrane (e.g. ePTFE) and good sealing
Conclusion
The CMC 61145 adhesive film from CMC Klebetechnik is an excellent solution for PEM fuel cells or PEM electrolysis cells. The high, hydrolysis-stable adhesive strength of the adhesive used and the resistance to solvents and deionized water with a low pH value make the adhesive ideally suited for fuel cell applications that must be particularly durable, such as in electric buses, in heavy goods transport or for decentralized power supplies. Instead of the PEN film used for price reasons, more hydrolysis-stable films such as PPS, PEEK or PVDF can also be used.
Contact us
If you have any questions, please contact:
Gerald Friederici, Application Technology
Phone: +49 6233 872 356
E-Mail: friederici@cmc.de
Further information
Further information and products for hydrogen technology can be found here:
Data sheet: CMC 61145 (German)
Further articles on application technology
Damage mechanisms in fuel cells using the example of subframes