TLDR

• Reducing Hydrogen Permeability in Thermoplastic Composites
• Challenges of Hydrogen Storage in Mobility Applications
• Experimental Investigation of Thermoplastic Composites
• Enhanced Liner Materials and Optimized Composite Structures

Why Reduce Hydrogen Permeability in Thermoplastic Composites?

Reducing hydrogen permeability in thermoplastic composites is crucial for several reasons, particularly in the context of hydrogen storage and transportation applications. Here are the key points:

1. Safety Concerns Hydrogen is a highly flammable gas, and its leakage can lead to severe safety hazards, including explosions and fires. Ensuring low hydrogen permeability in thermoplastic composites helps prevent such risks by minimizing the potential for hydrogen leaks.
2. Material Integrity and Durability Hydrogen permeation can degrade the mechanical properties of the composite materials. For instance, hydrogen can cause embrittlement, reducing the material's strength and increasing its brittleness, especially at low temperatures. This degradation can lead to the formation of microcracks and other defects, compromising the structural integrity and longevity of the materials used in hydrogen storage systems.
3. Efficiency and Performance High hydrogen permeability can result in significant hydrogen losses, reducing the efficiency of storage and transportation systems. By reducing permeability, the systems can maintain higher levels of stored hydrogen, improving overall performance and efficiency.
4. Economic Implications Leakage of hydrogen not only poses safety risks but also leads to economic losses due to the wastage of stored hydrogen. Ensuring low permeability helps in maintaining the economic viability of hydrogen as a fuel by reducing losses and ensuring that the maximum amount of hydrogen is available for use.
5. Regulatory Compliance There are stringent regulations and standards governing the storage and transportation of hydrogen. Materials used in these applications must meet specific permeability criteria to comply with safety and performance standards. Reducing hydrogen permeability in thermoplastic composites helps in meeting these regulatory requirements.
6. Technological Advancements Research and development in reducing hydrogen permeability can lead to innovations in material science, resulting in the creation of more advanced and efficient composite materials. These advancements can further enhance the capabilities of hydrogen storage and transportation systems, making them more reliable and cost-effective.
7. Environmental Impact Hydrogen is considered a clean energy source, and its effective use can significantly reduce greenhouse gas emissions. By improving the hydrogen barrier properties of thermoplastic composites, the adoption of hydrogen as a sustainable energy source can be accelerated, contributing to environmental conservation efforts

Challenges of Hydrogen Storage in Mobility Applications

Hydrogen storage in mobility applications faces significant technical challenges that impact both efficiency and safety. As the transportation industry moves towards zero-emission vehicles, the demand for effective hydrogen storage solutions grows. Fuel-cell electric vehicles (FCEVs) and other hydrogen-powered applications must address several key issues to become viable alternatives to battery electric vehicles (BEVs).

• High Hydrogen Permeation Rates:
  • Hydrogen's small molecular size makes it highly permeable through many materials, leading to gradual loss of stored hydrogen. This permeation reduces the vehicle's range and efficiency, necessitating frequent refueling.
• Material Degradation:
  • Hydrogen storage systems operate under high pressure (up to 70 MPa), which can cause material degradation over time. Polymer liners and composite structures must maintain their integrity under these conditions to prevent leaks and ensure safety.
• Structural Integrity:
  • Maintaining the structural integrity of hydrogen storage tanks is crucial. Microcracks, induced by mechanical loads or thermal stresses, can create leakage paths. These cracks change the mechanism from permeation to leakage, significantly increasing hydrogen loss.
• Space and Weight Efficiency:
  • Hydrogen storage systems must be designed to maximize volumetric energy density while minimizing weight. Bulky pressure tanks are not ideal for integration into vehicles where space is at a premium. Advanced designs, such as multi-cell and conformable vessels, aim to optimize space usage but introduce additional challenges in maintaining low permeation rates.
• Regulatory Requirements:
  • Compliance with stringent regulations, such as the United Nations Economic Commission for Europe's Regulation No. 134 (UN R134), is mandatory. These regulations set maximum allowable hydrogen discharge rates, influencing the design and materials used in storage systems.
• Cost and Manufacturing Complexity:
  • The development of advanced materials and complex designs often increases manufacturing costs. Cost-effective solutions that do not compromise performance or safety are essential for the widespread adoption of hydrogen-powered vehicles.

Addressing these challenges requires innovative materials and advanced design strategies. The study explored various thermoplastic matrix materials and continuous fiber-reinforced composites, along with high barrier films like EVOH, to identify configurations that reduce hydrogen permeation while maintaining structural integrity. Overcoming these hurdles is critical for the future of hydrogen mobility, ensuring efficient, safe, and cost-effective storage solutions.

Experimental Investigation of Thermoplastic Composites

To address the challenges of hydrogen permeability in storage systems, an extensive experimental investigation was conducted on various thermoplastic composites. The goal was to identify materials and configurations that can effectively reduce hydrogen permeation while maintaining structural integrity under high-pressure conditions. The study focused on a range of thermoplastic matrix materials and their reinforced composites, including PA6, PA12, PA410, PPA, and PPS.

Materials and Methods

1. Material Selection:
   1. Thermoplastic Matrices: The study selected commercially available thermoplastic matrix materials known for their potential in structural applications. These included polyamides (PA6, PA12, PA410), polyphtalamides (PPA), and polyphenylene sulfide (PPS).
   2. Fiber Reinforcements: Continuous fiber-reinforced thermoplastic composites were examined to assess the impact of fiber reinforcement on hydrogen permeability. The fibers provide mechanical strength and potentially reduce permeability by creating tortuous diffusion paths.
2. Liner Integration:
   1. EVOH Liners: Ethylene vinyl alcohol copolymers (EVOH), known for their excellent barrier properties, were applied as liners on PA6 composites. Two types of EVOH with different ethylene contents (24 mol% and 27 mol%) were tested. The EVOH layers were co-consolidated with a surface sealing layer of PA6 to enhance durability and prevent moisture absorption.
3. Manufacturing Process:
   1. Sample Preparation: Circular flat specimens with specific dimensions were prepared for permeation tests. The samples were manufactured using a high-temperature autoclave process to ensure high-quality laminates with minimal porosity and defects.
   2. Sealing Rings: To prevent in-plane leakage, a co-consolidation process was developed, integrating a matrix-compatible unreinforced or short-fiber reinforced polymer ring at the specimen edges.
4. Testing Methodology:
   1. Pressure Tightness Test: Before conducting permeation tests, a pressure tightness test was performed to detect major manufacturing defects. This qualitative test ensured that only leak-tight samples were subjected to permeation measurements.
   2. Permeation Tests: High-pressure hydrogen permeation tests were conducted according to the CSA/ANSI CHMC 2:19 standard. Samples were exposed to a hydrogen atmosphere at 10 MPa pressure in a thermal chamber set to 55°C. The amount of permeated hydrogen was measured using a marker fluid in a graduated capillary tubing system.

Key Findings

• Impact of Fiber Reinforcement: The study confirmed that fiber reinforcement significantly reduces hydrogen permeability compared to unreinforced polymers. This effect is attributed to the low solubility of hydrogen in carbon fibers, which creates longer and more tortuous diffusion paths.
• EVOH Liner Effectiveness: EVOH liners demonstrated excellent barrier properties, significantly reducing hydrogen permeation rates. The 27 mol% ethylene content EVOH (M100) exhibited superior performance, reducing permeation by a factor of eight compared to the PA6 composite without a liner.
• Textile Architecture: Non-crimp textile architectures were found to provide better permeation resistance compared to woven structures. However, manufacturing quality and the presence of microcracks were critical factors influencing the overall performance.

Enhanced Liner Materials and Optimized Composite Structures

The experimental investigation into thermoplastic composites and liner systems for hydrogen storage applications has yielded promising results. The study focused on developing materials and configurations that significantly reduce hydrogen permeability while maintaining the structural integrity necessary for high-pressure environments. Here, we explore the solutions identified through the research.

Key Solutions

1. EVOH Liners:
   1. Superior Barrier Properties: Ethylene vinyl alcohol copolymers (EVOH) were found to be highly effective in reducing hydrogen permeation. EVOH liners were integrated into PA6 composites, resulting in a substantial decrease in permeability.
   2. Ethylene Content Variants: Two types of EVOH with different ethylene contents (24 mol% and 27 mol%) were tested. The variant with 27 mol% ethylene content (M100) demonstrated the best performance, achieving an eight-fold reduction in permeability compared to unreinforced PA6.
2. Optimized Composite Structures:
   1. Fiber Reinforcement: Incorporating continuous fiber reinforcement in thermoplastic composites significantly reduced hydrogen permeability. The carbon fibers create a tortuous path for hydrogen molecules, effectively minimizing diffusion.
   2. Non-Crimp Textile Architecture: Non-crimp textile architectures in PA6 composites provided better permeability resistance compared to woven structures. The absence of fiber crimping reduces potential leakage paths and enhances structural integrity.
3. Advanced Manufacturing Techniques:
   1. High-Temperature Autoclave Process: The samples were manufactured using a high-temperature autoclave process, which ensured high-quality laminates with minimal porosity and defects. This process is crucial for maintaining the integrity of the composite structure under high-pressure conditions.
   2. Co-Consolidation of Liners: The integration of EVOH liners with a PA6 surface sealing layer was successfully achieved through co-consolidation. This method not only enhances the barrier properties but also protects the EVOH liner from mechanical damage and moisture absorption.
4. Multilayer Structures:
   1. Layer Configuration: The study demonstrated that multilayer structures consisting of a base composite layer, an EVOH barrier layer, and a protective PA6 layer are highly effective. This configuration combines the mechanical strength of the composite with the superior barrier properties of EVOH, resulting in an optimized solution for hydrogen storage.

Implementation and Benefits

• Improved Hydrogen Storage Systems: The integration of EVOH liners into thermoplastic composites offers a path to more efficient and safer hydrogen storage systems. These systems can achieve lower hydrogen permeation rates, enhancing the overall performance and safety of hydrogen-powered vehicles.
• Space and Weight Efficiency: The use of high-performance liners and optimized composite structures allows for the development of lighter and more compact hydrogen storage tanks. This is particularly beneficial for automotive and aerospace applications where space and weight are critical factors.
• Cost-Effective Manufacturing: The advanced manufacturing techniques developed in this study, including high-temperature autoclave processes and co-consolidation of liners, are scalable and cost-effective. These methods can be adopted by industry to produce high-quality hydrogen storage systems at a reasonable cost.

Conclusion

The solutions identified in this study represent a significant advancement in the field of hydrogen storage for mobility applications. By combining high-performance EVOH liners with optimized thermoplastic composites, it is possible to develop storage systems that meet the stringent requirements of modern hydrogen-powered vehicles. These innovations pave the way for the widespread adoption of hydrogen as a clean and efficient energy source, supporting the global transition to zero-emission transportation.

References

We would like to extend our heartfelt thanks to the authors of the study, Jan Condé-Wolter, Michael G. Ruf, Alexander Liebsch, Tobias Lebelt, Ilja Koch, Klaus Drechsler, and Maik Gude, for their invaluable contributions to the research detailed in the document "Hydrogen Permeability of Thermoplastic Composites and Liner Systems for Future Mobility Applications". Their meticulous work and innovative approaches have provided significant insights that pave the way for advancements in hydrogen storage solutions. The dedication and expertise demonstrated in this study are greatly appreciated and instrumental in driving the future of zero-emission mobility. Thank you for your exceptional efforts and for sharing your profound knowledge with the scientific community.

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