Recycling composite materials from high-pressure hydrogen storage tanks, particularly those made from carbon fiber-reinforced polymers (CFRPs), is a critical area of research as the hydrogen economy expands. These tanks are lightweight and durable, making them ideal for hydrogen storage, but their end-of-life management poses significant challenges. The recycling process must address the complex nature of CFRPs, which consist of high-performance carbon fibers embedded in a polymer matrix, often epoxy or thermoplastic resins. Three primary techniques—mechanical shredding, pyrolysis, and solvolysis—are currently employed to recover carbon fibers from these composites, each with distinct advantages and limitations.

Mechanical shredding is the most straightforward method for recycling CFRPs. The process involves physically breaking down the composite material into smaller pieces using grinding or cutting equipment. The resulting product is a mixture of fragmented fibers and resin particles, which can be reused as filler material in lower-grade applications, such as construction or automotive components. However, mechanical shredding severely reduces fiber length and degrades their mechanical properties, limiting their use in high-performance applications. The process also does not fully separate the fibers from the resin, leading to contamination that further diminishes the recycled material's quality. Despite these drawbacks, mechanical shredding is cost-effective and requires relatively simple infrastructure, making it a common choice for initial recycling efforts.

Pyrolysis is a thermal decomposition process that involves heating CFRPs in an oxygen-free environment to break down the polymer matrix into volatile gases and oils, leaving behind clean carbon fibers. The temperatures typically range between 400°C and 700°C, depending on the resin type. Pyrolysis effectively preserves fiber length and mechanical properties, making the recovered fibers suitable for reuse in high-value applications. However, the process requires significant energy input, and the residual char on the fibers may necessitate additional post-treatment, such as oxidation or washing, to restore their surface properties. Another challenge is the management of pyrolysis byproducts, including oils and gases, which must be properly handled to avoid environmental harm. Advances in pyrolysis technology, such as microwave-assisted heating, are being explored to improve efficiency and reduce energy consumption.

Solvolysis is a chemical recycling technique that uses solvents, often at elevated temperatures and pressures, to dissolve the polymer matrix and release the embedded carbon fibers. Common solvents include water (hydrothermal processing), alcohols, and acids. Solvolysis offers several advantages, including the ability to recover both fibers and resin components in reusable forms. The process operates at lower temperatures than pyrolysis, reducing energy consumption, and can achieve high purity in the recovered fibers. However, solvolysis faces challenges related to solvent selection, recovery, and potential toxicity. The process must also be tailored to specific resin chemistries, limiting its universality. Despite these hurdles, solvolysis is considered a promising method for achieving closed-loop recycling of CFRPs.

Resin removal is a critical challenge in all recycling methods. Incomplete removal can lead to fiber contamination, reducing their performance in subsequent applications. Pyrolysis and solvolysis are more effective at resin removal than mechanical shredding, but they may still leave residues that require additional processing. Fiber quality retention is another major concern, as any degradation in fiber strength, stiffness, or surface properties diminishes their value. Pyrolysis generally offers the best balance between resin removal and fiber preservation, but ongoing research aims to optimize all methods for minimal fiber damage.

Comparing recycling to landfill disposal and incineration highlights the environmental and economic benefits of recycling. Landfilling CFRP waste is the least sustainable option, as it occupies valuable space and does not recover any materials. Incineration can recover energy from the polymer matrix but destroys the carbon fibers and releases greenhouse gases and other pollutants. Recycling, despite its challenges, conserves resources by reintroducing carbon fibers into the manufacturing cycle, reducing the need for virgin materials and lowering the overall carbon footprint of hydrogen storage systems.

The choice of recycling method depends on multiple factors, including the desired quality of recovered fibers, economic feasibility, and environmental impact. Mechanical shredding is suitable for low-cost applications where fiber performance is not critical, while pyrolysis and solvolysis are better suited for high-performance reuse. As the hydrogen economy grows, developing efficient and scalable recycling processes for CFRPs will be essential to ensure sustainability. Continued advancements in recycling technologies, coupled with stricter regulations on composite waste, will drive progress in this field.

The challenges associated with recycling CFRPs from hydrogen storage tanks are significant but not insurmountable. Collaboration between industry, academia, and policymakers is necessary to establish standardized recycling protocols, improve process efficiencies, and create markets for recycled carbon fibers. By addressing these challenges, the hydrogen sector can move toward a more circular economy, minimizing waste and maximizing resource efficiency. The environmental benefits of recycling, combined with the economic potential of recovered materials, make this a critical area of focus for the sustainable development of hydrogen technologies.