Investigating thermal and mechanical properties of CFRP composites for hydrogen storage applications

Hydrogen fuel is gaining recognition as a key solution for achieving net-zero emissions by 2050. Hydrogen storage presents significant challenges due to its low volumetric energy density, requiring storage solutions that often involve high pressure and low temperatures. Currently, the space and energy industries use metallic tanks (Type I) for storing liquid hydrogen fuels because metals offer uniform expansion and contraction properties. However, these tanks are heavier, less gravimetrically efficient, and more brittle. A major focus has been the development of carbon fibre-reinforced plastic (CFRP) composites for hydrogen storage tanks due to their high strength and lightweight characteristics. However, challenges persist, particularly concerning the vulnerability of the fibre-matrix interface and the inevitable microcracking that occurs due to repeated refuelling cycles under the cryogenic condition with liquid hydrogen (- 253 degrees celsius). At the microscale, the formation of microcracks transversely across laminates can lead to the failure of the fibre-matrix interface as a result of poor interfacial shear strength (IFSS). As the load increases, these microcracks can grow, causing hydrogen leakage, which can have catastrophic consequences.

The fibre-matrix interface plays a vital role in composites failure. Strong interactions between fibres and the matrix are crucial for enhancing a composite's overall properties, with effective load transfer relying on strong interfacial adhesion. Recent research has revealed that this interface (more appropriately termed an interphase region) has a multiphase structure with distinct properties influenced by factors such as surface tension, electrostatic forces, chemical bonding, and mechanical adhesion. However, to date, the literature has focused on the supra-ambient performance of CFRP composites.

It is evident that some fibre/matrix combinations offer better performance at cryogenic temperatures (as evidenced by developing a higher IFSS, fewer microcracks, and greater longevity during extended thermal cycling), but the reasons for this and the factors that contribute most to the performance remain unclear. While experimental techniques to characterise the fibre-matrix interphase have improved significantly in recent years, directly investigating the molecular structure and response of this interphase region - especially at cryogenic temperatures - remains a challenge. In this context, molecular dynamics (MD) simulations can complement experimental efforts by providing valuable guidance for designing new, chemically functionalised fibre-matrix interphases optimised for cryogenic environments in CFRP composites.

The project's main focus is understanding the changes and failure of the fibre-matrix interface in CFRP composites for hydrogen storage applications. Key objectives of this project include:

- Manufacture CFRP composites with various matrices and fibre systems through different processes.
- Investigate the thermal characteristics of CFRP composites at cryogenic temperatures.
- Design and conduct rigorous testing to characterise the composite materials' performance for hydrogen storage applications, such as fatigue testing, cryogenic cycling, and hydrogen permeability.
- Apply molecular modelling to gain a deeper understanding of the system's conformational behaviour and associated physico-mechanical properties at cryogenic temperatures.

This research offers significant potential for enhancing sustainability in the transportation sector by utilising CFRP hydrogen storage tanks. The approach combines MD simulations with advanced experimental techniques to improve understanding of the fibre-matrix interphase, supporting the development of lighter, stronger, and more durable materials for cryogenic applications.

This project is supported by Syensqo and is carried out in collaboration with the University of Sheffield.