Molecular dynamics study on hydrogen permeation behavior and mechanism in polyethylene type IV hydrogen storage cylinder liner

Polyethylene (PE) and other thermoplastic polymers are commonly used as liners for type IV hydrogen storage cylinders but are prone to hydrogen permeation in high-pressure environments, which can cause material degradation and safety risks, such as hydrogen leakage. In this study, the atomic structures of PE and H2 are modeled using molecular dynamics simulations and grand canonical Monte Carlo methods. This research investigates the free volume distribution in PE and the mechanisms of hydrogen dissolution and diffusion under different temperature and pressure conditions. Solubility and diffusion coefficients were calculated from adsorption isotherms and mean squared displacement curves, respectively. The results show that solubility, diffusion, and permeability coefficients of H2 in PE increase with temperature but decrease with pressure. Higher temperature increases molecular chain movement, generating more free volume, while higher pressure compresses the molecular chains, reducing free volume. Hydrogen density maps indicate that H2 dissolves mainly in the free volume of PE. The diffusion mechanism follows an "oscillating + hopping" model, as shown in the H2 trajectory graphs. This study provides a microscopic understanding of hydrogen permeation in polymers, offering valuable insights for optimizing and ensuring the safe use of liner materials in type IV hydrogen storage tanks.