Hydrogen permeability of polyamide 6 as the liner material of Type IV hydrogen storage tanks: A molecular dynamics investigation

To attain a lightweight structure with high pressure and a high hydrogen storage mass ratio, the type IV hydrogen storage tank is the most popular choice for many fuel cell vehicle enterprises. Using polymer as the liner material prevents hydrogen embrittlement and hydrogen-induced damage of the tanks, but hydrogen permeation is inevitable. Within the service conditions (233-358 K, 0-87.5 MPa) of the Type-IV tanks, the dissolution and diffusion behaviors of H2 in polyamide 6 are comprehensively explored for the first time through the molecular dynamics and Grand Canonical Monte Carlo simulations in this study. The diffusion mechanism of H2 in polyamide 6 is further revealed. The results show that both the diffusion and permeability coefficients have positive correlations with temperature, whereas the opposite is true for the solubility coefficient. These relationships are all consistent with Arrhenius's law in the temperature range, although multiple Arrhenius regions are needed to describe them. A slight decrease in the diffusion and permeability coefficients is observed with increasing pressure. However, this effect is not as significant as that of temperature and can be ignored when the pressure range is narrow enough. The "hopping" mechanism can be used to describe the diffusion of H2 in polyamide 6, where the continuous vibrations and "hopping" allow the diffusion of H2 molecules. These findings will contribute to a more comprehensive understanding of the permeation behavior of H2 in polyamide 6 and evaluate the gas barrier property of the material in practical applications.