Molecular dynamics investigation into mechanisms of hydrogen diffusion in flexible-wall narrow carbon nanotubes

Understanding the interaction between hydrogen and carbon nanotubes is crucial in enhancing the performance of hydrogen storage and nanofluidic carbon-adsorbent systems. Accordingly, this study performs a series of molecular dynamics simulations to investigate the transport properties of hydrogen molecules confined within a flexible narrow carbon nanotube with a diameter of 10.8° at temperatures in the range 100~800 K and particle loadings ranging from 0.01~1 No/Å. The results show that the hydrogen molecules exhibit three distinct diffusion regimes, namely single-file, Fickian and ballistic, depending on the value of the Knudsen number. In addition, it is shown that at Knudsen numbers of less than 1, the thermal activation effect prompts a longitudinal wave slip and compression-expansion of the hydrogen molecule crowds within the CNT, which leads to a significant increase in the mean square displacement of the molecules.