Liquefaction of $H2$ molecules upon exterior surfaces of carbon nanotube bundles

We have used molecular dynamics simulations to investigate interaction of $H2$ molecules on the exterior surfaces of carbon nanotubes (CNTs): single and bundle types. At 80 K and 10 MPa, it is found that charge transfer occurs from a low curvature region to a high curvature region of the deformed CNT bundle, which develops charge polarization only on the deformed structure. The long-range electrostatic interactions of polarized charges on the deformed CNT bundle with hydrogen molecules are observed to induce a high local-ordering of $H2$ gas that results in hydrogen liquefaction. Our predicted heat of hydrogen liquefaction on the CNT bundle is $97.6kcalkg−1$⁠. On the other hand, hydrogen liquefaction is not observed in the CNT of a single type. This is because charge polarization is not developed on the single CNT as it is symmetrically deformed under the same pressure. Consequently, the hydrogen storage capacity on the CNT bundle is much higher due to liquefaction than that on the single CNT. Additionally, our results indicate that it would also be possible to liquefy $H2$ gas on a more strongly polarized CNT bundle at temperatures higher than 80 K.