Hydrogen adsorption and storage in porous single-wall carbon nanotubes

Efforts to increase the hydrogen capacity in carbon nanotubes (CNTs) point to find energetically favorable access to its inner space. Following this way, we investigate the incorporation of H$_2$ molecules inside CNTs through pores in their walls by density functional theory calculations and molecular dynamic simulations. The pores are constructed by multivacancies defects ($n$V) with $n$ missing atoms. CNTs of 1.1 and 1.4~nm in diameter were considered. We find that 16V has the limit size where the defect reconstruction is unlikely, preserving the unsaturated border. Under hydrogenation, the border is passivated by H$_2$ dissociative reactions, leaving an inert pore of about 6~\AA\ in diameter (16V-H).We verify that the incorporation and release of H$_2$ molecules through this pore occur barrierless and its stability in contact with a gas of 32 and 64 H$_2$ molecules for exohedral and endohedral adsorption is preserved at high temperatures up to 600~K. Our results show that at room temperature, the endohedral adsorption energy is high enough for a reversible adsorption-desorption process, suggesting that porous CNTs, as produced for instance by electron irradiation under a H$_2$ atmosphere, could be an effective H$_2$ storage medium, allowing the access to the CNT inner space.