Hydrogen adsorption in defected carbon nanotubes

Recently there has been lot of interest in the development of hydrogen storage in various systems for the large-scale application of fuel cells, mobiles and for automotive uses. Hectic materials research is going on throughout the world with various adsorption mechanisms to increase the storage capacity. It was observed that physisorption proves to be an effective way for this purpose. Some of the materials in this race include graphite, zeolite, carbon fibers and nanotubes. Among all these, the versatile material carbon nanotube (CNT) has a number of favorable points like porous nature, high surface area, hollowness, high stability and light weight, which facilitate the hydrogen adsorption in both outer and inner portions. In this work we have considered armchair (5,5), zig zag (10,0) and chiral tubes (8,2) and (6,4) with and without structural defects to study the physisorption of hydrogen on the surface of carbon nanotubes using DFT calculations. For two different H₂ configurations, adsorption binding energies are estimated both for defect free and defected carbon nanotubes. We could observe larger adsorption energies for the configuration in which the hydrogen molecular axis perpendicular to the hexagonal carbon ring than for parallel to C–C bond configuration corresponding to the defect free nanotubes. For defected tubes the adsorption energies are calculated for various configurations such as molecular axis perpendicular to a defect site octagon and parallel to C–C bond of octagon and another case where the axis perpendicular to hexagon in defected tube. The adsorption binding energy values are compared with defect free case. The results are discussed in detail for hydrogen storage applications.