An Investigation of hydrogen storage in carbon nanotubes via computational MM methods

Abstract

In this study, hydrogen storage behaviour of single wall carbon nanotubes wasinvestigated using moleculer mechanics. Accelerys̕ Discover Minimization module wasemployed with COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) forcefield. For the simulations, single wall CNT bundles,which have supercells of four nanotubes in triangular array with 8 layers long, were built inthe computer via using the code VISUALIZER. In order to study the relation betweennanotube size and hydrogen storage capacity, armchair nanotubes which have three different sizes, (5,5), (6,6) and (7,7), were used in the supercells. Simulations were carriedout firstly by varying the number of hydrogen molecules only inside the nanotubes andthen with different combinations of the amounts of hydrogen molecules inside nanotubestogether with the hydrogen molecules at interstitial spaces between nanotubes in the bundles. In all simulations, the hydrogen storage capacity that gives minimum total energyfor the system, i.e. nanotube bundle and the stored hydrogen molecules, was searched. Inthe simulations, chemisorption of hydrogen on nanotube walls was not considered; onlyphysisorption of hydrogen as free hydrogen molecules in nanotubes was counted. Simulation results of the systems were examined for searching the favorable hydrogenstorage according to change in the total energy of the system. Favorable hydrogen storageonly inside of nanotubes for (5,5), (6,6) and (7,7) CNTs are about 2.44, 5.00, 8.20 wt. percent, respectively. On the other hand, storage of hydrogen molecules at the interstitial spaces was found not favorable based on the total energy of the system. Stored hydrogen inall nanotubes showed a general trend; due to both the narrow diameter of nanotubes andstrong van der Waals forces led by short distance between molecules, hydrogen moleculesinside nanotubes condense to a molecular shell having the tube shape. It is concluded that (i) the repulsive forces determine the hydrogen storage capacity inside nanotubes and the stability of the nanotubes; (ii) hydrogen storage data showed that hydrogen storage islinearly dependent to radius of the nanotubes as expected. Considering the fact that thevolume and the number of hydrogen molecules increases with a square of the radius,whereas the number of carbon atoms increases linearly with the radius, the latter was anexpected result.