Hydrogen storage properties of carbon nanotubes and BN nanotubes

Abstract

Recently, hydrogen storage possibilities of carbon nanotubes (CNTs) have been actively studied because of an energy crisis. However this topic is very controversial now. Therefore it is the most important to clarify the hydrogen storage mechanism of the CNT for development of a practical hydrogen storage material. Due to the very small size of nano-materials such as CNT, it is not easy to examine the hydrogen storage mechanism of the nanotube with present experimental techniques. As the alternative way, the quantum and atomistic simulations may be very powerful. In this dissertation, hydrogen storage properties of CNTs and BN nanotubes (BNNTs) have been investigated using a combination of density functional theory (DFT) and molecular dynamics (MD) simulation. In performing molecular dynamics, a new simulation technique, reactive force field (ReaxFF), was used. Parameters of the reactive force field are fitted by only quantum calculations, e.g. ab-initio or DFT method. The ReaxFF for the H-C-Li system describes very well the interactions between Li and fullerenes and between hydrogen and Li-intercalated CNTs in comparing with experimental results. Also, the ReaxFF for the H-B-N system is able to predict the bond dissociation of various H-B-N clusters such as $BH_3$, $H_2B-NH_2$ and borazine ($B_3N_3H_6$) etc, and interaction between BNNTs and hydrogen. Especially, the H-B-N reactive force field can mimic accurately H binding energies on planar B-N system as a function of its out-of-plane degree in contrast to the previous empirical force field. According to DFT calculations of PW91 level, physisorption of $H_2$ outside the (10,0) single-walled CNT (SWCNT) is very weak, -0.79 kcal/mol. In the chemisorption of two H atoms the most stable state is above two adjacent carbon atoms of a hexagon with a C-H bond length of 1.10Å and one C-H bond energy of -45.76 kcal/mol. Based on these results, I have also investigated the transition state and the reaction pathway from ...