Tuning hydrogen storage of carbon nanotubes by mechanical bending: theoretical study

Abstract

The effects of mechanical bending on tuning the hydrogen storage of titanium functionalised (4,0) carbon nanotube have been assessed using density functional theory calculations with reference to the ultimate targets of the US Department of Energy (DOE). The assessment has been carried out in terms of physisorption, gravimetric capacity, projected densities of states, statistical thermodynamic stability and reaction kinetics. The Ti atom binds at the hollow site of the hexagonal ring. The average adsorption energies (−0.54 eV) per hydrogen molecule meet the DOE target for physisorption (−0.20 to −0.60 eV). The curvature attributed to the bending angle has no effect on the average adsorption energies per H2 molecule. With no metal clustering, the system gravimetric capacities are expected to be as large as 9.0 wt%. The reactions of the deformed (bent) carbon nanotube have higher probabilities of occurring than those of the un-deformed carbon nanotube. The Gibbs free energies, enthalpies and entropies meet the ultimate targets of the DOE for all temperatures and pressures. The closest reactions to zero free energy occur at (378.15 K/2.961 atm.) and reverse at (340 and 360 K/1 atm.). The translational component is found to exact a dominant effect on the total entropy change with temperature. Favourable kinetics of the reactions at the temperatures targeted by DOE are reported regardless of the applied pressure. The more preferable thermodynamic properties assigned to the bending nanotube imply that hydrogen storage can be improved compared to the nonbending nanotube.