Titanium-Decorated Carbon Nanotubes as a Potential High-Capacity Hydrogen Storage Medium

We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt % hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.

Figure 1

Energy vs reaction paths for successive dissociative and molecular adsorption of ${\mathrm{H}}_2$ over a single Ti-coated $(8,0)$ nanotube. (a) ${\mathrm{H}}_2+\mathrm{t}80\mathrm{Ti}\to {\mathrm{t}80\mathrm{TiH}}_2$. (b) $2{\mathrm{H}}_2+{\mathrm{t}80\mathrm{TiH}}_2\to {\mathrm{t}80\mathrm{TiH}}_2-2{\mathrm{H}}_2$. (c) ${\mathrm{H}}_2+{\mathrm{t}80\mathrm{TiH}}_2-2{\mathrm{H}}_2\to {\mathrm{t}80\mathrm{TiH}}_2-3{\mathrm{H}}_2$. The zero of energy is taken as the sum of the energies of two reactants. The relevant bond distances and binding energies ($E_B$) are also given.

Figure 2

(a) Two different views of the optimized structure of $\mathrm{t}80\mathrm{Ti}\mathrm{\text{−}}4{\mathrm{H}}_2$. The relevant structural parameters are $\mathrm{H}\mathrm{\text{−}}\mathrm{H}=0.84\AA$, $\mathrm{Ti}\mathrm{\text{−}}\mathrm{H}=1.9\AA$, $\mathrm{Ti}\mathrm{\text{−}}\mathrm{C}=2.17\AA$, $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{C}}^{\prime }=2.4\AA$. (b) The PDOS at the $\Gamma$ point contributed from Ti, four ${\mathrm{H}}_2$ molecules, and the six carbons of the hexagon on which Ti and ${\mathrm{H}}_2$ molecules are bonded. (c) The ${\sigma }^{*}$ antibonding orbital of four ${\mathrm{H}}_2$ complex; (d)–(f) isosurface of the state just below $E_F$ at three different values: at $\Psi =0.08$, it is mainly Ti-$d$ orbital; at $\Psi =0.04$ the hybridization between the $d$ orbital, two carbon $\pi$ orbitals, and $4{\mathrm{H}}_2$ ${\sigma }^{*}$ antibonding is apparent. At $\Psi =0.02$ it is clear that the other four carbon atoms are also involved in the bonding.

Figure 3

Two high-density hydrogen coverage on a Ti-coated $(8,0)$ nanotube.