Hydrogen storage capacity of Ti-doped boron-nitride and $\mathrm{B}∕\mathrm{Be}$-substituted carbon nanotubes

We investigate the hydrogen absorption capacity of two tubular structures, namely, $\mathrm{B}∕\mathrm{Be}$-substituted single-wall carbon nanotube (SWNT) and Ti covered single-wall boron nitride nanotube (SWBNT) using first-principles plane wave method. The interaction of ${\mathrm{H}}_2$ molecules with the outer surface of bare SWBNT, which is normally very weak, can be significantly enhanced upon functionalization by Ti atoms. Each Ti atom adsorbed on SWBNT can bind up to four ${\mathrm{H}}_2$ molecules with an average binding energy suitable for room temperature storage. While the substitution process of Be atom on SWNT is endothermic, the substituted Be strengthens the interaction between tube surface and ${\mathrm{H}}_2$ to hold one ${\mathrm{H}}_2$ molecule.

Figure 1

(Color online) Possible ${\mathrm{H}}_2$ absorption geometries on (8,0) SWBNT. [(a)–(c)] Various initial configurations of ${\mathrm{H}}_2$ absorbing at hollow site. (d) Final configuration of ${\mathrm{H}}_2$ after structure optimization.

Figure 2

(Color online) The configuration of ${\mathrm{H}}_2$ molecules absorbing to (8,0) $\mathrm{SWBNT}+\mathrm{Ti}$ system. (a) First ${\mathrm{H}}_2$ molecule dissociates into two H atom. (b) Subsequent three ${\mathrm{H}}_2$ are molecularly absorbed around Ti. (c) Possible (unrelaxed) symmetric configuration similar to that of $\mathrm{SWNT}+\mathrm{Ti}$ with four ${\mathrm{H}}_2$ molecularly absorbed around Ti (Ref. 8). (d) The resulting configuration occurring upon the relaxation of (c).

Figure 3

(Color online) Initial and final geometries of Ti covered (8,0) SWBNT. (a) Eight Ti per cell coverage (unrelaxed). (b) Eight Ti per cell coverage upon relaxation. (c) Four Ti per cell coverage (unrelaxed). (d) Four Ti per cell coverage upon relaxation.

Figure 4

(Color online) The structures of ${\mathrm{H}}_2$ absorption on Be (or B) substituted (8,0) SWNT. (a) The final geometry of $\mathrm{SWNT}\left({\mathrm{C}}_{31}X\right)$ system where one of the carbon atoms of (8,0) SWNT is replaced by $X$ ($X=\mathrm{B}$ or Be). (b) External and (c) internal absorptions of ${\mathrm{H}}_2$ molecule on $\mathrm{SWNT}\left({\mathrm{C}}_{31}X\right)$ system. ${\mathrm{H}}_2$ is molecularly absorbed for external case but dissociates into two atoms and binds to carbons for internal adsorption.