Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage Media

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of $\sim 14\mathrm{wt}\%$. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.

Figure 1

(a) One of the most stable structures of the $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{C}}_{60}$ complex where the Ti atom (black) is bonded to a double bond with four hydrogen molecules attached (dark gray). (b) The local structure of the $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{C}}_{60}$ double bond. (c) Replacing the end carbon atoms shown in (b) by H results in an ethylene molecule. This suggests that we may simply use the ethylene molecule to hold Ti atoms, which then binds multiple hydrogen molecules.

Figure 2

Optimized structure of an ethylene molecule ${\mathrm{C}}_2{\mathrm{H}}_4$ (a), ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{Ti}$ (b), and ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ (c). The panel (d) shows that the $\mathrm{Ti}\mathrm{\text{−}}{\mathrm{C}}_2{\mathrm{H}}_4$ bonding orbital results from the hybridization of the LUMO of the ${\mathrm{C}}_2{\mathrm{H}}_4$ and the Ti-$d$ orbital, in accord with Dewar coordination.

Figure 3

Atomic configurations of an ethylene molecule functionalized by two Ti atoms, holding (a) two ${\mathrm{H}}_2$ molecules which are dissociated, (b) six ${\mathrm{H}}_2$ molecules, and (c) eight ${\mathrm{H}}_2$ molecules. Panel (d) shows a configuration where ten ${\mathrm{H}}_2$ are bonded all molecularly. The spin-polarized calculations gave lower energies (in eV) by 1.5, 0.37, 0.16, and 0.06 for configurations shown in (a)–(d), respectively, suggesting a magnetic ground state for all cases with a moment of $\mu \approx 2{\mu }_B$. Panels (e) and (f) show the bonding orbital for the top (e) and side hydrogen molecules, respectively. Note that the hydrogen ${\sigma }^{*}$-antibonding orbitals are hybridized with $\mathrm{Ti}\mathrm{\text{−}}d$ orbitals, suggesting Kubas interaction for the ${\mathrm{H}}_2\mathrm{\text{−}}\mathrm{Ti}$ bonding.

Figure 4

(a) Atomic configuration and charge density plots of a dimer of ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ molecules linked by two Ti atoms one from either molecule. Large, medium, and small balls indicate titanium, carbon, and hydrogen atoms, respectively. Charge density plots (from left to right) correspond to HOMO spin-up, and two spin-down states. (b) Atomic configuration of ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ dimer holding 14 ${\mathrm{H}}_2$ molecules. (c) Polymer of ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ molecule with polymerization energy $E_P$. (d) Atomic configuration of the polymer holding 6 ${\mathrm{H}}_2$ molecule. (e) A ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ molecule adsorbed above the center of a hexagon in a ($3\times 3$) cell holding 7 ${\mathrm{H}}_2$. (f) Two Ti-ethylene complexes adsorbed above and below the center of the same hexagon holding 14 ${\mathrm{H}}_2$.