Hydrogen absorption properties of metal-ethylene complexes

Recently, we have predicted [Phys. Rev. Lett. 97, 226102 (2006)] that a single ethylene molecule can form stable complexes with light transition metals (TMs) such as Ti and the resulting ${\mathrm{TM}}_n$-ethylene complex can absorb up to $\sim 12$ and $14\mathrm{wt}\%$ hydrogen for $n=1$ and 2, respectively. Here we extend this study to include a large number of other metals and different isomeric structures. We obtained interesting results for light metals such as Li. The ethylene molecule is able to complex with two Li atoms with a binding energy of $0.7\mathrm{eV}∕\mathrm{Li}$ which then binds up to two ${\mathrm{H}}_2$ molecules per Li with a binding energy of $0.24\mathrm{eV}∕{\mathrm{H}}_2$ and absorption capacity of $16\mathrm{wt}\%$, a record high value reported so far. The stability of the proposed metal-ethylene complexes was tested by extensive calculations such as normal-mode analysis, finite temperature first-principles molecular-dynamics (MD) simulations, and reaction path calculations. The phonon and MD simulations indicate that the proposed structures are stable up to $500\mathrm{K}$. The reaction path calculations indicate about $1\mathrm{eV}$ activation barrier for the ${\mathrm{TM}}_2$-ethylene complex to transform into a possible lower energy configuration where the ethylene molecule is dissociated. Importantly, no matter which isometric configuration the ${\mathrm{TM}}_2$-ethylene complex possesses, the TM atoms are able to bind multiple hydrogen molecules with suitable binding energy for room-temperature storage. These results suggest that co-deposition of ethylene with a suitable precursor of TM or Li into nanopores of light-weight host materials may be a very promising route to discovering new materials with high-capacity hydrogen absorption properties.

Figure 1

(Color online) Various configurations of ${\mathrm{C}}_2{\mathrm{H}}_4{M}_n$ ($n=1$ and 2) complexes considered in this study. (a) ${\mathrm{C}}_2{\mathrm{H}}_4$ complexed with one metal atom. (b)–(e) ${\mathrm{C}}_2{\mathrm{H}}_4$ complexed with two metal atoms with different metal binding sites. Note that the bond-stick model is only used for clarity and should not be considered as an implication of the chemical covalent bonding between those atoms. For most metals, there is no classical chemical covalent bonding between the metal and carbon atoms. For a few metals (e.g., Fe), the complexes possess a structure, where $M$ and C are bonded more traditionally by covalent bonding, as shown in (d). (f) ${\mathrm{C}}_2{\mathrm{H}}_4{M}_2$ complex with dissociated $\mathrm{C}\mathrm{C}$ bond. Large, medium, and small balls represent $M$, C, and H atoms, respectively.

Figure 2

(Color online) Electronic density of states of ${\mathrm{C}}_2{\mathrm{H}}_4$, Li atom, and ${\mathrm{C}}_2{\mathrm{H}}_4+\mathrm{Li}$ complex. The isosurfaces of the relevant molecular orbitals are also shown. The hybridization of the $\mathrm{Li}\text{−}2p$ state and the LUMO of ${\mathrm{C}}_2{\mathrm{H}}_4$ is apparent. See text for further explanation.

Figure 3

(Color online) Various configurations that we considered in this study, for the hydrogen absorption on a metal center of a ${\mathrm{C}}_2{\mathrm{H}}_4{M}_n$ complex: (a) one ${\mathrm{H}}_2$ absorbed molecularly; (b) ${\mathrm{H}}_2$ dissociating with two $M\text{−}\mathrm{H}$ bond formed; (c) two atomic H and three ${\mathrm{H}}_2$ molecules; (d) five ${\mathrm{H}}_2$ absorbed as molecules. Large and small balls represent $M$ and H atoms, respectively.

Figure 4

(Color online) Hydrogen absorption configurations on (a) ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{Li}$ and (b) ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Li}}_2$ complexes. Note that in both cases, each Li can bind two ${\mathrm{H}}_2$, resulting in high absorption capacities. Large, medium, and small balls represent Li, C, and H atoms, respectively.

Figure 5

(Color online) Simulated neutron inelastic spectrum for various ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_n\text{−}{\mathrm{H}}_x$ configurations. Note that the $M\text{−}\mathrm{H}$ dynamics are unique and can be used as a probe to identify these structures. Thus these plots can provide a useful comparison to experiments when trying to synthesize these materials.

Figure 6

(Color online) (a) The minimum-energy path for the dissociation of the ${\mathrm{H}}_2$ molecule over the Ti atom complexed with ${\mathrm{C}}_2{\mathrm{H}}_4$. An energy barrier of $\approx 0.25\mathrm{eV}$ is found for the dissociation. A total of 21 images were used in the NEB calculations, five of which are shown on the top. Marked circles in the potential plot are the points corresponding to these five images. (b) The activation energy plot for the formation of titanol-molecule from ${\mathrm{C}}_2{\mathrm{H}}_4\mathrm{Ti}+{\mathrm{H}}_2$ complex, indicating a very low barrier of $\approx 0.15\mathrm{eV}$. Once the final product forms, the CCTi-bond angle is very soft, resulting in the zero-temperature structure shown in the inset.

Figure 7

(Color online) Hydrogen absorption on a titanol molecule. (a) One ${\mathrm{H}}_2$ binds molecularly to Ti, with a binding energy of $0.3\mathrm{eV}$. (b) One ${\mathrm{H}}_2$ is dissociated, yielding $\mathrm{Ti}{\mathrm{H}}_3$ structure. The corresponding binding energy is about $1.0\mathrm{eV}∕{\mathrm{H}}_2$. (c) Five ${\mathrm{H}}_2$ bind as molecules to the titanol molecule with an average binding energy of $0.4\mathrm{eV}∕{\mathrm{H}}_2$. Large, medium, and small balls represent Ti, C, and H atoms, respectively.

Figure 8

(Color online) (a) Bare ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ dimer-perp complex. (b) The complex with seven ${\mathrm{H}}_2$ absorbed. Note that there also exists other stable configurations that not discussed here. (c) The minimum-energy path for the transition from ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ sandwich configuration to dimer-perp configuration. The activation energy is about $0.55\mathrm{eV}$. Note that regardless of which isomer of ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ we have, the resulting complex is able to bind multiple hydrogen molecules.

Figure 9

(Color online) The minimum-energy paths for the transitions (a) from the ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ sandwich to the dissociated configuration and (b) from ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Ti}}_2$ dimer perp to the dissociated ${\mathrm{C}}_2{\mathrm{H}}_4$ configuration, respectively. In both cases, there are large energy barriers on the order of $1.1\mathrm{eV}$.

Figure 10

(Color online) First-principles MD results at $500\mathrm{K}$ for the ${\mathrm{C}}_2{\mathrm{H}}_4{\mathrm{Li}}_2+4{\mathrm{H}}_2$ complex. Shown are the time evolution of various quantities, including total energy (a) and temperature (b) of the system, C-C (c) and Li-C (d) bond distances, and Li-C-C-Li torsion angle (e). The bottom panel (f) shows the distance between the Li atom and the hydrogen center of mass, indicating successive desorption of ${\mathrm{H}}_2$ molecules along the simulation.

Figure 11

(Color online) First-principles MD results at $500\mathrm{K}$ for the ${\mathrm{C}}_2{\mathrm{H}}_4{(\mathrm{Ti}+5{\mathrm{H}}_2)}_2$ sandwich complex. Various quantities are shown, including C-C (a) and Ti-C (b) bond distances, and Ti-C-C-Ti torsion angle (c). The bottom panel (d) shows the number of ${\mathrm{H}}_2$ molecules that are within $2.2\mathrm{\AA }$ of Ti atoms. It indicates successive desorption of ${\mathrm{H}}_2$ molecules in the course of the simulation.

Figure 12

(Color online) Various quantities (same as in Fig. 11) obtained from MD simulations of the $\text{titanol}+5{\mathrm{H}}_2$ system at $500\mathrm{K}$.