High-Capacity Room-Temperature Hydrogen Storage in Carbon

E-mail: [email protected] (S.A.S.), [email protected] (Z.X.G.). ... the hydrogen storage properties of carbon-based materials: by providing multiple...
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J. Phys. Chem. C 2008, 112, 17456–17464

High-Capacity Room-Temperature Hydrogen Storage in Carbon Nanotubes via Defect-Modulated Titanium Doping S. A. Shevlin* and Z. X. Guo* Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom. ReceiVed: January 4, 2008; ReVised Manuscript ReceiVed: July 1, 2008

Carbon materials have been at the forefront of hydrogen storage research. However, without improvements in the hydrogen binding strength, as provided by transition-metal dopants, they will not meet practical targets. We performed ab initio density functional theory simulations on titanium-atom dopants adsorbed on the native defects of an (8,0) nanotube. Adsorption on a vacancy strongly binds titanium, preventing nanoparticle coalescence (a major issue for atomic dopants). The defect-modulated Ti adsorbs five H2 molecules with H2 binding energies in the range from -0.2 to -0.7 eV/H2, desirable for practical applications. Molecular dynamics simulations indicate that this complex is stable at room temperature, and simulation of a C112Ti16H160 unit cell finds that a structure with 7.1 wt % hydrogen storage is stable. Introduction The generation and utilization of clean and renewable energy is of major worldwide concern. Currently, energy consumption is intimately linked with CO2 emissions, a significant human contributor to climate change. Several solutions can ameliorate this problem including increases in efficiency, generation by renewable sources, and increasing use of nuclear power. However, fossil fuels with a high energy density will still be dominant in the near term. If CO2 emissions are to be greatly reduced, there should be a switch to one or more clean alternatives, of which hydrogen is a preferred candidate.1,2 Hydrogen, however, is difficult to store safely at low cost, low weight, and low volume. Thus, storage in solid materials that have fast H2 release and adsorption kinetics at 85 °C is desired.1-4 Materials that exhibit a fullerene structural motif, such as nanotubes, are especially promising in this respect. Carbon systems have been widely studied for hydrogen storage applications, with spectacular initial results indicating that nanotubes adsorb 5-10 wt % at low temperature and pressure.5 Later results, however, suggested that nanotubes do not adsorb significant amounts of hydrogen under these conditions.6-8 This is due to the nature of H2 binding to the substrate, as H2 physisorbs to the exterior of the nanotube via van der Waals interactions with a strength of -0.1 eV/H2;9-11 this is far too weak for room-temperature storage, which demands binding energies in the range from -0.2 to -0.7 eV/ H2. The maximum density of H2 that can be physisorbed is 70.58 kg/m3, so the maximum hydrogen storage capacity of carbon nanotubes (CNTs) is limited by surface area constraints to 3.3% by weight.12,13 Clearly, nanotubes must be modified if they are to be considered as realistic hydrogen storage materials, for example, by external pressure14 or doping.15-17 It has recently been proposed that the addition of transition-metal (TM) dopants can boost the hydrogen storage properties of carbon-based materials: by providing multiple sites for adsorption (increasing gravimetric storage), enhancing binding energy (allowing hydrogen storage at room temperature), and storing hydrogen in molecular form (speeding the kinetics of H2 adsorption/ desorption). First-principles simulations of Sc decoration on the * E-mail: [email protected] (S.A.S.), [email protected] (Z.X.G.).

5-fold faces of C48B12,15 Ti decoration on one of every two hexagons of an (8,0) CNT,16 and highly dispersed Ni atom decoration on a single-walled CNT18 all showed enhanced molecular H2 storage (g8 wt %). In all of these simulations, the dopants were dispersed uniformly and atomically on the surfaces of perfect carbon nanotubes. However, experiments have shown that nickel prefers to form ∼1-nm-diameter nanoparticles when adsorbed on CNTs,18 and simulations have indicated that, for atomic Ti deposition on C60, the preferred structure involves nanoparticle coalescence rather than widely dispersed TM atom dopants.19 Titanium on perfect nanotubes forms continuous chains,20 which is not ideal for hydrogen storage as the strong Ti-Ti interaction reduces the binding of hydrogen.16 To minimize metal-metal interactions, the dopants must be “pinned” in place on the carbon nanotube. An obvious way to do this is to adsorb dopants on native point defects of the nanotube, such as Stone-Wales defects and vacancies. Vacancies, in particular, are of interest because, in the limit of low TM concentration, they result in a material that is intermediate between carbon nanotubes and metallocarbohedrenes, which themselves are of possible interest for hydrogen storage applications.21,22 In this article, we present the results of density functional theory (DFT) simulations on the binding and hydrogenation properties of single-atom Ti dopants bound to carbon vacancies of an (8,0) carbon nanotube and consider generalizations to other diameters of zigzag nanotubes, as well as extensions to armchair nanotubes. We found that vacancies increase the binding of titanium, as the pinned dopants do not coalesce to form clusters. Additionally, the interplay of bonding between the Ti atom and the vacancy allows the defect as a whole to adsorb five H2 molecules, at odds with the 18-electron rule. The H2 binding energy is within the range required to meet U.S. Department of Energy (U.S. DOE) targets for hydrogen storage applications. Molecular dynamics simulations show that the maximally hydrogenated Ti-plus-vacancy system is stable at room temperature but releases H2 at 600 K. A C112Ti16H160 nanotube was found to retain full hydrogenation and be stable, showing that it is possible to store 7.1 wt % H2.

10.1021/jp800074n CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

High-Capacity Room-Temperature Hydrogen Storage in CNTs Computational Methods The adsorption of H2 on Ti-doped carbon nanotubes was modeled using unrestricted spin-polarized ab initio density functional theory23 with a plane-wave energy cutoff of 500 eV. Ultrasoft pseudopotentials were used to treat the core electrons, with the Ti pseudopotential treating semicore p states explicitly. Because it was recently shown by highly accurate quantum Monte Carlo simulations that the adsorption of hydrogen on carbon nanotubes24 and light-element-doped fullerenes10 is more accurately described by the local-density approximation (LDA)25 than the general gradient approximation (GGA),26 the majority of our simulations were performed using the LDA, although several calculations were checked using both functionals. We investigated two concentrations of Ti dopants (each involving the substitution of a single carbon atom by a single titanium atom). A dopant concentration of 1.56% was modeled using an orthorhombic simulation cell of 20.8 × 20.8 × 6a Å3 (the “highc”) structure, and a concentration of 0.78% was modeled using an orthorhombic simulation cell of 20.8 × 20.8 × 12a Å3 (the “low-c” structure), where a is oriented along the tube axis and set equal to the CsC bond length of graphite, i.e., 1.41 Å for the LDA and 1.42 for the GGA. For the smaller simulation cell, a (113) Monkhorst-Pack k-point net was needed for wellconverged geometries and structures, but for the larger unit cell, the Γ point was sufficient. For metallic nanotubes [(12,0) and (4,4)], a (114) k-point net is needed to obtain well-converged geometric and energetics. Molecular dynamics (MD) simulations were performed using the microcanonical-ensemble CarParrinello approach.27 Results and Discussion Interaction of Titanium with Carbon Nanotubes. We considered the interaction of Ti with three systems (Figure 1): pure defect-free nanotubes (1), nanotubes with Stone-Wales defects (2), and nanotubes with single-atom vacancies (3). We note that a bare carbon vacancy is unstable with respect to reconstruction to pentagon and nonagon rings (5,9), so we considered the Ti adsorption on this reconstructed defect structure. We considered several possible adsorption sites: (1) the center of the 5-fold ring, (2) the center of the 9-fold ring, (3) the [5,6] junction, (4) the [5,9] junction, and (5) the [9,6] junction. For all cases, the vacancy reconstructed such that the Ti atom inserted into the vacancy. There were no metastable states for adsorption on the vacancy defect itself. The adsorption of Ti on these reconstructed defects and insertion into the vacancy were found to be barrierless in our simulations, indicating that, in real systems, the stabilized vacancy structures should be easy to modify by Ti doping. Of two possible orientations for the 2 defect, the structure with the bond rotated normal to the tube axis was found to be more stable (by 0.51 eV), and of two possible structures for the 3 defect, the (5,9) ring aligned along the tube was found to be more stable (by 1.68 eV). For the 2 system, the Ti atom had a strong preference to reside centrally above the [7,7] junction, in agreement with previous work on Ni adsorption.28 For all systems, the Ti was found to reside above the nanotube, by 1.15 Å (3), 1.52 Å (1), and 1.91 Å (2); see Figure 1. Geometries were found to be similar for both small and large unit cells. The binding energy of titanium atoms to the substrate was determined as

EBind ) ETot(C + Ti) - ETot(C) - ETot(Ti) where ETot(C + Ti) is the total energy of the fully relaxed carbon nanotube plus Ti atom, ETot(C) is the total energy of the fully

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17457 relaxed carbon nanotube, and ETot(Ti) is the total energy of the Ti atom. If we take as the reference configuration for Ti the energy of an isolated atom, then EBind is -3.06, -2.74, and -8.02 eV/atom for the 1, 2, and 3 systems, respectively. All atoms are bound. However, if we take as the reference configuration the energy of a Ti atom in bulk hexagonal titanium (which is experimentally the more likely option for a reference state), then EBind is 3.36, 3.68, and -1.61 eV for the 1, 2, and 3 systems, respectively. The dopants in structures 1 and 2 are unstable with respect to Ti nanoparticle coalescence, whereas in structure 3, coalescence does not occur. Because it is known that the LDA can result in overbinding with respect to the GGA, we checked our calculations for the 1 and 3 systems. We found that using the atomic reference states gave binding energies of -2.11 eV (1) and -9.23 eV (3), whereas the bulk metal reference gave binding energies of 3.24 eV (1) and -3.87 eV (3). Vacancies pinned the Ti dopants to the carbon substrate, preventing coalescence. To gain insight into the nature of the titanium binding to the carbon substrate, we calculated the charge density difference (CDD)

∆F ) F(C + Ti) - F(C) - F(Ti) where F(C + Ti) is the charge density of the total system in the relaxed state, F(C) the charge density of the relaxed carbon nanotube, and F(Ti) is the charge density of the isolated Ti atom. Comparison of the ∆F values (see Figure 1) shows that the 1 system has a CDD in which charge density is depleted from a dxz orbital of the Ti atom into a dz2 orbital that has strong overlap with the π orbitals of the CNT, as expected from the Dewar analysis of Zhao et al.15 In contrast, the CDDs for both systems 2 and 3 do not present Dewar interactions. The 2 structure has a depletion of charge density from an orbital of d symmetry, but the hybridization with the π orbitals consists of a mixture of atomic orbitals. For the 3 structure, both depletion and accumulation of charge density involve a mixture of atomic orbitals. A Bader charge analysis28 was performed on all systems to determine the magnitude of charge transfer from the Ti atom to the carbon support. For the 1 system, the Ti donates 1.22e; for the 2 system, the Ti donates 0.83e; and for the 3 system, the Ti donates 1.42e. For all three systems, the carbon atoms that directly bind to the Ti are the ones that receive the greatest amount of charge. For the 1 system, this is in qualitative agreement with previous work for Ni29 and the respective Pauling electronegativities of the elements (2.55 for C, 1.54 for Ti). Unsurprisingly, the formation of strong bonds between the Ti and C atoms in the 3 system renders the Ti atom the most strongly cationic. Compared to the 1 system, the 3 system presents a different electrostatic environment and involves a Ti atom that does not have a depleted d orbital. Therefore, the hydrogen adsorption properties of this substitutional dopant (3) are expected to be different from those of the interstitial dopant (1). To investigate whether vacancies truly pin Ti atoms, thereby preventing coalescence, several simulations were performed on multiply defected (8,0) nanotubes using the low-c cell. Ti adsorption on a pair of widely separated vacancies versus Ti dimer formation on a perfect hexagon of the same nanotube was investigated. We found that that the vacancy-pinned Ti geometry is 12.68 eV lower in energy than the Ti dimer geometry. We also considered the formation of Ti clusters on a vacancy, comparing a quadruple vacancy with four pinned Ti atoms to a four-atom Ti cluster pinned on a single vacancy. The former structure is 7.77 eV lower in energy. Both results

17458 J. Phys. Chem. C, Vol. 112, No. 44, 2008 indicate a very strong thermodynamic driving force preventing nanoparticle coalescence. Hydrogen Adsorption on Ti Dopant. The molecular adsorption of hydrogen onto transition-metal dopants adsorbed on a carbon hexagon can be described using the 18-electron rule30

Shevlin and Guo

NH)18 - nv - 6 where 18 represents fully occupied s, p, and d levels; nv is the number of valence electrons of the TM, and 6 is the number of electrons contributed by the hexagon. Hydrogen binding in-

Figure 1. (Top) Structure (LDA) of three types of defects (gray and purple represent carbon and titanium, respectively): (a) 1, (b) 2, (c) and 3. In 3, Ti binds to two types of C atoms because of the effects of curvature, one aligned along the tube axis (C1) and one off the axis (C2). (Bottom) CDD plots for three different dopant structures. Red and blue represent electron density accumulation and electron density depletion, respectively.

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Figure 2. (Top) Relaxed geometries of the structure of 3 (LDA) upon initial and maximal hydrogenation. White represents hydrogen. (Bottom) Successive H2 binding energies for (A,B) low-c and (C,D) high-c structures. The hydrogen content in each graph increases toward the right. The dot-dashed line represents the H2 binding energy for physisorption on a defect-free nanotube.

volves the transfer of an electron from the H2 σ state to an empty d state. Therefore, TM dopants are able to adsorb hydrogen. However, to coordinate with H2, the dopant should have several strongly localized empty d states on and near the lowest unoccupied molecular orbital (LUMO). This is the case for the 1 system, which has been discussed by several authors15,16 and thus is not considered here. Test calculations on a Ti dopant adsorbed on a graphene 2 defect showed that it adsorbs only three H2 molecules, less than expected from the 18-electron rule. Thus, we considered only the hydrogenation of the 3 system, which has empty d states near the LUMO. Additionally, because the Ti atom binds to only three carbon atoms, it could bind more hydrogen. The strength of successive H2 binding to the Ti dopant was determined as

EBind(H2) ) ETot(X + H2) - ETot(X) - ETot(H2) where ETot(X + H2) is the total energy of the relaxed system upon hydrogenation, ETot(X) is the total energy of the relaxed system before hydrogenation, and ETot(H2) is the energy of an isolated hydrogen molecule in free space. We present the results for H2 adsorption on the exterior of the tube, as adsorption occurs on the exterior in molecular form.31-33 Additionally, if

hydrogen is present in the interior of the tube, it will chemisorb endohedrally to the carbon network and not be present in molecular form.34,35 The geometries and the binding energies of the hydrogenated states are shown in Figure 2. Initial hydrogen adsorption is on the Ti dopant and the C1 atom (see Figure 1), as this results in the least-stressed bond. This first hydrogenation is in atomic form: the dopant induces dissociation. We note that this does not induce a breaking of the Ti-C bond, as the Ti dopant binds to the σ band of the atom next to the carbon vacancy whereas the hydrogen atom binds to the π band of the nanotube that is oriented perpendicular to the tube surface. Structures in which one H is on the Ti and the other H is on C2 or C3 are higher in energy by 0.44 eV. If the hydrogen atoms are placed on the exterior of the tube so as to attack the carbon atoms that bind the Ti, then the resulting structure is 0.13 eV higher in energy. It is thus unlikely that CsH bond formation will replace CsTi bond formation. Initial hydrogenation of the Ti atom was found to be molecular in nature, but atomic adsorption on the Ti and C atoms was found to be energetically preferable by 0.44 eV. However, subsequent hydrogenation is molecular in nature, thus implying good kinetics for hydrogen storage and release.

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Figure 3. Electronic structure of the LDA five-H2 high-c structure. (A) Total DOS and projections onto the s orbitals of the H and the d orbitals of the Ti atoms. The dashed line represents the Fermi energy. Note that, for comparison purposes, the total DOS is reduced by a factor of 2. (B-D) Isosurfaces of the HOMO state at different values of Ψ. (B) For Ψ ) 0.04, there is a strong contribution from the π states of the CNT. (C) At Ψ ) 0.02, the Ti d state becomes visible. (D) At Ψ ) 0.01, there is an overlap between the Ti atom and the π states of the nanotube.

Both LDA and GGA high-c systems bind five H2, with four H2 molecules plus one H atom bound to the Ti and one H atom bound to the C atom. For the LDA, the bond length in the H2 molecule is stretched significantly (6-11%), whereas for the GGA, the H2 bond length is stretched only slightly (0-1%). This is reflected in the value of EBind: For the LDA simulation, EBind varies from -0.34 to -0.63 eV/H2, all in the range of desired binding energies for technological applications. However, for the GGA, this is not the case, as EBind ranges from -0.05 to -0.46 eV/H2. It is known that the LDA tends to overestimate binding energies compared to the GGA for most systems;36 indeed, we believe that this is the case for the initial atomic H adsorption on the titanium dopant. However, the additional molecular hydrogenation of the Ti atom is poorly described by the GGA. For the particular case of nondissociative H2 adsorption on carbon24 and on dopants with strongly localized empty states,10 it has been shown that the LDA describes H2 adsorption more accurately than the GGA. Therefore, we consider the LDA results to be more accurate than the GGA results (with the “true” EBind value somewhere between the two) for this particular system, which involves a peculiar combination of chemisorption and physisorption. We emphasize that, for both simulations, the binding of H2 to the Ti dopant is enhanced with respect to H2 binding to a pure carbon nanotube; that is, doping with titanium enhances the thermodynamics of hydrogen storage. For the low-c structure, the LDA still retains five H2 molecules, but the GGA retains only three. Finally, we note that the Ti atom directly binds four H2 molecules plus one H atom, not five H2 molecules plus one H atom as expected from the application of the 18-electron rule. We attribute this to the nature of the binding of the dopant to the carbon atoms, with the mixture of atomic orbitals involved preventing pure Kubas-type

liganding. However, the ligand binding for this system can be formally described by a 16-electron rule, although the structures we obtained are not square-planar and so would seem to exclude this possibility. To understand the details of binding of H2 to Ti, the density of states (DOS) as projected onto the pseudoatomic orbitals for the maximally hydrogenated five-H2 structure was calculated (see Figure 3). The Ti 4p orbitals are intact and lie 30 eV below the Fermi level. The Ti s orbitals are located approximately 10 eV below the Fermi level. There is significant d-orbital density on either side of the Fermi level. Surprisingly, however, whereas the LUMO state has a very strong overlap with the Ti d orbitals, the highest occupied molecular orbital (HOMO) state does not; instead, the HOMO (although it has d character) has a very strong overlap with the π states of the CNT. This is certainly because the Ti resides in the sp2 network of the nanotube. The hybridization of the Ti d states with the dangling-bond π states of the vacancy occurs 1.5 eV below the Fermi level. The H2 σ bonds form a band with the Ti d orbitals between 10 and 6 eV below the Fermi level: clearly, the H2 is bound to the dopant. This was further checked via calculation of the ∆F, as seen in Figure 4. The H2 donates charge density into the d states in both LDA and GGA simulations, albeit with different relative magnitudes. In the five-H2 structure, both the LDA and GGA simulations show the formation of a coordinate covalent bond between the H2 and the Ti. However, the magnitude of charge transfer is different: with the LDA, the H2 transfers 0.05e into the bond, whereas with the GGA, the H2 transfers 0.03e into the bond. This explains the low binding energies observed with the GGA.

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Figure 4. CDD for the LDA and GGA structures for addition of (A,C) two H2 molecules and (B,D) five H2 molecules. Red and orange represent accumulation in electron density and depletion in electron density, respectively.

Next, we considered the process of hydrogen storage and release. The free energy per metal atom upon successive hydrogenations is defined as

F(metal) ) [EBind(nH2) - nHµH(T,p)]/nm where nH is the number of hydrogen atoms; nm is the number of metal atoms; and µH is the chemical potential of hydrogen, determined by the method of Reuter et al.,37 at T ) 298.15 K. The free energies as a function of xc functional and Ti concentration as a function of pressure are shown in Figure 5. Full hydrogenation of the high-c structure occurs at 58.6 atm (64.3 atm for the GGA), whereas the low-c structure is fully hydrogenated at 63.6 atm (69.8 atm for the GGA, albeit for a loading of three H2 molecules). These pressures for hydrogenation are the same order of magnitude as those required for the hydrogenation of nanostructured carbon materials, but with much better thermodynamic properties. The stability of the C63Ti(H2)5 LDA system at room temperature was modeled by performing molecular dynamics calculations for 2 ps at 300 K, with only thermal fluctuations in energy and structure observed. In contrast, MD simulations at 600 K for 0.6 ps showed the structure losing three H2 molecules to free space. This system is stable at room temperature. This dopant structure can be considered as the building block for a hydrogen storage system. Therefore, several simulations on a C112Ti16 tube were performed (see Figure 6). The most

favorable structure has four Ti atoms distributed evenly around the circumference of the nanotube with the Ti atoms aligned in lines along the axis, as also observed for the case of titanium adsorption on defect-free nanotubes.20,38 The separation between Ti atoms is 4.23 Å along the axis and 6.13 Å around the circumference. An alternative in which the Ti atoms are rotated by 45° around the circumference for every increment along the tube axis (resulting in Ti-Ti distances of 5.36 Å) is 3.18 eV higher in energy. Each Ti atom induces changes on the Ti dopants around it, rearranging d-orbital electron density so that there is a stronger overlap, and thus binding, of the Ti dopants with the carbon network. As the line structure has the shorter Ti-Ti distance, and thus stronger additional stabilization, it is the more favorable structure. However, there is no direct Ti-Ti interaction and thus no tendency for Ti-Ti clustering; thus, the dopants are pinned to the tube. Five H2 molecules were placed on each of the dopants, with the resultant structure relaxing into a symmetric and stable ground state. All H2 molecules have stretched bonds, indicating that they are strongly bound. This C112Ti16H160 structure has a reversible gravimetric system storage capacity of 7.1 wt %, exceeding the U.S. DOE target for 2010. To determine whether the results presented to date are specific to (8,0) semiconducting nanotubes, we determined Ti binding energies and initial hydrogenation properties for larger-diameter [(11,0 and (12,0)] zigzag nanotubes and a (4,4) armchair

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Figure 5. Free energies for H2 adsorption as a function of gas overpressure. LDA structures in the (A) high-c and (C) low-c unit cells and GGA structures in the (C) high-c and (D) low-c structures.

nanotube. The (8,0) carbon nanotube that forms the basis for the majority of the work presented is semiconducting, as is the larger-diameter (11,0) nanotube, whereas the (12,0) nanotube is metallic. This comparison allows us to look at the effects of both metallicity and diameter on the hydrogen storage properties of Ti-doped vacancy-defected zigzag nanotubes. We found that the binding energy of the Ti atom (with respect to the atomic reference) is -7.91 eV/Ti for the (11,0) nanotube and -7.89 eV/Ti for the (12,0) nanotube, very close to that for the (8,0) nanotube. Thus, the energetics of Ti-atom insertion into the vacancy are not sensitive to nanotube diameter. Indeed, the electrostatic environment of the Ti atom is very similar for all three types of zigzag nanotubes studied, with all systems donating ∼1.45e to the tube. Moreover, the initial hydrogenation properties of all three nanotubes are similar: with EBind ) -0.42 eV/H2 for the (11,0) nanotube and EBind ) -0.37 eV/H2 for the (12,0) nanotube. As the Ti atom is in very similar electrostatic environments and has very similar initial hydrogenation properties, we conclude that our results are quite applicable to all small-diameter zigzag nanotubes. We also considered the interaction of a Ti atom with a vacancy of a metallic (4,4) armchair nanotube. As for the zigzag nanotubes, the Ti atom inserts itself into the carbon network with no appreciable barrier. The binding energy (with respect to the atom reference) is -9.82 eV/Ti, significantly higher than that of the (8,0) nanotube, indicating greater stability with respect to Ti nanoparticle coalescence. The Ti atom donates 1.41e to the nanotube, which is of similar magnitude to the result for the zigzag nanotubes. Initial hydrogenation of this structure is quite different, with the three C atoms neighboring the Ti atom preferentially adsorbing a single H atom first, before the Ti atom itself binds two H2 molecules plus one H atom. Although the Ti dopant itself binds less hydrogen than the (8,0) nanotube, the system as a whole has a hydrogen storage capacity of four H2 molecules, almost equal to that of the (8,0) nanotube.

Additionally, the successive binding energies for all adsorbed hydrogen are in the range from -0.67 to -0.35 eV/H2 and, thus, are in the range of technologically useful energies for applications. Our criterion for determining whether a H2 is stably bound to the dopant is 2-fold: that the binding energy should be negative (stability condition) and that the H2 bond is lengthened (binding condition). For the armchair system, four H2 molecules plus one H atom can bind to the dopant, although only two H2 molecules plus one H atom are stable. This indicates that, electronically, the maximum number of hydrogens that can bind to a Ti dopant pinned in a vacancy is governed by a 16electron rule, and combined with the information for the zigzag nanotubes, we suggest that this conclusion can be generalized to all carbon nanotubes. Conclusions We show by ab initio DFT simulations that the adsorption of atomic titanium on defect-free and Stone-Wales-defected metallic (8,0) carbon nanotubes is unstable with respect to nanoparticle coalescence, whereas the binding of Ti to vacancy defects pins the atoms. The nature of Ti-atom dopant binding to the defects is different from the binding to the pure carbon nanotube: significantly, there are no pure Dewar-type interactions, with the Ti hybridization with the nanotube caused by a mixture of atomic orbitals. The hydrogen storage properties of the vacancy-pinned Ti atom system were determined. A single Ti atom can bind five H2 molecules, with enhanced binding energies compared to the defect-free nanotube. The mechanism of binding was found to be similar to the Kubas interaction of H2 with metal atoms,15 with σ-bond donation into the localized empty d states of the Ti atom. However, the maximum number of H2 molecules that can be bound by the titanium is less than what is predicted by the 18-electron rule, which we ascribe to the nature of Ti

High-Capacity Room-Temperature Hydrogen Storage in CNTs

J. Phys. Chem. C, Vol. 112, No. 44, 2008 17463 within the range required to meet the U.S. DOE targets for near ambient storage. The stability of the hydrogen binding to the Ti defect was determined as a function of temperature and pressure. Molecular dynamics simulations show that the complex is stable at 300 K but not at 600 K, while calculations of the free energy as a function of hydrogen chemical potential (and thus implicitly a function of temperature and pressure) reveal that the H-rich structures are more stable with increasing pressure, with maximum hydrogenation at room temperature occurring at 48.5 atm. Hydrogen storage can thus be easily and smoothly controlled by varying the pressure of hydrogen gas. Additionally it was shown that the formation of a large-scale regularly doped C112Ti16H160 nanotube is both stable and binds hydrogen at a gravimetric storage capacity of 7.1 wt %, in line with U.S. DOE targets. Preliminary simulations on other zigzag nanotubes were also performed. The Ti adsorption energy and the electronic environment are not strongly modified by increasing the tube diameter. Our results are thus applicable to small-diameter zigzag nanotubes in general. A single dopant-modulated armchair nanotube was also simulated. We found that, although the specifics of the hydrogenation properties are different, the gross hydrogen storage properties are similar. In general, from electronic arguments, the maximum number of hydrogens that can bind to a TM dopant bound to a vacancy follows a 16-electron rule, although whether the H2 binding is stable or metastable is nanotube-dependent. Titanium-atom implantation in vacancies is a general method for improving the hydrogen storage properties of all types of carbon nanotubes. This system can be regarded as intermediate between carbon nanotubes and metallocarbohedrenes, exhibiting gravimetric and hydrogen binding properties that are similar to those of the latter, but structural and electronic properties that are similar to those of the former. The experimental formation of such a structure will be challenging, requiring atomic-scale control not only of defect creation but also of titanium implantation. However, novel synthesis techniques are advancing rapidly. Thus, it is of great interest to predict and design promising systems, so as to stimulate practical developments. We believe that this is a very promising system for room-temperature hydrogen storage and is thus of considerable significance and benefit to the wider hydrogen storage community. Acknowledgment. This work was supported by the EPSRC under the Sustainable Hydrogen Energy Consortium (GR/ S26965/01, EP/E040071/1) and a Platform Grant (GR/S52636/ 01, EP/E046193/1). References and Notes

Figure 6. Structures of a highly doped C112Ti16 nanotube: (A) lowestenergy doped nanotube, (B) higher-energy alternative, and (C) hydrogenated nanotube.

bonding to the carbon substrate. There are considerable differences in the H2 binding energies as found by the LDA or GGA, with the LDA binding more strongly than the GGA. As it has been observed that the nondissociative binding of H2 to dopants with strongly localized empty states is better described by the LDA, we opted to use this functional in our simulations. Although this approach has been used several times,39,40 it has not being tested at the post-Hartree-Fock level for transitionmetal dopants and is worthy of further investigation. The binding energies are all within the range from -0.2 to -0.7 eV/H2,

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