Enhanced Hydrogen-Storage Capacity and Structural Stability of an

Article Cite This: Chem. Mater. 2018, 30, 3028−3039

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Enhanced Hydrogen-Storage Capacity and Structural Stability of an Organic Clathrate Structure with Fullerene (C60) Guests and Lithium Doping Yesol Woo,†,# Byeong-Soo Kim,‡,# Jong-Won Lee,§ Jeasung Park,∥ Minjun Cha,⊥ Satoshi Takeya,¶ Junhyuck Im,□ Yongjae Lee,□ Tae-In Jeon,■ Hyeonhu Bae,○ Hoonkyung Lee,*,○ Sang Soo Han,● Byung Chul Yeo,● Dongseon Kim,†,△ and Ji-Ho Yoon*,†,‡,△ †

Department of Convergence Study on the Ocean Science and Technology, Ocean Science and Technology (OST) School, Department of Energy and Resources Engineering, and ■Division of Electrical and Electronics Engineering, Korea Maritime and Ocean University, Busan 49112, Korea § Department of Environmental Engineering, Kongju National University, Chungnam 31080, Korea ∥ Intelligent Sustainable Materials R&D Group, Korea Institute of Industrial Technology (KITECH), Chungnam 31056T, Korea ⊥ Department of Energy and Resources Engineering, Kangwon National University, Chuncheon 24341, Korea ¶ National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8569, Japan □ Department of Earth System Sciences, Yonsei University, Seoul 03722, Korea ○ Department of Physics, Konkuk University, Seoul 05029, Korea ● Computational Science Center, Korea Institute of Science and Technology, Seoul 02792, Korea △ Marine Chemistry & Geochemistry Research Center, Korea Institute of Ocean Science and Technology, Gyeonggi 15627, Korea

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S Supporting Information *

ABSTRACT: An effective combination of host and guest molecules in a framework type of architecture can enhance the structural stability and physical properties of clathrate compounds. We report here that an organic clathrate compound consisting of a fullerene (C60) guest and a hydroquinone (HQ) host framework shows enhanced hydrogen-storage capacity and good structural stability under pressures and temperatures up to 10 GPa and 438 K, respectively. This combined structure is formed in the extended β-type HQ clathrate and admits 16 hydrogen molecules per cage, leading to a volumetric hydrogen uptake of 49.5 g L−1 at 77 K and 8 MPa, a value enhanced by 130% compared to that associated with the β-type HQ clathrate. A close examination according to density functional theory calculations and grand canonical Monte Carlo simulations confirms the synergistic combination effect of the guest−host molecules tailored for enhanced hydrogen storage. Moreover, the model simulations demonstrate that the lithium-doped HQ clathrates with C60 guests reveal exceptionally high hydrogen-storage capacities. These results provide a new playground for additional fundamental studies of the structure−property relationships and migration characteristics of small molecules in nanostructured materials.

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INTRODUCTION Solid-state hydrogen storage is a promising alternative to overcome economic and safety issues for mobile applications.1 Much effort has been devoted to finding the best materials with chemically and physically active binding sites for hydrogen, which include metal hydrides, metal−organic frameworks (MOFs), and carbon materials.2−9 Carbon nanotubes, graphene, fullerene, and hybrid complexes of these materials are also attracting attention in the scientific community as potential materials for safe hydrogen storage.10−16 Although these materials have the potential to meet the DOE targets for © 2018 American Chemical Society

hydrogen storage, certain problems remain, such as the high levels of energy consumption associated with the release of hydrogen, before commercial applications can be realized. Clathrates, which are crystalline inclusion compounds stabilized by interactions between host and guest molecules in the host framework architecture, have been studied as practically beneficial and environmentally sustainable media for Received: February 20, 2018 Revised: April 16, 2018 Published: April 16, 2018 3028

DOI: 10.1021/acs.chemmater.8b00749 Chem. Mater. 2018, 30, 3028−3039

Article

Chemistry of Materials

Figure 1. Structural characteristics of C60−HQ clathrate. (a) Capped-stick representation of C60−HQ clathrate structure viewed along the (001) direction with C in gray, O in red, and H omitted for clarity. White lines define the unit cell. (b) Cage and guest (C60) structure viewed perpendicular to the c-axis. (c) Cuboctahedron structure formed by molecular centers of 12 HQ molecules around C60. (d) Rietveld refinement for C60−HQ clathrate: observed data (crosses), calculated profile (solid line), and difference (blue line). Green vertical ticks represent positions of calculated reflections. (e) Solid-sate 13C CP/MAS NMR spectra of C60−HQ clathrate.

gas storage over the past few decades.17−20 Clathrate structures possess suitably sized cages that can trap relatively small molecules such as CO2, CH4, and N2 and large hydrocarbons such as adamantine and methylcyclohexane in the hydrogenbonded framework. However, hydrogen is too small to be readily stored in the cages of clathrate structures under moderate conditions. Under high-pressure conditions, clathrate hydrates are capable of trapping hydrogen in the small and large cages of the structure II hydrate structure.21,22 When tetrahydrofuran molecules are introduced as a promoter, the pressure required to store hydrogen decreases to 10 MPa with an undesirable reduction in the hydrogen-storage capacity.23,24 Additionally, the use of these molecules may be restricted to a narrow range of temperatures near the melting point of ice, stemming from their stability regime in the temperature− pressure space. Organic clathrates are also promising materials for gas storage, particularly at relatively moderate pressures and in a wide range of temperatures. Hydroquinone (HQ), phenol, and Dianin’s compound have served as the backbone molecules for an extensively studied class of organic clathrates.19,25−29 Among these, the HQ clathrate comprises a flexible hydrogen-bonded organic framework (HOF) entrapping relatively small gaseous molecules such as Ar, Xe, CO2, and CH4 and large hydrocarbons such as methanol, formic acid, and formamide. HOFs can potentially adopt one-dimensional (1D) channel structures along the hexagonal c-axis, as defined by the hexagonal entrance of hydrogen-bonded host HQ molecules. The flexible hexagon allows the migration of guest molecules into the HOFs via a dynamic pore-widening process, leading to the controlled and reversible storage of small molecules.30,31 However, HQ clathrates are unsuitable for practical applications at high temperatures because of their poor thermal stability, which stems from their structural transformation and inherent sublimation characteristic.28,32,33 Another important drawback

pertaining to the structural stability of the HQ clathrates is that they undergo structural transformations under high pressures and exhibit a gradual release of guest molecules from the HOF framework.34−36 Here we report that the C60-loaded HQ (C60−HQ) clathrate reveals enhanced structural stability because of the presence of large guest C60 molecules templating the cages. We also show that the C60−HQ clathrate structure can trap hydrogen molecules in nanosized confinements to form hydrogenbonded two-dimensional (2D) hexagons and for space filling between guest and host molecules. A synergistic effect on the enhanced hydrogen-storage capacity of the host HQ and guest C60 is theoretically confirmed using density functional theory (DFT) calculations and grand canonical Monte Carlo (GCMC) simulations.

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RESULTS AND DISCUSSION High-Pressure Behavior of HQ Clathrates. HQ is known to exist in three crystalline forms, designated as the α-, β-, and γ-forms. The rhombohedral α-form HQ (α-HQ), which forms hydrogen-bonded double helices, is the most stable phase under ambient conditions.19 The crystal structure of the β-form arises from the hydrogen bonding between HQ molecules to form two interpenetrating networks. The CO2-, CH4-, N2-, and methanol-loaded HQ (CO2−HQ, CH4−HQ, N2−HQ, and MeOH−HQ) clathrates are categorized as β-form HQ clathrates.25,26 γ-Form HQ can be obtained by sublimation of α-HQ. The C60−HQ clathrate is a 3:1 complex of HQ and C60 which forms HOFs with supercages in a distorted cuboctahedron architecture; these can accommodate large C60 guest molecules (Figure 1a−c).37−39 Synchrotron powder X-ray diffraction (XRD) and Rietveld refinement results show that the crystal structure of the as-synthesized C60−HQ clathrate reveals rhombohedral space group R3̅m and hexagonal dimensions of a = 16.21636(6) Å and c = 13.84268(6) Å 3029


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Figure 2. Pressure-dependent structural stability. (a) Pressure-dependent synchrotron XRD patterns of C60−HQ clathrate. The Q value is the X-ray wave-scattering factor defined by Q = (4π sin θ)/λ = 2π/d. (b) Normalized unit cell volume of HQ clathrates as a function of pressure. Lines are the Birch−Murnaghan equation of state (BM-EoS) fit. Cage structures of C60−HQ clathrate at ambient pressure and 9.7(1) GPa are shown along the (001) and (010) directions. C60 molecule and hydrogen atoms in HQ molecules are omitted for clarity. Pressure-induced change in (c) the unit cell lengths and (d) the O−O length and O−O−O angle of the hexagon formed by six HQ molecules of C60−HQ clathrate.

(Figure 1d and Table S1). We propose to call an extended βform HQ in that the unit cell has 2.5 times longer c-axis compared to that of the β-form HQ. The XRD patterns show a clear difference between α-HQ and HQ clathrates and close structural similarity between the β-form and extended β-form HQ clathrates (Figure S1).37,40,41 Solid-state 13C crosspolarization/magic-angle spinning (CP/MAS) NMR spectroscopy is also useful to identify the crystal structure and hydrogen-bonding characteristics of HQ clathrates (Figure 1e and Figure S2). A sharp single resonance line at 142.7 ppm is assigned to the carbon atoms of the guest C60, which are all chemically equivalent.42,43 The chemical shift of carbon atoms attached to the hydroxyl groups of host HQ molecules appears

at 151.3 ppm. Interestingly, a single NMR signal at 118.4 ppm represents crystallographically equivalent nonsubstituted hydroxyl carbons, clearly different from the two split NMR signals of CO2−HQ and MeOH−HQ clathrates (Figure 1e, Figures S2 and S3).30,33,44 Fullerene (C60), a very stable solid, has a high-symmetry truncated icosahedral structure with a round hollow cage.45,46 Although the face-centered cubic (fcc) phase of bulk C60 (fullerite) shows an isothermal bulk modulus of K = 18.1 GPa and K′(dK/dP) = 5.7, the compressibility of the C60 molecule is significantly lower than that of the bulk fullerite solid.47 Therefore, the pseudospherical structure of C60 with a 3030


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Figure 3. Temperature-dependent structural stability. (a) Temperature-dependent solid-sate 13C CP/MAS NMR spectra of C60−HQ clathrate. Temperature-dependent Raman spectra of (b) CO2−HQ and (c) MeOH−HQ clathrates. (d) Graphical representation of temperature-dependent structural changes in β-form and extended β-form HQ clathrate structures. Hydrogen-bonding structures of α-form HQ, β-form HQ clathrate (CO2−HQ and MeOH−HQ clathrates), and extended β-form HQ clathrate (C60−HQ clathrate) are shown along the (001) and (110) directions. Oxygen atoms of the HQ molecule are shown as red spheres. Thick lines between the oxygen atoms represent hydrogen bonding. HQ molecule is simplified with a longer thin line along the O−O axis of the molecule for clarity and all guest molecules in the cages are omitted for clarity. Blue dashed lines are the unit cell.

diameter of 7.1 Å is stable under hydrostatic compression to ∼20 GPa at ambient temperatures.47,48 In contrast to the incompressibility of the C60 molecule, the C60−HQ clathrate exhibits pressure-dependent compressible behavior (Figure 2a). Clearly, this is caused by the inherent flexibility of the HOFs, which are composed of host HQ molecules. Synchrotron XRD patterns of the C60−HQ clathrate show significant peak broadening upon an increase in the

pressure, as typically found in compressible materials. In previous studies,34,35 CH4−HQ and N2−HQ clathrates, which show anisotropic axial compression behavior, transform into a new high-pressure phase at around 4 GPa and reveal pressureinduced partial amorphization (PIA) with a further increase in the pressure. Similar behavior was also observed with the MeOH−HQ clathrate, where a phase transition and subsequent PIA occur at pressures above 4 GPa (Figure 2b and Figure S4). 3031


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The in situ temperature-dependent solid-state 13C CP/MAS NMR spectra of the C60−HQ clathrate were measured in the temperature range of 353−423 K (Figure 3a). There are no dominant changes in the chemical shift with the variation of the temperature, indicating enhanced thermal stability of the C60− HQ clathrate frameworks. For the CO2−HQ and MeOH−HQ clathrates, similar attempts to acquire in situ 13C NMR measurements were not successful because of the bursting of the NMR rotor in the temperature range of 353−373 K. This clearly implies a structural transition of β-form HQ clathrates to nonporous α-HQ and a corresponding guest release, leading to an increase in the internal pressure of the NMR rotor. We note that several preliminary tests with a volatile fluid enabled us to confirm that the pressure resistance of the rotor materials applied only up to a value of 10 bar. Instead of NMR measurements, temperature-dependent Raman spectroscopic measurements of both β-form HQ clathrates were taken in an N2 environment at ambient pressure (Figure 3b,c and Figure S7). The two split Raman peaks at 1160 cm−1 are characteristic of α-form HQ (blue lines above 393 K), whereas both β-form HQ clathrates feature a single peak (black lines below 373 K).32,52 Particularly, at 373 K, the Raman spectra of the CO2−HQ clathrate show a partial split peak at 1160 cm−1, and the Raman peak at 1382 cm−1, evidence of guest CO2 molecules encapsulated in the cages of β-form HQ clathrate,30,52 is not observed. This implies that the resulting product is a solid mixture of α-HQ and guest-free βform HQ clathrate, accomplished by the complete CO2 release from the β-form HQ clathrate framework. Additionally, the shape and relative intensities of three unresolved Raman peaks at 1601, 1611, and 1625 cm−1 allow us to confirm the structural transformation of the CO2−HQ and MeOH−HQ clathrates to α-HQ at high temperatures above 393 K (Figure 3d). Time-resolved terahertz (THz) spectroscopy is also a useful tool for monitoring the structural transformation of solid compounds, especially in wide temperature ranges. The THz absorbance spectra of β-form HQ clathrates feature only one or two resonance peaks, whereas those of α-HQ show multiple resonance peaks.30,53 The THz spectra of the C60−HQ clathrate reveal two dominant resonance peaks in the temperature range of 77−438 K, exhibiting a substantial red shift with an increase in the temperature (Figure S8). These spectral results reflect the characteristics of the enhanced thermal stability and robustness of the extend β-form HQ clathrate, particularly covering a wide range of temperatures (Figure 3d). Hydrogen-Storage Capacity and GCMC Simulations. The α-HQ is known to trap hydrogen molecules in small cages inside interstitial 1D host channels. Considering the molar ratio of the HQ molecules to the cages (18:1) and complete occupancy, that is, one hydrogen molecule per cage, the ideal hydrogen-storage capacity of the α-HQ is 0.1 wt %.40 The experimental hydrogen content of α-HQ is known to be 0.05 wt % at 10 MPa and room temperature,54 indicating that 50% of the α-HQ cages is occupied by hydrogen molecules. For the H2-loaded β-form HQ (H2−HQ) clathrate, the hydrogenstorage capacity has been found to be 0.19 wt % at 10 MPa and 0.38 wt % at 35 MPa.55 A recent study reveals that the H2−HQ clathrate at high pressures shows triple H2 occupancy accompanied by negative compressibility at pressures above 3 GPa.56 In addition to these experimental results, molecular dynamic (MD) simulations suggest that the H2−HQ clathrate can be stabilized with three and four hydrogen molecules per

Under high pressures up to 10 GPa, however, the structural integrity of the C60−HQ clathrate retains the extended β-form architecture with a compressibility of the bulk modulus K = 10.4(7) GPa and with K′ = 15.3(6), calculated using the Birch− Murnaghan equation of state (BM-EoS).49 In terms of the unit cell volume as a function of the pressure, the enhanced structural stability of the C60−HQ clathrate is clearly different from the structural transition and PIA of CH4−HQ and MeOH−HQ clathrates (Figure 2b). For the CH4−HQ clathrate, the elastic axial anisotropy is drastic, resulting in K(a):K(c) = 11.8:1.34 Under high pressure, the structural transition and PIA of β-form HQ clathrates such as CH4−HQ and MeOH−HQ can be attributed to the dramatic axial compression anisotropy. For the C60−HQ clathrate, the linearized second-order BM-EoS is also refined to give the following axial elastic parameters: K(a) = 17(3) GPa and K′(a) = 113(2) for the a-axis and K(c) = 34(3) GPa and K′(c) = 16(2) for the c-axis (Figure 2c). Surprisingly, the elastic anisotropy of the C60−HQ clathrate has been determined to be K(a):K(c) = 1:2. This therefore shows quite different compression behavior. To clarify the observed contrast in the axial compression anisotropy, we investigate changes in the cage structure and corresponding hydrogen-bonding networks of the C60−HQ clathrate upon an increase in the pressure. Assuming that the pseudospherical structure of C60 is stable and incompressible up to 10 GPa, the O−O distance and O−O−O angle of the hydrogen bonding on the ab-plane to form hexagons on the top and bottom of the supercages are observed as a function of the pressure by Rietveld refinement (Figure 2d and Table S2). Under ambient conditions, the O−O distance of the hydrogen bonding in the C60−HQ clathrate (the extended β-form) is 2.809 Å, slightly stretched in comparison to that in β-form HQ clathrates (2.678 Å).41 Upon an increase in the pressure, the O−O distance gradually decreases to a minimum (2.505 Å) at 5.7(1) GPa and then gradually increases to 2.71 Å at 9.7(1) GPa. In contrast to the O−O distance, variation of the O−O− O angle of the hydrogen bonding in the C60−HQ clathrate is marginal up to 4.7(1) GPa. However, an additional increase in the pressure results in a gradual decrease in the O−O−O angle, leading to a large increase in the thickness of the hexagon due to the chairlike puckering of the hydrogen-bonding networks (Figure 2d and Figure S5).37 Similarly, the angle α between the ab-plane for the formation of hexagons and the intramolecular O−O axis of HQ gradually decreases with an increase in the pressure (Figure S6). We note that these considerable pressureinduced changes in the hydrogen-bonding networks and thus the HOFs are fully reversible. Clearly, the large guest C60 molecules immobilized in the supercages play an important role in the enhanced structural stability of the C60−HQ clathrate. Temperature-Dependent Phase Stability of HQ Clathrates. A thermal treatment is routinely used to remove contaminants and improve the properties of various materials. For clathrate inclusion compounds, however, processing at a high temperature usually leads to an undesirable phase transformation and release of the guest, causing these compounds to lose their nanostructural characteristics. Notably, most β-form HQ clathrates, such as the CH4−HQ clathrate, are known to exhibit structural transformations accompanying guest release at around 373 K,32−34,50,51 which is closely related to the sublimation characteristic of HQ molecules. This would be a fatal characteristic considering the heating process required to remove contaminants, in particular, water vapor. 3032


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Figure 4. Hydrogen storage capacity of HQ clathrates. (a) Hydrogen storage capacity of C60−HQ clathrates, α-HQ, and fullerite as a function of pressure at 77 K. (b) Comparison of hydrogen storage capacity between C60−HQ clathrate and other clathrate compounds. Black, blue, and red symbols stand for HQ clathrates, clathrate hydrates, and silicon clathrate, respectively: C60−HQ, C60−HQ clathrate at 8 MPa and 77 K; β-HQ(1), guest-free β-HQ clathrate at 35 MPa and 298 K;55 β-HQ(2), H2−HQ clathrate at 200 MPa and 296 K;58 β-HQ(3), guest-free β-HQ clathrate at 8 MPa and 77 K; β-HQ(4), H2−HQ clathrate at 3 GPa and 298 K;56 α-HQ, α-HQ at 8 MPa and 77 K; sII-CH, structure II H2 clathrate hydrate;22 sIITHF, structure II H2−THF clathrate hydrate;23,59 sII-CHONE, structure II H2−cyclohexanone clathrate hydrate;60 sII-DXN, structure II H2−1,4dioxane clathrate hydrate;61 sH-MTBE, structure H H2−methyl tert-butyl ether clathrate hydrate;62 sI-ETN, structure I H2−ethane clathrate hydrate;63 semi-TBAB, H2−tetra-n-butylammonium bromide semiclathrate hydrate;64 semi-TMA, H2−trimethylamine semiclathrate hydrate;65 sISC, structure type I silicon clathrate.66 (c) GCMC simulation results compared with experimental values for hydrogen storage capacity of C60−HQ 3033


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Chemistry of Materials Figure 4. continued

clathrate. The shaded regions indicate uncertainties in the GCMC simulation. Snapshots from GCMC simulations for hydrogen molecules stored (d) in the hexagon of six HQ molecules, (e) around C60 molecules, and (f) in the 1 × 1 × 2 supercell at 77 K and 8 MPa. C60 and HQ molecules are shown in capped-stick representation with C in gray, O in red, and H in white (H omitted for clarity in the 1 × 1 × 2 supercell). Hydrogen molecules stored in the hexagons and around C60 molecules are represented by dark yellow and sky-blue spheres, respectively.

cage at 20 K.57 On the other hand, it is known that fullerene C60 has suitable physicochemical properties that enable it to interact with hydrogen molecules.12−15 At room temperature, the molecular centers of C60 are arranged on the fcc lattice framework. Only interstitial octahedral sites in crystalline fcc C60 are capable of accommodating a hydrogen molecule.13 To the best of our knowledge, the hydrogen-storage capacity of fullerite is 0.1−0.2 wt % at room temperature and ∼0.7 wt % at 77 K.14,15 The volumetric hydrogen-storage capacity of the C60−HQ clathrate typically increases with an increase in the pressure, reaching 49.5 g L−1 at 77 K and 8 MPa (Figure 4a). This value is significantly higher than those of guest-loaded β-form HQ clathrates (CO2−HQ and MeOH−HQ clathrates), α-HQ, and fullerite (C60) and represents an enhancement of 130% in the hydrogen-storage capacity compared to that of the guest-free βform HQ clathrate (guest-free β-HQ). The enhanced hydrogen-storage capacity is attributable to the combined effect of the guest C60 and host HQ upon hydrogen adsorption in the extended β-form HQ clathrate framework. When compared to other clathrate compounds,22,23,55,56,58−66 the volumetric hydrogen-storage capacity of the C60−HQ clathrate is greater than those of all clathrate hydrates and greater than that of silicon clathrate as reported for hydrogen storage (Figure 4b). The relatively low gravimetric hydrogen-storage capacity (2.9 wt %) of the C60−HQ clathrate is due to its nonporous structural characteristics and high crystal density 1.659 g cm−3, which is nearly identical to that of C60 itself (1.678 g cm−3).46 The structural stability of the C60−HQ clathrate during repeated H2 uptake and release cycles was monitored by synchrotron XRD measurements. After 30 cycles of repeated use, the XRD patterns at 93 and 293 K showed only marginal degradation (Figure S9). With regard to hydrogen occupancy in clathrate cages, the guest-free β-HQ clathrate structure admits 2−3 hydrogen molecules per cage at 77 K and 8 MPa, which is consistent with earlier results for H 2 −HQ clathrates.56,57 Surprisingly, we found that 16 hydrogen molecules can be stored in each cage of the extended β-form HQ clathrate framework accompanying the C60 guest (Figure 4c), even though the cage volume of extended β-form HQ is ∼2.5 times larger than that of β-form HQ. An important feature to note is that while the clathrate structure does not appear to be fully porous, hydrogen molecules diffuse through the framework and become lodged in the void spaces. Although several experimental and theoretical studies have reported the migration of guest molecules in nonporous crystalline frameworks,67,68 it is difficult experimentally to determine how the hydrogen molecules bind to the active sites and how they travel through the crystal framework. We note, however, that the combination of C60 and HQ molecules to form the clathrate framework structure may favor intercage diffusion of hydrogen molecules, which then results in the highly enhanced loading capacity of the structure. To test this hypothesis, we undertook the GCMC simulations of the hydrogen-storage behavior of the C60−HQ clathrate. The simulation results for the hydrogen-storage

capacity are in reasonable agreement with the experimental values, though there is a small difference in the low-pressure region (Figure 4c). Snapshots of the GCMC simulations show an occupation outcome in which more than 16 hydrogen molecules can be stored in the cage with C60 in the HOFs of the extended β-form architecture (Figure 4d−f and Figure S10). Clearly, the simulation results indicate that most of the hydrogen molecules are stored in the void spaces between C60 and the 12 HQ molecules forming the distorted cuboctahedral confinements (Figures 1c and 4f). Surprisingly, the hexagons which represent the hydrogen bonding are fully occupied by a single hydrogen molecule (dark yellow spheres in Figure 4d and Figure S10). Along with the topological similarity of the βform and the extended β-form clathrate frameworks,37,41 an important clue pertaining to the occupation behavior of hydrogen molecules in the hexagons comes from two previous studies involving high-pressure experiments above 3 GPa and MD simulations at 20 K.56,57 Our DFT calculations show that the binding affinity of H••••C between the H2 and C(H) of HQ is stronger than that of H••••O between the H2 and O(H) of the hexagon; the binding energy of H••••C is calculated to be 0.41 kJ mol−1, greater than the corresponding value of 0.32 kJ mol−1 for H••••O (Table S3). Therefore, as shown in two previous studies,56,57 the hexagon formed by six HQ molecules is not a preferential binding site for hydrogen storage but is instead an accessible space for multiple hydrogen storage in the cages of the β-form clathrate framework. From a crystallographic perspective, however, the hexagon structure in the extended β-form clathrate framework is in somewhat of a different environment because the hexagons are sandwiched between two adjacent C60 molecules in the up and down positions.37 Similar environments for hydrogen storage are found in fullerites when they form an interstitial octahedral space between two C60 molecules in the fcc lattice.13 Therefore, it is likely that the hexagons as open 2D binding sites on the abplane are effectively stabilized by the neighboring C60 molecules along the c-axis perpendicular to the center of the hexagons. The increasing affinity of hydrogen toward the hexagons in the C60−HQ clathrate framework is strongly affected by the adjacent C60 molecules. More importantly, the synergistic combination of host HQ and guest C60 is certainly responsible for the enhanced hydrogen-storage capacity of the C60−HQ clathrate. The hydrogen-storage capacity of the C60−HQ clathrate is still far from the DOE targets and the test condition for hydrogen storage is far below the ambient temperature, but this is the first attempt, to the best of our knowledge, to realize hydrogen storage in a clathrate framework with large guest molecules of C60. For higher hydrogen-storage capacities and practical applications, it may be necessary to modify the host framework structure and/or guest functionalities, including cation-doped HQ frameworks, functionalized fullerenes, and open-cage fullerenes.16,69−71 For a new design strategy, we explore the hydrogen-storage behavior in Li-doped C60−HQ clathrates using GCMC simulations (Methods and Table S3). Clathrates are known to be nonstoichiometric compounds.17−19,25,26 This means that 3034


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Figure 5. GCMC simulations for hydrogen storage in Li-doped HQ clathrates. (a) Molecular structures of Li-doped HQ for (left) HQ:Li = 1:1 and (right) HQ:Li = 1:2. C, O, H, and Li atoms are represented in gray, red, black, and green spheres, respectively. (b) Hydrogen storage capacity in gravimetric units (wt %) for C60−HQ clathrates with partially empty cages. (c) Hydrogen storage capacity in gravimetric units (wt %) for Li-doped C60−HQ clathrates. (d) Snapshots from GCMC simulations for hydrogen molecules stored in (left) HQ3(C60)3/6 and (right) HQ3(C60)3/6Li6. C60 and HQ molecules are shown in capped-stick representation with C in gray, O in red, and H omitted for clarity. Li and H2 molecules stored in the C60−HQ clathrate framework are represented by green and sky-blue spheres, respectively. (e) Comparison of hydrogen storage capacity between C60−HQ clathrates and Li-doped C60−HQ clathrates.

HQ3(C60). Li-doped C60−HQ clathrates are constructed with partially empty C60−HQ clathrate frameworks of HQ3(C60)xLiy (x = 6/6, 5/6, 4/6, 3/6, and 2/6; y = 0, 3, and 6) doped with Li cations to bind at the center of the phenyl group of HQ molecules (Figure 5a). A previous study reported that Li atoms can activate the C−O bond between the graphene layer and organic linkers and prefer to cluster around the oxygen atoms, and then the building blocks collapse.74 Our DFT calculations show that the Li binding energy to the phenyl group of HQ molecules is ∼1.8 eV at the distance between the Li atom and HQ molecule of ∼2.0 Å, greater than ∼1.1 eV for the Li binding energy to the oxygen atoms of HQ molecules (Figure

hydrogen-bonded networks in clathrate structures can be constructed with both guest-loaded and guest-free (empty) cages. With regard to the CO2−HQ clathrate, the cage occupancy was found to be 0.74, leading to the resulting chemical formula of HQ3(CO2)0.74, with 26% of the cages empty in the clathrate structure. Notably, a complete guest-free β-form HQ clathrate can also be obtained by the release of guests from the CO2−HQ clathrate under controlled thermal processes30,72 and recrystallization from ethanol under controlled conditions.73 Assuming that all the cages in the C60−HQ clathrate framework are fully occupied by C60 molecules, the ideal formula of the C60−HQ clathrate is 3035


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HQ clathrate was prepared by the gas-phase synthesis method:32 Finely powdered α-HQ less than 45 μm in size was exposed to CO2 gas at ambient temperature and 5 MPa for 2 weeks. The guest-free βHQ clathrate was obtained by controlling the guest release of the assynthesized CO2−HQ clathrate.30 The C60−HQ clathrate was synthesized by dissolving α-HQ and C60 powders in benzene at 80 °C.37 This was followed by filtering and then gradually evaporating the solvent from the mixture at room temperature. The crystals formed at the top and sides of the flask bottom were collected and analyzed, and then the powdered samples with a size of

Enhanced Hydrogen-Storage Capacity and Structural Stability of an

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