Enhanced Hydrogen-Storage Capacity and Structural Stability of an

Subscriber access provided by Universitaetsbibliothek | Johann Christian Senckenberg

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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00749 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Enhanced Hydrogen-Storage Capacity and Structural Stability of an Organic Clathrate Structure with Fullerene (C60) Guests and Lithium Doping Yesol Woo,1, ‡ Byeong-Soo Kim,2, ‡ Jong-Won Lee,3 Jeasung Park,4 Minjun Cha,5 Satoshi Takeya,6 Junhyuck Im,7 Yongjae Lee,7 Tae-In Jeon,8 Hyeonhu Bae,9 Hoonkyung Lee,9,* Sang Soo Han,10 Byung Chul Yeo,10 Dongseon Kim,1,11 and Ji-Ho Yoon1,2,11,* 1

Department of Convergence Study on the Ocean Science and Technology, Ocean Science and

Technology (OST) School and 2Department of Energy and Resources Engineering, Korea Maritime and Ocean University, Busan 49112, Korea. 3Department of Environmental Engineering, Kongju National University, Chungnam 31080, Korea. 4Intelligent Sustainable Materials R&D Group, Korea Institute of Industrial Technology (KITECH), Chungnam 31056T, Korea. 5Department of Energy and Resources Engineering, Kangwon National University, Chuncheon 24341, Korea. 6National Institute of Advanced Industrial Science and Technology (AIST), Ibaraki 305-8569, Japan. 7Department of Earth System Sciences, Yonsei University, Seoul 03722, Korea. 8Division of Electrical and Electronics Engineering, Korea Maritime and Ocean University, Busan 49112, Korea. 9Department of Physics, Konkuk University, Seoul 05029, Korea.

10

Technology, Seoul 02792, Korea.

Computational Science Center, Korea Institute of Science and 11

Marine Chemistry & Geochemistry Research Center, Korea

Institute of Ocean Science and Technology, Gyeonggi 15627, Korea.

ACS Paragon Plus Environment

1

Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 34

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 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.


2

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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 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 hydrogen-bonded 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 introducing tetrahydrofuran molecules 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 Amongst 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 ACS Paragon Plus Environment

3


Page 4 of 34

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 due to 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 due to the presence of large guest C60 molecules templating the cages. We also show that the C60–HQ clathrate structure can trap hydrogen molecules in nano-sized confinements to form hydrogen-bonded 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.

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 super-cages in a distorted cuboctahedron architecture; these can accommodate large C60 guest molecules (Figures 1a–c).37–39 Synchrotron powder X-ray diffraction (XRD) and ACS Paragon Plus Environment

4

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Rietveld refinement results show that the crystal structure of the as-synthesized C60–HQ clathrate reveals rhombohedral space group R-3m and hexagonal dimensions of a = 16.21636(6) Å and c = 13.84268(6) Å (Figure 1d and Table S1). We propose to call an extended β-form HQ in that the unit cell has 2.5 times longer the c-axis compared to 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 βform HQ clathrates (Figure S1).37,40,41 Solid-state 13C cross-polarization/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 non-substituted hydroxyl carbons, clearly different from the two split NMR signals of CO2–HQ and MeOH–HQ clathrates (Figures 1e, S2, and S3).30,33,44


5


Page 6 of 34

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 grey, 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 twelve 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.

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 diameter of 7.1 Å is stable under hydrostatic compression to ~20 GPa at ambient temperatures.47,48


6

Chemistry of Materials 17.0

(a)

(c)

P decrease (GPa) 16.5

13.5

16.0

13.0

O

a-axis length (A)

Ambient 0.8 2.0 4.0

15.5

Scaled Intensity

5.4

12.5

c-axis length (A)

a-axis, K(a) = 17(3) GPa, K'(a) = 113(7) 7.7

c-axis, K(c) = 34(3) GPa, K'(c) = 16(2) 15.0

P increase (GPa)

0

2

4

6

8

12.0

10

Pressure (GPa) 9.7 2.90

7.1

(d)

O-O distance O-O-O angle

O

5.7 3.5 1.7 0.4 Ambient

0.5

1.0

1.5

2.0 O

-1

Q (A )

2.5

O-O distance (A)

2.85

120 115 110

2.80

105

2.75

100

2.70 95

2.65

90

2.60

85

2.55

80

2.50

75

0

2

4

6

8

10

Pressure (GPa)


7

O-O-O angle (degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14.0

O

Page 7 of 34


Page 8 of 34

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·sinθ)/λ = 2π/d. (b) Normalized unit cell volume of HQ clathrates as a function of pressure. Lines are the 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.

In contrast to the incompressibility of the C60 molecule, the C60–HQ clathrate exhibits pressuredependent 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). 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 βform 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 ACS Paragon Plus Environment

8

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 super-cages 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 chair-like 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 pressure-induced changes in the hydrogen bonding networks and thus the HOFs are fully reversible. Clearly, the large guest C60 molecules immobilized in the super-cages 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 ACS Paragon Plus Environment

9


Page 10 of 34

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. (a)

(b) α-form

extended β-form

423 K

423 K

393 K

β-form

418 K 373 K

413 K 353 K

408 K

298 K

403 K 398 K

1100

1200

388 K 383 K

1300

1400

1500

1600

1700

−1

393 K

Raman shift (cm )

(c) α-form

378 K

423 K

373 K

393 K

β-form

368 K

373 K

363 K

353 K

358 K 353 K

160

150

140

130

120

110

298 K

1100

1200

1300

1400

1500

1600

2850

3000

3150

−1

Raman shift (cm )

Chemical shift (ppm)


10

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Temperature-dependent structural stability. (a) Temperature-dependent solid-sate

13

C

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, βform 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

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

13

C NMR measurements were not successful due to 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 non-porous 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 only applied 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 (Figures 3b and 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 αHQ and guest-free β-form 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 ACS Paragon Plus Environment

11


Page 12 of 34

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, i.e., 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 are occupied by hydrogen molecules. For the H2-loaded β-form HQ (H2–HQ) clathrate, the hydrogen-storage 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 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


12

Page 13 of 34

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 60

C60−HQ

(a)

guest-free β-HQ 50

Volumetric H2 uptake (g/L)

Volumetric H2 Uptake (g/L)

60

MeOH−HQ CO2−HQ

40

α-HQ

fullerite (C60)

30 20 10

50

(b) C60−HQ

40 sI-ETN

30

sII-CH sI-SC

β-HQ(2)

2

4

6

0 0.0

8

β-HQ(4)

sII-CHONE

sH-MTBE sII-THF semi-TBAB semi-TMA sII-DXN α-HQ

10

0 0

β-HQ(3)

β-HQ(1)

20

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Gravimetric H2 uptake (wt %)

Pressure (MPa) 20


60

C60−HQ (experiment)

(c)

C60−HQ (GCMC simulation) 50

15

40 10

30 20

5 10

Number of H2 per cage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

0 0

2

4

6

8

Pressure (MPa)


13


Page 14 of 34

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 K55; β-HQ(2), H2–HQ clathrate at 200 MPa and 296 K58; 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 K56; α-HQ, α-HQ at 8 MPa and 77 K; sII-CH, structure II H2 clathrate hydrate22; sII-THF, structure II H2–THF clathrate hydrate23,59; sII-CHONE, structure II H2– cyclohexanone clathrate hydrate60; sII-DXN, structure II H2–1,4-dioxane clathrate hydrate61; sH-MTBE, structure H H2–methyl tert-butyl ether clathrate hydrate62; sI-ETN, structure I H2–ethane clathrate hydrate63; semi-TBAB, H2–tetra-n-butylammonium bromide semiclathrate hydrate64; semi-TMA, H2– trimethylamine semiclathrate hydrate65; sI-SC, structure type I silicon clathrate66. (c) GCMC simulation results compared with experimental values for hydrogen storage capacity of C60−HQ 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 grey, 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.

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 β-form 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 ACS Paragon Plus Environment

14

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


non-porous 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 H2–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 non-porous 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 (Figures 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 twelve 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 β-form and the extended β-form clathrate ACS Paragon Plus Environment

15


Page 16 of 34

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-from 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 ab-plane 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


16

Page 17 of 34

8

5

(c) Gravimetric H2 uptake (wt %)

(b) Gravimetric H2 uptake (wt %)

4

3

2 77 K, 8 MPa 77 K, 6 MPa 77 K, 4 MPa 77 K, 2 MPa

1

0 6/6

5/6

4/6

3/6

2/6

6

4

77 K, 8 MPa 77 K, 6 MPa 77 K, 4 MPa 77 K, 2 MPa

2

0 0

3

6

y of HQ3(C60)3/6Liy

x of HQ3(C60)x

(d)

(e)

85 80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


75 70 65 HQ3(C60)6/6

60

HQ3(C60)5/6 HQ3(C60)4/6

55

HQ3(C60)3/6

50

HQ3(C60)2/6 HQ3(C60)3/6Li3

45 40 3.0

HQ3(C60)3/6Li6

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Gravimetric H2 uptake (wt %)


17

7.0


Page 18 of 34

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 grey, 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 grey, 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.

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 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 βform 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 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 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 S11). This indicates that the Li atoms prefer to ACS Paragon Plus Environment

18

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bind to the phenyl group of HQ molecules and the optimized configuration of Li-doped C60–HQ clathrate frameworks in Figure 5a is suitable for practical use. Starting with HQ3(C60)x at y = 0, we undertook GCMC simulations to predict the hydrogen-storage capacity at 77 K (Figure 5b). At pressures above 4 MPa, the gravimetric H2 uptake increases with an increase in the fraction of empty cages, mainly due to the reduction of the crystal density caused by the removal of C60 molecules. The effect of Li doping on hydrogen storage in the C60–HQ clathrate was assessed by means of GCMC simulations (Figures 5c and d). Similar to the Li-doped MOFs70,75–77 and Li-doped graphene frameworks,74 Li-doped C60–HQ clathrates reveal significantly improved hydrogen-storage capacities, especially in both gravimetric and volumetric units (Figure 5e). Although the hydrogen-storage capacity of Li-doped C60–HQ clathrates at room temperature is far too small for practical use (Figure S12), we emphasize that the simulation results open up a new design strategy using carbon-organic hybrid clathrate compounds doped with electropositive metals for enhanced hydrogen storage. Our simulations also show that the hydrogen-storage capacity of HQ3(C60)3/6Li6 at 77 K and 8 MPa is 6.59 wt% in gravimetric units and 81.1 g L–1 in volumetric units. Importantly, the hydrogen density of 81.1 g L–1 is greater than the liquid hydrogen density of 70.8 g L–1. This is ascribed to the fact that the intermolecular distance of absorbed H2 in Li-doped HQ clathrates is approximately 2.0 Å, much shorter than the intermolecular distance, ~3.4 Å, in liquid hydrogen mediated by van der Waals interactions. This suggests that the cation doping of clathrate compounds is a promising means of enhancing the gas storage capacity.

Conclusions We have observed that hydrogen molecules can be adsorbed in an organic clathrate structure with large guest molecules of C60. The C60–HQ complex reveals the extended β-form HQ clathrate structure, as identified by XRD and solid-state 13C NMR. Under pressure levels up to 10 GPa, the C60–HQ clathrate retains its structural integrity, while β-form HQ clathrates such as CH4–HQ and MeOH–HQ show a structural transition and subsequent PIA above 4 GPa. Moreover, the C60–HQ clathrate reflects the ACS Paragon Plus Environment

19


Page 20 of 34

characteristics of enhanced thermal stability and robustness, notably covering a wide range of temperatures. The extended β-form HQ clathrate structure with C60 guests provides an ideal route for the inclusion and facile release of hydrogen molecules, greatly enhancing the hydrogen-storage capacity. Coupled with DFT calculations, the GCMC simulations conducted here confirm the synergistic combination effect of guest C60 and host HQ molecules on the hydrogen-storage capacities. Moreover, we explore an innovative design concept for exceptionally high hydrogen storage levels involving the doping of Li cations into clathrate cages. The present study provides a useful model for examining the structure-property relationships of host-guest inclusion compounds and tailoring carbon-organic hybrid materials in relation to the development of gas storage media.

Methods Synthesis of HQ clathrates Pure α-HQ with a purity of 99.9% was used without further purification. The MeOH−HQ clathrate was produced through a simple recrystallization process:25,26,28 α-HQ was dissolved in an n-propanol solution with methanol. This solution was then dried to obtain the crystalline MeOH−HQ clathrate. The CO2−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 two weeks. The guest-free β-HQ clathrate was obtained by controlling the guest release of the as-synthesized 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 powered samples with a size of

Enhanced Hydrogen-Storage Capacity and Structural Stability of an

Recommend Documents