Storage Capacity of Metal−Organic and Covalent

Jan 12, 2012 - materials that can store hydrogen for use in fuel cell vehicles.1. Ideally, these materials would ... An important open question remain...
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Storage Capacity of Metal−Organic and Covalent−Organic Frameworks by Hydrogen Spillover Eric Ganz* and Matthew Dornfeld Department of Physics, University of Minnesota, Minneapolis, Minnesota, United States ABSTRACT: We determine the saturation storage density for hydrogen on several metal−organic framework (MOF) and covalent−organic framework (COF) materials by spillover. We use density functional theory on periodic frameworks to achieve reliable and accurate predictions for these materials. We find that one hydrogen can be stored at each C atom of the linker, and an additional H for each CO2 group. For IRMOF-1 and IRMOF-8, we find reasonable agreement with the experimental results. For other materials, such as COF-1 and MOF-177, we find that the experiments could be dramatically improved. We also predict the gravimetric and volumetric storage densities for several new materials, including IRMOF-9, IRMOF-993, MIL-101, PCN-14, COF-1, and COF-5, which appear very promising. We find gravimetric storage densities up to 5.5 wt % and volumetric storage densities up to 44 g/L.

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INTRODUCTION Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Although several different approaches are being pursued, sorption onto a porous high-surface-area material is one contender. There is great interest in finding porous solid materials that can store hydrogen for use in fuel cell vehicles.1 Ideally, these materials would adsorb large amounts of hydrogen gas reproducibly at room temperature and moderate pressure. Recent experiments using the spillover method are operating at room temperature and are approaching the realworld 2010 gravimetric targets as set by the U.S. Department of Energy for potential use in fuel cell cars.2 The spillover process works using nanoscale metal catalysts distributed through the porous substrate material to break the molecular hydrogen gas into physisorbed atomic hydrogen.3 The atomic H then diffuses across and chemisorbs to the substrate. As H covers the nearby surface area, further H diffuses across the saturated areas and spills over onto remaining areas. The best results have been on substrates based on metal−organic framework materials.3 An important open question remains: how to design improved substrates for hydrogen spillover. The sample preparation for these hydrogen spillover experiments is quite complex, and there has been significant scatter in the experimental results. Without clear and accurate predictions for saturation storage capacities, it has been difficult to evaluate the experimental results. For this and other reasons, it has also been difficult to improve the early results. In this paper, accurate predictions for saturation storage density at room temperature for several metal−organic frameworks (MOFs)4 and covalent−organic frameworks (COFs)5 will be made using quantum chemistry calculations of binding energies for individual and multiple hydrogen atoms on full periodic models of the crystals. Instead of estimates © 2012 American Chemical Society

based on surface area, we count specific binding sites on the crystal surface. This work shows that many of the experimental results are a factor of 3 below theoretical predictions. Therefore, these materials require further improvement in sample preparation in order to achieve their full hydrogen storage potential. These MOF and COF materials are relatively easy to fabricate, porous, and lightweight and have extremely high surface areas. Hydrogen storage in general in MOF materials was recently reviewed by Murray et al.4 Spillover has been a topic of investigation for many years; we will concentrate only on hydrogen storage by spillover onto COF and MOF materials. Hydrogen storage by spillover has been reviewed by Wang and Yang.3 Li and Yang found that IRMOF-8 with bridged Pt catalysts can reversibly store 4 wt % hydrogen at room temperature and 100 bar pressure by spillover.2 Ingeniously, they used commercial Pt catalysts, mounted on amorphous carbon substrates (Pt/AC). These were then mixed with IRMOF-8 crystals, and sucrose, and annealed to form amorphous carbon bridges between the Pt/AC and the IRMOF-8. Yang’s group has also tested several other high-surface-area substrates, but none of these have yet reached the storage capacity of bridged IRMOF-8. Recently, Yang’s group has published the details of the bridging methods and tested variations.6 Independently, Tsao et al. have extended the IRMOF-8 result to 4.7 wt % in equilibrium at 70 bar.7 This is the largest value to date for hydrogen storage by spillover at room temperature and is close to the 2010 DOE gravimetric storage targets of 6 wt %. One issue for practical use of the IRMOF materials is that many of them are sensitive to water contamination.8 Wang and Received: November 11, 2011 Revised: January 10, 2012 Published: January 12, 2012 3661

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IRMOF-1, and, therefore, their result would only correspond to 2.0 wt % hydrogen storage by spillover. Cao et al. have extended this idea. They found that decoration of the BDC linker with electrophilic groups enhanced the hydrogen uptakes by spillover at low loading.18 Also, there has been a new study by Psofogiannakis and Froudakis proposing new theoretical explanations for hydrogen spillover.19 They predict 2.6 wt % excess storage for IRMOF-1. They conclude that only six H can be added to each linker. They did not include the 85% correction factor for the bridged samples. They have also studied spillover on graphite with O surface groups.20 One limitation of the IRMOF materials as one tries to increase the gravimetric storage density is that the Zn−O corners are relatively heavy. To overcome this problem, Cote et al. created a new series of covalent−organic frameworks.5 Two important members of this group are COF-1 and COF-5. These were found to have expanded porous graphitic layers with staggered (COF-1, P63/mmc) or eclipsed (COF-5, P6/ mmm) structures. Cote et al. found that COF-1 and COF-5 have high thermal stability (to temperatures up to 500−600 °C, respectively), permanent porosity, and high surface areas (711 and 1590 m2/g, respectively).

Yang have suggested that moisture-resistant materials, such as HKUST-19 or MIL-101,10 may be more durable. Another strategy is to modify the IRMOF materials to make them hydrophobic and, therefore, water resistant.11 One limitation of the bridged experiments is that the bridges are relatively fragile and do not survive multiple loading and unloading cycles. Therefore, it would be desirable to find an alternative to the bridging technique. Another concern is that some of the loading and unloading times are too long for practical use. One strategy to improve the kinetics is to deposit many tiny nanocatalysts directly on the substrate, and several strategies have been pursued to achieve this.3,12 Another idea would be to create a custom interface molecule to link the Pt catalysts to the MOF or COF substrate.

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PREVIOUS COMPUTATIONAL STUDIES BY OTHER AUTHORS There have been several theoretical studies of the spillover process. Li et al. published a study of the kinetics and mechanistics of this process on a metal−organic framework substrate using density functional theory (DFT).13 They used a large test molecule to calculate the binding energies for individual H atoms added to different sites. Their conclusion was that H atoms could bind by spillover at all sites studied (producing 6.5 wt % storage); however, at room temperature, the low binding energy sites would not be significantly occupied. Cheng et al. studied the energetics and binding energies of hydrogen spillover on several graphitic materials with Pt catalysts using DFT calculations. 14 They studied the dissociative chemisorption of gaseous H on a transition-metal catalyst, the migration of H atoms from the catalyst to the substrate, and the diffusion of H atoms on substrate surfaces. Miller et al. studied the addition of multiple hydrogen atoms to isolated benzenedicarboxylate and napthalenedicarboxylate linkers using DFT as models for hydrogen addition to IRMOF1 and IRMOF-8.15 They found that one can add up to 10 H to the isolated naphthalene molecule. We have previously published predictions for saturation hydrogen storage density at room temperature of IRMOF materials using DFT calculations of binding energies for individual and multiple hydrogen atoms on model individual molecules.16 In the earlier work, we were able to add up to six H atoms to each BDC linker on the C sites. However, the experimental work on IRMOF-8 suggested that more hydrogen was being loaded onto the framework. In the present paper, we extend these methods to a full periodic boundary condition calculation with a large repeating unit cell. This is a much more accurate and reliable approach to the real periodic crystal. This will allow us to add all of the required hydrogen to saturate the full periodic crystal. Recently, there has been a very interesting result by Lee et al. (using DFT in periodic cells) proposing a hole-mediated hydrogen spillover mechanism.17 They suggest that a population of Zn vacancies can dramatically change the energetics and diffusion barriers. This would provide a thermodynamic pathway for the addition of the first six H atoms to the IRMOF-1 framework. It will be interesting to see if experimental observation of these vacancies can be accomplished. They conclude that IRMOF-1 has an excess storage of 2.4 wt % hydrogen by spillover, with an additional 0.4 wt % H2 physisorbed. However, they did not include the fact that the bridged experimental samples were only 85%

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COMPUTATIONAL METHOD Density functional theory (DFT) calculations were performed in Materials Studio 5.5,21 using the DMol3 program.22,23 We have followed the method of Wang et al. developed for ZnO clusters.24 The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) parametrization25 was used to describe the exchange-correlation interaction. Density functional semicore pseudopotentials (DSPP) fitted to allelectron relativistic DFT results, and a double numerical basis set, including d-polarization functions (DND), were employed. The accuracy of this PBE/DSPP/DND scheme has been assessed via testing calculations on the ZnO molecule and wurtzite bulk by Wang et al.24 All calculations were performed using periodic boundary conditions. For IRMOF-1, the cell size was optimized.

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RESULTS The pure IRMOF-1 crystal has Fm3̅m space group symmetry. Six symmetry unique atoms were obtained from X-ray data by Eddaoudi et al.26 We added one hydrogen atom to get the seven symmetry unique atoms needed for pure IRMOF-1. The formula unit for IRMOF-1 is Zn4O(BDC)3. The 192 symmetry operations form a periodic cubic unit cell of dimension 25.67 Å, which includes 424 atoms. The converged periodic unit cell for IRMOF-1 is shown in Figure 1. As we start to add H to the crystal, the symmetry is reduced. Referring to the total number of H added per formula unit, IRMOF-1 + 12H is in F432 symmetry. For IRMOF-1 + 18H through IRMOF-1 + 36H, we used P432 symmetry. P432 provides 12-fold symmetry, which vastly speeds the calculations and was the best choice available. To run these models without symmetry was not possible due to limitations in human and computational resources and time. For the models in P432 symmetry, the interior benzenes were loaded with H, but the benzenes at the edges of the unit cell were not loaded due to conflict with the symmetry. These interior loaded structures can be extrapolated to the fully saturated structures. In the experiments, during loading, low 3662

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Figure 3. Image of periodic cell of the IRMOF-1 + 30H crystal. Six H atoms have been added to each of the central benzene linkers, and also two H to each of the central CO2 groups.

Figure 1. Image of periodic cell of the pure IRMOF-1 crystal.

loaded areas will be adjacent to higher loading areas. The pure crystals are strong enough to survive the strains of many loading and unloading cycles. The converged periodic unit cell for IRMOF-1 + 18H is shown in Figure 2.

loaded onto the IRMOF-1 crystal. This produces a stable and saturated crystal structure. For COF-5, the crystal was optimized using a fixed cell size. COF-5 is a layered material, and the standard DFT potentials available during the calculation were not capable of binding the layers together (van der Waals binding). We believe that using the experimental lattice constants for the pure crystal is a good approximation in this case. Pure COF-5 was optimized in P6MM space group symmetry. The original coordinates for the symmetry unique atoms were obtained from Cote et al.5 COF5 + 9H was created by adding hydrogen to each of the C atoms. COF-5 + 9H was optimized using P6 space group symmetry as shown in Figure 4. The next step is to calculate the binding energies for hydrogen atoms on metal−organic and covalent−organic frameworks. We do this by calculating the binding energy of periodic models with hydrogen relative to periodic models of the pure crystals. The results are presented relative to atomic hydrogen. The average relative energies [E(Crystal + NH) − E(Crystal)]/N − E(H) for N atoms are presented in Table 1. We find a binding energy of 2.8 eV/H for IRMOF-1 + 18H (adding six H to each benzene). For the saturation case of IRMOF-1 + 30H, we find 2.5 eV/H. We note that the high H2 gas pressure in the experiments drives the process forward to saturation, and, therefore, a certain range of binding energies can be accommodated.15 We note that the presence of zinc vacancies, or other hole doping, may ultimately contribute to the thermodynamic accessibility of these states, as discussed by Lee et al.17 For COF-5 + 9H, we find 2.4 eV/H. These values need to be relatively close to the binding energy of the H2 molecule (2.26 eV) for the spillover process to proceed smoothly and reversibly. By contrast, hydrogen storage on amorphous carbon or graphite, or graphene,27 will be difficult at room temperature due to insufficient binding energy.

Figure 2. Image of periodic cell of the IRMOF-1 + 18H crystal. H atoms have been added to saturate the central benzene linkers.

The converged periodic unit cell for IRMOF-1 + 30H is shown in Figure 3. We note that only half of the organic linkers were saturated with hydrogen in these models. The lattice constants were allowed to change, and the calculations were performed alternating fixed and free cell size in each case until the system converged. We tried extensively to create an IRMOF-1 + 36H crystal, but this was not successful. If the additional H atoms were added with short bond lengths, then the linker becomes an independent fully saturated molecule and drifts away from the rest of the crystal (i.e., the crystal slowly drifts apart). This shows that the fully saturated linker cannot be bound in the crystal, even in the easier case where only the interior benzenes are loaded in the unit cell. If the added H atoms were placed in a low-energy starting configuration with a longer bond length, then the result was an IRMOF-1 + 30H crystal with groups of expelled H atoms. We also attempted to add additional H atoms to the corners unsuccessfully. Therefore, a maximum of 30 H per formula unit can be

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SATURATION MODEL We can now estimate the saturation hydrogen storage density by spillover for various metal−organic and covalent−organic frameworks. This is an extension of our previous saturation model based on DFT calculations of small molecules.28 We count chemisorption binding sites based on our periodic calculations. For the IRMOF materials, we place one H on each 3663

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Figure 4. Image of periodic cell of COF-5 + 9H.

sucrose. This mixture was then processed to pyrolize the sugar to create bridges between the IRMOF-1 crystal and the AC/Pt catalysts. To make a comparison with experiment, we will keep only the carbon atoms in the sucrose and assume the 85:10:5 mixture of the final bridged IRMOF:AC/Pt:sucrose material. We will assume a 1 wt % loading for the amorphous carbon components. For this mixture, the theory predicts 3.3 wt % compared to the 3.0 wt % found experimentally at 100 bar.29 This is good agreement, as the experiment was not fully saturated. For IRMOF-8, we can add 42 hydrogen atoms per formula unit. The model predicts a saturation density of 4.4 wt % for IRMOF-8. For an 85:10:5 mixture, the predicted saturation density is 3.9 wt %. This can be compared to the unsaturated 4.0 wt % hydrogen storage at 100 bar experimental result of Li and Yang.2 We can also compare to the newer saturated result by Tsao et al. of 4.7 wt % in equilibrium at 70 bar.30 Our predictions are in good agreement with the Li and Yang results; however, our predictions are significantly lower than the Tsao et al. results. The Tsao et al. samples were processed to maximize the storage density, which led to many defects and a collapsed pore structure. These defects may include additional O atoms, potentially Zn vacancies, and also structural defects, such as broken bonds. These changes provided a 0.7 wt % increase in storage capacity compared to the earlier Li and Yang results.

Table 1. Average Relative Binding Energies per H for Several Hydrogen-Loading Models IRMOF-1 IRMOF-1 + 18H IRMOF-1 + 24H IRMOF-1 + 30H COF-5 COF-5 + 9H

space group

lat. (Å)

E (Ha)

Fm3m ̅ P432 P432 P432 P6MM P6

26.19 26.20 26.11 26.37 30.02 30.02

−23331.236 −23374.362 −23386.357 −23401.792 −3122.055 −3153.711

B.E./H (eV) −2.79 −2.12 −2.49 −2.44

of the C atoms and there is an additional H for each CO2 at one of the O sites. No hydrogen is added to the Zn4O corners (binding to the corners was examined in our previous paper16). For COF-1, hydrogen was added to the C atoms, but not to the boroxine corners for COF-1 (as shown in the previous paper16). For COF-5, nine H were added, as discussed above. The linkers for several MOF and COF materials are shown in Figure 5. By counting these binding sites for these MOF and COF materials, we predict the saturation storage densities presented in Table 2. For IRMOF-1, we find six hydrogen binding locations on the benzene ring at C sites, and two hydrogen binding locations on the CO2. This corresponds to 30 H atoms per formula unit. For IRMOF-1, the model predicts a saturation value of 3.8 wt %. The experiments used a 80:10:10 mixture of pure IRMOF-1 with Pt catalysts mounted on amorphous carbon (AC/Pt) and

Figure 5. Linkers for MOF and COF materials from references Côté et al.,5 Wong-Foy et al.,32 and Eddaoudi et al.26 3664

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Table 2. Predicted Saturation Hydrogen Storage Density for Various Materials Compared to Experimenta predicted material

formula unit

number of H per formula unit

IRMOF-1 IRMOF-8 IRMOF-9 IRMOF-10 IRMOF-15 IRMOF-16 MOF-177 HKUST-1 MIL-101 COF-1 COF-5 IRMOF993 MOF-505 PCN-14

Zn4O(BDC)3 Zn4O(NDC)3 Zn4O(BPDC)3 Zn4O(BPDC)3 Zn4O(TPDC)3 Zn4O(TPDC)3 Zn4O(BTB)2 Cu3(BTC)2 Cr3O(BDC)3 C6H4B2O2 C9H4BO2 Zn4O(ADC)3

30 42 54 54 66 66 60 30 36 6 9 54

3.8% 4.4% 5.3% 5.3% 5.3% 5.3% 5.0% 3.8% 5.1% 4.5% 5.5% 4.8%

23 20 35 17 22 11 21 37 23 44 32 39

Cu2(bptc) C30H18Cu2O10

20 34

4.3% 4.9%

40 40

experimental

gravimetric storage density (pure)

volumetric storage density (pure)

gravimetric storage density (85:10:5)

gravimetric result

3.3% 3.9%

3.0% 4.0, 4.7%

4.4% 3.4% 4.5% 4.0%

1.5% 1.1% 1.5% 0.7%

a

Gravimetric and volumetric storage densities are presented for pure materials, and also for the 85:10:5 mixture used in the experiments. Formula units and crystal parameters were extracted from the following references: IRMOF data from Wong-Foy et al.32 and Eddaoudi et al.,26 densities from Furukawa et al.,33 COF data from Cote et al.,5 PCN data from Ma et al.,34 MOF-505 data from Chen et al.,35 and HKUST-1 data from Chui et al.36 Volumetric storage density was estimated by multiplying gravimetric storage density by the pure crystal density.

44 g/L storage (the largest found in our study so far). This suggests that the COF materials are attractive targets for hydrogen storage by spillover. The binding energy for COF-5 + 9H relative to COF-5 was 2.44 eV per H. For COF-5 + 9H, a further two H were added symmetrically above and below the B atom. COF-5 + 11H was optimized using P6 space group symmetry with an additional binding energy of 1.36 eV/H for the last 2 atoms, which have longer B−H bond lengths of 1.6−2.0 Å. For COF-5, additional H atoms were added in pairs at the O sites, but were unable to bind without destroying the crystal. Our results indicate that 11 H per formula unit is the maximum amount of H that can be chemisorbed in the COF-5 crystal by spillover. This corresponds to a remarkable 6.7 wt % hydrogen storage. At a minimum, we will count nine H per formula unit chemisorbed at room temperature, leading to at least 5.5 wt % and 32 g/L storage under typical experimental conditions. Some additional hydrogen will bind at the B sites depending on the experimental details. This is the highest gravitational density of the materials that we have studied. The strong results for these two COF materials suggest that other COF materials should have even better hydrogen storage properties. The COF and IRMOF materials that we have been discussing range from 80% to 90% free volume, and, therefore, if one considers the total storage (as opposed to the excess storage we have been discussing), then additional hydrogen can be stored as H2 in the free space.26 As hydrogen is loaded into these structures, the lattice constants are changing. Some of the most successful experiments have used delicate bridges to connect the catalyst particles to the substrate. These bridges, in particular, may be damaged by strain on the crystal during loading and unloading cycles. One strategy to minimize this strain would be to use symmetric platelets mounted along the linker. IRMOF-993, shown in Figure 6, is an example of this strategy. After loading H, the molecule can bend, but the C−C distance across the central benzene will not change so much. Therefore, less strain per bound H will be transferred to the lattice. One can extend

Higher storage density is predicted for pure MOF-177, at 5.0 wt % (adding six H per benzene ring, and two H per CO2). This system and variants have attracted much interest as storage targets due to the enormous surface area of MOF-177, and that large quantities of MOF-177 are commercially available. Two experimental groups have already measured spillover onto MOF-177. Wang and Yang found 1.5 wt % storage at 100 bar, using a physical mixing of MOF-177 and Pt/ AC catalysts with bridge building.31 However, the particle size of MOF-177 may have been too large, or the bridge building may not have been optimized fully. Proch et al. fabricated samples by Pt chemical vapor deposition, but were only able to reproducibly load small amounts of H.12 Higher predicted storage density is also found for IRMOF15, IRMOF-16, and IRMOF-9 with 5.3 wt %. IRMOF-9 and IRMOF-15 are catenated and, therefore, will have double the volumetric storage density of their uncatenated partners. IRMOF-15 has a predicted volumetric storage density of 22 g/L. IRMOF-9 has a predicted gravimetric storage density of 5.3 wt %, and a high volumetric storage density of 35 g/L. IRMOF-9 should be a favorable candidate for storage by spillover. For HKUST-1, one can add 24 hydrogen per formula unit, leading to a gravimetric storage density of 3.8 wt % and a large 37 g/L volumetric storage density. It may be possible to add additional hydrogen to this lattice at the H2O sites (replacing the H2O with H). For PCN-14, we find a large predicted 4.9 wt % and 40 g/L volumetric storage density. We have not counted the Cu sites, as they have not been calculated. For MIL-101, we include 36 hydrogen per formula unit, leading to a large gravimetric storage density of 5.1 wt %. It may be possible to add additional hydrogen at the H2O sites, leading to increased storage density for MIL-101. For the covalent−organic framework COF-1, we put one H on each C atom, but no H on the B3O3 corners, for a total of six H per unit cell. We, therefore, predict a saturation storage density of 4.5 wt % for COF-1, much larger than the experimental value of 0.7 wt %. COF-1 has a large predicted 3665

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(10) Férey, G.; Mellot-Draznieks, C.; Serre, C.; Millange, F.; Dutour, J.; Surblé, S.; Margiolaki, I. Science 2005, 309, 2040−2042. (11) Nguyen, J. G.; Cohen, S. M. J. Am. Chem. Soc. 2010, 132, 4560− 4561. (12) Proch, S.; Herrmannsdörfer, J.; Kempe, R.; Kern, C.; Jess, A.; Seyfarth, L.; Senker, J. Chem.Eur. J. 2008, 14, 8204−8212. (13) Li, Y.; Yang, F. H.; Yang, R. T. J. Phys. Chem. C 2007, 111, 3405−3411. (14) Cheng, H.; Chen, L.; Cooper, A. C.; Shaa, X.; Peza, G. P. Energy Environ. Sci 2008, 1, 338−354. (15) Miller, M.; Wang, C. Y.; Merrill, G. J. Phys. Chem. C 2009, 113, 3222−3231. (16) Suri, M.; Dornfeld, M.; Ganz, E. J. Chem. Phys. 2009, 131, 174703. (17) Lee, K.; Kim, Y.; Sun, Y.; West, D.; Zhao, Y.; Chen, Z.; Zhang, S. Phys. Rev. Lett. 2010, 104, 263101. (18) Cao, W.; Li, Y.; Wang, L.; Liao, S. J. Phys. Chem. C 2011, 115, 13829−13836. (19) Psofogiannakis, G.; Froudakis, G. J. Phys. Chem. C 2011, 115, 4047−4053. (20) Psofogiannakis, G.; Froudakis, G. J. Am. Chem. Soc. 2009, 131, 15133−15135. (21) Materials Studio 5.5; Accelrys Corporation: San Diego, CA. (22) Delley, B. Chem. Phys. 1990, 92, 508−517. (23) DMol3; Accelrys Corporation: San Diego, CA. (24) Wang, B.; Wang, X.; Zhao., J. J. Phys. Chem. C 2010, 114, 5741− 5744. (25) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−3868. (26) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469−472. (27) Psofogiannakis, G.; Froudakis, G. J. Phys. Chem. C 2009, 113, 14908−14915. (28) Suri, M.; Dornfeld, M.; Ganz, E. J. Chem. Phys. 2009, 131, 174703. (29) Lachawiec, A. J.; Yang, R. T. Langmuir 2008, 24, 6159−6165. (30) Tsao, C. S.; Yu, M. S.; Chung, T. Y.; Wu, H. C.; Wang, C. Y.; Chang, K. S.; Chen, H. L. J. Am. Chem. Soc. 2009, 131, 1404−1406. (31) Wang, L.; Yang, R. Energy Environ. Sci 2008, 1, 268−279. (32) Wong-Foy, A. G.; Matzger, A. J.; Yaghi, O. M. J. Am. Chem. Soc. 2006, 128, 3494−3495. (33) Furukawa, H.; Yaghi, O. J. Am. Chem. Soc. 2009, 131, 8875− 8883. (34) Ma, S.; Sun, D.; Simmons, J. M.; Yuan, D.; Zhou, H.-C. Inorg. Chem. 2009, 48, 5263−5268. (35) Chen, B.; Contreras, D. S.; Ockwig, N.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745. (36) Chui, S.; Lo, S.; Charmant, J.; Orpen, A.; Williams, I. Science 1999, 283, 1148−1150. (37) Sagara, T.; Ortony, J.; Ganz, E. J. Chem. Phys. 2005, 123, 214707. (38) Duren, T.; Sarkisov, L.; Yaghi, O. M.; Snurr, R. Q. Langmuir 2004, 20, 2683−2689.

Figure 6. IRMOF-993 linker as proposed by Snurr for molecular hydrogen storage.38

this strategy by using even larger perpendicular linkers, such as IRMOF-M2, IRMOF-M3, IRMOF-M4, or others.37 The model predicts 4.8 wt % and 39 g/L volumetric storage density for IRMOF-993. Therefore, if IRMOF-993 were fabricated, it would have a combination of large gravimetric and large volumetric storage densities.

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SUMMARY In this paper, we have studied the saturation storage density for hydrogen on several MOF and COF materials by spillover. We used density functional theory on periodic frameworks to achieve reliable and accurate predictions for these materials. For IRMOF-1, we are in good agreement with the experimental results. We find that 10 H can be added per linker, in contrast to the 6 H predicted by Lee et al.17 and Psofogiannakis et al.19 For IRMOF-8, we also find good agreement with the experimental results, although it appears that extra processing to induce defects may increase hydrogen storage capacity. For other materials, such as COF-1 and MOF-177, we find that the experiments could be dramatically improved. We have also predicted the gravimetric and volumetric storage densities for several new materials, with IRMOF-9, IRMOF-993, MIL-101, PCN-14, COF-1, and COF-5 looking very promising. We find gravimetric storage densities up to 5.5 wt % and volumetric storage densities up to 44 g/L. These values are close to the 2010 DOE targets of 6 wt % and 45 g/L hydrogen storage. This suggests that MOF and COF materials should be desirable substrates for hydrogen storage by spillover.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the Minnesota Supercomputer Institute for Advanced Computational Research for providing computational resources and support. We would also like to thank Professor Laura Gagliardi for helpful suggestions.

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REFERENCES

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