Tetrahedral Silsesquioxane Framework: A Feasible Candidate for

Article pubs.acs.org/JPCC

Tetrahedral Silsesquioxane Framework: A Feasible Candidate for Hydrogen Storage Amol Deshmukh,†,‡,§ Yun-Wen Chen,*,† and Jer-Lai Kuo*,†,‡ †

Institute of Atomic and Molecular Sciences and ‡Molecular Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 11529, Taiwan § Department of Physics, National Central University, Jung-Li 32001, Taiwan S Supporting Information *

ABSTRACT: The search for new materials that can withstand the tough demands of practical hydrogen storage for use in automotive transportation is currently receiving a great deal of attention from the scientific community because of the urgency for replacements of traditional energy resources such as fossil fuels. In this work, transition metal (TM)-decorated boron-doped tetrahedral silsesquioxane frameworks (B-TSFs) for application in hydrogen storage are investigated using first-principles density functional theory calculations. We design this plausible hydrogen storage system based on the knowledge of previous works by other groups including metal atom decoration for quasi-molecular H2 adsorption, boron substitution into benzene rings to prevent metal clustering, and assembling modified benzene rings and tetrahedral silsesquioxane cages into the framework for this study. Boron substitution substantially enhances the TM binding energy to the linker of B-TSF to suppress metal clustering as well as maintain stable hydrogen adsorption energy to TMs. The average hydrogen adsorption energy energies in Sc-, Ti-, and V-decorated B-TSF are 0.29, 0.40, and 0.69 eV, respectively, with acceptable gravimetric density of 6.9, 5.6, and 4.15 wt %. Gibbs free energy calculations are also carried out to estimate the working temperature and pressure ranges for using B-TSF as a hydrogen storage system.

1. INTRODUCTION The increasing demand for energy supplies in a global civilization and the rising population require that humans explore more energy resources. Among them, fossil fuels are still the most popular for their well-developed technologies in a wide range of applications. However, environmentally unfriendly pollution and limited storage capacity for fossil fuels urge the investigation of other types of energy resources. It is strongly believed that using hydrogen energy can help to address the growing demand for energy and reduce global climate changes caused by using fossil fuels.1 The ideal hydrogen energy utility will emit no hazards, contain high energy density, be of light weight, and involve only water, hydrogen, and oxygen molecules in the reactions.2 The three key components of using hydrogen energy are the production, storage, and combustion of hydrogen. On the Earth, abundant water could be the source for hydrogen production via electrolysis-related processes such as photovoltaic-derived electrolysis or photocatalytic water splitting.3,4 On the other hand, hydrogen fuel cells are designed to convert hydrogencontaining fuels directly into electrical energy through the electrochemical reaction of hydrogen and oxygen, producing water.5 Both production and combustion of hydrogen are still actively developing fields to improve the efficiency of using hydrogen energy. Besides the two mentioned aspects, there are still some important challenges that need to be addressed prior to the hydrogen energy combustion system such as lack of © XXXX American Chemical Society

suitable media for hydrogen storage with sufficiently high system volumetric and gravimetric densities at ambient temperature.6 Targets set by DOE for on-board hydrogen storage systems applied to light duty vehicles up to 2015 are fast adsorption and desorption kinetics, gravimetric densities of 5.5 weight % (wt %), volumetric densities of 40 g/L, an operating temperature between −40 °C and 85 °C, a min/max delivery of pressure of 3/100 bar, and a 3.3 min refueling time for a 5 kg tank of H2.7 However, a hydrogen storage material has not yet been delivered to meet the above targets. Currently, the major facilities for hydrogen storage use compressed gas or cryogenic liquid tanks. However, both methods will waste a significant amount of energy during H2 storage and hence are not cost-effective enough for commercial application. The energy required to compress hydrogen from 20 to 700 bar consumes energy of 1.36 kWh/kg.8 On the other hand, the cryogenic method is more problematic in that the minimum energy required to produce liquid hydrogen is 3.3 kWh/kg, more than twice the energy required to compress hydrogen gas to 700 bar.8 Therefore, compression and the cryogenic method of hydrogen storage are not suitable for widespread commercial applications. Over the past decades, materials based on physisorption or chemisorption of hydrogen Received: July 7, 2015 Revised: August 27, 2015

A

DOI: 10.1021/acs.jpcc.5b06514 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

wt %. The name silsesquioxanes is derived from the one and one-half (1.5) ratio of oxygen bound to silicon with the general formula RSiO3/2; each silicon in silsesquioxane is trivalent toward oxygen, leaving its fourth valence to other substituents such as hydrogen or any organic group. To us as theoretical researchers, silsesquioxane cages and aromatic molecules could be used to build frameworks such as MOFs. Further modifications may make the frameworks applicable in hydrogen storage. Actually, an octahedral silsesquioxane (T8, R8Si8O12)based porous framework has already been synthesized.40−43 Tetrahedral silsesquioxane, on the other hand, is the simplest polyhedral silsesquioxane, which also has the potential for building low density porous frameworks. Li et al. have also suggested the use of TSF and related frameworks for energy storage design in their theoretical studies.31,32 Although tetrahedral silsesquioxane nanoclusters have not been synthesized yet, there are a few theoretical studies on the stability and other properties of tetrahedral silsesquioxane.44,45 Methyl tetrahedral silsesquioxane ([MeT]4) was expected to be stable via calculations;45 hence, it is possible that TSF could be synthesized. However, using TSF only, without TM-atom decoration, could only allow operation at low temperature and high pressure for high excess storage capacity because its weak interaction with hydrogen belongs to the physisorption category.31 With the knowledge from previous studies, we demonstrate a modified TSF hydrogen storage system to be suitable for operating near ambient conditions via density functional theory (DFT)46 calculations and Gibbs free energy corrections.47,48

have been considered as new strategies to achieve feasible hydrogen storage systems.9 Many types of hydrogen storage systems have been studied theoretically as well as tested experimentally to offer a practical solution. These include chemical hydrides,10−14 metal-functionalized carbon nanotubes,15,16 and metal organic frameworks (MOFs).17−24 All of these systems were developed to avoid problems with compressed gas or the cryogenic liquid tanks, but there are still issues that make them imperfect for hydrogen storage. Chemical hydride binds very strongly with hydrogen9 and hence can only operate at high temperature; in contrast, MOFs exhibit hydrogen interaction that is too weak,25 and metal clustering issues need to be improved in the metalfunctionalized nanostructures.26,27 After decades of studies, several groups have suggested the proper H2 adsorption energies for practical hydrogen storage systems. Bhatia et al. have shown that 15.1 kJ/mol (0.15 eV/H2) is the suitable H2 adsorption energy for hydrogen adsorption and desorption operation at ambient temperature. This adsorption energy value is optimal with respect to the affinity of hydrogen and strong enough to store large amounts of hydrogen at a charging pressure of 30 bar.28 Lochan et al. also have suggested that the ideal H2 adsorption energy for operating between −20 °C and 50 °C should vary between 20 and 40 kJ/mol (0.20−0.41 eV/ H2).29 Summarizing the past studies, Puru Jena suggested three types of hydrogen storage systems according to the strength of adsorption energies.9 The physisorption system only weakly binds hydrogen molecules within a few meV. In the case of chemisorption, hydrogen dissociates into individual atoms and binds much stronger to the surface with adsorption energy from 2 to 4 eV/H2. Between them, a quasi-molecular bonding system binds hydrogen with energy of 0.1 to 0.8 eV/H2.9 The quasi-molecular bonding system was suggested to be ideal for hydrogen storage under ambient temperature and pressure, which ensures reversibility and fast kinetics.9,29 In this study, we accept P. Jena’s classification. Specific porous materials such as MOFs,25 carbon nanotubes,30 tetrahedral silsesquioxane frameworks (TSFs),31,32 and also amorphous metal-decorated octahedral silsesquioxane33,34 have been suggested for use in hydrogen storage systems because of the advantage of having a large adsorption area. However, their molecular hydrogen adsorption energies belong to the physisorption category, which are too low for hydrogen storage applications. Various approaches to improve hydrogen adsorption in these porous materials were proposed, including the introduction of metal atoms into the pore of the crystal.15,35,36 Kubas interaction37 is recognized to explain the mechanism of TM atom-decorated hydrogen storage systems. Charge donation from the H2 molecule to the unfilled d orbitals of the TM atom and back-donation from the TM atom to the antibonding orbital of the H2 molecule is responsible for quasi-molecular bonding.37 However, metal clustering could be a potential problem in reducing the hydrogen storage capacity. In theory, Zou et al.23 and Dixit et al.38 have studied a borondoped benzene ring as a linker in MOF-5 for hydrogen storage applications. They have shown that boron doping in benzene rings substantially increases metal binding energy, thereby solving the metal clustering problem. Moreover, Sun et al. suggested a novel approach involving grafting of metaldecorated cyclopentadienyl on octahedral silsesquioxane (T8, R8Si8O12) cages for hydrogen storage applications.39 They have shown that Sc-decorated cyclopentadienyl on a single T8 cage can store hydrogen molecularly with a gravimetric density of 5

2. METHODOLOGY Boron-doped benzene as bridging linkers and silsesquioxane T4 cages as connector ligands are the building moieties to form the diamond-like tetrahedral silsesquioxane framework (B-TSF) as shown in Figure 1. Hence, the whole framework is built in a face-center (fcc) primitive cell containing formula unit (Si4O6(C4B2H4)2)2. Those linkers in B-TSF are decorated with three types of early transition metal atoms (Sc, Ti, and V) to functionalize the hydrogen storage system. The calculated BTSF lattice constant is 16.4 Å. Our T4 cage (Si4O6H4) structural parameters are consistent with the results derived from inelastic neutron scattering of various silsesquioxanes;49 the bond length of Si−H = 1.47 Å and Si−O = 1.66 Å as compared to experimentally derived Si−H = 1.48 Å and Si−O = 1.62 (1.61) Å. All related calculations are performed using Vienna Ab-initio Simulation Package (VASP)50,51 with spin-polarized electron wave functions expanded in plane wave basis sets. Energy cutoff is set to 400 eV. At the same time, the conjugated pseudopotentials constructed with the projected augmented wave (PAW) method52,53 are used. The electron exchange correlation energies are estimated using the Perdew, Burke, and Ernzerhof (PBE) generalized gradient approximation functional.54 The Grimme DFT-D3 method is applied to correct dispersion forces.55 Temperature dependence of adsorption energies with Gibbs free energy correction is also obtained to predict suitable operating temperature range. All the Gibbs free energies are estimated using thermochemistry formulas47,48 with the vibration frequencies calculated in VASP. The average metal binding energy and H2 adsorption energy are calculated with the following formulas. Metal binding energy ΔEbe (M): B


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ΔEthm‑X is the sum of thermal energies contributed from translational, rotational, and vibrational motion of X. R is the gas constant, T is temperature in Kelvin, and S−X is the sum of entropy due to translational, rotational, and vibrational motion of X. By combining ΔEthm‑X and TS−X, the contribution from rotational, translational, and vibrational motion can be summarized as Grot(X) = −RT ln(qr)

(5)

where qr is the rotational partition function, which will be a function of temperature and moment of inertia of the molecule. Gtran(X) = −RT n(qt) − RT

(6)

where qt is the translational partition function, which will be a function of temperature, pressure, and mass of the molecule. ⎡ Θv , k ⎤ Gvib(X) = R ∑ ⎢ + T ln(1 − e Θv ,k / T )⎥ ⎣ 2 ⎦ k

(7)

where Θv,k = ℏvk/KB is the vibrational temperature and vk is the vibration frequency of vibrational mode k. Hence, eq 4 can be rewritten as EG − X = E X + Grot(X) + Gtran(X) + Gvib(X) + RT ⎧ ⎡Θ ⎤ v ,k EG − X = E X + R ⎨∑ ⎢ + T ln(1 − e Θv ,k /T)⎥ ⎦ ⎩ k ⎣ 2 ⎫ − T ln(qr) − T ln(qt)⎬ ⎭ ⎪

⎪

Figure 1. (a) Optimized geometry of building moieties of B-TSF, with total energy noted, and (b) optimized B-TSF structure.

⎪

⎪

ΔE be(M) = {Ecomplex + nEM − EM + complex ]}/n

(1)

In addition, we ignore the translational and rotational contributions of ΔEthm‑X and TS−X of B-TSF (empty and with H2 adsorption) because the framework is in a periodic model. But all three contributions are included while calculating free energy correction of H2 molecules, assuming no interaction between different H2 molecules. To demonstrate how we construct this feasible B-TSF hydrogen storage system from small molecules with the knowledge of previous studies, our calculations are carried out in three successive steps. The first step demonstrates that the boron atom substitution greatly hinders metal atoms from clustering on the linker of the framework. Second, hydrogen adsorption on B-TSF with various transition metal decorations will be discussed. In the third step, the proper operating environments of the B-TSF hydrogen storage system will be demonstrated with Gibbs free energy estimation.

where Ecomplex stands for the total energy of complex without transition metal atom decorations. The complex could be one pure or boron-doped benzene molecule or a TSF. EM is the self-energy of one single Sc, Ti, or V atom. EM+complex is the total energy of complex with transition metal atom decorations, and n is the total number of metal atoms bound. H2 adsorption energy (ΔEad(H2)): ΔEad(H 2) = {EM + complex + m E H2 − EM + complex + H2}/m (2)

where EH2 is the total energy of one H2 molecule, EM+complex+H2 is the total energy of complex with transition metal decorations and H2 molecules adsorbed, and m is the total number of adsorbed H2 molecules. With the Gibbs free energy correction, we can extend eq 2 to estimate the proper operating conditions of the hydrogen storage system:

3. RESULTS AND DISCUSSION 3.1. Metal Adsorption on an Isolated Benzene Ring and TSF with Boron Substitution. Preventing metal clustering is one of the big issues for hydrogen storage materials based on metal-atom adsorption centers because it not only affects the structural stability but also decreases surface area for hydrogen holding. As studied by Krasnov et al., Sc atom clustering on single-walled carbon nanotubes (SWNT) is energetically favorable and kinetically permitted.26 Sun et al. and Lee et al. also showed that TMs and Be atoms energetically prefer to cluster on the pristine C60 surface, respectively.56,57 Fortunately, a solution has been found by modifying the carbon-based material surface as demonstrated in recent theoretical studies with boron substitution.23,38,58 The metal

ΔEG(H 2) = {EG − M + complex + mEG − H2 − EG − M + complex + H2}/m

(3)

where EG−M+complex, EG−M+complex+H2, and EG−H2 are the total energy with Gibbs free energy correction of M+complex, M +complex+H2, and only one H2 molecule, respectively. According to thermochemistry formulas,47,48 the Gibbs free energy correction EG−X is given as EG − X = E X + ΔEthm − X + RT − TS−X

(8)

(4)

where X represents M+complex, M+complex+H2, or only one H2 molecule. C


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The Journal of Physical Chemistry C aggregation can be effectively suppressed by introducing boron atoms into the carbon-based material surface, which leads to the metal atoms binding more strongly than in the pristine cases. In experiments, it has been realized that one can substitute up to three boron atoms into carbon-based rings.59 In theory, Zou et al. have shown that the two-boron-atom substitution for two carbon atoms of the benzene ring is the most stable configuration.60 In this study, we also draw from these previous studies by doping our benzene ring linkers with two boron atoms to prevent metal clustering. Three kinds of two-boron substitutions are available for the benzene ring, namely para (two boron atoms occupying exactly opposite sites in the ring), ortho (two boron atoms occupying nearest neighbor sites in the ring), and meta (two boron atoms occupying next nearest neighbor sites in the ring). To set up the framework in this study (Figure 1b), we first ignore metatype boron substitution because only para- and orthosubstituted organoboranes have been reported to be synthesized experimentally.59 Then we choose the one according to the most stable configuration among four possible cases of a boron-doped benezene ring connecting to two T4 cages (Figure 1a). Hereafter, we will take into consideration only the configuration of two-boron para substitution with two boron atoms not connecting to the silicon atoms of the T4 cage as the fourth case in Figure 1a. Optimized geometries of isolated boron-doped benzene linker C4B2H6M2 (M = Sc, Ti, V) with possible hybrid combinations are shown in Figure 2. It is observed that two

Figure 3. Energy eigen states of isolated pure benzene and borondoped benzene rings.

binding energies in both boron-doped isolated benzene linker and TSF. In Table 1, we compare the binding energies of Table 1. Calculated Average Metal Binding Energy on Isolated C6H6 and C4B2H6 Linker and Framework Structures (in eV) system

metal

isolated

framework

C6H6

Sc Ti V Ti−Sc V−Sc V−Ti Sc Ti V Ti−Sc V−Sc V−Ti

2.58 2.42 1.74 2.62 2.12 2.13 4.55 5.06 3.89 4.76 4.40 4.52

2.96 3.01 2.05 2.95 2.42 2.56 5.20 5.42 4.23 5.35 4.75 4.78

C4B2H6

transitions metals on a pristine, boron-doped benzene linker and their corresponding TSF, B-TSF. Sc, Ti, and V binding energies on isolated C6H6 as well as C6H6−TSF range between 1.74 and 3.01 eV/atom. Average binding energies of Sc, Ti, and V on isolated boron-doped benzene (C4B2H6) are 4.55, 5.06, and 3.89 eV/atom, respectively, and on B-TSF are 5.20, 5.42, and 4.23 eV/atom, respectively. The optimized geometry of Scdecorated B-TSF is shown in Figure 4a. To evaluate the metal clustering thermodynamically (in the long term) as well as kinetically (short period), we compare the binding energies with cohesive energies of the bulk metal as well as dimer and trimer formation energies, respectively (Table 2). Estimated average TM (Sc, Ti, and V) binding energies on both isolated C6H6 and C6H6-TSF linkers are found to be much less than their bulk cohesive energies and not much higher than the dimer and trimer formation energies. Consequently, clustering of TMs on C6H6 and C6H6−TSF linker surfaces is very likely to happen. On the other hand, isolated boron-doped benzene (C4B2H6) and B-TSF exhibit great enhancement in binding energies of metal atoms compared to the pure TSF. Because the calculated average binding energy of Sc atoms to B-TSF (Table 1) is higher than metal cohesive energy (Table 2), Sc is

Figure 2. Optimized structures of C4B2H6M2 where M is (a) Sc, (b) Ti, (c) V, (d) Ti−Sc, (e) V−Sc, or (f) Ti−V.

metal atoms can bind to the top and bottom sides of the hexagonal site of the C4B2H6 linker. Substituting boron for carbon changes electronic properties of the benzene ring by introducing new states above the highest occupied molecular orbital (HOMO) as shown in Figure 3. Our results are consistent with the observation by Park et al.61 They have shown that in boron-substituted graphene-like structures, new states appearing above the valence band maximum show an electron-deficient structure, which can easily accept extra electrons and suppress TM clustering by increasing metal binding energy. Bader charge analysis62,63 reflects the electrondeficient properties of the boron-doped benzene linker. Sc, Ti, and V atoms transfer 0.24, 0.33, and 0.25 more electrons to the boron-doped benzene linker than to the pure benzene linker, respectively. The boron substitutions greatly enhance the metal D


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Figure 4. Optimized geometry of (a) Sc-decorated B-TSF and (b) with maximum H2 adsorption.

Figure 5. H2 adsorption to TMs can be attributed to the Kubas charge interaction.37 Such electrostatic interactions will make

Table 2. Sc, Ti, and V Cohesive Energy in Bulk (in eV) as well as Dimer and Trimer Formation Energy/Atom (in eV) cohesive energy in bulk

a

formation energy

metal

experimental

PBE

dimer

trimer

Sc Ti V

3.90a 4.85a 5.31a

4.64 6.06 6.11

0.95 1.45 1.40

1.44 2.11 1.88

Reference 64.

thermodynamically stable binding to B-TSF. In contrast, Ti and V are not thermodynamically stable binding to B-TSF. The experimental cohesive energy of Ti64 is lower than the calculated one and may lead to an ambiguous explanation; but we do not want to judge the stability of Ti binding to BTSF here according to experimental values. However, the binding energies of all three metals surpass the dimer or trimer formation energies in Table 2. These results indicate that all of the TM (Sc, Ti, and V) adsorptions on B-TSF are kinetically stable, although Ti and V still prefer clustering in the long term. On the other hand, the metal binding energies to TSF are not much higher than the formation energies of metal trimers in Table 2. We predict metal clustering will be severe in pristine TSF. In both pristine TSF and boron-doped TSF, the metal binding energies show consistent enhancement compared to the binding energies in isolated pure and boron-doped benzene. We have analyzed the charge transfer and projected density of states of the systems in this study; however, we cannot offer a good explanation for the mechanism here. Further studies are needed to solve this problem and to possibly aid in the design of a new framework. 3.2. Hydrogen Adsorption on Metal-Decorated Complexes and B-TSF. Weck at al. reported that Sc- and Tidecorated benzene rings can adsorb a maximum of 4 H2 and 3 H2 molecules, respectively, and the first adsorbed H2 even prefers to dissociate to form dihydride.65 In this study we find that chemical modification of the linker by boron substitution results in prominent effects to both metal atom binding to aromatic linkers and hydrogen molecules adsorbed. As a consequence, controlling the charge states of metal binding complexes can modulate hydrogen−metal interactions. Optimized geometries of hydrogen adsorption on isolated C4B2H6 with different metal decorations (C4B2H6M2) are shown in

Figure 5. Optimized structure of C4B2H6M2 systems with maximum H2 adsorption. The metal atom decoration in each system is the same as in Figure 2.

small but consistent changes in H−H bond length of adsorbed H2 molecules. As the number of adsorbed H2 molecules increases, electron donation from the sigma bonding orbitals of each H2 to the d orbitals of TMs decreases due to the competition between more H2 molecules. Therefore, the number of H2 molecules adsorbed on early TMs is highly correlated to the number of empty d orbitals. The valence state d orbital electronic configuration of Sc, Ti, and V are 3d1, 3d2, and 3d3, respectively. Consequently, the number of empty d orbitals decreases from Sc to Ti and then V. In the cases of isolated complexes (C4B2H6M2), each Sc-, Ti-, and V-decorated C4B2H6 can adsorb maximum five, four, and three hydrogen molecules with an average adsorption energy 0.35, 0.45, and 0.71, respectively. The calculated average adsorption energies E


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The Journal of Physical Chemistry C ΔE by successive additions of H2 to C4B2H6M2 (M = Sc, Ti, V, Ti−Sc, V−Sc, and V−Ti) are summarized in Table 3. The Table 3. Calculated Average H2 Adsorption Energy for Isolated Metal-Decorated C4B2H6 (M = Sc, Ti, V, Ti−Sc, V− Sc, and V−Ti) Complexes (in eV) metal

2 H2

4 H2

6 H2

8 H2

Sc Ti V Ti−Sc V−Sc V−Ti

0.36 0.53 0.82 0.42 0.54 0.63

0.37 0.55 0.85 0.43 0.56 0.68

0.38 0.55 0.71 0.45 0.54 0.59

0.40 0.45 0.40 0.43 0.51

9 H2

10 H2 0.35

0.37

adsorption energies are within the ideal range of quasimolecular bonding9 for all the complexes. Moreover, all adsorbed H2 maintain molecular form. Hence, replacing C with B in the benzene linker not only allows one more H2 molecule to adsorb on the TMs but also results in stable H2 adsorption (in terms of fairly stable average H2 adsorption energies in Table 3). In this work, three frameworks (Sc−B-TSF, Ti−B-TSF, and V−B-TSF) can adsorb 40 H2, 32 H2, and 24 H2 molecules with average adsorption energies of 0.29, 0.40, and 0.69 eV, respectively. The optimized geometry of Sc-decorated B-TSF with maximum adsorbed H2 molecules is shown in Figure 4b. Although the average adsorption energies are somehow smaller than in the isolated linker cases (Table 3), they are all within quasi-molecular bonding range. We summarize the structural parameters of metal-decorated B-TSF (M = Sc, Ti, V, Ti−Sc, V−Sc, and V−Ti) at empty and full H2 adsorption in Figure 6. The average CC and C−B distances of B-TSF are abbreviated after successive H2 adsorptions. In contrast to this, the average C−M and B−M bond length in B-TSF increased after successive H2 adsorptions, respectively. At the same time, the average H−H distances in adsorbed coplanar H2 molecules on B-TSF (M = Sc, Ti, V, Ti−Sc, V−Sc, and V−Ti) are found to be within the elongated range of 0.81−0.89 Å. Actually, the H2 molecule adsorbed on the top site weakly binds to the Sc atom with only a slightly elongated H−H distance of 0.76 Å (the original bond length of H2 is 0.74 Å). The variation of structural parameters consists of observations of other groups applying Kubas interaction in hydrogen adsorption.38 Sc- and Ti-decorated B-TSF can reach gravimetric hydrogen uptake capacities up to 6.9 and 5.5 wt %, respectively. These numbers are well above the targets set by the US Department of Energy through 2015 (5.5 wt %). Nevertheless, V-decorated B-TSF can only reach up to 4.15 wt %. We have also tried to study the hybrid metal-decorated B-TSF with two different metal atoms adsorbed on the two sides of each linker in B-TSF. (Ti−Sc)−B-TSF, (V−Sc)−B-TSF, and (V−Ti)−B-TSF can reach gravimetric uptake capacity up to 6.28, 5.58, and 4.84 wt % with average H2 adsorption energy of 0.33, 0.44, and 0.49 eV, respectively. Though the uptake capacity for V-decorated BTSF does not meet the DOE target, still it may be useful in real applications. In the next section, we will review again metaldecorated B-TSF systems with respect to Gibbs free energy estimation. 3.3. Proper Operating Temperature and Pressure of B-TSF Hydrogen Storage Systems with Gibbs Free Energy Estimation. Operational characteristics such as

Figure 6. Calculated geometrical parameters before and after hydrogen is adsorbed (shaded bars) on B-TSF. (a) Average C−C and C−B bond length. (b) C−M and B−M bond length (M = Sc, Ti, V, Ti−Sc, V−Sc, and V−Ti).

temperature and pressure are crucial to the practical applications of hydrogen storage systems. The ideal operating environments should be near ambient conditions or within some criteria used in industry now. In this section, we will discuss the proper operating temperature and pressure of BTSF systems with Gibbs free energy correction estimation using thermochemistry formulas (eq 3)47,48 When the calculated ΔEG(H2) is positive at a particular temperature (pressure), H2 adsorption on the metal site is said to be energetically favorable; otherwise, the adsorption site tends to release hydrogen gas. The temperature dependence of ΔEG(H2) at certain pressures for M−B-TSF (M = Sc, Ti, V, Ti−Sc, V-Sc, V−Ti) with maximum H2 adsorption is shown in Figure 7. At low pressures such as 1 bar in Figure 7a, H2 adsorption is thermodynamically favorable up to a ceiling temperature (Tc) of 170 K for Sc−B-TSF. The Tc for other M−B-TSF systems varies between 170 to 460 K. Basically, a higher Tc of a system corresponds to a higher H2 average adsorption energy. Vanadium-decorated B-TSF could be used near 460 K without applying high pressure; however, this system has the lowest gravimetric density (4.15 wt %) among systems in this study. Its high Tc is also out of the suggested operation temperature range as set by DOE. On the other hand, the scandium-doped system possesses the highest gravimetric density (6.9 wt %), but its proper operation temperature is as low as 170 K. F


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Figure 7. Temperature-dependent curves of average adsorption energy with Gibbs free energy correction for M−B-TSF (M = Sc, Ti, and V) at (a) 1 bar, (b) 50 bar, (c) 100 bar, and (d) 150 bar pressure.

Figure 8. (a) Pressure-dependent ceiling temperature curves and (b) adsorption energy-dependent ceiling temperature curves for M−B-TSF (M = Sc, Ti, and V).

temperature-dependent ΔEG curves of M−B-TSF at pressures of 1 bar, 50 bar, 100 bar, and a slightly higher pressure 150 bar. Obviously, the ceiling temperature (Tc) rises with increasing

The other operational factor that we examine here is pressure. In the targets set by DOE7 the maximum pressure that can be accepted is 100 bar. Hence in Figure 7, we plot G


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pressure. The interaction between hydrogen and the porous materials in their studies represents physisorption; hence, the H2 adsorptions almost go to zero at 0 bar. The main contribution of absolute H2 adsorption at higher temperature and pressure will come from the accessible volume of their porous materials; the contribution from the hydrogen−porous material interaction will be negligible. X.-D. Li et al.31,32 have explicitly calculated the excess gravimetric capacities at 298 K which are much lower than 1 wt % (Figure 4 in refs 31, 32, and 66) for all systems. The excess capacities could be a result of hydrogen−porous material interaction as shown in eq 4 of refs 31 and 32. On the other hand, the OTiCl3 groups used in N. S. Suraweera’s study only offered physisorption of hydrogen, and hence the use of OTiCl3 groups only enhanced the hydrogen adsorption capacity slightly in experiment and even made it worse in theory.34 In our work, we use the early TM atom decorations to enhance the interaction of H2 to TSF, hence increasing the excess gravimetric capacity to at least 4.15 wt % (with V decoration). If the absolute H2 gravimetric capacity is considered (i.e., considering the accessible volume), the system studied in this work can at least reach gravimetric capacity of 6.15 wt % at 100 bar (take the reference TSF-1 in ref 31) at 298 K. Further higher capacity can be reached if a larger linker with higher accessible surface is considered to build the framework.31,32

pressure. In the Sc−B-TSF system, the proper operating temperature rises to 260 K at 100 bar, and about 275 K at 150 bar. Other M−B-TSF systems also present corresponding ceiling temperature shifts of ΔEG curves. Hence, regarding the four charts in Figure 7, we note that M−B-TSF systems (except the V−B-TSF system) are close to being feasible for hydrogen storage via manipulating temperature and pressure, approximating conditions applied in industry. Another possibility considered is the use of hybrid metal decorations in M−B-TSF to offer versatility in the operation of hydrogen storage systems in wider temperature and pressure ranges. In Figure 7, we also show the ΔEG curves of M−B-TSF systems with hybrid metal decorations. The Tc values vary correspondingly with the H2 average adsorption energies (Table 3) as in pure metal-decorated systems. Note that the Tc values in Figure 7 are determined with maximum H2 adsorption; those Tc values do not imply that hydrogen gas is entirely adsorbed under a ceiling temperature and that all is released above it. In practice, H2 should be released from the adsorption site gradually within a temperature range, because the successive adsorption energy of H2 varies at different filling stages (Table 3). In Figure 1 of Supporting Information, we offer a simple estimation of Tc values at different hydrogen storage states for Sc-, Ti-, and V-decorated isolated borondoped benzene ring systems at 1 bar. The Tc values vary about 60 K, 120 K, and 70 K in Sc-, Ti-, and V-decorated systems, which offers the possible operating range for the corresponding M−B-TSF systems. At the same time, ΔEG curves in Figure 7 also indicate that with hybrid metal decorations, M−B-TSF systems can be tuned to adapt to wider operating conditions and the H2 storage capacity can be adjusted. For example, a BTSF hybrid decorated with Sc(Ti) and V will raise the gravimetric capacity of the pure V−B-TSF system and also enable the system to be manipulated in a wider temperature window. Reorganizing the data in Figure 7, we get the ceiling temperature curves shown in Figure 8. In Figure 8a, the shaded region enclosed by the temperature/pressure criteria7 indicates the proper operation temperature and pressure ranges as set by DOE. The curve of Ti−B-TSF is almost within the shaded region (the estimated Tc at 100 bar is 366 K), which suggests that Ti−B-TSF will be the best choice for a hydrogen storage system, adapting to the widest DOE operating temperature and pressure ranges. The hybrid system, (Sc−Ti)-B-TSF, is also a very good choice, as most of the ceiling temperature curve is located within the shaded region. On the other hand, the systems decorated with V will be good hydrogen storage systems if the allowed operating temperature is raised to a higher range, between 450 K and 700 K. Figure 8b illustrates the relationship of H2 adsorption energies and ceiling temperatures at different pressures and could offer a guideline in the search for suitable operating temperature and pressure ranges for systems with known H2 adsorption energy. X.-D. Li et al. and N. S. Suraweera et al. have theoretically studied the hydrogen storage systems using crystalline and amorphous porous silsesquioxane materials.31−34 They have demonstrated that the absolute H2 adsorptions could get as high as 37 and 6 wt %, respectively, at the very low temperature of 77 K;32,34 however, this is not a good reference for H2 storage because the temperature is out of the suggested operation temperature range as set by DOE (−40 and 85 °C). On the other hand, the absolute H2 adsorptions dramatically decrease to 5.6 and 1.3 wt % around 298 K with 100 bar

4. CONCLUSION We have demonstrated a practical route for designing a hydrogen storage system starting from modifying small molecules via density functional theory calculations and Gibbs free energy corrections. In our investigation it was shown that boron substitution in linkers enhances transition metal interaction with linkers to suppress the metal clustering problem. Sc−B-TSF reaches the maximum hydrogen gravimetric capacity (6.9 wt %) with 5 H2 molecules adsorbed around each Sc atom. Calculated H2 adsorption energies are within the category of quasi-molecular bonding. Furthermore, estimated adsorption energies and temperature/pressure dependence of ΔEG suggest that Sc− and Ti−B-TSFs and their hybrid systems have the capacity to be an ideal hydrogen storage material, operating within DOE criteria except for the volumetric capacity. The maximum volumetric capacities of Sc-, Ti-, and V-decorated B-TSF in this study are 42.98, 34.38, and 25.79 g/L, respectively. Only the Sc-decorated system can meet the DOE 40 g/L criterion. However, this disadvantage could probably be overcome by using a larger aromatic link or octahedral silsesquioxane (T8) cages to construct the framework. The hybrid metal decoration in B-TSF systems can offer versatility to operate the hydrogen storage system in wider temperature and pressure ranges as well as to tune the gravimetric capacity. We believe this study could provide an initial prospective of transition metal-decorated silsesquioxanebased porous frameworks applied to hydrogen storage and provide key information to design silsesquioxane-based porous materials for hydrogen storage applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06514. Temperature-dependent curves of successive adsorption energy with Gibbs free energy correction for Sc-, Ti-, and H


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V-decorated isolated boron-doped benzene rings at 1, 50, and 100 bar (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by various grants from Academia Sinica and the Ministry of Science and Technology (NSC101-2113M-001-023-MY3 and MOST 104-2113-M-001-017). We appreciate the support from the National Center for Theoretical Sciences for various academic activities. We also appreciate the computational resources provided in part by the National Center for High Performance Computing.

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Tetrahedral Silsesquioxane Framework: A Feasible Candidate for

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