Tunable Gravimetric and Volumetric Hydrogen Storage Capacities in

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Tunable Gravimetric and Volumetric Hydrogen Storage Capacities in Polyhedral Oligomeric Silsesquioxane Frameworks Amol Deshmukh,†,‡,§ Cheng-chau Chiu,† 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: We study the hydrogen adsorption in porous frameworks composed of silsesquioxane cages linked via boron substituted aromatic structures by firstprinciples modeling. Such polyhedral oligomeric silsesquioxane (POSS) frameworks can be further modified by decorating them with metal atoms binding to the ring structures of the linkers. We have considered Sc- and Ti-doped frameworks which bind H2 via so-called Kubas interaction between hydrogen molecules and transition metal atoms. It will be demonstrated that the maximum H2 gravimetric capacity can be improved to more than 7.5 wt % by using longer linkers with more ring structures. However, the maximum H2 volumetric capacity can be tuned to more than 70 g/L by varying the size of silsesquioxane cages. We are optimistic that by varying the building blocks, POSS frameworks can be modified to meet the targets for the gravimetric and volumetric capacities set by the U.S. Department of Energy. KEYWORDS: hydrogen storage, gravimetric and volumetric capacities, porous frameworks, density functional theory, renewable energy

1. INTRODUCTION The development of sustainable energy supply systems has been an evolving field in the recent decades. This is because humans need not only to reduce the CO2 emissions to avoid the problems related to global warming but also to identify suitable substitutes for the limited fossil fuels resources.1−3 Hence, harvesting, storage, and utilization of renewable energy resources like solar energy are the main focuses of recent investigations in science and engineering. Among the possible directions considered for energy storage, using hydrogen gas (H2) as energy carrier, in particular as fuels for automobiles, is a clean way to store solar energy because only water, hydrogen, and oxygen are involved in the harvest and re-release of energy. One of the main challenges for the building of such a H2-based energy supply system is to develop cheap and safe H2 storage systems.4−7 Traditional H2 storage strategies based on compressing and liquefying H2 are not cheap enough for commercial use,4−7 because these processes consume a large amount of energy compared to the chemical energy stored in H2.8 Hence, scientists have put lots of effort in the identification of new possible materials for H2 storage, which can avoid large energy consumption during H2 charging and discharging processes. However, materials that fulfill the targets set by the U.S. Department of Energy (DOE)9 are yet to be found. The most recent targets include: usable gravimetric capacity higher than 7.5 wt %, volumetric capacity higher than 70 g/L, system cost less than US$266/kg H2, delivery temperatures between −40 and 85 °C, operating pressures between 3 and 100 bar, and a refueling rate of 5 kg H2 in less than 2.5 min. Various shortcomings are associated with different types of materials © XXXX American Chemical Society

that have been proposed in literature. Some of them can only operate at very high or very low temperature. Some others are too heavy; hence, the gravimetric capacities are too low. Still others have very low volumetric capacities or tend to degrade during the H2 charging and discharging processes.4−7 Among the studied materials, porous frameworks have been suggested to be a potential class of materials that can be used for H2 storage because they typically have large surface areas for capturing H2. At the same time, their large free volume also ensures a lower mass and thus a higher gravimetric capacity.4−6 Porous frameworks proposed for H2 storage comprise metal− organic frameworks (MOFs),10−22 covalent organic frameworks (COFs),23−27 porous aromatic frameworks (PAFs),28−34 polyhedral oligomeric silsesquioxane (POSS) frameworks,35−38 and some other structure types.39,40 However, those porous materials typically only physisorb H2 in a weakly exothermic process, if the framework is not further modified. In theoretical studies, the adsorption energies in many such systems have been estimated to be less than 0.06 eV/H2.15−17,21 Hence, such storage materials can only reach a high H2 capacity at very low t e m p e r a t u r e s , e . g . , 7 7 K a s r e p o r t e d in s o m e works. 5−7,11−13,15−18,21 Addressing this issue, Jena had suggested that H2 molecules should be bounded in a quasimolecular fashion, with adsorption energies ideally between 0.1 and 0.8 eV to allow efficient H2 storage under ambient pressures and temperatures.7 To raise the operating temperature to around 300 K, several theoretical studies have Received: May 25, 2016 Accepted: September 7, 2016

A

DOI: 10.1021/acsami.6b06245 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces suggested the doping of light metal atoms into porous materials to enhance the surface interaction between substrate and H2 molecules.15−22,33,34,38,39 Taking advantage of electric dipole and/or Kubas interactions,7,41 the H2 adsorption energies can be increased to more than 0.1 eV. The idea of using metal decoration to enhance the H2-framework interaction is not merely a theoretical concept; various such systems have already been realized experimentally.11,39 However, several theoretical studies have found that the clustering of introduced metal atoms could be a serious problem, as this might significantly reduce the H2 uptake.21,27,42 This issue has also been addressed in our previous study on the H2 storage in a POSS framework with metal atom decoration.38 In that study, the silsesquioxane cages are connected via benzene (C6H6) or boron-modified benzene (C4B2H6) units.38 It was observed that by substituting some carbon (C) atoms of the benzene linkers with boron (B) atoms, metal clustering can be effectively suppressed, while the H2 adsorption energies are only weakly affected. Here, the B atoms serve as electron depletion centers, which greatly enhance the binding between the linker and metal atoms as discussed in a previous computational study on lithium adsorption on B-doped graphene.43 Note that the effects of B substitution have only been investigated theoretically so far, and experiments are still need to confirm. Many theoretical and experimental works have studied MOFs as H2 storage materials.10−22 Various types of MOFs with different symmetries, even MOFs with metal decoration, have already been synthesized and successfully tested.10−14 It is interesting to note that a common strategy used in many studies to increase the gravimetric capacity is to synthesize porous frameworks with longer linkers.10,23,24,30−32,36 However, as we will show in this study, by using this approach, the volumetric capacity usually will be sacrificed for improving gravimetric capacity. In the present work, we evaluate the potential of POSS frameworks as H2 storage materials. Compared to MOFs, POSS frameworks based on silsesquioxane (HSiO1.5)2x cages featuring three-, four-, five-, and six-member rings44 are more stable against harsh operating conditions.45 Some POSS frameworks have already been successfully synthesized and are expected to be useful for various applications,46−49 among others as possible H2 storage systems.35−38 This study will deal with POSS frameworks that are doped with scandium (Sc) or titanium (Ti). We will not only discuss the H2 uptake in various POSS frameworks but also cover some issues on the stability of the metal decorated framework, e.g., the unwanted clustering of the doped metal. Furthermore, this study will demonstrate that the gravimetric and volumetric capacities of POSS frameworks can be tuned by combining silsesquioxane cages and linkers of different sizes.

Figure 1. Optimized structures of boron-modified aromatic linkers: (a) B-benzene, (b) B-biphenyl, (c) B-phenanthrene, (d) B-E-1,2diphenylethylene, and (e) B-diphenylacetylene. The C sites connected to the silsesquioxane cage in POSS frameworks are highlighted. Gray: C, pink: B, white: H. (abbreviated as B-benzene, B-biphenyl, etc.). We have constructed the POSS frameworks by connecting the silsesquioxane cages and the Bmodified linker molecules into 3D structures with diamond-like (using T4 cages), body-centered cubic (bcc, T8), and hexagonal symmetries (T12 and T6 combined). To evaluate the interaction between the doped metal atoms and the POSS framework, we calculate the average metal binding energy by: ΔE b = (Eframe + xEM − Eframe + x M)/x

(1)

where Eframe, Eframe+xM, and EM are the total energies of an empty POSS framework, the framework doped with x metal atoms, and one single metal atom. The average H2 adsorption energy of a metal-doped framework is calculated by:

ΔEad = (Eframe + x M + yE H2 − Eframe + x M + yH2)/y

(2)

where mframe+xM+yH2 and EH2 are total energies of a metal-decorated framework with y H2 adsorbed and a single gas-phase H2 molecule. In this paper, we also mention the metal binding energies for the isolated linker molecules as well as the corresponding H2 adsorption energies, which are defined in analogous fashion as in eqs 1 and 2, respectively. All energies in this study are evaluated within the framework of density functional theory (DFT) using Vienna Ab-initio Simulation Package (VASP).50,51 The spin-polarized wave functions of valence electrons are expanded in plane wave basis sets with 400 eV energy cutoff and pseudopotentials constructed with the projector augmented wave (PAW) method.52,53 Perdew−Burke−Ernzerhof type (PBE) generalized gradient approximation functional54 is used to calculate the electron exchange correlation energies. In addition, DFT-D3 correction as proposed by Grimme et al. is applied to account for dispersion forces.55 Most energies are calculated at the Γ point of the reciprocal space because the simulated unit cells are quite large. The only exception is made for the calculation of bulk energies of Sc and Ti, which are determined with a 1 × 1 × 1 hcp unit cell and a 15 × 15 × 15 Monkhorst−Pack k-points mesh.56 All structures are optimized until the forces on each relaxed atom are smaller than 0.01 eV/Å. The energies of isolated silsesquioxane cages, linker molecules, H2, and the single metal atoms in gas phase are evaluated in a cubic simulation box with a lattice constant of 30 Å. The symmetries and lattice constants of constructed frameworks will be addressed in the following paragraphs. To estimate the specific pore volumes (Vp) of each built framework, we follow the work of Myers and Monson57 and approximate it as the volume that is accessible for a helium (He) atom at temperature T = 298 K. For this, we use the following formula taken from ref 57 in combination with the Lennard-Jones (LJ) potentials reported by Li et al.:36,37

2. METHODOLOGY For the construction of POSS frameworks, we have used T4 (Td symmetry), T6 (D3h), T8 (Oh, cube shaped), and T12 (D6h) silsesquioxane cages, with T representing the silicon (Si) sites in tetrahedral coordination, as building units. Furthermore, we have considered five different linkers that can formally be derived from selected aromatic structures by substituting two C atoms with B atoms in the aromatic C6 rings. In particular, we have substituted the C atoms in benzene, biphenyl, phenanthrene, E-1,2-diphenylethylene, and diphenylacetylene (Figure 1). Though we are aware that the Bsubstituted structures are not aromatic anymore, we will for the sake of an easier notation refer to them as B-substituted aromatic species

Vp = B

1 mframe

∫ e−E(r)/kT dr

(3) DOI: 10.1021/acsami.6b06245 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

as in our earlier work.38 Analogue structures are considered for the larger linkers under study. Also here, each C4B2 ring interacts with two metal atoms (Figure 2). In order to investigate the metal clustering tendency on larger linkers, we also consider structures in which not all metal atoms are adsorbed at the ring sites as initial guesses for geometry optimization. For instance, we used structures in which two metal atoms are placed over/under the center of linker molecules, while the other two metal atoms remain at the C4B2 ring. Depending on the initial geometry, the optimized structures can be straight or strongly distorted. Some optimized structures are shown in Figures 2 and 3. A complete overview of the optimized structures is provided in Figure S1.

In eq 3, mframe is the mass of a framework without metal doping and H2 adsorption, E(r) is the potential between a He atom and the framework, and k stands for the Boltzmann constant. A 200 × 200 × 200 grid mesh is applied to get converged integrations of Vp. The parameters used for the LJ potentials are given in the Table S1. In addition, the specific volume of the total framework (Vtotal) defined as Vframe/mframe will also be calculated and compared with Vp. Vframe and mframe are the volume and mass of the unit cell of the framework used in simulation. For the sake of simplicity, we will approximate the volume of a metal-decorated framework by the corresponding value for the metal-free framework (Vframe) as we assume that the effect of metal decorations on the unit cell size of a framework can be neglected. The gravimetric (Dg) and volumetric (Dv) capacities are two important parameters used for evaluating the performance of H2 storage materials, which can be estimated by the following formulas:

Dg = (ymH2 /mframe + xM + yH2) × 100%

(4)

Dv = ymH2 /Vframe

(5)

where mframe+xM+yH2 and mH2 are the masses of a metal-decorated framework with y H2 molecules adsorbed and a single gas-phase H2 molecule. Note that we will estimate the maximum number (y) of H2 per unit cell from the number of H2 molecules that can bind to an isolated, metal-decorated B-benzene linker. However, as the discussion in sections 3.2 and 3.3 will show, there are issues like steric effects existing in some frameworks leading to a decrease of the number of H2 that a linker in a framework can bind. Thus, the Dg and Dv values reported in this work are to be understood as theoretical maximum values.

Figure 3. Optimized distorted structures of (a) Ti on B-biphenyl, (b) Sc on B-biphenyl, and (c) Sc on B-diphenylacetylene. Orange: Sc, blue: Ti, gray: C, pink: B, white: H.

With the exception of B-benzene, we have fixed the two terminal hydrogen atoms of linkers during the optimization to mimic the limited structural flexibility of the linkers in POSS frameworks. It is found that the adsorption of Sc and Ti atoms tends to bend or twist the structures of B-biphenyl and Bphenanthrene to reduce the metal−metal distances. As shown in Table 1, the distorted structures are more stable than the

3. RESULTS As shown in previous theoretical studies, substituting C atoms in aromatic linkers with B atoms can enhance the metal−linker binding energies to suppress the metal clustering.26,38 Therefore, we only consider B-modified linkers as shown in Figure 1 for this study. Furthermore, we will demonstrate the example of Sc- and Ti-doped POSS frameworks and how the H 2 gravimetric and volumetric capacities can be tuned, as well as the problems that may be encountered when modifying the POSS frameworks. 3.1. Metal Binding Energies on Linker Molecules and on Silsesquioxane Cages. Before investigating the H2 adsorption in the metal-doped POSS frameworks, we study the interaction between doped metal atoms and the building blocks of the POSS frameworks. We first consider the interaction between metal atoms and linker molecules. For the B-benzene linker, we consider a structure with two metal atoms adsorbed on the two sides of the C4B2 ring (Figure 2a),

Table 1. Average Metal Binding Energies (eV) on the Linker Molecules Shown in Figure 1 in the Most Stable Structure of Straight and Distorted (Bended or Twisted) Configurationsa straight B-benzene B-biphenyl B-phenanthrene B-E-1,2-diphenylethylene B-diphenylacetylene a

distorted

Sc

Ti

Sc

Ti

4.55b 4.64 4.30 4.47 4.52

5.06b 4.90 4.57 4.87 4.91

4.73 4.38 4.23 4.18

4.95 4.66 4.35 4.59

The more stable conformers are marked in bold. bFrom ref 38.

“straight” structures for B-biphenyl and B-phenanthrene with adsorbed metal. In contrast, the metal-decorated B-E-1,2diphenylethylene and B-diphenylacetylene maintain the straight structures (Figure 2d,e). Due to the tendency to distort upon adsorption of metal atoms as the adsorbed metal atoms interact with each other, structures like B-biphenyl and B-phenanthrene may not be suitable as linkers in POSS frameworks to be further modified via metal decoration. The problem of these linkers seems to be the small distance of only ∼4.5 Å between the C4B2 rings acting as metal docking sites (distance from ring center to ring center). In contrast, molecules like B-E-1,2-diphenylethylene

Figure 2. Optimized straight structures of Ti on (a) B-benzene, (b) Bbiphenyl, (c) B-phenanthrene, (d) B-E-1,2-diphenylethylene, and (e) B-diphenylacetylene. Blue: Ti, gray: C, pink: B, white: H. C


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ACS Applied Materials & Interfaces and B-diphenylacetylene with more distant metal docking sites (>6.9 Å) appear to be more suitable. The above finding probably applies for linkers in other types of frameworks, e.g., MOFs, with transition metal dopings as well. A second issue that should be checked before proceeding with the actual POSS frameworks is the interaction between a metal atom and the silsesquioxane cages to make sure that the doped metal atoms predominantly adsorbed at the linkers and not at the silsesquioxane cages. If the metal atoms prefer the silsesquioxane cages as adsorption sites, then the design of the POSS frameworks to achieve a high number of isolated metal sites for H2 adsorption likely has to be reconsidered. In Table 2,

Figure 4. Optimized structures of (a) 5 H2 adsorbed on Sc-doped Bbiphenyl and (b) 4 H2 adsorbed on Ti doped B-biphenyl.

Table 2. Ranges of Adsorption Energies of Sc and Ti Atoms on T4 and T8 Cages (eV) Sc Ti

T4

T8

0.26−0.61 0.44−0.55

0.19−0.40 0.37−0.47

remarks in the Supporting Information, section S4). The H2 expulsion could be related to the shorter distance between the metal docking sites in these to linkers, as mentioned in the previous paragraph. In contrast, the H2 expulsion is not observed for Sc-decorated B-E-1,2-diphenylethylene or Bdiphenylacetylene, which have larger distances between metal docking sites. While the H2 expulsion is only observed for Sc, it has not been observed for Ti. This can be attributed to the different metal−H2 interaction. Around Ti, which binds 4 H2 per metal atom with a more exothermic Ead compared to Sc, the Ti−H distances are between 1.82 and 2.12 Å. In contrast, the distances between Sc, which binds 5 H2, and the surrounding H atoms are significantly longer, between 1.98 and 2.61 Å. In other words, unlike in Sc-decorated systems, the H2 adsorbed to neighboring Ti centers are less strongly exposed to sterical stress which may trigger the expulsion of H2 molecules. In section 3.1 we have evaluated the stability of the metal atoms on the linkers by evaluating the metal binding energies as shown in Table 1. A question that arises here is whether the stability of metal sites changes upon the adsorption of H2. Therefore, we have calculated, in analogy to eq 1, how strongly the metal−(H2)x (x = 5 for Sc, x = 4 for Ti) complexes are bonded to the B-benzene linker. The energies required to desorb the metal−(H2)x complexes from B-benzene are 4.23 and 4.43 eV per Sc−(H2)5 and Ti−(H2)4, respectively. These two numbers are somehow lower than the Sc and Ti metal binding energies listed in Table 1 (4.55 and 5.06 eV). However, it is rather unlikely that the metal−H2 interaction will trigger the separation of the metal atoms from the linkers. Since H2 adsorption energies only range from 0.3 to 0.5 eV, H2 will desorb from the metal site and thus strengthen the metal-linker interaction again before the metal−(H2)x complexes can be desorbed from the linkers. 3.3. H2 Adsorption in POSS Frameworks. In the following, we will discuss the H2 adsorption in the 15 POSS frameworks built by connecting boron-modified linkers with different silsesquioxane cages. We will discuss the structural properties, as well as the expected gravimetric and volumetric capacities. 3.3.1. Frameworks Built with T4 Cages. The linkers and T4 cages can form frameworks which feature structures that correspond to the diamond structure with face-centered cubic ( fcc) symmetry (see Figure 5). All of them share a similar chemical formula: (SiO1.5)2x(linker)x with x = 4 for one unit cell of the simulation model used in this study (primitive fcc unit cell). As expected, frameworks using longer linkers have a larger specific volume (Vtotal) as well as a larger specific pore volume (Vp, Table 4 and Figure 6). Here, both Vtotal and Vp are

the ranges of the adsorption energies of Sc and Ti on different sites of hydrogen terminated, tetrahedral (T4), and cube-shaped (T8) silsesquioxane cages are listed. Figure S2 shows the structures and adsorption energies of Sc and Ti adsorptions on T4 and T8 cages. It is observed that the T4 cage adsorbs Sc and Ti atoms more strongly than T8. It is probably because of the fact that T4 is calculated to be less stable than T8 by 1.58 eV per formula unit H4Si4O6, in line with the value around 1.2 eV reported earlier.44 Compared to adsorption at the linkers shown in Table 1, those metal binding energies at the silsesquioxane cages (0.19−0.61 eV) are all much smaller. Hence, it can safely be assumed that metal atoms should mainly be adsorbed on the linkers. 3.2. Hydrogen Adsorption Energies on Isolated, Metal-Decorated Linkers. It has been shown in our previous study that each Sc site on a B-benzene linker can adsorb at most 5 H2 molecules, while a Ti site can only bind 4 H2.38 Thus, we only considered the structures of Sc and Ti decorated linker molecules with 5 and 4 H2 placed around each metal center, respectively. During the structure optimization, we have only considered such structures in which the linker molecules are straight, as the POSS framework structure is likely to enforce the straight geometry of the linkers. Table 3 Table 3. Average H2 Adsorption Energies (eV) on MetalDecorated Linkers under Study B-benzene B-biphenyl B-phenanthrene B-E-1,2-diphenylethylene B-diphenylacetylene a

Sc

Ti

0.35a 0.31 0.31 0.34 0.34

0.45a 0.43 0.44 0.44 0.44

From ref 38.

summarizes the average H2 adsorption energies. The Ead values for larger linkers, calculated under the assumption that 20 or 16 H2 (for 4 metal sites) are adsorbed, are very close to the values for B-benzene except the noticeable lower values for Scdecorated B-biphenyl and B-phenanthrene. In those exceptions, one H2 (out of 20) is expelled farther from the Sc sites with a Sc−H distance of over 4 Å, compared to Sc−H distances below 2.61 Å found for the other 19 H2 (Figure 4a, see also the D


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Figure 6. Specific volume Vtotal (the full height of each empty bar) as well as the specific pore volume Vp (filled bars) calculated for the empty POSS frameworks under study without metal decoration.

Figure 5. Optimized structure of Ti doped T4-B-diphenylacetylene framework (a) without/(b) with H2 adsorption.

calculated on the basis of the empty framework without metal doping. Under the assumption that each Sc and Ti center can bind 5 and 4 H2, respectively, we have estimated the corresponding gravimetric and volumetric capacities given in Table 4. It can be seen that those frameworks built from longer linkers with two C4B2 rings acting as metal binding sites have higher gravimetric capacities compared to B-benzene-based frameworks (raised by 14−16%). However, the volumetric capacities decrease significantly (dropped by ∼60%) upon substituting the Bbenzene by the longer linkers. Due to the slightly smaller size of the linkers, the frameworks built with B-biphenyl or B-phenanthrene are expected to feature higher volumetric capacities than frameworks with B-E1,2-diphenylethylene or B-diphenylacetylene linkers. Nevertheless, B-biphenyl or B-phenanthrene based frameworks are most likely not superior to the B-E-1,2-diphenylethylene- or Bdiphenylacetylene-based systems, since the metal centers on Bbiphenyl and B-phenanthrene tend to interact with each other, resulting in a distortion of the linkers, as discussed previously (Table 1). This may lead to unwanted clustering of the metal centers, which would reduce the H2 uptake capacity. Hence, we have skipped the explicit quantum chemical modeling of H2 adsorption in frameworks with B-biphenyl or B-phenanthrene as indicated in Table 4. The corresponding Dg and Dv values are estimated based on the number of H2 adsorbed to the isolated linker and merely included for the sake of completenes of the data. The average H2 adsorption energies for Sc- and Ti-decorated frameworks built of the T4 cages and B-E-1,2-diphenylethylene are 0.31 and 0.44 eV, respectively. The corresponding numbers

for B-diphenylacetylene-based frameworks are 0.32 and 0.44 eV. These adsorption energies are slightly higher, i.e., thermodynamically more favorable, than the results for the framework with B-benzene linkers from our previous study, 0.29 and 0.40 eV.38 In that work, it has been shown that at 100 bar the Gibbs free energy of adsorption in the Sc and the Ti decorated B-benzene framework becomes 0 at around 260 and 366 K, respectively. We expect that the frameworks with B-E1,2-diphenylethylene or B-diphenylacetylene linkers and the T4 cage can operate under similar temperatures and pressures as the framework with B-benzene. 3.3.2. Frameworks Built with T8 Cages. To improve the volumetric capacity, we have considered POSS frameworks based on silsesquioxane cages other than the T4 cage. This section will deal with the POSS frameworks based on T8 cages that have already been successfully synthesized.46−49 Using T8 cages with Oh symmetry44 as building blocks, one obtains a structure that has the same chemical formula ((SiO1.5)2x(linker)x with x = 4) as the T4 based framework but less free space. The structure of the formed frameworks corresponds to the bcc packing (Figure 7). We expect that the variation of the linkers in T8 frameworks will most likely show the same trend in Dg and Dv as what has been discussed above for T4 frameworks. The more interesting aspect here is how Dv changes upon switching from T4 to T8. Therefore, we will focus only on frameworks built with the B-benzene linker here (and also later for the discussion of the T12+T6 frameworks). For the sake of simplicity, we have only calculated the unit cell size for the other T8 frameworks built with longer linkers but omitted the explicit modeling of the H2 adsorption process.

Table 4. Chemical Formulas, Lattice Constants (a, Å), Specific Volumes (Vtotal, cm3/g), Specific Pore Volumes (Vp, cm3/g) as Well as Theoretical Maximum Gravimetric (Dg, wt%) and Volumetric Capacities (Dv, g/L) of Adsorbed H2 for Frameworks Built with T4 Cagesa Dg

Dv

linker

formulab

a

Vtotal

Vp

Sc

Ti

Sc

Ti

B-benzene (B-biphenyl)d (B-phenanthrene)d B-E-1,2-diphenylethylene B-diphenylacetylene

C16B8H16Si8O12 C32B16H32Si8O12 C40B16H32Si8O12 C40B16H40Si8O12 C40B16H32Si8O12

23.18c 33.92 33.96 39.16 39.68

2.64 5.84 5.34 8.14 8.53

2.20 5.38 4.93 7.67 8.06

7.0c 8.5 8.1 8.1 8.1

5.6c 6.8 6.5 6.4 6.5

43.0c 27.5 27.4 17.8 17.1

34.4c 22.0 21.9 14.3 13.7

a

Dg and Dv are estimated based on the assumption that each Sc and Ti center can bind 5 and 4 H2 molecules, respectively. bThe chemical formula is given in terms of the primitive fcc unit cell, which was used for the actual simulation. For the sake of an easier comparison, the lattice constant a is given in terms of the conventional cubic cell of fcc. cFrom ref 38. dThe metal-decorated frameworks are most likely not stable. Thus, neither the metal-decorated framework nor the H2 adsorbed systems has actually been modeled. The entries are shown for the sake of completeness of the data set. E


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per metal atom is, contrary to the initial assumption, not independent of the framework structure: We have explicitly modeled the H2 adsorption in T8−B-benzene framework and see that steric effects may play an important role. For the Scdecorated system, we are not able to find a reasonable arrangement for 5 H2 adsorbed per Sc. Instead, we have considered a structure with only 4 H2 per Sc. However, even in this case, the H2 molecules are not well-distributed around the Sc centers. As the Sc centers on neighboring B-benzene linkers are rather close, around 4.0 Å, some H2 are expelled from the coordination sphere of Sc during structure optimization. The expelled H2 either becomes unbound or is cleaved with one of its H atoms binding to the Sc center and the other one transferred to the linker in a hydrogen spillover process. In contrast to the Sc-decorated system, the maximum number of H2 molecules around each Ti is not affected by the structure of the T8 framework. Like the situation for Ti on a free B-benzene, the Ti centers in each of the considered T8 frameworks can adsorb four H2 (Figure 7a). The Ti centers on neighboring B-benzene linkers are separated by more than 4.73 Å because the Ti centers are bound with a smaller distance to C4B2 rings than in the corresponding Sc structures. Although the number of adsorbed H2 per Ti is unaffected, the steric effects seem to affect the H2 adsorption energy on Ti: The average value is reduced from 0.45 eV for Ti on free B-benzene to 0.36 eV in the T8 framework. Although we have not explicitly modeled the H2 adsorption in Ti-doped frameworks built from T8 cages and B-E-1,2-diphenylethylene or B-diphenylacetylene linkers, we expect that those structures will also be suitable H2 storage material, similar to the framework with B-benzene linkers. Compared to frameworks built from B-benzene, those frameworks are likely to feature lower volumetric capacities (Figure 8) but higher gravimetric H2 capacities (cf. Table 4). More specifications of these frameworks can be found in Table S2. 3.3.3. Frameworks Built with T12+T6 Cages. As the structure of the POSS frameworks with T8 cages did not affect the number of H2 bound to a Ti center, it seems reasonable to consider at least for the Ti-decorated system whether larger silsesquioxane cages can be used to build frameworks with even lower specific volumes Vtotal and thus higher volumetric capacity. In contrast to the T4 cage with Td symmetry, many of the larger silsesquioxane cages have already been realized in experiment.58,59 In this section, we consider hexagonal POSS frameworks composed of T12 and T644 cages and the five linker molecules (Figure 9). We should mention here that to our best knowledge similar to the T4 cage the T6 cage has not yet been

Figure 7. Optimized structure of (a) Ti-doped T8−B-benzene framework with H2 adsorption and (b) T8−diphenylacetylene framework without H2.

Since the T4 and the T8 frameworks have the same chemical formula, their theoretical maximum gravimetric H2 capacities are identical if using the same linker, assuming that the framework structure does not affect the number of bounded H2 (cf. Table 4). At variance, using T8 cages leads to a significant reduction of the specific volume Vtotal (Figure 6; the exact numbers are given in Table S2) and thus to an increase of the theoretical maximum volumetric capacities by more than a factor of 2 compared to the those of the T4 frameworks (Figure 8). However, it should be noted that number of adsorbed H2

Figure 8. Theoretical maximum volumetric capacity Dv for all frameworks under study. The values are estimated based on the assumption that each Sc and Ti center can bind 5 and 4 H2 molecules, respectively. Systems for which the H2 adsorption in the framework has explicitly been calculated are shown as full bars. The asterisks mark the systems for which the explicit modeling has revealed that the actual number of adsorbed H2 per metal is less than 5 and 4, respectively.

Figure 9. (a) Top view and (b) side view of T12+T6−B-diphenylacetylene framework. F


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consider as one of the most promising structures investigated here, there are no indications that a H2 expulsion will occur. Due to the lower mass of Sc compared to Ti and the ability to bind up to five H2 per metal, it will be a great improvement if the steric influences on the H2 adsorption at Sc in T8 or T12+T6 frameworks can be reduced. One possible solution to increase the “inter-linker” metal−metal distance could be applying further modifications to the linker molecules. Instead of linking the C4B2 rings directly to the silsesquioxane cages, one may consider linkers that have additional ethinyl groups (−CC−) at the C4B2 ring, as shown in Figure 10. This modification will

synthesized. Figure S3 shows the molecular structures of the T12 and T6 cages. Compared with the frameworks based on T8 or T4 cages, the theoretical maximum gravimetric capacities of T12+T6 frameworks do not change as their chemical composition is identical ((SiO1.5)2x(linker)x with x = 12), cf. Tables 4 and S3). The volumetric capacities increase again compared to those of T8 frameworks as shown in Figure 8. For the Sc -doped T12+T6 framework with B-benzene linkers, the theoretical maximum volumetric H2 uptake capacity is 104 g/L, corresponding to five H2 per Sc. However, as already demonstrated for the T8 framework, such a high H2 uptake is hindered due to steric effects. We have explicitly optimized the H2 adsorption in the T12+T6 framework with Ti-decorated B-benzene linkers. It is found that framework structure starts to affect the H 2 adsorption on Ti. This leads to the expulsion of some H2 from the Ti coordination sphere, which has not been observed in T8 and T4 frameworks. Table S3 shows more specifications of the T12+T6 frameworks.

4. DISCUSSION As shown by the results above, there are some general principles that determine the H2 uptake capacity of metaldecorated POSS frameworks as proposed in our study: (1) Using longer linkers could gradually enhance the theoretical maximum gravimetric capacities but dramatically reduce the volumetric capacities at the same time. (2) Switching to larger silsesquioxane cages, which can bind more linkers, does not affect the theoretical maximum gravimetric capacities but increases the volumetric capacities. However, steric influences, which may reduce the number of adsorbed H2 per metal and/ or the average H2 adsorption energy, become more pronounced in POSS frameworks with larger silsesquioxane cages. The results in Figure 8 suggest that there likely is an upper boundary for Dv beyond which the H2 expulsion induced by the steric effects between H2 occurs. This boundary likely is around 76 g/ L, the Dv value determined for Ti-decorated T8−B-benzene framework. However, these effects will need to be examined carefully in further studies. (3) The combination of different silsesquioxane cages and linker molecules offers the possibility of synthesizing H2 storage materials with tunable gravimetric and volumetric capacities. From our results, it seems that the Ti-doped T8 frameworks are a reasonable choice as H2 storage materials. While the volumetric capacity can be improved compared those of to T4 frameworks, the number of adsorbed H2 per metal is not affected by the framework structure, different from that of Scdecorated frameworks. Merely the average H2 adsorption energy is slightly reduced compared to the value for frameworks based on T4 cages, as shown at the example of T8 framework with Ti-decorated B-benzene linkers. We should note here that we are aware of the problem of our approach to compare the theoretical maximum volumetric and gravimetric capacities, which are estimated based on the number of H2 adsorbed on metal-decorated, isolated Bbenzene. By doing so we tend to overestimate the actual H2 capacities, as we do not account for the steric influences mentioned above, which may result in H2 expulsion from the metal coordination sphere making a framework structure unsuitable as H2 storage material. However, as we have pointed out in the previous section, this issue does not affect all frameworks under study. In particular, for the Ti-doped framework based on T8 cages and B-benzene, which we

Figure 10. Ethinyl-group modification to (a) B-benzene and (b) Bdiphenylacetylene. The C sites connected to the silsesquioxane cage in POSS frameworks are highlighted. Gray: C, pink: B, white: H.

push the rings farther away from the silsesquioxane cages and result in larger spacing between the metal atoms on adjacent linkers. However, such a modification is also expected to result in a decrease of the gravimetric and volumetric H2 capacities. Among the linkers considered in this study, B-benzene is the shortest linker and hence results in the smallest pore volumes together with the highest theoretical maximum volumetric capacities for the frameworks under study (Figures 6, and 8). In the particular case, the T8 framework with Ti-decorated Bbenzene linker could surpass the target for the volumetric H2 capacity of 70 g/L set by the U.S. DOE, reaching a value of 76.1 g/L. However, the corresponding gravimetric H2 capacity is only 5.6 wt %, which is still below the DOE target of 7.5 wt %. Nevertheless, we are optimistic that by varying the composition of the frameworks, i.e., the linkers, silsesquioxane cages, and also doped metals, suitable POSS frameworks can be found which will be able to fulfill the target for both the gravimetric and the volumetric H2 capacity. A further aspect that we mention here is also linked to the volumetric capacity/pore volume: As demonstrated by Li et al., the total amount of H2 stored in a porous structure does not only consist of those H2 adsorbed by the framework.36,37 They have applied grand canonical Monte Carlo (GCMC) simulations to show for porous frameworks without metal decoration that H2 molecules may also fill up the free space in the pores without directly interacting with the framework. If the framework−H2 interaction is weak, then such unbound H2 in the free space may become at elevated temperatures and pressures the dominating fraction of the H2 stored in the framework.36,37 The contribution of such unbound H2 is omitted in this study, as the interaction between H2 and framework is stronger in our system due to the metal decoration. It has been reported for the empty T4 framework with benzene molecules as linkers at 298 K and 100 bar that the unbound H2 contributes around 2.2 wt % and 8.8 g/L to the G



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gravimetric and volumetric H2 capacity.36,37 Hence, we assume that the POSS frameworks proposed in our study may benefit in a similar way from the H2 molecules in the free space of the framework. As visible from the discussion above, there is no easy answer for the question of whether a larger pore volume (and thus larger Vtotal) is beneficial or not. On the one hand, larger pore volumes lead to the reduction of the volumetric capacity for H2 bound at the framework sites; on the other hand, it allows a higher number of H2 to be stored in the free space of the framework. This shows that the effect of the pore volume on the H2 storage performance deserves to be carefully evaluated by experiments, as many aspects are affected by the pore size. A further issue that has not been mentioned here so far is the influence of the pore size on the gas diffusion rate and the total H2 uptake.12,14 Before proceeding with the conclusions, we want to give a small outlook on further potential methods to modify the POSS frameworks in order to improve their performance for H2 storage, which have not yet been addressed in this work. One option would be to vary the metal used for the decoration of the framework. It may not be necessary to restrict oneself to only one type of metal; frameworks with mixed metal decoration are also possible. A further possibility is to consider interpenetrating framework structures, similar to the IRMOF61 or IRMOF-62 MOFs studied by Tranchemontagne et al.14 Such structures may increase the density of metal sites and hence enhance the volumetric capacity, if the issue of steric effects can be excluded. Another possible way to increase the density of the metal sites would be to substitute the silsesquioxane cage by structures that can also act as anchor sites for metal atoms as well, e.g., C60 or C48B12 as reported by Gao et al.40

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 by the National Center for High Performance Computing.

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5. CONCLUSIONS We have proposed 15 POSS frameworks and evaluated their suitability for H2 storage based on DFT calculations. We see that frameworks using larger silsesquioxane cages (T8 and T12+T6) have the potential to enhance volumetric capacities while keeping the gravimetric capacities. However, the issue of steric effects should be carefully examined when doing such design for H2 storage materials. Among the POSS framework considered in this work, it is expected that the Ti-decorated frameworks based on T8 silsesquioxane cages will be the best choices for H2 storage. Our study suggests that metaldecorated, three-dimensional frameworks built from silsesquioxane cages and boron-substituted aromatic molecules may be promising H2 storage materials with tunable gravimetric and volumetric capacities which are worth further study.

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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/acsami.6b06245. Lennard-Jones parameters for the calculation of Vp, geometrical parameters and H2 capacities for the T8 based and T12+T6 based frameworks, optimized molecular structures of metal adsorption on linker molecules, T4, T8 cages, and the molecular structures of T6, T12 cages (PDF) H


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Tunable Gravimetric and Volumetric Hydrogen Storage Capacities in

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