- Email: [email protected]
Hydrogen storage by adsorption in porous materials: Is it possible?
Accepted Manuscript Title: Hydrogen storage by adsorption in porous materials: Is it possible? Author: Rafal Roszak Lucyna Firlej Szczepan Roszak Peter Pfeifer Bogdan Kuchta PII: DOI: Reference:
S0927-7757(15)30306-X http://dx.doi.org/doi:10.1016/j.colsurfa.2015.10.046 COLSUA 20254
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
26-5-2015 26-9-2015 26-10-2015
Please cite this article as: Rafal Roszak, Lucyna Firlej, Szczepan Roszak, Peter Pfeifer, Bogdan Kuchta, Hydrogen storage by adsorption in porous materials: Is it possible?, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.10.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrogen storage by adsorption in porous materials: is it possible? Rafal Roszak1, Lucyna Firlej2,3*, Szczepan Roszak1, Peter Pfeifer3, Bogdan Kuchta3,4 1
Advanced Materials Engineering and Modelling Group, Wrocław University of Technology,
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland 2
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS-Université de Montpellier, Montpellier,
F-France. 3
Department of Physics and Astronomy, University of Missouri, Columbia, USA
4
Laboratoire MADIREL, Aix-Marseille Université – CNRS UMR 7246, 13396 Marseille, France
*Corresponding address: [email protected]
GRAPHICAL ABSTRACT
Highlights
The adsorbing properties of graphene-based nanoporous carbons can be modulated by simultaneous fragmentation of graphene structure, substitution of carbons by electron deficient atoms, and functionalization of fragments’ edges.
Carbon substitution with beryllium dimer can substantially increase hydrogen binding to graphene-based surface. Hydrogen binding to Be2 dimer is mainly defined by a weak coordination bond.
Both C/Be2 substitution and functionalization of the graphene edges are necessary to increase the hydrogen gravimetric storage to the value that satisfies the DOE requirements.
Abstract The role of fundamental characteristics of porous systems (binding energy, specific surface area and multilayer adsorption) in designing an efficient hydrogen adsorbent is discussed. We analyze why the amount of hydrogen adsorbed in all known materials is much lower than required for mobile applications and what are possible strategies to increase it. Further we report new ab initio calculations demonstrating possible ways of chemical modification of graphene fragments which can lead to the substantial increase of hydrogen binding to the graphene-based surface. Such Open Carbon Frameworks, substituted and functionalized at the fragments’ edge may theoretically adsorb, at ambient temperature and relatively low pressure (60-100 bar), the amount of hydrogen necessary for mobile applications.
Keywords: hydrogen adsorption, hydrogen storage, graphene fragments, beryllium dimer
1. Introduction In recent years, great emphasis has been placed on developing the concepts and technology to power the vehicles using renewable energy. One potential solution is the use of hydrogen gas as an energy vector to power fuel cells. However, onboard hydrogen storage continues to be one of the most technically challenging barriers for the widespread commercialization of hydrogenfueled light duty vehicles that will be competitive with fossil fuel or hybrid vehicle markets. The major identified difficulty consists in designing hydrogen storage system capable of delivering a driving range of hundreds of kilometers. Among the four principal methods to store hydrogen: liquefaction, gas compression, chemical storage, and physical adsorption, the last one offers several advantages: rapid kinetics, reversibility and relatively high storage capacity. Nanoporous carbons are commonly considered as, potentially, the best hydrogen sorbents due to their low cost, light weight, high surface area and chemical stability. However, the quantity of hydrogen they can currently store is not sufficient for practical applications. This limitation is inherently related to the electronic properties of H2 molecule and the mechanism of hydrogen adsorption. New low pressure sorbents and material-oriented technologies have to be developed, to meet packing, cost, safety, and performance requirements. The main targets directly related to the system storage capacity are gravimetric and volumetric densities of the adsorbed hydrogen. According to the US DOE (Department of Energy) requirements, the future hydrogen tank should ultimately store 75 g of H2 per kg and 70 g per liter of storing system. Remembering that these parameters are system goals, the storing medium must greatly exceed these capacities. In the present paper we will use these numbers as reference values only.
Our main goal is to discuss a new approach to the chemical modification of carbon surface which will allow tuning the hydrogen binding in a wide range of values. The paper is organized as follow: in the first section we review the physical sources of limitations of existing hydrogen sorbents and indicate the directions for further material research in order to reduce these restraints. Then, we report ab initio calculations which suggest a new strategy of modifying the all-carbon surfaces and increase substantially their energy of hydrogen adsorption. Finally, we discuss the reported results in the context of their possible applications.
2. Search for optimized hydrogen sorbents. The state-of-the–art in the optimization of hydrogen sorbent has been summarized in many review articles [1-5] and in some critical analysis of the conditions required for hydrogen storage [6-8]. The conclusion coming out of all of these sources and our previous papers [9-12] is unique: despite more than 20 years of research, no material yet possesses adsorbing properties required for applications. All porous structures that exist today are far from being optimal when considered as hydrogen sorbents for mobile applications. Such sorbents include wide range of materials from traditional carbon-based structures (activated carbons, carbon nanotube-based structures, polyaromatic hydrocarbons) to currently researched open porous structures (metal organic frameworks (MOFs), covalent organic frameworks (COFs) or porous polymer networks (PPN)). It is well established today that it will be difficult to reach the high H2 storage in conventional high surface area adsorbents, because of their low average adsorption (binding) energy, heterogeneity of surfaces, mesoporosity, and poor volumetric packing. In carbon-based materials the main limitation comes from the relatively small binding energy on the graphene surface (4-6 kJ/mol) [1]. For these conventional porous carbons the hydrogen uptake, at ambient temperature, is proportional to their adsorbing surface. The importance of optimization of pore size distribution in order to maximize sorbent specific surface has been numerically estimated in our recent paper [8]. However, even if an ideal model of infinite, slitshaped pores is assumed our results have shown that graphene-based structures will never meet DOE goals, although their H2 storage at low temperature is very promising. Following this fact, other porous structures with exceptionally large surface areas (MOFs and COFs) have also been tested for the hydrogen storage [13,14]. These materials are synthesized from metal or metal oxide centers interconnected by organic linkers. In COFs, metal sites are replaced by light elements (mostly boron) to decrease the structure weight. Depending on choice of the building elements a variety of 3D structures with very high and tunable porosity has been prepared [15]. Many experimental and theoretical studies have found impressive storage capacity at cryogenic temperature and high pressure, especially in COFs (COF-108 exceed 20 wt% storage capacity already at 100 bar) [15]. However, an average adsorption energy of H2 in MOFs and COFs is even lower than that of activated carbons. In the consequence, their performance at ambient temperature and moderated pressure still falls far from the DOE targets.
Therefore, the next step in developing an optimal H2 sorbent should consist in searching for materials with higher binding energy [6-8, 10-12]. Assuming a hypothetical energy of adsorption up to 3 times larger than the energy of H2 binding on pristine graphene, it has been shown than graphene-based but chemically modified porous carbons could provide at room temperature the H2 storage capacity required by DOE [6,8]. One of the ways to control the adsorption energy of a given sorbent is a chemical modification of its surface, by doping or substituting carbons with other atoms. In particular, substitutional doping has attracted a lot of attention because it influences simultaneously sorbent topology (by introducing spatial heterogeneity) and the strength of intermolecular interactions. In the category of carbon-based sorbents of H2, substitution with boron has been promoted as the most promising. Early theoretical calculations suggested a non-dissociative adsorption of H2 on B and Be-doped fullerenes. The coupling between an empty, localized pz orbital of the dopant and the H2
s. In the case of B-
doping, H2 binding energy was raised to 15-35 kJ/mol [16], making boron-doped carbon an outstanding candidate for the high capacity H2 storage. Our recent Grand Canonical Monte Carlo simulations based on ab initio interaction model [9,17] reports an enhanced H2 uptake in graphene slit pores with walls containing boron atoms. The high capacity is not related to any particular crystallographic structure but depends only on the B/C substitution ratio. The 10 % substitution is necessary to rise the average binding energy to 10 kJ/mol, and to approach both gravimetric and volumetric directives of DOE for 2017 (Fig. 1), with almost 100% delivery of stored H2 simply by pressure sweep (without increasing sorbent temperature). It is not clear how the further increase of this limit could be achieved with a higher substitution. It was also shown that boron doping of graphene can indirectly contribute to the higher capacity of H2 storage of Ca-decorated carbons, via the stabilization of dispersed Ca atoms on the graphene surface. The Kubas interaction between H2 and Ca allows a stable binding of up to four H2 molecules to Bdoped graphene [18]. From simple geometrical considerations, double-sided Ca decorated graphene randomly doped with 12% of boron atoms can theoretically reach a gravimetric capacity of 8.38 wt% , but the average binding energy of 36,8 kJ/mol makes the H2 cycling difficult at ambient conditions [18]. Obviously, experimental confirmation of calculations cited above is needed for the validation of theoretical analysis. For that, substituted porous carbons have to be synthesized.
Recently, is has been demonstrated that the arc discharge between graphite electrodes, a technique developed to prepare large quantities of fullerenes and carbon nanotubes, can be optimized to synthesize graphene fragments composed of two to three layers. The method makes use of the fact that in the presence of hydrogen in the discharge apparatus graphene sheets do not roll to form cage-like structures [19]. Boron-stuffed electrodes or B2H6-containing discharge atmosphere has been used to prepare boron-doped graphene fragments of the lateral size of 20 to 60 nm. The presence of substitutional boron in the sp2 carbon structure has been confirmed by EELS, AFM, and Raman investigations. Similarly, nitrogen atoms have been successfully introduced to graphene sheets by the arc discharge in the presence of pyridine or ammonia [20]. Although the resulting substitution ratio was not high (up to 3% for the boron and 1.4% for nitrogen substitution), these preliminary results are promising as the arc discharge procedure could be easily optimized. The additional advantage of arc discharge technique is its ability to produce graphene fragments of nanometric size. This aspect of topology is extremely important, because the fragmentation of infinite pore walls into small patches creates an additional adsorption surface at patches’ edges. Recently we have shown that the fragmentation of infinite graphene walls into the patch-like structures increases hydrogen gravimetric storage capacity, by a factor of two [21]. The contribution of adsorption on fragment edges to the overall amount stored is independent of the fragment shape and increases when the fragment lateral size decreases. However, simultaneously, the energy of adsorption at edges rapidly decreases with a fragment size. Therefore, the final hydrogen uptake in pores of finite lateral dimensions will result from a competition between two factors: larger adsorption surface introduced by the pore edges and lower average adsorption energy and heterogeneity (structural and energetic) of the system. This result suggests that nanoporous carbons build up from interconnected fragments of graphene or polyaromatic hydrocarbons can show high hydrogen storage, assuming the fragment size and system topology can be optimized. This leads to redefined conditions, that nanoporous carbons optimized for hydrogen storage should fulfill: (i) First, the porous system must have an open geometry with considerable contribution of edge surface of building units (fragments of graphene sheets), (ii) the size of fragments should be wisely optimized: not too small because it leads to low energy of adsorption and decreasing uptake, and not too large to be able to take the
advantage of an important contribution from edges, (iii) the density of the structures cannot be too low because of the volumetric storage capacity requirements. Following these conclusions, we proposed a new class of carbon-based open high surface structures, Open Carbon Frameworks (OCFs) which, when synthesized, will greatly extend the current limits of hydrogen storage by the physisorption at room temperature [11,22]. However, the simultaneous optimization of both adsorbent surface and its adsorption will be necessary to attain an ultimate adsorption uptake. The analysis of extensive numerical database produced by Monte Carlo simulations of H2 adsorption in nanoporous carbons allowed us to deduce [12] the simple functional relation between material gravimetric capacity (Gc, in wt%), its specific surface (S, in m2), and average binding energy (E, in kJ/mol): Gc = f * (S/1000) *E where the coefficient f depends on the material’s topology, mainly on the pores width. The isoweight-capacity curves presented on Fig.1 show the pairs of values of (S, E) that are simultaneously needed to reach an assumed hydrogen storage capacity. In the past most of the research for optimal hydrogen sorbents have focused on improving one of two material parameters (S or E), keeping the other constant.
However, the functional
dependence presented on Fig.1 shows that using such approaches an ultimate DOE goals could be only achieved if the second characteristics is exceedingly improved (specific surface larger than 10,000 m2/g or binding energy close to 20 kJ/mol). Alternative approaches consisting in simultaneous optimization of both parameters (for example preparation of carbon-based adsorbent with improved but reasonable surface area (4000-5000 m2/g) and adsorption energy (8-10 kJ/mol) may be easier to achieve. In the following chapter we discuss one example of such an option. We show that the substitution of Be dimer into a finite size graphitic fragments may increase both parameters (S and E) simultaneously. Moreover, additional substitution of strong electronegative function close to the Be2 site may allow an additional modulation of adsorption energy of the material, depending on the nature and the localisation of substituent.
3. Graphene theoretical nano-engineering. Graphene, a two-dimensional sp2 bonded carbon sheet arranged in a hexagonal honeycomb lattice, with its high theoretical specific surface area of 2600 m2/g provides a rich platform for the surface chemistry. The modification of graphene has been extensively studied in the past in the aim of preparing new carbon-based sorbents with high binding energy. The most common functionalization strategy is a substitution of carbon by boron, nitrogen, or oxygen. The other possible way to increase binding energy is the deposition of metal clusters on the carbon surface [23]. The substitution of carbon by light metals was less explored. In particular, an introduction of beryllium into the carbon lattice is one of the least studied modifications. Beryllium can be incorporated into the graphene sp2 network either via the single atom substitution (C/Be) or by the substitution of carbon atom by the beryllium dimer (C/Be2). The knowledge regarding the properties of organoberyllium materials is very limited; some of them were recognized as promising hydrogen sorbents [24-26]. Therefore we have analyzed changes in the energy of hydrogen binding towards Be-substituted surfaces for both types of substitution and a variety of Be : C substitution ratios.
We have adopted the ovalene C32H14 as a model of graphene network and a starting point for the graphene substitution. To analyse the modulation of adsorption energy by the further functionalisation of organoberylium lattices, smaller units (benzene, tetracene and coronnene) have been employed. All structures were optimized at the density functional theory (DFT) level using the B3LYP functional [27]. No symmetry constraints were imposed during the geometry optimization: however, to model the extended structure of ovalene, the positions of carbon atoms in its external rings were frozen. We are aware that applied approach cannot reproduce the dispersion energy contribution; however, as hydrogen adsorption on Be centers is dominated by weak Kubas-like forces, the contribution of dispersion energy to the stabilization of such complexes can be neglected [28]. To calculate the energy of the DFT-optimized structures we have applied the Møller-Plesset second-order perturbation methodology with an aug-cc-pVDZ basis set [29]. The adsorption energy has been calculated as the difference between energy of H2-BeCn complex and isolated H2 and BeCn molecules: Ea = E(complex) – E(H2) – E(BeCn). All terms have been calculated consistently in the dimer-centered basis set (counterpoint correction) and are free from the basis set superposition error (BSSE). All calculations were carried out
using the Gaussian-09 code [30]. More detailed description of computation methodologies (MP2 and DFT) is presented in [28, 31]. The detailed study of structure of carbons substituted with single beryllium atoms is presented in our previous paper [31]. In general, if the substitution occurs as an isolated event, the energy of hydrogen adsorption remains comparable to that of pure graphene. The main consequence of the substitution is the perturbation of planarity of the system: the substituted Be is located ~0.56 Å above the carbon plane, and two of three adjacent carbon atoms being out of plane by 0.23 Å. When the substitution ratio increases, the substituted Be acts as an isolated adsorption center up to n =4, where n is the number of carbon atoms separating two nearest Be neighbours. In substituted ovalene C27BeC3BeH14 (n =3, see the insert on Fig.2, left panel) the energy of hydrogen bonding on each Be site increases to the value of ~12.8-13.7 kJ/mol. This value is interesting from the point of view of potential applications. Hovewer, the possibility of synthesis of OCF structures based ovalene units and showing high specifics surface areas, and as well as the stability of larger systems of such C5Be-type superstructure have still to be proven. If two adjacent carbons in ovalene are substituted with Be, the hydrogen binding on both sites increases to 46.5-49.5 kJ/mol, the values which are too high for reversible adsorption, even at room temperature. A particular case of topography is single carbon substitution by the Be dimer. Such substitution does not introduce any notable perturbation of hosting graphene structure. The center of the dimer remains in the graphene plane with Be atoms located on the perpendicular axis and separated by a distance of 1.7 Å (Fig2, right panel). Each Be dimer can accommodate 2 molecules of hydrogen. The binding energy of the first molecule amounts to ~23 kJ/mol. Theoretically, this is the perfect energy required for the optimal hydrogen adsorption medium [32]. Although the binding of the second molecule on the same center occurs with much weaker energy, ~ 8 kJ/mol, this value is still almost 50% higher than one for pure graphene. The modification of the energetic landscape for hydrogen adsorption is very localised and do not extend beyond centers of hexagons adjacent to the substitudes carbon (Fig.3a); the binding energy on Cα carbons is almost the same as that in graphene.
As for single atom substitution, this energetic picture evolves when the concetration of substitued sites increases. Three different situations can be distinguished in a function of number
n of carbon atoms separating substituted Be2 dimers: (i) n = 1: deformation of local structure is observed, with Be-Be axis not perpendicular to graphene plane and Cα carbons shifted out of plane. Such system can adsorb 3 hydrogen molecules, with binding energies of 24, 20, and 15 kJ/mol; (ii) n = 2 (two Be2 dimers are substituted in the same hexagon of graphene network): a local enhancement of the biding energy between dimers is observed
and the fragment
…Be2C2Be2… can adsorb 4 hydrogen molecules (Fig.3c) , (iii) n 3: the Be2 sites can be considered as isolated from the point of view of hydrogen adsorption (Fig.3c). The small (~ 10 %) further decrease of binding energy for n = 4 showed on Fig.3c is a consequence of frozen geometry of external carbons atoms in our ovalene model of graphene lattice during ab initio calculations and probably will not be observed in extended structures. We used the ab initio energies to parametrize the force field describing the interaction of H2 molecule with the Be2-substituted graphene pore walls, and to estimate the hypothetical hydrogen adsorption in such nanoporous carbons. The details of the procedure, based on Grand Canonical Monte Carlo simulations, are described in our previous paper [8]. Figure 4 shows the results of this evaluation for two particular situations: (i) for the sorbent constituted by infinite, undeformed and uniformly substituted graphene layers with the standard specific surface area S = 2600 m2/g (Fig.4a) and (ii) for a hypothetical sorbent consisting in nano-sized fragments of Be2-substituted graphene, forming locally slit-shaped pores of final lateral dimensions and the effective specific surface area S = 5000 m2/g [22]. In both cases we have limited the Be to C substitution ratio to 1:3, which corresponds to the substitution of no more than 2 carbon atoms within the same hexagon of graphene. Although the hydrogen uptake in infinite pores can be doubled by 25% substitution with Be2 dimer (Fig.4a), the simultaneous increase of material surface area is necessary to attain the 2017 DOE gravimetric target capacity of 55 g of H2 per kg of sorbent (Fig.4b). A significant additional improvement of binding energy and/or surface area will be needed to reach the ultimate (75 g/kg) goal. As the Be2 substitution does not induce notable structural perturbations of graphene lattice, the further engeeniring of Be2-Cn surfaces remains possible. In particular, the fonctionalisation of Be2-Cn fragments with electron withdrawing groups should decrease the electron density on Be atoms, and, in the consequence, increase the binding energy on the Be dimer. To be efficient, the functionalisation should take place in the vicinity of Be2 centers. Therefore, the Be2-substitution
should occur at the fragments’ edges. To analyze all the possible topographies of such susbtituted and functionalized structures in our calculation we have used the smalest graphene fragmenst containing both substituted Be2 moities and functionalized edges: benzene, tetracene, and ovalene. We have analyzed the variation of hydrogen binding energy upon functionalization of Be2 – containing fragment with the following light weight and low volume electronegative groups: CN, -NH2, -OH and –F. The effect of functional groups was initially tested on Be2-benzene (C5H6Be2), the smallest system that contains beryllium dimer and preserves structural and geometrical properties of largest sp2 carbon fragments. Figure.5 summarizes the results and indicates the relative variations of H2 binding energy upon functionalization. Among the tested substituents, the cyano (-CN) and fluorine (-F) groups have the largest impact on binding energy. The embedded Be2 dimer can still adsorb two hydrogen molecules. The binding energy of the first one increases by 5 – 50 %, depending on the function location. The effect is more pronounced for the adsorption of second molecule, for which the strength of binding increases by 60 – 150 %. The only exception from this rule is observed when the fluorine is attached in para position with respect to the Be2-center; in this case the binding energy of the first adsorbed H2 molecule increases and that of the second – decreases. The effect is not observed when C6H5Be2 is functionalized by two or more fluorine atoms: whichever is the location of functions with respect to Be2 moiety, both adsorption energies increase. The functionalization with amino (-NH2) and hydroxyl (-OH) groups has smallest effect. Depending on the distance to Be2 moiety, these substituents may increase (by ~ 10 – 25 %) or slightly decrease the binding of the first hydrogen molecule; unfortunately they generally lower the energy of the adsorption for the second molecule, in the worst cases by 75 %. Therefore, we conclude that -NH2 and -OH functions will not increase the sorption capacity of the system. Based on this result we selected -CN and -F groups for further analysis. As the effect of functionalization should be the most pronounced when Be2-substituted centers are close to the graphene fragment edges, graphene nanoribbons could potentially be good candidates for the hosting lattice. Tetracene (C18H12) can be considered as a model of an ultrathin graphene nanoribbon. In such system beryllium dimer may substitute either quaternary (Be2C3) or tertiary (Be2C2H) carbon (see the inserts in Fig.6). The energies of hydrogen binding by both
centers are different. Tertiary Be2 dimer has similar properties to those observed in Be2-benzene; however, due to electron withdrawing by neighboring aromatic rings the value of energy of hydrogen binding is higher (31.9 kJ/mol for the 1st H2 molecule and 18.6 kJ/mol for the second). The hydrogen binding on quaternary Be2 is much weaker: although the binding of the first hydrogen molecule remains strong (16.5 kJ/mol), the adsorption of the second molecules occurs with an energy ~2.5 kJ/mol only, two times lower than in all-carbon structures. The functionalization of both systems by –CN or –F groups has different consequences. For tertiary Be2-tetracene, as for previously described Be2-benzene, the functionalization with –CN increases the hydrogen binding on Be2 site, especially for second adsorbed molecule. If the function is attached to the ring containing Be2 dimer (para position, see Fig.6) the resulting binding energies are the strongest: 38.4 kJ/mol for the 1st hydrogen molecule and 29.1 kJ/mol for the second. The functionalization by -F group does not induce significant changes in the hydrogen binding by the tertiary Be2 dimer: depending on the function location, the binding energy can be tuned by +/- 0.6 kJ/mol for the first adsorbed hydrogen and +/- 2.3 kJ/mol for the second. For the Be2 dimer in the quaternary position, only two different carbon sites are available for the functionalization: carbons Cα, directly bonded to Be2 and Cα, located on the opposite side of the molecule. It is worth to note that only Cα positions will be available in ticker graphene ribbons, or in larger graphene fragments. The functionalization by the -CN group at the Cα position rises binding energy by 40 % and 220 % for first and second H 2 molecule (up to 23.8 kJ/mol and 7.9 kJ/mol), respectively. The effect is almost doubled for di-functionalization (at both Cα sites,) and the energy values rise to 29.1kJ/mol (1st H2 molecule) and 12.2 kJ/mol (2nd H2 molecule). The introduction of -CN group in the Cα position on the opposite side of nanoribbon does not cause considerable energy changes for the adsorption of the first H2 molecule (10 % and 20 % for mono- and di- substitution, respectively). The rise of the adsorption energy of the second molecule is larger; however, the strength of binding remains as low as for all-carbon sorbents (3.5 and 5.0 kJ/mol). The functionalization using the fluorine group has smaller influence on the binding energy that rises by 15 - 30 % for the first H2 molecule and by 65 - 100 % for the second. However, even in the best case the energy of adsorption of second molecule at the same Be2 center in not higher that one observed in carbon-only materials. In benzene and
tetracene the Be2 dimer is always located at the edge of the molecule. In real experiment, the substitution C/Be2 will occur at random and most of the substituted sites will be located inside carbon lattice. Therefore we have to answer the question whether the functionalization of edges will have any noticeable impact on the adsorption on the distant Be2 centers. Such situation has been investigated on Be2-coronene (C23H12Be2, see the insert on Fig.7). Two cases have been analyzed: i) the functionalization of 1, 2, or 4 carbons in rings containing Be2 center and ii) the functionalization of opposite side of coronene; in such a case the Be2 dimer and the function(s) are separated by 4 or 5 carbon atoms. In C23H12Be2 the energies for hydrogen binding on Be2 dimer are 32 kJ/mol and 8.4 kJ/mol, respectively for the first and the second adsorbed molecule. These values are smaller than those observed in Be2-benzene and Be2tetracene, and slightly larger than for Be2-ovalene; therefore we conclude that coronene can be considered as a model of small two-dimensional (2D) graphene patch. The functionalization by – CN group does not change significantly the already strong binding of the first hydrogen molecule; only small fluctuations (less than 4 % , i.e. +/- 1.2 kJ/mol) are observed, depending on the function location with respect to the Be2 dimer. The functionalization possesses more important effect on the binding of second hydrogen molecule: the energy rises systematically when 1, 2, or 4 carbons close to the Be2 dimer are functionalized, by 60 %, 100 % and 140 %, respectively. The effect is still noticeable (~ 40 % increase, to the value of 12 kJ/mol) when -CN groups are attached on opposite side of coronene. The functionalization by -F group has almost no effect on the bonding energy of the first hydrogen, wherever is the location of the function. In addition, when one or two –F groups are located in the para position close to Be2 dimer the binding of the second hydrogen molecule decreases. Substitution of all four carbon positions in the rings containing Be2 is necessary to slightly increase both binding energies (by ~10 % for the first and 20 % for the second adsorbed hydrogen). Therefore we conclude that functionalization by the -F group of extended graphene fragments will be efficient only in the case when occurring within the ring containing Be2 dimer; it implies that substituted Be2 must be located close to the graphene edge. To estimate the final impact of both: the Be2 substitution of graphene fragments and functionalization of fragment edges by electronegative groups we have estimated hydrogen uptake in hypothetical material of the specific surface of 5000 m2/g, containing 25% of carbon
sites substituted by Be2 dimers, and 25% of carbon sites at the fragment edges functionalized with a hypothetical electron withdrawing group. We assumed that the group will not increase the binding energy of the first hydrogen molecule absorbed over the edge Be2 dimer (and fixed at the value of 32 kJ/mol calculated for coronene), but improve the strength of the binding of the second molecule on the same site by, in average, 100 % (to the value of 16 kJ/mol). The results of this estimation are presented on Fig.8. The partial functionalization of pore edges (and the induced increase of the hydrogen adsorption energy on Be2 sites) doubles the gravimetric storage of Be2-substituted carbon. The improvement of the performance is such that both DOE goals (required by 2017 and the ultimate one) can be reached at relatively low pressure (60 bar and 110 bar, respectively). This perspective puts an important milestone on the way to define directions for further experimental search for ambient temperature hydrogen storage systems. 4. Discussion and Conclusions A possibility of adsorptive hydrogen storage for mobile applications has been studied for over 20 years and no practical solution has been found. In the meantime many claims were published but not confirmed by the following experiments. This redundant situation results today in skepticism towards new ideas. However, from purely scientific point of view, it remains important to explore non-conventional propositions of modifying hydrogen-sorbent interaction that could be explored experimentally in the future in the context of effective, large scale hydrogen storage. Therefore, the most important message of this work is the numerical proof that there is a way to tune the hydrogen adsorption energy in a wide range. The adsorbing surface is carbon-based and has graphene-like structure and is chemically modified by substitution of some carbon atoms with Be2 dimers acting as adsorbing centers. To tune the hydrogen adsorption energy this surface must be additionally functionalized, using small strongly electronegative groups. The results of our numerical studies demonstrate that both, specific surface area (S) and energy of hydrogen adsorption (E) of graphene-based nanoporous carbons can be modulated in a wide range of values by simultaneous substitution of graphene layer, fragmentation of its structure and functionalization of fragments’ edges. In such a way both parameters – S and E - are
simultaneously optimized and the required targets for gravimetric (and volumetric) hydrogen storage capacity can be reached without exceedingly improving only one parameter. For example, if the specific surface was increased from 2600 m2/g to 5000 m2/g, the (average) binding energy must be simultaneously increased to the value of 8 kJ/mol to reach 2017 DOE goal, and to 10 kJ/mol to reach the ultimate target (see Fig.1). These values are much lower than suggested by Bahtia et al. [6] for all-carbon infinite slit-shaped pores with the typical graphenelike specific surface. Increasing the surface available for adsorption is crucial for storage applications. The carbon structures with high surface areas were proposed by Kaneko [33], as ensembles of nanofragments of graphene. The idea of high surface resulting from the surface of the edges of nanofragments has been discussed by Yaghi [34, 35] and implemented to prepare MOFs. We showed in our previous papers [11, 22] that it is also possible to imagine nanoporous carbons having the surface higher than 5000 m2/g. Some of such structures have been proposed and discussed. To increase the hydrogen binding to carbon-derived sorbents, Ca, N, P, B and other substituents were considered in the past. Although in the resulting materials they all improved the hydrogen storage, none of them was able to approach the DOE target values. In this work we explored, for the first time, the effect of carbon substitution with beryllium dimer. We showed that this light atom can substantially increase the gravimetric storage of substituted sorbent (for 25% substitution ratio the storage capacity at room temperature is almost doubled). However, as the high energy of adsorption of Be2 dimer (23 kJ/mol for the first adsorbed H2 molecule and 8 kJ/mol for the second) is localized on the substitution site and does not modify substantially the adsorption on neighboring carbons, to further improve the hydrogen storage an additional functionalization of beryllium-containing sorbents with electron withdrawing groups is necessary.
According to our estimations a hypothetical sorbent with optimized specific surface (~5000 m2/g), with 25% of carbons substituted by Be2 dimers and partially (25%) functionalized pores’ edges would adsorb an amount of hydrogen that satisfies the gravimetric DOE storage requirements at room temperature and low pressure ( ~60 bar for 2017 goal and ~110 bar for the ultimate target). This conclusion is very promising and suggests the strategy one should adopt to
prepare carbon-based sorbents with open, high surface architecture and substantially increased energy of hydrogen adsorption. There are two consequences of the surface modifications that we do not discuss in this paper: variation of adsorbent density and that of delivery characteristics. It is well known, that the stronger interaction makes the delivery cycle less effective, that is, at the lowest working pressure the system can still retain a substantial amount of adsorbed hydrogen. This information can be easily obtained from the isotherm curve, if the operating temperature is constant. However, it will not necessarily be the case in the real storage tank. Remembering that adsorption is exothermic, and desorption endothermic, the concept of an ultimate tank should include system cooling at loading and its heating at delivery. Therefore, (most probably) the final system will not operate at constant temperature and the delivery cycle will critically depend on the working temperature range. Concerning the actual density of the adsorbent, as it is defined by the system topology (the distribution of the pores size), it will depend on the synthesis procedure.
ACNOWLEGEMENTS This work has been supported by ANR-14-CE05-0009 HYSTOR grant of French National Research Agency (ANR) and a statutory activity subsidy from Polish Ministry of Science and Technology of Higher Education for the Faculty of Chemistry of Wroclaw University of Technology and NCN grant No. 2012/07/B/ST4/01584. We also thank Wroclaw and Poznan Centers for Networking and Supercomputing, and Interdisciplinary Center for Mathematical Modeling of Warsaw University for providing generous computer time.
5. References 1. P.Benard, R.Chahine, Storage of hydrogen by physisorption on carbon and nanostructured materials, Scr.Mater. 56 (2007) 803-808 2. P. Benard, R. Chahine, Modeling of adsorption storage of hydrogen on activated carbons, Int. J. Hydrogen Energy 26 (2001) 849–855. 3. B. Panella, M. Hirscher, S. Roth, Hydrogen adsorption in different carbon nanostructures, Carbon 43 (2005) 2209–2214. 4. H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y.B. Go, M. Eddaoudi, A.J. Matzger, M.O’Keeffe, O.M. Yaghi, A route to high surface area, porosity and inclusion of large molecules in crystals, Nature 427 (2004) 523–527. 5. G.E.Froudakis, Hydrogen storage in nanotubes & nanostructures, Mat.Today 14 (2011) 324328. 6. S.K.Bhatia, A.L.Myers, Optimal conditions for adsorptive storage, Langmuir 22 (2006) 1688-1700 7. R. B. Getman, Y.-S. Bae, C. E. Wilmer, and R. Q. Snurr, Review and Analysis of Molecular Simulations of Methane, Hydrogen, and Acetylene Storage in Metal-Organic Frameworks. Chem. Rev. 112 (2012) 703–723 8. B. Kuchta, L. Firlej, P. Pfeifer and C. Wexler . Numerical estimation of physical limits of hydrogen storage by physisorption in microporous nanospaces. Carbon 48 (2010) 223 –231. 9. B. Kuchta, L. Firlej, R. Cepel, P. Pfeifer and C. Wexler, Structural and energetic factors in designing a nanoporous sorbent for hydrogen storage, Colloids and Surfaces A 357 (2010) 61-66. 10. B. Kuchta, L. Firlej, Sz. Roszak, P. Pfeifer, A review of boron enhanced nano-porous carbons for hydrogen adsorption: numerical perspective. Adsorption 16 (2010) 413-421. 11. B. Kuchta, L. Firlej, A.Mohammadhosseini, P.Boulet, M.Beckner, J.Romanos, P.Pfeifer, Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: open carbon frameworks, JACS 134, (2012) 15130–15137 12. L.Firlej, B.Kuchta,
P.Pfeifer, Understanding universal adsorption limits for hydrogen
storage in nanoporous systems, Adv.Mat. 25 (2013) 5971-5974,
13. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keefee and O. M. Yaghi, Hydrogen storage in microporous metal–organic frameworks, Science 300 (2003) 1127– 1129. 14. J. L. C. Rowsell and O. M. Yaghi, Strategies for hydrogen storage in metal–organic frameworks, Angew. Chem., Int. Ed.44, (2005) 4670– 4679. 15. H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-Cortes, A. P. Cote, R. E. Taylor, M. O’Keeffe and O. M. Yaghi, Designed synthesis of 3D covalent organic frameworks, Science 316 (2007) 268–272. 16. Kim, Y.-H., Zhao, Y., Williamson, A., Heben, M.J., Zhang, S.B.,
Nondissociative
adsorption of H2 molecules in light-element doped fullerenes, Phys.Rev.Lett. 96, (2006) 016102 17. L. Firlej, Sz. Roszak, B. Kuchta. P. Pfeifer and C. Wexler, Enhanced hydrogen adsorption in boron substituted carbon nanospaces . J.Chem.Phys. 131 (2009) 164702. 18. E.Beheshti, A.Nojeh, P.Servati, A first-principles study of calcium decorated, borondopedgraphene for high capacity hydrogen storage, Carbon 49 (2011) 1561-1567. 19. K.S.Subramanyan, L.S.Panchacarla, A.Govindaraj, C.N.R.Rao, Simple method of preparing graphene flakes by an arc-discharge method, J.Phys.Chem.C 113 (2009) 4257-4259. 20. L.S.Panchacarla,
K.S.Subramanyam,
S.K.Saha,
A.Govindaraj,
H.R.Krishnamurthy,
U.V.Waghmare, C.N.R.Rao, Synthesis, structure and properties of boron and nitrogendoped graphite, Adv.Mat. 21 (2009) 4726-4730. 21. L. Firlej, B. Kuchta, A.Lazarewicz and P. Pfeifer, Increased H2 gravimetric storage capacity in truncated carbon slit pores modeled by Grand Canonical Monte Carlo, Carbon 53 (2013) 208-215 22. B. Kuchta, L. Firlej, A.Mohammadhosseini, M.Beckner, J.Romanos, P.Pfeifer, Open carbon frameworks- a search for optimal geometry for hydrogen storage, J. Mol. Mod., 19 (2013) 4079-4087 23. D.Saha, S.Deng, Hydrogen Adsorption on Ordered Mesoporous Carbons Doped with Pd, Pt, Ni, and Ru, Langmuir 2009, 25, 12550-12560 24. K.Sumida, M.R.Hill, S.Horike, A.Dailly, J.R.Long, Synthesis and Hydrogen Storage Properties of Be12(OH)12(1,3,5-benzenetribenzoate)4, J.Am.Chem.Soc. 131(2009) 15120– 15121
25. H.Lee, B.Huang, W.Duan, J.Ihm Ab initio study of beryllium-decorated fullerenes for hydrogen storage, Journal of Applied Physics 107 (2010) 084304 26. Y.-H Kim, Y.Zhao, A.Williamson, M.J.Heben, S.B.Zhang Nondissociative Adsorption of H2 Molecules in Light-Element-Doped Fullerenes, Phys. Rev. Lett.,96 (2006) 016102 27. A.D.Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys., 98 (1993) 5648-52. 28. R.Roszak, Sz.Roszak s-Block metallabenzene: aromaticity and hydrogen adsorption, J.Mol.Model. (2015) 21-28. 29. T.H. Dunning Gaussian basis set for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen, J.Chem.Phys.90 (1989) 1007-1023. 30. Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. 31. R. Roszak, Sz. Roszak, D. Majumdar, L. Firlej, B. Kuchta and J. Leszczynski, Unique Bonding Nature of Carbon-Substituted Be2 Dimer inside the Carbon (sp2) Network. J. Phys. Chem.A 118 (2014) 5727–5733. 32. J. Li, T. Furuta, H. Goto, T. Ohashi, Y. Fujiwara, S. Yip, Theoretical evaluation of hydrogen storage capacity in pure carbon nanostructures, J. Chem. Phys. 119 (2003) 2376-2385. 33. K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons. Carbon 1992, 30(7), 1075−1088.
34. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Hydrogen Storage in Microporous Metal-Organic Frameworks. Science 2003, 300 (5622), 1127−1129 35. H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe, O.M. Yaghi A route to high surface area, porosity and inclusion of large molecules in crystals. Nature 427 (2004) 523–527
Figure 1. Iso-weight-capacity (constant Gc) curves for Gc values typical for high surface activated carbons and the DOE ultimate goal. The intersections of the dashed lines with Gc curves indicate the surfaces (horizontal lines) and average binding energy (vertical lines) required for adsorbent with typical graphene characteristics to achieve different Gc capacities.
Figure 2. (left) Planarity perturbation of single atom substituted ovalene C31BeH14. The insert shows a hypothetical C5Be superstructure. (right) Ovalene C31Be2H14 substituted with Be dimer. The equilibrium positions of two hydrogen molecules absorbed on booths sides of the ovalene are shown.
a)
b)
c)
Figure 3. (a) Energetic landscape of infinite graphene surface substituted by an isolated beryllium dimer, (b) Energetic landscape of infinite graphene surface substituted by two Be2 dimers separated by two carbon atoms (c) variation of hydrogen binding energy over substituted ovalene structure. The value of n denotes the number of carbon atoms separating the substituted dimers. Here the value of n = 0 corresponds to single substitution (isolated substitution site).
50
40
graphene 11% Be2
30
25% Be2
20 10 0
a)
Adsorption [gH2 /kg sorbent]
Adsorption [gH2 /kg sorbent]
50
40
20
40
60
80
Pressure [bar]
100
120
b)
25% Be2
20 10 2
SSA = 5000 m /g 0
0
graphene 11% Be2
30
0
20
40
60
80
100
120
Pressure [bar]
Figure 4. (a) Hypothetical room temperature storage capacity in infinite graphene-based slitshaped pores of the width of 1.1 nm, with 0%, 11% and 25 % of carbon sites substituted by Be2 dimers, (b) storage capacity in a hypothetical porous material with specific surface area of 5000 m2/g.
Figure 5. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer in Be2-benzene (C5H6Be2). Eref are the binding energy of H2 adsorbed on non-functionalized Be2benzene: Eref (1st H2) = 21 kJ/mol, Eref (2nd H2) = 8 kJ/mol. The abbreviations denote the position of the functional group with respect to Be2 dimer: (o)- ortho, (m) – meta, (p) – para; (per-F) stands for per-fluorinated Be2-benzene.
Figure 6. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer substituting the tertiary (left panel) or quaternary (right panel) carbon in
-CN and –F
functionalized Be2-tetracene (C17H12Be2). Eref are the binding energies of H2 adsorbed on Be2 center in corresponding non-functionalized Be2-tetracenes (see the text for the values).
Figure 7. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer in CN and –F functionalized Be2-coronene (C123H12Be2). Eref are the binding energies of H2 adsorbed on the Be2 center in corresponding non-functionalized Be2-coronene (Eref (1st H2) = 32 kJ/mol, Eref (2nd H2) = 8.4 kJ/mol.
Figure 8. The hypothetical room temperature storage capacity in hypothetical porous material with specific surface area of 5000 m2/g and the pore width of 1.1 nm: (open squares)- all carbon structure, (open circles)- 25% of carbon sites substituted by Be2 dimers, (close circles)- 25% of carbon sites substituted by Be2 dimers; 25% of them showing enhanced adsorption energy due to functionalization of pore edges. For comparison, the room temperature storage capacity in infinite graphene-based slit-shaped pores of the same width is also shown (black squares).
S0927-7757(15)30306-X http://dx.doi.org/doi:10.1016/j.colsurfa.2015.10.046 COLSUA 20254
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
26-5-2015 26-9-2015 26-10-2015
Please cite this article as: Rafal Roszak, Lucyna Firlej, Szczepan Roszak, Peter Pfeifer, Bogdan Kuchta, Hydrogen storage by adsorption in porous materials: Is it possible?, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.10.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hydrogen storage by adsorption in porous materials: is it possible? Rafal Roszak1, Lucyna Firlej2,3*, Szczepan Roszak1, Peter Pfeifer3, Bogdan Kuchta3,4 1
Advanced Materials Engineering and Modelling Group, Wrocław University of Technology,
Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland 2
Laboratoire Charles Coulomb (L2C), UMR 5221 CNRS-Université de Montpellier, Montpellier,
F-France. 3
Department of Physics and Astronomy, University of Missouri, Columbia, USA
4
Laboratoire MADIREL, Aix-Marseille Université – CNRS UMR 7246, 13396 Marseille, France
*Corresponding address: [email protected]
GRAPHICAL ABSTRACT
Highlights
The adsorbing properties of graphene-based nanoporous carbons can be modulated by simultaneous fragmentation of graphene structure, substitution of carbons by electron deficient atoms, and functionalization of fragments’ edges.
Carbon substitution with beryllium dimer can substantially increase hydrogen binding to graphene-based surface. Hydrogen binding to Be2 dimer is mainly defined by a weak coordination bond.
Both C/Be2 substitution and functionalization of the graphene edges are necessary to increase the hydrogen gravimetric storage to the value that satisfies the DOE requirements.
Abstract The role of fundamental characteristics of porous systems (binding energy, specific surface area and multilayer adsorption) in designing an efficient hydrogen adsorbent is discussed. We analyze why the amount of hydrogen adsorbed in all known materials is much lower than required for mobile applications and what are possible strategies to increase it. Further we report new ab initio calculations demonstrating possible ways of chemical modification of graphene fragments which can lead to the substantial increase of hydrogen binding to the graphene-based surface. Such Open Carbon Frameworks, substituted and functionalized at the fragments’ edge may theoretically adsorb, at ambient temperature and relatively low pressure (60-100 bar), the amount of hydrogen necessary for mobile applications.
Keywords: hydrogen adsorption, hydrogen storage, graphene fragments, beryllium dimer
1. Introduction In recent years, great emphasis has been placed on developing the concepts and technology to power the vehicles using renewable energy. One potential solution is the use of hydrogen gas as an energy vector to power fuel cells. However, onboard hydrogen storage continues to be one of the most technically challenging barriers for the widespread commercialization of hydrogenfueled light duty vehicles that will be competitive with fossil fuel or hybrid vehicle markets. The major identified difficulty consists in designing hydrogen storage system capable of delivering a driving range of hundreds of kilometers. Among the four principal methods to store hydrogen: liquefaction, gas compression, chemical storage, and physical adsorption, the last one offers several advantages: rapid kinetics, reversibility and relatively high storage capacity. Nanoporous carbons are commonly considered as, potentially, the best hydrogen sorbents due to their low cost, light weight, high surface area and chemical stability. However, the quantity of hydrogen they can currently store is not sufficient for practical applications. This limitation is inherently related to the electronic properties of H2 molecule and the mechanism of hydrogen adsorption. New low pressure sorbents and material-oriented technologies have to be developed, to meet packing, cost, safety, and performance requirements. The main targets directly related to the system storage capacity are gravimetric and volumetric densities of the adsorbed hydrogen. According to the US DOE (Department of Energy) requirements, the future hydrogen tank should ultimately store 75 g of H2 per kg and 70 g per liter of storing system. Remembering that these parameters are system goals, the storing medium must greatly exceed these capacities. In the present paper we will use these numbers as reference values only.
Our main goal is to discuss a new approach to the chemical modification of carbon surface which will allow tuning the hydrogen binding in a wide range of values. The paper is organized as follow: in the first section we review the physical sources of limitations of existing hydrogen sorbents and indicate the directions for further material research in order to reduce these restraints. Then, we report ab initio calculations which suggest a new strategy of modifying the all-carbon surfaces and increase substantially their energy of hydrogen adsorption. Finally, we discuss the reported results in the context of their possible applications.
2. Search for optimized hydrogen sorbents. The state-of-the–art in the optimization of hydrogen sorbent has been summarized in many review articles [1-5] and in some critical analysis of the conditions required for hydrogen storage [6-8]. The conclusion coming out of all of these sources and our previous papers [9-12] is unique: despite more than 20 years of research, no material yet possesses adsorbing properties required for applications. All porous structures that exist today are far from being optimal when considered as hydrogen sorbents for mobile applications. Such sorbents include wide range of materials from traditional carbon-based structures (activated carbons, carbon nanotube-based structures, polyaromatic hydrocarbons) to currently researched open porous structures (metal organic frameworks (MOFs), covalent organic frameworks (COFs) or porous polymer networks (PPN)). It is well established today that it will be difficult to reach the high H2 storage in conventional high surface area adsorbents, because of their low average adsorption (binding) energy, heterogeneity of surfaces, mesoporosity, and poor volumetric packing. In carbon-based materials the main limitation comes from the relatively small binding energy on the graphene surface (4-6 kJ/mol) [1]. For these conventional porous carbons the hydrogen uptake, at ambient temperature, is proportional to their adsorbing surface. The importance of optimization of pore size distribution in order to maximize sorbent specific surface has been numerically estimated in our recent paper [8]. However, even if an ideal model of infinite, slitshaped pores is assumed our results have shown that graphene-based structures will never meet DOE goals, although their H2 storage at low temperature is very promising. Following this fact, other porous structures with exceptionally large surface areas (MOFs and COFs) have also been tested for the hydrogen storage [13,14]. These materials are synthesized from metal or metal oxide centers interconnected by organic linkers. In COFs, metal sites are replaced by light elements (mostly boron) to decrease the structure weight. Depending on choice of the building elements a variety of 3D structures with very high and tunable porosity has been prepared [15]. Many experimental and theoretical studies have found impressive storage capacity at cryogenic temperature and high pressure, especially in COFs (COF-108 exceed 20 wt% storage capacity already at 100 bar) [15]. However, an average adsorption energy of H2 in MOFs and COFs is even lower than that of activated carbons. In the consequence, their performance at ambient temperature and moderated pressure still falls far from the DOE targets.
Therefore, the next step in developing an optimal H2 sorbent should consist in searching for materials with higher binding energy [6-8, 10-12]. Assuming a hypothetical energy of adsorption up to 3 times larger than the energy of H2 binding on pristine graphene, it has been shown than graphene-based but chemically modified porous carbons could provide at room temperature the H2 storage capacity required by DOE [6,8]. One of the ways to control the adsorption energy of a given sorbent is a chemical modification of its surface, by doping or substituting carbons with other atoms. In particular, substitutional doping has attracted a lot of attention because it influences simultaneously sorbent topology (by introducing spatial heterogeneity) and the strength of intermolecular interactions. In the category of carbon-based sorbents of H2, substitution with boron has been promoted as the most promising. Early theoretical calculations suggested a non-dissociative adsorption of H2 on B and Be-doped fullerenes. The coupling between an empty, localized pz orbital of the dopant and the H2
s. In the case of B-
doping, H2 binding energy was raised to 15-35 kJ/mol [16], making boron-doped carbon an outstanding candidate for the high capacity H2 storage. Our recent Grand Canonical Monte Carlo simulations based on ab initio interaction model [9,17] reports an enhanced H2 uptake in graphene slit pores with walls containing boron atoms. The high capacity is not related to any particular crystallographic structure but depends only on the B/C substitution ratio. The 10 % substitution is necessary to rise the average binding energy to 10 kJ/mol, and to approach both gravimetric and volumetric directives of DOE for 2017 (Fig. 1), with almost 100% delivery of stored H2 simply by pressure sweep (without increasing sorbent temperature). It is not clear how the further increase of this limit could be achieved with a higher substitution. It was also shown that boron doping of graphene can indirectly contribute to the higher capacity of H2 storage of Ca-decorated carbons, via the stabilization of dispersed Ca atoms on the graphene surface. The Kubas interaction between H2 and Ca allows a stable binding of up to four H2 molecules to Bdoped graphene [18]. From simple geometrical considerations, double-sided Ca decorated graphene randomly doped with 12% of boron atoms can theoretically reach a gravimetric capacity of 8.38 wt% , but the average binding energy of 36,8 kJ/mol makes the H2 cycling difficult at ambient conditions [18]. Obviously, experimental confirmation of calculations cited above is needed for the validation of theoretical analysis. For that, substituted porous carbons have to be synthesized.
Recently, is has been demonstrated that the arc discharge between graphite electrodes, a technique developed to prepare large quantities of fullerenes and carbon nanotubes, can be optimized to synthesize graphene fragments composed of two to three layers. The method makes use of the fact that in the presence of hydrogen in the discharge apparatus graphene sheets do not roll to form cage-like structures [19]. Boron-stuffed electrodes or B2H6-containing discharge atmosphere has been used to prepare boron-doped graphene fragments of the lateral size of 20 to 60 nm. The presence of substitutional boron in the sp2 carbon structure has been confirmed by EELS, AFM, and Raman investigations. Similarly, nitrogen atoms have been successfully introduced to graphene sheets by the arc discharge in the presence of pyridine or ammonia [20]. Although the resulting substitution ratio was not high (up to 3% for the boron and 1.4% for nitrogen substitution), these preliminary results are promising as the arc discharge procedure could be easily optimized. The additional advantage of arc discharge technique is its ability to produce graphene fragments of nanometric size. This aspect of topology is extremely important, because the fragmentation of infinite pore walls into small patches creates an additional adsorption surface at patches’ edges. Recently we have shown that the fragmentation of infinite graphene walls into the patch-like structures increases hydrogen gravimetric storage capacity, by a factor of two [21]. The contribution of adsorption on fragment edges to the overall amount stored is independent of the fragment shape and increases when the fragment lateral size decreases. However, simultaneously, the energy of adsorption at edges rapidly decreases with a fragment size. Therefore, the final hydrogen uptake in pores of finite lateral dimensions will result from a competition between two factors: larger adsorption surface introduced by the pore edges and lower average adsorption energy and heterogeneity (structural and energetic) of the system. This result suggests that nanoporous carbons build up from interconnected fragments of graphene or polyaromatic hydrocarbons can show high hydrogen storage, assuming the fragment size and system topology can be optimized. This leads to redefined conditions, that nanoporous carbons optimized for hydrogen storage should fulfill: (i) First, the porous system must have an open geometry with considerable contribution of edge surface of building units (fragments of graphene sheets), (ii) the size of fragments should be wisely optimized: not too small because it leads to low energy of adsorption and decreasing uptake, and not too large to be able to take the
advantage of an important contribution from edges, (iii) the density of the structures cannot be too low because of the volumetric storage capacity requirements. Following these conclusions, we proposed a new class of carbon-based open high surface structures, Open Carbon Frameworks (OCFs) which, when synthesized, will greatly extend the current limits of hydrogen storage by the physisorption at room temperature [11,22]. However, the simultaneous optimization of both adsorbent surface and its adsorption will be necessary to attain an ultimate adsorption uptake. The analysis of extensive numerical database produced by Monte Carlo simulations of H2 adsorption in nanoporous carbons allowed us to deduce [12] the simple functional relation between material gravimetric capacity (Gc, in wt%), its specific surface (S, in m2), and average binding energy (E, in kJ/mol): Gc = f * (S/1000) *E where the coefficient f depends on the material’s topology, mainly on the pores width. The isoweight-capacity curves presented on Fig.1 show the pairs of values of (S, E) that are simultaneously needed to reach an assumed hydrogen storage capacity. In the past most of the research for optimal hydrogen sorbents have focused on improving one of two material parameters (S or E), keeping the other constant.
However, the functional
dependence presented on Fig.1 shows that using such approaches an ultimate DOE goals could be only achieved if the second characteristics is exceedingly improved (specific surface larger than 10,000 m2/g or binding energy close to 20 kJ/mol). Alternative approaches consisting in simultaneous optimization of both parameters (for example preparation of carbon-based adsorbent with improved but reasonable surface area (4000-5000 m2/g) and adsorption energy (8-10 kJ/mol) may be easier to achieve. In the following chapter we discuss one example of such an option. We show that the substitution of Be dimer into a finite size graphitic fragments may increase both parameters (S and E) simultaneously. Moreover, additional substitution of strong electronegative function close to the Be2 site may allow an additional modulation of adsorption energy of the material, depending on the nature and the localisation of substituent.
3. Graphene theoretical nano-engineering. Graphene, a two-dimensional sp2 bonded carbon sheet arranged in a hexagonal honeycomb lattice, with its high theoretical specific surface area of 2600 m2/g provides a rich platform for the surface chemistry. The modification of graphene has been extensively studied in the past in the aim of preparing new carbon-based sorbents with high binding energy. The most common functionalization strategy is a substitution of carbon by boron, nitrogen, or oxygen. The other possible way to increase binding energy is the deposition of metal clusters on the carbon surface [23]. The substitution of carbon by light metals was less explored. In particular, an introduction of beryllium into the carbon lattice is one of the least studied modifications. Beryllium can be incorporated into the graphene sp2 network either via the single atom substitution (C/Be) or by the substitution of carbon atom by the beryllium dimer (C/Be2). The knowledge regarding the properties of organoberyllium materials is very limited; some of them were recognized as promising hydrogen sorbents [24-26]. Therefore we have analyzed changes in the energy of hydrogen binding towards Be-substituted surfaces for both types of substitution and a variety of Be : C substitution ratios.
We have adopted the ovalene C32H14 as a model of graphene network and a starting point for the graphene substitution. To analyse the modulation of adsorption energy by the further functionalisation of organoberylium lattices, smaller units (benzene, tetracene and coronnene) have been employed. All structures were optimized at the density functional theory (DFT) level using the B3LYP functional [27]. No symmetry constraints were imposed during the geometry optimization: however, to model the extended structure of ovalene, the positions of carbon atoms in its external rings were frozen. We are aware that applied approach cannot reproduce the dispersion energy contribution; however, as hydrogen adsorption on Be centers is dominated by weak Kubas-like forces, the contribution of dispersion energy to the stabilization of such complexes can be neglected [28]. To calculate the energy of the DFT-optimized structures we have applied the Møller-Plesset second-order perturbation methodology with an aug-cc-pVDZ basis set [29]. The adsorption energy has been calculated as the difference between energy of H2-BeCn complex and isolated H2 and BeCn molecules: Ea = E(complex) – E(H2) – E(BeCn). All terms have been calculated consistently in the dimer-centered basis set (counterpoint correction) and are free from the basis set superposition error (BSSE). All calculations were carried out
using the Gaussian-09 code [30]. More detailed description of computation methodologies (MP2 and DFT) is presented in [28, 31]. The detailed study of structure of carbons substituted with single beryllium atoms is presented in our previous paper [31]. In general, if the substitution occurs as an isolated event, the energy of hydrogen adsorption remains comparable to that of pure graphene. The main consequence of the substitution is the perturbation of planarity of the system: the substituted Be is located ~0.56 Å above the carbon plane, and two of three adjacent carbon atoms being out of plane by 0.23 Å. When the substitution ratio increases, the substituted Be acts as an isolated adsorption center up to n =4, where n is the number of carbon atoms separating two nearest Be neighbours. In substituted ovalene C27BeC3BeH14 (n =3, see the insert on Fig.2, left panel) the energy of hydrogen bonding on each Be site increases to the value of ~12.8-13.7 kJ/mol. This value is interesting from the point of view of potential applications. Hovewer, the possibility of synthesis of OCF structures based ovalene units and showing high specifics surface areas, and as well as the stability of larger systems of such C5Be-type superstructure have still to be proven. If two adjacent carbons in ovalene are substituted with Be, the hydrogen binding on both sites increases to 46.5-49.5 kJ/mol, the values which are too high for reversible adsorption, even at room temperature. A particular case of topography is single carbon substitution by the Be dimer. Such substitution does not introduce any notable perturbation of hosting graphene structure. The center of the dimer remains in the graphene plane with Be atoms located on the perpendicular axis and separated by a distance of 1.7 Å (Fig2, right panel). Each Be dimer can accommodate 2 molecules of hydrogen. The binding energy of the first molecule amounts to ~23 kJ/mol. Theoretically, this is the perfect energy required for the optimal hydrogen adsorption medium [32]. Although the binding of the second molecule on the same center occurs with much weaker energy, ~ 8 kJ/mol, this value is still almost 50% higher than one for pure graphene. The modification of the energetic landscape for hydrogen adsorption is very localised and do not extend beyond centers of hexagons adjacent to the substitudes carbon (Fig.3a); the binding energy on Cα carbons is almost the same as that in graphene.
As for single atom substitution, this energetic picture evolves when the concetration of substitued sites increases. Three different situations can be distinguished in a function of number
n of carbon atoms separating substituted Be2 dimers: (i) n = 1: deformation of local structure is observed, with Be-Be axis not perpendicular to graphene plane and Cα carbons shifted out of plane. Such system can adsorb 3 hydrogen molecules, with binding energies of 24, 20, and 15 kJ/mol; (ii) n = 2 (two Be2 dimers are substituted in the same hexagon of graphene network): a local enhancement of the biding energy between dimers is observed
and the fragment
…Be2C2Be2… can adsorb 4 hydrogen molecules (Fig.3c) , (iii) n 3: the Be2 sites can be considered as isolated from the point of view of hydrogen adsorption (Fig.3c). The small (~ 10 %) further decrease of binding energy for n = 4 showed on Fig.3c is a consequence of frozen geometry of external carbons atoms in our ovalene model of graphene lattice during ab initio calculations and probably will not be observed in extended structures. We used the ab initio energies to parametrize the force field describing the interaction of H2 molecule with the Be2-substituted graphene pore walls, and to estimate the hypothetical hydrogen adsorption in such nanoporous carbons. The details of the procedure, based on Grand Canonical Monte Carlo simulations, are described in our previous paper [8]. Figure 4 shows the results of this evaluation for two particular situations: (i) for the sorbent constituted by infinite, undeformed and uniformly substituted graphene layers with the standard specific surface area S = 2600 m2/g (Fig.4a) and (ii) for a hypothetical sorbent consisting in nano-sized fragments of Be2-substituted graphene, forming locally slit-shaped pores of final lateral dimensions and the effective specific surface area S = 5000 m2/g [22]. In both cases we have limited the Be to C substitution ratio to 1:3, which corresponds to the substitution of no more than 2 carbon atoms within the same hexagon of graphene. Although the hydrogen uptake in infinite pores can be doubled by 25% substitution with Be2 dimer (Fig.4a), the simultaneous increase of material surface area is necessary to attain the 2017 DOE gravimetric target capacity of 55 g of H2 per kg of sorbent (Fig.4b). A significant additional improvement of binding energy and/or surface area will be needed to reach the ultimate (75 g/kg) goal. As the Be2 substitution does not induce notable structural perturbations of graphene lattice, the further engeeniring of Be2-Cn surfaces remains possible. In particular, the fonctionalisation of Be2-Cn fragments with electron withdrawing groups should decrease the electron density on Be atoms, and, in the consequence, increase the binding energy on the Be dimer. To be efficient, the functionalisation should take place in the vicinity of Be2 centers. Therefore, the Be2-substitution
should occur at the fragments’ edges. To analyze all the possible topographies of such susbtituted and functionalized structures in our calculation we have used the smalest graphene fragmenst containing both substituted Be2 moities and functionalized edges: benzene, tetracene, and ovalene. We have analyzed the variation of hydrogen binding energy upon functionalization of Be2 – containing fragment with the following light weight and low volume electronegative groups: CN, -NH2, -OH and –F. The effect of functional groups was initially tested on Be2-benzene (C5H6Be2), the smallest system that contains beryllium dimer and preserves structural and geometrical properties of largest sp2 carbon fragments. Figure.5 summarizes the results and indicates the relative variations of H2 binding energy upon functionalization. Among the tested substituents, the cyano (-CN) and fluorine (-F) groups have the largest impact on binding energy. The embedded Be2 dimer can still adsorb two hydrogen molecules. The binding energy of the first one increases by 5 – 50 %, depending on the function location. The effect is more pronounced for the adsorption of second molecule, for which the strength of binding increases by 60 – 150 %. The only exception from this rule is observed when the fluorine is attached in para position with respect to the Be2-center; in this case the binding energy of the first adsorbed H2 molecule increases and that of the second – decreases. The effect is not observed when C6H5Be2 is functionalized by two or more fluorine atoms: whichever is the location of functions with respect to Be2 moiety, both adsorption energies increase. The functionalization with amino (-NH2) and hydroxyl (-OH) groups has smallest effect. Depending on the distance to Be2 moiety, these substituents may increase (by ~ 10 – 25 %) or slightly decrease the binding of the first hydrogen molecule; unfortunately they generally lower the energy of the adsorption for the second molecule, in the worst cases by 75 %. Therefore, we conclude that -NH2 and -OH functions will not increase the sorption capacity of the system. Based on this result we selected -CN and -F groups for further analysis. As the effect of functionalization should be the most pronounced when Be2-substituted centers are close to the graphene fragment edges, graphene nanoribbons could potentially be good candidates for the hosting lattice. Tetracene (C18H12) can be considered as a model of an ultrathin graphene nanoribbon. In such system beryllium dimer may substitute either quaternary (Be2C3) or tertiary (Be2C2H) carbon (see the inserts in Fig.6). The energies of hydrogen binding by both
centers are different. Tertiary Be2 dimer has similar properties to those observed in Be2-benzene; however, due to electron withdrawing by neighboring aromatic rings the value of energy of hydrogen binding is higher (31.9 kJ/mol for the 1st H2 molecule and 18.6 kJ/mol for the second). The hydrogen binding on quaternary Be2 is much weaker: although the binding of the first hydrogen molecule remains strong (16.5 kJ/mol), the adsorption of the second molecules occurs with an energy ~2.5 kJ/mol only, two times lower than in all-carbon structures. The functionalization of both systems by –CN or –F groups has different consequences. For tertiary Be2-tetracene, as for previously described Be2-benzene, the functionalization with –CN increases the hydrogen binding on Be2 site, especially for second adsorbed molecule. If the function is attached to the ring containing Be2 dimer (para position, see Fig.6) the resulting binding energies are the strongest: 38.4 kJ/mol for the 1st hydrogen molecule and 29.1 kJ/mol for the second. The functionalization by -F group does not induce significant changes in the hydrogen binding by the tertiary Be2 dimer: depending on the function location, the binding energy can be tuned by +/- 0.6 kJ/mol for the first adsorbed hydrogen and +/- 2.3 kJ/mol for the second. For the Be2 dimer in the quaternary position, only two different carbon sites are available for the functionalization: carbons Cα, directly bonded to Be2 and Cα, located on the opposite side of the molecule. It is worth to note that only Cα positions will be available in ticker graphene ribbons, or in larger graphene fragments. The functionalization by the -CN group at the Cα position rises binding energy by 40 % and 220 % for first and second H 2 molecule (up to 23.8 kJ/mol and 7.9 kJ/mol), respectively. The effect is almost doubled for di-functionalization (at both Cα sites,) and the energy values rise to 29.1kJ/mol (1st H2 molecule) and 12.2 kJ/mol (2nd H2 molecule). The introduction of -CN group in the Cα position on the opposite side of nanoribbon does not cause considerable energy changes for the adsorption of the first H2 molecule (10 % and 20 % for mono- and di- substitution, respectively). The rise of the adsorption energy of the second molecule is larger; however, the strength of binding remains as low as for all-carbon sorbents (3.5 and 5.0 kJ/mol). The functionalization using the fluorine group has smaller influence on the binding energy that rises by 15 - 30 % for the first H2 molecule and by 65 - 100 % for the second. However, even in the best case the energy of adsorption of second molecule at the same Be2 center in not higher that one observed in carbon-only materials. In benzene and
tetracene the Be2 dimer is always located at the edge of the molecule. In real experiment, the substitution C/Be2 will occur at random and most of the substituted sites will be located inside carbon lattice. Therefore we have to answer the question whether the functionalization of edges will have any noticeable impact on the adsorption on the distant Be2 centers. Such situation has been investigated on Be2-coronene (C23H12Be2, see the insert on Fig.7). Two cases have been analyzed: i) the functionalization of 1, 2, or 4 carbons in rings containing Be2 center and ii) the functionalization of opposite side of coronene; in such a case the Be2 dimer and the function(s) are separated by 4 or 5 carbon atoms. In C23H12Be2 the energies for hydrogen binding on Be2 dimer are 32 kJ/mol and 8.4 kJ/mol, respectively for the first and the second adsorbed molecule. These values are smaller than those observed in Be2-benzene and Be2tetracene, and slightly larger than for Be2-ovalene; therefore we conclude that coronene can be considered as a model of small two-dimensional (2D) graphene patch. The functionalization by – CN group does not change significantly the already strong binding of the first hydrogen molecule; only small fluctuations (less than 4 % , i.e. +/- 1.2 kJ/mol) are observed, depending on the function location with respect to the Be2 dimer. The functionalization possesses more important effect on the binding of second hydrogen molecule: the energy rises systematically when 1, 2, or 4 carbons close to the Be2 dimer are functionalized, by 60 %, 100 % and 140 %, respectively. The effect is still noticeable (~ 40 % increase, to the value of 12 kJ/mol) when -CN groups are attached on opposite side of coronene. The functionalization by -F group has almost no effect on the bonding energy of the first hydrogen, wherever is the location of the function. In addition, when one or two –F groups are located in the para position close to Be2 dimer the binding of the second hydrogen molecule decreases. Substitution of all four carbon positions in the rings containing Be2 is necessary to slightly increase both binding energies (by ~10 % for the first and 20 % for the second adsorbed hydrogen). Therefore we conclude that functionalization by the -F group of extended graphene fragments will be efficient only in the case when occurring within the ring containing Be2 dimer; it implies that substituted Be2 must be located close to the graphene edge. To estimate the final impact of both: the Be2 substitution of graphene fragments and functionalization of fragment edges by electronegative groups we have estimated hydrogen uptake in hypothetical material of the specific surface of 5000 m2/g, containing 25% of carbon
sites substituted by Be2 dimers, and 25% of carbon sites at the fragment edges functionalized with a hypothetical electron withdrawing group. We assumed that the group will not increase the binding energy of the first hydrogen molecule absorbed over the edge Be2 dimer (and fixed at the value of 32 kJ/mol calculated for coronene), but improve the strength of the binding of the second molecule on the same site by, in average, 100 % (to the value of 16 kJ/mol). The results of this estimation are presented on Fig.8. The partial functionalization of pore edges (and the induced increase of the hydrogen adsorption energy on Be2 sites) doubles the gravimetric storage of Be2-substituted carbon. The improvement of the performance is such that both DOE goals (required by 2017 and the ultimate one) can be reached at relatively low pressure (60 bar and 110 bar, respectively). This perspective puts an important milestone on the way to define directions for further experimental search for ambient temperature hydrogen storage systems. 4. Discussion and Conclusions A possibility of adsorptive hydrogen storage for mobile applications has been studied for over 20 years and no practical solution has been found. In the meantime many claims were published but not confirmed by the following experiments. This redundant situation results today in skepticism towards new ideas. However, from purely scientific point of view, it remains important to explore non-conventional propositions of modifying hydrogen-sorbent interaction that could be explored experimentally in the future in the context of effective, large scale hydrogen storage. Therefore, the most important message of this work is the numerical proof that there is a way to tune the hydrogen adsorption energy in a wide range. The adsorbing surface is carbon-based and has graphene-like structure and is chemically modified by substitution of some carbon atoms with Be2 dimers acting as adsorbing centers. To tune the hydrogen adsorption energy this surface must be additionally functionalized, using small strongly electronegative groups. The results of our numerical studies demonstrate that both, specific surface area (S) and energy of hydrogen adsorption (E) of graphene-based nanoporous carbons can be modulated in a wide range of values by simultaneous substitution of graphene layer, fragmentation of its structure and functionalization of fragments’ edges. In such a way both parameters – S and E - are
simultaneously optimized and the required targets for gravimetric (and volumetric) hydrogen storage capacity can be reached without exceedingly improving only one parameter. For example, if the specific surface was increased from 2600 m2/g to 5000 m2/g, the (average) binding energy must be simultaneously increased to the value of 8 kJ/mol to reach 2017 DOE goal, and to 10 kJ/mol to reach the ultimate target (see Fig.1). These values are much lower than suggested by Bahtia et al. [6] for all-carbon infinite slit-shaped pores with the typical graphenelike specific surface. Increasing the surface available for adsorption is crucial for storage applications. The carbon structures with high surface areas were proposed by Kaneko [33], as ensembles of nanofragments of graphene. The idea of high surface resulting from the surface of the edges of nanofragments has been discussed by Yaghi [34, 35] and implemented to prepare MOFs. We showed in our previous papers [11, 22] that it is also possible to imagine nanoporous carbons having the surface higher than 5000 m2/g. Some of such structures have been proposed and discussed. To increase the hydrogen binding to carbon-derived sorbents, Ca, N, P, B and other substituents were considered in the past. Although in the resulting materials they all improved the hydrogen storage, none of them was able to approach the DOE target values. In this work we explored, for the first time, the effect of carbon substitution with beryllium dimer. We showed that this light atom can substantially increase the gravimetric storage of substituted sorbent (for 25% substitution ratio the storage capacity at room temperature is almost doubled). However, as the high energy of adsorption of Be2 dimer (23 kJ/mol for the first adsorbed H2 molecule and 8 kJ/mol for the second) is localized on the substitution site and does not modify substantially the adsorption on neighboring carbons, to further improve the hydrogen storage an additional functionalization of beryllium-containing sorbents with electron withdrawing groups is necessary.
According to our estimations a hypothetical sorbent with optimized specific surface (~5000 m2/g), with 25% of carbons substituted by Be2 dimers and partially (25%) functionalized pores’ edges would adsorb an amount of hydrogen that satisfies the gravimetric DOE storage requirements at room temperature and low pressure ( ~60 bar for 2017 goal and ~110 bar for the ultimate target). This conclusion is very promising and suggests the strategy one should adopt to
prepare carbon-based sorbents with open, high surface architecture and substantially increased energy of hydrogen adsorption. There are two consequences of the surface modifications that we do not discuss in this paper: variation of adsorbent density and that of delivery characteristics. It is well known, that the stronger interaction makes the delivery cycle less effective, that is, at the lowest working pressure the system can still retain a substantial amount of adsorbed hydrogen. This information can be easily obtained from the isotherm curve, if the operating temperature is constant. However, it will not necessarily be the case in the real storage tank. Remembering that adsorption is exothermic, and desorption endothermic, the concept of an ultimate tank should include system cooling at loading and its heating at delivery. Therefore, (most probably) the final system will not operate at constant temperature and the delivery cycle will critically depend on the working temperature range. Concerning the actual density of the adsorbent, as it is defined by the system topology (the distribution of the pores size), it will depend on the synthesis procedure.
ACNOWLEGEMENTS This work has been supported by ANR-14-CE05-0009 HYSTOR grant of French National Research Agency (ANR) and a statutory activity subsidy from Polish Ministry of Science and Technology of Higher Education for the Faculty of Chemistry of Wroclaw University of Technology and NCN grant No. 2012/07/B/ST4/01584. We also thank Wroclaw and Poznan Centers for Networking and Supercomputing, and Interdisciplinary Center for Mathematical Modeling of Warsaw University for providing generous computer time.
5. References 1. P.Benard, R.Chahine, Storage of hydrogen by physisorption on carbon and nanostructured materials, Scr.Mater. 56 (2007) 803-808 2. P. Benard, R. Chahine, Modeling of adsorption storage of hydrogen on activated carbons, Int. J. Hydrogen Energy 26 (2001) 849–855. 3. B. Panella, M. Hirscher, S. Roth, Hydrogen adsorption in different carbon nanostructures, Carbon 43 (2005) 2209–2214. 4. H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y.B. Go, M. Eddaoudi, A.J. Matzger, M.O’Keeffe, O.M. Yaghi, A route to high surface area, porosity and inclusion of large molecules in crystals, Nature 427 (2004) 523–527. 5. G.E.Froudakis, Hydrogen storage in nanotubes & nanostructures, Mat.Today 14 (2011) 324328. 6. S.K.Bhatia, A.L.Myers, Optimal conditions for adsorptive storage, Langmuir 22 (2006) 1688-1700 7. R. B. Getman, Y.-S. Bae, C. E. Wilmer, and R. Q. Snurr, Review and Analysis of Molecular Simulations of Methane, Hydrogen, and Acetylene Storage in Metal-Organic Frameworks. Chem. Rev. 112 (2012) 703–723 8. B. Kuchta, L. Firlej, P. Pfeifer and C. Wexler . Numerical estimation of physical limits of hydrogen storage by physisorption in microporous nanospaces. Carbon 48 (2010) 223 –231. 9. B. Kuchta, L. Firlej, R. Cepel, P. Pfeifer and C. Wexler, Structural and energetic factors in designing a nanoporous sorbent for hydrogen storage, Colloids and Surfaces A 357 (2010) 61-66. 10. B. Kuchta, L. Firlej, Sz. Roszak, P. Pfeifer, A review of boron enhanced nano-porous carbons for hydrogen adsorption: numerical perspective. Adsorption 16 (2010) 413-421. 11. B. Kuchta, L. Firlej, A.Mohammadhosseini, P.Boulet, M.Beckner, J.Romanos, P.Pfeifer, Hypothetical high-surface-area carbons with exceptional hydrogen storage capacities: open carbon frameworks, JACS 134, (2012) 15130–15137 12. L.Firlej, B.Kuchta,
P.Pfeifer, Understanding universal adsorption limits for hydrogen
storage in nanoporous systems, Adv.Mat. 25 (2013) 5971-5974,
13. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keefee and O. M. Yaghi, Hydrogen storage in microporous metal–organic frameworks, Science 300 (2003) 1127– 1129. 14. J. L. C. Rowsell and O. M. Yaghi, Strategies for hydrogen storage in metal–organic frameworks, Angew. Chem., Int. Ed.44, (2005) 4670– 4679. 15. H. M. El-Kaderi, J. R. Hunt, J. L. Mendoza-Cortes, A. P. Cote, R. E. Taylor, M. O’Keeffe and O. M. Yaghi, Designed synthesis of 3D covalent organic frameworks, Science 316 (2007) 268–272. 16. Kim, Y.-H., Zhao, Y., Williamson, A., Heben, M.J., Zhang, S.B.,
Nondissociative
adsorption of H2 molecules in light-element doped fullerenes, Phys.Rev.Lett. 96, (2006) 016102 17. L. Firlej, Sz. Roszak, B. Kuchta. P. Pfeifer and C. Wexler, Enhanced hydrogen adsorption in boron substituted carbon nanospaces . J.Chem.Phys. 131 (2009) 164702. 18. E.Beheshti, A.Nojeh, P.Servati, A first-principles study of calcium decorated, borondopedgraphene for high capacity hydrogen storage, Carbon 49 (2011) 1561-1567. 19. K.S.Subramanyan, L.S.Panchacarla, A.Govindaraj, C.N.R.Rao, Simple method of preparing graphene flakes by an arc-discharge method, J.Phys.Chem.C 113 (2009) 4257-4259. 20. L.S.Panchacarla,
K.S.Subramanyam,
S.K.Saha,
A.Govindaraj,
H.R.Krishnamurthy,
U.V.Waghmare, C.N.R.Rao, Synthesis, structure and properties of boron and nitrogendoped graphite, Adv.Mat. 21 (2009) 4726-4730. 21. L. Firlej, B. Kuchta, A.Lazarewicz and P. Pfeifer, Increased H2 gravimetric storage capacity in truncated carbon slit pores modeled by Grand Canonical Monte Carlo, Carbon 53 (2013) 208-215 22. B. Kuchta, L. Firlej, A.Mohammadhosseini, M.Beckner, J.Romanos, P.Pfeifer, Open carbon frameworks- a search for optimal geometry for hydrogen storage, J. Mol. Mod., 19 (2013) 4079-4087 23. D.Saha, S.Deng, Hydrogen Adsorption on Ordered Mesoporous Carbons Doped with Pd, Pt, Ni, and Ru, Langmuir 2009, 25, 12550-12560 24. K.Sumida, M.R.Hill, S.Horike, A.Dailly, J.R.Long, Synthesis and Hydrogen Storage Properties of Be12(OH)12(1,3,5-benzenetribenzoate)4, J.Am.Chem.Soc. 131(2009) 15120– 15121
25. H.Lee, B.Huang, W.Duan, J.Ihm Ab initio study of beryllium-decorated fullerenes for hydrogen storage, Journal of Applied Physics 107 (2010) 084304 26. Y.-H Kim, Y.Zhao, A.Williamson, M.J.Heben, S.B.Zhang Nondissociative Adsorption of H2 Molecules in Light-Element-Doped Fullerenes, Phys. Rev. Lett.,96 (2006) 016102 27. A.D.Becke, Density-functional thermochemistry. III. The role of exact exchange, J. Chem. Phys., 98 (1993) 5648-52. 28. R.Roszak, Sz.Roszak s-Block metallabenzene: aromaticity and hydrogen adsorption, J.Mol.Model. (2015) 21-28. 29. T.H. Dunning Gaussian basis set for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen, J.Chem.Phys.90 (1989) 1007-1023. 30. Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009. 31. R. Roszak, Sz. Roszak, D. Majumdar, L. Firlej, B. Kuchta and J. Leszczynski, Unique Bonding Nature of Carbon-Substituted Be2 Dimer inside the Carbon (sp2) Network. J. Phys. Chem.A 118 (2014) 5727–5733. 32. J. Li, T. Furuta, H. Goto, T. Ohashi, Y. Fujiwara, S. Yip, Theoretical evaluation of hydrogen storage capacity in pure carbon nanostructures, J. Chem. Phys. 119 (2003) 2376-2385. 33. K. Kaneko, C. Ishii, M. Ruike, H. Kuwabara, Origin of superhigh surface area and microcrystalline graphitic structures of activated carbons. Carbon 1992, 30(7), 1075−1088.
34. N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe, O. M. Yaghi, Hydrogen Storage in Microporous Metal-Organic Frameworks. Science 2003, 300 (5622), 1127−1129 35. H.K. Chae, D.Y. Siberio-Perez, J. Kim, Y. Go, M. Eddaoudi, A.J. Matzger, M. O’Keeffe, O.M. Yaghi A route to high surface area, porosity and inclusion of large molecules in crystals. Nature 427 (2004) 523–527
Figure 1. Iso-weight-capacity (constant Gc) curves for Gc values typical for high surface activated carbons and the DOE ultimate goal. The intersections of the dashed lines with Gc curves indicate the surfaces (horizontal lines) and average binding energy (vertical lines) required for adsorbent with typical graphene characteristics to achieve different Gc capacities.
Figure 2. (left) Planarity perturbation of single atom substituted ovalene C31BeH14. The insert shows a hypothetical C5Be superstructure. (right) Ovalene C31Be2H14 substituted with Be dimer. The equilibrium positions of two hydrogen molecules absorbed on booths sides of the ovalene are shown.
a)
b)
c)
Figure 3. (a) Energetic landscape of infinite graphene surface substituted by an isolated beryllium dimer, (b) Energetic landscape of infinite graphene surface substituted by two Be2 dimers separated by two carbon atoms (c) variation of hydrogen binding energy over substituted ovalene structure. The value of n denotes the number of carbon atoms separating the substituted dimers. Here the value of n = 0 corresponds to single substitution (isolated substitution site).
50
40
graphene 11% Be2
30
25% Be2
20 10 0
a)
Adsorption [gH2 /kg sorbent]
Adsorption [gH2 /kg sorbent]
50
40
20
40
60
80
Pressure [bar]
100
120
b)
25% Be2
20 10 2
SSA = 5000 m /g 0
0
graphene 11% Be2
30
0
20
40
60
80
100
120
Pressure [bar]
Figure 4. (a) Hypothetical room temperature storage capacity in infinite graphene-based slitshaped pores of the width of 1.1 nm, with 0%, 11% and 25 % of carbon sites substituted by Be2 dimers, (b) storage capacity in a hypothetical porous material with specific surface area of 5000 m2/g.
Figure 5. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer in Be2-benzene (C5H6Be2). Eref are the binding energy of H2 adsorbed on non-functionalized Be2benzene: Eref (1st H2) = 21 kJ/mol, Eref (2nd H2) = 8 kJ/mol. The abbreviations denote the position of the functional group with respect to Be2 dimer: (o)- ortho, (m) – meta, (p) – para; (per-F) stands for per-fluorinated Be2-benzene.
Figure 6. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer substituting the tertiary (left panel) or quaternary (right panel) carbon in
-CN and –F
functionalized Be2-tetracene (C17H12Be2). Eref are the binding energies of H2 adsorbed on Be2 center in corresponding non-functionalized Be2-tetracenes (see the text for the values).
Figure 7. Variation (in %) of binding energy of H2 molecules adsorbed over the Be2 dimer in CN and –F functionalized Be2-coronene (C123H12Be2). Eref are the binding energies of H2 adsorbed on the Be2 center in corresponding non-functionalized Be2-coronene (Eref (1st H2) = 32 kJ/mol, Eref (2nd H2) = 8.4 kJ/mol.
Figure 8. The hypothetical room temperature storage capacity in hypothetical porous material with specific surface area of 5000 m2/g and the pore width of 1.1 nm: (open squares)- all carbon structure, (open circles)- 25% of carbon sites substituted by Be2 dimers, (close circles)- 25% of carbon sites substituted by Be2 dimers; 25% of them showing enhanced adsorption energy due to functionalization of pore edges. For comparison, the room temperature storage capacity in infinite graphene-based slit-shaped pores of the same width is also shown (black squares).