Pillared Graphene: A New 3-D NetworkNanostructure for Enhanced HydrogenStorageGeorgios K. Dimitrakakis,† Emmanuel Tylianakis,‡ and George E. Froudakis*,†

Department of Chemistry, and Materials Science and Technology Department,UniVersity of Crete, P.O. Box 2208, 71003 Heraklion, Crete, Greece

Received May 16, 2008; Revised Manuscript Received July 11, 2008

ABSTRACT

A multiscale theoretical approach was used to investigate hydrogen storage in a novel three-dimensional carbon nanostructure. This novelnanoporous material has by design tunable pore sizes and surface areas. Its interaction with hydrogen was studied thoroughly via ab initioand grand canonical Monte Carlo calculations. Our results show that, if this material is doped with lithium cations, it can store up to 41 g H2/Lunder ambient conditions, almost reaching the DOE volumetric requirement for mobile applications.

Hydrogen is considered to be one of the most promisingenergy fuels for automobiles, and its use can be furtherextended to smaller portable devices, like mobile phones andlaptops. It can be stored in either liquid or gas phase,provided that an efficient storage device exists. United States’Department of Energy (D.O.E.) has established requirementsthat have to be met by 2010, regarding the reversible storageof hydrogen according to which the required gravimetricdensity should be 6 wt % and the volumetric capacity shouldbe 45 g of H2/L.1 By moving in that direction, the appropriatematerial for hosting hydrogen has to be developed.

Initially, metal alloys, such as LaNi5, TiFe, and MgNi,were proposed as storage tanks since by chemical hydroge-nation they form metal hydrides. Hydrogen can then bereleased by dehydrogenation of the hydride. Regardingvehicle applications, metal hydrides can be distinguished intohigh or low temperature materials. This depends on thetemperature at which hydrogen absorption or desorption istaking place and if this is above or below 150 °C,respectively. La-based and Ti-based alloys are examples ofsome low temperature materials with their main drawbackas the very low gravimetric capacity (<2 wt %) theyprovide.2,3 Controversially, high temperature materials likeMg-based alloys can reach a theoretical maximum capacityof 7.6 wt %, suffering though from poor hydrogenation/dehydrogenation kinetics and thermodynamics.3-5 In everycase, metal alloys, besides the cost, are heavy for commercialproduction focused on mobile applications.

On the other hand, light nanoporous materials can storehydrogen by physisorption that allows fast loading andunloading. However, as the interaction between H2 and hostmaterial is dominated by weak van der Waals forces, only asmall amount of H2 can be stored at room temperature. Inthis case, high surface area and appropriate pore size arekey parameters for achieving high hydrogen storage. Nan-oporous carbon structures fulfill this criterion, placing themsince the beginning among of the best candidates forhydrogen storage media.6-9 After the synthesis of carbonnanotubes (CNTs),10 the scientific research focused on theirstorage capability. Initial studies showed indeed that CNTscan be considered as a good material for reversible hydrogenstorage,11-13 but it was revealed later that under ambientconditions, pristine CNTs are not that promising.14-16 Furtherstudies showed that doping CNTs with lithium atoms canconsiderably increase their hydrogen storage capacity.17-21

Despite this increase, D.O.E.’s target remains still unap-proachable, and innovative materials are needed to reach it.Lately, novel nanoporous materials like metal organicframeworks (MOFs) have been targeted to the hydrogenstorage problem.22,23 Their high surface area combined withthe increased adsorption sites leads to higher storage capaci-ties establishing them as better candidate materials thanCNTs.4,22 Nevertheless, carbon based materials possess asuperior structural stability and amenability to a wide rangeof processing conditions, keeping them in the race forcommercial applications. The only thing that is missing is away to increase their storage capacity. A possible route toachieve this is by synthesizing novel carbon-based architec-tures of large surface area, suitable for storage pores, likecarbon nanoscrolls (CNSs)24 and fullerene intercalated

* Corresponding author. Phone: +30-2810545055. Fax: +30-2810545001.E-mail: frudakis@chemistry.uoc.gr.

† Department of Chemistry.‡ Materials Science and Technology Department.

NANOLETTERS

2008Vol. 8, No. 10

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10.1021/nl801417w CCC: $40.75 © 2008 American Chemical SocietyPublished on Web 09/19/2008

graphite sheets.25 Calculations performed on these materialsby Mpourmpakis et al.26 and Kuc et al.27 revealed that theyare very promising since an enhancement on the storage ofhydrogen was observed. Nevertheless, despite the enhance-ment, the total amount of hydrogen stored in those structuresremains far from the D.O.E. targets. What is needed is tofind a way to further increase the amount of adsorbedmolecules. It has been shown that hydrogen’s adsorptiondepends on the porosity of the material.28-30 Thus, in orderto increase the amount of adsorbed molecules, we shouldhave high micropore volume30 and narrow micropore sizedistribution.28,30 Evidently, tailored porosity is the key aspectin the development of new promising carbon based materi-als,29,30 as the efficient use of space plays a significant role.28

By moving in this direction, our initial idea was to designa robust carbon nanoporous material of large surface areaand tunable pore size. Thus, it will be appropriate for manydifferent applications apart from hydrogen storage. This novelthree-dimensional (3-D) material proposed in this com-munication consists of parallel graphene layers at a variabledistance, stabilized by CNTs placed vertically to the grapheneplanes. In this way, CNTs support the graphene layers likepillars and assemble a 3-D building block reducing the emptyspace between the graphene layers by filling it with CNTs(Figure 1).

In an attempt to design and investigate the stability of thismaterial, theoretical ab initio calculations were performedusing Turbomole package.31 Because of computationallimitations, the system for these first principal calculationswas reduced to its building block, that is, one graphene sheetand one single walled nanotube (SWNT). At the beginning,a cylindrical hole is created on the graphene sheet. Thediameter of the gap is chosen carefully to fit the selectedCNT that in our case was an armchair (6,6) CNT (Figure2a).

By using generalized Euler’s rule for polygons on thesurface of a closed polyhedron, the bond surplus, that is,the excess in the number of polygonal sites compared with

normal value for the junction, can be found. According tothis rule, the number of faces, F, vertices, V, edges, E andgenus, G, obey the equation, F + V ) E + 2 - 2G or 6(E- F - V) ) 12(G - 1). The CNT and the graphene sheetcan be mated into a closed surface of genus 2, which meansthat a bond surplus of 12 should be shared between the twojunctions (6 per junction). Carbon nanostructures are purelyhexagonal when in sp2 state, so each pentagon causes a bondsurplus of -1, while heptagons and octagons contribute tobond surpluses of +1 and +2 each, respectively. In the finalstructure (Figure 2b), the number of heptagons is six inagreement with Euler’s rule. The dangling bonds of theresulted conformation were saturated with hydrogen atoms.The final structure was fully optimized at the densityfunctional theory (DFT) level resulting in the one shown inFigure 2b. This optimization was performed at the BLYP/SVP32,33 level of theory using the resolution of identity (RI)

Figure 1. Pillared graphene. A novel 3-D network nanostructureproposed for enhanced hydrogen storage.

Figure 2. (A) (6,6) CNT and graphene sheet ready to form a cluster.This is the initial structure which was optimized using DFT. (B)Optimized cluster of (6,6) CNT and graphene sheet; the beginningof the 3-D network nanostructure. The three different sites on whichhydrogen’s interaction was examined are heptagon (site A), thelower hexagon (site B), and the upper hexagon (site C).

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approximation.34-36 Then, an extensive periodic network wasconstructed, on the basis of the ab initio optimized buildingblock (Figure 1). In the second part, pillared graphene’shydrogen storage capacity was investigated using a multiscaletheoretical approach. Initially, first-principle quantum chemi-cal calculations were used to assess the H2 interaction withthe material. Then, the storage capacity was evaluated bygrand canonical Monte Carlo (GCMC) simulations.

The interaction between hydrogen molecules and CNTsor graphite sheets has been studied thoroughly in the pastby many groups.21,37-41 What has not been studied yet is theenergetics and interactions close to the junction, because ofthe novelty of this material. Hence, DFT calculations wereperformed using PBE functional42 with a mixed basis setschema, under the RI approximation.31,34-36,43 The calculationof the binding energy was corrected from basis set super-position error (BSSE) using the counterpoise (CP) method.44

The different sites where hydrogen’s interaction wasexamined are the heptagon (site A), the lower hexagon (siteB), and the upper hexagon (site C; Figure 2b). In every case,we left the H2 molecule free to relax starting from a verticalposition above the center of the ring, which has been shownto be the optimal conformation in similar cases.45 The relatedand all surrounding rings were also left free to relax, whilethe rest of the atoms were kept frozen. Aiming in the highestaffordable accuracy, a different basis set was applied to thesetwo different groups of atoms. For the frozen group atoms,single valence polarized (SVP) basis set36 was implemented,whereas for the rest, triple-! valence polarized (TZVP)43 wasused.

It was shown that the interaction of H2 with the curvedcarbon rings on the junction of the pillared material followsthe same trend as previous results have revealed for pristineCNTs. In this way, the binding energy of H2 above thehexagon (sites B and C) and heptagon (site A) rings wasfound to be 0.25 and 0.33 kcal/mol, respectively. Hence, itbecomes clear that heptagonal rings are more favorable thanthe hexagonal ones regarding H2 interaction on the junctionof the pillared material. In a previous study,41 Heine et al.reported a value of 0.23 kcal/mol for the H2 interaction withgraphite platelets, using the BSSE to correct the initiallycalculated 0.61 kcal/mol from MP2 calculations. Our resultsare in a good agreement, and the importance of BSSEcorrection is confirmed.21,40

The ability of the material to adsorb hydrogen was testedafterward by grand canonical Monte Carlo (GCMC) simula-tions, for a variety of thermodynamic states at ambient andcryogenic temperatures. Hydrogen molecules were modeledas single atoms, whereas the framework structure was keptfrozen. Interactions between fluid molecules and adsorbentatoms were described using the Lennard-Jones potentialtogether with Feynman-Hibbs effective potential in orderto take into consideration quantum effects. The parametersof this model were taken from literature.46 Particle displace-ment, creation, and destruction were attempted duringsimulation, each one with equal probability. For each state,a total amount of 2 ! 106 steps were carried out, from whichthe first 106 were used to reach equilibrium while the rest

were used to calculate the ensemble averages.Since GCMC simulations are computationally less expen-

sive than ab initio, the method was applied for varioussystems. Different combinations of nanotube curvature,intertube, and graphene sheet interlayer distances were tested,in an effort to achieve the best hydrogen uptake. In everycombination, it was found that the gravimetric uptake wasconstantly less than the corresponding of CNT or graphenestructure. This can be attributed to the fact that graphenesheets that are inserted to the CNT bundle increase signifi-cantly the total material weight. That increment is more thanthat of the additional adsorbed H2 molecules, which resultsin reduction of the gravimetric uptake. On the other hand,the volumetric uptake is increased in every combination,since it depends only on the adsorbed H2 molecules.

The results presented here are for the structure consistingof (6,6) SWNTs with intertube distance of 1.5 nm andgraphene sheets with 1.2 nm interlayer distance. Thesegeometric characteristics were chosen having as a criterionthe development of a structure that is energetically allowedto be built. For this specific case, the enhancement involumetric H2 uptake goes up to 25% at ambient temperature.For sake of comparison, similar simulations were alsoperformed for a bundle of (6,6) SWNTs and a series ofgraphene sheets. All of these aforementioned results arepresented in Figures 3 and 4, for the gravimetric andvolumetric uptake, respectively. A snapshot of the simula-tions at 77 K and 3 bar is presented in Figure 5A. From thisfigure, the load of hydrogen that can be achieved by thismaterial for the specific thermodynamic conditions can beobserved.

Figure 3. Gravimetric hydrogen uptake for graphene (diamonds),(6,6) carbon nanotubes (squares), pillared material (triangles), andLi-doped pillared (stars) at (a) 77 K and (b) 300 K.

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Since the total storage capacity of pillared graphene wasnot satisfactory, we tried to enhance it by doping. Previous

studies have shown that doping carbon materials with alkalimetals results in a significant increase of their hydrogenstorage capacity.17-21 This happens because of the chargeof the alkali metal that polarizes the H2 molecules. Phys-isorption of H2 is then dominated by this charge induceddipole interaction.17

Subsequently, similar calculations were carried out forpillared graphene, doped with lithium cations. First, thestructural properties and the energetics of the Li+ interactionwith our material were investigated by a series of ab initiocalculations. Lithium cations were placed above the centerof the different sites (A, B, and C) of Figure 2B. The lithiumcation, its respective ring, and the ring’s neighbors were leftfree to relax, whereas the rest of the atoms were kept frozen.Calculations were performed on the same level as be-fore31,34-36,42,43 and corrected for BSSE.44 The results indicatethat lithium prefers to bind on the defected heptagon with77 kcal/mol. The corresponding energy in the case ofhexagon was 62 kcal/mol. The same trend has been observedin haeckelites and has been explained thoroughly.47,48

Once the optimum positions for the lithium atoms wereobtained, H2 was placed perpendicular to the center of allnonequivalent neighboring rings, resulting to 10 differentconformations: 3 for the heptagon (site A), 3 for the lowerhexagon (site B), and 4 for the upper hexagon (site C). Thepartial optimization approach was used once again with H2,lithium, and their two corresponding rings, being free torelax. The binding energy per H2 per ring was found to be3.6 kcal/mol, 3.4 kcal/mol, and 3.5 kcal/mol when lithiumis above sites A, B, and C, respectively. The results clearlyindicate a significant enhancement on the binding of H2 whena lithium cation is present.

The enhancement of Li-doped structures to adsorb hydro-gen was also verified by GCMC simulations, at the samethermodynamic states that were examined in the case of theundoped material. The parameters of hydrogen-lithiuminteraction were taken by our previous ab initio calculations.26

The results of the simulation are presented in Figures 3 and4, together with the undoped cases, as clarified earlier inthis text. Both volumetric and gravimetric hydrogen uptakeare significantly increased compared with the undoped case.We have observed the same effect in the past, for othercarbon nanostructures, when doped by Li cations.26 Inparticular, for pressures below 10 bar, the gravimetricadsorption is enhanced by 76% at ambient temperature andby 88% at cryogenic. Additionally, the enhancement on thevolumetric adsorption is 30% and 38%, respectively. Besidesthese remarkable results, something else that is worthmentioning is that, for both temperature conditions, thesaturation pressure is reached very soon. This is an extrabenefit since low pressure is an important safety issue inmobile applications. The corresponding snapshot of thesesimulations at 77 K and 3 bar is shown in Figure 5b, wherethe increment on the load due to the inserted lithium cationsis obvious.

In conclusion, a multiscale theoretical investigation provedthat CNTs and graphene sheets can be combined to formnovel 3-D nanostructures, capable of enhancing hydrogen

Figure 4. Volumetric hydrogen uptake for graphene (diamonds),(6,6) carbon nanotubes (squares), pillared material (triangles), andLi-doped pillared (stars) at (a) 77 K and (b) 300 K.

Figure 5. (a) Snapshot from the GCMC simulations of pure pillaredstructure at 77 K and 3 bar. Hydrogen molecules are representedin green. (b) Snapshot from the GCMC simulations of lithium dopedpillared structure at 77 K and 3 bar. Hydrogen molecules arerepresented again in green while lithium atoms are in purple.

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storage. Ab initio calculations revealed that, even on thejunction of this material, hydrogen’s interaction remainsweak, comparable with already known hydrogen-carboninteraction values. The importance of the “charge induceddipole” in this type of interaction was verified, sincehydrogen bonds with a difference of more than 1 order ofmagnitude when a lithium cation is present. Those findingswere also supported by GCMC calculations. It was furtherproven that this novel material, when doped with lithiumcations, can reach D.O.E’s volumetric target for mobileapplications, under ambient conditions. Experimentalists arechallenged to fabricate this material and validate its storagecapacity.

Acknowledgment. Authors thank the Ministry of Devel-opment, General Secretariat for Research and Technology(GSRT) of the Greek government. Their financial supportthrough PENED2003 03ED548 (IIENE!200303E!548) ac-tion is gratefully acknowledged, since this paper is part ofthe 03ED375 research project, implemented within theframework of the “Reinforcement Programme of HumanResearch Manpower” (PENED) and cofinanced by Nationaland Community Funds (25% from the Greek Ministry ofDevelopment-General Secretariat of Research and Technol-ogy and 75% from E.U.-European Social Fund). Partialfunding by Interreg IIIA Greece - Cyprus collaborationK2301.004 and European Commission DG RTD (FP6Integrated Project NESSHY, Contract SES6-518271) aregratefully acknowledged.

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