Functionalization of hydrogenated graphene bypolylithiated species for efficient hydrogenstorage

Tanveer Hussain a,*, Abir De Sarkar a,b,c,*, Rajeev Ahuja a,b

aCondensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University,S-75120 Uppsala, SwedenbApplied Materials Physics, Department of Materials and Engineering, Royal Institute of Technology (KTH),S-100 44 Stockholm, SwedencCentral University of Rajasthan, Department of Physics, NH-8 (Jaipur-Ajmer Highway), Bandarsindri Dist.,Ajmer, Rajasthan 305801, India

a r t i c l e i n f o

Article history:

Received 7 June 2013

Received in revised form

16 November 2013

Accepted 20 November 2013

Available online 13 January 2014

Keywords:

Hydrogen storage

Functionalization

Polylithiated

Molecular dynamics

a b s t r a c t

The hydrogen (H2) storage capacity of defected graphane (CH) functionalized by poly-

lithiated species CLi3 and CLi4 has been investigated by means of first-principles DFT cal-

culations. The stability and electronic structures of these potential H2 storage materials

have also been studied. The binding of these lithium rich species (CLi3, CLi4) to the CH sheet

has been found to be strong enough to avoid clustering. The nature of bonding in CeLi and

CeC has been revealed by Bader charge analysis. It has been found that when both sides of

CH sheet are functionalized by polylithiated species, a storage capacity of more than 13 wt

% can be achieved with adsorption energies of H2 in the range of 0.25 eVe0.35 eV, which is

suitable for an efficient H2 storage.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

The energy demands across the world have increasedtremendously over the years, mounting pressure on the en-ergy supplies and causing a depleting effect on current re-sources of energy, especially the fossil fuels. At the same time,

the drastic changes in the environment caused by the esca-lating amount of CO2 emission due to the extensive use ofcarbon-based sources of energy have threatened life on earth[1]. This situation demands an urgent need for alternate,renewable, efficient and cleaner sources of energy. Hydrogen

(H2) can easily be the potential candidate for a perfect carrierof energy which could replace the traditional sources likefossil fuels owing to its abundant availability, zero emission,highest energy density low cost and environment friendliness

[2e4]. Unfortunately, the biggest challenge in the actualiza-tion of H2 economy is still the unavailability of a storagemediawhich is free from the constraints like low storage capacity,lack of reversibility, slow kinetics, safety and cost effective-ness [5,6]. Among all the H2 storage medias available at themoment, carbon based nanostructures (fullerenes, carbonnanotubes, graphene, graphane etc.) have attracted muchattention and considered to be an excellent choice because of

* Corresponding authors. Condensed Matter Theory Group, Department of Physics and Astronomy, Box 516, Uppsala University, S-75120Uppsala, Sweden.

E-mail addresses: tanveersaram@gmail.com, tanveer_20698@yahoo.com (T. Hussain), abirdesarkar@gmail.com, abirdesarkar@curaj.ac.in (A. De Sarkar).

Available online at www.sciencedirect.com

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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 2 5 6 0e2 5 6 6

0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.ijhydene.2013.11.083

their low cost and light weight. But, in their pristine form, the

carbon nanostructures are chemically too inert to be used forthe technological applications like hydrogen storage. To in-crease the reactivity of the carbon-based nanostructures andto be able to utilize them in specific applications like H2 stor-age, functionalization of their surfaces or interior is required[7] One of the most extensively adopted methodology for thefunctionalization of carbon-based nanostructures is thedecoration of their surface or interior with foreign atom,molecule or radicals. Metal-doped carbon nanostructurescarry great potential in many applications including H2 stor-age [8e10]. The commonly used metal dopants for function-

alization of H2 storage include transition metals (TM) [11e13],alkali metals (AM) [14e17], and alkaline earth metals (AEM)[18e21]. Regardless of the type of dopant and the purpose ofthe functionalization, the doped-material must fulfill thecriteria of (i) stability and (ii) efficiency simultaneously. Firstly,the dopant atom, molecule or the radical should bind stablyand remain intact within the host material. Secondly, thepurpose of functionalization should also be served optimally.Among all the elements mentioned above, the alkali metalsare preferred for the functionalization of carbon-nanostructures for efficient H2 storage materials, as the al-

kali metals are lighter than the transition metals. Moreover,the alkali metals have much lower cohesive energies, whichenable their uniform distribution over the host material.Whereas, the transition metal atoms tend to cluster togetherowing to their relatively higher cohesive energies, therebyreducing the storage capacity.

Graphane, (hydrogenated graphene, CH) is an interestingderivative of graphene, which has been predicted and syn-thesized, in the recent past [22,23]. This fascinating materialcan be prepared by exposing graphene sheet to the hydrogenplasma. Following the exposure to hydrogen plasma, the H

atoms bind to the each C atom on the planar graphene sheeton its either side alternately and thereby crumple the sheet.Despite its extraordinary electrical, mechanical and opticalproperties of CH, very few studies have been done recently onits different possible technological applications, especially H2

storage [7,14,24]. The advantage of using CH as H2 storagematerial is its nano-size, large stability and relatively strongerCH-metal binding. This would allow having uniform distri-bution of dopants over the surface, which is always preferredwhile functionalizing a nanostructure.

The molecules with high density of lithium atoms (Li) aretermed as polylithiated molecules. This class of species in-

cludes CLim and OLin (m ¼ 3e5 and n ¼ 1e4). There has beenmany theoretical as well as experimental studies predictingand verifying the existence of such Li rich species [25e27]. Thepolar nature of bonds in CeLi leaves a significant amount ofcharge on Li atoms, which in turn can polarize and adsorb H2

molecules, thus resulting in a good storage of H2 molecules. Inour recently published paper [14], we have substituted Hatoms of graphane with Li atoms to adsorb H2 in molecularform. The reason for preferring Li to the other members ofalkali or alkaline earthmetal is its lightweight, which not onlyensures its uniform distribution over the substrate but also

keeps the gravimetric density significantly high.Besides (AM), (AEM) and (TM) polylithiated structures such

as CLim and OLin can also be adsorbed on graphane resulting

into efficientmaterials to store reasonably good amount of H2.

The basic purpose of adsorbing these polylithiated structureson graphane is to avoid any kind of clustering among them,which could hinder the reversibility during H2 adsorbing/desorbing.

In thiswork two importantmembersof polylithiated familynamely CLi3 and CLi4 have been investigated for their effectiveuse as efficient H2 storage materials. It has been found thatbothCLi3 andCLi4 canaccommodate 12H2molecules eachwithan average binding energy of 0.25 eV and 0.27 eV respectively,which is ideal for their use at ambient conditions. In order toavoid any kind of clustering between these Li rich species and

to maintain the reversibility, both CLi3 and CLi4 have beenadsorbed on defected CH on both sides of the sheet.

2. Computational aspects

This study deals with the first-principles calculations on CLi3and CLi4 doped CH as a high capacity H2 storage materials.Structural optimizations and the total energy calculationshave been performed at the level of plane wave based density

functional theory (DFT) by means of Vienna ab initio simula-tion package (VASP) [28,29]. The projector-augmented wavepotentials (PAW) have been employed which treat H 1s, Li 1s,2s and C 2s and 2p as valence electrons [30,31].

An energy cutoff of 500 eV has been used. The Monkhorst-Pack methodology [32] has been employed for the generationof k-mesh. For structures optimizations and total energy cal-culations we have used 9 " 9 " 1 while for DOS calculationsthe 17 " 17 " 1 of k-points have been used. The geometrieshave been relaxed until the forces acting on each atomreached less than 0.005 eV/!A. The exchange and correlation

functional have been described by means of local densityapproximations (LDA) and the van der Waal’s interactionimplemented in the generalized gradient approximation(vdW-GGA). In case of weakly interacting systems andcoulomb type of interactions (Graphene-metal interaction),vdW-GGA and LDA provide better results. Few years ago, Chaet al. reported that the binding energies of H2 on Ca cationcenters are unphysical in comparison to correlated wavefunction theories when calculated by DFT [33]. Later Ohl et al.responded to this argument by reporting that the use of 6-311þþG** basis set which lacks the proper polarization was

causing the overestimations in the binding energies [34].

3. Results and discussions

3.1. Structural geometry

Before discussing the geometry of CLi3 and CLi4 doped CH, thestructure of pure and defected CH has been described first.The unit cell of CH considered here consists of four atoms (2C,

2H) with CeC and CeH bonds length of 1.54 !A and 1.12!Arespectively on optimization. We have taken a 3 " 3 " 1 supercell having 36 atoms (18C, 18H). A vacuum thickness of 15 !Ahas been inserted along (001) direction, perpendicular to theCH sheet, in order to avoid any kind of interaction between theperiodically repeated structures. For the functionalization of

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CH with the polylithiated species CLi3 and CLi4, 2 out of the

18 H atoms from super cell of CH has been removed to create 2vacancies, one from þZ and one from -Z direction in order tohave reasonable distance between them. The side and topview of the optimized structure of defective CH are shown inFigs. 1 and 2 respectively. First two CLi3 and then two CLi4radicals have been attached to these vacancies to function-alize the CH sheet. These are the starting materials to beinvestigated for efficient H2 storage medias. The optimizedstructures of these polylithiated functionalized CH (CLi3:CHand CLi4:CH) are shown in Figs. 3 and 4 respectively. Onoptimization, the C atoms of both CLi3 and CLi4 pull the C

atoms of CH by which they are bonded to CH (average CeCbond length 1.35 !A) out of the plane through a small distance(w0.4 !A), making the sheet little more crumpled. The averagedistance between the two-polylithiated species adsorbed onboth sides of CH is around 4.5 !A. We believe that this distanceis large enough to avoid the dopants from clustering andensure their uniform distribution over the CH sheet. In thisstudy, we have fully relaxed the atomic geometries of all thestructures allowing the variation in shape, while having heldthe volume fixed.

3.2. Binding energies of CLi3 and CLi4 on CH sheet

Binding energy DEb can be defined as the amount of energyrequired in dislodging the dopant from the substrate, in thiscase the energy required to separate polylithiated moleculesCLi3 or CLi4 from CH sheet in such a way that the interactionbetween them becomes negligibly small if not exactly zero. Toattain the uniform distribution of these lithium rich specieson CH sheet and to avoid the unwanted clustering amongthese dopants, which could reduce the H2 storage capacityand break the reversibility, DEb plays very important part. Thehigher value of DEb is always desired for the optimal utility offunctionalization. It can be calculated by the following

relation

DEb ¼ [E {n (CLi3/CLi4:CH)} $ E {n (CH)} $ E {n (CLi3/CLi4)}]/n (1)

where E (CLi3/CLi4 þ CH) is the energy of the polylithiateddoped CH, E (CH) is the energy of pure graphane and E (CLi3/

CLi4) are the energies of isolated polylithiated species. It

should beworthmentioning that the energy of isolated CLi3 orCLi4 has been estimated by putting these species inside a cubic

box of length 20 !A and employing the Gamma-point samplingof Brillouin zone.

The binding energies of both the polylithiated species CLi3and CLi4 have been calculated by using the above relation. Incase of CLi3, we have got very strong binding of 3.80 eV and3.24 eV with both LDA and Van der Waal’s induced calcula-tions respectively. The values of DEb in case of CLi4 on CH byLDA and van der Waal’s induced calculations are 2.50 eV and2.04 eV respectively, which are slightly on lower side ascompared to CLi3, but still large enough to ensure uniformdistribution of these polylithiated species on CH sheetwithout

clustering.Apart from the calculations of DEb the stability of CLi3/

CLi4:CH systems have also been confirmed by means of mo-lecular dynamics simulations. For this purpose Nose-thermostat algorithm has been employed. The stability ofCLi3 and CLi4 has been studied at temperature of 350 K. It hasbeen seen that even after 4ps both the structures namely

Fig. 1 e Front view of optimized structure of defected CH.The red and green balls represent carbon and hydrogenatoms respectively. (For interpretation of the references tocolor in this figure legend, the reader is referred to the webversion of this article.)

Fig. 2 e Top view of optimized structure of defected CH.

Fig. 3 e Front view of optimized structure of CLi3functionalized CH. The red, green and blue balls representcarbon, hydrogen and lithium atoms respectively. (Forinterpretation of the references to color in this figurelegend, the reader is referred to the web version of thisarticle.)

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CLi3:CH and CLi4:CH remained intact. This shows the stabilityof these structures.

3.3. Charge density analysis

In order to investigate the type on bonding in CHeCLi3,CHeCLi4, CeLi3 and CeLi4 and the mechanism of charge

transfer we have performed Bader charge analysis on ourdesigned structures [34,35]. As the electronegativity of C ishigher than that of Li, so the transfer of charge in both CLi3and CLi4 will be from Li to C. After careful analysis we havefound out that in CLi3 each Li donates w0.96 e of the chargeto the C atoms by which they are attached. In case of CLi4three out of the four Li transfer w0.96 e whereas, the fourth

Li donates w0.78 e of the charge to C. As these CLi3/CLi4species are attached with CH on either side of the sheet, soa strong covalent CeC bond exist between them. It shouldbe noted that C atoms capture the majority of chargetransferred from Li, but a small amount of charge is alsotransferred on H atoms, which are in close vicinity of Catoms. These H atoms gather w0.15 e of the charge. As aresult of this migration of charge the Li atoms on both CLi3and CLi4 attains a fractional positive where are the C atomsby which they are attached to CH sheet, attain fractionalnegative charge. For the further clarification, we haveplotted the iso-surface of differential charge densities

shown in Figs. 5 and 6.

3.4. Density of states

The total and partial densities of states for both the systemsunder investigation here, CLi3:CH and CLi4:CH have beenplotted and shown in Figs. 9 and 10 respectively.

CC is bonded to 4 C atoms, including atop C atom of CLi3.Its bonding with the 4 C atoms is purely covalent and istherefore, strong. As a result, CC contributes no localizedstates around the Fermi level (EF), as shown by the pro-jected density of states in Fig. 9. The CeLi bond is partiallyionic due to the difference in electronegativity between C

and Li atoms and is therefore not as strong as the CeCbond. This is testified by the localized states around the EFon both CLi and Li. The basic pattern in the DOS in Fig. 10remains the same in Fig. 9. The 4 Li atoms contributemore states than the 3 Li atoms. The states of Li hybridizewith that of C and as a result, the localized states on Li shiftslightly down in energy. Due to bond order conservation,the binding of C from CLi4 to the C atom of the underlyinggraphane sheet weakens as compared to the case of CLi3functionalization. The magnitude in this weakening is

Fig. 5 e Isosurface of differential charge density calculatedas Dr[ r (CH:CLi3) e r (CH) e r(CLi3). Cyan and yellow colorindicates the accumulation and depletion of chargerespectively. (For interpretation of the references to color inthis figure legend, the reader is referred to the web versionof this article.)

Fig. 6 e Isosurface of differential charge density calculatedas Dr [ r (CH:CLi4) e r (CH)e r(CLi4). Cyan and yellow colorindicates the accumulation and depletion of chargerespectively. (For interpretation of the references to color inthis figure legend, the reader is referred to the web versionof this article.)

Fig. 4 e Front view of optimized structure of CLi4functionalized CH. The red, green and blue balls representcarbon, hydrogen and lithium atoms respectively. (Forinterpretation of the references to color in this figurelegend, the reader is referred to the web version of thisarticle.)

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small, as shown by occurrence of partial density of states

on CC around EF.

3.5. Hydrogen adsorption in CH:CLi3 and CH:CLi4

The adsorption of H2 molecules on CH:CLi3 and CH:CLi4 isdiscussed here. In case of both the systems (CH:CLi3/CLi4), themechanism of H2 adsorption is the same. As discussed earlier,that due to lesser electronegativity of Li atoms as compare to C

atoms, the Li atoms bonded to the C atoms in each poly-lithiated species CLi3 and CLi4 transfer charge to C atoms andacquires partially positive charge to form Liþ. These Liþ ionspolarize the H2 molecules, which are adsorbed around themby means of van der Waal’s attractive forces. Because of thisfact we have incorporated van der Waal’s interaction in GGAin order to have reliable values of Eads in case of H2 adsorption.To have a maximum storage capacity and to avoid the stericrepulsion among H2 molecules, they have been adsorbedalmost vertically at physisorption distances, so that areasonable distance exists among them. It has been foundthat in case of CH:CLi3, each Li can adsorb a maximum of 4

whereas, in case of CH:CLi4, at the most 3H2 molecules couldbe adsorbed on each Li atom resulting into a storage capacityof 13.87 wt% and 13.33wt% respectively. The lesser H2 mole-cules on CH:CLi4 (3 instead of 4 of each Li) could be due to thesteric hindrance among the H2 molecules, which prevents theadsorption of fourth H2 molecule. But still in both the

polylithiated systems the H2 molecules adsorbed are the samethat is 24. The optimized structures of both.

CLi3:CH and CLi4:CH upon full coverage of H2 has beenshown in Figs. 7 and 8 respectively.

The adsorption energy Eads of H2 on CH:CLi3 and CH:CLi4systems have been calculated the relation

Fig. 7 e Front view of optimized structure of CLi3functionalized CH upon full coverage of H2. The red, greenand blue balls represent carbon, hydrogen and lithiumatoms respectively. (For interpretation of the references tocolor in this figure legend, the reader is referred to the webversion of this article.)

Fig. 8 e Front view of optimized structure of CLi3functionalized CH upon full coverage of H2. The red, greenand blue balls represent carbon, hydrogen and lithiumatoms respectively. (For interpretation of the references tocolor in this figure legend, the reader is referred to the webversion of this article.)

Fig. 9 e Total and partial density of states of CLi3 D CH.Fermi level EF is set to zero.

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Eads (n) ¼ {E (CH:CLi3/CLi4 þ nH2)} $ {E (CH:CLi3/CLi4 þ (n $ 1)H2)} $ n E (H2)

where Eads is the adsorption energy per H2, E (CH:CLi3/CLi4 þ nH2) and E (CH:CLi3/CLi4 þ (n $ 1) H2) are the energiesof the systems CH:CLi3/CLi4 upon adsorption of n and (n $ 1)H2 molecules. E (H2) is the energy of the single H2 molecule.

The complete results of the adsorption energies Eads of H2,the average HeH bond length and the storage capacity of thesystems CH:CLi3 and CH:CLi4 on different levels of hydroge-nation is given in Tables 1 and 2 respectively. As these are theweakly interacting systems, so the use of LDA would beappropriate in the calculations of Eads and HeH bond length.

However, the van der Waal’s induced calculations have also

been performed to bring more reliability into results pre-sented here. The average value of Eads and Dd in case ofCH:CLi3 calculated by van der Waal’s calculation as given inTable 1 are 0.229 (0.310) eV and 0.776 !A (0.802) respectively.The values in parenthesis give LDA results. In case of CH:CLi4the respective values of Eads and Dd are 0.270 eV (0.33) and0.776 !A (0.799). The values in parenthesis represent LDA re-sults, which show slight overestimation as expected. In boththe systems the storage capacity w13 wt% and the Eadswithin the range of 0.25e0.35 eV for H2 indicate that thesematerials can be the ideal ones for the practical storage

applications.

4. Conclusions

A detailed investigation based on first-principles DFT calcu-lations have been performed on the defected CH sheet func-tionalized by two polylithiated species namely, CLi3 and CLi4to serve as potential H2 storage materials. The nature of CeLibond has been found to be strongly ionic. Bader charge anal-ysis shows that the Li atoms on both CLi3 and CLi4 acquirepartially positive charge by donating their charge to the Catoms due to electropositivity of Li with respect to C atoms.These Liþ ions adsorb the H2 molecules around them bypolarizing them. The binding energies of CLi3 and CLi4 on CHsheet have been found to be 3.80 eV and 2.2 eV respectively,

which is large enough to avoid any kind of clustering amongthese species. The stability of the systems CH:CLi3 and CH:CLi4have also been confirmed by means of molecular dynamicssimulations at 350 K. It has been seen that with both sidescoverage of CLi3 and CLi4 on CH sheet, a maximum of 24H2

molecules can be adsorbed with average adsorption energywithin the ideal range of 0.25 eVe0.35 eV suitable for practical

Fig. 10 e Total and partial density of states of CLi4 D CH.Fermi level EF is set to zero.

Table 1 e Number of H2 molecules adsorbed (N), Adsorption energy (Eads), Average HeH bond length Dd and H2 storagecapacity of CLi3 doped CH.

No. of H2 molecules (N) Adsorptionenergy (Eads)

eV

Van der Waal’s HeH distance (!A) Storage capacity (wt% of H2)

LDA Dd Dd

6 0.338 0.810 0.270 0.770 3.8812 0.330 0.807 0.259 0.780 7.4518 0.285 0.798 0.221 0.774 10.7824 0.290 0.793 0.164 0.783 13.87

Table 2 e Number of H2 molecules adsorbed (N), Adsorption energy (Eads), Average HeH bond length Dd and H2 storagecapacity of CLi4 doped CH.

No. of H2 molecules Adsorptionenergy (Eads) eV

Van der Waal’s HeH distance (!A) Storage capacity (wt% of H2)

LDA Dd Dd

8 0.350 0.800 0.280 0.768 4.8816 0.330 0.801 0.270 0.778 9.3024 0.300 0.797 0.260 0.781 13.33

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applications and a very high storage capacity of more than

13wt% can be achieved.

Acknowledgment

TH is thankful to higher education commission of Pakistan for

doctoral fellowship. ADS is grateful to the Wenner-GrenFoundation and FORMAS for postdoctoral Fellowship. RA ac-knowledges Swedish Research Council (VR), FORMAS, SWECO,Wenner-Gren Foundations & Swedish Energy Agency forfinancial support and SNIC and UPPMAX for computing time.

r e f e r e n c e s

[1] Pacala S, Socolow R. Stabilization wedges: solving theclimate problem for the next 50 years with currenttechnologies. Science 2004;305:968e72.

[2] Schlapbacn L, Zuttel A. Hydrogen-storage materials formobile applications. Nature 2001;414:353e8.

[3] Balat M. Potential importance of hydrogen as a futuresolution to environmental and transportation problems. Int JHydrogen Energy 2008;33:4013e29.

[4] Durgun E, Ciraci S, Zhou W, Yildirim T. Transition-metal-ethylene complexes as high-capacity hydrogen-storagemedia. Phys Rev Lett 2006;97:226102e4.

[5] Maark TA, Hussain T, Ahuja R. Structural, electronic andthermodynamic properties of Al- and Si-doped a-, g-, and b-MgH2: density functional and hybrid density functionalcalculations. Int J Hydrogen Energy 2012;37:9112e22.

[6] Park HL, Yi SC, Chung YC. Hydrogen adsorption on Li metalin boron-substituted graphene: an ab initio approach. Int JHydrogen Energy 2010;35:3583e7.

[7] Hussain T, Pathak P, Ramzan M, Maark TA, Ahuja R. Calciumdoped graphane as a hydrogen storage material. Appl PhysLett 2012;100:183902e5.

[8] Kim G, SH J, Park N, Louie SG, Cohen ML. Optimization ofmetal dispersion in doped graphitic materials for hydrogenstorage. Phys Rev B 2008;78:085408e12.

[9] Durgun E, Ciraci S, Zhou W, Yildirim T. Transition-metal-ethylene complexes as high-capacity hydrogen-storagemedia. Phys Rev Lett 2006;92:226102e5.

[10] Gao Y, Zhao N, Li J, Liu E, He C, Shi C. Hydrogen spilloverstorage on Ca-decorated graphene. Int J Hydrogen Energy2012;37:11835e41.

[11] Shevlin SA, Guo ZX. High-capacity room-temperaturehydrogen storage in carbon nanotubes via defect-modulatedtitanium doping. J Phys Chem C 2008;112:17456e64.

[12] Bhattacharya A, Bhattacharya S, Majumder C, Das GP.Transition-metal decoration enhanced room-temperaturehydrogen storage in a defect-modulated graphene sheet. JPhys Chem C 2010;114:10297e301.

[13] Corral IL, German E, Volpe MA, Brizueela GP, Juan A. Tight-binding study of hydrogen adsorption on palladiumdecorated graphene and carbon nanotubes. Int J HydrogenEnergy 2010;35:2377e84.

[14] Hussain T, Sarkar AD, Ahuja R. Strain induced lithiumfunctionalized graphane as a high capacity hydrogen storagematerial. Appl Phys Lett 2012;101:103907e12.

[15] Liu W, Zhao YH, Li Y, Jiang Q, Lavernia EJ. Enhancedhydrogen storage on Li-dispersed carbon nanotubes. J PhysChem C 2009;113:2028e33.

[16] Er S, Wijs GA, Brocks G. Hydrogen storage by polylithiatedmolecules and nanostructures. J Phys Chem C2009;113:8997e9002.

[17] Bhattacharya A, Bhattacharya S, Das GP. Anti-Kubas typeinteraction in hydrogen storage on a Li decorated BHNHsheet: a first-principles based study. J Phys Chem C2011;116:3840e4.

[18] Beheshti E, Nojeh A, Servati P. A first-principles study ofcalcium-decorated boron-doped graphene for high capacityhydrogen storage. Carbon 2011;49:1561e7.

[19] Yoon M, Yang S, Hicke C, Wang E, Geohegan D, Zhang Z.Calcium as the superior coating metal in functionalization ofcarbon fullerenes for high-capacity hydrogen storage. PhysRev Lett 2008;100:206806e9.

[20] Yang X, Zhang RQ, Ni J. Stable calcium adsorbate on carbonnanostructures: applications for high-capacity hydrogenstorage. Phys Rev B 2009;79:075431e4.

[21] Ataca C, Akturk E, Ciraci S. Hydrogen storage of calciumatoms adsorbed on graphene: first-principles plane wavecalculations. Phys Rev B 2009;79:041406e9.

[22] Sofo JO, Chaudhari AS, Barber GD. Graphane: a two-dimensional hydrocarbon. Phys Rev B 2007;75:153401e4.

[23] Elias DC, Nair RR, Mohiuddin TMG, Morozov SV, Blake P,Halsall MP, et al. Control of graphene’s properties byreversible hydrogenation: evidence for graphane. Science2009;323:610e3.

[24] Antipina LY, Avramov PV, Sakai S, Naramoto H, Ohtomo M,Entani S, et al. High hydrogen-adsorption-rate materialbased on graphane decorated with alkali metals. Phys Rev B2012;86:085435e41.

[25] Schleyer PR, Wuerthwein EU, Pople JA. Effectivelyhypervalent first-row molecules. 1. Octet rule violations byOLi3 and OLi4. J Am Chem Soc 1982;104:5839e41.

[26] Janulis Jr EP, Arduengo AJ. Structure of an electronicallystabilized carbonyl ylide. J Am Chem Soc 1983;105:5929e30.

[27] Kudo H. Observation of hypervalent CLi6 by Knudsen-effusion mass spectrometry. Nature 1992;355:432e4.

[28] Kresse G, Hafner J. Ab initio molecular dynamics for liquidmetals. Phys Rev B 1993;47:558e61.

[29] Kresse G, Furthmuller J. Efficient iterative schemes forabinitio total-energy calculations using a plane-wave basis set.Phys Rev B 1996;54:11169e86.

[30] Blochl PE. Projector augmented-wave method. Phys Rev B1994;50:17953e79.

[31] Kresse G, Joubert D. From ultrasoft pseudopotentials to theprojector augmented-wave method. Phys Rev B1999;59:1758e75.

[32] Perdew JP, Chevary JA, Vosko SH, Jackson KA, Pederson MR,Singh DJ, et al. Atoms, molecules, solids, and surfaces:applications of the generalized gradient approximation forexchange and correlation. Phys Rev B 1992;46:6671e87.

[33] Cha J, Lim S, Choi CH, Cha MH, Park N. Inaccuracy of densityfunctional theory calculations for dihydrogen bindingenergetics onto Ca cation centers. Phys Rev Lett2009;103:216102e5.

[34] Bader RFW. Atoms in molecules- aquantum theory. Oxford:Oxford University Press; 1990.

[35] Tang W, Sanville E, Henkelman G. A grid-based Baderanalysis algorithm without lattice bias. J Phys Conden Matt2009;21:084204.

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