High-capacity hydrogen storage by metallized graphene

0.43 MB·English

by  C. Ataca

High-capacity hydrogen storage by metallized graphene
C. Ataca,1,2 E. Aktu¨rk,2 S. Ciraci,1,2,∗ and H. Ustunel3
1Department of Physics, Bilkent University, Ankara 06800, Turkey
2UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara 06800, Turkey
3Department of Physics, Middle East Technical University, Ankara 06531, Turkey
First-principles plane wave calculations predict that Li can be adsorbed on graphene forming a
uniform and stable coverage on both sides. A significant part of the electronic charge of the Li-
9 2s orbital is donated to graphene and is accommodated by its distorted π∗-bands. As a result,
0 semimetallic graphene and semiconducting graphene ribbons change into good metals. It is even
0 more remarkable that Li covered graphene can serve as a high-capacity hydrogen storage medium
2 witheachadsorbedLiabsorbinguptofourH2moleculesamountingtoagravimetricdensityof12.8
wt %.
n
a
J
Developing safe and efficient hydrogen storage is siteofgraphene(i.e. H1-siteabovethecenterofhexagon)

4 essential for hydrogen economy.1 Recently, much ef- is modelled by using (4×4) cell of graphene with 1.70
1 fort has been devoted to engineer carbon based ˚A minimum Li-graphene distance and with a minimum

nanostructures2,3,4,5 whichcanabsorbH moleculeswith Li-Li distance of 9.77 ˚A, resulting in a binding energy
] 2
ll high storage capacity, but can release them easily in the of EL=1.93 eV. Upon adsorption Li atom donates part
a course of consumption in fuel cells. Insufficient storage of the charge of its 2s state to the more electronegative
h
capacity, slow kinetics, poor reversibility and high dehy- carbon atoms at its proximity. Despite the ambiguities
-
s drogenationtemperatureshavebeenthemaindifficulties in determining the atomic charge, Lo¨wdin analysis esti-
e towards acceptable media for hydrogen storage. mates that Li becomes positively charged by donating
m
Recently, graphene, a single atomic plane of graphite, q ∼0.35 electrons (but q ∼ 0.9 electrons according to
. hasbeenproduced6showingunusualelectronicandmag- Bader analysis11). The energy barrier to the diffusion
t
a neticproperties. Inthisletter,wepredictthatmetallized of a single Li atom on the graphene sheet through top
m
graphenecanbeapotentialhigh-capacityhydrogenstor- (on-top of carbon atoms) and bridge (above the carbon-
- age medium. The process is achieved in two steps: Ini- carbonbond)sitesarecalculatedtobe∆Q=0.35eVand
d
tially,grapheneismetallizedthroughchargedonationby 0.14 eV, respectively.
n
∗
adsorbed Li atoms to its π - bands. Subsequently, each
o
c positively charged Li ion can absorb up to four H2 by
[ polarizing these molecules. At the end, the storage ca-

1 pacity up to the gravimetric density of gd=12.8 wt % is Lithium atoms can form a denser coverage on the
attained. These results are important not only because graphene with a smaller Li-Li distance of 4.92 ˚A form-
v
4 grapheneisfoundtobeahighcapacityhydrogenstorage ingthe(2×2)pattern. Owingtotherepulsiveinteraction
4 medium, butalsobecauseofits metallizationthroughLi between positively chargedLi atoms, the binding energy
9 coverage is predicted. of Li atom is smaller than that of the (4×4) cell. For
1 Our results have been obtained by performing H1 adsorption site (see Fig.1(a)), the binding energy is
.
1 first-principles plane wave calculations using ultra-soft calculated to be EL=0.86 eV. The binding energies are
0 pseudopotentials.7 We used Local Density Approxima- relatively smaller at the bridge and top sites, and are
9 tion (LDA), since the van der Waals contribution to 0.58and0.56eV,respectively. Thebindingenergyofthe
0 the Li-graphene interaction has been shown8 to be bet- second Li for the double sided adsorption with H1+H2
:
v ter accounted by LDA. Numerical results have been ob- and H1+H3 configurations described in Fig.1 (a), are
Xi tained by using VASP,9 which were confirmed by using EL=0.82 and 0.84 eV, respectively. The same binding
the PWSCF code.10 A plane-wave basis set with kinetic energiesforH1+H2,andH1+H3geometriesonthe(4×4)
ar energy cutoff ¯h2|k+G|2/2m = 380 eV has been used. cellarerelativelylargerduetoreducedrepulsiveLi-Liin-
In the self-consistent potential and total energy calcula- teraction,namelyEL=1.40eVand1.67eV,respectively.
tions the Brillouin zone has been sampled by (19x19x1) The coverage of Li on the (2×2) cell is Θ=12.5 % (i.e.
and(9x9x1)specialmeshpointsink-spacefor(2×2)and one Li for every 8 carbon atoms) for H1 geometry and
(4×4) graphene cells, respectively. Atomic positions in Θ=25%foreitherH1+H2orH1+H3geometries. Metal-
allstructuresareoptimized using the conjugate gradient licchargeaccumulatedbetweenLiandgrapheneweakens
method. Convergence is achieved when the difference of the interaction between Li atoms which are adsorbed at
thetotalenergiesoflasttwoconsecutivestepsislessthan different sites of graphene. Further increasing one-sided
10−6 eV and the maximum force allowed on each atom coverageofLitoΘ=25%withH1geometry(ortwo-sided
islessthan10−2 eV/˚A.Allconfigurationsstudiedinthis coverage to 50 % with H1+H2 or H1+H3 geometries)
work have also been calculated by using spin-polarized appears to be impossible due to strong Coulomb repul-
LDA,whichwereresultedinnon-magneticgroundstate. sion between adsorbed Li ions and results in a negative
Adsorptionofasingle(isolated)Liatomonthehollow binding energy (EL ∼ - 2.5 eV). On the other hand, the
2
FIG. 1: (Color online) (a) Variousadsorption sites H1,H2 and H3on the(2×2) cell (top panel) and energy band structureof
bare graphene folded to the (2x2) cell (bottom panel). (b) Charge accumulation, ∆ρ+, calculated for one Li atom adsorbed
to a single side specified as H1 (top) and corresponding band structure. (c) Same as (b) for one Li atom adsorbed to H1-site,
second Li adsorbed to H3-site of the(2×2) cell of graphene. Zero of band energy is set to theFermi energy,EF.
total binding energy of all Li atoms adsorbed on a(2×2) and without Li adsorbed segments can serve as a Schot-
cell with the H1+H2 (H1+H3) geometry corresponding tky barrier.
to Θ=25 % is 3.23 eV (3.12 eV) higher than that of Li
Sodium, a heavier alkali metal, can be bound to
atomadsorbedonthe(4×4)cellwiththesamegeometry graphene with Eb=1.09 eV at H1 site. However, the
correspondingtoΘ=6.25%. Hence,sincetheclusterfor- energy difference between top, bridge and H1 sites are
mation is hindered by the repulsive interaction between minute due to relatively larger radius of Na. Upon ad-
the adsorbed ions, a stable and uniform Li coverage on sorption, graphene and graphene nanoribbon are met-
both sides of graphene up to Θ= 25 % can be attained. allized. Nevertheless, Na is not suitable for hydrogen
storage because of its heavier mass and very weak bind-
ing to H molecules. Two dimensional BN-honeycomb
2
structure, being as a possible alternative to graphene,
The charge accumulation and band structure calcu-
has very weak binding to Li (∼ 0.13 eV) and hence it is
lated for the H1 and H1+H3 adsorption geometries are
not suitable for hydrogen storage.
presented in Fig.1 (b) and (c), respectively. Isosurface
plots of charge accumulation obtained by subtracting The absorption of H molecules by Li + graphene in
2
charge densities of Li and bare graphene from that of H1, H1+H2 and H1+H3 geometries. A summary of our
Li which is adsorbed to graphene, ∆ρ+, display positive results about the H absorption are presented in Fig.
2
values. As a result of Li adsorption, the charge donated 2. The binding energy of the first absorbed H , which
2
by Li is accumulated between Li and graphene and is prefers to be parallel to graphene, is generally small.
∗
accommodated by 2pπ -bonds of carbons. The empty However, when two or more H molecules are absorbed
2
∗
π -bandsbecomeoccupiedandeventuallygetdistorted. by the same Li atom, the binding geometry and mech-
∗
Occupation of distorted graphene-π bands gives rise to anism change and result in a relatively higher binding
the metallization of semimetallic graphene sheets. By energy. All H molecules are tilted so that one of two H
2
controlled Li coverage one can monitor the position of atoms of each absorbed H molecules becomes relatively
2
Fermienergyinthelinearregionofbandscrossingatthe closer to the Li atom. A weak ionic bond forms through
K-point of the Brillouin zone. Metallization is also im- asmallamountchange(>∼0.1electrons)transferredfrom
portant for zigzag and armchair graphene nanoribbons, Li and graphene to nearest H atoms of absorbed H
2
sincebotharesemiconductorswiththeirenergygapsde- molecules. At the end, H atoms receiving charge from
pending strongly on the widths of these ribbons.12 Seg- Libecomesnegativelychargedandthe covalentH bond
2
ments of these ribbons metallized by Li adsorption may becomespolarized. Weakionicbond,attractiveCoulomb
be interesting for their electronic andspintronic applica- interactionbetween positively chargedLi and negatively
tions. For example, a junction of two nanoribbons with charged H and weak van der Waals interaction are re-
3
FIG. 2: (Color online) Adsorption sites and energetics of Li adsorbed to the (2×2) cell of graphene and absorption of H2
molecules byLiatoms. EL isthebindingenergy ofLiatom adsorbedtoH1-site,which is aminimumenergysite. ForH1+H2
or H1+H3configuration corresponding to double sided adsorption, EL is the binding energy of second Li atom and EL is the
average bindingenergy. ForH1,H1+H2andH1+H3configurations, E1 isthebindingenergy of thefirst H2 absorbed byeach
Li atom; En (n=2-4) is the binding energy of the last nth H2 molecule absorbed by each Li atom; En is the average binding
energy of n H2 molecules absorbed bya Li atom. Shaded panel indicates themost favorable H2 absorbtion configuration.
sponsible for the formation of mixed weak bonding be- on graphene due to strong Coulomb repulsion between
tweenH moleculesandLi+graphenecomplex. Herethe adsorbedLi atoms. Moreover,Li coveredgraphene is re-
2
bonding interaction is different from the Dewar-Kubas sistant to clustering of adsorbed Li atoms. Earlier, Dur-
interaction13 found in H -Ti+C or carbon nanotube gun et al.5 has predicted that ethylen+Ti complex can
2 60
complexes.3 As the numberofabsorbedH2, n,increases, store H2 up to gd=14.4 wt % per molecule. Later, their
thepositivechargeonLiaswellastheminimumdistance resultshavebeenconfirmedexperimentally.17 Webelieve
betweenH andLislightlyincreases. Nomatterwhatthe that hydrogen storage by the Li covered graphene is in-
2
initialgeometryofabsorbedH moleculeswouldbe,they teresting,sinceitmaynotrequireencapsulingandhence
2
arerelaxedtothesamefinalgeometrypresentedinFig.2 may yield even higher effective gd.
for any given n. We found no energy barrier for a H
2
In conclusion, two crucial features of Li covered
moleculeapproachingtheabsorbedH whenn≤4. Note
2
graphene revealed in this paper may be of technological
that the dissociative absorption of H molecules do not
2
interest. These are high metallicity and high hydrogen
occur in the present system. The energy barrier for the
storagecapacityof graphene functionalized by Li atoms.
dissociationofH nearLitoformLi-Hbondis∼2eV.14
2
Graphene nanoribbons metallized through adsorbed Li
Moreover, dissociation of H to form two C-H bonds at
2
atoms can be used as interconnects between graphene
the graphene surface is energetically unfavorable by 0.7
based spintronic devices. Graphene functionalized by Li
eV.
can also serve as a medium of hydrogen storage. As far
MaximumnumberofabsorbedH per Liatomis four,
2 as efficiency in storage is concerned, graphene may be
and the maximum gravimetric density corresponding to
superior to carbon nanotubes, because its both sides are
H1+H2 geometry at Θ=25 % coverage is gd=12.8 wt readily utilized. Cell configurations formed by different
%. This is much higher than the limit (gd=6 wt %) set junctions of graphene18 functionalized by Li atoms are
for the feasible H storage capacity. Note that only for
2 expected to yield higher surface/volume ratio and hence
n = 4, H1+H2 absorption geometry has slightly lower
to provide efficient H storage in real applications.
2
energy than H1+H3 geometry. This is a remarkable re-
sult indicating anotherapplicationofgrapheneas a high ThisresearchwassupportedbytheScientificandTech-
capacity storage medium. Here Li+graphene complex nological Research Council of Turkey under the project
is superior to Ti+C or carbon nanotube complexes TBAG 104536. Part of computational resources have
60
since Li is lighter. Even though graphene by itself is been provided through a grant (20242007) by the Na-
stable15,16,morestableformisobtainedbyLiadsorption tionalCenterforHighPerformanceComputing,Istanbul
4
Technical University.
∗ Electronic address: [email protected] Mater. Sci. 36, 254-360 (2006).
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