Our study shows that the H2 storage media using Ni-dispersed fullerenes could be viable alternatives to reversible hydrogen storage. It is demonstrated that a single Ni coated on the fullerene surface can store up to three H2 molecules. Consequently, at high Ni coverage, Ni-dispersed fullerenes are considered to be the novel hydrogen storage media capable of storing ~6.8  wt  %  H2, thus exceeding the Department of Energy target (6.5 wt %) for automobile applications. Moreover, the H2 desorption activation barrier of 11.8  kcal/mol  H2 is ideal for many practical hydrogen storage applications

Goddard, William A., III

Kang, Jeung Ku

Shin, Weon Ho

Yang, Seong Ho

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS 88, 053111 2006Ni-dispersed fullerenes: Hydrogen storage and desorption properties
Weon Ho Shin and Seong Ho Yang
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Kusung-dong 373-1, Yusung-gu, Daejon, Republic of Korea 305-701
William A. Goddard III
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125
Jeung Ku Kanga
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Kusung-dong 373-1, Yusung-gu, Daejon, Republic of Korea 305-701
Received 10 October 2005; accepted 14 December 2005; published online 31 January 2006
Our study shows that the H2 storage media using Ni-dispersed fullerenes could be viable alternatives
to reversible hydrogen storage. It is demonstrated that a single Ni coated on the fullerene surface can
store up to three H2 molecules. Consequently, at high Ni coverage, Ni-dispersed fullerenes are
considered to be the novel hydrogen storage media capable of storing 6.8 wt % H2, thus
exceeding the Department of Energy target 6.5 wt % for automobile applications. Moreover, the
H2 desorption activation barrier of 11.8 kcal/mol H2 is ideal for many practical hydrogen storage
applications. © 2006 American Institute of Physics. DOI: 10.1063/1.2168775Hydrogen, which can be readily produced from renew-
able energy sources and mainly produces water as a byprod-
uct, contains much larger chemical energy per mass
142 MJ kg−1 Ref. 1 than any hydrocarbon fuel. Conse-
quently, it is considered as an ideal fuel to solve energy and
environmental problems in the earth that result from using
petroleum fuels. Many hydrogen storage methods have been
proposed, such as carbon-fiber reinforced high-strength
containers,2 liquid hydrogen,3 chemical hydrides,4 and car-
bon nanotubes CNTs.5 In particular, CNT has been sug-
gested as the most promising nanostructure due to its cylin-
drical structure and high surface-to-volume ratio, which
make it ideal for hydrogen storage. However, recent
studies5,6 indicate that the structure of a pristine CNT is not
relevant to satisfy the Department of Energy DOE target
6.5 wt % at ambient conditions ideal for many practical
applications. This is because the pristine CNT has only the
0.8 wt % hydrogen available at ambient conditions.5 How-
ever, we propose the use of nickel-doped fullerenes as the
hydrogen storage media capable of exceeding the DOE target
6.5 wt % at ambient conditions.
The recent experiment6 proved that Ni-doped multi-
walled MW CNTs with outer diameters of 20 nm could
release 2.8 wt % H2 at ambient conditions. We found that
each Ni dispersed on the CNT surfaces has the high affinity
for hydrogen by storing up to five H2 molecules at ambient
conditions. On the other hand, it was also determined that the
insertion of H2 into the interlayer of MWCNTs is thermody-
namically unstable since it is an endothermic reaction by
24.9 kcal/mol H2. Consequently, it is considered that Ni-
dispersed MWCNTs could release over the 3 wt % if Ni
nanoparticles could be dispersed on the surfaces of the CNTs
grown in vertical shapes with reduced numbers of inter-
layers. However, the problem is that not all the techniques to
control the growth directions and interlayers are available
yet. On the other hand, unlike the CNT, a fullerene has al-
aAuthor to whom correspondence should be addressed; electronic mail:
jeungku@kaist.ac.kr
0003-6951/2006/885/053111/3/$23.00 88, 05311
Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to ways a single wall and its surface is isotropic; thus there is
no need to reduce its interlayer and growth direction. Con-
sequently, the fullerene is considered as a better nanostruc-
ture for Ni dispersion to maximize the hydrogen storage ca-
pacity than that for the CNT.
A recent study by Barajas-Barraza et al.7 showed that up
to 19 H2 molecules could be stored in a C110 fullerene struc-
ture. Moreover, Narita et al.8 showed through a molecular
dynamic simulation study that H2 is stable inside the C60
cage at ambient conditions. On the other hand, they also
found that H2 could not escape from the cage due to the high
activation barrier of 369.1 kcal/mol H2 for penetration
through the cage; thus, using the inside of the cage as a
hydrogen storage site is considered as impractical for many
applications. On the other hand, the recent study6 showed
that Ni nanoparticles dispersed give the activation barrier of
8.7 kcal/mol relevant for hydrogen desorption at ambient
conditions. In addition, a theoretical study9 by Yildirim et al.
also found that metals-dispersed nanostructures could be
good hydrogen storage materials for practical applications.
To determine electronic wave functions, we use B3LYP
Ref. 10 and KMLYP Ref. 11 density functional theory
DFT methods with a split basis set of 6–31 G valence
double zeta basis sets12 for Ni and fullerene carbon atoms
and 6–31 G** split valence double zeta basis sets12 for hy-
drogen. The B3LYP is shown to accurately predict geom-
etries and thermochemical data compared to generalized gra-
dient approximation13–17 GGA and local density
approximation LDA,18,19 while the KMLYP has been
proven to be more accurate in predicting transition state
barriers11 than other DFT methods. In addition, here our en-
ergies include zero point-energy ZPE corrections deter-
mined at fully optimized geometry with the same basis sets.
We have chosen a C60 molecule with a diameter of 7.12 Å as
the specific system for our study, as shown in Fig. 1. The
fullerene has the two distinct bond lengths of 1.46 Å for C
−C bonds and 1.40 Å for C=C bonds, which compare to the
20experimental values of 1.43 Å and 1.39 Å, respectively.
© 2006 American Institute of Physics1-1
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
053111-2 Shin et al. Appl. Phys. Lett. 88, 053111 2006First, we have explored the hydrogen storage capacity on
the pristine C60. The physical adsorption reaction by a H2
molecule on the outside surface of the fullerene was found to
be unstable as it is endothermic by 0.7 kcal/mol H2, while its
corresponding chemical adsorption reaction was determined
to have the exothermic enthalpy of 15.2 kcal/mol H2. How-
ever, it was determined that chemical adsorption by H2 mol-
ecules on pristine fullerenes has a high activation energy of
83.1 kcal/mol H2. Consequently, using the computed parti-
tion functions for the reactants and the transition state, the
reaction rate k of the canonical rate equation21 is determined
by
k = T
kBT
h
QTS
QAQB
exp − E0
kBT
 , 1
where T is the thermal tunneling coefficient,22 QTS is the
partition functional for the transition state, QA and QB are the
partition functions for the reactants A and B, respectively,
and E0 is the barrier height. The resulting dependence of
chemisorption on pristine C60 is given by k=1.76
FIG. 1. The optimized geometries for C60. There are four stable positions
for Ni dispersion at: a the center of pentagonal ring, b the center of
hexagonal ring, c the edge between pentagonal and hexagonal rings, and
d the edge between two hexagonal rings, where gray colors are for C
atoms.Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to 1017 exp −83.1kcal/mol/RTR=2.0 kcal/mol K. In this
respect, chemisorption on a pristine fullerene should occur at
a very high temperature instead of at room temperature,
which is consistent with the experimental results of Loutfy et
al.23
Contrary to our previous study on the Ni-dispersed
CNTs,6 our theoretical calculations find that each Ni atom
dispersed on a fullerene could store up to three H2 molecules
at temperatures up to room temperature, where spin-
polarization effects were completely considered. Recently,
Zhao et al. have also studied hydrogen storage properties on
nickel-doped fullerenes.24 On the other hand, it was assumed
that the metal is the most stable on the center of the pentago-
nal rings of C60. However, our first-principles calculations
prove that the most stable sites for metal dispersion are edge
sites between hexagonal rings instead of their assumed center
of pentagonal sites. In fact, there are the four distinct sites for
Ni dispersion in a C60, as shown in Fig. 1: a the center of a
pentagonal ring, b the center of a hexagonal ring, c the
edge between hexagonal and pentagonal rings, and d the
edge between two hexagonal rings. The Gibbs energy of G
for Ni adsorption is calculated by a following equation:
GNi adsorption = H − TS , 2
where H and S are the enthalpy and entropy changes,
respectively. The lowest enthalpy structure is found to be for
the case d, with the case c 10.4 kcal/mol, the case a
15.7 kcal/mol, and the case b 16.6 kcal/mol higher. Con-
sequently, this indicates that the case d corresponds to the
global minimum state. On the other hand, the largest diffu-
sion barrier of Ni diffusion from the local minima in the
above to the global minimum state d is determined to be
very small 6.3 kcal/mol, thus implying that Ni in the local
minimum states could easily transform to the global mini-
mum state even at the very low temperatures larger than 109
K. In addition, the entropy difference of 0.03 kcal/mol K
between the global minimum d and the dissociated state
into a fullerene plus Ni atoms finds that the global minimum
state for a Ni-dispersed fullerene could be stable up to 1176
K. The exothermic adsorption energy for Ni at the most
stable position of d is 37.6 kcal/mol, and the distances be-
FIG. 2. Hydrogen adsorption energy
per H2 on the C60-Ni1-H2n cluster,
where n is 1 to 3.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
053111-3 Shin et al. Appl. Phys. Lett. 88, 053111 2006tween Ni atoms and their nearest C atom are 1.82 Å. A C60
includes two different CuC and CvC bond characters. A
CvC bond site in the edge between two hexagonal ring
sites is found to be more favorable for metal dispersion.
In addition, we find that the Mulliken charge on Ni is
0.51. Consequently, the resulting dipole moment of 5.15 D
from a Ni-C60 cluster, compared to the case of 0.002 D for
the pristine C60, is sufficient to enhance the van der Waal’s
interaction between hydrogen and Ni-C60. We also determine
the Ni adsorption energies of −47.5 kcal/mol Ni and
−54.5 kcal/mol Ni for Ni2 and Ni3 nanoparticles, respec-
tively. All Ni atoms in these cases were also found to be most
stable at the edges of the same hexagonal ring sites, as in the
case of the first Ni. The C-Ni and Ni-Ni bond distances for
C60-Ni2 are determined to be 1.82 and 2.52 Å, while the
CuNi bond and Ni-Ni bond distances for C60-Ni3 are 1.88
and 2.53 Å, respectively.
Figure 2 shows the hydrogen adsorption behaviors as
hydrogen molecules adsorbed in the C60-Ni1 cluster increase.
The physisorption energy is calculated by a following equa-
tion:
Hphysi = HtotH2 + fullerene − Htotfullerene − HtotH2
3
where HtotH2+fullerene is the enthalpy of the fullerene-H2
system at 298 K, Htot fullerene is the enthalpy for the
fullerene, and HtotH2 is the enthalpy for an optimized H2
molecule. The binding enthalpy change of the first hydrogen
molecule adsorbed by the nickel atom is determined to be
18.5 kcal/mol, which is lower than the result of Zhao et al.24
by 31.8 kcal/mol. Zhao et al. have reported that Ni could
store only one hydrogen molecule. However, our calculations
show that Ni is more likely to store up to three hydrogen
molecules rather than only one. This discrepancy appears to
be reasonable. Zhao et al. assumed that Ni is the most stable
at the center of a pentagonal ring in the fullerene, while our
results show the edge between two hexagonal rings for the
most stable site of the Ni atom, as mentioned above. More-
over, we find that three hydrogen molecules could be ad-
sorbed around each Ni with its exothermic enthalpy of
FIG. 3. Color online Hydrogenated Ni-dispersed C60. Three hydrogen
molecules could be adsorbed to a Ni atom. Carbon, nickel, and hydrogen
atoms are shown in silver, yellow, and black, respectively.Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to 11.8 kcal/mol H2. It should be noted that this H2-dispersed
fullerene is in a global minimum state. The bond distance
between hydrogen and Ni atoms ranges from 1.62 to 1.73 Å,
while HuH bonds slightly elongate with bond distances
ranging from 0.79 to 0.81 Å compared to the original H-H
bond distance of 0.73 Å, which indicates that the bonds be-
tween hydrogen and Ni have both physical and chemical
bond characters. In C60-Ni1, the dipole moment is calculated
to be 4.40 D. In the case of C60-Ni2, it is also determined that
three hydrogen molecules could be adsorbed around each Ni
atom. The adsorption energy for this case is determined to be
−9.7 kcal/mol H2, and the bond lengths between hydrogen
and Ni atoms range from 1.61 to 1.76 Å. In the C60-Ni3
cluster, each Ni atom also stores three hydrogen molecules
with the enthalpy change of −7.6 kcal/mol H2 and the bond
distances ranging from 1.60 to 1.77 Å. Consequently, these
results indicate that the Ni-dispersed C60 could store 90 hy-
drogen molecules attached to 30 Ni atoms, as shown in Fig.
3. Thus, this results in the hydrogen storage capacity is up to
6.8 wt %.
In conclusion, it was found that each Ni atom adsorbed
three hydrogen molecules. These results indicate that a Ni-
dispersed fullerene could have 6.8 wt % for its hydrogen
storage capacity, which exceeds the DOE target. This en-
hanced hydrogen storage capacity is attributed to the fact that
three H2 could be adsorbed around each polarized Ni involv-
ing the charge transfer from fullerenes.
The computational resources at the KAIST have been
supported by KOSEF Grant No. R012005000103330 and
by the KRF Grant No. 2005-005-J09703.
1L. Schlapbach and A. Züttel, Nature London 414, 353 2001.
2N. Takeichi, H. Senoh, T. Yokota, H. Tsuruta, K. Hamada, H. T. Takeshita,
H. Tanaka, T. Kiyobayashi, T. Takano, and N. Kuriyama, Int. J. Hydrogen
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10A. D. Becke, J. Chem. Phys. 98, 5648 1993.
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12A. D. McLean and G. S. Chandler, J. Chem. Phys. 72, 5639 1980; R.
Ditchfield, W. J. Hehre, and J. A. Pople, ibid. 54, 724 1971; M. J. Frisch,
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23R. O. Loutfy and E. M. Wexler, IEA Task 12 report, 2001 unpublished,
http://www.ieahia.org/pdfs/NN0094u.pdf.
24Y. Zhao, Y. H. Kim, A. C. Dillon, M. J. Heben, and S. B. Zhang, Phys.
Rev. Lett. 94, 155504 2005.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Ni-dispersed fullerenes: Hydrogen storage and desorption properties

APPLIED PHYSICS LETTERS 88, 053111 2006Ni-dispersed fullerenes: Hydrogen storage and desorption properties
Weon Ho Shin and Seong Ho Yang
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Kusung-dong 373-1, Yusung-gu, Daejon, Republic of Korea 305-701
William A. Goddard III
Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125
Jeung Ku Kanga
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology,
Kusung-dong 373-1, Yusung-gu, Daejon, Republic of Korea 305-701
Received 10 October 2005; accepted 14 December 2005; published online 31 January 2006
Our study shows that the H2 storage media using Ni-dispersed fullerenes could be viable alternatives
to reversible hydrogen storage. It is demonstrated that a single Ni coated on the fullerene surface can
store up to three H2 molecules. Consequently, at high Ni coverage, Ni-dispersed fullerenes are
considered to be the novel hydrogen storage media capable of storing 6.8 wt % H2, thus
exceeding the Department of Energy target 6.5 wt % for automobile applications. Moreover, the
H2 desorption activation barrier of 11.8 kcal/mol H2 is ideal for many practical hydrogen storage
applications. © 2006 American Institute of Physics. DOI: 10.1063/1.2168775Hydrogen, which can be readily produced from renew-
able energy sources and mainly produces water as a byprod-
uct, contains much larger chemical energy per mass
142 MJ kg−1 Ref. 1 than any hydrocarbon fuel. Conse-
quently, it is considered as an ideal fuel to solve energy and
environmental problems in the earth that result from using
petroleum fuels. Many hydrogen storage methods have been
proposed, such as carbon-fiber reinforced high-strength
containers,2 liquid hydrogen,3 chemical hydrides,4 and car-
bon nanotubes CNTs.5 In particular, CNT has been sug-
gested as the most promising nanostructure due to its cylin-
drical structure and high surface-to-volume ratio, which
make it ideal for hydrogen storage. However, recent
studies5,6 indicate that the structure of a pristine CNT is not
relevant to satisfy the Department of Energy DOE target
6.5 wt % at ambient conditions ideal for many practical
applications. This is because the pristine CNT has only the
0.8 wt % hydrogen available at ambient conditions.5 How-
ever, we propose the use of nickel-doped fullerenes as the
hydrogen storage media capable of exceeding the DOE target
6.5 wt % at ambient conditions.
The recent experiment6 proved that Ni-doped multi-
walled MW CNTs with outer diameters of 20 nm could
release 2.8 wt % H2 at ambient conditions. We found that
each Ni dispersed on the CNT surfaces has the high affinity
for hydrogen by storing up to five H2 molecules at ambient
conditions. On the other hand, it was also determined that the
insertion of H2 into the interlayer of MWCNTs is thermody-
namically unstable since it is an endothermic reaction by
24.9 kcal/mol H2. Consequently, it is considered that Ni-
dispersed MWCNTs could release over the 3 wt % if Ni
nanoparticles could be dispersed on the surfaces of the CNTs
grown in vertical shapes with reduced numbers of inter-
layers. However, the problem is that not all the techniques to
control the growth directions and interlayers are available
yet. On the other hand, unlike the CNT, a fullerene has al-
aAuthor to whom correspondence should be addressed; electronic mail:
jeungku@kaist.ac.kr
0003-6951/2006/885/053111/3/$23.00 88, 05311
Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to ways a single wall and its surface is isotropic; thus there is
no need to reduce its interlayer and growth direction. Con-
sequently, the fullerene is considered as a better nanostruc-
ture for Ni dispersion to maximize the hydrogen storage ca-
pacity than that for the CNT.
A recent study by Barajas-Barraza et al.7 showed that up
to 19 H2 molecules could be stored in a C110 fullerene struc-
ture. Moreover, Narita et al.8 showed through a molecular
dynamic simulation study that H2 is stable inside the C60
cage at ambient conditions. On the other hand, they also
found that H2 could not escape from the cage due to the high
activation barrier of 369.1 kcal/mol H2 for penetration
through the cage; thus, using the inside of the cage as a
hydrogen storage site is considered as impractical for many
applications. On the other hand, the recent study6 showed
that Ni nanoparticles dispersed give the activation barrier of
8.7 kcal/mol relevant for hydrogen desorption at ambient
conditions. In addition, a theoretical study9 by Yildirim et al.
also found that metals-dispersed nanostructures could be
good hydrogen storage materials for practical applications.
To determine electronic wave functions, we use B3LYP
Ref. 10 and KMLYP Ref. 11 density functional theory
DFT methods with a split basis set of 6–31 G valence
double zeta basis sets12 for Ni and fullerene carbon atoms
and 6–31 G** split valence double zeta basis sets12 for hy-
drogen. The B3LYP is shown to accurately predict geom-
etries and thermochemical data compared to generalized gra-
dient approximation13–17 GGA and local density
approximation LDA,18,19 while the KMLYP has been
proven to be more accurate in predicting transition state
barriers11 than other DFT methods. In addition, here our en-
ergies include zero point-energy ZPE corrections deter-
mined at fully optimized geometry with the same basis sets.
We have chosen a C60 molecule with a diameter of 7.12 Å as
the specific system for our study, as shown in Fig. 1. The
fullerene has the two distinct bond lengths of 1.46 Å for C
−C bonds and 1.40 Å for C=C bonds, which compare to the
20
experimental values of 1.43 Å and 1.39 Å, respectively.
© 2006 American Institute of Physics1-1
AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
053111-2 Shin et al. Appl. Phys. Lett. 88, 053111 2006First, we have explored the hydrogen storage capacity on
the pristine C60. The physical adsorption reaction by a H2
molecule on the outside surface of the fullerene was found to
be unstable as it is endothermic by 0.7 kcal/mol H2, while its
corresponding chemical adsorption reaction was determined
to have the exothermic enthalpy of 15.2 kcal/mol H2. How-
ever, it was determined that chemical adsorption by H2 mol-
ecules on pristine fullerenes has a high activation energy of
83.1 kcal/mol H2. Consequently, using the computed parti-
tion functions for the reactants and the transition state, the
reaction rate k of the canonical rate equation21 is determined
by
k = T
kBT
h
QTS
QAQB
exp − E0kBT  , 1
where T is the thermal tunneling coefficient,22 QTS is the
partition functional for the transition state, QA and QB are the
partition functions for the reactants A and B, respectively,
and E0 is the barrier height. The resulting dependence of
chemisorption on pristine C60 is given by k=1.76
FIG. 1. The optimized geometries for C60. There are four stable positions
for Ni dispersion at: a the center of pentagonal ring, b the center of
hexagonal ring, c the edge between pentagonal and hexagonal rings, and
d the edge between two hexagonal rings, where gray colors are for C
atoms.Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to 1017 exp −83.1kcal/mol/RTR=2.0 kcal/mol K. In this
respect, chemisorption on a pristine fullerene should occur at
a very high temperature instead of at room temperature,
which is consistent with the experimental results of Loutfy et
al.23
Contrary to our previous study on the Ni-dispersed
CNTs,6 our theoretical calculations find that each Ni atom
dispersed on a fullerene could store up to three H2 molecules
at temperatures up to room temperature, where spin-
polarization effects were completely considered. Recently,
Zhao et al. have also studied hydrogen storage properties on
nickel-doped fullerenes.24 On the other hand, it was assumed
that the metal is the most stable on the center of the pentago-
nal rings of C60. However, our first-principles calculations
prove that the most stable sites for metal dispersion are edge
sites between hexagonal rings instead of their assumed center
of pentagonal sites. In fact, there are the four distinct sites for
Ni dispersion in a C60, as shown in Fig. 1: a the center of a
pentagonal ring, b the center of a hexagonal ring, c the
edge between hexagonal and pentagonal rings, and d the
edge between two hexagonal rings. The Gibbs energy of G
for Ni adsorption is calculated by a following equation:
GNi adsorption = H − TS , 2
where H and S are the enthalpy and entropy changes,
respectively. The lowest enthalpy structure is found to be for
the case d, with the case c 10.4 kcal/mol, the case a
15.7 kcal/mol, and the case b 16.6 kcal/mol higher. Con-
sequently, this indicates that the case d corresponds to the
global minimum state. On the other hand, the largest diffu-
sion barrier of Ni diffusion from the local minima in the
above to the global minimum state d is determined to be
very small 6.3 kcal/mol, thus implying that Ni in the local
minimum states could easily transform to the global mini-
mum state even at the very low temperatures larger than 109
K. In addition, the entropy difference of 0.03 kcal/mol K
between the global minimum d and the dissociated state
into a fullerene plus Ni atoms finds that the global minimum
state for a Ni-dispersed fullerene could be stable up to 1176
K. The exothermic adsorption energy for Ni at the most
stable position of d is 37.6 kcal/mol, and the distances be-
FIG. 2. Hydrogen adsorption energy
per H2 on the C60-Ni1-H2n cluster,
where n is 1 to 3.AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
053111-3 Shin et al. Appl. Phys. Lett. 88, 053111 2006tween Ni atoms and their nearest C atom are 1.82 Å. A C60
includes two different CuC and CvC bond characters. A
CvC bond site in the edge between two hexagonal ring
sites is found to be more favorable for metal dispersion.
In addition, we find that the Mulliken charge on Ni is
0.51. Consequently, the resulting dipole moment of 5.15 D
from a Ni-C60 cluster, compared to the case of 0.002 D for
the pristine C60, is sufficient to enhance the van der Waal’s
interaction between hydrogen and Ni-C60. We also determine
the Ni adsorption energies of −47.5 kcal/mol Ni and
−54.5 kcal/mol Ni for Ni2 and Ni3 nanoparticles, respec-
tively. All Ni atoms in these cases were also found to be most
stable at the edges of the same hexagonal ring sites, as in the
case of the first Ni. The C-Ni and Ni-Ni bond distances for
C60-Ni2 are determined to be 1.82 and 2.52 Å, while the
CuNi bond and Ni-Ni bond distances for C60-Ni3 are 1.88
and 2.53 Å, respectively.
Figure 2 shows the hydrogen adsorption behaviors as
hydrogen molecules adsorbed in the C60-Ni1 cluster increase.
The physisorption energy is calculated by a following equa-
tion:
Hphysi = HtotH2 + fullerene − Htotfullerene − HtotH2
3
where HtotH2+fullerene is the enthalpy of the fullerene-H2
system at 298 K, Htot fullerene is the enthalpy for the
fullerene, and HtotH2 is the enthalpy for an optimized H2
molecule. The binding enthalpy change of the first hydrogen
molecule adsorbed by the nickel atom is determined to be
18.5 kcal/mol, which is lower than the result of Zhao et al.24
by 31.8 kcal/mol. Zhao et al. have reported that Ni could
store only one hydrogen molecule. However, our calculations
show that Ni is more likely to store up to three hydrogen
molecules rather than only one. This discrepancy appears to
be reasonable. Zhao et al. assumed that Ni is the most stable
at the center of a pentagonal ring in the fullerene, while our
results show the edge between two hexagonal rings for the
most stable site of the Ni atom, as mentioned above. More-
over, we find that three hydrogen molecules could be ad-
FIG. 3. Color online Hydrogenated Ni-dispersed C60. Three hydrogen
molecules could be adsorbed to a Ni atom. Carbon, nickel, and hydrogen
atoms are shown in silver, yellow, and black, respectively.sorbed around each Ni with its exothermic enthalpy of
Downloaded 24 Feb 2006 to 131.215.240.9. Redistribution subject to 11.8 kcal/mol H2. It should be noted that this H2-dispersed
fullerene is in a global minimum state. The bond distance
between hydrogen and Ni atoms ranges from 1.62 to 1.73 Å,
while HuH bonds slightly elongate with bond distances
ranging from 0.79 to 0.81 Å compared to the original H-H
bond distance of 0.73 Å, which indicates that the bonds be-
tween hydrogen and Ni have both physical and chemical
bond characters. In C60-Ni1, the dipole moment is calculated
to be 4.40 D. In the case of C60-Ni2, it is also determined that
three hydrogen molecules could be adsorbed around each Ni
atom. The adsorption energy for this case is determined to be
−9.7 kcal/mol H2, and the bond lengths between hydrogen
and Ni atoms range from 1.61 to 1.76 Å. In the C60-Ni3
cluster, each Ni atom also stores three hydrogen molecules
with the enthalpy change of −7.6 kcal/mol H2 and the bond
distances ranging from 1.60 to 1.77 Å. Consequently, these
results indicate that the Ni-dispersed C60 could store 90 hy-
drogen molecules attached to 30 Ni atoms, as shown in Fig.
3. Thus, this results in the hydrogen storage capacity is up to
6.8 wt %.
In conclusion, it was found that each Ni atom adsorbed
three hydrogen molecules. These results indicate that a Ni-
dispersed fullerene could have 6.8 wt % for its hydrogen
storage capacity, which exceeds the DOE target. This en-
hanced hydrogen storage capacity is attributed to the fact that
three H2 could be adsorbed around each polarized Ni involv-
ing the charge transfer from fullerenes.
The computational resources at the KAIST have been
supported by KOSEF Grant No. R012005000103330 and
by the KRF Grant No. 2005-005-J09703.
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Ni-dispersed fullerenes: Hydrogen storage and desorption properties

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