We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes (MWCNTs). The as-grown MWCNTs show continuous walls that do not provide sites for hydrogen storage under ambient conditions. However, after treating the nanotubes with oxygen plasma to create nanopores in the MWCNTs, we observed the appearance of a new hydrogen desorption peak in the 300–350 K range. Furthermore, the calculations of density functional theory and molecular dynamics simulations confirmed that this peak could be attributed to the hydrogen that is physically adsorbed inside nanopores whose diameter is approximately 1 nm. Thus, we demonstrated that 1 nm nanopores in MWCNTs offer a promising route to hydrogen storage media for onboard practical applications

Goddard, William A., III

Han, Kyu Sung

Han, Sang Soo

Kang, Jeung Ku

Kim, Hyun Seok

Lee, Hyuck Mo

Lee, Jai Young

van Duin, Adri C. T.

Woo, Seong Ihl

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS 87, 213113 2005Nanopores of carbon nanotubes as practical hydrogen storage media
Sang Soo Han, Hyun Seok Kim, Kyu Sung Han, Jai Young Lee,
Hyuck Mo Lee, and Jeung Ku Kanga
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea
Seong Ihl Woo
Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea
Adri C. T. van Duin and William A. Goddard III
Materials and Process Simulation Center, California Institute of Technology, California 91125
Received 2 June 2005; accepted 12 October 2005; published online 16 November 2005
We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes
MWCNTs. The as-grown MWCNTs show continuous walls that do not provide sites for hydrogen
storage under ambient conditions. However, after treating the nanotubes with oxygen plasma to
create nanopores in the MWCNTs, we observed the appearance of a new hydrogen desorption peak
in the 300–350 K range. Furthermore, the calculations of density functional theory and molecular
dynamics simulations confirmed that this peak could be attributed to the hydrogen that is physically
adsorbed inside nanopores whose diameter is approximately 1 nm. Thus, we demonstrated that 1 nm
nanopores in MWCNTs offer a promising route to hydrogen storage media for onboard practical
applications. © 2005 American Institute of Physics. DOI: 10.1063/1.2133928Since the first experimental report of Dillon et al.1 on
hydrogen storage in carbon nanotubes CNTs, there have
been many experimental and theoretical studies on the hy-
drogen uptake capacity in various CNT structures.2–4 Nano-
tubes with high surface-to-volume ratios are ideal for the fast
kinetics of hydrogenation and dehydrogenation. However,
despite this advantage, the use of pristine CNTs as hydrogen
storage media is costly; furthermore, due to the low adsorp-
tion temperature 80–150 K at 1 bar of hydrogen on pristine
CNTs, an intensive cryogenic process is needed for cooling.
Consequently, there remains a great challenge in storing hy-
drogen under ambient conditions that are ideal for practical
applications. Here, we report that 1 nm nanopores in CNTs
offer a promising solution to this challenge.
To synthesize multiwalled CNTs MWCNTs, we used
thermal chemical vapor deposition CVD and microwave
plasma-enhanced CVD PECVD. In the thermal CVD, we
synthesized the nanotubes by using a floating catalyst
method under a 0.04 g/ml solution of ferrocene in xylene.
After obtaining the MWCNTs on a SiO2 substrate, we etched
them immediately by using atmospheric plasma, and for the
etching gas, we added 2% of O2 to He. On the other hand,
for the PECVD, we first deposited a 50 nm thick cobalt layer
on a Si substrate by using rf-magnetron sputtering. For the
gas source, we used a mixture of H2 89.9%, CH4 0.1%,
and O2 10% with a pressure of 30 Torr and a temperature
of 750 °C. As a result, we formed open-tip MWCNTs.
We determined the hydrogen desorption properties by
analyzing thermal desorption spectra TDS. For this analy-
sis, we degassed the sample at 300 °C for 48 h under a
vacuum below 10−3 Torr, and then charged pure hydrogen
99.999% under 40 atm at 300 K. Next, we placed the
sample in a liquid nitrogen-cooled cryostat with a program-
mable heating supply apparatus. To detect the hydrogen that
evolved from the MWCNTs, we used a gas chromatograph
aAuthor to whom correspondence should be addressed; electronic mail:
jeungku@kaist.ac.kr
0003-6951/2005/8721/213113/3/$22.50 87, 21311
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject towith a temperature scanning range of 80–673 K and a rate of
3 K/min.
In addition, the density functional theory DFT5,6 and
the molecular dynamics simulation methods using a reactive
force field ReaxFF7–9 are used to determine hydrogen stor-
age and desorption properties of MWCNTs, where the MD
simulation was first performed under an NVT ensemble for
400 000 steps and then under an NVE ensemble for 400 000
steps.
Figures 1a before plasma etching and 1b after
plasma etching show hydrogen desorption spectra of the
open-tip MWCNTs, prepared by thermal CVD; the outer di-
ameters of the nanotubes are in the range of 20–50 nm and
the inner diameters are in the range of 10–20 nm. We found
that the plasma etching opens the MWCNTs tip. Before the
atmospheric plasma etching, we observed that the hydrogen
molecules evolved at only 100–150 K and that 4.9 wt % of
hydrogen was released from the peak. In contrast, as shown
in Fig. 1b, we observed an ambient temperature peak at
300–350 K after the plasma etching. In this case, 0.6 wt % of
hydrogen was released near an ambient temperature. Figure
1c shows hydrogen desorption spectra from MWCNTs syn-
thesized by PECVD with oxygen plasma. Two different
peaks occurred at temperature ranges of 100–200 K and
300–350 K. We obtained a total integrated amount of 1.8
wt % hydrogen from the first peak and 2.0 wt % hydrogen
from the second peak. In Figs. 1a and 1c, the peak in the
low-temperature range evolved at 100–150 K and the second
peak evolved in the range of 300–350 K, which consequently
implies that the hydrogen adsorption site for each peak is the
same irrespective of the synthesis method. And Fig. 1d
shows a high-resolution transmission electron microscope
HRTEM image of an open-tip MWCNT synthesized by
PECVD. The outer diameters ranged from 10 to 30 nm and
the inner diameters ranged from 5 to 15 nm. In our MWCNT
sample, the defective slits, shown as circles in Fig. 1d, were
estimated to be approximately 1 nm in the tube wall. Our
© 2005 American Institute of Physics3-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
circle
213113-2 Han et al. Appl. Phys. Lett. 87, 213113 2005observation of these nanopores is consistent with the findings
of Gao et al.10 and Hou et al.11
The TDS data of Figs. 1a–1c could not give atomistic
details for the hydrogen adsorption sites. Hence, we used
DFT and MD simulations to gain a better understanding of
the hydrogen desorption mechanisms of the nanopores in the
MWCNTs. The hydrogen that is chemisorbed with CNT can-
not be released near room temperature.3 Thus, the peaks in
Figs. 1a–1c involve hydrogen that has been physically
adsorbed in the MWCNT. Table I summarizes the physisorp-
tion energies of hydrogen on various sites of 10,0 CNT.
The exothermic enthalpy of about 1 kcal/mol indicates that,
from van’t Hoff’s equation,12 the hydrogen adsorption on or
in pristine CNTs is possible only at temperatures ranging
from 50–80 K at 0.1 MPa. In addition, the vacancy and edge
carbon atoms can enhance the exothermic enthalpies of hy-
drogen adsorption; thus, the adsorption temperatures in these
TABLE I. DFT results for the change of heat kcal/mol in hydrogen ad-
sorption on several sites of the CNT and estimated adsorption temperatures
K at 0.1 and 4 MPa. Cases A, B, C, D, and E mean physisorption
of H2 on the exterior nanotube wall, on the interior nanotube wall, on a
vacancy in the exterior wall, on a vacancy in the interior wall, and on edge
atoms on the CNT tip, respectively.
Case A B C D E
Adsorption energy –0.8 –1.4 –2.1 –2.6 –2.4
Temperature 0.1 MPa 47.0 82.0 124.0 153.0 141.0
Temperature 4.0 MPa 83.0 145.0 218.0 270.0 249.0
FIG. 1. TDS of MWCNTs synthesized by thermal CVD a before and b a
and d a HRTEM image for MWCNTs synthesized by PECVD where theDownloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject tosites increase to 120–150 K at 0.1 MPa. And, at 4.0 MPa,
hydrogen adsorption may occur on vacancy sites at tempera-
tures up to 200 K. Through MD simulations at different tem-
peratures, we determined the hydrogen adsorption and de-
sorption properties from the open tips of the CNTs. H2 is
incapable of being stored inside the 10,0 CNT tip up to 100
K, and this finding is consistent with the DFT results in Table
I. The H2 molecule, which is sucked into the open tip within
2.0 ps because of the nanocapillary phenomenon, moves
from the CNT tip to other sites via several movements, after
which it is finally stored inside the tube. The H2 molecule
stored in the open-tip does not escape from the nanotube,
tmospheric plasma treatment, c TDS of MWCNTs synthesized by PECVD
s indicate the nanopores with 1 nm size existing in the MWCNT wall.
FIG. 2. DFT results on hydrogen adsorption energies in the interlayer spacefter aof the graphite as a function of the interlayer distance.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
213113-3 Han et al. Appl. Phys. Lett. 87, 213113 2005even after 200 ps in the NVT ensemble condition; nor does it
escape after an additional 200 ps in the NVE ensemble. Re-
garding the desorption behavior of the hydrogen molecules
stored inside the CNT, we found that the hydrogen escapes
from the CNT at 150 K. To determine how the CNT diameter
affects the adsorption and desorption of hydrogen, we also
considered a 10,10 CNT with a diameter of 13.6 Å, which
is almost twice as large as the diameter of the 10,0 CNT.
The H2 molecule is absorbed inside the tube at 100 K and
desorbs at 140 K, thereby indicating that the hydrogen ad-
sorption and desorption temperatures are similar for various
CNT diameters whenever the diameters are much larger than
1 nm. In exploring the mechanisms of hydrogen adsorption
and desorption on the exterior wall, we found that H2 was
adsorbed on the exterior wall of the 10,0 SWCNT at 60 K
and released from the CNT at 80 K irrespective of the CNT
diameter. We conclude therefore that the first peak that was
experimentally observed at 100–200 K was mostly due to the
desorption of the hydrogen stored in the MWCNT hole. Our
analysis of Raman spectra also indicates that the crystallinity
degree of graphitization of the MWCNTs prepared by
PECVD is lower than that of the MWCNTs prepared by
thermal CVD. Thus, we consider that the pure graphene wall
area of the nanotubes synthesized by PECVD is smaller than
that synthesized by thermal CVD. This graphene area is pro-
portional to the amount of hydrogen adsorbed at the lower
peak. Consequently, as reflected by the first peak, the hydro-
gen storage capacity of the MWCNTs prepared by thermal
CVD is higher than that of MWCNTs prepared by PECVD.
In addition, the low-temperature peak of Fig. 1c is broader
than the corresponding peaks of Figs. 1a and 1b, possibly
because the velocity distribution of the hydrogen that
evolved from MWCNTs prepared by PECVD is more ran-
dom than the velocity distribution of the hydrogen that
evolved from MWCNTs prepared by thermal CVD.
We also investigated why the second peaks in Figs. 1b
and 1c occurred at around room temperature. Other re-
searchers have suggested that hydrogen molecules could be
stored in the intertube hollow of MWCNTs.2,13. In the DFT
calculations see Fig. 2, we used the graphite layers to
model the intertube layers in the MWCNT. At an interlayer
distance of 3.36 Å, we determined that hydrogen is thermo-
dynamically unstable because the binding energy of the hy-
drogen in the interlayer is endothermic by 59.94 kcal/mol.
This result implies that the H2 molecule cannot plausibly be
stored in the intertube of a normal MWCNT. Thus, we can-
not attribute the second peak near room temperature, as
shown in Figs. 1b and 1c, to the hydrogen stored in the
intertube of the MWCNT. Figure 2, on the other hand, shows
that hydrogen could be stable in the interlayer space of the
graphite or in the intertube of the MWCNT before the inter-
layer or intertube distances become 6 Å because the binding
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject toenergy of the hydrogen adsorption is exothermic by
2.84 kcal/mol.
As seen in Fig. 3a, we also used four layers of a graph-
ite structure trigonal with a=b=14.66 Å, c=40 Å, and 
=60° for the initial structure of the MD simulation, where a
nanopore of about 1 nm into the graphite was introduced. At
the start, we set 15 H2 molecules to be stored inside the
nanopore. To study the desorption temperature of the hydro-
gen molecules initially stored in the nanopore, we used an
NVT-MD simulation for the initial system of Fig. 3a at four
simulation temperatures of 200 K, 250 K, 300 K, and 350 K.
Below 250 K, as depicted in Figs. 3b and 3c, we found
that none of the hydrogen stored in the nanopore was re-
leased until 200 ps of the NVE-MD simulation; moreover,
the hydrogen only started to escape from the nanopore at 300
K. Figure 3 therefore indicates that the second peaks at room
temperature in Figs. 1b and 1c are attributed to the hy-
drogen desorption from the nanopores of about 1 nm.
In conclusion, we find that any nanopores of about 1 nm
that exist in MWCNT walls are available for hydrogen ad-
sorption sustainable up to room temperature. Thus, we have
demonstrated that nanopores of this size offer a promising
route to hydrogen storage sites for practical hydrogen storage
media.
This research was supported by the research program of
KOSEF Grant No. R012005000103330 and the Center for
Ultramicrochemical Process Systems CUPS sponsored by
KOSEF 2005.
1A. C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H. Klang, D. S. Bethune,
and M. J. Heben, Nature London 386, 377 1997.
2A. Chambers, C. Park, R. Terry, and K. Baker, J. Phys. Chem. B 102,
4254 1998.
3H. Lee, Y.–S. Kang, S.–H. Kim, and J.–Y. Lee, Appl. Phys. Lett. 80, 577
2002.
4S. S. Han and H. M. Lee, Carbon 42, 2169 2004.
5J. P. Perdew and Y. Wang, Phys. Rev. B 45, 13244 1992.
6B. Delly, J. Chem. Phys. 113, 7756 2000.
7A. C. T. van Duin, S. Dasgupta, F. Lorant, and W. A. Goddard III, J. Phys.
Chem. A 105, 9396 2001.
8K. D. Nielson, A. C. T. van Duin, J. Oxgaard, W.-Q. Deng, and W. A.
Goddard III, J. Phys. Chem. A 107, 3803 2003.
9S. S. Han, J. K. Kang, H. M. Lee, A. C. T. van Duin, and W. A. Goddard
III, Appl. Phys. Lett. 86, 203108 2005.
10H. Gao, X. B. Wu, J. T. Li, G. T. Wu, J. Y. Lin, K. Wu, and D. S. Xu,
Appl. Phys. Lett. 83, 3389 2003.
11P.-X. Hou, S.-T. Xu, Z. Ying, Q.-H. Yang, C. Liu, and H.-M. Cheng,
Carbon 41, 2471 2003.
12The van’t Hoff equation is given as ln p=−H /RT+S /R, where H is
the heat of adsorption, R is the gas constant, and S is the change in
entropy. We found that the S is about −0.02 kcal/molK at ambient con-
ditions.
13C. Park, P. E. Anderson, A. Chambers, C. D. Tan, R. Hidalgo, and N. M.
FIG. 3. Desorption behaviors of hydrogen stored in the
nanopore with size of 1 nm, obtained by MD simula-
tions. Here, configuration a represents the initial struc-
ture, while b and c show the structures after 200 ps
at 250 and 300 K, respectively.Rodriguez, J. Phys. Chem. B 103, 10572 1999.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


Nanopores of carbon nanotubes as practical hydrogen storage media

APPLIED PHYSICS LETTERS 87, 213113 2005Nanopores of carbon nanotubes as practical hydrogen storage media
Sang Soo Han, Hyun Seok Kim, Kyu Sung Han, Jai Young Lee,
Hyuck Mo Lee, and Jeung Ku Kanga
Department of Materials Science and Engineering, KAIST, Daejeon 305-701, Republic of Korea
Seong Ihl Woo
Department of Chemical and Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea
Adri C. T. van Duin and William A. Goddard III
Materials and Process Simulation Center, California Institute of Technology, California 91125
Received 2 June 2005; accepted 12 October 2005; published online 16 November 2005
We report on hydrogen desorption mechanisms in the nanopores of multiwalled carbon nanotubes
MWCNTs. The as-grown MWCNTs show continuous walls that do not provide sites for hydrogen
storage under ambient conditions. However, after treating the nanotubes with oxygen plasma to
create nanopores in the MWCNTs, we observed the appearance of a new hydrogen desorption peak
in the 300–350 K range. Furthermore, the calculations of density functional theory and molecular
dynamics simulations confirmed that this peak could be attributed to the hydrogen that is physically
adsorbed inside nanopores whose diameter is approximately 1 nm. Thus, we demonstrated that 1 nm
nanopores in MWCNTs offer a promising route to hydrogen storage media for onboard practical
applications. © 2005 American Institute of Physics. DOI: 10.1063/1.2133928Since the first experimental report of Dillon et al.1 on
hydrogen storage in carbon nanotubes CNTs, there have
been many experimental and theoretical studies on the hy-
drogen uptake capacity in various CNT structures.2–4 Nano-
tubes with high surface-to-volume ratios are ideal for the fast
kinetics of hydrogenation and dehydrogenation. However,
despite this advantage, the use of pristine CNTs as hydrogen
storage media is costly; furthermore, due to the low adsorp-
tion temperature 80–150 K at 1 bar of hydrogen on pristine
CNTs, an intensive cryogenic process is needed for cooling.
Consequently, there remains a great challenge in storing hy-
drogen under ambient conditions that are ideal for practical
applications. Here, we report that 1 nm nanopores in CNTs
offer a promising solution to this challenge.
To synthesize multiwalled CNTs MWCNTs, we used
thermal chemical vapor deposition CVD and microwave
plasma-enhanced CVD PECVD. In the thermal CVD, we
synthesized the nanotubes by using a floating catalyst
method under a 0.04 g/ml solution of ferrocene in xylene.
After obtaining the MWCNTs on a SiO2 substrate, we etched
them immediately by using atmospheric plasma, and for the
etching gas, we added 2% of O2 to He. On the other hand,
for the PECVD, we first deposited a 50 nm thick cobalt layer
on a Si substrate by using rf-magnetron sputtering. For the
gas source, we used a mixture of H2 89.9%, CH4 0.1%,
and O2 10% with a pressure of 30 Torr and a temperature
of 750 °C. As a result, we formed open-tip MWCNTs.
We determined the hydrogen desorption properties by
analyzing thermal desorption spectra TDS. For this analy-
sis, we degassed the sample at 300 °C for 48 h under a
vacuum below 10−3 Torr, and then charged pure hydrogen
99.999% under 40 atm at 300 K. Next, we placed the
sample in a liquid nitrogen-cooled cryostat with a program-
mable heating supply apparatus. To detect the hydrogen that
evolved from the MWCNTs, we used a gas chromatograph
aAuthor to whom correspondence should be addressed; electronic mail:
jeungku@kaist.ac.kr
0003-6951/2005/8721/213113/3/$22.50 87, 21311
Downloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject towith a temperature scanning range of 80–673 K and a rate of
3 K/min.
In addition, the density functional theory DFT5,6 and
the molecular dynamics simulation methods using a reactive
force field ReaxFF7–9 are used to determine hydrogen stor-
age and desorption properties of MWCNTs, where the MD
simulation was first performed under an NVT ensemble for
400 000 steps and then under an NVE ensemble for 400 000
steps.
Figures 1a before plasma etching and 1b after
plasma etching show hydrogen desorption spectra of the
open-tip MWCNTs, prepared by thermal CVD; the outer di-
ameters of the nanotubes are in the range of 20–50 nm and
the inner diameters are in the range of 10–20 nm. We found
that the plasma etching opens the MWCNTs tip. Before the
atmospheric plasma etching, we observed that the hydrogen
molecules evolved at only 100–150 K and that 4.9 wt % of
hydrogen was released from the peak. In contrast, as shown
in Fig. 1b, we observed an ambient temperature peak at
300–350 K after the plasma etching. In this case, 0.6 wt % of
hydrogen was released near an ambient temperature. Figure
1c shows hydrogen desorption spectra from MWCNTs syn-
thesized by PECVD with oxygen plasma. Two different
peaks occurred at temperature ranges of 100–200 K and
300–350 K. We obtained a total integrated amount of 1.8
wt % hydrogen from the first peak and 2.0 wt % hydrogen
from the second peak. In Figs. 1a and 1c, the peak in the
low-temperature range evolved at 100–150 K and the second
peak evolved in the range of 300–350 K, which consequently
implies that the hydrogen adsorption site for each peak is the
same irrespective of the synthesis method. And Fig. 1d
shows a high-resolution transmission electron microscope
HRTEM image of an open-tip MWCNT synthesized by
PECVD. The outer diameters ranged from 10 to 30 nm and
the inner diameters ranged from 5 to 15 nm. In our MWCNT
sample, the defective slits, shown as circles in Fig. 1d, were
estimated to be approximately 1 nm in the tube wall. Our
© 2005 American Institute of Physics3-1
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
circle
213113-2 Han et al. Appl. Phys. Lett. 87, 213113 2005observation of these nanopores is consistent with the findings
of Gao et al.10 and Hou et al.11
The TDS data of Figs. 1a–1c could not give atomistic
details for the hydrogen adsorption sites. Hence, we used
DFT and MD simulations to gain a better understanding of
the hydrogen desorption mechanisms of the nanopores in the
MWCNTs. The hydrogen that is chemisorbed with CNT can-
not be released near room temperature.3 Thus, the peaks in
Figs. 1a–1c involve hydrogen that has been physically
adsorbed in the MWCNT. Table I summarizes the physisorp-
tion energies of hydrogen on various sites of 10,0 CNT.
The exothermic enthalpy of about 1 kcal/mol indicates that,
from van’t Hoff’s equation,12 the hydrogen adsorption on or
in pristine CNTs is possible only at temperatures ranging
from 50–80 K at 0.1 MPa. In addition, the vacancy and edge
carbon atoms can enhance the exothermic enthalpies of hy-
drogen adsorption; thus, the adsorption temperatures in these
TABLE I. DFT results for the change of heat kcal/mol in hydrogen ad-
sorption on several sites of the CNT and estimated adsorption temperatures
K at 0.1 and 4 MPa. Cases A, B, C, D, and E mean physisorption
of H2 on the exterior nanotube wall, on the interior nanotube wall, on a
vacancy in the exterior wall, on a vacancy in the interior wall, and on edge
atoms on the CNT tip, respectively.
Case A B C D E
Adsorption energy –0.8 –1.4 –2.1 –2.6 –2.4
Temperature 0.1 MPa 47.0 82.0 124.0 153.0 141.0
Temperature 4.0 MPa 83.0 145.0 218.0 270.0 249.0
FIG. 1. TDS of MWCNTs synthesized by thermal CVD a before and b a
and d a HRTEM image for MWCNTs synthesized by PECVD where theDownloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject tosites increase to 120–150 K at 0.1 MPa. And, at 4.0 MPa,
hydrogen adsorption may occur on vacancy sites at tempera-
tures up to 200 K. Through MD simulations at different tem-
peratures, we determined the hydrogen adsorption and de-
sorption properties from the open tips of the CNTs. H2 is
incapable of being stored inside the 10,0 CNT tip up to 100
K, and this finding is consistent with the DFT results in Table
I. The H2 molecule, which is sucked into the open tip within
2.0 ps because of the nanocapillary phenomenon, moves
from the CNT tip to other sites via several movements, after
which it is finally stored inside the tube. The H2 molecule
stored in the open-tip does not escape from the nanotube,
tmospheric plasma treatment, c TDS of MWCNTs synthesized by PECVD
s indicate the nanopores with 1 nm size existing in the MWCNT wall.
FIG. 2. DFT results on hydrogen adsorption energies in the interlayer spacefter aof the graphite as a function of the interlayer distance.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
213113-3 Han et al. Appl. Phys. Lett. 87, 213113 2005even after 200 ps in the NVT ensemble condition; nor does it
escape after an additional 200 ps in the NVE ensemble. Re-
garding the desorption behavior of the hydrogen molecules
stored inside the CNT, we found that the hydrogen escapes
from the CNT at 150 K. To determine how the CNT diameter
affects the adsorption and desorption of hydrogen, we also
considered a 10,10 CNT with a diameter of 13.6 Å, which
is almost twice as large as the diameter of the 10,0 CNT.
The H2 molecule is absorbed inside the tube at 100 K and
desorbs at 140 K, thereby indicating that the hydrogen ad-
sorption and desorption temperatures are similar for various
CNT diameters whenever the diameters are much larger than
1 nm. In exploring the mechanisms of hydrogen adsorption
and desorption on the exterior wall, we found that H2 was
adsorbed on the exterior wall of the 10,0 SWCNT at 60 K
and released from the CNT at 80 K irrespective of the CNT
diameter. We conclude therefore that the first peak that was
experimentally observed at 100–200 K was mostly due to the
desorption of the hydrogen stored in the MWCNT hole. Our
analysis of Raman spectra also indicates that the crystallinity
degree of graphitization of the MWCNTs prepared by
PECVD is lower than that of the MWCNTs prepared by
thermal CVD. Thus, we consider that the pure graphene wall
area of the nanotubes synthesized by PECVD is smaller than
that synthesized by thermal CVD. This graphene area is pro-
portional to the amount of hydrogen adsorbed at the lower
peak. Consequently, as reflected by the first peak, the hydro-
gen storage capacity of the MWCNTs prepared by thermal
CVD is higher than that of MWCNTs prepared by PECVD.
In addition, the low-temperature peak of Fig. 1c is broader
than the corresponding peaks of Figs. 1a and 1b, possibly
because the velocity distribution of the hydrogen that
evolved from MWCNTs prepared by PECVD is more ran-
dom than the velocity distribution of the hydrogen that
evolved from MWCNTs prepared by thermal CVD.
We also investigated why the second peaks in Figs. 1b
and 1c occurred at around room temperature. Other re-
searchers have suggested that hydrogen molecules could be
stored in the intertube hollow of MWCNTs.2,13. In the DFT
calculations see Fig. 2, we used the graphite layers to
model the intertube layers in the MWCNT. At an interlayer
distance of 3.36 Å, we determined that hydrogen is thermo-
dynamically unstable because the binding energy of the hy-
drogen in the interlayer is endothermic by 59.94 kcal/mol.
This result implies that the H2 molecule cannot plausibly be
stored in the intertube of a normal MWCNT. Thus, we can-
not attribute the second peak near room temperature, as
shown in Figs. 1b and 1c, to the hydrogen stored in the
intertube of the MWCNT. Figure 2, on the other hand, shows
that hydrogen could be stable in the interlayer space of the
graphite or in the intertube of the MWCNT before the inter-
layer or intertube distances become 6 Å because the bindingDownloaded 14 Dec 2005 to 131.215.225.171. Redistribution subject toenergy of the hydrogen adsorption is exothermic by
2.84 kcal/mol.
As seen in Fig. 3a, we also used four layers of a graph-
ite structure trigonal with a=b=14.66 Å, c=40 Å, and 
=60° for the initial structure of the MD simulation, where a
nanopore of about 1 nm into the graphite was introduced. At
the start, we set 15 H2 molecules to be stored inside the
nanopore. To study the desorption temperature of the hydro-
gen molecules initially stored in the nanopore, we used an
NVT-MD simulation for the initial system of Fig. 3a at four
simulation temperatures of 200 K, 250 K, 300 K, and 350 K.
Below 250 K, as depicted in Figs. 3b and 3c, we found
that none of the hydrogen stored in the nanopore was re-
leased until 200 ps of the NVE-MD simulation; moreover,
the hydrogen only started to escape from the nanopore at 300
K. Figure 3 therefore indicates that the second peaks at room
temperature in Figs. 1b and 1c are attributed to the hy-
drogen desorption from the nanopores of about 1 nm.
In conclusion, we find that any nanopores of about 1 nm
that exist in MWCNT walls are available for hydrogen ad-
sorption sustainable up to room temperature. Thus, we have
demonstrated that nanopores of this size offer a promising
route to hydrogen storage sites for practical hydrogen storage
media.
This research was supported by the research program of
KOSEF Grant No. R012005000103330 and the Center for
Ultramicrochemical Process Systems CUPS sponsored by
KOSEF 2005.
1A. C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H. Klang, D. S. Bethune,
and M. J. Heben, Nature London 386, 377 1997.
2A. Chambers, C. Park, R. Terry, and K. Baker, J. Phys. Chem. B 102,
4254 1998.
3H. Lee, Y.–S. Kang, S.–H. Kim, and J.–Y. Lee, Appl. Phys. Lett. 80, 577
2002.
4S. S. Han and H. M. Lee, Carbon 42, 2169 2004.
5J. P. Perdew and Y. Wang, Phys. Rev. B 45, 13244 1992.
6B. Delly, J. Chem. Phys. 113, 7756 2000.
7A. C. T. van Duin, S. Dasgupta, F. Lorant, and W. A. Goddard III, J. Phys.
Chem. A 105, 9396 2001.
8K. D. Nielson, A. C. T. van Duin, J. Oxgaard, W.-Q. Deng, and W. A.
Goddard III, J. Phys. Chem. A 107, 3803 2003.
9S. S. Han, J. K. Kang, H. M. Lee, A. C. T. van Duin, and W. A. Goddard
III, Appl. Phys. Lett. 86, 203108 2005.
10H. Gao, X. B. Wu, J. T. Li, G. T. Wu, J. Y. Lin, K. Wu, and D. S. Xu,
Appl. Phys. Lett. 83, 3389 2003.
11P.-X. Hou, S.-T. Xu, Z. Ying, Q.-H. Yang, C. Liu, and H.-M. Cheng,
Carbon 41, 2471 2003.
12The van’t Hoff equation is given as ln p=−H /RT+S /R, where H is
the heat of adsorption, R is the gas constant, and S is the change in
entropy. We found that the S is about −0.02 kcal/molK at ambient con-
ditions.
13C. Park, P. E. Anderson, A. Chambers, C. D. Tan, R. Hidalgo, and N. M.
Rodriguez, J. Phys. Chem. B 103, 10572 1999.
FIG. 3. Desorption behaviors of hydrogen stored in the
nanopore with size of 1 nm, obtained by MD simula-
tions. Here, configuration a represents the initial struc-
ture, while b and c show the structures after 200 ps
at 250 and 300 K, respectively. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp


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Nanopores of carbon nanotubes as practical hydrogen storage media

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