hydrogen storage in carbon nanotubes revisited
TRANSCRIPT
C A R B O N 4 8 ( 2 0 1 0 ) 4 5 2 – 4 5 5
. sc iencedi rec t .com
avai lab le at wwwjournal homepage: www.elsev ier .com/ locate /carbon
Hydrogen storage in carbon nanotubes revisited
Chang Liu, Yong Chen, Cheng-Zhang Wu, Shi-Tao Xu, Hui-Ming Cheng *
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,
72 Wenhua Road, Shenyang 110016, PR China
A R T I C L E I N F O
Article history:
Received 6 March 2009
Accepted 17 September 2009
Available online 23 September 2009
0008-6223/$ - see front matter � 2009 Elsevidoi:10.1016/j.carbon.2009.09.060
* Corresponding author: Fax: +86 24 2390 312E-mail address: [email protected] (H.-M. C
A B S T R A C T
The reported hydrogen uptake of carbon nanotubes (CNTs) has been the subject of much
controversy. We have measured the hydrogen uptake capacity of different types of CNTs
using a volumetric measurement setup specifically-designed for CNTs. It was found that
under a pressure of �12 MPa and at room temperature, the hydrogen storage capacity of
the CNTs is less than 1.7 wt.%, which is far below the benchmark set for on-board hydrogen
storage systems by the US Department of Energy. These results suggest that it is no longer
worth investigating hydrogen uptake in pure CNTs for on-board applications. However, our
recent research indicates that CNTs can be an effective additive to some other hydrogen
storage materials to improve their kinetics.
� 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Carbon nanotubes (CNTs) possess a unique hollow tubular
structure, large surface area, and desirable chemical and ther-
mal stability. Therefore, they are considered as a promising
candidate for gas adsorption. In 1997, Dillon et al. reported
that single-walled CNTs (SWCNTs) could store �10 wt.%
hydrogen at room temperature, and predicted a possibility
to fulfill the benchmark set for on-board hydrogen storage
systems by the US Department of Energy (DOE) [1]. Soon after
this work, other optimistic results of hydrogen storage in
CNTs were reported [2–4], including our work in 1999 with a
4.2 wt.% capacity for purified SWCNTs prepared by a hydro-
gen arc discharge method measured under a pressure of 10–
12 MPa and at room temperature using a Sievelt apparatus
of the metal hydride laboratory at our institute [4]. In the
beginning, the hydrogen storage results from both theoretical
predictions and experimental studies were rather optimistic,
and there were great expectations of CNTs becoming an ideal
hydrogen carrier which had been sought for tens of years.
Nevertheless, a few years later, very low hydrogen storage
capacity of CNTs started to emerge, in particular, those exper-
imentally obtained at room temperature [5–8]. For example,
Tibbetts et al. studied the sorption of hydrogen in nine types
er Ltd. All rights reserved
6.heng).
of SWCNT, MWCNT, carbon fiber, and carbon filament sam-
ples, and found that their hydrogen storage capacity is lower
than 0.1 wt.% at room temperature and 3.5 MPa [5]. Actually,
the reproducibility of the reported high hydrogen capacity of
CNTs is poor, and the mechanism of how hydrogen is stored
in CNTs remains unclear. Thus, Baughman et al. pointed out
that ‘‘the application of CNTs in hydrogen storage is clouded
by controversy’’ [9]. Despite the ongoing debate, theoretical
and experimental studies on hydrogen storage of CNTs and
CNT-based hybrid structures have been continually con-
ducted very recently [10–14]. Fan et al. reported that CNT’s
curvature plays an important role in the physisorption of
hydrogen, and SWCNTs with diameters of 6–7 A are energet-
ically optimal candidates for physisorption of molecular
hydrogen [10]. Leonard et al. experimentally prepared a scaf-
fold structure by swelling SWCNT bundles and cross-linking
the open structures, and found that the SWCNT scaffold
physisorbs twice as much hydrogen per unit surface area as
do typical macroporous carbon materials [11]. In a review
on the topic published in 2005, we systematically analyzed
the possible factors that may influence the hydrogen storage
performance of CNTs and consequently cause the controver-
sies, and pointed out that it is necessary to investigate the
relations between the hydrogen storage performance and
.
C A R B O N 4 8 ( 2 0 1 0 ) 4 5 2 – 4 5 5 453
structures of CNTs and to perform cross-checking at different
laboratories using reliable measurement apparatus [15]. Fol-
lowing the above consideration, we have prepared various
CNT samples and measured their hydrogen storage capacity
using an equipment specially designed for CNT samples.
And the results obtained show that although a certain
amount of hydrogen can be stored in CNTs, the reliable
hydrogen storage capacity of CNTs is less than 1.7 wt.% under
a pressure of around 12 MPa and at room temperature, which
indicates that CNTs cannot fulfill the benchmark set for on-
board hydrogen storage systems by DOE. This work may shed
light on the realistic image of hydrogen storage in CNTs and
suggest that it is no longer worthy of further investigations
on hydrogen uptake in pure CNTs for on-board application.
2. Experimental
The SWCNTs and MWCNTs were synthesized by a hydrogen
arc discharge method [16] and by a floating catalyst chemical
vapor deposition method [17], respectively. The as-prepared
SWCNTs were purified using a multi-step method, which in-
volves mechanical crushing, nitric acid treatment, H2O2
refluxing, sonication in a NaOH solution, and de-ionized
water washing. The average diameter of the SWCNTs was
1.8 nm and the mean diameter of the MWCNTs used was
about 30 nm. Three different treatments for MWCNTs were
used: (a) the as-prepared MWCNTs were purified by air oxida-
tion at 900 �C followed by washing in hydrochloric acid; (b)
these purified MWCNTs were mixed with KOH powder and
heat-treated at 850 �C; (c) the as-prepared MWCNTs were
heat-treated at 850 �C under a CO2 atmosphere and then
washed with hydrochloric acid. The MWCNTs thus obtained
possess enriched structural defects and significantly im-
proved surface areas [18], which may enhance their hydrogen
storage capability. The detailed treatment procedure and
characterization of the samples are presented in a previous
paper [18].
The hydrogen storage capacities of the CNT samples were
investigated by a volumetric method at room temperature
and moderate high pressure (�12 MPa) using a specially de-
signed and built Sievelt apparatus. The amount of CNT sam-
ples used for each measurement was no less than 200 mg.
Fig. 1 – TEM images of th
3. Results and discussion
We examined the hydrogen storage properties of many types
of CNTs, both single-walled (SWCNTs) and multi-walled
(MWCNTs), in the as-prepared and different post-treatment
states. For simplicity, we here just give the results of some
representative CNT samples. The typical TEM images of the
purified SWCNTs are shown in Fig. 1. In Fig. 1a, we can see
that the SWCNTs exist as bundles with diameters of tens of
nanometers. Some SWCNTs at the periphery of the bundles
are damaged due to the etching effect of the H2O2 and nitric
acid treatment during the purification process, as shown in
Fig. 1b. The purity of the resultant sample was estimated to
be about 95% based on thermal gravimetric analysis and
transmission electron microscopy (TEM) observations. The
specific surface areas of the SWCNTs before and after purifi-
cation were measured to be 65 m2/g and 229 m2/g,
respectively.
Initially, many measurements of hydrogen storage prop-
erty of CNTs, including those by our group, were performed
using the apparatus built up for measuring metal hydrides.
A distinct difference between the measurements of CNT and
metal hydride samples is that the amount of the CNTs used
is much less than that of metal hydrides, due to the small
sample chamber volume for metal hydrides, and low packing
density and limited availability of CNTs. Thus, it is gradually
understood that system errors caused by temperature fluctu-
ation, pressure precision, and determination of sample vol-
ume are crucial for precisely determining the hydrogen
storage capacity of CNTs, although they may be negligible
for metal hydrides. To achieve a high measurement accuracy
for CNTs, the following points must be considered: the pres-
sure and temperature should be precisely monitored and
measured, the system should be leak free, and an enough
large amount of samples should be used. Therefore, we spe-
cially designed and built a Sievelt apparatus for CNT mea-
surements. We used pressure sensors (Rosemount 0.075%
FS) with a resolution of 10�4 MPa and temperature sensors
with a precision of ±0.01 K. The leakage rate of the valves
(Swagelok) was lower than 4 · 10�9 cm3/s. The testing was
controlled and programmed by a personal computer to
avoid possible errors caused by manual operations. Blank
e purified SWCNTs.
454 C A R B O N 4 8 ( 2 0 1 0 ) 4 5 2 – 4 5 5
experiments were performed by charging H2 into the system
to a pressure of about 12 MPa, and the pressure and tempera-
ture were monitored for a 120-h run. In Fig. 2a, we show the
typical curves of hydrogen pressure and temperature versus
time. We can see that both H2 pressure and temperature only
change very slightly. Since there was no adsorbent loaded in a
blank experiment, the pressure change is attributed to the
room temperature fluctuation. Based on the recorded pres-
sures and temperatures, we calculated the total amount of
hydrogen gas contained in the system, and the results are
shown in Fig. 2b. It can be seen that the amount of hydrogen
is almost unchanged. A total leakage rate of the whole system
was calculated from the linear fit to be 3.02 · 10�8 mmol/s,
which indicates that the error caused by leakage for a 10-h
test is less than 0.001%, and is therefore negligible. The equip-
ment was further calibrated by measuring LaNi5, a traditional
hydrogen storage material with a quite stable hydrogen stor-
age capacity. A result of 1.4 wt.% was obtained reproducibly,
which further confirms the precision and reliability of our
apparatus.
We list the experimental conditions and the results of rep-
resentative CNT samples in Table 1. Six samples, the as-pre-
pared SWCNTs, purified SWCNTs, as-prepared MWCNTs, air
oxidized MWCNTs, KOH activated MWCNTs, and CO2 oxi-
dized MWCNTs are included. We can see from Table 1 that
under a pressure of about 12 MPa and a temperature of
around 20 �C, the purified and post-treated CNT samples dis-
play hydrogen storage capacities of 0.9–1.7 wt.%. In fact, we
also measured three of our CNT samples using an apparatus
built for accurately determining the hydrogen storage capac-
ity of carbonaceous materials by Kiyobayashi et al. [19], and
the results were in the range of 0.7–0.9 wt.%. We also found
0 3 6 9 1212.06212.06412.06612.06812.07012.07212.07412.07612.07812.080
19.20
19.25
19.30
19.35
19.40
19.45
19.50
hydr
ogen
pre
ssur
e (M
Pa)
time (h)
P
T
(a)
Fig. 2 – (a) The recorded curves of hydrogen pressure and temper
gas contained in the Sievelt apparatus.
Table 1 – The experimental conditions and the hydrogen storag
Samples Temperature (�C) Equilpress
As-prepared SWCNTs 23.0 1Purified SWCNTs 19.0 1As-prepared MWCNTs 20.5 1Air-oxidized MWCNTs 20.1 1CO2-oxidized MWCNTs 20.1 1KOH-activated MWCNTs 19.9 1
that the hydrogen gas pressure has an evident influence on
the hydrogen uptake in CNTs, that is, the hydrogen storage
capacity of CNTs increases with increasing hydrogen pressure
in the range of 0–12 MPa.
From Table 1, it can be seen that the hydrogen storage
capacity of MWCNT samples increases with the increase of
their specific surface areas. For the SWCNT and MWCNT sam-
ples with similar specific surface areas, the SWCNTs show
higher hydrogen storage capacity. This result indicates that
specific surface area is not the decisive factor determining
the hydrogen storage capacity of CNTs. Structural configura-
tions, including diameter, length, cap-opening, de-bundling,
defects, interstitial voids, and functional groups at the surface
of CNTs may also affect their hydrogen storage performance.
A precise control of all these features of CNT structures is, at
present, impossible to realize, so the systematic relationship
between the structure and hydrogen storage capability of
CNTs is not clear yet.
The hydrogen storage results we obtained from different
CNT samples suggest that, though CNTs can adsorb or store
hydrogen, their hydrogen storage capacity is less than
1.7 wt.% under a moderately high pressure and room temper-
ature, far below the benchmark of 6.5 wt.% set by the DOE. In
consideration of the above results, our previous reported
hydrogen storage capacities were overestimated, mainly due
to the limited amount and ununiformity of CNT samples,
poor understanding on the intrinsic characteristics of CNTs
and the influence of temperature fluctuation and sample vol-
ume on the measured hydrogen storage capacity, and impro-
per measurement equipment and methodology employed at
the initial stage of the studies on hydrogen uptake in CNTs.
In fact, for the ten years since the first report on hydrogen
Tem
pera
ture
(Co )
20 40 60 80 100 120160
165
170
175
180
185
190 hydrogen content linear fit
hydr
ogen
con
tent
(mm
ol)
time (h)
(b)
ature versus time, and (b) the calculated amount of hydrogen
e capacity of representative CNT samples measured.
ibriumure (MPa)
Specific surfacearea (m2/g)
Hydrogen storagecapacity (wt.%)
2.10 65 0.52.20 229 1.72.04 66 0.22.05 270 0.92.02 429 1.01.96 785 1.2
1998 2000 2002 2004 2006 2008 2010
0
4
8
12
16H
ydro
gen
stor
age
capa
city
(wt.%
)
Year
Fig. 3 – A plot of the reported hydrogen storage capacities of
CNTs from the literature versus their year of publication.
C A R B O N 4 8 ( 2 0 1 0 ) 4 5 2 – 4 5 5 455
storage in CNTs, there is an obvious tendency that the re-
ported hydrogen storage capacity of CNTs from the literature
declines with the time extending (Fig. 3), which can be attrib-
uted to the improved CNT sample attainability and measure-
ment setup, methodology and accuracy. It is worth to note
that recent investigations from our group and others show
that CNTs are an effective additive to other hydrogen storage
materials such as metal hydride and complex compounds by
significantly improving their kinetics and capacity [20–22],
indicating that CNTs can be used for hydrogen storage in an
alternative way. These hybrid structures composed of CNTs
and materials with potential high hydrogen storage capacity
may present desirable overall hydrogen uptake performance,
where the synergistic effect of CNTs as additive, rather than
the function of pure CNTs, plays a key role.
4. Summary
Various CNT samples, including SWCNTs and MWCNTs, with
and without post-treatments, were employed for hydrogen
storage measurements by a volumetric method with a specif-
ically-designed equipment. Under a hydrogen gas pressure of
about 12 MPa and room temperature, the hydrogen storage
capacity of CNTs is less than 1.7 wt.%. These results suggest
that CNTs only have a hydrogen storage capacity far below
the DOE benchmark and be impractical for on-board hydro-
gen uptake systems. On the other hand, CNTs may be used
as an effective additive in other hydrogen storage material
systems such as metal hydrides or complex compounds by
significantly improving their kinetics and capacity.
Acknowledgements
We thank Dr. T. Kiyobayashi for his help and constructive dis-
cussions during the visit of C. Liu and S.T. Xu for measuring
some of our samples using his equipment.
R E F E R E N C E S
[1] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS,Heben MJ. Storage of hydrogen in single-walled carbonnanotubes. Nature 1997;386:377–9.
[2] Ye Y, Ahn CC, Witham C, Fultz B, Liu J, Rinzler AG, et al.Hydrogen adsorption and cohesive energy of single-walledcarbon nanotubes. Appl Phys Lett 1999;74:2307–9.
[3] Chen P, Wu X, Lin J, Tan KL. High H2 uptake by alkali-dopedcarbon nanotubes under ambient pressure and moderatetemperatures. Science 1999;285:91–3.
[4] Liu C, Fan YY, Liu M, Cong HT, Cheng HM, Dresselhaus MS.Hydrogen storage in single-walled carbon nanotubes at roomtemperature. Science 1999;286:1127–9.
[5] Tibbetts GG, Meisner GP, Olk CH. Hydrogen storage capacityof carbon nanotubes, filaments, and vapor-grown fibers.Carbon 2001;39:2291–301.
[6] Hirscher M, Becher M, Haluska M, Quintel A, Skakalova V,Choi YM, et al. Hydrogen storage in carbon nanostructures.J Alloys Compd 2002;330–332:654–8.
[7] Ritschel M, Uhlemann M, Gutfleisch O, Leonhardt A, Graff A,Taschner Ch, et al. Hydrogen storage in different carbonnanostructures. Appl Phys Lett 2002;80:2985–7.
[8] Kajiura H, Tsutsui S, Kadono K, Kakuta M, Ata M. Hydrogenstorage capacity of commercially available carbon materialsat room temperature. Appl Phys Lett 2003;82:1105–7.
[9] Baughman RH, Zakhidov AA, de Heer WA. Carbonnanotubes – the route toward applications. Science2002;297:787–92.
[10] Fan WJ, Zhang RQ, Teo BK, Aradi B, Frauenheim Th.Prediction of energetically optimal single-walled carbonnanotubes for hydrogen physisorption. Appl Phys Lett2009;95:013116.
[11] Leonard AD, Hudson JL, Fan H, Booker R, Simpson LJ, O’NeillKJ, et al. Nanoengineered carbon scaffolds for hydrogenstorage. J Am Chem Soc 2009;131:723–8.
[12] Nikitin A, Li X, Zhang Z, Ogasawara H, Dai H, Nilsson A.Hydrogen storage in carbon nanotubes through theformation of stable C–H bonds. Nano Lett 2008;8:162–7.
[13] Mishra A, Banerjee S, Mohapatra SK, Graeve OA, Misra M.Synthesis of carbon nanotube-TiO2 nanotubular materialfor reversible hydrogen storage. Nanotechnology2008;19:445607.
[14] Xu WC, Takahashi K, Matsuo Y, Hattori Y, Kumagai M,Ishiyama S, et al. Investigation of hydrogen storage capacityof various carbon materials. Intl J Hydrogen Energy2007;32:2504–12.
[15] Liu C, Cheng HM. Carbon nanotubes for clean energyapplications. J Appl Phys D 2005;38:R231–52.
[16] Liu C, Cong HT, Li F, Tan PH, Cheng HM, Lu K, et al. Semi-continuous synthesis of single-walled carbon nanotubes by ahydrogen arc discharge method. Carbon 1999;37:1865–8.
[17] Fan YY, Cheng HM, Wei YL, Su G, Shen ZH. Tailoring thediameters of vapor-grown carbon nanofibers. Carbon2000;38:921–7.
[18] Chen Y, Liu C, Li F, Cheng HM. Pore structures of multi-walledcarbon nanotubes activated by air, CO2 and KOH. J PorousMater 2006;13:141–6.
[19] Kiyobayashi T, Takeshita HT, Tanaka H, Takeichi N, Zuttel A,Schlapbach L, et al. Hydrogen adsorption in carbonaceousmaterials – how to determine the storage capacityaccurately? J Alloys Compd 2002;330–332:666–9.
[20] Wu CZ, Wang P, Yao X, Liu C, Chen DM, Lu GQ, et al.Hydrogen storage properties of MgH2/SWNT compositeprepared by ball milling. J Alloys Compd 2006;420:278–82.
[21] Chen Y, Wang P, Liu C, Cheng HM. Improved hydrogen storageperformance of Li–Mg–N–H materials by optimizingcomposition and adding single-walled carbon nanotubes.Intl J Hydrogen Energy 2007;32:1262–8.
[22] Berseth PA, Harter AG, Zidan R, Blomqvist A, Araujo CM,Scheicher RH, et al. Carbon nanomaterials as catalysts forhydrogen uptake and release in NaAlH4. Nano Lett2009;9:1501–5.