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Carbon Materials
M Hirscher, Max-Planck-Institut fur Metallforschung, Stuttgart, Germany
& 2009 Elsevier B.V. All rights reserved.
Introduction
The major bottleneck for commercializing fuel cell ve-
hicles is onboard hydrogen storage. The presently avail-
able systems are high-pressure tanks or liquefied
hydrogen in cryogenic vessels, which both possess severe
disadvantages. Storage in lightweight solids could be the
solution to this problem, because an onboard storage
system should possess a high gravimetric storage capacity
(US Department of Energy (DoE) system target of 6 and
9 wt% for 2010 and 2015, respectively). The gravimetric
storage capacity of the system is defined as mass of
hydrogen divided by the sum of the masses of storage
system and hydrogen given in percent, whereas forstorage materials often the wt% are based on the material
weight instead of the whole system weight. There are two
principal mechanisms: (1) adsorption of hydrogen mol-
ecules on surfaces, i.e., physisorption and (2) hydrogen
atoms dissolved or forming chemical bonds, i.e., chemi-
sorption. This article focuses on the interaction of
hydrogen with carbon materials. Carbon is one of the
very few light materials that are solid at room tempera-
ture. Furthermore, different carbon structures can be
produced with large inner surfaces, e.g., activated carbon,
fullerenes, carbon nanotubes, and carbon nanofibers.
Typically, at room temperature carbon only interacts
very weakly with hydrogen through van der Waals forces.
Only at elevated temperatures or strong mechanical
treatment, e.g., high-energy ball milling, are CH bonds
formed. These bonds are strong and hydrogen is only
released at temperatures far above 600 K, which is too
high for automotive applications. Activated carbon
structures with high surface areas of up to 3000 m2 g1
are commercially available and show, at liquid nitrogen
temperature, a sufficient hydrogen storage capacity by
physisorption of hydrogen molecules. The newly avail-
able nanostructured carbon materials (Figure 1), espe-
cially single-walled carbon nanotubes (SWNTs), have
given rise to speculation that capillary forces could en-
hance gas or hydrogen adsorption as well as drawing up
liquids by capillarity. The adsorption would be a con-
sequence of the attractive potential of the pore walls due
to the strong curvature. Unfortunately, however, the ef-
fect of a so-called nanocapillarity could be neither ex-
perimentally nor theoretically substantiated.This article addresses, first, the false disclosure of
spectacular results for novel carbon nanostructures and,
second, the capability of carbons possessing a high specific
surface area for hydrogen storage by cryo-adsorption.
Spurious Measurements of HydrogenStorage in Novel Carbon Nanostructures
The first publication concerning the hydrogen storage
capability of carbon nanotubes caused enormous interest.
This work claimed that hydrogen can condense to a high
density inside narrow SWNTs and estimated a gravi-
metric storage capacity of 510 wt%, which would be
highly attractive for automotive applications. The total
amount of hydrogen desorbed from the total sample with
only a small concentration of SWNTs and determined
with thermal desorption spectroscopy (TDS) was about
0.01 wt% considering the whole weight of the sample.
The reported large storage values were estimated after
attributing all desorbed hydrogen to the small weight
fraction of SWNTs present in the sample. Three years
later, the same authors affirmed that they had directly
measured a hydrogen storage capacity of about 7 wt% by
purifying the samples. This time, hydrogen was releasedat a significantly higher temperature than that required in
the first study. Furthermore, the SWNTs, which are
typically closed at the ends by fullerene-like caps, had
been opened. For opening the SWNTs the carbon
nanotubes were dispersed in HNO3and treated by high-
power ultrasonication using an ultrasonic horn, typically
made from a titanium alloy. After heating in high vac-
uum, the opened SWNTs could adsorb this high amount
of hydrogen within a few minutes at ambient pressure
and room temperature. In 2001, another laboratory found
5200 nm 3050 nm ~1 nm
Figure 1 Carbon nanostructures assembled from different
arrangements of graphene sheets. Left to right: graphitic
nanofibers, multiwalled carbon nanotubes, bundle of single-
walled carbon nanotubes.
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these results to be erroneous. They carefully investigated
the hydrogen uptake in various carbon materials after
high-power ultrasonication. The characteristic shape of
the TDS spectrum was more or less similar to the prior
data, but the storage capacity of the sonicated material
was reduced by a factor of three. Further investigations
revealed that owing to cavitation, the high-power ultra-
sonication transferred metal particles from the horn intothe samples. By using the titanium-alloy horn for soni-
cation, hydrogen desorption could be observed in all the
investigated carbon materials, e.g., purified SWNTs,
graphite, and even diamond powder. Thus, all hydrogen
storage of the sonicated samples could be attributed to
hydrogen storage in the titanium-alloy particles. Indeed,
the formation of titanium hydride was confirmed by X-
ray analysis. By contrast, no hydrogen storage was found
(i.e., below 0.005 wt%) when the SWNTs were sonicated
with a stainless-steel horn.
Very spectacular hydrogen storage capacities of up to
67 wt% were claimed for a certain type of carbon nanofiberin 1998. High hydrogen adsorption was measured for dif-
ferent carbon materials by applying a hydrogen pressure of
about 11 MPa at room temperature and monitoring for
24 h. The highest value of nearly 67 wt% was obtained for
herringbone-type carbon nanofibers, but even for graphite
itself a very high storage capacity of 4.5 wt% was reported,
measured at room temperature. To date, however, the ex-
tremely high values have not been confirmed in any other
laboratory. It should be noted that also in 1998, another
group found a capacity less than 0.2 wt% for comparable
carbon nanofibers examined by the same method under
similar experimental conditions.
In 1999, yet another group published promising data
obtained through a volumetric method, which measures
pressure changes at a constant volume that are due to
hydrogen adsorption or desorption of the sample. It was
claimed that multiwalled carbon nanotubes (MWNTs)
gave a hydrogen uptake of 1013 wt% at room tem-
perature and at a pressure of 11 MPa. The nanotubes had
to be boiled in hydrochloric acid. In a further publication,
however, the same group reduced the storage capacity of
the MWNTs by a factor of two. In the same year, other
workers used thermo-gravimetric analysis (TGA) to
measure the hydrogen adsorption of carbon nanofibers
and activated carbon at high pressure. They observed amaximum weight increase at 12.5 MPa and room tem-
perature that corresponded to a hydrogen uptake of
1.6 wt% in activated carbon and of 1.2wt% in fibrous
material. Furthermore, they found a linear correlation
between the specific surface area of the materials and the
maximum hydrogen uptake. Again in 1999, a group
measured SWNTs with large diameters treated with
hydrochloric acid and obtained a storage capacity of
4.2 wt% at room temperature. The purity of the samples
was about 5060%. High hydrogen storage capacities of
10 wt%, and later 15 wt% were claimed from another
laboratory for herringbone-type carbon nanofibers in
2001. But none of these high values has survived cross-
checking by independent research groups repeating the
measurements on the same sample in different labora-
tories. One US laboratory carefully examined different
carbon materials at high pressures but found no hydrogen
sorption appreciably above background at room tem-perature. Very low storage capacities, i.e., below 1 wt%,
have been independently observed at room temperature
and 8 MPa by several European laboratories. Latest re-
sults show values of storage capacity below 0.5 wt% at
room temperature for a large variety of carbon nanos-
tructures with specific surface areas up to 2500 m2 g1. In
essence, the measured isotherms are in agreement with
theoretical calculations based on the physisorption of
hydrogen molecules.
In 1999, a group from Asia announced storage capacities
of up to 20 wt% for lithium-treated MWNTs and even up
to 14 wt% for lithium-treated graphite. They tried tointercalate different carbon materials by mixing with alkali
salts and performing a so-called solid-state reaction by a
heat treatment under hydrogen flow. Subsequent gravi-
metric measurements performed under hydrogen flow
yielded dramatic and reversible weight changes of the al-
kali-treated carbon samples that were attributed to
hydrogen adsorption and release. The measurements were
revisited by two US laboratories and it was discovered that
the majority of the weight changes were due to the ad-
sorption and release of water. The gravimetric experiment
on lithium-treated graphite was reproduced under hydro-
gen flow in a European laboratory and yielded weight
changes very similar to those obtained in the first publi-
cation. The characterization of the intercalation product by
X-ray diffraction did not, however, provide any evidence of
intercalated graphite. The so-called solid-state reaction
yields a mixture of graphite, lithium oxide, and lithium
hydroxide. The lithium oxide is highly hygroscopic, and
the release and reabsorption of water of crystallization
leads to the observed weight changes.
Specific Surface Area andCryo-Adsorption Storage
The correlation between the specific surface of the carbon
materials and their ability to adsorb hydrogen was care-
fully examined in 2001. Applying the volumetric method,
different adsorbents were investigated for hydrogen at
77 K and atmospheric pressure. An approximately linear
correlation was found between the specific surface area
determined by BrunauerEmmetTeller (BET) model
and the hydrogen adsorption. The highest storage capacity
of about 2 wt% was obtained for activated carbon with a
surface area of about 2000m2 g1, whereas carbon
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nanofibers possessing lower surface areas showed the
storage capacity less than 1 wt%. Another group deter-
mined the hydrogen storage capacity for different carbon
nanostructures by applying an electrochemical method
that measures the electric charge of the carbon electrode
against a counterelectrode in a galvanostatic setup at room
temperature. They included these values in one plot
together with the low-temperature adsorption data. Allexperimental data may be described by a storage capacity
of 1.5 wt% per 1000 m2 g1 specific surface area. Slightly
higher values were obtained in later investigations of dif-
ferent nanostructured carbon materials at higher pressures
of 5 MPa and at 77 K. A Canadian group has reported the
hydrogen storage capacities of different types of activated
carbon at cryogenic temperature. The authors compare
regular-grade activated carbon with AX-21 activated car-
bon, which is produced by reaction with potassium hy-
droxide and has cage-like porosity and a very high surface
area of the order of 3000 m2 g1. AX-21 has a hydrogen
storage capacity of about 5 wt% at 77 K and 2 MPa, i.e.,twice as much as regular-grade activated carbon with
surface area ranging between 700 and 1800 m2 g1. These
data are in very good agreement with the results obtained
by volumetric measurements on a similar type of activated
carbon with a specific surface area of 2500 m2 g1. In this
recent study, the saturation values have been systematic-
ally investigated for hydrogen adsorption at 77 K on vari-
ous carbon nanostructures from different suppliers
worldwide. The results confirm the linear correlation be-
tween the specific surface area and the hydrogen storage
capacity independent of the type of carbon material
(Figure 2) with a slope of 1.9wt% per 1000m2 g1 of
specific surface area. This clearly indicates that the
hydrogen adsorption process is based on the local inter-
action between the hydrogen molecule and the surface; it
is independent of the long-range order, curvature, or
ordered arrays of graphene sheets. Therefore, the pre-
requisite to reach high gravimetric storage capacity for a
cryo-adsorption device is a porous material possessing a
high specific surface area.
The remaining problem is the volumetric storage
capacity that is limited physically by the density of liquid
hydrogen. Typically, the activated carbon materials with a
very high specific surface area are fine fluffy powders
showing a very low packing density. Recently, thisvolumetric problem was overcome by preparing activated
carbon monoliths (Figure 3), which show a volumetric
storage density of 29.7 kg H2 m3 at 77K and 4MPa.
Furthermore, if the compressed gas within the cavities of
the activated carbon monolith is included, a total volu-
metric storage density of 39.3 kg H2 m3 will be reached.
These results together with the fast kinetics and the total
reversibility, i.e., high cyclic stability, are very promising
for future applications of activated carbon as cryo-ad-
sorption storage devices. If an operating pressure of less
than 2 MPa can be achieved, it will be possible to use
conformable tank systems in contrast to high-pressure
cylinders.
BET specific surface area (m2g1)
Hydrogenstorage(wt%)
77 K
RT
5000
0
1
2
3
4
5
1000 1500 2000 2500 3000
Figure 2 Relationship between maximum hydrogen storage
capacity of different carbon nanostructures and their BET specific
surface area (determined by the BrunauerEmmetTeller (BET)
model) at 77 K and room temperature (RT). Reproduced from
Hirscher M and Panella B (2005) Nanostructures with high
surface area for hydrogen storage. Journal of Alloys and
Compound404406: 399401.
Figure 3 Examples of activated carbon monoliths with a high
micropore volume and a diameter of 4.5 and 1.6 cm, respectively.
Reproduced from Jorda-Beneyto M, Lozano-CastelloD, Suarez-
Garca F, Cazorla-Amoros D, and Linares-Solano A (2008)
Advanced activated carbon monoliths and activated carbons for
hydrogen storage. Microporous and Mesoporous Materials112:
235242.
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Concluding Remarks
The chemical bond between carbon and hydrogen is too
strong to be used for reversible hydrogen storage. On the
contrary, the interaction between hydrogen molecules
and the carbon surface through van der Waals forces is
typically weak. Unfortunately, however, the hopes placed
in novel carbon nanostructures proved falsely optimisticas none of the spectacular results that claimed high
storage capacities could be independently validated.
At low temperatures, carbon materials with high
specific surface areas show gravimetric hydrogen storage
capacities of around 5 wt%. Recently, the volumetric
storage capacity of these activated carbons has been im-
proved by preparing monoliths. Owing to the very fast
kinetics and total reversibility of the cryo-adsorption
process, very short refueling times and long cycle life-
times can be realized. Even carbon materials cannot meet
the gravimetric storage capacity and the operating tem-
perature for onboard hydrogen storage set by the DoE for2010; these materials are very promising for transporta-
tion applications because they meet the targets for ma-
terial cost, refueling rate, cycle lifetime, and can almost
reach the volumetric storage capacity.
Nomenclature
Abbreviations
BET Brunauer-Emmet-Teller
DoE US Department of Energy
MWNT multiwalled carbon nanotube
SWNT single-walled carbon nanotubeTDS thermal desorption spectroscopy
TGA thermo-gravimetric analysis
See also: Fuels Hydrogen Storage: MetalOrganic
Frameworks; Zeolites.
Further Reading
Ahn CC, Ye Y, Ratnakumar BV, Witham C, Bowman RC, and Fulz B
(1998) Hydrogen desorption and adsorption measurements ongraphite nanofibres. Applied Physics Letters 73: 3378--3380.
Chahine R and Bose TK (1994) Low pressure adsorption storage of
hydrogen. International Journal of Hydrogen Energy19: 161--164.
Chambers A, Park C, Baker R, and Rodriguez NM (1998) Hydrogen
storage in graphite nanofibres.Journal of Physical Chemistry B 102:
4253--4256.
Chen X, Haluska M, Dettlaff-Wegliskowska U, Hirscher M, Becher M,
and Roth S (2002). Pressure isotherms of hydrogen adsorption in
carbon nanostructures. Materials Research Society Symposia
Proceedings 706: Z9.11.1Z9.11.6.
Chen P, Wu X, Lin J, and Tan KL (1999) High H 2uptake by alkali-doped
carbon nanotubes under ambient pressure and moderate
temperatures.Science 285: 91--93.
Cheng HM, Liu C, Fan YY, et al. (2000) Synthesis and hydrogen storage
of carbon nanofibres and single-walled carbon nanotubes.
Zeitschrift fur Metallkunde91: 306--310.
Dillon AC, Gennett T, Alleman JL, Jones KM, Parilla PA, and Heben MJ
(2000). Carbon nano-tube materials for hydrogen storage.Proceedings
of the 2000 Hydrogen Program Review NREL/CP, 50728890.
Dillon AC, Jones KM, Bekkedahl A, Kiang CH, Bethune DS, and HebenMJ (1997) Storage of hydrogen in single-walled carbon nanotubes.
Nature386: 377--379.
Fan YY, Liao B, Liu M, Wie YL, Lu MQ, and Cheng HM (1999) Hydrogen
uptake in vapour-grown carbon nanofibres. Carbon37: 1649--1652.
Forcrand de M (1908) Recherches sur les oxydes de lithium, de
strontium et de baryum. Annals de Chimica Physique 15: 433--453.
Gupta BK and Srivastava ON (2001) Further studies on microstructural
characterization and hydrogenation behavior of graphitic nanofibres.
International Journal of Hydrogen Energy26: 857--862.
Hirscher M and Becher M (2003) Hydrogen storage in carbon
nanotubes. Journal of Nanoscience and Nanotechnology3: 3--17.
Hirscher M, Becher M, Haluska M, et al. (2001) Hydrogen storage in
sonicated carbon materials. Applied Physics A 72: 129--132.
Hirscher M and Panella B (2005) Nanostructures with high surface area
for hydrogen storage. Journal of Alloys and Compounds 404406:
399--401.
Jorda-Beneyto M, Lozano-Castello D, Suarez-Garca F, Cazorla-
Amoros D, and Linares-Solano A (2008) Advanced activated carbon
monoliths and activated carbons for hydrogen storage. Microporous
and Mesoporous Materials112: 235--242.
Jorda-Beneyto M, Suarez-Garca F, Lozano-Castello D, Cazorla-
Amoros D, and Linares-Solano A (2007) Hydrogen storage on
chemically activated carbons and carbon nanomaterials at high
pressures.Carbon 45: 293--303.
Liu C, Fan YY, Liu M, Cong HT, Cheng HM, and Dresselhaus MS (1999)
Hydrogen storage in single-walled carbon nanotubes at room
temperature.Science 286: 1127--1129.
Nijkamp MG, Raaymakers J, van Dillen AJ, and de Jong KP (2001)
Hydrogen storage using physisorption materials demands. Applied
Physics A 72: 619--623.
Panella B, Hirscher M, and Roth S (2005) Hydrogen adsorption in
different carbon nanostructures. Carbon 43: 2209--2214.
Park C, Anderson PE, Chambers A, Tan CD, Hidalgo R, and RodriguezNM (1999) Further studies of the interaction of hydrogen with
graphite nanofibres. Journal of Physical Chemistry103:
10572--10581.
Pinkerton FE, Wicke BG, Olk CH, et al. (2000) Thermogravimetric
measurement of hydrogen adsorption in alkali-modified carbon
materials. Journal of Physical Chemistry104: 9460--9467.
Ritschel M, Uhlemann M, Gutfleisch O, et al. (2002) Hydrogen storage
in different carbon nanostructures. Applied Physics Letters 80:
2985--2987.
Schlapbach L and Zuttel A (2001) Hydrogen storage materials for
mobile applications. Nature 414: 353--358.
Skakalova V, Quintel A, Choi YM, Roth S, Becher M, and Hirscher M
(2002) Chemical processes during solid state reaction of carbon with
alkali salts prepared for gravimetric hydrogen storage
measurements.Chemical Physics Letters 365: 333--337.
Strobel R, Garche J, Moseley PT, Jorissen L, and Wolf G (2006)
Hydrogen storage by carbon materials. Journal of Power Sources
159: 781--801.
Strobel R, Jorissen L, Schliermann T, et al. (1999) Hydrogen adsorption
on carbon materials. Journal of Power Sources 84: 221--224.
Tibbetts GG, Meisner GP, and Olk CH (2001) Hydrogen storage
capacity of carbon nanotubes, filaments, and vapor-grown fibres.
Carbon 39: 2291--2301.
Yang RT (2000) Hydrogen storage by alkali-doped carbon nanotubes
revisited.Carbon 38: 623--641.
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