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Carbon Nanotubes for Hydrogen StorageBy Chiao Ku

11/31/02

Submitted in partial fulfillment of course

Requirement for MatE 115, Fall 2002

Instructor: Professor G. Selvaduray

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TABLE OF CONTENTS

1.0 Introduction.3

1.1 Use of Carbon Nanotubes in Hydrogen Storage.....3

1.2 History of Carbon Nanotubes..4

1.3 Structure of Carbon Nanotubes...5

2.0 Principles / Mechanism...6

2.1 Specific Surface Area and Gas absorption......6

2.2 Experiments on Hydrogen Storage.8

3.0 Future Aspects...11

4.0 Summary / Conclusion..12

5.0 Reference...13

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1. Introduction1.1 Use of Carbon Nanotubes in Hydrogen Storage

With the accelerating demand for cleaner and more efficient energy sources,

hydrogen research has attracted more attention in the scientific community. The focus

of some of the research has been the development of hydrogen fuel cells,

electrochemical devices that are able power buildings, cars and portable electric

devices.

Hydrogen is an ideal fuel; it is abundant, renewable and its combustion produces

only water vapor and heat. Until now, full implementation of a hydrogen-based

energy system has been hindered in part by the challenge of storing hydrogen gas,

especially onboard an automobile. New techniques being researched may soon make

hydrogen storage more compact, safe and efficient.

These new methods use carbon as a storage medium and bring us a step closer to

the widespread use of hydrogen as a fuel source. Some scientists are using various

approaches to shape carbon into microscopic cylindrical structures known as

Nanotubes.

One of the critical factors in Nanotubes usefulness as a hydrogen storage

medium is the ratio of stored hydrogen to carbon. According to the US Department of

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Energy, a carbon material needs to store 6.5%[1] of its own weight in hydrogen to

make fuel cells practical in cars. Such fuel cell cars could then travel 300 miles

between refueling stops.

1.2History of Carbon Nanotubes

After Kroto and Smalley discovered Fullerene, one of carbon allotropes(a cluster

of 60 carbon atoms : C60) for the first time in 1985, Dr. Iijima[2], a researcher for this

new material, at the NEC laboratories in Japan. In 1991, he discovered this thin and

long straw-shaped carbon Nanotubes during a TEM analysis of carbon clusters. This

discovery was published in Naturefor the first time. The Nanotubes range in length

from a few tens of nanometers to several microns, and outer diameter from about 2.5

nm to 30 nm. A carbon atom in Nanotubes forms a hexagonal honeycomb lattice of

sp2 bond with 3 other carbon atoms. As the inner diameters of the tubes are extremely

thin down to about several nanometers, the tubes are called Nanotubes.

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1.3Structure of Carbon Nanotubes

Fig. 1 Structures of Carbon Nanotubes[3]

Carbon Nanotubes (CNTs) are formed by rolled graphite sheet[4], with a least

inner diameter of 0.7nm up to several nm and a length of 10-100 micron. Tubes

formed by only one single graphite layer are called single wall Nanotubes (SWNT,

Fig.1). For tubes contains multiple concentric graphite layers are called Multiwall

Nanotubes (MWNT, Fig.1)

The interlayer distance in MWNTs is closer then interlayer distance in graphic

which has a unit cell parameter c(0.5c= 0.3355nm). The diameter of SWNTs varies

from 0.671 to 3nm, whereas MWNTs show typical diameters of 30-50 nm. The

helicity of the Nanotubes is usually described by Hamada vector[5], which indicated

how the graphite sheet is rolled up along a lattice vector with components (n,m). The

value of n and m defines the geometry of SWNT. The Nanotubes with n = m are

referred as armchair, tubes with n = 0 or m = 0 are referred as zigzag. SWNTs

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adsorbate at a given pressure will lead to the saturation of one single monolayer[7].

In order to calculate the quantity of adsorbate in the monolayer we use the

density of the liquid adsorbate and compute the volume of the molecule:

Aadsorbate

adsorbateadsorbate

N

MV

= (1)

Where M stands for the molar mass of the adsorbate, " is the density of the

adsorbate and V is the volume for each molecule in liquid phase. With the general

assumption that the molecules are spherical and close-packed, the volume of the

sphere can be represented as:

Aadsorbate

adsorbate

M N

M

V

23=

(2)

Table 1 The properties of hydrogen as adsorbate*

Properties Hydrogen

M (g / mol) 2.0159

"(g / cm) 0.0708

Vm (nm) 0.0350

d (nm) 0.4059

Sm (nm) 0.1294

1 / S (mol / m) 1.2834e-05

1 / S (g / m) 2.5668e-05

* M is the molar mass, "is the density of the adsorbate at boiling point, Vm is the volume of the

molecule, d is the diameter of the molecules, Sm is the surface area occupied by the molecule. 1 / S is

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the amount of adsorbate per surface area in a monolayer.

From the volume of the spherical molecule, the diameter can be calculated under

the assumption of close packed molecules at the surface area Sm:

3326

Aadsorbate

adsorbatemm

N

MVd

== (3)

32

2

2

3

=

Aadsorbate

adsorbate

MN

MS

(4)

The above values parameters for hydrogen are calculated and presented in Table

1. The condensation of a monolayer of hydrogen on a graphite sheet with a specific

area of S(one side) = 1315 m / g leads to:

034.012

0159.2

)(

)( )(2===

A

adsorbate

M

oneside

N

M

S

S

Cm

Hm (5)

Therefore the theoretical maximum condensation is 3.4 wt%, and it is not enough

to meet out requirements for a hydrogen cell. However, the scientists have discovered

an overlap of these monolayers (SWNTs) can actually hold more hydrogen inside the

CNT. The details are explained in the experiments on hydrogen storage.

2.2 Experiments on hydrogen Storage

In 1997, Dillon et al.[8]discovered that SWNTs have a high reversible hydrogen

storage capacity. Thereafter, many research groups started to conduct hydrogen

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storage researches and have made remarkable progresses. Most of the current

experimental results of hydrogen storage in carbon Nanotubes are summarized in

Table 2.

Table 2

Summary of experimentally reported storage capacities in carbon Nanotubes

In their pioneering work, Dillon et al. showed that hydrogen can condense to

high capacity (estimated to 5~10 wt%) inside narrow SWNTs, and predicted that

SWNTs with diameters of 16.3 to 20 would come close to the target capacity of 6.5

wt%. The adsorption of H2in SWNT was investigated with temperature programmed

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desorption (TPD) spectroscopy and it suggested that physical adsorption of hydrogen

mainly occurred within the cavities of SWNTs.

According to Dillons prediction, this kind of SWNT may be promising as

hydrogen storage medium. Figure 2[9]shows the change of hydrogen pressures as a

function of time for SWNTs under an initial hydrogen pressure of 10 MPa in the first

adsorption cycle at room temperature. The hydrogen uptake is complete within few

hours. It has been proved that notable changes occur for pore structure in the course of

hydrogen uptake in SWNTs. All the above facts indicate that the inner hollow cavity

takes part in the hydrogen adsorption.

Fig. 2. The amount of H2in weight for SWNT samples, and the pressure change versus the adsorption

time. Sample 1 was used as synthesized. Sample 2 was soaked in 37% HCI acid for 48 h, rinsed with

deionized water, and dried at 423 K. Sample 3 was pretreated in the same way as sample 2, the vacuum

heat-treated for 2 h at 773 K

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Another striking experimental result on hydrogen storage is in graphite

nanofibers (GNFs) were reported by Chambers et al.[10] They claimed that tubular,

platelet, and herringbone forms of GNFs have the capacity of 11, 45, and 67 wt% H2,

respectively, at room temperature and 12MPa. Their recent research further explains

about the interaction between hydrogen and GNFs, and concluded that the GNFs have

special structure, which produces a material composed entirely of nano-pores that

hold hydrogen molecules, and the non-rigid pore walls can expand to accommodate

hydrogen molecules in a multiplayer conformation.

Chen et al.[11]reported in their TPD experiments that a high H2uptake of 20 and

14 wt% can be achieved in milligram quantities of Li-doped and K-doped MWNTs,

respectively, under room pressure. The K-doped MWNTs can adsorb H2 at room

temperature, but they are chemically unstable, whereas the Li-doped MWNTs are

chemically stable, but require elevated temperatures (473 to 673 K) for maximum

adsorption and desorption of H2.

3. Future Aspects

There are more and more reproducible evidences prove that carbon Nanotubes

are potential hydrogen storage medium.

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However, it is still unpractical due to the limitation on mass production and

utilization of carbon Nanotubes. And the following obstacles still needed to be solved

by Scientists[12]

:

1. Mass production of carbon nanostructures with a controlled microstructure at a

reasonable cost.

2. Purification of CNTs, development and optimization of pretreatment methods for

opening the caps at the tube ends to improve their hydrogen storage capacity.

3. Elucidation of the microstructure of Nanotubes, especially pore structure and

surface microstructure in the viewpoint of hydrogen adsorption/desorption.

4. Elucidation of volume storage capacity and how to improve it.

5. Further investigation of adsorption/desorption process, thermodynamics, kinetics

and cycling behaviors of carbon Nanotubes.

6. A more practical hydrogen adsorption model to design a CNT based hydrogen

storage medium.

4. Summary / Conclusion

In conclusion, hydrogen fuel is clean, versatile, efficient and safe, and it will play

an important role in the future world energy structure. Preliminary experimental

results and some of the theoretical predictions indicate that Carbon Nanostructures

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(CNTs and CNFs) can be a promising candidate for hydrogen storage, which may be

the solution hydrogen fuel cell-driven vehicles. Nevertheless, many efforts still have

to be made to reproduce and verify the hydrogen storage capacity of carbon

Nanotubes both theoretically and experimentally, to investigate their storage capacity,

absorption and desorption behaviors, and finally to clarify the feasibility of carbon

nanostructures as a practical hydrogen storage medium.

5. Reference

[1]http://www.energy.gov

[2]R. Saito, G. Dresselhaus and M.S. Dresselhaus. Physical Properties of CarbonNanotubes(Imperial College Press, London 1998) pp. 2-7

[3]http://www.iljinnanotech.co.kr

[4]S. Iijima, Nature, 1991;345;56-58

[5]A. Zuttel, P. Sudan, Ph. Mauron, T. Kiyobayashi, Ch. Emmenegger, L. Schlabach.Hydrogen storage in carbon nanostructures,(International Journal of HydrogenEnergy 27, 2002) pp 203-204.

[6]ibid, p 205-206.

[7]ibid, p 207.

[8]A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune and M.J.Heben, Storage of hydrogen in single-walled carbon nanotubes.Nature386(1997),

pp. 377379.

[9]C. Liu, Y.Y. Fan, M. Liu, H.T. Cong, H.M. Cheng and M.S. Dresselhaus,Hydrogen storage in single-walled carbon Nanotubes at room temperature. Science286(1999), pp. 11271129.

[10]A. Chambers, C. Park, R.T.K. Baker and N.M. Rodriguez, Hydrogen storage ingraphite nanofibers.J. Phys. Chem. B102(1998), pp. 42534256.

[11]P. Chen, X. Wu, J. Lin and K.L. Tan, High H2uptake by alkali-doped carbon

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Nanotubes under ambient pressure and moderate temperatures. Science285(1999),pp. 9193.

[12]T. Huang, Nanotechnology That Will Change the World,(Enlighten NoahPublishing, 2002) pp. 38-50