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The effects of bromine treatment on the hydrogen storageproperties of multi-walled carbon nanotubes

S. Mirershadi a, A. Reyhani b,*, S.Z. Mortazavi c, B. Safibonab a, M. Khabazian Esfahani d

aMaterial Research School, P.O. Box: 14395-836, Tehran, IranbDepartment of Physics, K.N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, IrancPhysics Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, IrandPolymer Group, Chemical Engineering Department, Tarbiat Modares University, P.O. Box 14115-114, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 12 June 2011

Received in revised form

23 August 2011

Accepted 10 September 2011

Available online 5 October 2011

Keywords:

MWCNTs

Hydrogen storage

Bromine

Volumetric technique

* Corresponding author. Tel.: þ98 21 6616 45E-mail address: reyhani@alum.sharif.edu

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.09.025

a b s t r a c t

The effects of bromine treatment on the properties of multi-walled carbon nanotubes

(MWCNTs) such as surface porosity, sp2 hybridization, functional groups and hydrogen

storage capacity were studied and compared with treated MWCNTs by HCl, HNO3 and

H2SO4 acids. The treatments affect the graphitization properties (sp2 hybridization) and

porous structures of MWCNTs by enlarging the specific surface area and the micro-pore

volume. In addition, the hydrogen storage capacity of the treated MWCNTs was also

investigated by volumetric technique. It is found that the destroying of sp2 hybridization of

bromine treated MWCNTs increases hydrogen adsorption sites and decreases hydrogen

desorption temperature.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction addition of metal on carbonmaterials play a major role on the

Nowadays there is a widespread concern over the emission of

greenhouse gases, meanwhile, hydrogen has attracted a lot of

attention as the future fuel due to its clean combustion

product. However, according to a major scientific challenge in

the storage of hydrogen by a safe, effective and cheap system,

it has not been used to a great extent as transportation fuel

and power generation. Since the report of Dillon et al. [1] on

possible hydrogen storage in single-walled carbon nanotubes

(SWCNTs), various structures of carbon materials have

attracted considerable research interest as a safe hydrogen

storage medium. Low density and high surface area are two

outstanding specifications of CNTs [2,3].

Recent reports show that defect sites (including nano-

pores and dangling bonds), structural variety and the

16; fax: þ98 21 6616 4536.(A. Reyhani).2011, Hydrogen Energy P

improvement of hydrogen storage capacity of CNTs [4e7]. In

fact, synthesized MWCNTs have relatively lower specific

surface area compared with other treated MWCNTs, which

hinders its application as hydrogen storage materials. Nowa-

days, full attention is focused on the surface texture of

carbonaceous materials in order to obtain high-capacity of

hydrogen storage via various activation methods [8]. Many

methods such as chemical etching [9,10], ultrasonic treatment

[11], mechanical treatment [12,13], high energy irradiation [14]

and microwave plasma etching [15] are used to obtain

MWCNTwith nano-sized pores and dangling bonds in order to

improve the hydrogen storage capacity of CNTs [16e22]. The

acidic treatment of MWCNTs has been suggested as a way to

open MWCNT tips and to enhance the defective cavities on

the surface, which can influence the hydrogen storage

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 1 15623

capacity [9,23]. Hydrogenmolecules can be adsorbed on defect

sites created by chemical activation of MWCNTs through van

der Waals forces [10]. Among chemical agents, bromine can

produce micro- and meso-pores on CNTs which prepares an

ideal media to store hydrogen.

In the prior works, in order to achieve MWCNTs with

characterized surface properties, theMWCNTswas treated by

various acids such as nitric, sulfuric and hydrochloric acids

and their electrochemical hydrogen storage properties were

investigated [23,24]. In this study, the role of bromine to

create micro- and meso-pores and defect sites (dangling

bonds) in order to improve the hydrogen storage properties of

MWCNTs is investigated. The results are compared with that

of the H2SO4, HNO3 and HCl treated MWCNTs. Crystalline

properties of MWCNTs by first and second-order Raman scat-

tering of the samples are also investigated. The morphology,

structure, graphitization, quality and adsorption character-

istics of the samples are evaluated. Finally, the hydrogen

storage capacity of the treated MWCNTs is studied by volu-

metric technique.

Fig. 1 e TEM micrographs of the (a) pristine MWCNTs and the t

bromine.

2. Experimental

2.1. Material preparation

Bimetallic catalyst supported by MgO with the proportion of

Fe/Ni/MgO (2/2/6 wt%) is prepared by wet chemical impreg-

nation method. After preparation, MWCNTs is grown on the

Fe/Ni/MgO catalyst by thermal chemical vapor deposition

(TCVD). The experimental details of catalyst preparation have

been reported elsewhere [25]. In brief, for MWCNTs growth,

high purity CH4 and H2 (with the ratio of 1/3) are introduced

into an electrical furnacewith a flow rate of 80 sccm for 40min

at 940 �C. Then, raw MWCNTs is placed in the boat, and

calcined in the furnace at 400 �C for 60 min under O2 atmo-

sphere at ambient pressure, and then cooled down to room

temperature. Then, MWCNTs is treated by HNO3, H2SO4 and

HCl at boiling temperature and with bromine at room

temperature according to the following conditions. The same

amount of MWCNTs is separately immersed in 5 M boiling

reated MWCNTs by: (b) HCl, (c) HNO3, (d) H2SO4 and (e)

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 115624

solutions of HNO3, H2SO4 and HCl for 3 h. For Bromine treat-

ment, 100 mg of MWCNTs was immersed in 60 ml of 3 M

bromine solution for 24 h at room temperature. Then, the

MWCNTs is washed out several times with deionized water

until the pH value of the solution becomes neutral. Finally, the

samples are dried at 130 �C for 24 h in ambient conditions.

2.2. Characterization techniques

Transmission electron microscopy (TEM) (XL, Philips) and

energydispersiveanalysisX-ray (EDX) (XL30,Philips)areusedto

determine the morphology, diameter and elemental composi-

tion of MWCNTs. Thermal gravimetric analysis (TGA) and

differential scanning calorimetry (DSC) (using Rometric Simul-

taneous Thermal Analysis) are employed to investigate

MWCNT’syieldandgraphitizationdegree.Theheatingprogram

isperformed ina temperature rangeof 30e850 �Cwithaheating

rate of 10 �C/min in ambient conditions. The structure of

MWCNTsand thepresenceofpossible impurities suchasFeeNi

and MgO are characterized by X-ray diffraction (XRD) (Cu Ka

Fig. 2 e Diameter distributions of the (a) pristine MWCNTs and

bromine obtained by statistical analysis of MWCNTs in TEM m

fitted by log-normal distribution.

X-ray radiation source) with 2qwithin the range of 20e80�. Thescanning speed and step interval are 1�/min and 0.02�, respec-tively. A micro-Raman spectrometer using 532 nm excitation

light (HR-800, Jobin-Yvon) is used to determine the quality and

structure of MWCNTs. The Raman spectra are recorded within

the range of 200e3000 cm�1 with 0.1 cm�1 resolution. A high-

performance volumetric physisorption apparatus at 77 K Bru-

nauer-Emmett-Teller (BET) is used to determine MWCNTs’

surface area. The pore size distribution of MWCNTs is also ob-

tained by Barret-Joyner-Halenda (BJH) equation using adsorp-

tion isotherm.Moreover, thevolumetric technique isperformed

to determine the hydrogen storage capacity ofMWCNTs. In this

experiment, 30 mg of treated MWCNTs is placed in a quartz

tube. In order to measure MWCNT’s temperature, a thermo-

couple is placed in the quartz tube adjacent to the boat.

MWCNTs is then heated to 300 �C under vacuum (w10�3 mbar)

and kept under these conditions for 3 h in order to degas and

remove any possible water or any other gases that may be

present within the bulk of the sample. MWCNTs is dosed with

hydrogen gas under pressure of 1 atm for 24 h at room

treated MWCNTs by: (b) HCl, (c) HNO3, (d) H2SO4 and (e)

icrographs. Experimental data of MWCNT’s diameter are

Table 2 e The results of DSC and TGA for the pristine andtreated MWCNTs.

MWCNT samples Onset, inflectionand offset

temperatures (�C)

Weightloss (%)

Pristine 548, 640 and 694 46.17

Treated by bromine 488, 619 and 667 53.23

Treated by H2SO4 560, 680 and 710 93.82

Treated by HNO3 500, 677 and 710 93.01

Treated by HCl 543, 672 and 710 92.31a

b

c

d

e

Fig. 3 e XRD pattern of the (a) pristine MWCNTs and the

treated MWCNTs by: (b) bromine, (c) H2SO4, (d) HNO3 and

(e) HCl.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 1 15625

temperature. Subsequently, it is cooled down to �150 �C by

liquid nitrogen and then heated to 300 �C with heating rate of

10 �C/min. A pressure gauge is used to measure the pressure

change in order to determine the amount of hydrogen desorp-

tion [26].

3. Results and discussion

3.1. Characterization of MWCNTs

TEM micrographs of the pristine and treated MWCNTs with

bromine, H2SO4, HNO3 and HCl are shown in Fig. 1. The

micrographs depict that the most of the metal catalysts and

impurities removed from MWCNTs by treatment with H2SO4,

HNO3 and HCl. However, it is noticeable that MWCNTs treated

by bromine has more metal catalysts and impurities comp-

ared with the other treated samples. Moreover, some defects

are found on the surface of MWCNTs after treating with

bromine. Using number of TEM micrographs, the diameter

distributions of the pristine and treated MWCNTs with

bromine and H2SO4, HNO3 and HCl are reported as histogram

plots and shown in Fig. 2. The histograms are then fitted to

a log-normal function. It is observed that the mean diameter

of the pristine, bromine treated, H2SO4 treated, HNO3 treated

and HCl treated MWCNTs is about 109.71, 99.69, 79.76, 79.66

and 77.21 nm, respectively. The standard deviation of diam-

eter for pristine and treated MWCNTs with bromine, H2SO4,

HNO3 and HCl is 51.64, 52.74, 33.20, 37.29 and 34.22,

Table 1 e The results of EDX analysis for the pristine andtreated MWCNTs.

MWCNT samples %C %O %Mg %Fe %Ni

Pristine 73.08 19.37 4.66 1.15 1.74

Treated by bromine 76.61 20.11 0.08 1.48 1.71

Treated by H2SO4 85.56 13.07 0.07 0.09 1.21

Treated by HNO3 83.18 15.44 0.07 0.56 0.75

Treated by HCl 84.46 13.58 0.08 0.74 1.14

respectively. This finding shows that rather other acids,

bromine cannot significantly decrease MWCNT’s diameter.

The XRD results of the pristine MWCNTs and treated

MWCNTs are shown in Fig. 3. The peak appeared at around

2q ¼ 26.3� is the characteristic of MWCNTs, the peaks at

2q ¼ 37.2�, 42.9� and 62.35� are characteristics of MgO, the peak

at 2q ¼ 53.7� indicates Fe3C and the peaks at 2q ¼ 43.9�, 51.05�

and 75.37� are indicative of FeeNi alloy. It could be observed

that with acid treatment, the peak intensities of FeeNi and

MgO are significantly decreased. According to our previous

work [23] and the XRD results in the present study, acid treat-

ment reduces the impurities such as MgO and metal catalysts.

However, a little amount of metal catalysts remains in the

samples after acid treatment. Moreover, these results indicate

Fig. 4 e TGA (a) and DSC (b) results of the (a) pristine

MWCNTs and the treated MWCNTs by: (b) bromine,

(c) H2SO4, (d) HNO3 and (e) HCl.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 115626

that MWCNTs treated by bromine has the highest amount of

FeeNi alloy as comparedwith the other acid-treatedMWCNTs.

As shown in Fig. 1, the FeeNi nanoparticles are encapsulated in

MWCNTs and the possible oxidation of FeeNi nanoparticles

during calcination process in the atmosphere of O20 is negli-

gible. It is worth noticing that the accompanying of metal

catalyst with MWCNTs can enhance the hydrogen storage

capacity. There are many literatures on the improvement of

hydrogen storage capacity of MWCNTs by the addition of

metals such as Fe, Pd and Ni [27e29]. Furthermore, in the

spectrum of MWCNTs treated by bromine, the (002) Bragg peak

is widened in comparison to the other samples. It may be

attributed to the increase of irregularity of the layer structures

Fig. 5 e First-order Raman spectra with the deconvolution band

(b) bromine, (c) H2SO4, (d) HNO3 and (e) HCl.

after bromine treatment [8]. The crystallite size in pristine and

treated MWCNTs with bromine, H2SO4, HNO3 and HCl is 11.5,

16.0, 18.9, 20.9 and 34.7, respectively.

EDX (XL30, Philips) analysis is employed to determine

elemental composition of pristine and treated MWCNTs as

summarized in Table 1. It depicts that the amount of Fe and Ni

atoms in MWCNTs treated by bromine are not decreased

noticeably in comparison to the other samples. Moreover, the

amount of oxygen atoms is the highest in MWCNTs treated by

bromine. These findings are also confirmed by XRD results.

In order to determine the carbon structure (the amorphous

carbon and CNTs) and other impurities, TGA and DSC anal-

yses are used. The percentages of the amorphous carbon and

s for (a) the pristine MWCNTs and the treated MWCNTs by:

Table 3 e The results of Raman spectroscopy for thepristine and treated MWCNTs.

MWCNT samples ID/IG ID0/IG IG0/IG

Pristine 0.47 0.757 1.35

Treated by bromine 0.39 0.507 0.85

Treated by H2SO4 0.28 0.324 1.30

Treated by HNO3 0.27 0.308 0.45

Treated by HCl 0.24 0.302 0.57

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 1 15627

the sp2 hybridization of carbon are obtained according to the

weight loss at 300e400 �C and 500e800 �C, respectively

[26,30e32]. The results are shown in Table 2 and Fig. 4. From

TGA, we found that lack of weight loss between 300 and 400 �C

Fig. 6 e Second-order Raman spectra with the deconvolution ba

by: (b) bromine, (c) H2SO4, (d) HNO3 and (e) HCl.

indicates that no amorphous carbon existed in the samples.

Furthermore, according to Table 2, the yield of MWCNTs for

pristine and treatedMWCNTswith bromine, H2SO4, HNO3 and

HCl is 46.1, 53.2, 93.8, 93.0 and 92.3%, respectively. The onset,

inflection and offset temperatures indicate the temperatures

at the initial, maximum, and the end of weight loss in DSC,

respectively.

Considering the XRD and EDX results, it can be concluded

that the bromine treated MWCNTs contained the highest

amount of FeeNi impurities compared with those treated by

other acids. DSC results (see Table 2 and Fig. 4) show that the

oxidation process for the bromine treated MWCNTs has

started and ended at lower temperatures as compared to

those treated by other acids. It is also illustrated that the

nds for (a) the pristine MWCNTs and the treated MWCNTs

0

5

10

15

20

25

30

35

40

45

50

0 0.2 0.4 0.6 0.8 1

pristineBromineH2SO4HNO3HCl

Relative Pressure, P/Po

Vol

ume

(cc/

g)

Pristine

Bromine

H SO

HNO

HCl

Fig. 7 e N2 adsorption and desorption isotherms of the

pristine and treated MWCNTs by bromine, H2SO4, HNO3

and HCl.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 5 10 15 20

0.010

Pristine

Bromine

H SO

HNO

HCl

dV(d

) (c

c/nm

/gr)

Pore Diameter (nm)

Fig. 8 e Pore size distribution of the pristine and treated

MWCNTs by bromine, H2SO4, HNO3 and HCl.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 115628

inflection temperature of the bromine treated MWCNTs is the

lowest. These results are attributed to the destructive effect of

bromine on the carbon sp2 hybridization which leads to the

formation of dangling bonds on the walls of MWCNTs. This

phenomenon is confirmed by XRD and TEM observations.

Raman spectroscopy is applied for the characterization of

graphitic structures. This method has historically played an

important role in the study and characterization of graphitic

materials. It gives the information about sp2 hybridization,

disorders in the sp2 network (diamond-like carbon, amorphous

carbon), nanostructure carbon and crystallite degree of carbon

materials [33]. All graphite-like materials, including MWCNTs

show the bands within the range of 1250e1450 (D), 1500e1600

(G) and 1610e1630 cm�1 (D0) corresponding to the first-order

Raman scattering and bands in the range of 2450e2705 cm�1

(G0) corresponding to the second-order Raman scattering. The

D, D0, G and G0 bands are related to lattice defects, finite crystal

size (nano-carbons), the tangential mode vibrations with a sp2

orbital structure of the C atoms and intrinsic property of well-

ordered sp2 carbons, respectively [29,34e36]. Fig. 5 shows the

deconvoluted bands of Raman spectra for the pristine and

treated MWCNTs. All spectra have a strong peak around

1580 cm�1 (G), which is attributed to high-frequency E2g first-

order mode; an additional band around 1350 cm�1 (D); and

aweakbandaround1620 cm�1 (D0) whichcanbe seenasa small

shoulder at the right-hand side of theG band in Fig. 5. There are

four and two vibration modes corresponding to D and G bands

in the whole spectra, respectively. Disordered structures of

MWCNTs are related to the ratio of D- to G-band intensities (ID/

IG). It is inferred that the ID/IG ratio for the pristine and treated

MWCNTswithbromine,H2SO4,HNO3andHClare indescending

order (see Table 3). Another useful ratio that is used for the

evaluation of disorder bands is ID0/IG. The decreasing ID0/IG ratio

is similar to ID/IG ratio in the spectrawhichmaybe related to the

increasingnano-cluster graphitization structures in the sample

under the treatment [35]. Therefore, bromineaffects theetching

of MWCNT walls resulting in the formation of dangling bonds

and carbon nano-clusters. The obtained results are in good

agreement with the previous analyses. Moreover, the

displacement of D0-band position is observed for the treated

MWCNTs. In literature, it is attributed to the size of nano-

carbons (corresponding to the stretching modes of the C]C

bonds) [35,36]. Also, it is reported that the carbon atoms are

generally arranged into clusters of different sizes [37e41]. The

vibration frequency (band gap) depends inversely on the cluster

size, the greater the cluster size, the lower the frequency of the

vibration [37e41]. According to this idea, MWCNTs treated by

HNO3havesmaller size of carbonclusters as comparedwith the

other samples. Furthermore, the peaks around 1275 and

1450 cm�1 are disappeared for MWCNT’s treated by HNO3. The

disappearance of these peaks can be attributed to the decrease

of the diamond-like structures.

The second-order Raman spectra with the deconvolution

bands for the pristine and treated MWCNTs are shown in

Fig. 6. The G0 bands are observed around 2450 cm�1(G10) and

2750 cm�1(G20) for all carbon structures [38,42]. Researches

indicate that the G10 band is correlated to the smaller crys-

tallite structures in the samples, while G20 band is attributed to

the larger crystallite structures. The intensity of 2700 cm�1

peak for bromine treated MWCNT’s is increased in

comparison to the other samples due to the destruction of sp2

hybridization of smaller crystallite structures.

To obtain detailed information about the pore size distri-

bution, specific surface area and pore volume, the N2

adsorption and desorption isotherms at 77 K are performed on

the samples. Fig. 7 depicts N2 adsorption isotherm of the

samples. The isotherms obviously indicate that the adsorp-

tion hysteresis behavior occurs in the range of P/P0 from 0.2 to

0.8. Therefore, all samples are mainly micro- and meso-pores

[43]. Pores are classified by diameter, macro-pores (diameter

above 50 nm), meso-pores (diameter between 2 and 50 nm)

andmicro-pores (diameter below 2 nm). In hysteresis diagram

shown in Fig. 7, the relative pressure ranges (P/P0) less than 0.4

is representing capillary condensation inmicro-pores (such as

carbon islands on the MWCNT surface) on the other hand, the

relative pressure range from 0.4 to 0.8 indicates meso-pores

(inner cavities of MWCNTs) and the relative pressure higher

than 0.8 is related to larger meso-pores (the corresponding

pore size is about 20e50 nm) [44,45]. Since the meso-pores in

range of 20e50 nm are low in the studied sample as compared

to micro-pores and meso-pores in range of 2e20 nm, the

hysteresis loop of MWCNT is only drawn for the relative

pressure range of 0.1e0.8 (Fig. 8). Fig. 8 shows the pore

Table 4 e Surface characterizations of the pristine andtreated MWCNTs determined by nitrogen physisorptionat 77 K.

MWCNTsamples

BET surfacearea

(m2 g�1)

Total porevolume(cm3 g�1)

Average porediameter

(nm)

Pristine 2.39 0.001 3.02

Treated by bromine 37 0.020 2.13

Treated by H2SO4 30 0.016 2.25

Treated by HNO3 29 0.016 2.27

Treated by HCl 24 0.014 2.30

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 1 15629

structures (BJH) of the samples. It is found that the pore size

distribution (PSD) of MWCNTs is within the range of

1.30e15.60 nm. The average pore diameter of the pristine and

treatedMWCNTswith bromine, H2SO4, HNO3 and HCl is about

3.02, 2.13, 2.25, 2.27 and 2.30 nm, respectively. The standard

0

0.2

0.4

0.6

0.8

1

-150 -100 -50 0 50 100 150

0

0.2

0.4

0.6

0.8

1

-150 -100 -50

-150 -100 -50 0 50 100 1500

0.2

0.4

0.6

0.8

1

Temperature (ºC)

Temper

Temperature (ºC)

dP/d

t(m

bar

/min

)dP

/dt(

mba

r/m

in)

dP/d

t(m

bar

/min

)

a

b

d

Fig. 9 e Volumetric data for the hydrogen storage properties of t

and HCl.

deviation for the average pore diameter of pristine and treated

MWCNTs with bromine, H2SO4, HNO3 and HCl is 0.00089,

0.0048, 0.0044, 0.0036 and 0.0038, respectively. It is observed

that after bromine treatment, the size distribution of pores is

noticeably decreased. The analytical results of surface char-

acterization for the pristine and treated MWCNTs are

summarized in Table 4. It is clearly observed that the specific

surface area and the total pore volume have significantly

increased by bromine treatment. It is worth mentioning that

according to Fig. 2 the diameter of the bromine treated

MWCNTs is not noticeably decreased, however, the surface

area is significantly increased. It is believed that the micro-

pores (1e2 nm) play an important role in adsorption

behavior of MWCNTs and improve the hydrogen storage

properties [23]. Therefore, bromine develops the micro-pore

surface area by increasing the defective cavities on the

surface of MWCNTs which results in an increasing of the

functional groups of MWCNT. Hysteresis loop shown in the

0

0.2

0.4

0.6

0.8

1

0 50 100 150

-150 -100 -50 0 50 100 150

-150 -100 -50 0 50 100 150

0

0.2

0.4

0.6

0.8

1

ature (ºC)

Temperature (ºC)

Temperature (ºC)

c

e

he pristine and treated MWCNTs by bromine, H2SO4, HNO3

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 5 6 2 2e1 5 6 3 115630

relative pressure range less than 0.4 may be related to carbon

islands induced from functional groups [23,44].

3.2. Hydrogen storage measurements

Hydrogen storage properties of the pristine MWCNTs and

treated MWCNTs were measured by volumetric technique as

shown in Fig. 9 and the results are tabulated in Table 5. The

hydrogen storage capacity of the pristine MWCNTs and treated

MWCNTswithHCl,HNO3,H2SO4 andbromine isabout 0.35, 0.62,

0.85, 0.41 and 1.15 wt%, respectively. The improvement of

hydrogen storage capacity of the bromine treatedMWCNTs is in

good agreement with the results obtained by BET and BJH anal-

yses (see Table 4 and Fig. 8). The enhancement of hydrogen

storage capacity of bromine treated MWCNTs can be related to

the increase of the micro-pore volume and the decrease of

micro- and meso-pores size of MWCNTs. Other studies also

show that the defect sites in CNTs can adsorb hydrogen mole-

cules with a considerable increase in both the adsorption

binding energy and hydrogen storage capacity in comparison to

ideal hexagonal structures of CNTs [46]. The presence of

dangling bands (defect sites) on the surface determined by

Raman spectroscopy and XPS played a significant role in

MWCNTs’ surface characteristics and enhancement of

hydrogen storage capacity. It is noticeable that the bromine

treated MWCNTs has the highest defect sites in comparison to

the other acid-treated MWCNTs. Moreover, the increase of

defective cavities (carbon islands) plays a significant role in the

improvement of hydrogen storage capacity. As Fig. 9 shows the

desorption temperature of the stored hydrogen on the pristine

MWCNTs is higher than the other treated MWCNTs. Having

increased the pore volume, hydrogen canmore easily enter into

the hollow core of MWCNTs and accumulate between the

graphite layers or desorbed from them [46]. Therefore, the

micro-pore volume and dangling bands (defect sites) on the

surface of MWCNTs play a significant role in MWCNTs’ surface

characteristics,which leadsto the improvementof thehydrogen

storage capacity and desorption temperature of adsorbed

hydrogen onMWCNTs. Moreover, the decrease of the hydrogen

desorption temperature for treated MWCNTs can be attributed

to the increasing pore volume on the surface. High-capacity

storage of hydrogen with a safe, effective and cheap system at

ambient temperature and under pressures below w100 bar is

essential in practical applications [1e5]. We believe that, in

addition to adsorption at the tips, hydrogen molecules are

Table 5 e Volumetric data for hydrogen storageproperties of the pristine and treated MWCNTs.

MWCNTsamples

Hydrogendesorptiontemperaturepeak (�C)

Hydrogen storagecapacity(wt%)

Pristine 54 0.35

Treated by bromine 30 1.15

Treated by H2SO4 52 0.41

Treated by HNO3 45 0.85

Treated by HCl 30 0.62

adsorbed on defect sites and transported through the spaces

between inner adjacent carbon layers of MWCNTs. Adsorb

hydrogen molecules are stored on the carbon structures and

FeeNi particles [27]. On the other hand, it is found that metal

atoms (such as Ti and Pd) can bond to the functional groups of

CNTs and increase their hydrogen storage capacity [27]. In

summary, Bromine treatment improves the hydrogen storage

capacity of MWCNTs by increasing the micro-pore volume and

the functional groups of MWCNTs surface. To our knowledge,

MWCNTs treated by bromine has high-capacity of hydrogen

storage in comparison with other acidic treatments [22,27e29].

4. Conclusion

Our results indicate that bromine has destructive effects

(etching MWCNT walls) on sp2 hybridization with smaller

crystallite properties which leads to the formation of dangling

bonds and nano-cluster on the wall of MWCNTs. It is also

observed that after bromine treatment, the size distribution of

pores is noticeably decreased and the specific surface area and

total micro-pore volume are significantly increased. It is

noticeable that the bromine treated MWCNTs has the highest

amount of the FeeNi alloy in comparison to the other acid-

treated MWCNTs. Moreover, the mean diameter of the

bromine treated MWCNTs is not significantly decreased in

spite of those treated with the other acids, whereas it is found

that the defective cavities (micro-pores and defect sites) on

the surface of MWCNTs are increased by treating with

bromine. The increase of defective cavities plays a significant

role in the improvement of hydrogen storage capacity and

decreases the hydrogen desorption temperature.

Acknowledgements

The authorswould like to thank theMaterials Research School

for their cooperation and supports.

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