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Synthesis of helical carbon nanofibres and its applicationin hydrogen desorption

Himanshu Raghubanshi*, M. Sterlin Leo Hudson, O.N. Srivastava

Nano-Science and Technology Unit, Department of Physics, Banaras Hindu University, Varanasi 221005, UP, India

a r t i c l e i n f o

Article history:

Received 14 September 2010

Received in revised form

30 December 2010

Accepted 31 December 2010

Available online 1 February 2011

Keywords:

Helical carbon nanofibres

Alloy

Oxidative dissociation

Polygonal shape

Catalytic activity

Sodium alanate

* Corresponding author. Tel.: þ91 9450392853E-mail address: hraghubanshi@rediffmai

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2010.12.139

a b s t r a c t

In this communication, we report the synthesis of helical carbon nanofibres (HCNFs) by

employing hydrogen storage intermetallic LaNi5 as the catalyst precursor. It was observed

that oxidative dissociation of LaNi5 alloy (2LaNi5 þ 3/2O2 / La2O3 þ 10Ni) occurred during

synthesis. The Ni particles obtained through this process instantly interacted with C2H2 and

H2 gases, and fragmented to nanoparticles of Ni (w150 nm) with polygonal shape. These

polygonal shapes of Ni nanoparticles were decisive for the growth of helical carbon nano-

fibres (HCNFs) at 650 �C.TEM, SAEDandEDAXstudieshave shown thatHCNFshave grownon

Ni nanoparticles. Typical diameter and length of the HCNFs are w150 nm and 6e8 mm

respectively. BET surface area of these typical HCNFs has been found to be 127 m2/g. It was

found that at temperature 750 �C, spherical shapes of Ni nanoparticles were produced and

decisive for the growth of planar carbon nanofibres (PCNFs). The diameter and length of the

PCNFs arew200 nm and 6e8 mmrespectively. In order to explore the application potential of

the present as-synthesized CNFs, they were used as a catalyst for enhancing the hydrogen

desorption kinetics of sodium aluminum hydride (NaAlH4). We have found that the present

as-synthesized HCNFs, with metallic impurities, indeed work as an effective catalyst. The

pristineNaAlH4 and 8mol%as-synthesizedHCNFs admixedNaAlH4, at 160 �Ce180 �Cand for

the duration of 5 h, liberate 0.8 wt% and 4.36 wt% of hydrogen, respectively. Thus there is an

enhancementofw5 times inkineticswhenas-synthesizedHCNFsareusedas thecatalyst. To

the best of our knowledge, the use of hydrogen storage alloy LaNi5 as the catalyst precursor

for the growth of HCNFs has not yet been done and thus represents a new feature relating to

the growth of HCNFs. Furthermore, we have shown that the as-synthesized HCNFs work as

aneffectivenewcatalyst for improving thedehydrogenationkineticsof thecomplexhydride,

NaAlH4.

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

reserved.

1. Introduction Helical carbon nanofibres (HCNFs) can be used as the rein-

Much attention has been paid toward the synthesis and

applications of helical carbon nanofibres because of their

exotic chiral morphologies and unique physical properties

[1,2]. Baker et al. [3] worked on the initial extensive and pio-

neering studies on the synthesis of carbon nanofibres (CNFs).

; fax: þ91 542 2368390.l.com (H. Raghubanshi).2011, Hydrogen Energy P

forcement materials, battery components, minisprings,

microwave absorbers, magnetic beam’s generators, catalyst

and catalyst support, etc. [4,5]. Some other advantageous

features of these HCNFs are its high magnetization and field

emission characteristics [6,7]. CNFs have the good conduc-

tivity and mechanical strength, which makes them suitable

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 ) 4 4 8 2e4 4 9 0 4483

for nanosized electronic and mechanical devices [8]. The

synthesis of HCNFs has been achieved by employing carbon

containing gases such as C2H2, CH4, etc., which are used as

feedstock gases [9e11]. The catalyst used corresponds to

specially prepared transition metal nanoparticles such as Fe

[6], Ni [9], In (iron coated) [10], Co [12], Cu [13], etc. As for

example, Tang et al. [6] synthesized HCNFs using Fe nano-

particles prepared by the combined solegel reductionmethod.

Synthesis of HCNFs through dissociation of hydrocarbon

directly on conventional catalyst particles is generally not

feasible. Therefore, very often chiral agent for tailoring the

catalyst particles and growth promoters are used. Some

recent such studies have employed sulfur embedded in thio-

phene or sulfuretted hydrogen together with catalytic parti-

cles [1,9]. Recently, Liu et al. [9] synthesizedmicro-coiled CNFs

on the graphite substrate using co-electrodeposition of nickel

and sulfur as catalysts. They have reported that the presence

of sulfur content in the catalyst affects morphology of CNFs.

Recently, water soluble catalysts such as alkali chlorides have

also been used for the synthesis of HCNFs [14]. It may be

mentioned that preparation method of nanoparticle catalysts

is an involved process.

Here, we report the synthesis of HCNFs by employing

hydrogen storage alloy LaNi5 as a catalyst precursor. Complete

oxidative dissociation of the hydrogen storage alloy LaNi5,

(2LaNi5 þ 3/2O2 / La2O3 þ 10Ni) occurred during synthesis.

This leads to availability of Ni particles. By the interaction of

C2H2 and H2 gas, these Ni particles fragmented into Ni nano-

particles. These Ni nanoparticles were decisive for the

formation of HCNFs. A comparatively interesting feature of

the present synthesis route is that the polygonal Ni nano-

particles have been obtained through the above said simple

oxidative process followed by the interaction with carrier and

feedstock gas, of a well known and readily available inter-

metallic material LaNi5. Also no growth promoter or chiral

agent formodification of Ni catalyst particles has been used in

the present synthesis of HCNFs. In addition to the synthesis,

we have elucidated the use of as-synthesized HCNFs which

contains some metal impurity, as an effective catalyst for

improving the desorption kinetics of the hydrogen storage

material NaAlH4. This is a potential new hydrogen storage

material for which a viable catalyst for enhancing the

desorption kinetics is being extensively investigated. The

complex hydride NaAlH4 has very poor desorption kinetics

and a catalyst, for which intensive studies are being made, is

needed for enhancing the desorption kinetics to make this

material a viable storage system [15]. In view of the fact that,

several of the carbon nano-phases are now known to possess

catalytic characteristics [16]. We have employed the as-

synthesized HCNFs and found that this indeed works as an

effective catalyst for enhancing desorption kinetics of NaAlH4.

2. Experimental details

2.1. Preparation of catalyst precursor

The precursor LaNi5 for the synthesis of HCNFs was prepared

in our hydrogen energy laboratory. The detailed synthesis of

LaNi5 alloy, by melting high purity La (99.90%) and Ni (99.99%)

in correct stoichiometric proportions using a radio-frequency

induction furnace (18 kW) are described in our earlier publi-

cation [17]. The alloy samples so prepared to be then ball-

milled to smaller particles of sizew6 mm. These particles were

used as the catalyst precursor for the growth of CNFs. The

lattice-structure of as prepared LaNi5 and carbonmaterial was

checked through X-ray diffraction (XRD).

2.2. Synthesis of HCNFs

In the present investigation, HCNFs have been synthesized by

employing catalytic thermal decomposition of acetylene

(C2H2). Acetylene was prepared through a unique and inex-

pensive process by employing calcium carbide (CaC2) stone

and water [CaC2 þ 2H2O / C2H2 þ Ca(OH)2]. Such a process

has been used by us for the preparation of planar CNFs by

using CueNi catalyst [18]. The thermal decomposition/

cracking has been carried out by filling C2H2 together with H2

inside a silica tube (75 cm long, 2.5 cm diameter) containing

inlet ports for C2H2, H2 and He gases at one end, the other end

being closed. We have employed 100 mg of the catalyst which

was found optimum for our experimental setup where C2H2

was thermally dissociated to form HCNFs. After this,

hydrogen and acetylene gas in the ratio 1:4, were filled in the

silica tube at a total pressure of 450 Torr (90 Torr for H2 and

360 Torr for C2H2). The cracking of the gases was achieved by

heating these gases in the presence of catalyst at 650 �C for 2 h

in a resistance heated furnace. The carbon deposition takes

place as a result of cracking of acetylene over the catalyst

particle. The tube was then allowed to cool at a rate of 5 �C/min. Before taking out the carbon deposits, the as-grown

material was flushed with 10%, He and air. The synthesis

temperature of CNFs reported in this paper is correct within

the range �3 �C.

2.3. Hydrogen desorption measurement

As received NaAlH4 (Aldrich, tech. 90%) has been usedwithout

further purification. We used the as-synthesized HCNFs and

PCNFs (with metallic impurities) in the present study to

explore its catalytic activity in NaAlH4. It was found that as-

synthesized CNFs concentration of 8 mol% led to optimum

results. NaAlH4 and 8 mol% as-synthesized HCNFs was

admixed together for 1 h under the inert atmosphere in

a chromeenickel stainless steel milling vial of volume 150 cc

with two stainless steel balls of 8.5 g each and one ball of

0.25 g, in a locally fabricated ball miller [19]. The milling vial

was purged with Ar gas several times before loading the

sample tomake it oxygen free. Thermal decompositions of the

as-synthesized CNFs admixed NaAlH4 sample were moni-

tored using computerized pressure composition isotherm

(PeCeI) measurement system supplied by Advanced Mate-

rials Corporation (AMC), U.S.A (standard deviation of data

derived from this machine: pressure <�0.1 bar, temperature

<�0.1 �C). About 0.5 g of the material was loaded in the PeCeI

evaluation holder supplied by AMC, which was evacuated and

inserted in the furnace. The machine was then programmed

tomonitor the amount of hydrogen liberated from the sample

after reaching the desired temperature (at 160 �C for 3 h and

then at 180 �C for 2 h).

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2.4. Characterization techniques

The structural characterization of the as-grown material has

been carried out using X-ray diffraction employing X’Pert PRO

PANalytical diffractometer equipped with a graphite mono-

chromator with a Cu source (l ¼ 1.54 A, CuKa operating at

45 kV and 40 mA). The microstructural analysis was carried

out by employing transmission electron microscopy (TECNAI

G20; 200 kV) in diffraction and imaging modes. Compositional

analysis was performed by an energy dispersive X-ray anal-

ysis (EDAX) coupled with TEM. BET surface area of CNFs, was

measured by Coulter Counter SA 3100 from N2 adsorp-

tionedesorption isotherms. Raman Spectra of the CNF sample

was recorded from Renishaw Raman Spectrometer (model no.

H 45517) using an argon ion laser l ¼ 514 nm. Average particle

size of catalyst precursor was measured by Particle Size

Analyzer CIS-50 (Ankersmid) and this was also confirmed by

TEM image analysis.

3. Results and discussion

The catalyst precursor LaNi5 and the as-grown carbon

deposits were characterized through X-ray Diffraction (XRD).

Fig. 1a shows XRD of primary catalyst precursor (LaNi5) phase.

Fig. 1b represents XRD pattern of the as-synthesized carbon

phase concurrently with oxidized dissociation of LaNi5, which

occurred during synthesis. Fig. 1c shows XRD pattern of the

acid (Conc. HNO3) treated as-grown products. As it can be seen

from Fig. 1c, the acid treatment employed for purification

removes the Ni catalyst particles. The (002) peak correspond-

ing to CNFs remains unaffected. This shows that acid treat-

ment does not affect the carbon nanophase. Analysis of the

XRD pattern (Fig. 1b) revealed the presence of Ni together with

La2O3, which is present in a nearly amorphous form and only

in small quantities. This is expected since in the LaNi5,

Fig. 1 e X-ray diffractogram of (a) LaNi5 alloy as the catalyst

precursor, (b) as-grown carbon nanophase and oxidized

dissociation of LaNi5, during synthesis and (c) Purified

carbon nanophase through acid treatment (Conc. HNO3)

indicating the removal of Ni catalyst particles.

compared to the amount of Ni, La, which reacts with O2 to

form La2O3 is comparatively small. It may be pointed that the

starting material LaNi5 particle sizes were w6 mm confirmed

from TEM images (Fig. not shown here). However, the Ni

particles which act as the active catalyst for the synthesis of

CNF is much smaller such as in nanometer range. Their sizes

are about w150 nm (shown in Fig. 2). This suggests that Ni

particles originating from oxidative dissociation of LaNi5(during synthesis) are instantaneously interacted with C2H2

and H2 gases, and the big Ni particles break into smaller ones

(nano range) and by the surface reconstruction phenomenon,

it adopted polygonal shapes. The most likely reason for this is

the interaction of C2H2 with bigger Ni particle. This may lead

to the formation of metastable NieC or NieH compound,

leading to breakage of bigger size Ni particles into smaller

nanoscale Ni particles [20], which work as the active catalyst

for the growth of HCNFs. It is known that oxidation of LaNi5takes place through the lattice dissolved oxygen or reaction

with direct oxygen [21]. In the present case required oxygen

for oxidation comes from the oxygen present in the residual

air due to vacuumof the order ofw10�3 Torr and some oxygen

with the acetylene gas (since here the production of acetylene

gas involved a unique and inexpensive process, see the

experimental part of synthesis of HCNFs). From Fig. 1b, in

addition to La2O3 and Ni the other dominant peak is at

0.342 nm. This corresponds to the known most intense (002)

XRD peak of graphite.

Extensive TEM microstructural analysis shows that the

polygonal Ni nanoparticles lead to the growth of carbon

nanostructures. Representative TEM micrograph is shown in

Fig. 2. Fig. 2a clearly brings out the polygonal Ni nanoparticle

at the tip of HCNF (see also Fig. 4b). Fig. 2b shows the micro-

structure of this catalyst particle at the higher magnification.

It again clearly reveals that the growth of carbon nanophase

takes place over polygonal Ni nanoparticles. The typical size

Fig. 2 e Representative TEM images (a) HCNF grown on

polygonal Ni nanoparticle, (b) polygonal Ni nanoparticles

as shown in (a) at the higher magnification. Inset of Fig. 2a

shows the four (002) type carbon diffraction spots revealing

the carbon nanophase to be nanofibre (Herringbone type).

Inset of Fig. 4b shows the diffraction pattern of the catalyst

particle along with that of HCNF.

Fig. 3 e (a) Raman spectra showing clear appearance of high intensity G-band over D-band exhibiting the quit good graphitic

nature of HCNFs and (b) EDAX pattern of the catalyst particle along with HCNF, shows the presence of carbon and nickel; (Cu

is due to EM grid).

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of the catalyst nanoparticle is found to be w150 nm. This is as

expected compatible with the diameter of the as-grown

nanostructures. Representative selected area electron

diffraction (SAED) pattern from the carbon nanophase con-

taining four (002) type arced spots are shown in the insets of

Fig. 2a. This is based on the revelation of known results that

the as-grown carbon nanostructures are of herringbone type

CNFs [22]. The inset of Fig. 2b shows the SAED pattern of the

CNF with catalyst particle, and the catalyst particles were

indexed with the known face-centered cubic structure of

Nickel (Ni).

Raman spectra of the CNFs are shown in Fig. 3a. The higher

intensity of G-band with respect to D-band reveals that quite

good graphitic nature is maintained in these CNFs. The

observed catalyst nanoparticles were also confirmed to be Ni

nanoparticles based on exploration through EDAX coupled to

the TEM. A representative EDAX pattern of the catalyst

particle with HCNFs is shown in Fig. 3b. It clearly reveals the

presence of Ni. The presence of Cu is due to E.M. copper grid

on which the samples are placed for analysis. The presence of

carbon is apparently due to carbon nanophase grown over the

Fig. 4 e Representative TEM images (a) HCNFs along with planar

that HCNF grown from the polygonal Ni nanoparticle (indicated

Ni nanoparticles. Thus it can be said that polygonal Ni nano-

particles work as the active catalyst for the growth of helical

carbon nanophase. Other examples of the HCNFs are shown

in Fig. 4a. This figure shows that the HCNFs are the dominant

type of CNFs with a yield ofw90%, the remainingw10% being

PCNFs. The observation of the polygonal nanoparticle is in

keeping with the known fact that the growth of HCNFs

requires the presence of polygonal nanoparticle catalyst [23].

The other example of polygonal catalyst particle is shown in

Fig. 4b for the growth of HCNF. Polygonal Ni nanoparticles

(Figs. 2 and 4b) were formed, presumably due to known

process of reconstruction of the particle surface with the well

defined crystallographic orientation on the interaction with

gases [24,25]. In support of this, Carneiro et al. [26] reported

that by the co-adsorption of carbon feedstock gases on the

metal surface, metals get the surface reconstruction

phenomenon and adopted some specific shape for the growth

of various types of CNFs. BET surface area was measured of

these HCNFs. Surface area was found to be 127 m2/g. It shows

that these fibers possess high surface area. The microstruc-

tures shown in Fig. 4 bring out the helical nature of the

CNFs with yield of w90% and w10% respectively (b) reveals

by arrow).

Fig. 5 e TEM images (a) as-grown HCNFs represent the long length of these fibers and (b) HCNF at higher magnification

showing its helical nature and pitch.

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nanofibres. We shall, therefore, denote the as-grown carbon

nanophase as HCNFs. TEM micrographs shown in Fig. 5

exhibit other examples of the as-grown HCNFs. Fig. 5a repre-

sent the long length of these fibers and Fig. 5b represent the

magnified TEM image, which indicates its helical nature and

pitch. Fig. 6(a and b) brought out the unique Y-structured

HCNFs. These were taken fromdifferent regions of the sample

which proves that the two HCNF branches have originated

from single HCNF. This unique type of Y-shaped HCNF gets

formed only rarely and reported in very few papers.

The effect of temperature on the catalyst shapes for the

formation of CNFs was also investigated. Keeping all the

above experimental conditions (for the synthesis of HCNFs)

fixed, we have increased the synthesis temperature from 650

to 750 �C. Interestingly, the spherical (approximately) shaped

Ni catalyst particles with average size of w200 nm gets

formed, which are decisive for the growth of PCNFs (indicated

by the arrow in Fig. 7). The diameters of the PCNFs were also

200 nm in consequence of the size of the catalyst particles.

This study also suggests that, by increasing the synthesis

temperature from 650 to 750 �C, the size of the catalyst particle

and hence diameter of the CNFs enhances from 150 to 200 nm,

and it can be understood by particle agglomeration phenom-

enon. Huang et al. [27] reported that the synthesis tempera-

ture dramatically affects the morphology and topography of

Fig. 6 e Typical TEM images of HCNFs; (a) and (b) shows Y-sha

the catalysts, which play an important role in the synthesis of

the various types of CNFs. In addition to this, we have also

obtained the interesting V-shape PCNFs at the synthesis

temperature of 750 �C (Fig. 7c and d). The nearly spherical

shaped Ni catalyst particles are located at the joint branch of

the V-shape PCNFs which is indicated by the arrow in Fig. 7(c

and d).

3.1. Application of helical carbon nanofibres (HCNFs)

One recent application of CNFswithmetallic impuritieswhich

has attracted considerable attention is its use as an effective

catalyst for MgH2 [28,29]. The new exotic hydrogen storage

material, the complex hydride NaAlH4 is known to possess the

high hydrogen storage capacity of >5 wt% [30]. It may be

pointed out that in sharp contrast to the intermetallic

hydrogen storagematerial (e.g. LaNi5, FeTi and ZrFe2, etc.), the

new complex hydride NaAlH4 is a built-in hydride compound.

The desorption kinetics of NaAlH4 atmanageable temperature

(w150 �C) is very slow [31] and needs to be improved. Here, the

main issue is to find suitable catalysts, which lead to

amenable desorption of hydrogen. In the pioneering work of

finding a catalyst for NaAlH4, Bogdanovic et al. [32] have used

Ti as a catalyst for improving hydrogen desorption from

NaAlH4. However, Ti is ametal, and it adds to theweight of the

pe HCNFs and taken from different regions of the sample.

Fig. 7 e TEM images of PCNFs synthesized at 750 �C, PCNFs grown over spherical shape catalyst particles, indicated by arrow

(a) and (b); (c) and (d) shows V-shape planar CNFs which were grown over nearly spherical shape Ni nanoparticles (indicated

by arrow).

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matrix. Furthermore, Ti is prone to oxidation. Themechanism

through which Ti works as a catalyst for NaAlH4 is not proven

so far [33]. Therefore, a continuous search for a more effective

catalyst is going on. In the light of the above and the fact that

CNFs also possesses catalytic activity [28,34,35], we explored

the use of as-synthesized HCNFs in the present investigation

for possible enhancement of the hydrogen desorption kinetics

of NaAlH4. One advantageous feature of CNF as compared to

Ti is that, it is much lighter and will not be incorporated in the

NaAlH4 lattice.

Fig. 8 e Desorption kinetics of 8 mol% as-synthesized HCNFs a

admixed NaAlH4 (curve B) and pristine NaAlH4 (curve C) at (a) 1

The desorption kinetics of 8 mol% as-synthesized HCNFs

admixed NaAlH4 has been carried out at 160 �C and 180 �C for

5 h (3 h at 160 �C and 2 h at 180 �C). The total hydrogen des-

orbed from 8 mol% as-synthesized HCNFs admixed NaAlH4 is

4.36 wt%. For the sake of comparison, the desorption kinetics

of 8mol% PCNFs (whichwere synthesized at 750 �C employing

the same catalyst precursor LaNi5) admixed NaAlH4 and

pristine NaAlH4 at 160 �C and 180 �C were also compared.

A representative desorption kinetic curve is shown in Fig. 8(a

and b). As is evident from Fig. 8, the total hydrogen desorbed

dmixed NaAlH4 (curve A), 8 mol% as-synthesized PCNFs

60 �C and (b) 180 �C.

Fig. 10 e Rehydrogenation kinetic curves of 8 mol% as-

synthesized HCNFs admixed NaAlH4 (curve A), 8 mol%

purified HCNFs admixed NaAlH4 (curve B) and pristine

NaAlH4 (curve C).

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from 8 mol% as-synthesized HCNFs admixed NaAlH4 and

pristine NaAlH4 is 4.36 wt% and 0.8 wt% hydrogen, respec-

tively. The desorption kinetics of 8 mol% as-synthesized

PCNFs admixed NaAlH4 (w3.1 wt%) is found to be less than

that of 8 mol% as-synthesized HCNFs admixed NaAlH4

(4.36 wt%). Thus admixing of 8 mol% as-synthesized HCNFs

improves the hydrogen liberation kinetics of pristine NaAlH4

by five times. The temperature programmed desorption (TPD)

curve as shown in Fig. 9 also confirms that as-synthesized

HCNFs is superior to as-synthesized PCNFs for improving the

low temperature dehydrogenation behavior of NaAlH4.

It should be pointed out that the as-synthesized CNFs

(helical and planar) incorporate metal impurities, particularly

Ni and La2O3 in a small amount from their preparation. We

have also used purified HCNFs as the catalyst for NaAlH4. The

catalytic effect of HCNFs with Ni particles (as-synthesized

HCNFs) has been estimated to bew10e20% higher than that of

Ni free HCNFs (purified HCNFs). It is also known that La2O3

possess catalytic activity for NaAlH4 system [36]. We have

therefore measured the rehydrogenation kinetics of 8 mol%

as-synthesized HCNFs admixed NaAlH4 under 90 atm H2

pressure at 120 �C for more than 3 h. For comparison, we have

monitored the rehydrogenation behavior of 8 mol% purified

HCNFs (without metal impurities) admixed NaAlH4 and pris-

tine NaAlH4 under similar condition. Fig. 10 shows the rehy-

drogenation kinetics of as-synthesized HCNFs admixed

NaAlH4, purified HCNFs admixed NaAlH4 and pristine NaAlH4.

It has been found that the rehydrogenation behavior of as-

synthesized HCNFs admixed NaAlH4 is superior to that of

purified HCNFs admixed NaAlH4 and pristine NaAlH4 (1.8 wt%

H2 for as-synthesized HCNFs admixed NaAlH4, 1.4 wt% H2 for

purified HCNFs admixed NaAlH4 and almost no reabsorption

for pristine NaAlH4). Thus the admixing of as-synthesized

Fig. 9 e Temperature Programmed Desorption (TPD) curves

of 8 mol% as-synthesized HCNFs admixed NaAlH4 (curve

A), 8 mol% as-synthesized PCNFs admixed NaAlH4 (curve

B) and pristine NaAlH4 (curve C). The red dotted line

represents the temperature profile of sample. (For

interpretation of the references to colour in this figure

legend, the reader is referred to the web version of this

article).

HCNFs with NaAlH4 markedly improves the desorption/reab-

sorption kinetics of NaAlH4. So it can be said that mixture

(composite) of CNFs with small metal impurities, acts like an

especially good catalyst for NaAlH4. Similar interesting

conclusion was made for MgH2 material admixed CNFs with

metal impurities [28,34].

Comparison of morphology and particle size of the pristine

and as-synthesized HCNFs admixed samples, as evidenced by

SEM micrographs, did not show any significant difference.

Therefore, enhancement in desorption kinetics for the pre-

sent case cannot be attributed tomicrostructural changes. It is

also reported that, CNFs with metal impurities gives better

results [28,34,37]. However, in present case the new feature is

that we have employed helical and planar CNFs with metallic

impurities in NaAlH4 system for the first time. There are two

reasons for the superior catalytic activity of as-synthesized

HCNFs over as-synthesized PCNFs. One reason corresponds to

curvature effect induced higher electronegativity of HCNFs in

comparison with PCNFs [38,39]. The other corresponds to the

Ni particles associated with HCNFs are smaller (150 nm) than

Ni particles associated with PCNFs (200 nm). It is well known

that Ni particles possess very good catalytic activity for the

hydrogen storage materials, and the smaller particle size

improves the catalytic activity [34]. In present case HCNFs and

PCNFs both contain metal impurities (as-synthesized),

however, we have obtained better results with as-synthesized

HCNFs. It can thus be said that as-synthesized HCNFs are an

effective catalyst for desorption of hydrogen from NaAlH4. On

the other hand, as-synthesized PCNFs possesses lower cata-

lytic activity when compared to as-synthesized HCNFs.

4. Conclusions

Helical carbon nanofibres (HCNFs) have been successfully

synthesized employing LaNi5 alloy as the catalyst precursor.

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The Ni particles are derived from the oxidative dissociation of

LaNi5 during synthesis. The bigger Ni particles interact with

C2H2 and H2 gases and get fragmented into small Ni nano-

particles of size 150 nmwith polygonal shape. These polygonal

shapes of Ni particles worked as the active catalyst for the

growth of HCNFs at 650 �C. Y-structured special HCNFs have

also been found in the present investigation. BET analysis

shows that the HCNFs possess high surface area (127 m2/g).

Temperature effect was also studied; it has been found that at

the synthesis temperature 750 �C spherical shaped Ni particles

gets produced, which are decisive for the growth of PCNFs. We

have checked the application aspect of as-synthesized HCNFs

and PCNFs with metal impurities by using them as a catalyst

for enhancing the hydrogen desorption kinetics of NaAlH4. It

has been found that 8 mol% as-synthesized HCNFs admixed

NaAlH4 and pristine NaAlH4, for the temperature range of

160 �Ce180 �C and for the duration of 5 h released the total

dehydrogenation capacity of 4.36 wt% H2 and 0.8 wt% H2

respectively. Thus there is an enhancement of hydrogen

desorption kinetics for as-synthesizedHCNFs admixed NaAlH4

by five times as compared to pristine NaAlH4. The present

studies related to the synthesis of HCNFs by using LaNi5 alloy

as catalyst precursor and the use of as-synthesized HCNFs as

the catalyst for enhancement of desorption kinetics in NaAlH4

is the first report of its type.

Acknowledgments

The authors are thankful to Late Prof. A.R. Verma, Dr. R. Chi-

dambaram, Prof. C.N.R. Rao, and Prof. D.P. Singh (VC BHU) for

their encouragement. The authors gratefully acknowledge to

Prof. R.S. Tiwari, Dr. M.A. Shaz, Dr. T.P Yadav, Mr. D Pukazh-

selvan and Mr. Rajesh Kumar Singh for their helpful discus-

sions. The authors express their sincere gratitude to Prof. J.

Kumar (IITK) for his kind help to carry out BET surface area

analysis and Prof. A.S.K Sinha (BHU IT) for help in particle size

analysis. Mr. Vijay Kumar and Mr. Vimal Kumar are

acknowledged for their technical support in EM and XRDwork

respectively. Thanks are also due to Prof. A.C. Pandey (Nano-

phosphor Center, Allahabad Univ.) for taking Raman Spectra

of HCNFs. Financial support from the DST (UNANST), UGC,

CSIR and MNRE are gratefully acknowledged.

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