Synthesis of helical carbon nanofibres and its application .... 2011 - Synthesis of helical...
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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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