Polyacrylonitrile fibers anchored cobalt/graphene sheet nanocomposite: A low-cost, high-performance and reusable catalyst for hydrogen generation

Article abstract of DOI:10.1166/jnn.2016.11659

Cobalt and its composites are known to be active and inexpensive catalysts in sodium borohydride (NaBH₄ ) hydrolysis to generate clean and renewable hydrogen energy. A novel fiber catalyst, cobalt/graphene sheet nanocomposite anchored on polyacrylonitrile fibers (Co/GRs-PANFs), which can be easily recycled and used in any reactor with different shapes, were synthesized by anchoring cobalt/graphene (Co/GRs) on polyacrylonitrile fibers coated with graphene (GRs-PANFs) at low temperature. The unique structure design effectively prevents the inter-sheet restacking of Co/GRs and fully exploits the large surface area of novel hybrid material for generate hydrogen. And the extra electron transfer path supplied by GRs on the surface of GRs-PANFs can also enhance their catalysis performances. The catalytic activity of the catalyst was investigated by the hydrolysis of NaBH₄ in aqueous solution with GRs-PANFs. GRs powders and Co powders were used as control groups. It was found that both GRs and fiber contributed to the hydrogen generation rate of Co/GRs-PANFs (3222 mL · min^-1 · g^-1), which is much higher than that of cobalt powders (915 mL · min^-1 · g^-1 ) and Co/GRs (995 mL ·min^-1 ·g^-1 ). The improved hydrogen generation rate, low cost and uncomplicated recycling make the Co/GRs-PANFs promising candidate as catalysts for hydrogen generation.

Full text of DOI:10.1166/jnn.2016.11659

Article
Journal of
Nanoscience and Nanotechnology
Vol. 16, 5627–5632, 2016
Copyright © 2016 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Polyacrylonitrile Fibers Anchored Cobalt/Graphene Sheet
Nanocomposite: A Low-Cost, High-Performance and
Reusable Catalyst for Hydrogen Generation
1
1
1
1ꢀ∗
1
2ꢀ∗
Fei Zhang , Guoji Huang , Chengyi Hou , Hongzhi Wang , Qinghong Zhang , and Yaogang Li
1State Key Laboratory for Modification of Chemical Fibres and Polymer Materials, College of Material Science and Engineering,
Donghua University, Shanghai, 201620, China
2
Engineering Research Center of Advanced Glasses Manufacturing Technology, MOE,
Donghua University, Shanghai, 201620, China
Cobalt and its composites are known to be active and inexpensive catalysts in sodium borohy-
dride (NaBH ) hydrolysis to generate clean and renewable hydrogen energy. A novel fiber catalyst,
4
cobalt/graphene sheet nanocomposite anchored on polyacrylonitrile fibers (Co/GRs-PANFs), which
can be easily recycled and used in any reactor with different shapes, were synthesized by anchor-
ing cobalt/graphene (Co/GRs) on polyacrylonitrile fibers coated with graphene (GRs-PANFs) at low
temperature. The unique structure design effectively prevents the inter-sheet restacking of Co/GRs
and fully exploits the large surface area of novel hybrid material for generate hydrogen. And the extra
Delivered by Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
electron transfer path supplied by GRs on the surface of GRs-PANFs can also enhance their catal-
IP: 191.101.55.64 On: Sat, 07 May 2016 05:56:26
ysis performances. The catalytic activity of the catalyst was investigated by the hydrolysis of NaBH
Copyright: American Scientific Publishers
4
in aqueous solution with GRs-PANFs. GRs powders and Co powders were used as control groups.
It was found that both GRs and fiber contributed to the hydrogen generation rate of Co/GRs-PANFs
−
1
−1
−1
−1
(
3222 mL·min ·g ), which is much higher than that of cobalt powders (915 mL·min ·g ) and
−1 −1
Co/GRs (995 mL·min ·g ). The improved hydrogen generation rate, low cost and uncomplicated
recycling make the Co/GRs-PANFs promising candidate as catalysts for hydrogen generation.
Keywords: Graphene Nanosheets, Cobalt, Fiber, Catalysis, Sodium Borohydride.
1
. INTRODUCTION
graphene-based metal nanocomposites.18–20 The graphene-
based novel hybrid materials’ catalytic performance were
improved due to the improved electrical conductivity and
high specific surface area of graphene. However, the struc-
ture deficiency of the hybrid materials has greatly limited
their catalytic performance. The graphene-based nanocom-
posites tend to aggregate or restack due to the strong
inter-sheet van der Waals attractions which decreases
the utilization of high specific surface area offered by
graphene supported catalysts. So it is still important to
develop novel hybrid catalyst with high specific surface
area and electric conductivity to increase their catalytic
performance. This is still a challenge that needs to be
addressed.
Hydrogen, as a renewable resource free of pollution,
is very promising for application in heating, electricity
generation, mechanical power and transportation. Sev-
eral ways have been used to obtain hydrogen, among
which the hydrolysis of NaBH is regarded as an efficient
way. Many catalysts were supported to control the hydro-
gen generation rate during the hydrolysis of NaBH4,
whereby their high cost appears to be a big impedi-
ment for their broad applications. At the same time, even
though low cost metal catalysts are also widely used in
the hydrolysis of NaBH4, their catalyst activity is limited
by their low specific surface area and electric conductivity.
Recently, owing to the unique physical and chemical prop-
1
2
–4
4
5
6
–10
1
1
1
2–17
erties of graphene and carbon materials,
have investigated the electrochemical performance of
many groups
In this work, cobalt/graphene sheets nanocomposite
anchored on polyacrylonitrile fibers (Co/GRs-PANFs)
were successfully synthesized. Their unique structure
effectively prevents the inter-sheet restacking of Co/GRs
∗
Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2016, Vol. 16, No. 6
1533-4880/2016/16/5627/006
doi:10.1166/jnn.2016.11659
5627

Polyacrylonitrile Fibers Anchored Cobalt/Graphene Sheet Nanocomposite
Zhang et al.
and fully exploits the large surface area of novel hybrid
material for generate hydrogen. In comparison with the
powder catalysts mentioned above, the hydrogen genera-
tion rate of the novel hybrid material was greatly improved
and the novel fiber catalyst can be easily recycled and
used in any reactor with different shapes. Based on the
synthesis method, different kinds of fiber catalysts can be
obtained by changing the fiber and the catalyst. The fabri-
cation detail, characterization, feasibility, and performance
of the novel fiber catalyst are reported in the following
sections.
2
. EXPERIMENTAL DETAILS
2
.1. Synthesis of GO
2
1
GO was prepared using the Hummers’ method. Natural
flake graphite powder (2 g) was added to 46 ml of cold
ꢀ
(
0 C) concentrated H SO . KMnO (6 g) was then added
2
4
4
gradually while cooling and stirring in an ice bath. Then
ꢀ
the mixture was stirred at 35 C for 2 h. Distilled water
(
92 mL) was slowly added to the mixture and the tem-
ꢀ
perature of the mixture was maintained below 98 C for
Figure 1. Schematic diagram showing the formation of polyacryloni-
trile fiber anchored cobalt/graphene sheet nanocomposite, and a con-
structed circuit using Co/GRs-PANFs as conductive wire.
1
5 min. Then, 280 mL of 30% H O solution was added
2 2
to the mixture. The product was collected by filtration and
washed with 1000 ml of 10% HCl aqueous solution to
remove metal ions, before a final wash with distilled water.
The brown yellow powder of GO thus obtained was dried
(
0.1 g APTES dissolved in 60 mL of alcoholic solution
with the volume ration of water and alcohol is 1:2) at
for hybrid Dp ree lpi va er ar tei od n b. y Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
ꢀ
IP: 191.101.55.64 On: Sat, r0o 7o mM at ey m2p 0e 1r a6 t u0 r 5e : f5 o6r : 21 62 h and dried at 70 C for 12 h to
Copyright: American S cc ioe mn tp i fl iec teP tuh be li rs eha ec rt is on between APTES and the hydroxyl
2
.2. Solvothermal Synthesis of Co/GRs Hybrids
on the surface of the hydroxylated PANFs.
c) GO-PANFs: Subsequently, the charge polarity on the
The Co/GRs hybrid prepared using the solvothermal
(
2
2
method according to our previous work. In detail, 18 mg
of GO sheets dispersed in 18 mL of ethylene glycol (EG)
was sonicated for 30 min. Then, 55 mg of Co(NO ꢀ ·
surface of the positively charged fibers was reversed by
the adsorption of a layer of negatively charged GO in
3
2
−1
ꢀ
1
mg ·L GO aqueous solution at 70 C for 2 h, followed
6
H O, 0.60 g of anhydrous sodium acetate and 0.1 mL of
2
by rinsing in water for 5 s. The brown yellow fibers were
dried at 40 C for 6 h. Repeat step (b) and (c), and the
polyethylene glycol (PEG, M = 200) were added to the
w
ꢀ
suspension, and stirred for 1 h. Then, the slurry was sealed
ꢀ
coating cycle was conducted for a total of three times.
in a Teflon-lined stainless steel bomb and held at 200 C
(
d) GRs-PANFs: After that, GO-PANFs were reduced by
for 12 h. On cooling to room temperature, the product was
collected by centrifugation and washed with distilled water
and ethanol for several times. The final product was dried
hydrazine hydrate. A few layers of graphene were format-
ted on the surface of the PANFs.
ꢀ
(e) Co/GRs-PANFs: 0.06 mg Co/GRs and 0.02 g GRs-
PANFs were immersed into 60 mL of distilled water, with
in vacuum at 50 C for 18 h.
ꢀ
continuous stirring at 80 C for 12 h.
2
.3. Polyacrylonitrile Fibers Anchored
Cobalt/Graphene Sheet Nanocomposite
ꢀ
The final product was dried at 40 C for 6 h. To show
the electron transfer ability of the Co/GRs-PANFs, we
connected three fibres in parallel and construct a circuit.
A LED lamp can be highly illuminated (Fig. 1(f)), show-
ing that the well contact of Co/GRs and electron transfer
ability of the composite.
(
Co/GRs-PANFs)
Figure 1 illustrates the procedure of Co/GRs-PANFs which
mainly contain five steps. In typical,
a) Hydroxylated PANFs: 0.1 g PANFs (Ziyuan Indus-
try and Trade Co., Ltd, Shanghai, China) were immersed
in 60 mL of sodium hydroxide (NaOH) aqueous solution
14 wt%) at 110 C for 30 min, and thoroughly rinsed with
(
ꢀ
(
2.4. Hydrolysis of Sodium Borohydride
−
distilled water to remove the residual OH on the surface
of the fibers. After then dried at 70 C for 6 h.
The catalytic hydrolysis of NaBH was performed at 20±
4
ꢀ
ꢀ
0ꢁ5 C. First, NaBH (568 mg) was dissolved in 100 mL
4
(
b) Aminated PANFs: The hydroxylated PANFs were then
of distilled water. Then 10 mg Co/GRs-PANFs were
immersed in the solution. The hydrogen gas generated was
soaked in 3-Triethoxysilylpropylamine (APTES) solution
5628
J. Nanosci. Nanotechnol. 16, 5627–5632, 2016

Zhang et al.
Polyacrylonitrile Fibers Anchored Cobalt/Graphene Sheet Nanocomposite
(
A)
(B)
Figure 2. XRD patterns of (A) GO and GRs-PANFs, (B) PANFs, hydroxylated PANFs and GRs-PANFs.
collected in the water column by a measuring cylinder.23
The amount of hydrogen was recorded with respect to
time. As a control group, GRs-PANFs, GRs powders and
Co powders were used in the hydrolysis experiment fol-
lowing the same procedure, respectively.
be attributed to the graphite structure (002) plane of the
graphene nanosheets, while the other two peaks match
well with the (010) and (300) planes of polyacrylonitrile
(JCPDS card No. 48–2119). Line b show that the sharp
ꢀ
peak of GO centered at around 2ꢃ = 10ꢁ8 , indicating that
2
3
GO forms a well-ordered layered structure. Furthermore,
the sharp peak of layered GO disappeared, which indicates
that the GO were well turned to GRs by the chemical
2
.5. Characterization
GRs-PANFs hybrids were analyzed using a laser Raman
microscope (InVia-Reflex, Renishaw) with a Raman shift
2
4–26
reduction.
In comparison, the intensity of the diffrac-
−1
from 100 to 2000 cm using an excitation wavelength of
85 nm. Powder X-ray diffraction (XRD) was carried out
tion peaks of hydroxylated PANFs (as shown in Fig. 2(B))
decreases, which indicates that the content of polyacryloni-
7
Delivered by Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
on a Rigaku D/max 2550 V X-ray diffractometer using Cu
trile in the GRs-PANFs is lower than that in PANFs. Thus,
Copyright: American S wc i ee ns tp i fe i cc u Pl a ut eb l ti sh ha te ra sfew layers of GRs were successfully
IP: 191.101.55.64 On: Sat, 07 May 2016 05:56:26
K irradiation (k = 1ꢁ5406 Å). The operating voltage and
ꢂ
current were kept at 40 kV and 300 mA, respectively. The
particle size and morphology of the as-prepared products
were determined at 20 kV by a JSM-6700F field emission
scanning electron microscopy (FESEM). The cobalt con-
tent in the samples was obtained by inductively coupled
plasma-atomic emission spectrometer (ICP-AES, Leeman
Prodigy).
coated on the surface of PANFs after step b and c.
The Raman spectra of PANFs, hydroxylated PANFs
and GRs-PANFs are shown in Figure 3(A). The peaks
of PANFs disappeared in the spectra of c, while two
−1
broad peaks displayed at 1346 and 1590 cm can be
27ꢄ28
assigned to the D and G vibrations of graphene.
In
Figure 3(B), Raman characterization demonstrates that the
intensity ratio (I /I ) of the D and G bands change from
D
G
3
. RESULTS AND DISCUSSION
0.98 (GO) to 1.42 (GRs-PANFs), this change clearly sug-
2
Figure 2(A) shows the XRD patterns of GO and the GRs-
PANFs. In line a, a new peak observed at 24.3 could
gests an increase in the average size of the sp domains
ꢀ
29ꢄ30
upon conversion from GO to graphene.
Figure 3. Raman spectra of (A) PANFs, hydroxylated PANFs and GRs-PANFs, (B) GO and GRs-PANFs.
J. Nanosci. Nanotechnol. 16, 5627–5632, 2016
5629

Polyacrylonitrile Fibers Anchored Cobalt/Graphene Sheet Nanocomposite
Zhang et al.
Figure 4. SEM images of (A) PANFs, (B) hydroxylated PANFs, (C) GRs-PANFs.
k3
Figure 4 shows the SEM images of (A) PANFs,
B) hydroxylated PANFs and (C) GRs-PANFs. After
−
−
BH +OD → BH OD
(4)
(5)
(6)
3
3
(
k4
treated by NaOH, the wale on the surface of the PANFs
become blurred (as showed in Fig. 4(B)) which can be
attributed to the hydrolysis of –CN and other functional
groups from the second and third monomers. During this
process, a large number of hydroxyl were obtained and
some of the second and third monomers were escaped
from the fibers which made the edge of the wale to be
corroded. It is clear to see that GRs were well coated on
the surface of the fiber (as showed in Fig. 4(C)), and the
wale can be still seen which clearly demonstrates that the
layers of the coated GRs were so few and the size is so
small that they can’t well shield the wale.
−
−
M+e +D O → MD+OD
M
2
k5
MH+MD → 2M+HD
the reaction begins with the attachment of the borohy-
dride ion to a catalytic site, forming two metal com-
plexes. Figure 6 shows the transformation of hydrogen
3
2
generation.
Figure 6 shows the volume of hydrogen generated ver-
sus time during the catalytic hydrolysis of a 150 mM
NaBH solution. The straight slope of the line show the
average hydrogen generation rate clearly, we calculated
the average hydrogen generation rate of Co/GRs-PANFs
4
Morphology of the as-prepared Co/GRs-PANFs is
shown in Figure 5. It can be seen that the Co/GRs com-
posite is well dispersed on the surface of PANFs (as shown
−1
−1
−1
−1
(
3222 mL · min · g or 53.7 mL · s · g ) accurately,
−1
−1
−1
−1
which was much higher than cobalt (915 mL·min ·g
Delivered by Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
in Fig. 5(A)). Figures 5(B) and (C) are high-magnification
−1
−1
−1
−1
or 15.25 mL · s · g ) and Co/GRs (995 mL · min · g
IP: 191.101.55.64 On: Sat, 07 May 2016 05:56:26
images of Figure 5(A), it exhibits that the GRs were well
Copyright: American S co ire n1 6t i .f 5i c8 Pmu Lb l·i ss he·r gs ). We also found that GRs-PANFs,
coated with Co nanoparticles, and the diameter of Co
nanoparticles are from 30 to 100 nm.
GRs had no catalysis for hydrogen generation from the
control group. There are three possible ways for enhanced
hydrogen generation rate of Co/GRs-PANFs (as shown in
Fig. 7).
Cobalt has been used as a low-cost catalyst in the gen-
eration of H by hydrolysis of NaBH for many years. The
2
4
chemical reaction is given by the following equation:
(
1) Cluster Co/GRs combine with GRS-PANFs. Due to
Co
the high specific surface area of graphene and the cobalt
nanoparticles, the Co/GRs catalysts aggregation exist in
the reaction system which can block the contact between
NaBH ꢅaqꢀ+2H Oꢅlꢀ NaBO ꢅaqꢀ+4H ꢅgꢀ (1)
4
2
2
2
According to Holbrook and Twist,31 who developed the
following reaction scheme based on tests using D O and
H O (M represents a surface site on the catalyst):
cobalt particles and NaBH molecule. In this work, in
4
2
order to decrease the aggregation of the catalyst, the
Co/GRs catalyst was anchored on the surface of GRS-
PANFs before used in the hydrogen generation. Thus,
when the Co/GRs-PANFs catalyst was immersed in the
2
k1
−
−
3
2
M+BH ⇔ MBH +MH
(2)
(3)
4
k−1
NaBH solution, cobalt particles can sufficiently react with
4
k2
−
−
NaBH and H O molecules which can help to improve the
MBH ⇔ BH +M+e
4
2
3
3
M
k−2
hydrogen generation rate.
Figure 5. FESEM images of Co/GRs-PANFs with different resolutions.
5630
J. Nanosci. Nanotechnol. 16, 5627–5632, 2016

Zhang et al.
Polyacrylonitrile Fibers Anchored Cobalt/Graphene Sheet Nanocomposite
3) Free Cobalt combine with GRS-PANFs. After the sec-
(
ond step in the experiment part, there might be some free
cobalt particles existed in Co/GRs powders. They were
absorbed on the surface of GRs-PANFs and formed new
Co-GRs catalyst which also enhanced the hydrogen gener-
ation rate. Moreover, recycling the catalyst is made easier
by anchoring the catalyst on the surface of GRs-PANFs.
No nanoparticle was found to be liberated from the GRs-
PANFs after the hydrogen generation reaction.
4
. CONCLUSIONS
In summary, a novel fiber catalyst was prepared by anchor-
ing Co/GRs on the surface of GRs-PANFs which was
obtained by a layer-by-layer process. It was found that the
PANFs were well coated by GRs which imply an opportu-
nity to anchor the Co/GRs on PANFs. The catalytic activ-
ity of the Co/GRs-PANFs catalyst was investigated in the
Figure 6. The volume of hydrogen versus time plots with different cat-
alysts (Co, Co/GRs and Co/GRs-PANFs) at constant sodium borohydride
concentration.
(
2) Separate Co/GRs combine with GRS-PANFs. In the
hydrolysis of NaBH . It has a considerable hydrogen gen-
4
−1
−1
Co/GRs nanocomposite, the cobalt nanoparticles were
coated on both side of GRs. Separate Co/GRs can be
anchored on the surface of GRs-PANFs to form a long
range ordered layer structure. The unique structure effec-
tively prevents the inter-sheet restacking of Co/GRs and
fully exploits the large surface area of Co/GRs-PANFs cat-
alyst. The cobalt nanoparticles were connected with the
GRs on the surface of the GRs-PANFs which can sup-
eration rate (3222 mL·min ·g ). The enhanced catalysis
activity is not only related to the block of Co/GRs aggre-
gation, but also related to the extra electron transfer path
supplied by GRs on the surface of GRs-PANFs.
Acknowledgments: We gratefully acknowledge the
financial support by NSF of China (No. 51172042),
MOE of China (IRT1221, No. 111-2-04,2232014A3-06),
Delivered by Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
STC of Shanghai (13JC1400200, 15ZR1401200), SRFDP
ply another electron transport path during the process that
IP: 191.101.55.64 On: Sat, 07 May 2016 05:56:26
−
(20110075130001), and Eastern Scholar.
the electron on the BH4 ion with on Ce oh py yd rr iog gh et n: Aa mt o me ri ci san Scientific Publishers
transferred to the adjacent metal atoms.
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Received: 20 January 2015. Accepted: 22 April 2015.
Delivered by Ingenta to: Chalmers Tekniska Hogskola (Chalmers University of Technology)
IP: 191.101.55.64 On: Sat, 07 May 2016 05:56:26
Copyright: American Scientific Publishers
5632
J. Nanosci. Nanotechnol. 16, 5627–5632, 2016