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Nano Res
1
Highly flexible conductive fabrics with hierarchically
nanostructured amorphous nickel tungsten tetraoxide
for enhanced electrochemical energy storage
Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1, and Jae Su Yu1 ()
Nano Res., Just Accepted Manuscript • DOI 10.1007/s12274-015-0874-z
http://www.thenanoresearch.com on August 4, 2015
© Tsinghua University Press 2015
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Nano Research
DOI 10.1007/s12274-015-0874-z
TABLE OF CONTENTS (TOC)
Highly flexible conductive fabrics with hierarchically
nanostructured amorphous nickel tungsten tetraoxide
for enhanced electrochemical energy storage
Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae
Su Yu1*
1Department of Electronics and Radio Engineering,
Institute for Wearable Convergence Electronics, Kyung
Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si,
Gyeonggi-do 446-701, Republic of Korea,
2Department of Environmental Science and Engineering,
Kyung Hee University, 1 Seocheon-dong, Giheung-gu,
Yongin-si, Gyeonggi-do 446-701, Republic of Korea
Burl-like
amorphous
NiWO4 NSs
Amorphous NiWO4
nanostructures
(i.e., NSs) on CF
Cu layer PET fiber
Hierarchical NSs
Amorphous NiWO4 NSs with burl-like morphologies were facilely
integrated on flexible CF fibers using ED method.
As a flexible and cost-effective electrode for psuedocapacitors, the
hierarchically nanostructured amorphous NiWO4/CF exhibited superior
electrochemical properties.
Highly flexible conductive fabrics with hierarchically
nanostructured amorphous nickel tungsten tetraoxide
for enhanced electrochemical energy storage
Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae Su Yu1 ()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
Amorphous NiWO4
nanostructures,
Conductive fabrics,
Electrochemical
deposition,
Electrochemical energy
storage properties
ABSTRACT
Amorphous nickel tungsten tetraoxide (NiWO4) nanostructures (NSs) are
successfully synthesized on flexible conductive fabric (CF) using a facile
one-step electrochemical deposition (ED) method. With an applied external
cathodic voltage (-1.8 V for 15 min), the amorphous NiWO4 NSs with burl-like
morphologies are well adhered on the seed coated CF substrate. The burl-like
amorphous NiWO4 NSs on CF (i.e., NiWO4 NSs/CF) are employed as a flexible
and binder-free electrode for psuedocapacitors, which exhibit remarkable
electrochemical properties with high specific capacitance (1190.2 F/g at 2 A/g),
excellent cyclic stability (92 % at 10 A/g), and good rate capability (765.7 F/g at
20 A/g) in 1 M KOH electrolyte solution. The superior electrochemical
properties can be ascribed to the advantageous properties of hierarchical and
large specific surface area of burl-like amorphous NiWO4 NSs/CF. This
cost-effective facile method for the synthesis of metal tungsten tetraoxide
nanomaterials on flexible CF could be a promising electrode for advanced
electronic and energy storage device applications.
1. Introduction
The growing interest in the development of
high-performance textile-based energy storage and
conversion devices has attracted widespread
attention due to their advanced feasibilities including
lightweight, high flexibility, low cost, and wearable
ability [1-5]. Among various flexible energy storage
devices, supercapacitors with irreplaceable
properties, such as high power density, fast
charge-discharge capacity, long lifetime, eco-friendly
operation, and low cost, have considered as the most
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DOI (automatically inserted by the publisher)
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Review Article/Research Article Please choose one
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2 Nano Res.
promising next-generation energy storage device to
satisfy the rapidly increasing power demand [6-10].
Accordingly, these supercapacitors have started to
play an indispensable role in the growth of
automotives and portable electronic systems
including hybrid electric vehicles, memory back-up
equipments, microelectromechanical systems, digital
cameras, and cellular phones [11-14]. With regard to
the energy storage mechanism, supercacpitors can be
classified into two types, i.e., electric double-layer
capacitors (EDLCs) and psuedocapacitors or redox
supercapacitors [15]. Typically, in EDLCs, the energy
can be stored by the electrostatic diffusion and
accumulation of ionic charges at the
electrode/electrolyte double layer interface [16]. In
contrast, psuedocapacitors can store more energy
than EDLCs due to the fast faradic charge transfer
reactions occurring between electroactive material
and electrolyte solution [17]. Therefore, intense
research studies have been devoted to the fabrication
of nanostructured single and mixed metal
oxides/hydroxide electroactive materials with
bespoke morphologies to increase the energy storage
abilities for psuedocapacitors [18-22]. During the
electrode preparation for psuedocapacitors, mixing
of polymer binders and conductive agents with
electroactive materials causes an increase of
dead-surface. This increased dead surface can
suppress the electron transfer as well as electrolyte
diffusion, which may limit their charge storage
properties. Therefore, the direct growth of
electroactive materials onto the conductive substrates
produces binder- and conductive additive-free
electrode for high-performance psuedocapacitors by
getting rid of the above mentioned limitations and
increases the electroactive sites for reversible faradic
redox reactions between nanomaterials and
electrolyte solution [23-26]. Various growth methods
including hydrothermal synthesis, solvothermal
method, physical deposition, and electrochemical
deposition (ED) have been employed to fabricate
electroactive materials directly on specific conductive
substrates based on conventional psuedocapacitor
electrodes [27-30]. Of these methods, the ED process
is versatile and simple to deposit various
nanostructured materials on the conductive
substrates by the help of an applied external cathodic
voltage in the growth solution. Moreover, the shape
and size of the nanomaterials can be readily tuned by
controlling the applied external cathodic voltage at
low growth temperature for short time [31].
Meanwhile, novel conductive substrates such as
nickel (Ni) foam, titanium (Ti) foil, carbon cloth, and
Ni foil have received much attention as a potential
candidate for psuedocapacitor electrodes [32-34]. As
a matter of fact, the cost of these conductive
substrates is highly expensive due to their
complicated manufacturing processes. Thus, the
conductive fabric (CF) substrates, fabricated by
simple electroless plating of metallic layer on
polyethylenterephthalate (PET) fibers, have recently
used as a low-cost conductive electrode to replace the
expensive current collectors in psuedocapacitors.
Owing to the process easiness, the CF substrate has
beneficial properties (e.g., good flexibility,
conductivity, and mechanical stability) similar to the
conventional psuedocapacitor electrodes [35, 36].
On the other hand, traditional single transition metal
oxides/hydroxides materials including MnO2 [37],
RuO2 [38], Co3O4 [39], NiO [40], Fe2O3 [41], TiO2 [42],
Co(OH)2 [43], and Ni(OH)2 [21] and nanostructures
based on transition binary metal oxides/hydroxides
like NiCo2O4 [12], NiMoO4 [44], MnCo2O4 [45], Ni-Co
[46], and Ni-Al layered double hydroxides [47] have
been extensively explored as pseudocapacitive
materials due to their multiple oxidation states for
reversible electrochemical reactions. Cobalt tungsten
tetraoxide (i.e., CoWO4) and nickel tungsten
tetraoxide (NiWO4) are important inorganic
functional materials for diverse potential applications
[48-50]. Additionally, these materials offer excellent
electrochemical properties as pseudocapacitive
materials in energy storage devices, which can be
ascribed to their good reversible faradic redox
reactions and good conductivity on the order of 10-7
to 10-3 S cm-1 by the incorporation of W atoms. And,
the amorphous nature of these metal tungsten
tetraoxide nanomaterials ensures unique energy
storage properties than their crystalline counterparts
[51, 52]. Thus, to further improve their
electrochemical energy properties, the direct growth
of these metal tungsten tetraoxide nanomaterials on
low-cost conductive substrates by facile growth
methods are highly required. However, there is no
such report up to now, which is based on simple ED
process of metal tungsten tetraoxide
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3 Nano Res.
pseudocapacitive materials on flexible textile
electrodes and their electrochemical properties. Here,
our objective is to grow the amorphous metal
tungsten tetraoxide nanomaterials on low-cost
flexible electrodes using a simple two-electrode
system based ED method, together with analysis of
their charge storage properties, for psuedocapacitors
without using any polymer binders and conductive
agents.
In this work, we successfully fabricated the burl-like
amorphous NiWO4 nanostructures (NSs) on CF
substrate (i.e., NiWO4 NSs/CF) via a facile ED
method and its application as a flexible and
cost-effective electrode for psuedocapacitors. The
burl-like amorphous NiWO4 NSs were densely and
abundantly coated on CF substrate with good
adhesion by applying an external cathodic voltage at
low growth temperature for short time. The
electrochemical properties reveal that the
as-prepared amorphous NiWO4 NSs/CF show high
specific capacitance and excellent capacity rate as
well as good cycling performance.
2. Experimental section
2.1. Materials
Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), sodium
tungstate dehydrate (Na2WO4·2H2O), nickel acetate
tetrahydrate (Ni(CH3CO2)2·4H2O), and sodium
dodecyl sulfate solution (CH3(CH2)11·OSO3Na) were
purchased from Sigma-Aldrich Co. Potassium
hydroxide (KOH) was obtained from DaeJung
Chemicals Ltd. All the chemicals were of analytical
grade and used without further purification in the
experiments.
2.2. Synthesis of burl-like amorphous NiWO4
NSs/CF
Burl-like amorphous NiWO4 NSs were synthesized
on the seed coated CT substate using a two-electrode
system based ED method. Here, we used a
commercially available and highly flexible CF as a
substrate, which is composed of non-woven copper
(Cu) plated PET (i.e., Cu/PET) fibers. Prior to the
synthesis, CF substrates (2 cm × 3 cm) were
ultrasonically cleaned in acetone, ethanol, and
de-ionized (DI) water for 5 min, respectively, and
air-dried. To prepare the seed solution, 10 mM of
Ni(CH3CO2)2·4H2O was dissolved in 100 ml of
n-propanol at room temperature and 750 µ l of
CH3(CH2)11·OSO3Na solution was slowly dropped
into the above solution using a micropipette. After 30
min with an intense magnetic stirring, the propanol
solution was changed to transparent green color.
Then, the CF pieces were soaked in the seed solution
for 15 min and placed in an oven at 120 C for 2 h to
ensure the strong adhesion of seed layer on the fibers
of CF. A simple two-electrode system with the seed
coated CF substrate as a working electrode and
platinum (Pt) mesh as a counter electrode were
utilized [53]. Meanwhile, the growth solution was
prepared by dissolving the 5 mM of Na2WO4·2H2O
and 5 mmol of Ni(NO3)2·6H2O in 900 ml of DI water
on hot plate at ~ 80-82 C. After stirring for 10 min,
the two electrodes were carefully immersed into the
aqueous growth solution with a distance of ~ 1 cm
between the electrodes. Then, the ED process was
performed at an external cathodic voltage of -1.8 V
for 15 min using a DC power supply. After
deposition, the sample was carefully removed, rinsed
with DI water, and subsequently dried at room
temperature. The weight of the as-grown sample on
CF substrate was calculated by its weight difference
before and after the ED process using an electronic
analytical balance (OHAUS DV214C), indicating ~ 0.6
± 0.03 mg/cm2.
2.3. Characterization
The surface morphology and structural properties of
the prepared samples were characterized by using a
field-emission scanning electron microscope
(FE-SEM; Carl Zeiss, LEO SUPRA 55, Reutlingen,
Germany) and transmission electron microscope
(TEM; JEM 200CX, JEOL, Tokyo, Japan) equipped
with energy dispersive X-ray spectroscopy (EDX).
The amorphous nature, chemical compositions and
surface electronic states of the prepared samples
were analyzed from the X-ray diffraction (XRD;
M18XHF-SRA, Mac Science Ltd., Yokohama, Japan)
pattern and X-ray photoelectron spectroscopy (XPS;
MultiLab2000, Thermo VG Scientific System, U.K).
The sheet resistance of the CF was measured using a
four-point probe system (FPP-RS8 Dasol Eng.).
2.4. Electrochemical Measurements
The burl-like amorphous NiWO4 NSs/CF substrate
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4 Nano Res.
was directly utilized as the working electrode for the
following electrochemical measurements by cyclic
voltammetry (CV), galvanic charge-discharge (GCD)
and electrochemical impedance spectroscopy (EIS)
conducted with IviumStat electrochemical
workstation (IVIUM Technologies, Eindhoven, The
Netherlands). The electrochemical tests were
performed in a conventional three-electrode cell
beaker with a working electrode, Pt wire and
Ag/AgCl electrode as the counter and reference
electrodes, respectively, at room temperature. The
freshly prepared 1 M KOH solution served as an
electrolyte. The CV measurement was analyzed
between -0.1 V to 0.6 V at different scan rates of 5 to
70 mV/s and the GCD analysis was carried out at
different current densities of 2 to 20 A/g, respectively.
The specific capacitance (F/g) and current density
(A/g) were calculated based on the mass of electrode
material, i.e., burl-like amorphous NiWO4 NSs
weight on CF substrate. The EIS measurement was
performed in the frequency range from 100 kHz to
0.01 Hz at the open circuit potential with an AC
voltage of 5 mV.
3. Results and discussion
Figure 1 illustrates the schematic diagram of the
fabrication process of burl-like amorphous NiWO4
NSs/CF using a facile and low-cost ED method: (a)
preparation and cleaning process of CF substrate, (b)
seed layer coated CF substrate, and (c) hierarchical
and burl-like amorphous NiWO4 NSs grown on the
seed-coated CF substrate. We had chosen a
commercially available CF substrate, which was
composed of non-woven Cu plated PET fibers (i.e.,
Cu/PET fibers). The sheet resistance of the CF was
measured to be very low of ~ 0.034 Ω/sq, which is
comparable with typical metal foils and carbon
textiles as shown in Figure S2. The highly conductive
nature and flexible properties of CF can be used as a
working electrode for the growth of various metal
hydroxide/oxide nanostructures using the ED
process as shown in Figure 1(a). In addition, the large
surface area and disorderly arranged non-woven
Cu/PET 3D fibrous framework of CF facilitate the
high-speed electron transport and serve as a
cost-effective and flexible electrode for wearable
energy storage devices. After immersing the
ultrasonically cleaned CF substrate into the seed
solution, followed by thermal treatment, the nickel
seed layer was uniformly coated on the surface of CF
(Figure 1b). It has been known that, in ED process,
the seed layer plays an important role in providing
the nuclei sites, indicating that the NSs could be
grown with good adhesion on CF substrate. As
shown in Figure 1(c), when the seed coated CF was
immersed into the electrochemical setup with an
applied external cathodic voltage to the working
electrode, the amorphous NiWO4 NSs with burl-like
morphologies were grown on the surface of CF. The
mechanism for the formation of amorphous NiWO4
NSs/CF may involve the electrochemical reactions
and subsequent precipitation as described by the
following equations:
NO3- + H2O + 2e- → NO2- + 2OH-,
Ni+2 + 2OH- → Ni(OH)2,
Na2WO4 → 2Na+ + WO4-2,
Ni(OH)2 + WO4-2 + 2Na+ → NiWO4 + 2NaOH
Due to the external cathodic voltage (-1.8 V for 15
min) in growth solution, the nitrate (NO3-) ions
pertained Ni(NO3)2·6H2O was electrochemically
reduced with water on the surface of seed coated CF
substrate accompanied with the production of
hydroxyl (OH-) ions. Afterwards, the nickel (Ni+2)
was diffused onto the seed layer by columbic
attraction under strong electric field and combined
with OH- ions, resulting in the formation of nickel
hydroxide (Ni(OH)2) units on working electrode [54].
In the meantime, the generated tungstate (WO4-2)
ions from the Na2WO4 rapidly reacted with Ni(OH)2
units, leading to the successful deposition of
amorphous NiWO4 NSs/CF substrate. Figure 2 shows
the FE-SEM images and EDX spectra of the
as-prepared amorphous NiWO4 NSs on the seed
coated CF substrate under the external cathodic
voltage of -1.8 V for 15 min. As shown in the
perspective view of FE-SEM image in Figure 2(a), the
CF substrate was weaved with irregularly arranged
non-woven Cu/PET fibers and they were fully
covered by the as-prepared sample. In addition, the
texture of CF substrate was kept unchanged even
after the ED process (see the bare FE-SEM image in
Figure S1). As shown in the inset of Figure 2(b), the
porous Cu/PET fibrous framework was uniformly
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5 Nano Res.
(a)Conductive fabric
(i.e., CF)
PET
fiber
Cu layer
Burl-like
NSs
Amorphous
NiWO4
nanostructures
(i.e., NSs) on CF
(b)
(c)
Seed coated CF ED process
Seed coating
& thermal treatment
Seed coated
CF fibers
Figure 1 Schematic diagram for the fabrication process of burl-like amorphous NiWO4 NSs on flexible CF substrate.
deposited over the entire surface with amorphous
NiWO4. Here, each fiber had an average diameter of
~ 20-22 m after the coating of the sample. From the
magnified view, the amorphous NiWO4 NSs were
densely and abundantly coated on Cu/PET fibers
with a burl-like morphology, as can be clearly
observed in Figure 2(b). The appearance of these NSs
on Cu/PET fibers was similar to the naturally grown
burls on trunk part of a tree. These burl-like
morphologies of amorphous NiWO4 NSs have an
average diameter of ~ 400-600 nm, as shown in the
magnified view of FE-SEM image in Figure 2(c).
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6 Nano Res.
(a) (b)
(c)
W
W W W
W
NiO
Ni
Cu
Cu Cu
C
(g)
Burl-like
amorphous
NiWO4 NSs
Burl-like
amorphous
NiWO4 NSs on
CF fibers
Hierarchical
NSs
(d)
(e) (f)
Bare CF
Amorphous
NiWO4 NSs
on CF
200 nm
10 m
50 nm
20 m100 m
Figure 2 (a-c) FE-SEM images of the burl-like amorphous NiWO4 NSs/CF synthesized at an external cathodic voltage of -1.8 V for 15
min, (d) schematic illustration of the burl-like amorphous NiWO4 NSs grown on CF fibers, and (e-f) photographic images of the bare CF
and amorphous NiWO4 NSs/CF substrates. (d) EDX spectrum of the corresponding NiWO4 NSs/CF sample.
Moreover, the amorphous NiWO4 NSs are composed
of ultrathin nanosheets with thickness of several
nanometers to form a hierarchical and
three-dimensional (3D) nanonetwork (inset of Figure
2(c)). In fact, such hierarchical NSs would
significantly increase the surface area of electrode
material and enable the fast diffusion of electrolyte
through the electrodes for enhanced electrochemical
properties including high specific capacitance and
excellent cyclic stability. The assembled hierarchical
amorphous NiWO4 NSs with burl-like morphologies
on the fibers of CF are schematically shown in Figure
2(c). From the photographic images of Figures 2(f)
and 2(e), the CF was uniformly coated with a
greenish-white color product after ED process. And,
the amorphous NiWO4 NSs/CF were highly flexible
and durable without formation of any cracks on its
surface, confirming the robust adhesion of sample on
CF (Figure 2(f)). The EDX spectrum in Figure 2(g)
demonstrates the basic elemental compositions for
the amorphous NiWO4 NSs/CF. In the EDX spectrum,
the elements of Cu and C from the CF substrate were
shown, while the Ni, W, and O elements were
observed for the amorphous NiWO4 NSs. Figure 3
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7 Nano Res.
1000 800 600 400 200 0
Ni 3s
Ni 3p
W 4f
(c)
W 4p
W 4d
C 1s
O 1s
Inte
nsit
y (
a.u
.)
Binding Energy (eV)
Ni 2p
10 20 30 40 50 60 70 80 90
(a)
Cu: #85-1326
(220)
(200)
Cu (111)
PET
Amorphous
NiWO4 NSs/CF
Bare CF
Inte
sit
y (
a.u
.)
2 (degree)
880 870 860 850
Ni 2p
Ni 2p1/2
"Sat""Sat"
Ni 2p3/2
Inte
nsit
y (
a.u
.)
Binding Energy (eV)
(d)
45 40 35 30
(e)
Inte
nsit
y (
a.u
.)
Binding Energy (eV)
W 4f7/2
W 4f5/2
W 4f
10 20 30 40 50 60 70 80 90
(04
1)(1
30
)(0
22
)- (10
2)
(00
2)(0
02
)(0
20
)
-(111)
(01
0)
(11
0)
(10
0)
Inte
nsit
y (
a.u
.)2 (degree)
NiWO4 NSs
JCPDS# 15-0755(b)
(01
1)
-(1
12
)
- (20
2)
- (11
3)- (3
11
)- (3
02
)
10 20 30 40 50 60 70 80 90
(041)(130)
(022)-
(102)(002)(0
02)
(020)
-(111)
(010)
(110)
(100)
Inte
nsit
y (
a.u
.)
2 (degree)
NiWO4 NSs
JCPDS# 15-0755
(b)
(011)
-(1
12)
-(2
02)
-(1
13)
-(3
11)
- (302)
540 535 530 525
295 290 285 280
Inte
ns
ity
(a
.u.)
Binding Energy (eV)
C 1s
O 1s
Inte
nsit
y (
a.u
.)
Binding Energy (eV)
(f)
Figure 3 (a-b) XRD patterns and (c-f) XPS spectra of the burl-like amorphous NiWO4 NSs/CF.
shows the XRD and XPS results of the as-prepared
sample. Figure 3(a) shows the XRD patterns of the
bare CF and the amorphous NiWO4 NSs/CF.
Obviously, the XRD patterns only revealed that CF
substrate peaks (JCPDS# 85-1326) and peaks for
NiWO4 were not observed in the XRD pattern of
NiWO4 NSs/CF which indicates the characteristic
amorphous nature of sample. As shown in the XRD
spectrum of Figure 3(b), the coated sample was
further scrapped off from the CF and calcined at 500
C for 2 h. The observed major diffraction peaks
located at 2θ degrees of 15.6, 19.27, 23.96, 24.9, 30.9,
36.5, 41.6, 52.3, 54.6, 62.3, 65.8, and 72.6 correspond
to the respective crystal planes of (010), (100), (011),
(110), (1 11), (002), (1 02), (130), (2 02), (1 13), (311), and
(302), respectively. This means that the material is
only composed of NiWO4 (JCPDS# 15-0755) without
any impurities. After calcination, the XRD spectra
and corresponding SAED pattern (Figure S3)
confirms the crystalline properties of the NiWO4 NSs.
For psuedocapacitor applications, Xing et al. and Niu
et al. recently reported that amorphous
characteristics of the metal tungsten tetraoxide
nanomaterials are beneficial for improving the
electrochemical properties, because the poor
crystalline material may exhibit faster charge transfer
reactions by providing the increased transport
channels in electrolyte ions than the crystalline one
[49, 50]. The surface bonding and element oxidation
states of the as-prepared sample were further
characterized by XPS spectra and the results were
plotted in Figures 3(c-f). As observed in Figure 3(c),
the XPS survey scan spectrum indicates the presence
of Ni, W, O and C elements with their respective
binding energies. Herein, C 1s peak appears mainly
due to the fortuitous carbon formed during
atmospheric air exposure. The high-resolution Ni 2p,
W 4f, and O 1s spectral curves were well-fitted with
Gaussian curves as shown in Figures 3(d-f). From the
Figure 3(c), the Ni 2p spectrum showed two-spin
orbit doublets and shake-up satellite peaks observed
in the range of 850-884 eV. The Ni 2p spectrum had
binding energies of 855.1 and 871.6 eV which are the
characteristic peaks of Ni 2p3/2 and Ni 2p1/2 spin-orbit
doublets, respectively. These results reveal that the
Ni species are in +2 oxidation state. Meanwhile, the
high-resolution XPS spectrum of W 4f appears as the
spin-orbit splitting of W 4f7/2 at 36.4 eV and W 4f5/2 at
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8 Nano Res.
34.6 eV, implying that the W is in +6 oxidation state
in the prepared product. Also, the O 1s spectrum
with a binding energy value of 530.8 eV was
associated to the bound O-bond with W and Ni
species in NiWO4 [23, 48]. Furthermore, the atomic
ratio of Ni to W was close to 1:1 with a stoichiometry
ratio of the amorphous NiWO4 NSs/CF. Combining
the XPS result with that XRD result confirms the
successful formation of amorphous NiWO4 NSs/CF
without any impurities.
The hierarchical structures of amorphous
NiWO4 NSs were further investigated by the TEM
images. To prepare the sample for TEM analysis,
amorphous NiWO4 NSs/CF substrate was cut into
small pieces and they were agitated in a small beaker
containing 10 ml of ethanol for 30 min by
ultrasonication. Then, the Cu grid mesh was
immersed into the ethanol solution and air-dried for
5 min before putting it into the TEM chamber. The
typical TEM images of hierarchical amorphous
NiWO4 NSs separated from the CF substrate were
shown in Figures 4(a-d). In TEM image of the Figure
4(a), amorphous NiWO4 NSs showed flower shaped
structures after detached from the CF and they were
agglomerated with each other. The average diameters
of these NSs were approximately 400-500 nm. The
TEM image of a single amorphous NiWO4 NS in
Figure 4(b) further reveals that the NSs were
well-assembled with 3D ultrathin nanosheets. Such
hierarchical features of NSs not only provide the
channels for diffusion of electrolyte ions into the
interior parts of the material, but also increase the
surface area for rapid faradic redox reactions, when
they were used as a pseudocapacitive material. The
high-resolution TEM image, as shown in Figure 4(c),
clearly shows no obvious lattice fringes on the
surface of NiWO4 NSs. Moreover, the corresponding
selected area electron diffraction (SAED) pattern was
taken from the area in Figure 4(c), confirming that
there are no crystalline ring patterns or dotted spots,
undeniably proving an amorphous phase of NiWO4
NSs, which agrees well with the XRD pattern of
Figure 3(a). The TEM-EDX mapping was performed
to illustrate the spatial distribution of the
corresponding elements in the burl-like amorphous
NiWO4 NSs. As shown in Figures 4(e-g), the EDX
mapping images clearly revealed the uniform
distribution of Ni (green), W (blue) and O (red)
atoms within the whole NiWO4 NSs.
The electrochemical properties of the burl-like
amorphous NiWO4 NSs/CF were investigated using
(a) (b) (c)
W NiO
(d)(e)(f)(g)
200 nm 100 nm 5 nm
5 1/nm
10 nm
Figure 4 (a-d) TEM images and (e-f) EDX elemental mapping images of the hierarchical amorphous NiWO4 NSs
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9 Nano Res.
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-60
-40
-20
0
20
40
60
Bare CF
Amorphous
NiWO4 NSs/CF
Scan rate: 30 mV.s-1
Cu
rren
t (m
A)
Potential (V, vs. Ag/AgCl)
(a)
0 5 10 15 20 25200
400
600
800
1000
1200
1400
Sp
ecif
ic C
ap
acit
an
ce (
F/g
)
Current Density (A/g)
(d)
0 100 200 300 400 500 600 7000.0
0.1
0.2
0.3
0.4
0.5
Po
ten
tia
l (V
, vs.
Ag
/Ag
Cl)
Time (sec)
2 A/g
4 A/g
6 A/g
8 A/g
10 A/g
15 A/g
20 A/g
(c)
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6
-100
-75
-50
-25
0
25
50
75
100
0 10 20 30 40 50 60 70-100
-75
-50
-25
0
25
50
75
100
Pe
ak
Cu
rre
nt
(mA
)
Scan Rate (mV/s)
Anodic current
Cathodic current
50 mV/s
70 mV/s
Cu
rren
t (m
A)
Potential (V, vs. Ag/AgCl)
5 mV/s
10 mV/s
20 mV/s
30 mV/s
40 mV/s
(b)
Figure 5 (a) CV curves of the bare CF and the amorphous NiWO4 NSs/CF measured at a scan rate of 30 mV/s. (b) CV curves of the
amorphous NiWO4 NSs/CF at different scan rates (5 to 70 mV/s). Inset of (b) shows the plot of redox peak currents with respect to scan
rate. (c) GCD curves of the amorphous NiWO4 NSs/CF at different current densities (2 to 20 A/g). (d) Calculated specific capacitance
values as a function of applied current density.
CV, GCD and EIS measurements in three-electrode
cell system with 1 M KOH aqueous solution. Figure
5(a) shows the CV curves measured for the bare CF
and the amorphous NiWO4 NSs/CF electrode at a
scan rate of 30 mV/s with a potential window of -0.1
to 0.6 V vs. Ag/AgCl electrode. Apparently, the area
under CV curve for the bare CF was very small,
which indicates that the contribution of bare CF to
the capacitance was negligible. Meanwhile, under the
same scan rate for the amorphous NiWO4 NSs/CF
electrode, relatively larger CV integrated area was
observed, exhibiting the superior capacitive behavior
of the materials. Moreover, the CV curve of the
amorphous NiWO4 NSs/CF electrode indicates the
presence of redox peaks, which is probably due to
the charge storage kinetics of NiWO4 NSs originating
from the pseudocapacitive behavior (Faradic redox
reactions). The corresponding redox process at the
NiWO4/electrolyte interface can be expressed as
follows [55]:
Ni+2 ↔ Ni+3 + e-.
Herein, the pseudocapacitance is mainly based on
the Faradic redox reactions of Ni ions in the NiWO4
nanomaterial. The W species in the product have
only contributed to the enhancement of electrical
conductivity, but not been involved in any redox
reactions [50]. Figure 5(b) shows the CV curves of
amorphous NiWO4 NSs/CF at different scan rates of
5, 10, 20, 30, 40, 50, and 70 mV/s in the potential
window of -0.1 to 0.6 V. As can be seen in the shape
of CV curves, the redox peaks were observed to be
mirror-image symmetric against each other for all the
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10 Nano Res.
scan rates, indicating the good electrochemical
reversibility of the as-prepared product [56]. As the
scan rate increased, the redox peak positions were
shifted slightly to the higher and lower potentials
with enhanced anodic and cathodic peak current
values. The linear relationship between increased
scan rates and current values of the redox peaks was
shown in the inset of Figure 5(b). This indicates that
the electrochemical process is a diffusion-controlled
process [57]. To estimate the superior electrochemical
performance in detail, the GCD measurements were
carried out in 1 M KOH electrolyte solution at room
temperature. Figure 5(c) shows the GCD curves of
the amorphous NiWO4 NSs/CF electrode at various
current densities. During the charging and
discharging phenomenon in all GCD curves, the
charge curves were almost similar to their respective
discharge counterpart, which was in well agreement
with the CV results. With regard to the GCD curves,
the specific capacitance of the amorphous NiWO4
NSs can be calculated using the following formula
[36, 39]:
C = (i×∆t)(m×∆V),
where C is the specific capacitance (F/g), i is the
discharge current (A), ∆t is the discharge time (sec),
m is the mass of amorphous NiWO4 NSs/CF
electrode (g), and ∆V is the potential window of
discharge curve (V). The calculated specific
capacitance values were 1190.2, 1065.1, 985.3, 921.6,
882.2, 790.5, and 765.7 F/g at the current densities of
2, 4, 6, 8, 10, 15, and 20 A/g, respectively. These
enhanced specific capacitance values were probably
attributed to the reliable adhesion of burl-like
amorphous NiWO4 NSs/CF substrate, which results
in an intimate contact and rapid electron transport
between the CF and each NS. Also, the porous
fibrous framework of CF substrate and high specific
surface area of the amorphous NiWO4 NSs possibly
initiated the hierarchical pathways for efficient
electrolyte ion transport. Moreover, the specific
capacitance values observed in the present work are
comparable to or even better than those of
previously reported metal tungsten tetraoxide
based electroactive materials such as
cauliflower-like amorphous NiWO4 NSs (586.2 F/g
at 0.5 A/g and 376.9 F/g at 8 A/g) [52], DNA
encapsulated chain-like NiWO4 NSs (173 F/g at 5
mV/s) [50], amorphous CoWO4 nanoparticles (403
F/g at 0.5 A/g and 298 F/g at 3 A/g) [51], and
rGO-CoWO4 nanocomposite (159.9 F/g at 5 mV/s)
[58], respectively. The specific capacitance values as
a function of applied current density (2 A/g to 20
A/g) were plotted in Figure 5(d). The burl-like
amorphous NiWO4 NSs/CF electrode exhibited
excellent capacity retention rates of 74.1 and 64.3%
at the current densities of 10 and 20 A/g as
compared with the initial current density of 2 A/g.
This decreased capacitance under high current
densities can be explained by the fact that the
electrolyte ions are only diffused to the outer
surface of the electroactive material at high applied
current densities. Meanwhile, at low current
densities, the electrolyte ions were easily diffused to
both inner and outer surface of the electroactive
material to be effectively involved in the reversible
electrochemical reactions.
The fundamental electrochemical behavior of
the as-prepared burl-like amorphous NiWO4 NSs/CF
was measured by the EIS analysis. The measurement
was performed in an open circuit potential at the
frequency range of 100 kHz to 0.01 Hz with an AC
voltage of 5 mV. As shown in Figure 6(a), the EIS
curve is a plot of the imaginary part (Z”) of the
impedance against the real part (Z’), which can be
characterized by two distinct parts: a negligible
semicircle in the high-frequency region followed by a
sloped line in the low-frequency region. The
diameter of the semicircle in the low-frequency
region corresponds to the Faradic reactions at the
electrode/electrolyte interface, whereas the sloped
line is associated with the faster diffusion of
electrolyte ions [59]. The inset of Figure 6(a) shows
the equivalent circuit used to fit the EIS curves,
where Rs, Rct, CPE, and Zw correspond to internal
resistance, interfacial charge transfer resistance, the
constant phase element, and Warburg impedance,
respectively. The low values of Rs and Rct were found
to be 1.85 and 2.36 Ω for the amorphous NiWO4
NSs/CF, indicating the good conductivity of
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
OH-
OH-
OH-
OH-
OH-
OH-
OH-
OH-
OH-
e-
e-e-
OH-
OH- OH-
OH-
Electrolyte
penetration
Electron
transfer
10 m
0 20 40 60 800
20
40
60
80
0 2 4 6
0
2
4
6
8
Z''(
oh
m)
Z'(ohm)
W
RS
CPE
CL
RctZW
Z''(o
hm
)
Z'(ohm)
(a)
0 100 200 300 400 500 600 700 8000.0
0.1
0.2
0.3
0.4
0.5
Po
ten
tia
l (V
, v
s.
Ag
/Ag
Cl)
Time (sec)
Current Density: 10 A/g(c) (d)
0 200 400 600 800 10000
200
400
600
800
1000
Sp
ec
ific
Ca
cit
an
ce
(F
/g)
Cycle Number
Current density: 10 A/g(b)
OH-
e-
Figure 6 EIS spectrum of the amorphous NiWO4 NSs/CF in 1 M KOH electrolyte solution. (b) Cycling performance of the burl-like
amorphous NiWO4 NSs/CF at a current density of 10 A/g and (c) corresponding GCD curves for the first 10 cycles. (d) Schematic
illustration of the electrolyte diffusion and electron transport path way of the burl-like amorphous NiWO4 NSs/CF. The inset of (b)
shows the FE-SEM image of the amorphous NiWO4 NSs/CF after 1000 cycles, revealing its structural stability
as-fabricated material. The long-term cycling
performance is also one of the key parameters in
determining the psuedocapacitors for practical
applications. Herein, the cycling stability was carried
by repeated GCD measurements at a constant current
density of 10 A/g for 1000 cycles in 1 M KOH
electrolyte solution, as shown in Figure 6(b). It was
found that the capacitance retention was observed to
be 92 % after 1000 cycles, which indicates the good
cyclic stability of the burl-like amorphous NiWO4
NSs/CF. Figure 6(c) shows the repeated GCDcurves
(first 10 cycles) for the amorphous NiWO4 NSs/CF at
a current density of 10 A/g. Visibly, all the GCD
curves remain undistorted and display a symmetrical
potential-time response behavior, exhibiting the
highly reversible characteristics of the active material.
After 1000 cycles, the burl-like amorphous NiWO4
NSs were kept intact on CF fibers (inset of Figure 6(b))
and the surface morphology of hierarchical
amorphous NiWO4 NSs appeared similar to its initial
morphology as shown in Figure S4 (a). Also, during
the cycling process in electrolyte solution, the
amount of NiWO4 NSs on CF still remains without
any mass loss or peeling as shown in Figure S4 (b),
which confirms the excellent stability and reliable
adhesion of burl-like amorphous NiWO4 NSs to the
CF. To further confirm the structural properties after
cycling process, the XPS analysis was carried out for
the burl-like amorphous NiWO4 NSs/CF. From the
XPS survey scan spectrum of Figure S4 (c), the
expected elements of Ni, W and O were well indexed
to their respective binding energy values, which
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12 Nano Res.
clearly reveals the reliable structural stability of the
burl-like amorphous NiWO4 NSs/CF even after
long-term cycling process. As schematically shown in
Figure 6(d), the burl-like amorphous NiWO4 NSs
entrapped on CF substrate using a facile ED process
possess several advantageous features and properties
to enhance the electrochemical properties as a
psuedocapacitor electrode: (i) an express pathway for
efficient electron transport particularly owing to an
intertwined fibrous framework and largely extended
surface area of flexible CF substrate; (ii) good
electrical conductivity, high specific surface area, and
hierarchically nanostructured burl-like amorphous
NiWO4 on CF as backbone for electron transport and
electrolyte diffusion by providing a greater number
of electroactive sites and ionic transportation
channels for reversible electrochemical reactions; (iii)
direct growth and robust adhesion of burl-like
amorphous NiWO4 NSs/CF substrate for the polymer
binder- and conductive additive-free integrated
electrode for psuedocapacitors, which reduces
additional interfacial resistances and defects, thus
initiating the electroactive material for the charge
storage. This facile fabrication of hierarchical
amorphous NiWO4 NSs with burl-like morphologies
on CF could be used as a promising electrode for
flexible energy storage devices.
The electrochemical performance of the
electrochemical cell was also investigated by
calculating energy density and power density of the
burl-like amorphous NiWO4 NSs/CF electrode.
Generally, a good electrochemical psuedocapacitor is
expected to provide high specific capacitance and
high energy density at rapid charging-discharging
rates. Using GCD curves at different current densities
in Figure 5, the energy density and power density of
the burl-like amorphous NiWO4 NSs/CF were
calculated using the following formulas [30]:
E = C∆V2,
P = E/∆t.
Here, E is the energy density (Wh/kg), C is the
specific capacitance (F/g), ∆V is the potential window
(V), P is the power density (W/kg), and Δt is the
discharge time (s). The obtained energy density and
power density values were plotted in Ragone chart as
shown in Figure 7(a). The maximum energy density
was 33.47 Wh/kg at a power density of 125.18 W/kg
and significantly, it still remains 18.93 Wh/kg at a
power density of 1173.84 W/kg, confirming that the
burl-like amorphous NiWO4 NSs/CF can be
promising electrode material for high-performance
psuedocapacitor applications. In order to show the
flexible property, the working electrode (0.5 × 3 cm2)
was cut and bended with a help of cotton thread as
depicted in the schematic and photographic images
of Figure 7(b). In the photographic images of Figure
7(c), the electrochemical properties of the burl-like
amorphous NiWO4 NSs/CF electrode under normal
and bending positions were carried out using a three
electrode system in 1 M KOH electrolyte solution.
Figure 7(d) shows the CV curves for the burl-like
amorphous NiWO4 NSs/CF under normal and
bending positions at a scan rate of 30 mV/s.
Apparently, the integral area under the CV curves
does not change and the redox peak positions were
similar in the normal and bending positions of the
pseudocapacitive electrode. This can be attributed to
the reliable adhesion and mechanical stability of the
burl-like amorphous NiWO4 NSs/CF electrode.
Therefore, the electrolyte ions can be easily diffused
into the active material for reversible electrochemical
reactions even under flexible condition. To further
study the electrochemical performance of the
burl-like amorphous NiWO4 NSs/CF electrode under
normal and bending conditions, GCD measurements
were conducted at current density of 5 A/g. As seen
in Figure 7(e), the charge-discharge times were
almost overlapping with each other, which clearly
indicates their charge storage performance was
constant in normal and bending conditions,
respectively. Moreover, the Rct values were kept
unchanged during normal and bending conditions
from the EIS curves (Figure 7(f)), which further
confirm the good electrochemical reliability of
flexible electrode. The higher mechanical flexibility
and good electrochemical performance of the burl-
like amorphous NiWO4 NSs/CF electrode ensures its
potential use for flexible and wearable energy storage
device applications.
Needle
Bended sample
Cotton thread
100 10001
10
100
En
erg
y D
en
sit
y (
Wh
/kg
)
Power Density (W/kg)
(a)
Cotton
thread
CF
(electrode)
NiWO4 NSs
0.5 cm2
0.5 2 cm2
(b)
0 40 80 120 160 2000.0
0.1
0.2
0.3
0.4
0.5
Bending
Normal
Po
ten
tia
l (V
, v
s.
Ag
/Ag
Cl)
Time (s)
(e)
0 20 40 60 80 1000
20
40
60
80
100
Z'(ohm)
Normal
Bending
Z''(
oh
m)
(f)
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6-60
-40
-20
0
20
40
60
Normal
Bending
Cu
rren
t (m
A)
Potential (V vs. Ag/AgCl)
(d)
Bending
Working
electrodeAg/AgCl
electrode
Pt wire
Normal
Working
electrodeAg/AgCl
electrode
(c)
Figure 7 (a) Ragone plot of the burl-like amorphous NiWO4 NSs/CF electrode obtained from GCD curves, (b) schematic and
photographic images of the preparation of working electrode for bending test and (c) photographic images of the burl-like amorphous
NiWO4 NSs/CF electrode under normal and bending conditions in three electrode cell system with 1 M KOH electrolyte solution. (d-f)
Electrochemical performance of the burl-like amorphous NiWO4 NSs/CF electrode under normal and bending conditions.
4. Conclusion
In summary, hierarchical and burl-like
amorphous NiWO4 NSs were facilely fabricated on
flexible CF substrate via a simple two-electrode
system based ED method. Herein, under the external
cathodic voltage (-1.8 V for 15 min), direct growth of
amorphous NiWO4 NSs/CF with burl-like
morphologies was carried out and it was used as the
binder- and conductive additive-free electrode for
psuedocapacitors. The designed flexible electrode in
this work exhibited the highest specific capacitance
of 1192.2 F/g at the current density of 2 A/g and the
capacitance loss was observed to be very low (8 %) at
a current density of 10 A/g after 1000 cycles, which
are higher among the NiWO4 NSs based electroactive
materials for psuedocapacitors. By the facile growth,
the amorphous NiWO4 NSs/CF based
psuedocapacitor electrode can be further expanded
to other metal tungsten tetroxide based materials for
enhanced energy storage and photocatalytic
applications.
Acknowledgements
This work was supported by the National Research
Foundation of Korea (NRF) grant funded by the
Korea government (MSIP) (No. 2014-069441).
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17 Nano Res.
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Electronic Supplementary Material: Supplementary material (FE-SEM images of the bare CF which shows the
conductive metallic layer of Cu on the PET fibers surface) is available in the online version of this article at
http://dx.doi.org/10.1007/s12274-***-****-* (automatically inserted by the publisher).
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Nano Res.
Electronic Supplementary Material
Highly flexible conductive fabrics with hierarchically
nanostructured amorphous nickel tungsten tetraoxide
for enhanced electrochemical energy storage
Goli Nagaraju1, Ramesh Kakarla2, Sung Min Cha1 and Jae Su Yu1 ()
Supporting information to DOI 10.1007/s12274-****-****-* (automatically inserted by the publisher)
Figure S1 FE-SEM images of the bare CF with different magnifications, where it was plated with metallic layer of Cu on PET fibers.
20 m 5 m
50 m
5 m
(a)
(b)
(c)
PET
fiber
Cu
layer
Bare conductive fabric (CF)
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Nano Res.
Figure S2. Sheet resistance of the various conductive electrodes measured using four-point probe system.
Address correspondence to [email protected]
Copper
foil
Nickel
foil
Conductive
fabric (CF)Graphite
sheet
Carbon
cloth
Copper foil Nickel foilConductive
fabric (CF)
Carbon
clothSubstrate
Graphite
sheet
28-28.5 28-28.7 34-35.5 41-42.1 1140-1270
Sheet
resistance
(mΩ/sq)
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Nano Res.
Figure S3. SAED pattern of the NiWO4 NSs heated at 500 C for 2 h, showing its crystalline properties.
5 1/nm
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Nano Res.
Figure S4. (a) FE-SEM images of the burl-like amorphous NiWO4 NSs/CF after 1000 cycles, (b) photographic images of the working
electrode during cycling process, showing the strong adhesion of burl-like amorphous NiWO4 NSs to the CF without any peeling and (c)
XPS analysis of the burl-like amorphous NiWO4 NSs/CF after long-term cycling process.
1000 800 600 400 200 0
Ni 3p
Ni 3s
C 1s
W 4dW 4f
W 4p
O 1s
Ni 2p
Binding Energy (eV)
Inte
nsit
y (
a.u
.)
(c)
50 nm
200 nm
(a) (b)
Working
electrode
Ag/AgCl
electrode
Pt wire