Enhanced electrochemical performance of nanoplate nickel cobaltite (NiCo2O4) supercapacitor applications

Article abstract of DOI:10.1039/c8ra09081e

Well-ordered, unique interconnected nanostructured binary metal oxides with lightweight, free-standing, and highly flexible nickel foam substrate electrodes have attracted tremendous research attention for high performance supercapacitor applications owing to the combination of the improved electrical conductivity and highly efficient electron and ion transport channels. In this study, a unique interconnected nanoplate-like nickel cobaltite (NiCo₂O₄) nanostructure was synthesized on highly conductive nickel foam and its use as a binder-free material in energy storage applications was assessed. The nanoplate-like NiCo₂O₄ nanostructure electrode was prepared by a simple chemical bath deposition method under optimized conditions. The NiCo₂O₄ electrode delivered an outstanding specific capacitance of 2791 F g^?1 at a current density of 5 A g^?1 in a KOH electrolyte in a three-electrode system as well as outstanding cycling stability with 99.1% retention after 3000 cycles at a current density of 7 A g^?1. The as-synthesized NiCo₂O₄ electrode had a maximum energy density of 63.8 W h kg^?1 and exhibited an outstanding high power density of approximately 654 W h kg^?1. This paper reports a simple and cost-effective process for the synthesis of flexible high performance devices that may inspire new ideas for energy storage applications.

Full text of DOI:10.1039/c8ra09081e

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Enhanced electrochemical performance of
nanoplate nickel cobaltite (NiCo2O4)
Cite this: RSC Adv., 2019, 9, 1115
supercapacitor applications†
*
Anil Kumar Yedluri
and Hee-Je Kim
Well-ordered, unique interconnected nanostructured binary metal oxides with lightweight, free-standing,
and highly flexible nickel foam substrate electrodes have attracted tremendous research attention for
high performance supercapacitor applications owing to the combination of the improved electrical
conductivity and highly efficient electron and ion transport channels. In this study,
a unique
interconnected nanoplate-like nickel cobaltite (NiCo2O4) nanostructure was synthesized on highly
conductive nickel foam and its use as a binder-free material in energy storage applications was assessed.
The nanoplate-like NiCo2O4 nanostructure electrode was prepared by a simple chemical bath deposition
method under optimized conditions. The NiCo2O4 electrode delivered an outstanding specific
capacitance of 2791 F gꢀ1 at a current density of 5 A gꢀ1 in a KOH electrolyte in a three-electrode
system as well as outstanding cycling stability with 99.1% retention after 3000 cycles at a current density
of 7 A gꢀ1. The as-synthesized NiCo2O4 electrode had a maximum energy density of 63.8 W h kgꢀ1 and
exhibited an outstanding high power density of approximately 654 W h kgꢀ1. This paper reports a simple
and cost-effective process for the synthesis of flexible high performance devices that may inspire new
ideas for energy storage applications.
Received 2nd November 2018
Accepted 26th December 2018
DOI: 10.1039/c8ra09081e
rsc.li/rsc-advances
capacitance and energy densities through rapid, reversible,
multi-electron, surface faradaic reactions.17–21 In particular,
Introduction
some low-cost transition metal oxides and hydroxides, such as
ZnO, Fe2O3, MnO2, Mn3O4 Co3O4, NiO, and Co(OH)2, have been
developed as candidates.22–26
The development of renewable energy conversion systems is an
urgent requirement to meet the needs of high power electronic
devices, electric vehicles, and electrochemical energy storage
devices as well as address ecological concerns.1–3 Among the
various electrochemical energy storage systems, super-
capacitors have attracted increasing attention in recent years
because they exhibit a good balance between energy density and
power density, excellent long-life cycling stability with a rapid
charge/discharge rate over batteries, and store more energy
than conventional capacitors.4–9
Electrode materials can be classied into two main groups
based on the charge storage mechanism: electric double-layer
capacitors (EDLCs) and pseudocapacitors (PCs).10,11 Generally,
EDLCs consist of conductive porous carbon materials, such as
carbon spheres, activated carbon, and graphene.12–16 On the
other hand, the practical applications of EDLCs have been
limited by their low specic capacitance and low energy density.
The main pseudocapacitive materials, such as metal oxides,
hydroxides and conducting polymers, exhibit higher specic
Among the various electroactive materials reported thus far,
nickel oxide (NiO) is a typical PC material owing to its strong
theoretical capacity, excellent reversibility, well-maintained and
fascinating morphology, suitable pore size, large specic area,
and excellent reliability.27–29 However, NiO as an active material
in PCs has been restricted because of its lower rate behavior,
poor cycling stability, and lower electrochemical activity.30–32 To
overcome this problem, further efforts have been made to
prepare nanocomposites that combine NiO with other electro-
active materials. Among the various PCs components, cobalt
oxides (Co3O4 and CoO) have become promising candidates for
supercapacitor electrode applications owing to their low
toxicity, low parity cost, facile preparation, and good corrosion
stability.33,34 Moreover, binary metal oxides exhibit better elec-
trochemical performance compared to single-component metal
oxides owing to their outstanding electrical conductivity and
multiple oxide states.
In particular, ternary nickel cobaltite (NiCo2O4) has attracted
considerable attention for its ultrahigh specic capacitance
because ternary NiCo2O4 combines the characteristics of simple
transition metal oxides, has high specic capacitance and
electronic conductivity, and better electron transport between
School of Electrical Engineering, Pusan National University, Busandaehak-ro
63beon-gil, Geumjeong-gu, Busan, 46241, Republic of Korea. E-mail: yedluri.anil@
gmail.com; yedluri.anil@pusan.ac.kr; Fax: +82 51 513 0212; Tel: +82 10 3054 8401
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra09081e
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the electrolyte and electrode surface area than binary metal homogeneous solution of all chemicals applied, such as 0.42 g
oxides of nickel oxide (NiO) and cobalt oxide (Co3O4).35,36 In of Ni (NO3)2$6H2O and 0.86 g of Co(NO2)3$6H2O, 0.22 g of
particular, NiCo2O4 has attracted considerable attention, owing CH4N2O and 1.80 g of urea were dissolved in 100 mL of
to its superior conductivity to other binary metal oxides, such as deionized water with constant magnetic stirring for 50 min.
ZnFe2O4, CoMoO4, MnMoO4, Zn3V2O8, and NiMnO3. Nickel Aer stirring, the homogeneous solution formed a clear pink
cobaltite (NiCo2O4) has excellent redox activity, natural abun- color solution, which indicates the active electrode of nickel
dance, low cost, environmentally benign characteristics, and cobaltite (NiCo2O4) material. The resulting clear solution con-
nontoxicity.37,38 Thus, the fabrication of nanoplate-structured taining a piece of nickel foam was transferred into a 100 mL
NiCo2O4 using a simple and novel method to enhance its elec- capped bottle and kept at 120 ꢂC for 12 h. Aer cooling to room
trochemical performance is the focus of this research. Until temperature, the as-prepared nickel foam was separated by
now, there are only a few reports on the use of NiCo2O4 as an centrifugation and rinsed with absolute ethanol and distilled
electrode material for supercapacitor applications. The chem- water. The as-prepared product was then dried with a dryer for
ical bath deposition approach is a more common method for 30 min. Finally, for the heat treatment step, the resulting
the preparation of functional materials than hydrothermal prepared productꢀw1as annealed under 400 ꢂC for 3 h at a heating
ꢂ
methods, microwave irradiation method, thermal annealing rate of 20 C min in air. The as-prepared samples supported
treatment approach, and mechano-chemical synthesis. Despite on nickel foam were obtained. The total average NiCo2O4
this, little attention has been paid to the synthesis of NiCo2O4 loading was 3 mg cmꢀ2
electrode materials. The production of high-performance
.
supercapacitors for nanostructured NiCo2O4 materials by Material characterization
a simple method remains a challenge.
The morphology, microstructure, and internal structural prop-
In this study, a nanostructure of NiCo2O4 nanoplate-like
structure was grown uniformly on highly conductive nickel
foam for high-performance supercapacitor applications
through a chemical bath deposition method. By rationally
controlling the growth kinetics of NiCo2O4, the electrode
exhibited a honeycomb nanostructure on a nickel foam surface,
which provided rapid channels for ion and electron transfer and
formed abundant electrochemically active sites exposed to the
electrolyte surface area. The NiCo2O4 nanoplate's electrode
exhibited a high specic capacitance of 2791 F gꢀ1 at a current
density of 5 A gꢀ1 and delivered an excellent energy density
63.8 W h kgꢀ1 and high power density of 654 W kgꢀ1 in a KOH
electrolyte. This might be because the unique interconnected
nanoplate's structure exhibits good reversibility and lower
charge-transfer resistance of 0.2 U during the faradaic process.
These supercapacitors also exhibited excellent long-life cycling
stability of approximately 99.1% retention aer 3000 cycles,
which might be because the nanoplate nanostructure allows
convenient ion transport within and between the combs.
erties of the samples were studied by eld emission scanning
electron microscopy (FESEM, S-4800, Hitachi) at an acceleration
voltage of 15 kV and eld emission transmission electron
microscopy (FETEM, JEOL, JEM-2100F) operated at 200 kV. The
crystalline phase structure of the as-prepared sample was
investigated by powder X-ray diffraction (XRD, D8 ADVANCE
Bruker) at a voltage of 40 kV and a current of 40 mA using Cu Ka
radiation (k ¼ 0.1542 nm). The elemental composition of the
material was determined by X-ray photoelectron spectroscopy
(XPS, VG Scientic ESCALAB 250) using Al Ka radiation
(Scheme 1).
Electrochemical measurements
Electrochemical characterization of the NiCo2O4 were carried
out in a 3 M aqueous KOH solution on an electrochemical
workstation (BioLogic-SP150 Korea) using a conventional three
electrode system. A three-electrode system containing Ag/AgCl
and platinum foil (Pt) as the reference and counter electrode,
respectively, was used. The as-prepared NiCo2O4 electrode were
directly used as the working electrode. Cyclic voltammetry (CV)
of the resulting product was performed over the potential range
from ꢀ0.2 to 0.5 V vs. Ag/AgCl at different scan rates. The gal-
vanostatic charge–discharge curves test of the prepared elec-
trode was performed over the potential range, 0 to 0.4 V vs. Ag/
AgCl, at various current densities (A). Electrochemical imped-
ance spectroscopy (EIS) was conducted over the frequency
range, 0.1 Hz to 100 kHz with an AC amplitude of 5 mV. The
specic capacitance (Csc, F gꢀ1) of the electrode from GCD plots,
energy density (E, W h kgꢀ1) and power density (P, W kgꢀ1) for
the three electrode system were calculated using the following
equations (eqn (1), (2), and (3)) given below:39
Experimental section
Preparation of the NiCo2O4 nanoplate material on nickel foam
All chemicals applied, such as nickel nitrate hexahydrate, cobalt
nitrate hexahydrate, urea, and ammonium uoride, were of
analytical grade and used as received. The nanoplate-like
NiCo2O4 nanostructure was synthesized by a simple chemical
bath deposition method. The process involved a solvothermal
reaction of nickel and cobalt ions as inorganic components with
urea and ammonium uoride to form
a precursor/self-
sacricing template, which was followed by heat treatment in
air to generate nanoplate-like structures. Prior to synthesis,
a piece of nickel (1 cm ꢁ 1 cm) foam was cleaned by being
immersed in 2 M HCl under ultrasonication in sequence for
15 min, and rinsed with absolute ethanol and deionized (DI)
water, respectively, which results in the removal of the surface
layer and greasing of Ni oxide. In particular, an aqueous
I ꢁ t
Csc
¼
(1)
m ꢁ v
0:5 ꢁ Csc ꢁ DV2
3:6
E ¼
(2)
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SEM, TEM and high-resolution TEM (HR-TEM), as shown in
Fig. 1. Fig. 1a–c shows the morphology of the nanoplate-like
NiCo2O4 at various magnications. The nickel foam is covered
uniformly by NiCo2O4 nanoplate structures, indicating that the
nanoplates are composed of many nanosheets. The ne nano-
plates had a mean diameter of 50–80 nm. As shown in Fig. 1c,
the internal structure of the as-prepared material has larger
pores because of the empty spaces between the plates. In
particular, this nanostructure has high specic surface areas
with a bimodal pore size distributions, which play a key role in
the high energy density and power density suitable for energy
E ꢁ 3600
Dt
P ¼
(3)
where I (A) is the discharge current, Dt (s) is the discharge time,
m (g) is the mass of the electroactive material of the active
electrode, and DV (V) is the voltage window drop, respectively.
Results and discussion
The morphology, interior structure, and crystalline properties
of the as-prepared NiCo2O4 nanomaterial was examined by
Scheme 1 Schematic of the preparation process of NiCo2O4 nanoplate's structure.
Fig. 1 (a–c) Low- and high-magnification FE-SEM images and (d and e) TEM and HR-TEM images of the NiCo2O4 nanoplate material.
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storage applications. Moreover, FE-SEM images of the working reaction of the redox couple in the KOH electrolyte and has
electrode showed that every nanoplate had grown uniformly on a larger surface area between the electrode and electrolyte. The
the nickel foam and had a rough surface area. The SEM image lattice fringes in the HR-TEM image separated by d ¼ 0.243 nm
(Fig. S1a†) of the NiCo2O4 at low magnication shows that were assigned to the (311) plane of NiCo2O4 (JCPDS) data, as
a layer of precursor is evenly covered on the surface of Ni foam. shown in Fig. 1e.
Aer the nanoplate's growth, the NiCo2O4 nanoplates covered
The crystallinity and phase composition of the as-prepared
the surface of nickel foam uniformly and also nanoplates grown NiCo2O4 nanoplate on nickel foam were examined by XRD, as
on nickel foam were uniformly distributed with no overlap. The shown in Fig. 2a. XRD showed that the NiCo2O4 material was
magnied image in the inset reveals shows a low magnication crystalline. The XRD peaks at 18.91ꢂ, 31.15ꢂ, 36.70ꢂ, 38.40ꢂ,
SEM image of the visible networks of Ni foam, in which we can 44.62ꢂ, 55.44ꢂ, 59.09ꢂ, 64.98ꢂ, and 68.31ꢂ 2q were indexed to the
see that the Ni foam has a porous network structure with (111), (220), (311), (222), (400), (422), (511), (440), and (531)
smooth surface and also no impurities. Fig. 1d and e presents planes, respectively, of the cubic spinal-like NiCo2O4 Joint
TEM and HR-TEM images taken on the surface of the NiCo2O4 Committee in Powder Diffraction Standards (JCPDS card no. 20-
nanoplates coated on the surface of nickel foam. As shown in 0781),40 which is consistent the TEM analysis. XPS provided
Fig. 1d, the internal structure of the active material clearly further evidence of the chemical composition and elemental
showed large pores and nanoplate regions. Atomic force oxidation states of the as-prepared NiCo2O4 sample. The XPS
microscopy (AFM) is a promising technique for studying some survey spectrum revealed nickel, cobalt, and oxygen in accor-
structural information and thickness measurements about the dance with the literature values for NiCo2O4 (Fig. 2b). EDX
as-obtained material. Fig. S2a and b† shows topology of the analysis of the as-prepared product showed nickel, cobalt and
surface of NiCo2O4 on Ni foil. It can be seen that a plate struc- oxygen (Fig. 2b), indicating a pure phase without impurities.
ture of NiCo2O4 is formed on Ni foil. The AFM images reveal The atomic percentage of Ni, Co, and O were 41.87%, 35.39%,
a uniform surface roughness, a smooth surface, and good and 22.73% (5 : 4 : 3), respectively. A Gaussian t of Ni 2p
adhesion to the substrate with a narrow particle size distribu- deconvoluted spectrum at 855.8, 861.8, 873.5, and 879.9 eV
tion. Also, it can be observed that the surface is converted to revealed Ni2+ 2p3/2, Ni3+ 2p3/2, Ni2+ 2p1/2 and Ni3+ 2p1/2
,
a relatively uniform size. Furthermore, AFM measurement respectively (Fig. 2d), indicating the presence of two spin–orbit
indicated that the average thickness of these NiCo2O4 nanoplate doublets and two shakeup satellites denoted as (“Sat.”) as well
was around ꢃ20 to 30 nm. Overall, AFM indicates that the as +2 and +3 oxidation states in the prepared NiCo2O4 mate-
NiCo2O4 contains more electrocatalytic activity sites for the rial.41 In the case of the Co 2p (Fig. 2e) deconvoluted spectrum,
Fig. 2 Spectroscopic and microscopic characterization of the nanoplate-like NiCo2O4 nanostructure. (a) XRD pattern. (b) XPS survey spectrum.
(c) EDX spectrum and high-resolution XP spectra of (d) Ni 2p, (e) Co 2p and (f) O 1s for the as-prepared NiCo2O4 sample.
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multiple peaks at 780.5, 789.6, 795.1 and 804.2 eV were assigned and redox reaction corresponding to an alkaline electrolyte
to Co2+ 2p3/2, Co2+ 2p3/2, Co3+ 2p1/2, and Co2+ 2p1/2, respec- according to the equations during the electrochemical
tively.42 The high resolution O 1s spectrum (Fig. 2f) was measurements.
deconvoluted into two main peaks, 530.2 and 530.9 eV, which
NiCo2O4 + H2O + OHꢀ 4 NiOOH + 2CoOOH + eꢀ
2CoOOH + OHꢀ 4 2CoO2 + H2O + eꢀ
(4)
(5)
conrmed the formation of metal–oxygen bonding in the
presence of O 1s.43 The data clearly show that the presence of
valence and mixed valence of metal ions, Ni2+/Ni3+, Co2+/Co3+,
and O 1s, on the surface of the as-prepared NiCo2O4 sample,
which is expected to provide sufficient active sites for the redox
reaction in the electroactive activity.
In addition, galvanic charge–discharge (GCD) measurements
were taken to evaluate the specic capacitance of the electro-
active material in a 3 M KOH aqueous electrolyte solution
between 0–0.5 V (vs. Ag/AgCl) at various current densities of 5–
10 A gꢀ1. The resulting curves were attributed to the faradaic
redox process and were close to battery type charge storage, as
shown in Fig. 3b. The voltage plateaus in the charge–discharge
plots were retained at high scan rates, which indicates the high
rate capability of the electrode material. According to eqn (1),
the NiCo2O4 electrode exhibited a specic capacitance of 2791,
2650, 2570, 2530, 2510, and 2490 F gꢀ1 at current densities of 5,
6, 7, 8, 8, 9, and 10 A gꢀ1, respectively (as shown in Fig. 3c). The
specic capacitance was also superior to those of recently re-
ported exible electrodes with NiCo2O4 agglomerated particles
(351 F gꢀ1 at a current density of 1 A gꢀ1),44 NiCo2O4 nanoat-
like particles (764 F gꢀ1 at the scan rate of 2 mV sꢀ1),45
NiCo2O4 ower-like nanostructure (658 F gꢀ1 at a current
The electrochemical performance of as-prepared NiCo2O4 as
an electrode material for supercapacitors was investigated using
a three-electrode system in a 3 M KOH aqueous solution as the
electrolyte. Fig. 3a presents the CV curves of the active electrode.
All CV curves showed a pair of redox peaks at various scan rates
from 10–100 mV sꢀ1 over the voltage range of ꢀ0.2 to 0.5 V,
indicating similar pseudocapacitive behavior and good rate
capability. In addition, with increasing scan rates, the redox
peaks current intensity areas become larger as the scan sweep
increased. Furthermore, due to polarization of the electroactive
sample, the anodic peaks shied enormously towards a positive
potential, whereas the cathodic peaks moved towards a negative
potential with increasing scan rate because of the low resistance
and fast ion and electron transfer rates. The strong peaks can be
closely related to the chemical composition and morphology of
the electrode material for the reversible faradaic redox reactions
Fig. 3 (a) CV curves of the NiCo2O4 nanoplate's obtained at various scan rates of 10–100 mV sꢀ1; (b) galvanostatic charge–discharge curves of
the NiCo2O4 at different current densities of 5–10 A gꢀ1; (c) specific capacitance of the three electrode material at various current densities.
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density of 1 A gꢀ1),46 NiCo2O4 nanosheets-like (899 F gꢀ1 at the
The long-term cycling stability and coulombic efficiency of
current density of 1 A gꢀ1),47 NiCo2O4 nanoakes (1270 F gꢀ1 at the as-prepared product as an electrode material was examined
the current density of 1 A gꢀ1),48 NiCo2O4 nanoneedles nano- by repeating the charge–discharge plots. As shown in Fig. 4c,
structure (1427 F gꢀ1 at the current density of 8 A gꢀ1),49 the NiCo2O4 nanoplate-like structure exhibited excellent long-
NiCo2O4 hollow microspheres (764 F gꢀ1 at a current density of term electrochemical stability and high rate stability with
2 A gꢀ1),50 and chain-like NiCo2O4 nanowires (1284 F gꢀ1 at the a very slight decrease to 99.1%, even aer 3000 cycles (only 0.9%
current density of 2 A gꢀ1).51
loss aer 3000 cycles) in a three-electrode system at a current
Fig. 4a shows the Nyquist plot of the device. The electro- density of 7 A gꢀ1. This is superior to those of previously re-
chemical characteristics of the electrode were examined ported 3D hierarchical ower-shaped NiCo2O4 microspheres
further by electrochemical impedance spectroscopy (EIS). The (93.2% retention aer 1000 cycles),56 hierarchical spinal
Nyquist plots of the electrode were prepared at the open NiCo2O4 nanowires (84.7% retention aer 500 cycles),57 NiCo2-
circuit potential over the frequency range, 0.01 Hz to 100 kHz. O4@MnO2 nanowire arrays (88% retention aer 2000 cycles),58
In the three electrodes, the Rs value was 0.2 U, which indicates NiCo2O4@NiO nanowire arrays (93.1% retention aer 3000
the low-frequency range. In addition, the NiCo2O4 electrode cycles),59 NiCo2O4@CoMoO4 nanowire/nanoplates arrays
showed an almost vertical line, indicating the diffusion of (74.1% retention aer 1000 cycles),60 ultrathin porous NiCo2O4
ions at the interface of the electrode/electrolyte, better nanosheet arrays (20% reduced aer 3000 cycles),61 and
conductivity and rapid electron transfer kinetics. Fig. 4b NiCo2O4@NiMoO4 nanowires (90.6% retention aer 2000
presents Ragone plots of the NiCo2O4 electrode of the energy cycles).62 TEM showed that the nanohoneycombs still existed in
and power densities. The device exhibited an excellent energy the NiCo2O4 electrode (inset in Fig. 4c) aer long term cycling,
density of 63.8 W h kgꢀ1 at a power density of 333 W kgꢀ1, even suggesting that there was no noticeable change in the
at a low energy density of 54.4 W h kgꢀ1 at a maximum power morphology of the sample. Such excellent cycling stability of the
density of 654 W kgꢀ1, which is superior to those reported NiCo2O4 electrode could be attributed to the following: the
previously, such as NiCo2O4-NC (28
8.5
kgꢀ1),52 NiCo2O4-GNF//AC (33.8
W
W
h
h
kgꢀ1 and unique honeycomb structure, which can alleviate the volume
W
kgꢀ1 and changes during the charge–discharge process to guarantee good
5 W kgꢀ1),53 NiCo2O4 mesoporous spinal-like (17.72 W h kgꢀ1 stability; gradual penetration of the electrolyte ions into the
and 25.42 W kgꢀ1),54 and NiCo2O4-rGO (23.32 W h kgꢀ1 and electroactive material; and reduced volume expansion resulting
324.9 W kgꢀ1).55
from the rapid and long-term faradaic reaction.
Fig. 4 (a) Nyquist plots of the NiCo2O4 electrode, (b) Ragone plots of the device and (c) cyclic performance of NiCo2O4 nanoplates during 3000
cycles at a scan rate of 7 A gꢀ1
.
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Conclusion
NiCo2O4 with a unique interconnected nanoplate-like structure
was fabricated on the surface of the highly conductive nickel
foam through a simple chemical bath deposition reaction. The
interconnected honeycomb nanostructure electrode can
enhance the pathway for electron transport originating from the
good electronic conductivity of the nickel foam. In addition, it
can reduce the transport distance of ions and enhance elec-
trode–electrolyte contact, which leads to higher material acti-
vation, and endow it with a high specic capacitance and
excellent long life cycling stability. The NiCo2O4 nanoplate
electrode delivered a high specic capacitance of 2791 F gꢀ1 at
a current density of 5 A gꢀ1 and exhibited a signicantly high
energy density of 63.8 W h kgꢀ1 and high power density of
654 W kgꢀ1, highlighting its potential for energy storage
applications. The nanoplate structure electrode material for
electrochemical capacitors displayed excellent cycling stability
of 99.1% retention aer 3000 cycles, even at a high current
density of 7 A gꢀ1. As a result, the NiCo2O4 electrode provides
a path for high-performance supercapacitors because of its low-
cost simple chemical bath deposition approach and environ-
mental friendliness.
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Conflicts of interest
23 N. Choudhary, C. Li, J. Moore, N. Nagaiah, L. Zhai, Y. Jung
and J. Thomas, Adv. Mater., 2017, 29, 1605336.
24 J. Zhao, Z. Li, M. Zhang, A. Meng and Q. Li, ACS Sustainable
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There are no conicts to declare.
Acknowledgements
25 L. Li, L. Tan, G. Li, Y. Zhang and L. Liu, Langmuir, 2017, 33,
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26 Y. A. Kumar, S. S. Rao, D. Punnoose, C. V. Thulasivarma,
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This work was nancially supported by BK 21 PLUS, Creative
Human Resource Development Program for IT Convergence
(NRF-2015R1A4A1041584), Pusan National University, Busan,
South Korea. We would like to thank KBSI, Busan for SEM, TEM,
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27 M. M. Yao, Z. H. Hu, Z. J. Xu, Y. F. Liu, P. P. Liu and Q. Zhang,
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