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lable at ScienceDirect
Electrochimica Acta 305 (2019) 81e89
Contents lists avai
Electrochimica Acta
journal homepage: www.elsevier .com/locate/electacta
Hierarchical CoNi2S4 nanosheet/nanotube array structure on
carbonfiber cloth for high-performance hybrid supercapacitors
Chen Su a, Shusheng Xu a, Lu Zhang c, Xinwei Chen a, Guoqin Guan
a, Nantao Hu a,Yanjie Su a, Zhihua Zhou a, Hao Wei a, Zhi Yang a,
*, Yong Qin b, **
a Key Laboratory of Thin Film and Microfabrication (Ministry of
Education), Department of Micro/Nano Electronics, School of
Electronic Information andElectrical Engineering, Shanghai Jiao
Tong University, Shanghai, 200240, PR Chinab Institute of
Nanoscience and Nanotechnology, School of Physical Science and
Technology, Lanzhou University, Gansu, 730000, PR Chinac School of
Advanced Materials and Nanotechnology, Xidian University, Xi'an,
710071, PR China
a r t i c l e i n f o
Article history:Received 24 December 2018Received in revised
form26 February 2019Accepted 3 March 2019Available online 4 March
2019
Keywords:CoNi2S4 nanosheet/nanotube arraysCarbon fiber
clothElectrodepositionHybrid supercapacitorEnergy density
* Corresponding author.** Corresponding author.
E-mail addresses: [email protected] (Z. Yang), q
https://doi.org/10.1016/j.electacta.2019.03.0130013-4686/© 2019
Elsevier Ltd. All rights reserved.
a b s t r a c t
Well-defined nanostructures are attractive due to their
excellent advantages in enhancing the perfor-mance of
electrochemical energy storage. In this work, a kind of hybrid
structure of hierarchical CoNi2S4nanosheet/nanotube array directly
assembled on carbon fiber cloth has been designed and developed
forhigh-performance supercapacitors. CoNi2S4 nanosheet/nanotube
arrays are fabricated through orderlyelectrodeposition of ZnO
nanorod arrays and CoNi2S4 nanosheets, followed by removing ZnO
nanorodstemplate. Benefiting from the unique hollow nanostructure
with abundant electrochemical active sites,the specific capacitance
of this electrode can reach up to 995.8 C g�1 at a current density
of 2 A g�1, alongwith excellent rate capability (740 C g�1 at 50 A
g�1). Moreover, the hybrid supercapacitors are preparedby using
hierarchical CoNi2S4 nanosheet/nanotube arrays as positive
electrode and reduced grapheneoxide-carbon nanotubes as negative
electrode for energy storage application, which demonstrate a
highenergy density of 35Wh kg�1 at a power density of 3 kW kg�1 and
excellent cycle stability with 96.9%capacitance retention after
10000 cycles. This work provides a feasible and a practical
approach tofabricate CoNi2S4 hollow nanostructures and its huge
potential in energy storage.
© 2019 Elsevier Ltd. All rights reserved.
1. Introduction
With the ever growing of energy crisis and environment
prob-lems, it is urgent to develop low cost and eco-friendly
energystorage devices [1e3]. Supercapacitors (SCs) are efficient
energystorage devices with excellent properties such as high
powerdensity, fast charge-discharge process and long cycle life
[4e6].However, compared with lithium ion batteries, the lower
energydensity of SCs limits its practical application. It is
expected that SCsshould have higher energy density and power
density. It has beenproved that hybrid supercapacitors (HSCs)
taking the advantages ofboth the electrochemical double-layer and
faradaic reactions tostore electrical energy are the promising
energy storage devices[7e9].
Transition metal oxides/hydroxides are the common used
active
[email protected] (Y. Qin).
electrode materials for battery-type faradaic electrode which
storeelectrical energy through the reversible faradaic reactions
[10].However, the performance of metal oxides/hydroxides
basedpesudocapacitors is limited due to the poor electrochemical
sta-bility and low conductivity. These problems can be solved
either bydesigning new electrode active materials with higher
conductivityor novel structures with more active sites exposed.
Recently,transition metal sulfides have been regarded as the
potentialelectrode active materials for pseudocapacitive energy
storage. Forexample, Xu and co-workers prepared Co3S4 nanosheets
(NSs) witha capacitance of 1037 F g�1 at 1 A g�1 [11]; Dai and
co-workers re-ported NiS2 hollow prisms with a high capacitance of
1725 F g�1 at5 A g�1 [12]; Guo and co-workers fabricated
double-shell CuSnanocages through anion exchange reaction, with
electrochemicalperformance of 843 F g�1 at 1 A g�1 [13]. It has
been reported thattransition metal sulfides such as Co3S4, NiS2,
and CuS have loweroptical band gap energy and higher conductivity
than transitionmetal oxides [14]. Since the electronegativity of
sulfur is lower thanthat of oxygen, the substitution of oxygenwith
sulfur can effectively
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C. Su et al. / Electrochimica Acta 305 (2019) 81e8982
prevent the structural damage caused by the elongation
betweenlayers and facilitate the electron transport in the
structure [15].Moreover, bimetallic NieCo sulfides show more active
sites for theredox reactions, higher electrical conductivity for
charge transferand more stable than their corresponding single
metal sulfides,which results in enhanced electrochemical
performance [16,17].Encouraged by these findings, various
nanostructures of NieCosulfides have been reported, such as CoNi2S4
nanoparticles (NPs)[18], CoNi2S4 nanowires [16], CoNi2S4 nanosheets
[19], CoNi2S4mushroom-like arrays [20], NiCo2S4 nanotube arrays
(NTAs) [17]and NiCo2S4 urchin-like nanostructures [14]. Among them,
well-defined NTAs have drawn great attention due to their high
sur-face area. It has been demonstrated that the well-oriented
NTAsalso contribute to the ion diffusion and substantially
facilitate theelectrolyte penetration [21]. Xiao and co-workers
reported NiCo2S4NTAs grew on carbon fiber paper [17]. However, the
preparationprocedure is complicated, including multistep of the
synthesis ofNieCo precursor, vulcanization of precursor at high
temperatureand acid etching to remove the interior metal oxide.
Xing and co-workers have synthesized Ni3S2 NTAs by sacrificial ZnO
nanorodarrays (NRAs) template [22], but this sample has the
contactproblems between the conductive substrate and active
materialwith the removal of ZnO seed layer. Therefore, it is
imperative todevelop a simple method to synthesize stable CoNi2S4
nano-structures with high surface area which show great potential
inenhancing the electrochemical performance. Here, inspired byChen
and co-workers’ work to synthesize CoNi2S4 NSs in one step[23], we
make use of electrodeposition method to synthesize aunique stable
hierarchical CoNi2S4 nanosheet/nanotube array (NS/NTA) structure
and further validate its application in hybridsupercapacitors.
In this work, we have successfully synthesized well aligned
hi-erarchical CoNi2S4 NS/NTAs by employing electrodeposited ZnONRAs
as template. This novel hybrid nanostructure of
hierarchicalNSs/NTAs intimately attached to carbon fiber cloth
(CFC) caneffectively improve the specific surface area and provide
moreactive sites which are benefit for improving specific
capacitance.This binder-free electrode has an ultrahigh specific
capacitance of
Fig. 1. (aed) Schematic illustration for the preparation of
CoNi2S4 NS/NTAs, and correspondinand (h) hierarchical CoNi2S4
NS/NTAs-5 on CFC.
995.8 C g�1 at 2 A g�1, superior rate capability (740 C g�1 at50
A g�1), and good cycling stability (can retain 77.2% after
2000charge-discharge cycles). Furthermore, HSCs based on the
hierar-chical CoNi2S4 NS/NTAs and reduced graphene
oxide-carbonnanotube (rGO-CNT) film was assembled and exhibited a
high en-ergy density of 35Wh kg�1 at a power density of 3 kWkg�1
withgood stability. This result outperforms most of the NieCo
basedmaterials, indicating that our new developed CoNi2S4 NS/NTAs
arethe promising candidate for energy storage devices.
2. Experimental section
2.1. Synthesis of ZnO NRAs on CFC
Two different methods including the electrodeposition and
thehydrothermal process were employed for the preparation of
ZnONRAs on CFC for comparison. CFC was treated in concentratedHNO3
for 30min at 60 �C in advance to improve the surface
hy-drophilicity. For the electrodeposition method, the electrolyte
wasprepared by dissolving 5mM zinc nitrate
hexahydrate(Zn(NO3)2$6H2O) and 5mM hexamethylenetetramine (HMTA)
into100mL deionized (DI) water. The electrodeposition was carried
outin a conventional three-electrode system, where CFC (1� 1
cm2)acted as the working electrode, 1� 1 cm2 platinum plate as
thecounter electrode and Ag/AgCl as the reference electrode,
respec-tively. ZnO NRAs were continuously deposited at a constant
po-tential of �1 V versus the reference electrode for 3600 s at 90
�C.For the hydrothermal method, firstly ZnO seed layer was
sputteredon CFC, then ZnO NRAs template was grown on CFC (1� 1 cm2)
in asolution of 0.01M Zn(NO3)2$6H2O and 0.01M HMTA at 90 �C for6
h.
2.2. Preparation of hierarchical CoNi2S4 NS/NTAs on CFC
Both of the two types ZnO NRAs were used as the templates forthe
electrodeposition of CoNi2S4. The electrodeposition of CoNi2S4on
ZnO NRAs were conducted by cyclic voltammetry (CV) scan atselected
scan rate and CV cycles in a potential range of �1.2 to 0.2 V
g SEM images of (e) bare CFC, (f) ZnO NRAs on CFC, (g)
ZnO/CoNi2S4 NS/NRAs-5 on CFC,
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C. Su et al. / Electrochimica Acta 305 (2019) 81e89 83
vs Ag/AgCl at 40 �C in a three-electrode system mentioned
above.The electrolyte was prepared by dissolving 5mM
CoCl2$6H2O,7.5mM NiCl2$6H2O and 0.75M thiourea (CS(NH2)2) into
100mL DIwater. The working electrodes were labeled as ZnO/CoNi2S4
NS/NRAs-x (x¼ number of CV cycles, i.e., 3, 5, 7 and 11 cycles).
Afterthe electrodeposition of CoNi2S4, the working electrodes
wereetched in 1M KOH for 6 h for the removal of ZnO NRAs
template,followed by drying at 60 �C for 10 h. The as-synthesized
sampleswere labeled as CoNi2S4 NS/NTAs-x, and directly used as
theworking electrode for the evaluation of energy storage
perfor-mance. The loading mass of the CoNi2S4 on CFC for optimal
samplewas 0.4mg cm�2 by comparing the mass of the CFC/CoNi2S4
NS/NTAs with that of bare CFC using a microbalance.
2.3. Characterizations
The morphologies and nanostructures of the
as-synthesizedelectrodes were observed by using a field emission
scanning
Fig. 2. Fine structure and chemical composition
characterization. (a) TEM image of CoNi2S4(cef) XPS analysis of the
electrodeposition growth of CoNi2S4 NT.
electronmicroscope (FE-SEM, Ultra Plus, Carl Zeiss, Germany) and
ahigh resolution transmission electron microscopy (HRTEM, FEITecnai
G2 F30). X-ray diffraction (XRD) patterns were recorded onan
advanced X-ray diffractometer (D8 Advance, Bruker, Germany)with
Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS,Japan Kratos
Axis UltraDLD spectrometer) was applied to analyzethe elemental
composition and chemical state of the as-synthesized materials. The
specific surface area was estimated bythe Brunauer-Emmett-Teller
(BET, Autosorb-iQ, Quantachrome,USA) through measuring N2
adsorption-desorption isothermals at77 K. The electrochemical
properties of as-synthesized materialswere measured with a CHI760E
electrochemical workstation. CV,galvanic charge-discharge (GCD) and
electrochemical impedancespectroscopy (EIS) measurements were
conducted using a three-electrode set-up in 6M KOH aqueous
electrolyte. A Pt plate andthe Hg/HgO were used as the counter
electrode and the referenceelectrode, respectively.
NT. The inset image is the SAED pattern. (b) HRTEM image of as
prepared CoNi2S4 NT.
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C. Su et al. / Electrochimica Acta 305 (2019) 81e8984
3. Results and discussion
The synthetic process of the hierarchical CoNi2S4 NS/NTAs
isillustrated in Fig. 1. CFC (Fig. 1a) was treated in concentrated
HNO3to improve the surface hydrophilicity and used as the support
forthe electrodeposition of ZnO NRAs (Fig. 1b). The ZnO NRAs
werethen used as the template for the electrodeposition of
CoNi2S4(Fig. 1c). After the removal of ZnO NRAs, hierarchical
CoNi2S4 NS/NTAs on CFC were obtained (Fig. 1d). As shown in Fig.
1f, theelectrochemical deposition method derived ZnO NRAs are
uni-formly coated on the CFC surface. With the further
electrochemicaldeposition of CoNi2S4, the smooth surface of ZnO
NRAs becomerather rough with conformally coated NSs (Fig. 1g).
There is nomorphology change observed with the removal of ZnO
NRAs(Fig. 1h). XRD patterns in Fig. S1 show no diffraction peaks of
ZnO,which means the complete removal of ZnO by the etching
process.
It must be mentioned that we adopted the electrodepositionmethod
for the preparation of the ZnO NRAs template rather thanthe common
used hydrothermal method. The hydrothermalmethod usually needs the
assistance of ZnO seed layer, which leadsto observable voids after
the removal of ZnO NRAs as evidenced bySEM images in Fig. S2a and
b. The ZnO NRAs from the electro-chemical deposition have intimate
contact with the CFC substrate(Fig. S2c and d). The Nyquist plots
in Fig. S3 indicate that theCoNi2S4 NS/NTAs prepared by using the
hydrothermal methodderived ZnONRAs template hasmuch higher
resistance than that ofthe sample from electrochemical deposition.
The electrochemicaldeposition of CoNi2S4 was optimized by changing
the scan rate ofthe CV scans. SEM images of the four samples with
different scanrates (5, 10, 15 and 20mV s�1) for 5 CV cycles are
shown in Fig. S4.The sample deposited at 15mV s�1 has the most NSs
on the surfaceof NRAs, implying its more active sites. And the
electrochemicalproperties showed in Fig. S5 indicate that the
optimal deposition
Fig. 3. Electrochemical characterization of CoNi2S4 NS/NTAs-5
electrode. (a) CV curves. (b) Gby using the discharge curves in
Fig. 3b. (d) Capacitance retention as a function of cycles a
scan rate is 15mV s�1.The nanotube (NT) structure of the CoNi2S4
is confirmed by TEM
characterization (Fig. 2a). The wall thickness of CoNi2S4 NT is
about20 nm. The insert image of the selected area electron
diffraction(SAED) pattern in Fig. 2a shows the ring-pattern,
indicating thepolycrystalline structure of the CoNi2S4 NT. The
diffraction rings areindexed to the (220), (311), (331) and (511)
planes of cubic phaseCoNi2S4 [20]. The measured distance of the
lattice fringes in theHRTEM image in Fig. 2b is 0.282 nm and 0.236
nm corresponding tothe (311) and (400) planes of CoNi2S4,
respectively [24]. Theelemental mapping images (Fig. S6) further
demonstrate the uni-form distribution of Ni, Co and S elements
throughout CoNi2S4 NT.As shown in Fig. S7, the specific surface
area of bare CFC is only1.23m2 g�1, and after grow CoNi2S4 NS/NTAs
on the CFC surface, thespecific surface area increased to 13.01m2
g�1. This further illus-trates that the specific surface area can
be greatly increased bysynthesizing this hierarchical
structure.
The composition and chemical states of CoNi2S4 was analyzedby
XPS, as shown in Fig. 2cef. The survey scan spectrum shows
theexistence of Ni, Co, S, C and O. The Ni 2p and Co 2p spectrum
isfurther studied by using peak-differentiation-imitating method.
Asshown in Fig. 2d, the well fitted Ni 2p3/2 orbit can be separated
intotwo peaks with binding energies of 856.3 and 857.2 eV, that
rep-resenting Ni2þ 2p3/2 and Ni3þ 2p3/2, respectively. Meanwhile,
the Ni2p1/2 can also be separated into two peaks at 874.1 and 875.9
eV,represent Ni2þ 2p1/2 and Ni3þ 2p1/2, respectively [25,26].
Similarly,in the Co 2p spectra (Fig. 2e), the peaks at 781.7 and
783.0 eVrepresenting Co3þ 2p3/2 and Co2þ 2p3/2, and the peaks at
796.9 and798.4 eV denoted Co3þ 2p1/2 and Co2þ 2p1/2 [27,28]. For
the S 2pspectrum in Fig. 2f, the peaks at 161.8 eV for S 2p1/2 and
163.1 eV forS 2p3/2 are associated with metalesulfur bonds (NieS
and CoeS),while the peak at 168.2 eV corresponds to the shakeup
satellite[29].
CD curves. (c) Specific capacitance and area capacitance as a
function of current densityt a current density of 40 A g�1.
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Fig. 4. Electrochemical characterization of as-prepared rGO-CNTs
electrode. (a) Cross-section SEM image of rGO-CNTs film. (b) CV
curves of rGO-CNTs at different scan rates. (c) GCDcurves of
rGO-CNTs at different current densities. (d) Specific capacitance
of rGO-CNTs at different current densities calculated from GCD
curves. (e) Nyquist plots of rGO-CNTs. (f)Cycle stability of
rGO-CNTs at 10 A g�1.
C. Su et al. / Electrochimica Acta 305 (2019) 81e89 85
The electrochemical performance was investigated and the
as-prepared hierarchical CoNi2S4 NS/NTAs on CFC were directly
usedas the binder-free supercapacitor electrode. The
electrochemicalmeasurements were conducted using a three-electrode
configura-tion in 6M KOH aqueous solution. It was found that the
perfor-mance was highly related to the film thickness (details seen
in thesupporting information). The bare CFC electrode shows
negligiblecapacitance (the black curve in Fig. S9a). While at 5
cycles, the as-prepared CoNi2S4 NS/NTAs sample shows the best
electro-chemical performance (the pink curve in Fig. S9a). Except
other-wise defined, the following results were obtained by using
thesample prepared at the optimal conditions, i.e., 15mV s�1 and
5cycles of CV scan, and the obtained results are shown in Fig. 3.
Theprofiles of the CV curves for at different scan rates in Fig. 3a
showsymmetrical anodic and cathodic peaks which are the
typicalcharacteristic of pesudocapacitors. The peaks are indexed to
theredox reactions of Ni2þ/Ni3þ and Co2þ/Co3þ in KOH solution
[23]:
CoNi2S4 þ 2OH�4CoS2xOH þ Ni2S4�2xOH þ 2e�
GCD curves for CoNi2S4 NS/NTAs-5 based on different
currentdensities are presented in Fig. 3b. Both of the charge and
thedischarge curves show symmetric profiles with voltage
plateauswhich is consistent with the results of CV scans, i.e.,
reversibleredox reactions are responsible for the energy storage.
It is foundthat the CoNi2S4 NS/NTAs-5 electrode shows high specific
capaci-tance of 995.8 C g�1 at a current density of 2 A g�1, and
even at ahigh current density of 50 A g�1, the specific capacitance
is still ashigh as 740 C g�1 (Fig. 3c). The energy storage
performance of ourmaterial is better than those of recently
developed materials(Table S1). The CoNi2S4 NS/NTAs-5 electrode also
displays goodlong-term stability, evidenced by the high capacitance
retention of77.2% after 2000 cycles of charge/discharge at the high
currentdensity of 40 A g�1 (Fig. 3d). SEM images in Fig. S10a and b
showthat the CoNi2S4 NS/NTAs-5 electrode also has excellent
structuralstability during the fast charge/discharge process. The
Nyquist plotsof CoNi2S4 NS/NTAs-5 electrode before and after cycle
test wereshown in Fig. S11. It shows low initial Rs, which
indicates excellentionic and electronic conductivity of CoNi2S4
NS/NTAs-5 electrode.
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C. Su et al. / Electrochimica Acta 305 (2019) 81e8986
Even after 2000 cycles the Rs is still 0.87U ensuring the high
cyclingstability of this electrode. These results demonstrate that
the as-prepared CoNi2S4 NS/NTAs is an excellent energy storage
materialwith high specific capacitance, good long-term stability
andstructural stability.
It has been well accepted that pesudocapacitors usually showhigh
energy density, while electrochemical double-layer super-capacitors
display high power density and fast charge/dischargeproperty. To
further increase the energy storage performance, wehave proposed to
assemble HSCs with rGO-CNT film which takesthe advantages of both
pesudocapacitors and electrochemicaldouble-layer capacitors. rGO is
widely used as the supercapacitorelectrode material due to its high
specific surface area, high con-ductivity and good stability [30].
However, in the reduction process,rGO sheets tend to restack
because of the strong Van Der Waalsinteraction, leading to an
obvious decrease of the specific capaci-tance [31]. Therefore, we
used CNTs as spacers to prevent rGO NSsfrom restacking, as
evidenced by the SEM image of rGO-CNT film(Fig. 4a). The detailed
preparation process of rGO-CNT film is shownin supporting
information.
Before the assembling of the HSCs, we have investigated
theelectrochemical energy storage performance of the rGO-CNT
filmusing a three-electrode set-up in 6M KOH solution (Fig. 4bef).
TheCV curves show rectangular profiles at all scan rates, and
thecharge/discharge curves display symmetrical triangle profiles at
allcurrent densities, indicating the good energy storage
performance.The specific capacitance calculated from the discharge
curves is160.3, 132.8, 119.6, 116, 109.6, 106 and 94 F g�1 at 1, 2,
4, 5, 8, 10 and20 A g�1, respectively. Fig. 4e shows the Nyquist
plot and theequivalent series resistance is as low as 0.794U. The
rGO-CNT filmalso shows high cycling stability supported by the high
retention of
Fig. 5. . Schematic illustration and electrochemical
characterization of the as-prepared CoNi2of CoNi2S4 NTAs-5 and
rGO-CNTs at a scan rate of 10mV s�1. (c) CV curves of HSCs
underpotential windows at a current density of 4 A g�1.
98.1% of the specific capacitance after 3000 cycles.Since the
excellent energy storage performance of both the
CoNi2S4 NS/NTAs and the rGO-CNTs, we assembled HSCs using
theCoNi2S4 NS/NTAs as the positive electrode and rGO-CNTs as
thenegative electrode (as shown in Fig. 5a). Based on the
measuredspecific capacitance of each electrode, the mass ratio of
the CoNi2S4NS/NTAs to the rGO-CNTs electrode is calculated to be
0.18. Theelectrolyte for the HSCs is 6M KOH. The voltage window has
criticalimportance to the energy density and powder density of
super-capacitors devices. The HSCs in this study consists of
electro-chemical double-layer capacitance and pseudocapacitance
inseries, as shown in Fig. 5b. Therefore, the voltage window is
thesuperposition of the voltage window of the
electrochemicaldouble-layer capacitor and the pseudocapacitor. To
avoid thesplitting of water in the aqueous electrolyte, we
investigated thevoltage window of our supercapacitor devices in 6M
KOH firstly.Fig. 5c shows the CV curves recorded at 30mV s�1 of the
HSCs atdifferent voltage windows. It is found that with the
increase ofvoltage window, the area inside CV curves increases.
Obviousoxidation current appears when the voltage window reaches
to1.6 V, implying the possible splitting of water at 1.6 V of the
voltagewindow. The CGD curves of the as-assembled HSC device
underpotential windows ranging from 1.1 to 1.5 V at 4 A g�1 were
testedto further explore the potential window. As shown in Fig. 5d,
noevident polarization appeared as the potential increased to 1.5
V.Therefore, the optimal voltage window is 1.5 V.
Fig. 6a shows the CV curves of the HSCs at different scan
rates.All CV curves exhibit quasi-rectangular shapes, which
ascribes tothe combined contributions of the electrochemical
double-layercapacitance and the pseudocapacitance. The GCD curves
in Fig. 6bexhibit nearly symmetrical triangular shape,
demonstrating the
S4 NS/NTAs//rGOeCNTs HSCs. (a) Schematic illustration of the HSC
device. (b) CV curvesvarious voltage window at 30mV s�1. (d) GCD
curves of HSCs measured at different
-
Fig. 6. Electrochemical characterization of the as-prepared
CoNi2S4 NS/NTAs//rGOeCNTs HSC. (a) CV curves of HSC device at
different scan rates. (b) GCD curves of HSCs at differentcurrent
densities. (c) Specific capacitance and area capacitance of the
HSCs as a function of current densities calculated from GCD curves.
(inset: optical photo shows a red LEDlighted powered by two aqueous
state HSCs of CoNi2S4 NS/NTAs//rGOeCNTs connected in series). (d)
Cycle performance of HSCs at 10 A g�1. Inset is the GCD curves of
last 10 cycles.(e) Nyquist plots of HSCs before and after 10000
cycle tests. (f) Ragone plots of HSCs. (For interpretation of the
references to colour in this figure legend, the reader is referred
to theWeb version of this article.)
C. Su et al. / Electrochimica Acta 305 (2019) 81e89 87
well-balanced loading mass of the active materials at both
elec-trodes. The specific capacitance of the HSCs calculated from
thedischarge curve is 112 F g�1 (277.5mF cm�2 in areal capacitance)
ata current density of 4 A g�1, and remains at 56 F g�1 (146.2mF
cm�2
in areal capacitance) at a high current density of 20 A g�1
(Fig. 6c).The performance of our device is better than the reported
values[32e34]. As shown in inset picture of Fig. 6c, two HSCs of
CoNi2S4NS/NTAs//rGOeCNTs connected in series can power a red
light-emitting-diode (LED), demonstrating their practical
applications.
The HSCs also show high cycling stability. The
charge/dischargecurves almost keep unchanged and the capacitance
retention is ashigh as 96.9% even after 10000 charge/discharge
cycles at a highcurrent density of 10 A g�1 (Fig. 6d), which
demonstrates theexcellent cycling stability. The Nyquist plots of
HSCs device beforeand after stability test were shown in Fig. 6e.
It shows that theinitial Rs is very low indicating the assembling
quality of the
supercapacitor device, and even after 10000 cycles the Rs is
still aslow as 3.9U ensuring the high cycling stability of our
device. Asshown in Fig. S12, the SEM images and EDS analysis of
CoNi2S4 NS/NTAs after 10000 cycles were done to further confirms
the goodcycle stability. Seen from the SEM images, the CoNi2S4
NS/NTAs stilltightly attached to CFC. The EDX spectras show that
the CoNi2S4were partially oxidized after long-time cycling
test.
Energy density and power density are two important parame-ters
for of supercapacitors. Fig. 6f shows the Ragone plots of theHSCs.
The highest gravimetric energy density is 35Wh kg�1 at apower
density of 3 kWkg�1, and remains 17.5Wh kg�1 at a powerdensity of
15 kWkg�1. These performances are higher than those ofpreviously
reported NieCo sulfides including NiCo2S4 NTs//rGOHSCs (16.6Wh kg�1
at 2.35 kWkg�1) [35], CoNi2S4 nanosheet ar-rays (NSAs)//AC HSCs
(27.2Wh kg�1 at 2.45 kWkg�1) [36], NiCo2S4NPs//AC HSCs (28.3Wh kg�1
at 245Wkg�1) [37], NiCo2S4
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C. Su et al. / Electrochimica Acta 305 (2019) 81e8988
NT@NiCo2S4 NSAs//rGO HSCs (24.9Wh kg�1 at 334Wkg�1) [38],and
NiCo2S4@NiO//AC HSCs (30.38Wh kg�1 at 288Wkg�1) [39].
4. Conclusions
In summary, the hierarchical CoNi2S4 NS/NTAs tightly
connectedwith CFC were synthesized through electrodeposition method
forthe purpose of providing more active sites and decreasing
thecontact resistance. The developed synthesis method ensures
theintimate contact of the hierarchical CoNi2S4 NS/NTAs with the
CFCsubstrate after the removal of the ZnO NRAs template. The
hierar-chical CoNi2S4 NS/NTAs on CFC were directly used as the
binder-free supercapacitor electrode which exhibits the ultrahigh
specificcapacitance of 995.8 C g�1 at a current density of 2 A g�1
and su-perior cycling stability. The hierarchical CoNi2S4 NS/NTAs
on CFCwere used to assemble HSCs with rGO-CNT film as the
counterelectrode. The HSCs display high specific capacitance of 112
F g�1 ata current density of 4 A g�1, with the energy density of
35Wh kg�1
at a power density of 3 kWkg�1. The present study provides
apromising way for constructing high performance energy
storagedevice.
Acknowledgments
We gratefully acknowledge the financial support of the
NationalNatural Science Foundation of China (61671299), Shanghai
Scienceand Technology Grant (16JC1402000 and 17ZR1414100), and
theProgram for Professor of Special Appointment (Eastern Scholar)
atShanghai Institutions of Higher Learning (GZ2016005). We
alsoacknowledge support from the Instrumental Analysis Center
ofShanghai Jiao Tong University and the Center for Advanced
Elec-tronic Materials and Devices of Shanghai Jiao Tong
University.
Appendix A. Supplementary data
Supplementary data related to this article can be found
athttps://doi.org/10.1016/j.electacta.2019.03.013.
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Supporting information
Hierarchical CoNi2S4 nanosheet/nanotube array structure on
carbon fiber cloth
for high-performance hybrid supercapacitors
Chen Sua, Shusheng Xua, Lu Zhangc, Xinwei Chena, Guoqin Guana,
Nantao Hua,
Yanjie Sua, Zhihua Zhoua Hao Weia, Zhi Yanga,*, Yong Qinb,*
aKey Laboratory of Thin Film and Microfabrication (Ministry of
Education),
Department of Micro/Nano Electronics, School of Electronic
Information and
Electrical Engineering, Shanghai Jiao Tong University, Shanghai
200240, P. R. China
bInstitute of Nanoscience and Nanotechnology, School of Physical
Science and
Technology, Lanzhou University, Gansu 730000, P. R. China
cSchool of Advanced Materials and Nanotechnology, Xidian
University, Xi’an 710071,
P. R. China
* To whom correspondence should be addressed:
[email protected], [email protected]
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1. Preparation of rGO-CNT film
The rGO-CNT film was prepared according to our previous report
[1].
Specifically, 50 mg GO and 10 mg CNTs were dispersed in 100 mL
DI water and
sonicated for 2 h. Then added 0.1 g KMnO4 into this solution and
continue stirred for
10 min. The mixed solution was heated by a microwave oven for 5
min. After cooled
to room temperature, the material was washed with oxalic acid,
hydrochloric acid (v/v,
1:1), and DI water, respectively. Then, the material was
dispersed in 90 mL DI water,
followed by adding 90 μL hydrazine hydrate and 350 μL ammonium
hydroxide into
this solution. The prepared solution was stirred for 20 min and
then putted into a
100 °C oven for 3 h. Finally the suspension was vacuum-filtrated
through a mixed
cellulose filter membrane to obtain rGO-CNTs film.
2. Calculation
The formulae for calculating the specific capacitance (C, C g–1)
of CFC/CoNi2S4
NS/NTAs electrode is given below [2]:
(1)C = IΔt / m
Where I (A) is current density, Δt (s) refers to discharge time,
m (g) is the mass of
active material.
To optimize the electrochemical performance of these HSCs, the
charges stored
between two electrodes should follow equations [3]:
(2)× × (3)
+ -
± ± ± ±
Q = QQ = m C ΔV
Where Q (C) is the quantity of electric charge, m (g) represents
the loading mass, C (F
g–1) stands for specific capacitance and ΔV (V) is the voltage
window.
The specific capacitance (C, F g–1), energy density (E, Wh kg−1)
and power
density (P, W kg−1) of the ASC device calculated from GCD curves
are given below
[4]:
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3
(4)× (5)
(6)
2
C = IΔt / mΔVW = CV / (2 3.6)P = 3600W / t
Where I (A) is current density, Δt (s) refers to discharge time,
m (g) is the mass of
active material, and V stands for the voltage window (V).
Fig. S1. XRD patterns of CFC/ZnO, CFC/ZnO/CoNi2S4 and
CFC/CoNi2S4 NS/NTAs.
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Fig. S2. Cross section SEM images of (a) ZnO/CoNi2S4 NSs/NRAs-5
and (b) CoNi2S4
NS/NTAs-5 synthesized by using hydrothermal ZnO NRAs as
template, (c) ZnO/CoNi2S4
NSs/NRAs-5 and (d) CoNi2S4 NS/NTAs-5 synthesized by using
electrodeposition ZnO NRAs as
template.
Fig. S3. Nyquist plots of CoNi2S4 NS/NTAs-5 synthesized by using
electrodeposition ZnO (black
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5
curve) and hydrothermal ZnO (red curve) as template.
Fig. S4. SEM images of CoNi2S4 NSs deposited on ZnO NRAs under
different scan rate of (a) 5
mV s−1, (b) 10 mV s−1, (c) 15 mV s−1 and (d) 20 mV s−1for 5 CV
cycles.
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Fig. S5. Electrochemical characterization of CoNi2S4 NS/NTAs
electrode deposited under
different scan rate for 5 CV cycles. (a) CV curves at 10 mV s–1.
(b) GCD curves at 10 A g–1. (c)
Specific capacitance as a function of current density. (d)
Nyquist plots.
Fig. S6. HRTEM images of (a) CoNi2S4 NS/NTAs-5 and (b) the
partial enlarged view of CoNi2S4
NS/NTAs-5. Elemental mapping images of (c) Ni, (d) Co, and (e)
S.
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Fig. S7. N2 adsorption-desorption isotherms curves for (a) bare
CFC and (b) CoNi2S4 NS/NTAs-5
electrode.
3. Optimization of the thickness for the electrodeposition of
CoNi2S4 NSs on ZnO
The electrochemical performances of hierarchical CoNi2S4 NS/NTAs
with
different electrodeposition cycles were also investigated to
determine the optimal
thickness. Fig. S8 is the SEM images of samples before and after
removing ZnO
NRAs template with 3, 7, 11 CV cycles, respectively. Fig. S9a
shows CV curves of
bare CFC, CoNi2S4 NSs, CoNi2S4 NS/NTAs-3 to CoNi2S4 NS/NTAs-11
at scan rate of
10 mV s–1. The CV curve of bare CFC electrode is almost a
straight line, which
indicates that the capacitance contributed from CFC is
negligible. It can be seen that
the CV area increases to a maximum value when the
electrodeposition cycles reach 5
and then decreases, indicating that the optimized
electrodeposition sample is CoNi2S4
NS/NTAs-5. GCD measurements for different samples mentioned
above are further
performed at a current density of 10 A g–1 as shown in Fig. S9b.
The results show that
CoNi2S4 NS/NTAs-5 electrode exhibits the longest discharge time
among these tested
electrodes, which agrees well with the comparison result of CV
areas. As seen in Fig.
S9c, CoNi2S4 NS/NTAs-3 electrode exhibits a poor rate
capability. This is mainly
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8
because that the ultrathin CoNi2S4 NSs covered on ZnO NRAs
incompletely after 3
CV cycles. Hence, after removing the ZnO NRAs template, there
are just a few
scattered NTs remained (as shown in Fig. S8b). With the
deposition cycles increased,
the thickness of deposited CoNi2S4 NSs will increase and the
synthesized CoNi2S4
NTs become more stable. As a consequence, the performance of
rate capacity and
specific capacity reaches maximum when the deposition cycles are
5 CV. However,
when the thickness of deposited CoNi2S4 NSs increased to 7 CV
and 11 CV cycles,
both specific capacity and rate capability decreased for the
reason that the top of NTs
congested together at some areas with the increase of CoNi2S4
NSs. There are even
some NSs clusters agglomerated on the top of NRAs, which may
hinder electrolyte
diffusing into CoNi2S4 NS/NTAs and in turn increase the bad
volume of active
materials. The specific capacitor of CoNi2S4 NS/NTAs-5 electrode
is higher than
CoNi2S4 NSs electrode that demonstrates hierarchical CoNi2S4
NSs/NTA structure can
efficiently increase the specific capacitor by increasing the
specific area. The
corresponding Nyquist plots are shown in Fig. S9d. In the
high-frequency region,
there is no distinct semicircle shape, which demonstrates the
fast charge transfer
inside the electrode materials during the charge-discharge
process. In the
low-frequency region, the straight lines lean more toward the
imaginary axis,
indicating all hierarchical CoNi2S4 nanostructure electrodes
have a lower diffusion
resistance, which can be ascribed to the electrolyte ions
diffusing and transporting
into the electrode materials.
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Fig. S8. SEM images of (a) ZnO/CoNi2S4 NS/NRAs-3, (b) CoNi2S4
NS/NTAs-3, (c) ZnO/CoNi2S4
NS/NRAs-7, (d) CoNi2S4 NS/NTAs-7, (e) ZnO/CoNi2S4 NS/NRAs-11,
and (f) CoNi2S4
NS/NTAs-11.
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Fig. S9. Electrochemical characterization of CoNi2S4 NSs,
CoNi2S4 NS/NTAs-3, CoNi2S4
NS/NTAs-5, CoNi2S4 NS/NTAs-7 and CoNi2S4 NS/NTAs-11 electrode.
(a) CV curves at 10 mV
s–1. (b) GCD curves at 10 A g–1. (c) Specific capacitance as a
function of current density. (d)
Nyquist plots.
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Table S1. Comparison of capacitance and retention rate
performance for hierarchical CoNi2S4
NS/NTAs and other reported active electrodes
Sample Capacitance Capacitance retention Electrolyte
Reference
Hierarchical
CoNi2S4 NS/NTAs
995.8 C g‒1
(2 A g‒1)
74.3%
(50 A g‒1) 6 M KOH This work
Porous Ni-Co-S NSs 521.6 C g‒1
(2 A g‒1)
85.6%
(20 A g‒1) 6 M KOH [5]
CoNi2S4 NSAs 709.0 C g‒1
(5 A g‒1)
90.6%
(100 A g‒1) 1 M KOH [6]
NiCo2S4 NTAs 313.2 C g‒1
(2 A g‒1)
73.6%
(32 A g‒1) 6 M KOH [7]
Reduced CoNi2S4
NSs
1117.0 C g‒1
(2 A g‒1)
78.0%
(40 A g‒1) 6 M KOH [8]
NiCo2S4/NCF 615.5 C g‒1
(2 A g‒1)
71.2%
(20 A g‒1) 6 M KOH [9]
NiCo2S4/RGO 749.0 C g‒1
(1 A g‒1)
72.3%
(40 A g‒1) 1 M KOH [10]
CoNi2S4–G–MoSe2 456.4 C g‒1
(1 A g‒1)
50.8%
(20 A g‒1) 6 M KOH [11]
Ni-Co-S/G 746.0 C g‒1
(1 A g‒1)
96.0%
(50 A g‒1) 6 M KOH [12]
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Fig. S10. SEM images of (a) and (b) CoNi2S4 NS/NTAs-5 after 2000
cycles test, (c) and (d)
CoNi2S4 NSs deposited under 10 mV s‒1 for 5 CV cycles.
Fig. S11. Nyquist plots of CoNi2S4 NS/NTAs-5 before and after
2000 cycles at room temperature.
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Fig. S12. (a) and (b) SEM images of CoNi2S4 NS/NTAs-5 after
10000 cycles test. (c) EDX spectra
of CoNi2S4 NS/NTAs-5 before cycles test. (d) EDX spectra of
CoNi2S4 NS/NTAs-5 after 10000
cycles test.
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Hierarchical CoNi2S4 nanosheet/nanotube array structure on
carbon fiber cloth for high-performance hybrid supercapacitors1.
Introduction2. Experimental section2.1. Synthesis of ZnO NRAs on
CFC2.2. Preparation of hierarchical CoNi2S4 NS/NTAs on CFC2.3.
Characterizations
3. Results and discussion4. ConclusionsAcknowledgmentsAppendix
A. Supplementary dataReferences