Facile hydrothermal synthesis of NiS hollow microspheres with mesoporous shells for high-performance supercapacitors

Article abstract of DOI:10.1039/c5nj02425k

Hollow nickel sulfide (NiS) microspheres with mesoporous shells have been successfully synthesized via a facile hydrothermal route using nickel chloride hexahydrate (NCH) as the nickel source and thiosemicarbazide (TSC) as the sulfur source. The NH₂-NH- group in the TSC molecule plays a crucial role in the formation of hollow microspheres. Ostwald ripening mechanism was suggested to explain the formation of hollow NiS microspheres with mesoporous shells. The resulting hollow NiS microspheres were further used as an electrode material for supercapacitors and found to exhibit a high specific capacitance of 1848.0 F g^-1 at a discharge current density of 1 A g^-1. About 74.3% of the capacity was retained as the current density increased from 1 to 10 A g^-1. Furthermore, the hollow NiS microspheres electrode also exhibited excellent cycling stability at a scan rate of 50 mV s^-1. The results indicate that the as-prepared hollow NiS microspheres with mesoporous shells are a promising electrode material for supercapacitors.

Full text of DOI:10.1039/c5nj02425k

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Facile hydrothermal synthesis of NiS hollow microspheres with
mesoporous shells for high-performance supercapacitors
Received 00th January 20xx,
Accepted 00th January 20xx
ab
b
ab
ab
Zhongchun Li, Aijun Gu , Jianhua Sun and Quanfa Zhou
DOI: 10.1039/x0xx00000x
Nickel sulfide (NiS) hollow microspheres with mesoporous shells have been successfully synthesized via a facile
hydrothermal route using nickel chloride hexahydrate (NCH) as nickel source and thiosemicarbazide (TSC) as sulfur source.
www.rsc.org/
2
The NH -NH- group in TSC molecule plays a crucial role in the formation of hollow microspheres. Ostwald ripening
mechanism was suggested to explain the formation of NiS hollow microspheres with mesoporous shells. The resulting NiS
hollow microspheres were further used as the electrode material for supercapacitors and found to exhibit a high specific
capacitance of 1848.0 F/g at a discharge current density of 1 A/g. About 74.3% of capacity was retained as the current
density increased from 1 to 10 A/g. Furthermore, NiS hollow microspheres electrode also presented excellent cycle
stability at a scan rate of 50 mV/s. The results indicate that the as-prepared NiS hollow microsphere with mesoporous
shells is a promising electrode material for supercapacitors.
diffusion paths for ions and electrons, resulting in reduced internal
6
, 21-23
1
. Introduction
resistance and improved high power capability
. Until now,
many methods have been developed to synthesize hollow
Recently, with the rapidly consumption of fossil fuels and ever-
increasing demand for energy, the effort to optimize the utilization
of present energy sources and search for advanced energy storage
2
4, 25
structures for supercapacitors, such as chemical precipitation
,
3
26, 27
5
electrospining , template
,
layer-by-layer self-assembly ,
1
9, 21
1
-3
solvothermal and hydrothermal methods
. For example, Shao et
devices has intensified dramatically . Supercapacitors, also known
as electrochemical capacitors or ultracapacitors have received
increasing attention due to their many advantages including high
power density, faster charge/discharge process, long cycle lifetime,
al. have reported that graphene hollow spheres with a specific
capacitance of 273 F/g at discharge current density of 0.5 A/g using
2
8
polystyrene spheres as templates . Hollow V
2 5
O spheres were
4
-6
synthesized by a solvothermal process, which exhibited a high
small size, and relatively low cost . Based on the energy storage
mechanisms, supercapacitors can be classified into electrical double
1
8
capacitance of 479 F/g at 5 mV/s .
4
, 7
As a new kind of electrode material, NiS has been extensively
applied in supercapacitors due to its low cost, high capacity and
electronic conduction, and environmental sustainability. For example,
Lou’s group has obtained hierarchical NiS hollow spheres with
specific capacitances of 927 F/g at a current density of 4.08 A/g by a
layer capacitors and pseudocapacitors . In electrical double layer
capacitors, capacitance arises from the charge separation at the
electrode/electrolyte interface, whereas in pseudocapacitors,
charge is stored and released in faradaic processes at/or near the
8, 9
electrode surface . Pseudocapacitors usually offer much higher
2
9
1
0, 11
template-engaged conversion method . Yang et al. reported a
hierarchical flower-like β-NiS architecture synthesized via
solvothermal method with specific capacitances of 857.76 F/ g at 2
A/g and 512.96 F/g at 5 A/g.
specific capacitances than electric double layer capacitors
.
It is well accepted that electrode materials play a vital role to
1
2, 13
improve the performance of supercapacitors
. And capability of
an electrode material is significantly influenced by its morphology.
An electrode material with uniform, and ordered structure is
Although a great deal of effort has been made and some
1
3, 29-32
1
4-17
successes have been achieved
, the realization of high
expected to exhibit superior performance for supercapacitors
.
capacitance with good rate performance and long cycle lifetime for
nickel sulfide still remains a significant challenge. Herein, we
applied a simple and efficient one-step hydrothermal route for the
synthesis of NiS hollow microspheres using nickel chloride
hexahydrate as nickel source and thiosemicarbazide (TSC) as sulfur
source. Neither surfactant nor template was used in our reaction
system. The substituent group in the sulfur source plays an
important role in the formation of NiS hollow microspheres.
Ostwald ripening mechanism is proposed to account for the
formation of NiS hollow microspheres. The NiS hollow microspheres
show impressive electrochemical properties with high specific
Among various micro/nanomaterials for supercapacitors, hollow
structured materials with low densities and high surface areas have
1
8-20
attracted much attention
. Hollow structured materials have
been proved to be an optimized architecture for boosting the
capacitive performance due to their high surface areas, short
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capacitance of 1848.0 F/g at a discharge current density of 1 A/g can be perfectly indexed to the (100), (101), (102), and (110)
and excellent cycling performance. reflections of hexagonal NiS with lattice constants a = 3.41 Å, b =
1
3, 33, 34
3
.41 Å, c = 5.317 Å (JCPDS No. 02-1280)
. In addition, energy
dispersive spectrum (EDS) was also employed to determine the
chemical composition. Both the peaks of the elements Ni and S
were detected, which further confirmed that the as-prepared
hollow microspheres were composed of NiS. Morphology of the
resulting NiS was investigated by SEM and TEM. Some holes are
found on the surfaces of NiS spherical particles in Fig. S1, clearly
demonstrating the hollow structure of NiS microspheres. Fig.1 c
shows a representative TEM image of NiS hollow microspheres. As
shown in Fig. 1c, hollow spheres with the diameter of about 2 µm
are observed. From the inset in Fig. 1c, we can see clearly that there
2
. Experimental Section
2
.1. Materials
Nickel chloride hexahydrate (NCH), thiosemicarbazide (TSC),
thiourea (TU), thioacetamide (TAA) and other reagents used were
purchased from Sinopharm Chemical Reagent Co. Ltd. and used
without further purification. The water used throughout the
experiments was distilled water.
2
.2. Synthesis of nickel sulfide hollow microspheres
Nickel sulfide hollow microspheres were synthesized by a facile are numerous gaps in the surface of hollow microspheres. And the
hydrothermal route. In a typical process, 1.0 mmol nickel chloride different brightness in TEM image can illuminate the hollow
hexahydrate was dissolved in 15.0 mL distilled water, followed by structure of NiS microspheres. The TEM image shows that the
addition of 2.0 mmol thiosemicarbazide. After stirring vigorously, microsphere consists of particles leading to textural pores. For
the mixture was transferred to a 25 mL Teflon-lined pressure vessel, further insight into the crystal structure of NiS hollow microspheres,
stainless steel autoclave. The autoclave was sealed and heated in an structural analyses were performed by HRTEM and fast Fourier
oven at 180°C for 24 h. The pressure vessel was cooled to room transform (FFT) pattern. Fig. 1d shows a typical HRTEM image of NiS
temperature naturally after reaction time. The precipitate was hollow microsphere. Lattice planes can be seen from Fig. 1d and a d
collected by centrifugation and washed with distilled water and spacing of 0.256 nm for adjacent lattice planes corresponds to the
ethanol for several times. Then the precipitate was dried at 60°C for (101) planes of hexagonal NiS. Meanwhile, the FFT was carried out,
1
2 h under vacuum condition.
shown as inset in Fig. 1d. The spot pattern implies that the as-
prepared NiS hollow microsphere is single crystal in nature.
2
.3. Characterization
X-ray diffraction (XRD) patterns were recorded on a Bruker D8
advance diffractometer with Cu radiation. Morphology
K
α
6
5
00
00
observations were done on Tecnai-12 transmission electron
microscopy (TEM) and HITACHI-3400s scanning electron microscopy
a
b
S
Ni
400
300
(
SEM). Samples for TEM experiments were deposited on copper
Ni
2
1
00
00
0
grids. High-resolution TEM (HRTEM) characterization was
performed with a FEI G2 F30 S-TWIN field emission transmission
electron microscopy operating at an accelerating voltage of 300 kV.
Ni
2
0
30
40
50
60
70
0
10
20
30
40
2
θ
/ degree
Energy (KeV)
N adsorption-desorption isotherm was tested using a Quadrasorb
2
SI analyzer.
2
.4. Electrochemical measurements
The working electrodes were prepared by mixing nickel sulfide,
acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio
of 80:10:10. The loading mass of hollow NiS microspheres is about
2
~3 mg. Isopropanol was added to the mixture to produce a
homogeneous paste, which was subsequently brush-coated onto
nickel foam and then dried at 60°C for 12 h in a vacuum oven. All
electrochemical tests were conducted on
a
CHI 660C
-1
2
.4
electrochemical workstation in 2.0 mol·L KOH solution with a
three electrode cell where a platinum foil serves as the counter
electrode and a standard calomel electrode (SCE) as the reference
electrode, respectively.
5
4
3
2
1
0
e
2.0
f2.9 nm
1
1
.6
.2
0.8
0.4
0.0
4.6 nm
13.3 nm
0
.0
0.2
0.4
0.6
0.8
1.0
0
4
8
12
16
20
3
. Results and discussion
P/P0
Pore diameter (nm)
3
.1. Morphology and structure of NiS hollow microspheres
The phase and composition of as-prepared hollow microspheres Fig.1 (a) XRD pattern; (b) EDS pattern; (c) TEM images; (d) HRTEM
were examined by X-ray diffraction (XRD) and energy dispersive image and inset FFT pattern; (e) N adsorption-desorption isotherm;
2
spectroscopy (EDS), as shown in Fig. 1a and 1b. Four diffraction and (f) pore diameter distribution of NiS hollow microspheres.
peaks were observed at 2θ = 30.2°, 34.7°, 45.9°, and 53.6°, which
2
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The pore size and specific surface area are very important for the 3.2 Effect of synthesis conditions on the morphology evolution
3
5
electrode materials in supercapacitors . Fig. 1e presents the N
2
To investigate the morphology evolution of NiS microstructures, the
influences of reaction parameters such as molar ratio of reactants,
reaction temperature and reaction time were studied, respectively.
Considering that morphology of the final product is usually affected
by the molar ratio of reactants in hydrothermal process. In the
present case, the influence of molar ratios of NCH to TSC on the
final product was investigated systematically. The evolutions of
compositions and morphologies were examined by XRD and TEM.
Fig. 2a represents the XRD patterns of products prepared at the
molar ratio of NCH/TSC ranging from 1:1 to 1:5. All diffraction peaks
agree well with the reported value (JCPDS No. 02-1280) when the
molar ratio of NCH/TSC is in the range of 1:1-1:4. No characteristic
peak due to impurity is detected, indicating that the as-prepared
products consist of a single phase of hexagonal NiS. It's worth
noting that the crystallinity becomes poorer and poorer as the
molar ratio of NCH to TSC decreased from 1:1 to 1:4. However,
when the molar ratio of TSC to NCH was increased to 5:1, the peaks
of NiS2 (JCPDS No. 65-3325) were found in the XRD pattern.
Therefore, the molar ratio of NCH to TSC is an important factor for
the formation of NiS hollow microspheres with high purity.
adsorption-desorption isotherm of NiS hollow microspheres. There
is a separation between adsorption branch and desorption branch
at a relative pressure in the range of 0.3-1.0, suggesting the
existence of mesoporous. The specific surface area of NiS hollow
2
microspheres is 5.3 m /g. The low specific surface area may be
ascribed to the fact that the diameter of NiS microsphere is very
large. The pore diameter distribution obtained from desorption
branch based on the NLDFT equilibrium model is shown in Fig. 1f.
The pore diameter mainly locates at 3.9 nm, 4.6 nm, and 13.3 nm.
The unique hollow structure with mesoporous shells of NiS hollow
microspheres can provide fast ion and electron transport, leading to
the excellent electrochemical capacitive properties.
Ni/S=1:1
Ni/S=1:3
*
Ni/S=1:4
Ni/S=1:5
*
*
*
Fig. 3 shows the TEM images of the products obtained at the
corresponding molar ratios. NiS hollow microspheres about 1.5-2.3
ꢀ
m in diameter were obtained when the molar ratio is in the range
of 1:1-1:4 (Fig. 3a-c). However, when the experiment was carried
out at the molar ratio of 1:5, only fragments of broken NiS hollow
microspheres were observed in Fig. 3d. In the present paper, we
choose the molar ratio of NCH to TSC is 1:2 in the subsequent
experiments.
20
30
40
50
60
70
2θ / degree
Fig. 4 represents the XRD patterns of products prepared at 120-
2
00°C when the molar ratio of NCH/TSC was 1:2. When the
Fig. 2 XRD patterns prepared at various molar ratios of NCH to
experiment was conducted at 120°C or 140°C, amorphous NiS was
obtained. When the reaction temperature was increased to 160°C,
a diffraction peak at 2θ = 53.6° emerged in XRD pattern, which is in
accordance with the (110) reflection of hexagonal NiS (JCPDS
No.02-1280). There are four diffraction peaks observed at 2θ =
TSC.
3
2
0.2°, 34.7°, 45.9°, and 53.6° when the temperature is raised to
00°C, which are in accordance with the (100), (101), (102), and
(
110) reflections of hexagonal NiS (JCPDS No. 02-1280).
120
140
160
200
10
20
30
40
50
60
70
2θ / degree
Fig. 3 TEM images obtained at various molar ratios of NCH to
TSC. (a) 1:1; (b) 1:3; (c) 1:4 and (d) 1:5.
Fig. 4 XRD patterns of products obtained at different temperatures.
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Fig. 5 shows the corresponding TEM images of samples obtained
Journal Name
at various reaction temperatures. When the experiment was
conducted at 120°C or 140°C, solid NiS microspheres could be
produced (Fig. 5a and 5b). As shown in Fig. 5c, NiS particles with
rough surfaces were obtained when the reaction was carried out at
1 h
4
8
1
h
h
6 h
1
2
60°C. When the reaction temperature was raised to 180°C or
00°C, significant change in the product shapes occurred. The final
products in Fig.1c and Fig.5d generally comprised NiS hollow
microspheres with diameters of 1.5-2.5 µm. The difference of NiS
morphology is just because of the reaction temperature, which may
be ascribed to the different growth ratio of NiS at different reaction
temperature. Hence, reaction temperature has direct effect on the
morphology of NiS samples. In this case, we chose 180°C as the
reaction temperature. In order to investigate the evolutions of NiS
hollow microspheres, a series of time-dependent experiments were
carried out. The evolutions of structures and morphologies were
examined by XRD and TEM, as shown in Fig. 6 and Fig. 7. At the
initial stage of reaction (1 h), solid spherical particles with
amorphous structure were obtained (Fig. 6 and 7a). After
hydrothermal treatment is prolonged to 4 h, (110) reflection of
hexagonal NiS (JCPDS No.02-1280) emerged in the XRD pattern. Fig.
10
20
30
40
50
60
70
2θ / degree
Fig. 6 XRD patterns of products obtained at different reaction time.
7
b shows that the sample collected at 4 h exhibits immature
spherical morphology, and it is integrated by masses of tiny thorns.
This could be explained by that the newly formed nuclei with a high
surface energy preferred to aggregate driven by the minimization of
surface energy and self-assembly controlled by the intrinsic crystal
3
6
orientation . After 8 h hydrothermal treatment, all XRD peaks were
well consistent with those of the final product obtained at 24 h. In
addition, the degree of crystallinity was enhanced with the
hydrothermal treatment time (shown in Fig. 6). When hydrothermal
treatment time was prolonged to 8 h, NiS hollow microspheres
were obtained as shown in Fig. 7c. The diameter of NiS hollow
microsphere was increased with the longer reaction time in Fig. 7d.
Fig. 7 TEM images of products obtained at different reaction time.
Reaction time (h): (a) 1; (b) 4; (c) 8; (d) 16.
TAA
TU
10
20
30
40
50
60
70
80
2θ / degree
Fig. 5 TEM images of products obtained at different temperatures.
Temperature (°C): (a) 120; (b) 140; (c) 160; (d) 200.
Fig. 8 XRD patterns of products obtained with TU and TAA.
4
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It is clear that the growth process was neither surfactant-assisted
nor template-directed, because no surfactant or template was used
in our reaction system. On the basis of the characterization results
from condition-dependent experiments, the growth process of NiS
hollow microspheres could be explained in terms of Ostwald
100
2000
1600
1200
2
mV/s
a
5 mV/s
10 mV/s
20 mV/s
b
5
0
4
0 mV/s
0
8
4
00
00
-50
3
7, 38
-100
ripening process
. This process is associated with a progressive
-0.2
0.0
0.2
0.4
0.6
0.8
0
10
20
30
40
Potential (V vs. SCE)
Scan rate (mv/s)
redistribution of matter from the interior to exterior. In the initial
stage, NiS crystal nuclei are produced and aggregate to solid
aggregates to decrease the surface energy. During the course of
reaction, the concentration of reactants in the solution decrease so
that supersaturation falls over time and a chemical equilibrium
between the solid-liquid interfaces is established. Compared with
the large and well-crystallized particles on the exteriors of the
spheres, the inner amorphous small crystallites have a higher
surface energy, which offers drive force for the Ostwald ripening
0
0
0
0
.5
.4
.3
.2
2000
1800
1600
1
2
4
6
8
1
A/g
A/g
A/g
A/g
A/g
c
d
0
A/g
0.1
0.0
1
1
400
200
0
500
1000
1500
2000
0
2
4
6
8
10
Time (s)
Current density (A/g)
Fig. 10 (a) CV curves and (b) the dependence of capacitance loss on
the scan rate; (c) charge/discharge curves and (d) specific
capacitance at various current densities.
3
9
process . Then, the inner crystallites tend to dissolve, resulting in
an interior void space. The exterior crystals will serve as starting
points to attract the smaller metastable crystals under the surface
layers, which is confirmed by the fact that the diameters of NiS
microspheres increase with the reaction time. As a consequence,
NiS hollow microspheres are formed.
3.3 Electrochemical properties of NiS hollow microspheres
In addition, we found the substituent group in the S source plays
an important role in formation of NiS hollow microspheres. If the
NiS hollow microspheres were further evaluated as an electrode
material for supercapacitors by cyclic voltammograms (CV) and
galvanostatic charge/discharge (GCD) techniques using a three
electrode system. Typical cyclic voltammograms at various scan
rates (2-40 mV/s) are shown in Fig. 10a. Each cyclic voltammogram
curve presents a pair of distinct redox peaks, which is different from
the rectangular shape for conventional electric double-layer
NH -NH- group was replaced by NH2- or CH3-, no hollow
2
microstructures were found at the same conditions except the S
source. As shown in Fig. 8, when TU was used as S source, the
product was composed of NiS (JCPDS No. 02-1280) and Ni S (JCPDS
3
4
No.43-1469). Rod-like structures were obtained (Fig. 9a). If S source
was replaced by TAA, the mixture of NiS (JCPDS No. 02-1280), NiS
40
capacitor . This result indicates that the energy storage of NiS
(
JCPDS No. 12-0041) and Ni S (JCPDS No. 43-1469) was achieved at
3 4
hollow microspheres were mainly ascribed to pseudocapacitance
the same experimental conditions (Fig. 9b). And morphology of the
product was irregular particles. It is well accepted that H S and N H
41, 42, 29
due to the following redox reaction
.
2
2
4
-
-
NiS + OH ↔ NiSOH + e
(4)
released gradually as the decomposition of thiosemicarbazide with
the increasing of temperature. Then, N H may adopt a complexant
The voltammetric charge integrated from a positive or negative
scan of cyclic voltammogram can be used as an effective signal in
determining the pseudocapacitance. Accordingly, the average
2
4
3
2
to bind nickel ions , forming Ni(N H ) Cl complex. Ni(N H ) Cl
2
2
4 3
2
2
4 3
complex and H S reacted to form NiS original nuclei, which can be
2
s
specific capacitance (C ) of the sample was calculated by the
expressed as follows:
4
3-45
following equation
.
NH CSNHNH + H O → H S + N H + CO + NH
3
(1)
(2)
(3)
q
1
2
2
2
2
2
4
2
C =
=
∫i(V )dV
(5)
s
2
+
2+
ꢁVm mvꢁV
Ni + 3N H4 → [Ni(N H ) ]
2
2
4 3
Where, m is the mass of NiS hollow microspheres and v is scan rate,
ΔV represents the potential window. The dependence of specific
capacitance loss on the scan rate from 2 to 40 mV/s is illustrated in
Fig. 10b. Increasing scan rate has a direct impact on the diffusion of
2
+
+
[
Ni(N H ) ] + H S → NiS + 2[N H ] + N H
2 4 3 2 2 5 2 4
-
46
-
OH into NiS hollow microspheres matrix . At low scan rate, OH
can easily diffuse into the electrode. However, at high scan rate,
-
OH will only approach the outer surface of the electrode, and the
-
effective interaction between OH and the electrode is greatly
reduced. The material located in the inner has little contribution to
the capacitance.
The rate capability of electrode material is
a significant
47
consideration for practical applications of supercapacitor . Fig. 10c
provides the GCD curves at different current densities in the voltage
range between 0 and 0.4 V (vs. SCE). Obviously, there are voltage
plateaus in all discharge curves, further supporting its typical
3
1,48
Faradaic redox pseudocapacitance
. The specific capacitance can
Fig. 9 TEM images of products obtained with different S sources. S
5
be calculated according to Equation 6 .
sources: (a) TU; (b) TAA.
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C
i × ꢁt
m ×ꢁV
solid microspheres. However, the specific capacitances are only
85.2% and 82.5% of those for NiS hollow microspheres at scan rate
C =
=
(6)
s
m
s
Where, C is the specific capacitance, i is the discharge current, Δt is of 2 mV/s and discharge current density at 1 A/g, respectively. This
the discharge time, ΔV represents the potential drop during result may be attributed to the unique hollow and mesoporous
discharge, and m is the mass of the active material within the structure of NiS hollow microspheres.
electrode. NiS hollow microspheres electrode delivers high specific
capacitance values of 1848.0, 1636.4, 1507.6, 1449.6, 1404.7, and
1
372.9 F/g at the discharge current densities of 1, 2, 4, 6, 8 and 10
8
6
4
00
00
00
A/g, respectively (Fig. 10d), which are more competitive than NiS
100
80
-1
with various morphologies, such as 927 F⋅g for NiS hollow
spheres , 857.76 F⋅g for flower-like NiS , and 893 F⋅g for NiS
nanoparticles . The specific capacitance gradually decreased with
the increase of current density due to the incremental IR drop and
insufficient active material involved in redox reaction at higher
60
2
9
-1
30
-1
40
49
20
0
0
20
40
60
80
100
Z' (ohm)
5
0, 51
-
200
0
current densities
. Besides, at higher current densities, OH ions
on the electrode decrease with the increase of current, whereas
initial value
after 1000 cycles
-
OH ions in the solution diffuse too slowly to meet the need of ions
52,53
near the interface during charge/discharge process
. 74.3% of
0
200
400
600
the specific capacitance was still retained when the discharge rate
increased from 1 to 10 A/g. Therefore, NiS hollow microspheres
electrode not only exhibits large specific capacitance but also
maintains them well at higher current densities. The outstanding
electrochemical performance of NiS hollow microspheres electrode
could be attributed to its unique structure.
Z' (ohm)
Fig. 11 Nyquist plots of NiS hollow microspheres electrode.
1000
Electrochemical impedance spectroscopy (EIS) was conducted to
evaluate the charge transfer and electrolyte diffusion in the
electrode/electrolyte interface and the corresponding Nyquist plots
are shown in Fig. 11. Obviously, the EIS of NiS hollow microspheres
electrode was composed of a semicircle in the high frequency
region and a straight line in the low frequency region. The internal
800
600
400
200
0
resistance (R
b
) can be obtained from the intercept of the plot with
value
the real axis in the high frequency region. After 1000 cycles R
b
0
200
400
600
800
1000
increased from 1.23 Ω to 1.26 Ω. The small semicircle corresponds
to the low charge transfer resistance (Rct) of the electrodes. The low
Cycle number
R
ct leads to better rate capability because Rct acts a limiting factor
54
for fast charge and discharge of supercapacitor . The Rct value
became larger after 1000 cycles. The increase of R
and Rct values Fig. 12 Electrochemical cycle stability of NiS hollow microspheres
b
may be attributed to the collapse of hollow microspheres. At low electrode at scan rate of 50 mV/s.
frequency region, the straight line represents the diffusive
resistance (Warburg impedance). NiS hollow microspheres
48
24
0
a
1600
b
electrode exhibits very low Warburg impedance before 1000 cycles,
-
1
200
800
400
which will facilitate the diffusion of OH ions into the electrode.
Therefore, NiS hollow microspheres with an impressive
electrochemical capacitive property could be achieved.
-24
-48
-
72
-0.4
Long cycle lifetime is another critical requirement for practical
-0.2
0.0
0.2
0.4
0.6
0.8
0
10
20
30
40
5
5,56
Potential (V vs.SCE)
Scan rate (mV/s)
applications of supercapacitors
. Cycle stability of NiS hollow
microspheres electrodes was examined by CV measurement.
Specific capacitance retention of NiS hollow microspheres electrode
over 1000 times of continuous cycling at a scan rate of 50 mV/s is
shown in Fig. 12. The specific capacitance is 480.3 F/g and 493.8 F/g
0.5
1800
600
c
1 A/g
2 A/g
4 A/g
1
d
0.4
0.3
0.2
0.1
0.0
6
A/g
1400
1200
8 A/g
10A/g
1
000
800
600
st
th
for the 1 cycle and 1000 cycle, respectively. Continuous CV
characterization indicates that NiS hollow microspheres electrode
has excellent electrochemical cycle stability.
0
400
800
1200
1600
0
2
4
6
8
10
Time (s)
Current density (A/g)
In order to highlight the superiority of hollow microspheres, the
electrochemical properties of NiS solid microspheres were also
performed by CV and GCD measurements (Fig. 13). Compared to
NiS hollow microspheres, the similar figures are obtained for NiS
Fig. 13 (a) CV curves and (b) the dependence of capacitance loss on
the scan rate; (c) charge/discharge curves and (d) specific
capacitance at various current densities of NiS solid microspheres.
6
| J. Name., 2012, 00, 1-3
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NP lee wa s eJ od uo r nn oa tl aod fj uC s ht em ma ri sg it nr ys
View Article Online
DOI: 10.1039/C5NJ02425K
Journal Name
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2 Y. Munaiah, B. G. Sundara Raj, T. Prem Kumar and P.
Ragupathy, J Mater Chem A, 2013, 1, 4300-4306.
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. Conclusions
In summary, NiS hollow microspheres with mesoporous shells were 23 C.-Y. Cao, W. Guo, Z.-M. Cui, W.-G. Song and W. Cai, J Mater
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successfully synthesized through a facile hydrothermal route using
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When tested in electrodes for supercapacitors, NiS hollow
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Acknowledgements
We thank for the support from the National Natural Scientific
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ACS Appl Mater Interfaces, 2013, 5, 7335-7340.
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New Journal of Chemistry
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DOI: 10.1039/C5NJ02425K
ARTICLE
Journal Name
5
6 H. Jiang, J. Ma and C. Li, Chem Commun, 2012, 48, 4465-
467.
4
8
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New Journal of Chemistry
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DOI: 10.1039/C5NJ02425K
Graphical Abstract
NiS hollow microspheres with excellent capacitive performance were prepared by a
facile hydrothermal route without any surfactant or template.