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Hydrothermal synthesis of flower-like
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molybdenum disulfide microspheres and their
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Cite this: RSC Adv., 2018, 8, 38945
application in electrochemical supercapacitors
Fangping Wang, * Guifang Li, Jinfeng Zheng, Jing Ma, Caixia Yang
and Qizhao Wang
Three-dimensional flower-like molybdenum disulfide microspheres composed of nanosheets were
prepared by a hydrothermal method using ammonium molybdate as the molybdenum source and
thiourea as the sulfur source. Structural and morphological characterizations were performed by X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-
dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrochemical
properties of MoS2 electrode were studied by performing cyclic voltammetry (CV), galvanostatic charge–
discharge analysis and electrochemical impedance spectroscopy (EIS). When used as an electrode
material for supercapacitor, the hybrid MoS2 showed a high specific capacity of 518.7 F g?1 at a current
density of 1 A g?1 and 275 F g?1 at a high discharge current density of 10 A g?1. In addition, a symmetric
supercapacitor composed of MoS2 as positive and negative electrodes was prepared, which exhibited
Received 22nd May 2018
Accepted 31st July 2018
a high energy density of 12.46 W h kg?1 at a power density of 70 W kg?1 and still maintains an impressive
energy density of 6.42 W h kg?1 at a large power density of 7000 W kg?1. The outstanding performance
DOI: 10.1039/c8ra04350g
of the MoS2 electrode material indicates its great potential for applications in high-performance energy
rsc.li/rsc-advances storage systems.
interest among researchers. At present, carbon materials (such
1. Introduction as activated carbon), transition metal oxides (nickel oxide, etc.),
Supercapacitors, also known as electrochemical capacitors, and conductive polymers are oen used as electrode materials
have gathered growing interest of researchers in the era of for supercapacitors.12,13 However, the growing demand for
miniaturization of devices.1,2 These present fascinating prop- energy storage devices has prompted researchers to develop
erties of higher energy density, higher power density, longer life, new types of electrode materials. Therefore, the research of
lower toxicity than batteries, and so on, compared with those of nanometer-scale metal sulde as the material of supercapacitor
traditional capacitors.3–6 According to the charge–discharge electrode has become a new eld. For example, cobalt sulde
mechanisms, SCs can be divided into electrical double-layer (CoS, CoS2), nickel sulde (NiS, NiS2, Ni3S2), molybdenum
capacitors (EDLCs) and pseudocapacitors.7,8 Pseudocapaci- sulde (MoS2), copper sulde (CuS, Cu2S), and vanadium
tance arises from reversible faradaic reactions of redox active sulde (VS, VS2) have been used as supercapacitors electrode
materials, such as transition metal oxides, hydroxides, and materials.14–16 In particular, MoS2 has aroused interest among
suldes. Among those materials, ruthenium oxide (RuO2) has other transition metal suldes due to its layered structure and
exhibited excellent pseudocapacitive performance, but the inherent conductivity,17 and it is considered to be a suitable
toxicity and high cost of RuO2 restrict its widespread commer- replacement for graphene and carbon nanotubes in energy
cial application.9,10 The low cost active material MnO2 can also storage applications. In addition, molybdenum-based materials
achieve a high specic capacitance; however, MnO2-based (such as MoO3, MoO2, and MoS2) exhibit various valences and
pseudocapacitors suffer from poor electrical conductivity and rich chemical properties, making them viable candidate mate-
cyclic stability.11 Application of nanometal suldes in the energy rials for electrochemical applications.18
storage devices, such as fuel cells, solar energy pools, lithium- MoS2 is a transition metal sulde with a layered structure,
ion batteries, and supercapacitors, have aroused widespread where a metal molybdenum layer is sandwiched between two
sulfur layers; the layers are connected by weak van der Waals
forces and the interlayer S–Mo–S atoms are strongly covalently
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education
linked.19–21 MoS2 possesses unique physicochemical properties
of China, Key Laboratory of Gansu Polymer Materials, College of Chemistry and
Chemical Engineering, Northwest Normal University, Lanzhou 730070, China.
due to its unique atomic and electronic structure. It is mainly
E-mail: wangfp@nwnu.edu.cn used in the solid lubricants, catalysts, supercapacitors and
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lithium-ion batteries.22–24 Among these, the research on the and heated at 200 ? C for different time periods (8 h, 16 h, and 24
application of MoS2 as a supercapacitor electrode material is the h). The obtained MoS2 was ushed with water and ethanol, in
most extensive. For example, Soon et al.25 found that the MoS2 sequence, and then dried at 70 ? C for 12 h. The MoS2 electrode
nano-lm presented an electric double layer capacitance materials were denoted as MoS2-8, MoS2-16, and MoS2-24,
behavior. Ma et al.26 reported that nano-MoS2 intercalated in according to the hydrothermal treatment time.
polypyrrole could improve its capacitance performance. Cao
et al.27 fabricated micro-supercapacitors using coated MoS2
2.3. Material characterization
nanolms, and showed that MoS2 has excellent electrochemical
performance in aqueous electrolytes. The morphology and microstructure of the samples were
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In particular, the structure of the electrode directly affects its characterized by eld-emission scanning electron microscopy
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electrochemical properties. Generally, the electrochemical (FESEM JSM-6701F, Japan), transmission electron microscopy
electrode is 2-dimensional and suffers from inadequate contact (TEM; JEOL, JEM-2010, Japan), and X-ray diffraction (XRD, D/
with electrolyte and low surface-area-utilization efficiency. Max-2400, Japan) with Cu Ka radiation (l ¼ 1.5418 Å) oper-
Numerous efforts have been made to design three-dimensional ating at 40 kV, 100 mA. X-ray photoelectron spectroscopy (XPS)
(3D) electrodes, such as MoS2/mesoporous carbon spheres. spectra were recorded on a PHI 5702 spectrometer using
Recently, there have been some reports related to NiCo2S4 and a standard Al Ka X-ray source of 300 W and an analyser pass
graphene oxide composites applied in supercapacitors. Krish- energy of 29.35 eV.
namoorthy et al.15 reported 92.85 F g?1 specic capacitance of
chemically prepared MoS2 nanostructure. Huang et al.28 re- 2.4. Electrode preparation and electrochemical
ported polyaniline/MoS2 composites as supercapacitor elec- characterization
trodes with the specic capacitance of 575 F g?1.
In this paper, the morphologically regular ower-like The electrochemical properties of the MoS2 nanostructures
molybdenum disulde microspheres were successfully synthe- were investigated in 1 M Na2SO4 solution using a three-
sized by a hydrothermal method (Fig. 1). The as-prepared MoS2 electrode system in an electrochemical work station
was directly used as a supercapacitor electrode and exhibited (CHI660E, Shanghai). Initially, 8 mg of MoS2-16 was dispersed
high specic capacitance (518.7 F g?1 at current density of in 400 mL of 0.5 wt% Naon solution by ultrasonication to
1 A g?1) and excellent cycling performance (88.2% retention obtain a well dispersed suspension. Then, 6 mL of the suspen-
aer 2500 cycles). In addition, a high performance symmetric sion was drop-casted onto the pre-treated glassy carbon elec-
supercapacitor was successfully fabricated by using MoS2 as trode (GCE) and le to dry at room temperature. Saturated
both positive electrode and negative electrode, which exhibited calomel electrode, platinum wire, and a loadable glassy carbon
a high energy density of 12.46 W h kg?1 at power density of electrode were respectively the reference, the counter, and the
70 W kg?1. working electrodes.29 Cyclic voltammetry (CV) in the range ?0.3
to 0.5 V was performed at different scan rates. Galvanostatic
charge–discharge curves were recorded in the potential range of
2. Experimental section ?0.3 to 0.5 V at different constant current density. The cycle life
2.1. Materials tests were performed by galvanostatic charge–discharge
measurements with a constant current density of 4 A g?1 for
Ammonium molybdate ((NH4)6Mo7O24$4H2O) and thiourea
2500 cycles. Electrochemical impedance spectroscopy (EIS) was
(CH4N2S) were obtained from Tianjin Kaixin Chemical Industry
performed in the frequency range of 0.01 Hz to 100 kHz with
Co. Ltd. All the chemical reagents were of analytical purity and
5 mV amplitude at current open circuit voltage.
used without any further purication.
A two-electrode symmetric supercapacitor cell was assem-
bled to measure the device performances. MoS2 was used as the
2.2. Synthesis of MoS2 positive electrode and negative electrode. The negative elec-
In a typical process, 0.8 g of ammonium molybdate and 5.12 g trode was prepared by the traditional slurry coating method.
thiourea were dissolved into 80 mL deionized water and stirred The mass loading of electroactive material in symmetric
until the solution was clear and transparent. The solution was supercapacitor was 0.3 mg. The specic capacitances (Cm) were
transferred into 100 mL PTFE-lined stainless steel autoclave calculated according to the following equations:30–32
Fig. 1 Schematic of the MoS2 synthesized by hydrothermal method.
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I ? Dt 403 cm?1, related to the characteristic vibrations of pure
Cm ¼ (1)
m ? DV metallic phase MoS2. The main peak associated with Mo–Mo
metallic vibration is located at 145 cm?1. Two characteristic
where Cm is the specic capacitance, I is the current of the
peaks are observed at 371 and 403 cm?1, which correspond to
charge–discharge, Dt (s) is the discharge time, DV is the voltage
the E2g1 and Ag1 modes of hexagonal MoS2, and are attributed to
window, and m is the mass of active materials.
the out-of-plane Mo–S phonon mode and the in-plane Mo–S
In the symmetrical supercapacitors, the corresponding
power density (P) and energy density (E) were calculated phonon mode, respectively.
according to the following equations.8 The chemical and surface states of the Mo and S elements in
the as-prepared MoS2-16 electrodes have been investigated via
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Cm ? DV 2 X-ray photoelectron spectroscopy. The XPS survey spectrum of
E¼
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(2)
2 the MoS2 electrodes is shown in Fig. 5a, which revealed the
presence of Mo 3d, Mo 3p, S 2p, C 1s and O 1s states.33 The C
E
P¼ (3) and O signals originated from the CO2 and H2O impurities, as
Dt
seen in many XPS analyses. The ne tted spectrum of Mo 3d is
shown in Fig. 5b, which revealed the presence of two major
peaks at around 228.5 and 232 eV, corresponding to the Mo4+
3. Results and discussion 3d5/2 and Mo4+ 3d3/2 states, respectively. Small peaks belonging
to S 2s in the vicinity of 226 eV are also observed.34 The ne
3.1. Characterization of MoS2 tted spectrum of S 2p (Fig. 5c) indicated the presence of two
Fig. 2 shows the SEM microstructures of the as-prepared MoS2- major peaks at around 161.5 and 162.9 eV, which corresponds
8, MoS2-16, and MoS2-24. It can be clearly observed from the to the S 2p3/2 and S 2p1/2 states, respectively.35 These studies
Fig. 2 that the as-prepared molybdenum disulde material has conrm the formation of MoS2 by the hydrothermal method.
a nanoower-like structure assembled from clear nanoakes.
Different hydrothermal treatment times had a great inuence
on the size of the molybdenum disulde nanoakes and the 3.2. Electrochemical and energy storage performance
size of the three-dimensional pore structure. Fig. 2a and b show Fig. 6a shows the cyclic voltammetry (CV) curves of MoS2-8,
the SEM microstructures of the MoS2-8 at low and high MoS2-16 and MoS2-24 at 10 mV s?1 in 1 M Na2SO4 solution,
magnications, respectively. It can be seen that the nanosheets obtained over the potential range between ?0.3 V and 0.5 V. In
are partially adhered together and contained a small amount of contrast, the CV curve area of the MoS2-16 electrode was larger
block-like structures, resulting in inconspicuous pore structure. than the electrode area of MoS2-8 and MoS2-24. Fig. 6b shows
MoS2-16 (Fig. 2c and d) possesses an evenly distributed larger the cyclic voltammetry (CV) curves of MoS2-16 at different scan
size of nanoakes, and forms highly open and relatively deep rates. On increasing the scanning speed from 10 mV s?1 to
porous nanostructures, making optimal use of the grain surface 100 mV s?1, the shape of the CV curve did not change signi-
readily accessible to the liquid electrolyte and providing effi- cantly, indicating that MoS2-16 presented better rate perfor-
cient channels for electron transport. Fig. 2e and f present the mance and small polarization.36,37 Galvanostatic charging–
low and high magnication SEM image of the MoS2-24 sample. discharging (GCD) technique was also applied to study the
Aer a 24 h long hydrothermal process, the nanosheets of MoS2- electrochemical capacitive properties of MoS2-8, MoS2-16 and
24 arranged regularly but too tightly, and some collapsed, MoS2-24 at a current density of 1 A g?1, as shown in Fig. 6c. The
resulting in a decrease or disappearance of the pore size in the longer discharge time of MoS2-16 electrode again conrmed its
material, which could degrade the electrochemical performance enhanced capacitance. Fig. 6d shows the galvanostatic charge–
of the electrode material. discharge curve (GCD) of MoS2-16 at various current densities
Fig. 3 shows the TEM images for MoS2-16. As shown in varying from 1 to 10 A g?1, with a potential window range from
Fig. 3a, the interconnected nanoakes consist of nano-owers. ?0.3 V to 0.5 V. Based on eqn (2), for MoS2-16 electrode, at
As seen in the magnied image (Fig. 3b), MoS2-16 nanoakes a discharge current of 1 A g?1, the specic capacitance reached
are very thin, leading to open and porous three-dimensional 518.7 F g?1, while at a high discharge current of 10 A g?1, the
structures, which are benecial to electrolyte access and elec- specic capacitance was as high as 275 F g?1. Using these GCD
tron transport during electrochemical reactions. These results curves, the specic capacitances of ve electrodes at various
are in accordance with the SEM images. current densities were calculated and depicted in Fig. 6e. The
The XRD patterns of MoS2-8, MoS2-16, and MoS2-24 hybrids calculated specic capacitances of MoS2-16 electrode were
are shown in Fig. 4a. The four diffraction peaks at 14.2? , 32.5? , calculated to be 518.7, 415, 363.7, 335, 318.7, and 275 F g?1 at
35.8? , and 55.4? correspond to the (002), (100), (102) and (106) discharge current densities of 1, 2, 3, 4, 5, and 10 A g?1,
planes of cubic phase MoS2 (JCPDS No. 75-1539). Energy- respectively, which are much higher than those for MoS2-8 and
dispersive X-ray (EDX) spectroscopy (Fig. 4b) demonstrates the MoS2-24 at the same current densities. Table 1 compares the
existence of Mo and S elements. The Raman spectrum of the as- electrochemical performance of the MoS2 electrode material
prepared MoS2 nanoowers was recorded in this study, as prepared in this study with that of the MoS2 electrode material
shown in Fig. 4c. At low wave numbers the Raman spectrum of reported in the literature. It can be seen that the electro-
the MoS2 sample showed peaks at 145, 227, 283, 371 and chemical performance of the MoS2 electrode material prepared
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Fig. 2 SEM images of (a and b) MoS2-8, (c and d) MoS2-16, and (e and f) MoS2-24.
Fig. 3 TEM images of MoS2-16.
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Fig. 4 (a) XRD pattern of the MoS2-8, MoS2-16, and MoS2-24; (b) EDX spectra of MoS2-16; (c) Raman spectra of MoS2-16.
in this experiment is superior. The superior electrochemical resistance. Nyquist plots based on the radius of the high
behaviors of MoS2-16 nanoower observed in this study should frequency arc on the real axis are shown in Fig. 6f. Clearly, the
be partially attributed to its ultrathin and porous features, semicircle over the high frequency range of the MoS2-16 elec-
which can offer even richer electroactive sites, and more effi- trode is smaller than that of others, indicating the smaller
cient and convenient electronic transport. charge-transfer resistance. Furthermore, the slope of the line
Electrochemical impedance spectroscopy (EIS) analysis is for MoS2-16 was larger than that of MoS2-8 and MoS2-24,
an important tool to examine the interface resistance of elec- implying a better capacitive behavior and a lower diffusion
trode materials for supercapacitors. For an ideal super- resistance of ions in the MoS2-16 electrode material. The
capacitor, the Nyquist plot comprises a vertical line, which can differences in the electrochemical properties of MoS2 material
be simulated by an equivalent circuit. The semicircle at high are mainly due to disparity in the material electrolyte interface
frequency region is indicative of interfacial charge transfer properties and electrolyte ion diffusion rates during the
resistance. In the equivalent circuit, the series resistance (R) charge–discharge processes, which are in good accordance
depends on electrolyte resistance and electrode electronic with its abovementioned electrochemical performance.
Fig. 5 (a) XPS survey scanning of MoS2-16. XPS spectra of (b) Mo 3d and (c) S 2p.
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Fig. 6 (a) Cyclic voltammograms of MoS2-8, MoS2-16, and MoS2-24 at 10 mV s?1. (b) CV curves of the MoS2-16 electrode at different scan rates. (c)
Galvanostatic discharge curves of MoS2-8, MoS2-16 and MoS2-24 at a current density of 1 A g?1. (d) GCD curves of MoS2-16 at various current
densities. (e) Specific capacitance as a function of the current density of the MoS2-8, MoS2-16, and MoS2-24. (f) Nyquist plots of MoS2-8, MoS2-16,
and MoS2-24 electrodes in the frequency range from 100 kHz to 0.01 Hz. (g) Cycle performance for the MoS2-16 electrodes at a current of 4 A g?1.
The cyclic stability of the electrode material is very current density of 4 A g?1 (Fig. 6g). Although the specic
important for practical supercapacitor applications. The capacitance gradually decreases with the increase of cycle
cycling performance of MoS2-16 electrode was tested by 2500 number, there is still 88.2% retention of the initial
cycles of continuous galvanostatic charge/discharge at the capacitance.
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Table 1 Comparison of electrochemical properties of different MoS2 electrode materials
Samples Electrolyte Current density Specic capacitance References
MoS2 nanosheets 1 M Na2SO4 1 A g?1 129.2 F g?1 38
Sphere like MoS2 1 M Na2SO4 5 mV s?1 106 F g?1 39
Spherically clustered MoS2 1 M H2SO4 5 mV s?1 113 F g?1 40
MoS2 nanospheres 1 M KCl 1 A g?1 122 F g?1 41
Hollow MoS2 nanospheres 1 M KCl 0.59 A g?1 144 F g?1 42
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MoS2 monolayers 6 M KOH 0.5 A g?1 366.9 F g?1 43
0.5 A g?1 268 F g?1
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MoS2/CMG 1 M Na2SO4 44
MoS2/MWCNT 1 M Na2SO4 1 A g?1 452.7 F g?1 45
MoS2-16 1 M Na2SO4 1 A g?1 518.7 F g?1 This work
Fig. 7 (a) CV curves of MoS2-16//MoS2-16 measured at different potential windows at a scan rate of 30 mV s?1. (b) CV curves of the SC measured
at different scan rates ranging from 20 to 120 mV s?1 in potential window of 0 to 1.6 V. (c) Galvanostatic charge/discharge curves of the SC
measure at different current densities from 1 to 10 A g?1. (d) Specific capacitance as a function of the current density of the symmetric
supercapacitor. (e) Ragone plots of the SC. (f) Cycling stability of the SC at 1.6 A g?1.
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3.3. Electrochemical performances of the MoS2-16//MoS2-16 supercapacitor was assembled using the MoS2-16 nanoower as
symmetric supercapacitor both positive electrode and negative electrode. This super-
capacitor delivered a maximum energy density of
To further evaluate the practical application potential of MoS2-
12.46 W h kg?1 at a power density of 70 W kg?1. Even at the
16 electrode, an aqueous SC was rst assembled using the
highest power density of 7000 W kg?1, the MoS2-16//MoS2-16
MoS2-16 electrode as both positive electrode and negative
device still maintained an energy density of 6.42 W h kg?1. Such
electrode. Fig. 7a shows a series of CV curves collected at 30 mV
outstanding capacitive behaviors imply the MoS2-16 nanoower
s?1 with an operating SC voltage ranging from 0.8 to 1.6 V to
obtain the best operating potential of MoS2-16//MoS2-16. Fig. 7b as a promising material for energy storage devices.
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shows typical CV curves for the SC device corresponding to
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different sweep rates. With the increment of sweep rate from 20 Conflicts of interest
to 120 mV s?1, all the curves presented similar shapes, revealing
the splendid high-rate charge–discharge performance of the There are no conicts to declare.
device.46–48 Fig. 7c shows the typical GCD curves of the cells at
various current densities with a potential window of 0–1.4 V.
During the charge and discharge processes, the charge curve of
Acknowledgements
MoS2-16//MoS2-16 (SSC) and its corresponding discharge curve We gratefully acknowledge the nancial support of this
are observed to be symmetrical, conrming that it has excellent research by the National Natural Science Foundation of China
electrochemical reversibility.49 The calculated specic capaci- (21065010, 21365019).
tance values based on the discharge curves are plotted in
Fig. 7d, which are 45.7, 43.7, 42.8, 42.3, 39.8, 36.9 and 23.57 F
g?1 at 0.1, 0.2, 0.3, 0.4, 1, 2, and 10 A g?1, respectively. Energy References
density and power density are important parameters to evaluate
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