One-step hydrothermal synthesis of a CoS2@MoS2 nanocomposite for high-performance supercapacitors

Article abstract of DOI:10.1016/j.jallcom.2018.01.324

A MoS₂-coated CoS₂ composite has been prepared successfully via a one-step hydrothermal method. This CoS₂@MoS₂ hybrid material has a distinct morphology with few-layer MoS₂ wrapping the CoS₂ nanoparticles. Serving as a supercapacitor electrode, the obtained CoS₂@MoS₂ hybrid material exhibits a remarkable specific capacitance of 1038 F/g, a high rate capability of 71.7%, and excellent cycling stability of 84.76% retention after 10000 cycles. The superior electrochemical properties of the CoS₂@MoS₂ nanocomposite are attributed to the synergistic effects between the layered MoS₂ and conductive CoS₂.

Full text of DOI:10.1016/j.jallcom.2018.01.324

Accepted Manuscript
One-step hydrothermal synthesis of a CoS @MoS nanocomposite for high-
2
2
performance supercapacitors
Feng Huang, Rongde Meng, Yanwei Sui, Fuxiang Wei, Jiqiu Qi, Qingkun Meng,
Yezeng He
PII:
S0925-8388(18)30338-4
10.1016/j.jallcom.2018.01.324
JALCOM 44788
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 17 January 2017
Revised Date: 23 January 2018
Accepted Date: 25 January 2018
Please cite this article as: F. Huang, R. Meng, Y. Sui, F. Wei, J. Qi, Q. Meng, Y. He, One-step
hydrothermal synthesis of a CoS @MoS nanocomposite for high-performance supercapacitors,
2
2
Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.01.324.
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ACCEPTED MANUSCRIPT
One-step hydrothermal synthesis of a CoS2@MoS2 nanocomposite for
high-performance supercapacitors
Feng Huang a, Rongde Meng b, Yanwei Sui a, Fuxiang Wei a, Jiqiu Qi a, Qingkun
Meng a, Yezeng He a,*
a
School of Materials Science and Engineering, China University of Mining and
Technology, Xuzhou 221116, People’s Republic of China
b Beijing Experimental School, Haidian, Beijing 100037, People's Republic of China
Abstract
A MoS2-coated CoS2 composite has been prepared successfully via a one-step
hydrothermal method. This CoS2@MoS2 hybrid material has a distinct morphology
with few-layer MoS2 wrapping the CoS2 nanoparticles. Serving as a supercapacitor
electrode, the obtained CoS2@MoS2 hybrid material exhibits a remarkable specific
capacitance of 1038 F/g, a high rate capability of 71.7%, and excellent cycling
stability of 84.76% retention after 10000 cycles. The superior electrochemical
properties of the CoS2@MoS2 nanocomposite are attributed to the synergistic effects
between the layered MoS2 and conductive CoS2.
Key words: MoS2; CoS2; flower-like structure; supercapacitors
1. Introduction
Supercapacitors have attracted worldwide attention due to their excellent
properties, such as rapid recharge ability, high power density, long cycle life,
eco-friendly nature and low-cost preparation [1-3]. There are two major types of
supercapacitors that are related to the different charge storage mechanisms. The first
type is a double-layer capacitor, which accumulates charge at the electrode-electrolyte
interface by establishing a Helmholtz double-layer [4-7], and the second type is a
pseudo-capacitor based on a redox capacitive mechanism [8-11].
Graphene, a two-dimensional carbon allotrope, has attracted considerable
attention due to its fascinating properties and broad applications in nanoelectronic
devices since its discovery in 2004 [12]. However, graphene materials also have
several disadvantages such as high costs, low yield and poor scalability [13,14]. MoS2
*Corresponding author, E-mail: hyz0217@hotmail.com

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is a transition metal sulfide with a layered structure. The ultrathin thickness and 2D
morphology enable MoS2 to achieve ultrahigh specific surface areas compared to its
bulk counterpart [15,16]. Mo atoms can easily prompt redox faradaic reactions in the
interlayer space due to oxidation states spanning a range from +2 to +6, and its
theoretical specific capacitance could be as high as 1000 F/g [17,18]. Therefore, the
layered structure and redox faradaic reactions play important roles in making the
MoS2 a promising electrode material used in the supercapacitors. To date, many
studies regarding the use of MoS2 in supercapacitors have been reported. For example,
Ramadoss et al. prepared a mesoporous MoS2 nanostructure by a hydrothermal
method and obtained maximum capacitance values of 376 and 403 F/g at a scan rate
of 1 mV/s in 1 M Na2SO4 and KCl electrolyte solutions, respectively [19]. Wang et al.
synthesized hierarchical MoS2 nanospheres for supercapacitors by a hydrothermal
method and demonstrated a capacitance of 142 F/g at a current density of 0.59 A/g
[20]. Zhou et al. reported flower-like MoS2 nanospheres that were synthesized by a
hydrothermal route and showed specific capacitances of 122 F/g at 1 A/g or 114 F/g at
2 mV/s [21]. Krishnamoorthy et al. synthesized few-layered MoS2 nanosheets by
using a mechanical milling method and demonstrated a MoS2 based wire type solid
state supercapacitors device with a specific capacitance of 119 ꢀF/cm-1 [22].
Nevertheless, the preparation of a high-performance supercapacitors using MoS2 as an
electrode material is still a challenging task due to the low electrical conductivity and
low number of accessible active sites in MoS2.
To overcome these difficulties, MoS2 should be combined with a conductive
material with high electrochemical activity to enhance the electrical conductivity and
increase content of accessible active sites [23,24]. As an important transition metal
sulfide, CoS2 has the best conductivity and thermal stability among the entire cobalt
sulfide family, such as CoS, Co3S4, Co4S3, Co2S3 and Co9S8 [25,26]. CoS2 has a
superior capacitive behavior because its negative Gibbs free energy value causes a
spontaneous thermodynamic reduction reaction that is useful for pseudo-capacitance
supercapacitors [27,28]. Xing et al. have successfully prepared a type of
octahedral-shaped CoS2 with a specific capacitance of 236.5 F/g at 1 A/g in 2 M KOH
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[29]. Ren et al. synthesized CoS2 nanowires deposited on a graphite disc using a
hydrothermal method with a specific capacitance of 828.2 F/g at 10 mV/s in 6 M
KOH [30].
In this work, we synthesized a novel CoS2@MoS2 nanocomposite by a facile
one-step hydrothermal technique. The CoS2@MoS2 nanocomposite electrode shows a
high specific performance of 1038 F/g at 1 A/g in 2 M KOH solution. In addition, the
CoS2@MoS2 nanocomposite electrode showed excellent cyclic stability and a high
rate capability contrasted with pure MoS2 and pure CoS2. The capacitance remained
approximately 84.76% after 10000 cycles of charge-discharge at a current density of 5
A/g. The synergistic effect between MoS2 and CoS2 in the composite is expected to
improve electrode performance when used as an electrode material for
supercapacitors.
2. Experimental section
2.1 Materials
All the reagents used in this experiment were of analytical grade and were used
without purification. Thiocarbamide (NH2CSNH2), sodium molybdatedihydrate
(Na2MoO4ꢁ2H2O), cobalt (II) nitrate hexahydrate (Co(NO3)2ꢁ6H2O) and anhydrous
ethanol (C2H6O) were purchased from Aladdin Industrial Corporation (ShangHai).
The water was purified through a Lab Pure Water System (18.25 mꢂ).
2.2 Synthesis of MoS2, CoS2 and CoS2@MoS2 nanocomposites
The CoS2@MoS2 nanocomposite was synthesized through a facile solvothermal
method. Initially, 2.5 mmol Na2MoO4ꢁ2H2O, 1.25 mmol Co(NO3)2ꢁ6H2O (at a Mo:Co
molar ratio of 2:1) and 6.25 mmol H2NCSNH2 were dissolved in a 30 ml mixed
solution of DI water and anhydrous ethanol (1:1 in volume). The precursor solution
was magnetically stirred by ultrasonication for 30 min. The mixture was then
transferred into a Teflon-lined autoclave, sealed, and heated in an oven at 180 °C for
48 hours. After natural cooling to room temperature, the resulting black solid product
was collected, then washed with DI water three times and ethanol three times before
drying in vacuum at 70 °C (referred to as CM-2). The number “2” refers to the molar
ratio of Na2MoO4ꢁ2H2O and Co(NO3)2ꢁ6H2O. CM-1 and CM-3 were synthesized by
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the addition of the corresponding quantity of Co(NO3)2ꢁ6H2O into the mixture with
the same amount of Na2MoO4ꢁ2H2O (2.5 mmol). As reference materials, pristine
MoS2 and CoS2 were also prepared, following the same procedure in the absence of
Co(NO3)2ꢁ6H2O and Na2MoO4ꢁ2H2O, respectively.
2.3 Characterization
The CoS2@MoS2 (CM-2) samples were characterized for structural properties
using X-ray diffraction (D8 Advance, Bruker, Germany) with Cu-Kα radiation in the
angle range from 5°-85° at a scan rate of 8 °/min. The surface area and nitrogen
absorption-desorption isotherm were analyzed using an ASAP 2020 analyzer. XPS
spectra were recorded on ESCALAB 250Xi using Al-Kɑ radiation as the X-ray source.
The morphology and structure of the products were characterized using field emission
SEM (Hitachi SU-8000) equipped with high-resolution TEM (HRTEM, Tecnai
G2F20).
2.4 Electrochemical measurements
Electrochemical measurements were performed on a CHI660D (Chenhua,
Shanghai, China) electrochemical workstation in a 2 M KOH aqueous electrolyte
solution using a three-electrode system that consists of a working electrode, a
platinum plate counter electrode, and a saturated calomel electrode (SCE, reference
electrode) at room temperature. The working electrode was prepared by mixing the
as-prepared active material, acetylene black and polyvinylidene fluoride (PVDF) in
N-methylpyrrolidinone (NMP) according to a ratio of 80:10:10. The resultant slurry
was pasted on nickel foam (1×2 cm2) serving as a current collector, which was then
dried at 80 °C for 12 h and pressed on 10 MPa. The mass of the activated material
was obtained by a weight difference method using a sensitive microbalance. Finally,
the loading masses of MoS2, CoS2 and CoS2@MoS2 on the Ni foam were 1.1, 0.78
and 0.9 mg/cm2, respectively. Cyclic voltammetry measurements were carried out in a
potential range from 0 to 0.65 V (vs. SCE) at different scan rates in a 2 M KOH
electrolyte. Galvanostatic charge-discharge measurements were performed at different
current densities (1, 2, 5, and 10 A·g-1) for the as-prepared electrodes. The specific
capacitance can be calculated from the galvanostatic charge-discharge curve
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according to Eq. (1)
ꢀ∆ꢁ
C = ꢂ∆ꢃ (1),
where C is the specific capacitance, I is the current, ∆t is the discharge time, ∆v is
the potential window, and m is mass of the electroactive material. Electrochemical
impedance spectroscopy (EIS) measurements were conducted by applying an AC
voltage with 5 mV amplitude in the frequency range from 0.01 Hz to 100 KHz.
3. Results and discussion
The CoS2@MoS2 (CM-1, 2, 3) nanostructures were prepared using a
low-temperature hydrothermal method. During the reaction, the active species (S2-
ions) are released from NH2CSNH2 according to Eq. (2). The S2- ions react with dilute
nitric acid to produce elemental sulfur (Eq. (3)). In addition, the elemental sulfur plays
an important role in the chemical reactions (Eq. (4)). Lastly, the S2- ions interacted
with MoO42- ions to produce MoS2 (Eq. (5)).
H2NCSNH2 + H2O → 2NH3 + 2H2S + CO2
6H2S + 4HNO3 → 6S +4NO +5H2O
H2S + S + Co2+ → CoS2 + 2H+
(2)
(3)
(4)
(5)
MoO42- + H2S → MoS2 + 3H2 + SO4
2-
The crystal structure and phase purity of the as-prepared samples were
characterized by XRD. Fig. 1a shows the XRD pattern of the as-prepared flowerlike
CoS2@MoS2 nanostructured sample. The curve shows several sharp peaks at 14.3°,
29.2°, 32.6°, 39.5°, 49.7°, 52.08°, 58.43°, 60.1°, 68.4°, 72.7° and 75.9° which can be
assigned to the (002), (004), (100), (103), (105), (018), (110), (008), (200), (203) and
(116) planes of hexagonal MoS2 (JCPDS 37-1942), respectively. The peaks at 32.3°,
36.23°, 39.8°, 46.3°, 60.2° and 69.8° correspond to the (200), (210), (211), (220), (230)
and (410) planes of CoS2 (JCPDS 65-3322), respectively [31,32]. Table 1 lists the
textural properties (pore volume, BET surface area and pore size) of MoS2,
CoS2@MoS2 and CoS2. Clearly, CoS2@MoS2 exhibits the largest pore volume, BET
surface and pore size. Due to the presence of flowerlike MoS2, the BET surface of
CoS2@MoS2 increases to 60.1 m2/g compared with a value of 51.2 for pure CoS2.
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Moreover, the pores were mainly distributed in the range of 2-20 nm. Such a
high-surface-area microstructure contains many active sites for electrochemical
interactions with electrolyte ions, which is very useful for energy storage applications.
Table 1 Textural properties of MoS2, CoS2@MoS2 and CoS2.
Pore volume
(cm3/g)
BET surface area
(m2/g)
Materials
Pore size (nm)
MoS2
CoS2@MoS2
(CM-2)
0.129
58.2
8.9
10.9
7.6
0.165
0.112
60.1
51.2
CoS2
XPS was carried out to obtain information regarding the structure and the
electronic state of Mo, Co and S of the as-prepared nanocomposite. As shown in Fig.
2a, the survey scan of the CoS2@MoS2 nanostructured electrode exhibits
characteristic peaks of the Mo 3d, Co 2p, S 2p, O 1s and C 1s levels at their
corresponding binding energies. Fig. 2b shows that the high-resolution scans of the
Mo 3d electrons exhibit two primary peaks at 228.9 and 232.2 eV that correspond to
the Mo 3d3/2 and 3d5/2 in MoS2, respectively, which further confirms the formation of
MoS2. The peak at 232.8 eV is attributed to the existence of a small amount of Mo6+
due to MoS2 exposure to the air. The emergence of a peak at 226.0 eV corresponds to
the S 2s orbitals of MoS2. Fig. 2c shows two dominating peaks at 779.8 eV and 794.7
eV that correspond to Co 2p3/2 and 2p1/2, respectively. The two peaks at 161.4 eV and
162.8 eV correspond to the S 2p3/2 and 2p1/2 levels of divalent sulfide ions (S2-). The
peak at 163.4 eV indicates the presence of bridging S22- or apical S2- [33,34].
The morphologies of the CoS2@MoS2 nanocomposites (the samples are referred
to as CM-1, CM-2 and CM-3) were examined by FESEM. As shown in Fig. 3, CM-2
presents a flowerlike morphology and is quite different from CM-1 and CM-3. In
addition, inductively coupled plasma (ICP) analysis was conducted to determine the
molar percentage between CoS2 and MoS2 in CM-1, CM-2 and CM-3, as shown in
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Table 2.
Table 2. The molar content of MoS2 and CoS2 in CM-1, CM-2 and CM-3.
Samples
CM-1
CM-2
CM-3
Component
MoS2
42.9%
57.1%
74.9%
25.1%
87.5%
12.5%
CoS2
Fig. 4 shows the morphologies of MoS2, CoS2 and CoS2@MoS2 (CM-2)
nanocomposites. The images indicate that the CoS2@MoS2 nanocomposite has a
similar morphology to MoS2. MoS2 (Fig. 4a,b) and CM-2 (Fig. 4e,f) are both
flowerlike, whereas CoS2 is sphere-like. The size of the CM-2 nanocomposite is much
smaller than that of either MoS2 or CoS2. Hence, the CM-2 nanocomposite has a large
surface area that provides plenty of active sites for an electrochemical reaction.
Fig. 5 shows the TEM images of the synthesized CM-2 composite. It can be seen
that the MoS2 sheets exhibit only a few layers located at the edge of CM-2 (Fig. 5a,b).
The interconnected nanosheets of MoS2 still maintain a flowerlike morphology, and
several CoS2 nanoparticles embedded among the flowerlike MoS2 nanosheets are
recognized in Fig. 5a (black area). The CoS2 nanoparticles serve as substrates for the
growth of MoS2 nanoplates and are highly likely to increase the conductivity of the
composites. The high-resolution TEM (HRTEM) images of CM-2 in Fig. 5 (c,d inset)
demonstrate that the interlayer spacing of MoS2 in the composite is 0.62 nm, and
interplanar spacing of the CoS2 sheets is 0.24 nm, which are both consistent with
previous reports [35,36].
To explore the potential applications of the CoS2@MoS2 nanocomposite for
supercapacitors, we have tested the electrochemical properties of the CM-1, CM-2
and CM-3 nanocomposite electrodes by cyclic voltammetry (CV) and galvanostatic
charge/discharge (GCD) measurements, as shown in Fig. 6a,b. The CV curves of
CM-1, CM-2 and CM-3 were obtained at a scan rate of 5 mV/s in the potential
window of 0.0-0.65 V versus SCE in 2 M KOH. Fig. 6b illustrates the
charge–discharge curves of CM-1, CM-2 and CM-3 in the potential range of 0–0.5 V
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at a current density of 1 A/g. The CM-2 nanocomposite shows the best charge storage
properties. The specific capacitance can be calculated by Eq. (1), and the specific
capacitance of the prepared CM-2 was calculated to be 1038 F/g, which is greater
than that of both CM-1 (508 F/g) and CM-3 (468 F/g). Next, CM-2 was taken as an
example for comparison with pure MoS2 and pure CoS2. As shown in Fig. 6c, the
significant increase in the integrated area indicates that the composite has a higher
capacity. For the curves of the CM-2 composite, the characteristic oxidation and
reduction positions (oxidation peak around +0.52 V and reduction peak around +0.24
V) are related to faradaic redox reactions between the Co2+ and Co3+ states of cobalt
in an alkaline KOH electrolyte [37,38]. The main reaction mechanism can be
described as follows,
ꢄꢅꢆꢇ + ꢈꢉ− ↔ ꢄꢅꢆꢇꢈꢉ + ꢉꢇꢈ + ꢊꢋ
MoSꢇ + ꢌꢍ + ꢊꢋ ↔ ꢎꢅꢆ − ꢆꢌꢍ
(6)
(7)
The supercapacitive characteristics can also be measured by a constant current
charge-discharge cycling test. Fig. 6d shows the GCD curves for MoS2, CoS2 and
CM-2 at a current density of 1 A/g. The specific capacitance values were calculated to
be 116.8 and 464.4 F/g for the MoS2 and CoS2, respectively, while the specific
capacitance of CM-2 composite attained 1038 F/g. Fig. 6e shows the specific
capacitances of the MoS2, CoS2 and CM-2 electrodes as a function of the current
density. The specific capacitance retention of CM-2 is higher than that of MoS2 and
CoS2, which can be up to 71.7% when the current density changes from 1 A/g to 10
A/g.
Typical impedance plots for this electrode are displayed in Fig. 7f. The
equivalent series resistance (Rs) of the MoS2, CoS2@MoS2 and CoS2 electrodes are
measured to be 0.56171, 0.34692 and 0.59254 Ω, respectively (Fig. 6f inset). The
charge transfer resistance (Rct) of the MoS2, CM-2 and CoS2 electrodes are 0.8686,
0.1303 and 0.46718 Ω, respectively. These results indicate that the conductivity of
CM-2 has increased significantly due to the existence of CoS2 [39,40]. The smaller Rs
and Rct values indicate a lower resistance and better capacitive behavior.
Cycle measurements were performed to further evaluate the electrochemical
performance of the MoS2, CoS2 and CM-2. The cycling stabilities of the MoS2, CoS2
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and the CM-2 electrodes were tested by repeating the GCD measurements between
0.0 and 0.5 V (vs. SCE) at a current density of 5 A/g for 10000 cycles. As shown in
Fig. 7, the capacitance retention values of MoS2, CoS2 and CM-2 in a 2 M KOH
electrolyte are 57.02%, 80.71% and 84.76% after 10000 cycles, which indicates that
CM-2 has the best cycling stability.
The excellent electrochemical properties of CM-2 are attributed to the synergistic
effect of MoS2 and CoS2: (1) Layered MoS2 forms ion-buffering reservoirs to improve
the ion diffusion rate within the prepared materials; (2) the nanoscale size of the
CM-2 composite increases the specific surface area and greatly shortens the diffusion
route through which both ions and electrons transfer during the charge-discharge
process; (3) the presence of MoS2 as a CoS2 substrate in the composite system not
only maintains a flower-like structure but also provides a path for the insertion and
extraction of ions within the composite to increase the reaction rate; and (4) the
conductive CoS2 nanoparticles could efficiently lower the internal resistance and
provide perfect continuous channels for electron transportation. Hence, the
electrochemical performance of CM-2 is better than that of pure MoS2 or CoS2 due to
the presence of two components with complementary properties.
4. Conclusions
In summary, the CoS2@MoS2 nanocomposite has been synthesized via a facile
one-step hydrothermal method. In contrast to pure MoS2 and pure CoS2, the specific
capacitance and rate capability of the CoS2@MoS2 (CM-2) nanocomposite has been
greatly enhanced. Furthermore, the CoS2@MoS2 (CM-2) nanocomposite displays an
excellent long-time cycling stability, retaining 84.76% of the initial capacitance after
10000 cycles at a current density of 5 A/g. These excellent results indicate that the
CoS2@MoS2 nanocomposite is a promising and suitable electrode material for
high-performance supercapacitors.
Acknowledgments
The authors would like to acknowledge the support by the National Natural Science
Foundation of China (Grant No. 51501221) and the China Postdoctoral Science
Foundation (No. 2016M591954).
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Figure Captions
Fig. 1 (a) XRD pattern of the CoS2@MoS2 (CM-2) nanocomposite. N2 sorption
isotherms of (b) MoS2; (c) CoS2@MoS2 (CM-2); and (d) CoS2. Inset: the pore size
distribution curve.
Fig. 2 XPS spectra of the CoS2@MoS2 nanocomposite: (a) survey; (b) Mo 3d; (c) Co
2p; and (d) S 2p.
Fig. 3 SEM images of a,d) CM-1, b,e) CM-2, and c,f) CM-3.
Fig. 4 SEM images of (a,b) pure MoS2; (c,d) pure CoS2; and the (e,f) CoS2@MoS2
nanocomposite.
Fig. 5 TEM (a,b) and HRTEM(c,d) of the CoS2@MoS2 nanocomposite.
Fig. 6 (a) Cyclic voltammograms curves of CM-1, CM-2 and CM-3. (b) Galvanostatic
charging/discharging curves of CM-1, CM-2 and CM-3. (c) Cyclic voltammograms of
MoS2, CoS2 and CoS2@MoS2. (d) Galvanostatic charging/discharging curves of MoS2,
CoS2 and CoS2@MoS2. (e) Rate capability curves of MoS2, CoS2 and CoS2@MoS2. (f)
Nyquist plots of MoS2, CoS2 and CoS2@MoS2 electrode.
Fig. 7 Cycling performance plots of MoS2, CoS2 and CoS2@MoS2.
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Figure 1
Figure 2
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Figure 3
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Figure 4
Figure 5
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Figure 6
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Figure 7
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Highlights:
1. A MoS2-coated CoS2 composite has been prepared successfully via a one-step
hydrothermal method.
2
.
The CoS2@MoS2 composite exhibits specific capacitance of 1038 F/g and rate
capability of 71.7%
.
3. The capacitance retention of CoS2@MoS2 composite equals 84.76% after 10000
cycles.