Controllable synthesis of hollow spherical nickel chalcogenide (NiS2and NiSe2) decorated with graphene for efficient supercapacitor electrodes

Article abstract of DOI:10.1039/d0ra10659c

New carbon-loaded nickel chalcogenide electrode materials (NiS₂/GO and NiSe₂/rGO) have been synthesized through an easy-to-operate process: NiSe₂was obtained based on NiS₂hollow spheres, and was successfully synthesized withl-cysteine assistance under the hydrothermal method at 120 °C. GO of different mass fraction was added together withl-cysteine. The electrochemical performance of NiS₂/GO and NiSe₂/rGO has been greatly improved because the formation of a carbon-loaded layer effectively increased the specific surface area and reduced the charge transport resistance. Compared with pure NiS₂and NiSe₂, NiS₂/GO and NiSe₂/rGO presented much better specific capacitance (1020 F g^?1and 722 F g^?1respectively at a current density of 1 A g^?1) and more superior rate capability (when the current density was raised to 5 A g^?1the specific capacitance remained at 569 F g^?1and 302 F g^?1). This work highlights the advantages of nickel compounds through a very simple experimental method, and contributes to providing a good reference for preparation of superior supercapacitor materials with high performance.

Full text of DOI:10.1039/d0ra10659c

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Controllable synthesis of hollow spherical nickel
chalcogenide (NiS2 and NiSe2) decorated with
graphene for efficient supercapacitor electrodes†
Cite this: RSC Adv., 2021, 11, 11786
a
a
a
Min Lu, Ming-yuan Sun,a Xiao-hui Guan,
Xue-mei Chen
*
*
b
*
and Guang-Sheng Wang
New carbon-loaded nickel chalcogenide electrode materials (NiS2/GO and NiSe2/rGO) have been
synthesized through an easy-to-operate process: NiSe2 was obtained based on NiS2 hollow spheres, and
was successfully synthesized with L-cysteine assistance under the hydrothermal method at 120 ꢀC. GO
of different mass fraction was added together with L-cysteine. The electrochemical performance of NiS2/
GO and NiSe2/rGO has been greatly improved because the formation of a carbon-loaded layer
effectively increased the specific surface area and reduced the charge transport resistance. Compared
with pure NiS2 and NiSe2, NiS2/GO and NiSe2/rGO presented much better specific capacitance (1020 F
gꢁ1 and 722 F gꢁ1 respectively at a current density of 1 A gꢁ1) and more superior rate capability (when
the current density was raised to 5 A gꢁ1 the specific capacitance remained at 569 F gꢁ1 and 302 F gꢁ1).
This work highlights the advantages of nickel compounds through a very simple experimental method,
and contributes to providing a good reference for preparation of superior supercapacitor materials with
high performance.
Received 22nd December 2020
Accepted 9th March 2021
DOI: 10.1039/d0ra10659c
rsc.li/rsc-advances
and cycle stability of energy storage device and further improve
the properties.
1 Introduction
As we all know, electrode materials, as the core component
of the device, play a decisive role in the development and
commercial application of supercapacitors. Moreover, their
composition, morphology and structure have a crucial impact
on the electrochemical performance of the materials in the
design of electrode materials.12–14 Nanostructured transition
metal chalcogenide, as electrode materials, are considered to be
inorganic functional materials with application prospects due
to the charge storage through Faraday reaction of metal ions.15
Nevertheless, a series of single nanomaterials with different
compositions and morphologies, have mostly been explored at
the research level as pseudo capacitors (PCs) electrode materials
in labs rather than at the industrial level for practical applica-
tion, due to their rapid degradation and low capacity retention
in high reversible ion adsorption or rapid redox reactions.16 For
instance, Pang etc. prepared nickel oxide nanostructures with
With the decline of fossil energy, there has been a positive
exploitation of alternative energy sources and high efficiency
energy storage devices.1,2 Supercapacitors (SCs),3,4 also called
ultracapacitors or electrochemical capacitors, store electrical
charge on high-surface-area conducting materials. As new
energy storage components, SCs are different from traditional
capacitors and batteries because of their advantages, including
high power density, high specic capacity, quick charge and
discharge, long cycle life and high safety capacity.5–9 Therefore,
SCs have a good application prospect in the elds of high-power
and high-energy power supply, clean energy, smart grid, trans-
portation, wireless communication, aerospace, military, elec-
tronic equipment and so on. However, the relatively low energy
density of supercapacitors seriously limits their commercial
application.10,11 At present, research teams at home and abroad
mainly develop new electrode materials, electrolytes and
asymmetric super capacitors to maintain high power density
different lengths and NiO nanowires with the longest length
have the largest specic capacitance of 180 F gꢁ1
.
Sun etc.
17
prepared NiS2 nanospheres via
a facile one-step poly-
vinylpyrrolidone assisted method which delivers a high revers-
ible specic capacity of 692 mA h gꢁ1
porous NiCo2O4 hollow spheres which showed specic capacity
as high as 183 C gꢁ1 19 Li etc. prepared graphene/NiS2 composite
.
Mondal etc. produced
18
aSchool of Chemical Engineering, Northeast Electric Power University, Jilin 132000, P.
R. China
bKey Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry
of Education, School of Chemistry, Beihang University, Beijing 100191, P. R. China.
E-mail: wanggsh@buaa.edu.cn
.
through a template-free solvothermal reaction using graphene
oxide shows a great value of 478.1 F gꢁ1 at a current density of
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10659c
0.5 A gꢁ1
.
However, it is noteworthy that the hollow spheres
20
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suldes with the large internal space are more likely showing
excellent physical and chemical properties such as low density,
large effective area and good mass permeability to have more
active sites and faster charge transfer ability and further
improve the electrochemical properties of materials.21,22
2.1.2 Preparation of NiS2 and NiS2/GO hollow spheres. All
of the chemicals are analytical grade and used without further
purication. In a typical synthesis,36 1 mmol Ni(NO3)2$6H2O
and 1 mmol urea were dissolved in 20 mL of deionized water
with magnetic stirring, then 4 mmol L-cysteine and 50 mL
deionized water were added. L-Cysteine is the source of sulfur.
Aer ultrasonic treatment, the homogeneous suspension was
transferred into a 100 mL teon-lined stainless steel autoclave
Compared with single nickel chalcogenide, nanocomposites
can make full use of the synergistic and complementary effects
among components, which makes them have greater advan-
tages in application.23–25 Among them, for the carbon based
transition metal compound composite, the introduction of
carbon material can improve the overall conductivity of the
material on the one hand, and effectively inhibit the agglom-
eration of small-size active substances, so that the composite
has a larger specic surface area, fully contact with the elec-
trolyte, thus improving its charge transfer and storage
capacity.26,27 On the other hand, the excellent structure and
chemical stability of carbon materials can buffer the volume
change of materials during charging and discharging, and
improve the cyclic stability of materials.28–30 When nickel
suldes/graphene composites are used as pseudocapacitor
electrode materials, the graphene can not only provide an
elastic space to buffer the volume change of the electrode
materials during the repetitive charge/discharge process, but
also facilitate the faster electron transport, resulting in an
enhanced electrochemical performance.31,32 It has been
demonstrated that the solvothermal synthesized NiS/graphene
composite33,34 and Ni3S2/N-doped graphene composite35
exhibited the enhanced electrochemical performances in
comparison with the NiS and Ni3S2. In particular, when
combined with graphene, the specic capacitance of NiO
increased from 47.2 F gꢁ1 to 187.53 F gꢁ1 at 10 mV sꢁ1 scan rate.
And the discharge–charge cycling stability of the Ni3S2@N-G
ꢀ
for solvothermal reaction at 120 C for 24 h. The black precip-
itates of hollow spherical NiS2 were washed several times with
distilled water and absolute ethanol and dried at 60 ꢀC for 12 h
for further characterization. To obtain NiS2/GO nano-
composites, GO of ten percent (10 wt%), een percent
(15 wt%), twenty percent (20 wt%), twenty ve percent (25 wt%)
and thirty percent (30 wt%) was added to the above mixed
solvent together with 4 mmol L-cysteine. To distribute GO evenly
in the solution, an additional stirring time of 30 minutes was
added. The following processing steps are consistent with the
method used to prepare NiS2.
2.1.3 Preparation of NiSe2 and NiSe2/rGO hollow spheres.
The 1 mmol above-prepared hollow spherical NiS2 was stably
dispersed in 40 mL of deionized water under magnetic stirring.
2 mmol H2SeO3 and 5 mL of N2H4$H2O were added in this
solution, and transferred to a 100 mL teon-lined stainless steel
ꢀ
autoclave and heated at 140 C for 24 h, then the solution was
cooled to room temperature. The products of NiSe2 were ob-
tained aer washing and drying. In composite preparation, GO
of one percent (1 wt%), three percent (3 wt%), ve percent
(5 wt%) and seven percent (7 wt%) was added to the above
mixed solvent together with N2H4$H2O. The following pro-
cessing steps are consistent with the method used to prepare
NiSe2 except the addition of graphite oxide.
and bare Ni3S2 was examined at
a current density of
50 mA gꢁ1 between 0.01 and 3.0 V delivers initial discharge–
charge capacities of 1739 and 729 mA h gꢁ1 respectively.
Herein, in this work, we prepared a series of controllable
morphology hollow spheres NiS2 and microspheres NiSe2
composited with graphene by a simple and effective hydro-
thermal process by adjusting the graphene oxide concentration,
and the result composites were characterized by XRD (X-ray
diffraction pattern), SEM (scanning electron microscopy),
FTIR (Fourier transformed infrared spectroscopy), TG (thermo-
gravimetric analysis) and XPS (X-ray photoelectron spectros-
copy). The electrochemical performances of NiS2/graphene
oxide (GO) composite and NiSe2/reduced graphene oxide (rGO)
composite were furtherly researched and compared in detail
based on cyclic voltammetry (CV), galvanostatic charge–
discharge (GCD) and electrochemical impedance spectroscopy
(EIS) techniques.
2.2 Characterization
The morphology and size of the products were obtained using
scanning electron microscopy (SEM) with a JSM-6510A micro-
scope and by sputtering with gold. The samples were examined
by X-ray diffraction (XRD-7000), recorded using a Shimadzu X-
ray powder diffractometer with Cu-Ka radiation (l ¼ 0.15405
nm). X-ray photoelectron spectroscopy (XPS, ESCALAB-250) with
an Al-Ka radiation source were used. Chemical bonding infor-
mation of the studied samples was gathered with Fourier
transformed infrared spectroscopy (FTIR, Nicolet iS50). Ther-
mogravimetric analysis (TG) was carried out using Met-
tlerToledo TGA/DSC1 apparatus.
2.3 Test of electrochemical properties
The electrochemical properties of the prepared-samples were
tested by an electrochemical workstation (Parstat 4000) under
a conventional three-electrode system in the electrolyte of 2.0 M
KOH aqueous solution (i.e. a saturated Hg/HgO reference elec-
trode, a platinum counter electrode and a working electrode).
2 Experimental
2.1 Sample preparation
2.1.1 Preparation of graphene oxides. Graphene oxides Among them, the working electrode was prepared using the
(GO) were synthesized via a modied Hummers method using active materials by the following procedure: the prepared active
pristine graphite powders as raw materials. The detail proce- material, acetylene black and a polytetrauoroethylene (PTFE)
dure are described in ESI.†
emulsion were mixed in ethanol with a weight ratio of 8 : 1 : 1;
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the slurry was coated on a nickel foam (1.0 ꢂ 1.0 cm2) current rGO. Fig. 1d and S2† show the corresponding SEM image of
collector and then pressed at 8 MPa for 30 s, and dried under NiSe2 decorated with rGO. It can be noticed that the more gra-
vacuum at 60 ꢀC for 12 h. Finally, the mass of the active loading phene is introduced, the greater thickness of the nanosheets of
on the nickel foam was about 10 mg. The average specic rGO is. It can be concluded that nickel chalcogenides on the
capacitance of the electrodes was calculated by eqn (1) based on surface of the graphene sheets can be used as a separator, which
the cyclic voltammogram curves.
could effectively prevent the aggregation of the spheres and the
restacking of the graphene oxide sheet to a certain extent.
However, with the increase of graphene oxide concentration,
agglomeration becomes more and more serious, which is also
one of the reasons for the decline of electrochemical
performance.
Ð
V
IðVÞdV
V0
C ¼
(1)
mvðV ꢁ V0Þ
where C (F gꢁ1), m (g), v (V sꢁ1), V–V0 (V), and I (V) are the specic
capacitance, mass of the active material on the working elec-
trode, potential scan rate, potential range and current density,
respectively. Besides, based on the galvanostatic charge–
discharge curves, the average specic capacitance of the elec-
trodes was calculated according to eqn (2).
To study the crystal structures of all samples, the XRD
patterns have been provided in Fig. 2 and S3.† It shows XRD
pattern of hollow spherical NiS2 and the diffraction peaks at
angles of 27.3ꢀ, 31.8ꢀ, 35.4ꢀ, 38.8ꢀ, 45.2ꢀ, 53.4ꢀ, 58.5ꢀand 61.1ꢀ
can be indexed to the (111), (200), (210), (211), (220), (311), (023),
(321) planes, respectively, which is a pure cubic phase NiS2
(JCPDS card no. 89-1495). The XRD pattern of NiSe2 all the
diffraction peaks can be indexed to the cubic phase NiSe2
(JCPDS card no. 11-0552). Because of no peaks for other impu-
rities in the above-mentioned XRD patterns, NiS2 and NiSe2
with the high purity and good crystallinity are synthesized
under the current experimental conditions.38 Moreover, the
XRD patterns of nickel nanocomposites decorated with GO are
almost the same as that of hollow spherical nickel chalcogen-
ides (Fig. 2 and S3†). As we all known, the diffraction peak
appears at 2q ¼ 10.6ꢀ corresponding to the (001) plane of GO.
Therefore, in theory the absence of the X-ray diffraction peak for
GO around 10.6ꢀ in these nanocomposites indicates that GO
can be reduced to rGO at 2q ¼ 26.5ꢀ effectively. However, this
characteristic peak of GO and rGO is not observed in the XRD
pattern of both 20%-NiS2/GO and 3%-NiSe2/rGO. It may be that
the strength of Ni peak is too high or the content of graphene is
too less to observe the broad peak of rGO.39
I ꢂ Dt
CS ¼
(2)
DV ꢂ m
where CS (F gꢁ1), I (A), Dt (s) and DV (V) are the specic capac-
itance, discharge current, discharge time and potential range of
discharge, respectively.
3 Results and discussion
3.1 Morphology and structure analysis of samples
As well shown in Fig. 1a and b, as-synthesized NiS2 and NiSe2
are of hollow spherical structure, which are consistent with our
previous report.37 And the diameters of these hollow spheres are
approximately 2.33 mm. Aer introducing GO (percentage
content like 20% and 3% was the amount of graphene oxide
added in the preparation process), from Fig. 1c and S1,† it was
found that nickel chalcogenides NiS2 coated by GO still retains
the hollow spherical structure. Meanwhile, the NiS2 hollow
spheres could effectively avoid agglomeration of GO nano-
sheets. The crinkled and exible GO surface is benecial to
increasing interface areas between GO sheets and NiS2 hollow
spheres. Due to the addition of hydrazine hydrate in the
synthesis process of NiSe2, the GO was chemically reduced to
Therefore, FT-IR spectrum was performed to certify the
existence of graphene. As shown in Fig. S4,† the stretching
vibration peak at 3400 cmꢁ1 for nanocomposites corresponds to
the hydroxyl group. The peaks of 20%-NiS2/GO at 1730 cmꢁ1
and 1644 cmꢁ1 are symmetric and antisymmetric stretching
vibrations of carboxyl groups in GO. And the stretching vibra-
tion peak at 700 cmꢁ1 correspond to C–O and C–O–C also
indicates the oxidation degree of graphite is ideal and it
contains oxygen-containing functional groups. Nevertheless,
Fig. 1 SEM images of as-prepared samples (a) NiS2; (b) NiSe2; (c) 20%-
NiS2/GO; (d) 3%-NiSe2/rGO.
Fig. 2 XRD patterns of the synthesized products.
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most of the oxygen-containing functional groups in the 3%-
NiSe2/rGO prepared by hydrazine hydrate reduction dis-
appeared and only a small amount of epoxy group existed,
indicating that GO was reduced to rGO.
The surface elemental composition and chemical state of the
nickel nanocomposites decorated with GO are investigated by X-ray
photoelectron spectroscopy (XPS). The survey spectrum (Fig. S5a†)
shows that the 20%-NiS2/GO and 3%-NiSe2/rGO are composed of
Ni, O, C, S and Ni, O, C, Se elements, respectively, without other
impurities. The high-resolution XPS spectrum of Ni 2p (Fig. S5b†)
illustrates two shake-up satellites and two characteristic peaks,
which can be assigned to Ni 2p3/2 and Ni 2p1/2 orbitals of Ni2+.
Furthermore, the spectrums of S 2p (Fig. S5c†) and Se 3d
(Fig. S5d†) display the presence of (S2)2ꢁ and Se2ꢁ. In Fig. 3, it can
be obviously found from the C 1s spectra that each of the two
composite materials has ve types of functional groups. Among
them, 20%-NiS2/GO has O–C]O (291 eV), C]O (288 eV), C–O
(286.8 eV), sp3 C–C (285.8 eV) and sp2 C]C (284.5 eV), while 3%-
NiSe2/rGO has C]O (288.4 eV), C–O (285.9 eV), sp3 C–C (284.8 eV),
sp2 C]C (284.2 eV) and C (graphite) (283.3 eV) respectively. Due to
the existence of reducing agents during the synthesis of NiSe2/rGO,
the strength of the oxygen-containing functional groups (C]O and
Scheme 1 Schematic description of the synthetic process of NiS2,
NiSe2 and their complexes.
H2NCONH2 + 3H2O / 2NH4+ + 2OHꢁ + CO2
ꢁ
Ni(NO3)2 / Ni2+ + 2NO3
Ni2+ + 2H2S + 2OHꢁ / NiS2 + H2O + H2
3.2 Electrochemical performances
C–O) are signicantly lower than that of NiS2/GO, as indicates that The electrochemical properties of NiS2, NiS2/GO, NiSe2 and
a reduction process occurs. NiSe2/rGO were evaluated using CV and GCD techniques using
The possible synthesis mechanism of the nickel chalcogen- a three-electrode cell in 2 M KOH aqueous electrolyte. Fig. S6
ides decorated with GO is proceeded as follows (Scheme 1). and S7† shows the typical CV curves of NiS2/GO and NiSe2/rGO
Firstly, Ni(NO3)2 is used as the nickel source and L-cysteine is at different scan rates with a potential window ranging from 0 V
used as the sulfur source which provides a chemical chain to 0.45 V, respectively. The CV measurements of NiS2 and NiSe2
mercaptan (–SH) with a strong tendency to coordinate with were also carried out for comparison. In Fig. S6 and S7,† each
inorganic ions and form Ni2+–L-cysteine complex. In this process, pair of peaks are visible in each voltammogram. In the cathodic
two complexes of Ni2+–L-cysteine react to form the original scanning, a small amount of charges would be storaged in the
nucleus of NiS2. Urea provides an alkaline medium through interphase between the electrode material and electrolyte,
hydrolysis during the self-assembly process. Meanwhile, GO can which occurs at different peaks. In the reverse anodic scanning,
be added to synthetise the homologous composites by the the stored charge would be released, which displays the
common thermal treatment of the as-prepared NiS2. While different peaks. When the scan rate is increased, the shapes of
hollow spherical NiSe2 is obtained by secondary hydrothermal the curves are maintained and the peaks current increase. The
process with a strong reductant N2H4$H2O, leading to the CV curves of all electrodes involving the reversible redox reac-
reduction of Se4+ and the substitution for S2ꢁ. Then we add GO tions are expected to exhibit good quasi-capacitance. We choose
based on as-prepared NiS2. In the hydrothermal process, hydra- three different weight ratios of GO and rGO to highlight the
zine hydrate reduces Se4+ to Se2ꢁ as well as GO to rGO.
differences of each electrode materials and the others are
The reaction route for the synthesis of NiS2 could be shown in ESI.† Fig. 4(a) and (b) show CV curves of NiS2 and
expressed as the following chemical equations according to ref. NiSe2 compared with their composites with different weight
40 and 41
:
ratios of rGO respectively at a scan rate of 10 mV sꢁ1, where the
well symmetry of all CV curves implying the high rate capacity.42
More importantly, form the larger integrated area of the CV
curves means the higher capacitance performance of the elec-
trode material in the line with eqn (1). Thereby, this work clearly
demonstrates different samples capacitance characteristics
under the same condition. Especially, NiS2 with 20 wt% of GO
(20%-NiS2/GO) and NiSe2 with 3 wt% of rGO (3%-NiSe2/rGO)
show the higher integrated area, implying the higher specic
capacitance, while the other composites exhibit the worse
pseudo-capacitance among these materials.
HSCH2CHNH2COOH + H2O / CH3COCOOH + NH3 + H2S
To further evaluate the electrochemical performances of
these products, GCD tests at different current densities are
conducted as shown in Fig. S8 and S9.† In order to make
a striking contrast, Fig. 4(c) and (d) shows the discharge proles
Fig. 3 XPS spectra for (a) C 1s of 20%-NiS2/GO; (b) C 1s of 3%-NiSe2/
rGO.
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with the increase of current density, the specic capacitance
retentions of NiS2, 20%-NiS2/GO, NiSe2 and 3%-NiSe2/rGO are
28.8%, 31.8%, 28.7% and 28.6%. The above results suggest that
the composites not only own higher charge storage capacity but
also own the higher rate capacity as the raw material. There
have been many recent studies on energy density.44–46 If the
energy density level is high, even an overall enhancement of
energy performance can be achieved. Among them, the energy
density of 20%-NiS2/GO is 28.7 W h kgꢁ1 and 3%-NiSe2/rGO is
20.3 W h kgꢁ1 at current density of 1 A gꢁ1. Fig. S10(a)† shows
the cyclic performance of these electrode materials at a current
density of 1 A gꢁ1 and the voltage range was from 0 V to 0.45 V.
The specic capacitance of NiS2 and 20%-NiS2/GO dropped
sharply in the rst 20 cycles, from 387 F gꢁ1 and 1020 F gꢁ1
declined to 144 F gꢁ1 and 511 F gꢁ1, respectively. And the
specic capacitance of NiSe2 and 3%-NiSe2/rGO dropped
sharply in the rst 20 cycles, from 333 F gꢁ1 and 722 F gꢁ1
declined to 202 F gꢁ1 and 378 F gꢁ1, respectively. The rapid
decrease of specic capacitance may be owing to the trans-
formation of the structure which was caused by the volume
expansion and contraction in the process of redox reactions.47
According to Fig. S10(b)† the retention rates of NiS2, 20%-NiS2/
GO, NiSe2 and 3%-NiSe2/rGO were 17.3%, 13.1%, 20% and
18.4% aer 100 cycles, respectively. Under the condition that
the capacitance retention rate of the composite product is
maintained at about the same level, the specic capacitance is
greatly improved.
Fig. 4 (a) and (b) CV curves of the four electrodes at a scan rate of
10 mV sꢁ1; (c) and (d) GCD curves of the four electrodes at a current
density of 1 A gꢁ1; (e) and (f) specific capacitances of the four elec-
trodes at various current densities.
To further illustrate the difference in electrochemical
performance between these four materials, we performed an
electrochemical impedance spectra (EIS) experiments. Fig. 5
demonstrates the Nyquist plots of the four electrodes to illus-
trate their impedance characteristics. In general, the Nyquist
diagram of electrode materials for redox supercapacitors should
include a semicircle related to the Faraday reaction in the high
frequency region and a straight line related to the Warburg
impedance in the low frequency region.48 From the equivalent
circuit model, it can be seen that the electrode systems con-
tained electrolyte solution resistance (Re), the faradaic charge
transfer resistance associated with the electron transfer (Rct),
Warburg impedance resistance in ions diffusion process (Zw)
and the double layer capacitance at the electrode/electrolyte
of the NiS2 and NiSe2 in contrast with their composites
respectively at a current density of 1 A gꢁ1, which agree well with
their CV curves traits. The obvious plateau regions demonstrate
faradic behaviors in the GCD curve of these electrodes, which is
caused by redox reactions. The specic capacitance calculations
display that samples NiS2 and its composites with 10 wt%,
20 wt%, 30 wt% of GO are 384 F gꢁ1, 661 F gꢁ1, 1020 F gꢁ1 and
600 F gꢁ1, respectively. 20%-NiS2/GO shows the highest specic
capacitance among these products. NiSe2 and its composites
with 1 wt%, 3 wt%, 5 wt% of rGO are 333 F gꢁ1, 386 F gꢁ1, 722 F
gꢁ1 and 672 F gꢁ1, and 3%-NiSe2/rGO is the highest. Moreover,
the specic capacities of the NiS2/rGO and NiSe2/rGO compos-
ites are calculated at current densities of 0.5, 1, 2 and 5 A gꢁ1
according to eqn (2) respectively,43 and the calculated capaci-
tances of the composites at corresponding current densities are
shown in Fig. 4(e) and (f). As the current density is increased to
5 A gꢁ1, the specic capacitances of NiS2 and its composites are
142 F gꢁ1, 284 F gꢁ1, 568 F gꢁ1and 302 F gꢁ1, respectively. NiSe2
and its composites are 106 F gꢁ1, 142 F gꢁ1, 302 F gꢁ1 and 248 F
gꢁ1. And when the current density is decreased to 0.5 A gꢁ1, the
specic capacitances of NiS2 and its composites increase to
495.56 F gꢁ1, 1010 F gꢁ1, 1790 F gꢁ1 and 737 F gꢁ1. NiSe2 and its
composites are 371 F gꢁ1, 387 F gꢁ1, 1056 F gꢁ1 and 868 F gꢁ1. In
conclusion, the specic capacities of the NiS2/GO and NiSe2/
rGO composites have an obvious improvement and show
conspicuously better electrochemical performances compared
with NiS2 and NiSe2, for which the reason is closely related to
the combination of NiS2 and GO or NiSe2 and rGO. In addition,
Fig. 5 Nyquist plots of NiS2, 20%-NiS2/GO, NiSe2 and 3%-NiSe2/rGO
electrodes, the inset is equivalent circuit model.
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interface (CPE). 49The impedance parameters could be matched
by Zview soware. The Rct of the NiS2, 20%-NiS2/GO, NiSe2 and
3%-NiSe2/rGO electrodes are 0.29 U, 0.11 U, 0.66 U, 0.22 U,
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4 Conclusion
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Conflicts of interest
There are no conicts to declare.
24 X. Li, J. Shen, N. Li and M. Ye, Mater. Lett., 2015, 139, 81–85.
25 F. Cai, R. Sun, Y. Kang, H. Chen, M. Chen and Q. Li, RSC
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Acknowledgements
This project was nancially supported by the National Natural
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Science and Technology Research Projects of the Education Electrochim. Acta, 2013, 111, 937–945.
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