J. Electrochem. Sci. Eng. 9(1) (2019) 55-62; DOI: http://dx.doi.org/10.5599/jese.

572

Open Access: ISSN 1847-9286

www.jESE-online.org
Original scientific paper

In-situ synthesis of mesoporous carbon/iron sulfide

nanocomposite for supercapacitors
Xiaolong Yu, Bogang Li
College of Materials Science and Engineering, Sichuan University, Chengdu 610065, China

Corresponding author: E-mail libogang@scu.edu.cn
Received: August 7, 2018; Revised: October 20, 2018; Accepted: October 21, 2018

Abstract
Mesoporous C@FexSy composite as a negative electrode for supercapacitors was
synthesized via a one-step hydrothermal treatment followed by an electrodeposition
process and its electrochemical properties were studied. Compared with bare carbon
sphere, the electrochemical performance of C@FexSy composite was significantly improved,
with a high specific capacitance (267.45 F/g), good rate performance (201.08 F/g at 2.5
A/g), and superior cycling stability (almost no capacitance degradation after 1000 cycles).
The results show that the obtained C@FexSy composite is a promising negative electrode
material for supercapacitors.
Keywords
Carbonaceous materials; porous structure; hydrothermal treatment; electrochemical properties;
energy storage

Introduction
Supercapacitors have attracted tremendous attention in recent years owing to their high power
density, short charging time, and good stability [1–3]. Carbonaceous materials have been widely
adopted as negative electrode materials for supercapacitors because of their low cost, good
electrical conductivity, and long cycle lifetime. However, carbonaceous materials exhibit limited
double-layer capacitance, which limits further development of high energy density supercapacitors
[4,5]. One of the effective approaches to enhance the energy density of supercapacitors is to
improve the specific capacitance of carbonaceous materials [6–8].
Carbon spheres (CS) with their large surface area, high porosity, and fine pore size is a promising
carbonaceous material for supercapacitors. Nevertheless, the low capacitance of CS limits its
practical applications [9,10]. To improve the capacitance, researchers have recently focused on
developing new negative electrode materials by activation pretreatment, doping modification, and
loading of metal-base materials [5,11–13]. Among the metal-base materials, Fe-based materials are

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J. Electrochem. Sci. Eng. 9(1) (2019) 55-62 MESOPOROUS CARBON/IRON SULFIDE NANOCOMPOSITE

promising candidates because of their suitable working window in negative potential, abundance,
low cost, large theoretical capacitance, and nontoxicity [14–17]. Fe-based materials are combined
with carbonaceous materials to explore an effective method to prepare composites; this not only
could improve the rate performance of Fe-based materials, but also enhance the specific
capacitance of carbonaceous materials.
In this work, CS was grown in situ on nickel foam (NF) using a one-step hydrothermal process,
and iron sulfide was selected as Fe-based material. A new approach was found to synthesize
C@FexSy composite by using the cyclic voltammetry (CV) electrochemical deposition method to
introduce iron sulfide onto CS. The electrical performance of C@FexSy was investigated, and the
results indicated that the C@FexSy composite exhibited a significantly improved electrochemical
performance than bare CS. The findings of this study would open up new possibilities for the design
of carbon-based composites for high-performance supercapacitors.

Experimental
Materials
Commercial NF was used as the current collector. Anhydrous dextrose, FeCl3·6H2O, thiourea, and
the other reagents in this experiment were used without further purification. Deionized water was
used to prepare solutions.
Synthesis of carbon spheres
The CS was prepared by hydrothermal treatment. NF (2×4 cm2) was treated with hydrochloric
acid (3 mol/L), acetone, and absolute ethanol in an ultrasound bath for 10 min, respectively, and
then washed with deionized (DI) water. Anhydrous dextrose (10.8 g) was dispersed in 60 ml of DI
water and stirred at 25 C until a clear solution was obtained. The solution was transferred into a
100 ml Teflon-lined autoclave. A piece of NF was placed vertically in the Teflon-lined stainless-steel
autoclave, soaked in the solution, at 180 C for 4 h. Subsequently, CS precursors were formed on
the NF. Then, the product was washed with DI water followed by vacuum drying at 60 C for 12 h
and heat treatment at 800 C for 1 h in a N2 atmosphere to remove oxygen-containing groups on
the surface of the CS.
Synthesis of C@FexSy composite
C@FexSy was obtained through an electrochemical deposition method. The experiment was
carried out using an electrochemical workstation with a three-electrode cell. CS grown on NF was
used as the working electrode, and a platinum plate and a saturated calomel electrode (SCE) were
used as the counter electrode and the reference electrode, respectively. FeCl 3·6H2O (1, 1.5, 2, 2.5,
and 3 mmol) and thiourea (5 mmol) dissolved in 100 ml DI water was used as the deposition bath.
The deposition process was carried out via CV at 5 mV/s for 6 cycles in the voltage range of -1.2 to
0.2 V. The as-prepared C@FexSy composites with 1, 1.5, 2, 2.5, and 3 mmol of FeCl3·6H2O are
henceforth referred to as C@FexSy-1, C@FexSy-1.5, C@FexSy-2, C@FexSy-2.5, and C@FexSy-3,
respectively. The as-prepared C@FexSy-n (n = 1, 1.5, 2, 2.5, and 3) was then carefully rinsed with DI
water and dried in a vacuum oven at 60 C for 12 h.
Characterization and electrochemical measurements
The morphology and the microstructure were observed by scanning electron microscopy (SEM,
Hitachi, S-4800). The crystal structure was characterized using an X-ray diffraction (XRD, Rigaku)
system with Cu K irradiation. The chemical composition of the sample was investigated by X-ray

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X. Yu and B. Li J. Electrochem. Sci. Eng. 9(1) (2019) 55-62

photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific). The structure of C@FexSy-2
was further explored by Raman spectroscopy (Renishaw Invia RM200). The Brunauer-Emmett-Teller
(BET; Gemini VII) specific surface areas of the samples were determined from N 2 adsorption data in
the relative pressure ranging from 0.1 to 1.0.
Electrochemical characterization was carried out using a CHI 660D electrochemical workstation
(Chenhua, Shanghai) at 25 C. The electrochemical characterization includes CV, galvanostatic
charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS), using a three-electrode
cell with 3.0 M KOH as the electrolyte, materials grown on NF (1×1 cm2) as the working electrode,
Hg/HgO as the reference electrode, and Pt foil as the counter electrode. CV was performed in the
voltage window from -1 V to 0 V at scan rates of 5, 10, 20, 30, and 50 mV/s. GCD was carried out
between -0.95-0 V at current densities of 0.25, 0.5, 1.0, 1.5, 2.0, and 2.5 A/g. EIS was evaluated in
the frequency range of 105 to 0.01 Hz. The DC potential and the AC amplitude for EIS measurements
are -0.01 V and 5 mV, respectively.

Results and discussion

Morphology and structural analysis
The SEM images of CS and C@FexSy-2 are presented in Figure 1. Highly dense CSs are distributed
on the NF substrate (Figure 1a). These CSs exhibit smooth surfaces and they are connected with
each other. This provides high volumetric specific surface area and good mass transport property,
which are useful for supercapacitor application. The size distribution of the carbon particles is within
the range of 1-2 μm. Figure 1b shows that the C@FexSy-2 composite maintains the spherical
structure, and many FexSy nanoparticles are dispersed on the surface of the carbon particles. This
provides a large surface area for easy diffusion of the electrolyte toward the electrode surface and
contributes to the enhancement of specific capacitance.

Figure 1. (a) SEM image of the carbon spheres/NF. (b) SEM image of C@FexSy-2/NF.

The XRD patterns of CS and C@FexSy-2 (Figure 2a) showed similar characteristics. A broad
diffraction peak is observed at approximately 2 = 23 besides peaks of Ni, corresponding to (002)
plane of graphitic structure. The decreased peaks for C@FexSy-2 indicate the amorphous nature of
FexSy. Figure 2b shows the Raman spectrum of the CS and C@FexSy-2 samples. Both of them exhibit
two characteristic peaks at 1345 and 1597 cm-1, corresponding to D peak from amorphous structure
of carbon and G peak from graphitic structure of carbon, respectively[18,19]. The ID/IG ratio of
C@FexSy-2 is 0.97, a little higher than 0.90 of CS, which indicates the C@FexSy-2 has a small number
of defects. The XPS data of C@FexSy-2 are shown in Figure 2c-d. The peaks at approximately 723 eV

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J. Electrochem. Sci. Eng. 9(1) (2019) 55-62 MESOPOROUS CARBON/IRON SULFIDE NANOCOMPOSITE

(Fe 2p1/2), 713 eV (Fe 2p3/2) and 164 eV in the Fe 2p spectra and the S 2p spectra are assigned to
amorphous iron sulfide [20,21]. These results are in good agreement with the SEM observations and
further confirm the presence of C@FexSy-2 on the composite electrode.

Figure 2. (a) XRD patterns of CS/NF and C@FexSy-2/NF electrode. (b) Raman spectra of CS and
C@FexSy-2. (c)-(d) Fe 2p and S 2p XPS spectra of C@FexSy-2, respectively.

The nitrogen adsorption-desorption isotherms and Brunauer-Joyner-Halenda (BJH) pore

diameter distribution of C@FexSy-2 are shown in Figure 3. The sample exhibits a typical type IV
isotherm and shows a hysteresis loop at medium relative pressure (Figure 3a), which reveals the
existence of relatively mesoporous pores. The hysteresis loop exhibits H2 type characteristics,
indicating that the sample has a wide pore size distribution. The BET surface area of C@Fe xSy-2 is
117.74 m2/g. The BJH pore diameter distribution of C@FexSy-2 (Figure 3b) shows that the pore
diameter distribution is within the range of 5-10 nm, which further demonstrates that the sample
has a mesoporous structure. From the above results, it is evident that the C@FexSy-2 electrode
material has a high specific surface area, and the mesoporous structure could facilitate infiltration
of the electrolyte and shorten electron and ion transport distances during energy storage. This
would increase the surface utilization of the active material and improve the capacitance of the
electrode material.

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X. Yu and B. Li J. Electrochem. Sci. Eng. 9(1) (2019) 55-62

Figure 3. (a) BET of C@FexSy-2. (b) Pore diameter distribution of C@FexSy-2.

Electrochemical performance analysis

The electrochemical performance of the composites was estimated by CV and GCD
measurements. Figure 4a shows the GCD curves of CS and C@Fe xSy-n (n = 1, 1.5, 2, 2.5, and 3) at a
current density of 0.25 A/g. The charge/discharge curves are nearly linear in shape, indicating ideal
electrical double-layer capacitive behavior. The symmetric triangle characteristics reveal the excel-
lent electrochemical reversibility of the samples with high Coulombic efficiency. The C@Fe xSy-2
sample exhibited the longest charge-discharge time, and hence, the largest specific capacitance. The
specific capacitance derived from galvanostatic tests can be determined using the following
equation [22]:
It
C (1)
mV
where C is the specific capacitance, I is the discharge current (A), ∆t is the discharge time, m is the
mass of active materials loaded on the working electrode, and ∆V is the potential window.

Figure 4. (a) GCD curves of CS and C@FexSy-n (n = 1, 1.5, 2, 2.5, and 3) at a current density of
0.25 A/g. (b) Specific capacitances of CS and C@FexSy-n based on the GCD curves. (c) CV curves
of CS and C@FexSy-2 at a scan rate of 20 mV/s.

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J. Electrochem. Sci. Eng. 9(1) (2019) 55-62 MESOPOROUS CARBON/IRON SULFIDE NANOCOMPOSITE

As plotted in Figure 4b, all the C@FexSy samples exhibit a higher specific capacitance than the CS
electrode (158.01 F/g); C@FexSy-2 has the highest specific capacitance of 267.45 F/g at 0.25 A/g. It
is considered that the loading mass of FexSy affected the capacitance, and C@FexSy-2 has the
optimum loading mass. The reason might be that FexSy deposited on the surface of CS provides a
porous structure, which further increases the specific surface area of the electrode material and
thus enhances the capacitance. However, owing to the poor conductivity of Fe-base materials, when
the loading mass of FexSy further increases, the capacitance of the electrode material decreases.
Figure 4c shows the CV curves of CS and C@FexSy-2 at a scan rate of 20 mV/s. As expected, the
C@FexSy-2 composite shows a much higher capacitance than CS.
The electrochemical performance of C@FexSy-2 for supercapacitor applications was investigated
in KOH solution with a three-electrode system. As shown in Figure 5a, CV test was carried out at
various scan rates of 5-50 mV/s. The quasi-rectangular shape of the CV curves with an obvious hump
shape indicates the effect of electrical double-layer capacitance. Moreover, the rectangular shape
of the curve even at a high scan rate of 50 mV/s suggests the outstanding capacitive behavior and
high ionic conductivity of C@FexSy-2. The inconspicuous redox peaks may be induced by FexSy. These
results demonstrate that C@FexSy-2 provides numerous accessible pores and paths for ion transfer.
Figure 5b-c show the discharge curves of C@FexSy-2 and CS at different current densities. All the
curves exhibited excellent linear response, implying the ideal electric double-layer capacitive
behavior of the samples. It is evident from Figure 5d that C@FexSy-2 has a much higher capacitance
than CS at all current densities. Remarkably, the C@FexSy-2 electrode exhibited capacitance
retention of 75 % when the current density was increased from 0.25 to 2.5 A/g, indicating its
outstanding rate capability.
The cycling stability of C@FexSy-2 was evaluated for 1000 cycles at a high charging-discharging
current density of 2.5 A/g. As shown in Figure 5e, the C@FexSy-2 electrode does not show
capacitance degradation, and the charge/discharge curves in the last ten cycles are almost identical
to those in the first ten cycles. After 1000 cycles, the capacitance retention of the C@Fe xSy-2
electrode is even 103 % of the initial capacitance, indicating its superior cyclic performance. The
reason for the slight increase in the capacitance may be the gradual entry of the electrolyte into the
inner micropores of the C@FexSy-2 electrode after several cycles, which increases the contact area
of the electrode and electrolyte, thus enhancing the capacitance.
EIS was used to gain a deep understanding of the rate capability of the C@Fe xSy-2 electrode.
Figure 5f shows the Nyquist plots of C@FexSy-2 and CS. It can be seen that the Nyquist plot is mainly
composed of two parts: a semicircle in the high frequency range and a vertical line in the low
frequency region. The inset in Figure 5f shows the magnified portion of the Nyquist plot near the
origin, providing more details on the electrode impedance behavior. The x-intercept of the high
frequency semicircle provides the value of equivalent series resistance (Rs) of the electrochemical
system [23,24]. It is observed that the Rs values of C@FexSy-2 and CS are 0.60 and 0.71 Ω,
respectively. The diameter of the high frequency semicircle corresponds to the charge transfer
resistance (Rct). The Rct of C@FexSy-2 and CS was calculated to be 0.12 and 0.50 Ω, respectively. As
for C@FexSy-2, the low resistances are associated with FexSy nanoparticles on the carbon substrate,
which provide a larger electrolyte/electrode interface, facilitating the electrode material wetting
and the ion transfer. The Nyquist plot of C@FexSy-2 exhibits an obvious Warburg 45° line region,
corresponding to the semi-infinite ion diffusion resistance, which reflects a lower ion diffusion rate
within the porous C@FexSy-2 electrode [25,26]. In the low-frequency region, the straight line
represents the good capacitive behavior of the electrode and the more parallel to the virtual axis,

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the closer to the double-layer capacitance [27]. The Nyquist plot in the low frequency range of
C@FexSy-2 shows a less vertical line, which is an indication that C@Fe xSy-2 has diffusion-controlled
faradaic processes due to the higher contribution of Warburg impedance [28-30].

Figure 5. (a) CV curves of C@FexSy-2 at different scan rates. (b)-(c) GCD curves of C@FexSy-2
and CS at different current densities, respectively. (d) Specific capacitance of C@FexSy-2 and CS
at different discharge current densities. (e) Cycling performance of the C@FexSy-2 electrode at
a current density of 2.5 A/g. (f) Electrochemical impedance spectra of C@FexSy-2 and CS.

Conclusions
In this study, a mesoporous negative electrode C@FexSy composite was successfully fabricated
on NF using a hydrothermal method followed by an electrodeposition treatment. The
electrochemical performance of the C@FexSy-2 electrode was significantly higher than that of bare

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 J. Electrochem. Sci. Eng. 9(1) (2019) 55-62 MESOPOROUS CARBON/IRON SULFIDE NANOCOMPOSITE

CS. The hybrid electrode demonstrated a high specific capacitance (up to 267.45 F/g), good rate
performance (201.08 F/g at 2.5 A/g), and superior cycling stability (almost no capacitance
degradation after 1000 cycles). The excellent electrochemical behavior could be attributed to the
unique hybrid electrode design and the enhancement in the surface area. The present study
provides an adaptable method for novel design of negative electrode materials for supercapacitors,
extending the potential applications of carbon-based devices.
Acknowledgements: The authors greatly appreciate the support of Xinyue Huang, Xingye Tong, and
Xiaoming Tu.

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