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Cite this: Nanoscale, 2016, 8, 11787
Received 11th February 2016,
Accepted 24th April 2016
DOI: 10.1039/c6nr01200k
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Amorphous MoSx thin-film-coated carbon fiber
paper as a 3D electrode for long cycle life
symmetric supercapacitors†
Suresh Kannan Balasingam,‡a,b Arun Thirumurugan,c Jae Sung Leed and
Yongseok Jun*b
Amorphous MoSx thin-film-coated carbon fiber paper as a binderfree 3D electrode was synthesized by a facile hydrothermal
method. The maximum specific capacitance of a single electrode
was 83.9 mF cm−2, while it was 41.9 mF cm−2 for the symmetric
device. Up to 600% capacitance retention was observed for 4750
cycles.
Supercapacitors (also known as electric double layer capacitors
or ultracapacitors) are a kind of electrochemical energy-storage
device, with a high power density, fast charge–discharge rate,
high capacitance, and superlong cycle life.1,2 Various carbonbased materials have been used in commercial supercapacitors
with high power density owing to the electric double layer
charge (EDLC)-storage mechanism. However, the energy
density value of such supercapacitors is still much lower than
those of commercial lithium ion batteries.3 In order to
increase the energy density, various pseudocapacitive materials
such as electrically conducting polymers, transition metal
oxides (TMOs), transition metal carbides (TMCs), transition
metal nitrides (TMNs), and transition metal dichalcogenides
(TMDCs) have been extensively investigated.4–6
Molybdenum disulfide (MoS2), a class of TMDCs, is one of
the most studied materials because of the following advantages: excellent electrocatalytic activity (next to noble metals,
as evidenced by the volcano curve), a 2D layered crystal structure that facilitates ion intercalation (Li+ or other alkali metal
ions), high surface area, and higher electronic conductivity
a
Department of Chemistry, School of Molecular Sciences, Ulsan National Institute of
Science and Technology (UNIST), Ulsan 44919, Republic of Korea
b
Department of Materials Chemistry & Engineering, Konkuk University, Seoul 05029,
Republic of Korea. E-mail: yjun@konkuk.ac.kr; Tel: +82-2450-0440
c
Institute of Physics, Bhubaneswar 751 005, India
d
School of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology (UNIST), Ulsan 44919, Republic of Korea
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
C6NR01200K
‡ Present Address: Department of Materials Science and Engineering, Faculty of
Natural Sciences and Technology, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway. E-mail: suresh.k.balasingam@ntnu.no
This journal is © The Royal Society of Chemistry 2016
than that of transition metal oxides (TMOs).7 Because of these
advantages, MoS2 has been utilized for various electrochemical
applications such as sensors, hydrogen evolution catalysts,
solar cells, and lithium/sodium ion batteries.8,9 Until 2007
when the capacitive behavior of MoS2 nanowall films was first
reported by Loh’s group, researchers did not think about using
TMDCs for application in supercapacitors.10 However, from
2007 onwards, a considerable number of articles have been
published on MoS2 and its composite (MoS2/conducting
polymer and MoS2/carbon)-based electrodes for supercapacitor
applications.11 Most of the MoS2 synthesis methods are hightemperature solution-based chemical methods or they involve
top-down exfoliation of bulk MoS2 crystals, forming highly
crystalline MoS2 materials.4,11 In general, the amorphous
phase of pseudocapacitive materials with low crystallinity
shows a higher specific capacitance (Cs) than their highly crystalline counterparts.12 This phenomenon has been proved by a
systematic study on the amorphous and crystalline NiO
column structure and its surface and bulk contribution to the
Cs value.13 Furthermore, Lu et al. experimentally verified that
the bulk contribution of an amorphous NiO electrode was
three times higher than that of its crystalline phase. Very
recently, Zhang and co-workers synthesized an amorphous
MoS2 shell over the crystalline Ni3S4 core (Ni3S4@MoS2 core–
shell structure), which showed better capacitive performance
than pure Ni3S4.14 In the present communication, we synthesized amorphous MoSx on a highly conductive 3-D carbon
fiber paper network (CFP/a-MoSx). To the best of our knowledge, this is the first report on using a CFP/a-MoSx binder-free
electrode material for supercapacitor applications.
Herein, we adopted a facile hydrothermal method to
deposit an amorphous MoSx thin layer on carbon fiber paper.
Carbon fiber paper is hydrophobic. Hence, prior to conducting
the hydrothermal growth of MoSx, the carbon fiber paper is
pretreated with oxygen plasma to induce hydrophilicity (see
the experimental details in the ESI†).8 The as-synthesized CFP/
a-MoSx electrode was subjected to X-ray diffraction (XRD) and
Raman spectroscopic analysis (see Fig. S1 and S2 in the ESI†).
The XRD pattern of CFP/a-MoSx is shown in Fig. S1.† Diffrac-
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tion peaks appearing at 2θ = 26.5° and 54.5° correspond to the
(002) and (004) planes of the carbon fiber paper (JCPDS no. 411487). Except carbon peaks, no characteristic peaks of molybdenum sulfide are observed, which infers that the as-deposited
MoSx is amorphous in nature. Fig. S2† shows the Raman spectrum of CFP/a-MoSx. The typical peaks observed at 1358 cm−1
and 1587 cm−1 correspond to the D and G bands of the carbon
fiber paper. Except D and G bands of the carbon material, no
characteristic peaks (E2g1 and A1g modes) of molybdenum
sulfide are observed. The absence of any characteristic peaks
of MoS2 from XRD and Raman spectra inferred that the asdeposited material is amorphous. Fig. 1 shows the scanning
electron microscopy (SEM) images of bare CFP (Fig. 1a and b)
and those of MoSx-coated CFP (Fig. 1c and d), both at low and
high magnifications. It is clearly evident that the high-magnification SEM image (Fig. 1d) shows a uniformly coated MoSx
thin layer on the 3D-CFP network. To examine the thickness of
the MoSx coating, a small portion of the sample was sliced
using a focused ion beam equipped with the SEM instrument
(FIB-SEM). To further confirm the structural information and
thickness, the as-prepared thin slice of CFP/a-MoSx was examined using high-resolution transmission electron microscopy
(HR-TEM). Fig. 1e shows the high-magnification TEM image of
CFP/a-MoSx, which shows that the average thickness of the
MoSx thin film was around 23 ± 1 nm. The absence of clear
lattice fringes and the corresponding fast Fourier transform
(FFT) image (inset of Fig. 1e) confirms the amorphous nature
of MoSx, which is in agreement with the XRD and Raman
measurements. Fig. 1f shows the high angle annular dark
field-scanning transmission electron microscopy (HAADFSTEM) image of the CFP/a-MoSx and Fig. 1g and S6 (see the
ESI†) show the corresponding energy dispersive spectrum
(EDS) of a-MoSx as marked in the small (red square) region of
Fig. 1f. From the magnified EDS spectrum (Fig. S7, see the
ESI†), the presence of molybdenum and sulfur has been confirmed. The detection of copper might be due to the TEM grid
Nanoscale
and the gallium impurity originates from the FIB milling of
TEM lamella sample preparation.
The chemical composition and oxidation state of the
elements were analyzed using X-ray photoelectron spectroscopy (XPS). Fig. 2a shows the Mo 3d spectra of amorphous
MoSx. The doublets of the 3d orbital clearly resolved into two
different peaks at 3d5/2 and 3d3/2 centered at 228.99 and
232.41 eV, respectively, which confirms the +4 oxidation state
of Mo (i.e., Mo(IV)). The sulfur 2s orbital appears at the
binding energy of 226.43 eV and a small hump of a Mo6+ peak
appears at a higher binding energy of 235.8 eV; the latter may
have formed because of the formation of oxides during the
sample-preparation process.15 Because of the amorphous
nature of MoSx, an unresolved broad hump of a sulfur 2p peak
is also observed (Fig. 2b). From the atomic concentration
table, the sulfur to molybdenum stoichiometric ratio was
estimated at around 2.17. The excess sulfur present in the
compound may be owing to the formation of
MoS3(Mo4+[S2−][S22−]), rather than MoS2. Upon deconvolution,
the doublet of S2− appears at lower binding energies and that
of S22− at higher binding energies. The peak position of each
orbital is tabulated in the ESI (Table S1 in the ESI†).
The electrochemical properties of the CFP/a-MoSx binderfree electrode material were investigated for supercapacitor
applications by using a symmetric two-electrode cell with an
aqueous 0.5 M sulfuric acid electrolyte. Fig. 3a represents the
cyclic voltammetry (CV) curves of the CFP/a-MoSx symmetric
cell at various scan rates in the potential range varying from
−0.3 to 0.6 V. At low scan rates, two pairs of prominent redox
peaks can be clearly seen, which are attributed to the redox
reaction of Mo atoms: Mo ↔ Mo(IV) ↔ Mo(VI).14 When the scan
rate was increased, an obvious shift was seen in the redox
peaks. Broad redox peaks are observed even at a higher scan
rate, which confirmed that a predominant charge-storage reaction occurs via a redox reaction ( pseudocapacitance). Apart
from pseudocapacitance, non-Faradaic processes owing to the
Fig. 1 SEM images of bare CFP (a and b); CFP/a-MoSx (c and d); HR-TEM cross-sectional image of CFP/a-MoSx (e), the inset shows the FFT pattern;
A HAADF-STEM cross-sectional image of CFP/a-MoSx (f); EDS spectrum of MoSx (g) (measured at the small portion of STEM image).
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Fig. 2
High-resolution XPS spectra of CFP/a-MoSx: Mo 3d (a); and S 2p (b).
adsorption and desorption of protons (or in some cases, alkali
metal ions) on the surface of MoS2, which involve the double
layer (capacitance) charge-storage mechanism, have also been
reported.10,16–20 In some cases, the Faradaic process is only
observable at a very lower scan rate; when the scan rate is
increased due to the kinetic limitation of cation diffusion, the
non-Faradaic process (EDLC behavior) becomes dominant.10,17
In our study, however, even at a higher scan rate of 125 mV s−1,
broad redox peaks were observed with distorted rectangular
CV curves, which confirmed that both the pseudocapacitance and EDLC-type charge-storage mechanism were
involved. Eqn (S1) and (S2†) were used to measure the cell
capacitance using CV and CD curves, and the corresponding
areal specific capacitance of a cell (Cm), and a single electrode
(Cs), was calculated using eqn (S3)–(S5) (see the experimental
methods in the ESI†). The relationship between the scan rates
and Cs is correlated (Fig. 3b). When the scan rate is increased
gradually from 1 to 125 mV s−1, the Cs value decreases, probably
owing to the insufficient time available for the insertion/extraction of protons into the interior (bulk) of the electrode.21 Based
on this observation, it can be inferred that the electrochemical
reaction kinetics is controlled by the protonic diffusion process
at higher scan rates. When the scan rate was 1 mV s−1, a single
electrode showed a Cs value of 83.9 mF cm−2 and the device
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showed a Cm value of 41.96 mF cm−2. The Cs value obtained in
our study using the two-electrode configuration is still higher
than that reported by Soon et al. (70 mF cm−2 @ 1 mV s−1,
obtained using the three-electrode configuration).10 When the
scan rate is increased to 5 mV s−1, Cm decreased to
20.4 mF cm−2; however, the Cm value obtained using our cell is
much higher than that reported by a recent study on exfoliated
MoS2 (1.83 mF cm−2) and graphene-exfoliated MoS2 composite
(4.29 mF cm−2) based symmetric devices at the same scan rate.20
On further increasing the scan rate to 10 mV s−1, the corresponding Cs value dropped to 33.6 mF cm−2 and the Cm value
decreased to 16.8 mF cm−2. However, the Cm value obtained in
the present work is eight times higher than that in a previous
report on exfoliated-MoS2 (2 mF cm−2 @ 10 mV s−1) and 33
times higher (0.5 mF cm−2 @ 10 mV s−1) than that obtained with
bulk MoS2-based symmetric devices.19
To further evaluate the electrochemical performance of the
CFP/a-MoSx electrode material, galvanostatic charge–discharge
measurements were carried out for the symmetric device at
current densities of 0.05–0.6 mA cm−2 (Fig. 3c). The nonlinear
shape of the discharge curves further confirmed the pseudocapacitive charge-storage behavior. The relationship between Cs
and the various current densities is shown in Fig. S3 (see the
ESI†) and the Ragone plot of the symmetric device showing
the areal energy and power density relationship is shown in
Fig. S4 (see the ESI†). When the discharge rate was 0.3 mA cm−2,
the corresponding Cs was 8.76 mF cm−2, which is more
than five times that of exfoliated MoS2 (1.65 mF cm−2 @
0.25 mA cm−2).20 The high specific capacitance of the CFP/
a-MoSx material may be due to the following reasons: (i) the
high surface area of the 3D porous carbon fiber network with
good electrical conductivity that increases the electron-transfer
reaction rate, even though the as-synthesized MoSx is poorly
crystalline. (ii) A thin amorphous MoSx film (thickness:
approximately 23 ± 1 nm, as determined from TEM measurements) comprised of high-density grain boundaries with fine
grains that act as diffusion channels for a more efficient intercalation of protons, which leads to an effective utilization of
bulk electrode materials when compared to their crystalline
counterparts.12 The higher areal specific capacitance of the
CFP/a-MoSx electrode with the previously reported MoS2 based
materials is compared in Table S2 (see the ESI†).
Electrochemical impedance spectroscopy is one of the ideal
tools to investigate the charge transfer mechanism at the electrode–electrolyte interface. Fig. 3d and S8 (see the ESI†) show
the Nyquist and Bode plots of the symmetric device, respectively. From the Nyquist plot, a high-frequency intercept on the
x-axis represents the electrochemical series resistance (ESR) of
the symmetric device. The semicircle arc at the high- to
medium-frequency range represents the charge transfer resistance (Rct), which is determined to be 0.5 Ω. A near vertical line
(almost perpendicular to the x-axis) is observed at the
medium-to-low frequency region, which represents the capacitive behavior of the device.
The long-term cyclability is one of the key criteria for the
real-time application of the device for commercial use. The
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Fig. 3 CV curves of CFP/a-MoSx at various scan rates ranging from 1 to 125 mV s−1 (a). The relationship of specific capacitance of the electrode vs.
scan rate (b). CD curves of CFP/a-MoSx at various current densities (c). EIS spectrum of the symmetric supercapacitor (d); the inset shows the highfrequency region. Long-term stability of the device measured as a function of capacitance retention vs. cycle number (e). CV curves of precycle and
post-4750 cycles of the long-term stability test (f ).
symmetric capacitor has been tested to investigate the continuous long-term charge–discharge process at a current density of
0.3 mA cm−2. The Cs value in the first cycle was around
8.46 mF cm−2, which increased to 50.4 mF cm−2 at the 4750th
cycle. Most of the previous reports on the long-term stability of
pseudocapacitive materials, including MoS2 electrode
materials, showed generally either a decrease in capacitance
retention or a stable value over many cycles.14,16–19,22,23 In contrast, the capacitance retention of CFP/a-MoSx synthesized in
our study continuously increased up to 600% for 4750 cycles
(Fig. 3e). This type of increase in capacitance is rarely observed
in binder-free graphene-based supercapacitors (using KCl
electrolyte) because of the “electroactivation” of electrode
materials via continuous (K+) ion intercalation/deintercalation,
which leads to a partial re-exfoliation process.24–26 Very
recently, Bissett et al. also observed the same trend on exfoliated MoS2 thin-film-based symmetric capacitors (using
Na2SO4 electrolyte), wherein the capacitance retention
increased up to 800% over the first 3000 cycles.20 The authors
investigated the charge storage mechanism by CV analysis
before and after long-term cycling. Before cycling, the CV
curves showed the EDLC-type charge-storage behavior;
however, after long-term cycling for over 10 000 cycles, the
shape of the post-CV curve distorted with enhanced current
density. Hence, the authors concluded that the electro-
11790 | Nanoscale, 2016, 8, 11787–11791
chemical charge-storage mechanism changes from EDLC to
pseudocapacitive nature because of the continuous ion intercalation/deintercalation process. Also, they proposed a
mechanism involving continuous intercalation/deintercalation
of relatively large Na+ ions over a long cycling process that
further increased the active surface area by partial re-exfoliation of MoS2 layers, and hence, led to enhanced current
density values in post-CV curves.
In our case, energy storage occurs predominantly by pseudocapacitance (from the first cycle to 4750th cycle and even for
high scan rate CV curves); hence, clear redox peaks are
observed from the CV curves. After long-term cycling, post-CV
analysis was done at the same scan rate, which shows characteristic redox peaks with an enhanced current density value
(Fig. 3f ). Moreover, Bissett et al. investigated liquid-phase exfoliation of crystalline MoS2 powder, and the resultant exfoliated
MoS2-flakes are also crystalline. However, in our case, the assynthesized MoSx is amorphous and has many defect sites.
Also, the electrolyte used in our study is 0.5 M sulfuric acid; H+
cations act as shuttle ions, which are much smaller than Na+
ions. The observed increase in the current density value, as
evident from the pre- and post-CV analyses, may be due to the
“electroactivation” process. However the re-exfoliation of amorphous MoSx by H+ ions is not yet understood. In general, Li+
ion intercalation into bulk MoS2 facilitates the formation of
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highly conducting 1T phase MoS2;27 however, a recent report
on MoS2 and rGO hybrids showed that a heterostructure comprising 1T MoS2 formed at the MoS2/rGO interface and 2H
MoS2 at the bulk.23 In our case, the as-synthesized material is
amorphous. Hence, the in-depth structural analysis of electrodes, both before and after long-term measurement, does not
give any detailed information (see Fig. S9,† post-mortem SEM
image). The trend of increasing capacitance retention is interesting to researchers working on electrochemical energy
storage devices. However, the increasing trend reaction mechanism is still unclear and presently under investigation.
In summary, a thin layer of amorphous MoSx on a highly
conducting 3-D network carbon fiber paper was synthesized by
a facile hydrothermal method. The as-synthesized binder-free
electrode material with excellent electrochemical energystorage capability and a symmetric device showed a high
specific device capacitance value of 41.96 mF cm−2. More
interestingly, because of “electroactivation”, the capacitance
retention increases up to 600% for amorphous MoSx in the
presence of a sulfuric acid medium. Although the exact
mechanism of the increasing trend in capacitance retention is
still under investigation, we believe that our primary observation and the obtained results may be interesting to the
energy storage research community and may provide futuristic
outcomes in this field.
The authors gratefully acknowledge the financial support provided by the National Research Foundation of Korea (NRF) funded
by the Korean government, MSIP/KEIT (2013R1A1A2059244,
2015M1A2A2056829, 2016R1A2B4007570, 10050509).
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