Few-layered MoSe2 nanosheets an an advanced...

Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2015, 44,
15491
Received 27th May 2015,
Accepted 8th July 2015
DOI: 10.1039/c5dt01985k
www.rsc.org/dalton
Few-layered MoSe2 nanosheets as an advanced
electrode material for supercapacitors
Suresh Kannan Balasingam,a
Jae Sung Leeb
and Yongseok Jun*c
We report the synthesis of few-layered MoSe2 nanosheets using a facile hydrothermal method and their
electrochemical charge storage behavior. A systematic study of the structure and morphology of the as-
synthesized MoSe2 nanosheets was performed. The downward peak shift in the Raman spectrum and the
high-resolution transmission electron microscopy images confirmed the formation of few-layered
nanosheets. The electrochemical energy-storage behavior of MoSe2 nanosheets was also investigated for
supercapacitor applications in a symmetric cell configuration. The MoSe2 nanosheet electrode exhibited a
maximum specific capacitance of 198.9 F g−1
and the symmetric device showed 49.7 F g−1
at a scan rate
of 2 mV s−1
. A capacitance retention of approximately 75% was observed even after 10 000 cycles at a
high charge–discharge current density of 5 A g−1
. The two-dimensional MoSe2 nanosheets exhibited a
high specific capacitance and good cyclic stability, which makes it a promising electrode material for
supercapacitor applications.
1. Introduction
Supercapacitors (also known as electrochemical capacitors or
ultracapacitors) are a class of electrochemical energy storage
devices with merits including high power density, long cycle
life, high reliability, short charging time, and low maintenance
cost.1,2
The bottleneck of the state-of-the-art electrochemical
capacitors (ECs) in the market is their low energy density com-
pared with that of commercial lithium-ion batteries
(120–170 Wh kg−1
).3
In general, ECs can be classified into two
main categories based on their charge storage mechanisms: (i)
double-layer capacitors, which are based on an electrostatic
charge storage mechanism (non-faradaic reaction at the inter-
face of capacitor electrodes) that leads to the regular double-
layer capacitance and (ii) pseudocapacitors, which are based
on a different charge storage mechanism, mainly faradaic in
origin (the passage of charge across the double layer). The
charge storage mechanism of pseudocapacitors involves elec-
trosorption of charged species (from the electrolyte) on the
electrode surface, which further leads to the fast redox reaction
at the electrode surfaces. Carbon-based materials are the best
known double-layer capacitor electrodes; however, their funda-
mental flaw is their low energy density values.4
To overcome
this issue, various pseudocapacitance materials such as con-
ducting polymers, transition metal oxides (TMOs), and tran-
sition metal dichalcogenides (TMDCs) have been intensively
investigated as electrode materials for supercapacitor appli-
cations. Of these materials, conducting polymers provide a
high energy density but exhibit lower cyclic stability, and the
transition metal oxides suffer from reduced electrical conduc-
tivity, which leads to poor device performance.
Recently, TMDCs have received much attention from
researchers because of their 2D sheet-like morphology, higher
electrical conductivity (than the oxides), high surface area and
the multivalent oxidation states of transition metal ions.5–7
Analogous to graphite, TMDCs consists of graphene-like layers
of MX2 sheets (where M can be any metal ions of groups IV, V
and VI transition elements and X is a chalcogen, X may be S,
Se and Te) which are stacked together by weak van der Waals
forces. Similar to graphene, a monolayer or few layers of MX2
can be exfoliated from their bulk counterpart, which has a very
distinct physical and chemical properties and also has a
diverse application in electronic devices, photo transistors,
sensors, catalysis, and electrochemical energy storage
systems.8–10
In 2007, the first report on the electrochemical
double-layer capacitance measurement of MoS2 nanowall films
was published by Soon and Loh,11
which opened up the field
of TMDC-based electrode materials for supercapacitor appli-
cations. Later, different morphologies of MoS2, WS2, and
their composite-based materials were investigated as super-
capacitor electrodes.12–25
Other sulfide materials such as, VS2
nanosheets were employed as an electrode material for in-
plane supercapacitors, CoS2 and NiS sheets were also demon-
a
Department of Chemistry, School of Natural Science, Ulsan National Institute of
Science and Technology (UNIST), Ulsan 689-798, Republic of Korea
b
School of Energy and Chemical Engineering, Ulsan National Institute of Science and
Technology (UNIST), Ulsan 689-798, Republic of Korea
c
Department of Materials Chemistry and Engineering, Konkuk University,
Seoul 143-701, Republic of Korea. E-mail: yjun@konkuk.ac.kr; Tel: +82-2450-0440
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strated as active materials for supercapacitor electrodes due to
their large surface area and enhanced structural stability.
When compared to sulfides, selenides are rarely studied for
capacitive properties.26–31
Moreover, recent reports on MoSe2
based anode materials for lithium ion and sodium ion bat-
teries showed them as promising electrodes for energy storage
applications. Shi et al.32
reported mesoporous crystalline
MoSe2 using silica SBA-15 as a hard template which showed a
highly reversible lithium storage capacity of 630 mA h g−1
, Ko
et al.33
synthesized yolk–shell structured MoSe2 microspheres
via selenization of MoO3 microspheres, which demonstrated a
high sodium ion storage capacity of 433 mA h g−1
. Wang
et al.34
also synthesized MoSe2 nanoplates using the thermal-
decomposition process, which could deliver high sodium ion
storage capacity of 513 mA h g−1
. The enhanced energy storage
capacity of the two dimensional MoSe2 electrode materials
render our attention to investigate this material for supercapa-
citor applications. When compare to the bulk MoSe2, few-
layered sheets show high surface area and unique electronic
(quasiparticle bandgap) properties due to interlayer coupling
and screening effects.35
Generally few-layered MoSe2
nanosheets are synthesized via a CVD or rapid thermal proces-
sing method which are very expensive and user-hostile tech-
niques for the synthesis of large quantity of electrode
materials.36–39
In this work, we have synthesized a large quan-
tity of few-layered MoSe2 nanosheets using a facile hydro-
thermal method, and their electrochemical capacitance was
measured in aqueous electrolyte (H2SO4) under a symmetric
cell (two-electrode) configuration. We obtained a maximal
specific capacitance of 199 F g−1
at a scan rate of 2 mV s−1
and
also for the long-term stability test, the capacitance retention
of approximately 75% was observed over 10 000 cycles, which
showed it as a suitable material for electrochemical capacitors.
2. Experimental
2.1. Material synthesis
The precursors, sodium molybdate (Na2MoO4·2H2O), selenium
powder (Se), and sodium borohydride (NaBH4) were purchased
from Sigma Aldrich (ACS grade) and used without further purifi-
cation. In a typical hydrothermal synthesis, precisely 1.32 g of
Na2MoO4·2H2O, 1.24 g of Se, and 0.2 g of NaBH4 were weighed
and dissolved in 80 ml of DI water. The mixture was continu-
ously stirred until a red-colored solution was obtained, which is
an indication of uniform distribution of the selenium metal
powder. The as-prepared precursor solution was transferred
into the autoclave, sealed, and then heated at 200 °C for 12 h in
an electric oven. The autoclave was then naturally cooled down
to room temperature. After that, the MoSe2 nanosheets were fil-
tered off and washed with DI water several times to remove the
residuals and then dried at 40 °C for a few hours.
2.2. Material characterization
The morphology of the as-prepared sample was examined
using high-resolution transmission electron microscopy
(HR-TEM) (JEOL). The crystal structure and elemental compo-
sition of MoSe2 was confirmed by X-ray diffraction (XRD,
Bruker) measurements using Cu Kα emission (λ = 1.5406 Å) in
10–90° range with a step size of 0.02°, Raman spectroscopy
(WITec) using 532 nm laser excitation, after calibrating the
Raman shift with a silicon reference at 521 cm−1
, and X-ray
photoelectron spectroscopy (XPS) (Thermo Fisher, UK).
2.3. Cell fabrication and electrochemical measurements
The as-synthesized electroactive material (MoSe2), carbon
black and poly(vinylidene fluoride) were mixed in a mass ratio
of 80 : 10 : 10 to obtain a slurry and then coated on the stain-
less steel (SS) substrate using a brush. An exactly 1 cm2
area of
MoSe2 nanosheet-coated SS substrate was used as a single elec-
trode. Mass loading of each electrode is around 4 mg cm−2
.
Two electrodes were sandwiched together with Whatman filter
paper as a separator. The assembled electrodes were placed in
a test cell rig, and a few drops of 0.5 M sulfuric acid were
added as the electrolyte. The test cell was sealed with an
O-ring and then left for a few minutes to ensure the uniform
soaking of the electrodes into the electrolyte solution before
the electrochemical measurements. Electrochemical experi-
ments were performed using a potentiostat/galvanostat (Bio-
logic/VSP) at room temperature. Cyclic voltammetry (CV)
curves were obtained at various scan rates (2, 5, 10, 25, 50, 75,
100, and 125 mV s−1
) in a potential window of 0–0.8 V. Electro-
chemical impedance spectroscopy (EIS) measurements were
performed over the frequency range of 0.1 Hz–100 kHz with an
AC amplitude of 10 mV. Galvanostatic charge–discharge (CD)
curves were recorded at various current densities (0.10, 0.25,
0.50, 0.75, 1, 2, 3, 4, and 5 A g−1
) in a potential window of
0–0.8 V.
2.4. Calculation of capacitance
The measured device capacitance (Cm, F g−1
) can be calculated
based on the CV measurements using the following equation:
Cm ¼
Ð
IðVÞdv
vmΔV
ðF gÀ1
Þ ð1Þ
where m is the total mass of the electroactive material in both
positive and negative electrodes (g), v is the scan rate (V s−1
),
ΔV is the potential window (V), and
Ð
IðVÞdv is the integral
area of the CV loop.
From the CD curves, the measured device capacitance (Cm)
can be computed as:
Cm ¼
IΔt
mΔV
ðF gÀ1
Þ ð2Þ
where I is the discharge current (A), and Δt is the discharge
time (s).
The specific capacitance (Cs) of the single electrode can be
determined as follows:
Cs ¼ 4 Â Cm ðF gÀ1
Þ ð3Þ
where Cm is obtained either from CV or CD curves given by
eqn (1) and (2).
Paper Dalton Transactions
15492 | Dalton Trans., 2015, 44, 15491–15498 This journal is © The Royal Society of Chemistry 2015
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3. Results and discussion
The crystal structure and phase purity of the as-prepared
MoSe2 nanosheets were characterized using X-ray diﬀraction.
Fig. 1 presents the XRD pattern of the MoSe2 nanosheets.
Diﬀraction peaks of the as-synthesized MoSe2 nanosheets
appear at 2θ = 13.3°, 31.6°, and 56°, which can be assigned to
the (002), (100), and (110) planes of the hexagonal phase of
MoSe2 (JCPDS no. 29-0914), respectively. The broadness of the
(002) peak indicates the formation of few-layered MoSe2
nanosheets.12,40
Raman spectroscopy was employed to further study the
structure of the as-synthesized MoSe2 nanosheets. The typical
peaks of MoSe2 nanosheets at 239 cm−1
and 287.11 cm−1
are
observed in the spectra (Fig. 2), which corresponds to the out-
of-plane A1g mode and in-plane E2g
1
mode of MoSe2, respect-
ively.41
For bulk MoSe2, the A1g mode appears at 242 cm−1
.
The downward red shift of approximately 3 cm−1
(A1g mode @
239 cm−1
) indicates the formation of few-layered MoSe2. The
E2g
1
mode of bulk MoSe2 appears at 286 cm−1
; however, the
blue shift of this in-plane mode observed in our study also
confirms the few-layered nanosheet formation.37,42
The chemical composition and oxidation state of the
elements were investigated using X-ray photoelectron spectro-
scopy. Mo 3d3/2 and Mo 3d5/2 peaks (Fig. 3a) appeared at
binding energies of 232 and 228.8 eV, respectively, which con-
firms the +4 oxidation state of molybdenum.43
In addition, the
3d peaks of selenium split into two well-defined peaks (Fig. 3b)
such as 3d3/2 and 3d5/2, which appeared at binding energies of
55.2 and 54.5 eV, confirming the −2 oxidation state of selenium
in MoSe2 nanosheets.44
From the atomic concentration table,
the mole ratio of Mo : Se was calculated to be 1 : 2.
The surface morphology of the as-synthesized MoSe2 was
investigated using HR-TEM. Fig. 4 presents HR-TEM images of
the as-synthesized MoSe2 nanostructure. The low-magnifi-
cation TEM images (Fig. 4a and b) show MoSe2 nanosheets.
Fig. 4c provides clear evidence of the formation of few-layered
Fig. 1 XRD spectrum of MoSe2 nanosheets.
Fig. 2 Raman spectrum of MoSe2 nanosheets showing two distinct A1g
and E2g
1
Raman modes.
Fig. 3 (a) High resolution XPS spectra of Mo 3d, (b) Se 3d regions of
MoSe2 nanosheets.
Dalton Transactions Paper
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nanosheets at higher magnification. In addition, the HR-TEM
images of the MoSe2 nanosheets reveal a layered crystal struc-
ture with clear crystal lattice fringes of approximately 0.62 ±
0.03 nm, corresponding to the standard d-spacing for the (002)
basal plane of the hexagonal crystal structure. Notably, the
porous nature of the MoSe2 nanosheet morphology provides a
high surface area for the electrochemical reaction.
The MoSe2-nanosheet-coated stainless steel substrates were
assembled in a two-electrode configuration (a symmetric cell
in a parallel-plate geometry, which is typically used in super-
capacitor measurements), with Whatman filter paper as the
separator and 0.5 M sulfuric acid as the electrolyte. The elec-
trochemical behavior of the MoSe2 electrodes was character-
ized using CV, EIS, and CD cycling measurements. Fig. 5a
shows the typical CV curves of a symmetric device (MoSe2
nanosheets coated on a stainless steel substrate as working
and counter electrodes) at various scan rates (2 mV s−1
to
125 mV s−1
). All the CV curves retain their nearly rectangular
shape at the various scan rates, which confirms the ideal
EDLC behavior (non-faradaic process, as indicated in eqn (4))
of this material with excellent reversibility. Moreover, the
broad peak appearing in both the anodic and cathodic scans
indicates the redox behavior of the MoSe2 nanosheets resulting
from the faradaic electrochemical process, as shown in eqn (5).
Fig. 4 TEM images of MoSe2 nanosheets: (a) and (b) low magnification,
and (c) high magnification; the inset shows the FFT pattern.
Fig. 5 (a) Cyclic voltammograms of MoSe2 nanosheets in 0.5 M H2SO4
electrolyte measured at different scan rates ranging from 2 to
125 mV s−1
. (b) Specific capacitance value calculated from the CV curves
of (a) at different scan rates.
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Notably, the redox peaks appeared even at higher scan rates
with the usual peak shift. The symmetric device exhibited a
measured device capacitance (Cm) of approximately 49.7 F g−1
at a scan rate of 2 mV s−1
. For the single electrode, a high
specific capacitance of (Cs) 198.9 F g−1
was calculated at the
same scan rate. The high Cs value may be attributed to the
combination of both faradaic and non-faradaic processes of
the 2D MoSe2 nanosheets:11
ðMoSe2Þ þ Hþ
þ eÀ
$ MoSe À SeHþ
ð4Þ
ðMoSe2Þ surface þ Hþ
þ eÀ
$ ðMoSe2 À Hþ
Þ surface ð5Þ
The detailed derivation of the specific capacitance calcu-
lation of a single electrode and device is given in eqn (1)–(3)
(Experimental section). Fig. 5b shows the relationship between
the specific capacitance of the MoSe2 nanosheets (a single
electrode) at various scan rates. By increasing the scan rate up
to 20 mV s−1
, a sudden drop of the Cs value is observed, and
upon further increasing the scan rate up to 125 mV s−1
, a
gradual decrease of the Cs value is recorded. This observation
is a common phenomenon in supercapacitors because at
higher scan rates, the mass-transport limitation of H+
ions
(from the electrolyte) to the interior (bulk) part of the electrode
limits the electrochemical performance, which leads to a lower
capacitance.45–48
The specific capacitance values observed for
few-layered MoSe2 electrodes were compared with the pre-
viously reported state-of-the-art MoS2 and WS2 based super-
capacitors. Using a similar two-electrode configuration,
Winchester et al.49
reported a specific capacitance of 2 mF
cm−2
@ 10 mV s−1
scan rate in 6 M KOH electrolyte for liquid
phase exfoliated MoS2 layers. The material showed poor
capacitance retention of 68% within 200 cycles in the same
electrolyte. Cao et al.25
synthesized MoS2 nanosheets for
micro-supercapacitors, and a high specific capacitance of 8 mF
cm−2
@ 10 mV s−1
scan rate was observed in 1 M NaOH;
however, the authors did not show the long-term cycle stability
of this material. Ratha and Rout12
synthesized WS2 nanosheets
by a facile hydrothermal method and reported a specific
capacitance of 70 F g−1
@ 2 mV s−1
scan rate. Using the same
scan rate, our material (few-layered MoSe2 nanosheets) showed
a specific capacitance of 198.9 F g−1
which is almost 2.8 times
higher than the WS2 based materials. Although a recent report
on metallic 1 T phase MoS2 electrode for supercapacitors
measured using a three-electrode configuration showed high
volumetric capacitance of around 650 F cm−3
@ 20 mV s−1
,
their gravimetric capacitance is approximately 190 F g−1
@
5 mV s−1
. However, using a two-electrode configuration, our
MoSe2 nanosheet electrodes showed a comparable gravimetric
capacitance to the previous report.50
Fig. 6a presents the galvanostatic CD curves of a symmetric
device at various current density values. The CD curves
are almost symmetric, which indicates the electrochemical
capacitive behavior with remarkable reversible redox reaction.
At the very low discharge current density of 0.1 A g−1
, a non-
linear shape of the CD curve is observed with the pseudocapa-
citive behavior of the electrode material. The symmetric super-
capacitor device exhibited a measured device capacitance of 10.4
F g−1
, and the single electrode exhibited a specific capacitance
of approximately 41.5 F g−1
at a discharge current density of
0.1 A g−1
. The rate capability of a single MoSe2 nanosheet elec-
trode was investigated under various current densities, as
shown in Fig. 6b. Consistent with the CV analysis, the Cs value
of the electrode material decreases at higher current density.
As explained in CV, the diffusion rate of the electrolyte ions is
limited at higher current density. Therefore, only the surface
area of the MoSe2 nanosheets is accessible at higher current
density. For a lower-current-density CD process, the electrolyte
ions could penetrate into the interior part of the electrode
surface, thereby resulting in a higher Cs value.
Long cycling life is another important criterion for the
application of this material in commercial supercapacitors.
The cyclic stability of MoSe2 nanosheet electrodes was
measured at a high current density of 5 A g−1
, as shown in
Fig. 7a. The capacitance retention is dramatically decreased in
the first few hundred cycles, which may be due to the active
site saturation of the surface of MoSe2 nanosheets during the
charge–discharge process.25
Thereafter, a gradual decrease is
Fig. 6 (a) Cyclic voltammograms of MoSe2 nanosheets in 0.5 M H2SO4
electrolyte measured at different scan rates ranging from 2 to
125 mV s−1
. (b) Specific capacitance value calculated from the CV curves
of Fig. 5a at different scan rates.
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observed until 10 000 cycles. Notably, a cycling performance of
approximately 75% capacitance retention is observed even
after 10 000 cycles, which indicates that MoSe2 nanosheets are
promising electrode materials for high-performance super-
capacitors. The remaining 25% capacitance loss may be due to
the chemical instability of MoSe2 in aqueous electrolyte, which
can be improved by the usage of organic/ionic liquid based
non-aqueous electrolytes. EIS was performed to analyze the
internal resistance and capacitance of the device. Fig. 7b pre-
sents a Nyquist plot of the symmetric device at the initial stage
and after 10 000 cycles. In the high-frequency region, the x-axis
intercept of the semi-circle on the z-real axis represents the
electrochemical series resistance (ESR) of the device, which
was determined to be 0.43 Ω. The depressed semi-circle in the
high-to-medium frequency range is modeled by the parallel
combination of an interfacial charge transfer resistance and
double-layer capacitance. In the mid-frequency region, a
typical 45° slope is observed for a short region, which is
known as the Warburg impedance due to the frequency depen-
dence of ion diffusion/transport in the electrolyte. A short
Warburg region on the plot indicates a short ion diffusion
path and, in turn, effective diffusion of electrolyte ions into
the electrode surface.51
In the low-frequency region, an almost
90° straight line is observed, which indicates the excellent
capacitive behavior of the device.52
EIS was performed after
10 000 cycles for the post-analysis of the device. The shape of
the spectrum after 10 000 cycles is similar to the initial shape,
except for the diameter of the semi-circle arc in the high-to-
medium frequency range. The semi-circle arc of the device
increased from 1 to 1.5 Ω after 10 000 cycles, which might be
due to the adhesion loss of some electroactive materials from
the stainless steel current collector or dissolution of some elec-
troactive materials during the repeated charge/discharge
process.53–55
The superior electrochemical performance of the
few-layered MoSe2 nanosheets may be attributed to the follow-
ing reasons: (1) the 2D sheet-like structure provides a high
surface area and more active sites for the fast reversible redox
reaction on the surface of the electrode and (2) the few-layered
MoSe2 nanosheets stacked together with van der Waals forces
allow the effective intercalation of H+
ions from the electrolyte.
4. Conclusions
In summary, this work reports the facile synthesis of few-
layered MoSe2 nanosheets and their capacitive properties in a
two-electrode configuration using aqueous electrolyte. The
symmetric device exhibited a measured capacitance of approxi-
mately 49.7 F g−1
, and the single electrode exhibited a specific
capacitance of approximately 199 F g−1
at a scan rate of 2 mV
s−1
. The excellent capacitive performance of the device is
mainly attributed to the graphene-like 2D layered crystal struc-
ture, which provides a high surface area and allows effective
diffusion of H+
ions for the fast reversible redox process. The
good cyclic stability and capacitance retention of approxi-
mately 75% are observed even after 10 000 cycles, which indi-
cates that MoSe2 nanosheets could be a potential candidate for
electrochemical capacitors.
Acknowledgements
This research was supported by the National Research Foun-
dation of Korea (NRF) funded by the Korean government,
MSIP/KEIT (2014064020, 10050509).
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