A 2H-MoS2/carbon cloth composite for high-performance all-solid-state supercapacitors derived from a molybdenum dithiocarbamate complex

Article abstract of DOI:10.1039/d1dt01643a

A molecular complex Mo₂O₂(μ-S)₂(Et₂dtc)₂(dtc = dithiocarbamate) is prepared and loaded onto carbon cloth (CC) through facile solvothermal treatment, followed by subsequent single-source pyrolysis. This results in a highly porous 2H-MoS₂/CC composite with a sponge-like stacked lamellar morphology. Due to its high porosity and unique nano/microstructure, the MoS₂/CC composite exhibits a specific capacitance of 550.0 F g^?1at 1 A g^?1, outperforming some 1T-MoS₂based electrodes. The composite is further assembled into a symmetric all-solid-state supercapacitor, which can be operated stably at a wide potential window and shows a specific capacitance of 127.5 F g^?1at 1 A g^?1. In addition, the device delivers a high energy density of 70.8 W h kg^?1at 1 kW kg^?1, which still remains 15.0 W h kg^?1at 18.0 kW kg^?1. 75% of the performance of the device can be retained after 8000 cycles. Such remarkable electrochemical performance is attributed to its novel nano/microstructures with a large surface area, convenient ion transport pathways, enhanced conductivity, and improved structural stability. Thus, this work demonstrates a highly promising dithiocarbamate-based single-precursor pyrolysis route towards the fabrication of metal sulfides/carbon composites for energy storage applications.

Full text of DOI:10.1039/d1dt01643a

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A 2H-MoS2/carbon cloth composite for high-per-
formance all-solid-state supercapacitors derived
from a molybdenum dithiocarbamate complex†
a
Cite this: Dalton Trans., 2021, 50,
11954
Zhishuo Yan,a Jixing Zhao,b Qingsheng Gao *a and Hao Lei
*
A molecular complex Mo2O2(μ-S)2(Et2dtc)2 (dtc = dithiocarbamate) is prepared and loaded onto carbon
cloth (CC) through facile solvothermal treatment, followed by subsequent single-source pyrolysis. This
results in a highly porous 2H-MoS2/CC composite with a sponge-like stacked lamellar morphology. Due
to its high porosity and unique nano/microstructure, the MoS2/CC composite exhibits a specific capaci-
tance of 550.0 F g−1 at 1 A g−1, outperforming some 1T-MoS2 based electrodes. The composite is further
assembled into a symmetric all-solid-state supercapacitor, which can be operated stably at a wide poten-
tial window and shows a specific capacitance of 127.5 F g−1 at 1 A g−1. In addition, the device delivers a
high energy density of 70.8 W h kg−1 at 1 kW kg−1, which still remains 15.0 W h kg−1 at 18.0 kW kg−1. 75%
of the performance of the device can be retained after 8000 cycles. Such remarkable electrochemical
performance is attributed to its novel nano/microstructures with a large surface area, convenient ion
transport pathways, enhanced conductivity, and improved structural stability. Thus, this work demon-
strates a highly promising dithiocarbamate-based single-precursor pyrolysis route towards the fabrication
of metal sulfides/carbon composites for energy storage applications.
Received 20th May 2021,
Accepted 23rd July 2021
DOI: 10.1039/d1dt01643a
rsc.li/dalton
Specifically, pseudocapacitors have additional advantages of
fast ion diffusion and intercalation, and thus they can store
1. Introduction
In the field of energy storage and conversion, devices includ- and release more electrons per mole material, which leads to a
ing batteries, fuel cells and supercapacitors are key candidates higher energy density than those which only have EDLC pro-
for sustainable energy solutions.1–3 Batteries have been widely cesses.10 On the other hand, wearable and portable electronics
used for centuries, but they possess relatively low power den- shows a big advantage for the development of flexible super-
sities. In contrast, supercapacitors have emerged as one of the capacitors with widespread demand.11,12 For instance, the
most important types of candidates for energy storage, due to Guan group reported a facile two-step solution method to
their higher power densities.4 The power densities of super- rationally design a novel electrode of hollow NiCo2O4 nanowall
capacitors are mainly determined by the nature of electrode arrays on flexible carbon cloth (CC), which exhibits a high
materials used and the potential window of the assembled capacitance of 1055.3 F g−1 at 2.5 mA cm−2 13
Featuring high
.
devices.5 Thus, finding a way to enhance the energy density conductivity and low cost, CC can serve as both a flexible
while still taking advantage of its intrinsic high power density current collector and a binder-free support for electroactive
remains a challenge for the exploitation and design of new materials.14
supercapacitor electrode materials. This mainly depends on
pseudocapacitive or battery-like performance rather than the performance transition-metal-based supercapacitor electrodes,
electrical double-layer capacitor (EDLC) and
process.6–9 including oxides,15 hydroxides,16,17 sulfides,18
To date, much effort has been devoted to investigate high-
selenides.19,20 For metal chalcogenides, the fabrication strat-
egy usually involves a separated sulfurization or selenation
step, utilizing an exogenous source of chalcogens. A less
explored alternative would be the direct single-source pyrolysis
of rationally designed molecular coordination complexes with
chalcogen-containing organic ligands.21 Since all the atoms in
the product come from the same molecular precursor, the
structures and properties of the product can be modulated
effectively through selection and modification of the coordi-
aCollege of Chemistry and Materials Science, Guangdong Provincial Key Laboratory
of Functional Supramolecular Coordination Materials and Applications, Jinan
University, Guangzhou 510632, China. E-mail: tleihao@jnu.edu.cn,
tqsgao@jnu.edu.cn
bAnalysis and Testing Center, Shihezi University, Shihezi, Xinjiang 832003, China
†Electronic supplementary information (ESI) available. CCDC 2072744. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1dt01643a
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nation complex.22 For instance, due to their intrinsic high
sulfur contents, metal complexes supported with dithiocarba-
mate (R1R2NCS2−) ligands have been employed as single-
2. Experimental
2.1. Preparation of the 2H-MoS2/CC electrode
source precursors for the synthesis of various transition metal Carbon cloth (CC) was heated in concentrated HNO3 at 150 °C
and main group element sulfide materials.23–27
and refluxed overnight. Then it was cleaned through ultra-
Layered transition metal disulfides (MS2, M = Mo, W, V, sonication in ethanol and deionized (DI) water each for
etc.) represent a large family of 2D-layered materials used as 15 minutes successively, and dried in a vacuum oven. A piece
catalysts, solid lubricants, supercapacitors, and so on.28,29 of well-cleaned CC (1.2 cm × 1.0 cm) was vertically immersed
MoS2 is especially attractive in terms of its layered in a mixed solution of MoO3 (14.4 mg, 0.10 mmol), Na(Et2dtc)
structure.30–33 In MoS2, each layer consists of a plane of molyb- (24.2 mg, 0.15 mmol) and DMF/H2O (1 : 1 in v/v, 10 mL). The
denum atoms sandwiched between sulfur layers and stacked solution was transferred to a Teflon-lined stainless steel auto-
together by van der Waals forces.34 The lattice formed is ben- clave and kept at 90 °C for 1 day. After solvothermal growth,
eficial for ion intercalation and de-intercalation. Thus, MoS2 the CC loaded with the Mo complex was taken out and care-
can crystallize in three different types of phases: the stable fully rinsed with DI water, and dried in a vacuum oven. The
semiconducting 2H phase, the metastable semiconducting 3R obtained Mo2O2(μ-S)2(Et2dtc)2/CC sample was placed in a
phase, and the metastable metallic 1T phase.35 Many previous quartz boat and put into the heating center of the furnace
reports using MoS2 for energy storage have focused on the 1T tube, and calcined at 600 °C under N2 for 1 h, with a heating
phase, possibly due to its much wider interlayer d spacing.36 rate of 2 °C min−1. After cooling down to room temperature
However, the over-expanded 1T-MoS2 is not beneficial for the naturally, 2H-MoS2/CC was obtained and used for further
pseudocapacitive performance owing to the competition investigation. Other synthetic details are included in the ESI.†
between the exfoliation and redox processes. More impor-
2.2. Characterization
tantly, pure 1T-MoS2 is difficult to prepare due to its meta-
stable nature, and MoS2 materials with a 2H/1T hybrid-phase Thermogravimetric analysis (TGA) was performed on
a
are commonly obtained.37,38 For example, hybridized 2H/1T NETZSCH STA 449 F3 thermal analyzer under N2 from 25 to
MoS2 with controlled 1T concentrations exhibits a specific 900 °C with a heating rate of 10 °C min−1. Powder X-ray diffrac-
capacitance of 259 F g−1 at 5 mV s−1 39
.
In another case, tion (PXRD) data were collected in the 2θ range of 5°–90° using
1T-MoS2 prepared in a magnetic field displays a higher specific a Bruker D8 FOCUS diffractometer. Raman spectra were
capacitance of 320 F g−1 at 1 A g−1 37
In contrast, electro- obtained on a HORIBA LabRAM HR Evolution spectrometer.
.
chemical charge storage using pure 2H-MoS2 receives much Scanning electron microscopy (SEM) with the corresponding
less attention, because of its relatively low conductivity and elemental mappings and energy-dispersive X-ray spectroscopy
small d-spacing. Therefore, boosting the conductivity and fine- (EDS) were performed using a ZEISS Ultra-55 microscope.
tuning the morphology and microstructures can be the poten- Transmission electron microscopy (TEM) with its corres-
tial solutions for achieving the high capacity and cycling stabi- ponding elemental mappings, high resolution transmission
lity of 2H-MoS2.
electron microscopy (HR-TEM) and selected area electron diffr-
Inspired by these designs above, we developed a facile two- action (SAED) were performed on a JEOL JEM-2100F instru-
step route for the fabrication of binder-free, freestanding ment. N2 adsorption–desorption isotherms were collected on a
sponge-like 2H-MoS2/CC electrodes. The easy-to-implement Quantachrome Instruments Autosorb AS-6B adsorption analy-
method consisted of solvothermal growth and thermolysis zer at 77 K, and the Brunauer–Emmett–Teller (BET) specific
steps. This approach circumvents the necessity of exogenous surface areas of the samples were calculated from adsorption
sulfur sources (e.g. S8, Na2S, thiourea and thioacetamide). The data. X-ray photoelectron spectroscopy (XPS) analyses were per-
unique nano/micro-architectures contributed to the increased formed on a Thermo ESCALAB 250 X-ray photoelectron
active sites and conductivity. The specific capacitance of spectrometer using monochromatic Al Kα radiation (1486.6
2H-MoS2/CC reaches as high as 550.0 F g−1 at 1 A g−1, outper- eV) as the X-ray source and the C 1s peak at 284.8 eV as an
forming the reported value for some 1T-MoS2 based internal reference for binding energies.
electrodes.37,38,40 More significantly, the symmetric all-solid-
state supercapacitor (ASSS) device assembled by two flexible
2.3. Electrochemical measurements
2H-MoS2/CC electrodes achieves a maximum potential in the In a three-electrode system, the electrochemical performances
range of −1 to 1 V, and displays a specific capacitance of 127.5 were evaluated by cyclic voltammetry (CV), galvanostatic
F g−1 at 1 A g−1. In addition, the device exhibits an impress- charge–discharge (GCD), and electrochemical impedance
ively high energy density of 70.8 W h kg−1 at a power density of spectra (EIS) measurements on a CHI-760E electrochemical
1.0 kW kg−1, which remains 15.0 W h kg−1 at 18.0 kW kg−1. At workstation using a platinum foil and a Ag/AgCl electrode
a current density of 8 A g−1, the specific capacitance of the (saturated with KCl) as the counter and reference electrodes,
ASSS device can be retained at about 75% after 8000 cycles. respectively. The 2H-MoS2/CC electrode with a testing loading
Thus, the flexible 2H-MoS2/CC electrode exhibits promising area of 1 × 1 cm2 was directly used as the working electrode
electrochemical
applications.
performance
towards
flexible
ASSS and the loading mass of the active material (2H-MoS2) is deter-
mined to be 4.2 mg by measuring the weight difference
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between pristine CC and the as-obtained 2H-MoS2/CC tangular cross sections, indicating the high crystalline nature
samples. EIS was conducted at the open circuit over a fre- of the materials.
quency range from 0.01 Hz to 100 kHz at an amplitude of
To evaluate the potential of Mo2O2(μ-S)2(Et2dtc)2 as a
5 mV. The CV curves were recorded in a potential range of single-source pyrolysis precursor towards MoS2, the thermal
−0.4–0.7 V vs. Ag/AgCl at scan rates from 5 to 100 mV s−1. The stability of the complex was investigated by TGA under a N2
GCD measurements were conducted in the potential range of atmosphere. As demonstrated in Fig. S3,† the TGA curve
−0.3–0.6 V vs. Ag/AgCl at current densities from 1 to 20 A g−1
,
showed a sharp weight loss at ca. 310 to 350 °C, in accordance
and the mass specific capacitance (C, F g−1) was calculated with the full dissociation and decomposition of the dithiocar-
based on the loading mass of 2H-MoS2 by the galvanostatic bamate ligands. Based on such an observation, single-source
discharge time Δt (s) according to previous literature reports.41 pyrolysis of Mo2O2(μ-S)2(Et2dtc)2 was carried out at 310 to
The electrochemical behavior of 2H-MoS2/CC was further eval- 350 °C (Fig. S4†), and the composition and phase purity of the
uated in a symmetric ASSS configuration, in which two products (denoted as MoS2-T, T equals pyrolysis temperature)
2H-MoS2/CC electrodes were directly utilized as the positive were determined by the PXRD study. In Fig. S5,† the diffraction
and negative electrodes, and the PVA-H2SO4 gel served as the pattern of MoS2-310 shows only one sharp peak for Mo2O2(μ-
electrolyte. CV and GCD were performed and the performances S)2(Et2dtc)2 at ca. 10.5°, and also a newly emerging diffraction
of the ASSS device were estimated using equations as pre- peak at 32.9° corresponding to the (100) facets of 2H-MoS2
viously reported (see the ESI† for details).42 In addition, the (JCPDS #75-1539). This suggests that a 2H-MoS2 phase begins
cycling stability of the 2H-MoS2/CC electrode was investigated to form as a result of the pyrolysis and sulfurization of Mo
through 5000 GCD cycles at 8 A g−1 in the range of −0.3–0.6 centers at 310 °C. Further increasing the heating temperature
V. Meanwhile, the stability of the ASSS device was evaluated by facilitates the full conversion of the Mo complex precursor into
performing 8000 GCD cycles at 8 A g−1 in the range of −1–1 V.
2H-MoS2, as evidenced by the PXRD patterns of MoS2-320 to
MoS2-350. They all demonstrate three characteristic diffraction
peaks located at 14.1, 32.9 and 58.9°, which are indexed to the
(002), (100) and (110) facets of 2H-MoS2 (JCPDS #75-1539),
respectively (Fig. S5†). The thermolysis of Mo2O2(μ-S)2(Et2dtc)2
was further performed at 400 to 900 °C, and the PXRD patterns
of the as-prepared MoS2-T (T = 400 to 900) are depicted in
3. Results and discussion
3.1. Preparation and characterization of the 2H-MoS2/CC
composite
The sulfido-bridged dimeric complex Mo2O2(μ-S)2(Et2dtc)2 was Fig. S6.† It can be clearly discerned that a broad hump at 39.4°
prepared by a facile solvothermal reaction of MoO3 with Na starts to appear for sample MoS2-500, and it gets more obvious
(Et2dtc) in a DMF/H2O mixture.43–45 The molecular structure for the samples synthesized at 600 °C (Fig. 2a) or higher. This
of the complex was established by single-crystal X-ray diffrac- new peak corresponds to the (103) plane of 2H-MoS2 (JCPDS
tion. The compound crystallizes in the monoclinic space #75-1539), and its appearance confirms the improving crystalli-
group of C2/c (Table S1†) and the asymmetric unit comprises a nity after high-temperature calcination. Notably, the variation
Mo2(μ-S)2 diamond core, with each Mo(V) center bonded with a of the pyrolysis temperature does not affect the domination of
terminal oxygen atom and a chelating dithiocarbamate ligand the 2H-phase in the prepared MoS2 materials, possibly due to
in a κ2-S,S coordinating fashion (Fig. 1). The sulfur-rich coordi- its much higher thermal stability in comparison with the 1T-
nation environment of molybdenum centers is beneficial for phase.46
single-source pyrolysis towards the preparation of pure phase
The morphology of 2H-MoS2 derived at different tempera-
MoS2. The phase purity of bulk Mo2O2(μ-S)2(Et2dtc)2 samples tures was further analyzed by SEM. As illustrated in Fig. S7,†
was also confirmed by PXRD (Fig. S1†). Furthermore, their the MoS2-300 sample is much coarser than that of the Mo pre-
morphology and microstructure were analyzed by SEM cursor (Fig. S2†), possibly due to the initiation of complex
imaging (Fig. S2†). The SEM images illustrated the inherent decomposition, although the stacked layers are still visible.
stacked plate morphology of the Mo2 complex with roughly rec- Sample MoS2-400 features a similar morphology while the
stacked microplates get smaller and more irregular in shape
(Fig. S8†). During pyrolysis, the material gradually becomes
porous and gaps are formed between different layers. As a
result, a sponge-like stacked lamellar morphology is developed
for MoS2-T (T = 500 to 700) (Fig. S9–S11†). Such highly porous
stacked lamellar structures were beneficial for electrolyte
storage and electrochemical performance enhancement.47 In
addition, the SEM images of MoS2-800 (Fig. S12†) and MoS2-
900 (Fig. S13†) reveal that further increasing the pyrolysis
temperature leads to a more densely packed structure and
much smaller inter-particulate pore sizes.
Fig. 1 X-ray crystal structure of Mo2O2(μ-S)2(Et2dtc)2. All hydrogen
atoms are omitted for clarity. Thermal ellipsoids are shown at the 50%
The distinct microstructures of 2H-MoS2 obtained at
probability level.
different temperatures would affect their surface area and pore
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Fig. 2 MoS2-600: (a) PXRD pattern; (b) Raman spectrum; (c) Mo 3d and (d) S 2p XPS spectra.
distribution. The specific surface area of the materials was be detected in the full XPS spectra, demonstrating the exist-
determined by Brunauer–Emmett–Teller (BET) theory and the ence of such elements at the surface of the MoS2-600 samples.
pore size was calculated by using the Brunauer–Joyner– The high-resolution Mo 3d spectrum of MoS2-600 (Fig. 2c) is
Halenda (BJH) model. As shown in Fig. S14a and S15,† all the fitted with three peaks at 235.5, 232.5 and 229.4 eV binding
samples exhibit a type IV isotherm with a H3 hysteresis loop, energies, corresponding to Mo6+ (Mo–O bond), Mo4+ 3d3/2 and
reflecting the existence of a mesoporous structure.48,49 The cal- Mo4+ 3d5/2 states, respectively. The presence of the Mo(VI)
culated specific surface areas are summarized in Table S2.† signal testifies the partial oxidation on the surface of MoS2-600
Consistent with the morphology revealed by SEM (vide supra), (the sampling depth of Al Kα radiation is ca. 6 nm), although
the MoS2-600 sample possesses the highest specific surface the dominant species formed from pyrolysis under N2 is MoS2.
area of 77.3 m2 g−1. Consequently, we focus on MoS2-600 for The Mo 3d spectrum also contains a broad S 2s photoelectron
further electrode assembly and characterization. As illustrated peak at 226.5 eV, which is indicative of the 2H-phase for MoS2
in Fig. S14b,† the pore sizes in MoS2-600 mainly distribute in consistent with the PXRD results.51 The high-resolution S 2p
the range of 2–12 nm peaked at ca. 2.5 nm. Such favorable spectrum (Fig. 2d) of MoS2-600 features two peaks at 163.4 and
specific surface areas and mesoporous structures are desirable 162.2 eV, which are ascribed to the 2p1/2 and 2p3/2 signals of
for ion diffusion during the energy storage process. As demon- the S2− anions, respectively. As displayed in Fig. S16b,† several
strated in Fig. 2b, the composition and phase purity of MoS2- C 1s peaks at 289.1 eV (OvC–O), 287.6 eV (CvO), 286.5 eV (C–
600 were further investigated by Raman spectroscopy. The O), 285.5 eV (C–N) and 284.7 eV (C–C/CvC) can be detected,
characteristic peaks at 383 and 407 cm−1 are attributed to the suggesting the presence of residual C at the surface of MoS2-
in-plane E1 and out-of-plane A1g vibration modes of the 2H 600. Similarly, the N 1s spectrum (Fig. S16c†) could be decon-
2g
phase, respectively (Δν = 24 cm−1).50 Moreover, D and G bands voluted into two types of N-containing species, i.e. graphitic N
for the carbon matrix were not detected in the Raman spec- (399.6 eV) and pyrrolic N (398.0 eV). The nearby peak at 395.3
trum, indicating the removal of the C element as volatile COx eV belongs to the Mo 3p3/2 state. In the O 1s spectrum
during the pyrolysis process, which is consistent with the (Fig. S16d†), the two peaks located at 532.9 and 531.8 eV are
results of the EDS experiment (vide infra).
assigned to chemisorbed O2 and the Mo–O bond, respectively.
XPS is a useful tool to give insights into the superficial
Scheme 1 illustrates the two-step process for the prepa-
chemical composition and valence state of MoS2-600. As seen ration of a freestanding 2H-MoS2/CC electrode via solvo-
in Fig. S16a,† the Mo 3d, S 2p, C 1s, N 1s and O 1s signals can thermal growth and subsequent pyrolysis of the Mo2O2(μ-
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(Fig. S18†). It is clear that all the peaks observed for 2H-MoS2/
CC can be indexed to the 2H-phase of MoS2 (JCPDS #75-1539),
except for the broad carbon cloth reflection at ca. 26°. Such
results are in line with the above-mentioned pyrolysis study of
the Mo2O2(μ-S)2(Et2dtc)2 precursor at various temperatures
(Fig. 2a and S6†). The detection of no other residues or impur-
ity phases suggests that the existence of the CC substrate does
not interfere with the solvothermal formation or thermal
decomposition of Mo2O2(μ-S)2(Et2dtc)2.
As depicted in the SEM images of Mo2O2(μ-S)2(Et2dtc)2/CC
(Fig. S19†), the Mo(V) precursor particles with an average size
of 15 to 30 μm were solvothermally formed and evenly distribu-
ted on the fibers of CC. After subsequent pyrolysis treatment at
600 °C, the obtained 2H-MoS2/CC exhibits a relatively uniform
distribution and a large-scale conformal morphology inherited
from its Mo complex precursor (Fig. 3a). The sample is binder-
Scheme 1 Schematic illustration for the two-step preparation of the
2H-MoS2/CC electrode.
free and self-supported, demonstrating
a highly porous
sponge-like stacked lamellar morphology penetrated by the
fibers of CC (Fig. 3b–e). Such microstructures might serve as
nano-reservoirs to preserve the electrolyte during the electro-
chemical processes of the assembled supercapacitor. Thus, it
can prolong the retention time of the electrolyte in the meso-
pores of the electrodes, which could result in superior electro-
chemical performance. The TEM images shown in Fig. S20†
show the coexistence of bulk 2H-MoS2 and exfoliated MoS2
nanosheets. In the HR-TEM image (Fig. 3f), the lattice fringes
with a d-spacing of 0.63 nm in accordance with the (002)
S)2(Et2dtc)2 complex in the presence of CC. First, the Mo(V)
complex precursor was in situ grown on the CC substrate
through solvothermal treatment. Secondly, the Mo2O2(μ-
S)2(Et2dtc)2/CC assembly was heated at 600 °C for a full conver-
sion to the sponge-like 2H-MoS2/CC electrode. This process
can be easily recognized by the color change of the sample-
loaded CC from yellow to black (Fig. S17†), which is also con-
firmed by the comparison of the PXRD patterns for the as-syn-
thesized 2H-MoS2/CC, Mo2O2(μ-S)2(Et2dtc)2/CC and CC
Fig. 3 2H-MoS2/CC: (a–e) SEM images at randomly selected areas with various magnifications. (f) HR-TEM image. (g) Elemental mapping.
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crystal planes of 2H-MoS2 were unambiguously observed, respectively. All the electrochemical tests were performed
demonstrating the existence of MoS2 in its 2H phase. This is using a 1 M aqueous H2SO4 solution as the electrolyte. This is
further confirmed by the detected SAED image (Fig. S21†) with due to the much smaller ionic radius of H+ in comparison
three distinct diffraction rings, corresponding to the (100), with that of Li+ (∼0.9 Å) or Na+ (∼1.16 Å), which will facilitate
(103) and (110) polycrystalline planes of 2H-MoS2.
the intercalation of H+ among the layered structure of
The element mapping (Fig. 3g) and EDS (Fig. S22†) of the 2H-MoS2.52 To the best of our knowledge, the small ionic
2H-MoS2/CC samples also corroborate the successful single- radius of H+ will result in the least lattice expansion, enhan-
precursor synthesis of 2H-MoS2/CC. Other than the predomi- cing the stability of 2H-MoS2 during the electrochemical
nant Mo, S, and C elements in the materials, only very limited process.
amounts of N and O elements were detected, indicating the
The CV curves of the 2H-MoS2/CC electrode (Fig. 4a) show a
quite high purity of the electrode materials. The observed dis- pattern of intercalation type with broad electrochemically
tribution of the C element is consistent with its origin from reversible redox peaks, which is caused by the intercalation of
CC rather than the organic component of the Mo precursor. charge-compensating ions.53 In this case, the redox peaks can
Likewise, only C (from CC), Mo and S peaks could be identi- be tentatively attributed to the MoS2 + H+ + e− ↔ MoS-SH reac-
fied in the energy dispersion spectrum. All these characteriz- tion as previously reported.54 In addition, there is no obvious
ation results unambiguously disclosed the advantage of the deformation in the shape of the CV curves with increasing
facile two-step, single-source process for the fabrication of free- scan rates, implying the relatively good electrochemical stabi-
standing MoS2/CC electrodes for electrochemical applications. lity of 2H-MoS2. Furthermore, CV tests at various scan rates
The key to such success might be the utilization of homoleptic (5–100 mV s−1) were conducted to probe the kinetic mecha-
molybdenum dithiocarbamate as a pyrolysis precursor.
nism (see the ESI† for details).55 As displayed in Fig. 4b, the
resulted b values for the linear fitting of the equation log(i) =
log(a) + b log(ν) at various potentials fall in the range of
0.62–0.74, suggesting the coexistence of capacitance and
diffusion contributions.56 The capacitive contributions at all
scan rates were calculated (Fig. 4c, d and S23†), and they
increased from 11.0% at 5 mV s−1 to 42.3% at 100 mV s−1, con-
firming the presence of both diffusion-controlled and capaci-
3.2. Electrochemical measurements
The electrochemical performance of the 2H-MoS2/CC assembly
was investigated by CV, GCD, and EIS measurements. In the
three-electrode system, 2H-MoS2/CC was employed as the
working electrode, while a platinum foil and a Ag/AgCl elec-
trode were used as the counter and reference electrodes,
Fig. 4 2H-MoS2/CC electrode: (a) CV curves at different scan rates. (b) Determination of b-values against scan rates at different voltages. (c) CV
partition analysis showing capacitive and diffusive contribution to total current density at the scan rate of 100 mV s−1. (d) Capacitive contribution
fraction at different scan rates.
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tive effects in the 2H-MoS2/CC electrode.57 In order to further activated process between the electrode and electrolyte.59
elucidate the capacitance mechanism of the 2H-MoS2/CC elec- Afterwards, the capacitance slowly decreases and achieves a
trode, the contributions of electrical double-layer capacitance satisfactory capacitance retention of 75.7% after 5000 GCD
(Cdl) and pseudocapacitance (Cp) were distinguished by calcu- cycles. Such normal capacitance loss might be a result of the
lating the plots of Q versus ν−1/2 and Q−1 versus ν1/2. A detailed consumption of active materials in 1 M H2SO4 solution, as
analysis (Fig. S24†) results in a total specific capacitance (CT) mentioned in previous reports.60 The inset in Fig. 5b illus-
of 909.1 F g−1, which is composed of Cdl (43.1 F g−1) and Cp trates the charge–discharge curves for the initial and final 10
(866.0 F g−1).58 This result indicates that the pseudocapaci- GCD cycles, and deformation in the shape of the GCD curves
tance (Cp) contribution dominates in the total specific capaci- could be observed for the last stage of the 5000 cycles. This is
tance, which is attributed to the rapid redox reactions near the further confirmed by the comparison of the CV (Fig. 5c) and
2H-MoS2 surface and intercalation in the highly porous struc- GCD (Fig. S26†) curves for the 1st and 5000th cycles, respect-
ture of 2H-MoS2.
ively, suggesting the partial electrochemical consumption of
As illustrated in Fig. 5a, the specific capacitances of 2H-MoS2 in aqueous solution using a three-electrode configur-
2H-MoS2/CC were determined to be 550.0, 255.6, 88.9, 77.8, ation and the corresponding intrinsic stability of MoS2 with a
and 66.7 F g−1 at discharge current densities of 1, 2, 5, 10 and semiconducting 2H phase.61
20 A g−1, respectively. Clearly, the 2H-MoS2/CC electrode pre-
At an open circuit voltage, EIS was carried out in a fre-
sented herein shows a much superior electrochemical per- quency range from 0.01 Hz to 100 kHz to further investigate
formance in comparison with some recently reported MoS2 the electrochemical and structural properties of 2H-MoS2/CC.
electrodes (Table S3†). GCD measurements on a pristine CC In the Nyquist plot (Fig. 5d), the 2H-MoS2/CC electrode fea-
electrode indicate that the capacitive contribution of CC is tures almost no semicircle in the high frequency region. In
minimal in this system (Fig. S25†). Fig. 5b depicts the cycling contrast, after 5000 GCD cycles, the Nyquist plot includes a
performance of the 2H-MoS2/CC positive electrode at 8 A g−1 semicircle in the high frequency region and a straight line
in a voltage window of −0.3 to 0.6 V versus Ag/AgCl in a three in the low frequency region. Equivalent series resistance (Rs)
electrode system. The specific capacitance of the 2H-MoS2/CC and charge-transfer resistance (Rct) in the high frequency
electrode gradually increases for the first ca. 650 cycles with a region and Warburg resistance (W) in the low frequency
peak value of 97.2 F g−1, possibly due to the enhanced wetting- region were extracted and the results are summarized in
Fig. 5 2H-MoS2/CC electrode: (a) GCD curves at different current densities. (b) Cycling performance at 8 A g−1 over 5000 cycles (the inset shows
the charge–discharge curves of the initial and final 10 cycles). (c) CV curves and (d) Nyquist plots before and after 5000 cycles (the inset of (d)
shows the equivalent circuit and the fitting curves with the measured ones in the high frequency region).
11960 | Dalton Trans., 2021, 50, 11954–11964
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Table S4.† The data suggest that the Rct value for the freshly
In recent years, flexible electronics have drawn widespread
synthesized 2H-MoS2/CC is very small, while that for the attention due to their potential daily life applications and
same sample after 5000 GCD cycles is 0.35 Ω. This is in economic impact.62 In order to further evaluate its energy
accordance with the above-mentioned partial electrochemical storage application potential, a flexible symmetric all-solid-
consumption of 2H-MoS2 in the electrolyte during the char- state supercapacitor (ASSS) device was constructed using both
ging/discharging process, as unveiled by the CV and GCD 2H-MoS2/CC electrodes in a PVA-H2SO4 gel electrolyte with a
study.
sandwich configuration. As shown in Fig. 6a, the ASSS device
Fig. 6 Electrochemical performance of a symmetric ASSS device assembled with 2H-MoS2/CC electrodes: (a) CV curves at different scan rates. (b)
GCD curves at different current densities. (c) Ragone plots. (d) Cycling durance at 8 A g−1 over 8000 cycles (the inset shows the charge–discharge
curves of the initial and final 10 cycles). (e) Digital photographs of four devices connected in series lighting up red LEDs.
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can achieve an operating potential in the range from −1.0 to 1.0 by a facile two-step method, i.e. solvothermal formation and
V. At all scan rates, the CV curves display several broad reversible deposition of Mo2O2(μ-S)2(Et2dtc)2 coupled with its subsequent
redox peaks, which correspond to the pseudocapacitive behavior pyrolysis. The obtained binder-free, self-supportive electrode
of the 2H-MoS2/CC electrode. In the GCD curves (Fig. 6b) provided efficient conduction and a unique nano/microstruc-
measured at various current densities (from 1 to 20 A g−1), the ture, which contributed to its enhanced electrochemical per-
assembled device exhibits a nonlinear potential–time relationship formances. Its specific capacitance reached 550.0 F g−1 at 1 A
in the discharge stage. This indicates that some faradaic g−1, which surpasses the electrochemical performance of some
reduction or oxidation of the active materials is occurring, imply- 1T-MoS2 based materials in a three-electrode configuration. In
ing the presence of pseudocapacitance in the device. On the basis addition, the symmetric ASSS device assembled by flexible
of discharge time, the specific capacitances of the device were cal- 2H-MoS2/CC electrodes achieved a maximum potential range
culated as 127.5, 78.0, 35.0, 30.0, and 27.0 F g−1 at current den- of −1 to 1 V, and exhibited a specific capacitance of 127.5 F
sities of 1, 2, 5, 10 and 20 A g−1, respectively. The Ragone plots g−1 at 1 A g−1. Impressively, it delivered a high energy density
(Fig. 6c) of the energy density vs. power density were employed to of 70.8 W h kg−1 at a power density of 1 kW kg−1. It also pos-
evaluate the energy storage performance of the 2H-MoS2/CC sessed satisfactory functioning stability with a retention of
based flexible ASSS device. Impressively, it delivers a high energy 75% after an 8000-time cycling test. Thus, this work provides a
density of 70.8 W h kg−1 at a power density of 1 kW kg−1, which new strategy for the rational synthesis of sponge-like 2H-MoS2
still remains 15.0 W h kg−1 at 18.0 kW kg−1. This demonstrates on flexible conductive substrates, which could be employed as
that it can supply much higher energy density than previously electrodes for high-performance all-solid-state symmetric
reported MoS2 electrodes in the literature.63–67
supercapacitors.
Moreover, the long-term cycling stability of the flexible ASSS
device was examined by repeating the GCD test within the poten-
tial window of −1.0 to 1.0 V at 8 A g−1 (Fig. 6d). The specific capaci-
tance retention reaches 75% after 8000 cycles, while the coulombic
Author contributions
efficiency is maintained at a high level of 97% throughout the test. Zhishuo Yan: Conceptualization, investigation, formal ana-
It is obvious that the specific capacitance of the device gradually lysis, writing – original draft. Jixing Zhao: Investigation, formal
increased during the first ca. 3300 cycles and then slowly declined analysis. Qingsheng Gao: Writing – review & editing, supervi-
until the end of the cycling operation. In addition, the inset in sion. Hao Lei: Conceptualization, writing – reviewing &
Fig. 6d displays the charge–discharge curves in the initial and last editing, supervision, funding acquisition.
10 GCD cycles. There is only very little deformation in the shape of
the GCD curves after 8000 GCD cycles, demonstrating the relatively
high cycling stability of the device.
Conflicts of interest
For practical applications, the flexible ASSS devices are
desirable to be connected in series to expand the operating There are no conflicts of interest to declare.
voltage window. Since the potential window of a single device is
1.0 V, the voltage window of four devices linked in series should
be increased to 4.0 V, and the shapes of the CV and GCD curves
for the 4-in-1 connected device are well maintained (Fig. S27†).
Acknowledgements
As demonstrated in Fig. 6e, red light emitting diodes (LEDs) This work was supported by the Natural Science Foundation of
could be successfully illuminated for a few minutes by the four Guangdong Province (No. 2018A0303130021), Pearl River S&T
flexible ASSS devices connected in series. As for the potential Nova Program of Guangzhou (No. 201610010118) and Jinan
charge storage mechanism of 2H-MoS2/CC, it is believed that University.
the highly porous structure generated through in situ growth
and pyrolysis of Mo2O2(μ-S)2(Et2dtc)2 on carbon cloth, can facili-
tate ion diffusions at the interface between H2SO4 and MoS2.
Consequently, it is beneficial for the electrode to achieve short
Notes and references
charge time by the fast electrosorption process. Hence, the
derived 2H-MoS2/CC electrode exhibits excellent pseudo-
capacitive behavior and high energy density in a three-electrode
system and flexible ASSS device, rendering itself as an outstand-
ing candidate for future energy storage applications.
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