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Metal Organic Framework Derived Core-shell Structured Co9S8@N-C@MoS2 Nanocubes for Supercapacitor Xiaocheng Hou, Yizhou Zhang, Qiuchun Dong, Ying Hong, Yunlong Liu, Wenjun Wang, Jinjun Shao, Weili Si, and Xiaochen Dong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00773 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018

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ACS Applied Energy Materials

Metal Organic Framework Derived Core-shell Structured Co9S8@N-C@MoS2 Nanocubes for Supercapacitor Xiaocheng Hou,†‡ Yizhou Zhang,†‡ Qiuchun Dong,‡ Ying Hong,‡ Yunlong Liu,§ Wenjun Wang,§ Jinjun Shao,‡ Weili Si,‡* Xiaochen Dong‡* ‡

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials

(IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China §

School of Physical Science and Information Technology, Liaocheng University,

Shandong, 252059, China

ABSTRACT：Transition metal sulfides with designed nanostructure have been attracted significant research interest as electrode materials for supercapacitors. In this work, core-shell structured Co9S8@N-C@MoS2 nanocubes have been successfully fabricated through a sulfuration process based on ZIF-67 precursor. Due to improved electrical conductivity and large surface area, Co9S8@N-C@MoS2 nanocubes with core-shell heterostructure exhibit better electrochemical performance in supercapacitors compared with Co9S8. Moreover, the morphology of core-shell structured Co9S8@N-C@MoS2 nanocubes can be well adjusted by tuning the ratio between Co9S8 and MoS2. The homogeneous core-shell structured Co9S8@N-C@MoS2 nanocubes (S-3) can be obtained with the mass ratio of Na2MoO4·2H2O and CH4N2S at 1:2. And the obtained core-shell

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structured Co9S8@N-C@MoS2 (S-3) delivers a high specific capacitance of 410.0 F g-1 at the current density of 10.0 A g-1 after 20000 cycles with an excellent cycling stability (101.7% of the initial value). The excellent electrochemical property is mainly due to the unique structure which induces synergistic effects between Co9S8 and MoS2. KEYWORDS：：Co9S8@N-C@MoS2, core-shell structure, ZIF-67, supercapacitor, electrode material, surface modification, vulcanization

1. INTRODUCTION Transition metal oxides or hydroxides, such as MnO2,1-3 NiO,4 Co3O4,5,

6

V2O5,7

Fe2O38 and Ni(OH)29 have been widely researched as pseudocapacitive electrode materials. However, their poor conductivity brings about inferior electrochemical properties as electrode materials of supercapacitors.10,11 Compared with their oxide counterparts, transition metal sulfides have appeared as a new promising class of supercapacitor electrode materials because of its high electrical conductivity, good mechanical

properties

and

thermal

stability,

which

result

in

enhanced

electrochemical performances.12,13 Particularly, molybdenum sulfide (MoS2) as a typical metal dichalcogenide with two-dimensional lamellar structure, has demonstrated its potential when used as supercapacitor electrode material on account of its distinctive physical and chemical properties.14-16 However, the electrical conductivity of MoS2 is inherently low, which is accompanied by severe particle aggregation during charge/discharge process.17-19 To solve these problems, various


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composites of MoS2 have been constructed by combining it with different conductive materials like polyaniline20,21 polypyrrole,15 polydopamine,22 porous carbon,23 carbon nanotube,24 graphene14,25 and carbon spheres.26 It is also an important method to prepare composites between doped carbon and MoS2 using metal organic frameworks (MOFs) as precursor due to the unique structures.27 MOFs can be used for preparing carbon materials, such as nanoporous carbon,28,29 carbon nanocages,30 hollow carbon nanobubbles,31 nanoporous carbon-cobalt-oxide hybrids,32 metal oxides and metal oxide/carbon composites,33,34 which is all synthesized by thermal conversion of zeolitic imidazolate frameworks (ZIFs) or MOF with controlled morphologies. Based on the above research foundation, three dimensional

ZIFs

(3D ZIFs) have received more extensive research interest as precursors for both metal compounds and their composites with nanoporous carbon, because the resulted products possess

not only ordered nanostructure, large surface area, tunable pore

sizes, but also abundant nitrogen content.35-37 For example, nitrogen-doped carbon dodecahedrons,38 MnS/MoS2/C,39 CoSnSx nanoboxes sheathed in N-doped carbon,40 Co-C@Co9S8 double-shelled nanocages41 and multifunctional Mo–N/C@MoS2 composites42 have been successfully synthesized, exhibiting promising performance in energy storage. As another important transition metal sulfide, Co9S8 possesses high conductivity, but its sluggish ion transport kinetics limits the electrochemical properties. From this perspective, a composite consisting of MoS2 and Co9S8 as well as doped carbon interlayer holds great promise as electrode material for supercapacitor. On the other hand, rationally designed hierarchical nanocomposite



can provide higher surface area and more reactive sites, which can lead to enhanced electrochemical performances, especially the core-shell nanostructured materials.43,44 Besides these advantages, higher energy and power densities can also be expected from core-shell nanostructured composites as supercapacitor electrode due to the increase of contact area of electrochemically active components and void space which can mitigate volume change during the discharging/charging cycles. However, there are only few reports about the design and synthesis of hybrid metal sulfides coupled with 3D MOF-derived porous nitrogen-doped carbon with nanopolyhedron morphology and core–shell structures. Herein, a facile approach to prepare core-shell structured Co9S8@N-C@MoS2 nanocubes using 3D ZIF-67 as precursor has been developed. The obtained product shows uniform size of around 800.0 nm and shell thickness of 20.0 nm. As electrode materials for supercapacitor, the Co9S8@N-C@MoS2 nanocubes (S-3) can also deliver a specific capacitance of 410.0 F g-1 at the current density of 10.0 A g-1even after 20000 cycles. This may be attributed to the merits of the heterostructure and core-shell structure with large surface area. 2. EXPERIMENTAL SECTION

2.1. Materials synthesis

Synthesis of ZIF-67. 2.0 mmol of Co(NO3)2·6H2O and 110.0 mmol of dimethyl imidazole were respectively dissolved in 20.0 mL of DI water containing 10.0 mg of hexadecyl trimethyl ammonium bromide (CTAB) and 140.0 mL of DI water. Then


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Co(NO3)2·6H2O solution was poured into dimethyl imidazole solution and the mixture was stirred vigorously for about 30.0 min at room temperature.45 Finally, purple solid powder (ZIF-67) can be obtained by filtering, washing and drying. Synthesis of Co/N-C. The as-synthesized ZIF-67 was distributed and immersed in a certain concentration of glucose solution firstly, after which the mixture was processed by centrifugation and drying to obtain the ZIF-67@G precursor. ZIF-67@G precursor was calcined at 600.0 °C in Ar atmosphere (Heating rate: 3.0 °C min-1, heating time: 2.0 h). The corresponding product was named as Co/N-C. Synthesis of core-shell structured Co9S8@N-C@MoS2 nanocubes. First, a certain amount of Na2MoO4·2H2O and CH4N2S were dispersed in 30.0 mL of DI water under strong stirring. Then add the obtained Co/N-C into the mixture and sonicate for 30 min. Transfer the resulting solution into Teflon-lined stainless-steel autoclave and keep at 200.0 °C for 24.0 h next. After the reaction, the product was rinsed with deionized water and ethanol and dried. Finally, the product was calcined at 600.0 °C for 2.0 h in Ar atmosphere to obtain Co9S8@N-C@MoS2 nanocubes. To investigate the influence of the atomic ratio between molybdenum, cobalt, and sulfur on the electrochemical properties of the products, the samples with different ratios between Na2MoO4·2H2O and CH4N2S were synthesized. The prepared core-shell structured Co9S8@N-C@MoS2 nanocubes were denoted as S-1, S-2, S-3, S-4 and S-5, respectively. The specific quantities of Na2MoO4·2H2O and CH4N2S used in each product are shown in Table S1. To compare the electrochemical performances



between sulfide hybrid and single sulfide, Co9S8 was synthesized using the same method without adding Na2MoO4·2H2O. Synthesis of Co9S8/N-C/MoS2 sulfide composites. To explore the effect of glucose surface modification on the composite

materials, Co9S8/N-C/MoS2 sulfide

composites have also been synthesized without glucose. The mass ratio of Na2MoO4·2H2O and CH4N2S is 2:1, and other reaction conditions remain unchanged. 2.2. Electrochemical measurements

The working electrode was fabricated via mingling 80% active material, 10% acetylene black and 10.0% polyvinylidene fluoride (PVDF) as binder in ethanol solvent. The mixture was coated on Ni foam (average loading of active substance about 1.0~2.0 mg). Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements of electrode materials were measured on CHI-660 electrochemical workstation (CH Instruments Inc., Shanghai in China) using three-electrode system in 3.0 M KOH aqueous electrolyte. Hg/HgO electrode was reference electrode, while Pt electrode was used as counter electrode. The potential window of CV was 0.0-0.6V at various scan rates (Scan rates: 10.0, 20.0, 30.0, 40.0, 50.0, 100.0 and 200.0 mV s-1). EIS tests were performed in the frequency range of 100.0 kHz to 0.01 Hz. Moreover, cyclic stability testing was recorded on NEWARE battery test system at the current density of 10.0 A g-1.


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On the basis of the time-potential curve of charge/discharge process, the specific capacitance of active material can be calculated. The calculation formula is as follows: Cs =

ூ∗td

(1)

m∗∆V

In this formula, I (A) is the charge/discharge current density, td (s) represents the discharge time, m (g) stands for the mass of electroactive materials, ∆V (V) means the discharge voltage range. 2.3. Characterization

The morphologies of ZIF-67, ZIF-67@G, Co/N-C and core-shell structured Co9S8@N-C@MoS2 nanocubes were characterized by field emission scanning electron microscopy (FESEM; Hitachi, S-4800), transmission electron microscopy (TEM; JEOL, JSM-2100F), and high-resolution transmission electron microscopy (HRTEM). X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advanced

X-Ray

Diffractometer

(Cu-Ka

radiation,

λ=1.5418

Å).

X-ray

photoelectron (XPS) spectra were collected on an ESCALAB 250Xi system (Thermo Scientific) probe spectrometer. Dynamic light scattering (DLS) was performed on NanoPlus-3 particle size analyzer. 3. RESULTS and DISCUSSION

Scheme 1 shows the synthesis of core-shell structured Co9S8@N-C@MoS2 nanocube. First, the ZIF-67 precursor was coated by a carbon layer, and then Co9S8@N-C@MoS2 nanocube was obtained through the vulcanization process.



The composition and morphology of ZIF-67 precursor were characterized by XRD, DLS and SEM. In the XRD pattern (Figure 1a), there are four typical diffraction peaks, which correspond to (011), (002), (112) and (222) planes of ZIF-67, respectively.46 DLS and SEM images (Figure 1b-d) show that ZIF-67 particles with uniform size of around 600.0 nm have regular cubic structure and smooth surface. This unique cubic structure is attributed to the CTAB surfactant, which can affect the growth rate of crystal grain on different surface.47,48 SEM images of ZIF-67 precursor with glucose as carbon source before and after calcination are shown in Figure 1e-f. The cubic structure still remains after calcination, and the thickness of covered carbon layer is approximately 50.0 nm. The carbon coating on the surface is crucial to the uniform growth of MoS2 on the cube.49 MoS2 nanosheets will be self-aggregated without the surface modification of glucose, the SEM images of corresponding sulfide compounds are shown in Figure S1. The XRD patterns of core-shell structured Co9S8@N-C@MoS2 nanocubes with different ratio of Na2MoO4·2H2O and CH4N2S are similar, all exhibiting main peaks of Co9S8 and MoS2 (Figure 2a). The diffraction peaks of Co9S8 are mainly located at 29.82°, 31.18°, 44.68°, 47.55°, 52.07°, and 68.04°, corresponding to (311), (222), (422), (511), (440) and (640) planes, respectively. Besides, the prominent diffraction peaks at 14.13°, 32.91°, 39.51°, 58.76° belong to (002), (100), (103) and (110) planes of MoS2. At the same time, SEM images of the corresponding sulfides are exhibited in Figure S2. The SEM image of Co9S8 shows cubic porous structure (Figure S2a). With different ratios of Na2MoO4·2H2O and CH4N2S, a series of


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Co9S8@N-C@MoS2 nanocubes from S-1 to S-5 have been prepared to adjust the morphology of the core-shell structure, as shown in Figure S2b-f. After treatment with carbon and MoS2, the Co9S8 cores are completely wrapped by MoS2 shells, and the average diameter of the whole cubic particle reaches to about 800.0 nm. It is clear that aggregation of the MoS2 nanosheets appears with the increasing of thiourea content. However, the less molybdenum concentration, the larger MoS2 nanosheets. Considering the structural advantages of S-3 nanocubes with more uniform morphology, larger surface area and more reactive sites, it was selected for further characterizations to improve the electrical performances.50,51 Figure 2b presents the Raman spectrum of S-3 nanocubes. D and G bands of amorphous carbon located at 1360.3 and 1583.9 cm-1 show the formation of carbonaceous composites. The D-band is resulted from disorder-induced phonon mode and the G-band corresponds to the E2g phonons of C sp2 atoms. The SEM images of S-3 nanocubes under different magnification are exhibited in Figure 2c-d. It can forecast that the well-formed Co9S8 nanoparticles coated with short and uniform MoS2 nanosheets can provide more active sites for the electrochemical reactions. TEM image of S-3 nanocubes in Figure 3a manifests its core-shell structure. And the diameter of the cube is about 800.0 nm. The HRTEM image further demonstrates the crystalline nature of Co9S8 and MoS2 (Figure 3b). The observed lattice spacing of 0.6265 nm is indexed to the (002) plane of MoS2, while the lattice fringe spacing of 0.2866 nm stands for the (222) plane of Co9S8. In addition, the crystal fringes of MoS2 were discontinuous as circled in Figure 3b, which demonstrates the existence



of defects in the crystals. On the one hand, structural defects in the crystals are favorable to increase the stability of MoS2 nanosheets by decreasing the surface energy. On the other hand, this structure is likely to produce more unsaturated active atoms, which can influence the electronic structure and in turns alter the activity of active sites. What is more, the defect structure also can help small ions permeate and travel. The high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) in Figure 3c demonstrates that Co9S8 cubes have been completely wrapped by MoS2 layer. As performed in EDX mappings (Figure 3d-h), cobalt, molybdenum and sulfur elements are uniformly dispersed in the cubes. By overlapping element signals, the hierarchical structure consisting of MoS2 shell, carbon interlayer and Co9S8 core is clear (Figure 3d). The XPS measurement was further conducted to investigate the chemical compositions of S-3 nanocubes. Co, Mo, C, S, N and O are detected in the survey spectrum, as shown in Figure 4a. The Co 2p XPS spectrum (Figure 4b) consists of six peaks mainly. Two obvious peaks located at 779.3 eV in Co2p3/2 and 794.2 eV in Co 2p1/2 originate from Co3+, while the peaks at 783.0 eV in 2p3/2 and 799.1 eV in 2p1/2 are derived from the spin−orbit characteristics of Co2+. In addition to these, the other two peaks at 787.1 and 804.5 eV can be indexed to satellite peaks of Co3+.52 As shown in the Mo 3d & S 2s spectra (Figure 4c), two peaks are located at 229.6 eV and 232.8 eV, corresponding to Mo 3d5/2 and Mo 3d3/2 of Mo4+ respectively. This further demonstrates the formation of MoS2 in the compound. The peak at 226.6 eV derives from S 2s in accordance with the chemical state of S species being bonded


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with Mo and Co ions. Meanwhile, the peak of Mo6+ at the binding energy of 235.9 eV can be attributed to partial surface oxidization from MoS2 to MoO3.52 In the S 2p spectrum (Figure 4d), two distinct peaks at 162.3 and 163.5 eV are ascribed to S 2p3/2 and S 2p1/2, indicating the existence of S2− in the hybrid. Besides, another satellite peak at 169.2 eV corresponds to SO42− species, which similarly results from the partial oxidation of sulfur species on the material surface by air.53 The C 1s spectrum in Figure 4e exhibits four obvious peaks with binding energies of 284.6 285, 285.7, and 287 eV, each of which can be indexed to C=C/C−C, C−S, C−N/O, and C=O bonds, respectively. The observed peaks of C−S and C−N/O moieties may be ascribed to the substitution of C with N and S atoms in the composite.52,54-56 The N 1s XPS spectrum (Figure 4f) can be divided into Mo 3p and three different types of N. The largest peak with binding energy of 395.6 eV corresponds to Mo 3p, while the other three fitted peaks at 398.4, 400.2, and 402.1 eV are ascribed to different types of nitrogen, including pyridinic, pyrrolic and graphitic nitrogen.52,57 Serving as electrode materials, the core-shelled S-3 nanocubes were evaluated for electrochemical performances. The cyclic voltammetry at different scan rates (Figure 5a) shows that all the curves have a pair of strong redox peaks, indicating the capacitance properties are mainly controlled by faradaic redox reactions.58 The position of redox peaks in the CV curves are consistent with the electrochemical redox peaks of Co9S8, proving the Faraday capacitance of the reaction is mainly produced by Co9S8.59,60 MoS2 in the materials displays the typical CV curve of



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electric-double-layer capacitor, as reported in literatures.61,62 The specific reversible Faradic reactions in the process are as follows:59 Co9S8 + OH– ↔ Co9S8OH + H2O + e–

(2)

Co9S8OH + OH– ↔ Co9S8O + H2O + e–

(3)

As the scan rate was gradually adjusted from 10 mV s-1 to 200 mV s-1, the peak current intensity and the area of CV curves also increase, suggesting the electron and ion transport rates are very rapid at the applied scan rates. Additionally, the oxidation peak and the reduction peak shifts towards high voltage and low voltage, respectively. This may be ascribed to the rapid embedding/stripping rates of proton and redox reactions on the electrode surface controlled by diffusion.63 Figure 5b is GCD curves of S-3 nanocubes at diverse current densities. The GCD curves are all highly symmetrical, implying the Faraday reactions are highly reversible.64 Figure 5c shows the cycling performance at the current density of 10.0 A g-1. The phenomenon that the specific capacitance increases upon cycling time may be attributed to the probable activation of materials in the process.61 After 20000 cycles, the specific capacitance of S-3 nanocubes electrode remained 101.7% of the initial value. To explore the composition changes before and after cycling, CV tests were performed on the electrode materials before and after cycling. As is shown in Figure S3, the position of redox peaks did not change, indicating the highly reversibility. Figure 5d exhibits the specific capacitances at diverse current densities. When the current density is adjusted from 0.5 to 20.0 A g-1, the specific capacitance of S-3 nanocubes


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can also maintain 79.8% of that of 0.5 A g-1, exhibiting an excellent rate performance. To further explore the electrochemical behaviors of S-3 nanocubes, the EIS tests before and after 20000 cycles were measured from 100.0 KHz to 0.01 Hz, as shown in Figure 5e. It can be observed that the Nyquist plots before and after 20000 cycles present a negligible semicircle shape in the high frequency region, indicating S-3 electrode is undergoing fast charge transfer efficiency on the interface.63 The slope of the lines almost remains the similarity, indicating the excellent reversibility of the electrode. The effect of MoS2 content on electrochemical performance was further studied. As shown in Figure S4a, the specific capacities of S-1 and S-3 nanocubes are 419.7 and 366.0 F g-1, both of which are higher than that of Co9S8 (313.2 F g-1). Also, the S-3 nanocubes exhibit a more vertical line in the EIS curve, suggesting the more efficient charge storage of the electrode (Figure S4b). The cycling performances of S-1, S-2, S-3, S-4 and S-5 nanocubes were also investigated at the current density of 10 A g-1. It can be concluded that the specific capacitance of the composite exhibits a degenerative trend with the increasing sulfur content (Figure S5). This is mainly due to the serious aggregation of MoS2 nanosheets as shells. The best electrochemical performance was achieved by S-3 nanocubes because of the appropriate MoS2 content and its stable structure. Additionally, performance comparison with other related works were compared. As shown

in

Table

S2,

core-shelled

sulfide

composite


shows

the

greatest


electrochemical advantage. Its excellent electrochemical performance may be due to the following reasons. Unique core-shell structure and large space between adjacent MoS2 nanosheets can not only store a large number of ions and electrolyte, but also make the transfer of ions in electrolyte easy during the reaction process. Besides, the unique structure with high specific surface area provides more reactive sites and corresponding double layer capacitance.65 What is more, the introduction of nitrogen doped carbon has also played a vital role in optimizing performance by improving electrical conductivity of the composite. 4. CONCLUSION

In summary, core-shell structured Co9S8@N-C@MoS2 nanocubes with uniform diameter of 800.0 nm are prepared by using 3D structured ZIF-67 as precursor through hydrothermal reaction, calcination, and vulcanization process. The composite shows a better electrochemical performance compared with Co9S8 as supercapacitor electrode. Among the composites with different ratio of molybdenum, cobalt, and sulfur, S-3 nanocubes exhibit the best sophisticated electrochemical performance. The specific capacitance of S-3 nanocubes can retain 410.0 F g-1 at the current density of 10.0 A g-1 during the 20000 cycles. This can be mainly ascribed to the synergistic effect between two metal sulfides facilitated by the nitrogen doped carbon interlayer, as well as the formation of the core-shelled structure. ASSOCIATED CONTENT

Supporting Information


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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.xxxxxxx.

SEM images of Co9S8/N-C/MoS2 sulfide composites synthesized without glucose and Co9S8@N-C@MoS2 sulfide composites, CV curves of S-3 nanocubes before and after cycling, comparison of the electrochemical properties of Co9S8, S-1 and S-3, cycling performances of Co9S8@N-C@MoS2 nanocubes prepared with different ratio of Na2MoO4·2H2O and CH4N2S, and performance comparison with other related works. Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The work was supported by the NNSF of China (61525402, 5161101159, 61775089)



Scheme 1 Schematic illustration of the synthetic process of core-shell structured Co9S8@N-C@MoS2 nanocube.


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Figure 1. (a) XRD pattern, (b) DLS size distribution, (c-d) SEM images of ZIF-67 precursors, (e) SEM image of ZIF-67@G, (f) SEM image of Co/N-C.



Figure 2. (a) XRD patterns of Co9S8@N-C@MoS2 nanocubes prepared with different ratio of Na2MoO4·2H2O and CH4N2S, (b) Raman spectrum of S-3 nanocubes, (c-d) SEM images of S-3 nanocubes under different magnifications.


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Figure 3. (a) TEM, (b) HRTEM, (c) HAADF-STEM, (d-h) EDX element mappings of core-shelled S-3 nanocubes, (e) S element, (f) Co element, (g) Mo element, (h) C element.



Figure 4. XPS spectra of core-shelled S-3 nanocubes. (a) Survey spectrum, (b) Co 2p spectrum, (c) Mo 3d & S 2s spectrum, (d) S 2P spectrum, (e) C 1s spectrum, (f) N 1s & Mo 3p spectrum.


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Figure 5. Electrochemical characterization of S-3 nanocubes. (a) CVs at various scan rates, (b) Charge/discharge curves at different current densities, (c) Cycling performance at current density of 10.0 A g-1, (d) Specific capacitances at diverse current densities, (e) Nyquist plots before and after 20000 cycles at current density of 10.0 A g-1.



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