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Poulami Hota
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Received: 2019-06-6
Revised: 2019-08-5
Accepted: 2019-08-29
Online: 2020-03-01
Copyright: 2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
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1Authors contributed equally to this work.
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Abstract
Amorphous materials have recently gained much attention as electrode materials in supercapacitor application due to the presence of larger amount active sites which can efficiently increase the storage capacity of the materials. Nano engineering is an elegant approach to fully utilize the advantages of the amorphous structure. Moreover, large surface area and high conductivity of reduced graphene oxide (RGO) can efficiently increase the storage capacity of the system. Exploiting this idea, in the present work, we have successfully synthesized amorphous MoS2 of two different sizes on reduced graphene oxide and thoroughly investigated the supercapacitor behavior of the system. The specific capacitance of the composite structures has been found to be largely increased with decreasing size of the amorphous nano particle. The specific capacitance of amorphous MoS2-RGO composite containing nearly 50 nm of MoS2 found to be 270 F/g whereas when the particle size is reduced to 5-7 nm, value of specific capacitance increases to 460 F/g. The large increase in specific capacitance with the tuning of the size of amorphous nano particle has been explained by the presence of a large number of active sulfur edges of ultra-small MoS2 nano structure along with the better charge transport which can effectively increase the storage capacity of the overall system. The retention in the capacitance of the material has been found to be 90 % after 5000 cycles.
Keywords：
Advancement of modern society needs to develop a new class of energy storage systems. Nowadays, supercapacitor becomes reliable energy storage device due to its high power density as well as excellent cyclic stability compared with the usual batteries [1]. Supercapacitors have a wide range of applications in portable electronics as well as in hybrid electrical vehicles. Supercapacitors store electrical energy by two mechanisms; they are electrochemical double-layer capacitance and pseudocapacitance. The former arises due to the reversible adsorption of ions at the electrode-electrolyte interfaces whereas the latter appears as a result of a Faradic redox reaction. However, one of the main challenges to improve supercapacitor performance is to improve its energy density and storage capacity. Several efforts have been made over the past few decades to explore new energy materials with high storage capacity [[2], [3], [4], [5]]. Therefore in the present situation exploration of new material with superior supercapacitive properties is very much desirable.
Large surface area and unique electronic properties make graphene and different metal chalcogenides as potential candidates for energy application. However, the main disadvantages of graphene-based devices are low volumetric capacitances and low gravimetric energy densities. To deal with these problems different graphene-based composite materials, for example, graphene-polymer composite, graphene-metal oxide/metal sulfide nanoparticles or composites of nanosheets have been prepared with improved storage capacity [3,[6], [7], [8], [9], [10]]. The specific capacitance of the material can be further improved by controlling the size and shape of the material. By decreasing the size of the material to their nano dimension several unique physical and chemical properties can be introduced compared to their bulk counterparts. Therefore, Nano engineering provides a new way to improve the electrochemical performance of energy materials.
Among the different transition metal dichalcogenides, Molybdenum disulfide (MoS2) has been considered as promising energy material due to its high conductivity and layered structure. The layer structure of MoS2 is composed of Molybdenum (Mo) atoms sandwiched between two sulfur(s) atoms held together via Vander wall interaction. MoS2 can efficiently store energy by reversible adsorption of electrolyte ions on MoS2 surfaces as well as faradaic charge transfer process involving the change in oxidation state of Mo and S edge centers. However, comparatively lower conductivity of MoS2 than graphene limits its storage capacity. Several efforts have been done to improve storage capacity of MoS2 either by its structural modulation or by introducing some conducting material (RGO) [[11], [12], [13], [14], [15]]. There are few pieces of literature available where amorphous structures show better electrochemical performance than their crystalline analogous [[16], [17], [18]]. Recently, it has been reported that amorphous MoS2-RGO-CNT composite shows better storage capacity, however, the storage capacity of amorphous MoS2 is not well explored [19].
To explore this, in the present work we have successfully synthesized amorphous MoS2 of two different sizes on conducting reduced graphene oxide (RGO) surface, namely, aMoS2(L)-RGO and aMoS2(S)-RGO for large and small nanoparticles respectively. Reduced graphene oxide increases electrical conductivity and available surface area of the system as well as higher mechanical strength of RGO helps to improve the cyclic stability of the composite structure. The excess sulfur sites present in amorphous MoS2 help to adsorb more number of H+ ion from the electrolyte solution. PVP (poly vinyl pyrrolidone) is used as capping agent which controls the size of MoS2 nanoparticles. It has been observed that decreasing the size of the amorphous MoS2 leads to the increase in number of active sulfur edges (S22-) which can effectively increase the storage capacity of the composite material. The aMoS2(L)-RGO composite shows specific capacitance of 270 F/g at current density of 1 A/g whereas aMoS2(S)-RGO composite shows specific capacitance of 460 F/g at the same current density. Therefore, tuning of the size of the amorphous nanoparticles provides a new way of increasing storage capacity of the composite material. In comparison, we have also measured the storage capacity of crystalline MoS2 quantum dots decorated RGO which shows much lower capacitance value than that of the aMoS2(S)-RGO composite.
Ultrafine graphite powder (Loba), sodium chloride (Merck), potassium permanganate (Merck), concentrated sulfuric acid (Merck, 98 % purity), hydrogen peroxide (30 %, Merck), hydrochloric acid (35 %, Merck), Ammonium tetrathiomolybdate ((NH4)2MoS4) (Sigma Aldrich), Polyvinylpyrrolidone (PVP) (Merck), Hydrazine hydrate (NH2NH2,H20) (Merck), ethylene glycol (Merck), Ethanol, deionized water.
Graphene oxide (GO) has been synthesized from natural graphite by the same modified Hummers method where KMnO4 is used in desired amount [20].
Amorphous MoS2(L)-RGO composite (aMoS2(L)-RGO) has been synthesized by a simple temperature controlled reflux method. In the First step, 5 ml of Graphene oxide (GO) solution is diluted to 20 ml with ethylene glycol and ultrasonicated to get a clear homogeneous brown solution. 0.1 mmol of Ammonium tetrathiomolybdate ((NH4)2MoS4) and 0.15 mmol of PVP have been dissolved separately in ethylene glycol and are ultrasonicated to get a clear solution. The clear solution of Ammonium tetrathiomolybdate is added drop wise to the GO solution taken in a double neck round bottom flask and allowed to stir for 30 min. Then the PVP solution is added to the mixture and again the mixture is allowed to stir for another 30 min. Finally, hydrazine hydrate is added to the mixture and the mixture is refluxed at 220 °C for 5 h under inert (Argon) atmosphere. The mixture is allowed to cool to room temperature and washed several times by water : ethanol (1:1) solution to get the final product.
To synthesis small amorphous MoS2 particle on Reduced graphene oxide (aMoS2(S)-RGO), above mentioned procedure is followed by changing the reflux time to 2 h.
Synthetic procedure of crystalline MoS2 decorated reduced graphene oxide (cMoS2 QDs-RGO) has been provided in supporting information.
The measurements of charging-discharging behavior and specific capacitance of the as-synthesized materials have been carried out by two electrode configuration. The two electrode supercapacitor has been fabricated by using two coin cell electrodes separated by a spacer (filter paper). 1 M aqueous solution of sulfuric acid solution is used as an electrolyte. The coin cell electrodes are prepared by drop casting homogeneous slurry onto the coin cell. The homogeneous slurry has been obtained by mixing 90 wt% of the as-synthesized active material, 5 wt% acetylene black and 5 wt% PVDF binder solution in NMP. Each of the electrodes contains about 3.8 mg of active material. The final coin cell electrode has been obtained after drying the as prepared electrodes. Supercapacitor measurements have been carried out by inserting the as prepared coin cell electrode within two thick copper plates. Schematic diagram of the devise fabrication of the two electrode configuration has been given in Scheme 1.
Scheme 1. Schematic diagram of the two electrode measurement system.
Microscopic structures of two composite systems are determined by Field Gun Transmission Electron Microscopy (FEGTEM), and energy-dispersive X-ray spectroscopy (EDS) to analyze the elemental composition. The TGA experiment is carried out using SDT Q600 V 20.5 Build 15 from 25 to 800 °C under inert atmosphere. BET measurements have been carried out using Quantachrome Instruments. X-ray diffraction (XRD) has been performed by X-ray diffractometer (RICHSEIFERT-XRD 3000 P) using an X-ray Generator-Cu, 10 kV, 10 mA, and wavelength of 1.5418 Å. X-ray photoelectron spectroscopy (XPS) is performed using an OMICRON-0571 system. FEGTEM is done using JEOL 2011 which is equipped with EDS tool. Electrochemical performance of the material is tested by cyclic voltammetry (Versastat 3, Princeton applied research) and NETWARE BTS battery tester is used to investigate galvanostatic charge/ discharge of the composites.
To increase the specific capacitance of MoS2, amorphous MoS2 nanoparticles of two different sizes have been grown on reduced graphene oxide surface. Ammonium tetrathiomolybdate [(NH4)2MoS4] has been used as a single precursor for both Mo and S. PVP has been used as capping agent as well as helps to grow the amorphous structure. Hydrazine hydrate is used as reducing agent. It has been observed that decrease in the size of the amorphous MoS2 nanoparticles leads to the increase in the number of sulfur edge sites which in turn help to increase the specific capacitance of the composite materials. The synthetic scheme for the formation of composites has been given in Scheme 2. Formation of RGO-aMoS2 composites is confirmed by different physical characterizations as mentioned earlier.
Scheme 2. Schematic diagram of the formation of amorphous MoS2-RGO composites.
Fig. 1(a) shows XRD pattern of the GO which shows the characteristic peak of Graphene oxide at 12.4°. Fig. 1(b) and (c) shows XRD patterns of the aMoS2(L)-RGO and aMoS2(S)-RGO composites. The XRD patterns show no characteristic peaks of MoS2 indicating the absence of any crystalline phase and the hump near 24° indicates the presence of RGO. Therefore XRD analysis reveals that the composite materials are completely amorphous in nature.
Fig. 1. (a) X-ray diffraction patterns of graphene oxide (GO), (b) aMoS2(L)-RGO composite and (c) aMoS2(S)-RGO composite, (d) full range XPS spectra of aMoS2(S)-RGO composite, (e) XPS spectrum of Mo 3d region of aMoS2(S)-RGO composite and (f) deconvoluted spectra of S 2p of aMoS2(S)-RGO composite.
X-ray photoelectron spectroscopy (XPS) has been employed to investigate the chemical state and composition of the aMoS2(S)-RGO composite. Fig. 1(d) shows full range XPS spectra of aMoS2(S)-RGO composite which shows the characteristic S 2s and 2p, C 1s, Mo 3d and O 1s core-level photoemission peaks near 226 eV, 161 eV, 286 eV, 229 eV, and 534 eV, respectively [21]. Fig. 1(e) represents low range XPS spectrum of Mo 3d region and S 2 s of the corresponding composite which shows three peaks. One peak appears at 226.8 eV which corresponds to the binding energy of S 2 s. Another two peaks appear at around 229.5 eV and 232.5 eV, representing the binding energy of Mo 3d5/2 and Mo 3d3/2 respectively, which indicate the presence of Mo (+IV) oxidation state in the composite structure. To realize the role of sulfur to increase the storage capacity of the material, S 2p region is thoroughly investigated. Fig. 1(f) shows high-resolution deconvoluted spectra of S 2p fitted by four components. Among the four components, the two intense peaks appear at around 161.8 eV and 163.7 eV corresponding to S 2p3/2 energy states. Lower binding energy peak (161.8 eV) is attributed to the binding energy of sulfide (S2-) ligand whereas the binding energy appears at 163.7 eV is attributed to the bridging disulfide (S22-) ligand or apical sulfide ligand. Apart from these peaks, two low intense peaks appear at around 162.8 eV and 164.8 eV which correspond to the binding energies of S 2p1/2 of sulfide and bridging disulfide ligand [22]. The XPS analysis of aMoS2(S)-RGO composite reveals the presence of sulfide (S2-) ligand and bridging disulfide (S22-) ligand or apical sulfide ligand with atomic percentage 3.3 % and 6.6 %, respectively. aMoS2(L)-RGO and cMoS2 QDs-RGO composites also have been characterized by the XPS analysis given in the Supporting information. In case of aMoS2(L)-RGO composite, the atomic percentage of sulfide (S2-) ligand and bridging disulfide (S22-) ligand or apical sulfide ligand is found to be 3.6 % and 2.9 % whereas for cMoS2 QDs-RGO composite the atomic percentage is obtained as 6.2 % and 1 % respectively. Therefore, XPS analysis reveals the presence of large amount of excess unsaturated bridging disulfide in the small amorphous MoSx nanostructure, which plays the key role to improve the storage capacity of the material.
The morphology and amorphous nature of the composite materials are further investigated by transmission electron microscopy analysis. Fig. 2(a) and (b) shows FEG-TEM images of aMoS2(L)-RGO composite from which it is seen that large MoS2 particles are formed on reduced graphene oxide surface. The MoS2 nanoparticles of about 50 nm size are grown on RGO surface as shown in Fig. 2(c). Selected area (electron) diffraction (SAED) patterns confirm the amorphous nature of the composite material as given in Fig. 2(d).
Fig. 2. (a-c) Low-resolution TEM images of aMoS2(L)-RGO composite, (d) SAED patterns of the aMoS2(L)-RGO composite and (e) the corresponding EDS spectrum of aMoS2(L)-RGO composite (the inset shows the atomic percentage of the composite).
The material is further investigated by energy-dispersive X-ray spectroscopy (EDS) analysis as given in Fig. 2(e). EDS spectrum confirms the presence of molybdenum (Mo) and sulfur (S) and the ratio of Mo:S is found to be 3.24:6.89. This indicates the presence of excess sulfur in the amorphous MoS2 phase as obtained from the XPS analysis.
Restricting the reaction time to 2 h results the formation of very small MoS2 nanoparticles as shown in Fig. 3(a). Fig. 3(b) shows high-resolution TEM images of aMoS2(S)-RGO composite which shows the formation of 5-7 nm particle. Fig. 3(c) shows the SAED pattern of the composite material. The SAED pattern does not contain any bright spot corresponding to the crystal lattice which confirms the amorphous nature of the composite material. EDS analysis of the composite confirms the presence of Mo and S as given in Fig. 3(d). The atomic percentages of Mo and S are found to be 3.52 % and 9.68 %, respectively. The atomic percentage indicates the presence of large amount of excess unsaturated sulfur ligand which is in good agreement with the XPS analysis. The presence of these large excess of sulfur ligands makes the material potential candidate for supercapacitor application. Microstructural analysis of crystalline MoS2 quantum dots decorated RGO has been given in the supporting information.
Fig. 3. (a) Low-resolution TEM images of aMoS2(S)-RGO composite, (b) high-resolution TEM image of aMoS2(S)-RGO composite, (c) SAED patterns of the aMoS2(S)-RGO composite and (d) the corresponding EDS spectrum of aMoS2(S)-RGO composite (the inset shows the atomic percentage of the elements).
In order to compare the content of MoS2 present in aMoS2(L)-RGO and aMoS2(S)-RGO composites, thermogravimetric analysis (TGA) has been performed in inert atmosphere for both the samples and the results are given in the supporting information. TGA analysis of both the composites show almost similar type of curves. The initial weight loss of both the composites is associated with the evaporation of adsorb solvent molecules. The mass loss in the region 100 °C-300 °C is attributed to the thermal reduction of the oxygen-containing functional groups of reduced graphene oxide (RGO) whereas above 300 °C the gradual weight loss occurs due to the oxidation of MoS2. The final weight maintained for aMoS2(L)-RGO and aMoS2(S)-RGO composites are at around 50.8 % and 50 %, respectively. cMoS2 QDs-RGO composite has been also characterized by thermogravimetric analysis (TGA) which shows final weight percentage at around 49 %. Therefore, the similar type of weight loss in the high-temperature region for all the three samples indicates the presence of almost similar content of MoS2 in the composite materials.
The aMoS2(L)-RGO and aMoS2(S)-RGO composites have been further investigated by BET (Brunauer-Emmett-Teller) analysis in order to get an idea about the specific surface area of the composites and the results are given in the supporting information. The specific surface area of aMoS2(L)-RGO and aMoS2(S)-RGO composites obtained from BET analysis are 185 and 241 m2/(g m) respectively. Therefore, the result suggests that the decrease in size of the amorphous MoS2 nanoparticle leads to the increase in specific surface area of the aMoS2-RGO composites. Moreover, the increased surface area provides more active site for charge storage application.
In order to realize the effect of unsaturated sulfide ligand on storage capacity, two composite materials with two different size of amorphous MoS2; aMoS2(S)-RGO and aMoS2(L)-RGO composites have been used as electrode material for supercapacitor application. To compare the storage capacity of aMoS2(S)-RGO composite with its crystalline analogous, another material cMoS2 quantum dots decorated RGO has been prepared for supercapacitive measurement. The materials are characterized by galvanostatic charging-discharging, cyclic voltammetry (CV) and electrical impedance spectroscopy (EIS) measurements. Fig. 4(a) represents charging-discharging behavior of aMoS2(L)-RGO composite. The charging-discharging behaviour of the composite has been measured at current density of 1 A/g. The specific capacitance of the material has been calculated from the charging-discharging curve by using the following equation:
Csp = 2(iΔt/(mΔV)) (1)
where i denotes discharge current (A), Δt is the discharging time (s), m is the mass of the active material of single electrode (g) and ΔV is the change in voltage window (V). Specific capacitance of the aMoS2(L)-RGO composite calculated by using Eq. (1) is 270 F/g. According to literature, crystalline MoS2 nanosheet/RGO composite shows specific capacitance value of 243 F/g at current density of 1 A/g [14]. Therefore, specific capacitance of the amorphous material is found to be slightly higher than the literature report. Fig. 4(b) shows cyclic voltammetry (CV) measurement of the aMoS2(L)-RGO composite. The CV curve shows nearly rectangular shape at a scan rate of 10 mV/s, and the rectangular shape is maintained upto scan rate of 25 mV/s. This is the characteristic of ideal double layer capacitor. Specific capacitance of the material is also calculated from CV by using the following equation:
$C(Fg^{-1})=\frac{1}{ mv(V_2-V_1)}\int_{V_1}^{V_2}I(V)dv$ (2)
where $\int_{V_1}^{V_2}$I(V)dv is the integrated area under the cyclic voltammetry curve, V1 and V2 are the operating voltage window and v is the scan rate. Specific capacitance of the material obtained from CV curves are 217 F/g at scan rate 10 mV/s, 195 F/g at scan rate 15 mV/s scan rate, 168 F/g at scan rate 20 mV/s, 139 F/g at scan rate 25 mV/s.
Fig. 4. (a) Galvanostatic charging-discharging behavior of aMoS2(L)-RGO and (b) cyclic voltammograms for the corresponding composite.
Fig. 5(a) and (b) shows the galvanostatic charging-discharging properties of aMoS2(S)-RGO composite at the current density of 1 A/g. The specific capacitance of the material calculated by Eq. (1) from the charging-discharging curve is found to be 460 F/g. Fig. 5(c) shows the variation of the charging-discharging behavior of the corresponding composite measured at different current densities. Increase in current density leads to the decrease in discharging time, resulting in a decrease in specific capacitance of the material. Longest discharging time obtained in case of aMoS2(S)-RGO composite is 230 s occurring at a current density of 1 A/g. This corresponds to the specific capacitance of 460 F/g which is much higher than the specific capacitance of aMoS2(L)-RGO composite. This suggests a greater extent of ion diffusions at the electrode-electrolyte interface in case of aMoS2(S)-RGO composite. Specific capacitance of the material has also been studied at current density of 1.5 A/g, 2 A/g, 2.5 A/g and 3 A/g, giving rise to the specific capacitance values of 435 F/g, 420 F/g, 395 F/g and 360 F/g respectively. The good symmetric nature in the discharge curves measured at different current densities indicates good capacitive behavior of the composite material. Capacitive response of the material has been further investigated by cyclic voltammogram measurement. Fig. 5(d) shows CV curves of the material measured at different scan rates of 10, 15, 20, 25 mV/s within the potential window 0-1 V. The material shows specific capacitance of 403 F/g at scan rate 10 mV/s obtained by equation (ii). Specific capacitance of the material measured at scan rates of 15, 20, 25 mV/s is 363 F/g, 302 F/g and 283 F/g respectively. The quasi-rectangular shape of CV suggests the small contribution of pseudocapacitive behavior compare to electric double layer capacitor (EDLC). Therefore, the material mainly follows double layer capacitive mechanism for charge storage. Additionally, the CV integrated area of aMoS2(S)-RGO composite is found to be greater than the aMoS2(L)-RGO composite measured at the same scan rate and the area under the curve increases with increasing scan rate. However, storage capacitance of the material decreases with increasing scan rate. This is due to the fact that increase in scan rate leads to the decrease in interaction between electrode and electrolyte ions causing the deviation of CV curve from rectangular shape.
Fig. 5. (a) Galvanostatic charging-discharging behavior of aMoS2(S)-RGO, (b) galvanostatic charging-discharging behavior of aMoS2(S)-RGO for single cycle, (c) charging-discharging behavior of aMoS2(S)-RGO composite at different current density and (d) cyclic voltammograms for the corresponding composite.
Galvanostatic charging-discharging properties and capacitive behavior of the cMoS2 QDs-RGO have been given in supporting information. The specific capacitance value calculated from charging discharging curve is found to be 224 F/g at current density of 1 A/g. CV measurements of the material give quasi-rectangular shape as obtained in the previous two materials aMoS2(S)-RGO composite and aMoS2(L)-RGO composite, however, the CV integrated area is found to be decreased in this composite. Specific capacitance of the material measured at scan rates of 10, 15, 20, 25 mV/s are 202 F/g, 176 F/g, 150 F/g and 130 F/g respectively. Specific capacitance of bare RGO has also been measured which shows specific capacitance 158 F/g at current density of 1 A/g calculated from charging discharging curve given in the supporting information. Therefore, specific capacitance of the cMoS2 QDs-RGO composite has been found to be lower than the amorphous composites. This may be due to the lack of active sulfur sites in the crystalline material rather than the amorphous analogous.
On the other hand, specific capacitance of the aMoS2(S)-RGO composite is found to be nearly two times higher than the aMoS2(L)-RGO composite. This increase in specific capacitance of the composite structure with the decrease in the size of amorphous MoS2 is attributed to the increase in the number of unsaturated sulfur edge state which can efficiently interact with the electrolyte ions to make the diffusion of ions much more effective. In addition to, decoration of less conducting ultrasmall amorphous particles on reduced graphene oxide surface creates a conducting channel for better electron transport making the system more conducting. Therefore, the synergistic effect of large anionic polarizability of unsaturated sulfide ligand present in ultrasmall amorphous MoS2 and the Л electron density of RGO offer efficient non-faradic proton adsorption at the electrode-electrolyte interfaces. Charge storage mechanism involves the following equation:
MoS2 (S2-/ S22- edges) + H+ ↔MoS2-H+(3)
The as-prepared device using the synthesized active material behaves much better than several supercapacitive materials reported in the literature. Table 1 represents specific capacitance of a series of active materials in comparison with aMoS2(S)-RGO composite and aMoS2(L)-RGO composite of our present work.
Table 1 Comparison of specific capacitance of the present work with previously reported literature.
| Active electrode material | Specific capacitance | Reference |
|---|---|---|
| MoS2/graphene | 243 F/g (at the current density of 1 A/(g m)) | Ref. [14] |
| 1T/2H Hybrid Ammoniated MoS2 | 346 F/g (at the current density of 1 A/(g m)) | Ref. [11] |
| Flower-like MoS2 | 122 F/g (at the current density of 1 A/(g m)) | Ref. [12] |
| α-Fe2O3/rGOcomposite | 255 F/g (at the current density of 1 A/(g m)) | Ref. [9] |
| Defect-rich MoS2 ultrathin nanosheets | 141.1 F/g (at the current density of 1 A/(g m)) | Ref. [13] |
| MoSx/GCNT/CP | 414 F/g (at the current density of 0.67 A/(g m)) | Ref. [19] |
| MnO@C | 578 F/g (at the current density of 1 A/(g m)) | Ref. [6] |
| aMoS2(S)-RGO composite | 460 F/g (at the current density of 1 A/(g m)) | This work |
| aMoS2(L)-RGO composite | 270 F/g (at the current density of 1 A/(g m)) | This work |
The materials have been further investigated by electrochemical impedance spectra (EIS) for better realization of the kinetic feature of the ion diffusion process on which storage capacity of the active material depends. Generally, in Nyquist plot, the intercept on the real axis in high-frequency region designates combined resistance (Rs) of the device consists of resistance of electrode material, an ionic resistance of electrolyte and contact resistance between the electrode material and current collector. The diameter of the semicircle corresponds to the charge transfer resistance. Fig. 6(a) shows the EIS spectra of aMoS2(S)-RGO and aMoS2(L)-RGO composite respectively. Rs value obtained from EIS spectra of aMoS2(S)-RGO composite is found to be 0.67 Ω which is much smaller than the Rs value of 2.6 Ω for aMoS2(L)-RGO composite. The Nyquist plots do not contain any semicircular portion indicating both the material have very low charge transfer resistance (Rct) in the electrolyte. Rct values for aMoS2(L)-RGO and aMoS2(S)-RGO composites are found to be 5.5 Ω and 1.7 Ω, respectively. Therefore, low Rs and Rct values make the aMoS2(S)-RGO composite much more electrochemically active than the aMoS2(L)-RGO composite.
Fig. 6. (a) Nyquist plot of aMoS2(S)-RGO composite and aMoS2(L)-RGO composite; Inset shows the zoomed view of the Nyquist plot in the high-frequency region and (b) cyclic performance of aMoS2(S)-RGO composite measured at a current density of 1 A/g over 5000 cycles.
Another important feature of supercapacitor material is its cyclic stability. Fig. 6(b) shows durability test of the device made by using aMoS2(S)-RGO composite as the electrode material. The durability test of the device has been performed for 5000 cycles at a current density of 1 A/g. After 5000 cycles the capacitance retention has been found to be about 90 % of the initial value indicating excellent cycle stability of the material.
In summary, we have successfully synthesized amorphous MoS2 nanoparticles for two different sizes on RGO surface and thoroughly investigated the effect of particle size on the storage capacity of the composite structures. In comparison storage capacity of crystalline MoS2 quantum dot decorated RGO has been investigated showing much lower storage capacity than the amorphous analogous. The storage capacity of the amorphous MoS2 decorated RGO composites have been found to be largely enhanced with decreasing size of the amorphous nanoparticles. The specific capacitance of aMoS2(L)-RGO composite has been found to be 270 F/g whereas that of aMoS2(S)-RGO composite has been found to be 460 F/g at the same current density. The decrease in the size of the amorphous MoS2 nanoparticle leads to the increase in the number of sulfur edge states which have been confirmed by XPS analysis. This excess number of unsaturated bridging sulfide ligand plays the key role to increase the storage capacity of the composite structure. Therefore, the synergistic effect of unsaturated bridging sulfide ligand of small amorphous MoS2 and high conductivity of RGO makes the aMoS2(S)-RGO composite as the efficient electrode material for supercapacitor application.
Poulami Hota acknowledges Council for Scientific and Industrial Research (CSIR); Milon Miah acknowledges DST-INSPIRE, Saptasree Bose and Diptiman Dinda acknowledge Indian Association for The Cultivation of Science (IACS) for awarding their fellowships. Uttam K Ghorai acknowledges West Bengal DST FIST and Central DST FIST programs for financial assistance. Shyamal K Saha acknowledges IACS and Department of Science and Technology (DST), Govt. of India for infrastructural facilities.
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