TWI668718B - A method for producing an ws2/rgo hybrid - Google Patents

Abstract

The invention provides a preparation of a well-structured tungsten disulfide/reduced graphene oxide (WS ₂ /RGO) nanosheet, which uses a simple molten salt method to use WS ₂ as an electroactive material of a super capacitor, since WS ₂ is in the second A large number of charge accumulation locations on the dimension plane and a synergistic effect of RGO which highly enhances conductivity and improves connectivity in the WS ₂ network, exhibiting a high specific capacitance of 2508.07F g ^-1 at a scan rate of 1 mV s ^-1 ( C _F ). For the supercapacitor electrode based on WS ₂ /RGO, it achieves excellent cycle stability of 98.6% retention after 5,000 cycles of charge/discharge, and the overall measured coulombic efficiency is close to 100%. The results show that combining two-dimensional metal sulfides and carbon materials as charge storage materials can solve energy problems and realize the possibility of sustainable society.

Description

Method for producing tungsten disulfide/reduced graphene oxide composite

The present invention relates to a supercapacitor electrode, in particular to the use of tungsten disulfide and reduced graphene oxide composites (WS ₂ /RGO hybrid) in supercapacitor electrodes.

Two-dimensional (2D) nanostructures such as graphene, reduced graphene oxide and metal chalcogenide, due to high active surface area and large planar conductivity, have been caused by many high-performance supercapacitor (SC) electrode electroactive materials. attention. Tungsten disulfide (WS ₂ ) is one of the attractive chalcogenide tungsten compounds for electrochemical applications, which is formed by 2D covalent bonding, and the SWS layers are separated by weak van der Waals gaps. The WS ₂ wide 2D nano-plane has a rich active position for charge accumulation. However, re-stacking between nanosheets, low conductivity, and relatively fragile WS ₂ limit their application. To solve this problem, highly conductive carbon materials are used to enhance the conductivity of the electrodes while avoiding the re-stacking of WS ₂ . Reduced graphene oxide (RGO) has been reported to have good ductility and high electrical conductivity, but its low theoretical capacity limits its capacitive properties. Therefore, it is desirable to build a uniform composite with a robust and conductive RGO nanosheet and a high capacitance 2D WS ₂ nanostructure to improve electrode conductivity, reduce WS ₂ nanosheet re-stacking, and during charge/discharge Providing a larger electrode/electrolyte interface also promotes electron transport and maintains volumetric changes during electrochemical ion insertion/extraction reactions.

Previous literatures have investigated the performance of WS ₂ and carbon composites in supercapacitor applications. Hu et al. prepared an amorphous carbon tube coated WS ₂ nanoparticle as a supercapacitor electrode material with a current density of 10 A g ^-1 . It reaches a value of 337F g ^-1 specific capacitance (C _F ) and has good cycle stability (B.Hu, X.Qin, AMAsiri, KAAlamry, AOAl Youbi, X.Sun, Electrochem.Commun., 28 (2013) 75-78). Ratha et al. synthesized a thin layered WS ₂ /RGO composite by simple hydrothermal method, and its supercapacitor obtained an enhanced specific capacitance value of 350 F g ^-1 , which was about 5 and 2.5 times higher than that of WS ₂ only. Supercapacitor electrodes with RGO nanosheets (S. Ratha, CSRout, ACS Appl. Mater. Interfaces, 5 (2013) 11427-11433).

Although WS ₂ /RGO complexes have been reported, the use of WS ₂ /RGO as a supercapacitor electrode charge accumulation material is limited, and its relative supercapacitor performance is far from expected, ie in previous studies. Only specific capacitance values below 350F g ^-1 are obtained. Therefore, in order to achieve a high-capacity supercapacitor electrode of a tungsten sulfide and carbon material composite, it is required to improve the exposed active position of the Faraday reaction and the conductivity of effective charge transport.

In order to solve the above problems, the present invention discloses a supercapacitor electrode which is manufactured by a simple molten salt method and which is based on a WS ₂ and RGO nanostructure 2D composite.

A first aspect of the present invention provides a supercapacitor electrode comprising a tungsten disulfide and a reduced graphene oxide composite (WS ₂ /RGO hybrid).

A second aspect of the present invention provides a supercapacitor comprising the supercapacitor electrode of the first aspect.

A third aspect of the present invention provides a method of preparing a WS ₂ /RGO composite comprising the steps of: (a) heating and reducing graphene oxide (RGO), tungstic acid (H ₂ WO ₄ ), and thiourea (CH ₄ N) ₂ ) a mixture of 500 to 800 ° C; (b) maintaining the mixture of step (a) at 500 to 800 ° C for 4 to 6 hours; and (c) filtering the WS ₂ /RGO composite by suction.

According to the present invention, a highly conductive 2D WS ₂ /RGO composite can be easily prepared by a simple molten salt method and used as an electroactive material for a supercapacitor electrode. The self-synthesized WS ₂ nano ladder and RGO nanosheets combine well to present a perfect 2D composite nanostructure. Using a CV diagram at a scan rate of 10 mV s ^-1 , a supercapacitor electrode based on a WS ₂ /RGO composite has a significantly enhanced specific capacitance compared to a supercapacitor electrode with a single composition of WS ₂ or RGO. The value is 1556.67F g ^-1 . EIS analysis showed that when WS ₂ / RGO super capacitor and the conductive electrodes to RGO higher as a base material, the charge transfer resistance at its super capacitor electrodes WS ₂ / RGO composite substrate is smaller. In the case of composite use, higher efficiency performance and higher energy and power can be obtained. In addition, the supercapacitor electrode based on the WS ₂ /RGO composite had a cycle stability retention of 98.6% after 5,000 charge/discharge cycles, and the overall coulombic efficiency was almost 100%. The use of WS ₂ /RGO composites for supercapacitor electrodes greatly enhances electrochemical performance, mainly due to the high redox reaction capability of WS ₂ nano ladders and the large electron transport capability of RGO nanosheets. effect.

The structure and advantages of the present invention will be more apparent from the following detailed description of the accompanying drawings, in which: FIG. 1 is an LED circuit and WS ₂ /RGO paper (enlarged view on the right) to test the conductivity of the composite paper. Sex. The photo above is the WS ₂ /RGO paper in the circuit and the photo below is the broken photo.

Figure 2(a) is an HRTEM image of WS ₂ ; (b) is an SEM image of WS ₂ ; (c) is an SEM image of RGO; (d) is an SEM image of the WS ₂ /RGO complex; e) is a TEM image of the WS ₂ /RGO complex.

FIG. 3 (a) is the EDX spectrum; (b) of WS ₂ / RGO composite tungsten, sulfur and carbon mapping corresponding to the respective SEM element pattern; (c) is a XRD pattern of WS _2, and WS ₂ / RGO complex .

Figure 4 (a) is a CV diagram at 10 mV s ^-1 measurement; (b) is a GC/ supercapacitance electrode with WS ₂ , RGO and WS ₂ /RGO composites as electroactive materials at 2A g ^-1 measurement D graph.

Figure 5 is a CV plot of a supercapacitor electrode measuring different substrates at different scan rates: (a) WS ₂ /RGO; (b) WS ₂ ; (c) RGO; and measuring different bases using different current densities GC/D plot of the supercapacitor electrode of the material; (d) WS ₂ /RGO; (e) WS ₂ ; (f) RGO.

Figure 6 is a graph of energy density-power density (Ragone) of a supercapacitor electrode with WS ₂ /RGO, WS ₂ and RGO as electroactive materials.

Fig. 7(a) is a Nyquist diagram; (b) is a supercapacitor electrode with WS ₂ /RGO, WS ₂ and RGO as electroactive materials, and the Nyquist diagram corresponds to an equivalent circuit.

Figure 8 is a graph showing the capacitance retention and coulombic efficiency of a supercapacitor electrode with different substrates as electroactive materials for charge/discharge cycles: (a) WS ₂ /RGO; (b) WS ₂ ; (c) RGO.

The present invention provides a supercapacitor electrode comprising a tungsten disulfide and a reduced graphene oxide composite (WS ₂ /RGO hybrid), and a method of producing the WS ₂ /RGO composite and a supercapacitor containing the composite.

The "supercapacitor" described herein is a type of electrochemical capacitor, which can be classified as an electric double layer capacitor (EDLC), which is charged and discharged by physical adsorption and desorption of the electrode surface, and can also be a pseudo capacitor. It is charged and discharged by a rapid and reversible redox reaction of the metal oxide.

A "composite" as used herein is a material composed of at least two or more components joined by one or more chemical bonds and having different functional and/or structural properties than its single composition. In this context, reduced graphene oxide (RGO) and tungsten disulfide (WS ₂ ) are bonded to each other by chemical bonds or van der Waals forces to produce a WS ₂ /RGO composite material having different functional and/or structural properties. RGO or WS ₂ consists of individual individual materials.

The "reduced graphene oxide (RGO)" described herein is a reduced state of graphene oxide, such as graphene oxide subjected to a reduction procedure, and thus is partially or substantially reduced.

In a preferred embodiment, the WS ₂ /RGO composite is contact deposited on the substrate to form an electrode. The base layer may be an extruded thermally expanded graphite film, a conductive porous film or more preferably a foamed nickel, but is not limited to the above materials. The foamed nickel is used as the base layer, and the number of holes is 80-160 holes/pores/inch (PPI) and the skeleton thickness is 0.5 to 3 mm, and the number of holes is 110 PPI and the thickness of the skeleton is 1.05 mm.

The electrolyte for the supercapacitor may be selected from potassium chloride (KCl), potassium sulfate (K ₂ SO ₄ ), lithium chloride (LiCl), lithium sulfate (Li ₂ SO ₄ ), sodium chloride (NaCl), and sulfuric acid. Sodium (Na ₂ SO ₄ ).

The method for preparing a WS ₂ /RGO composite of the present invention comprises the steps of: (a) heating and reducing a mixture of graphene oxide (RGO), tungstic acid (H ₂ WO ₄ ) and thiourea (CH ₄ N ₂ S); 500 to 800 ° C; (b) maintaining the mixture of the step (a) at 500 to 800 ° C for 4 to 6 hours; and (c) filtering the WS ₂ /RGO composite. Preferably, step (a) is to heat the mixture to 600 ° C at a heating rate of 1 ° C min ^-1 and maintain the mixture for 5 hours.

Hereinafter, the present disclosure will specifically describe the reference embodiments and the drawings, but the present disclosure is not limited to the following examples and drawings.

[Example 1] Preparation of WS ₂ /RGO composite and its supercapacitor electrode

To prepare the WS ₂ /RGO complex, 50 mg of RGO (P-HF10, Enerage Inc.) and 100 mg of H ₂ WO _{4 were} finely ground with 5 g of CH ₄ N ₂ S. The mixed sample was then heated to 600 ° C at a heating and cooling rate of 1 ° C min ^-1 and held at 600 ° C for 5 hours. The WS ₂ /RGO composite was further filtered by suction to form WS ₂ /RGO paper. The WS ₂ /RGO flexible paper has high electrical conductivity in the circuit test of the light emitting diode (LED) module, as shown in FIG.

To prepare the supercapacitor electrode, 10 mg of the WS ₂ /RGO complex was dispersed in 2 mL of EtOH solution. The solution was then deposited on foamed nickel (110 PPI, thickness 1.05 mm, Innovation Materials Co., Ltd., Taiwan) to contact the WS ₂ /RGO composite in contact with the foamed nickel as a specific example. On the material, the foamed nickel is used as a flexible substrate for the electrode using a titration coating technique. Finally, the electrode was dried under vacuum at 60 ° C for 12 hours.

[Comparative Example 1] Preparation of WS ₂ nano ladder and its supercapacitor electrode

Pure WS ₂ samples were synthesized in the same manner as in Example 1, except that RGO was not added throughout the procedure. The supercapacitor electrode was prepared in the same manner as in Example 1, except that the above WS ₂ /RGO complex was replaced with a pure WS ₂ sample.

[Comparative Example 2] Preparation of RGO and its supercapacitor electrode

A pure RGO sample was prepared in the same manner as in Example 1, and H ₂ WO ₄ was not added throughout the procedure. The supercapacitor electrode was prepared in the same manner as in Example 1, except that the above WS ₂ /RGO complex was replaced with a pure RGO sample.

[Test example]

The crystal lattice of the sample was examined using a high resolution transmission electron microscope (HRTEM) (Philips Tecnai F30 Field Emission Gun Transmission Micro-scope (FEG-TEM)). A field emission scanning electron microscope (FE-SEM, Nova Nano SEM 230, FEI, Oregon, USA) was equipped with an energy dispersive X-ray spectrometer (EDX) for studying the surface morphology and elemental composition of the product. The phase and structure of the product were determined using an X-ray diffractometer (XRD, X'Pert3 Powder, PANalytical) with Cu Kα radiation (λ = 1.5418 Å). Cyclic voltammetry (CV) and current charge/discharge curves (GC/D) were obtained using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco-Chemie, the Netherlands) with a three-electrode electrochemical system, where The prepared supercapacitor electrode is the working electrode, the platinum (Pt) wire is the counter electrode, and the Ag/AgCl/saturated KCl electrode is used as the reference electrode in the aqueous solution containing 1 M KOH and 0.5 M KCl. pole. The EIS analysis was performed using a potentiostat/galvanostat (PGSTAT 204, Autolab, Eco-Chemie, the Netherlands) equipped with an FRA2 module with a frequency range of 100,000 Hz to 0.01 Hz. The applied bias voltage is set to an open circuit voltage.

[Results] Morphology and composition characterization of WS ₂ /RGO, WS ₂ and RGO

The morphology of the 2D material is very important to determine the electrochemical performance of the associated supercapacitor electrode. Therefore, the self-synthesized WS ₂ structure was first examined to confirm the composition of the self-made product. Figure 2(a) shows the HRTEM image of the WS ₂ nanomaterial. It is observed that the stacking plane spacing of the WS ₂ is 0.62 nm, and the periodicity of the plane (100) lattice fringes can be observed in the inserted graph. The column spacing is 0.27 nm, providing evidence of this WS ₂ phase. In addition, the SEM images of the WS ₂ , RGO and WS ₂ /RGO complexes are shown in Figures 2(b), (c) and (d), respectively. The WS ₂ sample exhibited a plurality of sheet-like structures stacked in layers, and a large number of wrinkles were well distributed on the surface in a random direction, which contributed to the morphology of the nano ladder. The RGO sample also exhibited a 2D nanosheet structure with some wrinkles, but the wrinkle pattern on the RGO surface was very different from the WS ₂ surface in terms of distribution uniformity and orientation. In the RGO sample, the wrinkles grow in a parallel direction, but the distribution is relatively random. The wrinkle growth direction of the WS ₂ sample is not fixed, but its distribution is very average. It was concluded that the wrinkles in the RGO sample were caused by the folding of the 2D layer, while the wrinkles in the WS ₂ sample appeared to grow naturally on the surface rather than by the folding of the layers. On the other hand, the 2D WS ₂ /RGO composite exhibits a 2D layered structure with less wrinkles on the surface than the WS ₂ and RGO surfaces because the composite is composed of WS ₂ nano ladder and RGO nanosheets. The suction filtration is simply mixed, and the overlap of two different 2D nanostructures may cause the distribution of wrinkles on the surface to be more complicated. However, the wrinkles on the surface of the composite are relatively smooth compared to the WS ₂ and RGO surfaces, possibly because of the van der Waals force between the WS ₂ and RGO layered structures to reduce the degree of folding, thus smoothing the wrinkles of the two planes . In order to more clearly distinguish between the WS ₂ and RGO nanostructures, the TEM image of WS ₂ /RGO is shown in Figure 2(e). A light-colored and dark-colored layered structure can be observed, which can be inferred as the material of RGO and WS ₂ , respectively, since the molecular weight of RGO is smaller than that of WS ₂ . It was also observed that WS _{2 was} uniformly covered on the RGO, indicating that the 2D nanostructures of RGO and WS ₂ were successfully combined. To further confirm the composition of the composite, the corresponding EDX spectrum is as shown in Fig. 3(a), and the tungsten (W), sulfur (S) and carbon (C) signals can be clearly detected, indicating that the composite is successfully mixed. WS ₂ and RGO. In addition, the SEM images corresponding to the mapping spectra of individual elements of tungsten (W), sulfur (S) and carbon (C) are shown in Fig. 3(b), and tungsten (W), sulfur (S) and carbon (C) can be observed. Uniform distribution means that a homogeneous mixture of WS ₂ and RGO can be obtained by simple suction filtration.

To further confirm the composition of the sample, WS _2, and WS ₂ / XRD pattern of composite RGO FIG 3 (c), the sample ₂ which presented all the peaks WS ₂ WS hexagonal structure (JCPDS no.84- 1398) (S. Ratha, CSRout, ACS Appl. Mater. Interfaces, 5 (2013) 11427-1143), while the WS ₂ /RGO sample observed an additional diffraction peak at 24.21°, this peak (002) is RGO, the result It was shown that WS ₂ and RGO were successfully mixed in the nanocomposite.

[Result] Electrochemical performance of supercapacitor electrode with WS ₂ /RGO, WS ₂ and RGO as electroactive materials

As shown in Fig. 4(a), the capacitance performance of the corresponding supercapacitor electrode is evaluated by measuring the CV at a scan rate of 10 mV s ^-1 , and the specific capacitance value (C _F ) can be calculated according to the following equation (1): Where I is the current density, ∫ I dV is the integrated area of the CV curve, v is the scan rate, Δ V is the potential window, and m is the weight of the electroactive material in the supercapacitor electrode. Two pairs of redox peaks were observed on the supercapacitor electrode based on WS ₂ /RGO. The oxidation peaks were at potentials of 0.35 and 0.40 V, and the corresponding reduction peaks were at potentials of 0.22 and 0.17 V. On the other hand, a supercapacitor electrode based on WS ₂ or RGO alone observed only a pair of redox peaks of 0.35 and 0.25V. The CV curve clearly shows its pseudo-capacitance characteristics in all cases. The peak of the CV curve of the supercapacitor electrode based on WS ₂ corresponds to the redox reaction between W ⁶⁺ and W ⁴⁺ , while the peak of the supercapacitor electrode based on RGO may be derived from the oxygen on the surface of RGO. (O-containing) surface functional redox reaction. The supercapacitor electrode based on the WS ₂ /RGO composite exhibits two pairs of redox peaks, presumably the transition between W6+ and W4+ and the redox reaction of the oxygen-containing surface function on the surface of RGO. In addition to discussing the redox peaks of the sample, the curves in the CV plot are also important for indicating the capacitive performance of the corresponding supercapacitor electrodes. Compared with the supercapacitor electrode with WS ₂ or RGO alone, the C _F values are 398.5 and 119.9F g ^-1 , respectively, and the supercapacitor electrode based on WS ₂ /RGO composite can be significantly higher. The C _F value is 1356.67F g ^-1 . The supercapacitor electrode containing the WS ₂ /RGO composite achieved in the present invention has a C _F value which is one of the highest among the supercapacitor electrodes based on 2D metal chalcogenide. On the other hand, the supercapacitor electrode with WS ₂ , RGO and WS ₂ /RGO composites as electroactive materials has a charge/discharge capability estimated by measuring the GC/D curve at a current density of 2 A g ^-1 , as shown in the figure. 4(b), whose corresponding C _F value is calculated according to the following equation (2): Where Δ t is the discharge time and m is the weight of the electroactive material in the electrode. The WS ₂ /RGO composite supercapacitor electrode exhibits an approximately symmetrical charge and discharge curve with a maximum discharge time of 255 seconds, which corresponds to a high C _F value of 1275 _F g ^-1 , showing the outstanding charge/discharge performance of this example. The supercapacitor electrodes based on WS ₂ or RGO showed relatively short discharge times of 67 and 31 seconds, respectively, and the corresponding C _F values were 335 and 155 F g ^{-1 , respectively} . The supercapacitance electrode with WS ₂ /RGO composite as electroactive material has a large C _F value, indicating that the pseudo capacitor WS ₂ and the conductive RGO respectively provide a high capacitive redox reaction and a large amount of electron transport capability. Synergistic effect. In addition, with the help of RGO, the re-stacking of the WS ₂ nanosheets can be reduced, thus greatly enhancing the electroactive region between the electrodes and the electrolyte.

To further evaluate the reversibility of supercapacitor electrodes based on WS ₂ /RGO, WS ₂ and RGO, the CV curves measured at scan rates of 1, 5, 10, 20, 40, 60 and 80 mV s ^-1 See Figures 5(a), 5(b) and 5(c) respectively. All of the curves in Fig. 5(a) exhibited a similar shape, and no distortion occurred even when the scanning rate was increased by 80 times, demonstrating that this example has high reversibility. In addition, it was found that the separation of the redox peaks increases as the scanning rate increases, exhibiting a general phenomenon in electrochemistry. When 1mV s ^-1 to obtain the maximum value of C _F 2508.07F g ^-1, and the value decreases as the scan rate increases at a scan rate. The CV values measured at scan rates of 5, 10, 20, 40, 60, and 80 mV s ^-1 gave C _F values of 1891.07, 1606.68, 1355.67, 1066.00, 857.91, and 674.54 _F g ^-1 , respectively. When the scan rate was increased by up to 80 times, it was found to be only reduced by 73%, showing its high efficiency of capacitance performance. Figures 5(b) and 5(c) are CV plots of supercapacitor electrodes based on WS ₂ and RGO, respectively, which also show little distortion, and when measurements are applied at higher scan rates, these The redox peaks in the curve are still evident. The results show that the reversibility of the supercapacitor electrode can be maintained in combination with 2D WS ₂ and RGO nanomaterials, and the same characteristics are observed for supercapacitor electrodes using WS ₂ or RGO nanomaterials alone. Even if more interfaces can be produced by combining two different nanomaterials, in the case of the composite, the reversibility of the supercapacitor electrode can still be maintained due to the enhanced conductivity and capacitance. In addition, the high-rate charge/discharge capability of supercapacitor electrodes based on WS ₂ /RGO, WS ₂ and RGO is measured at current densities of 2, 4, 8 and 16A g ^{-1 over} a potential range of 0 to 0.4V. The GC/D spectra are shown in Figures 5(d), 5(e) and 5(f), respectively. The charge and discharge curves of all GC/D spectra in Figure 5(d) exhibit a highly symmetrical characteristic, indicating that the WS ₂ /RGO sample has high charge/discharge stability. C _F maximum value obtained 1285F g ^-1 at a current density of 2A g ^-1, while when the current density 4,8 16A ^-1 and g, C _F value is still maintained at 940,720 and 480F g ^-1. The current density increased by a factor of eight, and the C _F value was only reduced by 62%, again demonstrating the high rate operating conditions and excellent charge/discharge capability of this example. However, the GC/D spectra of supercapacitor electrodes based on WS ₂ and RGO alone are shown in Figures 5(e) and 5(f), respectively, and WS ₂ /RGO in Figure 5(d). Compared to a supercapacitor electrode, it exhibits a more asymmetrical shape with a significantly longer charge duration. In addition, when a higher current density is applied when measuring the GC/D curve, the supercapacitor electrode based on WS ₂ or RGO alone has a C _F value comparable to that of a supercapacitor electrode based on WS ₂ /RGO. Falling a lot. The results show that by combining WS ₂ and RGO as charge accumulation materials, the synergistic effect of the number of active sites and charge transfer conductivity can be enhanced, and the high rate charge/discharge capability can be greatly improved.

[Results] Energy and power density, charge transfer resistance and cycle stability of supercapacitor electrodes with WS ₂ /RGO, WS ₂ and RGO as electroactive materials

Further electrochemical analysis was carried out to study the supercapacitor properties of electrodes with WS ₂ /RGO, WS ₂ and RGO as electroactive materials. First, the Ragone diagram of the supercapacitor electrode based on WS ₂ /RGO, WS ₂ and RGO is shown in Figure 6, by GC/ in Figures 5(d), 5(e) and 5(f) respectively. D map data is calculated. Energy density (E) and power density (P) are estimated by equations (3) and (4), respectively:

Where Δ V is the potential window applied during GC/D (0.4V in this example) and Δ t is the discharge time. An energy density of 28.33 W h kg ^-1 was obtained at a current density of 2 A g ^{-1 for a} supercapacitor electrode with WS ₂ /RGO as an electroactive material. Under the same conditions, the supercapacitor electrodes based on WS ₂ or RGO showed significantly lower energy densities of 7.51 and 5.67 W h kg ^-1 , respectively. In addition, the super capacitor electrode WS ₂ / RGO as a base material at 400W kg ^-1 power density, high energy density 28.33W h kg ^-1, and ^-1 at high power densities 3200W kg, still can maintain its The energy density is 12.71 W h kg ^-1 . However, the super capacitor electrodes WS ₂ as a base material 400 and a power density of 3200W kg ^-1, the less energy density is obtained which, respectively, 7.51 and 1.34W h kg ^-1. RGO super capacitor electrode and as a base material 400 and a power density of 3200W kg ^-1, its energy density is obtained less, respectively, 5.67 and 1.33W h kg ^-1. Due to the synergistic effect between WS ₂ and RGO's powerful capacitive properties and conductive properties, the supercapacitor electrode with WS ₂ /RGO as the electroactive material can achieve high energy and power density, again showing its overwhelming electrochemical performance.

In addition, in order to investigate the transmission dynamics of the capacitance performance of the supercapacitor electrode, the Nyquist diagram is obtained by EIS technique as shown in Fig. 7(a), and the corresponding equivalent circuit is shown in Fig. 7(b). The intersection point on the x-axis can be used to estimate the internal resistance (R _S ) corresponding to conductivity, and the charge transfer resistance (Rct) between the electrolyte and the electrode can be evaluated using a semicircle in the high-frequency region map. The supercapacitor electrode based on WS ₂ has an RS value and a Rct value of 1.53 and 1740 Ω, respectively, and the WS ₂ /RGO electrode exhibits a smaller Rs value of 0.68 Ω, and the semicircle corresponds to the high frequency region. The smaller Rct value is 976Ω. On the other hand, the supercapacitance electrodes based on WS ₂ /RGO and RGO have smaller Rs values of 0.68 and 0.71 Ω, respectively, indicating that the high conductivity of the supercapacitor electrode based on WS ₂ /RGO is mainly It is a contribution from RGO nanomaterials. However, the RGO-based electrode found a significantly larger Rct value of 1650 Ω, indicating that it has poor electrical conductivity but poor electrical activity. The results show that the composite has higher conductivity and smaller charge transfer resistance, which is a synergistic effect from WS ₂ and RGO. In addition, for the WS ₂ /RGO electrode, the angle of tilting to the y-axis is greater than 45° in the low frequency region, and the supercapacitor electrode using WS ₂ and RGO alone is inclined to the y-axis at an angle of less than 45° in the low frequency region, The composite supercapacitor electrode behaves relative to an ideal capacitor.

To test the cycle stability of a supercapacitor electrode based on WS ₂ /RGO, WS ₂ and RGO at a current density of 2 A g ^-1 , 5,000 repetitive cycle charge/discharge cycles were performed. As shown in Figures 8(a), 8(b) and 8(c), respectively, are supercapacitor electrodes based on WS ₂ /RGO, WS ₂ and RGO, where the C _F value and the number of cycles corresponding thereto are The function, after repeating 5,000 charge/discharge cycles, compared with the corresponding first charge/discharge procedure, the supercapacitor electrodes based on WS ₂ /RGO, WS ₂ and RGO were 98.6%, 97.1, respectively. % and 100% C _F value retention. The C _F value retention rate is excellent in all cases, but only in the RGO-based supercapacitor electrode, after repeating 5,000 charge/discharge cycles, the C _F value retention rate is almost 100%, showing Carbon materials have good stability. The supercapacitor electrode based on WS ₂ /RGO has a slightly better cycle stability than the supercapacitor electrode based on WS ₂ , indicating that RGO is beneficial for enhancing its repeated charge/discharge performance. On the other hand, the time interval of charging and discharging is used to estimate the coulombic efficiency by the equation η=t _d /t _c x100% (where t _c and t _d represent the time intervals of charging and discharging, respectively), and the display is WS ₂ /RGO The supercapacitor electrode of the electroactive material has an overall measured η greater than 94%. However, the supercapacitor electrodes based on WS ₂ or RGO alone have an overall measured η of less than 80% and 85%, respectively. The results show that the coulombic efficiency can be greatly improved by mixing the electroactive WS ₂ and the highly conductive RGO to reduce the interfacial charge transfer resistance in the composite. Supercapacitor electrodes based on WS ₂ /RGO can achieve both good cycle performance and coulombic efficiency, which can be attributed to the insertion of RGO nanosheets between WS ₂ nano ladders, which can significantly reduce re-stacking during cycling Situation, and induces rapid transfer of electrons and ions during charge/discharge.

The WS ₂ /RGO complex of the present invention combines the characteristics of an electric double layer capacitor (EDLC) and a pseudo capacitor. First, the charge storage mechanism of the EDLC is discussed, and the charge accumulates on the surface of the capacitor material without participating in chemical energy. The EDLC is resorbed reversibly on the carbon surface to store energy at the electrolyte-carbon interface. The electrode attracts oppositely charged ions during charging, and the charge becomes free during discharge. Secondly, the charge storage mechanism of the pseudo-capacitor is discussed. The charge is stored in the capacitor material by the chemical energy of the chemical bond. As evidenced by the peaks of the CV curve in the foregoing discussion, this charge is stored in the pseudocapacitor via the redox reactions of W6+ and W4+ on WS ₂ and O-containing surface functions on the surface of RGO. In the charge/discharge procedure, ions in the electrolyte diffuse into the host material for redox reactions, a behavior similar to that in batteries. In summary, the charge can accumulate at the RGO/electrolyte interface for EDLC storage mechanisms, or it can be stored in the chemical bonds of the WS ₂ and RGO oxygen-containing surface functions to achieve pseudo-capacitance characteristics. In addition, RGO can not only act as a capacitive material for accumulating charge, but also enhance the conductivity of the composite to facilitate rapid transmission of electrons throughout the composite electrode material.

Although the exemplary embodiments are disclosed herein, it is to be understood that the invention may be modified and not limited to the spirit and scope of the exemplary embodiments of the present application, and all obvious modifications to those skilled in the art include Within the scope of the following request.

Claims (4)

A method of preparing a WS ₂ /RGO composite comprising the steps of: (a) heating a reduced mixture of graphene oxide (RGO), tungstic acid (H ₂ WO ₄ ), and thiourea (CH ₄ N ₂ S) from 500 Up to 800 ° C; (b) maintaining the mixture of step (a) at 500 to 800 ° C for 4 to 6 hours; and (c) filtering the WS ₂ /RGO complex. The method of claim 1, wherein the step (a) is heating the mixture to 600 °C. The method of claim 2, wherein the heating rate is 1 ° C min ^-1 . The method of claim 1, wherein the step (b) is to maintain the mixture of the step (a) at 600 ° C for 5 hours.