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Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets
Journal of Power Sources 320 (2016) 78e85
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets Chao-Chi Tu 1, Lu-Yin Lin*, 1, Bing-Chang Xiao, Yu-Shiang Chen Department of Chemical Engineering and Biotechnology, National Taipei University of Technology (Taipei Tech), Taipei 10608, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
WS2/reduced graphene oxide (RGO) nanosheets are made via a simple malten salt way. The nanosheets are used as the electroactive material for supercapacitors (SCs). A specific capacitance (CF) of 2508.07 F g1 at 1 mV s1 is got for the SCs. The 98.6% retention on the CF is got after 5000 cycles charge/discharge process. The Coulombic efficiency close to 100% for the entire measurement is achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2016 Received in revised form 6 April 2016 Accepted 18 April 2016 Available online 26 April 2016
Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/ reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g1 at the scan rate of 1 mV s1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society. © 2016 Elsevier B.V. All rights reserved.
Keywords: Cyclic voltammetry Electrochemical impedance spectroscopy Reduced graphene oxide Supercapacitor Two-dimensional Tungsten disulfide
1. Introduction
* Corresponding author. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. http://dx.doi.org/10.1016/j.jpowsour.2016.04.083 0378-7753/© 2016 Elsevier B.V. All rights reserved.
Two-dimensional (2D) nanostructures [1,2], graphene [3,4], reduced graphene oxide [5,6], and metal chalcogenides [7,8] etc., have attracted much attention as the electroactive material for the highly performed supercapacitor (SC) electrodes due to the high
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active surface area and large in-plane conductivity [2,9,10]. Tungsten disulfide (WS2) is one of the attractive tungsten chalcogenides for the electrochemical application, formed by the 2D covalently bonded SWS layers separated by a weak van der Waals gap, for which the large 2D nanoplane of WS2 plays as the abundant active sites for charge accumulation. However, the restacking between the nanosheets and the low electronic conductivity as well as the relative brittle of WS2 limits its applications. To solve this problem, highly conductive carbon materials have been applied to enhance the conductivity of the electrode and meanwhile prevent the restacking of WS2 [11e13]. Reduced graphene oxide (RGO) was reported to have good flexibility and high electric conductivity, but the low theoretical capacity limits its electrocapacitive performance [5,14e16]. Hence, constructing uniform hybrids with strong and conductive RGO nanosheets and highly capacitive 2D WS2 nanostructures is expected to improve the conductivity of the electrode, reduce the restacking of WS2 nanosheets, and hence to provide large electrode/electrolyte interfaces during the dischargecharge process as well as promote the electron transport and sustain the volume variation during the electrochemical ion insertion/ extraction reaction. Liu et al. assembled a WS2 and graphene oxide (GO) composite paper for Li-ion batteries to achieve a higher reversible capacity of 697.7 mA h g1 as compared with those of 88.5 and 60.2 mA h g1 respectively for the bare WS2 and RGO cases [12]. Liu et al. fabricated hybrid lamellar porous electrodes with an excellent electrochemical performance by homogeneously intercalating single-walled carbon nanotubes into the lamellar assembled WS2 nanosheets through the vacuum filtration [11]. Yang et al. synthesized the WS2/RGO nanosheet using a one-pot hydrothermal reaction process and demonstrated its good catalytic activity for the hydrogen evolution [17]. Previous literatures also studied the performance of WS2 and carbon hybrids on the application of SCs. Hu et al. prepared WS2 nanoparticleseencapsulated amorphous carbon tubes as the SC electrode material, and a specific capacitance (CF) value of 337 F g1 was achieved at a current density of 10 A g1 with a good cycle stability [13]. Ratha et al. synthesized layered WS2/RGO hybrids by a facile hydrothermal method for SCs to obtain an enhanced CF value of 350 F g1 which is about 5 and 2.5 times higher than those for the SC electrode with bare WS2 and RGO nanosheets, respectively [15]. However, the study of the SC electrodes using WS2/RGO hybrids as the charge-accumulating material is limited, and the related SC performance is far from expectation, i.e., the CF value of merely less than 350 F g1 was obtained in the previous studies [15]. Therefore, the exposed active sites for the Faradic reactions as well as the conductivity for the efficient charge transportation are necessary to be improved for achieving the highly efficient SC electrode based on the hybrid of metal sulfide and carbon materials. Here we report a SC electrode based on a 2D hybrid consisting of WS2 and RGO nanostructures fabricated by using a simple molten salt process. A higher CF value of 1355.67 F g1 was obtained for the WS2/RGO hybrid-based SC electrode, as compared with those of 398.5 and 119.9 F g1 respectively for the WS2 and RGO-based SC electrodes at a scan rate of 10 mV s1. A cycling stability of 98.6% retention after 5000 cycles charge/discharge process and almost 100% for the Coulombic efficiency in the entire measurement were obtained for this case. The charge-transfer resistances were also measured by the electrochemical impedance spectroscopy (EIS) to evaluate the kinetic behaviors of the electrodes.
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from Showa. Potassium chloride (laboratory reagent grade, KCl) and potassium hydroxide (KOH, analytical reagent grade) were brought from Fisher. Ethanol (EtOH) and sodium nitrate (99þ%, for analysis, NaNO3) were got from Acros. Sulfuric acid (H2SO4, ACS reagent, 95.0e98.0%) and tungstic acid (99%, H2WO4) were purchased from SigmaeAldrich. 2.2. Preparation of the WS2 nanoladder and the WS2/RGO hybrid as well as the corresponding supercapacitor electrodes To prepare the WS2/RGO hybrid, 50 mg RGO (P-HF10, Enerage Inc.) and 100 mg H2WO4 were finely ground with 5 g CH4N2S. Then the mixed sample was heated to 600 C at the heating and cooling rate of 1 C min1 and maintained at 600 C for 5 h. The WS2/RGO hybrid was further infiltrated by suction to form the WS2/RGO paper. The WS2/RGO paper is highly conductive as tested in the circuit for the light-emitting diode (LED) module, as presented in Fig. 1. This highly conductive and flexible paper can probably be used as the one-piece integrated substrate-electroactive material in our future work. The bare WS2 sample was also synthesized using the same method for preparing WS2/RGO hybrid but without the addition of RGO in the whole process. Similarly, the bare RGO sample was prepared using the same manner for synthesizing WS2/ RGO hybrid but without the addition of H2WO4 in the whole process. To prepare the SC electrodes, 10 mg of the WS2, RGO, or the WS2/ RGO hybrid were individually dispersed in a 2 mL EtOH solution. Then the solution was deposited on the nickel foam (110PPI, thickness ¼ 1.05 mm, Innovation Materials Co., Ltd, Taiwan) which is promising as the flexible substrate for the electrode using a dropcoating technique. Finally the electrodes were dried under vacuum at 60 C for 12 h. 2.3. Material characterization and electrochemical measurements The high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai F30 Field Emission Gun Transmission Micro-scope (FEG-TEM)) was utilized to examine the lattice of the samples. The field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA) equipped with the energy
2. Experimental 2.1. Materials Tungstic acid (H2WO4) and thiourea (CH4N2S) were obtained
Fig. 1. The circuit with a LED and the WS2/RGO paper (enlarged in the right side) for testing the conductivity of the hybrid paper. The upper photo is with the WS2/RGO paper in the circuit and the lower photo is the broken circuit.
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dispersive X-ray spectrometry (EDX) was used to investigate the surface morphology and element composition of the products. The phase and the structure of the products were determined by X-ray diffraction (XRD, X'Pert3 Powder, PANalytical) with Cu Ka radiation (l ¼ 1.5418 Å). The cyclic voltammetry (CV) and galvanic charge/ discharge curves (GC/D) were obtained using a potentiostat/galvanostat (PGSTAT 204, Autolab, EcoeChemie, the Netherlands) carried out with a three-electrode electrochemical system, where the prepared SC electrode was used as the working electrode, a Pt wire was used as the counter electrode, and an Ag/AgCl/saturated KCl electrode was used as the reference electrode in a aqueous solution containing 1 M KOH and 0.5 M KCl. The EIS was carried out using a potentiostat/galvanostat (PGSTAT 204, Autolab, EcoeChemie, the Netherlands) equipped with an FRA2 module, and the frequency range explored was 100,000 Hze0.01 Hz. The applied bias voltage was set at open-circuit voltage. 3. Results and discussion 3.1. Morphology and composition characterization of WS2/RGO, WS2, and RGO The morphology of the 2D materials is of the great importance to determine the electrochemical performance of the pertinent SC electrode. Therefore, the structure of the self-synthesized WS2 was firstly examined to confirm the composition of the self-made product. Fig. 2(a) presents the HRTEM image for the WS2 nanomaterial. The stacking of WS2 with the interplanar spacing of 0.62 nm was observed. Also, the spacing of 0.27 nm for the periodic arrays for the lattice fringe of (100) plane was obtained in the inserted figure, providing the evidence for the WS2 phase. In addition, the SEM images for the WS2, RGO, and the WS2/RGO hybrid were shown in Fig. 2(b)e(d), respectively. The WS2 sample presents multiple sheet-like structures stacked layer-by-layer, and numerous wrinkles were well distributed on the surface in random directions, suggesting the nanoladder morphology. The RGO sample also shows 2D nanosheet structure with several wrinkles on the surface, but the pattern of the wrinkles on the surface of RGO is very different from that on the surface of WS2, in terms of the distribution uniformity and the direction. The wrinkles grew in a relatively parallel direction but distributed more randomly for the RGO sample, while the growing direction is not fixed but the distribution of the wrinkles is very uniform for the WS2 sample. It is
inferred that the wrinkles in the RGO sample is resulting from the folding of 2D layers, while the wrinkles in the WS2 sample seems like grow naturally on the surface instead of being generated from the staking of the layers. On the other hand, the 2D WS2/RGO hybrid also presents 2D layered structure with fewer wrinkles on the surface, as compared with those on the surfaces of WS2 and RGO. Since the hybrid was prepared by simply mixing the WS2 nanoladders and the RGO nanosheets by suction filtration, the overlapping of two different 2D nanostructures may cause even more complex distribution of the wrinkles on the surface. However, the wrinkles on the surface of the hybrid are relatively smoother as compared with those on the surfaces of WS2 and RGO, probably due to the Van Der Waal force existing between the layered structures of WS2 and RGO to relieve the extent of the folding and therefore to smooth the wrinkles on both surfaces. To distinguish the WS2 and RGO nanostructures more clearly, the TEM image for the WS2/RGO was obtained as shown in Fig. 2(e). The layered structures with light and dark colors were observed, which can be respectively inferred to be the material of RGO and WS2 since the molecular weight for RGO is smaller than that for WS2. The uniform coverage of WS2 was observed on RGO, suggesting the successful combination of the 2D nanostructures of RGO and WS2. To further confirm the composition of the hybrid, the corresponding EDX spectrum was presented in Fig. 3(a). The signals for W, S, and C were clearly detected, suggesting the successful mixing of the hybrid with WS2 and RGO. In addition, the elemental mapping spectra for W, S, and C along with the corresponding SEM image were shown in Fig. 3(b), in which the uniform distribution of W, S, and C were observed, suggesting the even mixing of WS2 and RGO by the simple suction filtration method. To further confirm the composition of the samples, the XRD patterns of WS2 and WS2/RGO hybrid were shown in Fig. 3(c). The WS2 sample presents all the peaks indexed to be the hexagonal WS2 structure (JCPDS no.84-1398) [15], and an extra diffraction peak at 24.21 is observed for the WS2/RGO sample assigned to the (002) peak for RGO, suggesting the successful blending of the WS2 and RGO in the nanohybrid. 3.2. Electrochemical performance of the supercapacitor electrode with WS2/RGO, WS2, and RGO as the electroactive material After examining the morphology and the composition of WS2, RGO, and the WS2/RGO hybrid, the electrocapacitive performances
Fig. 2. (a) The HRTEM image of WS2, the SEM images of (b) WS2 (c) RGO and (d) WS2/RGO hybrid, and (e) the TEM image of WS2/RGO hybrid.
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Fig. 4. (a) The CV plots measured at 10 mV s1 and (b) the GC/D curves measured at 2 A g1 for the SC electrodes with WS2, RGO and WS2/RGO hybrid as the electroactive material.
Fig. 3. (a) the EDX spectrum and (b) the elemental mapping of W, S, and C elements along with the corresponding SEM image for the WS2/RGO hybrid, and (c) the XRD patterns for WS2 and the WS2/RGO hybrid.
of the corresponding SC electrodes were evaluated by using the CV measurement at a scan rate of 10 mV s1, as presented in Fig. 4(a). The value of the specific capacitance (CF) can be calculated according to Equation (1) as follows [18].
Z I dV CF ¼
n DV m
(1)
R where I is the current density, I dV is the integrated area of the CV curve, n is the scan rate, DV is the potential window, and m is the
weight of the electroactive material in the SC electrode. Two couples of the redox peaks were observed for the WS2/RGO-based SC electrode, with the oxidation peaks at the potentials of 0.35 and 0.40 V and the corresponding reduction peaks at the potentials of 0.22 and 0.17 V, while only one couple of the redox peaks at 0.35 and 0.25 V was obtained for the individual WS2 and RGO-based SC electrodes. The CV curves obviously suggested the pseudocapacitive characteristics for all the cases. The peaks in the CV curve for the WS2-based SC electrode corresponded to the redox reaction between W6þ and W4þ [13], while the peaks for the RGO-based SC electrode were probably contributed to the redox reactions of the O-containing surface functionalities on the surface of RGO, as also observed in the previously literatures [19,20]. The WS2/RGO hybrid-based SC electrode presents two couples of the redox peaks, which are inferred to be the redox reactions for the conversion between W6þ and W4þ and for the O-containing surface functionalities on the surface of RGO. Other than discussing the redox peaks for the samples, the current in the CV plots is also significant to index the electrocapacitive performance for the corresponding SC electrode. A much higher CF value of 1355.67 F g1 was obtained for the WS2/RGO-based SC electrode, as compared with those of 398.5 and 119.9 F g1 for the electrodes with WS2 and RGO, respectively. This CF value for the SC electrode with the WS2/RGO hybrid achieved in this study is considered to be one of the highest among the 2D metal chalcogenides-based SC electrodes reported till now [11,13,16,21,22]. On the other hand, the charge/discharge
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capability of the SC electrode with WS2, RGO, and WS2/RGO hybrid as the electroactive material was estimated by measuring the GC/D plots at a current density of 2 A g1 as shown in Fig. 4(b), and the corresponding CF values were calculated based on Equation (2) as follows [18].
CF ¼
I Dt m DV
(2)
where Dt is the discharge time and m is the weight of the electroactive material in the electrode. The WS2/RGO hybrid-based SC electrode presents nearly symmetric charge and discharge curves as well as the longest discharge time of 255 s which corresponds to a high CF value of 1275 F g1, indicating the outstanding charge/ discharge performance for this case, while the WS2 and RGO-based SC electrode shows relatively shorter discharging time of 67 and 31 s corresponding to the CF value of 335 and 155 F g1, respectively. The greatly enhanced CF value for the SC electrode with the WS2/ RGO hybrid as the electroactive material indicates the synergic effect of the psedocapacitive WS2 and conductive RGO to respectively provide highly capacitive redox reactions and the great electron transfer ability. Also, with the help of RGO the restacking of WS2 nanosheets can be reduced and hence the electroactive area between the electrode and the electrolyte can be greatly enhanced. The CF value achieved in this study is much higher than those reported in the previous literatures [13,15]. Hu et al. fabricated WS2 nanoparticles and encapsulated amorphous carbon tubes hybrid via two-step processes [13]. The resulting supercapacitor presents a CF value of 337 F g1 at a high current density of 10 A g1. The concept of this literature is to embed nanoparticles into the tube structures for utilizing the superior capacitive performance of the WS2 nanoparticle and the high rate of the charge transfer of the carbon nanotubes. However, in our system, 2D WS2 and 2D RGO were synthesized and combined via a molten salt method simultaneously. The layer-by-layer structure of the 2D hybrid material can enhance the contact between WS2 and RGO. The higher CF value achieved in our work is mainly due to the better capacitive performance of the 2D WS2 nanosheet synthesized in our system than that of the 3D WS2 nanoparticle made in the reference, and to the much better contact between the WS2 and RGO in the 2D structures for our system than the particle-tube combination made in the reference. [13] Ratha et al. synthesized WS2/RGO hybrids to exhibit an enhanced SC performance with the CF value of 350 F g1 at a scan rate of 2 mV s1 [15]. The RGO-based and WS2-based electrodes only show the CF value of 130 and 70 F g1, respectively. In our system, a higher CF value of 1355.67 F g1 was obtained for the WS2/RGO hybrid-based SC electrode, as compared with those of 398.5 and 119.9 F g1 respectively for the WS2-based and RGObased SC electrodes at a scan rate of 10 mV s1. It is clearly to find that the CF value of the WS2-based SC electrode is much higher in our work as compared with that obtained in the literature [15]. The CF value for the WS2/RGO hybrid-based SC electrode was enhanced by around 5 and 3.5 times as compared with those achieved by the corresponding WS2-based SC electrode, respectively in the literature and in our work. It is obviously to realize that when the RGO was incorporated in the WS2-based SC electrode, the electrocapacitive performance can be largely enhanced for both cases in the literature and in our work. Hence the main difference between the literature and our work is the performance of the SC electrode based on WS2. The WS2 was synthesized using a hydrothermal method in the literature by dissolving the precursors of WCl6 and C2H5NS in DIW and kept the solution at 265 C for 24 h in an autoclave. Whereas the WS2 was synthesized using a molten salt method in our work by grounding the precursors of H2WO4 and CH4N2S without adding any solvent in the mortar and kept the
mortar at 600 C for 5 h. The much higher CF value obtained by using the molten salt method in our work is probably due to the higher reaction temperature, which cannot be realized using the hydrothermal method. The higher reaction temperature is inferred to make higher crystallinity of WS2 and therefore cause the higher CF value for the pertinent SC electrode for our case. To further evaluate the reversibility of the WS2/RGO, WS2, and RGO-based SC electrodes, the CV plots at the scan rate of 1, 5, 10, 20, 40, 60, and 80 mV s1 were measured as respectively shown in Fig. 5(a)e(c). All the curves in Fig. 5(a) present similar shapes without distortion, even the scan rate was increased by 80-folds, demonstrating the high reversibility for this case. Also, the redox peak separation is found to increase with the scan rate increasing, presenting a common phenomenon in the electrochemistry. The largest CF value of 2508.07 F g1 was obtained at the scan rate of 1 mV s1, and the value declined with higher scan rates. The CF values of 1891.07, 1606.68, 1355.67, 1066.00, 857.91, and 674.54 F g1 were respectively obtained for the CV plots measured at the scan rate of 5, 10, 20, 40, 60, and 80 mV s1. Only a 73% decrease was found when the scan rate is enhanced by as high as 80-folds, indicating its high-rate capacitive behavior. The CV curves in Fig. 5(b) and (c) respectively for the WS2 and RGO-based SC electrodes also present nearly no distortion and the redox peaks in these curves maintained obvious when higher scan rates were applied on the measurement. The result indicates that the combination of the 2D WS2 and RGO nanomaterials can remain the reversibility of the SC electrode as also observed for the SC electrode with individual WS2 and RGO nanomaterials. Even more interfaces may be generated by combining two different nanomaterials, the reversibility of the SC electrode can still be maintained due the enhanced conductivity and electrocapacitive capability for the hybrid case. In addition, the GC/D plots of the WS2/RGO, WS2, and RGO-based SC electrodes were measured at the current densities of 2, 4, 8, and 16 A g1 in the potential range of 0e0.4 V to examine the high-rate charge/discharge ability, as shown in Fig. 5(d)e(f), respectively. The charge and discharge curves present highly symmetric features with their counter parts for all the GC/D plots in Fig. 5(d), suggesting the high charge/ discharge stability for the sample of WS2/RGO. The highest CF value of 1285 F g1 was obtained at the current density of 2 A g1, and the CF value still remains 940, 720, and 480 F g1 for the plots measured at the current density of 4, 8, and 16 A g1, respectively. The 8-folds increases on the current density results in only 62% decreases on the CF value, again demonstrating the high-rate operating conditions and the outstanding charge/discharge ability for this case. Nevertheless, the GC/D curves in Fig. 5(e) and (f) respectively for the SC electrode with WS2 and RGO present less symmetric shapes with much longer charge durations as compared with that for the WS2/RGO-based SC electrode in Fig. 5(d). Also, the declines on the CF value with higher current density applied on measuring the GC/ D curves are much larger for the SC electrodes with individual WS2 and RGO as the electrocapacitive material as compared with the WS2/RGO-based SC electrode. The results suggest that the high-rate charge/discharge capacity is greatly improved by combining WS2 and RGO as the charge-accumulating material with the synergic effects for enhancing the number of the active sites and the conductivity for charge transfer. 3.3. Energy and power densities, charge-transfer resistance, and the cycling stability of the supercapacitor electrode with WS2/RGO, WS2, and RGO as the electroactive material Further electrochemical analysis was applied to investigate the supercapacitor performance for the electrodes with WS2/RGO, WS2, and RGO as the electroactive material. Firstly, the Ragone plots for
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Fig. 5. The CV plots for the (a) WS2/RGO, (b) WS2, and (c) RGO-based SC electrodes measured at various scan rates, and the GC/D curves for the (d) WS2/RGO, (e) WS2, and (f) RGObased SC electrodes obtained using different current densities.
the WS2/RGO, WS2, and RGO-based SC electrodes obtained by calculating the data respectively from the GC/D plots in Fig. 5(d)e(f) were shown in Fig. 6. The energy density (E) and the power density (P) were estimated via Equation (3) and (4) as follows, respectively.
E¼
P¼
Fig. 6. The Ragone plots for the SC electrodes with WS2/RGO, WS2, and RGO as the electroactive material.
CF DV 2 2 E
Dt
(3)
(4)
where DV is the potential window employed for the GC/D process (0.4 V in this study) and Dt is the discharge time. The energy density of 28.33 W h kg1 was obtained at the current density of 2 A g1 for the SC electrode with WS2/RGO as the electroactive material, while the WS2 and RGO-based SC electrode shows much smaller energy densities respectively of 7.51 and 5.67 W h kg1 under the same condition. In addition, the WS2/RGO-based SC electrode has a high energy density of 28.33 W h kg1 at a power density of 400 W kg1, and at a high power density of 3200 W kg1 it can still remain an energy density of 12.71 W h kg1. However, the smaller energy densities of 7.51 and 1.34 W h kg1 were
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respectively obtained at a power density of 400 and 3200 W kg1 for the WS2-based SC electrode. Also, the even smaller energy densities of 5.67 and 1.33 W h kg1 were got at a power density of 400 and 3200 W kg1 for the RGO-based SC electrode, respectively. The high energy and power densities achieved for the SC electrode with WS2/RGO as the electroactive material again suggests its overwhelming electrochemical performance owing to the synergic effects from WS2 and RGO with their great electrocapacitive capability and conductive features. In addition, to explore the transport kinetics for the electrocapacitive behaviors of the SC electrodes, the Nyquist plots were attained using the EIS technique as shown in Fig. 7(a), and the corresponding equivalent circuit was shown in Fig. 7(b). The internal resistance (Rs) corresponding to the conductivity can be estimated by using the intersection on the x-axis. The chargetransfer resistance between the electrolyte and the electrode (Rct) can be evaluated by using the semicircle in the plot at the high frequency region [23]. The WS2/RGO electrode shows a smaller Rs value of 0.68 U and a semicircle corresponding to a smaller Rct value of 976 U at the high frequency region, as compared with those for the WS2-based SC electrode respectively with the Rs and Rct values of 1.53 and 1740 U. On the other hand, the similar Rs values of 0.68 and 0.71 U were obtained for the WS2/RGO and RGO-based SC electrodes, respectively, suggesting that the high conductivity of the WS2/RGO-based SC electrode is primarily contributed to the RGO nanomaterial. However, the much larger Rct value of 1650 U was found for the RGO-based electrode, indicating the worse electroactive capability for this case even if its conductivity is high. The results suggest the higher conductivity and smaller charge transfer resistance for the hybrid case benefitted by the synergic effects for WS2 and RGO. Additionally, the angle leaning to the y-
axis is more than 45 at the low frequency region for the WS2/RGO electrode, while the angles were found to be less than 45 at the low frequency region for the SC electrodes with individual WS2 and RGO, suggesting the relatively ideal capacitor behavior for the hybrid-based SC electrode [18,23,24]. Last but not least, the 5000 cycles repeated charge/discharge process was applied for testing the cycling stability of the WS2/RGO, WS2, and RGO-based SC electrodes at a current density of 2 A g1. The CF values and the Columbic efficiency as a function of the corresponding cycling number were presented in Fig. 8(a)e(c) for the WS2/RGO, WS2, and RGO-based SC electrodes, respectively. After 5000 repeated charge/discharge cycles, the 98.6%, 97.1%, and
Fig. 7. (a) The Nyquist plots, and (b) the corresponding equivalent circuit for the Nyquist plots of the SC electrodes with WS2/RGO, WS2, and RGO as the electroactive material.
Fig. 8. The capacitance retention and the Coulombic efficiency as the function of the charge/discharge cycle for the SC electrodes with (a) WS2/RGO, (b) WS2, and (c) RGO as the electroactive material.
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100% retention on the CF value were respectively obtained for the WS2/RGO, WS2, and RGO-based SC electrodes, as compared with those achieved at the corresponding first charge/discharge process. The CF retention is excellent for all the cases, but only the RGObased electrode presents almost 100% CF retention after 5000 cycles repeated charge/discharge process, demonstrating the better stability for the carbon material. The WS2/RGO-based SC electrode shows slightly better cycling stability than that for the WS2-based SC electrode, suggesting the help of RGO to enhance the repeated charge/discharge capability. On the other hand, the Coulombic efficiency (h) estimated by the charge and discharge intervals with the equation of h ¼ td/tc 100% (where tc and td presents the charge and discharge intervals, respectively) [25] shows higher than 94% for the entire measurement for the SC electrode with WS2/RGO as the electroactive material. However, the individual WS2 and RGO-based SC electrode respectively presents the h less than 80% and 85% for the entire measurement. The result suggests that the Coulombic efficiency can be greatly improved by mixing the electroactive WS2 and highly conductive RGO to reduce the interfacial charge-transfer resistance in the hybrid. The good cycling capability and Columbic efficiency were both achieved for the WS2/RGO-based SC electrode, which could be attributed to the intercalated RGO nanosheets between WS2 nanoladders for greatly reducing the restacking during the cycling process and inducing the fast electron and ion transfer upon the charge/discharge process. The carbonaceous materials usually help in improving the cycling stability of pseudocapacitive materials, as also observed in the previous literatures [26e28].
using a simple molten salt process and applied as the electroactive material for the SC electrode. The self-synthesized WS2 nanoladders and RGO nanosheets were well-combined to present the perfect 2D hybrid nanostructure. A greatly enhanced CF value of 1355.67 F g1 was obtained for the WS2/RGO-based SC electrode, as compared with those for the SC electrodes with single component of WS2 or RGO, obtained by using the CV plots at the scan rate of 10 mV s1. The EIS analysis indicates the higher conductivity for the WS2/RGO and RGO-based SC electrodes, as well as the smaller charge-transfer resistance for the WS2/RGO-based SC electrode. A better high-rate capability as well as higher energy and power densities were obtained for the hybrid case. Also, a 98.6% cycling stability retention after 5000 cycles charge/discharge process and the Coulombic efficiency of almost 100% for in the entire measurement were also obtained for the WS2/RGO-based SC electrode. The largely enhanced electrochemical performance for the SC electrode with the WS2/RGO hybrid is mainly due to the synergic effect from both materials with the highly capacitive redox reactions for WS2 nanoladders and the great electron transfer ability for RGO nanosheets.
3.4. The charge storage mechanism for the WS2/RGO electrode
References
After evaluating the electrochemical performance of the WS2/ RGO electrode, the charge storage mechanism was proposed as follows. The hybrid WS2/RGO system combines the characteristics for both the electrical double-layered capacitor (EDLC) and the pseudo-capacitors. First, to discuss the charge storage mechanism for the EDLC, in which the charges are accumulated on the surface of the capacitive materials without the participation of the chemical energy. The EDLC stores energy at the electrolyteecarbon interface through reversible ion adsorption onto the carbon surface. The electrode attracts the ions in the opposite electrical property at the charging process, while the charges become free conditions in the discharging process. Secondly, to discuss the charge storage mechanism for the pseudo-capacitor, the charges are stored in the capacitive materials by the chemical energy via the chemical bonds. The charges stored in the pseudo-capacitor via the redox reactions of W6þ and W4þ on WS2 as well as of the O-containing surface functionalities on the surface of RGO, as demonstrated in the previous discussion of the peaks in the CV curves. At the charge/ discharge processes, the ions in the electrolyte would diffuse into the bulk material to conduct the redox reactions. This behavior is similar to the mechanism for the batteries. In all, the charges can be accumulated at the RGO/electrolyte interface to conduct the EDLC storage mechanism and also be stored in the chemical bonds in the WS2 and the O-containing surface functionalities of the RGO to realize the pseudo-capacitor behavior. In addition, the RGO cannot only play as the capacitive material for accumulating charges but also can enhance the conductivity of the hybrids to facilitate fast transportation of electrons throughout the hybrid electrode material.
[1] H. Tang, J. Wang, H. Yin, H. Zhao, D. Wang, Z. Tang, Adv. Mater. 27 (2015) 1117e1123. [2] L. Jiang, S. Zhang, S.A. Kulinich, X. Song, J. Zhu, X. Wang, H. Zeng, 3 (2015) 177e183. [3] H.P. Cong, X.C. Ren, P. Wang, S.H. Yu, Energy Environ. Sci. 6 (2013) 1185e1191. [4] D.A.C. Brownson, C.E. Banks, Chem. Commun. 48 (2012) 1425e1427. [5] X. Cai, X. Shen, L. Ma, Z. Ji, L. Kong, RSC Adv. 5 (2015) 58777e58783. [6] R.R. Salunkhe, S.H. Hsu, K.C.W. Wu, Y. Yamauchi, Chem. Sus. Chem. 7 (2014) 1551e1556. [7] C. Xia, H.N. Alshareef, Chem. Mater. 27 (2015) 4661e4668. [8] X. Huang, Z. Zeng, H. Zhang, Chem. Soc. Rev. 42 (2013) 1934e1946. [9] C. Tan, H. Zhang, Nat. Commun. 6 (2015) 7873. [10] M. Acerce, D. Voiry, M. Chhowalla, Nat. Nanotech. 10 (2015) 313e318. [11] Y. Liu, W. Wang, H. Huang, L. Gu, Y. Wang, X. Peng, Chem. Commun. 50 (2014) 4485e4488. [12] Y. Liu, W. Wang, Y. Wang, X. Peng, 7 (2014) 25e32. [13] B. Hu, X. Qin, A.M. Asiri, K.A. Alamry, A.O. Al Youbi, X. Sun, Electrochem. Commun. 28 (2013) 75e78. [14] D.P. Dubal, R. Holze, P. Gomez Romero, Sci. Rep. 4 (2014) 7349. [15] S. Ratha, C.S. Rout, ACS Appl. Mater. Interfaces 5 (2013) 11427e11433. [16] E.G. da Silveira Firmiano, A.C. Rabelo, C.J. Dalmaschio, A.N. Pinheiro, E.C. Pereira, W.H. Schreiner, E.R. Leite, Adv. Energ. Mater. 4 (2014) 1301380. [17] J. Yang, D. Voiry, S.J. Ahn, D. Kang, A.Y. Kim, M. Chhowalla, H.S. Shin, Angew. Chem. Int. Ed. Engl. 52 (2013) 13751e13754. [18] L.Y. Lin, M.H. Yeh, J.T. Tsai, Y.H. Huang, C.L. Sun, K.C. Ho, J. Mater. Chem. A 1 (2013) 11237. [19] F. Su, C.K. Poh, J.S. Chen, G. Xu, D. Wang, Q. Li, J. Lin, X.W. Lou, Energy Environ. Sci. 4 (2011) 717e724. [20] D. Hulicova Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Adv. Func. Mater. 19 (2009) 438e447. [21] G. Ma, H. Peng, J. Mu, H. Huang, X. Zhou, Z. Lei, J. Power Sources 229 (2013) 72e78. [22] J. Wang, Z. Wu, K. Hu, X. Chen, H. Yin, J. Alloy. Compd. 619 (2015) 38e43. [23] K. Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Chem. Mater. 22 (2010) 1392e1401. [24] Z. Wu, X. Pu, X. Ji, Y. Zhu, M. Jing, Q. Chen, F. Jiao, Electrochim. Acta 174 (2015) 238e245. [25] M. Sun, J. Tie, G. Cheng, T. Lin, S. Peng, F. Deng, F. Ye, L. Yu, J. Mater. Chem. A 3 (2015) 1730e1736. [26] M. Xue, F. Li, J. Zhu, H. Song, M. Zhang, T. Cao, Adv. Funct. Mater. 22 (2012) 1284e1290. [27] L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie, G. Yu, Nano Lett. 13 (2013) 2151e2157. [28] M. Zhi, C. Xiang, J. Li, M. Li, N. Wu, Nanoscale 5 (2013) 72e88.
4. Conclusions A highly conductive 2D WS2/RGO paper was simply prepared by
Acknowledgements This work was supported in part by the Ministry of Science and Technology of Taiwan, under grant numbers: MOST 103-2218-E027-010-MY2 and MOST 103-2119-M-027-001-.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Highly efficient supercapacitor electrode with two-dimensional tungsten disulfide and reduced graphene oxide hybrid nanosheets Chao-Chi Tu 1, Lu-Yin Lin*, 1, Bing-Chang Xiao, Yu-Shiang Chen Department of Chemical Engineering and Biotechnology, National Taipei University of Technology (Taipei Tech), Taipei 10608, Taiwan
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
WS2/reduced graphene oxide (RGO) nanosheets are made via a simple malten salt way. The nanosheets are used as the electroactive material for supercapacitors (SCs). A specific capacitance (CF) of 2508.07 F g1 at 1 mV s1 is got for the SCs. The 98.6% retention on the CF is got after 5000 cycles charge/discharge process. The Coulombic efficiency close to 100% for the entire measurement is achieved.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2016 Received in revised form 6 April 2016 Accepted 18 April 2016 Available online 26 April 2016
Two-dimensional (2D) nanostructures with their high surface area and large in-plane conductivity have been regarded as promising materials for supercapacitors (SCs). Tungsten disulfide (WS2) is highly suitable for charge accumulation with its abundant active sites in the interspacing between the 2D structures and the intraspacing of each atomic layer, as well as on the tungsten centers with the charges generated by the Faradaic reactions. This study proposes the preparation of well-constructed WS2/ reduced graphene oxide (RGO) nanosheets using a simple molten salt process as the electroactive material for SCs, which presents a high specific capacitance (CF) of 2508.07 F g1 at the scan rate of 1 mV s1, because of the synergic effect of WS2 with its large charge-accumulating sites on the 2D planes and RGO with its highly enhanced conductivity and improved connections in the WS2 networks. The excellent cycling stability of 98.6% retention after 5000 cycles charge/discharge process and the Coulombic efficiency close to 100% for the entire measurement are also achieved for the WS2/RGO-based SC electrode. The results suggest the potential for the combination of the 2D metal sulfide and carbon materials as the charge storage material to solve the energy problems and attain a sustainable society. © 2016 Elsevier B.V. All rights reserved.
Keywords: Cyclic voltammetry Electrochemical impedance spectroscopy Reduced graphene oxide Supercapacitor Two-dimensional Tungsten disulfide
1. Introduction
* Corresponding author. E-mail address: [email protected] (L.-Y. Lin). 1 The authors contributed equally. http://dx.doi.org/10.1016/j.jpowsour.2016.04.083 0378-7753/© 2016 Elsevier B.V. All rights reserved.
Two-dimensional (2D) nanostructures [1,2], graphene [3,4], reduced graphene oxide [5,6], and metal chalcogenides [7,8] etc., have attracted much attention as the electroactive material for the highly performed supercapacitor (SC) electrodes due to the high
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active surface area and large in-plane conductivity [2,9,10]. Tungsten disulfide (WS2) is one of the attractive tungsten chalcogenides for the electrochemical application, formed by the 2D covalently bonded SWS layers separated by a weak van der Waals gap, for which the large 2D nanoplane of WS2 plays as the abundant active sites for charge accumulation. However, the restacking between the nanosheets and the low electronic conductivity as well as the relative brittle of WS2 limits its applications. To solve this problem, highly conductive carbon materials have been applied to enhance the conductivity of the electrode and meanwhile prevent the restacking of WS2 [11e13]. Reduced graphene oxide (RGO) was reported to have good flexibility and high electric conductivity, but the low theoretical capacity limits its electrocapacitive performance [5,14e16]. Hence, constructing uniform hybrids with strong and conductive RGO nanosheets and highly capacitive 2D WS2 nanostructures is expected to improve the conductivity of the electrode, reduce the restacking of WS2 nanosheets, and hence to provide large electrode/electrolyte interfaces during the dischargecharge process as well as promote the electron transport and sustain the volume variation during the electrochemical ion insertion/ extraction reaction. Liu et al. assembled a WS2 and graphene oxide (GO) composite paper for Li-ion batteries to achieve a higher reversible capacity of 697.7 mA h g1 as compared with those of 88.5 and 60.2 mA h g1 respectively for the bare WS2 and RGO cases [12]. Liu et al. fabricated hybrid lamellar porous electrodes with an excellent electrochemical performance by homogeneously intercalating single-walled carbon nanotubes into the lamellar assembled WS2 nanosheets through the vacuum filtration [11]. Yang et al. synthesized the WS2/RGO nanosheet using a one-pot hydrothermal reaction process and demonstrated its good catalytic activity for the hydrogen evolution [17]. Previous literatures also studied the performance of WS2 and carbon hybrids on the application of SCs. Hu et al. prepared WS2 nanoparticleseencapsulated amorphous carbon tubes as the SC electrode material, and a specific capacitance (CF) value of 337 F g1 was achieved at a current density of 10 A g1 with a good cycle stability [13]. Ratha et al. synthesized layered WS2/RGO hybrids by a facile hydrothermal method for SCs to obtain an enhanced CF value of 350 F g1 which is about 5 and 2.5 times higher than those for the SC electrode with bare WS2 and RGO nanosheets, respectively [15]. However, the study of the SC electrodes using WS2/RGO hybrids as the charge-accumulating material is limited, and the related SC performance is far from expectation, i.e., the CF value of merely less than 350 F g1 was obtained in the previous studies [15]. Therefore, the exposed active sites for the Faradic reactions as well as the conductivity for the efficient charge transportation are necessary to be improved for achieving the highly efficient SC electrode based on the hybrid of metal sulfide and carbon materials. Here we report a SC electrode based on a 2D hybrid consisting of WS2 and RGO nanostructures fabricated by using a simple molten salt process. A higher CF value of 1355.67 F g1 was obtained for the WS2/RGO hybrid-based SC electrode, as compared with those of 398.5 and 119.9 F g1 respectively for the WS2 and RGO-based SC electrodes at a scan rate of 10 mV s1. A cycling stability of 98.6% retention after 5000 cycles charge/discharge process and almost 100% for the Coulombic efficiency in the entire measurement were obtained for this case. The charge-transfer resistances were also measured by the electrochemical impedance spectroscopy (EIS) to evaluate the kinetic behaviors of the electrodes.
79
from Showa. Potassium chloride (laboratory reagent grade, KCl) and potassium hydroxide (KOH, analytical reagent grade) were brought from Fisher. Ethanol (EtOH) and sodium nitrate (99þ%, for analysis, NaNO3) were got from Acros. Sulfuric acid (H2SO4, ACS reagent, 95.0e98.0%) and tungstic acid (99%, H2WO4) were purchased from SigmaeAldrich. 2.2. Preparation of the WS2 nanoladder and the WS2/RGO hybrid as well as the corresponding supercapacitor electrodes To prepare the WS2/RGO hybrid, 50 mg RGO (P-HF10, Enerage Inc.) and 100 mg H2WO4 were finely ground with 5 g CH4N2S. Then the mixed sample was heated to 600 C at the heating and cooling rate of 1 C min1 and maintained at 600 C for 5 h. The WS2/RGO hybrid was further infiltrated by suction to form the WS2/RGO paper. The WS2/RGO paper is highly conductive as tested in the circuit for the light-emitting diode (LED) module, as presented in Fig. 1. This highly conductive and flexible paper can probably be used as the one-piece integrated substrate-electroactive material in our future work. The bare WS2 sample was also synthesized using the same method for preparing WS2/RGO hybrid but without the addition of RGO in the whole process. Similarly, the bare RGO sample was prepared using the same manner for synthesizing WS2/ RGO hybrid but without the addition of H2WO4 in the whole process. To prepare the SC electrodes, 10 mg of the WS2, RGO, or the WS2/ RGO hybrid were individually dispersed in a 2 mL EtOH solution. Then the solution was deposited on the nickel foam (110PPI, thickness ¼ 1.05 mm, Innovation Materials Co., Ltd, Taiwan) which is promising as the flexible substrate for the electrode using a dropcoating technique. Finally the electrodes were dried under vacuum at 60 C for 12 h. 2.3. Material characterization and electrochemical measurements The high-resolution transmission electron microscopy (HRTEM) (Philips Tecnai F30 Field Emission Gun Transmission Micro-scope (FEG-TEM)) was utilized to examine the lattice of the samples. The field-emission scanning electron microscopy (FE-SEM, Nova NanoSEM 230, FEI, Oregon, USA) equipped with the energy
2. Experimental 2.1. Materials Tungstic acid (H2WO4) and thiourea (CH4N2S) were obtained
Fig. 1. The circuit with a LED and the WS2/RGO paper (enlarged in the right side) for testing the conductivity of the hybrid paper. The upper photo is with the WS2/RGO paper in the circuit and the lower photo is the broken circuit.
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dispersive X-ray spectrometry (EDX) was used to investigate the surface morphology and element composition of the products. The phase and the structure of the products were determined by X-ray diffraction (XRD, X'Pert3 Powder, PANalytical) with Cu Ka radiation (l ¼ 1.5418 Å). The cyclic voltammetry (CV) and galvanic charge/ discharge curves (GC/D) were obtained using a potentiostat/galvanostat (PGSTAT 204, Autolab, EcoeChemie, the Netherlands) carried out with a three-electrode electrochemical system, where the prepared SC electrode was used as the working electrode, a Pt wire was used as the counter electrode, and an Ag/AgCl/saturated KCl electrode was used as the reference electrode in a aqueous solution containing 1 M KOH and 0.5 M KCl. The EIS was carried out using a potentiostat/galvanostat (PGSTAT 204, Autolab, EcoeChemie, the Netherlands) equipped with an FRA2 module, and the frequency range explored was 100,000 Hze0.01 Hz. The applied bias voltage was set at open-circuit voltage. 3. Results and discussion 3.1. Morphology and composition characterization of WS2/RGO, WS2, and RGO The morphology of the 2D materials is of the great importance to determine the electrochemical performance of the pertinent SC electrode. Therefore, the structure of the self-synthesized WS2 was firstly examined to confirm the composition of the self-made product. Fig. 2(a) presents the HRTEM image for the WS2 nanomaterial. The stacking of WS2 with the interplanar spacing of 0.62 nm was observed. Also, the spacing of 0.27 nm for the periodic arrays for the lattice fringe of (100) plane was obtained in the inserted figure, providing the evidence for the WS2 phase. In addition, the SEM images for the WS2, RGO, and the WS2/RGO hybrid were shown in Fig. 2(b)e(d), respectively. The WS2 sample presents multiple sheet-like structures stacked layer-by-layer, and numerous wrinkles were well distributed on the surface in random directions, suggesting the nanoladder morphology. The RGO sample also shows 2D nanosheet structure with several wrinkles on the surface, but the pattern of the wrinkles on the surface of RGO is very different from that on the surface of WS2, in terms of the distribution uniformity and the direction. The wrinkles grew in a relatively parallel direction but distributed more randomly for the RGO sample, while the growing direction is not fixed but the distribution of the wrinkles is very uniform for the WS2 sample. It is
inferred that the wrinkles in the RGO sample is resulting from the folding of 2D layers, while the wrinkles in the WS2 sample seems like grow naturally on the surface instead of being generated from the staking of the layers. On the other hand, the 2D WS2/RGO hybrid also presents 2D layered structure with fewer wrinkles on the surface, as compared with those on the surfaces of WS2 and RGO. Since the hybrid was prepared by simply mixing the WS2 nanoladders and the RGO nanosheets by suction filtration, the overlapping of two different 2D nanostructures may cause even more complex distribution of the wrinkles on the surface. However, the wrinkles on the surface of the hybrid are relatively smoother as compared with those on the surfaces of WS2 and RGO, probably due to the Van Der Waal force existing between the layered structures of WS2 and RGO to relieve the extent of the folding and therefore to smooth the wrinkles on both surfaces. To distinguish the WS2 and RGO nanostructures more clearly, the TEM image for the WS2/RGO was obtained as shown in Fig. 2(e). The layered structures with light and dark colors were observed, which can be respectively inferred to be the material of RGO and WS2 since the molecular weight for RGO is smaller than that for WS2. The uniform coverage of WS2 was observed on RGO, suggesting the successful combination of the 2D nanostructures of RGO and WS2. To further confirm the composition of the hybrid, the corresponding EDX spectrum was presented in Fig. 3(a). The signals for W, S, and C were clearly detected, suggesting the successful mixing of the hybrid with WS2 and RGO. In addition, the elemental mapping spectra for W, S, and C along with the corresponding SEM image were shown in Fig. 3(b), in which the uniform distribution of W, S, and C were observed, suggesting the even mixing of WS2 and RGO by the simple suction filtration method. To further confirm the composition of the samples, the XRD patterns of WS2 and WS2/RGO hybrid were shown in Fig. 3(c). The WS2 sample presents all the peaks indexed to be the hexagonal WS2 structure (JCPDS no.84-1398) [15], and an extra diffraction peak at 24.21 is observed for the WS2/RGO sample assigned to the (002) peak for RGO, suggesting the successful blending of the WS2 and RGO in the nanohybrid. 3.2. Electrochemical performance of the supercapacitor electrode with WS2/RGO, WS2, and RGO as the electroactive material After examining the morphology and the composition of WS2, RGO, and the WS2/RGO hybrid, the electrocapacitive performances
Fig. 2. (a) The HRTEM image of WS2, the SEM images of (b) WS2 (c) RGO and (d) WS2/RGO hybrid, and (e) the TEM image of WS2/RGO hybrid.
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Fig. 4. (a) The CV plots measured at 10 mV s1 and (b) the GC/D curves measured at 2 A g1 for the SC electrodes with WS2, RGO and WS2/RGO hybrid as the electroactive material.
Fig. 3. (a) the EDX spectrum and (b) the elemental mapping of W, S, and C elements along with the corresponding SEM image for the WS2/RGO hybrid, and (c) the XRD patterns for WS2 and the WS2/RGO hybrid.
of the corresponding SC electrodes were evaluated by using the CV measurement at a scan rate of 10 mV s1, as presented in Fig. 4(a). The value of the specific capacitance (CF) can be calculated according to Equation (1) as follows [18].
Z I dV CF ¼
n DV m
(1)
R where I is the current density, I dV is the integrated area of the CV curve, n is the scan rate, DV is the potential window, and m is the
weight of the electroactive material in the SC electrode. Two couples of the redox peaks were observed for the WS2/RGO-based SC electrode, with the oxidation peaks at the potentials of 0.35 and 0.40 V and the corresponding reduction peaks at the potentials of 0.22 and 0.17 V, while only one couple of the redox peaks at 0.35 and 0.25 V was obtained for the individual WS2 and RGO-based SC electrodes. The CV curves obviously suggested the pseudocapacitive characteristics for all the cases. The peaks in the CV curve for the WS2-based SC electrode corresponded to the redox reaction between W6þ and W4þ [13], while the peaks for the RGO-based SC electrode were probably contributed to the redox reactions of the O-containing surface functionalities on the surface of RGO, as also observed in the previously literatures [19,20]. The WS2/RGO hybrid-based SC electrode presents two couples of the redox peaks, which are inferred to be the redox reactions for the conversion between W6þ and W4þ and for the O-containing surface functionalities on the surface of RGO. Other than discussing the redox peaks for the samples, the current in the CV plots is also significant to index the electrocapacitive performance for the corresponding SC electrode. A much higher CF value of 1355.67 F g1 was obtained for the WS2/RGO-based SC electrode, as compared with those of 398.5 and 119.9 F g1 for the electrodes with WS2 and RGO, respectively. This CF value for the SC electrode with the WS2/RGO hybrid achieved in this study is considered to be one of the highest among the 2D metal chalcogenides-based SC electrodes reported till now [11,13,16,21,22]. On the other hand, the charge/discharge
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capability of the SC electrode with WS2, RGO, and WS2/RGO hybrid as the electroactive material was estimated by measuring the GC/D plots at a current density of 2 A g1 as shown in Fig. 4(b), and the corresponding CF values were calculated based on Equation (2) as follows [18].
CF ¼
I Dt m DV
(2)
where Dt is the discharge time and m is the weight of the electroactive material in the electrode. The WS2/RGO hybrid-based SC electrode presents nearly symmetric charge and discharge curves as well as the longest discharge time of 255 s which corresponds to a high CF value of 1275 F g1, indicating the outstanding charge/ discharge performance for this case, while the WS2 and RGO-based SC electrode shows relatively shorter discharging time of 67 and 31 s corresponding to the CF value of 335 and 155 F g1, respectively. The greatly enhanced CF value for the SC electrode with the WS2/ RGO hybrid as the electroactive material indicates the synergic effect of the psedocapacitive WS2 and conductive RGO to respectively provide highly capacitive redox reactions and the great electron transfer ability. Also, with the help of RGO the restacking of WS2 nanosheets can be reduced and hence the electroactive area between the electrode and the electrolyte can be greatly enhanced. The CF value achieved in this study is much higher than those reported in the previous literatures [13,15]. Hu et al. fabricated WS2 nanoparticles and encapsulated amorphous carbon tubes hybrid via two-step processes [13]. The resulting supercapacitor presents a CF value of 337 F g1 at a high current density of 10 A g1. The concept of this literature is to embed nanoparticles into the tube structures for utilizing the superior capacitive performance of the WS2 nanoparticle and the high rate of the charge transfer of the carbon nanotubes. However, in our system, 2D WS2 and 2D RGO were synthesized and combined via a molten salt method simultaneously. The layer-by-layer structure of the 2D hybrid material can enhance the contact between WS2 and RGO. The higher CF value achieved in our work is mainly due to the better capacitive performance of the 2D WS2 nanosheet synthesized in our system than that of the 3D WS2 nanoparticle made in the reference, and to the much better contact between the WS2 and RGO in the 2D structures for our system than the particle-tube combination made in the reference. [13] Ratha et al. synthesized WS2/RGO hybrids to exhibit an enhanced SC performance with the CF value of 350 F g1 at a scan rate of 2 mV s1 [15]. The RGO-based and WS2-based electrodes only show the CF value of 130 and 70 F g1, respectively. In our system, a higher CF value of 1355.67 F g1 was obtained for the WS2/RGO hybrid-based SC electrode, as compared with those of 398.5 and 119.9 F g1 respectively for the WS2-based and RGObased SC electrodes at a scan rate of 10 mV s1. It is clearly to find that the CF value of the WS2-based SC electrode is much higher in our work as compared with that obtained in the literature [15]. The CF value for the WS2/RGO hybrid-based SC electrode was enhanced by around 5 and 3.5 times as compared with those achieved by the corresponding WS2-based SC electrode, respectively in the literature and in our work. It is obviously to realize that when the RGO was incorporated in the WS2-based SC electrode, the electrocapacitive performance can be largely enhanced for both cases in the literature and in our work. Hence the main difference between the literature and our work is the performance of the SC electrode based on WS2. The WS2 was synthesized using a hydrothermal method in the literature by dissolving the precursors of WCl6 and C2H5NS in DIW and kept the solution at 265 C for 24 h in an autoclave. Whereas the WS2 was synthesized using a molten salt method in our work by grounding the precursors of H2WO4 and CH4N2S without adding any solvent in the mortar and kept the
mortar at 600 C for 5 h. The much higher CF value obtained by using the molten salt method in our work is probably due to the higher reaction temperature, which cannot be realized using the hydrothermal method. The higher reaction temperature is inferred to make higher crystallinity of WS2 and therefore cause the higher CF value for the pertinent SC electrode for our case. To further evaluate the reversibility of the WS2/RGO, WS2, and RGO-based SC electrodes, the CV plots at the scan rate of 1, 5, 10, 20, 40, 60, and 80 mV s1 were measured as respectively shown in Fig. 5(a)e(c). All the curves in Fig. 5(a) present similar shapes without distortion, even the scan rate was increased by 80-folds, demonstrating the high reversibility for this case. Also, the redox peak separation is found to increase with the scan rate increasing, presenting a common phenomenon in the electrochemistry. The largest CF value of 2508.07 F g1 was obtained at the scan rate of 1 mV s1, and the value declined with higher scan rates. The CF values of 1891.07, 1606.68, 1355.67, 1066.00, 857.91, and 674.54 F g1 were respectively obtained for the CV plots measured at the scan rate of 5, 10, 20, 40, 60, and 80 mV s1. Only a 73% decrease was found when the scan rate is enhanced by as high as 80-folds, indicating its high-rate capacitive behavior. The CV curves in Fig. 5(b) and (c) respectively for the WS2 and RGO-based SC electrodes also present nearly no distortion and the redox peaks in these curves maintained obvious when higher scan rates were applied on the measurement. The result indicates that the combination of the 2D WS2 and RGO nanomaterials can remain the reversibility of the SC electrode as also observed for the SC electrode with individual WS2 and RGO nanomaterials. Even more interfaces may be generated by combining two different nanomaterials, the reversibility of the SC electrode can still be maintained due the enhanced conductivity and electrocapacitive capability for the hybrid case. In addition, the GC/D plots of the WS2/RGO, WS2, and RGO-based SC electrodes were measured at the current densities of 2, 4, 8, and 16 A g1 in the potential range of 0e0.4 V to examine the high-rate charge/discharge ability, as shown in Fig. 5(d)e(f), respectively. The charge and discharge curves present highly symmetric features with their counter parts for all the GC/D plots in Fig. 5(d), suggesting the high charge/ discharge stability for the sample of WS2/RGO. The highest CF value of 1285 F g1 was obtained at the current density of 2 A g1, and the CF value still remains 940, 720, and 480 F g1 for the plots measured at the current density of 4, 8, and 16 A g1, respectively. The 8-folds increases on the current density results in only 62% decreases on the CF value, again demonstrating the high-rate operating conditions and the outstanding charge/discharge ability for this case. Nevertheless, the GC/D curves in Fig. 5(e) and (f) respectively for the SC electrode with WS2 and RGO present less symmetric shapes with much longer charge durations as compared with that for the WS2/RGO-based SC electrode in Fig. 5(d). Also, the declines on the CF value with higher current density applied on measuring the GC/ D curves are much larger for the SC electrodes with individual WS2 and RGO as the electrocapacitive material as compared with the WS2/RGO-based SC electrode. The results suggest that the high-rate charge/discharge capacity is greatly improved by combining WS2 and RGO as the charge-accumulating material with the synergic effects for enhancing the number of the active sites and the conductivity for charge transfer. 3.3. Energy and power densities, charge-transfer resistance, and the cycling stability of the supercapacitor electrode with WS2/RGO, WS2, and RGO as the electroactive material Further electrochemical analysis was applied to investigate the supercapacitor performance for the electrodes with WS2/RGO, WS2, and RGO as the electroactive material. Firstly, the Ragone plots for
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Fig. 5. The CV plots for the (a) WS2/RGO, (b) WS2, and (c) RGO-based SC electrodes measured at various scan rates, and the GC/D curves for the (d) WS2/RGO, (e) WS2, and (f) RGObased SC electrodes obtained using different current densities.
the WS2/RGO, WS2, and RGO-based SC electrodes obtained by calculating the data respectively from the GC/D plots in Fig. 5(d)e(f) were shown in Fig. 6. The energy density (E) and the power density (P) were estimated via Equation (3) and (4) as follows, respectively.
E¼
P¼
Fig. 6. The Ragone plots for the SC electrodes with WS2/RGO, WS2, and RGO as the electroactive material.
CF DV 2 2 E
Dt
(3)
(4)
where DV is the potential window employed for the GC/D process (0.4 V in this study) and Dt is the discharge time. The energy density of 28.33 W h kg1 was obtained at the current density of 2 A g1 for the SC electrode with WS2/RGO as the electroactive material, while the WS2 and RGO-based SC electrode shows much smaller energy densities respectively of 7.51 and 5.67 W h kg1 under the same condition. In addition, the WS2/RGO-based SC electrode has a high energy density of 28.33 W h kg1 at a power density of 400 W kg1, and at a high power density of 3200 W kg1 it can still remain an energy density of 12.71 W h kg1. However, the smaller energy densities of 7.51 and 1.34 W h kg1 were
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respectively obtained at a power density of 400 and 3200 W kg1 for the WS2-based SC electrode. Also, the even smaller energy densities of 5.67 and 1.33 W h kg1 were got at a power density of 400 and 3200 W kg1 for the RGO-based SC electrode, respectively. The high energy and power densities achieved for the SC electrode with WS2/RGO as the electroactive material again suggests its overwhelming electrochemical performance owing to the synergic effects from WS2 and RGO with their great electrocapacitive capability and conductive features. In addition, to explore the transport kinetics for the electrocapacitive behaviors of the SC electrodes, the Nyquist plots were attained using the EIS technique as shown in Fig. 7(a), and the corresponding equivalent circuit was shown in Fig. 7(b). The internal resistance (Rs) corresponding to the conductivity can be estimated by using the intersection on the x-axis. The chargetransfer resistance between the electrolyte and the electrode (Rct) can be evaluated by using the semicircle in the plot at the high frequency region [23]. The WS2/RGO electrode shows a smaller Rs value of 0.68 U and a semicircle corresponding to a smaller Rct value of 976 U at the high frequency region, as compared with those for the WS2-based SC electrode respectively with the Rs and Rct values of 1.53 and 1740 U. On the other hand, the similar Rs values of 0.68 and 0.71 U were obtained for the WS2/RGO and RGO-based SC electrodes, respectively, suggesting that the high conductivity of the WS2/RGO-based SC electrode is primarily contributed to the RGO nanomaterial. However, the much larger Rct value of 1650 U was found for the RGO-based electrode, indicating the worse electroactive capability for this case even if its conductivity is high. The results suggest the higher conductivity and smaller charge transfer resistance for the hybrid case benefitted by the synergic effects for WS2 and RGO. Additionally, the angle leaning to the y-
axis is more than 45 at the low frequency region for the WS2/RGO electrode, while the angles were found to be less than 45 at the low frequency region for the SC electrodes with individual WS2 and RGO, suggesting the relatively ideal capacitor behavior for the hybrid-based SC electrode [18,23,24]. Last but not least, the 5000 cycles repeated charge/discharge process was applied for testing the cycling stability of the WS2/RGO, WS2, and RGO-based SC electrodes at a current density of 2 A g1. The CF values and the Columbic efficiency as a function of the corresponding cycling number were presented in Fig. 8(a)e(c) for the WS2/RGO, WS2, and RGO-based SC electrodes, respectively. After 5000 repeated charge/discharge cycles, the 98.6%, 97.1%, and
Fig. 7. (a) The Nyquist plots, and (b) the corresponding equivalent circuit for the Nyquist plots of the SC electrodes with WS2/RGO, WS2, and RGO as the electroactive material.
Fig. 8. The capacitance retention and the Coulombic efficiency as the function of the charge/discharge cycle for the SC electrodes with (a) WS2/RGO, (b) WS2, and (c) RGO as the electroactive material.
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100% retention on the CF value were respectively obtained for the WS2/RGO, WS2, and RGO-based SC electrodes, as compared with those achieved at the corresponding first charge/discharge process. The CF retention is excellent for all the cases, but only the RGObased electrode presents almost 100% CF retention after 5000 cycles repeated charge/discharge process, demonstrating the better stability for the carbon material. The WS2/RGO-based SC electrode shows slightly better cycling stability than that for the WS2-based SC electrode, suggesting the help of RGO to enhance the repeated charge/discharge capability. On the other hand, the Coulombic efficiency (h) estimated by the charge and discharge intervals with the equation of h ¼ td/tc 100% (where tc and td presents the charge and discharge intervals, respectively) [25] shows higher than 94% for the entire measurement for the SC electrode with WS2/RGO as the electroactive material. However, the individual WS2 and RGO-based SC electrode respectively presents the h less than 80% and 85% for the entire measurement. The result suggests that the Coulombic efficiency can be greatly improved by mixing the electroactive WS2 and highly conductive RGO to reduce the interfacial charge-transfer resistance in the hybrid. The good cycling capability and Columbic efficiency were both achieved for the WS2/RGO-based SC electrode, which could be attributed to the intercalated RGO nanosheets between WS2 nanoladders for greatly reducing the restacking during the cycling process and inducing the fast electron and ion transfer upon the charge/discharge process. The carbonaceous materials usually help in improving the cycling stability of pseudocapacitive materials, as also observed in the previous literatures [26e28].
using a simple molten salt process and applied as the electroactive material for the SC electrode. The self-synthesized WS2 nanoladders and RGO nanosheets were well-combined to present the perfect 2D hybrid nanostructure. A greatly enhanced CF value of 1355.67 F g1 was obtained for the WS2/RGO-based SC electrode, as compared with those for the SC electrodes with single component of WS2 or RGO, obtained by using the CV plots at the scan rate of 10 mV s1. The EIS analysis indicates the higher conductivity for the WS2/RGO and RGO-based SC electrodes, as well as the smaller charge-transfer resistance for the WS2/RGO-based SC electrode. A better high-rate capability as well as higher energy and power densities were obtained for the hybrid case. Also, a 98.6% cycling stability retention after 5000 cycles charge/discharge process and the Coulombic efficiency of almost 100% for in the entire measurement were also obtained for the WS2/RGO-based SC electrode. The largely enhanced electrochemical performance for the SC electrode with the WS2/RGO hybrid is mainly due to the synergic effect from both materials with the highly capacitive redox reactions for WS2 nanoladders and the great electron transfer ability for RGO nanosheets.
3.4. The charge storage mechanism for the WS2/RGO electrode
References
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4. Conclusions A highly conductive 2D WS2/RGO paper was simply prepared by
Acknowledgements This work was supported in part by the Ministry of Science and Technology of Taiwan, under grant numbers: MOST 103-2218-E027-010-MY2 and MOST 103-2119-M-027-001-.