tungsten oxide nanohybrids for high performance supercapacitor with excellent rate capability

Accepted Manuscript Full Length Article Facile Synthesis of Reduced Graphene Oxide/Tungsten Disulfide/Tungsten Oxide Nanohybrids for High Performance Supercapacitor with Excellent Rate Capability Zhengchun Yang, Honghao Zhang, Bo Ma, Liqiang Xie, Yantao Chen, Zhihao Yuan, Kailiang Zhang, Jun Wei PII: DOI: Reference:

S0169-4332(18)32334-1 https://doi.org/10.1016/j.apsusc.2018.08.185 APSUSC 40222

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

5 June 2018 31 July 2018 21 August 2018

Please cite this article as: Z. Yang, H. Zhang, B. Ma, L. Xie, Y. Chen, Z. Yuan, K. Zhang, J. Wei, Facile Synthesis of Reduced Graphene Oxide/Tungsten Disulfide/Tungsten Oxide Nanohybrids for High Performance Supercapacitor with Excellent Rate Capability, Applied Surface Science (2018), doi: https://doi.org/10.1016/ j.apsusc.2018.08.185

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Facile Synthesis of Reduced Graphene Oxide/Tungsten Disulfide/Tungsten Oxide

Nanohybrids for High Performance Supercapacitor with Excellent Rate Capability Zhengchun Yanga,+, Honghao Zhanga,+, Bo Mab, Liqiang Xiea, Yantao Chenb,*, Zhihao Yuanb,*, Kailiang Zhanga and Jun Weia,c a

School of Electrical and Electronic Engineering, Tianjin Key Laboratory of Film Electronic &

Communication Devices, Tianjin University of Technology, Tianjin 300384, China b

School of Materials Science and Engineering, Tianjin Key Lab for Photoelectric Materials &

Devices, Tianjin University of Technology, Tianjin 300384, China c

Singapore Institute of Manufacturing Technology, Agency for Science, Technology and Research

(A*STAR),71 Nanyang Drive, 638075, Singapore

*Corresponding authors:

[email protected] (Y. Chen)

[email protected] (Z. Yuan)

+These authors contributed equally to this work.

Abstract

A facile synthesis strategy is developed for synthesizing nanohybrids of reduced graphene

oxide/tungsten disulfide/tungsten oxide (G/TS/TO). The as-prepared nanohybrids can effectively

combine the electrical double-layer capacitance and pseudo-capacitance when used as the

working electrode of supercapacitor. The specific capacitance value of the G/TS/TO-based working electrode is 148.5 mF cm-2 at current density of 0.1 mA cm-2 in the three-electrode setup.

Owing to the high electrical conductivity of tungsten oxide, large electrode/electrolyte interfaces

of tungsten disulfide and facile electron transfer as well as high surface area of reduced graphene

oxide, the G/TS/TO performs an excellent rate capability. A 90% capacitance retention after 3000

cycles of charge/discharge process is also obtained for the symmetrical all-solid-state

supercapacitor device fabricated by G/TS/TO. This study provides a new research strategy for

electrode materials based on tungsten-containing semiconductors in high rate capability

supercapacitors.

Keywords: tungsten disulfide; tungsten oxide; reduced graphene oxide; supercapacitor; rate

capability

1. Introduction

For the past two decades, supercapacitors have received considerable attention, owing to their

high power density, extremely long cycling life and fast charging/discharging rates, which are

desired by a wide spectrum of applications in load-leveling power sources, premium power

systems, fast switches, consumer electronics, battery-power operations and military devices [1-5].

Supercapacitors are broadly grouped into two types, depending on the charge storage

mechanisms. One mechanism is the electrical double-layered capacitor (EDLC),which is

common for materials with high specific surface area, such as mesoporous carbon [6, 7], carbon

nanotubes (CNTs) [8, 9], graphene [10-12] and so on. The other mechanism is the

pseudocapacitor, making use of the surface redox reaction of certain transitional metal oxides or

sulfides, such as ruthenium oxide (RuO2) [13-15], manganese dioxide (MnO2) [16-18], tungsten

disulfide (WS2) [19, 20], tungsten oxide [21, 22], and so on. In recent years, a large number of

studies have been conducted on the development of both types of supercapacitors. Although both

of the theoretical understanding and experimental fabrication have been made, the major

challenges are the poor overall performance, especially the poor rate capability and the high cost

of electrode materials, which have to be tackled in order to expand the applications of

supercapacitors [1, 3, 23, 24]. As a commonly used electrode material for EDLC, graphene has

attracted significant attention due to the high electrical conductivity and high theoretical specific surface area (2675 m2 g-1) [10, 25-27]. However, the generally reported specific capacitance of

graphene is lower than the theoretical value, which is attributed to the restacking of the graphene

sheets [28]. Thus, in order to improve the electrochemical performance of supercapacitors, the

graphene-based hybrids have been developed [5, 29-31]. Although the combination of EDLC and

pseudocapacitors can lead to improved performance in principle, the overall capacitance,

including the capacitance values and rate capability, reported so far for the graphene/transition

metal oxide hybrids is below the theoretical prediction. It would therefore be of great interest to

further investigate the novel hybrid structure consisting of graphene and transition metal oxide,

by developing new facile synthesis method and understanding the performance of

supercapacitors [32]. Since tungsten oxide is a highly electrical conductive n-type semiconductor

with different crystal structures for intercalation of ions [33, 34], the utilization of tungsten

oxide as the electrode material for supercapacitor draws research attention in recent years [33,

35-37]. In addition, as one of the most attractive two dimensional (2D) materials for the

electrochemical application, the 2D structure of WS 2 provides the abundant active sites and large

electrode/electrolyte interface for charge accumulation [38]. However, the poor electrical

conductivity, the restacking between the nanosheets of WS2 limits its applications in

supercapacitors [38].

Hence, a facile synthesis route to prepare the reduced graphene oxide/tungsten disulfide/tungsten

oxide nanohybrids as well as the supercapacitor properties are presented in this paper. The

combination of reduced graphene oxide (rGO) and WS2 is expected to prevent the restacking of

both graphene and WS2 for providing large electrode/electrolyte interfaces and minimizing the

volume variation during the electrochemical charge/discharge process, which will give rise to an

excellent rate capability [39, 40]. Moreover, the rGO and tungsten oxide would give rise to

improve the conductivity of the electrode, leading to an enhanced performance in supercapacitor

devices.

2. Experimental

2.1. Synthesis of rGO/tungsten disulfide/tungsten oxide (G/TS/TO) nanohybrids and tungsten

disulfide/tungsten oxide (TS/TO) control group

Graphene oxide (GO, 20 mg, Tianjin Plannano Energy Technologies Co. Ltd.) powder was

dispersed in 20 ml deionized (DI) water with the assistance of ultrasonication for 30 minutes to form a homogeneous suspension (1 mg ml-1). Ammonium tetrathiotungstate ((NH4)2WS4, 200

mg, Alfa Aesar) was well mixed with GO water suspension. The suspension was vigorously stirred for 30 minutes and then transferred into a vacuum oven set at 80 oC to evaporate the

solvent. The dried powder was placed in a tube furnace under Argon (Ar) atmosphere for thermal annealing. The furnace was heated with the rate of 10 oC⋅min-1 to 800 oC and maintained at 800 o

C for 1 hour. The furnace was then allowed to cool down to room temperature naturally and the

obtained powder is G/TS/TO nanohybrids. In order to demonstrate the role of rGO in the

nanohybrids, TS/TO is fabricated with the same procedure as before without GO.

2.2. Material Characterization

The scanning electron microscopy (SEM) was performed on an FEI Verios 460L with an

accelerating voltage of 2kV. The transmission electron microscopy (TEM), high-resolution

transmission electron microscopy (HRTEM), high angel annual dark field (HAADF) scanning

transmission electron microscopy (STEM) and elemental mapping were performed on an FEI

Talos F200X at 200kV. The Raman spectrum was conducted on a Horiba Evolution with 532 nm

laser source. The X-ray diffraction (XRD) patterns were recorded via a Rigaku SmartLab (9 kW)

with Cu Kβ radiation. The X-ray photoelectron spectroscopy (XPS) was conducted on a

ThermoFisher Scientific Escalab 250Xi. The nitrogen adsorption-desorption isotherms were

measured on a Quantachrome iQ-MP gas adsorption analyzer at 77K. The specific surface area

was calculated by Brunauer-Emmett-Teller (BET) method, and the pore size distribution was

calculated by applying density functional theory (DFT) on the adsorption and desorption data.

2.3. Electrochemical Measurement

Electrochemical performance of the as-prepared TS/TO and G/TS/TO as working electrodes in

supercapacitor was investigated. The working electrode was fabricated by coating the viscous

slurry of active materials (80 wt. %), carbon black (10 wt. %), and polyvinylidene fluoride (PVDF) (10 wt. %) in n-methyl pyrrolidone (NMP) onto a thin carbon paper with 1.51.1 cm2 area. After drying in a vacuum oven at 60 oC for 2 h, the as-prepared working electrodes were

tested for a series of electrochemical parameters, including the cyclic voltammetry (CV) and

galvanostatic charge/discharge, using both three-electrode and two-electrode setup on an

AMETEK VersaSTAT 3 Electrochemical System within the operation voltage of -1-0 V. In the

three-electrode system, potassium hydroxide (KOH) solution with 6.0 M concentration, Pt foil

and saturated calomel electrode (SCE) electrode were used as the electrolyte, counter electrode

and reference electrode, respectively. Moreover, the electrochemical impedance spectroscopy

(EIS) was conducted on an AMETEK VersaSTAT 3 Electrochemical System by sweeping the

frequency from 0.01 Hz to 100 kHz with the potential amplitude of 5 mV in the three-electrode

setup. In the two-electrode system, the two symmetric working electrodes were also tested by

using 6.0 M KOH solution as the electrolyte.

Furthermore, by using the same procedure as fabricating the working electrode, the symmetric

supercapacitor device was fabricated by using two working electrodes with G/TS/TO

nanohybrids as the active material. The gel electrolyte used within the symmetric supercapacitor

device was made from polyvinyl alcohol (PVA) and KOH. Typically, 4 g of PVA was dissolved into 50 mL DI water. After stirring under 95 oC until the solution became clear, the clear solution

was cooled down for further use. Moreover, 4 g KOH was dissolved into 50 mL DI water and

added into the as-prepared PVA solution under stirring. Finally, the syrupy gel electrolyte was

formed and pasted between the two working electrodes to complete the fabrication of symmetric

supercapacitor device. A series of electrochemical measurements of the final symmetric

supercapacitor device were performed, including the CV and galvanostatic charge/discharge by

using an AMETEK VersaSTAT 3 Electrochemical System within the operation voltage of -1-0 V,

while the cycling performance is evaluated by LAND CT2001A workstation.

3. Results and discussion

The fabrication process of G/TS/SO nanohybrids is displayed in Fig. 1. GO suspension (1 mg⋅ml-1) was obtained by dispersing 20 mg GO powder in DI water with the assistance of

ultrasonication. (NH4)2WS4 was added in the suspension with the concentration of 10 mg⋅ml-1.

The mixture was vigorously stirred and then placed in an oven with vacuum to remove the

solvent. The vacuum drying can facilitate the drying process by rapidly evaporating the solvents under negative pressure. The dried powder was placed in a heated tube furnace (800 oC) under

flowing Ar. The SEM is conducted to investigate the morphology of the nanohybrids, as shown

in Fig. 2a. Nanoparticles with size around 10~50 nm are distributed across the rGO sheets. TEM

was conducted to investigate the structure of the nanohybrids. In Fig. 2b, the nanoparticles were

decorated on the rGO sheets with very high density. Furthermore, HRTEM is performed on

G/TS/TO to reveal the structure of the nanoparticles on the rGO sheet (Fig. 2c). The

nanoparticles have lattice spacings of 0.38 nm and 0.27 nm, which are in good agreement with

(010) plane of WO2.72 and (100) plane of WS2, respectively [41, 42]. The STEM (Fig. 2d) and

elemental mapping (Fig. 2e) were performed on G/TS/TO to further reveal the structure and

elemental distribution of carbon (C), oxygen (O), sulfur (S) and tungsten (W). For comparison,

(NH4)2WS4 was dispersed in DI water alone, followed by vacuum drying and thermal annealing

under the same condition as G/TS/TO to fabricate TS/TO. As shown in Fig. S1 (Supplementary

Materials), TS/TO has a larger particle size than G/TS/TO, and there are no clear boundaries

between the particles in TS/TO, suggesting a sintering-like structure. The STEM and elemental

mapping of TS/TO were presented in Fig. S2. The STEM image further confirms that TS/TO has

a larger dimension than G/TS/TO.

The crystal structure and phase of G/TS/TO were investigated by XRD. The sharp peaks (Fig. 3a) at 23.6o and 48.2o, can be assigned to (010) and (020) planes of WO2.72 [43]. For TS/TO, the

diffraction peaks become sharper, suggesting TS/TO has a better crystallinity than G/TS/TO and

the presence of rGO may affect the crystallinity of G/TS/TO. The Raman spectrum (Fig. 3b) of G/TS/TO presents 7 peaks. The peak at 257 cm-1 corresponds to the O-W-O bending modes. The peaks at 704 cm-1 and 805 cm-1 can be assigned to the W-O stretching modes [44]. The peaks at 352 cm-1 and 417 cm-1 can be assigned to E2g and A1g vibration modes of WS2, respectively [45, 46]. The peaks at 1341 cm-1 and 1587 cm-1 correspond to the disorder-induced D band and

in-plane vibrational G band of carbon materials. The intensity ratio of D peak and G peak (I D/IG)

is determined as 1.22, suggesting the abundant defects in G/TS/TO. For comparison, the Raman

spectrum of TS/TO (Fig. S3) does not present the peaks of D and G bands, indicating the absence

of rGO. To compare the specific surface area of G/TS/TO and TS/TO, nitrogen

adsorption-desorption isotherms curves were displayed in Fig. 3c and identified as type IV. The values of the specific surface area for G/TS/TO and TS/TO are 71.988 m2g-1 and 4.39 m2g-1,

respectively, indicating the presence of rGO can effectively increase the surface area of G/TS/TO.

The DFT analysis (Fig. S4) suggests that G/TS/TO has a narrow distribution of mesopore size

(3-6 nm).

XPS analysis was carried out to characterize the elemental composition for better understanding

the structure of G/TS/TO. As shown in Fig. S5, four peaks corresponding to the binding energies

of W4f, S2p, C1s and O1s are labeled on the full survey XPS spectra of G/TS/TO. The XPS

spectra of W4f, S2p, O1s, together with the deconvolution results are presented in Fig. 4a-c. The W4+ (5p3/2 = 39.5 eV, 4f5/2 = 35 eV, 4f7/2 = 32.9 eV) peaks and S2- (2p1/2 = 163.7 eV, 2p3/2 = 162.5

eV) peaks demonstrate the presence of WS2 [47, 48]. The peaks located at 38 eV, 36.1 eV, and 41.5 eV correspond to the W6+ oxidation state [48]. The W5+ (4f7/2 = 34.1 eV, 4f5/2 = 36.4 eV)

peaks confirm the oxygen-deficient characteristics, indicating the formation of WO 2.72 [49, 50]. The peak at 530.6 eV corresponds to the O2- bonded to W, and the peak at 532.2 eV can be

assigned to the C-O bond or H-O bond [51].

The electrochemical performances of G/TS/TO and TS/TO as working electrodes in

supercapacitor are studied by using a three-electrode configuration. As shown in Fig. 5a, the CV

curves of TS/TO exhibit typical pseudocapacitive behavior in the potential range of -1-0 V at scan rates from 1 to 100 mVs-1, which demonstrate the electrochemical reactions in the as-tested

potential intervals between TS/TO and KOH electrolyte [28, 40]. In addition, due to the O

containing in the interface between WS2 and WO2.72, the charges could be stored in the pseudocapacitor via the redox reactions between W6+ and W4+ in the interface of WS2 and

WO2.72, as demonstrated in the redox reaction peaks in the CV curves in Fig. 5a [28, 40]. In addition, although the scan rate is increased from 1 to 100 mVs-1, the area of the CV curves increases and there are still diminished redox peaks at the scan rate of 100 mVs-1, which also

demonstrate that the working electrodes exhibit a desired capacitive behavior and excellent

electrochemical reversibility of Faradic reaction. The unobvious redox peak is caused by an

insufficient electrochemical reaction between the active material on the surface of the electrode

and the electrolyte as the scanning rate increases [38]. Moreover, as shown Fig. 5b, working

electrode fabricated by G/TS/TO nanohybrids combines the characteristics of both EDLC and pseudocapacitors in the potential range of -1-0 V at scan rates from 1 to 100 mVs-1. Comparing

the CV curves in Fig. 5b and a, at the lower scan rate, the redox reaction peaks are slightly

shifted to the positive direction, which can be attributed to the ions in the electrolyte diffusing

from the rGO layers into TS/TO interface to participate the redox reactions between WS2 and the

O in the WO2.72 and rGO. In addition, as the scan rate increases, the redox peaks are no longer

obvious, indicating there are more EDLC characteristics presented in the CV curves, which

further demonstrates that at higher scan rate, the ions in the electrolyte insufficiently diffuse from

the rGO layers into TS/TO interface, leading to an impeded redox reaction. The charges can be

accumulated at the rGO/electrolyte interface according to the EDLC storage mechanism at

higher scan rate [52]. In addition, the rGO in the working electrode made from G/TS/TO can

serve as the capacitive material for accumulating charges, as well as enhancing the conductivity

of the nanohybrids to facilitate the electron transfer throughout the nanohybrids, which would be

favorable for a higher specific capacitance [38]. As a result, as shown in the galvanostatic

charge/discharge behavior of the two types of working electrode under different current densities

in Fig. 5c and d, it is obvious that the capacitance of G/TS/TO is higher than that of TS/TO,

which indicates that the intimate contact between rGO and TS/TO can effectively improve the

electrochemical activity due to the enhanced electrical conductivity. According to Fig. 5c-d, and the equation of C = IΔt/(SΔV), where C (mFcm-2), S (cm2), I (mA), Δt (s), ΔV (V) represent the

as-calculated specific capacitance, area of working electrode, the current, time and operation

voltage during discharge process, respectively, the specific capacitance of the as-prepared

working electrodes under different current densities was calculated and shown in Fig. 5e. The specific capacitances of TS/TO are 103.1, 68.6, 51.4 and 46.2 mF cm-2 at 0.1, 0.2, 0.5, 1 mA cm-2, while the specific capacitances of G/TS/TO are 148.5, 120.4, 109.1 and 87.2 mF cm-2 at 0.1, 0.2, 0.5, 1 mA cm-2, respectively. The results indicated that the G/TS/TO nanohybrids has a higher

specific capacitance than TS/TO, which can be attributed to the excellent interfacial contact

between TS/TO and rGO, facile electron transfer and large specific surface area (see Fig. 3c).

These results agree well with the comparison between Fig. 5a and b. As shown in Fig. 5e, with

the increase of the testing current densities, the loss of the specific capacitance is 55 % for

TS/TO, while it is only 41% for G/TS/TO, indicating that the G/TS/TO has a better rate

capability. The rate capability, which is the specific capacitance retention from higher to lower

charging current densities, is an important parameter for the real applications of the

supercapacitors [2]. The G/TS/TO in this work displays an excellent rate capability (Table S1)

comparing with most of other similar hybrid structures, which can be attribute to the synergistic

effect from high electrical conductivity of WO2.72, large electrode/electrolyte interfaces of WS2

and facile electron transfer as well as high surface area of rGO. To study the resistive and

capacitive behavior of TS/TO and G/TS/TO, EIS analysis was performed and shown in Fig. 5f.

There are two distinct sections in the Nyquist plot, which is a semicircle at the high frequency

region and a relatively straight line in the low frequency region [40]. From the insertion part of

Fig. 5f, it can be concluded that the internal resistance of the TS/TO and G/TS/TO electrodes are 0.43 and 0.4 Ω, respectively, indicating that the internal resistance of G/TS/TO is slightly lower

than TS/TO. It can be seen from Fig. S6 that the charge transfer resistance (R 1) of TS/TO and G/TS/TO electrodes are 7.43 and 0.47 Ω, respectively, indicating that the charge transfer is

facilitated during electrochemical reaction for G/TS/TO, leading to a significant improvement in

electrochemical performance [53]. In addition, the almost straight line in the Nyquist plot of

impedance spectroscopy of G/TS/TO also indicated the excellent electrical conductivity of

G/TS/TO, leading to the enhanced specific capacitance in Fig. 5e [54].

To further evaluate the electrochemical performance of the as-prepared working electrodes in the

symmetrical supercapacitor devices, the electrochemical measurements of two-electrode setup of

TS/TO and G/TS/TO were shown in Fig. 6. In Fig. 6a and b, the CV curves of the working

electrodes measured in two-electrode setup exhibit a nearly rectangular shape at various scan

rates, which reveals the fast charge/discharge rate and ideal capacitive behavior [54, 55]. Since

entire voltammetric cycles, redox peaks are absent, which is different from the CV curves in Fig.

5a and b [54, 55]. Moreover, the rGO in the G/TS/TO nanohybrids give rise to the excellent

electrical conductivity, higher accessible surface area and additional EDLC characteristics, which

provides abundant ion adsorption, efficient ion intercalation/de-intercalation as well as facilitated

charge transfer. The comparison between Fig. 6c and d further confirmed the enhanced specific

capacitance of G/TS/TO nanohybrids. As calculated from galvanostatic charge/discharge

measurements in Fig. 6c and d, the specific capacitances of TS/TO in two-electrode setup are 21.3, 16.4, 13.8 and 12 mF cm-2 at 0.1, 0.2, 0.5, 1 mA cm-2, respectively, while the specific

capacitances of G/TS/TO in two-electrode setup are 36.4, 27.8, 23.2 and 20.5 mF cm-2 at 0.1, 0.2, 0.5, 1 mA cm-2, respectively. Thus, in Fig. 6e, the specific capacitance of G/TS/TO in a

symmetric two working electrode system is higher than that of TS/TO at each testing current

densities, which further indicates that the rGO in the nanohybrids would improve the

performance of supercapacitor. In order to understand the performance for energy density, the

ragone plot (Fig. 6f) is calculated based on the CV and galvanostatic charge/discharge results.

With a symmetric two working electrode system, the energy density of TS/TO varies from 0.003 to 0.001 mWhcm-2 with the power density varying from 0.05 to 0.5 mWcm-2. With a symmetric

two working electrode system, the energy density of G/TS/TO varies from 0.005 to 0.002 mWhcm-2 with the power density varying from 0.05 to 0.5 mWcm-2. These data further

demonstrate the enhanced capacitive performance of G/TS/TO nanohybrids within the aqueous

electrolyte.

According to the results in Fig. 6, a simple symmetrical all-solid-state supercapacitor device

based on G/TS/TO nanohybrids was assembled as shown in Fig. 7f. The PVA polymer-gel with

KOH was utilized as both separator and ionic electrolyte in as-fabricated supercapacitor device.

As shown in Fig. 7a, the CV curves of the supercapacitor device exhibit similar shape with that

in Fig. 6b, which reveals that there is no obvious change on the capacitive behavior by replacing

the aqueous electrolyte with gel electrolyte. The galvanostatic charge/discharge curves of the

as-prepared supercapacitor device in Fig. 7b were measured at different current densities,

revealing the fast charge/discharge rate and excellent capacitive behaviour [54, 55]. The areal

capacitances derived from Fig. 7b are shown in Fig. 7c, which are 25.0, 20.5, 14.5 and 7.2 mF cm-2 at 0.1, 0.2, 0.5, 1 mA cm-2, respectively. Comparing with the results in Fig. 6e, the slight

decrease of specific capacitance in Fig. 7c is attributed to the higher resistance of the gel

electrolyte. As shown in Fig. 7d, the energy density of the as-prepared supercapacitor device is 0.004 mWhcm-2 at a power density of 0.05 mWcm-2 while maintaining 0.001 mWhcm-2 at a power density of 0.5 mWcm-2, which is comparable to those previously reported results [54].

The cycling stability of the as-prepared supercapacitor device is evaluated by using the galvanostatic charge/discharge technique conducted at the current density of 0.5 mA cm-2. As

shown in Fig. 7e, the as-prepared supercapacitor device can retain 90% of the specific

capacitance after 3000 cycles of measurement, which demonstrates the excellent cycling stability.

In addition, the supercapacitor device could power a Light Emitting Diode (LED) group with 43

lights for 5 min, which is shown in the light digital photo in Fig. 7f, indicating the excellent rate

capability as the real application of the supercapacitor device

4. Conclusion

A facile synthesis strategy has been developed for preparing nanohybrids of rGO, WS2 and

WO2.72 by mixing, vacuum drying and annealing the mixture of GO and (NH4)2WS4. The

electrochemical performance of the as-prepared nanohybrids in supercapacitor is also

investigated. The overall supercapacitor performance of the working electrode fabricated by

G/TS/TO is better than that of TS/TO. As calculated from the galvanostatic charge/discharge

plots at each current density, the specific capacitance value of the G/TS/TO-based working

electrode is higher than that of the TS/TO-based working electrode. The EIS analysis indicates

the higher conductivity of G/TS/TO. An enhanced high-rate capability was observed for the

G/TS/TO-based working electrode and symmetrical all-solid-state supercapacitor device. Also, a

90% cycling stability retention after 3000 cycles of charge/discharge process were obtained for

the symmetrical all-solid-state supercapacitor device fabricated by G/TS/TO. The enhanced

electrochemical performance of the G/TS/TO-based supercapacitor is mainly due to the

synergistic

effect

originated

from

high

electrical

conductivity

of

WO2.72,

large

electrode/electrolyte interfaces of WS2 and facile electron transfer as well as high surface area of

rGO.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.

51702234 and 51502203), the Tianjin Young Overseas High-level Talent Plans (Grant No.

01001502), the Tianjin Science and Technology Foundation (Grant No. 17ZXZNGX00090) and

Tianjin Development Program for Innovation and Entrepreneurship.

Supplementary Materials

The SEM image, STEM image, elemental mapping and Raman spectrum of TS/TO, pore size

distribution and XPS survey spectra of G/TS/TO, the equivalent circuits for the Nyquist plots,

and table for comparison of rate capability of rGO/tungsten-based semiconductor nanohybrids

are included in the Supplementary Materials.

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Fig. 1. Schematic illustration of fabricating the G/TS/TO nanohybrids. GO and (NH 4)2WS4 are well mixed first and then dried under vacuum at 80 oC. The powder is annealed at 800 oC for 1

hour under Ar atmosphere.

Fig. 2 (a-d) SEM, TEM, HRTEM and STEM images of the as-prepared G/TS/TO nanohybrids.

(e) Elemental mapping of C, O, S and W in (d).

Fig. 3. (a) XRD patterns of G/TS/TO and TS/SO. (b) Raman spectrum of G/TS/TO. (c) Nitrogen

adsorption-desorption isotherms of G/TS/TO and TS/TO.

Fig. 4. (a) W4f and W5p, (b) S2p and (c) O1s XPS spectra obtained from the G/TS/TO sample.

Fig. 5. Electrochemical performance of the working electrodes in the three-electrode setup. CV

curves of the supercapacitor working electrode made of TS/TO (a) and G/TS/TO (b) under

different scan rates. Galvanostatic charge/discharge of the supercapacitor working electrodes

made of TS/TO (c) and G/TS/TO (d) under different current densities. Variation in specific

capacitance against different current densities (e) and EIS spectra (f) of the two types of

supercapacitor working electrodes.

Fig. 6. Electrochemical performance of the working electrodes in the two-electrode setup. CV

curves of the supercapacitor working electrode made of TS/TO (a) and G/TS/TO (b) under

different scan rates. Galvanostatic charge/discharge of the supercapacitor working electrodes

made of TS/TO (c) and G/TS/TO (d) under different current densities. Variation in specific

capacitance against different current densities (e) and the ragone plot (d) of the two types of

supercapacitor working electrodes.

Fig. 7. Electrochemical performance of the symmetric supercapacitor device fabricated by

G/TS/TO. (a) CV curves of the as-prepared symmetric supercapacitor device under different scan

rates. (b) Galvanostatic charge/discharge of the as-prepared symmetric supercapacitor device

under different current densities. Variation in specific capacitance against different current densities (c), the ragone plot (d), cycling performance at current density of 0.5 mA cm-2 (e) and

the digital photo of the charged LED light (f) by the as-prepared symmetric supercapacitor

device.

Graphical abstract

Highlights 

Reduced graphene oxide/tungsten disulfide/tungsten oxide composite was synthesized



The supercapacitor based on the hybrids with high performance was fabricated



The synergistic effect from the hybrids generated excellent rate capability