Electrochemical Self-Assembly of 3D Interpenetrating Porous Network

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C: Physical Processes in Nanomaterials and Nanostructures

Electrochemical Self-Assembly of 3D Interpenetrating Porous Network PEDOT-PEG-WS Nanocomposite for High-Efficient Energy Storage 2

Aiqin Liang, Yingying Zhang, Fengxing Jiang, Weiqiang Zhou, Jingkun Xu, Jian Hou, Yanli Wu, Yong-Bo Ding, and Xuemin Duan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05227 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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The Journal of Physical Chemistry

Electrochemical Self-Assembly of 3D Interpenetrating Porous Network PEDOT-PEG-WS2 Nanocomposite for High-Efficient Energy Storage Aiqin Liang,†,║ Yingying Zhang,†, ║ Fengxing Jiang,† Weiqiang Zhou,†,* Jingkun Xu,†,‡,* Jian Hou,§ Yanli Wu,† Yongbo Ding,† and Xuemin Duan† †

Jiangxi Engineering Laboratory of Waterborne Coatings, Jiangxi Science and

Technology Normal University, Nanchang 330013, People’s Republic of China ‡

College of Chemistry and Molecular Engineering, Qingdao University of Science and

Technology, Qingdao 266042, China. §

State key laboratory for marine corrosion and protection, Luoyang ship material

research institute, Qingdao, 266101, China

1

ACS Paragon Plus Environment

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ABSTRACT: Like interchange bridges used in traffic, 3D interpenetrating porous network (3D IPN) nano-/micro materials are of great significance in the field of energy storage. Here, we developed a 3D IPN poly(3,4-ethyleenedioxythiophene)poly(ethylene

glycol)-WS2

(PEDOT-PEG-WS2)

nanocomposite

through

the

electrochemical self-assembly of EDOT and WS2 nanosheets with assistance of PEG. The SEM, EDS and XPS results explicitly demonstrated the formation of 3D IPN PEDOT-PEG-WS2 nanocomposite. The electrochemical results indicated that the PEDOT-PEG-WS2 nanocomposite had high specific capacitance of 236.5 mF cm-2 at 5 mV s-1, which was about 3.1 times higher than PEDOT-PEG (77.5 mF cm-2). The symmetric supercapacitors based on PEDOT-PEG-WS2 presented a high specific energy of 78 Wh m-2 at 900 W m-2 and high cycle stability of 91% after 5000 cycles. It was proposed that the enhanced performances were attributed to the unique 3D IPN structures and the synergistic effect between WS2 and PEDOT, which made it promising candidate as high efficient electrode in electrochemical energy storage devices.

2


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1. INTRODUCTION The evolution of energy storage devices such as supercapacitors largely rests with the performance of electrode materials. For the advancement of electrode materials such as carbons, metal oxides and conducting polymers (CPs), many strategies have been proposed including nanostructure, pore-structure, surface modification and composition optimization, etc.. However, the nano-/micro structures like interchange bridges used in traffic rarely appear. It is well-known that such 3D interpenetrating porous network (3D IPN) can make full use of limited space and substantially increase ions transport volume, which can significantly promote the energy storage capacity. Presently, few 3D IPN confined to metal-organic frameworks or coordination polymers are mainly used in hydrogen storage.1,2 Among the electrode materials for supercapacitors, this especial 3D IPN structure was barely reported.3 CPs are class of potential electrochemical energy storage materials because of their superior redox behaviors. Among them, poly(3.4-ethylenedioxythiophene) (PEDOT) is an excellent candidate because of its chemical structure stability, high rate capability, excellent conductivity, fast and stable redox property and long cycling life relative to others CPs such as polyaniline, polypyrrole and polythiophene.4,5 Generally, pristine PEDOT displayed featureless structures and showed lower specific capacitance of about 120 F g-1 or 40 mF cm-2 below.6,7 By the reasonable design of PEDOT nanotubes, nanocapsules and mesocellular foams, a higher specific capacitance of 130-170 F g-1 was achieved.8,9 In recent years, there have been a number of reports on the preparation of the PEDOT-based nanocomposites for the capacitance advancement, e.g. PEDOT/CNT-graphene,10 PEDOT/RGO,11 PEDOT/MnO2/MWNT,12 MoS2/PEDOT7 and WS2/PEDOT:PSS.13 These nanocomposites still showed a lower specific capacitance of about 150 mF cm-2 or 150 F g-1 below. In order to enhance their 3



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capacitance performance, Liu and Lee prepared array MnO2@PEDOT nanowires by electrodeposition in a porous alumina template and its specific capacitance reached to 210 F g-1.14 The enhanced capacitance is mainly ascribed to the increase in the effective specific surface area, the nanostructural passageway benefiting ion transport and the synergistic effect between ingredients. As far as we know, the preparation and capacitance of 3D IPN PEDOT composites has not been reported. Tungsten disulfide (WS2) with a W layer sandwiched between two layers of S atoms has attracted considerable attention because W atoms have a wide range of oxidation states varying from +2 to +6, revealing a typical pseudocapacitance behavior.15 For instance, interwoven WS2 nanosheets supported on carbon fiber cloth showed a high specific capacitance of 399 F g-1 at 1 A g-1, however, with increasing to 15 A g-1, the capacitance retention of WS2 nanosheets was only about 31%.16 The low rate capability possibly stemmed from the low electronic/ionic conductivity between two adjacent S-W-S sheets. Hence, for enhancing the capacitance performance of WS2, a few WS2-based nanocomposites have been also researched, such as WS2/SWCNT,17 WS2/RGO,15

Fe3O4/WS2,18

and

[email protected]

The

fabrication

of

these

nanocomposites mainly adopted the hydrothermal method and physical mixing method. The polyvinylidene fluoride (PVDF) binder will be used when those nanocomposites prepared by the above methods are transferred to collector, which inevitably causes the performance degradation of nanocomposites due to the introduction of the nonconducting PVDF. Electrodeposition has been considered as one of the most ideal approaches for onestep fabrication of the nanocomposite electrodes of CPs and transitional metal oxides/sulfides

avoiding

the

non-conducting

binder

additives.

Using

the

electrodeposition method, Xiao et al. prepared 1.1 nm thickness PEDOT/WS2 coating 4


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on nanoporous gold from EDOT and (NH4)2WS4 solution, which exhibited excellent electro-catalytic activity for the hydrogen evolution.20 In this case, an issue of concern is that a thick composite film would be obtained difficultly in this system of EDOT and (NH4)2WS4 solution. However, an alternative strategy for the fabrication of a thick PEDOT/WS2 film is the electrochemical polymerization of EDOT with the presence of the prefabricated WS2 nanosheets. It is well-known that the negative charge compound in solution can migrate to the surface of working electrode to insert into the growing CPs films during the electrodeposition of CPs.7 Our previous work showed the chemically exfoliated 2D WS2 nanosheet was negative charge material,13 which would be expected to combine with CPs through the electrodeposition. Due to the low conductivity of WS2 nanosheets, additional electrolytes need to be added into the system in the process of electrodeposition. However, in experimenting it will bring about a serious problem that WS2 nanosheets aggregate quickly once the electrolytes are added.21 Therefore, it is a challenge to prepare well-defined nanocomposites based on WS2 nanosheets and CPs by the electrodeposition method. In this work, a PEG surfactant was used as the stabilizing agent of WS2 nanosheets under the coexistation of additional electrolytes. Under the electrochemical driving forces, a 3D IPN PEDOT-PEG-WS2 nanocomposite was fabricated from the mixed solution containing EDOT, WS2 nanosheets, 0.1 M LiClO4 and 5% PEG. As-prepared 3D IPN nanocomposite showed a high areal capacitance of 236.7 mF cm-2 with 91% of cycle stability after 2000 cycles, which was about 3.1 times higher than PEDOT-PEG (77.5 mF cm-2). This work could provide a feasible way to develop high-performance 3D IPN PEDOT-based nanocomposites. 2. EXPERIMENTAL SECTION 2.1 Materials. 5



EDOT and n-butyllithium were obtained from Energy Chemical Co., Ltd (Shanghai, China), WS2 power was from Sigma-Aldrich. Ltd. n-Hexane was supplied by J&K Chemical Reagent Co.,Ltd. Acetonitrile (ACN) was bought from Shanghai Lingfeng Chemical Reagent Co, Ltd. Anhydrous lithium perchlorate (LiClO4) power was supplied by Xiya Reagent Research Center (Shandong, China). Polythylene glycol 400 (PEG) was from Tianjin Kermel chemical development center. 2.2 Fabrication of the 3D IPN WS2/PEDOT Nanocomposite. The 3D IPN PEDOT-PEG-WS2 nanocomposite was prepared by electrochemical method, as shown in Scheme 1. In brief, the synthesis was carried out in a compartment of electrochemical cell which was made up of a glassy carbon electrode (GCE, 3 mm diameter) as the working electrode, platinum wire as the counter electrode and saturated calomel electrode (SCE) as the reference electrode. For the deposition, cyclic voltammetry (CV) was carried out from 0 to 1.2 V at 100 mV s-1 at room temperature. The ACN/water (2/5) solution was consist of EDOT, WS2 and 0.1 M LiClO4 with 5% PEG (v/v) by volume, in which the preparation of few-layers WS2 nanosheets referred to our previous work.13 The thickness of WS2 nanosheet measured by AFM was about 3~4 nm (Figure S1 in the Supporting Information). For comparison, PEDOT-PEG was also fabricated by same method without WS2 nanosheets.

Scheme 1. Schematic diagrams of the electrochemical deposition of 3D IPN PEDOT6


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PEG-WS2 nanocomposite. 2.3 Characterizations Scanning electron microscopy (SEM, ZEISS SUPRA 40, German) was utilized to measure the morphologies of the samples. The thickness of the WS2 nanosheet was measured by atomic force microscopy (AFM, Asylum Research, MFP-3D). Nitrogen adsorption-desorption measurements were carried out on a physical adsorber (tristar II Plus 2.02, Micromeritics instrument corp) using physical adsorption/desorption of N2 at 77 K. X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Al Ka, Thermo Fisher Scientific) was used to investigate the element composition of the film. The surface morphology and elementary composition of the product was determined by energy dispersive X-ray spectrometry (EDS, X-Max, Oxford). The electrochemical measurements were carried out using CHI660E (Chenhua, Shanghai, China) electrochemical working station with a three-electrode electrochemical cell. The electrochemical impedance spectroscopy (EIS) was performed on the frequency range of 100 kHz to 0.01 Hz at an open-circuit voltage of -0.3 V. 3. RESULTS AND DISCUSSION

7



Figure 1. Photographs of WS2 nanosheets aqueous respectively containing 0.1 M LiClO4, Bu4NBF4, HCl, H2SO4 and KCl in the absence (top) or presence (down) of 5% PEG. Although as-prepared WS2 nanosheets were dispersed well in water, they rapidly aggregated and deposited to the bottom when slats were added (Figure 1, top). In the presence of 5% PEG, it was found that WS2 nanosheets could disperse stably only in 0.1 M LiClO4 solution among different slats or acids as conventional electrolytes (Figure 1, down). This indicated that LiClO4 electrolyte did not affect the dispersion of WS2 nanosheets in 5% PEG solution. In the ACN-water solution (5 ml) containing 0.1 M LiClO4, 1.34 mg ml-1 WS2 nanosheets and 5% PEG, the electrochemical polymerization of EDOT was faster than that in the absence of WS2 nanosheets. This was originated from on one hand the solution conductivity increased due to the conductive WS2 nanosheets in itself, on the other hand the WS2 nanosheets adnexed on 8


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electrode could further increase the surface area used for the electrochemical deposition of PEDOT, which was reflected in the increase of current densities per cycles in the continuous CVs (Figure 2).

Figure 2. CV curves of 0.48 mmol EDOT + 5% PEG (a), 0.48 mmol EDOT + 5% PEG + 1.34 mg ml-1 WS2 nanosheets (b) respectively in ACN-water solution + 0.1 M LiClO4. (c) The plot of anodic current density at 0.6 V from (a) and (b) against scan numbers.

9



Figure 3. SEM images of PEDOT-PEG (a&b) prepared from EDOT + PEG + LiClO4, and PEDOT-WS2 (c&d) prepared from EDOT + WS2 + LiClO4, PEDOT-PEG-WS2 (e&f) prepared from EDOT + WS2 + PEG + LiClO4, via CV for 50 cycles, and the sketch map (g) of the formation mechanism of 3D IPN PEDOT-PEG-WS2 composites. The microstructure of hybrid material is of the great significance to decide the electrochemical property.22,23 The images of PEDOT-PEG prepared from EDOT + PEG 10


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+ LiClO4 displayed a wrinkled and compact structures (Figure 3a&b). Additionally, PEDOT-WS2 composite prepared from EDOT + WS2 + LiClO4 were composed of cluster WS2 particles embedded PEDOT nanofibrous films (Figure 3c&d). The cluster WS2 particles were mainly produced because of the aggregation of WS2 nanosheets in the LiClO4 electrolytes. However, when WS2 was added into the solution of PEG + LiClO4, as demonstrated in Figure 3e&f, the as-prepared PEDOT-PEG-WS2 nanocomposite exhibited a 3D IPN structure. This was possible that on one hand the insertion of WS2 made the compact film change the porous film, on the other hand PEG still retained the wrinkled structures of films. So the integration of two aspects promoted the formation of 3D IPN structure. The formation mechanism of 3D IPN PEDOT-PEG-WS2 nanocomposite could be proposed according to the sketch map (Figure 3g). Namely, the PEG firstly were coated on the surface of WS2, because of the chelating-coordinating effect between the lone pair electrons of oxygen of PEG and the unoccupied 5d orbital of tungsten,24 and then EDOT was bonded to PEG through the hydrogen bond between the oxygen atoms of EDOT monomers and terminal hydroxy groups of PEG moleculars. Finally, EDOT-PEG-WS2 were electrochemically polymerized into PEDOT-PEG-WS2, forming the 3D IPN structure. Moreover, the electronic conductivity of the samples measured by four-point probe were about 197 S cm-1 for PEDOT-PEG-WS2, 246 S cm-1 for PEDOT-PEG and 4 S cm-1 for WS2.

11



Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b) of PEDOT-PEG-WS2 and PEDOT-PEG nanocomposites. The specific surface area and pore distribution of PEDOT-PEG-WS2 and PEDOTPEG nanocomposites were analyzed by N2 adsorption/desorption isothermal curves and calculated according to the Brunauer-Emmett-Teller (BET) method. As shown in Figure 4a, a slow increase in nitrogen uptake at low and intermediate relative pressures suggested negligible micropores and the sharp increasing adsorption at high relative pressure region indicated the presence of mesopores or macropores.25,26 The corresponding BET specific surface areas of PEDOT-PEG-WS2 and PEDOT-PEG nanocomposite were 8.5 m2 g-1 and 5.2 m2 g-1, respectively. In the pore size distribution curves (Figure 4b), the PEDOT-PEG-WS2 nanocomposite showed abundant mesopores and macropores compared with PEDOT-PEG. And the average pore diameter and pore volume of PEDOT-PEG-WS2 nanocomposite were 7.1 nm and 0.014 cm3 g-1, which were larger than those (3.7 nm and 0.004 cm3 g-1) of PEDOT-PEG. These results indicated that the addition of WS2 created a porous structure and an increased BET specific surface area during the process of the electrochemical polymerization.

12


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Figure 5. EDS (a) of 3D IPN PEDOT-PEG-WS2 nanocomposite, C (b), O (c), S (d) and W (e) elements mappings in 3D IPN PEDOT-PEG-WS2 nanocomposite. The elements of the 3D IPN PEDOT-PEG-WS2 nanocomposite were confirmed by EDS and element mappings (Figure 5). The C, O, S and W elements were detected by EDS (Figure 5a). The detected W element showed the existence of WS2 in the PEDOT-PEG-WS2 nanocomposite. The EDS mappings displayed the uniform distribution of C, O, S and W elements in the 3D IPN PEDOT-PEG-WS2 nanocomposite (Figure 5b-e). 13



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Figure 6. XPS survey spectra (a), S 2p (b), O 1s (c) and W 4f (d) of 3D IPN PEDOTPEG-WS2 nanocomposite. To further verify the chemical composition and bonding sate of the fabricated 3D IPN PEDOT-PEG-WS2 nanocomposite, the XPS was carried out. In Figure 6a, the C 1s, S 2p, O 1s and W 4f were explicitly detected, which proved well the acquisition of 3D IPN PEDOT-PEG-WS2 nanocomposite. According to the integral ratio of peaks in XPS, the W content was calculated to about 8.1 wt%, so the WS2 in the PEDOT-PEGWS2 nanocomposite was about 10.9 wt%. After deducting the content of S element in WS2, the surplus S content belonging to PEDOT was about 19.5 wt%. According to literature, the typical molecular weight of PEDOT is in the range of 1000 to 2500 Da (6–18 repeating units) and the S content in PEDOT is about 22.8 wt %,

27,28

thus the

PEDOT in PEDOT-PEG-WS2 nanocomposite was calculated to about 85.5%. Based on 14


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the above results, it was inferred that the ratio of PEDOT:PEG:WS2 was about 85.5:3.6:10.9. Furthermore, in Figure 6b, the peaks located at 163.8 eV, 164.7 eV and 166.5 eV were attributed at S 2p1/2, C-S 2p3/2 and C-S 2p1/2.24 The characteristic peaks of C 1s located at 284.6 eV and 285.9 eV were assigned to C-C/C-H and C=O (Figure S2 in the Supporting Information).29 From the Figure 6c, the core-level peaks of O 1s were exhibited at 532.7 eV and 533.7 eV, which were assigned to the oxygen ether group C-O-C and C=C-O.30 In Figure 6d, there were two characteristic peaks centered at 35.2 eV and 37.4 eV, which were attributed to W 4f5/2 of the W4+ oxidation state and W 5p3/2 of W6+.15, 24, 29 The presence of W6+ indicated that the W4+ could be partially oxidized during the electrochemical cycle. In comparison with W6+, the stronger signal of W4+ indicated that the W4+ was dominant in 3D IPN PEDOT-PEG-WS2 nanocomposite.

15



Figure 7. CV curves at 5 mV s-1 (a). GCD curves at 1mA cm-2 (b). Specific capacitance of electrodes as a function of scan rates (c) and as a function of current densities (d). Nyquist plot (e) and cycle stability (f) of PEDOT-PEG and PEDOT-PEG-WS2 nanocomposite. The preparation of PEDOT-PEG-WS2 nanocomposite was optimized by adjusting the WS2 contents and CV cycles. According to their CV area and specific capacitance of every sample, it was obviously found that as-prepared PEDOT-PEG-WS2 16


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nanocomposite could be obtained the maximum specific capacitance as the addition of 2.7% WS2 in solution and electrodeposition for 50 cycles (Figure S3&4 in the Supporting Information). Next, we studied the electrochemical properties of the optimized 3D IPN PEDOT-PEG-WS2 nanocomposite and compared with PEDOT-PEG. The electrochemical performances of the optimized 3D IPN PEDOT-PEG-WS2 nanocomposite were tested in 1.0 M H2SO4 solution by CV, GCD and EIS, and compared with PEDOT-PEG. In Figure 7a, the CV curve area of the PEDOT-PEG-WS2 nanocomposite was larger than that of PEDOT-PEG, and the PEDOT-PEG-WS2 nanocomposite appeared a pair of redox peaks originating from the reaction of W active atoms from WS2 layers with H+ ions (WS2 + H+ + e- = WS-SH).31 Additionally, the discharge time of PEDOT-PEG-WS2 nanocomposite was also longer in comparison with PEDOT-PEG (Figure 7b), indicative of the high energy storage capability of PEDOT-PEG-WS2 nanocomposite. The areal capacitances of PEDOT-PEG-WS2 nanocomposite and PEDOT-PEG were calculated from their corresponding CVs and GCD curves at different scan rates and current densities (Figure S5 in the Supporting Information) using eqs (S1, S2) shown in Supporting Information. At 5 mV s-1, the 3D IPN PEDOT-PEG-WS2 nanocomposite showed a specific capacitance of 236.5 mF cm-2 (Figure 7c), which was about 3.1 times higher than PEDOT-PEG (77.5 mF cm-2) and about 4.1 times as high as the average value of PEDOT and WS2 (Figure S6 in the Supporting Information), namely, (77.5 + 38)/2 ≈ 57.8 mF cm-2, which indicated the synergistic effect of both components. At the same time, with increasing current density from 1 mA cm-2 to 15 mA cm-2, the specific capacitances of PEDOT-PEG and 3D IPN 17



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PEDOT-PEG-WS2 nanocomposite were declined from 72.1 mF cm-2 to 56.3 mF cm-2 with about 21.9% losing ratio and 236.9 mF cm-2 to 176.2 mF cm-2 with about 25.6% losing rate (Figure 7d), respectively. Note that the retention rate of 3D IPN PEDOTPEG-WS2 nanocomposite was close to that of PEDOT-PEG, indicating composites had an enhanced specific capacitance hardly deteriorating their high rate capability. The enhanced specific capacitance of PEDOT-PEG-WS2 nanocomposite was mainly from the unique 3D IPN structures with large specific surface areas and facilitating ion transport tunnel as well as the synergistic effect between WS2 and PEDOT. Furthermore, the capacitive behaviors of PEDOT-PEG-WS2 nanocomposite were compared with other reported electrode materials as listed in Table 1. These results revealed that 3D IPN PEDOT-PEG-WS2 nanocomposite owned a preferable capacitance performance. Table 1. Comparison of Capacitive Properties of PEDOT-PEG-WS2 Nanocomposite with Other Electrode Materials Areal Current density Material

electrolyte

or scan rate

capacitance mF

cm-2

cm-2

Capacitance Retention

Ref.

79.5 – 61.3

77%

13

WS2/PEDOT:PSS film

1 M H2SO4

1 – 4.5 mA

ULGO/WS2

1 M H2SO4

0.25 – 0.5 mA cm-2

80.5 - 51.5

64.3%

22

h-WO3/WS2

0.1 M Na2SO4

5 - 10 mV s-1

47.5 - 33

69.5%

32

PEDOT(BF4-)/MoS2

1.0 M KCl

1 - 15 mA cm-2

150 - 100

67%

7

ITO/(PDDA/MoS2)20

1.0 KCl

0.04 - 0.1 mA cm-2

1.1 - 0.8

73%

33

MoS2/CNT

1 M H2SO4

20 - 100 mV s-1

37.7 – 27.6

73.2%

34

GO/PEDOT-CNTs

1.0 M KCl

0.5 – 10 mA cm-2

100.3 - 75

74.8%

35

WS2NPs@PANI

0.5 M H2SO4

0.014 - 0.113 mA cm-2

0.69 - 0.2

29.9%

36

PEDOT-PEG-WS2

1 M H2SO4

1 - 15 mA cm-2

236.9 – 176.2

74.4%

This work

ULGO: ultralarge graphene oxide. CNT: carbon nanotube. NPs: nanoparticles. GO: graphene oxide

The conductivity and ionic mobility of electrode materials could reflect the rate capacity. The Nyqusit plots of PEDOT-PEG and 3D IPN PEDOT-PEG-WS2 18


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nanocomposite were tested at -0.3 V as shown in Figure 7e. The intersection of the curve with the real axis was about 21 ohm for PEDOT-PEG-WS2 nanocomposite and 37 ohm for PEDOT-PEG at high frequency owing to the resistances of both the electrolyte and the electrode (Rs), and the charge transfer resistance (Rct, the diameter of the semicircle) at the electrode material/electrolyte interface was 3 ohm for PEDOTPEG-WS2 nanocomposite and 8 ohm for PEDOT-PEG. In the low frequency region, the straight 45° sloped line corresponded to the Warburg diffusion process (W), reflecting the ion diffusion into the electrode materials.37 The ions (H+ or SO42- ) diffusion coefficient (D) was calculated using the following eqs (1,2):38,39 Z   Rs  Rct   0.5

(1)

D  R 2T 2 / (2 S 2 F 4  2C 2）

(2)

Where Z′ is the real part of the impedance, ω is the angular frequency (ω = 2πf), R is the gas constant (8.314 J K-1 mol-1), T is the absolute temperature (298.15 K), S is the surface area of GCE (0.071 cm2). F is the Faraday constant (96485 A s mol-1), C is the molar concentration of ions in the active materials (0.007 mol cm-3),40 and  is the Warburg factor, which is related to the Z′–ω-0.5 dependence obtained from the slopes of the lines (Figure S7 in the Supporting Information) and calculated based on eq 1. The D value calculated by eq 2 for PEDOT-PEG-WS2 nanocomposite was more than about 4.0 times that of PEDOT-PEG according to the ratio of D in the two electrodes, implying the nanopores of PEDOT-PEG-WS2 nanocomposite could boost the ions diffusion coefficient.

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Electrochemical stability of 3D IPN PEDOT-PEG-WS2 nanocomposite was also evaluated by successive GCD at 15 mA cm-2. In Figure 7f, at the first 1000 cycles, the specific capacitance of the PEDOT-PEG-WS2 nanocomposite was slightly decreased to 155 mF cm-2 from 175 mF cm-2, which was attributed to the instability of the outermost WS2 nanosheets. After that, the specific capacitance was increased owing to activation of the PEDOT-PEG-WS2 nanocomposite. Finally, the 3D IPN PEDOT-PEG-WS2 composites retention was 91.5% of specific capacitance after 2000 cycles, which was slightly higher than that of PEDOT-PEG (89%). This implied that 3D IPN PEDOTPEG-WS2 nanocomposite had good cycle stability.

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Figure 8. CV (a), GCD (b), specific capacitances as a function of the scan rate (c), Ragone plots (d), Nyquist plot (e) and cycle stability (f) of supercapacitors based on PEDOT-PEG-WS2 nanocomposite electrodes. In order to further assess the capacitive performance of nanocomposite, a supercapacitor based on two identical PEDOT-PEG-WS2 electrodes was tested in 1.0 M H2SO4. The CV curves of the device were nearly rectangular in Figure 8a and the GCD curves at different current densities presented approximately triangular shapes in 21



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Figure 8b, indicating the supercapacitor had good capacitance performance. The areal specific capacitance of supercapacitor was 159.4 mF cm-2 at 10 mV s-1, which was calculated using eq (S3) in the Supporting Information. When the scan rates increased to 200 mV s-1, the specific capacitances of supercapacitor was 138.3 mF cm-2 with about 87% capacitance retention (Figure 8c). In addition, the supercapacitor delivered a high specific energy of 78 Wh m-2 at 900 W m-2 (Figure 8d) according to the eqs (S5, S6) in the Supporting Information. For Nyquist plots (Figure 8e), the Rs and Rct values were about 42 ohm and 3 ohm, respectively. In Figure 8f, the initial specific capacitance of device still remained 91% after 5000 cycles at 10 mA cm-2, indicative of good cycling stability. 4. CONCLUSION In summary, a novel 3D IPN PEDOT-PEG-WS2 nanocomposite has been fabricated by simple electrodeposition from ACN/water solution containing EDOT + WS2 + 5% PEG + 0.1 M LiClO4. The results of EDS and XPS proved the formation of PEDOT-PEG-WS2

nanocomposite.

As-obtained

3D

IPN

PEDOT-PEG-WS2

nanocomposite exhibited a high specific capacitance of 236.5 mF cm-2 at 5 mV s-1, higher than the 77.5 mF cm-2 of PEDOT-PEG. The enhanced performances were attributed to the unique 3D IPN structures and the synergistic effect between WS2 and PEDOT. Additionally, the assembled device using 3D IPN PEDOT-PEG-WS2 nanocomposite showed an areal specific capacitance of 159.4 mF cm-2 at 10 mV s-1, cycling stability of 91% after 5000 cycles and specific energy of 78 Wh m-2 at 900 W m-2. These results meant that 3D IPN PEDOT-PEG-WS2 nanocomposite would be 22


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prospective in highly efficient energy storage areas. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Calculation eqs about capacitance performances. Thickness of WS2 nanosheet. XPS spectra of C 1s of 3D IPN PEDOT-PEG-WS2 nanocomposite. Electrochemical behaviors of PEDOT-PEG, PEDOT-PEG-WS2 composites and WS2 by dipping on the GCE electrode. The Z′–ω-0.5 dependence for PEDOT-PEG and PEDOT-PEG-WS2 nanocomposite (PDF) AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. ORCID Weiqiang Zhou: 0000-0001-9649-468X Fengxing Jiang: 0000-0001-8907-9445 Jingkun Xu: 0000-0003-3492-5450 Author Contributions ║Aiqin

Liang and Yingying Zhang contributed equally to this work.

Notes The authors declare no competing financial interest. 23



ACKNOWLEDGMENTS This work was supported by The National Natural Science Foundation of China (Grant Nos. 51662012, 51862011, 51863009, 51762020), the Natural Science Foundation of Jiangxi Province (20171BAB206013), Jiangxi Outstanding Young Talent Fund Projects (20171BCB23076), Innovation Driven "5511" Project of Jiangxi Province (20165BCB18016), Scientific Research Projects of Jiangxi Science and Technology Normal University (2016QNBJRC001). Jiangxi Provincial Department of Education (GJJ160762). REFERENCE (1) Kesanli, B.; Cui, Y.; Smith, M. R.; Bittner, E. W.; Bockrath, B. C.; Lin, W. Highly Interpenetrated Metal-Organic Frameworks for Hydrogen Storage. Angew. Chem. Int. Edit. 2005, 44 (1), 72-75. (2) Du, M.; Jiang, X. J.; Zhao, X. J. Direction of Unusual Mixed-ligand Metal-Organic Frameworks: a New Type of 3-D Polythreading Involving 1-D and 2-D Structural Motifs and a 2-Fold Interpenetrating Porous Network. Chem. Commun. 2005, (44), 5521. (3) Chen, Z.; Qin, Y.; Weng, D.; Xiao, Q.; Peng, Y.; Wang, X.; Li, H.; Wei, F.; Lu, Y. Design and Synthesis of Hierarchical Nanowire Composites for Electrochemical Energy Storage. Adv. Funct. Mater. 2009, 19 (21), 3420-3426. (4) Ge, Y.; Jalili, R.; Wang, C.; Zheng, T.; Chao, Y.; Wallace, G. G. A Robust FreeStanding MoS2/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Film for Supercapacitor Applications. Electrochim. Acta 2017, 235, 348-355. 24


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TOC Graphic

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Electrochemical Self-Assembly of 3D Interpenetrating Porous Network

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