MXene composite as high performance electrode for supercapacitors

MXene composite as high performance electrode for supercapacitors

Journal Pre-proof WO3 Nanorods/MXene composite as high performance electrode for supercapacitors Chang Peng, Zeyuan Kuai, Tianqin Zeng, Yong Yu, Zefan...

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Journal Pre-proof WO3 Nanorods/MXene composite as high performance electrode for supercapacitors Chang Peng, Zeyuan Kuai, Tianqin Zeng, Yong Yu, Zefan Li, Jiangtao Zuo, Shu Chen, Shuaijun Pan, Ling Li PII:

S0925-8388(19)33168-8

DOI:

https://doi.org/10.1016/j.jallcom.2019.151928

Reference:

JALCOM 151928

To appear in:

Journal of Alloys and Compounds

Received Date: 25 April 2019 Revised Date:

10 August 2019

Accepted Date: 17 August 2019

Please cite this article as: C. Peng, Z. Kuai, T. Zeng, Y. Yu, Z. Li, J. Zuo, S. Chen, S. Pan, L. Li, WO3 Nanorods/MXene composite as high performance electrode for supercapacitors, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.151928. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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WO3 Nanorods/MXene composite as high performance electrode for

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supercapacitors

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Chang Peng a , Zeyuan Kuai a, Tianqin Zeng a, Yong Yu a, Zefan Li a, Jiangtao Zuo a,

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Shu Chen b, Shuaijun Pan b, Ling Li a,*

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a

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b

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410082, China.

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* Corresponding author: Ling Li

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E-mail: [email protected]

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College of Science, Hunan Agricultural University, Hunan 410082, P.R. China. College of Chemistry and Chemical Engineering, Hunan University, Changsha

Abstract

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Tungsten oxide (WO3) as electrode material has the drawbacks including poor

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rate capability and low capacitance. We firstly report a facile strategy to prepare

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WO3/MXene composite by intimately electrostatic attraction between the positively

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charged WO3 nanorods (WNRs) and the negatively charged transition metal carbides

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(MXene). This type composite shows higher specific capacitance (297 F·g−1)

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compared with pure WNRs (121 F·g−1) at a current density of 1 A·g−1 in 0.5 M H2SO4

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aqueous electrolyte. In addition, the as-prepared composite electrode displays good

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retention rate (82.2 % retention at 5 A·g−1) and cyclic stability (73.4 % even after 5000

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cycles at 4 A·g−1). The attractive electrochemical performance of WNRs/MXene may

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be attributed to the main reasons as follows: The MXene acts as electrons collector

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effectively improving the electrical conductivity and supplying more electrons to

1

1

participate redox reactions happened on the surface and interior of WNRs; The

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electrostatic self-assembly improves the stability of WNRs/MXene.

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Keywords: MXene; WO3 nanorods; Electrode; Specific capacitance; Stability

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1. Introduction

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With the development of technology for energy, supercapacitor is the promising

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ideal equipment due to its reliable charge-discharge rate and high energy capacitance

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[1-2]

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been investigated widely for the application of supercapacitor. Among various

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pseudocapacitive transition metal compounds, tungsten oxide (WO3) has been attracted

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extensive attention due to its multiple oxidation state, low cost, and special tunnel

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structure for ion insertion

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WO3 can allow more protons or ions to adequately insert

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capacitive performance and low electrical conductivity have greatly limited the

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application of WO3 for supercapacitor [7-8]. If the conductivity of WO3 can be increased,

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the specific capacitance can be improved due to the low charge transfer resistance.

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Thus, in order to overcome these drawbacks, some exciting researches have been

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reported that the performance of WO3 can be improved by incorporating with other

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highly conductive materials. Thus, one of most widely used electrode materials is

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carbon materials with active sites for the redox reactions, high conductivity, long cycle

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performance at high current rates

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graphene are the promising candidates due to the high conductivity and surface area,

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which can effectively utilize the redox reaction and enhance specific capacitance of

. The electrode materials such as pseudocapacitive transition metal oxides have

[3-4]

. Especially, the large number of tunnels in hexagonal

[9]

[5-6]

. However, the poor

. For example, the 2D carbon materials such as

2

1

WO3 [10-11].

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Recently, a novel 2D transition metal carbide (MXene) that combined hydrophilic

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surface and high metallic conductivity has been reported [12]. This new 2D MXene can

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be produced by etching aluminium from its ‘MAX’ phase. The MXene with high

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conductivity is considered as “inorganic graphene”

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for graphene in the application of supercapacitor

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nanowhiskers/MXene composite has been fabricated by direct chemical synthesis for

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enhanced dupercapacitive performance

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may also be improved considerably upon using the MXene as the template for the

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immobilization of WO3. However, the use of MXene for fabricating WO3/MXene

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composite has not been reported. More importantly, the simple physical mixing of

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WO3 and MXene may limit the interfacial contact area between the two type

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nanomaterials. It is desirable to harness the excellent electron conductivity of MXene

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through the close interfacial contact, by which the supercapacitive performance of

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WO3 can be improved. Thus, it is worth studying and developing a strategy to control

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the WO3 position and density during the synthesis of WO3/MXene composite.

[17]

[13-15]

, which is an ideal substitute

[16-17]

. For example, the MnO2

. Thus, the specific capacitance of the WO3

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In this research, a novel WO3/MXene composite was designed and fabricated

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from WO3 nanorods (WNRs) and MXene nanosheets by an electrostatic self-assembly

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strategy. There are two steps in the fabrication procedure: 1) A cationic surfactant,

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3-aminopropyl triethoxysilane (APTES), was used to modify the WNRs and make the

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surface of WNRs to be positive; 2) The positively charged APTES-WNRs were

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adsorbed and stabilized on the negatively charged surface of MXene via electrostatic

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1

attraction. The cationic APTES could help WNRs to distribute more uniformly on the

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MXene surface. The intimate contact between the MXene and WNRs improves the

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charge transfer at the interface. Then, the electrochemical behaviors of WNRs/MXene

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electrode were also studied in three-electrode system, and the test results show higher

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specific capacitances, better capacitances retention and rate capability comparing to the

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WNRs.

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2. Materials and Characterizations

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Synthesis of the APTES-WNRs

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WO3 nanorods (WNRs) were synthesized by a hydrothermal process. In a typical

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synthesis, 0.825 g of Na2WO4·2H2O and 0.290 g of NaCl were dissolved in 20 mL of

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deionized water. Subsequently, 3 M HCl was slowly dropped into the solution with

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stirring until the pH = 2.0. Then, the solution was transferred into a Teflon-lined 45 mL

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capacity autoclave. Hydrothermal reaction was carried out at 180 °C for 24 h. After

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cooling to room temperature, a white product was collected after repeatedly washing

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by water. At last, WNRs were obtained after drying under vacuum at 40 °C.

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The 0.4 g of as-prepared WNRs was dispersed in 200 ml ethanol by sonication for

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30 min. Then, the cationic surfactant APTES (2 ml) was added and refluxed for 5 h.

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After cooling to room temperture, the mixture was washed with ethanol to remove the

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unbound APTES. The APTES-WNRs was obtained after drying under vacuum at

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40 °C.

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Synthesis of the MXene nanosheets

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Multilayered Ti3C2Tx was prepared by etching the Ti3AlC2. 1g LiF (98.5%) was

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added into 10 mL HCl solution (9 M), and 1 g of Ti3AlC2 was added to the above

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solution under stirring for 24 h at 35 °C. Then, the mixture was washed by water using

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centrifugation until the pH of the supernatant was above 5. The multilayered Ti3C2Tx

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was obtained by freeze drying.

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The multilayered Ti3C2Tx (1 g) was added in 250 mL deionized water and

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sonicated for 1 h under Ar flow. Then, the dispersion solution was centrifuged 1 h at

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3500 rpm. A dark green supernatant was collected to obtain the delaminated MXene

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suspension. This MXene suspension was filtered and freeze dried to measure the

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concentration of the delaminated MXene.

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Synthesis of the WNRs/MXene composite

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The WNRs/MXene composite was prepared by electrostatic attraction between

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the positively charged APTES-WNRs and the negatively charged MXene nanosheets.

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The excess MXene suspension (100 mL, 0.3 mg/mL) was added into the

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APTES-WNRs dispersion (20 mL, 0.3 mg/mL) and sonicated for 1 h under Ar flow.

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Then, the mixture solution was centrifuged 1 h at 3500 rpm. The green supernatant

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attributed to the unbound MXene nanosheets was removed from the mixture solution.

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The residual product was freeze dried to obtain the WNRs/MXene composite.

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Characterizations

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Malvern Zetasizer Nano ZS90 (Malvern Instruments, UK) was used to measure

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all zeta potential of samples. The scanning electron microscopy (SEM) (JSM-6700F,

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Hitachi Ltd) and transmission electron microscopy (TEM) (JEM-2100F, JEOL) were

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used to inspect the morphology of materials. The energy dispersive spectroscopy (EDS)

5

1

results were acquired by using the TEM. The thickness of materials were measured by

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atomic force microscope (AFM) (SPA400, Seiko Instruments Inc,16 JP). For each

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sample 10-15 different spots were measured, and the averages were reported. The

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X-ray diffraction (XRD) instrument (Rigaku, Ltd., JP) was used to measure the

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crystallinity of materials. The PHI 5000c ESCA photoelectron spectrometer was used

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to record the X-ray photoelectron spectroscopy (XPS) of materials.

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The CHI 760C workstation (CH Instruments Inc.) with the three-electrode system

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was used to measure the cycle voltammetry (CV) and galvanostatic charge-discharge

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(GCD). The working electrode was prepared by mixing the active electrode material,

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acetylene black, and polyvinylidene fluoride with the mass ratio of 85:10:5 to form a

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slurry, and then the slurry was coated onto the stainless steel foil. The active material

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loading on the stainless steel foil, which served as the working electrode, was at 5

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mg·cm−2. The stainless steel foil was ultrasonicated in H2SO4 (0.5 M) for 10 minutes

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before use, and rinsed with acetone and deionized water followed by vacuum drying.

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All electrochemical experiments were carried out in 0.5 M H2SO4. The platinum wire

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and Ag/AgCl were used as counter and reference electrodes, respectively. The

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gravimetric capacitances can be calculated by the equations (1) and (2): =

∫ ∆

( )

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where C, I, v, ∆V, and m are the gravimetric capacitance (F·g-1), response current (A),

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potential scan rate (V·s−1), actual potential window in CV (V), and mass of

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electroactive materials in the electrodes (g) respectively. =

∆ ( ) ∆

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1

where C, m, I, ∆V, and ∆t are the gravimetric capacitance (F·g-1), mass of electroactive

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materials in the electrodes (g), discharge current (A), actual potential window in GCD

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(V), and discharge time (s) respectively. The electrochemical impedance spectroscopy

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(EIS) was tested at the open-circuit voltage using a sinusoidal signal of 5 mV from

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0.01 Hz to 100 kHz.

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3. Results and Discussion

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Figure 1a shows the process of electrostatic self-assembly between the

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APTES-WNRs and MXene for fabricating the WNRs/MXene composite. In our work,

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the APTES not only makes a positively charged WNRs surface but also improves the

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surface reaction between the WNRs and MXene. The APTES-WNRs was obtained

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through the surface modification of WNRs by APTES, and the zeta potential of the

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APTES-WNRs was +19.9 mV by measuring with Zetasizer Nano ZS (Figure 1b). This

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result indicates the APTES modification resulting in a positively charged WNRs

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surface due to the role of cationic APTES that originate from the amine (NH2) group

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by protonation (NH3+)

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exfoliating the multilayered Ti3C2Tx. Because of the many functional groups (COO-)

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on the layered MXene, the zeta potential of the negatively charged MXene was -28.8

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mV (Figure 1c). After the negative MXene dispersion was added into the positive

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APTES-WNRs dispersion, the WNRs/MXene composite was formed via electrostatic

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self-assembly, and the zeta potential of the WNRs/MXene was measured to be +9.35

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mV (Figure 1d). These results demonstrate that the APTES-WNRs effectively attached

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on the surface of MXene by the electrostatic attraction, which is an efficient approach

[18]

. The MXene was prepared by etching the Ti3AlC2 and

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1

for the synthesis of WNRs/MXene composite.

2 3

Figure (1) (a) Illustration of the synthesis for WNRs/MXene; Zeta potentials of (b)

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APTES-WNRs, (c) MXene, and (d) WNRs/MXene

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The morphology of the MXene, WNRs, and WNRs/MXene composite were

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investigated using scanning electron microscope (SEM) and transmission electron

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microscopy (TEM). The SEM of MXene presents accordion-like multilayer

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microstructure that the layered structure can be finely separated and clearly identified

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(Figure 2a). In addition, the dimension of the delaminated MXene with the range of

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hundred to thousand nanometres appears thin and transparent, that observed from the

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TEM (Figure 2b). Furthermore, per sheet of MXene that calculated from the

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cross-sectional TEM (Figure 2c) is about 1.33 nm, which matches well with the

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previous literature [19]. The AFM indicates that the thickness of MXene is about 1-4 nm

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(Figure 2d). The morphology results suggest that MXene is a well functional material

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1

at nanoscale thickness for assembly with other nanomaterials. The SEM and TEM of

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WNRs present the nanorod-shaped structure with diameters of ∼50 nm (Figure 2e-f),

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indicating that the WNRs were successfully synthesized. The SEM and TEM images of

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WNRs/MXene (Figure 2g-h) shows the presence of WNRs uniformly attached to the

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surface of MXene nanosheets, in which the two type microstructures exhibit lattice

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fringes with a d-spacing of 0.39 nm reflected (001) plane of WNRs [20], while the other

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exhibits lattice fringes with a d-spacing of 0.31 nm reflected (006) plane of MXene

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nanosheets

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was verified by the energy dispersive spectroscopy (EDS) obtained from TEM (Figure

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2j). These results indicate that the WNRs are intimately assembled on MXene due to

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electrostatic self-assembly.

[17]

(Figure 2i). In addition, the elementary composition of WNRs/MXene

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1 2

Figure (2) (a) SEM, (b) TEM, (c) Cross-sectional TEM, and (d) AFM images of

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MXene; (e) SEM and (f) TEM of WNRs; (g) SEM, (h) TEM, (i) HRTEM images, and

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(j) EDS of WNRs/MXene.

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The chemical structure of WNRs/MXene composite was measured by X-ray

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1

diffraction (XRD). As shown in the XRD of MXene, the diffraction peak (002) at 7.1°

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[21]

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XRD of the WNRs and WNRs/MXene contain the characteristic peaks of hexagonal

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phase of WO3 at 2θ=13.98° (100), 22.75° (001), 28.15° (200), 36.57° (201), 49.88°

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(220), 55.46° (202) respectively (JCPDS No.33-1387) [20]. Meanwhile, comparing with

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WNRs, the characteristic peak of MXene at 2θ = 7.1° (002) also appears in the XRD of

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WNRs/MXene. This result indicates that the WNRs have deposited on the surface of

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MXene. To further investigate the elemental composition, chemical bonding states, and

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grafting degree, the X-ray photoelectron spectroscopy (XPS) of WNRs/MXene

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composite was conducted. In the XPS of survey scan (Figure 3b), the WNRs/MXene is

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found to contain W, Si, and N after grafting the APTES-WNRs onto the MXene,

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whereas no W, Si, and N are found in pure MXene, indicating the existence of W

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attributing to the WNRs and Si, N attributing to the APTES, respectively. Table 1

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shows the atomic ratios of WNRs/MXene obtained from XPS, indicting the high

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grafting degree of WNRs in WNRs/MXene. Furthermore, the results of high-resolution

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XPS for WNRs/MXene are shown in Figure 3c-f. Obviously, five characteristic peaks

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at 455.2 eV, 457.1 eV, 459.0 eV, 461.4 eV, and 464.8 eV identified as Ti-C, Ti-OH,

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Ti-O (2p3/2), Ti-F, and Ti-O (2p1/2) [22] in Figure 3c. The C1s of the composite can be

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deconvoluted into C-Ti at 281.5 eV, C-C at 284.5 eV, C-N at 285.8 eV, C-O at 286.4 eV,

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and O–C=O at 288.8 eV, respectively [19, 23]. The C-N group is attributed to the APTES

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that contains nitrogen (Figure 3d). Moreover, high-resolution XPS of N1s and W4f for

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WNRs/MXene are also detected in Figure 3e-f. The W4f reveals the presence of two

has been detected in Figure 3a, which represents the pure MXene. In addition, the

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[24]

1

peaks, corresponding to W4f5/2 at 37.6 eV and W4f7/2 at 35.6 eV, respectively

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(Figure 3f), indicating that the W is in the +6 oxidation state. These results of XPS also

3

indicate that the WNRs are present in WNRs/MXene composite due to the electrostatic

4

self-assembly. It is believed that the conductivity of the WNRs can be effectively

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promoted due to the well contact between the WNRs and MXene.

6

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1

Figure (3) (a) XRD of MXene, WNRs, and WNRs/MXene; (b) XPS of survey scan for

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MXene and WNRs/MXene; High-resolution of XPS including (c) Ti2p, (d) C1s, (e)

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N1s, and (f) W4f for WNRs/MXene

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5 6 7

Table (1) Atomic ratios and binding energies for various elements in WNRs/MXene by XPS

a

atom % of each element calculated from the following sensitivity factors (RSF) by high-resolution

data of XPS

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The electrochemical behaviors of WNRs/MXene electrode including cycle

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voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical

10

impedance spectroscopy (EIS) were conducted with three-electrode system in 0.5 M

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H2SO4. In Figure 4a, the WNRs/MXene electrode exhibits the redox peak at 0.25 V

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because of the pseudocapacitive effect of WNRs. Moreover, the area under CV curve

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of WNRs/MXene is greater than those of WNRs and MXene, which indicates the

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superior capacitive property of WNRs/MXene electrode. It is proving that the surface

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reactions between WNRs and MXene can be improved by the electrostatic

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self-assembly. Furthermore, the currents of the oxidation waves for WNRs/MXene

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electrode linearly increase with the scan rate even up to 125 mV·s−1, and the specific

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capacitances calculated by CV are 309 F·g−1 at 25 mV·s−1, 284 F·g−1 at 50 mV·s−1, 275

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F·g−1 at 75 mV·s−1, 271 F·g−1 at 100 mV·s−1, and 265 F·g−1 at 125 mV·s−1 respectively,

20

indicating the favorable reversibility and rate capability (Figure 4b). This is because

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the redox process of the immobilized WNRs is confined on the surface of MXene, and

13

1

the ion can be fastly diffused into the electrode surface. The specific capacitance of

2

WNRs/MXene electrode (297 F·g−1) obtained from GCD is higher than those of

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WNRs (121 F·g−1) and MXene (112 F·g−1) at 1 A·g−1 (Figure 4c). Besides, the specific

4

capacitances values of WNRs/MXene, MXene, and WNRs retain 82.2 %, 79.3 %, and

5

55.3 % of their initial value when GCD current densities increase from 1 A·g−1 to 5

6

A·g−1 (Figure 4d), indicating the good capacitance retention and higher specific

7

capacitance of WNRs/MXene electrode. In addition, the GCD was performed with

8

5000 cycles at 4 A·g−1 to investigate the cycling life for WNRs/MXene electrode. The

9

specific capacitances values of WNRs/MXene, MXene, and WNRs retain 73.4 %,

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70.0 % and 41.8 % of the initial value respectively (Figure 4e), indicating that the

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WNRs/MXene electrode has the good cycling performance even after the 5000 GCD

12

cycle. The EIS was tested to further investigate the electrochemical behaviors. In high

13

frequency, the juncture of the axis under the curve of Nyquist plots acts as the

14

resistance for the active material based electrode. The resistance of WNRs/MXene is

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lower than that of WNRs and MXene, reflecting the better conductivity of

16

WNRs/MXene (Figure 4f). As seen in the corresponding equivalent circuit (Inset of

17

Figure 4f ), the larger semicircle shows poor electrical conductivity of the material.

18

Thus, the WNRs/MXene electrode has the less Rct indicating the good conductivity. In

19

addition, the straight line of WNRs/MXene is more vertical than those of WNRs and

20

MXene at low frequency region, indicating the faster ion diffusion and more ideal

21

capacitive performance for WNRs/MXene electrode. Thus, the WNRs/MXene

22

composite with high supercapacitive performance could be applied as a promising

14

1

electrode material. The attractive electrochemical performance of WNRs/MXene may

2

be attributed to the main reasons as follows: (1) The MXene acts as electrons collector

3

effectively improving the electrical conductivity. (2) The MXene supplies more

4

electrons to participate redox reactions happened on the surface and interior of WNRs

5

leading to the enhancement of specific capacitance for WNRs. (3) The electrostatic

6

self-assembly improves the stability of WNRs/MXene. Thus, the good electrochemical

7

behaviors of this composite can be understood by the synergetic effect and sufficient

8

interfacial contact between WNRs and MXene.

15

1 2

Figure (4) (a) CV curves of MXene, WNRs, and WNRs/MXene at 50 mV·s−1; (b) CV

3

curves of WNRs/MXene at various scan rates. Inset is the oxidation peak current

4

versus scan rate plot; (c) GCD curves of MXene, WNRs, and WNRs/MXene at 1 A·g−1;

5

(d) GCD curves of WNRs/MXene at various current densities. Inset is the specific

6

capacitances of MXene, WNRs, and WNRs/MXene at various current densities; (e)

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Cycle lifetimes of MXene, WNRs, and WNRs/MXene at 4 A·g−1 after 5000 GCD

16

1

cycles; (f) Nyquist plots of MXene, WNRs, and WNRs/MXene at open circuit

2

potential. Inset is the high frequency region after magnification and corresponding

3

equivalent circuit mode

4

4. Conclusion

5

A novel WNRs/MXene composite is prepared firstly through electrostatic

6

self-assembly between WNRs and MXene. The specific capacitance and stability of

7

the composite electrode achieve significant improvement due to the moderate doped

8

MXene nanosheets in the WNRs materials. This interfacial modification as an effective

9

approach could finely control the structure for the WNRs/MXene, and the enhanced

10

electrochemical performance suggest that the WNRs/MXene composite could be

11

employed as a promising electrode material for supercapacitors.

12

Conflicts of interest

13 14

There are no conflicts to declare. Acknowledgements

15

This work was supported by the research funds for the National Natural Science

16

Foundation of China (21606081, 21707031, and 51703056), the Platform Program of

17

Hunan Provincial Department of Science (Grant No. 2018TP2003 and 2018JJ3028)

18

and China Postdoctoral Science Foundation (2016M602402) .

19

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applications. Acs Applied Materials & Interfaces 2016, 8, 18806-18814.

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Highlights

• WO3/MXene composite is fabricated by electrostatic attraction for the first time. • The electrostatic self-assembly improves the stability of the composite. • This composite has highly supercapacitive performance.