Polyaniline nanowire arrays on three-dimensional hollow graphene balls for high-performance symmetric supercapacitor

Journal Pre-proof Polyaniline nanowire arrays on three-dimensional hollow graphene balls for highperformance symmetric supercapacitor Teng Zhang, Hongyan Yue, Xin Gao, Fei Yao, Hongtao Chen, Xinxin Lu, Yuanbo Wang, Xinrui Guo PII:

S1572-6657(19)30842-2

DOI:

https://doi.org/10.1016/j.jelechem.2019.113574

Reference:

JEAC 113574

To appear in:

Journal of Electroanalytical Chemistry

Received Date: 19 September 2019 Revised Date:

15 October 2019

Accepted Date: 15 October 2019

Please cite this article as: T. Zhang, H. Yue, X. Gao, F. Yao, H. Chen, X. Lu, Y. Wang, X. Guo, Polyaniline nanowire arrays on three-dimensional hollow graphene balls for high-performance symmetric supercapacitor, Journal of Electroanalytical Chemistry (2019), doi: https://doi.org/10.1016/ j.jelechem.2019.113574. 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.

Polyaniline nanowire arrays on three-dimensional hollow graphene balls for high-performance symmetric supercapacitor Teng Zhang1, Hongyan Yue1,*, Xin Gao1, Fei Yao2, Hongtao Chen1, Xinxin Lu1, Yuanbo Wang1, Xinrui Guo1 1

School of Materials Science and Engineering, Harbin University of Science and

Technology, Harbin 150040, People’s Republic of China 2

Department of Materials Design and Innovation, University at Buffalo, North

Campus, Buffalo 14260, USA Abstract Three-dimensional graphene-based hybrid is a promising supercapacitor electrode

material.

Herein,

a

novel

hybrid

of

polyaniline

nanowire

array/three-dimensional hollow graphene balls (PANI NWA/3D HGBs) is prepared. Three-dimensional hollow graphene balls (3D HGBs) are first fabricated by template-oriented carbon segregation and then polyaniline nanowire arrays (PANI NWAs) are grown on their surface by in-situ polymerization of aniline monomers. The results show that PANI NWAs with the length of ～200 nm are vertically grown on 3D HGBs with a diameter of ～500 nm. 3D HGBs act as the stable skeleton during long-term cyclic testing, and PANI NWAs provide a larger contact area with the electrolyte and a direct ion diffusion path. The hybrid exhibits a high specific capacitance of 635 F⋅g-1 at 1 A⋅g-1. When it’s assembled into a symmetric 1

supercapacitor, it exhibits an energy density of 25.3 Wh⋅kg-1 at a power density of 553.4 W⋅kg-1 and maintains a good cycle stability of 89% after 5000 cycles at 1.0 A⋅g-1. Keywords: three-dimensional hollow graphene balls; polyaniline nanowire arrays; in situ polymerization; supercapacitor.

1. Introduction The electrical double-layer capacitor (EDLC) made of carbon materials usually provides the excellent stability and rate capability, however, its capacitance is relatively poor compared with pseudocapacitor made of metal oxides or conductive polymers [1]. Combining carbon materials with metal oxides or conductive polymers is an effective method to overcome these problems [2-6]. Graphene, a two-dimensional structure with a single atomic thick layer, is a promising supercapacitor electrode material because it has a large specific surface area, excellent chemical stability and high conductivity [7]. But the strong π-π interactions results in its lower experimental specific surface area than the theoretical value (2630 m2/g) [8]. The structure of three-dimensional hollow graphene balls (3D HGBs) can effectively avoid the agglomeration of graphene nanosheets. Meanwhile, 3D HGBs can be prepared with low cost and in a large scale at low temperature. So 3D HGBs have shown excellent electrochemical stability and application prospects in

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the field of supercapacitors [9-12]. Moreover, the surface of 3D HGBs provides a platform for bonding with other nanomaterials. Polyaniline (PANI), as an excellent conductive polymer, has high theoretical specific capacitance (2.0×103 F⋅g-1), good flexibility and easy synthesis [13, 14]. Among many different morphologies of PANI, the nanowire arrays (NWAs) have a direct ion or electron diffusion path and large contact areas with the electrolyte. Although it is still a challenge and it needs tremendous efforts to prepare long PANI NWAs, it is crucial for improving the electrochemical performance of supercapacitors. Herein, a novel hybrid of PANI NWA/3D HGBs was prepared. Nickel nanoparticles (Ni NPs) were firstly synthesized by a simple reduction process. Then, the 3D HGBs were fabricated by template-oriented carbon segregation. Finally, PANI NWAs were vertically grown on the 3D HGBs by in-situ polymerization of aniline monomer. The integration of 3D HGBs and PANI NWAs plays a synergistic effect to effectively improve the electrochemical properties of supercapacitors. The hybrid shows a high specific capacitance of 635 F⋅g-1 at 1 A⋅g-1. When it was assembled into a symmetric supercapacitor, it exhibits an energy density of 25.3 Wh⋅kg-1 at a power density of 553.4 W⋅kg-1 and maintains a good cycling stability of 89% after 5000 cycles at 1.0 A⋅g-1.

2. Experimental 3

2.1 Synthesis of Ni NPs and 3D HGBs Ni NPs were fabricated by reducing nickel chloride by hydrazine hydrate [15]. The prepared Ni NPs were used as the template to prepare 3D HGBs. In a typical synthesis[16], the appropriate amount of Ni NPs were transferred into triethylene glycol solution (40 mL) containing 0.3 mL 50% NaOH. Then, the system was refluxed for 12 h at 220°C to obtain carbon-coated Ni NPs, followed by heat treatment at 500°C for 1 h under Ar atmosphere to form the stable graphene layer on Ni NPs. Finally, the 3D HGBs were obtained by etching Ni NPs in 3M hydrochloric acid (HCl) solution. 2.2 Preparation of PANI NWA/3D HGBs PANI NWAs were vertically grown on the above prepared 3D HGBs by in-situ polymerization of aniline. Firstly, The 5.5 mg 3D HGBs were immersed in 1 M sulfuric acid (H2SO4, 40 mL) containing aniline (AN, 364.6 µL) and stirred vigorously at -5°C for 10 h to completely adsorb aniline on the surface of 3D HGBs. Then, the same volume pre-cooled H2SO4 (1 M) containing ammonium peroxydisulfate (APS, 0.2282 g) was added into the reaction system. The molar ratio of AN and APS was 4:1. The PANI NWA/3D HGBs were obtained after reaction at -5°C 12 h. PANI nanowires were also fabricated without 3D HGBs under the same condition.

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2.3 Characterizations The samples were characterized by scanning electron microscopy (SEM, FEI Sirion-200) ， high-resolution transmission electron microscopy (HRTEM; JEOL JEM2100F)，X-ray diffraction (XRD, Philips X’Pert diffractometer) and Raman spectroscopy (Renishaw, RM1000-In Via). 2.4 Electrochemical measurements Mixing PANI NWA/3D HGBs (80 wt.%), conductive carbon black (10 wt.%) and PVDF (10 wt.%) and stir in NMP to form a homogeneous mixture, the mixture was coated on the 316 stainless steel, followed by drying at 120°C for 24 h to prepare the working electrode. The mass of active substance at each electrode is about 2 mg/cm2. All electrochemical tests were carried out on a VMP3 electrochemical workstation (France). The electrochemical performance of PANI NWA/3D HGBs were studied by three-electrode system. Active material, platinum wire and Ag/AgCl (std. KCl) electrode were used as the working electrode, counter electrode and reference electrode, respectively. The 1 M H2SO4 was used as electrolyte. The electrochemical performance of symmetric supercapacitor were tested under a two-electrode cell. A symmetric supercapacitor was constructed using two 316 stainless steels containing the same mass of PANI NWA/3D HGBs as electrodes. The

5

glass cellulose membrane as separator and 1 M H2SO4 was used as electrolyte.

3. Results and discussion 3.1 Morphology and structure characterizations The schematic of the preparation of PANI NWA/3D HGBs electrode and assembly of the symmetric supercapacitor are shown in Figure 1 and S1. Firstly, Ni NPs were fabricated by a simple reduction reaction. Then, the Ni NPs were added in triethylene glycol solution, which boiled by heating reflux at 220°C for 12 h, so that carbon atoms infiltrate into Ni NPs. The carburized Ni NPs are heat-treated by the segregation of carbon atoms in the Ni NPs to be formed on the surface of the Ni NPs, and the 3D HGBs are obtained after etching the Ni NPs. Finally, PANI NWAs were vertically grown on the 3D HGBs by in-situ polymerization at -5°C for 24 h in 1 M H2SO4. After filtering and freeze drying, the PANI NWA/3D HGB were obtained. Figure 2 shows the images and structures of the PANI NWA/3D HGBs. Ni NPs with the diameter of ～500 nm was prepared (Fig. 2a) and the influnce factors of Ni NPs on the diameter are investigated (Fig. S2). The average diameter of the 3D HGBs is ～500 nm，which is similar to that of the Ni NPs (Fig. 2b). The hollow structure of 3D HGBs with the graphene thickness of ～1.5 nm can be seen, indicating graphene is made up of several layers (Fig. 2c). Because of the high carbon solubility in Ni NPs, graphene prepared on Ni NPs are multilayered. PANI nanowires without 3D HGBs as

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the substance is interconnected short rod (Fig. S3). The SEM images of the PANI NWA/3D HGBs at different polymerization time are shown in Fig. 2d-2f and Fig. S4. With increasing the polymerization time, the length of PANI NWAs increase first and decrease later. After polymerization for 12 h, their lengths are largest. Fine-tipped PANI NWAs are tightly and vertically grown on 3D HGBs and the length of PANI NWAs is ～200 nm (Fig. 2g). The growth mechanism and influnce factors are investigated (Fig. S4). The structures of the PANI NWA/3D HGBs are investigated by XRD and Raman. A broad diffraction peak of 3D HGBs is shown at 2θ=25.9°, corresponding to the (002) basal plane of graphite[17]. The typical diffraction peaks of PANI can be observed at 9.1°, 14.8°, 20.8° and 25.4°, corresponding to the (001), (011), (020) and (200) planes of PANI nanowires. For PANI NWA/3D HGBs, the curve is similar to that of PANI nanowires and coincides with the (200) plane of 3D HGBs (Fig. 2h). The 3D HGBs show two prominent peaks at 1351 and 1592 cm-1, corresponding to the D and G bands of 3D HGBs. The typical prominent peaks at 1174, 1241, 1338, 1409, 1505 and 1604 cm-1 corresponds to C-H bending of quinoid ring, C-H bending of benzenoid ring, C-N stretching of benzenoid ring, C-C bending of quinoid ring, C=N stretching of quinoid ring and C=C stretching of benzenoid ring of PANI NWAs[18,19]. The curve of PANI NWA/3D HGBs is similar to that of PANI and coincides with the D and G bands of 3D HGBs (Figure 2i).

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3.2 Electrochemical properties The electrochemical properties of the prepared samples were characterized, as shown in Figure 3. Fig. 3a presents the Cyclic voltammetry (CV) curves of 3D HGBs, PANI nanowires and PANI NWA/3D HGBs electrodes at the scan rate of 10 mV s-1. The curve of the 3D HGBs is close to rectangle (Fig. S5a) and shows the feature of the EDLC. However, the PANI nanowires and PANI NWA/3D HGBs, the CV curves exhibit two pairs of redox peaks, which relates to the pseudocapacitance characteristic and reversible charge-discharge behavior of PANI nanowires. The pair of peaks A1/B1 is attributed to the redox transition of PANI nanowires between a semiconducting state

and

a

conducting

state.

Another

pair

of

peaks

A2/B2

are

the

emeraldine-pernigraniline transformation[20]. The peak current of PANI NWA/3D HGBs electrode is highest compared with that of 3D HGBs and PANI nanowires (Fig. 3a and S5). Fig. 3b exhibits the CV curves of PANI NWA/3D HGBs at various scan rates (from 10 to 100 mV⋅s-1). As the scan rate increases, the oxidation peak shifts positively and the reduction peaks shifts negatively, and the redox current densities increase. This indicates that the electrode has a good rate capacity. However, the redox peaks are not obviously separated at a higher scanning rate because of the influence of the resistance of the electrode material. The galvanostatic charge/discharge (GCD) curves of 3D HGBs, PANI nanowires,

8

PANI NWA/3D HGBs at 1 A⋅g-1 are shown in Fig.3c. The 3D HGBs electrode shows a quasi-triangular shape, indicating the characteristic of the EDLC. Deviation from linearity indicates that the capacitance of PANI nanowires and PANI NWA/3D HGBs electrodes mainly come from pseudocapacitance. It can also be seen that the discharge time of PANI NWA/3D HGBs is longer than that of 3D HGBs and PANI nanowires, showing its higher specific capacitance. Fig. 3d shows the GCD curves of PANI NWA/3D HGBs at various current densities with well symmetric, indicating a well reversibility of capacitance behavior. The specific capacitance (Cs) is calculated by integrating the data obtained. The Cs of 3D HGBs, PANI nanowires, PANI NWA/3D HGBs are shown in Fig. 3e. As the current density increases, the Cs of 3D HGBs is lowest and basically unchanged, indicating that it has an excellent stability. The Cs of PANI nanowires decreases sharply, which indicates that the stability of PANI nanowires is very poor. For PANI NWA/3D HGBs, the Cs decreases steadily, indicating that the stability of PANI NWA/3D HGBs is improved compared with PANI nanowires. The Cs of PANI NWA/3D HGBs (635 F⋅g-1) is higher than that of PANI nanowires (411 F⋅g-1) and 3D HGBs (46.6 F⋅g-1) at 1 A⋅g-1. Even at 10 A⋅g-1, the Cs of PANI NWA/3D HGBs (440 F⋅g-1) is much higher than that of PANI nanowires (141 F⋅g-1) and 3D HGBs (47.8 F⋅g-1). To further investigate the internal resistance of the samples, the EIS tests were

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conducted in a frequency range from 10 mHz to 100 kHz at an open-circuit potential with an ac perturbation of 5 mV, as shown in Fig. 3f. The modeled equivalent circuit is shown in Fig. 3g. The conductivity of 3D HGBs is best and the conductivity of PANI nanowires is worst. The conductivity of PANI NWA/3D HGBs is improved compared to PANI nanowires and it is betwen 3D HGBs and PANI NWA/3D HGBs. The C1 is the double layer capacitance and C2 is the Faradic capacitance. The intersection of curve and real impedance axis is the equivalent series resistance (Rs), including electrolyte internal resistance and active material/current collector contact resistance. As seen from the Nyquist plots, the Rs of the three electrodes are about 3.0 Ω, mainly due to the internal resistance of the collector. The charge transfer resistance (Rct) at the interface of electrode/solution, which can be represented by a semicircle diameter in the high frequency region. According to the curves, the Rct of PANI NWA/3D HGBs is about 6 Ω, less than that of a pure PANI nanowires (～8 Ω), indicting that the conductive properties of the PANI NWA/3D HGBs is improve compared with pure PANI nanowires, which results in a good electrochemical performance. Zw represents the Warburg resistance, at low frequency, PANI NWA/3D HGBs exhibits a more vertical line than pure PANI nanowires, indicating a faster diffusion of ions in the electrolyte. Fig. 3h shows the cycle tests of 3D HGBs, PANI nanowires and PANI NWA/3D

10

HGBs at 1.0 A·g-1. After 5000 cycles, the Cs of 3D HGBs retains 94.4%, which indicates that it has excellent stability. While, the Cs of PANI nanowires decreses to 75.8%, indicating its poor stability. For PANI NWA/3D HGBs, its capacity retention is 89%, indicating that the stability of PANI is significantly improved when it is combined with 3D HGBs. Figure 3i shows the schematic of increasing contact area between PANI NWAs length and electrolyte. When the length of the PANI NWAs is doubled, the contact area of the hybrid with the electrolyte increased about three times. The length of PANI NWAs is longest in PANI NWA/3D HGBs after polymerization for 12 h, therefore, its electrochemical properties are best (Fig. S6). Furthermore, the investigation of the charge storage kinetics in this hybrid also shows the similar result (Fig. S7). The excellent electrochemical performance of PANI NWA/3D HGBs electrode is mainly reflected in the following four aspects. (i) 3D HGBs with excellent mechanical properties may provide an additional mechanical support and leave room for the volume expansion and contraction of PANI NWAs. (ii) The strong π-electron interaction between 3D HGBs and PANI NWAs with aromatic/quinone can provide a strong interface adhesion and promote the electron transport. (iii) The vertical PANI NWAs with a larger length on 3D HGBs greatly increases the contact area between the hybrid electrode and electrolyte, and facilitates the diffusion of ions from the

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bulky electrolyte to the PANI NWA/3D HGBs surface. (iiii) The PANI NWAs morphology shortens the electron and ion diffusion path in the hybrid electrode. Therefore, the integration of PANI NWAs and 3D HGBs effectively exerts the synergestic effect to increase the electrochemical performance of the PANI NWA/3D HGBs. The symmetric supercapacitor cell was assembled using PANI NWA/3D HGBs as electrode active material (Fig. 4a). Fig. 4b depicts the CV curves in the potential window from 0 to 1.0 V at various scan rates. The symmetric supercapacitor cell maintains a good rate performance as the scan rate increases. The rate capability of the supercapacitor is also confirmed by GCD curves (Fig. 4c). The Cs of the symmetric supercapacitor reaches 165.5 F g-1 at 1 A⋅g-1 and 52.0% of capacitance retention even at 10 A⋅g-1 (Fig. 4d). The cycle life of the symmetric supercapacitor cell was conducted at 1.0 A⋅g-1. As displayed in Fig. 4e, the Cs of the supercapacitor cell retained 89% after 5000 cycles. The Cs reduction of the PANI NWA/3D HGBs electrode is caused by degradation of the polymer due to expansion and contraction during long-term cycling. The inset in Fig. 4e is a photograph showing that three symmetric supercapacitors in series can successfully light up a red LED. The energy density and power density are calculated from the GCD curves Fig. 4f depicts the Ragone plots based on two-electrode symmetric supercapacitor. When the P is 553.4

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W⋅kg-1, the E of the PANI NWAs/3D HGBs symmetric supercapacitor is 25.3 Wh⋅kg-1, and even at the high P of 6124.4 W⋅kg-1, the E of 14.3 Wh⋅kg-1 is maintained. The maximum E of our work is higher than that of most previously reported PANI-based supercapacitors[21-28]. All of the above results prove that PANI NWA/3D HGBs are excellent supercapacitors electrode material.

4. Conclusions In summary, Ni NPs are used as the template to fabricate 3D HGBs by template-oriented carbon segregation. Then, PANI NWAs are vertically grown on 3D HGBs by in-situ polymerization to obtain PANI NWA/3D HGBs. The results show that the diameter of the 3D HGBs is ～500 nm, and the length of the PANI NWAs are ～200 nm. Owing to the synergistic effect of 3D HGBs and PANI NWAs, it exhibits a high specific capacitance of 635 F⋅g-1 at 1 A⋅g-1. When assembled into a symmetric supercapacitor, it exhibits an energy density of 25.3 Wh⋅kg-1 at a power density of 553.4 W⋅kg-1 and the electrode maintains a good cycle stability of 89% after 5000 cycles at 1.0 A⋅g-1.

Acknowledgements This work is supported by the fundamental research foundation for University of Heilongjiang province (LGYC2018JQ012) and the innovative talent fund of Harbin city (2016RAQXJ185).

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doi:10.1016/j.jpowsour.2015.09.018 List of Figure Captions Figure 1 Schematic of the preparation of PANI NWA/3D HGBs electrode and assembly of the symmetric supercapacitor. Figure 2 Images and structures of the PANI NWA/3D HGBs. SEM images of (a) Ni NPs and (b) 3D HGBs, (c) TEM image of 3D HGBs (inset exhibits the HRTEM image), SEM images of PANI NWA/3D HGBs obtained at different reaction time (d) 6 h, (e) 12 h and (f) 24 h, (g) TEM image of PANI NWA/3D HGBs obtained at 12 h, (h) XRD patterns and (i) Raman spectra of the 3D HGBs, PANI NWs and PANI NWA/3D HGBs. Figure 3 Electrochemical performance of the PANI NWA/3D HGBs in three-electrode system. (a) CV curves of 3D HGBs, PANI NWs and PANI NWA/3D HGBs at the scan rate of 10 mV·s-1, (b) CV curves of PANI NWA/3D HGBs at

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various scan rates, (c) GCD curves of 3D HGBs, PANI NWs and PANI NWA/3D HGBs at 1.0 A·g-1, (d) GCD curves of PANI NWA/3D HGBs at various current densities, (e) Specific capacitance plots of 3D HGBs, PANI NWs and PANI NWA/3D HGBs at different current densities, (f) The Nyquist plots (the inset is the enlargement of the high-frequency region) of 3D HGBs, PANI NWs and PANI NWA/3D HGBs, (g) Cycle tests of 3D HGBs, PANI NWs and PANI NWA/3D HGBs at 1.0 A·g-1, (h) modeled equivalent circuit of electrochemical impedance spectroscopy, (i) Schematic diagram of increasing contact area between PANI NWA/3D HGBs and electrolyte. Figure 4 The electrochemical properties of symmetric supercapacitor. (a) Schematic of the assembled supercapacitor, (b) CV curves within the potential window from 0 to 1.0 V at various scan rates, (c) GCD curves at various current densities, (d) Specific capacitance plot at different current densities, (e) Cycle test at 1.0 A·g-1 and the inset is a photograph showing that three symmetric supercapacitors in series can successfully light up a red LED, (f) Ragone plot based on two-electrode symmetric supercapacitor.

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Highlights 1. Vertical polyaniline nanowire arrays (PANI NWAs) were successfully grown on three-dimensional hollow graphene balls (3D HGBs) to obtain a novel electrode material (PANI NWA/3D HGBs). 2. The electrode exhibits a high specific capacitance of 643 F⋅g-1 at 1 A⋅g-1 in 1 M H2SO4 in three-electrode system. 3. The symmetric supercapacitor exhibits an energy density 26.4 Wh⋅kg-1 at a power density of 563.5 W⋅kg-1 and 89% capacitance is retained after 5000 cycles at 1 A⋅g-1.

Declaration of Interest Statement; The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Hong Yan Yue