HrGO composite films for high-performance symmetric solid-state supercapacitors

Accepted Manuscript Hierarchically porous PDAA@rGO/HrGO composite films for high-performance symmetric solid-state supercapacitors

Juanli Liu, Qi Wang, Du Pengcheng, Niu Jingye, Peng Liu PII: DOI: Reference:

S0169-4332(19)31058-X https://doi.org/10.1016/j.apsusc.2019.04.066 APSUSC 42374

To appear in:

Applied Surface Science

Received date: Revised date: Accepted date:

28 February 2019 2 April 2019 5 April 2019

Please cite this article as: J. Liu, Q. Wang, D. Pengcheng, et al., Hierarchically porous PDAA@rGO/HrGO composite films for high-performance symmetric solid-state supercapacitors, Applied Surface Science, https://doi.org/10.1016/j.apsusc.2019.04.066

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ACCEPTED MANUSCRIPT Hierarchically porous PDAA@rGO/HrGO composite films for high-performance symmetric solid-state supercapacitors Juanli Liu,1,2 Qi Wang,1 Du Pengcheng,1 Niu Jingye,1 Peng Liu1,* 1

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and

College of Chemical Engineering, Northwest Minzu University, Lanzhou 730030,

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2

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Chemical Engineering, Lanzhou University, Lanzhou 730000, China

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China Abstract

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Hierarchically porous PDAA@rGO/HrGO composite films were fabricated as

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self-supporting flexible film electrodes for high-performance symmetric solid-state supercapacitors, via facile vacuum filtration of the mixed dispersion containing the

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well-defined PDAA@rGO nanocomposite and HrGO nanosheets as building blocks.

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The electrochemical property of the proposed hierarchically porous PDAA@rGO/HrGO composite film electrodes was optimized in a two-electrode

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configuration, by adjusting the feeding ratios and the total solid content in the mixed

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dispersion. The optimized one, the PDAA@rGO/HrGO-4 composite film electrode, possessed a high gravimetric specific capacitance of 409 F/g and areal specific capacitance of 644 mF/cm2 at 0.5 A/g, with a high capacitance retention of 98% after 10000 cycles at 5 A/g. The symmetric solid-state supercapacitor fabricated with the

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Corresponding author at: State Key Laboratory of Applied Organic Chemistry, College of Chemistry and

Chemical Engineering, Lanzhou University, Lanzhou 730000, China. Tel./Fax: +86 931 8912582. E-mail address: [email protected] (P. Liu). 1

ACCEPTED MANUSCRIPT PDAA@rGO/HrGO-4 composite film electrodes, three in series, could light a red LED.

Keywords: Symmetric solid-state supercapacitors; flexible electrode,

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poly(1,5-diaminoanthraquinone); hierarchically porous composite film; holey

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graphene

1. Introduction

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As a kind of energy storage devices, supercapacitors have attracted increasing

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attentions in hybrid vehicles, portable electronics and military devices, owing to their advantages of high power density, fast charging/discharging rate, and excellent

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cycling stability [1]. The electrode materials are the most important component

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determining the energy storage performance of the supercapacitors [2], by physical charge accumulation at the electrode/electrolyte interface as electrical double-layer

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capacitors (EDLCs) or direct storing charges during the charging/discharging

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processes through redox reaction as Faradaic capacitors (FCs) or pseudocapacitors. It has been reported that the hybrid supercapacitors combining EDLCs and pseudocapacitors could deliver higher specific capacitance in comparison with the single charge storage mechanism [3]. Graphene, the one-atom-thick sheet of sp2-hybridized carbon atoms, is a promising kind of electrode materials for EDLCs with high theoretical specific capacitance [4]. The heteroatom doping, surface functionalities, hole-formation and composites with 2

ACCEPTED MANUSCRIPT pseudocapacitance materials have been widely investigated to enhance the specific capacitance of the graphene-based supercapacitors [5]. In recent years, binder-free self-supporting graphene-based electrodes have been developed without insulating binders, which decrease the electrode conductivity and suppress electron transport,

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subsequently declining electrochemical performance of the supercapacitors. The

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self-supporting graphene-based film electrodes could be obtained by coating, printing,

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or vacuum filtration [6]. However, the restacking of graphene sheets via the sheet-to-sheet van der Waals interaction limited their practical applications. To avoid

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the problem, the hybrid film electrodes have been designed with other

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pseudocapacitance materials as spacers, such as transition metal oxide and conducting polymer nanoparticles [7].

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Poly(1,5-diaminoanthraquinone) (PDAA) has been recognized as promising

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candidate for supercapacitor electrode because of enhanced electroactivity and cycling stability, and broadened potential window [8]. The PDAA/rGO hybrids have

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been also prepared as electrode materials for supercapacitors with excellent

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electrochemical performance: high specific capacitance of 617 F/g and superior cycle life of 124% after 15000 cycles [9]. However, maybe due to the rigidity of the well-defined sheet-shaped PDAA/rGO hybrids, self-supporting film could not be obtained via vacuum filtration of their dispersion. Compared with graphene, holey reduced graphene oxide (HrGO) shows great improved electrochemical performance owing to excellent ion diffusion rate and shortcuts between the graphene layers [10]. It is expected as an ideal building block 3

ACCEPTED MANUSCRIPT for self-supporting graphene-based film electrodes for high performance supercapacitors. In the present work, hierarchically porous PDAA@rGO/HrGO composite films were fabricated as self-supporting flexible film electrodes for high-performance symmetric solid-state supercapacitors, via facile vacuum filtration

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of the mixed aqueous dispersion of the well-defined PDAA@rGO nanocomposite and

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HrGO nanosheets (Scheme 1). The solid-state supercapacitors, three in series, could

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light a red LED.

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2. Experimental section

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2.1. Materials and reagents

Graphite powder was obtained from Huatai Lubricant Sealing S&T Co. Ltd., Qingdao,

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China. 1,5-Diaminoanthraquinone (DAA, 97%) was bought from Alfa Aesar

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Chemicals Co., Ltd., Shanghai, China. CH3CN, N, N’-dimethyl formamide (DMF) and C2H5OH (95%) were purchased from Tianjin Damao Chemical Reagent (Tianjin,

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China). Hydrazine hydrate (N2H4∙H2O, 80%), perchloric acid (HClO4, 72%),

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ammonium persulfate (APS, 98%) and ammonium hydroxide (NH3∙H2O, 25~28%) were purchased from Kelong Chemical Reagent Company (Chengdu, China). Deionized water was used throughout the experiments.

2.2. Preparation of holey graphene oxide (HGO) Graphene oxide (GO) was prepared from graphite powder using a modified Hummer’s method [9]. For the HGO, 100 mg of GO was dispersed in 50 mL of water 4

ACCEPTED MANUSCRIPT with sonication for 30 min, then 5 mL of H2O2 was added into the dispersion, followed with refluxing at 100 °C for 4 h. After cooling to room temperature, the dispersion was transferred to dialysis for 48 h with deionized water, by replacing

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water every 8 h. Finally, the HGO was collected by lyophilization.

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2.3. Preparation of PDAA@GO nanocomposite

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The PDAA@GO nanocomposite was prepared via the in-situ chemical oxidation polymerization of DAA in the presence of GO [9]. Typically, 20 mg of GO was

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dispersed in 19 mL of DMF with sonication for 30 min, then 19 mL of acetonitrile

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containing 238.2 mg of DAA was added into the dispersion with sonication for another 30 min, followed with refluxing at 90 °C for 8 h. After that, 280 μL of HClO4

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and 1 mL of APS (342.3 mg) solution were added and the mixture was kept at 25 °C

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with stirring for 48 h. The resultant PDAA@GO nanocomposite was centrifuged and

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rinsed with DMF and water for three times, respectively.

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2.4. Fabrication of PDAA@rGO/HrGO composite films The dispersions of PDAA@GO (2.5 mg/mL) and HGO (1.5 mg/mL) were mixed with different volume ratios and diluted to 80 mL with deionized water (Table 1). Then 40 μL of hydrazine hydrate and 480 μL of NH3H2O were added and the mixture was heated to 95 °C with refluxing for 1 h. After reduction, the PDAA@rGO/HrGO composite films were obtained by vacuum filtration, and rinsed with water and ethanol. 5

ACCEPTED MANUSCRIPT For comparison, the HrGO film was also prepared with the same method without the PDAA@GO nanocomposite.

2.5. Analysis and characterization

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FT-IR spectra were recorded on a Fourier transform infrared spectrometer (NEXUS

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670, Nicolet, Germany) in the range of 400-4000 cm-1, using KBr pellet method.

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Raman spectra were recorded on a Fourier Transform Raman spectrometer (HORIBA Jobin Yvon LabRAM HR 800, France), employing a 633 nm laser beam.

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XRD patterns were collected on an X-ray diffraction diffractometer (X’Pert PRO,

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Holland) at the Cu Ka (k = 0.15406 nm) radiation in the range of 10~80°at 40 kV and 40 mA.

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X-ray photoelectron spectroscopy (XPS) analysis was performed on a VG

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Scientific ESCALAB 250Xi-XPS photoelectron spectrometer with an Al Kα X-ray resource. The binding energies were calibrated by the C1s binding energy of 284.7 eV.

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The morphologies of the samples were recorded on JEM-1230 transmission

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electron microscope (TEM, JEOL, Tokyo, Japan), high resolution transmission electron microscope (HRTEM, Tecnai F30, Philips-FEI, Holland) and scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The electrical conductivities of the PDAA@rGO/HrGO composite films were measured using a RTS-2 four-point probe conductivity tester (Guangzhou four-point probe Technology Co., Ltd, Guangdong, China) at ambient temperature.

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ACCEPTED MANUSCRIPT 2.6. Electrochemical measurements The composite films were tailored with an area of 1.01.0 cm2 and immersed in 1 M H2SO4 for 12 h prior to electrochemical tests. Two-electrode configuration was used to fabricate the PDAA@rGO/HrGO composite film based symmetric supercapacitor

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for the electrochemical evaluation on a CHI660B electrochemical workstation (CHI,

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Shanghai, China) at room temperature in a potential window of 0 to 0.8 V. The cyclic

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voltammetry (CV) curves were carried out at different scanning rates ranging from 5 to 100 mV/s. The galvanostatic charge/discharge (GCD) tests were measured at

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different current densities from to 0.5 to 10 A/g. The electrochemical impedance

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spectroscopy (EIS) measurement was carried out in the frequency from 100 kHz to 0.01 Hz with the potential amplitude of 5 mV. The cycling life was tested at the

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current density of 5 A/g for 10000 cycles.

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The gravimetric specific capacitance (Cm) and areal specific capacitance (Cs) of the proposed composite film electrodes were calculated from the GCD tests with the

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following equations:

Cm = 4I*t/(m*V) Cs = I*t/(S*V)

where I is the current (A), t is the discharge time (s), m is the total mass of the two electrodes (g), S is the geometric area of the supercapacitor device (cm2), and V is the voltage window, respectively. The energy density (E) and power density (P) of the as-prepared SSC were calculated based on the following equations: 7

ACCEPTED MANUSCRIPT E=

1 1 (𝑊ℎ 𝑘𝑔−1 ) × 𝐶𝑠𝑚 × ∆𝑉 2 × 8 3.6 P=

𝐸 × 3600 (𝑊 𝑘𝑔−1 ) ∆𝑡

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2.7. Fabrication of symmetric solid-state supercapacitor The symmetric solid-state supercapacitors were fabricated using two pieces of

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PDAA@rGO/HrGO-4 (1.01.0 cm2) as electrodes with a conductive carbon plate

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(thickness: 1.0 mm) as mechanical support to avoid the destruction of the composite

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film during the fabrication, sandwiched with the cellulose separator (NKK TF4030),

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with PVA-H2SO4 gel electrolyte.

3. Results and discussion

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3.1. Preparation of HrGO and PDAA@rGO nanocomposite The GO was prepared by the oxidation of graphene, and then the holey graphene

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oxide (HGO) was obtained by refluxing GO with H2O2, as illustrated in Scheme 1. Then the reduced graphene oxide (rGO) and holey reduced graphene oxide (HrGO)

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were obtained after reduction with hydrazine hydrate. The yield for the modified graphene samples were determined by the gravimetric method. A yield of 92% and 54% was achieved for the GO sample after oxidation of graphite and the HGO sample after hole-formation of GO, respectively. Clearly, the hole-formation of GO by etching carbon atoms from the graphitic plane resulted in a low yield for the HGO sample. As for the reduced samples, a yield of 85% and 83% was achieved for the rGO and HrGO respectively, due to the removal of the oxygen-containing groups such as 8

ACCEPTED MANUSCRIPT carboxyl and carbonyl groups. As shown in Figure 1, the thin sheets of rGO were obtained with little wrinkles (Figure 1a). The HrGO showed the similar morphology as the rGO from the TEM analysis (Figure 1c), however, small nano-holes could be clearly seen in the HRTEM image (Figure 1d).

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In the XPS analysis (Figure 2a), a strong signal of O 1s appeared at 530 eV in the

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XPS survey of the GO and HGO samples, owing to the introduction of

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oxygen-containing groups. Compared with the GO sample, the HGO sample possessed a higher surface O atomic concentration (Table 2), which was attributed to

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the introduction of the oxygen-containing groups at the hole edge in HGO. Besides,

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the surface S atom should be caused by the adsorption of the SO42- ion. In the FT-IR spectra (Figure 2b), both the rGO and HrGO exhibited the characteristic absorbance

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peaks of the –OH and CH2 groups at 3438 cm-1 and 1638 cm-1, but the peak intensity

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of the HrGO was much higher than the rGO, due to the defects introduced in graphene during the hole-formation, the oxygen-containing functional groups. Although the

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similar crystal structure with X-ray patterns at 24° and 43° of the (002) and (100)

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planes of graphitic carbon respectively (Figure 2c) [11], the holey-formation in the HrGO could also be revealed by the Raman analysis (Figure 2d). Both the rGO and HrGO showed the prominent Raman shifts at 1341 and 1578 cm-1 ascribed to the D and G bands of the sp3 and sp2 hybridized carbon, respectively. The ID/IG ratio of the HrGO was 1.28, much higher than that of the rGO of 1.05, due to the defects introduced in graphene during the hole-formation [12]. The PDAA@GO nanocomposite was prepared via the in-situ chemical oxidation 9

ACCEPTED MANUSCRIPT polymerization of DAA in the presence of GO. The surface N atomic concentration increased while those of C and O decreased (Table 2), indicating the successful deposition of PDAA onto the surface of the GO sheets. Finally, the PDAA@rGO nanocomposite was obtained by the reduction of the PDAA@GO nanocomposite with

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hydrazine hydrate. It showed well-defined sheet-shaped with PDAA nanoparticles

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uniformly immobilized on the rGO (Figure 1b). The characteristic absorbance peaks

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of PDAA, such as 1593 cm-1, 1488 cm-1 and 1255 cm-1 corresponding to the C=C of N=Q=N and N-Q-N rings, C-NH-C bonds respectively [8], could be seen in the FT-IR

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spectrum of the PDAA@rGO nanocomposite (Figure 2b). It showed a broad strong

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pattern from 15 to 35°, due to the overlapping of the (002) plane of rGO and the broad peak from 15 to 40° of the amorphous PDDA [8] (Figure 2c). Furthermore, the ID/IG

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ratio of the PDAA@rGO nanocomposite increased to 1.47 (Figure 2d), also revealing

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the successful preparation of the PDAA@rGO nanocomposite, in which some covalent linkages might be formed between PDAA and the functional groups in the

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GO such as epoxy groups [13].

3.2. Fabrication of hierarchically porous PDAA@rGO/HrGO composite films Then the hierarchically porous PDAA@rGO/HrGO composite films were fabricated via the vacuum filtration of the mixed dispersion of HrGO nanosheets and PDAA@rGO nanocomposite with different feeding ratios. Clearly, the surface of the PDAA@rGO/HrGO composite films became smoother and smoother with increasing the amount of the PDAA@rGO nanocomposite with the same total mass of 20 mg 10

ACCEPTED MANUSCRIPT (HrGO film, PDAA@rGO/HrGO-1, PDAA@rGO/HrGO-2 and PDAA@rGO/HrGO-3 composite films) (Figure 1 e, g, I and k). Under the same feeding ratio of HrGO nanosheets and PDAA@rGO nanocomposite, the PDAA@rGO/HrGO composite films showed the similar surface morphology with

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increasing the solid content in the mixed dispersion (Figure k, m and o), from

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PDAA@rGO/HrGO-3 to PDAA@rGO/HrGO-5 composite film with total mass of 30

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mg.

The inner structure of the films could be observed with the cross-section SEM

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images. The PDAA@rGO/HrGO-1 and PDAA@rGO/HrGO-2 composite films

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prepared with lower contents of PDAA@rGO nanocomposite showed the near uniformly layered structure with micropores (Figure 1 h and j), as the HrGO film

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(Figure 1 f). However, the non-homogeneous sites could be seen in the

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PDAA@rGO/HrGO-3, PDAA@rGO/HrGO-4 and PDAA@rGO/HrGO-5 composite films prepared with a high content of PDAA@rGO nanocomposite (Figure 1 l, n and

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p), maybe due to the aggregation of the PDAA@rGO nanocomposite. Such

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aggregation led to bigger pores in the composite films. The hierarchically porous structure, containing the nano-holes in the HrGO and pores in the composite films, is beneficial for shortening the diffusion pathways and accelerating the ion diffusion speed [14]. The thickness of the fabricated films was measured with a spiral-micrometer and the results are summarized in Table 1. The thickness of the PDAA@rGO/HrGO-1, PDAA@rGO/HrGO-2 and PDAA@rGO/HrGO-3 composite films declined with 11

ACCEPTED MANUSCRIPT increasing the feeding ratio of the PDAA@rGO nanocomposite and all the thickness values were much thinner than the HrGO film with the same solid content in the dispersion (Table 1). It might be due to the strong interaction between the HrGO nanosheets and PDAA@rGO nanocomposite, which caused an oriented packing,

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showing layered structure from the cross-section SEM images. With the same feeding

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ratio of the PDAA@rGO nanocomposite, the thickness of the PDAA@rGO/HrGO-3

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composite film increased from 54 μm to 70 μm of the PDAA@rGO/HrGO-5 composite film, by increasing the solid content of the mixed dispersion from 20 mg to

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30 mg.

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The electrical conductivities of the fabricated films are also compared in Table 1. All the PDAA@rGO/HrGO composite films showed the lower electrical conductivity

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than the HrGO film. Furthermore, the electrical conductivity decreased slightly with

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increasing the feeding ratio of the PDAA@rGO nanocomposite and increasing the thickness of the composite films. Such results should be resulted from the fact that the

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PDAA@rGO nanocomposite disrupted the conductive network structure formed by

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the HrGO nanosheets, due to the strong interaction between the two components. Furthermore, both the non-homogeneous structure and bigger pores were formed by increasing the thickness of the composite films, also leading a slight decrease in the electrical conductivity, from the PDAA@rGO/HrGO-3 to the PDAA@rGO/HrGO-5 composite film.

3.3. Electrochemical performance of hierarchically porous PDAA@rGO/HrGO 12

ACCEPTED MANUSCRIPT composite films The electrochemical capacitive performance of the proposed hierarchically porous PDAA@rGO/HrGO composite films was evaluated within a voltage range of 0 – 0.8 V using a two-electrode configuration. The cyclic voltammograms (CV) at a scanning

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rate of 5.0 mV/s and the galvanostatic charge/discharge (GCD) curves at a current

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density of 0.5 A/g are presented in Figure 3. The HrGO film electrode showed a

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rectangular-shaped CV curve of typical EDLCs [15], while the PDAA@rGO/HrGO composite film electrodes showed the quasi-rectangular-shaped CV curves with

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obvious broad faradic redox peaks (Figure 3a), due to the pseudocapacitive PDAA.

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The loop area of the composite film electrodes was much larger than that of the HrGO film electrode, indicating the incorporation of the PDAA@rGO nanocomposite could

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significantly enhance the capacitive property of the film electrode. Among all the

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PDAA@rGO/HrGO composite film electrodes, the PDAA@rGO/HrGO-4 composite film electrode possessed the largest loop area, indicating the highest capacitance.

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All the film electrodes showed near symmetrical triangular-shaped GCD curves

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with slight deviation, demonstrating the hybridization of EDLCs and pseudocapacitors [16]. Furthermore, the equal charge/discharge durations for each electrode indicated reversible behavior, high columbic efficiency, and an ideal capacitor performance. The negligible voltage drop in the GCD curves indicated a fast ion and electron transport through the electrodes, owing to the hierarchically porous structure [14]. The times to accomplish one charge–discharge cycle for the PDAA@rGO/HrGO composite film electrodes were much longer than that of the 13

ACCEPTED MANUSCRIPT HrGO film electrode, implying their highest capacitance consistent with the CV results. The gravimetric specific capacitance (Cm) and areal specific capacitance (Cs) of the proposed composite film electrodes were calculated from the GCD analysis and summarized in Table 1, with the highest Cm for the PDAA@rGO/HrGO-4 composite

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film electrode of 409 F/g. Its Cs was lower than that of the PDAA@rGO/HrGO-5

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composite film electrode because of its thickness. So the PDAA@rGO/HrGO-4

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composite film electrode was selected for the further electrochemical investigation. Figure 4a d illustrated the CV curves of the PDAA@rGO/HrGO-4 composite film

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electrode at different scan rates. Obviously, the peak current increased gradually with

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increasing the scan rate from 5 to 100 mV/s. All CV curves displayed a quasi-rectangular shape even under 100 mV/s, indicating fast and reversible

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charge-discharge ability owing to the hierarchically porous structure [17]. The GCD

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curves of the PDAA@rGO/HrGO-4 composite film electrode maintained the symmetrical triangular-shaped, with a rate capacity of 65% from 0.5 to 10 A/g. The

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rate capability was lower than that of the HrGO film electrode (Figure 4c), due to the

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PDAA@rGO nanocomposite limited the fast ion diffusion at higher current density. The electrochemical impedance spectrum (EIS) was also used to evaluate the electrochemical property of the PDAA@rGO/HrGO composite film electrodes. All the plots present a similar shape with a semi-circle in the middle frequency region and a nearly vertical line in the low frequency region (Figure 4d). The charge transfer resistance (Rct) and equivalent series resistance (Rs) can be obtained from the intercept of the X-axis and the diameter of the semicircle, respectively. The Rct value of the 14

ACCEPTED MANUSCRIPT PDAA@rGO/HrGO composite film electrodes increased with increasing the amount of the PDAA@rGO nanocomposite, from PDAA@rGO/HrGO-1 to PDAA@rGO/HrGO-3 composite film electrode, as well as increasing the thickness of the PDAA@rGO nanocomposite, from PDAA@rGO/HrGO-3 to

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PDAA@rGO/HrGO-5 composite film electrode. The results indicated that the charge

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transfer was facilitated with more HrGO with nano-holes in the hierarchically porous

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PDAA@rGO/HrGO composite film electrodes [18] and thinner thickness of the film electrodes. The equivalent series resistance (Rs, equivalent circuit as insert in Figure

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4d) generally means the resistance of the electrolyte combined with the intrinsic

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resistance of the electrode. All the PDAA@rGO/HrGO composite film electrodes showed higher Rs than the HrGO film electrode (0.63 Ω), while the value of the

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PDAA@rGO/HrGO-4 composite film electrode was relatively lower (0.72 Ω),

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indicating its better charge/discharge performance. The energy density (E) and power density (P) of the symmetric supercapacitor were

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calculated from GCD measurement at a cell voltage of 0.8 V over various current

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densities ranging from 0.5 A/g to 10 A/g and plotted as the Ragone curve shown in Figure 4e, to describe the relationship between energy density and power density. Energy density of 3.3-4.2 Wh/kg was achieved for the HrGO film electrode-based supercapacitor at a power density of 0.2-3.9 kW/kg, comparative or higher than the reported graphene-based supercapacitors [19-23]. Even though, the energy density of the PDAA@rGO/HrGO-4 composite film electrode-based one reached 9.1 Wh/kg at a current density of 0.5 A/g, while the power density was 200 W/kg. At the same power 15

ACCEPTED MANUSCRIPT density range, the energy density of the PDAA@rGO/HrGO-4 composite film electrode-based one (5.9-9.2 Wh/kg) was much higher than the HrGO film electrode-based supercapacitor, suggesting that the power density of supercapacitors can be highly improved by introducing pseudocapacitive PDAA into the

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graphene-based electrodes [24].

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The electrochemical cyclic stability of the PDAA@rGO/HrGO-4 composite film

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electrode was compared with the HrGO film electrode by the GCD technique for 10000 cycles at a current density of 5 A/g, as illustrated in Figure 5a. The

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PDAA@rGO/HrGO-4 composite film electrode showed excellent electrochemical

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cyclic stability with obious capacitance increasement in the first 2000 CV cycles to 107.2%, due to the activation of the inner electroactive materials, mainly the

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PDAA@rGO nanocomposite. While the specific capacitance of the HrGO film

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electrode increased in the first 500 cycles to 102.4%, exhibiting a faster activation due to the nano-holes. After 10000 cycles, the capacitance retention was 98% and 93% for

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the PDAA@rGO/HrGO-4 composite and HrGO film electrodes, respectively. The

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better cycling stability of the PDAA@rGO/HrGO-4 composite film electrode should be resulted from the rigid PDAA@rGO nanocomposite, which restricting the mechanical deformation of the film electrode undertaken in the charge discharge process [25].

3.4. Symmetric solid-state supercapacitor (SSCs) The symmetric solid-state supercapacitors (SSCs) were fabricated with two pieces of 16

ACCEPTED MANUSCRIPT the PDAA@rGO/HrGO-4 composite film electrodes with size of 1.01.0 cm2 with conductive carbon mechanical support, sandwiched with the cellulose separator (NKK TF4030), with PVA-H2SO4 gel electrolyte, to preliminarily evaluate their potential application. The CV and GCD curves of single and tandem supercapacitors (two and

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three devices connected in series) were shown in Figure 5 a and b. The output voltage

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was 0.8 V for the single device, while it increased to 1.6 V and 2.4 V for the two

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series and three series, respectively. In addition, the tandem supercapacitors showed nearly the same charge-discharge time as that of the single one, indicating that the

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performance of each supercapacitor in tandem supercapacitors was well kept as that in

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the single supercapacitors [26]. Furthermore, the tandem supercapacitors were assembled with three SSCs in series connection to power a red LED. As shown in

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Figure 5c, the red LED could be lighted by three SSCs in series. These results

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indicated potential application of the SSCs based on the proposed self-supporting flexible hierarchically porous PDAA@rGO/HrGO-4 composite electrode for high

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performance energy storage devices.

4. Conclusions

In summary, a facile approach was established to fabricate the self-supporting flexible PDAA@rGO based film electrodes for supercapacitors, by facile vacuum filtration of the aqueous dispersion of the well-defined PDAA@rGO nanocomposite with conductive HrGO nanosheets as binder. Owing to the hierarchically porous structure, the resultant PDAA@rGO/HrGO composite films possessed good comprehensive 17

ACCEPTED MANUSCRIPT electrochemical performance, with a high gravimetric and areal specific capacitance of 409 F/g and 644 mF/cm2 at 0.5 A/g, and a high capacitance retention of 98% after 10000 cycles at 5 A/g. Such features make it a promising self-supporting flexible

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electrode for high performance energy storage devices.

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References

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[1] L. Zhang, X.S. Hu, Z.P. Wang, F.C. Sun, D.G. Dorrell, A review of supercapacitor modeling, estimation, and applications: A control/management perspective,

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Renew. Sustainable Energy Rev. 81 (2018) 1868–1878.

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[2] G.P. Wang, L. Zhang, J.J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828.

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[3] A. Muzaffar, M.B. Ahamed, K. Meshmukh, J. Thirumalai, A review on recent

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advances in hybrid supercapacitors: Design, fabrication and applications, Renew. Sustainable Energy Rev. 101 (2019) 123–145.

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[4] G.P. Wang, L. Zhang, J.J. Zhang, Graphene for batteries, supercapacitors and

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beyond, Nat. Rev. Mater. 1 (2016) 16033. [5] S.I. Wong, J. Sunarso, B.T. Wong, H. Lin, A.M. Yu, B.H. Jia, Towards enhanced energy density of graphene-based supercapacitors: Current status, approaches, and future directions, J. Power Source 396 (2018) 182–206. [6] F. Torrisi, T. Carey, Graphene, related two-dimensional crystals and hybrid systems for printed and wearable electronics, Nanotoday 23 (2018) 76–97. [7] X.T. Guo, S.S. Zhang, G.X. Zhang, X. Xiao, X.R. Li, Y.X. Xu, H.G. Xue, H. Pang, 18

ACCEPTED MANUSCRIPT Nanostructured graphene-based materials for flexible energy storage, Energy Storage Mater. 9 (2017) 150–169. [8] J.L. Liu, P. Liu, Synthesis and electrochemical properties of various dimensional poly(1,5-diaminoanthraquinone) nanostructures: Nanoparticles, nanotubes and

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nanoribbons, J. Colloid Interface Sci. 542 (2019) 1–7.

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[9] J.L. Liu, P. Liu, Well-defined poly(1,5-diaminoanthraquinone)/reduced graphene

SC

oxide hybrids with superior electrochemical property for high performance electrochemical capacitors, J. Colloid Interface Sci. 542 (2019) 33–44.

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[10] X. Zhao, C.M. Hayner, M.C. Kung, H.H. Kung, Flexible holey graphene paper

Nano 5 (2011) 8739–8749.

MA

electrodes with enhanced rate capability for energy storage applications, ACS

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[11] H.J. Liu, W.L. Zhu, D.F. Long, J.L. Zhu, G. Pezzotti, Porous V2O5

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nanorods/reduced graphene oxide composites for high performance symmetric supercapacitors, Appl. Surf. Sci. 478 (2019) 383–392.

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[12] W.Q. Kong, X.D. Duan, Y.J. Ge, H.T. Liu, J.W. Hu, X.F. Duan, Holey graphene

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hydrogel with in-plane pores for high performance capacitive desalination, Nano Res. 9 (2016) 2458–2466. [13] E. Jokar, S. Shahrokhian, A. Irajizad, Electrochemical functionalization of graphene nanosheets with catechol derivatives as an effective method for preparation of highly performance supercapacitors, Electrochim. Acta 147 (2014) 136–142. [14] Q.H. Bai, Q.C. Xiong, C. Li, Y.H. Shen, H. Uyama, Hierarchical porous carbons 19

ACCEPTED MANUSCRIPT from a sodium alginate/bacterial cellulose composite for high-performance supercapacitor electrodes, Appl. Surf. Sci. 455 (2018) 795–807. [15] F. Dang, P.F. Yang, W. Zhao, J.Z. Liu, H.P. Wu, A.P. Liu, Y.L. Liu, Tuning capacitance of graphene films via a robust routine of adjusting their hierarchical

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structures, Electrochim. Acta 298 (2019) 254–264.

RI

[16] J.S. Sanchez, A. Pendashteh, J. Palma, M. Anderson, R. Marcilla, Synthesis and

SC

application of NiMnO3-rGO nanocomposites as electrode materials for hybrid energy storage devices, Appl. Surf. Sci. 460 (2018) 74–83.

NU

[17] J. Shen, P. Man, Q.H. Zhang, B. He, Z.Y. Zhou, C.W. Li, X.N. Wang, J.B. Guo,

MA

J.X. Zhao, L.Y. Xie, Q.W. Li, J. Sun, G. Hong, Y.G. Yao, Hierarchically-structured Co3O4 nanowire arrays grown on carbon nanotube fibers as novel cathodes for

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Sci. 447 (2018) 795–801.

D

high-performance wearable fiber-shaped asymmetric supercapacitors, Appl. Surf.

[18] Z.C. Yang, H.H. Zhang, B. Ma, L.Q. Xie, Y.T. Chen, Z.H. Yuan, K.L. Zhang, J.

CE

Wei, Facile synthesis of reduced graphene oxide/tungsten disulfide/tungsten oxide

AC

nanohybrids for high performance supercapacitor with excellent rate capability, Appl. Surf. Sci. 463 (2019) 150–158. [19] H.L. Wang, Y.Y. Liang, T. Mirfakhrai, Z. Chen, H.S. Casalongue, H.J. Dai, Advanced asymmetrical supercapacitors based on graphene hybrid materials, Nano Res. 4 (2011) 729–736. [20] Q.X. Xie, R.R. Bao, C. Xie, A.R. Zheng, S.H. Wu, Y.F. Zhang, R.W. Zhang, P. Zhao, Core-shell N-doped active carbon fiber@graphene composites for aqueous 20

ACCEPTED MANUSCRIPT symmetric supercapacitors with high-energy and high-power density, J. Power Sources 317 (2016) 133–142. [21] Y.M. Jia, L. Zhou, J.Z. Shao, Electrochemical performance of flexible graphene-based fibers as electrodes for wearable supercapacitors, Synth. Met. 246

PT

(2018) 108–114.

RI

[22] S.S. Balaji, M. Karnan, M. Sathish, Supercritical fluid processing of N-doped

Hydrogen Energy 43 (2018) 4044–4057.

SC

graphene and its application in high energy symmetric supercapacitor, Int. J.

NU

[23] E. Senthikumar, V. Sivasankar, B.R. Kohakade, K. Thileepkumar, M. Ramya,

MA

G.S. Sundari, S. Raghu, R.A. Kalaivani, Synthesis of nanoporous graphene and their electrochemical performance in a symmetric supercapacitor, Appl. Surf. Sci.

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460 (2018) 17–24.

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[24] G.S. Augusto, J. Scarminio, P.R.C. Silva, A. Siervo, C.S. Rout, F. Rouxinol, R.V. Gelamo, Flexible metal-free supercapacitors based on multilayer graphene

CE

electrodes, Electrochim. Acta 285 (2018) 241–253.

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[25] S. Bilal, M. Fahim, I. Firdous, A.A. Shah, Insight into capacitive performance of polyaniline/graphene oxide composites with ecofriendly binder, Appl. Surf. Sci. 435 (2018) 91–101. [26] Z.Q. Song, W.Y. Li, Y. Bao, Z.H. Sun, L.F. Gao, M.H. Nawaz, D.X. Han, L. Niu, A new route to tailor high mass loading all-solid-state supercapacitor with ultra-high volumetric energy density, Carbon 136 (2018) 46–53.

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Table 1. Effect of the feeding ratios of HGO and PDAA@GO on the capacitive

PDAA@GO

Thickness

Conductivity

Cm

Cs

(mg)

(mg)

(μm)

(S/cm)

(F/g)

(mF/cm2)

HrGO

20

0

8113

0.040

188

237

PDAA@rGO/HrGO-1

12

8

7312

0.035

341

430

PDAA@rGO/HrGO-2

10

10

7016

376

459

PDAA@rGO/HrGO-3

8

12

5416

0.030

405

514

PDAA@rGO/HrGO-4

10

15

6711

0.028

409

644

PDAA@rGO/HrGO-5

12

18

7012

0.025

389

733

0.032

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Samples

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HGO

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property of the PDAA@rGO/HrGO composite film electrodes.

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Table 2. Surface atomic concentrations of the modified graphene samples from the XPS survey. N%

O%

S%

Graphite

93.44

1.98

4.58

GO

65.78

2.45

28.71

HGO

65.91

2.53

PDAA@GO

74.94

8.07

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C%

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Samples

3.03

30.76

0.80

15.17

1.82

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ACCEPTED MANUSCRIPT Figure captions Scheme 1. Schematic illustration of the preparation of the hierarchically porous PDAA@rGO/HrGO composite films. Figure 1. TEM images of rGO (a), PDAA@rGO (b), HrGO (c) and HRTEM of

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HrGO (d); SEM surface and cross-section images of HrGO film (e and f),

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PDAA@rGO/HrGO-1 composite film (g and h), PDAA@rGO/HrGO-2 composite

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film (i and j), PDAA@rGO/HrGO-3 composite film (k and l),

PDAA@rGO/HrGO-4 composite film (m and n) and PDAA@rGO/HrGO-5

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composite film (o and p).

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Figure 2. XPS (a), FT-IR (b), XRD (c), and Raman (d) analysis of the modified graphene samples.

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Figure 3. (a) CV curves at a scanning rate of 5.0 mV/s and (b) GCD curves at a

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current density of 0.5 A/g of the composite film electrodes. Figure 4. (a) CV curves at a scanning rate of 5, 10, 20, 50 and 100 mV/s, (b) GCD

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curves at a current density of 0.5, 1, 2.5, 5 and 10 A/g of the

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PDAA@rGO/HrGO-4; (c) Specific capacitance of the HrGO and PDAA@rGO/HrGO-4 film electrodes at different current densities; (d) EIS curves of the film electrodes; (e) Ragone plots of the supercapacitors based on the HrGO and PDAA@rGO/HrGO-4 film electrodes; and (f) Cycle performance of the HrGO and PDAA@rGO/HrGO-4 electrodes at a current density of 5 A/g. Figure 5. (a) CV curves at a scanning rate of 20 mV/s, (b) GCD curves at a current density of 10 A/g for a single, 2-serise and 3-serise SSC of the 24

ACCEPTED MANUSCRIPT PDAA@rGO/HrGO-4, (c) digital photograph of a red LED powered by the

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3-series SSC.

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Scheme 1. Schematic illustration of the preparation of the hierarchically porous

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PDAA@rGO/HrGO composite films.

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Figure 1. TEM images of rGO (a), PDAA@rGO (b), HrGO (c) and HRTEM of HrGO (d); SEM surface and cross-section images of HrGO film (e and f), 28

ACCEPTED MANUSCRIPT PDAA@rGO/HrGO-1 composite film (g and h), PDAA@rGO/HrGO-2 composite film (i and j), PDAA@rGO/HrGO-3 composite film (k and l), PDAA@rGO/HrGO-4

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composite film (m and n) and PDAA@rGO/HrGO-5 composite film (o and p).

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b PDAA@GO

Intensity (a.u.)

N1s

O1s

rGO

Transmittance (a.u.)

C1s

HGO

GO

PDAA

PDAA@rGO HrGO PDAA@rGO/HrGO

Graphite

200

400

600

4000

800

3000

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a

2000

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c Intensity (a.u.)

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Intensity (a.u.)

HrGO PDAA@rGO rGO

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PDAA DAA

40

60

80

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2 θ ( degree )

ID/IG=1.47

PDAA@rGO/HrGO

PDAA@rGO/HrGO

20

1000

Wavenumber (cm-1 )

Binding Energy (eV)

ID/IG=1.28

HrGO

ID/IG=1.12

PDAA@rGO

ID/IG=1.05 rGO

500

1000

1500

2000

Raman shift (cm-1)

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Figure 2. XPS (a), FT-IR (b), XRD (c), and Raman (d) analysis of the modified

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graphene samples.

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3

b

0.8

1

0

HrGO PDAA@rGO/HrGO-1 PDAA@rGO/HrGO-2 PDAA@rGO/HrGO-3 PDAA@rGO/HrGO-4 PDAA@rGO/HrGO-5

0.6 0.4 0.2

-1

0.0 0.2

0.4 Potential (V)

0.6

0

0.8

100

200 Time (s)

300

RI

0.0

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Current density (A/g)

2

HrGO PDAA@rGO/HrGO-1 PDAA@rGO/HrGO-2 PDAA@rGO/HrGO-3 PDAA@rGO/HrGO-4 PDAA@rGO/HrGO-5

Potential (V)

a

400

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Figure 3. (a) CV curves at a scanning rate of 5.0 mV/s and (b) GCD curves at a

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current density of 0.5 A/g of the composite film electrodes.

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20

b

0.8

0.5 A/g 1 A/g 2.5 A/g 5 A/g 10 A/g

10

Potential (V)

Current density (A/g)

a

0 5 mV/s 10 mV/s 20 mV/s 50 mV/s 100 mV/s

-10

0.6

0.4

0.2

-20

0.0

0.0

0.2

0.4

0.6

0.8

0

100

200

Potential (V)

Specific capacitance (F/g)

500

-Z"(ohm)

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40

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HrGO PDAA@rGO/HrGO-4

4

20

100

d

HrGO PDAA@rGO/HrGO-1 PDAA@rGO/HrGO-2 PDAA@rGO/HrGO-3 PDAA@rGO/HrGO-4 PDAA@rGO/HrGO-5

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60 300

2

0

0

2

4

0

0

e

Capacitance retention (%)

HrGO

0

10

D

103

4 6 8 Current density (A/g)

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104

2

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0

PDAA@rGO/HrGO-4

102

101

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Power density (W kg-1)

400

RI

80

c

400

200

300 Time (s)

101

20

40

60

80

Z'(ohm)

120

f

100 PDAA@rGO/ HrGO-4 HrGO

80 60 40 20 0

102

0

2000

Energy density (Wh kg-1)

4000

6000

8000

10000

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Cycle number

Figure 4. (a) CV curves at a scanning rate of 5, 10, 20, 50 and 100 mV/s, (b) GCD curves at a current density of 0.5, 1, 2.5, 5 and 10 A/g of the PDAA@rGO/HrGO-4; (c) Specific capacitance of the HrGO and PDAA@rGO/HrGO-4 film electrodes at different current densities; (d) EIS curves of the film electrodes; (e) Ragone plots of the supercapacitors based on the HrGO and PDAA@rGO/HrGO-4 film electrodes; and (f) Cycle performance of the HrGO and PDAA@rGO/HrGO-4 electrodes at a current density of 5 A/g. 32

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a

b

2.5 Single 2 Series 3 Series

Potential (V)

2

0

-2

Single 2-Series 3-Series

2.0 1.5 1.0 0.5 0.0

-4 0.5

1.0 1.5 Potential (V)

2.0

0

2.5

100

200

300

400

500

Time (s)

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0.0

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Current density (A/g)

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Figure 5. (a) CV curves at a scanning rate of 20 mV/s, (b) GCD curves at a current

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density of 10 A/g for a single, 2-series and 3-series SSC of the PDAA@rGO/HrGO-4,

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(c) digital photograph of a red LED powered by the 3-series SSC.

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100 80

PT

60

RI

40 20 0 0

2000

4000

6000

SC

Capacitance retention (%)

Graphical Abstract

8000

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Cycle number

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10000

ACCEPTED MANUSCRIPT Highlights 1. Hierarchically porous PDAA@rGO/HrGO film was designed as flexible electrode. 2. It showed gravimetric and areal specific capacitance 409 F/g and 644 mF/cm2.

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3. It possessed capacitance retention of 98% after 10000 cycles.

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