CFC Electrodes

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High-Performance LBL Self-Assembly PANI/GQD-rGO/CFC Electrodes for Flexible Solid-State Supercapacitor by a Facile Spraying Technique Su-Min Wang, Jingwen Shen, Qiguan Wang, Yaru Fan, Lu Li, Kai Zhang, Lei Yang, Wenzhi Zhang, and Xinhai Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01631 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Applied Energy Materials

High-Performance LBL Self-Assembly PANI/GQD-rGO/CFC Electrodes for Flexible Solid-State Supercapacitor by a Facile Spraying Technique Sumin Wang†, Jingwen Shen†, Qiguan Wang†, Yaru Fan†, Lu Li†, Kai Zhang†, Lei Yang†, Wenzhi Zhang†, Xinhai Wang‡ †Shaanxi Key Laboratory of Photoelectric Functional Materials and Devices, School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an 710032, China ‡ School of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China KEYWORDS: Flexible supercapacitor, GQD-rGO, Polyaniline, LBL self-assembly, Spraying technique ABSTRACT: A flexible solid-state supercapacitor based on carbon fibre cloth is constructed, on which the graphene quantum dot-reduced graphene oxide solution and the aqueous polyaniline solution are alternatively sprayed. An acidic gel is used both as the electrolyte and the binder, by which two plates of electrodes are glued together, forming a symmetrical capacitor. The graphene quantum dot-reduced graphene oxide modifies the hydrophobicity nature of carbon fibre cloth, which enhances the interactions between carbon fibre cloth and polyaniline. Additionally, under the strong electrostatic forces between the electropositive polyaniline and the electronegative graphene quantum dot-reduced graphene oxide, a self-assembled three dimensional fiber-like network on carbon fibre cloth is observed. The fabricated solid-state supercapacitor shows a maximum capacitance of 82.9 mF cm–2 (1036 F g–1) at 0.1 mA cm–2 current density referred to the active materials on the electrodes, with high stability of 97.7% retention after 10000 charge and discharge cycles, because of the strong interaction between polyaniline and the modified carbon fibre cloth. Moreover, the capacitance is unchanged upon bending. This facile layer-by-layer self-assembly spraying technique shows great advantage of easy operation, good controllability and versatility, which possesses universality for the fabrication of foldable and wearable photoelectric devices.

ACS Paragon Plus Environment

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1. Introduction Nowadays, the electrochemical capacitors (ECs) of so-called supercapacitors have been considered as one category of promising energy sources, owing to high specific capacitance, rapid charge–discharge processes, low maintenance cost, and long period of cycling life.1–4 Moreover, in order to meet the increasingly demanded market in portable and foldable electric equipment, development of deformable and flexible EC devices has been attracting more and more attention.5–8 In regard to the fabrication of a flexible EC device, attachment of pseudocapacitive materials such as conductive polymers (CPs) or transition metal oxides on the freestanding carbonous materials with electrical double-layer capacitance (EDLC) has attracted much research interest.9–14 Based on this hybridized route, the synergistic electrochemical effect from high power density and good cycling stability of carbon and the high energy density of pseudocapacitive materials achieves. At present, the commercial carbon fibre cloth (CFC) has been a promising material for fabrication of simplified, lightweight and flexible ECs,15–21 based on the realization of large scale industrial production by spinning and weaving of carbon fibers. Also, the CFCs show notable mechanical features, compared to the CNT and graphene based paper. By using an anodic electrodeposition technique, Chen et al.22 prepared MnO2 nanosheet/CFC as flexible EC electrodes, in which MnO2 nanosheets were attached on porous CFC. The flexible electrodes displayed large capacitance value of 425 F g–1 and good cycling stability. However, the poor electrical conductivity of transition metal oxides limits the electron transfer and ion diffusion, which hinders the enhancement of electrochemical properties. Compared to transition metal oxides, the conductive polymers such as polyaniline (PANI), polypyrrole (PPy) and polythiophene (PTh) showing relatively high conductivity possess advantages of higher specific capacitance and energy density.23 Thus, to develop flexible and deformable energy devices, many CP/CFC based supercapacitors have been fabricated. For the preparation of CP/CFC composites, an in–situ route is commonly used, in which the aniline monomer is polymerized on CFC surface by chemical or electro-induced method.24,25 For example, by using an in–situ chemical oxidative polymerization, the PANI deposited on carbon


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nanofiber paper prepared by Yan et al.26 showed specific capacitance (SC) value of 638 F g–1 at 2 A g–1. Meng et al.27 used electro-polymerization method to obtain a flexible electrode composed of bearded CFC decorated with pinhole PANI. The PANI/CFC showed a higher SC value of 808 F g–1, and the optimal flexible electrodes achieved a high area-normalized capacitance of 3.3 F cm–2. In measurements, the CFC/PANI-based flexible electrodes can remain above 90% of the initial SC value after 1000 charging and discharging cycles. However, they face risks of significant SC decline in the applications requiring much longer cycles, due to the possible exfoliation of PANI from CFC, resulted from the weak interaction between CFC and PANI. As is known, the CFC surface is high hydrophobic. Therefore, during the PANI/CFC synthesis, the hydrophilic aniline cation is easy to be self-clustered on hydrophobic CFC. This not only leads to difficulty in control of PANI microstructure, but results in weak interactions between CFC and PANI. It may easily induce the exfoliation of PANI from CFC and affect the cycling stability in practical applications. In order to enhance the interactions between carbon nanofiber (CNF) and PANI, Xu et al.28 firstly employed Friedel–Crafts acylation reaction to prepare carboxyl functionalized CNFs. Then PANI was grafted onto the modified carbons covalently linked by octa-aminophenylsilsesquioxane by an in-situ polymerization method. After vacuum filtration, the obtained flexible CNF-PANI composite electrodes showed higher SC value (167 F g–1) than the pure CNFs (2.5 F g–1). Although the strong interactions between CNF and PANI can be realized by means of such a grafting route, but at the cost of damaging skeleton structure and electric conductivity of CNF. Moreover, the insulated grafted group of aminophenylsilsesquioxane, which is a non-carbon unit, also affects the conductivity and limits potential applications of the composite. Therefore, how to enhance carbon/PANI interactions without influencing the intrinsic structures of carbon is still a challenge. From the viewpoint of surface property, modification of CFC surface from hydrophobic to hydrophilic may be a feasible route to enhance the interaction between CFC and PANI. Considering supramolecular self-assembly technique is an effective method to modify the physical and chemical properties of a solid



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surface by growing one monolayer of functional molecules,29,30 herein, a surface modification route by self-assembly was proposed. As is known, the reduced graphene oxide (rGO) has generally been used as the additive to enhance the electrical properties of matrix.3133 Therefore, to avoid the conductivity decrease of CFC, the self-assembly modifier of rGO with allotropic carboneous structures, was chosen. However, in preparation of aqueous rGO solution, the insulated surfactants or dispersants are usually used.34,35 Obviously, the presence of surfactant can decrease the electrical conductivity, which is disadvantageous for the electrochemical property. Therefore, in this work, we find a new allotropic carboneous surfactant of graphene quantum dot (GQD) for rGO. Because the carboxyl groups on GQD make the large carbon plane show good amphiphilicity, thus, the homogeneously dispersed solution of rGO was prepared by using GQD as the surfactant. Afterwards, the obtained GQD-rGO solution was coated and self-assembled on the CFC surface through a solution spraying method. The ionic groups on GQD endow CFC good hydrophilicity, and rGO ensures CFC high level of electric conductivity. According to this spraying route, one different layer of conductive PANI was subsequently coated and self-assembled on the GQD-rGO modified CFC. Furthermore, by repetition of the self-assembly spraying processes alternatively, electrostatic interacted multilayers of PANI/GQD-rGO on CFC were obtained. This layer-by-layer (LBL) spraying method effectively modified CFC surface from hydrophobic to hydrophilic and enhanced the interaction between PANI and CFC without damaging the skeleton structure of CFC. Thus, a high stable flexible solid-state CFC capacitor with high performance was constructed by using the H2SO4/PVA gel as both the electrolyte and the binder (as illustrated in Figure 1).


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Figure 1. Schematic illustration of PANI/GQD-rGO/CFC preparation by the LBL self-assembly spraying technique. Inset: Electrostatic interaction between GQO-rGO and PANI within the LBL film. 2. Experimental section 2.1 Materials All chemicals, unless stated otherwise, were purchased from Aladdin Chemical Co., China and used without further purification. Aniline was used after distilled. Carbon fibre cloth was purchased from Changzhou Best Photoelectric Technology Co., China. The dialysis bag with molecular weight cutoff of 3500 Da was obtained from Beijing RuiDaHengHui Science  Technology Development Co, LTD, China. 2.2 Preparation of graphene Graphite oxide (GO) was prepared by a method modified from Hummer’s report.36 Firstly 2 gram of natural flake graphite was dispersed in the mixture composed of 45 ml of H2SO4 and 5 ml of H3PO4 under sonication in an ice bath, during which 2 gram KMnO4 was slowly added. Afterwards, the mixture was heated to 40 oC and stirred for 1 h. Then, 100 ml of high-purity water was added. After stirred for 3 h, the acidic mixture was moved to an oil bath and heated at 85 oC for 1 h, and 4 ml of H2O2 was then added. The formed graphite oxide was immersed in the 1.0 M HCl solution for 24 h and filtered. After thoroughly washing with deionized water, the powdered graphite oxide was obtained by freeze drying. Upon heating at 600 oC in nitrogen flow, the graphite oxide was thermally reduced to graphene.



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2.3 Synthesis of graphene quantum dot Graphene quantum dots were synthesized from the method of carbon fiber exfoliation by mixed acids.37 Briefly, 0.1 gram of carbon fibers were added to the mixture of 60 ml of concentrated H2SO4 and 20 ml of concentrated HNO3, and stirred for 24 h at 100 oC. Afterwards, the mixture solution was slowly poured into 800 ml of water and then was neutralized by aqueous sodium hydroxide solution (1.0 M). After thorough dialysis in a dialysis bag with molecular weight cutoff of 3500 Da to remove small-sized ions, the GQD powders were obtained by a distillation method. 2.4 Preparation of self-assembly spraying solution of GQD-rGO Generally rGO alone cannot form stable dispersion in water because of the hydrophobic nature. Thus the amphiphilic GQD was used as a surfactant to assist rGO dispersed in water by the strong π–π interactions between them. The self-assembly spraying solution of GQD-rGO was prepared by addition of 15 mg of rGO and 15 mg of GQD in aqueous diluted HCl solution (15 ml, pH=3.0). After sonication, a significantly stable dispersion was obtained. 2.5 Polyaniline synthesis Polyaniline was synthesized by adding 5.0 gram of ammonium persulfate to 30 ml of aqueous HCl solution (1.0 M) containing 2.0 gram of aniline monomer.38 The mixture was reacted at 0–5 oC for 24 h. Then the product was filtered and washed by diluted aqueous HCl solution, ethanol and water. After immersed in ammonia solution for 10 h, the obtained emeradine was dried under vacuum at 50 oC. Measured by the gel permeation chromatography (GPC) method, the number average molecular weight (Mn), weight average molecular weight (Mw) and polydispersity index (PDI) of the synthesized PANI are: 22788, 56238 and 2.47, respectively (Figure S1). 2.6 Preparation of self-assembly spraying solution of conducting polyaniline By adding 100 mg of emeradine polyaniline in 5.0 ml of N,N-dimethyl acetamide, a blue homogeneous solution was obtained after stirred for 2 h. Subsequently, the aqueous conducting polyaniline solution can be gained by adding above blue solution in 50 ml of diluted aqueous HCl solution with pH=3.0, which


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was stored for LBL self-assembly spraying. 2.7 Procedure of LBL spraying film The CFC used as the flexible conductive substrate was cut into slices with dimension of 1.0×5.0 cm, and then were cleaned in ethanol, acetone and water and dried under vacuum at 50 oC prior to use. Multilayer films of PANI/GQD-rGO were coated on CFC by using a LBL-spraying technique. Typically, a slice of CFC was firstly fixed on a stainless steel mesh by a clip. The stainless steel mesh was then hung between two upright rods by clamps to allow airflow through the CFC during spray deposition. All solutions were atomized into spray and manually delivered by a commercial pressurized spray can. A single layer of GQD-rGO/PANI was deposited on the CFC according to the following steps. Self-assembly GQD-rGO solution was sprayed toward CFC for 6 times (approximate 0.3 ml per spray) and drained for 15 s. Subsequently, the CFC was rinsed by spraying 2 ml of water and drained for 15 s. Then the GQD-rGO/CFC film was dried by nitrogen flow. This half cycle was repeated for aqueous conducting PANI solution to complete one bilayer of GQD-rGO/PANI film, which was defined as (PANI/GQD-rGO)1/CFC. The cycle number of n was adjusted to result in a target number for (PANI/GQD-rGO)n/CFC. To find the slight weight change induced by one bilayer of PANI/GQD-rGO, three pieces of (PANI/GQD-rGO)25/CFC were weighed to calculate the averaged value. After sprayed by 25 layers, the average weight increase of CFC is found to be 1.0 mg. Therefore, one bilayer of active material loading of PANI/GQD-rGO is about 8.0×10–3 mg cm–2 on CFC. 2.8 Preparation of solid polymeric ionic gel The solid polymeric ionic gel was prepared by a casting method. To a flask containing 24 ml of water, 1.6 gram of polyvinyl alcohol (PVA) was added. After stirred for 1 h at 85 oC, the PVA was dissolved to create aqueous PVA solution. Addition of 5.6 ml H2SO4 solution (2.0 M) to the PVA solution and well agitated at 85 oC for 0.5 h. Afterwards the system was cooled to room temperature and 75 μl of glutaraldehyde solution (25 % weight ratio) was added. The conductivity of the H2SO4/PVA gel is 1.2 S cm–1 determined from the complex impedance measurements.39



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2.9 Fabrication of flexible solid (PANI/GQD-rGO)n/CFC supercapacitor The resulting polymeric ionic gel was coated on the surface of (PANI/GQD-rGO)n/CFC (mass loading of 0.16 mg cm–2) by using a glass bar. Then, two (PANI/GQD-rGO)n/CFC slices were glued using the PVA gel as the binder in a parallel way. Thus a symmetrical flexible solid supercapacitor sandwiched by polymeric ionic gel achieved, in which the contact area between the two (PANI/GQD-rGO)n/CFC slices is 0.5×1.0 cm–2. 2.10 Characterization The surface chemistry of (PANI/GQD-rGO)n/CFC was measured by a Kratos AXIS X-ray photoelectron

spectrometer

(XPS).

For

UV−visible

spectra

analysis,

the

samples

of

(PANI/GQD-rGO)n/CFC were firstly cut into pieces and grinded to powders by a ball mill, which were then dispersed in N,N-dimethyl formamide (DMF) and measured on a Shimadzu 1901 spectrophotometer. The molecular masses of PANI are measured by using a GPC system with a Waters 610 fluid pump, Waters 2410 RI detector, set of columns PSS GRAM and polystyrene standards. Sheet resistance was detected at room temperature using a four-point probe system. Five measurements were performed and averaged on each sample to give the final data with the standard deviation. Morphologies of the surfaces of LBL (PANI/GQD-rGO)n/CFC were examined using a Hitachi S–4800 scanning electron microscopy (SEM) operating at 3 and 5 kV. By sessile drop method, contact angle mearsurements were performed on an optical contact angle machine (OCA15Pro, DataPhysics, Filderstadt, Germany). 2.11 Electrochemistry measurements The cyclic voltammetry (CV) characterization of LBL (PANI/GQD-rGO)n/CFC films was performed on a CHI 660 electrochemical system in a conventional three‒electrode cell, using a platinum foil and an Ag/AgCl

electrode

as

the

counter

electrode

and

reference

electrode,

respectively.

The

(PANI/GQD-rGO)n/CFC film was used as working electrode, the H2SO4 solution (1.0 M) as the electrolyte. The CV curves were obtained at the scan rate of 10~500 mV s‒1. Capacitance measurement of the flexible solid LBL (PANI/GQD-rGO)n/CFC supercapacitor was performed on a Neware CT3008W


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battery test machine. The measured current density 0.1~0.5 (mA cm‒2) and the calculated capacitance (mF cm‒2, F g‒1) are all referred to the active materials of PANI/GQD-rGO loaded on the contact surface between the two CFC slices. 3. Results and discussion Figure 2a and 2b show liquid shapes of water on the pure CFC and GQD-rGO/CFC surface, measured by the sessile drop method. The liquid on original CFC surface exhibited spherical shape with static contact angle of 135.6o (Figure 2a), while the liquid on GQD-rGO/CFC samples illustrated spherical shape with contact angle of 61.3o (Figure 2b). This means that the hydrophobic surface of CFC was changed hydrophilic after spraying one layer of GQD-rGO, due to the presence of the carboxyl groups on GQD (inset of Figure 1). The hydrophobicity-hydrophilicity transition on CFC surface can greatly enhance the adsorption strength of CFC for polar molecules, and the ionic groups on GQD support the channels to construct strong interaction with other charged molecules.

Figure 2. Contact angle measurement results of water droplet on bare CFC (a) and GQD-rGO modified CFC surface (b) measured by a digital microscope, wide scan XPS spectrum (c) and the deconvoluted N 1s spectrum (d) of (PANI/GQD-rGO)1/CFC, UVvisible spectra (e) of CFC, PANI/CFC, GQD-rGO/CFC and (PANI/GQD-rGO)20/CFC, electrical conductivity (f) of pure CFC, PANI/CFC, GQD-rGO/CFC and (PANI/GQD-rGO)n/CFC with different layers of PANI/GQD-rGO.



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After sprayed by one layer of conductive PANI, the self-assembled hybrid film of (PANI/GQD-rGO)1/CFC was formed linked by the electrostatic forces between the electropositive PANI and the electronegative GQD-rGO (as shown in inset of Figure 1). The wide scan XPS spectrum of (PANI/GQD-rGO)1/CFC in Figure 2c showed the successful anchoring of PANI/GQD-rGO, discerned from the C 1s, N 1s and O 1s XPS signals. In addition, the deconvoluted N 1s spectrum in Figure 2d showed the presence of ionic nitrogen. The N 1s XPS spectrum was divided into three peaks located at 398.6 eV, 399.5 eV and 400.5 eV, corresponding to imine, amine and radical cation nitrogen, respectively.40 The presence of radical cation nitrogen ensured the electrostatic linkage between the electropositive PANI and the electronegative GQD-rGO. Figure 2e demonstrates the UV–vis absorption spectra of bare CFC, GQD-rGO/CFC, (PANI/GQD-rGO)20/CFC and PANI/CFC. It is shown that no distinct features were found on the bare CFC (blue curve in Figure 2e), and a characteristic peak at 345 nm was observed for GQD-rGO/CFC (black curve in Figure 2e) due to the presence of carbonyl groups on GQD. The UV–vis absorption spectrum of PANI/CFC (dark cyan curve in Figure 2e) showed typical features of PANI emeraldine salt located around 328 nm (π–π* transition) and a polaron band 619 nm (π–polaron transition). However, in the case of (PANI/GQD-rGO)20/CFC (red curve in Figure 2e), the characteristic absorption peak of PANI at 619 nm was red shifted for 12 nm to 631 nm. This displayed the π–conjugated electrons on PANI backbone were increased under the electrostatic interactions between PANI and GQD within PANI/GQD-rGO/CFC (inset of Figure 1).41 Similar red-shift is also found for the π–π* transition absorption peak at 348 nm for (PANI/GQD-rGO)20/CFC, compared to PANI/CFC at 328 nm.


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Figure 3. (a) Galvanostatic charge-discharge plots of (PANI/GQD-rGO)n/CFC with different layers of PANI/GQD-rGO at a current density of 0.1 mA cm–2, (b) Comparative capacitance of (PANI/GQD-rGO)n/CFC with different layers of PANI/GQD-rGO at different current density, (c) Ragone plot of (PANI/GQD-rGO)20/CFC at different current density, (d) Galvanostatic charging-discharging plots of (PANI/GQD-rGO)20/CFC upon bending, (e) Curves of relationship between capacitance retention and charge/discharge cycle number of (PANI/GQD-rGO)n/CFC at 0.1 mA cm–2 current density, (f) Curves of relationship

between

capacitance

retention

and

charge/discharge

cycle

number

of

(PANI/GQD-rGO)20/CFC soaked in water at 0.1 mA cm–2 current density. The electrochemical behavior of the flexible (PANI/GQD-rGO)n/CFC electrodes was evaluated by galvanostatic charging/discharging measurements on a two-electrode symmetrical supercapacitor using solid H2SO4/PVA gel as the binder and electrolyte at 0.1 mA cm–2 current density. The obtained results are depicted by the curves in Figure 3. Based on the discharging time, three kinds of specific capacitance values for the (PANI/GQD-rGO)n/CFC can be calculated, referred to the effective area, the mass of active materials and the total mass of the whole device including active material, CFC and all layers of gel/electrolyte, which are abbreviated as Cea, Cma and Ctm, respectively. From Figure 3a, the specific capacitance value of the original CFC was estimated to be only 3.7 mF cm–2, due to the EDLC nature of carbon fibers. Surprisingly, it was increased to 20 mF cm–2 (Cma=1000 F g–1 and Ctm=95.2 F g–1) for the



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sample of CFC self-assembled by five layers of PANI/GQD-rGO, much higher than that of pure CFC. This is mainly resulted from the large pseduocapacitance of PANI. In addition, the attachment of GQD-rGO on CFC surface enhances the surface area and facilitates ion diffusion within the electrode materials,42,43 which also leads to the capacitance increase. On the other hand, with the increase of the sprayed layer numbers, the SC value of the flexible electrodes was gradually increased. It was increased from 20 mF cm–2 to 34.3 mF cm–2 (Cma=857.5 F g–1 and Ctm=137.2 F g–1) with altering sprayed layer number from 5 to 10. In fact, as the layer numbers increase, the electrostatic interactions between PANI and GQD can be accordingly enhanced, which makes the self-assemblied PANI/GQD-rGO film correspondingly change denser and tighter on CFC, to form a three-dimensional (3D) conductive network. Therefore, such LBL self-assembly structures showed superiority of allowing rapid and significant uptake of electrolyte ions, which facilitates quick insertion and de-intercalation of active species, and provides abundant pathways for charge transportation. For (PANI/GQD-rGO)20/CFC, the SC value reaches as high as 82.9 mF cm–2 (Cma=1036 F g–1 and Ctm=251.2 F g–1). It is rather higher than the state of the art PANI/CFC (808 F g–1)27 prepared by electro-polymerization method. However, as the self-assembly layer numbers further increase, such as the (PANI/GQD-rGO)25/CFC, the SC value was surprisingly decreased to 65.4 mF cm–2 (Cma=654 F g–1 and Ctm=176.8 F g–1) (Figure 3a). This is probably because the loaded multilayer of PANI/GQD-rGO on CFC in this case is too dense to allow the electrolyte well penetrate across the interface between PANI/GQD-rGO and CFC. Similarly, the columbic efficiency of (PANI/GQD-rGO)20/CFC calculated from the charge and discharge time showed the

maximum

value

of

98%,

higher

than

that

of

(PANI/GQD-rGO)5/CFC

(89%),

(PANI/GQD-rGO)10/CFC (91%) and (PANI/GQD-rGO)25/CFC (95%), probably because of the easy ion diffusion and fast electron transportation in the conductive networks of (PANI/GQD-rGO)20/CFC.


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Figure


4.

SEM

images

of

CFC

(a),

GQD-rGO/CFC

(b),

(PANI/GQD-rGO)5/CFC

(c),

(PANI/GQD-rGO)20/CFC (d), (PANI/GQD-rGO)25/CFC (f), (e) and (g) show the high-magnification images of (d) and (f), respectively. Cross section images of carbon fibers of CFC (h) and (PANI/GQD-rGO)20/CFC (i).



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Figure 4 illustrates the morphological changes of CFC surface during the LBL self-assembly process observed by SEM images. As shown by Figure 4a, the commercial CFC shows a smooth and clean surface with carbon fiber diameter of around 7~10 μm (Figure 4h). After self-assembled by one bilayer of GQD-rGO/PANI, the CFC surface changed coarser, wrapped by plate-like sheets of rGO and random particles of PANI (see the arrows in Figure 4b). As the self-assembly layers number increases, a dense film was formed under the electrostatic interactions between PANI and GQD-rGO, such as the (PANI/GQD-rGO)10/CFC shown in Figure 4c. With the further increase of layer number and mass loading of PANI/GQD-rGO on CFC, such as the sample of (PANI/GQD-rGO)20/CFC (Figure 4d), the random PANI and layered GQD-rGO were self-assembled to be thin fibers and interconnected to give a 3D network on CFC, among which masses of micropores were found (see the arrows in Figure 4e). From the cross section image shown in Figure 4i, the thickness of (PANI/GQD-rGO)20 film is around 2.5 μm. It is noted that the specific porous structure of (PANI/GQD-rGO)20/CFC can seriously increase the surface area, which is beneficial for the uptake and penetration of electrolyte ion. Therefore, the (PANI/GQD-rGO)20/CFC electrodes showed the maximum SC value, as illustrated in Figure 3a. Unfortunately, the loose porous structure disappeared as further

attachment

of

PANI/GQD-rGO

layer

on

CFC.

For

the

samples

of

(PANI/GQD-rGO)25/CFC, the SEM image (Figure 4f) showed that the self-assembled PANI/GQD-rGO layers changed exceedingly dense (Figure 4g), under the strong electrostatic interactions between PANI and GQD-rGO. Therefore, the penetration of electrolyte ions in (PANI/GQD-rGO)25/CFC was seriously hindered, which gave a lower capacitance than that of (PANI/GQD-rGO)20/CFC electrodes. Additionally, the interconnected 3D fiber networks in (PANI/GQD-rGO)20/CFC lead to synergistic effect on the electrical conductivity. From Figure 2f, the electrical conductivity of (PANI/GQD-rGO)20/CFC is 40.5 S cm−1, higher than that of GQD-rGO/CFC (7.8 S cm−1), PANI/CFC (24.9 S cm−1) and pure CFC (20.1 S cm−1). The abundant mesopores support (PANI/GQD-rGO)20/CFC samples excellent rate capability. ACS Paragon Plus Environment

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As shown in Figure 3b, (PANI/GQD-rGO)20/CFC samples maintain 89.7% of the initial capacitance with growing current densities from 0.1 to 0.5 mA cm−2, higher than that for (PANI/GQD-rGO)5/CFC (54.5%), (PANI/GQD-rGO)10/CFC (77%), (PANI/GQD-rGO)15/CFC (66.1%) and (PANI/GQD-rGO)25/CFC (26.5%). Figure 3c depicts the Ragone plot of (PANI/GQD-rGO)20/CFC from energy density as a function of power density.44 With increase in discharge current density from 0.1 mA cm–2 to 0.5 mA cm–2, the specific energy density decreased from 34.2 Wh kg–1 (5.610–3 Wh cm–2) to 30.6 Wh kg–1 (5.110–3 Wh cm–2) and the specific power density increased from 424.4 W kg–1 (69.510–3 W cm–2) to 2.1 kW kg–1 (3.510–1 W cm–2), respectively. Thus, an excellent energy density (30.6 Wh kg–1) and corresponding power density (2118.5 W kg–1) achieves. The feasibility of the layered flexible (PANI/GQD-rGO)20/CFC was tested by bending at different angles. As shown in Figure 3d, the flexible capacitor of (PANI/GQD-rGO)20/CFC showed nearly unchanged SC value, bended at 60o (Cea=82.1 mF cm–2, Cma=1026 F g–1 and Ctm=248.8 F g–1) and 90o (Cea=79.2 mF cm–2, Cma=990 F g–1 and Ctm=240 F g–1), compared with that of original value (Cea=82.9 mF cm–2, Cma=1036 F g–1 and Ctm=251.2 F g–1). Meanwhile, the energy supply value of (PANI/GQD-rGO)20/CFC referred to the mass of active materials during bending at 60o and 90o is 69.8 Wh kg–1 (or 17.0 Wh kg–1 per mass of the whole device) and 67.4 Wh kg–1 (or 16.4 Wh kg–1 per mass of the whole device), respectively ， compared with that of original value 70.5 Wh kg–1 (or 17.1 Wh kg–1 per mass of the whole device),

which

is

also

nearly

unchanged.

This

implies

the

layered

flexible

(PANI/GQD-rGO)20/CFC has the good structural stability, mainly owing to the specific self-assembled 3D structures from the strong electrostatic interactions between PANI layer and GQD-rGO layer. The structural stability endowed such LBL self-assembled (PANI/GQD-rGO)n/CFC electrodes high charge-discharge cycling performance. Figure 3e shows the SC variance of



Page 16 of 27

flexible solid-state (PANI/GQD-rGO)n/CFC capacitors in the process of charging and discharging cycles at 0.1 mA cm–2 current density. It can be seen the LBL self-assembled (PANI/GQD-rGO)n/CFC electrodes all showed high SC retention after 10000 cycles. This shows that the strong interactions between PANI and the GQD-rGO modified CFC can effectively hinder the exfoliation of PANI from CFC and benefit holding the structural skeleton. After 10000 cycles, the LBL self-assembled (PANI/GQD-rGO)20/CFC electrodes illustrated almost

unchanged

SC

(PANI/GQD-rGO)5/CFC

value

(97.7%

(78.9%),

of

the

original

value),

(PANI/GQD-rGO)10/CFC

compared (80%)

with and

(PANI/GQD-rGO)25/CFC electrodes (82.4%). This is because the abundant porous structures from the specific self-assembled 3D structures within (PANI/GQD-rGO)20/CFC allowed the significant volumetric change of active materials, and the embedded GQD-rGO layers effectively released the internal stress during charge and discharge cycles. Even so, the internal stress between different layers may still exist due to the interfacial interaction, which could be one reason for the slight capacitance decrease during charging/discharging process. On the other hand, (PANI/GQD-rGO)20/CFC also showed relatively strong abrasion resistance. Abrasion tests were carried out according to a reported method.45 The electrodes were placed onto 2000 grit sandpaper upside down. Then, a weight of 100 gram was applied to the electrodes. The electrodes were moved for 15 cm at a speed of 30 mm s–1. After this abrasion process, the capacitance of the (PANI/GQD-rGO)20/CFC showed slightly decrease from 82.9 mF cm–2 to 81.6 mF cm–2 (Figure S2). For evaluation of the application of (PANI/GQD-rGO)20/CFC

electrodes

in

the

wet

environment,

stability

of

the

(PANI/GQD-rGO)20/CFC soaked in water is examined at 0.1 mA cm–2 current density. As shown in Figure 3f, the capacitance value showed around 5% decay after 3800 cycles upon water soaking. However, a capacitance increase from 95% to 100% between 3800 and 5100 cycles was found. This is because the absorbed water in the sample can carry the electrolyte to the deep sites of the electrode, which activates those oxygen-containing groups and partial ACS Paragon Plus Environment

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PANI chains embedded within the conductive PANI/GQD-rGO networks. Over 5100 cycles, the capacitance value showed continuous decrease and the SC retention is ultimately 78.6% after 10000 cycles. To find their potential applications for flexible supercapacitors, four slides of all solid-state supercapacitor based on the (PANI/GQD-rGO)20/CFC were connected in series as a demonstration. A light-emitting diode (LED) with starting voltage of 2.5V can be lighted by using the (PANI/GQD-rGO)20/CFC as the power supply. The results demonstrate the superior potential of such a LBL self-assembly technique for fabricating flexible solid-state CFC-based capacitors with high mechanical and electrochemical properties.

Figure

5.

(a)

Cyclicvoltammogram

of

PANI/CFC,

GQD-rGO/CFC

and

(PANI/GQD-rGO)n/CFC in 1.0 M H2SO4 solution scanned at 50 mV s–1, (b) Enlarged drawing of cyclic voltammogram of PANI/CFC, GQD-rGO/CFC in 1.0 M sulfuric acid solution, (c) Capacitance plot of PANI/CFC, GQD-rGO/CFC and (PANI/GQD-rGO)n/CFC calculated from the CV data, (d–f) CV curves of (PANI/GQD-rGO)10/CFC, (PANI/GQD-rGO)20/CFC, (PANI/GQD-rGO)25/CFC respectively in 1.0 M H2SO4 solution at different scanning rates. Cyclic voltammetry data show the electrochemical performance of (PANI/GQD-rGO)n/CFC in 1.0 M H2SO4 solution. As seen from Figure 5a, each sample illustrated four characteristic ACS Paragon Plus Environment


Page 18 of 27

couples of redox peaks at 0.19/0.04, 0.45/0.41, 0.53/0.49 and 0.76/0.67 V. The first characteristic pair of peaks corresponds to the redox reaction of the fully reduced leucoemeraldine oxidation state, and the last pair of redox peaks is resulted from the fully oxidized pernigraniline oxidation state. The middle two couples of peaks observable at about 0.45 and 0.53 V are assignable to influence of different acidic dopants on emeraldine oxidation state,46 from the protons in sulfuric acid and the carboxyl groups on GQD. For the GQD-rGO/CFC sample, it showed no obvious features in the CV curve, compared with the PANI/CFC (Figure 5b). From the integrated area of the CV curve, the SC values of (PANI/GQD-rGO)n/CFC were calculated47 to be 20.7, 33.4, 50.1, 63.5, 83.9 and 66.5 mF cm–2 at the scan rate of 50 mV s–1, when the number of self-assembly layers is 1, 5, 10, 15, 20 and 25, respectively. From Figure 5c, it can be found that the capacitance of (PANI/GQD-rGO)n/CFC was gradually enhanced with the increase of self-assembly layer numbers. However, it is decreased for (PANI/GQD-rGO)25/CFC, due to the hindrance of electrolyte penetration by the exceedingly dense

surface,

similar

to

the

results

from

the

measurement

on

solid-state

(PANI/GQD-rGO)n/CFC shown in Figure 3. Moreover, it can be found from Figure 5d–f, the CV curves of (PANI/GQD-rGO)20/CFC electrodes showed best shape retention as the scan rate was increased from 10~500 mV s–1, compared with (PANI/GQD-rGO)10/CFC and (PANI/GQD-rGO)25/CFC. This also showed that the

(PANI/GQD-rGO)20/CFC

electrodes

possess

the

better

rate

capacity

than

(PANI/GQD-rGO)10/CFC and (PANI/GQD-rGO)25/CFC electrodes. This is because the porous conductive self-assembled 3D networks in (PANI/GQD-rGO)20/CFC support easy uptake of electrolyte and rapid transfer of electron, which ensured high rate performance. 4. Conclusion A flexible solid-state supercapacitor based on carbon fibre cloth (CFC) was fabricated by


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alternatively spraying the graphene quantum dot-reduced graphene oxide (GQD-rGO) solution and the aqueous solution of polyaniline (PANI) on CFC. The self-assembly layer of GQD-rGOs modified the hydrophobicity nature of CFC surface, and enhanced the interaction between CFC and PANI. Repetition of this facile environment-friendly spraying technique leads to the formation of multiple layers of PANI/GQD-rGO self-assembled on CFC. By using H2SO4/PVA gel as both the electrolyte and the binder, the prepared symmetrical PANI/GQD-rGO/CFC capacitor showed a maximum capacitive value of 82.9 mF cm–2 (1036 F g–1) referred to the active materials on the electrodes at 0.1 mA cm–2 current density. Due to the strong interactions between PANI and the modified CFC, the PANI/GQD-rGO/CFC capacitor shows high cycling stability, with 97.7% retention after 10000 charge and discharge cycles. In addition, this flexible PANI/GQD-rGO/CFC capacitor showed nearly unchanged capacitance under large angle bending condition. In comparison with other approaches, this layer-by-layer spraying technique possesses great advantage of easy operation, good controllability and versatility, as well as excellent universality for the fabrication of foldable and wearable photoelectric devices. AUTHOR INFORMATION Corresponding Author * Email: [email protected]. Phone: +86 29 86173324. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes



Page 20 of 27

Any additional relevant notes should be placed here. ACKNOWLEDGMENT S.M. acknowledges the National Natural Science Foundation of China (Grant No. 21772152, 21103133); Shaanxi Provincial Education Department Program (Grant No.18JK0384); the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. ASSOCIATED CONTENT Supporting Information available: GPC chromatographs for polyaniline samples; capacitance change of the (PANI/GQD-rGO)20/CFC electrode induced by abrasion.

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High-Performance LBL Self-Assembly PANI/GQD-rGO/CFC Electrodes for Flexible Solid-State Supercapacitor by a Facile Spraying Technique Sumin Wang, Jingwen Shen, Qiguan Wang, Yaru Fan, Lu Li, Kai Zhang, Lei Yang, Wenzhi Zhang and Xinhai Wang


CFC Electrodes

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