Conjunction of Conducting Polymer Nanostructures with Macroporous

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Conjunction of Conducting Polymer Nanostructures with Macroporous Structured Graphene Thin Films for High-performance Flexible Supercapacitors Mushtaque A. Memon, Wei Bai, Jinhua Sun, Muhammad Imran, Shah Nawaz Phulpoto, Shouke Yan, Yong Huang, and Jianxin Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01879 • Publication Date (Web): 25 Apr 2016 Downloaded from http://pubs.acs.org on May 1, 2016

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ACS Applied Materials & Interfaces

Conjunction of Conducting Polymer Nanostructures with Macroporous Structured Graphene Thin Films for High-performance Flexible Supercapacitors

Mushtaque A. Memon,†,‡ Wei Bai,† Jinhua Sun,† Muhammad Imran,† Shah Nawaz Phulpoto,†,‡ Shouke Yan,‡ Yong Huang,† Jianxin Geng†,*

†

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East

Road, Haidian District, Beijing 100190, China, E-mail: [email protected] ‡

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology,

15 Beisanhuan East Road, Chaoyang District, Beijing 100029, China

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Abstract: Fabrication of hybridized structures is an effective strategy to promote the performances of graphene-based composites for energy storage/conversion applications. In this work, macroporous structured graphene thin films (MGTFs) are fabricated on various substrates including flexible graphene papers (GPs) through an ice crystal-induced phase separation process. The MGTFs prepared on GPs (MGTF@GPs) are recognized with remarkable features such as interconnected macroporous configuration, sufficient exfoliation of the conductive RGO sheets, and good mechanical flexibility. As such, the flexible MGTF@GPs are demonstrated as a versatile conductive platform for depositing conducting polymers (CPs), e.g., polyaniline (PAn), polypyrrole, and polythiophene, through in situ electropolymerization. The contents of the CPs in the composite films are readily controlled by varying the electropolymerization time. Notably, electrodeposition of PAn leads to the formation of nanostructures of PAn nanofibers on the walls of the macroporous structured RGO framework (PAn@MGTF@GPs): thereafter, the PAn@MGTF@GPs display a unique structural feature that combine the nanostructures of PAn nanofibers and the macroporous structures of RGO sheets. Being used as binder-free electrodes for flexible supercapacitors, the PAn@MGTF@GPs exhibit excellent electrochemical performance, in particular a high areal specific capacity (538 mF cm−2), high cycling stability, and remarkable capacitive stability to deformation, due to the unique electrode structures.

Keywords: Macroporous structured graphene thin films, graphene papers, conducting polymers, electropolymerization, flexible supercapacitors

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Introduction Graphene, a one-atom-thick layer of sp2 carbon atoms, has appeared as the most promising carbon material in the last decades due to its exceptional physical and chemical properties.1 Assembly of graphene sheets into three dimensional (3D) architectures is of significant importance to carry out highorder applications. Graphene sheets tend to form irreversible agglomerates due to the strong π−π stacking and van der Waals interactions during their assembling process. Fabrication of graphene into porous structures is therefore an effective strategy to overcome the stacking behavior of graphene sheets and to obtain graphene materials with high surface areas.2,3 Due to the unique structures, 3D graphene architectures exhibit outstanding properties such as a large accessible surface area, excellent electrical conductivity, good flexibility as well as fast ion transport kinetics.4,5 The past few years have witnessed the boom in fabrication of various 3D graphene architectures. A number of synthesis methods to fabricate 3D graphene architectures based on the strategies of either direct growth from a carbon source or the assembly of graphene oxide (GO) have been developed. For instance, graphene networks were produced through chemical vapor deposition (CVD) using porous Ni as template.6−8 However, the CVD process requires high processing temperature and etching process. In addition to the CVD method, selfassembly of GO has been developed for construction of 3D graphene architectures with advantages such as low cost, high yield, easy scalability, and adjustability. Through using GO, a variety of approaches, including chemical reduction,9 electrochemical reduction,10,11 hydrothermal processes,12,13 and various assembly methods,14−18 have been developed to fabricate 3D graphene architectures. Among these reports, chemical or physical cross-linkers, such as organic binders,19 DNA molecules,20 ion linkages,21 and ion coordination,3 are used to prepare 3D graphene architectures. However, 3D graphene architectures as flexible thin films, which can be directly used as porous electrodes, have rarely been reported. Most recently, we developed an ice crystal-induced phase separation process for construction

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of macroporous structured graphene thin films (MGTFs) and demonstrated the applications of the MGTFs as versatile conductive platforms.22 In this process, the macropores in the surfaces of the films can be either open or closed depending on the manners of ice crystal growth, which are determined by the thickness of the wet GO films. On the other hand, conducting polymers (CPs) and their nanostructures have become a growing field of interest for energy storage/conversion devices due to their attractive properties such as tunable electrical conductivity, high flexibility, and unique redox behavior.22,23 Regardless of a large number of the advantages, conversely, inferior properties of CPs, e.g., low electrochemical stability and fragile mechanical strength, often thwart their practical usage.24 The call for overcoming these drawback has led to a rising attention in hybridization of CPs by other additives.24 For example, various nanostructured carbon materials (e.g., carbon nanotubes, mesoporous carbon, and graphene) have been integrated with CPs to fabricate composites with improved properties including enhanced thermal stability, electrochemical stability, and mechanical strength.24 The combination of graphene and CPs has revealed more advanced properties than those displayed by the composites composed of CPs and other carbon materials due to the remarkable properties of graphene and the synergistic effect between graphene and CPs.24,25 Recently, graphene/CP composites have been prepared by in situ chemical oxidative polymerization,24,25 electropolymerization,26,27 and interfacial polymerization28 for application in electrochemical supercapacitors. Among these methods, the electropolymerization shows several advantages including short reaction time, being free of any oxidant, as well as avoidance of any binders or conductive fillers in the preparation of the composites. Here, we report construction of nanostructures of conjugated polymers inside the macroporous structures of RGO framework through an ice crystal-induced phase separation process for fabrication of MGTFs, followed by in situ electropolymerization of CPs using the MGTFs as working electrodes. The

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use of flexible substrates, graphene papers (GPs), enabled the resultant composite films to be successfully applied in flexible supercapacitors. As flexible film electrodes, the polyaniline (PAn) coated composite films (PAn@MGTF@GPs) exhibited an areal specific capacity (538 mF cm−2) that is higher than the values displayed by other PAn-graphene based film supercapacitors reported previously.26,29,30 The influence of PAn content of the composite films on the areal specific capacitance were investigated by varying the period of electropolymerization time. The outstanding electrochemical performances were ascribed to the superior structural characteristics of our PAn@MGTF@GPs, including the efficiently exfoliated feature of the RGO sheets in the MGTFs, the stable conductive framework of the MGTFs, as well as the conjugation of the PAn nanostructures with the macroporous structures of the MGTFs.

Experimental Section Materials: The GO used in this study was synthesized by oxidation of natural graphite powder following a modified Hummers method.31 After oxidation, it was purified by cycles of centrifugation for 30 min at 15000 RPM and washing with deionized (DI) water. Finally, a GO hydrogel was obtained after the pH value of the supernatant was close to 6. GPs were prepared from reduced GO (RGO) using polytetrafluoroethylene (PTFE) membranes through a facile filtration method.31 Electrical conductivity of the PTFE-supported GPs was measured to be up to 116 S m−1. This value is comparable to that obtained in our previous research.32 The monomers including aniline, pyrrole, and thiophene were purchased from Aladdin Industrial Corp. All the other chemicals were purchased from Sinopharm Chemical Co. Ltd. and used as obtained. Preparation of MGTFs: The concentration of the as-prepared GO hydrogel was determined to be ca. 2.0 wt% by drying a small amount of the hydrogel in a vacuum oven at 40 °C overnight. GO

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hydrogels with different concentrations (from 2 to 7 mg mL−1) were obtained by diluting the as-prepared hydrogel with DI water. Regular glass, indium-tin oxide (ITO) coated glass, and GPs were used as substrates for preparation of the MGTFs. In typical procedures, GO hydrogel thin films were coated on substrates using the GO hydrogels at a spin coating speed of 300, 600, 900, 1200, 1500, or 1800 RPM for 9 s. The GO hydrogel thin films were immediately frozen in liquid N2 and then transferred into a lyophilize machine for freeze drying, leading to formation of macroporous structured GO thin films. Finally, the macroporous structured GO thin films prepared on regular glass and ITO glass were converted into MGTFs by thermal reduction at 400 °C for 2 h in a tube furnace under Ar atmosphere. The MGTFs prepared on ITO glass substrates were designated as MGTF@ITOs. The macroporous structured GO thin films prepared on GPs were converted into MGTFs by adopting a previously reported thermal reduction conditions (i.e., at 200 °C for 6 h, heating rate 2 °C min−1).32 The resultant porous films were designated as MGTF@GPs. Electropolymerization of conducting polymers: All the electropolymerization was carried out in a three-electrode electrochemical cell using MGTF@ITOs or MGTF@GPs as working electrodes, a Pt plate as counter electrode, and a saturated calomel electrode (SCE) as reference electrode at room temperature. PAn was potentiostatically electropolymerized at 0.8 V versus SCE in a deaerated aqueous solution containing aniline (0.1 M) and H2SO4 (1 M).28 For comparison, PAn was also electrodeposited on GPs for the same period of polymerization time. After electropolymerization, the PAn films in the MGTF@ITOs (PAn@MGTF@ITOs), in the MGTF@GPs (PAn@MGTF@GPs), and on GPs (PAn@GPs) were washed with DI water and dried. Electropolymerization of thiophene was potentiostatically performed at 1.9 V versus SCE in a deaerated acetonitrile solution containing thiophene (0.1 M) and tetrabutylammonium hexafluorophosphate (0.1 M).33 After electropolymerization, the polythiophene (PTh) films in the MGTF@ITOs (PTh@MGTF@ITOs) were rinsed with acetonitrile

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and then dried. Electropolymerization of pyrrole was potentiostatically performed at 0.9 V versus SCE in a deaerated aqueous solution containing pyrrole (0.1 M) and HCl (0.1 M).34 After electropolymerization, the polypyrrole (PPy) films in the MGTF@ITOs (PPy@MGTF@ITOs) were rinsed with DI water and then dried. The quantities of deposited CPs on the substrates were controlled by variation of the period of electropolymerization time. Preparation of MGTFs was optimized using ITO substrate because of its commercial availability. To show application of the MGTFs in flexible devices, MGTF@GPs were used for electropolymerization of PAn and the morphology of the deposited PAn layers were investigated as function of electropolymerization time. Finally, PAn@MGTF@GPs were used for electrochemical characterization. Electrochemical test: Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic (GV) charge/discharge tests were performed using a CHI 660E electrochemical work station. The working electrodes were fabricated by cutting the MGTF@GPs into strips with area of ca. 1×2 cm2 and then PAn was electrodeposited on an area of ca. 1×1 cm2, while the remaining part of the GPs was used for connection. A Pt foil and a SCE were used as counter and reference electrodes, respectively. EIS spectra were recorded in the frequency range of 0.01 to 105 Hz with an ac perturbation of 5 mV. The areal specific capacitance (Ca) was calculated using the equation Ca= I×∆t/S×∆V, where I is the constant discharge current, ∆t is the discharge time, ∆V is the discharge potential drop (excluding IR drop), and S is the area of the MGTFs deposited with PAn, i.e., 1×1 cm2. Characterization: Surface morphologies of MGTFs were observed by Hitachi S-4800 microscope operated under an accelerating voltage of 10 kV. Raman spectra were recorded on a Renishaw inVia Reflex confocal Raman microscope with an excitation wavelength of 532 nm. FT-IR spectra were recorded on an Excalibur 3100 spectrometer from 4000 to 400 cm−1 by accumulation of 32 scans at a resolution of 0.2 cm−1.

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Results and Discussion Figure

1

depicts

the

fabrication

of

MGTFs

from

GO

hydrogels

and

subsequent

electropolymerization of CPs using the MGTF@ITOs or MGTF@GPs as working electrodes. The fabrication of MGTFs consisted of several operations, including spin coating GO hydrogels on substrates, freeze drying the wet GO hydrogel thin films to obtain macroporous structured GO thin films, and converting the macroporous structured GO thin films to MGTFs through thermal reduction. With this method, the thickness of the MGTFs can be readily tuned by changing the concentration of the GO hydrogels and the spin coating speed. In addition, this facile process is applicable to various substrates. Our previous study indicated that the MGTFs are electrically conductive because of the presence of a RGO framework and the formation of a continuous RGO film on the surface of the substrate.22 Finally, CPs were deposited into the MGTFs through electropolymerization using MGTF@ITOs or MGTF@GPs as working electrodes.

Figure 1. Schematic presentation of fabrication of MGTFs using GO hydrogels and subsequent electropolymerization of CPs into MGTFs using the MGTF@ITOs or MGTF@GPs as working electrodes.

The morphologies of MGTFs were investigated through scanning electron microscopy (SEM). Figure 2a presents the SEM image of the MGTF prepared on glass substrate from a GO hydrogel with C

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= 2 mg mL−1 at a spin coating speed of 900 RPM. It is seen that the substrate was not fully covered by the macroporous structures of RGO. Increasing the concentration of the GO hydrogel to 3 mg mL−1 led to formation of continuous macroporous structures (Figure 2b), with only some areas of the substrate being seen through pores. The formation of ice crystals in the freezing process caused the GO sheets dispersed in the hydrogel to be expelled to the boundaries between the ice crystals, and subsequent freeze drying resulted in formation of the macroporous structures. The rapid crystallization process of the water in the hydrogel thin film prevented aggregation of the GO sheets. Further increasing the concentration of the GO hydrogel (to C = 4, 5, and 6 mg mL−1) led to thicker and thicker MGTFs that consisted of interconnected and overlapped pores (Figure 2c−2e). Finally, when a GO hydrogel with C = 7 mg mL−1 was used, some pores in the surface of the MGTFs were found to be closed (Figure 2f). The closed pores in the surface of the MGTFs were resulted from the temperature gradient existing in a thicker wet GO hydrogel film, which drove the ice crystals to grow from the surface of the GO hydrogel film to the inner of the film.22 To further verify the controllability on the thickness of the MGTFs, transparency spectra of the aforementioned MGTFs were collected. The transparencies of the MGTFs at 550 nm were found to decrease as the concentration of the used GO hydrogels increased (the third column of the points in Figure 2g). Meanwhile, the transparencies of the MGTFs prepared with other spin coating speeds followed the same changing tendency as a function of the concentration of the GO hydrogels used. Therefore, the preparation of the MGTFs can be controlled by adjusting the concentration of the GO hydrogels and the spin coating speed.

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Figure 2. SEM images of the MGTFs prepared from GO hydrogels with different concentrations of (a) 2 mg mL−1, (b) 3 mg mL−1, (c) 4 mg mL−1, (d) 5 mg mL−1, (e) 6 mg mL−1, and (f) 7 mg mL−1 at a spin coating speed of 900 RPM. (g) The plot of transparencies at 550 nm of the MGTFs as function of the spin coating speed and the concentration of the GO hydrogels.

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The interconnected macroporous structures enable the MGTFs prepared on conductive substrates to be used as porous electrodes. Next, we manifest electropolymerization and deposition of CPs into the MGTFs using the MGTF@ITOs or MGTF@GPs as working electrodes. Figure 3a shows an SEM image of the MGTF@ITO prepared through optimized parameters, i.e., a GO hydrogel of 5 mg mL−1 and a spin coating speed of 900 RPM, exhibiting a well-defined and uniform 3D RGO framework with all the pores open in the surface. The pore sizes in the framework were found to be several micrometers. Observations at higher magnifications revealed that the walls of the MGTF consisted of very thin and wrinkled RGO layers (Figure 3b). Such interconnected macroporous structures effectively prevented agglomeration of the RGO sheets, and resulted in mechanical flexibility of the walls as well as high conductivity of the whole MGTF. Energy-dispersive X-ray spectroscopy (EDS) element mapping indicated uniform distribution of carbon throughout the whole area of the MGTF (Figure 3c). Figures 3d, 3g, and 3j show SEM images of the PAn@MGTF@ITO, the PPy@MGTF@ITO, and the PTh@MGTF@ITO prepared by electrodeposition of the respective CPs with typical parameters, i.e., 0.8 V and 300 s, 0.9 V and 200 s, and 1.9 V and 120 s, respectively. Comparison of the SEM images obtained before and after electropolymerization indicated that deposition of CPs did not fracture the macroporous structures of the MGTFs. The well-arranged RGO sheets in the MGTFs enhanced the stability of the macroporous framework and prevented shrinkage or collapse of the 3D RGO structures during electrodeposition of CPs. Figure 3e, 3h, and 3k display enlarged SEM images of the PAn@MGTF@ITO, the PPy@MGTF@ITO, and the PTh@MGTF@ITO, respectively. Compared with the as-prepared MGTF@ITO (Figure 3b), the walls of the macroporous structures became thicker after electrodeposition of the CPs. Close inspections under SEM revealed that each CP formed a specific microstructure, i.e., thorn-like protuberances for PAn, grains for PPy, and a smooth surface for PTh (Figure S1). The thorn-like protuberances comprised plenty of nanoscale pores, which potentially made

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the PAn@MGTF a good electrode for supercapacitors due to the large specific area and favorable electrolyte transportation. No matter which kinds of CP microstructures were formed, they all had uniform distributions in the MGTFs as verified by EDS mappings (Figure 3f, 3i, and 3l). Raman spectra of the MGTF@ITOs before and after electropolymerization confirmed the successful deposition of the conducting polymers into the MGTFs (Figure S2).

Figure 3. SEM observations and EDS characterization of the as-prepared MGTF@ITO and the MGTF@ITOs electrodeposited with CPs. (a−c) A MGTF@ITO prepared using a GO hydrogel of 5 mg mL−1 at spin coating speed of 900 RPM. (d−f) A PAn@MGTF@ITO. (g−i) A PPy@ MGTF@ITO. (j−l) A PTh@MGTF@ITO. The EDS elemental maps of (c) carbon, (f) nitrogen, (i) nitrogen, and (l) sulfur were collected from the entire areas shown in panels (b), (e), (h), and (k), respectively.

In order to utilize MGTFs in flexible devices, GPs were used as substrates for fabrication of the MGTFs. Figure 4a shows a photograph of a round GP with diameter of ca. 3.5 cm prepared by a

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filtration method.31 The MGTF@GP looked slightly darker than a GP (Figure 4b). It showed that adhesion of the MGTF layer to the surface of the GP was robust so the MGTF layer did not peel off under repeated bending (Figure 4c). In addition to the unique microstructure of the coated PAn layer as aforementioned, PAn exhibits structural transitions between leucoemeraldine/emeraldine and emeraldine/pernigraniline at a specific electrochemical potential, which provides pseudocapacitance.35 Therefore, PAn was used as an electrode material for supercapacitors, as discussed below. GP strips with width of ca. 1 cm were used as working electrodes for electropolymerization of PAn. Deposition of PAn was perceived by appearance of green material layers on the surfaces of the working electrodes (Figure 4d).

Figure 4. Optical photographs of (a) a round GP with diameter of ca. 3.5 cm, (b) a MGTF@GP, (c) a bent MGTF@GP fabricated on a GP strip with width of ca. 1 cm, and (d) a PAn@MGTF@GP prepared by electrodeposition of PAn for 900 s at 0.8 V. SEM images of the PAn@MGTF@GPs prepared by electrodeposition of PAn for different periods of time: (e) 300 s, (f) 600 s, (g) 900 s, and (h) 1200 s.

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To verify the controllability of preparation of the PAn@MGTF@GPs, electrodeposition of PAn in MGTF@GPs was performed for different periods of time. Figure 4e shows a PAn@MGTF@GP that was obtained by electrodeposition for 300 s. It is noticeable that the deposited thorn-like PAn protuberances formed a uniform layer on the RGO sheets of the MGTFs. The uniform deposition was attributed to the π−π interactions between the aromatic units of PAn and the conjugated surface of RGO sheets.36 Increasing the electrodeposition time to 600 s resulted in the formation of PAn nanofibers on the surface of the PAn layer (Figure 4f). Further increasing the electrodeposition time to 900 s led to full coverage of the RGO sheets by the PAn nanofibers, with nanoscale pores (Figure 4g). Such porous structures formed by the PAn nanofibers will be substantially beneficial for enhancing the capacitive performance of the PAn@MGTF@GPs, as discussed later. However, when the electrodeposition time was increased to 1200 s, the PAn nanofibers aggregated and the porous structures formed by them started to disappear (Figure 4h).

Figure 5. Raman spectra of the MGTF@GP, the PAn@ITO, and the PAn@MGTF@GPs prepared by electrodeposition for 300, 600, 900, and 1200 s.

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Raman spectroscopy was used for further study about interactions between PAn and the substrate, as well as the composition of the PAn@MGTF@GPs as a function of electropolymerization time (Figure 5). The spectrum of the MGTF@GP showed two dominant peaks at ca. 1351 and 1602 cm−1, which corresponded to the D and G bands of RGO respectively.31 The PAn@ITO exhibited characteristic signals at ca. 576, 815, 1186, 1355, 1504, and 1625 cm−1, which can be attributed to the out-of-plane C–H deformation of the quinonoid rings,37 imine deformation vibrations, in-plane C−H bending vibrations, vibrations of the semiquinone radicals, C=N stretching vibrations of quinoid, and C−C stretching vibrations of benzenoid, respectively.38,39 As expected, the Raman signals of both RGO and PAn were detected in the Raman spectra of PAn@MGTF@GPs. All the PAn characteristic signals, except one at 1355 cm−1 that overlapped with the D band of RGO, were found to increase in intensity with proceeding electrodeposition of PAn, which recommended the controllable deposition of PAn into the MGTF@GP electrodes.

Figure 6. (a) CV curves of the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes at a scan rate of 10 mV s−1 in a 1 M H2SO4 electrolyte. (b) GV charge/discharge curves of the MGTF@GP, PAn@GP, 15



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and PAn@MGTF@GP electrodes at a current density of 0.1 mA cm−2. (c) Nyquist plots of the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes. (d) The plot of the areal specific capacities of the PAn@MGTF@GPs prepared with different periods of electropolymerization time measured at a current density of 0.1 mA cm−2. (e) The plots of the areal specific capacitance for the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes (prepared by electropolymerization time of 900 s) versus current density (from 0.1 to 2 mA cm−2). (f) Cycling performance of the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes (prepared by electropolymerization time of 900 s) at 1 mA cm−2 over 3000 cycles.

To explore the benefits of the PAn@MGTF@GPs as electrodes for flexible supercapacitors, electrochemical behaviors were measured by CV, GV charge/discharge tests, EIS, and cycling stability in 1 M H2SO4 aqueous solution in a three-electrode system. CV curves were collected in a potential range of −0.2 to 0.8 V versus SCE at a scan rate of 10 mV s−1 (Figure 6a). The MGTF@GP showed a nearly rectangular CV loop, with no obvious redox peaks, indicating an ideal double-layer capacitance behavior of the MGTF prepared on a GP. But, the displayed capacitance was small because the area of the CV loop was very small. In contrast, the PAn@GP and PAn@MGTF@GP (prepared by electropolymerization time of 900 s) as electrodes exhibited redox peaks in their CV curves, which resulted from the leucoemeraldine/emeraldine and emeraldine/pernigraniline transitions of PAn and contributed pseudocapacitance.35 Evidently, the PAn@MGTF@GP composite electrode showed a CV loop with much larger area than MGTF@GP and PAn@GP electrodes, indicating a higher specific capacitance of the PAn@MGTF@GP composite electrode. It is noteworthy that the capacitance of the PAn@MGTF@GP composite electrode is predominantly contributed by the PAn component because its CV loop is much greater than that of the MGTF@GP electrode. The excellent electrochemical

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capacitance of the PAn@MGTF@GP as electrode for supercapacitors can be ascribed to its unique structures: the MGTF contains interconnected macropores, which lead to uniform distribution of the PAn nanofibers on the walls of the MGTF. One significant role of the macroporous structures formed by RGO sheets is to keep the PAn nanofibers in a nanoscale distribution. Therefore, the coexistence of the macroporous structured RGO framework and the PAn nanostructures facilitates easy access of PAn nanofibers to electrolyte. In addition, the conductive RGO framework is responsible for fast electron transportation. Furthermore, the advantage of the PAn@MGTF@GPs as electrodes for highperformance supercapacitors is also reflected in good rate capability because the shape of the CV curves and the aforementioned redox peaks can be properly maintained even at a high scan rate up to 50 mV s−1 (Figure S3). Electrochemical performance of the PAn@MGTF@GP composite electrode was also evaluated using GV charge/discharge tests, which were measured at various current densities from 0.1 to 2 mA cm−2 over a potential window of 0 − 0.8 V. The GV charge/discharge curves of the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes were collected at a current density of 0.1 mA cm−1 and summarized in Figure 6b. Consistent with the CV data, the MGTF@GP electrode exhibited a small but ideal double-layer capacitance because it showed symmetric triangular charge/discharge curves and the charge/discharge times derived from the curves were short. In contrast, the PAn@GP electrode and PAn@MGTF@GP composite electrode exhibited shoulders between 0.4 and 0.6 V in the charge/discharge curves because of the redox reactions of PAn. Due to the presence of the macroporous structures, which maintained the nanostructures formed by the PAn nanofibers, the PAn@MGTF@GP composite electrode displayed longer charge/discharge time than the PAn@GP electrode, indicating a higher areal specific capacitance of the former than the latter. The charge/discharge processes of the PAn@MGTF@GP composite electrode were reversible since the charge/discharge curves persisted

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identical shapes during cycling. Furthermore, the GV charge/discharge tests also indicated the good rate capability, since the electrochemical capacitive features could be recognized in the GV charge/discharge curves obtained at a high current density (Figure S4). In order to further understand the electrode dynamics, EIS measurement was performed from a frequency range of 0.01 to 105 Hz. The Nyquist plots of the MGTF@GP, PAn@GP, and PAn@MGTF@GP electrodes are displayed in Figure 6c. Using a commonly adopted equivalent circuit (Figure S5), the Nyquist plots were fitted to give charge-transfer resistance (Rct) and serial resistance (Rs) for the different electrodes. As summarized in Table S1, PAn@MGTF@GP composite electrode exhibited a lower Rct1 than the other two electrodes at the interfaces of electrode materials and electrolyte (8.5 Ω for PAn@MGTF@GP, 11.7 Ω for MGTF@GP, and 19.7 Ω for PAn@GP). Such prominent feature can be ascribed to the wetting surfaces of the PAn nanofibers as well as the PAn nanostructures formed on the walls of the conductive RGO framework. Meanwhile, the lowest Rs for the PAn@MGTF@GP composite electrode among the three samples was attributed to the conductive RGO frame work and the robust adhesion of the PAn nanofibers to the surfaces of RGO sheets through π−π interactions, which facilitated electron transfer at the interfaces. In addition, a relatively large Rct2 was obtained probably due to confined transport of infiltrated electrolyte in the GP.30 Next, the electrochemical capacitive performance of the PAn@MGTF@GPs as electrodes was also evaluated by changing the electropolymerization time. Figure 6d shows that the areal specific capacitance increased as the eletropolymerization time increased, with a maximum value (538 mF cm−2) achieved from the PAn@MGTF@GP that was prepared by electropolymerization for 900 s. Conversely, further increasing the electropolymerization time resulted in decrease of the areal specific capacitance. Retrospection of the morphological evolution of PAn@MGTF@GPs as electropolymerization time revealed that the areal specific capacitances of the PAn@MGTF@GP composite electrodes were related

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to their morphologies. The content of the PAn nanofibers increased in PAn@MGTF@GPs as the electropolymerization time increased. However, the nanostructures formed by the PAn nanofibers could be well maintained in the samples obtained with an electropolymerization time less than 900 s (Figure 4e−4g), whereas the PAn nanofibers started to aggregate when the electropolymerization time was longer than 900 s (Figure 4h). Aggregation of the PAn nanofibers led to reduced specific area and increased diffusion pathway for charges. Therefore, the combination of the nanostructures formed by the PAn nanofibers and the macroporous structures of RGO framework is one key factor for the PAn@MGTF@GPs as electrodes to obtain a maximum areal capacitance. The maximum value of areal specific capacitances exhibited by our PAn@MGTF@GPs is higher than those exhibited by other PAngraphene based flexible supercapacitors reported previously.29,30,40 The thickness of the PAn@MGTF composite layer prepared with an electropolymerization time of 900 s was measured to be ca. 70 µm from its cross-sectional image (Figure S6). Therefore, the volume specific capacitance was measured to be ca. 77 F cm−3. Furthermore, the deposited PAn was calculated to be ca. 1.6 mg from the mass differences of the electrode before and after electropolymerization. Thus, mass specific capacitance was calculated to be ca. 336 F g−1. Dependence of the areal specific capacitance on current density was also measured by changing the charge/discharge current density from 0.1 to 2 mA cm−2. As summarized in Figure 6e, the PAn@MGTF@GP composite electrode always exhibited higher areal capacitances than the PAn@GP and MGTF@GP electrodes at all current densities. The significant difference in the capacitances displayed by the MGTF@GP and the PAn@MGTF@GP revealed that the areal capacitance of the PAn@MGTF@GP composite electrode was predominantly contributed by the PAn nanofibers. Although the areal capacitance exhibited by the MGTF@GP electrode is negligible compared with that of the PAn@MGTF@GP composite electrode in this research, the value obtained for the MGTF@GP

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electrode is higher than those displayed by other graphene flexible films reported previously.41 This fact demonstrates the benefit of our MGTFs in electrodes for the application of supercapacitors. The enhanced areal capacitance of the PAn@MGTF@GP composite electrode with respective to that of the PAn@GP electrode can be ascribed to the formation of the nanostructures of the PAn nanofibers on the walls of the macroporous RGO structures. In addition, the PAn@MGTF@GP composite electrode also maintained high areal capacitances at high current densities. The cycling stability of the PAn@MGTF@GP composite electrode was evaluated by running the charge/discharge cycles over 3000 cycles (Figure 6f). In control test, the MGTF@GP electrode exhibited a gradual decrease in capacitance within the first 350 cycles, followed by an increase of capacitance due to improved ion accessibility in the 3D graphene framework during cycling process.42,43 After 1000 cycles, 99% of its initial areal capacitance was retained. The PAn@GP electrode exhibited a marked drop in areal capacitance within the first 350 cycles and only 76% of its initial areal capacitance was remained after 1000 cycles. The severe decay of capacitance is ascribed to structural changes of PAn due to swelling and shrinking during charge/discharge process.40,44 By contrast, the PAn@MGTF@GP composite electrode remained 87 and 83% of its initial capacitance after 1000 and 3000 cycles, respectively, implying a significantly enhanced cycling stability compared with the PAn@GP electrode. The enhanced cycling stability might be ascribed to the uniform distribution of the PAn nanofibers on the walls of the conductive RGO framework so that the morphological and electrochemical property changes of PAn nanofibers induced by charge/discharge cycling were substantially alleviated.24

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Figure 7. CV curves of the flexible PAn@MGTF@GP composite electrode measured at different conditions including straight, bent, and rolled forms at a scan rate of 10 mV s−1.

The excellent electrochemical performance of the PAn@MGTF@GPs as electrodes for supercapacitors can be ascribed to the following factors that stem from the design of the hybridized electrode structures. First, the conjunction of the macroporous structured RGO framework with the PAn nanostructures facilitates full access of the electrochemically active materials to electrolyte. Second, the MGTFs serve as a robust 3D substrate to immobilize electrochemically active PAn nanofibers: the intimate interactions between them prevent the loss of PAn due to structure and volume changes during repeated redox reactions. Therefore, the PAn@MGTF@GP composite electrode exhibit enhanced cycling stability compared with the PAn@GP electrode. Third, the conductive RGO framework benefits efficient transfer of electrons. In this way, the decreased ion-diffusion and charge-transfer resistance lead to the good rate capability. Fourth, the flexible feature of the MGTF@GP substrates enable their versatile applications. As shown in Figure 7, a PAn@MGTF@GP composite electrode displayed identical CV curves when measured under different conditions, including straight electrode film, bent electrode film with a 90-degree angle, and rolled electrode film in tube-like shape. The stability of the PAn@MGTF@GP composite electrode to deformation of the electrode is stemmed from the good 21



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mechanical properties of the MGTF@GPs and the robust adhesion of the PAn nanofibers to the walls of the MGTFs.

Conclusions MGTFs are successfully fabricated on flexible GPs through an ice crystal-induced phase separation process. The resultant MGTF@GPs own prominent properties such as sufficient exfoliation of the RGO sheets, interconnected porous channels, high electrical conductivity, and mechanical flexibility. These features enable the MGTF@GPs ideal current collectors as well as the substrate for loading electrochemically active materials (PAn, PPy, or PTh) by a green in situ electropolymerization method. Notably, electrodeposition of PAn leads to formation of nanostructures of PAn nanofibers, which results in conjunction of the macroporous structured RGO framework with the PAn nanostructures in the PAn@MGTF@GPs. As binder-free electrodes for flexible supercapacitors, the PAn@MGTF@GPs exhibit a high areal capacity (538 mF cm−2), high cycling stability, and remarkable capacitive stability to electrode deformation. EIS analysis confirms the low Rct and Rs, which are ascribed to the unique structures and the remarkable properties of the PAn@MGTF@GPs. We expect that the methodology and electrode structures developed in this research can contribute an important step forward to bringing porous graphene-based nanomaterials to various applications such as flexible supercapacitors, lithium ion batteries, solar cells, biosensors, and information storage.

Acknowledgment: This work was supported by the “Hundred Talents Program” of the Chinese Academy of Sciences and the National Natural Science Foundation of China (21274158, 91333114).

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Supporting Information Available: SEM images of the PAn@MGTF@ITO, the PPy@MGTF@ITO, and the PTh@MGTF@ITO recorded at a high magnification, Raman spectroscopy for conducting polymer-coated MGTFs, CV curves of the PAn@MGTF@GP composite electrode measured at different scan rates, GV charge/discharge curves of the PAn@MGTF@GP composite electrode measured at various current densities, the equivalent circuit used for fitting the EIS curves and summarization of the kinetic parameters, and a cross-sectional image of the PAn@MGTF composite. This material is available free of charge via the Internet at http://pubs.acs.org.

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Liu,

X.;

Wen,

N.;

Wang,

X.;

Zheng,

Y.

A

High-Performance

Hierarchical

Graphene@Polyaniline@Graphene Sandwich Containing Hollow Structures for Supercapacitor Electrodes. ACS Sustainable Chem. Eng. 2015, 3, 475−482.

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Conjunction of Conducting Polymer Nanostructures with Macroporous

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