Polymer-Coated Graphene Aerogel Beads and Supercapacitor

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Polymer-Coated Graphene Aerogel Beads and Supercapacitor Application An Ouyang, Anyuan Cao, Song Hu, Yan-Hui Li, Ruiqiao Xu, Jinquan Wei, Hongwei Zhu, and Dehai Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01965 • Publication Date (Web): 08 Apr 2016 Downloaded from http://pubs.acs.org on April 9, 2016

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ACS Applied Materials & Interfaces

Polymer-Coated

Graphene

Aerogel

Beads

and

Supercapacitor

Application An Ouyang1, Anyuan Cao2*, Song Hu3, Yanhui Li3, Ruiqiao Xu4, Jinquan Wei4, Hongwei Zhu4, Dehai Wu1 1

Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China

2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P.

R. China 3 College of Electromechanical Engineering, Qingdao University, Qingdao 266071, P. R. China 4

School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China

Corresponding authors: [email protected]

Abstract Graphene aerogels are highly porous materials with many energy and environmental applications; tailoring the structure and composition of pore walls within the aerogel is the key to those applications. Here, by freeze casting the graphene oxide sheets, we directly fabricated freestanding porous graphene beads containing radially oriented through channels from the sphere center to its surface. Furthermore, we introduced pseudo-polymer to make reinforced, functional composite beads with a unique pore morphology. We showed that polymer layers can be coated smoothly on both sides of the pore walls, as well as on the junctions between adjacent pores, resulting in uniform polymer-graphene-polymer sandwiched structures (skeletons) throughout the bead. These composite beads significantly improved the electrochemical properties, with specific capacitances up to 669 F/g and good cyclic stability. Our results indicate that controlled fabrication of homogeneous hierarchical structures is a potential route toward high performance composite electrodes for various energy applications. Keywords: Graphene beads, freeze casting, sandwich structure, composite beads, supercapacitor.

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ACS Paragon Plus Environment


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Introduction Recently, there has been tremendous interest in the synthesis and applications of graphene-based aerogels. 1-5 These are highly porous materials consisting of a three-dimensional graphene network, which is mechanically stable and electrically conductive. The porous nature and conductive network of graphene aerogels enable many environmental and energy applications such as oil adsorption, composites, electrodes and sensors.6-11 Typically, graphene aerogels have been synthesized by assembling dispersed graphene oxide (GO) sheets, which can be obtained in large-scale by exfoliation

of

expanded

graphite

(so-called

Hummer’s

method).12-15

Especially,

liquid

nitrogen-assisted freeze-casting, a convenient and efficient method to manufacture various porous structures from ceramic materials and polymers, can also be used to prepare graphene aerogels.16-19 Within the aerogel, the highly porous structure is formed due to the self-assembly of GO sheets and formation of interconnected skeletons (pore walls) during the freeze-casting process. Therefore, the pore walls are made from an aggregation of stacked (or cross-linked) GO sheets, which are usually very thin and flexible. In general, pure GO or reduced GO (rGO) aerogels do not have sufficient mechanical strength and chemical activity for many applications, and composite structures need be constructed for enhancing the performance. Tailoring the structure and composition of the pore walls can not only enhance the mechanical property of GO aerogels, but also lead to functional composite structures that are important for applications. To this end, a variety of GO-based composite aerogels have been fabricated by introducing foreign materials such as carbon nanotubes (CNTs), nanowires, graphene nanoribbons, oxides and pseudo-polymers.20-26 For example, GO/CNT composite aerogels, in which the CNT

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skeletons were embedded into the pore walls, showed elastic volume recovery after large-strain compression, compared with considerable plastic deformation found in pure GO aerogels.27 Also, GO aerogels deposited by some pseudo-polymer acted as compression-tolerant supercapacitor electrodes with simultaneously improved specific capacitance.28-33 Those results have demonstrated that the controlling of the structure and composition of pore walls is critical for achieving high performance aerogels. Previously, columnar graphene-polymer monoliths have been prepared through freeze casting with well-ordered micro channels oriented along the freezing direction (longitudinal direction along the column, and rGO sheets were dispersed into poly(vinylalcohol) to improve the mechanical strength.26 Our group also fabricated graphene-chitosan core shell beads by pasting a porous chitosan shell around the graphene core via a two-step freeze-casting process.17 However, bead-shape graphene aerogels and tailoring of their porous structures have not been studied in detail. Also, applications of these porous conductive graphene beads in areas such as energy devices have rarely been explored. Here, we report template-free freeze-casting of porous graphene beads containing radially oriented channels from the sphere center to the bead surface, that are different from previous columnar monoliths. We also deposited pseudo-polymers into these aerogels to make composite beads in which the GO sheets were coated by smooth polymer layers on both sides, resulting in a sandwiched pore wall structure. We showed good control on the thickness of the polymer coating over a wide range (from several to nearly 165 nm), leading to tailored microstructure and improved properties. We have also investigated the electrochemical properties of graphene-pseudo-polymer composite beads, and found significant improvement in the specific capacitance, proving potential applications as porous electrodes. 3



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Results and discussion Fabrication of rGO and rGO/PANI beads We fabricated graphene aerogels with sandwiched pore walls by two steps, as illustrated in Figure 1a (see Experimental for details). First, porous GO aerogels in the form of small spherical beads were synthesized by freeze casting method; the bead shape was generated naturally when droplets of GO solution entered liquid nitrogen. These GO beads were then reduced thermally to render conductive rGO beads. Second, a pseudo-polymer, polyaniline (PANI), was electro-deposited into the porous beads to make rGO/PANI composite beads. The as-synthesized freestanding rGO beads have diameters of 1.5-3 mm depending on the droplet size and solution concentration, and the rGO/PANI beads retain the same size but the color has changed to deep-green due to surface coating of PANI (Fig. 1b, 1c). The porous nature of rGO beads was demonstrated by mechanical deformation and capillarity induced shrinking. For example, a single rGO bead placed in air can be compressed to a disc shape without collapse and then recover original spherical shape, indicating certain mechanical stability and elasticity. Such shape recovery is also observed in liquid environment; an acid-treated (hydrophilic) bead subjected to compression can recover by absorbing water into the pores (Fig. 1d). In another experiment, we immersed a wetting rGO bead in water droplet, and used a filter paper to suck away water. At this time, the bead shrinks into a much smaller volume due to loss of water and capillarity induced pore densification. The bead expands again to original shape once a second water droplet is fed (Fig. 1e). We observe that rGO/PANI beads with PANI loadings of 38.0 wt% to 92.0 wt% shrink to different degrees after loss of water; the bead with the lowest PANI loading shrink most

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(close to rGO beads) while the bead with 92.0 wt% PANI shows negligible volume reduction (Fig. 1f). This phenomenon indicates that the PANI deposition can strengthen the graphene network and make the beads more resistant to deformation. Structural characterization of rGO beads We have characterized the microstructure of rGO and rGO/PANI beads by scanning electron microscopy (SEM). The surface of the rGO bead is very rough at microscale with a lot of openings, indicating an open-porous structure (Fig. 2a). In the cross-sectional view, radial channels emitting from the sphere center to its surface can be distinguished (Fig. 2b). Since the GO solution is fed in droplets during freeze-casting, each droplet is subjected to a large temperature gradient immediately after it is immersed and surrounded by liquid nitrogen. This temperature difference prompt the growth of ice crystals from the sphere surface toward the center, resulting in observed open pores connecting radial channels. These are continuous channels with widths of 15-20 µm in the majority part, which are also interconnected to each other through side pores (Fig. 2c). The channel width can be varied by changing the initial GO solution concentration (from 2.6 to 7.6 mg/mL) (Supporting Information, Fig. S1). Enlarged view reveals that the pore walls composed of rGO sheets are very thin, semi-transparent, and flexible (Fig. 2d). This indicates that the starting GO solution is very uniform, which could produce highly porous structures with thin pore walls during freeze-casting. We have also characterized structure and composition of rGO beads before and after thermal reduction by other techniques including Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD) (Figure S2). The electrochemical properties of those rGO beads were measured in a three-electrode

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configuration between 0 to 0.8 V with 2 M KCl aqueous electrolyte (a single bead was tested in one time). Rectangular cyclic voltammetry (CV) curves at scan rates of 2 to 200 mV/s are obtained, due to the electrical double-layer capacitor behavior for pure rGO (Fig. 2e). As the scan rate increases from 200 to 10000 mV/s, the CV curves tend to be near rectangular (Fig. S3a). Compare to the capacitance at original scan rate of 2 mV/s, the rate performance shows 43.4 % capacitance retention at a scan rate of 200 mV/s (Fig. S3b). After that, the capacitance continues to decrease with the scan rate increasing to 10000 mV/s. Also, the galvanostatic charge-discharge curves at current densities of 5 to 25 A/g display a nearly symmetric triangular shape, indicating reversible charge and discharge processes as a supercapacitor electrode (Fig. 2f). In the Nyquist plot, the electrochemical impedance spectroscopy (EIS) shows a slight semicircular arc at high frequency, suggesting relatively low charge-transfer resistance of the rGO beads (Fig. S3c). For this individual bead tested in 0 to 0.8 V (with more symmetric galvanostatic charge-discharge curves than tested in 0 to 1V), the corresponding specific capacitance is about 139 F/g at a scan rate of 2 mV/s (Fig. S4). Based on this, introducing pseudo-polymer to make rGO/PANI beads (as shown in Fig.1) could be a strategy to further improve the capacitive properties, as will be discussed below. Structural Characterization of rGO/PANI beads We have investigated the microstructure and morphology of rGO/PANI beads by cross-sectional SEM images (Fig. S5a-5d). It is shown that the PANI deposition only occurs on the GO sheets and the 3D porous structure is still maintained. This is because during the electro-deposition process, aniline monomers were first adsorbed onto the pore walls and then polymerized by appropriate CV cycles. The amount of deposited PANI increases consistently due to continuous polymerization with more CV cycles. Deposition is rather uniform throughout the bulk sphere; the PANI coating in the 6


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sphere center is slightly thinner than that approaching the outer surface. The loadings of PANI in the resulting rGO/PANI beads are calculated to be 63.5 to 93.5 wt%, indicating that the original rGO beads are very light and the weight of final composite beads is dominated by PANI. High-magnification SEM images reveal many interesting structures in the rGO/PANI composite beads, characterized by the PANI-rGO-PANI sandwiched pore walls. We selected samples with relatively thick polymer coatings (from ∼110 to 150 nm) in which the sandwiched structures could be clearly observed (Fig. 3). The PANI coatings are deposited uniformly and the pore walls become much thicker compared with the thin and flexible rGO sheets in original beads (Fig. 3a). We can see the PANI coating is quite smooth and some GO sheets protrude from the fractured section of the pore wall (Fig. 3b). Previous PANI-grafted graphene or carbon nanotube aerogels have similar sandwiched structures, but their PANI layers show different morphology (deposited as needles or rods).34 For thick coatings, we frequently observe broken pore walls detached from the original 3D framework (Fig. 3c). Initially the rGO sheets are very flexible, but with thicker PANI coating, cracks are more easily to occur through both the polymer layer and rGO sheets, resulting in brittle fracture behavior in the pore walls. The PANI-rGO-PANI sandwich structure consists of a rGO middle layer and two PANI layers coated on the top and bottom sides (Fig. 3d, Fig.3f). It shows that during electro-deposition the PANI is coated on all exposed (therefore accessible) surfaces of the 3D rGO framework, with equal thickness on both sides. At a local region where three pore walls meet, the PANI coating shows smooth transition along the curved rGO sheets and has fixed this junction (Fig. 3e, Fig.3f). We attribute the strengthening effect in the rGO/PANI beads (as shown in Fig. 1f) to two structural factors: the thickening of rGO pore walls and the welding of the wall junctions. Thus the sandwiched wall structure is favorable for improving the mechanical properties of porous aerogels. 7



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Furthermore, we can tailor the thickness of PANI layer by controlling the number of electro-deposition cycles (see Experimental). PANI coatings with thickness of ∼34 to 165 nm can be measured from rGO/PANI beads electro-deposited by 50 to 200 scan cycles, and the corresponding PANI contents are calculated as well (Figure S6 and Figure S7). Successful deposition of PANI has been confirmed by additional measurements. First, XPS spectra of original and composite beads have revealed the evolution of surface states, in which the intensities of N1s (400.8 eV) and O1s (533.5 eV) peaks increase considerably after PANI deposition (Fig. 4a). The N1s band shows several peaks corresponding to quinoid imine (-N=) (398.4 eV), benzenoid amine (-NH-) (399.5 eV), charged imine in the bipolaron state (400.8 eV), and 402.2 eV refer to protonated amine in the polaron state (402.2 eV), respectively (Fig. 4b). Appearance of those peaks demonstrates the polymerization of PANI inside the rGO beads. Consistently, the FIIR spectrum of rGO/PANI has beads revealed a group of typical bands of PANI including 1564 cm-1 ( C=C stretching of the quinonoid ring), 1491 cm-1 (C=C stretching of the benzenoid ring), 1298 cm-1 (C-N stretching of secondary aromatic amines), and 1108 cm-1 (C-H bending of the quinonoid ring) (Fig. 4c), showing that PANI has been conjugated to the graphene framework.35, 36 Raman spectrum of the rGO/PANI beads also shows a series of characteristic peaks of PANI, including two bands at 1328 and 1386 cm-1 associated with the C-N stretching vibration of quinonediimine and benzene diamine, respectively, and two peaks at 817 and 1165 cm-1 corresponding to the C-H bending in the quinonoid ring and benzenoid ring, respectively (Fig. 4d). Capacitive performances of rGO/PANI beads as supercapacitor electrodes The CV curves of the rGO and rGO/PANI beads were tested in a three-electrode system (See the

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Experimental for details) at scan rates of 2 to 200 mV/s. Pure rGO beads show an nearly electric double layer capacitance of 219 F/g in H2SO4 (Fig. S8). In comparison, the CV curves of rGO/PANI beads with different PANI loadings (38.0 to 90.2 wt%) all have redox peaks, revealing the pseudocapacitance behavior of PANI (Fig. 5a, Fig. S9a). When the scan rate increases to 10000 mV/s, the CV curve turns from irregular to spindle shape (Fig. S9b). At a scan rate of 2 mV/s, the rGO/PANI beads with PANI contents in 38.0, 63.5 and 90.2 wt% have specific capacitances of 568, 669 and 420 F/g, respectively (Fig. 5b). Their specific capacitances decrease as scan rate increases, and the bead with best rate performance (rGO/PANI (63.5 wt%)) shows 50.2 % capacitance retention at a scan rate of 200 mV/s compared to the original scan rate of 2 mV/s. To evaluate the contribution of PANI at different loadings, rGO/PANI composite samples with 25.0 wt% to 90.2 wt% PANI were prepared by 13 to 150 electro-deposition cycles, and their CV characteristics and specific capacitances were tested systematically (Fig. 5c). It is interesting to see that an increase of PANI loading resulted in enhancement of specific capacitance, reaching a maximum value of 669 F/g at 63.5 wt% PANI. After that, the capacitance started to drop (e.g. 420 F/g at 90.2 wt% PANI) when the PANI loading kept increasing further. The existence of an optimal (or appropriate) PANI loading was also reported in other rGO-pseudo polymer systems.29,31 Here, in our sandwiched structure, this appropriate PANI loading corresponds to a relatively thin PANI layer thickness of ~ 34 nm, which is a critical factor to reduce the internal resistance as well as improve the specific capacitance. We have tested the specific surface area and pore size distribution of rGO and rGO/PANI (60.3 wt%) beads (Figure S10). The result shows that the rGO/PANI beads owe larger specific surface area of 151.3 m2/g than the rGO beads (117.7 m2/g), with more percentage of pores in the range of 2-100 nm. It reveals that the thin PANI layer can not only act as pseudo coating 9



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but also can increase the specific surface area of the rGO beads, resulting in improved specific capacitance. Continual deposition of PANI from 63.5 wt% to 90.2 wt% leads to a relatively thick PANI layer of ~89 nm. The Nyquist plots of EIS spectra for rGO/PANI (38.0, 63.5 and 90.2 wt%) beads are shown in Figure 5d, with each plot composed of two semicircles at high and mid-frequency, straight line at low frequency. When the PANI loading increases to 90.2 wt%, the plot shows a larger diameter of the first semicircle, corresponding to an increased charge-transfer resistance (Rct).This indicates that the relatively thick PANI coating is not favorable for efficient charge transfer between the electrode and electrolyte, resulting in reduced capacitance. The second semicircle at the mid-frequency region looks more like a line, which is associated with the electrolyte penetration within the porous rGO/PANI electrode.37, 38 In addition, in the low frequency part of the plot, lines of rGO/PANI beads tend to be vertical along the imaginary axis, indicating an ideally capacitive behavior due to the fast redox reaction of PANI. Compared to the nearly electric double layer capacitance of pure rGO beads, the enhanced capacitance of rGO/PANI beads derives from the faradaic reactions of PANI layer on the electrode/electrolyte surface. The CV curve of rGO/PANI (63.5 wt%) bead at scan rate of 10 mV/s shows three sets of redox peaks (C1/A1, C2/A2 and C3/A3), which are attributed to the leucoemeraldine/emeraldine (peaks between 0 and 0.4 V) and emeraldine/pernigraniline (peaks between 0.4 and 0.7 V) transitions of PANI, during the charge and discharge processes (Fig. S9d). Such pseudo-capacitance behavior stems from the uniform PANI coating on the graphene skeleton throughout the composite beads, which is beneficial for improving the specific capacitance. When the scan rate increases from 2 to 200 mV/s, the redox peaks shifted gradually and became less 10


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distinct, likely due to insufficient redox reactions within the porous bead at fast scan rates (Fig. 5e). Consistently, the galvanostatic charge-discharge curves at a current density of 5 A/g exhibits nearly symmetrical triangular curves (Fig. S9e). In addition, we have measured the cycling behavior of the rGO/PANI beads, which shows capacitance retention of 87.2 % and 72.6 % after 1000 and 3000 charge and discharge cycles tested at a scan rate of 100 mV/s, respectively. The capacitance becomes stable after 2000 cycles (Fig. 5f). Conclusions In summary, we demonstrated direct fabrication of open-porous rGO beads containing center-to-surface radial channels by freeze-casting GO solution in liquid nitrogen. Using the rGO beads as a 3D template for electro-deposition of pseudo-polymers, we further constructed rGO/PANI composite beads by depositing smooth PANI layers on both sides of rGO sheets. Our graphene based composite beads showed high specific capacitance and excellent cycling stability, indicating potential application as porous electrodes for energy devices. The sandwiched pore wall structure also could be formed in graphene aerogels with versatile shapes made by different methods (e.g. sol-gel). Many pseudo-polymers or other materials might be explored to make bead-shape graphene-based composite aerogels with energy and environmental applications.

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Experimental Materials Graphene oxide (GO) sheets were synthesized via the modified Hummers’ method.39 Aniline (99.5%), sulfuric acid (98 % H2SO4) and KCl were purchased from Sinopharm Chemical Reagent Co., Ltd. Liquid nitrogen (-196 °C, degree of purity) was used as cooling medium for freeze casting. Synthesis of rGO beads and rGO/PANI beads The rGO beads were fabricated by freeze casting method. First, aqueous solutions of GO sheets with concentration of 5 mg/ml were injected through a syringe needle (diameter of 0.7 mm) into liquid nitrogen, respectively. Then the frozen GO droplets were freeze dried to make porous GO beads, followed by thermal reduction at 800 °C. In comparison, rGO beads were also prepared from GO solution with concentration of 2.6 and 7.6 mg/ml. Acid treatment was conducted by adding several rGO beads into 30 vol% HNO3 aqueous solution for 10 min. Then these beads were taken out and washed in deionized water until the pH was about 7. The rGO/PANI beads were prepared by electro-deposition method in a three-electrode system with aqueous electrolyte of 0.05 M aniline and 1 M H2SO4, in a potential range of -0.2 V to 0.8 V at a scan rate of 50 mV/s. The electro-polymerization was performed by three-electrode method using rGO beads, platinum plate and saturated calomel electrode (SCE) as the working electrode, counter electrode and reference electrode, respectively. The weight content of PANI in the rGO/PANI beads was controlled by the number of deposition cycles. We use deposition cycles from 13-200 to control the PANI content (25.0 wt% to 93.5 wt%) in the rGO/PANI beads. After deposition, the rGO/PANI beads were washed in deionized water until the pH was about 7, and finally freeze dried for 12 h. Sample characterization Sample morphology and structure were characterized by SEM (LEO-1530 field emission electron microscope) and XRD spectroscope (Bruker-D8 Discover). XPS spectra were obtained using PHI Quantera SXM and FTIR spectra were obtained using Nicolet 6700 FTIR. The XPS spectra were calibrated to the C-C peak at 284.8 eV. Raman spectra of the samples were obtained by a RM 200 Microscopic Confocal Raman Spectrometer (Renishaw PLC, England) with a 532 nm laser. The surface area and pore size distribution of rGO and rGO/PANI were measured by the Brunner-Emmet-Teller method and Barrett-Joyner-Halen method, respectively. Electrochemical Measurement 12


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The CV, galvanostatic charge-discharge and EIS curves were measured using three-electrode method in a CHI660D electrochemical workstation. A single rGO or rGO/PANI bead was tested one time as working electrode. The bead was clamped by two polymeric blocks, and the blocks were twisted by platinum wire connected to the beads as electrode collector. A platinum plate and SCE electrode were used as the counter electrode and reference electrode, respectively. Before the deposition of PANI, the CV curves of rGO beads were measured at scan rates of 2-10000 mV/s, and all electrochemical measurements were carried out in 2 M KCl aqueous electrolyte at room temperature. After the PANI deposited, the CV curves of rGO/PANI beads were measured at scan rates of 2-10000 mV/s between -0.2-0.8 V, and all electrochemical measurements were carried out in in a 1 M H2SO4 aqueous electrolyte. For comparison, the rGO beads were measured in 1 M H2SO4 electrolyte between 0-1 V. EIS measurements were tested in the frequency range of 100 KHz to 0.01 Hz at open circuit potential. The specific capacitance was calculated through the corresponding CV curve. The cycling stability was tested at a scan rate of 100 mV/s for 1000 cycles. The specific capacitance of the rGO and rGO/PANI beads were calculated from the CV curves according to the following equation.

C=‫׬‬

ூ ௠

ܸ݀/‫( ܸ∆ݒ‬1)

Where I is the response current (A), m is the mass of a rGO or rGO/PANI bead (g), ∆V is the potential range in the CV (V), v is the potential scan rate (mV/s).

Acknowledgement. A. Cao acknowledges the National Natural Science Foundation of China (NSFC) under the Grant No. 51325202. Supporting Information Available: SEM of rGO beads with different initial GO concentrations; FTIR, Raman, XPS and XRD results; Supercapacitor performance of rGO beads in 2 M KCl and 1 M H2SO4 electrolyte; low-magnification and High-magnification SEM images of the pore walls of rGO/PANI beads; Change of content and thickness of PANI with different electro-deposition cycles; Supercapacitor performance of rGO/PANI beads with 38.0 wt%, 63.5 wt% and 90.2 wt% PANI contents; BET results of rGO and rGO/PANI beads; This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. Fabrication of rGO and rGO/PANI composite beads. (a) Illustration of the freeze-casting process to make rGO beads and subsequent electro-deposition of rGO/PANI beads. (b) Photos of rGO beads and rGO/PANI beads. (c) Optical images of rGO beads and rGO/PANI beads. Red arrows point to the compression and recovery direction. (d) Mechanical deformation of rGO beads in air and under water. (e) Recovery of the shrunk rGO beads in water. Red arrows point to the shrinking and recovery direction. (f) Photo showing the rGO and rGO/PANI beads with different PANI contents shrinking to different degrees after loss of water.

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Figure 2. Structural characterization and electrochemical properties of the rGO beads. (a) SEM image of the surface morphology in a rGO bead. Inset shows the entire bead. (b) Cross-section of a bead showing radial channels emitting from the sphere center to its surface (dotted lines showing radial orientation). (c) SEM image the cross-section showing parallel and uniform channels. (d) Enlarged view of the pore walls composed of thin and flexible rGO sheets. (e) CV curves of rGO beads at scan rates of 2 to 200 mV/s in 2 M KCl electrolyte with potential range of 0 V to 0.8 V. (f) Galvanostatic charge-discharge curves of rGO beads at current densities of 5 to 25 A/g.

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Figure 3. PANI-rGO-PANI sandwiched structure in rGO/PANI beads. (a) SEM image showing pore walls of rGO/PANI beads with thick PANI layers. (b) Uniform PANI coating on rGO sheets. White arrow points to the exposed rGO sheets from the fracture section. (c) A broken pore wall (see white arrow) in the rGO/PANI bead. (d) Fracture sections of rGO/PANI pore walls. (e) A junction area of three adjacent pores coated by PANI layers. White arrow points to the pore walls. (f) Illustration of the sandwich structure and the PANI-coated junction structure.

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Figure 4. XPS, FTIR and Raman characterization of rGO/PANI beads. (a) XPS results obtained from the rGO and rGO/PANI beads. (b) N 1s spectra for rGO/PANI beads. (c) FTIR spectra and (d) Raman spectra for rGO and rGO/PANI beads.

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Figure 5. Supercapacitor performance of rGO/PANI beads. (a) CV curves of rGO/PANI (38.0, 63.5 and 90.2 wt%) beads measured at a scan rate of 10 mV/s. (b) Specific capacitances of rGO/PANI (38.0, 63.5 and 90.2 wt%) beads at different scan rates. (c) Change of weight content of PANI in the rGO/PANI beads after different electro-deposition cycles (13-150 cycles), and the corresponding specific capacitances. (d) EIS of rGO/PANI beads (38.0, 63.5 and 90.2 wt%) beads. (e) CV curves of rGO/PANI (63.5 wt%) bead measured at a scan rate of 2 to 200 mV/s. (f) Cycling test showing capacitance retention of 87.2 % and 72.6 % after 1000 and 3000 charge and discharge cycles at a scan rate of 100 mV/s, respectively. Inset shows selected CV curves of every 200 cycles.

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Table of Contents:

Graphic for manuscript. Based on the porous graphene beads with radial channels, the polymer-coated graphene/PANI beads have been fabricated, owning sandwiched pore walls with thickness-tailored PANI layers. The composite beads show improved supercapacitor performance.

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Polymer-Coated Graphene Aerogel Beads and Supercapacitor

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