Polyaniline Nanocomposite with

Article pubs.acs.org/IECR

Preparation of Graphene Oxide/Polyaniline Nanocomposite with Assistance of Supercritical Carbon Dioxide for Supercapacitor Electrodes Guiheng Xu, Nan Wang, Junyi Wei, Leilei Lv, Jianan Zhang, Zhimin Chen, and Qun Xu* College of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450052, People's Republic of China S Supporting Information *

ABSTRACT: We report a facile strategy to prepare graphene oxide (GO)/polyaniline (PANI) nanocomposite by in situ polymerization with the assistance of supercritical carbon dioxide (SC CO2). The morphology and chemical structure of the synthesized samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Xray diffraction (XRD), FT-IR, Raman, and UV−vis spectrophotometry. As a result, PANI nanoparticles uniformly cover the GO sheets with the help of SC CO2, and the formation mechanism is suggested. The morphologies of GO/PANI nanocomposites can be controlled through adjusting the concentration of aniline. The GO/PANI nanocomposite exhibits better specific capacitance and cycle stability than pure PANI and GO owing to the synergistic effect of GO and PANI nanoparticles, making it promising application in electrochemical devices.

1. INTRODUCTION Supercapacitors, as important devices in energy storage and conversion systems, have attracted great attention in recent years due to their high power density and long cycle life compared with rechargeable batteries.1−5 Recently, great effort has been focused on developing advanced supercapacitors by exploring novel electrode materials with high performance.6−8 At present, carbon-based materials (active carbon,9,10 carbon nanotubes,11 and graphene12,13) are usually employed as electrode materials for electrical double layer capacitors because of their excellent electrical conductivity and large surface area. Transition metal oxides14,15 and lightweight conducting polymers16−18 with high specific capacitance are usually used as electrode materials of pseudocapacitors. In order to obtain high performance eletrode materials, nanocomposites of conducting polymers and carbonbased materials have been investigated because they combine the unique properties of individual material and show their special synergistic effects that can be used in electrode materials of supercapactiors. Graphene, a two-dimensional form of graphite, has attracted great attention because of its high surface area, excellent mechanical properties, and conductivity.19 Graphene oxide (GO) is a single sheet of graphite oxide bearing oxygen functional groups (i.e., epoxide, hydroxyl, carboxyl groups) on their basal planes and edges, and it can be synthesized by exfoliation of graphite oxide.20 The tunable oxygen functional groups and good compatibility with polymer have made GO a promising material to synthesize functional nanocomposites.21 Among conducting polymers, such as poly(3,4ethylenedioxythiophene)s (PEDOTs), polyanilines (PANIs), polypyrroles (PPYs), and polythiophenes (PThs), PANIs have received a great deal of attention because of their low cost, ease of synthesis, good environmental stability, and high specific capacitance from multiple redox states.22−24 However, the relatively poor cycle life restricts their practical application in supercapacitors. To solve this problem, great effort has been © 2012 American Chemical Society

made to combine carbon materials with PANI, which is an effective approach to obtain electrode materials with high capacitance performance and good cycle stability.25,26 Small nanoscaled and nanostructured PANI can reduce the diffusion length, enhance the electroactive regions, and further increase the capacitive performance of nanocomposites.27 Therefore, combining nanometer-sized and nanostructured PANI with carbon materials has been extensively studied. For instance, Fan et al. fabricated hierarchical nanocomposites of polyaniline nanorods grown on the surface of carbon nanotubes for highperformance supercapacitor electrodes.28 Xu et al. prepared hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets for enengy storage.29 Li et al. synthetized polyaniline nanofibers/graphene nanaosheets composites as electrode for supercapacitors.30 In this paper, we report a facile method to prepare such a nanocomposite of PANI nanoparticles uniformly covering GO sheets with the assistance of supercritical carbon dioxide (SC CO2). Compared with the traditional preparation method of PANI/GO or PANI/GE, this newly introduced method is simpler and easy to control the microstructure or morphology of the obtained nanocomposite. SC CO2 possesses many advantages (such as gas-like diffusivity, liquid-like density, negligible surface tension, and environmentally benign properties, etc.), which have been widely studied for polymerization reactions.31 Continuous precipitation polymerization of vinylidene fluoride and acrylic acid in SC CO2 has been studied by DeSimone and co-workers.31,32 However, to the best of our knowledge, the polymerization of conducting polymers in SC CO2 has not been reported. The polymerizations of aniline on the surface of GO with the assistance of supercritical fluid Received: Revised: Accepted: Published: 14390

July 1, 2012 October 13, 2012 October 17, 2012 October 17, 2012 dx.doi.org/10.1021/ie301734f | Ind. Eng. Chem. Res. 2012, 51, 14390−14398

Industrial & Engineering Chemistry Research

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Figure 1. SEM images of (a) GO, (b,c) pure PANI, and PANI/GO nanocomposites with different conccentration of aniline: (d,e) 0.05 M; (f,g) 0.08 M; (h,i) 0.1 M. Other conditions: aniline/APS = 3/1 (molar ratio), GO = 0.3 mg/mL, and the reaction was carried out in SC CO2 (40 °C, 12 MPa, 3 h).

technology is reported for the first time. A series of GO/PANI nanocomposites at different aniline concentrations have been prepared. The obtained various GO/PANI nanocomposites exhibit remarkably enhanced electrochemical performance and excellent cycle stability compared with GO and pure PANI, making it a promising application of electrochemical capacitive energy storage.

gradually under stirring and cooling to prevent the temperature of the mixture from exceeding 20 °C. The ice-bath was then removed and the mixture was stirred at 35 °C for 2 h. Successively, deionized water (184 mL) was slowly added into the mixture and the diluted suspension was maintained at 98 °C for 15 min. After that, deionized water (560 mL) and 30% hydrogen peroxide (10 mL) were added into the suspension to end the reaction, after which the suspension turned bright yellow. The brilliant-yellow mixture was centrifuged and washed with 10% HCl aqueous solution (1 L) to remove the residual metal ions followed by deionized water to remove the acid until the pH of filtrate was neutral. The resulting solid was dried in vacuum at 40 °C. Exfoliation was achieved by sonicating oxidized graphite dispersion for 30 min at 240 W. The prepared GO was dried in a vacuum at 40 °C for 24 h. Preparation of Polyaniline-Graphene Oxide Composites. In a typical procedure, 3 mg of GO was added into 9 mL ethanol solution and ultrasonicated until GO was uniformly dispersed. A certain amount of aniline monomers was then added into the above solution and sonicated for 30 min at room temperature to form a homogeneous mixture of GO and aniline monomers. The oxidant, ammonium persulfate (APS) was dissolved in 1 mL of 1 mol/L aqueous HCl (the molar ratio of

2. EXPERIMENTAL SECTION Preparation of Graphene Oxide (GO). GO was prepared from graphite powder by a modified Hummers method.33,34 In a typical synthesis, graphite powder (10 g) was put into an 80 °C solution of concentrated H2SO4 (40 mL), K2S2O8 (8.4 g), and P2O5 (8.4 g). The dark blue mixture was kept at 80 °C for 4.5 h. The mixture was cooled to room temperature, diluted with deionized water (90 mL), transferred to a large beaker, and left overnight. The mixture was then carefully filtered and washed with deionized water using a 0.22 μm polycarbonate filter until the pH value of the rinsewater became neutral. The product was dried at 40 °C for 24 h under vacuum. The preoxidized graphite was then subjected to oxidation by Hummers’ method. The preoxidized graphite powder (4 g) was added into cold (0 °C) concentrated H2SO4 (92 mL). KMnO4 (12 g) was then added 14391

dx.doi.org/10.1021/ie301734f | Ind. Eng. Chem. Res. 2012, 51, 14390−14398


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Figure 2. TEM images of (a) GO, and PANI/GO nanocomposites with different conccentration of aniline: (b,c) 0.05 M; (d−f) 0.08 M; (g−i) 0.1 M. Other conditions: aniline/APS = 3/1 (molar ratio), GO = 0.3 mg/mL, and the reaction was carried out in SC CO2 (40 °C, 12 MPa, 3 h).

27 FTIR spectrometer (Bruker) in the absorption mode with resolution of 2 cm−1. The Raman measurements were carried out on a Renishaw Microscope System RM2000 with a 50 mW Ar+ laser at 514.5 nm, UV−vis spectra was performed on a Shimadzu UV-240/PC with a scanning speed at 200 nm/min and a bandwidth 0.1 nm. Electrochemical Performance Measurement. The test electrodes were prepared by mixing 85 wt % active material, 10 wt % acetylene black, and 5 wt % poly(tetrafluoroethylene) (used as a binder, PTFE 60% dispersion in H2O, Sigma Aldrich) to form a uniform mixture. The mixture was coated and then pressed onto a stainless steel (10 MPa). Electrochemical measurements were carried out in 1 M H2SO4 using twoelectrode system with a separator between the two symmetrical working electrodes. The electrochemical performances of prepared electrodes were tested by cyclic voltammetry (CV) and galvanostatic charge/discharge tests. The potential range for CV tests was −0.2 to 0.8 V, and the scan rate was 5, 10, 20, 50, and 100 mV s−1. Galvanostatic charge/discharge measurements were done from 0 to 0.96 V at different current densities. The experiments were performed using a CHI 660D electrochemical workstation controlled by a computer.

aniline to APS is 3:1), and the APS solution was then added rapidly into the solution of GO and aniline monomers. The mixture was quickly transferred into the SC CO2 apparatus to polymerize at the condition of suitable temperature and pressure under magnetic stirring. The time was controlled to 3 h for all samples and then the system was slowly depressurized. The nanocomposites were collected by centrifugation and repetitively washed with water and ethanol for several times until the supernatant became colorless. The product was dried at 60 °C for 24 h under vacuum. For comparison, the pure PANI was synthesized chemically at 0.1 M of aniline in the absence of graphene oxide via the similar procedure above. Characterization. A Digital Instruments MultiMode scanning probe microscope with a Nanoscope IIIA controller in tapping mode was used for the AFM measurements. Fieldemission scanning electron microscope (FE-SEM, JEOR JSM6700F) was used to characterize the morphology of the samples, Transmission electron microscope measurements (TEM, FEI Tecnai G2 20) were conducted with an accelerating voltage of 120 kV. X-ray diffraction (XRD) patterns of samples were measured on a Y-2000 X-ray Diffractometer with copper Kα radiation (λ = 1.5406 Å) operating at 40 kV and 40 mA. Fourier transform infrared spectra (FTIR) were recorded on a TENSOR 14392



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Scheme 1. Schematic Illustration of the Process for Preparation of GO/PANI Nanocomposite with Assistance of Supercritical CO2

3. RESULTS AND DISCUSSIONS Morphology Analysis and Formation Mechanism. The morphology of the GO is characterized by TEM and atomic force microscope (AFM) in a tipping mode. From TEM images in Figure S1a of the Supporting Information, SI, the layer-like morphology of GO sheets with the size of several micrometers is clearly shown. The thin nanoplate motif of the GO sheet was also confirmed by AFM (Figure S1b of the SI). The AFM micrograph of GO shows that the thickness of the resulting GO sheets is about 3.3 nm, suggesting that the GO film consists of two or three atomic layers thickness in structure (Figure S1c of the SI). The surface morphology and structure of GO, pure PANI, and GO/PANI nanocomposites with different concentration of aniline are observed by FE-SEM and TEM, and the results are shown in Figures 1 and 2, respectively. The PANI prepared in the absence of GO shows irregular stacking spherical morphology with the diameter of about 150 nm (Figure 1b,c). The synthesized GO/PANI nanocomposites are completely different from pure irregular spherical PANI. Figure 1d−i shows the FESEM images of GO/PANI nanocomposite with different concentration of aniline, PANI nanoparticles uniformly coated on GO sheets can be clearly observed for all of the nanocomposites. In addition, some spherical PANI agglomerates are also observed on the surface of GO. The morphologies of GO/PANI nanocomposites are affected by the concentration of aniline monomer. The thickness of the nanocomposites increases with the concentration of aniline from 0.05 to 0.1 M, and the thickness can reach about 100 nm when the concentration of aniline is 0.1 M. The TEM images also reveal that the GO sheets are compactly covered by PANI nanoparticles, from the high resolution TEM images (Figure 2f), it can be observed clearly that the PANI nanoparticles and their sizes are about 12 to 15 nm, and the PANI nanoparticles become denser with the increase of aniline concentration. According to the experimental results above, a formation mechanism about GO/PANI nanocomposites is illustrated in Scheme 1. The in situ polyemerization method with the assistance of SC CO2 is an effeciant route to prepare GO/ PANI nanocomposites. And ethanol in this system is indispensable because it not only improved the dispersion GO,

but also facilitated the diffusion of aniline monomer on the surface of GO.17 Because of the excellent compatibility of supercritical CO2 and ethanol, CO2 can help to further improve the dispersion of monomer in the solution system, so compared the pure ethanol system, the addition of SC CO2 can enhance the dispersion of the formed PANI nanoparticles. So SC CO2 plays an important role in coating GO with PANI nanoparticles. The zero surface tension of SC CO2 makes the ethanol better wet basal planes of GO and helps the anilinium ions easily absorb on the surface of GO. In addition, at the beginning of the chemical oxidation polymerization of PANI, GO with a great number of oxygen functional groups on its basal planes and edges, can supply numbers of active sites for the uniform adsorption of anilinium ions due to the eletrostatic attraction.35 When APS solution is added into the GO/aniline suspension, the anilinium ions absorbed on GO sheets begin to polymerize from the active sites, forming the structure of PANI nanoparticles uniform coating GO sheets and the phenomena can be observed from Figure 2f,I with high resolution TEM. So PANI is coated on the GO sheet due to the π−π interaction. This structure not only makes full use of the large specific area of GO, but also increases the contact of PANI nanoparticles with electrolyte, and thus may be favorable for increasing the electrochemical performance of GO/PANI nanocomposite. Structure Characterization of GO/PANI Nanocomposite. To confirm the structure of GO/PANI nanocomposite, the GO, pure PANI, and GO/PANI nanocomposite are subjected to XRD, FT-IR, Raman, and UV−vis spectroscopy analysis. Figure 3 shows the XRD patterns of GO, pure PANI, and GO/PANI nanocomposite. The XRD pattern of GO exhibits a strong, sharp diffraction peak at 2θ = 12.53°, corresponding to interlayer spacing of 0.63 nm of GO sheets.36 For GO/PANI nanocomposite, it can be observed that the diffraction peaks assigned to the interlayer distance between the GO sheets gradually moves from 12.53° to a smaller angle at 8.75°, corresponding to an interspacing distance of 1.02 nm. Such an expansion of layer distance is due to the intercalation of PANI between the GO sheets.37 For pure PANI, the two broad peaks centered at 2θ = 19.92 and 25.24° are attributed to (020) and (200) crystal planes of PANI in its emealdine salt form.38 The XRD pattern of the 14393



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summarizes all of the important absorption bands and their assignments. For GO, the bands at 3426, 1727, 1621, and 1400− 1052 cm−1 are attributed to the stretching vibration of the OH, the stretching vibration of CO in carboxyl groups, the stretching vibration of CC in the quinoid ring, and the stretching vibration of C−O in C−OH/C−O−C functional groups, respectively.23,29 For pure PANI, the characteristic bands located at 1568 and 1484 cm−1 are ascribed to stretching vibration of CC in quinoid and benzene rings, respectively, indicating the presence of oxidation state of PANI (ES).39 The bands at 1298, 1241, 1123, and 819 cm−1 are attributed to the C− N stretching vibration of the secondary aromatic amine, stretching vibration of C−N·+ in protonic acid doped PANI, CN stretching vibration (−NquinoidN−) and out of plane bending vibrations of C−H band in the aromatic ring, respectively.40 And the bands between 800 and 500 cm−1 are owing to the C−H vibration of benzene rings.40 The resulting GO/PANI nanocomposite has similar characteristic bands as compared to the bands of pure PANI. However, some differences from pure PANI are noted. The peaks at 1123, 1241, 1484, 1568 cm−1 in pure PANI shift to higher frequencies of 1140, 1248, 1494, 1582 cm−1 in GO/PANI nanocomposite, respectively. It can be explained that the interaction between polyaniline chains and GO sheets restrict the modes of vibrations in polyaniline, which is reflected in the related high wavenumber of the FTIR peaks. Raman spectroscopy is employed to further confirm the interaction between PANI and GO, which is a powerful technique for investigating the interfacial interaction between polymer and carbon-based materials. Figure 5 shows the Raman

Figure 3. XRD patterns of (a) GO, (b) pure PANI, and (c) GO/PANI nanocomposite.

GO/PANI nanocomposite exhibits similar crystalline peaks compared to that observed from pure PANI, indicating that no additional crystalline order being introduced into the GO/PANI nanocomposite. The structure of the nanocomposite is further investigated by FT-IR measurements. The FT-IR spectra of GO, pure PANI, and GO/PANI nanocomposite are shown in Figure 4. Table 1

Figure 4. FTIR spectra of (a) GO, (b) pure PANI, and (c) GO/PANI nanocomposite.

Table 1. FT-IR Absorption Bands and Their Assignment of GO, PANI, and GO/PANI Nanocomposites (4000−400 cm−1) frequency (cm−1)

assignment

material

3426 1727 1621 1400 1052 1568 → 1582 1484 → 1494 1298 1241 → 1248 1123 → 1140 819 800−500

V(OH) V(CO) δ(OH) V(C−O/COOH) V(C−O/C−O-C) V(CC/Q) V(CC/B) V(C−N)) V(C−N·+) V(CNH+) Δ(C−H/B) (C−H/B)

GO GO GO GO GO PANI, GO/PANI PANI, GO/PANI PANI, GO/PANI PANI, GO/PANI PANI, GO/PANI PANI, GO/PANI PANI, GO/PANI

Figure 5. Raman spectra of (a) GO, (b) pure PANI, and (c) GO/PANI nanocomposite.

spectra of GO, pure PANI, and GO/PANI nanocomposite. The pure GO shows two characteristic peaks at 1364 and 1598 cm−1, corresponding to the D-band (C−C, the disordered graphite structure) and G-band (CC, sp2 -hybridized carbon), respectively.41 For pure PANI and GO/PANI nanocomposite, the bands at 1160, 1233, 1400, 1484, and 1592 cm−1 are assigned to in plane C−H bending of quinoid ring, in plane C−H bending of benzenoid ring, C−C stretching of quinoid ring, CC stretching of quinoid ring, and CC stretching of benzenoid ring, respectively, indicating the presence of doped PANI structure.42 However, compared to the spectra of pure PANI, the C−N·+ stretching peak shifts to the higher wavenumber from 1332 to 1341 cm−1 in GO/PANI nanocomposite, which can be explained by the electrostatic interaction between the C−N+ 14394



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species of PANI and the COO− of GO.40 The electrostatic interaction strongly restricts the C−N+ stretching vibrations in the PANI molecule, which leads to higher wavenumber band shifts. The UV−vis spectra of GO, pure PANI and GO/PANI nanocomposite are shown in Figure 6. An absorption peak at 225

nm from GO is observed. GO/PANI nanocomposite shows three characteristic absorption bands at around 360, 450, and 860 nm, which are attributed to the formation of oxidation state of PANI (ES). The absorption peak at 360 nm corresponds to the π−π* electron transition of benzenoid rings, the absorption peak at 450 and 860 nm are attributed to the polaron-π* transition and π-polaron transition, respectively.43 Therefore, the PANI in the GO/PANI nanocomposite is in the highly doped state, which are supposed to possess high capacitance performance. Electrochemical Properties. In order to evaluate the electrochemical performance of GO/PANI nanocomposite as electrode materials for supercapacitor, cyclic voltammetry (CV), and galvanostatic charge/discharge are employed to fully characterize the electrochemical performance. Figure 7a shows the CVs of GO, pure PANI, and GO/PANI nanocomposite. In the CV curves, GO shows very low current density response probably due to the low conductivity. There are a couple of redox peaks appeared for pure PANI and GO/PANI nanocomposite, which are attributed to the redox transition of PANI between a semiconducting-state (leucoemeraldine form) and a conductingstate (polaronic emeraldine form).27,35 Therefore, different from electric double-layer capacitance of carbon-based materials, the capacitance of PANI/GO nanocomposites mainly comes from redox reactions of PANI at the electrode/electrolyte surface. It can be observed that the current density response of GO/PANI is apparently larger than that of pure PANI, indicating higher

Figure 6. UV−visible spectra of (a) GO, (b) PANI, and (c) GO/PANI nanocomposite.

Figure 7. Electrochemical performance of GO/PANI nanocomposite prepared at 0.1 M concentration of aniline. (a) CV curves of GO, pure PANI, and GO/PANI nanocomposit at 10 mV s−1 in 1 M H2SO4 solution. (b) CV curves of GO/PANI nanocomposite at different scan rates of 5, 10, 20, 50, and 100 mV s−1. (c) Galvanostatic charge-discharging curves of pure PANI and GO/PANI nanocomposite at current density of 200 mA g−1. (d) Specific capacitance of pure PANI and GO/PANI nanocomposite at different current densities. 14395



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0.2 A/g, respectively. Even at 2 A/g, the specific capacitance of GO/PANI nanocomposite still remain at 196 F g−1, while only 87 F g−1 remain for pure PANI. Table 2 summarizes specific capacitances (F g−1) of pure PANI and GO/PANI nanocomposites with different aniline concentration at different current densities. The cycle stability of pure PANI and GO/PANI nanocomposite was measured by long-term charge/discharge cycling at a current density of 1 A/g (Figure 8). The capacitance

specific capacitance. Besides, the specific capacitance of GO/ PANI nanocomposite is enhanced to different degrees as the concentration of aniline changes (Table 2). The increased Table 2. Specific Capacitances (F g−1) of Pure PANI and GO/ PANI Nanocomposites with Different Aniline Concentration at Different Current Densities in 1M H2SO4 samples

0.2 (A/g)

0.6 (A/g)

1.0 (A/g)

2.0 (A/g)

PANI 0.05M 0.08M 0.1M

378 404 421 425

161 290 315 325

150 250 260 280

87 141 158 196

specific capacitance may be attributed to the morphology changes of PANI and the combining with GO. After combining, PANI coats uniformly on GO sheets in the form of nanoscale particles (Figure 1), ensuring full utilization of specific surface area of electrode materials. Thus the specific capacitance is significantly improved. So the introduction of GO into the nanocomposite and the high specific surface area of PANI nanoparticles play an important role in enhancing electrochemical performance of GO/PANI nanocomposite Figure 7b shows the CV curves of GO/PANI nanocomposite at different scan rates from 5 to 100 mV s−1. It can be noted that with an increase of scan rate, the cathodic peaks shift positively and the anodic peaks shift negatively, which is mainly owing to the resistance of the electrode.6 In addition, the peak current density obviously increases with rising scan rate and the shape of CV curve does not change evidently below 20 mV s−1, indicating a good rate capability for GO/PANI nanocomposite electrode.28 Figure 7c shows the charge/discharge curves of all samples at a current density of 0.2 A g−1 with a potential range of 0−0.96 V. For this symmetrical two electrode system, the specific capacitance (Cs) of electrode materials can be calculated according to the following equation;44 Cs = 2C /m = 2I Δt /ΔVm

Figure 8. Cycle stability of pure PANI and GO/PANI nanocomposite during the long-term charge/discharge process.

retention of GO/PANI nanocomposite still keeps 83% of its original value after 500 cycle tests, while pure PANI keeps only 72% of its original value. These results indicate that the GO/ PANI nanocomposite exhibits better cycle stability and lifetime than pure PANI. We hope to enhance the cycle stability of GO/ PANI nanocomposite further in our following study.

(1) −1

4. CONCLUSIONS

where Cs is specific capacitance (F g ), I is the discharge current (A), Δt is discharge time (s), ΔV is the potential drop (V), and m is the mass of single electrode materials (g). On the basis of the above equation, the specific capacitance of GO/PANI nanocomposite (425 F g−1) is much higher than pure PANI (378 F g−1) at 0.2A/g. We believe that the relative high specific capacitance of GO/PANI nanocomposites is owing to the following two factors. First, PANI nanoparticles with smaller size facilitate better utilization of PANI as electrode materials. Second, nanosized PANI shorten the charge diffusion distance during the charge/discharge process.6 It is also found that the specific capacitance of GO/PANI nanocomposites is highly affected by the concentration of aniline monomer. The specific capacitance of GO/PANI nanocomposites increases with the concentration of aniline from 0.05 to 0.1 M, until reaches the maximum value (425 F g−1) at 0.1 M aniline (Table 2). Figure 7d shows the specific capacitance of pure PANI and GO/PANI nanocomposites at different aniline concentration as a function of different current densities. It is obvious that the specific capacitance of pure PANI and GO/PANI nanocomposites decreases with the increase of discharge current densities, but the specific capacitance of GO/PANI nanocomposites is always higher than that of pure PANI at the same current density. The specific capacitance of pure PANI and GO/ PANI nanocomposites is 378 and 425 F g−1 at current density of

In summary, a GO/PANI nanocomposite with PANI nanoparticles uniformly coated on GO sheets has been successfully prepared with the assistance of supercritical carbon dioxide (SC CO2). The morphologies of GO/PANI nanocomposites can be controlled through adjusting the concentration of aniline. XRD verifies the intercalation of PANI between the GO sheets. The electrostatic interaction between GO and PANI nanoparticles are verified by FT-IR and Raman. GO/PANI nanocomposite with aniline concentration at 0.1 M exhibits high specific capacitance (425 F g−1) at a current density of 0.2 A g−1. The excellent electrochemical capacitance and cycle stability is due to the synergistic effect between the small nanosized PANI nanoparticles and GO with high specific surface area. This study introduces a facile strategy to prepare the nanocomposites of conducting polymer nanoparticles into a uniform coating of carbon materials, and these nanocomposites may have promising applications in energy storage devices.

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ASSOCIATED CONTENT

* Supporting Information S

The TEM and tapping-mode AFM images of GO. This material is available free of charge via the Internet at http://pubs.acs.org. 14396



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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 371 67767827; fax: +86 371 67767827; e-mail: [email protected] . Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We are grateful for the National Natural Science Foundation of China (Nos. 51173170, 50955010, and 20974102), the financial support from the Innovation Talents Award of Henan Province (114200510019) and the Program for New Century Excellent Talents in University (NCET).

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