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Fabrication of highly ordered polyaniline nanocone on pristine graphene for high-performance supercapacitor electrodes
Journal of Physics and Chemistry of Solids 115 (2018) 148–155
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Fabrication of highly ordered polyaniline nanocone on pristine graphene for high-performance supercapacitor electrodes Ningning Song, Wucong Wang, Yue Wu, Ding Xiao, Yaping Zhao * School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Hybrids Supercapacitor Polyaniline Nanocone
The hybrids of pristine graphene with polyaniline were synthesized by in situ polymerizations for making a highperformance supercapacitor. The formed high-ordered PANI nanocones were vertically aligned on the graphene sheets. The length of the PANI nanocones increased with the concentration of aniline monomer. The specific capacitance of the hybrids electrode in the three-electrode system was measured as high as 481 F/g at a current density of 0.1 A/g, and its stability remained 87% after constant charge-discharge 10000 cycles at a current density of 1 A/g. This outstanding performance is attributed to the coupling effects of the pristine graphene and the hierarchical structure of the PANI possessing high specific surface area. The unique structure of the PANI provided more charge transmission pathways and fast charge-transfer speed of electrons to the pristine graphene because of its large specific area exposed to the electrolyte. The hybrid is expected to have potential applications in supercapacitor electrodes.
1. Introduction A supercapacitor is a promising energy device for electrical vehicles, mobile electronic devices, and grid energy storage due to its high power density, long cycle life and fast charging rate [1,2]. Carbon-based materials, transition metal oxides and conducting polymers are commonly used as electrode materials in supercapacitor [3]. Graphene has attracted increasing attention for making the supercapacitors with fast charge-discharge rate and excellent cycle stability because of its superior electrical conductivity, high specific surface area, and outstanding mechanical performance [4,5]. The composite of graphene with a conductive polymer or a transition metal oxide was preferred [6–8]. Polyaniline (PANI), a conductive polymer, has been widely studied as a supercapacitor electrode because it has high capacitive characteristics and low cost [9]. The PANI can form a π-π conjugated structure with graphene via their rich π-electrons. Thus, the graphene/polyaniline (G/PANI) hybrid is regarded as a promising candidate material for a supercapacitor electrode [10–12]. Recently, various methods have been proposed to fabricate the G/PANI hybrid [13–17] using graphene oxide (GO) or chemically reduced GO (RGO). However, the performance of the supercapacitor electrode [18] made from GO or RGO was inadequate because the GO contains oxygen-containing groups and the RGO could not recover to its original graphene structure. Cao et al. [19]
synthesized a microsphere composite of the graphene/PANI to improve the supercapacitor performance by adopting pristine graphene. But the improved performance was not too much because the graphene used was restacked and the loading amount of the PANI on the graphene was small also. To solve this issue, Cao et al. [20] prepared a graphene nanomesh by a chemical vapor deposition method (CVD), and then let an aniline monomer polymerized on the resulted graphene via chemical oxidative polymerization. In this way, the PANI nanorod arrays were formed on the graphene, and the performance of the supercapacitor electrode made accordingly was improved considerably. Unlike the PANI nanorod, Yu et al. [21] synthesized a polyaniline nanocone array on the three-dimensional (3D) graphene network in which the PANI nanocone arrays offered a large area exposing to the electrolyte, and the space between the nanocones provided a free-diffusion path for electrolyte ions. This structure of the nanocone array and the 3D graphene facilitated the charge transfer between the electrode and the electrolyte resulting in enhanced performance. However, the synthesis method was complicated and expensive. To our best knowledge, there are few published papers on the PANI nanocone. Moreover, reports on fabricating the hybrid of the nanocone PANI with the pristine graphene are much less. Therefore, it is worth to explore the fabrication of the PANI nanocones with the pristine graphene for boosting the performance of supercapacitor electrodes.
* Corresponding author. E-mail address: [email protected] (Y. Zhao). https://doi.org/10.1016/j.jpcs.2017.12.022 Received 21 October 2017; Received in revised form 8 December 2017; Accepted 17 December 2017 Available online 19 December 2017 0022-3697/© 2017 Published by Elsevier Ltd.
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Journal of Physics and Chemistry of Solids 115 (2018) 148–155
ICON), Raman spectroscopy (DXR), Fourier Transform Infrared Spectroscopy (FTIR, Spectrum 100, Perkin Elmer, Inc., USA) and Thermogravimetric Analyzer (TGA7, Perkin Elmer, Inc., USA), respectively.
The purpose of this research is to prepare a hybrid of the nanocone polyaniline with the pristine graphene to enhance the performance of the supercapacitor electrode using a novel and facile approach. The strategy is to make aniline monomer in situ polymerized on the fresh exfoliated pristine graphene sheets. The aniline monomer absorbed onto the surface of the pristine graphene via π-π interaction not only provided active nucleation sites for the PANI nanocones but also avoided the aggregation of the graphene sheets. The length of the PANI nanocones could be controlled via the amount of the aniline monomer initially used. The morphology and structure of the as-resulted G/PANI composites were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The specific capacitance and the cycle stability of the hybrid were investigated by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.
2.4. Electrochemical measurements
Graphite powder (99.85%, CP), aniline (AR), ammonium persulfate (AR) and sulfuric acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.. Carbon dioxide (99.90%) was obtained from Shanghai Chemical Co., Ltd. Absolute ethanol was purchased from Changshu Yangyuan Chemical Co., Ltd. Deionized water was used in all experiments.
A three-electrode system was carried out to evaluate the electrochemical performance of the samples. The preparation procedure of constructing the three-electrode was as follows: 85 wt% G/PANI composite, 10 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) were mixed for 2 h. Then the mixture was pressed onto a stainlesssteel grid served as a current collector (5 cm 1 cm) at 2 MPa and dried at 60 C for 12 h. The electrochemical measurement was carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Company, China) using a three-electrode cell, in which the platinum and Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. An aqueous solution of 1 M H2SO4 was used as the electrolyte. Electrochemical characterization of the prepared materials was performed as follow: cyclic voltammetry (CV) curves were carried out from 0.2–0.8 V at the scanning rates of 10, 20 and 50 mV/s, respectively; galvanostatic charge/discharge (GCD) tests were measured at the current densities of 0.1, 0.5, 1, 2 and 5 A/g with a scanning range of 0.2–0.7 V, respectively; electrochemical impedance spectra (EIS) was recorded at the open circuit voltage amplitude of 5 mV at a frequency ranging from 0.01 Hz to 100 kHz.
2.2. Preparation of pristine graphene and G/PANI composites
3. Results and discussion
The pristine graphene was prepared by exfoliating graphite using a fluid dynamic force in supercritical CO2 as reported in a previously published paper [22]. At first, an amount of graphite was put into the reactor made from stainless steel 304, and then carbon dioxide was pumped into the reactor by a manual pump. When the pressure and the temperature reached to 15 MPa and 45 C, the shear mixer started to run at the desired speed of 3000 rpm for 3 h. The as-exfoliated graphene was collected in a 200 mL beaker filled with a certain amount of ethanol to obtain the ethanol dispersion of the graphene. The G/PANI composites were synthesized by a modified method [23]. Typically, a certain amount of aniline monomer and 9.8 g H2SO4 were slowly poured into 100 mL graphene dispersion (1 mg/mL) in aqueous ethanol solution (20 wt %) and stirred for 1 h. Then, another 100 mL ethanol aqueous solution (20 wt %) containing ammonium persulfate was added to the above mixture solution under stirring at 0 C. The reaction lasted for 6 h. The blackish-green G/PANI formed was filtered and repeatedly washed with deionized water and absolute ethanol for several times, and finally was dried under vacuum at 60 C for 24 h. A series of the G/PANI composites were prepared corresponding to the concentration (CAN) of the aniline monomer 0.01, 0.02, 0.03, 0.04 and 0.05 M, respectively (named as G/PANI1, G/PANI2, G/PANI3, G/PANI4 and G/PANI5, respectively). The mass ratio of the graphene to the PANI in the composites was determined via their mass change after high-temperature calcination (Fig. S1). They were 1:0.24, 1:0.49, 1:0.73, 1: 0.98 and 1: 1.22, respectively. The PANI without graphene was prepared at CAN ¼ 0.05 M using the same process to the method above, which was as a contrast sample.
3.1. Morphology and structure
2. Experimental 2.1. Materials
The characterization of the pristine graphene sheets is shown in Fig. 1. We can see from Fig. 1a–b that the graphene sheets are irregular ultrathin and semitransparent, and the lateral size is about 4 μm. The sharp shape and position (2683 cm1) of the 2D peak shown in Fig. 1c indicates that the layer number of the graphene is few [24]. Fig. 1d suggests that a typical graphene flake is 3 layers according to its thickness of about 1.5 nm. We measured over 100 sheets of the graphene, and the mean thickness is approximately 1.5 nm. It indicates that the average layer number of the graphene is around three. The G/PANI hybrids with different weight ratios of the PANI to the G were prepared by changing the CAN while fixing the amount of the graphene sheets as 100 mg. SEM images in Fig. 2a–e displays that the surface of the graphene sheets was entirely covered with the PANI made at all conditions. The highly-ordered PANI array was formed on both sides of the graphene sheets. It can be ascribed to the π-π interaction between the PANI and the graphene. The PANI was guided by π-conjugated graphene to grow on the graphene sheets. The length of the PANI increased from 70 nm to 440 nm with increasing from 0.01 M to 0.05 M. However, the disorder PANI nanowires were formed when the CAN was 0.05 M as shown in Fig. 2e. The excessive aniline monomer might produce these isolated PANI nanowire clusters which were formed via self-nucleation and polymerization [25] in the ethanol solution rather than on the graphene sheets. The aligned PANI on the graphene sheets was created at the optimum concentration of 0.04 M as shown in Fig. 2a–e, and its length is about 400 nm (Fig. 2c). Fig. 3a–b indicates that the pure PANI consisted of randomly stacked nanowires which are different with that in the hybrids. The SEM image of the G/PANI4 shown in Fig. 3c suggests that all of the PANI are vertically arranged on both sides of the graphene sheets, and the PANI has a nanocone structure as shown in Fig. 3d. It might be attributed to the interactions of the graphene and the PANI. FTIR analysis can illustrate the combination of the PANI and the pristine graphene sheets in the hybrids. The characteristic peaks of the PANI at 1570, 1486, 1296, 1108 and 801 cm1 suggest the presence of – C stretching vibration of a quinoid structure, and a benzene ring, the C– – N, and C–H, respectively [26,27], as stretching vibration of C–N, C–
2.3. Characterization The graphene concentration (C: mg/mL) was analyzed using a UV–vis spectrophotometer at a 660 nm wavelength. It was calculated from the formula A/l ¼ αC based on the Lambert-Beer law. Herein, A/l (m1) is the absorbance per cell length, and α is the absorption coefficient, which is 528 mL mg1 m1 (Fig. S2). The as-prepared samples were characterized by a field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 450, FEI Company, USA), Transmission Electron Microscope (TEM, JEM-2100 JEOL Ltd., Japan), Atomic Force Microscopy (AFM, Bruker 149
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Fig. 1. Graphene sheets. (a) SEM, (b) TEM, (c) Raman spectrum, (d) AFM.
– C stretching vibration of the quinoid shown in Fig. 4a. However, the C– structure and the benzene ring in the PANI/G were blue-shifted from 1570 cm1, 1486 cm1 to 1580 cm1 and 1491 cm1, respectively. This blue-shifted phenomenon resulted from the conjugational π-π interactions between the graphene sheets and the PANI [13]. We applied Raman spectroscopy to analyze the microstructure of the graphene, the PANI and the G/PANI further. As shown in Fig. 4b, the peaks of the PANI/G hybrid are different with that of either the pure PANI or the graphene in terms of the position and peak numbers. There are more peaks in the G/PANI hybrid than in the graphene. The peaks of the pure PANI located at 1485 cm1, 1163 cm1, and 806 cm1 were redshifted to 1499 cm1, 1172 cm1, and 812 cm1, respectively, in the G/ – C and C–H stretching vibration PANI. These peaks correspond to the C– of the quinoid ring and the deformational benzene ring. Similar to the reported results [28], the peak at 1343 cm1 corresponding to the C–N stretching in the G/PANI hybrid was red-shifted from 1328 cm1 in the pure PANI. The red-shifted of these peaks further demonstrate mutually with FTIR the combination of the PANI with the graphene via π-π conjugated structure in the composite, which would improve the charge-carrier transport properties of the composites [29]. We applied BET analyzer (Surface Area and Porosimetry analyzer, Micromeritics Instrument Corp., USA) to analyze the microstructure of G/PANI hybrids. The nitrogen adsorption and desorption of the two G/ PANI hybrids were studied. As shown in Fig. 5, the nitrogen adsorption/ desorption isotherms of the G/PANI hybrids are the Type Ⅳ with a small hysteresis loop at a relatively high pressure. It suggests that the composites possessed abundant mesopores and slit-shaped pores between the layers [30]. The specific surface area of the G/PANI1 and the G/PANI4 are 18.3 m2/g and 40.0 m2/g, respectively. The large area of the G/PANI increased its exposures to the electrolyte and benefited its electrical-chemical performance. The higher surface area of the G/PANI4 is attributed to the longer length of the PANI nanocones grown on the graphene sheets. Also, the PANI can reduce the stacking between the graphene sheets to a greater extent.
The fabrication process of the G/PANI hybrids can be illustrated in Fig. 6. At first, the pristine graphene was exfoliated from graphite by supercritical CO2. Such graphene sheets facilitate the adsorption of the aniline monomers via a π-π connection under stirring. The aniline monomers adsorbed on the graphene sheets, in turn, prevent them aggregation. When the ammonium persulfate was added to initiate a reaction, the PANI nucleus would be formed due to the polymerization of the aniline monomers absorbed on the graphene sheets, which provided nucleation centers for the extending growth of the PANI nanocone arrays. The length of the PANI arrays increased with increasing the concentration of aniline from 0.01 M to 0.04 M as shown in Fig. 2. However, when the concentration was up to 0.05 M, some isolated PANI nanowire clusters were generated because the excessive aniline monomers in the ethanol solution rather than on the graphene sheets would be formed into the disordered PANI wires via self-nucleation and polymerization. 3.2. Electrochemical performance To evaluate the electrochemical performance of the G/PANI composite, the cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy were measured. As shown in Fig. 7a, there were three pairs of redox peaks for the CV curves of the G/ PANI and the pure PANI. They could be attributed to the two redox transitions of the PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form), and the emeraldine-pernigraniline transformation [31,32]. But the CV curve of the graphene did not have a peak, and the shape was close to a typical double-layer capacitance rectangular shape. It indicates that the capacitance of the G/PANI mainly comes from Faradaic reactions of the PANI in the electrode. Also, we can see that the area of the CV curve in the G/PANI increased considerably compared with that in the pure PANI. This enhanced capacitance resulted from the coupling effects between the graphene and the ordered PANI nanocone arrays. The redox current of the G/PANI increased with the increase of the scanning rate as shown 150
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Fig. 2. SEM images of G/PANI hybrids with a various mass ratio of graphene to PANI. (a) G/PANI1, (b) G/PANI2, (c) G/PANI3, (d) G/PANI4 and (e) G/PANI5.
Fig. 3. (a) SEM image of pure PANI, (b) TEM image of pure PANI, (c) SEM image of G/PANI4 hybrid and (d) TEM image of G/PANI4 hybrid.
in Fig. 7c, the specific capacitance increased with the increase of concentration from 0 to 0.04 M. This might be attributed to the growing length of the PANI nanocones and the large area of the nanocones exposed to the electrolyte, which facilitated ion diffusion and charge transmission. However, the capacitance would not always increase with
in Fig. 7b. It indicates that the G/PANI hybrid has a good rate ability and a fast response to redox reactions [33]. To further systematically explore the influence of the aniline concentration on the capacitance performance, the specific capacitance was investigated by the galvanostatic charge/discharge at 0.1 A/g. As shown 151
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Fig. 4. (a) FTIR spectra of pure PANI and G/PANI, (b) Raman spectra of graphene, pure PANI and G/PANI hybrid.
the increase of the concentration. Fig. 7c indicates that the capacitance at the concentration of 0.04 M (481 F/g) is higher than that at 0.05 M (377 F/g). This phenomenon might result from the disordered PANI nanowires extra generated when the concentration was beyond 0.04 M. The disordered PANI nanowires decreased the utilization of the active materials [34]. Fig. 7d shows the galvanostatic charge/discharge curves of the G/ PANI4 at different current densities with a potential window from 0.2–0.7 V. The specific capacitance (Cs) was calculated from the discharge process according to the equation: Cs ¼ I Δt/(m ΔV). Wherein, I is the charge-discharge current (A), Δt is the discharge time, m is the mass of the active material in a single electrode (g), and ΔV is the potential charge during the discharging process (V). The Cs of the G/PANI4 at the current density of 0.1, 0.5, 1, 2 and 5 A/g was 481, 367, 341, 329 and 314 F/g, respectively (Fig. 7e). We can see from Fig. 7e that Cs reached the maximum value (481 F/g) when a current density was 0.1 A/g. If deducting part affected by the graphene mass in the composite (ratio: 50.66%), the Cs attributed to the ordered aligned PANI phase should be as high as 972 F/g, which is much higher than the Cs (373 F/g) of the pure PANI. The capacitance of the G/PANI exceeded the reported RGO/PANI composite (408 F/g) [35]. It illustrates that the graphene phase and the PANI phase possessed an excellent synergistic effect. The nanocone structure of the PANI and the pristine graphene in the hybrids
Fig. 5. The nitrogen adsorption/desorption isotherms of the G/PANI hybrids.
Fig. 6. A scheme illustrating the preparation process of G/PANI hybrid materials. 152
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Fig. 7. The electrochemical capacitance behavior of PANI, graphene and G/PANI hybrids. (a) CV curves of different samples with the potential window 0.2–0.8 V, (b) CV curves of the G/ PANI4 composite with the potential window 0.2–0.8 V at 10, 20 and 50 mV/s scan rate; (c) specific capacitance of samples prepared at different CAN at the current density of 0.1 A/g; (d) galvanostatic charge-discharge curves of G/PANI4 at different current densities; (e) specific capacitance of G/PANI4 at different current densities.
77 over the low-frequency region was attributed to the Warburg resistance, resulting from the frequency dependency of ion diffusion and transport in the electrolyte [37]. The large gradient implies a short path of ion diffusion and small obstruction of ion movement [38,39]. The specific capacitance of the G/PANI hybrids remarkably increased due to the lower ions diffuse resistance and charge transfer resistance. It should be attributed to the hierarchical structure of the G/PANI composites and the ordered-aligned PANI nanocones on both sides of the graphene sheets, which much benefit to the diffusion and transport for the ions between the electrode and the electrolyte. The cycling stability is also an essential parameter in the evaluation of supercapacitor electrode materials. The charge-discharge cycling at a current density of 1 A/g is shown in Fig. 8b. The discharge capacitance
shortened the charge transport distance and ionic diffusion path within the composite and facilitated the diffusion of ions from the electrolyte to the PANI. With increasing the current density from 0.1 to 5 A/g, the capacitance retention was still about 65% (the Cs reduces from 481 to 314 F/g). It suggests that the hybrid possesses excellent rate capability and has potential application as a supercapacitor electrode material [36]. The electrochemical behavior of the hybrid materials was evaluated in terms of the electrodes by EIS analyzation. The interfacial chargetransfer resistance (R) of the pure PANI over the high-frequency region was about 13.5 Ω shown in Fig. 8a (inset), while the R of all the composite materials was less than 3.3 Ω. It can be attributed to the real connection between the PANI and the graphene, which decreased the internal resistance of the hybrids considerably. The gradient of more than 153
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Fig. 8. (a) Nyquist plots of G/PANI hybrids electrodes in the frequency range of 100 kHz to 0.01 Hz, inset is the enlarge plots if the high-frequency region. (b) Cycling stability of pure PANI and G/PANI4 electrodes at 1 A/g for 10000 cycles.
Conflicts of interest
retention of the G/PANI composite materials was about 87% after 10000 cycles. But the pure PANI showed only 47% of initial capacitance in the same condition. The improved cycle stability resulted from the synergistic effect of the graphene sheet and the ordered PANI nanocones. The graphene sheets as the substrate matrix can prevent the mechanical change caused by the shrinking and swelling of the PANI during the longterm redox reactions. Therefore, the G/PANI hybrid exhibited an excellent cycling stability. The high-performance of the pristine graphene/PANI hybrid can be attributed to the particular structure of the PANI nanocone arrays and the superior electrical conductivity of the pristine graphene. In the electrochemical process for the supercapacitor, the charge transports from electrolyte to the PANI phase and then to the graphene, finally return to the electrolyte. Therefore, the speed of the charge transport affects the performance of the supercapacitor significantly. Pristine graphene can make the charge transport rapidly during the processing of chargedischarge because it possesses superior electronic conductivity [40]. Also, the nanocone arrays have more and shorter electronic transmission path compared with the PANI short rods obtained at low concentration and the disorder PANI wires obtained at high concentration. Moreover, both the surface and inner phase of the PANI can carry electron transfer too, and the space between the nanocones could also conduct charge transmission due to their high specific area exposed to the electrolyte. Therefore, the coupling effects of the pristine graphene and the hierarchical structure of the PANI nanocones enable the hybrid to have the excellent electrochemical performance for supercapacitor.
The authors declare that they have no conflict of interest. Prime novelty statement The hybrid of highly ordered polyaniline nanocones on the pristine graphene as outstanding supercapacitor electrode was fabricated for the first time, whose specific capacitance was as high as 481 F/g at a current density of 0.1 A/g, and the capacitance maintained 87% after 10000 cycles at 1 A/g. Acknowledgments We are thankful for the financial support of the National Natural Science Foundation of China (Grants No. 21576165), and Ms. Huiqin Li of Instrumental Analysis Center of SJTU for AFM analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi. org/10.1016/j.jpcs.2017.12.022. References [1] D. Liu, S. Yu, Y. Shen, H. Chen, Z. Shen, S. Zhao, S. Fu, Y. Yu, B. Bao, Polyaniline coated boron doped biomass derived porous carbon composites for supercapacitor electrode materials, Ind. Eng. Chem. Res. 54 (2015) 12570–12579. [2] A. Sanger, A. Kumar, A. Kumar, P.K. Jain, Y.K. Mishra, R. Chandra, Silicon carbide nanocauliflowers for symmetric supercapacitor devices, Ind. Eng. Chem. Res. 55 (2016) 9452–9458. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [4] J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene double-layer capacitor with ac line-filtering performance, Science 329 (2010) 1637–1639. [5] M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb carbon: a review of graphene, Chem. Rev. 110 (2010) 132–145. [6] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, A nanostructured graphene/polyaniline hybrid material for supercapacitors, Nanoscale 2 (2010) 2164–2170. [7] M. Moussa, M.F. El-Kady, Z. Zhao, P. Majewski, J. Ma, Recent progress and performance evaluation for polyaniline/graphene nanocomposites as supercapacitor electrodes, Nanotechnology 27 (2016), 442001. [8] A. Majeed, W. Ullah, A.W. Anwar, F. Nasreen, A. Sharif, G. Mustafa, A. Khan, Graphene-metal oxides/hydroxide nanocomposite materials: fabrication advancements and supercapacitive performance, J. Alloy. Comp. 671 (2016) 1–10. [9] S. Palaniappan, S.L. Devi, Novel chemically synthesized polyaniline electrodes containing a fluoroboric acid dopant for supercapacitors, J. Appl. Polym. Sci. 107 (2008) 1887–1892. [10] M. Mahmoud, F.E.-K. Maher, Z. Zhiheng, M. Peter, M. Jun, Recent progress and performance evaluation for polyaniline/graphene nanocomposites as supercapacitor electrodes, Nanotechnology 27 (2016), 442001.
4. Conclusions The G/PANI hybrid for the supercapacitor electrode has been prepared successfully by in situ polymerization technique. The highly ordered array PANI nanocones were grown on both sides of the pristine graphene sheets. They not only offer a large surface area to the electrolyte for facilitating the charge diffusion and transfer between the electrolyte and electrode but also entirely prevent the pristine graphene from restacking. In the same time, the pristine graphene not only improves the electrical conductivity of the composites but also prevents the mechanical change of the PANI, such as shrinking and swelling. The specific capacitance and cycling stability of the hybrids are improved substantially compared to the pure PANI and the graphene. The specific capacitance is as high as 481 F/g at a current density of 0.1 A/g, and the capacitance maintains 87% after 10000 cycles at 1 A/g. The combination of the pristine graphene and the ordered PANI nanocones grown on the former has a potential application in the supercapacitors.
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Fabrication of highly ordered polyaniline nanocone on pristine graphene for high-performance supercapacitor electrodes Ningning Song, Wucong Wang, Yue Wu, Ding Xiao, Yaping Zhao * School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Graphene Hybrids Supercapacitor Polyaniline Nanocone
The hybrids of pristine graphene with polyaniline were synthesized by in situ polymerizations for making a highperformance supercapacitor. The formed high-ordered PANI nanocones were vertically aligned on the graphene sheets. The length of the PANI nanocones increased with the concentration of aniline monomer. The specific capacitance of the hybrids electrode in the three-electrode system was measured as high as 481 F/g at a current density of 0.1 A/g, and its stability remained 87% after constant charge-discharge 10000 cycles at a current density of 1 A/g. This outstanding performance is attributed to the coupling effects of the pristine graphene and the hierarchical structure of the PANI possessing high specific surface area. The unique structure of the PANI provided more charge transmission pathways and fast charge-transfer speed of electrons to the pristine graphene because of its large specific area exposed to the electrolyte. The hybrid is expected to have potential applications in supercapacitor electrodes.
1. Introduction A supercapacitor is a promising energy device for electrical vehicles, mobile electronic devices, and grid energy storage due to its high power density, long cycle life and fast charging rate [1,2]. Carbon-based materials, transition metal oxides and conducting polymers are commonly used as electrode materials in supercapacitor [3]. Graphene has attracted increasing attention for making the supercapacitors with fast charge-discharge rate and excellent cycle stability because of its superior electrical conductivity, high specific surface area, and outstanding mechanical performance [4,5]. The composite of graphene with a conductive polymer or a transition metal oxide was preferred [6–8]. Polyaniline (PANI), a conductive polymer, has been widely studied as a supercapacitor electrode because it has high capacitive characteristics and low cost [9]. The PANI can form a π-π conjugated structure with graphene via their rich π-electrons. Thus, the graphene/polyaniline (G/PANI) hybrid is regarded as a promising candidate material for a supercapacitor electrode [10–12]. Recently, various methods have been proposed to fabricate the G/PANI hybrid [13–17] using graphene oxide (GO) or chemically reduced GO (RGO). However, the performance of the supercapacitor electrode [18] made from GO or RGO was inadequate because the GO contains oxygen-containing groups and the RGO could not recover to its original graphene structure. Cao et al. [19]
synthesized a microsphere composite of the graphene/PANI to improve the supercapacitor performance by adopting pristine graphene. But the improved performance was not too much because the graphene used was restacked and the loading amount of the PANI on the graphene was small also. To solve this issue, Cao et al. [20] prepared a graphene nanomesh by a chemical vapor deposition method (CVD), and then let an aniline monomer polymerized on the resulted graphene via chemical oxidative polymerization. In this way, the PANI nanorod arrays were formed on the graphene, and the performance of the supercapacitor electrode made accordingly was improved considerably. Unlike the PANI nanorod, Yu et al. [21] synthesized a polyaniline nanocone array on the three-dimensional (3D) graphene network in which the PANI nanocone arrays offered a large area exposing to the electrolyte, and the space between the nanocones provided a free-diffusion path for electrolyte ions. This structure of the nanocone array and the 3D graphene facilitated the charge transfer between the electrode and the electrolyte resulting in enhanced performance. However, the synthesis method was complicated and expensive. To our best knowledge, there are few published papers on the PANI nanocone. Moreover, reports on fabricating the hybrid of the nanocone PANI with the pristine graphene are much less. Therefore, it is worth to explore the fabrication of the PANI nanocones with the pristine graphene for boosting the performance of supercapacitor electrodes.
* Corresponding author. E-mail address: [email protected] (Y. Zhao). https://doi.org/10.1016/j.jpcs.2017.12.022 Received 21 October 2017; Received in revised form 8 December 2017; Accepted 17 December 2017 Available online 19 December 2017 0022-3697/© 2017 Published by Elsevier Ltd.
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ICON), Raman spectroscopy (DXR), Fourier Transform Infrared Spectroscopy (FTIR, Spectrum 100, Perkin Elmer, Inc., USA) and Thermogravimetric Analyzer (TGA7, Perkin Elmer, Inc., USA), respectively.
The purpose of this research is to prepare a hybrid of the nanocone polyaniline with the pristine graphene to enhance the performance of the supercapacitor electrode using a novel and facile approach. The strategy is to make aniline monomer in situ polymerized on the fresh exfoliated pristine graphene sheets. The aniline monomer absorbed onto the surface of the pristine graphene via π-π interaction not only provided active nucleation sites for the PANI nanocones but also avoided the aggregation of the graphene sheets. The length of the PANI nanocones could be controlled via the amount of the aniline monomer initially used. The morphology and structure of the as-resulted G/PANI composites were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The specific capacitance and the cycle stability of the hybrid were investigated by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy.
2.4. Electrochemical measurements
Graphite powder (99.85%, CP), aniline (AR), ammonium persulfate (AR) and sulfuric acid (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.. Carbon dioxide (99.90%) was obtained from Shanghai Chemical Co., Ltd. Absolute ethanol was purchased from Changshu Yangyuan Chemical Co., Ltd. Deionized water was used in all experiments.
A three-electrode system was carried out to evaluate the electrochemical performance of the samples. The preparation procedure of constructing the three-electrode was as follows: 85 wt% G/PANI composite, 10 wt% acetylene black and 5 wt% polytetrafluoroethylene (PTFE) were mixed for 2 h. Then the mixture was pressed onto a stainlesssteel grid served as a current collector (5 cm 1 cm) at 2 MPa and dried at 60 C for 12 h. The electrochemical measurement was carried out on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Company, China) using a three-electrode cell, in which the platinum and Ag/AgCl electrode were used as a counter electrode and a reference electrode, respectively. An aqueous solution of 1 M H2SO4 was used as the electrolyte. Electrochemical characterization of the prepared materials was performed as follow: cyclic voltammetry (CV) curves were carried out from 0.2–0.8 V at the scanning rates of 10, 20 and 50 mV/s, respectively; galvanostatic charge/discharge (GCD) tests were measured at the current densities of 0.1, 0.5, 1, 2 and 5 A/g with a scanning range of 0.2–0.7 V, respectively; electrochemical impedance spectra (EIS) was recorded at the open circuit voltage amplitude of 5 mV at a frequency ranging from 0.01 Hz to 100 kHz.
2.2. Preparation of pristine graphene and G/PANI composites
3. Results and discussion
The pristine graphene was prepared by exfoliating graphite using a fluid dynamic force in supercritical CO2 as reported in a previously published paper [22]. At first, an amount of graphite was put into the reactor made from stainless steel 304, and then carbon dioxide was pumped into the reactor by a manual pump. When the pressure and the temperature reached to 15 MPa and 45 C, the shear mixer started to run at the desired speed of 3000 rpm for 3 h. The as-exfoliated graphene was collected in a 200 mL beaker filled with a certain amount of ethanol to obtain the ethanol dispersion of the graphene. The G/PANI composites were synthesized by a modified method [23]. Typically, a certain amount of aniline monomer and 9.8 g H2SO4 were slowly poured into 100 mL graphene dispersion (1 mg/mL) in aqueous ethanol solution (20 wt %) and stirred for 1 h. Then, another 100 mL ethanol aqueous solution (20 wt %) containing ammonium persulfate was added to the above mixture solution under stirring at 0 C. The reaction lasted for 6 h. The blackish-green G/PANI formed was filtered and repeatedly washed with deionized water and absolute ethanol for several times, and finally was dried under vacuum at 60 C for 24 h. A series of the G/PANI composites were prepared corresponding to the concentration (CAN) of the aniline monomer 0.01, 0.02, 0.03, 0.04 and 0.05 M, respectively (named as G/PANI1, G/PANI2, G/PANI3, G/PANI4 and G/PANI5, respectively). The mass ratio of the graphene to the PANI in the composites was determined via their mass change after high-temperature calcination (Fig. S1). They were 1:0.24, 1:0.49, 1:0.73, 1: 0.98 and 1: 1.22, respectively. The PANI without graphene was prepared at CAN ¼ 0.05 M using the same process to the method above, which was as a contrast sample.
3.1. Morphology and structure
2. Experimental 2.1. Materials
The characterization of the pristine graphene sheets is shown in Fig. 1. We can see from Fig. 1a–b that the graphene sheets are irregular ultrathin and semitransparent, and the lateral size is about 4 μm. The sharp shape and position (2683 cm1) of the 2D peak shown in Fig. 1c indicates that the layer number of the graphene is few [24]. Fig. 1d suggests that a typical graphene flake is 3 layers according to its thickness of about 1.5 nm. We measured over 100 sheets of the graphene, and the mean thickness is approximately 1.5 nm. It indicates that the average layer number of the graphene is around three. The G/PANI hybrids with different weight ratios of the PANI to the G were prepared by changing the CAN while fixing the amount of the graphene sheets as 100 mg. SEM images in Fig. 2a–e displays that the surface of the graphene sheets was entirely covered with the PANI made at all conditions. The highly-ordered PANI array was formed on both sides of the graphene sheets. It can be ascribed to the π-π interaction between the PANI and the graphene. The PANI was guided by π-conjugated graphene to grow on the graphene sheets. The length of the PANI increased from 70 nm to 440 nm with increasing from 0.01 M to 0.05 M. However, the disorder PANI nanowires were formed when the CAN was 0.05 M as shown in Fig. 2e. The excessive aniline monomer might produce these isolated PANI nanowire clusters which were formed via self-nucleation and polymerization [25] in the ethanol solution rather than on the graphene sheets. The aligned PANI on the graphene sheets was created at the optimum concentration of 0.04 M as shown in Fig. 2a–e, and its length is about 400 nm (Fig. 2c). Fig. 3a–b indicates that the pure PANI consisted of randomly stacked nanowires which are different with that in the hybrids. The SEM image of the G/PANI4 shown in Fig. 3c suggests that all of the PANI are vertically arranged on both sides of the graphene sheets, and the PANI has a nanocone structure as shown in Fig. 3d. It might be attributed to the interactions of the graphene and the PANI. FTIR analysis can illustrate the combination of the PANI and the pristine graphene sheets in the hybrids. The characteristic peaks of the PANI at 1570, 1486, 1296, 1108 and 801 cm1 suggest the presence of – C stretching vibration of a quinoid structure, and a benzene ring, the C– – N, and C–H, respectively [26,27], as stretching vibration of C–N, C–
2.3. Characterization The graphene concentration (C: mg/mL) was analyzed using a UV–vis spectrophotometer at a 660 nm wavelength. It was calculated from the formula A/l ¼ αC based on the Lambert-Beer law. Herein, A/l (m1) is the absorbance per cell length, and α is the absorption coefficient, which is 528 mL mg1 m1 (Fig. S2). The as-prepared samples were characterized by a field-emission scanning electron microscope (FE-SEM, Nova NanoSEM 450, FEI Company, USA), Transmission Electron Microscope (TEM, JEM-2100 JEOL Ltd., Japan), Atomic Force Microscopy (AFM, Bruker 149
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Fig. 1. Graphene sheets. (a) SEM, (b) TEM, (c) Raman spectrum, (d) AFM.
– C stretching vibration of the quinoid shown in Fig. 4a. However, the C– structure and the benzene ring in the PANI/G were blue-shifted from 1570 cm1, 1486 cm1 to 1580 cm1 and 1491 cm1, respectively. This blue-shifted phenomenon resulted from the conjugational π-π interactions between the graphene sheets and the PANI [13]. We applied Raman spectroscopy to analyze the microstructure of the graphene, the PANI and the G/PANI further. As shown in Fig. 4b, the peaks of the PANI/G hybrid are different with that of either the pure PANI or the graphene in terms of the position and peak numbers. There are more peaks in the G/PANI hybrid than in the graphene. The peaks of the pure PANI located at 1485 cm1, 1163 cm1, and 806 cm1 were redshifted to 1499 cm1, 1172 cm1, and 812 cm1, respectively, in the G/ – C and C–H stretching vibration PANI. These peaks correspond to the C– of the quinoid ring and the deformational benzene ring. Similar to the reported results [28], the peak at 1343 cm1 corresponding to the C–N stretching in the G/PANI hybrid was red-shifted from 1328 cm1 in the pure PANI. The red-shifted of these peaks further demonstrate mutually with FTIR the combination of the PANI with the graphene via π-π conjugated structure in the composite, which would improve the charge-carrier transport properties of the composites [29]. We applied BET analyzer (Surface Area and Porosimetry analyzer, Micromeritics Instrument Corp., USA) to analyze the microstructure of G/PANI hybrids. The nitrogen adsorption and desorption of the two G/ PANI hybrids were studied. As shown in Fig. 5, the nitrogen adsorption/ desorption isotherms of the G/PANI hybrids are the Type Ⅳ with a small hysteresis loop at a relatively high pressure. It suggests that the composites possessed abundant mesopores and slit-shaped pores between the layers [30]. The specific surface area of the G/PANI1 and the G/PANI4 are 18.3 m2/g and 40.0 m2/g, respectively. The large area of the G/PANI increased its exposures to the electrolyte and benefited its electrical-chemical performance. The higher surface area of the G/PANI4 is attributed to the longer length of the PANI nanocones grown on the graphene sheets. Also, the PANI can reduce the stacking between the graphene sheets to a greater extent.
The fabrication process of the G/PANI hybrids can be illustrated in Fig. 6. At first, the pristine graphene was exfoliated from graphite by supercritical CO2. Such graphene sheets facilitate the adsorption of the aniline monomers via a π-π connection under stirring. The aniline monomers adsorbed on the graphene sheets, in turn, prevent them aggregation. When the ammonium persulfate was added to initiate a reaction, the PANI nucleus would be formed due to the polymerization of the aniline monomers absorbed on the graphene sheets, which provided nucleation centers for the extending growth of the PANI nanocone arrays. The length of the PANI arrays increased with increasing the concentration of aniline from 0.01 M to 0.04 M as shown in Fig. 2. However, when the concentration was up to 0.05 M, some isolated PANI nanowire clusters were generated because the excessive aniline monomers in the ethanol solution rather than on the graphene sheets would be formed into the disordered PANI wires via self-nucleation and polymerization. 3.2. Electrochemical performance To evaluate the electrochemical performance of the G/PANI composite, the cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy were measured. As shown in Fig. 7a, there were three pairs of redox peaks for the CV curves of the G/ PANI and the pure PANI. They could be attributed to the two redox transitions of the PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form), and the emeraldine-pernigraniline transformation [31,32]. But the CV curve of the graphene did not have a peak, and the shape was close to a typical double-layer capacitance rectangular shape. It indicates that the capacitance of the G/PANI mainly comes from Faradaic reactions of the PANI in the electrode. Also, we can see that the area of the CV curve in the G/PANI increased considerably compared with that in the pure PANI. This enhanced capacitance resulted from the coupling effects between the graphene and the ordered PANI nanocone arrays. The redox current of the G/PANI increased with the increase of the scanning rate as shown 150
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Fig. 2. SEM images of G/PANI hybrids with a various mass ratio of graphene to PANI. (a) G/PANI1, (b) G/PANI2, (c) G/PANI3, (d) G/PANI4 and (e) G/PANI5.
Fig. 3. (a) SEM image of pure PANI, (b) TEM image of pure PANI, (c) SEM image of G/PANI4 hybrid and (d) TEM image of G/PANI4 hybrid.
in Fig. 7c, the specific capacitance increased with the increase of concentration from 0 to 0.04 M. This might be attributed to the growing length of the PANI nanocones and the large area of the nanocones exposed to the electrolyte, which facilitated ion diffusion and charge transmission. However, the capacitance would not always increase with
in Fig. 7b. It indicates that the G/PANI hybrid has a good rate ability and a fast response to redox reactions [33]. To further systematically explore the influence of the aniline concentration on the capacitance performance, the specific capacitance was investigated by the galvanostatic charge/discharge at 0.1 A/g. As shown 151
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Fig. 4. (a) FTIR spectra of pure PANI and G/PANI, (b) Raman spectra of graphene, pure PANI and G/PANI hybrid.
the increase of the concentration. Fig. 7c indicates that the capacitance at the concentration of 0.04 M (481 F/g) is higher than that at 0.05 M (377 F/g). This phenomenon might result from the disordered PANI nanowires extra generated when the concentration was beyond 0.04 M. The disordered PANI nanowires decreased the utilization of the active materials [34]. Fig. 7d shows the galvanostatic charge/discharge curves of the G/ PANI4 at different current densities with a potential window from 0.2–0.7 V. The specific capacitance (Cs) was calculated from the discharge process according to the equation: Cs ¼ I Δt/(m ΔV). Wherein, I is the charge-discharge current (A), Δt is the discharge time, m is the mass of the active material in a single electrode (g), and ΔV is the potential charge during the discharging process (V). The Cs of the G/PANI4 at the current density of 0.1, 0.5, 1, 2 and 5 A/g was 481, 367, 341, 329 and 314 F/g, respectively (Fig. 7e). We can see from Fig. 7e that Cs reached the maximum value (481 F/g) when a current density was 0.1 A/g. If deducting part affected by the graphene mass in the composite (ratio: 50.66%), the Cs attributed to the ordered aligned PANI phase should be as high as 972 F/g, which is much higher than the Cs (373 F/g) of the pure PANI. The capacitance of the G/PANI exceeded the reported RGO/PANI composite (408 F/g) [35]. It illustrates that the graphene phase and the PANI phase possessed an excellent synergistic effect. The nanocone structure of the PANI and the pristine graphene in the hybrids
Fig. 5. The nitrogen adsorption/desorption isotherms of the G/PANI hybrids.
Fig. 6. A scheme illustrating the preparation process of G/PANI hybrid materials. 152
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Fig. 7. The electrochemical capacitance behavior of PANI, graphene and G/PANI hybrids. (a) CV curves of different samples with the potential window 0.2–0.8 V, (b) CV curves of the G/ PANI4 composite with the potential window 0.2–0.8 V at 10, 20 and 50 mV/s scan rate; (c) specific capacitance of samples prepared at different CAN at the current density of 0.1 A/g; (d) galvanostatic charge-discharge curves of G/PANI4 at different current densities; (e) specific capacitance of G/PANI4 at different current densities.
77 over the low-frequency region was attributed to the Warburg resistance, resulting from the frequency dependency of ion diffusion and transport in the electrolyte [37]. The large gradient implies a short path of ion diffusion and small obstruction of ion movement [38,39]. The specific capacitance of the G/PANI hybrids remarkably increased due to the lower ions diffuse resistance and charge transfer resistance. It should be attributed to the hierarchical structure of the G/PANI composites and the ordered-aligned PANI nanocones on both sides of the graphene sheets, which much benefit to the diffusion and transport for the ions between the electrode and the electrolyte. The cycling stability is also an essential parameter in the evaluation of supercapacitor electrode materials. The charge-discharge cycling at a current density of 1 A/g is shown in Fig. 8b. The discharge capacitance
shortened the charge transport distance and ionic diffusion path within the composite and facilitated the diffusion of ions from the electrolyte to the PANI. With increasing the current density from 0.1 to 5 A/g, the capacitance retention was still about 65% (the Cs reduces from 481 to 314 F/g). It suggests that the hybrid possesses excellent rate capability and has potential application as a supercapacitor electrode material [36]. The electrochemical behavior of the hybrid materials was evaluated in terms of the electrodes by EIS analyzation. The interfacial chargetransfer resistance (R) of the pure PANI over the high-frequency region was about 13.5 Ω shown in Fig. 8a (inset), while the R of all the composite materials was less than 3.3 Ω. It can be attributed to the real connection between the PANI and the graphene, which decreased the internal resistance of the hybrids considerably. The gradient of more than 153
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Fig. 8. (a) Nyquist plots of G/PANI hybrids electrodes in the frequency range of 100 kHz to 0.01 Hz, inset is the enlarge plots if the high-frequency region. (b) Cycling stability of pure PANI and G/PANI4 electrodes at 1 A/g for 10000 cycles.
Conflicts of interest
retention of the G/PANI composite materials was about 87% after 10000 cycles. But the pure PANI showed only 47% of initial capacitance in the same condition. The improved cycle stability resulted from the synergistic effect of the graphene sheet and the ordered PANI nanocones. The graphene sheets as the substrate matrix can prevent the mechanical change caused by the shrinking and swelling of the PANI during the longterm redox reactions. Therefore, the G/PANI hybrid exhibited an excellent cycling stability. The high-performance of the pristine graphene/PANI hybrid can be attributed to the particular structure of the PANI nanocone arrays and the superior electrical conductivity of the pristine graphene. In the electrochemical process for the supercapacitor, the charge transports from electrolyte to the PANI phase and then to the graphene, finally return to the electrolyte. Therefore, the speed of the charge transport affects the performance of the supercapacitor significantly. Pristine graphene can make the charge transport rapidly during the processing of chargedischarge because it possesses superior electronic conductivity [40]. Also, the nanocone arrays have more and shorter electronic transmission path compared with the PANI short rods obtained at low concentration and the disorder PANI wires obtained at high concentration. Moreover, both the surface and inner phase of the PANI can carry electron transfer too, and the space between the nanocones could also conduct charge transmission due to their high specific area exposed to the electrolyte. Therefore, the coupling effects of the pristine graphene and the hierarchical structure of the PANI nanocones enable the hybrid to have the excellent electrochemical performance for supercapacitor.
The authors declare that they have no conflict of interest. Prime novelty statement The hybrid of highly ordered polyaniline nanocones on the pristine graphene as outstanding supercapacitor electrode was fabricated for the first time, whose specific capacitance was as high as 481 F/g at a current density of 0.1 A/g, and the capacitance maintained 87% after 10000 cycles at 1 A/g. Acknowledgments We are thankful for the financial support of the National Natural Science Foundation of China (Grants No. 21576165), and Ms. Huiqin Li of Instrumental Analysis Center of SJTU for AFM analysis. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi. org/10.1016/j.jpcs.2017.12.022. References [1] D. Liu, S. Yu, Y. Shen, H. Chen, Z. Shen, S. Zhao, S. Fu, Y. Yu, B. Bao, Polyaniline coated boron doped biomass derived porous carbon composites for supercapacitor electrode materials, Ind. Eng. Chem. Res. 54 (2015) 12570–12579. [2] A. Sanger, A. Kumar, A. Kumar, P.K. Jain, Y.K. Mishra, R. Chandra, Silicon carbide nanocauliflowers for symmetric supercapacitor devices, Ind. Eng. Chem. Res. 55 (2016) 9452–9458. [3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [4] J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene double-layer capacitor with ac line-filtering performance, Science 329 (2010) 1637–1639. [5] M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb carbon: a review of graphene, Chem. Rev. 110 (2010) 132–145. [6] H. Wang, Q. Hao, X. Yang, L. Lu, X. Wang, A nanostructured graphene/polyaniline hybrid material for supercapacitors, Nanoscale 2 (2010) 2164–2170. [7] M. Moussa, M.F. El-Kady, Z. Zhao, P. Majewski, J. Ma, Recent progress and performance evaluation for polyaniline/graphene nanocomposites as supercapacitor electrodes, Nanotechnology 27 (2016), 442001. [8] A. Majeed, W. Ullah, A.W. Anwar, F. Nasreen, A. Sharif, G. Mustafa, A. Khan, Graphene-metal oxides/hydroxide nanocomposite materials: fabrication advancements and supercapacitive performance, J. Alloy. Comp. 671 (2016) 1–10. [9] S. Palaniappan, S.L. Devi, Novel chemically synthesized polyaniline electrodes containing a fluoroboric acid dopant for supercapacitors, J. Appl. Polym. Sci. 107 (2008) 1887–1892. [10] M. Mahmoud, F.E.-K. Maher, Z. Zhiheng, M. Peter, M. Jun, Recent progress and performance evaluation for polyaniline/graphene nanocomposites as supercapacitor electrodes, Nanotechnology 27 (2016), 442001.
4. Conclusions The G/PANI hybrid for the supercapacitor electrode has been prepared successfully by in situ polymerization technique. The highly ordered array PANI nanocones were grown on both sides of the pristine graphene sheets. They not only offer a large surface area to the electrolyte for facilitating the charge diffusion and transfer between the electrolyte and electrode but also entirely prevent the pristine graphene from restacking. In the same time, the pristine graphene not only improves the electrical conductivity of the composites but also prevents the mechanical change of the PANI, such as shrinking and swelling. The specific capacitance and cycling stability of the hybrids are improved substantially compared to the pure PANI and the graphene. The specific capacitance is as high as 481 F/g at a current density of 0.1 A/g, and the capacitance maintains 87% after 10000 cycles at 1 A/g. The combination of the pristine graphene and the ordered PANI nanocones grown on the former has a potential application in the supercapacitors.
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