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polyaniline spherical electrode material for supercapacitors
Electrochimica Acta 146 (2014) 511–517
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Tremella-like graphene/polyaniline spherical electrode material for supercapacitors Haiyan Liu, Wei Zhang, Huaihe Song *, Xiaohong Chen, Jisheng Zhou, Zhaokun Ma State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2014 Received in revised form 22 August 2014 Accepted 2 September 2014 Available online 22 September 2014
A novel tremella-like graphene/polyaniline composite was achieved from self-assembly of graphene nanosheets during polymerization of aniline in H2O/N, N-dimethylformamide solution, and was further employed as an electrode for supercapacitors. This graphene/polyaniline composite demonstrates a spherical tremella-like structure and a large specific capacity of 497 F g1 at a current density of 0.5 A g1. Particularly, an outstanding rate capability of 456 F g1 under 5 A g1 after 1000 cycles was obtained. Scanning electron microscopy showed that polyaniline nanoparticles were uniformly deposited on freestanding graphene nanosheets, and self-assembled to a spherical tremella-like structure. Therefore, this unique nanostructure is promising for high-performance electrochemical applications. ã 2014 Elsevier Ltd. All rights reserved.
Key words: Graphene polyaniline Tremella-like Self-assembly Supercapacitors
1. Introduction Supercapacitors or electrochemical capacitors are considered as very promising candidates for future power storage devices due to their high power density and outstanding cycling performance [1,2]. There are two energy storage mechanisms for supercapacitors: the electric double-layer (EDL) capacitance and the pseudo-capacitance [3]. The EDL capacitance arises from the charge separation at the electrode/electrolyte interface, but the energy densities of EDL capacitors are too low for many important applications [4]. The high capacitances of pseudocapacitors are derived from fast, reversible redox reactions on the surfaces or in the bulk phase of active materials [5]. Polymers are usually used as pseudocapacitor active materials [6]. Among all of the developed materials, electronically conducting polymers such as polypyrrole, polythiophene and especially polyaniline (PANI), are widely investigated because of their easy synthesis, relative high specific capacitance and low fabrication cost than many other electrode materials [7]. However, severe volume swelling and shrinkage during charge-discharge processes lead to the mechanical degradation of these conductive polymers, which causes the obvious capacity fading [8,9]. In addition, low electrical conductivity can aslo influence the pseudocapacitive
* Corresponding author. Tel.: +86 10 64434916; fax: +86 10 64434916. E-mail address: [email protected] (H. Song). http://dx.doi.org/10.1016/j.electacta.2014.09.083 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
performance of PANI when structured as electrode material of supercapacitors [10]. Graphene, a two-dimensional all-sp2-hybridized carbon, which was first obtained in 2004 by Geim [11], has attracted a great attention due to its extraordinary electric, mechanical properties and thermal properties [12–16]. These properties suggest wide applications, such as fabricating electronic devices, lithium-ion batteries and supercapacitors [12,17,18]. The combination of PANI with graphene has been proved to be attractive to reinforce the stability and the electrical conductivity of PANI as well as maximize the capacitance value [19,20]. Therefore, high dispersion of PANI on a support material with excellent conductivity and high surface area should be a promising way to improve the capacitive properties of PANI. Recently, many attempts have been devoted to the use of graphene/PANI as electrode materials in supercapacitors [21–30]. Xu et al. [31] constructed a hierarchical nanocomposite containing PANI nanowires and graphene oxide (GO) nanosheets by dilute polymerization, and found that this architecture material showed a capacitance of 227 F g1 at the current of 2 A g1. Covalentlygrafted graphene/PANI nanocomposites with capacitance of 338 F g1 at the scan of 5 mv s1 were synthesized by diazotization chemistry to enhance the three dimensional functionalization of graphene with p-aminobenzoic acid [32]. Kumar et al. [20] synthesized polyaniline-grafted reduced graphene oxide composites with capacitance of 250 F g1 by functionalizing graphite oxide with 4-aminophenol via acyl chemistry, where a concomitant reduction of GO takes place.
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However, up to now, much attention has been paid on the preparation method of PANI/graphene materials; few researches investigated the influence of the constructed architecture by graphene nanosheets (GNS) on the electrochemical performance for supercapacitors. In addition, graphene/PANI composites with a sheet-like morphology have a strong tendency to restack during solvent evaporation, which may hinder the diffusion of electrolytes [33]. Therefore, designing a three-dimensional structure which is conducive to charge transfer and diffusion of electrolyte can improve the electrochemical properties of composites effectively. In this paper, a novel tremella-like graphene/polyaniline (TGP) composite was achieved from self-assembly of GNS during polymerization of aniline in H2O/N, N-dimethylformamide (DMF) solution. The composites have a three-dimensional porous structure which is conducive to electron transfer and diffusion of electrolyte. An excellent specific capacitance as high as 456 F g1 at a current density of 5 A g1 was obtained in the TGP electrode; good rate performance and cyclic stability were also exhibited in this electrode. The results demonstrate that the synergistic effects between PANI and GNS significantly affect the electrochemical performance of supercapacitor electrodes.
2.4. Electrochemical measurements The TGP material, carbon black, and polyvinylidene fluoride (PVDF) as binder, were mixed in a mass ratio of 80:10:10 and dispersed into a certain volume in NMP. Then the resulting mixture was coated onto the nickel foam substrate, which was followed by drying at 120 C for 12 h in a vacuum oven. The mass of each electrode is about 3 mg, including conducting agent and binder. The surface area of each electrode is 100 mm2. All electrochemical measurements were done in a standard three-electrode system: nickel foil electrode with the active material as the working electrode, nickel hydroxide electrode as a counter electrode, and an Hg/HgO as a reference electrode. The measurements were carried out in a KOH aqueous solution (30 wt. %). Galvanostatic charge/discharge test was taken by CT2001A Battery Program Controlling Test System (China-Land Comp. Ltd.) within the voltage range of 0.8–0 V. The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 660B electrochemical working station. For the cyclic voltammetric measurements, the sweep rate ranged from 1 to 50 mV s1 within a potential range of 0.8 to 0 V. For the EIS measurements, the frequency range was from 1 Hz to 10 kHz.
2. Experimental 3. Results and discussion 2.1. Preparation of graphite oxide 3.1. Morphology and structure of PANI, GNS and TGP GO was prepared by a modified Staudenmaier's method [34]. KClO3 was employed as the oxidants to obtain GO. Firstly, 87.5 ml sulfuric acid (98%), 45.0 ml nitric acid (68%, all analytical pure grade) and 5 g natural graphite (47 mm, provided by Jing Yuan Co. Ltd. Baotou, China) were mixed together in an ice-water bath. After 15 min, certain amounts of KClO3 (analytical pure, 99.5%) were gradually added into the obtained hydrated colloid, and the mixture was laid at room temperature for 96 h. Then the GO obtained by filtration was washed with deionized water until neutralization, and dried at 40 C for 24 h. Finally, GNS were obtained after the treated GO was put into a muffle oven preheated to 1050 C for 30 s. 2.2. Preparation of TGP composites The TGP composite was synthesized via an in situ polymerization. First, 0.01 g GNS powders obtained above were dispersed in 10 ml DMF solution by sonication for 20 minutes, and 0.2 g aniline was added into the above suspension with sonication for another 1 h. The temperature was then cooled to 5 C. Ammonium persulfate (0.4 g) in 100 ml deionized water was added into the as-prepared mixture and kept at 5 C for 10 h. After the reaction the composite washed with deionized water and ethanol, respectively, and dried in an oven at 40 C for 24 h. Finally, the resulting product was collected by washing and drying, and labeled as TGP (the weight ratio of PANI in the TGP composite is calculated to be 44%). For comparison, pure PANI was also prepared with the same chemical process. 2.3. Characterization The microstructures of the samples were observed by scanning electron microscope (SEM, ZEISS SUPRA 55) and high resolution transmission electron microscope (HRTEM, JEOL 3010). X-ray diffraction (XRD, Rigaku D/max-2500B2+/PCX system) using Cu Ka radiation (l = 1.5406 Å) over the range of 5–90 (2u ) at room temperature. The functional groups were measured by Fourier transform infrared spectroscopy instrument (FTIR, Nicolet Nexus 670).
The morphologies of the obtained GNS were observed by TEM and HRTEM and their images are shown in Fig. 1(a) and (b). Fig. 1(a) exhibits the TEM image of GNS with thin wrinkled structure that graphene owns intrinsically [35]. As is shown in Fig. 1(b), the thickness of the GNS is approximately 3–6 nm and is composed of approximately 5–10 wrinkled individual monoatomic graphene layers. Fig. 1(c) shows that the PANI fiber woven into pieces looks like weaving seats. As for the TGP composites in Fig. 1(d), a spherical morphology is observed instead of the sheet morphology of PANI or GNS. The diameter of the sphere assembled by nanosheets is about 10 mm from Fig. 1(e). The nanosheets with the thickness of about 50 nm are perpendicular to the centre of sphere. High magnification SEM images shown in Fig. 1(f) reveal that PANI attached on graphene sheets exhibits a granulated morphology. That could be ascribed to the affinity between two components both having conjugated structures. Plenty of holes have been formed between nanosheets, which are not only beneficial to the electrolyte infiltration but also good for the ion transmission. On the basis of the above experimental results, formation mechanisms of PANI and TGP are illustrated in Fig. 2. In the chemical oxidation polymerization process of aniline, two possible nucleation sites, bulky solution and solid substrates [31], compete with each other. GNS bring negative charges due to their residual carboxylic groups, [36] Therefore, aniline monomer can be adsorbed on the surface of GNS under ultrasonication. Under this condition of the reaction, most active nucleation sites were generated on the graphene nanosheet surface at the beginning of the polymerization by heterogeneous nucleation. These active sites would minimize the interfacial energy barrier between the solid surface and bulk solution, which is beneficial to the subsequent growth of PANI on the solid substrates [31,37]. PANI nanoparticles would further grow along the initial nuclei, and therefore, layered PANI nanoparticles on the graphene nanosheets were produced. After that, the soft nanosheets will self-assemble in the principle of minimizing surface energy, and finally form a spherical morphology with holes, like tremellas (Fig. 1c,d). In the process of the preparation of pure PANI, nucleation will take place after the initial nucleation on the solid surface [38]. Consequently, the nucleation will result connected PANI nanowires by using
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Fig. 1. TEM image of GNS (a), HRTEM image of GNS (b), SEM images of PANI (c) and TGP (d, e, and f). Inset is the magnification image of TGP (c).
aniline micelles as “soft template” and self-assembled into sheets (Fig. 1b). Further evidence in the coexistence of GNS and PANI in the nanocomposite is provided by the Fourier-transform infrared spectroscopy (FT-IR) spectra shown in Fig. 3. The typical FT-IR spectrum of GNS in Fig. 3 is in good agreement with previous work [15]. The bands centered at 3426 and 1397 cm1 are attributed to deformation of OH bond of GNS and CO H groups, respectively. The stretching vibration of the carbonyl or carboxyl groups is observed at 1724 cm1. The FT-IR spectrum of PANI shows the
strong N H stretching vibrations at 3265 cm1 and 3191 cm1. The weak shoulder at 3047 cm1 is attributable to the sp2 C H stretching on aromatic ring [39]. The CQC stretching deformation of the quinoid ring and benzenoid rings appear at 1582 cm1 and 1508 cm1, respectively [40]. The absorption peaks located at 1298 cm1 correspond to C N stretching of the secondary aromatic amine and at 695 cm1 is due to the C H bending vibrations [39]. The downmost curve in Fig. 3 shows the spectrum of TGP. It is found that the typical peaks of hydroxyl and carbonyl groups in GNS and the CQC stretching deformation of the quinoid
Fig. 2. Illustration of the preparation processes of PANI and TGP.
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Fig. 3. FT-IR spectra of GNS, PANI and TGP. Fig. 4. XRD patterns of GNS, PANI and TGP.
ring and benzenoid rings in PANI can be observed. Therefore, it is concluded that graphene/PANI nanocomposite is successfully obtained based on both SEM and FT-IR data. The PANI nanoparticle loaded on the GNS could be further confirmed by XRD. As shown in Fig. 4, one unusual sharp peak at 2u = 6.4 , which can be assigned to the periodic distance between the dopant and nitrogen atom on adjacent main chains [41,42], is observed for PANI specimens. Two characteristic peaks centered at 2u = 18.4 and 25.6 are ascribed to periodicity parallel and perpendicular to the polymer chains of PANI, respectively [43]. For GNS sample, one broad reflection peak centered at 2u = 24.6 was observed in the XRD pattern. For the composites, the peak at 2u = 6.4 indicates that the dopant and nitrogen atom on adjacent main chains have periodic distance. The new peaks between the graphene sheets overlap with the diffractions from the PANI, and usually broad intense peaks around 2u = 25 together some weak reflections are observed for TGP. 3.2. Electrochemical performances The samples were used as the electrode materials in supercapacitors and were tested for their electrochemical performances via CV in a three-electrode system. The curves of GNS electrodes exhibit an approximately rectangular shape that is characteristic of the electrical double-layer capacitance. As for the CV curve of PANI in Fig. 5(a) inset, two couples of redox peaks (C1/A1 and C2/A2) are observed, corresponding to redox transition fleucoemeraldine form (semiconductor)/polaronic emeraldine form (conductor) and Faradic transformation of emeraldine/pernigraniline, respectively [7]. The TGP composite electrodes exhibit a behaviour of a combination of both EDLC and pseudocapacitor. The peak positions of PANI and TGP change a little, which exhibit the synergetic performance of PANI and GNS. It is commonly believed that the electrochemical capacitance is proportional to the area under the CV curve. The areas under the CV curves of the TGP are larger than those of the parallel samples of pure PANI, indicating that the introduction of GNS improves the capacity of TGP electrode efficiently by improving the conductivity and stability of the materials. The three-dimensional nanostructure increases the electrode/electrolyte contact area and shortens the path lengths for electronic or electrolyte ion transport, which is valuable for enhancing the use of active materials [7,44,45]. Fig. 5(b) shows the CV curves of the TGP composites at different scan rates. It can be noted that the peak current density increases as the scan rate
increases and the CV curves almost maintain the same shape. Simultaneously the cathodic peaks shift positively and the anodic peaks shift negatively with the increase of potential scan rates from 1 to 50 mV s1, which is mainly due to the resistance of the electrode [7,46,47]. Furthermore, the obvious increase of current with scan rates means a good rate capability for TGP composite electrode. The electrochemical performances of the samples were aslo tested via galvanostatic charge/discharge in a three-electrode system, and the results were shown in Fig. 6a. Their specific capacitances (Cm) can be calculated with equation (1) Cm ¼
C It ¼ m DV m
(1), 1
where Cm is the specific capacitance (F g ), I is the charge/discharge current (A), DV is 0.8 V, and m is the mass of active material (GNS, PANI and TGP) within the electrode [48]. The galvanostatic charge/discharge curves of samples at a current density of 0.5 A g1 are shown in Fig. 6a. By calculation, the specific capacitance of TGP is as high as 497 F g1 at a current density of 0.5 A g1, and those of pure PANI and GNS are 36 and 123 F g1, respectively. As shown in Fig. 6a, the charge/discharge curve of GNS clearly demonstrates a symmetrical triangle, corresponding to the typical characteristics of EDLC. However, the curve of the pure PANI exhibits the platform which originated from the faradaic reactions. The charge/discharge curve of TGP clearly demonstrates that the charge and discharge time is longer than PANI and GNS electrodes, which can be indicated that the energy consumed by the internal resistance is reduced and the effective energy storage is improved. The reasons for the improved properties could be ascribed to the reduced inner resistance of the electrode when GNS are present [9]. The stable specific capacitances after 1000 time cycles at every current density are summarized in Fig. 6(b). The specific capacitance for TGP at 0.5 A g1 is 497 F g1, which is much higher than those of similar morphology composite electrodes. Zhou et al. [33] prepared graphene microspheres with a 3-D porous structure by the spray drying method where graphene nanosheets suspensions were used as the template to synthesize PANI/modified graphene sheets composite. The composite displayed a maximum specific capacitance value of 261 F g1 at 0.5 A g1 in 1 M H2SO4 electrolyte. Zhang's group [49] found a high specific capacitance of 480 F g1 for graphene/PANI nanofiber composite at a discharge current density of 0.1 Ag1 in 1 M H2SO4 electrolyte. However, at the current density of 0.5 A g1, the specific
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Fig. 5. (a) Cyclic voltammetry (CV) curves of GNS, PANI and TGP electrodes at a scan rate of 20 mV s1 in 30 wt% KOH solution; (b) CV curves of TGP electrode at different scan rate of 1 mV s1, 10 mV s1, 20 mV s1 and 50 mV s1, respectively. Inset is the magnification curve of PANI (a).
capacitance decreased to 260 F g1. Wu et al. [50] fabricated graphene and PANI nanofiber based composite electrode which displayed a specific capacitance of 210 F g1 at a discharge rate of 0.3 Ag1 in 1 M H2SO4 aqueous electrolyte. Even at the high current density of 5.0 A g1, the specific capacitance of our TGP hybrid (456 F g1) is much higher than those of the graphene/polyaniline composites [31,51]. The uniform dispersion of individual graphene sheet within the polymer nanoparticles matrix improved the kinetics for both charge transfer and ion transport throughout the electrode and consequently produced higher specific
capacitance. The long cycling performance at 5.0 A g1 were shown in Fig. 6(c). Under the successive charging/discharging cycles, the speficific capacitance of TGP decreases from 542 to 440 F g1 in the initial 50 cycles, which corresponds to 81% of its initial value, then levels off gradually in the following cycles and 78% of its initial value is finally remained after 1000 cycles. It is inferred that the partial structural damage or detachment of PANI nanoparticles occurs over the initial 50 cycles causes the decay in specific capacitance, subsequently, the structure tends to be stable, resulting in an approximately stable capacitance in the following
Fig. 6. (a) The galvanostatic charge/discharge curves at 0.5 A g1, (b) the specific capacitances at different current densities, (c) the cycling performance at 5.0 A g1 and (d) Nyquist plots for PAIN, GNS and TGP electrodes.
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cycles [27]. The high performance comes down to the particular structure of composites, the unique slice layer structure and the holes benefit to electrolyte infiltration and the ion transmission. In composite materials, GNS can improve the electrical conductivity of the electrode material and inhibit the volume change of PANI effectively. Electrochemical impedance spectroscopy (EIS) analysis is a powerful and informative technique to evaluate the properties of conductivity, structure and charge transport in the electrolyte interface. The impedance behavior might be dominated by three major processes occurring in the high, medium and low frequency regions [52]. From the Nyquist plots shown in Fig 6d, the radius of semicircle in TGP located between PANI and GNS. The result conforms to the composite theory. The radius of semicircle in TGP is smaller than PANI, indicating that the polarization resistance of TGP is lower than that of PANI, owing to the highly electrical conductive 3D network inside the composites. And the slope of 45 portion of the curve indicates typical Warburg impedance resulting from the frequency dependence of ion diffusion in the electrode structure. The larger Warburg region of GNS than TGP indicates greater variations in ion diffusion path lengths which increases the obstruction of ion movement. The results also indicate that the designed 3D network of TGP preponderates over GNS in term of making full use of the conductivity of graphene. 4. Conclusions A novel TGP composite was achieved from self-assembly of GNS during polymerization of aniline in H2O/DMF solution. This TGP composite possesses a spherical tremella-like structure and a large specific capacity of 497 F g1 at a current density of 0.5 A g1. Particularly, an outstanding rate capability of 456 F g1 under 5 A g1 after 1000 cycles was obtained. This superior rate performance is attributed to the 3D interconnected structure of TGP, which allowed electrolyte ions to pass through quickly during the rapid charge/discharge process. Therefore, the TGP composite is quite a suitable and promising electrode material for supercapacitors. Acknowledgments This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001). References [1] M.Y. Kodama, J. Yamashita, Y. Soneda, H. Hatori, K. Kamegawa, Preparation and electrochemical characteristics of N-enriched carbon foam, Carbon 45 (2007) 1105–1107. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845–854. [3] B. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (POD), Kluwer Academic, Plenum. New York, 1999. [4] C. Peng, S. Zhang, D. Jewell, G.Z. Chen, Carbon nanotube and conducting polymer composites for supercapacitors, Progress in Natural Science 18 (2008) 777–788. [5] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angewandte Chemie International Edition 47 (2008) 373–376. [6] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Advanced Materials 22 (2010) E28–E62. [7] Y.G. Wang, H.Q. Li, Y. Xia, Ordered whiskerlike polyaniline grown on the surface of mesoporous carbon and its electrochemical capacitance performance, Advanced Materials 18 (2006) 2619–2623. [8] L.Z. Fan, Y.S. Hu, J. Maier, P. Adelhelm, B. Smarsly, M. Antonietti, High electroactivity of polyaniline in supercapacitors by using a hierarchically porous carbon monolith as a support, Advanced Functional Materials 17 (2007) 3083–3087.
[9] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors, Journal of Power Sources 195 (2010) 3041–3045. [10] X. Zhao, B.M. Sánchez, P.J. Dobson, P.S. Grant, The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices, Nanoscale 3 (2011) 839–855. [11] K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [12] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano letters 8 (2008) 3498–3502. [13] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.B.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282–286. [14] O. Leenaerts, B. Partoens, F. Peeters, Adsorption of small molecules on graphene, Microelectronics Journal 40 (2009) 860–862. [15] X. Du, P. Guo, H. Song, X. Chen, Graphene nanosheets as electrode material for electric double-layer capacitors, Electrochimica Acta 55 (2010) 4812–4819. [16] M. Lazzari, F. Soavi, M. Mastragostino, High voltage, asymmetric EDLCs based on xerogel carbon and hydrophobic IL electrolytes, Journal of Power Sources 178 (2008) 490–496. [17] C. Rao, K. Biswas, K. Subrahmanyam, A. Govindaraj, Graphene, the new nanocarbon, Journal of Materials Chemistry 19 (2009) 2457–2469. [18] M. Liang, L. Zhi, Graphene-based electrode materials for rechargeable lithium batteries, Journal of Materials Chemistry 19 (2009) 5871–5878. [19] Y. Li, X. Zhao, P. Yu, Q. Zhang, Oriented arrays of polyaniline nanorods grown on graphite nanosheets for an electrochemical supercapacitor, Langmuir 29 (2012) 493–500. [20] N.A. Kumar, H.-J. Choi, Y.R. Shin, D.W. Chang, L. Dai, J.-B. Baek, Polyanilinegrafted reduced graphene oxide for efficient electrochemical supercapacitors, ACS Nano 6 (2012) 1715–1723. [21] Z. Zhou, X.-F. Wu, Graphene-beaded carbon nanofibers for use in supercapacitor electrodes: synthesis and electrochemical characterization, Journal of Power Sources 222 (2013) 410–416. [22] Z.Zhou,X.-F.Wu,High-performanceporouselectrodesforpseudosupercapacitors based on graphene-beaded carbon nanofibers surface-coated with nanostructured conducting polymers, Journal of Power Sources 262 (2014) 44–49. [23] N.B. Trung, T.V. Tam, H.R. Kim, S.H. Hur, E.J. Kim, W.M. Choi, Three-dimensional hollow balls of graphene-polyaniline hybrids for supercapacitor applications, Chemical Engineering Journal (2014) . [24] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A.I. Minett, V.G. Gomes, Hierarchical assembly of graphene/polyaniline nanostructures to synthesize free-standing supercapacitor electrode, Composites Science and Technology 98 (2014) 1–8. [25] C. Yan, Y.W. Kanaththage, R. Short, C.T. Gibson, L. Zou, Graphene/Polyaniline nanocomposite as electrode material for membrane capacitive deionization, Desalination 344 (2014) 274–279. [26] F. Yang, M. Xu, S.-J. Bao, H. Wei, H. Chai, Self-assembled hierarchical graphene/ polyaniline hybrid aerogels for electrochemical capacitive energy storage, Electrochimica Acta 137 (2014) 381–387. [27] Z. Gao, F. Wang, J. Chang, D. Wu, X. Wang, X. Wang, F. Xu, S. Gao, K. Jiang, Chemically grafted graphene-polyaniline composite for application in supercapacitor, Electrochimica Acta 133 (2014) 325–334. [28] B. Mu, W. Zhang, A. Wang, Template synthesis of graphene/polyaniline hybrid hollow microspheres as electrode materials for high-performance supercapacitor, Journal of Nanoparticle Research 16 (2014) 1–12. [29] Z. Tong, Y. Yang, J. Wang, J. Zhao, B.-L. Su, Y. Li, Layered polyaniline/graphene film from sandwich-structured polyaniline/graphene/polyaniline nanosheets for high-performance pseudosupercapacitors, Journal of Materials Chemistry A 2 (2014) 4642–4651. [30] Y. Liu, X.Y. Su, S.Y. Guang, H.Y. Xu, Polyaniline Grafted Amino-Functionalized Graphene Nanocomposite with Excellent Electrochemical Performance for Supercapacitor Electrode Materials, Materials Science Forum, Trans Tech Publ, 2014, pp. 12–17. [31] J. Xu, K. Wang, S.Z. Zu, B.H. Han, Z. Wei, Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage, ACS Nano 4 (2010) 5019–5026. [32] M. Kumar, K. Singh, S.K. Dhawan, K. Tharanikkarasu, J.S. Chung, B.-S. Kong, E.J. Kim, S.H. Hur, Synthesis and characterization of covalently-grafted graphene– polyaniline nanocomposites and its use in a supercapacitor, Chemical Engineering Journal 231 (2013) 397–405. [33] H. Cao, X. Zhou, Y. Zhang, L. Chen, Z. Liu, Microspherical polyaniline/graphene nanocomposites for high performance supercapacitors, Journal of Power Sources (2013) . [34] L. Staudenmaier, Ber. Dtsch. Chem. Ges 31 (1898) 1481. [35] J.C. Meyer, A.K. Geim, M. Katsnelson, K. Novoselov, T. Booth, S. Roth, The structure of suspended graphene sheets, Nature 446 (2007) 60–63. [36] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nature Nanotechnology 3 (2008) 101–105. [37] N.-R. Chiou, C. Lu, J. Guan, L.J. Lee, A.J. Epstein, Growth and alignment of polyaniline nanofibres with superhydrophobic, superhydrophilic and other properties, Nature Nanotechnology 2 (2007) 354–357. [38] N.R. Chiou, A.J. Epstein, Polyaniline nanofibers prepared by dilute polymerization, Advanced Materials 17 (2005) 1679–1683.
H. Liu et al. / Electrochimica Acta 146 (2014) 511–517 [39] X. Yan, Z. Han, Y. Yang, B. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sensors and Actuators B: Chemical 123 (2007) 107–113. [40] R. Cruz-Silva, J. Romero-García, J. Angulo-Sanchez, E. Flores-Loyola, M. Farías, F. Castillon, J. Díaz, Comparative study of polyaniline cast films prepared from enzymatically and chemically synthesized polyaniline, Polymer 45 (2004) 4711–4717. [41] C. Wang, Z. Wang, M. Li, H. Li, Well-aligned polyaniline nano-fibril array membrane and its field emission property, Chemical Physics Letters 341 (2001) 431–434. [42] H. Chaudhari, D. Kelkar, X-ray diffraction study of doped polyaniline, Journal of Applied Polymer Science 62 (1996) 15–18. [43] J. Pouget, M. Jozefowicz, X. A. e. a. Epstein, Tang, A. MacDiarmid, X-ray structure of polyaniline, Macromolecules 24 (1991) 779–789. [44] P.-L. Taberna, S. Mitra, P. Poizot, P. Simon, J.-M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nature Materials 5 (2006) 567–573. [45] B.K. Kuila, B. Nandan, M. Böhme, A. Janke, M. Stamm, Vertically oriented arrays of polyaniline nanorods and their super electrochemical properties, Chemical Communications (2009) 5749–5751.
517
[46] K. Zhang, L. Mao, L.L. Zhang, H.S. On Chan, X.S. Zhao, J. Wu, Surfactantintercalated, chemically reduced graphene oxide for high performance supercapacitor electrodes, Journal of Materials Chemistry 21 (2011) 7302– 7307. [47] L. Mao, K. Zhang, H.S.O. Chan, J. Wu, Surfactant-stabilized graphene/polyaniline nanofiber composites for high performance supercapacitor electrode, Journal of Materials Chemistry 22 (2012) 80–85. [48] K. Y.-g. Wang, Y.-y. Xia, Hybrid aqueous energy storage cells using activated carbon and lithium-lntercalated compounds I. The C/Li Mn2O4 system, Journal of The Electrochemical Society 153 (2006) A450–A454. [49] Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chemistry of Materials 22 (2010) 1392–1401. [50] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/ polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963–1970. [51] J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance, Carbon 48 (2010) 487–493. [52] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nature Nanotechnology 4 (2009) 217–224.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Tremella-like graphene/polyaniline spherical electrode material for supercapacitors Haiyan Liu, Wei Zhang, Huaihe Song *, Xiaohong Chen, Jisheng Zhou, Zhaokun Ma State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 July 2014 Received in revised form 22 August 2014 Accepted 2 September 2014 Available online 22 September 2014
A novel tremella-like graphene/polyaniline composite was achieved from self-assembly of graphene nanosheets during polymerization of aniline in H2O/N, N-dimethylformamide solution, and was further employed as an electrode for supercapacitors. This graphene/polyaniline composite demonstrates a spherical tremella-like structure and a large specific capacity of 497 F g1 at a current density of 0.5 A g1. Particularly, an outstanding rate capability of 456 F g1 under 5 A g1 after 1000 cycles was obtained. Scanning electron microscopy showed that polyaniline nanoparticles were uniformly deposited on freestanding graphene nanosheets, and self-assembled to a spherical tremella-like structure. Therefore, this unique nanostructure is promising for high-performance electrochemical applications. ã 2014 Elsevier Ltd. All rights reserved.
Key words: Graphene polyaniline Tremella-like Self-assembly Supercapacitors
1. Introduction Supercapacitors or electrochemical capacitors are considered as very promising candidates for future power storage devices due to their high power density and outstanding cycling performance [1,2]. There are two energy storage mechanisms for supercapacitors: the electric double-layer (EDL) capacitance and the pseudo-capacitance [3]. The EDL capacitance arises from the charge separation at the electrode/electrolyte interface, but the energy densities of EDL capacitors are too low for many important applications [4]. The high capacitances of pseudocapacitors are derived from fast, reversible redox reactions on the surfaces or in the bulk phase of active materials [5]. Polymers are usually used as pseudocapacitor active materials [6]. Among all of the developed materials, electronically conducting polymers such as polypyrrole, polythiophene and especially polyaniline (PANI), are widely investigated because of their easy synthesis, relative high specific capacitance and low fabrication cost than many other electrode materials [7]. However, severe volume swelling and shrinkage during charge-discharge processes lead to the mechanical degradation of these conductive polymers, which causes the obvious capacity fading [8,9]. In addition, low electrical conductivity can aslo influence the pseudocapacitive
* Corresponding author. Tel.: +86 10 64434916; fax: +86 10 64434916. E-mail address: [email protected] (H. Song). http://dx.doi.org/10.1016/j.electacta.2014.09.083 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
performance of PANI when structured as electrode material of supercapacitors [10]. Graphene, a two-dimensional all-sp2-hybridized carbon, which was first obtained in 2004 by Geim [11], has attracted a great attention due to its extraordinary electric, mechanical properties and thermal properties [12–16]. These properties suggest wide applications, such as fabricating electronic devices, lithium-ion batteries and supercapacitors [12,17,18]. The combination of PANI with graphene has been proved to be attractive to reinforce the stability and the electrical conductivity of PANI as well as maximize the capacitance value [19,20]. Therefore, high dispersion of PANI on a support material with excellent conductivity and high surface area should be a promising way to improve the capacitive properties of PANI. Recently, many attempts have been devoted to the use of graphene/PANI as electrode materials in supercapacitors [21–30]. Xu et al. [31] constructed a hierarchical nanocomposite containing PANI nanowires and graphene oxide (GO) nanosheets by dilute polymerization, and found that this architecture material showed a capacitance of 227 F g1 at the current of 2 A g1. Covalentlygrafted graphene/PANI nanocomposites with capacitance of 338 F g1 at the scan of 5 mv s1 were synthesized by diazotization chemistry to enhance the three dimensional functionalization of graphene with p-aminobenzoic acid [32]. Kumar et al. [20] synthesized polyaniline-grafted reduced graphene oxide composites with capacitance of 250 F g1 by functionalizing graphite oxide with 4-aminophenol via acyl chemistry, where a concomitant reduction of GO takes place.
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However, up to now, much attention has been paid on the preparation method of PANI/graphene materials; few researches investigated the influence of the constructed architecture by graphene nanosheets (GNS) on the electrochemical performance for supercapacitors. In addition, graphene/PANI composites with a sheet-like morphology have a strong tendency to restack during solvent evaporation, which may hinder the diffusion of electrolytes [33]. Therefore, designing a three-dimensional structure which is conducive to charge transfer and diffusion of electrolyte can improve the electrochemical properties of composites effectively. In this paper, a novel tremella-like graphene/polyaniline (TGP) composite was achieved from self-assembly of GNS during polymerization of aniline in H2O/N, N-dimethylformamide (DMF) solution. The composites have a three-dimensional porous structure which is conducive to electron transfer and diffusion of electrolyte. An excellent specific capacitance as high as 456 F g1 at a current density of 5 A g1 was obtained in the TGP electrode; good rate performance and cyclic stability were also exhibited in this electrode. The results demonstrate that the synergistic effects between PANI and GNS significantly affect the electrochemical performance of supercapacitor electrodes.
2.4. Electrochemical measurements The TGP material, carbon black, and polyvinylidene fluoride (PVDF) as binder, were mixed in a mass ratio of 80:10:10 and dispersed into a certain volume in NMP. Then the resulting mixture was coated onto the nickel foam substrate, which was followed by drying at 120 C for 12 h in a vacuum oven. The mass of each electrode is about 3 mg, including conducting agent and binder. The surface area of each electrode is 100 mm2. All electrochemical measurements were done in a standard three-electrode system: nickel foil electrode with the active material as the working electrode, nickel hydroxide electrode as a counter electrode, and an Hg/HgO as a reference electrode. The measurements were carried out in a KOH aqueous solution (30 wt. %). Galvanostatic charge/discharge test was taken by CT2001A Battery Program Controlling Test System (China-Land Comp. Ltd.) within the voltage range of 0.8–0 V. The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 660B electrochemical working station. For the cyclic voltammetric measurements, the sweep rate ranged from 1 to 50 mV s1 within a potential range of 0.8 to 0 V. For the EIS measurements, the frequency range was from 1 Hz to 10 kHz.
2. Experimental 3. Results and discussion 2.1. Preparation of graphite oxide 3.1. Morphology and structure of PANI, GNS and TGP GO was prepared by a modified Staudenmaier's method [34]. KClO3 was employed as the oxidants to obtain GO. Firstly, 87.5 ml sulfuric acid (98%), 45.0 ml nitric acid (68%, all analytical pure grade) and 5 g natural graphite (47 mm, provided by Jing Yuan Co. Ltd. Baotou, China) were mixed together in an ice-water bath. After 15 min, certain amounts of KClO3 (analytical pure, 99.5%) were gradually added into the obtained hydrated colloid, and the mixture was laid at room temperature for 96 h. Then the GO obtained by filtration was washed with deionized water until neutralization, and dried at 40 C for 24 h. Finally, GNS were obtained after the treated GO was put into a muffle oven preheated to 1050 C for 30 s. 2.2. Preparation of TGP composites The TGP composite was synthesized via an in situ polymerization. First, 0.01 g GNS powders obtained above were dispersed in 10 ml DMF solution by sonication for 20 minutes, and 0.2 g aniline was added into the above suspension with sonication for another 1 h. The temperature was then cooled to 5 C. Ammonium persulfate (0.4 g) in 100 ml deionized water was added into the as-prepared mixture and kept at 5 C for 10 h. After the reaction the composite washed with deionized water and ethanol, respectively, and dried in an oven at 40 C for 24 h. Finally, the resulting product was collected by washing and drying, and labeled as TGP (the weight ratio of PANI in the TGP composite is calculated to be 44%). For comparison, pure PANI was also prepared with the same chemical process. 2.3. Characterization The microstructures of the samples were observed by scanning electron microscope (SEM, ZEISS SUPRA 55) and high resolution transmission electron microscope (HRTEM, JEOL 3010). X-ray diffraction (XRD, Rigaku D/max-2500B2+/PCX system) using Cu Ka radiation (l = 1.5406 Å) over the range of 5–90 (2u ) at room temperature. The functional groups were measured by Fourier transform infrared spectroscopy instrument (FTIR, Nicolet Nexus 670).
The morphologies of the obtained GNS were observed by TEM and HRTEM and their images are shown in Fig. 1(a) and (b). Fig. 1(a) exhibits the TEM image of GNS with thin wrinkled structure that graphene owns intrinsically [35]. As is shown in Fig. 1(b), the thickness of the GNS is approximately 3–6 nm and is composed of approximately 5–10 wrinkled individual monoatomic graphene layers. Fig. 1(c) shows that the PANI fiber woven into pieces looks like weaving seats. As for the TGP composites in Fig. 1(d), a spherical morphology is observed instead of the sheet morphology of PANI or GNS. The diameter of the sphere assembled by nanosheets is about 10 mm from Fig. 1(e). The nanosheets with the thickness of about 50 nm are perpendicular to the centre of sphere. High magnification SEM images shown in Fig. 1(f) reveal that PANI attached on graphene sheets exhibits a granulated morphology. That could be ascribed to the affinity between two components both having conjugated structures. Plenty of holes have been formed between nanosheets, which are not only beneficial to the electrolyte infiltration but also good for the ion transmission. On the basis of the above experimental results, formation mechanisms of PANI and TGP are illustrated in Fig. 2. In the chemical oxidation polymerization process of aniline, two possible nucleation sites, bulky solution and solid substrates [31], compete with each other. GNS bring negative charges due to their residual carboxylic groups, [36] Therefore, aniline monomer can be adsorbed on the surface of GNS under ultrasonication. Under this condition of the reaction, most active nucleation sites were generated on the graphene nanosheet surface at the beginning of the polymerization by heterogeneous nucleation. These active sites would minimize the interfacial energy barrier between the solid surface and bulk solution, which is beneficial to the subsequent growth of PANI on the solid substrates [31,37]. PANI nanoparticles would further grow along the initial nuclei, and therefore, layered PANI nanoparticles on the graphene nanosheets were produced. After that, the soft nanosheets will self-assemble in the principle of minimizing surface energy, and finally form a spherical morphology with holes, like tremellas (Fig. 1c,d). In the process of the preparation of pure PANI, nucleation will take place after the initial nucleation on the solid surface [38]. Consequently, the nucleation will result connected PANI nanowires by using
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Fig. 1. TEM image of GNS (a), HRTEM image of GNS (b), SEM images of PANI (c) and TGP (d, e, and f). Inset is the magnification image of TGP (c).
aniline micelles as “soft template” and self-assembled into sheets (Fig. 1b). Further evidence in the coexistence of GNS and PANI in the nanocomposite is provided by the Fourier-transform infrared spectroscopy (FT-IR) spectra shown in Fig. 3. The typical FT-IR spectrum of GNS in Fig. 3 is in good agreement with previous work [15]. The bands centered at 3426 and 1397 cm1 are attributed to deformation of OH bond of GNS and CO H groups, respectively. The stretching vibration of the carbonyl or carboxyl groups is observed at 1724 cm1. The FT-IR spectrum of PANI shows the
strong N H stretching vibrations at 3265 cm1 and 3191 cm1. The weak shoulder at 3047 cm1 is attributable to the sp2 C H stretching on aromatic ring [39]. The CQC stretching deformation of the quinoid ring and benzenoid rings appear at 1582 cm1 and 1508 cm1, respectively [40]. The absorption peaks located at 1298 cm1 correspond to C N stretching of the secondary aromatic amine and at 695 cm1 is due to the C H bending vibrations [39]. The downmost curve in Fig. 3 shows the spectrum of TGP. It is found that the typical peaks of hydroxyl and carbonyl groups in GNS and the CQC stretching deformation of the quinoid
Fig. 2. Illustration of the preparation processes of PANI and TGP.
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Fig. 3. FT-IR spectra of GNS, PANI and TGP. Fig. 4. XRD patterns of GNS, PANI and TGP.
ring and benzenoid rings in PANI can be observed. Therefore, it is concluded that graphene/PANI nanocomposite is successfully obtained based on both SEM and FT-IR data. The PANI nanoparticle loaded on the GNS could be further confirmed by XRD. As shown in Fig. 4, one unusual sharp peak at 2u = 6.4 , which can be assigned to the periodic distance between the dopant and nitrogen atom on adjacent main chains [41,42], is observed for PANI specimens. Two characteristic peaks centered at 2u = 18.4 and 25.6 are ascribed to periodicity parallel and perpendicular to the polymer chains of PANI, respectively [43]. For GNS sample, one broad reflection peak centered at 2u = 24.6 was observed in the XRD pattern. For the composites, the peak at 2u = 6.4 indicates that the dopant and nitrogen atom on adjacent main chains have periodic distance. The new peaks between the graphene sheets overlap with the diffractions from the PANI, and usually broad intense peaks around 2u = 25 together some weak reflections are observed for TGP. 3.2. Electrochemical performances The samples were used as the electrode materials in supercapacitors and were tested for their electrochemical performances via CV in a three-electrode system. The curves of GNS electrodes exhibit an approximately rectangular shape that is characteristic of the electrical double-layer capacitance. As for the CV curve of PANI in Fig. 5(a) inset, two couples of redox peaks (C1/A1 and C2/A2) are observed, corresponding to redox transition fleucoemeraldine form (semiconductor)/polaronic emeraldine form (conductor) and Faradic transformation of emeraldine/pernigraniline, respectively [7]. The TGP composite electrodes exhibit a behaviour of a combination of both EDLC and pseudocapacitor. The peak positions of PANI and TGP change a little, which exhibit the synergetic performance of PANI and GNS. It is commonly believed that the electrochemical capacitance is proportional to the area under the CV curve. The areas under the CV curves of the TGP are larger than those of the parallel samples of pure PANI, indicating that the introduction of GNS improves the capacity of TGP electrode efficiently by improving the conductivity and stability of the materials. The three-dimensional nanostructure increases the electrode/electrolyte contact area and shortens the path lengths for electronic or electrolyte ion transport, which is valuable for enhancing the use of active materials [7,44,45]. Fig. 5(b) shows the CV curves of the TGP composites at different scan rates. It can be noted that the peak current density increases as the scan rate
increases and the CV curves almost maintain the same shape. Simultaneously the cathodic peaks shift positively and the anodic peaks shift negatively with the increase of potential scan rates from 1 to 50 mV s1, which is mainly due to the resistance of the electrode [7,46,47]. Furthermore, the obvious increase of current with scan rates means a good rate capability for TGP composite electrode. The electrochemical performances of the samples were aslo tested via galvanostatic charge/discharge in a three-electrode system, and the results were shown in Fig. 6a. Their specific capacitances (Cm) can be calculated with equation (1) Cm ¼
C It ¼ m DV m
(1), 1
where Cm is the specific capacitance (F g ), I is the charge/discharge current (A), DV is 0.8 V, and m is the mass of active material (GNS, PANI and TGP) within the electrode [48]. The galvanostatic charge/discharge curves of samples at a current density of 0.5 A g1 are shown in Fig. 6a. By calculation, the specific capacitance of TGP is as high as 497 F g1 at a current density of 0.5 A g1, and those of pure PANI and GNS are 36 and 123 F g1, respectively. As shown in Fig. 6a, the charge/discharge curve of GNS clearly demonstrates a symmetrical triangle, corresponding to the typical characteristics of EDLC. However, the curve of the pure PANI exhibits the platform which originated from the faradaic reactions. The charge/discharge curve of TGP clearly demonstrates that the charge and discharge time is longer than PANI and GNS electrodes, which can be indicated that the energy consumed by the internal resistance is reduced and the effective energy storage is improved. The reasons for the improved properties could be ascribed to the reduced inner resistance of the electrode when GNS are present [9]. The stable specific capacitances after 1000 time cycles at every current density are summarized in Fig. 6(b). The specific capacitance for TGP at 0.5 A g1 is 497 F g1, which is much higher than those of similar morphology composite electrodes. Zhou et al. [33] prepared graphene microspheres with a 3-D porous structure by the spray drying method where graphene nanosheets suspensions were used as the template to synthesize PANI/modified graphene sheets composite. The composite displayed a maximum specific capacitance value of 261 F g1 at 0.5 A g1 in 1 M H2SO4 electrolyte. Zhang's group [49] found a high specific capacitance of 480 F g1 for graphene/PANI nanofiber composite at a discharge current density of 0.1 Ag1 in 1 M H2SO4 electrolyte. However, at the current density of 0.5 A g1, the specific
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Fig. 5. (a) Cyclic voltammetry (CV) curves of GNS, PANI and TGP electrodes at a scan rate of 20 mV s1 in 30 wt% KOH solution; (b) CV curves of TGP electrode at different scan rate of 1 mV s1, 10 mV s1, 20 mV s1 and 50 mV s1, respectively. Inset is the magnification curve of PANI (a).
capacitance decreased to 260 F g1. Wu et al. [50] fabricated graphene and PANI nanofiber based composite electrode which displayed a specific capacitance of 210 F g1 at a discharge rate of 0.3 Ag1 in 1 M H2SO4 aqueous electrolyte. Even at the high current density of 5.0 A g1, the specific capacitance of our TGP hybrid (456 F g1) is much higher than those of the graphene/polyaniline composites [31,51]. The uniform dispersion of individual graphene sheet within the polymer nanoparticles matrix improved the kinetics for both charge transfer and ion transport throughout the electrode and consequently produced higher specific
capacitance. The long cycling performance at 5.0 A g1 were shown in Fig. 6(c). Under the successive charging/discharging cycles, the speficific capacitance of TGP decreases from 542 to 440 F g1 in the initial 50 cycles, which corresponds to 81% of its initial value, then levels off gradually in the following cycles and 78% of its initial value is finally remained after 1000 cycles. It is inferred that the partial structural damage or detachment of PANI nanoparticles occurs over the initial 50 cycles causes the decay in specific capacitance, subsequently, the structure tends to be stable, resulting in an approximately stable capacitance in the following
Fig. 6. (a) The galvanostatic charge/discharge curves at 0.5 A g1, (b) the specific capacitances at different current densities, (c) the cycling performance at 5.0 A g1 and (d) Nyquist plots for PAIN, GNS and TGP electrodes.
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cycles [27]. The high performance comes down to the particular structure of composites, the unique slice layer structure and the holes benefit to electrolyte infiltration and the ion transmission. In composite materials, GNS can improve the electrical conductivity of the electrode material and inhibit the volume change of PANI effectively. Electrochemical impedance spectroscopy (EIS) analysis is a powerful and informative technique to evaluate the properties of conductivity, structure and charge transport in the electrolyte interface. The impedance behavior might be dominated by three major processes occurring in the high, medium and low frequency regions [52]. From the Nyquist plots shown in Fig 6d, the radius of semicircle in TGP located between PANI and GNS. The result conforms to the composite theory. The radius of semicircle in TGP is smaller than PANI, indicating that the polarization resistance of TGP is lower than that of PANI, owing to the highly electrical conductive 3D network inside the composites. And the slope of 45 portion of the curve indicates typical Warburg impedance resulting from the frequency dependence of ion diffusion in the electrode structure. The larger Warburg region of GNS than TGP indicates greater variations in ion diffusion path lengths which increases the obstruction of ion movement. The results also indicate that the designed 3D network of TGP preponderates over GNS in term of making full use of the conductivity of graphene. 4. Conclusions A novel TGP composite was achieved from self-assembly of GNS during polymerization of aniline in H2O/DMF solution. This TGP composite possesses a spherical tremella-like structure and a large specific capacity of 497 F g1 at a current density of 0.5 A g1. Particularly, an outstanding rate capability of 456 F g1 under 5 A g1 after 1000 cycles was obtained. This superior rate performance is attributed to the 3D interconnected structure of TGP, which allowed electrolyte ions to pass through quickly during the rapid charge/discharge process. Therefore, the TGP composite is quite a suitable and promising electrode material for supercapacitors. Acknowledgments This work was supported by the National Natural Science Foundation of China (51202009 and 51272016), and Foundation of Excellent Doctoral Dissertation of Beijing City (YB20121001001). References [1] M.Y. Kodama, J. Yamashita, Y. Soneda, H. Hatori, K. Kamegawa, Preparation and electrochemical characteristics of N-enriched carbon foam, Carbon 45 (2007) 1105–1107. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845–854. [3] B. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications (POD), Kluwer Academic, Plenum. New York, 1999. [4] C. Peng, S. Zhang, D. Jewell, G.Z. Chen, Carbon nanotube and conducting polymer composites for supercapacitors, Progress in Natural Science 18 (2008) 777–788. [5] D.W. Wang, F. Li, M. Liu, G.Q. Lu, H.M. Cheng, 3D aperiodic hierarchical porous graphitic carbon material for high-rate electrochemical capacitive energy storage, Angewandte Chemie International Edition 47 (2008) 373–376. [6] C. Liu, F. Li, L.P. Ma, H.M. Cheng, Advanced materials for energy storage, Advanced Materials 22 (2010) E28–E62. [7] Y.G. Wang, H.Q. Li, Y. Xia, Ordered whiskerlike polyaniline grown on the surface of mesoporous carbon and its electrochemical capacitance performance, Advanced Materials 18 (2006) 2619–2623. [8] L.Z. Fan, Y.S. Hu, J. Maier, P. Adelhelm, B. Smarsly, M. Antonietti, High electroactivity of polyaniline in supercapacitors by using a hierarchically porous carbon monolith as a support, Advanced Functional Materials 17 (2007) 3083–3087.
[9] J. Yan, T. Wei, Z. Fan, W. Qian, M. Zhang, X. Shen, F. Wei, Preparation of graphene nanosheet/carbon nanotube/polyaniline composite as electrode material for supercapacitors, Journal of Power Sources 195 (2010) 3041–3045. [10] X. Zhao, B.M. Sánchez, P.J. Dobson, P.S. Grant, The role of nanomaterials in redox-based supercapacitors for next generation energy storage devices, Nanoscale 3 (2011) 839–855. [11] K. Novoselov, A. Geim, S. Morozov, D. Jiang, Y. Zhang, S. Dubonos, I. Grigorieva, A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [12] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors, Nano letters 8 (2008) 3498–3502. [13] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D. Piner, S.B.T. Nguyen, R.S. Ruoff, Graphene-based composite materials, Nature 442 (2006) 282–286. [14] O. Leenaerts, B. Partoens, F. Peeters, Adsorption of small molecules on graphene, Microelectronics Journal 40 (2009) 860–862. [15] X. Du, P. Guo, H. Song, X. Chen, Graphene nanosheets as electrode material for electric double-layer capacitors, Electrochimica Acta 55 (2010) 4812–4819. [16] M. Lazzari, F. Soavi, M. Mastragostino, High voltage, asymmetric EDLCs based on xerogel carbon and hydrophobic IL electrolytes, Journal of Power Sources 178 (2008) 490–496. [17] C. Rao, K. Biswas, K. Subrahmanyam, A. Govindaraj, Graphene, the new nanocarbon, Journal of Materials Chemistry 19 (2009) 2457–2469. [18] M. Liang, L. Zhi, Graphene-based electrode materials for rechargeable lithium batteries, Journal of Materials Chemistry 19 (2009) 5871–5878. [19] Y. Li, X. Zhao, P. Yu, Q. Zhang, Oriented arrays of polyaniline nanorods grown on graphite nanosheets for an electrochemical supercapacitor, Langmuir 29 (2012) 493–500. [20] N.A. Kumar, H.-J. Choi, Y.R. Shin, D.W. Chang, L. Dai, J.-B. Baek, Polyanilinegrafted reduced graphene oxide for efficient electrochemical supercapacitors, ACS Nano 6 (2012) 1715–1723. [21] Z. Zhou, X.-F. Wu, Graphene-beaded carbon nanofibers for use in supercapacitor electrodes: synthesis and electrochemical characterization, Journal of Power Sources 222 (2013) 410–416. [22] Z.Zhou,X.-F.Wu,High-performanceporouselectrodesforpseudosupercapacitors based on graphene-beaded carbon nanofibers surface-coated with nanostructured conducting polymers, Journal of Power Sources 262 (2014) 44–49. [23] N.B. Trung, T.V. Tam, H.R. Kim, S.H. Hur, E.J. Kim, W.M. Choi, Three-dimensional hollow balls of graphene-polyaniline hybrids for supercapacitor applications, Chemical Engineering Journal (2014) . [24] M. Hassan, K.R. Reddy, E. Haque, S.N. Faisal, S. Ghasemi, A.I. Minett, V.G. Gomes, Hierarchical assembly of graphene/polyaniline nanostructures to synthesize free-standing supercapacitor electrode, Composites Science and Technology 98 (2014) 1–8. [25] C. Yan, Y.W. Kanaththage, R. Short, C.T. Gibson, L. Zou, Graphene/Polyaniline nanocomposite as electrode material for membrane capacitive deionization, Desalination 344 (2014) 274–279. [26] F. Yang, M. Xu, S.-J. Bao, H. Wei, H. Chai, Self-assembled hierarchical graphene/ polyaniline hybrid aerogels for electrochemical capacitive energy storage, Electrochimica Acta 137 (2014) 381–387. [27] Z. Gao, F. Wang, J. Chang, D. Wu, X. Wang, X. Wang, F. Xu, S. Gao, K. Jiang, Chemically grafted graphene-polyaniline composite for application in supercapacitor, Electrochimica Acta 133 (2014) 325–334. [28] B. Mu, W. Zhang, A. Wang, Template synthesis of graphene/polyaniline hybrid hollow microspheres as electrode materials for high-performance supercapacitor, Journal of Nanoparticle Research 16 (2014) 1–12. [29] Z. Tong, Y. Yang, J. Wang, J. Zhao, B.-L. Su, Y. Li, Layered polyaniline/graphene film from sandwich-structured polyaniline/graphene/polyaniline nanosheets for high-performance pseudosupercapacitors, Journal of Materials Chemistry A 2 (2014) 4642–4651. [30] Y. Liu, X.Y. Su, S.Y. Guang, H.Y. Xu, Polyaniline Grafted Amino-Functionalized Graphene Nanocomposite with Excellent Electrochemical Performance for Supercapacitor Electrode Materials, Materials Science Forum, Trans Tech Publ, 2014, pp. 12–17. [31] J. Xu, K. Wang, S.Z. Zu, B.H. Han, Z. Wei, Hierarchical nanocomposites of polyaniline nanowire arrays on graphene oxide sheets with synergistic effect for energy storage, ACS Nano 4 (2010) 5019–5026. [32] M. Kumar, K. Singh, S.K. Dhawan, K. Tharanikkarasu, J.S. Chung, B.-S. Kong, E.J. Kim, S.H. Hur, Synthesis and characterization of covalently-grafted graphene– polyaniline nanocomposites and its use in a supercapacitor, Chemical Engineering Journal 231 (2013) 397–405. [33] H. Cao, X. Zhou, Y. Zhang, L. Chen, Z. Liu, Microspherical polyaniline/graphene nanocomposites for high performance supercapacitors, Journal of Power Sources (2013) . [34] L. Staudenmaier, Ber. Dtsch. Chem. Ges 31 (1898) 1481. [35] J.C. Meyer, A.K. Geim, M. Katsnelson, K. Novoselov, T. Booth, S. Roth, The structure of suspended graphene sheets, Nature 446 (2007) 60–63. [36] D. Li, M.B. Muller, S. Gilje, R.B. Kaner, G.G. Wallace, Processable aqueous dispersions of graphene nanosheets, Nature Nanotechnology 3 (2008) 101–105. [37] N.-R. Chiou, C. Lu, J. Guan, L.J. Lee, A.J. Epstein, Growth and alignment of polyaniline nanofibres with superhydrophobic, superhydrophilic and other properties, Nature Nanotechnology 2 (2007) 354–357. [38] N.R. Chiou, A.J. Epstein, Polyaniline nanofibers prepared by dilute polymerization, Advanced Materials 17 (2005) 1679–1683.
H. Liu et al. / Electrochimica Acta 146 (2014) 511–517 [39] X. Yan, Z. Han, Y. Yang, B. Tay, NO2 gas sensing with polyaniline nanofibers synthesized by a facile aqueous/organic interfacial polymerization, Sensors and Actuators B: Chemical 123 (2007) 107–113. [40] R. Cruz-Silva, J. Romero-García, J. Angulo-Sanchez, E. Flores-Loyola, M. Farías, F. Castillon, J. Díaz, Comparative study of polyaniline cast films prepared from enzymatically and chemically synthesized polyaniline, Polymer 45 (2004) 4711–4717. [41] C. Wang, Z. Wang, M. Li, H. Li, Well-aligned polyaniline nano-fibril array membrane and its field emission property, Chemical Physics Letters 341 (2001) 431–434. [42] H. Chaudhari, D. Kelkar, X-ray diffraction study of doped polyaniline, Journal of Applied Polymer Science 62 (1996) 15–18. [43] J. Pouget, M. Jozefowicz, X. A. e. a. Epstein, Tang, A. MacDiarmid, X-ray structure of polyaniline, Macromolecules 24 (1991) 779–789. [44] P.-L. Taberna, S. Mitra, P. Poizot, P. Simon, J.-M. Tarascon, High rate capabilities Fe3O4-based Cu nano-architectured electrodes for lithium-ion battery applications, Nature Materials 5 (2006) 567–573. [45] B.K. Kuila, B. Nandan, M. Böhme, A. Janke, M. Stamm, Vertically oriented arrays of polyaniline nanorods and their super electrochemical properties, Chemical Communications (2009) 5749–5751.
517
[46] K. Zhang, L. Mao, L.L. Zhang, H.S. On Chan, X.S. Zhao, J. Wu, Surfactantintercalated, chemically reduced graphene oxide for high performance supercapacitor electrodes, Journal of Materials Chemistry 21 (2011) 7302– 7307. [47] L. Mao, K. Zhang, H.S.O. Chan, J. Wu, Surfactant-stabilized graphene/polyaniline nanofiber composites for high performance supercapacitor electrode, Journal of Materials Chemistry 22 (2012) 80–85. [48] K. Y.-g. Wang, Y.-y. Xia, Hybrid aqueous energy storage cells using activated carbon and lithium-lntercalated compounds I. The C/Li Mn2O4 system, Journal of The Electrochemical Society 153 (2006) A450–A454. [49] Zhang, L.L. Zhang, X.S. Zhao, J. Wu, Graphene/polyaniline nanofiber composites as supercapacitor electrodes, Chemistry of Materials 22 (2010) 1392–1401. [50] Q. Wu, Y. Xu, Z. Yao, A. Liu, G. Shi, Supercapacitors based on flexible graphene/ polyaniline nanofiber composite films, ACS Nano 4 (2010) 1963–1970. [51] J. Yan, T. Wei, B. Shao, Z. Fan, W. Qian, M. Zhang, F. Wei, Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance, Carbon 48 (2010) 487–493. [52] S. Park, R.S. Ruoff, Chemical methods for the production of graphenes, Nature Nanotechnology 4 (2009) 217–224.