CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400147
Large-Scale Synthesis of Reduced Graphene Oxides with Uniformly Coated Polyaniline for Supercapacitor Applications Rahul R. Salunkhe,[a] Shao-Hui Hsu,[b] Kevin C. W. Wu,[b] and Yusuke Yamauchi*[a] We report an effective route for the preparation of layered reduced graphene oxide (rGO) with uniformly coated polyaniline (PANI) layers. These nanocomposites are synthesized by chemical oxidative polymerization of aniline monomer in the presence of layered rGO. SEM, TEM, X-ray photoelectron spectroscopy (XPS), FTIR, and Raman spectroscopy analysis results
demonstrated that reduced graphene oxide–polyaniline (rGO– PANI) nanocomposites are successfully synthesized. Because of synergistic effects, rGO–PANI nanocomposites prepared by this approach exhibit excellent capacitive performance with a high specific capacitance of 286 F g1 and high cycle reversibility of 94 % after 2000 cycles.
Introduction Conjugated polymers (CPs), which are well-known intrinsically conductive polymers, are widely used in various applications due to their conductivity and optical and electronic properties.[1] Examples of conventional CPs are polyaniline (PANI),[2] polypyrrole (PPy),[3] polythiophenes (PTs),[4] and their derivatives, which have been extensively studied due to their intrinsic conductivities. Among the above mentioned polymers, PANI is one of the most promising CPs for commercial applications due to its facile synthesis, low cost, and environmental stability.[5] In emeraldine base form, PANI becomes electrically conductive when doped with an acid. The doping level and the electrical conductivity of PANI can be controlled by the pH of the dopant acid solution. Presently, polyaniline (PANI) is successfully used as electrode materials for electrochemical energy storage, because of its high conductivity and fast redox electroactivity. However, continuous swelling/shrinkage of the interlaced polymer chains during charge/discharge processes usually causes inadequate drainage of ionic carriers, leading to poor cycling stability.[6] To overcome this shortcoming, the construction of PANI-based composites with ordered carrier passages was proposed as a potential strategy.
[a] R. R. Salunkhe, Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) E-mail: [email protected] Homepage: http://www.yamauchi-labo.com [b] S.-H. Hsu, K. C. W. Wu Department of Chemical Engineering National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei 10617 (Taiwan) Part of a Special Issue for the 6th Asia-Pacific Catalysis Congress (APCAT6). A link to the full Table of Contents will appear here. Supporting Information for this article is available on the WWW under http://dx.doi.org/ 10.1002/cssc.201400147.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
On the other hand, graphene has become a main research focus for developing high performance supercapacitors.[7] Graphene is an atomically thick, two-dimensional (2D) sheet consisting of sp2-carbon atoms in the honeycomb network of sixmember rings.[8] This material can be considered as the basic unit for building all graphitic materials in various forms such as zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs), and the stacking of graphene sheets into three-dimensional (3D) graphite. In addition, the graphene has shown high electrical conductivity, mechanical strength, and optical absorption properties that are obviously different from those of activated carbons (ACs), CNTs, and fullerenes, leading to strong research interests in physics and chemistry.[9] Other advantages of graphene include its exceptionally high theoretical specific surface area of over 2600 m2 g1, sufficient porosity, superior conductivity, a broad potential window, and rich surface chemistry.[10] Therefore, graphene-based materials have been developed for high-power and high-energy supercapacitors in order to circumvent the issue of limited power capability due to electrode kinetics such as ion transport within the long micropores of AC. In spite of such ideal capacitor properties, however, graphene-based supercapacitors possess low capacitances in the range of 100–200 F g1, in various aqueous and non-aqueous electrolytes.[11] Uniformly coated or coaxial structures synthesized by various deposition methods have been shown to be promising electrode materials for supercapacitor applications.[12] These nanocomposites have multiple functionalities by combining physical properties of different materials. In particular, nanocomposites consisting of both faradaic materials such as PANI and carbon-based materials that function as an electrical double layer capacitor (EDLC) such as reduced graphene oxide (rGO), carbon nanotube, activated carbon (AC), and nanoporous carbon are of technological importance. These materials have the advantages of a dual storage mechanism (pseudocapacitor and EDLC) and fast charge-transfer. There have been ChemSusChem 0000, 00, 1 – 7
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CHEMSUSCHEM FULL PAPERS a few reports on graphene-based nanocomposites. Some of them have been obtained by utilizing electropolymerization,[13a] vacuum filtration,[13b] oxidative polymerization,[13e] dilute polymerization,[13c] electrochemical synthesis,[13d] and so on. Currently, considerable efforts are being devoted to developing inexpensive and facile synthesis methods for obtaining graphene-based nanocomposites with high specific capacitance. In this paper, we have adopted a one-step, simple, and cost effective oxidative polymerization method for the development of rGO–PANI nanocomposites. The oxidative polymerization of aniline was carried out in the presence of rGO at optimized concentrations of aniline and rGO, resulting in a uniform coating of PANI onto the rGO surface. This method resolves the problem of the stacking of rGO sheets. Therefore, the coating of PANI is expected to maximize the unique properties of rGO as a supercapacitor electrode material, as shown in the schematic (Figure 1). This nanocomposite combines the fast redox property of PANI, with the mechanical strength and conductivity of rGO, thereby realizing an excellent gravimetric capacitance of 286 F g1 and a negligible loss of 6 % after 2000 cycle.
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Figure 2. SEM image of a) rGO, b) randomly connected PANI nanowires prepared without rGO, and c, d) rGO–PANI nanocomposites.
Results and Discussion The morphological observation of the film was carried out using scanning electron microscope (SEM). Figure 2 a shows the morphology of rGO, where we can see the four different flakes that are not attached to each other. Figure 2 b shows an SEM image of the morphology of bare PANI prepared without rGO, in which interconnected nanowires were mostly observed. The morphologies of rGO–PANI nanocomposites at two different areas are shown in Figure 2 c and d. Here, we can clearly observe that PANI is uniformly coated in the inner and outer layers of rGO. The coating does not contain the PANI nanowire morphology which is observed in Figure 2 b. From these results, it can be predicted that the rGO surface provides abundant nucleation sites that are favorable for continuous PANI layer growth. Another reason for successful coating might be that the optimized concentration of polyaniline (0.05 m) allows a uniform coating to the rGO surface without further growth of PANI nanowires. We can also observe that there is effective spacing between these layers, which is very useful for electrochemical reactions by providing reaction sites (Figure 2 c and d), as ions can reach deep inside the electrode surface. A TEM image of the rGO–PANI nanocomposites is shown in Figure 3. Thin PANI layers are coated over the entire
Figure 3. TEM image of rGO–PANI nanocomposites, which confirms the uniform coating of PANI on rGO surface. The inset selected area ED pattern clearly reveals the presence of two or more rGO flakes.
rGO surface. The inset selected-area electron diffraction (ED) patterns show several intense dots at several places, clearly confirming the existence of two or more rGO–PANI flakes. The typical FTIR spectra of rGO, PANI nanowires, and rGO– PANI nanocomposites were measured. The characteristic peaks of PANI were observed at 1552 and 1475 cm1 (C=C stretching deformation of the quinonoid and benzenoid ring in the emeraldine salt, respectively), 1290 cm1 (CN stretching of the secondary aromatic amine, -N-benzenoid-N-), 1116 cm1 (aromatic CH in-plane bending), and 791 cm1 (CH out of the plane of
Figure 1. Schematic depiction of growth of uniformly coated PANI on rGO flakes. The advantage of this system is the dual storage mechanism by the synergy of two materials. The space between rGO flakes provides deep electrolyte penetration and the rGO flakes offer fast charge transfer channels.
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CHEMSUSCHEM FULL PAPERS the 1, 4-substituted aromatic ring stretching).[13d, 14] The peaks between 800 and 500 cm1 were assigned to the vibration of CH bands in the benzene rings. For rGO, the OH, C=O in COOH, intercalated water, and CO in COH/COC (epoxy) functional groups are identified from the peaks located at 3409, 1729, and 1399–1064 cm1, respectively.[13d, 15] Compared to the spectrum of rGO, the spectrum of rGO–PANI showed several new peaks attributed to PANI.[16] Compositional analysis of rGO–PANI nanocomposites was performed by means of XPS (X-ray photoelectron spectroscopy, Figure 4 a). The XPS analysis confirms the presence of C, N, O, and S elementals in the obtained product. The high resolution XPS spectrum of C 1s core-level can be decomposed into six peaks of 284.2 (C1, C=C), 284.4 (C2, CC), 285.6 (C3, CN), 286.6 (C4, CO), 288.1 (C5, C=O), and 291.2 (C6, OC=O), as shown in Figure 4 b.[17] The N 1s region exists in three different electronic states; the benzenoid amine with binding energy (BE) centered at 399.7 eV (N2), the quinoid amine with BE at 398.8 eV (N1), and the nitrogen cationic radical (NC + ) with BE at 401.4 eV (N3),[18] as shown in Figure 4 c. Moreover, Raman scattering was measured to give a more complete picture of the chemical bonding structure. As shown in Figure 5, significant structural changes occurred during the chemical processing from rGO, and the formation of rGO–PANI was reflected in the Raman spectra. As expected, the spectrum of the as-prepared rGO displays two prominent peaks at 1350 and 1590 cm1, which correspond to the well-documented D mode or the phonon mode corresponding to the conversion of a sp2-hybridized carbon to a sp3-hybridized carbon and the G mode related to the vibration of a sp2-hybridized carbon. Apart from the D and G bands of rGO, one new peak at 1440 cm1 appears, which corresponds to the PANI structure.[13a, 19] For confirmation of the merits of rGO–PANI nanocomposites, the electrochemical properties of PANI, rGO, and rGO–PANI were examined by means of cyclic voltammetry (CV) and cycle stability studies. The CV curves for PANI and rGO–PANI show two pairs of redox peaks which are attributed to redox transitions of PANI (i.e., leucoemeraldine–emeraldine transition and the emeraldine–pernigraniline transition), while the CV curve for rGO has no clear peaks (Figure 6 a). Thus, the shape of the CV curve of rGO–PANI nanocomposites is mainly attributed to faradaic reactions of PANI at the electrode/electrolyte interface, which is different from the CV curve shape of electric double layer capacitance of carbon materials. Figure 6 b shows CV curves of rGO–PANI nanocomposites at various scan rates. Symmetry during the oxidation and reduction cycles is confirmed. The specific capacitance values obtained are 286, 90, and 78 F g1 for rGO–PANI, rGO, and PANI electrodes, respectively. The specific capacitance values of rGO, PANI, and rGO– PANI at various scan rates are summarized in Table 1. The specific capacitances of all three samples decrease on increasing the applied scan rate. Compared to disordered PANI nanowires prepared without rGO, PANI coated on rGO surface in nanocomposites have optimized ionic transport pathways between rGO–PANI flakes, leading to improved capacitance. Comparison of specific capacitance values obtained in the present study 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. a) XPS survey and b, c) 1high resolution XPS spectra of rGO–PANI nanocomposites for b) C 1s and c) N 1s XPS spectra.
Table 1. Specific capacitance values of rGO, PANI, and rGO–PANI nanocomposites obtained at various scan rates. Scan rate [mV s1] rGO 5 20 40 60 80 100
90 64 56 51 47 44
Specific capacitance [F g1] PANI rGO–PANI 78 60 54 51 48 46
286 194 155 132 117 105
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Figure 5. Raman spectra of rGO and rGO–PANI nanocomposites.
with previous reports on graphene-based nanocomposites is given in Table S1. It has generally known that conducting polymers often suffer from limited long-term stability during cycling,[6] because the swelling and shrinking of polymers lead to degradation in capacitance, which restricts these low-cost supercapacitor electrode materials from the viewpoint of commercial applications. However, our rGO–PANI nanocomposites can overcome this problem. The electrochemical stability of rGO–PANI was examined in 1 m H2SO4 aqueous electrolyte by consecutive cyclic voltammetry cycles at a scan speed of 50 mV s1. Interestingly, rGO–PANI nanocomposites are highly stable with 94 % retention (a negligible loss of 6 %) even after 2000 cycles (Figure S1). The higher stability of rGO–PANI comes from the synergistic cooperation of rGO and PANI. The effective support of rGO can undergo some mechanical deformation in the redox process of PANI, thus avoiding destruction of electrode material and giving better stability. For further practical demonstration of our rGO–PANI nanocomposites, we constructed a full cell supercapacitor using 1 m
www.chemsuschem.org H2SO4 as electrolyte. Figure 7 a shows CV curves of symmetric supercapacitor in various potential windows at a sweep rate of 60 mV s1. The device was cycled with upper cell voltage varied from 1.0 to 1.8 V. The CV curves show typical a symmetric shape in the potential window of 0.0 to 1.6 V. However, for further increase in the potential window of 0.0 to 1.8 V, there is a slight hump in current which is due to irreversible chemical reactions. Thus, the optimum potential window for operation of the symmetric supercapacitor is 0.0 to 1.6 V. Figure 7 b shows CV curves of the symmetric supercapacitor in a fixed potential window of 0.0 to 1.6 V at various scan rates. To further evaluate the capacitance of the symmetric capacitor, galvanostatic charge–discharge studies were also carried out at current densities varied from 2.0 to 6.0 A g1. Figure S2 shows a charge–discharge curve of the symmetric supercapacitor at current density of 5.0 A g1. The capacitance value calculated from charge–discharge studies shows capacitance value of 53 F g1 (at a current density of 2.0 A g1) and decreases to 18 F g1 (at a current density of 6.0 A g1). Moreover, a specific energy of 19.02 W h kg1 at a power density of 1599 W kg1 is achieved for a symmetric supercapacitor using rGO–PANI nanocomposites (at current density of 2.0 A g1). The above results demonstrate that our rGO–PANI nanocomposites are useful as promising electrodes for future supercapacitor applications.
Conclusions Uniform coating of PANI on rGO has been achieved through a simple oxidative polymerization. The uniform coating is controlled by adjusting the ratio of aniline concentration and rGO quantity. The uniform coating of PANI is advantageous because this structure allows electrolyte ions to reach deep inside the electrode material, which results in better utilization of the electrode surface for electrochemical reactions. The electrochemical studies showed a synergistic effect in the rGO–PANI nanocomposites. The capacitance of our rGO–PANI nanocomposites was more than twice that of industrial rGO and PANI.
Figure 6. a) Comparative CVs of PANI, rGO, and rGO-PANI nanocomposites at a scan rate of 60 mV s1, and b) CVs of rGO-PANI nanocomposites at a scan rate of 5, 20, 40, 60, 80, and 100 mV s1.
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Figure 7. a) CVs of symmetric supercapacitor based on rGO–PANI nanocomposites. The device is cycled varying the upper cell voltage from 1.0 to 1.8 V at a scan rate of 60 mV s1, and b) CVs of the symmetric supercapacitor using a fixed potential window of 0.0–1.6 V at various scan rates ranging from 20 to 100 mV s1.
Moreover, these nanocomposites exhibited very high cycle stability with negligible losses. The method developed in this work can open up a general route to prepare a wide range of rGO-based nanocomposite materials with a broad range of potential applications.
Experimental Section Synthesis of reduced graphene oxide (rGO): Nanographite platelets (N008-100-N) of 100 nm thickness were used as raw material to prepare graphene oxide (GO). The preparation of GO solutions followed the modified Hummers’ method. The procedures are described briefly as follows. Sodium nitrate (0.30 g) was dissolved in sulfuric acid (10 mL). N008-100-N carbon source was added to this solution, which was stirred for 30 min. KMNO4 salt (0.30 g) was added to the solution, which was stirred again for 1 h. Then, H2O2 (10 mL) was added to the solution with stirring. Finally, the solution was centrifuged 3 times at different time intervals such as 10, 30, and 45 min. Then, the material was extracted by adding water, mixing with methanol, and keeping for further processing. The reduced graphene oxide (rGO) was prepared by reduction of graphene oxide with hydrazine hydrate. GO was dissolved in 100 mL of water and 0.12 mL of hydrazine hydrate was added to the above solution. The mixture was reacted at 90 8C for 1 h under constant stirring. rGO was obtained by filtering and drying the product in vacuum. Synthesis of polyaniline coated reduced graphene oxide (rGO– PANI): Polyaniline (PANI) was coated onto the rGO surface by oxidative polymerization. The rGO quantity was varied between 9, 10, 11, and 12 g, but it was found that 9 g of rGO polyaniline resulted in a uniform coating without any PANI aggregation. First, rGO (9 g, optimized quantity) was added into HClO4 (1 m, 15 mL) solution. This mixture was carefully ultrasonicated. After that, ethanol (5 mL) was added to the above solution. Aniline monomer (0.05 m) was added to the above solution and stirred for 30 min at 10 8C. In another beaker, oxidant NH4S2O8 (APS) was dissolved in 5 mL aqueous HClO4 solution (molar ratio of aniline/APS = 1:5) and cooled at 10 8C. These two solutions were mixed and stirred at 10 8C for 24 h. Finally, the precipitate of greenish black color was obtained. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Preparation of electrodes: The graphite substrates were first polished using a fine polisher in water flow. They were then rinsed with deionized water, etched in a HCl (0.1 m) solution at room temperature for 10 min, and finally rinsed with deionized water in an ultrasonic bath for 30 min. The masses of the electrodes were measured using an ultra-microbalance (METTLER TOLEDO). Each electrode contained 0.5 mg cm2 of electroactive material. The rGO– PANI composite was mixed with poly (vinylidine difluoride) (PVDF, 20 %) in N-methylpyrolidinone (NMP) solvent. The resulting slurry was homogenized by ultrasonication and coated onto a graphite substrate, which was followed by drying at 80 8C for 2 h in a vacuum oven. Electrochemical analysis: The electrochemical measurements were conducted in a three-electrode electrochemical cell with a Pt counter electrode and Ag/AgCl as the reference electrode in a 1 m H2SO4 solution. The graphite substrate coated with rGO–PANI was used as the working electrode. Cyclic voltammetry measurements were obtained using an electrochemical workstation (CHI 660E CH Instruments, USA) in a scan range of 0.1 to 0.9 V. For every experiment, the typical area under consideration was 1 1 cm2. The specific capacitance value was calculated from cyclic voltammetry using Equation (1), C¼
1 msðVf Vi Þ
Z
Vf
ð1Þ
IðVÞdv Vi
where m is the mass of active electrode material, s is the potential scan rate, Vf and Vi are the integration limits of the voltammetric curve V, and I(V) is the response current density. For the symmetric supercapacitor applications, two electrodes of same charge capacity were employed as positive and negative electrode. The specific capacitance (C, F g1), specific energy (SE, W h Kg1) and specific power (SP, kW Kg1) calculated from chronopotentiometric curves using following Equations (2)–(4), C¼
I Dt m DV 1
SE ¼ 2
ð2Þ
CV 2 3:6
ð3Þ
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3600 SE t
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where I is charge/discharge current at a discharge time t(s), DV is the potential window, and m is the mass of electrode.
Keywords: nanoporous material · polyaniline · reduced graphene oxide · supercapacitors · synergic effect [1] a) C. Li, H. Bai, G. Q. Shi, Chem. Soc. Rev. 2009, 38, 2397 – 2409; b) H. D. Tran, D. Li, R. B. Kaner, Adv. Mater. 2009, 21, 1487 – 1499; c) D. Li, J. X. Huang, R. B. Kaner, Acc. Chem. Res. 2009, 42, 135 – 145; d) M. X. Wan, Adv. Mater. 2008, 20, 2926 – 2932. [2] a) D. S. Dhawale, R. R. Salunkhe, V. S. Jamadade, T. P. Gujar, C. D. Lokhande, Appl. Surf. Sci. 2009, 255, 8213 – 8216; b) C. C. Hu, C. H. Chu, Mater. Chem. Phys. 2000, 65, 329 – 338; c) V. Gupta, N. Miura, Electrochem. Solid-State Lett. 2005, 8, A630 – A632; d) F. Fusalba, P. Gouerec, D. Belanger, J. Electrochem. Soc. 2001, 148, A1. [3] a) E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Beguin, J. Power Sources 2006, 153, 413 – 418; b) K. Jurewicz, S. Delpeux, V. Bertaggna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 2001, 347, 36 – 40. [4] a) A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, P. J. Ferraris, J. Power Sources 1994, 47, 89 – 107; b) A. Laforgue, P. Simon, C. Sarrazin, J. F. Fauvarque, J. Power Sources 1999, 80, 142 – 148. [5] a) E. T. Kang, K. G. Neoh, K. L. Tan, Prog. Polym. Sci. 1998, 23, 277 – 324; b) K. S. Ryu, K. M. Kim, N. G. Park, Y. J. Park, S. H. Chang, J. Power Sources 2002, 103, 305 – 309. [6] K. Zhang, L. L. Zhang, X. S. Zhao, J. Wu, Chem. Mater. 2010, 22, 1392 – 1401. [7] a) B. Luo, S. Liu, L. Zhi, Small 2012, 8, 630 – 646; b) Y. Huang, J. Liang, Y. Chen, Small 2012, 8, 1805 – 1834. [8] a) C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. 2009, 48, 7752 – 7777; Angew. Chem. 2009, 121, 7890 – 7916; b) A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191.
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Received: January 29, 2014 Revised: March 19, 2014 Published online on && &&, 0000
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FULL PAPERS R. R. Salunkhe, S.-H. Hsu, K. C. W. Wu, Y. Yamauchi* && – &&
Not just a flash in the PANI: An effective route for the preparation of layered reduced graphene oxide (rGO) with uniformly coated polyaniline (PANI) layers is reported. The nanocomposites are synthesized by chemical oxidative poly-
merization of aniline monomer in the presence of layered rGO. Through synergistic effects, the rGO–PANI nanocomposites exhibit excellent capacitive performance with a high specific capacitance and high cycle reversibility.
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Large-Scale Synthesis of Reduced Graphene Oxides with Uniformly Coated Polyaniline for Supercapacitor Applications
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Large-Scale Synthesis of Reduced Graphene Oxides with Uniformly Coated Polyaniline for Supercapacitor Applications Rahul R. Salunkhe,[a] Shao-Hui Hsu,[b] Kevin C. W. Wu,[b] and Yusuke Yamauchi*[a] We report an effective route for the preparation of layered reduced graphene oxide (rGO) with uniformly coated polyaniline (PANI) layers. These nanocomposites are synthesized by chemical oxidative polymerization of aniline monomer in the presence of layered rGO. SEM, TEM, X-ray photoelectron spectroscopy (XPS), FTIR, and Raman spectroscopy analysis results
demonstrated that reduced graphene oxide–polyaniline (rGO– PANI) nanocomposites are successfully synthesized. Because of synergistic effects, rGO–PANI nanocomposites prepared by this approach exhibit excellent capacitive performance with a high specific capacitance of 286 F g1 and high cycle reversibility of 94 % after 2000 cycles.
Introduction Conjugated polymers (CPs), which are well-known intrinsically conductive polymers, are widely used in various applications due to their conductivity and optical and electronic properties.[1] Examples of conventional CPs are polyaniline (PANI),[2] polypyrrole (PPy),[3] polythiophenes (PTs),[4] and their derivatives, which have been extensively studied due to their intrinsic conductivities. Among the above mentioned polymers, PANI is one of the most promising CPs for commercial applications due to its facile synthesis, low cost, and environmental stability.[5] In emeraldine base form, PANI becomes electrically conductive when doped with an acid. The doping level and the electrical conductivity of PANI can be controlled by the pH of the dopant acid solution. Presently, polyaniline (PANI) is successfully used as electrode materials for electrochemical energy storage, because of its high conductivity and fast redox electroactivity. However, continuous swelling/shrinkage of the interlaced polymer chains during charge/discharge processes usually causes inadequate drainage of ionic carriers, leading to poor cycling stability.[6] To overcome this shortcoming, the construction of PANI-based composites with ordered carrier passages was proposed as a potential strategy.
[a] R. R. Salunkhe, Y. Yamauchi World Premier International (WPI) Research Center for Materials Nanoarchitectonics (MANA) National Institute for Materials Science (NIMS) 1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan) E-mail: [email protected] Homepage: http://www.yamauchi-labo.com [b] S.-H. Hsu, K. C. W. Wu Department of Chemical Engineering National Taiwan University No. 1, Sec. 4, Roosevelt Road, Taipei 10617 (Taiwan) Part of a Special Issue for the 6th Asia-Pacific Catalysis Congress (APCAT6). A link to the full Table of Contents will appear here. Supporting Information for this article is available on the WWW under http://dx.doi.org/ 10.1002/cssc.201400147.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
On the other hand, graphene has become a main research focus for developing high performance supercapacitors.[7] Graphene is an atomically thick, two-dimensional (2D) sheet consisting of sp2-carbon atoms in the honeycomb network of sixmember rings.[8] This material can be considered as the basic unit for building all graphitic materials in various forms such as zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs), and the stacking of graphene sheets into three-dimensional (3D) graphite. In addition, the graphene has shown high electrical conductivity, mechanical strength, and optical absorption properties that are obviously different from those of activated carbons (ACs), CNTs, and fullerenes, leading to strong research interests in physics and chemistry.[9] Other advantages of graphene include its exceptionally high theoretical specific surface area of over 2600 m2 g1, sufficient porosity, superior conductivity, a broad potential window, and rich surface chemistry.[10] Therefore, graphene-based materials have been developed for high-power and high-energy supercapacitors in order to circumvent the issue of limited power capability due to electrode kinetics such as ion transport within the long micropores of AC. In spite of such ideal capacitor properties, however, graphene-based supercapacitors possess low capacitances in the range of 100–200 F g1, in various aqueous and non-aqueous electrolytes.[11] Uniformly coated or coaxial structures synthesized by various deposition methods have been shown to be promising electrode materials for supercapacitor applications.[12] These nanocomposites have multiple functionalities by combining physical properties of different materials. In particular, nanocomposites consisting of both faradaic materials such as PANI and carbon-based materials that function as an electrical double layer capacitor (EDLC) such as reduced graphene oxide (rGO), carbon nanotube, activated carbon (AC), and nanoporous carbon are of technological importance. These materials have the advantages of a dual storage mechanism (pseudocapacitor and EDLC) and fast charge-transfer. There have been ChemSusChem 0000, 00, 1 – 7
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CHEMSUSCHEM FULL PAPERS a few reports on graphene-based nanocomposites. Some of them have been obtained by utilizing electropolymerization,[13a] vacuum filtration,[13b] oxidative polymerization,[13e] dilute polymerization,[13c] electrochemical synthesis,[13d] and so on. Currently, considerable efforts are being devoted to developing inexpensive and facile synthesis methods for obtaining graphene-based nanocomposites with high specific capacitance. In this paper, we have adopted a one-step, simple, and cost effective oxidative polymerization method for the development of rGO–PANI nanocomposites. The oxidative polymerization of aniline was carried out in the presence of rGO at optimized concentrations of aniline and rGO, resulting in a uniform coating of PANI onto the rGO surface. This method resolves the problem of the stacking of rGO sheets. Therefore, the coating of PANI is expected to maximize the unique properties of rGO as a supercapacitor electrode material, as shown in the schematic (Figure 1). This nanocomposite combines the fast redox property of PANI, with the mechanical strength and conductivity of rGO, thereby realizing an excellent gravimetric capacitance of 286 F g1 and a negligible loss of 6 % after 2000 cycle.
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Figure 2. SEM image of a) rGO, b) randomly connected PANI nanowires prepared without rGO, and c, d) rGO–PANI nanocomposites.
Results and Discussion The morphological observation of the film was carried out using scanning electron microscope (SEM). Figure 2 a shows the morphology of rGO, where we can see the four different flakes that are not attached to each other. Figure 2 b shows an SEM image of the morphology of bare PANI prepared without rGO, in which interconnected nanowires were mostly observed. The morphologies of rGO–PANI nanocomposites at two different areas are shown in Figure 2 c and d. Here, we can clearly observe that PANI is uniformly coated in the inner and outer layers of rGO. The coating does not contain the PANI nanowire morphology which is observed in Figure 2 b. From these results, it can be predicted that the rGO surface provides abundant nucleation sites that are favorable for continuous PANI layer growth. Another reason for successful coating might be that the optimized concentration of polyaniline (0.05 m) allows a uniform coating to the rGO surface without further growth of PANI nanowires. We can also observe that there is effective spacing between these layers, which is very useful for electrochemical reactions by providing reaction sites (Figure 2 c and d), as ions can reach deep inside the electrode surface. A TEM image of the rGO–PANI nanocomposites is shown in Figure 3. Thin PANI layers are coated over the entire
Figure 3. TEM image of rGO–PANI nanocomposites, which confirms the uniform coating of PANI on rGO surface. The inset selected area ED pattern clearly reveals the presence of two or more rGO flakes.
rGO surface. The inset selected-area electron diffraction (ED) patterns show several intense dots at several places, clearly confirming the existence of two or more rGO–PANI flakes. The typical FTIR spectra of rGO, PANI nanowires, and rGO– PANI nanocomposites were measured. The characteristic peaks of PANI were observed at 1552 and 1475 cm1 (C=C stretching deformation of the quinonoid and benzenoid ring in the emeraldine salt, respectively), 1290 cm1 (CN stretching of the secondary aromatic amine, -N-benzenoid-N-), 1116 cm1 (aromatic CH in-plane bending), and 791 cm1 (CH out of the plane of
Figure 1. Schematic depiction of growth of uniformly coated PANI on rGO flakes. The advantage of this system is the dual storage mechanism by the synergy of two materials. The space between rGO flakes provides deep electrolyte penetration and the rGO flakes offer fast charge transfer channels.
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CHEMSUSCHEM FULL PAPERS the 1, 4-substituted aromatic ring stretching).[13d, 14] The peaks between 800 and 500 cm1 were assigned to the vibration of CH bands in the benzene rings. For rGO, the OH, C=O in COOH, intercalated water, and CO in COH/COC (epoxy) functional groups are identified from the peaks located at 3409, 1729, and 1399–1064 cm1, respectively.[13d, 15] Compared to the spectrum of rGO, the spectrum of rGO–PANI showed several new peaks attributed to PANI.[16] Compositional analysis of rGO–PANI nanocomposites was performed by means of XPS (X-ray photoelectron spectroscopy, Figure 4 a). The XPS analysis confirms the presence of C, N, O, and S elementals in the obtained product. The high resolution XPS spectrum of C 1s core-level can be decomposed into six peaks of 284.2 (C1, C=C), 284.4 (C2, CC), 285.6 (C3, CN), 286.6 (C4, CO), 288.1 (C5, C=O), and 291.2 (C6, OC=O), as shown in Figure 4 b.[17] The N 1s region exists in three different electronic states; the benzenoid amine with binding energy (BE) centered at 399.7 eV (N2), the quinoid amine with BE at 398.8 eV (N1), and the nitrogen cationic radical (NC + ) with BE at 401.4 eV (N3),[18] as shown in Figure 4 c. Moreover, Raman scattering was measured to give a more complete picture of the chemical bonding structure. As shown in Figure 5, significant structural changes occurred during the chemical processing from rGO, and the formation of rGO–PANI was reflected in the Raman spectra. As expected, the spectrum of the as-prepared rGO displays two prominent peaks at 1350 and 1590 cm1, which correspond to the well-documented D mode or the phonon mode corresponding to the conversion of a sp2-hybridized carbon to a sp3-hybridized carbon and the G mode related to the vibration of a sp2-hybridized carbon. Apart from the D and G bands of rGO, one new peak at 1440 cm1 appears, which corresponds to the PANI structure.[13a, 19] For confirmation of the merits of rGO–PANI nanocomposites, the electrochemical properties of PANI, rGO, and rGO–PANI were examined by means of cyclic voltammetry (CV) and cycle stability studies. The CV curves for PANI and rGO–PANI show two pairs of redox peaks which are attributed to redox transitions of PANI (i.e., leucoemeraldine–emeraldine transition and the emeraldine–pernigraniline transition), while the CV curve for rGO has no clear peaks (Figure 6 a). Thus, the shape of the CV curve of rGO–PANI nanocomposites is mainly attributed to faradaic reactions of PANI at the electrode/electrolyte interface, which is different from the CV curve shape of electric double layer capacitance of carbon materials. Figure 6 b shows CV curves of rGO–PANI nanocomposites at various scan rates. Symmetry during the oxidation and reduction cycles is confirmed. The specific capacitance values obtained are 286, 90, and 78 F g1 for rGO–PANI, rGO, and PANI electrodes, respectively. The specific capacitance values of rGO, PANI, and rGO– PANI at various scan rates are summarized in Table 1. The specific capacitances of all three samples decrease on increasing the applied scan rate. Compared to disordered PANI nanowires prepared without rGO, PANI coated on rGO surface in nanocomposites have optimized ionic transport pathways between rGO–PANI flakes, leading to improved capacitance. Comparison of specific capacitance values obtained in the present study 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. a) XPS survey and b, c) 1high resolution XPS spectra of rGO–PANI nanocomposites for b) C 1s and c) N 1s XPS spectra.
Table 1. Specific capacitance values of rGO, PANI, and rGO–PANI nanocomposites obtained at various scan rates. Scan rate [mV s1] rGO 5 20 40 60 80 100
90 64 56 51 47 44
Specific capacitance [F g1] PANI rGO–PANI 78 60 54 51 48 46
286 194 155 132 117 105
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Figure 5. Raman spectra of rGO and rGO–PANI nanocomposites.
with previous reports on graphene-based nanocomposites is given in Table S1. It has generally known that conducting polymers often suffer from limited long-term stability during cycling,[6] because the swelling and shrinking of polymers lead to degradation in capacitance, which restricts these low-cost supercapacitor electrode materials from the viewpoint of commercial applications. However, our rGO–PANI nanocomposites can overcome this problem. The electrochemical stability of rGO–PANI was examined in 1 m H2SO4 aqueous electrolyte by consecutive cyclic voltammetry cycles at a scan speed of 50 mV s1. Interestingly, rGO–PANI nanocomposites are highly stable with 94 % retention (a negligible loss of 6 %) even after 2000 cycles (Figure S1). The higher stability of rGO–PANI comes from the synergistic cooperation of rGO and PANI. The effective support of rGO can undergo some mechanical deformation in the redox process of PANI, thus avoiding destruction of electrode material and giving better stability. For further practical demonstration of our rGO–PANI nanocomposites, we constructed a full cell supercapacitor using 1 m
www.chemsuschem.org H2SO4 as electrolyte. Figure 7 a shows CV curves of symmetric supercapacitor in various potential windows at a sweep rate of 60 mV s1. The device was cycled with upper cell voltage varied from 1.0 to 1.8 V. The CV curves show typical a symmetric shape in the potential window of 0.0 to 1.6 V. However, for further increase in the potential window of 0.0 to 1.8 V, there is a slight hump in current which is due to irreversible chemical reactions. Thus, the optimum potential window for operation of the symmetric supercapacitor is 0.0 to 1.6 V. Figure 7 b shows CV curves of the symmetric supercapacitor in a fixed potential window of 0.0 to 1.6 V at various scan rates. To further evaluate the capacitance of the symmetric capacitor, galvanostatic charge–discharge studies were also carried out at current densities varied from 2.0 to 6.0 A g1. Figure S2 shows a charge–discharge curve of the symmetric supercapacitor at current density of 5.0 A g1. The capacitance value calculated from charge–discharge studies shows capacitance value of 53 F g1 (at a current density of 2.0 A g1) and decreases to 18 F g1 (at a current density of 6.0 A g1). Moreover, a specific energy of 19.02 W h kg1 at a power density of 1599 W kg1 is achieved for a symmetric supercapacitor using rGO–PANI nanocomposites (at current density of 2.0 A g1). The above results demonstrate that our rGO–PANI nanocomposites are useful as promising electrodes for future supercapacitor applications.
Conclusions Uniform coating of PANI on rGO has been achieved through a simple oxidative polymerization. The uniform coating is controlled by adjusting the ratio of aniline concentration and rGO quantity. The uniform coating of PANI is advantageous because this structure allows electrolyte ions to reach deep inside the electrode material, which results in better utilization of the electrode surface for electrochemical reactions. The electrochemical studies showed a synergistic effect in the rGO–PANI nanocomposites. The capacitance of our rGO–PANI nanocomposites was more than twice that of industrial rGO and PANI.
Figure 6. a) Comparative CVs of PANI, rGO, and rGO-PANI nanocomposites at a scan rate of 60 mV s1, and b) CVs of rGO-PANI nanocomposites at a scan rate of 5, 20, 40, 60, 80, and 100 mV s1.
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Figure 7. a) CVs of symmetric supercapacitor based on rGO–PANI nanocomposites. The device is cycled varying the upper cell voltage from 1.0 to 1.8 V at a scan rate of 60 mV s1, and b) CVs of the symmetric supercapacitor using a fixed potential window of 0.0–1.6 V at various scan rates ranging from 20 to 100 mV s1.
Moreover, these nanocomposites exhibited very high cycle stability with negligible losses. The method developed in this work can open up a general route to prepare a wide range of rGO-based nanocomposite materials with a broad range of potential applications.
Experimental Section Synthesis of reduced graphene oxide (rGO): Nanographite platelets (N008-100-N) of 100 nm thickness were used as raw material to prepare graphene oxide (GO). The preparation of GO solutions followed the modified Hummers’ method. The procedures are described briefly as follows. Sodium nitrate (0.30 g) was dissolved in sulfuric acid (10 mL). N008-100-N carbon source was added to this solution, which was stirred for 30 min. KMNO4 salt (0.30 g) was added to the solution, which was stirred again for 1 h. Then, H2O2 (10 mL) was added to the solution with stirring. Finally, the solution was centrifuged 3 times at different time intervals such as 10, 30, and 45 min. Then, the material was extracted by adding water, mixing with methanol, and keeping for further processing. The reduced graphene oxide (rGO) was prepared by reduction of graphene oxide with hydrazine hydrate. GO was dissolved in 100 mL of water and 0.12 mL of hydrazine hydrate was added to the above solution. The mixture was reacted at 90 8C for 1 h under constant stirring. rGO was obtained by filtering and drying the product in vacuum. Synthesis of polyaniline coated reduced graphene oxide (rGO– PANI): Polyaniline (PANI) was coated onto the rGO surface by oxidative polymerization. The rGO quantity was varied between 9, 10, 11, and 12 g, but it was found that 9 g of rGO polyaniline resulted in a uniform coating without any PANI aggregation. First, rGO (9 g, optimized quantity) was added into HClO4 (1 m, 15 mL) solution. This mixture was carefully ultrasonicated. After that, ethanol (5 mL) was added to the above solution. Aniline monomer (0.05 m) was added to the above solution and stirred for 30 min at 10 8C. In another beaker, oxidant NH4S2O8 (APS) was dissolved in 5 mL aqueous HClO4 solution (molar ratio of aniline/APS = 1:5) and cooled at 10 8C. These two solutions were mixed and stirred at 10 8C for 24 h. Finally, the precipitate of greenish black color was obtained. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Preparation of electrodes: The graphite substrates were first polished using a fine polisher in water flow. They were then rinsed with deionized water, etched in a HCl (0.1 m) solution at room temperature for 10 min, and finally rinsed with deionized water in an ultrasonic bath for 30 min. The masses of the electrodes were measured using an ultra-microbalance (METTLER TOLEDO). Each electrode contained 0.5 mg cm2 of electroactive material. The rGO– PANI composite was mixed with poly (vinylidine difluoride) (PVDF, 20 %) in N-methylpyrolidinone (NMP) solvent. The resulting slurry was homogenized by ultrasonication and coated onto a graphite substrate, which was followed by drying at 80 8C for 2 h in a vacuum oven. Electrochemical analysis: The electrochemical measurements were conducted in a three-electrode electrochemical cell with a Pt counter electrode and Ag/AgCl as the reference electrode in a 1 m H2SO4 solution. The graphite substrate coated with rGO–PANI was used as the working electrode. Cyclic voltammetry measurements were obtained using an electrochemical workstation (CHI 660E CH Instruments, USA) in a scan range of 0.1 to 0.9 V. For every experiment, the typical area under consideration was 1 1 cm2. The specific capacitance value was calculated from cyclic voltammetry using Equation (1), C¼
1 msðVf Vi Þ
Z
Vf
ð1Þ
IðVÞdv Vi
where m is the mass of active electrode material, s is the potential scan rate, Vf and Vi are the integration limits of the voltammetric curve V, and I(V) is the response current density. For the symmetric supercapacitor applications, two electrodes of same charge capacity were employed as positive and negative electrode. The specific capacitance (C, F g1), specific energy (SE, W h Kg1) and specific power (SP, kW Kg1) calculated from chronopotentiometric curves using following Equations (2)–(4), C¼
I Dt m DV 1
SE ¼ 2
ð2Þ
CV 2 3:6
ð3Þ
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3600 SE t
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where I is charge/discharge current at a discharge time t(s), DV is the potential window, and m is the mass of electrode.
Keywords: nanoporous material · polyaniline · reduced graphene oxide · supercapacitors · synergic effect [1] a) C. Li, H. Bai, G. Q. Shi, Chem. Soc. Rev. 2009, 38, 2397 – 2409; b) H. D. Tran, D. Li, R. B. Kaner, Adv. Mater. 2009, 21, 1487 – 1499; c) D. Li, J. X. Huang, R. B. Kaner, Acc. Chem. Res. 2009, 42, 135 – 145; d) M. X. Wan, Adv. Mater. 2008, 20, 2926 – 2932. [2] a) D. S. Dhawale, R. R. Salunkhe, V. S. Jamadade, T. P. Gujar, C. D. Lokhande, Appl. Surf. Sci. 2009, 255, 8213 – 8216; b) C. C. Hu, C. H. Chu, Mater. Chem. Phys. 2000, 65, 329 – 338; c) V. Gupta, N. Miura, Electrochem. Solid-State Lett. 2005, 8, A630 – A632; d) F. Fusalba, P. Gouerec, D. Belanger, J. Electrochem. Soc. 2001, 148, A1. [3] a) E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Beguin, J. Power Sources 2006, 153, 413 – 418; b) K. Jurewicz, S. Delpeux, V. Bertaggna, F. Beguin, E. Frackowiak, Chem. Phys. Lett. 2001, 347, 36 – 40. [4] a) A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, P. J. Ferraris, J. Power Sources 1994, 47, 89 – 107; b) A. Laforgue, P. Simon, C. Sarrazin, J. F. Fauvarque, J. Power Sources 1999, 80, 142 – 148. [5] a) E. T. Kang, K. G. Neoh, K. L. Tan, Prog. Polym. Sci. 1998, 23, 277 – 324; b) K. S. Ryu, K. M. Kim, N. G. Park, Y. J. Park, S. H. Chang, J. Power Sources 2002, 103, 305 – 309. [6] K. Zhang, L. L. Zhang, X. S. Zhao, J. Wu, Chem. Mater. 2010, 22, 1392 – 1401. [7] a) B. Luo, S. Liu, L. Zhi, Small 2012, 8, 630 – 646; b) Y. Huang, J. Liang, Y. Chen, Small 2012, 8, 1805 – 1834. [8] a) C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam, A. Govindaraj, Angew. Chem. Int. Ed. 2009, 48, 7752 – 7777; Angew. Chem. 2009, 121, 7890 – 7916; b) A. K. Geim, K. S. Novoselov, Nat. Mater. 2007, 6, 183 – 191.
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Received: January 29, 2014 Revised: March 19, 2014 Published online on && &&, 0000
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FULL PAPERS R. R. Salunkhe, S.-H. Hsu, K. C. W. Wu, Y. Yamauchi* && – &&
Not just a flash in the PANI: An effective route for the preparation of layered reduced graphene oxide (rGO) with uniformly coated polyaniline (PANI) layers is reported. The nanocomposites are synthesized by chemical oxidative poly-
merization of aniline monomer in the presence of layered rGO. Through synergistic effects, the rGO–PANI nanocomposites exhibit excellent capacitive performance with a high specific capacitance and high cycle reversibility.
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Large-Scale Synthesis of Reduced Graphene Oxides with Uniformly Coated Polyaniline for Supercapacitor Applications
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