Polyaniline

Article pubs.acs.org/EF

A New Partially Reduced Graphene Oxide Nanosheet/Polyaniline Nanowafer Hybrid as Supercapacitor Electrode Material Zan Gao,† Wanlu Yang,† Jun Wang,*,†,‡ Bin Wang,† Zhanshuang Li,† Qi Liu,† Milin Zhang,†,‡ and Lianhe Liu†,‡ †

Key Laboratory of Superlight Material and Surface Technology, Ministry of Education, Harbin Engineering University, Harbin 150001, PR China ‡ Institute of Advanced Marine Materials, Harbin Engineering University, Harbin 150001, PR China ABSTRACT: A special partially reduced graphene oxide nanosheet/polyaniline nanowafer (GNS/PANI) hybrid for supercapacitor electrode material was fabricated by an in situ polymerization method. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results revealed that PANI nanowafers (50 nm) homogeneously grew onto the surfaces of GNSs as spacers to keep the neighboring sheets separate. The electrochemical performance was analyzed by cyclic voltammetry (CV), galvanostatic charge/discharge, and electrochemical impedance spectrometry (EIS). The composite exhibited a maximum specific capacitance of 329.5 F/g and excellent cycle life, indicating a positive synergistic effect of GNS and PANI for the improvement of electrochemical performance. Our investigation highlighted the importance of anchoring of small PANI nanowafers on graphene sheets for maximum utilization of electrochemically active PANI and graphene in supercapacitors, presenting a promising application of GNS/PANI composites as electrode materials for energy storage.

1. INTRODUCTION In the wake of increasing air pollution, global warming and depletion of traditional energy resource, the development of renewable energy production and hybrid electric vehicles with low CO2 emission have stimulated intense research on energy storage and use of alternative energy sources.1 Supercapacitors (SCs), also called electrochemical supercapacitors (ESCs) or ultracapacitors, have attracted considerable attention for higher power density, longer cycle life, lower maintenance cost, and environmental friendliness. Such outstanding properties make them promising energy storage devices in a wide range of applications, such as portable electronics, mobile communications, hybrid electric vehicles, memory backup systems, large industrial equipments, and military devices.2−4 According to the charge−discharge mechanisms, supercapacitors can be divided into two categories: (i) electrical double-layer capacitors (EDLCs), where the electrical charge is stored at the interface between the electrode and the electrolyte, (ii) pseudocapacitors, where capacitance arises from reversible Faradaic reactions taking place at the electrode/electrolyte interface. Carbon materials,5−10 transition metal oxides and hydroxides,11−15 and conducting polymers16−20 have been extensively employed as the most promising materials for supercapacitor electrodes. Electrode materials are the main determinants of performance of SCs. However, each material has its unique advantages and disadvantages for SC application. EDLCs based on carbon materials can provide a long cycle life (>105 cycles) but with relatively low specific capacitances. Pseudocapacitors based on metal oxides and conducting polymer materials have much higher specific capacitances due to their redox properties but with relatively low mechanical stability and cycle life. Considerable effort has been exerted to combine the unique advantages of different capacitive materials for SCs.15,21−23 © 2012 American Chemical Society

Conducting polymer polyaniline (PANI) is one of the most promising potential electrode materials for pseudocapacitor applications because of its low cost, high conductivity in doped form, fast and stable transition between doped and reduced states, and facile synthesis using chemical or electrochemical methods.24−27 However, PANI exhibits the disadvantage of a low cycle life because swelling and shrinkage may occur during doping/dedoping processes, thus leading to mechanical degradation of the electrodes and fading of electrochemical performance. To solve this problem, research has focused on synthesizing electrode materials with highly electro-active regions by controlling the microstructure such as grain size, thickness, specific surface area, and pore character.28−31 Recently, many researchers have experimented on increasing the utilization of the electro-active material by anchoring PANI on carbon-based materials, such as activated carbon, carbon nanotubes (CNTs), porous carbon, and graphene/reduced graphene oxide, because of their improved stability (Table 1).30,32−37 Graphene, as a two-dimensional monolayer of sp2-bonded carbon atoms, has attracted much attention because of its exceptional thermal, mechanical, and electrical properties.38 Graphene also exhibits great promise for potential applications in the fields of nanoelectronics, sensors, batteries, hydrogen storage, and supercapacitors. Graphene-based supercapacitors have high specific capacitances ranging from 135 to 264 F/g in aqueous solution.39−43 However, the true capacity of graphene material is not realized because graphene nanomaterials are prone to stacking, which blocks their electroactive sites. To exploit the potential of graphene-based materials for superReceived: May 16, 2012 Revised: December 10, 2012 Published: December 15, 2012 568

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Table 1. Recent Published Papers about GO/PANI and GNS/PANI ref

method

33

in situ polymerization

34 35 36 37 49

in situ polymerization in situ polymerization in situ polymerization vacuum filtration in situ polymerization-reduction/dedopingredoping in situ polymerization in situ polymerization in situ polymerization electrostatic adsorption in situ polymerization

50 51 52 53 our paper

morphology of PANI in composite

specific capacitance of composite (F/g)

capacitance retention

short fibrillar and granular agglomerate worm-like agglomerates fiber nanofiber nanofibre fibrous

627

73% after 500 cycles

1046 526 1130 569 1126

not given not given 87% after 1000 cycles 96% after 1000 cycles 84% after 1000 cycles

nanofiber rod-like agglomerations not given nanofiber wafer-like

480 1046 547 301 328

not given 67% after 1000 cycles not given 67% after 1000 cycles 94% after 1000 cycles

mass fraction of PANI, calculated from the weight of GNS before and after polymerization, was 65%. For comparison, pure PANI powders were also prepared by the above-mentioned chemical process without GNS. 2.2. Characterization Methods. The crystallographic structures of the materials were determined by a powder X-ray diffraction (XRD) system (Rigaku TTR-III) equipped with Cu Kα radiation (λ = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) measurements were performed using a PHI 5700 ESCA spectrometer with monochromated Al Kα radiation (hν = 1486.6 eV). All XPS spectra were corrected by the C1s line at 284.5 eV. Raman measurements were carried out using a Jobin Yvon HR800 micro-Raman spectrometer at 457.9 nm. The laser beam was focused with a 50× objective lens to about 1 lm spot on the surface of the sample. The microstructure of the samples was investigated by atomic force microscopy (AFM; Nanoscope IIIa), scanning electron microscopy (SEM; JEOL JSM6480A microscope), and transmission electron microscopy (TEM; Philips CM 200 FEG, 160 kV). Surface area measurements were carried out by physical adsorption of N2 at 77 K (Micromeritics ASAP 2010) and obtained by the Brunauer−Emmett−Teller (BET) method. 2.3. Preparation of Electrodes and Electrochemical Characterization. The working electrodes were fabricated by a reported procedure.23 Briefly, the as-prepared materials, acetylene black, and poly(tetrafluoroethylene) (PTFE) were mixed in a mass ratio of 80:15:5 and dispersed in ethanol to produce a homogeneous paste, which was then pressed onto the stainless steel grid (1 cm × 1 cm). The as-prepared materials, acetylene black, and PTFE were used as the electroactive material, conductive agent, and binder, respectively. Finally, the fabricated electrodes were dried at 60 °C for 2 h in a vacuum oven. All electrochemical tests were carried out in a conventional three-electrode electrochemical cell. The stainless steel grid coated with the GNS/PANI composite was used as the working electrode. Platinum foil and a saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. The mass of the mixture onto the electrode, which included the electro-active material, conducting agent, and binder, was 9.2, 8.3, and 8.5 mg for GNS, PANI, and GNS/PANI electrode, respectively. The measurements were carried out in 1 M H2SO4 aqueous electrolyte at room temperature. Cyclic voltammograms, galvanostatic charge/discharge curves, and electrochemical impedance spectroscopy (EIS) were carried out by a CHI 660D electrochemical workstation. Cyclic voltammetry (CV) tests were done between −0.2 and 1.0 V (vs SCE) at scan rates of 5, 10, 20, 40, 80, and 120 mV/s. Galvanostatic charge/ discharge curves were measured in the potential range of −0.20 to 1.0 V (vs SCE) at different current densities. EIS measurements were carried out in the frequency range from 100 kHz to 0.05 Hz at open circuit potential with an AC perturbation of 5 mV.

capacitors, one promising prospect is to utilize a single-atomthick GNS as substrate to anchor nanomaterials, which then act as spacers in the formation of new nanocomposites.44−46 To date, extensive efforts and many approaches have been developed to prepare graphene-based composites,47,48 especially GO/PANI or graphene/PANI composite powders with PANI nanofiber or nanowires anchored onto the surfaces of GO/GNS.29,49−54 The electrochemical properties of the composite are improved significantly by the synergistic effects between GNS and conducting polymers because the ultrathin and flexible graphene matrix not only increases the conductivity of the composite but also improves the stability of the polymer during the charge/discharge process. GNS/PANI composite is, therefore, a promising electrode material for energy storage. With little research about nanowafer PANI as supercapacitor electrode material, we report here a facile method to synthesize GNS composite doped with PANI by an in situ polymerization method. Unique PANI nanowafers were homogeneously anchored onto graphene sheets, which acted as spacers to keep neighboring sheets separate. The composites embraced desirable electrode properties, namely good chemical stability because of the reduced graphene oxide matrix, low electron transfer resistance owing to the layer conductive network, and high pseudocapacitive properties due to the presence of active species PANI. Our experimental results showed a high specific capacitance of 329.5 F/g for GNS/PANI composite, as well as improved rate performance and excellent cycle ability. The effect of microstructure on the electrochemical performance of the composite was also investigated.

2. EXPERIMENTAL SECTION 2.1. Synthesis of the GNS/PANI Composite. All chemicals are of analytical grade and were used without further purification. Graphite oxide was prepared from natural graphite by a modified Hummers method.55 GNS was prepared by reduction of graphite oxide with glucose.56 The GNS/PANI composite was synthesized using an in situ polymerization method in the presence of GNS suspension and aniline monomer. Aniline was first distilled under vacuum to remove the oxidation impurities. Then, 12.5 mmol purified aniline (An) monomers were dispersed in 200 mL of GNS (4 mg/mL) suspension and sonicated for 30 min. While maintaining vigorous stirring in an ice−water bath, 50 mL of ammonium persulfate (APS) aqueous solution (solute, 6.25 mmol; solvent, 0.1 M HCl) was quickly poured into the above mixture. The above system, with a pH of about 3, was maintained at 0−4 °C for 6 h under N2 atmosphere protection. Finally, the obtained GNS/PANI composite was washed with distilled water and ethanol and dried in a vacuum oven at 80 °C for 12 h. The

3. RESULTS AND DISCUSSION 3.1. Material Characterization. Figure 1 shows the XRD patterns of GO, GNS, PANI, and the as-prepared GNS/PANI 569

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Figure 1. Typical XRD patterns of GO (a), the GNS (b), PANI (c), and the prepared GNS/PANI composite (d).

composites. The diffraction peak of exfoliated GO at 10.8° (001) features a basal spacing of 0.82 nm, showing the complete oxidation of graphite to the graphite oxide due to the introduction of oxygen-containing functional groups onto the graphite sheets (Figure 1a).57 For the XRD pattern of GNS (Figure 1b), which was reduced from GO by glucose, the peak located at 10.8° disappears, while a broad diffraction peak (002) appears at a 2θ of about 24.5°, revealing high reduction of GO and the exfoliation of the layered GNS.58,59 The interlayer spacing of the GNS decreases from 0.8 nm for GO to 0.37 nm, which is still a little larger than that of natural graphite (0.34 nm). The small amount of oxygen-containing groups and hydrogen remaining may be the main reason for this difference, indicating incomplete reduction of GO to graphene. These residual oxygenated functional groups most likely involve the intercalation and adsorption of cations onto the surfaces of GNSs. For pure PANI, the diffraction peaks appear at 2θ = 15.6°, 20.7°, and 25.3°, corresponding to (0 1 1), (0 2 0), and (2 0 0) crystal planes of PANI in its emeraldine salt form, respectively (Figure 1c).60 The X-ray data of the GNS/PANI composites present crystalline peaks similar to those obtained from pure PANI, the (002) peak of the layered GNS has almost disappeared, and no other characteristic peaks are observed. The result suggests that the composites have not acquired additional crystalline structure, and the restacking of the graphene sheets is effectively prevented from a complete exfoliation state of graphite in the hybrid GNS/PANI material. This corresponds to the TEM image in Figure 5f in which PANI wafer nanoplatelets (about 50 nm) are located onto the surfaces of the GNSs. Figure 2 shows the Raman spectra of GO, GNS, PANI, and the GNS/PANI composite. A broad D band (1350 cm−1) and G band (1575 cm−1) are observed in all of the Raman spectra. The Raman spectrum of graphene is usually characterized by two main features: the G band arising from the first-order scattering of the E1g phonon of sp2 C atoms (usually observed at 1575 cm−1) and the D band arising from a breathing mode of point photons of A1g symmetry (1350 cm−1).61 Compared with GO (Figure 2a), an increased ID/IG intensity ratio for the GNS (Figure 2b) is observed, indicating a decrease in the size of the in-plane sp2 domains62,63 and the removal of the oxygen

Figure 2. Raman spectra of GO (a), GNS (b), pure PANI (c), and the GNS/PANI composite (d).

functional groups in the graphite oxide sheet. As well as the G band and D band in the Raman spectrum of the GNS/PANI composite (Figure 2d), a C−N+ stretching vibration at 1344 cm−1 and CN stretching vibration at 1452 cm−1 are observed, revealing the presence of PANI in the composite.64 The basic chemical structure of PANI molecule is shown in Scheme 1a. The intrinsic oxidation state of PANI ranges from Scheme 1. (a) Structure of PANI (n + m = 1); (b) Schematic Representation of the Formation Process of the GNS/PANI Composite

the fully oxidized pernigraniline (n = 0, m = 1), through half oxidized emeraldine (n = m = 0.5), to fully reduced leucoemeraldine (n = 1, m = 0).65,66 The chemical and electrical properties of PANI are closely associated with its intrinsic structure, which is the distribution of quinonoid amine (−N), benzenoid amine (−NH−) as well as the positively charged nitrogen. XPS analysis was employed to identify the chemical nature of the PANI formed on the surfaces of GNSs. Figure 3a shows the XPS spectra of GNS and the GNS/PANI composite. Compared with that of GNS, the XPS spectrum of the GNS/PANI composite not only exhibits a relatively low O 1s peak and C 1s peak but also exhibits a peak at 400 eV 570

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groups on the surfaces. The ideal single-atom-thick GNS, as a support to anchor functional nanomaterials in the formation of new composites, is highly promising. Figure 5 shows the morphology and structure of the asobtained products. The corrugated and scrolled graphene

Figure 5. SEM (a) and TEM (b) images of pure GNSs. SEM (c) and TEM (d) images of pure PANI. SEM (e) and TEM (f) images of GNS/PANI composites (inset: corresponding high magnification TEM images).

Figure 3. XPS survey spectra of GNS and the GNS/PANI composite (a). N 1s XPS spectra of the GNS/PANI composite (b).

corresponding to N 1s, which confirms the presence of PANI in the composite. As seen in Figure 3b, the deconvolution of N1s spectra results in three peaks: benzenoid amine centered at 399.4 eV, quinonoid amine at 398.2 eV, and nitrogen cationic radical (N+) at 401 eV.65 The peak at 401 eV is indicative of the doping level of the polymer, an important factor affecting the electrical and other properties of PANI.67,68 On the basis of quantitative analysis of the deconvoluted N1s spectra, a N+/N ratio of 0.34 was obtained, indicating a relative high proton doping for the anchored PANI on the surfaces of GNSs, which is desirable in electrochemical applications. Figure 4 gives a typical AFM image of an exfoliated GO dispersion in water after their deposition on a freshly cleaved mica surface. Compared with the theoretical value of 0.78 nm for single-layer graphene, the average thickness of as-prepared GO is about 1.1 nm, which corresponds to a single layer GO. The higher thickness of GO may arise from oxygen-containing

sheets resemble crumpled silk-like waves (Figure 5a). GNSs agglomerate with each other through van der Waals interactions of the remaining oxygen-containing functionalities on the surfaces of the sheets. The TEM images show that GNS has corrugations and scrollings on the sheet edges (Figure 5b). From SEM and TEM images (Figure 5c,d), pure PANI consists of irregular wafers, with a mean lateral size of 50 nm, stacked to form layer agglomerates. For the as-obtained GNS/PANI composites, the thin PANI wafers exhibit loose flower-like structures (Figure 5e,f). The special wafer-like structure is attributed to a low temperature, relatively high PH of the reaction system, and a high mol ratio of [An]/[ASP]. As previously reported, in the early polymerization stage, oligoanilines usually showed a sheet-like microstructure, which acted as templates for further growth of PANI nanostructures.69 Because of insufficient acidity in the reaction system, PANI nanosheets could not be transformed into PANI nanotube or nanofiber through a rolling mechanism.70 Instead, in our study, wafer-like PANIs anchored onto GNS sheets to form a loose structure, which is desirable for supercapacitor application because both surfaces containing thin PANI wafers are effective in contributing pseudocapacitance to the total energy storage. At the same time, the densely distributed PANI wafers effectively prevent the restacking of graphene nanosheets, resulting in an open, loose structure favorable for the improvement of electrochemical performance as an electrode for supercapacitors (the BET surface areas of the GNS, pure PANI, and the GNS/PANI composite were 30.77, 16.18, and 76.63 m2/g, respectively).

Figure 4. AFM image of exfoliated GO sheets on mica surface with height profile. 571

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Figure 6. (a) CV curves of GNS, pure PANI, and the GNS/PANI composite at the scan rate of 5 mV/s. (b) CV curves of the GNS/PANI composite at different scan rates of 5, 10, 20, and 40 mV/s in 1.0 M H2SO4 solution.

Figure 7. (a) First charge/discharge curves of GNS, pure PANI, and GNS/PANI composite electrodes in 1.0 M H2SO4 solution at a galvanostatic current density of 10 mA/cm2. (b) Galvanostatic charge/discharge tests of GNS, pure PANI, and GNS/PANI composite within the potential window −0.2 to 1.0 V (vs SCE) at different current densities.

On the basis of the above results, Scheme 1b illustrates the preparation procedure of the GNS/PANI composite. As shown from previous studies, GO sheets have their basal planes covered mostly with epoxy and hydroxyl groups, while carbonyl and carboxyl groups are located at the edges.71 From one perspective, these functional groups act as anchor sites and enable in situ formation of nanostructures onto the surfaces and edges of GO sheets. On the other hand, these oxygencontaining functional groups impair the conductivity of GO sheets so that they are not suitable for electrode materials. After the reduction process by glucose, most of the oxygencontaining groups are removed and the conductivity of GNS is recovered, which improves the electrochemical property of the GNS-based composite. When the as-prepared GNS was soaked in aniline acid solution, positively charged aniline radical cation was easily adsorbed onto the surfaces of GNSs due to electrostatic force. Finally, polymerization was initiated by ammonium persulfate under an ice−water bath, and PANI nanowafers in situ grew onto the surfaces of graphene nanosheets. The PANI nanowafers, which intercalated into the graphene nanosheets, produced an open structure, keeping a highly active surface area and improving the capacity and cyclic performance of the composites. 3.2. Electrochemical Behavior. To explore the potential application of the as-synthesized GNS/PANI composite, the

samples were fabricated as supercapacitor electrodes and characterized by CV, EIS, and galvanostatic charge/discharge measurements. Figure 6a shows CV curves of GNS, pure PANI, and the GNS/PANI composite at a scan rate of 5 mV/s. The CV curve of GNS is quasi-rectangular along the current−potential axis without obvious redox peaks, indicating a desirable capacitive behavior in SCs. For the CV curves of PANI and GNS/PANI composite, two pairs of peaks can be seen: the first redox peaks at around 0.25 V (B/B′) are related to the transition of PANI from its semiconducting-state (leucoemeraldine form) to a conducting state (polaronic emeraldine form). The second redox peaks (A/A′) are related to the emeraldine−pernigranline transition.24,72 Compared with GNS/PANI composite, the A/A′ redox peaks of pure PANI are not obvious as those of GNS/PANI, indicating the complete redox reaction of PANI anchored onto GNS substrate. In addition, the area surrounded by CV curves for the GNS/PANI electrode is apparently larger than that of the pure PANI and GNS electrode at the same scan rate (Figure 6b), implying a higher specific capacitance for the composite electrode. The unique microstructure may be the main factor for this difference. The loose-formed, open structure reduces diffusion and migration length of the electrolyte ions during the fast charge/discharge process and increases electrochemical utilization of PANI. In addition, fast 572

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electron transfer is facilitated due to excellent conductivity of GNS in the composite. With an increase of scan rate (Figure 6b), the anodic peak shifts toward positive potential and the cathodic peak shifts toward negative potential, due mainly to resistance of the electrode. With increased scan rates, internal resistance limits the electrochemical performance of the GNS/ PANI composite because of relatively poor conductivity of sheet-like PANI, as a result of weak protonation in a reaction system with pH = 3.70 Further research is needed to improve conductivity of the wafer-like PANI. Galvanostatic charge/discharge was performed to calculate the specific capacitance of the electrode according the following equation: Csp =

i×t Δv × m

(1)

where i, t, Δv, and m are the constant current (A), discharge time (s), the total potential deviation (V), and the weight of active materials (g), respectively. Figure 7a shows the first charge/discharge curves of the GNS, pure PANI. and GNS/PANI electrodes in 1.0 M H2SO4 solution at a galvanostatic current density of 10 mA/cm2. The charge/discharge curve of GNS exhibits a triangular shape, implying an ideal capacitor character, and the discharge curves of GNS/PANI electrode consist of two clear voltage stages: a fast potential drop (from 1.0 to 0.5 V) and a slow potential decay (from 0.5 to −0.2 V). The former, shorter discharge results from the electric double-layer capacitance of the electrode, while the latter, longer discharge is ascribed to a combination of electric double-layer capacitance and Faradaic capacitance, representing the pseudocapacitive feature of the electrode. The first discharge specific capacitances of GNS, pure PANI, and GNS/PANI composite electrodes are 112.6, 203.2, and 289.2 F/g, respectively (calculated from eq 1), which indicate that the specific capacitance of the hybrid material is obviously enhanced compared with the corresponding pure PANI. The larger capacitance for GNS/PANI may be caused by the combination of electric double-layer capacitance and Faradaic pseudocapacitance. In addition, the loose open structure system of the composite improves the contact between the electrode and the electrolyte, thus making full use of the electrochemical active material. These results are in accordance with those deduced from the CV tests. Figure 7b shows the charge/discharge curves of GNS, pure PANI, and GNS/PANI composite electrodes at different current densities. The specific capacitance values of the GNS/PANI composite electrode obtained from the discharge curves are 329.5, 289.2, 257.4, 222.8, and 183.2 F/g at the current densities of 5, 10, 20, 50, and 100 mA/cm2, respectively (calculated from eq 1). Compared with pure PANI, the GNS/PANI composite exhibits a desirable rate capability, attributed to the introduction of GNS in the composite. The decreased capacitance of GNS/ PANI composite with the increase of discharge current densities is likely caused by the resistance of electrode and the insufficient Faradaic redox reaction of the active material under higher discharge current densities. Figure 8 shows the Nyquist plots of GNS, PANI, and GNS/ PANI composite electrodes, recognized as a principal method to examine fundamental behavior of electrode materials for supercapacitors. Each impedance spectrum has a semicircular arc and a straight line. The high-frequency arc corresponds to the charge transfer resistance (Rct) caused by the Faradaic reactions and the double-layer capacitance (Cdl) at the contact

Figure 8. Nyquist plots of GNS, PANI, and GNS/PANI composite electrodes. Inset: magnified high-frequency regions.

interface between electrode and electrolyte solution. Rct can be directly measured as the semicircular arc diameter. The 45° slope portion of the curve is the Wurburg resistance (Zw), which is a result of the frequency dependence of ionic diffusion/transport in the electrolyte and to the surface of the electrode. The values of Rct for GNS, PANI, and GNS/PANI composite electrodes are 0.27, 1.01, and 0.18 Ω, respectively. The equivalent series resistance (Re) can be obtained from the X-intercept of the Nyquist plots, which is a combined resistance comprising ionic resistance of electrolyte, intrinsic resistance of substrate, and contact resistance at the active material/current collector interface. Re, as shown in Figure 8, is 1.52 Ω for GNS, 1.81 Ω for pure PANI, and 1.27 Ω for GNS/PANI composite, respectively. Clearly, the Rct and Re of GNS/PANI composite electrode are smaller than those of GNS and PANI electrode, which demonstrates that the addition of graphene enhances conductivity and improves charge transfer performance of GNS/PANI composite electrode. At the same time, anchored PANI nanowafers prohibit aggregation of graphene sheets and the loose-formed open structure facilitates fast electron transfer between the active materials and the charge collector. The length of cycle life of a supercapacitor is a crucial parameter for its application. Figure 9a shows the cyclic performance of the GNS/PANI composite electrode examined by galvanostatic charge/discharge tests for 1000 cycles. Each charge/discharge cycle approximately has a similar potentialtime response behavior (Inset Figure 9b), implying that the charge/discharge process of the GNS/PANI composite electrode is reversible. Interestingly, the specific capacitance of GNS/PANI decreases slightly in the initial cycles; however, the specific capacitance stabilizes after 350 cycles, indicating an improved cycle life. The initial decrease of capacitance is ascribed to PANI on the outer surfaces of the composite electrode, which had not come in close contact with GNS and which led to swelling and shrinkage during the doping/ dedoping processes. After repetitive charge/discharge cycling, the electrochemically active PANIs anchored on the surfaces of GNS produce a synergistic effect with GNS, which improves cyclic stability of the composite electrode. On comparing with a previous report that GNS/PANI nanofibers composite gave a specific capacitance of 210 F/g,54 this unique GNS/PANI nanowafer composite shows excellent cycle stability and higher 573

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eration special project (S2013ZR0649), and Special Innovation Talents of Harbin Science and Technology (2012RFXXG104).

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Figure 9. (a) Cyclic performance of the GNS/PANI composite electrode. (b) The charge−discharge curves of the GNS/PANI composite electrode, which were performed at 50 mA/cm2 in solution 1.0 M H2SO4 solution.

specific capacitance (329.5 F/g). We attribute these improved qualities to the flexibility of GNS in the composite not only forming an open, loose structure but also improving electrical conductivity of the overall electrode due to the high conductivity of graphene. The connection between the active material and electrolyte is improved, and full use is made of pseudocapacitive PANI in the composite electrode. The unique GNS/PANI nanowafer composite will be a promising electrode material for supercapacitors in their application.

4. CONCLUSION In summary, we report a facile process to fabricate a new GNS/ PNAI composite containing nanowafer PANI anchored onto the surfaces of GNS for supercapacitor electrode material. The surface morphology, structure, and capacitive behaviors of the GNS/PANI composite were thoroughly investigated. The incorporation of special PANI nanowafers onto the surfaces of GNS prevents the restacking of graphene sheets and improves the capacitance of the composite electrode, the electrolyte/electrode accessibility as well as conductivity, indicating a positive synergistic effect for GNS and PANI on the improvement of overall electrochemical performance. The prepared GNS/PANI composite exhibited a high specific capacitance (329.5 F/g at 5 mV/s) and excellent long cycle life, suggesting a highly promising prospective for SCs. The facile method of synthesis can be readily adapted to prepare other high-performance electrode materials containing graphene as a conducting additive.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 451 8253 3026. Fax: +86 451 8253 3026. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Special Innovation Talents of Harbin Science and Technology (2011RFQXG016), Fundamental Research Funds of the Central University (HEUCFZ), Key Program of the Natural Science Foundation of Heilongjiang Province, Program of International S&T Coop574

dx.doi.org/10.1021/ef301795g | Energy Fuels 2013, 27, 568−575

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