Conducting Polymers Directly Coated on Reduced Graphene Oxide

Article pubs.acs.org/JPCC

Conducting Polymers Directly Coated on Reduced Graphene Oxide Sheets as High-Performance Supercapacitor Electrodes Jintao Zhang† and X. S. Zhao*,†,‡ †

Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 School of Chemical Engineering, The University of Queensland, St Lucia, Brisbane, QLD 4072, Australia

‡

S Supporting Information *

ABSTRACT: In this work, conducting polymers poly(3,4ethylenedioxythiophene) (PEDOT), polyaniline (PANi), and polypyrrole (PPy) were directly coated on the surface of reduced graphene oxide (RGO) sheets via an in situ polymerization process to prepare conducting-polymer-RGO nanocomposites with different loadings of the conducting polymers. Experiment results showed that ethanol played an important role in achieving a uniform coating of the polymers on RGO sheets. The electrochemical capacitive properties of the composite materials were investigated by using cycle voltammetry and charge/discharge techniques. The composite consisting of RGO and PANi (RGO-PANi) exhibited a specific capacitance of 361 F/g at a current density of 0.3 A/g. The composites consisting of RGO and PPy (RGO-PPy) and PEDOT (RGO-PEDOT) displayed specific capacitances of 248 and 108 F/g, respectively, at the same current density. More than 80% of initial capacitance retained after 1000 charge/discharge cycles, suggesting a good cycling stability of the composite electrodes. The good capacitive performance of the conducting−polymer-RGO composites is contributed to the synergic effect of the two components.

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INTRODUCTION The increasing demand for clean and sustainable energy has driven intensive research efforts toward the development of energy storage and delivery systems. Supercapacitors, as one of the important energy storage devices, attract growing attentions because of their important features, such as high power density, fast charging/discharging processes (within seconds), and long cycle life.1−3 Recent research efforts have been focused on improving the energy density of supercapacitors by exploring novel electrode materials.3 Conducting polymers (CPs) can store charges not only in the electrical double layer (EDL) but also through the rapid faradic charge transfer (pseudocapacitance). As a result, the specific capacitance of CP electrodes is higher than that of EDL capacitors based on carbon electrodes.1−3 However, one of the drawbacks for CPs as supercapacitor electrodes is their poor cycling stability because CPs are usually brittle and weak in mechanical strengths. Coupling CPs to a carbon material has been shown to be an effective approach to improving the cycling stability of the CPs.4−7 For example, a composite electrode consisting of nanodiamond (ND) and polyaniline (PANi) exhibited dramatically improved cycle stability and high capacitance retention compared with pure PANi electrodes.5 Reduced graphene oxide (RGO) sheets can be prepared in large quantity through chemical reduction of graphene oxide (GO).8−11 The RGO materials possess a good electrical conductivity and high surface area, thus holding great promises for energy storage applications.12,13 CPs have been coupled with RGO to prepare supercapacitor electrodes.14−16 A RGO-PANi © 2012 American Chemical Society

composite paper prepared using an anodic electropolymerization method displayed an electrochemical capacitance of ∼233 F/g.17 However, the agglomerated layerlike structure resulted in the inhomogenerous coating of PANi on the 3D structure of RGO paper. An RGO sheet/PANi composite was synthesized by oxidative polymerization of aniline monomers in a suspension of RGO sheets.18 It was found that RGO sheets provided the nucleation sites for PANi as well as an excellent pathway for electron transfer, leading to good capacitive performance. Composite films consisting of sulfonated RGO (SG) and polypyrrole (PPy) were deposited on a Pt foil by an electrochemical polymerization process.19 With the assistance of dodecylbenzene sulfonic acid (DBSA), the amorphous PPy component formed thick coatings on the SG surface. A uniform coating of PANi on GO sheets has been achieved by using a dilute polymerization method at a low temperature (−10 °C).20 It should be noted that this method is only suitable for the preparation of conducting composites with a small amount of GO because of the insulating property of GO.21,22 Subsequent studies revealed that the electrical conductivity of the resultant GO-PANi composite can be enhanced by chemical reduction of GO.23,24 However, the complex postprocessing usually resulted in CPs in a partially agglomerated form on RGO sheets. Direct coating of CPs on RGO sheets is difficult to implement because the dispersion of RGO sheets in water is poor,15,25 Received: November 29, 2011 Revised: February 6, 2012 Published: February 8, 2012 5420

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especially for those polymer monomers with a low water solubility, such as 3,4-ethylenedioxythiophene. A sulfonated RGO with a good water solubility was synthesized and used to prepare a composite material consisting of SG and PEDOT.26 However, no investigation was conducted to reveal whether PEDOT was uniformly coated on SG sheets. As a result, it is desirable to develop the easier and more convenient routes to fabricate RGO-CP nanocomposites for their electrochemical applications in different areas. In this Article, we demonstrate an in situ polymerization method for the preparation of RGO−CP composite materials. In the polymerization process, the presence of an acid and the subsequent doping of the anion counterpart of the acid (e.g., SO42− for acid H2SO4) on the CP chain are the key steps to modulate the electrical properties of the CP.27,28 The in situ polymerization method is an effective approach to implementing the doping process of CPs in acidic aqueous solutions. In the present work, CPs PEDOT, PANi, and PPy were directly coated on RGO sheets at loadings as high as 90 wt % by using the in situ polymerization method. Ethanol was found to be indispensable in the preparation system because it not only improved the dispersion of RGO sheets but also facilitated the diffusion and growth of polymer monomers on the surface of RGO sheets. The capacitive performance of these composite materials was evaluated as supercapacitor electrode materials, exhibiting high specific capacitance and improved cycling stability.

(HRTEM, JEOL-2100F) were employed to characterize the morphology of samples. Electrochemical Measurement. An Autolab PGSTAT302N was used to measure the electrochemical properties of the samples at ambient temperature (∼22 °C). The electrochemical cell had a three-electrode configuration with a bright Pt plate as the counter electrode and an Ag/AgCl electrode as the reference electrode. The working electrodes were prepared by mixing an active material, acetylene black, and polytetrafluoroethylene (PTFE) with a mass ratio of 85:10:5. A small amount of ethanol was added to obtain a slurry, which was subsequently pressed on a gold foil and dried at 95 °C for 4 h. The mass loading of active materials in each electrode was ∼2 mg.

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RESULTS AND DISCUSSION The XPS survey spectra of RGO, RGO-PEDOT, RGO-PANi, and RGO−PPy are shown in Figure S1 of the Supporting Information. The XPS spectrum of RGO (Figure S1a of the Supporting Information) shows the presence of C 1s and O 1s signals, indicating the presence of residual oxygenate groups on the RGO sheets.31 For the RGO-PEDOT sample, apart from the C 1s and O 1s signals, typical S 2p signal can be seen (Figure S1b of the Supporting Information), indicating that PEDOT had been coated on the RGO sheets. For samples RGO-PANi and RGO-PPy, the obvious N 2p signals indicated the presence of CPs PANi and PPy in the samples, respectively. The core-level N 2p and S 2p XPS spectra are shown in Figure 1. The S 2p region

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EXPERIMENTAL SECTION Preparation of Conducting-Polymer-Coated RGO Sheets. RGO sheets used in this work were prepared by reducing GO via a microwave-assisted method.29,30 The preparation of CP-coated RGO sheets is illustrated in Scheme 1. Scheme 1. Illustration of Preparation Process of Conducting Polymer-Coated RGO Sheets

RGO (50 mg) was dispersed in deionic water with ethanol. After sonication for 2 h, a given amount of aniline or pyrrole monomer dissolved in deionic water with concentrated H2SO4 (2 mL) was added. The mixture was continuously sonicated for another 2 h. After cooling to 10 °C, 0.95 g of (NH4)2S2O8 dissolved in 30 mL of deionic water was added. The mixture was stirred overnight. The resulting precipitates were washed with deionic water and ethanol and dried at 60 °C. For monomer 3,4-ethylenedioxythiophene, ethanol was added in the solution to improve its solubility. The CP-RGO composites are designated as RGO-PANi, RGO-PPy, and RGO-PEDOT, respectively. Characterization. X-ray photoelectron spectroscopy (XPS) spectra were collected on a PHI-5300 ESCA spectrometer (PerkinElmer) with an energy analyzer working in the pass energy mode at 35.75 eV. An Al Kα line was used as the excitation source. Fourier transform infrared (FTIR) spectra were recorded on an IRPrestige-21 (Shimadzu, Japan) infrared spectrophotometer with a KBr pellet technique. Field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL, Japan) and high-resolution transmission electron microscope

Figure 1. XPS core-level spectra of RGO-PEDOT (a), RGO-PANi (b), and RGO−PPy (c).

(Figure 1a) shows the presence of sulfur spin-split doublet of PEDOT at around 164.0 eV (S 2p3/2) and 165.1 eV (S 2p1/2).32 The higher binding energy doublet at around 168.2 and 169.5 eV would be ascribed to sulfur spin-split coupling from PEDOT+SO42− due to the incorporation of counterion SO42− into PEDOT, suggesting the formation of doped PEDOT.33 For the RGO-PANi and RGO-PPy samples, the N 1s signals were deconvoluted to three component peaks, that is, the imine nitrogen (N=) at 398.2 eV for RGO-PANi and RGO-PPy, the 5421

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amine nitrogen (NH) at 399.0 eV for RGO-PANi and 399.4 eV for RGO-PPy, and nitrogen cationic radical (N+) at 400.5 eV for RGO-PANi and 401.5 eV for RGO-PPy.34 The presence of the last peaks suggested the formation of doped PANi and PPy.35 Figure 2 shows the FTIR spectra of samples. The featureless FT-IR spectrum of RGO shown in Figure 2a indicated that the

Figure 3. Raman spectra of RGO (a), RGO−PANi (b), RGO−PPy (c), and RGO−PEDOT (d). Figure 2. FT-IR spectra of RGO (a), RGO-PEDOT (b), RGO-PPy (c), and RGO−PANi (d).

can be seen, suggesting the presence of PANi in sample RGOPANi. For sample RGO-PPy, the broad D band with weak satellites would be attributed to the interaction of PPy with RGO sheets. For sample RGO-PEDOT, the band at ∼989 cm−1 was assigned to oxyethylene ring deformation. The bands at about 1515 and 1434 cm−1 were attributed to CC stretching. The small red shift in comparison with the reported data would be due to the π−π interactions interaction of PEDOT chains with RGO sheets,40 suggesting the formation of RGO-PEDOT composite. The FESEM image (Figure S2a of the Supporting Information) shows that RGO sheets look like crumpled and curved papers. Wrinkles were observed on the surface of RGO sheets (Figure S2b of the Supporting Information), indicating the sheets were very thin. After coating CPs, the paper-like morphology is still seen (Figure 4). However, the thickness of samples is obviously increased. Figure 4b shows an enlarged FESEM image of RGO-PEDOT. The surface of RGO sheets is covered with lots of protuberances, attributed to the PEDOT coated on the surface. The enlarged FESEM image of sample RGO-PANi exhibits a worm-like structure (Figure 4d), which is attributed to the PANi coating on RGO sheets. Figure 5 shows the TEM images of the composite materials. All samples exhibit a paper-like morphology, and wrinkles are seen on surfaces, as indicated by arrows. The enlarged TEM image of RGO-PEDOT (Figure 5b) reveals that the surface is covered with many nanowhiskers, whereas the image of RGOPANi (Figure 5d) shows a porous structure. However, a flat surface is observed on sample RGO-PPy (Figure 5f). The loading of the CP on the RGO sheets was easily controlled by adding different amounts of monomers. The RGO-CPs with mass ratio (CP to RGO) of about 67 and 80 wt % were synthesized, respectively. The FESEM and TEM images shown in Figures S3−S5 of the Supporting Information confirmed that the CPs (PEDOT, PANi, and PPy) were uniformly coated on the surface of RGO. The surface was smooth at low mass loadings without aggregated polymer particles. Here ethanol played an important role in the coating process. Our experimental results

chemical reduction of GO was relatively complete with few oxygen-containing groups. For the FTIR spectrum of RGOPEDOT (Figure 2b), the vibrational bands at about 1517 and 1340 cm−1 were attributed to the CC and C−C stretching vibrations of the quininoid structure of the thiophene ring, respectively. The bands at about 1200, 1142, and 1088 cm−1 were due to the C−O−C bond stretching in the ethylene dioxy (alkylenedioxy) group. Additionally, the C−S bond in the thiophene ring was evidenced by the presence of bands at about 981 and 838 cm−1.36 The series of bands suggested the formation of RGO-PEDOT. Figure 2c shows the FTIR spectrum of RGO-PPy. The peaks at 1549 and 1468 cm−1 were due to the fundamental stretching vibrations of pyrrole rings. The bands at 1305 and 1043 cm−1 were ascribed to the C−N stretching and C−H deformation vibrations, respectively. The presence of strong bands at about 1190 and 914 cm−1 indicated the formation of doped PPy.37 For RGO-PANi sample, the characteristic bands at 1584 and 1484 cm−1 (Figure 2d) were ascribed to CC stretching vibration in quinoid and benzene rings, respectively,4,38 whereas the bands at 1295 and 1240 cm−1 were related to the C−N and CN stretching modes.35 The presence of these bands suggested the successful coating of PANi on RGO sheets. The Raman spectra of samples RGO, RGO-PANi, RGO-PPy, and RGO-PEDOT are shown in Figure 3. For sample RGO, the two typical bands at about 1340 and 1590 cm−1 were contributed to the D-band and G-band of RGO, respectively. The D-band is associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of amorphous carbon films, whereas the G-band due to the E2g mode is closely related to the vibration of sp2-bonded carbon atoms in a 2-D hexagonal lattice, such as in a graphene layer.39 For sample RGO-PANi, apart from the D and G bands, two bands at about 1164 and 1470 cm−1 assigned to C−H vibrations in quinoid/ phenyl groups and semiquinone radical cation structure in PANi 5422

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Figure 4. FESEM images of samples RGO-PEDOT (a,b), RGO-PANi (c,d), and RGO-PPy (e).

Figure 5. TEM images of RGO-PEDOT (a,b), RGO-PANi (c,d), and RGO-PPy (e,f).

revealed that substantial aggregation of polymers occurred in the absence of ethanol under otherwise the same experimental conditions (Figure S6 of the Supporting Information). Therefore, it is believed that ethanol assisted the dispersion of RGO sheets in water. This facilitated diffusion of polymer monomers to the nucleation sites and subsequent deposition.

According to the experimental observations, a mechanism accounting for the formation of CP coated RGO sheets is proposed. When a monomer was added to a RGO suspension, a kind of weak charge-transfer complex could be formed because RGO sheets are an electron acceptor and polymer monomers aniline, pyrrole, and 3,4-ethylenedioxythiophene are 5423

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an electron donor.41 Therefore, the monomer could immediately adsorb on the surface of RGO sheets due to the electrostatic interactions. As the dispersion of RGO sheets in water is poor because of the removal of oxygenate groups,15,42 it is difficult to form a homogenerous polymer coating on RGO surfaces in aqueous solutions,25 especially for 3,4-ethylenedioxythiophene with a low water solubility, which would result in the aggregation of the polymer. Ethanol can facilitate the diffusion of polymer monomers to the nucleation sites on RGO. After oxidative polymerization, CP is coated on the RGO surface due to the strong π−π interactions between the CP and the RGO sheets.43,44 The nucleation and polymer growth occur on the surface of RGO sheets at low mass loadings, leading to a uniform coating of the CPs (Figures S3−S5 of the Supporting Information). With increasing the mass loading of polymers (∼80 wt %), a large number of protuberances appeared on the surface of RGO sheets (Figures S3−S5 of the Supporting Information), indicating further growth of CPs along the initial nuclei. Further increasing the mass ratio (∼90%) lead to the formation of PEDOT nanowhiskers on RGO sheets, whereas a porous structure formed for the sample of RGO-PANi. For sample RGO-PPy, the homogenerous growth of PPy on RGO sheets resulted in a smooth surface. Figure 6 shows the cyclic voltammograms (CVs) of RGOPEDOT, RGO-PPy, and RGO-PANi at different scan rates. All

the transport of counterions into and out of the polymer.45 It is also seen that the redox current increased with increasing scan rate, indicating a good rate capability. Kelly et al.46 observed that a PEDOT-carbon composite electrode displayed a distorted rectangular CV, indicating an uncompensated resistance in the system due to the poor electrical conductivity. In the present study, the presence of RGO sheets in the RGO-PEDOT composite may largely reduce the conductive resistance, leading to the observed good capacitive behavior. The CVs of electrode RGO-PPy exhibited a nearly rectangle shape with slight redox peaks. These peaks were due to the oxidation and reduction of PPy. The quasi-rectangle area of each CV curve of RGO-PPy was larger than that of RGO-PEDOT, suggesting a larger capacitance than that of the latter.47,48 The CVs of RGO-PANi (Figure 6c) display two couples of redox peaks. The redox peaks in the range of +0.3 to 0 V were due to the redox transition of PANi forms between the semiconducting-state form (leucoemeraldine) and the conductive form (emeraldine).49,50 Additional oxidation peaks appeared in the range of +0.5 to 0.4 V, which were due to the redox processes associated with the overoxidation of the polymer, followed by hydrolysis to quinone-type species.51 Figure 7 shows the charge/discharge curves of RGO-PEDOT, RGO-PPy, and RGO-PANi, respectively. All curves exhibit

Figure 7. Charge/discharge curves of RGO-PEDOT (a), RGO-PPy (b), and RGO-PANi (c) at different current densities.

Figure 6. Cyclic voltammograms of RGO-PEDOT (a), RGO-PPy (b), and RGO-PANi (c).

equilateral triangle shape, featuring that the potential of charge/ discharge is a linear response to time, indicating a good reversibility during the charge/discharge processes. Obviously, the charge/discharge times are different at a constant current density for electrodes RGO-PEDOT, RGO-PPy, and RGO-PANi,

CV curves observed from electrode RGO-PEDOT exhibit a rectangular shape, suggesting the good capacitive properties of RGO-PEDOT. This good capacitive behavior can be contributed to the successive redox reactions of PEDOT along with 5424

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increased. As we have discussed above, the effective utilization of the active species is crucial in the realization of pseudocapacitance and rate capability of the composite materials. Excess PEDOT coating on RGO sheets may slow down the charge transport to the underlayer of the RGO sheets, leading to a poor rate retention. Therefore, the RGO−CP composites with ∼80 wt % CPs were taken as the samples for investigating their capacitive properties. The instability of CPs during the long-term charge/discharge processes is a detrimental issue in supercapacitor applications.7 PANi fibers were synthesized in the absence of RGO sheets under otherwise the same experimental conditions. (See Figure S8 of the Supporting Information for the morphology of the PANi fibers.) The capacitive retention of the pristine PANi fibers quickly decreased to 68% after 600 charge/discharge cycles (Figure 8). However, under the same measurement conditions,

suggesting different specific capacitances. On the basis of the charge/discharge curves, the specific capacitance (Csp) of a single electrode can be calculated according to the following equation52,53 Csp(F /g ) = I Δt /ΔEm

(1)

where I in amperes is the discharge current, Δt in seconds is the discharge time, m in grams is the mass of the active electrode material, and ΔE in volts is the voltage range. The specific capacitances for electrodes RGO-PEDOT, RGO-PPy, and RGOPANi are summarized in Table 1. It can be seen that at a current Table 1. Specific Capacitances of RGO-PEDOT, RGO-PPy, and RGO-PANi at Different Current Densities specific capacitance (F/g) samples

0.3 A/g

0.5 A/g

1.0 A/g

2.0 A/g

RGO-PEDOT RGO-PPy RGO-PANi

108 249 361

104 229 349

102 204 323

99 189 305

density of 0.3 A/g, electrodes RGO-PEDOT, RGO-PPy, and RGO-PANi had specific capacitances of about 108, 249, and 361 F/g, respectively. When the current density was increased to 2.0 A/g, the specific capacitance of electrode RGO-PANi reached ∼305 F/g. This excellent capacitive behavior of RGO-PANi is ascribed to the good pseudocapacitance of PANi. The rate performance of RGO-PEDOT, RGO-PPy, and RGO-PANi was evaluated by charging/discharging at different current densities (Table 1). For RGO-PEDOT, the specific capacitance decreased to 99 F/g with increasing current density to 2.0 A/g. Electrode RGO-PPy maintained 76% capacitance as the current density increased from 0.3 to 2.0 A/g. The capacitive retention of electrode RGO-PANi was 84% in the same current loading range. These observations revealed that coating of different polymers on RGO sheets led to different electrochemical capacitive performances, which would be understood by the synergic effect of RGO sheets and CPs.16,20,53,54 The uniform coating of CPs on RGO sheets prevents them from aggregating, leading to improved electrical double-layer capacitance. The pseudocapacitance of polymers PEDOT, PPy, and PANi in the composites is believed to be the main contribution to the charge storage. For RGO-PANi, the typical redox peaks were observed on the CVs (Figure 5c). The pseudocapacitances of composite materials were enhanced by the incorporation of good conductive RGO sheets into polymers, which enhanced the redox reaction of polymer component. To investigate the effect of the mass loading of CP in the composites materials on capacitive performance, we tested RGO-PEDOT with PEDOT loadings of 67, 80, 90 wt % as electrode materials, respectively. The CV curves (Figure S7a of the Supporting Information) all exhibited a rectangle shape with a gradual increase in current density with increasing PEDOT content. The charge/discharge curves with an equilateral triangle shape demonstrated that the charge/discharge time increased with increasing PEDOT loading (Figure S7b of the Supporting Information), indicating enhancement of capacitance. The rate retentions of the RGO-PEDOT electrodes with PEDOT loadings of 67, 80, and 90 wt % were estimated to be about 93, 92, and 82%, respectively, when the current density was increased from 0.3 to 2.0 A/g. It can be clearly seen that the rate retention was decreased when the loading of PEDOT was

Figure 8. Cycle stability of PANi fibers, RGO-PANi, RGO-PEDOT, and RGO-PPy during the long-term charge/discharge process.

the capacitive retention of RGO-PANi was ∼82% after 1000 cycles. The capacitive retentions for RGO-PEDOT and RGOPPy were about 88 and 81%, respectively. The results indicate a good cycling ability of the composite materials. The RGO sheets provided a robust support for the CPs, thus enhancing the mechanical strength of the composites and preventing the CPs from swelling and shrinking during the long-term cycling. Therefore, the composite materials exhibited a better stability compared with the pristine polymers.

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CONCLUSIONS In summary, direct coating of CPs PEDOT, PANi, and PPy on RGO sheets was achieved by using an in situ polymerization method in the presence of ethanol. This method provides a simple and efficient way to prepare RGO-based nanocomposites. The electrochemical properties revealed a superior capacitive performance of the RGO-conducting-polymer composites. For example, the specific capacitance of sample RGO-PANi was measured to be as high as 361 F/g at a current density of 0.3 A/g. The capacitive retention was ∼82% after 1000 cycles, which is much better than that of pristine PANi fibers. The good capacitive performance of the RGOconducting-polymer composites is attributed to the contributions of both EDL capacitance and pseudocapacitance, together with the good electrical conductivity of the composites.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed description on XPS, Raman spectra, TEM image, and CV and charge/discharge curves. This material is available free of charge via the Internet at http://pubs.acs.org. 5425

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61-7-33469997. Fax: +61-7-33654199. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Ministry of Education (Tier 2 grant number MOE2008-T2-1-004), The Australian Research Council (ARC) Future Fellow Program (grant number FT100100879), and The University of Queensland is acknowledged.

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dx.doi.org/10.1021/jp211474e | J. Phys. Chem. C 2012, 116, 5420−5426