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graphene oxide nanoribbon nanocomposite as electrode material for high performance supercapacitors
Synthetic Metals 198 (2014) 188–195
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Enhanced capacitance of one-dimensional polypyrrole/graphene oxide nanoribbon nanocomposite as electrode material for high performance supercapacitors Feng-Hao Hsu, Tzong-Ming Wu * Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402 Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 August 2014 Received in revised form 18 September 2014 Accepted 13 October 2014 Available online xxx
High-performance polypyrrole/graphene oxide nanoribbon (PPy/GONR) nanocomposites are synthesized via in situ chemical oxidation polymerization of pyrrole in the presence of GONR and cetyltrimethylammonium bromide. The molecular structure and morphology of the fabricated nanocomposites are characterized by Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The FESEM and TEM images show a one-dimensional structure for the PPy/GONR nanocomposites. The FTIR spectra of the nanocomposites demonstrate p–p interaction between the PPy backbone and GONR. The electrochemical properties are analyzed by cyclic voltammetry, which reveals that the highest specific capacitance of 747 F g1 is obtained at a scan rate of 5 mV s1. The cyclic stability of these nanocomposites is enhanced and the initial specific capacitance decays by only 12% after 1000 cycles. Galvanostatic charge/discharge and electrochemical impedance spectroscopy results for the PPy/GONR nanocomposites are also discussed. ã 2014 Elsevier B.V. All rights reserved.
Keyword: Polypyrrole Graphene oxide nanoribbon Electrochemical properties Supercapacitors
1. Introduction In recent decades, extensive research has been devoted toward the development of various types of energy-storage devices. Among these, supercapacitors have attracted considerable attention because of their higher power density and lower cost compared to those of conventional capacitors and batteries. According to previous investigations, high specific capacitance and long life cycle are the key performance criteria for a supercapacitor [1–3]. Therefore, obtaining high performance for a supercapacitor is a good challenge and objective in the study of energy storage. Based on their charge-storage mechanism, supercapacitors can be divided into two categories: electrical doublelayer capacitors (EDLCs) and pseudocapacitors. The charge-storage of EDLCs is mainly ascribed to the formation of an electrical double layer at the electrode/electrolyte interface [4]. The higher specific surface areas of carbon materials, such as activated carbon and carbon nanotubes (CNTs), would, therefore, accumulate charge at the electrode/electrolyte interface and these have attracted a lot of interest in EDLC research [5]. Pseudocapacitors, also called faradic capacitors, store charge by fast redox reactions by electrochemical
* Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). http://dx.doi.org/10.1016/j.synthmet.2014.10.016 0379-6779/ ã 2014 Elsevier B.V. All rights reserved.
doping–dedoping in intrinsically conducting polymers and transition-metal oxides [4,6]. Normally, EDLCs exhibit good electrochemical cycle stability, but their capacitance is comparatively low. Most pseudocapacitors exhibit high electrical capacitance but relatively poor cycle stability. Therefore, an effective approach to combine both EDLC and pseudocapacitor components can improve the performance of supercapacitors [3,7]. Carbon materials such as zero-dimensional carbon black, onedimensional CNTs, and two-dimensional graphene can provide high electron transport and reduce device volume, owing to their small sizes and high electrical conductivity [8–10]. In particular, graphene, a monolayer of carbon atoms, can be obtained via mechanical exfoliation of bulk graphite, as reported by Novoselov et al. [11]. Graphene possesses excellent electrical conductivity, thermal stability, specific surface area, and mechanical properties [12–18]. Therefore, it is extensively used in supercapacitors, solar cells, and biosensors [19–24]. Graphene nanoribbon (GNR), a strip of graphene with a high length-to-width ratio and straight edge, is a new morphology of graphene and has attracted a lot of attention in the last few years. Electron confinement within these nanoribbons results in a width dependence of the electronic properties that transform from semimetallic to semiconducting as the width decreases [25]. GNRs have been fabricated by various processes, including exfoliation of bulk graphite, chemical vapor deposition, and plasma etching of multi-walled carbon nanotubes (MWCNT)
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[26–28]. A high-yield and efficient process for production of singlelayer GNRs by chemical oxidation of MWCNT in sulfuric acid was recently reported [29]. This chemical oxidation process is similar to that for the preparation of graphene oxide (GO), so these materials are designated as graphene oxide nanoribbons (GONRs). GONRs also possess oxygen functional groups at their edges and surface, which can be readily dispersed in water or polar organic solvents. In addition, GONRs can be chemically reduced with hydrazine to reduce the number of functional groups and to restore the honeycomb structure. Therefore, the conductivity of chemically reduced GONR can be significantly recovered compared to that of GONR. Intrinsically conducting polymers (ICPs) have recently received a lot of attention because of their good electrical conductivity, physical properties, and high energy-storage capacities. Among ICPs, polypyrrole (PPy) is widely used in electronic devices, sensors, and solid electrodes for capacitors because of its ease of synthesis, high electrical conductivity, and environmental stability [30–32]. The addition of carbon-based materials into ICPs has been extensively investigated with the aim of creating high-performance supercapacitors [33–35]. PPy/CNT and PPy/graphene nanocomposites with good electrocapacity and cycle stability have been used as electrode materials for supercapacitors. Paul et al. prepared a PPy/CNT nanocomposite via in situ polymerization with a specific capacitance of 167 F g1 [9]. Qian et al. prepared a highly dispersed PPy/CNT nanocomposite containing a core/shell structure, which exhibited high specific capacitance of 276.3 F g1 [36]. Xu et al. synthesized a hierarchical PPy/graphene nanocomposite via in situ polymerization, exhibiting excellent electrochemical capacitance of 318.6 F g1 and good cyclic stability [37]. Wang et al. prepared graphene-supported PPy nanoparticles via in situ chemical oxidative polymerization in the presence of sodium dodecyl sulfonate, which served as both surfactant and dopant, with a specific capacitance of 294 F g1 at a current density 10 mA cm2 [38]. Nevertheless, there are no reports of the preparation of one-dimensional GONR-based materials used in supercapacitor applications.
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Herein, we report a simple process to synthesize a onedimensional nanocomposite with high electrochemical capacitance and good cyclic stability. The GONR and intrinsically conducting polymer PPy were selected as the EDLC and pseudocapacitor, respectively, to fabricate one-dimensional PPy/ GONR nanocomposites via in situ chemical oxidation polymerization in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. The formation mechanism of PPy/GONR nanocomposites is shown in Scheme 1. In the present work, the morphology, electrochemical, and capacitance properties of PPy/GONR nanocomposites were also investigated. 2. Experimental 2.1. Materials Multi-wall carbon nanotube was obtained from XinNano Materials. Pyrrole monomer (98%, Aldrich Chemical Co.) was purified by distillation under reduced pressure. Other reagents, including potassium permanganate (KMnO4), iron(III) nitrate, cetyltrimethylammoniumbromide, were used without further purification. 2.2. Preparation of graphene oxide nanoribbon Graphene oxide nanoribbon was prepared by the following method described previously [29]. 0.05 g MWCNT was mixed with 10 mL H2SO4 (98%) and the mixture was stirred for 1 h at room temperature. 0.25 g KMnO4 was added slowly into the mixture and reacted for 1 h at room temperature. Then the deionized (DI) water was very slowly added to the solution at 90 C for 1 h. The color of solution was changed to brown. After 1 h of reaction, 5% H2O2 solution was added into solution and kept it for 24 h. The product was then filtered and washed several times with ethanol and DI water until the solution became acid free. Finally, the product dried under vacuum at 60 C for 24 h.
Scheme 1. Conceptual diagram of the formation mechanism of PPy/GONR nanocomposites.
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2.3. In situ synthesis of PPy/GONR nanocomposite
2.5. Electrochemical measurements
The PPy/GONR nanocomposites were synthesized using in situ chemical oxidative polymerization. The weight ratio of GONR used in this study was 1, 3, 5, 10, 15, and 20 wt%. The resulting PPy/GONR nanocomposites were designated as x%-PPy/GONR, where x means the weight ratio of GONR. In a typical synthesis experiment, various weight ratio of GONR were added into 40 mL 1 M HCl solution in a reaction vessel and sonicated for 1 h. 0.183 g CTAB powder was added into GONR solution and stirred for 30 min at 25 C. After 30 min, 0.08 mL pyrrole monomer was added into GONR dispersion solution and stirred for 30 min. The 0.023 g of iron(III) nitrate was first dissolved in 10 mL 1 M HCl solution and then was dropped into the pyrrole monomer/GONR solution. Therefore, the reaction was carried out for 3 h at 25 C. The PPy/ GONR product was added into large amount of methanol to stop reaction. Finally, the product was filtered and washed with DI water and methanol for several times. The product was dried at 60 C for 24 h.
The electrochemical properties of the active electrodes were assembled into a three-electrode cell system by cyclic voltammetry (CV), galvanostatic charge/discharge test and electrochemical impedance spectroscopy (EIS) using a CHI 6271D electrochemical analysis instrument. The active electrode was prepared according to the following steps: 1 mg composite was dispersed in 2 mL ethanol solution containing 0.01 mL nafion solution by sonication for 1 h. Then, 0.02 ml solution was dropped onto the glassy carbon electrode and dried before electrochemical analysis and using 1 M H2SO4 solution as an electrolyte. Then, the potential range was measured from 0.2 V to 0.8 V (vs. Ag/AgCl) and the platinum was used as counter electrode. Galvanostatic charge/discharge curves were measured in the potential range of 0.2 V to 0.8 V at a current density of 5 A g1. The EIS test was analyzed in the range from 106 to 1 Hz of frequency with alternate current amplitude of 5 mV. The specific capacitance (Csp) of the pure PPy and PPy/GONR nanocomposites was calculated from CV curves by using the following equation [6]: Z IdV C sp ¼ (1) 2mnV
2.4. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a PerkinElmer spectrum one spectrometer with the resolution of 4 cm1. The samples were pressed into tablets with potassium bromide (KBr). Raman spectra were recorded on a Jobin Yvon TRIAX 550 spectrometer with the scan range 1000– 2000 cm1 using He–Ne laser having a wavelength of 633 nm. The morphology was observed by field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM). FESEM measurements were conducted at 3 kV using a JEOL JSM6700F field-emission instrument. The samples for TEM measurement were prepared by casting a drop of the sample suspended in ethanol on a copper grid covered with carbon.
where Csp is the specific capacitance (F g1), n is the potential scan rate (mV s1), m is the mass of active sample on the glassy carbon R electrode, I dV term is the area of CV curve, V is the scan potential range. 3. Results and discussion One-dimensional unzipped GONRs were synthesized using sulfuric acid via a chemical oxidation method. Fig. 1 shows FESEM images of the pristine MWCNTs and GONRs. The MWCNT
Fig. 1. FESEM images (a) MWCNT, (b) GONR, and HRTEM images of (c) MWCNT, (d) GONR.
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diameters were about 40 nm and they had a length of several micrometers. Compared to the pristine MWCNTs, the diameter of unzipped GONRs increased by 90 10 nm. Furthermore, the surface of the MWCNTs was very smooth before chemical oxidation. Closer inspection of HRTEM images of MWCNTs, shown in Fig. 1c, reveals their tube-like structure. After chemical oxidation, the tube-like structure was completely unzipped because the oxidant attacks the MWCNT axial structure to form GONR; therefore, the GONR edges may contain many functional groups [29]. The Raman spectra of MWCNT and GONR are shown in Fig. 2. For the pristine MWCNT, the two bands at about 1320 cm1 and 1580 cm1 are contributed to D-band and G-band, respectively. The D-band is associated with the disorder carbon atoms in the MWCNT structure, and G-band shows the sp2 bonding in a 2-D hexagonal lattice, such as graphite layer structure. It is clearly seen that the ID/IG ratio of GONR increases after chemical oxidation. This result suggested that the MWCNT was subjected to chemical oxidation to form GONR. The FTIR spectra of PPy, MWCNT, GONR, and PPy/GONR nanocomposites are shown in Fig. 3. Compared to MWCNT (curve (i) in Fig. 3), the GONR (curve (h) in Fig. 3) absorption peak located at 1715 cm1 is due to the C¼O stretching vibration, suggesting that there are many oxygen-containing functional groups on the GONR surface after the chemical oxidation process. The absorption band at 1580 cm1 is attributed to the stretching vibration of C¼C bonds in the GONR backbone. For pure PPy, the peak at 1555 cm1 contributed to the C—C stretching vibration in the pyrrole ring. Additionally, the peak at 1050 cm1 is assigned to C—H deformation and two absorption peaks at 1195 and 930 cm1 contribute to the formation of doped PPy [1,39]. Compared to pure PPy, the absorption peak at 1555 cm1 shifted to 1570 cm1 when the GONR was added into PPy. This may be attributed to a p–p interaction between the PPy backbone and GONR [40,41]. Furthermore, the peak intensity of the C¼O stretching vibration increased as the GONR weight ratio increased, indicating that the oxygen-containing functional groups of the nanocomposites increased with increasing GONR content. According to previous investigations, the oxygen-containing functional groups are good electron acceptors and the molecular structure of PPy is a good electron donor [2,38,42,43]. Therefore, the structure of PPy/GONR nanocomposites containing oxygen-containing functional groups could enhance nanocomposite charge-transfer properties. Fig. 4 shows the FESEM and TEM images of pure PPy, 5%-PPy/ GONR, and 20%-PPy/GONR nanocomposites. For pure PPy (Fig. 4a
Fig. 2. Raman spectrum of pristine MWCNT and GONR.
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Fig. 3. FTIR spectrum of PPy/GONR nanocomposites with various weight ratios of (b) 1%, (c) 3%, (d) 5%, (e) 10%, (f) 15%, (g) 20% GONR. For comparison, the spectra of (a) PPy, (h) GONR, and (i) MWCNT were also shown in this figure.
and d), aggregated nanoparticles were observed with diameters in the range of 20–40 nm. As shown in Fig. 4b, aggregates of onedimensional PPy/GONR nanocomposite and PPy nanoparticles were obtained in 5%-PPy/GONR nanocomposite. This morphology is also clearly observed in TEM image, which is shown in Fig. 4e. In Fig. 4c and f, the PPy nanoparticles on the surface of the 20%-PPy/ GONR nanocomposite had almost disappeared. This may be attributed to the high amounts of GONR interacting with more pyrrole monomer to form the one-dimensional PPy/GONR nanocomposite. Additionally, the diameters of 20%-PPy/GONR nanocomposite were larger than those of the 5%-PPy/GONR nanocomposite. This result indicated that the coating of PPy on the GONR surface in 20%-PPy/GONR nanocomposite is nearly uniform. To assess their use in supercapacitor applications, the electrochemical performance of the fabricated composites was analyzed by CV using a three-electrode system. Fig. 5 shows the CV curves of pure PPy and the PPy/GONR nanocomposites at a scan rate of 20 mV s1. It can be seen that the current response and area of the CV curve for the nanocomposites are larger than those of pure PPy, indicating that the electrochemical performance and electric capacitance of PPy/GONR nanocomposites are higher than those of pure PPy. Furthermore, the current response and CV curve area increased as the weight ratio of GONR increased, suggesting that the electrochemical performance and electric capacitance were enhanced as the loading of GONR increased. Fig. 6a and b presents the CV curves of pure PPy and 20%-PPy/GONR nanocomposite, respectively, using scan rates from 5 to 200 mV s1. The CV curves of the 20%-PPy/GONR nanocomposite exhibit an almost rectangular shape at scan rates from 5 to 200 mV s1. In contrast, the CV curves for pure PPy are deformed. These results demonstrate the 20%-PPy/GONR nanocomposite has better charge propagation behavior and ion response than pure PPy [44,45]. The specific capacitances at various scan rates for pure PPy and PPy/GONR nanocomposite are shown in Fig. 7 and the calculated data are summarized in Table 1. The specific capacitance of the 1%PPy/GONR nanocomposite is very close to that of pure PPy at various scan rates. When the weight ratio of GONR increased to 3 wt%, the specific capacitance significantly increased. The specific capacitance increased with increasing weight ratio of GONR and the highest specific capacitance at a scan rate of 5 mV s1 was 747 F g1 for the 20%-PPy/GONR nanocomposite. Compared to pure PPy, the specific capacitance of the 20%-PPy/GONR nanocomposite
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Fig. 4. FESEM images (a) pure PPy, (b) 5%-PPy/GONR, (c) 20%-PPy/GONR, and TEM images of (d) pure PPy, (e) 5%-PPy/GONR, (f) 20%-PPy/GONR.
at a scan rate of 5 mV s1 was almost thirty times higher in magnitude than that of pure PPy. The improvement in specific capacitance was probably due to the following reasons: first, the synergistic effect between PPy and GONR may increase the electrochemical properties of nanocomposites. Second, GONR possesses many oxygen-containing functional groups that enhance the charge-transfer properties. The specific capacitance decreased as the scan rate increased from 5 mV s1 to 200 mV s1. This may be attributed to the electrode material suffering from high iondiffusion resistance and large electrochemical polarization at high scan rates, which decrease the specific capacitance [4,46]. Fig. 8 shows the galvanostatic charge/discharge measurements of pure PPy and PPy/GONR nanocomposites at a current density of 5 A g1. Based on the charge/discharge curve, the specific capacitance can be calculated from the following equation [2]: C sp ¼ I
Fig. 5. CV curves of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios.
Dt m
DV;
(2)
where I is the discharge current (A), Dt is the discharge time (s), DV is the potential range during the discharge process (V), and m is the
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Table 1 Specific capacitance of pure PPy and PPy/GONR nanocomposites at various scan rates. Sample
Pure PPy 1%-PPy/GONR 3%-PPy/GONR 5%-PPy/GONR 10%-PPy/GONR 15%-PPy/GONR 20%-PPy/GONR
Specific capacitance (F g1) at various scan rates 5 mV s1
20 mV s1
50 mV s1
100 mV s1
200 mV s1
26 28 62 123 320 518 747
22 26 59 69 177 425 575
16 17 48 60 139 346 502
12 16 50 65 101 318 415
9 13 44 59 103 317 398
Fig. 8. Galvanostatic charge/discharge curve of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios at a current density of 5 A g1.
mass of electroactive material (g). The charge/discharge time significantly increased with increasing GONR weight ratio, indicating that the calculated specific capacitance increased. The highest specific capacitance at a current density of 5 A g1 was
715 F g1 for the 20%-PPy/GONR nanocomposite. This trend of improved specific capacitance is similar to the CV results, and may be ascribed to the superior charge-transfer properties of PPy/GONR nanocomposites and a synergistic effect between PPy and GONR. Electrochemical impedance tests were conducted to evaluate charge transfer and ion diffusion in the electrode/electrolyte interface [47]. Fig. 9 shows the Nyquist plots of pure PPy, GONR, and 20%-PPy/GONR nanocomposite. The intercepts of real axis (solution resistance, Rs) are 108 V, 6.2 V, and 12.1 V for PPy, GONR,
Fig. 7. Specific capacitance value of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios at scan rates in the range of 5–200 mV s1.
Fig. 9. Nyquist plots from EIS test of pure PPy, GONR, and 20%-PPy/GONR nanocomposites in 1 M H2SO4 solution. The inset is the enlarged Nyquist plots.
Fig. 6. CV curves of (a) pure PPy and (b) 20%-PPy/GONR nanocomposite with various scan rates.
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PPy/GONR nanocomposite are attributed to the synergistic effect between PPy and GONR and good structural stability of GONR. Therefore, we can conclude that the PPy/GONR nanocomposite is a promising material for supercapacitor electrode applications. Acknowledgment The financial support provided by National Science Council through the project NSC102-2221-E-005-093 is greatly appreciated. References
Fig. 10. Cyclic life of pure PPy and 20%-PPy/GONR nanocomposite.
and 20%-PPy/GONR nanocomposites, respectively. In the highfrequency region, the pure PPy shows a larger semicircle than that of 20%-PPy/GONR nanocomposite. Larger semicircles reflect higher interfacial resistance of electro-active materials with poor chargetransfer behavior [5]. The semicircle is not observed in the plots for GONR and 20%-PPy/GONR nanocomposite because of its lower interfacial resistance and better charge-transfer behavior. In addition, the 45 slopped portion at the low-frequency region is the Warburg resistance, which reflects the ion-diffusion resistance and capacitive behavior of the electrode [3,5]. For the GONR and 20%-PPy/GONR nanocomposite, the slope is also steeper than that for pure PPy, indicating better capacitive behavior and lower iondiffusion resistance. Therefore, the incorporation of GONR could improve the charge-transfer properties of PPy/GONR nanocomposites due to the presence of oxygen-containing functional groups. Long life cycle of supercapacitors is a crucial consideration in their application; therefore, the cycle lives of pure PPy and 20%PPy/GONR nanocomposite were investigated by repeating the CV test at a scan rate of 100 mV s1, as shown in Fig. 10. The PPy/GONR nanocomposite exhibits excellent cycle stability and its specific capacitance decreases by only 12% after 1000 cycles, which is lower than that of pure PPy under similar conditions. The decrease in specific capacitance in the cycling life test can be attributed to the swelling and shrinkage of the molecular chain of PPy, which results in the decay of its specific capacitance properties during the repeated CV test. Consequently, this result demonstrates that the presence of GONRs can inhibit the swelling and shrinkage of the PPy molecular chain by the strong p–p interaction between GONRs and PPy, thereby, improving the cycle stability [48–51]. 4. Conclusions In summary, high-performance one-dimensional PPy/GONR nanocomposites were synthesized via in situ chemical oxidation polymerization of pyrrole in the presence of GONR and CTAB surfactant. The MWCNT was subjected to chemical oxidation to unzip its structure and then to form GONR. FESEM images show that the fabricated PPy/GONR composites are one-dimensional structures. The FTIR spectra of the nanocomposites demonstrate the presence of a p–p interaction between PPy and GONR. The specific capacitances of the nanocomposites increase with increasing GONR weight ratio and the highest specific capacitance of 747 F g1 is obtained by CV test at a scan rate of 5 mV s1 for 20%PPy-GONR nanocomposites. The 20%-PPy/GONR nanocomposite retains 88% of its initial specific capacitance after 1000 cycles (i.e., a 12% decay). The capacitance and cyclic stability performance of the
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Enhanced capacitance of one-dimensional polypyrrole/graphene oxide nanoribbon nanocomposite as electrode material for high performance supercapacitors Feng-Hao Hsu, Tzong-Ming Wu * Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Road, Taichung 402 Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 August 2014 Received in revised form 18 September 2014 Accepted 13 October 2014 Available online xxx
High-performance polypyrrole/graphene oxide nanoribbon (PPy/GONR) nanocomposites are synthesized via in situ chemical oxidation polymerization of pyrrole in the presence of GONR and cetyltrimethylammonium bromide. The molecular structure and morphology of the fabricated nanocomposites are characterized by Fourier transform infrared spectroscopy (FTIR), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). The FESEM and TEM images show a one-dimensional structure for the PPy/GONR nanocomposites. The FTIR spectra of the nanocomposites demonstrate p–p interaction between the PPy backbone and GONR. The electrochemical properties are analyzed by cyclic voltammetry, which reveals that the highest specific capacitance of 747 F g1 is obtained at a scan rate of 5 mV s1. The cyclic stability of these nanocomposites is enhanced and the initial specific capacitance decays by only 12% after 1000 cycles. Galvanostatic charge/discharge and electrochemical impedance spectroscopy results for the PPy/GONR nanocomposites are also discussed. ã 2014 Elsevier B.V. All rights reserved.
Keyword: Polypyrrole Graphene oxide nanoribbon Electrochemical properties Supercapacitors
1. Introduction In recent decades, extensive research has been devoted toward the development of various types of energy-storage devices. Among these, supercapacitors have attracted considerable attention because of their higher power density and lower cost compared to those of conventional capacitors and batteries. According to previous investigations, high specific capacitance and long life cycle are the key performance criteria for a supercapacitor [1–3]. Therefore, obtaining high performance for a supercapacitor is a good challenge and objective in the study of energy storage. Based on their charge-storage mechanism, supercapacitors can be divided into two categories: electrical doublelayer capacitors (EDLCs) and pseudocapacitors. The charge-storage of EDLCs is mainly ascribed to the formation of an electrical double layer at the electrode/electrolyte interface [4]. The higher specific surface areas of carbon materials, such as activated carbon and carbon nanotubes (CNTs), would, therefore, accumulate charge at the electrode/electrolyte interface and these have attracted a lot of interest in EDLC research [5]. Pseudocapacitors, also called faradic capacitors, store charge by fast redox reactions by electrochemical
* Corresponding author. Tel.: +886 4 2287 2482; fax: +886 4 2285 7017. E-mail address: [email protected] (T.-M. Wu). http://dx.doi.org/10.1016/j.synthmet.2014.10.016 0379-6779/ ã 2014 Elsevier B.V. All rights reserved.
doping–dedoping in intrinsically conducting polymers and transition-metal oxides [4,6]. Normally, EDLCs exhibit good electrochemical cycle stability, but their capacitance is comparatively low. Most pseudocapacitors exhibit high electrical capacitance but relatively poor cycle stability. Therefore, an effective approach to combine both EDLC and pseudocapacitor components can improve the performance of supercapacitors [3,7]. Carbon materials such as zero-dimensional carbon black, onedimensional CNTs, and two-dimensional graphene can provide high electron transport and reduce device volume, owing to their small sizes and high electrical conductivity [8–10]. In particular, graphene, a monolayer of carbon atoms, can be obtained via mechanical exfoliation of bulk graphite, as reported by Novoselov et al. [11]. Graphene possesses excellent electrical conductivity, thermal stability, specific surface area, and mechanical properties [12–18]. Therefore, it is extensively used in supercapacitors, solar cells, and biosensors [19–24]. Graphene nanoribbon (GNR), a strip of graphene with a high length-to-width ratio and straight edge, is a new morphology of graphene and has attracted a lot of attention in the last few years. Electron confinement within these nanoribbons results in a width dependence of the electronic properties that transform from semimetallic to semiconducting as the width decreases [25]. GNRs have been fabricated by various processes, including exfoliation of bulk graphite, chemical vapor deposition, and plasma etching of multi-walled carbon nanotubes (MWCNT)
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[26–28]. A high-yield and efficient process for production of singlelayer GNRs by chemical oxidation of MWCNT in sulfuric acid was recently reported [29]. This chemical oxidation process is similar to that for the preparation of graphene oxide (GO), so these materials are designated as graphene oxide nanoribbons (GONRs). GONRs also possess oxygen functional groups at their edges and surface, which can be readily dispersed in water or polar organic solvents. In addition, GONRs can be chemically reduced with hydrazine to reduce the number of functional groups and to restore the honeycomb structure. Therefore, the conductivity of chemically reduced GONR can be significantly recovered compared to that of GONR. Intrinsically conducting polymers (ICPs) have recently received a lot of attention because of their good electrical conductivity, physical properties, and high energy-storage capacities. Among ICPs, polypyrrole (PPy) is widely used in electronic devices, sensors, and solid electrodes for capacitors because of its ease of synthesis, high electrical conductivity, and environmental stability [30–32]. The addition of carbon-based materials into ICPs has been extensively investigated with the aim of creating high-performance supercapacitors [33–35]. PPy/CNT and PPy/graphene nanocomposites with good electrocapacity and cycle stability have been used as electrode materials for supercapacitors. Paul et al. prepared a PPy/CNT nanocomposite via in situ polymerization with a specific capacitance of 167 F g1 [9]. Qian et al. prepared a highly dispersed PPy/CNT nanocomposite containing a core/shell structure, which exhibited high specific capacitance of 276.3 F g1 [36]. Xu et al. synthesized a hierarchical PPy/graphene nanocomposite via in situ polymerization, exhibiting excellent electrochemical capacitance of 318.6 F g1 and good cyclic stability [37]. Wang et al. prepared graphene-supported PPy nanoparticles via in situ chemical oxidative polymerization in the presence of sodium dodecyl sulfonate, which served as both surfactant and dopant, with a specific capacitance of 294 F g1 at a current density 10 mA cm2 [38]. Nevertheless, there are no reports of the preparation of one-dimensional GONR-based materials used in supercapacitor applications.
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Herein, we report a simple process to synthesize a onedimensional nanocomposite with high electrochemical capacitance and good cyclic stability. The GONR and intrinsically conducting polymer PPy were selected as the EDLC and pseudocapacitor, respectively, to fabricate one-dimensional PPy/ GONR nanocomposites via in situ chemical oxidation polymerization in the presence of cetyltrimethylammonium bromide (CTAB) surfactant. The formation mechanism of PPy/GONR nanocomposites is shown in Scheme 1. In the present work, the morphology, electrochemical, and capacitance properties of PPy/GONR nanocomposites were also investigated. 2. Experimental 2.1. Materials Multi-wall carbon nanotube was obtained from XinNano Materials. Pyrrole monomer (98%, Aldrich Chemical Co.) was purified by distillation under reduced pressure. Other reagents, including potassium permanganate (KMnO4), iron(III) nitrate, cetyltrimethylammoniumbromide, were used without further purification. 2.2. Preparation of graphene oxide nanoribbon Graphene oxide nanoribbon was prepared by the following method described previously [29]. 0.05 g MWCNT was mixed with 10 mL H2SO4 (98%) and the mixture was stirred for 1 h at room temperature. 0.25 g KMnO4 was added slowly into the mixture and reacted for 1 h at room temperature. Then the deionized (DI) water was very slowly added to the solution at 90 C for 1 h. The color of solution was changed to brown. After 1 h of reaction, 5% H2O2 solution was added into solution and kept it for 24 h. The product was then filtered and washed several times with ethanol and DI water until the solution became acid free. Finally, the product dried under vacuum at 60 C for 24 h.
Scheme 1. Conceptual diagram of the formation mechanism of PPy/GONR nanocomposites.
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2.3. In situ synthesis of PPy/GONR nanocomposite
2.5. Electrochemical measurements
The PPy/GONR nanocomposites were synthesized using in situ chemical oxidative polymerization. The weight ratio of GONR used in this study was 1, 3, 5, 10, 15, and 20 wt%. The resulting PPy/GONR nanocomposites were designated as x%-PPy/GONR, where x means the weight ratio of GONR. In a typical synthesis experiment, various weight ratio of GONR were added into 40 mL 1 M HCl solution in a reaction vessel and sonicated for 1 h. 0.183 g CTAB powder was added into GONR solution and stirred for 30 min at 25 C. After 30 min, 0.08 mL pyrrole monomer was added into GONR dispersion solution and stirred for 30 min. The 0.023 g of iron(III) nitrate was first dissolved in 10 mL 1 M HCl solution and then was dropped into the pyrrole monomer/GONR solution. Therefore, the reaction was carried out for 3 h at 25 C. The PPy/ GONR product was added into large amount of methanol to stop reaction. Finally, the product was filtered and washed with DI water and methanol for several times. The product was dried at 60 C for 24 h.
The electrochemical properties of the active electrodes were assembled into a three-electrode cell system by cyclic voltammetry (CV), galvanostatic charge/discharge test and electrochemical impedance spectroscopy (EIS) using a CHI 6271D electrochemical analysis instrument. The active electrode was prepared according to the following steps: 1 mg composite was dispersed in 2 mL ethanol solution containing 0.01 mL nafion solution by sonication for 1 h. Then, 0.02 ml solution was dropped onto the glassy carbon electrode and dried before electrochemical analysis and using 1 M H2SO4 solution as an electrolyte. Then, the potential range was measured from 0.2 V to 0.8 V (vs. Ag/AgCl) and the platinum was used as counter electrode. Galvanostatic charge/discharge curves were measured in the potential range of 0.2 V to 0.8 V at a current density of 5 A g1. The EIS test was analyzed in the range from 106 to 1 Hz of frequency with alternate current amplitude of 5 mV. The specific capacitance (Csp) of the pure PPy and PPy/GONR nanocomposites was calculated from CV curves by using the following equation [6]: Z IdV C sp ¼ (1) 2mnV
2.4. Characterization Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a PerkinElmer spectrum one spectrometer with the resolution of 4 cm1. The samples were pressed into tablets with potassium bromide (KBr). Raman spectra were recorded on a Jobin Yvon TRIAX 550 spectrometer with the scan range 1000– 2000 cm1 using He–Ne laser having a wavelength of 633 nm. The morphology was observed by field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM). FESEM measurements were conducted at 3 kV using a JEOL JSM6700F field-emission instrument. The samples for TEM measurement were prepared by casting a drop of the sample suspended in ethanol on a copper grid covered with carbon.
where Csp is the specific capacitance (F g1), n is the potential scan rate (mV s1), m is the mass of active sample on the glassy carbon R electrode, I dV term is the area of CV curve, V is the scan potential range. 3. Results and discussion One-dimensional unzipped GONRs were synthesized using sulfuric acid via a chemical oxidation method. Fig. 1 shows FESEM images of the pristine MWCNTs and GONRs. The MWCNT
Fig. 1. FESEM images (a) MWCNT, (b) GONR, and HRTEM images of (c) MWCNT, (d) GONR.
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diameters were about 40 nm and they had a length of several micrometers. Compared to the pristine MWCNTs, the diameter of unzipped GONRs increased by 90 10 nm. Furthermore, the surface of the MWCNTs was very smooth before chemical oxidation. Closer inspection of HRTEM images of MWCNTs, shown in Fig. 1c, reveals their tube-like structure. After chemical oxidation, the tube-like structure was completely unzipped because the oxidant attacks the MWCNT axial structure to form GONR; therefore, the GONR edges may contain many functional groups [29]. The Raman spectra of MWCNT and GONR are shown in Fig. 2. For the pristine MWCNT, the two bands at about 1320 cm1 and 1580 cm1 are contributed to D-band and G-band, respectively. The D-band is associated with the disorder carbon atoms in the MWCNT structure, and G-band shows the sp2 bonding in a 2-D hexagonal lattice, such as graphite layer structure. It is clearly seen that the ID/IG ratio of GONR increases after chemical oxidation. This result suggested that the MWCNT was subjected to chemical oxidation to form GONR. The FTIR spectra of PPy, MWCNT, GONR, and PPy/GONR nanocomposites are shown in Fig. 3. Compared to MWCNT (curve (i) in Fig. 3), the GONR (curve (h) in Fig. 3) absorption peak located at 1715 cm1 is due to the C¼O stretching vibration, suggesting that there are many oxygen-containing functional groups on the GONR surface after the chemical oxidation process. The absorption band at 1580 cm1 is attributed to the stretching vibration of C¼C bonds in the GONR backbone. For pure PPy, the peak at 1555 cm1 contributed to the C—C stretching vibration in the pyrrole ring. Additionally, the peak at 1050 cm1 is assigned to C—H deformation and two absorption peaks at 1195 and 930 cm1 contribute to the formation of doped PPy [1,39]. Compared to pure PPy, the absorption peak at 1555 cm1 shifted to 1570 cm1 when the GONR was added into PPy. This may be attributed to a p–p interaction between the PPy backbone and GONR [40,41]. Furthermore, the peak intensity of the C¼O stretching vibration increased as the GONR weight ratio increased, indicating that the oxygen-containing functional groups of the nanocomposites increased with increasing GONR content. According to previous investigations, the oxygen-containing functional groups are good electron acceptors and the molecular structure of PPy is a good electron donor [2,38,42,43]. Therefore, the structure of PPy/GONR nanocomposites containing oxygen-containing functional groups could enhance nanocomposite charge-transfer properties. Fig. 4 shows the FESEM and TEM images of pure PPy, 5%-PPy/ GONR, and 20%-PPy/GONR nanocomposites. For pure PPy (Fig. 4a
Fig. 2. Raman spectrum of pristine MWCNT and GONR.
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Fig. 3. FTIR spectrum of PPy/GONR nanocomposites with various weight ratios of (b) 1%, (c) 3%, (d) 5%, (e) 10%, (f) 15%, (g) 20% GONR. For comparison, the spectra of (a) PPy, (h) GONR, and (i) MWCNT were also shown in this figure.
and d), aggregated nanoparticles were observed with diameters in the range of 20–40 nm. As shown in Fig. 4b, aggregates of onedimensional PPy/GONR nanocomposite and PPy nanoparticles were obtained in 5%-PPy/GONR nanocomposite. This morphology is also clearly observed in TEM image, which is shown in Fig. 4e. In Fig. 4c and f, the PPy nanoparticles on the surface of the 20%-PPy/ GONR nanocomposite had almost disappeared. This may be attributed to the high amounts of GONR interacting with more pyrrole monomer to form the one-dimensional PPy/GONR nanocomposite. Additionally, the diameters of 20%-PPy/GONR nanocomposite were larger than those of the 5%-PPy/GONR nanocomposite. This result indicated that the coating of PPy on the GONR surface in 20%-PPy/GONR nanocomposite is nearly uniform. To assess their use in supercapacitor applications, the electrochemical performance of the fabricated composites was analyzed by CV using a three-electrode system. Fig. 5 shows the CV curves of pure PPy and the PPy/GONR nanocomposites at a scan rate of 20 mV s1. It can be seen that the current response and area of the CV curve for the nanocomposites are larger than those of pure PPy, indicating that the electrochemical performance and electric capacitance of PPy/GONR nanocomposites are higher than those of pure PPy. Furthermore, the current response and CV curve area increased as the weight ratio of GONR increased, suggesting that the electrochemical performance and electric capacitance were enhanced as the loading of GONR increased. Fig. 6a and b presents the CV curves of pure PPy and 20%-PPy/GONR nanocomposite, respectively, using scan rates from 5 to 200 mV s1. The CV curves of the 20%-PPy/GONR nanocomposite exhibit an almost rectangular shape at scan rates from 5 to 200 mV s1. In contrast, the CV curves for pure PPy are deformed. These results demonstrate the 20%-PPy/GONR nanocomposite has better charge propagation behavior and ion response than pure PPy [44,45]. The specific capacitances at various scan rates for pure PPy and PPy/GONR nanocomposite are shown in Fig. 7 and the calculated data are summarized in Table 1. The specific capacitance of the 1%PPy/GONR nanocomposite is very close to that of pure PPy at various scan rates. When the weight ratio of GONR increased to 3 wt%, the specific capacitance significantly increased. The specific capacitance increased with increasing weight ratio of GONR and the highest specific capacitance at a scan rate of 5 mV s1 was 747 F g1 for the 20%-PPy/GONR nanocomposite. Compared to pure PPy, the specific capacitance of the 20%-PPy/GONR nanocomposite
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Fig. 4. FESEM images (a) pure PPy, (b) 5%-PPy/GONR, (c) 20%-PPy/GONR, and TEM images of (d) pure PPy, (e) 5%-PPy/GONR, (f) 20%-PPy/GONR.
at a scan rate of 5 mV s1 was almost thirty times higher in magnitude than that of pure PPy. The improvement in specific capacitance was probably due to the following reasons: first, the synergistic effect between PPy and GONR may increase the electrochemical properties of nanocomposites. Second, GONR possesses many oxygen-containing functional groups that enhance the charge-transfer properties. The specific capacitance decreased as the scan rate increased from 5 mV s1 to 200 mV s1. This may be attributed to the electrode material suffering from high iondiffusion resistance and large electrochemical polarization at high scan rates, which decrease the specific capacitance [4,46]. Fig. 8 shows the galvanostatic charge/discharge measurements of pure PPy and PPy/GONR nanocomposites at a current density of 5 A g1. Based on the charge/discharge curve, the specific capacitance can be calculated from the following equation [2]: C sp ¼ I
Fig. 5. CV curves of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios.
Dt m
DV;
(2)
where I is the discharge current (A), Dt is the discharge time (s), DV is the potential range during the discharge process (V), and m is the
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Table 1 Specific capacitance of pure PPy and PPy/GONR nanocomposites at various scan rates. Sample
Pure PPy 1%-PPy/GONR 3%-PPy/GONR 5%-PPy/GONR 10%-PPy/GONR 15%-PPy/GONR 20%-PPy/GONR
Specific capacitance (F g1) at various scan rates 5 mV s1
20 mV s1
50 mV s1
100 mV s1
200 mV s1
26 28 62 123 320 518 747
22 26 59 69 177 425 575
16 17 48 60 139 346 502
12 16 50 65 101 318 415
9 13 44 59 103 317 398
Fig. 8. Galvanostatic charge/discharge curve of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios at a current density of 5 A g1.
mass of electroactive material (g). The charge/discharge time significantly increased with increasing GONR weight ratio, indicating that the calculated specific capacitance increased. The highest specific capacitance at a current density of 5 A g1 was
715 F g1 for the 20%-PPy/GONR nanocomposite. This trend of improved specific capacitance is similar to the CV results, and may be ascribed to the superior charge-transfer properties of PPy/GONR nanocomposites and a synergistic effect between PPy and GONR. Electrochemical impedance tests were conducted to evaluate charge transfer and ion diffusion in the electrode/electrolyte interface [47]. Fig. 9 shows the Nyquist plots of pure PPy, GONR, and 20%-PPy/GONR nanocomposite. The intercepts of real axis (solution resistance, Rs) are 108 V, 6.2 V, and 12.1 V for PPy, GONR,
Fig. 7. Specific capacitance value of pure PPy and PPy/GONR nanocomposites with various GONR weight ratios at scan rates in the range of 5–200 mV s1.
Fig. 9. Nyquist plots from EIS test of pure PPy, GONR, and 20%-PPy/GONR nanocomposites in 1 M H2SO4 solution. The inset is the enlarged Nyquist plots.
Fig. 6. CV curves of (a) pure PPy and (b) 20%-PPy/GONR nanocomposite with various scan rates.
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PPy/GONR nanocomposite are attributed to the synergistic effect between PPy and GONR and good structural stability of GONR. Therefore, we can conclude that the PPy/GONR nanocomposite is a promising material for supercapacitor electrode applications. Acknowledgment The financial support provided by National Science Council through the project NSC102-2221-E-005-093 is greatly appreciated. References
Fig. 10. Cyclic life of pure PPy and 20%-PPy/GONR nanocomposite.
and 20%-PPy/GONR nanocomposites, respectively. In the highfrequency region, the pure PPy shows a larger semicircle than that of 20%-PPy/GONR nanocomposite. Larger semicircles reflect higher interfacial resistance of electro-active materials with poor chargetransfer behavior [5]. The semicircle is not observed in the plots for GONR and 20%-PPy/GONR nanocomposite because of its lower interfacial resistance and better charge-transfer behavior. In addition, the 45 slopped portion at the low-frequency region is the Warburg resistance, which reflects the ion-diffusion resistance and capacitive behavior of the electrode [3,5]. For the GONR and 20%-PPy/GONR nanocomposite, the slope is also steeper than that for pure PPy, indicating better capacitive behavior and lower iondiffusion resistance. Therefore, the incorporation of GONR could improve the charge-transfer properties of PPy/GONR nanocomposites due to the presence of oxygen-containing functional groups. Long life cycle of supercapacitors is a crucial consideration in their application; therefore, the cycle lives of pure PPy and 20%PPy/GONR nanocomposite were investigated by repeating the CV test at a scan rate of 100 mV s1, as shown in Fig. 10. The PPy/GONR nanocomposite exhibits excellent cycle stability and its specific capacitance decreases by only 12% after 1000 cycles, which is lower than that of pure PPy under similar conditions. The decrease in specific capacitance in the cycling life test can be attributed to the swelling and shrinkage of the molecular chain of PPy, which results in the decay of its specific capacitance properties during the repeated CV test. Consequently, this result demonstrates that the presence of GONRs can inhibit the swelling and shrinkage of the PPy molecular chain by the strong p–p interaction between GONRs and PPy, thereby, improving the cycle stability [48–51]. 4. Conclusions In summary, high-performance one-dimensional PPy/GONR nanocomposites were synthesized via in situ chemical oxidation polymerization of pyrrole in the presence of GONR and CTAB surfactant. The MWCNT was subjected to chemical oxidation to unzip its structure and then to form GONR. FESEM images show that the fabricated PPy/GONR composites are one-dimensional structures. The FTIR spectra of the nanocomposites demonstrate the presence of a p–p interaction between PPy and GONR. The specific capacitances of the nanocomposites increase with increasing GONR weight ratio and the highest specific capacitance of 747 F g1 is obtained by CV test at a scan rate of 5 mV s1 for 20%PPy-GONR nanocomposites. The 20%-PPy/GONR nanocomposite retains 88% of its initial specific capacitance after 1000 cycles (i.e., a 12% decay). The capacitance and cyclic stability performance of the
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