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multiwalled carbon nanotube film as a high performance electrode material for supercapacitors
Electrochimica Acta 56 (2011) 5115–5121
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors Xiangjun Lu a , Hui Dou a,∗ , Bo Gao a , Changzhou Yuan b , Sudong Yang a , Liang Hao a , Laifa Shen a , Xiaogang Zhang a,∗ a b
College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu, PR China School of Materials Science and Engineering, Anhui University of Technology, Maànshan 243002, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2010 Received in revised form 27 February 2011 Accepted 11 March 2011 Available online 12 April 2011 Keywords: Graphene Carbon nanotube Hybrid film Supercapacitor Flexible electrode
a b s t r a c t A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flowdirected assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g−1 at 0.1 A g−1 and a good rate capability (49% capacity retention at 50 A g−1 ), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical bendable energy conversion/storage devices have received increasing attention in the past ten years for their functional applications in portable electronic devices, including wearable devices, implantable medical devices, mobile phones, roll-up displays, active RFID tags/cards, computers [1–5]. Significantly, compared with conventional energy conversion/storage resources, the flexible freestanding electrode-based manufacturing process can be simplified without involving the conducting additives and binders. Although the emerging field of flexible batteries has the potential to achieve the goal of high energy characteristics in future applications [6–8], an energy source with high-power capability is still necessary. Flexible supercapacitors with desirable properties of large power density, long cycle life, light weight and good operational safety have been proposed as next-generation power devices [9–11]. Making flexible energy conversion/storage devices requires the development of pliable electrode-active materials. Owing to fracture under slight tensile strain, transition metal oxides and conducting polymers are impractical to be incorporated
∗ Corresponding authors. Tel.: +86 025 52112918; fax: +86 025 52112626. E-mail addresses: dh [email protected] (H. Dou), [email protected], [email protected] (X. Zhang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.066
directly into flexible electronic devices [12–15]. Carbon materials such as carbon nanotubes (CNTs) [16–18], carbon fibers [19] and graphene (GN) sheets [20–22] would be better candidates as flexible freestanding electrodes due to their exceptional structural, mechanical and electrical properties. Among them, GN sheets have been considered as one of the most promising flexible electrode materials because of the low cost [23,24], high specific surface area [25] and extraordinary pliable property [26]. Therefore, considerable researches have been done on the preparation of GN film for supercapacitor applications [27–29]. Although a maximum specific capacitance of 140 F g−1 has been achieved [29], GN film exhibits a lower electrochemical capacity than powder-based GN electrodes with specific capacitance values ranging from 191 to 279 F g−1 [30–33]. Consequently, it is significant and necessary to further improve the electrochemical performance of GN film. It has been already confirmed that the specific capacitance of GN sheets is directly correlated to their effective specific surface area [34], that is, GN sheets with less stacking should exhibit better supercapacitive performance. So far, one of the most successful approaches to prepare GN sheets has involved in-solution reduction of graphite oxide (GO) prepared by Hummers method with hydrazine [35–38]. However, this process currently results in severe agglomeration of GN sheets if there is no proper force to prevent. In addition, the strategy is also not befitting to prepare GN film because GO film immersed into the hydrazine aqueous
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solution is broken up to small GN fragments [39]. Recently, Wang et al. reported a novel gas-based hydrazine reduction to restore the conducting carbon network of GN sheets which agglomerate not as severely as that prepared by direct annealing or solution reduction [30]. As a result, a specific capacitance as high as 205 F g−1 is achieved. Significantly, this gas–solid reduction process is suitable for the reduction of GO film to obtain GN film [39]. Recently, the combination of one-dimensional CNTs and two-dimensional GN sheets to design hierarchically structured composites has been extensively studied in lithium ion batteries [40], supercapacitors [41,42] and transparent conductors [43–45]. Great improvements in performance have been observed in these materials as CNTs can bridge the defects for electron transfer and expand the layer distance between GN sheets. Therefore, the inclusion of CNTs is expected to improve the unique potential of GN film as a freestanding electrode for supercapacitors. In this work, we demonstrate the preparation of a GN/MWCNT (16 wt.% MWCNT) film by gas-based hydrazine reduction of the precursor, GO/MWCNT film, obtained by filtration of the complex dispersion of GO and MWCNTs. Electrochemical data demonstrate that the GN/MWCNT film is capable of delivering a specific capacitance of 265 F g−1 at 0.1 A g−1 and a specific capacitance of 130 F g−1 even at 50 A g−1 , and shows a capacitance degradation of 3% after 2000 cycles.
The microstructure of the samples was investigated by scanning electron microscopy (SEM, LEO1530). X-ray diffraction (XRD) measurements were performed on a Bruker D8 advance-X diffrac˚ Fourier transformation tometer with Cu K␣ radiation ( = 1.5418 A). infrared (FTIR) spectra were recorded with a Model 360 Nicolet AVATAR. Thermogravimetric (TG) analysis measurements were carried out under nitrogen flow with a NETZSCH STA 409 PC system TG Analyzer at a heating rate of 10 ◦ C min−1 . Electrical conductivity was measured by the conventional four-probe DC method (SDY-5 Four-Point probe meter). The N2 adsorption–desorption isotherms were obtained on a Micromeritics ASAP 2010 instrument. The samples were degassed for 4 h at 200 ◦ C before measurements. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area of the samples. The pore size distribution curves were obtained from the analysis of N2 desorption isotherms by using the BJH method.
2. Experimental
2.3. Electrochemical measurements
2.1. Preparation of GN/MWCNT film
All electrochemical measurements were investigated by a threeelectrode system equipped with a sample working electrode (5 mg), a standard calomel reference electrode (SCE) and a platinum foil counter electrode. Cyclic voltammetry and galvanostatic charge/discharge tests were performed by a CHI 660 C electrochemical workstation system in 6 M KOH electrolyte.
GO was synthesized from natural graphite flakes by a modified Hummers method [46]. MWCNTs were purchased from Nanotech Port Co., Ltd. (Shenzhen, China) and purified by refluxing them in nitric acid at 25 ◦ C for 4 h to remove the catalyst impurities. The GN/MWCNT film was prepared by the following process: GO (30 mg) was dispersed in distilled water (50 mL) by ultrasonic treatment for 1 h. MWCNTs (3 mg) were added and then sonicated for another 0.5 h. The resulting complex dispersion was filtered by a vacuum filter equipped with a 0.2 m porous PTFE membrane to produce a GO/MWCNT film. Subsequently, gas-based hydrazine was applied in a reaction chamber under ambient conditions for 3 days to reduce the GO in the GO/MWCNT film [30]. After reduction, the film maintained the original integrity and still had good flexibility (Fig. 1), and the mass of the film decreased to 18.7 mg. The 16 wt.% of mass load of MWCNTs in composite was evaluated due to the constant mass of MWCNTs during the reduction process. Parallel experiments were carried out just by changing the amount of MWCNTs in the starting mixture to obtain other GN/MWCNT
Fig. 1. Photograph of the flexible GN/MWCNT film (16 wt.% MWCNTs).
composites with different mass ratio of MWCNTs. For comparison, a pure GN film was prepared in the absence of MWCNTs by the same procedure described above. The average thickness of the GN and GN/MWCNT (16 wt.% MWCNTs) films was ca. 49.3 and 50.1 m, respectively. 2.2. Material characterizations
3. Results and discussion 3.1. GO-assisted dispersion of pristine MWCNTs in water Fig. 2 shows the photograph of the corresponding dispersibility of (a) GO, (b) MWCNTs and (c) GO/MWCNT complex in water. As seen in Fig. 2b, MWCNTs display poor dispersion in water because of their strong van der Waals interactions and high aspect ratio, as noticed by immediate precipitation of the aggregates of MWCNTs at the bottom of cuvette. When the rufous aqueous colloidal suspension of GO is mixed with MWCNTs, the solution turns black after
Fig. 2. Photograph of the corresponding dispersibility of (a) GO, (b) MWCNTs and (c) GO/MWCNT complex in water solution.
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Table 1 Electrical conductivity of GO, GO/MWCNT (10:1), GN and GN/MWCNT (16 wt.% MWCNTs). Samples
Conductivity (S cm−1 )
GO GO/MWCNT GN GN/MWCNT MWCNT
Very low 0.6 2.4 11.5 88.7
sonication (Fig. 2c), indicating that MWCNTs have been dispersed in aqueous media. This can be attributed to that the aromatic regions of GO sheets interact with the sidewalls of MWCNTs through – supramolecular interactions, while the hydrophilic oxygen groups retain the water dispersion of the GO-MWCNT complex [47,48]. The GO/MWCNT suspension remains stable even after two months deposition, which favors the assembly of a highly ordered hierarchical structure under vacuum filtration-induced directional flow [20–22,28]. In this study, it should be noted that the MWCNTs refluxed in nitric acid still maintains the highly graphitized MWCNTs with a negligible amount of defect sites, which has been verified in our previous work [49,50]. Hence, the noncovalent interactions between GO and MWCNTs mentioned above avoid the destruction of the long range conjugation of MWCNTs, which makes MWCNTs retain their high conductivity. MWCNTs have demonstrated their capability as fillers in diverse functional nanocomposites. The enhancement of electrical conductivity by several orders of magnitude at low percolation thresholds (<1 wt.%) of MWCNTs in polymeric and inorganic material has been observed in many literatures [51,52]. Therefore, the pristine MWCNTs can play an important role to enhance the conductivity of composites, which can be confirmed from the conductivity data listed in Table 1.
Fig. 4. TG curves of (a) GO, (b) GN and (c) MWCNT.
Fig. 3 shows the FTIR spectra of (a) GO, (b) GO/MWCNT (10:1), (c) GN and (d) GN/MWCNT (16 wt.% MWCNTs). The FTIR spectra of GO and GO/MWCNT mainly exhibit the characteristic peaks of hydrophilic oxygen groups. The broad peak at 3407 cm−1 is related to O–H stretching vibrations. The peaks at 1728, 1623, 1224 and 1057 cm−1 can be assigned to C O stretching motions of carboxylic acid and carbonyl moieties, C C skeletal vibrations of unoxidized graphitic domains, C–OH stretching vibrations and C–O–C stretching vibrations, respectively. After reduction by hydrazine, the decreasing peaks in intensity at 3407, 1728 and 1623 cm−1 and the absence of the peaks at 1224 and 1057 cm−1 indicate that a
majority of functional groups have been removed. Fig. 4 exhibits the TG curves of (a) GO, (b) GN and (c) MWCNT. GO shows a significant decrease of relative mass due to the loss of adsorbed water and decomposition of the labile oxygenated functional groups. After reduction, GN undergoes a smaller mass loss compared with the GO. However, it still has a mass loss about 20%, revealing that certain oxygenated functional groups remain in carbon sheets, which can not only deliver pseudo-capacitance but also improve the surface wettability of the GN sheets to ensure an efficient utilization of the exposed surfaces for energy storage [30]. The slight massloss region of MWCNT, from 30 to 210 ◦ C, is related to removal of physisorbed water. The XRD patterns of (a) GO, (b) GN, (c) GO/MWCNT (10:1) and (d) GN/MWCNT (16 wt.% MWCNTs) are shown in Fig. 5. The interlayer spacing between the GO sheets is 0.78 nm calculated from (0 0 1) diffraction peak centered at 11.5◦ . For the GO/MWCNT film, (0 0 1) diffraction peak shifts to 10.2◦ and the calculated interlayer spacing between the GO sheets becomes widened (0.87 nm). After reduction, both GN and GN/MWCNT films display a broad reflection peak, revealing that GN sheets are loosely stacked in samples that can be further supported by the following SEM images. The interlayer spacing of GN and GN/MWCNT films was calculated to be 0.34 and 0.36 nm, respectively. From the cross-section SEM images of GO/MWCNT (10:1) film (Fig. 6), a layered structure can be observed at low magnification (Fig. 6a and b), caused by the flow assembly effect of GO during filtration [28]. Close-up view reveals that MWCNTs are uniformly sandwiched between the GO sheets (marked by arrows in
Fig. 3. FTIR spectra of (a) GO, (b) GO/MWCNT (10:1), (c) GN and (d) GN/MWCNT (16 wt.% MWCNTs).
Fig. 5. XRD patterns of (a) GO, (b) GN, (c) GO/MWCNT (10:1) and (d) GN/MWCNT (16 wt.% MWCNTs).
3.2. Structural characterization and morphology analysis
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Fig. 7. Cross-section SEM images of GN/MWCNT (16 wt.% MWCNTs).
Fig. 6. Cross-section SEM images of GO/MWCNT (10:1).
Fig. 6c). After reduction, the obtained GN/MWCNT (16 wt.% MWCNTs) film with low magnification (Fig. 7a) shows obvious expansion in film thickness and a highly loose architecture, similar to the GN sheets prepared by microwave treatment of GO [32]. This should be attributed to the gas intercalation and gas release during reduction process [39]. The GN/MWCNT film also exhibits a layered structure with MWCNTs distributed between the GN sheets (Fig. 7b and c). These results indicate that the GN/MWCNT film produced by the gas–solid reduction process with hydrazine has a lower degree of agglomeration, which is favorable for electrolyte ion diffusing into the electrode to contact the surfaces of GN sheets. For comparison, the cross-section SEM images of the pure GN film are displayed in Fig. 8. Clearly, the obtained GN film shows smaller basal spacing compared with GN/MWCNT due to the absence of MWCNTs. In addition, the specific surface area of the GN and GN/MWCNT samples measured by the N2 absorption BET method is 207 and 261 m2 g−1 , respectively, which further confirms that the rigid MWCNTs between the GN sheets can inhibit the stacking of flex-
ible GN sheets. Fig. 9 presents the (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution of the GN/MWCNT and GN. In terms of shape, the isotherms with pronounced desorption step can be characterized as type IV according to the IUPAC classification, suggesting their mesoporous characteristics. The BJH pore size distributions in Fig. 9b obtained from the N2 desorption isotherms further indicate that the porosity of these materials is essentially made up of mesopores. It is to be pointed out that the GN/MWCNT displays broader pore size distribution in the range of 2–100 nm compared with GN, which should be attributed to the spacer effect of MWCNTs to increase the basal spacing between the GN sheets. 3.3. Electrochemical characterization The specific capacitance of the electrodes can be calculated as: C=
I × t V
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Fig. 9. (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution of GN and GN/MWCNT (16 wt.% MWCNTs).
Fig. 8. Cross-section SEM images of GN.
ment of pristine GN film will be investigated in detail in the next section. The cyclic voltammetry curves of GN and GN/MWCNT (16 wt.% MWCNTs) within the potential window −1 to 0 V at different scan rates are shown in Fig. 11. It can be found that the cyclic voltammetry curves of both electrodes at low scan rates present good mirror images with respect to the zero-current line and symmetric I–E response. As the scan rate increasing, the curves of GN tend to display the shuttle-shaped, as shown in Fig. 11a. However, the curves of GN/MWCNT are still rectanglelike even at scan rate of 100 mV s−1 (Fig. 11b), suggesting a quick charge/discharge process in GN/MWCNT composite film. In addition, compared with GN, the GN/MWCNT has larger voltam-
where C is the specific capacitance of electrode (F g−1 ), I is the discharge current density (A g−1 ), t is the discharge time (s) and V is the potential window (V). In order to obtain the optimum capacitive characteristics of GN/MWCNT composite, the relationship between the specific capacitance and the weight percent of MWCNTs can be studied in Fig. 10. From the curve, it is clear that there is an increase in the specific capacitance of GN/MWCNT composite with increasing amount of MWCNTs and the largest value of 265 F g−1 is obtained with 16 wt.% MWCNTs within the potential window −1 to 0 V at a current density of 0.1 A g−1 . However, further improvement on the weight percent of MWCNTs leads to a decrease in specific capacitance. The less specific capacitance of MWCNTs (26 F g−1 at 0.1 A g−1 ) is believed to be responsible for this deterioration, indicating that GN is the main capacitance source in the GN/MWCNT film. Herein, the optimum loading of MWCNTs in the composite film for the electrochemical performance has been obtained. Therefore, the GN/MWCNT film with 16 wt.% MWCNTs and the contrast experi-
Fig. 10. The specific capacitance of GN/MWCNT composite film with different weight percent of MWCNTs within the potential window −1 to 0 V (vs. SCE) at a current density of 0.1 A g−1 in 6 M KOH.
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Fig. 11. Cyclic voltammetry of (a) GN and (b) GN/MWCNT (16 wt.% MWCNTs) electrodes within the potential window −1 to 0 V (vs. SCE) at different scan rates in 6 M KOH.
metric current response, revealing higher energy storage for this composite. Fig. 12 shows the galvanostatic charge/discharge curves of GN and GN/MWCNT (16 wt.% MWCNTs) at different current densities with voltage between −1 and 0 V. The specific capacitance of 265 F g−1 obtained for the GN/MWCNT composite at the current density of 0.1 A g−1 has a 65 F g−1 higher specific capacitance than that obtained from the GN electrode. The enhanced capacity should be attributed to the unique structure of GN/MWCNT composite with MWCNTs uniformly sandwiched between the GN sheets, which increases electrolyte/electrode contact area and facilitates transportation of electrolyte ion and electron into the inner region of electrode. Compared with the theoretical value of GN (2630 m2 g−1 ) and the values of powdery GN/MWCNT composites (539 m2 g−1 and 700 m2 g−1 ) in the published literatures [53,54], the GN/MWCNT film shows lower specific surface area due to the ordered stacking of GN sheets. However, the GN/MWCNT film possesses satisfactory specific capacitance, which is significantly higher than that of CNT-based flexible electrodes and comparable to that of activated carbon [55–57], ordered mesoporous carbon [58–60]. In addition, the specific capacitance per surface area of GN/MWCNT (1.02 F m−2 ) is higher than that of powdery GN/MWCNT composite (0.61 F m−2 ) [54], suggesting that the GN/MWCNT film electrode may be favorable for energy storage because insulating binders, which add extra contact resistance and reduce the electrochemical reaction rate, are not necessary. Rate capability is a significant parameter in the application and development of supercapacitors in power applications. An excellent flexible energy storage device is required to deliver high energy density at a high charge/discharge rate. Fig. 13 shows the specific capacitances of GN and GN/MWCNT (16 wt.% MWCNTs) electrodes as a function of discharge current density. Impressively, compared
Fig. 12. Galvanostatic charge/discharge curves of (a) GN and (b) GN/MWCNT (16 wt.% MWCNTs) electrodes with voltage between −1 and 0 V (vs. SCE) at different current densities in 6 M KOH.
to GN, the GN/MWCNT film not only displays high specific capacitance values but also retains them well at high current density. Specifically, the specific capacitance of GN/MWCNT film drops 51% (from 265 to 130 F g−1 ) as current density increases from 0.1 to 50 A g−1 , while the pure GN electrode loses 75% of its capacitance (from 200 to 50 F g−1 ) under the same conditions. The improvement of the rate capability of GN/MWCNT film can be attributed to the effective intercalation and distribution of MWCNTs between the GN sheets. Chemical reduction of GO prepared by Hummers method with hydrazine inevitably remains functional groups on the GN sheets, which disrupts the long range conjugation of GN sheets, resulting in the decreased conductivity of GN sheets. However, for the GN/MWCNT film, the existence of pristine MWCNTs can bridge the gaps between GN sheets and facilitate charge-transfer, result-
Fig. 13. Specific capacitance of (a) GN/MWCNT (16 wt.% MWCNTs) and (b) GN electrodes as a function of discharge current density.
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Fig. 14. Cycle-life of GN/MWCNT (16 wt.% MWCNTs) electrode at a constant current density of 6 A g−1 .
ing in a small ohmic drop, especially at high current density. In addition, the wide basal spacing of GN sheets caused by MWCNTs, due to its “ion-buffering reservoirs” [61,62], affords a robust sustentation of electrolyte ions, which could reduce the diffusion and migration length of the electrolyte ions into the interior surfaces of electrode at high current density for energy storage. Long cycling life is another important requirement for practical application. The specific capacitance of the GN/MWCNT (16 wt.% MWCNTs) film as a function of cycle number at a current density of 6 A g−1 is presented in Fig. 14. As shown, the specific capacitance only decreases from 217 F g−1 to 211 F g−1 after 2000 charge/discharge cycles. The specific capacitance retention of 97% indicates the good electrochemical stability of the GN/MWCNT film. 4. Conclusions A simple approach was developed for preparing a flexible GN/MWCNT (16 wt.% MWCNTs) film with MWCNTs uniformly sandwiched between GN sheets. The approach involves gas-based hydrazine reduction of the precursor, GO/MWCNT film, obtained by filtration of the complex dispersion of GO and MWCNTs. GN/MWCNT film shows high specific capacitance (265 F g−1 at 0.1 A g−1 ), good rate capability (49% capacity retention at 50 A g−1 ) and excellent electrochemical stability (3% capacity loss after 2000 cycles). Such performance indicates that the GN/MWCNT film could be an important electrode in the fabrication of flexible energy storage devices. Acknowledgements This work was supported by National Basic Research Program of China (973 Program) (No. 2007CB209703), National Natural Science Foundation of China (No. 20633040, No. 20873064). References [1] M. Kaltenbrunner, G. Kettlgruber, C. Siket, R. Schwödiauer, S. Bauer, Adv. Mater. 22 (2010) 2065. [2] P. Hiralal, S. Imaizumi, H.E. Unalan, H. Matsumoto, M. Minagawa, M. Rouvala, A. Tanioka, G.A.J. Amaratunga, ACS Nano 4 (2010) 2730. [3] L.B. Hu, M. Pasta, F. La-Mantia, L.F. Cui, S. Jeong, H.D. Deshazer, J.W. Choi, S.M. Han, Y. Cui, Nano Lett. 10 (2010) 708. [4] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885. [5] M. Rasouli, L.S.J. Phee, Expert Rev. Med. Devices 7 (2010) 693. [6] S.Y. Chew, S.H. Ng, J.Z. Wang, P. Novak, F. Krumeich, S.L. Chou, J. Chen, H.K. Liu, Carbon 47 (2009) 2976. [7] P. Hiralal, S. Imaizumi, H.E. Unalan, H. Matsumoto, M. Minagawa, M. Rouvala, A. Tanioka, G.A.J. Amaratuga, ACS Nano 4 (2010) 2730.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors Xiangjun Lu a , Hui Dou a,∗ , Bo Gao a , Changzhou Yuan b , Sudong Yang a , Liang Hao a , Laifa Shen a , Xiaogang Zhang a,∗ a b
College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, Jiangsu, PR China School of Materials Science and Engineering, Anhui University of Technology, Maànshan 243002, PR China
a r t i c l e
i n f o
Article history: Received 11 November 2010 Received in revised form 27 February 2011 Accepted 11 March 2011 Available online 12 April 2011 Keywords: Graphene Carbon nanotube Hybrid film Supercapacitor Flexible electrode
a b s t r a c t A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flowdirected assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g−1 at 0.1 A g−1 and a good rate capability (49% capacity retention at 50 A g−1 ), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Mechanical bendable energy conversion/storage devices have received increasing attention in the past ten years for their functional applications in portable electronic devices, including wearable devices, implantable medical devices, mobile phones, roll-up displays, active RFID tags/cards, computers [1–5]. Significantly, compared with conventional energy conversion/storage resources, the flexible freestanding electrode-based manufacturing process can be simplified without involving the conducting additives and binders. Although the emerging field of flexible batteries has the potential to achieve the goal of high energy characteristics in future applications [6–8], an energy source with high-power capability is still necessary. Flexible supercapacitors with desirable properties of large power density, long cycle life, light weight and good operational safety have been proposed as next-generation power devices [9–11]. Making flexible energy conversion/storage devices requires the development of pliable electrode-active materials. Owing to fracture under slight tensile strain, transition metal oxides and conducting polymers are impractical to be incorporated
∗ Corresponding authors. Tel.: +86 025 52112918; fax: +86 025 52112626. E-mail addresses: dh [email protected] (H. Dou), [email protected], [email protected] (X. Zhang). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.066
directly into flexible electronic devices [12–15]. Carbon materials such as carbon nanotubes (CNTs) [16–18], carbon fibers [19] and graphene (GN) sheets [20–22] would be better candidates as flexible freestanding electrodes due to their exceptional structural, mechanical and electrical properties. Among them, GN sheets have been considered as one of the most promising flexible electrode materials because of the low cost [23,24], high specific surface area [25] and extraordinary pliable property [26]. Therefore, considerable researches have been done on the preparation of GN film for supercapacitor applications [27–29]. Although a maximum specific capacitance of 140 F g−1 has been achieved [29], GN film exhibits a lower electrochemical capacity than powder-based GN electrodes with specific capacitance values ranging from 191 to 279 F g−1 [30–33]. Consequently, it is significant and necessary to further improve the electrochemical performance of GN film. It has been already confirmed that the specific capacitance of GN sheets is directly correlated to their effective specific surface area [34], that is, GN sheets with less stacking should exhibit better supercapacitive performance. So far, one of the most successful approaches to prepare GN sheets has involved in-solution reduction of graphite oxide (GO) prepared by Hummers method with hydrazine [35–38]. However, this process currently results in severe agglomeration of GN sheets if there is no proper force to prevent. In addition, the strategy is also not befitting to prepare GN film because GO film immersed into the hydrazine aqueous
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solution is broken up to small GN fragments [39]. Recently, Wang et al. reported a novel gas-based hydrazine reduction to restore the conducting carbon network of GN sheets which agglomerate not as severely as that prepared by direct annealing or solution reduction [30]. As a result, a specific capacitance as high as 205 F g−1 is achieved. Significantly, this gas–solid reduction process is suitable for the reduction of GO film to obtain GN film [39]. Recently, the combination of one-dimensional CNTs and two-dimensional GN sheets to design hierarchically structured composites has been extensively studied in lithium ion batteries [40], supercapacitors [41,42] and transparent conductors [43–45]. Great improvements in performance have been observed in these materials as CNTs can bridge the defects for electron transfer and expand the layer distance between GN sheets. Therefore, the inclusion of CNTs is expected to improve the unique potential of GN film as a freestanding electrode for supercapacitors. In this work, we demonstrate the preparation of a GN/MWCNT (16 wt.% MWCNT) film by gas-based hydrazine reduction of the precursor, GO/MWCNT film, obtained by filtration of the complex dispersion of GO and MWCNTs. Electrochemical data demonstrate that the GN/MWCNT film is capable of delivering a specific capacitance of 265 F g−1 at 0.1 A g−1 and a specific capacitance of 130 F g−1 even at 50 A g−1 , and shows a capacitance degradation of 3% after 2000 cycles.
The microstructure of the samples was investigated by scanning electron microscopy (SEM, LEO1530). X-ray diffraction (XRD) measurements were performed on a Bruker D8 advance-X diffrac˚ Fourier transformation tometer with Cu K␣ radiation ( = 1.5418 A). infrared (FTIR) spectra were recorded with a Model 360 Nicolet AVATAR. Thermogravimetric (TG) analysis measurements were carried out under nitrogen flow with a NETZSCH STA 409 PC system TG Analyzer at a heating rate of 10 ◦ C min−1 . Electrical conductivity was measured by the conventional four-probe DC method (SDY-5 Four-Point probe meter). The N2 adsorption–desorption isotherms were obtained on a Micromeritics ASAP 2010 instrument. The samples were degassed for 4 h at 200 ◦ C before measurements. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area of the samples. The pore size distribution curves were obtained from the analysis of N2 desorption isotherms by using the BJH method.
2. Experimental
2.3. Electrochemical measurements
2.1. Preparation of GN/MWCNT film
All electrochemical measurements were investigated by a threeelectrode system equipped with a sample working electrode (5 mg), a standard calomel reference electrode (SCE) and a platinum foil counter electrode. Cyclic voltammetry and galvanostatic charge/discharge tests were performed by a CHI 660 C electrochemical workstation system in 6 M KOH electrolyte.
GO was synthesized from natural graphite flakes by a modified Hummers method [46]. MWCNTs were purchased from Nanotech Port Co., Ltd. (Shenzhen, China) and purified by refluxing them in nitric acid at 25 ◦ C for 4 h to remove the catalyst impurities. The GN/MWCNT film was prepared by the following process: GO (30 mg) was dispersed in distilled water (50 mL) by ultrasonic treatment for 1 h. MWCNTs (3 mg) were added and then sonicated for another 0.5 h. The resulting complex dispersion was filtered by a vacuum filter equipped with a 0.2 m porous PTFE membrane to produce a GO/MWCNT film. Subsequently, gas-based hydrazine was applied in a reaction chamber under ambient conditions for 3 days to reduce the GO in the GO/MWCNT film [30]. After reduction, the film maintained the original integrity and still had good flexibility (Fig. 1), and the mass of the film decreased to 18.7 mg. The 16 wt.% of mass load of MWCNTs in composite was evaluated due to the constant mass of MWCNTs during the reduction process. Parallel experiments were carried out just by changing the amount of MWCNTs in the starting mixture to obtain other GN/MWCNT
Fig. 1. Photograph of the flexible GN/MWCNT film (16 wt.% MWCNTs).
composites with different mass ratio of MWCNTs. For comparison, a pure GN film was prepared in the absence of MWCNTs by the same procedure described above. The average thickness of the GN and GN/MWCNT (16 wt.% MWCNTs) films was ca. 49.3 and 50.1 m, respectively. 2.2. Material characterizations
3. Results and discussion 3.1. GO-assisted dispersion of pristine MWCNTs in water Fig. 2 shows the photograph of the corresponding dispersibility of (a) GO, (b) MWCNTs and (c) GO/MWCNT complex in water. As seen in Fig. 2b, MWCNTs display poor dispersion in water because of their strong van der Waals interactions and high aspect ratio, as noticed by immediate precipitation of the aggregates of MWCNTs at the bottom of cuvette. When the rufous aqueous colloidal suspension of GO is mixed with MWCNTs, the solution turns black after
Fig. 2. Photograph of the corresponding dispersibility of (a) GO, (b) MWCNTs and (c) GO/MWCNT complex in water solution.
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Table 1 Electrical conductivity of GO, GO/MWCNT (10:1), GN and GN/MWCNT (16 wt.% MWCNTs). Samples
Conductivity (S cm−1 )
GO GO/MWCNT GN GN/MWCNT MWCNT
Very low 0.6 2.4 11.5 88.7
sonication (Fig. 2c), indicating that MWCNTs have been dispersed in aqueous media. This can be attributed to that the aromatic regions of GO sheets interact with the sidewalls of MWCNTs through – supramolecular interactions, while the hydrophilic oxygen groups retain the water dispersion of the GO-MWCNT complex [47,48]. The GO/MWCNT suspension remains stable even after two months deposition, which favors the assembly of a highly ordered hierarchical structure under vacuum filtration-induced directional flow [20–22,28]. In this study, it should be noted that the MWCNTs refluxed in nitric acid still maintains the highly graphitized MWCNTs with a negligible amount of defect sites, which has been verified in our previous work [49,50]. Hence, the noncovalent interactions between GO and MWCNTs mentioned above avoid the destruction of the long range conjugation of MWCNTs, which makes MWCNTs retain their high conductivity. MWCNTs have demonstrated their capability as fillers in diverse functional nanocomposites. The enhancement of electrical conductivity by several orders of magnitude at low percolation thresholds (<1 wt.%) of MWCNTs in polymeric and inorganic material has been observed in many literatures [51,52]. Therefore, the pristine MWCNTs can play an important role to enhance the conductivity of composites, which can be confirmed from the conductivity data listed in Table 1.
Fig. 4. TG curves of (a) GO, (b) GN and (c) MWCNT.
Fig. 3 shows the FTIR spectra of (a) GO, (b) GO/MWCNT (10:1), (c) GN and (d) GN/MWCNT (16 wt.% MWCNTs). The FTIR spectra of GO and GO/MWCNT mainly exhibit the characteristic peaks of hydrophilic oxygen groups. The broad peak at 3407 cm−1 is related to O–H stretching vibrations. The peaks at 1728, 1623, 1224 and 1057 cm−1 can be assigned to C O stretching motions of carboxylic acid and carbonyl moieties, C C skeletal vibrations of unoxidized graphitic domains, C–OH stretching vibrations and C–O–C stretching vibrations, respectively. After reduction by hydrazine, the decreasing peaks in intensity at 3407, 1728 and 1623 cm−1 and the absence of the peaks at 1224 and 1057 cm−1 indicate that a
majority of functional groups have been removed. Fig. 4 exhibits the TG curves of (a) GO, (b) GN and (c) MWCNT. GO shows a significant decrease of relative mass due to the loss of adsorbed water and decomposition of the labile oxygenated functional groups. After reduction, GN undergoes a smaller mass loss compared with the GO. However, it still has a mass loss about 20%, revealing that certain oxygenated functional groups remain in carbon sheets, which can not only deliver pseudo-capacitance but also improve the surface wettability of the GN sheets to ensure an efficient utilization of the exposed surfaces for energy storage [30]. The slight massloss region of MWCNT, from 30 to 210 ◦ C, is related to removal of physisorbed water. The XRD patterns of (a) GO, (b) GN, (c) GO/MWCNT (10:1) and (d) GN/MWCNT (16 wt.% MWCNTs) are shown in Fig. 5. The interlayer spacing between the GO sheets is 0.78 nm calculated from (0 0 1) diffraction peak centered at 11.5◦ . For the GO/MWCNT film, (0 0 1) diffraction peak shifts to 10.2◦ and the calculated interlayer spacing between the GO sheets becomes widened (0.87 nm). After reduction, both GN and GN/MWCNT films display a broad reflection peak, revealing that GN sheets are loosely stacked in samples that can be further supported by the following SEM images. The interlayer spacing of GN and GN/MWCNT films was calculated to be 0.34 and 0.36 nm, respectively. From the cross-section SEM images of GO/MWCNT (10:1) film (Fig. 6), a layered structure can be observed at low magnification (Fig. 6a and b), caused by the flow assembly effect of GO during filtration [28]. Close-up view reveals that MWCNTs are uniformly sandwiched between the GO sheets (marked by arrows in
Fig. 3. FTIR spectra of (a) GO, (b) GO/MWCNT (10:1), (c) GN and (d) GN/MWCNT (16 wt.% MWCNTs).
Fig. 5. XRD patterns of (a) GO, (b) GN, (c) GO/MWCNT (10:1) and (d) GN/MWCNT (16 wt.% MWCNTs).
3.2. Structural characterization and morphology analysis
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Fig. 7. Cross-section SEM images of GN/MWCNT (16 wt.% MWCNTs).
Fig. 6. Cross-section SEM images of GO/MWCNT (10:1).
Fig. 6c). After reduction, the obtained GN/MWCNT (16 wt.% MWCNTs) film with low magnification (Fig. 7a) shows obvious expansion in film thickness and a highly loose architecture, similar to the GN sheets prepared by microwave treatment of GO [32]. This should be attributed to the gas intercalation and gas release during reduction process [39]. The GN/MWCNT film also exhibits a layered structure with MWCNTs distributed between the GN sheets (Fig. 7b and c). These results indicate that the GN/MWCNT film produced by the gas–solid reduction process with hydrazine has a lower degree of agglomeration, which is favorable for electrolyte ion diffusing into the electrode to contact the surfaces of GN sheets. For comparison, the cross-section SEM images of the pure GN film are displayed in Fig. 8. Clearly, the obtained GN film shows smaller basal spacing compared with GN/MWCNT due to the absence of MWCNTs. In addition, the specific surface area of the GN and GN/MWCNT samples measured by the N2 absorption BET method is 207 and 261 m2 g−1 , respectively, which further confirms that the rigid MWCNTs between the GN sheets can inhibit the stacking of flex-
ible GN sheets. Fig. 9 presents the (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution of the GN/MWCNT and GN. In terms of shape, the isotherms with pronounced desorption step can be characterized as type IV according to the IUPAC classification, suggesting their mesoporous characteristics. The BJH pore size distributions in Fig. 9b obtained from the N2 desorption isotherms further indicate that the porosity of these materials is essentially made up of mesopores. It is to be pointed out that the GN/MWCNT displays broader pore size distribution in the range of 2–100 nm compared with GN, which should be attributed to the spacer effect of MWCNTs to increase the basal spacing between the GN sheets. 3.3. Electrochemical characterization The specific capacitance of the electrodes can be calculated as: C=
I × t V
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Fig. 9. (a) N2 adsorption–desorption isotherm and (b) BJH pore size distribution of GN and GN/MWCNT (16 wt.% MWCNTs).
Fig. 8. Cross-section SEM images of GN.
ment of pristine GN film will be investigated in detail in the next section. The cyclic voltammetry curves of GN and GN/MWCNT (16 wt.% MWCNTs) within the potential window −1 to 0 V at different scan rates are shown in Fig. 11. It can be found that the cyclic voltammetry curves of both electrodes at low scan rates present good mirror images with respect to the zero-current line and symmetric I–E response. As the scan rate increasing, the curves of GN tend to display the shuttle-shaped, as shown in Fig. 11a. However, the curves of GN/MWCNT are still rectanglelike even at scan rate of 100 mV s−1 (Fig. 11b), suggesting a quick charge/discharge process in GN/MWCNT composite film. In addition, compared with GN, the GN/MWCNT has larger voltam-
where C is the specific capacitance of electrode (F g−1 ), I is the discharge current density (A g−1 ), t is the discharge time (s) and V is the potential window (V). In order to obtain the optimum capacitive characteristics of GN/MWCNT composite, the relationship between the specific capacitance and the weight percent of MWCNTs can be studied in Fig. 10. From the curve, it is clear that there is an increase in the specific capacitance of GN/MWCNT composite with increasing amount of MWCNTs and the largest value of 265 F g−1 is obtained with 16 wt.% MWCNTs within the potential window −1 to 0 V at a current density of 0.1 A g−1 . However, further improvement on the weight percent of MWCNTs leads to a decrease in specific capacitance. The less specific capacitance of MWCNTs (26 F g−1 at 0.1 A g−1 ) is believed to be responsible for this deterioration, indicating that GN is the main capacitance source in the GN/MWCNT film. Herein, the optimum loading of MWCNTs in the composite film for the electrochemical performance has been obtained. Therefore, the GN/MWCNT film with 16 wt.% MWCNTs and the contrast experi-
Fig. 10. The specific capacitance of GN/MWCNT composite film with different weight percent of MWCNTs within the potential window −1 to 0 V (vs. SCE) at a current density of 0.1 A g−1 in 6 M KOH.
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Fig. 11. Cyclic voltammetry of (a) GN and (b) GN/MWCNT (16 wt.% MWCNTs) electrodes within the potential window −1 to 0 V (vs. SCE) at different scan rates in 6 M KOH.
metric current response, revealing higher energy storage for this composite. Fig. 12 shows the galvanostatic charge/discharge curves of GN and GN/MWCNT (16 wt.% MWCNTs) at different current densities with voltage between −1 and 0 V. The specific capacitance of 265 F g−1 obtained for the GN/MWCNT composite at the current density of 0.1 A g−1 has a 65 F g−1 higher specific capacitance than that obtained from the GN electrode. The enhanced capacity should be attributed to the unique structure of GN/MWCNT composite with MWCNTs uniformly sandwiched between the GN sheets, which increases electrolyte/electrode contact area and facilitates transportation of electrolyte ion and electron into the inner region of electrode. Compared with the theoretical value of GN (2630 m2 g−1 ) and the values of powdery GN/MWCNT composites (539 m2 g−1 and 700 m2 g−1 ) in the published literatures [53,54], the GN/MWCNT film shows lower specific surface area due to the ordered stacking of GN sheets. However, the GN/MWCNT film possesses satisfactory specific capacitance, which is significantly higher than that of CNT-based flexible electrodes and comparable to that of activated carbon [55–57], ordered mesoporous carbon [58–60]. In addition, the specific capacitance per surface area of GN/MWCNT (1.02 F m−2 ) is higher than that of powdery GN/MWCNT composite (0.61 F m−2 ) [54], suggesting that the GN/MWCNT film electrode may be favorable for energy storage because insulating binders, which add extra contact resistance and reduce the electrochemical reaction rate, are not necessary. Rate capability is a significant parameter in the application and development of supercapacitors in power applications. An excellent flexible energy storage device is required to deliver high energy density at a high charge/discharge rate. Fig. 13 shows the specific capacitances of GN and GN/MWCNT (16 wt.% MWCNTs) electrodes as a function of discharge current density. Impressively, compared
Fig. 12. Galvanostatic charge/discharge curves of (a) GN and (b) GN/MWCNT (16 wt.% MWCNTs) electrodes with voltage between −1 and 0 V (vs. SCE) at different current densities in 6 M KOH.
to GN, the GN/MWCNT film not only displays high specific capacitance values but also retains them well at high current density. Specifically, the specific capacitance of GN/MWCNT film drops 51% (from 265 to 130 F g−1 ) as current density increases from 0.1 to 50 A g−1 , while the pure GN electrode loses 75% of its capacitance (from 200 to 50 F g−1 ) under the same conditions. The improvement of the rate capability of GN/MWCNT film can be attributed to the effective intercalation and distribution of MWCNTs between the GN sheets. Chemical reduction of GO prepared by Hummers method with hydrazine inevitably remains functional groups on the GN sheets, which disrupts the long range conjugation of GN sheets, resulting in the decreased conductivity of GN sheets. However, for the GN/MWCNT film, the existence of pristine MWCNTs can bridge the gaps between GN sheets and facilitate charge-transfer, result-
Fig. 13. Specific capacitance of (a) GN/MWCNT (16 wt.% MWCNTs) and (b) GN electrodes as a function of discharge current density.
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Fig. 14. Cycle-life of GN/MWCNT (16 wt.% MWCNTs) electrode at a constant current density of 6 A g−1 .
ing in a small ohmic drop, especially at high current density. In addition, the wide basal spacing of GN sheets caused by MWCNTs, due to its “ion-buffering reservoirs” [61,62], affords a robust sustentation of electrolyte ions, which could reduce the diffusion and migration length of the electrolyte ions into the interior surfaces of electrode at high current density for energy storage. Long cycling life is another important requirement for practical application. The specific capacitance of the GN/MWCNT (16 wt.% MWCNTs) film as a function of cycle number at a current density of 6 A g−1 is presented in Fig. 14. As shown, the specific capacitance only decreases from 217 F g−1 to 211 F g−1 after 2000 charge/discharge cycles. The specific capacitance retention of 97% indicates the good electrochemical stability of the GN/MWCNT film. 4. Conclusions A simple approach was developed for preparing a flexible GN/MWCNT (16 wt.% MWCNTs) film with MWCNTs uniformly sandwiched between GN sheets. The approach involves gas-based hydrazine reduction of the precursor, GO/MWCNT film, obtained by filtration of the complex dispersion of GO and MWCNTs. GN/MWCNT film shows high specific capacitance (265 F g−1 at 0.1 A g−1 ), good rate capability (49% capacity retention at 50 A g−1 ) and excellent electrochemical stability (3% capacity loss after 2000 cycles). Such performance indicates that the GN/MWCNT film could be an important electrode in the fabrication of flexible energy storage devices. Acknowledgements This work was supported by National Basic Research Program of China (973 Program) (No. 2007CB209703), National Natural Science Foundation of China (No. 20633040, No. 20873064). References [1] M. Kaltenbrunner, G. Kettlgruber, C. Siket, R. Schwödiauer, S. Bauer, Adv. Mater. 22 (2010) 2065. [2] P. Hiralal, S. Imaizumi, H.E. Unalan, H. Matsumoto, M. Minagawa, M. Rouvala, A. Tanioka, G.A.J. Amaratunga, ACS Nano 4 (2010) 2730. [3] L.B. Hu, M. Pasta, F. La-Mantia, L.F. Cui, S. Jeong, H.D. Deshazer, J.W. Choi, S.M. Han, Y. Cui, Nano Lett. 10 (2010) 708. [4] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885. [5] M. Rasouli, L.S.J. Phee, Expert Rev. Med. Devices 7 (2010) 693. [6] S.Y. Chew, S.H. Ng, J.Z. Wang, P. Novak, F. Krumeich, S.L. Chou, J. Chen, H.K. Liu, Carbon 47 (2009) 2976. [7] P. Hiralal, S. Imaizumi, H.E. Unalan, H. Matsumoto, M. Minagawa, M. Rouvala, A. Tanioka, G.A.J. Amaratuga, ACS Nano 4 (2010) 2730.
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