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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19550 Received 24th September 2013, Accepted 4th October 2013 DOI: 10.1039/c3cp54017k
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Boosting supercapacitor performance of carbon fibres using electrochemically reduced graphene oxide additives† Yachang Cao,a Miao Zhu,ab Peixu Li,c Rujing Zhang,a Xinming Li,ad Qianming Gong,a Kunling Wang,a Minlin Zhong,a Dehai Wu,c Feng Linc and Hongwei Zhu*ab
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Modifying conventional materials with new recipes represents a straightforward yet efficient way to realize large-scale applications of new materials. Electrochemically reduced graphene oxide (ERGO) coated carbon fibres (CFs), prepared as fibre-like supercapacitor electrodes, exhibited excellent electrochemical energy storage performance. Upon addition of only a small amount (B1 wt%) of ERGO, the hybrid fibres showed superior electrochemical capacitances (nearly three orders of magnitude enhanced) compared to pure CFs in both aqueous and gel electrolytes. Meanwhile, the energy density did not decrease notably as the power density increased. The superior capacitive performance could be attributed to the synergistic effect between wrinkled and porous ERGO sheets and highly conductive CFs. This fibre electrode material also offered advantages such as easy operation, mass production capability, mechanical flexibility and robustness, and could have an impact on a wide variety of potential applications in energy and electronic fields.
Introduction Currently, one dimensional (1D) and two dimensional (2D) energy storage devices have attracted great attention due to the revolutionary idea of flexible electronics, which are considered as the future direction of electronic equipment.1–8 Previous studies explored practical possibilities of this novel concept using 1D a
School of Materials Science and Engineering, Lab for Advanced Materials Processing Technology, Tsinghua University, Beijing 100084, P. R. China. E-mail: [email protected] b Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P. R. China c Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China d National Center for Nanoscience and Technology, Zhongguancun, Beijing 100190, P. R. China † Electronic supplementary information (ESI) available: Current–time curve for the electrochemical deposition process (Fig. S1); Raman and XPS spectra of GO and ERGO (Fig. S2 and S3); the Nyquist plot of the impedance of an ERGO@CF electrode (Fig. S4) and length specific capacitance with respect to current density (Fig. S5). See DOI: 10.1039/c3cp54017k
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electrode materials, such as pen ink,9 reduced graphene oxide,3,10 carbon nanotubes,11,12 conductive polymers13,14 and metal oxides.15,16 These 1D electrodes usually took advantage of high specific surface areas of nanomaterials or the Faradaic charge transfer process to enhance the capacitive performance. However, in the real cases of production and application, there are remaining challenges to be addressed, such as capacitive performance, sufficiency of strength and flexibility, especially the capability for mass production. Because of its considerably high specific surface area17 and excellent electrical conductivity,18 graphene has become one of the ideal active materials as a supercapacitor electrode material. However, conventional graphene materials have lower strength than their theoretical ones,10 leading to limited applications of graphene as an electrode material. To address this limitation, certain substrates, such as Au wire,3 Ni fibre,19 and polymethyl methacrylate (PMMA) fibre,6 have been applied to explore the application of graphene coating as a supercapacitor material. However, high cost, poor capacitive behavior and flexibility of these hybrid fibres rendered them far away from being the ideal candidates for commercial purposes. Carbon fibres (CFs) with high strength, sufficient electrical conductivity and acceptable price have been widely used in our daily life. As thought to be unique electrode materials,20 CFs can be used as a basement for graphene coating. Thus, the synthesis of hybrid materials, combining the advantages of CF, a conventional and commercialized carbon material, and graphene, a new carbon nanomaterial, has attracted our attention. Herein, we demonstrated one kind of hybrid fibre as a supercapacitor electrode based on CFs and graphene additives by a synchronous electrochemical reduction and deposition method. The electrochemically reduced graphene oxide coated CF (ERGO@CF) with inherent high tensile strength displayed high double-layer capacitance. The resulting hybrid fibre was flexible as well as could be easily mass-produced, and most importantly shows superior electrochemical energy storage performance (three orders of magnitude enhanced) compared
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Results and discussion
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Preparation of ERGO@CF The process for fabricating ERGO@CF hybrid fibres is shown in Fig. 1A. A certain amount of CFs was immersed in a 3 mg mL 1 graphene oxide (GO) aqueous dispersion containing 0.1 M lithium perchlorate (LiClO4) which was electrochemically deposited at a constant potential of 1.2 V for 10 min (Fig. S1†).10,21–23 In this process, GO sheets were gradually coated on the surface of CFs, meanwhile most oxygen containing functional groups on GO sheets were removed, indicating that GO sheets were electrochemically reduced.3,21 Mass production can be easily carried out by this simple approach. Long ERGO@CF–H hybrid fibres were continuously prepared with a home-made setup using the same processing parameters (Fig. 1B). Electrochemical deposition time can be easily controlled by adjusting the speed of the motor. This method has the potential for large-scale production and applications of the hybrid fibres.
PCCP Characterization of GO and ERGO Scanning electron microscopy (SEM) was employed to characterize the surface structure of ERGO@CF. In Fig. 1C–E, the porous structure of ERGO coating (usually thinner than 200 nm) can be obviously seen. The mass ratio of ERGO is less than 1.0 wt%, statistically estimated from SEM images based on the volume and density of ERGO and CFs. Compared to pure CFs (Fig. 1C), this surface porous structure can cause a considerable increase of the specific surface area, thus greatly enhancing the electrochemical double layer capacitive performance. The chemical structures of GO and ERGO were studied by Raman and X-ray photoelectron spectroscopy (XPS). In Raman spectra of GO and ERGO (Fig. S2†), the ratio of the D band to the G band (ID/IG) is different before and after electrochemical deposition. GO has an ID/IG ratio of about 0.93 (o1), while the exact ratio for ERGO is about 1.15, indicating that more defects formed when oxygenated groups were removed after electrochemical deposition.24 XPS spectra of GO and ERGO provide further evidence that GO sheets have been notably reduced after electrochemical deposition,25 as demonstrated in Fig. S3.† The C/O ratio increases dramatically from 0.48 to 2.30 after electrochemical deposition. Thus, the electrical conduction of
Fig. 1 Preparation and structure of ERGO@CF. (A) Schematic diagram of the process of fabricating ERGO@CF hybrid fibres. CF was electrochemically deposited in the GO aqueous dispersion, in which GO coating and reduction actually took place simultaneously. (B) Schematic illustration and digital photograph of a home-made setup for continuous preparation of long ERGO@CF–H hybrid fibres. 1.2 V was applied using a direct-current (DC) power supply with a graphite rod as the anode and CF as the cathode. (C) SEM image of a single pure CF. (D) SEM image of a single ERGO@CF–H hybrid fibre. The folded, wrinkled structure of ERGO coating can be obviously observed. (E) SEM image of the gully-like porous structure of ERGO on the surface of CF. Scale bars in (C), (D) and (E) are 2 mm, 2 mm and 1 mm, respectively.
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ERGO has been significantly improved after reduction, contributing to the favored capacitive behavior of ERGO additives.
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Capacitive performance of ERGO@CF in an aqueous electrolyte To characterize the capacitive performance of ERGO@CF, it was first tested with a three-electrode system in 1 M Na2SO4 aqueous solution as an electrolyte using a platinum wire as a counter electrode and Ag/AgCl as a reference electrode. The cyclic voltammetry (CV) curves remain approximately rectangular even when the scan rate was up to 10 V s 1 with a potential window from 0 to 1 V (Fig. 2A). This is attributed to the electrochemical double layer mechanism that accessible surface area of ERGO additives is quite large and the charge transfers fast before the interface of the electrode and the electrolyte. The galvanostatic charge–discharge curves are approximately triangular (Fig. 2B), demonstrating ideal double layer capacitive performance of the hybrid fibre.26 The measured length specific capacitance of a single ERGO@CF is B3.4 mF cm 1 at a scan rate of 10 mV s 1, which is over hundred times higher than that of a single pure CF (0.02–0.03 mF cm 1 at the same scan rate). The capacitance remains >95% after 5000 charge– discharge cycles, suggesting excellent capacitive stability of this hybrid fibre electrode (Fig. 2C). The capacitance waves at high values are worth noting, presumably because the ERGO sheets slightly move when nearby electrolyte ions are being transferred.27
Equivalent series resistance (ESR) of a single ERGO@CF in 1 M Na2SO4 aqueous solution is relatively large (Fig. S4†). The problem can be solved if a bundle of CFs are used. Impedance spectroscopy of B250 ERGO@CF with a frequency from 0.01 to 106 Hz shows that the ESR value is less than 10 O cm 1 (Fig. 2D). The phase angle is close to 901 at low frequency, suggesting an ideal capacitive behavior. In order to further enhance the capacitive properties of ERGO@CF, CFs were treated with ultrasound in H2SO4–HNO3 (3 : 1 in volume) for 10 h before electrochemical deposition.28 This process made the CF surface rough and hydrophilic, which was beneficial for the deposition on GO sheets. As a result, a wrinkled ERGO coating could be formed with better quality. The resulting single fibre was denoted as ERGO@CF–H, and the same capacitance characterization as for ERGO@CF was applied. Although the CV curves of a single ERGO@CF–H fibre have a certain deviation from rectangular shape, the area of CV curves continually increases indicating that the capacitance is further increased (Fig. 2E). The length specific capacitance of ERGO@CF–H reaches 22.6 mF cm 1 (scan rate: 10 mV s 1) corresponding to a surface area specific capacitance of 10.3 mF cm 2, which is comparable to those of the previously reported fibre-like supercapacitors.5,9,29 The capacitance still remained 82–93% after being ultrasonically cleaned in deionized water for 10 min, which indirectly indicated that the ERGO additives were stably attached to the surface of CFs.
Fig. 2 Electrochemical performance in an aqueous electrolyte. (A) Cyclic voltammetry (CV) curves of a single ERGO@CF at different scan rates, which remained approximately rectangular even when the scan rate was up to 10 V s 1. (B) Galvanostatic charge–discharge curves of ERGO@CF at different current densities, which are approximately rectangular indicating good capacitive performance. (C) The capacitance stability of ERGO@CF. The capacitance waves at high values, which remained >95% after 5000 cycles. (D) Impedance spectroscopy of B250 ERGO@CF with a frequency from 0.01 Hz to 106 Hz displaying ESR less than 10 O cm 1. (E) CV curves of CF, ERGO@CF and ERGO@CF–H at a scan rate of 100 mV s 1. The capacitances of ERGO@CF and ERGO@CF–H are hundred times higher than that of pure CF due to the high specific surface area beneficial for the porous structure of ERGO. (F) The length specific capacitance increases linearly at different scan rates when more CFs are used.
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Capacitive performance of ERGO@CF–H using a gel electrolyte To explore the practical performance of the hybrid fibres in flexible and wearable devices, we used about 250 CFs (140–160 mm of the overall diameter estimated from optical microscopic images) to be electrochemically deposited with ERGO and characterized as symmetric supercapacitor electrodes using a gel electrolyte (PVA–H3PO4). Two ERGO@CF–H electrodes are coated with the PVA–H3PO4 gel electrolyte, respectively, before combining together with a distance of less than 1 mm comparable to that of cotton threads in clothes (Fig. 3A and B). The total length
PCCP specific capacitance of such a ERGO@CF–H/PVA–H3PO4 supercapacitor is measured to be up to B13.5 mF cm 1 (B307 mF cm 2 calculated using the surface area), which is derived from galvanostatic charge–discharge curves at a current density of 0.05 mA cm 1 (Fig. 3C) and remains B85% of the initial value after 5000 cycles (Fig. 3D). The same phenomenon of slightly fluctuating capacitance is observed as in the above-mentioned aqueous electrolyte. When the current density increases to 0.5 mA cm 1, the overall length specific capacitance decreases to B6 mF cm 1, which is still relatively high (Fig. S5†). Such a charged supercapacitor is capable of lighting up a red lightemitting diode (LED) as shown in the left photograph in Fig. 3B. The long charge–discharge time signifies high capacitance that is attributed to the large surface area for adsorption of electrolyte ions. High electrical conductivity and close connection between the CF and ERGO additives led to a low ESR of B5 O cm 1, which is obtained from the Nyquist plot of the impedance (Fig. 3E). The ESR mainly stems from the electrolyte between
Fig. 3 Preparation and electrochemical performance in a gel electrolyte. (A) Schematic diagram of the process of fabricating a solid ERGO@CF–H/PVA–H3PO4 supercapacitor. (B) The solid fibre-like supercapacitor can light up a LED and can be woven into textiles. The inset digital photograph shows the lighted LED. (C) Galvanostatic charge–discharge curves of a solid ERGO@CF–H/PVA–H3PO4 supercapacitor at different current densities. (D) Cyclic stability of a solid ERGO@CF–H/ PVA–H3PO4 supercapacitor demonstrating that after 5000 cycles the capacitance remains B85% of the initial value. (E) Nyquist plot of the impedance of a solid ERGO@CF–H supercapacitor at the high frequency range with an ESR of B5 O cm 1. (F) Ragone plot of such a supercapacitor, which indicates that the energy density does not decrease significantly when the power density increases. (G) CV curves of such a supercapacitor at a scan rate of 10 mV s 1 at different bending angles, which indicate that the length specific capacitance does not decrease when such a solid supercapacitor is bent. (H) Galvanostatic charge–discharge curves of a single solid ERGO@CF–H/PVA–H3PO4 supercapacitor and two in series at the same current density of 0.1 mA cm 1.
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the electrodes as well as the contact resistance between ERGO and CFs, explaining the IR drop in galvanostatic charge–discharge curves in Fig. 3C and H. Energy density and power density are two key parameters to determine the quality of a capacitor. The energy density of our solid supercapacitor is calculated to be B1.9 mW h cm 1 at a power density of 27.2 mW cm 1 (Fig. 3F). The maximum power density can reach 748.6 mW cm 1 without a great loss of energy density, which decreases to 1.2 mW h cm 1, corresponding to 8.5 mW cm 2 of power density and 13.1 mW h cm 2 of energy density, which is higher than those of fibre-like supercapacitors previously reported by other groups.10,11,15 Flexible or portable electronic devices are required to be bendable and easily packed in series. Our string-like solid supercapacitor is flexible enough to be easily woven into textiles as shown in the right panel of Fig. 3B. The capacitance does not drop when an identically prepared supercapacitor is bended from 01 to 1801 (Fig. 3G). This is attributed to the intrinsic flexible nature of CFs. Two solid ERGO@CF–H/PVA–H3PO4 supercapacitors connected in series have been tested (Fig. 3H). As expected, these two supercapacitors display a charge–discharge potential window of 2 V, which is twice that of a single supercapacitor at the same current density and within nearly the same charge–discharge time. Thus, the supercapacitors can be used in combination appropriately if necessary. Our fibre-like solid supercapacitors exhibit higher energy and power performance than those of previously reported ones, such as GF@3D-G,10 pen ink,9 ZnO nanowires15 and carbon nanotube (CNT) coated CFs11 (Fig. 4). Considering the capacitive performance, intrinsic high strength and great flexibility together, our solid hybrid fibre supercapacitor is one of best options to be produced in large scale and practically used. We have noted that although the mass content of ERGO additives is less than 1.0 wt%, the length specific capacitance of ERGO@CF–H is nearly three orders of magnitude higher than that of pure CFs. A lower amount of ERGO additives will inevitably be less effective while outer sheets will be badly chipped during cycling if the amount of ERGO is excessively abundant. This significant improvement can be attributed to
Fig. 5 Schematic diagram of the mechanism that, by stably attaching ERGO additives, increased specific surface area featuring a unique wrinkled structure for adsorption of electrolyte ions contributing to the excellent electrochemical double-layers. (A) Pure CF electrode. (B) ERGO@CF electrode.
the relatively high specific surface area of ERGO sheets. ERGO sheets, stably attached to the fibre surface, featured a unique wrinkled yet porous structure with increasingly available surface area, which is beneficial for the adsorption of electrolyte ions to produce excellent electrochemical double-layers (Fig. 5).22 With the satisfied electrical conductivity of all the electrode materials and the ability to tightly coat to CF surfaces of ERGO additives, ultrahigh capacitance, low inner resistance, outstanding energy and power properties are thus obtained.
Conclusions In summary, we have developed a simple yet efficient approach for the synchronous electrochemical reduction and deposition of ERGO on CFs as supercapacitor electrodes, which displayed excellent electrochemical performance in both aqueous and gel electrolytes. The specific capacitance of a single hybrid fibre was as high as 22.6 mF cm 1 (10.3 mF cm 2) in an aqueous electrolyte at a scan rate of 10 mV s 1. The specific capacitance of a solid supercapacitor that combined two B250 hybrid fibres together using gel electrolyte was also measured to be up to 13.5 mF cm 1 (307 mF cm 2) at a current density of 0.05 mA cm 1. The maximum power density of such a solid supercapacitor could reach 0.74 mW cm 1 (8.5 mW cm 2) and the maximum energy density was measured to be 1.9 mW h cm 1 (21.4 mW h cm 2), representing the highest values reported thus far. The hybrid fibres also possessed intrinsic high strength and desirable flexibility, which could benefit from the wrinkled and porous structure of ERGO additives. This 1D supercapacitor could be used alone or embedded in 2D and 3D architectures, which might endow it with new promising applications.
Experimental Materials Fig. 4 Comparison of energy and power densities of different fibre-like solid supercapacitors. Our supercapacitor shows clear advantages compared with others.
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Carbon fibres (T700, tensile strength: 4.9 GPa) were purchased from TORAY, and ultrasonically cleaned in deionized water
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before being used. GO materials were purchased from XF-NANO, Inc. A GO aqueous solution (3 mg mL 1) containing 0.1 M LiClO4 was prepared by ultrasonic dissolution for electrochemical deposition. Polyvinyl alcohol (PVA-124) was purchased from Sinopharm Chemical Reagent Co., Ltd.
Acknowledgements
Preparation of ERGO@CF electrodes
Notes and references
A CHI660D electrochemical workstation was used for the electrochemical deposition as well as the characterization of electrochemical performance. A typical current–time curve for the electrochemical deposition process using a 2 cm single CF at a constant potential of 1.2 V for 10 min is shown in Fig. S1.† After electrochemical deposition, ERGO@CF was washed 2–3 times in deionized water to remove residual GO sheets and ions, and then dried naturally for 10 h. Preparation of solid state ERGO@CF–H/PVA–H3PO4 supercapacitors About 250 CFs were firstly ultrasonically treated in H2SO4– HNO3 (3 : 1 in volume) for 10 h, and then ERGO sheets were electrochemically deposited on fibre surfaces using the same parameters just as for a single CF. 2 g polyvinyl alcohol (PVA) and 2 mL H3PO4 were dissolved in 20 mL deionized water. The mixture was stirred at 90 1C and 900 rotations per min for >3 h with a magnetic heating stirrer until it is completely mixed. The as-prepared PVA–H3PO4 gel was used as the solid electrolyte and a separator. SEM, Raman and XPS spectra of GO and ERGO Structure characterization of materials was conducted using a scanning electron microscope (SEM, Leo1530) at an acceleration voltage of 10 kV. Raman spectra were recorded using a Microscopic Confocal Raman Spectrometer (Renishaw RM2000) with a 514.5 nm Ar laser at a power density of 1.2 mW. X-ray photoelectron spectra (XPS) analysis was performed using a PHI-5300 ESCA photoelectron spectrometer. Calculation methods When a single electrode was tested in an aqueous electrolyte, the capacitance (C) was calculated from the CV curve by using the equation: C = S/(2DVn), where S, DV and n represent the area of the CV curve, the potential window and the scan rate, respectively. For a two-electrode system using gel electrolyte PVA–H3PO4, the capacitance C was calculated from the galvanostatic charge– discharge curves by using the equation: C = I/(DV/Dt), where I is the constant current density, DV and Dt are the discharge voltage and the discharge time after the voltage drop at the beginning of discharge, respectively. The length specific capacitance CL was calculated by dividing C with the length L of electrodes: CL = C/L. The surface area specific capacitance Cs was obtained using Cs = C/(2pDL), where D represents the diameter (B140 mm) of one electrode containing B250 hybrid fibres. The energy densities were obtained from the equations: E = CLV2/2 and E = CsV2/2, where V is the operating potential that is 1 V. The power density can be obtained by using the equation: P = E/Dt.
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This work was supported by the Beijing Natural Science Foundation (2122027), the National Program on the Key Basic Research Project (2011CB013000) and the Tsinghua University Initiative Scientific Research Program (2012Z02102).
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Cite this: Phys. Chem. Chem. Phys., 2013, 15, 19550 Received 24th September 2013, Accepted 4th October 2013 DOI: 10.1039/c3cp54017k
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Boosting supercapacitor performance of carbon fibres using electrochemically reduced graphene oxide additives† Yachang Cao,a Miao Zhu,ab Peixu Li,c Rujing Zhang,a Xinming Li,ad Qianming Gong,a Kunling Wang,a Minlin Zhong,a Dehai Wu,c Feng Linc and Hongwei Zhu*ab
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Modifying conventional materials with new recipes represents a straightforward yet efficient way to realize large-scale applications of new materials. Electrochemically reduced graphene oxide (ERGO) coated carbon fibres (CFs), prepared as fibre-like supercapacitor electrodes, exhibited excellent electrochemical energy storage performance. Upon addition of only a small amount (B1 wt%) of ERGO, the hybrid fibres showed superior electrochemical capacitances (nearly three orders of magnitude enhanced) compared to pure CFs in both aqueous and gel electrolytes. Meanwhile, the energy density did not decrease notably as the power density increased. The superior capacitive performance could be attributed to the synergistic effect between wrinkled and porous ERGO sheets and highly conductive CFs. This fibre electrode material also offered advantages such as easy operation, mass production capability, mechanical flexibility and robustness, and could have an impact on a wide variety of potential applications in energy and electronic fields.
Introduction Currently, one dimensional (1D) and two dimensional (2D) energy storage devices have attracted great attention due to the revolutionary idea of flexible electronics, which are considered as the future direction of electronic equipment.1–8 Previous studies explored practical possibilities of this novel concept using 1D a
School of Materials Science and Engineering, Lab for Advanced Materials Processing Technology, Tsinghua University, Beijing 100084, P. R. China. E-mail: [email protected] b Center for Nano and Micro Mechanics, Tsinghua University, Beijing 100084, P. R. China c Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P. R. China d National Center for Nanoscience and Technology, Zhongguancun, Beijing 100190, P. R. China † Electronic supplementary information (ESI) available: Current–time curve for the electrochemical deposition process (Fig. S1); Raman and XPS spectra of GO and ERGO (Fig. S2 and S3); the Nyquist plot of the impedance of an ERGO@CF electrode (Fig. S4) and length specific capacitance with respect to current density (Fig. S5). See DOI: 10.1039/c3cp54017k
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electrode materials, such as pen ink,9 reduced graphene oxide,3,10 carbon nanotubes,11,12 conductive polymers13,14 and metal oxides.15,16 These 1D electrodes usually took advantage of high specific surface areas of nanomaterials or the Faradaic charge transfer process to enhance the capacitive performance. However, in the real cases of production and application, there are remaining challenges to be addressed, such as capacitive performance, sufficiency of strength and flexibility, especially the capability for mass production. Because of its considerably high specific surface area17 and excellent electrical conductivity,18 graphene has become one of the ideal active materials as a supercapacitor electrode material. However, conventional graphene materials have lower strength than their theoretical ones,10 leading to limited applications of graphene as an electrode material. To address this limitation, certain substrates, such as Au wire,3 Ni fibre,19 and polymethyl methacrylate (PMMA) fibre,6 have been applied to explore the application of graphene coating as a supercapacitor material. However, high cost, poor capacitive behavior and flexibility of these hybrid fibres rendered them far away from being the ideal candidates for commercial purposes. Carbon fibres (CFs) with high strength, sufficient electrical conductivity and acceptable price have been widely used in our daily life. As thought to be unique electrode materials,20 CFs can be used as a basement for graphene coating. Thus, the synthesis of hybrid materials, combining the advantages of CF, a conventional and commercialized carbon material, and graphene, a new carbon nanomaterial, has attracted our attention. Herein, we demonstrated one kind of hybrid fibre as a supercapacitor electrode based on CFs and graphene additives by a synchronous electrochemical reduction and deposition method. The electrochemically reduced graphene oxide coated CF (ERGO@CF) with inherent high tensile strength displayed high double-layer capacitance. The resulting hybrid fibre was flexible as well as could be easily mass-produced, and most importantly shows superior electrochemical energy storage performance (three orders of magnitude enhanced) compared
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Results and discussion
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Preparation of ERGO@CF The process for fabricating ERGO@CF hybrid fibres is shown in Fig. 1A. A certain amount of CFs was immersed in a 3 mg mL 1 graphene oxide (GO) aqueous dispersion containing 0.1 M lithium perchlorate (LiClO4) which was electrochemically deposited at a constant potential of 1.2 V for 10 min (Fig. S1†).10,21–23 In this process, GO sheets were gradually coated on the surface of CFs, meanwhile most oxygen containing functional groups on GO sheets were removed, indicating that GO sheets were electrochemically reduced.3,21 Mass production can be easily carried out by this simple approach. Long ERGO@CF–H hybrid fibres were continuously prepared with a home-made setup using the same processing parameters (Fig. 1B). Electrochemical deposition time can be easily controlled by adjusting the speed of the motor. This method has the potential for large-scale production and applications of the hybrid fibres.
PCCP Characterization of GO and ERGO Scanning electron microscopy (SEM) was employed to characterize the surface structure of ERGO@CF. In Fig. 1C–E, the porous structure of ERGO coating (usually thinner than 200 nm) can be obviously seen. The mass ratio of ERGO is less than 1.0 wt%, statistically estimated from SEM images based on the volume and density of ERGO and CFs. Compared to pure CFs (Fig. 1C), this surface porous structure can cause a considerable increase of the specific surface area, thus greatly enhancing the electrochemical double layer capacitive performance. The chemical structures of GO and ERGO were studied by Raman and X-ray photoelectron spectroscopy (XPS). In Raman spectra of GO and ERGO (Fig. S2†), the ratio of the D band to the G band (ID/IG) is different before and after electrochemical deposition. GO has an ID/IG ratio of about 0.93 (o1), while the exact ratio for ERGO is about 1.15, indicating that more defects formed when oxygenated groups were removed after electrochemical deposition.24 XPS spectra of GO and ERGO provide further evidence that GO sheets have been notably reduced after electrochemical deposition,25 as demonstrated in Fig. S3.† The C/O ratio increases dramatically from 0.48 to 2.30 after electrochemical deposition. Thus, the electrical conduction of
Fig. 1 Preparation and structure of ERGO@CF. (A) Schematic diagram of the process of fabricating ERGO@CF hybrid fibres. CF was electrochemically deposited in the GO aqueous dispersion, in which GO coating and reduction actually took place simultaneously. (B) Schematic illustration and digital photograph of a home-made setup for continuous preparation of long ERGO@CF–H hybrid fibres. 1.2 V was applied using a direct-current (DC) power supply with a graphite rod as the anode and CF as the cathode. (C) SEM image of a single pure CF. (D) SEM image of a single ERGO@CF–H hybrid fibre. The folded, wrinkled structure of ERGO coating can be obviously observed. (E) SEM image of the gully-like porous structure of ERGO on the surface of CF. Scale bars in (C), (D) and (E) are 2 mm, 2 mm and 1 mm, respectively.
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ERGO has been significantly improved after reduction, contributing to the favored capacitive behavior of ERGO additives.
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Capacitive performance of ERGO@CF in an aqueous electrolyte To characterize the capacitive performance of ERGO@CF, it was first tested with a three-electrode system in 1 M Na2SO4 aqueous solution as an electrolyte using a platinum wire as a counter electrode and Ag/AgCl as a reference electrode. The cyclic voltammetry (CV) curves remain approximately rectangular even when the scan rate was up to 10 V s 1 with a potential window from 0 to 1 V (Fig. 2A). This is attributed to the electrochemical double layer mechanism that accessible surface area of ERGO additives is quite large and the charge transfers fast before the interface of the electrode and the electrolyte. The galvanostatic charge–discharge curves are approximately triangular (Fig. 2B), demonstrating ideal double layer capacitive performance of the hybrid fibre.26 The measured length specific capacitance of a single ERGO@CF is B3.4 mF cm 1 at a scan rate of 10 mV s 1, which is over hundred times higher than that of a single pure CF (0.02–0.03 mF cm 1 at the same scan rate). The capacitance remains >95% after 5000 charge– discharge cycles, suggesting excellent capacitive stability of this hybrid fibre electrode (Fig. 2C). The capacitance waves at high values are worth noting, presumably because the ERGO sheets slightly move when nearby electrolyte ions are being transferred.27
Equivalent series resistance (ESR) of a single ERGO@CF in 1 M Na2SO4 aqueous solution is relatively large (Fig. S4†). The problem can be solved if a bundle of CFs are used. Impedance spectroscopy of B250 ERGO@CF with a frequency from 0.01 to 106 Hz shows that the ESR value is less than 10 O cm 1 (Fig. 2D). The phase angle is close to 901 at low frequency, suggesting an ideal capacitive behavior. In order to further enhance the capacitive properties of ERGO@CF, CFs were treated with ultrasound in H2SO4–HNO3 (3 : 1 in volume) for 10 h before electrochemical deposition.28 This process made the CF surface rough and hydrophilic, which was beneficial for the deposition on GO sheets. As a result, a wrinkled ERGO coating could be formed with better quality. The resulting single fibre was denoted as ERGO@CF–H, and the same capacitance characterization as for ERGO@CF was applied. Although the CV curves of a single ERGO@CF–H fibre have a certain deviation from rectangular shape, the area of CV curves continually increases indicating that the capacitance is further increased (Fig. 2E). The length specific capacitance of ERGO@CF–H reaches 22.6 mF cm 1 (scan rate: 10 mV s 1) corresponding to a surface area specific capacitance of 10.3 mF cm 2, which is comparable to those of the previously reported fibre-like supercapacitors.5,9,29 The capacitance still remained 82–93% after being ultrasonically cleaned in deionized water for 10 min, which indirectly indicated that the ERGO additives were stably attached to the surface of CFs.
Fig. 2 Electrochemical performance in an aqueous electrolyte. (A) Cyclic voltammetry (CV) curves of a single ERGO@CF at different scan rates, which remained approximately rectangular even when the scan rate was up to 10 V s 1. (B) Galvanostatic charge–discharge curves of ERGO@CF at different current densities, which are approximately rectangular indicating good capacitive performance. (C) The capacitance stability of ERGO@CF. The capacitance waves at high values, which remained >95% after 5000 cycles. (D) Impedance spectroscopy of B250 ERGO@CF with a frequency from 0.01 Hz to 106 Hz displaying ESR less than 10 O cm 1. (E) CV curves of CF, ERGO@CF and ERGO@CF–H at a scan rate of 100 mV s 1. The capacitances of ERGO@CF and ERGO@CF–H are hundred times higher than that of pure CF due to the high specific surface area beneficial for the porous structure of ERGO. (F) The length specific capacitance increases linearly at different scan rates when more CFs are used.
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Capacitive performance of ERGO@CF–H using a gel electrolyte To explore the practical performance of the hybrid fibres in flexible and wearable devices, we used about 250 CFs (140–160 mm of the overall diameter estimated from optical microscopic images) to be electrochemically deposited with ERGO and characterized as symmetric supercapacitor electrodes using a gel electrolyte (PVA–H3PO4). Two ERGO@CF–H electrodes are coated with the PVA–H3PO4 gel electrolyte, respectively, before combining together with a distance of less than 1 mm comparable to that of cotton threads in clothes (Fig. 3A and B). The total length
PCCP specific capacitance of such a ERGO@CF–H/PVA–H3PO4 supercapacitor is measured to be up to B13.5 mF cm 1 (B307 mF cm 2 calculated using the surface area), which is derived from galvanostatic charge–discharge curves at a current density of 0.05 mA cm 1 (Fig. 3C) and remains B85% of the initial value after 5000 cycles (Fig. 3D). The same phenomenon of slightly fluctuating capacitance is observed as in the above-mentioned aqueous electrolyte. When the current density increases to 0.5 mA cm 1, the overall length specific capacitance decreases to B6 mF cm 1, which is still relatively high (Fig. S5†). Such a charged supercapacitor is capable of lighting up a red lightemitting diode (LED) as shown in the left photograph in Fig. 3B. The long charge–discharge time signifies high capacitance that is attributed to the large surface area for adsorption of electrolyte ions. High electrical conductivity and close connection between the CF and ERGO additives led to a low ESR of B5 O cm 1, which is obtained from the Nyquist plot of the impedance (Fig. 3E). The ESR mainly stems from the electrolyte between
Fig. 3 Preparation and electrochemical performance in a gel electrolyte. (A) Schematic diagram of the process of fabricating a solid ERGO@CF–H/PVA–H3PO4 supercapacitor. (B) The solid fibre-like supercapacitor can light up a LED and can be woven into textiles. The inset digital photograph shows the lighted LED. (C) Galvanostatic charge–discharge curves of a solid ERGO@CF–H/PVA–H3PO4 supercapacitor at different current densities. (D) Cyclic stability of a solid ERGO@CF–H/ PVA–H3PO4 supercapacitor demonstrating that after 5000 cycles the capacitance remains B85% of the initial value. (E) Nyquist plot of the impedance of a solid ERGO@CF–H supercapacitor at the high frequency range with an ESR of B5 O cm 1. (F) Ragone plot of such a supercapacitor, which indicates that the energy density does not decrease significantly when the power density increases. (G) CV curves of such a supercapacitor at a scan rate of 10 mV s 1 at different bending angles, which indicate that the length specific capacitance does not decrease when such a solid supercapacitor is bent. (H) Galvanostatic charge–discharge curves of a single solid ERGO@CF–H/PVA–H3PO4 supercapacitor and two in series at the same current density of 0.1 mA cm 1.
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the electrodes as well as the contact resistance between ERGO and CFs, explaining the IR drop in galvanostatic charge–discharge curves in Fig. 3C and H. Energy density and power density are two key parameters to determine the quality of a capacitor. The energy density of our solid supercapacitor is calculated to be B1.9 mW h cm 1 at a power density of 27.2 mW cm 1 (Fig. 3F). The maximum power density can reach 748.6 mW cm 1 without a great loss of energy density, which decreases to 1.2 mW h cm 1, corresponding to 8.5 mW cm 2 of power density and 13.1 mW h cm 2 of energy density, which is higher than those of fibre-like supercapacitors previously reported by other groups.10,11,15 Flexible or portable electronic devices are required to be bendable and easily packed in series. Our string-like solid supercapacitor is flexible enough to be easily woven into textiles as shown in the right panel of Fig. 3B. The capacitance does not drop when an identically prepared supercapacitor is bended from 01 to 1801 (Fig. 3G). This is attributed to the intrinsic flexible nature of CFs. Two solid ERGO@CF–H/PVA–H3PO4 supercapacitors connected in series have been tested (Fig. 3H). As expected, these two supercapacitors display a charge–discharge potential window of 2 V, which is twice that of a single supercapacitor at the same current density and within nearly the same charge–discharge time. Thus, the supercapacitors can be used in combination appropriately if necessary. Our fibre-like solid supercapacitors exhibit higher energy and power performance than those of previously reported ones, such as GF@3D-G,10 pen ink,9 ZnO nanowires15 and carbon nanotube (CNT) coated CFs11 (Fig. 4). Considering the capacitive performance, intrinsic high strength and great flexibility together, our solid hybrid fibre supercapacitor is one of best options to be produced in large scale and practically used. We have noted that although the mass content of ERGO additives is less than 1.0 wt%, the length specific capacitance of ERGO@CF–H is nearly three orders of magnitude higher than that of pure CFs. A lower amount of ERGO additives will inevitably be less effective while outer sheets will be badly chipped during cycling if the amount of ERGO is excessively abundant. This significant improvement can be attributed to
Fig. 5 Schematic diagram of the mechanism that, by stably attaching ERGO additives, increased specific surface area featuring a unique wrinkled structure for adsorption of electrolyte ions contributing to the excellent electrochemical double-layers. (A) Pure CF electrode. (B) ERGO@CF electrode.
the relatively high specific surface area of ERGO sheets. ERGO sheets, stably attached to the fibre surface, featured a unique wrinkled yet porous structure with increasingly available surface area, which is beneficial for the adsorption of electrolyte ions to produce excellent electrochemical double-layers (Fig. 5).22 With the satisfied electrical conductivity of all the electrode materials and the ability to tightly coat to CF surfaces of ERGO additives, ultrahigh capacitance, low inner resistance, outstanding energy and power properties are thus obtained.
Conclusions In summary, we have developed a simple yet efficient approach for the synchronous electrochemical reduction and deposition of ERGO on CFs as supercapacitor electrodes, which displayed excellent electrochemical performance in both aqueous and gel electrolytes. The specific capacitance of a single hybrid fibre was as high as 22.6 mF cm 1 (10.3 mF cm 2) in an aqueous electrolyte at a scan rate of 10 mV s 1. The specific capacitance of a solid supercapacitor that combined two B250 hybrid fibres together using gel electrolyte was also measured to be up to 13.5 mF cm 1 (307 mF cm 2) at a current density of 0.05 mA cm 1. The maximum power density of such a solid supercapacitor could reach 0.74 mW cm 1 (8.5 mW cm 2) and the maximum energy density was measured to be 1.9 mW h cm 1 (21.4 mW h cm 2), representing the highest values reported thus far. The hybrid fibres also possessed intrinsic high strength and desirable flexibility, which could benefit from the wrinkled and porous structure of ERGO additives. This 1D supercapacitor could be used alone or embedded in 2D and 3D architectures, which might endow it with new promising applications.
Experimental Materials Fig. 4 Comparison of energy and power densities of different fibre-like solid supercapacitors. Our supercapacitor shows clear advantages compared with others.
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Carbon fibres (T700, tensile strength: 4.9 GPa) were purchased from TORAY, and ultrasonically cleaned in deionized water
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before being used. GO materials were purchased from XF-NANO, Inc. A GO aqueous solution (3 mg mL 1) containing 0.1 M LiClO4 was prepared by ultrasonic dissolution for electrochemical deposition. Polyvinyl alcohol (PVA-124) was purchased from Sinopharm Chemical Reagent Co., Ltd.
Acknowledgements
Preparation of ERGO@CF electrodes
Notes and references
A CHI660D electrochemical workstation was used for the electrochemical deposition as well as the characterization of electrochemical performance. A typical current–time curve for the electrochemical deposition process using a 2 cm single CF at a constant potential of 1.2 V for 10 min is shown in Fig. S1.† After electrochemical deposition, ERGO@CF was washed 2–3 times in deionized water to remove residual GO sheets and ions, and then dried naturally for 10 h. Preparation of solid state ERGO@CF–H/PVA–H3PO4 supercapacitors About 250 CFs were firstly ultrasonically treated in H2SO4– HNO3 (3 : 1 in volume) for 10 h, and then ERGO sheets were electrochemically deposited on fibre surfaces using the same parameters just as for a single CF. 2 g polyvinyl alcohol (PVA) and 2 mL H3PO4 were dissolved in 20 mL deionized water. The mixture was stirred at 90 1C and 900 rotations per min for >3 h with a magnetic heating stirrer until it is completely mixed. The as-prepared PVA–H3PO4 gel was used as the solid electrolyte and a separator. SEM, Raman and XPS spectra of GO and ERGO Structure characterization of materials was conducted using a scanning electron microscope (SEM, Leo1530) at an acceleration voltage of 10 kV. Raman spectra were recorded using a Microscopic Confocal Raman Spectrometer (Renishaw RM2000) with a 514.5 nm Ar laser at a power density of 1.2 mW. X-ray photoelectron spectra (XPS) analysis was performed using a PHI-5300 ESCA photoelectron spectrometer. Calculation methods When a single electrode was tested in an aqueous electrolyte, the capacitance (C) was calculated from the CV curve by using the equation: C = S/(2DVn), where S, DV and n represent the area of the CV curve, the potential window and the scan rate, respectively. For a two-electrode system using gel electrolyte PVA–H3PO4, the capacitance C was calculated from the galvanostatic charge– discharge curves by using the equation: C = I/(DV/Dt), where I is the constant current density, DV and Dt are the discharge voltage and the discharge time after the voltage drop at the beginning of discharge, respectively. The length specific capacitance CL was calculated by dividing C with the length L of electrodes: CL = C/L. The surface area specific capacitance Cs was obtained using Cs = C/(2pDL), where D represents the diameter (B140 mm) of one electrode containing B250 hybrid fibres. The energy densities were obtained from the equations: E = CLV2/2 and E = CsV2/2, where V is the operating potential that is 1 V. The power density can be obtained by using the equation: P = E/Dt.
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This work was supported by the Beijing Natural Science Foundation (2122027), the National Program on the Key Basic Research Project (2011CB013000) and the Tsinghua University Initiative Scientific Research Program (2012Z02102).
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