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Cite this: New J. Chem., 2017, 41, 11792
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A simple and large-scale method to prepare flexible hollow graphene fibers for a high-performance all-solid fiber supercapacitor† Degang Jiang,a Jizhen Zhang,b Chenwei Li,a Wenrong Yang Jingquan Liu *a
*b and
Herein, we develop a spray deposition process for the production of flexible and conductive hollow graphene fibers (HGFs). Firstly, a graphene oxide suspension is spray-coated on silk fibers, which act as a template, followed by the reduction of GO into RGO using HI as the reductant. This simple method gets rid of the picky conditions and complicated process for the fabrication of graphene fibers (GFs) which possess good flexibility, conductivity and a hollow structure. A flexible all-solid hollow graphene fiber supercapacitor (HGFS) is assembled using the as-prepared HGFs and shows an excellent specific capacitance of 76.1 F g1 (127.4 mF cm2, 48.5 F cm3) at a current density of 1 A g1, excellent rate
Received 8th June 2017, Accepted 30th August 2017
capability (over 87% retention at 5 A g1) and high cycling stability with only 9.5% capacitance decay
DOI: 10.1039/c7nj02042b
over 2000 recycles at a scan rate of 100 mV s1. This simple large-scale template method for the preparation of flexible and conductive HGF electrodes could promise broad prospects for high-performance
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energy storage applications, particularly for next-generation wearable electronic devices.
1. Introduction One-dimensional graphene fibers are a new class of fiber materials with practical importance, which specifically meet the demands for flexible and wearable energy storage devices.1–5 They integrate the unique properties of individual graphene sheets such as high mechanical strength, excellent electrical and thermal conductivities, and light weight.6–11 In order to fulfill these potential applications, a lot of methods have been explored to produce flexible and conductive graphene fibers (GFs). For example, Gao et al. prepared graphene fibers using the wet-spinning method for the fabrication of high-performance graphene fiber supercapacitors,9 graphene coaxial fiber supercapacitors12 and fiber-based asymmetric micro-supercapacitors,7 Peng’s group developed composite fibers based on graphene and carbon nanotubes with high tensile strength, electrical conductivity and electrocatalytic activity. Although these kinds of graphene fibers are favorable candidates for flexible electrodes, wearable optoelectronics and energy devices, the fabrication of high-performance graphene fibers is still a great challenge. Among different preparation methods, such as wet a
College of Materials Science and Engineering, Institute for Graphene Applied Technology Innovation, Qingdao University, Ningxia Road 308, Qingdao 266071, China. E-mail:
[email protected] b School of Life and Environmental Sciences, Deakin University, VIC 3217, Australia. E-mail:
[email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj02042b
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spinning of GO liquid crystal phases,13–21 CVD growth of graphene tubes,22 self-assembly and reduction of GO23,24 and film conversion,25 the wet-spinning process is the most scalable way to produce continuous long graphene fibers. However, three main challenges impede their large scale production. The first one is to achieve continuous production of graphene fibers with uniformity and stability (such as the diameter and profile of their cross-section). Due to the low concentration of the GO suspension, the volume reduction severely affects the final surface appearance of the graphene fibers, which is of great benefit for the production of uniform graphene fibers.20,21,26,27 Secondly, the process of spinning is determined by both the lateral size of the GO sheets and the precise control of the fiber movement speed.19,28,29 The spinnability of the GO suspension is usually considered in the liquid-crystalline spinning theory,30 which means that there is a concentration limit (critical concentration) for a certain-sized GO.29 The third one is the effective control of the structure of graphene fibers for high-performance energy storage applications. The surface morphology and inner structure are critical factors to determine the mechanical, electrical and electrochemical performances of graphene fibers.16,29,31 For example, the tensile strength of graphene fibers could increase from 50 to 350 MPa, just by adopting different coagulation baths.21,29 To find an alternative approach, we developed a simple, stable and scalable spray coating method to prepare continuous, flexible and conductive hollow graphene fibers (HGFs) which were utilized for the fabrication of a flexible all-solid hollow graphene fiber
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supercapacitor (HGFS) with outstanding electrochemical performance. The composition, morphology and properties of the as-prepared HGFs were investigated by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), X-ray diffraction (XRD), and field-emission scanning electron microscopy (SEM). This spray-coating method was convenient to prepare flexible hollow graphene fibers, especially on a large scale. Therefore, the HGFs used as electrodes for the fabrication of high performance energy storage devices offer enormous promising prospects, particularly for next-generation wearable electronics.
2. Experimental part 2.1
Materials
Natural graphite powder (40 mm) was purchased from Qingdao Tianyuan Co., Ltd. Silkworm cocoons were purchased from Taobao.com. Sulfuric acid (AR), phosphoric acid and hydrochloric acid (AR) were purchased from Yantai Shuangshuang Chemical Co., Ltd. Potassium permanganate (AR), sodium carbonate (AR) and hydroiodic acid (AR) were purchased from Sinopharm chemical reagent Co., Ltd. Ethanol and hydrogen peroxide (30%, AR) were purchased from Tianjin Fuyu Chemical Co., Ltd. Poly(vinyl alcohol) (PVA) was purchased from Tianjin Heowns Biochem LLC. 2.2
Pre-treatment of silk fibers
The silkworm cocoons were firstly degummed in a 0.5 wt% Na2CO3 solution at 100 1C for 30 min to remove the sericin on the silk fibers, and then rinsed thoroughly with distilled water three times and dried at 60 1C for 12 hours to obtain purified silk fibers. 2.3
Preparation and purification of GO
A graphene oxide (GO) suspension was prepared by a modified Hummers method.32 Firstly, natural graphite flakes (2 g) were added into concentrated H2SO4 (200 mL), followed by mechanical agitation for 24 hours. KMnO4 (10 g) was slowly added to the mixture which was chilled to 0 1C using an ice bath under continuous stirring for another 24 hours. And then, the ice bath was removed and deionized water (200 mL) was added to the mixture with moderate stirring for 24 hours. After that, H2O2 (30%) solution was added to the mixture until the entire solution became golden yellow. Subsequently, the mixture was rinsed using HCl solution (150 mL) and centrifuged at 5000 rpm 3 times, followed by dialysis using deionized water for 72 hours. Finally, the obtained GO suspension was kept in a fridge at about 0 1C. The quality of the GO was characterized by AFM and TEM, as shown in Fig. S1 and S2 (ESI†). It can be seen that the average size of the GO was found to be about 16 mm. The large-size of the GO sheets should contribute to the outstanding electrical and mechanical properties of the as-prepared hollow graphene fibers.29,33–35
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suspension of graphene oxide (0.2 mg mL1) was injected into a spraying device, followed by spray deposition of the GO dispersion on the silk template to obtain the GO@silk composite fiber (GOF), and then the GOF was kept at room temperature for further drying. When the GOF was completely dried, the as-obtained GOF was reduced by HI at 60 1C for one hour to prepare hollow graphene fibers (HGFs). The HGFs were then washed with distilled water and methanol three times to afford purified HGFs. To fabricate a flexible all-solid hollow fiber supercapacitor (HGFS), two parallel HGFs (2 cm in length) were connected to copper tape with silver paste, and then transferred onto a PET film, coated with a H3PO4/polyvinyl alcohol (PVA)/H2O (1/1/10 in weight) gel electrolyte, dried at room temperature and finally sealed by covering with another strip of PET film. 2.5
Electrochemical measurements
A CHI 760D electrochemical workstation (CHI Instruments Inc., Austin, TX) was used to test and record the electrochemical performance of the all-solid HFGS. Cyclic voltammetry (CV) was measured at scan rates from 2 to 100 mV s1 and the galvanostatic charge–discharge (GCD) test was conducted at current densities from 0.3 to 5 A g1. Electrochemical impedance spectroscopy (EIS) was conducted at open circuit potential with a range of 5 mV for frequencies from 100 kHz to 0.1 Hz. The specific capacitance of the HGFS was calculated according to the galvanostatic discharge curve via the formula: CS ¼
2IDt mDU
where I is the discharge current and m is the total mass for one HGF electrode, Dt is the discharge time and DU is the voltage change during the discharge process.22,36 The energy density and power density of the symmetrical supercapacitor systems were calculated using the following equations: E¼
CDU 2 2 3:6
P¼
E t
2.4 Fabrication of hollow graphene fibers (HGFs) and assembly of the fiber supercapacitor The preparation of flexible and conductive hollow graphene fibers (HGFs) is schematically shown in Scheme 1. Briefly, a
Scheme 1 Illustration of the preparation of flexible and conductive hollow graphene fibers (HGFs) and the fabrication of the all-solid hollow graphene fiber supercapacitor (HGFS).
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where E (W h kg1) is the energy density, P (W kg1) is the power density of the HGFS, and C (F g1) is the specific capacitance of the whole HGFS device, which is equal to CS/4. DU (V) is the voltage window and t (h) is the discharge time.
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2.6
Characterization
A JEOL 6701 field-emission scanning electron microscope (SEM) (JEOL 6701, Japan) was used to characterize the morphology of the as-obtained silk, GOF and HGF samples. The atomic force microscopy (AFM) images of graphene oxide were acquired by using a SPI3800N microscope (Seiko Instruments Inc.) operating in tapping mode. A Perkin-Elmer PHI-5702 multifunctional X-ray photoelectron spectroscope (XPS, Physical Electronics, USA) was used to examine the chemical elements of the as-prepared nature silk, GOF and HGF samples. X-ray diffraction (XRD) measurements of the nature silk, GOF and HGF samples were measured with CuKa radiation (Rigaku D/max-2500, l = 1.5405 Å). The Fourier transform infrared (FTIR) and Raman spectra of the silk, GOF and HGF samples were recorded on a Nicolet 5700 FTIR spectrometer with an FT-Raman module (Thermo Electron Corporation, USA).
3. Results and discussion 3.1 Preparation of the GO@silk composite fiber (GOF) and hollow graphene fibers (HGFs) Natural silk, which is spun by domesticated Bombyx mori, is well known and has been used for at least 5000 years due to its wide applications such as in textile fibers. Recently, investigative interest in natural silk has been aroused due to its excellent mechanical properties and biocompatibility.37 Herein, HGFs were produced by spray deposition of a GO dispersion onto natural silk. As shown in Scheme 1, a solution of graphene oxide (0.2 mg mL1) was spray-coated onto the natural silk (Fig. 1a), which was applied as a scaffold for surface self-assembly of GO sheets. The as-prepared GO@silk composite fiber (GOF) was then kept at room temperature for further drying. The asobtained GOF was reduced by HI at 60 1C for one hour to obtain the HGFs. The GOF and HGFs were first characterized by
Fig. 1 (a) SEM image of the natural silk. The inset is a magnified image of one single silk fiber. (b) SEM image of the GO@silk composite fiber (GOF), (c) SEM image of a fractured GOF, (d) SEM image of a hollow graphene fiber (HGF) and (e) SEM image of the surface of the HGF at high magnification. (f) Cross-sectional SEM image of the HGF.
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scanning electron microscopy (SEM). The SEM image of a typical as-prepared GOF is shown in Fig. 1b, in which the slightly rough surface of the GOF is observed. When the GOF is fractured, the cut surface is shown in Fig. 1c, the interior part is the silk template and the external part is the GO layer, which confirms that the GO sheets have been successfully coated on the silk surface. In the reduction process, HI acted as a favorable reductant which could effectively remove the oxygen-containing groups from the GO sample.38 In order to compare the reductive effect of various reducing agents, three control experiments were carried out. Fig. S2 (ESI†) shows the visual changes of the GO films immersed in different reductive agents for different times. As shown in Fig. S2a (ESI†), the GO films maintained the original shape when they were reduced in HI and N2H4 H2O. However, when the GO films were reduced in a NaBH4 aqueous solution, numerous bubbles could be observed and the GO films started to disintegrate. After two hours of reduction, the GO films immersed in N2H4 solution were broken up and then floated on the solution surface (Fig. S2b, ESI†). But the GO films which were immersed in HI acid remained intact and no bubbles were observed. Subsequently, the bottles were roughly shaken to test the stability of the RGO films (Fig. S2c, ESI†). Obviously, the films immersed in both the NaBH4 and N2H4 aqueous solutions were broken into small graphene fragments, while the films immersed in the HI solution maintained their integrity. Therefore, HI could not only efficiently reduce the GO films but also protect them from breaking. More importantly, as HI is a kind of strong acid causing corrosion, the internal silk of the GOF can be corroded by HI to obtain a hollow structure during the reductive process. Fig. S3 (ESI†) shows a mat of pure silk before and after immersion in HI for 1 h at 60 1C. As shown in Fig. S3a–d (ESI†), the immersed part of the pure silk disappeared after 1 h, which confirmed that silk can be etched in the presence of HI. All of these results reveal that HI is a favorable reductant to prepare hollow graphene fibers. An SEM image of a HGF is shown in Fig. 1d, in which it can be seen that the HGF shares a similar shape to the GOF. A magnified SEM image of the HGF is shown in Fig. 1e. The surface of the HGF is more rough and wrinkled than that of the GOF, which increases the specific area of electric double layers for the supercapacitor. As shown in Fig. 1f, some obvious interior holes could be seen in the cross-sectional SEM image of the HGF, which might have resulted from the corrosion of silk during the reductive process. In the high-resolution TEM (HRTEM) images of a cross-sectional HGF shown in Fig. S5 (ESI†), the layered structure which is orderly stacked due to the p–p interactions among the graphene sheets could be observed, and the interlayer spacing between two graphene sheets is about 0.38 nm. In order to further confirm that the hollow structure is caused by HI, the side-sprayed GO@silk composite was reduced by HI. As shown in Fig. S6(a and b) (ESI†), only the RGO shell can be observed, revealing that the hollow structure of the HGF was successfully generated by the removal of the silk template via the corrosion process by HI.
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3.2 Characterization of the natural silk fiber, GO@silk composite fiber (GOF) and hollow graphene fibers (HGFs) The XPS spectra of the GOF and HGFs are shown in Fig. 2(a and b). Before reduction (Fig. 2a), the XPS spectra of the GOF showed five different peaks at 285.4, 284.5, 286.6, 287.8, and 288.8 eV, corresponding to C–C, CQC in aromatic rings, C–O (epoxy and alkoxy), CQO, and O–CQO groups, respectively. After reduction with HI (Fig. 2b), although similar peaks were observed in the spectra of the HGFs, the intensities of C–O (epoxy and alkoxy), CQO, and O–CQO groups decreased dramatically, revealing that most oxygen containing functional groups were removed during the reductive process. The XRD patterns of the natural silk fiber, GOF and HGFs are presented in Fig. 2c. The main diffraction peak of the nature silk fiber appeared at around 20.41 (2y). However, a new peak of the GOF was observed at approximately 11.361 (2y), which confirmed that GO had been coated on the silk surface successfully. After reduction for 1 hour, the peak of the silk fiber (20.41) and GO (11.361) disappeared. However, a new broad diffraction peak of the HGFs appeared at around 24.011 which is close to the typical diffraction peak of graphite (26.611), revealing the successful reduction of the GOF and the removal of the silk fiber template to obtain the hollow graphene fibers. Fig. 2d shows the FTIR spectra of the natural silk fiber, GOF and HGFs. For the silk fiber, the strong absorption peaks at 1647 cm1 and 1539 cm1 can be attributed to the stretching vibrations of the CQO and CQN groups of amide I and amide II, respectively. The FTIR spectrum of the GOF also revealed the formation of the silk@GO composite by the observation of the related oxygenous functional groups such as –OH groups (3413 cm1), epoxy C–O groups (1198 cm1), carboxyl CQO groups and C–O groups (1635 and 1405 cm1) and alkoxy C–O groups (1054 cm1). However, after the reductive process, these related oxygenous groups disappeared, while a new peak at 1565 cm1 which reflects the skeletal vibration of RGO sheets appeared. All of these results indicated that the HGFs
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were prepared by the spray deposition process and reduced by HI successfully. 3.3
The mechanical properties of the HGFs
The HGFs can be prepared with different diameters and lengths as shown in Fig. 3a. It is worth noting that the diameter and length of the HGFs can be controlled by either simply manipulating the spray-deposition time or using natural silk with different lengths. Therefore, this method will be more convenient for producing graphene fibers than the wet-spinning method or other template methods.22,39–41 Furthermore, the HGFs can be rolled onto rods, which display good flexibility. The HGFs not only have good flexibility but also excellent mechanical strength. The GOF was measured to have a tensile strength of up to 176 MPa (Fig. 3b). However, after reduction by HI at 60 1C for 1 h, a higher strength of about 289 MPa was observed for the HGFs. The higher tensile strength might be attributed to the strong p–p stacking interactions among the RGO sheets. Interestingly, the HGFs exhibited an elastic behaviour before breaking, which presumably arose from the possible displacement of the graphene sheets within the fibers.40 The ultrahigh tensile strength of the HGFs promised an enormous potential for wearable electronics. 3.4
The electrical properties of the HGFs
As shown in Fig. 4, the conductivity of the GOF and HGFs with a length of 1 cm and outer diameter of 100 mm was about 28 and 13 973 S m1, respectively. The conductivity of the HGFs was improved 508 times compared to that of the GOF, confirming that the HGFs can be a good candidate for the fabrication of fiber supercapacitors. As shown in Fig. 4c and d, a simple circuit was fabricated using the HGFs as part of the wire, through which a LED could be controlled on or off by a switch, which revealed the excellent conductivity of HGFs. 3.5
Electrochemical performance of the HGF supercapacitor
To investigate the electrochemical behaviour of the as-prepared hollow graphene fibers (HGFs), the CV curves of the HGF electrodes were also tested at different scan rates using a threeelectrode system (Fig. S7a, ESI†). It can be seen that they also exhibit excellent double-layer supercapacitor performance. Furthermore, for practical applications, an all-solid flexible symmetric hollow graphene fiber supercapacitor (HGFS) was assembled using the HGFs as electrodes and PVA/H3PO4 gel as the electrolyte. As a typical electrical double-layer supercapacitor, the cyclic voltammetry (CV)
Fig. 2 Characterization of the natural silk fiber, GO@silk composite fiber (GOF) and hollow graphene fiber (HGF). XPS spectra: (a) C 1s spectra of the GOF and (b) C 1s spectra of the HGF. (c) XRD patterns and (d) FTIR spectra of the natural silk fiber, GOF and HGF.
Fig. 3 (a) A photograph of HGFs with different diameters and lengths. (b) Typical stress–strain curves of a single GOF and HGF.
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Fig. 4 Electrical conductivity of the GOF and HGF. (a and b) Photographs of the resistance of the GOF and HGF. (c and d) A simple circuit (on or off) fabricated using the HGF.
shape of the all-solid flexible symmetric HGFS should be rectangular.42,43 As shown in Fig. 5a, the HGFS exhibited a rectangular-like shape at various scan rates from 2 mV s1 to 100 mV s1. For comparison, the CV data of a solid graphene fiber supercapacitor (GFS), which was prepared using the reported method, are shown in Fig. S8a (ESI†). The GFS also exhibited a rectangular-like shape. However, the current density of the GFS was much smaller than that of the HGFS, which indicated that the charge propagation within the interface of the HGF electrodes and the electrolyte was faster than that of the GF electrodes. Fig. 5b shows the galvanostatic charge–discharge (GCD) curves for the HGFS at current densities of 0.3, 0.6, 0.8, 1 and 5 A g1, respectively. The typical triangular shape could be observed, which can be attributed to the rapid charge transport between the two fiber electrodes. The galvanostatic charge– discharge (GCD) profiles of the HGFS also exhibited linear slopes between potential and time, which was similar to the electric double-layer capacitance behaviour. The Coulombic efficiency of the HGFS between the charge and discharge curves was about 69%, which was higher than that of the GFS (61.5%, Fig. S8, ESI†), indicating the good reversibility of the HGFS. Furthermore, slight voltage drops could be observed in the GCD curves, which might be caused by the internal resistance of the HGFS. The specific capacitances of the HGFS calculated through the discharge process were about 76.1 F g1 (127.4 mF cm2, 48.5 F cm3 in Fig. S9a and c, ESI†) at 1 A g1 and 66.2 F g1 (110.8 mF cm2, 42.3 F cm3) at 5 A g1, which were much higher than those of the GFS (Fig. S8b, ESI†). The GCD curves (Fig. S7b, ESI†) of the HGF electrode were also tested using a three-electrode system. As shown in Fig. S7c (ESI†), the specific capacitance of the HGF electrode was slightly higher than that
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Fig. 5 (a) CV curves of the HGFS at different scan rates. (b) The galvanostatic charge–discharge curves of the HGFS at different current densities. (c) Nyquist plot for the GFS and HGFS in the frequency range from 100 kHz to 0.1 Hz. High frequency range data (inset) are shown by the semicircle, indicating the faradic charge-transfer resistance. (d) The charge–discharge cyclic stability of the fiber supercapacitor at a charge and discharge current density of 1 A g1. The inset shows the CV curves of the fiber supercapacitor after cycling 2000 times at a scan rate of 20 mV s1. (e) Specific capacitance of the GFS and HGFS at different current densities. (f) Ragone plots of the GFS and HGFS derived from the charge–discharge curves at different current densities.
of the HGFS device, which indicated that the HGF electrode is a potential material for the fabrication of energy storage devices. For comparison, some reported all-solid fiber-shaped supercapacitors are summarized in Table 1. It can be seen that the HGFS exhibited superior or comparable performance to most others. The improved performance of the as-fabricated HGFS should have resulted from the hollow structure of the HGF electrode. Electrochemical impedance spectroscopy (EIS) of the HGFS and GFS was conducted over the frequency range from 100 kHz to 0.1 Hz. The Nyquist plots obtained from electrochemical impedance spectroscopy measurements are shown in Fig. 5c, and the insets are the high-frequency region of the Nyquist plots. It is well known that EIS analysis can be used as one of the typical methods to investigate the fundamental electrochemical behaviour of different electrode materials for supercapacitors.44,45 The knee frequency of the Nyquist plot is regarded as the point at which a supercapacitor begins to show its capacitive performance. The knee frequency of the supercapacitor based on the HGF electrode was measured to be 147 Hz, which was much higher that of the supercapacitor based on the GF electrode (76 Hz). The high frequency might reflect the high conductivity and fast electrolyte diffusion through the HGF. The equivalent series resistance (ESR) which is the X-intercept of the Nyquist plot in the high-frequency region shows the internal resistance of the
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Table 1
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Electrode materials
Ca (mF cm2)
Ref.
This work Graphene fiber Graphene–PANI fiber RGO–GO–RGO ZnO NWs Carbon microfiber bundles coated with carbon nanotubes Electrochemically RGO MnO2/graphene/graphene fiber Bismuth oxide nanotubes–graphene fiber
127.4 2.4 3.8 1.2 2.0 86.8
— 44
0.73 9.6 69.3
45 46 47 48 49 50
electrode and the contact resistance between electrode and electrolyte. The ESR of the HGFS was derived to be 1.0 O while that of the GFS was about 2.7 O from the expanded image of the Nyquist plot in the high-frequency region. The smaller resistance of the HGFS might result from the high conductivity and large surface area of the HGF electrodes which provided a larger accessible interface between the two fiber electrodes and the electrolyte. Furthermore, the results also agreed with the negligible voltage drops at the beginning of the discharge curves as shown in Fig. 5b. On the other hand, the capacitive behaviour of the HGFS was related to the verticality of the sloped line in the low-frequency region. The high slope of the HGFS indicated that ion diffusion could rapidly proceed within the HGF electrode. These results showed that the HGFs exhibited high conductivity, a large surface area and good ion diffusion, which are important factors for excellent fiber electrode materials. As shown in Fig. 5d, the galvanostatic charge–discharge test was conducted to analyse the cyclic stability of the HGFS; a capacitance retention of 90.5% was achieved for the HGFS after scanning for 2000 cycles at a current density of 1 A g1. Furthermore, the CV loop at a scan rate of 20 mV s1 obtained at the 2000th cycle was still similar to that at the 1st cycle, indicating the high-rate performance and good stability of the HGFS. As shown in Fig. 5e, the specific capacitance of the GFS and HGFS decreased with increasing current density. But it can be seen that the specific capacitance of the HGFS was much higher than that of the GFS, confirming that the hollow structure of the fiber electrodes made a great contribution to the electrochemical performance. The Ragone plots of the GFS and HGFS are shown in Fig. 5f. The specific energy density of the HGFS was 2.64 W h kg1 (3.98 W h cm2 and 1.52 W h cm3, Fig. S9b and d, ESI†), while that of the GFS was only 0.77 W h kg1 at a current density of 1 A g1. The specific energy density of the HGFS is almost equal to that of the commercial supercapacitor (less than 3 W h kg1). Therefore, these results revealed the possible commercial application of the HGF electrodes. To further evaluate the potential for flexible energy storage, the bending test for the HGFS device was implemented under different bending conditions. As shown in Fig. 6a, it can be seen that bending up to 1201 had no effect on the HGF electrodes and the whole device. Furthermore, the electrochemical stability was also analyzed and the results are shown in Fig. 6(b–d). It can be seen that the CV, GCD curves and capacitance retention were almost unchanged at different bending degrees (01, 301, 601,
Fig. 6 (a) The bending scheme and the photograph of the HGFS device, (b) the CV curves, (c) GCD curves and (d) the capacitance retention of the HGFS tested at different bending degrees.
901, and 1201), which shows that the HGFS had excellent flexibility and electrochemical stability. In addition, the HGFS device had a low leakage current of about 12 mA and a low opencircuit potential decay as shown in Fig. S10(a and b) (ESI†), which indicated the good electrochemical performance of the assembled supercapacitor. These results clearly show the great potential of the as-prepared HGF electrodes for the fabrication of flexible and wearable energy storage devices. In order to further explain the effect of the HGF structure on its electrochemical performance, structure models of the solid fiber51 (Fig. 7A) and the HGF (Fig. 7B) were designed to illustrate the ion diffusion between the interface of the fiber electrodes and the electrolyte. For a typical electrical double-layer supercapacitor, the specific capacitance is proportional to the interface between the two fiber electrodes and the electrolyte. Therefore, the capacitance of a typical electrical double-layer supercapacitor can be calculated via the formula: C¼
eS 4pkd
Here, e, p, k and d represent the fundamental constants related to the electrode materials, and S is the interface between the two fiber electrodes and the electrolyte. As shown in Fig. 7, the
Fig. 7 Schematic illustration of the charge distribution on the solid fiber (A) and HGFs (B).
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supercapacitor fabricated using HGFs (B) as electrodes has a much larger interface than that fabricated using solid fibers (A) between the electrode and electrolyte, which should make a significant contribution to the enhanced specific capacitance.
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4. Conclusions In summary, flexible and conductive hollow graphene fibers have been successfully prepared and used as electrodes for fabricating a flexible fiber supercapacitor with excellent electrochemical properties and good flexibility. The as-fabricated fiber supercapacitor exhibited a high specific capacitance of 76.1 F g1 (127.4 mF cm2, 48.5 F cm3), which is comparable or superior to that of other fiber-shaped supercapacitors. Considering the combined merits of cost-efficiency, the large-scale availability of chemically prepared graphene and the simple, green and controllable fabrication process, these flexible, hollow and conductive graphene fibers are particularly promising for the fabrication of flexible and wearable electronics.
Conflicts of interest There are no conflicts of interest to declare.
Acknowledgements This work was supported by the Natural Science Foundation of China (grant no. 51173087) and Qingdao (12-1-4-2-2-jch). JL also acknowledges the Taishan Scholars Program.
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