carbon fiber and porous

Volume 3 Number 21 7 June 2015 Pages 11139–11670

Journal of

Materials Chemistry A Materials for energy and sustainability www.rsc.org/MaterialsA

ISSN 2050-7488

PAPER Shuai Wang et al. Hierarchically structured MnO2/graphene/carbon fiber and porous graphene hydrogel wrapped copper wire for fiber-based flexible all-solid-state asymmetric supercapacitors

Journal of

Published on 17 April 2015. Downloaded by Huazhong University of Science & Technology on 25/05/2015 04:37:55.

Materials Chemistry A View Article Online

PAPER

Cite this: J. Mater. Chem. A, 2015, 3, 11215

View Journal | View Issue

Hierarchically structured MnO2/graphene/carbon fiber and porous graphene hydrogel wrapped copper wire for fiber-based flexible all-solid-state asymmetric supercapacitors† Zheye Zhang, Fei Xiao and Shuai Wang* Recent progress in fiber-based supercapacitors has attracted tremendous attention due to the tiny volume, high flexibility and weavability of the fibers, which are required for the development of high-performance fiber electrodes. In this work, we report for the first time, the design and fabrication of two types of core–shell fiber-based electrodes, i.e. hierarchically structured manganese dioxide (MnO2)/graphene/ carbon fiber (CF) and three-dimensional (3D) porous graphene hydrogel (GH) wrapped copper wire (CW), and their practical application in a fiber-architectured flexible all-solid-state supercapacitor. Taking advantage of the synergistic effects of the different components in the hierarchically structured nanohybrid fiber electrodes and the merits of the proposed synthesis strategies, the assembled

Received 31st March 2015 Accepted 17th April 2015

asymmetric supercapacitor device using MnO2/graphene/CF as the positive electrode and GH/CW as the negative electrode could be cycled reversibly in a high-voltage region of 0–1.6 V, delivering a high areal energy density of 18.1 mW h cm2 and volumetric energy density of 0.9 mW h cm3. Furthermore, our

DOI: 10.1039/c5ta02331a

fiber-based flexible supercapacitor also shows a good rate capability, excellent flexibility and high long

www.rsc.org/MaterialsA

term cyclability, which makes it a promising power source for flexible energy-related devices.

Introduction The emergence of ber-based supercapacitors has attracted tremendous attention as a result of the integration of a tiny volume, high exibility and weavability, making them intriguing candidates for the next generation of wearable electronic devices.1–12 However, compared with other energy storage devices such as batteries, supercapacitors still suffer from a much lower energy density, limiting their practical applications. Technically, enhancements in energy density can be achieved by increasing either the specic capacitance (C) or the operating voltage (V), since the energy stored is proportional to CV2. Recent advances in the scalable synthesis of a hierarchically structured carbon microber made of a carbon nanotube/graphene sheet interconnected network architecture also show that these ber-based supercapacitors exhibit high volumetric energy densities comparable to those of thin-lm lithium batteries.6,7 However, most of the reported ber supercapacitors are based on a symmetric device conguration with two identical ber electrodes, which suffers from a narrow operating voltage window of less than 1 V and thus this limits their energy

School of Chemistry & Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) supplementary gures. See DOI: 10.1039/c5ta02331a

This journal is © The Royal Society of Chemistry 2015

available:

Additional

densities. Therefore, the asymmetric supercapacitor design is considered to be an effective approach for extending the operating voltage window and enhancing the energy density.13–16 For example, the ber shaped all-solid-state asymmetric supercapacitor based on metal ber/Co3O4 nanowire and carbon ber (CF)/graphene electrodes can be operated up to 1.5 V and can achieve a maximum volumetric energy density of 0.62 mW h cm3.8 In all cases, however, it is still a challenge to develop a relatively simple and cost-effective approach to fabricate ideal ber-based electrodes and increase the energy density of berbased exible supercapacitors without sacricing other electrochemical characteristics, such as power density or cyclability. Two-dimensional (2D) graphene nanomaterials are of particular current interest because of their large specic surface areas, high electrical conductivities, high mechanical strengths as well as their intrinsic exibilities and chemical stabilities, which are promising for energy storage applications.17–24 Furthermore, individual graphene nanosheets can be assembled into 3D macroscopic structures and these assemblies possess several promising properties, such as macro/mesoporosity, large surface area, multi-dimensional conductivity and high stability, which are highly desirable for the fabrication of high performance energy storage devices.25,26 However, supercapacitors based on pure graphene materials still cannot meet the requirements for practical applications because of their lower energy densities compared to pseudocapacitors based on

J. Mater. Chem. A, 2015, 3, 11215–11223 | 11215

View Article Online


Journal of Materials Chemistry A

conductive polymers or transition metal oxides. Hence, the integration of graphene and pseudocapacitor materials is highly desirable for achieving a battery-level energy density for graphene-based materials. In this work, we have developed two facile and cost-effective strategies to synthesize the as-mentioned nanohybrid ber electrodes. As illustrated in Fig. 1, for the synthesis of hierarchically structured MnO2/graphene/CF, three key steps are involved in the fabrication process. First, the CF was wrapped with an ultrathin GO lm via a dip-coating method (step 1). Second, GO partially restored the 2D ordered structure of graphene27–29 to form a reduced GO (RGO) nanosheet wrapped CF (RGO/CF) by a facile and green electrochemical reduction, according to our previous work (step 2).15 Third, MnO2 nanostructures were grown in situ on the RGO/CF scaffold by a wellcontrolled template-free electrochemical deposition method (step 3). For the fabrication of the GH wrapped CW (GH/CW) electrode, the CW was directly immersed in an aqueous GO suspension, upon which a spontaneous redox reaction between the CW and GO nanosheets occurs (step 4), leading to the formation of the 3D porous GH/CW electrode. Beneting from the synergistic effects of different components in the hierarchically and porous structured nanohybrid ber electrodes and the merits of the proposed synthesis strategies, the resultant ber-architectured exible all-solid-state asymmetric supercapacitor device based on MnO2/RGO/CF and GH/CW electrodes can be reversibly charged/discharged over a wide voltage range of 1.6 V, delivering a high areal energy density of 18.1 mW h cm2 and volumetric energy density of 0.9 mW h cm3. Furthermore, our ber-based exible supercapacitor also shows a good rate capability, excellent exibility and high long term cyclability (over 90% device capacitance retention over 10 000 cycles).

Experimental section Preparation of the samples GO was synthesized from graphite powder based on a modied Hummer's method.30 The concentration of the prepared GO solution was about 5 mg mL1. A three electrode system was used for the electrochemical reduction of the GO coated CFs and the deposition of MnO2. For the fabrication of the MnO2/ RGO/CF electrode, a bundle of CFs (1 mg cm3, with a diameter of 400 mm) was dipped into the GO suspension and immediately removed. Then, the obtained GO/CF was used as a

Fig. 1

Paper

working electrode in 1.0 M Na2SO4 aqueous solution with a potential of 1.5 V for 300 s, and the product was denoted as RGO/CF. The electrochemical deposition of MnO2 nanoakes on RGO/CF was performed in 0.1 M Mn(OAc)2 aqueous solution with a constant potential of 1.0 V for 5 min. For the fabrication of GH/CW, a CW (61.8 mg cm3, with a diameter of 600 mm) was immersed into the GO suspension at 60 C for a desired period of time (0.5–8 h). Then, the GH grown on the CW surface was thoroughly washed with deionized water to remove physisorbed GO platelets. Fabrication of the ber-based all-solid-state asymmetric supercapacitor A potassium polyacrylate (PAAK)/KCl gel was prepared by adding 1.0 g of PAAK into 10 mL of KCl solution (1.0 M). The mixture was stirred until the solution became clear, and this was used as both the ionic electrolyte and separator. Next, the GH/CW electrode was immersed in the PAAK/KCl gel solution for 5 min, and then carefully entangled with the MnO2/RGO/CF electrode (the spacing between the two electrodes was about 1 mm). The length of the positive electrode and the negative electrode was xed on the basis of charge balance theory. The device was nally solidied at room temperature to vaporize the excess water. Materials characterization The morphologies and structures of the as-prepared ber electrodes were characterized with a eld-emission scanning electron microscope (SEM, FEI, Nova NanoSEM 450) and transmission electron microscope (TEM, FEI, Tecnai G2 20). Xray photoelectron spectroscopy (XPS) measurements were performed on a Kratos-Axis spectrometer with monochromatic Al Ka (1486.71 eV) X-ray radiation (15 kV and 10 mA) and a hemispherical electron energy analyzer. XRD patterns were recorded using a diffractometer (X0 Pert PRO, PANalytical B.V., Netherlands) equipped with a Cu Ka radiation source (l ¼ ˚ Raman spectra were measured on a confocal laser 1.5406 A). micro-Raman spectrometer (Thermo Fisher DXR, USA) equipped with a He–Ne laser with an excitation of 532 nm. Electrochemical measurements All the electrochemical measurements were performed with a CHI 760E electrochemical workstation (CH Instruments Inc. US). For single electrode tests, three electrodes set up in 1.0 M

Schematic illustration of the design and fabrication process of the fiber-based asymmetric supercapacitor.

11216 | J. Mater. Chem. A, 2015, 3, 11215–11223


View Article Online


Paper


Na2SO4 were used, with gauze platinum and a saturated calomel electrode (SCE) as the counter and reference electrode, respectively. The areal capacitance (CA) and volume capacitance (CV) were calculated from galvanostatic charge/discharge tests. In detail, the CA value of the bers in a three-electrode cell was calculated according to the equation: CA ¼ I t/U S, where I is the charge/discharge current, t is the discharge time, U represents the potential window and S is the surface area of the ber electrode. The CV value of the bers was derived from the equation: CV ¼ CA S/V, where V is the volume of the ber electrode. The areal and volume capacitances of the asymmetric supercapacitor (Ccell) were calculated according to the equations: Ccell,A ¼ I t/U Acell and Ccell,V ¼ CA S/Vcell, where Acell and Vcell refer to the total device area and volume of the prepared asymmetric supercapacitor, respectively. The areal energy density (EA) and areal power density (PA) of the asymmetric supercapacitor can be obtained from the equations: EA ¼ Ccell,A U2/(2 3600) and PA ¼ EA/t. The volumetric energy density (EV) and volumetric power density (PV) can be obtained from: EV ¼ Ccell,V U2/(2 3600) and PV ¼ EV/t.

Results and discussion For the fabrication of the MnO2/RGO/CF electrode, RGO/CF was rst prepared (Experimental section), as shown in the scanning electron microscopy (SEM) images. Aer being wrapped with a layer of RGO lm, the surface of the CF became very rough (Fig. 2a), and wrinkled features of the graphene nanosheets can clearly be observed from the high-magnication SEM image (Fig. 2b). The RGO/CF was then coated with MnO2 nanomaterials (Experimental section). The SEM images of MnO2/ RGO/CF show that MnO2 nanomaterials are axially grown on the CF to form a hierarchically porous nanostructure (Fig. 2c–e), which is favorable for fast transportation of electrons and diffusion of electrolyte ions during the supercapacitor application. The transmission electron microscopy (TEM) image indicates that the MnO2 nanostructures are highly porous and composed of many tiny ultrathin nanoakes (Fig. 2f). The selected area electron diffraction (SAED) pattern (Fig. 2f, inset) demonstrates that the as-formed MnO2 nanoakes are polycrystalline, which was further conrmed by the X-ray diffraction (XRD) pattern (Fig. S1†). The elemental characterization of MnO2/RGO/CF was performed by X-ray photoelectron spectroscopy (XPS). The C 1s spectrum of MnO2/RGO/CF demonstrates the successful reduction of GO during the electrochemical reduction process (Fig. S2†). The deconvoluted Mn 2p XPS spectrum shows two peaks centered at 642.6 and 654.2 eV, which can be assigned to the binding energy of Mn 2p3/2 and Mn 2p1/2, respectively, revealing that Mn4+ ions were dominant in the product. The peaks in the O 1s band at 530.2 and 532.0 eV are assigned to Mn–O–Mn and Mn–O–H, respectively (Fig. 2g), which is consistent with previously reported results, indicating the formation of MnO2 (ref. 31 and 32) on RGO/CF. For the fabrication of the GH/CW electrode, we developed a novel method based on the spontaneous redox reaction between a CW and GO. Upon being immersed in the GO


Fig. 2 SEM images of (a and b) RGO/CF and (c–e) MnO2/RGO/CF composites. (f) TEM image of the MnO2 nanostructures that are detached from MnO2/RGO/CF, the inset is the corresponding SAED pattern. (g) XPS survey of the Mn 2p and O 1s spectra for MnO2/RGO/ CF.

dispersion, a 3D GH was generated and spontaneously wrapped around the CW surface, as shown in Fig. 3a and b. This phenomenon can be explained by an interfacial redox reaction between GO and active metals, as in previous studies.33–36 In the rst stage, only a thin layer of GO that is in direct contact with the CW surface can be spontaneously reduced and form a deposited graphene lm on the CW. Then, more and more insulating GO nanosheets in the GO suspension can harvest electrons from the deposited graphene lm through the

J. Mater. Chem. A, 2015, 3, 11215–11223 | 11217

View Article Online



Paper

Fig. 3 Optical images of a (a) CW, (b) GH/CW electrode, (c) standing GH/CW spring and (d) dry GH/CW with a length of 22 inches. (e–g) SEM images of the GH/CW composite. (h) Deconvoluted C 1s spectra of GO and GH. (i) XRD patterns of GO and GH.

continuous transfer of electrons from the CW surface and form graphene, while the reduction and deposition process proceeds, leading to the formation of a 3D porous graphene network wrapped CW. The as-formed GH/CW spring is mechanically stable, exible and stretchable (Fig. 3c). This spontaneous growth is also arbitrary scalable, for example a CW with a length of 22 inches could be entirely wrapped with a layer of GH materials (Fig. 3d). More importantly, we found that a GH layer on a CW exhibits a typical 3D cross-linking porous structure (Fig. 3e and f), with pore sizes within the network on the micrometer scale (Fig. 3g), which dramatically increases the accessible electroactive surface area of the ber-based electrode materials. The chemical structure of GH/CW was characterized by XPS spectroscopy and XRD spectroscopy, along with a GO sample as a control. The deconvoluted C 1s XPS spectrum of GH/CW (Fig. 3h) shows three peaks for the graphitic structure (C]C/C–C at 284.7 eV), the hydroxyl/epoxy groups (C–O at 286.7 eV), and the carbonyl groups (O–C]O at 288 eV). Compared with the C 1s XPS spectrum of GO, the signals for the oxygen-containing groups in GH/CW decrease signicantly, demonstrating a high degree of deoxygenation and the successful reduction of GO

11218 | J. Mater. Chem. A, 2015, 3, 11215–11223

during the deposition process.37 It is worth noting that reversible redox reactions can take place among the residual hydroxyl, carbonyl, carboxyl, and lactone groups on the graphene nanosheets, which not only enhance the surface wettability, but also provide high pseudocapacitance.38,39 Fig. 3i exhibits the XRD patterns of GO and GH/CW aer etching the CW with a FeCl3 solution. The GO exhibits one sharp peak centered at 10.5 , corresponding to the (002) reection of stacked GO sheets with ˚ Aer reduction, the peak posian interlayer spacing of 8.42 A. tion for the GH shis from 10.5 to 24.1 and the interlayer ˚ further conrming the removal of spacing decreases to 3.69 A, oxygen-containing groups on the GO nanosheets. The electrochemical characteristics of the as-prepared MnO2/RGO/CF composite were investigated by using it as a working electrode in a three electrode system in 1.0 M aqueous Na2SO4 electrolyte, with gauze platinum and SCE as the counter and reference electrodes, respectively. It can be observed that the area surrounded by the CV curve for the MnO2/RGO/CF electrode is larger than that for the MnO2/CF and RGO/CF electrodes at a scan rate of 50 mV s1 (Fig. 4a), which is due to the double layer contribution of graphene along with the pseudocapacitive contribution of MnO2. When the scan rate


View Article Online


Paper


Fig. 4 (a) CV curves for the RGO/CF, MnO2/CF and MnO2/RGO/CF electrodes measured at a scan rate of 50 mV s1. (b) CV curves for the MnO2/

RGO/CF electrode at different scan rates (10, 20, 50 and 100 mV s1). (c) Galvanostatic charge/discharge curves for the RGO/CF, MnO2/CF and MnO2/RGO/CF electrodes at a current density of 0.5 mA cm2. (d) Dependence of the volumetric and areal capacitances on the charge/ discharge current density for the MnO2/RGO/CF (1, 3), MnO2/CF (2, 4) and RGO/CF (5, 6) electrodes.

increases from 10 to 100 mV s1, the rate dependent CV curves for the MnO2/RGO/CF electrode (Fig. 4b) show an almost rectangular shape, indicating nearly ideal supercapacitor behavior. To reveal the electrochemical capacitive performance of the MnO2/RGO/CF electrode, galvanostatic charge/discharge measurements were carried out along with the RGO/CF and MnO2/CF electrodes as controls. As shown in Fig. 4c, the charge storage capacity for the MnO2/RGO/CF electrode was signicantly improved, with 15.4% increase in discharge time over the MnO2/CF electrode and 127.55 times increase over the RGO/ CF electrode. The areal and volumetric capacitances, which are recognized to be more reliable parameters than the gravimetric parameter,10 are usually utilized to evaluate the charge storage capacity of ber-based electrodes. According to the charge/ discharge curves, the specic volumetric capacitance and areal capacitance of the MnO2/RGO/CF electrode are calculated to be 13.7 F cm3 and 205.7 mF cm2, respectively, both of which are much higher than those of RGO/CF (0.1 F cm3, 1.6 mF cm2) and MnO2/CF (11.8 F cm3, 178.3 mF cm2), indicating the synergistic effect of the RGO nanosheets and MnO2 nanoakes in the MnO2/RGO/CF electrode as well as the interactions between them, which signicantly improve the overall capacitive performance of the resultant electrode. The rate capability is also an important parameter to evaluate the electrochemical performance of electrodes.40,41 Therefore, we plotted the specic capacitance of different nanohybrid electrodes at various current densities (Fig. 4d) on the basis of the charge/discharge curves (Fig. S3†). As the current density increased from 0.5 to 10 mA cm2, the MnO2/ RGO/CF electrode preserved 73% of its specic capacitance (from 13.7 to 10 F cm3). In comparison, the specic


volumetric capacitance of MnO2/CF decreased quickly from 11.8 to 7.2 F cm3 – only 61% of the specic capacitance was maintained. The well-maintained specic capacitance of MnO2/RGO/CF can be attributed to the unique hierarchical structure of MnO2/RGO/CF as well as the synergistic contribution from different components in it. The RGO layer wrapped CF possesses a large surface area and high electrical conductivity, and can act as an ideal scaffold to host MnO2 nanomaterials and provide more conductive channels to facilitate the electron transfer and electrolyte ion diffusion during the charge/discharge process (Fig. S4†). Subsequently, we explored the capacitive performance of the prepared GH/CW electrodes. For the fabrication of GH/CW via the immersion process (Experimental section), the immersion time of the CW in the GO suspension is the key parameter and needs to be optimized. Consequently, the relationship between the immersion time and the performance of the device was studied in detail. Fig. 5a shows the CV curves of the GH/CW samples produced with different immersion times (i.e. 0.5, 1, 2, 3, 4, 6 and 8 h) at a scan rate of 100 mV s1. At rst, the area surrounded by the CV curve increases with immersion time, indicating that more active materials deposited on the CW contribute to the charge storage process. However, the situation changes when the immersion time further increases from 6 to 8 h. Correspondingly, the areal capacitance of GH/CW increases from 17.2 to 150.3 mF cm2 and then decreases to 138.4 mF cm2 (Fig. 5b). This phenomenon can be ascribed to the excessive mass loading of active materials, which increases the resistance of the electrode and blocks charge transportation. As a result, the optimal immersion time of the CW in the GO suspension was 6 h.

J. Mater. Chem. A, 2015, 3, 11215–11223 | 11219

View Article Online



Paper

Fig. 5 (a) CV curves for the GH/CW electrode at a scan rate of 100 mV s1 with different GH deposition times of 0.5, 1, 2, 3, 4, 6 and 8 h. (b) The relationship between the areal capacitance of the GH/CW electrode and the deposition time of GH. (c) CV curves for the GH/CW (6 h) electrode at different scan rates (10, 20, 50 and 100 mV s1). (d) Dependence of the areal and volumetric capacitances on the charge/discharge current density for the GH/CW (6 h) electrode. (e) Nyquist plots of the GH/CW (6 h) electrode carried out in a frequency range from 10 mHz to 100 kHz (the inset shows a Bode plot of the phase angle versus frequency). (f) Cycling life of the GH/CW (6 h) electrode.

Fig. 5c shows the CV curves of the GH/CW electrode with a deposition time of 6 h (GH/CW (6 h)) at different scan rates from 10 to 100 mV s1. They display near mirror-like images and relatively rectangular-like shapes, demonstrative of ideal capacitive behavior and a good reversibility. The galvanostatic charge/discharge behaviour of GH/CW (6 h) was also tested, and shows that all charge/discharge curves are close to a triangular shape, conrming the formation of efficient electric double layers and good charge propagation across the electrode (Fig. S5†). Furthermore, a large specic areal capacitance of 171 mF cm2 (7.3 F cm3 for the specic volumetric capacitance) can be obtained at a current density of 0.5 mA cm2. Even at a higher current density of 10 mA cm2, the specic capacitance of GH/CW was still 119 mF cm2 (Fig. 5d). The large specic capacitance and good rate capability can be attributed to the porous network structure of the GH, which effectively increases the surface area of GH/CW. Moreover, the considerable residual oxygen containing functionalities on the graphene nanosheets facilitate the wetting of the electrodes with aqueous electrolyte. They synergistically facilitate the electron transfer and electrolyte ion accessibility to the electrode surface, and improve the capacitive performance of the GH/CW electrode. The fast ion transport in the GH/CW can be further conrmed by electrochemical impedance spectroscopy (EIS), as shown in Fig. 5e. The plot features a vertical curve, indicating nearly ideal capacitive behavior. Furthermore, the capacitor response frequency, f0, is about 2.2 Hz (Fig. 5e inset) and the corresponding time constant, s0 (the inverse of the characteristic frequency at which 45 is reached in the Bode phase plot), is calculated to be about 0.46 s by the equation: s0 ¼ 1/f0, which

11220 | J. Mater. Chem. A, 2015, 3, 11215–11223

is relatively low compared to the values of previously reported porous GH.42 Moreover, the cycling performance of GH/CW can retain 87.5% of its initial capacitance aer 5000 cycles (Fig. 5f), suggestive of an excellent cycling stability. For the fabrication of the exible ber-based all-solid-state asymmetric supercapacitor, the GH/CW (as the negative electrode) was rst immersed in the PAAK/KCl gel solution and then carefully entangled around a bundle of MnO2/RGO/CFs (as the positive electrode). Fig. 6a shows the CV behavior of the MnO2/ RGO/CF and GH/CW electrodes at 100 mV s1. The potential windows of the GH/CW and MnO2/RGO/CF electrodes are 0.8–0 V and 0–0.8 V, respectively. Accordingly, the maximum operating potential for the proposed asymmetric supercapacitor is anticipated to reach 1.6 V. The CV curves of the asymmetric supercapacitor were measured between 0 and 1.6 V at scan rates ranging from 10 to 200 mV s1. They exhibited nearly rectangular shapes without obvious redox peaks (Fig. 6b). Galvanostatic charge/discharge testing was also performed with different operating voltages from 0–0.8 V to 0–1.6 V (Fig. S6a†) and different current densities in a voltage window of 0–1.6 V (Fig. S6b†). The discharge curves are almost symmetrical to the corresponding charge curves, indicating excellent reversibility and good charge propagation between the two ber electrodes. Fig. 6c demonstrates the calculated specic capacitance and current densities based on the discharge curves at different current densities from 0.2 to 5 mA cm2. The specic volumetric and areal capacitances of the as-prepared asymmetric supercapacitor can reach their maximum values of 2.54 F cm3 and 50.8 mF cm2 at a current density of 0.2 mA cm2. These values exceed those of other solid-state asymmetric


View Article Online


Paper


Fig. 6 (a) CV curves for the MnO2/RGO/CF and GH/CW electrodes at a scan rate of 100 mV s1. (b) CV curves for the MnO2/RGO/CF//GH/CW asymmetric supercapacitor device with a cell voltage of 1.6 V at different scan rates (10, 20, 50, 100 and 200 mV s1). (c) Dependence of volumetric and areal capacitances on the charge/discharge current density for the MnO2/RGO/CF//GH/CW device. (d) Ragone plots of the MnO2/RGO/CF//GH/CW device compared with a commercially available supercapacitor (SC) and selected previously reported fiber-shaped supercapacitors with a solid electrolyte. (e) Capacitance retention of the flexible MnO2/RGO/CF//GH/CW device after 500 cycles with up to 90 bending angles, the inset shows the capacitance retention after bending inwards at different angles and a photograph of a bent device. (f) Cycling life of the MnO2/RGO/CF//GH/CW device at a current density of 5 mA cm2, the inset shows a photograph of a blue LED lit by tandem devices.

supercapacitors based on VOx//VN (1.35 F cm3),43 MnO2//Fe2O3 (1.5 F cm3),16 and Co3O4/metal ber//graphene/CFs (2.1 F cm3).8 The proposed ber-based asymmetric supercapacitor also shows a good rate performance with 70% of the volumetric capacitance retained as the current density increases from 0.2 to 5 mA cm2. A Ragone plot, representing the relationship between the energy density and power density, is usually used to evaluate the performance of a supercapacitor device.41,44 Fig. 6d shows the Ragone plot of the MnO2/RGO/CF//GH/CW asymmetric supercapacitor device, which exhibits a high volumetric energy density of 0.9 mW h cm3 and an areal energy density of 18.1 mW h cm2. Moreover, a high power density of 0.2 W cm3 could be obtained when the energy density was as high as 0.63 mW h cm3 at a discharge current density of 5 mA cm2. The energy density values, calculated based on the device volume, are higher than those of commercially available supercapacitors (2.75 V/44 mF and 5.5 V/100 mF,