Porous Graphene-Carbon Nanotube Scaffolds for Fiber

Research Article www.acsami.org

Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Porous Graphene-Carbon Nanotube Scaffolds for Fiber Supercapacitors Hun Park,†,∥ Rohan B. Ambade,†,∥ Sung Hyun Noh,†,∥ Wonsik Eom,† Ki Hwan Koh,† Swapnil B. Ambade,†,‡ Won Jun Lee,§ Seong Hun Kim,†,‡ and Tae Hee Han*,†,‡ Department of Organic and Nano Engineering and ‡The Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea § Department of Fiber System Engineering, Dankook University, Yongin 16890, Republic of Korea Downloaded via UNIV OF GOTHENBURG on January 23, 2019 at 14:18:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: Fiber nanomaterials can become fundamental devices that can be woven into smart textiles, for example, miniaturized fiberbased supercapacitors (FSCs). They can be utilized for portable, wearable electronics and energy storage devices, which are highly prospective areas of research in the future. Herein, we developed porous carbon nanotube−graphene hybrid fibers (CNT−GFs) for all-solid-state symmetric FSCs, which were assembled through wetspinning followed by a hydrothermal activation process using environmentally benign chemicals (i.e., H2O2 and NH4OH in deionized water). The barriers that limited effective ion accessibility in GFs were overcome by the intercalation of CNTs in the GFs which enhanced their electrical conductivity and mechanical properties as well. The all-solid-state symmetric FSCs of a precisely controlled activated hybrid fiber (a-CNT−GF) electrode exhibited an enhanced volumetric capacitance of 60.75 F cm−3 compared with those of a pristine CNT−GF electrode (19.80 F cm−3). They also showed a volumetric energy density (4.83 mWh cm−3) roughly 3 times higher than that of untreated CNT−GFs (1.50 mWh cm−3). The excellent mechanical flexibility and structural stability of a miniaturized a-CNT−GF are highlighted by the demonstration of negligible differences in capacitance upon bending and twisting. The mechanism of developing porous, large-scale, low-cost electrodes using an environmentally benign activation method presented in this work provides a promising route for designing a new generation of wearable, portable miniaturized energy storage devices. KEYWORDS: fiber supercapacitors, graphene fibers, carbon nanotubes, wet spinning, surface activation, environmentally benign process

1. INTRODUCTION

FSCs that store energy using reversible ion adsorption are more advantageous8,9 than pseudocapacitor-based FSCs that store energy using a faradaic surface redox mechanism. Carbon nanotubes (CNTs) and graphene are recognized as outstanding candidates among many other carbon-based EDLC materials such as activated carbon. Graphene has exceptional intrinsic features such as high electrical conductivity, thermal conductivity, high specific surface area (SSA), excellent mechanical properties, and high intrinsic capacitance due to its unique 2D structure.10−13 The onedimensional (1D) assembled macroscopic architecture of graphene, known as graphene fibers (GFs), has prompted immense research interest in textile capacitive energy storage devices. GFs possess high electrical conductivity due to their dimensional configuration and stable mechanical properties

The miniaturized form of supercapacitors (SCs), i.e., fiberbased flexible SCs (FSCs), is being considered as promising energy storage devices compared to conventional threedimensional (3D) and two-dimensional (2D) SCs. FSCs show great promise due to their outstanding mechanical flexibility, excellent deformability, low cost, ability to be woven into smart textile fabrics, and applications in modern portable wearable electronics.1−4 However, the low energy density of FSCs remains a major challenge before they can be considered as alternative energy storage devices that are comparable to batteries. Theoretically, the energy density of SCs is proportional to the capacitance in the operating voltage.5−7 The volumetric energy density can be enhanced by improving the capacitance of the electrode materials by using compact materials with a highly porous architecture, which allows maximum utilization of the limited volume. In the development of highly stable FSCs that can deliver high energy density, the electrical double layer capacitor (EDLC)-based © XXXX American Chemical Society

Received: November 8, 2018 Accepted: January 8, 2019

A

DOI: 10.1021/acsami.8b17908 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

volumetric energy density of 4.83 mWh cm−3 while maintaining their power density and rate capability. The devices also demonstrated exceptional cycling stability of over 10 000 cycles. Furthermore, the a-CNT−GF FSCs showed high flexibility and bendability with good mechanical properties. Overall, our proposed green activation strategy is facile, cost-effective, and provides a highly porous structure in the vicinity of the fiber surface. This structure results in a substantial SSA increase and facilitates ion accessibility, thereby improving the electrochemical performance of GFbased FSCs. These attributes make a-CNT−GFs promising candidates for high-performance flexible, portable, wearable electronics.

(i.e., flexibility and stretchability), which are prerequisites for FSC electrodes. Moreover, the flexible nature of GFs is attractive compared to rigid and bulky metal and metal alloy wires; thus, GFs are being explored as substrates for scaffolding active electrode materials in FSCs.14−18 However, the low accessible surface area of GFs (due to their compact restacking of graphene sheets in GFs) compared to the theoretical value often limits the actual applications of GFs as electrode materials in FSCs.19 This compact nature inevitably limits ion diffusion paths and thereby severely reduces the electrical double-layer capacitance of GFs. Thus, developing superior GFs for FSCs demands increased porosity and precise control over the restacking of the sheets.9,20,21 An effective way to overcome the previously mentioned restacking problem in GFs is to use spacers like CNTs between the graphene sheets.22,23 The CNT spacers can increase the interlayer distance, contribute to effective ion accessibility of GFs, and adequately increase the device performance. Second, for augmentation of porosity to increase SSA, the inaccessibility to the inner surface area of GFs can be overcome by an activation process that creates pores to facilitate ion diffusion and improve effective surface areas in GFs.24,25 Of the many chemical activation methods, KOH activation is an effective way to produce high porosity, large SSA, and high capacitance in bulk carbon materials such as graphene, CNTs, and carbon nanofibers.26,27 Xu et al. demonstrated an activation technique to generate pores on graphene sheets and create 3D hierarchical porous structures for SC electrodes.28,29 These hole-containing graphene films exhibited an enhanced specific capacitance of 283 F g−1, which was 38% higher than that of pristine graphene films (205 F g−1). Despite the appreciable electrochemical performance of KOH-activated graphene films, this activation process is inevitably accompanied by the decomposition of carbon materials; thus, it is likely to bring about the unintended degradation of other crucial properties related to realizing high-performance FSCs, such as mechanical properties and electrical conductivity. Moreover, it is noteworthy that the corrosive and toxic nature of KOH limits its large-scale applications.30 To date, less attention has been paid to obtain highly porous GF-based materials using activation processes. To resolve these issues, activation using environmentally benign chemicals, such as hydrogen peroxide (H2O2)31 and ammonium hydroxide (NH4OH),32 is an effective approach to achieve highly promising electrode materials with hierarchical porous morphology for highperformance FSCs. In this work, we developed a facile one-step environmentally benign hydrothermal activation method for CNT−graphene hybrid fiber electrodes (CNT−GFs) for all-solid-state symmetric FSCs. The activation of CNT−GFs (i.e., a-CNT− GFs) was carried out hydrothermally via a solution etching process using environmentally friendly H2O2 and NH4OH, and resulted in hierarchical porous morphology with increased SSA. To the best of our knowledge, the activation of GFs for all-solid-state FSCs using environmentally benign chemicals is unprecedented. The presence of CNTs in a-CNT−GFs improved their electrical conductivity and mechanical properties as compared to activated GFs (a-GFs). Furthermore, the activation time was precisely optimized because excessive activation had significant adverse effects on their properties and device performance. The all-solid-state FSCs based on aCNT−GF15 (activated for 15 min) exhibited a high volumetric capacitance of 60.75 F cm−3 and a superior

2. EXPERIMENTAL SECTION 2.1. Materials. Graphene oxide (GO) was synthesized from natural graphite flake (grade 2012, Asbury Carbons) via the modified Hummers’ method according to previously reported literature.33,34 CNTs were purchased from Carbon Solutions Inc. (P3-SWNT). Hydrogen peroxide (H2O2, 30%, GR grade) was purchased from Junsei Chemical Co. Ltd. Poly(vinyl alcohol) (PVA, MW: 146−186 K), ammonium hydroxide (NH4OH, 28−30%, ACS Reagent), and potassium hydroxide (KOH, ACS Reagent) were purchased from Sigma-Aldrich. Anhydrous calcium chloride (CaCl2, practical grade) was obtained from Duksan Pure Chemicals Co. Hydroiodic acid (HI, EP grade) was purchased from Yakuri Pure Chemicals Co. Ltd. Water was purified from a Millipore Direct Q3 Ultrapure Water System to obtain deionized water (DIW). All chemicals purchased were used as received without further purification. 2.2. Preparation of CNT−GFs. An aqueous CNT−GO (0.6 wt %; 1:5 w/w) dispersion was spun through a spinneret (diameter: 400 μm) at a fixed injection rate of 10 mL h−1 into a coagulation bath containing 5 wt % CaCl2. Extruded CNT−GO gel fibers in the bath were collected continuously on a reel and dried in a 60 °C oven for 30 min. The dried CNT−GO fibers were chemically reduced with HI (30 wt %) for 6 h at 80 °C in an oil bath to obtain CNT−GFs. After the reduction reaction, the CNT−GFs were washed with DIW and ethanol to remove the remaining acid. For comparison, GFs were also prepared in the same manner by using GO dispersion (0.5 wt %) without CNTs. 2.3. Surface Activation Process. The prepared CNT−GFs were immersed in 10 mL of DIW, 1 mL of H2O2, and 0.5 mL of NH4OH in a Teflon reactor (50 mL). The reactor was placed inside a steel autoclave and left for 15 or 30 min at 150 °C. After the activation process, the fibers were collected and subsequently washed with DIW and then dried at 80 °C in an oven for 1 h. GFs and graphene films were also activated following the same manner. 2.4. Materials Characterization. The surface morphologies of the samples were observed with field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and atomic force microscopy (AFM, Park Systems, XE-70). X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Co., Theta Probe Base System) measurements were obtained by using monochromated Al Kα radiation with a flood gun. Raman spectra were obtained by using an NRS-3100 Raman spectrometer (JASCO) at an excitation wavelength of 532 nm. The tensile properties of fibers were investigated by using a universal testing machine (Instron 5966) equipped with a 10 N load cell operating at a loading rate of 10% min−1 and referring to the tensile measurement of single ultrafine fibers, as previously reported.35,36 Fibers were loaded onto a rectangular frame with a gauge length of 25 mm, and melting adhesive was used to prevent the samples from slipping. The mechanical strength of the GFs was calculated from the force divided by the cross-sectional area. The Brunauer−Emmett−Teller (BET) surface area was analyzed by nitrogen adsorption (microtracBEL, BELSORP-max), and the samples were degassed at 200 °C overnight prior to each measurement. The electrical conductivity of the fibers was measured through a two-probe method with a uniform 3 mm B


Research Article


Figure 1. (a) Schematic of the assembly process of CNT−GFs from the aqueous CNT−GO dispersion. (b) Meter-long CNT−GFs reeled on a bobbin. (c) Photograph of GO (0.05 wt %) and CNT−GO (0.06 wt %; 1:5 w/w, CNT/GO) solutions taken between two crossed polarizers. (d) Photograph of the continuous wet-spinning assembly of fibers. (e) Polarized optical microscopy images of GO (top) and CNT−GO (bottom) LC gel fibers. SEM images of the (f) surface, (g) cross-section, and (h) high magnification cross-section of GF. SEM images of the (i) surface, (j) crosssection, and (k) high magnification cross-section of CNT−GF. spacing at room temperature using a potentiostat (Bio-Logic, SP200). 2.5. Electrochemical Measurements. All electrochemical measurements were carried out with a potentiostat (Bio-Logic, SP200). A conventional three-electrode cell, comprised of a counter electrode (Pt wire), a single fiber with an effective length of 2 cm as a working electrode, and a reference electrode (Hg/HgO) in a 6 M KOH aqueous solution, was used for the characterization. For the preparation of all-solid-state symmetric FSCs, two fibers were aligned in parallel, homogenously coated with the PVA−KOH gel electrolyte, and allowed to dry at room temperature. The two electrodes were packed in a transparent poly(vinyl chloride) (PVC) tube (length: 1 cm) after the PVA−KOH gel solidified. The PVA−KOH gel electrolyte was prepared by dissolving 3 g of KOH and 6 g of PVA in 60 mL DIW. This mixture was stirred at 80 °C until its appearance became clear. Electrochemical impedance spectroscopy (EIS) measurements were carried out at the open circuit potential with a sinusoidal signal over a frequency range of 100 kHz to 10 mHz with an amplitude of 10 mV. The volumetric capacitance in the three-

electrode system was calculated from the cyclic voltammetry (CV) measurements as follows C=

1 2Vvπ

∫ I(V )dV

(1)

where the integral term refers to the area of the CV curve and V is the potential sweep range starting from 0 to −0.8 V. ν is the scan rate and π refers to the volume of the active electrode materials. Additionally, the galvanostatic charge and discharge (GCD) curves were used to calculate the capacitance as follows C = I Δt /πΔV

(2)

where I, ΔV, and Δt are the discharge current, voltage range excluding the IR drop, and discharge time, respectively. The equivalent series resistance (ESR; RES) value was calculated from the GCD curves as follows RES = ΔVIR /2I C

(3) DOI: 10.1021/acsami.8b17908 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article


Figure 2. (a) Schematic description of the fiber activation process. AFM images of the graphene film (b) before and (c) after activation (15 min). The corresponding height profiles (bottom) were measured along the white line in the images. where I is the applied constant current and ΔVIR is the voltage of the IR drop. The volumetric energy density (E) was calculated as follows E=

I 3600π

∫ V (t )dt

texture of the interwoven dark and bright brushes implies a nematic LC phase.37 This colloidal LC phase indicates that the GO sheets were completely dissolved in water without forming aggregates.37−40 The birefringent LC patterns were not disturbed even after the addition of CNTs at a weight ratio of 1:5. It is noteworthy that CNTs (surfactant free) were dispersible in aqueous media with the aid of surface-active GO.41 Eventually, the two components successfully dispersed together in DIW even at a higher concentration of 0.6 wt % (CNT/GO, 1:5 w/w). The LC nature of the colloids with anisotropic shapes promoted macroscopic assemblies that form fibers due to their spontaneous ordering.42 Figure 1d shows a photograph of the wet-spinning process using an aqueous CNT−GO dispersion (0.6 wt %; 1:5 w/w). The CNT−GO dope was spun through a spinneret into an aqueous coagulation bath containing 5 wt % CaCl2. In the bath, the dope solution transformed into CNT−GO LC gel fibers. As observed with a polarized optical microscope (Figure 1e), the GO (top) and CNT−GO (bottom) gel fibers showed clear Schlieren textures (evidence of the retained two-phase LC) even after the concentrated dispersion was gelled into a fibershaped structure.43 The low brightness of the CNT−GO gel fibers was due to the light absorption of CNTs. After drying, meter-long CNT−GO fibers reeled on a graphite bobbin were chemically reduced to CNT−GFs using HI (Figure 1b).

(4)

where V(t) is the discharge voltage as a function of discharge time from the GCD curves. The volumetric power density (P) was calculated as follows

P=

V02 4RESπ

(5)

where V0 is the operating voltage window of the GCD curve.

3. RESULTS AND DISCUSSION 3.1. Hybrid Assembly of Graphene and CNTs into Fibers and Surface Functionalization of Fibers via an Environmentally Benign Activation Process. The preparation of CNT−GF hybrids from the aqueous CNT−GO dispersion by the wet-spinning process is schematically shown in Figure 1a. The liquid crystalline (LC) phase of the aqueous GO−CNT dispersions was characterized by optical birefringent morphologies. After mild sonication of GO powder in DIW at a concentration of 0.05 wt %, a stable aqueous GO solution was obtained and was kept stationary between two crossed polarizers, as shown in Figure 1c. The birefringent D


Research Article


Figure 3. SEM images of (a, d) GF, (b, e) a-GF15, (c, f) a-GF30, (g, j) CNT−GF, (h, k) a-CNT−GF15, and (i, l) a-CNT−GF30.

The surface morphologies of the wet-spun GFs and CNT− GFs were studied using SEM. Figure 1f,g shows the surface and cross-sectional SEM images of a GF having a diameter of 34 μm, respectively. The stacked graphene sheets were successfully aligned to the axis of the fiber. A magnified image (Figure 1h) clearly shows highly stacked layers due to the strong van der Waals interactions between graphene sheets. Figure 1i,j shows the surface and cross-sectional SEM images of a CNT− GF with a slightly increased diameter of 36 μm, respectively. The CNTs are clearly visible and were uniformly distributed between graphene sheets as shown in the magnified image (Figure 1k). The morphology of the CNT−GF shown in the SEM images (Figure 1j,k) appears entirely distinctive with highly porous and rugged surfaces as compared to that of the pristine GF. Activation was performed on pristine GFs and CNT−GFs to develop porous and highly conducting FSCs. The fabrication

method for large-scale one-step activated CNT−GFs is schematically illustrated in Figure 2a. In the simple hydrothermal method, CNT−GFs were inserted in a sealed Teflon autoclave containing DIW (10 mL), H2O2 (1 mL), and NH4OH (500 μL) and were heated at 150 °C for 15 min (aCNT−GF15) or 30 min (a-CNT−GF30). Similarly, GFs without CNTs activated for various times (a-GF15 and aGF30) were also prepared for comparison. The mild activation performed using benign chemicals like H2O2 and NH4OH in DIW resulted in the formation of perforated graphene, which effectively removed the amorphous regions of GO sheets as observed in our previous studies.44,45 Graphene films were activated similarly using a hydrothermal method identical to GF to directly observe the activation performance on graphene macrostructures. Graphene films were prepared with the vacuum-assisted filtration of GO and subsequent reduction with HI. AFM was used to study the surface morphology of the E


Research Article


Figure 4. (a) XPS C 1s spectra and (b) Raman spectra of GF (top) and CNT−GF (bottom) samples. (c) Stress−strain curves of GF (top) and CNT−GF (bottom) samples. (d) Electrical conductivities of GF and CNT−GF samples as a function of the activation time.

pristine and activated graphene films because the fibers are very small with high surface curvature and roughness. Figure 2b,c shows the surface morphologies of graphene films observed with AFM before and after the activation reaction, respectively. Unlike the pristine graphene film (Figure 2b), the activated graphene film exhibited a widespread porous surface morphology, which was also evidenced by a corresponding height profile (Figures 2c and S1). The effectiveness of the mild activation process was verified using N2 adsorption−desorption isotherms with activated graphene films (Figure S2). The surface area of the pristine graphene films was measured to be 5.7 m2 g−1 and gradually increased from 7.4 to 8.9 m2 g−1 as the activation time increased from 15 to 30 min. The hydrothermal activation created pores that are favorable for the diffusion of N2 molecules into the stacked layers.24 Also, the total pore volume of graphene films (0.028 cm3 g−1) increased to 0.034 and 0.039 cm3 g−1 after 15 and 30 min of activation, respectively. To understand the effect of activation on the fibers, the surface morphologies were investigated using SEM (Figure 3). The as-prepared GF with a diameter of ∼34 μm showed compact and stacked graphene sheets as observed in Figure 3a,d. To increase the porosity, the GF was activated using environmentally benign chemicals by the simple hydrothermal route with different activation times as shown in Figure 3b,e

(a-GF15) and Figure 3c,f (a-GF30). Also, CNTs were incorporated as spacers between the graphene sheets to increase the electrochemical performance of GFs (Figure 3g,j). CNTs reduce dense stacking and enhance the porosity, which can increase the ion accessibility of GFs for high-performance FSC electrodes.23 The activation strategy was applied to the CNT−GF to further increase the porosity with different activation times as shown in Figure 3h,k (a-CNT−GF15) and Figure 3i,l (a-CNT−GF30). The activation process resulted in a definite change of the smooth surface morphology to become porous and rough. This could enhance the ion-accessible surface area of the fiber electrodes and ion-transport rate through the electrodes, and thus become beneficial for improving the electrochemical performance of the FSCs. 3.2. Chemical and Mechanical Properties of CNT− Graphene Hybrid Fibers. XPS and Raman analyses were used to confirm the hydrothermal activation of CNT−GFs. XPS spectra of the fibers before and after the activation were obtained to elucidate their surface composition and the results are presented in Figure 4a. The XPS C 1s spectra show a characteristic peak at 284.5 eV originating from the C−C and CC bonds in the carbon backbones of both graphene and CNTs. The minor peaks at 286.2, 287.8, and 289.0 eV are due to the presence of C−O, CO, and O−CO functionalities, respectively. The intensities of oxygen-related peaks gradually increased with the activation time due to the formation of F


Research Article


Figure 5. CV curves of (a) GF and (b) CNT−GF samples at a scan rate of 20 mV s−1 for a three-electrode configuration in a 6 M KOH electrolyte. Volumetric capacitance of (c) GF and (d) CNT−GF samples at different scan rates. GCD curves of (e) GF and (f) CNT−GF samples at a current density of 0.8 A cm−3. Nyquist plots of (g) GF and (h) CNT−GF samples (insets: the high-frequency regions of the plots).

corresponds to the presence of defective amorphous carbon structures.47,48 The intensity of the D band deteriorated with the activation time, which may have been due to the removal of the topological defects (e.g., Stone-Wales defects) during the activation process. The tensile properties of the fibers were measured before and after activation and the results are summurized in Table S1, including the tensile strength, Young’s modulus, elongation at break, and toughness. As shown in Figure 4c, the typical

pores resulting from the introduction of the oxygen functionalities into the samples.46 A similar trend was observable in XPS O 1s spectra of GF and CNT−GF samples (Figure S3). Furthermore, Raman analysis was used to verify the formation of pores with our hydrothermal activation method. Figure 4b displays the Raman spectra with two prominent peaks at 1340 and 1580 cm−1 corresponding to the D and G bands, respectively. The G band corresponds to the crystalline sp2-hybridized regions, whereas the D band G


Research Article


Figure 6. (a) Schematic illustration of all-solid-state FSCs which consist of two symmetric a-CNT−GF15 electrodes coated with a gel electrolyte. (b) CV curves of a-CNT−GF15 FSCs at different scan rates. (c) GCD curves of a-CNT−GF15 FSCs at different current densities. (d) Volumetric capacitance of a-CNT−GF15 FSCs at various current densities. (e) Nyquist plot of a-CNT−GF15 FSC recorded in the frequency range of 100 kHz to 10 mHz (inset: the high-frequency region of the plot). (f) Cycle life of a-CNT−GF15 FSC measured at a current density of 0.7 A cm−3.

CNT−GFs was attributed to their increased electrical conductivity.52 Interestingly, the redox reaction of residual oxygen functionalities on CNTs contributed to the asymmetric shape of the CV curves. After the activation, the CV curves of a-GF15 and a-CNT−GF15 became less oblique with larger total area than pristine GF and CNT−GF samples. However, in the cases of a-GF30 and a-CNT−GF30, excessive activation degraded their capacitive performance resulting in reduced current densities that were inferior to those of the pristine fibers. Figure 5c,d presents the changes in the volumetric capacitance of GF and CNT−GF samples depending on scan rates, respectively. When the scan rate increased from 10 to 100 mV s−1, a-CNT−GF15 retained 63% of its initial volumetric capacitance (120.49 F cm−3), which is higher than that of pristine CNT−GFs (51%, 102.53 F cm−3). GCD curves of the fiber electrodes were measured at various current densities of 1.6−0.7 A cm−3 (Figure S4g−l). Figure 5e,f displays the GCD curves of GF and CNT−GF samples at a current density of 0.8 A cm−3, respectively. The discharge times of a-GF15 and a-CNT−GF15 increased, and their corresponding volumetric capacitances were 95.79 and 139.49 F cm−3, respectively. These values are 1.3 and 1.2 times greater than those of pristine GF and CNT−GF samples, respectively. The ESR value of each fiber was derived from the voltage (IR) drops at the initial stage of the discharge curves. These values are listed in Table S2. The inherent resistance of electrodes and ion diffusion resistance were considered as the main contributors to the ESR.53 Thus, the reduced ESR values of aGF15 and a-CNT−GF15 were due to the faster ion diffusion rate into the porous microstructure than those of the nonporous pristine GF and CNT−GF electrodes. In contrast, the relatively high ESR values of a-GF30 and a-CNT−GF30

stress−strain curves of GF and CNT−GF samples show a gradual decrease of the elongation at break with activation time. Surprisingly, CNT−GFs exhibited enhanced overall mechanical properties compared to GFs due to the intercalated CNTs. Interlayer frictional forces between components inside fibers can be increased by filling voids with CNTs; this can often be found in the mixture of small and large GOs.49,50 The improvement of elongation at break with the addition of CNTs may be due to the partially aligned CNTs.51 The presence of CNTs in the a-CNT−GFs provides a significant improvement in mechanical properties, indicating superior ductility. The electrical properties were studied to understand the effect of CNTs and activation on CNT−GFs. The intercalation of CNTs in GFs played a critical role in significantly enhancing the electrical conductivity (133.3 S cm−1), which increased 6fold compared to that of pristine GFs (22.6 S cm−1) (Figure 4d). Longer activation times had an adverse effect on the mechanical and electrical properties of a-GFs and a-CNT−GFs due to the formation of pores which acted as defects for the mechanical performance and as trap sites for the transport of conduction electrons. 3.3. Electrochemical Properties of CNT−Graphene Hybrid Fibers. The electrochemical performance of different fiber electrodes was tested using a three-electrode configuration in a 6 M KOH aqueous electrolyte. CV curves were measured at various scan rates of 10−100 mV s−1 (Figure S4a−f). Figure 5a,b shows CV curves of GF and CNT−GF samples at a fixed scan rate of 20 mV s−1, respectively. GFs showed a compressed but symmetric CV curve with a streamlined shape, which implies a resistive nature and slow ion diffusion into its compact layered structure.51 As shown in Figure 5b, the incorporation of CNTs increased the total area of the CV curve. This enhanced capacitive performance of H


Research Article


Figure 7. (a) Capacitance retention before and after twisting of a-CNT−GF15 FSCs showing almost unchanged CV (at a scan rate of 25 mV s−1) and (b) GCD curves (at a current density of 0.07 A cm−3) (inset: photograph of a-CNT−GF15 FSC after twisting). (c) Capacitance retention at various bending angles (inset: photograph of a-CNT−GF15 FSC during bending). (d) Ragone plot of volumetric energy density versus volumetric power density for CNT−GF and a-CNT−GF15 FSCs.

GFs (Figure S6a), indicating that activation enhances the charge storage capability of the device due to the high porosity and the larger accessible surface area, which facilitate ion accessibility. The CV curves of CNT−GF and a-CNT−GF15 FSCs at various scan rates from 10 to 100 mV s−1 show a rapid current response with good symmetry (Figure S6b,c). Furthermore, GCD measurements were carried out at different current densities to evaluate the electrochemical performance of the optimized activated electrode. The GCD curves at different current densities for a-CNT−GF15 FSCs are shown in Figure 6c. The excellent symmetry of GCD curves with a minimal IR drop for a-CNT−GF15 FSCs suggests good charge storage properties due to low internal resistance and the short ion-diffusion distance required for FSCs. The highest volumetric capacitance of a-CNT−GF15 FSCs was 60.75 F cm−3, which was higher than that of CNT− GFs (19.80 F cm−3) at 0.05 A cm−3. The volumetric capacitance decreased with increasing current densities and retained 50% for a-CNT−GF15 FSCs at a current density of 1.0 A cm−3 (Figure 6d). EIS and cycling stability are essential for evaluating the performance of SCs. EIS studies were performed to understand the effect of activation on a-CNT−GF15 for all-solid-state symmetric FSCs (Figure 6e). The Nyquist plots were obtained in the frequency range of 100 kHz to 10 mHz. The porous aCNT−GF15 electrode showed small intrinsic resistance and vertical straight line in the low-frequency region, depicting its good capacitive performance (inset Figure 6e). The minor loop region shows the fast ion diffusion and the excellent electrical conductivity of the a-CNT−GF15 electrode. Allsolid-state symmetric FSCs based on a-CNT−GF15 exhibited excellent electrochemical stability above 94% after 10 000 cycles measured at a current density of 0.7 A cm−3, as is often

were attributed to their high electrode resistance despite their developed porous structures. Figure 5g,h shows the Nyquist plots of GF and CNT−GF samples obtained over a frequency range of 100 kHz to 10 mHz. The a-CNT−GF15 showed the highest slope of the vertical line in the low-frequency region compared to CNT− GF or CNT−GF30. In the high-frequency 45° Warburg region (inset Figure 5h), a-CNT−GF15 showed the shortest length with a low ion diffusion resistance. As shown in Figure S5, the Warburg impedance (Zw) values of GF and CNT−GF electrodes were derived by fitting the EIS results (Figure 5g,h) with the equivalent circuit of the inset. Indeed, the aCNT−GF15 exhibited the lowest Zw value (1.1 kΩ s−1/2) compared to CNT−GF (1.6 kΩ s−1/2) and a-CNT−GF30 (1.4 kΩ s−1/2), indicating the facile ion diffusion through the porous surface structure of a-CNT−GF15. In addition, a similar trend was observed for GF electrodes; Zw values of GF, a-GF15, and a-GF30 were 2.9, 2.7, and 2.8 kΩ s−1/2, respectively. The tendency of Zw values is in good agreement with that of our ESR results from the GCD analysis (Table S2). The optimized a-CNT−GF15 was chosen to evaluate the performance of the all-solid-state symmetric FSCs using a twoelectrode cell configuration. Two strands of a-CNT−GF15 were aligned in parallel to form both the anode and cathode for all-solid-state symmetric FSCs. Furthermore, each fiber was coated with a thin layer of the PVA−KOH gel electrolyte, acting both as a separator and an electrolyte, and they were then packed in a PVC tube (Figure 6a). The CV curves of allsolid-state symmetric a-CNT−GF15 FSCs at different scan rates (10−100 mV s−1) are shown in Figure 6b. The CV curves show a rectangular shape with typical EDLC behavior. The area under the CV curve and current density of a-CNT−GF15 FSCs are comparatively more significant than those of CNT− I


Research Article

ACS Applied Materials & Interfaces observed in the typical carbon-based EDLC SCs (Figure 6f).54−56 This stable capacitive performance of the fiber electrodes demonstrates their excellent potential for applications in miniaturized portable and wearable electronics. Flexibility and bendability are essential factors for an electrode to be used in all-solid-state symmetric FSCs. CV and GCD curves were measured before and after twisting the FSCs (Figure 7a,b) to verify their flexibility. The CV and GCD curves showed a negligible change. In addition, Figure 7c shows the bendability test results at various bending angles. It is clear that the all-solid-state symmetric FSCs retained almost 100% of the initial capacitance even when they are bent at 180°. The device was initially bent to 180°, then to 90°, followed by 45° before finally recovering to 180°. The above results indicate the stable capacitive performance of a-CNT− GF15 under deformation conditions for wearable FSC applications. Energy density and power density are shown in a Ragone plot (Figure 7d) to evaluate the efficiency of our all-solid-state symmetric FSCs based on our facile activation strategy. Energy density and power density were obtained from GCD curves using eqs 4 and 5. The a-CNT−GF15 FSCs exhibited an energy density of 4.83 mWh cm−3 at a power density of 18.10 mW cm−3, which is an almost 3-fold overall improvement compared to CNT−GF FSCs (1.50 mWh cm−3 at 17.11 mW cm−3). The energy densities of preveiously reported FSCsare compared and summarized in Table S3. The high energy density of our a-CNT−GF15 FSCs is due to synergistic effects of the high electrical conductivity and mobile ion permeability as a result of the porous morphology which paved the way for the electron and ion pathways to enhance the electrochemical performance. Enhancements in the electrochemical performance of our aCNT−GF15 FSCs hinge on the following factors. First, the intercalation of CNTs in GFs as spacers improved the electrical conductivity and mechanical properties as GFs have restacked graphene sheets with a compact nature, which hinders their EDLC performance. Second, an increase in porosity due to the activation technique helped in increasing the SSA by facilitating ion diffusion and improving active surface areas in GFs. Third, the activation of CNT−GFs was precisely controlled by optimizing the activation conditions to overcome degradations in the mechanical and electrical properties and capacitive performance. We believe that this simple activation technique to prepare highly porous and conductive electrodes for 1D-based FSCs will provide excellent materials for designing environmentally friendly, stable, and wearable SC electrodes in the near future.

conductivity. These mechanically robust and porous fibers consisting of CNTs and perforated graphene sheets can increase the accessible surface area and facilitate ion diffusion, which is extremely important when serving as electrodes for FSCs. Miniature FSCs based on a-CNT−GF15 will be promising electrodes for next-generation environmentally friendly, wearable portable electronics.

4. CONCLUSIONS In summary, high-performance flexible all-solid-state FSCs based on the facile activation of CNT−GFs were successfully developed through a simple hydrothermal method using environmentally benign chemicals like H2O2 and NH4OH. The optimized a-CNT−GF15 electrodes showed an enhanced capacitance with a volumetric capacitance of 60.75 F cm−3, which was higher than that of the pristine CNT−GF (19.80 F cm−3). Also, a-CNT−GF15 exhibited a high energy density (4.83 mWh cm−3) that was ∼3 times higher than that of the untreated CNT−GFs (1.50 mWh cm−3) while maintaining its power density, rate capability, and cycling stability. In addition, the a-CNT−GFs showed high flexibility and bendability while maintaining good mechanical properties and electrical

■

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b17908.

■

Large-area AFM images of graphene films before and after activation; BET isotherms of graphene and activated graphene films; XPS O 1s spectra of GF and CNT−GF samples; CV curves of GF and CNT−GF samples; Warburg impedance values of GF and CNT− GF electrodes; various comparisons of CV and GCD curves; table of summary of the mechanical and electrical properties of GFs and CNT−GFs; and table of summary of ESR values in the liquid KOH electrolyte (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tae Hee Han: 0000-0001-5950-7103 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Author Contributions ∥

H.P., R.B.A., and S.H.N. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation (NRF2017R1A2B4010771 and 2016R1A6A1A03013422), the Nano Material Technology Development Program (NRF2016M3A7B4905609), and the program for fostering nextgeneration researchers in engineering (NRF2017H1D8A2032495) of the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning. REFERENCES

(1) Peng, H. Fiber-Shaped Energy Harvesting and Storage Devices; Springer: Berlin, 2015. (2) Dubal, D. P.; Chodankar, N. R.; Kim, D-H.; Gomez-Romer, P. Towards Flexible Solid-State Supercapacitors for Smart and Wearable Electronics. Chem. Soc. Rev. 2018, 47, 2065−2129. (3) Yu, D.; Qian, Q.; Wei, L.; Jiang, W.; Goh, K.; Wei, J.; Zhang, J.; Chen, Y. Emergence of Fiber Supercapacitors. Chem. Soc. Rev. 2015, 44, 647−662. (4) Gulzar, U.; Goriparti, S.; Miele, E.; Li, T.; Maidecchi, G.; Toma, A.; Angelis, F. D.; Capiglia, C.; Zaccaria, R. P. Next-Generation Textiles: From Embedded Supercapacitors to Lithium Ion Batteries. J. Mater. Chem. A 2016, 4, 16771−16800. J


Research Article

ACS Applied Materials & Interfaces (5) Ambade, R. B.; Ambade, S. B.; Salunkhe, R. R.; Malgras, V.; Jin, S-H.; Yamauchi, Y.; Lee, S-H. Flexible-Wire Shaped All-Solid-State Supercapacitors Based on Facile Electropolymerization of Polythiophene with Ultra-High Energy Density. J. Mater. Chem. A 2016, 4, 7406−7415. (6) Allagui, A.; Freeborn, T. J.; Elwakil, A. S.; Maundy, B. J. Reevaluation of Performance of Electric Double-layer Capacitors from Constant-current Charge/Discharge and Cyclic Voltammetry. Sci. Rep. 2016, 6, No. 38568. (7) Singh, A.; Chandra, A. Enhancing Specific Energy and Power in Asymmetric Supercapacitors - A Synergetic Strategy Based on the Use of Redox Additive Electrolytes. Sci. Rep. 2016, 6, No. 25793. (8) Shi, M.; Yang, C.; Song, X.; Liu, J.; Zhao, L.; Zhang, P.; Gao, L. Recoverable Wire-Shaped Supercapacitors with Ultrahigh Volumetric Energy Density for Multifunctional Portable and Wearable Electronics. ACS Appl. Mater. Interfaces 2017, 9, 17051−17059. (9) Purkait, T.; Singh, G.; Kumar, D.; Singh, M.; Dey, R. S. HighPerformance Flexible Supercapacitors Based on Electrochemically Tailored Three-Dimensional Reduced Graphene Oxide Networks. Sci. Rep. 2018, 8, No. 640. (10) Cong, H.-P.; Chen, J.-F.; Yu, S.-H. Graphene-Based Macroscopic Assemblies and Architectures: An Emerging Material System. Chem. Soc. Rev. 2014, 43, 7295−7325. (11) Shao, Y.; El-Kady, M. F.; Wang, L. J.; Zhang, Q.; Li, Y.; Wang, H.; Mousavi, M. F.; Kaner, R. B. Graphene-Based Materials for Flexible Supercapacitors. Chem. Soc. Rev. 2015, 44, 3639−3665. (12) Wang, Q.; Yan, J.; Fan, Z. Carbon Materials for High Volumetric Performance Supercapacitors: Design, Progress, Challenges and Opportunities. Energy Environ. Sci. 2016, 9, 729−762. (13) Miller, J. R.; Outlaw, R. A.; Holloway, B. C. Graphene DoubleLayer Capacitor with ac Line-Filtering Performance. Science 2010, 329, 1637−1639. (14) Xu, Z.; Gao, C. Graphene Chiral Liquid Crystals and Macroscopic Assembled Fibres. Nat. Commun. 2011, 2, No. 571. (15) Xu, Z.; Liu, Y. J.; Zhao, X. L.; Peng, L.; Sun, H. Y.; Xu, Y.; Ren, X. B.; Jin, C. H.; Xu, P.; Wang, M.; Gao, C. Ultrastiff and Strong Graphene Fibers via Full-Scale Synergetic Defect Engineering. Adv. Mater. 2016, 28, 6449−6456. (16) Chen, L. L.; Liu, Y.; Zhao, Y.; Chen, N.; Qu, L. T. GrapheneBased Fibers for Supercapacitor Applications. Nanotechnology 2016, 27, No. 032001. (17) Huang, G. J.; Hou, C. Y.; Shao, Y. L.; Wang, H. Z.; Zhang, Q. H.; Li, Y. G.; Zhu, M. F. Highly Strong and Elastic Graphene Fibres Prepared from Universal Graphene Oxide Precursors. Sci. Rep. 2014, 4, No. 4248. (18) Kou, L.; Huang, T.; Zheng, B.; Han, Y.; Zhao, X.; Gopalsamy, K.; Sun, H.; Gao, C. Coaxial Wet-Spun Yarn Supercapacitors for High-Energy Density and Safe Wearable Electronics. Nat. Commun. 2014, 5, No. 3754. (19) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-Based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558−1565. (20) Zheng, X.; Zhang, K.; Yao, L.; Qiu, Y.; Wang, S. Hierarchically Porous Sheath−Core Graphene-Based Fiber-Shaped Supercapacitors with High Energy Density. J. Mater. Chem. A 2018, 6, 896−907. (21) Wang, S.; Pei, B.; Zhao, X.; Dryfe, R. A. W. Highly Porous Graphene on Carbon Cloth as Advanced Electrodes for Flexible AllSolid-State Supercapacitors. Nano Energy 2013, 2, 530−536. (22) Beidaghi, M.; Wang, C. Micro-Supercapacitors Based on Interdigital Electrodes of Reduced Graphene Oxide and Carbon Nanotube Composites with Ultrahigh Power Handling Performance. Adv. Funct. Mater. 2012, 22, 4501−4510. (23) Lu, Z.; Foroughi, J.; Wang, C.; Long, H.; Wallace, G. G. Superelastic Hybrid CNT/Graphene Fibers for Wearable Energy Storage. Adv. Energy Mater. 2018, 8, No. 1702047. (24) Zhu, Y. W.; Murali, S.; Stoller, M. D.; Ganesh, K. J.; Cai, W. W.; Ferreira, P. J.; Pirkle, A.; Wallace, R. M.; Cychosz, K. A.; Thommes,

M.; Su, D.; Stach, E. A.; Ruoff, R. S. Carbon-Based Supercapacitors Produced by Activation of Graphene. Science 2011, 332, 1537−1541. (25) Zhang, L. L.; Zhao, X.; Stoller, M. D.; Zhu, Y. W.; Ji, H. X.; Murali, S.; Wu, Y. P.; Perales, S.; Clevenger, B.; Ruoff, R. S. Highly Conductive and Porous Activated Reduced Graphene Oxide Films for High-Power Supercapacitors. Nano Lett. 2012, 12, 1806−1812. (26) Xu, J.; Tan, Z.; Zeng, W.; Chen, G.; Wu, S.; Zhao, Y.; Ni, K.; Tao, Z.; Ikram, M.; Ji, H.; Zhu, Y. A Hierarchical Carbon Derived from Sponge-Templated Activation of Graphene Oxide for HighPerformance Supercapacitor Electrodes. Adv. Mater. 2016, 28, 5222− 5228. (27) Tang, J.; Wang, J.; Shrestha, L. K.; Hossain, M. S. A.; Alothman, Z. A.; Yamauchi, Y.; Ariga, K. Activated Porous Carbon Spheres with Customized Mesopores through Assembly of Diblock Copolymers for Electrochemical Capacitor. ACS Appl. Mater. Interfaces 2017, 9, 18986−18993. (28) Xu, Y. X.; Lin, Z. Y.; Zhong, X.; Huang, X. Q.; Weiss, N. O.; Huang, Y.; Duan, X. F. Holey Graphene Frameworks for Highly Efficient Capacitive Energy Storage. Nat. Commun. 2014, 5, No. 4554. (29) Xu, Y. X.; Chen, C. Y.; Zhao, Z. P.; Lin, Z. Y.; Lee, C.; Xu, X.; Wang, C.; Huang, Y.; Shakir, M. I.; Duan, X. F. Solution Processable Holey Graphene Oxide and Its Derived Macrostructures for HighPerformance Supercapacitors. Nano Lett. 2015, 15, 4605−4610. (30) Sevilla, M.; Ferrero, G. A.; Fuertes, A. B. Beyond KOH Activation for the Synthesis of Superactivated Carbons from Hydrochar. Carbon 2017, 114, 50−58. (31) Boronat, M.; Corma, A.; Renz, M.; Sastre, G.; Viruela, P. M. A Multisite Molecular Mechanism for Baeyer−Villiger Oxidations on Solid Catalysts Using Environmentally Friendly H2O2 as Oxidant. Chem. - Eur. J. 2005, 11, 6905−6915. (32) Gatemala, H.; Pienpinijtham, P.; Thammacharoen, C.; Ekgasit, S. Rapid Fabrication of Silver Microplates under an Oxidative Etching Environment Consisting of O2/Cl−, NH4OH/H2O2, and H2O2. CrystEngComm 2015, 17, 5530−5537. (33) Kim, F.; Luo, J. Y.; Cruz-Silva, R.; Cote, L. J.; Sohn, K.; Huang, J. X. Self-Propagating Domino-like Reactions in Oxidized Graphite. Adv. Funct. Mater. 2010, 20, 2867−2873. (34) Eom, W.; Kim, A.; Park, H.; Kim, H.; Han, T. H. GrapheneMimicking 2D Porous Co3O4 Nanofoils for Lithium Battery Applications. Adv. Funct. Mater. 2016, 26, 7605−7613. (35) Chuang, T. J.; Anderson, P. M.; Wu, M. K.; Hsieh, S. Nanomechanics of Materials and Structures; Springer: the Netherlands, 2006. (36) Bazbouz, M. B.; Stylios, G. K. The Tensile Properties of Electrospun Nylon 6 Single Nanofibers. J. Polym. Sci., Part B: Polym. Phys. 2010, 48, 1719−1731. (37) Kim, J. E.; Han, T. H.; Lee, S. H.; Kim, J. Y.; Ahn, C. W.; Yun, J. M.; Kim, S. O. Graphene Oxide Liquid Crystals. Angew. Chem., Int. Edit. 2011, 50, 3043−3047. (38) Han, T. H.; Kim, J.; Park, J. S.; Park, C. B.; Ihee, H.; Kim, S. O. Liquid Crystalline Peptide Nanowires. Adv. Mater. 2007, 19, 3924− 3927. (39) Song, W. H.; Kinloch, I. A.; Windle, A. H. Nematic Liquid Crystallinity of Multiwall Carbon Nanotubes. Science 2003, 302, No. 1363. (40) Davis, V. A.; Ericson, L. M.; Parra-Vasquez, A. N. G.; Fan, H.; Wang, Y.; Prieto, V.; Longoria, J. A.; Ramesh, S.; Saini, R. K.; Kittrell, C.; Billups, W. E.; Adams, W. W.; Hauge, R. H.; Smalley, R. E.; Pasquali, M. Phase Behavior and Rheology of SWNTs in Superacids. Macromolecules 2004, 37, 154−160. (41) Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R.; Huang, J. X. Graphene Oxide Sheets at Interfaces. J. Am. Chem. Soc. 2010, 132, 8180−8186. (42) Xu, Z.; Gao, C. Graphene in Macroscopic Order: Liquid Crystals and Wet-Spun Fibers. Acc. Chem. Res. 2014, 47, 1267−1276. (43) Xu, Z.; Liu, Z.; Sun, H. Y.; Gao, C. Highly Electrically Conductive Ag-Doped Graphene Fibers as Stretchable Conductors. Adv. Mater. 2013, 25, 3249−3253. K


Research Article

ACS Applied Materials & Interfaces (44) Han, T. H.; Huang, Y. K.; Tan, A. T. L.; Dravid, V. P.; Huang, J. X. Steam Etched Porous Graphene Oxide Network for Chemical Sensing. J. Am. Chem. Soc. 2011, 133, 15264−15267. (45) Park, H.; Noh, S. H.; Lee, J. H.; Lee, W. J.; Jaung, J. Y.; Lee, S. G.; Han, T. H. Large Scale Synthesis and Light Emitting Fibers of Tailor-Made Graphene Quantum Dots. Sci. Rep. 2015, 5, No. 14163. (46) Zhao, X.; Hayner, C. M.; Kung, M. C.; Kung, H. H. Flexible Holey Graphene Paper Electrodes with Enhanced Rate Capability for Energy Storage Applications. ACS Nano 2011, 5, 8739−8749. (47) Casiraghi, C.; Hartschuh, A.; Qian, H.; Piscanec, S.; Georgi, C.; Fasoli, A.; Novoselov, K. S.; Basko, D. M.; Ferrari, A. C. Raman Spectroscopy of Graphene Edges. Nano Lett. 2009, 9, 1433−1441. (48) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on Carbon Nanotubes and Graphene Raman Spectroscopy. Nano Lett. 2010, 10, 751−758. (49) Xin, G. Q.; Yao, T. K.; Sun, H. T.; Scott, S. M.; Shao, D. L.; Wang, G. K.; Lian, J. Highly Thermally Conductive and Mechanically Strong Graphene Fibers. Science 2015, 349, 1083−1087. (50) Guo, F.; Kim, F.; Han, T. H.; Shenoy, V. B.; Huang, J. X.; Hurt, R. H. Hydration-Responsive Folding and Unfolding in Graphene Oxide Liquid Crystal Phases. ACS Nano 2011, 5, 8019−8025. (51) Ding, X. T.; Zhao, Y.; Hu, C. G.; Hu, Y.; Dong, Z. L.; Chen, N.; Zhang, Z. P.; Qu, L. T. Spinning Fabrication of Graphene/ Polypyrrole Composite Fibers for All-Solid-State, Flexible Fibriform Supercapacitors. J. Mater. Chem. A 2014, 2, 12355−12360. (52) Ma, Y. W.; Li, P.; Sedloff, J. W.; Zhang, X.; Zhang, H. B.; Liu, J. Conductive Graphene Fibers for Wire-Shaped Supercapacitors Strengthened by Unfunctionalized Few-Walled Carbon Nanotubes. ACS Nano 2015, 9, 1352−1359. (53) Rose, M.; Korenblit, Y.; Kockrick, E.; Borchardt, L.; Oschatz, M.; Kaskel, S.; Yushin, G. Hierarchical Micro- and Mesoporous Carbide-Derived Carbon as a High-Performance Electrode Material in Supercapacitors. Small 2011, 7, 1108−1117. (54) Xu, P.; Gu, T. L.; Cao, Z. Y.; Wei, B. Q.; Yu, J. Y.; Li, F. X.; Byun, J. H.; Lu, W. N.; Li, Q. W.; Chou, T. W. Carbon Nanotube Fiber Based Stretchable Wire-Shaped Supercapacitors. Adv. Energy Mater. 2014, 4, No. 1300759. (55) Han, X. G.; Funk, M. R.; Shen, F.; Chen, Y. C.; Li, Y. Y.; Campbell, C. J.; Dai, J. Q.; Yang, X. F.; Kim, J. W.; Liao, Y. L.; Connell, J. W.; Barone, V.; Chen, Z. F.; Lin, Y.; Hu, L. B. Scalable Holey Graphene Synthesis and Dense Electrode Fabrication toward High-Performance Ultracapacitors. ACS Nano 2014, 8, 8255−8265. (56) Wang, T.; Wang, L. X.; Wu, D. L.; Xia, W.; Jia, D. Z. Interaction between Nitrogen and Sulfur in Co-Doped Graphene and Synergetic Effect in Supercapacitor. Sci. Rep. 2015, 5, No. 9591.

L


Porous Graphene-Carbon Nanotube Scaffolds for Fiber

Recommend Documents