Hierarchically porous sheath–core graphene-based fiber-shaped supercapacitors with high energy density

Xianhong Zheng ab, Kun Zhang *ab, Lan Yao ab, Yiping Qiu abc and Shiren Wang d
aKey Laboratory of Textile Science & Technology, Donghua University, Ministry of Education, Shanghai 201620, PR China. E-mail: kun.zhang@dhu.edu.cn
bCollege of Textiles, Donghua University, Shanghai 201620, PR China
cCollege of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, PR China
dDepartment of Industrial and Systems Engineering, Texas A&M University, College Station, TX 77843, USA

Received 21st September 2017 , Accepted 6th November 2017

First published on 6th November 2017


Owing to their high power density, fast charge/discharge rate, ultralong cycling life and safe operating conditions, all-carbon fiber-shaped supercapacitors (FSSCs) hold great promise for powering wearable electronics. However, their low energy density hinders them from practical application, due to the absence of effective approaches for highly electrically conductive fiber electrodes with high specific capacitance. Herein, we develop a scalable and cost-effective strategy towards hierarchically porous core–sheath graphene-based fiber electrodes with a rational PSD (88–97% micropores, 0–8.3% mesopores and 1.9–4.2% macropores) and high SSA (up to 416.4 m2 g−1). The hierarchical architecture is achieved by simply decorating graphene fiber with carbonized phenol formaldehyde (CPF) resin containing small-size (∼156 nm in diameter) graphene (SG), wherein the incorporation of CPF and SG synergistically provides ultrahigh micro-porosity with narrowed micro-/meso-PSDs and enhanced electrical conductivity, facilitating ion storage and transport. The assembled FSSCs exhibit an ultrahigh specific areal capacitance of 391.2 mF cm−2 in polyvinyl alcohol/H2SO4 electrolyte at 0.1 mA cm−2 in a two-electrode cell, which is 17 times that of graphene fibers. The entire-device energy density Ecell,A is 8.7 μW h cm−2 in aqueous electrolyte and 66.4 μW h cm−2 at an areal power density of 0.54 mW cm−2 in organic electrolyte. Moreover, the FSSCs also show ultralong cycling life (98.9% capacitance retention after 7000 cycles) and good flexibility. All these results have made it one of the best ever reported all-carbon FSSCs to date. This work may shed light on mass-manufacturing low-cost but high-performance wearable fiber-shaped energy storage devices.


Introduction

In recent years, widespread and concerted efforts have been made to develop smart textiles integrated with diverse functionalities such as self-powering, active/adaptive sensing, antenna and display, etc.1–5 Fiber-shaped supercapacitors (FSSCs) hold great promise as wearable and weavable energy storage devices for smart textiles, owing to their high power density, long cycling life, fast charge–discharge rate and safe operating conditions.6,7

The fabrication of FSSCs involves the usage of pseudocapacitive active materials and/or carbon materials. Pseudocapacitors containing transition metal oxides and/or conducting polymers (theoretical capacitance, 210–3560 F g−1) possess specific capacitance as high as 834 mF cm−2.8–13 However, most of them still suffer from high cost, low electrical conductivity (10−6–1 S cm−1), inferior rate performance and poor cycling stability.14–17 Owing to their high power density, fast charge/discharge rate, ultralong cycling life and safe operating conditions, all-carbon based fibers have great potential for FSSCs, which mainly refer to carbon nanotube (CNT), graphene and carbon fiber-based supercapacitors.18–29 Among them, graphene fibers are very promising because of their low cost, easy functionalization and tunable structures.30–33 However, due to strong π–π interaction, the restacking of graphene sheets in graphene fibers leads to a much smaller SSA (13.4–35.8 m2 g−1) than the theoretical SSA of graphene (2630 m2 g−1).34–36 Therefore, it dramatically diminishes the ion-accessible surface area for solvated ions and thus hinders ion transport and storage.

Several strategies have been developed to increase the SSA and improve the electrochemical (EC) performances of graphene fibers, which include increasing the mesoporosity by mixing graphene with other carbonaceous materials (i.e. carbon black, CNTs, etc.35–39). For instance, mesoporous carbon black/graphene composite fibers (pore size, 2–200 nm) exhibited a high SSA (254.6 m2 g−1) and a high specific capacitance of 97.5 F cm−3.35 CNT/pristine or nitrogen-doped graphene composite FSSCs showed high specific capacitance (38.8–300 F cm−3), owing to the increased SSA and mesoporosity (pore size, 2–18 nm).36–39 Besides, pseudocapacitive active materials/graphene hybrid fibers were also fabricated to achieve better electrochemical performance. For example, MnO2 nanostructure and conducting polymer (i.e. polyaniline and polypyrrole) decorated graphene-based FSSCs showed significant improvements in specific capacitance owing to the introduced faradaic pseudo-capacitance.40–43

Although notable achievements have been made in improving the electrochemical performance of graphene-based FSSCs, intense research efforts have been mainly focused on improving the mesoporous SSA or introducing pseudo-capacitance. However, for all-carbon graphene-based FSSCs, few studies on increasing the microporous SSA for better electrochemical performance have been reported. It is well known that ideal carbon electrodes should have a hierarchically porous structure with abundant micropores (<2 nm) for facilitating charge storage, a moderate amount of mesopores (2–50 nm) for promoting ion transport and a few macropores (>50 nm) as ion-buffering reservoirs.14,44–52 Hence, it is of key importance to design hierarchically micropore-rich carbon-based electrodes for FSSCs.

Herein, we report all-carbon, core–sheath graphene-based fibers with a hierarchically porous architecture, containing 88–97% micropores, 0–8.3% mesopores and 1.9–4.2% macropores. These graphene-based fiber electrodes were fabricated by a facile, scalable and cost-effective wet-spinning/dip-coating method with high-temperature carbonization. The assembled SCs show a record high area specific areal capacitance (391.2 mF cm−2 measured at 0.1 mA cm−2), high entire-device energy density (8.7 μW h cm−2 for the PVA/H2SO4 electrolyte and 66.4 μW h cm−2 for organic electrolyte) at high power density, ultralong cycling life, and good flexibility and cycling stability.

Experimental

Preparation of small-size graphene oxide (SGO)

Graphene oxide (GO) was prepared by a modified Hummers' method.53 In detail, a mixture of 3 g graphite (50 mesh) and 18 g potassium permanganate (KMnO4) was added into a solution of 360 ml sulfuric acid (H2SO4, 98 wt%) and 40 ml phosphoric acid (H3PO4, 85 wt%). The mixture was kept at 50 °C, stirred for 10 hours, and subsequently diluted with 500 ml deionized (DI) water. Hydrogen peroxide (H2O2, 30 wt%) was added into the reaction mixture until the solution color changed to light yellow. The solution was then filtered and washed with 200 ml hydrochloric acid (30 wt%) and abundant DI water. The as-synthesized graphite oxide was ultrasonicated for 1 h and subsequently centrifuged at 3000 rpm for 10 min to remove the unexfoliated graphite oxide. To remove the small size graphene oxide, the supernatant was centrifuged at 8000 rpm and 4000 rpm, respectively, and subsequently condensed and used for spinning graphene oxide fibers (GOFs).

Small size graphene oxide (SGO) was prepared with a high-power ultrasonication instrument (Biosafer, 312 W) with 10 hour ultrasonication to break graphene oxide (∼30 microns) into SGO (∼150 nm). The obtained dispersion was then subjected to 30 min centrifugation at 3000 rpm to remove the unexfoliated graphite oxide. Finally, the SGO dispersion was thermally concentrated to high concentration for further usage. Note: in order to distinguish it from SGO, we denoted the as-synthesized GO as LGO.

Preparation of graphene fibers

Graphene oxide fiber (GOF) was wet-spun using the method described in our previous work.54 Typically, 15 mg ml−1 GO aqueous dispersion was injected into an ethanol/water (1[thin space (1/6-em)]:[thin space (1/6-em)]3 v/v) coagulation bath containing 5 wt% CaCl2 through a 300 μm inner diameter spinneret. The resultant GOF was then thoroughly washed with water and ethanol to remove any residual coagulants. Graphene fiber (GF) was obtained by thermally annealing GOF at 800 °C for 3 hours under argon protection.

Preparation of porous core–sheath graphene-based fibers (SG-CPF@GF)

SG-CPF@GF was fabricated by a “dip-coating” method as described below. A certain amount of SGO was mixed into an aqueous solution of 50 wt% phenol formaldehyde (PF) resin, and the SGO concentration was set to 0.5 wt%, 1 wt%, 1.5 wt% and 2 wt% with respect to solid PF content. The mixture was homogenized at 10[thin space (1/6-em)]000 rpm for 15 min. The as-prepared GOF was immersed in the SGO-PF mixture and then dried and solidified at 80 °C. The product is denoted as SGO-PF@GOF. The uncarbonized fibers (SGO-PF@GOF) containing 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt% SG in an SGO-PF sheath are denoted as PF@GOF, 0.5%-SGO-PF@GOF, 1%-SGO-PF@GOF, 1.5%-SGO-PF@GOF, and 2%-SGO-PF@GOF, respectively.

Porous SG-CPF@GFs were prepared by pre-oxidizing the SGO-PF@GOF with temperature steps at 120 °C, 150 °C and 180 °C for 1 h, respectively. Subsequent thermal annealing was carried out at 800 °C for 3 h under argon protection. The carbonized composite fibers (SG-CPF@GF) containing 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, and 2 wt% SMG in an SMG-PF sheath are denoted as CPF@GF, 0.5%-SG-CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF, and 2%-SG-CPF@GF, respectively.

Assembly of porous graphene composite FSSCs

The process for the fabrication of FSSCs in aqueous electrolyte is as follows. 1 g polyvinyl alcohol (PVA, 80% hydrolyzed) was dissolved in 10 ml DI water by stirring at 90 °C for 30 min. 0.98 g H2SO4 (98 wt%) was then added to form the gel electrolyte (H2SO4/PVA). To fabricate all-solid-state fiber-based SCs, two fiber electrodes were immersed in PVA/H2SO4 gel electrolyte and then placed parallel to each other on a PET substrate and the end of each fiber was electrically glued to a polished copper foil with silver paste, respectively. The assembled FSSCs were solidified overnight at room temperature. FSSCs were also assembled and tested in organic electrolyte. In detail, for the preparation of organic electrolyte, 189 mg polyvinylidene fluoride-co-hexafluoropropylene (PVDF) was added into 10 ml DMF and stirred for 1 h. 1.98 g 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) was added into the solution to form the gel electrolyte.

Characterization

The polarized optical microscopy images of GOFs and SG-CPF@GFs were obtained using a Nikon ECLIPSE LV100POL. The lateral size of SGO was measured with a transmission electron microscope (TEM, JEM-2100). The lateral size distribution of GO sheets was measured based on more than 300 pieces of GO sheets obtained from TEM images. The equivalent diameter of regular GO sheets was estimated using ImageJ software. High-resolution TEM images (HRTEM) were obtained with a JEOL JEM2100F microscope. The surface morphology of GFs and SG-CPF@GFs was characterized using a low-/high-resolution scanning electron microscope (SEM, HITACHI, TM3000 and SU5000). X-ray diffraction (XRD) patterns of the samples were obtained with an X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) (RIGAKU D/Max-2550 PC). Fourier transform infrared spectroscopy (FTIR) was conducted on a Nicolet NEXUS-670 to characterize the chemical structure of fibers. Atomic force microscopy (AFM) was carried out using an Agilent 5500. X-ray photoelectron spectroscopy (XPS) spectra were collected on an ESCALAB250Xi.

Raman spectra were collected with a Raman spectrometer with a 633 nm laser line (Via-Reflex) to provide structural fingerprints of GF and SG-CPF@GFs. The crystalline domain size (La) was determined using the empirical formula La (nm) = (2.4 × 10−10) × λ4(IG/ID), where λ is the laser wavelength and IG/ID is the integrated intensity ratio of G band and D band.55,56 Nitrogen (N2) adsorption–desorption isotherms were measured at 77 K with a Micromeritics ASAP 2020 static volumetric gas adsorption instrument. Prior to testing, fibers were degassed at 300 °C under high vacuum for 10 h.43 The SSA was calculated based on the Brunauer–Emmett–Teller (BET) technique. The proportion of micropores, mesopores and macropores is calculated as Vmicro/Vtotal, Vmeso/Vtotal and Vmacro/Vtotal, respectively, where Vmicro, Vmeso, Vmacro and Vtotal are the micropore, mesopore, macropore and total pore volume, respectively. The tensile properties were measured using a single fiber tension tester (XQ-2, Shanghai New Fiber Instrument). The linear density (unit, g m−1) of the fiber was measured by using a Sartorius BT125D electronic balance to test the weight of the fiber at a given length.

The electrochemical performances of the fiber-based SCs, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectra (EIS) (0.01 Hz to 100 kHz), were measured with an electrochemical workstation (CHI 660E, CH Instruments Inc.) in a two-electrode configuration. The cycling performance was investigated with a CT 2001A battery program controlling test system (China-Land Co., Ltd). The specific capacitance, energy density and power density were calculated as reported in the literature.8,36 The capacitance of the supercapacitors (Ccell) in a two-electrode cell was calculated from the galvanostatic charge/discharge curves at different current densities using Ccell = i/(dV/dt), where i and dV/dt are the current and the corresponding slope of the discharge curve, respectively. Ccell can also be calculated by the cyclic voltammetry method, using the equationimage file: c7ta08362a-t1.tif, where Q, V1 and V2, ν and i(V) are the total charge, low and high potentials, scan rate and current, respectively. The areal and volumetric specific capacitance of single fiber electrodes was calculated using CA = 2Ccell/Asingle and CV = 2Ccell/Vsingle, respectively, where Asingle and Vsingle are the surface area and volume of a single fiber. In detail, A = πDL and V = πD2L/4, where D and L are the diameter and length of a fiber, respectively. The length of the fiber electrode is the active part, which was measured with a ruler. The diameter of the fiber electrode was measured with a SEM to measure the fiber electrode area. The entire-device areal energy density (Ecell,A) and volumetric energy density (Ecell,V) can be calculated using Ecell,A = CAV2/(8 × 3600) and Ecell,V =CVV2/(8 × 3600), respectively, where CA, CV and V are the areal specific capacitance, volumetric specific capacitance and operating voltage window. The areal power density (Pcell,A) and volumetric power density (Pcell,V) can be calculated using Pcell,A = Ecell,A × 3600/tdischarge and Pcell,V = Ecell,V × 3600/tdischarge, respectively, where Ecell,A, Ecell,V and tdischarge are the areal energy density, volumetric energy density and discharge time. Specifically, the areal energy density (EA) and power density (PA) of a single electrode can be calculated using EA = 4Ecell,A and PA = 4Pcell,A, respectively. The volumetric energy density (EV) and power density (PV) of a single electrode can be calculated using EV = 4Ecell,V and PV = 4Pcell,V, respectively.

Results and discussion

The core–sheath graphene-based composite fibers (SG-CPF@GFs) with different SG loadings in a PF sheath were fabricated by a simple wet-spinning/dip-coating method coupled with thermal annealing (Fig. 1). In brief, the core GO fiber was wet-spun by using concentrated LGO solution. The sheath precursor SGO-PF was prepared by mixing PF with SGOs, where SGOs were synthesized by pulverizing LGOs with the assistance of 10 hour ultrasonication. Then the SGO-PF slurry was successfully dip-coated onto the GO fiber due to the hydrophilic characteristics of GOF, and subsequent high-temperature carbonization was conducted to form the designed core–sheath, hierarchically porous graphene-based composite fiber. Finally, the FSSCs were assembled by immersing two pieces of SG-CPF@GF in gel electrolyte for further electrochemical (EC) characterization.
image file: c7ta08362a-f1.tif
Fig. 1 Schematic illustration showing the roadmap for producing SG-CPF@GF electrodes and the assembled FSSC. The hierarchically porous core–sheath graphene-based fiber electrodes were simply prepared by coating graphene oxide fiber with aqueous PF resin mixing with a certain amount of small-size graphene and with subsequent high temperature annealing. The porous structure has been further proved by SEM and N2 adsorption/desorption in the following sections.

Fig. S1c shows the high-resolution transmission electron microscopy (HRTEM) image of GO. The lattice fringes of graphene oxide layers are clearly seen. In Fig. S1d, reduced graphene oxide with less than 10 layers can be found after thermal annealing at 800 °C. Fig. S2a shows the XRD patterns of GO and reduced graphene oxide. The diffraction peak of GO is located at 2θ = 9.78°, corresponding to an interlayer distance of 9.0 Å. After thermal annealing at 800 °C, the (002) diffraction peak shifts to 2θ = 26°, corresponding to a d-spacing of 3.4 Å. The decreased interlayer distance should be attributed to the removal of oxygen-containing groups in GO layers by thermal annealing. To further confirm the thermal reduction of GO, the FTIR patterns of GO and graphene were also analyzed (Fig. S2b). GO shows a strong characteristic infrared absorption peak at 1730 cm−1, which is ascribed to the C[double bond, length as m-dash]O stretching vibration. However, the peak disappears after thermal annealing at 800 °C, indicating that the GO has been successfully reduced into graphene. Fig. S2c and d show the AFM image and the corresponding height profile of the reduced graphene oxide; the height is 5.7 nm, corresponding to 6–8 layer graphene (the thickness of monolayer graphene ranges from 0.34 to 1 nm, depending on the content of surface functional groups, such as oxygen-containing groups).57,58

XPS was carried out to examine the carbon and oxygen contents in the GO and reduced graphene oxide (Fig. S3a–d). There are two peaks, C1s and O1s, exhibited by GO and reduced graphene oxide in the wide-scan XPS spectra (Fig. S3a and c). GO shows a predominant O1s peak (Fig. S3a). The high resolution C1s core-level spectra were deconvolved (Fig. S3b). The binding energies of C–OH, C[double bond, length as m-dash]O and COOH are located at 286.7, 287.6 and 289.2 eV, respectively. The peaks at 284.7 and 285.5 eV correspond to C–C and C–H bonds, respectively. In contrast, reduced graphene oxide shows a dominant C1s peak (Fig. S3c), implying the successful reduction of GO. The increased peak intensity at 284.7 eV and decreased peak intensity at 286.7 eV (Fig. S3d) further confirm that the GO has been reduced to graphene, which is in accord with the FTIR result. In addition, the carbon atom ratio was also increased from 62.2% to 75.5%. Furthermore, the Raman spectra of GO and reduced graphene oxide (Fig. S3e and f) show that the intensity ratio between G peak and D peak (IG/ID) was also increased from 0.44 to 0.76 after thermal annealing.

Fig. 2 shows the clean and wrinkled surface of neat GF (∼50 μm in diameter) with highly aligned graphene sheets along the fiber axis, which is caused by the drawing force during wet spinning. The wrinkled structure may lead to the formation of mesopores in GF, which can be proved by further BET analysis. In addition, tightly packed graphene sheets can be observed in the transverse section of GF (Fig. 2c). As shown in Fig. 2d and e, the decoration of the SGO-PF sheath endows the core GO fiber with a very smooth surface and a larger diameter of ∼85 μm, indicating the successful coating of SGO-PF. Additionally, apparent interfaces between the core and the sheath can be found from the transverse section of the composite fiber (Fig. 2f). The core fiber is ∼45 μm (comparable with that of neat GF in Fig. 2a–d) and the sheath is ∼20 μm. After carbonization at 800 °C, 1.5%-SG-CPF@GF shows a highly porous surface with micron-scale open-pores (Fig. 2g) and macropores (Fig. 2h). The formation of a porous structure should be associated with the incorporation of SG and the pyrolysis of PF resin at high temperature, resulting from the decomposition of phenolic hydroxyl groups and breakage of molecular backbones.59–61 In addition, 1.5%-SG-CPF@GF shows a circular, core–sheath and compact cross section (Fig. 2i). It is worth noting that the interface between the core and the sheath becomes fuzzy after carbonization at 800 °C (inset in Fig. 2i), which should be attributed to the enhanced graphitization of CPF after the incorporation of SG and the possible C–C cross-linking between graphene sheets from the core and the sheath.61,62


image file: c7ta08362a-f2.tif
Fig. 2 SEM images of GF (a–c), 1.5%-SGO-PF@GOF (d–f), and 1.5%-SG-CPF@GF (g–i). The side (a and b) and cross-sectional (c) views of GF. The inset of (c) is the magnified SEM image of GF. The side (d and e) and cross-sectional (f) views of the as-prepared 1.5%-SGO-PF@GOF. The inset of (f) shows the distinguishable interface between the core and the sheath. (g and h) The typical porous structure of 1.5%-SG-CPF@GF along the fiber axis proving the existence of mesopores and macropores. The cross-sectional (i) view and the inset of (i) of 1.5%-SG-CPF@GF present the core–sheath structure. In contrast to (f), the inset here shows a relatively fuzzy interface between the core and the sheath.

To investigate the electrochemical properties of the porous core–sheath graphene-based fiber electrodes, we carried out cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests in a 2-electrode cell by using 1 M H2SO4/PVA gel electrolyte (Fig. 3). We calculated the areal specific capacitance (CA) and the single-fiber energy density (EA) and power density (PA) of these graphene-based fibers from the CV and GCD results and analyzed their electrochemical performance with the aid of the BET SSA, pore size distribution (PSD), electrical conductivity, Raman spectra and electrochemical impedance spectra (EIS).


image file: c7ta08362a-f3.tif
Fig. 3 Electrochemical performances of GF, CPF@GF and SG-CPF@GF measured in a 2-electrode cell using H2SO4/PVA gel electrolyte. (a) CV curves measured at a scan rate of 5 mV s−1. (b) GCD curves recorded at a current density of 0.1 mA cm−2. (c) Areal specific capacitances (CA) based on GCD tests with respect to measured current density. (d) The Ragone plots of the FSSCs with the graphene-based fiber electrodes.

Fig. 3a shows the CV curves of all graphene-based FSSCs at 5 mV s−1. All CV curves present a rectangular shape, indicating a typical electrical double layer capacitance behavior. Interestingly, CPF@GF shows the smallest enclosed area of its CV curve (almost a line), even smaller than that of GF. Notably, 1%-SG-CPF@GF and 1.5%-SG-CPF@GF based FSSCs show surprisingly enlarged CV areas, suggesting their better charge-storage capability as fiber electrodes. However, further increasing the SG content to 2 wt% results in deteriorated electrochemical performance because the CV area dramatically diminishes, which however is still larger than those of GF and CPF@GF.

Fig. 3b shows the GCD curves of all fiber electrodes at 0.1 mA cm−2. It can be seen that a negligible electrode-potential drop (IR drop) was found for 1%-SG-CPF@GF (5.3 mV), 1.5%-SG-CPF@GF (10 mV) and 2%-SG-CPF@GF (14 mV), probably owing to the low ion-transport resistance and short diffusion distance, which improves the charge storage capability of FSSCs.63 However, the GF, CPF@GF and 0.5%-SG-CPF@GF show a relatively high IR drop of 30 mV, 164 mV and 40 mV, respectively. The discharge time obtained for GF, CPF@GF, 0.5%-SG-CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF is 84.4 s, 1.8 s, 89.6 s, 1523 s, 1565 s and 264.1 s at 0.1 mA cm−2, respectively. Therefore, 1%-SG-CPF@GF and 1.5%-SG-CPF@GF may show higher CA than the other electrodes. Moreover, as displayed in Fig. S6, it can be found that the charge curves of GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF are symmetric to their corresponding discharge counterparts even at high current density (1 mA cm−2), demonstrating the high reversibility of FSSCs and the rapid charge transport between fiber electrodes.

Fig. 3c shows the CA based on GCD curves relative to the current density from 0.1 to 1 mA cm−2. The neat GF in this work exhibits a CA value of 21.1 mF cm−2 at 0.1 mA cm−2, which is comparable with previously reported values.40,41,64 However, CPF@GF shows the lowest CA of 0.46 mF cm−2, and the CA is negligible with the increase of current density because of the failed charge–discharge process, which may result from the low electrical conductivity. The addition of SG into the PF sheath significantly enhances the CA to 29.2 mF cm−2, 380.6 mF cm−2 and 391.2 mF cm−2 at 0.1 mA cm−2 for 0.5%-SG-CPF@GF, 1%-SG-CPF@GF and 1.5%-SG-CPF@GF, respectively. To the best of our knowledge, the 1.5%-SG-PF@PF shows one of the highest CA values among all reported all-carbon FSSCs to date,24–26,37,39,64 which is, for example, 121% higher than that of RGO + CNT@CMC (177 mF cm−2),37 and 71.6% higher than that of surface porous graphene fibers.24 Moreover, the CA of 1.5%-SG-CPF@GF is even higher than those of most of the hybrid FSSCs;8,40–43,65,66 more specifically, it is 28% higher than that of hollow graphene/PEDOT:PSS fiber (304.5 mF cm−2)8 and 83% higher than that of vanadium nitride/CNT yarn (213.5 mF cm−2).65 Upon further increasing the SG loading to 2 wt%, the CA of 2%-SG-CPF@GF suddenly decreases to 70.6 mF cm−2. Similar trends can be found for all FSSCs at different current densities (0.1 to 1 mA cm−2, Fig. 2c). In addition, for 1%-SG-CPF@GF and 1.5%-SG-CPF@GF, the gravimetric specific capacitance was also calculated based on GCD tests. 1%-SG-CPF@GF and 1.5%-SG-CPF@GF show gravimetric specific capacitances of 121.4 F g−1 and 196 F g−1 (Fig. S7a), respectively. These are larger than those of other reported graphene-based fiber-shaped supercapacitors including GF@3D-G (38 F g−1)25 and CB/rGO (125.8 F g−1).35 The capacitance retention is 40%, 17.1%, 59.2%, 50.1% and 29.2% for GF, 0.5%-SG-CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF, respectively.

As shown in Fig. 3d, we constructed the Ragone plots of the FSSCs for the graphene-based fiber electrodes. It can be seen that the CPF@GF has the lowest EA (0.04 μW h cm−2), lower than that of GF (1.88 μW h cm−2). The EA is improved to 2.6 μW h cm−2 by adding 0.5 wt% SG into the sheath for 0.5%-SG-CPF@GF. Further increasing the SG percentage to 1–1.5 wt% significantly increased the EA of SG-CPF@GF based FSSCs to 34.8 μW h cm−2 (1.5%-SG-CPF@GF) at the same PA of 80 μW cm−2, which is 17.5 fold higher than that of the GF based one (1.88 μW h cm−2). More importantly, the Ragone plots of EAversus PA for SG-CPF@GF based FSSCs are relatively flat, demonstrating that the high EA of the core–sheath FSSCs can be maintained with the increase of PA. In addition, the mechanical properties of GF were not deteriorated after the decoration of SG@PF, as the tensile force of the 1%-SG-CPF@GF and 1.5%-SG-CPF@GF was slightly higher than that of GF (Fig. S2d). Furthermore, the mass-specific energy density and power density of the entire device were also calculated for 1%-SG-CPF@GF and 1.5%-SG-CPF@GF (Fig. S7b). 1%-SG-CPF@GF and 1.5%-SG-CPF@GF FSSCs show an energy density of 2.7 W h kg−1 and 4.4 W h kg−1 at a power density of 6.4 W kg−1 and 10 W kg−1, respectively.

To elucidate the impact of SG incorporation on the enhancement of the electrochemical performance of the SG-CPF@GF based FSSCs, we conducted a series of tests including N2 adsorption/desorption, electrical conductivity and Raman spectra. The BET SSA and PSD were characterized by N2 adsorption/desorption (Fig. 4a and b). The isotherms of GF present typical Type-IV isotherms with H3 hysteresis, indicating the mesoporous structure of GF (Fig. S8), while the SG-CPF@GFs show Type-I isotherms, suggesting their micropore-rich characteristics. According to Barrett–Joyner–Halenda (BJH) and density functional theory (DFT) calculations, GF shows a mesopore-rich structure while SG-CPF@GF possesses a micropore-rich structure (Fig. 4b). As summarized in Fig. 4c, all graphene-based fiber electrodes including GF possess a relatively constant macroporosity (∼4%), but show a simultaneously increased microporosity and reduced mesoporosity with the addition of SG into the sheath. Though the mechanism is still unclear, we suggest that the incorporation of nanoscale SG (mean Ø ∼ 156 nm) plays a crucial role in splitting mesopores into micropores, leading to a tunable micro-/meso-porosity ratio. Besides that, the fierce ultrasonication for the fabrication of SG may introduce more defects (i.e. holes) in the SG surface for enhanced microporosity. Similar results have been reported by Cheng et al.61 The high meso-porosity (90.3%, pore size of 3–20 nm, Fig. 4b) and low BET SSA (40.85 m2 g−1) of GF cause it to have a low CA of 22.7 mF cm−2 at 0.1 mA cm−2, but a good capacitance retention (40%, Fig. 3c) probably due to the sufficient amount of mesopores. After the decoration of the CPF sheath, the core–sheath structured CPF@GF shows a high BET SSA of 266.7 m2 g−1 and hierarchical pores comprising 88% micropores (0.5–1.7 nm, Fig. 3b), 8.3% mesopores (2–50 nm, Fig. 4b) and 3.7% macropores as shown in Fig. 4c, which should be correlated with the porous feature of CPF after pyrolysis. Moreover, owing to the amorphous sheath, the electrical conductivity (1783 S m−1, Fig. 4e) of CPF@GF is much lower than that of GF (1.2 × 104 S m−1). This can be proved by its Raman spectrum which shows a low intensity ratio (IG/ID = 0.31, Fig. 4e) and a broad disorder-induced D-band peak (FWHM = 180.5 cm−1, Fig. S9). This severely prevents the electron transport in electrodes, leading to a large IR drop (164 mV) and equivalent series resistance (ESR, 1160 Ω) (Fig. 4f). All these results account for the ultralow specific capacitance (0.46 mF cm−2 at 0.1 mA cm−2) of CPF@GF based FSSCs.


image file: c7ta08362a-f4.tif
Fig. 4 (a) N2 adsorption/desorption isotherms and (b) the corresponding PSD of CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF. (c) BET SSA and micropore, mesopore and macropore volume percentage of CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF. (d) Pore volume percentage of CPF@GF, 1%-SG-CPF@GF, 1.5%-SG-CPF@GF and 2%-SG-CPF@GF with pores in the size range of 0.5–1 nm, 1–2 nm, 2–10 nm, and 10–50 nm. (e) Electrical conductivity, microcrystalline size (La) and intensity ratio between G band and D band (IG/ID) of SG-CPF@GF versus SG percentage. (f) Nyquist plots of FSSCs using 1 M H2SO4/PVA gel electrolyte with increasing frequencies from 0.01 to 100 kHz. The inset shows the magnified plot of 1%-SG-CPF@GF, 1.5%-SG-PF and 2%-SG-CPF@GF.

In contrast to CPF@GF, adding SG leads to enhanced CA. For instance, 1%-SG-CPF@GF possesses a comparable SSA of 233.1 m2 g−1 but a different PSD containing 92.2% micropores, 3.6% mesopores and 4.2% macropores. Surprisingly, the CA of 1%-SG-CPF@GF reaches 380.6 mF cm−2 at 0.1 mA cm−2, which is two orders of magnitude higher than that of CPF@GF. This extraordinarily high CA is believed to be strongly associated with the higher microporosity (92.2%) with a narrowed micropore size range (0.5–1.4 nm) and enhanced electrical conductivity (3107 S m−1). So it is of key importance to explore the mechanism through which the hierarchical porous structure and electrical conductivity play their roles in CA enhancement.

For 1%-SG-CPF@GF, the narrowed micropore size (0.5–1.4 nm) based on the DFT model and enlarged microporosity (92.2%) indicate that an additional micropore population with a smaller size appears after the addition of SG. It is further proved that there are >65.4% micropores with a pore size of 0.5–1 nm (Fig. 4d), which is slightly larger than that of the bare ion SO42− (Ø ∼ 0.294 nm in diameter). It is well known that more subnanometer pores close to the size of bare ions will substantially enhance specific capacitance as reported by Gogotsi et al.45,67 Additionally, the mesoporosity of 1%-SG-PF@GF is reduced to 3.6%, but shows a narrower size distribution (9 to 28 nm). These reasonable mesopores (>10 nm) of 1 wt%-SG-CPF@GF will shorten the ion transport length, reduce the ion scattering and facilitate the ion transfer from the electrolyte to the electrode surface.14,63

More importantly, the addition of 1 wt% SG in the CPF sheath increases the electrical conductivity of 1%-SG-CPF@GF to 3107 S m−1. This enhancement should account for the higher graphitization of CPF and the percolated highly conductive networks after the addition of SG.61 As displayed in Fig. 4e and S9, the enlarged IG/ID peak ratio (0.58), large microcrystalline size (La, 22.3 nm) and narrowed D-band FWHM (89.8 cm−1) of 1%-SG-CPF@GF all suggest an increased graphitization, demonstrating the critical role of SG in enhancing the graphitization level. Larger La values are deemed to facilitate ion transport, as reported by H. M. Cheng and N. Wu et al.14,51 On the other hand, it can be seen that the core–sheath interface is fuzzy (Fig. 1i) after thermal annealing, indicating the interconnected structure between the core and the sheath. Thus it can be expected that the electrical resistance at core–sheath interfaces should be largely reduced owing to the probable C–C cross-linking between graphene from the core and the sheath.62,68 All of this results in the enhanced electrical conductivity of 1%-SG-CPF@GF and thus a tiny IR drop (5.3 mV) and small ESR (292.9 Ω) (Fig. 4f) for the assembled FSSCs. Moreover, compared with other FSSCs, the higher slope (d(−Z′′)/dZ′) at low frequency for 1%-SG-CPF@GF FSSCs (Fig. 4f) also proves the better charge storage behavior of 1%-SG-CPF@GF. Herein, we can affirm that the narrowed micro-/meso-PSDs with higher microporosity and enhanced electrical conductivity synergistically strengthen CA and improve the rate capability of 1%-SG-CPF@GF.

1.5%-SG-CPF@GF shows the highest SSA (416.4 m2 g−1) among all the graphene-based core–sheath fiber electrodes with 96.4% micropores (pore size, 0.5–1.8 nm), 1.7% mesopores (2–25 nm) and 1.9% macropores. Moreover, it exhibits the highest electrical conductivity (3934 S m−1, Fig. 4e). Similar to 1%-SG-CPF@GF, the largest IG/ID ratio (1.41), small FWHM (33.2 cm−1) and largest La (54.3 nm) cause the maximum graphitization and thus the highest electrical conductivity in 1.5%-SG-CPF@GF. Unquestionably, 1.5%-SG-CPF@GF shows the highest CA of 391.2 mF cm−2 at 0.1 mA cm−2, which is confirmed by the comparably large slope at low frequency in the Nyquist plot of the 1.5%-SG-CPF@GF FSSC (Fig. 4f). However, the capacitance retention is slightly worse than that of the 1%-SG-CPF@GF based FSSC with increasing current density. We believe that this is related to the lower meso-porosity and diminished mesopores ranging from 10 to 50 nm (ref. 14 and 51) (Fig. 4d) in 1.5%-SG-CPF@GF as compared with that of 1%-SG-CPF@GF, because micropores can strengthen charge storage and mesopores (especially >10 nm) maintain high capacitance retention at high rates by providing small ion-transport resistance and shortening the ion diffusion distance. Though 2%-SG-CPF@GF has a comparable BET SSA (399.1 m2 g−1) and microporosity (96.9%, 0.5–1.6 nm) with 1.5%-SG-CPF@GF, there are few detectable mesopores, if at all, which immensely prevents the diffusion of ions into the inner area of the electrodes and weakens the charge transport and storage. Therefore, 2%-SG-CPF@GF SCs showed a relatively low CA (70.6 mF cm−2).

We further enhanced the energy density of 1%-SG-CPF@GF and 1.5%-SG-CPF@GF by utilizing organic ionic liquid electrolyte; it is expected that the EA can be further improved due to its high voltage window of up to 3.5 V (i.e. tetraethylammonium tetrafluoroborate (TEABF4) dissolved in acetonitrile or propylene carbonate (PC) solvent), which is much higher than that of water (1.23 V).17 We used 1 M 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)/polyvinylidene fluoride (PVDF)/N,N-dimethylformamide (DMF) gel electrolyte. The CV results of the solid-state 1.5%-SG-CPF@GF based FSSC are shown in Fig. S10. It can be seen that the working voltage window is extended to 2.2 V. 1.5%-SG-CPF@GF shows a high CA as large as 395 mF cm−2 (Cm, 176.5 F g−1; scan rate, 5 mV s−1), which is higher than that of 1%-SG-CPF@GF (272 mF cm−2; 149.8 F g−1 at a scan rate of 5 mV s−1). This is because all the sizes of bare ions in organic and aqueous electrolytes (BF4, Ø ∼ 0.45 nm,67 EMIM+, Ø ∼ 0.43 nm (ref. 67 and 69) and SO42−Ø ∼ 0.294 nm) are well below 1 nm and close to the lower bound of micro-PSD (0.5–1.8 nm) for 1.5%-SG-CPF@GF. It can be seen that the voltage window is extended to 2.2 V, which is almost 3-fold that in aqueous gel electrolyte (0.8 V). The high CA combined with the high voltage window endow 1%-SG-CPF@GF and 1.5%-SG-CPF@GF FSSCs with a high EA of 45.7 μW h cm−2 (Em, 25.2 W h kg−1) and 66.4 μW h cm−2 (Em, 29.7 W h kg−1), respectively (Fig. S7b).

As shown in Fig. 5a, 1%-SG-CPF@GF and 1.5%-SG-CPF@GF based FSSCs exhibit excellent charge storage capability and stability with an ultrahigh entire-device Ecell,A of 8.7 μW h cm−2 (34.8 μW h cm−2 for single-fiber EA) when the areal power density Pcell,A is 0.02 mW cm−2. To the best of our knowledge, the Ecell,A of 1.5%-SG-CPF@GF based FSSCs at a given Pcell,A surpasses those of one of the best ever reported all-carbon FSSCs and most of the hybrid FSSCs containing metal oxides and conductive polymers.8,25,27,37,66,70–72 For instance, it is 50 fold higher than that of all-graphene core–sheath FSSCs (0.17 μW h cm−2),25 3 fold higher than that of carbon fiber@pen ink FSSCs (2.7 μW h cm−2),27 and more than twice as high as that of RGO + CNT@CMC yarn-based SCs (3.84 μW h cm−2).37 The Ecell,A is further increased to 66.41 μW h cm−2 when the power density Pcell,A is 0.54 mW cm−2 for the 1.5%-SG-CPF@GF FSSC using the EMIMBF4/PVDF/DMF electrolyte. Fig. 5b shows the volumetric Ragone plots, where 1%-SG-CPF@GF and 1.5%-SG-CPF@GF based FSSCs possess a very stable volumetric energy density Ecell,V of 2–3.4 mW h cm−3 with aqueous electrolyte, which is comparable with that of Li thin film batteries (0.3–10 mW h cm−3).73 It is also much higher than those of commercialized SCs74,75 and previously reported FSSCs.37,75–79 In addition, the EV with organic electrolyte reaches up to 25.1 mW h cm−3 at a PV of ∼0.2 W cm−3, which is much higher than that of Li-ion thin film batteries, and even comparable with that of lead–acid batteries.80


image file: c7ta08362a-f5.tif
Fig. 5 Areal (a) and volumetric (b) Ragone plots of our 1%-/1.5%-SG-CPF@GF based FSSCs and other previously reported solid-state all-carbon FSSCs, some commercially available state-of-the-art SCs, Lead–acid batteries and Li-ion thin film batteries. Note: all energy and power densities are based on the whole device. The power/energy density of the whole FSSC is more meaningful for evaluating the energy storage performance of our FSSCs than the gravimetric power/energy density based on single fiber electrodes.

Fig. 6a shows that the as-assembled 1%-SG-CPF@GF based FSSC retains 98.9% of its initial capacitance after 500 bending cycles at 90° (Fig. 6a), demonstrating its remarkable performance stability under cyclic bending. In Fig. 6b, the assembled FSSC also exhibits excellent cycling performance with 98.9% capacitance retention after 7000 GCD cycles, suggesting its outstanding stability with a long cycle life. The GCD curves exhibit a symmetric isosceles triangle-like shape (Fig. S11d), indicating the high reversibility of the assembled FSSCs after long time cycling. To demonstrate the practical application of the SG-CPF@GF based FSSCs as an efficient energy storage system for electronics, we assembled 4 pieces of 1%-SG-CPF@GF based FSSCs in series to provide a high voltage window. The fiber-shaped SCs could be readily connected in series to provide a higher power output. As shown in Fig. 6c and d, the integrated FSSC device connected in series can easily light up 36 red LEDs with a logo of “DHU”.


image file: c7ta08362a-f6.tif
Fig. 6 (a) The capacitance retention of the 1%-SG-CPF@GF FSSC after 500 bending cycles. Inset: the photo of the bent FSSC at 90°. (b) Cycle life of the all-solid-state 1%-SG-CPF@GF FSSC at a current density of 1 mA cm−2. The inset shows the GCD curve after 7000 cycles between 0 and 0.8 V at 1 mA cm−2. Photos of the four FSSCs connected in series before (c) and after (d) being connected to 36 LEDs, respectively. The corresponding videos can be found in the ESI.

Conclusions

We developed a facile but robust route to hierarchically porous graphene-based FSSCs with an ultrahigh areal specific capacitance (391.2 mF cm−2) and entire-device energy density (8.7 μW h cm−2 in PVA/H2SO4 electrolyte and 66.41 μW h cm−2 in EMIMBF4/PVDF electrolyte). We added small sized graphene oxide into the PF sheath to tune the micropore-rich BET SSA (up to 416.4 m2 g−1) and pore size distribution (micro-/meso-porosity ratio) for strengthened areal specific capacitance and rate capability. It is found that the hierarchically porous micropore-domain architecture and high electrical conductivity synergistically impact the electrochemical performance of these graphene based composite fiber electrodes. The as-assembled FSSCs also exhibit excellent cycling stability and good flexibility. To the best of our knowledge, the excellent performance of the FSSCs in this work surpasses those of one of the best ever reported all-carbon FSSCs and most of the hybrid FSSCs containing metal oxides and conductive polymers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the funding support from the “DHU Distinguished Young Professor Program”, the National Natural Science Foundation of China (No. 51603036), and the Key Laboratory of Textile Science & Technology (Donghua University), Ministry of Education (KLTST201606). We also acknowledge Prof. Xin Ding and Mr Wenqi Nie of Donghua University for their valuable discussion on electrochemical measurements. We also thank Ms Jing Zhang and Ms Jie Meng for their kind help with XPS measurements and data analysis.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ta08362a

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