reduced graphene oxide macrofibers for flexible all-solid-state supercapacitors

Accepted Manuscript Polypyrrole@TEMPO-oxidized bacterial cellulose/reduced graphene oxide macrofibers for flexible all-solid-state supercapacitors Nan Sheng, Shiyan Chen, Jingjing Yao, Fangyi Guan, Minghao Zhang, Baoxiu Wang, Zhuotong Wu, Peng Ji, Huaping Wang PII: DOI: Reference:

S1385-8947(19)30422-X https://doi.org/10.1016/j.cej.2019.02.173 CEJ 21086

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 November 2018 23 January 2019 23 February 2019

Please cite this article as: N. Sheng, S. Chen, J. Yao, F. Guan, M. Zhang, B. Wang, Z. Wu, P. Ji, H. Wang, Polypyrrole@TEMPO-oxidized bacterial cellulose/reduced graphene oxide macrofibers for flexible all-solid-state supercapacitors, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.02.173

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Polypyrrole@TEMPO-oxidized

bacterial

graphene

for

oxide

macrofibers

cellulose/reduced

flexible

all-solid-state

supercapacitors

Nan Sheng a, Shiyan Chen a,*, Jingjing Yao a, Fangyi Guan a, Minghao Zhang a, Baoxiu Wang a, Zhuotong Wu a, Peng Ji b,*, Huaping Wang a,* a

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Materials Science and Engineering, Donghua University, Shanghai 201620, PR China b Co-Innovation

Center for Textile Industry, Donghua University, Shanghai 201620, PR China

Email addresses: [email protected]; [email protected]; [email protected]

Abstract The emerging fiber-based supercapacitors with high energy and power density are highly desirable for they provide strong support for the development of portable and wearable devices. Here, a flexible, binder-free high-performance fiber-based supercapacitors is structured based on hierarchical polypyrrole@TEMPO-oxidized bacterial cellulose/reduced graphene oxide macrofibers by wet spinning and in situ polymerization polypyrrole. For the synergistic effects of three components, the excellent specific capacitance of the electrode with 391 F g-1 (373 F cm-3) at the current density of 0.5 A g-1 (0.48 A cm-3) and the fiber-based supercapacitor made of the electrodes with 259 F g-1 (258 F cm-3) at the current density of 0.2 A g-1 (0.199 A cm-3)

are achieved. Moreover, the fiber-based supercapacitor exhibits a high energy density of 8.8 mWh cm-3 at the power density of 49.2 mW cm-3 and a high-power density (429.3 mW cm-3 at the energy density of 4.1 mWh cm-3), which is better than most previously reported graphene fiber-based supercapacitors. In these devices, we realize a desirable combination of excellent electrochemical performance and good flexibility, which will be significant for satisfying the requirement of the energy and power in various portable, miniaturized, and wearable electronic devices.

Keywords Bacterial cellulose nanofibers; Graphene; Polypyrrole; Fiber-based supercapacitors

1. Introduction Supercapacitors (SCs) have attracted much attention as a new energy storage system between traditional capacitors and lithium-ion batteries. Compared with secondary batteries, SCs have the characteristics of faster charge and discharge, high power density, long cycling life, high coulombic efficiency and broad application prospects. Commercial SCs are mainly based on rigid batteries which can’t meet the flexibility requirements [1−3]. Replacing rigid electrodes with flexible or stretchable electrodes will be the development trend of SCs in the future. As a basic two-dimensional carbon material, graphene with large surface area, high electrical conductivity and excellent electrochemical stability is a potential candidate material for SC. However, poor electrolyte infiltration and restacking of

graphene caused by the strong π-π interactions and van der Waals force between the planar basal planes of the graphene leads to poor performance of SC [4−6]. So far, researchers have made many efforts to improve these problems and the first strategy is to intercalate some “spacers”, such as metal oxides [7−10], carbon nanotubes (CNTs) [11−13] and nanodiamond (ND) [14] between graphene sheets. The capacitance of which can be significantly improved, but the interaction of the interface between graphene and nanoparticles leads to poor mechanical properties and hydrophilic problem. Some researchers have also added some hydrophilic substances, such as polyvinyl alcohol (PVA) [15] and cellulose nanocrystals (CNCs) [5]. However, since the PVA insulating coating formed on the surface of the rGO hinders the ion transport between the graphene sheets, the conductivity of the hybrid fibers is greatly reduced, which in turn affected the rate performance[15]. The CNCs can improve the mechanical properties and hydrophilic, however, they increase the layer spacing of rGO sheets, and thus cause a minor decrease in conductivity. Therefore, it is necessary to improve the hydrophilicity and prevent the graphene from self-stacking to ensuring the electrical conductivity and mechanical properties. Cellulose nanofibers with nanoscale diameters and microscale lengths are a kind of promising polymers which can be used as surfactant due to their amphiphilicity [16,17]. Cellulose nanofibers contain electrolyte absorption properties and can diffuse them into energy storage materials while providing good diffusion channels for electrolyte solution and enhancing ion transportation to the active materials [18−21], also they can be used as a mechanical buffer layer [22,23] or binder [24]. As a special

type of cellulose, bacterial cellulose (BC) is a promising flexible material with high aspect ratio, high water retention capacity, excellent mechanical strength and low cost [25−28]. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized bacterial cellulose (TOBC) has attracted extensive attention owing to its ultrafine nanofibers, high aspect ratio, high tensile strength and high elastic modulus [25,26,29]. In our previous work, it was confirmed that TOBC not only effectively prevented the aggregation of CNTs, but also provided excellent wet-spinnability, while significantly improved the electrolyte infiltration to fabricate high-performance fiber-based supercapacitors (FSCs) [30]. Additionally, combination of graphene with conducting polymers (PANI [31], PPy [32] and PEDOT:PSS [33]) and metal oxides (MnO2 [7,34,35], Co3O4 [36] and Bi2O3 [37]) can provide a better performance for SCs. PPy is a potential electrode material with high theoretical capacitance, low cost, easy synthesis, controllable electrical conductivity and environmental stability [38−40]. However, the poor cycling stability of PPy is another obstacle for practical application which can be improved by introduction of conductive carbon materials. Herein, we have developed a facile and scalable method for the preparation of flexible, binder-free high performance all-solid-state FSCs based on the polypyrrole@TEMPO-oxidized

bacterial

cellulose/reduced

graphene

oxide

(PPy@TOBC/rGO) macrofibers. The TOBC/GO hybrid fibers were first extruded from the needle into the coagulation bath by wet spinning and then dried. PPy improved electrochemical performance by in-situ polymerization on TOBC/GO hybrid fibers,

wherein rGO was used as electrical double layer capacitance active material after hydroiodic acid (HI) reduction, and PPy acts as pseudo-capacitance active material to further

improve

the

specific

capacitance.

Significantly,

the

resulting

PPy@TOBC30/rGO70 electrodes delivered an outstanding specific capacitance. Furthermore, highly flexible all-solid-state FSCs using PPy@TOBC30/rGO70 as the electrodes and PVA/H2SO4 hydrogel as solid-state electrolytes were fabricated which exhibited high specific capacitance and high energy density while maintained high power density. This study used an industrially viable wet-spinning process to provide a promising approach to the development of environmentally friendly energy storage devices. 2. Experimental section 2.1 Materials. The preparation method of TOBC nanofibers was derived from our previous study [30,41,42]. GO was purchased from Jiaxing Zhuorong New Energy Technology Co. Ltd. Iron (III) chloride hexahydrate (FeCl3·6H2O), HI (45 %), acetone, acetic acid and PVA were purchased from Sinopharm Chemical Reagent Co. Ltd. Pyrrole were purchased from Shanghai Aladdin Bio-Chem Technology Co. Ltd. 2.2 Preparation of TOBC/rGO conductive fibers 10 mg mL-1 GO dispersion was prepared by adding 1 g GO into 99 g deionized water, followed by sonicating with a probe sonicator (JX-950D, China) for 2 h. Then the GO dispersion was mixed with TOBC at different TOBC/GO weight ratios of 70/30, 50/50, 30/70 and 0/100 wt%/wt%, followed by mixing at 1500 rpm for 1 h to get

uniform spinning dispersions. The obtained dispersions were evaporated to 40 mg mL-1 at 60 ºC water bath, and the pH of the suspensions was adjusted to 7.0 by adding 10 M NaOH to get the non-liquid-crystal suspension [5,15,43]. The suspensions were continuously spun into a mixed rotating coagulation bath from a 2.5 mL syringe with a 22G needle (Internal diameter of the needle is 0.4 mm) at a speed of 2 m min-1. The mixed coagulation bath is composed of acetone and acetic acid, and its ratio was the same as the weight ratio of TOBC and GO. The obtained fibers were then dried at 50 ºC. Finally, the fibers were reduced in an aqueous HI solution at 85 ºC for 8 h, followed by washing with ethanol and deionized water and drying at 60 ºC for 12 h. The TOBC/GO and TOBC/rGO fibers are named as TOBCx/GOy and TOBCx/rGOy fibers, respectively, where x and y are percent weight ratio of TOBC and GO, respectively. Pre-spinning dispersions with different components are shown in Table S1. 2.3 Preparation of the PPy@TOBC/rGO fiber electrodes Polymerization of PPy in the TOBC/GO macrofibers by in-situ polymerization process. The pyrrole (1.0 M) was first dissolved in 50 mL deionized water, and TOBC30/GO70 fibers were soaked in the solution at 4 ºC for 1 h. FeCl3 (0.5 M) in 25 mL of deionized water was added to the solution. The polymerization was initiated immediately and the reaction was allowed to continue for 30, 60, 120, 240 min at 4 ºC. The obtained samples were washed several times with alcohol and distilled water and then dried at 50 ºC. Finally, the fibers were reduced in an aqueous HI solution at 85 ºC for 8 h, followed by washing with ethanol and deionized water and drying at 60 ºC for 12 h. The obtained macrofibers were named as PPy@TOBC30/rGO70.

2.4 Fabrication of the flexible all-solid-state FSCs The PVA/H2SO4 gel electrolyte was obtained by adding 1 g PVA powder into 10 mL deionized water with magnetic stirring under 95 ºC to get a homogeneous solution. After cooling to the room temperature, 1 g H2SO4 was added to the PVA solution and stirred until the solution became clear and transparent. The average diameter of PPy@BC30/rGO70 electrodes for 60 min polymerization was about 100 μm. The active mass loading of each fiber was about 75 μg cm-1. The thickness of the gel of electrodes was controlled by the time of immersing the electrodes in PVA/H2SO4 solution. The conductive fibers except the electrode parts were immersed into PVA/H2SO4 solution for 24 h, and then picked out and dried at room temperature. Two PPy@TOBC30/rGO70 macrofibers were arranged in parallel on the polyethylene terephthalate (PET) film and the middle overlapping portion part (~1 cm) was coated with some extra PVA/H2SO4 gel electrolyte and air-dried at room temperature. Then each electrode portion (~0.2 cm) of the fiber was adhered to a conductive copper foil by conductive silver paste and finally a flexible all-solid-state FSC was fabricated. 2.5 Characterization The morphology of GO was characterized using JEM-2100 transmission electron microscopy (TEM, JSM-6700F, Japan). The morphologies of the samples were carried out on a S-4800 field emission scanning electron microscope (FE-SEM, S-480, Hitachi, Japan). The samples were sprayed with a thin gold layer before observation. The fourier transform infrared spectroscopy (FT-IR) was recorded on an FT-IR spectrometer (Nicolet 6700, Thermo Fisher) with the pressing potassium bromide troche method.

The fibers were solidified with epoxy resin, then soaked in liquid nitrogen and broken to obtain the cross section for observation. The X-ray diffraction (XRD) spectroscopy was obtained in a Rigaku D/MAX-2550PC X-ray diffractometer with the Cu Kα irradiation at a scanning rate of 2 s-1. The Raman spectra were recorded with a confocal Raman microscope equipped with a laser having an excitation wavelength of 532 nm (InVia Reflex). The average diameter of the fiber was measured by 10 samples using optical microscope. The linear density of fibers was obtained by calculating the average of the ratio of the mass and length of fibers, where the mass was weighed with an XS microbalance (Mettler Toledo, USA, accurate of up to 0.00001g). The conductivity (σ, S/cm) of the fiber was calculated based on length (L, cm), resistance (R, Ω), and crosssectional area (S, cm2): σ = L/(R*S), S = πD2/4, where D is calculated by averaging the diameter of at least ten fibers using optical microscope images. The tensile tests of fibers were tested in a fiber mechanical strength tester (XQ-2, China) with a strain rate of 20 mm/min at a test length of 20 mm. Tensile strength, modulus and elongation at break were calculated as the average of at least 10 measurements. 2.6 Electrochemical measurements Electrochemical measurements including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves were tested on CHI 660E (Shanghai Chenhua), and electrochemical impedance spectroscopy (EIS) were carried out using Zennium CIMPS-1 (Zahner, German) electrochemical workstation. For the fiber electrodes, the tests of CV (from −0.4 V to 0.8 V at scan rates range from 5 to 100 mV s-1), GCD (current densities from 0.5 to 2 A g-1), and EIS (AC

amplitude of 5 mV, frequencies between 10 mHz and 100 kHz) was performed in 1 M H2SO4 with a three-electrode configuration. The fiber electrode was used as working electrode, Ag/AgCl as reference electrode and platinum wire as counter electrode. For the FSC, the two electrodes of the CV (from 0 V to 1 V at scan rates range from 5 to 100 mV s-1) and GCD (current densities from 0.2 to 2 A g-1) cycling tests were performed for the flexible all-solid-state FSCs. 2.7 Calculations The specific capacitance of the fiber electrodes was calculated from the CV and GCD curves according to the following equation: C =

dU/v△U, C = It/(△U-IR),

where C (F) is the capacitance of the fiber electrodes, I (A) is the discharge current, v (V s-1) is the scanning rate, △U (V) is the potential window, t (s) is the discharge time, IR (V) is the potential drop. The gravimetric specific capacitance (Cm, F g-1) is Cm = C/m, the volumetric specific capacitance (Cv, F cm-3) is Cv = C/V, V = π(D/2)2L, where m (g) is the mass loading of active material, D (cm) is the diameter and L (cm) is the length of the fiber electrodes in the H2SO4 electrolyte solution. The specific capacitance of FSC was calculated from the GCD curves according to the following equations: Cm,

electrode

= 2It/m(△U-IR), Cv,

electrode

= 2It/V(△U-IR),

where m (g) is the mass loading of active material in a single electrode, V (cm3) is the volume of a single electrodes. The loading of PPy was calculated from the mass differences of fibers before and after polymerization. Therefore, the total load of active materials (rGO and PPy) is calculated by the following equations: Mactive = (MPPy@TOBC

/rGO70

30

− MTOBC

/rGO70)

30

+ MTOBC

/rGO70×70%.

30

The specific capacitance of the

device was calculated from Ccell = Celectrode/4. The energy density (Em, Wh kg-1; Ev, mWh cm-3) and power density (Pm, W kg-1 ; Pv, mW cm-3) of the supercapacitor cell were calculated according to the equations: Ecell = Ccell(△U−IR)2/2 ×3.6 and Pcell = 3600×Ecell/t, respectively [44]. 3. Results and discussion Scheme 1 shows the fabrication procedures of PPy@TOBC30/rGO70 macrofibers. Due to electrostatic repulsion, GO and TOBC can achieve a uniform pre-spinning dispersing solution. TOBC can prevent irreversible self-accumulation between graphene nanosheets to obtain excellent electrochemical properties [18,45]. Because of the presence of hydroxyl and carboxylate groups on the surface of TOBC and GO, pyrrole monomers can penetrate into the inside of the macrofibers, resulted in the PPy generation inside the fibers. The electrolyte infiltration can be improved by introducing of the hydrophilic TOBC nanofibers. At the same time, PPy provides pseudocapacitance, and its unique hierarchical structure provides better electrochemical stability.

Scheme 1. Schematic illustration of the fabrication process and structure of

PPy@TOBC/rGO macrofibers.

3.1 Fabrication and characterization of TOBC/rGO hybrid fiber electrode Fig. 1a shows the optical images of GO, TOBC and TOBCx/GOy composites with a concentration of 5 mg mL-1. As the images shown, by introducing charged carboxyl groups on the surface of TOBC, the obtained TOBC solution is transparent and stable [46]. After adding TOBC nanofibers into GO dispersing solution, the dispersibility of GO is obviously improved. After 48 hours, aggregations are observed in neat GO, while others still show homogeneous dispersion, indicating that GO dispersion can be obviously improved by the completely individualized TOBC nanofibers [30,47]. The electrostatic repulsion between charged carboxyl groups on the TOBC helps to stabilize the GO sheets (Fig. 1b). Also, it partly comes from steric stabilization mechanism by the polymer to stabilize 2D flakes by May et al. [48]. Therefore, the TOBC nanofibers assisting to disperse GO sheets may be related to both electrostatic repulsion and steric hindrance of nanofibers formation, resulting in the formation of stable dispersions. The morphologies of GO and TOBC50/GO50 with a concentration of 0.01 mg mL-1 are shown in Fig. 1c and d. As a result of ionization of carboxylic acid and phenolic hydroxyl groups on the GO sheets, the surfaces of GO sheets are highly negatively charged when dispersed in water [49]. They could easily form a uniform solution when mixed with TOBC due to a large amount of oxygen atoms [21,50]. TEM images indicate that the size of GO nanosheets with ~500 nm in width (Fig. 1c) and the diameter of these nanofibers is about 20~80 nm (Fig. 1d). With an increasing content

of GO, viscosity of the TOBC/GO composites increases, which indirectly proved that the interactions between the hydroxyl groups of TOBC and the oxygen-containing groups of GO sheets [51]. GO attaches more oxygen-containing hydrophilic functional groups, such as −COOH and −OH [52,53], which generate more cross-linking points of hydrogen bond with TOBC. Continuous neat GO and TOBC/GO hybrid fibers were successfully obtained via wet-spinning with extrusion unit, rotating coagulation bath and a winder at 2 m min-1. We adopted a mixed coagulation bath of acetone and acetic acid which can achieve the rapid dehydration and solidification of TOBC/GO hybrid fibers. In addition, the obtained TOBC/rGO hybrid fibers still maintained continuity, integrity and flexibility after reducing in HI solution.

Fig. 1. (a) Optical images of TOBC, GO and TOBC/GO composites with 30 wt%, 50 wt% and 70 wt% content of TOBC. (b) Schematic diagram of the interaction between TOBC and GO sheets. TEM images of GO sheets (c) and TOBC50/GO50 suspensions

(d) from a diluted dispersion.

The microscopic morphologies of the TOBC/rGO macrofibers containing different content of rGO were characterized using FE-SEM (Fig. 2a-f). The average diameter of TOBC/rGO macrofibers is approximately 60 μm after the reduction of HI solution and drying. The surfaces of TOBC/rGO macrofibers are extremely rough with numerous wrinkles and voids (Fig. 2d-f), which were caused by the dehydration reaction, shrinkage in the process of the HI reduction and drying process. As the content of rGO increases, the wrinkles displayed on the surface of the fibers become more. The high-wrinkled and perforated structure endows the fibers a large surface area, which would be beneficial for its application as SCs [54].

Fig. 2. FE-SEM images of the surface of TOBC/rGO macrofibers at different concentrations of rGO: (a, d) 30 wt%, (b, e) 50 wt%, (c, f) 70 wt%.

Fig. 3a shows the XRD patterns of TOBC, rGO and TOBC/rGO macrofibers, which confirms that rGO and TOBC are both present in the macrofibers. The diffraction

__

peaks at 14.5º, 16.7º, 22.6º belong to the (11(__)10), (110) and (200) plane of TOBC, respectively [41]. The crystallinity index of BC before and after oxidation was almost constant (Fig. S1), and it can be assigned to the diffraction of cellulose Iα (JCPDS: 502241). The diffraction peak at around 24.4ºin rGO and hybrid fibers is ascribed to the graphite-like structure (002) face in graphene [55]. Also, it can be observed that the XRD diffraction intensity of TOBC obviously decreases with the increase of rGO content. Two main reasons are involved, one of which is from the decrease of TOBC content, and the other is from the deposition of rGO sheets on the surface of TOBC fibers [25]. Raman spectra of rGO and TOBC/rGO macrofibers are shown in Fig. 3b, which reveals the interaction between rGO and TOBC. As shown in Fig. 3b, two characteristic peaks at 1348 cm-1 (D band) and 1595 cm−1 (G band) are observed for TOBC/rGO and rGO, respectively. The D band is associated with the disordered and defective graphite and G band represents the in-plane vibration of sp2 bonded carbon atoms. Comparing with rGO, the ID/IG of TOBC30/rGO70 hybrid fibers decreased from 1.48 to 1.33, which suggested the formation of new sp2 graphitic domains.

Fig. 3. (a) XRD and (b) Raman spectra of TOBC and TOBC/rGO macrofibers. As free-standing electrode materials, the mechanical properties of the TOBC/rGO

macrofibers are very important. As shown in Fig. S2. Compared with the values of neat rGO macrofiber, the mechanical properties of TOBC/rGO macrofibers with different TOBC content are all improved and the maximum stress of TOBC30/rGO70 reaches 85 MPa since TOBC acts as a “enhancer” and provides abundant intermolecular and intramolecular hydrogen bonds [56], which creates a strong interaction between the components in the fibers and ultimately improves the mechanical properties. The electrical conductivity and electrochemical performance of rGO macrofibers and TOBC/rGO macrofibers are shown in Fig. 4. The conductivity of hybrid fibers increases with the increase of rGO concentration and reaches 100.2 S cm-1 with 70% rGO concentration (Fig. 4a). The excellent conductivity of the electrodes would build a fast electron transport network for good rate performance [7]. Chen et al. [15] fabricated hydrophilic rGO/CNC hybrid fibers by non-liquid-crystal spinning. CNC would increase the layer spacing of rGO sheets, and decreased the electron hopping among rGO sheets, causing a minor decrease in conductivity. However, the electrical conductivity of hybrid fibers in our work is only affected by the content of graphene, and the addition of TOBC do not cause an additional decrease of conductivity, which is consistent with the unchanging interlayer spacing (~3.65 Å) of hybrid fibers (24.4°) in XRD. Although organic electrolytes with wide voltage windows are the basic choice for commercial SCs in terms of energy density, aqueous electrolytes are inexpensive and easily handled in the laboratory without special conditions [57]. Moreover, in most studies, EDLCs obtained a higher specific capacitance in the acidic electrolyte than in the neutral

electrolytes [58−62]. As one of pseudo-capacitive materials, polypyrrole is generally stable in H2SO4 electrolytes [32,44,63]. As mentioned above, we have selected H2SO4 electrolytes commonly used for carbon-based electrodes. The electrochemical performance of neat rGO and TOBC/rGO hybrid fibers was tested in a three-electrode system in 1 M H2SO4 aqueous solution. The operational voltage window was optimized using the CV curves of BC30/rGO70 electrode at the scan rate of 5 mV s-1 by setting different voltage windows (Fig. S3). A suitable voltage window of −0.4~0.8 V was selected for a significant peak under −0.2~1.0 V voltage window caused by oxygen reduction. The CV curves of rGO and TOBC/rGO hybrid fibers at the scan rate of 10 mV s-1 were compared in Fig. 4b. We can see that the curve of TOBC30/rGO70 hybrid fiber exhibit the highest area owing to the high conductivity and good infiltration. From the CV curves at the different scan rate, the Cm and Cv of rGO and TOBC/rGO hybrid fibers are calculated shown in Fig. 4c. The mass-specific capacitance for TOBC30/rGO70 hybrid fiber can reach to 172 F g-1 (170 F cm-3) at scan rate of 5 mV s1.

The values are far higher than the neat rGO fiber (Cm= 52 F g-1, Cv= 69 F cm-3). So,

the more detailed characterizations were performed for the TOBC30/rGO70 fiber electrode. Fig. 4d and 4e show the CV curves of the electrode at different scan rates and the GCD curves at different current densities, respectively. The GCD curves exhibit a good symmetrical triangle shape, revealing the excellent charge transport within the electrode. From the specific capacitance calculated from the GCD curve, the value is as high as 248 F g-1 (246 F cm-3) at a current density of 0.5 A g-1 (0.496 A cm-3). It is much higher than other electrodes, such as PVA/rGO hybrid fiber (216 F g-1 at 0.2 A

g-1, in 1 M H2SO4) [15], CNT-coated carbon fiber (~11 F g-1 at 2 mV s-1, in 1 M H2SO4) [64], rGO/CB-1.5 film (96 F g-1 at 5 mV s-1, in 1 M H2SO4) [65], and CNFs (105 F g-1 at 5 mV s-1, in 2 M KOH) [66]. Even at high current density of 5 A g-1 (4.96 A cm-3), the specific capacitance of the electrode remains at 132 F g-1 (131 F cm-3) due to high conductivity, good infiltration and porous structure of electrode material, which may promote the transport of ions and shorten the diffusion distance from the external electrolyte to the interior surface [26,67].

Fig. 4. (a) Conductivity of neat rGO and TOBC/rGO hybrid fibers. (b) CV curves of TOBC/rGO electrodes with different concentration of rGO at the same scan rate of 5 mV s-1 in 1 M H2SO4. (c) Comparison of gravimetric capacitance at different scan rates for TOBC/rGO electrodes with different concentration of rGO. (d) CV curves of TOBC30/rGO70 fiber at different scan rates. (e) GCD curves of TOBC30/rGO70 fiber at different current densities. (f) Specific capacitance at different current densities of TOBC30/rGO70 fiber.

3.2 Fabrication and characterization of PPy@TOBC/rGO fiber electrode

Electric double layer capacitors (EDLCs) typically possess high power density and excellent rate performance, while pseudo-capacitors exhibit the advantages of high energy density and high capacity [68−72]. Therefore, this study combines the doublelayer capacitor and the pseudo-capacitor charging/discharging mechanism. The pseudo-capacitance property was obtained by polymerizing pyrrole on the electrode for further study. By immersing TOBC/GO hybrid fibers in the aqueous solution of pyrrole for 1 h, the pyrrole monomers were permeable to the inner network of fibers, and the phenomenon of fibers swelling was observed. The carboxylate groups and the hydroxyl groups of TOBC and GO could interact with the imine groups of pyrrole to form hydrogen bonds [56,73,74], which ensured that the PPy layer can be uniformly polymerized on both TOBC nanofibers and GO nanosheets. This method provides a porous hierarchical structure with a large electrolyte-accessible surface area. After different polymerization time, the diameter of fiber is about 94~110 μm, which

is

bigger

than

the

original

diameter

(60

μm),

confirming

that

PPy@TOBC30/rGO70 were successfully achieved by in situ polymerization. Fig. 5a-l show the representative surface and cross-sectional morphologies of TOBC30/rGO70 and PPy@TOBC30/rGO70 macrofibers for 60 min polymerization. It is observed that the surfaces of the hybrid fibers are fully covered by PPy which uniformly grew on the fibers after in situ polymerization (Fig. 5d-f). The rougher cross-section images and more obviously porous structure of PPy@TOBC30/rGO70 fibers (Fig. 5j-l) are observed than those of TOBC30/rGO70 fibers (Fig. 5g-i) which confirm the uniform PPy layer inside and outside of the macrofibers.

The TOBC30/rGO70 and PPy@TOBC30/rGO70 hybrid fibers were identified by Raman spectra (Fig. S4a). The Raman spectrum of PPy@TOBC30/rGO70 hybrid fiber exhibits typical characteristic peaks of PPy and rGO. Two typical peaks at 1352 cm-1 and 1582 cm-1 of PPy are assigned to the C=C backbone stretching and ring stretching modes [75], which are overlapped by D and G bands of rGO. As evident from the Raman spectrum of the PPy@TOBC30/rGO70 hybrid fiber, the D and G bands broaden, corroborating the interaction between the PPy and rGO [76,77]. The peaks of TOBC are not visible in the PPy@TOBC/rGO fibers, which probably due to Raman signal is more sensitive to the conjugated structure [44]. Fig. S4b shows the FTIR spectra of TOBC30/rGO70 and PPy@TOBC30/rGO70 macrofibers. The peak at 1180 cm-1 corresponds to the C-N stretching vibration of the pyrrole ring [78]. The obvious characteristic peaks at around 1555 cm-1 (asymmetric ring stretching) correspond to the C–C ring vibration and 1454 cm-1 (symmetric ring stretching) correspond to C–N inplane ring vibrations of pyrrole ring [39,40], which confirm the formation of PPy. The effective loading of PPy was further confirmed by calculation of linear densities of the PPy@TOBC30/rGO70 hybrid fibers, which were about 77, 89, 93 and 98 μg cm-1 with PPy polymerization times of 30, 60, 120, and 240 min, respectively. Although loading PPy in the macrofibers slightly decreases the mechanical properties for the inherent brittleness of PPy (Fig. S5)[79], the stress of PPy@BC30/rGO70 electrodes is about 60 MPa for 60 min PPy polymerization which is close to that of rGO@CMC (73 MPa) [13], rGO-GO (80 MPa) [80] and PEDOT:PSS (80 ± 5 MPa) [81].

Fig. 5. FE-SEM images of surface of TOBC30/rGO70 (a, b, c) and PPy@TOBC30/rGO70 (d, e, f) hybrid fiber for 60 min PPy polymerization at various magnification. FE-SEM images of cross section of TOBC30/rGO70 (g, h, i) and PPy@TOBC30/rGO70 (j, k, l) hybrid fiber for 60 min PPy polymerization at various magnification.

The electrochemical properties of the PPy@TOBC30/rGO70 fiber electrodes were measured in 1 M H2SO4 solution with a three-electrode system (Fig. 6). The CV curves of hybrid fibers with different polymerization time in pyrrole at the scan rate of 5 mV s-1 were compared in Fig. 6a. A pair of redox peaks corresponding to PPy appeared, indicating the pseudo-capacitance of the electrode belonging to the Faradaic redox transition [30,44,82]. The capacitance of the electrodes with different reaction times calculated from the GCD curves at the current density of 0.5 A g-1 are shown in Fig. 6b. It can be concluded that the specific capacitance initially increases with PPy polymerization time, while it declines when polymerization time further increased, due

to the excess active material (PPy) and higher resistivity [82,83]. At the current density of 0.5 A g-1 (0.48 A cm-3) with the reaction time of 60 min, the most specific capacitance of PPy@TOBC30/rGO70 can reach 391 F g-1 (373 F cm-3). Fig. 6c gives the EIS diagram of TOBC30/rGO70 and PPy@TOBC30/rGO70 fiber electrodes (Inset displays the equivalent circuit). The Nyquist plots consist of an arc in the high frequency and a line in the low frequency, which can reflect the ion transmission and electrochemical performance information of the electrode and electrolytes. Comparing the intercept of the real axis, the PPy@TOBC30/rGO70 electrode (14.5Ω) shows a larger the equivalent series resistance (Rs) than the TOBC30/rGO70 electrode (11.7Ω) in the high frequency. It is related to the electrolyte resistance, intrinsic resistance of the active materials, and contact resistance at the interface between the current collector and the active materials [44,84]. The increased Rs after the polymerization of pyrrole may be ascribed to the slightly slower redox reaction of PPy in the electrolyte and the poorly conductive PPy [85]. The charge transfer resistance (Rct) at the interface can be reflected by the diameter of the semicircles and mainly relies on the infiltration. The larger diameter of the semicircles, the greater the Rct at the interface. As Fig. 6c shown, almost no semicircle is detected for the TOBC30/rGO70 electrodes and only a small semicircle is observed after introduction of PPy into the electrodes, indicating significantly low Rct, which was attributed to the superior infiltration of TOBC nanofibers [23,47]. The line in the low frequency is the Warburg impedance (Zw) and the steeper it is, the superior capacitive behavior the electrode exhibits [86,87]. Both TOBC30/rGO70 and PPy@TOBC30/rGO70 electrodes exhibit straight and nearly vertical straight lines, indicating the superior

capacitive behavior. In order to evaluate the cycling stability of PPy@BC30/rGO70 electrodes, 5000 cycles were continuously scanned at the rate of 100 mV s-1 between −0.4~ 0.8 V. The impedance of electrodes after 1st and 5000th cycle was measured (Fig. S6). After 5000 cycles, the Rs of PPy@BC30/rGO70 electrodes increased from 14.5 Ω to 20.2 Ω, which may be due to the loss of adhesion of some active material or some electrochemical degradation of PPy [88,89]. In addition, Zw is almost invariable due to the good diffusion and migration of electrolyte ions in the charging/discharging process. Fig. 6d shows the CV curves at different scan rates which show shuttle like shape after polymerization of pyrrole due to the high electrical resistivity of PPy. The PPy@TOBC30/rGO70 electrode has a longer discharge time and shows small slight deviations from the ideal triangular shape, which can be attributed to the pseudocapacitive contribution of PPy and oxygen-containing groups in the GCD curves (Fig. 6e) [90,91]. Fig. 6f shows the specific capacitance diagram of the fiber electrodes calculated by the GCD curves. The GCD of PPy@TOBC30/rGO70 fiber electrodes with 391 F g-1 (373 F cm-3) at the current density of 0.5 A g-1 (0.48 A cm-3) is much higher than that of CNT-PPy (50%) fibers (302 F g-1 at 5 mV s-1, in 1 M H2SO4) [80], cellulose/rGO/MWCNT/SnO2/Co3O4 (214 F g-1 at 0.2 A g-1, in 6 M KOH) [92], graphene/MnO2 composite (130 F g-1 at 2 mV s-1, in 0.5 M Na2SO4) [4], CNF/GNS paper (197 F g-1 at 1.25 A g-1, in 6 M KOH) [93] and graphene/MnO2/CNTs film (372 F g-1 at 5 mV s-1, in 1 M Na2SO4) [94]. The excellent electrochemical performance of PPy@TOBC30/rGO70 fiber electrodes is attributable to the synergism of three components (TOBC, rGO and PPy): i) the use of high conductivity rGO greatly

facilitates electron transport during the charge–discharge process. ii) Both TOBC and GO are flexible substrates with large specific surface area, which is beneficial to the PPy polymerization. Further, as a hydrophilic material, TOBC improves the infiltration of electrolyte of the electrode, shortens the ions diffusion distance, and can be as a mechanical buffer layer. iii) the specific capacitance is improved by combining the high pseudo-capacitance of PPy and double-layer capacitance of rGO.

Fig. 6. Electrochemical performance of PPy@TOBC30/rGO70 electrodes with different polymerization time in liquid electrolyte of 1 M H2SO4 aqueous solution. (a) CV curves and (b) Specific capacitances of PPy@TOBC30/rGO70 electrodes with different polymerization time. (c) Nyquist plots of TOBC30/rGO70 and PPy@TOBC30/rGO70 electrodes. Inset: the equivalent circuit. (d) CV curves of PPy@TOBC30/rGO70 fiber at different scan rates. (e) GCD curves of PPy@TOBC30/rGO70 fiber at different current densities. (f) Specific capacitance at different current densities of PPy@TOBC30/rGO70 fiber.

3.3 Fabrication and electrochemical performance of PPy@TOBC/rGO all-solid-state

supercapacitors Based on the superior electrochemical performance of the fiber electrodes, a allsolid-state FSC can be further assembled with the two PPy@TOBC30/rGO70 electrodes and PVA/H2SO4 gel polyelectrolyte (Fig. 7a). The CV curves of the FSC at various scan rates are shown in Fig. 7b. They all show relatively rectangular shapes, revealing good rate performance. Fig. 7c presents the GCD curves of FSC at different current densities. The curves were approximately triangular with a low potential drop region. The variation of FSC specific capacitance at different current densities calculated from the GCD curves are shown in Fig. 7d. The specific capacitance values of the device are 259 F g-1 (258 F cm-3) and 230 F g-1 (229 F cm-3) at the current density of 0.2 A g-1 (0.199 A cm-3) and 0.5 A g-1 (0.498 A cm-3), respectively. These values are much better than symmetric FSC based on CNY@PPy@rGO fiber (93 F g-1 at 2 mV s-1) [79], rGO/PANI fiber (76 F cm-3 at 0.1 mA cm-2) [31], rGO/CNC-20 fiber (117 F g-1 at 0.2 A g-1) [5] and rGO-MoS2 (2.2 wt%) hybrid fiber (~27 F cm-3) [95]. For practical integrated applications, the flexibility of the FSC devices is another significant indicator in addition to specific capacitance. The flexibility of FSC under different bending angles (0º, 45º, 90º, 135º, 180º, inset in Fig. 7e) was further evaluated. The capacitance retention shows no obvious change, indicating that the performance of the FSC based on PPy@TOBC/rGO fibers is stable in the bending state. The pictures inset in Fig. 7e show that the capacitance retention is still good when the FSC was bent at 180º for 500 times. Cycling stability plays a vital role in real applications for supercapacitors. As shown in Fig. 7f, cycling measurements were carried out at the

current density of 1.0 A g-1. The result shows good electrochemical performance with high cycling stability of 79% capacitance retention after 5000 cycles. With similar discharge time, two or three devices were connected in series and the voltage window can reach 2.0 V or 3.0 V from the previous voltage of 1 V (Fig. 7g). We have successfully lit a red LED with a working potential of 3 V by connecting three FSCs in series (inset of Fig. 7g and video S1). The CV curves of the FSCs connected in parallel are shown in Fig. 7h. The integral area could be elevated two or three times of single device by connecting two or three devices in parallel, respectively. The energy density and power density of the flexible devices are two key parameters in the practical application of SCs. The Ragone curves of the FSC and some commercial supercapacitors, other energy storage systems are compared in Fig. 7i. Our fabricated FSC shows an energy density as high as 8.8 mWh cm-3 at the power density of 49.2 mW cm-3, which are higher than those of commercially available SCs (2.75 V/44 mF and 5.5 V/100 mF) [12]. The maximum energy density of our FSC is equal to Li thin film battery (4 V/500 μAh), and the power density is much higher than it [96]. The volumetric energy density value is also higher than that of various rGO-based SCs such as rGO/CNT//rGO/MnO2 FSC [97], CNT@MnO2//CNT@PPy [98] and CNF/rGO/MoOxNy SC [47]. This study demonstrates that the structurally superior flexible FSC would have a great potential in wearable energy storage devices.

Fig. 7. Electrochemical performance of all-solid-state PPy@TOBC30/rGO70 FSCs. (a) Schematic diagram of the FSC structure. (b) CV curves. (c) GCD curves. (d) Specific capacitance. (e) Capacitance stability during bending from 0º to 180º. Inset: variation of FSC at different bending angles and cyclic stability of the device bending at 180º for 500 times. (f) Cyclic stability at the current density of 1.0 A g-1. Inset: GCD curves of the 4990st to the 5000st cycle at 1.0 A g-1. (g) GCD curves of single, two and three FSCs connected in series at the current density of 0.2 A g-1. Inset shows the lighted LED by connecting three FSCs in series. (h) CV curves of single, two and three FSCs connected in parallel at the scan rate of 5 mV s-1. Inset: the corresponding schematics. (i) Ragone plots of the FSC and the other reported supercapacitors. 4. Conclusions In summary, a facile and feasible route was developed to fabricate flexible and binder-free PPy@TOBC/rGO FSC fiber-shaped all-solid-state supercapacitors with

fabrication of TOBC/rGO by wet spinning and in situ polymerization PPy. The TOBC nanofibers interacts strongly with both the PPy and rGO, which enhance the entire electrode while improve electrolyte infiltration and contain a sufficient number of mesopores to promote charge and ion transport. The excellent specific capacitance of PPy@TOBC/rGO electrode with 391 F g-1 (373 F cm-3) at the current density of 0.5 A g-1 (0.48 A cm-3) and FSC made of PPy@TOBC/rGO electrodes with 259 F g-1 (258 F cm-3) at the current density of 0.2 A g-1 (0.199 A cm-3) are achieved due to the synergistic effects of three components. Moreover, FSC exhibits a high energy density of 8.8 mWh cm-3 at the power density of 49.2 mW cm-3, which is better than most previously reported graphene FSCs. In addition, high electrochemical performance is maintained after cycling and bending. This study provides a scalable method for the design and manufacture of flexible FSCs for hybrid fibers for use in future energy storage devices.

Acknowledgements This work was supported by the National Natural Science Foundation of China (51573024 and 51273043), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.

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Highlights 

TOBC nanofibers are used as “spacer” in rGO sheets and mechanical buffer layer

in macrofibers. 

The highly-winkled TOBC/rGO fibers has high electrical conductivity, which can reach 100.2 S cm-1 with 70% rGO concentration.



PPy@TOBC/rGO electrode delivers high capacitance of 391 F g-1 at 0.5 A g-1.



The FSC exhibits an energy density of 8.8 mWh cm-3 with high cycling and bending stability.