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Core-sheath Porous Polyaniline Nanorods/Graphene Fiber-Shaped Supercapacitors with High Specific Capacitance and Rate Capability Xianhong Zheng, Lan Yao, Yiping Qiu, Shiren Wang, and Kun Zhang ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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ACS Applied Energy Materials

Core-sheath Porous Polyaniline Nanorods/Graphene Fiber-Shaped Supercapacitors with High Specific Capacitance and Rate Capability Xianhong Zhenga, Lan Yaoa, Yiping Qiua,b, Shiren Wangc, Kun Zhanga,* a

Key Laboratory of Textile Science & Technology of Ministry of Education, College of

Textiles, Donghua University, Shanghai 201620, China b

College of Textiles and Apparel, Quanzhou Normal University, Fujian 362000, China

c

Department of Industrial and Systems Engineering, Texas A&M University, College

Station, TX 77843, United States * Corresponding author. E-mail address: [email protected] (K. Zhang)

Abstract Fiber and/or yarn-shaped supercapacitors (FSSCs) have tremendous potential applications in portable and wearable electronics owing to their light weight, good flexibility and weavability. However, FSSCs usually show the low energy density, which hinders their wide applications in wearable electronics. It remains challenging for the FSSCs to enhance their energy densities without sacrificing the flexibility and mechanical properties. Herein, we develop a chemical polymerization strategy to fabricate core-sheath porous polyaniline nanorods/graphene fibers, which are used as the FSSCs electrode and show excellent electrochemical performances. The assembled polyaniline nanorods/graphene FSSCs exhibit ultrahigh capacitance of 357.1 mF/cm2, high energy density of 7.93 μWh/cm2 (5.7 mWh/cm3), and power density of 0.23 mW/cm2 (167.7 mW/cm3). In addition, the FSSCs show ultralong cycling life (3.8% capacitance loss, 5000 charge-discharge tests), good rate capability (78.9% capacitance retention) and flexibility. The electrochemical performance of polyaniline nanorods/graphene FSSCs exceeds most reported hybrid FSSCs containing conducting

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polymers and/or metal oxide. This work may pave the way in structure design for portable and wearable energy storage devices. Key words: supercapacitor, graphene fiber, polyaniline nanorods, core-sheath, cycling life, rate capability

Introduction Nowadays, tremendous efforts have been devoted to wearable electronics owing to their potential applications in flexible display, health monitoring, energy harvester, etc.1-3 Therefore, it is important and urgent to develop flexible power sources. Among various flexible power sources,4-9 fiber and/or yarn-shaped supercapacitors (FSSCs) show great potential because of their notable features, including high power density, tiny volume, light weight, excellent cycling performance, good flexibility, weavability and fast charge-discharge speed.10-13 Carbonaceous fibers (CNT yarns, carbon fibers, graphene fibers, etc.) are usually considered as the promising electrodes for FSSCs owing to their high electrical conductivity.14-17 Compared with other fibers, graphene fibers exhibit many outstanding advantages, such as tunable structure, low cost, good flexibility,

easy

functionalization.18-21

However,

the

pure

graphene

fiber

supercapacitors usually show low specific capacitance and energy density, as the graphene aggregates easily and results in low surface (13.4-35.8 m2/g),22-24 which impedes the transport of solvated ions and charge storage. Researchers have developed many strategies to increase the specific capacitance of graphene fiber supercapacitors, including surface coating and/or inside imbedding pseudocapacitive active materials to fabricate hybrid FSSCs.23,

25-28

For instance,

polyaniline (PANI) coated graphene fibers demonstrated the specific capacitance up to 66.6 mF/cm2, which is 20.2 times that of graphene fibers (3.3 mF/cm2);26 MnO2 nanoflowers and/or nanorods/graphene hybrid fibers exhibited the capacitance of 9.6~ 82.6 mF/cm2;

23, 25

Mxene/graphene fibers showed the specific capacitance of 372.2

mF/cm2 owing to the ultrahigh loading of Mxene (90%) and the induced


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pseudocapacitance.29 Many attempts have been made to fabricate high performance graphene fiberbased electric double layer capacitor by increasing the mesopore porosity and/or micropore porosity, through combining the graphene with other carbonaceous materials (CNT, carbon dots, nanosized graphene, etc.24, 30-36) For example, CNT/graphene fiber displayed the high specific capacitance (32.6-177 mF/cm2), resulting from the increased mesopore porosity.30,

33,

36

Carbon black/graphene fibers demonstrated better

electrochemical performance (97.5 F/cm3) than pure graphene fibers due to the higher mesopore porosity and surface area (254.6 m2/g).24 In addition, it has been proved that increasing the micropore porosity was also an efficient way to improve the electrochemical performance of graphene fibers, which resulted from the their ultrahigh micropore porosity (97%), high electrical conductivity and surface area (416.4 m2/g). 31

Although the electrochemical performance has been significantly increased for the graphene fibers, their specific capacitances are increased by sacrificing other performances, including the mechanical properties, cycling life and rate capability. For example, pseudocapacitive active materials decorated graphene fibers showed short cycling life and worse rate capability, resulting from the partly irreversible redox reactions and irreversible volume expansion during the charge-discharge cycles.37-40 Mixing graphene with other carbonaceous materials usually leaded to the low strength of graphene fibers (less than 200 MPa), owing to the high inside porosity and low density.24, 30, 32, 36 Hence, it still remains challenging to fabricating graphene fiber-based FSSCs with excellent electrochemical performance without sacrificing the mechanical properties of graphene fibers. In principle, depositing one-dimensional polyaniline (PANI) nanorods onto the surface of graphene fibers can improve the electrochemical performance of graphene fibers. The advantages of this novel structure are as follows: (ⅰ) PANI nanorods sheath will contribute a large fraction of faradic pseudocapacitance due to the ultrahigh specific capacitance of PANI; (ⅱ) the pores induced by PANI nanorods will favor the solvated ions transport from the electrolyte into the inside of electrodes. Moreover, high electrically conductive graphene fiber will facilitate the



electron transport along the fiber axis. Thus, the porous structure combined with the high electrical conductivity of graphene fiber will benefit for the rate capability; (ⅲ) one-dimensional PANI nanorods will relieve their volume expansion under cycling tests, rendering the hybrid fibers good cycling performance. Herein, we developed a in situ chemical polymerization strategy to depositing polyaniline nanorods on graphene fibers. The polyaniline nanorods/graphene fibers had the core-sheath porous structure, containing the polyaniline nanorods (diameter 30 nm, length 500 nm) as the sheath, and the graphene fiber as the core. The fabricated polyaniline nanorods/graphene fibers showed comparable mechanical properties with graphene fibers. Most importantly, the assembled polyaniline nanorods/graphene FSSCs exhibited ultrahigh capacitance (357.1 mF/cm2) and energy density (5.7 mWh/cm3 or 7.93 μWh/cm2) at 0.23 mW/cm2 (167.7 mW/cm3). Moreover, our FSSCs demonstrate excellent cycling performance (3.8% capacitance loss, 5000 chargedischarge tests), rate capability (78.9% capacitance retention) and good flexibility.

Experimental Synthesis of large graphene oxide (LGO) The improved Hummers method was adopted to synthesize the graphene oxide.41 Typically, 40 mL H3PO4 (85 wt%) was poured into 360 ml concentrated H2SO4 (98 wt%) and 3 g graphite (50 meth) was added into the acid mixture. 18 g KMnO4 was dissolved in the above mixture and kept stirring for 10 h (50 ºC). Subsequently, the reaction system was diluted with 1 L deionized water. The reaction was dropped with 3 mL 30 wt% H2O2. The obtained suspension was vacuum filtrated, and the precipitate was collected and washed by 200 mL 30 wt% HCl to remove the metal ions. Then, the precipitate was washed by deionized water for 3~4 times. Gradient centrifugation was conducted for selecting large graphene oxide according to our previous work.31 In detail, ultrasonication was used to exfoliate the as-synthesized graphite oxide, and the unexfoliated multilayer graphene oxide was removed by centrifugation method (3000


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rpm). The obtained graphene oxide dispersion was then centrifuged at 8000 rpm, and the obtained precipitate was collected and redispersed into deionized water for further centrifugation at 4000 rpm. The precipitate was collected and concentrated in 15 mg/mL for wet spinning. Preparation of Graphene fibers and GF@PANI fibers Wet spinning and thermal annealing were adopted in this work to prepare graphene fibers, and the specific process are described as follows.18 15 mg/mL large size graphene oxide (LGO) aqueous dispersion was spun into 5 wt% CaCl2 coagulation bath (spinneret diameter, 300 μm). Graphene fibers were obtained by thermal annealing of GOF at 800 °C for 3 h (99.999% argon protection) and the obtained fibers were labeled as GF. Polyaniline nanorods/graphene fibers were prepared as follows. 186 μL aniline (>99.5 wt%) was dropped into 40 mL 1M H2SO4 solution to prepare 0.05 M aniline/H2SO4 solution. Then 114 mg ammonium persulfate (APS, >98 wt%) was dissolved into 40 mL 1M H2SO4 solution. The APS/H2SO4 solution was cooled down in ice bath and stirred for 10 min. The prepared APS/H2SO4 solution was quickly mixed with aniline solution, GF were fixed on the PET plate and immersed into the prepared solution for 24 h under slight stirring (0 ºC). The obtained fibers were labeled as GF@PANI. Assembly of all-solid-state FSSCs 1g PVA (Polyvinyl alcohol) was dissolved into 10 mL deionized water and the prepared PVA solution was cooled down to room temperature and added in 0.98 g concentrated sulfuric acid (98 wt%) to form the 1M H2SO4/PVA gel electrolyte. The prepared gel electrolyte firstly coated the fibers and the obtained fiber electrodes were twisted together and then coated with the H2SO4/PVA gel electrolyte. Finally, the allsolid-state FSSCs can be obtained. The prepared FSSCs were tested in two electrode configurations for electrochemical characterizations.



Characterization Nikon ECLIPSE LV100POL was used to observe the liquid crystal of graphene oxide spinning dope and birefringence of gel GO fibers. HITACHI TM3000 and SU5000 were used to observe the morphology of GFs and GF@PANIs. Fourier transform infrared spectroscopy (FTIR, Nicolet NEXUS-670) was conducted to analyze the functional groups in the fibers (KBr pellet technique). The weight of GF (m1) used for polymerizing PANI and GF@PANI hybrid fibers (m2) were measured by using the MS105DU (Mettler Toledo) microbalance. The mass loading (η) of active substance PANI was calculated using the following equation: η=(m2-m1)/m1×100%. Thermogravimetric analysis (TGA) was performed on TGA 4000 analyzer (Perkin Elmer). V-sorb 2800P was used to test the nitrogen adsorption/desorption isotherms of fibers (temperature, 77K), and the specific surface area was calculated based on Brunauer-Emmett-Teller (BET) method. High vacuum treatment for 12 h was firstly conducted to degas the fibers before measurement. The pore size distribution of fibers was calculated based on the Barrett-Joyner-Halenda (BJH) model according to the desorption curve. The electrochemical performance tests, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) and electrochemical impedance spectra (EIS), were performed on CHI 660E electrochemical workstation (CH Instruments Inc.). Areal and volumetric specific capacitance (CA, CV, respectively), energy density (Ecell,A, Ecell,V, respectively), and power density (Pcell,A, Pcell,V, respectively) were calculated according to the equations in supporting information, which are also described in our previous work.31

Results and discussion As shown in Figure 1, We developed a novel, scalable and facile strategy to fabricate polyaniline nanorods/ graphene hybrid fibers with core-sheath porous structure, which contained the wet spinning, thermal annealing and in-situ chemical polymerization process. Graphene oxide fibers (GOF) were firstly spun by the wet


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spinning of LGO dispersion. Subsequently, the prepared GO fibers were reduced into graphene fibers (GF) by thermal annealing at 800 ºC. The prepared GFs were immersed into the aniline/ammonium persulfate/H2SO4 solution for polymerizing PANI nanorods on their surface. The PANI loading was tunable through polymerization time.42-46 In addition, It has been reported that polymerization time affected the morphology of PANI,42-45 and longer polymerization time resulted in higher specific capacitance.47 Longer polymerization time, typically 24 h, usually results in higher loading of PANI and formation of nanostructured PANI (PANI nanoarray or nanorods) with higher specific surface area, which provides more redox active sites and channels for fast ion diffusion and transport. Therefore, polymerization time of 24 h usually leads to better electrochemical performance of the supercapacitors, and we also set the polymerization time as 24 h based on the above factors. It is expected that the graphene fiber after polymerization of aniline for 24 h will exhibit good electrochemical performance owing to ultrahigh loading of PANI nanorods. The fabricated fibers were assembled into allsolid-state FSSCs by using the H2SO4/PVA gel electrolyte.

Figure 1. Schema of fabrication process of GF@PANI, the assembled FSSCs and the structure of GF@PANI.

Figure 2 shows the morphology of GF and GF@PANI. Wrinkles and trenches can be clearly observed on the surface of GF as shown in Figure 2a and 2b (diameter, ~35



μm). Most of the graphene sheets are highly oriented, which results from drawinginduced orientation of graphene during wet spinning process. Furthermore, the densely stacked graphene sheets can be clearly observed in Figure S2a and S2b. Hence, the prepared GF exhibited high tensile stress and electrical conductivity of 484 MPa and 3.2×104 S/m (Figure S3), respectively, which exceeded most of GF.18, 20, 48-49 After the polymerization of PANI, the electrical conductivity was decreased to 1.4×104 S/m due to the fluffy PANI sheath. The GF@PANI shows the rough and porous surface as shown in Figure 2c, and the diameter increased to 90 μm, indicating the surface of GF has been covered by PANI. High magnification SEM image of GF@PANI further reveals that interconnected PANI nanorods (diameter 30 nm, length ~500 nm) was the main structure of the sheath in GF@PANI (Figure 2d and Figure S4), which may be attributed to the synthetical interactions of heterogeneous and homogeneous nucleation.45 Moreover, GF@PANI shows open pores with diameter ranging from 10 nm to 70 nm, which are resulted from gaps between interconnected PANI nanorods. Figure S2c and S2d show that polyaniline nanorods were evenly grown on the surface of GF, implying the core-sheath porous structure of the prepared GF@PANI. It is worthwhile mentioning that GF@PANI shows the comparable mechanical performance with GF (Figure S5d), suggesting the chemical polymerization did not deteriorate the mechanical properties of GF. To get the surface area and pore size distribution of GF and GF@PANI, all the fibers were measured by using the N2 adsorption/desorption method (Figure S6). The mesoporous structure of GF can be confirmed by its type-Ⅳ isotherms (Figure S6a). In addition, GF shows the specific surface area of 57.2 m2/g (BET SSA). The BJH pore size distribution of GF ranges from 2 nm to 189.6 nm (Figure S6b), implying the mesopore-majority hierarchically porous structure of GF. Notably, most of the pores were concentrated at 3.8 nm (pores in the GF) and 7.5~11.8 nm (wrinkles and pores on the surface of GF). After the polymerization of PANI nanorods on the GF, GF@PANI shows the same type isotherms with GF (Figure S6c), which indicates that GF@PANI contains many mesopores. However, BET specific surface area was decreased to 34.8 m2/g. Figure S6d shows the BJH pore size distribution of GF@PANI, and GF@PANI


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shows a narrow BJH pore size distribution and most pores was concentrated at 3.8 nm, and the disappearance of 7.5~11.8 nm mesopores may be attributed to the pores on the GF surface were blocked and replaced by the PANI nanorods.

Figure 2. Morphologies of GF and GF@PANI. (a) and (b) longitudinal SEM image of GF, (c) and (d) longitudinal SEM image of GF@PANI. To further confirm the successful deposition of PANI on the GF, we analyzed the chemical structure of the GF@PANI based on the FTIR (Figure 3). GF shows almost no characteristic infrared absorption peaks, indicating the successful reduction of GF. In contrast, GF@PANI exhibits 7 characteristic peaks at 778 cm-1, 866 cm-1, 1032cm-1, 1134 cm-1, 1294 cm-1, 1444 cm-1 and 1554 cm-1, which is similar with that of pure PANI. The peaks at 778 cm-1 and 866 cm-1 correspond to the vibration of C-H stretching.50 The peak at 1032 cm-1 is associated with S=O stretching vibration resulting from the residuals of H2SO4. The peak at 1134 cm-1 is associated with in-plane bending of C-H in aromatic rings.51 The characteristic peak of C-N stretching vibration can be found at 1294 cm-1, which directly proves the PANI has been deposited onto the surface of GF.51 Two characteristic peaks of C=C stretching vibration in PANI can be also found at 1444 cm-1 and 1554 cm-1, respectively.28, 52 The similar FTIR spectrum of GF@PANI and PANI demonstrated the sheath of GF@PANI was the PANI nanorods.



Figure 3. (a) FTIR spectra of GF, PANI, and GF@PANI, (b) Enlarged FTIR (500-2000 cm-1). We characterized the electrochemical properties of fibers by CV and GCD test, which are tested in two-electrode configuration using H2SO4/PVA gel electrolyte (Figure 4 and Figure S8). GF shows almost rectangular CV curves (Figure 4a), suggesting its electronic double layer capacitor (EDLC) charge storage behavior. On the contrary, GF@PANI displays the pseudocapacitive charge storage behavior due to the existence of a reduction peak at about 0.3 V (Figure 4a). In addition, GF@PANI exhibits larger enclosed CV area compared with that of GF, implying its better charge storage capability. The better electrochemical performance of GF@PANI can be also verified by the GCD test (Figure 4b). The charge curves of GF and GF@PANI are symmetric to their corresponding discharge counterparts, suggesting their capacitance behaviors. The symmetric characteristic of their GCD curves can be also proved by their coulomb efficiencies, which is the ratio of discharge capacity divided by the charge capacity. It can be seen almost all the coulomb efficiencies are above 90% (Figure S9) for both GF and GF@PANI, indicating the symmetry of their GCD curves, high structure stability and reversibility. The discharge time for GF and GF@PANI is 90.6 s and 1413 s (current density, 0.1 mA/cm2), respectively, which also proves the GF@PANI has higher specific capacitance. The CV curves are nearly rectangle shape at low scan rate for GF@PANI, and changes from nearly rectangle into shuttle as shown in Figure 4c, which is associated with the mismatching of charge and electron transport.36 Furthermore, GF@PANI has


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the highest capacitance of 314.5 mF/cm2 (or 226.3 F/cm3 or 5.49 mF/cm), which is 12.1 times higher than that of GF (24.0 mF/cm2). The better electrochemical performance of GF@PANI was associated with the high loading PANI (up to 73.1 wt%). Furthermore, All the GCD curves of GF@PANI are symmetric even at 2 mA/cm2, implying fast charge transport between GF@PANI electrodes and high reversibility of our assembled GF@PANI FSSCs (Figure 4d). Figure 4e shows the specific capacitance of GF@PANI and other reported EDLC FSSCs and hybrid fibers FSSCs containing pseudocapacitance active materials. GF shows the CA of 22.6 mF/cm2 (0.1 mA/cm2), which is in accord with CV results and comparable with previously reported value of GF.26, 30-31 However, after the decoration of polyaniline nanorods, the CA was greatly increased to 357.1 mF/cm2 for GF@PANI, which is superior than the majority all-carbon EDLC FSSCs and pseudocapacitive FSSCs such as RGO@Au (0.726 mF/cm2),53 RGO (205 mF/cm2),54 RGO+CNT@CMC (177 mF/cm2),36 CNT@Co3O4 (52.6 mF/cm2),55 CNT@PANI (38 mF/cm2),56 hollow RGO/PEDOT:PSS (304.5 mF/cm2)27 and other FSSCs.25-26,

55, 57-63

The ultrahigh

specific capacitance of our GF@PANI is attributed to the contribution of pseudocapacitance of high loading PANI nanorods. In addition, Figure 4e shows that the capacitance retention is 78.9% for the GF@PANI FSSC (current density, 0.1-1 mA/cm2), demonstrating GF@PANI has excellent rate capability. In contrast, GF shows 50.8% capacitance retention (current density, 0.1-1 mA/cm2). The excellent rate capability of our GF@PANI FSSC are summarized as follows: (ⅰ) the high electrical conductivity of GF (3.2×104 S/m) facilitates the electron transport along fiber axis;64 (ⅱ) the mesopores (diameter, >10 nm) induced by the polyaniline nanorods facilitated the transport of solvated ions along fiber diameter, leading to the low charge transfer resistance and ion diffusion resistance.65 Hence, fast electron transport along fiber axis and charge transport along fiber diameter provides GF@PANI FSSCs with excellent rate capability. The better electrochemical properties of GF@PANI can be confirmed by their electrochemical impedance spectrum (Figure 4f). Although GF@PANI has a similar equivalent series resistance (ESR, 0.85 KΩ) with GF (0.76 KΩ), the slope is larger in



low frequency for GF@PANI FSSCs, suggesting the lower ion diffusion resistance.66 In addition, no semicircle in Nyquist plot can be found for the GF@PANI, indicating the fast electron transport between the GF and PANI nanorods and low charge transport resistance.67 These results proved that GF@PANI has lots of interconnected channels for ion transport and strong electrical connection between the GF and PANI nanorods for electrons transport.

Figure 4. Electrochemical performance of GF and GF@PANI. (a) CV curves (5 mV/s), (b) GCD curves (0.1 mA/cm2), (c) CV curves of GF@PANI (5~50 mV/s), (d) GCD curves of GF@PANI (0.1-2 mA/cm2), (e) Specific capacitance of our FSSCs compared with the reported FSSCs, (f) Nyquist plots of GF FSSC and GF@PANI FSSC (frequency, 0.01~100 KHz). As shown in Figure 5a, GF@PANI FSSC can be bended at 90º and 180º, and there


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is almost no change of the CV curves even bended at 180º, indicating that bending shows negligible effects on the capacitance of GF@PANI FSSCs. Furthermore, discharge curves of GF@PANI FSSCs are parallel with each other (Figure 5b), implying almost no decrease of specific capacitance. After 500 bending cycles, there is still a 99.8% capacitance retention of our GF@PANI FSSC (Figure S10), implying the excellent flexibility of GF@PANI FSSCs. We connected 3 GF@PANI FSSCs in series, which displayed the voltage of 2.4 V and easily power a red LED as shown in Figure 5c. We also investigated the cycling performance of GF@PANI FSSCs (Figure 5d). There is still 96.2% capacitance retention for GF@PANI FSSC after 5000 GCD cycles, which is slightly lower than that of GF (98.3%), and greatly higher than that of other conducting polymer-based FSSCs including CNT@PANI FSSC (91% for 800 cycles),56 Gamma-irradiated CNT@PANI FSSC (89% for 1000 cycles),68 Gammairradiated CNT@PEDOT/PSS FSSC (91% for 600 cycles),69 CNT@PPy FSSC (90% for 1000 cycles).70 Furthermore, the GCD curves of our GF@PANI FSSC still display the isosceles triangle shape from 4996 to 5000 cycles, implying the high reversibility and good capacitance behavior of GF@PANI FSSC. The excellent cycling performance of GF@PANI FSSCs may be associated with the following factors: (ⅰ) the all-solidstate electrolyte enables the structure integrity of electrode and prevents the volume expanding of PANI nanorods; (ⅱ) the interconnected PANI nanorods contact tightly in the sheath, and the GF acts as a solid skeleton to keep a structural foundation; (ⅲ) one-dimensional PANI nanorods relieve the volume expansion during the chargedischarge cycles.38, 71-72



Figure 5. CV curves (a) and GCD curves (b) of all-solid-state GF@PANI FSSC bended with 0º, 90º and 180º, (c) GCD curves of single and 3 GF@PANI FSSCs connected in series, (d) Cycling life of GF and GF@PANI FSSCs (1 mA/cm2). Inset in (a) is the images of GF@PANI FSSC bended with 0º, 90º and 180º, inset in (c) is 3 GF@PANI FSSCs are powering a red LED, and inset in (d) is GCD curves of GF@PANI FSSC from 4996 to 5000 cycles Figure 6a is areal Ragone plots of our FSSCs and other FSSCs. At Pcell,A of 0.02 mW/cm2, the Ecell,A is 0.5 μWh/cm2 for GF FSSC, which exceeds previously reported pure GF and CNT FSSCs, including all-graphene core-sheath FSSCs (0.17 μWh/cm2)63 and CNT FSSCs (0.19 μWh/cm2).73 After the modification of PANI nanorods, the GF@PANI FSSC exhibits a stable and high energy density, indicating the high energy density can be retained even at high power density. GF@PANI FSSC has the energy density of 7.93 μWh/cm2, which exceeds most of the pseudocapacitance hybrid FSSCs and all-carbon EDLC FSSCs. For example, the energy density of GF@PANI FSSC is 2.9 times that of CF@pen ink FSSC,74 2.1 times that of RGO+CNT FSSC,36 7.2 times that of CNT@Co3O4 FSSC,73 and 16.7% higher than hollow RGO/PEDOT:PSS FSSC.27 Moreover, GF@PANI FSSC even shows higher energy density than the RGO@MnO2 asymmetric FSSC (6.7 μWh/cm2).30 The maximum power density of our


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supercapacitor is 0.23 mW/cm2, which is also comparable with above FSSCs. We also summarized the Ecell,V of our FSSCs with commercial supercapacitors, lithium thin film battery and other reported FSSCs. At Pcell,V of 14.5 mW/cm3, the Ecell,V is 5.7 mWh/cm3 for GF@PANI FSSC, which is greatly higher than commercial capacitors (more than 10 times),75-76 26 times that of CF@MnO2 FSSC (0.22 mWh/cm3),77 2.8 times that of CF@PANI FSSC (2 mWh/cm3),50 4 times that of CNT@PEDOT FSSC (1.4 mWh/cm3),62 162.9% that of RGO+CNT FSSC36 and CNT@MnO2 FSSC (3.52 mWh/cm3),78 even comparable with the lithium thin film battery (0.4-8 mWh/cm3)79, CF@PPy@MnO2 FSSC (6.16 mWh/cm3)80 and MnO2@RGO FSSC (5.8 mWh/cm3).23 GF@PANI FSSC shows the maximum Pcell,V of 167.7 mW/cm3, which greatly exceeds the lithium thin-film batteries,76,

79

and

comparable with most FSSCs and some commercial capacitors. We connected 4 GF@PANI FSSCs in series to increase the output voltage up to 3.2 V, and the series GF@PANI FSSCs can light 3 red LEDs (Figure 6c and 6d), which also proves the high energy density of our GF@PANI FSSCs.

Figure 6 Areal Ragone plots of GF@PANI FSSCs compared with other FSSCs, (b)



volumetric Ragone plots of GF@PANI FSSCs compared with other FSSCs, commercial capacitors and lithium thin film battery. (c) and (d) Photographs of 4 GF@PANI FSSCs before and after connected to power 3 LEDs, respectively. We summarized the reasons for the excellent electrochemical properties of GF@PANI, which are as follows: (1) The GF@PANI composite fiber has the coresheath structure with the high electrically conductive graphene fiber as the core, and high loading PANI nanorods as the sheath. High electrically conductive graphene fiber facilitates the electron transport along the fiber axis. Besides, graphene fiber contributes electrical double-layer capacitance. The sheath composed of high loading PANI nanorods can provide most of the faradic pseudocapacitance due to the ultrahigh specific capacitance of PANI. Therefore, the GF@PANI shows ultrahigh specific capacitance. (2) The GF@PANI has hierarchically porous structure. The mesopores induced by the interconnected PANI nanorods favor the solvated ions transport from the electrolyte into the inside of electrodes and facilitate the ions transport along the fiber diameter direction. Thus, the mesopores combined with the high electrical conductivity of GF leads to the excellent rate capability of GF@PANI FSSCs. (3) Onedimensional PANI nanorods can reduce the volume expansion, favoring for the cycling performance of GF@PANI FSSCs. Furthermore, ion diffusion path will be shortened for the porous PANI nanorods sheath, resulting in low ion diffusion resistance. As a result, the GF@PANI fiber-shaped supercapacitors display excellent cycling stability.

Conclusions

We developed a facile, scalable and novel strategy to manufacture core-sheath polyaniline nanorods/graphene fibers by wet spinning, thermal annealing and in-situ chemical polymerization method. The fabricated polyaniline nanorods/graphene fibers showed hierarchically porous structure. The assembled polyaniline nanorods/graphene FSSCs exhibited high capacitance of 357.1 mF/cm2, ultrahigh energy density (7.93 μWh/cm2 or 5.7 mWh/cm3), ultralong cycling life (3.8% capacitance loss, 5000 chargedischarge tests) and excellent rate capability (78.9% capacitance retention, 0.1-1


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mA/cm2). In addition, GF@PANI FSSCs showed good flexibility. The electrochemical performances of GF@PANI FSSCs are superior than the majority of reported pseudocapacitance hybrid and EDLC FSSCs. This article will pave the way in structure design for portable and wearable energy storage devices.

ASSOCIATED CONTENT

Supporting information Calculation of specific capacitance, energy density and power density, POM images, SEM images, typical stress-strain curve, Statistic distribution of PANI nanorods diameter, N2 adsorption/desorption isotherms and the corresponding pore size distribution, TGA curves, CV curves, GCD curves, Coulomb efficiency, bending performance, electrochemical performance comparison of GF@PANI FSSCs with other reported FSSCs.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

The authors thank the financial support from the Fundamental Research Funds for the Central Universities (19D110106), Young Elite Scientists Sponsorship Program by CAST (2017QNRC001), the National Natural Science Foundation of China (No.51603036), and the “DHU Distinguished Young Professor Program”.

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