Metal–Organic Framework Derived Spindle-like Carbon Incorporated

Sep 7, 2018 - Metal–Organic Framework Derived Spindle-like Carbon Incorporated α-Fe2O3 Grown on Carbon Nanotube Fiber as Anodes for ...
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Metal-Organic Framework Derived Spindle-like Carbon Incorporated #-Fe2O3 Grown on Carbon Nanotube Fiber as Anodes for High-Performance Wearable Asymmetric Supercapacitors Zhenyu Zhou, Qichong Zhang, Juan Sun, Bing He, Jiabin Guo, Qiulong Li, Chaowei Li, Liyan Xie, and Yagang Yao ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04336 • Publication Date (Web): 07 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Metal-Organic Framework Derived Spindle-like Carbon Incorporated α-Fe2O3 Grown on Carbon

Nanotube

Fiber

as

Anodes

for

High-Performance

Wearable

Asymmetric

Supercapacitors Zhenyu Zhou+a,b,c, Qichong Zhang+b, Juan Sunb, Bing Heb, Jiabin Guob, Qiulong Lib, Chaowei Lib, Liyan Xieb and Yagang Yao* b,c a. Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China. b. Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, Joint Key Laboratory of Functional Nanomaterials and Devices, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China c. Division of Nanomaterials, Suzhou Institute of Nano-Tech and Nano-Bionics, Nanchang, Chinese Academy of Sciences, Nanchang 330200, China.

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ABSTRACT Iron oxide (Fe2O3) has drawn much attention because of its high theoretical capacitance, wide operating potential window, low cost, natural abundance, and environmental friendliness. However, the inferior conductivity and insufficient ionic diffusion rate of simple Fe2O3 electrode leading to the low specific capacitance and poor rate performance of supercapacitors have impeded its applications. In this work, we report a facile and cost-effective method to directly grow MIL-88-Fe metal-organic framework

(MOF)

derived

spindle-like

α-Fe2O3@C on

oxidized

carbon

nanotube

fiber

(S-α-Fe2O3@C/OCNTF). The S-α-Fe2O3@C/OCNTF electrode is demonstrated with high areal capacitance of 1232.4 mF/cm2 at a current density of 2 mA/cm2 and considerable rate capability with capacitance retention of 63% at a current density of 20 mA/cm2, and is well matched with cathode of the Na-doped MnO2 nanosheets on CNTF (Na-MnO2 NSs/CNTF). The electrochemical test results show that the S-α-Fe2O3@C/OCNTF//Na-MnO2 NSs/CNTF asymmetric supercapacitors possess a high specific capacitance of 201.3 mF/cm2 and an exceptional energy density of 135.3 µWh/cm2. Thus, MIL-88-Fe MOF derived S-α-Fe2O3@C will be a promising anode for applications in next-generation wearable asymmetric supercapacitors. KEYWORDS: metal-organic framework, spindle-like α-Fe2O3@C, fiber-shaped, asymmetric supercapacitors,wearable electronic

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As a specific type of energy storage device for the emerging portable and wearable electronic products, fiber-shaped supercapacitor (FSC) has attracted widespread attention due to its high power density, fast charge-discharge rate, superior flexibility and excellent weavability.1-13 However, the low energy density has limited its further application in various energy-consuming devices. Constructing hybrid and asymmetric supercapacitors has been widely explored as a promising strategy to increase their energy density.14-17 Although the energy density of hybrid supercapacitors can be considerably increased by different charge-storage mechanisms and the extended operating potential window in aqueous electrolytes, the low specific capacitance of the commonly used carbon-based electrode materials with double layer energy storage mechanism have severely restricted their maximum energy density. Therefore, increasing research interests have been focused on the development of highly pseudocapacitive anodes and cathodes to develop high-energy-density fiber-shaped asymmetric supercapacitors (FASCs).18-25 Unfortunately, compared with the considerable advancement achieved for the cathodes, the lack of high performance anodes seriously obstructs further improvement of their energy density. Thus, it is urgent to develop ultrahigh-performance fibrous anodes for high-energy-density FASCs. Up to now, a considerable number of anode pseudocapacitive materials have been studied in the search for relatively large specific capacitance and good rate performance.26-28 Among them, Fe2O3 has drawn much attention because of its high theoretical capacitance, wide operating potential window, low cost, natural abundance and environmental friendliness.29-31 However, the inferior conductivity and insufficient ionic diffusion rate of simple Fe2O3 electrodes leading to low specific capacitance and poor rate performance have hindered their further application in FASCs. To address these issues, intensive efforts have been devoted to synthesizing Fe2O3 nanostructures and hierarchical core-shell structures with high surface area and short ion diffusion paths,32-34 yet their specific capacitances remain far 3

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below the theoretical value. Thus, the development of significantly capacitive Fe2O3-based fibrous anodes still remains a great challenge. To optimize the performance of Fe2O3-based fibrous anodes, it is necessary to controllably synthesize nanostructured Fe2O3 with abundant surface area on conductive fiber substrate to maximize the redox kinetics. Metal-organic frameworks (MOFs), which are compounds constructed from metal ions or clusters and organic linkers, contain nano-cavities and open channels, and have thus been considered as ideal sacrificial templates to synthesize nanomaterials with large surface areas and suitable structures.35-38 MIL-88-Fe, a representative Fe-based MOF, has been used as the template for fabricating a porous α-Fe2O3 anode powder material for lithium-ion batteries with superior specific capacity and high rate capability.39,40 Despite great advances have been achieved in nanostructured α-Fe2O3,31,41-43 their specific surface area effectively involved in the actual reaction is yet unsatisfactory because of the inferior conductivity of Fe2O3. Coating conductive polymer or nanocarbon on the surface of α-Fe2O3 is a very effective way to improve its conductivity,34, 44-47 however, leading to the reduced reactive sites of α-Fe2O3. To date, direct growth of nanostructured MIL-88-Fe MOF-derived carbon incorporated α-Fe2O3 (S-α-Fe2O3@C) on conductive fibrous substrates still remains a major challenge. Meanwhile, carbon nanotube fibers (CNTFs) are regarded as the most promising candidates for conductive fibrous substrates due to their light weight, good flexibility, high electrical conductivity, and mechanical strength.48 To further improve the compatibility between the surface of CNTFs and electroactive materials, we have developed a simple and effective electrochemical method to fabricate highly hydrophilic oxidized CNTFs (OCNTFs). Here, we have successfully fabricated S-α-Fe2O3@C on OCNTF with increased electrical conductivity and large specific surface area. Contributed by the ultrahigh surface area and intriguing 4

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nanostructure, the S-α-Fe2O3@C/OCNTF electrode exhibits a superior specific capacitance of 1232.4 mF/cm2 at a current density of 2 mA/cm2 and an excellent rate capability with capacitance retention of 63% at a current density of 20 mA/cm2. Encouragingly, our FASC device consisting of S-α-Fe2O3@C/OCNTF anode and Na-doped MnO2 nanosheets/CNTF (Na-MnO2 NSs/CNTF) cathode achieves a high specific capacitance of 201.3 mF/cm2 and an exceptional energy density of 135.3 µWh/cm2. RESULTS AND DISCUSSION Figure1a-d illustrate the fabrication process of the S-α-Fe2O3@C/OCTNF electrode and the details are presented in the experimental section. Typically, the OCNTF (Figure 1b) is achieved via oxidizing pristine CNTF (Figure 1a) with an electrochemical cyclic voltammetric (CV) method according to our previously published report. Then, MIL-88-Fe is densely coated on the surface of the OCNTF to form a MIL-88-Fe/OCNTF composite fiber (Figure 1c). Finally, S-α-Fe2O3@C/OCNTF (Figure 1d) hybrid fiber was obtained by annealing MIL-88-Fe/OCNTF (Figure 1c) under air atmosphere. The magnified scanning electron microscopy (SEM) images in Figure 1e-f exhibit that MIL-88-Fe uniformly covers the entire OCNTF surface and the shape of single MIL-88-Fe is regular dodecahedron. In the subsequent step, all of the MIL-88-Fe was completely converted to α-Fe2O3 with a homogeneous spindle-like nanostructure (Figure 1g). As MOFs are made up of metal ion centers coordinated to organic linkers, their controlled heating in air can give rise to porous transition metal oxides due to the oxidation of the metal ions and the release of gaseous. Compared to MIL-88-Fe (Figure 1f), the S-α-Fe2O3@C composite (Figure1g) showed angular shape and its lateral size decreased apparently, we attribute this to the decomposition and contraction when being heated. The microstructure of S-α-Fe2O3 is further characterized by transmission electron microscopy (TEM). Evidently, the low-magnification image and corresponding energy-dispersive spectroscopy (EDX) mapping results (Figure 1h) not only 5

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further verify its intriguing nanostructure resulting from the ideal MIL-88-Fe-based sacrificial templates, but also confirm uniform distribution of the elements Fe, O in the S-α-Fe2O3@C and where the C signal is attributed to the organics from MIL-88-Fe precursor being annealed. The high-resolution image in Figure 1i demonstrates that the lattice fringes have a spacing of 0.27 nm, which corresponds to the (014) plane of α-Fe2O3. Furthermore, in the X-ray diffraction (XRD) spectrum, most of the diffraction peaks can be assigned to α-Fe2O3 (Figure 1j, JCPDS card No.33-0064) and the (*) can be attribute to C, which in good agreement with the high-resolution TEM result. As shown in Figure S1, the BET surface area of the S-α-Fe2O3@C is estimated to be 116.5 m2/g and the BJH adsorption pore diameter of the S-α-Fe2O3@C is 19.2 nm, which confirm the large specific surface area and mesoporous structure. Simultaneously, the low-magnification image of TEM in figure S2 also illustrated the porosity of the spindle-like S-α-Fe2O3@C. Furthermore, the existence of carbon can be confirmed by the mapping (Figure 1h), XRD (Figure 1j) and the Raman spectra (Figure S3) images, which can improve conductivity of S-α-Fe2O3. These results provide strong evidence for the successful fabrication

of

S-α-Fe2O3@C/CNTFs,

which

were

subsequently

investigated

for

use

in

high-performance energy storage devices. The chemical compositions and valence states of the S-α-Fe2O3@C/OCNTFs were analyzed by X-ray photoelectron spectroscopy (XPS) (Figure 2). The full XPS spectrum in Figure 2a contains the signals of C, O, and Fe, manifesting their coexistence in the S-α-Fe2O3@C/OCNTFs hybrid fiber. As shown in Figure 2b, the spectrum of Fe2p contains two distinct peaks at the binding energy of 711.9 eV and 725.4 eV, assigned to Fe2p3/2 and Fe2p1/2, and two shake-up satellites at 719.5 eV and 733.2 eV, which are consistent with previously reported XPS spectra for Fe2O3.34, 49 The O1s spectrum presented in Figure 2c is divided into two peaks centered at 530.7 eV and 532.8 eV. The low-binding-energy component at 530.7 eV is attributed to the O2- ions in Fe2O3, while the latter peak 6

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is assigned to C-O. The deconvoluted peaks (Figure 2d) of the C1s spectrum are resolved into two components, centered at 284.8 eV and 285.6 eV, which can be assigned to C in nonoxygenated carbon groups (CC) and carbonyl groups (CO), respectively. To evaluate the electrochemical performance of the material, the S-α-Fe2O3@C/OCNTFs was used as the anode in a three-electrode system using 1 M Na2SO4 solution as the electrolyte, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. Figure 3a compares the CV curves of the pristine CNTF, OCNTF, S-α-Fe2O3@C/CNTF, and S-α-Fe2O3@C/OCNTF electrodes at 50 mV/s with the same fiber length. Evidently, the OCNTF electrode exhibits a larger area under the CV curve than the pristine CNTF electrode, indicating an enhanced specific capacitance of the CNTF after the activation process. Furthermore, the CV curve area of the S-α-Fe2O3@C/CNTFs is much higher than for the OCNTFs, which suggests that the pseudocapacitive material S-α-Fe2O3@C makes a considerably higher contribution to the capacitance than does the activation of the CNTFs. More importantly, compared with S-α-Fe2O3@C/CNTF electrode, the S-α-Fe2O3@C/OCNTF electrode exhibits nearly three times CV curve area, demonstrating that the mass load of electrochemical active substance on the surface of OCNTF is significant larger than on the pristine CNTF. Furthermore, the galvanostatic charge-discharge (GCD) curves of the pristine CNTF, OCNTF, S-α-Fe2O3@C/CNTF and S-α-Fe2O3@C/OCNTF electrodes at a current density of 4 mA/cm2 are compared together in Figure 3b. As can clearly be seen, all four curves display excellent symmetry with triangular shapes; meanwhile, the S-α-Fe2O3@C/OCNTFs electrode has the longest discharge time, which again confirms its superior capacitive behavior and the very high interface-compatibility between OCNTFs and S-α-Fe2O3@C. Figure 3c displays the CV curves of the S-α-Fe2O3@C/OCNTF electrode in a potential window from -1 to 0 V at different scan rates between 10 and100 mV/s. All the curves exhibit quasi-rectangular shapes, implying typical psedocapacitive behavior. Figure 3d shows the nearly symmetric triangular-shaped 7

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GCD curves of the S-α-Fe2O3@C/OCNTF electrode, which have no obvious iR drop even current densities increased to 20 mA/cm2 , suggesting excellent electrochemical capacitance and small internal resistance. As presented in Figure 3e, the S-α-Fe2O3@C/OCNTF electrode achieves a maximum areal specific capacitance of 1232.4 mF/cm2 at a discharge current density of 2 mA/cm2, and still retains a prominent rate capability of 63% at a high discharge current of 20 mA/cm2. The areal, longitudinal, volumetric and gravimetric capacitance of the S-α-Fe2O3@C/OCTNF electrode were provided in Figure S4. Figure 3f demonstrates the electrochemical impedance spectroscopy (EIS) curves of the S-α-Fe2O3@C/OCNTF electrode in the frequency range between 0.01 Hz and 100 kHz. In the high frequency region, the intercepts of the Nyquist plots on the real axis almost reaches zero, indicating that the electrolyte contact resistance (Re) is very small. The relatively small radius of are on the real axis presents the low charge transfer resistance (Rct). Moreover, the low frequency line is nearly vertical in shape for the S-α-Fe2O3@C/OCNTF electrode, suggesting the fast ion diffusion behavior. Figure S5 illustrates the long-term cycling performance of the S-α-Fe2O3@C/OCNTF at a current density of 3 mA/cm2. A capacitance loss of only 2.4% was observed after 4000 cycles, demonstrating its outstanding cycling stability. Figure 3g compares the specific capacitance and rate capability of S-α-Fe2O3@C/OCNTF electrode with the values reported for other Fe2O3-based electrodes. As shown in the columnar comparison chart, the blue columns represent the maximum specific capacitance (C0 F/g) and the pink ones display the capacitance retention at five times curent density ((C5/C0)%). Impressively, the areal capacitance and ((C5/C0)%) of the S-α-Fe2O3@C/OCNTF electrode reaches 1538 F/g (at 2.5 A/g) and 70%, which is much higher than that of previously reported works including Fe3O4nanoparticles@graphene (220.1 mF/cm2)/(59%),50 C3N4/Fe2O3 (260 F/g)/(52%),51 Fe3O4 (510 F/g),52 Fe2O3 particles/graphene (908 F/g)/(69%),44 FeOOH (100.6 F/g)/(67.1),53 graphene/porous Fe2O3 (1095 F/g)/(55%),54 Fe2O3/CNT nanocomposites (580.6 F/g)/(69.4%),55 iron oxide/graphene 8

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(326 F/g),56 Fe2O3 (64.5 F/g)/(53%),41 Ti-Doped Fe2O3@PEDOT Core/Shell (311.6 F/g)/(66.7%),45 Porous α-Fe3O4 (343.7 F/g)/(38%),46 nanoporousFe3O4 (102 F/g)/(30%),57 Fe3O4 (207.7 F/g),58 Fe3O4 nanoparticles@graphene (368 F/g)/(60%),59 Fe2O3 (143 F/g)/(65.3%),60 FeOx (84 F/g).61 The admirable electrochemical performance of our as-fabricated S-α-Fe2O3@C is ascribed to the increased electrical conductivity and large specific surface area of well-designed MOF-derived electrode materials. Additionally, to demonstrate the superior electrochemical performance of the S-α-Fe2O3@C/OCNTF electrode, different sizes of α-Fe2O3@C/OCNTF electrodes have been synthesized, the detail method and characterizations were shown in Figure S6 and Figure S7. To demonstrate the practical applicability of the as-established S-α-Fe2O3@C/OCNTF electrode, a flexible solid-state twisted FASC device based on S-α-Fe2O3@C/OCNTF anode and Na-MnO2 NSs/CNTF cathode was fabricated (Figure 4a). MnO2 NSs were coated on the pristine CNTF, then sodium ions were inserted to form Na-MnO2 NSs (details see Experimental Section).62-64 Furthermore, the related SEM/TEM images and XPS spectrums are displayed in Figure S8, S9 and S10. Figure S11 presents the electrochemical performance of Na-MnO2 NSs electrode such as CV, GCD and rate capability curves. Obviously, the as-prepared Na-MnO2 NSs/CNTF electrode attains a large areal capacitance of 845 mF/cm2 at a discharge current density of 2 mA/cm2, and possesses ideal symmetry of quasi-rectangular CV and nearly triangular shaped GCD curves. Moreover, it possesses an excellent rate capability because the capacitance still retains 78.5% at a current density of 20 mA/cm2. These results convincingly exhibit that the as-obtained Na-MnO2 NSs/CNTF owns excellent electrochemical performance. As shown in Figure 4b, the operating potential windows of S-α-Fe2O3@C/OCNTF and Na-MnO2 NSs/CNTF are -1-0 V and 0-1.2 V, respectively, from which we can infer that the maximum operating voltage for the as-assembled device can achieve 2.2 V. As expected, the CV curve of the as-assembled S-α-Fe2O3@C/OCNTF//Na-MnO2 NSs/CNTF ASC device collected at 10 mV/s given in 9

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Figure 3c still maintains rectangular-like shape even at a high potential window up to 2.2 V. Furthermore, the almost symmetrical triangular GCD curves of the device in Figure 4d at a current density of 4 mA/cm2 again confirm its ideal capacitive characteristics with a rapid I-V response and low equivalent series resistance. Figure 4e exhibits the areal specific capacitance and energy density of the FASC, computed from discharge curves at a current density of 4 mA/cm2. Impressively, both the areal specific capacitance and energy density of the FASC device are significantly enhanced while the operation potential increasing from 0.6 to 2.2 V. Notably, the energy densities of the FASC device increase from 7 to 119.5 µWh/cm2 with an enhancement of more than 1700%. Figure 5a shows the CV curves of the FASC device at different scan rates under the potential range of 0-2.2 V, and no obvious change is observed in the quasi-rectangular CV curves even at a high scan rate of 100 mV/s, accordingly demonstrating that the as-prepared device exhibits fast electron transfer and excellent capacitive performance. This is further supported by the nearly triangular-shaped GCD curves at various current densities from 2 to 20 mA/cm2 (Figure 5b). Based on these GCD curves, we calculate the specific capacitances of the FASC device and depict them in Figure 5c, where the device achieves high specific capacitance values of 201.3 mF/cm2 and 136.4 mF/cm2 at the current densities of 2 mA/cm2 and 20 mA/cm2, respectively, implying an exceptional rate capability. Figure 5d shows the Ragone plot (energy density and power density) for the S-α-Fe2O3@C/OCNTF//Na-MnO2 NSs/CNTF FASC device. The device exhibits an ultrahigh areal energy density of 135.3 µWh/cm2 at a power density of 2199.9 µW/cm2 and still maintains a high energy density of 91.7 µWh/cm2 even at a high power density of 21998.4 µW/cm2. These levels are substantially superior to previous reported FASC devices, such as ultrathin MnO2 nanosheet/carbon fiber (1.428 µWh/cm2, 51.4 µW/cm2),65 MnO2/graphene yarn (8.2 µWh/cm2, 930 µW/cm2),66 polyelectrolyte-wrapped graphene/ carbon nanotube

core-sheath

fibers

(3.84-2.2

µWh/cm2,

17-165

µW/cm2),2 10

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Ni/NiO/Ni(OH)2/PEDOT//Ni/CMK-3 (11 µWh/cm2, 330 µW/cm2),67 ss/PEDOT@MnO2//ss/C@Fe3O4 (33.5 µWh/cm2, 600 µW/cm2),68 CNT/ZnO-NWs@MnO2//CNT (13.25 µWh/cm2, 210 µWh/cm2),69 NiCo-DHs/CNT//Pen ink/CNT (9.57 µWh/cm2, 492.17 µW/cm2).70 The superior performance of the S-α-Fe2O3@C/OCNTF//Na-MnO2 NSs/CNTF FASC device indicates that the S-α-Fe2O3@C/OCNTF electrode is an ideal anode candidate for energy storage in practical supercapacitors. The Nyquist plot of the electrochemical impedance and cycling stability of our FASC is presented in Figure S12. As shown in Figure 5e, the GCD curves undergo only negligible changes when the bending angle is varied from 0° to 180° at a current density of 4 mA/cm2, exhibiting the outstanding flexibility of the device. Furthermore, after bending at 90° for more than 4000 cycles, the device still retains 97.1% of its specific capacitance (Figure 5f), further confirming its mechanical stability. Figure S13 exhibits the size of electrodes/cell and the application of the FASC device. To display the excellent weavability of the device, a longer device was weaved into a piece of cloth (Figure S14), and the electrochemical performance of the textile was shown in Figure S15. CONCLUSIONS In summary, we have successfully fabricated specific MOF-derived S-α-Fe2O3@C/OCTNF anodes via a facile and cost-effective method. Due to the superior structure of MOFs-derived α-Fe2O3@C, S-α-Fe2O3@C/OCTNF electrodes deliver a superior areal capacitance of 1232.4 mF/cm2 at a current density of 2 mA/cm2 and superior rate capability with capacitance retention of 63% at a current density of 20 mA/cm2. Furthermore, a high-performance FASC device with maximum operating voltage of 2.2 V was assembled by matching the anode with Na-MnO2 NSs/CNTF cathodes. Our FASC device exhibits a high specific capacitance of 201.3 mF/cm2 and an exceptional energy density of 135.3 µWh/cm2, which exceeds most previously reported state-of-the-art FSCs. Thus, this work provides a cost-effective and scalable route to develop high-capacity anode materials for 11

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next-generation wearable energy storage devices. EXPERIMENTAL METHODS Synthesis of S-α-Fe2O3/OCNTF electrode. To obtain OCNTF, we used a standard three-electrode configuration to oxidize the CNTF at room temperature with 0.8 M H2SO4 aqueous solution as the electrolyte, a graphite sheet as the counter electrode and Ag/AgCl electrode as the reference electrode. Where the CV method was adopted with the potential window ranging from 1 to 2 V at a scan rate of 25 mV/s for 15 cycles. Then, the OCNTF was rinsed with plenty of deionized water and dried at 60 oC in a vacuum oven. The as-fabricated OCNTFs were then immersed in an DMF solution (40 ml) containing 6.75 mmol ferric chloride hexahydrate and 4.5 mmol terephthalic acid had been stirred for 60 min prior to transfer into a Teflon-lined stainless steel autoclave (50 ml inner volume). The autoclave was sealed and kept at 100 oC in a vacuum oven for 15 h. After cooling to room temperature, the obtained OCNTFs were washed with distilled water and dried overnight at 60 o

C. Finally, they were annealed into S-α-Fe2O3@C/OCNTF at 350 oC in air for 2 h with a heating rate

of 2 oC/min. Synthesis of Na-MnO2 NSs/CNTF electrode. The CNTFs were pretreated with concentrated hydrochloric acid for 10 min, followed by rinsing with plenty of deionized water. Typically, 0.23 g of KMnO4 and 0.75ml g of concentrated hydrochloric acid were mixed in 45 ml of distilled water and stirred for 10min to form a homogeneous solution. The solution was then transferred into a 50 ml Teflon-lined stainless steel autoclave and the CNTFs were immersed in the solution. The autoclave was then sealed and maintained at 85 °C for 40 min. After natural cooling to room temperature,the resulting CNTFs were washed repeatedly with distilled water and dried 6 h at 60 oC. Finally, the Na-MnO2 NSs were obtained by electrochemical oxidizing via CV scans between 0 and 1.3 V (vs Ag/AgCl) in saturated Na2SO4 electrolyte for 500 cycles. 12

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Assembly of S-α-Fe2O3/OCNTF//Na-MnO2 NSs/CNTF FASC. The gel electrolyte was prepared by adding 10 g Na2SO4 and 6 g carboxymethyl cellulose sodium (CMC) into 100ml of distilled water under quick stirring at 85

o

C for 3 h until the gel became transparent. Thereafter,

S-α-Fe2O3@C/OCNTF and Na-MnO2 NSs/CNTF were then soaked in Na2SO4-CMC gel electrolyte for 10 min and annealed at 70 °C for 20 min. Finally, the FASC device was prepared successfully by twisting two electrodes together and leaving it overnight until the electrolyte was solidified.

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FIGURES

Figure 1 (a-d) Schematic of the synthesis process of S-α-Fe2O3@C/OCTNF electrode. (a) Schematic of pristine CNTF. (b) Schematic of OCNTF. (c) Schematic of MIL-88-Fe/OCNTF composite. (d) Schematic of S-α-Fe2O3@C/OCTNF electrode. (e,f) SEM image of MIL-88-Fe/OCNTF at increasing magnification. (g) SEM image of S-α-Fe2O3@C/OCTNF electrode. (h) Low magnification TEM image and EDX mappings of different elements of Fe, O and C recorded from an individual S-α-Fe2O3@C. (i) Higher magnification of an individual S-α-Fe2O3@C. (j) XRD spectrum of the S-α-Fe2O3@C.

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Figure 2 XPS survey scans of (a) full spectrum, (b) Fe2p, (c) O1s, and (d) C1s regions for the S-α-Fe2O3@C.

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Figure 3 (a) Comparison of CV curves of pristine CNTF, OCNTF, S-α-Fe2O3@C/CTNF and S-α-Fe2O3@C/OCTNF electrodes measured at a scan rate of 25 mV/s. (b) Comparison of GCD curves of pristine CNTF, OCNTF, S-α-Fe2O3@C/CTNF and S-α-Fe2O3@C/OCTNF electrodes obtained at a current density of 5 A/g (4 mA/cm2). (c) CV curves of S-α-Fe2O3@C/OCTNF at different scan rates. (d) GCD curves of the S-α-Fe2O3@C/OCTNF electrode at different current densities. (e) Gravimetric specific capacitance of the S-α-Fe2O3@C/OCTNF electrode calculated from the GCD curves as a 16

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function of current density. (f) Nyquist plots of the S-α-Fe2O3@C/OCTNF electrode. (g) Comparision of the specific capacitance and rate of C5/C0 of our S-α-Fe2O3@C/OCTNF electrode with recently reported Fe2O3-based electrodes (the rate capability of [52], [56], [58] and [61] was not reported).

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Figure 4 (a) Schematic illustration of our FASC device. (b) Comparative CV curves of the S-α-Fe2O3@C/OCNTF and Na-MnO2 NSs/CNTF at a scan rate of 25 mV/s. (c) CV curves of the S-α-Fe2O3@C/OCNTF//Na-MnO2 NSs/CNTF device measured at different operating voltages at a constant scan rate of 25 mV/s. (d) GCD curves of the as-fabricated FASC device collected over different voltages from 0.6 to 2.2 V at a current density of 4 mA/cm2. (d) Areal specific capacitance and energy density calculated based on GCD curves obtained at 4 mA cm-2.

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Figure 5 (a) CV curves measured at different scan rates between 0 and 2.2 V. (b) GCD CV curves of the as-fabricated FASC measured at different current densities between 0 and 2.2 V.

(c) Areal

specific capacitances calculated from the charge-discharge curves as a function of the current density. (d) Areal energy and power densities of our device in comparison with previously reported FASC. (e) CV curves of the as-prepared FASC device measured at a current density of 4 mA cm-2 under different bending angles. (f) Normalized capacitances of the as-obtained FASC with a bending angle of 90° for 4000 cycles.

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ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Electrochemical Performance Measurements; N2 absorption-desorption isotherm of the α-Fe2O3@C electrode

materials;

Areal,

longitudinal,

volumetric

and

gravimetric

capacitance

of

the

S-α-Fe2O3@C/OCTNF electrode. Cycling stability of the S-α-Fe2O3@C/OCTNF electrode; Experiment conditions of different heteromorphic MIL-88-Fes; The corresponding SEM images and electrochemical performance of different MIL-88-Fes/α-Fe2O3@C with diffrtrnt experiment conditions ; SEM image of Na-MnO2/CTNF electrode at increasing magnification; Low magnification TEM image and X-ray elemental mappings of different elements of Mn, O and Na recorded from an individual Na-MnO2; Higher magnification of the Na-MnO2;The weight/atomic percentage of Na, Mn and O;XPS survey scans of full spectrum for the Na-MnO2;XPS survey scans of Na1s, Mn2p and O1s for the Na-MnO2. CV curves at various scan rates;GCD curves at different current densities of the Na-MnO2/CTNF electrode measured in 1 M of Na2SO4; Areal specific capacitance of the Na-MnO2/CTNF electrode calculated from the GCD curves as a function of current density. Nudist plots and cycling stability of the FASC device; The size of electrode and cell its application exhibition; Optical image of the FASC textiles and its application exhibition; The electrochemical performance of the FSAC textiles; Cycling stability of the S-α-Fe2O3@C/OCTNF electrode. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Author Contributions 20

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+

Z.Z. and Q.Z. contributed equally to this work.

Conflict of Interest The authors declare no conflict of interest. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51522211, 51602339, 51703241 and U1710122), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China (Nos. 2016M601905 and 2017M621855), the Natural Science Foundation of Jiangsu Province, China (Nos. BK20160399), and the Science and Technology Project of Nanchang (2017-SJSYS-008). REFERENCES (1) Yu, D.; Goh, K.; Wang, H.; Wei, L.; Jiang, W.; Zhang, Q.; Dai, L.; Chen, Y., Scalable Synthesis of Hierarchically Structured Carbon Nanotube-Graphene Fibres for Capacitive Energy Storage. Nat.

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