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Functional Inorganic Materials and Devices

Highly Stretchable Waterproof Fiber Asymmetric Supercapacitors in an Integrated Structure Kai Guo, Xianfu Wang, Lintong Hu, Tianyou Zhai, Huiqiao Li, and Neng Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05676 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Applied Materials & Interfaces

Highly Stretchable Waterproof Fiber Asymmetric Supercapacitors in an Integrated Structure Kai Guo,† Xianfu Wang,‡ Lintong Hu,§ Tianyou Zhai,§ Huiqiao Li*,§ and Neng Yu*,† †

Jiangxi Province Key Laboratory of Polymer Micro/Nano Manufacturing and Devices, School

of Chemistry, Biology and Materials Science, East China University of Technology, Nanchang 330013, P. R. China ‡

College of Physics, Optoelectronics and Energy, Suzhou Key Laboratory of Advanced Carbon

Materials and Wearable Energy Technology & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, P. R. China §

State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials

Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China KEYWORDS: waterproof, stretchable, asymmetric, fiber supercapacitor, wearable electronics

ABSTRACT: Fiber supercapacitors have attracted tremendous attention as promising power source candidates for next generation of wearable electronics, which are flexible, stretchable, and washable. Although asymmetric fiber supercapacitors with a high energy density have been achieved, their stretchability is no more than 200%, and they still face mechanical instability and

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unreliable waterproof structure. This work develops a highly integrated structure for waterproof, highly stretchable and asymmetric fiber-shaped supercapacitor, which is assembled by integrating a helix-shaped asymmetric fiber supercapacitor into a bifunctional polymer. The asymmetric fiber supercapacitor demonstrates a working voltage of 1.6 V, a high energy density of 2.86 mW h/cm3, remains unchanged capacitance after immersed in water for 50 h and retains 95% of its initial capacitance after 3000 cycles of stretching-releasing at a maximum strain of 400%. The extraordinary waterproof capability, the large stretching strain, and excellent stretching stability are attributed to the highly integrated structure design, which can also simplify the assembly process of stretchable, waterproof fiber supercapacitors.

1. INTRODUCTION Next generation of wearable electronics is emerging as fascinating products because they are light-weighted, deformable, and washable.1-3 Their matching power sources should have a high energy density, good flexibility, large stretchability, and good waterproof capability.4-6 Fiber supercapacitors are one of the most promising power source candidates because of superior merits, including high power density, long life, small diameters, lightweight, and extraordinary flexibility.7-8 Asymmetric fiber supercapacitors have attracted tremendous attention due to a larger working voltage compared with symmetric counterparts.9-10 The energy density (E) of fiber supercapacitors is determined by the equation of E = 1/2 CV2 (C is the specific capacitance and V is the working voltage). Thus, widening the working voltage range is a highly efficient route to enhance the energy density, which has been proved by several asymmetric fiber supercapacitors.11-14 Although a great progress has been achieved in asymmetric fiber


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supercapacitors, their stretchability and waterproof performance require urgent improvements to meet the demands of wearable electronics. Asymmetric stretchable fiber supercapacitors are rare and their stretchability is no more than 200%.15-19 The limited stretching strain is due to the limitation of the electrode and assembly technologies.20-21 Stretchable fiber supercapacitors are typically assembled by winding fiber electrodes on elastic substrates or by putting helical elastic fiber electrodes together,16, 22 which may result in mechanical and electrochemical instability due to unsatisfactory structure design: the winded fiber electrodes on the elastic substrate can slide axially due to the friction force from the elastic substrate during elongation or shrinkage,15 while the helical electrodes can separate from each other during the stretching and releasing process.23 In addition, as a necessary feature of next-generation wearable electronics, the waterproof performance has recently aroused research attention.24-25 Two waterproof fiber supercapacitors have been developed very recently through coating the as-prepared fiber supercapacitors with a layer of polymer or being inserted into a thermally-shrinkable tube, but they are not stretchable.26-27 Moreover, the additional waterproof layer or component not only extends fabrication process but also increases the waterproof malfunction risk due to an unintegrated structure: the additional waterproof layer or component may detach with the fiber supercapacitors during the stretching process. Thus, a highly elastic and reliable structure for stretchable, waterproof fiber supercapacitors is highly demanded. Herein, a highly integrated structure is developed for stretchable, waterproof, and asymmetric fiber-shaped supercapacitors (SWAFS). The SWAFS is assembled by embedding a helix-like asymmetric fiber supercapacitors into a bifunctional polymer, which serve as both an elastic substrate and a waterproof packaging material. The rational structure can simplify the fabrication


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process by realizing the stretching and waterproof functions in one step and can largely reduce waterproof malfunction risk of stretchable, waterproof fiber supercapacitors through increasing the integration degree. The asymmetric fiber supercapacitors work at a voltage range of 0-1.6V and demonstrate a large energy density of 2.86 mWh/cm3. Due to the protection of polymer encapsulation, the SWAFS shows excellent waterproof performance during a 50 h test under water, a superior stretching strain of 400% and a capacitance retention of more than 95% during the 3000 cycles of stretching-releasing at a strain of 400%. The extraordinary waterproof capability, the large stretching strain, and excellent stretching stability are attributed to the highly integrated structure design. 2. EXPERIMENTAL SECTION 2.1 Synthesis of MnO2 on the SW All of the chemicals were of analytical grade and used without further purification. The MnO2 nanofibers were electroplated on the SW under 0.6 V (vs. Ag/AgCl) in the solution containing 0.1 mol/l Na2SO4 and 0.1 mol/l Mn(CH3COO)2 for 10min, 20min, 30min, and 1h at ambient temperature. The samples are noted as SW/MnO2-10, SW/MnO2-20, SW/MnO2-30, and SW/MnO2-60, respectively. 2.2 Synthesis of RGO on the SW Aqueous GO solution was prepared by dispersing 10 mg GO in 100 mL water with the aid of ultra-sonication for 30 min, followed by addition of 0.1 M LiClO4. Pt fiber, Ag/AgCl, and SW fibers were used as a counter, reference electrodes, working electrodes (current collectors)


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respectively. A constant potential of -1.2 V was then applied on the SW, and the deposition lasted for 2 h. 2.3 Assembly of Waterproof Stretchable Fiber Supercapacitors One SW/MnO2 and two SW/RGO fiber electrodes were coated with polyvinyl alcohol (PVA)/LiCl hydrogel electrolyte, separately. After drying at 40 ℃ for 10 min to remove excess water, the three fiber electrodes were placed together and coated with a small amount of PVA/LiCl gel to glue them together to make a fiber supercapacitor. The fiber supercapacitor was then winded around a steel fiber to form a helix-like shape. After the steel fiber was pulled away, the helix-like fiber supercapacitor was inserted into a tube. The Ecoflex (Smooth-on, USA) polymer precursor was injected into and filled the interspace of the tube by a syringe before curing at 50 ℃ for 2 h. Finally, the cured polymer was pulled out of the tube with the helix-like fiber supercapacitor incorporated inside. The PVA/LiCl gel electrolyte was prepared as follows: 2.5 g of LiCl and 6 g of PVA powder were dissolved in 60 mL of deionized water and then stirring the mixture at 90°C until it became clear. 2.4 Material Characterizations The materials were characterized by X-ray diffraction (XRD, X'Pert PRO, PANalytical B.V., the Netherlands) with a Cu target (Kα, λ = 0.15406 nm), field emission scanning electron microscopy (FESEM, JEOL JSM-6700F, 5 kV), transmission electron microscopy (TEM, JEOL, JEM-2010 HT), Raman spectra (Horiba JobinYvon, LabRAM HR800), nitrogen adsorption– desorption isotherms (Micromeritics, ASAP2020M) using the Brunauer–Emmett–Teller (BET)


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method and X-ray photoelectron spectroscopy (XPS, VG Multilab 2000 system with a monochromatic Al KR X-ray source). 2.5 Electrochemical Measurements All electrochemical measurements including CV curves, galvanostatic charge-discharge curves, and cycling performance were conducted on an electrochemical workstation (CHI 760D, CH Instruments Inc., Shanghai). The electrochemical performance of the SW/MnO2 and SW/RGO fiber electrodes were evaluated with a 1 mol/l LiCl electrolyte solution in a threeelectrode system (fiber electrodes, a platinum plate, and an Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively). 3. RESULTS AND DISCUSSION The fabrication process of the SWAFS is demonstrated in the schematic Figure 1. MnO2 nanofibers and graphene nanosheets are first deposited on SW to make the positive and negative fiber electrodes followed by coating LiCl/polyvinyl alcohol (PVA) hydrogel electrolyte on the surface, respectively. After the excessive water of the fiber electrode is removed, one SW/MnO2 fiber and two SW/RGO fibers are placed together and are stuck together with a little hydrogel electrolyte to make a fiber supercapacitor. Then, the fiber supercapacitor is winded on a steel fiber and the steel wire core is removed to form a helix-like shape. The helix-like fiber supercapacitor is inserted into a tube and then the tube is fulfilled with a precursor solution of Ecoflex polymer by a syringe. After curing, the polymer is crosslinked and pulled out of the tube with the helix-like fiber supercapacitor inside. The as-prepared device is waterproof and can be reversibly stretched at a strain of 400%. The structure of the stretchable waterproof asymmetric fiber supercapacitors is highly integrated by using only two components: a fiber supercapacitor


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and an elastic polymer material, free from the use of additional waterproof materials, which is believed to reduce the mechanical instability risk of the stretchable fiber supercapacitor. And the stretchable and waterproof polymer is molded in one step, which can simplify the assembly process of stretchable, waterproof fiber supercapacitors.

Figure 1. The schematic fabrication process of a SWAFS.

Figure 2a shows a typical X-ray diffraction (XRD) spectrum of the MnO2 film deposited on the SW. There are four weak diffraction peaks at 2θ = ∼22.3°, ∼36.6°, 55.4 and ∼65.2°, which

can be assigned to the (101), (210), (212) and (020) crystal planes of γ-MnO2 (JCPDS 44-

0142).28-29 The MnO2 on the SW are uniform, and the diameter of SW reaches 42.6 μm after 30 min of deposition (Figure S1a). The high-resolution scanning electron microscopy (SEM) image in Figure 2b shows the MnO2 film on the SW are made of interconnected nanofibers. And the thickness of the MnO2 nanofibers after 30 min deposition is about 2 μm, as displayed in Figure S1b. The transmission electron microscopy (TEM) image in Figure 2c clearly confirms the MnO2 film are composed of interconnected nanofibers with a diameter of 10~20 nm. High-


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resolution TEM image in Figure S1c displays crystalline planes with an interlayer distance of 0.24 nm, which is assigned to the (210) planes of γ-MnO2.29 The deposited MnO2 is further confirmed by X-ray photoelectron spectroscopy (XPS) spectra of Mn and O elements. The Mn 3s spectrum in Figure S1d is deconvoluted into two distinct peaks at 83.7 and 88.7 eV.30 The separation of the two peaks is 5.0 eV, which is between 4.78 and 5.41 eV for Mn4+ and Mn3+, respectively. The Mn 2p spectrum in Figure S1e displays two peaks at 642.1 and 653.9 eV (binding energy gap: 11.8 eV), corresponding to the Mn 2p3/2 and Mn 2p1/2 orbits of MnO2.31-33 The O1s spectrum (Figure S1f) are fitted with three peaks at 529.7, 531.2, and 532.5 eV, which are corresponding to the Mn-O-Mn bond for the tetravalent oxide, the Mn-O-H bond for the hydrated trivalent oxide, and the H-O-H bond for the residual water, respectively. Based on the O1s spectrum, the chemical valence of Mn is estimated to be 3.8, which is in good agreement with the analysis result of Mn 3s spectrum.34


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Figure 2. (a) XRD pattern, (b) FESEM image, and (c) TEM image of the as-synthesized MnO2 on the SW. The electrochemical performance characterization of the SW/MnO2 electrode: (d) CV curves, (e) GCD curves at different current densities, and (f) the specific capacitance at different scan rates.

The specific surface area and porosity of the MnO2 nanofiber film are evaluated by the N2 adsorption-desorption isotherms and BJH pore size distribution analysis, as shown in Figure S2. The MnO2 nanofiber film shows a specific surface area of 93.2 m2/g through the Brunauer– Emmett–Teller (BET) calculation method and a typical type Ⅳ adsorption isotherm with a hysteresis at a relative pressure between 0.4 and 1.0 (Figure S2a), suggesting the presence of mesostructure. The pore distribution analysis results in Figure S2b show a wide peak and centered at around 3 nm in the range of 2-30 nm, further indicating abundant mesopores in the MnO2 film. The large specific surface area can provide abundant electrochemical reaction sites and mesopores among MnO2 nanofibers can facilitate ion diffusion. The morphologies of MnO2 deposited on SWs for 10, 20, and 60 min are all nanowires, similar to the 30 min samples, while the MnO2 film becomes thicker with longer deposition time, as shown in Figure S3. Obvious cracks appear in the MnO2 film with a 60 min deposition, which may lead to peeling off of MnO2 upon deformation.35-36 The electrochemical performance of SW/MnO2 is optimized by varying the deposition time of MnO2 for 10, 20, 30, and 60 min, which are noted as SW/MnO2-10, SW/MnO2-20, SW/MnO230, and SW/MnO2-60. The quasi-rectangular cyclic voltammetry (CV) curves of SW/MnO2-30 in Figure 2d at a scan rate of 10-500 mV/s in the potential range of 0-0.8 V indicates dominant pseudocapacitive behavior. The galvanostatic charge-discharge (GCD) plots in Figure 2e are in a symmetric triangle shape at the charging-discharging current density of 0.002-0.02 mA/cm, revealing highly reversible electrochemical reactions. The SW/MnO2-30 fiber electrode displays


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a maximum specific capacitance of 0.6 mF/cm or 42.1 F/cm3 at the scan rate of 10 mV/s and can still retain 44.0% of initial specific capacitance when the scan rate increases to 500 mV/s, indicating good rate performance (Figure 2f). The large specific capacitance and good rate performance of SW/MnO2 can be attributed to the mesoporous nanofiber film morphology. The SW/MnO2-30 fiber electrode also displays excellent cycling performance. More than 90% initial capacitance is retained after 5000 charging-discharging cycles at 0.01 mA/cm, as shown in Figure S4. The SW/MnO2-10, SW/MnO2-20, and SW/MnO2-60 also show typical pseudocapacitive behaviors as shown by the quasi-rectangular CV curves in Figure S5a-S5c, respectively. The specific capacitance of the SW/MnO2 increases while the rate performance becomes worse with longer deposition time (Figure S5d). The SW/MnO2-30 fiber electrodes were selected for assembly of fiber supercapacitors The SW/RGO negative electrodes for fiber supercapacitors are fabricated by simultaneously electro-depositing and reducing graphene oxide (GO) nanosheets on the SWs. The surface of SW/RGO fiber is wrinkled with a diameter of ~45 μm, as shown in Figure 3a. Magnified SEM image in Figure S6a shows wrinkles are made of stacking RGO sheets, which is confirmed by the TEM image in Figure 3b. The SAED analysis of the RGO sheets (Figure S6b) shows bright spots in dim circular rings, suggesting crystallized graphene sheets. In order to estimate the quality of RGO sheets, XPS and Raman analysis of GO and RGO were performed. The relative intensity of carbon-oxygen bond peaks in the C 1s XPS spectrum of RGO is much smaller than that of GO (Figure S6c), suggesting GO are highly reduced through electrochemical reduction.37 The reduction of GO is also confirmed by the Raman spectra in Figure S6d. The intensity ratio of D peak and G peak in the Raman spectrum increases from 1.01 to 1.4 due to the increase of sp2 domains, which is similar to other reports.38-39


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The electrochemical performance of SW/RGO electrode is characterized by a three-electrode system. The SW/RGO electrode shows typical electric-double-layer capacitance, while the bare SW shows no obvious capacitance (Figure S7a). The CV curves of SW/RGO in Figure 3c remain quasi-rectangular shape at the scan rate of 10-100 mV/s in the potential range of -0.8~0 V, revealing good rate performance. The maximum specific capacitance of SW/RGO is 0.27 mF/cm or 16.8 F/cm3 at a scan rate of 10 mV/s, and 74.1% of the specific capacitance is retained at the scan rate of 100 mV/s (Figure 3d), indicating good rate performance. Figure S7b shows the SW/RGO is electrochemically stable with a capacitance retention of more than 90% after 5000 cycling test at 50 mV/s.


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Figure 3. (a) SEM image of the SW/RGO fiber. (b) TEM image of the RGO deposited on the SW. (c) CV curves at various scan rates, and (d) the specific capacitance at different scan rates for the SW/RGO electrode.

Prior to the assembly of fiber supercapacitor, the charge balance of positive and negative electrodes is based on the CV test result of SW/MnO2 and SW/RGO, as displayed in Figure 4a. According to the calculation result, an SW/MnO2 fiber is paired with two SW/RGO fibers to make an all-solid-state fiber supercapacitor. The SWAFS shows quasi-rectangular CV curves (Figure 4b) and triangular GCD curves (Figure 4c) at a working voltage range from 0-0.8 V to 0-1.6 V, suggesting the maximum working voltage of fiber supercapacitor is no less than 1.6 V, much higher than that of MnO2 based symmetric fiber supercapacitors (0.8 or 1.0 V).40-41 Figure 4d shows the specific capacitance of SWAFS increases 26.7% when the working voltage is broadened from 0.8 to 1.6 V. As a result, the energy density of the SWAFS is increased 324.4%, according to the equation E = 1/2 CV2.


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Figure 4. (a) The CV curve of SW/MnO2-30 and the CV curve of SW/RGO at 100 mV/s in a three-electrode system. (b) CV curves at 100 mV/s, (c) GCD curves at 0.008 mA/cm, and (d) the specific capacitance at 0.008 mA/cm in different voltage windows for the SWAFS.

When the scan rate increases from 10 to 100 mV/s, the CV curves of the SWAFS remain a quasi-rectangular shape (Figure 5a). The GCD curves in Figure 5b are in a symmetric triangle shape, indicating reversible electrochemical reactions. The SWAFS has a maximum specific capacitance of 0.47 mF/cm, 10.1 mF/cm2 or 8.21 F/cm3 at 0.008 mA/cm and retains 62.4% of capacitance when the current density increases to 0.08 mA/cm (Figure 5c), revealing good rate performance. During 10000 charging-discharging cycling test, the fiber supercapacitor remains almost unchanged in capacitance, demonstrating high electrochemical stability (Figure 5d). Although the specific capacitance is smaller than some asymmetric fiber supercapacitors, 42 43 44 the maximum energy density of the fiber supercapacitor is 3.52 μWh/cm2 or 2.86 mWh/cm3, which is comparative or even superior to recent results for stretchable fiber supercapacitors, such as symmetric CNT fiber electrodes (0.226 μWh/cm2, 0.6 mWh/cm3),43 symmetric CNT fiber electrodes (0.629 mWh/cm3),44 symmetric PPy/RGO/MWCNT fiber electrodes (0.94 mWh/cm3),42 symmetric MnO2/CNT fiber electrodes (2.6 μWh/cm2, 0.53 mWh/cm3),23 asymmetric CNT@ MnO2//CNT@PPy fiber electrodes (2.98 mWh/cm3),16 and asymmetric CFT@PANI//FCFT fiber electrodes (2.0 mWh/cm3).19 The high energy density of the SWAFS is attributed to a high voltage of 1.6 V for the asymmetric device and a large specific capacitance, as shown in Table S1.


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Figure 5. The electrochemical performance characterization of the SWAFS: (a) CV curves, (b) GCD curves, (c) specific capacitance at different current densities, and (d) capacitance retention during a 10000 chargingdischarging test at a current density of 0.08 mA/cm.

The waterproof capability is a necessary function of wearable electronics. The fiber supercapacitor is wrapped by the Ecoflex polymer, which renders the SWAFS waterproof capability. In order to evaluate the waterproof performance, the electrochemical performance of SWAFS is measured underwater, as shown in Figure 6a. The CV curves (Figure 6b) curves of the SWAFS in air condition and underwater almost overlap, similar to the GCD curves (Figure 6c), indicating water does insignificant influence on the electrochemical performance of the SWAFS. And the long-time waterproof performance of the SWAFS was evaluated by a ~50 h cycling test, as shown in Figure 6d. The fiber supercapacitor remains very stable


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electrochemical performance during the cycling test underwater and retains more than 95% of initial capacitance after 10000 charging-discharging cycles. The extraordinary waterproof has not ever been achieved in other fiber supercapacitors, which can be attributed to complete waterproof encapsulation structure.

Figure 6. (a) Schema of a SWAFS immersed in water during the electrochemical characterization. Electrochemical performance comparison of the SWAFS in air and underwater: (b) CV curves at 100 mV/s, (c) GCD curves at 0.008 mA/cm in air and underwater, and (d) cycling test underwater at 0.08 mA/cm.

The flexibility is a primary but important property for flexible supercapacitors. The SWAFS deforms extensively by bending or knotting for flexibility test, as the schemes in Figure S8a shows. Figure S8b shows the CV curves of the SWAFS at the fold and the knot states nearly


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coincide with that at a natural state. And the overlapped GCD curves of the SWAFS at three different states (Figure S8c) further confirms that bending or knotting makes an insignificant impact on the electrochemical performance, suggesting excellent flexibility of the SWAFS. Stretching capability is an advanced and critical feature for actual applications of wearable electronics. The SWAFS is stretched to different states with a maximum strain up to 400% to estimate the stretching capability. At stretching states, the SWAFS elongates in the axial direction and contracts in the radial direction, but the SWAFS keeps intact and is still tightly encapsulated by the polymer substrate, as shown in Figure 7a and Figure S9. After the tensile is released, the stretched Ecoflex polymer spontaneously recovers its initial shape and the embedded helix-like fiber supercapacitor is forced to deform along with the polymer substrate. As a result, the SWAFS can be reversibly stretched. The CV curves of the SWAFS almost overlap after being stretched, even when the strain is 400% (Figure 7b). The capacitance change of the SWAFS is less than 7% at different stretching states according to the GCD curves in Figure 7c. Stretching-releasing tests at five different strains of from 50% to 400% for 3000 cycles are also conducted on the SWAFS to further estimate the stretching stability. The capacitance of the SWAFS remains almost unchanged during 3000 cycles of stretchingreleasing at the strain of 50%~300%. Although the capacitance slightly fluctuates at the strain of 400%, more than 95% of the initial capacitance is retained after the cycling test (Figure 7d), suggesting the excellent mechanical stability of SWAFS. Compared with current stretchable fiber supercapacitors, the SWAFS shows a comparative energy density and a larger stretching strain, such as the asymmetric MnO2/CNT//CNT (1.44 mWh/cm3, 100%),18 asymmetric CFT@PANI//FCFT (2 mWh/cm3, 100%),19 asymmetric CNT@MnO2//CNT@PPy (2.98 mWh/cm3, 50%),16 and MnO2/CNT//PEDOT/CNT (2.13 μWh/cm2, 200%),17 as shown in


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Figure 7e and Table S1. The large stretching strain of the SWAFS and its stable electrochemical stability at a large strain can be attributed to the rational mechanical structure. The helix-shaped fiber supercapacitor can be substantially stretched hundreds of percent by reducing the diameter and increasing the spacing of the helix. By embedded and tightly fixed by the elastic polymer, the electrodes are pressed together and is forced to keep the same deformation rate with the polymer substrate, which avoids the separation of fiber electrodes or the relative displacement of fiber electrodes and the elastic substrate and thus increase the mechanical stability of the whole device. By using the bifunctional polymer as a waterproof encapsulating material as well as an elastic substrate, the number of components can be reduced and the assembly process can be simplified for waterproof, stretchable fiber supercapacitors. The waterproof malfunction risk during severe deformation can also be highly reduced since there is no detachment of waterproof component with fiber supercapacitor.


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Figure 7. (a) Photos of a SWAFS stretched at a strain of 0%, 200%, and 400%, respectively. (b) The CV curves at 100 mV/s and (c) the GCD curves at 0.008 mA/cm of the SWAFS at different stretching states. (d) The capacitance retention of the SWAFS during 3000 stretching cycles at different strains. (e) The maximum stretching strain and energy density of the recent stretchable fiber supercapacitors.

4. CONCLUSIONS


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In summary, a highly elastic and reliable structure is developed for fiber supercapacitors through encapsulating a helix-shaped fiber supercapacitor inside a bifunctional elastic polymer substrate. The SWAFS demonstrates a working voltage of 1.6 V and a high energy density of 2.86 mWh/cm3. In particular, the SWAFS shows very stable electrochemical performance even immersed in water for about 50 h, displaying excellent waterproof performance. The SWAFS exhibits great flexibility and almost unchanged electrochemical performance under folded and knotted states. Significantly, the SWAFS showed excellent stretchability of 400% without significant capacitance decay even after 3000 cycles of stretching-releasing tests, which is superior to most of the reported stretchable fiber supercapacitors. The long-time waterproof performance, high flexibility, and a 400% stretching stain are attributed to the highly integrated polymer encapsulated helix-shaped fiber supercapacitor structure, which can also help reduce the number of components and simplify the assembly process of stretchable, waterproof fiber supercapacitors by realizing the stretching and waterproof functions in one step. The SWAFSs are very promising energy storage devices for next-generation wearable electronics. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website, including the following information: Calculation methods for electrochemical performance of electrodes and devices, SEM images and XPS spectra of the SW/RGO fiber, N2 adsorption-desorption isotherm and BJH pore distribution analysis result of MnO2 nanowires, SEM images, galvanostatic chargedischarge cycling result and CV curves of SW/MnO2 fibers, SEM image, SAED pattern,


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XPS and Raman spectrum of RGO peeled from the SW/RGO fiber, CV curves of the bare SW and SW/RGO fiber, cycling performance of the SW/RGO fiber, table of performance of recent stretchable fiber supercapacitors, schemes of a SWAFS at different deformation states and corresponding CV and GCD curves, photos of a SWAFS stretched from 0% to 400%. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51702048, 21603157), the National Basic Research Program of China (2015CB932600), the Education Department of Jiangxi Province (GJJ170459, GJJ170457), and the Science and Technology Department of Jiangxi province (20171BBE50031). REFERENCES (1) Gates, B. D. Materials Science. Flexible Electronics. Science 2009, 323, 1566-1567. (2) Wu, Z. S.; Zheng, Y.; Zheng, S.; Wang, S.; Sun, C.; Parvez, K.; Ikeda, T.; Bao, X.; Mullen,

K.; Feng, X. Stacked-Layer Heterostructure Films of 2D Thiophene Nanosheets and Graphene


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