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Wrapping Aligned Carbon Nanotube Composite Sheets around Vanadium Nitride Nanowire Arrays for Asymmetric Coaxial Fiber-Shaped Supercapacitors with Ultrahigh Energy Density Qichong Zhang, Xiaona Wang, Zhenghui Pan, Juan Sun, Jingxin Zhao, Jun Zhang, Cuixia Zhang, Lei Tang, Jie Luo, Bin Song, Zengxing Zhang, Weibang Lu, Qingwen Li, Yuegang Zhang, and Yagang Yao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00854 • Publication Date (Web): 28 Mar 2017 Downloaded from http://pubs.acs.org on March 29, 2017

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Wrapping Aligned Carbon Nanotube Composite Sheets around Vanadium Nitride Nanowire Arrays for Asymmetric Coaxial Fiber-Shaped Supercapacitors with Ultrahigh Energy Density Qichong Zhang+a,b, XiaonaWang+b, Zhenghui Panb, Juan Sunb, Jingxin Zhaob, Jun Zhangb, Cuixia Zhangb, Lei Tangb,Jie Luob, Bin Songb,Zengxing Zhang*a, Weibang Lub, Qingwen Lib,Yuegang Zhangb and Yagang Yao*b a. Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Science and Engineering, Tongji University, Shanghai 200092, P. R. China b. Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and Applications, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Suzhou 215123, P. R. China [*] Email:[email protected] [email protected] [+] These authors contribute equally to this work. The emergence of fiber-shaped supercapacitors (FSSs) has led to a revolution in portable and wearable electronic devices. However, obtaining high energy density FSSs for practical applications is still a key challenge. This article exhibits a facile and effective approach to directly growth well-aligned three-dimensional vanadium nitride (VN) nanowire arrays (NWAs) on carbon nanotube (CNT) fiber with an ultrahigh specific capacitance of 715 mF/cm2 in a three-electrode system. Benefiting from their intriguing structural features, we successfully fabricated a prototype asymmetric coaxial FSS (ACFSS) with a maximum

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operating voltage of 1.8 V. From core to shell, this ACFSS consists of a CNT fiber core coated with VN@C NWAs as the negative electrode, Na2SO4 polyvinyl alcohol (PVA) as the solid electrolyte, and MnO2/conducting polymer/CNT sheets as the positive electrode. The novel coaxial architecture not only fully enables utilizationof the effective surface area and decreases the contact resistance between the two electrodes but also, more importantly, provides a short pathway for the ultrafast transport of axial electrons and ions. The electrochemical results show that the optimized ACFSS exhibits a remarkable specific capacitance of 213.5 mF/cm2 and an exceptional energy density of 96.07 µWh/cm2, the highest areal capacitance and areal energy density yet reported in FSSs. Furthermore, the device possesses excellent flexibility in that its capacitance retention reaches 96.8% after bending 5,000 times, which further allows it to be woven into flexible electronic clothes with conventional weaving techniques. Therefore, the asymmetric coaxial architectural design allows new opportunities to fabricate high-performance flexible FSSs for future portable and wearable electronic devices. KEYWORDS: carbon nanotubes, fibers, vanadium nitride, coaxial, asymmetric supercapacitors


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With the rapid development of portable and wearable electronic devices, flexible supercapacitors have attracted tremendous attention for their high power density, rapid charge/discharge capability, long cycle life, lightweight, extraordinary flexibility, and outstanding safety.1-10 As the most promising energy-storage device for next-generation wearable power textiles, fiber-shaped supercapacitors (FSSs) with a tiny volume, extraordinary flexibility, and remarkable stitchability are particularly intriguing.11-27 In the past few years, considerable efforts have been made to develop high-performance FSSs with parallel and twisted structures. Unfortunately, parallel structures usually have a large volume and are not suitable for large-scale integration, and the two fiber electrodes that comprise twisted structures are easily separated by bending. FSSs with a coaxial structure have recently been successfully fabricated, providing an effective means to develop high-performance, ultra-flexible, wearable energy-storage devices.28-30 However, the low energy density of FSSs limits their practical applications. To improve their energy density, some recent studies have focused on improving their specific capacitance, but asymmetric-architecture coaxial FSSs with a larger voltage window are rarely studied.31-37 Compared with carbon-based anode nanomaterials that rely on an electrical double layer to store energy, pseudocapacitive materials are known to possess higher specific capacitance and energy density as a result of their reversible and rapid redox reactions.38, 39 Recent studies have proved that metal oxide-based anodes such as MoO3-x and iron oxide deliver much higher energy density than carbon-based materials.40-42 However, they suffer from limited power density due to their poor electrical conductivity. In this regard, vanadium nitride (VN) holds great promise as an advanced anode for asymmetric super capacitors due to its large


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specific capacitance (1,340 F/g) and superb electrical conductivity (106/Ωm).43-45 A plausible method for acquiring the high specific capacitance of VN is to enlarge its specific area, which would not only increase the electric double-layer capacitance, but also accommodate a large amount of electrolytes to participate in the Faradaic redox reactions.26, 46 Here, a facile and effective method is successfully developed to directly grow well-aligned three-dimensional VN nanowire arrays (NWAs) on carbon nanotube (CNT) fibers, forming a VN NWA/CNT fiber binder-free electrode. Such a composite structure is expected to effectively avoid contact resistance and maximize the specific surface area of VN active materials. In this study, we describe the rational design and fabrication of high-performance asymmetric coaxial FSS (ACFSS) by wrapping aligned CNT composite sheets onto VN@C NWAs CNT fibers. The aligned VN structure favored a rapid charge transport with high electrochemical activity, and the coaxial structure could make full use of the effective surface area and decrease the contact resistance between the two electrodes. As a new type of wearable energy-storage device, the as-prepared ACFSS exhibits a high specific capacitance of 213.5 mF/cm2 and an exceptional energy density of 96.07 µWh/cm2, the highest areal capacitance and areal energy density yet reported in an FSS. In addition, its capacitance retention reaches 96.8% after bending 5,000 times, which demonstrates its excellent flexibility. As expected, the ACFSS is lightweight, flexible, highly integrated, and wearable, and it could be woven into energy-storage textiles for large-scale applications with high efficiency and low cost. The fabrication process of the ACFSS is schematically illustrated in Figure 1a. First, VN NWAs with an ultrathin carbon shell were grown directly on CNT fiber. Here, the VN NWAs


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were synthesized by facile hydrothermal synthesis and post-annealing in ammonia at 600°C for 2h, and the as-prepared VN NWAs were coated with a thin carbon shell by carbonization of the thin glucose layer at high temperature. The modified fiber served as the negative electrode (inner electrode) and was coated with a thin layer of polyvinyl alcohol (PVA) gel electrolyte. A vacuum treatment was used to improve the infiltration of the electrolyte into the VN@C NWAs/CNT fiber. The aligned CNT sheets were then directly drawn from spinnable CNT arrays and simultaneously wrapped around the modified CNT fiber, followed by coating with PEDOT:PSS, and electrodeposition of MnO2 onto the PEDOT:PSS; after these procedures, the positive electrode (inner electrode) came into being. Finally, the second layer of PVA gel electrolyte was coated to obtain the desired ACFSS. Figure 1b shows a schematic of the cross-sectional structure of the ACFSS. The wrapping of the aligned CNT sheet around the modified CNT fiber is shown schematically in Figure 1c. The two ends of the modified CNT fiber were fixed on two motors, and a spinnable CNT array was fixed onto a motorized translation stage. The CNT sheet was drawn directly from the spinnable CNT array and carefully attached to the modified CNT fiber, and the two motors and motorized translation stage were run simultaneously. The thickness of the aligned CNT sheet on the modified CNT fiber was controlled by the helical angle, the width of the CNT sheet, and the number of wrapping repetitions.29, 47-50


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Figure 1.Schematic illustrations of (a) the fabrication of the ACFSS, (b) the cross-sectional structure of the ACFSS, and (c) wrapping of an aligned CNT sheet around a modified CNT fiber. A CNT fiber fabricated by twisting a CNT strip via a fast and scale process (Figure S1) was used for current collection. VN NWAs were densely grown on the CNT fiber after the hydrothermal synthesis and annealing process, as shown in Figure 2a. The high-magnification scanning electron microscopic (SEM) image in Figure 2b further reveals that the VN NWAs were well distributed and highly aligned in the hybrid fiber. It is known that the unique alignment of nanostructures can provide a large electrochemically active surface and short paths for rapid ion diffusion and electron transport so as to enhance the specific capacitance and energy density. Figure S2 compares the typical stress-strain curves of pristine CNT, VOx/CNT, and VN/CNT fibers under the same experimental conditions. It can be clearly


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observed that VN/CNT fibers have an excellent tensile strength of 370 MPa, which is of great importance

to

further

wrapping

aligned

CNT

composite

sheet electrodes.

The

energy-dispersive X-ray spectrometry (EDX) mapping results in Figure 2c clearly suggest the homogeneously distributed elements of C, N, and V, which can be attributed to the CNT fiber and VN NWAs. The core-shell structure of the VN@C NWAs is further characterized by high-resolution transmission electron microscopy (TEM). The lattice resolved TEM image (Figure 2d) reveals that the VN NWAs were covered by a continuous amorphous carbon shell, with an average thickness of 10 nm. Furthermore, the typical high-resolution TEM image (Figure 2e) discloses lattice fringes with spacings of 0.24 and 0.21 nm, which agreed well with the spacings of (111) and (200) on the VN plane. The crystal structures of the as-prepared VN NWAs on the CNT fiber are also recorded by X-ray diffraction (XRD) in Figure 2f. Note that the VN NWAs are cubic-phase crystallites, with a calculated crystallite size of about 15.9 nm. The chemical compositions and valence states of the VN NWAs were further investigated with X-ray photoelectron spectroscopy (XPS) (Figure S3).43,

51

After

incorporation of the PVA gel electrolyte, the as-obtained hybrid fiber possessed a smooth outer surface (Figure 2g). When the aligned CNT sheets were wrapped around the PVA gel electrolyte at certain helical angles, the CNTs were loosely attached on the surface and remained highly aligned (Figure 2h and inset). The aligned structure of the CNTs in the sheet originated from the vertical array (Figure S4 and S5). During the preparation of the positive electrode, a conductive PEDOT:PSS polymer layer was uniformly coated on the aligned CNT sheets (Figure 2i). Figure S6 demonstrates the XRD and Raman characterization of the PEDOT:PSS. Next, MnO2 was grown onto the PEDOT:PSS/CNT surface via an


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electrochemical deposition process to form a hierarchically porous nanostructure, as shown in Figure 2j; this structure was favorable for rapid diffusion of electrolyte ions during supercapacitor charging/discharging. XRD and Raman spectroscopy of the as-prepared MnO2 was carried out and is shown in Figure S7. Furthermore, the EDX mapping shows the uniform distribution characteristic of Mn, O, and S elements in the MnO2/PEDOT:PSS/CNT composite sheets in Figure 2k. To further elucidate the structural and chemical composition of the MnO2 sheets, XPS measurement was performed (Figure S8).12, 27 To verify the coaxial structure, the cross-section of the as-prepared ACFSS was further analyzed using SEM (Figure 2l). Two concentric cycles were clearly observed from the cross-section, corresponding

to

the

inner

VN@C

NWA/CNT

hybrid

MnO2/PEDOT:PSS/CNT composite sheet region.


fiber

and

the

outer

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Figure 2. (a-b) SEM images of VN NWAs on carbon fiber with increasing magnifications; (c) SEM image and corresponding EDX elemental mappings of C, N, and V for the VN NWAs on CNT fiber. (d) High-resolution TEM images of VN@C NWAs. (e) Higher magnification of the orange rectangle in (d). (f) XRD pattern of VN NWAs (g) SEM images of the VN@C NWAs/CNT fiber after it was coated with the PVA gel electrolyte. (h) SEM images of aligned CNT sheets covered on (g). (i) SEM images of CNT/PEDOT:PSS composite sheets. (j) SEM images of MnO2nanostructure grown on the surface of (i). (k) SEM image and corresponding


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EDX elemental mapping of Mn, O, and S for CNT/PEDOT:PSS/MnO2 composite sheets covered on (h). (l) SEM images for a cross section of the as-prepared ACFSS. The electrochemical properties of as-prepared VN@C NWAs on CNT fiber were examined in a three-electrode system using 1 M Na2SO4 aqueous solution as the electrolyte, a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. Figure S9b presents the cyclic voltammogram (CV) curves for VN@C NWAs/CNT hybrid fiber electrode in a potential window between −1 and −0.2 V at different scan rates. These CV curves display quasi-rectangular geometry as the scan rate from 10 to 100 mV/s, implying the good capacitive behavior of the hybrid VN@C NWAs/CNT hybrid fiber electrode. In comparison, it can be clearly observed in Figure S10 that the capacitance of bare CNT fiber is negligible, which suggests that the active layer of VN@C NWAs makes the main contribution to the capacitance of the hybrid fiber electrode. The galvanostatic charge/discharge curves of VN@C NWAs/CNT fiber electrodes are shown in Figure S9c within the voltage range of −0.2 to −1V. All of the curves are distorted from the ideal asymmetric triangle shape, which confirms the pseudocapacitive behavior of the hybrid fiber electrode. As shown in Figure S9d, the VN@C NWA/CNT fiber electrode delivers an impressive areal specific capacitance of 715 mF/cm2 at a current density of 1.0 mA/cm2, substantially higher than that of conventional carbon-based electrodes.23,

26, 27,52

Furthermore, the VN@C NWA/CNT fiber electrode

exhibits an excellent rate capability, with a retention rate of 68.8% when the current density increases from 1.0 mA/cm2 to 10 mA/cm2. Early researches suggested that the poor cycling stability of VN, which arose from irreversible oxidation reactions, seriously restricted its practical applications in high-performance asymmetric supercapacitors. To solve this problem,


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researchers have combined VN with more stable materials to form core-shell structures, including carbon coating, CNT encapsulation, and graphene wrapping.

44, 53-55

As shown in

Figure S11, compared with the pure VN NWAs, the VN@C NWAs endow long-term cycling stability because their carbon shell suppresses electrochemical oxidation and dissolution of the inner VN NWAs. Thus, VN is a most promising candidate electrode material for next-generation supercapacitors. To obtain a high-energy-density FSS for practical applications in portable and wearable electronics, an asymmetric electrode configuration was designed by combining two materials with different potential windows in the same electrolyte. It was necessary to find a good supercapacitor positive electrode material to match the aforementioned VN@C NWAs on the CNT fiber electrode with excellent capacitive behavior. Peng and co-workers reported a ternary hybrid positive electrode composed of MnO2nanosheets grown on a skeleton of conducting poly-coated aligned CNT sheets that exhibited excellent specific capacitance, good rate performance, and high cyclic stability.56 We thus developed a prototype ACFSS device that consisted of VN@C NWAs on CNT fiber as a core electrode and MnO2/PEDOT:PSS/CNT composite sheets as an outer electrode. To investigate the device’s total voltage, we measured the CV curves collected in a three-electrode system from the VN@C NWAs/CNT fiber negative electrode and MnO2/PEDOT:PSS/CNT positive electrode in 1 M Na2SO4 electrolyte. The operating potential windows for VN@C NWAs/CNT fiber and MnO2/PEDOT:PSS/CNT sheet hybrid electrodes were −1.0 to −0.2 V and 0 to 0.8 V (Figure 3a), respectively. Thus, it was anticipated that the operating voltage for the assembled ACFSS could reach 1.8 V. Figure 3b shows the CV profiles of the VN@C


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NWAs/CNT fiber//MnO2/PEDOT:PSS/CNT composite sheets ACFSS at a scan rate of 25 mV/s in different potential windows. As expected, the voltage of the as-fabricated ACFSS device presented rectangular-like CV curves without obvious redox peaks, even at a high potential window up to 1.8 V. Moreover, the charge-discharge curves of the ACFSS at a current density of 2 mA/cm2 were still nearly symmetric at an operating potential as high as 1.8 V (Figure 3c), suggesting that the device exhibited ideal capacitive characteristics with a rapid I–V response and low equivalent series resistance.21 Figure 3d shows the areal specific capacitances and energy density (based on the area of the entire device) of the ACFSS device based on galvanostatic charge-discharge curves collected at 2 mA/cm2 as a function of the potential window. It is worth mentioning that the calculated areal specific capacitance significantly increased from 74 to 133.8 mF/cm2 when the voltage windows were extended from 0.8 to 1.8V. Accordingly, the areal energy density of the ACFSS device was also greatly improved from 6.58 to 60.21 µWh/cm2, an enhancement of more than 815%.


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Figure 3.Electrochemical characterization of ACFSS. (a) Comparative CV curves obtained for the VN@C NWAs/CNT fiber and MnO2/PEDOT:PSS/CNT composite sheets at a scan rate of 25 mV/s in a three-electrode system. (b) CV curves measured at different operating voltages at a constant scan rate of 25 mV/s. (c) Galvanostatic charge-discharge curves of the VN@C NWA/CNT fiber//MnO2/PEDOT:PSS/CNT sheet composite device collected over different voltages from 0.8 to 1.8 V at a current density of 2 mA/cm2. (d) Areal specific capacitance and energy density calculated based on galvanostatic charge-discharge curves obtained at 2 mA/cm2 as a function of the potential window. Figure 4a shows the CV curves of the ACFSS device measured with a voltage window of 0 to 1.8 V at scan rates from 10 to 200 mV/s. All of the CV curves exhibit quasi-rectangular shapes without obvious redox peaks, even at a high scan rate of 200 mV/s, suggesting the


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ideal capacitive behavior and desirable rapid charge/discharge property of our device. To further evaluate the performance of the ACFSS device, we measured the galvanostatic charge-discharge curves at different current densities from 0.3 to 3 mA/cm2, as shown in Figure 4b. These symmetric triangular curves again confirm the excellent capacitive behavior and good reaction reversibility of the device. The areal specific capacitances of the as-prepared ACFSS device were calculated from the galvanostatic charge-discharge curves. As shown in Figure 4c, it achieved a high capacitance of 213.5mF/cm2 at a current density of 0.3 mA/cm2 and remained at 114.7 mF/cm2 at the higher current density of 3 mA/cm2, indicating a good rate capability. The volume specific capacitances and length specific capacitances are shown in Figure S12. In addition, energy and power density are important parameters for evaluating the performance of energy-storage devices. The Ragone plots in Figure 4d compare the areal performance of our ACFSS to those of previously reported high-performance fiber-shaped supercapacitors. An energy density as high as 96 µWh/cm2 can be achieved at a power density of 270 µW/cm2, and an energy density of 52 µWh/cm2 is maintained even at a high power density of 2,700 µW/cm2. These values are substantially higher than those reported for other fiber-shaped supercapacitors, such as the coaxial MWNT/carbon fiber (9.8 µWh/cm2,189.4 µW/cm2),31ultrathin MnO2 nanosheet/carbon fiber (1.428 µWh/cm2, 51.4 µW/cm2),26 MnO2-modified nanoporous gold wire with coaxial structure (5.4 µWh/cm2, 284 µW/cm2),36 MnO2/graphene yarn (8.2 µWh/cm2, 930 µW/cm2),57 all-graphene coaxial fiber (17.5 µWh/cm2, 819 µW/cm2),34 and three-dimensional graphene core-sheath fiber (0.17 µWh/cm2,100 µW/cm2).16 The ultrahigh energy density of the as-fabricated ACFSS could be ascribed to its unique structural features. Specifically, the VN


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NWAs and outer CNT sheet have large surface areas and are highly aligned. More importantly, the integrated coaxial architecture can make full use of the effective surface area and decrease the contact resistance between the two electrodes. Figure 4e further compares the electrochemical performance of the recently reported coaxial fiber-shaped supercapacitors.28, 30, 31, 33, 34, 36, 37

To the best of our knowledge, the as-prepared ACFSS represents the highest

areal capacitance and energy density in coaxial FSSs to date. Electrochemical impedance spectroscopy (EIS) was carried out over a frequency range of 10−2 to 105 Hz, as shown in Figure S13.The equivalent series resistance of our ACFSS was as low as 100.5 Ω, and the inclination of nearly 90° in the Nyquist plot indicated ideal capacitive behavior. In addition, the capacitance retention of our ACFSS could reach 90% after 6,000 cycles, demonstrating its excellent cyclic ability (Figure S14).


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Figure 4. (a) CV curves of the as-fabricated ACFSS measured at different scan rates from 0 to 1.8V. (b) Galvanostatic charge-discharge curves collected at different current densities. (c) Areal specific capacitances calculated from the charge-discharge curves as a function of the current density. (d) Ragone plots of our ACFSS in comparison with previously reported FSSs (

_

Ref.34,

_

Ref.16,

_

Ref.16,

_

Ref.31,


_

Ref.57,

_

Ref.36). (e)

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Comparison of the electrochemical performance of our ACFSS with recently reported coaxial FSSs. The capability to withstand harsh bending is an important requirement for portable and wearable energy-storage devices. Therefore, a series of mechanical flexibility tests was performed on our ACFSS. In Figure 5a, no change can be observed in the charge-discharge curves at 2 mA/cm2, when the as-prepared ACFSS device is bent at different angles from 0° to 180°, indicating that the structures of our device were maintained well and thus possessed exceptional flexibility. Meanwhile, in Figure 5b, the specific capacitance can remain at 96.8% after bending at 90° for more than 5,000 cycles, further confirming the robust mechanical property of our device. For practical applications in portable and wearable electronics, a single device can be integrated in series and in parallel to achieve higher operating voltages and output currents, respectively. As shown in Figures5c and 5d, the operating voltage and discharge times are doubled when two devices are connected in series and in parallel, respectively. In addition, the shapes of the voltage profiles were perfectly retained, indicating that the integrated devices are stable. Figure 5e shows that the as-prepared ACFSS device could be easily tied into a knot with super flexibility. To further demonstrate the potential applications for the newly developed FSSs as efficient energy-storage components for wearable devices, these ACFSS devices were woven into the flexible powering textile shown in Figure 5f to demonstrate the favorable stitchability of our ACFSS devices.


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Figure 5. (a) Galvanostatic charge-discharge curves of the as-prepared ACFSS bent at various angles at a current density of 2 mA/cm2; insets are digital photos of ACFSSs at different bending angles. (b) Normalized capacitances of the as-prepared ACFSS bent 90° for 5,000 cycles. (c) Charge-discharge curves of two ACFSSs connected in series. (d) Charge-discharge curves of two ACFSSs connected in parallel. (e) SEM image of the ACFSS tied into a knot. (f) Photograph of the as-prepared ACFSS woven into flexible textiles.


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In summary, we fabricated a novel ACFSS with a maximum operating voltage of 1.8 V by wrapping aligned carbon nanotube composite sheets around aligned VN@C NWA/CNT fibers. Due to the unique coaxial and aligned structure and superior electrode nanomaterials, the prepared ACFSS exhibited a remarkable specific capacitance of 213.5 mF/cm2 and an exceptional energy density of 96.07 µWh/cm2, the highest areal capacitance and areal energy density yet reported in an FSS. The ACFSS device retained almost invariant electrochemical performance after bending 5,000 times, which further allowed it to be woven into flexible electronic clothes with conventional weaving techniques. Therefore, this study further provided a general and effective strategy in the development of FSSs with ultrahigh energy density for next-generation wearable electronic devices. Synthesis of VN@C NWAs/CNT fiber The aligned VN NWAs on CNT fibers were fabricated using a two-step approach. First, before the experiment, CNT fibers were treated in O2 plasma for 5 min at 150 W. Typically, 2.4 g V2O5 and 5 g H2C4O4•2H2O were dissolved in 80 ml of distilled water with vigorous stirring at 80°C for 5 h to form a clear blue solution (VOC2O4). A 10 ml portion of the as-fabricated VOC2O4 solution was then added into a 50 ml beaker, followed by the addition of 2 ml H2O2 (35 wt%) under stirring. After 10 min, 30 ml of ethanol was added, and the obtained solution was stirred for another 5 min. The solution was then transferred into a 60 ml Teflon-lined stainless steel autoclave, and the CNT fibers were immersed in the solution. The autoclave was then sealed and maintained at 180 °C for 24 h. After cooling to room temperature naturally, the CNT fibers were removed and washed with ethanol several times and dried at 60 °C in a vacuum overnight. Second, the as-fabricated VOx/CNT fibers were


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directly annealed under 100 sccm NH3 and 100 sccm Ar gas flow at 600 °C for 2 h with a heating rate of 5 °C/min to obtain aligned VN NWAs/CNT fibers. Furthermore, VN NWAs were modified with one thin layer of carbon. In a typical process, the VN NWA/CNT fiber was immersed in a 0.04 M aqueous glucose solution for 10 h, followed by carbonization at 450 °C in Ar gas for 2 h. Assembling ACFSS The gel electrolyte was prepared by mixing 10 g Na2SO4 and 10 g PVA in 100 mL distilled water under vigorous stirring at 95°C for 2 h until the solution became clear. An ACFSS was fabricated by first coating the PVA gel electrolyte onto the as-fabricated aligned VN@C NWAs/CNT fiber and maintained at 60 °C overnight to evaporate the excess water in the electrolyte and to improve the infiltration of the electrolyte into aligned VN@C NWAs. After the PVA gel electrolyte was dried, the aligned CNT sheets were wrapped, and the wrapped aligned CNT sheets were arranged in 50 layers. To prepare the homogeneous PEDOT:PSS layer, a unique heat-assisted reciprocation translation stage coating setup was used. In brief, aligned CNT sheets/PVA/VN@C NWA/CNT fiber was first placed above a heat stage, and a PEDOT:PSS droplet was then attached to a capillary tube that boned onto a reciprocation translation stage. During the cycle coating process, the aligned CNT sheets/PVA/VN@C NWA/CNT fiber passed through the PEDOT:PSS droplet driven by the reciprocation translation stage, the temperature of the heat stage was set to 80 °C to 100 °C, and the speed of the reciprocation translation stage was fixed at 40 mm/min.A uniform PEDOT:PSS could be obtained after 20 cycles, after which the PEDOT:PSS layer was annealed at 120 °C for 2 h. Thereafter, MnO2 nanomaterials were grown onto the


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PEDOT:PSS/CNT composite sheets via a facile electrodeposition method in a mixed aqueous solution of 0.05 M MnSO4, 0.05 M CH3COONa, and 10% volethanol with a current density of 5 mA/cm2. In a three-electrode system, the as-fabricated PEDOT:PSS/CNT hybrid sheet, platinum wire, and saturated calomel electrode were used as the working, counter, and reference electrodes, respectively. The loading of MnO2 could be easily adjusted by changing the deposition time. Finally, the second PVA gel electrolyte was coated onto the ACFSS. ■AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected],[email protected] Notes The authors declare no competing financial interest. Acknowledgements This work was supported by National Natural Science Foundation of China (Nos. 51522211, 51372265, 51528203, and 51602339), the National Key Research and Development Program of China (No.2016YFA0203301), the Key Research Program of Frontier Science of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031),Natural Science Foundation of Shanghai (Nos.16ZR1439400 and 17ZR1447700), the Thousand Youth Talents Plan, the Postdoctoral Foundation of China(No. 2016M601905), the Natural Science Foundation of Jiangsu Province, China (Nos.BK20160399 and BK20140392), the Transformation of Scientific and Technological Achievements in Jiangsu Province (No.BA2016026), the Postdoctoral Foundationof Jiangsu Province (No.1601065B), and the Science and Technology Project of Suzhou, China (Nos. SZS201508,ZXG201428, and ZXG201401).


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Supporting Information Available: Materials; Preparation and characterization of aligned CNT sheet; Material characterizations; XPS spectra of as-prepared VN; Electrochemical performance measurements of the VN; Balance the charge of electrodes in ACFSS device; XRD and Raman spectroscopy of PEDOT:PSS; XPS, XRD and Raman spectroscopy of MnO2; Volume specific capacitances, length specific capacitances, Nyquist plot and cycling performance of ACFSS device. This materials is available free of charge via the internet at http://pubs.acs.org.

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Wrapping Aligned Carbon Nanotube Composite ... - ACS Publications

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