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Wire-shaped Supercapacitor with Organic Electrolyte Fabricated via Layer-by-Layer Assembly Kayeon Keum, Geumbee Lee, Hanchan Lee, Junyeong Yun, Heun Park, Soo Yeong Hong, Changhoon Song, Jung Wook Kim, and Jeong Sook Ha ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07113 • Publication Date (Web): 13 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Wire-shaped Supercapacitor with Organic Electrolyte Fabricated via Layer-by-Layer Assembly Kayeon Keum,a Geumbee Lee,b Hanchan Lee,a Junyeong Yun,a Heun Park,a Soo Yeong Hong,a Changhoon Song,a Jung Wook Kim,a and Jeong Sook Ha a.b,* a

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seoul, 02841, Republic of Korea b

KU-KIST Graduate School of Converging Science and Technology, 145 Anam-ro, Seoul, 02841, Republic of Korea

*

Corresponding author. Tel: +82-2-3290-3303. E-mail: [email protected] (Jeong Sook Ha)

Keywords: wire-shaped supercapacitor, layer-by-layer assembly, organic electrolyte, MWCNT, e-textile

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Abstract Wire-shaped supercapacitor (WSS) has structural advantages of high flexibility and ease of incorporation into conventional textile substrates. In this work, we report a thin, reproducible WSS fabricated via layer-by-layer (LbL) assembly of multi-walled carbon nanotubes (MWCNTs), combined with organic electrolyte of propylene carbonate (PC) – acetonitrile (ACN) – lithium perchlorate (LiClO4) – poly-(methyl methacrylate) (PMMA) that extends the voltage window to 1.6 V. MWCNTs were uniformly deposited on a curved surface of a thin Au wire using LbL assembly technique, resulting in linearly increased areal capacitance of the fabricated WSS. Vanadium oxide was coated on the LbL assembled MWCNTs electrode to induce pseudo-capacitance, hence enhancing overall capacitance of the fabricated WSS. Both cyclic stability of WSS and viscosity of the electrolyte could be optimized by controlling the mixing ratio of propylene carbonate (PC) to acetonitrile (ACN). As a result, the fabricated WSS exhibits areal capacitance of 5.23 mF cm-2 at 0.2 mA cm-2, energy density of 1.86 µWh cm-2, and power density of 8.5 mW cm-2, in addition to the high cyclic stability with 94% capacitance retention after 10,000 galvanostatic charge-discharge cycles. This work demonstrates a great potential of the fabricated scalable WSS in the application to high performance textile electronics as an integrated energy storage device.


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1. Introduction As wearable and portable devices are widely used nowadays, extensive studies have been conducted on developing an energy storage device that can be easily integrated onto conventional substrates and to operate active devices integrated with the storage device itself. Supercapacitors are considered to be promising energy storage devices since they can provide higher power density (> 10 kW kg-1), better cyclic stability, and faster charge-discharge rate than batteries.1-2 Thus, they can operate active devices directly, or can act as auxiliary devices to batteries in wearable devices, supplying instant power to the active device when the batteries cannot. Supercapacitors can be classified into two categories based on their charge storage mechanism: electrical double-layer capacitors (EDLC) that store charges by reversible ion adsorption and desorption on the surface of the electrode, and pseudocapacitors that accumulate charges by fast Faradaic processes, i.e., redox reaction.3 Among various forms of supercapacitors, wire-shaped supercapacitor (WSS) is receiving widespread interest due to its 1dimensional structure and advantages associated with it. It is flexible by nature while planar or stacked supercapacitors have limited flexibility depending on the electrode material.4 WSS can be easily incorporated into textiles like a thread, or can form a textile comprised of several WSSs.5-7 Various wires or fibers can be used as electrode material of WSS. Pu et al. coated reduced graphene oxide (rGO) and Nickel on commercial polyester yarn,8 and Yu et al. built a WSS based on graphene fiber.9 Asymmetric WSS based on carbon fiber with porous carbon and hydrated copper hexacyanoferrate was fabricated by Senthilkumar et al.10 Yuan et al. fabricated a PEDOT:PSS WSS through electrospinning,11 and Wang et al. designed a self-healable WSS with graphene oxide-carbon nanotubes (GO-CNT) composite fiber and carboxylated polyurethane.12



Others utilized metal wires as core material and current collector for WSSs due to their high conductivities and mechanical strengths.5, 13 If a highly conductive core such as metal wire is utilized, WSS can reduce loss of charge, and transmit electrical power and store charge simultaneously.14-15 Although considerable research has been conducted on WSS, it still has some limitations due to its shape. It is hard to deposit electrode materials onto wires, or to pattern the wire surface with conventional, scalable, industry-applicable methods such as e-beam evaporation or photolithography.4, 16 Also, fabricated WSS should have small radius to be easily integrated into conventional textile without awkwardness. In this work, we built a WSS with 100 µm-thick Au wire, coated with multi-walled carbon nanotubes (MWCNTs) and vanadium oxide (VOx). Cu or Ni wire was commonly reported as a current collector and a structural core of the supercapacitor.17-19 However, though the electrical resistivity of Cu wire is significantly low (1.7 x 10-8 Ω m), it suffers severe deterioration over positive potential window due to the oxidation of Cu in a 3-electrode system (E/V vs. Ag/AgCl).20 On the other hand, Ni wire has almost 3 times higher electrical resistivity (7.1 x 108

Ω m) than Cu or Au wire (2.3 x 10-8 Ω m),21 which limits its performance as a current collector.

Thus we selected Au wire as a current collector and as a core material to enable supercapacitor operate stably over wide voltage window and to enhance charge transfer from the electrode materials. In order to reproducibly deposit electrode materials onto the curved metal wire surface, we utilized layer-by-layer assembly (LbL) technique. LbL method enables sequential deposition of single layers of both positively charged and negatively charged MWCNTs via electrostatic interaction.22-23 By depositing two oppositely charged single MWCNT layers per cycle, it is simple to control the thickness and the amount of the electrode material deposited compared to typical dipping method.24-25 Moreover, a porous structure can be created by


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regularly stacking 1-dimensional MWCNTs onto each other, which can improve electron and ion transport to electrode surface, hence enhancing energy density and power density.26-28 We believe that we are the first group to deposit electrode material via LbL assembly onto a metal wire to build scalable and reproducible WSS. Vanadium oxide (VOx) was subsequently coated onto the surface of MWCNT-coated Au wire to enhance the overall supercapacitor performance via pseudo-capacitance. Though carbon-based materials show stable electrical double-layer capacitance (EDLC) performance and high cyclic stability, its capacitance is limited.3, 29 Use of pseudo-capacitive material can significantly increase charge-discharge time compared to EDLC, and induce redox peaks which increases the total capacitance.30 VOx was chosen as a pseudocapacitive material among numerous metal oxides due to its various oxidation state, facile synthesis methods, and ability to induce pseudo-capacitance in both amorphous and crystalline forms.31-33 Combined with MWCNT, VOx can overcome its shortcomings such as low conductivity and cyclic stability since carbon-based materials can facilitate electron transport and hinder VOx from coagulation.34-36 Furthermore, we also tried to enhance the energy density of the supercapacitor by utilizing organic electrolyte. We obtained the optimal proportion between two electrolyte solvent materials (propylene carbonate (PC) and acetonitrile (ACN)) for the supercapacitor to expand the voltage window up to 1.6 V. To easily integrate the fabricated WSS into textile, the overall thickness was controlled to be under 1 mm. As a result, we developed a WSS with high electrochemical performance of areal capacitance of 5.23 mF cm-2 at 0.2 mA cm2

, energy density of 1.86 µWh cm-2, and power density of 8.5 mW cm-2, in addition to high cyclic

stability retaining 94% of initial capacitance after 10,000 charge/discharge cycles.

2. Experimental Section



2.1 Fabrication of Au/MWCNT/VOx electrode The surface of Au wire (Alfa Aesar, diameter 100 µm) was modified with 3-aminopropyl triethoxysilane (Sigma-Aldrich) before LbL deposition of MWCNTs to enhance the surface reactivity by adding positively charged functional group. The surface-functionalized Au wire (APTES-Au wire) was dipped into MWCNT-COOH solution (1 mg/mL) for 30 minutes. After rinsing the wire with distilled (DI) water, the wire was dipped into MWCNT-NH2 solution (1 mg/mL) for another 30 minutes. After sequential dipping and rinsing, MWCNT layer was formed on the Au wire. Then vanadium oxide was coated on MWCNT-coated Au wire by chemical bath deposition method.32 400 µL of 1 M NaOH solution was added dropwise to 0.1 M of vanadium oxide sulfate (VOSO4, Alfa Aesar), and the MWCNT-coated wire was placed in the solution. The solution was heated up to 60 ˚C for 3 hours, and the wire was rinsed with DI water. 2.2 Fabrication of Organic Electrolyte with Varying Compositions The organic electrolyte used in this work consists of acetonitrile (Sigma-Aldrich), propylene carbonate (Sigma-Aldrich), lithium perchlorate (Sigma-Aldrich, battery grade), and poly (methyl methacrylate) (Sigma-Aldrich). For electrolyte with high stability, 1 g of ACN and 2 g of PC were mixed thoroughly, then 0.3 g of lithium perchlorate was added to the mixture. Then 0.233 g of poly-(methyl methacrylate) was added to enhance viscosity, and the mixture was heated up to 70 ˚C for 6 hours. To form a solid-state electrolyte, 7 g of acetonitrile, 3 g of propylene carbonate, 0.3 g of lithium perchlorate, and 0.7 g of poly (methyl methacrylate) were used during the same synthesis process. 2.3 Fabrication of the wire-shaped supercapacitor The wire-shaped supercapacitor was fabricated with two Au/MWCNT/VOx electrodes and the solid-state organic electrolyte. Each electrode was wetted with the electrolyte and was dried for


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20 minutes. Then, the two electrodes were twisted together, and were wetted once more to form a wire-shaped supercapacitor. Finally, the WSS was encapsulated with Ecoflex (Smooth-on, ecoflex 0030) by dip-coating method. 2.4 Characterization To ensure uniform deposition of MWCNT layers, absorbance and frequency change of quartz crystal was measured with UV-visible spectroscopy (UV-vis) (Shimadzu, UV-1800) and quartz crystal microbalance (QCM) (QCM 200, Stanford Research Systems), respectively. UV-visible spectra were taken at wavelengths from 900 nm to 300 nm, and the frequency output in QCM was 5 MHz. The sensitivity factor for the crystal used (Cf) was 56.6 Hz µg-1 cm2 at room temperature. The surface morphology and the cross-sectional view of the fabricated electrode were obtained by scanning electron microscopy (SEM) (Quanta 250 FEG, FEI), whereas energy dispersive X-ray spectroscopy (EDS) mapping was also conducted in the same device to assess if the electrode materials are coated uniformly. Canon EOS 7D was used to acquire optical images. The oxidation states of Vanadium were investigated via X-ray photoelectron spectroscopy (XPS) (X-TOOL, ULVAC-PHI). Electrochemical measurements including cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) were conducted using an electrochemical analyzer (Iviumsoft Technologies, CompactStat). EIS was evaluated in the frequency range from 300 kHz to 10 mHz at a potential amplitude of 10 mV.



Scheme 1 Schematic illustration of the fabricated supercapacitor. (Left: top) The wire-shaped supercapacitor (WSS) consisting of two wire electrodes and organic electrolyte whose components are presented. The inset shows the optical image of the fabricated WSS. (Left: bottom) Cross-sectional view of the layer-by-layer (LbL) assembly process of MWCNTs on 3-aminopropyl triethoxysilane (APTES) treated Au wire. (Right) Flowchart of the electrode fabrication process.

3. Results and Discussion The schematic illustration and detailed fabrication method of the WSS is shown in Scheme 1. First, the surface of Au wire is functionalized with –NH2 functional groups of 3-aminopropyl triethoxysilane (APTES). Then it is first dipped into negatively charged MWCNT-COOH solution, and then rinsed with pH-adjusted DI water to flush away residual MWCNT-COOHs not


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Figure 1 (a) UV-visible absorbance spectra of MWCNT film assembled on quartz through LbL assembly. The inset shows dependence of absorbance to the number of MWCNT bilayers deposited on quartz surface. (b) Frequency change in accordance with MWCNT layers deposited, measured using quartz crystal microbalance (QCM). (c) Areal capacitance of the electrode in a three-electrode configuration in proportion to number of MWCNT layers deposited, shown with error bar. bonded to –NH2 functional groups on the Au wire. As a result, only a single layer of MWCNTCOOH is coated onto positively functionalized surface of the Au wire by electrostatic attraction.37 Then, when the Au wire is dipped into positively charged MWCNT-NH2 solution and rinsed with DI water, a single layer of MWCNT-NH2 is layered on top of a MWCNT-COOH layer. This process is repeated until sufficient amount of MWCNT is stacked onto the Au wire surface as shown in the lower part of Scheme 1. Then, the Au wire covered with MWCNTs is dipped into VOSO4 solution for 3 h at 60 ˚C to be coated with VOx. After fabrication of the electrodes, each electrode was dipped into PC-ACN-LiClO4-PMMA electrolyte. Two electrodes were then twisted to form a WSS. The inset shows an optical image of the fabricated WSS, with total thickness of less than 1 mm. Uniform coating of the electrode material is critical to ensure enhanced utilization of the electrode surface for electrochemical reactions.38 MWCNT layers were deposited on a quartz



Figure 2 (a) Optical image of a single wire electrode. Energy dispersive X-ray Spectroscopy (EDS) mapping images were taken on three sites on a wire. (b) Respective SEM image and EDS mapping images on one of the sites. (c) Atomic percentage of vanadium and carbon on each site. (d) X-ray photoelectron spectroscopy (XPS) analysis on vanadium oxide (VOx) encapsulated on MWCNT. The XPS spectra shows the oxidation states of vanadium between 510 eV to 528 eV. (e) Cross-sectional SEM image of MWCNT-VOx coated Au wire. The inset shows a magnified view of blue dotted box. The thickness of the electrode material was measured to be 3 µm. crystal to confirm controllable deposition via LbL method by taking UV-visible absorption spectra in Figure 1(a). As oppositely charged MWCNTs were stacked alternately, the absorbance of quartz crystal increased accordingly as shown in the inset. These results demonstrate that the amount of adsorbed MWCNT per layer is uniform.24, 39 To further evaluate the controllability of the deposited MWCNT layers, quartz crystal microbalance (QCM) was employed (Figure 1(b)). The frequency of the gold-coated quartz crystal decreased in proportion


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to the number of layers deposited due to the change in mass.40 We could confirm that the mass loading of the MWCNT per bilayer was consistent since the frequency change per each bilayer was linear. The frequency shift per single bilayer in 30 minute-interval LbL deposition was about 140 Hz per bilayer. By Sauerbrey equation (∆f = - Cf · ∆m), we could calculate that the deposited amount was 2.47 µg cm-2 per each bilayer. Lastly, 3-electrode electrochemical measurement was conducted with 3 Au wires each after 1, 4, 8, 12, 16, and 20 cycles of LbL assembly of MWCNT to confirm if the layers of MWCNT can be deposited reproducibly on Au wire (Figure 1(c)). The measurement on MWCNT coated Au (AM) wires was done in 1 M LiClO4 solution with Pt wire as counter electrode and Ag/AgCl as reference electrode, respectively. The cell capacitance was evaluated from galvanostatic charge-discharge (GCD) curves shown in the inset following Eq. (1): = ∙∆ ∆

(1)

where I, ∆t, and ∆V are the the discharge current, discharge time, and discharge voltage, respectively. The surface area (S) of the electrode was calculated as S = πdl, where d is diameter of the wire electrode and l is the length of the WSS. The areal capacitance was calculated by dividing cell capacitance by surface area of the electrode. The areal capacitance of AM wire increased linearly as the number of deposition cycles increased with standard deviation less than 0.018, indicating that MWCNT layers are stacked uniformly in accordance with the number of deposited layers. After confirming the uniform coating of MWCNT layers onto Au wire, we proceeded with vanadium oxide encapsulation. An optical image of AM wire encapsulated with vanadium oxide (AMV) is shown in Figure 2(a). To verify if electrode materials sufficiently and evenly covered the surface of the entire electrode, energy dispersive X-ray spectroscopy (EDS) mapping on



Figure 3 (a) Cyclic voltammetry (CV) and (b) galvanostatic charge-discharge (GCD) curves of Au wire-MWCNT (AM) electrode and Au wire-MWCNT-VOx (AMV) electrode in 1 M LiClO4

solution with Pt wire as a counter electrode, and Ag/AgCl as a reference electrode. (c) Nyquist plot of AMV electrode. (d) b-value calculation with CV curves of MWCNT_VOx measured in 1M LiClO4. b-value was calculated by acquiring the slope of log i vs. log ν. (e) Optical image

demonstrating the viscosity of electrolyte with varying compositions of the electrolyte materials: propylene carbonate (PC) and acetonitrile (ACN) in 2:1, 1:1, and 1:2 ratios, respectively. (f) Capacitance retention (orange circles) and coulombic efficiency (blue circles) of the WSS

fabricated with AM electrodes as a function of GCD cycles, with different electrolyte compositions. various sites on MWCNT-coated Au wire with vanadium oxide (AMV) was conducted as illustrated in Figure 2(b). The coverage by vanadium was found to be uniform, showing average atomic percentage of 1.61% with standard deviation value of 0.13 throughout the wire, whereas carbon showed average atomic percentage of 82.45% with standard deviation value of 1.77


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(Figure 2(c)). EDS mapping on other sites on Au wire are presented in Figure S1. Vanadium oxide coated on the wire was found to be a mixture of vanadium (IV) oxide (VO2) and vanadium (V) pentoxide (V2O5), which was verified by X-ray photoelectron spectroscopy (XPS) shown in Figure 2(d). XPS results shows V2p3/2 peaks at 516.7 eV and 518.0 eV, and V2p1/2 peaks at 523.4 eV and 525.1 eV, respectively, which matches with previous studies.41-42 XPS survey scan of the electrode Is provided in Figure S2(a). To further analyze the deposited VOx, we conducted X-ray diffraction (XRD) on MWCNT and MWCNT_VOx as shown in Figure S2(b). However, the XRD spectrum of MWCNT_VOx exhibits similar peaks as MWCNTs, perhaps because the amount of VOx deposited on MWCNT was too small to accurately analyze its crystallinity, as shown in EDS data. Even though we are not certain if VOx grown on MWCNT is crystalline (M1) or amorphous, both forms can exhibit pseudo-capacitance to enhance the overall capacitance.31, 43-45 A cross-sectional SEM image was obtained as presented in Figure 2(e), to assure uniform deposition around the curved surface of the electrode and to measure the thickness of the deposited electrode layer. The inset image shows a magnified view of the blue dotted box, indicating the thickness to be 3 µm. Next, electrochemical performance of the fabricated electrode was measured in a 3-electrode configuration. Cyclic voltammetry (CV) curves of AM and AMV electrodes in 1 M LiClO4 solution were measured at a scan rate of 0.1 V s-1 as presented in Figure 3(a). By coating vanadium oxide, the capacitance increased significantly compared to that of AM electrode due to pseudo-capacitance. The pseudo-capacitive peaks can be observed in CV curves of AMV electrode: the oxidation peak at around 0.5 V and the reduction peak at around 0.36 V agree with the peaks caused by redox reaction of vanadium oxide as previously reported.33, 46 To compare charge-discharge behaviors of AM and AMV electrodes, GCD curves were evaluated as shown



in Figure 3(b). The capacitance of AMV electrode calculated from GCD curve was 2 times larger than that of AM electrode. Both GCD curves exhibit symmetric triangular shapes, which reflects ideal capacitive behavior, 31, 47 and the coulombic efficiency of AM and AMV electrode was 97% and 93%, respectively. The CV curves and GCD curves of AMV electrode at various scan rates and current densities are shown in Figure S3. The CV curves of AMV show semirectangular shape with redox peaks at various scan rate as mentioned above, and GCD curves also exhibit symmetric triangular shape with coulombic efficiency varying from 95 to 96%. The Nyquist plot (Figure 3(c)) shows time-dependent impedance spectra of AMV electrodes taken over a frequency range from 300 kHz to 10 mHz. Nyquist plot of AM electrode is shown in figure S4. Both graphs show steep angle which reflects ideal capacitive behavior, and the Nyquist plot of AMV electrode at higher frequencies shows an angle smaller than 90° and less steeper than that of AM electrode, confirming its pseudo-capacitive behavior.48 The capacitance of the fabricated supercapacitor may originate from two distinct mechanisms based on charge transfer kinetics, where one is capacitive effect associated with Li+ ion accumulation and adsorption on the electrode surface, and the other is diffusion-controlled process contributed by Li+ ion diffusion or intercalation into VOx matrix.49-51 These effects can be analyzed according to this equation: i = ανb (2) where i is the current at a fixed potential and ν is a sweep rate. b-value indicates whether capacitance arises due to capacitive effect (b = 1.0) or diffusion-controlled effect (b = 0.5). Whereas battery-type materials such as LiFePO4, graphite, Ni(OH)2 has b-value around 0.5, pseudo-capacitive materials such as MnO2 and RuOx show b-value around 1.52 the b-value of AMV electrode was configured using CV curves at scan rates from 10 mV s-1 to 100 mVs-1 as


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shown in Figure 3(d). the b-value was calculated to be around 1 in both cathodic (0.41 V) and anodic peak (0.35 V), indicating that the capacitance is dominated by surface-controlled reaction, both ion accumulation (EDLC) and adsorption (redox pseudo-capacitance on electrode surface), but not by diffusion-controlled process caused by Li+ intercalation.53-54 Since the amount of VOx deposited onto electrode is very small and is grown on top of MWCNT surface, it’d be tough to say the VOx matrix would grow on top of MWCNT. Thus we can say that the capacitance of the fabricated supercapacitor is attributed to surface reactions, and almost no Li+ intercalation happens in the fabricated MWCNT_VOx. After confirming the performance of the electrode, we analyzed the electrolyte. To induce intercalative pseudo-capacitance of vanadium oxide, LiClO4 was used as a conducting salt. Since the aqueous electrolyte limits the voltage window of the supercapacitor (< 1.23 V) due to decomposition of water (strictly speaking, due to oxygen evolution at positive electrode), it is useful to apply organic electrolyte or ionic liquid-based electrolyte to widen the voltage window.3, 55 Ionic liquid based-electrolyte exhibits high theoretical potential window of 3–4 V when used under vacuum, but in practice, 1) unwanted electrochemical reaction may occur due to moisture contents in air, and 2) capacitance of a supercapacitor may decrease due to parasitic reactions of impurities or functional groups on the surface of the electrode.56-57 Thus, we chose organic electrolyte to extend voltage window. Among various organic solvents, propylene carbonate (PC) and acetonitrile (ACN) are commonly used. PC exhibits high electrochemical stability but low specific capacitance, whereas ACN shows low electrochemical stability but high conductivity, leading to higher specific capacitance and power density.55 By combining these two solvents, we were able to design a suitable organic electrolyte for the supercapacitor. Yun et al. developed a solid-state electrolyte that utilizes PC and ACN as organic solvents of the



Figure 4 (a) Electrochemical measurements of AMV electrode in PC-ACN-LiClO4-PMMA electrolyte in various voltage windows. (b) CV curves and (c) GCD curves of the supercapacitor with AMV electrodes at scan rates from 0.01 V s-1 to 0.2 V s-1 and at current density from 0.2 mA cm-2 to 2 mA cm-2, respectively. The length of the fabricated supercapacitor was 3.8 cm. (d) Dependence of areal capacitance on increasing current density from 0.2 mA cm-2 to 11 mA cm-2.

(e) Capacitance retention (orange circles) and the coulombic efficiency (blue circles) of the fabricated WSS with AMV electrodes as a function of GCD cycles. (f) Ragone plot comparing the energy density and power density of the fabricated WSS to those of previous works. electrochromic supercapacitor.58 However, when we actually tested the electrolyte on AMV electrodes, the degradation of cyclic stability was too fast. We conjecture that this phenomenon is caused by gas evolution and degradation of electrolyte at the surface of the electrode over the suitable voltage window.59 To enhance the stability of the electrolyte, we altered the mixing ratio of PC to ACN. We expected that the overall cyclic stability would be enhanced if the amount of PC increase relative to that of ACN, since PC exhibits higher electrochemical stability. Hence,


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we evaluated the stability of PC-ACN-LiClO4-PMMA electrolytes with different mixing ratios of PC to ACN, 2:1, 1:1, and 1:2. Here, AM wires were used as electrodes to exclude the effect of pseudo-capacitance. Figures 3(e) shows an optical image of electrolyte, where inkjet printer ink (Epson, EP-T664) was added to make the viscosity of the electrolyte visible to bare eyes. The viscosity decreased with increasing amount of PC, hindering the formation of solid-state electrolytes. On the contrary, as proportion of ACN increased, the electrolyte instantly attached itself to the surface, not falling any droplets. Figure 3(f) presents the capacitance retention and Coulombic efficiency over 10,000 GCD cycles with different composition of electrolyte. As shown in Figure S5, the capacitance retention gradually decreased from 97% to 88% with increasing amount of ACN. Finally, we came to conclusion that the best electrochemical performance can be achieved with the mixing ratio of PC to ACN to be 2:1, while the solid state electrolyte is formed with the ratio of 1:2. With AMV electrodes and PC-ACN-LiClO4-PMMA electrolyte in optimized composition, a symmetric WSS was fabricated. To find the optimal voltage window, electrochemical performance was measured with variation of voltage windows. As shown in Figure 4(a), CV curves with voltage window over 1.6 V showed unstable performance such as asymmetric charge and discharge currents and sharp edges at the maximum voltage. The GCD curves for various voltage windows are presented in Figure S6(a). The GCD curves with voltage window under 1.6 V exhibit coulombic efficiency over 98%, whereas those with voltage window of 2.0 V and 2.4 V showed decreased coulombic efficiency of 94% and 90%, respectively. These results demonstrate that the optimal operating voltage window for the fabricated WSS is 1.6 V. Next, we compared the CV curves of WSS with AMV electrodes to those of AM electrodes, as shown in Figure S6(b). The capacitance increased significantly with vanadium oxide encapsulation,



Figure 5 (a) Cell capacitance and length capacitance of the WSSs fabricated in different lengths of 1 cm, 3 cm, 5 cm and 7 cm at current of 0.05 mA. The inset shows GCD curves of WSS with varying lengths, respectively. Capacitance retention of the fabricated WSS (b) at different bending radii and (c) under bending repetition, respectively. The inset in (c) presents the GCD curves before and after 1,000 bending cycles. and the profile showed symmetric shape indicating ideal capacitive behavior. The capacitances of these two supercapacitors were compared with GCD curves in Figure S6(c). The capacitance of the WSS with AMV electrode was 2.33 times larger than that with AM electrode, and the WSSs showed semi-triangular shape of GCD curves, indicating the ideal capacitive behavior in both electrodes. The electrochemical characteristics of the WSS fabricated with AMV electrode were further investigated at various scan rates and current densities, as presented in Figures 4(b) and 4(c). CV curves showed symmetric charging and discharging, as well as revealing pseudocapacitive peaks. GCD curves showed a quasi-triangular shape and the maximum areal capacitance was estimated to be 5.23 mF cm-2 at a current density of 0.2 mA cm-2. The areal capacitance obtained from the fabricated WSS is comparable to or superior to other WSSs based on all-graphene fiber (0.443 mF cm-2)9, wet-spun graphene-MWCNT composite fiber (2.38 mF cm-2)60, rGO coated Au wire (6.49 mF cm-2)61, and CNT and TiO2 grown on Ti wire (3.32 mF cm-2).62 The change of capacitance with current density is given in Figure 4(d). It decreased


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from 5.23 to 2.48 mF cm-2 as the current density increased from 0.2 mA cm-2 to 10 mA cm-2. The cyclic stability was also evaluated for the WSS (Figure 4(e)). The Volumetric capacitance of the fabricated WSS varies from 1.73 F cm-3 to 3.65 F cm-3 at current density of 7.47 A cm-3 to 139 mA cm-3, and the volume (V) of the WSS was calculated by the equation (V=πR2L), excluding the volume of Au wire. The WSS exhibited 94% of capacitance retention and 100% Coulombic efficiency after 10,000 GCD cycles at 0.5 mA cm-2, indicating the stable operation of the fabricated WSS over numerous charge-discharge cycles. The Nyquist plot of the fabricated WSS over a frequency range from 300 kHz to 10 mHz is presented in Figure S7(b). The equivalent series resistance (ESR) of the WSS was found to be 98 Ω, and the plot exhibited ideal capacitive behavior, similar to EDLC behavior with angle near 90°. As the energy density and power density are the key factors to evaluate the performance of the fabricated supercapacitor, we calculated the areal energy density (EA) and power density (PA) according to the following equations: = =

∙∆

∙ ∆

(3) (4)

where ∆V is the voltage difference at the end of the discharge after the IR drop and ∆t is the discharge time. The energy density of the fabricated WSS was 1.86 µWh cm-2 at a power density of 0.16 mW cm-2, and the power density was 8.5 mW cm-2 at an energy density of 0.88 µWh cm2

. The Ragone plot of the WSS is shown in Figure 4(f). These results are superior to or

comparable to WSS based on Ni wire coated with MWCNT and MnO2 (3.1 µWh cm-2, 1.9 mW cm-2)19, rGO coated Ni yarn (1.60 µWh cm-2, 2.42 mW cm-2)8, as well as graphene (0.0039 µWh cm-2, 1.75 mW cm-2)9 and carbon ink coated Ni wire (2.7 µWhcm-2, 9.07 mW cm-2)63. This finding demonstrated the high possibility of applying the fabricated WSS to various functional



Figure 6 (a) CV curves of a single WSS and two WSSs connected in series at a scan rate of 0.1 V s-1. (b) GCD curve of a single WSS and two WSSs connected in parallel at a current density of 1.0 mA cm-2. (c) Optical image of two WSSs connected in series after integration into a knitted sweater with lit green (3.0 V), red (2.1 V), and blue (3.2 V) LEDs, respectively. (d) Optical images of a green LED operated by two WSSs connected in series with time passage. The length of the fabricated WSSs shown here was 6 cm. devices. Finally, to examine if the encapsulation of the WSS with Ecoflex affects the electrochemical performance, CV and GCD curves were compared before and after the encapsulation as shown in Figure S8. There appeared no noticeable difference due to Ecoflex, confirming its role as a protective layer of the fabricated WSS. Since WSS possesses a 1-dimensional shape, it is easy to elongate the WSS and fabricate a long WSS so that its integration into existing devices and enhancing the capacitance becomes facile as shown in Figure 5(a) with error bar. The length dependent GCD curves are presented in


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the inset. The length capacitance of the WSS measured at 0.05 mA was almost constant, and the areal capacitance increased linearly in accordance with the length of the WSS with standard deviation value less than 0.089, which enables scalable elongation of the WSS. We tried to measure the mass of VOx deposited on MWCNTs, but the weight was too small to be accurately measured with conventional methods such as QCM. Though we cannot figure out the exact mass loading of VOx, we can say that the amount of VOx is uniform along the wire since the capacitance increase in the fabricated supercapacitor is linear as shown in Figure 5(a). To evaluate the flexibility of the fabricated WSS, the capacitance was measured when the supercapacitor is flat, bent with different radii, and folded (Figure 5(b)). The GCD curves of the WSS under various deformation at 1 mA cm-2 showed a constant shape and charge-discharge time, as presented in Figure S9. In addition, the capacitance retention under shape deformation varied from 97% to 104%, reasonably constant and equivalent to the original capacitance. Moreover, to ensure that the fabricated WSS can endure constant shape deformation, for instance, shape modifications occurring in practical textiles, bending repetition was performed as shown in Figure 5(c). The capacitance of the WSS retained 95 % of its original value and showed Coulombic efficiency of 93 % after 1,000 bending cycles with bending radius of 1.8 cm, confirming its durability to repeated deformation. Finally, to assess if capacitance and voltage window of the fabricated WSS can be modified to fit the daily needs, changes in capacitance and voltage window was recorded when the WSSs were connected in series and in parallel. Figures 6(a) and 6(b) show that the voltage window doubled while the capacitance is reduced to half when two WSSs are connected in series. The charge-discharge time doubled with two WSSs connected in parallel. This result demonstrates that the connected WSSs can meet the capacitance and voltage requirements on demand.



Furthermore, we successfully operated LEDs (Digi-key, USA) by WSSs connected in series that are integrated into a knitted sweater. Due to small overall thickness of the fabricated WSS (~ 1 mm), the WSSs could be weaved together with commercial threads and be easily integrated into conventional textiles. As shown in the magnified image in Figure 6(c), two WSSs were connected in series (2S) to light up variously colored LEDs. Since 2S can function up to 3.2 V, blue (3.2 V), green (3.0 V), and red (2.1 V) LEDs could be operated. Figure 6(d) shows a magnified view of the WSSs integrated into a knitted sweater and LEDs. 2S operated green LED for 52 seconds when charged up to 3.0 V in a current density of 0.05 mA, which showed possibility of applying the fabricated WSS in the e-textile industry.

4. Conclusion In summary, we designed a reproducible, industry-applicable WSS fabricated via LbL assembly, combined with organic electrolyte that extends the voltage window to 1.6 V. Au wire was utilized as a current collector for fast charge transfer and as a backbone of the flexible WSS, and MWCNT was coated onto Au wire via LbL method. Vanadium oxide was thoroughly coated to enhance the capacitance of the overall WSS by pseudo-capacitance. The fabricated WSS exhibited an areal capacitance of 5.23 mF cm-2, energy density of 1.86 µWh cm-2, and power density of 8.5 mW cm-2. Furthermore, it showed excellent capacitance retention after 10,000 GCD cycles. Our work demonstrates a great potential in the application of our scalable WSSs to the e-textile.

Associated content


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The Supporting Information is available free of charge on the ACS Publications website at DOI: EDS mapping (Figure S1) and XPS spectrum (Figure S2(a)) of MWCNT_VOx; XRD spectra of MWCNT and MWCNT_VOx (Figure S2(b)); Electrochemical properties of AMV electrode in 1 M LiClO4 solution (Figure S3); Nyquist plot of AM electrode (Figure S4); Capacitance retention with variation of mass fraction of PC in electrolyte (Figure S5); Additional electrochemical properties of WSS fabricated with two AMV electrode (Figure S6); Nyquist plot of WSSs based on AM and AMV electrodes, respectively (Figure S7); Electrochemical properties of WSS before and after ecoflex encapsulation (Figure S8); Galvanostatic chargedischarge curves of WSS at various bending radius (Figure S9).

Author information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements



This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government (MEST) (Grant No. NRF-2016R1A2A1A05004935). The authors also thank the KU-KIST graduate school program of the Korea University.


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Table of Contents Figure




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Scheme 221x178mm (150 x 150 DPI)



Figure 1 381x101mm (150 x 150 DPI)


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Figure 2 165x137mm (150 x 150 DPI)



Figure 4 365x182mm (150 x 150 DPI)


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Figure 5 386x101mm (150 x 150 DPI)



Figure 6 260x227mm (150 x 150 DPI)


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Wire-shaped Supercapacitor with Organic ... - ACS Publications

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