All-Transparent Stretchable ElectrochromicSupercapacitor Wearable Patch DeviceTae Gwang Yun,†,⊥ Minkyu Park,†,⊥ Dong-Ha Kim,† Donghyuk Kim,† Jun Young Cheong,†

Jin Gook Bae,† Seung Min Han,*,†,‡ and Il-Doo Kim*,†,§

†Department of Materials Science and Engineering and ‡Graduate School of EEWS, Korea Advanced Institute of Science andTechnology, Daejeon 305-701, Republic of Korea§Advanced Nanosensor Research Center, KAIST Institute for Nanocentury, Daejeon 305-701, Republic of Korea

*S Supporting Information

ABSTRACT: Flexible and stretchable electrochromic super-capacitor systems are widely considered as promisingmultifunctional energy storage devices that eliminate theneed for an external power source. Nevertheless, theperformance of conventional designs deteriorates significantlyas a result of electrode/electrolyte exposure to atmosphere aswell as mechanical deformations for the case of flexiblesystems. In this study, we suggest an all-transparentstretchable electrochromic supercapacitor device with ultra-stable performance, which consists of Au/Ag core−shellnanowire-embedded polydimethylsiloxane (PDMS), bis-tacked WO3 nanotube/PEDOT:PSS, and polyacrylamide(PAAm)-based hydrogel electrolyte. Au/Ag core−shell nano-wire-embedded PDMS integrated with PAAm-based hydrogel electrolyte prevents Ag oxidation and dehydration whilemaintaining ionic and electrical conductivity at high voltage even after 16 days of exposure to ambient conditions andunder application of mechanical strains in both tensile and bending conditions. WO3 nanotube/PEDOT:PSS bistackedactive materials maintain high electrochemical−electrochromic performance even under mechanical deformations.Maximum specific capacitance of 471.0 F g−1 was obtained with a 92.9% capacity retention even after 50 000 charge−discharge cycles. In addition, high coloration efficiency of 83.9 cm2 C−1 was shown to be due to the dual coloration andpseudocapacitor characteristics of the WO3 nanotube and PEDOT:PSS thin layer.KEYWORDS: electrochromic supercapacitor, transparent stretchable, hydrogel electrolyte, dual coloration, wearable patch device

Amultifunctional wearable electrochromic device usingcathodic electrochromic materials that operates simul-taneously as electrochemical energy storage is expected

to overcome the problematic reliance on an external powersource and limited range of application to conventionalelectrochromic devices. Such a wearable electrochromicsupercapacitor device has the potential to be applied aswearable electronics for daily applications. Nevertheless, theperformance of previously reported electrochromic super-capacitor systems deteriorates in conventional designs whenthe active material becomes exposed on the surface after beingsubjected to repeated mechanical deformations and electro-chemical cycling,1−10 which highlights the need for optimiza-tion of transparent stretchable electrode and electrolytesystems. To operate wearable electrochromic supercapacitordevices under mechanical deformations and exposure toambient conditions, two factors must be considered: (1)highly reliable electrochromic−electrochemical performanceeven under mechanical deformations and application of high

external voltage and (2) electrolyte that exhibits no leakageand solvent evaporation even if repeated stretching andbending are imposed on the electrolytes in ambient conditions.First, in order to solve the deterioration of electrochromic

supercapacitor performance caused by the delamination ofactive materials as a result of mechanical deformations, variouswrapping layers such as carbon,11,12 Al2O3,

13 and Fe2O314 have

been used as flexible and stretchable current collectorscomposed of Ag nanowire-coated polyethylene terephthalate(PET), nanopaper,15−21 polydimethylsiloxane (PDMS),22−24

and polyurethane (PU).25 Although these flexible andstretchable electrodes effectively prevent the delamination ofactive materials, they are merely used as buffer layers withoutadditional contribution to the electrochemical performance.Moreover, they are unsuitable for stretching/bending-based

Received: November 9, 2018Accepted: February 19, 2019Published: February 19, 2019

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wearable applications due to their brittle characteristics. Inaddition, Ag nanowire networks have the benefit of beinghighly reliable under cyclic bendings, where the electricconductance is maintained over a large number of cycles.However, the operation lifetime is short for Ag nanowirenetworks due to the delamination of the Ag nanowire/wrapping layer from the surface of the substrate and loss ofelectric conductivity by Ag nanowire oxidation. These reasonsled to deterioration of electrochemical−electrochromic proper-ties, thus such an electrochromic supercapacitor system cannotbe applied for wearable applications. Therefore, a stretchabletransparent current collector with high resistance againstoxidation and mechanical deformation must be developed torealize a wearable multifunctional electrochromic device.Another key issue that needs to be addressed is electrolyte

evaporation and leakage as a result of mechanical deforma-tions. The supercapacitor lifetime is significantly affected in thecase of electrolyte evaporation, especially under ambientconditions where the electrolyte leakage can occur, whichthen leads to a significant decrease in the electrochemicalperformances.26 Incorporating liquid electrolytes into apolymer matrix to make gel electrolytes has been attempted;gel electrolytes, however, still possess ionic conductivity (1−50mS cm−1) inferior to that of aqueous electrolytes.26−30 Inaddition, application as wearable supercapacitors furtheremphasized the problematic leakage issue as a result ofincreased frequency of mechanical deformation. Furthermore,the loss of electrolyte due to the evaporation of solvent uponexposure to ambient conditions and the narrow operatingvoltage range due to constraints imposed by electrolytedissociation are the issues of current electrolyte systems.Under the circumstances, a new electrolyte is needed to solvethe poor ionic conductivity, evaporation, leakage, and narrowoperation voltage range problems.In this study, we have demonstrated ultrastable operation of

a wearable patch device by devising an all-transparentstretchable electrochromic supercapacitor (hereafter, all-TSES) composed of a PEDOT:PSS thin wrapping layer andpolyacrylamide (PAAm) electrolytes. WO3 nanotubes on alow-density Au/Ag core−shell nanowire-embedded PDMSsubstrate were coated with a thin PEDOT:PSS layer. ThePAAm electrolyte shows high ionic conductivity as well asstretchability of up to 80%. Thus, the incorporation of PAAmelectrolyte into the device allows significant stretchability andflexibility, unlike conventional devices that use liquid and/orgel polymer electrolyte. In addition, the PEDOT:PSS/WO3nanotube composite enhances electrochromic−electrochem-ical performances. The all-TSES wearable patch device showsexcellent electrochromic supercapacitor performance andreliability even under repeated stretching−bending motions,which substantiates its suitability for wearable applications.

RESULTS AND DISCUSSIONDesigns of an All-Transparent Stretchable Electro-

chromic Supercapacitor Device. The designs and compo-nents of all-TSES devices are schematically shown in Figure 1.To overcome the problems of conventional Ag nanowire-coated PDMS current collector, especially the conductivity lossdue to oxidation24 (0.15 V vs Pt counter electrode and Ag/AgCl reference electrode) and poor adhesion, the Au/Agcore−shell nanowire with high oxidation resistance and Agnanowire network were embedded into PDMS by a moldingprocess. The detailed fabrication process of the Au/Ag core−

shell and Ag nanowire-embedded PDMS is described in theExperimental Section and is also shown in Figure S1. The Au/Ag core−shell and Ag nanowire-embedded PDMS were fixedto a highly reliable transparent stretchable counter/workingcurrent collector that maintains the electrical conductivity evenwith repeated deformations under tensile/bending conditionsand applied positive voltage (0.9 V vs Pt counter electrode/Ag/AgCl reference electrode), as shown in Figures S2 and S3.Transparent stretchable hydrogel electrolyte was integratedwith a current collector to solve the leakage, evaporation,electrode contact, ionic conductivity, and dehydration issuesthat arise when adopting liquid electrolytes in a flexiblestretchable device. When PAAm polymer matrix, which candissolve transparently in water, becomes a hydrogel after UVcuring, it has a high resistance toward dehydration and highvoltage as it contains water molecules inside the polymerchain.31,32

The structure of the ultrastable WO3 nanotube andPEDOT:PSS thin layer composite demonstrates high electro-chromic−electrochemical performance. WO3 was used as thecathodic electrochromic material as it simultaneously under-goes electrochemical energy storage and cathodic colorationupon Li+ ion intercalation. The WO3 nanotube structure iscomposed of densely packed nanoparticles, which enhancesthe electrochemical energy and power density by enhancingthe contact efficiency with the Ag nanowires. To address thedelamination issue, conductive polymer PEDOT:PSS wasadopted as the wrapping layer. The PEDOT:PSS layerenhances the cyclic lifetime by prevention of WO3 nanotubedelamination. In addition, PEDOT:PSS undergoes cathodiccoloration, as it is a pseudocapacitor material with highcapacity and can participate in the electrochromic coloration−electrochemical energy storage in tandem with the WO3nanotubes. Both PEDOT:PSS and WO3 nanotubes undergocathodic coloration from transparent to blue, and thesynergistic electrochromic reaction stemming from dualcomposite layers further enhances the coloration efficiency ofthe PEDOT:PSS-WO3 nanotube electrode.

Fabrication of Electrospun WO3 Nanotube andPEDOT:PSS Composite Layer. The fabrication process isshown schematically in Figure 2a, where the electrospun WO3nanotube is coated with a PEDOT:PSS thin layer and drop-coated onto the Ag nanowire-embedded PDMS substrate. To

Figure 1. Schematic illustration of all transparent stretchableelectrochromic supercapacitor device. High oxidation resistance ofAu/Ag core−shell and Ag nanowire-embedded PDMS, PAAm-based transparent stretchable hydrogel electrolyte, and WO3nanotube and PEDOT:PSS wrapping layer.

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fabricate the WO3 nanotube, a cellulose nanocrystal (CNC)with the length of 100 nm and width of 20 nm was used as asacrificial template and was incorporated in electrospinningsolution containing W precursor [(NH4)6H2W12O40·xH2O]/polyvinylpyrrolidone (PVP) dissolved in DI water. Duringelectrospinning, CNC agglomerates due to the abundance ofhydroxyl groups, which results in the phase separation of theas-spun nanofiber scaffold, resulting in a CNC-rich core and Wprecursor/PVP-rich shell. After calcination at 600 °C, theagglomerated CNC core is completely decomposed, whereasthe W precursor is oxidized to form the WO3 nanotube.Detailed fabrication conditions are described in the Exper-imental Section.The synthesized WO3 nanotubes were then dispersed in

isopropyl alcohol (IPA) to make a 0.5 wt % solution and drop-casted onto the Ag nanowire-embedded PDMS. A 1.0 wt %solution of PEDOT:PSS was diluted to 0.1 wt % and drop-casted to form a thin capping layer on the WO3 nanotube-coated Ag nanowire-embedded PDMS. Surface scanningelectron microscope (SEM) and transmission electron micro-scope (TEM) images of the Ag nanowire-embedded PDMSand WO3 nanotube−PEDOT:PSS thin layer are shown inFigure 2b−f. The SEM image of the Ag nanowire-embeddedPDMS shows that the Ag nanowire network is embedded wellin the PDMS and partially exposed on the surface. This

partially exposed, low-density Ag nanowire network isbeneficial for enhanced adhesion between the Ag nanowireand PDMS, while maintaining the electric conductivity andmaking it suitable for use as a transparent stretchable currentcollector.SEM images of the drop-casted WO3 nanotube with an

average diameter of 300−400 nm and a length of 5 μm are ingood contact with the Ag nanowire network, as shown inFigure 2c. The surface morphology of the 5 μm long WO3nanotube can be seen in Figure 2d, which reveals that the WO3nanotube has a polycrystalline structure composed of nano-particles of around 30−50 nm in a nanotube structure. ThePEDOT:PSS thin film uniformly covers the WO3 nanotubesvia a simple drop-coating (Figure 2e). Cross sectional TEMimage in Figure 2f shows that the average thickness of thePEDOT:PSS is about 20 nm. Energy-dispersive spectroscopyand Raman spectroscopy analyses confirmed that the thinoverlayer coated on the WO3 nanotubes is PEDOT:PSS, asshown in Figures S4 and S5. X-ray diffraction pattern analysisrevealed that WO3 nanotubes with a monoclinic crystalstructure were synthesized after calcination (Figure 2g). AsCNC is prepared by chemical modification with sodiumhydroxide and sulfuric acid, CNC includes partial sulfatefunctional molecule groups as well as traces of sodiumimpurity. Therefore, a negligible amount of triclinic crystal

Figure 2. (a) Schematic illustration of fabrication process of WO3 nanotube and PEDOT:PSS layer. Characterization of Ag nanowire-embedded PDMS substrate and WO3 nanotube−PEDOT:PSS thin layer. Scanning electron microscope images of (b) Ag nanowire-embedded PDMS, (c) WO3 nanotube coated on Ag nanowire-embedded PDMS, (d) cross section image of WO3 nanotube, and (e)PEDOT:PSS thin layer coated on WO3 nanotube. (f) Transmission electron microscope cross section image of a PEDOT:PSS thin layercoated on WO3 nanotube. (g) X-ray diffraction results of the WO3 nanotube.

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structure was found, which originates from Na2W4O13 by theuse of the CNC with Na+. CNC residue of Na2W4O13 has alayered structure and can also contribute to the electro-chemical reaction with the Li+ ion33 but was concluded to havea negligible impact on the electrochromic−electrochemicalperformance.Electrochromic Performance of a Transparent

Stretchable Electrode. To accurately measure the trans-mittance variance due to dual electrochromic coloration effect,the change in transmittance was measured in a three-electrodesystem including the electrolyte composed of 1 M LiClO4 saltin propylene carbonate (PC) (Figure 3). As the aqueouselectrolyte can undergo dehydration at a voltage higher than1.2 V, organic electrolyte was chosen, which could maintainionic conductivity at a voltage higher than 2 V. Theelectrochromic properties were characterized by applyingpulse-type charge−discharge voltages over 20 000 cycles from−1.5 V (vs Pt counter electrode (Ag/AgCl)) colored statewhen the Li+ ions are intercalated into the crystal structure to−0.1 V (vs Pt counter electrode (Ag/AgCl)) bleached statewhen the Li+ ions are deintercalated. The change intransmittance was measured in real time and calculated viathe light power change detected by the 635 nm laser module.Four cycles of in situ transmittance change measurements ofthe WO3 particles, WO3 nanotube, and PEDOT:PSS-coatedelectrodes are shown in Figure 3a. The coloration contrast(Tb−Tc), which is the difference between the bleached state(Tb) and colored state (Tc), of the WO3 nanoparticle and theWO3 nanotube is 35.3 and 37.7%, respectively. In addition, theswitch from the colored state to the bleached state occurred

much faster in the WO3 NT electrode. The enhancement wasalso seen in the PEDOT:PSS thin layer coated WO3 NTelectrode where the WO3 NT electrode showed colorationcontrast 3.2% higher than that on the WO3 NP electrode.These results are attributed to the improved contact efficiencyof the WO3 NT with the transparent current collector. Thecoloration contrast of the WO3 nanotubes was higher than thatof WO3 nanoparticles (∼100 nm) because 1D WO3 nanotubesoffer enhanced contact with the low-density Ag nanowirecompared with 0D WO3 nanoparticles.34,35 However, theoverall transmittance of the electrode decreases after coatingthe PEDOT:PSS overlayer on the WO3 nanotubes, and thedual coloration results in the improvement of the colorationcontrast by 12.0% in comparison with the electrode withoutPEDOT:PSS coating. The enhancement is attributed to theeffect of the dual electrochromic coloration of PEDOT:PSSand WO3. Ex situ transmittance measurements over the visuallight range (400−800 nm) was conducted to compare thebleached state and colored state of the electrodes with andwithout PEDOT:PSS coating layer (Figure 3b). For wave-lengths in the range of 340−500 nm, there is a rapid increasein the transmittance due to the coloration of WO3 andPEDOT:PSS (Figure S6), and thus it is more appropriate touse wavelength of 635 nm for reliable transmittance analysis.The cyclic life enhancement of the PEDOT:PSS-coated all-

TSES was confirmed by in situ transmittance measurementover 20 000 charge−discharge cycles. The normalizedcoloration contrast (ΔT/ΔT0) decreased to 30.8% after20 000 cycles for the case of the WO3 nanotube only electrode.By contrast, the WO3 nanotube electrode with the

Figure 3. Electrochromic properties of WO3 nanoparticle and nanotube coated on Ag nanowire-embedded PDMS and PEDOT:PSS layer(PL) added WO3 nanoparticle and nanotube-coated Ag nanowire-embedded PDMS. (a) In situ transmittance measurement of bleached (Tb)and colored state (Tc) electrodes at 635 nm wavelength. (b) Transmittance measurement of bleached (Tb) and colored state (Tc) electrodesat visible light wavelength range (400−800 nm). (c) Electrochromic cycle reliability testing up to 20 000 cycles in LiClO4−PC liquidelectrolyte. (d) Evaluation of coloration efficiency by optical density change measurement with charge density. (e) Coloration contrastchange measurement of all-TSES integrated with hydrogel electrolyte.

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PEDOT:PSS overlayer maintained 60.8% of its originalcoloration contrast (Figure 3c). Therefore, the adoption ofthe PEDOT:PSS thin layer enhanced the electrochromic cyclicreliability of the all-TSES wearable patch device. To quantifythe enhancement of the coloration efficiency contributed bythe PEDOT:PSS overlayer, the change in optical density in logscale with respect to charge density was plotted for electrodeswith and without a PEDOT:PSS thin layer (Figure 3d).Coloration efficiency was obtained from the slope of the linearplots, which for the case of PEDOT:PSS layer coated all-TSESpatch electrode was 83.9 cm2 C−1, a 20.4% improvementcompared with the 66.8 cm2 C−1 of the pristine WO3 nanotubeelectrode without the PEDOT:PSS layer. The colorationefficiency of the WO3 nanotube and PEDOT:PSS thin layerelectrode increased as both WO3 and PEDOT:PSS have thesame cathodic electrochromic coloration mechanism thatchanges to a blue color. Therefore, the contrast change iscompounded that leads to the increase in coloration efficiency

and is referred to as the dual coloration mechanism. Therefore,use of the WO3 nanotube and PEDOT:PSS thin layer thathave the dual coloration mechanism is beneficial for enhancedcoloration efficiency as quantitatively compared in Figure 3d.

Electrochemical Performance of Transparent Stretch-able Electrodes. Electrochemical properties were measuredto confirm the effect of the WO3 nanotube/PEDOT:PSS thinlayer composite on the electrochemical properties. Electro-chemical properties were measured in the three-electrodesystem based on the −2.4 to −0 V (ensures the stability of thePC) with respect to the Pt counter electrode (Ag/AgCl) usingthe identical electrolyte. Charge−discharge tests were con-ducted between current densities of 1−16 A g−1, and CV testswere conducted at scan rate conditions of 0.1 and 1 V s−1 asshown in Figures S7 and S8. Figure 4a plots charge−dischargetests of different electrodes at a current density of 4 A g−1. Lesssteep discharge slope indicates a higher capacity, which isattributed to the addition of the WO3 nanotube and

Figure 4. Electrochemical properties of all-TSES electrodes. (a) Charge−discharge, (b) cyclic voltammetry test, and (c) specific capacitancenormalized by the mass for active materials. (d) Electrochemical cycle reliability testing of 50 000 cycles for an all-TSES wearable patchdevice. (e) Schematic illustration of electrochemical property enhancement by WO3 nanotubes. (f) Electrochemical performance evaluatedby the Ragone plot.

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PEDOT:PSS thin layer. IR drop does not occur in the charge−discharge curve for the electrode with the PEDOT:PSS thinlayer based on this result. The result confirms that thePEDOT:PSS thin layer improves the Coulombic efficiency byenhancing the conductivity of WO3 active materials.In addition, the CV test conducted at a scan rate of 1 V s−1

in Figure 4b shows that the enclosed area of the CV curve,which indicates the capacity, increases with the addition of theWO3 nanotube and PEDOT:PSS overlayer. Therefore, thecharge−discharge and CV tests qualitatively confirm that thecapacity increases with the use of WO3 nanotubes andPEDOT:PSS. Normalized capacitance with respect to changesin current density is plotted in Figure 4c. The maximumspecific capacitances of the WO3 nanoparticle, WO3 nanotube,and WO3 nanotube−PEDOT:PSS thin layer were 298.0, 375.4,and 471.0 F g−1, respectively. Specific capacitance increased by20.6% with the use of WO3 nanotubes and a further 20.3%increase for the WO3 nanotube−PEDOT:PSS thin layer.Figure 4e is a schematic illustration explaining the mechanismbehind the increase in specific capacitance. A low-density Agnanowire network was embedded to maximize transmittance,which leaves the majority of WO3 nanoparticles disconnectedfrom the Ag nanowire network. The disconnected WO3nanoparticles do not participate in the electrochromic andelectrochemical energy storage reaction, thereby resulting inreduced specific capacitance. By contrast, the WO3 nanotubesoffer good contact with the Ag nanowire network and thereforeresult in superior electrochromic supercapacitor performance.To examine the cyclic lifetime of the electrode using thin

layer of PEDOT:PSS, charge−discharge tests up to 50 000cycles were conducted to characterize the change incapacitance retention. Figure 4d shows the electrochemicalcyclic reliability test, which indicates that the WO3 nano-particle and nanotube electrodes without the PEDOT:PSS

wrapping layer retained 71.6 and 72.4% of its original capacity.On the other hand, the PEDOT:PSS-coated WO3 nanotubeelectrode retained 79.5% of its original capacity. ThePEDOT:PSS wrapping layer improved the cyclic reliabilityby 7.1% due to the effective prevention for delamination of theWO3 nanotubes. The energy and power densities of the all-TSES wearable patch device are compared with othertransparent energy storage systems in Figure 4f. Flexibletransparent device using MoO3

36 as the active materialexhibited power and energy densities of 0.7 kW kg−1 and22.89 Wh kg−1, respectively. Another flexible electrochromicsupercapacitor using poly[4,7-bis(3,6-dihexyloxythieno[3,2-b]-thiophen-2-yl)]-benzo[c][1,2,5]thiadiazole (PBOTT-BTD)37

demonstrated energy and power densities of 0.6−8.8 kWkg−1 and 3.5−6.3 Wh kg−1. In addition, a device using thePEDOT analogue38 as the active material reported a powerdensity of 0.8−3.3 kW kg−1 and energy density of 4.0−18.0Wh kg−1. The fabricated all-TSES wearable patch deviceexhibits superior power density of 7.7−19.1 kW kg−1 andenergy density of 44.67−52.6 Wh kg−1 due to the use of WO3nanotube−PEDOT:PSS thin film composite. It is important tonote that the CNT/MnO2 macrofilm stretchable energystorage systems,39 which is not transparent, still demonstrateslower power and energy densities compared with our all-transparent stretchable device. These results highlight that theAg nanowire-embedded PDMS-WO3 nanotube−PEDOT:PSSthin layer based all-TSES wearable patch device offer highsuitability for wearable applications requiring high electro-chemical performance.

Dehydration Properties of Transparent StretchableHydrogel Electrolytes. To confirm the enhancement andretention of ionic conductivity of transparent stretchablehydrogel electrolyte in exposure to ambient conditions,hydrogel electrolyte was exposed to the environment with a

Figure 5. (a) Impedance test results of PAAm-based hydrogel electrolyte after exposure to ambient conditions of relative humidity of 30%for 16 days. (b) Water contents and ionic conductivity change of hydrogel electrolyte after exposure in ambient conditions of relativehumidity of 30% for 16 days. (c) Charge−discharge test of stainless steel electrodes using PAAm-based hydrogel electrolyte. (d) Cyclicvoltammetry test result of Au/Ag core−shell nanowire-embedded PDMS symmetric electrode and PAAm hydrogel electrolyte.

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low relative humidity of 30% for 16 days, and the changes inionic conductivity and water content (%) were measured. Inorder to evaluate the change in ionic conductivity, animpedance test was conducted by a two-electrode systeminserting the hydrogel electrolyte between stainless steel discselectrodes, as shown in Figure 5a and Figure S9. The ionicconductivity of the hydrogel electrolyte is 265 mS cm−1

(Figure 5b) and thus has superior ionic conductivity incomparison to that of the conventional gel electrolytes used insupercapacitors such as zwitterionic poly(propylsulfonatedimethylammonium propylmethacrylamide) (PPDP) gel elec-trolyte26 (17 mS cm−1), PVDF-co-HFP gel polymer27 (9 mScm−1), and polyacrylamide cross-linked by vinyl hybrid silicananoparticles hydrogel electrolyte28 (17 mS cm−1). Inaddition, 94% of ionic conductivity (mS cm−1) was maintained(from 265 to 249 mS cm−1) and 93% of water content wasmaintained, minimizing the performance loss (Figure 5b) evenafter exposure to ambient conditions of relative humidity of30% for 16 days. As a result, PAAm-based hydrogel electrolytecan solve evaporation and leakage problems in mechanical

deformations and the high performance of the electrochromicsupercapacitor can be maintained by integration with thePAAm-based hydrogel electrolyte in ambient conditions evenafter repeated stretching−bending deformations.To demonstrate the high resistance to dehydration of the

PAAm-based transparent stretchable hydrogel electrolyte whena bias is applied, charge−discharge test was conducted by atwo-electrode system inserting the hydrogel electrolytebetween symmetric electrodes using stainless steel discs. Asshown in Figure 5c, dehydration did not occur at 5 V, and theperformance was maintained in the hydrogel electrolyte, whichconfirmed that the device would be stable under 2 V, which isthe operating voltage for a wearable electrochemical−electro-chromic reaction. Furthermore, a cyclic voltammetry test wasconducted in order to demonstrate the wider operationpotential range of Au/Ag core−shell nanowire-embeddedPDMS integrated high dehydration resistance hydrogelelectrolyte. A hydrogel electrolyte integrated Au/Ag core−shell nanowire-embedded sandwich electrode was used in atwo-electrode system. Unlike the conventional Ag nanowire

Figure 6. Demonstration of all-TSES wearable patch device. (a) Schematic illustration of an all-TSES wearable patch device. (b) Real imageof bleached and colored state of an all-TSES wearable patch device. (c) Demonstration of an all-TSES wearable patch device. (d) Normalizedcoloration contrast change and capacitance change of an all-TSES wearable patch device after exposure in ambient conditions for 14 days.(e) Cyclic voltammetry result of 20% stretch imposed on all-TSES device. (f) Normalized capacitance change of 20% stretch imposed on anall-TESE wearable patch device.

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current collector, the developed Au/Ag core-shell nanowirePDMS current collector operated stably in the range of −0.9 to0.9 V and also −1.5 to 1.5 V, which confirmed the resistance tooxidation, as confirmed in Figures S3 and 5d. Therefore, thePAAm-based hydrogel electrolyte demonstrates high resistanceto dehydration that enables long cyclic lifetime in ambientcondition even after repeated stretching−bending deforma-tions and applied external voltage.Demonstration of All-Transparent Stretchable Elec-

trochromic Supercapacitor Wearable Patch Device. Thefabricated all-TSES was further integrated into a wearablepatch device that performs as an electrochromic colorationdevice and an electrochemical energy storage device. Thearrangement of the current collectors in the all-TSES deviceconsisted of Ag nanowire-embedded PDMS working electrodeand Au/Ag core−shell nanowire-embedded PDMS counterelectrode, as shown in Figure 6a. Ag nanowire-embeddedPDMS current collector which could operate at relativelynegative voltage with respect to Pt counter electrode was usedas the working electrode. The Ag nanowire in the counterelectrode, which is susceptible to oxidation under relativepositive voltage, was also replaced with a Au/Ag core−shellnanowire-embedded in PDMS. Two electrodes, one loadedwith active materials and another embedded only with Au/Agcore−shell nanowire network, were sandwiched with thehydrogel electrolyte in the middle. The hydrogel electrolyte,which physically separates the two electrodes provides highconduction path for Li+ ion diffusion and good adhesion withthe Cu foil. Both the electrode and electrolyte are transparentand solid, hence eliminating issues associated with electrolyteevaporation/leakage as well as electrode shorting duringstretching−bending deformation.The performance of the fabricated all-TSES wearable patch

device was tested while fixed onto the wrist, as organized inFigure 6b,c. The image in Figure 6b confirms reversiblecolored (−1.5 V) into bleached state (−0.1 V) of the all-TSESwearable device. In addition, the all-TSES wearable device wasfixed onto the wrist, placed under constant stretching−bendingstate and put through electrochromic and electrochemicalreactions. Stable operation on the wrist means that the patchdevice will be stable when applied to other areas of the body asa wearable device. Figure 6c shows reversible coloration at−1.5 V and transparent state by returning to −0.1 V of thepatch device. Application on the wrist imposes 15% tensilestrain followed by bending of the device with bending radius of4 cm for 0.2 cm substrate thickness. As a result, the total strainimposed in the device on the surface of the substrate issummation of 15% tensile and 5% bending strains. The patchdevice demonstrated stable electrochromic and electrochem-ical properties when subjected to prolonged stretching−bending condition. Figure 6d shows that the patch deviceretains 95.0 and 97.6% of its normalized transmittance changeand normalized specific capacitance even after being exposedto the ambient conditions for 2 weeks. Furthermore, toevaluate the suitability of a fabricated all-TSES device forwearable application, the change in capacitance was measuredin Figure 6e,f while applying 20% total strain that showed98.6% retention of capacitance under 20% total strain.Therefore, the all-TSES wearable device suitable for wearableapplications was fabricated, which demonstrates high trans-mittance and stable operation even under stretching−bendingdeformations.

CONCLUSION

In this study, the all-TSES system that demonstrates enhancedperformance and reliability even under tensile and bendingstrains. The improved electrochromic supercapacitor perform-ance is attributed to the dual coloration and pseudocapacitivecharacteristics of the WO3 nanotube and PEDOT:PSS activematerial. The coloration efficiency improved by 20.4% to 83.9cm2 C−1 and specific capacity improved by 38.6% to 471.0 Fg−1 with the introduction of PEDOT:PSS wrapping layer onthe WO3 nanotube electrode. The improved electrochemicalcapacity also had an effect on enhanced energy and powerdensities with the fabricated device, exhibiting maximumpower density of 19.1 kW kg−1 and maximum energy densityof 52.6 Wh kg−1. In addition, the electrochromic super-capacitor cycle reliability also improved due to thePEDOT:PSS wrapping layer, resulting in 30.0 and 7.1%enhancements in electrochromic reliability and electrochemicalenergy storage reliability compared to pristine WO3 nanotubeelectrode, respectively. The fabricated system demonstratedstable performance and reversible coloration from transparentto colored state even under mechanical bending−stretchingdeformation and in atmospheric conditions. A rationallydesigned an all-TSES system consisted of 1D oxide materialcombined with conductive polymeric overlayer is highlysuitable for wearable applications.

EXPERIMENTAL SECTIONSample Preparation. Monoclinic WO3 nanoparticles purchased

from Sigma-Aldrich (<100 nm) and WO3 nanotubes synthesizedusing electrospinning method were used as electrochromic super-capacitor coupling materials. Electrospinning solution was preparedwith 0.2 g of ammonium metatungstate hydrate [(NH4)6H2W12O40·xH2O], 0.25 g of polyvinylpyrrolidone (PVP,Mw = 1300000 g mol−1),and 2 g of 5.0 wt % cellulose nanocrystal dispersed DI water. Thesolution was vigorously stirred at 500 rpm at room temperature for 8h. Then, electrospinning was carried out at a constant voltage of 17kV between the stainless steel, which served as the collector, and thesyringe needle (25 gauge) with a feeding rate of 0.1 mL min−1. Theas-spun W precursor/PVP composite nanofibers were calcined at 600°C for 1 h with a ramping rate of 5 °C min−1 in air to obtain a WO3nanotube. As sodium naturally doped CNC was utilized in this work,a minor Na2W4O13 phase was partially formed during high-temperature calcination, forming a composite nanotube withheterojunctions between Na2W4O13 and WO3. In addition, a 1.0 wt% PEDOT:PSS solution (Clevios PH 1000) was diluted to 0.1 wt %and drop-casted onto the WO3nanotube-coated Ag nanowire-embedded PDMS to coat a thin layer of PEDOT:PSS.

Fabrication of a Au/Ag Core−Shell Nanowire. PVP (1.1 g,MW = 55000 g mol−1, Sigma-Aldrich), 88 mg of L-ascorbic acid (AA,Sigma-Aldrich), and 40 mg of NaOH (Sigma-Aldrich) were dissolvedin 30 mL of DI water.40 Then, 100 μL of a 0.5 wt % Ag nanowireaqueous solution (Nanopyxis, Korea) was dispersed in PVP solution.Six milliliters of aqueous solution of HAuCl4 (0.15 Mm, Sigma-Aldrich) was slowly stirred and injected to AgNW-dispersed solutionfor 6 h. After the injection process, the Au/Ag core−shell NWs werewashed by DI water twice and collected in ethanol.

Fabrication of Au/Ag Core−Shell and Ag Nanowire-Embedded PDMS. Hydrophobic treatment was performed to Siwafer by (perfluorooctyltrichlorosilane) vapor deposition. Au/Agcore−shell and Ag nanowire were uniformly coated on hydrophobicSi wafer by air-spray. After the air-spray process, PDMS (Sylard 184,Sigma-Aldrich), with a ratio of cross-linking agent to monomer of1:10 wt %, was poured onto the Si wafer with the air-sprayed Au/Agcore−shell and Ag nanowire network. Then, the nanowire-embeddedPDMS was achieved by detachment of cured PDMS.

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Synthesis of PAAm-Based Stretchable Transparent Hydro-gel Electrolyte. The stretchable transparent hydrogel electrolyte wassynthesized using a polyacrylamide polymer matrix. Acrylamide(Sigma-Aldrich), N,N-methylenebisarylamide (Sigma-Aldrich), am-monium persulfate (Sigma-Aldrich), and (N,N,N′,N′-tetramethyle-thylenediamine) were used as polymer matrix monomer, cross-linker,photoinitiator, and cross-linking accelerator, respectively. A highconcentration of LiCl salt (Sigma-Aldrich) 8 M was dissolved in DIwater, and 2.2 M of acrylamide monomer (Sigma-Aldrich) was addedto the Li-dissolved solution. To N,N-methylenebisarylamide andammonium persulfate were added 0.0006 and 0.0017 the weight ofacrylamide monomer. The cross-linking accelerator N,N,N′,N′-tetramethylethylenediamine (0.0025 the weight of acrylamidemonomer) was dropped into the solution and cured using ultravioletlight treatment for 30 min. Lastly, the stretchable transparent hydrogelelectrolyte was achieved.Electrochromic Property Measurements. Electrochromic

properties were measured by using a three-electrode system. Thethree-electrode system was used with a Ag/AgCl reference electrode(Fisher Scientific), a platinum wire counter electrode (FisherScientific), and transparent stretchable electrode working electrode.LiClO4 salt (1 M, Sigma-Aldrich) in propylene carbonate (Sigma-Aldrich) electrolyte was used for electrochromic characterization.Pulse-type voltage was applied between −1.5 and −0.1 V (vs Ptcounter electrode (Ag/AgCl)) and change in transmittance wasmeasured in real time (WONWOO Co.). Coloration efficiency wascalculated using the following equation:

λ λ = ΔT Tlog( ( )/ ( )) optical densityb c (1)

= Δcoloration efficiency (optical density)/(charge density) (2)

Electrochemical Properties Measurement. The 1 M of LiClO4salt (Sigma-Aldrich) in propylene carbonate (Sigma-Aldrich) electro-lyte was used for electrochemical characterization. The three-electrode system was used with a voltage window of −2.4 to 0 V(vs Pt counter electrode (Ag/AgCl)). Specific capacitance (Cs) wascalculated with reference to the discharge time, and the equationsbelow were used to calculate energy density and power density.Equivalent series resistance (ESR) value was obtained from theNyquist plot and used to calculate the power density.

= Δ ΔC I V t/( / )s m (3)

= ΔE CT V12

( )2(4)

= ΔP V m( ) /(4 (ESR))2 (5)

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.8b08560.

Experimental details of sample preparation, fabricationof Au/Ag core−shell nanowire, fabrication of Au/Agcore−shell and Ag nanowire-embedded PDMS, syn-thesis of PAAm-based stretchable transparent hydrogelelectrolyte, electrochromic property measurements, andelectrochemical property measurements of transparentstretchable electrode (PDF)

AUTHOR INFORMATIONCorresponding Authors*E-mail: smhan01@kaist.ac.kr.*E-mail: idkim@kaist.ac.kr.ORCIDSeung Min Han: 0000-0001-9434-6405Il-Doo Kim: 0000-0002-9970-2218

Author Contributions⊥T.G.Y. and M.P. contributed equally to this work.

NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the Korea CCS R&D Center(KCRC) grant funded by the Korean government (Ministry ofS c i e n c e , I C T & F u t u r e P l a n n i n g ) ( N o .NRF2014M1A8A1049303), NRF (National Research Foun-dation of Korea), and Wearable Platform Materials Technol-ogy Center (WMC) (NR2016R1A5A1009926).

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