Skin-Inspired Surface-Microstructured Tough Hydrogel Electrolytesfor Stretchable SupercapacitorsLvye Fang,†,# Zefan Cai,†,# Zhengqing Ding,†,# Tianyi Chen,† Jiacheng Zhang,† Fubin Chen,†

Jiayan Shen,‡ Fan Chen,‡ Rui Li,§ Xuechang Zhou,‡ and Zhuang Xie*,†

†School of Materials Science and Engineering and Key Laboratory for Polymeric Composite and Functional Materials of Ministry ofEducation, Sun Yat-sen University, Guangzhou 510275, P. R. China‡College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518055, P. R. China§School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, P. R. China

*S Supporting Information

ABSTRACT: Double-network tough hydrogels have raisedincreasing interest in stretchable electronic applications as wellas electronic skin (e-skin) owing to their excellent mechanicalproperties and functionalities. While hydrogels have beenextensively explored as solid-state electrolytes, stretchableenergy storage devices based on tough hydrogel electrolytesare still limited despite their high stretchability and strength. Akey challenge remains in the robust electrode/electrolyteinterface under large mechanical strains. Inspired by the skinstructure that involves the microstructured interface for thetight connection between the dermis and epidermis, wedemonstrated that a surface-microstructured tough hydrogelelectrolyte composed of agar/polyacrylamide/LiCl (AG/PAAm/LiCl) could be exploited to allow stretchable supercapacitors with enhanced mechanical and electrochemicalperformance. The prestretched tough hydrogel electrolyte was treated to generate surface microstructures with a roughness oftens of micrometers simply via mechanical rubbing followed by the attachment of activated carbon electrodes on both sides torealize the fabrication of the stretchable supercapacitor. Through investigating the properties of the tough hydrogel electrolyteand the electrochemical performance of the as-fabricated supercapacitors under varied strains, the surface-microstructuredhydrogel electrolyte was shown to enable robust adhesion to electrodes, improving electrochemical behavior and capacitance, aswell as having better performance retention under repeated stretching cycles, which surpassed the pristine hydrogel with smoothsurfaces. Our approach could provide an alternative and general strategy to improve the interfacial properties between theelectrode and the hydrogel electrolyte, driving new directions for functional stretchable devices based on tough hydrogels.

KEYWORDS: double-network tough hydrogel, hydrogel electrolyte, microstructured surface, supercapacitor, stretchable electronics

■ INTRODUCTION

The emergence of electronic skin (e-skin) to mimic the skinwith soft and lightweight thin film electronic devices has driventhe rapid progress in intelligent human-machine interfaces,wearable medical devices, soft robotics, and bioinspiredsensing and computing systems.1−3 As one of the mostpromising materials appealing to e-skin, hydrogels, withintrinsic properties resembling to biological tissues, havebeen extensively exploited to establish ion-based soft electronicdevices for e-skin (or hydrogel ionotronics).4−6 In particular,double-network tough hydrogels have raised increasing interestto function as ideal candidates for wearable applications owingto their excellent mechanical properties and functionalities.7,8

These tough hydrogels consist of two polymer networks, eitherphysically or chemically cross-linked, leading to enhancedtensile strength and high fracture energy and toughness whilemaintaining exceedingly high stretchability over 2000% strain,

which surpass the conventional single-network hydrogelshaving a weak and brittle nature. Moreover, the inherentporous property of hydrogels to hold large amounts of aqueoussolution affords tunable ion conductivity, good biocompati-bility, mass transport capability, and so on. Therefore, toughhydrogels have been applied in a wide range of stretchabledevices including stretchable substrates,9−11 fluidic chan-nels,9,12 wearable force and temperature sensors,13−15 dis-plays,16 triboelectric generators,17 and electrodes.18−20

To date, the request for an integrated energy storage devicehas become an urgent issue for fully autonomous wearablefunctions. Among them, supercapacitors can serve as fastcharging, safe, and convenient power units for wearable

Received: February 23, 2019Accepted: May 24, 2019Published: May 24, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 21895−21903

© 2019 American Chemical Society 21895 DOI: 10.1021/acsami.9b03410ACS Appl. Mater. Interfaces 2019, 11, 21895−21903

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applications in which solid-state, gel-like polymers containingaqueous electrolytes are the key elements to realize all-solid-state devices with flexibility.21,22 Thus, in addition to thedesign of electrode layers with buckled or other stretchablearchitectures to afford the device stretchability,23−25 elastichydrogel electrolytes have attracted rising attention forstretchable supercapacitors since they can ensure efficient iontransport without electrolyte leakage while allowing highmechanical flexibility.26 For example, intrinsically stretchableand compressible supercapacitors have been fabricated basedon hydrogel networks cross-linked by SiO2 nanoparticles orpolyurethane.27−29 Hydrogel electrolytes can be also designedto realize self-healable and cold-resistant devices.30−36 Moreimportantly, double-network tough gels have introducedunique functions for energy storage applications recently, forexample, for the fabrication of super-tough, bendable, andcompressible supercapacitors with adjustable performance37−39

as well as safe energy storage devices.40,41 Nevertheless, whiletough hydrogels possess high stretchability and strength,stretchable energy storage devices based on tough hydrogelelectrolytes have yet to be fully investigated.A key challenge for the applications of tough hydrogels in

stretchable devices is the interfacial bonding between the geland a variety of materials, which relies on the chemicallyadhesive interlayers in previous efforts.10,42 Inspired by the skinstructure in which the dermis has an outer papillary layer forthe tight connection with the epidermis (Figure 1A),43,44 wehypothesized that a microstructured tough hydrogel surfacewith high surface area could provide an alternative and generalapproach to improve the interfacial properties between theelectrode and electrolyte and enable stretchable super-capacitors with enhanced mechanical stability. Herein, weexamined a tough hydrogel electrolyte made of agar/polyacrylamide (AG/PAAm) double networks involving LiClsalts for the fabrication of stretchable supercapacitors in whichthe tough hydrogel simultaneously served as both the solid-state polymer electrolyte and the stretchable supportingsubstrate. The prestretched tough hydrogel electrolyte wasroughened to generate microstructured surfaces through astraightforward rubbing method followed by direct coating ofactivated carbon (AC) electrodes on both sides to assemble aprototype monolithic and stretchable supercapacitor device(Figure 1B). By investigating the electrochemical performanceof our devices under varied strains, we demonstrated that thetough hydrogel in combination with the surface structuringstrategy could enhance the mechanical and electrochemical

performance of the stretchable supercapacitor, in addition tothe proof-of-concept demonstration of the hydrogel-basedpower source to drive LEDs.

■ EXPERIMENTAL SECTIONSynthesis and Characterization of AG/PAAm/LiCl Tough

Hydrogel Electrolytes. All the chemicals were purchased fromMacklin Inc. and used as received. The AG/PAAm/LiCl double-network hydrogel electrolyte was synthesized via a one-pot methodfollowing the literature.45 Typically, 3.6 g of acrylamide, 225 μL ofN,N′-methylene-bisacrylamide (MBA, 10 mg mL−1 aqueous sol-ution), 105 mg of photoinitiator 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959), 0.63 g of LiCl (1 mol kg−1),and 0.3 g of agar (gel strength 1000−1200 g cm−2) were mixed in 15mL of H2O and ultrasonicated for 20 min followed by degassing in avacuum chamber. The degassed mixture was then heated at 90 °Cuntil it turned into a clear solution. The homogeneous solution waspoured out into a plastic mold, covered with a plastic thin film, andcooled for 30 min to form the first agar gel network. After that, thewhole gel was illuminated by UV light (365 nm, 48 W) for 2 h tofinish the polymerization and cross-linking of the second PAAmnetwork and obtain the hydrogel electrolyte. The tensile test of the as-prepared hydrogel electrolyte was carried out using SANS CMT4204.For the tensile test, a 3.5 mm wide and 2.5 mm thick dumbbellhydrogel electrolyte sample with a gauge length of 20 mm was used,and the tensile rate was 100 mm min−1. The ionic conductivities σ(mS cm−1) of the sample under various strains were measured byelectrochemical impedance spectroscopy (EIS) using an electro-chemical workstation (CHI 660E, Shanghai Chenhua) in which thesample was sandwiched between two steel plate electrodes with a testarea of 1.77 cm2. The conductivity was calculated according to thefollowing formula

HR A

σ =· (1)

where H (cm), A (cm2), and R (Ω) are thickness, electrode area, andbulk resistance obtained by the intercept with the x axis in Nyquistplots, respectively. The morphology of the tough hydrogel wascharacterized with a stereo optical microscope (Leica DVM6).

Fabrication and Electrochemical Characterization ofStretchable Supercapacitors. A piece of hydrogel electrolyte wascut to ∼3 × 3 cm2 (thickness ∼3 mm) and prestretched to a certainstrain. The surfaces on both sides of the hydrogel were mechanicallyroughened by slightly rubbing with a piece of sandpaper or atoothbrush. Then active electrode materials (4 mg) consisting ofactivated carbon (YEC-8A, ∼10 μm particle size, Fuzhou YihuanCarbon Inc.), acetylene black, and polyvinylidene fluoride with aweight ratio of 8:1:1 were made into slurry and coated on each side ofthe hydrogel electrolyte followed by the release of the device to therelaxed state. Finally, the obtained activated charcoal electrodes with a

Figure 1. (A) Schematic illustration of the cross-sectional view of the skin structure and the skin-inspired design of a stretchable supercapacitorbased on the surface-microstructured tough hydrogel electrolyte. (B) Schematic illustration of the agar/polyacrylamide/LiCl (AG/PAAm/LiCl)tough hydrogel electrolyte for the fabrication of stretchable supercapacitor utilizing the prestretching and release approach.

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mass loading of about 1 mg cm−2 were dried in air for 30 min beforetesting of the supercapacitor performance.All the electrochemical measurements of the supercapacitors were

performed in a two-electrode system at 25 °C on the electrochemicalworkstation (CHI 660E, Shanghai Chenhua), including cyclicvoltammetry (CV), galvanostatic charge−discharge (GCD) measure-ment, and EIS. The potential range was set as 0−0.8 V for CV andGCD tests. EIS was performed in the frequency range from 105 to10−1 Hz with a root-mean-square amplitude of 5 mV at open-circuitpotential. The specific capacitance for single electrode (C, F g−1) wascalculated from CV and GCD tests according to the followingformulas

CIt

Vm2=

Δ (2)

Cm U U

i U1

( )d

U

U

max minmin

max∫ν=

− (3)

where I (A), t (s), ΔV (V), Umax (V), Umin (V), i (A), ν (V s−1), andm (g) are discharge current, discharge time, voltage change aftervoltage drop during the discharging process in the GCD curve,maximum voltage, minimum voltage, current during CV, scan rate ofthe CV curve, and mass of activated carbon on one electrode (4 mg),respectively.

■ RESULTS AND DISCUSSIONTo prepare an intrinsically stretchable and tough hydrogelelectrolyte, we selected agar (AG) as the first physically cross-linked network and polyacrylamide (PAAm) as the secondchemically cross-linked network, in combination with 1 molkg−1 LiCl salts to provide the ionic conductivity. All the AG,PAAm, and AG/PAAM double-network hydrogels have beenused as polymer electrolytes for supercapacitors before,showing good flexibility and conductivity.38,46,47 A facile one-pot synthesis approach was employed to obtain the double-network hydrogel electrolyte according to the literature,

including a heating−cooling cycle followed by UV-inducedpolymerization and cross-linking.38,45 The as-prepared toughhydrogel electrolyte was a transparent elastomeric thin filmwith superior stretchability and mechanical strength, allowinguniaxially and biaxially stretching, bending, twisting, compress-ing, and knotting (Figure 2A,B and Figure S1). As seen fromFigure 2A, a piece of hydrogel electrolyte thin film (∼2 × 2cm2 area and ∼2 mm thickness) can be manually stretched toabove 1000% strain. In addition, it can withstand themechanical deformation during the puncture utilizing acentrifuge tube without breakage (Figure 2B). The mechanicalproperties were then examined in detail from the stress−straincurve (Figure 2C) using a dumbbell-shaped AG/PAAm/LiClelectrolyte sample (3.5 mm in width and 2.5 mm in thickness).It indicated that the as-prepared tough hydrogel electrolytetypically had a fracture stress of 0.38 MPa, a fracture strain of2185%, an elastic modulus of 0.18 MPa, and a toughness of 4.4MJ m−3, in well accordance with previously reportedresults.38,45 The mechanical properties of the tough hydrogelelectrolyte could be adjusted by varying the mass ratio betweenAG and PAAm, the cross-linker content, and the watercontent, with the maximum strain varying from ∼500 to∼2300%.45To evaluate the performance of AG/PAAm/LiCl hydrogel

as a stretchable solid-state electrolyte, we then investigated itsionic conductivity (σ) along with mechanical stretching from 0to 500% strain. After stretching the sample to a certain strain,the hydrogel thin film with an initial thickness of ∼3 mm wastightly sandwiched between two steel electrodes for electro-chemical impedance spectroscopy (EIS) measurement. Utiliz-ing the bulk resistance (R) from the measured Nyquist plots,the measured film thickness (H), and the electrode area (A),the σ can be calculated by σ = H/(R × A). Different from theelectronic conductors, the resistance of the hydrogel electrolyteexhibited almost constant values with decreasing thickness

Figure 2. (A) Photos of the as-prepared AG/PAAm/LiCl tough hydrogel (∼2 mm thickness) upon stretching to ∼1000% strain. (B) Photodemonstration of the hydrogel thin film resistant to mechanical puncture. (C) Typical stress−strain curve of a 2.5 mm thick tough hydrogel sample,showing a break strain of ∼2185%. (D) Plot of ionic conductivity of the tough hydrogel electrolyte containing 1 mol kg−1 LiCl versus the appliedstrain from 0 to 500%. Statistics were obtained from three samples.

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when stretching the hydrogel thin film (Figure S2A).Therefore, the conductivity varied with the same trend asthe film thickness. As seen from Figure 2D, it was found thatthe σ decreased from 13 ± 0.8 to 6.4 ± 1.2 mS cm−1 within theinitial strain range between 0 and 300%, and then it remainedat ∼6−8 mS cm−1 independent of the following strain variationup to 500%. Since the tough hydrogel electrolyte contains alarge portion of aqueous solution inside the porous polymernetwork structures, the ion transport behavior is similar toliquid-phase conductors in which cations and anions oppositelymigrate toward the cathode and anode through the solventunder an electric field.48 The ionic conductivity is almostidentical to the LiCl solution, and the strain-inducedconductivity variation is also consistent with previous reportedresults on the PAAm hydrogel,48 possibly related to the volumeexpansion of the hydrogel networks. Such high ionicconductivity was sufficient for supercapacitor applicationsand could be further increased by using other salts. We alsotested its conductivity under repeated stretching cycles andfound that the conductivity of the tough hydrogel electrolytewas unchanged after stretching for 500 cycles (Figure S2).Based on the mechanical performance and good ionicconductivity upon stretching, the AG/PAAm/LiCl hydrogelcan be considered as both the elastic supporting substrate andthe ion-conducting polymer electrolyte with high durability indevice applications.Next, a supercapacitor can be fabricated by simply coating

AC electrodes on both sides of the AG/PAAm/LiCl hydrogelelectrolyte to form a sandwiched configuration (Figure 1B, seethe Experimental Section for details). In order to firmly attachthe electrodes to the hydrogel surface, prior to coating, weintentionally roughened the surfaces through gently rubbing

with a piece of sandpaper or a toothbrush. After rubbing, thepristine smooth surfaces of the tough hydrogel (Figure 3A)were turned into microstructured surfaces exhibiting irregularmicroscale ridges and cavities (100−150 μm in width and 10−40 μm in height), as characterized by stereo optical microscopy(Figure 3B and Figure S3), which were in a similar length scaleto the AC microparticles with a size of ∼1−10 μm.Remarkably, the surface-microstructured hydrogel electro-

lyte could afford several benefits for stretchable devices. First,for a variety of tough hydrogels, it was found that the firstformed polysaccharide network tended to produce a nonstickyand stiff polysaccharide thin shell at the outer surface (FigureS3). Thus, in order to improve the adhesion between theelectrodes and electrolyte, mechanical removal of such asurface layer was an efficient way, which introduced microscaleroughness as well as exposed the inner hydrogel componentsthat were wetter, softer, and stickier. As proof-of-conceptdemonstration, we performed tape tests to peel off the ACelectrodes coated on the microstructured and the pristinesmooth hydrogel surfaces. It was clearly shown in Figure 3Cthat most of the carbon particles remained on the micro-structured surface, while a large portion of the electrodes wasdetached from the smooth one, leaving a semitransparentsurface. Furthermore, the microstructured surface alsoincreased the surface area at the electrode/electrolyte interface,which was believed to not only allow tighter contact betweencarbon microparticles and the hydrogel but also enhance thecapacitive behavior. Figure 3D presents the cross-sectionalview of the microstructured hydrogel embedded with carbonmicroparticles, indicating the tight connections with enhancedcontact areas. To investigate the interfacial charge transport,the Nyquist plots of the AC-coated hydrogel electrolytes with

Figure 3. (A) Stereo optical microscope image of the pristine AG/PAAm/LiCl hydrogel. (B) Stereooptical microscope image of the surface-microstructured tough hydrogel electrolyte by rubbing. The bottom shows the typical cross-sectional profile from the optical image. (C) Photo ofthe tape peeling test results of two carbon-coated tough hydrogel samples with microstructured (left) and pristine (right) surfaces. The image onthe right shows a remarkable loss of carbon particles on the pristine smooth surface after tape peeling. (D) Cross-sectional optical (top) andscanning electron microscope (bottom) images of the surface-microstructured hydrogel electrolyte coated with activated carbon (AC) electrodes.(E) Nyquist plots of the pristine and surface-microstructured tough hydrogel electrolytes tested with AC electrodes on both sides. (F) Photos ofthe prestretching and release process for the fabrication of stretchable supercapacitor. (G) 3D optical topography image of the wrinkled ACelectrode surface in the as-fabricated supercapacitor.

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microstructured and pristine surfaces were compared, as shownin Figure 3E. At high frequencies, the interception of the curveat the real part represented the Ohmic resistance (Rs) thatreflects charge transport in both the electrolyte and electrodematerials, while the diameter of the semicircle-like curveindicated the charge transfer resistance (Rct) related to theinterfacial resistance between the electrode and electrolyte.38

Both Rs (2.8 Ω) and Rct (1.4 Ω) of the sample with themicrostructured surfaces were relatively smaller than thosewith the pristine ones (Rs = 3.3 Ω and Rct = 3.4 Ω), revealingbetter performance of the microstructured interface for thecharge transport between the electrolyte and electrodes.Moreover, the slopes of the lines at the mid- and lowfrequencies can reflect the ion diffusion resistance and thecapacitive characteristics. The microstructured sample alsoexhibited larger slopes than the pristine one, suggesting betterionic diffusion and a behavior closer to the ideal electric doublelayer capacitor. In addition to the interfacial contact, thesurface microstructures could also be beneficial to theimmobilization of carbon microparticles and improve thestability upon mechanical deformation owing to the micro-cavity topography as well as the interlocking effect.49,50

Importantly, we found that the carbon electrode layer on thepristine surface could be easily cracked and delaminated fromthe hydrogel surface when bending or stretching the device

(Figure S4A), while losing the film conductivity. Incomparison, the electrode layer on the roughened surface didnot show notable changes under an optical microscope evenwith a 2-fold elongation, and the film resistance of ACelectrodes remained unchanged within the initial 50% strain,indicating that the attached layer could be preserved to benearly intact for electronic conduction. In addition, the roughsurface was stable after stretching for 1000 cycles (Figure S4B),and the carbon electrodes on the microstructured hydrogelwere still attached after cycling.Knowing that the surface-microstructured tough hydrogel

electrolyte could enable the electrode/electrolyte interfacewith good mechanical and electrochemical properties, we thenfabricated monolithic and stretchable supercapacitors thataccommodate to large strains through a prestretching andrelease process. Typically, a piece of AG/PAAm/LiCl hydrogelof 3 × 3 cm2 (∼3 mm thickness) was uniaxially prestretched toa certain strain, roughened by rubbing, and then coated withthe AC electrodes on both sides (4 mg for each side, ∼2 × 2cm2 after releasing). The employed prestrain was limited to∼600% for the fabrication of stretchable supercapacitors sincethe tough hydrogel electrolyte with a thickness less than 0.5mm at higher strains could undergo rapid dehydrationaccompanied by a loss of elasticity during the fabricationprocess. Still, it can well satisfy the stretchability requirement

Figure 4. (A) Cyclic voltammetry (CV) measurements of the supercapacitor prestretched to 350% strain under various strains (ε) from 0 to 350%.The scan rate was 0.1 V s−1. (B) Galvanostatic charge−discharge (GCD) curves of the device under various strains at a constant current of 0.25 Ag−1. (C) Comparison of the capacitance obtained under various strains and between the microstructured and pristine hydrogel electrolytes at 0%strain. The errors were derived from the statistics of three devices. (D) Plots of specific capacitance with discharging current at 0 and 350% strains.The inset shows the corresponding GCD curves under a strain of 350%. (E) Nyquist plots for devices under various strains. (F) GCD curves of thedevice that was biaxially prestreched to 150% strain. The inset shows the photo of the stretched device. (G) Capacitance retention under repeatedstretching cycles for devices based on the surface-microstructured AG/PAAm electrolyte (red square) and pristine PAM electrolyte (black circle).The specific capacitance was measured at 1.25 A g−1 and 150% strain. (H) Photo of the yellow LED light powered by three supercapacitor devicesin series at the released (left) and stretched (right) states (ε = 150%).

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for practical wearable applications (>100% strain). Thefollowing release process resulted in wrinkled electrode layerswith a periodicity of ∼200−300 μm (Figure 3F,G) in therelaxed state, which could be harnessed to render the devicehighly stretchable while retaining electronic functions withinthe range of prestrain.The electrochemical performance of the as-fabricated

supercapacitor was then measured using two pieces of carboncloth as the current collectors. As shown in Figure 4A, wetested a supercapacitor fabricated at a 350% prestrain using aseries of cyclic voltammetry (CV) measurements at a scan rateof 0.1 V s−1 and varying strains of 0, 75, 150, 250, and 350%.As expected, all the CV curves under different strains presentedthe characteristic rectangular-like shapes in the potential rangeof 0−0.8 V. The CV curves had almost the same shapes thatwere difficult to distinguish, showing constant electrochemicalperformance upon stretching. Figure S5 further shows that therectangular-like shape of CV curves could be maintained atvarious scan rates even as high as 0.2 V s−1 for devices underthe relaxed state (0% strain) and the fully stretched state(350% strain), indicating the fast ion diffusion in theelectrodes. Tests on devices with 600% prestrain were alsoachieved, showing comparable capacitance between the relaxedand stretched states (Figure S5D).Next, galvanostatic charge−discharge (GCD) experiments

were carried out to investigate the energy storage performanceof the stretchable supercapacitor. First, at a current of 0.25 Ag−1, the GCD curves obtained under various strains wereexamined. Again, it demonstrated that the charge−dischargeprocess and the specific capacitance remained almost the samewithin the strain range from 0 to 350% in which thecapacitance upon stretching showed a small variation of 94−105% related to the relaxed state (Figure 4B,C). Even withslight overstretching to 400% strain, the capacitance onlydropped to ∼90% compared with the relaxed one (FigureS6A). Notably, we also compared the GCD performancesbetween devices based on the microstructured and pristinehydrogel electrolytes without stretching. The pristine hydrogelwas shown to result in a capacitance of ∼20−30% less thanthat of the microstructured one (Figure 4C and Figure S6B),suggesting that the microstructured electrode/electrolyteinterface could enhance the capacitance with the increasedcontact area at the microscale. In addition, as the currentaltered from 0.125 to 5 A g−1, the capacitance of ourstretchable supercapacitor still remained similar at the relaxedand stretched states, even though a slight decrease at highercurrent was observed for the fully stretched device at 350%strain (Figure 4D). For instance, the specific capacitance couldreach 90 F g−1 at 0.125 A g−1 for devices under both 0 and350% strains, while at 5 A g−1, it remained around 52% of thecapacitance (47 F g−1) at 0% strain and 47% (42 F g−1) at350% strain. The capacitance was similar to the reported valueusing AG/PAAm/LiCl and AC electrodes elsewhere.38 Theperformance could be improved by increasing the liquidelectrolyte penetration to the whole electrode materials, forinstance, using thinner and more porous electrode layers.To further illustrate the interfacial charge transport affected

by stretching, impedance spectra of the stretchable super-capacitor were measured with varying strains (Figure 4E). Itwas found that the Rs showed slight variations of ±0.5 Ωbetween 0 and 75% strains in different batches of devices.Within this range, there was possible stress release of thehydrogel electrolytes under the wrinkled electrodes, and the

hydrogel electrolyte might not be elongated too much.However, when the strain exceeded 75%, the Rs significantlyincreased by 1.5-fold or greater (e.g., from 2.5 to 3.5 Ω),possibly due to the strain effect on the ionic conductivity aswell as the Ohmic contact. On the other hand, the Rct and theline slopes in all Nyquist curves were similar, indicating littleadverse effect of the strain on the charge transfer, ion diffusion,and capacitive behavior. Besides, it is also worth mentioningthat the hydrogel electrolyte can be biaxially prestretched toendow the supercapacitor with the biaxially stretchability. As aproof of concept, we tested the GCD performance of thedevice with 150% prestrain in both the x and y directions,which showed comparable discharging times between thestretched and the relaxed states (Figure 4F). Overall, theseresults confirmed that the electrochemical performance of thesupercapacitor could be well maintained during stretching andreleasing based on the surface-microstructured hydrogelelectrolyte and the wrinkled electrode layers.The surface-microstructured tough hydrogel electrolyte

could also afford durable performance. First, the electro-chemical cycling stability is demonstrated in Figure S7, with∼80% capacitance retention over 6000 GCD cycles at aconstant current of 1.25 A g−1 and 150% strain (Figure S7).More importantly, the device was tested to undergo repeatedstretching cycles with 150% strain for 800 times. Wediscovered that, for the first 300 cycles, the device couldmaintain 90% capacitance, and after 800 stretching cycles, itstill preserved ∼80% capacitance (Figure 4G). In comparison,with the pristine single-network PAAm electrolyte, whichpresented adhesive surfaces similar to the microstructured AG/PAAm hydrogel, the device could only withstand cycles of 500times and was damaged thereafter, associated with a significantcapacitance deterioration to ∼60%. Clearly, it verified that thetough hydrogel electrolyte with microstructured surfaces couldimprove the mechanical stability of the stretchable super-capacitor while retaining the capacitance, regardless of theinherent adhesion of the hydrogel surface. Nevertheless, therepeated stretching cycle did lead to changes in the electrodes.As seen from the Nyquist plots of the device after differentstretching cycles (Figure S8), the slopes of the line in the low-frequency regime gradually decreased, indicating the increasein the diffusion resistance of ions to the electrode surface.Meanwhile, the long-term stability of the electrolyte could beanother major issue (Figure S2), which included the loss ofwater inside the hydrogel during either the mechanical orelectrochemical cycling process lasting for several hours toseveral days in air (humidity <50%) without encapsulation.Also, the decrease in ionic conductivity could result from theirreversible elongation of the tough hydrogel. Therefore, toimprove the device performance based on the tough hydrogelelectrolyte, it is better to improve the stability of the electrodeand electrolyte structures as well as employ encapsulation andanti-drying additives in future studies.Following the performance investigation to evaluate the

surface-microstructured tough hydrogel electrolyte for stretch-able supercapacitors, we further explored supercapacitordevices connected in series to drive an LED as the proof-of-concept demonstration of its practical application. Thin filmsof conductive carbon nanotube composites (10−30 μm inthickness) were utilized as the current collectors, which werecoated with AC electrodes prior to assembly. The electrode-coated thin films were pasted onto the prestretched toughhydrogel surfaces and formed wrinkles to enable the

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stretchable applications. The corresponding electrochemicalperformance was stable with the applied strain, similar to thatwith carbon cloth (Figure S9). The serially connected devicescould supply a voltage of 2.0 V or higher for several minutes,allowing the shining of yellow LED light at both the releasedand stretched states (Figure 4H and Figure S9). In addition,not only limited to the above demonstrated device, a similartransfer printing approach could be compatible with oursurface-microstructured hydrogel for the fabrication of hydro-gel-based microdevices. For example, a patterned micro-supercapacitor was produced by transfer printing of theprepatterned AC microelectrodes from a PDMS donorsubstrate onto the microstructured surface of the hydrogelelectrolyte (Figure S10). Such a microdevice also showed goodelectrochemical performance as well as mechanical flexibility,which would be investigated in detail by further studies.

■ CONCLUSIONS

In summary, we demonstrated a skin-inspired strategy toexploit double-network tough hydrogel electrolyte in combi-nation with the microstructured surface for the construction ofstretchable supercapacitors, which significantly improved themechanical robustness and electrochemical performance of thedevice. The high stretchability, high toughness, and high ionicconductivity of the as-prepared tough hydrogel electrolyteallowed the stretchable devices to withstand diverse mechan-ical deformations, while the microstructured electrode/electro-lyte interface was demonstrated as an essential innovation toenable the robust adhesion of electrode materials andimproved interfacial charge transport and capacitance, as wellas the performance retention under stretching. This approachcan be general and allows further development to realize novelfunctional stretchable devices. For example, tough hydrogelswith other functionalities such as self-healing, anti-freezing, orshape-adaptive properties51−53 could be explored in stretchableelectronics. The microstructured surface might be rationallydesigned with lithographic techniques to establish a 3Dinterface54,55 with tunable adhesion, surface area, or opticalproperties.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b03410.

Additional photos and optical microscope images oftough hydrogel electrolytes and carbon electrodes afterstretching, electrochemical measurements of the toughhydrogel electrolyte, stretchable supercapacitors, photosand measurements of devices in series and microdevices(PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: xiezhuang@mail.sysu.edu.cn.

ORCIDRui Li: 0000-0001-5480-8089Xuechang Zhou: 0000-0002-1558-9423Zhuang Xie: 0000-0002-2211-7141

Author Contributions#L.F., Z.C., and Z.D. contributed equally. The manuscript waswritten through contributions of all authors. All authors havegiven approval to the final version of the manuscript.

FundingSun Yat-sen University and National Natural ScienceFoundation of China (No.21802171).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work is supported by Sun Yat-sen University (“HundredTalent Plan” Grant and Lab Open Fund Project forUndergraduates) and National Natural Science Foundationof China (No. 21802171). The authors would like to thankProf. Xuelin Tian at Sun Yat-sen University for the assistancein characterization and Prof. Zijian Zheng at The Hong KongPolytechnic University for helpful discussions.

■ REFERENCES(1) Chortos, A.; Liu, J.; Bao, Z. Pursuing Prosthetic Electronic Skin.Nat. Mater. 2016, 15, 937−950.(2) Someya, T.; Bao, Z.; Malliaras, G. G. The Rise of PlasticBioelectronics. Nature 2016, 540, 379−385.(3) Choi, J.; Ghaffari, R.; Baker, L. B.; Rogers, J. A. Skin-InterfacedSystems for Sweat Collection and Analytics. Sci. Adv. 2018, 4,No. eaar3921.(4) Yang, C.; Suo, Z. Hydrogel ionotronics. Nat. Rev. Mater. 2018, 3,125−142.(5) Lee, H.-R.; Kim, C.-C.; Sun, J.-Y. Stretchable Ionics-a PromisingCandidate for Upcoming Wearable Devices. Adv. Mater. 2018, 30,1704403.(6) Yuk, H.; Lu, B.; Zhao, X. Hydrogel bioelectronics. Chem. Soc.Rev. 2019, 48, 1642−1667.(7) Sun, J.-Y.; Zhao, X.; Illeperuma, W. R. K.; Chaudhuri, O.; Oh, K.H.; Mooney, D. J.; Vlassak, J. J.; Suo, Z. Highly Stretchable andTough Hydrogels. Nature 2012, 489, 133−136.(8) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. Double-Network Hydrogels with Extremely High Mechanical Strength. Adv.Mater. 2003, 15, 1155−1158.(9) Lin, S.; Yuk, H.; Zhang, T.; Parada, G. A.; Koo, H.; Yu, C.; Zhao,X. Stretchable Hydrogel Electronics and Devices. Adv. Mater. 2016,28, 4497−4505.(10) Wirthl, D.; Pichler, R.; Drack, M.; Kettlguber, G.; Moser, R.;Gerstmayr, R.; Hartmann, F.; Bradt, E.; Kaltseis, R.; Siket, C. M.;Schausberger, S. E.; Hild, S.; Bauer, S.; Kaltenbrunner, M. InstantTough Bonding of Hydrogels for Soft Machines and Electronics. Sci.Adv. 2017, 3, No. e1700053.(11) Kim, S. H.; Jung, S.; Yoon, I. S.; Lee, C.; Oh, Y.; Hong, J.Ultrastretchable Conductor Fabricated on Skin-like Hydrogel-Elastomer Hybrid Substrates for Skin Electronics. Adv. Mater. 2018,30, 1800109.(12) Yuk, H.; Zhang, T.; Parada, G. A.; Liu, X.; Zhao, X. Skin-Inspired Hydrogel-Elastomer Hybrids with Robust Interfaces andFunctional Microstructures. Nat. Commun. 2016, 7, 12028.(13) Kim, C. C.; Lee, H. H.; Oh, K. H.; Sun, J. Y. Highly Stretchable,Transparent Ionic Touch Panel. Science 2016, 353, 682−687.(14) La, T.-G.; Li, X.; Kumar, A.; Fu, Y.; Yang, S.; Chung, H.-J.Highly Flexible, Multipixelated Thermosensitive Smart WindowsMade of Tough Hydrogels. ACS Appl. Mater. Interfaces 2017, 9,33100−33106.(15) Wu, J.; Han, S.; Yang, T.; Li, Z.; Wu, Z.; Gui, X.; Tao, K.; Miao,J.; Norford, L. K.; Liu, C.; Huo, F. Highly Stretchable andTransparent Thermistor Based on Self-Healing Double NetworkHydrogel. ACS Appl. Mater. Interfaces 2018, 10, 19097−19105.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b03410ACS Appl. Mater. Interfaces 2019, 11, 21895−21903

21901

(16) Larson, C.; Peele, B.; Li, S.; Robinson, S.; Totaro, M.; Beccai,L.; Mazzolai, B.; Shepherd, R. Highly Stretchable ElectroluminescentSkin for Optical Signaling and Tactile Sensing. Science 2016, 351,1071−1074.(17) Lee, Y.; Cha, S. H.; Kim, Y. W.; Choi, D.; Sun, J. Y. Transparentand Attachable Ionic Communicators Based on Self-CleanableTriboelectric Nanogenerators. Nat. Commun. 2018, 9, 1804.(18) Darabi, M. A.; Khosrozadeh, A.; Mbeleck, R.; Liu, Y.; Chang,Q.; Jiang, J.; Cai, J.; Wang, Q.; Luo, G.; Xing, M. Skin-InspiredMultifunctional Autonomic-Intrinsic Conductive Self-Healing Hydro-gels with Pressure Sensitivity, Stretchability, and 3D Printability. Adv.Mater. 2017, 29, 1700533.(19) Liu, Y.; Liu, J.; Chen, S.; Lei, T.; Kim, Y.; Niu, S.; Wang, H.;Wang, X.; Foudeh, A. M.; Tok, J. B. H.; Bao, Z. Soft and ElasticHydrogel-Based Microelectronics for Localized Low-Voltage Neuro-modulation. Nat. Biomed. Eng. 2019, 3, 58−68.(20) Zhao, Y.; Chen, S.; Hu, J.; Yu, J.; Feng, G.; Yang, B.; Li, C.;Zhao, N.; Zhu, C.; Xu, J. Microgel-Enhanced Double NetworkHydrogel Electrode with High Conductivity and Stability forIntrinsically Stretchable and Flexible all-Gel-State Supercapacitor.ACS Appl. Mater. Interfaces 2018, 10, 19323−19330.(21) Lu, X.; Yu, M.; Wang, G.; Tong, Y.; Li, Y. Flexible Solid-StateSupercapacitors: Design, Fabrication and Applications. EnergyEnviron. Sci. 2014, 7, 2160−2181.(22) Li, L.; Lou, Z.; Chen, D.; Jiang, K.; Han, W.; Shen, G. RecentAdvances in Flexible/Stretchable Supercapacitors for WearableElectronics. Small 2018, 14, 1702829.(23) Yu, C.; Masarapu, C.; Rong, J.; Wei, B.; Jiang, H. StretchableSupercapacitors Based on Buckled Single-Walled Carbon NanotubeMacrofilms. Adv. Mater. 2009, 21, 4793−4797.(24) Hu, L.; Pasta, M.; La Mantia, F.; Cui, L.; Jeong, S.; Deshazer,H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Stretchable, Porous, andConductive Energy Textiles. Nano Lett. 2010, 10, 708−714.(25) Yang, Z.; Deng, J.; Chen, X.; Ren, J.; Peng, H. A HighlyStretchable, Fiber-Shaped Supercapacitor. Angew. Chem., Int. Ed.2013, 52, 13453−13457.(26) Wang, Z.; Li, H.; Tang, Z.; Liu, Z.; Ruan, Z.; Ma, L.; Yang, Q.;Wang, D.; Zhi, C. Hydrogel Electrolytes for Flexible Aqueous EnergyStorage Devices. Adv. Funct. Mater. 2018, 28, 1804560.(27) Huang, Y.; Zhong, M.; Huang, Y.; Zhu, M.; Pei, Z.; Wang, Z.;Xue, Q.; Xie, X.; Zhi, C. A Self-Healable and Highly StretchableSupercapacitor Based on a Dual Crosslinked Polyelectrolyte. Nat.Commun. 2015, 6, 10310.(28) Huang, Y.; Zhong, M.; Shi, F.; Liu, X.; Tang, Z.; Wang, Y.;Huang, Y.; Hou, H.; Xie, X.; Zhi, C. An Intrinsically Stretchable andCompressible Supercapacitor Containing a Polyacrylamide HydrogelElectrolyte. Angew. Chem., Int. Ed. 2017, 56, 9141−9145.(29) Tang, Q.; Chen, M.; Wang, G.; Bao, H.; Saha, P. A FacilePrestrain-Stick-Release Assembly of Stretchable SupercapacitorsBased on Highly Stretchable and Sticky Hydrogel Electrolyte. J.Power Sources 2015, 284, 400−408.(30) Guo, Y.; Zhou, X.; Tang, Q.; Bao, H.; Wang, G.; Saha, P. ASelf-Healable and Easily Recyclable Supramolecular HydrogelElectrolyte for Flexible Supercapacitors. J. Mater. Chem. A 2016, 4,8769−8776.(31) Tao, F.; Qin, L.; Wang, Z.; Pan, Q. Self-Healable and Cold-Resistant Supercapacitor Based on a Multifunctional HydrogelElectrolyte. ACS Appl. Mater. Interfaces 2017, 9, 15541−15548.(32) Guo, Y.; Zheng, K.; Wan, P. A Flexible Stretchable HydrogelElectrolyte for Healable all-in-One Configured Supercapacitors. Small2018, 14, 1704497.(33) Shang, Y.; Wei, J.; Wu, C.; Wang, Q. Extreme Temperature-Tolerant Organohydrogel Electrolytes for Laminated Assembly ofBiaxially Stretchable Pseudocapacitors. ACS Appl. Mater. Interfaces2018, 10, 42959−42966.(34) Li, X.; Liu, L.; Wang, X.; Ok, Y. S.; Elliott, J. A. W.; Chang, S.X.; Chung, H. J. Flexible and Self-Healing Aqueous Supercapacitorsfor Low Temperature Applications: Polyampholyte Gel Electrolyteswith Biochar Electrodes. Sci. Rep. 2017, 7, 1685.

(35) Huang, Y.; Liu, J.; Wang, J.; Hu, M.; Mo, F.; Liang, G.; Zhi, C.An Intrinsically Self-Healing NiCo||Zn Rechargeable Battery with aSelf-Healable Ferric-Ion-Crosslinking Sodium Polyacrylate HydrogelElectrolyte. Angew. Chem., Int. Ed. 2018, 57, 9810−9813.(36) Li, H.; Lv, T.; Sun, H.; Qian, G.; Li, N.; Yao, Y.; Chen, T.Ultrastretchable and Superior Healable Supercapacitors Based on aDouble Cross-Linked Hydrogel Electrolyte. Nat. Commun. 2019, 10,536.(37) Liu, X.; Wu, D.; Wang, H.; Wang, Q. Self-Recovering ToughGel Electrolyte with Adjustable Supercapacitor Performance. Adv.Mater. 2014, 26, 4370−4375.(38) Lin, T.; Shi, M.; Huang, F.; Peng, J.; Bai, Q.; Li, J.; Zhai, M.One-Pot Synthesis of a Double-Network Hydrogel Electrolyte withExtraordinarily Excellent Mechanical Properties for a HighlyCompressible and Bendable Flexible Supercapacitor. ACS Appl.Mater. Interfaces 2018, 10, 29684−29693.(39) Liu, Z.; Liang, G.; Zhan, Y.; Li, H.; Wang, Z.; Ma, L.; Wang, Y.;Niu, X.; Zhi, C. A Soft Yet Device-Level Dynamically Super-ToughSupercapacitor Enabled by an Energy-Dissipative Dual-CrosslinkedHydrogel Electrolyte. Nano Energy 2019, 58, 732−742.(40) Wu, H.; Cao, Y.; Su, H.; Wang, C. Tough Gel Electrolyte UsingDouble Polymer Network Design for the Safe, Stable Cycling ofLithium Metal Anode. Angew. Chem., Int. Ed. 2018, 57, 1361−1365.(41) Duan, J.; Xie, W.; Yang, P.; Li, J.; Xue, G.; Chen, Q.; Yu, B.;Liu, R.; Zhou, J. Tough Hydrogel Diodes with Tunable InterfacialAdhesion for Safe and Durable Wearable Batteries. Nano Energy 2018,48, 569−574.(42) Yuk, H.; Zhang, T.; Lin, S.; Parada, G.; Zhao, X. ToughBonding of Hydrogels to Diverse Non-porous Surfaces. Nat. Mater.2016, 15, 190−196.(43) Liu, Y.; He, K.; Chen, G.; Leow, W. R.; Chen, X. Nature-Inspired Structural Materials for Flexible Electronic Devices. Chem.Rev. 2017, 117, 12893−12941.(44) Wang, S.; Oh, J. Y.; Xu, J.; Tran, H.; Bao, Z. Skin-InspiredElectronics: An Emerging Paradigm. Acc. Chem. Res. 2018, 51, 1033−1045.(45) Chen, Q.; Zhu, L.; Zhao, C.; Wang, Q.; Zheng, J. A Robust,One-Pot Synthesis of Highly Mechanical and Recoverable DoubleNetwork Hydrogels Using Thermoreversible Sol-Gel Polysaccharide.Adv. Mater. 2013, 25, 4171−4176.(46) Moon, W. G.; Kim, G. P.; Lee, M.; Song, H. D.; Yi, J. ABiodegradable Gel Electrolyte for Use in High-Performance FlexibleSupercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 3503−3511.(47) Li, H.; Lv, T.; Li, N.; Yao, Y.; Liu, K.; Chen, T. Ultraflexible andTailorable all-Solid-State Supercapacitors Using Polyacrylamide-BasedHydrogel Electrolyte with High Ionic Conductivity. Nanoscale 2017,9, 18474−18481.(48) Keplinger, C.; Sun, J.-Y.; Foo, C. C.; Rothemund, P.;Whitesides, G. M.; Suo, Z. Stretchable, Transparent, IonicConductors. Science 2013, 341, 984−987.(49) Liu, Z.; Wang, X.; Qi, D.; Xu, C.; Yu, J.; Liu, Y.; Jiang, Y.;Liedberg, B.; Chen, X. High-Adhesion Stretchable Electrodes Basedon Nanopile Interlocking. Adv. Mater. 2017, 29, 1603382.(50) Guo, R.; Yu, Y.; Zeng, J.; Liu, X.; Zhou, X.; Niu, L.; Gao, T.; Li,K.; Yang, Y.; Zhou, F.; Zheng, Z. Biomimicking TopographicElastomeric Petals (E-Petals) for Omnidirectional Stretchable andPrintable Electronics. Adv. Sci. 2015, 2, 1400021.(51) Fan, H.; Wang, J.; Jin, Z. Tough, Swelling-Resistant, Self-Healing, and Adhesive Dual-Cross-Linked Hydrogels Based onPolymer−Tannic Acid Multiple Hydrogen Bonds. Macromolecules2018, 51, 1696−1705.(52) Chen, F.; Zhou, D.; Wang, J.; Li, T.; Zhou, X.; Gan, T.;Handschuh-Wang, S.; Zhou, X. Rational Fabrication of Anti-Freezing,Non-Drying Tough Organohydrogels by One-Pot Solvent Displace-ment. Angew. Chem., Int. Ed. 2018, 57, 6568−6571.(53) Zhou, X.; Li, T.; Wang, J.; Chen, F.; Zhou, D.; Liu, Q.; Li, B.;Cheng, J.; Zhou, X.; Zheng, B. Mechanochemical Regulated Origamiwith Tough Hydrogels by Ion Transfer Printing. ACS Appl. Mater.Interfaces 2018, 10, 9077−9084.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b03410ACS Appl. Mater. Interfaces 2019, 11, 21895−21903

21902

(54) Xue, M.; Xie, Z.; Zhang, L.; Ma, X.; Wu, X.; Guo, Y.; Song, W.;Li, Z.; Cao, T. Microfluidic Etching for Fabrication of Flexible and all-Solid-State Micro Supercapacitor Based on MnO2 Nanoparticles.Nanoscale 2011, 3, 2703−2708.(55) Xie, Z.; Chen, C.; Zhou, X.; Gao, T.; Liu, D.; Miao, Q.; Zheng,Z. Massively Parallel Patterning of Complex 2D and 3D FunctionalPolymer Brushes by Polymer Pen Lithography. ACS Appl. Mater.Interfaces 2014, 6, 11955−11964.

ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.9b03410ACS Appl. Mater. Interfaces 2019, 11, 21895−21903

21903