Highly stretchable, compressible and arbitrarily deformable all-hydrogel soft supercapacitors

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Highly stretchable, compressible and arbitrarily deformable all-hydrogel soft supercapacitors ⁎

Juan Zenga, Liubing Dongb, Wuxin Shaa, Lu Weia, , Xin Guoa,

⁎

a State Key Laboratory of Material Processing and Die & Mould Technology, Laboratory of Solid State Ionics, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China b Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

supercapacitor with arbitrarily • Soft deformable capability is constructed. electrode and electrolyte contain • Both the same dual-network hydrogel matrix.

all-in-one architecture endows the • The device with superb mechanical stabi-

•

lity. The device delivers favourable energy output under severe deformation conditions.

A R T I C LE I N FO

A B S T R A C T

Keywords: Soft supercapacitor Hydrogel Stretching Compressing Deformation

Most developed flexible energy storage devices lack sufficient softness and toughness to tolerate various deformations, such as stretching, compressing, twisting, folding and puncturing, meanwhile guarantee a stable energy output required for the soft human-machine interfaces and intelligent wearable electronics. In this work, all-hydrogel soft supercapacitors consisting of reversibly deformable hydrogel electrodes and electrolyte are constructed without using additional stretchable substrate or separator membrane. Both electrode and electrolyte contain the same polyacrylamide/sodium alginate dual-network hydrogel matrix, making them possess superb self-adhesion due to hydrophilic interaction/hydrogen bonds, and highly softness/toughness thanks to the energy-dissipative mechanism. After adding carbon nanotube conductive network/electrode active material and electrolyte salt/redox couple in the hydrogel matrix, the newly developed supercapacitor with all-in-one architecture is intrinsically highly stretchable/compressible and can even be deformed arbitrarily under various severe stress-strain deformation conditions at device level, simultaneously deliver high areal capacitance (232 mF cm−2 at 5 mV s−1 and 128 mF cm−2 at 1 mA cm−2) and maintain stable energy output. The simple device architecture, novel structural components, steady mechanical properties combined with excellent electrochemical properties make the soft supercapacitors promising for truly wearable applications.

1. Introduction Human-machine interfaces (HMIs) are developed for human

⁎

interacting with machines/robots or monitoring human health conditions. To adapt to the musculoskeletal deformations, soft HMIs based on flexible/stretchable electronics, such as soft tactile sensors in electronic

Corresponding authors at: School of Materials Science and Engineering, Luoyu Road 1037, Wuhan 430074, China. E-mail addresses: [email protected] (L. Wei), [email protected] (X. Guo).

https://doi.org/10.1016/j.cej.2019.123098 Received 27 June 2019; Received in revised form 26 September 2019; Accepted 6 October 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Juan Zeng, et al., Chemical Engineering Journal, https://doi.org/10.1016/j.cej.2019.123098

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low interface resistance; iii) neither binder nor separator is required, which simplifies the device structure; iv) the introduction of ferricyanide/ferrocyanide redox couple into the hydrogel electrolyte improves its physicochemical properties, which enables extra Faradaic capacitance from the electrolyte [35]. The above characteristics endow the soft supercapacitor not only excellent mechanical property, but also high specific capacitance, good power density/energy density and long cycle life.

skins, motion sensors, nervous sensors, electro-physiology sensors and feedback stimulators, can fulfil the requirements of the next-generation intelligent wearable electronics [1,2]. To match and power such advanced electronic devices, novel energy storage devices with the abilities to be stretched, compressed, bent, twisted and even deformed into arbitrary shapes have to be considered and developed [3]. Among them, innovative stretchable/compressible supercapacitors with high power density, fast charge-discharge rate and long cycle life have attracted great attention [4,5]. The key to the fabrication of stretchable/compressible supercapacitors lies on the design of appropriate electrodes and electrolytes. The general strategy for preparing stretchable electrodes is to deposit electrochemically active materials on stretchable substrates, such as poly(dimethylsiloxane) (PDMS) membrane [6,7], carbon nanotube (CNT) film [8,9], CNTs/PDMS film [10], elastic fibers [11,12] and fabrics [13,14]. However, the stretchability (especially compressibility) of the assembled supercapacitors is limited, severely restricted by the elastic deformation abilities of the substrates. Moreover, most of these substrates provide low conductivity or capacitances, which is not conducive to obtain high energy density or power density. Compression is also one of the important factors influencing the electrochemical performances of supercapacitors when they are subjected to external pressure in actual applications [15]. Common flexible electrodes based on polyethylene terephthalate or polyimide substrate and conventional polyvinyl alcohol (PVA)-based gel electrolytes are not compressible [16–20]. Few attempts have been dedicated to the investigation of compressible electrodes or electrolytes. Polymeric hydrogels with cross-linked polymer chains and tunable amounts of water can withstand large strains and reversible deformations [21]. They have received widespread attentions in the areas of tissue engineering [22], ionic skins [23], stretchable electroluminescent devices [24], soft robotic actuators [25] and drug delivery systems [26]. Recently, polymeric hydrogels with intrinsically stretchable and compressible properties have been studied as electrodes, such as α-cyclodextrin/ polyacrylamide (PAM)/polyaniline (PANI) hybrid hydrogel [27], PANI/graphene oxide [28] and poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS)/PAM/polypyrrole [29], or electrolytes, such as Na2SO4-anionic polyurethane acrylates/ PAM hydrogel [30], H3PO4-vinyl hybrid silica nanoparticles (VSNPs)/ polyacrylic acid hydrogel [31], H3PO4-VSNPs/PAM hydrogel [32], Li2SO4-Al-alginate/PAM [33] and poly(2-acrylamido-2-methylpropane sulfonic acid-co-N,N-dimethylacrylamide) hydrogel [34], for advanced supercapacitors. However, soft supercapacitors with both electrode and electrolyte intrinsically highly stretchable/compressible and even twistable, foldable and rollable are rarely reported up to now. Two challenges remain unsettled for current flexible energy storage devices. One is that most developed devices lack sufficient softness and toughness to tolerate various deformations, especially, severe mechanical stresses at device level; the other is that they can hardly guarantee a stable energy output when suffering severe and repeated deformations. In this work, all-hydrogel soft supercapacitors with arbitrary deformation capability and delivering steady energy output at the same time are developed for the first time. The implementation of the deformation function is based on a PAM/sodium alginate (SA) dual-network hydrogel matrix. To obtain the hydrogel electrode, CNTs and PEDOT:PSS are introduced in the PAM/SA matrix to increase the electronic conductivity and electrochemical activity; to prepare the hydrogel electrolyte, Na2SO4 electrolyte salt and potassium ferricyanide/potassium ferrocyanide [K3Fe(CN)6/K4Fe(CN)6] redox-couple are added in the PAM/SA matrix to enhance the ionic conductivity and redox-activity. The all-hydrogel supercapacitor presents the following advantages: i) no extra elastic substrate is required, and both electrode and electrolyte are intrinsically stretchable and reversibly compressible to tolerate various deformations; ii) by using the same hydrogel as matrix, strong self-adhesion between the electrode and electrolyte is achieved due to hydrophilic interaction and hydrogen bonds, ensuring

2. Experimental 2.1. Materials Acrylamide monomer (AM, 98 wt%), sodium alginate (SA, 180–220 mPa·s in viscosity), N,N′-methylenebisacrylamide (MBAA, 98%), N,N,N′,N′-tetramethylethylenediamine (TEMED, Mw = 116.2), ammonium persulfate (APS, Mw = 228.2, 98 wt%), Na2SO4 (99 wt%), K3Fe(CN)6 (99.5 wt%) and K4Fe(CN)6 (99.5 wt%) were all purchased from ALADDIN Chemical Co., Ltd. CNT aqueous suspension (Model: NTP2021) was obtained from Shenzhen Nanotech Port Co., Ltd. The CNTs possess length of 5 to 15 μm, diameter of 15 to 25 nm and specific surface area of 150–210 m2 g−1. PEDOT:PSS aqueous suspension (Clevios PH1000, 15–50 mPa·s in viscosity) was purchased from Hereaus. 2.2. Preparation of hydrogel electrode AM (2.1 g) was dissolved in diluent CNT aqueous suspension (1.0 wt %, 21 mL), and SA powder (0.263 g) was slowly added into the mixed solution under magnetic stirring for 8 h to obtain a homogeneous mixture. Then, PEDOT:PSS suspension was added into the mixture in a volume ratio of 1:1 and stirred evenly. Next, cross-linker MBAA (0.01 wt%), accelerator TEMED (0.09 wt%) and initiator APS (0.1 wt%) were added into the blended solution successively and mixed uniformly. The obtained mixture was poured into a glass mold and sealed for crosslinking/polymerization of AM up to 12 h. To promote the further polymerization of PEDOT:PSS, the hydrogel with sealing was then placed in an oven at 120 °C for 30 min. 2.3. Preparation of hydrogel electrolyte AM (4.2 g) and SA (0.525 g) powders were dissolved in deionized water (21 mL) with fast stirring for 6 h to obtain a uniform solution. Then, MBAA (0.01 wt%), TEMED (0.09 wt%) and APS (0.1 wt%) were added into the solution successively and mixed uniformly. After degassing by ultrasonic treatment and vacuum, the obtained solution was poured into a glass mold with sealing and placed in an oven at 60 °C overnight for polymerization. Afterwards, the derived polymer gel was soaked in 0.5 M Na2SO4 aqueous solution containing 0.05 M [K3Fe (CN)6]/[K4Fe(CN)6] for 24 h in dark. To explore the influence of alginate incorporation, PAM hydrogel was prepared likewise but without adding SA. 2.4. Device assembly The hydrogel electrode and electrolyte were cut into rectangular slices with size of 9 × 9 × 0.45 mm3 and 20 × 15 × 0.6 mm3, respectively. Two pieces of identical electrode were separated by a piece of electrolyte, and copper foil (0.01 mm in thickness) was attached to each electrode as current collector. During the tensile test, we pre-stretched the integrated electrodes/electrolyte, fixed them and attached copper foils on the surfaces of the two electrodes. Due to the strong adhesion on the hydrogel electrode surface, the copper foil can be tightly combined with the electrode after applying pressure. As for compression and other deformation tests, copper foils were pressed on electrodes before deformations. The device was encapsulated with PDMS film 2

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Fig. 1. Graphical illustration of the composition and preparation process for all-hydrogel soft supercapacitors.

(200 μm in thickness).

Both the electrode and electrolyte hydrogels can be deformed into desired shapes and sizes by using different 3D molds, such as cylinder, floriform, triangle, heart-shape and star (Fig. 2a and c). The hydrogel electrode and electrolyte present excellent stretchability and good reversible compressibility (Fig. S2, Supporting Information). When being undergone compressive strains, they could recover to their original shape after removing the pressure (Fig. S2c and d, Supporting Information). The microstructure of the freeze-dried hydrogel electrode shows interconnected reticular porous structure (Fig. 2b), similar to that of hydrogel electrolyte (Fig. 2d). As shown in Fig. S3 (Supporting Information), the integrated all-hydrogel supercapacitor presents perfect interface combination between the electrode and electrolyte layers. The arbitrary deformation capability of the soft supercapacitor is shown in Fig. 2e-l. The device (a piece of hydrogel eletrolyte sandwiched between two hydrogel electrodes) can be easily stretched to 1000% strain without any obvious cracking, signifying its excellent deformation capability. Meanwhile, it is reversibly compressible, and can be twisted, knotted, folded, rolled or crumpled, demonstrating its excellent softness. Moreover, the supercapacitor is tough and can withstand a heavy load of 50-fold weight of the whole supercapacitor. It is worth mentioning that such a soft and tough supercapacitor has rarely been reported. The mechanical properties of the hydrogel electrode and electrolyte were measured by tensile and compression tests. As shown in Fig. 2m, the electrolyte possesses a better elongation and can be easily stretched to 2400% strain. The high tensile strength stems from the dual-network structure of the hydrogel matrix. The network of covalently crosslinked PAM can bridge cracks and stabilize deformation; the unzipping of the SA network can dissipate energy and reduce the stress concentration [37]. Such synergistic mechanism endows the hydrogel with superb toughness, stretchability, compressibility and recoverability. The compressive curves (Fig. 2n) reveal that the electrolyte reaches a compressive stress of 952 kPa at the compressive deformation of 90%, higher than those of pure PAM hydrogel (734 kPa) and hydrogel electrode (336 kPa) at the same compressive deformation, being consistent with the results of tensile tests. For the electrode, insoluble CNTs and PEDOT were added in the hydrogel. These solid phase components could whittle the compactness and weaken the mechanical properties of the dual-network crosslinked hydrogel matrix. Thus, the stretchability/

2.5. Characterizations The characterizations on the morphologies, chemical compositions and mechanical properties of the hydrogels, electrochemical measurements and calculations are provided in the Electronic Supplementary Information (ESI). 3. Results and discussion Fig. 1 graphically illustrates the hydrogel preparation process and the construction of the soft supercapacitor. The AM contains carboncarbon double bonds and amide bifunctional groups. SA is a biological material derived from sea algae. The SA chain is composed of linear, non-branched polysaccharides, which contains varying amounts of (1 → 4)-linked β-D-mannuronic acid (M unit) and α-L-guluronic acid (G unit) residues. When binding with water, SA forms a viscous gel, however the SA hydrogel is not stretchable (Fig. S1a and b, Supporting Information). In the AM/SA aqueous solution, the AM monomers form covalently crosslinked polymer chains through N,N-methylenebisacrylamide; SA chains randomly dispersed in the covalently crosslinked network of PAM. As macromolecule and thickener, long-chain SA swells in the solution to form a three-dimensional (3D) network by molecular chain entanglement and interaction between molecular chains (such as hydrogen bonds), which increases the flow resistance of the solution and acts as a mechanical support matrix [36]. In PAM/SA hybrid hydrogel, the two types of polymer chains are intertwined, and physically entangled alginate chains could strengthen the mechanical robustness on PAM chain basis [33]. Therefore, the tensile and compression properties of the PAM/SA hydrogel are superior to those of pure PAM hydrogel (Fig. S1c and d, Supporting Information). The conductivity and electrochemical activity of the hydrogel electrode can be enhanced by introducing CNTs as connected conductive networks and PEDOT:PSS as active material; the ionic conductivity of the hydrogel electrolyte and charge storage can be boosted through incorporating Na2SO4 electrolyte salt and K3[Fe(CN)6]/K4[Fe(CN)6] redox-couple in the eletrolyte hydrogel matrix. The resultant hydrogel electrode and electrolyte contain 3D crosslinked network structures. 3

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Fig. 2. a) Electrode hydrogel with different geometries, b) SEM image of the freeze-dried electrode hydrogel. c) Electrolyte hydrogel with different geometries, d) SEM image of the freeze-dried electrolyte hydrogel. The integrated all-hydrogel supercapacitor under variant deformations: e) stretching, f) compressing and recovering, g) twisting, h) knotting, i) folding, j) rolling, k) crumpling and l) hanging weight. m) Tensile and n) compressive stress-strain curves of PAM, electrolyte and electrode hydrogels.

attributed to the symmetric C–O stretching [40]. The results manifest that the PAM and SA keep their individual chain structures, signifying physical interpenetration of the two components in the 3D dual network matrix. After adding CNTs and PEDOT:PSS in the PAM/SA matrix, the extra introduced peaks could be attributed to the –C–S bonds (813 and 958 cm−1) and –C–O–C– stretching (1274 cm−1) from the PEDOT:PSS [41]. The electrochemical performances of the all-hydrogel supercapacitors were evaluated in two-electrode symmetric configuration in the cell voltage range of 0 to 1.0 V. Devices for tests were encapsulated by PDMS films (Fig. S5, Supporting Information). The charge storage

compressibility of the electrode hydrogel is inferior to electrolyte hydrogel and even PAM hydrogel. The chemical compositions of the hydrogels were investigated by Fourier Transform Infrared spectroscopy (FTIR) (Fig. S4, Supporting Information). For pure PAM, the peaks at 3330 and 3178 cm−1 are related to the –NH2 group (one for symmetrical and the other for antisymmetrical stretching of two –N–H bonds) [38], and the peaks at 1649 and 1599 cm−1 are induced by the amide I and II bonds [39]. After introducing SA, additional peaks present in the spectrum of PAM/ SA hydrogel. The peak at 1411 cm−1 belongs to the C–H deformation with secondary alcohols, and one small sharp peak at 1031 cm−1 is 4

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provides relatively high pseudocapacitance, without adding CNT conductive network, the capacitance of the device is still low due to the lack of continuous electronic paths. Therefore, CNTs are responsible for providing continuous conductive network and PEDOT supplies high capacitance; the two components complement each other. Thanks to the synergistic effect of CNTs and PEDOT, the optimal specific capacitance is achieved for the device based on the electrode of PAM/SA/ CNTs/PEDOT:PSS. The Nyquist plot (Fig. S8b, Supporting Information) also validates its optimized capacitive behavior with low equivalent series resistance at high frequency (0.35 Ω), and more vertical line at low frequencies compared with the other two electrodes. Fig. 4b shows the galvanostatic charge-discharge (GCD) profiles of the all-hydrogel supercapacitor at various current densities ranging from 1 to 8 mA cm−2. Considering the slight polarization at the maximum voltage of 1 V in the CV curve (Fig. 4a), we chose the cell voltage range of 0 to 0.9 V for GCD and long-term GCD cycling tests. The GCD curves present a gentle slope from 0 to 0.2 V, which is typical for the pseudocapacitive effect of redox couple [44]. According to the GCD tests, the areal capacitance of the device reaches 128 mF cm−2 (0.85F cm−3 based on the device) at a current density of 1 mA cm−2, higher than the values for stretchable supercapacitors based on stretchable substrates or helical fibers [6,7,13,14,46–49]. However, the coulombic efficiency of our device is inferior to that of common supercapacitors due to the self-discharge effect induced by the redox couple, especially at low current density (50% at 1 mA cm−2 and 90% at 8 mA cm−2). The areal capacitances of the device as a function of scan rate or current density are presented in Fig. 4c. When the current density increases to a high value of 8 mA cm−2, the supercapacitor still holds a specific capacitance of 64 mF cm−2, demonstrating its good rate capability. To further explore the conductivity and ion transport characteristics of the all-hydrogel supercapacitor, electrochemical impedance spectroscopy (EIS) measurement was carried out. From the Nyquist plot (Fig. 4d), it can be seen that the device owns a small equivalent series resistance (Rs = 0.35 Ω) and low charge transfer resistance (Rct = 0.22 Ω). The low impedance comes from the following facts: 1) the hydrogel electrode with highly conductive active materials, continuous conductive pathways and 3D porous network structure provides fast and effective electron/ion transport; 2) the hydrogel electrolyte has a high ionic conductivity of 14.9 mS cm−1, higher than that of PAM (0.45 mS cm−1), PAM/SA (0.56 mS cm−1), PAM/SA/CNT/PEDOT (0.48 mS cm−1) hydrogels (Fig. S9 and S10, Supporting Information), and comparable to the ionic conductivity of PVA/H3PO4 gel electrolyte [50]; 3) the strong self-adhesion between electrode and electrolyte hydrogels endows the cell with low interface resistance. The charge-discharge cycling stability of the all-hydrogel supercapacitor was evaluated by a long-term GCD cycling test at a current density of 2 mA cm−2. As presented in Fig. 4e, the areal capacitance of the supercapacitor retains 78% of its initial value and the coulombic efficiency holds ~96% after 5000 cycles. Generally, the cycling stability of the PEDOT conductive polymer electrode is poor due to the large volumetric changes during doping/de-doping processes. It undergoes swelling, shrinking, cracking and pulverization gradually decreasing its conductivity, and the electrode generally presents capacitance retention of ~80% only after 2000 cycles [51–53]. In our hydrogel electrode, the addition of CNTs enhances not only the conductivity of the electrode, but also its mechanical stability and weakens the deformations of PEDOT during the long-term GCD cycling, thus leading to improved electrochemical stability for the electrode. In the Ragone plots (Fig. 4f), the energy and power densities based on unit area of our all-hydrogel supercapacitor are compared with those of previously reported results. It is worth mentioning that the working voltage window of 0.9 V is relatively high compared with those of previously reported stretchable supercapacitors using hydrogel electrolytes [28,29,32,34]. The high areal capacitance (128 mF cm−2) and voltage window lead to a high energy density of 3.6 μWh cm−2 (24 μWh cm−3) at a power density of 0.2 mW cm−2 (1.33 mW cm−3). It still maintains 1.8 μWh cm−2

Fig. 3. Illustration of the charge storage mechanism of the all-hydrogel supercapacitor.

mechanism of the all-hydrogel supercapacitor is depicted in Fig. 3. During charging, in the positive electrode, PEDOT loses electrons and turns into PEDOT+ owing to p-type doping [42]. To keep charge equilibrium in the molecular chain, electrolyte anions are absorbed at the interface between PEDOT and electrolyte, meanwhile, Fe(CN)64− ions in the electrolyte migrate to the positive electrode (involved in electrical double-layer capacitance), and then are oxidized to Fe (CN)63− (Eq. (1)):

Fe(CN)64 − ⇌ Fe(CN)36 − + e−

(1)

In the negative electrode, PEDOT obtains electrons turning into PEDOT-, and electrolyte cations (Na+ and K+) are absorbed on the surface of PEDOT, at the same time, Fe(CN)63- ions closed to the negative electrode are reduced to Fe(CN)64-. During discharge, the reverse process occurs. The CV curves of the all-hydrogel supercapacitors (Fig. 4a) confirm the charge storage mechanism. They comprise a couple of redox peaks and rectangular shapes. The oxidation peak near 0.1 V can be ascribed to the oxidation process of Fe(CN)64-. The incomplete reduction peak during discharge is attributed to the slight self-discharge of the redox couple, a common feature for redox couples [43,44]. The rectangular part from 0.2 to 1.0 V is mainly contributed by the fast Faradaic pseudocapacitive reaction of PEDOT [45] and electrical double-layer capacitance from CNTs. The maximum areal capacitance of the allhydrogel supercapacitor achieves 232 mF cm−2 (1.55F cm−3 based on the device) at a scan rate of 5 mV s−1, far higher than that of the supercapacitor without adding redox couple (Fig. S6, Supporting Information). The CV curves of the all-hydrogel supercapacitor without adding redox couple display almost rectangular shapes and GCD curves exhibit isosceles triangle shapes (Fig. S7, Supporting Information), indicating the device presents fast nondiffusion-controlled capacitive storage behavior. However, the specific capacitance is relatively low, only 20 mF cm−2 at 0.1 mA cm−2. To further explore the effect of CNTs and PEDOT components in hydrogel electrode, control experiments were conducted. Fig. S8a (Supporting Information) shows the CV curves of the all-hydrogel supercapacitors with electrodes based on different components at a scan rate of 10 mV s−1; without introducing CNTs or PEDOT, the supercapacitors present significantly reduced specific capacitances. In addition to contribute finite electrical double-layer capacitance, CNTs mainly act as conductive network in the electrode. Although PEDOT 5

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Fig. 4. Electrochemical performance of the all-hydrogel supercapacitor under flat state. a) CV curves at different scan rates. The inset shows the encapsulated allhydrogel supercapacitor. b) GCD curves at different current densities. c) Areal capacitances and rate capability. d) Nyquist plot. e) Cycling performance at 2 mA cm−2. The inset shows the CD curves of the first cycle and the 5,000th cycle. f) Ragone plots. The Ragone plots of the other advanced supercapacitors based on stretchable electrodes or electrolytes reported in literatures are provided for comparison. g) Four cells connected in series powering an E-Ink display for more than 180 s.

(12 μWh cm−3) at 1.64 mW cm−2 (10.89 mW cm−3). The energy densities of our all-hydrogel soft supercapacitor are superior to those of previously reported stretchable supercapacitors. They are generally almost based on stretchable electrodes (with nonstretchable PVA-based electrolytes) or stretchable electrolytes (with nonstretchable electrodes), such as CNTs electrode (with poly(ethylene glycol) diacrylate/ 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide as electrolyte) [54], CNTs/MnO2 electrode (with PVA/LiCl as electrolyte) [55], CNTs/PANI electrode (with PVA/H2SO4 as electrolyte) [56], CNTs/PANI electrode (with poly(methyl methacrylate)/LiClO4/propylene carbonate as electrolyte) [6], graphene fiber/3D-graphene electrode (with PVA/H2SO4 as electrolyte) [57], PEDOT:PSS electrode (with PVA/polymethacrylic acid/H3PO4 as electrolyte) [58], CNTs/PPy electrode (with PAM/SA/Li2SO4 as electrolyte) [33] and CNTs/PANI (with PVA/H2SO4 as electrolyte) [59]. To demonstrate the application of the all-hydrogel supercapacitor, four cells connected in series were fabricated to power an E-Ink display. As shown in Fig. 4g, the device was charged at 1 mA cm−2 to 3.0 V, and powered the E-Ink display for more than 180 s. Both the hydrogel electrode and electrolyte are soft and deformable, the assembled all-hydrogel supercapacitors hold excellent mechanical properties. To investigate its electrochemical performance under tensile condition, the supercapacitor was stretched at various tensile strains

and the relevant CV and GCD curves were recorded. As revealed in Fig. 5a, the GCD curves of the soft supercapacitor at 0, 200, 400, 600 and 800% strains, are nearly overlapped, and there is no downward trend in the capacitance retention. The slightly increased capacitance may result from the reduced thickness of the device during stretching, which favors the diffusion and transport of electrolyte ions. The CV curves at different tensile strains and the corresponding capacitance retentions keep the tendency similar to the GCD results (Fig. S11a, Supporting Information). The supercapacitor was further tested under repeated stretching (Fig. 5b and Fig. S11b, Supporting Information), and the capacitance retention maintains nearly 100% after 400 stretching cycles at 200% strain, confirming the unexceptionable tensile stability of the device base on the dual-network hydrogel. Compressibility is another important mechanical deformation for soft supercapacitors. The capacitance retention as a function of compression strain and the corresponding CV and GCD curves are shown in Fig. S11c (Supporting Information) and Fig. 5c. The roughly superposed CV and GCD curves at 50%, 65% and 75% compression strains demonstrate excellent mechanical and electrochemical stability of the soft supercapacitor. To evaluate whether the device could withstand a sudden puncture, a possible damage suffered for wearable electronics in practical applications, pinning test was conducted on the supercapacitor. As displayed in Fig. S12 (Supporting Information), after 6

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Fig. 5. Electrochemical stability of the all-hydrogel supercapacitor under variant deformations. a) Capacitance retention as a function of tensile strain and corresponding GCD curves. b) Capacitance retention as a function of stretching cycle number and corresponding GCD curves under repetitive stretching from 0% to 200% strain. c) Capacitance retention as a function of compression strain and corresponding GCD curves. d) Capacitance retention as a function of hole number under pinning test and corresponding GCD curves. e) Capacitance retentions under different deformations. f) GCD curves at 2 mA cm−2 under different deformations. g) Schematic illustration of the stable network structure for the integrated all-hydrogel supercapacitor during the stress–strain process (F = Force).

suffering severe and repeated mechanical stresses. This impressive electrochemical stability of the all-hydrogel supercapacitor mainly stems from its unique all-in-one architecture (Fig. 5g). The same hydrogel matrix for the electrode and electrolyte and strong self-adhesion between them offer tight bonding to avoid structural misalignment or delamination during severe mechanical deformations. Moreover, the intertwined CNTs in the hydrogel electrode can offer stable electronic paths under various strain states, and the crosslinked points formed among SA chains, PAM chains and CNTs can act as buttons to keep the stable corsslinked network structure; the hydrogel electrolyte can simultaneously act as supertough separator to protect the electrode when undergoing harsh conditions. Thus, the all-hydrogel supercapacitor could keep original performances under complex deformations.

pricking penetrating holes with a hole diameter of ~0.2 mm one by one in the device, the all-hydrogel supercapacitor shows self-recovering ability. When stretching it after pricking 400 holes, the device remains structurally stable. The CV (Fig. S11d, Supporting Information) and GCD tests (Fig. 5d) show that the capacitance retentions are satisfactory with increasing hole numbers, which reflects that the pinning effect on the all-hydrogel supercapacitor is insignificant even after severe penetrating destruction. The supercapacitor could retain 79% of the initial capacity even after pricking 800 holes per square centimeter according to the GCD test (Fig. 5d). Arbitrary deformation is not just embodied in extensibility and compressibility, other more severe deformations such as twisting, knotting, folding, rolling and crumpling may also be involved in practical applications. The capacitance retentions of the supercapacitor under these deformations were also investigated by GCD tests (Fig. 5e and f). The capacitances of the device remain stable under all the deformation status, demonstrating its steady energy output ability when

4. Conclusions We successfully developed novel soft supercapacitors based on 7

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intrinsically deformable and recoverable hydrogel electrode and electrolyte without using stretchable substrate or separator. The 3D crosslinked dual-network structures of the electrode and electrolyte hydrogels endow the supercapacitor with superb mechanical flexibility, allowing it to deform arbitrarily; the high ionic conductivity of the electrolyte and pseudocapacitive storage mechanism induced by PEDOT and redox-couple at the interface between the electrode and electrolyte endue the supercapacitor with superior specific capacitance (232 mF cm−2 at 5 mV s−1 and 128 mF cm−2 at 1 mA cm−2), energy density (3.6 μWh cm−2), fast rate capability and long cycle life (over 5000 cycles) among reported stretchable and/or compressible supercapacitors. The unique device structure, stable mechanical and electrochemical properties of the all-hydrogel supercapacitors is a new breakthrough for advanced supercapacitors, and would shed light on the future design of soft supercapacitors for the applications of intelligent wearable electronics and soft HMIs.

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