A Triblock Copolymer Design Leads to Robust Hybrid Hydrogels for

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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36301-36310

A Triblock Copolymer Design Leads to Robust Hybrid Hydrogels for High-Performance Flexible Supercapacitors Guangzhao Zhang,†,‡ Yunhua Chen,*,§ Yonghong Deng,*,† and Chaoyang Wang*,‡ †

Department of Materials Science & Engineering, South University of Science and Technology of China, Shenzhen 518055, China Research Institute of Materials Science, South China University of Technology, Guangzhou 510640, China § School of Materials Science and Engineering, South China University of Technology, Guangzhou 510006, China ‡

S Supporting Information *

ABSTRACT: We report here an intriguing hybrid conductive hydrogel as electrode for high-performance flexible supercapacitor. The key is using a rationally designed water-soluble ABA triblock copolymer (termed as IAOAI) containing a central poly(ethylene oxide) block (A) and terminal poly(acrylamide) (PAAm) block with aniline moieties randomly incorporated (B), which was synthesized by reversible additional fragment transfer polymerization. The subsequent copolymerization of aniline monomers with the terminated aniline moieties on the IAOAI polymer generates a three-dimensional cross-linking hybrid network. The hybrid hydrogel electrode demonstrates robust mechanical flexibility, remarkable electrochemical capacitance (919 F/g), and cyclic stability (90% capacitance retention after 1000 cycles). Moreover, the flexible supercapacitor based on this hybrid hydrogel electrode presents a large specific capacitance (187 F/g), superior to most reported conductive hydrogel-based supercapacitors. With the demonstrated additional favorable cyclic stability and excellent capacitive and rate performance, this hybrid hydrogel-based supercapacitor holds great promise for flexible energystorage device. KEYWORDS: triblock copolymer, conductive hydrogel, flexible supercapacitor, PANI, high electrochemical performance

1. INTRODUCTION Energy storage devices, especially those with mechanical flexibility, are strongly demanded for the applications of next-generation portable electronics.1−3 Compared with the traditional energystorage devices, which are typically rigid, the flexible ones can be bent freely and incorporated appropriately with clothes or attached to human bodies,4−7 making them portable and convenient to customers. Although numerous efforts have been devoted onto this field in the past decades, it is still a big challenge to make efficient flexible energy-storage devices with superior energy and power density. One of the biggest problems is that most active materials serving as electrodes are metal,8 metal oxide,9 conductive polymer, or carbon-based materials,10,11 and the inherent fragile character of these materials brings fracture and easy separation from current collectors when bending, resulting in dramatic decrease of electrochemistry behavior and cycling performance of the electronic devices. Therefore, the fabrication of flexible electrodes and introduction of bendable current collectors are the keys of making promising flexible electronic devices. Recently, remarkable progress has been made on the preparation of flexible lithium ion batteries in which active electrode materials were smartly incorporated with bendable conductive current collector to replace traditional rigid electrodes.12,13 Besides lithium ion batteries, electrochemical © 2017 American Chemical Society

supercapacitors, which are also widely considered as a significant class of energy storage devices due to the high power density, outstanding cycling performance, and the great potential to achieve high energy density, have been intensively studied in the past few years.14,15 Currently, numerous all-solidstate flexible supercapacitors with high performance have been successfully prepared by coating electrochemically active materials (such as conductive polymer,11 graphene,16 MoS217) onto conductive carbon fibers or carbon nanotube (CNT)/ graphene papers, opening another door to the practical application of portable electronic devices. However, many reported flexible supercapacitors are not able to bend in large angles (commonly less than 90°), and the preparation procedures also suffer from high cost of carbon-based materials or noble metals,13,17 limiting their further commercialization in large scale. Alternatively, polymeric hydrogels,18,19 which are composed of chemically or physically cross-linked three-dimensional (3D) hydrophilic networks and have been widely applied to many important fields, such as the drug delivery,20 electrolyte,21−23 soft robots,24,25 tissue regeneration,26 biosensors,27−29 and adhesives,30 Received: August 3, 2017 Accepted: September 25, 2017 Published: September 25, 2017 36301

DOI: 10.1021/acsami.7b11572 ACS Appl. Mater. Interfaces 2017, 9, 36301−36310

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ACS Applied Materials & Interfaces

monomers could be well-dissolved in this polymer solution. After adding ammonium persulfate (APS), the aniline monomers were polymerized to generate linear PANI chains, and aniline moieties on the IAOAI block copolymer can be simultaneously polymerized into the polyaniline chains, resulting in numerous bridges that connected PANI and the ABA copolymer chains thus forming a 3D hybrid conductive hydrogel network (Figure 1). The PANI plays two important roles in this hydrogel system: (a) as conductive agent and electrochemical active material for supercapacitor and (b) as cross-linker of hydrogel to chemically connect rigid PANI and soft IAOAI polymers at molecular level. Owing to this design, the prepared conductive hybrid hydrogels exhibit robust mechanical property as well as excellent electrochemical behavior, which make it one of the most promising candidates as electrodes for flexible supercapacitors.

could display excellent mechanical performance including high stretchable character, tunable strength, and flexibility. Therefore, hydrogels might serve as ideal structural materials to assemble highly flexible supercapacitors.31,32 However, the direct use of conventional hydrogels as electrodes is hardly possible due to the electrochemical inactivity of most hydrophilic polymers and the intrinsic electrical insulation of hydrogels. Much different from the hydrogel, most conductive polymers as mentioned above, including polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTP), are rigid, fragile, and hydrophobic. Therefore, considering the intrinsic characters of both hydrogels and conductive polymers, a rational strategy to make promising flexible capacitors will be incorporating rigid conductive polymers into hydrogel matrix through chemical or physical interactions. Some beautiful pioneering explorations have been performed based on this concept;33−35 for example, the physical interactions between PANI and α-cyclodextrin and nitrogen protonation of PANI or PPy by phytic acid have been applied to prepare conductive hydrogels.10,36,37 More recently, stable chemical bridges between PANI and hydrophilic poly(vinyl alcohol) (PVA) have been built to produce flexible conductive hydrogel with robust mechanical character and excellent electrochemical properties.38,39 Inspired by these elegant approaches, we demonstrate in this report an intriguing hybrid conductive hydrogel (HCH) with high flexibility and remarkable electrochemical behavior by chemically integrating PANI chains into a specific hydrogel matrix at molecular level. The key of preparing this hybrid hydrogel is using a novel water-soluble ABA triblock copolymer containing a central poly(ethylene oxide) block (A) and terminal poly(acrylamide) (PAAm) block with aniline moieties randomly incorporated (B), which was synthesized by reversible additional fragment transfer (RAFT) polymerization (termed as IAOAI, Figure 1, Scheme S2). The middle poly(ethylene oxide) (PEO) block renders IAOAI polymer hydrophilic, while the terminated PAAm segment could contribute good mechanical performance of the resulting hybrid hydrogel. The IAOAI polymer was first dissolved in water and formed a homogeneous polymer solution. With the assistance of hydrochloric acid (HCl), aniline

2. EXPERIMENTAL SECTION 2.1. Materials. The macro-RAFT agent CTA-PEO445-CTA, where CTA is 2-methyl-2-[(dodecylsulfanylthiocarbonyl)sulfanyl]propanoic acid, was synthesized according to the reference reported previously.19 3-Aminobenzylamine, 2-isocyanatoethyl methacrylate, aniline, and poly(vinyl alcohol) (PVA, degree of polymerization ∼1750) were purchased from Aladdin Bio-Chem Technology Co., LTD (Shanghai, China). Dichloromethane (CH2Cl2), acrylamide (AAm), azodiisobutyronitrile (AIBN), N,N-dimethylformamide (DMF), n-hexane, and APS were obtained from Guangzhou Chemical Reagent Factory (Guangzhou, China). AIBN was recrystallized from methanol and stored at 4 °C. Hydrophilic carbon cloth (thickness: 0.36 mm) was purchased from Shanghai HESEN Electric Co., Ltd., China. Hydrophilic nylon fiber separator with pore size of 0.45 μm was purchased from Tianjin Yilong Equipment Co., Ltd. All other reagents are of analytical grade and used as received without further retreatment unless specified otherwise. Deionized water was used in all experiments. 2.2. Synthesis of 2-(3-(3-Aminobenzyl)ureido)ethyl Methacrylate. The functional monomer 2-(3-(3-aminobenzyl)ureido)ethyl methacrylate (termed as IM) was synthesized for the first time, and the typical synthetic protocol was described as follows (Scheme S1): 2.0 g of 3-aminobenzylamine was dissolved into 50 mL of anhydrous CH2Cl2 followed by adding 2.54 g of 2-isocyanatoethyl methacrylate. The mixture was allowed to react under ambient temperature for 6 h

Figure 1. Schematic illustration of the gelation of ABA triblock copolymer and PANI toward hybrid conductive hydrogel. 36302


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The flexible symmetric hydrogel supercapacitor was obtained by assembling two pieces of hydrogel-coated carbon cloth (2 cm × 1 cm) together with the electrolyte-saturated separator sandwiched between. For the convenience of electrochemical tests, the symmetric supercapacitor was fixed with tape. 2.7. Characterization. Characterization of the Functional Monomer and Synthetic ABA Copolymer. The chemical structures of the synthetic monomer and copolymer were characterized by nuclear magnetic resonance (1H NMR; 600 MHz, Bruker); DMSO-d6 was used as the deuterium solvent. Gel permeation chromatography (GPC) test was performed on a Waters 2414 (United States) with poly(ethylene glycol) (PEG) as standard for characterizing the molecular weight of the triblock copolymer. Characterization of Hybrid Hydrogel. Fourier transform infrared spectroscopy (FT-IR) spectra of CTA-PEO-CTA, polymer IAOAI, and dehydrated hydrogel were collected with a KBr pellet method from 4000 to 400 cm−1 using a Perkin-Elmer Spectrum 100 at ambient temperature. The micromorphology of the dehydrated hybrid hydrogel was observed with scanning electron microscope (SEM) characterization. The specific surface area of the hydrogel was measured by N2 sorption analyses determined on BELSORP-max. The X-ray diffraction (XRD) data of the dehydrated hydrogel was measured by a Philips X’Pert Pro Super X-ray diffractometer equipped with graphite monochromatized Cu Ka radiation (λ = 1.541 841 Å). The polymer content of the hydrogel was measured by freeze-drying in which the weight of completely freeze-dried hydrogel was compared with the wet hydrogel. The conductivity of bulk hydrogel was measured according to the electrochemical impedance spectroscope (EIS) determined by an electrochemical station. For the ease of test, a disk-shaped hydrogel sample with a diameter of 10 mm (5 mm thickness) was sandwiched between two copper plates. Then the impedance between twoterminal copper plate electrodes was measured within the frequency range from 0.01 Hz to ∼100 kHz. 2.8. Mechanical Test of the Hybrid Hydrogel. The mechanical properties of the conductive hydrogel were measured on a Shimadzu autograph AG-Xplus 1KN. For the tensile strain−stress test, the hydrogel was made into a rod sample with a diameter of 4.2 mm and a length of 30 mm. Tensile measurements were then performed by uniaxially stretching the hydrogel strip at a strain rate of 100 mm/min. For the stress−strain test, the conductive hydrogel was made into a cylinder with a diameter of 12.7 mm and 15 mm long. The compress measurements were performed by uniaxially compressing the hydrogel sample at a strain rate of 2 mm/min. 2.9. Electrochemical Measurements on the Prepared Hydrogel Electrode. The electrochemical behavior of the above prepared hydrogel electrode was then characterized by EIS, cyclic voltammogram (CV), and galvanostatic charge−discharge (GCD) tests. All tests above were performed with a three-electrode system on a Solarton analytical electrochemical workstation (model 1470E, England). The reference electrode, work electrode, and counter electrode were Ag/AgCl electrode (Shanghai Leici Instrument Co., Ltd.), hydrogel electrode (coating area 0.25 cm2 ), and platinum electrode, respectively. The H2SO4 solution with a concentration of 1 M was employed as the electrolyte for the tests. Note that the CV test was performed within the potential range from −0.2 to 0.8 V versus Ag/AgCl under a scan rate of 10−100 mV/s. The GCD tests were performed by scanning the potential from 0 to 0.8 V at constant current density varying from 0.5 to 20 A/g. The EIS measurements were performed on the Solarton electrochemical workstation (1260A) with the frequency range from 0.01 Hz to 10 kHz while keeping the amplitude at 10 mV referring to the open circuit. The charge− discharge behavior of the hydrogel electrode was evaluated by the GCD measurements at the current density of 20 A/g. 2.10. The Electrochemical Tests of the Hydrogel Supercapacitor. The electrochemical property of the hydrogel supercapacitor was performed with two-electrode system on the same electrochemical workstation mentioned above. For the CV tests, the applied potential was varied from 0 to 0.8 V under the scanning rates of 5−100 mV. The charge−discharge curves were obtained from the GCD tests, while the scanning potential was changed between 0 and

with continuous stirring. Then, 20 mL of n-hexane was added for precipitation followed by filtering, and a whitish powder was obtained. The white product (4.35g, 96%) was then washed with n-hexane three times and stored in a brownish glass bottle under 4 °C after drying in vacuum. The 1H NMR and 13C NMR of IM were performed using deuterated dimethyl sulfoxide (DMSO-d6) as solvent. Mass spectral analysis was also performed using methanol as solvent. 2.3. Synthesis of the ABA Triblock Copolymer IAOAI. The hydrophilic triblock copolymer was synthesized by a RAFT polymerization. Briefly, 0.3 g of monomer IM (1.08 mmol), 0.518 g of CTAPEO-CTA (0.025 mmol), and 1.07 g of acrylamide (15 mmol) were dissolved into 3 mL of DMF with the assistance of sonication, followed by adding 2 mg of AIBN as initiator. Then, the mixture was allowed to polymerize at 70 °C for 24 h after three freeze−pump−thaw cycles. The polymerization was quenched by decreasing the temperature to 0 °C followed by adding 12 mL of deionized water to dissolve the product. The block copolymer solution was dialyzed against pure water for 7 d, and a slight yellow product (IAOAI) was obtained after freezedrying. The value of m and n (the degrees of polymerization of IM and AAM, respectively) in the structure of IAOAI was determined using 1 H NMR, which was shown in Table S1. Other ABA copolymers with varied IM incorporations were synthesized to investigate the influence of functional monomer to the subsequent polymer gelation. 2.4. Preparation of the Hybrid Conductive Hydrogel. The typical fabrication protocol of the conductive hybrid hydrogel is detailed as follows (Scheme S3): First, 0.8 g of APS was dissolved in 1 mL of deionized water to form an initiator solution (tag solution A). Then, 0.3 g of copolymer IAOAI was dissolved into 2.7 mL of water to make a polymer solution with a concentration of 10 wt % at room temperature. After the complete dissolving of IAOAI, 0.5 mL of HCl solution (with a concentration of 6 M) was added to mix homogeneously. Aniline (0.2 mL) was then added followed by stirring vigorously to form solution B. Solution A and solution B were cooled to 0 °C in a refrigerator and then mixed quickly in an ice bath. The mixture was allowed to react at room temperature for 5 h to generate conductive hydrogel. To remove the residual ions, excessive aniline, and byproducts, the obtained hydrogel was immersed in excessive amount of deionized water for 24 h. The effect of ABA copolymer concentration and the mass ratio of copolymer to aniline on the gelation properties were also studied. The related parameters and results are summarized in Table 1.

Table 1. Effect of ABA Copolymer Concentration and Mass Ratio of Copolymer to Aniline on the Hydrogel Formation code

copolymer

initial copolymer concentration (wt %)

mass ratio of polymer to aniline

gelation

1 2 3 4 5 6 7

IAOAI-18 IAOAI-18 IAOAI-18 IAOAI-8 IAOAI-8 IAOAI-8 AOA

10 5 5 10 10 5 10

3:2 3:3 3:2 3:3 3:2 3:3 3:3

gel gel no gel no no no

2.5. Preparation of Hydrogel Electrode. The hydrogel electrode was prepared by in situ copolymerization of IAOAI and aniline onto carbon cloth collector. First, the carbon cloth was immersed into concentrated nitric acid for 24 h followed by washing with ethanol and deionized water three times. Solution A and solution B were cooled to 0 °C and mixed quickly followed by application on the carbon cloth immediately. After the complete polymerization of aniline (∼5 h), the prepared electrode was then immersed in deionized water overnight to remove any excess ions and unreacted monomers. 2.6. The Assembly of Conductive Hydrogel-Based Supercapacitor. The PVA/H2SO4 electrolyte was first prepared by dissolving 1.0 g of PVA into 10 mL of 10% H2SO4 solution at 90 °C for half an hour. After it cooled to room temperature, a hydrophilic nylon fiber separator was immersed into the electrolyte solution until saturation. 36303


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where the I, t, m, and V are the discharge current, the discharge time, the total mass of active material in two electrodes and the discharge voltage change during discharging in the GCD curves (excluding the IR drop), respectively. The energy density and power density of the supercapacitor were obtained based on Equations 5 and 6, respectively:

0.8 V at constant current density range from 0.25 to 2.5 A/g. The EIS of the supercapacitor was measured with frequency increasing from 0.01 Hz to 10 kHz at an amplitude of 10 mV referring to the open circuit. The specific capacitance of the supercapacitor was calculated according to the discharge profile. The flexibility of the prepared supercapacitor was tested by measuring the CV behaviors when the supercapacitor was folded under different bending angles. 2.11. Calculation. The conductivity calculation of the bulk hydrogel: EIS profile was employed to evaluate the conductivity of bulk hydrogel, which was calculated according to the following equation: σ = L /(R × S)

(2)

(3)

in which the I, t, m, and V terms are the discharge current, the discharge time, the mass of active material in the electrode, and the discharge voltage change during discharging in the GCD curves, respectively. Specific capacitance of the hydrogel supercapacitor: The specific capacitance of the prepared hydrogel supercapacitor was obtained from the discharge curves according to the following equation: C p = (I × t )/(m × v)

P = E /t

(6)

3. RESULT AND DISSCUSSION 3.1. Synthesis of the Monomer 2-(3-(3-Aminobenzyl)ureido)ethyl Methacrylate and Triblock Copolymer IAOAI. The methacrylate monomer with aniline functional group, which can be used as specific cross-linker for hydrogels, was designed and synthesized by the coupling reaction of 2-isocyanatoethyl methacrylate with 3-aminobenzylamine for the first time. The chemical shifts at 4.95 and 5.75 ppm in the 1 H NMR spectrum are assigned to the protons from aniline and CC double bonds, respectively, implying the successful synthesis of IM monomer (Figure 2). The 13C NMR and mass spectrometry (MS) data of the monomer IM were also given in Figures S4 and S5. The monomer was then copolymerized with acrylamide by RAFT polymerization, while CTA-PEO-CTA was applied as the macro-transfer agent. The aniline proton peak at 5.0 and the amide proton peaks around 6.5−7.5 are clearly observed in the 1H NMR spectrum of IAOAI copolymer (Figure 2), and the degrees of polymerization of IM and AAM (m and n, respectively) were evaluated and summarized in Table S1.

where the I, t, S, and V terms are the discharge current, the discharge time, the area of hydrogel electrode, and the discharge voltage change during discharging in the GCD curves, respectively. Specific capacitance: the specific capacitance of the hydrogel electrode was also calculated according to the GCD curves from the following equation: C p = (I × t )/(m × v)

(5)

where E, C, V, P, and t are the energy density, cell specific capacitance, voltage change upon discharging (excluding the IR drop), power density, and discharge time, respectively.

(1)

where the L and S are the length and the bottom area of the cylindrical hydrogel sample, respectively; R is the resistance at the frequency where the phase angle approached to zero (Figure S1). Area capacitance of the hydrogel electrode: the area capacitance was calculated from the GCD profiles according to the following equation:

Cs = (I × t )/(S × V )

E = C p/2 × V 2

(4)

Figure 2. 1H NMR of the synthetic monomer IM and copolymer IAOAI using DMSO-d6 as solvent. 36304


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ACS Applied Materials & Interfaces 3.2. Characterization of the Conductive Hybrid Hydrogel. It should be pointed out that the introduction of aniline moieties on the IAOAI is indispensable to form conductive hydrogel. Without or too little aniline moieties incorporated, the polymer solution is not able to gel because of the insufficient cross-linking points (Table 1; IAOAI-18 means the value of m is 18 according to 1H NMR, and AOA means the copolymer without aniline moieties). Besides, the concentration of IAOAI copolymer and the amount of aniline monomer are also found to be important regarding the gelation of the polymer hybrid. The optimized HCH sample was prepared by using IAOAI-18 (m = 18) polymer with an initial concentration of 10 wt %, while the mass ratio of polymer to aniline was 3:2 (Code 1 in Table 1). After the complete purification by washing with deionized water, the obtained HCH sample was stored. The total polymer content (IAOAI + PANI) of HCH was determined to be 10.5 wt % by lyophilization and gravimetry after immersing in deionized water after 24 h, in which case the hydrogel has already approached swelling equilibrium, and the swelling ratio of our hydrogel sample was ∼40 wt % (Code 1). Glass vial tilting confirms the gelation of polymer hybrid, as shown in Figure 3A. The gelation took place in a few minutes after adding the oxidant APS at room temperature, indicating the fast cross-linking between IAOAI polymer and PANI chains. After the complete oxidative polymerization, the fabricated hydrogel can be easily demolded from the glass vial and kept well intact (Figure 3B). It is well-known that the microstructure of electrode is vital to prepare high-performance supercapacitors with high specific capacitance and excellent cycling property. SEM was used to observe the microstructure of our conductive hydrogels. Figure 3C shows the internal morphology

of dehydrated hydrogel, and well-ordered macroporous structures with an average pore size of 30 μm can be clearly observed, indicating the large water space present in the hydrogel, which could facilitate the mass transfer and the free moving of electrolyte. Intriguingly, after further enlarging the structures, we can see that the hydrogel scaffold is composed of numerous nanoparticles with a diameter less than 50 nm (Figure 3D). A possible explanation for this phenomenon is the crystallization of PANI, which will be confirmed by the X-ray diffraction. These nanoparticles could considerably increase the specific area of hydrogel, which are conducive to the storage of charges and subsequently offer more active sites for the redox reactions. The chemical structure of hybrid hydrogel was characterized by FT-IR spectroscopy, while the spectra of IAOAI polymer and the macro-RAFT agent were also given for comparison. As shown in Figure 4A, the absorption peaks at 1564 and 1486 cm−1 are assigned to the benzene ring vibration of PANI in HCH hydrogels. The peak appearing at 1654 cm−1 in the spectrum of HCH is attributed to the stretching absorption of CO from polyacrylamide portion, which also appears in the spectrum of IAOAI. The sharp peaks at 1109 cm−1 appearing in the three spectra belong to the symmetrical stretching vibration of C−O−C, attributed from the PEO parts. Through the comparison of three spectra, the obtained HCH proves having PANI, PAAm, and PEO segments in present. To further analyze the composition of HCH, X-ray diffraction (XRD) was also conducted. As described in Figure 4B, two apparent narrow peaks appear at 21° and 25°, assigned to the crystallization of PAAm and PANI, respectively, suggesting that the HCH possesses partially ordered structures at nanometer scale. Because of the copolymerization with acrylamide, the crystallization behavior of PEO was inhibited, and no apparent

Figure 3. (A) Gelation test via simple tilting method. (B) The prepared hybrid hydrogel could keep its initial form intact after fetching from the glass bottle. (C) The SEM image of internal porous structures of the dehydrated hydrogel. (D) The enlarged observation reflects that the hydrogel scaffold is composed of nanoparticles resulting from the crystallization of PANI, and these nanoparticles are conducive to the storage of charges and redox reaction.

Figure 4. (A) FT-IR of the dehydrated hybrid hydrogel, copolymer IAOAI, and macro-RAFT agent PEO-CTA. (B) XRD profile of the dehydrated hybrid conductive hydrogel. 36305


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Figure 5. Mechanical behaviors of the hybrid conductive hydrogel. The prepared hydrogel is flexible to twine round (A) a glass rod and (B) be made into a knot. (C) Photographs showing the hybrid hydrogel could recover to its initial performance after compression. (D) The flexible hydrogel can keep intact after 1000 cycles bending into 90° and 180°. (E) The tensile stress−strain profile and (F) compressive stress−stain profile of the conductive hydrogel.

then evaluated, and the result was shown in Figure 5F. Clearly, the hydrogel could be compressed over 90% without fracture, and the compressive strength approaches to 65 kPa, suggesting excellent compressive property. 3.4. Electrochemical Behaviors of the Conductive Hydrogel-Based Electrodes. To characterize the electrochemical performance of the prepared HCH, the hydrogel electrode was obtained by in situ gelation of HCH on a piece of hydrophilic carbon cloth which serves as flexible current collector. Then the electrochemical tests including EIS, CV, and GCD were performed on the prepared hydrogel electrode in a three-electrode system using 1 M H2SO4 solution as electrolyte. The electrochemical kinetics and ionic resistance of the hydrogel electrodes with different areal mass loading of PANI (2 mg/cm2 and 4 mg/cm2) were studied by EIS. As shown in Figure 6A, the small semicircles of the EIS plot in high-frequency range reflects a very small charge transfer resistance in electrode system (0.8 and 0.3 Ω for electrodes with 2 and 4 mg/cm2 PANI loading, respectively), suggesting a favorable charge-transfer kinetics. Small series resistances are also observed in the high-frequency region from the EIS profile, which are 2 and 1.1 Ω for electrodes with 2 and 4 mg/cm2 PANI loading, respectively. The slopes of both EIS curves are close to 90° in the low-frequency region, indicating nearly ideal capacitive behavior. Figure 6B reflects the rate-dependent CV curves of the prepared electrode. The characteristic redox peaks of PANI can be clearly observed from the CV profile, which indicates the transformation of PANI between different redox

corresponding peaks were observed in the XRD pattern. The conductivity of the bulky hybrid hydrogel, which is of great significance to supercapacitors, was determined to be ∼0.15 S/cm at room temperature by EIS, which is much higher than most reported conductive hydrogels (typically within the range from 1 × 10−4 to 1 × 10−1 S/cm).36,37 3.3. Mechanical Property of the Conductive Hybrid Hydrogel. As a potential soft electrode for portable supercapacitors, the flexible property of conductive hydrogel is vital. To confirm this feature, the hybrid hydrogel was prepared into cylindrical strip shape, and it was found that the hydrogel strip could easily twine around a small glass rod or be made into a knot (Figure 5A,B). Because of this robust mechanical performance, the hybrid hydrogel can also be stretched or compressed. To exam its compression characteristics, we conducted a simple demonstration by compressing the hydrogel to certain content with a finger (Figure 5C). Interestingly, the hybrid hydrogel could be fully recovered to its initial state without any change after the compressive force release. For practical uses, the flexible supercapacitors will be bent into different angles inevitably. To mimic the bending process of the supercapacitors, we bend the strip sample into 90° and 180° to check the flexibility of the hydrogel, and remarkably, the hydrogel sample can keep intact even after 1000 bending cycles (Figure 5D). The stretchable property of the conductive hydrogel was investigated by a tensile test. It shows the hydrogel sample could be stretched to 112% with a tensile strength of 11 kPa (Figure 5E). The compressive strength of hybrid hydrogel was 36306


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Figure 6. Electrochemical tests of the hybrid hydrogel electrode using a three-electrode system. (A) EIS of the hydrogel electrode with active material PANI loading of 2 and 4 mg/cm2 (from 0.01 Hz to 100 kHz). (B) CV of the hydrogel electrode (2 mg/cm2) with the scan rate range from 10 to 100 mV. (C) GCD curves at various current density of 0.5−5 A/g. (D) Specific capacitance vs current density of the hydrogel electrode with different active material loading. (E) Areal capacitance vs current density of the hydrogel electrode with different active material loading. (F) Capacitance retention during GCD cyclic test at a current density of 20 A/g.

Figure 7. Electrochemical tests of the hydrogel supercapacitor using a two-electrode system. (A) EIS of the hydrogel supercapacitor with the frequency varies from 0.01 Hz to 100 kHz. (inset) Schematic structure of the sandwich supercapacitor. (B) CV curves of the hydrogel supercapacitor with the scan rate range from 5 to 100 mV. (C) CV plots of the supercapacitor at a scan rate of 10 mV/s under different bending angles. (D) Discharge curves of the hydrogel supercapacitor at various current density of 0.25−2.5 A/g. (E) Specific capacitance of the hydrogel supercapacitor at varied GCD current densities. (F) Capacitance retention during GCD cyclic tests at a current density of 2.5 A/g.

nearly symmetrical at large current density (1−20 A/g; Figure 6C and Figure S3), indicating the excellent reversible charge− discharge behavior. Negligible voltage drops (iR, commonly caused by resistance) are observed from the GCD curves, which is attributed from the high conductivity of both hybrid hydrogel and hydrophilic carbon cloth. The specific capacitance of the as-prepared HCH electrodes with different PANI loading were calculated according to eq 3 based on the GCD profiles (Figure 6C and Figure S3). As depicted in Figure 6D, the specific capacitance reaches to 919 and 844 F/g at the current density

status during the charge−discharge process. Nearly central symmetric CV curves also suggest favorable redox reversibility of the hydrogel electrode. In addition, the current densities rise along with elevated scan rates, indicating the good rate performance. The CV test of bare hydrophilic carbon cloth was also performed for comparison, which shows a negligible areal capacitance (Figure S2). Figure 6C exhibits the charge− discharge properties of the as-prepared hydrogel electrode (PANI loading: 2 mg/cm2) at different current densities from 0.5 to 5 A/g. Clearly, the GCD curves of HCH electrode are 36307


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during charge−discharge process. In the current range above, the highest energy density of the flexible supercapacitor is 13.1 Wh/kg at the current density of 0.25 A/g, and the corresponding power density is 89 W/kg (Figure 8), which is comparable to the previously reported conductive hydrogel-based supercapacitors.37

of 0.5 A/g for hydrogel electrode with PANI loading of 2 and 4 mg/cm2, respectively, higher than most reported conductive polymer-based hydrogels.32,36,37 With the increase of current density from 0.5 to 1 A/g, the specific capacitance decreases sharply and then reaches to a steady value of ∼400−600 F/g at large current density, revealing good rate performance. No large decline is observed from the curves where the PANI loading increased from 2 to 4 mg/cm2, indicating the favorable capacitive behavior of the electrode. Areal capacitance of HCH electrode was also calculated according to eq 2 based on GCD curves. The highest areal capacitance was obtained from the electrode with 4 mg/cm2 of PANI loading, reaching 2702 mF/cm2 (Figure 6E), which is much higher than most reported conductive polymer-based electrodes.40 When the PANI loading is 2 mg/cm2, the areal capacitance reaches to 1833 mF/cm2 at the current density of 0.5 A/g, also higher than the previously reported results (listed in Table S2). Electrodes derived from conductive polymers often suffer poor long-term cyclic stability due to the swelling and shrink of polymer during the charge−discharge process. However, our HCH-based electrode could keep 90% of specific capacitance retention even after 1000 charge− discharge cycles (Figure 6F), revealing the excellent chemical stability of the prepared conductive hydrogel, which is possibly attributed to its own flexible property of the hydrogel. 3.5. Electrochemical Properties of the HCH Electrodes-Based Supercapacitors. With the pursuit of highly flexible energy-storage equipment for practical application, we assembled a symmetric supercapacitor device by using two HCH electrodes with an electrolyte saturated nylon separator sandwiched between (Figure 7A inset; see detail in Experimental Section). PVA−H2SO4 gel was exploited as electrolyte for supercapacitor. The electrochemical performance of the obtained flexible supercapacitor was examined by using EIS, CV, and GCD methods. As shown in Figure 7A, the small charge-transfer resistance from the EIS profile suggests a good electrochemical kinetics in this supercapacitor. The plot tail with high slope in low-frequency region indicates favorable capacitive character. The CV curves at different scan rates in Figure 7B present central symmetric shape, also implying the excellent capacitive performance of the supercapacitor. To study the electrochemical stability of the hydrogel supercapacitor under bending conditions, the CV tests of supercapacitor at different bending angles (0°, 90°, 180°) were performed while keeping the scan rate at 10 mV/s. As presented in Figure 7C, the CV curves obtained at different bending angles are almost identical to each other, reflecting the outstanding flexible capability of the prepared supercapacitor. Figure 7D presents the discharge curves of the hydrogel supercapacitor at different current densities. The specific capacitance of the supercapacitor was calculated from the discharge profile and plotted in Figure 7E. We can see that our flexible supercapacitor demonstrates high specific capacitance of 187 F/g at the discharge rate of 0.25 A/g, higher than previously reported conductive polymer-based supercapacitors.37 With the increase of current density from 0.25 to 2.5 A/g, a 91% capacitance retention was observed from Figure 7E, revealing the excellent rate performance of the hydrogel supercapacitor. The long-term cyclic performance of as-prepared supercapacitor was also checked by charge− discharge cycling test. As depicted in Figure 7F, ∼80% capacitance retention is observed after 1000 GCD cycles at a current density of 2.5 A/g, showing good electrochemical stability. This could be also attributed to the flexible property of the hydrogel, which relieves the swelling and shrink of electrodes

Figure 8. Ragone plot of the HCH-based flexible supercapacitor.

4. CONCLUSIONS In conclusion, we developed a promising soft hybrid conductive hydrogel by integrating a water-soluble ABA copolymer with rigid PANI. The PANI chains serve not only as conductive networks but also as cross-linkers for the gelation of hybrid hydrogel. This new hydrogel presents robust mechanical flexibility and recovery. Electrodes based on this hydrogel exhibit outstanding electrochemical properties including high specific capacitance (919 F/g at 0.5 A/g, 2 mg/cm2), areal capacitance (2702 mF/cm2, 4 mg/cm2), excellent cyclic stability, and outstanding flexible capability, which were superior or comparable with other PANI-based electrodes. The supercapacitor assembled from these electrodes also shows superior capacitance (178 F/g), flexibility, and capacitive behavior. With this outstanding performance, we believe the HCH hydrogel offers great promise flexible energy-storage devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b11572. The synthetic schemes, characterization, and supporting analytical data (PDF) The bending process of the hydrogel supercapacitor (AVI)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (Y.C.) *E-mail: [email protected]. (Y.D.) *E-mail: [email protected]. (C.W.) ORCID

Yunhua Chen: 0000-0002-5543-8152 Chaoyang Wang: 0000-0002-7270-5451 Author Contributions

All authors have given approval to the final version of the manuscript. 36308


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21474032 and 21604025), Natural Science Foundation of Guangdong Province (2016A030310461), China Postdoctoral Science Foundation (2016M592486 and 2017T100624), and the Fundamental Research Funds for the Central Universities (2017MS074).

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ABBREVIATIONS IM, 2-(3-(3-aminobenzyl)ureido)ethyl methacrylate IAOAI, the triblock copolymer HCH, hybrid conductive hydrogel EIS, electrochemical impedance spectroscope CV, cyclic voltammetry GCD, galvanostatic charge−discharge

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A Triblock Copolymer Design Leads to Robust Hybrid Hydrogels for

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