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Enhancing the Properties of Conductive Polymer Hydrogels by Freeze-thaw Cycles for High-performance Flexible Supercapacitors Wanwan Li, Han Lu, Ning Zhang, and Mingming Ma ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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ACS Applied Materials & Interfaces

Enhancing the Properties of Conductive Polymer Hydrogels by Freeze-thaw Cycles for High-performance Flexible Supercapacitors Wanwan Li, Han Lu, Ning Zhang,* Mingming Ma* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China ABSTRACT: We report that a post-synthesis physical process (freeze-thaw cycles) can reform the microstructure of conductive polymer hydrogels from clustered nanoparticles to interconnected nanosheets, leading to enhanced mechanical and electrochemical properties. The polyaniline-polyvinyl alcohol hydrogel after 5 freeze-thaw cycles (PPH-5) showed a remarkable tensile strength (16.3 MPa), large elongation at break (407%) and high electrochemical capacitance (1053 F·g-1). The flexible supercapacitor based on PPH-5 provided a large capacitance (420 mF·cm-2 and 210 F·g-1) and high energy density (18.7 Wh·kg-1), whose robustness was demonstrated by its 100% capacitance retention after 1000 galvanostatic charge-discharge cycles or after 1000 mechanical folding cycles. The outstanding performance enables PPH-5 based supercapacitor as a promising power device for flexible electronics, which also demonstrates the merit of freeze-thaw cycles for enhancing the performance of functional hydrogels. KEYWORDS: conducting polymers hydrogels, polyaniline, freeze-thaw cycles, flexible supercapacitors, supramolecular self-assembly 1 ACS Paragon Plus Environment


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1. INTRODUCTION Flexible electronic devices are promising for applications in wearable systems, wireless sensors, and implantable systems.1-2 Mechanically flexible power devices with large capacity, high stability and reliability, and good mechanical properties are desired for powering flexible electronics devices.3-5 Various flexible supercapacitors (SCs) have been designed by using carbon-based materials,6-8 metal oxides,9 conducting polymers,10-11 and the composites of these materials12-14 as electroactive materials. To make flexible SC electrodes, these electroactive materials are typically coated as a thin layer on elastic but electrochemically non-active substrates, such as polydimethylsiloxane,15 rubber fiber,16 and cotton sheet17. The electrochemically non-active substrates occupy a significant amount of weight and volume in the final SC devices.4-5 In contrast, conducting polymer-based hydrogels (CPHs) are both electro-active and flexible, which are promising to achieve compact SCs with higher specific capacitances.10 A few CPHs have been studied for SCs development, such as CPH based on PEDOT-PSS,18 polyaniline,10, 19 and polypyrrole.20 However, the mechanical performance and electrochemical stability of current CPHs need to be enhanced to meet the requirement of flexible SCs.11 Therefore, strong and robust CPHs are desired for high-performance flexible SCs, but remain as a big challenge. In previous work, we developed a general strategy to achieve strong and robust functional polymer composite by crosslinking a soft polymer and a rigid polymer through supramolecular interactions, for applications such as actuators and nanogenerators,21 bioelectrodes22, stretchable conductors23 and flexible supercapacitors.24 Particularly, we designed a supramolecular CPH by using boronate ester bonds to crosslink soft polyvinyl alcohol (PVA) and rigid polyaniline (PANI) to form polyaniline-polyvinyl alcohol hydrogels (PPH, Figure 1). Boronic acid groups was covalently installed on PANI chain by copolymerization of 3-aminophenylboronic acid (ABA) and aniline (AN) (Figure 1a), which crosslink PANI and PVA through dynamic boronate bonds to give PPH (Figure 1b).24 Besides 2 ACS Paragon Plus Environment

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our PPH, Huang and coworkers also reported a CPH based on PANI and PVA,25 which was made by a 2-step procedure: aniline was polymerized in a PVA solution to form a suspension of PANI nanoparticles in the PVA solution; the suspension was then frozen and thawed to form hydrogel due to the gelation of PVA,26 with PANI nanoparticles physically trapped in the hydrogel.

Figure 1. (a) The synthesis of boronate-bearing polyaniline by co-polymerization of 3aminophenylboronic acid and aniline. (b) Polyvinyl alcohol (PVA) and boronate-bearing polyaniline (PANI) are crosslinked by boronate ester bond to form hydrogel. (c) The freezethaw cycles would reform the microstructure of polyaniline-polyvinyl alcohol hydrogel.

It is known that freeze-thaw (FT) cycles can cause gelation of PVA solution and improve the mechanical properties of PVA hydrogels.26 Typically, PVA solutions or hydrogels are frozen at -20 °C and then thawed back to room-temperature for several times. Crystallization of PVA and phase seperation occur during the FT process, which are referred as the primary

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mechanism responsible for the gelation and enhanced mechanical properties.27 On the other hand, previous studies on freeze casting have demonstrated that solid nanoparticles can be forced to accumulate between the growing ice microcrystals to form a continuous 3D network.28 During the water freezing process, a large internal mechanical force is generated and applied onto the nanoparticles due to the volume expansion of ice. In our PPH, PANI and PVA are crosslinked by boronate bonds to form polymer nanoparticles,24 which could be mechanically reformed by the internal force generated during water freezing process. Based on these two phenomena during FT process: crystallization of PVA and re-organization of nanoparticles, we propose that FT process could reform the structure of PPH and further improve its properties (Figure 1c). In fact, we have found that FT process can generate a spongy-bone-like microstructure inside PPH, leading to significantly enhanced mechanical and electrochemical properties. Comparing with the PPH without FT processing (referred as PPH-0), the PPH after 5 FT cycles (referred as PPH-5) showed superior tensile strength (16.3 MPa vs 5.3 MPa), better flexibility (elongation at break of 407% vs 250%) and larger electrochemical capacitance (1053 F·g-1 vs 928 F·g-1). As another comparison, Huang's CPH with PANI nanoparticles physically trapped in PVA hydrogel showed a moderate capacitance in the range of 100-200 F/g.25 The flexible SC based on PPH-5 provided a large capacitance (420 mF·cm-2 and 210 F·g-1) and high energy density (18.7 Wh·kg-1), which is superior to the SC based on PPH-0 (306 mF·cm-2, 153 F·g-1, 13.6 Wh·kg-1).24 The robustness of the PPH-5 based SC was demonstrated by its 100% capacitance retention after 1000 galvanostatic charge-discharge cycles or 1000 mechanical folding cycles. To the best of our knowledge, the performance of PPH-5 based SC is superior to that of other hydrogel-based SCs,18-20 and among the best of flexible SCs based on polyaniline (see Table S1 and S2 for detailed comparison).10, 14, 29

2．EXPERIMENTAL SECTION 4 ACS Paragon Plus Environment

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2.1. Materials. Aniline (AN), ammonium persulfate (APS) and polyvinyl alcohol (99% hydrolyzed, degree of polymerization was 1750) were analytical grade and purchased from Shanghai Chemical Reagent Co. Ltd. 3-Aminophenylboronic acid hydrochloride (ABA) was purchased from Sigma-Aldrich. Aniline was distilled under reduced pressure before use. All the other chemicals were used as received without further purification. Carbon cloths (CC, thickness: 0.36 mm) were purchased from Shanghai HESEN Electric Co., Ltd., China. Polyester mesh (thickness: 55 µm) was purchased from Spectrum Labs.

2.2. Preparation of PPH-0, PPH-3 and PPH-5: PPH was synthesized according to the literature.24 In a typical process, solution A was prepared by dissolving 2 mmol APS in 1 mL deionized (DI) water. 1.5 mmol aniline, 3 mL polyvinyl alcohol solution (10% wt/wt in DI water) and 0.105 mmol 3-aminophenylboronic acid hydrochloride (ABA) were dissolved in 835 µL 6 M HCl and 225 µL DI water to form solution B. Solution A and B were cooled to 0 o

C in an ice-water bath. Add 808 µL solution A to solution B, mix them quickly and react for

6 h. In this reaction system, the molar ratio of APS to (ABA+AN) was 1:1, and the final concentration of HCl was 1 M. For the freeze and thaw cycle process, PPH sample was frozen at -20 oC for 9 h and thawed at room temperature for 3 h, which is referred as one FT cycle. PPH-0, PPH-3 and PPH-5 refer to PPH samples that were treated through 0, 3, 5 FT cycles, respectively. All samples were immersed in DI water for 24 h to remove excess ions, byproducts and excess PVA.

2.3. Preparation of PPH electrodes: Hydrophilic carbon cloth was prepared by immersing commercial carbon cloth in concentrated HNO3 at room temperature for 3 days, followed by thorough washing with water and ethanol. As described in PPH preparation method, solutions A and B were mixed at 0 oC and immediately applied onto the hydrophilic carbon cloth (coating area of 0.5 cm×0.5 cm) and react for 6 h. For the freeze and thaw cycle process, PPH electrodes were frozen at -20 oC for 9 h and thawed at room temperature for 3 h, as one FT cycle. PPH-0, PPH-3 and PPH-5 electrodes refer to electrodes that were treated through 0, 3 5 ACS Paragon Plus Environment


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and 5 FT cycles, respectively. All electrodes were immersed in DI water for 24 h to remove excess ions, by-products and excess PVA.

2.4. Preparation of PPH-based flexible solid-state supercapacitors: H2SO4/PVA electrolyte was prepared by dissolving 1 g PVA in 10 mL 10% H2SO4 solution at 85°C, and coated onto the surface of PPH electrodes (coating area of 2 cm×1 cm). A piece of polyester mesh (serving as the separator) saturated by H2SO4/PVA electrolyte and two same sample electrodes coated with H2SO4/PVA electrolyte were sandwiched together to give the flexible solid-state SCs.

2.5. Characterization of PPH samples. The morphology of PPH samples was examined by using SEM (JSM-6700F) at an acceleration voltage of 5 kV. The chemical structure of the dehydrated PPH samples were analyzed by FTIR on a Thermo Scientific OMNIC spectroscopy (Nicolet iS5) from 4000 to 400 cm-1 at room temperature. The Raman spectra of PPH samples were recorded on an Olympus Optical Raman Microscope employing a 514 nm laser beam, by using a 50× objective at a low power (0.5 mW) and accumulated two times for 60 s each. X-Ray diffraction (XRD) data were measured by a Philips X'Pert Pro Super X-ray diffractometer equipped with graphite monochromatized Cu K radiation. The specific surface area of PPH was examined by N2 sorption analysis on BELSORP-Max.

2.6. Mechanical test. The tensile and compression properties of all PPH samples were measured by using an Instron 3342 Universal Tester. For tensile measurement, all PPH films were cut into stripes with 3.5 mm wide, 10 mm long (the effective length is 5 mm) and 40 µm thick. Tensile measurements were performed by uniaxially stretching the stripes at a strain rate of 50 mm·min-1. In the compression tests, all samples were mold into cylindrical samples with 9 mm diameter and 5 mm thickness, which were set on the lower plate and compressed by the upper plate at a strain rate of 2 mm·min-1. The compression tests were stopped when reaching the upper limit of stress sensor (100 Newton), while all samples were not broken.


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2.7. Electrochemical characterization. The electrochemical performances of all PPH electrodes were characterized by cyclic voltammogram (CV), galvanostatic charge–discharge (GCD) tests, and electrochemical impedance spectroscopy (EIS) measurements in a threeelectrode system by using a CHI 660E electrochemical workstation. The working electrode, reference electrode and counter electrode were PPH electrode (coating area of 0.5 cm×0.5 cm), an Ag/AgCl electrode (Shanghai Yuphoton Optoelectronics Co., Ltd.) and a Titanium plate, respectively. The electrolyte was 1 M H2SO4 aqueous solution. CV tests were performed in the potential range of -0.2 to 0.8 V vs. Ag/AgCl under a scan rate of 10-100 mV·s−1. GCD tests were performed by scanning from 0 to 0.8 V at current densities from 0.5 A·g-1 to 20 A·g-1. EIS measurements were done with a frequency range of 100 kHz to 0.01 Hz with an alternate voltage amplitude of 5 mV at an open-circuit potential. The charge-discharge cyclic stability of PPH electrodes was carried by GCD tests with 1000 cycles at 20 A·g-1. The specific capacitance (Cp) (F·g-1) of all electrodes were calculated from their GCD curves and derived from the equation (1): Cp = I × t / (m × V)

(1)

where I, t, m and V are the discharge current, discharge time from GCD curves, the mass of active electrode material and the voltage change upon discharging (excluding the IR drop), respectively. The electrochemical performances of flexible solid-state supercapacitors (coating area of 2 cm × 1 cm) were evaluated in a two-electrode system by using a CHI 660E electrochemical workstation. CV tests were conducted at scan rates from 5 to 100 mV·s-1. GCD tests were conducted at current densities from 0.25 to 2.5 A·g-1. EIS measurements were conducted in the frequency range of 0.01 Hz to 100 kHz with an alternate voltage amplitude of 5 mV. The charge-discharge cyclic stability of these flexible solid-state supercapacitors was carried by GCD tests through 1000 cycles at 2.5 A·g-1. The flexibility of these supercapacitors was examined by measuring the CV performances when the supercapacitors were folded to 7 ACS Paragon Plus Environment


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different bending angles. The robustness of these supercapacitors was examined by measuring the specific capacitance during 1000 mechanical folding cycles. The cell specific capacitance (Ccell) (F·g-1) of all supercapacitors devices were calculated from their GCD curves according to the equation (2): Ccell = I × t / (M × V)

(2)

where I, t, M and V are the discharge current, discharge time from GCD curves, the mass of two pieces of active electrode material and the voltage change upon discharging (excluding the IR drop), respectively. The energy density and power density of the supercapacitors were obtained based on Equations (3) and (4), respectively: Ecell = (1/2) × Ccell × V2

(3)

Pcell = Ecell / t

(4)

where Ecell, Ccell, V, Pcell and t are the energy density, cell specific capacitance, the voltage change upon discharging (excluding the IR drop), power density and discharge time, respectively. 3．RESULTS AND DISCUSSION

3.1. Mechanical properties of PPH samples. Three PPH samples were prepared in parallel with the same water content of 70%. One sample was purified as usual and noted as PPH-0; the other two was further treated by 3 or 5 cycles of FT process, and noted as PPH-3 or PPH-5, respectively. The mechanical properties of these PPH samples were studied by tensile and compressive tests (Figure 2a and 2b), which were summarized in Table 1. The tensile modulus, tensile strength and elongation at break of PPH-3 and PPH-5 were significantly enhanced over that of PPH-0.


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Figure 2. Mechanical properities of PPH samples. (a) The tensile stress-strain curves and (b) compression stress-strain curves of PPH-0, PPH-3 and PPH-5 samples. (c) Photographs of PPH-5 samples being bent or stretched.

Table 1. Mechanical properties of PPH samples. sample

Tensile modulus (MPa)

Tensile strength (MPa)

Elongation at break

PPH-0

27.9

5.3

250%

PPH-3

30.4

12.3

340%

PPH-5

40.6

16.3

407%

Under the compressive test, the PPH-0 sample can sustain a large strain of 98% and a high stress of 30 MPa without being broken. The PPH-3 and PPH-5 samples were clearly more resistant to compression than the PPH-0 sample, which reach the same stress of 30 MPa at a smaller strain of 95.6% and 92.6% (Figure 2b), respectively. Taking PPH-5 as one example (Figure 2c), it can be bent to 180° or stretched to 300% elongation, and returns to the initial shape when the stress is relieved. These tensile and compressive test results demonstrate that FT cycles can greatly enhance the mechanical properties of PPH. The tensile strength of these PPH samples is much higher than that of previously reported hydrogels (typically in the range of 10-2-100 MPa).30-32 Particularly, the tensile strength of PPH-3 and PPH-5 is comparable to that of vulcanized natural rubber,33 despite the 70% water content in these PPH samples. The reason of the exceptional strength of PPH samples after FT cycles was attributed to the reformed microstructure of PPH samples, as discussed in section 3.3.



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Figure 3. Characterization of the chemical structure of PPH samples. (a) FTIR spectra, (b) Raman spectra and (c) XRD patterns of PPH-0, PPH-3 and PPH-5 samples.

3.2. Chemical structure of PPH samples. To probe the reason for the exceptional strength of PPH samples after FT cycles, we characterized the three PPH samples (PPH-0, PPH-3 and PPH-5) by using Infrared spectroscopy, Raman spectroscopy and X-ray diffraction (Figure 3). In IR spectra of PPH samples, the peaks at 1579 and 1491 cm-1 were assigned to the benzene ring vibration of PANI, and the peaks at 3230, 2911, and 1081 cm-1 were attributed to the vibrations of O-H, C-H, and C-O bonds of PVA, respectively. In Raman spectra of PPH samples, the peaks at 1602 and 1188 cm-1 were assigned to the benzene ring and quinoid ring vibration of PANI, and the peaks at 1340 cm-1 were attributed to the stretching vibrations of C-N bonds of PANI. In XRD patterns of PPH samples, the broad peak at 2θ around 20° and 26° were corresponding to PVA27 and PANI,24 respectively. As expected, there was little difference on the IR, Raman spectra and XRD patterns of these three PPH samples, which indicates that the FT process caused not much change on the chemical structure of PPH samples.


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Figure 4. SEM images show the microstructure of PPH-0, PPH-3 and PPH-5 samples.

3.3. Microstructure of PPH samples. SEM images of PPH-0 show a porous structure consisted by clustered nanoparticles (Figure 4). In contrast, SEM images of PPH-5 show a porous structure consisted by interconnected thin sheets (Figure 4), similar to that of spongy bone,34 which is obviously different from the microstructure of PPH-0. The reformation of microstructure is mainly attributed to the internal mechanical force generated during the water freezing process,28 which forces the globular nanoparticles (as observed in PPH-0) to fuse together to form compact thin nanosheets (as observed in PPH-5). At the meantime, PVA could serve as a perfect glue to bond these nanosheets together, which makes PPH-5 mechanically much stronger and tougher than PPH-0. The SEM images of PPH-3 show mainly compact nanosheets, with some clustered nanoparticles, which indicates that PPH-3 is an intermediate between PPH-0 and PPH-5. This structure feature of PPH-3 correlates well with the mechanical properties of PPH-3, which is also between that of PPH-0 and PPH-5 (Figure 2a). The SEM images of PPH samples on the carbon cloth (Figure S1) also show that the FT process reforms the microstructure of PPH samples, which is consistent with above results. The Brunauer-Emmett-Teller (BET) specific surface areas of dehydrated PPH-0, 11 ACS Paragon Plus Environment


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PPH-3 and PPH-5 (Figure S2) are 90.2 m2 g-1, 58.0 m2 g-1 and 41.1 m2 g-1. Since the nanoparticles in PPH-0 were gradually converted into compact nanosheets in PPH-3 and PPH-5, the specific surface area of PPH samples decreases upon multiple FT treatments.

Figure 5. The electrochemical performance for PPH-0, PPH-3 and PPH-5 electrodes. (a) CV curves at the scan rate of 10 mV·s-1. (b) Impedance plot in the frequency range of 0.01Hz to 100 kHz. (c) GCD curves at the current density of 0.5 A·g-1. (d) and (e) Speciﬁc capacitance and areal capacitance at different current densities. (f) The capacitance retention during GCD cyclic test at a current density of 20 A·g-1.

3.4. Electrochemical Properties of PPH electrodes. The electrochemical properties of these PPH electrodes with the same polyaniline loading (2 mg·cm−2) was investigated in 1 M H2SO4 electrolyte by using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge (GCD) methods in a three-electrode system (Figure 5). The CV curves of three PPH samples measured at 10 mV·s-1 are shown in Figure 5a. Two pairs typical PANI redox peaks are observed, which are attributed to the conversion between three redox states of PANI. It is notable that the redox peaks for the PPH5 electrode are closer to each other than those for the PPH-0 electrode and PPH-3 electrode 12 ACS Paragon Plus Environment

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(Figure 5a), which implies a more reversible redox kinetics for the PPH-5 electrode. Based on the CV area, the specific capacitance of PPH-5 and PPH-3 electrodes is 25% and 15% larger than that of the PPH-0 electrode, respectively. The CV curves of all three PPH samples measured at 20-100 mV·s-1 measured are shown in Figure S3a-c, while PPH-5 electrode shows the largest capacitance at all scan rates. Figure 5b shows the Nyquist plots of PPH-0, PPH-3 and PPH-5 electrodes. The equivalent series resistances (Rs) of PPH-0, PPH-3 and PPH-5 electrodes are similar (0.86 Ω·cm2), indicating a low ohmic contact between the electrode and electrolyte. The charge transfer resistance (Rct) was calculated as 1.15 Ω·cm2, 0.92 Ω·cm2 and 0.47Ω·cm2 for PPH-0, PPH-3 and PPH-5 electrodes, respectively. The decreasing Rct value with increasing FT cycles clearly indicate the important effect of FT cycles on facilitating the ion transfer and ion exchange process during charge-discharge cycles. Figure 5c and Figure S3d-f show the charge-discharge curves at a current density of 0.5-5 A·g−1, where the PPH-5 electrode displays a significantly larger capacitance than the PPH-3 and PPH-0 electrodes. The specific capacitance and areal capacitance of PPH-0, PPH-3 and PPH-5 electrodes are calculated based on the GCD data at different current densities are shown in Figure 5d-e. Particularly, the specific capacitance of PPH-0, PPH-3 and PPH-5 electrodes at 0.5 A·g−1 are 928 F·g−1 (1856 mF·cm−1), 997 F·g−1 (1994 mF·cm−1) and 1053 F·g−1 (2106 mF·cm−1), which is considerably higher than that of previous reported PANI-based electrodes.10 The full CV and GCD curves of PPH-3 and PPH-5 electrodes are shown in Figure S4 and S5. All different electrodes show good rate performance, with 84% (PPH-0 and PPH-3) and 82% (PPH-5) capacitance retention, when the current density increases from 1 A·g−1 to 10 A·g−1. Their performance is better than previous reported PANI based electrodes, which typically show ~60% capacitance retention at a 10-fold higher current density.35 As shown in Figure 5e, PPH-5 and PPH-3 electrodes show a very good stability, with 90% and 89% capacitance retention after 1000 GCD cycles, which is better than that of PPH-0 electrode (86% 13 ACS Paragon Plus Environment


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capacitance retention). In addition, the Coulombic efficiency of PPH-3 and PPH-5 electrodes remains 100% during the GCD cycling test (Figure S4d and S5d). The electrochemical performance of PPH-3 and PPH-5 electrode is significantly enhanced over that of PPH-0 electrode, which is attributed to the compact nanosheets structure in PPH3 and PPH-5. The FT process converts globular nanoparticles in PPH-0 into thinner nanosheets in PPH-3 and PPH-5, which would facilitate ion exchange by decreasing ion diffusion distance during charge-discharge cycles, leading to enhanced specific capacitance for PPH-3 and PPH-5. The low stability of PANI-based hydrogel during charge-discharge cycles is typically attributed to the damage of hydrogel microstructure caused by PANI volume change upon the doping/dedoping of PANI.24 The interconnected nanosheets structure of PPH-3 and PPH-5 is similar to that of spongy bone, which can transfer stress and avoid stress concentration,34 and would reduce the damage of hydrogel microstructure during the charge-discharge cycles. This result indicates the beneficial effect of microstructure reformation on enhancing the electrochemical performance of PPH electrodes.


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Figure 6. Electrochemical characterization of PPH-0, PPH-3 and PPH-5 based solid-state SCs. (a) The fabrication of solid-state SCs. (b) CV curves at a scan rate of 5 mV·s−1. (c) Impedance plot in the frequency range of 0.01 Hz to 100 kHz. (d) GCD curves at the current density of 0.25 A·g-1. (e) Specific capacitance of the SCs at different discharge current densities. (f) Capacitance retention after GCD cycling tests at 2.5 A·g-1. (g) The energy density vs power density profile.

3.5. Electrochemical Properties of PPH based supercapacitors. As shown in Figure 6a, two pieces of PPH electrodes were assembled into solid-state flexible supercapacitors using a PVA-H2SO4 gel electrolyte and a polyester mesh separator film. The CV curves (Figure 6b and Figure S6) at 5 mV·s-1 and 10-20 mV·s-1 for PPH-0, PPH-3 and PPH-5 based solid-state SCs present essentially similar and symmetric shape, suggesting the good capacitive behavior of all the PPH based SCs. Meanwhile, the increase areas of CV curves demonstrate that PPH5 and PPH-3 based SCs possesses a large capacitance of than that of the PPH-0 based SC. 15 ACS Paragon Plus Environment


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Figure 6c show EIS curves of PPH-0, PPH-3 and PPH-5 based SCs. In the low frequency region, more vertical tail of the PPH-3 and PPH-5 based SCs implies a better capacitive character of these SCs than the PPH-0 based SC. Figure 4d and Figure S7b shows the chargedischarge curves at 0.25 A·g−1 and 0.5 A·g−1, with minimal IR drop. The specific capacitance and areal capacitance of PPH-0, PPH-3 and PPH-5 based SCs at different current densities are shown in Figure 6e and Figure S7a. At 0.25 A·g−1, the specific capacitance of PPH-0, PPH-3 and PPH-5 based SCs are 153 F·g−1 (306 mF·cm−2), 188 F·g−1 (376 mF·cm−2) and 210 F·g−1 (420 mF·cm−2), respectively. The specific capacitance of PPH-3 and PPH-5 based SCs is improved by 23% and 37% compared to that of PPH-0 based SC. The rate performance (retention of capacitance when current density was increased by a factor of 10) of PPH-0, PPH-3 and PPH-5 based SC are 87%, 88% and 92.3%, respectively, where PPH-5 based SC provides the best rate performance. The full CV and GCD curves of PPH-3 and PPH-5 based SCs are shown in Figure S8a-b and Figure S9a-b. Cyclic test with 1000 GCD cycles for all SCs are shown in Figure 6f. PPH-5 based SC showed a small capacitance increase at the beginning and then remained stable. PPH-3 and PPH-0 based SC showed 5.4% and 10% capacitance loss after 1000 GCD cycles. In fact, PPH-5 based SC can retain its capacitance with no loss after 2000 GCD cycles (Figure S10), indicating the enhanced electrochemical stability of PPH-5 by FT processing. The electrochemical stabiliy of PPH-based SCs is higher than PPH in the three-electrode system. The improvement of electrochemical stability of PPH-based SCs is due to the quasi-solid-state gel electrolytes, which can protect the active PPH and avoid the PPH delamination from the current collector. Figure 6g and Figure S7b show the Ragone plots of all SCs. PPH-5 based SC provides the highest energy density of 18.7 Wh·kg-1 at current density of 0.25 A·g−1, which are significantly higher than most of solid-state SCs. The CV curves of PPH-3 and PPH-5 based SCs (Figure S8c and Figure S9c) obtained at various bending angles of 0o, 90o and 180° coincide with each other, which demonstrate the excellent flexibility of these SCs. After 1000 cycles of folding, the specific 16 ACS Paragon Plus Environment

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capacitance of PPH-3 and PPH-5 based SCs retains 100% of their initial values (Figure S8d and Figure S9d), and the SEM image (Figure S11) of PPH-5 after 1000 mechanical folding cycles shows that the mechanical folding caused not much change on the microstructure of PPH-5, which proves the great robustness of these SCs. 4．CONCLUSION In summary, we have demonstrated a simple post-synthesis physical approach of freezethaw cycles can reform the microstructure of conductive polymer hydrogels, leading to significantly enhanced mechanical and electrochemical properties. Particlarly, the polyaniline-polyvinyl alcohol hydrogel after 5 FT cycles (PPH-5) showed a remarkable tensile strength (16.3 MPa), good flexibility (elongation at break of 407%) and high electrochemical capacitance (1053 F·g-1). The flexible supercapacitor based on PPH-5 showed a large capacitance (420 mF·cm-2 and 210 F·g-1) and high energy density (18.7 Wh·kg-1), whose robustness was confirmed by its 100% capacitance retention after 1000 galvanostatic charge-discharge cycles or after 1000 mechanical folding cycles. The remarkable performance enables PPH-5 based SC a promising power device for flexible electronics, underscoring the merit of freeze-thaw cycles for enhancing the performance of hydrogel-based materials.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx xxx. The characterization of PPH samples and the electrochemical performance of PPH electrodes and PPH-based supercapacitors, Figure S1-S11, Table S1-S2. AUTHOR INFORMATION

Corresponding Authors E-mail: [email protected], [email protected]. 17 ACS Paragon Plus Environment


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Notes The authors declare no competing ﬁnancial interests.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21474094, 81401531), the Natural Science Foundation of Anhui Province (1508085QH154).

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