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Self-Recovering Tough Gel Electrolyte with Adjustable Supercapacitor Performance Xinhua Liu, Dongbei Wu, Huanlei Wang, and Qigang Wang* Flexible devices are a mainstream direction in modern electronics and related multidisciplinary fields.[1] Concerning flexible capacitors and batteries, the current research is mainly focused on the fabrication of flexible electrode materials;[2] however, electrolyte development is also a critical factor for attaining highly flexible devices.[3] The development of flexible energy devices even soft robots has quickly increased the requirement of soft electrolyte with mechanical robustness and facile ion or solute transport.[4] Ionic conducting gels are emerging as the main building blocks for these functional devices due to their high conductivity, semi-solid state and flexiblility.[5] In general, compressive pressure is a convenient source of mechanical deformation for an electric device. Sensitive flexible pressure sensors, such as electronic skins, have been fabricated via the conversion of pressure to electrical signals through various transduction methods, like capacitance, piezoelectricity, optics, and wireless antennas.[6] The electrochemical behaviors and electrode/electrolyte interfacial properties in supercapacitors with aqueous electrolyte solutions also have been investigated by applying pressure and tuning ionic size.[7] However, a fundamental understanding of pressure effect on the performance of a gel electrolyte based supercapacitor has not been tackled. Since the correlation between the pressure and the electrochemical performance is very important for guiding the packaging process and exploring the pressure-adjustable energy storage devices, gel electrolytes should have enough compressive pressure and flexibility to fabricate the desired flexible devices.[8] Current ionic conducting gels, which mainly consist of polyvinyl alcohol hydrogels and polymer ion liquid gels prepared by simple mixing or the covalent polymerization of vinyl polymer, are brittle and exhibit poor mechanical strength. Thus, these polymers cannot exhibit any pressure-dependent performance as the electrolyte in an energy storage device because they become damaged after coming into contact with a high compressive pressure. Recently, supramolecular materials have demonstrated a superior self-healing ability and high
X. Liu, Dr. D. Wu, Prof. Q. Wang Department of Chemistry and Advanced Research Institute Tongji University Shanghai 200092, P. R. China E-mail: [email protected] Dr. H. Wang Institute of Materials Science and Engineering Ocean University of China Qingdao 266100, P. R. China
DOI: 10.1002/adma.201400240
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mechanical strength due to their non-covalent interactions.[9] Tough and self-healable hydrogels have been prepared by the multiple supramolecular interactions of clay nanosheets with dendritic polymers or in situ-formed polymer.[10] Many workers have studied ionic gels formed by the supramolecular effect of ionic liquids and block copolymers with/without SiO2 nanofiller, which also demonstrate the high mechanical modulus.[11] Here, we employ a mild self-initiated UV polymerization to prepare an ionic conducting polymer gel, whose non-covalent crosslinking interaction can endow the conducting gel compressive toughness and self-recovering ability. Our system consists of four components: 1-ethyl-3-methylimidazolium chloride (EMIMCl), hydroxyethyl methacrylate (HEMA), chitosan (CS) and water, whose molecular structures have been shown in Figure 1A. At first, CS and HEMA are dissolved in EMIMCl via heating and cooling process, which forms a viscous solution from the solid of EMIMCl (Figures S1a and 1C). The destroy of crystal structure of EMIMCl is the main reason to form such homogeneous solution, which is caused by the dissolution of CS and HEMA through hydrogen bond.[12] Then, tough EMIMCl gels can be formed by irradiating the asprepared homogeneous solution with UV light (Figures S1b and S2; Figure 1D). The UV-generated Cl radicals from EMIMCl (Figure 1B) are the initiator for the polymerization of HEMA.[13] The physical gelation of our EMIMCl gel without crosslinker should be ascribed to the hydrogen bonding interactions between the hydroxyl group in PHEMA, the amine group of CS and imidazolium group in solvent. Interesting is that our EMIMCl gel is miscible with water, which has a slightly decrease strength but a sharply increase ionic conductivity (Figure 1E). As shown in Figure S3, water exists in the form of free water and bound water, and is relatively stable at room temperature (Figure S4). The final EMIMCl/water gel exhibits the water and pressure controlled ionic conductivity, which is suitable as an appropriate electrolyte and separator in flexible supercapacitors. The detailed preparation, mechanism and properties are shown as followed. The EMIMCl gels can be formed by irradiating the as-prepared homogeneous solution under 45 minutes UV light with intensity of 22.4 mw/cm2 at 365 nm. The component ratio of the conducting gel is optimized by the comprehensive evaluation of the electrochemical performances and mechanical strength. Finally, a varying amount of water was injected into the EMIMCl gel to form EMIMCl/water gels with 5 to 50 wt% (by mass) water content (Figure 1E). If the water was added in advance, the final gel could not be obtained. The EMIMCl/water gel shows high toughness when they underwent large deformations and show good strength under compression (Figure 2A), puncture (Figure S5a), and stretch (Figures 2B and S5b). Our
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COMMUNICATION Figure 1. A) Molecular structures of EMIMCl, HEMA, and CS. B) the self-initiated mechanism under UV irradiation. C) The precursor’s solution of three components. D) The EMIMCl gels formed by the UV irradiation. E) The EMIMCl/water gels after injecting water.
gel electrolyte has about 200 times enhancement in compressive strength comparing with the common used PVA/H3PO4 gel electrolytes (Figure S6). The tough gels can be used to fabricate sandwiched structure supercapacitors with activated charcoal electrodes (Figure 2C). A morphological analysis of the cryo-dried sample using scanning electron microscopy (SEM) indicates that porous microstructures occur ubiquitously in the EMIMCl gels and EMIMCl/water gels. It can be observed that the pore size increases from 1–2 µm to 4–10 µm in diameter after the addition of water (Figures 2D and 2E). The greater
swollen network in gels thus enables lower strength and higher ionic conductivity. This polymerization of HEMA and further gelation is enabled by a self-initiated mechanism. According to literatures,[14] the photo-induced ionization of the alkylimidazolium type ILs yields two types of primary trapped-charge species: trapped holes and trapped electrons. The holes (electron deficiencies) are preferentially trapped by the constituent anions (Cl−), yielding the corresponding neutral Cl radicals (Cl•). The electrons are possible entrapped by various EMIM cation and cavities in ionic
Figure 2. A) Compression demo of EMIMCl/water gel. B) Tensile property of EMIMCl/water gel. C) The sandwiched structure of the symmetrical cell of a supercapacitor with EMIMCl/water gel and activated charcoal electrodes. SEM images of the composite EMIMCl/water gels after supercritical drying D) without water and E) with 20 wt% water.
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Figure 3. A) Compressive test of the EMIMCl/water gels with 0 to 50 wt% water content. B) Compression recovery of the composite EMIMCl/water gel with 20 wt% water through 500 cycles with 80% strain. C) The ionic conductivities of EMIMCl/water gels with various water contents. D) The ionic conductivities of EMIMCl/water gel with 20 wt% water under various strains.
liquid. The oxidative Cl• can be added to the monomer to form the starting carbon radical. Figure 1B demonstrates the UV photolysis of from EMIMCl to form Cl radical and the solvated electron. The existence of the chlorine radical has been proven by electron paramagnetic resonance (EPR) spectroscopy after alpha-phenyl-tert-butyl-nitrone (PBN) adduction (Figure S7). The successful radical polymerization has been proved by the long-lived propagating radicals observed in the EPR spectrum of the UV irradiated precursors (Figure S8). The final conversion of the monomer is more than 95% after 45 min of UV irradiation (Figures S9 and S10), and the molecular weight distribution of PHEMA is about 104–106 (Figure S11). The mechanical properties of our gels are shown in Figure 3A. The compressive strength of the EMIMCl gel with 17 wt% HEMA and 3 wt% CS is 1706 kPa at 95% strain. On the contrary, the EMIMCl gel with 20 wt% HEMA and 0% CS can achieve 361 kPa compressive strength. The hydrogen bonding interaction of CS can reinforce the 3-dimensional (3-D) network of the gel and improve the final compressive strength. The EMIMCl/water gels exhibit a slow expansion relative to a nonhydrated EMIMCl gel (Figure S12). The swollen volume ratio of EMIMCl/water gel with 50 wt% water content is about 1.52 times than that of the pure EMIMCl gel. As a result, the value of compressive strength decreases from 1706 kPa to 1290 kPa as the amount of water is increased from 0% to 50 wt%, and the Young’s modulus decreases from 28 kPa to 14 kPa, respectively (Table S1). The swollen effect and the enlarged network in SEM picture should be the reason of their slightly low compressive strength relative to the waterless one. As shown in Figure 3B, this gel exhibits a rubber-like recovery when subjected to a fatigue cyclic compression test (at ε = 80%) with at least 500 successive loading/unloading cycles. The
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thickness reduction after the fatigue test is only approximately 10.5%. The gelation via hydrogen bond endows them with high strength and self-recovering ability, thus ensuring adjustable pressure. Ionic conductivity was determined from the high frequency plateau of the real part of the complex conductivity (Figures S13 and S14). The experimental details about this in-situ conductivity measurement are illustrated in Figures S15 and S16. The conductivity of the electrolyte is calculated according to the Equation σ = l/(ARb), where Rb is the resistance of the bulk electrolyte, l is the thickness of the gel and A is the area of electrode covered by gel.[15] Figure 3C presents the plots of the ionic conductivities of EMIMCl/water gels versus various water contents. The ionic conductivity significantly increases from 0.4 mS/cm to 23.2 mS/cm after the addition of 50 wt% H2O. According to liteatures,[16] the conductivity (σ) of an electrolyte given by σ = nqµ, depends upon the carrier concentration (n) and mobility (μ). The ion mobility is inversely related to the viscosity (η) of the electrolyte as μ = q/6πrη (q is the electronic charge on each charge carrier; r is the effective hydrodynamic radius). The water-indcued enhancement of ionic conductivity is due to the followed reasons: 1) The addition of water to the gel electrolyte results in a decrease in the viscosity (as shown in Figure S17), which increases mobility and hence conductivity. 2) The more swollen network of the EMIMCl/water gel proved by SEM image can provide the larger conducting pathways for ion transport.[17] The compressive stress also has a positive effect on the conductivity of EMIMCl/water gels. As shown in Figure 3D, the ionic conductivity quickly increases from 19.4 mS/cm to 40.1 mS/cm for an EMIMCl/water gel with 20 wt% water under 90% compression. The pressure-induced enhancement
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obtained from the cyclic voltammetry (CV) curves at 10 mV/s. Figure 4B shows an increase in the Csp value from 9.0 F/g to 43.5 F/g as the water content from 0 to 50 wt%. The improved performance is determined by its high ion mobility due to the adding water and convenient ion transferring from electrolyte to electrode. Amazing, the shape of the CV curves shown in Figure 4A becomes further rectangular if the 20 wt% water gel-based device ensures 90% compression. Since the specific capacity changed slightly at various thickness of gels (Figure S26), the influence of the thickness during compression can be ignored in our devices. The pressure-induced low viscosity and the pressure-accelerated ionic transfer should be the two reasons of the excellent performance of supercapacitors under compression. On one hand, the decreasing viscosity of gel under pressure can lead to the higher ionic mobility in gel electrolyte. On another hand, the applied pressure between the electrodes can improve the performance of the capacitor due to the increasing ionic transferring between electrode/electrolyte interfaces. Wei’s group has reported that the increasing pressure can significantly increase performance of supercapacitor with liquid electrolyte due to the same reason.[7] In one word, the compressive stress can increase the ionic mobility within gel electrolyte and the ionic transferring from electrolyte to the surface of porous carbon, which both contribute to their higher capacities. Inspired by this result, we prepared tight supercapacitors under 90% compression with EMIMCl/water gels having 0 to 50 wt% water as the flexible electrolyte and separator. The selfrecovering ability makes our gels suitable for this type of application. The galvanostatic charge–discharge curves are shown in Figure 4C. The conducting gel-based supercapacitors
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of ionic conductivity is due to the change of viscosity. Since the ion mobility is inversely related to the viscosity (η) of the electrolyte as µ = q/6πrη, the decrease in the viscosity (Figure S18) can increase mobility and ionic conductivity under compressive strain. The gel state can remain even under 90% compression with a decreased storage modulus and viscosity (Figures S19 and S20). The decreasing storage modulus of gel under compressive strain is the main reason of the decreased viscosity. The pressure-induced weaken of storage modulus is due to the possible breaking of the non-covalent bonds. The ionic conductivity under a certain compression can be permanent even within 48 hours (Figure S21), which reflects the remained gel structure against compression. The ionic conductivity can be changed with the adjusting pressure. As shown in Figure S22, the compressive stress and viscosity are recoverable during a compression-release programmed procedure, which is determined by the self-recovering gel with the reversible non-covalent bonds. At the same time, ionic conductivity of the gel electrolyte can also be enhanced through elongation (Figure S23). Therefore, the as-prepared EMIMCl/water gels with adjustable ionic conductivity and self-restoring mechanical property could be considered an excellent electrolyte candidate for ideal electrochemical performance. Figure 4A demonstrates that the conducting gel-based supercapacitor with 20 wt% water exhibits larger CV curves than those without water. The electrode material of our supercapacitor is the commercial activated charcoal (YP 80F, 2100 m2/g) with 0.8–1.35 nm microspore;[18] which has size matching with the EMIM+ ions (DEMIM+) of 0.76 nm (Figure S24). The corresponding specific capacitances (Csp) (Figure S25) calculated using the charge-discharge profiles agree well with those
Figure 4. A) Cyclic voltammetry curves tested at 10 mV/s from 0 to 90% compression of EMIMCl/water gel with 20 wt% water. B) The specific electrode capacitance of supercapacitors with different water content under 0 and 90% compression. C) Galvanostatic charge–discharge curves with water content from 0 to 50 wt% under 90% compression. D) The Csp values of an EMIMCl/water gel based supercapacitor with 20 wt% water under various compressive stresses.
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demonstrate ideal electrochemical performance and comparable Csp values, when the water content is greater than 20 wt%. This indicates that 20 wt% water content in EMIMCl/ water gel is enough to ensure fast ion transport and diffusion at 10 mV/s. Therefore, the gel with 20 wt% water is selected to fabricate the flexible supercapacitor for the latter electrochemical test. It is notable that the gel-based supercapacitor (20 wt% water) has a high capacitance of 136 F/g at 0.5 A/g, which is much higher than the value of 17 F/g at 0.1 A/g for the anhydrous device measured. Under pressure, higher water content in EMIMCl/water gels for supercapacitors still provide bigger Csp. As shown in Figures 4B and 4C, the Csp value under 90% strain increases from 13 F/g to 140 F/g with water amount from 0 to 50 wt%. Figure 4D shows the Csp values of 20 wt% water gel-based supercapacitor increase with the corresponding compressive stress. Remarkably, there is a significant improvement in the specific capacitance from 43 F/g to 98 F/g during the early stage (strain
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Self-Recovering Tough Gel Electrolyte with Adjustable Supercapacitor Performance Xinhua Liu, Dongbei Wu, Huanlei Wang, and Qigang Wang* Flexible devices are a mainstream direction in modern electronics and related multidisciplinary fields.[1] Concerning flexible capacitors and batteries, the current research is mainly focused on the fabrication of flexible electrode materials;[2] however, electrolyte development is also a critical factor for attaining highly flexible devices.[3] The development of flexible energy devices even soft robots has quickly increased the requirement of soft electrolyte with mechanical robustness and facile ion or solute transport.[4] Ionic conducting gels are emerging as the main building blocks for these functional devices due to their high conductivity, semi-solid state and flexiblility.[5] In general, compressive pressure is a convenient source of mechanical deformation for an electric device. Sensitive flexible pressure sensors, such as electronic skins, have been fabricated via the conversion of pressure to electrical signals through various transduction methods, like capacitance, piezoelectricity, optics, and wireless antennas.[6] The electrochemical behaviors and electrode/electrolyte interfacial properties in supercapacitors with aqueous electrolyte solutions also have been investigated by applying pressure and tuning ionic size.[7] However, a fundamental understanding of pressure effect on the performance of a gel electrolyte based supercapacitor has not been tackled. Since the correlation between the pressure and the electrochemical performance is very important for guiding the packaging process and exploring the pressure-adjustable energy storage devices, gel electrolytes should have enough compressive pressure and flexibility to fabricate the desired flexible devices.[8] Current ionic conducting gels, which mainly consist of polyvinyl alcohol hydrogels and polymer ion liquid gels prepared by simple mixing or the covalent polymerization of vinyl polymer, are brittle and exhibit poor mechanical strength. Thus, these polymers cannot exhibit any pressure-dependent performance as the electrolyte in an energy storage device because they become damaged after coming into contact with a high compressive pressure. Recently, supramolecular materials have demonstrated a superior self-healing ability and high
X. Liu, Dr. D. Wu, Prof. Q. Wang Department of Chemistry and Advanced Research Institute Tongji University Shanghai 200092, P. R. China E-mail: [email protected] Dr. H. Wang Institute of Materials Science and Engineering Ocean University of China Qingdao 266100, P. R. China
DOI: 10.1002/adma.201400240
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mechanical strength due to their non-covalent interactions.[9] Tough and self-healable hydrogels have been prepared by the multiple supramolecular interactions of clay nanosheets with dendritic polymers or in situ-formed polymer.[10] Many workers have studied ionic gels formed by the supramolecular effect of ionic liquids and block copolymers with/without SiO2 nanofiller, which also demonstrate the high mechanical modulus.[11] Here, we employ a mild self-initiated UV polymerization to prepare an ionic conducting polymer gel, whose non-covalent crosslinking interaction can endow the conducting gel compressive toughness and self-recovering ability. Our system consists of four components: 1-ethyl-3-methylimidazolium chloride (EMIMCl), hydroxyethyl methacrylate (HEMA), chitosan (CS) and water, whose molecular structures have been shown in Figure 1A. At first, CS and HEMA are dissolved in EMIMCl via heating and cooling process, which forms a viscous solution from the solid of EMIMCl (Figures S1a and 1C). The destroy of crystal structure of EMIMCl is the main reason to form such homogeneous solution, which is caused by the dissolution of CS and HEMA through hydrogen bond.[12] Then, tough EMIMCl gels can be formed by irradiating the asprepared homogeneous solution with UV light (Figures S1b and S2; Figure 1D). The UV-generated Cl radicals from EMIMCl (Figure 1B) are the initiator for the polymerization of HEMA.[13] The physical gelation of our EMIMCl gel without crosslinker should be ascribed to the hydrogen bonding interactions between the hydroxyl group in PHEMA, the amine group of CS and imidazolium group in solvent. Interesting is that our EMIMCl gel is miscible with water, which has a slightly decrease strength but a sharply increase ionic conductivity (Figure 1E). As shown in Figure S3, water exists in the form of free water and bound water, and is relatively stable at room temperature (Figure S4). The final EMIMCl/water gel exhibits the water and pressure controlled ionic conductivity, which is suitable as an appropriate electrolyte and separator in flexible supercapacitors. The detailed preparation, mechanism and properties are shown as followed. The EMIMCl gels can be formed by irradiating the as-prepared homogeneous solution under 45 minutes UV light with intensity of 22.4 mw/cm2 at 365 nm. The component ratio of the conducting gel is optimized by the comprehensive evaluation of the electrochemical performances and mechanical strength. Finally, a varying amount of water was injected into the EMIMCl gel to form EMIMCl/water gels with 5 to 50 wt% (by mass) water content (Figure 1E). If the water was added in advance, the final gel could not be obtained. The EMIMCl/water gel shows high toughness when they underwent large deformations and show good strength under compression (Figure 2A), puncture (Figure S5a), and stretch (Figures 2B and S5b). Our
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COMMUNICATION Figure 1. A) Molecular structures of EMIMCl, HEMA, and CS. B) the self-initiated mechanism under UV irradiation. C) The precursor’s solution of three components. D) The EMIMCl gels formed by the UV irradiation. E) The EMIMCl/water gels after injecting water.
gel electrolyte has about 200 times enhancement in compressive strength comparing with the common used PVA/H3PO4 gel electrolytes (Figure S6). The tough gels can be used to fabricate sandwiched structure supercapacitors with activated charcoal electrodes (Figure 2C). A morphological analysis of the cryo-dried sample using scanning electron microscopy (SEM) indicates that porous microstructures occur ubiquitously in the EMIMCl gels and EMIMCl/water gels. It can be observed that the pore size increases from 1–2 µm to 4–10 µm in diameter after the addition of water (Figures 2D and 2E). The greater
swollen network in gels thus enables lower strength and higher ionic conductivity. This polymerization of HEMA and further gelation is enabled by a self-initiated mechanism. According to literatures,[14] the photo-induced ionization of the alkylimidazolium type ILs yields two types of primary trapped-charge species: trapped holes and trapped electrons. The holes (electron deficiencies) are preferentially trapped by the constituent anions (Cl−), yielding the corresponding neutral Cl radicals (Cl•). The electrons are possible entrapped by various EMIM cation and cavities in ionic
Figure 2. A) Compression demo of EMIMCl/water gel. B) Tensile property of EMIMCl/water gel. C) The sandwiched structure of the symmetrical cell of a supercapacitor with EMIMCl/water gel and activated charcoal electrodes. SEM images of the composite EMIMCl/water gels after supercritical drying D) without water and E) with 20 wt% water.
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Figure 3. A) Compressive test of the EMIMCl/water gels with 0 to 50 wt% water content. B) Compression recovery of the composite EMIMCl/water gel with 20 wt% water through 500 cycles with 80% strain. C) The ionic conductivities of EMIMCl/water gels with various water contents. D) The ionic conductivities of EMIMCl/water gel with 20 wt% water under various strains.
liquid. The oxidative Cl• can be added to the monomer to form the starting carbon radical. Figure 1B demonstrates the UV photolysis of from EMIMCl to form Cl radical and the solvated electron. The existence of the chlorine radical has been proven by electron paramagnetic resonance (EPR) spectroscopy after alpha-phenyl-tert-butyl-nitrone (PBN) adduction (Figure S7). The successful radical polymerization has been proved by the long-lived propagating radicals observed in the EPR spectrum of the UV irradiated precursors (Figure S8). The final conversion of the monomer is more than 95% after 45 min of UV irradiation (Figures S9 and S10), and the molecular weight distribution of PHEMA is about 104–106 (Figure S11). The mechanical properties of our gels are shown in Figure 3A. The compressive strength of the EMIMCl gel with 17 wt% HEMA and 3 wt% CS is 1706 kPa at 95% strain. On the contrary, the EMIMCl gel with 20 wt% HEMA and 0% CS can achieve 361 kPa compressive strength. The hydrogen bonding interaction of CS can reinforce the 3-dimensional (3-D) network of the gel and improve the final compressive strength. The EMIMCl/water gels exhibit a slow expansion relative to a nonhydrated EMIMCl gel (Figure S12). The swollen volume ratio of EMIMCl/water gel with 50 wt% water content is about 1.52 times than that of the pure EMIMCl gel. As a result, the value of compressive strength decreases from 1706 kPa to 1290 kPa as the amount of water is increased from 0% to 50 wt%, and the Young’s modulus decreases from 28 kPa to 14 kPa, respectively (Table S1). The swollen effect and the enlarged network in SEM picture should be the reason of their slightly low compressive strength relative to the waterless one. As shown in Figure 3B, this gel exhibits a rubber-like recovery when subjected to a fatigue cyclic compression test (at ε = 80%) with at least 500 successive loading/unloading cycles. The
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thickness reduction after the fatigue test is only approximately 10.5%. The gelation via hydrogen bond endows them with high strength and self-recovering ability, thus ensuring adjustable pressure. Ionic conductivity was determined from the high frequency plateau of the real part of the complex conductivity (Figures S13 and S14). The experimental details about this in-situ conductivity measurement are illustrated in Figures S15 and S16. The conductivity of the electrolyte is calculated according to the Equation σ = l/(ARb), where Rb is the resistance of the bulk electrolyte, l is the thickness of the gel and A is the area of electrode covered by gel.[15] Figure 3C presents the plots of the ionic conductivities of EMIMCl/water gels versus various water contents. The ionic conductivity significantly increases from 0.4 mS/cm to 23.2 mS/cm after the addition of 50 wt% H2O. According to liteatures,[16] the conductivity (σ) of an electrolyte given by σ = nqµ, depends upon the carrier concentration (n) and mobility (μ). The ion mobility is inversely related to the viscosity (η) of the electrolyte as μ = q/6πrη (q is the electronic charge on each charge carrier; r is the effective hydrodynamic radius). The water-indcued enhancement of ionic conductivity is due to the followed reasons: 1) The addition of water to the gel electrolyte results in a decrease in the viscosity (as shown in Figure S17), which increases mobility and hence conductivity. 2) The more swollen network of the EMIMCl/water gel proved by SEM image can provide the larger conducting pathways for ion transport.[17] The compressive stress also has a positive effect on the conductivity of EMIMCl/water gels. As shown in Figure 3D, the ionic conductivity quickly increases from 19.4 mS/cm to 40.1 mS/cm for an EMIMCl/water gel with 20 wt% water under 90% compression. The pressure-induced enhancement
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obtained from the cyclic voltammetry (CV) curves at 10 mV/s. Figure 4B shows an increase in the Csp value from 9.0 F/g to 43.5 F/g as the water content from 0 to 50 wt%. The improved performance is determined by its high ion mobility due to the adding water and convenient ion transferring from electrolyte to electrode. Amazing, the shape of the CV curves shown in Figure 4A becomes further rectangular if the 20 wt% water gel-based device ensures 90% compression. Since the specific capacity changed slightly at various thickness of gels (Figure S26), the influence of the thickness during compression can be ignored in our devices. The pressure-induced low viscosity and the pressure-accelerated ionic transfer should be the two reasons of the excellent performance of supercapacitors under compression. On one hand, the decreasing viscosity of gel under pressure can lead to the higher ionic mobility in gel electrolyte. On another hand, the applied pressure between the electrodes can improve the performance of the capacitor due to the increasing ionic transferring between electrode/electrolyte interfaces. Wei’s group has reported that the increasing pressure can significantly increase performance of supercapacitor with liquid electrolyte due to the same reason.[7] In one word, the compressive stress can increase the ionic mobility within gel electrolyte and the ionic transferring from electrolyte to the surface of porous carbon, which both contribute to their higher capacities. Inspired by this result, we prepared tight supercapacitors under 90% compression with EMIMCl/water gels having 0 to 50 wt% water as the flexible electrolyte and separator. The selfrecovering ability makes our gels suitable for this type of application. The galvanostatic charge–discharge curves are shown in Figure 4C. The conducting gel-based supercapacitors
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of ionic conductivity is due to the change of viscosity. Since the ion mobility is inversely related to the viscosity (η) of the electrolyte as µ = q/6πrη, the decrease in the viscosity (Figure S18) can increase mobility and ionic conductivity under compressive strain. The gel state can remain even under 90% compression with a decreased storage modulus and viscosity (Figures S19 and S20). The decreasing storage modulus of gel under compressive strain is the main reason of the decreased viscosity. The pressure-induced weaken of storage modulus is due to the possible breaking of the non-covalent bonds. The ionic conductivity under a certain compression can be permanent even within 48 hours (Figure S21), which reflects the remained gel structure against compression. The ionic conductivity can be changed with the adjusting pressure. As shown in Figure S22, the compressive stress and viscosity are recoverable during a compression-release programmed procedure, which is determined by the self-recovering gel with the reversible non-covalent bonds. At the same time, ionic conductivity of the gel electrolyte can also be enhanced through elongation (Figure S23). Therefore, the as-prepared EMIMCl/water gels with adjustable ionic conductivity and self-restoring mechanical property could be considered an excellent electrolyte candidate for ideal electrochemical performance. Figure 4A demonstrates that the conducting gel-based supercapacitor with 20 wt% water exhibits larger CV curves than those without water. The electrode material of our supercapacitor is the commercial activated charcoal (YP 80F, 2100 m2/g) with 0.8–1.35 nm microspore;[18] which has size matching with the EMIM+ ions (DEMIM+) of 0.76 nm (Figure S24). The corresponding specific capacitances (Csp) (Figure S25) calculated using the charge-discharge profiles agree well with those
Figure 4. A) Cyclic voltammetry curves tested at 10 mV/s from 0 to 90% compression of EMIMCl/water gel with 20 wt% water. B) The specific electrode capacitance of supercapacitors with different water content under 0 and 90% compression. C) Galvanostatic charge–discharge curves with water content from 0 to 50 wt% under 90% compression. D) The Csp values of an EMIMCl/water gel based supercapacitor with 20 wt% water under various compressive stresses.
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demonstrate ideal electrochemical performance and comparable Csp values, when the water content is greater than 20 wt%. This indicates that 20 wt% water content in EMIMCl/ water gel is enough to ensure fast ion transport and diffusion at 10 mV/s. Therefore, the gel with 20 wt% water is selected to fabricate the flexible supercapacitor for the latter electrochemical test. It is notable that the gel-based supercapacitor (20 wt% water) has a high capacitance of 136 F/g at 0.5 A/g, which is much higher than the value of 17 F/g at 0.1 A/g for the anhydrous device measured. Under pressure, higher water content in EMIMCl/water gels for supercapacitors still provide bigger Csp. As shown in Figures 4B and 4C, the Csp value under 90% strain increases from 13 F/g to 140 F/g with water amount from 0 to 50 wt%. Figure 4D shows the Csp values of 20 wt% water gel-based supercapacitor increase with the corresponding compressive stress. Remarkably, there is a significant improvement in the specific capacitance from 43 F/g to 98 F/g during the early stage (strain
