Poly(3,4-ethylenedioxythiophene) Based Solid-State Polymer Supercapacitor with Ionic Liquid Gel Polymer Electrolyte

In this work, solid-state polymer supercapacitor (SSC) was assembled using poly(3,4-ethylenedioxythiophene/carbon paper (PEDOT/CP) as an electrode and ionic liquid (1-butyl-3-methylimidazole tetrafluoroborate)/polyvinyl alcohol/sulfuric acid (IL/PVA/H2SO4) as a gel polymer electrolyte (GPE). The GPE was treated through freezing–thawing (F/T) cycles to improve the electrochemical properties of PEDOT SSC. Cyclic voltammetry (CV), galvanostatic charge–discharge measurements (GCD) and electrochemical impedance spectroscopy (EIS) techniques and conductivity were carried out to study the electrochemical performance. The results showed that the SSC based on ionic liquid GPE (SSC-IL/PVA/H2SO4) has a higher specific capacitance (with the value of 86.81 F/g at 1 mA/cm2) than the SSC-PVA/H2SO4.The number of F/T cycles has a great effect on the electrochemical performance of the device. The energy density of the SSC treated with 3 F/T cycles was significantly improved, reaching 176.90 Wh/kg. Compared with the traditional electrolytes, IL GPE has the advantages of high ionic conductivity, less volatility, non-flammability and wider potential window. Moreover, the IL GPE has excellent elastic recovery and self-healing performance, leading to its great potential applications in flexible or smart energy storage equipment.

Electrolyte and electrode materials are the important components of SCs. Different types of electrolytes, including liquid, solid state electrolytes, and gel polymer electrolyte, have been widely used for SCs. Compared with the conventional liquid electrolyte, solid-state polymer electrolytes (SPEs) and gel polymer electrolytes (GPEs) have the advantages of avoiding liquid leakage and corrosion problems, having a simple packaging and manufacturing process, good electrochemical stability, etc. [14][15][16]. However, SPEs are restrictively applied, due to their low conductivity and poor elastic or flexible properties, which limit the applications of SPEs in flexible or stretchable devices. GPEs are basically a liquid electrolyte (an ionic salt dissolved into an organic solvent or ionic liquids (ILs)) trapped inside a polymer network, which possess both the properties of solids and the

Preparation of PEDOT/CP Electrode Material
CP substrate (2 × 1 cm) was soaked in ethanol absolute and deionized water, respectively, followed by ultrasonic treatment 30 min before electrodeposition to remove the impurities on the surface of CP. After that, 0.01 M EDOT monomer was added to 0.1 M LiClO 4 ·3H 2 O electrolyte, stirring for 60 min. The three electrodes were inserted into the above solution, with pretreated CP as the working electrode, platinum plate (Pt) as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. PEDOT film was coated on CP by unipolar pulse electro-deposition [38]. Finally, the PEDOT/CP electrode was rinsed several times with deionized water and dried at 50 • C for 24 h. Parameters of the unipolar pulse electro-deposition were set as: voltage value: 1.1 V, on time: 0.1 s, off time: 1.0 s, pulse time: 700 times. The mass of PEDOT active materials on CP substrate is 0.8-1.0 mg cm −2 , which was measured with an AL104 electronic balance (Shanghai Mettler Toledo Instrument Co. Ltd., Shanghai, China).

Fabrication of Gel Polymer Electrolytes and Solid-State Supercapacitors
IL/PVA/H 2 SO 4 electrolyte was prepared by the following method [39]: H 2 SO 4 (2 g) was added to deionized water (20 mL) with magnetic stirring, then PVA (2 g) was added, stirring for 1 h at 85 • C to form a homogeneous, clear and viscous solution. Subsequently, 50 wt% BMIMBF 4 (relative to the mass of PVA) was added, stirring for 15 min. After cooling to RT (20 • C), it was frozen at −20 • C for 10 min. A pipette was used to evenly distribute IL/PVA/H 2 SO 4 GPE on one side of the PEDOT/CP electrodes, After 10 min, the SSC were assembled by facing two symmetrical PEDOT/CP electrodes distributed with GPE, followed by freezing at −20 • C and thawing at RT for 24 h with different numbers of F/T cycles, recorded as F/T-n (n = 1, 2, 3, 4). PEDOT SSC based on gel electrolyte without BMIMBF 4, i.e., PVA/H 2 SO 4 , was fabricated using the same method for comparison.

Electrochemical Performance Test
The electrochemical tests of PEDOT SSC were performed on multi-channel VMP3 electrochemical workstation controlled by EC-Lab software (Princeton Applied Research, Princeton, NJ, USA). The mass of PEDOT films were measured by AL104 electronic balance (Shanghai Mettler Toledo Instrument Co., Ltd., Shanghai, China). The electrochemical performances of SSC were evaluated by CV, GCD, and EIS test in ambient condition. The mass specific capacitance (C s ) value of SSC was calculated from charge/discharge curves, according to Equation (1) where, I (A) is the discharge current, m (g) is the mass of electroactive materials, t d (s) is the discharge time and ∆V (V) represents the operating voltage window of the device. Energy density (E, Wh/kg), power density (P, kW/kg) were calculated from the following equations The electrical conductivity of the IL/PVA/H 2 SO 4 GPE films was measured at RT by the SZT-2A four-point probes resistivity measurement system (Suzhou Tongchuang Electronics Co. Ltd., Suzhou, China). For each sample, at least five replicates were tested, and the results were presented as the average.

Differential Scanning Calorimetry (DSC)
DSC analysis was conducted on a NETZSCH DSC 204 (Selb, German) under a nitrogen atmosphere. IL/PVA/H 2 SO 4 GPE were dried at 80 • C under vacuum oven; small pieces were then sliced for the DSC tests. Measurements were carried out from -60 to 290 • C at a heating rate of 10 • C/min. Figure 1A,D show macrographs of the CP and the PEDOT/CP electrode. The detail preparation of PEDOT/CP electrode was stated in our previous work [38]. The long and smooth fibers of CP orient randomly and are densely packed, forming the porous structure ( Figure 1B,C) [3,40], which is help for the coating of PEDOT and provides the channel for the ion migration. Figure 1E,F shows that the CP coated with PEDOT films (PEDOT/CP) retained the loose, porous microstructure of electrode and that there are lots of spindle-like multi-sized PEDOT distributing uniformly over individual carbon fibers. In 0.5 M H 2 SO 4 liquid electrolyte, the prepared PEDOT/CP electrode presents a high specific capacitance of 126.24 F/g at 1 mA/cm 2 and good cycling stability, with 80.57% retention of its initial capacitance over 5000 cycles. Thus, we choose the PEDOT/CP electrode to assemble SSC using GPE and study its electrochemical properties. As illustrated in the macroscopical digital images and the illustration in Figure 1G, the two GPE-coated electrodes were assembled face to face into SSC. Its electrochemical performances were studied using CV, GCD, and EIS in the following.  Figure 2A shows the CV curves of SSC based on two different electrolytes at 5 mV/s. The rectangular shapes of the CV curves indicate their excellent capacitive behaviors. The SSC-IL/PVA/H2SO4 has larger CV area than SSC-PVA/H2SO4, which indicates that embedding BMIMBF4 to PVA/H2SO4 can effectively improve the capacitive performance of the device. Figure 2B shows that the CV area gradually increases when the voltage window expands from 1.6 V (−0.8~0.8 V) to 2.0 V (−1.0~1.0 V); the shape of all CV curves are close to rectangular. However, there are distinct and sharp peaks at the end of the CV curves at a maximum operating voltage of 1.2 V; this indicates that higher electrochemical potential will cause the decomposition of water [17,[41][42][43]. Thus, we choose −1.0-1.0 V as the optimal operating voltage window for SSC. The rectangular shape is generally retained even when the scan rate is increased to 100 mV/s, which indicates the good rate performance of SSC.  Figure 2A shows the CV curves of SSC based on two different electrolytes at 5 mV/s. The rectangular shapes of the CV curves indicate their excellent capacitive behaviors. The SSC-IL/PVA/H 2 SO 4 has larger CV area than SSC-PVA/H 2 SO 4 , which indicates that embedding BMIMBF 4 to PVA/H 2 SO 4 can effectively improve the capacitive performance of the device. Figure 2B shows that the CV area gradually increases when the voltage window expands from 1.6 V (−0.8~0.8 V) to 2.0 V (−1.0~1.0 V); the shape of all CV curves are close to rectangular. However, there are distinct and sharp peaks at the end of the CV curves at a maximum operating voltage of 1.2 V; this indicates that higher electrochemical potential will cause the decomposition of water [17,[41][42][43]. Thus, we choose −1.0-1.0 V as the optimal operating voltage window for SSC. The rectangular shape is generally retained even when the scan rate is increased to 100 mV/s, which indicates the good rate performance of SSC.

The Galvanostatic Charge-Discharge Test
GCD tests were carried out to further evaluate the performance of SSC based on the two kinds of electrolytes, and the results are shown in Figure 3. It can be seen that the discharging time of SSC-IL/PVA/H2SO4 is obviously longer than that of SSC-PVA/H2SO4, indicating the former has a better supercapacitive performance [14]. The IL GPE can provide electro-active ions (BMIM + and BF4 − ) for the electrode and electrolyte interface, thereby enhancing specific capacitance [20]. The GCD curves in Figure 3B,C indicate that the charge-discharge time is approximately equal and the coulombic efficiency is close to 100%, which means the two SSCs have good electrochemical reversibility [9,44,45]. The specific capacitances calculated from the GCD curves according to Equation (1) are plotted as a function of the discharge current density ( Figure 3D). Compared with the SSC-PVA/H2SO4, the SSC-IL/PVA/H2SO4 has better electrochemical behavior; the specific capacitance increases by two times with the introduction of IL. The capacitance of SSC-IL/PVA/H2SO4 decreases from 86.81 to 52.15 F/g with the gradual increase in current density from 1 to 5 mA/cm 2 . The slight fading of the capacitance up to a high current density is attributed to the fast redox at the electrode/electrolyte interfaces.

The Galvanostatic Charge-Discharge Test
GCD tests were carried out to further evaluate the performance of SSC based on the two kinds of electrolytes, and the results are shown in Figure 3. It can be seen that the discharging time of SSC-IL/PVA/H 2 SO 4 is obviously longer than that of SSC-PVA/H 2 SO 4 , indicating the former has a better supercapacitive performance [14]. The IL GPE can provide electro-active ions (BMIM + and BF 4 − ) for the electrode and electrolyte interface, thereby enhancing specific capacitance [20]. The GCD curves in Figure 3B,C indicate that the charge-discharge time is approximately equal and the coulombic efficiency is close to 100%, which means the two SSCs have good electrochemical reversibility [9,44,45]. The specific capacitances calculated from the GCD curves according to Equation (1) are plotted as a function of the discharge current density ( Figure 3D).

The Electrochemical Impedance Spectroscopy Test
The EIS results of the two SSCs are shown in Figure 4, and the corresponding impedance fitting data are listed in Table 1. Rs is the internal resistance related to the ionic resistance of the electrolyte, and the contact resistance at the interface of the electrode/electrolyte. Rct is the charge transfer resistance occurring at the electrode/electrolyte interface during the charging-discharging process. The Rs and Rct values are calculated from the results EIS. Compared with SSC-PVA/H2SO4, the Rs and Rct of SSC-IL/PVA/H2SO4 is relatively smaller because IL can be used as an ion source to obtain more carriers in IL-PVA/H2SO4, including imidazole cations BMIM + and BF4 − apart from H + and SO4 2− [14]. The four-point probe method result also confirms that the conductivity of IL/PVA/H2SO4 (90.09 mS/cm) polymer gel is better than that of PVA/H2SO4 (38.36 mS/cm). With the recent great progress in polymer gel, some work focuses on how to improve its conductivity. Peng [46] reported the graphite oxide (GO) doped PVA/KCl/GO hydrogel had a high ionic conductivity of 47.5 mS/cm (2.3 wt% GO) compared with bare B-PVA/KCl hydrogel (32.6 mS/cm). Ramesh group [47] showed the ionic conductivity of solid polymer electrolytes based on PVA incorporated with sodium salt and ionic liquid improved the conductivity from 4.87 × 10 −3 to 2.31 mS/cm. Hashaikeh and coworkers [48] showed the sulfated cellulose/PVA composites as proton-conducting electrolyte for capacitors activated with 0.25M H2SO4 show a high ionic conductivity of 250 mS/cm. These studies suggest that the introduction of IL or H2SO4 has a great contribution to the ionic conductivity of the PVA-based gel. Therefore, the high conductivity of SSC-IL/PVA/H2SO4 can be attributed to the coefficient of all ions, including BMIM + , BF 4− , H + and SO4 2− .

The Electrochemical Impedance Spectroscopy Test
The EIS results of the two SSCs are shown in Figure 4, and the corresponding impedance fitting data are listed in Table 1. R s is the internal resistance related to the ionic resistance of the electrolyte, and the contact resistance at the interface of the electrode/electrolyte. R ct is the charge transfer resistance occurring at the electrode/electrolyte interface during the charging-discharging process.

The Energy Density and Power Density Test
Energy density (E) is an important influence parameter for SSC in practice. The E and power density (P) of SSC based on the two electrolytes are evaluated and the results are shown in Figure  5A. The SSC-IL/PVA/H2SO4 shows a much higher E and P than SSC-PVA/H2SO4. The maximum P is 21.27 kW/kg at a current density of 5 mA/cm 2 , and the optimal energy density is up to 176.90 Wh/kg at 1 mA/cm 2 , which is about 3.1 times higher than that of SSC-PVA/H2SO4 (57.11 Wh/kg). This indicates that adding BMIMBF4 to PVA/H2SO4 electrolyte can improve the capacitive behavior. The values of E and P are comparable to many recently reported PEDOT electrodes-based SCs. For example, Pandey and coworkers studied the electrochemical energy storage performance of SCs fabricated using PEDOT-coated carbon fiber paper electrodes and IL GPE in acetone [13]. It had a maximum specific capacitance of 154.4 F/g and the specific power and energy values were found to be 11.3 and 6.5 Wh/kg, respectively. Compared to this, the specific power and specific energy in this work have been improved. Moreover, we use aqueous IL-GPE rather than organic solvent base IL-GPE; the assembled SSC-IL/PVA/H2SO4 is nontoxic, green or eco-friendly, and more promising. The cycling stability is one of the most important parameters for the supercapacitor. As Figure 5B shows, the capacitive retention rate of SSC-IL/PVA/H2SO4 decreases to 71.61% after 1000 cycles. The capacitance fading is mainly attributed to the slightly decreased electroactivity of PEDOT.

The Energy Density and Power Density Test
Energy density (E) is an important influence parameter for SSC in practice. The E and power density (P) of SSC based on the two electrolytes are evaluated and the results are shown in Figure 5A. The SSC-IL/PVA/H 2 SO 4 shows a much higher E and P than SSC-PVA/H 2 SO 4 . The maximum P is 21.27 kW/kg at a current density of 5 mA/cm 2 , and the optimal energy density is up to 176.90 Wh/kg at 1 mA/cm 2 , which is about 3.1 times higher than that of SSC-PVA/H 2 SO 4 (57.11 Wh/kg). This indicates that adding BMIMBF 4 to PVA/H 2 SO 4 electrolyte can improve the capacitive behavior. The values of E and P are comparable to many recently reported PEDOT electrodes-based SCs. For example, Pandey and coworkers studied the electrochemical energy storage performance of SCs fabricated using PEDOT-coated carbon fiber paper electrodes and IL GPE in acetone [13]. It had a maximum specific capacitance of 154.4 F/g and the specific power and energy values were found to be 11.3 and 6.5 Wh/kg, respectively. Compared to this, the specific power and specific energy in this work have been improved. Moreover, we use aqueous IL-GPE rather than organic solvent base IL-GPE; the assembled SSC-IL/PVA/H 2 SO 4 is nontoxic, green or eco-friendly, and more promising. The cycling stability is one of the most important parameters for the supercapacitor. As Figure 5B shows, the capacitive retention rate of SSC-IL/PVA/H 2 SO 4 decreases to 71.61% after 1000 cycles. The capacitance fading is mainly attributed to the slightly decreased electroactivity of PEDOT. be 11.3 and 6.5 Wh/kg, respectively. Compared to this, the specific power and specific energy in this work have been improved. Moreover, we use aqueous IL-GPE rather than organic solvent base IL-GPE; the assembled SSC-IL/PVA/H2SO4 is nontoxic, green or eco-friendly, and more promising. The cycling stability is one of the most important parameters for the supercapacitor. As Figure 5B shows, the capacitive retention rate of SSC-IL/PVA/H2SO4 decreases to 71.61% after 1000 cycles. The capacitance fading is mainly attributed to the slightly decreased electroactivity of PEDOT.

The Effect of Freezing-Thawing Cycles on the Electrochemical Properties of SSC
Due to the regularly arranged hydroxyl groups along the PVA chains, PVA hydrogels are commonly prepared through the strong physical crosslinking points generated by the repeated F/T method [18,49]. The physical crosslinking is constructed through the crystallite region and the hydrogen bond interor intro-PVA chains. Therefore, it is expected that the two conducting polymer electrodes can be assembled as SSCs using a PVA-based ionic liquid gel as the electrolyte. The number of F/T cycles is one important condition that must be taken into account for the structure and properties of PVA hydrogel. A higher number of F/T cycles will increase the physically crosslinked PVA macromolecules, which are bound into a network, thus improving the stability and elastic properties of the hydrogel [18,50,51]. To obtain the excellent properties, herein we study the effect of the number of F/T cycles on the SSC. Figure 6A shows that the CV curves of SSC-IL/PVA/H 2 SO 4 applied with different F/T cycles have quasi-rectangular shapes. The CV area and the response current slightly increase firstly and then decrease with the higher F/T cycles. The F/T-3 has the optimum performance. The Nyquist curves in Figure 6B show that the internal resistance of SSC depends on the number of F/T cycles. The device subjected to three F/T cycles has the minimum internal resistance, as listed in Table 2, because the stable network of GPE with three F/T cycles can provide effective channels for the ions' transportation. The corresponding R ct is relatively large, which is due to the formation of ion layers on the electrode surface during the embedding/embedding process, resulting in the increasing impedance [52]. However, too many physical crosslinking points will generate in the GPE when the F/T is more than three cycles, which will hinder the ion transportation. Figure 6C shows that all GCD curves display good symmetry, indicating that the SSCs have excellent electrochemical reversibility. In addition, the nearly linear discharge behavior exhibits the good capacitive performance of SSCs. The charge-discharge time and specific capacitance gradually increase, and then decrease with the F/T cycles, reaching a maximum value of 53.73 F/g at three F/T cycles.

The Effect of Freezing-Thawing Cycles on the Thermal Properties of SSC-IL/ PVA/H2SO4
The intermolecular interaction increases between pendant hydroxyl groups, forming a crystallite region, and generating a three-dimensional polymer gel network, where the crystallites and the polymer chains entanglement act as knots [53,54], while the pores are filled by the ionic liquid, H2SO4 and free water. The polymer network is not stable or perfect when the SSC is subjected one F/T cycle. Upon increasing the number of F/T cycles to three, the numbers of hydrogen bond in the gel increases, and the physical cross-linking points also increase due to the polymer chains' entanglement, forming a gel with stable crosslinking network as shown in Figure 7A [55], which allows ionics to transfer in the electrolyte. A higher degree of crystallinity, present in the PVA hydrogen with higher F/T cycles, is confirmed by DSC thermograms of IL/PVA/H2SO4 GPE subjected to one or three F/T cycles. For sample F/T-1, the peak observed at 100 °C is considered as a chain movement caused by water or acid, IL molecules release or movement. The peak at 175.19 °C is the melting temperature caused by the limited crystal region. For sample F/T-3, the peak present at 171.15 °C becomes weak and the main melting peak shifts to a higher temperature, implying that the increasing degree of crystallization is due to the increasing F/T cycles.  The intermolecular interaction increases between pendant hydroxyl groups, forming a crystallite region, and generating a three-dimensional polymer gel network, where the crystallites and the polymer chains entanglement act as knots [53,54], while the pores are filled by the ionic liquid, H 2 SO 4 and free water. The polymer network is not stable or perfect when the SSC is subjected one F/T cycle. Upon increasing the number of F/T cycles to three, the numbers of hydrogen bond in the gel increases, and the physical cross-linking points also increase due to the polymer chains' entanglement, forming a gel with stable crosslinking network as shown in Figure 7A [55], which allows ionics to transfer in the electrolyte. A higher degree of crystallinity, present in the PVA hydrogen with higher F/T cycles, is confirmed by DSC thermograms of IL/PVA/H 2 SO 4 GPE subjected to one or three F/T cycles. For sample F/T-1, the peak observed at 100 • C is considered as a chain movement caused by water or acid, IL molecules release or movement. The peak at 175.19 • C is the melting temperature caused by the limited crystal region. For sample F/T-3, the peak present at 171.15 • C becomes weak and the main melting peak shifts to a higher temperature, implying that the increasing degree of crystallization is due to the increasing F/T cycles. Polymers 2020, 12, x FOR PEER REVIEW 11 of 15

The Mechanical Properties and Function of Ionic Liquid-Based Polymer Gel
The GPE was sandwiched between the two PEDOT/CP electrodes. It is not convenient to study the appearance, elasticity recovery, mechanical properties or other functional characteristics, so we fabricated the film and rod shape of the GPE sample using the same preparation method to directly observe the mechanical properties. The mechanical property of the GPE is the critical parameter that will affect the applications [56]. Figure 8A,B shows the IL/PVA/H2SO4 gel film or road sample is soft and flexible, and it has excellent recovery elasticity. It can be easily stretched at least two times or twist under extra force and fully recover to its original shape just like rubber, indicating that GPE has excellent mechanical properties. What is interesting is the IL GPE has excellent self-healing properties, as shown in Figure 8C; the road sample was cut in the middle and it can recover to its original state after 5 min in air at RT, and we can pick up the self-healed sample with tweezers. The self-healing can be attributed to the hydrogen bonding interaction during the formation of the IL polymer gel. Although the hydrogel electrolytes have made great progress, they rarely present self-healing behavior [46]. Furthermore, it still faces great challenges to assemble the flexible SCs with good specific capacity, long life cycles, high ionic conductivity and excellent self-healing when suffering damage [57]. This will open a window for studying the flexible and self-healing smart supercapacitor or other multi-functional electrical devices. Future work will focus on this and will be carried out continuously in this field.

The Mechanical Properties and Function of Ionic Liquid-Based Polymer Gel
The GPE was sandwiched between the two PEDOT/CP electrodes. It is not convenient to study the appearance, elasticity recovery, mechanical properties or other functional characteristics, so we fabricated the film and rod shape of the GPE sample using the same preparation method to directly observe the mechanical properties. The mechanical property of the GPE is the critical parameter that will affect the applications [56]. Figure 8A,B shows the IL/PVA/H 2 SO 4 gel film or road sample is soft and flexible, and it has excellent recovery elasticity. It can be easily stretched at least two times or twist under extra force and fully recover to its original shape just like rubber, indicating that GPE has excellent mechanical properties. What is interesting is the IL GPE has excellent self-healing properties, as shown in Figure 8C; the road sample was cut in the middle and it can recover to its original state after 5 min in air at RT, and we can pick up the self-healed sample with tweezers. The self-healing can be attributed to the hydrogen bonding interaction during the formation of the IL polymer gel. Although the hydrogel electrolytes have made great progress, they rarely present self-healing behavior [46]. Furthermore, it still faces great challenges to assemble the flexible SCs with good specific capacity, long life cycles, high ionic conductivity and excellent self-healing when suffering damage [57]. This will open a window for studying the flexible and self-healing smart supercapacitor or other multi-functional electrical devices. Future work will focus on this and will be carried out continuously in this field.

Conclusions
In this paper, the symmetrical SSC was fabricated using PEDOT/CP prepared by the unipolar pulse method as electrodes, GPE based on IL/PVA/H 2 SO 4 as GPE. IL/PVA/H 2 SO 4 GPEs were prepared by F/T cycle. The electrochemical performances of SSC were tested by CV, GCD, and EIS. Compared with the SSC-PVA/H 2 SO 4 , the mass specific capacitance of SSC-IL/PVA/H 2 SO 4 was improved by two times, reaching 86.81 F/g. And its energy density was also significantly improved to reach 176.90 Wh/kg. In addition, the SSC-IL/PVA/H 2 SO 4 had good cycle stability, which retained 71.61% of the initial capacitance after 1000 GCD cycles. Therefore, adding IL (BMIMBF 4 ) to PVA/H 2 SO 4 GPE can effectively improve the capacitive performance and energy density of SSC. Furthermore, IL/PVA/H 2 SO 4 GPE also exhibits good elastic recovery and self-healing, which gives it significant application value in light, thin and wearable flexible devices due to its excellent mechanical properties.