Construction of Polymer Electrolyte Based on Soybean Protein Isolate and Hydroxyethyl Cellulose for a Flexible Solid-State Supercapacitor

Supercapacitors are a very active research topic. However, liquid electrolytes present several drawbacks on security and packaging. Herein, a gel polymer electrolyte was prepared based on crosslinked renewable and environmentally friendly soybean protein isolate (SPI) and hydroxyethyl cellulose (HEC) with 1.0 mol L−1 Li2SO4. Highly hydrophilic SPI and HEC guaranteed a high ionic conductivity of 8.40 × 10−3 S cm−1. The fabricated solid-state supercapacitor with prepared gel polymer electrolyte exhibited a good electrochemical performance, that is, a high single electrode gravimetric capacitance of 91.79 F g−1 and an energy density of 7.17 W h kg−1 at a current density of 5.0 A g−1. The fabricated supercapacitor exhibited a flexible performance under bending condition superior to liquid supercapacitor and similar electrochemical performance at various bending angles. In addition, it was proved by an almost 100% cycling retention and a coulombic efficiency over 5000 charge–discharge cycles. For comparison, supercapacitors assembled with commercial aqueous PP/PE separator, pure SPI membrane, and crosslinked SPI membrane were also characterized. The obtained gel polymer electrolyte based on crosslinked SPI and HEC may be useful for the design of advanced polymer electrolytes for energy devices.


Introduction
Supercapacitor (SC) is considered as an electrochemical energy storage device with a great potential due to its high-power, long cycle life, low environmental impact, and high safety at present [1]. Recently, the development of flexible solid-state supercapacitors fabricated with polymer electrolytes that can be used in wearable electronic devices has attracted more attention.
It is well known that the energy density of a supercapacitor is determined by the specific capacitance and squared operating voltage window [2,3]. To improve the energy density, tremendous efforts have been devoted to developing novel electrode materials to elevate the specific capacitance [4][5][6][7][8][9][10]. Apart from the electrodes, electrolyte is one of the key components of a supercapacitor, that can significantly affect the operating voltage as well as lifespan and safety of supercapacitors [11][12][13]. The supercapacitors assembled with liquid electrolytes have the disadvantages of electrolyte leakage, In this study, a series of membranes was prepared by SPI as the polymer matrix, blended with HEC and crosslinked by ethylene glycol diglycidyl ether (EGDE) which is an epoxy compound with low toxicity. The crosslinked membranes were then saturated with 1.0 M Li 2 SO 4 electrolyte with a large operating potential window to form gel polymer electrolytes (GPEs) and applied in solid-state supercapacitors (SSCs). The comprehensive properties of the membranes, GPEs, and SSCs were systematically investigated. The membrane based on the crosslinked SPI and HEC not only maintains its water resistance, but also improves the ionic conductivity of GPE and electrochemical performance of supercapacitor.

Preparation of Gel Polymer Electrolyte Based on Crosslinked SPI and HEC
SPI powder was dispersed in deionized water and stirred constantly to form a suspension. Glycerol was added and the pH of suspension was adjusted to 10, after which the suspension was stirred for 30 min at room temperature followed by being heated at 85 • C under constant stirring for 30 min. The suspension was then cooled down to room temperature and ultrasonicated for 10 min. Finally, a 5 wt % SPI solution was obtained. The HEC powder was dissolved in deionized water to form a 5 wt % solution simultaneously. The SPI solution and HEC solution were mixed for 2 h. The crosslinker EGDE was dropped into the solution and stirred for 1 h. In order to maintain a similar degree of crosslinking of each membrane, equal amount of crosslinker EGDE was added in the membranes of the same quantity. The homogeneous solution was cast onto a dry flat glass plate and dried at 50 • C for 24 h. The membrane based on crosslinked SPI and HEC was peeled off and recorded as MCM-H. SPI membrane named MCM-S and crosslinked SPI membrane named MCM-E were prepared in a similar process described above. For comparison, the commercial hydrophilic PP/PE membrane was used as a separator and named MCM-C. The component information was listed in Table 1. All the membrane samples were immersed in 1.0 M Li 2 SO 4 electrolyte to form GPEs named GPE-H, GPE-S, GPE-E, and GPE-C, respectively.

Fabrication of SSCs
Firstly, the AC electrodes were prepared by 80 wt % AC powder, 10 wt % acetylene black and 10 wt % PTFE, and each electrode was 4.1 mg. The SSCs were then assembled as the following sandwiched structure: AC electrode//GPE//AC electrode and simply packaged. The process was described as shown in Figure 1. For comparison, a SSC with GPE-C was also fabricated by the same method and recorded as SSC-C. The flexibility of supercapacitor was named SSC-H' and tested by assembling polymer electrolyte and electrodes with area of 3 cm 2 (1 × 3 cm).
Polymers 2019, 11, x FOR PEER REVIEW 4 of 17 described as shown in Figure 1. For comparison, a SSC with GPE-C was also fabricated by the same method and recorded as SSC-C. The flexibility of supercapacitor was named SSC-H' and tested by assembling polymer electrolyte and electrodes with area of 3 cm 2 (1 × 3 cm).

Characterization of Membrane Samples
The ATR-FTIR spectra of all the samples were recorded from 500 to 4000 cm −1 using a Spectrum One FTIR spectrophotometer (Nicolet Co., Madison, WI, USA). XRD of samples were evaluated with a D/max-2 200 diffractometer (Rigaku International Corporation, Tokyo, Japan). X-ray radiation was generated from a Cu-Kα source at an accelerating voltage of 40 kV and a current of 30 mA. The diffraction data were collected from 5° to 50° with a step interval of 0.02°. Thermogravimetric analysis (TGA) of samples were analyzed using a Netzsch 209 F3 TGA instrument (NET Scientific Instruments Trading (Shanghai) Co., Ltd., Shanghai, China) under a Nitrogen atmosphere from 30 °C to 600 °C at a heating rate of 10 °C min −1 .

Electrolyte Uptake of Membranes
Membrane samples (1.5 × 4.0 cm) were dried at 80 °C for 24 h to a constant weight which was recorded as Wdry. The dry membrane was immersed in 1.0 M Li2SO4 for 30 min at room temperature, followed by wiping with filter paper for removal of excess Li2SO4 solution on the surface, the weight of the wet membrane was recorded as Wwet. The electrolyte uptake (EU) was calculated by the equation.

Ionic Conductivity of GPEs
The ionic conductivity (σ, S cm −1 ) of the GPEs can be calculated by the equation.
= × where L (cm) is the thickness of the GPE, S (cm 2 ) is the effective interface area between GPE and stainless-steel electrode. Rb (ohm) is the bulk resistance of the GPE and measured by electrochemical impedance spectroscopy (EIS) measurement with a potential amplitude of 10 mV and a frequency range from 1 Hz to 100 kHz by using electrochemical workstation (CHI660B, Shanghai, China). In order to establish the relationship between the ionic conductivity of GPE and the electrochemical

Characterization of Membrane Samples
The ATR-FTIR spectra of all the samples were recorded from 500 to 4000 cm −1 using a Spectrum One FTIR spectrophotometer (Nicolet Co., Madison, WI, USA). XRD of samples were evaluated with a D/max-2 200 diffractometer (Rigaku International Corporation, Tokyo, Japan). X-ray radiation was generated from a Cu-Kα source at an accelerating voltage of 40 kV and a current of 30 mA. The diffraction data were collected from 5 • to 50 • with a step interval of 0.02 • . Thermogravimetric analysis (TGA) of samples were analyzed using a Netzsch 209 F3 TGA instrument (NET Scientific Instruments Trading (Shanghai) Co., Ltd., Shanghai, China) under a Nitrogen atmosphere from 30 • C to 600 • C at a heating rate of 10 • C min −1 .

Electrolyte Uptake of Membranes
Membrane samples (1.5 × 4.0 cm) were dried at 80 • C for 24 h to a constant weight which was recorded as W dry . The dry membrane was immersed in 1.0 M Li 2 SO 4 for 30 min at room temperature, followed by wiping with filter paper for removal of excess Li 2 SO 4 solution on the surface, the weight of the wet membrane was recorded as W wet . The electrolyte uptake (EU) was calculated by the equation.
Five replicates were performed for each membrane.

Ionic Conductivity of GPEs
The ionic conductivity (σ, S cm −1 ) of the GPEs can be calculated by the equation.
where L (cm) is the thickness of the GPE, S (cm 2 ) is the effective interface area between GPE and stainless-steel electrode. R b (ohm) is the bulk resistance of the GPE and measured by electrochemical impedance spectroscopy (EIS) measurement with a potential amplitude of 10 mV and a frequency range from 1 Hz to 100 kHz by using electrochemical workstation (CHI660B, Shanghai, China). In order to establish the relationship between the ionic conductivity of GPE and the electrochemical performance of the corresponding SSC, only one set of data for each GPE and corresponding solid-state supercapacitor is used and discussed.

Electrochemical Characterization of SSCs
The electrochemical performance of the SSCs with different GPEs was investigated in a two-electrode system by EIS, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) techniques by using electrochemical workstations (CHI660B, Shanghai, China). The EIS spectrum was used to analyze the impedance behavior of the single cell with frequency range from 0.01 Hz to 100 kHz at the amplitude of open-circuit voltage. CV tests of the solid-state supercapacitors were conducted within a stable potential window (0-1.5 V) at different scan rate (10, 20, 50, 100, 200 mV s −1 ). GCD characterization was conducted within a stable potential window (0-1.5 V) at different current density (1, 2, 5, 10 A g −1 ). GCD measurement was used to determine the capacitance and the cycle performance of cells. The single electrode gravimetric specific capacitance (C sp , F g −1 ) was calculated from the GCD curves according to the equation.
where I is the constant current (A), m is the AC mass of two electrodes (g), dv/dt (V s −1 ) is the slope of the fitting straight line to the discharge curve excluding the IR drop. The energy density (E cell , W h kg −1 ) and power density (P cell , W kg −1 ) of SSC were calculated as.
where V is the cell voltage without voltage drop, ∆t is the discharge time.
The electrochemical cycling life of the SSC-H was investigated for 5000 charge and discharge cycles at a current density of 2 A g −1 by using LAND test equipment (LAND, CT3001A, Wuhan, China).

Structural Analysis of Membrane Samples
The ATR-FTIR spectra of HEC powder, MCM-S, MCM-E, and MCM-H are shown in Figure 2. The adsorption band at 3000-3500 cm −1 and about 2900 cm −1 were attributed to the free bound -OH and -CH 3 and -CH 2 -in the prepared membranes, respectively. The absorption peaks of characteristic group of protein were observed at 1626 cm −1 , 1537 cm −1 , and 1223 cm −1 and correspond to amide I (C=O stretching vibration), amide II (N-H bending vibration), and amide III (C-N stretching and N-H bending vibration), which were consistent with a previous report [43]. The COO-and -C-NH 2 absorption peaks could be observed at 1396 cm −1 and 1036 cm −1 , respectively [44,45]. From Figure 2b, it can be observed that the introduction of HEC, the absorption peak of amide II (1537 cm −1 ) shifted to 1556 cm −1 , which was due to the hydrogen bond interaction between HEC and SPI. XRD patterns of pure SPI power and membrane samples are shown in Figure 3. Characteristic diffraction peaks at 2θ ≈ 9.3° and 19.3° of the pure SPI power represent the α-helix and β-sheet structures of SPI secondary structure, respectively [46]. It is obvious that these two characteristic diffraction peaks can be found in MCM-S. With the addition of EGDE, the characteristic absorption peak at 2θ ≈ 9.3° of MCM-E disappeared, which indicates that a chemical reaction occurred between EGDE and SPI. The XRD curves of MCM-E and MCM-H are almost the same, indicating that high compatibility between SPI and HEC.

Thermal Stability
Thermal stability of samples was investigated using TGA. The TGA curves recorded from 30 °C to 600 °C are shown in Figure 4. Only one decomposition stage can be observed for thermal decomposition of the SPI powder. The initial degradation and the maximum decomposition rate of SPI powder occurred at around 200 °C and 310 °C. Initial decomposition of modified SPI membranes occurred at around 125 °C, which is lower than that of the SPI powder, which may be due to partial degradation of the SPI molecular structure caused by KOH and glycerol. Decomposition temperature of MCM-H is higher than MCM-S and MCM-E from the beginning to the end of thermal XRD patterns of pure SPI power and membrane samples are shown in Figure 3. Characteristic diffraction peaks at 2θ ≈ 9.3 • and 19.3 • of the pure SPI power represent the α-helix and β-sheet structures of SPI secondary structure, respectively [46]. It is obvious that these two characteristic diffraction peaks can be found in MCM-S. With the addition of EGDE, the characteristic absorption peak at 2θ ≈ 9.3 • of MCM-E disappeared, which indicates that a chemical reaction occurred between EGDE and SPI. The XRD curves of MCM-E and MCM-H are almost the same, indicating that high compatibility between SPI and HEC. XRD patterns of pure SPI power and membrane samples are shown in Figure 3. Characteristic diffraction peaks at 2θ ≈ 9.3° and 19.3° of the pure SPI power represent the α-helix and β-sheet structures of SPI secondary structure, respectively [46]. It is obvious that these two characteristic diffraction peaks can be found in MCM-S. With the addition of EGDE, the characteristic absorption peak at 2θ ≈ 9.3° of MCM-E disappeared, which indicates that a chemical reaction occurred between EGDE and SPI. The XRD curves of MCM-E and MCM-H are almost the same, indicating that high compatibility between SPI and HEC.

Thermal Stability
Thermal stability of samples was investigated using TGA. The TGA curves recorded from 30 °C to 600 °C are shown in Figure 4. Only one decomposition stage can be observed for thermal decomposition of the SPI powder. The initial degradation and the maximum decomposition rate of SPI powder occurred at around 200 °C and 310 °C. Initial decomposition of modified SPI membranes occurred at around 125 °C, which is lower than that of the SPI powder, which may be due to partial degradation of the SPI molecular structure caused by KOH and glycerol. Decomposition temperature of MCM-H is higher than MCM-S and MCM-E from the beginning to the end of thermal

Thermal Stability
Thermal stability of samples was investigated using TGA. The TGA curves recorded from 30 • C to 600 • C are shown in Figure 4. Only one decomposition stage can be observed for thermal decomposition of the SPI powder. The initial degradation and the maximum decomposition rate of SPI powder occurred at around 200 • C and 310 • C. Initial decomposition of modified SPI membranes occurred at around 125 • C, which is lower than that of the SPI powder, which may be due to partial degradation of the SPI molecular structure caused by KOH and glycerol. Decomposition temperature of MCM-H is higher than MCM-S and MCM-E from the beginning to the end of thermal decomposition. Maximum decomposition rate temperature of MCM-E is significantly higher than MCM-S and is even higher than that of SPI powder. Although the maximum decomposition rate temperature of MCM-H is slightly lower than that of MCM-E, it is still higher than that of MCM-S, which indicates that the crosslinked structure can significantly improve the thermal stability of MCM-H membrane.
Polymers 2019, 11, x FOR PEER REVIEW 7 of 17 decomposition. Maximum decomposition rate temperature of MCM-E is significantly higher than MCM-S and is even higher than that of SPI powder. Although the maximum decomposition rate temperature of MCM-H is slightly lower than that of MCM-E, it is still higher than that of MCM-S, which indicates that the crosslinked structure can significantly improve the thermal stability of MCM-H membrane.

Electrolyte Uptake, Water Resistance of Membrane Samples, and Ionic Conductivities of GPEs
The Nyquist plots of GPEs are shown in Figure 5a,b. The Ionic resistance (Rb) can be determined by the intersection of the Nyquist plot and the Z' axis in high frequency area. The ionic conductivities of GPEs can be calculated and displayed in Table 2. The lower ionic conductivity of GPE-E resulted from the consumption of the hydrophilic amino groups in the SPI via the crosslinking reaction with EGDE. GPE-H has a higher ionic conductivity and electrolyte uptake than GPE-S and GPE-E. The addition of hydrophilic HEC compensates for the loss of hydrophilic functional groups due to crosslinking reaction and due to HEC facilitating the transport of electrolyte ions, which can be obviously observed from the electrolyte uptake results in Figure 5c. SPI membrane has a good ionic conductivity due to the good hydrophilicity as well as ionic property to facilitate the ion transport of the polymer electrolyte. This is an important factor in the selection of SPI as a polymer matrix of polymer electrolyte. Furthermore, comparison between GPE-H and GPE-C was carried out. GPE-C exhibits a higher ionic conductivity due to the high porosity facilitating the transport of ions.
As a natural polymer material, SPI has poor water resistance. To study the water resistance of SPI membranes, the membrane samples were submerged in deionized water for 24 h as shown in Figure 5d. MCM-S has been excessively swollen and almost lost its mechanical strength. Conversely, MCM-E and MCM-H exhibited better stability in deionized water.

Electrolyte Uptake, Water Resistance of Membrane Samples, and Ionic Conductivities of GPEs
The Nyquist plots of GPEs are shown in Figure 5a,b. The Ionic resistance (R b ) can be determined by the intersection of the Nyquist plot and the Z' axis in high frequency area. The ionic conductivities of GPEs can be calculated and displayed in Table 2. The lower ionic conductivity of GPE-E resulted from the consumption of the hydrophilic amino groups in the SPI via the crosslinking reaction with EGDE. GPE-H has a higher ionic conductivity and electrolyte uptake than GPE-S and GPE-E. The addition of hydrophilic HEC compensates for the loss of hydrophilic functional groups due to crosslinking reaction and due to HEC facilitating the transport of electrolyte ions, which can be obviously observed from the electrolyte uptake results in Figure 5c. SPI membrane has a good ionic conductivity due to the good hydrophilicity as well as ionic property to facilitate the ion transport of the polymer electrolyte. This is an important factor in the selection of SPI as a polymer matrix of polymer electrolyte. Furthermore, comparison between GPE-H and GPE-C was carried out. GPE-C exhibits a higher ionic conductivity due to the high porosity facilitating the transport of ions.

Electrochemical Performance of SSCs
The electrochemical performance of SSCs was explored by EIS, CV and GCD techniques. For comparison, the electrochemical performance of SSC with GPE-C was also investigated. Nyquist plots of SSC-X (SSCs with different GPEs) in the frequency range of 0.01 Hz-100 kHz was shown in Figure 6. The intercept with real axis (Z' axis) in the high frequency region (Z" = 0) reflects equivalent series resistance (Rs) of SSC cells, which is associated with the ionic resistance of the electrolyte, the intrinsic resistance of the electrode material and the contact resistance at the interface of electrode material [47][48][49]. An approximate semicircular behavior at the high to medium frequency region refers to the interfacial charge transfer resistance (Rct), which is related to the process at the electrolyte/electrode interface [50]. A slope line at medium frequency region is related to diffusion of the electrolyte, its length depends on the frequency of electrolyte ions transporting between electrode and polymer electrolyte [51]. The straight line nearly vertical to the axis at low frequency region indicates excellent capacitive behavior of the SSC cells [52]. Electrostatic diffusion capacity and overall capacitance behavior of SSCs assembled by SPI membranes are superior to SSC assembled with GPE-C. Various electrical parameters of SSC-E associated with bulk properties of electrolyte and  As a natural polymer material, SPI has poor water resistance. To study the water resistance of SPI membranes, the membrane samples were submerged in deionized water for 24 h as shown in Figure 5d. MCM-S has been excessively swollen and almost lost its mechanical strength. Conversely, MCM-E and MCM-H exhibited better stability in deionized water.

Electrochemical Performance of SSCs
The electrochemical performance of SSCs was explored by EIS, CV and GCD techniques. For comparison, the electrochemical performance of SSC with GPE-C was also investigated. Nyquist plots of SSC-X (SSCs with different GPEs) in the frequency range of 0.01 Hz-100 kHz was shown in Figure 6. The intercept with real axis (Z' axis) in the high frequency region (Z" = 0) reflects equivalent series resistance (R s ) of SSC cells, which is associated with the ionic resistance of the electrolyte, the intrinsic resistance of the electrode material and the contact resistance at the interface of electrode material [47][48][49]. An approximate semicircular behavior at the high to medium frequency region refers to the interfacial charge transfer resistance (R ct ), which is related to the process at the electrolyte/electrode interface [50]. A slope line at medium frequency region is related to diffusion of the electrolyte, its length depends on the frequency of electrolyte ions transporting between electrode and polymer electrolyte [51]. The straight line nearly vertical to the axis at low frequency region indicates excellent capacitive behavior of the SSC cells [52]. Electrostatic diffusion capacity and overall capacitance behavior of SSCs assembled by SPI membranes are superior to SSC assembled with GPE-C. Various electrical parameters of SSC-E associated with bulk properties of electrolyte and electrode-electrolyte interface in different frequency ranges are worse than SSC-S. This is because crosslinked structures consume a large number of hydrophilic groups in the SPI and reduce the electrolyte uptake of MCM-E. Conversely, SSC-H exhibits a similar R s with SSC-S and shows a better capacitive behavior than SSC-S. HEC in GPE-H is beneficial for absorption of liquid electrolyte and the transport of ions, and the moderate swelling of GPE-H caused by HEC improves the interface performance and promotes the transport of ions at the electrode-electrolyte interface. Compared with SSC-H, SSC-C assembled with GPE-C shows a low R s due to its high ionic conductivity, however, SSC-C exhibits a larger ion transfer resistance between GPE-C and electrodes than SSC-H since MCM cannot be swelled by electrolyte, which results in a worse electrochemical performance than SSC-H.
Polymers 2019, 11, x FOR PEER REVIEW 9 of 17 electrode-electrolyte interface in different frequency ranges are worse than SSC-S. This is because crosslinked structures consume a large number of hydrophilic groups in the SPI and reduce the electrolyte uptake of MCM-E. Conversely, SSC-H exhibits a similar Rs with SSC-S and shows a better capacitive behavior than SSC-S. HEC in GPE-H is beneficial for absorption of liquid electrolyte and the transport of ions, and the moderate swelling of GPE-H caused by HEC improves the interface performance and promotes the transport of ions at the electrode-electrolyte interface. Compared with SSC-H, SSC-C assembled with GPE-C shows a low Rs due to its high ionic conductivity, however, SSC-C exhibits a larger ion transfer resistance between GPE-C and electrodes than SSC-H since MCM cannot be swelled by electrolyte, which results in a worse electrochemical performance than SSC-H. CV curves of SSCs at scan rates of 20 and 100 mV s −1 are shown in Figure 7a,b, respectively. All curves of SSCs have similar rectangular profiles, symmetric with respect to the X axis and without redox peaks. The CV curves of SSC-C and SSC-E deviate significantly from the rectangle, while those of SSC-S and SSC-H still maintain a good rectangular at high scan rates. In addition, integral area of the curves of SSC-S and SSC-H are almost overlapping, which are far greater than those of SSC-E and SSC-C, demonstrating that the SSC-S and SSC-H exhibit similar capacitance behaviors better than SSC-E and SSC-C. The result further illustrates that the introduction of HEC in GPE-H improved the electrolyte uptake and accelerated the transport of electrolyte ions at the electrode-electrolyte interface, which elevated the capacitance of SSC-H. CV curves of SSC-H at different scan rates (10-200 mV s −1 ), as depicted in Figure 7c, showed only a slight deviation from perfect rectangular even at high scan rate of 200 mV s −1 , which indicated that the capacitive behavior of SSC-H belongs to electrical double-layer formation and SSC-H had lower internal resistance. HEC in MCM-H accelerates transport of the electrolyte ions and reduces the equivalent series resistance of the GPE-H, which endows SSC with a higher specific capacitance and a better power performance. CV curves of SSCs at scan rates of 20 and 100 mV s −1 are shown in Figure 7a,b, respectively. All curves of SSCs have similar rectangular profiles, symmetric with respect to the X axis and without redox peaks. The CV curves of SSC-C and SSC-E deviate significantly from the rectangle, while those of SSC-S and SSC-H still maintain a good rectangular at high scan rates. In addition, integral area of the curves of SSC-S and SSC-H are almost overlapping, which are far greater than those of SSC-E and SSC-C, demonstrating that the SSC-S and SSC-H exhibit similar capacitance behaviors better than SSC-E and SSC-C. The result further illustrates that the introduction of HEC in GPE-H improved the electrolyte uptake and accelerated the transport of electrolyte ions at the electrode-electrolyte interface, which elevated the capacitance of SSC-H. CV curves of SSC-H at different scan rates (10-200 mV s −1 ), as depicted in Figure 7c, showed only a slight deviation from perfect rectangular even at high scan rate of 200 mV s −1 , which indicated that the capacitive behavior of SSC-H belongs to electrical double-layer formation and SSC-H had lower internal resistance. HEC in MCM-H accelerates transport of the electrolyte ions and reduces the equivalent series resistance of the GPE-H, which endows SSC with a higher specific capacitance and a better power performance. GCD curves of SSCs at the current density of 1 A g −1 are shown in Figure 8a. The curves of SSC-S and SSC-H exhibited an ideal linear profile, the charge curves were almost perfectly symmetrical with their corresponding discharge curve within the potential window (0-1.5 V). However, the GCD curves of SSC-E and SSC-C deviate from the ideal symmetrical triangle, which indicates that SSC-H has superior electrical double layer capacitance characteristics and reversibility of charge and discharge over SSC-E and SSC-C. The GCD curves of SSC-H at different current densities (1-10 A g −1 ) are shown in Figure 8b. The GCD curve of SSC-H still maintains a symmetrical linear shape even at high current density, which further demonstrates that SSC-H has a good reversibility charge and discharge. Csp and voltage drop (Vdrop, V) of the SSC-X were calculated according to the discharge profile and demonstrated in Figure 8c. The Csp of SSC-S and SSC-H decreased slightly and the Vdrop increased with the increase of current density due to internal resistance of SSCs. However, the Csp of SSC-E and SSC-C are greatly reduced and the Vdrop increase substantially with the increase of current density. This demonstrates that SSC-H has a better reversibility of charge and discharge and small internal resistance, which indicates that GPE-H can accelerates the transport of electrolyte ions and reduces the internal resistance of SSC-H due to the introduction of HEC. The Csp value of SSC-H was 98.91 F g −1 which is much higher than that of SSC-C (86.22 F g −1 ) at a current density of 1 A g −1 . Even at high current densities of 5 A g −1 , the Csp of SSC-H can reach 91.79 F g −1 , whereas the sharp decay of Csp of SSC-C with commercial membrane MCM-C is only 69.9 F g −1 . The Csp, Ecell, and Pcell of SSC-H were calculated at different current densities and listed in Table 3. GCD curves of SSCs at the current density of 1 A g −1 are shown in Figure 8a. The curves of SSC-S and SSC-H exhibited an ideal linear profile, the charge curves were almost perfectly symmetrical with their corresponding discharge curve within the potential window (0-1.5 V). However, the GCD curves of SSC-E and SSC-C deviate from the ideal symmetrical triangle, which indicates that SSC-H has superior electrical double layer capacitance characteristics and reversibility of charge and discharge over SSC-E and SSC-C. The GCD curves of SSC-H at different current densities (1-10 A g −1 ) are shown in Figure 8b. The GCD curve of SSC-H still maintains a symmetrical linear shape even at high current density, which further demonstrates that SSC-H has a good reversibility charge and discharge. C sp and voltage drop (V drop , V) of the SSC-X were calculated according to the discharge profile and demonstrated in Figure 8c. The C sp of SSC-S and SSC-H decreased slightly and the V drop increased with the increase of current density due to internal resistance of SSCs. However, the C sp of SSC-E and SSC-C are greatly reduced and the V drop increase substantially with the increase of current density. This demonstrates that SSC-H has a better reversibility of charge and discharge and small internal resistance, which indicates that GPE-H can accelerates the transport of electrolyte ions and reduces the internal resistance of SSC-H due to the introduction of HEC. The C sp value of SSC-H was 98.91 F g −1 which is much higher than that of SSC-C (86.22 F g −1 ) at a current density of 1 A g −1 . Even at high current densities of 5 A g −1 , the C sp of SSC-H can reach 91.79 F g −1 , whereas the sharp decay of C sp of SSC-C with commercial membrane MCM-C is only 69.9 F g −1 . The C sp , E cell , and P cell of SSC-H were calculated at different current densities and listed in Table 3.  Flexibility is a crucial parameter for SSCs applied in wearable electronic devices and portable devices. The electrochemical performance of SSC-H' was systematically measured under different bending angles as shown in Figure 9. The resulting CV curves, GCD curves and EIS curves with the bending angles of 0°, 30°, 45°, 90°, and 180° are shown in Figure 9a-e, respectively. Figure 9f is physical pictures of the various angles in the experiment. From the Nyquist plots in Figure 9a,b, it is known that at different angles, there is a small equivalent series resistance in the high frequency region, indicating that the bend has little effect on the equivalent series resistance; a vertical line close to 90° is shown in the low frequency region, indicating that the bend has almost no effect on the interface change between GPE-H and the electrode. The almost overlap CV and GCD curves for SSC-H' at different bending angles are obtained from the Figure 9c-e. Figure 9b presents the analogous CV responses and area. The as-measured GCD curves in Figure 9d and e show a negligible change at different bending angles at a current density of 1.0 A g −1 and 5.0 A g −1 . The approximate EIS, CV, and  Flexibility is a crucial parameter for SSCs applied in wearable electronic devices and portable devices. The electrochemical performance of SSC-H' was systematically measured under different bending angles as shown in Figure 9. The resulting CV curves, GCD curves and EIS curves with the bending angles of 0 • , 30 • , 45 • , 90 • , and 180 • are shown in Figure 9a-e, respectively. Figure 9f is physical pictures of the various angles in the experiment. From the Nyquist plots in Figure 9a,b, it is known that at different angles, there is a small equivalent series resistance in the high frequency region, indicating that the bend has little effect on the equivalent series resistance; a vertical line close to 90 • is shown in the low frequency region, indicating that the bend has almost no effect on the interface change between GPE-H and the electrode. The almost overlap CV and GCD curves for SSC-H' at different bending angles are obtained from the  Two SSC-H cells connected in series and parallel were evaluated by CV and GCD measurements as shown in Figure 10a,b. Two SSC-H cells could be connected in series or in parallel and obtain a device that outputted approximately twice the potential, or twice the current density and chargedischarge time of the single SSC-H. As shown in Figure 10c, the 2 V light-emitting diode (LED) could be lighted up by three SSC-H cells in series connection, which indicated their good practical prospects.
The long cycle life of SSCs is one of the most important factors with regard to its practical application. To investigate the cycling stability, the SSC-H cell was tested at a constant current density Two SSC-H cells connected in series and parallel were evaluated by CV and GCD measurements as shown in Figure 10a,b. Two SSC-H cells could be connected in series or in parallel and obtain a device that outputted approximately twice the potential, or twice the current density and charge-discharge time of the single SSC-H. As shown in Figure 10c, the 2 V light-emitting diode (LED) could be lighted up by three SSC-H cells in series connection, which indicated their good practical prospects. efficiency calculated from the ratio of the discharge capacitance to the charge capacitance, maintained almost as high as 100% for 5000 cycles, which indicates that SSC-H has good charge and discharge reversibility. Even though the cycle retention shows a slight fluctuation in the early stage of chargedischarge, it remained almost 100% even after 5000 cycles. The GCD curves still maintain a good symmetrical and linear shape and are almost the same as the initial GCD curves indicating an excellent electric double layer capacitive behavior and cycle stability of SSC-H.   The long cycle life of SSCs is one of the most important factors with regard to its practical application. To investigate the cycling stability, the SSC-H cell was tested at a constant current density of 2 A g −1 over 5000 continuous cycles. Figure 11 exhibits the variation of the coulombic efficiency and cycling retention as a function of the charge-discharge cycle numbers and the first 10th and last 10th charge-discharge curves of SSC-H are displayed in the inset of Figure 11. The coulombic efficiency calculated from the ratio of the discharge capacitance to the charge capacitance, maintained almost as high as 100% for 5000 cycles, which indicates that SSC-H has good charge and discharge reversibility. Even though the cycle retention shows a slight fluctuation in the early stage of charge-discharge, it remained almost 100% even after 5000 cycles. The GCD curves still maintain a good symmetrical and linear shape and are almost the same as the initial GCD curves indicating an excellent electric double layer capacitive behavior and cycle stability of SSC-H. Figure 10. (a) CV curves of two SSC-Hs connected in series and in parallel at a scan rate of 50 mV s −1 , (b) GCD curves of two SSC-Hs connected in series and in parallel at current density of 1 A g −1 , (c) photograph of the LED powered by three SSC-Hs connected in series. Figure 11. Cycle stability of SSC-H at a current density of 2.0 A g −1 . Figure 11. Cycle stability of SSC-H at a current density of 2.0 A g −1 .

Conclusions
A transparent, flexible, biodegradable membrane based on crosslinked SPI and hydrophilic HEC was successfully prepared, and the GPE-H was obtained by the membrane saturated with 1 M Li 2 SO 4 . The water resistance of the MCM-E is significantly improved due to the crosslinked structure of the membranes. However, the ionic conductivity of GPE-E is reduced. The addition of the HEC not only improves the water resistance of the MCM-H, but also facilitates the transport of electrolyte ions for GPE-H. The electrochemical performance of SSCs assembled from SPI composite membranes were higher than that of SSC assembled by commercial aqueous PP/PE membrane. SSC-H assembled with GPE-H exhibits a high capacitance of 91.79 F g −1 and an energy density of 7.17 W h kg −1 at a current density of 5 A g −1 , which is much higher than SSC-E assembled with GPE-E. According to the experiments at various angles, it was proved that SSC-H' fabricated with GPE-H has good flexibility. The solid-state supercapacitors also show considerable application performance at series a parallel connection condition. Electrochemical cycle performance of the SSC-H indicated almost 100% cycling retention and near 100% coulombic efficiency over 5000 charge-discharge cycles at 2 A g −1 . The results obtained in this work confirm that GPEs based on renewable, degradable biomass material are interesting candidates for replacing liquid electrolytes for flexible solid-state devices. Further investigations on regulating composition structure of gel polymer electrolyte based on SPI/HEC system to optimize the mechanical stability and improve the electrochemical performance such as energy density and specific capacitance are currently in progress.