Cross-Linked Polyacrylic-Based Hydrogel Polymer Electrolytes for Flexible Supercapacitors

Hydrogel polymer electrolytes (GPEs), as an important component of flexible energy storage devices, have gradually received wide attention compared with traditional liquid electrolytes due to their advantages of good mechanical, bending, and safety properties. In this paper, two cross-linked GPEs of poly(acrylic acid-co-acrylamide) or poly(acrylic acid-co-N-methylolacrylamide) with NaNO3 aqueous solution (P(AA-co-AM)/NaNO3 or P(AA-co-HAM)/NaNO3) were successfully prepared using radical polymerization, respectively, using acrylic acid (AA) as the monomer, N-methylolacrylamide (HAM) or acrylamide (AM) as the comonomer, and N, N-methylenebisacrylamide (MBAA) as the cross-linking agent. We investigated the morphology, glass transition temperature (Tg), ionic conductivities, mechanical properties, and thermal stabilities of the two GPEs. By comparison, P(AA-co-HAM)/NaNO3 GPE exhibits a higher ionic conductivity of 2.00 × 10−2 S/cm, lower Tg of 152 °C, and appropriate mechanical properties, which are attributed to the hydrogen bonding between the -COOH and -OH, and moderate cross-linking. The flexible symmetrical supercapacitors were assembled with the two GPEs and two identical activated carbon electrodes, respectively. The results show that the flexible supercapacitor with P(AA-co-HAM)/NaNO3 GPE shows good electrochemical performance with a specific capacitance of 63.9 F g−1 at a current density of 0.2 A g−1 and a capacitance retention of 89.4% after 3000 charge–discharge cycles. Our results provide a simple and practical design strategy of GPEs for flexible supercapacitors with wide application prospects.


Introduction
Over the past decade, with the rapid development of portable electronics and electric vehicles, batteries and supercapacitors as electrical energy storage devices have received increasing attention from both academia and industry [1].Batteries as traditional energy storage devices own high energy density, but they have low power outputting capability [2][3][4][5][6].In contrast, supercapacitors are promising for applications because of their higher power density and efficiency, as well as higher cycle durability, along with faster charging and discharging capability and lower cost [7][8][9].In supercapacitors, electrolytes play a key role in determining the electrochemical and mechanical properties of supercapacitors.Especially, some supercapacitors require flexibility and resistance to deformation to meet the demands of applications under different conditions [10][11][12][13][14].However, conventional liquid electrolytes have serious limitations in that there is the risk of accidental leakage when the supercapacitor is repeatedly bent or compressed.Therefore, considering the high ionic conductivity, tunable mechanical property, flexibility, and dimensional stability, the hydrogel polymer electrolytes (GPEs) are the ideal candidate for flexible all-solid-state supercapacitors [15,16].Nowadays, many efforts have been devoted to the development of GPEs as an alternative to liquid electrolytes, aiming to enable supercapacitors to be used under more demanding mechanical conditions [17,18].
focuses on the effect of hydrogen bonding interactions on the Tg, ionic conductivities and mechanical properties of the GPEs and electrochemical performance of the assembled supercapacitors.The synthesized P(AA-co-HAM)/NaNO3 GPE has a wide range of application prospects for flexible, wearable, and smart energy storage devices.

Preparation of Electrodes and Assembly of Supercapacitors
The electrode was prepared as shown below.The carbon cloth was first soaked in a 3:1 mixture of 10% HNO3 and 10% H2SO4 for 12 h for hydrophilic treatment, washed with deionized water three times before being ultrasonically cleaned with deionized water for 30 min, and then dried to remove the residual HNO3 and H2SO4 from the carbon cloth.Appropriate amounts of activated carbon (AC), graphite, PVDF binder with the mass ratio of 8:1:1, and an adequate amount of ethanol were added to a beaker, and, after stirring for 30 min, a well-mixed electrode slurry was obtained.The carbon cloth was cut to a suitable size (10 mm × 30 mm), and the slurry was uniformly coated on the cleaned carbon cloth (the loaded active substance was about 5 mg), placed in a vacuum drying oven at 80 °C for 24 h, and then cooled to room temperature to obtain an activated carbon electrode.
The prepared GPE was sandwiched between two activated carbon electrodes, packaged with PET film, and then sealed using hot melt adhesive to obtain a supercapacitor device.Figure 3 shows the preparation process of P(AA-co-HAM)/NaNO3 GPE, activated carbon electrode, and the assembly of a supercapacitor.

Preparation of Electrodes and Assembly of Supercapacitors
The electrode was prepared as shown below.The carbon cloth was first soaked in a 3:1 mixture of 10% HNO 3 and 10% H 2 SO 4 for 12 h for hydrophilic treatment, washed with deionized water three times before being ultrasonically cleaned with deionized water for 30 min, and then dried to remove the residual HNO 3 and H 2 SO 4 from the carbon cloth.Appropriate amounts of activated carbon (AC), graphite, PVDF binder with the mass ratio of 8:1:1, and an adequate amount of ethanol were added to a beaker, and, after stirring for 30 min, a well-mixed electrode slurry was obtained.The carbon cloth was cut to a suitable size (10 mm × 30 mm), and the slurry was uniformly coated on the cleaned carbon cloth (the loaded active substance was about 5 mg), placed in a vacuum drying oven at 80 • C for 24 h, and then cooled to room temperature to obtain an activated carbon electrode.
The prepared GPE was sandwiched between two activated carbon electrodes, packaged with PET film, and then sealed using hot melt adhesive to obtain a supercapacitor device.Figure 3 shows the preparation process of P(AA-co-HAM)/NaNO 3 GPE, activated carbon electrode, and the assembly of a supercapacitor.

Characterization
Fourier transform infrared (FTIR) spectra of samples were collected with a spectrometer (Bruker Tyskland, Equiox 55, Bruker, Germany) in the range of 400-4000 cm −1 with a resolution of 6 cm −1 for 32 scans of GPEs using the KBr pellet technique.A integrated ther-

Characterization
Fourier transform infrared (FTIR) spectra of samples were collected with a spectrometer (Bruker Tyskland, Equiox 55, Bruker, Germany) in the range of 400-4000 cm −1 with a resolution of 6 cm −1 for 32 scans of GPEs using the KBr pellet technique.A integrated thermal analyzer (Beijing Hengjiu, HCT-4, Beijing, China) was used to assess T g and thermal stability of GEPs from room temperature to 400 • C under N 2 atmosphere at a heating rate of 5 • C/min.A scanning electron microscope (SEM, Zeiss, Gemini SEM 300, Shanghai, China) was used to investigate the morphology and microstructure of GPEs.The mechanical properties of GPEs (30 mm × 15 mm × 2 mm) were tested at a constant tensile speed of 2 mm/min at room temperature using a tensile tester (Wenteng Testing Instruments, XLW (PC)-500N, Jinan, China).The ionic conductivity of GPEs was determined at room temperature using a four-probe conductivity meter (Guangzhou Four Probe Technology Co., RTS-9, Guangzhou, China) [27,28].The electrochemical performance of supercapacitors were investigated by the following methods.Cyclic voltammetry (CV) tests were performed by a computerized electroanalytical system (Tianjin Lannico, LK98B II, Tianjin, China) at room temperature in the potential window range of 0 to 1 V at different scan rates from 10 mV/s to 100 mV/s.Charge-discharge (GCD) measurements were performed using an electrochemical workstation (LAND Wuhan Jinnuo, CT2001A, Wuhan, China) at current densities from 0.2 A g −1 to 1.0 A g −1 .EIS measurements were performed over a frequency range of 0.01 Hz to 100 kHz with an amplitude of 10 mV−0.1 V at the open-circuit potential of −0.1 V with the electrochemical workstation (Shanghai Chenhua, CHI660E-B19775, Shanghai, China).The specific capacitance (C g , F g −1 ) of supercapacitors was calculated based on GCD curves with the following Equation (1).
The specific energy (E, Wh kg −1 ) and specific power (P, W kg −1 ) of supercapacitors were calculated with the following Equation ( 2).
where I (A) is the current density, m is the total mass of active substances on both electrodes (g), ∆t is the discharge time (s), and ∆v is the voltage range after the IR drop during the discharge process (V) [29].

Infrared Analysis
The compositions of AA, HAM, and P(AA-co-HAM) were studied using FTIR spectra, as shown in Figure 4a.The spectrum of AA shows three main significant vibration peaks in the range of 500-4000 cm −1 : stretching vibration peak of O-H at 3430 cm −1 , the stretching vibration peak of C=O and C=C at 1695 and 1635 cm −1 in the -COOH, respectively [30][31][32].The main characteristic peaks of HAM appear at 3443 cm −1 (N-H asymmetric stretching vibration), 2935 cm −1 (-CH 2 asymmetric stretching vibration), 1737 cm −1 (C=O stretching vibration), 1640 cm −1 (C=C stretching vibration), 1274 cm −1 (N-C stretching vibration), and 1021 cm −1 (C-O stretching vibration) [30,32,33].In contrast, P(AA-co-HAM) show the characteristic peaks of AA and HAM, and the C=C stretching vibration peaks of AA and HAM at 1635 and 1640 cm −1 disappear after P(AA-co-HAM) was synthesized, confirming the synthesis of P(AA-co-HAM)/NaNO 3 GPE.Figure 4b shows the FTIR spectra of AA, AM, and P(AA-co-AM).The main characteristic peaks of AM appear in the regions of 3100 cm −1 to 3500 cm −1 (N-H band), 1650 cm −1 (C=O stretching vibration), and 1618 cm −1 (C=C stretching vibration) [32,34].Similarly, P(AA-co-AM) show the characteristic peaks show the characteristic peaks of AA and HAM, and the C=C stretching vibration peaks of AA and HAM at 1635 and 1640 cm −1 disappear after P(AA-co-HAM) was synthesized, confirming the synthesis of P(AA-co-HAM)/NaNO3 GPE. Figure 4b shows the FTIR spectra of AA, AM, and P(AA-co-AM).The main characteristic peaks of AM appear in the regions of 3100 cm −1 to 3500 cm −1 (N-H band), 1650 cm −1 (C=O stretching vibration), and 1618 cm −1 (C=C stretching vibration) [32,34].Similarly, P(AA-co-AM) show the characteristic peaks of AA and AM, and the C=C stretching vibration peaks of AA and AM at 1635 and 1618 cm −1 disappear after P(AA-co-AM) was synthesized, confirming the synthesis of P(AA-co-AM)/NaNO3 GPE.

Mechanical Properties Analysis
The mechanical strength of GPEs is the essential property for the application of flexible supercapacitors.The stress-strain curves of P(AA-co-HAM)/NaNO3 GPE and P(AAco-AM)/NaNO3 GPE are presented in Figure 5. From the curves, it can be seen that both GPEs exhibits the characteristics of elastomers.The maximum stress of P(AA-co-HAM)/NaNO3 GPE and P(AA-co-AM)/NaNO3 GPE are 9.6 KPa and 13.9 KPa, respectively.The fracture strain of P(AA-co-AM)/NaNO3 GPE and P(AA-co-HAM)/NaNO3 GPE are 26% and 20%, respectively.By comparison, P(AA-co-HAM)/NaNO3 GPE shows slightly lower fracture stress and strain than P(AA-co-AM)/NaNO3 GPE, which could be due to the fact that before polymerization -COOH in AA monomer can form stronger hydrogen bonding with -OH in HAM monomer than -NH2 in AM monomer, resulting in the slightly lower cross-linking degree and mechanical properties of synthesized P(AA-co-HAM)/NaNO3 GPE than those of P(AA-co-AM)/NaNO3 GPE.

Mechanical Properties Analysis
The mechanical strength of GPEs is the essential property for the application of flexible supercapacitors.The stress-strain curves of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE are presented in Figure 5. From the curves, it can be seen that both GPEs exhibits the characteristics of elastomers.The maximum stress of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE are 9.6 KPa and 13.9 KPa, respectively.The fracture strain of P(AA-co-AM)/NaNO 3 GPE and P(AA-co-HAM)/NaNO 3 GPE are 26% and 20%, respectively.By comparison, P(AA-co-HAM)/NaNO 3 GPE shows slightly lower fracture stress and strain than P(AA-co-AM)/NaNO 3 GPE, which could be due to the fact that before polymerization -COOH in AA monomer can form stronger hydrogen bonding with -OH in HAM monomer than -NH 2 in AM monomer, resulting in the slightly lower cross-linking degree and mechanical properties of synthesized P(AA-co-HAM)/NaNO 3 GPE than those of P(AA-co-AM)/NaNO 3 GPE.

Thermal Stability Analysis
Tg and thermal stabilities of GPEs are very important for their applications.The lower Tg of GPEs means that it has more flexible chains, which helps the transport of electrolyte ions.So we carried out the DSC and TG measurements to determine the Tg and thermal stability properties of the synthesized GPE.
Figure 6 shows DSC and TG curves of P(AA-co-HAM)/NaNO3 GPE and P(AA-co-AM)/NaNO3 GPE.It can be clearly seen that the Tg of P(AA-co-HAM)/NaNO3 GPE and P(AA-co-AM)/NaNO3 GPE are ~152 °C and ~194 °C, respectively.By comparison, P(AAco-HAM)/NaNO3 GPE shows a lower Tg with the promoted movement of polymer seg-

Thermal Stability Analysis
T g and thermal stabilities of GPEs are very important for their applications.The lower T g of GPEs means that it has more flexible chains, which helps the transport of electrolyte ions.So we carried out the DSC and TG measurements to determine the T g and thermal stability properties of the synthesized GPE.
Figure 6 shows DSC and TG curves of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE.It can be clearly seen that the T g of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE are ~152 • C and ~194 • C, respectively.By comparison, P(AA-co-HAM)/NaNO 3 GPE shows a lower T g with the promoted movement of polymer segments and increased flexibility, which greatly contributes to the transport of electrolyte ions, thus endows its supercapacitor superior electrochemical performance.It also can be seen that there is no significant difference in the thermal stabilities of the two GPEs.There are mainly three thermal decomposition steps.An initial weight loss of 5-10 wt.% in the temperature range of 50-150 • C is due to the evaporation of absorbed water.In the temperature range of 150-250 • C, the two GPEs show significant weight loss of 10-25 wt.%, which is due to the decomposition of groups in the polymer backbone.A rapid weight loss of 25-50 wt.% takes place over the temperature range of 250-400 • C, which is attributed to the degradation of polymer chains.

Thermal Stability Analysis
Tg and thermal stabilities of GPEs are very important for their applications.The lower Tg of GPEs means that it has more flexible chains, which helps the transport of electrolyte ions.So we carried out the DSC and TG measurements to determine the Tg and thermal stability properties of the synthesized GPE.
Figure 6 shows DSC and TG curves of P(AA-co-HAM)/NaNO3 GPE and P(AA-co-AM)/NaNO3 GPE.It can be clearly seen that the Tg of P(AA-co-HAM)/NaNO3 GPE and P(AA-co-AM)/NaNO3 GPE are ~152 °C and ~194 °C, respectively.By comparison, P(AAco-HAM)/NaNO3 GPE shows a lower Tg with the promoted movement of polymer segments and increased flexibility, which greatly contributes to the transport of electrolyte ions, thus endows its supercapacitor superior electrochemical performance.It also can be seen that there is no significant difference in the thermal stabilities of the two GPEs.There are mainly three thermal decomposition steps.An initial weight loss of 5-10 wt.% in the temperature range of 50-150 °C is due to the evaporation of absorbed water.In the temperature range of 150-250 °C, the two GPEs show significant weight loss of 10-25 wt.%, which is due to the decomposition of groups in the polymer backbone.A rapid weight loss of 25-50 wt.% takes place over the temperature range of 250-400 °C, which is attributed to the degradation of polymer chains.

Ionic Conductivity Analysis
The ionic conductivities of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE at room temperature for different periods (0, 7, and 14 days) are given in Table 1.At 0 day, 7 days, and 14 days, the conductivities of P(AA-co-HAM)/NaNO 3 GPE are 2.00 × 10 −2 S/cm, 1.96 × 10 −2 S/cm and 1.82 × 10 −2 S/cm, respectively, while those of P(AA-co-AM)/NaNO 3 GPE are 6.13 × 10 −3 S/cm, 5.95 × 10 −3 S/cm, and 5.56 × 10 −3 S/cm, respectively.Both the two GPEs display high ionic conductivities, and with the increase of time, their conductivities do not decay significantly, indicating that they have good electrolyte retention capacities.By comparison, the ionic conductivities of the prepared GPEs, especially that of P(AA-co-HAM)/NaNO 3 GPE, are higher than those of the reported GPEs (as shown in Table S1).It is due to the fact that P(AA-co-HAM)/NaNO 3 GPE has lower T g (as shown in Figure 6a) and richer pore structure (as shown in Figure S1), which are conducive to the migration and rapid diffusion of electrolyte ions [35,36].It is expected that the supercapacitor with P(AA-co-HAM)/NaNO 3 GPE will exhibit a higher electrochemical properties.
Their electrochemical properties were investigated.The CV curves of the two AC-based symmetrical supercapacitors with different GPEs at 100 mV s −1 are given in Figure 7a.It can be found that CV curves of two supercapacitors show rectangular shapes under the voltage window from 0 V to 1.0 V, indicating good double layer capacitance behavior.By comparison, the AC//P(AA-co-HAM)/NaNO 3 //AC has a rectangular shape with the largest closed area, showing the best capacitive behavior.In Figure 7b, the GCD curves of the two supercapacitors with different GPEs show the symmetrical triangular shapes at 0.2 A g −1 .By comparison, the charge-discharge curve of AC//P(AAco-HAM)/NaNO 3 //AC has the longest discharge time.Based on the Equation (1), the C g value of AC//P(AA-co-HAM)/NaNO 3 //AC is 63.9 F g −1 at 0.2 A g −1 , which is higher than that of AC//P(AA-co-AM)/NaNO 3 //AC (50.1 F g −1 ).Moreover, as shown in Figure 7c and Figure S2a, even at a scan rate of 100 mV s −1 , the CV curves of AC//P(AA-co-HAM)/NaNO 3 //AC and AC//P(AA-co-AM)/NaNO 3 //AC show no distortion, implying their good rate performance [37,38].Figure 7d and Figure S2b show their GCD curves from 0.2 A g −1 to 1 A g −1 .As the current density increases, it can be seen that their GCD curves remain symmetrically triangular, indicating typical reversible charging and discharging behavior [39], further confirming its good rate performance.In order to compare the performance of the GPEs and conventional aqueous electrolytes, the conventional aqueous supercapacitor was assembled with 3 mol/L NaNO 3 aqueous solution as the electrolyte.As can be seen from its CV curves (Figure 7e) and GCD curve (Figure 7f), the conventional aqueous supercapacitor also shows a rectangular shape and symmetrical triangle, and its calculated C g value is 34.8 F g −1 at 0.2 A g −1 , obviously lower than those of the two supercapacitors with GPEs.It indicates that the synthesized GPEs, especially P(AA-co-HAM)/NaNO 3 GPE, are superior to conventional aqueous electrolytes.
Polymers 2024, 16, 800 9 of 13 bled with 3 mol/L NaNO3 aqueous solution as the electrolyte.As can be seen from curves (Figure 7e) and GCD curve (Figure 7f), the conventional aqueous supercap also shows a rectangular shape and symmetrical triangle, and its calculated Cg va 34.8 F g −1 at 0.2 A g −1 , obviously lower than those of the two supercapacitors with G indicates that the synthesized GPEs, especially P(AA-co-HAM)/NaNO3 GPE, are su to conventional aqueous electrolytes.The C g values of two supercapacitors with different GPEs at different current densities are shown in Figure 8a.When the current density increases from 0.2 A g −1 to 1.0 A g −1 , the C g value of AC//P(AA-co-HAM)/NaNO 3 //AC decreases from 63.9 F g −1 to 37.6 F g −1 , showing a better rate capability with a capacitance retention of 58.8% than AC//P(AA-co-AM)/NaNO 3 //AC (50.2%).Figure 8b shows Nyquist plots for two symmetrical supercapacitors with different GPEs.Both diagrams include a semicircle and sloping line.In the low-frequency region, the Nyquist plot of AC//P(AA-co-HAM)/NaNO 3 //AC is closer to the imaginary axis than that of AC//P(AA-co-AM)/NaNO 3 //AC, which indicates that AC//P(AA-co-HAM)/NaNO 3 //AC has the faster diffusion rate of electrolyte ions.
Polymers 2024, 16, 800 10 of 13 Furthermore, it is clear at high frequencies the semicircular diameter of AC//P(AA-co-HAM)/NaNO 3 //AC is smaller than that of AC//P(AA-co-AM)/NaNO 3 //AC, which indicates its smaller charge transfer resistance, contributing to its better capacitive performance.The cycling stabilities of the two symmetrical supercapacitors with different GPEs are given in Figure 8c.After 3000 charge-discharge cycles, the capacitance retention of AC//P(AA-co-HAM)/NaNO 3 //AC is 89.4%, which is higher than AC//P(AA-co-AM)/NaNO 3 //AC (83.9%), showing its better cycling stability.It is attributed to the cross-linking structure stability of P(AA-co-HAM)/NaNO 3 GPE because of the stronger hydrogen bonding interactions between -COOH and -OH in P(AA-co-HAM)/NaNO 3 GPE than those between -COOH and -NH 2 in P(AA-co-AM)/NaNO 3 GPE.Figure 8d shows Ragone plots of the two supercapacitors with different GPEs.The AC//P(AAco-HAM)/NaNO 3 //AC achieves a specific energy of 7.83 Wh kg −1 at a specific power of 93.98 W kg −1 , which is superior to that of AC//P(AA-co-AM)/NaNO 3 //AC (a specific energy of 4.24 Wh kg −1 at a specific power of 81.30W kg −1 ).Based on the above analysis, it is concluded that the synthesized P(AA-co-HAM)/NaNO 3 GPE is superior to P(AA-co-AM)/NaNO 3 GPE and conventional aqueous electrolyte for supercapacitor applications.To further investigate the mechanical flexibility of the optimal AC//P(AA-co-HAM)/NaNO 3 //AC, its CV tests were performed under various bending conditions.As shown in Figure 8e,f, at different bending angles (90 • and 180 • ) and repeat bending cycles from 0 to 100 cycles, all the CV curves display a similar shape and area, indicating that the bending conditions do not obviously influence the electrochemical performance and the synthesized P(AA-co-HAM)/NaNO 3 GPE is suitable for flexible supercapacitors with high performance.

Conclusions
In conclusion, two cross-linked hydrogel polymer electrolytes of P(A HAM)/NaNO3 and P(AA-co-AM)/NaNO3 were successfully developed by r polymerization at room temperature.By comparison, due to stronger hydrogen bo between the -COOH and -OH, as well as moderate cross-linking, P(AA-co-HAM)/N hydrogel polymer electrolyte exhibits a higher ionic conductivity of 2.00 × 10 −2 S/cm, glass transition temperature of 152 °C, and appropriate mechanical properties.The a bled AC//P(AA-co-HAM)/NaNO3//AC symmetrical supercapacitor show the speci pacitance of 63.9 F g −1 at 0.2 A g −1 , capacitance retention of 89.4% after 3000 chang charge cycles, and the power density of 93.98 W kg −1 at an energy density of 7.83 Wh

Conclusions
In conclusion, two cross-linked hydrogel polymer electrolytes of P(AA-co-HAM)/NaNO 3 and P(AA-co-AM)/NaNO 3 were successfully developed by radical polymerization at room temperature.By comparison, due to stronger hydrogen bonding between the -COOH and -OH, as well as moderate cross-linking, P(AA-co-HAM)/NaNO 3 hydrogel polymer electrolyte exhibits a higher ionic conductivity of 2.00 × 10 −2 S/cm, lower glass transition temperature of 152 • C, and appropriate mechanical properties.The assembled AC//P(AA-co-HAM)/NaNO 3 //AC symmetrical supercapacitor show the specific capacitance of 63.9 F g −1 at 0.2 A g −1 , capacitance retention of 89.4% after 3000 change-discharge cycles, and the power density of 93.98 W kg −1 at an energy density of 7.83 Wh kg −1 , which is obviously higher than those of supercapacitors assembled with P(AA-co-AM)/NaNO 3 GPE and a conventional aqueous electrolyte.Thus, P(AA-co-HAM)/NaNO 3 GPE would be a promising electrolyte for flexible and high-performance supercapacitor devices.
Author Contributions: Methodology, investigation, visualization, validation, and writing-original draft preparation, L.S.; writing-review and editing, formal analysis and methodology, P.J.; investigation, P.Z.; data curation, N.D.; investigation, Q.L.; conceptualization, project administration, funding acquisition, and resources, C.Q.All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the National Natural Science Foundation of China (21971057) and the Joint guidance project of Natural Science Foundation of Heilongjiang Province (LH2020E103).
Institutional Review Board Statement: Not applicable.

2. 2 . 1 .
Synthesis of P(AA-co-HAM)/NaNO3 GPE An amount of 1 mL of AA, 1.01 g of HAM, 20 mg of MBAA, 30 mg of APS, and 4 mL of NaNO3 solution (3 mol/L) were sequentially added to a 10 mL beaker, mixed, and stirred for 2 h at room temperature.The obtained solution was poured into a PTFE mold, placed in a vacuum oven, and held at 60 °C for 1.5 h.It was then cooled to room temperature and demolded to obtain a solid GPE (40 mm × 20 mm × 5 mm).The synthesis route of P(AA-co-HAM)/NaNO3 GPE is shown in Figure1.

Figure 1 .
Figure 1.The synthesis route of P(AA-co-HAM)/NaNO3 GPE.Figure 1.The synthesis route of P(AA-co-HAM)/NaNO 3 GPE.2.2.2.Synthesis of P(AA-co-AM)/NaNO 3 GPE 0.71 g of AM were added instead of 1.01 g of HAM.The other regents and synthesis process are the same as those for the preparation of P(AA-co-HAM)/NaNO 3 GPE.The synthesis route of P(AA-co-AM)/NaNO 3 GPE is shown in Figure 2.

2 .
Synthesis of P(AA-co-AM)/NaNO3 GPE 0.71 g of AM were added instead of 1.01 g of HAM.The other regents and synthesis process are the same as those for the preparation of P(AA-co-HAM)/NaNO3 GPE.The synthesis route of P(AA-co-AM)/NaNO3 GPE is shown in Figure2.

Polymers 2024 , 14 Figure 3 .
Figure 3. Schematic representation of the preparation process of P(AA-co-HAM)/NaNO3 GPE, activated carbon electrode, and the assembly of a supercapacitor.

Figure 3 .
Figure 3. Schematic representation of the preparation process of P(AA-co-HAM)/NaNO 3 GPE, activated carbon electrode, and the assembly of a supercapacitor.
Polymers 2024,16, 800 6 of 13 of AA and AM, and the C=C stretching vibration peaks of AA and AM at 1635 and 1618 cm −1 disappear after P(AA-co-AM) was synthesized, confirming the synthesis of P(AA-co-AM)/NaNO 3 GPE.

Figure 7 .
Figure 7. Electrochemical performance of AC-based symmetrical supercapacitors with d GPEs: (a) CV curves at 100 mV s −1 and (b) GCD curves at 0.2 A g −1 ; electrochemical perform AC-based symmetrical supercapacitors with P(AA-co-HAM)/NaNO3 GPE: (c) CV curves at ent scan rates, and (d) GCD curves at different current densities; electrochemical performa

Figure 7 .
Figure 7. Electrochemical performance of AC-based symmetrical supercapacitors with different GPEs: (a) CV curves at 100 mV s −1 and (b) GCD curves at 0.2 A g −1 ; electrochemical performance of ACbased symmetrical supercapacitors with P(AA-co-HAM)/NaNO 3 GPE: (c) CV curves at different scan rates, and (d) GCD curves at different current densities; electrochemical performance of conventional aqueous supercapacitor with NaNO 3 aqueous solution: (e) CV curves at different scan rates, and (f) GCD curves at different current densities.

Figure 8 .
Figure 8. Electrochemical performance of AC-based symmetrical supercapacitors with different GPEs: (a) specific capacitances at different current densities, (b) Nyquist plots, and (c) cycling stability measured at 1 A g −1 for 3000 cycles; (d) Ragone plot; CV curves of AC-based symmetrical supercapacitors with P(AA-co-HAM)/NaNO 3 GPE; (e) with different bending angles; and (f) with different bending cycles, inset: photos under the bending conditions.

Table 1 .
Ionic conductivity of P(AA-co-HAM)/NaNO 3 GPE and P(AA-co-AM)/NaNO 3 GPE at room temperature for different periods.