Effect of plasma jet on electrochemical properties of silk fibroin hydrogel doped with PEDOT:PSS

In this study, by injecting cold plasma into the silk fibroin (SF) solution, and adding poly (3,4‐ethylenedioxythiophene): polystyrene sulfonic acid (PEDOT:PSS) into the SF solution, the conductive SF‐PEDOT:PSS hydrogel modified by plasma was successfully prepared. The changes of the electrochemical parameters and structure of SF‐PEDOT:PSS hydrogels after plasma modification were observed with plasma discharge time (60, 120, 180 s) and the type of working gas (Ar, air, He) as variables. The results of electrochemical impedance spectroscopy show that both argon plasma and helium plasma can reduce the impedance of SF‐PEDOT:PSS hydrogel in the extremely low‐frequency range (10−2–10° Hz), and a longer discharge time shrinks the impedance, while the impact of air plasma on the impedance of hydrogel is not obvious. The capacitance results show that both argon and helium plasma can make the capacitance of hydrogel larger, and the longer discharge time leads to the larger capacitance. While air plasma can reduce the capacitance of hydrogel. The results of Fourier infrared absorption spectroscopy and X‐ray diffraction spectrum show that the secondary structure of SF is mainly β‐fold. The results of Raman show that PEDOT:PSS is successfully incorporated into hydrogel, and there exists random coil conformation of SF in hydrogel. The results of X‐ray photoelectron spectroscopy show that the SF‐PEDOT:PSS hydrogel mainly contains C, O, and N elements.

the field of energy storage has rarely been reported. In the field of energy storage, supercapacitor 11 is a very important concept. Because of its soft, comfortable, excellent mechanical properties, biocompatibility, and many other advantages, it is more and more popular among researchers. The supercapacitor is mainly composed of two active electrodes 12 and a hydrogel electrolyte. 13 The hydrogel electrolyte is sandwiched between the two active electrodes to transfer ions. From the perspective of raw materials, hydrogels are mainly divided into synthetic hydrogels and natural hydrogels. 14 Synthetic hydrogel materials mainly include polyacrylamide, 15 polyvinyl alcohol, 16 polyethylene glycol, 17 and so forth. Natural hydrogel materials mainly include sodium alginate, 18 chitosan 19 silk fibroin (SF), 20 and so forth. Although the mechanical properties of synthetic hydrogels are good, they lack biological activity. However, natural hydrogels have been studied a lot due to their excellent biocompatibility. 21 Among them, SF hydrogel has good biocompatibility, mechanical properties, and degradability. It shows a good development trend in some fields in recent years. In the field of biomedical science, SF hydrogel can be used for drug release, 22 in vivo stent, 23 and skin repair. 24 In the field of energy device, SF hydrogel can be used as flexible electrode 25 and bionic sensor. 26 The SF hydrogel can be given conductivity by doping some conductive substances, such as graphene, 27 sodium chloride, PEDOT:PSS. Traditionally, the electrochemical properties of hydrogels were improved by changing the content of doped conductive materials, which inevitably resulted in the loss of mechanical properties. 28 On the premise of not damaging the mechanical properties of materials, cold plasma can have impact on biological materials. 29 In the field of supercapacitor, plasma was mainly used to treat active electrodes to improve the electrochemical performance of supercapacitor, 30 while plasma treatment of hydrogel electrolyte was less studied. Therefore, the effect of cold plasma on the electrochemical performance of hydrogel electrolyte was studied.
In this work, taking the plasma discharge time and the type of plasma working gas as variables, the SF solution was modified by injecting cold plasma, then conductive polymer poly (3,4-ethylenedioxythiophene): polystyrene sulfonic acid (PEDOT:PSS) and surfactant sodium dodecyl sulfate 31 (SDS) were added to prepare SF-PEDOT:PSS conductive hydrogel. PEDOT: PSS was selected as the conductive material because it is a liquid polymer with high electrical conductivity, which can reach 500 S cm −1 . It is liquid at room temperature and can be uniformly mixed with SF solution. 32 The electrochemical measurement results show that cold plasma can affect the electrochemical properties (including impedance and capacitance) of SF-PEDOT:PSS conductive hydrogel. This research result can provide some guidance value for the future application of cold plasma in the field of energy storage.

| Design of cold plasma jet device
The cold plasma jet device adopts a double electrode structure. 33 Copper clips are used to connect the copper tube with the CTP-2000K plasma generator high-voltage output terminal, the copper tube as the positive electrode. The inner diameter of the copper tube is 3 mm. The upper copper tube is connected to the gas supply device (argon cylinder, air compressor, helium cylinder). The flow rate is regulated by the rotor flowmeter. The gas flow rate in this study is set to 2 L/ min. Regulate the voltage regulator and output frequency adjustment knob of the plasma generator to make highvoltage ionized gas flow in copper tube, and generate cold plasma jet at the copper tube outlet. The plasma ionization parameters are set in Table 1. The plasma discharge time is 60, 120, and 180 s. For safety, a glass tube is sheathed on the outside of the copper tube. The inner diameter of the glass tube is 4 mm and the thickness is 2 mm. The outlet of the glass tube is a thin glass tube with an inner diameter of 3 mm and a height of 10 mm. The negative electrode of the device is a square copper sheet with a side length of 65 mm, which is placed under the copper tube and connected to the low-voltage output terminal of the plasma generator with a copper clip. The plasma generator shall be grounded. Figure 1 displays plasma jet device.

| Experimental method
The preparation of SF solution is carried out according to the traditional method of degumming-dissolutiondialysis. 34 The solution of ternary mixed system of CaCl 2 :C 2 H 5 OH:H 2 O (molar ratio is 1:2:8) is used to dissolve silk. The concentration of regenerated SF solution in this study is 3.9085%. With the plasma discharge time (60, 120, 180 s) and plasma working gas (argon, air, helium) as variables, inject cold plasma into the SF solution. The SF solution without plasma modification is used as the control group. Ten groups of SF solution samples are labeled blank, Ar60s, Ar120s, Ar180s, air60s, air120s, air180s, He60s, He120s, and He180s. Prepare 5% sodium dodecyl sulfate (SDS) solution. Add 600 μL SDS solution, 575 μL PEDOT:PSS solution with the concentration of 1.05% to 6 mL SF solution of each sample, SDS as the gel accelerator, PEDOT:PSS as the conductive material, then stir evenly. The homogenous solution obtained is allowed to stand, and the gels will naturally form within 15 min. Then the gel samples of each group were electrochemical tested and characterized.

| Impedance characterization
The impedance characteristics of the hydrogel electrolyte were characterized by the AC impedance method 35 (EIS) of the electrochemical workstation. The electrodes were two square platinum plates with a spacing of 17 mm, a side length of 5 mm, and a thickness of 0.1 mm. The initial voltage was set as 0 V, the frequency range was 0.01 Hz to 1 MHz, and the scanning speed was 0.1 V/s. The equivalent circuit for AC impedance test of hydrogel electrolyte is shown in Figure 2.
Wherein, R Ω is the gel internal resistance, R ct is the charge transfer resistance, Z w is the Warburg impedance, and C d is the double-layer capacitance.
The impedance Z of the equivalent circuit for AC impedance test can be expressed as follows, The impedance Z is a complex number, which can be expressed by the real part Z′ and the imaginary part Z″, T A B L E 1 Plasma ionization parameter settings.

Working gases Argon Air Helium
Excitation voltage/kV 20 26 wherein,  idV is the absolute area enclosed by the CV curve, v is the voltage scanning rate, and V is the scanning voltage range.

| Fourier infrared absorption spectroscopy 37 (FTIR)
FTIR is the spectrum produced by the energy level transition caused by the molecular rotation and vibration, which is produced by the light wave irradiation of the sample. It is used to analyze the material structure. Grind the freeze-dried hydrogel samples of each group to a diameter of less than 80 μm powder, then prepare samples by KBr compression. Conduct FTIR test, and measure the absorbance in the range of 400-4000 cm −1 .

| Raman spectrum 38 (Raman)
Raman spectroscopy mainly uses the Raman scattering of incident light on the sample to reflect the molecular structure of the tested sample, which is a supplement to FTIR spectroscopy. Raman test was carried out on the freeze-dried hydrogel powder samples of each group using a laser Raman spectrometer system. The excitation wavelength was 633 nm and the scanning range was 400-2000 cm −1 .

| X-ray diffraction spectrum 39 (XRD)
X-ray diffraction technology is a research means to obtain information about the composition, structure, and morphology of materials by X-ray diffraction. It is the main method to study the phase and crystal structure of materials. The multifunctional rotating X-ray diffractometer was used to conduct XRD test on the freeze-dried hydrogel powder samples of each group. The scanning range was 2θ = 5°-45°.
2.4.6 | X-ray photoelectron spectroscopy 40 (XPS) The XPS irradiates the tested sample with X-ray, excites the inner layer electrons and valence electrons of molecules or atoms in the sample, and draws the photoelectron spectrogram of the material by measuring the energy of the emitted photoelectrons, to obtain the element composition, element content, chemical structure, and other information of the tested sample. The elemental composition and content of the freeze-dried hydrogel powder samples were determined by X-ray photoelectron spectroscopy.

| RESULTS AND DISCUSSION
This study aims to explore the influence of cold plasma jet on the electrochemical performance and structure of hydrogel electrolyte. The electrochemical parameters include impedance and capacitance. It will support the application of cold plasma technology in the field of energy storage in the future. Among the plasma parameters that affect the electrochemical performance and structure of hydrogels, plasma discharge time and working gas type are two of the most important. Therefore, in this study, the plasma discharge time (60, 120, 180 s) and working gas type (Ar, air, He) were used as variables, and the hydrogel without plasma modification was used as the control group, to investigate the effect of cold plasma on the electrochemical performance and structure of the SF-PEDOT:PSS hydrogel.
3.1 | The effect of discharge time (60, 120, 180 s) on the electrochemical properties and structure of hydrogels when the working gas is argon The influence of argon plasma discharge time (60, 120, 180 s) on the impedance of SF-PEDOT:PSS hydrogel is shown in Figure 3. From normalized impedance modulus |Z| diagram and phase φ diagram, it can be seen that the argon plasma has no effect on the impedance of SF-PEDOT:PSS hydrogel in the range of medium and high frequencies (10 2 -10 5 Hz), and the internal resistance of the hydrogel at the highest frequency (10 5 Hz) has no change.
The impedance curve in the high-frequency range is a horizontal line, because the resistive impedance is frequency independent. In the low frequency (10 0 -10 2 Hz) range, the electric double-layer capacitance effect is generated between the hydrogel electrolyte and the electrodes, the phase angle develops to −90°, and the slope of the normalized impedance modulus curve starts to become negative. In the extremely low frequency (10 −2 -10 0 Hz) range, it is obvious that the argon plasma jet can reduce the SF-PEDOT:PSS hydrogel impedance by more than 50%, and a longer argon plasma's discharge time shrinks the hydrogel impedance. This is because when argon gas is ionized, a large number of charged particles can be generated, and then the ion concentration in the hydrogel is increased, so the ion diffusion effect is enhanced and the hydrogel impedance is reduced. The longer the argon plasma discharge time is, the more charged particles are generated, the higher the ion concentration in the hydrogel is, and the lower the impedance is. 41 According to formula (5), the capacitance of SF-PEDOT:PSS hydrogel is calculated by the area surrounded by the cyclic voltammetric characteristic curve. The capacitance of the hydrogels modified by argon plasma with different discharge time is shown in Table 2. It can be concluded that the argon plasma can make the capacitance of SF-PEDOT:PSS hydrogel larger, and the longer argon plasma's discharge time leads to the larger hydrogel capacitance. This is because the argon plasma generates a large number of active substances, which make the SF-PEDOT:PSS hydrogel grafted with many hydrophilic functional groups, so that the wettability between the SF-PEDOT:PSS hydrogel and electrodes becomes better, the contact area is larger, and more charges can be stored. 42 Hence, the argon plasma can make the capacitance of the SF-PEDOT:PSS hydrogel become larger. The FTIR spectra of SF-PEDOT:PSS hydrogels with argon plasma modification with different discharge time are shown in Figure 4. The FTIR curves of blank group and each Ar group have strong IR absorption peaks at 1234, 1525, and 1628 cm −1 wavelengths, which respectively correspond to the β-fold structure in the amide III, II, and I bands of SF. 43 This shows that the SF secondary structure of SF-PEDOT:PSS hydrogel after argon plasma modification has not changed, mainly β-fold structure. However, compared with blank group, the infrared absorption peaks of Ar group at 1234, 1526, and 1628 cm −1 wavelengths are somewhat weakened, indicating that argon plasma can make SF β-fold content decrease slightly, but is not affected by the discharge time. In addition, the peak at 1061 cm −1 corresponds to aliphatic ether, the peak at 2920 cm −1 corresponds to C-H stretching vibration, and the peak at 3284 cm −1 corresponds to O-H stretching vibration. The peak at 1443 cm −1 corresponds to alkyl, indicating the presence of sodium dodecyl sulfate in the hydrogel.
The Raman spectra of SF-PEDOT:PSS hydrogels after argon plasma modification with different discharge time are shown in Figure 5. The Raman spectra of SF-PEDOT:PSS hydrogels after argon plasma treatment show no change in the position of the characteristic peak. The peaks of 439, 576, 699, 862, 991, 1092, 1371, and 1423 cm −1 in Raman spectrum belong to PEDOT:PSS conductive polymer, 44 which proves the successful doping of PED-OT:PSS. The main peak of 1423 cm −1 represents the stretching vibration of C α = C β in PEDOT molecule. The PEDOT molecule is mainly quinone structure, which is conducive to the efficient transport of electrons. The Raman characteristic peak at 1250 cm −1 corresponds to the random coil conformation 45 of the amide Ⅲ band of SF, indicating that the random coil conformation of SF exists in SF-PEDOT:PSS hydrogel. The Raman spectra of the hydrogels treated with argon plasma were not as sharp as before, indicating that argon plasma reduced the crystallinity of SF-PEDOT:PSS hydrogels, the random coil conformation of SF, and the content of PEDOT:PSS.
The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels after argon plasma modification with different discharge time are shown in Figure 6. The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels after argon plasma treatment show no change in the position of the characteristic peak, and the position of the main peak is 2θ = 20.3°corresponding to the Bragg angle of SF β-fold, 46 shows that argon plasma does not change the secondary structure of SF, β-fold is dominant. The X-ray diffraction peaks of SF-PEDOT:PSS hydrogels before and after argon plasma modification have little difference.

|
The effect of discharge time (60, 120, 180 s) on the electrochemical properties and structure of hydrogels when the working gas is air The influence of air plasma discharge time (60, 120, 180 s) on SF-PEDOT:PSS hydrogel impedance is shown in Figure 7. From normalized impedance modulus |Z| diagram and phase φ diagram, it can be seen that, on the whole, air plasma has little influence on the impedance of SF-PEDOT:PSS hydrogel. Above 10 0 Hz, air plasma has no effect on SF-PEDOT:PSS hydrogel impedance. In the extremely low frequency (10 −2 -10 0 Hz) range, the air plasma jet makes the impedance of SF-PEDOT:PSS hydrogel slightly reduce, and the impedance is not affected by the air plasma discharge time. This is because the active substances produced by air plasma react with PEDOT:PSS molecules in the hydrogel, 47 which fails to increase the ion concentration in the hydrogel, so the impedance of the hydrogel changes little.
The capacitance of SF-PEDOT:PSS hydrogels modified by air plasma with different discharge time is shown in Table 3. It can be concluded that the air plasma makes the SF-PEDOT:PSS hydrogel capacitance smaller, and the longer air plasma's discharge time leads to the smaller hydrogel capacitance. This is because the air plasma jet slightly reduces the impedance of SF-PEDOT:PSS hydrogel, making it easier for the current to pass through, leaving no electricity, so the capacitance becomes slightly smaller. 48 F I G U R E 6 X-ray diffraction spectra of hydrogels modified by Ar plasma with different discharge time (60, 120, 180 s). The Fourier infrared spectra of SF-PEDOT:PSS hydrogels after air plasma modification with different discharge time are shown in Figure 8. The FTIR curves of blank group and air group have strong infrared absorption peaks at 1234, 1525, and 1628 cm −1 wavelengths, which respectively correspond to the β-fold structure in the amide III, II, and I bands of SF. This shows that the secondary structure of SF in SF-PEDOT:PSS hydrogel after air plasma modification has not changed, mainly β-fold. However, compared with blank group, the infrared absorption peaks at 1234, 1526, and 1628 cm −1 wavelengths in air group were weakened, indicating that air plasma can make SF β-fold content decrease slightly, but was not affected by the discharge time. In addition, the peak at 1061 cm −1 corresponds to aliphatic ether, the peak at 2920 cm −1 corresponds to C-H stretching vibration, and the peak at 3284 cm −1 corresponds to O-H stretching vibration. The peak at 1443 cm −1 corresponds to alkyl, indicating the presence of sodium dodecyl sulfate in the hydrogel.
The Raman spectra of SF-PEDOT:PSS hydrogels after air plasma modification with different discharge time are shown in Figure 9. The Raman spectra of SF-PEDOT:PSS hydrogels after air plasma treatment do not show the change of the position of the characteristic peak. The peaks of 439, 576, 699, 862, 991, 1092, 1371, and 1423 cm −1 in Raman spectrum belong to PEDOT:PSS conductive polymer, which proves the successful doping of PEDOT:PSS. The main peak of 1423 cm −1 represents the stretching vibration of C α = C β in PEDOT molecule. The PEDOT molecule is mainly quinone structure, which is conducive to the efficient transport of electrons. The Raman characteristic peak at 1250 cm −1 corresponds to the random coil conformation of the amide Ⅲ band of SF, indicating that the random coil conformation of SF exists in SF-PEDOT:PSS hydrogel. The Raman spectra of the hydrogels treated by air plasma were not as sharp as before, indicating that the air plasma reduced the crystallinity of SF-PEDOT:PSS hydrogels, the random coil conformation of SF and the content of PEDOT:PSS.
The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels after air plasma modification with different discharge time are shown in Figure 10. The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels after air plasma treatment do T A B L E 3 Electric capacity of hydrogels modified by air plasma for 60, 120, and 180 s.

Discharge time
Blank air60s air120s air180s Electric capacity C/10 −5 F F I G U R E 9 Raman spectra of hydrogels modified by air plasma with different discharge time (60, 120, 180 s).
F I G U R E 10 X-ray diffraction spectra of hydrogels modified by air plasma with different discharge time (60, 120, 180 s). not show the change in the position of the characteristic peak, and the main peak position is 2θ = 20.3°corresponding to the Bragg angle of SF β-fold. It shows that the air plasma does not change the secondary structure of the SF, β-fold is dominant. The X-ray diffraction peaks of SF-PEDOT:PSS hydrogels before and after air plasma modification have little difference.
3.3 | The effect of discharge time (60, 120, 180 s) on the electrochemical properties and structure of hydrogels when the working gas is helium The influence of helium plasma discharge time (60, 120, 180 s) on SF-PEDOT:PSS hydrogel impedance is shown in Figure 11. From normalized impedance modulus |Z| diagram and phase φ diagram, it can be seen that the influence of helium plasma on the impedance of SF-PEDOT:PSS hydrogel is also concentrated in the extremely low frequency (10 −2 -10 0 Hz) range. Above 10 0 Hz, helium plasma has no influence on the impedance of SF-PEDOT:PSS hydrogel. In the extremely low frequency (10 −2 -10 0 Hz) range, helium plasma jet can reduce the impedance of SF-PEDOT:PSS hydrogel by more than 30%, and a longer helium plasma's discharge time shrinks the hydrogel impedance. This is because when helium gas is ionized, a large number of charged particles can be produced, 49 and then the ion concentration in the hydrogel is increased, which enhances the ion diffusion effect and reduces the impedance of the hydrogel. The longer the helium plasma discharge time is, the more charged particles are generated, the higher the ion concentration in the hydrogel is, and the lower the impedance is. The capacitance of SF-PEDOT:PSS hydrogels modified by helium plasma with different discharge time is shown in Table 4. It can be concluded that, on the whole, the helium plasma makes the SF-PEDOT:PSS hydrogel capacitance tend to increase, and the longer helium plasma's discharge time leads to the larger hydrogel capacitance. This is because the helium plasma generates a large number of active substances, which make the SF-PEDOT:PSS hydrogel grafted with many hydrophilic functional groups, so that the wettability between the SF-PEDOT:PSS hydrogel and electrodes becomes better, the contact area is larger, and more charges can be stored. Hence, the helium plasma can make the capacitance of the SF-PEDOT:PSS hydrogel become larger.
The FTIR spectra of SF-PEDOT:PSS hydrogels after helium plasma modification with different discharge time are shown in Figure 12. The FTIR curves of blank group and He group have strong IR absorption peaks at 1234, 1525, and 1628 cm −1 wavelengths, which respectively correspond to the β-fold structure in the amide III, II, and I bands of SF. This shows that the secondary structure of SF in SF-PEDOT:PSS hydrogel modified by helium plasma has not changed, mainly β-fold structure. However, compared with blank group, the infrared absorption peaks of He group at 1234, 1526, and 1628 cm −1 wavelengths were weakened, indicating that helium plasma can make SF β-fold content decrease slightly, but was not affected by the discharge time. In addition, the peak at 1061 cm −1 corresponds to aliphatic ether, the peak at 2920 cm −1 corresponds to C-H stretching vibration, and the peak at 3284 cm −1 corresponds to O-H stretching vibration. The peak at 1443 cm −1 corresponds to alkyl, indicating the presence of sodium dodecyl sulfate in the hydrogel.
The Raman spectra of SF-PEDOT:PSS hydrogels after helium plasma modification with different discharge time are shown in Figure 13. The Raman spectra of SF-PEDOT:PSS hydrogels after helium plasma treatment do not show the change of the position of the characteristic peak. The peaks of 439, 576, 699, 862, 991, 1092, 1371, and 1423 cm −1 in Raman spectrum belong to PEDOT:PSS conductive polymer, which proves the successful doping of PEDOT:PSS. The main peak of 1423 cm −1 represents the stretching vibration of C α = C β in PEDOT molecule. The PEDOT molecule is mainly quinone structure, which is conducive to the efficient transport of electrons. The Raman characteristic peak at 1250 cm −1 corresponds to the random coil conformation of the amide Ⅲ band of SF, indicating that the random coil conformation of SF exists in SF-PEDOT:PSS hydrogel. The Raman spectra of the hydrogels treated with helium plasma were not as sharp as before, indicating that helium plasma reduced the crystallinity of SF-PEDOT:PSS hydrogels, the random coil conformation of SF and the content of PEDOT:PSS.
The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels modified by helium plasma with different T A B L E 4 Electric capacity of hydrogels modified by helium plasma for 60, 120, and 180 s.

Discharge time Blank He60s
He120s He180s Electric capacity C/10 −5 F discharge time are shown in Figure 14. The X-ray diffraction spectra of SF-PEDOT:PSS hydrogels after helium plasma treatment do not show the change in the position of the characteristic peak, and the main peak position is 2θ = 20.3°corresponding to the Bragg angle of SF β-fold, shows that helium plasma does not change the secondary structure of the SF, β-fold is dominant. The Xray diffraction peaks of SF-PEDOT:PSS hydrogels before and after helium plasma modification have little difference.

| XPS analysis
To study the effect of plasma on the elements and chemical bonds of SF-PEDOT:PSS hydrogel, XPS was carried out. As shown in Figure 15A, the SF-PEDOT:PSS hydrogel mainly contains C, O, and N elements, as well F I G U R E 14 X-ray diffraction spectra of hydrogels modified by helium plasma with different discharge time (60, 120, and 180 s). as S elements. The type and content of elements are not affected by plasma. Figure 15B shows three C1s main peaks at 284.79, 286.30, and 288.15 eV, which correspond to C-C, C-O, and C═O chemical bonds respectively. In SF-PEDOT:PSS hydrogel, C-C accounts for the most (43.43%), C-O takes the second place (32.26%), and C═O accounts for least (24.31%). Figure 15C shows two O1s main peaks at 531.61 and 532.90 eV, respectively corresponding to C═O and C-O chemical bond. 50 The O element in SF-PEDOT:PSS hydrogel mainly exists in the form of C═O chemical bond, accounting for 77.85%, and the rest is C-O structure. Figure 15D shows two N1s main peaks at 400.02 and 401.68 eV, respectively corresponding to pyrrole nitrogen (N-5) and quaternary nitrogen (N-Q). 51 The N element in SF-PEDOT:PSS hydrogel mainly exists in the form of N-5, accounting for 93.54%, and a little is N-Q structure. The effect of plasma on the elements and chemical bonds of SF-PEDOT:PSS hydrogel is not much obvious.

| CONCLUSIONS
In this study, the effects of cold plasma jet on the electrochemical properties and structure of SF-PEDOT:PSS conductive hydrogel were studied from the perspectives of plasma discharge time (60, 120, 180 s) and plasma working gas types (Ar, air, He). Both argon plasma and helium plasma can reduce the impedance of SF-PEDOT:PSS hydrogels in the extremely low frequency (10 −2 -10 0 Hz) range, and a longer discharge time shrinks the impedance. Argon plasma can reduce the impedance of SF-PEDOT:PSS hydrogel by more than 50% in the extremely low-frequency range, and helium plasma can reduce the impedance by more than 30%, while air plasma has no obvious effect on the impedance of SF-PEDOT:PSS hydrogel. The impedance change rule is: Ar group < He group < air group ≈ blank. This is due to the fact that argon plasma and helium plasma can generate a large number of charged particles, which increases the ion concentration in SF-PEDOT:PSS hydrogels and reduces the impedance, while the charged particles generated by air plasma are consumed by PEDOT:PSS molecules. Both argon plasma and helium plasma can increase the capacitance of SF-PEDOT:PSS hydrogels, and the capacitance of the hydrogel modified by argon plasma increases the most, and the longer argon plasma or helium plasma's discharge time leads to the larger capacitance, while air plasma can reduce the capacitance of SF-PEDOT:PSS hydrogel. The change rule of capacitance is: Ar group > He group > blank > air group. The results of FTIR and XRD showed that the secondary structure of SF in SF-PEDOT:PSS hydrogels modified by plasma of any kind of working gas remained unchanged, mainly β-fold. Raman test results proved that PED-OT:PSS was successfully doped into SF hydrogel, and found that SF-PEDOT:PSS hydrogel had random coil conformation of SF. XPS results show that SF-PEDOT:PSS hydrogel mainly contains C, O, and N elements. These results prove that the cold plasma can have impact on the electrochemical performance of the hydrogel electrolyte, and can provide some guidance value for the practical application of cold plasma in the field of energy storage in the future.