Effects of plasma on electrochemical performance of carbon cloth-based supercapacitor

In this work, the surface of carbon cloth is treated by plasma jet to improve its hydrophilicity. The symmetrical carbon cloth-based supercapacitor is assembled with the carbon cloth treated by plasma as the active electrodes and sodium chloride solution as the electrolyte. With the discharge time (1 min, 2 min, 3 min) and working gas types (argon, air, helium) of plasma as variables, the effects of different plasma on the hydrophilicity of carbon cloth are observed, and the changes of the electrochemical properties of the supercapacitors with single or double carbon cloth electrodes treated by different plasma are studied. The contact angle test results show that the plasma of different working gases can weaken the hydrophobicity of carbon cloth, and the helium plasma can make the carbon cloth change from hydrophobicity to hydrophilicity. The electric capacity calculated by cyclic voltammetry shows that plasma can increase the electric capacity of carbon cloth-based supercapacitors. The electric capacity of carbon cloth-based supercapacitors with two carbon cloth electrodes treated by plasma is larger than that of single carbon cloth electrode treated by plasma. The argon and helium plasma with longer discharge time can significantly improve the electric capacity of carbon cloth-based supercapacitors. The galvanostatic charge-discharge curve shows that different working gases of plasma can make carbon cloth-based supercapacitors obtain pseudocapacitance, increase the charge-discharge time and electric capacity. From the AC impedance spectrum, it can be concluded that the plasma of any kind of working gas can reduce the impedance and charge transfer resistance of the carbon cloth-based supercapacitor. The longer plasma discharge time lead to the smaller impedance, and the impedance of the supercapacitor with both carbon cloth electrodes treated by plasma is smaller.


Introduction
Energy shortage is the most important issue in the world today. Energy storage is a key section in alleviating energy shortage. Supercapacitor is attracting the attention of researchers in the field of energy storage, because of its high power density, long service life and fast charge-discharge rate [1]. Supercapacitor is mainly composed of active electrodes and electrolyte [2]. Active electrode materials mainly include carbon materials [3], transition metal oxides [4] and conductive polymers [5]. The working principle of supercapacitors using transition metal oxides and conductive polymers as active electrode materials is mainly based on Faraday quasi-capacitance mechanism [6]. Although its electric capacity is large, due to the high degree of oxidation-reduction reaction between the active electrode and the electrolyte, the electrode volume will shrink and expand, so the life of Faraday supercapacitors is short, the cycle performance is poor [7], and it is unstable. Supercapacitors using carbon materials as active electrodes have excellent cycle performance, long service life and stable electrochemical properties [8], because their double-layer capacitor working principle [9] does not involve redox reaction. Carbon cloth [10] is the most common carbon material used in the active electrode of supercapacitors. However, the number of functional groups on the surface of carbon cloth is small, and carbon cloth is hydrophobic [11]. Its contact area with the electrolyte is limited. Therefore, carbon cloth-based supercapacitors [12] only store a small number of charges and have a small electric capacity. In order to improve the electric capacity of carbon cloth-based supercapacitors, it is necessary to improve the wettability between carbon cloth electrode and electrolyte. Common methods used to improve the hydrophilicity of carbon cloth include heat treatment [13], wet chemical treatment [14] and electrochemical oxidation [15]. Heat treatment requires high temperature conditions, high energy consumption, and it is difficult to control the reaction process [16]. The chemical reagents used in wet chemical treatment may be toxic, not environmentally friendly, and the cost is high [17]. Electrochemical oxidation generally needs to be carried out in strong acidic solution, which the requirement of equipment and operation is higher [18]. As a green and simple technology, atmospheric pressure low-temperature plasma [19] contains a large number of high-energy charged particles and active substances, such as HO · , O · , HO 2 · , H 2 O 2 and O 3 , etc [20]. When the plasma jet is sprayed on the surface of the substrate, hydrophilic oxygen-containing functional groups can be grafted on the surface of the substrate, effectively improving the hydrophilicity of the substrate surface, and will not damage the main properties of the material [21]. Therefore, this paper uses plasma jet to improve the wettability between carbon cloth electrode and electrolyte, and improve the energy storage capacity of carbon cloth-based supercapacitors.
In this work, the discharge time (1 min, 2 min, 3 min) and working gas types (argon, air, helium) of plasma are used as variables, spray plasma onto the surface of carbon cloth for modification. The symmetrical carbon cloth-based supercapacitor [22] was assembled with single or double carbon cloth treated by plasma as the active electrode and sodium chloride solution as the electrolyte [23]. The experimental results show that plasma can enhance the hydrophilicity of carbon cloth and have a certain impact on the electrochemical performance (including electric capacity and impedance) of carbon cloth-based supercapacitors. This work can provide more guidance value for plasma application in the field of supercapacitors.

Materials and equipments
Deionized water was purchased from HonghuangZhili flagship store, sodium chloride (analytical grade, molecular weight is 58.5) was purchased from Sinopharm Chemical Reagent Co. Ltd, and Taiwan Carbon Energy WOS-1011 carbon cloth (thickness is 0.36 mm, area specific impedance < 5 mΩ·cm 2 ) was purchased from Suzhou Keshenghe metal material enterprise. FA2004 electronic balance, ControlECO digital heating magnetic stirrer, CTP-2000K low-temperature plasma experimental power supply, GDS-2104 digital storage oscilloscope, JY-PHC contact angle measuring instrument, CHI660E electrochemical workstation.

Design of plasma jet device
The plasma jet was excited in a copper tube with an inner diameter of 3 mm. The copper tube was sheathed with a layer of glass tube with an inner diameter of 4 mm and an outer diameter of 8 mm as a protective layer. The exit of glass tube was a small section of thin glass tube with an inner diameter of 3 mm and a height of 10 mm. The inlet of copper pipe was connected with gas source device, which included argon (Ar) cylinder, air compressor and helium (He) cylinder. The gas flow rate was controlled by a rotameter. In this study, the gas flow rate was set at 2 l min −1 . Connected the copper tube to the high-voltage output terminal of the CTP-2000K low-temperature plasma experimental power supply with a copper clip, and the whole copper tube was used as the positive pole. By adjusting the voltage regulator and the frequency adjusting knob, the gas passing through the copper tube was ionized to produce a plasma jet. The petri dish containing the carbon cloth sample was placed at the outlet of the copper tube with a distance of 10 mm, and the plasma jet was sprayed on the surface of the carbon cloth for modification. The low-temperature plasma experimental power supply needs to be grounded. The plasma jet device is shown in figure 1. The plasma discharge parameters of different working gases (argon, air, helium) in this study are shown in table 1.

Experimental method
Taking the discharge time and working gas types of plasma as variables, the time for plasma treatment of carbon cloth surface with different working gases (Ar, air, He) was set as 1 min, 2 min, 3 min according to the gradient. The carbon cloth without plasma treatment was used as the control group. The carbon cloth samples in each group were labeled as blank, Ar1min, Ar2min, Ar3min, air1min, air2min, air3min, He1min, He2min, He3min. Using 1 M sodium chloride solution as neutral electrolyte, each group of supercapacitor assembled with 1 cm × 1 cm carbon cloth as active electrodes was marked as Double blank, Ar1min+blank, Ar2min+blank, Ar3min+blank, Double Ar1min, Double Ar2min, Double Ar3min; air1min+blank, air2min+blank, air3min +blank, Double air1min, Double air2min, Double air3min; He1min+blank, He2min+blank, He3min+blank, Double He1min, Double He2min, Double He3min. For example, two carbon cloth without plasma treatment were used as active electrodes to assemble a supercapacitor, which was marked as Double blank. A carbon cloth treated with argon plasma for 1 min plus another carbon cloth without plasma treatment as the active electrodes to assemble a supercapacitor, which was marked as Ar1min+blank. Two carbon cloth treated with argon plasma for 1 min were used as active electrodes to assemble the supercapacitor, which was marked as Double Ar1min. Then the contact angle of each carbon cloth sample and the electrochemical characterization of carbon clothbased supercapacitors were carried out.

Contact angle test
Contact angle can measure whether a substrate is hydrophilic or hydrophobic [24]. Add 5 μl water drop slowly on the surface of modified carbon cloth, if the contact angle θ between the water drop and the surface of modified carbon cloth is smaller than 90°, the modified carbon cloth is hydrophilic. The smaller contact angle θ proves the stronger hydrophilicity of carbon cloth. If the contact angle θ between the water drop and the surface of the modified carbon cloth is bigger than 90°, the modified carbon cloth is hydrophobic. The larger contact angle θ proves the stronger hydrophobicity of carbon cloth. The contact angle of is analyzed by automatic method and tangent method [25].

Cyclic voltammetry (CV)
Cyclic voltammetry [26] can be used to analyze the electric capacity of supercapacitor. Set the operating voltage window range to −0.5 V ∼ 0.5 V, the voltage scanning rate to 0.1 V s −1 , and the sensitivity to 10 −5 A V −1 . Using the area surrounded by the CV curve, the electric capacity C is calculated according to the following formula [27], Where, ò idV is the absolute area surrounded by the cyclic voltammetric curve, v is the voltage scanning rate, and V is the operating voltage window range.

Galvanostatic charge-discharge (GCD)
The galvanostatic charge-discharge method [28] is used to charge-discharge supercapacitors under constant current conditions, which can analyze the electric capacity of supercapacitors. Set the charge-discharge current to 0.2 mA and the charge -discharge potential window to 0 ∼ 1.3 V. The electric capacity C can be calculated according to the following formula using GCD curve [29], In the formula, I is the charge-discharge current, !t is the discharge time in the GCD curve, !U is the charge-discharge potential window.

AC impedance spectrum (EIS)
AC impedance spectrum [30] can be used to analyze the impedance characteristics of supercapacitor. Set the initial voltage to 0 V, the frequency range to 0.01 Hz ∼ 1 MHz, and the voltage scanning rate to 0.1 V s −1 . The impedance Z of the supercapacitor can be expressed by the following formula [31], In the formula, R Ω is the internal resistance of electrolyte, C d is the double-layer electric capacity, and R ct is the charge transfer resistance.
The complex number Z can be divided into real part Z ′ and imaginary part Z ″, The phase angle j of the impedance is:

Results and discussion
The purpose of this study is to explore the effect of plasma jet on the electrochemical performance of carbon cloth-based supercapacitor, including electric capacity and impedance, so as to promote the application of plasma technology in the field of supercapacitor. Among many plasma parameters, the discharge time and type of working gas are two predictable items that can improve the electrochemical performance of supercapacitor [32]. Therefore, this work takes the discharge time (1 min, 2 min, 3 min) and the type of working gas (Ar, air, He) of plasma as variables, uses plasma jet to treat the single or double carbon cloth electrodes of the supercapacitor, and sets the carbon cloth-based supercapacitor without plasma modification as the control group, to study the effect of plasma on the electrochemical performance of the carbon cloth-based supercapacitor.

Effects of argon plasma with different discharge time (1 min, 2 min, 3 min) on the electrochemical performance of carbon cloth-based supercapacitors
The hydrophilic and hydrophobic degree of carbon cloth determines the contact area between carbon cloth and electrolyte. Argon plasma treatment can bring some hydrophilic functional groups to carbon cloth and improve the wettability of carbon cloth electrode in NaCl electrolyte. The effect of argon plasma with different discharge time (1 min, 2 min, 3 min) on the water contact angle of carbon cloth is shown in figure 2. The water contact angle of carbon cloth in blank group is 135°, and the water contact angle of carbon cloth treated with argon plasma for 1 min has little change. As the time of argon plasma treatment of carbon cloth becomes longer, more hydrophilic functional groups are generated and grafted on the surface of carbon cloth, making the contact angle between carbon cloth and water drop gradually smaller, reaching 119°, indicating that the hydrophilicity of carbon cloth has been improved.
The electric capacity of carbon cloth-based supercapacitors is analyzed by the cyclic voltammetric characteristic curve. With the area surrounded by the cyclic voltammetric characteristic curve, the electric capacity is calculated according to the formula (1). The electric capacity of the supercapacitors assembled by carbon cloth treated with argon plasma at different discharge time is shown in figure 3. It can be concluded that compare with the electric capacity (0.707 * 10 −5 F) of the Double blank group, the argon plasma can make the electric capacity of the carbon cloth-based supercapacitor larger. The wettability of carbon cloth treated with argon plasma for 1 min has not been significantly improved, so the electric capacity of Ar1min+blank group carbon cloth-based supercapacitor has little change. As the time of carbon cloth treated by argon plasma becomes longer, the hydrophilicity of carbon cloth is gradually improved, the contact area between carbon cloth electrode and NaCl electrolyte becomes larger, and more charges can be collected, so the electric capacity of carbon cloth-based supercapacitors treated by argon plasma increases. Moreover, compare with the single carbon cloth electrode treated by argon plasma, the supercapacitor assembled by two carbon cloth electrodes treated by argon plasma has larger electric capacity, electric capacity enhancement effect is obvious. This is because the hydrophilicity of the two carbon cloth electrodes treated by argon plasma has been improved. Both carbon cloth electrodes can be in well contact with NaCl electrolyte, Na + and Cl − ions can be well transferred  between the electrode and the electrolyte, so the electric capacity will also increase more significantly. However, the electric capacity of supercapacitor with only a single carbon cloth electrode treated by argon plasma is not as large as the supercapacitor with both carbon cloth electrodes treated by argon plasma, because the hydrophilicity of the other carbon cloth electrode is still very poor, which will affect its charge storage capacity. With the increase of the time when the two carbon cloth electrodes are treated by argon plasma, the electric capacity of the carbon cloth-based supercapacitor increases by 9 ∼ 12 times.
The galvanostatic charge-discharge method can also analyze the electric capacity of carbon cloth-based supercapacitors. The GCD curves of carbon cloth-based supercapacitors treated by argon plasma with different discharge time are shown in figure 4. From figure 4, it can be seen that the GCD curve of the carbon cloth-based supercapacitor without argon plasma treatment presents a symmetrical triangular profile, indicating its doublelayer capacitance characteristics [33]. As the treatment time of argon plasma increases, the GCD curves of carbon cloth-based supercapacitors gradually deviate from the symmetrical triangular profile, and the potential shows a nonlinear relationship with the charge-discharge time, indicating that argon plasma brings a pseudocapacitance for carbon cloth-based supercapacitors [34]. This is because argon plasma introduces some hydrophilic functional groups into the carbon cloth electrode, which undergo redox reactions and form pseudocapacitance [35]. Calculate the electric capacity of carbon cloth-based supercapacitors based on formula (2) from the GCD curve. Compare with the electric capacity of the Double blank group (3.08 * 10 −5 F), argon plasma can increase the discharge time and electric capacity of carbon cloth-based supercapacitors. And the supercapacitors with two carbon cloth electrodes treated with argon plasma have a larger electric capacity. This is because argon plasma not only brings additional pseudocapacitance to carbon cloth-based supercapacitors, but also increases the contact area between the carbon cloth electrode and the electrolyte, allowing carbon clothbased supercapacitors to store more charges. In general, argon plasma with longer processing time can continuously increase the electric capacity of carbon cloth-based supercapacitors, and the electric capacity of the Double Ar3min group can reach 2.77 * 10 −4 F. AC impedance spectrum (EIS) can be used to analyze the change impedance of the supercapacitor with the frequency (10 −2 Hz ∼ 10 5 Hz) [36]. The effect of argon plasma with different discharge time (1 min, 2 min, 3 min) on the impedance of the carbon cloth-based supercapacitor is shown in figure 5. It can be seen from Nyquist curve that the impedance of each carbon cloth-based supercapacitor presents a semicircle shape. Compare with Double blank group of carbon cloth-based supercapacitor, the impedance of each group of carbon cloth-based supercapacitor treated by argon plasma shows a decreasing trend. The semicircle region at high frequency in Nyquist curve corresponds to the charge transfer resistance R ct of the ions between the electrolyte and the electrode. The semicircle diameter of the carbon cloth-based supercapacitors treated by argon plasma is smaller in the AC impedance spectrum, which indicates that the argon plasma can reduce the charge transfer resistance R ct [37], and the electrolyte ion can contact the electrode material more fully, so the carbon cloth-based supercapacitor has higher electric capacity and electrochemical performance. Because the hydrophilic property of carbon cloth does not change significantly after argon plasma treatment for 1 min, NaCl electrolyte and carbon cloth electrode still could not contact fully, the impedance of Ar1min+blank group carbon cloth-based supercapacitor does not decrease significantly, the charge transfer resistance in the high frequency region is also large, and the charge storage capacity of carbon cloth-based supercapacitor is not improved. However, as the time of argon plasma treatment of carbon cloth increases, the impedance at each frequency and charge transfer resistance R ct of carbon cloth-based supercapacitors decrease, which is conducive to the improvement of the charge storage capacity of carbon cloth-based supercapacitors, this further explains the reason why the extended time of argon plasma treatment can make the electric capacity of carbon clothbased supercapacitors larger. Moreover, compare with the supercapacitor with single carbon cloth electrode treated by argon plasma, the impedance of the supercapacitor with two carbon cloth electrodes treated by argon plasma is smaller, and the charge transfer resistance R ct is also smaller. This is due to the larger contact area between the two carbon cloth electrodes treated by argon plasma and NaCl electrolyte, so the electric capacity of the supercapacitor with two carbon cloth electrodes treated by argon plasma is larger. It can be seen from the normalized impedance modulus | Z | figure that, the argon plasma can reduce the impedance of the carbon cloth-based supercapacitor in the low frequency range (10 −2 ∼10°Hz), and the longer the discharge time is, the lower the impedance of the supercapacitors. The impedance of the supercapacitors with both carbon cloth electrode treated by the argon plasma is smaller.

Effects of air plasma with different discharge time (1 min, 2 min, 3 min) on the electrochemical performance of carbon cloth-based supercapacitors
The carbon cloth is hydrophobic, the wettability of carbon cloth in aqueous electrolyte can be improved through post-treatment. The air plasma contains many hydrophilic oxygen-containing functional groups, such as hydroxy OH − , O − ion, etc [38]. Using air plasma to treat carbon cloth can make carbon cloth grafted with some hydrophilic oxygen-containing functional groups and improve the wettability of carbon cloth electrode in NaCl electrolyte. The influence of air plasma with different discharge time (1 min, 2 min, 3 min) on the water contact angle of carbon cloth is shown in figure 6. The water contact angle of carbon cloth in blank group is 135°. The air plasma can reduce the water contact angle of carbon cloth, but the decrease of water contact angle of carbon cloth after 2 min treatment with air plasma is small. As the time of treating carbon cloth with air plasma becomes longer, more hydrophilic oxygen-containing functional groups are generated and grafted on the surface of carbon cloth, so the contact angle between carbon cloth and water drop gradually decreases to 123°, indicating that air plasma can improve the hydrophilicity of carbon cloth.
Cyclic voltammetry can be used to analyze the electric capacity of carbon cloth-based supercapacitors. According to formula (1), the electric capacity of carbon cloth-based supercapacitors treated with air plasma at different discharge time is calculated by the area surrounded by the cyclic voltammetric characteristic curve. The results are shown in figure 7. It can be concluded that compare with the electric capacity (0.707 * 10 −5 F) of the Double blank group, the air plasma can increase the electric capacity of the carbon cloth-based supercapacitor. The wettability of carbon cloth treated with air plasma for 2 min is improved a little, and the contact area  between carbon cloth and NaCl solution is still limited, so the increasing electric capacity of air2min+blank and Double air2min group carbon cloth-based supercapacitors is limited. As the time of treating carbon cloth with air plasma becomes longer, more hydrophilic oxygen-containing functional groups are grafted onto the surface of carbon cloth, which improves the hydrophilicity of carbon cloth. The contact area between carbon cloth electrode and NaCl electrolyte becomes larger, and more charges can be collected. Therefore, the electric capacity of carbon cloth-based supercapacitors treated with air plasma increases. In addition, compare with only single carbon cloth electrode treated by air plasma, the supercapacitor assembled by two carbon cloth electrodes treated by air plasma has larger electric capacity. This is because the hydrophilicity of the two carbon cloth electrodes treated by air plasma has been improved, which can improve the contact area between the two carbon cloth electrodes and NaCl electrolyte. Na + and Cl − ions can also well transfer between the electrode and the electrolyte, and the charge storage capacity of the carbon cloth-based supercapacitor will become stronger. However, for the supercapacitor with only one carbon cloth treated by air plasma, the hydrophilicity of the other carbon cloth electrode is still very poor, which will affect its charge storage capacity, and its electric capacity is not as good as that of the supercapacitor with both carbon cloth electrodes treated by air plasma. With the increase of the time when the two carbon cloth electrodes are treated by air plasma, the electric capacity of the carbon clothbased supercapacitor increases by three times.
The GCD curves of carbon cloth-based supercapacitors treated by air plasma with different discharge time are shown in figure 8. From figure 8, it can be seen that the shape of the Double blank group's charge-discharge curve is a symmetrical triangle, so the carbon cloth-based supercapacitor mainly stores charges through a double-layer capacitance mechanism. As the time of treating carbon cloth with air plasma increases, the chargedischarge curves of carbon cloth-based supercapacitors gradually deviate from the symmetrical triangular profile, indicating that air plasma can graft some oxygen-containing functional groups onto the carbon cloth electrode, which participate in electrochemical reactions and form pseudocapacitance. Generally, air plasma can increase the electric capacity of carbon cloth-based supercapacitors, but the GCD curves of air1min+blank and air2min+blank groups overlap with that of Double blank group, and the electric capacity does not increase. This is because the process of ionizing air to generate plasma is unstable, resulting in a limited number of active groups. The pseudocapacitance and hydrophilicity of carbon cloth electrodes have not been significantly improved. As the air plasma treatment time increases, the electric capacity of carbon cloth-based supercapacitors gradually increases, and the electric capacity of the air3min+blank group reaches 6.15 * 10 −5 F. The supercapacitor with two carbon cloth electrodes treated with air plasma has a larger electric capacity, and the electric capacity of the Double air2min group reaches 2.00 * 10 −4 F.
The influence of air plasma with different discharge time (1 min, 2 min, 3 min) on the impedance of carbon cloth-based supercapacitor is shown in figure 9. It can be seen from Nyquist curve that the carbon cloth-based supercapacitor presents a semicircular impedance curve. Compare with the Double blank group of carbon clothbased supercapacitor, the impedance of each carbon cloth-based supercapacitor treated by air plasma is generally decreasing. The semicircle of the high-frequency region in the Nyquist curve corresponds to the charge transfer resistance R ct of the ions between the electrolyte and the electrode. Compare with the Double blank group, the impedance and charge transfer resistance R ct of carbon cloth-based supercapacitors in air1min+blank, air2min +blank and Double air2min groups do not change much. This is because the air plasma treatment time of carbon cloth is not enough, the hydrophilicity of carbon cloth has not been significantly enhanced, the contact between carbon cloth electrode and NaCl electrolyte is still difficult, and the charge storage capacity of carbon cloth-based supercapacitors is limited. As the time of treating carbon cloth with air plasma increases, the semicircular diameter in the high frequency region decreases gradually, the charge transfer resistance R ct and impedance at various frequencies of carbon cloth-based supercapacitors decrease gradually, which is conducive to the migration of Na + and Cl − ions between the carbon cloth electrode and the electrolyte, and the electric capacity will also increase. Moreover, compare with the carbon cloth-based supercapacitor with single carbon cloth electrode treated by air plasma, the impedance of the supercapacitor with two carbon cloth electrodes treated by air plasma is smaller, and the charge transfer resistance R ct is also smaller, which is due to the air plasma makes the contact area between the two carbon cloth and NaCl electrolyte larger, the migration rate of Na + and Cl − ions between the carbon cloth electrode and the electrolyte will also be faster [39], the barrier of ions in the migration path is smaller. It can be seen from the normalized impedance modulus | Z | figure that, on the whole, air plasma can reduce the impedance of carbon cloth-based supercapacitors in the low frequency range (10 −2 ∼ 10°Hz). The impedance of air1min +blank and air2min+blank groups of carbon cloth-based supercapacitors has no obvious decreasing trend, which is related to the fact that short discharge time of air plasma does not improve the hydrophilicity of carbon cloth much. The longer the discharge time of air plasma is, the lower the impedance of the supercapacitor is. The impedance of the supercapacitor with both carbon cloth electrodes treated by air plasma is smaller.
3.3. Effects of helium plasma with different discharge time (1 min, 2 min, 3 min) on the electrochemical performance of carbon cloth-based supercapacitors The effect of helium plasma with different discharge time (1 min, 2 min, 3 min) on the water contact angle of carbon cloth is shown in figure 10. The effect of using helium plasma to improve the hydrophilicity of carbon cloth is very excellent, which can change the hydrophobicity of carbon cloth into hydrophilicity, this effectively increase the contact area between carbon cloth electrode and NaCl electrolyte. Helium plasma contains a lot of hydrophilic functional groups, and the carbon cloth can be grafted with a lot of hydrophilic functional groups after treatment. 1 min of helium plasma treatment can make the carbon cloth change from hydrophobic to hydrophilic, and the contact angle is 76°. As the time of treating carbon cloth with helium plasma increases, more hydrophilic functional groups can be grafted on the surface of carbon cloth, and the contact angle between carbon cloth and water drop decreases to 33°, the hydrophilicity of carbon cloth is greatly improved.
According to formula (1), the electric capacity of the carbon cloth-based supercapacitor is calculated by the area surrounded by the cyclic voltammetric characteristic curve. The electric capacity of the supercapacitors assembled by carbon cloth treated with helium plasma at different discharge time is shown in figure 11. It can be concluded that compare with the electric capacity (0.707 * 10 −5 F) of the Double blank group, the helium plasma can significantly increase the electric capacity of the carbon cloth-based supercapacitor. Helium plasma can make the water contact angle of carbon cloth change from obtuse angle to acute angle, the hydrophilicity of carbon cloth is significantly improved, so the carbon cloth treated by helium plasma has a larger contact area with NaCl electrolyte, and the carbon cloth-based supercapacitor can also collect more charges, and the electric capacity becomes larger. With the increase of helium plasma discharge time, more hydrophilic functional groups further enhance the hydrophilicity of carbon cloth, and the energy storage capacity of carbon cloth-based supercapacitors also continues to grow. Moreover, when both carbon cloth electrodes of carbon cloth-based supercapacitors are treated with helium plasma, their electric capacity enhancement effect is far better than that of single carbon cloth electrode treated with helium plasma. This is because the helium plasma can convert two hydrophobic carbon cloth electrodes into hydrophilic electrodes, the larger area of NaCl electrolyte can be attached to the carbon cloth electrode to carry out the charge transfer process, and the Na + and Cl − ions can be well transferred between the electrode and the electrolyte, and the electric capacity will also increase more obviously. As the other carbon cloth electrode is still hydrophobic, the charge storage capacity of the supercapacitor with only one carbon cloth treated by helium plasma is not as good as that of the supercapacitor with both carbon cloth electrodes treated by helium plasma. As the time of treating two carbon cloth electrodes with helium plasma increases, the electric capacity of carbon cloth-based supercapacitors can even increase by 18 times.
The GCD curves of carbon cloth-based supercapacitors treated by helium plasma with different discharge time are shown in figure 12. From figure 12, it can be seen that the helium plasma makes the charge-discharge curves of the carbon cloth-based supercapacitors gradually deviate from the symmetrical triangular shape, indicating that the helium plasma brings additional pseudocapacitance to the carbon cloth electrode. This is because the functional groups generated by the helium plasma can cause the carbon cloth electrode to undergo redox reactions, forming pseudocapacitance. Helium plasma can increase the charge-discharge time and electric capacity of carbon cloth-based supercapacitors. The GCD curves of He1min+blank, He2min+blank, and He3min+blank groups are almost coincident. The electric capacity of the supercapacitors with a single carbon cloth electrode treated by helium plasma is not affected by the treatment time, which is consistent with the results of cyclic voltammetry. According to formula (2), the electric capacity of the supercapacitors treated with helium plasma on a single carbon cloth electrode is 7.69 * 10 −5 F. The supercapacitors with two carbon cloth electrodes treated by helium plasma have a larger electric capacity, and the electric capacity increases with increasing treatment time. The electric capacity of the Double He3min group can reach 2.00 * 10 −4 F.
The influence of helium plasma with different discharge time (1 min, 2 min, 3 min) on the impedance of carbon cloth-based supercapacitor is shown in figure 13. It can be seen from Nyquist curve that the impedance of each carbon cloth-based supercapacitor presents a semicircle shape. Compare with Double blank group of carbon cloth-based supercapacitor, the impedance of each carbon cloth-based supercapacitors treated by helium plasma is basically decreasing. The semicircle region at high frequency in Nyquist curve corresponds to the charge transfer resistance R ct of electrolyte ion between electrolyte and electrode. The semicircle diameter of the carbon clothbased supercapacitor treated with helium plasma is smaller in the AC impedance spectrum, which indicates that the helium plasma can reduce the charge transfer resistance R ct of the supercapacitor, and the electrolyte ion can contact the electrode material more fully, so the carbon cloth-based supercapacitor has higher electric capacity and electrochemical performance. The impedance of carbon cloth-based supercapacitors of He1min+blank group and He2min+blank group does not drop significantly, and the charge transfer resistance in the high frequency region is also large, which may be due to the short time of helium plasma treatment of carbon cloth. However, as the time of treating carbon cloth with helium plasma increases, the impedance at all frequencies and charge transfer resistance R ct of carbon cloth-based supercapacitors decrease, which is beneficial to the improvement of the charge storage capacity of carbon cloth-based supercapacitor, proving that the helium plasma with longer treatment time can make the electric capacity of carbon cloth-based supercapacitors larger. Moreover, compare with the supercapacitor with single carbon cloth electrode treated by helium plasma, the impedance of the supercapacitor with two carbon cloth electrodes treated by helium plasma is smaller, and the charge transfer resistance R ct is also smaller. This is due to the larger contact area between the two carbon cloth electrodes treated by helium plasma and NaCl electrolyte, so the electric capacity of the supercapacitor with two carbon cloth electrodes treated by helium plasma is larger. It can be seen from the normalized impedance modulus | Z | figure that the helium plasma with longer discharge time can continuously reduce the impedance of carbon cloth-based supercapacitors in the low frequency range (10 −2 ∼ 10 0 Hz), and the impedance of supercapacitors with both carbon cloth electrode treated by helium plasma is significantly reduced.

Conclusions
In this study, from the perspective of plasma's discharge time (1 min, 2 min, 3 min) and working gas types (Ar, air, He), the effects of plasma jet on the electrochemical performance of carbon cloth-based supercapacitor were studied. Argon plasma can strengthen the hydrophilicity of carbon cloth and increase the electric capacity of carbon cloth-based supercapacitors. Moreover, the electric capacity of carbon cloth-based supercapacitors increases with the prolongation of argon plasma discharge time, and the electric capacity enhancement effect of both carbon cloth electrodes treated by argon plasma is more obvious, which can reach 9 ∼ 12 times of the electric capacity of carbon cloth-based supercapacitor without plasma treatment. Air plasma can improve the hydrophilicity of carbon cloth. When air plasma is used to treat two carbon cloth electrodes, the longer treatment time can increase the electric capacity of carbon cloth-based supercapacitor by three times. Helium plasma can change the carbon cloth from hydrophobic to hydrophilic, make the contact area between carbon cloth electrode and electrolyte larger, and enhance the charge storage capacity of supercapacitors. When both carbon cloth electrodes of the carbon cloth-based supercapacitor are treated by helium plasma for a long time, its electric capacity can be increased by 18 times. The plasma of the three working gases can reduce the impedance at various frequencies and charge transfer resistance of the carbon cloth-based supercapacitor, which is conducive to the migration of ions between the carbon cloth electrode and the electrolyte, and make the energy storage of the supercapacitor stronger. These results show that plasma can improve the electrochemical performance of carbon cloth-based supercapacitor on the basis of improving the hydrophilicity of carbon cloth, and can provide some guidance value for the application of plasma in the field of energy storage.