Enhancement of the electrochemical properties of commercial coconut shell-based activated carbon by H2O dielectric barrier discharge plasma

Commercial coconut shell-based activated carbon (CSAC) has low specific capacitance and specific capacitance retention owing to its undeveloped pore structure and low proportion of heteroatoms. In this study, dielectric barrier discharge plasma was used to enhance the specific capacitance and rate capability of CSAC. H2O was used as an excited medium to introduce oxygen functional groups. The physico-chemical properties of CSAC and CSAC modified by H2O plasma (HCSAC) were revealed by automated surface area and pore size analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. Electrochemical work was applied to investigate the electrochemical properties of CSAC and HCSAC. The results obtained showed that plasma modification improved the specific capacitance of CSAC by 64.8% (current density, 1 A g−1; electrolyte, 6 M KOH solution) within 100 s. This result is ascribed to the oxygen functional groups introduced to the surface of CSAC. It can also improve the hydrophilicity and wettability of the carbon surface leading to an increase from 76.7% to 84.6% in specific capacitance retention. Furthermore, H2O plasma modification can introduce oxygen functional groups without destroying the initial pore structures of CSAC. In summary, we provide a simple, fast, environment-friendly modification method to enhance the electrochemical properties of CSAC.


Manuscript ID: RSOS-180872
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) The paper of Wang et al. deals with the Enhancement on the electrochemical properties of commercial coconut shell-based activated carbon by H2O dielectric barrier discharge plasma. One may deplore that, finally, only one single material has been prepared and investigated. There is neither repetition of the same, in order to check the repeatability, nor synthesis of a series of materials in which one parameter would have been varied with some impact on the final properties. As a result, this is a quite short studies with a very limited content for a full-length paper. Detailed comments follow: 1. Page1, line 32; line 52, 55, the states are contradict each other; 2. Page 2, section 3.3, This section must be considerably improved because of the dramatic mack of details. Examples are: " the temprature for degass" 3. section4.1, line 31, the isotherm type, please make sure. The BET method is known to overestimate the surface area of microporous materials. If the pore size distribution has been obtained by DFT method, why not having applied the same for determining the surface area? The corresponding result should be different and more reliable. The same also applies to the micropore volume, VDR being always overestimated. 6. Authors should provide the details of determination of O and N content by XPS. 7. Authors should rewrite the conclusions showing parameters of the final material, which are more important.
8. How much of weight loss during plasma treatment. 9. this paper paye close attention on the plasma midification, so only the C1s deconvoluted in Figure 6 is not enough, the N1 and O1 should also be done as well. one more thing, XPS analysis should have allowed a quantitative assignment of each kind of nitrogen/carbon/oxygen instead of just a qualitative approach. 10. Did the authors study the reusability for electrochemical properties? It is important to cost down the process. 11. It is necessary to compare the results with some recent literatures. 12. Deeper discussion of the results should be provided.
I would encourage the authors to reconsider all of these points and, after a careful review I would encourage a resubmission.

Reviewer: 2
Comments to the Author(s) Comments: In this work, carbon treated by water vapor plasma is reported for EDLcs electrodes application. The treated carbon based electrodes present a much better capacitance performance than that of untreated. The method is creative, however, the explanation of plasma working mechanism is not clear enough, either the enhancement of capacitance. Upon reviewing, major revision is suggested. A few comments are as following: Typos like missing space between value and unit (70℃, line 46, P2, 5.0mm, line 47, P2 etc.), capital letter (Rct, line 46, P4) are detected. Check carefully! Abbreviations should be denoted at their first appearance in main text, e.g. CSAC. EDLC is commonly the abbreviation of "electrical double layer capacitor" rather than electrical double layer capacitance.
The Fig. numbers are in mess, do not match with the description in text. Fig. 7d (Fig. 6 in manuscript) is not mentioned. Looks like it is the capacitance performance test at different current density, however, more cycles, e.g. 1000 cycles for each current density, should be provided. Otherwise, it is not sufficient to prove the good performance at different current densities.
In electrochemical properties section, the explanation of improved capacitance performance is concerned. It is claimed by authors that the improvement is attributed to pseudocapacitance owing of oxygen functional groups on carbon surface (line 22-23, P4) besides double layer capacitance. How did the pseudo capacitance of oxygen functional groups work? What is the the possible working mechanism? And any supporting references?
Contact angel should be measured to support the claimed improved hydrophilicity and wettability after plasma treatment; How is the plasma working condition come up, e.g. 160 W, 100s, 30KPa? How did these factors affect the treatment and consequently to capacitance performance? In my experience, the treatment time is highly related to the structure evolution.
In the XPS analysis result ( Comments to the Author(s) I am satisfied with the modifications made by authors according to my comments, and it can be accepted on condition the supplementary figures, discussions and references are wrapped up with the manuscript. Otherwise, the manuscript is quite short, hardly to be a full-length research paper with few substantial contents. Besides, the response to comment 9 of reviewer #1 is concerned. Did the authors measure the mass of samples before and after plasma treatment? Besides, 160 W is not low and there will be mass loss based on my experience on plasma processing.

03-Jan-2019
Dear Dr Wang: Title: Enhancement on the electrochemical properties of commercial coconut shell-based activated carbon by H<sub>2</sub>O dielectric barrier discharge plasma Manuscript ID: RSOS-180872.R1 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email.
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Comments to the Author: (There are no comments.) ************************************** Reviewer comments to Author: Reviewer: 2 Comments to the Author(s) I am satisfied with the modifications made by authors according to my comments, and it can be accepted on condition the supplementary figures, discussions and references are wrapped up with the manuscript. Otherwise, the manuscript is quite short, hardly to be a full-length research paper with few substantial contents. Besides, the response to comment 9 of reviewer #1 is concerned. Did the authors measure the mass of samples before and after plasma treatment? Besides, 160 W is not low and there will be mass loss based on my experience on plasma processing.

14-Jan-2019
Dear Dr Wang: Title: Enhancement on the electrochemical properties of commercial coconut shell-based activated carbon by H<sub>2</sub>O dielectric barrier discharge plasma Manuscript ID: RSOS-180872.R2 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.
Thank you for your fine contribution. Appendix A

Responses to reviewer's comments
We appreciate the detailed and helpful comments and suggestions which are very helpful to improve the quality of our manuscript. The manuscript (RSOS-180872) has been carefully revised as required. All new and revised contents are highlighted with yellow in our revised version. We hope that the correction will meet with your approval. The point-by-point answers to the comments and suggestion were listed below.

Reviewer #1
[1] Only one single material has been prepared and investigated. There is neither repetition of the same, in order to check the repeatability, nor synthesis of a series of materials in which one parameter would have been varied with some impact on the final properties. As a result, this is a quite short studies with a very limited content for a full-length paper.
Response: According to your helpful suggestion, we have provided more electrochemical performance data of samples prepared under different time (50 s,100 s,150 s,200 s,300 s) and power (50 W, 100 W, 130 W, 160 W, 200 W), please see the figure below. We have conducted a serious of preliminary experiment on electrochemical performance to select the optimum condition (100 s, 160 W). From the results shown below, it can be concluded that the H2O dielectric barrier discharge (DBD) plasma has ability to improve electrochemical performance of commercial coconut shell-based activated carbon (CSAC) in a short time. The exhaustive study is very meaningful that we are going to investigate in the future. The electrochemical performance of CSAC modified with H2O plasma under different time and different power is shown below (Fig. 1). The cyclic voltammetry (CV) plots at the scan rate at 10 mV s -1 of CSAC and CSAC modified with H2O plasma are shown in Fig. 1(a-b). The integrated area of the sample D-100 W-100 s (D means DBD plasma modification, the first number is modification power, the second number is modification time) is larger than other sample in the same power, indicating that 100 s is a more suitable modification time. With modification power changing from 50 W to 200 W, the sample D-160 W-100 s exhibits the largest integrate area among all the samples at the same modification time of 100 s. The largest integrate area of D-160 W-100 s indicates that the sample shows the larger specific capacitance than other samples. Furthermore, the galvanostatic charge-discharge (GCD) curves at the current density of 1 A g -1 of CSAC and CSAC modified with H2O plasma are shown in Fig. 1(c-d). After calculation by the formula below, we come up the conclusion that D-160 W-100 s exhibits the excellent specific capacitance (specific capacitance of 155.2 F g -1 at the current density of 1 A g -1 ) among all the samples, which is consistent with CV curves. The Nyquist plots of CSAC and CSAC modified with different power exhibit similar shapes, shown in Fig. 1e. The internal resistance (Ri) of samples were below 0.4 Ω, demonstrating a good electrical conductivity of materials. The increased Ri of the sample D-160 W-100 s can be ascribed to the presence of surface oxides and thus increase the ohmic resistance along the axial direction of micropores. Also, the smaller diameter in the semicircle of the sample D-160 W-100 s illustrates that the lower charge transfer resistance (Rct) at electrolyte interfaces, owing to the higher hydrophilicity after oxygen functional groups introduced. Fig. 1f demonstrates the rate capability (capacitance retention from the current density of 0.5 A g -1 to 10 A g -1 ) of CSAC and CSAC modified under different power. The higher rate capability indicates that the sample is capable of maintaining excellent charge and discharge characteristics at high current density. In summary, we perform other detailed characterization of CSAC modified by H2O plasma under modification time of 100 s and power of 160 W (HCSAC) based on the electrochemical performance. We keep the pressure of the reactor chamber to 30 KPa, mainly aiming to pump water into the reactor chamber instead of maintaining the vacuum environment. Based on the above results, we select the conditions (discharge power, 160 W; treating time, 100 s; the pressure, 30 KPa) to make detailed analysis. We supply the result in supporting information.
The specific capacitance of the electrodes (C), was calculated by the following equation: Where C is specific capacitance (F g -1 ), I is the constant charge-discharge current (A), ∆t is the discharge time (s), ∆V is the total change in voltage (V), m is the mass of the active material in an electrode (g) Fig. 1 (a-b) CV curves of CSAC and CSAC modified with DBD H 2 O plasma in a three-system with 6 M KOH aqueous electrolyte at the scan rate of 10 mV s -1 . (c-d) GCD curves of CSAC and CSAC modified with DBD H 2 O plasma in a three-electrode system with 6 M KOH aqueous electrolyte at the current density of 1 A g -1 . (e) Nyquist plots of CSAC and CSAC modified at different power electrodes. The inset is the detail with enlarged scale (f) Rate capability of CSAC and CSAC modified at different power at the current density from 0.5 to 10 A g -1 .
[2] Page1, line 32; line 52, 55, the states are contradict each other. Response: Based on your significant suggestion, I realize that the 'Introduction' section is incorrectly in expression and revised below. The sentence of 'It should be noted that activated carbon synthesized from coconut shell is considered better because of its mesoporous structure which makes it suitable for its application in supercapacitor as electrode materials' has been revised into 'It should be noted that coconut shell is considered to be a suitable precursor owing to its availability, low price and ability to create mesoporous structure in carbon materials'.
Page1, line 55 ---'It should be noted that coconut shell is considered to be a suitable precursor owing to its availability, low price and ability to create mesoporous structure in carbon materials.' Response: After we consider your significant suggestion, we make sure that the isotherm show a combined type I and IV isotherm with small H4 type hysteresis according to the IUPAC classification. We have revised the isotherm type as shown below [1,2] .  Fig. 7a shows the CV plots of the CSAC and HCSAC electrodes at a sweep rate of 10mV s -1 .' Page 4, line 51 ---'The GCD curves (Fig. 7b) of CSAC and HCSAC at a current density of 1A g-1 show typical triangular shapes.' Page 4, line 56 ---'To further comprehend the capacitive behavior of SCAC and HSCAC, EIS test was performed over a frequency range from 10 kHz to 10 mHz (Fig.  7c).' [6] The BET method is known to overestimate the surface area of microporous materials. If the pore size distribution has been obtained by DFT method, why not having applied the same for determining the surface area? The corresponding result should be different and more reliable. The same also applies to the micropore volume, VDR being always overestimated. Response: Based on your significant suggestion, we realized that density functional theory (DFT) method is more suitable than Brunauer-Emmett-Teller (BET) method for obtaining the surface area especially for the case that we have already applied DFT method to study the pore size distribution of carbon samples. We provide you with the surface area revealed by DFT method. The DFT surface area of CSAC and HCSAC is 644.0 and 726.8 m 2 g -1 , respectively. (The BET surface area of CSAC and HCSAC is 779.8 and 846 m 2 g -1 , respectively.) We choose BET method instead of DFT method, mainly owing to the fact that BET method is used widely in other reports [1][2][3] . It is convenient for us to compare our data with the value in other reports, so that we can understand the activated carbon performance clearly. Reference:

[7] Authors should provide the details of determination of O and N content by XPS.
Response: After we considered your helpful suggestions, the details of determination of O and N by X-ray photoelectron spectroscopy (XPS) are provided. The XPS spectra of CSAC and HCSAC is shown in Fig. 2. As presented in the figure below, the relative proportion of surface oxygen, nitrogen and carbon elements are calculated by the integrate area of each element divided by the integrate area of all the elements (C, O, N), respectively. Also, we have added the details in the 'Materials and Methods' section. Response: Based on your significant suggestion, we have revised the 'Conclusion' section and added parameters of the final materials. We add the detailed properties of HCSAC. The related revised content is presented below.
Page 5, line 20 ---'The BET surface area (846.0 m 2 g -1 ) of HCSAC shows unremarkable change in comparation with the initial material.' Page 5, line 21 ---'A 60.4% higher surface oxygen content is observed as compared to the untreated materials, indicating massive free radicals are generated and introduced to the carbon surface during H2O plasma modification.' Page 5, line 23 ---'Quinolyl (-C=O) exhibits an 74.5% increment after modification, which is capable of facilitating electrochemical redox activity and improve hydrophilicity and wettability of material surface.'

[9] How much of weight loss during plasma treatment.
Response: There is no weight loss of CSAC during plasma treatment owing to the low discharge power (160W) utilized in our experiment [1,2] . The BET surface area, total pore volume and average pore size exhibit unremarkable changes after H2O plasma modification. The physical structure without significant changes indicates that H2O plasma treatment can hardly cause weight loss of CASC.
Reference: [1]  [10] This paper pays close attention on the plasma modification, so only the C1s deconvoluted in Figure 6 is not enough, the N1 and O1 should also be done as well. one more thing, XPS analysis should have allowed a quantitative assignment of each kind of nitrogen/carbon/oxygen instead of just a qualitative approach. Response: According to your helpful suggestion, we present the quantitative assignment of O1s and N 1s as shown below ( Fig. 3 and Table 1). As shown in Fig. 3(ab), the O 1s spectrum of CSAC and HCSAC can be resolved into three individual peaks based on the previous literature (531.3 eV, carbonyl and/or quinone; 532.7 eV, hydroxyl and/or ether; 534.0 eV, chemisorbed oxygen and/or water) [1] . As shown in Fig. 3(c-d), the N1s spectrum can be fitted by three peaks (398.2 eV, pyridinic nitrogen; 399.9 eV, pyrrolic nitrogen; 400.9 eV, quaternary nitrogen) [2] . The nitrogen content of HCSAC show slightly changes in comparison with CSAC. Table 1 presents the composition of the oxygen and nitrogen chemical functional groups of CSAC and HCSAC. The obvious increment of carbonyl and carboxyl can be ascribed to the ions excited from the H2O molecule during H2O plasma modification. The results from the deconvoluted C1s spectrum is more significant in this manuscript than that from O1s and N1s. However, according to your suggestion, we supply the results of deconvoluted O1s and N1s in supporting information.  [11] Did the authors study the reusability for electrochemical properties? It is important to cost down the process. Response: After taking your helpful suggestion into account, we supply the life cycles of HCSAC electrode at different current density for 1000 cycles, shown in Fig. 4. The capacitance retention of CSAC modified by H2O plasma (HSCAC) electrodes maintain over 97% in the 6.0 M KOH electrolyte. The high capacitance retention indicates that ideal interconnected porous structure generating high ion transmission efficiency in HCSAC. Also, we supply this figure in supporting information. [12] It is necessary to compare the results with some recent literatures.
Response: According to your helpful suggestion, we provide the electrochemical properties (specific capacitance and rate capability (capacitance retention at the current density from 0.5 A g -1 to 10 A g -1 )) of activated carbon in some recent literatures (shown in Table 2). The CSAC utilized in our experiment exhibits low specific capacitance can be ascribed to commercial activated carbon is generally physically activated by steam or carbon dioxide. The biomass derived carbon materials prepared from harsh experimental environment commonly adopt chemical activation (KOH activation adopted in the references below), listed in Table 2 [1][2][3][4] . The rate capability of CSAC was improved from 76.7% to 84.6% and an increment of 64.8% in specific capacitance after H2O plasma modification. Furthermore, we are capable of applying this H2O plasma modification in other activated carbon electrode materials to improve specific capacitance and rate capability. Furthermore, the nitric acid treatment introducing oxygen containing functional group to activated carbon need long heating duration, extra energy consumption and utilization of chemical reagents [5,6] . In comparison to the reported modifying method, the H2O plasma modification used in our experiment can improve the electrochemical properties of CSAC with a short time (100 s) and pollution-free media (H2O). [13] Deeper discussion of the results should be provided.
Response: Based on your significant suggestion, we supply the three-dimensional schematic model of the functional groups of HCSAC (shown in Fig. 5). The oxygen functional groups and nitrogen functional groups are presented in the figure below. The additional pseudo-capacitance is attributed to oxygen functional groups. Previous studies demonstrate that some oxygen functional groups can directly participate in faradaic reactions not only in acidic medium [1] but alkaline medium [2] . The increased oxygen functional groups in HCSAC play an important role in the enhancement of capacitance via reversible redox actions in 6 M KOH electrolyte. The inductive effects of oxygen containing functional groups' bonds structure is capable of causing the electrons redistribution and some bonds polarization. The electric potential induced redox reactions of polarized sites proceed through the simultaneously reversible gaining/losing of electrons and adsorption/desorption of protons, respectively [3,4] . This deeper discussion has been added to the 'Result and discussion' section. The supporting references are listed below.
Reference:  [4] Fig. 7d (Fig. 6 in manuscript) is not mentioned. Looks like it is the capacitance performance test at different current density, however, more cycles, e.g. 1000 cycles for each current density, should be provided. Otherwise, it is not sufficient to prove the good performance at different current densities.
Response: According to your kind remind, we realize that Fig. 7d (Fig. 6 in manuscript) has been analyzed, but we forget to mention this figure number. we add the figure number 'As shown in Fig. 7d' before explanation. After considering your significant suggestions, we add the life cycles of HCSAC electrode at different current density (from 0.5 A g -1 to 10 A g -1 ) for 1000 cycles, shown in Fig. 1. The capacitance retention of CSAC modified by H2O plasma (HSCAC) electrode maintains over 97% in the 6.0 M KOH electrolyte. The high capacitance retention indicates that ideal interconnected porous structure generating high ion transmission efficiency in HCSAC. Also, we supply this figure in supporting information. Fig. 1 Life cycles of HCSAC electrode at different current density for 1000 cycles.
[5] In electrochemical properties section, the explanation of improved capacitance performance is concerned. It is claimed by authors that the improvement is attributed to pseudo-capacitance owing of oxygen functional groups on carbon surface (line 22-23, P4) besides double layer capacitance. How did the pseudo capacitance of oxygen functional groups work? What is the possible working mechanism? And any supporting references? Response: Based on your significant suggestions, we provide the possible working mechanism of oxygen functional groups in the manuscript. The additional pseudocapacitance is attributed to oxygen functional groups. Previous studies demonstrate that some oxygen functional groups can participate directly in faradaic reactions not only in acidic medium [1] but alkaline medium [2] . The increased oxygen functional groups in HCSAC play an important role in the enhancement of capacitance via reversible redox actions in 6 M KOH electrode. The inductive effects of oxygen containing functional groups' bonds structure is capable of causing the electrons redistribution and some bonds polarization. In 6 M KOH electrolyte, the electric potential induced redox reactions of polarized sites proceed through the simultaneously reversible gaining/losing of electrons and adsorption/desorption of protons, respectively [3,4] . This deeper discussion has been added to the 'Result and discussion' section. The supporting references are listed below.
Page 4, line 38 ---'The additional pseudo-capacitance is attributed to oxygen functional groups. Previous studies demonstrate that some oxygen functional groups can directly participate in faradaic reactions not only in acidic medium but alkaline medium. The increased oxygen functional groups play an important role in the enhancement of capacitance via reversible redox actions in alkaline medium. The inductive effects of oxygen containing functional groups' bonds structure is capable of causing the electrons redistribution and some bonds polarization. In 6 M KOH electrolyte, the electric potential induced redox reactions of polarized sites proceed through the simultaneous reversible gaining/losing of electrons and adsorption/desorption of protons, respectively'.
Reference: [1]  [6] Contact angel should be measured to support the claimed improved hydrophilicity and wettability after plasma treatment; Response: Based on your helpful suggestions, we try to carry on the contact angle measurement. We have failed in pressing the powder into full tablet and thus the test results are not convincing. However, we have found supporting references to support the claim that hydrophilicity and wettability are improved after plasma modification. The oxygen functional groups can provide an extra pseudo-capacitance through revisable redox reaction and improved wettability between the electrodes and electrolytes [1][2][3] . The presence of surface oxygen functional groups is advantageous for improving the hydrophilicity of carbon material which would increase the surface area accessible to aqueous electrolyte [4][5][6] .
[7] How is the plasma working condition come up, e.  (Fig. 2). The cyclic voltammetry (CV) plots at the scan rate of 10 mV s -1 of CSAC and CSAC modified with H2O plasma are shown in Fig. 2(a-b). The integrated area of the sample D-100 W-100 s (D means DBD plasma modification, the first number is modification power, the second number is modification time) is larger than other sample in the same power, indicating that 100 s is a more suitable modification time. With modification power changing from 50 W to 200 W, the sample D-160 W-100 s exhibits the largest integrate area among all the samples in the same modification time of 100 s. The largest integrate area of D-160 W-100 s indicates that the sample shows the larger specific capacitance than other samples. Furthermore, the galvanostatic charge-discharge (GCD) curves of CSAC and CSAC modified with H2O plasma at the current density of 1 A g -1 are shown in Fig. 2(c-d).
After calculation by the formula below, we come up the conclusion that D-160 W-100 s exhibits the excellent specific capacitance among all the samples, which is consistent with CV curves. The Nyquist plots of CSAC and CSAC modified with different power exhibit similar shapes, shown in Fig. 2e. The internal resistance (Ri) of samples were below 0.4 Ω, demonstrating a good electrical conductivity of materials. The increased Ri of the sample D-160 W-100 s can be ascribed to the presence of surface oxides and thus increase the ohmic resistance along the axial direction of micropores. Also, the smaller diameter in the semicircle of the sample D-160 W-100 s illustrates that the lower charge transfer resistance (Rct) at electrolyte interfaces, owing to the higher hydrophilicity after oxygen functional groups introduced. Fig. 2f demonstrates the rate capability (capacitance retention from the current density of 0.5 A g -1 to 10 A g -1 ) of CSAC and CSAC modified under different power. The higher rate capability indicates that the sample is capable of maintaining excellent charge and discharge characteristics at high current density. In summary, we perform other detailed characterization of CSAC modified by H2O plasma under modification power of 160 W and time of 100 s (HCSAC) based on the electrochemical performance. We keep the pressure of the reactor chamber to 30 KPa, mainly aiming to pump water into the reactor chamber instead of maintaining the vacuum environment. Based on the above results, we select the conditions (discharge power, 160 W; treating time, 100 s; the pressure, 30 KPa) to make detailed analysis. We supply the result in supporting information.
The specific capacitance of the electrodes (C), was calculated by the following equation: Where C is specific capacitance (F g -1 ), I is the constant charge-discharge current (A), ∆t is the discharge time (s), ∆V is the total change in voltage (V), m is the mass of the active material in an electrode (g) Fig. 2 (a-b) CV curves of CSAC and CSAC modified with H 2 O plasma in a three-system with 6 M KOH aqueous electrolyte at the scan rate of 10 mV s -1 . (c-d) GCD curves of CSAC and CSAC modified with H 2 O plasma in a three-electrode system with 6 M KOH aqueous electrolyte at the current density of 1 A g -1 . (e) Nyquist plots of CSAC and CSAC modified at different power electrodes. The inset is the detail with enlarged scale (f) Rate capability of CSAC and HCSAC modified at different power at the current density from 0.5 to 10 A g -1 .
[8] In the XPS analysis result (Table 2), why is the atomic composition of N increasing? Response: The definite parameters of our experiment are determined by a serious of preliminary experiments. The results shown in this article were obtained by one experiment. According to your helpful suggestion, we consider that the increasing nitrogen content may be ascribed to experimental error. We perform X-ray photoelectron spectroscopy (XPS) test to CSAC and HCSAC for five times, respectively. Afterwards, we delete smallest and largest values and take the average of the three times results. The nitrogen atomic composition is listed below (Table 1), the average value of CSAC and HCSAC is 1.04% and 1.58%, respectively (the value in our manuscript of CSAC and HCSAC is 1.6% and 2.8%, respectively). The nitrogen content is much lower when compared with oxygen and carbon content. So, nitrogen play a relatively minor role in this experiment.

Responses to reviewer's comments
We appreciate the detailed and helpful comments and suggestions which are very helpful to improve the quality of our manuscript. The manuscript (RSOS-180872.R2) has been carefully revised as required. All new and revised contents are highlighted with yellow in our revised version. We hope that the correction will meet with your approval. The point-by-point answers to the comments and suggestion were listed below. Response: According to your helpful suggestion, we supplied the part of supplementary figures, discussions and references into our revised manuscript. For example, the quantitative assignment of O1s and N 1s, the three-dimensional schematic model of the functional groups of HCSAC (commercial coconut shell-based activated carbon modified with H2O plasma) and the life cycles of HCSAC electrode at different current density for 1000 cycles were supplied in the revised manuscript, shown in Fig.1, Fig.   2, Fig. 3 and Table below. (Listed as Fig. 6, Fig. 7, Fig. 9 and Table 2  cycles to prove the reusability of HCSAC can be seen in Fig. 9. The capacitance retention of CSAC…'.     Response: Based on your significant suggestion, we re-weighed the mass of samples before and after treatment.

Reviewer
The weight loss rate of sample (W), was calculated by the following equation:

= −
Where is the weight of the whole reactor chamber including the sample before plasma treatment (g), is the weight of the whole reactor chamber including the sample after plasma treatment (g), is the weight of the sample.
After weighing for three times, the average of was 138.4310 g, the average of was 138.4309 g, the weight of sample is 0.5 g, the difference between and is too small and beyond their accuracy. The results indicate that 160 W can hardly cause weight loss of activated carbon.